Group X Secretory Phospholipase A2 Regulates the Expression of Steroidogenic Acute Regulatory Protein (StAR) in Mouse Adrenal Glands

Boris B. Boyanovsky,Michael H. Gelb,Rob C. Oslund,Preetha Shridas,William M. Bailey,Nancy R. Webb

doi:10.1074/jbc.m109.090423

Boris B. Boyanovsky, Michael H. Gelb + Show 4 more

Open Access

https://doi.org/10.1074/jbc.m109.090423

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Abstract

We developed C57BL/6 mice with targeted deletion of group X secretory phospholipase A(2) (GX KO). These mice have approximately 80% higher plasma corticosterone concentrations compared with wild-type (WT) mice under both basal and adrenocorticotropic hormone (ACTH)-induced stress conditions. This increased corticosterone level was not associated with increased circulating ACTH or a defect in the hypothalamic-pituitary axis as evidenced by a normal response to dexamethasone challenge. Primary cultures of adrenal cells from GX KO mice exhibited significantly increased corticosteroid secretion compared with WT cells. Conversely, overexpression of GX secretory phospholipase A(2) (sPLA(2)), but not a catalytically inactive mutant form of GX sPLA(2), significantly reduced steroid production 30-40% in Y1 mouse adrenal cell line. This effect was reversed by the sPLA(2) inhibitor, indoxam. Silencing of endogenous M-type receptor expression did not restore steroid production in GX sPLA(2)-overexpressing Y1 cells, ruling out a role for this sPLA(2) receptor in this regulatory process. Expression of steroidogenic acute regulatory protein (StAR), the rate-limiting protein in corticosteroid production, was approximately 2-fold higher in adrenal glands of GX KO mice compared with WT mice, whereas StAR expression was suppressed in Y1 cells overexpressing GX sPLA(2). Results from StAR-promoter luciferase reporter gene assays indicated that GX sPLA(2) antagonizes StAR promoter activity and liver X receptor-mediated StAR promoter activation. In summary, GX sPLA(2) is expressed in mouse adrenal glands and functions to negatively regulate corticosteroid synthesis, most likely by negatively regulating StAR expression.

Highlights

The secretory phospholipase A22 family represents a group of structurally related calcium-dependent enzymes that hydrolyze glycerophospholipids at the sn-2 position to liberate lysophospholipids and free fatty acids
GX secretory phospholipase A2 (sPLA2) Deficiency Leads to Hypercorticosteronemia—To elucidate the physiological function of GX sPLA2 in vivo, we recently developed C57BL/6 mice with targeted deletion of the GX sPLA2 gene (GX KO mice)
A structural analogue of indoxam devoid of inhibitory properties failed to alter the amount of phospholipase activity or progesterone secreted by either Y1 or Y1-GX cells. These results provide evidence that GX sPLA2 enzymatic activity plays a role in the regulation of adrenal steroidogenesis. As another approach to investigate whether GX sPLA2 negatively regulates steroidogenesis through a mechanism that requires phospholipid hydrolysis, we developed Y1 cells stably expressing an active site mutant of GX sPLA2 whereby a histidine residue in the active site was replaced by glutamine

Summary

Introduction

The secretory phospholipase A2 (sPLA2)2 family represents a group of structurally related calcium-dependent enzymes that hydrolyze glycerophospholipids at the sn-2 position to liberate lysophospholipids and free fatty acids. Our data indicate that GX sPLA2 negatively regulates corticosterone production by altering the expression of steroidogenic acute regulatory protein (StAR), likely by suppressing LXR-mediated StAR promoter activation.

Results

Conclusion