Insulin-Like Growth Factor-Binding Protein-3 Mediates High Glucose-Induced Apoptosis by Increasing Oxidative Stress in Proximal Tubular Epithelial Cells

IGF-binding protein-3 (IGFBP-3) is the major circulating carrier protein for IGF, and also acts as a potent antiproliferative agent in various cell types. Recently, IGFBP-3 was reported to mediate high glucose-induced apoptosis in mesangial cells and podocytes. In this study, we investigated the role of IGFBP-3 in high glucose-induced apoptosis in proximal tubular epithelial cells (PTEC). Expression of IGFBP-3 protein and mRNA in a porcine PTEC line (LLC-PK1 cells) was measured after exposure to either standard (5.5 mM) or high-glucose (30 mM) medium. We quantified apoptosis after treatment with small interfering RNA against IGFBP-3 (siRNA:IGFBP-3) in high-glucose medium or in cells that overexpressed IGFBP-3. Oxidative stress was measured in high-glucose medium, in the presence of siRNA:IGFBP-3, or in IGFBP-3-overexpressing cells. IGFBP-3 protein and mRNA expression in LLC-PK1 cells was higher in high-glucose medium than in standard-glucose medium. Exposure to high-glucose medium increased apoptosis, and high-glucose-induced apoptosis was abolished by siRNA:IGFBP-3. IGFBP-3 overexpression induced apoptosis in LLC-PK1 cells. Both high-glucose medium and IGFBP-3 overexpression increased reactive oxygen species, and siRNA:IGFBP-3 reduced this increase. Antioxidant treatment decreased IGFBP-3 expression and apoptosis, whereas oxidative stress from hydrogen peroxide increased IGFBP-3 expression, suggesting that oxidative stress increases IGFBP-3 expression. Our results suggest that increased IGFBP-3 expression by high glucose mediates high-glucose-induced apoptosis in PTEC. Increased oxidative stress from high glucose enhances IGFBP-3 expression, inducing apoptosis. Increased expression of IGFBP-3 by high glucose induces additional oxidative stress, which may result in amplification of hyperglycemic damage.