Bidirectional regulation of adenosine 5'-monophosphate-activated protein kinase activity by berberine and metformin in response to changes in ambient glucose concentration.

Both berberine and metformin are well-known antihyperglycemic agents for diabetes treatment. Adenosine monophosphate (AMP)-activated protein kinase (AMPK) activation is often considered as the most important molecular mechanism although the mechanism has been challenged recently. Up to now, when the ambient glucose level changes dynamically, the interaction between AMPK activity and the glucose-lowering effects of the agents remains largely unknown. To address this issue, HepG2 hepatocytes and C2C12 myotubes were preincubated at normal (5.6 mM), moderate (15 mM), or high (30 mM) glucose concentrations followed by moderate-glucose incubation plus berberine or metformin treatment. Preincubation at high glucose concentration followed by moderate-glucose incubation activated the AMPK pathway, but the activation was abolished with berberine or metformin treatment. In contrast, alteration from normal glucose to moderate glucose concentration in the medium suppressed AMPK activity, which was activated by berberine or metformin. Both metformin and berberine decreased the intercellular adenosine triphosphate content, enhanced glucose consumption, and lactate release under all three preincubation glucose concentrations regardless of AMPK activity. In conclusion, AMPK activated by glucose reduction is inhibited by berberine or metformin. The elevation of glucose level led to suppressed AMPK activity, which was activated with the addition of agents. The potent glucose-lowering effects with minimal hypoglycemia of berberine and metformin may be partially due to their bidirectional regulation of the AMPK signaling pathway. Berberine and metformin promote glucose metabolism via stimulation of glycolysis, which may not be related to AMPK activity.