Waves of gene regulation suppress and then restore oxidative phosphorylation in cancer cells

Abstract

We posit the following hypothesis: Independently of whether malignant tumors are initiated by a fundamental reprogramming of gene expression or seeded by stem cells, "waves" of gene expression that promote metabolic changes occur during carcinogenesis, beginning with oncogene-mediated changes, followed by hypoxia-induced factor (HIF)-mediated gene expression, both resulting in the highly glycolytic "Warburg" phenotype and suppression of mitochondrial biogenesis. Because high proliferation rates in malignancies cause aglycemia and nutrient shortage, the third (second oncogene) "wave" of adaptation stimulates glutaminolysis, which in certain cases partially re-establishes oxidative phosphorylation; this involves the LKB1-AMPK-p53, PI3K-Akt-mTOR axes and MYC dysregulation. Oxidative glutaminolysis serves as an alternative pathway compensating for cellular ATP. Together with anoxic glutaminolysis it provides pyruvate, lactate, and the NADPH pool (alternatively to pentose phosphate pathway). Retrograde signaling from revitalized mitochondria might constitute the fourth "wave" of gene reprogramming. In turn, upon reversal of the two Krebs cycle enzymes, glutaminolysis may partially (transiently) function even during anoxia, thereby further promoting malignancy. The history of the carcinogenic process within each malignant tumor determines the final metabolic phenotype of the selected surviving cells, resulting in distinct cancer bioenergetic phenotypes ranging from the highly glycolytic "classic Warburg" to partial or enhanced oxidative phosphorylation. We discuss the bioenergetically relevant functions of oncogenes, the involvement of mitochondrial biogenesis/degradation in carcinogenesis, the yet unexplained Crabtree effect of instant glucose blockade of respiration, and metabolic signaling stemming from the accumulation of succinate, fumarate, pyruvate, lactate, and oxoglutarate by interfering with prolyl hydroxylase domain enzyme-mediated hydroxylation of HIFα prolines.