Defective uptake of triglyceride-associated fatty acids in adipose tissue causes the SREBP-1c-mediated induction of lipogenesis

Lipoprotein lipase (LPL) is the only known enzyme in the capillary endothelium of peripheral tissues that hydrolizes plasma triglycerides and provides fatty acids (FAs) for their subsequent tissue uptake. Previously, we demonstrated that mice that express LPL exclusively in muscle develop essentially normal fat mass despite the absence of LPL and the deprivation of nutritionally derived FAs in adipose tissue (AT). Using this mouse model, we now investigated the metabolic response to LPL deficiency in AT that enables maintenance of normal AT mass. We show that the rate of FA production was 1.8-fold higher in LPL-deficient AT than in control AT. The levels of mRNA and enzymatic activities of important enzymes involved in FA and triglyceride biosynthesis were induced concomitantly. Increased plasma glucose clearing and (14)C-deoxyglucose uptake into LPL-deficient mouse fat pads indicated that glucose provided the carbon source for lipid synthesis. Leptin expression was decreased in LPL-deficient AT. Finally, the induction of de novo FA synthesis in LPL-deficient AT was associated with increased expression and processing of sterol regulatory element binding protein 1 (SREBP-1), together with an increase in INSIG-1 expression. These results suggest that in the absence of LPL in AT, lipogenesis is activated through increased SREBP-1 expression and processing triggered by decreased availability of nutrition-derived FAs, elevated insulin, and low leptin levels.     

Brain insulin controls adipose tissue lipolysis and lipogenesis

White adipose tissue (WAT) dysfunction plays a key role in the pathogenesis of type 2 diabetes (DM2). Unrestrained WAT lipolysis results in increased fatty acid release, leading to insulin resistance and lipotoxicity, while impaired de novo lipogenesis in WAT decreases the synthesis of insulin-sensitizing fatty acid species like palmitoleate. Here, we show that insulin infused into the mediobasal hypothalamus (MBH) of Sprague-Dawley rats increases WAT lipogenic protein expression, inactivates hormone-sensitive lipase (Hsl), and suppresses lipolysis. Conversely, mice that lack the neuronal insulin receptor exhibit unrestrained lipolysis and decreased de novo lipogenesis in WAT. Thus, brain and, in particular, hypothalamic insulin action play a pivotal role in WAT functionality.     

Lipocalin prostaglandin D synthase and PPARγ2 coordinate to regulate carbohydrate and lipid metabolism in vivo

Mice lacking Peroxisome Proliferator-Activated Receptor γ2 (PPARγ2) have unexpectedly normal glucose tolerance and mild insulin resistance. Mice lacking PPARγ2 were found to have elevated levels of Lipocalin prostaglandin D synthase (L-PGDS) expression in BAT and subcutaneous white adipose tissue (WAT). To determine if induction of L-PGDS was compensating for a lack of PPARγ2, we crossed L-PGDS KO mice to PPARγ2 KO mice to generate Double Knock Out mice (DKO). Using DKO mice we demonstrated a requirement of L-PGDS for maintenance of subcutaneous WAT (scWAT) function. In scWAT, DKO mice had reduced expression of thermogenic genes, the de novo lipogenic program and the lipases ATGL and HSL. Despite the reduction in markers of lipolysis in scWAT, DKO mice had a normal metabolic rate and elevated serum FFA levels compared to L-PGDS KO alone. Analysis of intra-abdominal white adipose tissue (epididymal WAT) showed elevated expression of mRNA and protein markers of lipolysis in DKO mice, suggesting that DKO mice may become more reliant on intra-abdominal WAT to supply lipid for oxidation. This switch in depot utilisation from subcutaneous to epididymal white adipose tissue was associated with a worsening of whole organism metabolic function, with DKO mice being glucose intolerant, and having elevated serum triglyceride levels compared to any other genotype. Overall, L-PGDS and PPARγ2 coordinate to regulate carbohydrate and lipid metabolism.     

Short term voluntary overfeeding disrupts brain insulin control of adipose tissue lipolysis

Insulin controls fatty acid (FA) release from white adipose tissue (WAT) through direct effects on adipocytes and indirectly through hypothalamic signaling by reducing sympathetic nervous system outflow to WAT. Uncontrolled FA release from WAT promotes lipotoxicity, which is characterized by inflammation and insulin resistance that leads to and worsens type 2 diabetes. Here we tested whether early diet-induced insulin resistance impairs the ability of hypothalamic insulin to regulate WAT lipolysis and thus contributes to adipose tissue dysfunction. To this end we fed male Sprague-Dawley rats a 10% lard diet (high fat diet (HFD)) for 3 consecutive days, which is known to induce systemic insulin resistance. Rats were studied by euglycemic pancreatic clamps and concomitant infusion of either insulin or vehicle into the mediobasal hypothalamus. Short term HFD feeding led to a 37% increase in caloric intake and elevated base-line free FAs and insulin levels compared with rats fed regular chow. Overfeeding did not impair insulin signaling in WAT, but it abolished the ability of mediobasal hypothalamus insulin to suppress WAT lipolysis and hepatic glucose production as assessed by glycerol and glucose flux. HFD feeding also increased hypothalamic levels of the endocannabinoid 2-arachidonoylglycerol after only 3 days. In summary, overfeeding impairs hypothalamic insulin action, which may contribute to unrestrained lipolysis seen in human obesity and type 2 diabetes.     

Proposed involvement of adipocyte glyceroneogenesis and phosphoenolpyruvate carboxykinase in the metabolic syndrome

Elevated concentration of plasma non-esterified fatty acids (NEFA) is now recognized as a key factor in the onset of insulin-resistance and type 2 diabetes mellitus. During fasting, circulating NEFAs arise from white adipose tissue (WAT) as a consequence of lipolysis from stored triacylglycerols. However, a significant part of these FAs (30-70%) is re-esterified within the adipocyte, so that a recycling occurs and net FA output is much less than < true > lipolysis. Indeed, a balance between two antagonistic processes, lipolysis and FA re-esterification, controls the rate of net FA release from WAT. During fasting, re-esterification requires glyceroneogenesis defined as the de novo synthesis of glycerol-3-P from pyruvate, lactate or certain amino acids. The key enzyme in this process is the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C; EC 4.1.1.32). Recent advance has stressed the role of glyceroneogenesis and of PEPCK-C in FA release from WAT. Results indicate that glyceroneogenesis is indeed important to lipid homeostasis and that a disregulation in this pathway may have profound pathophysiological effects. The present review focuses on the regulation of glyceroneogenesis and of PEPCK-C gene expression and activity by FAs, retinoic acids, glucocorticoids and the hypolipidemic class of drugs, thiazolidinediones.     

Spexin: A novel regulator of adipogenesis and fat tissue metabolism

Spexin (SPX, NPQ) is a novel peptide involved in the regulation of energy metabolism. SPX inhibits food intake and reduces body weight. In obese humans, SPX is the most down-regulated gene in fat. Therefore, SPX might be involved in the regulation of lipid metabolism. Here, we study the effects of SPX on lipolysis, lipogenesis, glucose uptake, adipogenesis, cell proliferation and survival in isolated human adipocytes or murine 3T3-L1 cells. SPX and its receptors, GALR2 and GALR3, are present at mRNA and protein levels in murine 3T3-L1 cells and human adipocytes. SPX inhibits adipogenesis and down-regulates mRNA expression of proadipogenic genes such as Pparγ, C/ebpα, C/ebpβ and Fabp4. SPX stimulates lipolysis by increasing the phosphorylation of hormone sensitive lipase (HSL). Simultaneously, SPX inhibits lipogenesis and glucose uptake in human adipocytes and murine 3T3-L1 cells. SPX has no effect on murine 3T3-L1 cell proliferation and viability. Moreover, our research showed that the SPX effect on adipocytes metabolism is mediated via GALR2 and GALR3 receptors. SPX is a novel regulator of lipid metabolism in murine 3T3-L1 and human adipocytes.     

De novo lipogenesis and desaturation of fatty acids during adipogenesis in bovine adipose-derived mesenchymal stem cells

Adipose-derived mesenchymal stem cells (ADSCs) are useful cell model to study adipogenesis and energy metabolism. However, the biological characteristics of bovine ADSCs (bADSCs) remain unclear. This study aimed to isolate and identify bADSCs and further investigate fatty acid (FA)-related gene expression and composition of FAs during adipogenesis. The growth curve showed the bADSCs of P5 cells had rapid proliferation superior to P10–P50. The colony formation assay showed colony number of P5 cells was higher than that of P50 cells (51.67 ± 3.06 vs 35.67 ± 6.43, P < 0.05). The immunofluorescence showed that bADSCs were positive for CD13, CD44, CD49d, CD90, CD105, and Vimentin while negative for CD34. The multipotential towards adipocyte, osteocyte, and chondrocyte was confirmed by specific histological staining and lineage gene expression. During adipogenic induction, the genes related to lipogenesis and lipolysis were assessed by real-time PCR and the FA composition was detected by GC-MS. Expression of lipogenesis-related genes showed coordinated regulation as peaking on day 7 and declining until induction ended, including PPARγ, SREBP1, ACC1, FAS, ELOVL6, SCD1, and FABP4. FA deposition-related genes (DGAT1 and ACAT1) increased until day 14. Lipolysis genes (CPT-1A, VLCAD, and ACO) showed a variant expression pattern. The profile of FAs showed that proportion of the FAs (C4–C15, ≥ C22) increased, but proportion of long-chain fatty acids (C16–C20) reduced after induction. And saturated FAs (SFA) decreased while monounsaturated FAs (MUFA) and polyunsaturated FAs (PUFA) increased during adipogenesis. These data suggest that bADSCs possess the characteristics of mesenchymal stem cells and have active de novo lipogenesis (DNL) and desaturation of FAs during adipogenesis.     

Acute central infusion of leptin modulates fatty acid mobilization by affecting lipolysis and mRNA expression for uncoupling proteins

Chronic administration of leptin has been shown to reduce adiposity through energy intake and expenditure. The present study aims to examine how acute central infusion of leptin regulates peripheral lipid metabolism, as assessed by markers indicative of their mobilization and utilization. A bolus infusion of 1 microg/rat leptin into the third cerebroventricle increased the expression of mRNA for hormone-sensitive lipase (HSL), an indicator of lipolysis, in white adipose tissue (WAT). This was accompanied by elevation of plasma levels of glycerol, but not of free fatty acids, as compared to the saline control (P < 0.03). The same treatment with leptin decreased plasma insulin levels but did not affect the plasma glucose level (P < 0.05 for insulin). Among the major regulators of the transportation or utilization of energy substrates, leptin treatment increased expression of mRNA for uncoupling protein 1 (UCP1) in brown adipose tissue (BAT), UCP2 in WAT, and UCP3 in quadriceps skeletal muscle, but not those for fatty acid-binding protein in WAT, carnitine phosphate transferase-1, a marker for beta oxidation of fatty acids in muscle, nor glucose transporter 4 in WAT and muscle (P < 0.01 for HSL, P < 0.05 for UCP1, and P < 0.005 for UCP2 and UCP3). These results indicate that, even in a single bolus, leptin may regulate the mobilization and/or utilization of energy substrates such as fatty acids by affecting lipolytic activity in WAT and by increasing the expression of UCPs in BAT, WAT, and muscle.     

Lipid metabolism and adipokine levels in fatty acid-binding protein null and transgenic mice

Fatty acid-binding proteins (FABPs) facilitate the diffusion of fatty acids within cellular cytoplasm. Compared with C57Bl/6J mice maintained on a high-fat diet, adipose-FABP (A-FABP) null mice exhibit increased fat mass, decreased lipolysis, increased muscle glucose oxidation, and attenuated insulin resistance, whereas overexpression of epithelial-FABP (E-FABP) in adipose tissue results in decreased fat mass, increased lipolysis, and potentiated insulin resistance. To identify the mechanisms that underlie these processes, real-time PCR analyses indicate that the expression of hormone-sensitive lipase is reduced, while perilipin A is increased in A-FABP/aP2 null mice relative to E-FABP overexpressing mice. In contrast, de novo lipogenesis and expression of genes encoding lipoprotein lipase, CD36, long-chain acyl-CoA synthetase 5, and diacylglycerol acyltransferase are increased in A-FABP/aP2 null mice relative to E-FABP transgenic animals. Consistent with an increase in de novo lipogenesis, there was an increase in adipose C16:0 and C16:1 acyl-CoA pools. There were no changes in serum free fatty acids between genotypes. Serum levels of resistin were decreased in the E-FABP transgenic mice, whereas serum and tissue adiponectin were increased in A-FABP/aP2 null mice and decreased in E-FABP transgenic animals; leptin expression was unaffected. These results suggest that the balance between lipolysis and lipogenesis in adipocytes is remodeled in the FABP null and transgenic mice and is accompanied by the reprogramming of adipokine expression in fat cells and overall changes in plasma adipokines.     

Adipose triglyceride lipase and hormone-sensitive lipase are the major enzymes in adipose tissue triacylglycerol catabolism

The mobilization of free fatty acids from adipose triacylglycerol (TG) stores requires the activities of triacylglycerol lipases. In this study, we demonstrate that adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) are the major enzymes contributing to TG breakdown in in vitro assays and in organ cultures of murine white adipose tissue (WAT). To differentiate between ATGL- and HSL-specific activities in cytosolic preparations of WAT and to determine the relative contribution of these TG hydrolases to the lipolytic catabolism of fat, mutant mouse models lacking ATGL or HSL and a mono-specific, small molecule inhibitor for HSL (76-0079) were used. We show that 76-0079 had no effect on TG catabolism in HSL-deficient WAT but, in contrast, essentially abolished free fatty acid mobilization in ATGL-deficient fat. CGI-58, a recently identified coactivator of ATGL, stimulates TG hydrolase activity in wild-type and HSL-deficient WAT but not in ATGL-deficient WAT, suggesting that ATGL is the sole target for CGI-58-mediated activation of adipose lipolysis. Together, ATGL and HSL are responsible for more than 95% of the TG hydrolase activity present in murine WAT. Additional known or unknown lipases appear to play only a quantitatively minor role in fat cell lipolysis.     

Hormone-sensitive lipase modulates adipose metabolism through PPARγ

Hormone-sensitive lipase (HSL) is rate limiting for diacylglycerol and cholesteryl ester hydrolysis in adipose tissue and essential for complete hormone-stimulated lipolysis. Gene expression profiling in HSL-/- mice suggests that HSL is important for modulating adipogenesis and adipose metabolism. To test whether HSL is required for the supply of intrinsic ligands for PPARγ for normal adipose differentiation, HSL-/- and wild-type (WT) littermates were fed normal chow (NC) and high-fat (HF) diets supplemented with or without rosiglitazone (200 mg/kg) for 16 weeks. Results show that supplementing rosiglitazone to an NC diet completely normalized the decreased body weight and adipose depots in HSL-/- mice. Additionally, rosiglitazone resulted in similar serum glucose, total cholesterol, FFA, and adiponectin values in WT and HSL-/- mice. Furthermore, rosiglitazone normalized the expression of genes involved in adipocyte differentiation, markers of adipocyte differentiation, and enzymes involved in triacylglycerol synthesis and metabolism, and cholesteryl ester homeostasis, in HSL-/- mice. Supplementing rosiglitazone to an HF diet resulted in improved glucose tolerance in both WT and HSL-/- animals and also partial normalization in HSL-/- mice of abnormal WAT gene expression, serum chemistries, organ and body weight changes. In vitro studies showed that adipocytes from WT animals can provide ligands for activation of PPARγ and that activation is further boosted following lipolytic stimulation, whereas adipocytes from HSL-/- mice displayed attenuated activation of PPARγ, with no change following lipolytic stimulation. These results suggest that one of the mechanisms by which HSL modulates adipose metabolism is by providing intrinsic ligands or pro-ligands for PPARγ.     

Biological activity of a fragment of insulin

Insulin controls or alters glucose, protein, and fat metabolism as well as other cellular functions. Insulin binds to a specific receptor on the cell membrane initiating a protein phosphorylation cascade that controls glucose uptake and metabolism and long-term effects such as mitogenesis. This process also initiates insulin uptake and ultimate cellular metabolism in all insulin sensitive cells. The effects of insulin on other cellular metabolic properties have not been clearly related to this mechanism. Here we show that intracellular metabolism of insulin may be related to some aspects of insulin actions, specifically control of fat metabolism. A normal intracellular degradation product of insulin has been synthesized and tested for actions on fat turnover in cultured adipocytes. This 7-peptide, B-chain fragment (HLVEALY) inhibits both basal and stimulated lipolysis as measured by glycerol release, but does not inhibit FFA release because of a lack of effect on FFA reesterification in the adipocyte. HLVEALY also enhances insulin's effects on lipogenesis. This study shows that a fragment of insulin produced by the action of the insulin-degrading enzyme has both independent biological effects and interactions with insulin. This supports a biologically important effect of insulin metabolism and insulin degradation products on insulin action on non-glucose pathways.     

No difference in lipolysis or glucose transport of subcutaneous fat cells between moderate-fat and low-fat hypocaloric diets in obese women.

The objective of the present study was to evaluate the effect of two different diets on lipolysis and lipogenesis in subcutaneous fat cells from obese women. In a ten-week nutritional intervention study, forty women were randomly assigned to a hypoenergetic-2,514 kJ (- 600 kcal/day) diet of either moderate-fat/moderate-carbohydrate or low-fat/high-carbohydrate content. Body weight was equally reduced by approximately 7.5 % in both diet groups (p = 0.58). A subcutaneous adipose tissue biopsy was obtained for subsequent measurement of triglyceride breakdown (lipolysis) using drugs active at different steps of the lipolytic signaling cascade, and lipid synthesis (glucose transport) before and after intervention. No difference was found between the two diet groups at the maximum rate of either lipolysis or adrenoceptor sensitivity (p-values: 0.14 - 0.97). Inhibition of lipolysis by insulin was also similar in both diet groups before and after intervention. Finally, insulin-stimulated glucose transport did not show any changes that could be attributed to the type of diet. In conclusion, our data suggest that macronutrient diet composition has no major influence on glucose transport or mobilization of triglycerides in human subcutaneous fat cells of obese women.     

Lipogenesis and lipolysis: The pathways exploited by the cancer cells to acquire fatty acids

One of the most important metabolic hallmarks of cancer cells is enhanced lipogenesis. Depending on the tumor type, tumor cells synthesize up to 95% of saturated and mono-unsaturated fatty acids (FA) de novo in spite of sufficient dietary lipid supply. This lipogenic conversion starts early when cells become cancerous and further expands as the tumor cells become more malignant. It is suggested that activation of FA synthesis is required for carcinogenesis and for tumor cell survival. These observations suggest that the enzymes involved in FA synthesis would be rational therapeutic targets for cancer treatment. However, several recent reports have shown that the anti-tumor effects, following inhibition of endogenous FA synthesis in cancer cell lines may be obviated by adding exogenous FAs. Additionally, high intake of dietary fat is reported to be a potential risk factor for development and poor prognosis for certain cancers. Recently it was reported that breast and liposarcoma tumors are equipped for both de novo fatty acid synthesis pathway as well as LPL-mediated extracellular lipolysis. These observations indicate that lipolytically acquired FAs may provide an additional source of FAs for cancer. This review focuses on our current understanding of lipogenic and lipolytic pathways in cancer cell progression.