Phosphoserine/threonine phosphatases in the rat adrenal cortex: a role in the control of steroidogenesis?

Abstract

The involvement of protein kinases in the signal transduction pathways controlling adrenal steroidogenesis is well established, and the phosphorylation of substrates by cAMP-dependent protein kinase is a major mechanism in ACTH action. However, the possibility that protein phosphatases (PPs) might also be involved in this process has not been investigated. The aim of this study was, therefore, to measure the function, expression and enzymic activity of PPs in zona glomerulosa (ZG) and zona fasciculata/reticularis (ZFR) tissue from the rat adrenal cortex. Immunoblot analysis using specific antisera demonstrated the presence in whole adrenals and capsules of PP type 1 (PP1) migrating with an apparent molecular mass of 37 kDa, and PP type 2A (PP2A) migrating with apparent molecular masses of 38 and 31 kDa. The PP inhibitors, okadaic acid (OA), calyculin A (CA), tautomycin and microcystin RR, caused a reduction in PP activity in vitro, at doses between 1 nM and 1 microM. In addition, treatment of ZG cells with the adenylate cyclase stimulator, forskolin (10 microM) resulted in a significant reduction in PP activity. The effects of CA and OA on steroid secretion by ZG and ZFR cells were also investigated. Neither CA nor OA had any effect on basal steroid secretion or on yields of steroid obtained from 22R-hydroxycholesterol at doses between 1 and 100 nM. However, both OA and CA (10 and 100 nM respectively) significantly reduced ACTH-stimulated aldosterone and corticosterone production by ZG and ZFR cells. CA and OA (10 and 100 nM respectively) also reduced steroid secretion by cells stimulated by forskolin (10 microM) or dibutyryl cAMP (200 microM). These results suggest that PPs may be involved in the intracellular mechanisms through which adrenocortical steroidogenesis is regulated, acting at a point after cAMP generation and action, but proximal to the side-chain cleavage of cholesterol.