14-3-3ε Mediates the Cell Fate Decision-Making Pathways in Response of Hepatocellular Carcinoma to Bleomycin-Induced DNA Damage

Background: Lack of understanding of the response of hepatocellular carcinoma (HCC) to anticancer drugs causes the high mortality of HCC patients. Bleomycin (BLM) that induces DNA damage is clinically used for cancer therapy, while the mechanism underlying BLM-induced DNA damage response (DDR) in HCC cells remains ambiguous. Given that 14-3-3 proteins are broadly involved in regulation of diverse biological processes (BPs)/pathways, we investigate how a 14-3-3 isoform coordinates particular BPs/pathways in BLM-induced DDR in HCC. Methodology/Principal Findings: Using dual-tagging quantitative proteomic approach, we dissected the 14-3-3 epsilon interactome formed during BLM-induced DDR, which revealed that 14-3-3 epsilon via its associations with multiple pathway-specific proteins coordinates multiple pathways including chromosome remodeling, DNA/RNA binding/processing, DNA repair, protein ubiquitination/degradation, cell cycle arrest, signal transduction and apoptosis. Further, "zoom-in" investigation of the 14-3-3 epsilon interacting network indicated that the BLM-induced interaction between 14-3-3 epsilon and a MAP kinase TAK1 plays a critical role in determining cell propensity of apoptosis. Functional characterization of this interaction further revealed that BLM triggers site-specific phosphorylations in the kinase domain of TAK1. These BLM-induced changes of phosphorylations directly correlate to the strength of the TAK1 binding to 14-3-3 epsilon, which govern the phosphorylation-dependent TAK1 activation. The enhanced 14-3-3 epsilon-TAK1 association then inhibits the anti-apoptotic activity of TAK1, which ultimately promotes BLM-induced apoptosis in HCC cells. In a data-dependent manner, we then derived a mechanistic model where 14-3-3 epsilon plays the pivotal role in integrating diverse biological pathways for cellular DDR to BLM in HCC. Conclusions: Our data demonstrated on a systems view that 14-3-3 epsilon coordinates multiple biological pathways involved in BLM-induced DDR in HCC cells. Specifically, 14-3-3 epsilon associates with TAK1 in a phosphorylation-dependent manner to determine the cell fate of BLM-treated HCC cells. Not only individual proteins but also those critical links in the network could be the potential targets for BLM-mediated therapeutic intervention of HCC.