Three members of the
progestin and adipoQ receptor (PAQR) family, PAQR-7, PAQR-8, and PAQR-5 [membrane
progesterone (P4) receptor (PR) (mPR)α, mPRβ, and mPRγ], function as plasma mPRs coupled to
G proteins in mammalian cells, but the characteristics of two other members, PAQR6 and PAQR9 (mPRδ and mPRε), remain unclear, because they have only been investigated in yeast expression systems. Here, we show that recombinant human mPRδ and mPRε expressed in MDA-MB-231
breast cancer cells display specific, saturable, high-affinity [(3)H]-P4 binding on the plasma membranes of transfected cells with equilibrium dissociation constants (K(d)s) of 2.71 and 2.85 nm, respectively, and low affinity for
R5020, characteristics typical of mPRs. P4 treatment increased cAMP production as well
as [(35)S]-
guanosine 5'-triphosphate (
GTP)γS binding to transfected cell membranes, which was immunoprecipitated with a stimulatory
G protein antibody, suggesting both mPRδ and mPRε activate a
stimulatory G protein (Gs), unlike other mPRs, which activate an
inhibitory G protein (Gi). All five mPR mRNAs were detected in different regions of the human brain, but mPRδ showed greatest expression in many regions, including the forebrain, hypothalamus, amygdala, corpus callosum, and spinal cord, whereas mPRε was abundant in the pituitary gland and hypothalamus.
Allopregnanolone and other
neurosteroids bound to mPRδ and other mPRs and acted as agonists, activating second messengers and decreased
starvation-induced cell death and apoptosis in mPRδ-transfected cells and in hippocampal neuronal cells at low nanomolar concentrations. The results suggest that mPRδ and mPRε function as mPRs coupled to
G proteins and are potential intermediaries of nonclassical antiapoptotic actions of
neurosteroids in the central nervous system.