Ectopically-Expressed Pro-Group X Secretory Phospholipase A2 is Proteolytically Activated in Mouse Adrenal Cells by Furin-like Proprotein Convertases: Implications for the Regulation of Adrenal Steroidogenesis [Lipids]

January 26th, 2015 by Layne, J. D., Shridas, P., Webb, N. R.

Group X secretory phospholipase A2 (GX sPLA2) hydrolyzes mammalian cell membranes, liberating free fatty acids and lysophospholipids. GX sPLA2 is produced as a proenzyme (pro-GX sPLA2) that contains an N-terminal 11 amino acid propeptide ending in a dibasic motif, suggesting cleavage by a furin-like proprotein convertase (PC). While propeptide cleavage is clearly required for enzymatic activity, the protease(s) responsible for pro-GX sPLA2 activation have not been identified. We previously reported that GX sPLA2 negatively regulates adrenal glucocorticoid production, likely by suppressing liver X receptor (LXR)-mediated activation of steroidogenic acute regulatory protein (StAR) expression. In this study, using a FLAG epitope-tagged pro-GX sPLA2 expression construct (FLAG-pro-GX sPLA2) we determined that adrenocorticotropic hormone (ACTH) enhances FLAG-pro-GX sPLA2 processing and phospholipase activity secreted by Y1 adrenal cells. ACTH increased the expression of furin and PCSK6, but not other members of the PC family, in Y1 cells. Overexpression of furin and PCSK6 in HEK 293 cells significantly enhanced FLAG-pro-GX sPLA2 processing, whereas siRNA-mediated knockdown of both PCs almost completely abolished FLAG-pro-GX sPLA2 processing in Y1 cells. Expression of either furin or PCSK6 enhanced the ability of GX sPLA2 to suppress LXR reporter activity. The PC inhibitor Dec-RVKR-cmk significantly suppressed FLAG-pro-GX sPLA2 processing and sPLA2 activity in Y1 cells, and significantly attenuated GX sPLA2-dependent inhibition of StAR expression and progesterone production. These findings provide strong evidence that pro-GX sPLA2 is a substrate for furin and PCSK6 proteolytic processing, and define a novel mechanism for regulating corticosteroid production in adrenal cells.
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