Differential effects of agonists of aldosterone secretion on steroidogenic acute regulatory phosphorylation

The steroidogenic acute regulatory (StAR) protein mediates cholesterol transport within the mitochondria, and its phosphorylation is believed to be required for steroidogenesis. Increased extracellular potassium concentrations (K⁺), angiotensin II (AngII), and adrenocorticotropic hormone (ACTH) induce aldosterone secretion from bovine adrenal glomerulosa cells. We hypothesized that, although these agonists act via different signaling pathways, StAR phosphorylation should be common to their action. We studied the effects of K⁺, AngII, and ACTH, at concentrations that yield comparable secretory responses, on StAR phosphorylation. All three agents induced significant increases in StAR phosphorylation although the response to ACTH was less than that of AngII and K⁺. In cells stimulated with the protein kinase C (PKC) agonist 12-tetradecanoylphorbol 13-acetate (TPA), the Ca²⁺ channel agonist BAY K8644, and the adenylate cyclase agonist forskolin, TPA caused a small but statistically significant increase in StAR phosphorylation while BAY K8644 and forskolin had no significant effect. Interestingly, the combination of TPA and BAY K8644 produced a larger increase in StAR phosphorylation than the agents alone. We conclude that in cultured bovine adrenal glomerulosa cells the PKC signaling pathway is most effective at inducing StAR phosphorylation but that there is no simple correlation between this event and aldosterone production.