Metformin Redirects Autophagy from Bulk Turnover to Mitochondrial Clearance

Metformin is the most widely prescribed antidiabetic drug and an active candidate for repurposing in oncology. How it engages autophagy - a pathway central to both its metabolic and its anti-tumor effects - has remained unresolved, with reports of induction, suppression, and no effect. Here we show that metformin reroutes rather than induces or inhibits autophagy in human cancer cells: at therapeutic concentrations, it suppresses bulk cytosolic turnover by selectively blocking WIPI2-mediated phagophore tethering, while the ULK1 initiation complex relocates toward mitochondria and engages selective mitochondrial clearance. We trace this redirection to mitochondrial complex I inhibition, registered as a shift in the NAD⁺/NADH ratio before any change in the adenylate pool, and to a non-canonical reprogramming of the ULK1 complex that operates independently of mTORC1 and of the proposed PEN2-lysosomal route. AMPK is engaged in a subunit-specific manner that restrains ATG13 at initiation and enables WIPI2 displacement at maturation. The ULK1 complex is therefore the node at which metformin sets autophagic substrate selection, with direct implications for combination therapy in diabetes and cancer.