The perilipins are a family of proteins, first identified in this laboratory, that are expressed on in adipocytes and in steroidogenic cells, where they coat the surfaces of intracellular neutral lipid droplets. The hydrolysis of lipids in these cells is initiated by protein kinase A-mediated phosphorylation hormone-sensitive lipase (HSL) in adipocytes and the closely related cholesteryl esterase in steroidogenic cells. Protein kinase A also polylphosphorylates the perilipins, which combined with their tissue distribution and subcellular location strongly imply a role for these proteins in the hydrolysis of lipids by the HSL/Ease class of enzymes. By confocal immunocytochemistry, we find that acute stimulation of lipolysis in adipocytes leads to a dispersion of the perilipin from the surface of larger lipid storage droplets. Concomitantly, HSL translocates from the cytoplasm to the surface of the lipid droplets. The dispersion of perilipin and translocation of HSL are reversed upon removal of the lipolytic stimulus. These data suggest that the perilipins act as a barrier to protect lipids from inappropriate hydrolysis by HSL. The appearance of perilipin in differentiating adipocytes is closely linked temporally to the deposition of lipid, and previously we found that fatty acid synthesis was required for perilipin expression. We have studied the effects of lipid loading on perilipins in steroidogenic Y-1 adrenal cortical cells, which are more suitable for examining the effect of exogenous lipids than adipocytes. First, exogenous fatty acids increase the expression of the rnRNA for the A but not the C isoform of perilipin, suggesting that perilipin A may be important in the early stages of lipid accretion. Second, increased lipid depositions upon lipid loading of cells dramatically increases perilipins by stabilizing the proteins. Thus, lipids regulate the content of perilipin by two modes. The cytokine, interleukin-6 (IL-6) has been implicated in several types of disease-associated cachexia. Previously, we found that IL-6 increases the tyrosine phosphorylation MAP kinases, the transcription factor, STAT3, plus several other proteins in 3T3-L1 adipocytes. We have identified two of these proteins as JAK-1 and JAK-2, members of the Janus kinase family that phosphorylate the STATs; this cytokine receptor/JAK/STAT scheme is bought to constitute a """"""""short circuit"""""""" for cytokine activation of gene transcription. However, the serine/threonine kinase inhibitor H-7 inhibits IL-6 induced tyrosine phosphorylation of the JAKs without affecting STAT3 tyrosine phosphorylation. Moreover, H-7 inhibited the tyrosine phosphorylation of nearly all other proteins by IL-6. These data point to an upstream serine/threonine kinase early in IL-6 signalling, and raise questions about the role of the JAKs in tyrosine phosphorylation of STATs in adipocytes.
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