INDUCTION OF PROSTAGLANDIN ENDOPEROXIDE SYNTHASE-2 BY HUMAN CHORIONIC-GONADOTROPIN IN BOVINE PREOVULATORY FOLLICLES IN-VIVO

Gonadotropins have recently been shown to induce prostaglandin endoper oxide synthase-2 (PGHS-P) in rat preovulatory follicles before ovulati on, but hormonal induction of this enzyme has not yet been reported in preovulatory follicles of any other species. To determine whether the selective induction of PGHS-8 by the ovulatory gonadotropin surge is a molecular process that has been conserved in other mammalian species with different reproductive cycles, the regulation of PGHS isoforms w as studied in bovine preovulatory follicles isolated 0, 6, 12, 18, 20, 22, 24, and 26 h after an ovulatory dose of hCG. Each follicle was di ssected into three preparations: the intact follicle wall (Le. theca i nterns with attached granulosa cells), theca interna, and isolated gra nulosa cells. Solubilized cell extracts were obtained from each prepar ation and analyzed by Western blots, using polyclonal antibodies that selectively recognize PGHS-2 and PGHS-1 isoforms. RNA extracts were pr epared and analyzed by Northern blots, using a mouse PGHS-2 complement ary DNA. The results indicated that the expression of PGHS-2 messenger RNA (mRNA) and protein, but not that of PGHS-1, was regulated by hCG in bovine preovulatory follicles before ovulation. Levels of PGHS-2 pr otein (mol wt, 74,000) in the follicle wall were undetectable at 0, 6, and 12 h, but increased dramatically after 18 h to reach maximal leve ls 24 h post-hCG treatment. Northern blots revealed a similar temporal induction of PGHS-2 mRNA (4.0 kilobases). Analyses of isolated prepar ations of theca interna and granulosa cells clearly showed that PGHS-2 was expressed exclusively in granulosa cells and not in theca interna . Significant increases in follicular fluid concentrations of prostagl andins E(2) and F were associated with the induction of follicular PGH S-2 protein and mRNA. Thus, this study establishes that the induction by gonadotropin of PGHS-(2) in granulosa cells before ovulation appear s to be a conserved mechanism by which the synthesis of prostaglandins necessary for the ovulation process is regulated in different species . However, the striking difference between the time course of PGHS-2 i nduction previously reported in rat follicles (4 h post-hCG treatment) and that observed in bovine follicles (18 h post-hCG treatment) sugge sts that the control of PGHS-2 expression could be one of the determin ants involved in dictating the species-specific progression (length) o f the ovulation process.