MAP Kinase Pathway is Involved in IGF-1-Stimulated Proliferation of Human Retinal Pigment Epithelial Cells (hRPE)

Purpose: To investigate the mitogenic activity of insulin-like growth factor-1 (IGF-1) on the proliferation of human retinal pigment epithelial cells (hRPE) and to elucidate the role of vascular endothelial growth factor (VEGF) and MAP kinase (MAPK) in the IGF-1 signaling cascade. Methods: Human RPE specimens were obtained from postmortem non-pathological eyes and cultured in vitro through several passages. Cellular proliferation in the presence of increasing concentrations of IGF-1 and IGF-1 + PD98059 (a known MAPK inhibitor) was measured by [H-3] thymidine incorporation; trypan blue exclusion studies (T) verfied cell viability. Under the same experimental conditions, synthesis of VEGF was measured utilizing [C-14] methionine immunoprecipitation and immunocytochemical methods as well as Western blot analysis. Results: IGF-1 stimulated hRPE proliferation, as demonstrated by [H-3] thymidine incorporation. There was also an IGF-1-induced increase in VEGF synthesis as measured quantitatively by [C-14] methionine-VEGF immunoprecipitation. This was qualitatively confirmed by immunocytochemistry and Western blotting. PD98059 suppressed both IGF-1-induced cell proliferation as well as IGF-1-stimulated VEGF production. Conclusions: These studies suggest that IGF-1 is a mitogen for hRPE cells and also stimulates production of the angiogenic factor, VEGF. Additionally, PD98059 inhibits the production of VEGF, suggesting that the MAP kinase pathway is involved in IGF-1-mediated angiogenesis.