The Role of ATPase Inhibitor Protein, IF1 in the Regulation of Both Mitochondrial Funcion and Ca2+ Homeostasis in Tumor Cells

Tumor cells exhibit metabolic differences with respect to non-transformed cells. An important feature of cancer cells is the switch towards glycolysis to compensate the increased energy demand. Many biochemical mechanisms contribute to enhancing the glycolysis rate, one of which is the inhibition of the mitochondrial F1FO-ATPase by its endogenous inhibitor, IF1. When the electron transport is impaired, the mitochondrial electrochemical gradient collapses leading to the activation of ATP synthase hydrolytic activity in order to restore the proton gradient essential for cell function. Under these circumstances, the acidification of the mitochondrial matrix induces IF1 to inhibit the ATP synthase blocking the reversal of the enzyme and saving ATP. Furthermore, a putative binding site for calmodulin in IF1 sequence has been uncovered, suggesting that IF1 may contribute to the regulation of Ca2+ levels. Therefore, the intracellular Ca2+ handling was analysed in scrambled and IF1-silenced HeLa cells: the data obtained showed that mitochondrial Ca2+ uptake was higher in IF1 KD cells with respect to controls upon stimulation with Thapsigargin. This result was related with both the higher membrane potential and MCU expression found in IF1 KD cells, suggesting a role of IF1 in Ca2+ signalling regulation. In the light of the above, tumor cells bioenergetics were addressed in the presence or the absence of IF1 in order to shed light on its role in cancer cells bioenergetics. For this purpose, the inner membrane potential was investigated in IF1 silenced 143B cells under normoxic (21% O2) and hypoxic (0.5% O2) conditions, and related to ATP content, respiration rate, OXPHOS enzymes levels: our results showed that in normoxia IF1 may actively contribute to enhancing OXPHOS efficiency, whereas in hypoxia, a selective advantage concerning cell growth and proliferation was detected in IF1 expressing cells, suggesting that IF1 plays important role(s) in cells exposed to hypoxia/anoxia.