Expression of cyclooxygenase genes and production of prostaglandins during ovulation in the ovarian follicles of Xenopus laevis

Involvement of cyclooxygenase (COX) and prostaglandin (PG) synthesis during ovulation in Xenopus laevis ovarian follicles was investigated. X. laevis COX cDNAs were isolated from ovarian tissues by using reverse transcription-polymerase chain reaction (RT-PCR) followed by rapid amplification of cDNA ends (RACE). In X. laevis ovary, expression of COX-2 but not COX-1 mRNA was up-regulated during gonadotropin-induced oocyte maturation and ovulation in vitro. Elevation of PGF2α synthesis was directly correlated with the up-regulation of COX-2 mRNA. Synthesis of PGE2 also increased during periovulatory period, however, the concentrations were much lower than those of PGF2α. Progesterone (P4) also induced up-regulation of COX-2 mRNA as well as ovulation. Actinomycin D (ActD) blocked P4-induced ovulation. The inhibition of ovulation by Act D was rescued by co-treatment with exogenous PGF2α in a dose dependent manner. A non-selective COX inhibitor (indomethacin) and selective COX-2 inhibitor (NS398) strongly inhibited the hCG- and P4-dependent production of PGF2α. Inhibitory effects of selective COX-1 inhibitor (SC560) on PGF2α production were lower than that of other inhibitors. Indomethacin and NS398 blocked P4-induced ovulation. NS398 did not block hCG-induced ovulation although it strongly suppressed PGF2α production. These results suggest that (1) in Xenopus ovarian follicles, PGF2α is synthesized during periovulatory period, similar to that in mammals, (2) PGF2α synthesis is regulated by de novo transcription of COX-2 but not COX-1, (3) PGF2α is necessary for P4-induced ovulation but may not be essential for hCG-induced ovulation, and other factor(s) may be involved in the hCG-induced ovulation.