Background: FSP27 depletion increases both basal and stimulated lipolysis. Results: FSP27 interacts with ATGL via amino acids 120 -220 to regulate lipolysis and triglyceride storage in human adipocytes. Conclusion: FSP27 inhibits ATGL-mediated lipolysis and protects adipocytes against free fatty acid-impaired insulin signaling. Significance: The novel lipolytic regulation shown here may lead to new treatments for insulin resistance.

Background: FSP27 is a lipid droplet-associated protein.Results: Expression of FSP27 in human adipocytes reversely correlates with ATGL levels. Mechanistically, FSP27 increases the inhibitory effect of Egr1 on the ATGL promoter. Conclusion: FSP27 controls lipolysis by regulating ATGL transcription. Significance: Our study provides a new model of regulation of lipolysis in adipocytes. Current epidemics of metabolic diseases, such as type 2 diabetes, cardiac dysfunction, hypertension, hepatic steatosis, etc., are largely caused by widespread obesity. Although obesity can affect human health via several different mechanisms (1), the best established connection between obesity and metabolic disease is abnormal levels of circulating fatty acids (FA). 3 FA play important physiological roles in energy production and the synthesis of most lipids; nonetheless, their oversupply is highly detrimental as it leads to insulin resistance, oxidative stress, and other pathophysiological effects via mechanisms that are currently under intense investigation (1-5). Lipolysis in fat tissueNormally, dietary FA are partitioned into adipose tissue, converted into triglycerides, and stored in lipid droplets (LDs) that represent dynamic intracellular organelles consisting of a core of triglycerides and cholesterol esters, surrounded by a monolayer of phospholipids. Several proteins are associated with this monolayer, notably the PAT family proteins, PLIN 1-5 (6, 7), and fat-specific protein 27 (FSP27, also known as CIDEC) (8 -10). The latter protein plays an essential role in the regulation of LD morphology. Depletion of FSP27 in adipocytes leads to fragmentation of LDs (11,12), whereas overexpression of FSP27 increases the size of LDs while decreasing their number (8, 9, 12) by promoting LD fusion (13) and exchanging lipids from one droplet to another (14).It has also been demonstrated by us and others that FSP27 has anti-lipolytic activity (8,9,12,(15)(16)(17)(18). Lipolysis in adipose tissue is the major source of circulating . Correspondingly, unrestricted lipolysis in adipose tissue represents a serious metabolic defect and a causative factor of insulin resistance, diabetes mellitus, and other metabolic diseases (3,(25)(26)(27). As the mechanism of the anti-lipolytic activity of FSP27 is not completely clear, we decided to focus on this problem.Our recent study showed that FSP27 directly interacts with ATGL and regulates its lipase activity (18). Another study predicted that FSP27-mediated fusion of LDs might limit the access of intracellular lipases to the LD surface due to decreased surface area of larger LDs (17). As ATGL represents a major lipolytic enzyme (28), reducing its contact with LDs may suppress lipolysis.Here, we report another mechanism of the anti-lipolytic action of FSP27 in human adipocytes. We have found that FSP27 inhibits expression of ATGL at the level of transcription by stimulating the effect of its transcriptional repressor Egr1. EXPERIMENTAL PROCEDURESCell Culture-Human preadipocytes were procured from the ...

In adipocytes, lipolysis is a highly regulated process involving hormonal signals, lipid droplet‐associated proteins and lipases. The discovery of new lipid droplet‐associated proteins added complexity to the current model of lipolysis. In the present study, we used cultured human adipocytes to demonstrate that Fat Specific Protein 27 (FSP27), an abundantly expressed protein in adipocytes, regulates both basal and stimulated lipolysis by interacting with adipose triglyceride lipase (ATGL). We identified a core domain of FSP27, aa 120‐220, that interacts with ATGL to inhibit its lipolytic function and promote triglyceride storage. We also defined the role of FSP27 in free fatty acid‐induced insulin resistance in adipocytes. FSP27 depletion in human adipocytes increased lipolysis and inhibited insulin signaling by decreasing AKT phosphorylation. However, reducing lipolysis by either depletion of ATGL or expression of exogenous full length FSP27 or aa 120‐220 protected human adipocytes against the adverse effects of free fatty acids on insulin signaling. In embryonic fibroblasts derived from ATGL‐KO mice, exogenous free fatty acids did not affect insulin sensitivity. Our results demonstrate a crucial role for FSP27‐ATGL interactions in regulating lipolysis, TG accumulation, and insulin signaling in human adipocytes. Grant Funding Source: Supported by NIH‐NIDDK R56DK094815, NIH‐NIDDK P30DK046200, KL‐2 (8KL2TR000158)