FOXO3a-dependent mechanism of E1A-induced chemosensitization

Gene therapy trials in human breast, ovarian, and head and neck tumors indicate that adenovirus E1A can sensitize cancer cells to the cytotoxic effects of paclitaxel in vitro and in vivo. Resistance to paclitaxel has been reported to occur in cells expressing lowlevels of the Forkhead transcription factor FOXO3a. In this article, wereport that FOXO3a is critical for E1A-mediated chemosensitization to paclitaxel. RNAinterference-mediated knockdown of FOXO3a abolished E1A-induced sensitivity to paclitaxel. Mechanistic investigations indicated that E1A indirectly stabilized FOXO3a by acting at an intermediate step to inhibit a ubiquitin-dependent proteolysis pathway involving the E3 ligase βTrCP and the FOXO3a inhibitory kinase IKKβ. E1Aderepressed this inhibitory pathway by stimulating expression of the protein phosphatase 2A (PP2A)/C protein phosphatases,which by binding to the TGF-β-activated kinase TAK1, inhibited its ability to activate IKKβ and, thereby, to suppress βTrCP-mediated degradation of FOXO3a. Thus, by stimulating PP2A/C expression, E1A triggers a signaling cascade that stabilizes FOXO3a and mediates chemosensitization. Our findings provide a leap forward in understanding paclitaxel chemosensitization by E1A, and offer a mechanistic rational to apply E1A gene therapy as an adjuvant for improving therapeutic outcomes in patients receiving paclitaxel treatment.