A 48-hour exposure of pancreatic islets to calpain inhibitors impairs mitochondrial fuel metabolism and the exocytosis of insulin

Calpain and Cardiometabolic Diseases

Calpain is defined as a member of the superfamily of cysteine proteases possessing the CysPC motif within the gene. Calpain-1 and -2, which are categorized as conventional isozymes, execute limited proteolysis in a calcium-dependent fashion. Accordingly, the calpain system participates in physiological and pathological phenomena, including cell migration, apoptosis, and synaptic plasticity. Recent investigations have unveiled the contributions of both conventional and unconventional calpains to the pathogenesis of cardiometabolic disorders. In the context of atherosclerosis, overactivation of conventional calpain attenuates the barrier function of vascular endothelial cells and decreases the immunosuppressive effects attributed to lymphatic endothelial cells. In addition, calpain-6 induces aberrant mRNA splicing in macrophages, conferring atheroprone properties. In terms of diabetes, polymorphisms of the calpain-10 gene can modify insulin secretion and glucose disposal. Moreover, conventional calpain reportedly participates in amino acid production from vascular endothelial cells to induce alteration of amino acid composition in the liver microenvironment, thereby facilitating steatohepatitis. Such multifaceted functionality of calpain underscores its potential as a promising candidate for pharmaceutical targets for the treatment of cardiometabolic diseases. Consequently, the present review highlights the pivotal role of calpains in the complications of cardiometabolic diseases and embarks upon a characterization of calpains as molecular targets.

Calpain-10 regulates actin dynamics by proteolysis of microtubule-associated protein 1B

Calpain-10 (CAPN10) is the calpain family protease identified as the first candidate susceptibility gene for type 2 diabetes mellitus (T2DM). However, the detailed molecular mechanism has not yet been elucidated. Here we report that CAPN10 processes microtubule associated protein 1 (MAP1) family proteins into heavy and light chains and regulates their binding activities to microtubules and actin filaments. Immunofluorescent analysis of Capn10^−/− mouse embryonic fibroblasts shows that MAP1B, a member of the MAP1 family of proteins, is localized at actin filaments rather than at microtubules. Furthermore, fluorescence recovery after photo-bleaching analysis shows that calpain-10 regulates actin dynamics via MAP1B cleavage. Moreover, in pancreatic islets from CAPN10 knockout mice, insulin secretion was significantly increased both at the high and low glucose levels. These findings indicate that deficiency of calpain-10 expression may affect insulin secretion by abnormal actin reorganization, coordination and dynamics through MAP1 family processing.

The novel Aryl hydrocarbon receptor inhibitor biseugenol inhibits gastric tumor growth and peritoneal dissemination

Biseugenol (Eug) is known to antiproliferative of cancer cells; however, to date, the antiperitoneal dissemination effects have not been studied in any mouse cancer model. In this study, Aryl hydrocarbon receptor (AhR) expression was associated with lymph node and distant metastasis in patients with gastric cancer and was correlated with clinicolpathological pattern. We evaluated the antiperitoneal dissemination potential of knockdown AhR and Biseugenol in cancer mouse model and assessed mesenchymal characteristics. Our results demonstrate that tumor growth, peritoneal dissemination and peritoneum or organ metastasis implanted MKN45 cells were significantly decreased in shAhR and Biseugenol-treated mice and that endoplasmic reticulum (ER) stress was caused. Biseugenol-exposure tumors showed acquired epithelial features such as phosphorylation of E-cadherin, cytokeratin-18 and loss mesenchymal signature Snail, but not vimentin regulation. Snail expression, through AhR activation, is an epithelial-to-mesenchymal transition (EMT) determinant. Moreover, Biseugenol enhanced Calpain-10 (Calp-10) and AhR interaction resulted in Snail downregulation. The effect of shCalpain-10 in cancer cells was associated with inactivation of AhR/Snail promoter binding activity. Inhibition of Calpain-10 in gastric cancer cells by short hairpin RNA or pharmacological inhibitor was found to effectively reduced growth ability and vessel density in vivo. Importantly, knockdown of AhR completed abrogated peritoneal dissemination. Herein, Biseugenol targeting ER stress provokes Calpain-10 activity, sequentially induces reversal of EMT and apoptosis via AhR may involve the paralleling processes. Taken together, these data suggest that Calpain-10 activation and AhR inhibition by Biseugenol impedes both gastric tumor growth and peritoneal dissemination by inducing ER stress and inhibiting EMT.

Role of calpain-10 in the development of diabetes mellitus and its complications

Calpain activity has been implicated in several cellular processes such as cell signaling, apoptosis, exocytosis, mitochondrial metabolism and cytoskeletal remodeling. Evidence has indicated that the impairment of calpain expression and the activity of different calpain family members are involved in diverse pathologies. Calpain-10 has been implicated in the development of type 2 diabetes, and polymorphisms in the CAPN10 gene have been associated with an increased risk of developing this disease. The present work focused on the molecular biology of calpain-10, supporting its key participation in glucose metabolism. Current knowledge regarding the role of calpain-10 in the development of type 2 diabetes mellitus and diabetes-related diseases is additionally reviewed.

Microfluidic array with integrated oxygenation control for real-time live-cell imaging: effect of hypoxia on physiology of microencapsulated pancreatic islets

In this article, we present a novel microfluidic islet array based on a hydrodynamic trapping principle. The lab-on-a-chip studies with live-cell multiparametric imaging allow understanding of physiological and pathophysiological changes of microencapsulated islets under hypoxic conditions. Using this microfluidic array and imaging analysis techniques, we demonstrate that hypoxia impairs the function of microencapsulated islets at the single islet level, showing a heterogeneous pattern reflected in intracellular calcium signaling, mitochondrial energetic, and redox activity. Our approach demonstrates an improvement over conventional hypoxia chambers that is able to rapidly equilibrate to true hypoxia levels through the integration of dynamic oxygenation. This work demonstrates the feasibility of array-based cellular analysis and opens up new modality to conduct informative analysis and cell-based screening for microencapsulated pancreatic islets.

Association of diabetes susceptibility gene calpain-10 with pancreatic cancer among smokers

OBJECTIVE

The objective of this study was to test the association between calpain-10 (CAPN10), a diabetes susceptibility gene, with risk of pancreatic cancer (PC).

METHODS

DNA samples from 83 incident exocrine PC cases and 166 controls, all of whom were smokers, were genotyped for four markers of CAPN10 in a nested case-control study based on the Beta-Carotene and Retinol Efficacy Trial (CARET), a randomized chemoprevention trial of subjects at high risk of lung cancer. Controls were matched on sex, race, age, CARET intervention arm, duration of exposure to asbestos, and smoking history. Conditional logistic regression was used for statistical analyses.

RESULTS

The minor allele of SNP-43 (rs3792267) in intron 3 was associated with increased risk of PC with an odds ratio of 1.57 (95%CI 1.03-2.38, p = 0.035) per allele. The three markers of the highest risk haplotype had an odds ratio of 1.98 (95%CI 1.12-3.49, p = 0.019) for risk of PC compared to the most common haplotype. There was no evidence of interaction between either of these associations by diabetes status.

CONCLUSION

These results suggest that variation in CAPN10 may be associated with increased risk of PC among smokers. Thus, studies of genes associated with diabetes risk in PC are warranted in a larger population.

Transcriptome-wide identification of preferentially expressed genes in the hypothalamus and pituitary gland

To identify preferentially expressed genes in the central endocrine organs of the hypothalamus and pituitary gland, we generated transcriptome-wide mRNA profiles of the hypothalamus, pituitary gland, and parietal cortex in male mice (12-15 weeks old) using serial analysis of gene expression (SAGE). Total counts of SAGE tags for the hypothalamus, pituitary gland, and parietal cortex were 165824, 126688, and 161045 tags, respectively. This represented 59244, 45151, and 55131 distinct tags, respectively. Comparison of these mRNA profiles revealed that 22 mRNA species, including three potential novel transcripts, were preferentially expressed in the hypothalamus. In addition to well-known hypothalamic transcripts, such as hypocretin, several genes involved in hormone function, intracellular transduction, metabolism, protein transport, steroidogenesis, extracellular matrix, and brain disease were identified as preferentially expressed hypothalamic transcripts. In the pituitary gland, 106 mRNA species, including 60 potential novel transcripts, were preferentially expressed. In addition to well-known pituitary genes, such as growth hormone and thyroid stimulating hormone beta, a number of genes classified to function in transport, amino acid metabolism, intracellular transduction, cell adhesion, disulfide bond formation, stress response, transcription, protein synthesis, and turnover, cell differentiation, the cell cycle, and in the cytoskeleton and extracellular matrix were also preferentially expressed. In conclusion, the current study identified not only well-known hypothalamic and pituitary transcripts but also a number of new candidates likely to be involved in endocrine homeostatic systems regulated by the hypothalamus and pituitary gland.

Impaired insulin secretion in transgenic mice over-expressing calpastatin in pancreatic β-cells

Calpains are a family of calcium-activated proteases involved in a number of cellular functions including cell death, proliferation and exocytosis. The finding that variation in the calpain-10 gene increases type 2 diabetes risk in some populations has increased interest in determining the potential role of calpains in pancreatic β-cell function. In the present study, transgenic mice (Cast (RIP)) expressing an endogenous calpain inhibitor, calpastatin, in pancreatic β-cells were used to dissect the role of the calpain system in the regulation insulin secretion in vivo and in vitro. Glucose concentrations after the administration of intraperitoneal glucose were significantly increased in Cast (RIP) mice compared with wildtype littermate controls. This was associated with a reduction in glucose-stimulated insulin secretion in vivo. Using pancreas perfusion, static islet incubation and islet perifusion, it was demonstrated that Cast (RIP) islets hypersecreted insulin at low glucose, but exhibited significantly impaired insulin responses to high glucose. Examination of insulin release and calcium signals from isolated islets indicated that distal components of the insulin exocytotic pathway were abnormal in Cast (RIP) mice. Cast (RIP) islets had modestly reduced expression of Rab3a and other critical components in the late steps of insulin exocytosis. These studies provide the first evidence that blocking endogenous calpain activity partially impairs insulin release in vivo and in vitro by targeting distal components of the insulin exocytotic machinery.

Calpain-mediated degradation of reversibly oxidized protein-tyrosine phosphatase 1B

Protein-tyrosine phosphatases (PTPs) are regulated by reversible inactivating oxidation of the catalytic-site cysteine. We have previously shown that reversible oxidation upon UVA irradiation is followed by calpain-mediated PTP degradation. Here, we address the mechanism of regulated cleavage and the physiological function of PTP degradation. Reversible oxidation of PTP1B in vitro strongly facilitated the association with calpain and led to greatly increased calpain-dependent inactivating cleavage. Both oxidation-induced association and cleavage depended exclusively on the presence of the catalytic (reversibly oxidized) cysteine residue 215. A major cleavage site was identified preceding amino acid position Ala77. In calpain-deficient cells, insulin signaling was apparently diminished, consistent with a possible role for calpain in removing a negative regulator of insulin signaling. Reversibly oxidized PTP1B may be a target of calpain in this context.

Calpain-10 expression is elevated in pancreatic islets from patients with type 2 diabetes

BACKGROUND

Calpain-10 was the first gene to be identified influencing the risk of type 2 diabetes (T2D) by positioning cloning. Studies in beta-cell lines and rodent islets suggest that calpain-10 may act as a regulator of insulin secretion. However, its role in human pancreatic islets remains unclear. The aim of this study was to examine if calpain-10 expression is altered in islets from patients with T2D and if the transcript level correlates with insulin release. We also tested if polymorphisms in the CAPN10 gene are associated with gene expression and insulin secretion in vitro.

METHODOLOGY/PRINCIPAL FINDINGS

Calpain-10 mRNA expression was analysed in human pancreatic islets from 34 non-diabetic and 10 T2D multi-organ donors. CAPN10 SNP-43 and SNP-44 were genotyped and related to gene expression and insulin release in response to glucose, arginine and glibenclamide. The mRNA level of calpain-10 was elevated by 64% in pancreatic islets from patients with T2D compared with non-diabetic donors (P = 0.01). Moreover, the calpain-10 expression correlated positively with arginine-stimulated insulin release in islets from non-diabetic donors (r = 0.45, P = 0.015). However, this correlation was lost in islets from patients with T2D (r = 0.09; P = 0.8). The G/G variant of SNP-43 was associated with reduced insulin release in response to glucose (P</=0.04) in non-diabetic donors.

CONCLUSIONS

While calpain-10 expression correlates with insulin release in non-diabetic human islets, this correlation is lost in T2D suggesting that a stimulatory effect of calpain-10 could be lost in patients with T2D.

Calpain activation contributes to endotoxin-induced diaphragmatic dysfunction

Calpain activation occurs in skeletal muscle in response to infection, but it is unknown if calpain inhibition improves muscle functional capacity. We hypothesized that infection induces diaphragm calpain activation, that calpain activation results in cleavage of important diaphragm cytoskeletal proteins, and that inhibition of calpain attenuates infection-induced diaphragm dysfunction. Mice (n = 4-6/group) were given: (1) saline (intraperitoneal); (2) endotoxin (12 mg/kg intraperitoneal); (3) calpain inhibitor peptide III (12 mg/kg intraperitoneal); and (4) endotoxin (12 mg/kg) plus calpain inhibitor peptide III (12 mg/kg). At 24 hours, diaphragms were removed and the following determined: (1) calpain activity by fluorogenic assay; (2) calpain I and II protein levels; (3) talin protein levels; and (4) the force-frequency relationship. Endotoxin significantly increased diaphragm calpain activity (P < 0.001), active calpain I protein (P < 0.001), active calpain II protein (P < 0.01), levels of a calpain-specific cleavage talin degradation product (P < 0.003), and reduced diaphragm force (P < 0.001). Calpain inhibitor III administration prevented endotoxin-induced increases in calpain activity, reduced talin degradation, and attenuated reductions in diaphragm force. Diaphragm-specific force at 150 Hz stimulation was significantly higher in control, endotoxin plus calpain inhibitor III, and calpain inhibitor III alone groups (23 +/- 1, 20 +/- 1 and 23 +/- 1 N/cm(2), respectively) than in the endotoxin alone group (15 +/- 1 N/cm(2)) (P < 0.01). This model of sepsis results in significant diaphragm calpain activation and calpain-dependent diaphragm cytoskeletal protein cleavage. Moreover, calpain inhibition attenuates endotoxin-induced diaphragm weakness, suggesting that such inhibitors may be a potential treatment to improve respiratory muscle function in infected patients.

Identification and optimization of a novel inhibitor of mitochondrial calpain 10

Calpain 10 has been localized to the mitochondria and is a key mediator of Ca(2+) induced mitochondrial dysfunction. A peptide screen followed by a series of modifications identified the homodisulfide form of CYGAK (CYGAK)(2) as an inhibitor of calpain 10 while showing no inhibitory activity against calpain 1. Methylation or truncation of the N-terminal cysteine significantly reduced the inhibitory activity of (CYGAK)(2) and inhibition was reversed by reducing agents, suggesting that CYGAK forms a disulfide with a cysteine near the active site. Data suggests CYGAK may be a P' calpain inhibitor and may achieve its specificity through this mechanism. CYGAK inhibited calpain activity in intact mitochondria, renal cells, and hepatocytes, prevented Ca(2+) induced cleavage of NDUFV2, and blocked Ca(2+) induced state III dysfunction. (CYGAK)(2) is the first P' specific calpain inhibitor and will be a valuable tool to prevent Ca(2+) induced mitochondrial dysfunction and explore the function of calpain 10.

Replication of calpain-10 genetic association with carotid intima-media thickness

OBJECTIVE

Diabetes and atherosclerosis may share common genetic determinants. A prior study in Hispanics found association of haplotypes in the diabetes gene calpain-10 (CAPN10) with carotid artery intima-media thickness (CIMT). This study sought to replicate this association in an independent cohort.

METHODS

Four CAPN10 SNPs were genotyped and haplotypes determined in 487 Hispanic Americans from 143 families ascertained via an index case with hypertension. CIMT was measured from B-mode ultrasound, and glycemic traits quantified from euglycemic clamps. Association of SNPs and haplotypes with CIMT was determined.

RESULTS

The minor alleles of SNP-56 and SNP-63 were associated with increased CIMT in dominant and additive models. The association of haplotype 1112 with increased CIMT was replicated. No associations with fasting insulin, insulin secretion, or insulin sensitivity were observed.

CONCLUSIONS

CAPN10 association with CIMT was replicated, further supporting its role as a common genetic determinant of diabetes and atherosclerosis in Hispanics.

Microfluidic device for multimodal characterization of pancreatic islets

A microfluidic device to perfuse pancreatic islets while simultaneously characterizing their functionality through fluorescence imaging of the mitochondrial membrane potential and intracellular calcium ([Ca(2+)](i)) in addition to enzyme linked immunosorbent assay (ELISA) quantification of secreted insulin was developed and characterized. This multimodal characterization of islet function will facilitate rapid assessment of tissue quality immediately following isolation from donor pancreas and allow more informed transplantation decisions to be made which may improve transplantation outcomes. The microfluidic perfusion chamber allows flow rates of up to 1 mL min(-1), without any noticeable perturbation or shear of islets. This multimodal quantification was done on both mouse and human islets. The ability of this simple microfluidic device to detect subtle variations in islet responses in different functional assays performed in short time-periods demonstrates that the microfluidic perfusion chamber device can be used as a new gold standard to perform comprehensive islet analysis and obtain a more meaningful predictive value for islet functionality prior to transplantation into recipients, which is currently difficult to predict using a single functional assay.

CHOP T/C and C/T haplotypes contribute to early-onset type 2 diabetes in Italians

Type 2 diabetes (T2D) is characterized by impaired insulin secretion, insulin insensitivity and decreased beta-cell mass. Multiple genes contribute to T2D. The chromosome 12q13.1 region is in linkage to T2D in different populations, including our Italian dataset. CHOP is a candidate gene for the linkage, as it is located in the chromosome 12q13.1 region, and may contribute to T2D by increasing beta-cell apoptosis susceptibility and by impairing insulin sensitivity. Our goal was to identify any potential CHOP gene variants contributing to T2D in our Italian early-onset T2D families, which show linkage to the CHOP region. We directly sequenced the CHOP gene in 28 Italian probands of the linked T2D families and in 115 control subjects. We performed genotype and haplotype association tests with T2D of the identified single nucleotide polymorphisms (SNPs). We performed model-free and parametric association haplotype tests with T2D. We identified three SNPs [5'UTR-c.279T > C, 5'UTR-c.120A > G and + nt30C > T (F10F)] in CHOP. These SNPs are in complete linkage disequilibrium. The genotype association test showed an association trend with T2D of TT (F10F) and AG (-c.120A > G). The haplotype association test provided significant results for the haplotypes T/C (frequency = 0.33) and C/T (frequency = 0.01) (at 5'UTR-c.279T > C and + nt30C > T, respectively) under non-parametric analysis (P-value = 0.0000), recessive model (P-value = 0.0000) and additive model (P-value = 0.0014). Our data show that CHOP described haplotypes T/C and C/T, as an additive and as a homozygous variant, contribute significantly to T2D in our Italian early-onset group. We conclude that the CHOP T/C and C/T haplotype contributes to our T2D linkage signal on chromosome 12q13.1.

The caspase selective inhibitor EP1013 augments human islet graft function and longevity in marginal mass islet transplantation in mice

OBJECTIVE

Clinical islet transplantation can provide insulin independence in patients with type 1 diabetes, but chronic graft failure has been observed. This has been attributed in part to loss of >or=60% of the transplanted islets in the peritransplant period, resulting in a marginal implant mass. Strategies designed to maximize survival of the initial islet mass are likely to have major impact in enhancing long-term clinical outcomes. EP1013 (N-benzyloxycabonyl-Val Asp-fluoromethyl ketone [zVD-FMK]), is a broad-spectrum caspase selective inhibitor with no observed toxicity in rodents.

RESEARCH DESIGN AND METHODS

The therapeutic benefit of EP1013 was examined in a syngeneic rodent islet transplant model using deceased donor human islets to determine whether the amount of tissue required to restore euglycemia in diabetic animals could be reduced.

RESULTS

EP1013 (combined pretransplant islet culture for 2 h and in vivo treatment for days 0-5 posttransplant) significantly improved marginal islet mass function following syngeneic islet transplantation in mice, even at lower doses, compared with previous studies using the pan-caspase inhibitor N-benzyloxycabonyl-Val Ala-Asp-fluoromethyl ketone (zVAD-FMK). EP1013 supplementation in vitro improved human islet yields following prolonged culture and reversed diabetes following implantation of a marginal human islet mass (80-90% reduction) into mice.

CONCLUSIONS

Our data suggest that EP1013 therapy will markedly reduce the islet mass required in clinical islet transplantation, improving insulin independence rates following single-donor infusion.

Characterization of endogenous and recombinant human calpain-10

Calpain-10 is a novel ubiquitous calpain family member that has been implicated as a susceptibility gene for type 2 diabetes. One of the major challenges is that the function of calpain-10 is not yet known. To address this problem, we purified human calpain-10 from different sources, including the endogenous and the recombinant calpain-10 from HeLa S3 and 293F cells, respectively. Both endogenous and recombinant calpain-10 were present as two major forms with different origins. Interestingly, radiolabeled calpain-10 was found to be efficiently cleaved at the N-terminal region by calpain-2, but not by other proteases. None of these calpain-10 proteins have putative proteolytic activity under in vitro conditions when examined using different peptide substrates, including more than 70 in vitro translated, radiolabeled oligopeptides. Our results raise the possibility that calpain-10 may require a special intracellular localization or interacting partner(s) to acquire proteolytic activity, or it functions by interacting with other proteins rather than through its proteolytic activity.

Linkage studies for T2D in Chop and C/EBPbeta chromosomal regions in Italians

The genes causing type 2 diabetes (T2D), a complex heterogeneous disorder, differ and/or overlap in various populations. Among others there are two loci in linkage to T2D, the chromosomes 20q12-13.1 and 12q15. These two regions harbor two genes, C/EBPbeta and CHOP, which are excellent candidate genes for T2D. In fact, C/EBPbeta protein cooperates with HNF4alpha (MODY1, monogenic form of diabetes) and 1alpha (MODY3, monogenic form of diabetes). C/EBPbeta mediates suppression of insulin gene transcription in hyperglycemia and may contribute to insulin-resistance. It interacts in a complex pathway with the CHOP protein. CHOP may play a role in altered beta-cell glucose metabolism, in beta-cell apoptosis, and in lack of beta-cell replication. Thus, both C/EBPbeta and CHOP genes may independently and interactively contribute to T2D. The chromosomal regions targeting C/EBPbeta and CHOP genes have never been previously explored in T2D. We planned to identify their potential contribution to T2D in Italians. We have genotyped a group of affected siblings/families with both late- and early-onset T2D around the C/EBPbeta and the CHOP genes. We have performed non-parametric linkage analysis in the total T2D group, in the late-onset and the early-onset group, separately. We have identified a suggestive linkage to T2D in the CHOP gene locus in the early-onset T2D group (P = 0.04). We identified the linkage to T2D in the chromosome 12q15 region in the early-onset T2D families and specifically target the CHOP gene. Our next step will be the identification of CHOP gene variants, which may contribute to the linkage to T2D in Italians.

Mitochondrial calpain 10 activity and expression in the kidney of multiple species

Calpains, Ca(2+)-activated cysteine proteases, have been implicated in the progression of multiple disease states. We recently identified calpain 10 as a mitochondrial calpain that is involved in Ca(2+)-induced mitochondrial dysfunction. The goals of this study were to characterize the expression and activity of renal mitochondrial calpain 10 in rabbit, mouse, and rat. Using shRNA technology and immunoblot analysis three previously postulated splice variants of calpain 10 were identified (50, 56, and 75kDa). SLLVY-AMC zymography and immunoblot analysis was used to directly link calpeptin-sensitive calpain activity to calpain 10 splice variants. Rabbit, mouse, and rat kidney mitochondria contained 75kDa (calpain 10a), 56kDa (calpain 10c or 10d), and 50kDa (calpain 10e) splice variants. Interestingly, zymography yielded distinct bands of calpain activity containing multiple calpain 10 splice variants in all species. These results provide evidence that several previously postulated splice variants of calpain 10 are localized to the mitochondria in kidneys of rabbits, rats, and mice.

Calpain proteolysis of insulin-like growth factor binding protein (IGFBP) -2 and -3, but not of IGFBP-1

Calpains are cytoplasmic Ca(2+)-regulated cysteine proteases that may regulate insulin-like growth factor (IGF)-independent actions of insulin-like growth factor binding proteins (IGFBPs) through IGFBP proteolysis. In this study, [(125)I]-labeled IGFBP-2 and -3, but not IGFBP-1, were proteolyzed by Ca(2+)-activated m-calpain in vitro. Degradation of higher concentrations of the recombinant proteins IGFBP-2 and -3 by m-calpain was dose-dependent, but was terminated within 20 min by autolysis. By subjecting proteolytic fragments to N-terminal amino acid sequence analysis, the primary cleavage sites in IGFBP-2 and -3 were localized to the non-conserved central linker regions. Using the biosensor technique, in vitro binding of m-calpain to IGFBP-3 was demonstrated to be a Ca(2+)-dependent reaction with a rapid on/off rate.

The activity of calpains in lymphocytes is glucose-dependent and is decreased in diabetic patients

Calpains are nonlysosomal calcium-dependent cysteine proteases that participate in insulin secretion and action. Polymorphisms in the calpain-10 gene have been shown to increase the risk for type 2 diabetes. Since white blood cells have been used to study glucose homeostasis, the present study was carried to find out if calpains have different activity and/or expression in accessible cells such as lymphocytes of individuals with or without type 2 diabetes. Fasting blood glucose concentration was significantly higher in diabetic subjects, whereas the difference in the activity of calpains evaluated in basal and stimulating extracellular glucose concentration was significantly higher in the lymphocytes from the control group. The mRNA expression of calpain-10 was similar in the lymphocytes of both patients and controls. The protein blots showed four bands that ranged between 75 and 50 kDa; however, no statistical differences were observed in the expression of the calpain-10 isoforms between controls and patients. Data obtained showed that human lymphocytes express calpain-10 mRNA and protein, showing a similar expression between diabetic and control subjects, nevertheless in the diabetic group calpain activity was less glucose-sensitive.

Targeted suppression of calpain-10 expression impairs insulin-stimulated glucose uptake in cultured primary human skeletal muscle cells

Calpain-10 was identified as a novel type 2 diabetes susceptibility gene, although the mechanisms by which it increases susceptibility to type 2 diabetes remain unclear. As skeletal muscle is the principal site of the peripheral insulin resistance for glucose disposal in type 2 diabetes, we investigated whether targeted suppression of calpain-10 expression directly affects insulin action in cultured human skeletal muscle cells. Short interfering RNAs (siRNAs) were employed to specifically suppress CAPN10 gene expression. Suppression was seen at both the transcript and protein level, as assessed by quantitative PCR and Western blotting. Suppression of CAPN10 mRNA expression (75% decrease compared to untransfected myotubes) was associated with a significant decrease (p=0.04) in insulin-stimulated glucose uptake (1.03+/-0.06 [mean+/-SEM]-fold increase over basal) compared to the untransfected myotubes (1.43+/-0.16-fold increase). In contrast, decreased suppression of calpain-10 expression did not affect insulin-stimulated glycogen synthesis nor insulin-stimulated phosphorylation of protein kinase B, a key component of the insulin-signalling pathway. This study confirms that calpain-10 plays a role in insulin-stimulated glucose uptake in human skeletal muscle cells. Suppression of calpain-10 expression did not affect insulin-stimulated glycogen synthesis nor insulin-signalling via PKB, suggesting that calpain-10 may exert a direct regulatory effect upon the glucose uptake mechanism.

Calpains and their multiple roles in diabetes mellitus

Type 2 diabetes mellitus (T2DM) can lead to death without treatment and it has been predicted that the condition will affect 215 million people worldwide by 2010. T2DM is a multifactorial disorder whose precise genetic causes and biochemical defects have not been fully elucidated, but at both levels, calpains appear to play a role. Positional cloning studies mapped T2DM susceptibility to CAPN10, the gene encoding the intracellular cysteine protease, calpain 10. Further studies have shown a number of noncoding polymorphisms in CAPN10 to be functionally associated with T2DM while the identification of coding polymorphisms, suggested that mutant calpain 10 proteins may also contribute to the disease. Here we review recent studies, which in addition to the latter enzyme, have linked calpain 5, calpain 3, and its splice variants, calpain 2 and calpain 1 to T2DM-related metabolic pathways along with T2DM-associated phenotypes, such as obesity and impaired insulin secretion, and T2DM-related complications, such as epithelial dysfunction and diabetic cataract.

Nutritional regulation of proteases involved in fetal rat insulin secretion and islet cell proliferation

Epidemiological studies have indicated that malnutrition during early life may programme chronic degenerative disease in adulthood. In an animal model of fetal malnutrition, rats received an isoenergetic, low-protein (LP) diet during gestation. This reduced fetal β-cell proliferation and insulin secretion. Supplementation during gestation with taurine prevented these alterations. Since proteases are involved in secretion and proliferation, we investigated which proteases were associated with these alterations and their restoration in fetal LP islets. Insulin secretion and proliferation of fetal control and LP islets exposed to different protease modulators were measured. Lactacystin and calpain inhibitor I, but not isovaleryl-l-carnitine, raised insulin secretion in control islets, indicating that proteasome and cysteinyl cathepsin(s), but not μ-calpain, are involved in fetal insulin secretion. Insulin secretion from LP islets responded normally to lactacystin but was insensitive to calpain inhibitor I, indicating a loss of cysteinyl cathepsin activity. Taurine supplementation prevented this by restoring the response to calpain inhibitor I. Control islet cell proliferation was reduced by calpain inhibitor I and raised by isovaleryl-l-carnitine, indicating an involvement of calpain. Calpain activity appeared to be lost in LP islets and not restored by taurine. Most modifications in the mRNA expression of cysteinyl cathepsins, calpains and calpastatin due to maternal protein restriction were consistent with reduced protease activity and were restored by taurine. Thus, maternal protein restriction affected cysteinyl cathepsins and the calpain–calpastatin system. Taurine normalised fetal LP insulin secretion by protecting cysteinyl cathepsin(s), but the restoration of LP islet cell proliferation by taurine did not implicate calpains.

Identifying susceptibility variants for type 2 diabetes

The etiology of type 2 diabetes (T2D) is complex and remains poorly understood. Differences in individual susceptibility to this condition reflect the action of multiple variants, each of which confers a modest effect, and their interactions with a variety of environmental exposures. Several complementary approaches to the identification of the etiological variants have been adopted, though, for all, association analyses provide the final common pathway. The genes and/or chromosomal regions studied have been selected on the basis of their presumed biological relevance to diabetes, known involvement in monogenic forms, or animal models of the condition and/or signals arising from whole-genome linkage scans. These association studies have featured a wide variety of designs and analytical approaches, but reliable biological insights have been few, largely because of difficulties in obtaining reproducible findings. However, in recent years, several examples of robustly replicated associations have emerged, largely as a result of an emphasis on the need for improved power and more appropriate analysis and interpretation. New strategies for the large-scale identification of T2D susceptibility variants are now becoming possible, including the prospect of genuine genome-wide association scans, but caution in their design, analysis, and interpretation remains essential.

Calpain facilitates actin reorganization during glucose-stimulated insulin secretion

Calpain-10 (CAPN10) has been identified as a diabetes susceptibility gene. Previous studies have shown that alterations in calpain activity alter both glucose uptake and insulin secretion. In this report, we investigated the role of calpain activity in the actin reorganization required for glucose-stimulated insulin secretion. In pancreatic INS-1 cells, acute exposure to a high glucose environment stimulated CAPN10 gene expression with a concomitant increase in calpain activity. However, high glucose did not significantly alter expression of the two major ubiquitously expressed calpain family members, CAPN1 and CAPN2. Furthermore, glucose stimulation resulted in the reorganization of actin and inhibition of calpain activity impaired this reorganization in INS-1 cells. Finally, we identified a 54 kDa isoform as the major CAPN10 isoform that associates with the actin cytoskeleton. Based on our findings, we propose that calpain plays a role in facilitating the actin reorganization required for glucose-stimulated insulin secretion in INS-1 cells.

The therapeutic potential of the calpain family: new aspects

The calpain family is a group of cysteine proteases unique in their dependency on calcium to attain functionally active forms. Calpains are involved in a wide range of cellular calcium-regulated functions, including signal transduction, cell proliferation and differentiation, and apoptosis. Moreover, altered calpain activity has been observed in several human diseases. Specific calpain inhibitors hold promise for the treatment of neuromuscular and neurodegenerative diseases in which calpains have been shown to be upregulated (e.g. Parkinson's disease and Duchenne muscular dystrophy). Conversely, calpain activators could be a useful approach for those diseases where reduced calpain activity has been observed, such as type 2 diabetes or metabolic syndrome.

Haplotype combination of Calpain-10 gene polymorphism is associated with metabolic syndrome in type 2 diabetes

Patients with metabolic syndrome are at increased risk of developing cardiovascular disease. The combinations of the haplotype created by the alleles of three single nucleotide polymorphisms (SNPs): SNP-43, -19, and -63 of the Calpain-10 gene (CAPN10), have been reported to be associated with the risk of type 2 diabetes in many populations. The aim of this study was to examine the association of the CAPN10 polymorphism with metabolic syndrome in patients with type 2 diabetes in Korea. Overall, 382 patients with type 2 diabetes were enrolled in this study. All the subjects were genotyped according to CAPN10 SNP-43, -19, and -63. The restriction fragment length polymorphism method was used for the three SNPs. The baseline presence of components of metabolic syndrome was determined. Two hundred and sixty-five (69.4%) patients had metabolic syndrome. Patients with the 111/121 haplotype combination showed a higher risk of hypertension than the other haplotype combinations (odd ratio (OR)=2.334, P=0.010). Patients with the 111/121 haplotype combination had a significantly high risk of metabolic syndrome (OR=1.927, P=0.042). The results of this study suggest that a novel 111/121 haplotype combination created by the CAPN10 SNP-43, -19, and -63 increases the susceptibility to the metabolic syndrome in patients with type 2 diabetes.

A novel 111/121 diplotype in the Calpain-10 gene is associated with type 2 diabetes

Genetic variations in the Calpain-10 gene, CAPN10, have been reported to be associated with the risk of type 2 diabetes mellitus (T2DM) in Mexican-Americans and Northern Europeans whereas these variations are not associated with T2DM in other populations. The aim of this study was to determine whether there is an association between specific CAPN10 diplotype (SNP-43, -19, and -63) and T2DM in the Korean population. Overall, 454 Korean patients with T2DM (male 230, female 224) and 236 non-diabetic controls (male 124, female 112) with no family history of diabetes were enrolled in this study. All the subjects were genotyped according to CAPN10 SNP-43, -19, and -63. The restriction fragment length polymorphism method was used for the three SNPs. There were eight estimated haplotype allelic variations. After adjusting for gender and age, the 111 haplotype was associated with a high risk of T2DM (P <0.0001). The 111/121 diplotype was associated with a high risk of T2DM (odds ratio =2.580, 95% confidence interval =1.602-4.155, P =0.001). The high-risk haplotype (112/121) in Mexican-Americans was not significant in our study population. In conclusion, we found that a novel 111/121 diplotype in Calpain-10 gene is associated with T2DM in the Korean population.

Evidence for a role of the ubiquitin-proteasome pathway in pancreatic islets

The ubiquitin-proteasome pathway is crucial for protein turnover. Part of the pathway involves deubiquitination, which is carried out by cystein proteases known as ubiquitin COOH-terminal hydrolases. The isoform Uch-L1 was found to be present in large amounts in rat islets by immunostaining, Western blot analysis, and RT-PCR. Culturing islets in high glucose concentrations (16.7 mmol/l) for 24 h led to decreased gene expression. Exposure to chronic hyperglycemia following 90% partial pancreatectomy also led to reduced Uch-L1 expression. Expression of other members of the ubiquitin-proteasome pathway studied after culturing islets at high glucose concentrations revealed little change except for modest declines in parkin, human ubiquitin-conjugating enzyme 5 (UbcH5), and beta-TRCP (transducin repeat-containing protein). With the pancreatectomy model, expression of polyubiquitin-B and c-Cbl were increased and E6-associated protein was reduced. Further insight about the proteasome pathway was obtained with the proteasome inhibitor lactacystin, which in short-term 2-h experiments enhanced glucose-induced insulin secretion. An important role for the ubiquitin-proteasome pathways in beta-cells is suggested by the findings that changes in glucose concentration influence expression of genes in the pathway and that blockade of the proteasome degradation machinery enhances glucose-stimulated insulin secretion.

Synergy of glucose and growth hormone signalling in islet cells through ICA512 and STAT5

Nutrients and growth hormones promote insulin production and the proliferation of pancreatic beta-cells. An imbalance between ever-increasing metabolic demands and insulin output causes diabetes. Recent evidence indicates that beta-cells enhance insulin gene expression depending on their secretory activity. This signalling pathway involves a catalytically inactive receptor tyrosine phosphatase, ICA512, whose cytoplasmic tail is cleaved on glucose-stimulated exocytosis of insulin secretory granules and then moves into the nucleus, where it upregulates insulin transcription. Here, we show that the cleaved cytosolic fragment of ICA512 enhances the transcription of secretory granule genes (including its own gene) by binding to tyrosine phosphorylated signal transducers and activators of transcription (STAT) 5 and preventing its dephosphorylation. Sumoylation of ICA512 by the E3 SUMO ligase PIASy, in turn, may reverse this process by decreasing the binding of ICA512 to STAT5. These findings illustrate how the exocytosis of secretory granules, through a retrograde pathway that sustains STAT activity, converges with growth hormone signalling to induce adaptive changes in beta-cells in response to metabolic demands.

Calpain-10: from genome search to function

Calpain-10 (CAPN10) is the first diabetes gene to be identified through a genome scan. Many investigators, but not all, have subsequently found associations between CAPN10 polymorphism and type 2 diabetes (T2D) as well as insulin action, insulin secretion, aspects of adipocyte biology and microvascular function. However, this has not always been with the same single nucleotide polymorphism (SNP) or haplotype or the same phenotype, suggesting that there might be more than one disease-associated CAPN10 variant and that these might vary between ethnic groups and the phenotype under study. Our understanding of calpain-10 physiological action has also been greatly augmented by our knowledge of the calpain family domain structure and function, and the relationship between calpain-10 and other calpains is discussed here. Both genetic and functional data indicates that calpain-10 has an important role in insulin resistance and intermediate phenotypes, including those associated with the adipocyte. In this regard, emerging evidence would suggest that calpain-10 facilitates GLUT4 translocation and acts in reorganization of the cytoskeleton. Calpain-10 is also an important molecule in the beta-cell. It is likely to be a determinant of fuel sensing and insulin exocytosis, with actions at the mitochondria and plasma membrane respectively. We postulate that the multiple actions of calpain-10 may relate to its different protein isoforms. In conclusion, the discovery of calpain-10 by a genetic approach has identified it as a molecule of importance to insulin signaling and secretion that may have relevance to the future development of novel therapeutic targets for the treatment of T2D.

Clustering of metabolic comorbidity in schizophrenia: a genetic contribution?

People with schizophrenia are more likely to develop type 2 diabetes than the general population. Although an increased risk of diabetes has been attributed to environmental determinants such as diet, lifestyle and antipsychotic drugs, the association between these two disorders was noticed well before the advent of current lifestyles and pharmacological interventions, raising the possibility of a shared genetic basis. Schizophrenia and type 2 diabetes are common diseases with a complex mode of inheritance which includes both genetic factors and environmental determinants. As susceptibility genes for both type 2 diabetes and schizophrenia are beginning to be identified there is increasing interest in the possibility of shared susceptibility loci between the two conditions. This article reviews the genetic basis to schizophrenia and type 2 diabetes and discusses the potential for shared loci between both conditions.

Genetic and nongenetic regulation of CAPN10 mRNA expression in skeletal muscle

The gene encoding calpain-10 (CAPN10) has been identified as a candidate gene for type 2 diabetes. Our aim was to study the impact of genetic (heritability and polymorphisms) and nongenetic (insulin, free fatty acids, and age) factors on CAPN10 mRNA expression in skeletal muscle using two different study designs. Muscle biopsies were obtained before and after hyperinsulinemic-euglycemic clamps from 166 young and elderly monozygotic and dizygotic twins as well as from 15 subjects with normal (NGT) or impaired glucose tolerance (IGT) exposed to an Intralipid infusion. We found hereditary effects on both basal and insulin-exposed CAPN10 mRNA expression. Carriers of the type 2 diabetes-associated single nucleotide polymorphism (SNP)-43 G/G genotype had reduced CAPN10 mRNA levels compared with subjects carrying the SNP-43 A-allele. Age had no significant influence on CAPN10 mRNA levels. Insulin had no significant effect on CAPN10 mRNA levels, neither in the twins nor in the basal state of the Intralipid study. However, after a 24-h infusion of Intralipid, we noted a significant increase in CAPN10 mRNA in response to insulin in subjects with NGT but not in subjects with IGT. In conclusion, we provide evidence that mRNA expression of CAPN10 in skeletal muscle is under genetic control. Glucose-tolerant but not glucose-intolerant individuals upregulate their CAPN10 mRNA levels in response to prolonged exposure to fat.

Abnormal glucose homeostasis and pancreatic islet function in mice with inactivation of the Fem1b gene

Type 2 diabetes mellitus is a disorder of glucose homeostasis involving complex gene and environmental interactions that are incompletely understood. Mammalian homologs of nematode sex determination genes have recently been implicated in glucose homeostasis and type 2 diabetes mellitus. These are the Hedgehog receptor Patched and Calpain-10, which have homology to the nematode tra-2 and tra-3 sex determination genes, respectively. Here, we have developed Fem1b knockout (Fem1b-KO) mice, with targeted inactivation of Fem1b, a homolog of the nematode fem-1 sex determination gene. We show that the Fem1b-KO mice display abnormal glucose tolerance and that this is due predominantly to defective glucose-stimulated insulin secretion. Arginine-stimulated insulin secretion is also affected. The Fem1b gene is expressed in pancreatic islets, within both beta cells and non-beta cells, and is highly expressed in INS-1E cells, a pancreatic beta-cell line. In conclusion, these data implicate Fem1b in pancreatic islet function and insulin secretion, strengthening evidence that a genetic pathway homologous to nematode sex determination may be involved in glucose homeostasis and suggesting novel genes and processes as potential candidates in the pathogenesis of diabetes mellitus.

Inhibition of calpain blocks pancreatic beta-cell spreading and insulin secretion

In addition to promoting insulin secretion, an increase in cytosolic Ca(2+) triggered by glucose has been shown to be crucial for spreading of beta-cells attached on extracellular matrix (804G matrix). Calpains are Ca(2+)-dependent cysteine proteases involved in an extended spectrum of cellular responses, including cytoskeletal rearrangements and vesicular trafficking. The present work aimed to assess whether calpain is also implicated in the process of Ca(2+)-induced insulin secretion and spreading of rat pancreatic beta-cells. The results indicate calpain dependency of beta-cell spreading on 804G matrix. Indeed, treatment with three distinct calpain inhibitors (N-Ac-Leu-Leu-norleucinal, calpeptin, and ethyl(+)-(2S,3S)-3-[(S)-3-methyl-1-(3-methylbutylcarbamoyl)butyl-carbamoyl]-2-ox-iranecarboxylate) inhibited cell spreading induced by glucose and KCl, whereas cell attachment was not significantly modified. Calpain inhibitors also suppressed glucose- and KCl-stimulated insulin secretion without affecting insulin synthesis. Washing the inhibitor out of the cell culture restored spreading on 804G matrix and insulin secretory response after 24 h. In addition, incubation with calpeptin did not affect insulin secretory response to mastoparan that acts on exocytosis downstream of intracellular calcium [Ca(2+)]i. Finally, calpeptin was shown to affect the [Ca(2+)]i response to glucose but not to KCl. In summary, the results show that inhibition of calpain blocks spreading and insulin secretion of primary pancreatic beta-cells. It is therefore suggested that calpain could be a mediator of Ca(2+)-induced-insulin secretion and beta-cell spreading.

Calpain 10 and type 2 diabetes: are we getting closer to an explanation?

PURPOSE OF REVIEW

A variation in the gene encoding the cysteine protease calpain 10 (CAPN10) was recently linked and associated with type 2 diabetes by positional cloning. This positional cloning was a follow-up investigation to the identification of a diabetes-linked region on human chromosome 2 identified by genome-wide scanning a few years earlier. In this paper we give a general background on the genetic studies performed on CAPN10 to date, and review the most recent studies on the functional role of calpain 10.

RECENT FINDING

A haplotype or haplotype combination comprising three intronic single nucleotide polymorphisms (UCSNP-43, 19, and 63) were associated with a threefold increased risk of type 2 diabetes in the population in which linkage was first found. Another polymorphism, UCSNP-44, which is in linkage disequilibrium with a coding single-nucleotide polymorphism (Thr504Ala), has subsequently been associated with type 2 diabetes in extensive meta-analyses. Meanwhile, initial studies probing the possible role of calpain-10, completely unknown at the time, are now being pursued, both in isolated cells and humans.

SUMMARY

The positional cloning of CAPN10 as a candidate gene for type 2 diabetes has been particularly fruitful. Not only has it identified an important and surprising piece of the puzzle underlying the development of diabetes, but it has also modelled and paved the way for investigations concerning complex genetic diseases other than type 2 diabetes.

Calpain inhibition and insulin action in cultured human muscle cells

Variation in the calpain 10 gene has been reported to increase susceptibility to type 2 diabetes. Part of this susceptibility appears to be mediated by a decrease in whole body insulin sensitivity. As skeletal muscle is the primary tissue site of the peripheral insulin resistance in type 2 diabetes, the aim of this study was to use a human skeletal muscle cell culture system to explore the effects of calpain inhibition on insulin action. Calpain 10 mRNA and protein expression was examined in cultured myoblasts, myotubes, and whole skeletal muscle from non-diabetic subjects using RT-PCR and Western blotting. Changes in insulin-stimulated glucose uptake and glycogen synthesis in response to the calpain inhibitors ALLN and ALLM were measured. Calpain 10 expression was confirmed in cultured human myoblasts, myotubes, and native skeletal muscle. Insulin-stimulated glucose uptake was significantly decreased following preincubation with ALLN [404+/-40 vs 505+/-55 (mean+/-SEM)pmol/mg/min; with vs without ALLN: p = 0.04] and ALLM [455+/-38 vs 550+/-50 pmol/mg/min; with vs without ALLM: p = 0.025] in day 7 fused myotubes, but not in myoblasts. Neither ALLN nor ALLM affected insulin-stimulated glycogen synthesis in myoblasts or myotubes. These studies confirm calpain 10 expression in cultured human muscle cells and support a role for calpains in insulin-stimulated glucose uptake in human skeletal muscle cells that may be relevant to the pathogenesis of the peripheral insulin resistance in type 2 diabetes.

Evidence that an isoform of calpain-10 is a regulator of exocytosis in pancreatic beta-cells

Calpain-10 (CAPN10) is the first type 2 diabetes susceptibility gene to be identified through a genome scan, with polymorphisms being associated with altered CAPN10 expression. Functional data have been hitherto elusive, but we report here a corresponding increase between CAPN10 expression level and regulated insulin secretion. Pancreatic beta-cell secretory granule exocytosis is mediated by the soluble N-ethylmaleimide-sensitive fusion protein attachment receptor protein complex of synaptosomal-associated protein of 25 kDa (SNAP-25), syntaxin 1, and vesicle-associated membrane protein 2. We report, for the first time, direct binding of a calpain-10 isoform with members of this complex. Furthermore, SNAP-25 undergoes a Ca2+-dependent partial proteolysis during exocytosis, with calpain protease inhibitor similarly suppressing both insulin secretion and SNAP-25 proteolysis. Based upon these findings, we postulate that an isoform of calpain-10 is a Ca2+-sensor that functions to trigger exocytosis in pancreatic beta-cells.

RyR2 and calpain-10 delineate a novel apoptosis pathway in pancreatic islets

Cells are programmed to die when critical signaling and metabolic pathways are disrupted. Inhibiting the type 2 ryanodine receptor (RyR2) in human and mouse pancreatic beta-cells markedly increased apoptosis. This mode of programmed cell death was not associated with robust caspase-3 activation prompting a search for an alternative mechanism. Increased calpain activity and calpain gene expression suggested a role for a calpain-dependent death pathway. Using a combination of pharmacological and genetic approaches, we demonstrated that the calpain-10 isoform mediated ryanodine-induced apoptosis. Apoptosis induced by the fatty acid palmitate and by low glucose also required calpain-10. Ryanodine-induced calpain activation and apoptosis were reversed by glucagon-like peptide or short-term exposure to high glucose. Thus RyR2 activity seems to play an essential role in beta-cell survival in vitro by suppressing a death pathway mediated by calpain-10, a type 2 diabetes susceptibility gene with previously unknown function.

Glucose sensitivity and metabolism-secretion coupling studied during two-year continuous culture in INS-1E insulinoma cells

Rat insulinoma-derived INS-1 cells constitute a widely used beta-cell surrogate. However, due to their nonclonal nature, INS-1 cells are heterogeneous and are not stable over extended culture periods. We have isolated clonal INS-1E cells from parental INS-1 based on both their insulin content and their secretory responses to glucose. Here we describe the stable differentiated INS-1E beta-cell phenotype over 116 passages (no. 27-142) representing a 2.2-yr continuous follow-up. INS-1E cells can be safely cultured and used within passages 40-100 with average insulin contents of 2.30 +/- 0.11 microg/million cells. Glucose-induced insulin secretion was dose-related and similar to rat islet responses. Secretion saturated with a 6.2-fold increase at 15 mm glucose, showing a 50% effective concentration of 10.4 mm. Secretory responses to amino acids and sulfonylurea were similar to those of islets. Moreover, INS-1E cells retained the amplifying pathway, as judged by glucose-evoked augmentation of insulin release in a depolarized state. Regarding metabolic parameters, INS-1E cells exhibited glucose dose-dependent elevations of NAD(P)H, cytosolic Ca(2+), and mitochondrial Ca(2+) levels. In contrast, mitochondrial membrane potential, ATP levels, and cell membrane potential were all fully activated by 7.5 mm glucose. Using the perforated patch clamp technique, 7.5 and 15 mm glucose elicited electrical activity to a similar degree. A K(ATP) current was identified in whole cell voltage clamp using diazoxide and tolbutamide. As in native beta-cells, tolbutamide induced electrical activity, indicating that the K(ATP)conductance is important in setting the resting potential. Therefore, INS-1E cells represent a stable and valuable beta-cell model.