HARPΔ111-136 enhances radiation-induced apoptosis of U87MG glioblastoma by induction of the proapoptotic protein CHOP.

We previously demonstrated, using the glioblastoma cell line U87MG as an experimental model, that the adenoviral mediated overexpression of the truncated protein HARPΔ111-136 inhibits the proliferation of these cells in vitro as well as tumor growth and angiogenesis in vivo. This study focused on identifying the underlying mechanisms for the observed antitumoral effect. The present study demonstrated that HARPΔ111-136 induced the ATF4/ATF3/CHOP cascade resulting in a strong expression of the proapoptotic protein CHOP, leading to tumor cell apoptosis as demonstrated by PARP cleavage and FACS analysis. siRNA-mediated CHOP gene silencing abolished Ad-HARPΔ111-136 induced apoptosis. Moreover, Ad-HARPΔ111-136 increased the expression of the death receptor DR5 and enhanced U87MG cells sensitivity in vitro to TRAIL a DR5 ligand with subsequent activation of caspase 8. Infection of U87MG cells with Ad-HARPΔ111-136 also enhanced radiation-induced apoptosis. In vivo, the combination of Ad-HARPΔ111-136 and radiation therapy resulted in a striking inhibition (92%) of the growth of U87MG xenografts, resulting from the potent effect on tumor angiogenesis and tumor cell apoptosis as determined by TUNEL analysis. Taken together, our results indicated that the inhibitor HARPΔ111-136 sensitized U87MG cells to apoptosis.