Abstract A07: MNT-dependent suppression of branched chain amino acid catabolism: A window into the role of MNT:MAX and MNT:MLX complexes in controlling MYC activity

The MYC family of transcription factors participate in a variety of physiological process and can be oncogenic when dysregulated. A key function of MYC is to reprogram metabolic pathways to meet the biomass needs for both normal the cancer cell proliferation. We previously showed that either too much or too little of the MAX-interacting bHLHZip transcriptional repressor MNT can suppress MYC induced oncogenesis. To begin to investigate the complicated relationship between MYC and MNT, we employed microarray and RNAseq analyses to investigate the gene expression profiles between WT and MNT deficient cells during log phase cell proliferation and during the process of serum stimulated cell cycle entry. Key genes found to be significantly upregulated in MNT deficient cells included ones that promote glycolysis, glutaminolysis, polyamine synthesis, and branched chain amino acid (BCAA) catabolism. Data from metabolomic analysis of MNT-deficient cells was also supportive of elevated glycolysis, glutaminolysis and BCAA catabolism. The high glycolysis and glutaminolysis profiles were similar to MYC-induced metabolic reprograming in tumor cells and T cell activation, but BCAA catabolic activity appeared to be unique to MNT deficient cells. Branched chain α-keto acid dehydrogenase E1α (BCKDH E1α), a subunit of the rate-limit enzyme complex in BCAA catabolism, was found to be one of the most upregulated genes in MNT deficient cells. BCKDH E1α protein was also strongly upregulated in MNT deficient cells, and MNT was found to bind a highly conserved Ebox element in the BCKDH E1αgene and repress its transcription. In contrast to MNT, MYC was found to not regulate BCKDH E1α. Replacing the MNT bHLHZip with the MYC bHLHZip region negated repression and replacing the MYC bHLHZip region with the MNT bHLHZip allowed MYC to activate BCKDH E1α transcription. In contrast to BCKDH E1α, either an engineered Ebox reporter or proximal promoters for specific E-box-containing MYC target genes involved in glycolysis and glutaminolysis that were activated by MYC were repressed by MNT. Protein interaction studies suggest that the differential target gene specificity of MNT compared to MYC is related to the ability of MNT to interact with both MAX and with MLX. Finally, since BCKDH E1α complex was identified as one of the major sources of Reactive Oxygen Species (ROS) in cells and MNT deficient cells have high levels of ROS that sensitizes them to apoptosis (Link et al. PNAS, 2012), we investigated the role of BCKDH E1α in the high ROS in MNT deficient cells and in inhibition of MYC-induced cell transformation. Our findings suggest that elevated BCKDH E1α and BCKDH E1α-induced BCAA catabolism and ROS production significantly contribute to the anti-oncogenic effect of MNT deficiency. Together, our findings suggest that MNT and MYC function as antagonists in some key metabolic pathways, but not in the BCAA catabolism pathway and that the BCAA catabolism pathway may be a potential target for anti-cancer drug development. Citation Format: Guang Yang, Jason M. Link, Peter J. Hurlin, Peter J. Hurlin. MNT-dependent suppression of branched chain amino acid catabolism: A window into the role of MNT:MAX and MNT:MLX complexes in controlling MYC activity. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr A07.