Objectives: Glucose uptake, glycolysis and respiration are regulated in cancer cells in a coordinated fashion that allow development of new targets for tumor therapy based on acute acidification and oxygen consumption. Here we report that differences in mitochondrial density may help explain heterogeneity of metabolism observed between tumors.

Methods: Early passage human melanoma lines DB1 and DB8 were cultured in α-MEM+10% FBS, at pH 7.3 or 6.7, with T_c of 24-42 h. Lactate production rates were determined in full medium, while oxygen consumption rates were determined in HEPES/MES buffer+salts. The maximum rate of respiration was determined with the uncoupler, 100µM dinitrophenol. Mitochondrial mass per cell was determined with Mitotracker Green and flow cytometry. DB1 is an example of an oxidative cell line and DB8 a glycolytic cell line.

Results: In DB1 compared to DB8 cells at pH 7.3, the O₂ consumption rate was greater by a factor of 2.1, lactate production rate less by 1.7, and DNP increased the O₂ consumption rate by 1.7 in DB1 compared to only 1.2 in DB8 cells. Mitochondrial density was 2.3 times greater in DB1 than in DB8 cells. When DB1 cells were cultured at pH 6.7 rather than at pH 7.3, O₂ consumption rate reduced by a factor of 0.23, lactate production rate increased by 1.3, and DNP increased the O₂ consumption rate in cells growing at pH 7.3 by 1.7 compared to only 1.2 in cells growing at pH 6.7. Mitochondrial density was reduced by 0.5 in DB1 cells growing at pH 6.7 compared to pH 7.3.

Conclusion: Mitochondrial mass per cell may explain differences between the oxidative DB1 and glycolytic DB8 melanoma cell lines and explain changes in metabolism in DB1 cells adapted to a tumor-like growth at pH 6.7, including perhaps the Crabtree Effect (Cancer Res. 61:5630-35, 2001). These results also suggest why a respiratory inhibitor such as metaiodobenzylguanidine (MIBG) plus glucose will acutely acidify and oxygenate a predominantly oxidative tumor like DB1, whereas a more glycolytic tumor like DB8 is more responsive to an inhibitor of the monocarboxylic transporter such as lonidamine. (Supported in part by USPHS P01 CA56690).