Stepwise activation of the gonadotropic signal transduction pathway, and the ability of prostaglandin F2α to inhibit this activated pathwayto inhibit this activated pathway

Through selective activation of the gonadotropic signal transduction pathway, we have determined the probable site of the antigonadotropic effects of prostaglandin F2α (PGF2α) in the human granulosa-luteal cell (hGLC). The gonadotropic signal transduction pathway was activated at the level of the receptor (luteinizing hormone and β-adrenergic), stimulatory G protein (Gs), adenylate cyclase (AC), and protein kinase A (PKA) by human chorionic gonadotropin (hCG) and isoproterenol (Iso), cholera toxin (CTX), forskolin, and dibutryl cAMP (Db cAMP), respectively. Concomitantly, the ability of PGF2α to inhibit progesterone production in response to the activation of this cascade at these different levels was examined. hGLCs were obtained from in vitro fertilization patients and were precultured for 8 d in Medium 199 supplemented with fetal bovine serum (M199; 10% FBS). Following the preculture period, cells were treated with either vehicle or one of the above activators of the gonadotropic pathway, either in the absence or presence of PGF2α (in M199; No FBS). Following the treatment period, media were collected and assayed for progesterone by RIA. Prostaglandin F2α (10−6M) significantly inhibited hCG (1 IU/mL), Iso (10−5M), CTX (1 µg/mL), and forskolin- (10−5M) stimulated progesterone production. Conversely, PGF2α did not inhibit progesterone production stimulated by a saturating concentration of Db cAMP (10−6M). The ability of PGF2α to inhibit hCG- or CTX-stimulated progesterone production was attenuated by pertussis toxin (PTX; 50 ng/mL). In conclusion, through a pertussis toxinsensitive G protein, PGF2α inhibits progesterone production at a level below AC, and above the activation of PKA by cAMP.