Differential agonist-induced signal transduction cascades and their correlation with MARCKS phosphorylation, StAR phosphorylation, StAR protein synthesis, and aldosterone secretion in cultured bovine adrenal glomerulosa cells

Abstract

Aldosterone is a steroid hormone secreted by the cells of the _zona glomerulosa of the adrenal gland in response to increases in serum potassium (K+) concentrations, angiotensin II (Angll), and adrenocorticotropic hormone (ACTH). Although all of these agonists stimulate Ca2+ entry, which is required but not sufficient for aldosterone secretion, they generate other intracellular signals that are unique to each agent. In the first part of this study we addressed the possible involvement of Protein Kinase C (PKC) in the actions of these agonists, as measured by the phosphorylation of a specific endogenous ~ substrate of PKC: the myristoylated alanine-rich C-kinase substrate (MARCKS). Both Angll and K+ induced an increase in, MARCKS phosphorylation, while ACTH inhibited this response. We conclude that PKC activation is involved in aldosterone secretion stimulated by either Angll or K+, but not by ACTH .. Although these three agonists act via different signaling pathways, it seems likely that at some point, the transducing events should converge. The _ transfer of cholesterol from the outer mitochondrial membrane to the inner mitochondrial membrane - is the limiting step in steroidogenesis. The steroidogenic· g_cute .regulatory (StAR) protein is thought to be a principal mediator of this transfer, with its acute synthesis and phosphorylation thought to be required for steroid production. Thus, StAR activation should be common to the actions of all three agonists. The second part- of this study determined 1) the effect of these agonists on StAR protein synthesis and protein phosphorylation, and 2) how these events relate to the secretory. response. Stimulation with Angll significantly increased StAR protein synthesis and StAR protein phosphorylation whereas stimulation with K+ significantly increased StAR ' ' protein phosphorylation but did not affect StAR protein synthesis. Finally, ACTH significantly increased . both events but the increase in StAR protein phosphorylation was less than that for Angll or K+. We conclude that these agonists differentially regulate StAR protein synthesis and protein phosphorylation in cultured bovine adrenal glomerulosa cells. In addition, there is no simple correlation between these events and aldosterone production. These results suggest that StAR may not be the only factor regulating intramitochondrial cholesterol transport and steroid synthesis.