For more than 50 years, metformin (N, N-dimethylbiguanide) has been a frontline drug for treatment of type 2 diabetes. It is a reliable antihyperglycemic agent that inhibits hepatic glucose production. However, despite its obvious clinical success, its cellular mechanism of action remained unclear.
A new study published in Nature Medicine establishes how metformin blocks glucose production by the liver. The research team included David Wasserman, Ph.D., Annie Mary Lyle Professor of Molecular Physiology and Biophysics at Vanderbilt University School of Medicine, and his departmental colleagues Louise Lantier, Ph.D., and Curtis C. Hughey, Ph.D.
While previous studies suggested ways metformin may inhibit mitochondrial respiration, the actual signaling pathways involved were controversial. “This study defines the site-specific mechanism of action of metformin on blood glucose control,” Wasserman said.
Teasing Apart Glucose Biosynthesis
Wasserman collaborated with an international team of researchers led by Kei Sakamoto, Ph.D., of the Nestlé Institute in Lausanne, to investigate the acute effects of metformin on hepatic glucose production.
Making glucose is energetically costly, note the authors in the new study: “Hepatocytes must balance this energy demand with production, thereby maintaining energy homeostasis.”
“This study defines the site-specific mechanism of action of metformin on blood glucose control.”
Hepatocytes are equipped with enzymes that help balance energy costs, including fructose-1,6-bisphosphatase. The researchers investigated whether this enzyme may also underlie how metformin keeps glucose production in check.
The rate-limiting enzyme functions downstream of another enzyme — AMP-activated protein kinase, or AMPK — that is central to mitochondrial respiration. Some groups have suggested metformin works by modulating AMPK, or its activator, AMP. Others say it works independent of either.
Wasserman and his colleagues discovered metformin indeed increases AMP levels in the liver, causing mild energy stress, but that these increases in turn inhibit fructose-1,6-bisphosphatase. As a result, metformin effectively inhibits gluconeogenesis through fructose-1,6-bisphosphatase. This has the effect of limiting the rate that glucose is released from the liver and preventing elevated blood glucose.
Developing New Tools for Diabetes Research
The researchers showed fructose-1,6-bisphophatase is a metformin therapeutic target using diabetic mouse models. Mice genetically engineered to lack a functioning fructose-1,6-bisphosphatase enzyme couldn’t fully benefit from metformin. Their blood glucose levels remained high.
Wasserman, who is also director of the National Institutes of Health-funded Mouse Metabolic Phenotyping Center (MMPC) at Vanderbilt University Medical Center, and his colleagues designed a “metformin glucose clamp” to elucidate the drug’s effects in the genetically engineered mice.
Metformin causes a fall in blood glucose, which results in physiological changes that complicate experiments trying to isolate metformin action. Unique tools developed in the MMPC prevented this fall in blood glucose in genetically engineered mice by “clamping” blood glucose at fasting concentrations. “This, in combination with isotope tracers, permitted a detailed pathway-specific analysis of liver metformin sensitivity,” Wasserman explained.
Translating Results to Patients
The study has revealed a new therapeutic target — fructose-1,6-bisphosphatase — that could be leveraged by drug developers to help treat type 2 diabetes. Since the enzyme underlies the antihyperglycemic effects of metformin, it may also be the target of other natural compounds used to treat diabetes. Metformin itself is derived from the plant Galenga officinalis (also known as goat’s rue or French Lilac).
“This might indeed be the reason for the apparent glucose-lowering effect of many biologically active metabolites identified in traditional Chinese medicine and edible natural products,” Sakamoto, the paper’s corresponding author, said in a news release.
The researchers are now looking into ways to modulate the newly-identified target enzyme. Although more studies are needed, the alternative target could help avoid side effects associated with metformin, including diarrhea and abdominal discomfort.
