Hormone-sensitive Lipase Deficiency in Mice Causes Diglyceride Accumulation in Adipose Tissue, Muscle, and Testis

Abstract

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...

A computational model of adipose tissue metabolism: Evidence for intracellular compartmentation and differential activation of lipases

“…The intracellular rates of lipolysis by ATGL, HSL, and MGL were estimated from the difference between AVDs of glycerol and TG such that ~15% of the produced FA are re-utilized inside adipose tissue (Frayn et al, 1994;Coppack et al, 1990). Finally, fluxes through individual lipase reaction were estimated based on the 10-fold higher activity that HSL has for DG than for TG and MG (Shen et al, 1998;Haemmerle et al, 2002). Thus, the flux rate for DG breakdown by HSL was calculated first and then those for TG and MG breakdown by HSL.…”

“…However, recently, it has been shown that the HSL deficient mice retain the basal lipolysis rate and respond to the beta-adrenergic stimulation, although the response was quantitatively less than in the wild type (Okazaki et al, 2002;Zechner et al, 2005;Haemmerle et al, 2002). The accumulation of diglycerides (DG) in the adipose tissue of HSL knockout mice suggests that HSL is the rate limiting enzyme for the hydrolysis of DG and not TG (Haemmerle et al, 2002). Adipose TG lipase (ATGL) has been suggested to be the key enzyme involved in TG hydrolysis in the adipose tissue (Schweiger et al, 2006;Haemmerle et al, 2006;Zimmermann et al, 2004).…”

PPARγ agonism increases rat adipose tissue lipolysis, expression of glyceride lipases, and the response of lipolysis to hormonal control

“…Lipolytic hormone-stimulated, protein kinase A-mediated phosphorylation of both HSL and perilipin results in hydrolysis of triglyceride into NEFAs and monoglyceride, which is further converted to NEFA and glycerol by monoglyceride lipase (MGL) [8]. Recent studies showing that adipocytes from HSL knockout mice retain a marked basal and adrenergically stimulated lipolysis and accumulate diglycerides after stimulation of lipolysis demonstrated that HSL is not the sole lipase involved in triglyceride catabolism [9][10][11]. Three independent groups have discovered a protein-alternatively termed adipose triglyceride lipase (ATGL) [12], desnutrin [13] or calcium-independent phospholipase A 2 /lipase ζ (iPLA 2 ζ) [14]-that fulfils all the predicted characteristics of this new lipase: a preference for the hydrolysis of the first triglyceride ester bond, drastically reduced lipolysis following antibody or antisense ATGL inhibition, and enhanced basal and isoproterenol-stimulated lipolysis following ATGL overexpression [15].…”

Human adipose triglyceride lipase (PNPLA2) is not regulated by obesity and exhibits low in vitro triglyceride hydrolase activity

“…This enzyme possesses tri-and diglyceride hydrolase activities and is regulated by the major lipolysis-regulating hormones, which in man are insulin, catecholamines and natriuretic peptides [3]. However, HSL knock-out mice have residual lipolytic activity [4,5], which may be attributed to an additional lipase with high affinity for triglycerides [6,7]. Accordingly, a novel adipocyte-specific lipase termed adipose triglyceride lipase (ATGL, now known as patatin-like phospholipase domain containing 2 [PNPLA2]), the product of Pnpla2, and alternatively designated iPLA 2 ζ or desnutrin, was recently identified using different strategies [8][9][10].…”