The ability to switch energy production from oxidative phosphorylation to glycolysis in the presence of oxygen, the Warburg effect, has been observed in many tumours and is thought to represent a major biochemical switch associated with malignant transformation. Cells devoid of mitochondrial DNA that are deficient in aerobic respiration (ρo cells) have been used in our laboratory to model glycolytic switching. In culture, these cells are characterized by reduced growth, increased NADH levels, increased survival under hypoxic conditions, reduced reactive oxygen species (ROS) production and drug resistance in vitro. Here we investigate the effect of mitochondrial DNA (mtDNA) deletion on tumour progression and malignant transformation in vivo.
B16ρo melanoma cells were generated and characterised, and injected either subcutaneously or intravenously into syngeneic C57BL/6 or NOD/scid mice. When injected subcutaneously into C57BL/6 mice, B16ρo tumours showed delayed growth and grew at a slower rate than tumours from parental B16 cells. Following intravenous injection, B16ρo cells failed to form lung tumors even after a prolonged period of observation (65 days). Similar results were observed when cells were injected into immunocompromised NOD/scid mice except that B16ρo tumour growth was further delayed. Interestingly, B16ρo cells, showed distinct changes in tumorigenic phenotype in vitro, including loss of ability to grow as anchorage-independent colonies in soft agar and as spheres in serum-free stem cell medium. Similar in vitro results were obtained with human LN18ρo glioblastoma cells but parental LN18 cells were poorly tumourigenic in NOD/scid mice precluding in vivo analysis.
These findings indicate that although glycolytic metabolism may be a common characteristic of tumor cells, imposing glycolytic metabolism on B16 melanoma cells slows optimum growth and compromises their ability to seed in the lung and form tumors. Whether failure to form lung tumours relates to altered expression of adhesion molecules is presently under investigation.