Insulin-like growth factor binding protein -2 and -4 messenger ribonucleic acid expression in bovine ovarian follicles: effect of gonadotropins and developmental status.

This work is concerned with the role of insulin-like growth factor binding protein (IGFBP)-2 and -4 in the regulation of IGF bioactivity in bovine follicles during the development of dominance. We measured the expression of IGFBP-2 and -4 messenger RNA (mRNA) in small (1-4 mm) gonadotropin-sensitive follicles and medium (4-8 mm) and large (>8 mm) gonadotropin-dependent follicles using in situ hybridization. In healthy nonatretic bovine follicles, IGFBP-2 and -4 mRNA expression was confined to granulosa and theca tissue, respectively. Moreover, during the development of follicular atresia, there were distinct changes in the temporal and spatial expression of these genes. IGFBP-2 immunoactivity was localized in granulosa tissue and the basement membrane of healthy preantral follicles, whereas IGFBP-4 immunoactivity was localized in both theca and granulosa tissue. Of particular interest was the lack of IGFBP-2 mRNA expression in large (>8 mm) gonadotropin-dependent follicles, an observation that was confirmed by the lack of immunoreactive IGFBP-2 in these follicles. The regulation of IGFBP-2 and -4 mRNA expression in granulosa and theca cells was analyzed using a serum-free cell culture system. FSH inhibited the expression of IGFBP-2 mRNA in granulosa cells, whereas LH stimulated IGFBP-4 mRNA expression in theca cells. Our results provide evidence for the existence of different roles for IGFBP-2 and -4 in the developing follicle.