Failure of hCG/LH receptors to stimulate the transmembrane effector adenylyl cyclase in human endometrium

The functional significance of the endometrial hCG/ LH receptors has been related to a rapid release of prostaglandins. However, as compared to gonads and myometrium, in-endometrium mechanisms of transmembrane signalling of the hCG/LH receptors are probably not conventional and remain unclear. Here we investigated, in vivo, the potential of hCG to interact with, and stimulate the membrane effector enzyme, adenylyl cyclase (AC), in human endometrium. Hormonal and nonhormonal activation of AC was tested in membrane fractions prepared from endometrial biopsies obtained from patients undergoing evaluation cycles for hormone replacement therapy (HRT) and controlled ovarian hyperstimulation (COH). AC activity was determined by the direct conversion of the substrate ATP into cAMP under unstimulated conditions and in the presence of the non-hormonal activators guanyl nucleotide and forskolin. Also AC activity was tested in the presence of hCG under conditions allowing maximal enzyme stimulation. Isoproterenol and prostaglandin E2 (PGE2) were included for comparison. Immunoblot analyses demonstrated the presence of hCG/LH receptors and Gsα protein and other members of the G protein family in the membrane fractions. Endometrial membranes also exhibited high levels of AC activity compared to luteal membranes used as control. Stimulation by GMP-P(NH)P alone was 196 ± 63 (n = 8) (pmol/mg/ min ± SD). Neither hCG nor isoproterenol showed stimulation of endometrial AC (210 ± 65, and 197 ± 53, respectively; n = 66 assays). But PGE2 stimulated the enzyme system significantly (264 ± 63, p < 0.05; n = 66 assays). These data show that membrane fractions from human endometrium express all the AC system components, namely, hCG/LH receptors, Gsα protein and AC; however, hCG does not stimulate the endometrial AC system. Our data indicate that, in great contrast to gonadal receptors, endometrial hCG/ LH receptors are not coupled to the transmembrane AC effector. The well known release of eicosanoids in response to hCG suggests that these receptors are functional in human endometrium but throughout a signalling system different from AC. This enzyme is certainly coupled to and directly activated by eicosanoids and other embryonic signals.