Hypoxia-induced VEGF and collagen I expressions are associated with angiogenesis and fibrogenesis in experimental cirrhosis

TLDR

Cirrhosis consists of hepatocyte nodules surrounded by highly vascularized fibrous tissue. The authors aimed to determine whether hepatocellular hypoxia occurs in diethylnitrosamine‑induced cirrhosis and whether it directly regulates VEGF, VEGF receptors, and type I/IV collagen expression in hepatic stellate cells and VEGF expression in hepatocytes. They employed a DEN‑induced cirrhosis rat model and exposed activated hepatic stellate cells and hepatocytes to hypoxia in vitro to measure changes in VEGF, VEGF receptor, and collagen expression. The results showed that in DEN‑treated rats, fibrosis progression is accompanied by hepatocellular hypoxia and angiogenesis, with VEGF and Flt‑1 levels rising and correlating with microvessel density; in vitro, hypoxia induced VEGF, Flt‑1, and type I collagen in stellate cells and VEGF in hepatocytes, and type I collagen induction can occur independently of TGF‑β1, indicating that hypoxia directly promotes fibrogenesis alongside angiogenesis.

Abstract

Cirrhosis consists of hepatocyte nodules surrounded by a highly vascularized fibrous tissue. We previously showed that the development of biliary cirrhosis in the rat is associated with the occurrence of hepatocellular hypoxia and the induction of hepatic angiogenesis. We herein examined the occurrence of hypoxia in an experimental model of diethylnitrosamine (DEN)-induced cirrhosis. We also determined whether hypoxia directly affects the expression of vascular endothelial growth factor (VEGF), of VEGF receptors (Flt-1, Flk-1), and of type I and type IV collagens in activated hepatic stellate cells (HSCs) and the expression of VEGF in hepatocytes. Our results show that in DEN-treated rats, although the progression of liver fibrosis is associated with hepatocellular hypoxia and angiogenesis, VEGF and Flt-1 expressions in the liver are increased and correlated with the density of microvessels. In vitro, hypoxia induces the expression of VEGF, Flt-1, and type I collagen in activated HSCs and that of VEGF in hepatocytes. In addition, we show that hypoxia-induced type I collagen expression in HSCs may occur independently of transforming growth factor beta1 (TGF-beta1) overexpression. In conclusion, the present study provides further evidence that hepatocellular hypoxia and angiogenesis progress together with fibrogenesis after liver injury and that hypoxia directly contributes to the progression of liver fibrosis.