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Dynamics of Lipid Droplet-Associated Proteins during Hormonally Stimulated Lipolysis in Engineered Adipocytes: Stabilization and Lipid Droplet Binding of Adipocyte Differentiation-Related Protein/Adipophilin

Department of Biochemistry (D.N.G., T.H., E.C.P., P.F.P.), Boston University School of Medicine, Boston, Massachusetts 02118; and Jean Mayer-U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University (H.M., H.-H.Z., S.S., M.O., A.S.G.), Boston, Massachusetts 02111

Address all correspondence and requests for reprints to: Paul F. Pilch, Boston University School of Medicine, Department of Biochemistry 715 Albany Street, Boston, Massachusetts 02118. E-mail: ppilch{at}bu.edu.

In mature adipocytes, triglyceride is stored within lipid droplets, which are coated with the protein perilipin, which functions to regulate lipolysis by controlling lipase access to the droplet in a hormone-regulatable fashion. Adipocyte differentiation-related protein (ADRP) is a widely expressed lipid droplet binding protein that is coexpressed with perilipin in differentiating fat cells but is minimally present in fully differentiated cultured adipocytes. We find that fibroblasts ectopically expressing C/EBP (NIH-C/EBP cells) differentiate into mature adipocytes that simultaneously express perilipin and ADRP. In response to isoproterenol, perilipin is hyperphosphorylated, lipolysis is enhanced, and subsequently, ADRP expression increases coincident with it surrounding intracellular lipid droplets. In the absence of lipolytic stimulation, inhibition of proteasomal activity with MG-132 increased ADRP levels to those of cells treated with 10 µM isoproterenol, but ADRP does not surround the lipid droplet in the absence of lipolytic stimulation. We overexpressed a perilipin A construct in NIH-C/EBP cells where the six serine residues known to be phosphorylated by protein kinase A were changed to alanine (Peri A 1–6). These cells show no increase in ADRP expression in response to isoproterenol. We propose that ADRP can replace perilipin on existing lipid droplets or those newly formed as a result of fatty acid reesterification, under dynamic conditions of hormonally stimulated lipolysis, thus preserving lipid droplet morphology/structure.

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