University of Groningen Metformin Protects Rat Hepatocytes against Bile Acid-Induced Apoptosis

Background: Metformin is used in the treatment of Diabetes Mellitus type II and improves liver function in patients with non-alcoholic fatty liver disease (NAFLD). Metformin activates AMP-activated protein kinase (AMPK), the cellular energy sensor that is sensitive to changes in the AMP/ATP-ratio. AMPK is an inhibitor of mammalian target of rapamycin (mTOR). Both AMPK and mTOR are able to modulate cell death. Aim: To evaluate the effects of metformin on hepatocyte cell death. Methods: Apoptotic cell death was induced in primary rat hepatocytes using either the bile acid glycochenodeoxycholic acid (GCDCA) or TNFa in combination with actinomycin D (actD). AMPK, mTOR and phosphoinositide-3 kinase (PI3K)/Akt were inhibited using pharmacological inhibitors. Apoptosis and necrosis were quantified by caspase activation, acridine orange staining and Sytox green staining respectively. Results: Metformin dose-dependently reduces GCDCA-induced apoptosis, even when added 2 hours after GCDCA, without increasing necrotic cell death. Metformin does not protect against TNFa/ActD-induced apoptosis. The protective effect of metformin is dependent on an intact PI3-kinase/Akt pathway, but does not require AMPK/mTOR-signaling. Metformin does not inhibit NF-kB activation. Conclusion: Metformin protects against bile acid-induced apoptosis and could be considered in the treatment of chronic liver diseases accompanied by inflammation. Citation: Woudenberg-Vrenken TE, Conde de la Rosa L, Buist-Homan M, Faber KN, Moshage H (2013) Metformin Protects Rat Hepatocytes against Bile AcidInduced Apoptosis. PLoS ONE 8(8): e71773. doi:10.1371/journal.pone.0071773 Editor: Justin L. Mott, University of Nebraska Medical Center, United States of America Received August 22, 2012; Accepted July 7, 2013; Published August 12, 2013 Copyright: 2013 Woudenberg-Vrenken 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: This work was supported by the Dutch Digestive Foundation and the J.K. de Cock Foundation. 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. * E-mail: a.j.moshage@umcg.nl ¤a Current address: Department of Pathology and Medical Biology, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands ¤b Current address: Department of Cell Death and Proliferation, Institute of Biomedical Research of Barcelona, Spanish National Research Council (IIBB-CSIC), Barcelona, Spain


Introduction
Metformin is a drug primarily used in the treatment of Diabetes Mellitus type II where it suppresses glucose production by the liver. Recently, metformin was shown to have beneficial effects in patients with (non-alcoholic) fatty liver diseases (NAFLD) and poly-cystic ovarian syndrome (PCOS) [1,2]. In patients and in vivo models of non-alcoholic steatohepatitis (NASH), metformin reduced leptin secretion and aminotransferase levels and decreased liver size. Moreover, metformin treatment improved hepatocyte viability in fatty livers [3][4][5][6][7][8]. In addition, metformin protected hepatocytes from cell death induced by saturated fatty acids [9].
Metformin is known to stimulate AMP-activated protein kinase (AMPK) activity both in whole liver, primary hepatocytes, and a hepatoma cell line [10][11][12]. Among the 5 members of the AMPK family are AMPK-a1 and -a2 that are activated by metformin [10,13]. AMPK consists of a catalytic a subunit and two regulatory subunits (b, c; [10,14]. AMPK is involved in insulin signaling, energy homeostasis, and becomes activated upon a rise in cellular AMP concentration or changes in the AMP/ATP-ratio. Furthermore, AMPK can be activated by stimuli that do not affect the AMP/ATP-ratio, like hyperosmotic stress, hypoxia, oxidative stress or pharmacological compounds [12,[14][15][16][17][18][19][20]. AMPK activity is dependent on the phosphorylation of Thr172 in the a subunit [21]. Activation of AMPK using the cell permeable adenosine analogue 5-aminoimidazole-4-carboxamide 1-b-D-ribofuranoside (AICAR) was shown to be pro-apoptotic, via activation of JNK and caspase-3 in liver cells [22]. Also, in a rat hepatoma cell line AMPK activity stimulated apoptosis, and in pancreatic bcells both metformin and AICAR induced apoptosis. In contrast, AMPK activation reduced apoptosis in astrocytes and endothelial cells [23]. Moreover, in DLD-1 cells, Ark5, another AMPK family member, was protective against Fas-mediated cell death. Ark5 directly inhibited one of the effector caspases, caspase-6, and Ark5 activity was shown to be controlled by Akt, a key regulator in survival signaling [11,15,[24][25][26][27]. In whole liver, AMPK activity represses signaling via mammalian target of rapamycin (mTOR), a downstream target of Akt and phosphoinositide-3 kinase (PI3K). mTOR is a key player in transcription, translation, cytoskeletal arrangement, and protein degradation [14,16,26,[28][29][30][31][32][33]. Akt was found to suppress apoptosis in various cell types, including liver cells. In a rat hepatoma cell line, constitutive activation of PI3K blocks GCDCA-induced apoptosis. In primary rat hepatocytes, the protection of tauroursodeoxycholic acid (TUDCA) against GCDCA-induced apoptosis was abolished when the PI3K/Akt survival pathway was inhibited [34][35][36][37][38][39]. Several important survival pathways next to PI3K/Akt are present in hepatocytes, like the transcription factor nuclear factor-kB (NF-kB) and the mitogen activated protein (MAP) kinases [40]. Activation of NF-kB leads to the induction of survival genes and subsequently inhibition of apoptosis. In cholestatic livers, NF-kB is activated, and reduces liver injury [41], and glycochenodeoxycholic acid (GCDCA)-induced apoptosis was reduced by NF-kB activation in primary rat hepatocytes in vitro [42]. Furthermore, inhibition of both the PI3K pathway and one of the MAP-kinases p38 or ERK enhanced bile acid induced cell death in rat hepatocytes [43].
Considering the controversy about the role of metformin and its downstream target AMPK in apoptosis and the apparent beneficial effect of the drug in the treatment of NAFLD, we investigated the effect of metformin in two different models of hepatocyte damage. We studied an in vitro model of acute liver damage triggered by cytokines and a model of chronic liver disease induced by bile acids. We investigated whether metformin has effects on hepatocyte survival pathways and whether downstream targets of metformin modulate hepatocyte cell death. We describe a hepatoprotective action of metformin against bile acid-induced apoptosis that is independent of AMPK activation, but dependent on an intact PI3K/Akt signaling pathway.

Animals
Specified pathogen-free male Wistar rats (200-250 g) were purchased from Charles River Laboratories Inc (Wilmington, MA, USA). Rats were housed under standard laboratory conditions with free access to standard laboratory chow and water. Prior to isolation, rats were fasted overnight, and anesthetized using a combination of medetomidine/ketamine (i.p. injection). Experiments were approved by the Institutional Animal Care and Use Committee of the University of Groningen (IACUC-RuG) and were performed following their guidelines.

Rat Hepatocyte Isolation
Hepatocytes were isolated from rats by a two step collagenase perfusion as described before [44]. Cell viability was greater than 85% as determined by Trypan Blue exclusion. After isolation, 112,500 cells per cm 2 were plated on VitrogenH (Cohesion Technologies Inc., Palo Alto, CA, USA) coated plates in William's E medium (Invitrogen, Breda, The Netherlands) supplemented with 50 mg/mL gentamycin (Invitrogen) and penicillin-streptomycin-fungizone (Lonza, Verviers, Belgium). During the attachment period (4 hrs) 50 nmol/L dexamethasone (Dept. of Pharmacy, UMCG, Groningen, The Netherlands) and 5% fetal calf serum (Invitrogen) were added to the medium. Cells were cultured in a humidified incubator at 37uC and 5% CO 2 .
Cells were harvested at the indicated time points after the addition of the apoptotic stimuli, and washed three times with ice cold phosphate buffered saline (PBS) before the addition of hypotonic cell lysis buffer (protein analysis, caspase-3 assay), 2times concentrated sample buffer (Western blot analysis) or Trireagent (RNA isolation, Sigma-Aldrich) as described previously [43].

Caspase Enzyme Activity Assays
Caspase-3 like activity was assayed as described previously [43]. The arbitrary fluorescence unit (AFU) was corrected for the amount of protein in the cell lysate. Caspase-6 like activity was assayed according to the manufacturer's instructions (Biovision, Mountain View, CA, USA).

Sytox Green and Acridine Orange Nuclear Staining
To determine necrotic cell death at the indicated time points, hepatocytes were incubated for 15 minutes with Sytox green (Invitrogen) nucleic acid stain. Sytox green can only enter cells with compromised plasma membranes, and cannot cross the membranes of viable cells or apoptotic bodies. Hepatocytes exposed to 5 mmol/L H 2 O 2 (Merck Chemicals Ltd, Nottingham, Table 1. Sequences of primers and probes used for quantitative PCR analysis. UK) for 6 hrs served as positive control for necrosis. Apoptotic nuclei were visualized with acridine orange (Sigma-Aldrich) as described previously [44]. Fluorescent nuclei were visualized using an Olympus CKX41 microscope at 450-490 nm. Necrosis and apoptosis were quantified by counting fluorescent nuclei (necrotic or apoptotic cells) and the total number of cells in 3 randomly chosen high power fields.

Western Blot Analysis
Western blot analysis of cell lysates was performed by SDS-PAGE followed by semi dry-blotting to transfer the proteins to Hybond ECL nitrocellulose membrane (Amersham Biosciences, Piscataway, NJ, USA). Ponceau S 0.1% w/v (Sigma-Aldrich) staining was used to ensure electrophoretic transfer. Activation of AMPK was detected using the polyclonal antibody against phosphorylated AMPKa (Thr 172, Invitrogen) at a dilution of 1:500. Hepatocytes exposed to 250 mmol/L AICAR (Biomol Research Laboratories Inc, Plymouth Meeting, PA, USA) for 60 minutes served as positive control for phosphorylation and activation of AMPK. Akt activation was detected using the polyclonal antibody against phosphorylated Akt (Ser473, Cell Signaling Technology, Danvers, MA, USA) at a dilution of 1:1000. After Western blot analysis, blots were stripped as described previously [44] and incubated with a monoclonal antibody against b-actin (Sigma-Aldrich, dilution 1:10000) or GAPDH (Calbiochem, dilution 1:50000).
The blots were analyzed in a ChemiDoc XRS system (Bio-Rad, Hercules, CA, USA). Protein band intensities were quantified by Quantity One software (Bio-Rad).

RNA Isolation and Quantitative PCR
RNA isolation was performed as described previously [45]. RNA concentration was determined with the Ribogreen RNA quantitation reagent and kit (Invitrogen). Reverse transcription PCR (RT-PCR) was carried out on 2.5 mg of total RNA using random primers in a total volume of 50 mL using Moloney murine leukemia virus (M-MLV) reverse transcriptase system (Sigma-Aldrich) according to the manufacturer's instruction.
Quantitative PCR was performed on 4 mL 20-times diluted complementary DNA in a final volume of 20 mL [45]. Fluorescence was measured using the ABI PRISM 7700 Sequence Detector version 1.7 software (Applied Biosystems, Nieuwerkerk a/d IJssel, The Netherlands) starting with 10 minutes at 95uC, followed by 40 cycles of 15 seconds at 95uC and 1 minute at 60uC. Details of primers and probes are listed in Table 1. Each sample was analyzed in duplicate. 18S mRNA levels were used as endogenous control.

Statistical Analysis
All numerical results are reported as the mean of at least 3 independent experiments 6 standard error of the mean. For each experiment, the results were analyzed using the Kruskal-Wallis test to verify the significance. If P,0.05 for the Kruskal-Wallis test, a Mann-Whitney U test was used to determine the significance of differences between experimental groups. A P-value smaller than 0.05 was considered to be statistically significant. Figure 1. Metformin dose-dependently reduces bile acid-induced caspase-3 activity, but not TNFa/ActD-induced apoptosis. Primary rat hepatocytes were exposed to metformin (0.1-2 mmol/L) added 10 minutes prior to (A) GCDCA (50 mmol/L, 4 hrs) or (B) TNFa/ActD (20 ng/ml, 200 ng/ml, 16 hrs). Caspase-3 like activity is shown as fold induction compared to control values; control values were set at one. (C) Metformin prevents GCDCA (4 hrs) induced nuclear condensation as demonstrated by acridine orange staining, but has no effect on TNFa/ActD (16 hrs) induced nuclear condensation. Percentages represent condensed nuclei. Magnification 20X. (D) Metformin also reduces GCDCA-induced caspase-6 activity. Caspase-6 like activity was measured in primary rat hepatocytes exposed to GCDCA (50 mmol/L, 4 hrs) and/or metformin (1 mmol/L) and is shown as fold induction compared to control values; control values were set at one. N = 4 for each experiment. Statistical analysis: * p,0.05 or & p,0.01 compared to control. # p,0.05 compared to GCDCA-treated cells. doi:10.1371/journal.pone.0071773.g001

Metformin Dose-dependently Reduces Bile Acid Induced Apoptosis
To examine a potential hepatoprotective effect of metformin, two different models for apoptosis were used: bile acid (GCDCA)induced apoptosis and TNFa/ActD-induced apoptosis. GCDCAinduced caspase-3 activity peaks at 4 hrs [43], while for TNFa/ ActD-induced apoptosis, caspase-3 activity is maximal between 12-16 hrs (data not shown). Therefore, these time points were chosen to study the beneficial effect of metformin. Metformin inhibited GCDCA-induced caspase-3 activity dose-dependently from 24% inhibition at 0.1 mmol/L to an almost complete block (84% inhibition) at a concentration of 2 mmol/L (Fig. 1A). In contrast, 1 mmol/L metformin, a concentration that blocks GCDCA-induced caspase-3 activity for 63%, had no effect on TNFa/ActD-induced caspase-3 activity (Fig. 1B). Metformin (0.1-2 mmol/L) alone did not induce caspase-3 activity. However, higher concentrations (5 mmol/L and higher) were toxic to hepatocytes (data not shown). Acridine orange staining confirmed the results obtained with the caspase-3 assay. Condensed nuclei are visible in 47% of GCDCA-treated hepatocytes (Fig. 1C), whereas in combination with metformin this number of apoptotic cells is reduced by half to 23%. However, the TNFa/ActD- induced nuclear condensation is hardly affected by metformin (Fig. 1C). Apoptotic cells were not observed in control or metformin-exposed hepatocytes (Fig. 1C).
To demonstrate that metformin inhibits and not delays bile acid induced apoptosis, a time course study was performed. At the indicated time points, from four to nine hrs after addition of GCDCA, metformin significantly inhibited GCDCA-induced caspase-3 activity (Fig. 2A). After 24 hrs, the caspase-3 activity levels of GCDCA and GCDCA plus metformin treated cells were both comparable to control. Acridine orange staining was performed to ensure that the peak of caspase-3 activity of hepatocytes treated with GCDCA plus metformin was not between any of the time points analyzed. Both 4 and 24 hr after exposure, GCDCA-induced nuclear condensation was reduced by metformin by 72% (Fig. 2B).

Metformin does not Block the Uptake of GCDCA at the Cell Membrane
Next, we examined whether metformin only protects cells when added prior to GCDCA, or also after bile acid exposure. Metformin was added either 10 minutes before, simultaneously with, or 1, 2 or 3 hrs after addition of the bile acid. Metformin significantly decreased GCDCA induced caspase-3 activity when added up to 2 hrs after GCDCA (Fig. 3A). No difference in reduction of caspase-3 activity between a pre-incubation of 10 minutes with metformin or addition at the same time as GCDCA was detected (data not shown). Furthermore, equal uptake of fluorescent bile acids in hepatocytes treated with or without metformin was observed ( Figure 3B). These results indicate that the protective effect of metformin is not due to reduced uptake of GCDCA into the cell.

Metformin Reduces Apoptosis without Increasing Necrotic Cell Death
Subsequently, we studied whether the reduction in apoptotic cell death by metformin results in a shift towards necrotic cell death. At the peak of caspase-3 activity, the percentage of necrotic cells was determined. GCDCA caused necrosis in 11% of the cells, whereas 7% of the hepatocytes exposed to GCDCA and metformin were necrotic (Fig. 4). Also after 16 hours exposure, no significant differences between cultures treated with GCDCA or GCDCA plus metformin (1 mmol/L) could be observed (Fig. 4). No significant necrosis was observed in control cultures or cultures exposed to metformin alone (Fig. 4).

The Anti-apoptotic Effects of Metformin are Dependent on PI3K/Akt Signaling, but not on AMPK/mTOR Signal Transduction
The involvement of the known downstream targets of metformin, the AMPK/mTOR pathway was examined. AMPK was blocked using a pharmacological inhibitor (59-iodotubercidin), and mTOR activation was prevented by the specific inhibitor rapamycin. As shown in figure 5A and E, blocking either AMPK or mTOR had no effect on the anti-apoptotic actions of metformin and both inhibitors did not change GCDCA-induced caspase-3 activity or nuclear condensation. Hepatocytes treated with the inhibitors alone or in combination with metformin showed caspase-3 values similar to control cells (data not shown). Interestingly, Western blot analysis demonstrated that hepatocytes treated for 1 hr with either GCDCA or metformin caused a slight induction (2-fold) of phosphorylated AMPKa compared to control cells (Fig. 5B). Cells exposed to both GCDCA and metformin showed a further 1.5-fold increase of AMPKa phosphorylation compared to control hepatocytes, while cells treated with the known AMPK activator AICAR (5-aminoimidazole-4-carboxamide-1-b-d-ribofuranoside) induced an 8-fold induction of AMPKa phosphorylation.
Previous results showed the involvement of the PI3K survival pathway in the protective effect of TUDCA against GCDCAinduced apoptosis [43]. Thus, we investigated whether the PI3K pathway could be involved in the anti-apoptotic effects of metformin, using the specific PI3K inhibitor, LY294002. Metformin reduced the GCDCA-induced caspase-3 like activity with 52% (Fig. 5C). The protective effect of metformin against apoptosis was completely abolished when the PI3K pathway was blocked (Fig. 5C, E). In addition, the protein levels of phosphorylated Akt, the downstream target of PI3K, increased in time after exposure to metformin (Fig. 5D). As described previously [43], a significant increase in GCDCA-induced apoptosis was observed when the PI3K pathway was inhibited in hepatocytes ( Figure 5C).

Metformin is not an Inhibitor of NF-kB
Since inhibition of NF-kB sensitizes hepatocytes to TNFainduced cell death [37], it is important to determine the effect of metformin on NF-kB activation, prior to its use in inflammatory (TNFa-mediated) liver diseases. Therefore, we studied the involvement of NF-kB signaling in the protective effects of metformin. Previous studies demonstrated strong NF-kB-dependent cytokine-induced expression of inducible nitric oxide synthase (iNOS) in hepatocytes [37]. Cytokine mixture induces a 200-fold induction of iNOS expression compared to control cells (Fig. 6A), which was not affected by metformin. As expected, the transcriptional inhibitor actinomycin D completely blocked cytokineinduced iNOS expression (Fig. 6A). Metformin alone did not induce iNOS expression and showed expression levels similar to control hepatocytes ( Figure 6A). The qPCR results were confirmed using the caspase-3 assay. TNFa alone, or in combination with 1 mmol/L metformin did not induce caspase-3 activation (Fig. 6B). In contrast, caspase-3 activity was strongly induced in hepatocytes exposed to TNFa in combination with actinomycin D. Metformin exposed hepatocytes showed caspase-3 levels similar to control cells (data not shown).

Discussion
In this study we investigated the hepatoprotective effects of metformin in models of TNFa-and bile acid-induced apoptosis in primary rat hepatocytes. Other studies described that metformin improves the viability of hepatocytes in fatty livers and reduced serum levels of ALT and AST in NASH [6][7][8]. We show that metformin is protective against bile acid-induced cell death, while it has no effect on TNFa-induced apoptosis. Metformin reduces, but does not delay, GCDCA-induced caspase activity and nuclear condensation, but has no effect on TNFa/ActD-induced hepatocyte apoptosis. The opposite effects of metformin in these two models of hepatocyte apoptosis is most likely due to the involvement of different signaling pathways in cytokine and bile acid induced apoptosis. Cytokines like TNFa exert their effects via death receptors at the cell membrane, while bile acids need to be taken up by the bile acid transporter, Ntcp [46][47][48]. Furthermore, bile acid-induced apoptosis has been demonstrated to involve activation of the Fas pathway via EGF-receptor phosphorylation [49]. TNFa is known to activate the transcription factor NF-kB, while we have previously shown that GCDCA has no effect on NF-kB signaling [42].
To elucidate the protective mechanism of metformin, we first excluded the possibility that metformin is hepatoprotective via inhibition of GCDCA-uptake at the cell membrane. The results obtained with the fluorescent bile acids clearly demonstrated that metformin has no effect on bile acid uptake. Furthermore, metformin was able to reduce apoptosis when added up to 2 hrs after bile acid exposure. These data suggest that metformin does not interfere with bile acid uptake and activates survival pathways very rapidly. In this respect, the protective effect of metformin resembles the protective effect of TUDCA, TUDCA was protective against GCDCA-induced cell death when added up to 2 hrs after the bile acid [43]. We confirm previous findings [43], that 2 hours after GCDCA exposure, caspase-3 activation is still Figure 6. Metformin does not inhibit NF-kB activation. qPCR analysis for iNOS using cDNA of primary rat hepatocytes exposed for 4 hrs to cytokine mixture with or without 1 mmol/L metformin. The transcriptional inhibitor actinomycin D (200 ng/ml) served as positive control for NF-kB inhibition. (A). Metformin does not reduce cytokine mixture induced iNOS mRNA expression. (B) Metformin does not sensitize hepatocytes to TNFainduced apoptosis. Caspase-3 like activity was measured in primary rat hepatocytes exposed to TNFa (20 ng/ml, 16 hrs) with or without metformin (1 mmol/L) or actinomycin D (200 ng/ml, positive control for induction of TNFa-induced caspase-3 activity). Caspase-3 activity is shown as fold induction compared to control values; control values were set at one. N = 5 for each experiment. Statistical analysis: Fig. 6A * p,0.05 or & p,0.01 compared to control. # p,0.01 compared to cytokine mixture-treated cells. # p,0.01 compared to cytokine mixture plus metformin-treated cells. Fig. 6B  minimal, and apoptosis can be prevented, while beyond this threshold the apoptotic machinery cannot be reversed anymore.
Previously, we demonstrated that under particular conditions the mode of cell death can shift from apoptosis to necrosis [44]. In this study, metformin does not cause a switch from apoptotic to necrotic cell death. The percentage of necrotic cells is similar for GCDCA and GCDCA plus metformin treated hepatocytes. Furthermore, metformin does not significantly reduce GCDCAinduced necrosis. These results demonstrate that metformin only protects against apoptotic cell death, but not against necrotic cell death.
Since the AMPK/mTOR pathway is an important downstream target of metformin, we investigated the involvement of these pathways in the protective effect of metformin. Our results demonstrate that the AMPK/mTOR signaling pathway is not involved in the anti-apoptotic actions of metformin [16,17,26,50]. Both known inhibitors for AMPK and mTOR, 59-iodotubercidin and rapamycin, had no effect on the reduction in GCDCAinduced apoptosis caused by metformin. Although AMPK does not appear to be involved in the protective effect, it is of interest to note that GCDCA slightly increased the phosphorylation and activation of AMPK. The mechanism of this effect and its relevance remains to be elucidated. Since AMPK is not involved in the protective actions of metformin, the drug must exert its antiapoptotic actions via other pathways. Our results demonstrate that the PI3K/Akt pathway plays an important role in the protective effect of metformin. The protective effect of metformin against GCDCA-induced apoptosis is dependent on an intact PI3K pathway. These data are in accordance with other reports emphasizing the importance of PI3K/Akt in the protection against bile acid-induced apoptosis [34][35][36]38,39,43]. On the contrary, our data imply that members of the MAP kinase family are not involved in the anti-apoptotic signaling of metformin, since inhibitors of the anti-apoptotic MAP kinases ERK and p38 do not reduce the protective effect of metformin (data not shown).
The possibility that GCDCA sensitizes hepatocytes to NF-kB activation can be excluded, since metformin, either alone or in combination with GCDCA, did not induce the expression of the NF-kB-dependent gene iNOS. One study reported caspaseregulated Bcl-xl expression, via caspases downstream of ERK1/ 2. Using the specific ERK1/2 inhibitor U0126, Bcl-xl expression was reduced and apoptosis was induced. This U0126-induced down-regulation of Bcl-xl was reversed by the pancaspase inhibitor Z-VAD-FMK [51]. Although a different cell type was used, caspase-dependent Bcl-xl expression could explain our results: in our model GCDCA-induced caspase activity is reduced by metformin and this could subsequently result in an additional increase in Bcl-xl expression.
Our results clearly demonstrate that metformin does not inhibit NF-kB activity. Hepatocytes are not sensitized to TNFa-induced apoptosis by metformin, and metformin does not affect cytokine induced NF-kB-dependent gene transcription. Our data are conflicting with some reports describing AMPK-mediated inhibition of NF-kB by metformin [52,53]. However, these studies are not performed in hepatocytes but in endothelial cells, which could explain the discrepancy. Furthermore, these reports show the involvement of AMPK in metformin signaling, while in our study AMPK is not involved in the anti-apoptotic actions of metformin.
In summary, we have shown that metformin is protecting hepatocytes against GCDCA-induced apoptosis, while metformin has no effect on TNFa/ActD-induced apoptosis. The PI3K/Akt survival pathway is required for the anti-apoptotic effect of metformin; however the protection is independent of the AMPK/ mTOR signaling pathway. In addition, we have demonstrated that metformin does not inhibit NF-kB activity and hence does not sensitize hepatocytes to TNFa-induced apoptosis. The latter finding would make metformin suitable in the treatment of chronic liver diseases, in which an inflammatory component is present.