Protein Phosphorylation Changes Ligand-binding Efficiency of Cytochrome P450c17 (CYP17) and Accelerates Its Proteolytic Degradation: Putative Relevance for Hormonal Regulation of CYP17 Activity

Two novel mechanisms of protein kinase function in the complex gonadotropic regulation of the bifunctional cytochrome P450c17 (CYP17), the rate-limiting enzyme of androgen synthesis within the smooth endoplasmic reticulum of gonadal endocrine cells, are reported. In microsomal membranes from rat testes, the maximal type I optical difference spectrum induced by the physiological CYP17 substrate, progesterone, as a measure of spin state transition due to hydrophobic ligand-protein interaction is enhanced by 24% within 15 minutes in the presence of MgATP; the dissociation constant decreases from 71 to 43 nM. Testicular cytosol does not modify this effect which is completely abolished by the protein kinase inhibitor, bisindolylmaleimide, and which does not occur with ketoconazole as ligand. Furthermore, CYP17 degradation by cytosolic protease(s) is 2.5-fold accelerated by ATP; this action is completely reversed by the protein kinase inhibitors bisindolylmaleimide (half-maximal protective concentration 2.04 microM) and KT5720 (99 nM). The former compound also prevents human choriogonadotropin-induced testicular CYP17 inactivation in situ. It is concluded that protein kinase A-catalyzed target phosphorylation integrates the known biphasic steroidogenic response upon hormonal stimulation by initial improvement of substrate accommodation followed by counter-regulatory promotion of CYP17 proteolysis.