Targeting Mitochondrial Respiratory Complex I: A Novel Anticancer Strategy

Tumor cells exhibit profound bioenergetic changes compared to non-transformed cell types. One of the main driving mechanism leading to such a metabolic alteration is triggered by hypoxia. Hypoxia is experienced by cancer cells during tumor progression and leads to a significant enhancement of glycolysis in order to sustain tumor growth and survival. The transcription factor hypoxia-inducible factor-1 (HIF1) is the master regulator of cell adaptation to low oxygen condition, and thus of tumor progression. Our group described the lack of HIF1α in low-proliferative oncocytic tumors, which are characterized by the loss of the mitochondrial respiratory Complex I (CI). Hence, we hypothesized that severe CI dysfunctions prevent HIF1α stabilization and impair tumor adaptation to low oxygen levels. Using the zing finger nucleases technology, we generated NDUFS3-deficient cancer cells that display acute CI deficiency. Engineered CI-defective cancer cells showed a lack of HIF1α stabilization in hypoxic condition, likely due to higher intracellular oxygen concentration and increased levels of the metabolite α-ketoglutarate, which both foster HIF1α proteasomal degradation. In vivo experiments showed that the lack of CI was associated to an impairment in the growth of the xenografts, together with the lack of HIF1 activation and a significant reduction of the expression of HIF1-responsive genes involved in the glycolytic machinery and tumor vascularization. Further analysis showed that CI-deficient xenografts may have developed a tailored structure to compensate the metabolic restriction caused by the lack of CI and HIF1α. Using a Tet-Off expression system, we thus generated NDUFS3-knock out inducible clones, allowing us to target CI during xenografts growth. Removing NDUFS3 from xenografts have successfully recapitulated key oncocytic features. CI loss induced HIF1α destabilization, the accumulation of defective mitochondria and xenografts growth arrest. Overall, these data showed that cancer cells are unable to sustain proliferation when functional CI is lost during tumor progression.