C/EBPβ-LIP dually activates glycolysis and the malate aspartate shuttle to maintain NADH/NAD+ homeostasis

Oncogene-induced metabolic reprograming supports cell growth and proliferation. Yet, it also links cancer cell survival to certain metabolic pathways and nutrients. In order to synthesise amino acids and nucleotides de novo for growth and proliferation, cancer cells depend on glycolysis, the cytoplasmic oxidation of glucose, which generates necessary metabolic intermediates and ATP. During glycolysis, NAD+ is used as the oxidizing agent and is thereby reduced into NADH. To ensure high glycolysis rates and maintain NADH/NAD+ homeostasis, cytoplasmic NAD+ has to be regenerated. The mitochondria are the major sites of NADH reoxidation into NAD+ where NADH-derived electrons enter the electron transport chain for ATP production. Since NADH/NAD+ cannot cross membranes, the malate-aspartate shuttle (MAS) or the glycerol-3-phosphate shuttle (GPS) are used as intermediate electron carriers. In addition, cytoplasmic NAD+ is generated by NADH-electron transfer to pyruvate, reducing it to lactate (the Warburg effect). NADH/NAD+ homeostasis plays a pivotal role in cancer cell survival, but our knowledge about the involved regulatory mechanisms is still limited. Here, we show that the proto-oncogenic transcription factor C/EBP{beta}-LIP stimulates both glycolysis and the MAS. Inhibition of glycolysis with ongoing C/EBP{beta}-LIP-induced MAS activity results in NADH depletion and apoptosis that can be rescued by inhibiting either the MAS or other NADH-consuming processes. Therefore, beyond the discovery of C/EBP{beta}-LIP as a dual activator of glycolysis and the MAS, this study indicates that simultaneous inhibition of glycolysis and lowering of the NADH/NAD+ ratio may be considered to treat cancer.