Correlation between binding affinities of C21 steroids for the maturation-inducing steroid membrane receptor in spotted seatrout ovaries and their agonist and antagonist activities in an oocyte maturation bioassay

Abstract

The relative binding affinities of steroids for the maturation-inducing steroid (MIS) plasma membrane receptor in spotted seatrout (Cynoscion nebulosus) ovaries were compared to their relative potencies in inducing final oocyte maturation (FOM) of seatrout oocytes in vitro. The MIS receptor is specific for C21 steroids (pregnenes) lacking ketone or hydroxyl (OH) groups at the 11 position. The addition of single OH groups at the 17, 20, or 21 positions and two OHs at the 17, 20 and at the 17, 21 positions of both progesterone and pregnenolone derivatives decreased binding affinity. In contrast, the combination of OH at the 20β and 21 positions increased affinity, and greatest binding affinity for a progesterone derivative was observed with three OHs at positions 17, 20β, and 21 (17,20β,21-trihydroxy- 4-pregnen-3-one, 20β-S), the natural MIS in this species. Germinal vesicle breakdown (GVBD) bioassays showed that 1-min exposure to 290 nM 20β-S or 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) in vitro was sufficient to induce final maturation of follicle-enclosed oocytes of seatrout and a closely related species, Atlantic croaker (Micropogonias undulatus), whereas the other steroids tested were ineffective. 20β-S was more potent than 17,20β-P in inducing GVBD after 1-min exposure at lower steroid concentrations (5 nM-100 nM), even though the follicular uptake of the two steroids was similar. Coincubation of seatrout oocytes for 1 min with other steroids at concentrations capable of displacing more than 80% of the bound 20β-S from its receptor effectively blocked the induction of GVBD by 5 nM 20β-S. The binding affinities of steroids for the MIS receptor correlated in general with either their agonist or their antagonist activities in the GVBD bioassays. These findings strongly support the concept that induction of FOM by the MIS in spotted seatrout is mediated solely through the ovarian plasma membrane 20β-S receptor.