PS2 - 194 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 tumour glioblastoma (GBM). It has been previously shown that epidermal growth factor receptors (EGFR) up-regulate Mcl-1 expression 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 over-expressed in glioma cells, it induces cell death. The reasons for this discrepancy are unclear. METHODS: Using malignant glioma cell lines +/- hypoxia, gain and/or loss of function assays of BNIP3 or Mcl-1 were performed. BNIP3 and MCL-1 expression was assessed in GBM tumours from adult patients and human gliomas grown as xenografts in immunocompromised mice. RESULTS: 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 augmented 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, GBM tumours and in xenograft glioma tumours expressing mutant EGFR (EGFRvIII). CONCLUSION: These results support that Mcl-1 can block the ability of BNIP3 to induce cell death under hypoxia in GBM tumours