Galloflavin suppresses lactate dehydrogenase activity and causes MYC downregulationin Burkitt lymphoma cells via NAD/NADH-dependent inhibition of sirtuin-1

Activation of the myc oncogene in cancer cells up regulates lactate dehydrogenase A (LDH-A) expression, leading to a sustained glycolytic flux that is needed to produce ATP under hypoxic conditions. We studied the effects of galloflavin (GF), a recently identified LDH inhibitor, on myc over-expressing Burkitt lymphoma (BL) cells. EBV infected lymphoblasts were used as a non-neoplastic control. Our results showed that myc over-activation induced a 2- to 7-fold increase in LDH-A expression in BL cells compared to non-neoplastic lymphoblasts; this result is consistent with previously reported data. Moreover, GF treatment suppressed LDH activity and inhibited BL cell replication but did not affect lymphoblast viability. Surprisingly, we found that increased levels of the MYC and LDH-A proteins did not lead to a metabolic shift of BL cells toward glycolytic ATP generation. BL cells were treated with GF at doses that achieved 50% inhibition of cell growth and lactate production, and ATP levels were scarcely affected following GF treatment. The same results were also obtained by suppressing LDH activity with oxamate, a specific LDH inhibitor. Our data suggest that LDH activity is important for maintaining a correct NAD/NADH balance in BL cells. LDH inhibition led to decreased NAD cellular levels, which resulted in sirtuin-1 inhibition. Confirming previous studies, sirtuin-1 inhibition caused a reduction in MYC protein levels, depriving BL cells of their most important survival signal. This study further describes the biological functions of the LDH enzyme and suggests that LDH inhibition could be useful for the treatment of cancer.