A New Mechanism of Action for Metformin

Abstract

Metformin activates AMP Kinase (AMPK) through an unknown mechanism. Using L6 muscle cells, we replicated the stimulation of AMPK phosphorylation by metformin, as well as the stimulation of glucose transport and fatty acid oxidation. We examined the leading hypothesis for metformin action, inhibition of mitochondrial respiration. It is difficult to rationalize this hypothesis with the established metformin stimulation of fatty acid oxidation. We conducted rotenone titrations on L6 muscle cell cultures for both glucose transport and fatty acid oxidation and compared this with metformin. Rotenone stimulated glucose transport, but inhibited fatty acid oxidation at all levels of the inhibitor. A similar divergence was found using agonists for peroxynitrite. Knockout experiments using siRNA suggested that the upstream kinase for AMPK was not involved. We formed a novel hypothesis: inhibition of the AMP destruction through the inhibition of AMP deaminase. We found metformin (and phenformin) inhibited purified AMP deaminase. Moreover, a known inhibitor of AMP deaminase stimulated glucose uptake and fatty acid oxidation. We have recently used several inhibitors of AMP deaminase, and found that they replicate metformin action. We conclude that AMP deaminase inhibition may be the link between metformin and AMPK activation.