Lipolysis In Human Fat Cells Research Articles

Effects of Chemical Structures Interacting with Amine Oxidases on Glucose, Lipid and Hydrogen Peroxide Handling by Human Adipocytes.

Benzylamine is a natural molecule present in food and edible plants, capable of activating hexose uptake and inhibiting lipolysis in human fat cells. These effects are dependent on its oxidation by amine oxidases present in adipocytes, and on the subsequent hydrogen peroxide production, known to exhibit insulin-like actions. Virtually, other substrates interacting with such hydrogen peroxide-releasing enzymes potentially can modulate lipid accumulation in adipose tissue. Inhibition of such enzymes has also been reported to influence lipid deposition. We have therefore studied in human adipocytes the lipolytic and lipogenic activities of pharmacological entities designed to interact with amine oxidases highly expressed in this cell type: the semicarbazide-sensitive amine oxidase (SSAO also known as PrAO or VAP-1) and the monoamine oxidases (MAO). The results showed that SZV-2016 and SZV-2017 behaved as better substrates than benzylamine, releasing hydrogen peroxide once oxidized, and reproduced or even exceeded its insulin-like metabolic effects in fat cells. Additionally, several novel SSAO inhibitors, such as SZV-2007 and SZV-1398, have been evidenced and shown to inhibit benzylamine metabolic actions. Taken as a whole, our findings reinforce the list of molecules that influence the regulation of triacylglycerol assembly/breakdown, at least in vitro in human adipocytes. The novel compounds deserve deeper investigation of their mechanisms of interaction with SSAO or MAO, and constitute potential candidates for therapeutic use in obesity and diabetes.

High doses of tyramine stimulate glucose transport in human fat cells.

Among the dietary amines present in foods and beverages, tyramine has been widely studied since its excessive ingestion can cause catecholamine release and hypertensive crisis. However, tyramine exerts other actions than depleting nerve endings: it activates subtypes of trace amine associated receptors (TAARs) and is oxidized by monoamine oxidases (MAO). Although we have recently described that tyramine is antilipolytic in human adipocytes, no clear evidence has been reported about its effects on glucose transport in the same cell model, while tyramine mimics various insulin-like effects in rodent fat cells, such as activation of glucose transport, lipogenesis, and adipogenesis. Our aim was therefore to characterize the effects of tyramine on glucose transport in human adipocytes. The uptake of the non-metabolizable analogue 2-deoxyglucose (2-DG) was explored in adipocytes from human subcutaneous abdominal adipose tissue obtained from women undergoing reconstructive surgery. Human insulin used as reference agent multiplied by three times the basal 2-DG uptake. Tyramine was ineffective from 0.01 to 10µM and stimulatory at 100µM-1mM, without reaching the maximal effect of insulin. This partial insulin-like effect was not improved by vanadium and was impaired by MAO-A and MAO-B inhibitors. Contrarily to benzylamine, mainly oxidized by semicarbazide-sensitive amine oxidase (SSAO), tyramine activation of glucose transport was not inhibited by semicarbazide. Tyramine effect was not dependent on the Gi-coupled receptor activation but was impaired by antioxidants and reproduced by hydrogen peroxide. In all, the oxidation of high doses of tyramine, already reported to inhibit lipolysis in human fat cells, also partially mimic another effect of insulin in these cells, the glucose uptake activation. Thus, other MAO substrates are potentially able to modulate carbohydrate metabolism.

1165-P: In Vitro Actions of a Novel Biguanide (BIG), NT1195, That Support Its Use in T2D

Metformin, a frontline treatment for T2D, is a BIG drug with low potency and limiting side effects. To overcome these limitations, we examined a compound library of 265 novel BIGs by screening these agents against five organic cationic transporters (OCTs) (OCTs 1, 2 and 3, PMATE and MATE1), an approach based on the need of OCTs for BIG cell entry and action. mRNA expression data from vervet monkey liver, fat and muscle showed OCT1, OCT3, PMAT and MATE1 were the predominate transporters and were used to guide our search for more potent and selective BIGs. BIG transport activity was measured by a decrease in oxygen consumption rate (OCR) in HEK 293 cells that express a single OCT and the results reported as EC50 values. Following the screening, we identified NT1195 from its EC50 transport values of 25, 5, 12, 3 and 57 uM respectively for OCT1, 2 and 3, PMAT and MATE1 when compared to those for metformin of 409, 320, 133, 288 and 5,993 uM. Other cellular effects of NT1195 were then examined in human adipocytes and myotubes, and in H4-lle liver cells. AMPK activation, a basic feature of BIG action, was increased by 6-fold over basal by NT1195 in fat cells versus a 3-fold increase by metformin; the EC50 values were respectively 78 uM and 4,928 uM. NT1195 stimulated AMPK in myotubes by 3-fold with an EC50 of 1,994 uM; metformin stimulation AMPK by 1.8-fold with an EC50 of 7,817 uM. Both agents produced a 2-fold activation of AMPK in H4-II e cells but with different EC50 values of 641 uM for NT1195 and 5,576 uM for metformin. Glucose production by H4-IIe cells was inhibited 90% by both BIGs, whereas the EC50 values were 32 nM for NT1195 and 285 uM for metformin. NT1195 inhibited lipolysis in human fat cells stimulated by isoproterenol by 90% with an EC50 of 286 uM while metformin inhibited lipolysis by 66% with an EC50 of 4,360 uM. Therefore overall, these findings demonstrate the much greater potency of NT1195 over metformin and supports its potential as an improved therapeutic for T2D. Disclosure J.N. Livingston: Consultant; Self; CohBar, Inc. Employee; Self; NovoTarg Therapeutics. J.E. Cobb: Consultant; Self; NovaTarg Therapeutics. K.K. Brown: None. K. Kavanagh: Research Support; Self; Takeda Development Centre Europe Ltd. K.W. Batchelor: None. Funding National Institutes of Health

MicroRNAs Regulate Human Adipocyte Lipolysis: Effects of miR-145 Are Linked to TNF-α

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression and have multiple effects in various tissues including adipose inflammation, a condition characterized by increased local release of the pro-lipolytic cytokine tumor necrosis factor-alpha (TNF-α). Whether miRNAs regulate adipocyte lipolysis is unknown. We set out to determine whether miRNAs affect adipocyte lipolysis in human fat cells. To this end, eleven miRNAs known to be present in human adipose tissue were over-expressed in human in vitro differentiated adipocytes followed by assessments of TNF-α and glycerol levels in conditioned media after 48 h. Three miRNAs (miR-145, -26a and let-7d) modulated both parameters in parallel. However, while miR-26a and let-7d decreased, miR-145 increased both glycerol release and TNF-α secretion. Further studies were focused therefore on miR-145 since this was the only stimulator of lipolysis and TNF-α secretion. Time-course analysis demonstrated that miR-145 over-expression up-regulated TNF-α expression/secretion followed by increased glycerol release. Increase in TNF-α production by miR-145 was mediated via activation of p65, a member of the NF-κB complex. In addition, miR-145 down-regulated the expression of the protease ADAM17, resulting in an increased fraction of membrane bound TNF-α, which is the more biologically active form of TNF-α. MiR-145 overexpression also increased the phosphorylation of activating serine residues in hormone sensitive lipase and decreased the mRNA expression of phosphodiesterase 3B, effects which are also observed upon TNF-α treatment in human adipocytes. We conclude that miR-145 regulates adipocyte lipolysis via multiple mechanisms involving increased production and processing of TNF-α in fat cells.

Natriuretic peptides enhance the oxidative capacity of human skeletal muscle

Cardiac natriuretic peptides (NP) are major activators of human fat cell lipolysis and have recently been shown to control brown fat thermogenesis. Here, we investigated the physiological role of NP on the oxidative metabolism of human skeletal muscle. NP receptor type A (NPRA) gene expression was positively correlated to mRNA levels of PPARγ coactivator-1α (PGC1A) and several oxidative phosphorylation (OXPHOS) genes in human skeletal muscle. Further, the expression of NPRA, PGC1A, and OXPHOS genes was coordinately upregulated in response to aerobic exercise training in human skeletal muscle. In human myotubes, NP induced PGC-1α and mitochondrial OXPHOS gene expression in a cyclic GMP-dependent manner. NP treatment increased OXPHOS protein expression, fat oxidation, and maximal respiration independent of substantial changes in mitochondrial proliferation and mass. Treatment of myotubes with NP recapitulated the effect of exercise training on muscle fat oxidative capacity in vivo. Collectively, these data show that activation of NP signaling in human skeletal muscle enhances mitochondrial oxidative metabolism and fat oxidation. We propose that NP could contribute to exercise training-induced improvement in skeletal muscle fat oxidative capacity in humans.

Effects of pain controlling neuropeptides on human fat cell lipolysis

Neuropeptides NPFF and NPSF are involved in pain control, acting through the G-protein coupled receptors (GPR)74 (high affinity for NPFF) and GPR147 (equal affinity for NPFF and NPSF). GPR74 also inhibits catecholamine-induced adipocyte lipolysis and regulates fat mass in humans. The aim of this study was to compare the effects of NPFF and NPSF on noradrenaline-induced lipolysis and to determine the expression of their receptors in human fat cells. Adipose tissue was obtained during surgery. Adipocytes were prepared and kept in primary culture. Lipolysis, protein expression and gene expression were determined. NPFF counteracted noradrenaline-induced lipolysis, which was more marked after 48 h than after 4 h exposure and was solely attributed to inhibition of beta-adrenoceptor signalling. NPSF counteracted noradrenaline-induced lipolysis maximally after 4 h of exposure, which was attributed to a combination of inhibition of beta-adrenoceptor signalling and decreased activation of the protein kinase-A hormone sensitive lipase complex by cyclic AMP. Both neuropeptides were effective in nanomolar concentrations. NPFF and NPSF had no effects on the expression of genes involved in catecholamine signal transduction. Both GPR74 and GPR147 were expressed at the protein level in fat cells from various adipose regions. GPR74 mRNA levels were higher in adipose tissue from obese as compared with non-obese subjects. High gene expression of either receptor correlated with low noradrenaline-induced lipolysis (P<0.05). Pain controlling neuropeptides NPFF and NPSF may be important for the regulation of lipolysis in man probably acting through GPR74 and GPR147. At low concentrations they inhibit catecholamine-induced lipolysis through rapid and long-term post-transcriptional effects at several steps in adrenoceptor signalling in fat cells.

The common rs9939609 gene variant of the fat mass- and obesity-associated gene FTO is related to fat cell lipolysis

We investigated the rs9939609 single nucleotide polymorphism of the FTO gene in relation to fat cell function and adipose tissue gene expression in 306 healthy women with a wide range in body mass index (18-53 kg/m(2)). Subcutaneous adipose tissue biopsies were taken for fat cell metabolism studies and in a subgroup (n = 90) for gene expression analyses. In homozygous carriers of the T-allele, the in vitro basal (spontaneous) adipocyte glycerol release was increased by 22% (P = 0.007) and the in vivo plasma glycerol level was increased by approximately 30% (P = 0.037) compared with carriers of the A allele. In contrast, there were no genotype effects on catecholamine-stimulated lipolysis or basal or insulin-induced lipogenesis. We found no difference between genotypes for adipose tissue mRNA levels of FTO, hormone-sensitive lipase, adipose triglyceride lipase, perilipin, or CGI-58. Finally, the adipose tissue level of FTO mRNA was increased in obesity (P = 0.002), was similar in subcutaneous and omental adipose tissue, was higher in fat cells than in fat tissue (P = 0.0007), and was induced at an early stage in the differentiation process (P = 0.004). These data suggest a role of the FTO gene in fat cell lipolysis, which may be important in explaining why the gene is implicated in body weight regulation.

The role of neutral lipases in human adipose tissue lipolysis

The aim of this article is to describe the relative roles of hormone sensitive lipase and adipose triglyceride lipase in human fat cell lipolysis. Until recently, only hormone sensitive lipase was considered important for the regulation of lipolysis within fat cells. Recent rodent studies have suggested that adipose triglyceride lipase may, however, be more important. The few human adipose triglyceride lipase studies that have been published point to species differences between humans and rodents. Selective inhibition of hormone sensitive lipase in human fat cells completely counteracts hormone-activated lipolysis, though there is a considerable (>>50%) residual nonhormonal (basal) lipolysis. In rodents, adipose triglyceride lipase enzyme activity is stimulated by a cofactor termed CGI-58. In the absence of CGI-58, lipase activity in fat cells is much higher for hormone sensitive lipase than adipose triglyceride lipase. Hormone sensitive lipase expression is regulated by obesity and body weight reduction (decreased and increased, respectively), but this is not the case for adipose triglyceride lipase. A role of adipose triglyceride lipase in human lipolysis is suggested by studies of gene polymorphisms. Two lipases the 'old' hormone sensitive lipase and the 'new' adipose triglyceride lipase are of importance for the regulation of lipolysis in rodent fat cells. In humans, adipose triglyceride lipase seems essential for maintaining basal lipolytic activity, while hormone sensitive lipase is the enzyme most responsive to stimulated lipolysis.

Obesity and Atherogenic Dyslipidemia

Obesity is associated with an increased risk of coronary heart disease, in part due to its strong association with atherogenic dyslipidemia, characterized by high triglycerides and low high-density lipoprotein (HDL) cholesterol. There has been substantial research effort focused on the mechanisms of the link between obesity and atherogenic dyslipidemia, both in the absence and presence of insulin resistance. After a brief overview of the epidemiology of atherogenic dyslipidemia, this article details the known molecular mechanisms of adipocyte function and its relationship to apoB-containing lipoprotein assembly and metabolism, both in the healthy as well as in the obese states. We also discuss the pathophysiology of low HDL cholesterol in obesity and the implications for cardiovascular disease risk.

NF-κB is important for TNF-α-induced lipolysis in human adipocytes

Tumor necrosis factor-alpha (TNF-alpha) promotes lipolysis in mammal adipocytes via the mitogen-activated protein kinase (MAPK) family, resulting in reduced expression/function of perilipin (PLIN). The role of another pivotal intracellular messenger activated by TNF-alpha, nuclear factor-kappaB (NF-kappaB), has not been recognized. We explored the role of NF-kappaB in TNF-alpha-induced lipolysis of human fat cells. Primary cultures of human adipocytes were incubated in the presence of a cell-permeable peptide that inhibits NF-kappaB signaling (WP). Incubation with WP, but not with a biologically inactive peptide (MP), abolished the nuclear translocation of NF-kappaB and effectively abrogated TNF-alpha-induced lipolysis in a concentration-dependent manner. Western blot analysis demonstrated that although TNF-alpha per se reduced mainly PLIN protein expression, TNF-alpha in the presence of WP resulted in a pronounced combined reduction of both hormone-sensitive lipase (HSL) and PLIN protein. The expression of a set of other lipolytic or adipocyte-specific proteins was not affected. The regulation was presumably at the transcriptional level, because mRNA expression for HSL and PLIN was markedly reduced with TNF-alpha in the presence of NF-kappaB inhibition. This was confirmed in gene reporter assays using human PLIN and HSL promoter constructs. We conclude that in the presence of NF-kappaB inhibition, TNF-alpha-mediated lipolysis is reduced, which suggests that NF-kappaB is essential for retained human fat cell lipolysis.

Comparative studies of the role of hormone-sensitive lipase and adipose triglyceride lipase in human fat cell lipolysis

Hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) regulate adipocyte lipolysis in rodents. The purpose of this study was to compare the roles of these lipases for lipolysis in human adipocytes. Subcutaneous adipose tissue was investigated. HSL and ATGL protein expression were related to lipolysis in isolated mature fat cells. ATGL or HSL were knocked down by RNA interference (RNAi) or selectively inhibited, and effects on lipolysis were studied in differentiated preadipocytes or adipocytes derived from human mesenchymal stem cells (hMSC). Subjects were all women. There were 12 lean controls, 8 lean with polycystic ovary syndrome (PCOS), and 27 otherwise healthy obese subjects. We found that norepinephrine-induced lipolysis was positively correlated with HSL protein levels (P < 0.0001) but not with ATGL protein. Women with PCOS or obesity had significantly decreased norepinephrine-induced lipolysis and HSL protein expression but no change in ATGL protein expression. HSL knock down by RNAi reduced basal and catecholamine-induced lipolysis. Knock down of ATGL decreased basal lipolysis but did not change catecholamine-stimulated lipolysis. Treatment of hMSC with a selective HSL inhibitor during and/or after differentiation in adipocytes reduced basal lipolysis by 50%, but stimulated lipolysis was inhibited completely. In contrast to findings in rodents, ATGL is of less importance than HSL in regulating catecholamine-induced lipolysis and cannot replace HSL when this enzyme is continuously inhibited. However, both lipases regulate basal lipolysis in human adipocytes. ATGL expression, unlike HSL, is not influenced by obesity or PCOS.

Human fat cell lipolysis: Biochemistry, regulation and clinical role

Release of fatty acids (FAs) from adipose tissue through lipolysis in fat cells is a key event in many processes. FAs are not only energy substrates but also signalling molecules and substrates for lipoprotein production by the liver. Fat cells consist of>95% triglycerides that are hydrolysed during lipolysis to glycerol and FAs. The major rate-limiting factor for lipolysis is hormone-sensitive lipase, but additional lipases such as adipose tissue triglyceride lipase may also play a role. The regulation of human fat cell lipolysis is, in many ways, species unique. Only catecholamines, insulin and natriuretic peptides have pronounced acute effects. Catecholamines influence lipolysis through four different adrenoceptor subtypes, in contrast to rodents where only one subtype (beta(3)) is of major importance. There are regional variations in adipocyte lipolysis leading to more release of FAs from the visceral than subcutaneous adipose tissue during hormone stimulation (insulin, catecholamines). Since, only visceral fat is linked to the liver (by the portal vein), alterations in visceral adipocyte tissue lipolysis have direct effects on the liver through portal FA release. The regional variations in lipolysis are further enhanced in obesity and polycystic ovarian syndrome, and are of importance for dyslipidaemia, hyperinsulinaemia and glucose intolerance in these conditions. There is a marked elevation of circulating FA levels among the obese, which may be due to enhanced production of tumour necrosis factor alpha in adipose tissue. This cytokine stimulates lipolysis through so-called MAP kinases. Pharmacological agents in clinical practice such as nicotinic acid and glitazones exert lipid-lowering and glucose-lowering effects, respectively, by decreasing FA output from the adipose tissue. This review covers the biochemistry, regulation and clinical aspects of human fat cell lipolysis.

Benzylamine exhibits insulin-like effects on glucose disposal, glucose transport, and fat cell lipolysis in rabbits and diabetic mice.

Benzylamine, a substrate of semicarbazide-sensitive amine oxidase (SSAO), stimulates glucose transport in rat adipocytes and improves glucose disposal in diabetic rats only in the presence of vanadate. These effects have been described to result from a synergism between the hydrogen peroxide formed during amine oxidation and vanadate, via the generation of pervanadate, a powerful insulin mimicker. However, it has also been reported that benzylamine alone can stimulate glucose uptake and inhibit lipolysis in human fat cells. In this work, we therefore investigated whether benzylamine on its own was able to induce both in vivo and in vitro insulin-like responses in animal models other than rat. In rabbits, the i.v. infusion of 7 micromol/kg benzylamine before a glucose tolerance test resulted in a net reduction of the hyperglycemic response without a change in insulin secretion. Benzylamine also improved glucose tolerance and reduced lipid mobilization in hyperglycemic/obese mice. In vitro, 0.1 mM benzylamine stimulated glucose transport and inhibited lipolysis in mouse and rabbit adipocytes. These effects were blocked by previous treatments with semicarbazide, a SSAO inhibitor. Levels of benzylamine oxidation were more elevated in mouse than in rabbit adipose tissues, whereas the reverse was observed for skeletal muscles. Finally, benzylamine was unable to stimulate insulin secretion by isolated pancreatic islets from both species and SSAO activity was hardly detectable in pancreas. Together, our results bring evidence that benzylamine on its own can improve glucose tolerance in rabbit and mouse, likely by stimulating glucose uptake via amine oxidase activation in insulin-sensitive tissues.

Evidence for an important role of perilipin in the regulation of human adipocyte lipolysis.

We investigated the role of the adipocyte-specific protein perilipin for lipolysis in humans. Perilipin protein content and lipolysis rates were measured in human subcutaneous fat cells of non-obese (n=10) and obese (n=117) women. Single nucleotide polymorphisms in the perilipin gene were examined in obese subjects. Basal and noradrenaline-induced rates of lipolysis were two to fourfold increased (p<0.01) and perilipin protein content decreased 50% (p=0.005) in adipocytes of the obese women. In subjects matched for body mass index and fat-cell volume, a high rate of lipolysis was associated with a low adipocyte content of perilipin (p=0.01). Adipocyte content of perilipin was inversely correlated with the circulating concentrations of glycerol (r=0.62) and non-esterified fatty acids (n=0.49). A gene polymorphism (rs891460 A/G) in intron 6 was common. In AA subjects basal and noradrenaline induced lipolysis were 50 to 100% times more rapid (p</=0.01) and the adipocyte perilipin content was about 80% reduced (p=0.005) as compared to GG carriers. Intermediate values were found in AG carriers. Perilipin seems important for the regulation of lipolysis in human fat cells. Obesity and a polymorphism in the perilipin gene associate with decreased protein content of perilipin and increased basal (unstrained) and noradrenaline-induced lipolysis. Low perilipin content also associate with high in vivo lipolytic activity. Perilipin could be a factor behind impaired lipolysis in insulin-resistant conditions.

Effect of the (C825T) Gbeta(3) polymorphism on adrenoceptor-mediated lipolysis in human fat cells.

A common Gbeta(3) gene polymorphism (C825T) influences G protein receptor-mediated signal transduction. We investigated whether this polymorphism influences lipolysis in isolated subcutaneous fat cells from 114 healthy obese subjects. The Gbeta(3) protein content was markedly decreased in adipocytes of TT carriers, but the alternatively spliced short form of Gbeta(3) previously shown in platelets of 825T carriers was not detected. Fat cells of TT carriers showed a significant 10-fold decrease in the half-maximum effective concentration of agonists selective for lipolytic beta(1)- and beta(2)-adrenoceptors as well as for the antilipolytic alpha(2)A-adrenoceptor. In TT carriers, maximum beta-adrenoceptor agonist-stimulated lipolysis was decreased, but the maximum antilipolytic effect of alpha(2)-adrenoceptors was less marked. Norepinephrine induced adipocyte lipolysis and circulating fasting levels of free fatty acids and glycerol were reduced by half in TT carriers. The polymorphism did not influence the adipocyte content of alpha(2)A-adrenoceptors, beta(2)-adrenoceptors, Galpha(i), or Galpha(s). In conclusion, the C825T variant of Gbeta(3) influences lipolysis. Adipocytes of TT carriers have a lower Gbeta(3) protein content and a decreased function of native G(s)- as well as G(i)-coupled adrenoceptors, which reduces the lipolytic effect of catecholamines. These data differ from those obtained in other cell systems that have shown increased expression of an alternative spliced Gbeta(3) variant and enhanced G protein signaling in 825T carriers, indicating that the polymorphism has cell type-specific effects that may be of importance for type 2 diabetes and other insulin-resistant conditions.

Polymorphism in the Calpain 10 gene influences glucose metabolism in human fat cells.

A common G to A polymorphism ( UCSNP-43) in the Calpain 10 gene was recently found to be associated with Type II (non-insulin-dependent) diabetes mellitus and variations in post-absorptive and insulin stimulated glucose metabolism in vivo. We aimed to study the influence of Calpain 10 polymorphism on insulin action in fat cells. Calpain 10 polymorphism ( UCSNP-19, -43 or -63) were set in relation to lipolysis and lipogenesis in isolated subcutaneous adipocytes of 46 apparently healthy non-obese subjects. For UCSNP-43 the G/G genotype had twofold higher basal and insulin stimulated rates as compared with AA/AG genotypes. However, there was no genotype effect on basal or insulin inhibited lipolysis rates in fat cells. The protein amount of GLUT 4 in adipocytes was not influenced by the polymorphism. Fat cells expressed mRNA for the Calpain 10 gene at a relatively high concentration, about 4 amol/microg RNA, which is similar to that of uncoupling protein-2. Neither a UCSNP-19 nor a UCSNP-63 polymorphism in the Calpain 10 gene was found to be associated with basal or insulin-induced adipocyte lipolysis and lipogenesis. None of the polymorphisms influenced body mass index or fasting plasma concentrations of insulin and glucose in 693 non-obese healthy subjects. The Calpain 10 gene could be involved in the regulation of glucose metabolism but not lipolysis in human fat cells, although it does not involve adipocyte GLUT-4 protein content. It is possible that the Calpain 10 gene predisposes to diabetes by influencing the glucose metabolism.

Mapping of Early Signaling Events in Tumor Necrosis Factor-α-mediated Lipolysis in Human Fat Cells

Tumor necrosis factor-alpha (TNF-alpha) is a pleiotropic cytokine with a proposed role in obesity-related insulin resistance. This could be mediated by increased lipolysis in adipose tissue resulting in elevated free fatty acid levels. The early intracellular signals entailed in TNF-alpha-mediated lipolysis are unknown but may involve members of the mitogen-activated protein kinase (MAPK) family. We investigated the possible contribution of MAPK in TNF-alpha-induced lipolysis in human preadipocytes. TNF-alpha activated the three mammalian MAPK, p44/42, JNK, and p38, in a distinct time- and concentration-dependent manner. TNF-alpha also induced a concentration-dependent stimulation of lipolysis with a more than 3-fold increase at the maximal dose. Lipolysis was completely inhibited by blockers specific for p44/42 (PD98059) and JNK (dimetylaminopurine) but was not affected by the p38 blocker SB203580. Use of receptor-specific TNF-alpha mutants showed that activation of MAPK is entirely mediated by the TNFR1 receptor. The results in human preadipocytes differed from those obtained in murine 3T3-L1 adipocytes in which all three MAPK were constitutively active. Thus, studies of intracellular signaling pathways obtained in different cellular contexts should be interpreted with caution. In conclusion, although TNF-alpha activates all three known MAPK in human preadipocytes, only p44/42 and JNK appear to be involved in the regulation of lipolysis.

Genetic variance and lipolysis regulation: implications for obesity

Catecholamines are the major lipolytic hormones in human fat cells, and lipolytic catecholamine resistance is described in obesity. Studies on twins and in rare genetic disorders suggest a strong heredity component of catecholamine-induced lipolysis. Polymorphisms in catecholamine receptor signalling pathways have been described, several of which associate with obesity. Many polymorphisms in adrenoceptor genes are functional in transfected cell lines. The importance of polymorphisms in catecholamine signalling pathways for lipolysis regulation is discussed. A Trp64Arg polymorphism in the β2-receptor, which associates with obesity, is accompanied by changes in lipolytic sensitivity of the receptor in human fat cells. Similarly, a Gln 16Glu and an Arg 16411e variation in the β2-adrenoceptor cause marked variations in the lipolytic sensitivity of this receptor in human adipocytes. Furthermore, β2-adrenoceptor gene polymorphisms associate with obesity. A dinucleotide (CA) intron repeat in hormone-sensitive lipase gene is linked to obesity and markedly decreases the ability of catecholamines to activate the lipase and thereby lipolysis in human fat cells. However, an Arg 389Gly polymorphism in the β2-adrenoceptor, which alters receptor function in transfected cell lines, has no effect on lipolysis in human fat cells and is not associated with obesity. Thus, polymorphism in human genes that are involved in catecholamine signal transduction have effects on fat cell lipolysis and also relate to obesity. The lipolysis effects of these polymorphisms cannot always be predicted from gene transfer experiments on artificial cell lines. It is possible that genetic variance in catecholamine signalling pathways, through alterations in adipocyte lipolysis, may promote obesity.

Expression of nitric oxide synthases in subcutaneous adipose tissue of nonobese and obese humans

Studies have shown evidence of production of nitric oxide (NO) in adipose tissue, as well as inhibition of lipolysis by NO. We have analyzed nitric oxide synthase (NOS) expression in subcutaneous adipose tissue from 13 nonobese and 18 obese male subjects. Using a competitive reverse transcription polymerase chain reaction method, endothelial (eNOS) and inducible (iNOS), but not neuronal (nNOS), nitric oxide synthase mRNA expression was detected in isolated fat cells and pieces of adipose tissue. Tissue mRNA levels for eNOS were 3,814 +/- 825 and 5,956 +/- 476 amol/mg RNA (P = 0.043), and for iNOS 306 +/- 38 and 332 +/- 48 amol/mg RNA, for nonobese and obese individuals, respectively. Western blotting revealed similar eNOS protein levels in isolated fat cells and adipose tissue pieces. Protein levels for eNOS in nonobese and obese individuals, respectively, were (in optical density [OD] units per mm(2) per 100 microgram of total protein) 0.11 +/- 0.08 and 2.80 +/- 1.30 (P = 0.043). iNOS protein was detectable, but not measurable, at low levels in a subset of obese patients (3 of 10). iNOS protein levels could not be detected in nonobese individuals. Hormone-sensitive lipase (HSL), the key regulating enzyme in lipolysis, is reduced in obesity. The expression of HSL protein in subcutaneous adipose tissue was studied in the same subset of patients; in agreement with previous results, HSL levels were reduced in obese subjects: 4.64 +/- 1.10 and 1.27 +/- 0.35 (P = 0.012) in nonobese and obese subjects, respectively. In conclusion, this study shows that eNOS and iNOS, but not nNOS, are present in human subcutaneous adipose tissue. Gene expression and protein levels of eNOS are increased, whereas HSL protein levels are decreased in obesity. It is speculated that increased NO production, preferably by eNOS, and decreased HSL levels may cause decreased subcutaneous adipose tissue lipolysis in obesity. synthases in subcutaneous adipose tissue of nonobese and obese humans.

Hormone-sensitive lipase expression and activity in relation to lipolysis in human fat cells

Hormone-sensitive lipase (HSL) catalyzes the rate-limiting step in adipocyte lipolysis. The activity of HSL is thought to be primarily regulated by reversible phosphorylation. However, the regulation of HSL activity by pretranslational mechanisms has been poorly studied. The present studies were undertaken to explore the relationship between the levels of HSL protein and mRNA expressions and the lipolytic capacity. The study was performed in human abdominal subcutaneous adipocytes with identical sizes but having either a high (HL) or low (LL) lipolytic capacity (n = 16). Basal and maximal lipolysis induced by catecholamines, an adenylyl cyclase activator forskolin, and a cyclic AMP analogue dibutyryl cAMP were 50% lower in LL- in comparison with HL-fat cells (P < 0.05 or better). No differences in drug sensitivity were found. HSL activity and quantity were about 50% lower in LL- compared with HL-fat cells (P < 0.05). Moreover, the mRNA ratio between HSL and γ-actin was 35% lower in LL-compared with HL-fat cells (P < 0.05). There was a strong linear correlation between the protein and enzymatic HSL measurements (r2 = 0.91). In addition, the maximum lipolytic capacity was significantly correlated with HSL activity (r2 = 0.75) and HSL protein amount (r2 = 0.64).▪ It is concluded that hormone-sensitive lipase (HSL) expression, measured either as total HSL protein by Western blot analysis or as total amount of activatable HSL enzyme, is a major determinant of the maximum lipolytic capacity of human fat cells. In addition, HSL protein expression is at least, in part, determined by HSL mRNA expression.—Large, V., P. Arner, S. Reynisdottir, J. Grober, V. Van Harmelen, C. Holm, and D. Langin. Hormone-sensitive lipase expression and activity in relation to lipolysis in human fat cells.