A potential role for insulin-like growth factor binding protein-4 proteolysis in the establishment of ovarian follicular dominance in cattle

A critical transition in ovarian follicular development is the selection of a dominant follicle, capable of ovulating, from a cohort of synchronously growing antral follicles. However, little is known about mechanisms and fac tors that regulate the selection and growth of dominant ovarian follicles. We have investigated whether a component of the insulin-like growth factor (IGF) system, namely IGFBP-4 protease, is associated with the establishment of follicular dominance in cattle. IGFBP proteases degrade IGFBPs, freeing IGFs to interact with their receptors. In experiment 1, follicular fluid f rom preovulatory follicles (n 4) degraded about 80 % of the added recombina nt human (rh) IGFBP-4 within 18 h of incubation, The IGFBP-4 protease exhib ited optimal activity at neutral/basic pH and its sensitivity to various pr otease inhibitors suggested a metalloprotease. The decline in the intensity of the band corresponding to intact rh-IGFBP-4 was accompanied by the appe arance of immunoreactive fragments of molecular weights similar to 18 and 1 4 kDa, which were not detectable by ligand blot analysis. In experiment 2, follicular fluid samples were collected from dominant and subordinate folli cles on Day 2 or 3 of the first follicular wave, after ovariectomy (experim ent 2a, n = 3/day) or by ultrasound-guided follicular aspiration (experimen t 2b, n = 4-5/day). Estradiol concentrations in follicular fluid from domin ant vs. subordinate follicles confirmed their identities and indicated that the dominant follicle had been selected by Day 2 of the follicular wave. I n both experiments 2a and 2b, IGFBP-4 proteolytic activity was 2- to 3.5-fo ld (P < 0.05) and 5-fold (P < 0.01) higher in follicular fluid from dominan t than subordinate follicles on Days 2 and 3 of the follicular wave, respec tively. The finding that IGFBP-4 proteolytic activity is higher in dominant , estrogen-active follicles than in subordinate follicles of the same cohor t, as early as Day 2 of the follicular wave, strongly suggests a role for I GFBP-4 protease in the establishment of ovarian follicular dominance.