Angiotensin II stimulates α3(IV) collagen production in mouse podocytes via TGF-β and VEGF signalling:: implications for diabetic glomerulopathy

Background. The podocyte is bathed in an angiotensin II (AngII)-rich ultrafiltrate, but the impact of AngII on podocyte pathobiology is not well known. Because podocytes play a direct role in the glomerular basement membrane (GBM) thickening of diabetes, the alpha 3(IV) collagen chain was examined. Podocyte expression of alpha 3(IV) collagen may involve the transforming growth factor-beta (TGF-beta) and vascular endothelial growth factor (VEGF) systems. Methods. Cultured mouse podocytes were treated with various doses of AngII for selected periods of time, with or without inhibitors of TGF-beta and VEGF signalling, SB-431542 and SU5416, respectively. TGF-beta 1 and VEGF were assayed by enzyme-linked immunosorbent assay (ELISA); alpha 3(IV) collagen, TGF-beta type II receptor and phospho-Smad2 were assayed by immunoblotting. Results. AngII >= 10(-10) M was found to stimulate the production of alpha 3(IV) collagen significantly in as short a time as 3h. The expression of alpha 3(IV) collagen was influenced by the TGF-beta system, but AngII did not increase the podocyte's production of TGF-beta 1 ligand; rather, it increased the expression of the TGF-beta type II receptor and activated the TGF-beta signalling system through Smad2. Despite the TGF-beta receptor upregulation, synergy between AngII and TGF-beta 1 to boost alpha 3(IV) collagen production was not observed. However, blockade of TGF-beta signalling with SB-431542 prevented AngII from stimulating alpha 3(IV) collagen production. Podocyte expression activity of VEGF. Podocytes were stimulated to secrete VEGF by 10(-10)M or higher AngII after 48h. Blockade of the endogenous VEGF activity by SU5416 prevented AngII-stimulated alpha 3(II) collagen production. Conclusions. AngII stimulates the podocyte to produce alpha 3(IV) collagen protein via mechanisms involving TGF-beta and VEGF signalling. Alterations in alpha 3(IV) collagen production may contribute to GBM thickening and perhaps proteinuria in diabetes.