Abstract 1419: Oncogenic Myc disrupts NAMPT circadian oscillation in mouse hepatocellular carcinoma cell line

MYC, which is overexpressed in a variety of human cancers, drives aerobic glycolysis, also called the Warburg effect. The Warburg effect results in regeneration of NAD+ from increased conversion of pyruvate to lactate, such that decreased lactate dehydrogenase A (LDHA) activity results in NAD+ depletion and can result in cell death. Nicotinamide phosphoribosyltransferase (NAMPT), a rate-limiting enzyme that is involved in the NAD+ salvage pathway, was reported to be directly upregulated by Myc. Increased NAD+ synthesis thus helps to replenish NAD+ along with LDHA. In this regard, we have previously reported the synergy of inhibiting LDHA and NAMPT in causing lymphoma xenograft regression. Targeting NAMPT by its inhibitor FK866 has been tested in clinical trials. However, the toxicity of FK866 in normal cells is not negligible, with thrombocytopenia being reported. Toxicity, however, could be diminished if therapy is administered at specific times of the day (termed chronotherapy) when normal cells are least dependent on NAMPT, whose levels oscillate every 24 hours. We have previously found that Myc upregulates Rev-erbα, which inhibits and disrupts oscillation of the circadian rhythm gene ARNTL (protein name Bmal1) in the osteosarcoma cell line U2OS. However, although NAMPT was shown oscillating in normal mouse liver and other tissues, how the oscillation of NAMPT is affected in cancer especially under Myc overexpression is still poorly understood. Here we show Myc induces Nampt mRNA and protein in mouse hepatocellular carcinoma (mHCC) cell line that has been engineered tetracycline inducible Myc transgene expression vector. Inhibition of NAMPT by FK866 induces perturbation of circadian gene expression in mHCC cells, including Rev-erbα, Per1 and Cry1. The alteration of circadian gene expression can be rescued by nicotinamide, suggesting that the circadian clock is affected by NAD+/NADH ratio. Interestingly, Nampt mRNA oscillates in mHCC cell lines in the absence of Myc and the oscillation was lost when Myc is induced, suggesting a therapeutic opportunity could exist in the window between circadian regulation of the ebb-and-flow of normal cell NAMPT level and the sustained, non-circadian cancer cell NAMPT level. Our observation that Myc disrupts the oscillation its target gene NAMPT provides a conceptual framework for metabolic chronotherapy that could potentially lead to better cancer treatment strategies that reduce side effects. We thank the following fou