Differential expression of rat pancreatic islet beta-cell glucose transporter (GLUT 2), proinsulin and islet amyloid polypeptide genes after prolonged fasting, insulin-induced hypoglycaemia and dexamethasone treatment

Prolonged insulin-induced hypoglycaemia reduces ß-cell activity rather than number in pancreatic islets in non-diabetic rats

Pancreatic β-cells have an extraordinary ability to adapt to acute fluctuations in glucose levels by rapid changing insulin production to meet metabolic needs. Although acute changes have been characterised, effects of prolonged metabolic stress on β-cell dynamics are still unclear. Here, the aim was to investigate pancreatic β-cell dynamics and function during and after prolonged hypoglycaemia. Hypoglycaemia was induced in male and female rats by infusion of human insulin for 8 weeks, followed by a 4-week infusion-free recovery period. Animals were euthanized after 4 or 8 weeks of infusion, and either 2 days and 4 weeks after infusion-stop. Total volumes of pancreatic islets and β-cell nuclei, islet insulin and glucagon content, and plasma c-peptide levels were quantified. Prolonged hypoglycaemia reduced c-peptide levels, islet volume and almost depleted islet insulin. Relative β-cell nuclei: total pancreas volume decreased, while being unchanged relative to islet volume. Glucagon: total pancreas volume decreased during hypoglycaemia, whereas glucagon: islet volume increased. Within two days after infusion-stop, plasma glucose and c-peptide levels normalised and all remaining parameters were fully reversed after 4 weeks. In conclusion, our findings indicate that prolonged hypoglycaemia inactivates β-cells, which can rapidly be reactivated when needed, demonstrating the high plasticity of β-cells even following prolonged suppression.

Unfolding Novel Mechanisms of Polyphenol Flavonoids for Better Glycaemic Control: Targeting Pancreatic Islet Amyloid Polypeptide (IAPP)

Type 2 diabetes (T2D) is characterised by hyperglycaemia resulting from defective insulin secretion, insulin resistance, or both. The impact of over-nutrition and reduced physical activity, evidenced by the exponential rise in obesity and the prevalence of T2D, strongly supports the implementation of lifestyle modification programs. Accordingly, an increased consumption of fruits and plant-derived foods has been advocated, as their intake is inversely correlated with T2D prevalence; this has been attributed, in part, to their contained polyphenolic compounds. Over the last decade, a body of work has focussed on establishing the mechanisms by which polyphenolic compounds exert beneficial effects to limit carbohydrate digestion, enhance insulin-mediated glucose uptake, down-regulate hepatic gluconeogenesis and decrease oxidative stress; the latter anti-oxidative property being the most documented. Novel effects on the inhibition of glucocorticoid action and the suppression of amylin misfolding and aggregation have been identified more recently. Amyloid fibrils form from spontaneously misfolded amylin, depositing in islet cells to elicit apoptosis, beta cell degeneration and decrease insulin secretion, with amyloidosis affecting up to 80% of pancreatic islet cells in T2D. Therefore, intervening with polyphenolic compounds offers a novel approach to suppressing risk or progression to T2D. This review gives an update on the emerging mechanisms related to dietary polyphenol intake for the maintenance of glycaemic control and the prevention of T2D.

Glucose Metabolism Abnormalities in Cushing Syndrome: From Molecular Basis to Clinical Management

An impaired glucose metabolism, which often leads to the onset of diabetes mellitus (DM), is a common complication of chronic exposure to exogenous and endogenous glucocorticoid (GC) excess and plays an important part in contributing to morbidity and mortality in patients with Cushing syndrome (CS). This article reviews the pathogenesis, epidemiology, diagnosis, and management of changes in glucose metabolism associated with hypercortisolism, addressing both the pathophysiological aspects and the clinical and therapeutic implications. Chronic hypercortisolism may have pleiotropic effects on all major peripheral tissues governing glucose homeostasis. Adding further complexity, both genomic and nongenomic mechanisms are directly induced by GCs in a context-specific and cell-/organ-dependent manner. In this paper, the discussion focuses on established and potential pathologic molecular mechanisms that are induced by chronically excessive circulating levels of GCs and affect glucose homeostasis in various tissues. The management of patients with CS and DM includes treating their hyperglycemia and correcting their GC excess. The effects on glycemic control of various medical therapies for CS are reviewed in this paper. The association between DM and subclinical CS and the role of screening for CS in diabetic patients are also discussed.

Control of Insulin Secretion by Production of Reactive Oxygen Species: Study Performed in Pancreatic Islets from Fed and 48-Hour Fasted Wistar Rats

Mitochondria and NADPH oxidase are important sources of reactive oxygen species in particular the superoxide radical (ROS) in pancreatic islets. These molecules derived from molecular oxygen are involved in pancreatic β-cells signaling and control of insulin secretion. We examined the involvement of ROS produced through NADPH oxidase in the leucine- and/or glucose-induced insulin secretion by pancreatic islets from fed or 48-hour fasted rats. Glucose-stimulated insulin secretion (GSIS) in isolated islets was evaluated at low (2.8 mM) or high (16.7 mM) glucose concentrations in the presence or absence of leucine (20 mM) and/or NADPH oxidase inhibitors (VAS2870–20 μM or diphenylene iodonium—DPI—5 μM). ROS production was determined in islets treated with dihydroethidium (DHE) or MitoSOX Red reagent for 20 min and dispersed for fluorescence measurement by flow cytometry. NADPH content variation was examined in INS-1E cells (an insulin secreting cell line) after incubation in the presence of glucose (2.8 or 16.7 mM) and leucine (20 mM). At 2.8 mM glucose, VAS2870 and DPI reduced net ROS production (by 30%) and increased GSIS (by 70%) in a negative correlation manner (r = -0.93). At 16.7 mM glucose or 20 mM leucine, both NADPH oxidase inhibitors did not alter insulin secretion neither net ROS production. Pentose phosphate pathway inhibition by treatment with DHEA (75 μM) at low glucose led to an increase in net ROS production in pancreatic islets from fed rats (by 40%) and induced a marked increase (by 144%) in islets from 48-hour fasted rats. The NADPH/NADP⁺ ratio was increased when INS-1E cells were exposed to high glucose (by 4.3-fold) or leucine (by 3-fold). In conclusion, increased ROS production through NADPH oxidase prevents the occurrence of hypoglycemia in fasting conditions, however, in the presence of high glucose or high leucine levels, the increased production of NADPH and the consequent enhancement of the activity of the antioxidant defenses mitigate the excess of ROS production and allow the secretory process of insulin to take place.

Glucocorticoid treatment and endocrine pancreas function: implications for glucose homeostasis, insulin resistance and diabetes

Glucocorticoids (GCs) are broadly prescribed for numerous pathological conditions because of their anti-inflammatory, antiallergic and immunosuppressive effects, among other actions. Nevertheless, GCs can produce undesired diabetogenic side effects through interactions with the regulation of glucose homeostasis. Under conditions of excess and/or long-term treatment, GCs can induce peripheral insulin resistance (IR) by impairing insulin signalling, which results in reduced glucose disposal and augmented endogenous glucose production. In addition, GCs can promote abdominal obesity, elevate plasma fatty acids and triglycerides, and suppress osteocalcin synthesis in bone tissue. In response to GC-induced peripheral IR and in an attempt to maintain normoglycaemia, pancreatic β-cells undergo several morphofunctional adaptations that result in hyperinsulinaemia. Failure of β-cells to compensate for this situation favours glucose homeostasis disruption, which can result in hyperglycaemia, particularly in susceptible individuals. GC treatment does not only alter pancreatic β-cell function but also affect them by their actions that can lead to hyperglucagonaemia, further contributing to glucose homeostasis imbalance and hyperglycaemia. In addition, the release of other islet hormones, such as somatostatin, amylin and ghrelin, is also affected by GC administration. These undesired GC actions merit further consideration for the design of improved GC therapies without diabetogenic effects. In summary, in this review, we consider the implication of GC treatment on peripheral IR, islet function and glucose homeostasis.

Relations between bone status and glucose metabolism with progression of insulin resistance

Az elhízás, a metabolikus szindróma, a 2-es típusú cukorbetegség és a csontritkulás előfordulása világszerte növekszik, vagyis a világméretű diabéteszjárványt az az elhízás „hajtja”, amely nők esetében erősebb csontokat eredményez. Vizsgálatunk során a glükózanyagcsere-zavar korai időszakában kerestük a csontállapot és a metabolikus paraméterek közötti összefüggéseket. A vizsgálatban 20 egészséges és 51 glükózintoleráns (49 ± 9 év) nőbeteg vett részt. Mértük a szénhidrát-, lipid- és csontanyagcsere paramétereit, a csontok denzitását (lumbális 1–4 csigolyákon és a femur nyakon); cukorterheléses és hyperinsulinaemiás-normoglykaemiás klamp vizsgálatot végeztünk. A csontok denzitása a két csoport között nem különbözött. Az egészségesek denzitása szoros kapcsolatban volt az egésztest-cukorfelhasználással (inzulinérzékenység) (gerinc r = –0,4921, p < 0,05, femur: r = –0,4972, p < 0,05), de a romló glükóztoleranciával ez a kapcsolat megszűnt (gerinc: r = –0,022, ns; femur: r = –0,3136, ns). Az adipokinek közül csak az adiponectin korrelált a denzitással, amíg ez a kapcsolat a cukoranyagcsere romlásával megmaradt a gerincen ( r = –0,5081, p < 0,05; –0,2804, p < 0,05), eltűnt a femuron ( r = –0,6742, p < 0,01; –0,1723, ns). A formációs és reszorpciós markerből képzett „reszorpciós hányados” növekedése a glükózanyagcsere romlásával csökkenő csontreszorpciót jelezte. Adataink az inzulinrezisztencia „gold standard” mérőmódszerét használva szoros kapcsolatot igazoltak a glükózanyagcsere, inzulinérzékenység és a csontok állapota között az egészséges, változó korban lévő nőkben, mely a glükóztolerancia romlásával és az inzulinrezisztencia kialakulásával megbomlik. Az egészségesekben észlelhető, de az inzulinrezisztencia kialakulásával romló, negatív adiponectin-csont kapcsolat értelmezése további vizsgálatokat igényel.

Tissue Expression and Secretion of Amylin

Amylin and insulin are co-localized within the same secretory granules of pancreatic beta-cells. Acutely, the secreted ratio of amylin:insulin is comparatively invariant, but long-standing hyperglycemia may favor induction of amylin synthesis and secretion over that of insulin. Amylin is also found in much lesser quantities in the gut and other tissues. In humans, both type 1 diabetes mellitus and the later stages of type 2 diabetes mellitus are characterized by deficiency of both insulin and amylin secretion. The severity of amylin deficiency appears to correlate with the severity of insulin deficiency. This concordance of deficiencies in amylin and insulin secretion observed with the progression of diabetes mellitus is consistent with their co-localization in pancreatic beta-cells. Amylin is cleared mainly by proteolytic degradation at the kidney. The terminal t1/2 for rat amylin in rats is approximately 13 min, and that for pramlintide in humans is approximately 20-45 min.

Mutation at position -132 in the islet amyloid polypeptide ( IAPP) gene promoter enhances basal transcriptional activity through a new CRE-like binding site

AIMS/HYPOTHESIS

Mutations in the islet amyloid polypeptide ( IAPP) gene may play a potential role in the abnormal regulation or expression of the peptide. The aim of this study was to determine the functional role of the -132 G/A mutation reported in the promoter region of the IAPP gene in a population of Spanish Type 2 diabetic patients.

METHODS

We investigated the transcriptional activity using MIN6 cells and luciferase reporter plasmids in several culture conditions. Key regulatory elements of the IAPP promoter region were also analysed by electrophoretic mobility shift assays (EMSA).

RESULTS

The mutant construct doubled IAPP transcriptional activity ( p<0.001). Both constructs showed severely reduced promoter activity (four-fold decrease) in the presence of verapamil and diazoxide. In contrast, IAPP promoter activity was doubled after incubation with forskolin or dexamethasone, regardless of the glucose concentrations in the culture media. EMSA revealed that the -132 G/A mutation increased the binding affinity through two DNA-protein complexes. In addition, a cAMP-responsive element binding protein (CREB) was identified by super-shift EMSA.

CONCLUSIONS/INTERPRETATION

Our studies show that the wild-type and the mutant constructs are regulated in a similar pattern under all conditions, strongly indicating that the -132 G/A mutation increases basal but not inducible transcription. These results may be explained by new binding to the mutant region through CREB and other transcription factors not yet identified.

Insulin receptors and insulin action in the brain: review and clinical implications

Insulin receptors are known to be located on nerve cells in mammalian brain. The binding of insulin to dimerized receptors stimulates specialized transporter proteins that mediate the facilitated influx of glucose. However, neurons possess other mechanisms by which they obtain glucose, including transporters that are not insulin-dependent. Further, insulin receptors are unevenly distributed throughout the brain (with particularly high density in choroid plexus, olfactory bulb and regions of the striatum and cerebral cortex). Such factors imply that insulin, and insulin receptors, might have functions within the central nervous system in addition to those related to the supply of glucose. Indeed, invertebrate insulin-related peptides are synthesized in brain and serve as neurotransmitters or neuromodulators. The present review summarizes the structure, distribution and function of mammalian brain insulin receptors and the possible implications for central nervous system disorders. It is proposed that this is an under-studied subject of investigation.

Glucose transporters: structure, function and consequences of deficiency

There are two mechanisms for glucose transport across cell membranes. In the intestine and renal proximal tubule, glucose is transported against a concentration gradient by a secondary active transport mechanism in which glucose is cotransported with sodium ions. In all other cells, glucose transport is mediated by one or more of the members of the closely related GLUT family of glucose transporters. The pattern of expression of the GLUT transporters in different tissues is related to the different roles of glucose metabolism in different tissues. Primary defects in glucose transport all appear to be extremely rare and not all possible deficiencies have been identified. Deficiency of the secondary active sodium/glucose transporters result in glucose/galactose malabsorption or congenital renal glycosuria. GLUT1 deficiency produces a seizure disorder with low glucose concentration in cerebrospinal fluid and GLUT2 deficiency is the basis of the Fanconi-Bickel syndrome, which resembles type I glycogen storage disease.

Glucose stimulation of pancreatic beta-cell lines induces expression and secretion of dynorphin

To investigate adaptive responses of pancreatic beta-cells to hyperglycemia, genes induced by glucose stimulation were identified by subtraction cloning. Among 53 clones representing differentially expressed genes, 20 encoded the endogenous opioid precursor, prodynorphin. The amino acid sequence of murine prodynorphin is identical to the rat protein in sequences comprising the opioid peptides and 86% identical in the remainder of the molecule. Stimulation of MIN6 cells increased prodynorphin RNA levels to more than 20-fold in proportion to physiological glucose concentrations. Similar induction levels were observed in murine betaTC3 and rat Rinm5F beta-cell lines. Prodynorphin RNA expression increased within 1 h of glucose stimulation, achieved maximal levels by 4 h, and remained elevated for at least 24 h. By using RIA, MIN6 cells were shown to contain and secrete increased amounts of dynorphin-A following glucose stimulation. Treatment of MIN6 cells with KCl, forskolin, or isobutyl-methyl-xanthine strongly induced prodynorphin RNA expression, suggesting that induction may be related to secretion-coupled signaling pathways. The induction of prodynorphin in several beta-cell lines is consistent with previous demonstrations of beta-cell synthesis of other endogenous opioids, including beta-endorphin, and suggests that opioids may have a potentially significant role in regulating beta-cell secretion.

Validation of nonradioactive in situ hybridization as a quantitative approach of messenger ribonucleic acid variations: a comparison with northern blot

The recent improvement of in situ hybridization (ISH) procedures, the increased sensitivity of immunohistochemical detection systems, and the development of assisted image analysis now enable the quantification of specific messenger RNAs (mRNAs) detected by nonisotopic probes on histological sections. However, the reliability and accuracy of this type of mRNA quantification are still to be determined. To this end, we compared in an experimental model of rat malnutrition the densitometric analysis of proinsulin mRNA detected by nonradioactive ISH with those obtained from radioactive Northern blot hybridization (NBH). Proinsulin gene expression was quantified by ISH and by NBH in the pancreatic islets of normally fed rats, rats fasted for 3 days, and rats refed for 8, 24, and 48 h after fasting. Starvation decreased the pancreatic proinsulin mRNA signal by 34% and 38%, as evaluated by ISH and NBH, respectively. Also, with both methods, mRNA levels returned to normal after refeeding. Taken together, the results derived from nonradioactive quantitative ISH were closely correlated to those obtained by quantitative NBH (r = 0.975, p < 0.005). It is thus possible to evaluate variations of mRNA content accurately by quantitative ISH as is currently done by NBH, but with the invaluable advantage of integrating the data with a morphological analysis.

Dexamethasone induces posttranslational degradation of GLUT2 and inhibition of insulin secretion in isolated pancreatic beta cells. Comparison with the effects of fatty acids

GLUT2 expression is strongly decreased in glucose-unresponsive pancreatic beta cells of diabetic rodents. This decreased expression is due to circulating factors distinct from insulin or glucose. Here we evaluated the effect of palmitic acid and the synthetic glucocorticoid dexamethasone on GLUT2 expression by in vitro cultured rat pancreatic islets. Palmitic acid induced a 40% decrease in GLUT2 mRNA levels with, however, no consistent effect on protein expression. Dexamethasone, in contrast, had no effect on GLUT2 mRNA, but decreased GLUT2 protein by about 65%. The effect of dexamethasone was more pronounced at high glucose concentrations and was inhibited by the glucocorticoid antagonist RU-486. Biosynthetic labeling experiments revealed that GLUT2 translation rate was only minimally affected by dexamethasone, but that its half-life was decreased by 50%, indicating that glucocorticoids activated a posttranslational degradation mechanism. This degradation mechanism was not affecting all membrane proteins, since the alpha subunit of the Na+/K+-ATPase was unaffected. Glucose-induced insulin secretion was strongly decreased by treatment with palmitic acid and/or dexamethasone. The insulin content was decreased ( approximately 55 percent) in the presence of palmitic acid, but increased ( approximately 180%) in the presence of dexamethasone. We conclude that a combination of elevated fatty acids and glucocorticoids can induce two common features observed in diabetic beta cells, decreased GLUT2 expression, and loss of glucose-induced insulin secretion.

Signals related to glucose metabolism regulate islet amyloid polypeptide (IAPP) gene expression in human pancreatic islets

Intracellular pathways involved in glucose stimulation of IAPP gene expression were studied in human pancreatic islets. Glucose (16.7 mM), but not mannose, caused a 2.3-fold increase in IAPP mRNA levels; this effect was inhibited by actinomycin D. In the presence of the non-metabolizable 6-deoxyglucose (16.7 mM) IAPP mRNA levels were markedly depleted. Both mannoheptulose and verapamil blocked glucose-induced stimulation of the IAPP gene. The magnitude of the insulin gene response to glucose was smaller (1.3-fold); none of the above-mentioned agents had significant effects on insulin mRNA content. Tunicamycin elicited a 2.4- and 2.7-fold increase in IAPP mRNA levels in the low and high glucose media, respectively; however, it did not change insulin mRNA. It had no effect on rat IAPP or insulin mRNAs, either. We conclude that IAPP gene expression is regulated by signals derived from glucose metabolism and that intracellular calcium may be involved in this response. IAPP and insulin genes are not co-regulated in cultured human pancreatic islets.

Islet amyloid polypeptide (amylin) and insulin are differentially expressed in chronic diabetes induced by streptozotocin in rats

Islet amyloid polypeptide (IAPP) is overexpressed relative to insulin under several experimental conditions relevant to diabetes mellitus, including the immediate phase (7 days) following induction of streptozotocin diabetes. In the present study, IAPP and insulin gene expression were examined in chronic streptozotocin diabetes (3 weeks) in rats. Quantitative in situ hybridization, determining grain areas and optical densities of mRNA labelling, revealed that IAPP and insulin expression were reduced at the islet level at both low and high streptozotocin doses, partly due to reduced beta-cell mass. In contrast, the cellular levels of IAPP mRNA were either increased or unaffected at the low and high streptozotocin doses, respectively, whereas those of insulin mRNA were unaffected or reduced. When dexamethasone was administered to rats given the low streptozotocin dose, IAPP expression was increased, whereas that of insulin was markedly reduced. Immunocytochemistry revealed that IAPP predominantly occurred in insulin cells and to a lesser extent in somatostatin cells at all treatments examined. Our findings demonstrate that IAPP and insulin gene expression are differentially regulated; the over-expression of IAPP relative to insulin is augmented when the beta-cell insult is aggravated, in our experiments represented by massive beta-cell destruction (high streptozotocin dose) or a combination of moderate beta-cell damage and peripheral insulin resistance (low streptozotocin dose and dexamethasone). An over-expression of IAPP relative to insulin may therefore be involved in diabetes pathogenesis, contributing to its metabolic perturbations, possibly through the capacity of IAPP to restrain insulin release and action and to form islet amyloid.

Effect of adrenalectomy on the development of a pancreatic islet lesion in fa/fa rats

Adrenalectomy prevents development of obesity and hyperinsulinaemia in obese (fa/fa) Zucker rats, thereby implicating the hypothalamo- pituitary-adrenal axis in the pathogenesis of obesity. In this study glucose-induced insulin secretion and glucokinase activity were investigated in isolated islets from adrenalectomized and control obese and lean female rats. Islets from control fa/fa rats were more sensitive to glucose with a half-maximal effective concentration (EC50) of 6.1 +/- 2.0 mmol. 1(-1) compared with 10.6 +/- 2.7 mmol. 1(-1) for adrenalectomized fa/fa rat islets. Adrenalectomy did not alter the islet sensitivity to glucose in the lean rats (EC50 of 9.4 +/- 1.5 mmol.1(-1) and 9.3 +/- 2.0 mmol. 1(-1) for adrenalectomized and control lean rats respectively). Mannoheptulose did not inhibit insulin secretion from control obese rats; however at concentrations of 1.0 mmol. 1(-1) or more it significantly inhibited glucose-induced insulin secretion in adrenalectomized obese and lean, and control lean rat islets (P < 0.05). In adrenalectomized fa/fa islets the glucokinase Km was increased twofold compared with the control fa/fa rats (9.5 +/- 1.5 mmol. 1(-1) vs 5.0 +/- 1.5 mmol. 1(-1), respectively), but there was no significant change in glucokinase Km in the lean rat islets after adrenalectomy. Mannoheptulose (10 mmol.1(-1) caused a significant reduction in glucose phosphorylation in disrupted islets of adrenalectomized fa/fa and lean, and of control lean rats, but not of control fa/fa rats. These data demonstrate that development of abnormal regulation of glycolysis in pancreatic islet beta cells of fa/fa rats, as indicated by the insulin response to manno-heptulose and glucokinase activity, is dependent on an intact hypothalamo-pituitary-adrenal axis.

Mechanism of impaired glucose-potentiated insulin secretion in diabetic 90% pancreatectomy rats. Study using glucagonlike peptide-1 (7-37)

Chronic hyperglycemia causes a near-total disappearance of glucose-induced insulin secretion. To determine if glucose potentiation of nonglucose secretagogues is impaired, insulin responses to 10(-9) M glucagonlike peptide-1 (GLP-1) (7-37) were measured at 2.8, 8.3, and 16.7 mM glucose with the in vitro perfused pancreas in rats 4-6 wk after 90% pancreatectomy (Px) and sham-operated controls. In the controls, insulin output to GLP-1 was > 100-fold greater at 16.7 mM glucose versus 2.8 mM glucose. In contrast, the increase was less than threefold in Px, reaching an insulin response at 16.7 mM glucose that was 10 +/- 2% of the controls, well below the predicted 35-40% fractional beta-cell mass in these rats. Px and control rats then underwent a 40-h fast followed by pancreas perfusion using a protocol of 20 min at 16.7 mM glucose followed by 15 min at 16.7 mM glucose/10(-9) M GLP-1. In control rats, fasting suppressed insulin release to high glucose (by 90%) and to GLP-1 (by 60%) without changing the pancreatic insulin content. In contrast, in Px the insulin response to GLP-1 tripled in association with a threefold increase of the insulin content, both now being twice normal when stratified for the fractional beta-cell mass. The mechanism of the increased pancreas insulin content was investigated by assessing islet glucose metabolism and proinsulin biosynthesis. In controls with fasting, both fell 30-50%. In Px, the degree of suppression with fasting was similar, but the attained levels both exceeded those of the controls because of higher baseline (nonfasted) values. In summary, chronic hyperglycemia is associated with a fasting-induced paradoxical increase in glucose-potentiated insulin secretion. In Px rats, the mechanism is an increase in the beta-cell insulin stores, which suggests a causative role for a lowered beta-cell insulin content in the impaired glucose-potentiation of insulin secretion.

Non-parallelism of islet amyloid polypeptide (amylin) and insulin gene expression in rats islets following dexamethasone treatment

Islet amyloid polypeptide (IAPP), a novel islet hormone candidate, has been reported to be over-expressed relative to insulin in rats following dexamethasone treatment. In order to investigate the expression of IAPP and insulin following dexamethasone treatment of rats for 12 days, we applied in situ hybridization and immunocytochemistry, allowing us to evaluate islet changes in gene expression and morphology. Tissue concentrations of IAPP and insulin were measured by radioimmunoassay. A low dose of dexamethasone (0.2 mg/kg daily) increased the islet levels of IAPP and insulin mRNA to 249 +/- 13% and 150 +/- 24% of controls, respectively (p < 0.001 and p < 0.01). A high dose of dexamethasone (2.0 mg/kg daily) increased the islet levels of IAPP and insulin mRNA to 490 +/- 13% and 203 +/- 9% of controls, respectively (p < 0.001 and p < 0.001). The pancreatic concentration of IAPP increased more than that of insulin (p < 0.05). Morphometric analysis revealed that dexamethasone treatment induced both hyperplasia and hypertrophy of insulin cells. Changes in the cellular localization of IAPP and insulin mRNA were not observed. Thus, we conclude that the increased level of IAPP mRNA is due to both an increase at the cellular level as well as hyperplasia/hypertrophy of insulin cells. In contrast, the increased level of insulin mRNA appears to be due to hyperplasia/hypertrophy of insulin cells, since insulin gene expression decreased at the cellular level (p < 0.001 vs controls). These observations provide further evidence that IAPP and insulin gene expression are regulated in a non-parallel fashion, which may be relevant to the pathogenesis of non-insulin-dependent diabetes mellitus.

Pancreatic beta-cell function and islet-cell proliferation: effect of hyperinsulinaemia

Pancreatic beta-cell function was studied in adult female rats, in which endogenous insulin demand was fully met by SC infusion of human insulin (4.8 IU/24 h) for 6 days, resulting in hyperinsulinaemia and severe hypoglycaemia. The amount of pancreatic endocrine tissue declined by 40%, (pro)insulin mRNA, as determined by in situ hybridisation by 95%, and the amount of stored insulin by 90%. Islet-cell proliferation as determined by 24 h of BrdU infusion declined by 60%. Basal glucose levels normalized within 2 days after the insulin treatment was ended, whereas about 1 week was needed to restore the amount of pancreatic insulin, glucose-induced insulin release, and glucose tolerance to normal values. The amount of endocrine tissue recovered within 48 h and mRNA abundance within 96 h after discontinuation of the insulin infusion, whereas at that time islet-cell proliferation still showed a sixfold increase, before returning to control levels after 1 week. These results show that after a period of suppression of beta-cell function, recovery of insulin synthetic capacity does not immediately result in normalization of insulin stores and insulin release. Under these conditions, episodes of hyperglycaemia may occur, which may act as a stimulus for islet-cell proliferation.

Islet amyloid polypeptide: does it play a pathophysiological role in the development of diabetes?

There is suggestive evidence that amylin acts physiologically in an autocrine manner within the islet to restrain insulin secretion, but conversely there is little indication that this action of amylin plays any role in the development of NIDDM. Deposition of amylin within pancreatic islets is a feature in patients with NIDDM but is of sufficient degree to disrupt beta-cell function in only a small minority of individuals. Current evidence suggests that amylin does not have any physiologically important extra-islet metabolic effects. The potential exists for the development of amylin antagonists as pharmacological agents to enhance insulin secretion in NIDDM but antagonism of systematic CGRP would need to be avoided. There is little, if any, indication that either replacement of amylin or treatment with amylin agonists are likely to have any beneficial role in patients with IDDM.

Facilitative glucose transporters

Facilitative glucose transport is mediated by members of the Glut protein family that belong to a much larger superfamily of 12 transmembrane segment transporters. Six members of the Glut family have been described thus far. These proteins are expressed in a tissue- and cell-specific manner and exhibit distinct kinetic and regulatory properties that reflect their specific functional roles. Glut1 is a widely expressed isoform that provides many cells with their basal glucose requirement. It also plays a special role in transporting glucose across epithelial and endothelial barrier tissues. Glut2 is a high-Km isoform expressed in hepatocytes, pancreatic beta cells, and the basolateral membranes of intestinal and renal epithelial cells. It acts as a high-capacity transport system to allow the uninhibited (non-rate-limiting) flux of glucose into or out of these cell types. Glut3 is a low-Km isoform responsible for glucose uptake into neurons. Glut4 is expressed exclusively in the insulin-sensitive tissues, fat and muscle. It is responsible for increased glucose disposal in these tissues in the postprandial state and is important in whole-body glucose homeostasis. Glut5 is a fructose transporter that is abundant in spermatozoa and the apical membrane of intestinal cells. Glut7 is the transporter present in the endoplasmic reticulum membrane that allows the flux of free glucose out of the lumen of this organelle after the action of glucose-6-phosphatase on glucose 6-phosphate. This review summarizes recent advances concerning the structure, function, and regulation of the Glut proteins.