Hormonally stimulated lipolysis occurs by activation of cyclic AMP-dependent protein kinase (PKA) which phosphorylates hormone-sensitive lipase (HSL) and increases adipocyte lipolysis. Evidence suggests that catecholamines not only can activate PKA, but also the mitogen-activated protein kinase pathway and extracellular signal-regulated kinase (ERK). We now demonstrate that two different inhibitors of MEK, the upstream activator of ERK, block catecholamine-and ␤ 3 -stimulated lipolysis by ϳ30%. Furthermore, treatment of adipocytes with dioctanoylglycerol, which activates ERK, increases lipolysis, although MEK inhibitors decrease dioctanoylglycerol-stimulated activation of lipolysis. Using a tamoxifen regulatable Raf system expressed in 3T3-L1 preadipocytes, exposure to tamoxifen causes a 14-fold activation of ERK within 15-30 min and results in ϳ2-fold increase in HSL activity. In addition, when differentiated 3T3-L1 cells expressing the regulatable Raf were exposed to tamoxifen, a 2-fold increase in lipolysis is observed. HSL is a substrate of activated ERK and site-directed mutagenesis of putative ERK consensus phosphorylation sites in HSL identified Ser 600 as the site phosphorylated by active ERK. When S600A HSL was expressed in 3T3-L1 cells expressing the regulatable Raf, tamoxifen treatment fails to increase its activity. Thus, activation of the ERK pathway appears to be able to regulate adipocyte lipolysis by phosphorylating HSL on Ser 600 and increasing the activity of HSL.

-ichi Osuga, Shun Ishibashi, and Fredric B. Kraemer. Resistance to high-fat diet-induced obesity and altered expression of adipose-specific genes in HSL-deficient mice. Am J Physiol Endocrinol Metab 285: E1182-E1195, 2003. First published September 3, 2003 10.1152/ajpendo.00259.2003.-To elucidate the role of hormone-sensitive lipase (HSL) in diet-induced obesity, HSLdeficient (HSL Ϫ/Ϫ ) and wild-type mice were fed normal chow or high-fat diets. HSL Ϫ/Ϫ mice were resistant to diet-induced obesity showing higher core body temperatures. Weight and triacylglycerol contents were decreased in white adipose tissue (WAT) but increased in both brown adipose tissue (BAT) and liver of HSL Ϫ/Ϫ mice. Serum insulin levels in the fed state and tumor necrosis factor-␣ mRNA levels in adipose tissues were higher, whereas serum levels of adipocyte complement-related protein of 30 kDa (ACRP30)/adiponectin and leptin, as well as mRNA levels of ACRP30/adiponectin, leptin, resistin, and adipsin in WAT, were lower in HSL Ϫ/Ϫ mice than in controls. Expression of transcription factors associated with adipogenesis (peroxisome proliferator-activated receptor-␥, CAAT/enhancer-binding protein-␣) and lipogenesis (carbohydrate response element-binding protein, adipocyte determination-and differentiation-dependent factor-1/sterol regulatory element-binding protein-1c), as well as of adipose differentiation markers (adipocyte lipid-binding protein, perilipin, lipoprotein lipase), lipogenic enzymes (glycerol-3-phosphate acyltransferase, acyl-CoA:diacylglycerol acyltransferase-1 and -2, fatty acid synthase, ATP citrate lyase) and insulin signaling proteins (insulin receptor, insulin receptor substrate-1, GLUT4), was suppressed in WAT but not in BAT of HSL Ϫ/Ϫ mice. In contrast, expression of genes associated with cholesterol metabolism (sterol-regulatory element-binding protein-2, 3-hydroxy-3-methylglutaryl-CoA reductase, acyl-CoA:cholesterol acyltransferase-1) and thermogenesis (uncoupling protein-2) was upregulated in both WAT and BAT of HSL Ϫ/Ϫ mice. Our results suggest that impaired lipolysis in HSL deficiency affects lipid metabolism through alterations of adipose differentiation and adipose-derived hormone levels. adipocyte; differentiation; insulin; leptin; fatty liver HORMONE-SENSITIVE LIPASE (HSL) mediates the cytosolic hydrolysis of triacylglycerols (lipolysis) and cholesteryl esters (12). HSL is expressed in various tissues, including white (WAT) and brown adipose tissues (BAT), cardiomyocytes, adrenocortical cells, and gonads (11). Because HSL is responsible for the release of free fatty acids (FFA) from stored triacylglycerols in adipose tissues, the enzyme has been proposed to play an essential role in the regulation of body weight and fat mass. We previously reported, however, that the body weight of HSL-deficient (HSL Ϫ/Ϫ ) mice generated by homologous recombination fed a normal chow diet did not differ from that of wild-type (HSL ϩ/ϩ ) mice despite the presence of a markedly suppressed hydrolysis of triacylglycerols and cholester...

BackgroundWhile an increase in bone marrow adiposity is associated with age-related bone disease, the function of bone marrow adipocytes has not been studied. The aim of this study was to characterize and compare the age-related gene expression profiles in bone marrow adipocytes and epididymal adipocytes.ResultsA total of 3918 (13.7%) genes were differentially expressed in bone marrow adipocytes compared to epididymal adipocytes. Bone marrow adipocytes revealed a distinct gene profile with low expression of adipocyte-specific genes peroxisome proliferator-activated receptor gamma (PPARγ), fatty acid binding protein 4 (FABP4), perilipin (Plin1), adipsin (CFD) and high expression of genes associated with early adipocyte differentiation (CCAAT/enhancer binding protein beta (C/EBPβ), regulator of G-protein signaling 2 (RGS2). In addition, a number of genes including secreted frizzled related protein 4 (SFRP4), tumor necrosis factor α (TNFα), transforming growth factor beta 1(TGFβ1), G-protein coupled receptor 109A (GPR109A) and interleukin 6 (IL-6), that could affect adipose-derived signaling to bone are markedly increased in bone marrow adipocytes. Age had a substantial effect on genes associated with mitochondria function and inflammation in bone marrow adipocytes. Twenty seven genes were significantly changed with age in both adipocyte depots. Among these genes, IL6 and GPR109A were significantly reduced with age in both adipocyte depots.ConclusionsOverall, gene profiling reveals a unique phenotype for primary bone marrow adipocytes characterized by low adipose-specific gene expression and high expression of inflammatory response genes. Bone marrow and epididymal adipocytes share a common pathway in response to aging in mice, but age has a greater impact on global gene expression in epididymal than in bone marrow adipocytes. Genes that are differentially expressed at greater levels in the bone marrow are highly regulated with age.

This article is available online at http://www.jlr.org to the release of FFA, the process known as lipolysis. The control of lipolysis represents a complex process involving multiple regulatory events ( 1, 2 ). These broadly include lipolytic (ACTH, ␤ -adrenergic agonists, etc.) and anti-lipolytic (insulin, adenosine, etc.) hormones, their cognate receptors and signaling pathways, and lipid droplet-associated proteins, such as perilipins, cytosolic lipid binding proteins, and lipases. The complexity of the regulation of lipolysis has further increased in recent years with the identifi cation of a large number of proteins that are found associated with the lipid droplets stored within adipocytes and with the realization of the dynamic nature of the proteins that populate lipid droplets.The major lipid droplet-associated protein in differentiated adipocytes is perilipin (Plin) A, which is also the most highly phosphorylated protein in adipose cells following cyclic AMP-dependent protein kinase (PKA) activation ( 3 ). Plin A plays an essential role in regulating lipolysis, as evidenced by the 3-fold elevation in basal lipolysis and the absence of PKA-stimulated lipolysis in adipocytes from Plin A null mice ( 4, 5 ). Moreover, using model cell systems, it has been shown that Plin A promotes PKA-stimulated lipolysis ( 6-8 ). Thus, it appears that Plin A has dual functions of acting as a barrier to suppress basal or constitutive lipolysis and of enhancing PKA-stimulated lipolysis.Intracellular lipases are instrumental in lipolysis, and there are many proteins that are predicted to have lipase or esterase activity that are expressed in adipose tissue. It had long been considered that hormone-sensitive lipase (HSL) ( 9, 10 ) was the rate-limiting enzyme for lipolysis; however, examination of the nonobese HSL null mouse indicated the existence and importance of other lipases The major contribution of adipose tissue to whole-body metabolism is the storage of energy in the form of triacylglycerols and the mobilization of this stored energy leading