RESUMO
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.
Assuntos
Células Estreladas do Fígado/metabolismo , Hepatite C Crônica/genética , Hepatócitos/metabolismo , Hipóxia/genética , RNA Mensageiro/genética , Receptores Acoplados a Proteínas G/genética , Regulação para Cima , Receptores de Apelina , Western Blotting , Células Cultivadas , Progressão da Doença , Células Estreladas do Fígado/patologia , Hepatite C Crônica/metabolismo , Hepatite C Crônica/patologia , Hepatócitos/patologia , Humanos , Hipóxia/metabolismo , Fígado/metabolismo , Fígado/patologia , Receptores Acoplados a Proteínas G/biossíntese , Reação em Cadeia da Polimerase Via Transcriptase ReversaRESUMO
UNLABELLED: Apelin is a peptide with relevant functions in angiogenesis and inflammation. Recent studies have demonstrated that apelin is overexpressed in hepatic stellate cells (HSCs) of cirrhotic rats. Moreover, patients with cirrhosis show high circulating levels of this peptide. We evaluated the role of endogenous apelin system in fibrogenesis-related gene induction in human HSCs. Messenger expression and immunolocalization of apelin were analyzed in human cirrhotic liver and in control samples. Apelin expression was analyzed in a human HSC line (LX-2) under hypoxic conditions or in the presence of proinflammatory or profibrogenic stimuli. LX-2 cells were stimulated with apelin, and a selected profile of fibrogenesis-related genes was quantified. In vivo inactivation of apelin was analyzed in the liver of fibrotic rats after administrating specific blockers of the molecules triggering apelin induction. Apelin was overexpressed in HSCs from human cirrhotic liver. Neither hypoxia nor proinflammatory substances induced the expression of apelin in LX-2. By contrast, both profibrogenic molecules angiotensin II (AII) and endothelin-1 (ET-1) enhanced apelin expression in these cells. Apelin increased the synthesis of collagen-I and platelet-derived growth factor receptor ß (PDGFRß) in LX-2. AII and ET-1 stimulated collagen-I and PDGFRß expression, and this induction was drastically reduced when apelin receptor was blocked in these cells. In accordance, AII or ET-1 receptor antagonists reduced the hepatic synthesis of apelin, collagen-I, and PDGFRß in fibrotic rats. CONCLUSIONS: apelin mediates some of the fibrogenic effects triggered by AII and ET-1, thus suggesting that apelin could be an important mediator of fibrogenesis in human liver disease.
