Although research has made significant progress in metabolic disorders such as diabetes, obesity and liver disease, the importance of glucagon has been overlooked. Glucagon is a hormone with a critical role in regulating blood glucose by stimulating the liver to release glucose. Researchers have now developed a test to measure glucagon resistance – a risk marker for developing diabetes among people who have obesity and liver disease (MASLD).
Although science has made considerable progress in understanding metabolic disorders such as diabetes, overweight and liver disease, researchers have consistently underestimated the importance of a well-known hormone – glucagon, best known for having the opposite effect of insulin. Insulin removes glucose from the blood, and glucagon stimulates the liver to release glucose into the blood.
“Insulin resistance is well studied, but glucagon resistance is unexplored. This lack of knowledge has motivated us to develop a new diagnostic test to measure glucagon resistance among people with obesity and metabolic dysfunction–associated steatotic liver disease,” explains Sasha A.S. Kjeldsen, Postdoctoral Fellow, Department of Clinical Biochemistry, Bispebjerg and Frederiksberg Hospital, Copenhagen, Denmark.
“Our recent research and that of others has revealed the important role of glucagon for metabolism of protein and lipids. It has shed light on a complex feedback loop between the liver and the alpha cells in the pancreas that produce glucagon. It is known as the liver–alpha cell axis and appears to be closely related to both diabetes and MASLD,” adds Consultant Nicolai J. Wewer Albrechtsen, Department of Clinical Biochemistry, Bispebjerg and Frederiksberg Hospital, Copenhagen and Associate Professor, Novo Nordisk Foundation Center for Protein Research, University of Copenhagen, Denmark.
Enormous problem
Researchers and clinicians have known for many years that insulin resistance – cells’ reduced response to insulin – leads to high blood glucose and is therefore key in developing metabolic disorders, including type 2 diabetes, in which insulin does not have the same effect as for healthy people. In contrast, the understanding of glucagon resistance has been limited.
“It has also been known that glucagon is related to glucose metabolism, but the problem is that if healthy people get an injection of glucagon, the effect on glucose lasts for about 20 minutes and then dissipates. In contrast, the effects on fat and protein metabolism last longer, as found in clinical studies using glucagon,” explains Nicolai J. Wewer Albrechtsen.
Today, one in four people has liver disease, so the problem is enormous. Many of these people also have type 2 diabetes, but although the cause is unknown, it could result from a hormonal imbalance.
“We had discovered that the effect of glucagon on protein and fat metabolism was reduced in mice that had MASLD, indicating glucagon resistance. A test for human glucagon resistance could therefore provide an opportunity to better understand the link between liver diseases and the development of diabetes,” notes Sasha A.S. Kjeldsen.
Investigating how protein is metabolised
The researchers in Nicolai J. Wewer Albrechtsen’s group at the Department of Clinical Biochemistry, led by Sasha A.S. Kjeldsen and Michael M. Richter, therefore developed a glucagon sensitivity test (GLUSENTIC), based on the same principles as Yoshitaka Matsuda’s insulin sensitivity test from 1999, the clinical standard for measuring insulin resistance and thus cardiometabolic diseases.
“Our study involved three groups: a control group without MASLD, a group with the disease and a group with type 1 diabetes. The method included two tests: a glucagon injection and an amino acid infusion,” says Michael M. Richter, PhD Student, Department of Clinical Biochemistry, Bispebjerg and Frederiksberg Hospital, Copenhagen, Denmark.
Whereas Yoshitaka Matsuda’s insulin resistance test is based on measuring glucose and insulin levels, GLUSENTIC measures changes in the blood levels of amino acids (the building blocks of protein) and glucagon to quantify glucagon resistance and compare it across groups.
“On the first day of the experiment, the participants received an intravenous injection of 0.2 mg of glucagon, and blood samples were taken at specific times over a two-hour period, during which we measured the amino acids in the blood. On the second day, they received an infusion of protein, and we measured both glucagon and amino acids in the blood to determine how the body metabolises this protein,” explains Michael M. Richter.
Sensitivity index
The study found that people with MASLD had a weaker response to glucagon injections than the control group.
“Our results clearly show that people with MASLD have reduced ability to metabolise amino acids after receiving glucagon. This indicates significant glucagon resistance, which may affect their metabolic health,” elaborates Sasha A.S. Kjeldsen.
The glucagon resistance was further confirmed through the amino acid infusion test, in which the people with MASLD had higher levels of glucagon and amino acids than the control group.
“We calculated a glucagon sensitivity index (GLUSENTIC index). Comparing the quantities of glucagon and amino acids in the blood shows that people with liver disease have more glucagon resistance than healthy people,” says Sasha A.S. Kjeldsen.
Giving alpha cells an extra nudge
The new study thus focuses on the fact that glucagon’s role in relation to blood glucose – which has already been well described – is far from the only feature that is important. The connection to glucagon and amino acids in the blood may prove even more important to understand. But why is this important to understand?
“When you have eaten a meal, stabilising metabolites such as glucose, protein and fat in the blood is important. The protein in the blood should also be metabolised. Glucagon is crucial for this in the liver, and this has been known since the 1980s, but there has not been as much focus on it,” explains Sasha A.S. Kjeldsen.
The researchers now theorise that fat accumulation in the liver causes diabetes.
“We have also shown this connection epidemiologically. We have found that glucagon resistance causes glucagon to not work as well on the liver to break down amino acids. The amino acids accumulate in the blood and give an extra boost to the alpha cells, which then produce more glucagon to compensate for the glucagon resistance. This releases more glucose from the liver, and thus glucagon resistance promotes diabetes,” adds Sasha A.S. Kjeldsen.
More effective weight-loss medicine
The development of the GLUSENTIC test provides researchers a new and potentially useful tool to clinically identify individuals at high risk of diabetes and other cardiometabolic diseases because of glucagon resistance.
“Our new clinical test provides a more accurate picture of glucagon resistance. With GLUSENTIC, we now have a tool that can help doctors to identify people at high risk of developing diabetes. This can lead to earlier interventions and better treatments to prevent disease progression,” says Sasha A.S. Kjeldsen.
The next step is to test this method on people with type 2 diabetes and to take liver biopsies while the person is receiving glucagon to investigate whether glucagon resistance and insulin resistance can exist together and separately and the molecular causes that lead to glucagon resistance. The researchers will therefore now start investigating the underlying mechanisms behind glucagon resistance. This will help to develop more targeted treatments for people with metabolic disorders.
“We believe that glucagon resistance can have even greater effects, and we are striving to understand all the mechanisms behind it. Clinical trials have shown that combining existing weight-loss medicines with glucagon can create even more effective medicines for weight loss, and glucagon’s effects on the liver are particularly interesting. The liver becomes healthier by receiving glucagon,” concludes Nicolai J. Wewer Albrechtsen.
