Beginning of a new era in glucagon research: Breakthrough by the new glucagon assay

Abstract

There is a new concept of diabetes as a “comprehensive nutrition disorder”, caused due to both insulin and glucagon dysregulation. Dysregulated glucagon secretion in α‐cells exacerbates multiple metabolic disorders: glycemic control and amino acid metabolism, together with insulin deficiency.

Incretin‐based therapies were introduced a decade ago in the treatment of diabetes. They have dramatically changed the strategy of diabetes treatments and also reiterated the significance of glucagon and α‐cells in the pathophysiology of diabetes. First, glucagon and α‐cells have been extensively studied in the field of basic medical research. For example, our previous studies provided molecular evidence for the role of insulin signaling in α‐cells in the regulation of glucagon secretion ¹ and its involvement in the excessive secretion of glucagon in the hyperglycemic state ² . Furthermore, the strong association between glucagon and amino acids has recently been shown. Glucagon enhances amino acid metabolism in the liver, and amino acids enhance α‐cell proliferation ³ , ⁴ . Recent advances highlight the significance of α‐cells in the maintenance of islet quality and quantity ⁵ . Together, glucagon and α‐cells are now back under the spotlight.

In contrast to the progress in the field of basic medical research, clinical studies targeting glucagon were one step behind. The gap could be attributed to the unavailability of a reliable clinical assay for glucagon. Recent advances in molecular biology have been useful in addressing the problems of widely used conventional glucagon radioimmunoassay (RIA), including cross‐reactivity of antibodies with proglucagon‐related peptides and non‐specific effects of serum impurities on the assay. Recently developed double‐antibody “sandwich” enzyme‐linked immunosorbent assay (ELISA) resolved these limitations. High‐performance liquid chromatography results have shown that this ELISA improves the accuracy of the assay, when compared with RIA ⁶ . Then, the ELISA is routinely applied to clinical diabetology, and is rapidly becoming the standard method for glucagon estimation. Hence, previous observations about glucagon are now being re‐examined by the new system, which has led to publication of multiple reports reconfirming and/or discovering critical findings about the clinical significance of glucagon. Journal of Diabetes Investigation also contributes to the progress of clinical research about glucagon through publishing a number of clinical research papers.

One of the milestones in confirming the significance of glucagon is by Matsuo et al. ⁷ , who reported abnormal glucagon response to oral glucose tolerance tests in patients with type 2 diabetes. The study also compares the specificity of ELISA and RIA. The ELISA data recorded a significant difference in the glucagon response pattern to glucose load between patients with diabetes and healthy controls, whereas the RIA data did not show such a difference in glucagon response over time. Additionally, the absolute values of plasma glucagon levels estimated by ELISA were much lower than those estimated by RIA. Horie et al. ⁸ also examined the glucagon response during the oral glucose tolerance test in young healthy volunteers, both men and women ⁸ . The data from that study showed significant suppression of glucagon levels in response to glucose load, thereby confirming the findings by Matsuo et al. ⁷ Additionally, glucagon suppression was significantly larger in men as compared with that in women. The application of ELISA and physiological data from these studies ⁷ , ⁸ has contributed significantly to the scientific progress in the field of glucagon research.

A study by Takahashi et al. ⁹ reported a critical observation regarding the significance of glucagon unresponsiveness in hypoglycemia, observed in a patient with fulminant type 1 diabetes. The authors reported a sharp increase of glucagon in the arginine stimulation test just after the onset of the disease. However, the glucagon response of the same patient became defective at 6 months after the onset. Several research groups responded immediately to this report, and reported “preserved” glucagon response in their studies of patients with fulminant type 1 diabetes, and there were active discussions regarding this topic as well ¹⁰ , ¹¹ , ¹² , ¹³ . These discussions are clinically important for interaction and dispersal of clinical data, experiences, and knowledge within the medical community. Usually, the introduction of new drugs and therapeutic approaches enhances the progress of both clinical medicine and basic research. These studies and discussions show that the new assay system could be a great catalyst for progress in the field of diabetes. Other reports also include important argument about diabetes in the context of glucagon dysregulation, in addition to insulin ¹⁴ , ¹⁵ . These reports in the Journal of Diabetes Investigation left a significant scientific impact on the field of clinical diabetology, as they provided crucial evidence in the form of experimental data about the clinical significance of glucagon.

We also provided clinical evidence for the dysregulation of glucagon secretion in patients with type 1 diabetes, by estimating plasma glucagon levels by the new ELISA system ¹⁶ . The plasma glucagon levels were widely dispersed, and were not associated with the plasma glucose levels at the time of sampling. The glucagon levels were not elevated when the plasma glucose levels were low, and they were also not suppressed when the glucose levels were high. Importantly, glucagon levels in patients with hypoglycemia unawareness were significantly lower than those in patients without hypoglycemia unawareness, regardless of plasma glucose levels at the time of blood sampling. These data suggest a role for glucagon dysregulation in the pathophysiology of the disease: excessive (non‐suppressive) secretion leading to exacerbation of hyperglycemia and unresponsiveness leading to hypoglycemic events. Additionally, the plasma glucagon levels were statistically associated only with serum blood urea nitrogen levels, but not with other parameters, such as glycemic control parameters and liver function markers. The annual comparison of glucagon levels in the same patients showed a significant correlation between both years ¹⁷ . We also confirmed a positive correlation between plasma glucagon levels and serum blood urea nitrogen levels, which established a functional link between glucagon and amino acid metabolism. Amino acids stimulate α‐cell glucagon secretion and cellular proliferation, whereas glucagon enhances amino acid metabolism in the liver ³ , ⁴ . Thus, our clinical result could clinically validate the correlation between glucagon and amino acids found by basic glucagon studies ¹⁸ . From these findings, I propose a new concept of diabetes as a “comprehensive nutrition disorder”, caused by both insulin and glucagon dysregulation (Figure 1, see legend for details). The new glucagon assay is expected to be a powerful tool for clinical evaluation, and could be the beginning of a new era in glucagon research.

Figure 1.

New concept of diabetes as a “comprehensive nutrition disorder”, caused by both insulin and glucagon dysregulation. Pancreatic β‐cell dysfunction and diabetes induce dysregulated glucagon secretion in α‐cells in addition to systemic hyperglycemia caused by insulin shortage. Excessive glucagon secretion in the hyperglycemic state and defects in the hypoglycemic state exacerbate glycemic control disorder; both in hyper‐ and hypoglycemia. Excessive glucagon induces abnormal amino acid metabolism in the liver. Absolute and relative insulin shortage exacerbate glycemic control and abnormal amino acid metabolism. Dysregulation in insulin and glucagon also induces dysregulated lipid metabolism.

Disclosure

The author declares no conflict of interest.

J Diabetes Investig 2020; 11: 1123–1125