Transcriptional regulation of human genes for steroidogenic enzymes

Steroid hormones are synthesized from cholesterol in the adrenals, gonads, and placenta by a complex series of reactions. The human genes encoding each of these biosynthetic enzymes have been cloned, permitting study of their regulation. Tropic hormones, such as corticotropin and the gonadotropins, exert their chronic effects on steroidogenesis by increasing the amounts of steroidogenic enzymes; this in turn occurs primarily through increased gene transcription. Our studies have emphasized the cholesterol side-chain cleavage enzyme P450scc, which catalyzes the first and rate-limiting step in steroidogenesis, and P450c17, which determines what class of steroids is synthesized. By fusing the promoters of the genes for these enzymes to readily assayed reporter genes and transiently transfecting cultured cells with these constructions, we have identified the regions of each promoter that confer basal expression, induction by cAMP, and repression by activators of protein kinase C. Different segments of the P450scc promoter are used for each of these purposes in different cell types, indicating that the regulation of this gene is very complex. Transcription is not the only level at which steroidogenesis is regulated. The abundance of mRNA for adrenodoxin reductase, a flavoprotein needed for P450scc activity, is post-transcriptionally regulated by cAMP.