Combination of Hypoglycemia and Metformin Impairs Tumor Metabolic Plasticity and Growth by Modulating the PP2A-GSK3β-MCL-1 Axis

Tumor cells may adapt to metabolic challenges by alternating between glycolysis and oxidative phosphorylation (OXPHOS). To target this metabolic plasticity, we combined intermittent fasting, a clinically feasible approach to reduce glucose availability, with the OXPHOS inhibitor metformin. In mice exposed to 24-h feeding/fasting cycles, metformin impaired tumor growth only when administered during fasting-induced hypoglycemia. Synergistic anti-neoplastic effects of the metformin/hypoglycemia combination were mediated by glycogen synthase kinase 3 beta (GSK3 beta) activation downstream of PP2A, leading to a decline in the pro-survival protein MCL-1, and cell death. Mechanistically, specific activation of the PP2A-GSK3 beta axis was the sum of metformin-induced inhibition of CIP2A, a PP2A suppressor, and of upregulation of the PP2A regulatory subunit B56 delta by low glucose, leading to an active PP2A-B568 delta complex with high affinity toward GSK3 beta.