Role of activating transcription factor 3 on TAp73 stability and apoptosis in paclitaxel-treated cervical cancer cells

Taxol (paclitaxel) is a potent anticancer drug that has been found to be effective against several tumor types, including cervical cancer. However, the exact mechanism underlying the antitumor effects of paclitaxel is poorly understood. Here, paclitaxel induced the apoptosis of cervical cancer HeLa cells and correlated with the enhanced activation of caspase-3 and TAp73, which was strongly inhibited by TAp73 beta small interfering RNA (siRNA). In wild-type activating transcription factor 3 (ATF3)-overexpressed cells, paclitaxel enhanced apoptosis through increased a and 0 isoform expression of TAp73; however, these events were attenuated in cells containing inactive COO H-terminal-deleted ATF3 [ATF3(Delta C)] or ATF3 siRNA. In contrast, paclitaxel-induced ATF3 expression did not change in TAp73 beta-overexpressed or TAp73 beta siRNA-cotransfected cells. Furthermore, paclitaxel-induced ATF3 translocated into the nucleus where TAp73 beta is expressed, but not in ATF3(Delta C) or TAp73 beta siRNA-transfected cells. As confirmed by the GST pull-down assay, ATF3 bound to the DNA-binding domain of p73, resulting in the activation of p2l or Bax transcription, a downstream target of p73. Overexpression of ATF3 prolonged the half-life of TAp73 beta by inhibiting its ubiquitination and thereby enhancing its transactivation and proapoptotic activities. Additionally, ATF3 induced by paclitaxel potentiated the stability of TAp73, not its transcriptional level. Chromatin immunoprecipitation analyses show that TAp73 beta and ATF3 are recruited directly to the p2l and Bax promoter. Collectively, these results reveal that overexpression of ATF3 potentiates paclitaxel-induced apoptosis of HeLa cells, at least in part, by enhancing TAp73 beta's stability and its transcriptional activity. The investigation shows that ATF3 may function as a tumor-inhibiting factor through direct regulatory effects on TAp73 beta, suggesting a functional link between ATF3 and TAp73 beta.