Fine metabolism reprogramming of citrate in cancer cells

Citrate is the first intermediate of the tricarboxylic cycle in the mitochondria and is also a key metabolic regulator for glycolysis, gluconeogenesis and fatty acid synthesis. Within the cytosol, citrate is cleaved by ATP citrate lyase (ACLY) into oxaloacetate (OAA) and acetyl-CoA, which is the precursor of some processes, including the synthesis of fatty acids and histones acetylation, required to sustain the rapid division and the epigenetic metabolic reprogramming of cancer cells. We recently evaluated the effect of different concentrations of citrate in hepatoma cells (HepG2) and in non-tumor immortalized human hepatocyte (IHH). Our results suggested a complex effect of exogenous citrate: at low concentration both lipid deposition and Histone H4 acetylation increased, while they decreased at high concentration. In the same condition, ACLY expression decreased in HepG2 cells, while it remained unchanged in IHH cells, thus suggesting that cancer cells, through the epigenetic regulation of ACLY, are able to adapt their metabolism for specific cellular requirements. We proposed a different fate for exogenous citrate, compared to that deriving from the TCA: considering the strong demand for acetyl-CoA but not for OAA, citrate acts as a Trojan horse for cancer cells releasing OAA in the cytoplasm, which can be only removed after reduction to malate by NADH produced during the glycolisys1. We performed further experiment to prove that hypothesis. What would it happen if NADH was not the limiting factor for the removal of oxaloacetate from the cytoplasm? To answer this question we repeated experiment in presence of inhibitors of lactate dehydrogenase, which consumes NADH to reduce pyruvate deriving from glycolysis. Results partially confirm what stated, and suggest a fine biochemical regulation of metabolism in cancer cell