Effects of Cellular Mediator Agonists on Cortisol and Steroid Acute Regulatory (StAR) Protein in Bovine Zona Fasciculata (ZF) Cells

Abstract

Primary cultures of bovine ZF cells were incubated for 1h or 6h with the agonists 8-bromo-cyclic AMP (8-Br-cAMP) an activator of protein kinase A (PKA), the phorbol ester phorbol 12-myristate 13-acetate (PMA) a protein kinase C (PKC) activator, the Ca²⁺ ionophore, A23187, or the L-type Ca²⁺ channel agonist Bay K8644. Both cortisol secretion (determined by radioimmunoassay of cell medium) and cellular StAR protein levels (quantified by western blotting) were significantly increased at 6h, by all agonists. However, while all agonists stimulated cortisol secretion at 1 h, StAR protein levels remained unchanged by these treatments. We conclude that in bovine ZF cells, StAR protein synthesis can be regulated by mechanisms involving activation of PKA, PKC and Ca²⁺ influx. However, a net increase in cellular StAR protein does not appear to be essential for the initiation for the first stage of acute steroidogenesis.