Tolbutamide controls glucagon release from mouse islets differently than glucose: involvement of K(ATP) channels from both α-cells and δ-cells

We evaluated the role of ATP-sensitive K⁺ (K(ATP)) channels, somatostatin, and Zn²⁺ in the control of glucagon secretion from mouse islets. Switching from 1 to 7 mmol/L glucose inhibited glucagon release. Diazoxide did not reverse the glucagonostatic effect of glucose. Tolbutamide decreased glucagon secretion at 1 mmol/L glucose (G1) but stimulated it at 7 mmol/L glucose (G7). The reduced glucagon secretion produced by high concentrations of tolbutamide or diazoxide, or disruption of K(ATP) channels (Sur1(-/-) mice) at G1 could be inhibited further by G7. Removal of the somatostatin paracrine influence (Sst(-/-) mice or pretreatement with pertussis toxin) strongly increased glucagon release, did not prevent the glucagonostatic effect of G7, and unmasked a marked glucagonotropic effect of tolbutamide. Glucose inhibited glucagon release in the absence of functional K(ATP) channels and somatostatin signaling. Knockout of the Zn²⁺ transporter ZnT8 (ZnT8(-/-) mice) did not prevent the glucagonostatic effect of glucose. In conclusion, glucose can inhibit glucagon release independently of Zn²⁺, K(ATP) channels, and somatostatin. Closure of K(ATP) channels controls glucagon secretion by two mechanisms, a direct stimulation of α-cells and an indirect inhibition via somatostatin released from δ-cells. The net effect on glucagon release results from a balance between both effects.

Mechanisms of control of the free Ca2+ concentration in the endoplasmic reticulum of mouse pancreatic β-cells: interplay with cell metabolism and [Ca2+]c and role of SERCA2b and SERCA3

OBJECTIVE

Sarco-endoplasmic reticulum Ca(2+)-ATPase 2b (SERCA2b) and SERCA3 pump Ca(2+) in the endoplasmic reticulum (ER) of pancreatic β-cells. We studied their role in the control of the free ER Ca(2+) concentration ([Ca(2+)](ER)) and the role of SERCA3 in the control of insulin secretion and ER stress.

RESEARCH DESIGN AND METHODS

β-Cell [Ca(2+)](ER) of SERCA3(+/+) and SERCA3(-/-) mice was monitored with an adenovirus encoding the low Ca(2+)-affinity sensor D4 addressed to the ER (D4ER) under the control of the insulin promoter. Free cytosolic Ca(2+) concentration ([Ca(2+)](c)) and [Ca(2+)](ER) were simultaneously recorded. Insulin secretion and mRNA levels of ER stress genes were studied.

RESULTS

Glucose elicited synchronized [Ca(2+)](ER) and [Ca(2+)](c) oscillations. [Ca(2+)](ER) oscillations were smaller in SERCA3(-/-) than in SERCA3(+/+) β-cells. Stimulating cell metabolism with various [glucose] in the presence of diazoxide induced a similar dose-dependent [Ca(2+)](ER) rise in SERCA3(+/+) and SERCA3(-/-) β-cells. In a Ca(2+)-free medium, glucose moderately raised [Ca(2+)](ER) from a highly buffered cytosolic Ca(2+) pool. Increasing [Ca(2+)](c) with high [K] elicited a [Ca(2+)](ER) rise that was larger but more transient in SERCA3(+/+) than SERCA3(-/-) β-cells because of the activation of a Ca(2+) release from the ER in SERCA3(+/+) β-cells. Glucose-induced insulin release was larger in SERCA3(-/-) than SERCA3(+/+) islets. SERCA3 ablation did not induce ER stress.

CONCLUSIONS

[Ca(2+)](c) and [Ca(2+)](ER) oscillate in phase in response to glucose. Upon [Ca(2+)](c) increase, Ca(2+) is taken up by SERCA2b and SERCA3. Strong Ca(2+) influx triggers a Ca(2+) release from the ER that depends on SERCA3. SERCA3 deficiency neither impairs Ca(2+) uptake by the ER upon cell metabolism acceleration and insulin release nor induces ER stress.

Modeling the asymmetric evolution of a mouse and rat-specific microRNA gene cluster intron 10 of the Sfmbt2 gene

Background

The total number of miRNA genes in a genome, expression of which is responsible for the miRNA repertoire of an organism, is not precisely known. Moreover, the question of how new miRNA genes arise during evolution is incompletely understood. Recent data in humans and opossum indicate that retrotranspons of the class of short interspersed nuclear elements have contributed to the growth of microRNA gene clusters.

Method

We studied a large miRNA gene cluster in intron 10 of the mouse Sfmbt2 gene using bioinformatic tools.

Results

Mice and rats are unique to harbor a 55-65 Kb DNA sequence in intron 10 of the Sfmbt2 gene. This intronic region is rich in regularly repeated B1 retrotransposons together with inverted self-complementary CA/TG microsatellites. The smallest repeats unit, called MSHORT1 in the mouse, was duplicated 9 times in a tandem head-to-tail array to form 2.5 Kb MLONG1 units. The center of the mouse miRNA gene cluster consists of 13 copies of MLONG1. BLAST analysis of MSHORT1 in the mouse shows that the repeat unit is unique for intron 10 of the Sfmbt2 gene and suggest a dual phase model for growth of the miRNA gene cluster: arrangment of 10 MSHORT1 units into MLONG1 and further duplication of 13 head-to-tail MLONG1 units in the center of the miRNA gene cluster. Rats have a similar arrangment of repeat units in intron 10 of the Sfmbt2 gene. The discrepancy between 65 miRNA genes in the mouse cluster as compared to only 1 miRNA gene in the corresponding rat repeat cluster is ascribed to sequence differences between MSHORT1 and RSHORT1 that result in lateral-shifted, less-stable miRNA precursor hairpins for RSHORT1.

Conclusion

Our data provides new evidence for the emerging concept that lineage-specific retroposons have played an important role in the birth of new miRNA genes during evolution. The large difference in the number of miRNA genes in two closely related species (65 versus 1, mice versus rats) indicates that this species-specific evolution can be a rapid process.

Pdx1- and Ngn3-Cre-mediated PLAG1 expression in the pancreas leads to endocrine hormone imbalances that affect glucose metabolism

Pleomorphic adenoma gene-like 1 (PLAGL1) has been linked to transient neonatal diabetes mellitus. Here, we investigated the role of the related pleomorphic adenoma gene 1 (PLAG1) in glucose homeostasis. PLAG1 transgenic mice in which expression of the PLAG1 transgene can be targeted to different organs by Cre-mediated modulation were crossed with Pdx1-Cre or Ngn3-Cre mice, resulting in double transgenic P1-Pdx1Cre or P1-Ngn3Cre mice, respectively. P1-Pdx1Cre and P1-Ngn3Cre mice developed hyperplasia of pancreatic islets due to increased β- and δ- but not α-cell proliferation. In young P1-Pdx1Cre mice (less than 15 weeks) there was a balanced increase in the pancreatic content of insulin and somatostatin, which was associated with normoglycemia. In older P1-Pdx1Cre mice the pancreatic somatostatin content far exceeded that of insulin, leading to the progressive development of severe hypoglycemia beyond 30 weeks. In contrast, in older P1-Ngn3Cre mice the relative increase of the pancreatic insulin content exceeded that of somatostatin and these mice remained normoglycemic. In conclusion, forced expression of PLAG1 under the control of the Pdx1 or Ngn3 promoter in murine pancreas induces different degrees of endocrine hormone imbalances within the pancreas, which is associated with hypoglycemia in P1-Pdx1Cre mice but not P1-Ngn3Cre mice. These results suggest that once stem cell-derived islet transplantations become possible, the appropriate balance between different hormone-producing cells will need to be preserved to prevent deregulated glucose metabolism.

Toll-like receptor 2-deficient mice are protected from insulin resistance and beta cell dysfunction induced by a high-fat diet

AIMS/HYPOTHESIS

Inflammation contributes to both insulin resistance and pancreatic beta cell failure in human type 2 diabetes. Toll-like receptors (TLRs) are highly conserved pattern recognition receptors that coordinate the innate inflammatory response to numerous substances, including NEFAs. Here we investigated a potential contribution of TLR2 to the metabolic dysregulation induced by high-fat diet (HFD) feeding in mice.

METHODS

Male and female littermate Tlr2(+/+) and Tlr2(-/-) mice were analysed with respect to glucose tolerance, insulin sensitivity, insulin secretion and energy metabolism on chow and HFD. Adipose, liver, muscle and islet pathology and inflammation were examined using molecular approaches. Macrophages and dendritic immune cells, in addition to pancreatic islets were investigated in vitro with respect to NEFA-induced cytokine production.

RESULTS

While not showing any differences in glucose homeostasis on chow diet, both male and female Tlr2(-/-) mice were protected from the adverse effects of HFD compared with Tlr2(+/+) littermate controls. Female Tlr2(-/-) mice showed pronounced improvements in glucose tolerance, insulin sensitivity, and insulin secretion following 20 weeks of HFD feeding. These effects were associated with an increased capacity of Tlr2(-/-) mice to preferentially burn fat, combined with reduced tissue inflammation. Bone-marrow-derived dendritic cells and pancreatic islets from Tlr2(-/-) mice did not increase IL-1beta expression in response to a NEFA mixture, whereas Tlr2(+/+) control tissues did.

CONCLUSION/INTERPRETATION

These data suggest that TLR2 is a molecular link between increased dietary lipid intake and the regulation of glucose homeostasis, via regulation of energy substrate utilisation and tissue inflammation.

Free fatty acids induce a proinflammatory response in islets via the abundantly expressed interleukin-1 receptor I

Islets of patients with type 2 diabetes mellitus (T2DM) display features of an inflammatory process including elevated levels of the cytokine IL-1beta, various chemokines, and macrophages. IL-1beta is a master regulator of inflammation, and IL-1 receptor type I (IL-1RI) blockage improves glycemia and insulin secretion in humans with T2DM and in high-fat-fed mice pointing to a pivotal role of IL-1RI activity in intra-islet inflammation. Given the association of dyslipidemia and T2DM, we tested whether free fatty acids (FFA) promote the expression of proinflammatory factors in human and mouse islets and investigated a role for the IL-1RI in this response. A comparison of 22 mouse tissues revealed the highest IL-1RI expression levels in islets and MIN6 beta-cells. FFA induced IL-1beta, IL-6, and IL-8 in human islets and IL-1beta and KC in mouse islets. Elevated glucose concentrations enhanced FFA-induced proinflammatory factors in human islets. Blocking the IL-1RI with the IL-1R antagonist (IL-1Ra) strongly inhibited FFA-mediated expression of proinflammatory factors in human and mouse islets. Antibody inhibition of IL-1beta revealed that FFA stimulated IL-1RI activity via the induction of the receptor ligand. FFA-induced IL-1beta and KC expression in mouse islets was completely dependent on the IL-1R/Toll-like receptor (TLR) docking protein Myd88 and partly dependent on TLR2 and -4. Activation of TLR2 in purified human beta-cells and islets stimulated the expression of proinflammatory factors, and IL-1RI activity increased the TLR2 response in human islets. We conclude that FFA and TLR stimulation induce proinflammatory factors in islets and that IL-1RI engagement results in signal amplification.

Insulin storage and glucose homeostasis in mice null for the granule zinc transporter ZnT8 and studies of the type 2 diabetes-associated variants

OBJECTIVE

Zinc ions are essential for the formation of hexameric insulin and hormone crystallization. A nonsynonymous single nucleotide polymorphism rs13266634 in the SLC30A8 gene, encoding the secretory granule zinc transporter ZnT8, is associated with type 2 diabetes. We describe the effects of deleting the ZnT8 gene in mice and explore the action of the at-risk allele.

RESEARCH DESIGN AND METHODS

Slc30a8 null mice were generated and backcrossed at least twice onto a C57BL/6J background. Glucose and insulin tolerance were measured by intraperitoneal injection or euglycemic clamp, respectively. Insulin secretion, electrophysiology, imaging, and the generation of adenoviruses encoding the low- (W325) or elevated- (R325) risk ZnT8 alleles were undertaken using standard protocols.

RESULTS

ZnT8(-/-) mice displayed age-, sex-, and diet-dependent abnormalities in glucose tolerance, insulin secretion, and body weight. Islets isolated from null mice had reduced granule zinc content and showed age-dependent changes in granule morphology, with markedly fewer dense cores but more rod-like crystals. Glucose-stimulated insulin secretion, granule fusion, and insulin crystal dissolution, assessed by total internal reflection fluorescence microscopy, were unchanged or enhanced in ZnT8(-/-) islets. Insulin processing was normal. Molecular modeling revealed that residue-325 was located at the interface between ZnT8 monomers. Correspondingly, the R325 variant displayed lower apparent Zn(2+) transport activity than W325 ZnT8 by fluorescence-based assay.

CONCLUSIONS

ZnT8 is required for normal insulin crystallization and insulin release in vivo but not, remarkably, in vitro. Defects in the former processes in carriers of the R allele may increase type 2 diabetes risks.

Cluster analysis of rat pancreatic islet gene mRNA levels after culture in low-, intermediate- and high-glucose concentrations

AIMS/HYPOTHESIS

Survival and function of insulin-secreting pancreatic beta cells are markedly altered by changes in nutrient availability. In vitro, culture in 10 rather than 2 mmol/l glucose improves rodent beta cell survival and function, whereas glucose concentrations above 10 mmol/l are deleterious.

METHODS

To identify the mechanisms of such beta cell plasticity, we tested the effects of 18 h culture at 2, 5, 10 and 30 mmol/l glucose on the transcriptome of rat islets pre-cultured for 1 week at 10 mmol/l glucose using Affymetrix Rat 230 2.0 arrays.

RESULTS

Culture in either 2-5 or 30 mmol/l instead of 10 mmol/l glucose markedly impaired beta cell function, while little affecting cell survival. Of about 16,000 probe-sets reliably detected in islets, some 5,000 were significantly up- or downregulated at least 1.4-fold by glucose. Analysis of these probe-sets with GeneCluster software identified ten mRNA profiles with unidirectional up- or downregulation between 2 and 10, 2 and 30, 5 and 10, 5 and 30 or 10 and 30 mmol/l glucose. It also identified eight complex V-shaped or inverse V-shaped profiles with a nadir or peak level of expression in 5 or 10 mmol/l glucose. Analysis of genes belonging to these various clusters using Onto-express and GenMAPP software revealed several signalling and metabolic pathways that may contribute to induction of beta cell dysfunction and apoptosis after culture in low- or high- vs intermediate-glucose concentration.

CONCLUSIONS/INTERPRETATION

We have identified 18 distinct mRNA profiles of glucose-induced changes in islet gene mRNA levels that should help understand the mechanisms by which glucose affects beta cell survival and function under states of chronic hypo- or hyperglycaemia.

Evidence for co-evolution between human microRNAs and Alu-repeats

This paper connects Alu repeats, the most abundant repetitive elements in the human genome and microRNAs, small RNAs that alter gene expression at the post-transcriptional level. Base-pair complementarity could be demonstrated between the seed sequence of a subset of human microRNAs and Alu repeats that are integrated parallel (sense) in mRNAs. The most common target site coincides with the evolutionary most conserved part of Alu. A primate-specific gene cluster on chromosome 19 encodes the majority of miRNAs that target the most conserved sense Alu site. The individual miRNA genes within this cluster are flanked by an Alu-LINE signature, which has been duplicated with the clustered miRNA genes. Gene duplication events in this locus are supported by comparing repeat length variations of the LINE elements within the cluster with those in the rest of the chromosome. Thus, a dual relationship exists between an evolutionary young miRNA cluster and their Alu targets that may have evolved in the same time window. One hypothesis for this dual relationship is that these miRNAs could protect against too high rates of duplicative transposition, which would destroy the genome.

Acute nutrient regulation of the unfolded protein response and integrated stress response in cultured rat pancreatic islets

AIMS/HYPOTHESIS

Inadequate chaperone function relative to client protein load in the endoplasmic reticulum triggers an adaptive unfolded protein response (UPR), including the integrated stress response (ISR), the latter being also activated by other types of stresses. It is well established that pancreatic beta cells, which synthesise and secrete insulin upon nutrient stimulation, are markedly affected by pathological disruption or excessive activation of the UPR. However, whether and how physiological nutrient stimulation affects the beta cell UPR has been little investigated.

MATERIALS AND METHODS

We compared the effects of increasing glucose concentrations and of endoplasmic reticulum Ca(2+) emptying with thapsigargin on the UPR (X-box binding protein [Xbp1] mRNA splicing and XBP1/activating transcription factor [ATF] 6-target gene expression) and ISR (eukaryotic translation initiation factor 2A phosphorylation, ATF4 protein levels and target gene expression) in isolated rat islets.

RESULTS

Thapsigargin strongly increased both UPR and ISR. In comparison, glucose moderately increased the UPR between 5 and 30 mmol/l, but exerted complex effects on the ISR as follows: (1) marked reduction between 2 and 10 mmol/l; (2) moderate increase parallel to the UPR between 10 and 30 mmol/l. These glucose effects occurred within 2 h, were mimicked by other metabolic substrates, but were independent of changes in Ca(2+) influx or insulin secretion. Remarkably, attenuating the glucose stimulation of protein synthesis with a low concentration of cycloheximide prevented UPR activation but not ISR reduction by high glucose.

CONCLUSIONS/INTERPRETATION

Nutrient stimulation acutely activates rat islet UPR in a manner dependent on protein synthesis, while exerting complex effects on the ISR. These effects may contribute to nutrient-induced maintenance of the beta cell phenotype.

Glucose activates a protein phosphatase-1-mediated signaling pathway to enhance overall translation in pancreatic beta-cells

Both the rate of overall translation and the specific acceleration of proinsulin synthesis are known to be glucose-regulated processes in the beta-cell. In this study, we propose that glucose-induced stimulation of overall translation in beta-cells depends on a protein phosphatase-1-mediated decrease in serine-51 phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha), a pivotal translation initiation factor. The decrease was rapid and detectable within 15 min and proportional to the range of glucose concentrations that also stimulate translation. Lowered net eIF2alpha phosphorylation was not associated with a detectable decrease in activity of any eIF2alpha kinase. Moreover, okadaic acid blocked glucose-induced eIF2alpha dephosphorylation, suggesting that the net effect was mediated by a protein phosphatase. Experiments with salubrinal on intact cells and nuclear inhibitor of protein phosphatase-1 (PP1) on cell extracts suggested that this phosphatase was PP1. The net effect contained, however, a component of glucose-induced folding load in the endoplasmic reticulum because coincubation with cycloheximide further amplified the effect of glucose on eIF2alpha dephosphorylation. Thus, the steady-state level of eIF2alpha phosphorylation in beta-cells is the result of a balance between folding-load-induced phosphorylation and PP1-dependent dephosphorylation. Because defects in the pancreatic endoplasmic reticulum kinase-eIF2alpha signaling system lead to beta-cell failure and diabetes, deregulation of the PP1 system could likewise lead to cellular dysfunction and disease.

Probe-independent and direct quantification of insulin mRNA and growth hormone mRNA in enriched cell preparations

Task division in multicellular organisms ensures that differentiated cell types produce cell-specific proteins that fulfill tasks for the whole organism. In some cases, the encoded mRNA species is so abundant that it represents a sizeable fraction of total mRNA in the cell. In this study, we have used a probe- and primer-free technique to quantify such abundant mRNA species in order to assess regulatory effects of in vitro and in vivo conditions. As a first example, we were able to quantify the regulation of proinsulin mRNA abundance in beta-cells by food intake or by the glucose concentration in tissue culture. The second example of application of this technique is the effect of corticosteroids on growth hormone mRNA in enriched somatrotrophs. It is anticipated that other examples exist in which measurement of very abundant mRNAs in dedicated cells will help to understand biological processes, monitor disease states, or assist biotechnological manufacturing procedures.

Anaplerosis via pyruvate carboxylase is required for the fuel-induced rise in the ATP:ADP ratio in rat pancreatic islets

AIMS/HYPOTHESIS

The molecular mechanisms of insulin release are only partially known. Among putative factors for coupling glucose metabolism to insulin secretion, anaplerosis has lately received strong support. The anaplerotic enzyme pyruvate carboxylase is highly expressed in beta cells, and anaplerosis influences insulin secretion in beta cells. By inhibiting pyruvate carboxylase in rat islets, we aimed to clarify the hitherto unknown metabolic events underlying anaplerotic regulation of insulin secretion.

METHODS

Phenylacetic acid (5 mmol/l) was used to inhibit pyruvate carboxylase in isolated rat islets, which were then assessed for insulin secretion, fuel oxidation, ATP:ADP ratio, respiration, mitochondrial membrane potential, exocytosis and ATP-sensitive K(+) channel (K(ATP)-channel) conductance.

RESULTS

We found that the glucose-provoked rise in ATP:ADP ratio was suppressed by inhibition of pyruvate carboxylase. In contrast, fuel oxidation, respiration and mitochondrial membrane potential, as well as Ca(2+)-induced exocytosis and K(ATP)-channel conductance in single cells, were unaffected. Insulin secretion induced by alpha-ketoisocaproic acid was suppressed, whereas methyl-succinate-stimulated secretion remained unchanged. Perifusion of rat islets revealed that inhibition of anaplerosis decreased both the second phase of insulin secretion, during which K(ATP)-independent actions of fuel secretagogues are operational, as well as the first and K(ATP)-dependent phase.

CONCLUSIONS/INTERPRETATION

Our results are consistent with the concept that anaplerosis via pyruvate carboxylase determines pyruvate cycling, which has previously been shown to correlate with glucose responsiveness in clonal beta cells. These processes, controlled by pyruvate carboxylase, seem crucial for generation of an appropriate ATP:ADP ratio, which may regulate both phases of fuel-induced insulin secretion.

Redox control of exocytosis: regulatory role of NADPH, thioredoxin, and glutaredoxin

Cellular redox state is an important metabolic variable, influencing many aspects of cell function like growth, apoptosis, and reductive biosynthesis. In this report, we identify NADPH as a candidate signaling molecule for exocytosis in neuroendocrine cells. In pancreatic beta-cells, glucose acutely raised the NADPH-to-NADP+ ratio and stimulated insulin release in parallel. Furthermore, intracellular addition of NADPH directly stimulated exocytosis of insulin granules. Effects of NADPH on exocytosis are proposed to be mediated by the redox proteins glutaredoxin (GRX) and thioredoxin (TRX) on the basis of the following evidence: 1) Expression of GRX mRNA is very high in beta-cells compared with other studied tissues, and GRX protein expression is high in islets and in brain; 2) GRX and TRX are localized in distinct microdomains in the cytosol of beta-cells; and 3) microinjection of recombinant GRX potentiated effects of NADPH on exocytosis, whereas TRX antagonized the NADPH effect. We propose that the NADPH/GRX/TRX redox regulation mediates a novel signaling pathway of nutrient-induced insulin secretion.

Control of mRNA translation preserves endoplasmic reticulum function in beta cells and maintains glucose homeostasis

Type 2 diabetes is a disorder of hyperglycemia resulting from failure of beta cells to produce adequate insulin to accommodate an increased metabolic demand. Here we show that regulation of mRNA translation through phosphorylation of eukaryotic initiation factor 2 (eIF2alpha) is essential to preserve the integrity of the endoplasmic reticulum (ER) and to increase insulin production to meet the demand imposed by a high-fat diet. Accumulation of unfolded proteins in the ER activates phosphorylation of eIF2alpha at Ser51 and inhibits translation. To elucidate the role of this pathway in beta-cell function we studied glucose homeostasis in Eif2s1(tm1Rjk) mutant mice, which have an alanine substitution at Ser51. Heterozygous (Eif2s1(+/tm1Rjk)) mice became obese and diabetic on a high-fat diet. Profound glucose intolerance resulted from reduced insulin secretion accompanied by abnormal distension of the ER lumen, defective trafficking of proinsulin, and a reduced number of insulin granules in beta cells. We propose that translational control couples insulin synthesis with folding capacity to maintain ER integrity and that this signal is essential to prevent diet-induced type 2 diabetes.

Plasticity of the beta cell insulin secretory competence: preparing the pancreatic beta cell for the next meal

It is well established that the acute rise in plasma glucose and in the incretin hormones glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (7-36) amide (GLP-1), as occurs during a meal, is of pivotal importance in regulating the minute-to-minute output of insulin from pancreatic beta cells. In addition to this well studied acute effect, both glucose and incretin hormones have been recently observed to determine the future secretory responsiveness of the cells. Such plasticity of the insulin secretory competence would imply that glucose and incretins not only act during the present meal, but also help to prepare the beta cells to function during the subsequent meal. Evidence supporting this hypothesis is growing as a result of physiological studies of cultured beta cells (either primary cells or beta cell lines), as well as from an increasing number of large-scale gene expression studies, exploring transcriptional and post-transcriptional events in genes regulated by glucose and incretins. On the basis of this hypothesis, one can speculate that genetic or environmental disturbances of plasticity of the insulin secretory competence is one aspect of beta cell dysfunction that can contribute to the aetiology of type 2 diabetes.

Physiological role of AMP-activated protein kinase (AMPK): insights from knockout mouse models

AMP-activated protein kinase (AMPK) is viewed as a fuel sensor for glucose and lipid metabolism. To understand better the physiological role of the catalytic AMPK subunit isoforms, we generated two knockout mouse models with the alpha1 (AMPK alpha 1(-/-)) and alpha 2 (AMPK alpha 2(-/-)) catalytic subunit genes deleted. No defect in glucose homoeostasis was observed in AMPK alpha 1(-/-) mice. On the other hand, AMPK alpha 2(-/-) mice presented high plasma glucose levels and low plasma insulin concentrations in the fed period and during the glucose tolerance test. Nevertheless, in isolated AMPK alpha 2(-/-) pancreatic islets, glucose-stimulated insulin secretion was not affected. Surprisingly, AMPK alpha 2(-/-) mice were insulin-resistant and had reduced muscle glycogen synthesis as assessed in vivo by the hyperinsulinaemic euglycaemic clamp procedure. Reduction of insulin sensitivity and glycogen synthesis were not dependent on the lack of AMPK in skeletal muscle, since mice expressing a dominant inhibitory mutant of AMPK in skeletal muscle were not affected and since insulin-stimulated glucose transport in incubated muscles in vitro was normal in AMPK alpha 2(-/-) muscles. Furthermore, AMPK alpha 2(-/-) mice have a higher sympathetic tone, as shown by increased catecholamine urinary excretion. Increased adrenergic tone could explain both decreased insulin secretion and insulin resistance observed in vivo in AMPK alpha 2(-/-) mice. We suggest that the alpha2 catalytic subunit of AMPK plays a major role as a fuel sensor by modulating the activity of the autonomous nervous system in vivo.

The AMP-activated protein kinase alpha2 catalytic subunit controls whole-body insulin sensitivity

AMP-activated protein kinase (AMPK) is viewed as a fuel sensor for glucose and lipid metabolism. To better understand the physiological role of AMPK, we generated a knockout mouse model in which the AMPKalpha2 catalytic subunit gene was inactivated. AMPKalpha2(-/-) mice presented high glucose levels in the fed period and during an oral glucose challenge associated with low insulin plasma levels. However, in isolated AMPKalpha2(-/-) pancreatic islets, glucose- and L-arginine-stimulated insulin secretion were not affected. AMPKalpha2(-/-) mice have reduced insulin-stimulated whole-body glucose utilization and muscle glycogen synthesis rates assessed in vivo by the hyperinsulinemic euglycemic clamp technique. Surprisingly, both parameters were not altered in mice expressing a dominant-negative mutant of AMPK in skeletal muscle. Furthermore, glucose transport was normal in incubated isolated AMPKalpha2(-/-) muscles. These data indicate that AMPKalpha2 in tissues other than skeletal muscles regulates insulin action. Concordantly, we found an increased daily urinary catecholamine excretion in AMPKalpha2(-/-) mice, suggesting altered function of the autonomic nervous system that could explain both the impaired insulin secretion and insulin sensitivity observed in vivo. Therefore, extramuscular AMPKalpha2 catalytic subunit is important for whole-body insulin action in vivo, probably through modulation of sympathetic nervous activity.

IA-2 autoantibodies predict impending type I diabetes in siblings of patients

AIMS/HYPOTHESIS

Multiple islet autoantibody positivity is currently believed to best predict progression to Type I (insulin-dependent) diabetes mellitus. We compared its predictive value with that of positivity for a particular type of islet autoantibody, directed against the IA-2 antigen.

METHODS

Autoantibodies against islet cell cytoplasm (ICA), insulin (IAA), GAD (GADA) and IA-2 (IA-2A) were measured at initial sampling in 1724 non-diabetic siblings (median age [range]:16 [0-39] years) of Type I diabetic patients with a median follow-up of 50 months.

RESULTS

On initial sampling 11% of siblings were positive for one antibody type or more and 2.1% for three of more types. During follow-up, 27 antibody-positive siblings developed diabetes. Using survival analysis, the risk for clinical onset within 5 years was 34% in subjects positive for three or more types compared with 13% in those with one type or more. Progression to diabetes amounted to 12% within 5 years among siblings positive for IAA, 20% for ICA, 19% for GADA but 59% for IA-2A (p<0.001 vs absence of the respective antibody). IA-2A were detected in 1.7% of all siblings and in 56% of the prediabetic subjects on first sampling. Initial positivity for two or three antibody markers was associated with a higher progression rate in IA-2A positive as compared to IA-2A negative siblings (p=0.001). In absence of IA-2A initial positivity for another antibody (IAA, ICA or GADA) conferred a low (<10% within 5 years) risk of diabetes compared to subjects lacking this antibody.

CONCLUSIONS/INTERPRETATION

In siblings of Type I diabetic patients, IA-2A positivity is a more direct predictor of impending clinical onset than multiple antibody positivity per se. Assessment of IA-2A status allows us to select subjects with homogeneously high risk of diabetes for participation in prevention trials.

Critical role for cataplerosis via citrate in glucose-regulated insulin release

The molecular mechanisms mediating acute regulation of insulin release by glucose are partially known. The process involves at least two pathways that can be discriminated on basis of their (in)dependence of closure of ATP-sensitive potassium (K+(ATP)) channels. The mechanism of the K+(ATP) channel-independent pathway was proposed to involve cataplerosis, the export of mitochondrial intermediates into the cytosol and in the induction of fatty acid-derived signaling molecules. In the present article, we have explored in fluorescence-activated cell sorter (FACS)-purified rat beta-cells the molecular steps involved in chronic glucose regulation of the insulin secretory response. When compared with culture in 10 mmol/l glucose, 24 h culture in 3 mmol/l glucose shifts the phenotype of the cells into a state with low further secretory responsiveness to glucose, lower rates of glucose oxidation, and lower rates of cataplerosis. Microarray mRNA analysis indicates that this shift can be attributed to differences in expression of genes involved in the K+(ATP) channel-dependent pathway, in cataplerosis and in fatty acid/cholesterol biosynthesis. This response was paralleled by glucose upregulation of the transcription factor sterol regulatory element binding protein 1c (SREBP1c) (ADD1) and downregulation of peroxisome proliferator-activated receptor (PPAR)-alpha and PPAR-beta (PPARdelta). The functional importance of cataplerosis via citrate for glucose-induced insulin release was further supported by the observation that two ATP-citrate lyase inhibitors, radicicol and (-)-hydroxycitrate, block part of glucose-stimulated release in beta-cells. In conclusion, chronic glucose regulation of the glucose-responsive secretory phenotype is associated with coordinated changes in gene expression involved in the K+(ATP) channel-dependent pathway, in cataplerosis via citrate and in acyl CoA/cholesterol biosynthesis.

Relation between disease phenotype and HLA-DQ genotype in diabetic patients diagnosed in early adulthood

We investigated inaugural disease phenotype in relation to the presence or absence of diabetes-associated autoantibodies and human leukocyte antigen (HLA) DQ risk genotypes in adult-onset diabetic patients. Blood samples and questionnaires were obtained from 1584 recent-onset Belgian Caucasian patients (age 15-39 yr at diagnosis of primary diabetes) who were recruited by the Belgian Diabetes Registry over an 11-yr period. At clinical diagnosis, antibody-positive patients (n = 1198) were on average younger and had more symptoms, a more acute disease onset, lower body mass index, and random C-peptide levels, but higher insulin needs, glycemia, and prevalence of ketonuria, HLA-DQ, and 5' insulin gene susceptibility genotypes (P < 0.001 vs. antibody-negative patients; n = 386). In antibody-positive patients, these characteristics did not differ according to HLA-DQ genotype. However, in antibody-negative subjects, we found that patients were younger (P = 0.001); had a lower body mass index (P < 0.001), higher insulin needs (P = 0.014), and amylasemia (P = 0.001); and tended to have a higher glycemia and lower C-peptide in the presence of susceptible HLA-DQ genotypes. Differences according to HLA-DQ genotype subsisted after careful age-matching. In conclusion, we found no relation between initial disease phenotype and HLA-DQ genotype in antibody-positive diabetic young adults. In contrast, antibody-negative patients displayed more type 1-like features when carrying susceptible HLA-DQ genotypes known to promote the development of antibody-positive diabetes. The overrepresentation of these susceptibility genotypes in antibody-negative patients suggests the existence of an immune-mediated disease process with as yet unidentified immune markers in a subgroup of seronegative patients.

Relative and absolute HLA-DQA1-DQB1 linked risk for developing type I diabetes before 40 years of age in the Belgian population: implications for future prevention studies

HLA-DQ genotyping remains the cornerstone of genetic risk stratification in type I diabetes prediction and prevention studies. We developed a genetic screening strategy for predisposition to type I diabetes in the Belgian population based upon HLA-DQA1-DQB1 typing and taking into account the age at clinical onset. A group of 1866 autoantibody-positive type I patients below age 40 years recruited by the Belgian Diabetes Registry and a group of 750 control subjects were DQA1-DQB1 genotyped. In the total study population 16 different DQA1-DQB1 haplotypes were revealed, allowing the stratification of 81 genotypes in ten different genotype groups. Apart from the highest risk DQA1*-DQB1* genotype 0301-0302/0501-0201 (odds ratio 21; absolute risk 6%), three other genotype groups conferred a highly significant disease risk (p < 10(-6)). Altogether, these susceptibility genotypes were carried by 9% of the control subjects versus 60% of the patients diagnosed before age 40 years and up to 70% of those under age 5 years. All other genotypes were protective, neutral, infrequent or associated with a moderate protection or susceptibility. A strong, although not absolute protection was conferred by DQB1*0602-positive haplotypes (odds ratio = 0.03). This study in a large cohort of autoantibody-positive patients shows that a DQA1-DQB1-based genotyping strategy allows the identification of a subgroup representing less than 10% of the Belgian population but harbouring the majority of future type I patients arising in childhood or early adulthood. Future prediction and prevention studies should take into account the age dependency of this HLA-DQ associated risk.

Glucagon-like peptide 1 receptor signaling influences topography of islet cells in mice

Glucagon-like peptide 1 (GLP-1) amplifies glucose-induced insulin release in vivo and in vitro. Activation of GLP-1 receptor (GLP-1R) signaling leads to differentiation of exocrine cells towards a beta-cell phenotype in vitro and stimulation of islet cell proliferation in vitro and in vivo, suggesting a potential role for GLP-1 in the modulation of islet growth and differentiation. To determine whether basal levels of GLP-1R signaling are essential for islet development, we examined islet cell composition and topography in GLP-1R-/- mice. Total beta-cell volume and number are not altered, but the topography of beta cells is markedly different in GLP-1R-/- mice compared with GLP-1R+/+ controls. The distribution of beta cells is shifted from large to small and medium-sized islets in the absence of GLP-1R signaling (large islets: 50 +/- 3% in GLP-1R+/+ vs 28 +/- 4% in GLP- 1R-/-, P < 0.01 and medium islets: 32 +/- 2% in GLP- 1R+/+ vs 48 +/- 3% in GLP-1R-/-, P < 0.001). Furthermore, GLP-1R-/- islets exhibit abnormalities in cell topography, with two to threefold more centrally located alpha cells detected in GLP-1R-/- islets. These alterations in alpha- and beta-cell topography indicate that basal levels of GLP-1 signaling in the normal rodent are involved in the normal cellular organization of the endocrine pancreas.

Male-to-female excess in diabetes diagnosed in early adulthood is not specific for the immune-mediated form nor is it HLA-DQ restricted: possible relation to increased body mass index

AIMS/HYPOTHESIS. We investigated whether the reported HLA-DQ/DR restricted male-to-female (M:F) excess in Type I (insulin-dependent) diabetes mellitus also exists in Belgian patients, is specific for immune-mediated diabetes, remains genotype-restricted after adjustment for age at diagnosis, and is associated with sex-dependent environmental factors.

METHODS

Autoantibodies, HLA-DQ and 5'INS (5'insulin gene) polymorphisms were assessed in 2,532 diabetic patients (all phenotypes) diagnosed under 40 years of age. Autoantibodies and body mass index (expressed as a standard deviation score by comparison to age-matched and sex-matched control subjects; SDS-BMI) were measured in 1986 siblings or offspring of Type I diabetes patients (0-39 years).

RESULTS

In patients aged 15-39 years at diagnosis, the male-to-female ratio was 1.5 or more regardless of their antibody status and significantly higher (p < 0.001) than that in the age-matched Belgian general population. There was no sex bias in patients under 15 years of age. Overall, the male-to-female ratio was significantly higher in patients without HLADQA1*0301-DQB1*0302 (p < or = 0.003) but stratification in age groups and multivariate analysis identified age as the major determinant of male-to-female ratio. The SDS-BMI increased (p < 0.01) in male antibodypositive relatives (n = 103) but not in female antibody-positive (n = 92) or in antibody-negative relatives (n = 1,791). This phenomenon tended to be restricted to male relatives who were positive only for glutamate decarboxylase antibodies (n = 44).

CONCLUSIONS/INTERPRETATION

The male-to-female excess in Belgian diabetic patients diagnosed in early adulthood is not specific for immune-mediated Type I diabetes and not HLA-DQ or 5'INS restricted. Our data suggest that, similar to Type II (non-insulin-dependent) diabetes mellitus, the metabolic burden of obesity and insulin resistance could preferentially precipitate postpubertal clinical onset in male subjects with slowly progressive subclinical (immune-mediated) diabetes.

Glucose sensing in pancreatic beta-cells: a model for the study of other glucose-regulated cells in gut, pancreas, and hypothalamus

Nutrient homeostasis is known to be regulated by pancreatic islet tissue. The function of islet beta-cells is controlled by a glucose sensor that operates at physiological glucose concentrations and acts in synergy with signals that integrate messages originating from hypothalamic neurons and endocrine cells in gut and pancreas. Evidence exists that the extrapancreatic cells producing and secreting these (neuro)endocrine signals also exhibit a glucose sensor and an ability to integrate nutrient and (neuro)hormonal messages. Similarities in these cellular and molecular pathways provide a basis for a network of coordinated functions between distant cell groups, which is necessary for an appropriate control of nutrient homeostasis. The glucose sensor seems to be a fundamental component of these control mechanisms. Its molecular characterization is most advanced in pancreatic beta-cells, with important roles for glucokinase and mitochondrial oxidative fluxes in the regulation of ATP-sensitive K+ channels. Other glucose-sensitive cells in the endocrine pancreas, hypothalamus, and gut were found to share some of these molecular characteristics. We propose that similar metabolic signaling pathways influence the function of pancreatic alpha-cells, hypothalamic neurons, and gastrointestinal endocrine and neural cells.

Dehydroepiandrosterone sulfate and beta-cell function: enhanced glucose-induced insulin secretion and altered gene expression in rodent pancreatic beta-cells

Administration of dehydroepiandrosterone (DHEA), or its sulfated form (DHEAS), controls hyperglycemia in diabetic rodents without directly altering insulin sensitivity. We show that DHEAS enhanced glucose-stimulated insulin secretion when administered in vivo to rats or in vitro to beta-cell lines, without changing cellular insulin content. Insulin secretion increased from 3 days of steroid exposure in vitro, suggesting that DHEAS did not directly activate the secretory processes. DHEAS selectively increased the beta-cell mRNA expression of acyl CoA synthetase-2 and peroxisomal acyl CoA oxidase in a time-dependent manner. Although DHEAS is a peroxisomal proliferator, it did not alter the mRNA expression of peroxisomal proliferator-activated receptor (PPAR) alpha or beta, or enhance the activity of transfected PPAR alpha, beta, or gamma in vitro. Thus, DHEAS directly affected the beta-cell to enhance glucose-stimulated insulin secretion and increased the mRNA expression of specific beta-cell mitochondrial and peroxisomal lipid metabolic enzymes. This effect of DHEAS on insulin secretion may contribute to the amelioration of hyperglycemia seen in various rodent models of diabetes.

Associations of GAD65- and IA-2- autoantibodies with genetic risk markers in new-onset IDDM patients and their siblings. The Belgian Diabetes Registry

OBJECTIVE

To investigate the association of GAD (65-kDa) autoantibodies (GAD65-Abs) and IA-2 autoantibodies (IA-2-Abs) with human leukocyte antigen (HLA)-DQ and insulin gene (INS) risk markers in patients with recent-onset IDDM and their siblings.

RESEARCH DESIGN AND METHODS

Blood was sampled from 608 recent-onset IDDM patients and 480 siblings, aged 0-39 years and consecutively recruited by the Belgian Diabetes Registry, to determine GAD65- and IA-2-Ab (radiobinding assay), HLA-DQ- (allele-specific oligonucleotyping), and INS-genotypes (restriction fragment length polymorphism analysis; siblings, n = 439).

RESULTS

At the onset of IDDM, GAD65-Abs were preferentially associated with two populations at genetic risk but only in the 20- to 39-year age-group: 1) their prevalence was higher in carriers of DQA1*0301-DQB1*0302 (88 vs. 73% in non[DQA1*0301-DQB1*0302], P = 0.001), and 2) an association was found in patients lacking this haplotype but carrying DQA1*0501-DQB1*0201, together with INS I/I (87 vs. 54% vs. non[INS I/I], P = 0.003). Siblings of IDDM patients also presented the association of GAD65-Abs with DQA1*0301-DQB1*0302 (13 vs. 2% non[DQA1*0301-DQB1*0302], P < 0.001), while associations with the second genetic risk group could not yet be assessed. At the onset of IDDM, IA-2-Ab prevalence was higher in carriers of DQA1*0301-DQB1*0302 (69 vs. 39% non[DQA1*0301-DQB1*0302], P < 0.001) but not of DQA1*0501-DQB1*0201 or INS I/I. This association was present in both the 0- to 19- and the 20- to 39-year age-groups. It was also found in siblings of IDDM patients (4 vs. 0% non[DQA1*0301-DQB1*0302], P < 0.001).

CONCLUSIONS

Both GAD65- and IA-2-Abs exhibit higher prevalences in presence of HLA-DQ- and/or INS-genetic risk markers. Their respective associations differ with age at clinical onset, suggesting a possible usefulness in the identification of subgroups in this heterogeneous disease.

Epidemiology, clinical aspects, and biology of IDDM patients under age 40 years. Comparison of data from Antwerp with complete ascertainment with data from Belgium with 40% ascertainment. The Belgian Diabetes Registry

OBJECTIVE

To compare the incidence rate of IDDM in the age-groups 0-14 and 15-39 years in Antwerp, Belgium, and to compare demographic, clinical, and biological data from Antwerp IDDM patients with 92% ascertainment with those from a larger Belgian patient group with 40% ascertainment.

RESEARCH DESIGN AND METHODS

Incident cases of IDDM were reported by physicians of the Belgian Diabetes Registry and in Antwerp by several other sources. In Antwerp, completeness of ascertainment was calculated by the capture-recapture method. Demographic and clinical data were collected by questionnaire. Blood was sampled for HLA-DQ genotyping and, in new-inset patients, for autoantibodies.

RESULTS

In Antwerp, the age- and sex-standardized IDDM incidence rates were similar in both age-groups (0-14 years: 11.8/100,000; 15-39 years: 8.9/100,000). The incidence rate decreased in girls above age 15 years (6.9/100,000; P = 0.003) but not in boys (11.0/100,000). Both in Antwerp and Belgium, IDDM was diagnosed more frequently in the 15-39 years age-group (60% of all cases) than under age 15 years, with a lower prevalence of acute symptoms, ketonuria, high-risk HLA-DQ genotype, and autoantibodies against insulin, islet cells, and IA-2, but with a higher prevalence of GAD65 autoantibodies.

CONCLUSIONS

In Antwerp, the incidence rate of IDDM under age 15 years is intermediately high compared with the rates in other European regions. It is similar in the 15-39 years age-group, but with a marked male predominance. Demographic, clinical, and biological data show the same age-dependent heterogeneity as the data collected nationwide, with 40% ascertainment indicating the representativeness of the latter.

CTLA-4 gene polymorphism confers susceptibility to insulin-dependent diabetes mellitus (IDDM) independently from age and from other genetic or immune disease markers. The Belgian Diabetes Registry

Apart from genes in the HLA complex (IDDM1) and the variable number of tandem repeats in the 5' region of the insulin gene (INS VNTR, IDDM2), several other loci have been proposed to contribute to IDDM susceptibility. Recently, linkage and association have been shown between the cytotoxic T lymphocyte-associated protein 4 (CTLA-4) gene on chromosome 2q and IDDM. In a registry-based group of 525 recent-onset IDDM patients <40 years old we investigated the possible interactions of a CTLA-4 gene A-to-G transition polymorphism with age at clinical disease onset and with the presence or absence of established genetic (HLA-DQ, INS VNTR) and immune disease markers (autoantibodies against islet cell cytoplasm (ICA); insulin (IAA); glutamate decarboxylase (GAD65-Ab); IA-2 protein tyrosine phosphatase (IA-2-Ab)) determined within the first week of insulin treatment. In new-onset IDDM patients. G-allele-containing CTLA-4 genotypes (relative risk (RR)= 1.5; 95% confidence interval (CI) = 1.2-2.0; P < 0.005) were not preferentially associated with age at clinical presentation or with the presence of other genetic (HLA-DR3 or DR4 alleles; HLA-DQA1*0301-DQB1*0302 and/or DQA1*0501-DQB1*0201 risk haplotypes; INS VNTR I/I risk genotype) or immune (ICA, IAA, IA-2-Ab, GAD65-Ab) markers of diabetes. For 151 patients, thyrogastric autoantibodies (anti-thyroid peroxidase, anti-thyroid-stimulating hormone (TSH) receptor, anti-parietal cell, anti-intrinsic factor) were determined, but association between CTLA-4 risk genotypes and markers of polyendocrine autoimmunity could not be demonstrated before or after stratification for HLA- or INS-linked risk. In conclusion, the presence of a G-containing CTLA-4 genotype confers a moderate but significant RR for IDDM that is independent of age and genetic or immune disease markers.

Effects of chronically elevated glucose levels on the functional properties of rat pancreatic beta-cells

This study examines the effects of chronically elevated glucose levels on the survival and function of purified rat beta-cells. Prolonged exposure (9 days) of beta-cell aggregates to 20 mmol/l glucose did not lead to cell losses, but reduced the amount of insulin secreted in response to glucose. This decrease was not caused by cellular desensitization but resulted from the lower cellular insulin content after a prolonged imbalance between stimulated rates of insulin synthesis and release. Virtually all beta-cells exhibited a state of metabolic and biosynthetic activation, which was maintained for at least 2 h in glucose-depleted media. Their rates of protein and insulin synthesis were amplified by glucose, reaching (half-) maximal stimulation at lower glucose concentrations (2 and 5 mmol/l, respectively) than control cells cultured at 10 mmol/l glucose (5 and 10 mmol/l, respectively). As for insulin release, the net glucose effect on insulin synthesis was markedly reduced as compared with that in control cells. This was also the case after culture at 6 mmol/l glucose. In the latter condition, the lower glucose-inducible activities were caused by cellular desensitization, with 50% of the beta-cells unresponsive to glucose and the other 50% responding with a lower sensitivity (half-maximal stimulation at 7 mmol/l glucose). Comparison of beta-cells cultured at the three glucose concentrations indicated that prolonged exposure to elevated glucose levels increases the number of degranulated cells, of cells with a high proportion of immature insulin granules, and of cells with glycogen deposition-morphologic features previously described in conditions of hyperglycemia. It is concluded that chronic exposure (9 days) of rat beta-cells to elevated glucose levels induces a prolonged state of beta-cell activation and glucose hypersensitivity rather than a glucotoxicity or glucose desensitization. This shift in the functional state of the beta-cell population is responsible for a reduced insulin release in response to glucose, as observed in other conditions of prolonged exposure to high glucose levels.

Abnormal circulating pancreatic enzyme activities in more than twenty-five percent of recent-onset insulin-dependent diabetic patients: association of hyperlipasemia with high-titer islet cell antibodies. Belgian Diabetes Registry

Pancreatic amylase and lipase activities were measured in sera of 307 Caucasian insulin-dependent diabetes mellitus patients (IDDM) at clinical onset, 303 nondiabetic siblings of registered patients, and 207 control subjects under age 40 years. In all subject groups lipasemia and pancreatic (but not salivary) amylasemia increased with age and were significantly correlated. Using age-dependent reference ranges, reduced pancreatic enzyme levels were measured in 18% of patients, 6% of siblings, and only 2% of control subjects (p < 0.001). Increased lipase levels were noted in 10% of patients and in only 3% of siblings and 2% of control subjects (p < 0.001). Using both univariate and multivariate statistical analysis, elevated lipase activities at clinical onset were associated with higher titers of autoantibodies against islet cell cytoplasmic antigens and glucagon, but not against insulin or the 65-kDa isoform of glutamic acid decarboxylase (GAD65-Ab), or with markers of genetic predisposition or metabolic dysregulation. These findings indicate the presence of modest, but statistically significant, variations in circulating pancreatic enzyme levels in 28% of IDDM patients at clinical onset (p < 0.001 vs. 5% in control subjects). Increased lipase levels may express a form or a stage of the disease with exocrine cell damage; their association with higher titers of islet cell and glucagon autoantibodies is not yet explained. Lower lipase and isoamylase levels are thought to result from the reduced acinar cell function in the vicinity of insulin-depleted islets. It must be tested whether pancreatic enzyme activities in serum can also be altered during the preclinical stage and can thus be considered as an additional marker for the disease process in the pancreas.

Factors determining the glucose sensitivity and glucose responsiveness of pancreatic beta cells

Functional, metabolic and molecular studies using purified beta cells (beta-cell have contributed to our understanding how insulin synthesis and release at regulated by glucose. Individual rat islet beta-cells are heterogeneous in the threshold sensitivity to glucose, so that the physiological graded glucose induced response of the pancreatic beta-cell population can be explained--at least in part--by dose-dependent recruitment of cells. beta-Cell threshold sensitivity to glucose is correlated to glucokinase gene expression rather than glucose transport, reinforcing the concept that glucokinase is directly involved in beta-cell glucose sensing. This idea is further supported by observing major species differences in islet GLUT2 expression, whereas islet cell glucokinase expression and function are strongly conserved. Studies on pure rat beta-cell have also shown that cyclic AMP acts--in addition to its well-known potentiator function of glucose-induced insulin release--as a competence factor which is absolutely required for normal beta-cell responsiveness to glucose. Intraislet glucagon appears to be a paracrine regulator of cyclic AMP production in vitro, but this signalling pathway can be an artifact of the islet isolation procedure. In rat beta-cells, expression and functional activity can be demonstrated on receptors recognising glucagon, glucagon-like peptide I and glucose-dependent insulinotropic peptide. Whether this diversity in signalling reflects another form of beta-cell heterogeneity, functional complementation or biological redundancy, remains to be investigated.

Influence of age on the associations among insulin autoantibodies, islet cell antibodies, and HLA DAQ1*0301-DQB1*0302 in siblings of patients with type 1 (insulin-dependent) diabetes mellitus. Belgian Diabetes Registry

In recent-onset type 1 (insulin-dependent) diabetes mellitus (IDDM), insulin autoantibodies (IAA) and islet cell antibodies (ICA) occur preferentially in young (< 10 yr) patients with the HLA DQA1*0301-DQB1*0302 risk haplotype. We investigated whether this association also exists in siblings of IDDM patients. In our group of 310 siblings, aged 0-39 yr, 6% were positive for IAA, 7% for ICA 12 Juvenile Diabetes Foundation units (JDFU) or more, 5% for ICA 20 JDFU or more, and 2% for high titer ICA (> or = 80 JDFU). The occurrence of IAA and ICA (> or = 20 JDFU) was significantly associated, with a preferential relationship to the HLA DQA1*0301-DQB1*0302 susceptibility haplotype. In the present group of siblings, IAA and DQA1*0301-DQB1*0302 were significantly associated under age 10 yr (26% positivity for IAA vs. 4% in relatives without this haplotype). In this age group, IAA were more prevalent than (high titer) ICA (6%) in the presence of the haplotype. The association between (high titer) ICA and DQA1*0301-DQB1*0302 was not restricted to subjects under age 10 yr. High titer ICA (n = 5) occurred exclusively in homozygotes for the latter haplotype and in carriers of the heterozygous DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201 high risk genotype, mostly under age 20 yr (four of five). The preferential occurrence of IAA in DQA1*0301-DQB1*0302-positive siblings under age 10 yr was caused by their high prevalence (47%) in subjects with the heterozygous high risk genotype in this age group. As in patients at onset, IAA and high titer ICA are preferentially associated with the DQA1*0301-DQB1*0302 haplotype in siblings of IDDM patients, but, unlike at onset, these associations are observed with specific genotypes only and are more pronounced in subjects under age 10 yr for IAA only. Longitudinal analysis in first degree relatives and other normal controls carrying the DQA1*0301-DQB1*0302 haplotype should assess the hypothesis that IAA qualify as earlier predictive markers for IDDM than high titer ICA.

Insulin autoantibodies and high titre islet cell antibodies are preferentially associated with the HLA DQA1*0301-DQB1*0302 haplotype at clinical type 1 (insulin-dependent) diabetes mellitus before age 10 years, but not at onset between age 10 and 40 years. The Belgian Diabetes Registry

Demographic and biological data were collected from all Caucasian Type 1 diabetic patients (n = 279) who were recruited at clinical onset by the Belgian Diabetes Registry over 34 months. The male/female ratio was significantly higher for onset between age 20 and 40 years (2.4) than before age 20 years (1.0); no age-or sex-differences were noticed in serum fructosamine concentration. Total and high concentrations of insulin autoantibodies and islet cell antibodies were preferentially associated with the HLA DQA1*0301-DQB1*0302 susceptibility haplotype. The occurrence of both types of antibodies was also correlated, irrespective of haplotype. At onset before age 10 years, the high risk genotype DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201 was more prevalent than all other DQA1-DQB1 genotypes taken together, leading to a higher prevalence of the DQA1*0301-DQB1*0302 haplotype in this age group (75%) than in the 10-39 years age group (54%). Under age 10 years, the presence of DQA1*0301-DQB1*0302 was strongly associated with insulin autoantibodies (90%) and islet cell autoantibodies (92% with 85% of high titre), whereas patients without this haplotype were less frequently positive for insulin autoantibodies (31%) or islet cell autoantibodies (38% high titre). In the group with onset at age 10-39 years, the DQA1*0301-DQB1*0302 haplotype presented a lower association with insulin autoantibodies (approximately 40%) and islet cell autoantibodies (50 to 65% high titre), prevalences which no longer differed from those in subjects lacking this haplotype.(ABSTRACT TRUNCATED AT 250 WORDS)

5' insulin gene polymorphism confers risk to IDDM independently of HLA class II susceptibility

The polymorphic variable number of tandem repeats in the 5' upstream region of the human insulin gene is a well-known non-human leukocyte antigen locus contributing to genetic susceptibility to IDDM. Controversy exists about the question as to whether INS susceptibility haplotypes are or are not preferentially inherited together with HLA-DR4 haplotypes. We investigated whether genetic interaction between INS and the HLA complex can be better defined using DQ genotypic and phenotypic markers in addition to DR serology. The 5' INS 1/1 genotype was positively associated with IDDM both in non-DR4 subjects (relative risk = 4.3; 95% confidence interval, 1.6-11.5) and DR4 subjects (relative risk = 4.2; 95% confidence interval, 1.9-9.0). Further subdivision of IDDM patients and matched control subjects according to HLA-DQA1 and HLA-DQB1 genotype or phenotype also failed to show any association between 5' INS and HLA class II genes in diabetic patients. The 5' INS and HLA class II polymorphisms therefore provide independent risk markers, which may both contribute to the genetic screening of a high-risk population among nondiabetic individuals.

Complementation of HLA-DQA and -DQB genes confers susceptibility and protection to insulin-dependent diabetes mellitus

Lack of an aspartic acid 57 in the HLA-DQ beta chain was introduced as a genetic marker of insulin-dependent diabetes mellitus (IDDM). Because 25% of the control population carries the same marker, we analyzed the DQ locus for the presence of more specific disease susceptibility markers, taking into account a possible role for the polymorphic DQA gene. We thereby identified the DQA3-DQB3.2/DQA4.1-DQB2 (DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201) genotype which was detected in 30% of the 268 typed IDDM patients and only in 1% of the 331 typed healthy controls, resulting in a relative risk of 35. This genetic marker was more frequent in patients with clinical onset before age 18 years (36%) than in patients diagnosed between age 18 and 40 years (22%) and was not observed in patients with non-IDDM. The new susceptibility genotype DQA3-DQB3.2/DQA4.1-DQB2 (DQA1*0301-DQB1*0302/DQA1*0501-DQB1*0201) may explain the well-known excess of DR3/DR4 heterozygous IDDM patients and is expected to help identify individuals at risk for developing the disease.

Measuring the balance between insulin synthesis and insulin release

The absolute rates of hormone synthesis and release were determined in purified pancreatic B cells. Newly synthesized proteins were labeled with L-[3,5-3H]tyrosine or L-[2,5-3H]histidine. When medium glucose was less than or equal to 10 mM, the production of insulin exceeded or equaled its release. Raising the glucose levels above 10 mM did not further increase the rate of insulin synthesis (67 +/- 10 fmol/10(3) cells/2 hour) but elevated that of insulin release up to 3-fold the production rates (181 +/- 10 fmol/10(3) cells/2 hour). In the presence of glucagon or of the phorbol ester 12-O-tetradecanoylphorbol 13-acetate the cells also released 3-fold more hormone that they synthesized; release was however reduced to 25% of the rate of production in the presence of epinephrine. It is concluded that glucose as well as hormonal regulators of islet B cells can influence, bi-directionally, the balance between the rates of insulin synthesis and release.

Preferential stimulation by glucose of its oxidation relative to glycolysis in purified insulin-producing cells

A rise in extracellular D-glucose concentration increases to a greater relative extent the conversion of both D-[5-3H]glucose to 3HOH and D-[6-14C]glucose to 14CO2 in rat purified insulin-producing cells than previously observed in pancreatic islets. In the pure B-cells, the ratio between D-[6-14C]glucose oxidation and D-[5-3H]glucose utilization increases, in a sigmoidal manner, as a function of the hexose concentration. The preferential stimulation by D-glucose of mitochondrial oxidative events is proposed to represent an unusual but essential feature of the metabolic and, hence, functional response of these fuel-sensor cells.

Sensitivity of rat pancreatic A and B cells to somatostatin

Islet A and B cells were purified from the rat pancreas and examined for their respective sensitivity to somatostatin. Both somatostatin-14 (S14) and -28 (S28) inhibited glucagon and insulin release through direct interactions with the corresponding cell types. A dose-dependent suppression of the secretory activities was paralleled by a reduction in cellular cyclic AMP formation with similar ED50 values for both actions. The somatostatin effects on pancreatic hormone release may thus be mediated via an inhibition of adenylate cyclase activity. In pancreatic A cells, S14 and S28 were equally potent inhibitors with ED50 values ranging from 2 x 10(-12) to 2 x 10(-11) mol/l. Pancreatic B cells exhibited a similar sensitivity to S28 as the A cells (ED50 of 2 to 5 x 10(-11) mol/l), but not to S14 (ED50 of 2 x 10(-9) mol/l). Extrapolation of these in vitro sensitivities of islet A and B cells to the in vivo situation suggests that both cell types can respond to circulating S28 levels and that A cells are sensitive to both locally and distally released S14. Islet B cells appear insensitive to the normal peripheral S14 levels but could respond to locally released somatostatin. The marked difference in the sensitivities of islet A and B cells to S14 suggest that these cell types are equipped with different somatostatin receptors. This notion was further supported by the cell-selective actions of the synthetic S14 analogues [D-Trp8, D-Cys14]S14 and desAsn5[D-Trp8, D-Ser13]S14.

Interaction of sulfonylureas with pancreatic beta-cells. A study with glyburide

In this study on purified rat pancreatic beta-cells, we show that the second-generation sulfonylurea glyburide stimulates insulin release through a direct interaction with the beta-cells. During static incubations, 2 microM glyburide releases 0.16 pg insulin per beta-cell, which corresponds to a half-maximal glucose stimulation. This effect occurs independently from the glucose-recognition unit, being detectable at both nonstimulatory and stimulatory glucose concentrations and proceeding without alterations in the rate of glucose oxidation. The secretagogue action of glyburide appears not to be mediated through cAMP but is potentiated by cAMP-generating substances such as glucagon (10(-8) M; 0.31 pg insulin released per beta-cell). Its 10-fold higher potency in isolated islets is attributed to the markedly higher cAMP levels that are maintained in islet beta-cells under the influence of locally released glucagon. Perifused pancreatic beta-cells respond to glyburide with a biphasic insulin release. After removal of the drug, the cells continue to secrete insulin at the same rate for greater than or equal to 30 min. This prolonged secretory activity coincides with a cellular accumulation of the drug, primarily in association with membranes of secretory vesicles and mitochondria. Tolbutamide also stimulates insulin release from pure beta-cells, but it is less powerful on a molar basis and does not lead to a sustained hormone release after its removal from the extracellular medium. We conclude that the hypoglycemic action of glyburide is at least partly the result of a direct interaction with pancreatic beta-cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose stimulates proinsulin biosynthesis by a dose-dependent recruitment of pancreatic beta cells

Glucose is a well-known stimulus of proinsulin biosynthesis. In purified beta cells, the sugar induces a 25-fold increase in the synthesis of insulin immunoreactive material over 60-min incubation. Autoradiographic analysis of the individual cells shows that this effect is achieved via dose-dependent recruitment of pancreatic beta cells to biosynthetic activity. Recruitment of beta cells is also seen in isolated islets exposed to glucose. The sigmoidal dose-response curve for glucose-induced proinsulin biosynthesis thus reflects a heterogeneous responsiveness of pancreatic beta cells rather than a progressively increasing activity of functionally homogeneous cells. Dose-dependent recruitment of functionally diverse cells may be a ubiquitous mechanism in tissue function.

Differences in adrenergic recognition by pancreatic A and B cells

The adrenergic control of glucose homeostasis is mediated in part through variations in the release of pancreatic hormones. In this study, purified pancreatic A and B cells were used to identify the recognition and messenger units involved in the adrenergic regulation of glucagon and insulin release. Catecholamines induced beta-adrenergic receptor activity in A cells and alpha 2-adrenergic receptor activity in B cells. The two recognition units provoked opposite variations in the production of cellular cyclic adenosine monophosphate, the beta-adrenergic unit enhancing the nucleotide's permissive effect on amino acid-induced glucagon release and the alpha 2-adrenergic unit inhibiting that upon glucose-induced insulin release. In both cell types, catecholamines interact powerfully with the synergistic control of hormone release by nutrient- and (neuro)hormone-driven messenger systems.

A new in vitro model for the study of pancreatic A and B cells

A method is developed for the preparation of single, pure, and viable rat pancreatic A and B cells in numbers sufficient for in vitro analysis. Islet isolation and dissociation techniques have been modified to increase the yield in islet cells per pancreas and per experiment. Islet cells are separated on the basis of their light scatter activity and flavin adenine dinucleotide autofluorescence into single non-B cells, single B cells, and structurally coupled B cells. Islet non-B cells are further purified into single A cells by autofluorescence-activated sorting according to the cellular nicotinamide adenine dinucleotide phosphate content at 20 mM glucose. Apart from offering the advantage of separating cells according to their functional characteristics, this procedure succeeds in the simultaneous isolation of 95-100% pure A and B cells. More than 50% of the cells in the initial islet preparation are recovered as single purified cells which can be maintained in culture. The isolated pancreatic A and B cells have been defined in terms of their cell volume, DNA and hormone content, and ultrastructural characteristics. The availability of pure pancreatic A and B cells is expected to contribute to our understanding of the regulation of glucagon and insulin release.

Interplay of nutrients and hormones in the regulation of glucagon release

The role of nutrients and hormones in the regulation of glucagon release is investigated in pancreatic A cells purified by autofluorescence-activated cell sorting. Purified A cells lack secretory activity in 1-h incubation at 1.4 mM glucose. Their release mechanism can be activated by arginine, alanine, and glutamine, alone or in combination. Glucose inhibits amino acid-induced glucagon release through a direct insulin-independent action upon pancreatic A cells. Nutrient-induced glucagon release is suppressed by somatostatin and amplified by (Bu)2cAMP or epinephrine. The epinephrine stimulus is inhibited by 10(-11) M somatostatin and abolished by 10(-10) M of this peptide. The effects of somatostatin and epinephrine are associated with parallel changes in cellular cAMP levels, which is not the case for the variations induced by amino acids or glucose. It is confirmed that calcium is an essential requirement for glucagon release. In contrast to its exquisite sensitivity for somatostatin, the glucagon release process is relatively insensitive to insulin during a 1-h exposure. The hormone affects solely epinephrine-induced glucagon release and its inhibitory action is partial and only observed at 10(-7) M. This suppressive effect of insulin is not attributable to variations in glucose handling but appears associated with the stimulatory effect of epinephrine. It is concluded that a nutrient-induced signal interacts with a hormone-inducible cAMP signal to activate the secretory process in pancreatic A cells.

Regulation of adenosine 3',5'-monophosphate levels in the pancreatic B cell

The poor glucose-induced insulin release from single purified B cells has been attributed, in part, to the low cellular cAMP levels. The present study demonstrates that isolated B cells exhibit a markedly lower cAMP formation than B cells lodged in intact islets and suggest that this deficiency is caused by their separation from glucagon-containing A cells. cAMP levels in purified B cells are rapidly and potently elevated by glucagon from 10(-10) M on, reaching the values of islet B cells at 10(-9) M. In contrast, exogenous glucagon stimulates cAMP formation in isolated islets only from 3.10(-9) M on, suggesting that endogenously released glucagon is mainly responsible for the higher cAMP levels in islet B cells. Somatostatin counteracts glucagon-induced cAMP production in purified B cells and, therefore, has also the potential to mediate an intra-islet regulation of B cell functions. Neither insulin nor pancreatic polypeptide affect cAMP formation in pancreatic B cells. Glucose alone does not influence cAMP levels in purified B cells, but enhances glucagon-induced cAMP formation in these cells. The glucose-dependent increase in islet cAMP is therefore not considered as the nutrient-induced mediator for hormone release but as a minor amplification of the glucagon-dependent signal. Experiments on reaggregated islet cells permit the reconstruction of the events which regulate cAMP levels in isolated islets. Further support is hereby given to the hypothesis that a normal glucose-induced insulin release from intact islets requires the simultaneous synarchic participation of a nutrient-dependent and a hormone-dependent messenger system.

Interplay of nutrients and hormones in the regulation of insulin release

Single pancreatic B cells are purified by autofluorescence-activated cell sorting, and their secretory activity is measured after overnight culture. Compared to intact islets, the isolated cells release 2-fold more insulin under basal conditions and 5-fold less during nutrient stimulation. Their secretory activity can be induced by glucose, leucine, or arginine, but only 0.3-1.7% of their hormone content is liberated at 20 mM nutrient concentrations. This poor nutrient-induced insulin release from purified B cells is attributed to their low cAMP levels and is markedly increased after addition of (Bu)2cAMP, of glucagon, or of pancreatic A cells. These results strongly support the concept that the potent in vivo insulin-releasing action of glucose and leucine is not only dependent on their fuel capacity in pancreatic B cells but also on the concurrent cAMP levels in these cells. In isolated islets, endogenously released glucagon apparently determines the cAMP production in B cells and thus participates in the nutrient-induced secretory process. Somatostatin and epinephrine were shown to exert their suppressive effects via the glucagon-dependent messenger system. It is concluded that nutrients and hormones interact with two different messenger systems which amplify each others' stimulatory effect upon insulin release. cAMP might represent the hormone-induced messenger which sets the B cell's sensitivity and secretory capacity for nutrient stimuli such as glucose. The higher insulin secretory response observed after reaggregation of single B cells could not be attributed to an altered activity in the nutrient or hormonal regulatory units, raising the possibility that the aggregated state of the cells is rather responsible for a better organization or cooperation of the secretory effector unit.