Abstract
Cancer cells rely on mitochondrial oxidative phosphorylation (OXPHOS) and the noncanonical tricarboxylic acid (TCA) cycle. In this paper, we shed light on the vital role played by the noncanonical TCA cycle in a host-side concession to mitochondria, especially in highly energy-demanding malignant tumor cells. Inhibition of ATP-citrate lyase (ACLY), a key enzyme in the noncanonical TCA cycle, induced apoptosis by increasing reactive oxygen species levels and DNA damage while reducing mitochondrial membrane potential. The mitochondrial membrane citrate transporter inhibitor, CTPI2, synergistically enhanced these effects. ACLY inhibition reduced cytosolic citrate levels and CTPI2 lowered ACLY activity, suggesting that the noncanonical TCA cycle is sustained by a positive feedback mechanism. These inhibitions impaired ATP production, particularly through OXPHOS. Metabolomic analysis of mitochondrial and cytosolic fractions revealed reduced levels of glutathione pathway-related and TCA cycle-related metabolite, except fumarate, in mitochondria following noncanonical TCA cycle inhibition. Despite the efficient energy supply to the cell by mitochondria, this symbiosis poses challenges related to reactive oxygen species and mitochondrial maintenance. In conclusion, the noncanonical TCA cycle is indispensable for the canonical TCA cycle and mitochondrial integrity, contributing to mitochondrial domestication.