Insulin-Like Growth Factor-Binding Protein (IGFBP)-1, -2, -3, -4, -5, and -6 and IGFBP-Related Protein 1 during Rainbow Trout Postvitellogenesis and Oocyte Maturation: Molecular Characterization, Expression Profiles, and Hormonal Regulation

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In the present study we report the full coding sequence of rainbow trout IGF-binding protein-1 (IGFBP1), -2, -3, -5, and -6 and IGFBP-related protein-1 (IGFBP-rP1) mRNAs as well as the partial coding sequence of IGFBP-4 mRNA. To our knowledge, this is the first report of IGFBP4, IGFBP6, and IGFBP-rP1 in a nonmammalian species. The tissue distribution of all mRNAs was studied, and the ovarian expression profiles of IGFBP2 to -6 and IGFBP-rP1 between late vitellogenesis and oocyte maturation were characterized. In addition, in vitro hormonal regulation by the maturation-inducing steroid 17,20β-dihydroxy-4-pregnen-3-one (17,20βP), gonadotropin, and estradiol were studied. We observed that besides IGFBP1, which was only found in liver, IGFBP2 to -6 and IGFPB-rP1 were expressed in the preovulatory ovary. IGFBP3 was also detected in liver, trunk, kidney, skin, and gills, whereas IGFBP2 to -6 and IGFBP-rP1 exhibited a wider tissue distribution. In the preovulatory ovary, IGFBP3 was strongly down-regulated during the postvitellogenesis period, whereas IGFBP5 exhibited a limited up-regulation. In addition, IGFBP6 and IGFBP-rP1 were up-regulated during oocyte maturation. Hormonal regulation data indicated that all ovarian IGFBPs and IGFBP-rP1 transcripts are regulated under gonadotropic stimulation at a concentration that induced 100% oocyte maturation. In addition, IGFBP2 to -5 transcripts are regulated by 17,20βP and estradiol. Together, our observations strongly suggest that during final oocyte maturation, a down-regulation of IGFBP3, -4, and -5 occurs in the oocyte in response to gonadotropic and 17,20βP (IGFBP3 and -5) stimulation, whereas an up-regulation of IGFBP2 and -6 occurs in follicular layers or extrafollicular tissue in response to gonadotropic stimulation.

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