INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEINS IN FETAL-RAT MESENCEPHALIC CULTURES - REGULATION BY FIBROBLAST GROWTH-FACTOR AND INSULIN-LIKE GROWTH-FACTOR-I

In rat ventral mesencephalic cultures, IGF-I and bovine fibroblast gro wth factor (bFGF) act cooperatively to support the survival of dopamin ergic neurons. To determine the potential role of IGFBPs in modulating the actions of IGF-I in the ventral mesencephalon, we identified the IGFBPs present in ventral mesencephalic cultures and examined their re gulation by IGF-I and bFGF. In the absence of added growth factors, th e major binding protein secreted from these cultures was IGFBP-2. Smal l amounts of IGFBP-3 and IGFBP-4 were also detected. Addition of bFGF to the cultures increased the amounts of IGFBP-3 and IGFBP-4 released from the cells by 4.4 +/- 2.6-fold (P < 0.1) and 11.5 +/- 3.5-fold (P < 0.05), respectively. IGF-I, by itself, had little effect on the prod uction of IGFBPs, but when added together with bFGF increased the leve ls of IGFBP-3 and IGFBP-4 by 12.4 +/- 5.1-fold (P < 0.05) and 27.4 +/- 5.3-fold (P < 0.02), respectively. The stimulatory effect of bFGF and IGF-I on IGFBP production was apparent after a 2- to 3-day exposure o f the mesencephalic cultures to the peptides. IGFBP-4, the most abunda nt IGFBP present in the cultures after 7 days of growth factor treatme nt, was immunocytochemically localized primarily to neurons, of which a subset were dopaminergic neurons. The addition of purified rat IGFBP -4 to the cultures in the absence of added growth factors had no effec t on the survival of dopaminergic neurons, but when added with IGF-I p otentiated the effect of IGF-I on neuronal survival. We propose that t he up-regulation of IGFBP-4 by IGF-I and bFGF may serve to localize IG F-I to sites of action in the nervous system and thereby potentiate th e neurotrophic actions of IGF-I.