Glucagon: The Opposing Force to Insulin
Glucagon, the predominant product of α-cells within islets, was originally identified in 1923 during efforts to purify insulin, where it was identified as a contaminant hyperglycemic factor (1). Further research determined that the hyperglycemic action of glucagon was mediated by increased hepatic glycogenolysis and gluconeogenesis, thereby increasing endogenous glucose production (2). This aspect of glucagon biology has been leveraged for pharmacological treatment of insulin-induced hypoglycemia in patients with diabetes (3). This historical progression has positioned insulin and glucagon as opposing hormones with respect to glycemic control (4), with imbalances in the insulin-to-glucagon ratio predicted to disrupt euglycemia. Diabetic hyperglycemia is often described to arise from both impaired insulin action and inappropriately elevated levels of glucagon (5–7). The perceived hyperglycemic effects of glucagon were reinforced by studies demonstrating that reduction of glucagon receptor (GCGR) activity blunts hyperglycemia in rodent models of insulinopenic diabetes (8,9). These observations have fostered the development of GCGR antagonists (GRAs) for the treatment of hyperglycemia (10). Consequently, the evolution of glucagon biology has fostered the general perspective that glucagon is a hypoglycemia/fasting-induced hormone that has the primary action of increasing glycemia. This perception has limited the interest in or investigation into the potential benefits of glucagon agonism for the treatment of type 2 diabetes (T2D). Here, we highlight a selection of findings that we believe contribute to a repositioning of glucagon away from its classical dogma of being a hypoglycemia-responsive opposing hormone to insulin. We also discuss how these somewhat paradoxical, underappreciated aspects of glucagon biology can be leveraged for the pharmacological treatment of T2D.
Does Glucagon Only Exist to Prevent Hypoglycemia?
Reevaluation of historical data, along with a number of recent advances in our understanding of glucagon biology, has questioned whether the primary role of glucagon is to guard against hypoglycemia. First, it is important to consider the relationship between glycemia and glucagon levels. Although glucagon levels initially rise following the onset of a fast, concurrent with decreasing glycemia, with prolonged fasting (>3 days) circulating glucagon levels fall progressively to postprandial values despite persistent low blood glucose (11). Moreover, the administration of glucagon in hypoglycemic humans that have been fasting for >3 days does not produce any meaningful changes in glycemia (12), likely because of depleted glycogen stores. Hence, low plasma glucose does not always associate with elevated glucagon levels, raising the possibility that hypoglycemia is not the primary driver of α-cell secretory function. Furthermore, blocking glucagon action with genetic interruption (13,14) or pharmacological antagonism in mice (15), nonhuman primates (16), or humans (17,18) lowers glucose but does not necessary alter the susceptibility to hypoglycemia. On the other hand, a much tighter relationship is seen between glucagon levels and a subset of amino acids (19). Clinical studies often utilize amino acids, primarily arginine, as a α-cell secretagogue, which induces significant increases in circulating glucagon regardless of ambient glycemia. Alanine infusion also induces a robust increase in glucagon secretion (20), while branch-chain amino acids do not have a direct effect on glucagon secretion (21). Reducing glucagon action in hepatocytes drives hyperaminoacidemia, which is the precipitating factor for α-cell hyperplasia and hyperglucagonemia (14,16,22,23). We have found in isolated mouse and human islets that the amino acids glutamine, arginine, and alanine are potent inducers of glucagon secretion concentrations (21). Furthermore, physiological concentrations (0.5–1.0 mmol/L) of these amino acids can increase glucagon secretion up to 10-fold, while changes in glucose within physiological ranges only modify glucagon secretion twofold (Fig. 1). This observation that α-cells are more responsive to amino acids compared with changes in glucose concentrations questions the primacy of glycemia for glucagon secretion. In fact, it is unclear whether glucose serves as a direct signal for α-cells. It is difficult to dissociate any potential direct effects of glucose on α-cells from those mediated indirectly through either β- or δ-cells. Interestingly, isolated α-cells paradoxically increase glucagon secretion in response to glucose (24), which would indicate that the inhibitory actions of glucose on α-cells are indirect. Glucose increases β-cell activity and the secretion of products (insulin, zinc, γ-aminobutyric acid) that dampen α-cell function through direct paracrine inhibition mediated by β- to α-cell signaling. Similarly, δ-cell secretory activity also inhibits glucagon through somatostatin. Thus, the ability for glucose to modulate glucagon secretion is likely to be primarily driven by indirect paracrine interactions originated from β- and δ-cells. This model also suggests that the role of ambient glucose is to dictate the tone of α-cells, rather than serving as a direct, dose-related stimulus for glucagon secretion. In support of this, stimulation of glucagon secretion by amino acids is greater at low glucose, where inhibitory tone is low, compared with high glucose, where inhibitory tone is high (Fig. 2). Whether glycemic levels dictate the α-cell tone and response to other key regulators of α-cell function (25) such as fatty acids or CNS input is unknown.
A second factor that challenges the dogma that glucagon is primarily driven by hypoglycemia is the consistent observations that plasma glucagon levels increase postprandially, coinciding with an increase in glycemia. Postprandial rises in glucagon seen in T2D (26) have been described as pathogenic and a cause of hyperglycemia (27). Yet similar rises in postprandial glucagon are seen in individuals without diabetes (28,29). The amino acid component of a mixed meal is likely a major contributor to postprandial increases in glucagon secretion; however, both healthy individuals and individuals with T2D often display a modest increase immediately following oral glucose alone (30–32). The effect of oral glucose to stimulate glucagon is more pronounced in people with T2D, generally of short duration, and typically followed by a decrease in glucagon levels after 30 min. Furthermore, the mechanisms by which oral glucose stimulates α-cells are unknown but potentially involve enteroendocrine hormones such as GIP (33), a peptide that tends to have higher circulating concentrations among persons with T2D. The importance of postprandial rises in glucagon for metabolic homeostasis has not been extensively tested.
α-Cell Hyperplasia: Metabolic Adaptation Versus Pathogenic?
A key question in this revisionary model of α-cell function is as follows: why would metabolism evolve to increase α-cell function in response to metabolic stress and hyperglycemia if the primary actions of glucagon were to enhance endogenous glucose production? The argument has been made that this is a precipitating event that drives metabolic dysfunction (27) rather than an adaptation to help correct it. However, this argument does not align with the recent demonstrations that α-cells are essential for determining β-cell (21,34,35) and glycemic tone (36). Interruption of proglucagon input to β-cells drastically dampens the magnitude of nutrient-stimulated insulin secretion. The insulinotropic properties of glucagon are essential for β-cell function, which establishes a direct and critical relationship between α- and β-cells that is manifested in both mouse (21,34,35) and human (21,36) islets. These new studies raise the possibility that the hyperglucagonemia present in T2D is a compensatory mechanism to enhance β-cell function, but this has yet to be formally tested. However, this hypothesis provides an alternative perspective to position α-cell hyperplasia and increased glucagon secretion observed in T2D as a mechanism to correct, rather than induce, dysregulated homeostasis. This is similar to the well-accepted observation that insulin resistance drives hyperinsulinemia. The α-cell model whereby α-cell function in subjects with diabetes is a compensatory response to metabolic stress that stimulates β-cell function to maintain homeostasis is plausible based on available data. Mice chronically fed a high-fat diet demonstrate increased α-cell mass (37), supporting the hypothesis that α-cell hypertrophy compensates for metabolic stress; whether this occurs in humans is difficult to test. People with T2D consistently demonstrate an elevated α-cell–to–β-cell mass ratio (38), but this may result from decreased β-cell mass rather than an increase in α-cell mass (39). However, it is interesting to note that metabolic stress increases glucagon concentrations, while there is little evidence to support that undernutrition or chronic fasting impacts α-cell function. Finally, in addition to increased α-cell mass and glucagon secretion, metabolic stress also alters α-cell function to produce GLP-1 by increasing the expression of the prohormone convertase (PC)1 isoform (40–42). This enables the processing of proglucagon peptide to generate GLP-1, which is a much more potent insulin secretagogue compared with glucagon (21) (Fig. 3).
How can the perspective that increased α-cell activity is a compensatory response to metabolic stress be reconciled with the data demonstrating that pharmacological or genetic reductions in glucagon action consistently lower glycemia? GRAs lower glycemia in humans with T2D (43) and in preclinical models of hyperglycemia (15). This effect of blocking glucagon activity would argue against the essential contribution of the α-cell toward postprandial glucose metabolism. However, while blocking glucagon activity in hepatocytes reduces endogenous glucose production, it is unclear whether this is the primary mechanism to facilitate glucose lowering in response to GRAs. A universal outcome of inhibiting glucagon action, either globally or specifically in hepatocytes, is α-cell hyperplasia and pharmacological levels of circulating glucagon and GLP-1 (14,22,44,45). Both glucagon and GLP-1 elevate β-cell tone and insulin secretion primarily through the GLP-1 receptor (GLP-1R) (21) (Fig. 3 [discussed in detail below]). Thus, GRAs would be expected to enhance β-cell tone and insulin secretion by substantially increasing activity at the GLP-1R (Fig. 3). Whether this is a mechanism that can account for the glucose-lowering response to GRA therapy has not been formally tested. However, a number of studies have demonstrated that the glucose lowering in response to a GRA or following genetic deletion of the GCGR is severely diminished in the absence of GLP-1R signaling (44,46–48). Moreover, pharmacological or genetic elimination of glucagon action requires some level of endogenous β-cell function in order to lower glycemia (49). Thus, there is evidence that β-cell GLP-1R activity and insulin secretion meaningfully contribute to the metabolic effects following inactivation of the GCGR, suggesting that the mechanism of glucagon lowering in response to GRAs potentially expands beyond reductions in hepatic glucose production.