Increased Glucose Uptake and Metabolism in Mesangial Cells Overexpressing Glucose Transporter 1 Increases Interleukin-6 and Vascular Endothelial Growth Factor Production: Role of AP-1 and HIF-1α
Previous results indicate that enhanced glucose transporter (GLUT)1 expression mediates the deleterious effects of metabolic and hemodynamic perturbations leading to diabetic kidney disease. First screening for altered gene expression in GLUT1 overexpressing cells (GT1) by Affymetrix microarray analysis revealed upregulation of interleukin-6 (IL-6) and vascular endothelial growth factor (VEGF) expression, which was verified by RT-PCR. Subsequently, IL-6 and VEGF protein production was more than 3-fold increased in the GT1 cells. This upregulation was independent from each other. Studies on the underlying transcriptional mechanisms by gelshift assays and siRNA approach implicated activation of AP-1 in the increased expression of both, IL-6 and VEGF. We found also increased nuclear protein levels of hypoxia-inducible factor (HIF)-1α and enhanced DNA binding activity to a hypoxia responsible element located in the VEGF promoter. Knock-down of HIF-1α reduced the VEGF expression to 50% with an additive effect of AP-1 gene silencing down to 24%. The IL-6 expression was not affected by reducing HIF-1α. In conclusion our results link increased GLUT1 levels leading to excess glucose metabolism under normoglycemic conditions and altered gene expression of pathogenetic factors involved in diabetic kidney disease.