Redox state and altered pyruvate metabolism contribute to a dose-dependent metformin-induced lactate production of human myotubes

Metformin-induced lactate production can lead to lactate acidosis as life-threatening side effect, whereas lactate in a physiological range might also be involved in the therapeutic effect. How metformin increases systemic lactate concentrations is only partly understood. Since human skeletal muscle has a high capacity to produce lactate, we aim to elucidate the potential contribution of skeletal muscle and the dose-dependent regulation of metformin-induced lactate production in primary human myotubes after 48 h of metformin treatment (16-776 µM). At 78 µM, metformin induced lactate production and glucose consumption. Investigating the cellular redox state by mitochondrial respirometry, we found metformin to inhibit the respiratory chain complex I (776 µM, p<0.01) along with a decreased [NAD+]:[NADH] ratio (776 µM, p<0.001). RNA sequencing and phospho-immunoblot data indicate inhibition of pyruvate oxidation mediated through phosphorylation of PDH complex (39 µM, p<0.01). In vivo in human skeletal muscle, we found pyruvate metabolism altered by exercise and not by metformin. Nonetheless, blood lactate levels in the pre-exercise condition are increased under metformin treatment (p<0.05). Metformin-induced inhibition of pyruvate oxidation combined with altered cellular redox state, both shift the equilibrium of the LDH reaction leading to enhanced lactate production of human myotubes. Overall design: Primary human myoblasts treated with 16 – 776 µM metformin for 48 h.