Novel regulation of pregnant human myometrial smooth muscle cell gap junctions by human chorionic gonadotropin

Abstract

Human myometrium contains hCG/LH receptors. There are fewer of these receptors during labor compared to no labor at preterm or term deliveries. Exogenous hCG can directly inhibit oxytocin-stimulated human myometrial contractions. These findings suggest that hCG may directly maintain myometrial quiescence during pregnancy. As maintenance of uterine quiescence may involve down-regulation of myometrial gap junctions, we investigated the effect of hCG on connexin-43 (CX-43) gene expression from RNA to protein and morphological gap junctions. The addition of 5 or 10 nM highly purified hCG to subconfluent cultures of pregnant myometrial smooth muscle cells resulted in a significant decrease in CX-43 protein levels. Higher hCG concentrations (100 and 1000 nM), however, had no effect. The maximal effect of hCG was seen at 4-8 h of culture, followed by recovery after a longer duration of culture. hCG treatment also concomitantly decreased CX-43 messenger RNA and morphological gap junctions. The hCG effect on CX-43 protein levels is hormone specific and mediated by protein kinase-A signaling. Estradiol and oxytocin increased, whereas progesterone decreased, CX-43 protein levels and morphological gap junctions. The oxytocin-induced increase was reversed by cotreatment with hCG. Although RU 486 alone had no effect on CX-43 protein levels, it prevented the down-regulating action of hCG and progesterone. In summary, our results demonstrate that hCG can directly decrease CX-43 messenger RNA, protein, and morphological gap junctions in cultured pregnant human myometrial smooth muscle cells. The hCG action is concentration and time dependent, hormone specific, and mediated by protein kinase-A signaling and appears to involve progesterone receptors. These data lend support to the concept that hCG could be one of the hormones responsible for maintaining uterine quiescence by down-regulating myometrial gap junctions during pregnancy.