A Regulatory Role Of Wnt/β-catenin Signaling In Apoptosis Of Lung Epithelial Cells Induced By BCG

Mycobacterium tuberculosis (MTB) is an airborne pathogen that causes tuberculosis (TB) the second leading infectious death worldwide after the HIV infection, and a co-infection of MTB and HIV even causes more deaths than an infection of either of these pathogens. At least there are two major concerns need to be facing to the emerging of global MTB infection, (1) increasing incidence of co-infections of HIV and MTB, and (2) increasing emergence of multidrug resistant MTB, which have led a difficulty for the prevention of MTB infection. The innate immunity and adaptive immunity of host are two consistent steps for protection from an MTB infection, in which macrophages are recognized as primary immune cells in response to MTB infection. Increasing numbers of studies revealed that alveolar epithelial cells also play a key role during the pathogenesis of TB after an MTB infection. There are at least three consequences may occur when a target cell is infected by MTB, survive, apoptotic death and necrotic death. Apoptosis and necrosis are two distinct programmed cell deaths, of which an apoptosis leads a cell death with an intact cytomembrane and promotes MTB degraded in its host cells; in contrast, a necrotic cell death leads the lysis of infected cells, release of pathogens from their host cells that infect new target cell. Wnt signaling is a widespread pathway from deuterostome to mammal, it affects the proliferation, differentiation, migration, apoptosis, even the immunity plication of cells. An increasing numbers of evidence recently suggested that Wnt signaling pathway, including the canonical Wnt and noncanonical Wnt signaling pathways have an impact on cell apoptosis through various mechanisms. In order to investigate the underlying mechanism of Wnt signaling in pathogen-induced apoptotic cell death, the interaction between canonical Wnt signaling pathway and apoptosis signaling pathways was explored by activation or inhibition of signaling pathways in A549 alveolar epithelial cells Mycobacterium bovis Bacillus Calmette-Guerin (BCG) strain. These results may lay a foundation for understanding a mechanism by which the Wnt signaling regulates cell apoptosis upon pathogen infections.By flow cytometry and Western blotting, we found that Wnt/β-catenin signaling regulates the BCG induced expression of Wnt target genes cyclinDl and c-myc, reduces the BCG-induced cell apoptosis and ROS production. The alterations of the expression of apoptosis inducing factor (AIF) and proapoptotic protein BAX were in line with the Wnt/β-catenin signaling pathways, irrespective of an addition of Wnt/β-catenin signaling pathway inhibitor XAV939 or broad-spectrum caspase inhibitor Z-vad. In contrast, there was no obvious changes of caspase 3 protein expression was found during the entire process. In addition to further confirm our previous finding that BCG can inhibit the canonical Wnt signaling in alveolar epithelial cells, the major findings including (1) the BCG-induced apoptosis of alveolar epithelial cells can be either inhibited or induced by an activation of canonical Wnt signaling; (2) the inhibition of BCG-induced apoptosis in alveolar epithelial cells mediated by Wnt/β-catenin signaling is through a caspase independent pathway; and (3) the inhibitory role of Wnt/β-catenin signaling in BCG-induced apoptosis of alveolar epithelial cells is controlled by apoptosis inducing factor (AIF)-BAX signaling. Therefore, several conclusions can be drawn from this study:(1) a balance between the AIF and caspase signaling pathways may contribute to maintenance of the homeostasis mitochondrial transmembrane potential in alveolar epithelial cells upon BCG infection; and (2) in addition to the regulation of AIF/BAX signaling pathway, the canonical Wnt signaling may regulate the BCG-induced alveolar epithelial cell apoptosis through other undefined signaling pathways.