Dual Role of IGF-II in Oocyte Maturation in Southern Flounder: Up-Regulation of Membrane Progesterone Receptor Alpha (mPRalpha) and Resumption of Meiosis.

Abstract

Increasing evidence suggests a regulatory role for the insulin-like growth factor (IGF) system in teleost oocyte maturation (OM). The likely maturation-inducing steroid (MIS) in southern flounder and the mechanisms by which IGF-I or –II may be involved in the OM process were investigated. The most abundant final product of ovarian steroidogenesis assays eluted at the position of 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S). 20beta-S was also a more sensitive and potent inducer than the other teleost MIS, 17,20beta-dihydroxy-4-pregen-3-one, of germinal vesicle breakdown (GVBD) of maturationally-competent oocytes. IGF-II (100nM) induced maturational competence, as greater GVBD was induced after incubation with IGF-II + 20beta-S compared to that of the 20beta-S +20beta-S or IGF-II + N/T group. Interestingly, incubation with IGF-II (100nM) for 4-8h increased ovarian membrane progestin receptor alpha (mPRalpha or Paqr7b) mRNA levels 12-15% and mPRalpha protein levels 75-101%. Further, the IGF-II-induced increase in mPRalpha protein concentrations was partially blocked by pretreatment with Wortmannin, a Pik3 inhibitor, and PD 098,059, a Mapk inhibitor. Both IGF-I and –II (100nM) induced meiotic resumption in maturationally-competent oocytes which was blocked by incubation with cycloheximide. Incubation with DL-Aminoglutethimide decreased IGF-II-induced GVBD but had no effect on IGF-I induced GVBD. IGF-I and –II were also able to induce meiotic resumption in maturationally-incompetent oocytes, and elicited 75 and 135% greater GVBD, respectively, than hCG + 20beta-S at100nM. In conclusion, we show that 20beta-S is the likely MIS in this species and that IGF-I and –II are also able to induce meiotic resumption. Further, IGF-II not only induces OMC but also up-regulates ovarian mPRalpha mRNA and protein through Pik3- and Mapk-dependent pathways. This is the first demonstration of mPRalpha regulation by an IGF in any vertebrate species.