Immunoneutralization of endogenous glucagon reduces hepatic glucose output and improves long-term glycemic control in diabetic ob/ob mice

Benefits and limitations of reducing glucagon action for the treatment of type 2 diabetes

Glucagon is secreted from the alpha-cells of the pancreatic islets and regulates glucose homeostasis through modulation of hepatic glucose production. As elevated glucagon levels contribute to the pathophysiology of hyperglycemia in subjects with type 2 diabetes, reduction of glucagon receptor gene (Gcgr) activity represents a potential target for the treatment of T2DM. Herein, we review current concepts of glucagon action in hepatic and extrahepatic tissues and evaluate the therapeutic potential, mechanisms of action, and safety of reducing Gcgr signaling for the treatment of T2DM.

Fully human monoclonal antibodies antagonizing the glucagon receptor improve glucose homeostasis in mice and monkeys

Antagonizing the glucagon signaling pathway represents an attractive therapeutic approach for reducing excess hepatic glucose production in patients with type 2 diabetes. Despite extensive efforts, there is currently no human therapeutic that directly inhibits the glucagon/glucagon receptor pathway. We undertook a novel approach by generating high-affinity human monoclonal antibodies (mAbs) to the human glucagon receptor (GCGR) that display potent antagonistic activity in vitro and in vivo. A single injection of a lead antibody, mAb B, at 3 mg/kg, normalized blood glucose levels in ob/ob mice for 8 days. In addition, a single injection of mAb B dose-dependently lowered fasting blood glucose levels without inducing hypoglycemia and improved glucose tolerance in normal C57BL/6 mice. In normal cynomolgus monkeys, a single injection improved glucose tolerance while increasing glucagon and active glucagon-like peptide-1 levels. Thus, the anti-GCGR mAb could represent an effective new therapeutic for the treatment of type 2 diabetes.

New hepatic targets for glycaemic control in diabetes

Type-2 diabetes is associated with impaired glucose clearance by the liver in the postprandial state, and with elevated glucose production in the post-absorptive state. New targets within the liver are currently being investigated for development of antihyperglycaemic drugs for type-2 diabetes. They include glucokinase, which catalyses the first step in glucose metabolism, the glucagon receptor, and enzymes of gluconeogenesis and/or glycogenolysis such as glucose 6-phosphatase, fructose 1,6-bisphosphatase and glycogen phosphorylase. Preclinical studies with candidate drugs on animal models or cell-based assays suggest that these targets have the potential for pharmacological glycaemic control. Data from clinical studies is awaited. Further work is required for better understanding of the implications of targeting these sites in terms of possible side-effects or tachyphylaxis. The advantage of combined targeting of two or more sites within the liver for minimizing side-effects and tachyphylaxis caused by single-site targeting is discussed.

A switch from prohormone convertase (PC)-2 to PC1/3 expression in transplanted alpha-cells is accompanied by differential processing of proglucagon and improved glucose homeostasis in mice

OBJECTIVE

Glucagon, which raises blood glucose levels by stimulating hepatic glucose production, is produced in alpha-cells via cleavage of proglucagon by prohormone convertase (PC)-2. In the enteroendocrine L-cell, proglucagon is differentially processed by the alternate enzyme PC1/3 to yield glucagon-like peptide (GLP)-1, GLP-2, and oxyntomodulin, which have blood glucose-lowering effects. We hypothesized that alteration of PC expression in alpha-cells might convert the alpha-cell from a hyperglycemia-promoting cell to one that would improve glucose homeostasis.

RESEARCH DESIGN AND METHODS

We compared the effect of transplanting encapsulated PC2-expressing alpha TC-1 cells with PC1/3-expressing alpha TCDeltaPC2 cells in normal mice and low-dose streptozotocin (STZ)-treated mice.

RESULTS

Transplantation of PC2-expressing alpha-cells increased plasma glucagon levels and caused mild fasting hyperglycemia, impaired glucose tolerance, and alpha-cell hypoplasia. In contrast, PC1/3-expressing alpha-cells increased plasma GLP-1/GLP-2 levels, improved glucose tolerance, and promoted beta-cell proliferation. In GLP-1R(-/-) mice, the ability of PC1/3-expressing alpha-cells to improve glucose tolerance was attenuated. Transplantation of PC1/3-expressing alpha-cells prevented STZ-induced hyperglycemia by preserving beta-cell area and islet morphology, possibly via stimulating beta-cell replication. However, PC2-expressing alpha-cells neither prevented STZ-induced hyperglycemia nor increased beta-cell proliferation. Transplantation of alpha TCDeltaPC2, but not alpha TC-1 cells, also increased intestinal epithelial proliferation.

CONCLUSIONS

Expression of PC1/3 rather than PC2 in alpha-cells induces GLP-1 and GLP-2 production and converts the alpha-cell from a hyperglycemia-promoting cell to one that lowers blood glucose levels and promotes islet survival. This suggests that alteration of proglucagon processing in the alpha-cell may be therapeutically useful in the context of diabetes.

Targeting glucagon receptor signalling in treating metabolic syndrome and renal injury in Type 2 diabetes: theory versus promise

Pancreatic bi-hormones insulin and glucagon are the Yin and Yang in the regulation of glucose metabolism and homoeostasis. Insulin is synthesized primarily by pancreatic beta-cells and is released in response to an increase in blood glucose levels (hyperglycaemia). By contrast, glucagon is synthesized by pancreatic alpha-cells and is released in response to a decrease in blood glucose (hypoglycaemia). The principal role of glucagon is to counter the actions of insulin on blood glucose homoeostasis, but it also has diverse non-hyperglycaemic actions. Although Type 1 diabetes is caused by insulin deficiency (insulin-dependent) and can be corrected by insulin replacement, Type 2 diabetes is a multifactorial disease and its treatment is not dependent on insulin therapy alone. Type 2 diabetes in humans is characterized by increased insulin resistance, increased fasting blood glucose, impaired glucose tolerance and the development of glomerular hyperfiltration and microalbuminuria, ultimately leading to diabetic nephropathy and end-stage renal disease. Clinical studies have suggested that an inappropriate increase in hyperglycaemic glucagon (hyperglucagonaemia) over hypoglycaemic insulin (not insulin deficiency until advanced stages) plays an important role in the pathogenesis of Type 2 diabetes. However, for decades, research efforts and resources have been devoted overwhelmingly to studying the role of insulin and insulin-replacement therapy. By contrast, the implication of glucagon and its receptor signalling in the development of Type 2 diabetic metabolic syndromes and end-organ injury has received little attention. The aim of this review is to examine the evidence as to whether glucagon and its receptor signalling play any role(s) in the pathogenesis of Type 2 diabetic renal injury, and to explore whether targeting glucagon receptor signalling remains only a theoretical antidiabetic strategy in Type 2 diabetes or may realize its promise in the future.

The role of alpha-cell dysregulation in fasting and postprandial hyperglycemia in type 2 diabetes and therapeutic implications

The hyperglycemic activity of pancreatic extracts was encountered some 80 yr ago during efforts to optimize methods for the purification of insulin. The hyperglycemic substance was named "glucagon," and it was subsequently determined that glucagon is a 29-amino acid peptide synthesized and released from pancreatic alpha-cells. This article begins with a brief overview of the discovery of glucagon and the contributions that somatostatin and a sensitive and selective assay for pancreatic (vs. gut) glucagon made to understanding the physiological and pathophysiological roles of glucagon. Studies utilizing these tools to establish the function of glucagon in normal nutrient homeostasis and to document a relative glucagon excess in type 2 diabetes mellitus (T2DM) and precursors thereof are then discussed. The evidence that glucagon excess contributes to the development and maintenance of fasting hyperglycemia and that failure to suppress glucagon secretion contributes to postprandial hyperglycemia is then reviewed. Although key human studies are emphasized, salient animal studies highlighting the importance of glucagon in normal and defective glucoregulation are also described. The past eight decades of research in this area have led to development of new therapeutic approaches to treating T2DM that have been shown to, or are expected to, improve glycemic control in patients with T2DM in part by improving alpha-cell function or by blocking glucagon action. Accordingly, this review ends with a discussion of the status and therapeutic potential of glucagon receptor antagonists, alpha-cell selective somatostatin agonists, glucagon-like peptide-1 agonists, and dipeptidyl peptidase-IV inhibitors. Our overall conclusions are that there is considerable evidence that relative hyperglucagonemia contributes to fasting and postprandial hyperglycemia in patients with T2DM, and there are several new and emerging pharmacotherapies that may improve glycemic control in part by ameliorating the hyperglycemic effects of this relative glucagon excess.

Alpha-cells of the endocrine pancreas: 35 years of research but the enigma remains

Glucagon, a hormone secreted from the alpha-cells of the endocrine pancreas, is critical for blood glucose homeostasis. It is the major counterpart to insulin and is released during hypoglycemia to induce hepatic glucose output. The control of glucagon secretion is multifactorial and involves direct effects of nutrients on alpha-cell stimulus-secretion coupling as well as paracrine regulation by insulin and zinc and other factors secreted from neighboring beta- and delta-cells within the islet of Langerhans. Glucagon secretion is also regulated by circulating hormones and the autonomic nervous system. In this review, we describe the components of the alpha-cell stimulus secretion coupling and how nutrient metabolism in the alpha-cell leads to changes in glucagon secretion. The islet cell composition and organization are described in different species and serve as a basis for understanding how the numerous paracrine, hormonal, and nervous signals fine-tune glucagon secretion under different physiological conditions. We also highlight the pathophysiology of the alpha-cell and how hyperglucagonemia represents an important component of the metabolic abnormalities associated with diabetes mellitus. Therapeutic inhibition of glucagon action in patients with type 2 diabetes remains an exciting prospect.