Antagonistic relationship between insulin and glucagon pathways

Antagonistic Hormones

antagonistic relationship between insulin and glucagon pathways

Glucagon and insulin are antagonist to one another When blood glucose Glucagon has a G-protein coupled receptor and utilizes the cAMP signaling pathway. Lack of insulin and excess glucagon exists in diabetes I. The ratio of insulin to. In this mini-review, we summarize the antagonistic effects of insulin signaling and by the opposing actions of insulin signaling and glucagon signaling pathways. Furthermore, dephosphorylation of CBP also leads to its association with. Antagonistic hormones go against each other's actions; so, when the level of one hormone is high, This explains the relationship between insulin and glucagon.

During early fasting, glucagon stimulates hepatic glycogenolysis, in which stored glycogen in the liver is broken down into glucose and released into the bloodstream to maintain euglycemia. Moreover, glucagon-stimulated hepatic gluconeogenesis plays a dominant role in maintaining euglycemia during prolonged fasting [ 10 ].

Pancreatic regulation of glucose homeostasis

Glucagon Signaling Stimulates Gluconeogenesis in the Liver. This event recruits the formation of the CREB cAMP response-element-binding protein co-activators complex and initiates the transcription of gluconeogenic genes containing CRE sites cAMP response elementsincluding G6pc glucosephosphatase and Pck1 phosphoenolpyruvate carboxykinase1. Hence, CRTC2 re-localizes into the nucleus. Together, these events result in the formation of the CREB co-activator complex to promote gluconeogenic gene expression and hepatic glucose production.

Antagonistic Hormones

In addition, inhibition of P histoneacetyl transferase activity decreased hepatic FOXO1 protein levels as well as blood glucose levels. Since FOXO1 also up-regulates gluconeogenic expression by binding to insulin response sequences located in the promoters of G6pc and Pck1, the induction of the Foxo1 gene by cAMP-PKA, thus, fully activates the gluconeogenic program and maintains euglycemia in the fasted state [ 11 ].

Suppression of Hepatic Gluconeogenic Gene Expression by Insulin Signaling in the Fed State Insulin signaling is crucial for the suppression of glucose production in the liver. Mice with liver specific insulin receptor knockout exhibited marked increase of hepaticglucose production and extreme hyperglycemia [ 17 ]. In the fed and postprandial states, the suppression of hepatic glucose production by insulin is complicated and involves many transcription factors and signaling mediators that have been reported.

antagonistic relationship between insulin and glucagon pathways

The gluconeogenic engine, CREB co-activator complex, first needs to be turned off to reduce endogenous glucose production. When assembled, the CREB co-activators complex upregulates the transcription of hepatic gluconeogenic related genes, such as G6pc and Pck1, and increases hepatic glucose production.

To sufficiently control the blood glucose levels in the fed and postprandial states, several mechanisms have been proposed. Importantly, a mouse model with a germline-mutation of this CBP phosphorylation site SA exhibits inappropriate activation of gluconeogenesis [ 1618 ].

antagonistic relationship between insulin and glucagon pathways

This enzyme, in turn, activates phosphorylase kinasewhich then phosphorylates glycogen phosphorylase b PYG bconverting it into the active form called phosphorylase a PYG a. Phosphorylase a is the enzyme responsible for the release of glucosephosphate from glycogen polymers. Additionally, the coordinated control of glycolysis and gluconeogenesis in the liver is adjusted by the phosphorylation state of the enzymes that catalyze the formation of a potent activator of glycolysis called fructose-2,6-bisphosphate.

Glucagon - Wikipedia

This covalent phosphorylation initiated by glucagon activates the former and inhibits the latter. This regulates the reaction catalyzing fructose-2,6-bisphosphate a potent activator of phosphofructokinase-1, the enzyme that is the primary regulatory step of glycolysis [12] by slowing the rate of its formation, thereby inhibiting the flux of the glycolysis pathway and allowing gluconeogenesis to predominate.

This process is reversible in the absence of glucagon and thus, the presence of insulin. Glucagon stimulation of PKA also inactivates the glycolytic enzyme pyruvate kinase in hepatocytes. Studies of fasting patients with type 1 diabetes who were maintained at near euglycemia with insulin infusions demonstrated that plasma glucose and ketone levels increased rapidly after termination of the insulin infusion; importantly, this was paralleled by increases of plasma glucagon concentrations 9.

Furthermore, if somatostatin which strongly inhibits glucagon secretion was infused simultaneously, the rise in not only glucagon but also plasma glucose and ketones could be strongly reduced, but not when glucagon was replaced 1, 9.

This proposal caused considerable debate, particularly regarding the pathophysiology of type 1 diabetes; the existing dogma—that the clinical features of the disease were entirely due to lack of insulin—was not easily abandoned.

Pancreatic regulation of glucose homeostasis

The strongest support for the traditional belief was from demonstrations of missing or inappropriately low secretion of insulin in patients with diabetic ketoacidosis and the observation of immediate relief of the condition upon insulin administration.

InBarnes et al. This finding suggested that extrapancreatic secretion of glucagon might not occur in people. However, Ravazzola et al.

  • How Insulin and Glucagon Work

Subsequent research with even more sophisticated techniques has now demonstrated that pancreatectomized patients may have remarkably large amounts of glucagon secreted from the gastrointestinal tract Therefore, it remains possible that glucagon can contribute to the diabetic phenotype, even in patients with total pancreatectomy. For many years, it was tacitly accepted that glucagon might play a role, but that the lack of insulin was believed to be the predominant hyperglycemic factor in diabetes, and this remained textbook dogma.

However, the evidence for a role for glucagon was intriguing enough to spur interest in developing antagonists of glucagon action, and many generally futile attempts at this were made. Initially, these were mainly peptide-derived glucagon receptor antagonists that were invariably partial agonists, and the antagonistic effect was not sufficiently robust However, evidence started to mount that glucagon might also play a role in the hyperglycemia of type 2 diabetes, including important pioneering studies from the laboratories of Alain Baron 4 and Robert Rizza To provide proof of concept for the use of glucagon antagonism in diabetes, Brand et al.

The same antibody could also normalize glycemia in intermediate-dose streptozotocin STZ -diabetic rats a type 2 modelbut it was ineffective in animals with more severe diabetes caused by high-dose STZ a type 1 model This supported the general concept regarding the role of glucagon in contributing to hyperglycemia but also indicated that there may be a minimal insulin requirement for glucagon antagonism to be effective.

These and other studies provided further impetus to develop glucagon receptor antagonists, and several pharmaceutical companies became engaged in the hunt.