Neuroendocrine hormone amylin in diabetes

Abstract

The neuroendocrine hormone amylin, also known as islet amyloid polypeptide, is co-localized, co-packaged and co-secreted with insulin from adult pancreatic islet β cells to maintain glucose homeostasis. Specifically, amylin reduces secretion of nutrient-stimulated glucagon, regulates blood pressure with an effect on renin-angiotensin system, and delays gastric emptying. The physiological actions of human amylin attribute to the conformational α-helix monomers whereas the misfolding instable oligomers may be detrimental to the islet β cells and further transform to β-sheet fibrils as amyloid deposits. No direct evidence proves that the amylin fibrils in amyloid deposits cause diabetes. Here we also have performed a systematic review of human amylin gene changes and reported the S20G mutation is minor in the development of diabetes. In addition to the metabolic effects, human amylin may modulate autoimmunity and innate inflammation through regulatory T cells to impact on both human type 1 and type 2 diabetes.

Key Words: Amylin, Neuroendocrine hormone, Diabetes

Core tip: This is a systematic review to describe amylin as a neuroendocrine hormone. Besides the glucose homeostasis and cytotoxicity of amylin, we tried to perform that the S20G mutation of human amylin is also minor in the pathogenesis of diabetes. In addition to the metabolic effects, human amylin may have impact on autoimmunity, implicating a potential as the immunosuppressor to improve autoimmunity conditions in the future therapy of diabetes, allergic diseases and immune rejection.

INTRODUCTION

Amylin, or islet amyloid polypeptide (IAPP), is a neuroendocrine hormone co-localized, co-secreted and co-packaged with insulin from pancreatic β cells[1,2]. Abnormalities in human amylin folding, secretion and action have detrimental effects on islet function and glucose regulation by islet amyloidosis and β cell dysfunction in type 2 diabetes (T2D)[3-5]. The molecules of amylin polypeptide fold to form the α-helix monomers and oligomers and the β-sheet fibrils. The amylin-aggregated amyloid fibrils are thought to form through smaller cell toxic intermediates and deposited amyloid disrupts normal islet architecture[6]. However, amylin plays a critical role in metabolism homeostasis[7] as a neuroendocrine hormone that carries a targeted signal to the brain. Several actions of amylin that impact glucose regulation have been identified, including the effects on nutrient-stimulated glucagon secretion[8], on nutrient delivery from the stomach to the small intestine for absorption[9], on renin-angiotensin system (RAS)[10] and on food intake by delaying gastric emptying[11].

FUNCTIONAL AMYLIN

Amylin functions as part of the neuroendocrine pancreas and contributes to glucose homeostasis with other two pancreatic islet hormones insulin and glucagon. The amylin and insulin is a pair of synergistic partner genes co-expressed by a common promoter[12], and regulates the levels of glucose by complex endocrine and neuronal pathways. In physiological state, the simultaneous release of amylin and insulin from the secretory granules results in a parallel pattern in the islet β-cells in response to glucose stimulation[13]. However, concentration of plasma amylin and insulin decreased in advanced T2D[7]. Glucagon commonly increases blood glucose when nutrients are not available; while insulin and amylin primarily decrease the post-meal glucose by stimulating the uptake of glucose from circulation into muscle and fat cells for storage and by inhibiting the endogeneous glucose output from liver. Complimentary to insulin, amylin regulates postprandial glycaemia by suppressing postmeal glucagon secretion from islet α-cells[8], which is possibly mediated by signals from the vagus nerve at the pancreatic islets. Amylin and insulin also coordinate storage of carbohydrate to transfer triglyceride into muscle glycogen in skeletal muscles[14] probably by phosphorylase activation[15] (Table 1 and Figure 1).

Table 1 Physiological actions of amylin.

Neuroendocrine effects

Inhibiting insulin secretion in a high concentration

Inhibiting glucagon secretion at mealtime

Delaying nutrient delivery from stomach to the small intestine

Reducing food intake by a signal from the central nervous system

Metabolic effects

Co-regulating glucose with insulin and glucagon

Inhibiting muscle glycogen synthesis[15]

Stimulating oxidative responses and low density lipoprotein uptake in insulin-producing cells

Inhibiting bone resorption

Lipolytic-like effects

Renal effects

Renin ↑

Angiotensin II ↑

Regulating renal growth

Regulating water-sodium homeostasis

Haemodynamic effect

Aldosterone ↑

Hypocalcaemia

Vasodilation

Figure 1 Overview of physiological actions of amylin. (1) Amylin suppresses glucagon secretion from islet alpha cells at mealtime and thus, inhibits glucagons-induced glucose release from the liver; (2) Amylin delays nutrient delivery from the stomach to the small intestine for absorption; (3) Amylin reduces food intake by a signal mediated through the central nervous system; (4) Renal amylin may stimulate Renin-Angiotensin System; and (5) Amylin and insulin coordinate storage of carbohydrate.

As a neuroendocrine hormone, amylin also acts in the central nervous system to produce satiety through brainstem-localized receptors, which have been found at several locations in the brain, including the nucleus accumbens, the dorsal raphe and the area postrema in rat brain[16]. The area postrema may be an important site for amylin action. This area does not have a blood brain barrier and allows access to circulatory peptides. Lesioning studies have indicated that some of amylin’s actions are mediated at this site. The suppression of neuronal amylin on food intake and gastrointestinal motility[17] to slow down the absorption and to limit the rate at which glucose enters the circulation[18] has been found in human. Gastric emptying is considered to be a typical pathological phenomenon and a crucial reason for the postprandial hyperglycemia in T1D. It is believed that most of amylin deficiency in T1D may be pathogenically significant in the gastric behavior[19]. Thus, high plasma amylin concentration in young with newly-diagnosed T1D[20], which may result in a delay in gastric emptying that markedly improved postprandial glucose excursions in new T1D patients[19]. Amylin deficiency significantly affects the lack of delay in gastric emptying in response to hyperglycemia in T1D[19], and is further supported by the highly potent protective effects of amylin on glucose homeostasis[21].

Furthermore, a physiological effect of amylin on the RAS has been implicated in the hemodynamic regulation of blood pressure[22] and kidney function[10]. Inhibition of angiotensin-converting enzyme (ACE) is associated with the reducing density of amylin binding in the renal cortex[10]. Pharmacokinetics pattern of amylin closely resembles that of C-peptide[23-25]. Excreted in the urine, when glomerular filtration decreases, amylin cumulates in the blood stream. Therefore, patients with renal failure may have high levels of circulating amylin[26]. These patients also have a higher than normal prevalence of islet amyloid in the absence of diabetic symptoms[27] (Table 1 and Figure 1).

KINETICS OF AMYLIN

Human amylin is derived from a larger precursor proamylin, coding sequence with 89 amino acids residue. The flanking peptides at N-terminal and C-terminal of mature amylin are removed by a proteolytic enzyme, which is also responsible for proinsulin to insulin conversion in the β cells[28]. The prohormone convertases PC2 and PC3 involved in processing proinsulin are likely responsible for amylin processing as well[5].

Measuring the concentration of circulating amylin is challenging. The minimum detectable concentration of amylin in 50 mL plasma is 0.5 to 2 pmol/L, and the dynamic range is 2 to 100 pmol/L[29]. The basal plasma concentrations of amylin in human in the 2-15 pmol/L range, with an insulin/amylin ratio of 10-100:1[30,31]. In healthy subjects, circulating amylin rises in response to the glucose challenge[32]. The amylin to insulin molar ratio is similar at all time points despite of high-frequency oscillations and inter-race differences in circulating amylin concentrations[33,34].

Circulating amylin levels are increased in individuals with obesity, hypertension, positive family history of insulin resistance in line with hyperinsulinaemia[35-37]. An exaggerated amylin response has been documented in subjects with obesity and impaired glucose tolerance[32,36]. Moreover, the amylin to insulin ratio is consistent in insulin-resistant persons at all time points, although large interindividual variations (0.2% to 1.6%) in amylin/insulin secretory ratios have been documented[35]. Thus, hyperamylinaemia due to insulin resistance precedes the occurrence of T2D[36]. However, in later-stage T2D, the secretion of both amylin and insulin becomes deficient[32]. In diabetic patients on insulin treatment, amylin/insulin is detectable but the response to the glucose challenge is negligible, reflecting functional failure of the islet β cells[38]. Interestingly, diabetes is also characterised by an excess of glucagon, in particularly after mealtimes. The net effect of insulin and amylin deficiency and glucagon excess is an increased postmeal glucose level. Furthermore, prolonged exposure of pancreatic islets to hyperglycaemia favours selective amylin secretion, increasing the risk of islet amyloid formation and β cell apoptosis[39,40].

AMYLIN-DERIVED AMYLOIDOSIS

Amyloidosis is a generic term for aggregation state of amyloid polypeptide with β-sheet conformation that bounds to each other by certain chemical bonds[41]. Amylin is encoded by calcitonin mRNA from a gene made up of three introns on the 12^th chromosome[42]. Besides amylin, more than 25 proteins in human are known by their fibrillate aggregation[43,44]. Many of them have similar protein structures with amyloid-like properties and characteristic occurrences in metabolic disturbances, such as amylin, amyloid light-chain and β-amyloid[45,46]. In the islet of T2D patients, amylin fibrils commonly contribute to the form of islet amyloid. In addition, amylin is also found to deposit in brain[16], plays the potential role in the development of Alzheimer’s disease (AD) and cerebrovascular disease (CVD) pathology with β-amyloid, or might impair brain function independently of β-amyloid pathology[47].

Certain gene mutations, amino acid sites in amylin protein and minor components are more or less associated with amyloid deposits. It has been reported some mutations in the human amylin gene leading to amino acid substitutions, such as S20G. S20G is an important amylin gene mutation resulting in a glycine for serine substitution at position 20 of the mature IAPP molecule (Figure 2A). In vitro studies indicate that the S20G mutation amylin is more cytotoxic in forming amyloid and inducing apoptosis in COS-1 cells[48]. A low prevalence (< 5%) of the S20G has been reported in T2D Japanese[49-51], Hong Kong Chinese[52,53], Taiwanese[54], and Mainland Chinese. All the cases with the mutation S20G are heterozygous. Although the heterozygous mutation S20G is more common in diabetic patients than in normal control (2.6% vs 0.9%, P < 0.0001), linkage analysis reveals that mutation in or near amylin gene is unlikely a major course of T2D[55] (Table 2).

Table 2 Prevalence of the amylin gene mutation S20G in Asian populations (%) n.

	Ref.	Type 2 diabetes	Impaired glucose tolerance	Type 1 diabetes	Control
Japanese	Seino et al[50]	2.6 (1538)	-	-	0.8 (1108)
Japanese	Yamada et al[51]	4.7 (86)	-	-	1.6 (182)
Japanese	Sakagashira et al[49]	4.1 (294)	-	0 (59)	0 (187)
Hong Kong Chinese	Ng et al[53]	1.5 (462)	-	-	0 (126)
Taiwanese	Chuang et al[54]	1.6 (182)	4.2 (24)	1.6 (122)	4.4 (99)
Mainland Chinese	Lee et al[52]	2.1 (94)	-	-	0 (106)

Figure 2 Amino acid sequence and diagrammatic representations of human amylin and pramlintide. A: Amino acid sequence alignment of human (WT, S20G), rat, mouse amylin and pramlintide. Only the amino acids that differ are shown. The sequence between amino acids 20 to 29 represents the amyloidogenic domain; B: The synthetic amylin analog pramlintide differs from human amylin at three amino acid sites (proline at 25, 28, and 29) and this molecule overcomes these disadvantages of human amylin.

Many molecular chaperones, like apolipoprotein E (apoE) and heat shock protein (HSP) family, may relates to amylin deposits[56-58], and is generally considered as a major genetic modulator of β-amyloid deposition and risk of AD[59,60]. The apoE ε4 allele particularly affects the increased risk for atherosclerosis[61], brain plaque[61] and islet amyloidosis. In T2D, apoE plays a critical role in lipid metabolism, amylin fibril formation[62] and is a probable link to atherosclerosis[57]. HSP is identified within highly purified β cell granules derived from INS-1E islet β cells such as insulin and amylin[63]. Vita demonstrates the existence of direct functional interactions involved HSP70, can suppress the misfolding of human amylin[58], which is also proved to limit the toxicity of β-amyloid[64]. These chaperones may contribute the AD-diabetes link at the pathophysiological level, including the interactive amyloid of β-amyloid and human amylin[65,66].

CONFORMATIONS OF AMYLIN

The conformation of amylin is considered a significant factor in that abnormal accumulation of amylin fibrils in organs may lead to amyloidosis in T2D. Human amylin is subtyped into three different conformations: Monomers, oligomers and fibrils (Figure 3). Monomers are unfolded random-coiled peptides physiologically. These molecules can be misfolding with α-helix structures, and aggregated into the pathologic oligomers, the soluble amyloid intermediates, which include spherical particles of 2.7 to 4.2 nm in diameter[67]. Amylin fibrils formation is a self-driven process accumulating the misfolded oligomeric proteins with a β-sheet fibrillar structure into insoluble islet amyloid deposits. Islet amylin-amyloid is the pathologic hallmark of most individuals with T2D[22,68].

Figure 3 Three conformations of human amylin. Monomers of amylin with physiological functions mainly contribute to glucose and lipid homeostasis and tend to misfold into the cytotoxic oligomers. A self-driven process is accumulating the misfolded oligomers into insoluble nontoxic amylin fibrils with a β-sheet structure.

In physiological state, the toxic oligomers can be rescued into amyloid fibrils by chaperones or eliminated by the ubiquitin-proteasome system[69]. Concentration of amylin fibrils in the intracytoplasmic organelles of human beta cells far exceed the in vitro concentration required for amyloid formation, so there must be an underlying mechanism in normal beta cells to induce the aggregation of amylin monomers into fibrils. Mechanisms that may associate with amylin-amyloid include an acid pH[70], the presence of chaperon proteins (HSP), or the presence of other proteins[58]. Then these possible factors for amylin aggregation may derive from hyperglycaemia, high-fat diet, or low-grade chronic inflammation[3], which are considered as the cardinal symptoms of T2D. The amylin monomer has its special function in endocrine system, or further polymerise to amyloid fibrils, which may play an important role in cell informational transfer, memory and survival prolonging[71].

Mature fibrillar aggregate of amylin has been considered to be nontoxic, and even these small amyloid deposits seen widely located in islets or other organs in T2D, may not have significant contribution to organs damage. Therefore, the formation of fibrils from cytotoxic oligomers can be considered as a protective mechanism of transforming a dynamic protein into inert amyloid. Here we have to be curious whether this process of fibrillar formation initially acts as a rescuer in the pathway of cell failure[71].

The amino acid sequence of amylin derived from islet amyloid in T2D is identical to that present in healthy humans, and amylin from human insulinoma tissue. Moreover, amylin structure exhibits close sequence homology among all species in both the amino terminal (residues 1 to 19) and the carboxy terminal (residues 20 to 29) regions. In contrast, residues 20 to 29 show considerable divergence among species and have been implicated in the conversion of the peptide’s secondary structure from a predominantly α helical one to a β sheet structure[72]. This assignment is based on the comparison of the sequences of human amylin, which is highly amyloidogenic, with those of cat amylin, which is moderately amyloidogenic, and with rat and mouse amylin, which does not aggregate to form amyloid[72,73].

AMYLIN TOXICITY

Mechanisms of islet β cell depletion by amyloid include mechanical replacement, apoptosis, necrosis and β cell membrane damage. Human amylin has been clearly shown toxic to insulin-producing β cells of the adult pancreas of rats and humans[74]. The cytotoxic action of amylin in insulin-producing cells is paralleled by increased oxidative responses and low density lipoprotein (LDL) uptake, suggesting that cytotoxic mechanisms of amylin in insulin-producing cells involve changes in pathways of cellular oxidative stress systems and lipid homeostasis[75].

Soluble oligomer of amylin is recently reported to contribute the primary toxicity in T2D but not unsoluble fibril in the amyloid diseases[76-78]. Different misfolded oligomers of amylin with a conformation-dependent structure suggest that they share a common mechanism of pathogenesis[79]. Like oligomer of β amyloid protein playing an important role in the pathogenesis of AD and CVD[80], oligomic amylin is also a central subject in the risk of the islet β-cell lesion in T2D through formation of toroidal (ion-leaking) pores inserted into membranes[81,82].

THERAPEUTIC APPLICATION AND PROSPECTIVE

Human amylin has a tendency to aggregate, form insoluble particles and stick to surfaces. Learnt from non-amyloidogenic rat amylin, the peptide structure is broken by substituting the positions 25 alanine, 28 and 29 serines into proline residues (Figure 2B). This analog of human amylin, named “pramlintide”, is used for the potential prevention of complications of T1D as an adjunct with insulin and a single agent for T2D[2]. This soluble, stable synthetic analog amylin avoids aggregation of amyloid relating the development of β-cell dysfunction[83]. Like wild-type human amylin, pramlintide can adjust postprandial glucagon release and gastric emptying rate in individuals with T1D and T2D[84-87]. In clinical therapy of diabetes, pramlintide as an assistant treatment of insulin usually decreases postprandial glucose without rising insulin level[88,89]. In T1D, pramlintide therapy significantly reduced 4.4-6.6 mmol/mol haemoglobin a1c at 26 wk vs placebo[90,91]. And mean body weight was significantly reduced (-0.8 to -1.3 kg at week 26 or 29) vs placebo[90-92]. Then in T2D, pramlintide therapy resulted in significant reductions in haemoglobin a1c (-7.7 to -8.7 mmol/mol after 16 or 26 wk) and mean body weight (-1.4 to -1.6 kg after 16 or 26 wk) vs placebo[93-95]. According to these functions, the pramlintide is manufactured and designed as injection in pen injector. Since the pH value of pramlintide buffer is incompatible with most insulin products, pramlintide is recommended not to mix with insulin in the same syringe (shown in Symlin^® Package Insert). This analog has the same functions of blood glucose regulation and gastric emptying delay as wild-type human amylin.

Human amylin no doubt plays a significant role in neuroendocrine contribution to glucose homeostasis. Treatment with non-fibrillar pramlintide improves glycaemic control and weight management without adverse events of severe hypoglycaemia in T1D and T2D[96]. However, whether the toxicity of fibrillar amylin contributes significantly to pathogenesis of diabetes is yet unconvincing. Studies data indicate that microscopically evident fibrillar amylin is neither necessary nor sufficient to cause diabetes, but rather that it is positively correlated with protection[97,98].

It is worth noting that amylin may regulate the inflammatory response and immune factor secretion[99,100]. Mouse amylin was reported that can trigger a broad autoimmune response by CD4⁺ effector T cells in NOD mice[101]. Recent study shows that human amylin can induce CD4⁺CD25⁺FoxP3⁺ Regulatory T cells and reduce risk of autoimmune diabetes[102]. It firstly demonstrates autoimmune inhibition by human amylin. All these findings suggest a novel approach to restore glucose homeostasis and improve autoimmunity conditions such as autoimmune diseases, allergic diseases, immune rejection of organ transplantation and graft vs host reaction (GVHR).