Progesterone receptor isoforms: Function and regulation

Progesterone participates in the regulation of several physiological proces ses in mammals. The biological response to progesterone is mediated by two forms of the progesterone receptor (PR) denominated PR-A and PR-B. The diff erence between them is that 164 amino acids of N-terminal of PR-B are absen t in PR-A. Both PR isoforms are derived from a single gene but are generate d from either alternative transcriptional or translational start sites, and are regulated by different estrogen-induced promoters. PR-B acts as a tran scriptional activator in different cellular contexts whereas PR-A functions as a strong inhibitor of transcriptional activity. PR isoforms expression and function vary among target tissues such as the uterus, the mammary glan d and the brain. The Knowledge of the molecular mechanisms involved in the regulation of expression and function of PR isoforms will contribute to the understanding of fundamental biological processes such as sexual behavior and reproduction, and it will open the possibility of alternative therapies in fertility control as well as in the treatment of breast, endometrial an d cerebral tumors.