Biology of Reproduction, Vol 44, 315-320, Copyright © 1991 by Society for the Study of Reproduction

ARTICLES

Characterization of insulin-like growth factor-binding proteins of porcine ovarian follicular fluid

JS Mondschein, TD Etherton and JM Hammond
Department of Medicine, Milton S. Hershey Medical Center, Pennsylvania State University, Hershey 17033.

Using competitive ligand-binding studies, ligand blotting, and immunoprecipitation, we have characterized the insulin-like growth factor (IGF)-binding proteins (BPs) of porcine follicular fluid. Competitive ligand-binding studies revealed a preference of ovarian IGFBPs for IGF-II over IGF-I. Follicular fluid from small, 1-3-mm follicles had nearly twice the binding capacity for IGFs as that from large, 6-10-mm follicles. Ligand blots of porcine follicular fluid resolved 5 major bands of IGF-binding activity having apparent molecular sizes of 44, 40, 34, 29, and 22 kDa. The 40-44-kDa bands were immunoprecipitated by an antibody to porcine IGFBP-3, the acid-stable subunit of the 150-kDa growth hormone-dependent IGF-binding protein complex of porcine serum. The 34-kDa band was immunoprecipitated by an antibody to rat IGFBP-2, the major IGF-binding protein found in fetal rat serum. To date we have been unable to immunoprecipitate the 29- and 22-kDa bands with any of the antibodies tested, including a panel of monoclonal antibodies to human IGFBP-1, the amniotic fluid IGF-binding protein. The 40-44-kDa species (IGFBP-3) was the predominant form and was equally abundant in fluid from large and small follicles. In contrast, the smaller forms, including IGFBP-2 and the 29- and 22-kDa forms were significantly more prominent in fluid from small follicles. In view of other studies indicating a significant effect of IGFBPs on ovarian cell function, follicular IGFBPs may play an important role in the IGF autocrine/paracrine regulatory system of the ovary.

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