Abstract
Chemoresistance in cancer has previously been attributed to gene mutations or deficiencies. Bax or p53 deficiency can lead to resistance to cancer drugs. We aimed to find an agent to overcome chemoresistance induced by Bax or p53 deficiency. Here, we used immunoblot, flow-cytometry analysis, gene interference, etc. to show that genistein, a major component of isoflavone that is known to have anti-tumor activities in a variety of models, induces Bax/p53-independent cell death in HCT116 Bax knockout (KO), HCT116 p53 KO, DU145 Bax KO, or DU145 p53 KO cells that express wild-type (WT) Bak. Bak knockdown (KD) only partially attenuated genistein-induced apoptosis. Further results indicated that the release of AIF and endoG also contributes to genistein-induced cell death, which is independent of Bak activation. Conversely, AIF and endoG knockdown had little effect on Bak activation. Knockdown of either AIF or endoG alone could not efficiently inhibit apoptosis in cells treated with genistein, whereas an AIF, endoG, and Bak triple knockdown almost completely attenuated apoptosis. Next, we found that the Akt-Bid pathway mediates Bak-induced caspase-dependent and AIF- and endoG-induced caspase-independent cell death. Moreover, downstream caspase-3 could enhance the release of AIF and endoG as well as Bak activation via a positive feedback loop. Taken together, our data elaborate the detailed mechanisms of genistein in Bax/p53-independent apoptosis and indicate that caspase-3-enhanced Bid activation initiates the cell death pathway. Our results also suggest that genistein may be an effective agent for overcoming chemoresistance in cancers with dysfunctional Bax and p53.
Highlights
Numerous studies have demonstrated a critical role for Bax in determining tumor cell sensitivity to drug induction and in tumor development
The apoptosis-inducing factor (AIF) and endonuclease G (endoG) double knockdown substantially decreased apoptosis (Figures 4d and e). These results indicate that the release of mitochondrial AIF and endoG is necessary for genistein-induced Bax/p53-independent cell apoptosis
Our study revealed that Bak could induce the release of Cyt c from the mitochondria to the cytosol; we detected whether Bak induced the release of AIF and endoG during apoptosis
Summary
Numerous studies have demonstrated a critical role for Bax in determining tumor cell sensitivity to drug induction and in tumor development. Few studies have focused on the role of Bak in tumor cell apoptosis and cancer development. Bak mutations have only been shown in gastric and colon cancer cells.[32] Some studies have revealed that Bak is a determinant of cancer cell apoptosis.[33,34] Some studies have even demonstrated that Bak renders Bax KO cells sensitive to drug induction.[33,35] In this study, we are the first group to show that tBid induces Bak activation and the release of AIF and endoG in colon cancer cells, which causes cellular apoptosis independent of Bax/p53. We reveal that Akt upregulates apoptosis progression. Our experiments may provide a theoretical basis for overcoming chemoresistance in cancer cells
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