Regulation of Glomerular Epithelial Cell Production of Fibronectin and Transforming Growth Factor-β by High Glucose, Not by Angiotensin II

Abstract

Accumulation of matrix proteins is a prominent feature of diabetic nephropathy. Glomerular visceral epithelial cells (GVECs) are important contributors to extracellular matrix (ECM) production in the glomerulus. Factors involved with increased accumulation of ECM proteins are high glucose, angiotensin II (ANG II), and transforming growth factor (TGF)-β. Therefore, we investigated the effects of high glucose and ANG II on fibronectin and TGF-β production by human GVECs in vitro. We found that ANG II had no effect on the production of fibronectin and TGF-β by GVECs. Using reverse transcriptase–polymerase chain reaction analysis, no ANG II receptor could be detected on these cells. However, high glucose induced a twofold increase in fibronectin (P < 0.01) and a three- to sixfold increase in TGF-β (P < 0.001) production. Similar results were obtained by analyzing the mRNA levels of fibronectin (increased 2.7-fold) and TGF-β (increased 3.5-fold). Addition of increasing concentrations of rTGF-β to control cells resulted in increased fibronectin production. Neutralizing antibodies against TGF-β significantly reversed the increase in fibronectin protein and mRNA caused by high glucose back to control levels. We conclude that high glucose concentrations stimulate the synthesis of fibronectin and that this effect is mediated by induction of TGF-β. These results suggest that in diabetic nephropathy, high glucose levels play a role in changing the matrix composition of the glomerular basement membrane through induction of TGF-β. Our results indicate that a contribution to this process by an effect of ANG II on GVECs seems unlikely.