Hypoxia inducible factor-1α mediates the profibrotic effect of albumin in renal tubular cells

Proteinuria is closely associated with the progression of chronic kidney diseases (CKD) by producing renal tubulointerstitial fibrosis. Over-activation of hypoxia inducible factor (HIF)-1 alpha has been implicated in the progression of CKD. The present study tested the hypothesis that HIF-1 alpha mediates albumin-induced profibrotic effect in cultured renal proximal tubular cells. Incubation of the cells with albumin (40 mu g/ml) for 72 hrs significantly increased the protein levels of HIF-1 alpha, tissue inhibitor of metalloproteinase (TIMP)-1 and collagen-I, which were blocked by HIF-1 alpha shRNA. Albumin also stimulated an epithelial-mesenchymal transition (EMT) as indicated by the decrease in epithelial marker E-cadherin, and the increase in mesenchymal markers alpha-smooth muscle actin and fibroblast-specific protein 1. HIF-1 alpha shRNA blocked albumin-induced changes in these EMT markers as well. Furthermore, albumin reduced the level of hydroxylated HIF-1 alpha, indicating an inhibition of the activity of prolyl-hydroxylases, enzymes promoting the degradation of HIF-1 alpha. An anti-oxidant ascorbate reversed albumin-induced inhibition of prolyl-hydroxylase activity. Overexpression of prolyl-hydroxylase 2 (PHD2) transgene, a predominant isoform of PHDs in renal tubules, to reduce HIF-1 alpha level significantly attenuated albumin-induced increases in TIMP-1 and collagen-I levels. These results suggest that albumin-induced oxidative stress inhibits PHD activity to accumulate HIF-1 alpha, which mediates albumin-induced profibrotic effects in renal tubular cells.