Müller Cell Production of Insulin-like Growth Factor–Binding Proteins In Vitro: Modulation with Phenotype and Growth Factor Stimulation

Abstract

Müller cells are present in diabetic fibrocontractive ocular tissues and generate tractional forces in response to insulin-like growth factors. Recent studies indicate that diabetes-associated increases in vitreous insulin-like growth factor activity are, in part, attributable to changes in insulin-like growth factor binding proteins (IGFBPs). The objectives of this study were to evaluate Müller cells as a source of IGFBPs and characterize changes associated with cell phenotype and growth factor stimuli known to be present in diabetic vitreous. Müller cells isolated from normal porcine retina were maintained in culture for 1 and 5 weeks, yielding phenotypes described as proliferative and myofibroblastic. RNA preparations from porcine liver, retina, and Müller cell cultures were evaluated by RT-PCR and Northern blot analysis. IGFBP production was verified by Western ligand and Western blot analysis of Müller-cell-conditioned media and detergent-extracted proteins. Molecular biological analyses of RNA from normal retina and from proliferative and myofibroblastic Müller cells did not detect message for IGFBP-1, but revealed progressive increases in message abundance for IGFBP-2, -3, -4, and -6. IGFBP-5 message was detected in all samples, but was least abundant in myofibroblastic Müller cells. Stimulation of myofibroblastic Müller cells by IGF-I and -II, but not PDGF, further increased message abundance and production of IGFBP-2, -4, -5, and -6, but not IGFBP-3. Müller cell production of IGFBPs changes with phenotype and, in most cases, is highest in the cells most likely to participate in fibrocontractive retinal disease. IGFBP production by these cells is further increased by IGF-I and -II, growth factors known to be present and active in proliferative vitreoretinal disorders, suggesting that Müller cells represent a potential source of vitreous IGFBPs in disorders involving this cell type.