GLUT1 regulation of the pro-sclerotic mediators of diabetic nephropathy

Diabetic glomerulosclerosis is characterized by accumulation of extracellular matrix proteins, mesangial expansion, and tubulointerstitial fibrosis. Hyperglycemia accelerates development of the disease, a direct result of increased intracellular glucose availability. The facilitative glucose transporter GLUT1 mediates mesangial cell glucose flux which leads to activation of signaling cascades favoring glomerulosclerosis, including pathways mediated by angiotensin II (Ang II), transforming growth factor β (TGF-β), connective tissue growth factor (CTGF), and vascular endothelial growth factor (VEGF). Ang II has both hemodynamic and metabolic effects directly inducing GLUT1 and/or matrix protein synthesis through diacyl glycerol (DAG) or protein kinase C (PKC) induction, mesangial cell stretch, and/or through transactivation of the epidermal growth factor receptor, the platelet-derived growth factor receptor, and the insulin-like growth factor-1 receptor, all of which may stimulate GLUT1 synthesis via an ERK-mediated pathway. Conversely, inhibition of Ang II effects suppresses GLUT1 and cellular glucose uptake. GLUT1-mediated glucose flux leads to metabolism of glucose via glycolysis, with induction of DAG, PKC, TGF-β1, CTGF and VEGF. VEGF in turn triggers both GLUT1 and matrix synthesis. New roles for GLUT1-mTOR and GLUT1-mechano-growth factor interactions in diabetic glomerulosclerosis have also recently been suggested. Recent mouse models confirmed roles for GLUT1 in vivo in stimulating glomerular growth factor expression, growth factor receptors and development of glomerulosclerosis. GLUT1 may therefore act in concert with cytokines and growth factors to induce diabetic glomerulosclerosis. Further clarification of the pathways involved may prove useful for the therapy of diabetic nephropathy. New directions for investigation are discussed.