Cancer cells with defective oxidative phosphorylation require endoplasmic reticulum-to-mitochondria Ca 2+ transfer for survival

Spontaneous Ca signaling from the InsP R intracellular Ca 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 Ca uptake in regulating bioenergetics in these cells, we used OXPHOS-competent and OXPHOS-defective cells. Inhibition of InsP R activity or mitochondrial Ca uptake increased α-ketoglutarate (αKG) abundance and the NAD /NADH ratio, indicating that constitutive endoplasmic reticulum (ER)-to-mitochondria Ca transfer promoted optimal αKG dehydrogenase (αKGDH) activity. Reducing mitochondrial Ca inhibited αKGDH 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 αKGDH and impaired autophagic flux in OXPHOS-defective cells resulted in pronounced cell death in response to interruption of constitutive flux of Ca 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 Ca regulation of αKGDH activity playing a pivotal role. Inhibition of ER-to-mitochondria Ca transfer may represent a general therapeutic strategy against cancer cells regardless of their OXPHOS status.