MTOR and cancer: Functional and energized mitochondria are essential for the energy-intensive process of cell proliferation [73] and metformin-induced decreases in mitochondrial ATP synthesis may directly cause cell death in cancer cells intolerant of energetic stress, as well as have an anti-proliferative effect as the result of energy-conserving responses, mediated by the activation of AMPK and the inhibition of mTOR (mammalian target of rapamycin) [4].