Hormone-sensitive lipase (HSL) is the predominant lipase effector of catecholamine-stimulated lipolysis in adipocytes. HSL-dependent lipolysis in response to catecholamines is mediated by protein kinase A (PKA)-dependent phosphorylation of perilipin A (Peri A), an essential lipid droplet (LD)-associated protein. It is believed that perilipin phosphorylation is essential for the translocation of HSL from the cytosol to the LD, a key event in stimulated lipolysis. Using adipocytes retrovirally

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... ineered from murine embryonic fibroblasts of perilipin null mice (Peri؊/؊ MEF), we demonstrate by cell fractionation and confocal microscopy that up to 50% of cellular HSL is LD-associated in the basal state and that PKA-stimulated HSL translocation is fully supported by adenoviral expression of a mutant perilipin lacking all six PKA sites (Peri A⌬1-6). PKA-stimulated HSL translocation was confirmed in differentiated brown adipocytes from perilipin null mice expressing an adipose-specific Peri A⌬1-6 transgene. Thus, PKA-induced HSL translocation was independent of perilipin phosphorylation. However, Peri A⌬1-6 failed to enhance PKA-stimulated lipolysis in either MEF adipocytes or differentiated brown adipocytes. Thus, the lipolytic action(s) of HSL at the LD surface requires PKA-dependent perilipin phosphorylation. In Peri؊/؊ MEF adipocytes, PKA activation significantly enhanced the amount of HSL that could be cross-linked to and co-immunoprecipitated with ectopic Peri A. Notably, this enhanced cross-linking was blunted in Peri؊/؊ MEF adipocytes expressing Peri A⌬1-6. This suggests that PKA-dependent perilipin phosphorylation facilitates (either direct or indirect) perilipin interaction with LD-associated HSL. These results redefine and expand our understanding of how perilipin regulates HSL-mediated lipolysis in adipocytes. The enzymatic hydrolysis of stored neutral lipid in adipocytes is an exquisitely regulated process that maintains whole body energy home-ostasis in response to physiological demands. In both white and brown adipose tissue (BAT) 4 basal (constitutive) rates of adipocyte lipolysis are rapidly and dramatically up-regulated by lipolytic hormones such as catecholamines (1, 2). In white adipose tissue, catecholamine-induced lipolysis provides fatty acids as fuel to peripheral tissues during times of energy need, such as fasting and exercise (3-5). In BAT of human newborns and rodents, catecholamine-stimulated lipolysis provides fatty acids for heat production (via ␤-oxidation and mitochondrial uncoupling) in response to hypothermia (i.e. adaptive thermogenesis) (2, 6). Fatty acids that are released during adipocyte lipolysis also function as modulators of glucose and insulin action and insulin production (7, 8). Moreover, the dysregulated release of fatty acids from adipocytes that occurs in obesity is implicated in the etiology of obesity-related complications, including type 2 diabetes (8 -10). Thus, in addition to its role in whole body energy homeostasis, the regulation of adipocyte lipolysis is vital to metabolic health. Catecholamines stimulate lipolysis by binding to ␤-adrenergic receptors on adipocytes, resulting in up-regulation of adenyl cyclase and activation of PKA (11). PKA activity is thought to increase lipolysis simultaneously by phosphorylating two key substrates, 1) the LD-associated protein, Peri A (12), and 2) HSL, the major lipase in adipocytes with significant hydrolytic action against triacylglyceride (TAG) and diacylglyceride (13-15). HSL phosphorylation by PKA increases HSL catalytic activity, although this increase (2-3-fold) cannot account for the up to a 100-fold increase in adipocyte lipolysis observed after PKA stimulation by catecholamines (15-17). More importantly, PKA-dependent phosphorylation of HSL also promotes HSL translocation from the cytosol to the LD, and the resulting interaction between catalytically active HSL and neutral lipid stores at the LD surface is thought to account for the preponderance of catecholamine-stimulated (versus basal) lipolysis (16, 18) . However, the molecular mechanisms by which HSL accumulates at the LD remain unelucidated. Recent studies suggest that HSL translocation requires PKA-dependent phosphorylation of Peri A (19). Peri A (the predominant perilipin isoform in adipocytes) is the most prevalent PKA substrate in adipocytes and is a key regulator of both basal and PKA-stimulated lipolysis.