CA2+-REGULATED EXPRESSION OF STEROID HYDROXYLASES IN H295R HUMAN ADRENOCORTICAL-CELLS

Although changes in the expression of key steroidogenic enzymes such as cytochrome P450 cholesterol side-chain cleavage, 17 alpha-hydroxylase (P450c17), aldosterone synthase, and 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) in the human adrenal cortex are known to be controlled by factors activating the protein kinase A or protein kinase C signaling pathways, little is known concerning the effects of increased intracellular Ca2+. In this study we describe the effects of K+, an agent known to increase intracellular Ca2+ through the opening of voltage-sensitive Ca2+ channels, on steroidogenesis in H295R human adrenocortical cells and corresponding changes in expression of these vital steroidogenic enzymes. Treatment of cells for 48 h with K+ (14 mM) resulted in an increase in aldosterone (3.5-fold) as well as the 17 alpha-hydroxylated steroids cortisol (2.9-fold) and dehydroepiandrosterone (DHEA; 3.7-fold). This action of K+ was accompanied by a dose-dependent (P < 0.05 at 6 mM K+ or above) and time-dependent (P < 0.05 at 24 h and beyond) increase in expression of P450c17 and, to a lesser extent, cytochrome P450 cholesterol side-chain cleavage messenger RNA (mRNA). Treatment with K+ also caused a time-dependent increase in aldosterone synthase mRNA levels, which were detectable by 12 h. Treatment with K+, however, was without effect on 3 beta HSD expression. These effects contrast with those of (Bu)(2)cAMP, which stimulated a greater increase in cortisol and DHEA secretion as well as P450c17 expression. The effects of K+ treatment also differ from those of AII, which promoted a greater aldosterone secretory response (5,7-fold), but a lesser effect on DHEA secretion (2.2-fold) and P450c17 expression. Although AII and TPA (known activators of protein kinase C) as well as forskolin and (Bu)(2)cAMP (known activators of protein kinase A) increased the expression of 3 beta HSD mRNA, K+ treatment was without effect, suggesting that elevation of [Ca2+](i) in response to K+ did not activate the protein kinase C or protein kinase A signaling pathways. Furthermore, the effects of K+ on steroid secretion and 17 alpha-hydroxylase activity were reproduced by the voltage-sensitive Ca2+ channel activator BAYK 8644, and increases in P450c17 mRNA in response to K+ were reversed by the Ca2+ channel antagonist, nifedipine. We conclude that K+ can modulate the expression of key steroidogenic enzymes in H295R cells through the Ca2+ signaling pathway without involvement of the protein kinase A or protein kinase C pathways. These findings reveal a further level of complexity in the control of adrenocortical expression of steroid-metabolizing enzymes.