Repositioning the Alpha Cell in Postprandial Metabolism

Endocrinology, 2020, Vol. 161, No. 11, 1–11 doi:10.1210/endocr/bqaa169 Mini-Review

Mini-Review Repositioning the Alpha Cell in Postprandial

Metabolism Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 Kimberley El,1 Megan E. Capozzi, 1 and Jonathan E. Campbell1,2,3

1Duke Molecular Physiology Institute, Duke University, Durham, North Carolina, 27701; 2Department of Pharmacology and Cancer Biology, Duke University, Durham, North Carolina, 27708; and 3Department of Medicine, Division of Endocrinology, Duke University, Durham, North Carolina, 27708

ORCiD number: 0000-0003-4358-6331 (J. E. Campbell).

Received: 7 August 2020; Accepted: 17 September 2020; First Published Online: 23 September 2020; Corrected and Typeset: 24 October 2020.

Abstract Glucose homeostasis is maintained in large part due to the actions of the pancreatic islet hormones insulin and glucagon, secreted from β- and α-cells, respectively. The his- torical narrative positions these hormones in opposition, with insulin primarily responsible for glucose-lowering and glucagon-driving elevations in glucose. Recent progress in this area has revealed a more complex relationship between insulin and glucagon, highlighted by data demonstrating that α-cell input is essential for β-cell function and glucose homeostasis. Moreover, the common perception that glucagon levels decrease following a nutrient challenge is largely shaped by the inhibitory effects of glucose administration alone on the α-cell. Largely overlooked is that a mixed nutrient challenge, which is more representative of typical human feeding, actually stimulates glucagon secretion. Thus, postprandial metabolism is associated with elevations, not decreases, in α-cell activity. This review discusses the recent advances in our understanding of how α-cells regulate metabolism, with a particular focus on the postprandial state. We highlight α- to β-cell communication, a term that describes how α-cell input into β-cells is a critical axis that regulates insulin secretion and glucose homeostasis. Finally, we discuss the open questions that have the potential to advance this field and continue to evolve our understanding of the role that α-cells play in postprandial metabolism. Key Words: alpha-cell, beta-cell, glucagon, GLP-1, GIP, glucose homeostasis

Glucagon, secreted by the pancreatic α-cells, has long been glucagon and insulin, highlighting paracrine actions to in- studied under the perception that it has the primary role of hibit glucagon secretion. raising blood glucose to prevent hypoglycemia. A number The aim of this review is to present an alternative and of reviews of glucagon secretion have been published since updated perspective of how glucagon regulates metab- the 1970s and resurfaced in the 2000s. The first reviews olism, with a focus on the postprandial state, derived from alluded to a role for glucagon in insulin secretion, but con- recent evidence that the α-cell is essential for the regula- clusions were centered on glucose-dependent changes in tion of insulin secretion and for glucose metabolism. This

ISSN Online 1945-7170 © The Author(s) 2020. Published by Oxford University Press on behalf of the Endocrine Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and https://academic.oup.com/endo 1 reproduction in any medium, provided the original work is properly cited. Endocrinology, 2020, Vol. 161, No. 11 2 review will reference original and reviewed research that but only a ~2-fold increase in GLP-1 (11). To this end, it highlight the transitioning view of glucagon from its his- has been shown that intestinal preproglucagon (Gcg) ex- torical role as a counterregulatory hormone to having a pression, which is responsible for the production of GLP-1, more significant role in postprandial metabolism. is dispensable for the hyperglycemic effects of GLP-1R antagonism in mice (15), suggesting that gut-derived GLP-1 has minimal activity at the GLP-1R in β-cells. A separate The Incretin Effect set of studies demonstrated that loss of Gcg in the gut does Much of nutrient absorption occurs in the proximal duo- not impair oral glucose-stimulated insulin secretion in mice denum, which subtends the pancreas and receives bile from (16), reinforcing that gut-derived GLP-1 minimally inter- the liver (1). Enteroendocrine cells in the intestine, like K- acts with the β-cell GLP-1R. The modestly impaired glucose

and L-cells, sense nutrients in the lumen at different sections tolerance in this model was attributed to elevated gastric Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 in the intestine: K-cells are predominantly located in the emptying, supporting a role for gut-derived GLP-1 in post- duodenum, while the highest density of L-cells are found in prandial glucose metabolism, just not at the level of the the ileum and colon (2, 3). Absorption of nutrients into the islet. These recent papers raise the question of what ligand intestinal cells stimulates the release of glucose-dependent is responsible for the insulinotropic effects of β-cell GLP-1R insulinotropic polypeptide (GIP) and glucagon-like pep- signaling. The hyperglycemic effects of GLP-1R antag- tide 1 (GLP-1) from K- and L-cells, respectively (4). Termed onism were present in mice with select expression of Gcg in incretins, or insulin secretins (5), GIP and GLP-1 fulfill this α-cells (15), suggesting that the β-cell GLP-1R is stimulated definition by relaying nutrient intake to enhanced insulin primarily from paracrine factors originating in the islet. secretion through endocrine actions mediated by incretin Conventionally, the α-cell processes preproglucagon specif- receptors located on β-cells. The GIP receptor (GIPR) and ically to generate glucagon, not GLP-1 (17). However, there GLP-1 receptor (GLP-1R) are class B G-protein coupled re- is evidence showing the potential for stress factors such as ceptors that potentiate glucose-stimulated insulin secretion streptozotocin or interleukin-6 to induce changes in the in β-cells. Both GIP and GLP-1 require elevated glucose to α-cell that enable differential processing of preproglucagon potentiate insulin secretion, and have little to no action on to yield GLP-1 (18–20). These data postulate that the pri- insulin secretion at low glucose (6, 7). The dependency on mary source of GLP-1 that engages the β-cell is from the elevated glucose allows the incretin axis to fine-tune insulin α-cell, not the gut. Complicating this conclusion is the ob- secretion to reflect the magnitude of nutrient ingestion. In servation that glucagon is also a ligand for the GLP-1R other words, a larger meal provokes greater insulin secre- (21), although with less activity than GLP-1. In mouse is- tion to maintain euglycemia in large part due to the incretin lets, glucagon is secreted at levels that are 100–300x more axis. The incretin effect is defined as the greater insulin secre- than GLP-1, but GLP-1 is 100–300x more potent at stimu- tory response to glucose administered orally than to glucose lating insulin secretion compared with glucagon (6). Thus, given intravenously (8), which encapsulates the importance more work is needed to determine if and when GLP-1 or of these hormones for postprandial glucose metabolism. glucagon acts as the primary ligand for the β-cell GLP-1R. A number of recent observations have challenged the con- Still, the recent observations supporting the notion that the ventional model of the incretin axis. It is clear that both the α-cell is the primary source of this ligand has forced a recon- GIPR and GLP-1R are the primary mediators of the incretin sideration of the incretin axis. It seems that GIP is the pri- action in β-cells. Glucose-dependent insulinotropic polypep- mary incretin derived from the intestine that signals to the tide continues to be considered a gut-derived peptide that islets through the GIPR, whereas the factors that stimulate behaves in an endocrine manner to stimulate the GIPR on GLP-1R signaling come from the α-cell. β-cells, although there is some evidence of GIP production locally in the islets (9, 10). On the other hand, the source of the ligand for the GLP-1R is less clear (11). The location What Do We Actually Eat? of GLP-1 secreting L-cells in the distal gut, far from nu- Diabetes is defined as inappropriately elevated blood glucose trient absorption, suggests its function in the sensing and re- due to insufficient insulin production. The gold-standard to sponding to nutrients is secondary and that GIP may be the diagnose type 2 diabetes (T2D) in patients is the oral glu- primary gut-derived incretin. However, a number of mech- cose tolerance test (OGTT), where blood glucose levels are anisms have been proposed to enable the secretion of GLP-1 monitored in response to ingested glucose. The current con- through signals originating in the proximal gut, including a cept of the incretin axis was originally defined by the ele- neuroendocrine loop mediated by the vagus nerve and GIP vated insulin response to orally ingested glucose versus an (12–14), demonstrating that nutrients do not need to reach isoglycemic load given intravenously (5). The insulinotropic the distal gut in order to stimulate GLP-1 secretion. Still, actions of incretins in β-cells are described as glucose- nutrient intake stimulates an 8- to 10-fold increase in GIP, dependent, and much of the research that has led to the 3 Endocrinology, 2020, Vol. 161, No. 11 development of antihyperglycemic drugs, including incretin combined with glucose? Incretins are secreted in response mimetics, has focused on these glucose-lowering proper- to lipids and amino acids, as well as to glucose. Introducing ties. As such, the diabetes field has comfortably adapted to amino acids during a mixed meal tolerance test (MTT) in- using glucose as a surrogate for nutrient intake. Yet, typical duces markedly more GIP than an OGTT, whereas GLP-1 human feeding does not consist of consuming strictly carbo- levels are increased marginally (23, 24). Intuitively, studies hydrates as the primary source of nutrients. In fact, a mixed of incretin action in the islet have focused on the β-cell and nutrient intake versus a pure carbohydrate stimulus pro- insulin secretion, but incretins also affect the α-cell and glu- duces dramatically different effects on insulin secretion and cagon secretion. Glucagon-like peptide 1 inhibits glucagon the subsequent glycemic excursion. As stated earlier, glucose secretion likely through indirect effects on the β-cell and given orally versus parentally produces a much more robust δ-cell from intra-islet signals (25, 26), although recently

insulin secretion effect, generally attributed to the incretin it was suggested that GLP-1 suppressed glucagon directly Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 axis. When the same bolus of glucose is given intravenously through GLP-1R on the murine α-cell and that GLP-1 can versus orally, the glycemic excursion in response to oral glu- also stimulate glucagon secretion at low glucose levels (27). cose is much lower. This is in part due to the delay caused Yet, the observation that GLP-1 receptors are present on by gastric emptying and intestinal absorption of glucose, the α-cell is unsubstantiated by most studies in both ro- but oral glucose also leads to increased insulin secretion. dents and humans (28–31), and most recently by islet cell However, when isocaloric amounts of glucose are combined populations sorted into enriched individual cell types by with other macronutrients, typically in the form of a mixed fluorescence-activated cell sorting (FACS) using a validated meal challenge, there is a further enhancement of insulin se- GLP-1R antibody, and then analyzed by reverse transcrip- cretion and glycemic lowering (22). In other words, a meal tion polymerase chain reaction (RT-PCR) (32). Additionally, challenge provides a more robust stimulus on the β-cell to while both the GLP-1R and GIPR are expressed in β-cells, enhance insulin secretion compared to oral glucose alone, only the GIPR is expressed in α-cells (33). GIPR activation even though the carbohydrate load is the same (Fig. 1). in α-cells stimulates glucagon secretion, but how this dir- What factors contribute to enhanced insulin secre- ectly contributes to β-cell function and insulin secretion has tion and glucose lowering when additional nutrients are not been tested.

Figure 1. A comparison of the glucagon, insulin, and glycemic response to various nutrient challenges. Glucagon and insulin secretion increase from IPGTT → OGTT → MTT, while the glycemic response decreases from IPGTT → OGTT → MTT. All 3 challenges contain the same glucose load. Abbreviations: GIP, glucose-dependent insulinotropic peptide; IPGTT, intraperitoneal glucose tolerance test; OGTT, oral glucose tolerance test; MTT, mixed meal tolerance test. Endocrinology, 2020, Vol. 161, No. 11 4

The GIPR varies with different nutrient stimuli. that a gut-derived factor counters the inhibitory actions Generally, administration of oral glucose induces a 2-fold of glucose alone on the α-cell, pointing to a potential role increase in GIP levels in humans (34–39), while the levels of the incretin axis. GLP-1R agonists are potent inhibitors of GIP are increased by 3-fold in response to lipid intake of glucagon secretion (55), while GIP stimulates glucagon (40–44) and 4-fold in response to protein intake (24, 43, secretion (56). Could the stimulation of GIP by oral glu- 45, 46). Thus, the enhanced β-cell effect of a mixed nu- cose be the factor that limits the suppression of glucagon trient stimuli could be explained in part by a more robust secretion during an oral glucose challenge compared with activation of the incretin axis. Furthermore, both lipids an intravenous glucose challenge? Studies directly testing and amino acids have direct effects on β-cell function that this hypothesis with loss-of-function interventions for the can serve to amplify insulin secretion independent of the GIPR are needed to answer this question. In healthy hu-

incretin axis (47). However, central to this review is the mans given a mixed nutrient meal, the GIPR antagonists Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 stark difference on the α-cell invoked by glucose alone GIP(3–30) led to a modest decrease in glucagon levels that versus a mixed-nutrient stimulus (Fig. 1). A glucose chal- were not statistically different from control (57). However, lenge exerts a negative effect on α-cell function and re- GIP(3–30) also significantly elevated the glycemic excur- duces glucagon secretion (48). This is more potently seen sion and lowered the insulin response during this test, with intraperitoneal/intravenous glucose compared with making it difficult to assess the direct effects of GIPR an- oral glucose, possibly due to the effects of GIP on the tagonism on the α-cell. α-cell during an OGTT (49). Glucose stimulus alone ac- Compared to an OGTT, a mixed nutrient meal pro- tivates both the β-cell and δ-cell, with a subsequent eleva- duced a greater response in the α-cell, leading to enhanced tion in the inhibitory tone on the α-cell and a decrease in glucagon secretion. It is possible that the presence of add- glucagon secretion (50). Loss of β-cell function in diabetes itional macronutrients in the gut lumen leads to a greater decreases this inhibitory tone, and the glucose stimulation GIP response and a greater stimulation of the α-cell. As is less effective at reducing glucagon levels (51). Because stated earlier, amino acids and lipids are potent regulators of this, T2D is often described as presenting with inappro- of GIP secretion compared with glucose alone. However, priately high glucagon levels following a glucose tolerance there is also evidence that, unlike glucose that confers test (49). However, a mixed nutrient stimulus incorporates an inhibitory signaling on α-cells, amino acids and lipids potent activators of the α-cell, including glucagonotropic stimulate glucagon secretion. Administration of intralipid amino acids and fatty acids (24), overcoming the inhibi- in healthy humans robustly stimulated glucagon secretion tory paracrine actions and leading to an increase in glu- (44). Interestingly, when comparing orally versus intra- cagon secretion (52). Interestingly, direct glucagonotropic venously administered intralipid, at doses that produce effects of GIP have been reported during hypoglycemia and the same concentrations of circulating triglycerides, oral euglycemia, but not hyperglycemia (53), unless there was administration produced a greater rise in glucagon, GIP, underlying metabolic disease (54). The physiological rele- and GLP-1 levels, hinting at an interaction between gut vance of GIP-stimulated glucagon during hypoglycemia is peptides and lipids on α-cell function. The mechanism by unclear, since GIP is secreted in response to nutrients and which lipids enhance glucagon secretion is still unclear. G is expected to be at low levels during hypoglycemia. In re- protein-coupled receptors (GPCRs) that bind fatty acids sponse to a typical meal, α-cell function is enhanced in the are expressed in α-cells (33), and agonism of Gpr119 (58), postprandial state and glucagon levels become elevated. Gpr120 (59), and Gpr40 (60) have been shown to induce Thus, if stimulation of glucagon secretion in the postpran- glucagon secretion. On the other hand, the intracellular dial state is the expected physiological response, how can metabolism of lipids and the generation of adenosine tri- glucagon levels be inappropriately elevated? phosphate (ATP) has also been shown to drive glucagon secretion (61). Many of these studies have been conducted in the context of low glucose, where enhancing α-cell function Stimulation of Glucagon Secretion is anticipated to counter hypoglycemia. Thus, the contri- What are the factors that lead to a rise in glucagon se- bution of lipids to α-cell function in the postprandial state cretion following a mixed nutrient stimulus? Isoglycemic remains less clear. intravenous glucose lowers plasma glucagon more ro- Amino acids have long been known to potently in- bustly than oral glucose in healthy subjects, a phenom- duce glucagon secretion, although not all amino acids enon that is magnified in patients with T2D, potentially are equal in their glucagonotropic activities (62, 63). due to an enhancement in the factors that limit glucagon Historically, arginine has been the most commonly used suppression during an oral glucose challenge (49). This amino acid to induce both insulin and glucagon secre- difference between intravenous and oral glucose suggests tion; however, it is commonly used at concentrations that 5 Endocrinology, 2020, Vol. 161, No. 11 far exceed physiological circulating levels. Indeed, most α- to β-Cell Communication in Fed Conditions studies that have defined the glucagonotropic activities Glucagon has long been known to stimulate insulin secre- of individual amino acids have used supraphysiological tion, despite glucagon being considered a counterregulatory concentrations (62, 63), which limit the extension of hormone that opposes insulin action (74, 75). In patients these findings to the context of a postprandial setting. given an intravenous bolus of glucagon, insulin levels rose Nonetheless, it is generally accepted that branch chain within a minute (74). Moreover, a dose of glucagon given in amino acids (leucine, isoleucine, valine) do not stimulate a hyperglycemic state led to greater insulin secretion (74). the α-cell (62, 64), while arginine, alanine, and glycine This demonstrated a glucose-dependent insulinotropic do when used at physiological concentrations. The fac- action of glucagon, similar to that observed with incretins. tors that mediate the differential glucagonotropic prop- The proximity of α-cells and β-cells positions β-cells to re- erties of specific amino acids remains unresolved (65). In Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 spond to the large paracrine signal from islet α-cells. particular, alanine potently stimulates glucagon secretion α- to β-cell communication and the subsequent enhance- at concentrations as low as 0.5 mM (66), which aligns ment of insulin secretion are lost in mice with deletion of with plasma concentrations of alanine (67). The intimate proglucagon peptides in the α-cell or if the receptors for relationship between amino acids and the α-cell is high- proglucagon peptides (GLP-1R and glucagon receptor lighted by the liver–α-cell axis (68). This axis was origin- [GCGR]) are silenced (6, 15, 71). However, these experi- ally identified by the substantialα -cell hyperplasia that ments do not clarify which proglucagon peptide is respon- occurs following interruption of glucagon signaling in sible for α- to β-cell communication. On one hand, some hepatocytes (69), which was subsequently attributed to studies argue that GLP-1 may be produced in the α-cell in the dramatic rise in amino acids that were no longer being meaningful amounts to affect insulin secretion and thereby catabolized due to the lack of hepatic glucagon signaling systemic glycemia (15, 76, 77). Furthermore, the produc- (67). This phenomenon strikingly resembles the degree of tion of GLP-1 may be amplified during metabolic stress β-cell hyperplasia ensuing after chronically blocking the through increased PC1/3 expression in α-cells (18, 78) and insulin receptor and inducing hyperglycemia (70). This GLP-1 is a more potent insulin secretegogue than glucagon strong relationship between amino acids and the α-cell (6, 79, 80). However, there remains an ongoing debate on has spurred the idea that the primary role for α-cell is the whether α-cells produce bona fide, bioactive GLP-1. First, regulation of amino acid metabolism, not glucose metab- many assays are unable to distinguish the bioactive form olism (65). Interestingly, perifusion of mouse or human of GLP-1 (GLP-1 (7-36)) from the inactive 9–36 or 1–36 islets with the amino acids arginine, glutamine, or ala- isoforms, which have been argued to be the predominant nine produces a biphasic pattern of glucagon secretion, forms of GLP-1 from α-cells (81). A mouse model of α-cell denoted by a strong first phase and followed by a sus- deletion of PC1/3 was used to argue for GLP-1 production tained second phase at a lower rate, remarkably similar in α-cells (78); however, the validity of the knockout was in- to glucose-stimulated insulin secretion (6, 71). Could completely described and the reduction in measured GLP-1 amino acids be the major secretagogue that stimulates was modest. Deletion of intestinal Gcg drastically reduced glucagon secretion, very similar to the primacy of glu- plasma levels of active GLP-1, but only modestly reduced cose for insulin secretion? Directly testing this is diffi- total GLP-1 levels (16). This suggests that α-cells do not cult given the numerous and promiscuous transporters contribute meaningfully to circulating concentrations of ac- for amino acids. However, a reduction in mammalian tive GLP-1. Finally, further complicating the debate is the re- target of rapamycin (mTOR) signaling in α-cells, which cent observations that glucagon stimulates insulin secretion is a central node for intracellular amino acid metabolism primarily through the GLP-1R (6, 71, 81). Although some (72), impairs glucagon secretion and strongly implicates activity is noted at the GCGR, the majority of glucagon- the importance of amino acids for α-cell function (73). stimulated insulin secretion is mitigated by antagonism or Finally, the reduction in glycemic excursion and enhanced deletion of the GLP-1R. This complicates the dissociation insulin and glucagon secretion produced by a mixed nu- of glucagon versus GLP-1 activity in β-cells, as both ligands trient challenge, relative to an intraperitoneal glucose engage the same receptor. Re-examining the previous work tolerance test, can be replicated by the addition of just utilizing Ex9 to argue for α-cell GLP-1 (15) dampens the alanine to an intraperitoneal glucose tolerance test (71). enthusiasm for this evidence, as all of these data could be at- Therefore, if glucose is the primary fuel for the β-cell, it tributed to glucagon. Therefore, α- to β-cell communication seems plausible that amino acids are the primary fuel for is attributed broadly to proglucagon products until further α-cells. Moreover, activation of the α-cell by amino acids studies can specify the extent to which glucagon or GLP-1 is necessary for insulin secretion, a term we have called uniquely contribute to intra-islet communication. α- to β-cell communication (6, 52, 71). Endocrinology, 2020, Vol. 161, No. 11 6

β-cells express both the GLP-1R and GCGR, which a GRA with GLP-1R antagonist Ex9 reduced efficacy of utilize cyclic adenosine monophosphate (cAMP) as a sec- a GRA to lower glucose (86). Furthermore, the hypogly- ondary messenger to stimulate insulin secretion (6). The cemic phenotype of Gcgr-/- mice in response to a glucose GCGR is also highly expressed in the liver (68), while the tolerance test is absent in Gcgr:Glp1r-/- mice (87). The hep- GLP-1R is not (82). While it is clear that glucagon signals atic contribution of glucagon signaling clearly contributes through the hepatic GCGR to exert effects on glycemia, the to the lowered glycemic phenotype in models with loss of role of the GCGR in the β-cell is more complex. GCGR pos- function for glucagon signaling, evident by the phenotype sesses a high degree of homology with GIPR and GLP-1R of Gcg-/- mice that lack glucagon and GLP-1 (15). However, (83), congruent with its insulinotropic actions. However, the enhanced α- to β-cell communication that results from an early study using the GCGR inhibitor, des-His1-[Glu9] blockade of glucagon signaling is an underappreciated con-

glucagon-amide, showed that unlike hepatocytes, blockade tributor that warrants further investigation (80). Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 of GCGR in rat β-cells had little effect on glucagon-induced Recent work demonstrates the importance of the nutri- cAMP induction (21). However, the GLP-1R inhibitor, tional state in dictating the glycemic impact of glucagon. Exendin 9 (Ex9), partially blocked glucagon-stimulated When 1 mg/kg glucagon was administered to fed mice, there insulin secretion. These findings have since been corrob- was a significant induction of insulin that led to an overall orated (6, 81, 84, 85), and found to be more pronounced lowering of glycemia (71). Alanine produced a similar with physiological levels of glucagon stimulation in mouse effect to lower glycemia while increasing circulating glu- pancreas (81), mouse islets (6, 85), and human islets (6, 84). cagon (71), demonstrating that the glucose-lowering effects These studies showed that both GCGR and GLP-1R are can be achieved by endogenous α-cell activity. Moreover, important for glucagon-stimulated insulin secretion, but the glucose lowering effect of exogenous or endogenous that GLP-1R predominates for the insulinotropic actions. glucagon activity requires β-cells and insulin secretion, as Moreover, recent studies have shown the crucial role for β-cell–specific deletion of Glp1r and Gcgr prevented glu- the α-cell in determining the tone of islet insulin secretion cose lowering in response to either exogenous glucagon (6, 84); the dynamics of insulin secretion are maintained or alanine (71). Additionally, an approach utilizing a with loss of proglucagon products, but the magnitude of in- Gi-coupled designer receptor exclusively activated by de- sulin secretion is substantially dampened. It is now appre- signer drugs (to lower glucagon secretion from the α-cell ciated that in isolated islets, proglucagon tone is necessary showed that a significant loss of plasma glucagon levels co- for the full extent of nutrient-stimulated insulin secretion. incided with a drop in plasma insulin and glucose intoler- The results of ex vivo studies have been supported by ance in an intraperitoneal glucose tolerance test (IPGTT) several in vivo studies using a range of techniques to dem- (85). We have also recently validated that the insulinotropic onstrate the importance of α- to β-cell communication. actions of glucagon prevail using ketone production from Interestingly, it was recently shown that islet grafts carry the the liver as an output of hormone action (88). Indeed, glycemic setpoint in a species-dependent manner, and that treatment with exogenous glucagon significantly lowered this relies on glucagon content of the donor (84). In these ketone production within 20 minutes, in line with insulin studies, immunodeficient mice made diabetic with strepto- action on ketosis (88). The ability for glucagon to stimulate zotocin received islet xenografts in the eye or kidney that ketone production only prevailed when insulin secretion generated a glycemic set point in line with the species of the was completely removed with the combination of strepto- transplant. Importantly, human-specific glucagon receptor zotocin, to delete the majority of β-cells, and treatment blockade by the antagonist L-168,049 in mice with human with an insulin receptor antagonist. Overall, these studies islet xenografts raised systemic glycemia (84). Further sup- demonstrate that α- to β-cell communication is necessary port for the importance of α- to β-cell communication is for setting the tone of insulin secretion and overall glycemia observed in mice lacking the GLP-1R and receiving glu- in normal physiology. cagon receptor antagonist (GRA). Glucagon receptor antagonist consistently lower glycemia, which has primarily been attributed to a reduction in hepatic glucose output. α- to Hepatocyte Communication in Fed A limitation to clinical GRA use, however, is that it leads Conditions to significantα -cell hyperplasia and elevation of circulating The canonical view of glucagon focuses on its hepatic ac- glucagon. However, with the perspective that glucagon can tions as a hypoglycemia-responsive hormone functioning in act through the β-cell GLP-1R to lower glycemia, it would the absence of ingested mixed nutrients. Indeed, glucagon be expected that large elevations in glucagon could lead to potently activates glycogenolysis and gluconeogenesis significant increases in insulin that may be equally important through activation of GCGR and cAMP/protein kinase A for lowering glycemia. Indeed, pre-treatment of mice given (PKA) signaling in fasting conditions. Yet, glucagon levels 7 Endocrinology, 2020, Vol. 161, No. 11 actually decrease following prolonged fasting (>3 days) hypoglycemic hormone has limited the study of glucagon (89), and the addition of glucagon to patients during a and insulin costimulation of hepatocytes in a prandial prolonged fast did not elevate glycemia (90). These find- state. While there have been studies of bihormonal actions ings question the notion that glucagon’s primary role is in the liver, they have largely been studied in the context of to coordinate the glycemic responses to a prolonged fast. glucagon rescue for insulin-induced hypoglycemia, which However, it is important to note that prolonged fasting would not mimic the secretory dynamics of the 2 hormones. significantly reduces hepatic glycogen content, potentially There is some evidence to suggest that glucagon and in- minimizing the ability for glucagon to enhance hepatic glu- sulin have cooperative actions in hepatocytes. Interestingly, cose output. Nonetheless, these data show that additional pretreating mice with a glucagon receptor agonist led mechanisms other than glucagon maintain glycemic levels to enhanced glucose lowering during a subsequent

in the context of sustained fasting. In fact, an overnight fast intraperitoneal glucose tolerance test (103). This effect was Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 in Gcg-/- mice produces the same reduction of glycemia as attributed in part to increased hepatic insulin sensitivity de- control mice (88). On the other hand, several recent studies noted by increased phosphorylation of AKT (pAKT473), in liver suggest a role for glucagon that extend beyond the an integral step of hepatic insulin signaling (103). While prevention of hypoglycemia and into the control of nu- it is possible that glucagon agonism stimulated insulin se- trient metabolism during the fed state. cretion in β-cells, this effect has recapitulated in isolated The culmination of early landmark studies and recent hepatocytes, suggesting a specific action of glucagon in work supports the notion that glucagon is a master regu- the liver. Notably, liver-specific GCGR deletion mitigated lator of amino acid metabolism. Amino acids provide a insulin-induced pAKT in hepatocyte cultures (103), sug- carbon source for several catabolic processes, while the ni- gesting a direct cross-talk of glucagon and insulin at the trogen backbone is removed through urea. Maintenance of liver that remains undefined. A recent study using whey amino acid levels and, similarly, ureagenesis are processes protein feeding in healthy human subjects reported induc- highly responsive to glucagon. In both human studies tion of both insulin and glucagon, along with increased en- (91–93) and animal models (94–97), glucagon treatment dogenous glucose production (104). This finding suggests drastically modulates urea synthesis and amino acid levels. that the glycemic lowering that results from the combin- Furthermore, infusion of glucagon above basal levels, with ation of insulin and glucagon is not only because insulin glucose, amino acids, and insulin to mimic a postprandial overcomes the ability for glucagon to stimulate hepatic state, promoted increased amino acid utilization in catheter- glucose production. The hepatic signature of insulin and ized dogs (96). Studies using GCGR deletion or blockade glucagon costimulation remains an open question in the in mice have profound increases in circulating amino acids, field, with important implications for understanding the eliciting α-cell hyperplasia and hyperglucagonemia (69, hormonal control of glucose homeostasis. 98–101). Conversely, patients with glucagonomas, char- acterized by hyperglucagonemia, have substantially elevated ureagenesis and lowered circulating amino acids. Conclusion In fact, glucagonomas present with a skin rash, necrolytic The recent progress in our understanding of how the migratory erythema, which is a direct result of severe α-cell contributes to postprandial metabolic homeostasis hypoaminoacidemia (102). These findings demonstrate a has forced a reconsideration for the role of proglucagon critical liver– α-cell axis communicated through circulating peptides in response to food intake. While the GLP-1R in amino acid levels. Indeed, the drastic changes in amino acid β-cells continues to be a key regulator of insulin secretion levels following glucagon manipulation support the notion and glucose homeostasis, it is less clear what activates this that glucagon is a master regulator of hepatic protein me- receptor. The emerging evidence suggests that gut-derived tabolism. As such, glucagon’s hyperglycemic actions may GLP-1 does not interact with the GLP-1R, but rather that be a secondary mechanism to promote conversion of amino GLP-1 might originate from the α-cell. Alternatively, glu- acids to glucose in order to maintain appropriate circu- cagon enhances insulin secretion via the β-cell GLP-1R, lating concentrations. questioning the need for GLP-1 as a β-cell ligand altogether. In a mixed nutrient meal, glucagon responds to amino Future studies are needed to delineate which hormone is acid levels to regulate protein metabolism, while insulin is primarily responsible for governing insulin secretion, and secreted to manage carbohydrate metabolism. In this case whether this changes with the progression of metabolic of normal physiology in response to meal ingestion, the dysfunction. Still, the concept that the α-cell is required various nutrients would cause glucagon and insulin secre- for β-cell function has re-oriented the relationship between tion into the portal circulation simultaneously. However, α- and β-cells. Indeed, rather than carrying out opposing the conventional understanding of glucagon being a roles in the balance of glucose homeostasis, α- and β-cells Endocrinology, 2020, Vol. 161, No. 11 8 cooperate in a postprandial setting to optimally regulate 2. Gutierrez-Aguilar R, Woods SC. Nutrition and L and the metabolic control of nutrients. K-enteroendocrine cells. Curr Opin Endocrinol Diabetes Obes. The question remains as to why glucagon is abnor- 2011;18(1):35–41. 3. Berghöfer P, Peterson RG, Schneider K, Fehmann HC, Göke B. mally elevated in patients with diabetes and does not Incretin hormone expression in the gut of diabetic mice and lower in response to a carbohydrate load in these patients. rats. Metabolism. 1997;46(3):261–267. We believe that recent data suggests that glucagon eleva- 4. Baggio LL, Drucker DJ. Biology of incretins: GLP-1 and GIP. tion is an adaptive reaction as opposed to an early causa- Gastroenterology. 2007;132(6):2131–2157. tive step in hyperglycemia, similar to the accepted view 5. Rehfeld JF. The origin and understanding of the incretin con- that the hyperinsulinemia induced by insulin resistance cept. Front Endocrinol (Lausanne). 2018;9:387. is an adaptive function of β-cells. The root cause driving 6. Capozzi ME, Svendsen B, Encisco SE, et al. β cell tone is defined by proglucagon peptides through cAMP signaling. JCI Insight. hyperglucagonemia remains unknown. Candidate mechan- Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 2019:4(5):1–15. isms include a reduction in inhibitory tone as β-cell func- 7. Miki T, Minami K, Shinozaki H, et al. Distinct effects of tion diminishes, α-cell hyperplasia/hypertrophy in response glucose-dependent insulinotropic polypeptide and glucagon- to metabolic stress, enhanced GIP levels that increase α-cell like peptide-1 on insulin secretion and gut motility. Diabetes. activity, and an increase in amino acid tone on the α-cell 2005;54(4):1056–1063. due to either enhanced amino acid flux or decreased amino 8. Nauck MA, Homberger E, Siegel EG, et al. Incretin effects of acid catabolism. increasing glucose loads in man calculated from venous in- The α-cell is a powerful and central regulator of meta- sulin and C-peptide responses. J Clin Endocrinol Metab. 1986;63(2):492–498. bolic homeostasis. To fully appreciate the role that α-cell 9. Yanagimachi T, Fujita Y, Takeda Y, et al. Pancreatic glucose- products have on postprandial nutrient metabolism, the dependent insulinotropic polypeptide (GIP) (1-30) expression is field has begun to expand beyond the role of glucagon as upregulated in diabetes and PEGylated GIP(1-30) can suppress a counterregulatory hormone that works to limit hypogly- the progression of low-dose-STZ-induced hyperglycaemia in cemia. Furthermore, expanding these investigations beyond mice. Diabetologia. 2016;59(3):533–541. glucose metabolism enables a better appreciation for the 10. Fujita Y, Wideman RD, Asadi A, et al. Glucose-dependent additional mechanisms that govern metabolic control in a insulinotropic polypeptide is expressed in pancreatic islet alpha- postprandial state. Understanding these mechanisms can cells and promotes insulin secretion. Gastroenterology. 2010;138(5):1966–1975. better identify the compliment of pathophysiological com- 11. D’Alessio D. Is GLP-1 a hormone: whether and When? J plications that ultimately drive the metabolic dysfunction Diabetes Investig. 2016;7(Suppl 1):50–55. that leads to diabetes. By incorporating the α-cell into the 12. Anini Y, Hansotia T, Brubaker PL. Muscarinic receptors control equation, the field is moving towards a more complete pic- postprandial release of glucagon-like peptide-1: in vivo and in ture of metabolic homeostasis. vitro studies in rats. Endocrinology. 2002;143(6):2420–2426. 13. Brubaker PL, Anini Y. Direct and indirect mechanisms regulating secretion of glucagon-like peptide-1 and glucagon-like Additional Information peptide-2. Can J Physiol Pharmacol. 2003;81(11):1005–1012. 14. Rocca AS, Brubaker PL. Role of the vagus nerve in mediating Correspondence: Jonathan E. Campbell, 300 N Duke Street, Dur- proximal nutrient-induced glucagon-like peptide-1 secretion. ham, North Carolina 27701, USA. E-mail: jonathan.campbell@ Endocrinology. 1999;140(4):1687–1694. duke.edu. 15. Chambers AP, Sorrell JE, Haller A, et al. The role of pancre- Disclosure Summary: K.E. and M.E.C. are funded by fellowships atic preproglucagon in glucose homeostasis in mice. Cell Metab. from the National Institutes of Health (NIH) (T32 DK007012, F32 2017;25(4):927–934.e3. DK116542). J.E.C. is funded by the American Diabetes Association 16. Song Y, Koehler JA, Baggio LL, Powers AC, Sandoval DA, (ADA) (1-18-JDF-017) and NIH (R01 DK123075, R01 DK125353) and is a Borden Scholar. The Campbell lab receives funding for basic Drucker DJ. Gut-proglucagon-derived peptides are essen- science projects unrelated to this work from Eli Lilly and Novo tial for regulating glucose homeostasis in mice. Cell Metab. Nordisk. J.E.C. has received a speaker honorarium from Merck in 2019;30(5):976–986.e3. the past 12 months. 17. Campbell JE, Drucker DJ. Islet α cells and glucagon–crit- Data Availability: Data sharing is not applicable to this article, ical regulators of energy homeostasis. Nat Rev Endocrinol. as no datasets were generated or analyzed during the current study. 2015;11(6):329–338. 18. Marchetti P, Lupi R, Bugliani M, et al. A local glucagon-like peptide 1 (GLP-1) system in human pancreatic islets. Diabetologia. References 2012;55(12):3262–3272. 19. Ellingsgaard H, Hauselmann I, Schuler B, et al. Interleukin-6 1. Collins JT, Nguyen A, Badireddy M. Anatomy, abdomen and enhances insulin secretion by increasing glucagon-like pep- pelvis, small intestine. In StatPearls [Internet]. Treasure Island tide-1 secretion from L cells and alpha cells. Nat Med. (FL); 2020. 2011;17(11):1481–1489. 9 Endocrinology, 2020, Vol. 161, No. 11

20. Nie Y, Nakashima M, Brubaker PL, et al. Regulation of pan- glucose loading are associated with different factors in Japanese creatic PC1 and PC2 associated with increased glucagon-like subjects. J Diabetes Investig. 2011;2(3):193–199. peptide 1 in diabetic rats. J Clin Invest. 2000;105(7):955–965. 38. Limb C, Tamborlane WV, Sherwin RS, Pederson R, Caprio S. 21. Moens K, Flamez D, Van Schravendijk C, Ling Z, Pipeleers D, Acute incretin response to oral glucose is associated with stimu- Schuit F. Dual glucagon recognition by pancreatic beta-cells lation of gastric inhibitory polypeptide, not glucagon-like pep- via glucagon and glucagon-like peptide 1 receptors. Diabetes. tide in young subjects. Pediatr Res. 1997;41(3):364–367. 1998;47(1):66–72. 39. Theodorakis MJ, Carlson O, Muller DC, Egan JM. Elevated 22. Douros JD, Niu J, Sdao S, et al. Sleeve gastrectomy rapidly en- plasma glucose-dependent insulinotropic polypeptide associates hances islet function independently of body weight. JCI Insight. with hyperinsulinemia in impaired glucose tolerance. Diabetes 2019;4(6):1–13. Care. 2004;27(7):1692–1698. 23. Koopman ADM, Rutters F, Rauh SP, et al. Incretin responses to 40. Elliott RM, Morgan LM, Tredger JA, Deacon S, Wright J, oral glucose and mixed meal tests and changes in fasting glucose Marks V. Glucagon-like peptide-1 (7-36)amide and glucose- Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 levels during 7 years of follow-up: the Hoorn Meal Study. Plos dependent insulinotropic polypeptide secretion in response to One. 2018;13(1):1–13. nutrient ingestion in man: acute post-prandial and 24-h secre- 24. Carrel G, Egli L, Tran C, et al. Contributions of fat and pro- tion patterns. J Endocrinol. 1993;138(1):159–166. tein to the incretin effect of a mixed meal. Am J Clin Nutr. 41. Yamane S, Harada N, Inagaki N. Mechanisms of fat-induced 2011;94(4):997–1003. gastric inhibitory polypeptide/glucose-dependent insulinotropic 25. Müller TD, Finan B, Bloom SR, et al. Glucagon-like peptide 1 polypeptide secretion from K cells. J Diabetes Investig. (GLP-1). Mol Metab. 2019;30:72–130. 2016;7(Suppl 1):20–26. 26. Gromada J, Rorsman P. New insights into the regulation of glu- 42. Falko JM, Crockett SE, Cataland S, Mazzaferri EL. Gastric in- cagon secretion by glucagon-like peptide-1. Horm Metab Res. hibitory polypeptide (GIP) stimulated by fat ingestion in man. J 2004;36(11-12):822–829. Clin Endocrinol Metab. 1975;41(2):260–265. 27. Zhang Y, Parajuli KR, Fava GE, et al. GLP-1 receptor in pancre- 43. Carr RD, Larsen MO, Winzell MS, et al. Incretin and islet hor- atic α-cells regulates glucagon secretion in a glucose-dependent monal responses to fat and protein ingestion in healthy men. bidirectional manner. Diabetes. 2019;68(1):34–44. Am J Physiol Endocrinol Metab. 2008;295(4):E779–E784. 28. Muraro MJ, Dharmadhikari G, Grün D, et al. A single- 44. Lindgren O, Carr RD, Deacon CF, et al. Incretin hormone and cell transcriptome atlas of the human pancreas. Cell Syst. insulin responses to oral versus intravenous lipid administration 2016;3(4):385–394.e3. in humans. J Clin Endocrinol Metab. 2011;96(8):2519–2524. 29. Segerstolpe Å, Palasantza A, Eliasson P, et al. Single-cell tran- 45. Lindgren O, Pacini G, Tura A, Holst JJ, Deacon CF, Ahrén B. scriptome profiling of human pancreatic islets in health and Incretin effect after oral amino acid ingestion in humans. J Clin type 2 diabetes. Cell Metab. 2016:24(4):593–607. Endocrinol Metab. 2015;100(3):1172–1176. 30. Waser B, Blank A, Karamitopoulou E, Perren A, Reubi JC. 46. Salehi A, Gunnerud U, Muhammed SJ, et al. The insulinogenic Glucagon-like-peptide-1 receptor expression in normal effect of whey protein is partially mediated by a direct effect and diseased human thyroid and pancreas. Mod Pathol. of amino acids and GIP on β-cells. Nutr Metab (Lond). 2015;28(3):391–402. 2012;9(1):1–7. 31. Tornehave D, Kristensen P, Rømer J, Knudsen LB, Heller RS. 47. Prentki M, Matschinsky FM, Madiraju SR. Metabolic Expression of the GLP-1 receptor in mouse, rat, and human signaling in fuel-induced insulin secretion. Cell Metab. pancreas. J Histochem Cytochem. 2008;56(9):841–851. 2013;18(2):162–185. 32. Gray SM, Xin Y, Ross EC, et al. Discordance between GLP-1R 48. Unger RH, Aguilar-Parada E, Müller WA, Eisentraut AM. gene and protein expression in mouse pancreatic islet cells. J Studies of pancreatic alpha cell function in normal and diabetic Biol Chem. 2020;295(33):11529–11541. subjects. J Clin Invest. 1970;49(4):837–848. 33. DiGruccio MR, Mawla AM, Donaldson CJ, et al. Comprehensive 49. Knop FK, Vilsbøll T, Madsbad S, Holst JJ, Krarup T. alpha, beta and delta cell transcriptomes reveal that ghrelin se- Inappropriate suppression of glucagon during OGTT but not lectively activates delta cells and promotes somatostatin release during isoglycaemic i.v. glucose infusion contributes to the re- from pancreatic islets. Mol Metab. 2016;5(7):449–458. duced incretin effect in type 2 diabetes mellitus. Diabetologia. 34. Cataland S, Crockett SE, Brown JC, Mazzaferri EL. Gastric in- 2007;50(4):797–805. hibitory polypeptide (GIP) stimulation by oral glucose in man. J 50. Watts M, Ha J, Kimchi O, Sherman A. Paracrine regulation of Clin Endocrinol Metab. 1974;39(2):223–228. glucagon secretion: the β/α/δ model. Am J Physiol Endocrinol 35. Gjesing AP, Ekstrøm CT, Eiberg H, et al. Fasting and oral Metab. 2016;310(8):E597–E611. glucose-stimulated levels of glucose-dependent insulinotropic 51. Godoy-Matos AF. The role of glucagon on type 2 diabetes at a polypeptide (GIP) and glucagon-like peptide-1 (GLP- glance. Diabetol Metab Syndr. 2014;6(1):1–5. 1) are highly familial traits. Diabetologia. 2012;55(5): 52. Finan B, Capozzi ME, Campbell JE. Repositioning glucagon 1338–1345. action in the physiology and pharmacology of diabetes. 36. Bagger JI, Knop FK, Lund A, Vestergaard H, Holst JJ, Vilsbøll T. Diabetes. 2020;69(4):532–541. Impaired regulation of the incretin effect in patients with type 2 53. Christensen M, Vedtofte L, Holst JJ, Vilsbøll T, Knop FK. diabetes. J Clin Endocrinol Metab. 2011;96(3):737–745. Glucose-dependent insulinotropic polypeptide: a bifunctional 37. Harada N, Hamasaki A, Yamane S, et al. Plasma gastric in- glucose-dependent regulator of glucagon and insulin secretion hibitory polypeptide and glucagon-like peptide-1 levels after in humans. Diabetes. 2011;60(12):3103–3109. Endocrinology, 2020, Vol. 161, No. 11 10

54. Christensen MB, Calanna S, Holst JJ, Vilsbøll T, Knop FK. 73. Bozadjieva N, Blandino-Rosano M, Chase J, et al. Loss of Glucose-dependent insulinotropic polypeptide: blood glu- mTORC1 signaling alters pancreatic α cell mass and impairs cose stabilizing effects in patients with type 2 diabetes. J Clin glucagon secretion. J Clin Invest. 2017;127(12):4379–4393. Endocrinol Metab. 2014;99(3):E418–E426. 74. Samols E, Marri G, Marks V. Interrelationship of glucagon, 55. Campbell JE, Drucker DJ. Pharmacology, physiology, insulin and glucose. The insulinogenic effect of glucagon. and mechanisms of incretin hormone action. Cell Metab. Diabetes. 1966;15(12):855–866. 2013;17(6):819–837. 75. Turner DS, McIntyre N. Stimulation by glucagon of insulin release 56. El K, Campbell JE. The role of GIP in α-cells and glucagon se- from rabbit pancreas in vitro. Lancet. 1966;1(7433):351–352. cretion. Peptides. 2020;125:1–6. 76. Campbell SA, Golec DP, Hubert M, et al. Human islets contain 57. Gasbjerg LS, Helsted MM, Hartmann B, et al. GIP and GLP-1 a subpopulation of glucagon-like peptide-1 secreting α cells that receptor antagonism during a meal in healthy individuals. J Clin is increased in type 2 diabetes. Mol Metab. 2020;39:1–11. Endocrinol Metab. 2020;105(3):e725–e738. 77. Fava GE, Dong EW, Wu H. Intra-islet glucagon-like peptide 1. J Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 58. Li NX, Brown S, Kowalski T, et al. GPR119 agonism increases Diabetes Complications. 2016;30(8):1651–1658. glucagon secretion during insulin-induced hypoglycemia. 78. Traub S, Meier DT, Schulze F, et al. Pancreatic α cell-derived Diabetes. 2018;67(7):1401–1413. glucagon-related peptides are required for β cell adaptation and 59. Suckow AT, Polidori D, Yan W, et al. Alteration of the glucagon glucose homeostasis. Cell Rep. 2017;18(13):3192–3203. axis in GPR120 (FFAR4) knockout mice: a role for GPR120 in 79. Finan B, Yang B, Ottaway N, et al. A rationally designed mono- glucagon secretion. J Biol Chem. 2014;289(22):15751–15763. meric peptide triagonist corrects obesity and diabetes in ro- 60. Flodgren E, Olde B, Meidute-Abaraviciene S, Winzell MS, dents. Nat Med. 2015;21(1):27–36. Ahrén B, Salehi A. GPR40 is expressed in glucagon produ- 80. Finan B, Capozzi ME, Campbell JE. Repositioning glucagon cing cells and affects glucagon secretion. Biochem Biophys Res action in the physiology and pharmacology of diabetes. Commun. 2007;354(1):240–245. Diabetes. 2020;69(4):532–541. 61. Briant LJB, Dodd MS, Chibalina MV, et al. CPT1a-dependent 81. Svendsen B, Larsen O, Gabe MBN, et al. Insulin secre- long-chain fatty acid oxidation contributes to maintaining tion depends on intra-islet glucagon signaling. Cell Rep. glucagon secretion from pancreatic islets. Cell Rep. 2018;25(5):1127–1134.e2. 2018;23(11):3300–3311. 82. Panjwani N, Mulvihill EE, Longuet C, et al. GLP-1 receptor ac- 62. Rocha DM, Faloona GR, Unger RH. Glucagon-stimulating tivation indirectly reduces hepatic lipid accumulation but does activity of 20 amino acids in dogs. J Clin Invest. not attenuate development of atherosclerosis in diabetic male 1972;51(9):2346–2351. ApoE(-/-) mice. Endocrinology. 2013;154(1):127–139. 63. Galsgaard KD, Jepsen SL, Kjeldsen SAS, Pedersen J, 83. Brubaker PL, Drucker DJ. Structure-function of the glucagon re- Wewer Albrechtsen NJ, Holst JJ. Alanine, arginine, cysteine, ceptor family of G protein-coupled receptors: the glucagon, GIP, GLP- and proline, but not glutamine, are substrates for, and acute 1, and GLP-2 receptors. Recept Channels. 2002;8(3-4):179–188. mediators of, the liver-α-cell axis in female mice. Am J Physiol 84. Rodriguez-Diaz R, Molano RD, Weitz JR, et al. Paracrine inter- Endocrinol Metab. 2020;318(6):E920–E929. actions within the pancreatic islet determine the glycemic set 64. Kaneto A, Kosaka K. Effects of leucine and isoleucine in- point. Cell Metab. 2018;27(3):549–558.e4. fused intrapancreatically on glucagon and insulin secretion. 85. Zhu L, Dattaroy D, Pham J, et al. Intra-islet glucagon signaling Endocrinology. 1972;91(3):691–695. is critical for maintaining glucose homeostasis. JCI Insight. 65. Dean ED. A primary role for α-cells as amino acid sensors. 2019;4(10):1–15. Diabetes. 2020;69(4):542–549. 86. Gu W, Winters KA, Motani AS, et al. Glucagon receptor 66. Gray SM, Niu J, Zhang A, et al. Intraislet ghrelin signaling antagonist-mediated improvements in glycemic control are de- does not regulate insulin secretion from adult mice. Diabetes. pendent on functional pancreatic GLP-1 receptor. Am J Physiol 2019;68(9):1795–1805. Endocrinol Metab. 2010;299(4):E624–-E632. 67. Dean ED, Li M, Prasad N, et al. Interrupted glucagon signaling 87. Ali S, Lamont BJ, Charron MJ, Drucker DJ. Dual elimination of reveals hepatic α cell axis and role for L-glutamine in α cell pro- the glucagon and GLP-1 receptors in mice reveals plasticity in liferation. Cell Metab. 2017;25(6):1362–1373.e5. the incretin axis. J Clin Invest. 2011;121(5):1917–1929. 68. Wewer Albrechtsen NJ, Pedersen J, Galsgaard KD, et al. 88. Capozzi ME, Coch RW, Koech J, et al. The limited role of glu- The liver-α-cell axis and type 2 diabetes. Endocr Rev. cagon for ketogenesis during fasting or in response to SGLT2 2019;40(5):1353–1366. inhibition. Diabetes. 2020;69(5):882–892. 69. Longuet C, Robledo AM, Dean ED, et al. Liver-specific disrup- 89. Marliss EB, Aoki TT, Unger RH, Soeldner JS, Cahill GF Jr. tion of the murine glucagon receptor produces α-cell hyper- Glucagon levels and metabolic effects in fasting man. J Clin plasia: evidence for a circulating α-cell growth factor. Diabetes. Invest. 1970;49(12):2256–2270. 2013;62(4):1196–1205. 90. Fisher M, Sherwin RS, Hendler R, Felig P. Kinetics of glu- 70. Yi P, Park JS, Melton DA. Betatrophin: a hormone that controls cagon in man: effects of starvation. Proc Natl Acad Sci U S A. pancreatic β cell proliferation. Cell. 2013;153(4):747–758. 1976;73(5):1735–1739. 71. Capozzi ME, Wait JB, Koech J, et al. Glucagon lowers glycemia 91. Almdal TP, Jensen T, Vilstrup H. Increased hepatic efficacy of when beta-cells are active. JCI Insight. 2019;4(16):1–12. urea synthesis from alanine in insulin-dependent diabetes mel- 72. Sabatini DM. Twenty-five years of mTOR: Uncovering the litus. Eur J Clin Invest. 1990;20(1):29–34. link from nutrients to growth. Proc Natl Acad Sci U S A. 92. Hamberg O, Vilstrup H. Regulation of urea synthesis by glucose 2017;114(45):11818–11825. and glucagon in normal man. Clin Nutr. 1994;13(3):183–191. 11 Endocrinology, 2020, Vol. 161, No. 11

93. Boden G, Rezvani I, Owen OE. Effects of glucagon on plasma exposing a possible liver-alpha-cell axis. Am J Physiol amino acids. J Clin Invest. 1984;73(3):785–793. Endocrinol Metab. 2018;314(1):E93–E103. 94. Almdal TP, Petersen KF, Hansen BA, Vilstrup H. Increased 100. Kim J, Okamoto H, Huang Z, et al. Amino acid transporter capacity of urea synthesis in streptozotocin diabetes in rats. Slc38a5 controls glucagon receptor inhibition-induced Diabetologia. 1986;29(11):812–816. pancreatic alpha cell hyperplasia in mice. Cell Metab. 95. Almdal TP, Vilstrup H. Exogenous hyperglucagonaemia in in- 2017;25(6):1348–1361.e8. sulin controlled diabetic rats increases urea excretion and ni- 101. Watanabe C, Seino Y, Miyahira H, et al. Remodeling of hep- trogen loss from organs. Diabetologia. 1988;31(11):836–841. atic metabolism and hyperaminoacidemia in mice deficient 96. Kraft G, Coate KC, Winnick JJ, et al. Glucagon’s effect on liver in proglucagon-derived peptides. Diabetes. 2012;61(1): protein metabolism in vivo. Am J Physiol Endocrinol Metab. 74–84. 2017;313(3):E263–E272. 102. Eldor R, Glaser B, Fraenkel M, Doviner V, Salmon A, Gross DJ. 97. Snodgrass PJ, Lin RC, Müller WA, Aoki TT. Induction of Glucagonoma and the glucagonoma syndrome - cumulative Downloaded from https://academic.oup.com/endo/article/161/11/bqaa169/5910252 by guest on 01 October 2021 urea cycle enzymes of rat liver by glucagon. J Biol Chem. experience with an elusive endocrine tumour. Clin Endocrinol 1978;253(8):2748–2753. (Oxf). 2011;74(5):593–598. 98. Galsgaard KD, Winther-Sørensen M, Pedersen J, et al. Glucose 103. Kim T, Holleman CL, Nason S, et al. Hepatic glucagon receptor and amino acid metabolism in mice depend mutually on glu- signaling enhances insulin-stimulated glucose disposal in ro- cagon and insulin receptor signaling. Am J Physiol Endocrinol dents. Diabetes. 2018;67(11):2157–2166. Metab. 2019;316(4):E660–E673. 104. Ang T, Bruce CR, Kowalski GM. Postprandial aminogenic in- 99. Galsgaard KD, Winther-Sørensen M, Ørskov C, et al. Disruption sulin and glucagon secretion can stimulate glucose flux in hu- of glucagon receptor signaling causes hyperaminoacidemia mans. Diabetes. 2019;68(5):939–946.