Abstract 1178: Group X Secretory Phospholipase A2 Negatively Regulates Adrenal Glucocorticoid Synthesis
Introduction: Group X secretory phospholipase A2 (GX sPLA2) has been implicated in a number of important biological processes, some of which appear to be independent of its lipolytic activity. These effects of GX sPLA2 may be in part due to its high affinity binding to the M-type sPLA2 receptor. To elucidate the physiological functions of this enzyme in vivo, we recently developed C57BL/6 mice with targeted deletion of GX sPLA2 (GX KO mice).
Results: Unexpectedly, GX KO mice have ~50% higher plasma corticosterone level compared to wild-type (WT) mice. Real-time RT-PCR and immunoblot analyses showed that expression of Steroidogenic Acute Regulatory Protein (StAR), a rate limiting protein in steroid production, is 2–5-fold higher in adrenals of GX KO mice compared to WT mice. GX KO mice responded normally to dexamethasone, indicating no impairment of the hypothalamic-pituitary axis. To provide additional evidence for a direct effect of Group X sPLA2 on adrenal corticosteroid production, we performed studies in mouse adrenocortical tumor (Y-1) cells, which endogenously express GX sPLA2 and the M-type receptor. Over-expression of GX sPLA2, or incubations with GX sPLA2, resulted in a significant reduction in progesterone production and StAR expression in Y-1 cells. Indoxam, an inhibitor of GX sPLA2 activity and potent blocker of sPLA2 binding to the M-type receptor, resulted in a significant increase in progesterone production and StAR expression in both control and GX sPLA2-overexpressing cells. Because several studies have implicated the extracellular signal-regulated kinase (ERK) cascade in the regulation of steroidogenesis, we investigated whether GX sPLA2 altered adrenal ERK1/2 activation. Over-expression of GX sPLA2 in Y-1 cells resulted in significantly increased ERK1/2 phosphorylation, whereas deficiency of GX sPLA2 was accompanied by a significant decrease in phosphorylated ERK1/2 in mouse adrenals.
Conclusions: We provide the novel finding that GX sPLA2 has a direct effect on adrenal cells to regulate glucocorticoid production. Our in vivo and in vitro data indicate that Group X sPLA2 down-regulates the expression of StAR, possibly through M-type receptor-mediated activation of ERK1/2.