Battle of the kinases:: Integration of adrenal responses to cAMP, DG and Ca2+ at the level of steroidogenic cytochromes P450 and 3βHSD expression in H295R cells.

While ACTH receptors (activating the protein kinase A pathway) are expressed throughout the human/bovine/ovine zona glomerulosa (zg) and zona fasciculata (zf), there are clear zonal differences in AII Type-1 receptor levels (activating protein kinase C/Ca2+), as well as resting membrane potential. Thus zg is most responsive to AII and K+ (Ca2+ signalling), while zf is less responsive to Ail with no K+ response. Zonal function in turn requires differential expression of CYP17/3 beta HSD and CYP11B2/CYP11B1. We have used the H295R cell to study how differential activation of kinase A, kinase C and Ca2+/calmodulin (CaM) kinases may alter the relative expression of the steroidogenic P450s and 3 beta HSDII. While CYP11A, CYP17, 3 beta HSDII, CYP21, and CYP11B1 are all induced by increases in cAMP, studies with TPA alone or in combination with forskolin reveal subsets of steroidogenic enzymes regulated either positively (CYP21, 3 beta HSDII) or negatively (CYP17, CYP11A) by protein kinase C. Thus adrenal 3 beta HSDII and CYP21 expression is high in zg and zf, but CYP17 is not expressed in the zg where AII activation of kinase C is highest. In turn both K+ and AII-induced elevation of Ca2+ strongly induces CYP11B2 but not CYP11B1, consistent with preferential expression of CYP11B2 in the zg. We conclude that differential signaling through kinase C and CaM kinases in addition to kinase A underlies zonal differences in both the early and late pathways involved in steroid hormone production within the adrenocortical zones.