Insulin-like growth factor binding proteins modulate Muller cell responses to insulin-like growth factors

Purpose. Muller cells are consistently identified in diabetic fibrocontractive ocular tissues and, in response to insulin-like growth factor I, generate tractional forces of the type that cause retinal detachment. Recent studies suggest that diabetes-associated increases in vitreous insulin-like growth factor activity cannot be attributed to simple increases in concentration alone, suggesting that more complex biochemical changes in vitreous growth factor control mechanisms are involved. The goal of this study was to evaluate the contributions of vitreous insulin-like growth factor-binding proteins (IGFBPs) toward control of growth factor activity. Methods. Native and recombinant IGFBPs effects were evaluated on IGF-I- and -II-stimulated Muller cells in tissue culture assays that involved cell incubation on three-dimensional collagen gels and that monitored progressive matrix condensation. IGFBP degradation by Muller cell-secreted proteases was assessed in Western ligand blots, and direct stimulatory effects were evaluated by incubating cells with IGFBPs alone. Results. IGFBP direct stimulatory effects on Muller cells were significant, but relatively modest, and IGFBP modulation through Muller cell-secreted proteases was undetectable. In contrast, IGFBP inhibitory effects on IGF-I and -II were highly variable and, in some cases, profound. IGFBP-3 effectively inhibited IGF-I and -II stimulation with detectable effects at concentrations equimolar to the growth factor. IGFBP-1, -2, -4, and -5 were of intermediate effectiveness as inhibitors, 3- to 11-fold less active than IGFBP-3. IGFBP-6 had virtually no inhibitory effects on IGF-I, but was moderately effective against IGF-II. Conclusions. IGFBP effects on IGF-I- and -II-stimulated Muller cells are primarily inhibitory with only modest direct stimulatory effects of limited physiologic relevance. IGFBP-2 and -3, the major binding proteins identified in vitreous, most likely function as the vitreous growth factor sink and control ligand activity through sequestration.