Cancer cells with defective oxidative phosphorylation require endoplasmic reticulum-to-mitochondria Ca2+ transfer for survival

Spontaneous Ca2+ signaling from the InsP(3)R intracellular Ca2+ release channel to mitochondria is essential for optimal oxidative phosphorylation (OXPHOS) and ATP production. In cells with defective OXPHOS, reductive carboxylation replaces oxidative metabolism to maintain amounts of reducing equivalents and metabolic precursors. To investigate the role of mitochondrial Ca2+ uptake in regulating bioenergetics in these cells, we used OXPHOS-competent and OXPHOS-defective cells. Inhibition of InsP(3)R activity or mitochondrial Ca2+ uptake increased alpha-ketoglutarate (alpha KG) abundance and the NAD(+)/NADH ratio, indicating that constitutive endoplasmic reticulum (ER)-to-mitochondria Ca2+ transfer promoted optimal alpha KG dehydrogenase (alpha KGDH) activity. Reducing mitochondrial Ca2+ inhibited alpha KGDH activity and increased NAD(+), which induced SIRT1-dependent autophagy in both OXPHOS-competent and OXPHOS-defective cells. Whereas autophagic flux in OXPHOS-competent cells promoted cell survival, it was impaired in OXPHOS-defective cells because of inhibition of autophagosome-lysosome fusion. Inhibition of alpha KGDH and impaired autophagic flux in OXPHOS-defective cells resulted in pronounced cell death in response to interruption of constitutive flux of Ca2+ from ER to mitochondria. These results demonstrate that mitochondria play a fundamental role in maintaining bioenergetic homeostasis of both OXPHOS-competent and OXPHOS-defective cells, with Ca2+ regulation of alpha KGDH activity playing a pivotal role. Inhibition of ER-to-mitochondria Ca2+ transfer may represent a general therapeutic strategy against cancer cells regardless of their OXPHOS status.