Mitochondrial p53 phosphorylation induces Bak-mediated and caspase-independent cell death

// Jinjing Wang 1, * , Wenhao Guo 1, * , Hang Zhou 2, * , Na Luo 3 , Chunlai Nie 1 , Xinyu Zhao 1 , Zhu Yuan 1 , Xinyu Liu 1 , Yuquan Wei 1 1 Department of Abdominal Oncology, State Key Laboratory of Biotherapy and Cancer Center, West China Hospital, Sichuan University and Collaborative Innovation Center for Biotherapy, Chengdu 610041, P. R. China 2 Department of Chemotherapy, Sichuan Cancer Hospital, Chengdu, Sichuan 610041, P. R. China 3 Nankai University School of Medicine/Collaborative Innovation Center of Biotherapy, Tianjin 300071, P. R. China * These authors have contributed equally to this work Correspondence to: Chunlai Nie, e-mail: niecl1022@hotmail.com Xinyu Liu, e-mail: xinyuliu20142014@163.com Keywords: Akt, Bak, p53, caspase-independent Received: January 19, 2015      Accepted: April 10, 2015      Published: April 23, 2015 ABSTRACT Chemoresistance in cancer has previously been attributed to gene mutations or deficiency. Caspase mutations or Bax deficiency can lead to resistance to cancer drugs. We recently demonstrated that Bak initiates a caspase/Bax-independent cell death pathway. We show that Plumbagin (PL) (5-hydroxy-2-methyl-1,4-napthoquinone), a medicinal plant-derived naphthoquinone that is known to have anti-tumor activity in a variety of models, induces caspase-independent cell death in HCT116 Bax knockout (KO) or MCF-7 Bax knockdown (KD) cells that express wild-type (WT) Bak. The re-expression of Bax in HCT116 Bax KO cells fails to enhance the PL-induced cell death. Additionally, Bak knockdown by shRNA efficiently attenuates PL-induced cell death. These results suggest that PL-induced cell death depends primarily on Bak, not Bax, in these cells. Further experimentation demonstrated that p53 Ser15 phosphorylation and mitochondrial translocation mediated Bak activation and subsequent cell death. Knockdown of p53 or a p53 Ser15 mutant significantly inhibited p53 mitochondrial translocation and cell death. Furthermore, we found that Akt mediated p53 phosphorylation and the subsequent mitochondrial accumulation. Taken together, our data elaborate the role of Bak in caspase/Bax-independent cell death and suggest that PL may be an effective agent for overcoming chemoresistance in cancer cells with dysfunctional caspases.