Inhibition of the reversible deactivation of chicken adipose tissue hormone-sensitive lipase by heat-stable proteins from adipose tissue and skeletal muscle.

The reversible deactivation of chicken adipose tissue hormone-sensitive lipase is catalyzed by a lipase phosphatase. Heat-stable protein preparations from rat epididymal fat pads, chicken adipose tissue, and rabbit skeletal muscle inhibited lipase phosphatase activity. Phosphatase inhibitor preparations from rat adipose tissue did not inhibit the protein kinase-catalyzed activation of hormone-sensitive lipase, whereas inhibitor preparations from rabbit skeletal muscle were contaminated with protein kinase inhibitor.     

Growth hormone-mediated breakdown of body fat: effects of GH on lipases in adipose tissue and skeletal muscle of old rats fed different diets.

Lipid storage and breakdown is mainly controlled by lipoprotein lipase and hormone-sensitive lipase. The aim of this work was to elucidate whether growth hormone mediated loss of adipose tissue involves a concerted action on tissue lipases, and to what degree such events are modulated by dietary regimen. Twelve-month-old rats fed first a high-fat diet or a low-fat diet for 14 weeks were injected with saline or growth hormone (4 mg/kg/d) for four days or three weeks in different combinations with either high- or low-fat diets. In adipose tissue, growth hormone generally inhibited lipoprotein lipase and also attenuated the inhibiting effect of insulin on hormone-sensitive lipase activity. Growth hormone treatment combined with restricted high-fat feeding reduced the activity of both lipases in adipose tissue and stimulated hormone-sensitive lipase in muscle. Generally, plasma levels of free fatty acids, glycerol and cholesterol were reduced by growth hormone, and in combination with restricted high-fat feeding, triglyceride levels improved too. We conclude that growth hormone inhibits lipid storage in adipose tissue by reducing both lipoprotein lipase activity and insulin's inhibitory action on hormone-sensitive lipase. We also propose that growth hormone's effects on tissue lipases and blood lipids are modulated by dietary regimen.     

Changes in the expression of uncoupling proteins and lipases in porcine adipose tissue and skeletal muscle during feed deprivation*(1)

The hormone-sensitive and lipoprotein lipases are critical determinants of the metabolic adaptation to starvation. Additionally, the uncoupling proteins have emerged with potential roles in the metabolic adaptations required by energy deficiency. The objective of this study was to evaluate the expression (mRNA abundance) of uncoupling proteins 2 and 3 and that of hormone-sensitive and lipoprotein lipase in the adipose tissue and skeletal muscle of the pig in relationship to feed deprivation. Thirty-two male castrates (87 kg +/- 5%) were assigned at random to fed and feed-deprived treatment groups. After 96 hr, the pigs were euthanized and adipose and skeletal muscle tissue obtained for total RNA extraction and nuclease protection assays. Feed deprivation increased uncoupling protein 3 mRNA abundance 103-237% (P < 0.01) in longissimus and red and white semitendinosus muscle. In contrast, the increase in uncoupling protein 3 mRNA in adipose tissue was only 23% (P < 0.06), and adipose uncoupling protein 2 mRNA was not influenced (P > 0.66) by feed deprivation. The increased abundance of uncoupling protein 2 mRNA in the longissimus muscle of feed-deprived pigs was small (22%), but significant (P < 0.04). The expression of hormone-sensitive lipase was increased 46% and 64% (P < 0.04) in adipose tissue and longissimus muscle, respectively, by feed deprivation, whereas adipose lipoprotein lipase expression was reduced (P < 0.01) to 20% of that of the fed group. Longissimus lipoprotein lipase expression in the feed-deprived group was 37% of that of the fed group (P < 0.01), and similar reductions were detected in red and white semitendinosus muscle. Overall, these findings indicate that uncoupling protein 3 expression in skeletal muscle is quite sensitive to starvation in the pig, whereas uncoupling protein 2 changes are minimal. Furthermore, we conclude that hormone-sensitive lipase is upregulated at the mRNA level with prolonged feed deprivation, whereas lipoprotein lipase is downregulated.     

Lipolysis: pathway under construction

PURPOSE OF REVIEW: The lipolytic catabolism of stored fat in adipose tissue supplies tissues with fatty acids as metabolites and energy substrates during times of food deprivation. This review focuses on the function of recently discovered enzymes in adipose tissue lipolysis and fatty acid mobilization. RECENT FINDINGS: The characterization of hormone-sensitive lipase-deficient mice provided compelling evidence that hormone-sensitive lipase is not uniquely responsible for the hydrolysis of triacylglycerols and diacylglycerols of stored fat. Recently, three different laboratories independently discovered a novel enzyme that also acts in this capacity. We named the enzyme 'adipose triglyceride lipase' in accordance with its predominant expression in adipose tissue, its high substrate specificity for triacylglycerols, and its function in the lipolytic mobilization of fatty acids. Two other research groups showed that adipose triglyceride lipase (named desnutrin and Ca-independent phospholipase A2zeta, respectively) is regulated by the nutritional status and that it might exert acyl-transacylase activity in addition to its activity as triacylglycerol hydrolase. Adipose triglyceride lipase represents a novel type of 'patatin domain-containing' triacylglycerol hydrolase that is more closely related to plant lipases than to other known mammalian metabolic triacylglycerol hydrolases. SUMMARY: Although the regulation of adipose triglyceride lipase and its physiological function remain to be determined in mouse lines that lack or overexpress the enzyme, present data permit the conclusion that adipose triglyceride lipase is involved in the cellular mobilization of fatty acids, and they require a revision of the concept that hormone-sensitive lipase is the only enzyme involved in the lipolysis of adipose tissue triglycerides.     

Developmental regulation of hormone-sensitive lipase mRNA in the rat: changes in steroidogenic tissues.

The hydrolysis of triglycerides and cholesteryl esters stored within cells is mediated by the enzyme, hormone-sensitive lipase. In adipose tissue and heart, hormone-sensitive lipase primarily hydrolyzes stored triglycerides to free fatty acids, while in steroidogenic tissues, it principally converts cholesteryl esters to free cholesterol for steroid hormone production. To determine whether hormone-sensitive lipase is under tissue-specific, developmental regulation, the steady state levels of hormone-sensitive lipase mRNA were determined in normal rats from late fetal life through 2 years of age. Hormone-sensitive lipase mRNA levels did not appear to vary in adipose tissue from epididymal fat pads obtained from animals between 3 weeks and 2 years of age. In heart, hormone-sensitive lipase mRNA levels were lowest in the fetus increased rapidly within the first day postnatally, and then gradually increased to stable adult levels by 2 months that were 3-fold higher than observed in fetal rats. Steady state mRNA levels of hormone-sensitive lipase in the adrenals were lowest in fetal rats, increased 4-fold during the first day and peaked at levels that were 9-fold higher by the end of the first week. Thereafter, levels fell and remained 3- to 4-fold higher than at birth throughout adult life. Hormone-sensitive lipase mRNA was undetectable in testes before 4 weeks of age and increased 25-fold to stable adult levels between 4 and 12 weeks. Thus, hormone-sensitive lipase is differentially expressed and regulated in a tissue-specific fashion during development and aging.     

Hormone-sensitive lipase and monoacylglycerol lipase are both required for complete degradation of adipocyte triacylglycerol

The respective roles of monoacylglycerol lipase and hormone-sensitive lipase in the sequential hydrolysis of adipose tissue triacylglycerols have been examined. An adipose tissue preparation, containing both lipases in approximately the same proportion as in the intact tissue, hydrolyzed emulsified tri- or dioleoylglycerol to fatty acids and glycerol, with little accumulation of di- or monooleoylglycerol. Selective removal of the monoacylglycerol lipase by immunoprecipitation markedly reduced the glycerol release. Isolated hormone-sensitive lipase hydrolyzed acylglycerols with a marked accumulation of monoacylglycerol in accordance with the positional specificity of this enzyme (Fredrikson, G. and Belfrage, P. (1983) J. Biol. Chem. 258, 14253-14256). Addition of increasing amounts of isolated monoacylglycerol lipase led to a corresponding increase in glycerol release, due to hydrolysis of the monoacylglycerols formed. The reaction proceeded to completion when the relative proportion of the two lipases was similar to that in the intact tissue. These findings indicate that hormone-sensitive lipase catalyzes the hydrolysis of triacylglycerol in the rate-limiting step of adipose tissues lipolysis, and of the resulting diacylglycerol, whereas the action of monoacylglycerol lipase is required in the final hydrolysis of the 2-monoacylglycerols produced.     

Stimulation of the hormone-sensitive triacylglycerol lipase from adipose tissue by phosphatidylethanolamine

The activity of a pigeon adipose tissue hormone-sensitive triacylglycerol lipase preparation was increased from 2- to 5-fold by the presence of phosphatidylethanolamine in assays with three different methods of preparing triolein substrates. Phosphatidylethanolamine from egg yolk produced the greatest stimulation of lipase activity; the stimulation was concentration-dependent but was not time-dependent. A comparable increase in triacylglycerol lipase activity due to phosphatidylethanolamine was also observed with enzyme preparations from chicken and rat adipose tissue. Phosphatidylcholine, phosphatidylserine, phosphatidylinositol, phosphatidic acid, cardiolipin, sphingomyelin, Triton X-100 and sodium dodecyl sulfate all inhibited enzyme activity. Phosphatidylethanolamine had no effect on acid lipase activity in the pigeon adipose tissue preparation. Preincubation of the pigeon adipose tissue lipase with ATP, cyclic AMP and protein kinase resulted in a 2.15-fold activation of hydrolase activity determined in the absence of phosphatidylethanolamine. In contrast, non-activated and protein kinase-activated forms of the lipase were characterized as having very nearly the same activity in assays with substrate preparations containing phosphatidylethanolamine. The phosphatidylethanolamine-dependent stimulation of lipase activity was characterized kinetically as being due to an increase in maximal velocity. The modulation of the adipose tissue hormone-sensitive lipase activity by phospholipids could be involved in the hormonal regulation of lipolysis.     

Letting lipids go: hormone-sensitive lipase

PURPOSE OF REVIEW: Despite their pathophysiological importance, the molecular mechanisms and enzymatic components of lipid mobilization from intracellular storage compartments are insufficiently understood. The aim of this review is to evaluate the role of hormone-sensitive lipase in this process. RECENT FINDINGS: Hormone-sensitive lipase exhibits a broad specificity for lipid substrates such as triglycerides, diglycerides, cholesteryl esters, and retinyl esters and the enzyme is in a wide variety of tissues. The high enzyme activity in adipose tissue was considered rate-limiting in the degradation of stored triglycerides. This view of a single enzyme controlling the catabolism of stored fat was challenged by recent findings that in hormone-sensitive lipase deficient mice adipose tissue triglycerides were still hydrolyzed and that these animals were leaner than normal mice. These results indicated that in adipose tissue hormone-sensitive lipase cooperates with other yet unidentified lipases to control the mobilization of fatty acids from cellular depots and that this process is coordinately regulated with lipid synthesis. Induced mutant mouse lines that overexpress or lack hormone-sensitive lipase also provided evidence that hormone-sensitive lipase-mediated cholesteryl ester hydrolysis is involved in steroid-hormone production in adrenals and affects testis function. Finally, hormone-sensitive lipase deficiency in mice results in a lipoprotein profile characterized by low triglyceride and VLDL levels and increased HDL cholesterol concentrations. SUMMARY: The 'anti-atherosclerotic' plasma lipoprotein profile and the fact that hormone-sensitive lipase deficient animals become lean identifies the inhibition of hormone-sensitive lipase as a potential target for the treatment of lipid disorders and obesity.     

Nutritional regulation of lipoprotein lipase in guinea pig tissues

Glucose transport in guinea pig adipocytes has been shown to be markedly resistant to stimulation by insulin. Lipoprotein lipase is another transport catalyst in adipose tissue which is believed to be regulated by insulin. We have therefore studied how feeding-fasting affects lipoprotein lipase activity in guinea pig tissues. There was an even more marked decrease in adipose tissue lipoprotein lipase activity on fasting in guinea pigs (10-20 fold) than in rats or mice (4-5 fold). In adipocytes, the activity decreased only 2.5-4.5 fold; most of the change was in extracellular lipoprotein lipase. On glucose refeeding, the activity was rapidly restored. In the first 4 hours after glucose administration extracellular lipoprotein lipase activity increased to more than 10 times the amount present in adipocytes. After cycloheximide, lipoprotein lipase activity decreased with a half-life of 22 min. It is concluded that lipoprotein lipase is rapidly produced and turned over in guinea pig adipose tissue, and that the system is quite sensitive to feeding-fasting. In contrast to adipose tissue, there was no significant change in lipoprotein lipase activity in any other tissue on fasting. There was a strong correlation between the activities in heart and diaphragm muscle, but this correlation was independent of feeding-fasting.     

Hormone-sensitive lipase of adipose tissue.

Some physiologic aspects of the mobilization and fate of free fatty acids are reviewed. The molecular mechanism of the activation of hormone-sensitive lipase in adipose tissue is then discussed. Recent evidence established that hormone-sensitive lipase, concerned with fat mobilization, is both functionally and immunochemically distinct from lipoprotein lipase, concerned with uptake of plasma triglycerides. Lipoprotein lipase activity is not altered by cyclic AMP-dependent protein kinase. The latter enzyme enhances not only triglyceride hydrolase but also monoglyceride, diglyceride and cholesterol ester hydrolase activities in chicken adipose tissue. Finally, it is shown that the activation of all four acyl hydrolases is reversible, the deactivation being magnesium-dependent. Protein phosphatase fractions from heart and liver active against phosphorylase a can reversibly deactivate adipose tissue hormone-sensitive lipase, implying a low degree of substrate specificity for lipase phosphatase.     

Hormone-sensitive lipase from swine adipose tissue: identification and some properties

Swine adipose tissue hormone-sensitive lipase, purified 475-fold to 10% protein purity, has been identified as a polypeptide of Mr = 84,000. The enzyme has high specific activity against tri-, di- and monoacylglycerols, as well as cholesterol esters, and is inhibited by millimolar NaF, and micromolar HgCl2 and DFP. The enzyme polypeptide serves as a substrate for cyclic AMP-dependent protein kinase. The characteristics of the hormone-sensitive lipase from swine adipose tissue are similar to those reported previously for the enzyme from rat. They differ from those reported for the lipase from chicken adipose tissue, and possible reasons for these differences are discussed.     

Restoration of lipoprotein lipase activity in insulin-deficient rats by insulin infusion is tissue-specific

The activity of lipoprotein lipase was measured in white and brown adipose tissues, red vastus lateralis muscle, and heart of rats that have been insulin deficient (streptozotocin, 75 mg.kg-1) for 2 weeks, and that have then received implants of insulin-delivering minipumps (17 U.kg-1.day-1) for 1 or 4 days. Normal glycemia was restored in insulin-deficient animals after 4 days of insulin treatment. Hypertriglyceridemia, but not hypercholesterolemia, was reversed after 4 days of insulin infusion. After 2 weeks of insulin deficiency, fasting lipoprotein lipase activity was lowered in all tissues studied. In white adipose tissue, lipoprotein lipase decreased to 50% of control values. After a single day of insulin infusion, even if tissue weight has not yet been greatly affected, total activity was completely restored to control levels. Enzyme activity in brown adipose tissue was also depressed in deficient animals, and insulin infusion was followed by a slow recovery of activity, to a level intermediate between those of control and insulin-deficient groups. Insulin status had milder effects on lipoprotein lipase activity in vastus lateralis muscle than in the adipose tissues. Deficient rats displayed 60% less activity than controls, and 4 days of hormone infusion only partially restored enzyme activity. There was a large loss of lipoprotein lipase in the heart following 2 weeks of insulin depletion, which was not counteracted by hormone infusion. Thus the speed and extent of recovery of lipoprotein lipase activity following hormone replacement in insulin-deficient animals varied widely among tissues. These findings suggest that insulin is part of the factors that determine the tissue specificity of lipoprotein lipase regulation.     

Combined lipase deficiency (cld/cld) in mice. Demonstration that an inactive form of lipoprotein lipase is synthesized

Combined lipase deficiency, cld, is a recessive mutation within the T/t complex of mouse chromosome 17. Mice homozygous for this defect display severe functional deficiencies of lipoprotein lipase and the related hepatic lipase. They develop massive hyperchylomicronemia and die within 3 days when allowed to suckle. Heart, diaphragm muscle, and brown adipose tissue of 1-day-old cld/cld and unaffected mice incorporated in vivo [35S]methionine into a protein that could be immunoprecipitated by antilipoprotein lipase serum. The immunoprecipitated protein in all tissues had the same Mr as bovine lipoprotein lipase as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The proportion of radioactivity in the lipoprotein lipase band to that in total protein was 0.02% in tissues of cld/cld mice and 0.01% in tissues of unaffected mice. There was 2-6 times more lipoprotein lipase-like protein (determined by immunoassay) in tissues of defective mice than in those of unaffected mice. These findings indicate that the cld mutation did not cause deletion of the structural gene for lipoprotein lipase. Lipoprotein lipase activity in heart, diaphragm muscle, brown adipose tissue, and lung of cld/cld mice was less than 5% of that in tissues of unaffected mice. This low activity could be inhibited more than 85% by antilipoprotein lipase serum, but not by nonimmune serum. It is concluded that tissues in cld/cld mice synthesize a lipoprotein lipase-like protein which has subnormal catalytic activity.     

Expression of biologically active hormone-sensitive lipase in mammalian (COS) cells.

cDNAs encoding rat adipose tissue hormone-sensitive lipase were expressed in COS cells, under the control of the SV40 promoter to half the level in rat adipocytes, the richest native source of the enzyme. A cDNA lacking most of the long 5'-untranslated region of the full-length rat hormone-sensitive lipase cDNA was, with regard to the lipase activity, on the average 70% more efficiently expressed that the full-length cDNA. The recombinant protein was almost identical to hormone-sensitive lipase of rat adipose tissue with respect to specific activity, susceptibility to inhibitors, molecular size, phosphorylation and activation by cyclic AMP-dependent protein kinase. The described eukaryotic expression system will allow analysis of effects of amino acid substitutions introduced into the lipase molecule by site-directed mutagenesis.     

Evaluation of the role of AMP-activated protein kinase and its downstream targets in mammalian hibernation

Mammalian hibernation requires an extensive reorganization of metabolism that typically includes a greater than 95% reduction in metabolic rate, selective inhibition of many ATP-consuming metabolic activities and a change in fuel use to a primary dependence on the oxidation of lipid reserves. We investigated whether the AMP-activated protein kinase (AMPK) could play a regulatory role in this reorganization. AMPK activity and the phosphorylation state of multiple downstream targets were assessed in five organs of thirteen-lined ground squirrels (Spermophilus tridecemlineatus) comparing euthermic animals with squirrels in deep torpor. AMPK activity was increased 3-fold in white adipose tissue from hibernating ground squirrels compared with euthermic controls, but activation was not seen in liver, skeletal muscle, brown adipose tissue or brain. Immunoblotting with phospho-specific antibodies revealed an increase in phosphorylation of eukaryotic elongation factor-2 at the inactivating Thr56 site in white adipose tissue, liver and brain of hibernators, but not in other tissues. Acetyl-CoA carboxylase phosphorylation at the inactivating Ser79 site was markedly increased in brown adipose tissue from hibernators, but no change was seen in white adipose tissue. No change was seen in the level of phosphorylation of the Ser565 AMPK site of hormone-sensitive lipase in adipose tissues of hibernating animals. In conclusion, AMPK does not appear to participate in the metabolic re-organization and/or the metabolic rate depression that occurs during ground squirrel hibernation.     

Cholesterol esterase in rat adipose tissue and its activation by cyclic adenosine 3':5'-monophosphate-dependent protein kinase.

A high level of cholesterol esterase activity, comparable to that of hormone-sensitive triglyceridase, has been demonstrated in rad adipose tissue. Essentially all of the activity was in the isolated adipocytes, primarily in the 100,000 times g supernatant fraction of the adipocytes. Cholesterol esterase activity in the 100,000 times g supernatant fraction was increased 40 plus or minus 16% by incubation with ATP (0.5 mM), Mg-2+ (1.25 mM), and cyclic adenosine 3':5'-monophosphate (cyclic AMP) (10 muM), conditions which also activated hormone-sensitive triglyceridase. Protein kinase inhibitor (rabbit skeletal muscle) blocked activation, and activation was restored by the addition of excess protein kinase (bovine skeletal muscle). In extracts prepared from adipocytes first incubated for 5 min with 10 muM epinephrine and 1 mM theophylline, there was no cyclic AMP-dependent cholesterol esterase activation, implying that the enzyme had been activated by a similar mechanism in the intact cell. The physiological role of this high level of cholesterol esterase activity in adipose tissue is unclear. Its relationship to hormone-sensitive triglyceride lipase, with which it extensively co-fractionates, and its possible involvement in fat mobilization remain to be determined.     

Regulation of triglyceride metabolism. IV. Hormonal regulation of lipolysis in adipose tissue

Triacylglycerol (TAG) stored in adipose tissue can be rapidly mobilized by the hydrolytic action of lipases, with the release of fatty acids (FA) that are used by other tissues during times of energy deprivation. Unlike synthesis of TAG, which occurs not only in adipose tissue but also in other tissues such as liver for very-low-density lipoprotein formation, hydrolysis of TAG, lipolysis, predominantly occurs in adipose tissue. Until recently, hormone-sensitive lipase was considered to be the key rate-limiting enzyme responsible for regulating TAG mobilization. However, recent studies on hormone-sensitive lipase-null mice have challenged such a concept. A novel lipase named desnutrin/ATGL has been recently discovered to play a key role in lipolysis in adipocytes. Lipolysis is under tight hormonal regulation. Although opposing regulation of lipolysis in adipose tissue by insulin and catecholamines is well understood, autocrine/paracrine factors may also participate in its regulation. Intricate cooperation of these endocrine and autocrine/paracrine factors leads to a fine regulation of lipolysis in adipocytes, needed for energy homeostasis. In this review, we summarize and discuss the recent progress made in the regulation of adipocyte lipolysis.     

Human adipose tissue hormone-sensitive lipase: identification and comparison with other species

The mRNA for human hormone-sensitive lipase (HSL) was identified using Northern blot analysis and a cDNA-probe for rat HSL. As in the rat, human adipose tissue expresses a single mRNA species of 3.3 kb. Using Western blotting with a polyclonal rabbit antibody towards rat adipose tissue HSL, the corresponding enzyme in human adipose tissue was identified with an apparent 88 kDa polypeptide, thus slightly larger than the rat and bovine 84 kDa, and the mouse and guinea-pig 82 kDa species. Additional evidence for the identification was provided by the inhibition of HSL diacylglycerol lipase activity by the anti-rat HSL antibody, and by NaF, DFP and Hg2+, known inhibitors of HSL. The concentration of the enzyme, as reflected by its activity per g tissue and the specific activity was about two thirds of that in the rat adipose tissue (200 g rats). The identification of the human enzyme protein made it possible to directly demonstrate its phosphorylation by cAMP-dependent protein kinase, thus extending the previous report regarding activation of the lipase with this kinase and ATP-Mg2+ in human adipose tissue extracts (Khoo, J.C., Aquino, A.A. and Steinberg, D. (1974) J. Clin. Invest. 53, 1124-1131).     

The significance of lipoprotein lipase in rat skeletal muscles.

Lipoprotein lipase was assayed in extracts of acetone-ether powders of rat skeletal muscles. Enzyme activity in soleus had typical characteristics of lipoprotein lipase in other tissues: inhibition by molar NaCl and protamine sulfate and activation by the human apolipoprotein, R-glutamic acid. Activity in muscles with predominantly red fibers (soleus, diaphragm, lateral head of gastrocnemius and anterior band of semitendinosus) was higher than in those with predominantly white fibers (body of gastrocnemius and posterior band of semitendinosus). No effect of a 24 hour fast upon enzyme activity was observed in ten skeletal muscles, but activity decreased substantially in four adipose tissue depots and increased slightly in heart muscle with fasting. Four minutes after intravenous injection of labeled lymph chylomicrons, skeletal muscles with predominantly red fibers incorporated several times more chylomicron triglyceride fatty acids than thos with predominantly white fibers. Estimated lipoprotein lipase activity in total skeletal muscle was about two-thirds that in total adipose tissue of rats fed ad libitum. After a 24 hour fast, total activity in skeletal muscle was about twice that in adipose tissue. These data suggest that a substantial fraction of lipoprotein lipase is in skeletal muscle of rats and that this tissue, especially its red fibers, is an important site of removal of triglycerides from the blood.     

Adipose triglyceride lipase and the lipolytic catabolism of cellular fat stores

Fatty acids (FAs) are essential components of all lipid classes and pivotal substrates for energy production in all vertebrates. Additionally, they act directly or indirectly as signaling molecules and, when bonded to amino acid side chains of peptides, anchor proteins in biological membranes. In vertebrates, FAs are predominantly stored in the form of triacylglycerol (TG) within lipid droplets of white adipose tissue. Lipid droplet-associated TGs are also found in most nonadipose tissues, including liver, cardiac muscle, and skeletal muscle. The mobilization of FAs from all fat depots depends on the activity of TG hydrolases. Currently, three enzymes are known to hydrolyze TG, the well-studied hormone-sensitive lipase (HSL) and monoglyceride lipase (MGL), discovered more than 40 years ago, as well as the relatively recently identified adipose triglyceride lipase (ATGL). The phenotype of HSL- and ATGL-deficient mice, as well as the disease pattern of patients with defective ATGL activity (due to mutation in ATGL or in the enzyme's activator, CGI-58), suggest that the consecutive action of ATGL, HSL, and MGL is responsible for the complete hydrolysis of a TG molecule. The complex regulation of these enzymes by numerous, partially uncharacterized effectors creates the "lipolysome," a complex metabolic network that contributes to the control of lipid and energy homeostasis. This review focuses on the structure, function, and regulation of lipolytic enzymes with a special emphasis on ATGL.