Basic concepts and recent developments in human steroid hormone biosynthesis

The biosynthesis of steroid hormones requires the coordinated expression of the enzymes that comprise the pathways via which specific hormones are synthesized. These pathways and their associated enzymes are typically subject to regulation consisting of trophic hormone stimuli and feedback mechanisms. Very few tissues contribute substantially to de novo steroidogenesis, primarily the adrenal glands, the gonads, and the placenta. Both the embryonic origins and the signaling mechanisms for the adrenals and gonads are similar, and steroid synthesis in these two glands are the major focus of this review. We will further describe peripheral steroid metabolism and the regulation of steroid hormone potency in target tissues. In addition, we will briefly discuss the congenital adrenal hyperplasias to illustrate the principles developed in the initial sections. Finally, we will discuss some recent developments in steroidogensis, focusing on cytochrome P450 oxidoreductase deficiency and the alternate or "backdoor" pathway to dihydrotestosterone. We will conclude with a description of aberrant signaling mechanisms observed in adrenal tumors as a further example of how these pathways can be disturbed in pathologic states.