| The glycolytic pathway is one of the most ancient cellular metabolic processes and has been highly conserved throughout evolution. In the presence of oxygen, pyruvate generated from glucose by glycolysis, is metabolized through the TCA cycle to produce NADH and FADH2, which then donate high-energy electrons to the electron transfer chain. In hypoxia, pyruvate derived from glycolysis is instead converted to lactate, which is termed the Pasteur effect, and has been considered as a key metabolic mechanism of cellular adaptation to hypoxia. Hypoxia-induced glycolysis requires transcriptional activation through hypoxia inducible factor (HIF) of genes encoding glucose transporters and glycolytic enzymes. In contrast to normal cells, cancer cells convert glucose to lactic acid even in the presence of oxygen, termed the Warburg effect. This aberrantly enhanced aerobic glycolysis was observed almost eight decades ago and has been repeatedly verified for many cancers, yet the exact molecular mechanisms underlying the Warburg effect remain to be fully understood. Autonomous oncogenic activation of Akt and MYC has been proposed to contribute to aerobic tumor glycolysis. MYC is one of the classic oncogenic transcription factors. Deregulated expression of, or mutations in, MYC has been implicated in the development of many types of human tumors. Since MYC exerts its biological functions by regulating the transcription of its target genes, delineation of a regulatory network of MYC and its target glycolytic enzyme genes is essential to our understanding the underlying mechanisms of MYC-induced aerobic IT tumor glycolysis. Using phylogenetic footprinting analysis and chromatin immunoprecipitation (ChIP), we comprehensively dissected the transcriptional regulation of glycolytic enzyme genes through MYC in the human Burkitt's lymphoma cell line, P493-6. Phylogenetic footprinting analysis identified conserved canonical E-boxes in ENO1, HK2 and LDHA. MYC bound these conserved regions and induced expression of these genes. We also determined whether Myc could bind non-conserved canonical E-boxes found in the remaining human glycolytic genes. MYC bound PFKM, but it did not significantly bind GPI, PGK1 and PKM2. Binding to BPGM, PGAM2 and PKLR was absent. Both GAPD and TPI1 do not have conserved E-boxes, but are induced and bound by Myc through regions with non-canonical E-boxes. Since HIF-1 also promotes glycolysis through transactivation of glycolytic enzyme genes during hypoxia, and its DNA binding site (5'-RCGTG-3') is similar to the MYC binding site (5'-CACGTG-3'), we attempted to determine if HIF-1 cooperates with MYC activation to allow cells to further enhance cellular glycolysis in response to hypoxic microenvironments. (Abstract shortened by UMI.)... |
