Cancer cells overexpress IFsub.1, the endogenous protein that inhibits the hydrolytic activity of ATP synthase when mitochondrial membrane potential (Δμsub.H sup.+) falls, as in ischemia. Other roles have been ascribed to IFsub.1, but the associated molecular mechanisms are still under debate. We investigated the ability of IFsub.1 to promote survival and proliferation in osteosarcoma and colon carcinoma cells exposed to conditions mimicking ischemia and reperfusion, as occurs in vivo, particularly in solid tumors. IFsub.1-silenced and parental cells were exposed to the FCCP uncoupler to collapse Δμsub.H sup.+ and the bioenergetics of cell models were validated. All the uncoupled cells preserved mitochondrial mass, but the implemented mechanisms differed in IFsub.1-expressing and IFsub.1-silenced cells. Indeed, the membrane potential collapse and the energy charge preservation allowed an increase in both mitophagy and mitochondrial biogenesis in IFsub.1-expressing cells only. Interestingly, the presence of IFsub.1 also conferred a proliferative advantage to cells highly dependent on oxidative phosphorylation when the uncoupler was washed out, mimicking cell re-oxygenation. Overall, our results indicate that IFsub.1, by allowing energy preservation and promoting mitochondrial renewal, can favor proliferation of anoxic cells and tumor growth. Therefore, hindering the action of IFsub.1 may be promising for the therapy of tumors that rely on oxidative phosphorylation for energy production.