The Lipolytic Proteome of Mouse Adipose Tissue

Abstract

In animals, plant seeds, and fungi, excessive amounts of lipids are stored in the form of intracellular triacylglycerol (TG) 1 and steryl ester deposits. Mammals store TG in adipose tissue as the primary source of energy during periods of starvation and increased energy demand. The balance of lipid storage and mobilization is tightly regulated to ensure whole body energy homeostasis. Stored fat is mobilized by activation of lipolytic enzymes, which degrade adipose TG and cholesterol esters and release non-esterified (free) fatty acids into the circulation. Variation in the concentration of circulating free fatty acids is an established risk factor for the development of insulin resistance in type 2 diabetes and related disorders (1-4). Cholesteryl esters represent the stored esterified form of cholesterol, which is an important constituent of membranes and a precursor of bile acid and steroid hormones. TG lipases and cholesteryl esterases are key enzymes in lipid mobilization, and respective lipolytic activities have been described in various tissues. However, only some of them have been identified on the molecular level. Many characterized lipases are secretory proteins (5-8), whereas the hormonesensitive lipase (HSL) (9), the monoglyceride lipase (MGL) (10), and the triacylglycerol hydrolase (TGH) (11) are intracellular enzymes. HSL was shown to hydrolyze TG and cholesteryl esters and was thought to catalyze the rate-limiting step in TG hydrolysis in adipose tissue. Yet in HSL knock-out mice, TG hydrolysis in adipose tissue as well as cholesteryl ester hydrolysis in macrophages were not abolished (9). MGL is ubiquitously expressed and rather specific for monoacylglycerol hydrolysis (10). TGH appears to be involved in TG mobilization in liver and is also expressed in adipose tissue where its role is not clear (11,12).The present study was aimed at the molecular identification of the lipolytic proteome of mouse adipose tissue using activity labels designed in our laboratory (Fig. 1). Basically an activity label is a molecule consisting of (i) a recognition site targeting a certain enzyme species, (ii) a properly positioned reactive site that forms a covalent bond with the target, and (iii) a tag for visualization and/or purification of the covalently bound target (13-16). The active site for lipases and esterases typically consists of a catalytic triad formed by Ser-His-(Asp/ Glu), which is common for most serine hydrolases (17). A feature specific for many lipases and esterases, however, is the ␣/␤-hydrolase fold consisting of a series of parallel ␤-sheets and a number of helices that flank the sheets on both sides (18,19). Most lipases contain a lid controlling the access of substrates to the hydrophobic active site. The same structural features are found in esterases except for the lid. Therefore, in contrast to lipases, most esterases do not show activation at lipid/water interfaces. However, it has to be emphasized that the borderline between lipases and esterFrom the ‡Institute

Rectification of impaired adipose tissue methylation status and lipolytic response contributes to hepatoprotective effect of betaine in a mouse model of alcoholic liver disease

“…Although adipocyte lipolysis is regulated by a variety of proteins exhibiting lipase activity (Birner-Gruenberger et al, 2005), HSL plays an important role in this process and, together with ATGL, is responsible for more than 95% of lipase activity in murine white adipose tissue (Schweiger et al, 2006). A recent study from Zhou's group elegantly showed that chronic alcohol feeding was associated with increased ATGL expression and HSL activation in adipose tissue .…”

Regulation of Lipolysis in Adipocytes

“…Others have subsequently confirmed the presence of triacylglycerol hydrolase (TGH; carboxylesterase 3; EC 3.1.1.1) in adipose tissue (10). TGH is a 60 kDa microsomal lipase that contains a catalytic triad with an active site serine located in the GXSXG motif (68).…”

Adipose Triglyceride Lipase (ATGL) and Hormone-Sensitive Lipase (HSL) Deficiencies Affect Expression of Lipolytic Activities in Mouse Adipose Tissues

“…Abhydrolase domain-containing protein 11 is encoded on a chromosomal region that is often deleted in the human developmental disorder Williams-Beuren syndrome (38). Earlier experiments using substrates with various polarities led to the conclusion that this protein may rather be an esterase than a lipase (19). Enolase 1, which is also a serine hydrolase, catalyzes the stereochemically specific anti-elimination of water from 2-phosphoglycerate to form P-enolpyruvate in a stepwise reaction (39).…”

“…Expression in other tissues is rather low. Currently it cannot be excluded, that other lipases also exist that are important for lipid catabolism (19). Recent functional proteomic screens in various mouse tissues led to the identification of enzyme candidates that are currently subject to functional characterization (unpublished data).…”