Bcl-2 family member Mcl-1 expression is reduced under hypoxia by the E3 ligase FBW7 contributing to BNIP3 induced cell death in glioma cells

Abstract

Mcl-1 is an anti-apoptotic Bcl-2 family member that is often over-expressed in the malignant brain tumor glioblastoma (GBM). It has been previously shown that epidermal growth factor receptors up-regulate Mcl-1 contributing to a cell survival response. Hypoxia is a poor prognostic marker in glioblastoma despite the fact that hypoxic regions have areas of necrosis. Hypoxic regions of GBM also highly express the pro-cell death Bcl-2 family member BNIP3, yet when BNIP3 is overexpressed in glioma cells, it induces cell death. The reasons for this discrepancy are unclear. Herein we have found that Mcl-1 expression is reduced under hypoxia due to degradation by the E3 ligase FBW7 leading to increased hypoxia induced cell death. This cell death is reduced by EGFR activation leading to increased Mcl-1 expression under hypoxia. Conversely, BNIP3 is over-expressed in hypoxia at times when Mcl-1 expression is decreased. Knocking down BNIP3 expression reduces hypoxia cell death and Mcl-1 expression effectively blocks BNIP3 induced cell death. Of significance, BNIP3 and Mcl-1 are co-localized under hypoxia in glioma cells. These results suggest that Mcl-1 can block the ability of BNIP3 to induce cell death under hypoxia in GBM tumors.

Keywords:

• BNIP3
• EGFR
• glioma cells
• hypoxia and
• Mcl-1

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

We would like to thank Niki Vegh for injecting the mice in the xenograft study. The BNIP3-null mouse model was kindly provided by Dr. Gerald Dorn (Washington University School of Medicine). Epsita Shome was funded by an Alberta Cancer Foundation summer studentship. This study is supported by the CancerCare Manitoba Foundation and the Brain Tumor Foundation of Canada. Dr. Spencer Gibson is the Margaret A. Sellers Chair in Cell Biology.

Supplemental Material

Supplemental data for this article can be accessed on the publisher's website.

Authors' Contributions

YC carried out the analysis of Mcl-1 and BNIP3 in glioma cell lines by Western blot, immunofluorescence and cell death assays and drafted a portion of the manuscript. TB conducted experiments in (Fig. 1). EH participated in the experiments requiring confocal microscopy. MA participated in the animal experiments and immunohistochemistry and western blotting of isolated tissue (Fig. 6). WX conducted co-localization experiments in (Fig. 9). MQ transfected BNIP3 and Mcl-1 in cell lines and conducted cell death and ROS production experiments. ES and DE conducted the GBM tumor histochemical staining and analysis. SG conceived of the study, wrote the manuscript and participated in its design and coordination. All authors read and approved the final manuscript.