Metabolic Regulation of Steroidogenesis in Isolated Adrenal Cells of Rat

Abstract

ACTH in the physiological concentration range stimulates the synthesis of cyclic[c]GMP, protein kinase activity and steroidogenesis in a concentration-dependent manner without detectable rise in the levels of cAMP. Millimolar concentrations of cAMP and cGMP, which stimulate corticosterone synthesis, also activate kinase activity and steroidogenesis in a sigmoid concentration-response manner. The process of phosphorylation activated by ACTH, cAMP and cGMP is not inhibited by cycloheximide or actinomycin D. It is therefore proposed that the hormonal responses mediated by cGMP and cAMP are via the protein kinase enzymatic steps, and the inhibitory effect of cycloheximide and actinomycin D in ACTH-stimulated steroidogenesis follows this step. In conjunction with previous observations that the biosynthetic steps from (20S)-20-hydroxycholesterol to corticosterone are neither inhibited by cycloheximide nor affected by cGMP, it is inferred that the rate-limiting step of adrenal steroidogenesis is the transformation of cholesterol to (20S)-20-hydroxycholesterol and this very step is regulated by cGMP and cAMP. Of further significance are the findings that micromolar concentrations of cAMP and cGMP, which do not stimulate steroidogenesis, effectively stimulate protein kinase activity in a concentration-dependent manner. All cyclic-nucleotide-dependent protein kinase activities of the cell are apparently not necessarily related to steroidogenesis.