Hepatocyte-specific disruption of Bcl-xL leads to continuous hepatocyte apoptosis and liver fibrotic responses

Background & Aims: Recent research has suggested that apoptosis could be involved in the development of fibrosis, although it is generally considered to be a mechanism of cell removal without consequences to the tissue. Bcl-x(L), an antiapoptotic member of the Bcl-2 family, is expressed in hepatocytes and up-modulated during various pathologic conditions. The aim of this study was to explore the function of Bcl-x(L) in hepatocytes using the Cre-loxP system and to analyze the consequences of long-term apoptosis in hepatocytes. Methods: Hepatocytes isolated from mice homozygous for a floxed bcl-x allele (bcl-x fl/fl) were infected with recombinant adenovirus expressing the Cre recombinase gene (AdexCre). Bcl-x fl/fl mice were crossed with Alb-Cre transgenic mice, which express Cre under regulation of the albumin gene promoter to generate hepatocyte-specific Bcl-x(L)-deficient mice. Results: On AdexCre infection, primary cultured bcl-x fl/fl hepatocytes reduced their expression of Bcl-x(L) and rapidly underwent apoptosis associated with mitochondrial damage. In vivo hepatocyte-specific disruption of Bcl-x(L) resulted in spontaneous apoptosis of hepatocytes for more than 6 months. The Bcl-x(L-) deficient mice showed liver fibrosis with advanced age that was preceded by an increase in hepatic transforming growth factor beta production. In vitro, macrophages and hepatocytes produced transforming growth factor beta on exposure to apoptotic hepatocytes. Conclusions: The present study identified Bcl-x(L) as a critical apoptosis antagonist in hepatocytes. Furthermore, it offers proof that persistent apoptosis of parenchymal cells is sufficient to induce fibrotic responses and suggests a mechanistic link between apoptosis and fibrosis.