542 | NATURE | VOL 510 | 26 JUNE 2014

Metformin is considered to be one of the most effective therapeutics for treating type 2 diabetes because it specifically reduces hepatic gluconeogenesis without increasing insulin secretion, inducing weight gain or posing a risk of hypoglycaemia1, 2. For over half a century, this agent has been prescribed to patients with type 2 diabetes worldwide, yet the underlying mechanism by which metformin inhibits hepatic gluconeogenesis remains unknown. Here we show that metformin non-competitively inhibits the redox shuttle enzyme mitochondrial glycerophosphate dehydrogenase, resulting in an altered hepatocellular redox state, reduced conversion of lactate and glycerol to glucose, and decreased hepatic gluconeogenesis. Acute and chronic low-dose metformin treatment effectively reduced endogenous glucose production, while increasing cytosolic redox and decreasing mitochondrial redox states. Antisense oligonucleotide knockdown of hepatic mitochondrial glycerophosphate dehydrogenase in rats resulted in a phenotype akin to chronic metformin treatment, and abrogated metformin-mediated increases in cytosolic redox state, decreases in plasma glucose concentrations, and inhibition of endogenous glucose production. These findings were replicated in whole-body mitochondrial glycerophosphate dehydrogenase knockout mice. These results have significant implications for understanding the mechanism of metformin’s blood glucose lowering effects and provide a new therapeutic target for type 2 diabetes.

Clin Med Res. 2007 Oct;5(3):193-202

Alcohol-induced hypoglycemia has traditionally been attributed to the amount of ethanol consumed rather than any inherent decline in glucose output capacity by the gluconeogenic organs and/or an increase in skeletal muscle glucose uptake. Further, while the potential for sex differences that might impact glucose homeostasis following chronic alcohol consumption has been recognized, direct evidence has been noticeably absent. This paper will provide a brief review of past and present reports of the potential for sex differences in glucose homeostasis following chronic ethanol consumption. This paper will also provide direct evidence from our laboratory demonstrating sex differences from chronic alcohol consumption resulting in a decrement in glucose appearance and more importantly, a specific decline in hepatic gluconeogenic (HGN) capacity in the absence and presence of ethanol. All our studies involved 8 weeks of chronic alcohol consumption in male and female Wistar rats, as well as a 24 to 48 hour fast to deplete hepatic glycogen stores. Under the conditions of chronic alcohol consumption and an acute dose of ethanol, we provide in vivo evidence of an early decline in whole body glucose appearance in females fed an ethanol diet compared to controls. While the decline was also observed in males fed the alcohol diet, it occurred much later compared to ethanol fed females. The site for the decline in whole body glucose production (i.e., either the kidneys or the liver) was beyond the scope of our prior in vivo study. In a follow-up study using the in situ perfused liver preparation, we provide additional evidence for a specific reduction in HGN capacity from lactate in ethanol fed females compared to ethanol fed males in the absence of alcohol in the perfusion medium. Finally, employing the isolated hepatocyte technique, we report decrements in HGN from lactate in ethanol fed females compared to ethanol fed males in the presence of ethanol in the incubation medium. The mechanism for the specific decline in HGN within the liver of ethanol fed females remains to be determined. To the extent that our observations in animals can be extrapolated to humans, we conclude that alcoholic women are more susceptible to ethanol-induced hypoglycemia compared to alcoholic men.