Induction of Sphk1 activity in obese adipose tissue macrophages promotes survival

During obesity, adipose tissue macrophages (ATM) are increased in concert with local inflammation and insulin resistance. Since the levels of sphingolipid (SLs) in adipose tissue (AT) are altered during obesity we investigated the potential impact of SLs on ATMs. For this, we first analyzed expression of SL metabolizing genes in ATMs isolated from obese mice. A marked induction of sphingosine kinase 1 (Sphk1) expression was observed in obese ATM when compared to lean ATM. This induction was observed in both MGL-ve (M1) and MGL1+ve (M2) macrophages from obese WAT. Next, RAW264.7 cells were exposed to excessive palmitate, resulting in a similar induction of Sphk1. This Sphk1 induction was also observed when cells were treated with chloroquine, a lysosomotropic amine impacting lysosome function. Simultaneous incubation of RAW cells with palmitate and the Sphk1 inhibitor SK1-I promoted cell death, suggesting a protective role of Sphk1 during lipotoxic conditions. Interestingly, a reduction of endoplasmic reticulum (ER) stress related genes was detected in obese ATM and was found to be associated with elevated Sphk1 expression. Altogether, our data suggest that lipid overload in ATM induces Sphk1, which promotes cell viability.


Introduction
White adipose tissue (WAT) acts as the major body storage for fat from which free fatty acids (FFAs) can be mobilized [1]. FFAs are used as energy supply during starvation and for heat production, but also act as building blocks for various essential lipids such as steroids, eicosanoids, (phospho)glycerolipids and SLs [2]. Over-nutrition and sedentary life promote an increase of WAT mass and adipocyte size, favouring obesity and imbalanced homeostasis of metabolism. During obesity, adipocyte necrosis takes place due to excessive ER stress and hypoxia [3]. Subsequently, excessively FFAs are released into the circulation together with chemokines and pro-inflammatory cytokines, triggering further local recruitment of inflammatory cells in the adipose tissue. This cascade promotes a pro-inflammatory environment, which drives insulin resistance (IR) and type-2 diabetes [4]. The adaptive and innate immune system both contribute to the chronic low grade inflammatory status of obese WAT. Changes in virtually all immune cells have been associated to obese adipose tissue derailment, including T cells,  [5][6][7][8][9][10][11][12][13][14]. The most dramatically in numbers increased immune cell in obese WAT is the ATM [15]. Moreover, during obesity the ATM undergoes a change in phenotype. Obese ATMs are polarized towards a more classically activated (M1) state and are associated with lipid accumulation, whereas in lean WAT alternatively activated (M2) ATMs prevail [13,14,16]. Interestingly, treatment with rosiglitazone in mice induced a redistribution of lipids towards adipocytes and reduced the presence of M1 polarized ATM [16]. Recently a potential role for lysosomes in the phenotypic change of obese ATMs has been recognized. Induction of lysosome biogenesis in these cells has been observed, a process also excited by lysosomal stress [17]. In line with this, we recently found glycoprotein nonmetastatic melanoma protein B (Gpnmb) to be induced in obese ATM as a consequence of lipid-induced lysosomal stress [18]. Of interest, lysosomal stress also occurs in inherited disorders in intralysosomal sphingolipid catabolism, either caused by a deficiency in a lysosomal glycosphingolipid hydrolase such as glucocerebrosidase in Gaucher disease or by a generalized lysosome dysfunction as in Niemann-Pick type C disease. Both in Gaucher disease and Niemann-Pick type C disease Gpnmb has also been found to be markedly induced [19][20][21]. These findings suggest that impaired lysosomal sphingolipid metabolism induces lysosome biogenesis and might be considered to also occur in obese ATMs.

neutrophils, Natural Killer (NK) cells, dendritic cells, NK T cells, invariant NK T cells, eosinophils, mast cells and B cells
There exist numerous SL species with various functions, ranging from structural membrane components to bioactive lipid mediators [22]. SL synthesis universally begins with the condensation of serine to palmitoyl-CoA at the cytosolic leaflet of the ER generating sphinganine (Spha) [23]. Next, Spha acylation is carried out by ceramide synthases (CerS), generating dihydroceramides (DHCer) that are converted into ceramides (Cer) by the action of a desaturase (DES). Excessive Cer is known to induce programmed cell death. Normally, it serves as building block for more complex SLs such as sphingomyelin (SM), ceramide 1-phosphate (Cer-1-P) and the carbohydrate containing glycosphingolipids (GSLs) [24]. The catabolism of complex GSLs resides largely in lysosomes and depends on stepwise hydrolysis of the terminal sugar moieties [23]. The formed Cer is next deacylated by acid ceramidase to generate the bioactive molecule sphingosine (Spho), which is either transported to the ER and re-enters the SL cycle being transformed to ceramide (salvage pathway) or phosphorylated by sphingosine kinases (Sphk) 1 or 2 to generate sphingosine-1-phosphate (S1P) [25][26][27]. S1P can be exported or dephosphorylated again by a phosphatase to Spho. However, most S1P is transformed by S1P lyase (SPL1) to phosphoethanolamine and trans-2-hexadecenal [28]. Oxidation of the latter by the long change aldehyde dehydrogenase (ALDH3a2) renders palmitoleic acid that is further catabolized by mitochondrial fatty acid beta-oxidation.
To study the role of SL in observed ATM changes during obesity, we first analyzed gene expression profiles of different SL synthesizing -and degrading enzymes in sorted ATM from both genetic and diet-induced obese (DIO) mouse models. Sphk1 gene expression was found to be strikingly induced in obese ATM from both models. This induction was observed in both MGL-ve (M1) and MGL1+ve (M2) macrophages from obese WAT. Concomitantly, Sphk1 activity was found to be elevated. Exposing cultured RAW 264.7 macrophage-like cells to high amounts of palmitate also induced Sphk1 gene expression and activity. Finally, we provide evidence that Sphk1 plays a critical role in macrophage survival during lipotoxic conditions. BSA Fraction V fatty acid-free (Roche) as described previously [18] and used at 250 and 500 μmol/L. Interleukin (IL)-4 (R&D Systems), IL-10 (PeproTech), and interferon-γ (IFN-γ; PeproTech) were used at 50 ng/mL; lipopolysaccharide (LPS) from Salmonella minnesota R 595 (Alexa) was used at 100 ng/mL; chloroquine (CQ) was used at 40 μmol/L (Sigma); tunicamycin at 10 μg/mL (Sigma); the Sphk1 inhibitor, SK1-I 12.5 μmol/L (Enzo Life Sciences).

Animals
12-week-old male C57BL/6J control mice (WT) and leptin-deficient obese (ob/ob) mice (C57BL/6J background) were purchased from Harlan laboratories (Horst, Netherlands). The animals were fed ad libitum commercial chow diet (AM-II) (Arie Blok BV, Woerden, the Netherlands). 6-week-old male C57Bl/6J mice were fed ad libitum a high fat diet (HFD; D12492, Research Diets Inc.; consisting of 20 kcal % protein, 20% carbohydrates, and 60% fat) or low-fat diet (LFD; D12450B, Research Diets Inc.; consisting of 20 kcal % protein, 70% carbohydrates, and 10% fat) for 16 weeks. Principles of laboratory animal care were followed. All animal protocols were approved by the Institutional Animal Welfare committee of the Academic Medical Center in the Netherlands (#DBC101943). The mice (n = 6 per group unless stated otherwise) were housed in a temperature-controlled room on a 12-h light-dark cycle.
To obtain CD11b + cells, SVFs were incubated with anti-CD11b conjugated magnetic microbeads and enriched according to the manufacturer's protocol (Miltenyi Biotec). CD11b + selected cells were labelled using anti-mouse F4/80 fluorescein isothiocyanate and analyzed using a FACSCalibur instrument (BD Biosciences) to assess macrophage purity.

Sphingolipid quantification and Sphk1 activity assay
Ceramide (Cer) and glycosphingolipid (GSL) levels were determined by HPLC as described previously [29]. Shortly, after adding C17-sphinganine (Avanti) as internal standard, lipids were extracted according to Bligh/Dyer et al [30]. Subsequently, the chloroform phase was thoroughly dried, and deacylation of lipids was performed in 0.5 mL of 0.1 M NaOH in methanol in a microwave (CEM microwave Solids/Moisture System SAM-255) for 60 min. Deacylated lipids were derivatized on-line for 30 min with Phtaldialdehyde (Sigma/Aldrich) and separated with a Ultra High Pressure Liquid Chromatography system with fluorescence detection (UHPLC Thermo Scientific) and a Alltima C 18 3 μm, 150 mm x 4.6 mm reverse-phase column (Grace Davison Discovery Science Inc., USA). All samples were measured in triplicate, unless otherwise stated, and in every run a reference sample was included.
Sphingosine-1-phosphate (S1P) detection and quantification was performed by liquid chromatography coupled to tandem mass spectrometry (LC-MS-MS). C17 base S1P (Avanti) was used as internal standard. After a total extraction and protein precipitation with chloroformmethanol 1:2 (v/v), the solvents were evaporated at 30˚C by Eppendorf concentrator 5301. The residue was extracted with butanol/water 1:1 (v/v), the upper phase was dried and dissolved in 100μL methanol.
Sphk1 activity was measured as described previously [32]. Briefly, after CD11b + enrichment, or culture of RAW 264.7, cells were washed three times with PBS, and disrupted by one freeze/thaw cycle and 20 passages thought a 25 gauge needle in Sphk1 assay buffer (20 mM Tris-HCl (pH 7.4), 1 mM EDTA, 20% glycerol, 1 mM 2-mercaptoethanol, 1 mM Na-orthovanadate, 40 mM β-glycerophosphate, 15 mM NaF, 0.5 mM 4-deoxypyridoxine, and protease inhibitor cocktail (Roche Applied Science, EDTA free)). In low salt and 0.25% Triton X-100, Sphk2 activity is negligible [26]. After protein determination the assay was performed in a glass tube using 60 μgr of sample, which was incubated with 50 μM Sphingosine (Avanti), 0.25% Triton X-100, 10 mM MgCl 2 and 1.1 mM ATP (Roche) for 60 minutes at 37˚C. The reaction was stopped by adding chloroform:methanol, a different extraction method was used to remove the Spho excess. After protein precipitations, lipids were extracted from the supernatant the supernatant according to Blight and Dyer method [30]. Phase separation extraction with chloroform:methanol:water; 1:1:0.9 (v/v/v) was required. The upper phase was dried and further extracted by butanol:water; 1:1 (v/v), the butanol phase was taken to the dryness and the residue was dissolved in 100μL methanol. Finally, S1P was determined by LC-MS-MS as previously described.

RNA isolation and Q-PCR analysis
Total RNA was extracted from FACS-sorted ATMs, or CD11b + enriched cells using TRIzol (Invitrogen) reagent and the NucleoSpin II extraction kit following manufacturer's instructions (Macherey Nagel). NanoDrop spectrophotometer (NanoDrop Technologies) was used to determine RNA concentrations. Equal amounts of RNA were used to synthesize cDNA according to the manufacturer's method (Invitrogen). iCycler MyiQ system (Bio-Rad) was used for real-time Q-PCR to analyse gene expression. Expression levels in mouse were normalized to those of acidic ribosomal phosphoprotein 36B4, also referred to as P0. Analysis of the relative changes in gene expression was performed using the 2 -ΔΔCT method. List of primers used for the Q-PCR are in the S1 Table. Gene expression analysis for the various markers could not be followed up by confirmation at the level of protein due to the limited yield after cell enrichment procedures.

Western blot analysis
After stimulations of cells with palmitate, or BSA vehicle alone, cells were washed three times with PBS and total cell lysates were prepared in RIPA lysis buffer (150 mmol/L NaCl, 50 mmol/L Tris-HCl pH 7.4, 2 mmol/L EDTA, 0.5% deoxycholaat, 1 mmol/L Na 3 VO 4 , 20 mmol/ L NaF, 1 mM PMSF and 0.5% Triton X-100) supplemented with protease inhibitor cocktail (Roche). Lysates were incubated for 30 min on ice and subsequently centrifuged at 4˚C for 10 min at 14.000x g. Samples were separated on a 7.5% SDS gel and transferred to nitrocellulose membranes. Primary antibodies used: rabbit anti-PARP (Cell signalling; #9542) and mouse anti-alpha tubulin (Cedarlane Laboratories Limited).

Cell viability assay
Cell viability was determined using the WST-1 assay (Roche) following the manufacturer's instructions. Absorbance at 450 nm was determined using an ELISA plate reader (Synergy; Bio Tek). WST-1 is a metabolic activity indicator and therefore should be interpreted with caution.

Heat-map and cluster analysis
Hierarchical cluster analysis was performed using relative gene expression levels, which were transformed to Z-score values (mean centered and scaled by standard deviation), using the HierarchicalClustering module from GenePattern software [33]. Genes were clustered using the pairwise complete linkage method, in which distance are the calculated Pearson's correlation coefficient. Fold change (log2) values of lipids in CD11b + enriched were represented in a heat map using Genepattern software.

Statistical analysis
All results are expressed as mean ± S.D. The statistical significance of differences between groups was analyzed using unpaired Student t-test or Whitney-Mann as indicated.

Sphk1 expression is increased in obese ATM from HFD and ob/ob mice
To get insight in the role of GSL metabolism in obese ATM, a selected set of genes involved in GSL synthesis, breakdown (S1 Fig) and transport was analyzed by quantitative PCR (QPCR) in sorted ATM populations isolated as described previously [14,18]. LFD fed mouse ATM (F4/80 + MGL1 + : 'M2') and HFD fed mouse ATM (F4/80 + MGL1 -: 'M1') and (F4/80 + MGL1 + : 'M2') were studied. Hierarchical cluster analysis of the gene expression z-scores revealed that three clustered families can be observed in the mice challenged with HFD and LFD (Fig 1A). The first group comprises 9 genes, which are elevated in HFD F4/80 + MGL1 + ATM and reduced in F4/80 + MGL1 -ATM from HFD and F4/80 + MGL1 + ATM from LFD fed mice. The second group consists of genes displaying elevated expression in LFD F4/80 + MGL1 + ATM and reduced expression in both ATM populations isolated from the HFD mice. The third group is composed of two genes, Sphk1 and phosphatidic acid phosphatase type 2A (Ppap2a), which are highly elevated in HFD fed mice F4/80 + MGL1 -ATM. Considering that Sphk1 was the gene most prominently induced in sorted obese macrophages compared to lean/LFD models and given the pivotal role of Sphk1 activity for instance in pancreatic β-cell survival during lipotoxic conditions, we focussed on Sphk1 regulation and role in obese ATM [34,35].
The relative expression levels of Sphk1 in the ATM populations are depicted in more detail in a graph ( Fig 1B). Remarkably, the isoenzyme Sphk2 behaves in the opposite direction in the sorted ATM of HFD animals (Fig 1C).
A comparable gene expression analysis was performed focussing on obese ATM populations isolated from the genetic, leptin-deficient ob/ob mouse model. The hierarchical cluster analysis is depicted in Fig 1D. In sorted ATM from lean mice a comparable profile was observed, when compared to the LFD group. Importantly, in sorted ob/ob F4/80 + MGL1 -ATM Sphk1 was highly expressed as well (Fig 1D). The relative gene expression of Sphk1 in sorted ob/ob ATM is depicted in the graph in Fig 1E. As for the HFD ATM populations Sphk2 levels are also in the opposite direction, so higher in the lean group and reduced in ob/ob ATM populations (Fig 1F).

Sphk1 activity is increased in ob/ob WAT enriched CD11b + cells
Next we addressed if the increase in Sphk1 gene expression also translated into an increase in Sphk1 activity. To increase ATM yield we enriched for macrophages by positively selecting CD11b + cells from the SVF of lean and obese ob/ob WAT using anti-CD11b antibodies coated with magnetic beads (MACS) [36]. First we analyzed the phenotype of the CD11b + enriched cells. FACS analysis revealed that the CD11b + populations where >90% positive for F4/80 indicating that the majority of the cells are macrophages. Moreover, QPCR gene expression analysis revealed that the CD11b + enriched cells and sorted ATM displayed similar induction of Gpnmb and CD11c expression (Fig 2A and 2B). ob/ob ATM (Fig 2C and 2D). The enrichment for CD11b + cells provided sufficient yield to determine Sphk1 enzyme activity and analysis of lipid content.
In line with the increase in Sphk1 gene expression in FACS sorted ob/ob ATM, we found a marked increase of Sphk1 activity in CD11b + enriched cells isolated from ob/ob SVF (Fig 3A). Additionally, lipid analysis revealed that the Sphk1 substrate and S1P precursor, Spho, was significantly elevated in ob/ob CD11b + cells (Fig 3B). S1P levels from ob/ob macrophages were in the same range as the lean cells (Fig 3C).
Interestingly, further lipid analysis showed that ob/ob ATM present a different lipid signature compared to WT. The sphingoid bases, Spho, sphingosine-diene and Spha are increased together with the GSL lactosylceramide (LacCer) and glycosylceramide-diene (GlyCer-diene) (Fig 3D). The GSL globotriaosylceramide (Gb3) was significantly reduced in ob/ob CD11b + ATM, whereas several analyzed lipid species were not significantly changed when compared to the lean cells (Fig 3D).

Sphk1 gene expression and activity are induced in macrophages by palmitate and CQ
It is known that various factors during obesity are altered. Not only the inflammatory status of the AT switches from a T helper cell type 2(Th2)-like (T cell-originated IL-10 and eosinophil IL-4) to a Th1-like environment (T cell-derived IFN-γ), but also metabolic changes occur such as hypoxia, ER stress, lipotoxicity induced by FFA and more recently described lysosomal stress [18,37,38]. To get more insight in the regulation of Sphk1 expression and its activity in obesity, we systematically tested above conditions using the mouse macrophage-like RAW264.7 cells. We first studied inflammation by investigating the anti-inflammatory cytokines IL-4 and IL-10 and the pro-inflammatory factors IFN-γ and LPS. (Fig 4A). In line with earlier work we found that LPS induced Sphk1 gene expression [39]. Efficacy of the inflammatory stimuli was verified by analysing TNF-α and arginase-1 (S2 Fig). Neither hypoxia, induced by using CoCl2, nor ER stress, induced using tunicamycin (TM) resulted in elevation of Sphk1 gene expression (Fig 4A). Efficacy of the stimuli was again confirmed. CoCl2 potently induced GLUT1 expression and CHOP was strongly induced with TM. We recently found that mimicry of lipotoxicity using palmitate induced Gpnmb, a marker induced upon lysosomal stress [18]. Potent induction of Sphk1 gene expression was found by both palmitate and the lysosomal stressor chloroquine (CQ) (Fig 4A). Efficacy of the stimuli was verified by Gpnmb induction (S2 Fig). In parallel Sphk2 gene expression was also determined and was found not to be induced in the conditions tested (Fig 4B).
In agreement with the gene expression data, Sphk1 activity was increased in RAW264.7 challenged with palmitate and LPS (Fig 5A). Next we analyzed S1P levels under the same conditions and observed that palmitate and LPS induced S1P levels ( Fig 5B). Interestingly, also other (G)SL species analyzed showed an increase upon palmitate treatment (Fig 5C).

Sphk1 protects macrophages from lipid induced apoptosis
Lipotoxicity induces ER stress amongst others characterized by the activation of the C/EBP homologous protein (CHOP)-dependent unfolded protein response pathway [40]. Furthermore, previous work showed that dysregulation of sphingolipid metabolism induces ER stress via CHOP upregulation [41]. We observed an increased Spho versus S1P ratio in obese ATM (Fig 3D). Furthermore, gene expression analysis in FACS sorted ATM revealed that the ratio of CHOP/Sphk1 is decreased in both the HFD fed and ob/ob ATM populations (Fig 6A and  6B), suggesting that obese macrophages may be protected from ER stress and associated apoptosis. To test if Sphk1, thus reducing Ceramide and/or Spho, contributes to survival, RAW264.7 were challenged with BSA coupled palmitate in the presence of the Sphk1 inhibitor SK1-I. Analysis of S1P levels showed that levels are indeed decreased in cells incubated with SK1-I (Fig 6C). By using different assays it is demonstrated that Sphk1 activity is crucial for survival upon challenge with palmitate. In the presence of SK1-I, thus no Sphk1 activity, cell survival is severely reduced after overnight incubation using WST-1 as a measure for cell survival ( Fig 6D). Furthermore, it was found that PARP was already processed after 8h when SK1-I was used to block Sphk1 activity (Fig 6E). In the presence of CQ, SK1-I also perturbed  Taken together the data this suggests that Sphk1 activity is required for protection of ATM against FFA-induced cell death.

Discussion
It is widely accepted that the risk of developing insulin resistance during obesity is associated with altered SL metabolism [42,43]. Changes in the innate and adaptive immune system during obesity also are known to cause inflammation of adipose tissue [44]. Macrophages in the adipose tissue spectacularly increase in number and show a more M1 prone phenotype [15,45]. The present study aimed to identify a possible link between abnormal SL metabolism and ATM during obesity. For this, we first comparatively analyzed in sorted ATM populations from normal, ob/ob and HFD fed mouse the expression of genes involved in SL metabolism.
Obese ATM showed down regulation of most SL metabolism genes as compared to lean M2 ATM. However expression Sphk1 mRNA was most prominently induced in obese ATM in comparison to lean ATM. Moreover, we confirm increased Sphk1 activity in obese CD11b + enriched ATM. In line with this finding, elevated Sphk1 expression in total AT in obese subjects and mice has previously been reported [42,46,47]. Interestingly, a lack of differential expression of Sphk1 in ob/ob M1 and M2 macrophages was observed, whereas M1 ATM from HFD presented an increased expression compared to M2. Even though the genetic background between all the groups were the same, they present intrinsic differences. ob/ob are leptin-deficient, younger and have a different diet compared to the HFD group. Other studies reported that aging alone already has an impact on sphingolipid composition in organs such as brain [48]. Furthermore, HFD model animals increase weight during a defined period when the diet is offered, whereas the ob/ob mice have been dealing with their condition since pups, allowing for adaptive mechanisms. Alternatively the diet itself is strongly associated with obesity and can have a considerable impact on metabolic regulation, possibly affecting Sphk1 regulation in M1 and M2 macrophages. Interestingly, the Sphk1 isoform, Sphk2, was found down regulated in obese ATM from both models. Currently it is unknown by what mechanism this is triggered, but several studies also describe that reduced Sphk2 is accompanied by induced Sphk1. Gao et al. showed in tumour cells that Sphk1 ablation did not display any effects on Sphk2 expression and S1P levels [49]. Other studies using different models, namely, tumour cells, acute colitis and knock out mice in which Sphk2 expression was depleted, showed an increase of Sphk1 expression, activity and S1P levels [46,47,50,51]. The Spiegel group demonstrated that the molecular mechanism of Sphk1 induction by Sphk2 downregulation is histone deacetylase HDAC1/2 activity and c-Jun dependent [51]. In addition, it has been shown in RAW264.7 that palmitate reduced HDAC activity and induced c-Jun [52,53]. Altogether these reports suggest that the Sphk1 increase observed in obese ATM might in part be due to a reduction of the Sphk2 expression levels. The observed induction of Sphk1 in obese ATM was recapitulated in macrophage-like RAW264.7 cells exposed to a high amount of palmitate, hence mimicry of a lipotoxic environment. This was not accompanied by a decrease in Sphk2 expression. The reason for this is not understood, but possibly the ratio of Sphk1 vs Sphk2 is relevant in this context. It has been previously reported that Sphk1 expression is also induced by palmitate in C2C12 myocytes, primary hepatocytes and HepG2 challenged with FFA [54,55]. In parallel, we confirmed that palmitate feeding to RAW264.7 cells also promotes Sphk1 activity. Of note, we observed in obese ATM an elevation of Spho, the substrate of sphingosine kinases. No concomitant increase in S1P was detected, suggesting that either S1P is rapidly secreted, or degraded through S1P lyase activity followed by beta-oxidation, or both. Earlier, Samad et al. demonstrated that plasma S1P levels were increased in ob/ob mice, yet S1P levels were hardly detectable in AT [42]. Further studies reported that S1P levels are increased in human plasma from diabetic obese individuals [42,56]. The majority of the circulating S1P is bound to the apolipoprotein M (ApoM) in the high-density lipoprotein (HDL) [57]. Furthermore, secretion of S1P by the obese AT and down regulation of S1P degrading enzymes has been reported, again suggesting that great part of the S1P generated in the tissue is secreted [56]. In our gene expression analysis from the sorted ATM, we show that crucial genes involved in S1P degradation; SPL1 and aldh3a2 are down regulated during obese conditions (Fig 1A and 1D). Taken all together, our results suggest that in vivo most of the S1P generated by the cells is likely to be secreted and released into the extracellular compartment, rather than being degraded. Interestingly, it has been reported in obese mice that treatment with rosiglitazone induced changes in ceramide levels in ATM and triggered redistribution of lipids towards adipocytes, improving insulin sensitivity in obese animals and a reduced presence of M1 polarized ATM, underscoring the association between lipid alterations and M1 ATM polarization [16]. Lastly, we explored the biological role of Sphk1 activity in macrophages undergoing a high FFA pressure, hence mimicking local adipocyte-derived FFA elevation in obesity. Unlike Cer and Spho, a relative increase in S1P levels promotes cell survival and inhibits apoptosis [58]. The control of cell fate depends on the balance between S1P on one hand, and on ceramide and Spho on the other hand, which is known as the sphingolipid biostat/rheostat [59]. It is reported that increased Sphk1 activity and consequently increase S1P/Cer-Spho, plays a critical role in pancreatic β-cell survival under lipotoxic conditions [34,35]. In addition, it recently has been demonstrated in dendritic cells that S1P generated by Sphk2 activity is uncoupled from cell survival, since Sphk2 activity failed to rescue Sphk1 deficient cells that were exposed to LPS [60]. Moreover, it has been shown that Sphk1 overexpression in muscle counteracts Cer levels and ameliorates insulin resistance in HFD mice corroborating the role of Sphk1 in cell survival [61]. Furthermore, Boslem et al. demonstrated that glucosylceramide synthase overexpression protects pancreatic β-cells from palmitate driven CHOP induction and apoptosis by increasing the ratio of glucosylceramide/Cer, ergo reducing Cer and Spho levels [62]. Interestingly, the above discussed rosiglitazone effects in ob/ob mice coincided with reduced ceramide species containing palmitate as a fatty acid group (Cer(d18:1/16:0)) [16]. In line with those studies, our data on increased gene expression and Sphk1 activity in vivo, suggests a shift towards S1P production counteracting the apoptosis supposedly promoted by Spho and Cer. Additionally, our gene analysis showed a reduction of CHOP expression in the obese ATM, indicating that those cells are protected from the lipid spill over by the adipocytes. Importantly, when RAW264.7 cells were treated simultaneously with the Sphk1 inhibitor (SK1-I) and palmitate, cell death induction was not prevented, suggesting an essential protective role of Sphk1 during lipotoxic conditions in macrophages. Consistently, Hill et al. showed recently that obese ATM present an enhanced survival phenotype through a mechanism involving NF-kB activation [63]. Interestingly, it has also been demonstrated that Sphk1 seems not to be required for inflammatory responses in macrophages, reinforcing the idea of Sphk1 playing an essential pro-survival role in obese ATM [64].
In summary, our data suggest that lipid-mediated lysosomal stress induction promotes Sphk1 activity in obese ATM, favouring macrophage survival, thus allowing the generation of a local lipid sink in AT and thereby contributing to the prevention of acute systematic lipotoxicity.