Autophagy has been emerging as a novel cytoprotective mechanism to increase tumor cell survival under conditions of metabolic stress and hypoxia as well as to escape chemotherapy-induced cell death. To elucidate whether autophagy might also protect cancer cells from the growth inhibitory effects of targeted therapies, we evaluated the autophagic status of preclinical breast cancer models exhibiting auto-acquired resistance to the anti-HER2 monoclonal antibody trastuzumab (Tzb). We first examined the basal autophagic levels in Tzb-naive SKBR3 cells and in two pools of Tzb-conditioned SKBR3 cells (TzbR), which optimally grow in the presence of Tzb doses as high as 200 µg/ml Tzb. Fluorescence microscopic analyses revealed that the number of punctate LC3 structures -a hallmark of autophagy- was drastically higher in Tzb-refractory cells than in Tzb-sensitive SKBR3 parental cells. Immunoblotting analyses confirmed that the lipidation product of the autophagic conversion of LC3 was accumulated to high levels in TzbR cells. High levels of the LC3 lipidated form in Tzb-refractory cells were accompanied by decreased p62/sequestosome-1 protein expression, a phenomenon characterizing the occurrence of increased autophagic flux. Moreover, increased autophagy was actively used to survive Tzb therapy as TzbR pools were exquisitely sensitive to chemical inhibitors of autophagosomal formation/function. Knockdown of LC3 expression via siRNA similarly resulted in reduced TzbR cell proliferation and supra-additively interacted with Tzb to re-sensitize TzbR cells. Sub-groups of Tzb-naive SKBR3 parental cells accumulated LC3 punctate structures and decreased p62 expression after treatment with high-dose Tzb, likely promoting their own resistance. This is the first report showing that HER2-overexpressing breast cancer cells chronically exposed to Tzb exhibit a bona fide up-regulation of the autophagic activity that efficiently works to protect breast cancer cells from the growth-inhibitory effects of Tzb. Therapeutic targeting autophagosome formation/function might represent a novel molecular avenue to reduce the emergence of Tzb resistance in HER2-dependent breast carcinomas.
Citation: Vazquez-Martin A, Oliveras-Ferraros C, Menendez JA (2009) Autophagy Facilitates the Development of Breast Cancer Resistance to the Anti-HER2 Monoclonal Antibody Trastuzumab. PLoS ONE 4(7): e6251. https://doi.org/10.1371/journal.pone.0006251
Editor: Joseph Najbauer, City of Hope Medical Center, United States of America
Received: April 11, 2009; Accepted: June 17, 2009; Published: July 16, 2009
Copyright: © 2009 Vazquez-Martin et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Alejandro Vazquez-Martin is the recipient of a “Sara Borrell” post-doctoral contract (CD08/00283, Ministerio de Sanidad y Consumo, Fondo de Investigación Sanitaria -FIS-, Spain). Javier A. Menendez is the recipient of a Basic, Clinical and Translational Research Award (BCTR0600894) from the Susan G. Komen Breast Cancer Foundation (Texas, USA). This work was supported in part by Instituto de Salud Carlos III (Ministerio de Sanidad y Consumo, Fondo de Investigación Sanitaria -FIS-, Spain, Grants CP05-00090, PI06-0778 and RD06-0020-0028 to Javier A. Menendez). Javier A. Menendez was also supported by a Grant from the Fundación Científica de la Asociación Española Contra el Cáncer (AECC, Spain). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Significant amount of research has been dedicated to elucidate molecular mechanisms that could explain de novo and acquired resistance to the anti-HER2 monoclonal antibody trastuzumab (Tzb; Herceptin®), the first immunotherapeutic drug for the successful treatment of breast carcinomas overexpressing the HER2 (erbB-2) oncogene –. Proposed mechanisms for innate or acquired resistance to Tzb include steric inhibition of Tzb binding to the extracellular domain (ECD) of the HER2 tyrosine kinase receptor imposed by other extracellular factors such as the glycoprotein mucin 4 (MUC-4) , , molecular changes in the target receptor itself (e.g. HER2 mutations –; accumulation of a proteolyzed HER2 fragment –p95HER2- lacking the extracellular Tzb binding epitope but retaining ligand-independent TK activity –), and cross-talk with other transduction cascades such as the insulin growth factor (IGF)-1, estrogen receptor (ER) and vascular endothelial growth factor (VEGF) pathways that could compensate for attenuated HER2 signaling , –. Alterations in the regulation of HER2 downstream signaling components, including sub-cellular localization of the cyclin-dependent kinase (CDK)-inhibiting protein p27Kip1 and independent attenuation of PI-3′K/AKT/mTOR-mediated apoptosis through downregulation of the phosphatase and tensin homolog (PTEN) tumor suppressor have also been implicated as potential sources of resistance to HER2-targeted therapies –. Increased activation of PI-3′K and its downstream effector AKT has also been associated with Tzb resistance in HER2-dependent breast carcinoma cells –.
To date, the survival pathway of macroautophagy (also referred as autophagy) has not been implicated in Tzb resistance. Autophagy –lysosomal degradation, or eating (phagy), of part of the cell's self (auto)- is a catabolic process of organelle digestion that generates ATP during periods on nutrient limitation –. Autophagy optimizes nutrient utilization in rapidly growing cells when faced with hypoxic or metabolic stresses and, hence it contributes to normal and cancer cell survival. During autophagy, macroautophagosomes (also referred as autophagosomes) are formed as double membrane-bound vesicles which engulf cytoplasm and/or cytoplasmic organelles. Then, autophagosomes fuse with lysosomes to degrade the contents of the autophagic vesicle and provide essential building blocks, such as amino acids back to cell. Because this mechanism may be decreased in tumor cells compared with normal cells, initial studies appreciated autophagy as a tumor suppressor mechanism , . Indeed, cancer cells may undergo autophagic cell death (APCD; also referred to as active cell death II [ACDII]) following extreme autophagic degradation associated with exposure to several cancer therapies . However, the autophagic response can also function as a protective mechanism allowing the recycling of proteins and cellular components to survive cell injuries induced by cytotoxic agents. Although it is well established that autophagy can protect cancer cells against various stressors, including chemotherapeutics –, it remains largely unknown whether “protective autophagy” might also defend cancer cells from the growth inhibitory effects of targeted therapies such as monoclonal antibodies, tyrosine kinase inhibitors, etc.
In this study, we investigated the autophagic status in preclinical breast cancer models exhibiting auto-acquired resistance to Tzb that were obtained by continuously growing Tzb-sensitive HER2-overexpressing SKBR3 breast cancer cells in the presence of clinically relevant concentrations of Tzb for more than 10 months. We followed complementary criteria to accurately monitor autophagy in SKBR3-derived Tzb-refractory cells . First, we monitored the level of intermediary structures of the autophagic pathway, e.g. the level of endogenous LC3-II/LC3-I (-microtubule-associated protein 1 light chain 3 beta- a specific and sensitive autophagosome marker extensively used to monitor autophagic activity) and the level of p62 protein (also known as SQSTM1 [sequestosome-1]), which serves as a link between LC3 and ubiquitinated substrates destined for autophagic degradation –. Second, to unambiguously establish the pro-survival role of an increased catabolic flux through the autophagic pathway was critical to the development of acquired Tzb resistance, we assessed how chemical autophagy inhibitors or siRNA-induced knockdown of LC3 altered cell proliferation in TzbR cells. Under experimental conditions described here, we report for the first time that induction of autophagy is closely related to the cell survival system acquired by HER2-overexpressing breast cancer cells chronically exposed to Tzb.
We established Tzb-resistant HER2-positive breast cancer cells by exposing Tzb-naive SKBR3 parental cells to incremental increases of Tzb. Tzb-resistance selection continued until the SKBR3 cell population could sustain cell viability and proliferate when challenged with 200 µg/ml Tzb. Under these experimental conditions, two pools of Tzb-refractory cells (TzbR POOL1 and TzbR POOL2) were obtained upon exposure of SKBR3 parental cells for a minimum of 10 months before starting any experimental procedure. We confirmed resistance to Tzb by performing MTT-based cell viability assays. When the concentrations of Tzb needed to decrease optical density by 50% were calculated from the percentage of viable cells after exposure to graded concentrations of Tzb, the Inhibitory Concentration 50 (IC50) value for SKBR3 parental cells was as low as 2 µg/ml Tzb. Treatment with Tzb at concentrations as high as 200 µg/ml Tzb likewise failed to significantly decrease cell viability in TzbR POOLs (Figure 1, left panel). TzbR POOLs exhibited HER2 expression levels comparable or slightly higher than those naturally occurring in SKBR3 parental cells (data not shown).
Left. The metabolic status of Tzb-naive SKBR3 parental cells and Tzb-refractory TzbR POOLs treated with graded concentrations of Tzb was evaluated using a MTT-based cell viability assays and constructing dose-response graphs as % of untreated cells (dashed line = 100% cell viability). Results are means (circles) and 95% confidence intervals (bars) of three independent experiments performed in triplicate. Treatment with Tzb concentrations as high as 100 µg/mL Tzb failed to significantly decrease cell viability in TzbR POOLs whereas a 10 times lower concentration of Tzb drastically reduced cell viability (>50%) in SKBR3 parental cells. Right: Representative microphotographs of untreated and experimental cell cultures following 72 hours treatment with different concentrations of Tzb, as specified. Examination of microphotographs reveals the smaller size of Tzb-refractory TzbR POOLs when compared to Tzb-naive SKBR3 parental morphology. Scale bar = 10 µm.
Tzb-refractory Cells Exhibit Increased Autophagosome Formation
During Tzb selection of the TzbR POOLs, light microscopy examination consistently showed an increase in the number of dark, cytosolic granules in surviving, smaller cells (Figure 1, right panel). Because these granules were not related to senescent granules as confirmed by β-galactosidase staining (data not shown), we sought to determine if they may correspond to increased cytosolic levels of autophagosomes. Immunoblotting assessment of LC3 expression is an easy method to predict autophagic activity of mammalian cells because the amount of LC3-II –i.e. the product of the autophagic posttranslational modification of LC3- correlates with the number of autophagosomes –. The product of this autophagic conversion of LC3, LC3-II, tightly associates with the autophagosome membrane and migrates faster than LC3-I on SDS-PAGE. Therefore, LC3 immunoblotting may detect two bands: LC3-I with an apparent mobility of 18 kDa and LC3-II (16 kDa). When we utilized this property of LC3 to initially monitor changes in the dynamics of the autophagic process in Tzb-sensitive and in Tzb-refractory cancer cells, both the total amount and, particularly, the lipidation status of LC3 (i.e. LC3-I is converted by lipidation to the phosphatidyl-ethanolamine conjugated form LC3-II) were drastically up-regulated in Tzb-unresponsive TzbR POOLs whereas LC3-I/LC3-II proteins were hardly detectable in Tzb-naive SKBR3 parental cells (Figure 2). To further confirm that autophagosome formation was increased in Tzb-refractory cells, both the expression and the sub-cellular compartmentalization of LC3 in individual cells and in whole cell cultures were monitored by indirect immunofluorescence using an automated confocal-imaging approach. Untreated SKBR3 parental cells showed a homogenous but weak cytoplasmic staining of LC3, consistent with the distribution of LC3-I and typical of low-level or no autophagosome formation. Consistent with the dramatic increase in LC3 processing assessed by immunoblotting procedures, LC3 localization dramatically changed from diffuse to punctate or dotted pattern in Tzb-refractory TzbR POOLs (Figure 2). High content-imaging of whole cell populations growing in individual wells (captured as 4×4 montages) clearly revealed that >90% of TzbR cells exhibited an intense punctate LC3 fluorescence (the medium number of autophagosomes per cell was ∼50) whereas a significantly lower percentage of SKBR3 parental cells (∼5%) revealed LC3-containing vesicles (Figure 3). Video confocal microscopy also revealed a robust accumulation of endogenous LC3 in the cytosol of Tzb-refractory cells was, consistent with the distribution of LC3-II-positive bodies representing isolation membranes and autophagosomes (Supplemental Video S1 –SKBR3-, Supplemental Video S2 –TzbR POOL1- and Supplemental Video S3 –TzbR POOL2-). Importantly, this high level of autophagosomes accompanied with healthy-appearing nuclei and high proliferation rates in Tzb-refractory cells.
Immunoblotting. Autophagosome formation in whole cell lysates of Tzb-naive SKBR3 parental cells and Tzb-refractory TzbR POOLs was detected with Western blot analysis using a LC3 antibody. Top band (18 kDa) represents LC3-I. Bottom band (16 kDa) represents LC3-II, a typical marker of autophagosomes. Autophagosome formation is robust in Tzb-refractory TzbR cells when compared to low to undetectable levels of LC3-I/LC3-II expression in Tzb-sensitive SKBR3 parental cells. Figure shows a representative immunoblotting analysis. Equivalent results were obtained in three independent experiments. Immunofluorescence. After fixation and permeabilization, cellular distribution of autophagosome marker LC3 was assessed following staining with a LC3 antibody and Hoechst 33258 for nuclear counterstaining. SKBR3 parental cells display a homogenous but weak cytoplasmic LC3 staining, which is typical of absent or low-level autophagosome formation. TzbR POOLs show a marked contrast enhancement in the punctated pattern of endogenous LC3 expression, which is characteristic of autophagosome formation. Of note, Tzb exposure further increases autophagosome LC3 pattern. Images show representative portions of SKBR3 and TzbR cell cultures captured in different channels for LC3 (red) and Hoechst 33258 (blue) with a 20× objective, and merged on BD Pathway™ 855 Bioimager System using BD Attovision™ software. Scale bar = 25 µm.
Top panels. SKBR3 parental cells and SKBR3-derived Tzb-refractory POOLs were plated in 24-well plates at a density of 10,000 cells/well and cultured with regular medium in the absence or presence of Tzb (10 µg/mL Tzb in SKBR3 parental cells; 100 µg/mL Tzb in TzbR POOLs). The data presented are mean of number cells ×105/well (circles) and 95% confidence intervals (bars) of three independent experiments made in duplicate obtained after 0, 3, 6 and 9 days. Statistically significant differences (one-factor ANOVA analysis) between experimental conditions and unsupplemented control cells are shown. All statistical tests were two-sided. No statistically significant differences were observed in the number of TzbR cells growing in the presence of 100 µg/mL Tzb up to 9 days whereas 10 µg/mL Tzb significantly reduced cell proliferation in SKBR3 parental cells as early as 3 days after Tzb exposure. Slope of the growth curves clearly denotes a faster proliferation of TzbR POOLs regardless Tzb exposure. Bottom panels. Images show representative whole populations of SKBR3 cells and TzbR cells growing in individual wells that were captured using different channels for LC3 (red) and Hoechst 33258 (blue) as a 4×4 montage with a 20× objective on BD Pathway™ 855 Bioimager System, and merged using BD Attovision™ software. As discussed in Figure 2, TzbR POOLs notably exhibit a marked contrast enhancement in the punctated pattern of endogenous LC3 expression, which is typical for autophagosomal formation. Scale bar = 200 µm.
Tzb-refractory Cells Exhibit Increased Basal Autophagy
Since the lipidation status of LC3 has been proposed to accurately reflect autophagic activity, our findings revealing an increase in LC3-II expression as assessed by immunoblotting along with an increase in LC3 fluorescent puncta, strongly suggested that augmented autophagosome formation due to increases in autophagic activity may represent a previously unrecognized pro-survival pathway underlying acquired auto-resistance to Tzb. However, autophagy is a dynamic, multi-step process that can be modulated at several steps, both positively and negatively. Therefore, the striking accumulation of autophagosomes as measured by LC3 lipidation on a western blot or fluorescent LC3 dots could also reflect a reduced turnover of autophagosomes in TzbR cells. To determine whether the increase in autophagosome-related LC3 was due to an increase in autophagosome formation or was a secondary consequence of lysosome dysfunction preventing autophagosome clearance in TzbR cells, we initially performed indirect immunofluorescence staining for the lysosomal marker lysosomal-associated membrane protein-1 (LAMP-1). We found that lysosome abundance was increased relative in each Tzb-refractory pool relative to Tzb-naive SKBR3 parental cells (data not shown), thus suggesting that diminished lysosomal function did not contribute to autophagosome accumulation in Tzb-resistant SKBR3 pools. Although these data were consistent with an increase in flux through autophagic clearance pathways, we took additionally two complementary experimental strategies -namely measurement of sequestosome 1 (SQSTM1/p62) protein expression and pharmacological inhibition of autophagy- to further distinguish whether increases in the level of phosphatidylethanolamine-modified LC3 (LC3-II) reflected the induction of autophagy or inhibition of autophagosome clearance upon development of Tzb resistance.
When high levels of LC3 lipidated form associate with impairment in autophagosome maturation, this phenomenon is accompanied by a marked increase in the level of p62. Conversely, increased LC3-II levels together with a reduction of p62 protein levels characterize the occurrence of a real autophagic flux increase , –. The latter scenario was observed in Tzb-refractory TzbR POOLs when compared to Tzb-naive SKBR3 parental cells, thus suggesting that acquired Tzb auto-resistance of Tzb-conditioned cells is accompanied by increased autophagy flux. As shown in Figure 4, immunoblotting procedures detected a slight but significant reduction in the total p62 protein content in TzbR-derived whole cell lysates when compared to p62 protein expression status in Tzb-naive SKBR3 parental cells. High content-imaging of whole cell populations growing in individual wells (captured as 4×4 montages) further confirmed that >80% of TzbR cells exhibited a significant decrease in content of p62 per individual cell when compared to Tzb-naive SKBR3 parental cells. Because p62/SQSTM1 is a selective substrate of autophagy and, hence, monitoring endogenous p62 protein levels indirectly measures autophagic degradation, we concurrently monitored how acquisition of breast cancer auto-resistance to Tzb affected cellular distribution of both p62 and LC3 (Figure 5). As steady state, the p62 level in Tzb-naive SKBR3 parental cells was much higher than in Tzb-refractory SKBR3 TzbR POOLs. This significant decrease in p62 content per individual cell along with the formation of some punctate vesicular sub-cellular distributions of p62 (p62 bodies) was accompanied by the accumulation of intense punctate LC3 fluorescence in TzbR cells. When comparing overlapping fluorescence of LC3 and p62 -which may be viewed as a good indicative of p62 presenting ubiquitinated protein bodies to the autophagic machinery via LC3- it become obvious that Tzb-refractory cells exhibit an exacerbated autophagic clearance compared to Tzb-sensitive SKBR3 parental cells (Figure 5).
Immunoblotting. Autophagic degradation in whole cell lysates of Tzb-naive SKBR3 parental cells and Tzb-refractory TzbR POOLs was detected with Western blot analysis using a p62 antibody. Immunoblotting bands (64 kDa) represent p62/SQSTM1, a selective substrate of autophagy. Autophagic degradation (i.e. down-regulation of endogenous p62 protein expression) is significantly increased in Tzb-refractory TzbR cells when compared to Tzb-sensitive SKBR3 parental cells. Figure shows a representative immunoblotting analysis. Equivalent results were obtained in three independent experiments. Immunofluorescence. After fixation and permeabilization, cellular distribution of p62 was assessed following staining with a p62 antibody and Hoechst 33258 for nuclear counterstaining. SKBR3 parental cells display a homogenous and strong cytoplasmic p62 staining, which is typical of absent or low-level of autophagic degradation. TzbR POOLs show a significant decrease in the cytoplasmic distribution of p62 that appears somewhat vesiculated, which is typical of enhanced autophagic degradation. Images show representative whole populations of SKBR3 and TzbR cells growing in individual wells that were captured using different channels for p62 (green) and Hoechst 33258 (blue) as a 4×4 montage with a 20× objective on BD Pathway™ 855 Bioimager System, and merged using BD Attovision™ software. Scale bar = 200 µm.
Since increased LC3-II levels together with a reduction of p62 protein levels characterize the occurrence of increased autophagic flux, SKBR3 parental cells and TzbR cells (untreated and treated with 100 µg/mL Tzb) were triple stained with antibodies against LC3 and p62 and with Hoechst 33258 for nuclear counterstaining. Tzb-refractory cells exhibit an exacerbated autophagic clearance compared to Tzb-sensitive SKBR3 parental cells when considering overlapping fluorescence of LC3 and p62 as an indirect marker of p62 presenting ubiquitinated protein bodies to the autophagic machinery in a LC3-dependent manner. Images show representative portions of SKBR3 and TzbR cell cultures that were captured using different channels for LC3 (red), p62 (green) and Hoechst 33258 (blue) with a 20× objective and merged on BD Pathway™ 855 Bioimager System using BD Attovision™ software. Scale bar = 25 µm.
Blockade of Macroautophagosome Formation/Function Enhances Tzb Efficacy in Tzb-refractory Cells
To pharmacologically evaluate whether Tzb-refractory cells actively used increased basal autophagy to survive Tzb therapy, we finally assessed the growth inhibitory effects of autophagy inhibitors . Inhibition of the formation of pre-autophagosomal structure upon treatment with 3-methyladenine (3-MA) notably reduced cell viability in Tzb-refractory TzbR POOLs but not in Tzb-naive SKBR3 parental cells (Figure 6). Since 3-MA treatment may induce off-target effects, we further confirmed that 3-MA treatment was efficient at reducing the number of LC3-positive autophagosomes in TzbR cells. Representative immuno-confocal images of Tzb-refractory TzbR POOLs cultured in the absence or presence of 3-MA are shown in Figure 6. Cultures of TzbR cells treated with 3-MA likewise showed a significant increase in the number of cells containing <20 autophagosomes per cell. These findings, altogether, strongly suggest that increased macroautophagy actively provides a survival function to Tzb-refractory cells. This notion was further supported when similar studies were carried out in the presence of 2-(4-morpholinyl)-8-phenylchromone (LY294002). In terms of cell viability, Tzb-refractory cells were exquisitely sensitive to this agent that blocks phosphatidylinositol 3-kinase activity and prevents autophagic sequestration when compared to SKBR3 parental cells (Figure 6). Bafilomycin A1-prevented fusion of the autophagosomes and lysosome, and was highly cytotoxic to TzbR cells. Although less markedly, cell viability of Tzb-naive SKBR3 parental cells was also significantly reduced in the presence of bafilomycin A1 (Figure 6).
The metabolic status of SKBR3 parental cells and Tzb-refractory TzbR POOLs treated with autophagy inhibitors including 3-MA, LY294002 and bafilomycin A1 was evaluated using MTT-based cell viability assays and constructing dose-response graphs as % of untreated cells (dashed line = 100% cell viability). Results are means (columns) and 95% confidence intervals (bars) of three independent experiments made in triplicate. Figure panels display results obtained with representative doses of each autophagy inhibitor, as specified. Pharmacologically-induced loss of autophagosome formation/function is highly cytotoxic to TzbR cells (A, B) compared to cell viability effects in Tzb-naive SKBR3 parental cells (C). Statistically significant differences (one-factor ANOVA analysis) between experimental conditions (i.e. treatment with autophagy inhibitors) and unsupplemented control cells are shown. All statistical tests were two-sided. To confirm that 3-MA-induced changes in cell viability related to 3-MA-induced changes in the number of autophagosomes in TzbR cells, cellular distribution of autophagosome marker LC3 was assessed following 72 hours exposure to 5 mM 3-MA. The total number of LC3-positive autophagosomes per cell is notably decreased in 3-MA-treated TzbR cells. Images show representative portions of SKBR3 and TzbR cell cultures captured in different channels for LC3 (red) and Hoechst 33258 (blue) with a 20× objective, and merged on BD Pathway™ 855 Bioimager System using BD Attovision™ software. Scale bar = 25 µm.
To provide additional evidence that autophagy plays a critical survival role in enabling Tzb-insensitive high-rates of cell proliferation in Tzb-refractory cells and to avoid any off-target side effects that may confound interpretation of the results obtained with autophagy inhibitors, we used the potent and highly sequence-specific mechanism of RNA interference (RNAi) to block LC3-dependent autophagosome formation. Tzb-refractory TzbR POOLs transiently transfected with sequence-specific double-stranded RNA oligonucleotides targeting Atg8/LC3 gene demonstrated hypersensitivity to high-dose Tzb. Following sequential exposure to LC3 siRNA and 200 µg/ml Tzb, TzbR cells were extremely fragile and many of the cells died before the 216-hours harvest (data not shown), thus precluding analyses of the nature of interaction. Interestingly, supra-additive (synergistic) growth inhibitory interactions occurred at late time points (i.e. 144 and 216 hours) when RNAi-induced knock-down of LC3 was followed by exposure to 10 µg/ml Tzb, an ineffective low-dose of Tzb when used as single agent in TzbR cells (Figure 7). These findings, altogether, clearly establish that hyperactivation of basal autophagy plays an essential survival role in Tzb-refractory TzbR cells re-challenged with Tzb.
Tzb-refractory TzbR POOLs were mock transfected, transfected with a non-specific control siRNA Pool (negative control) or transfected with siRNA-targeting LC3. 72 h after transfection, one set of mock-, non-specific negative control-, and RNAi LC3-transfected cells were used for immunoblotting analyses of LC3 expression. A second set of cells were harvested, re-cultured in 24-well plates at a density of 10,000 cells/well and treated with regular medium in the absence or presence of Tzb (1 µg/mL Tzb in SKBR3 parental cells and 10 µg/mL Tzb in TzbR POOLs). There were no significant changes in cell numbers when TzbR cells were treated with Tzb as single agent (omitted). siRNA-induced blockade of LC3 expression significantly reduces cell proliferation rates in TzbR POOLs. More importantly, supra-additive growth inhibitory interactions occur in LC3-depleted Tzb-treated TzbR POOLs. The data presented are mean of number cells ×105/well (circles) and 95% confidence intervals (bars) of three independent experiments made in duplicate after 3, 6 and 9 days. Statistically significant differences (one-factor ANOVA analysis) between experimental conditions (i.e. LC3 siRNA±Tzb) and control cells (i.e. siRNA [-]) are shown. All statistical tests were two-sided.
Activation of Autophagy Might Protect HER2-overexpressing Breast Cancer Cells Against Tzb
We finally speculated that macroautophagosome formation/function may contribute to cell survival of Tzb-naive HER2-positive breast cancer cells challenged to Tzb at first, thus limiting its activity and promoting further resistance. Of note, few groups of SKBR3 parental cells were found to accumulate some delicate LC3 punctate structures following treatment with low-dose Tzb (1 µg/ml Tzb). This increase in the number of autophagosomes per cell was significantly higher in isolated “SKBR3 clones” capable to survive 72 hours treatment with high-dose (100 and 200 µg/ml) Tzb (Figure 8). In line with this, we found that p62 expression was concomitantly decreased after Tzb treatment in this small fraction of SKBR3 cells. Tzb-promoted dynamics of autophagosomal formation (i.e. punctate pattern of LC3) and increased autophagic flux (i.e. p62 degradation) were confirmed with Western blot analyses (Figure 8). Similar to our findings when employing SKBR3 cells, autophagic activity was significantly elevated in Tzb-naive HER2-dependent BT474 breast carcinoma cells, whereas Tzb treatment failed to modulate autophagosome-related LC3 expression, lysosomal function and p62 expression in HER2-negative MCF-7 cells, Together, these data point to HER2 as required element in the cascade of events leading to autophagy following exposure to the anti-HER2 monoclonal antibody Tzb. As previously noted in Tzb-refractory pools, the nuclei of the adherent, autophagic surviving SKBR3 parental cells seemed viable. Although we did not evaluate if the detached cells treated with Tzb showed irregular chromatin condensation in the nucleus –which is typical of cells dying by APCD (ACDII)-, the fact that Tzb-induced macroautophagy was concomitant with a reduction in SKBR3 cell number provides evidence that Tzb-induced cell death is not an obligatory outcome of Tzb-induced macroautophagy. Therefore, macroautophagy appears to facilitate survival in Tzb-naive HER2-overexpressing human breast cancer cells, likely promoting their own resistance if Tzb-induced HER2 blockade persists.
Tzb-naive SKBR3 parental cells were exposed to graded concentrations of Tzb (0, 10, 100 and 200 µg/mL Tzb) for 72 hours. Immunofluorescence. After fixation and permeabilization, cells were triple stained with LC3 and p62 antibodies and counterstained with Hoechst 33342 to visualize cell nuclei. Untreated control SKBR3 cells likewise show homogenous cytoplasmatic staining of LC3 and p62 (i.e. absent or low-level autophagosome formation). Surviving SKBR3 cells following exposure to graded concentrations of Tzb notably exhibit a marked contrast enhancement in the punctated pattern of LC3 (i.e. autophagosome formation) concurrently accompanied by p62 down-regulation (i.e. enhanced autophagic flux). Images show representative portions of SKBR3 and TzbR cell cultures captured in different channels for LC3 (red) and Hoechst 33258 (blue) with a 20× objective, and merged on BD Pathway™ 855 Bioimager System using BD Attovision™ software. Scale bar = 25 µm. Immunoblotting. Autophagosome formation and autophagic degradation in whole cell lysates of Tzb-treated Tzb-naive SKBR3 cells was confirmed with Western blot analyses using LC3 and p62 antibodies. High-dose Tzb notably increases LC3-II while reducing p62 expression, thus revealing an enhanced autophagic flux in the surviving fraction of Tzb-treated SKBR3 cells. Figure shows a representative immunoblotting analysis. Equivalent results were obtained in three independent experiments.
We here report for the first time that the induction of autophagy is closely related to the cell survival system triggered by HER2-gene amplified human breast cancer cells in response to the anti-HER2 monoclonal antibody Tzb. Because a key point that needs to be emphasized when monitoring macroautophagy is that there is a difference between measurements that examine the numbers of autophagosomes versus those that measure flux through the autophagy pathway, we followed the “Guidelines for the use and interpretation of assays for monitoring autophagy in higher eukaryotes” recently presented by Klionsky and colleagues . Thus, to unambiguously demonstrate that enhanced basal autophagy causally functioned to protect HER2-dependent human breast cancer cells from cell death upon chronic exposure to Tzb, we firstly measured autophagosome accumulation by fluorescence microscopy of endogenous LC3 (i.e. increase in punctate LC3 -a hallmark of autophagy because it is the first protein identified on the autophagosomal membrane-) and LC3-II immunoblotting (i.e. increase in the amount of the lipidation product of the autophagic conversion of LC3) –. Autophagy flux was confirmed by fluorescence microscopy and immunoblotting of p62/sequestosome-1 protein, which serves as a link between LC3 and ubiquitinated substrates destined for autophagic degradation , –. To determine that autophagy induced by Tzb provided an indispensable role in cell survival and facilitated the development of acquired resistance to Tzb, we pharmacologically impaired formation/function of macroautophagosomes by using small-molecule autophagy inhibitors. Because it is now known that pharmacological inhibitors of autophagy are not completely specific to the autophagic process and thus may produce secondary or off-target effects , we finally included the use of RNAi technology for specific inhibition of autophagy. These combined studies not only demonstrated that activation of basal autophagy was causally related to the acquisition of Tzb resistance but further confirmed an active role of “protective autophagy” in the maintenance of Tzb insensitivity.
The typical punctate staining that accompanies the translocation of LC3 II from the cytosol to the autophagosome membrane was detected at high levels detected in Tzb-refractory TzbR cells. Formation of autophagosomes was further enhanced in the presence of Tzb, thus suggesting that Tzb-refractory TzbR cells are uniquely characterized by their ability to sustain high levels of Tzb-induced macroautophagy without induction of cell death. Because the polyubiquitin-binding protein p62/sequestosome-1 recognizes long-lived ubiquitinated protein bodies and presents these to the autophagic machinery via LC3 (i.e. a large fraction of p62-formed protein aggregates are degraded by autophagy –), the fact that p62/SQSTM1 protein expression was reduced in Tzb-refractory cells supported the notion that the catabolic function of activated basal autophagy was playing a pro-survival role in Tzb-refractory cells. Since p62 downregulation was maintained in Tzb-refractory HER2-overexpressing cells chronically exposed to Tzb (i.e. twice weakly for a minimum of 5 months), it could be argued that reduced levels of p62/SQSTM1 may arise from reduced gene transcription or reduced translation of this protein, rather than from autophagic degradation . However, p62 expression has been shown also to remain significantly decreased for several months in the chronic autophagic maladaptive response that accompanies premature aging in Zmpste24(−/−) mice , . Moreover, immunofluorescence microscopy analyses indicated that p62 downregulation was concomitant with reduced immunoreactive for ubiquitin-positive bodies (data not shown), further suggesting that Tzb-refractory cells exhibit a significantly enhanced turnover of autophagic substrates , including potentially toxic aggregate-prone ubiquitinated proteins. This Tzb-resistance phenotype consistent with a chronic increase in flux through autophagic clearance pathways is in marked contrast with a newly discovered role for LC3 in nonautophagic cytoplasmic vacuolation death of cancer cells . In this latter scenario, up-regulation and processing of the autophagic marker LC3 is accompanied by a marked increase in p62/SQSTM1 expression and dilation of endoplasmic reticulum due to accumulation of ubiquitinated proteins. Both chemical and genetic inhibition of autophagy demonstrated that development of acquired resistance to Tzb was due, at least in part, to activation of Autophagy in HER2-overexpressing breast cancer cells chronically cultured in the presence of Tzb. Three chemicals (3-MA, LY294002 and bafilomycin A1), which are routinely used to inhibit autophagy at different stages of the autophagosome maturation , significantly reduced cell viability in TzbR POOLs as measured by MTT assays. Besides the chemical autophagy inhibitors, knockdown of the autophagosome membrane protein LC3 by siRNA similarly decreased TzbR cell viability as measured by cell numbers. In contrast to either agent alone, Tzb and autophagy LC3 gene-siRNA showed a profound combinatorial (supra-additive) effect, greatly increasing the growth inhibitory properties of Tzb. In this regard, it was noteworthy that Tzb treatment significantly increased autophagosome levels in the cytosol of viable Tzb-naive SKBR3 parental cells, thus revealing that ACDII is not an obligatory outcome of Tzb-induced macroautophagy. While further studies are clearly required to definitely uncover the ultimate molecular pathways involved (e.g. it could be relevant to address whether Tzb-induced non viable cells show higher levels of autophagosomes), our results suggest that autophagy may represent a general mechanism responsible for circumventing and/or delaying Tzb-induced cell death. If the magnitude of autophagy may determine the fate of Tzb-treated HER2-positive breast cancer cells (i.e. macroautophagy is fully induced as a survival mode but fail to rescue Tzb-sensitive cells), the ability of HER2-positive breast cancer to switch Tzb-induced autophagy to a mechanism of tumor cell survival may further explain the degree of primary (inherent) resistance to Tzb. Thresholds may exist within HER2-positive cancer cells for the magnitude of Tzb-induced autophagy that leads to cell death versus the magnitude of Tzb-induced autophagy that leads to cell survival. This notion would support recent findings indicating that primary Tzb resistance is not synonymous with lack of responsiveness to Tzb .
Autophagy is known to display a dual contrasting function in cancer cell biology. On the one hand, autophagy is a tumor suppressor mechanism and, therefore, the activation of autophagy could reverse the neoplastic phenotype. On the other hand, autophagy may contribute to tumor progression as a protective mechanism against stressful microenvironmental conditions including anti-cancer therapies . From a clinical perspective, this debate is crucial in order to preferentially promote the development of therapeutic interventions that can either inhibit or enhance autophagy in tumor cells. In the cellular response to cancer therapy, a number of clinically available cancer therapeutics and experimental anticancer treatment modalities, including DNA-damaging chemotherapeutics, endocrine therapies (e.g. tamoxifen) and radiation therapy have been found to induce autophagy in cell culture and animal models –. Recent investigations have found also the presence of autophagic structures in response to molecular cancer therapies such as the TKI imatinib mesylate (Gleevec®) -the first approved drug to directly turn off the signal of a protein known to cause a cancer- and autophagy/autophagic cell death have been suggested as next targets for elimination of the resistance to imatinib in chronic myelogenous leukaemia (CML) and gastrointestinal stromal tumors (GIST) –. For instance, imatinib-resistant cell lines undergo cell death when concurrently treated with imatinib and the autophagy inhibitor chloroquine. Further expanding the cytoprotective role of autophagy following exposure to anti-cancer therapies, we now add the anti-HER2 monoclonal antibody as a novel molecularly targeted therapy that can trigger a pro-survival function of autophagy in HER2-dependent human breast carcinoma cells. Under experimental conditions described here, increased basal autophagy related to an increased proliferative capacity of Tzb-refractory cells translated into a significant decrease in the doubling time of Tzb-refractory cells, whereas inhibition of autophagy accelerated Tzb-induced cell death. Because recent studies have uncovered significant interactions between autophagic, apoptotic and proliferative signaling pathways , , a potential contribution of enhanced autophagic activity to efficiently maintain energy homeostasis and confer a selective growth advantage under stress conditions imposed by molecularly targeted therapies such as the anti-HER2 monoclonal antibody Tzb strongly suggests that specific inhibition of autophagic machinery may have a therapeutic role not only in HER2-positive breast cancer patients with advanced disease refractory to Tzb but also in the prevention or delay of Tzb resistance in early HER2-positive breast cancer disease.
In summary, we provide compelling data that increased autophagosome formation and function (i.e. enhanced autophagic flux) induced by Tzb treatment plays a critical role in HER2-positive breast cancer cell survival. HER2-overexpressing breast cancer cells chronically exposed to Tzb exhibit a bona fide up-regulation of the autophagic activity that efficiently works to protect themselves from the growth-inhibitory effects of Tzb. Our working model is that macroautophagosome formation and catabolic function contributes to HER2-dependent breast cancer survival and facilitates a rapid development of Tzb resistance, whereas blockade of autophagosome formation/function significantly helps to enhance the growth inhibitory activity of Tzb toward Tzb-refractory breast cancer cells. To our knowledge, these are the first examples demonstrating a synergistic nature of combining Tzb with autophagy inhibition, thus highlighting the importance of investigating autophagy knock-down as a novel means to sensitize Tzb-resistant HER2-positive breast carcinomas to the growth inhibitory actions of Tzb. Chloroquine, a drug initially developed for the treatment of malaria in the 1930s, and recently tested as autophagy inhibitor in experimental models –, may be used in future clinical trials in combination with Tzb, which should help clarify the importance of manipulating autophagy for enhancing the therapeutic benefit of Tzb in HER2-dependent breast carcinomas.
Materials and Methods
Trastuzumab (Tzb; Herceptin®) -kindly provided by Hospital Universitari de Girona Dr. Josep Trueta Pharmacy (Girona, Spain)- was solubilized in bacteriostatic water for injection (USP, a sterile, nonpyrogenic preparation of water for injection containing 1.1%–1.1 mg/mL- of benzyl alcohol added as a bacteriostatic preservative)-, stored at 4°C (stock solution at 21 mg/mL) and used within one month. 3-methyladenine (3-MA) was purchased from Sigma-Chemicals (St. Louis, MO, USA) and solubilized (stock solution at 1 M) in phosphate buffered saline (PBS). Bafilomycin A1 and LY294002 were purchased from Sigma-Chemicals (St. Louis, MO, USA) and Cell Signaling Technology, respectively, and reconstituted in dimethyl sulphoxide (DMSO; stock solution at 1 mM). Rabbit anti-light-chain 3 (LC3) polyclonal antibody was purchased from MBL International Corporation (Woburn, MA, USA; PD014). Mouse anti-SQSTM1/p62 monoclonal antibody was purchased from Abcam plc. (Cambridge, UK; ab56416).
SKBR3 breast cancer cells were obtained from the American Type Culture Collection (ATCC) and were routinely grown in Improved MEM (IMEM; Biosource International; Invitrogen S.A., Barcelona, Spain) supplemented with 10% fetal bovine serum (FBS) and 2 mM L-glutamine. Cells were maintained at 37°C in a humidified atmosphere of 95% air and 5% CO2. Cells were screened periodically for Mycoplasma contamination. For experimental use Tzb, 3-MA, bafilomycin A1 and LY294002 were prepared freshly from stock solutions and diluted with growth medium. Control cells were cultured in medium containing the same concentration (v/v) as the experimental cultures with treatments. The vehicle solutions had no noticeable influence on the proliferation of experimental cells.
Establishment of Tzb-acquired Auto-resistance in HER2-positive SKBR3 breast Cancer Cells
To establish SKBR3/TzbR pools exhibiting secondary resistance to the anti-HER2 monoclonal antibody Tzb, Tzb-naive SKBR3 parental cells were exposed to increasing concentrations of Tzb for a minimum of 10 months. Briefly, SKBR3 cells were initially exposed to 20 µg/mL Tzb for 3 months (4 treatments weekly) followed by 185 µg/mL Tzb for 2 months (twice weekly). Two pools selected for further study (i.e. TzbR POOL1 and TzbR POOL2), resisted continuous growth in 200 µg/mL Tzb (cells were passaged at 70% confluence and Tzb-containing medium was replaced twice weakly for a minimum of 5 months). The resistant pools were maintained in medium without Tzb for at least 2 days before each experiment.
Metabolic Status Assessment (MTT-based Cell Viability Assays)
Cell viability was determined using a standard colorimetric MTT (3-4,5-dimethylthiazol-2-yl-2, 5-diphenyl-tetrazolium bromide) reduction assay. Cells in exponential growth were harvested by trypsinization and seeded at a concentration of ∼2.5×103 cells/200 µL/well into 96-well plates, and allowed an overnight period for attachment. Then the medium was removed and fresh medium along with Tzb, 3-MA, LY294002, or bafilomycin A1 was added to cultures, as specified. Control cells without agents were cultured in parallel using the same conditions with comparable media changes. Agents were not renewed during the entire period of cell exposure. Following treatment (4–5 days), the medium was removed and replaced by fresh drug-free medium (100 µL/well), and MTT (5 mg/mL in PBS) was added to each well at a 1/10 volume. After incubation for 2–3 hr at 37°C, the supernatants were carefully aspirated, 100 µL of DMSO were added to each well, and the plates agitated to dissolve the crystal product. Optical density (OD) was measured at 570 nm using a multi-well plate reader (Model Anthos Labtec 2010 1.7 reader). Cell viability after exposure of cells to drugs was analyzed as percentages of the control cell absorbances, which were obtained from control wells treated with appropriate concentrations of the agents' vehicles that were processed simultaneously. For each treatment, cell viability was evaluated as a percentage using the following equation: (OD570 of treated sample/OD570 of untreated sample) ×100.
Transient transfection of siRNAs
The siRNA sequences used for targeted silencing of human LC3 (MAP LC3β siRNA (h): sc-43390) were supplied by Santa Cruz Biotechnology (Santa Cruz, CA, USA) as ready-to-use pools of 3 to 5 target-specific 19–25 nt double-stranded siRNAs designed to efficiently knock down the expression of LC3 gene. siRNA A (sc-37007), which consists of a scrambled sequence that will not lead to the specific degradation of any known cellular mRNA, was employed as negative control for experiments using LC3-targeted siRNA transfection. Transfections were performed as described in Santa Cruz technical bulletin. Briefly, cells at a confluence of 60 to 80% were transfected with the selected siRNAs using Santa Cruz Biotechnology's siRNA Transfection Reagent (sc-29528) and siRNA Transfection Medium (sc-36868) following the manufacturer's instructions.
Cell proliferation assays
Cells were trypsinized and re-plated in 24-well plates at a density of 10,000 cells/well. Cells were incubated for 18 h to allow for attachment, after which a zero time point was determined. Cells were then cultured in regular medium containing 5% FBS) in the absence or presence of Tzb, and counted at days 0, 3, 6 and 9 with a Coulter Counter (Coulter Electronics, Inc., Hialeah, FL, USA). All assays were performed at least three times in duplicate.
Immunofluorescence staining and high-content confocal imaging
Cells were seeded at approximately 5,000 cells/well in 96-well clear bottom imaging tissue culture plates (Becton Dickinson Biosciences; San Jose, CA, USA) optimized for automated imaging applications. Triton® X-100 permeabilization and blocking, primary antibody staining (1∶50 dilution), secondary antibody staining using Alexa Fluor® 488/594 goat anti-rabbit/mouse IgGs (Invitrogen, Molecular Probes, Eugene, Oregon, USA) and counterstaining (using Hoechst 33258; Invitrogen) were performed following BD Biosciences protocols. Images were captured in different channels for Alexa Fluor® 488 (pseudo-colored green), Alexa Fluor® 594 (pseudo-colored red) and Hoechst 33258 (pseudo-colored blue) on a BD Pathway™ 855 Bioimager System (Becton Dickinson Biosciences, San Jose, California, USA) with 20× or 40× objectives (NA 075 Olympus). Merged images, confocal Z stack acquisition and 3D visualization were obtained according to the Recommended Assay Procedure using BD Attovision™ software.
Cells were washed twice with cold-PBS and then lysed in buffer (20 mM Tris pH 7.5, 150 mM NaCl, 1 mM EDTA, 1 mM EGTA, 1% Triton® X-100, 2.5 mM sodium pyrophosphate, 1 mM β-glycerolphosphate, 1 mM Na3VO4, 1 µg/mL leupeptin, 1 mM phenylmethylsulfonylfluoride, and complete protease inhibitor cocktail [Sigma-Chemicals; St. Louis, MO, USA]) for 30 minutes on ice. The lysates were cleared by centrifugation in an Eppendorff tube (15 minutes at 14,000×g, 4°C). Protein content was determined against a standardized control using the Pierce Protein Assay Kit (Rockford, IL, USA). Equal amounts of protein (i.e. 50 µg) were resuspended in 5× Laemmli sample buffer (10 min at 70°C), resolved by electrophoresis on 10% SDS-PAGE, and transferred onto nitrocellulose membranes. Non-specific binding on the nitrocellulose filter paper was minimized by blocking for 1 h at RT with TBS-T buffer [25 mM Tris-HCl (pH 7.5), 150 mM NaCl, 0.05% Tween 20] containing 5% (w/v) nonfat dry milk. The treated filters were washed in TBS-T and then incubated with anti-LC3 (1∶1000 dilution) or anti-p62 (1∶1000 dilution) antibodies, as specified, in 5% w/v BSA, 1× TBS-T buffer, 0.1% Tween-20 at 4°C with gentle shaking, overnight. The membranes were washed in TBS-T, horseradish peroxidase-conjugated secondary anti-mouse/rabbit IgGs in TBS-T was added for 1 h, and immunoreactive bands were detected by chemiluminiscence reagent (Pierce, Rockford, IL, USA). Experiments involving immunoblotting procedures were repeated at least three times and blots were re-probed with an antibody for β-actin to control for protein loading and transfer. Densitometric values of proteins bands were quantified using the Scion Image software (Scion Corporation, Frederick, MD, USA).
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Conceived and designed the experiments: AVM JAM. Performed the experiments: AVM COF. Analyzed the data: AVM COF JAM. Contributed reagents/materials/analysis tools: JAM. Wrote the paper: JAM.
- 1. Pegram MD, Konecny G, Slamon DJ (2000) The molecular and cellular biology of HER2/neu gene amplification/overexpression and the clinical development of herceptin (trastuzumab) therapy for breast cancer. Cancer Treat Res 103: 57–75.MD PegramG. KonecnyDJ Slamon2000The molecular and cellular biology of HER2/neu gene amplification/overexpression and the clinical development of herceptin (trastuzumab) therapy for breast cancer.Cancer Treat Res1035775
- 2. Lan KH, Lu CH, Yu D (2005) Mechanisms of trastuzumab resistance and their clinical implications. Ann N Y Acad Sci 1059: 70–75.KH LanCH LuD. Yu2005Mechanisms of trastuzumab resistance and their clinical implications.Ann N Y Acad Sci10597075
- 3. Nahta R, Esteva FJ (2006) Herceptin: mechanisms of action and resistance. Cancer Lett 232: 123–138.R. NahtaFJ Esteva2006Herceptin: mechanisms of action and resistance.Cancer Lett232123138
- 4. Nahta R, Yu D, Hung MC, Hortobagyi GN, Esteva FJ (2006) Mechanisms of disease: understanding resistance to HER2-targeted therapy in human breast cancer. Nat Clin Pract Oncol 3: 269–280.R. NahtaD. YuMC HungGN HortobagyiFJ Esteva2006Mechanisms of disease: understanding resistance to HER2-targeted therapy in human breast cancer.Nat Clin Pract Oncol3269280
- 5. Nahta R, Esteva FJ (2006) HER2 therapy: molecular mechanisms of trastuzumab resistance. Breast Cancer Res 8: 215.R. NahtaFJ Esteva2006HER2 therapy: molecular mechanisms of trastuzumab resistance.Breast Cancer Res8215
- 6. Nahta R, Esteva FJ (2007) Trastuzumab: triumphs and tribulations. Oncogene 26: 3637–3643.R. NahtaFJ Esteva2007Trastuzumab: triumphs and tribulations.Oncogene2636373643
- 7. Menendez JA, Lupu R (2007) Transphosphorylation of kinase-dead HER3 and breast cancer progression: a new standpoint or an old concept revisited? Breast Cancer Res 9: 111.JA MenendezR. Lupu2007Transphosphorylation of kinase-dead HER3 and breast cancer progression: a new standpoint or an old concept revisited?Breast Cancer Res9111
- 8. Jin Q, Esteva FJ (2008) Cross-talk between the ErbB/HER family and the type I insulin-like growth factor receptor signaling pathway in breast cancer. J Mammary Gland Biol Neoplasia 13: 485–498.Q. JinFJ Esteva2008Cross-talk between the ErbB/HER family and the type I insulin-like growth factor receptor signaling pathway in breast cancer.J Mammary Gland Biol Neoplasia13485498
- 9. Price-Schiavi SA, Jepson S, Li P, Arango M, Rudland PS, et al. (2002) Rat Muc4 (sialomucin complex) reduces binding of anti-ErbB2 antibodies to tumor cell surfaces, a potential mechanism for herceptin resistance. Int J Cancer 99: 783–791.SA Price-SchiaviS. JepsonP. LiM. ArangoPS Rudland2002Rat Muc4 (sialomucin complex) reduces binding of anti-ErbB2 antibodies to tumor cell surfaces, a potential mechanism for herceptin resistance.Int J Cancer99783791
- 10. Nagy P, Friedländer E, Tanner M, Kapanen AI, Carraway KL, et al. (2005) Decreased accessibility and lack of activation of ErbB2 in JIMT-1, a herceptin-resistant, MUC4-expressing breast cancer cell line. Cancer Res 65: 473–482.P. NagyE. FriedländerM. TannerAI KapanenKL Carraway2005Decreased accessibility and lack of activation of ErbB2 in JIMT-1, a herceptin-resistant, MUC4-expressing breast cancer cell line.Cancer Res65473482
- 11. Altundag O, Altundag K, Ozcakar B, Silay YS (2005) HER2/neu intragenic kinase domain mutations may be major determinant of response to trastuzumab or specific kinase inhibitors in non-small cell lung cancer patients. Lung Cancer 49: 279–280.O. AltundagK. AltundagB. OzcakarYS Silay2005HER2/neu intragenic kinase domain mutations may be major determinant of response to trastuzumab or specific kinase inhibitors in non-small cell lung cancer patients.Lung Cancer49279280
- 12. Cappuzzo F, Bemis L, Varella-Garcia M (2006) HER2 mutation and response to trastuzumab therapy in non-small-cell lung cancer. N Engl J Med 354: 2619–2621.F. CappuzzoL. BemisM. Varella-Garcia2006HER2 mutation and response to trastuzumab therapy in non-small-cell lung cancer.N Engl J Med35426192621
- 13. Wang SE, Narasanna A, Perez-Torres M, Xiang B, Wu FY, et al. (2006) HER2 kinase domain mutation results in constitutive phosphorylation and activation of HER2 and EGFR and resistance to EGFR tyrosine kinase inhibitors. Cancer Cell 10: 25–38.SE WangA. NarasannaM. Perez-TorresB. XiangFY Wu2006HER2 kinase domain mutation results in constitutive phosphorylation and activation of HER2 and EGFR and resistance to EGFR tyrosine kinase inhibitors.Cancer Cell102538
- 14. Molina MA, Codony-Servat J, Albanell J, Rojo F, Arribas J, et al. (2001) Trastuzumab (herceptin), a humanized anti-Her2 receptor monoclonal antibody, inhibits basal and activated Her2 ectodomain cleavage in breast cancer cells. Cancer Res 61: 4744–4749.MA MolinaJ. Codony-ServatJ. AlbanellF. RojoJ. Arribas2001Trastuzumab (herceptin), a humanized anti-Her2 receptor monoclonal antibody, inhibits basal and activated Her2 ectodomain cleavage in breast cancer cells.Cancer Res6147444749
- 15. Liu X, Fridman JS, Wang Q, Caulder E, Yang G, et al. (2006) Selective inhibition of ADAM metalloproteases blocks HER-2 extracellular domain (ECD) cleavage and potentiates the anti-tumor effects of trastuzumab. Cancer Biol Ther 5: 648–656.X. LiuJS FridmanQ. WangE. CaulderG. Yang2006Selective inhibition of ADAM metalloproteases blocks HER-2 extracellular domain (ECD) cleavage and potentiates the anti-tumor effects of trastuzumab.Cancer Biol Ther5648656
- 16. Scaltriti M, Rojo F, Ocaña A, Anido J, Guzman M, et al. (2007) Expression of p95HER2, a truncated form of the HER2 receptor, and response to anti-HER2 therapies in breast cancer. J Natl Cancer Inst 2007 99: 628–638.M. ScaltritiF. RojoA. OcañaJ. AnidoM. Guzman2007Expression of p95HER2, a truncated form of the HER2 receptor, and response to anti-HER2 therapies in breast cancer.J Natl Cancer Inst 200799628638
- 17. Lu Y, Zi X, Zhao Y, Mascarenhas D, Pollak M (2001) Insulin-like growth factor-I receptor signaling and resistance to trastuzumab (Herceptin). J Natl Cancer Inst 93: 1852–1857.Y. LuX. ZiY. ZhaoD. MascarenhasM. Pollak2001Insulin-like growth factor-I receptor signaling and resistance to trastuzumab (Herceptin).J Natl Cancer Inst9318521857
- 18. Lu Y, Zi X, Pollak M (2004) Molecular mechanisms underlying IGF-I-induced attenuation of the growth-inhibitory activity of trastuzumab (Herceptin) on SKBR3 breast cancer cells. Int J Cancer 108: 334–341.Y. LuX. ZiM. Pollak2004Molecular mechanisms underlying IGF-I-induced attenuation of the growth-inhibitory activity of trastuzumab (Herceptin) on SKBR3 breast cancer cells.Int J Cancer108334341
- 19. Nahta R, Yuan LX, Zhang B, Kobayashi R, Esteva FJ (2005) Insulin-like growth factor-I receptor/human epidermal growth factor receptor 2 heterodimerization contributes to trastuzumab resistance of breast cancer cells. Cancer Res 65: 11118–11128.R. NahtaLX YuanB. ZhangR. KobayashiFJ Esteva2005Insulin-like growth factor-I receptor/human epidermal growth factor receptor 2 heterodimerization contributes to trastuzumab resistance of breast cancer cells.Cancer Res651111811128
- 20. Esparís-Ogando A, Ocaña A, Rodríguez-Barrueco R, Ferreira L, Borges J, et al. (2008) Synergic antitumoral effect of an IGF-IR inhibitor and trastuzumab on HER2-overexpressing breast cancer cells. Ann Oncol 19: 1860–1869.A. Esparís-OgandoA. OcañaR. Rodríguez-BarruecoL. FerreiraJ. Borges2008Synergic antitumoral effect of an IGF-IR inhibitor and trastuzumab on HER2-overexpressing breast cancer cells.Ann Oncol1918601869
- 21. Ropero S, Menéndez JA, Vázquez-Martín A, Montero S, Cortés-Funes H, et al. (2004) Trastuzumab plus tamoxifen: anti-proliferative and molecular interactions in breast carcinoma. Breast Cancer Res Treat 86: 125–137.S. RoperoJA MenéndezA. Vázquez-MartínS. MonteroH. Cortés-Funes2004Trastuzumab plus tamoxifen: anti-proliferative and molecular interactions in breast carcinoma.Breast Cancer Res Treat86125137
- 22. Ocaña A, Cruz JJ, Pandiella A (2006) Trastuzumab and antiestrogen therapy: focus on mechanisms of action and resistance. Am J Clin Oncol 29: 90–95.A. OcañaJJ CruzA. Pandiella2006Trastuzumab and antiestrogen therapy: focus on mechanisms of action and resistance.Am J Clin Oncol299095
- 23. du Manoir JM, Francia G, Man S, Mossoba M, Medin JA, et al. (2006) Strategies for delaying or treating in vivo acquired resistance to trastuzumab in human breast cancer xenografts. Clin Cancer Res 12(3 Pt 1): 904–916.JM du ManoirG. FranciaS. ManM. MossobaJA Medin2006Strategies for delaying or treating in vivo acquired resistance to trastuzumab in human breast cancer xenografts.Clin Cancer Res12(3 Pt 1)904916
- 24. Nahta R, Takahashi T, Ueno NT, Hung MC, Esteva FJ (2004) P27(kip1) down-regulation is associated with trastuzumab resistance in breast cancer cells. Cancer Res 64: 3981–3986.R. NahtaT. TakahashiNT UenoMC HungFJ Esteva2004P27(kip1) down-regulation is associated with trastuzumab resistance in breast cancer cells.Cancer Res6439813986
- 25. Nagata Y, Lan KH, Zhou X, Tan M, Esteva FJ, et al. (2004) PTEN activation contributes to tumor inhibition by trastuzumab, and loss of PTEN predicts trastuzumab resistance in patients. Cancer Cell 6: 117–127.Y. NagataKH LanX. ZhouM. TanFJ Esteva2004PTEN activation contributes to tumor inhibition by trastuzumab, and loss of PTEN predicts trastuzumab resistance in patients.Cancer Cell6117127
- 26. Lu CH, Wyszomierski SL, Tseng LM, Sun MH, Lan KH, et al. (2007) Preclinical testing of clinically applicable strategies for overcoming trastuzumab resistance caused by PTEN deficiency. Clin Cancer Res 13: 5883–5888.CH LuSL WyszomierskiLM TsengMH SunKH Lan2007Preclinical testing of clinically applicable strategies for overcoming trastuzumab resistance caused by PTEN deficiency.Clin Cancer Res1358835888
- 27. Park BH, Davidson NE (2007) PI3 kinase activation and response to Trastuzumab Therapy: what's neu with herceptin resistance? Cancer Cell 12: 297–299.BH ParkNE Davidson2007PI3 kinase activation and response to Trastuzumab Therapy: what's neu with herceptin resistance?Cancer Cell12297299
- 28. Berns K, Horlings HM, Hennessy BT, Madiredjo M, Hijmans EM, et al. (2007) A functional genetic approach identifies the PI3K pathway as a major determinant of trastuzumab resistance in breast cancer. Cancer Cell 12: 395–402.K. BernsHM HorlingsBT HennessyM. MadiredjoEM Hijmans2007A functional genetic approach identifies the PI3K pathway as a major determinant of trastuzumab resistance in breast cancer.Cancer Cell12395402
- 29. Serra V, Markman B, Scaltriti M, Eichhorn PJ, Valero V, et al. (2008) NVP-BEZ235, a dual PI3K/mTOR inhibitor, prevents PI3K signaling and inhibits the growth of cancer cells with activating PI3K mutations. Cancer Res 68: 8022–8030.V. SerraB. MarkmanM. ScaltritiPJ EichhornV. Valero2008NVP-BEZ235, a dual PI3K/mTOR inhibitor, prevents PI3K signaling and inhibits the growth of cancer cells with activating PI3K mutations.Cancer Res6880228030
- 30. Eichhorn PJ, Gili M, Scaltriti M, Serra V, Guzman M, et al. (2008) Phosphatidylinositol 3-kinase hyperactivation results in lapatinib resistance that is reversed by the mTOR/phosphatidylinositol 3-kinase inhibitor NVP-BEZ235. Cancer Res 68: 9221–9230.PJ EichhornM. GiliM. ScaltritiV. SerraM. Guzman2008Phosphatidylinositol 3-kinase hyperactivation results in lapatinib resistance that is reversed by the mTOR/phosphatidylinositol 3-kinase inhibitor NVP-BEZ235.Cancer Res6892219230
- 31. Lum JJ, DeBerardinis RJ, Thompson CB (2005) Autophagy in metazoans: cell survival in the land of plenty. Nat Rev Mol Cell Biol 6: 439–448.JJ LumRJ DeBerardinisCB Thompson2005Autophagy in metazoans: cell survival in the land of plenty.Nat Rev Mol Cell Biol6439448
- 32. Huang J, Klionsky DJ (2007) Autophagy and human disease. Cell Cycle 6: 1837–1849.J. HuangDJ Klionsky2007Autophagy and human disease.Cell Cycle618371849
- 33. Høyer-Hansen M, Jäättelä M (2008) Autophagy: an emerging target for cancer therapy. Autophagy 4: 574–580.M. Høyer-HansenM. Jäättelä2008Autophagy: an emerging target for cancer therapy.Autophagy4574580
- 34. Jin S, White E (2008) Tumor suppression by autophagy through the management of metabolic stress. Autophagy; 4: 563–566.S. JinE. White2008Tumor suppression by autophagy through the management of metabolic stress.Autophagy;4563566
- 35. Mizushima N, Levine B, Cuervo AM, Klionsky DJ (2008) Autophagy fights disease through cellular self-digestion. Nature 451: 1069–1075.N. MizushimaB. LevineAM CuervoDJ Klionsky2008Autophagy fights disease through cellular self-digestion.Nature45110691075
- 36. Kundu M, Thompson CB (2008) Autophagy: basic principles and relevance to disease. Annu Rev Pathol 3: 427–455.M. KunduCB Thompson2008Autophagy: basic principles and relevance to disease.Annu Rev Pathol3427455
- 37. Galluzzi L, Morselli E, Vicencio JM, Kepp O, Joza N, et al. (2008) Life, death and burial: multifaceted impact of autophagy. Biochem Soc Trans 36(Pt 5): 786–790.L. GalluzziE. MorselliJM VicencioO. KeppN. Joza2008Life, death and burial: multifaceted impact of autophagy.Biochem Soc Trans36(Pt 5)786790
- 38. Jin S, White E (2008) Tumor suppression by autophagy through the management of metabolic stress. Autophagy 4: 563–566.S. JinE. White2008Tumor suppression by autophagy through the management of metabolic stress.Autophagy4563566
- 39. Jones RG, Thompson CB (2009) Tumor suppressors and cell metabolism: a recipe for cancer growth. Genes Dev 23: 537–548.RG JonesCB Thompson2009Tumor suppressors and cell metabolism: a recipe for cancer growth.Genes Dev23537548
- 40. Kondo Y, Kondo S (2006) Autophagy and cancer therapy. Autophagy 2: 85–90.Y. KondoS. Kondo2006Autophagy and cancer therapy.Autophagy28590
- 41. Hippert MM, O'Toole PS, Thorburn A (2006) Autophagy in cancer: good, bad, or both? Cancer Res 66: 9349–9351.MM HippertPS O'TooleA. Thorburn2006Autophagy in cancer: good, bad, or both?Cancer Res6693499351
- 42. Moretti L, Yang ES, Kim KW, Lu B (2007) Autophagy signaling in cancer and its potential as novel target to improve anticancer therapy. Drug Resist Updat 10: 135–143.L. MorettiES YangKW KimB. Lu2007Autophagy signaling in cancer and its potential as novel target to improve anticancer therapy.Drug Resist Updat10135143
- 43. Rubinsztein DC, Gestwicki JE, Murphy LO, Klionsky DJ (2007) Potential therapeutic applications of autophagy. Nat Rev Drug Discov 6: 304–312.DC RubinszteinJE GestwickiLO MurphyDJ Klionsky2007Potential therapeutic applications of autophagy.Nat Rev Drug Discov6304312
- 44. Amaravadi RK, Thompson CB (2007) The roles of therapy-induced autophagy and necrosis in cancer treatment. Clin Cancer Res 13: 7271–7279.RK AmaravadiCB Thompson2007The roles of therapy-induced autophagy and necrosis in cancer treatment.Clin Cancer Res1372717279
- 45. Klionsky DJ, Abeliovich H, Agostinis P, Agrawal DK, Aliev G, et al. (2008) Guidelines for the use and interpretation of assays for monitoring autophagy in higher eukaryotes. Autophagy 4: 151–175.DJ KlionskyH. AbeliovichP. AgostinisDK AgrawalG. Aliev2008Guidelines for the use and interpretation of assays for monitoring autophagy in higher eukaryotes.Autophagy4151175
- 46. Tanida I, Minematsu-Ikeguchi N, Ueno T, Kominami E (2005) Lysosomal turnover, but not a cellular level, of endogenous LC3 is a marker for autophagy. Autophagy 1: 84–91.I. TanidaN. Minematsu-IkeguchiT. UenoE. Kominami2005Lysosomal turnover, but not a cellular level, of endogenous LC3 is a marker for autophagy.Autophagy18491
- 47. Mizushima N, Yoshimori T (2007) How to interpret LC3 immunoblotting. Autophagy 3: 542–545.N. MizushimaT. Yoshimori2007How to interpret LC3 immunoblotting.Autophagy3542545
- 48. Tanida I, Yamaji T, Ueno T, Ishiura S, Kominami E, et al. (2008) Consideration about negative controls for LC3 and expression vectors for four colored fluorescent protein-LC3 negative controls. Autophagy 4: 131–134.I. TanidaT. YamajiT. UenoS. IshiuraE. Kominami2008Consideration about negative controls for LC3 and expression vectors for four colored fluorescent protein-LC3 negative controls.Autophagy4131134
- 49. Kimura S, Fujita N, Noda T, Yoshimori T (2009) Monitoring autophagy in mammalian cultured cells through the dynamics of LC3. Methods Enzymol 452: 1–12.S. KimuraN. FujitaT. NodaT. Yoshimori2009Monitoring autophagy in mammalian cultured cells through the dynamics of LC3.Methods Enzymol452112
- 50. Bjørkøy G, Lamark T, Brech A, Outzen H, Perander M, et al. (2005) p62/SQSTM1 forms protein aggregates degraded by autophagy and has a protective effect on huntingtin-induced cell death. J Cell Biol 171: 603–614.G. BjørkøyT. LamarkA. BrechH. OutzenM. Perander2005p62/SQSTM1 forms protein aggregates degraded by autophagy and has a protective effect on huntingtin-induced cell death.J Cell Biol171603614
- 51. Bjørkøy G, Lamark T, Johansen T (2006) p62/SQSTM1: a missing link between protein aggregates and the autophagy machinery. Autophagy 2: 138–139.G. BjørkøyT. LamarkT. Johansen2006p62/SQSTM1: a missing link between protein aggregates and the autophagy machinery.Autophagy2138139
- 52. Pankiv S, Clausen TH, Lamark T, Brech A, Bruun JA, et al. (2007) p62/SQSTM1 binds directly to Atg8/LC3 to facilitate degradation of ubiquitinated protein aggregates by autophagy. J Biol Chem 282: 24131–24145.S. PankivTH ClausenT. LamarkA. BrechJA Bruun2007p62/SQSTM1 binds directly to Atg8/LC3 to facilitate degradation of ubiquitinated protein aggregates by autophagy.J Biol Chem2822413124145
- 53. Ichimura Y, Kominami E, Tanaka K, Komatsu M (2008) Selective turnover of p62/A170/SQSTM1 by autophagy. Autophagy 4: 1063–1066.Y. IchimuraE. KominamiK. TanakaM. Komatsu2008Selective turnover of p62/A170/SQSTM1 by autophagy.Autophagy410631066
- 54. Shvets E, Elazar Z (2008) Autophagy-independent incorporation of GFP-LC3 into protein aggregates is dependent on its interaction with p62/SQSTM1. Autophagy 4: 1054–1056.E. ShvetsZ. Elazar2008Autophagy-independent incorporation of GFP-LC3 into protein aggregates is dependent on its interaction with p62/SQSTM1.Autophagy410541056
- 55. Mariño G, Ugalde AP, Salvador-Montoliu N, Varela I, Quirós PM, et al. (2008) Premature aging in mice activates a systemic metabolic response involving autophagy induction. Hum Mol Genet 17: 2196–2211.G. MariñoAP UgaldeN. Salvador-MontoliuI. VarelaPM Quirós2008Premature aging in mice activates a systemic metabolic response involving autophagy induction.Hum Mol Genet1721962211
- 56. Mariño G, López-Otín C (2008) Autophagy and aging: new lessons from progeroid mice. Autophagy 4: 807–809.G. MariñoC. López-Otín2008Autophagy and aging: new lessons from progeroid mice.Autophagy4807809
- 57. Kirkin V, McEwan DG, Novak I, Dikic I (2009) A role for ubiquitin in selective autophagy. Mol Cell 34: 259–269.V. KirkinDG McEwanI. NovakI. Dikic2009A role for ubiquitin in selective autophagy.Mol Cell34259269
- 58. Kar R, Singha PK, Venkatachalam MA, Saikumar P (2009) A novel role for MAP1 LC3 in nonautophagic cytoplasmic vacuolation death of cancer cells. Oncogene. R. KarPK SinghaMA VenkatachalamP. Saikumar2009A novel role for MAP1 LC3 in nonautophagic cytoplasmic vacuolation death of cancer cells.Oncogene(doi: 10.1038/onc.2009.118). (doi: 10.1038/onc.2009.118).
- 59. Klionsky DJ, Cuervo AM, Seglen PO (2007) Methods for monitoring autophagy from yeast to human. Autophagy 31: 181–206.DJ KlionskyAM CuervoPO Seglen2007Methods for monitoring autophagy from yeast to human.Autophagy31181206
- 60. Narayan M, Wilken JA, Harris LN, Baron AT, Kimbler KD, et al. (2009) Trastuzumab-induced HER reprogramming in “resistant” breast carcinoma cells. Cancer Res 69: 2191–2194.M. NarayanJA WilkenLN HarrisAT BaronKD Kimbler2009Trastuzumab-induced HER reprogramming in “resistant” breast carcinoma cells.Cancer Res6921912194
- 61. Kanzawa T, Germano IM, Komata T, Ito H, Kondo Y, et al. (2004) Role of autophagy in temozolomide-induced cytotoxicity for malignant glioma cells. Cell Death Differ 11: 448–457.T. KanzawaIM GermanoT. KomataH. ItoY. Kondo2004Role of autophagy in temozolomide-induced cytotoxicity for malignant glioma cells.Cell Death Differ11448457
- 62. Katayama M, Kawaguchi T, Berger MS, Pieper RO (2007) DNA damaging agent-induced autophagy produces a cytoprotective adenosine triphosphate surge in malignant glioma cells. Cell Death Differ 14: 548–558.M. KatayamaT. KawaguchiMS BergerRO Pieper2007DNA damaging agent-induced autophagy produces a cytoprotective adenosine triphosphate surge in malignant glioma cells.Cell Death Differ14548558
- 63. Abedin MJ, Wang D, McDonnell MA, Lehmann U, Kelekar A (2007) Autophagy delays apoptotic death in breast cancer cells following DNA damage. Cell Death Differ 14: 500–510.MJ AbedinD. WangMA McDonnellU. LehmannA. Kelekar2007Autophagy delays apoptotic death in breast cancer cells following DNA damage.Cell Death Differ14500510
- 64. Carew JS, Nawrocki ST, Kahue CN, Zhang H, Yang C, et al. (2007) Targeting autophagy augments the anticancer activity of the histone deacetylase inhibitor SAHA to overcome Bcr-Abl-mediated drug resistance. Blood 110: 313–322.JS CarewST NawrockiCN KahueH. ZhangC. Yang2007Targeting autophagy augments the anticancer activity of the histone deacetylase inhibitor SAHA to overcome Bcr-Abl-mediated drug resistance.Blood110313322
- 65. Samaddar JS, Gaddy VT, Duplantier J, Thandavan SP, Shah M, et al. (2008) A role for macroautophagy in protection against 4-hydroxytamoxifen-induced cell death and the development of antiestrogen resistance. Mol Cancer Ther 7: 2977–2987.JS SamaddarVT GaddyJ. DuplantierSP ThandavanM. Shah2008A role for macroautophagy in protection against 4-hydroxytamoxifen-induced cell death and the development of antiestrogen resistance.Mol Cancer Ther729772987
- 66. Qadir MA, Kwok B, Dragowska WH, To KH, Le D, et al. (2008) Macroautophagy inhibition sensitizes tamoxifen-resistant breast cancer cells and enhances mitochondrial depolarization. Breast Cancer Res Treat 112: 389–403.MA QadirB. KwokWH DragowskaKH ToD. Le2008Macroautophagy inhibition sensitizes tamoxifen-resistant breast cancer cells and enhances mitochondrial depolarization.Breast Cancer Res Treat112389403
- 67. Schoenlein PV, Periyasamy-Thandavan S, Samaddar JS, Jackson WH, Barrett JT (2009) Autophagy facilitates the progression of ERalpha-positive breast cancer cells to antiestrogen resistance. Autophagy 5: 400–403.PV SchoenleinS. Periyasamy-ThandavanJS SamaddarWH JacksonJT Barrett2009Autophagy facilitates the progression of ERalpha-positive breast cancer cells to antiestrogen resistance.Autophagy5400403
- 68. Samudio I, Kurinna S, Ruvolo P, Korchin B, Kantarjian H, et al. (2008) Inhibition of mitochondrial metabolism by methyl-2-cyano-3,12-dioxooleana-1,9-diene-28-oate induces apoptotic or autophagic cell death in chronic myeloid leukemia cells. Mol Cancer Ther 7: 1130–1139.I. SamudioS. KurinnaP. RuvoloB. KorchinH. Kantarjian2008Inhibition of mitochondrial metabolism by methyl-2-cyano-3,12-dioxooleana-1,9-diene-28-oate induces apoptotic or autophagic cell death in chronic myeloid leukemia cells.Mol Cancer Ther711301139
- 69. Mishima Y, Terui Y, Mishima Y, Taniyama A, Kuniyoshi R, et al. (2008) Autophagy and autophagic cell death are next targets for elimination of the resistance to tyrosine kinase inhibitors. Cancer Sci 99: 2200–2208.Y. MishimaY. TeruiY. MishimaA. TaniyamaR. Kuniyoshi2008Autophagy and autophagic cell death are next targets for elimination of the resistance to tyrosine kinase inhibitors.Cancer Sci9922002208
- 70. Miselli F, Negri T, Gronchi A, Losa M, Conca E, et al. (2008) Is autophagy rather than apoptosis the regression driver in imatinib-treated gastrointestinal stromal tumors? Transl Oncol 1: 177–86.F. MiselliT. NegriA. GronchiM. LosaE. Conca2008Is autophagy rather than apoptosis the regression driver in imatinib-treated gastrointestinal stromal tumors?Transl Oncol117786
- 71. Shingu T, Fujiwara K, Bögler O, Akiyama Y, Moritake K, et al. (2009) Inhibition of autophagy at a late stage enhances imatinib-induced cytotoxicity in human malignant glioma cells. Int J Cancer 124: 1060–1071.T. ShinguK. FujiwaraO. BöglerY. AkiyamaK. Moritake2009Inhibition of autophagy at a late stage enhances imatinib-induced cytotoxicity in human malignant glioma cells.Int J Cancer12410601071
- 72. Shingu T, Fujiwara K, Bögler O, Akiyama Y, Moritake K, et al. (2009) Stage-specific effect of inhibition of autophagy on chemotherapy-induced cytotoxicity. Autophagy 5: 537–539.T. ShinguK. FujiwaraO. BöglerY. AkiyamaK. Moritake2009Stage-specific effect of inhibition of autophagy on chemotherapy-induced cytotoxicity.Autophagy5537539
- 73. Zakeri Z, Melendez A, Lockshin RA (2008) Detection of autophagy in cell death. Methods Enzymol 442: 289–306.Z. ZakeriA. MelendezRA Lockshin2008Detection of autophagy in cell death.Methods Enzymol442289306
- 74. Yoshioka A, Miyata H, Doki Y, Yamasaki M, Sohma I, et al. (2008) LC3, an autophagosome marker, is highly expressed in gastrointestinal cancers. Int J Oncol 33: 461–468.A. YoshiokaH. MiyataY. DokiM. YamasakiI. Sohma2008LC3, an autophagosome marker, is highly expressed in gastrointestinal cancers.Int J Oncol33461468
- 75. Carew JS, Nawrocki ST, Cleveland JL (2007) Modulating autophagy for therapeutic benefit. Autophagy 3: 464–467.JS CarewST NawrockiJL Cleveland2007Modulating autophagy for therapeutic benefit.Autophagy3464467
- 76. Amaravadi RK, Yu D, Lum JJ, Bui T, Christophorou MA, et al. (2007) Autophagy inhibition enhances therapy-induced apoptosis in a Myc-induced model of lymphoma. J Clin Invest 117: 326–336.RK AmaravadiD. YuJJ LumT. BuiMA Christophorou2007Autophagy inhibition enhances therapy-induced apoptosis in a Myc-induced model of lymphoma.J Clin Invest117326336
- 77. Amaravadi RK (2008) Autophagy-induced tumor dormancy in ovarian cancer. J Clin Invest 118: 3837–3840.RK Amaravadi2008Autophagy-induced tumor dormancy in ovarian cancer.J Clin Invest11838373840