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Nutrient regulation of pancreatic beta-cell function in diabetes: problems and potential solutions.

Increasing prevalence of obesity combined with longevity will produce an epidemic of Type 2 (non-insulin-dependent) diabetes in the next 20 years. This disease is associated with defects in insulin secretion, specifically abnormalities of insulin secretory kinetics and pancreatic beta-cell glucose responsiveness. Mechanisms underlying beta-cell dysfunction include glucose toxicity, lipotoxicity and beta-cell hyperactivity. Defects at various sites in beta-cell signal transduction pathways contribute, but no single lesion can account for the common form of Type 2 diabetes. Recent studies highlight diverse beta-cell actions of GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). These intestinal hormones target the beta-cell to stimulate glucose-dependent insulin secretion through activation of protein kinase A and associated pathways. Both increase gene expression and proinsulin biosynthesis, protect against apoptosis and stimulate replication/neogenesis of beta-cells. Incretin hormones therefore represent an exciting future multi-action solution to correct beta-cell defect in Type 2 diabetes.

Amino Acid Sequence↗

Impaired cell volume regulation in intestinal crypt epithelia of cystic fibrosis mice.

Cystic fibrosis is a disease characterized by abnormalities in the epithelia of the lungs, intestine, salivary and sweat glands, liver, and reproductive systems, often as a result of inadequate hydration of their secretions. The primary defect in cystic fibrosis is the altered activity of a cAMP-activated Cl- channel, the cystic fibrosis transmembrane conductance regulator (CFTR) channel. However, it is not clear how a defect in the CFTR Cl- channel function leads to the observed pathological changes. Although much is known about the structural properties and regulation of the CFTR, little is known of its relationship to cellular functions other than the cAMP-dependent Cl- secretion. Here we report that cell volume regulation after hypotonic challenge is also defective in intestinal crypt epithelial cells isolated from CFTR -/- mutant mice. Moreover, the impairment of the regulatory volume decrease in CFTR -/- crypts appears to be related to the inability of a K+ conductance to provide a pathway for the exit of this cation during the volume adjustments. This provides evidence that the lack of CFTR protein may have additional consequences for the cellular function other than the abnormal cAMP-mediated Cl- secretion.

Animals↗

Mutational analysis of regulated exocytosis in Tetrahymena.

Genetic analysis of regulated exocytosis can be accomplished in ciliates, since mutants defective in stimulus-dependent secretion of dense-core vesicles can be identified. In Tetrahymena thermophila, secretion in wild-type cells can result in their encapsulation by the proteins released from vesicle cores. Cells with defects in secretion were isolated from mutagenized homozygous cells that were generated using a highly efficient method. Screening was based both on a visual assay for encapsulation, and on a novel panning step using differential centrifugation to take advantage of the selective mobility of mutants that fail to encapsulate upon stimulation. 18 mutants with defects in several ordered steps have been identified. Defects in a set of these could be localized to three stages: granule formation, transport to cell surface docking sites, and exocytosis itself. Mutants with defects in this last stage can be ordered into successive steps based on several criteria, including their responsiveness to multiple secretagogues and Ca2+ ionophores. The results of both somatic and genetic complementation on selected pairs also help to characterize the defective factors.

Animals↗

Aggregation and retention of human urokinase type plasminogen activator in the yeast endoplasmic reticulum.

BACKGROUND: Secretion of recombinant proteins in yeast can be affected by their improper folding in the endoplasmic reticulum and subsequent elimination of the misfolded molecules via the endoplasmic reticulum associated protein degradation pathway. Recombinant proteins can also be degraded by the vacuolar protease complex. Human urokinase type plasminogen activator (uPA) is poorly secreted by yeast but the mechanisms interfering with its secretion are largely unknown. RESULTS: We show that in Hansenula polymorpha overexpression worsens uPA secretion and stimulates its intracellular aggregation. The absence of the Golgi modifications in accumulated uPA suggests that aggregation occurs within the endoplasmic reticulum. Deletion analysis has shown that the N-terminal domains were responsible for poor uPA secretion and propensity to aggregate. Mutation abolishing N-glycosylation decreased the efficiency of uPA secretion and increased its aggregation degree. Retention of uPA in the endoplasmic reticulum stimulates its aggregation. CONCLUSIONS: The data obtained demonstrate that defect of uPA secretion in yeast is related to its retention in the endoplasmic reticulum. Accumulation of uPA within the endoplasmic reticulum disturbs its proper folding and leads to formation of high molecular weight aggregates.

Journal Article↗

Altered insulin secretory responses to glucose in diabetic and nondiabetic subjects with mutations in the diabetes susceptibility gene MODY3 on chromosome 12.

One form of maturity-onset diabetes of the young (MODY) results from mutations in a gene, designated MODY3, located on chromosome 12 in band q24. The present study was undertaken to define the interactions between glucose and insulin secretion rate (ISR) in subjects with mutations in MODY3. Of the 13 MODY3 subjects, six subjects with normal fasting glucose and glycosylated hemoglobin and seven overtly diabetic subjects were studied as were six nondiabetic control subjects. Each subject received graded intravenous glucose infusions on two occasions separated by a 42-h continuous intravenous glucose infusion designed to prime the beta-cell to secrete more insulin in response to glucose. ISRs were derived by deconvolution of peripheral C-peptide levels. Basal glucose levels were higher and insulin levels were lower in MODY3 subjects with diabetes compared with nondiabetic subjects or with normal healthy control subjects. In response to the graded glucose infusion, ISRs were significantly lower in the diabetic subjects over a broad range of glucose concentrations. ISRs in the nondiabetic MODY3 subjects were not significantly different from those of the control subjects at plasma glucose levels <8 mmol/l. As glucose rose above this level, however, the increase in insulin secretion in these subjects was significantly reduced. Administration of glucose by intravenous infusion for 42 h resulted in a significant increase in the amount of insulin secreted over the 5-9 mmol/l glucose concentration range in the control subjects and nondiabetic MODY3 subjects (by 38 and 35%, respectively), but no significant change was observed in the diabetic MODY3 subjects. In conclusion, in nondiabetic MODY3 subjects insulin secretion demonstrates a diminished ability to respond when blood glucose exceeds 8 mmol/l. The priming effect of glucose on insulin secretion is preserved. Thus, beta-cell dysfunction is present before the onset of overt hyperglycemia in this form of MODY. The defect in insulin secretion in the nondiabetic MODY3 subjects differs from that reported previously in nondiabetic MODY1 or mildly diabetic MODY2 subjects.

Adolescent↗

Mutants of Aspergillus nidulans blocked at an early stage of sporulation secrete an unusual metabolite.

Mutants of Aspergillus nidulans defective in conidiation (asexual sporulation) can be classified according to whether they are blocked before or after induction of conidiation. Mutants blocked before induction (preinduction mutants) appear to be unable to respond to the inducing stimulus and thus are defective in one of the earliest events in the sporulation process. Three preinduction mutants have been isolated and characterized. Each was found to exhibit the same pleiotropic phenotype: they also were defective in sexual sporulation and secreted a set of phenolic metabolites at a level much higher than did wild type or mutants blocked at later stages of conidiation. One of the metabolites has been identified as the antibiotic diorcinal (3,3'-dihydroxy-5,5'-dimethyldiphenyl ether) which is known to be involved in the synthesis of certain farnesyl phenols of unknown function. These results suggest that preinduction mutants are blocked in a phenolic metabolic pathway, one or more product of which participates in the initiation of sporulation.

Aspergillus nidulans↗

Defective 5-HT 1-receptor-mediated neurotransmission in the control of growth hormone secretion in Parkinson's disease.

In order to gain a better insight in the serotonergic disorder affecting the parkinsonian brain, the growth hormone (GH) response to the 5-HT 1 serotonergic receptor agonist sumatriptan was tested. Sumatriptan was injected subcutaneously in 10 de novo parkinsonian patients (aged 58-69 years) and in 9 age-matched normal controls. On different occasions, subjects were also tested with GH-releasing hormone (GH-RH; 1 micrograms/kg body weight in an intravenous bolus) and L-arginine (30 g in 50 ml normal saline over 30 min), which releases GH from somatostatin inhibition, to determine whether GH secretion in response to alternate secretagogues is preserved in Parkinson's disease. In addition, a control test with the administration of normal saline instead of drug treatments was performed. Plasma GH levels were recorded over 2 h in all tests. Placebo administration did not change plasma GH levels in any subject. Similar GH responses were observed in normal controls and parkinsonian patients when GH-RH or arginine were administered. A significant GH increase was observed in normal controls after sumatriptan injection; in contrast, GH secretion was not modified by sumatriptan administration in parkinsonian patients. These data show that Parkinson's disease is associated with an impairment in the 5-HT1-receptor-mediated serotonergic transmission in the control of GH secretion, suggesting that this specific defect might alter other serotonergic-mediated mechanisms in the parkinsonian brain.

Aged↗

Influence of the mutation "diabetes" on insulin release and islet morphology in mice of different genetic backgrounds.

Mice, 7-8-mo old, of the C57BL/KsJ-db strain and homozygotic for the mutant gene db, exhibited marked hyperglycemia and moderately elevated serum insulin levels. Light and electron microscopy provided evidence of a slightly decreased proportion of beta cells in the pancreatic islets, irregular islet architecture with intraislet ducts, and degenerative as well as hypertrophic changes in the individual beta cells. As a rule, islets microdissected from these mice did not release insulin in response to glucose, theophylline, iodoacetamide, or chloromercuribenzene-p-sulphonic acid. The absence of secretory responses was not simply due to lack of insulin. Although the islet content of insulin was decreased in C57BL/KsJ-db/db mice, the remaining amount was severalfold larger than that released from stimulated islets of normal controls. Another mutation, db(2J), an allele of db with identical phenotypic expressions in the C57BL/KsJ strain, was studied on the genetic background C57BL/6J. In contrast to the severely diabetic C57BL/KsJ-db/db animals, the C57BL/6J-db(2J)/db(2J) mice were characterized by highly elevated serum insulin levels and only moderate hyperglycemia. Their endocrine pancreas was enlarged and showed an increased proportion of beta cells. Like the islets of normal mice, those of C57BL/6J-db(2J)/db(2J) mice responded to glucose and chloromercuribenzene-p-sulphonic acid, the glucose-induced responses being potentiated by theophylline or iodoacetamide. C57BL/KsJ-db/db mice should provide a valuable model for studying defects in insulin secretion in relation to diabetes mellitus. Mice of the C57BL/6J strain offer a control material that may help to elucidate the dependence of the insulin secretory defect on the background genome.

Animals↗

Parathyroid hormone secretion after operation for primary hyperparathyroidism.

BACKGROUND: Primary hyperparathyroidism (pHPT) is associated with a defective regulation of the secretion of parathyroid hormone (PTH). Thus in pHPT, higher than normal calcium concentrations are required to inhibit PTH release. However, it is not known if this defective regulation is normalized by removal of the parathyroid adenoma (i.e., whether the regulation of PTH secretion is normal in the remaining glands). In this study we therefore investigated the PTH secretion in patients operated on for parathyroid adenoma 1 year after operation. METHODS: Na2 ethylenediamine tetraacetic acid and CaCI2 were infused at constant rates in six patients operated on for parathyroid adenoma and six healthy individuals. Serum levels of intact PTH and ionized calcium were determined during the infusions. RESULTS: No significant differences between the two groups were found in baseline levels of serum ionized calcium and PTH. Furthermore, no significant differences between patients and control subjects were found in the maximum serum PTH levels during the hypocalcemic infusion of ethylenediamine tetraacetic acid or in the minimum serum PTH levels during the calcium infusion. In contrast, the set point (the calcium concentration required for half-maximal inhibition of PTH secretion) was significantly lower in the patients (1.20 +/- 0.01 mmol/L) compared with control subjects (1.22 +/- 0.01 mmol/L; p < 0.05). CONCLUSIONS: We conclude that the elevation of set point in patients with parathyroid adenoma is corrected by successful operation. This suggests a monoclonal origin of parathyroid adenomas.

Adenoma↗

End-organ response to adrenocorticotropin, thyrotropin, gonadotropin-releasing hormone, and glucagon in hypocalcemic magnesium deficient patients.

We have evaluated the responsiveness of hypocalcemic magnesium-deficient patients to ACTH, TRH, gonadotropin-releasing hormone, and glucagon as determined by the rise in serum cortisol, TSH, LH, and plasma cAMP concentrations, respectively. It was previously been shown that the hypocalcemia of magnesium deficiency is secondary to impaired secretion of parathyroid hormone (PTH) along with renal and skeletal resistance to the action of PTH. Since PTH secretion and action are though to be effected through the intermediary action of cAMP, and magnesium is a required cofactor for adenylate cyclase, defective generation of cAMP could account for the observed defects in PTH secretion and action. Other hormonal systems requiring the intermediary action of cAMP may be similarly affected by magnesium deficiency. The results of the present study, however, demonstrate normal responsiveness of the adrenal cortex, thyrotrophs, gonadotrophs, and liver to their respective trophic hormones in hypocalcemic magnesium-deficient patients. The reason why these responses are intact while PTH secretion and action are impaired is unknown but may be accounted for by differing magnesium requirements of the adenylate cyclase complex in these tissues.

Adrenocorticotropic Hormone↗

Mechanisms of impaired fasting glucose and glucose intolerance induced by an approximate 50% pancreatectomy.

Impaired fasting glucose (IFG) and impaired glucose tolerance (IGT) often coexist and as such represent a potent risk factor for subsequent development of type 2 diabetes. beta-Cell mass is approximately 50% deficient in IFG and approximately 65% deficient in type 2 diabetes. To establish the effect of a approximately 50% deficit in beta-cell mass on carbohydrate metabolism, we performed a approximately 50% partial pancreatectomy versus sham surgery in 14 dogs. Insulin secretion was quantified from insulin concentrations measured in the portal vein at 1-min sampling intervals under basal conditions, after a 30-g oral glucose, and during a hyperglycemic clamp. Insulin sensitivity was measured by a hyperinsulinemic-euglycemic clamp combined with isotope dilution. Partial pancreatectomy resulted in IFG and IGT. After partial pancreatectomy both basal and glucose-stimulated insulin secretion were decreased through the mechanism of a selective approximately 50 and approximately 80% deficit in insulin pulse mass, respectively (P < 0.05). These defects in insulin secretion were partially offset by decreased hepatic insulin clearance (P < 0.05). Partial pancreatectomy also caused a approximately 40% decrease in insulin-stimulated glucose disposal (P < 0.05), insulin sensitivity after partial pancreatectomy being related to insulin pulse amplitude (r = 0.9, P < 0.01). We conclude that a approximately 50% deficit in beta-cell mass can recapitulate the alterations in glucose-mediated insulin secretion and insulin action in humans with IFG and IGT. These data support a mechanistic role of a deficit in beta-cell mass in the evolution of IFG/IGT and subsequently type 2 diabetes.

Animals↗

Effect of the amino acid alanine on glucagon secretion in non-diabetic and type 1 diabetic subjects during hyperinsulinaemic euglycaemia, hypoglycaemia and post-hypoglycaemic hyperglycaemia.

AIMS/HYPOTHESIS: The aim of our study was to establish whether the well-known defective or absent secretion of glucagon in type 1 diabetes in response to hypoglycaemia is selective or includes lack of responses to other stimuli, such as amino acids. MATERIALS AND METHODS: Responses of glucagon to hypoglycaemia were measured in eight patients with type 1 diabetes and six non-diabetic subjects during hyperinsulinaemic (insulin infusion 0.5 mU kg(-1) min(-1)) and eu-, hypo- and hyperglycaemic clamp studies (sequential steps of plasma glucose 5.0, 2.9, 5.0, 10 mmol/l). Subjects were studied on three randomised occasions with infusion of low- or high-dose alanine, or saline. RESULTS: With saline, glucagon increased in hypoglycaemia in non-diabetic subjects but not in diabetic subjects. Glucagon increased further with low-dose (181 +/- 16 ng l(-1) min(-1)) and high-dose alanine (238 +/- 20 ng l(-1) min(-1)) in non-diabetic subjects, but only with high-dose alanine in diabetic subjects (area under curve 112 +/- 5 ng l(-1) min(-1)). The alanine-induced glucagon increase in diabetic subjects paralleled the spontaneous glucagon response to hypoglycaemia in non-diabetic subjects not receiving alanine. The greater responses of glucagon to hypoglycaemia with alanine infusion were offset by recovery of eu- or hyperglycaemia. CONCLUSIONS/INTERPRETATION: In type 1 diabetes, the usually deficient responses of glucagon to hypoglycaemia may improve after increasing the concentration of plasma amino acids. Amino acid-enhanced secretion of glucagon in response to hypoglycaemia remains under physiological control since it is regulated primarily by the ambient plasma glucose concentration. These findings might be relevant to improving counter-regulatory defences against insulin-induced hypoglycaemia in type 1 diabetes.

Adolescent↗

Gallbladder emptying, plasma levels of estradiol and progesterone, and cholecystokinin secretion in liver cirrhosis.

Defective gallbladder emptying has been proposed as a possible accessory pathogenetic factor to explain the increased prevalence of gallstones in liver cirrhosis. In this study we have evaluated the fasting volume and the meal-stimulated emptying of the gallbladder, the plasma levels of estradiol and progesterone, and the basal and postprandial secretion of cholecystokinin in Child A cirrhotic patients compared to normal subjects. Basal (42.2 +/- 27 vs 22.8 +/- 8.4 ml) (P < 0.002) and residual (8.4 +/- 8.7 vs 4.6 +/- 3.8 ml) (P < 0.05) gallbladder volumes were higher in cirrhotics but neither the integrated gallbladder response to meal nor the maximal percentage of emptying was significantly different. Circulating estradiol and progesterone was slightly increased in only 1/13 and 5/13 cirrhotics, respectively. In eight cirrhotics and seven normals taken from the overall populations, the secretion of cholecystokinin was also measured. The fasting plasma level of cholecystokinin was higher in the cirrhotics (6.71 +/- 5.08 vs 2.02 +/- 0.46 pmol/liter) (P < 0.01). The meal-stimulated integrated plasma cholecystokinin response also was greater in cirrhotics (438.5 +/- 615 pmol/liter/270 min) than in normals (153 +/- 170.4 pmol/liter/270 min), but this difference was not significant because of the small study population. In spite of a normal kinetics of postprandial emptying, cirrhotic patients show increased fasting gallbladder volume and increased plasma levels of basal and postprandial cholecystokinin. Circulating estradiol and progesterone do not seem to be responsible for the large gallbladder volume found in liver cirrhosis.

Adult↗

Therapy of insulin-independent diabetes mellitus: general considerations.

Rational approaches to the therapy of patients with insulin-independent diabetes should be based on correcting the underlying defects. The major metabolic abnormalities present in patients with insulin-independent diabetes are: (1) impaired insulin action, (2) defective glucose-mediated insulin secretion, and (3) hyperglucagonemia. All of these defects could be due to a generalized plasma membrane abnormality that affects insulin and glucose receptors or to a generalized disturbance in postreceptor insulin action. Insulin action may be increased by sulfonylureas, possibly by alterations in intracellular cyclic AMP content, or by weight-reducing diets. Insulin secretion may be increased by gastrointestinal hormones, sulfonylureas, serotonin antagonists, or weight-reducing diets. Individual patients with insulin-independent diabetes have varying degrees of insulin resistance or impaired glucose-mediated insulin secretion. The use of therapeutic agents to correct the predominant defect should provide the most effective means of controlling the hyperglycemia and glucose intolerance of the individual patient.

Blood Glucose↗

Somatic mutation of TRbeta can cause a defect in negative regulation of TSH in a TSH-secreting pituitary tumor.

In patients with TSH-secreting tumors (TSHomas), serum TSH is poorly suppressed by thyroid hormone. The mechanism for this defect in negative regulation of TSH secretion is not known. To investigate the possibility of a somatic mutation of TR causing this defect, we performed mutational analysis of TRbeta by RT-PCR using RNA obtained from five surgically resected TSHomas. In one TSHoma, we identified a somatic mutation in the ligand-binding domain of TRbeta that caused a His to Tyr substitution at codon 435 of TRbeta1 corresponding to codon 450 of TRbeta2. Interestingly, this mutation occurred in the same codon as two mutations (TRbetaH435L and H435Q) previously identified in patients with the syndrome of resistance to thyroid hormone. This mutant TRbeta had impaired T3 binding and T3-mediated negative regulation. It also blocked the negative regulation by wild-type TRbeta2 on glycoprotein hormone alpha-subunit and TSHbeta reporter genes in cotransfection studies. Our results demonstrate that somatic mutation of TRbeta occurred in a TSHoma and was probably responsible for the defect in negative regulation of TSH by thyroid hormone in the tumor.

Adult↗

Human glucagon-like peptide-1 receptor gene. Localization to chromosome band 6p21 by fluorescence in situ hybridization and linkage of a highly polymorphic simple tandem repeat DNA polymorphism to other markers on chromosome 6.

Glucagon-like peptide-1 is a fragment of proglucagon secreted by intestinal L-cells. It has potent glucose-dependent insulin secretory effects and also suppresses gastric acid secretion in the stomach. The biological actions of GLP-1 are mediated by the GLP-1 receptor, the structure of which has recently been determined. Defects in insulin secretion are a common feature of NIDDM and as such the GLP-1 receptor is a candidate for contributing to the development of this clinically and genetically heterogeneous disorder. As a first step in determining the role of the GLP-1 receptor in the development of NIDDM, we have isolated the human GLP-1 receptor gene and mapped it to chromosome 6, band p21.1, using the technique of fluorescence in situ hybridization. We also identified a simple tandem repeat DNA polymorphism in the human GLP-1 receptor gene of the form (TG)n. This DNA polymorphism has 14 alleles and a heterozygosity of > 0.8. We have used this DNA polymorphism to localize the GLP-1 receptor gene within the genetic map of the short arm of chromosome 6. This DNA polymorphism will facilitate genetic studies of the contribution of the GLP-1 receptor gene to impaired beta-cell function and NIDDM.

Base Sequence↗

Evidence of NK cell dysfunction in SIV-infected rhesus monkeys: impairment of cytokine secretion and NKG2C/C2 expression.

Defects in the adaptive immune response have been extensively characterized in human immunodeficiency virus type-1 (HIV-1)-infected individuals; however, much less is known about the function of natural killer (NK) cells during the course of HIV-1 infection. In the present study, we demonstrate that the NK cells from simian immunodeficiency virus (SIV)-infected rhesus monkeys are significantly impaired in their ability to secrete IFN-gamma, TNF-alpha, and IL-2, while NK cell function in SIV-infected long-term non-progressor monkeys is similar to that of normal monkeys. These findings suggest that abnormal NK cell activity may contribute to the global immune dysfunction observed in HIV-1-infected individuals. NK cell function is modulated by several families of cell surface receptors, including the CD94/NKG2 family. We evaluated the messenger RNA levels of these inhibitory and activating NKG2 molecules in SIV-infected rhesus monkeys. These experiments demonstrate that the activating molecules NKG2C and NKG2C2 are significantly down-regulated in peripheral blood mononuclear cells of SIV-infected rhesus monkeys, suggesting that the dysregulation of these molecules may contribute to the abnormal NK cell function observed in the setting of infection.

Animals↗

Increased expression of Epstein-Barr virus receptor on lymphoblastoid cell lines from subsets of patients with rheumatoid arthritis.

We studied the expression of Epstein-Barr virus receptor (EBVR/CR2) on lymphoblastoid cell (LCL) lines established from 23 patients with rheumatoid arthritis and the T cell suppression of IgM secretion by EBV activated B cells. Ten patients had normal T cell suppression of IgM secretion, whereas 13 patients had defective suppressor T cell function. EBVR expression on LCL was assessed using a rabbit anti-gp 140 IgG; Raji cells, used as reference cell line expressed 50,000 EBVR, a 140 k glycoprotein (gp 140). Patients with defective T cell suppression of IgM secretion by EBV activated B cells had a significantly higher EBVR expression on LCL than patients with normal T cell suppression (mean +/- SD; 50.8 +/- 23.8 vs 29.5 +/- 13.2, respectively; p less than 0.05). These data suggest a relationship between the T cell suppression defect and an increased EBVR expression on LCL from patients with RA.

Arthritis, Rheumatoid↗