An Insulin-like growth factor (IGF) is one of the regulators of development of ovarian follicles and its effects are thought to be regulated by several IGF-binding proteins (IGFBPs) at different stages of follicle development. IGFBPs contain an IGF-binding domain on the N-terminal side, and a thyroglobulin-type 1 (Tg-1) domain on the C-terminal side. Although the primary function of IGFBPs is to control the availability of IGF through their binding, the question has arisen whether Tg-1 domain, whose function is unknown, affects follicle development in any way. In this study, we investigated which domains are involved in follicle development at different stages by using recombinant proteins.
Five recombinant IGFBPs (1, 2, 4, 5, 6) and two chimeric proteins composed of N-terminal half of IGFBP 1 and C-terminal half of IGFBP 2 (IGFBP 1/2) and inversed order (IGFBP 2/1) were made by an E. coli expression system. To see the effects on follicle development, these recombinant proteins were added to cultures of granulosa cells or follicles in the presence and absence of FSH.
As a result, IGFBPs 1 and 5, but not 2, 4, and 6, additively promoted granulosa cell proliferation with FSH. To compare their effects on follicle development, we selected IGFBPs 1 and 2, which have different effects on granulosa cells, and found that IGFBP 2 promoted follicle development more than IGFBP 1. To clarify the role of each domain, we compared the effects of IGFBP 1/2 and 2/1 on granulosa cell proliferation and follicle development. As a result, IGFBP 2/1 showed an additive effect with FSH on granulosa cell proliferation and also promoted follicle development. Taken together, it is suggested that IGFBPs regulate follicle development by controlling the availability of IGF via N-terminal domain, in concerted with by controlling granulosa cell proliferation via C-terminal Tg-1 domain.