Perilipins, the major structural proteins coating the surfaces of mature lipid droplets of adipocytes, play an important role in the regulation of triacylglycerol storage and hydrolysis. We have used proteomic analysis to identify CGI-58, a member of the ␣/␤-hydrolase fold family of enzymes, as a component of lipid droplets of 3T3-L1 adipocytes. CGI-58 mRNA is highly expressed in adipose tissue and testes, tissues that also express perilipins, and at lower levels in liver, skin, kidney, and heart. Both endogenous CGI-58 and an ectopic CGI-58-GFP chimera show diffuse cytoplasmic localization in 3T3-L1 preadipocytes, but localize almost exclusively to the surfaces of lipid droplets in differentiated 3T3-L1 adipocytes. The localization of endogenous CGI-58 was investigated in 3T3-L1 cells stably expressing mutated forms of perilipin using microscopy. CGI-58 binds to lipid droplets coated with perilipin A or mutated forms of perilipin with an intact C-terminal sequence from amino acid 382 to 429, but not to lipid droplets coated with perilipin B or mutated perilipin A lacking this sequence. Immunoprecipitation studies confirmed these findings, but also showed co-precipitation of perilipin B and CGI-58. Remarkably, activation of cAMP-dependent protein kinase by the incubation of 3T3-L1 adipocytes with isoproterenol and isobutylmethylxanthine disperses CGI-58 from the surfaces of lipid droplets to a cytoplasmic distribution. This shift in subcellular localization can be reversed by the addition of propanolol to the culture medium. Thus, CGI-58 binds to perilipin Acoated lipid droplets in a manner that is dependent upon the metabolic status of the adipocyte and the activity of cAMP-dependent protein kinase.Lipid droplets, organelles that store neutral lipids, are found in nearly all types of eukaryotic cells. The most highly studied lipid droplet-associated proteins are members of the PAT 1 (for perilipin, adipophilin (also called adipose differentiation-related protein or ADRP), and TIP47) family of proteins (1) that includes perilipin, adipophilin, TIP47, and a related but structurally divergent protein, S3-12 (2, 3). Three protein isoforms of perilipins, named perilipins A, B, and C, are translated from alternatively spliced forms of mRNA. Perilipin A and S3-12 localize to lipid droplets in adipocytes (3-5), where they are the most abundant structural proteins of large mature and small nascent lipid droplets, respectively. In contrast, most other types of cells have small lipid droplets covered with adipophilin (6, 7) and TIP47 (1, 8, 9). Perilipins A and C, and adipophilin, coat the lipid droplets of steroidogenic cells (6) that store cholesterol ester precursors for steroid hormone synthesis.The members of the PAT family comprise the major structural proteins of lipid droplets. Studies in perilipin knockout mice have established an important role for perilipins in regulating lipolysis in adipocytes, and hence, controlling the mass of triacylglycerol deposited in adipose tissue (10, 11). The mechanisms by which per...

Perilipin A is a key regulator of triacylglycerol storage and hydrolysis in adipocytes; phosphorylation of perilipin A by protein kinase A facilitates maximal lipolysis. Chronic stimulation of lipolysis in 3T3-L1 adipocytes causes large perinuclear lipid droplets to fragment into myriad dispersed perilipin A-covered microlipid droplets. In cultured fibroblasts stably expressing ectopic perilipin A, clustered lipid droplets disperse throughout the cytoplasm upon incubation of the cells with forskolin and isobutylmethylxanthine (IBMX) to elevate levels of cAMP and activate protein kinase A, mirroring events observed in adipocytes. Furthermore, diethylumbelliferyl phosphate inhibits stimulated lipolysis but not the dispersion of lipid droplets, suggesting that products of lipolysis are not required for this remodeling process. We hypothesized that protein kinase A-mediated phosphorylation of perilipin A triggers the remodeling of lipid droplets. The mutation of serine 492 of perilipin A to alanine prevented the dispersion of clustered lipid droplets in fibroblasts stably expressing the mutated perilipin upon incubation with forskolin and IBMX. In contrast, the substitution of serines 81, 222, 276, or 433 with alanine, either singly or in combinations, did not affect the protein kinase A-mediated remodeling of lipid droplets. Interestingly, substitution of serines 433, 492, and 517 of perilipin A with glutamic acid residues blocked the dispersion of clustered lipid droplets in cells incubated with forskolin and IBMX, indicating that the addition of a negative charge does not mimic a phosphate group. We conclude that protein kinase A-mediated phosphorylation of serine 492 of perilipin A drives the fragmentation and dispersion of lipid droplets.Adipose tissue contains the largest energy reserve in the body, stored as triacylglycerol in the intracellular lipid droplets of adipocytes. Triacylglycerol stores are hydrolyzed to mobilize energy during fasting and extended exercise. Catecholamines bind to ␤-adrenergic receptors on the plasma membranes of adipocytes to initiate a G-protein-mediated signaling cascade that activates adenylyl cyclase, thus increasing intracellular cAMP levels. Protein kinase A (or cAMP-dependent protein kinase) is then activated and phosphorylates several proteins required for the hydrolysis of triacylglycerol and consequent mobilization of fatty acids. Following phosphorylation by protein kinase A, hormone-sensitive lipase translocates from the cytosol onto lipid droplets (1-5), where it hydrolyzes triacylglycerol and diacylglycerol (6 -8). Perilipin A is another major substrate for protein kinase A (9, 10).Perilipin A localizes to lipid droplets in adipocytes and plays roles in facilitating both the storage and hydrolysis of triacylglycerol. In adipocytes, two isoforms of perilipin are derived from the alternate splicing of mRNA transcribed from a single gene (10, 11); perilipin A is the predominant protein isoform, whereas perilipin B is a minor isoform. Perilipins A and B share a common sequ...

Germ-line mutations in the BRCA1 gene strongly predispose women to breast cancer (lifetime risk up to 80%). Furthermore, the BRCA1 protein is absent or present at very low levels in about one third of sporadic breast cancers. However, the mechanisms underlying BRCA1 somatic inactivation appear multiple and are still not fully understood. We report here the involvement of miR-146a and miR-146b-5p that bind to the same site in the 3′UTR of BRCA1 and down-regulate its expression as demonstrated using reporter assays. This was further confirmed with the endogenous BRCA1 gene by transfecting microRNA (miRNA) precursors or inhibitors in mammary cell lines. This down-regulation was accompanied by an increased proliferation and a reduced homologous recombination rate, two processes controlled by BRCA1. Furthermore, we showed that the highest levels of miR-146a and/or miR-146b-5p are found in basal-like mammary tumour epithelial cell lines and in triple negative breast tumours, which are the closest to tumours arising in carriers of BRCA1 mutations. This work provides further evidence for the involvement of miRNAs in sporadic breast cancer through down-regulation of BRCA1.