Inhibition of eIF2α dephosphorylation enhances TRAIL-induced apoptosis in hepatoma cells

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is an inducer of cancer cell death that holds promise in cancer therapy. Cancer cells are more susceptible than normal cells to the cell-death-inducing effects of TRAIL. However, a variety of cancer cells are resistant to TRAIL through complex mechanisms. Here, we investigate the effects of inhibition of eukaryotic initiation factor 2 subunit a (eIF2 alpha) dephosphorylation on TRAIL-induced apoptosis in hepatoma cells. Treatment of hepatoma cells with salubrinal, an inhibitor of eIF2 alpha dephosphorylation, enhances TRAIL-induced eIF2 alpha phosphorylation, CCAAT/enhancer-binding protein homologous protein (CHOP) expression and caspase activation. Salubrinal enhances TRAIL-induced apoptosis, which could be abrogated by caspase inhibitor. Overexpression of phosphomimetic eIF2 alpha (S51D) enhances TRAILinduced CHOP expression, caspase 7 and PARP cleavage and apoptosis. By contrast, overexpression of phosphodeficient eIF2 alpha (S51A) abrogates the stimulation of TRAIL-induced apoptosis by salubrinal. Moreover, knockdown of growth arrest and DNA damage-inducible protein 34 (GADD34), which recruits protein phosphatase 1 to dephosphorylate eIF2 alpha, enhances TRAILinduced eIF2a phosphorylation, CHOP expression, caspase activation and apoptosis. Furthermore, the sensitization of hepatoma cells to TRAIL by salubrinal is dependent on CHOP. Knockdown of CHOP abrogates the stimulation of TRAIL-induced caspase activation and apoptosis by salubrinal. Combination of salubrinal and TRAIL leads to increased expression of Bim, a CHOP-regulated proapoptotic protein. Bim knockdown blunts the stimulatory effect of salubrinal on TRAIL-induced apoptosis. Collectively, these findings suggest that inhibition of eIF2 alpha dephosphorylation may lead to synthetic lethality in TRAIL-treated hepatoma cells.