Endocrine Abstracts

Rapid activation of diverse signal transduction pathways is thought to play an integral role in the cellular responses to many steroid hormones, including progesterone. This sex steroid is essential for normal reproduction in nearly all species. The recent cloning of a G-protein coupled receptor with the properties of a membrane progestin receptor (mPR) in fish and the subsequent identification of three mammalian homologues (mPR-alpha, -beta, and -gamma) raised the possibility of defining the relevance of non-genomic progesterone actions in normal reproduction and disease. Phylogenetic and sequence analyses demonstrated that the mPRs belong to a larger, highly conserved family of proteins, termed ‘progestin and adeponectin receptors’ (PAQRs). Furthermore, a comparison of the expression of mPR transcripts in human reproductive tissues revealed markedly divergent expression levels and profiles. Interestingly, mPR-alpha is most highly expressed in the placenta and its relative expression in many tissues correlates inversely with that of the nuclear PR. For functional analysis, we transiently and stably transfected a panel of cell lines with expression vectors encoding for either wild-type or tagged human mPRs. Confocal microscopy studies revealed that transfected mPRs localize to the endoplasmic reticulum but not plasma membrane or mitochondriae. Most notably, endogenous mPR-alpha could be demonstrated in the MDA-MB-231 and BeWo cells, displaying the same pattern of distribution. Treatment of MDA-MB-231 cells transfected with the fish mPR has been reported to result in a rapid decrease in cAMP levels and an increase in MAPK (p44/p42) phosphorylation. However, progesterone treatment did not modulate either cAMP content, calcium signalling, or MAPK activation in cell lines transiently or stably transfected with human or mouse mPR-alpha, -beta, or -gamma. Our findings suggest that the role of the fish mPR has not been evolutionarily conserved. The mammalian homologues could be currently considered orphan receptors with as yet undefined roles in reproduction.