Evidence for a Common Mechanism Regulating Distinct Lipases
Hormone-sensitive lipase (HSL) is presumed to be essential for lipolysis, which is defined as the mobilization of free fatty acids from adipocytes. In the present study, we investigated the effects of various lipolytic hormones on the lipolysis in adipocytes derived from mouse embryonic fibroblasts (MEF adipocytes) prepared from HSL-deficient mice (HSL−/−). HSL−/− MEF differentiated into mature adipocytes in a manner indistinguishable from that of wild-type mice. Both isoproterenol (ISO) and tumor necrosis factor (TNF)-α stimulated the rate of lipolysis in HSL−/− MEF adipocytes, although to a lesser extent than in wild-type cells, and these lipolytic activities were inhibited by H-89, a cAMP-dependent protein kinase inhibitor, and troglitazone, respectively. Thus, the responses of the residual lipolytic activity to lipolytic hormones and TNF-α were well conserved in the absence of HSL. Extracts from HSL−/− MEF adipocytes hydrolyzed triacylglycerol (TG) but not cholesterol ester, indicating that the residual lipolytic activity was mediated by another TG-specific lipase. The TG lipase activity, which was decreased in cytosolic fraction in response to ISO, was increased in fat cake fraction. Therefore, translocation of the TG lipase may explain, at least partially, the ISO-stimulated lipolysis in HSL−/− adipocytes. In conclusion, lipolysis is mediated not only by HSL but also by the non-HSL TG lipase, whose responses to lipolytic hormones are similar to those of HSL. We propose that both lipases are regulated by common mechanism of lipolysis.
Address correspondence and reprint requests to Shun Ishibashi, MD, Division of Endocrinology and Metabolism, Department of Internal Medicine, Jichi Medical School, 3311-1 Yakushiji Minamikawachi-machi, Kawachi-gun, Tochigi 329-0498 Japan. E-mail:.
Received for publication 18 October 2000 and accepted in revised form 23 August 2002.
aP2, adipocytes fatty acid-binding protein; BCA, bicinchoninic acid; CE, cholesterol ester; C/EBP, CAAT enhancer-binding protein; DEX, dexamethasone; FFA, free fatty acid; HODE, hydroxyoctadecadienoic acid; HSL, hormone-sensitive lipase; IBMX, 3-isobutyl-2-methylxanthine; ISO, isoproterenol; MEF, mouse embryonic fibroblasts; NCEH, neutral cholesterol ester hydrolase; PIA, (−)-N6-(2-phenylisopropyl)-adenosine; PKA, protein kinase A; PPAR-γ, peroxisome proliferator-activated receptor-γ; TG, triacylglycerol; TNF, tumor necrosis factor; TZD, thiazolidinedione; WAT, white adipose tissue.