Viability of HepG2 and MCF-7 Cells is not Correlated with Mitochondrial Bioenergetics

Alterations in metabolism is a hallmark of cancer. It is unclear, however, if oxidative phosphorylation (OXPHOS) is required for tumor cell survival. We investigated the effect of severe hypoxia, site-specific inhibition of respiratory chain (RC) components, and uncouplers on the survival of HepG2 and MCF-7 2D cultured cells. Comparable respiratory complex activities were observed in both cell lines, but HepG2 cells exhibited much higher oxygen consumption rates (OCR) and respiratory capacity than the MCF-7 cells. Significant non-mitochondrial OCR was found in MCF-7 cells that was insensitive to acute combined inhibition of complexes I and III. However, pre-treatment of either cell line with RC inhibitors for 24-72 hours abolished respective complex activities and OCRs completely, and this was associated with a time-dependent decrease in citrate synthase activity, suggesting mitophagy. HepG2 cells viability was mostly unaffected by any pharmacological treatment or severe hypoxia as temporally recorded from high-content automated microscopy. Conversely, MCF-7 cells viability exhibited strong sensitivity to CIV or CV inhibition, severe hypoxia, and uncoupling, but were only moderately affected by CI, CII and CIII inhibition. CII, CIII and CIV-inhibitor mediated MCF-7 cell death were partially abrogated by aspartate. The data show that OXPHOS activity and viability are uncorrelated in these cell lines indicating that a linkage of OXPHOS to cancer cell survival must be cell- and condition-defined.