Reproductive Functions of the Mitochondrial Progesterone Receptor (PR-M)

Classic transcriptional regulation by progesterone via the nuclear progesterone receptors A and B (PR-A, PR-B) has been recognized for decades. Less attention has been given to a mitochondrial progesterone receptor (PR-M) responsible for non-nuclear activities. PR-M is derived from the progesterone receptor (PR) gene from an alternate promoter with the cDNA encoding a unique 5' membrane binding domain followed by the same hinge and hormone-binding domain of the nPR. The protein binds to the mitochondrial outer membrane and functions to increase cellular respiration via increased beta-oxidation and oxidative phosphorylation with resulting adenosine triphosphate (ATP) production. Physiologic activities of PR-M have been studied in cardiac function, spermatozoa activation, and myometrial growth, all known to respond to progesterone. Progesterone via PR-M increases cardiomyocyte cellular respiration to meet the metabolic demands of pregnancy with increased contractility. Consequential gene changes associated with PR-M activation include production of proteins for sarcomere development and for fatty acid oxidation. Regarding spermatozoa function, progesterone via PR-M increases cellular energy production necessary for progesterone-dependent hyperactivation. A role of progesterone in myometrial and leiomyomata growth may also be explained by the increase in necessary cellular energy for proliferation. Lastly, the multi-organ increase in cellular respiration may contribute to the progesterone-dependent increase in metabolic rate reflected by an increase in body temperature through compensatory non-shivering thermogenesis. An evolutionary comparison shows PR-M expressed in humans, apes, and Old World monkeys, but the necessary gene sequence is absent in New World monkeys and lower species. The evolutionary advantage to PR-M remains to be defined, but its presence may enhance catabolism to support the extended gestation and brain development found in these primates.

Automated summary

The classic transcriptional regulation by nuclear progesterone receptors A and B has been well-studied, but the mitochondrial progesterone receptor (PR-M) responsible for non-nuclear activities has received less attention. PR-M, derived from the progesterone receptor gene, binds to the mitochondrial outer membrane and increases cellular respiration to meet the metabolic demands of pregnancy, spermatozoa activation, and myometrial growth. Its presence may play a role in extended gestation and brain development in primates.