Hypoxia and proinflammatory factors upregulate apelin receptor expression in human stellate cells and hepatocytes.

BACKGROUND: The activation of the apelin receptor (APJ) plays a major role in both angiogenic and fibrogenic response to chronic liver injury. However, the mechanisms that govern the induction of APJ expression have not been clarified so far.
METHODS: The regulation and the role of APJ in cultured human liver cells were investigated. Tissular expression of APJ and α-smooth muscle actin was analysed by immunocolocalisation in human cirrhotic liver and in control samples. mRNA and protein expression of APJ were analysed in two cell lines, LX-2 (as hepatic stellate cells, HSCs) and HepG2 (as hepatocytes), under hypoxic conditions or after exposure to proinflammatory or profibrogenic factors. Additionally, both hepatic cell lines were stimulated with apelin to assess cell survival and the expression of angiogenic factors.
RESULTS: The APJ-positive signal was negligible in control livers. In contrast, APJ was highly expressed in HSCs and slightly expressed in hepatocytes of human cirrhotic liver. Sustained hypoxia and lipopolysaccharide stimulated the expression of APJ in LX-2 cells. Moreover, hypoxia, tumour necrosis factor α and angiotensin II induced the expression of APJ in HepG2 cells. Activation of APJ stimulated angiopoietin-1 expression and cell survival in LX-2 cells and, in turn, triggered the synthesis of vascular endothelial growth factor type A and platelet-derived growth factor-BB in HepG2 cells.
CONCLUSIONS: These results suggest that hypoxia and inflammatory factors could play a major role in the activation of the hepatic apelin system leading to angiogenic and fibroproliferative response occurring in chronic liver disease.