Changes in insulin-like growth factor-I (IGF-I), IGF-II, and their binding proteins during growth and atresia of ovine ovarian follicles

Abstract

Levels of insulin-like growth factor-I (IGF-I), IGF-II, and IGF-binding proteins (IGFBPs) were studied in ovine follicular fluid and serum with a view to determining their respective endocrine and/or paracrine roles in ovarian folliculogenesis. Ovarian follicles of Ile-de-France ewes were dissected and measured individually, and their follicular fluid collected. Follicles were classified according to size and degree of atresia, assessed on the basis of microscopical examination of smears of granulosa cells. Follicular fluid and serum samples were assayed for IGF-I and IGF-II. Free IGF-binding activity was also determined, and the IGFBP profiles in serum and follicular fluid were examined by Western ligand blotting [44- to 42-kilodalton (kDa) doublet and 35-kDa, 28.5- to 32-kDa, and 24-kDa bands], followed by densitometric analysis of the autoradiographs. Finally, the effects of follicular fluid IGFBPs on granulosa cell responses to IGF-I were studied in vitro. The size and atretic stage of the follicles had little influence on the IGF-I concentrations in follicular fluid, but IGF-II concentrations were approximately 1.5 times higher in small than in large follicles (P < 0.01). IGF-I levels were lower in fluid from large normal (highly vascularized) follicles than in serum (P < 0.01). Follicular fluid and serum IGF-I levels were positively correlated (r = 0.55; P < 0.05). No significant difference was found between follicular fluid and serum IGF-II levels. Follicular growth was accompanied by a decrease in free IGF-binding activity (P < 0.0001), a slight increase in the intensity of the 44- to 42-kDa IGFBP doublet (P < 0.05), and a clear decrease in the intensities of the 35-kDa (P < 0.0001) and 24-kDa bands. By contrast, follicular atresia was characterized by a marked increase in free IGF-binding activity (P < 0.0001) and strongly increased intensities of the 35-kDa, 28.5- to 32-kDa, and 24-kDa bands (P < 0.0001). Low mol wt IGFBPs, particularly the 24-kDa species, were clearly more abundant in serum than in follicular fluid from large normal follicles. In vitro experiments showed IGF-I to be less active on granulosa cells in the presence of follicular fluid from atretic than from normal follicles. The action of the IGF-I analog [Gln3,Ala4,Tyr15,Leu16]IGF-I, which has a weak affinity for the IGFBPs, was, however, similar whether atretic or normal follicular fluid was tested.