Hormone-sensitive lipase (HSL) is expressed predominantly in white and brown adipose tissue where it is believed to play a crucial role in the lipolysis of stored triglycerides (TG), thereby providing the body with energy substrate in the form of free fatty acids (FFA). From in vitro assays, HSL is known to hydrolyze TG, diglycerides (DG), cholesteryl esters, and retinyl esters. In the current study we have generated HSL knock-out mice and demonstrate three lines of evidence that HSL is instrumental in the catabolism of DG in vivo. First, HSL deficiency in mice causes the accumulation of DG in white adipose tissue, brown adipose tissue, skeletal muscle, cardiac muscle, and testis. Second, when tissue extracts were used in an in vitro lipase assay, a reduced FFA release and the accumulation of DG was observed in HSL knock-out mice which did not occur when tissue extracts from control mice were used. Third, in vitro lipolysis experiments with HSL-deficient fat pads demonstrated that the isoproterenol-stimulated release of FFA was decreased and DG accumulated intracellularly resulting in the essential absence of the isoproterenolstimulated glycerol formation typically observed in control fat pads. Additionally, the absence of HSL in white adipose tissue caused a shift of the fatty acid composition of the TG moiety toward increased long chain fatty acids implying a substrate specificity of the enzyme in vivo. From these in vivo results we conclude that HSL is the rate-limiting enzyme for the cellular catabolism of DG in adipose tissue and muscle. Hormone-sensitive lipase (HSL)1 is thought to be a key enzyme for the mobilization of triglycerides (TG) deposited in adipose tissue. Human HSL is composed of 775 amino acids that are encoded by a 2.9-kb mRNA transcribed from a single gene composed of 9 exons (1, 2). The mouse and the human genes are similar in size and share a high degree of sequence homology. A tissue-specific size variation has been observed in testis where an additional exon 13 kb upstream of exon 1 in adipose tissue gives rise to a 3.9-kb mRNA and a 1076-amino acid protein (3). The molecular basis of size variations in HSL mRNA in muscle, macrophages, and ovaries is unknown.HSL-mediated lipolysis is strictly controlled by hormones. The enzyme is activated by catecholamines and other lipolytic hormones upon phosphorylation by the cAMP-dependent protein kinase A and the lipotransin-mediated translocation of the enzyme from the cytoplasm to the lipid droplet (4 -6). Insulin, the major antilipolytic hormone, inhibits HSL through phosphodiesterase-3-dependent cAMP degradation and interference with the lipotransin-mediated enzyme translocation. Accordingly, mice with elevated protein kinase A activity exhibit increased lipolysis and a lean phenotype (7). Absence of perilipin in mice also resulted in leanness through constitutive activation of HSL (8). Conversely, mice that lack the insulin receptor substrate 2 become obese (9). These results imply that imbalances between lipid accumulation and fat mobilizati...

Abstract-The role of hepatic lipase as a multifunctional protein that modulates lipoprotein metabolism and atherosclerosis has been extensively documented over the last decade. Hepatic lipase functions as a lipolytic enzyme that hydrolyzes triglycerides and phospholipids present in circulating plasma lipoproteins. Hepatic lipase also serves as a ligand that facilitates lipoprotein uptake by cell surface receptors and proteoglycans, thereby directly affecting cellular lipid delivery. Recently, another process by which hepatic lipase modulates atherogenic risk has been identified. Bone marrow transplantation studies demonstrate that hepatic lipase present in aortic lesions markedly alters aortic lesion formation even in the absence of changes in plasma lipids. These multiple functions of hepatic lipase, which facilitate not only plasma lipid metabolism but also cellular lipid uptake, can be anticipated to have a major and complex impact on atherogenesis. Consistently, human and animal studies support proatherogenic and antiatherogenic roles for hepatic lipase. Key Words: transgenic mouse models Ⅲ lipolytic enzyme Ⅲ ligand-binding function Ⅲ macrophages Ⅲ bone marrow transplantation Ⅲ aortic atherosclerosis C oronary artery disease (CAD) is a major cause of mortality in advanced societies. 1-3 Multiple factors contribute to the formation of lesions that ultimately lead to CAD. One of the initial events in the development of atherosclerosis is the accumulation of cells containing excess lipids within the arterial wall. 4 Plasma lipoproteins play a major role in the deposition and removal of lipids that accumulate in atherosclerotic lesions. Apolipoprotein B (apoB)-containing lipoproteins and high-density lipoprotein (HDL) have opposite effects on CAD and are independent risk factors for this disease. [5][6][7] Both classes of lipoproteins have been major targets for the development of new therapeutic approaches for treatment of CAD.During the last decade, a great deal of interest has focused on hepatic lipase and its impact on lipoprotein metabolism, including intermediate-density lipoproteins (IDLs), chylomicron remnants and HDLs, and atherogenesis. Hepatic lipase has been shown in several studies to modulate atherogenic risk; however, its role as either a protective or proatherogenic agent remains unclear. Published human and animal studies support proatherogenic and antiatherogenic functions for hepatic lipase. 8 -14 In humans, low hepatic lipase activity has been associated with increased risk of CAD. [15][16][17][18] Furthermore, premature CAD has been reported in patients with complete hepatic lipase deficiency, 19 although the manner in which these very few individuals have been identified raises the issue of ascertainment bias. Other studies have concluded that decreased hepatic lipase activity does not influence susceptibility to CAD. 20 Finally, increased hepatic lipase activity has been reported in patients with CAD. 21,22 A proatherogenic role for hepatic lipase has been suggested from the inverse corre...

It has been observed previously that hormonesensitive lipase-deficient (HSL-ko) mice have reduced white adipose tissue (WAT) stores compared to control mice. These findings contradict the expectation that the decreased lipolytic activity in WAT of HSL-ko mice would cause accumulation of triglycerides (TGs) in that tissue. Here we demonstrate that the cellular TG synthesis in HSL-deficient WAT is markedly reduced due to downregulation of the enzymatic activities of glycerophosphate acyltransferase, dihydroxyacetonphosphate acyltransferase, lysophosphatidate acyltransferase, and diacylglycerol acyltransferase. Fatty acid de novo synthesis is also decreased due to reduced cellular glucose uptake, reduced glucose incorporation into adipose tissue lipids, and reduced activities of acetyl:CoA carboxylase and fatty acid synthase. Finally, the activities of phosphoenolpyruvate carboxykinase (PEPCK), acyl:CoA synthetase (ACS), and glucose 6-phosphate dehydrogenase, the enzymes that provide glycerol-3-phosphate, acyl-CoA, and NADPH for TG synthesis, respectively, are decreased in HSL-ko mice. The reduced expression of the peroxisome proliferator-activated receptor ␥ (PPAR ␥ ) target genes PEPCK, ACS, and aP2, as well as reduced mRNA levels of PPAR ␥ itself, suggest the involvement of this transcription factor in the downregulation of lipogenesis. Taken together, these results establish that in the absence of HSL, the reduced NEFA production is counteracted by a drastic reduction of NEFA reesterification that provides sufficient quantities of NEFA for release into the circulation. These metabolic adaptations result in decreased fat mass in HSL-ko mice. Storage and mobilization of metabolic energy in mammals are coordinated by tight hormonal control of the synthesis and the catabolism of triglycerides (TGs) in white adipose tissue (WAT). Imbalances in these anabolic and catabolic processes might be involved in dysregulation of body weight control and the pathogenesis of obesity and related disorders. Hormone-sensitive lipase (HSL) is considered to be the central enzyme in the mobilization of WAT-TG stores. This multifunctional enzyme has been shown to hydrolyse TGs, diglycerides, and monoglycerides, as well as cholesteryl ester and other small water-soluble substrates (1). During periods of increased energy demand, HSL is activated by hormones such as catecholamines, which leads to an increase in the intracellular cAMP levels, resulting in the activation of protein kinase A (PKA) and phosphorylation of HSL (2). In parallel, activation of PKA leads to the phosphorylation of perilipin A, which elicits the translocation of phosphorylated HSL from the cytoplasm to the lipid droplet (3), a process that might also involve lipotransin (4). Once activated and translocated, HSL can hydrolyze lipid droplet-associated TG, and the mobilized nonesterified fatty acids (NEFAs) are released from WAT into the circulation.Hormone-sensitive lipase-deficient (HSL-ko) mice exhibited a marked decrease of acylglyceride hydrolase activity...

The presence of a rapidly metabolizable carbon source, especially glucose, results in a coordinated change of metabolic functions in many bacteria. Regulation is achieved by altering the activity of a number of proteins, which then leads to the differential expression of operons encoding metabolic enzymes, a process termed carbon catabolite repression (CR). In Escherichia coli (for reviews, see references 47-49), the regulatory cascade starts with the glucose-specific EIIA protein (EIIA Glc ) of the phosphoenolpyruvate:sugar phosphotransferase system (PTS) as the allosteric effector. Nonphosphorylated EIIA Glc prevents the uptake of alternative carbon sources, either indirectly by inhibition of a catabolic enzyme (glycerol kinase) or directly by inhibition of sugar permeases (e.g., lactose and melibiose permease), a phenomenon designated inducer exclusion. In both cases, sugar-specific induction of the respective catabolic genes is hindered. Phosphorylated EIIA Glc , however, activates adenylate cyclase, the cyclic AMP (cAMP)-generating enzyme. An enhanced level of cAMP serves as the signal for a global alteration of gene expression, which is finally exerted by the catabolite gene activator protein, also called cAMP receptor protein.In gram-positive bacteria with low GC content, one central regulatory pathway also depends on a PTS component, the phosphocarrier protein HPr (42, 43). HPr, phosphorylated at a serine residue by an ATP-dependent, metabolite-activated protein kinase (13), allosterically controls sugar permeases in Lactobacillus brevis (64, 66) and a sugar-phosphate phosphatase of Lactococcus lactis (65), which results in inducer exclusion and inducer expulsion, the rapid efflux of preaccumulated sugars or sugar metabolites (41). HPr(serine-phosphate) [HPr(Ser-P)] also interacts with a pleiotropic regulator (11), the catabolite control protein CcpA, first recognized in