An overview of incretin hormones

Professor, Department of Biomedical Sciences, The Panum Institute, University of Copenhagen, Copenhagen, Denmark

Private Practice, Excelsior Springs, Missouri

Dr Holst receives salary from Merck & Co., Inc., and Novo Nordisk Inc.; is on the advisory boards for Merck & Co., Inc, Novo Nordisk Inc., and Roche; is on the speakers bureau for Merck & Co., Inc, and Novo Nordisk Inc.; and is engaged in contracted research with Merck & Co., Inc, and Novartis Pharmaceuticals Corporation.

Dr LaSalle is on the advisory boards for astraZeneca, GlaxoSmithKline, Novartis pharmaceuticals corporation, and Novo Nordisk, Inc.; and is on the speakers bureaus for AstraZeneca, GlaxoSmithKline, and Novo Nordisk Inc.

 

The pathophysiology of type 2 diabetes

Type 2 diabetes (T2DM) is often viewed as a disease characterized by insulin resistance and pancreatic β-cell dysfunction. Although both of these defects play central roles in T2DM, research conducted over the past 4 decades makes it clear that other processes play integral roles in this disease as well. Consequently, this article will discuss the multifactorial pathophysiology of T2DM and the role of the incretin system. By gaining a better understanding of the pathophysiology at work, family physicians will be better able to select appropriate therapeutic options at each stage of the disease based on the individual patient’s therapeutic needs.

In addition, the importance of targeting both fasting plasma glucose (FPG) and postprandial plasma glucose (PPG) will be emphasized. Although FPG and glycosylated hemoglobin (A1C) levels are the usual treatment targets, elevation of PPG is increasingly being recognized as significant in the development of atherogenesis and the all-cause mortality associated with T2DM.¹ PPG has also been found to be an important contributor to elevated A1C levels and is therefore an important glycemic target in addition to FPG. Whereas FPG is the primary determinant of A1C levels >8.5%, PPG is the primary determinant of A1C levels <8.5%.²

 

insulin resistance and pancreatic β-cell dysfunction

Insulin resistance plays an early and central role in the pathogenesis of T2DM.³ Prevailing evidence suggests that low-grade chronic inflammation and oxidative stress, perhaps mediated by inflammatory cytokines, play key roles in the development of skeletal muscle insulin resistance.⁴ However, it is not clear if insulin resistance precedes or is a result of pancreatic β-cell dysfunction.⁵

The progression of T2DM evolves over 5 stages, with each stage distinguished by alterations in β-cell mass, phenotype, and function.⁶ In stage 1, glucose-stimulated insulin secretion increases as does pancreatic β-cell mass, probably as a consequence of insulin resistance. Stage 2 occurs as FPG levels rise to approximately 116 mg/dL and is characterized by a stable state of β-cell adaptation, wherein the first-phase insulin response is lost but the second-phase response is mostly preserved. As a consequence, normal glucose levels are no longer maintained but are elevated. This is likely due to a decline in β-cell function and/or an increase in insulin resistance. Although patients may remain in this prediabetic stage for years, at some point β-cell mass becomes inadequate, allowing glucose levels to rise rapidly. During stage 3, fluctuation in blood glucose levels results in variable β-cell function and less efficient insulin secretion. By the time the patient reaches stage 4, the FPG has risen to approximately 285 mg/dL. Further, β-cell mass is reduced by approximately 50%, probably as a result of increased β-cell apoptosis, despite normal islet cell formation and β-cell replication.⁷ At stage 4, insulin secretion is generally sufficient to prevent ketoacidosis, despite clinically important hyperglycemia. Stage 5, severe decompensation, is characterized by profound pancreatic β-cell failure and ketosis. Unlike patients with type 1 diabetes who progress rapidly to stage 5, patients with T2DM can remain in stage 4 for many years.

It should be noted that although the importance of β-cell dysfunction in T2DM is well established, directly measuring β-cell function is problematic and can only be accomplished in animal models or in vitro using human cells. Variables include β-cell tissue mass, neogenesis, apoptosis, amyloid deposition, and cell morphology, among others. Indirect measures of β-cell function in humans include first- and second-phase insulin secretion, proinsulin:insulin ratio, C-peptide level, insulinogenic index, arginine-induced insulin secretion, basal insulin secretion, and the homeostasis model of assessment (HOMA). Improvements in these indirect variables may result from reductions in A1C (glucotoxicity), lipids (lipotoxicity), or inflammation, as well as the changes that can be directly measured in animal models or in vitro.^8,9

 

Glucose release and disposal

Considerable insight into the role of the liver and other processes as they relate to PPG results from research by Woerle et al.^10,11 In a study of 26 patients with T2DM and 15 healthy controls, postprandial endogenous glucose release was elevated approximately 2-fold in individuals with T2DM vs healthy controls. This increase in glucose release is in part due to uncontrolled gluconeogenesis and, to a lesser extent, glycogenolysis (FIGURE 1).¹⁰ Gluconeogenesis was elevated preprandially and did not decrease after a meal, remaining elevated 2-fold in the 6-hour postprandial period in those with T2DM vs healthy controls. Glycogenolysis was also elevated preprandially but decreased substantially following a meal. Over the 6-hour period following a meal, glycogenolysis was more than 2-fold greater in patients with T2DM than in those without T2DM. Preprandial glucagon levels were comparable in both groups; however, glucagon levels increased postprandially in patients with T2DM but decreased in those without T2DM such that postprandial glucagon levels were 60% higher in patients with T2DM. Of note, insulin levels in diabetic individuals increased significantly less during the first 90 minutes postprandially (179 vs 290 pM; P=.016) than in nondiabetic individuals, despite being significantly greater preprandially (77 vs 52 pM; P=.015). Total postprandial tissue glucose uptake was comparable between the diabetic and nondiabetic groups.

In addition to glucose synthesis and release, abnormalities of glucose disposal are important factors contributing to T2DM. Such abnormalities include increased nonoxidative glycolysis, reduced postprandial glucose oxidation, and indirect pathway glycogen storage.^10,11

 

The roles of the pancreatic α-cell and glucagon

Although often overlooked, the pancreatic α-cell also plays an important role in glucose homeostasis, as it provides a physiologic balance to the pancreatic β-cell.¹² Most of the glucoregulatory actions of insulin are opposed by glucagon. Normally, the pancreatic α-cell protects against hypoglycemia by increasing glucagon secretion when glucose levels drop, thus stimulating glycogenolysis. Conversely, the α-cell protects against hyperglycemia by reducing glucagon secretion when glucose levels increase.^13,14 In patients with T2DM, α-cells have impaired glucose sensing, as do β-cells. Fasting plasma glucagon levels are generally higher in patients with T2DM, but they are present in the context of higher ambient glucose levels, which contributes to increased glucose production. The reduction in glucagon secretion in response to hyperglycemia is blunted in patients with T2DM. In fact, hyperglycemia may stimulate a paradoxical increase in glucagon secretion in T2DM, especially in those with advanced disease. Conversely, the secretion of glucagon may be blunted during episodes of hypoglycemia.¹³

 

The gastrointestinal system

The gastrointestinal system is involved in regulating glucose in several ways. First, the gastric emptying rate in some patients with T2DM may be twice as fast as in nondiabetic individuals.¹⁵ Despite the more rapid rate of gastric emptying, PPG levels of patients with T2DM peak ≥1 hour later than the 30- to 45-minute peak for nondiabetic individuals. The reason for the delay in reaching the PPG peak is not clear but may result from insulin resistance and a delayed or missing first-phase insulin response.

An understanding of the role of the gastrointestinal system in glucose homeostasis came initially from the observation that enteral nutrition provided a stronger insulinotropic stimulus than did intravenous administration of isoglycemic glucose.¹⁶ Subsequent investigation led to the discovery of the incretin hormones—peptide hormones secreted from the gut that were found to be responsible for augmented insulin secretion in response to oral nutrients. Among these incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) appear to be the most important.

 

The incretin system

Glucose-dependent insulinotropic polypeptide

GIP, the first incretin polypeptide identified, is secreted from K-cells in the gut, with the highest density of K-cells being found in the proximal small intestine. The GIP receptor is expressed in pancreatic islet cells, adipose tissue, heart, pituitary, adrenal cortex, and several regions of the brain, although its function in many of these tissues is unknown. Secretion of GIP is stimulated by intake of absorbable carbohydrates and lipids and results in a 10- to 20-fold increase in plasma concentration above baseline within minutes of oral food ingestion. The major pathway regarding the effect of GIP on insulin secretion involves direct activation of receptors on islet β-cells (FIGURE 2).^17,18

Glucagon-like peptide-1

GLP-1 is secreted throughout the day by L-cells in the intestinal mucosa in response to nutrients in the gut. One of the most potent stimulants of insulin release, GLP-1 is responsible for a significant part of the insulin response to oral glucose. However, because of its potent inhibition of gastric emptying, administration of a 3-hour infusion of GLP-1 to healthy, adult men after an overnight fast actually caused a significant decrease in postprandial insulin (P=.01) and glucose (P=.007) levels compared with saline infusion.¹⁹ Similar to GIP, one pathway of the glucose-dependent insulinotropic activity of GLP-1 is direct activation of receptors on islet β-cells (FIGURE 2).¹⁷ GLP-1 causes anabolic actions on the synthesis of insulin in β-cells by stimulating all steps of insulin biosynthesis and insulin gene transcription (FIGURE 3).¹⁷ Consequently, GLP-1 provides continued and augmented release of insulin for secretion in response to glucose without overproduction that could lead to hypoglycemia.^18,20 Insulin secretion is also stimulated by GLP-1 through the central nervous system following activation of sensory afferents of the vagus nerve in the gut wall as well as in the portal vein and the liver. This may be its most important stimulatory pathway, since circulating GLP-1 is rapidly inactivated (see below).

GLP-1 also acts on islet α-cells, causing strong inhibition of postprandial glucagon secretion, unlike GIP, which weakly stimulates glucagon secretion.^19,21 However, GLP-1 does not suppress glucagon secretion at plasma glucose levels below approximately 65 mg/dL, thereby preserving an important hypoglycemia counterregulatory mechanism.²²

GLP-1 also acts by 2 other important mechanisms (FIGURE 3). First, GLP-1 slows gastric emptying and promotes early satiety with reduced food intake. In a study of 24 healthy male volunteers randomized to a single 30-minute infusion of GLP-1 or placebo, the fraction of a radiolabeled nutrient meal that had been emptied from the stomach at the end of the infusion was significantly lower for the GLP-1 group than for the placebo group (7% vs 23%, respectively; P=.008).²³ By 1 hour after the end of the infusion, the fraction of the meal that had been emptied from the stomach was similar in each group (21% vs 35%, respectively; P=.28). The mechanism of delayed gastric emptying is likely to involve long vagovagal reflexes. GLP-1 also induces dose-dependent early satiety and a feeling of fullness, and has been shown to reduce food intake.²⁴ In vitro studies and studies in rodents have shown that GLP-1 also has trophic effects on β-cells that result in β-cell proliferation and increased differentiation of new β-cells from pancreatic progenitor cells.^25,26 GLP-1 also prolongs β-cell survival through reduced apoptosis, as shown in various in vitro models.²⁷ However, improvement in β-cell function may not be sustained. Twelve-month administration of the GLP-1 receptor agonist exenatide to 36 patients, followed by a 12-week washout period, showed that all β-cell function parameters were not significantly different from pretreatment values.²⁸

Dipeptidyl peptidase-4

Within minutes of secretion or exogenous administration, GIP—and in particular GLP-1—are rapidly degraded by dipeptidyl peptidase-4 (DPP-4).²⁹ DPP-4 is found in many body tissues, including liver, renal, and intestinal brush-border membranes; lymphocytes; and endothelial cells. As a consequence of the action of DPP-4, GLP-1 is rapidly truncated in vivo and, although retaining affinity for the GLP-1 receptor, it lacks efficacy. In fact, GLP-1 becomes an antagonist.³⁰ In addition to GIP and GLP-1, many gastrointestinal hormones, neuropeptides, cytokines, and chemokines are substrates for DPP-4.³¹

 

Diminished incretin effect in type 2 diabetes

The incretin system is impaired in patients with T2DM, which, as a consequence of its insulinotropic actions, contributes to fasting and postprandial hyperglycemia. GLP-1 levels are significantly elevated in patients with T2DM in the fasting state compared with patients with healthy controls (P=.037), whereas levels of GIP are comparable in the fasting state in these populations (P=.13).³² However, in response to a mixed meal of solids and liquids, the secretion of GLP-1 is significantly impaired in most patients with T2DM (P<.001 vs healthy controls), whereas the secretion of GIP is only slightly impaired (P=.047).^18,32 The impairment of GLP-1 and GIP secretion varies directly with the degree of insulin resistance; those who are more insulin resistant have a lower rise in GLP-1 and GIP in response to a meal.³³ Furthermore, there is an attenuated early GLP-1 response and a diminished GIP response later in the postprandial period.³³ Although the potency of GLP-1 with respect to insulin secretion is reduced, supraphysiological concentrations can actually normalize insulin secretion.^34,35 In contrast, the insulinotropic activity of GIP is almost completely lost regardless of GIP dose.^34-36

 

enhancing incretin action in type 2 diabetes

Native GLP-1 and GLP-1 analogs

Initial attempts at enhancing the action of the incretin system involved the administration of both GLP-1 and GIP. An early study by Nauck et al involved patients with T2DM (mean FPG, 140 mg/dL) and healthy volunteers.³⁵ Patients and volunteers received separate 4-hour infusions of GLP-1, GIP, and placebo under hyperglycemic clamp conditions. GLP-1 augmented insulin secretion in a dose-dependent manner, but the effect of GIP in stimulating insulin secretion in patients with T2DM was dramatically reduced compared to healthy volunteers. GLP-1 led to a significant reduction (~40%) in glucagon secretion, whereas GIP did not, which suggests that GIP is not as useful in the management of T2DM.

In another study of patients with T2DM, a 4-hour infusion of GLP-1 resulted in a complete normalization of FPG concentrations, while decreasing glucagon secretion and increasing insulin secretion.³⁷ In a subsequent study, GLP-1 also enhanced satiety (P=.026) and a feeling of fullness (P=.028), with a 29% reduction in food (P=.034) and a 27% reduction in caloric (P=.034) consumption.³⁸

In a proof-of-concept study by Zander et al, patients with T2DM were administered either GLP-1 or saline via continuous subcutaneous infusion for 6 weeks.³⁹ Whereas there was no change in FPG or PPG in the saline group, patients who received GLP-1 experienced a mean decrease in FPG and 8-hour PPG levels of approximately 80 mg/dL and 100 mg/dL (P<.0001 vs saline for both), respectively. PPG excursions were significantly reduced (P<.0001 vs saline), while A1C levels decreased from 9.2% at baseline to 7.9% (P=.003). Gastric emptying was prolonged and appetite reduced, resulting in a mean reduction in body weight of 1.9 kg from baseline (P=.02). Direct analysis demonstrated that β-cell function and insulin sensitivity improved significantly at the end of treatment versus baseline. The results of this study demonstrated the potential therapeutic value of GLP-1 in patients with T2DM. However, the need for continuous infusion made the use of GLP-1 problematic. Therefore, analogs of GLP-1 that are resistant to the actions of DPP-4 are currently being developed for clinical use. A stable GLP-1 receptor agonist, exendin-4, was discovered in the saliva of the Gila monster. The synthetic form (exenatide) has been available in the United States since 2005.

Dipeptidyl peptidase-4 inhibitors

The inactivation of GLP-1 by DPP-4 occurs rapidly so that the elimination half-life of GLP-1 is only 1 to 2 minutes in vivo. Inhibiting DPP-4 has been reported to increase the levels of circulating GLP-1 and GIP and to enhance glucose homeostasis.^40-42 Administration of valine-pyrrolidide, a DPP-4 inhibitor, in anesthetized pigs at a dose that reduced plasma DPP-4 activity >90% increased 3-fold (P<.05) the concentration of GLP-1 in the basal state and 5-fold (P<.0001) the proportion remaining undegraded. Administration of a glucose load resulted in a 3-fold increase (P<.05) in insulin secretion.⁴⁰ Results of other preclinical studies demonstrate stimulation of insulin secretion and inhibition of glucagon secretion. Pancreatic β-cell mass is increased by both proliferation and inhibition of apoptosis. However, the rate of gastric emptying is unchanged, as is body weight.^31,42 Whether all of the actions of DPP-4 inhibitors are mediated through GLP-1 is presently unknown, but DPP-4 inhibitors had no effect in mice with knockout of both GIP and GLP-1 receptors.^43,44 DPP-4 inhibitors are now available in the United States (sitagliptin), with more under review by the FDA (alogliptin) or in phase 3 clinical trials (vildagliptin and saxagliptin).

 

summary

T2DM results from a complex pathophysiologic process that includes pancreatic α-and β-cell dysfunction, peripheral insulin resistance, and impaired function of the incretin hormones. Incretin hormones, which are found in the gastrointestinal system and other tissues, are integral to glucose homeostasis, increasing insulin secretion, reducing glucagon secretion, slowing gastric emptying, and enhancing early satiety. Enhancement of the incretin system with GLP-1 receptor agonists and DPP-4 inhibitors is effective in reestablishing glucose homeostasis.

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