6.7. Progesterone in Men
Defined in the majority of the literature as a female hormone, the importance of PROG in the male endocrine system has remained largely neglected. Testicular and adrenal PROG has been regarded as a physiologically unimportant by-product of steroidogenesis. However, in several conditions, including aging, the serum PROG/androgen ratio increases. The average reference range for serum PROG in healthy men is generally considered under 1 ng/mL, which is similar to in postmenopausal women [
210].
The male-specific actions of PROG are predominately membrane-dependent, including a rapid increase in Ca2+ resulting in a sperm capacitation/acrosome reaction, LH receptor suppression and the subsequent influence of testosterone biosynthesis in Leydig cells, increased classical PR expressions in the prostate (benign prostatic hyperplasia as well as prostate cancer), interactions with the GABAA receptor complex in the CNS, including sedative and anesthetic actions, and interactions in adipose tissue and the kidneys. PROG-binding membrane proteins have been identified in the liver, sperm and lens epithelial cells [
210]. PROG is also one of the steroid hormones that affect spermatogenesis. It has been found that the co-administration of progestins in androgen-based contraceptive pills for men augments the induction of gonadotropin-induced spermatogenic suppression [
211]. One study found that the inhibition of sperm production is not caused indirectly by affecting the hypothalamus, but rather the effect of PROG was confirmed directly in the testes [
212]. Adding PROG to androgens reduces circulatory concentrations of inhibin and alters the expression of some germ cell-specific genes in human testes [
213]. Clinical studies have shown that sperm obtained from oligospermic men had reduced responses to PROG stimulation, suggesting that this membrane effect of PROG can be crucial for the development and fertilizing capacity of sperm [
214,
215]. It was also reported that PRs may play a role in the regulation of spermatogenesis in humans and that a lack of PR expression in germ cells may be linked to impaired spermatogenesis and could be one possible cause of male infertility [
216,
217]. PROG is therefore considered to directly act on the testes to regulate spermatogenesis.