Inhibition of fatty acid oxidation by etomoxir impairs NADPH production and increases reactive oxygen species resulting in ATP depletion and cell death in human glioblastoma cells.

Normal differentiated cells rely primarily on mitochondrial oxidative phosphorylation to produce adenosine triphosphate (ATP) to maintain their viability and functions by using three major bioenergetic fuels: glucose, glutamine and fatty acids. Many cancer cells, however, rely on aerobic glycolysis for their growth and survival, and recent studies indicate that some cancer cells depend on glutamine as well. This altered metabolism in cancers occurs through oncogene activation or loss of tumor suppressor genes in multiple signaling pathways, including the phosphoinositide 3-kinase and Myc pathways. Relatively little is known, however, about the role of fatty acids as a bioenergetic fuel in growth and survival of cancer cells. Here, we report that human glioblastoma SF188 cells oxidize fatty acids and that inhibition of fatty acid ß-oxidation by etomoxir, a carnitine palmitoyltransferase 1 inhibitor, markedly reduces cellular ATP levels and viability. We also found that inhibition of fatty acid oxidation controls the NADPH level. In the presence of reactive oxygen species scavenger tiron, however, ATP depletion is prevented without restoring fatty acid oxidation. This suggests that oxidative stress may lead to bioenergetic failure and cell death. Our work provides evidence that mitochondrial fatty acid oxidation may provide NADPH for defense against oxidative stress and prevent ATP loss and cell death.

Increased OXPHOS activity precedes rise in glycolytic rate in H-RasV12/E1A transformed fibroblasts that develop a Warburg phenotype.

BACKGROUND: The Warburg phenotype in cancer cells has been long recognized, but there is still limited insight in the consecutive metabolic alterations that characterize its establishment. We obtained better understanding of the coupling between metabolism and malignant transformation by studying mouse embryonic fibroblast-derived cells with loss-of-senescence or H-RasV12/E1A-transformed phenotypes at different stages of oncogenic progression. RESULTS: Spontaneous immortalization or induction of senescence-bypass had only marginal effects on metabolic profiles and viability. In contrast, H-RasV12/E1A transformation initially caused a steep increase in oxygen consumption and superoxide production, accompanied by massive cell death. During prolonged culture in vitro, cell growth rate increased gradually, along with tumor forming potential in in vitro anchorage-independent growth assays and in vivo tumor formation assays in immuno-deficient mice. Notably, glucose-to-lactic acid flux increased with passage number, while cellular oxygen consumption decreased. This conversion in metabolic properties was associated with a change in mitochondrial NAD+/NADH redox, indicative of decreased mitochondrial tricarboxic acid cycle and OXPHOS activity. CONCLUSION: The high rate of oxidative metabolism in newly transformed cells is in marked contrast with the high glycolytic rate in cells in the later tumor stage. In our experimental system, with cells growing under ambient oxygen conditions in nutrient-rich media, the shift towards this Warburg phenotype occurred as a step-wise adaptation process associated with augmented tumorigenic capacity and improved survival characteristics of the transformed cells. We hypothesize that early-transformed cells, which potentially serve as founders for new tumor masses may escape therapies aimed at metabolic inhibition of tumors with a fully developed Warburg phenotype.