Submitted to: Journal of Lipid Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2010
Publication Date: 11/20/2010
Citation: Ranjit, S., Boutet, E., Gandhi, P., Prot, M., Tamori, Y., Chawla, A., Greenberg, A., Puri, V., Czech, M. 2010. Regulation of fat specific protein 27 by isoproterenol and TNF-alpha to control lipolysis in murine adipocytes. Journal of Lipid Research. 52(2):221-236. Interpretive Summary: In both animals and humans fat is stored within fat cell cells in a specific compartment within the cell called lipid droplets. Our laboratory discovered a protein called perilipin that coats the surface of lipid droplets and regulates the breakdown of stored fat. FSP27 is one of several proteins that coats the surface of lipid droplets. The expression of several hormones, called interleukin-beta, tumor necrosis factor, and interferon gamma are increased in fat within animals and humans with infection and cancer. These hormones are associated with loss of fat mass. We treated fat cells that were cultured in the laboratory with these hormones and found that the hormones decreased FSP27 expression and increased fat breakdown. We also used genetic techniques to reduce FSP27 expression and found that this also resulted in increased rates of fat breakdown. When animals or human fasted, the nerves in fat tissue release factors called catecholamines that promote fat breakdown within fat cells. When we treated fat cells in culture with catecholamines we found that the expression of FSP27 protein was increased even thought rates of fat breakdown were increased. These observations indicate that the regulation of fat breakdown within fat cells is complex and needs further study. However it is clear that if FSP27 protein expression is reduced fat breakdown occurs.
Technical Abstract: The lipid droplet-associated fat specific protein 27 (FSP27) suppresses lipolysis and thereby enhances triglyceride accumulation in adipocytes. We and others have recently found FSP27 to be a remarkably short-lived protein (half-life, 15 min) due to its rapid ubiquitination and proteasomal degradation. Thus, we tested the hypothesis that lipolytic agents such as tumor necrosis factor-a (TNF-alpha) and isoproterenol modulate FSP27 levels to regulate FFA release. Consistent with this concept, we showed that the lipolytic actions of TNF-alpha, interleukin-1beta (IL-1beta), and IFN-gamma are accompanied by marked decreases in FSP27 expression and lipid droplet size in mouse adipocytes. Similar depletion of FSP27 using short interfering RNA (siRNA) mimicked the lipolysis-enhancing effect of TNF-alpha, while maintaining stable FSP27 levels using expression of hemagglutinin epitope-tagged FSP27 blocked TNF-alpha-mediated lipolysis. In contrast, we show the robust lipolytic action of isoproterenol is paradoxically associated with increases in FSP27 levels and a delayed degradation rate corresponding to decreased ubiquitination. This catecholamine-mediated increase in FSP27 abundance, probably a feedback mechanism for restraining excessive lipolysis by catecholamines, is mimicked by forskolin or 8-bromo-cAMP treatment and is prevented by the protein kinase A (PKA) inhibitor KT5720 or by PKA depletion using siRNA. Taken together, these data identify the regulation of FSP27 as an important intermediate in the mechanism of lipolysis in adipocytes in response to TNF-alpha and isoproterenol.