Heterologous expression of human mPRα, mPRβ and mPRγ in yeast confirms their ability to function as membrane progesterone receptors

The nuclear progesterone receptor (nPR) mediates many of the physiological effects of progesterone by regulating the expression of genes, however, progesterone also exerts non-transcriptional (non-genomic) effects that have been proposed to rely on a receptor that is distinct from nPR. Several members of the progestin and AdipoQ-Receptor (PAQR) family were recently identified as potential mediators of these non-genomic effects. Membranes from cells expressing these proteins, called mPR alpha, mPR beta and mPR gamma, were shown to specifically bind progesterone and have G-protein coupled receptor (GPCR) characteristics, although other studies dispute these findings. To clarify the role of these mPRs in non-genomic progesterone signaling, we established an assay for PAQR functional evaluation using heterologous expression in Saccharomyces cerevisiae. Using this assay, we demonstrate unequivocally that mPR alpha, mPR beta and mPR gamma can sense and respond to progesterone with EC50 values that are physiologically relevant. Agonist profiles also show that mPR alpha, mPR beta and mPR gamma are activated by ligands, such as 17 alpha-hydroxyprogesterone, that are known to activate non-genomic pathways but not nPR. These results strongly suggest that these receptors may indeed function as the long-sought-after membrane progesterone receptors. Additionally, we show that two uncharacterized PAQRs, PAQR6 and PAQR9, are also capable of responding to progesterone. These mPR-like PAQRs; have been renamed as mPR delta (PAQR6) and mPR epsilon (PAQR9). Additional characterization of mPR gamma and mPRa indicates that their progesterone-dependent signaling in yeast does not require heterotrimeric G-proteins, thus calling into question the characterization of the mPRs; as a novel class of G-protein coupled receptor. (c) 2008 Elsevier Inc. All rights reserved.