EGFR Mutation-Induced Alternative Splicing of Max Contributes to Growth of Glycolytic Tumors in Brain Cancer

Abstract

SUMMARY Alternative splicing contributes to diverse aspects of cancer pathogenesis including altered cellular metabolism, but the specificity of the process or its consequences are not well understood. We characterized genome-wide alternative splicing induced by the activating EGFRvIII mutation in glioblastoma (GBM). EGFRvIII upregulates the heterogeneous nuclear ribonucleoprotein (hnRNP) A1 splicing factor, promoting glycolytic gene expression and conferring significantly shorter survival in patients. HnRNPA1 promotes splicing of a transcript encoding the Myc-interacting partner Max, generating Delta Max, an enhancer of Myc-dependent transformation. Delta Max, but not full length Max, rescues Myc-dependent glycolytic gene expression upon induced EGFRvIII loss, and correlates with hnRNPA1 expression and downstream Myc-dependent gene transcription in patients. Finally, Delta Max is shown to promote glioma cell proliferation in vitro and augment EGFRvIII expressing GBM growth in vivo. These results demonstrate an important role for alternative splicing in GBM and identify Delta Max as a mediator of Myc-dependent tumor cell metabolism.

Compensatory glutamine metabolism promotes glioblastoma resistance to mTOR inhibitor treatment

“…mTORC1 regulates c-MYC activity through splicing of the MYC-interacting protein MAX, and mTORC2 regulates MYC activity by controlling its levels through a FOXO-acetylation-dependent mechanism (20,21). The results presented here underscore the importance of mTOR-mediated regulation of glutamine metabolism by showing that GBM cells adapt to mTOR inhibitors by increasing GLS expression to elevate glutamate levels.…”

“…Glutamine uptake also appears to be critical for lipid synthesis and carbon supply to operate the TCA cycle. We overexpressed the EGFR-activating mutation (EGFRvIII) in the U87 glioma cell line, which has been demonstrated to increase both mTORC1 and mTORC2 signaling (20,21). Using gas chromatography-mass spectroscopy (GC/MS) of U87 and U87/EGFRvIII cells treated with mTOR inhibitors (rapamycin or PP242) for 48 hours, we identified 91 metabolites whose levels significantly changed in response to the allosteric mTOR inhibitor rapamycin or the ATPcompetitive mTOR inhibitor PP242 ( Figure 2 and Supplemental Table 1).…”

An Overview of MYC and Its Interactome

“…3C) (Blackwood and Eisenman 1991;Makela et al 1992). Induction of an alternative splicing factor, following activating EGFR mutation in glioblastoma, generates DMAX, which dimerizes with MYC and augments MYCtransforming activity (Makela et al 1992;Babic et al 2013). The mechanism underlying the elevated MYC activity is unknown but we surmise that the carboxyl terminus of MAX normally permits association of a negative regulatory factor with the heterodimer.…”

EGFR signaling and autophagy dependence for growth, survival, and therapy resistance

“…In relation, gene expression arrays showed upregulation of genes involved in regulation of the cell metabolism, e.g., glucose transporter 1 (GLUT1) and GLUT3, Hexokinase2 (HK2), and pyruvate dehydrogenase kinase (PDK1). 99 In general, EGFRvIII-expressing tumors require upregulation of cell metabolism proteins and require increased glucose uptake to maintain their elevated growth rate. This might explain why these tumors may display increased dependence on autophagy for their energy supply in a tumor microenvironment that is low in glucose or deprived of oxygen.…”