Perilipin 5: putting the brakes on lipolysis

Lipid droplets serve a vital function in many types of cells. In most cells of the body, cholesterol esters stored in lipid droplets are thought to serve as a source of cholesterol for membrane synthesis and repair when dietary cholesterol is scarce (1). In steroidogenic cells, cholesterol derived from the hydrolysis of stored cholesterol esters provides substrate for steroid hormone synthesis. In other cells, the cores of lipid droplets store primarily triacylglycerols that, following lipolysis, provide a source of fatty acids to drive ATP production; fatty acids and partially hydrolyzed neutral lipids also support the synthesis of phospholipids for membranes. In heart and skeletal muscle, fatty acids are a major energy substrate and triacylglycerols stored in lipid droplets are an important source of fatty acids during fasting. Stored neutral lipids are hydrolyzed by cytosolic neutral lipid lipases. Lipolysis has been best characterized in adipocytes, where the release of fatty acids from triacylglycerols is orchestrated by the coordinated activity of three lipases and several accessory factors (2). Adipose triglyceride lipase (ATGL) is the major triacylglycerol hydrolase; its activity is modulated by a coactivator, CGI-58, and an inhibitory protein, G0S2. The hydrolysis of triacylglycerol releases a fatty acid and diacylglycerol, which is further cleaved by hormone-sensitive lipase (HSL). The final fatty acid is released by monoglyceride lipase. The same trio of lipases can be detected in myocytes of heart and skeletal muscle; however, the identity of relevant lipases remains unclear for many tissues where neutral lipids are stored on a relatively small scale. The hydrolysis of stored neutral lipids is finely tuned by the interaction of lipid droplet-associated proteins with these cytosolic lipases. In chordates, some of the most abundant lipid droplet-associated proteins are perilipins. Perilipins are encoded by 5 genes, and are numbered according to the order of discovery (3). Lipid droplets in most tissues are coated by two or more members of the perilipin family of proteins. Perilipins 2 (formerly adipophilin or ADRP) and 3 (formerly TIP47) are ubiquitously expressed and hence, components of lipid droplets in most tissues (1). Perilipin 1 expression is restricted primarily to adipocytes of white and brown adipose tissue and to a lesser extent, steroidogenic cells of adrenal cortex, testes, and ovaries. Perilipin 4 (formerly S3-12) is expressed primarily in adipocytes of white adipose tissue. Perilipin 5 (formerly OXPAT, MLDP, or LSDP5) is expressed in myocytes of heart and skeletal muscle, and adipocytes of brown adipose tissue; each of these cells relies upon lipolysis to provide fatty acids to mitochondria for β-oxidation to drive the production of either ATP or heat. The perilipin composition of lipid droplets within a tissue is an important component of the regulation of lipolysis. In the current issue of the Journal of Lipid Research, two articles written by Wang et al. (4) and Pollak et al. (5) provide important insight into the control of lipolysis by perilipin 5 in cardiac myocytes. In each study, perilipin 5 was overexpressed in cardiac myocytes of mice, using the α-myosin heavy chain promoter. Increased perilipin 5 expression in these transgenic mice led to storage of excessive triacylglycerol in lipid droplets and cardiac steatosis (4, 5). Pollak et al. (5) noted that the observed steatosis resembled that of mice with a genetic deletion of ATGL (6), suggesting impaired hydrolysis of triacylglycerol; however, while ATGL-null mice succumb to early death due to heart failure, the hearts of perilipin 5 transgenic mice function relatively normally and the mice have normal life spans (4, 5). Investigation of the mechanisms behind the steatosis revealed that, while ATGL-null mice have reduced cytosolic lipase activity, lysates of cardiac tissue from perlipin 5 transgenic mice have elevated triacylglycerol hydrolase activity with corresponding elevated protein levels of ATGL and its coactivator CGI-58 (5). Further investigation revealed that lipid droplets isolated from Cos7 cells expressing perilipin 5 are less susceptible to the activity of either ATGL or HSL than lipid droplets from control cells (coated with perilipin 2), suggesting that perilipin 5 provides effective barrier function to reduce lipase access to lipid droplets (5). These studies are consistent with several previous studies characterizing reduced lipolysis in cultured cells in which perilipin 5 has been overexpressed (7–9). Conversely, the phenotype of perilipin 5-null mice reveals decreased triacylglycerol storage in cardiac myocytes accompanied by increased oxidation of fatty acids and increased oxidative stress (10), suggesting increased lipolysis. All of these studies support the idea that perilipin 5 functions similarly to perilipin 1 in reducing lipolysis. Thus, perilipin 5 may keep lipolysis in check to sequester potentially lipotoxic fatty acids and reduce β-oxidation. Stored triacylglycerols provide fuel for cardiac myocyte function, so a major remaining question is, “how does perilipin 5 permit or facilitate lipolysis in times of need?” One possible mechanism is through control of CGI-58 availability to activate ATGL. Previous studies have shown that perilipin 5 binds both ATGL and CGI-58 via carboxyl terminal binding sites (9, 11) while inhibiting lipolysis (9); however, both proteins cannot bind to the same molecule of perilipin 5 at the same time (11). This protein binding may serve to sequester CGI-58, preventing interaction with and activation of ATGL. Moreover, incubation of cells expressing ectopic perilipin 5 with forskolin to activate adenylyl cyclase and consequently protein kinase A increases lipolysis (9). Phosphorylation of perilipin 5 increases following the addition of forskolin to cells (9); however, the functional consequence of perilipin 5 phosphorylation is as yet unknown. It is tempting to speculate that phosphorylation of perilipin 5 may be part of the mechanism by which lipolysis is increased, perhaps by triggering the release of CGI-58 and ATGL from their binding sites on perilipin 5, in turn, facilitating interaction of the released proteins. The current work extends previous observations that perilipin 5 links lipid droplets to mitochondria. Histology of cardiac muscle from perilipin 5 transgenic mice reveals that perilipin 5-coated lipid droplets are closely surrounded by mitochondria (4, 5). Previous studies have revealed that a highly conserved carboxyl terminal sequence of perilipin 5 is required for recruitment of mitochondria to lipid droplets (12). The physical nature of the linking mechanism is as yet unknown, as are the consequences of the linkage. Importantly, Wang et al. (4) reveal that perilipin 5 overexpression impairs mitochondrial function, reducing the function of mitochondrial enzymes, and altering mitochondrial respiration, despite increases in mitochondrial size. The functional significance of the link between perilipin 5, lipid droplets, and mitochondria requires further study. Finally, both recent studies reveal that changes in gene expression accompany perilipin 5 overexpression in cardiac muscle (4, 5). The expression of peroxisome proliferator-activated receptor (PPAR)α, PPARγ coactivator 1α PGC1α, and PGC1β were decreased, as were mRNA levels for downstream target genes, including components of mitochondrial fatty acid import and oxidation. Prior studies have revealed that lipolysis of stored triacylglycerols provides ligands for PPARα, activating gene expression (13, 14). When lipolysis is decreased by reducing ATGL (13) or increasing perilipin 5 (4, 5), PPAR-mediated gene expression is impaired. Changes in the expression of genes encoding mitochondrial proteins are likely responsible for the observed diminished mitochondrial function in perilipin 5 transgenic mice (4); similar changes were observed in the hearts of ATGL-null mice and could be reversed by treatment of animals with a PPARα agonist (13). Interestingly, genes encoding proteins that mitigate oxidative stress responses were upregulated despite decreased β-oxidation (4). Protein levels of NF-E2-related factor 2, a transcription factor that induces antioxidant mechanisms in response to stress, were elevated in cardiac tissue of perilipin 5 transgenic mice, as were mRNA levels of downstream target genes. Thus, despite decreased lipolysis and flux of fatty acids to mitochondria, the cells show signs of increased oxidative stress. Further study is needed to elucidate these mechanisms. In summary, these studies have added to a growing body of evidence that perilipin 5 is a negative regulator of lipolysis in cardiac myocytes. By putting the brakes on lipolysis, perilipin 5 is an important component of the mechanisms controlling energy homeostasis.