This was associated with an increase in glucose disposal and muscle glycogen deposition. It is suggested that the increase in AMPK a2 activity was due to a decrease in the energy status since both ATP and phosphocreatine levels were lower after metformin treatment. These findings suggest that the metabolic effects of metformin in type 2 diabetes may be mediated by activation of AMPK a2.

Comment
The efficacy of metformin, an antidiabetic drug for the treatment of type 2 diabetes, was demonstrated in the United Kingdom Prospective Diabetes Study [1]. Its glucose-lowering effect (without a corresponding increase in body weight) is due to decreased glucose production and increased glucose utilization. In addition, metformin decreases plasma triglycerides and reduces cardiovascular risk [1]. On the basis of in vitro studies using pharmacological concentrations of metformin, it was reported that metformin: (1) decreases hepatic glucose production (inhibition of gluconeogenesis) [2, 3]; (2) increases glucose uptake (translocation of GLUT4 glucose transporters) and glycogen synthesis in skeletal muscles; and (3) inhibits adipose tissue lipolysis and hepatic VLDL production. In addition, two studies have reported that metformin inhibits complex I of the mitochondrial respiratory chain [4, 5].
A recent study by the same group [6] proposed a unique mechanism to explain the different effects of metformin on glucose and lipid metabolism. They proposed that AMPK, a protein kinase activated by AMP, was a potential target of metformin action. AMPK is a heterotrimeric protein of the family of serine/threonine kinases. It is composed of one catalytic subunit (a-subunit) and two regulatory subunits (b- and g-subunits), generated by different genes. The binding of AMP to the g-subunit activates AMPK whereas ATP inhibits it. All the situations in which intracellular ATP decreases and cellular AMP increases (hypoxia, inhibition of oxidative phosphorylation, physical exercise) are accompanied by AMPK activation and conservation of ATP by the blockade of anabolic pathways (synthesis of cholesterol and fatty acids) and the activation of catabolic pathways (fatty acid oxidation). Indeed, AMPK phosphorylates and inhibits the enzymes controlling cholesterol and fatty acid synthesis: hydroxymethyl glutaryl-coenzyme A (CoA) reductase and acetyl-CoA carboxylase. Inactivation of acetyl-CoA carboxylase by AMPK induces a decrease in malonyl-CoA concentration and an increase in the activity of carnitine palmitoyltransferase-1 and fatty acid oxidation. Zhou et al. [6] showed that metformin activated (phosphorylation) AMPK in primary cultures of rat hepatocytes by decreasing acetyl-CoA carboxylase activity and lipogenesis and by increasing fatty acid oxidation (secondary to a decrease in malonyl-CoA levels). Metformin also inhibited glucose production (in the presence of glucagon). The effects of metformin on glucose and fatty acid metabolism were markedly reduced by an inhibitor of AMPK. Metformin also activated AMPK in muscle cells, inducing an increase in glucose uptake, and inhibited the expression of a transcription factor, SREBP-1c (sterol regula-tory element binding protein 1c), implicated in the regulation of the expression of several genes encoding enzymes of lipogenesis (e.g. fatty acid synthase). Metformin had no effect on purified AMPK, indicating that its effects are indirect, possibly resulting from an increase in intracellular AMP. The in vitro effects of metformin were compatible with all the effects demonstrated in vivo: decrease in plasma glucose, triglycerides and insulin.
The present paper extends to humans the observations made in the rat. However, the mechanism of action of metformin is far from being elucidated. The hypothesis that activation of AMPK by metformin could be secondary to inhibition of the mitochondrial respiratory chain and to an increase in cellular AMP has recently been challenged. In muscle cells, it was shown that two different antidiabetic drugs, metformin and rosiglitazone, activated AMPK via different mechanisms [7]. Rosiglitazone activated AMPK secondarily to an increased AMP/ATP ratio, whereas metformin activated AMPK in the absence of change in the AMP/ATP ratio [7]. Other experiments, using different cell lines, have also established that activation of AMPK by metformin was not a consequence of depletion of the cellular energy charge. Thus, AMPK can be activated by mechanisms other than changes in cellular AMP/ATP ratio [8]. These mechanisms need to be clarified. Nevertheless, the development of molecules capable of modulating in vivo the activity of AMPK would be of great benefit in the treatment of type 2 diabetes.

References
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Summary and Comment:
Jean Girard, Paris, France