CHANGES IN INSULIN-LIKE GROWTH FACTOR-II MANNOSE-6-PHOSPHATE RECEPTORDURING GROWTH AND ATRESIA OF OVINE OVARIAN FOLLICLES

To assess a potential role of insulin-like growth factor (IGF)-II/mann ose-6-phosphate (M6P) receptor in the ewe ovary, [I-125]IGF-II binding was assessed on whole ovarian membranes and on ovarian sections. Comp etition studies with IGF-I, IGF-II, and insulin were performed to esti mate the specificity of[I-125]IGF-II binding. On the ovarian sections, labeling was quantified after microautoradiography by image analysis. In addition, immunohistochemistry experiments using a specific polycl onal antibody raised against the IGF-II/M6P receptor were performed on ovarian sections. Specific and high-affinity binding sites for IGF-II were present on whole ovarian membranes and on ovarian sections. On m embranes, the K-d value was close to 0.2 nM. IGF-I was approximately 1 000-fold less potent than IGF-II in displacing binding of [I-125]IGF-I I from whole membranes as well as from granulosa and thecal cells on o varian sections. Insulin did not displace [I-125]IGF-II binding. Cross -linking experiments followed by sDS-PAGE under reducing conditions re vealed that [I-125]IGF-II binding predominantly involved a 260-kDa pro tein, the size of which is compatible with that of the IGF-II/M6P rece ptor. Furthermore, both autoradiography and immunohistochemistry exper iments demonstrated that IGF-II/M6P receptor was present at high level s in granulosa of atretic follicles and in theca of healthy follicles. Conversely, very low levels were found in granulosa of healthy follic les and in theca of atretic follicles. The involvement of the IGF-II/M 6P receptor in ovarian tissue remodeling and in mediating IGF-Il actio n during the processes of folliculogenesis and atresia remains to be d etermined.