Calpain facilitates GLUT4 vesicle translocation during insulin-stimulated glucose uptake in adipocytes

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.

Single-nucleotide polymorphisms as important risk factors of diabetes among Middle East population

Diabetes is a chronic metabolic disorder that leads to the dysfunction of various tissues and organs, including eyes, kidneys, and cardiovascular system. According to the World Health Organization, diabetes prevalence is 8.8% globally among whom about 90% of cases are type 2 diabetes. There are not any significant clinical manifestations in the primary stages of diabetes. Therefore, screening can be an efficient way to reduce the diabetic complications. Over the recent decades, the prevalence of diabetes has increased alarmingly among the Middle East population, which has imposed exorbitant costs on the health care system in this region. Given that the genetic changes are among the important risk factors associated with predisposing people to diabetes, we examined the role of single-nucleotide polymorphisms (SNPs) in the pathogenesis of diabetes among Middle East population. In the present review, we assessed the molecular pathology of diabetes in the Middle East population that paves the way for introducing an efficient SNP-based diagnostic panel for diabetes screening among the Middle East population. Since, the Middle East has a population of 370 million people; the current review can be a reliable model for the introduction of SNP-based diagnostic panels in other populations and countries around the world.

Cryptic splicing events result in unexpected protein products from calpain-10 (CAPN10) cDNA

Calpain-10 (CAPN10) belongs to the calpain superfamily. Genetic polymorphisms of the CAPN10 gene are associated with susceptibility to develop type 2 diabetes mellitus. Although the role of CAPN10 in the pathophysiology of diabetes has been extensively investigated, its biochemical properties are largely unknown. In this report, we made the surprising discovery that CAPN10 cDNA transcripts are subject to cryptic splicing and unexpected protein products were expressed. The same set of splicing products was reproducibly detected in four types of cultured cells including the primary culture of mouse myoblast. At least, one of the products was identical to a natural splicing variant. Sequence analysis of the splicing potential of CAPN10 cDNA, together with mutagenesis studies, resulted in the identification of a powerful splicing acceptor site at the junction of the sequences encoded by exons 9 and 10. We successfully extended the analysis to create expression construct resistant to splicing for both human and mouse CAPN10. The construct allowed us to analyze two major CAPN10 isoforms and reveal their difference in substrate proteolysis and potential cell functions. These results demonstrate that proteins produced from cDNA do not necessarily reflect the original nucleotide sequence. We provide insight into the property of recombinantly expressed CAPN10 proteins in cultured cells circumventing unexpected protein products.

In vitro and in vivo antihyperglycemic activity of the ethanol extract of Heritiera fomes bark and characterization of pharmacologically active phytomolecules

OBJECTIVE

This study aimed to demonstrate the mechanistic basis of Heritiera fomes, which has traditionally been used to treat diabetes.

METHODS

Clonal pancreatic β-cells and primary islets were used to measure insulin release. 3T3-L1 cells were used to analyse insulin action, and in vitro systems were used to measure further glucose-lowering activity. In vivo assessment was performed on streptozotocin (STZ)-induced type-2 diabetic rats and reversed-phase-HPLC followed by liquid chromatography mass spectrometry (LC-MS) to detect bioactive molecules.

KEY FINDINGS

Ethanol extract of Heritiera fomes (EEHF) significantly increased insulin release with stimulatory effects comparable to 1 µM glucagon-like peptide 1, which were somewhat reduced by diazoxide, verapamil and calcium-free conditions. Insulin release was stimulated by tolbutamide, isobutyl methylxanthine and KCl. EEHF induced membrane depolarization and increased intracellular Ca2+ levels. EEHF enhanced glucose uptake in 3T3L1 cells and decreased protein glycation. EEHF significantly inhibited postprandial hyperglycaemia following sucrose loading and inversely elevated unabsorbed sucrose concentration in the gut. It suppressed glucose absorption during in situ gut perfusion. Furthermore, EEHF improved glucose tolerance, plasma insulin and gut motility, and decreased plasma dipeptidyl peptidase IV activity. Procyanidins, epicatechin and proanthocyanidins were some of the identified bioactive constituents that may involve in β-cell actions.

CONCLUSIONS

This study provides some evidence to support the use of H. fomes as an antidiabetic traditional remedy.

Effects of functionally diverse calpain system on immune cells

Calpains are a family of nonlysosomal cysteine proteases, which play important roles in numerous physiological and pathological processes. Locations of them dictates the functions so that they are classified as ubiquitously expressed calpains and tissue-specific calpains. Recent studies are mainly focused on conventional calpains (calpain-1,2) in development and diseases, and increasing people pay attention to other subtypes of calpains but may not been summarized appropriately. Growing evidence suggests that calpains are also involved in immune regulation. However, seldom articles review the regulation of calpains on immune cells. The aim of this article is to review the research progress of each calpain isozyme and the effect of calpains on immune cells, especially the promotion effect of calpains on the immune response of macrophage, neutrophils, dendritic cells, mast cells, natural killed cells, and lymphocytes. These effects would hold great promise for the clinical application of calpains as a practicable therapeutic option in the treatment of immune related diseases.

Calpain Activity in Leukocytes Is Associated with Diabetes Biochemical Markers

BACKGROUND AND AIMS

CAPN10 gene is associated with type 2 diabetes (T2D). Specific members of the calpain system (CAPN1, CAPN2 and CAPN10) are implicated in glucose metabolism. The aim of this study was to evaluate the calpain activity in leukocytes of control subjects and patients with T2D and its association with the calpain family members involved in glucose metabolism and with biochemical parameters that are altered in T2D.

METHODS

Calpain activity under extracellular glucose concentrations (70-280 mg/dL) was evaluated in leukocytes from subjects with and without T2D. Protein and mRNA levels of CAPN1, CAPN2 and CAPN10 were evaluated. Calpain inhibitors assays were performed in leukocytes from subjects without T2D to evaluate glucose uptake. Calpain activity at 100 mg/dL glucose was correlated with biochemical parameters by multivariate regression.

RESULTS

Calpain activity in control subjects increased with extracellular glucose concentration in a dose-dependent manner, showing a negative association with HbA1c levels and total amount of CAPN10 protein. In contrast, calpain activity is decreased in patients with T2D and do not respond to changes in glucose concentration. A reduction of CAPN1 autolytic fragments were observed in the subjects with diabetes. Calpain inhibitors decreased calpain activity but did not altered glucose uptake in leukocytes.

CONCLUSIONS

Calpain activity induced by glucose in leukocytes was associated with biochemical markers of glucose metabolism and with CAPN10 protein abundance. Calpain activity is low in subjects with T2D. Thus, calpain activity induced by extracellular glucose in leukocytes could be a potential marker for T2D early risk detection.

Arsenic impairs GLUT1 trafficking through the inhibition of the calpain system in lymphocytes

Exposure to arsenic is associated with increased risk of developing insulin resistance and type 2 diabetes. The proteases calpain-1 (CAPN1), calpain-2 (CAPN2) and calpain-10 (CAPN10) and their endogenous inhibitor calpastatin (CAST) regulate glucose uptake in skeletal muscle and adipocytes. We investigated whether arsenic disrupts GLUT1 trafficking and function through calpain inhibition, using lymphocytes as a cell model. Lymphocytes from healthy subjects were treated with 0.1 or 1 μM of sodium arsenite for 72 h and challenged with 3.9 or 11.1 mM of glucose. Our results showed that arsenite inhibited GLUT1 trafficking, glucose uptake, and calpain activity in the presence of 11.1 mM of glucose. These correlated with a decrease in the autolytical fragment of 50 kDa of CAPN1 and increased levels of CAST, but there were no changes in CAPN2 and CAPN10. We used a cell-free system to evaluate the effect of arsenite over CAPN1, finding that arsenite induced CAPN1 autolysis. To confirm that calpains are involved in GLUT1 trafficking and glucose uptake in lymphocytes, we generated stable CAPN1 or CAPN10 knockdowns in Jurkat cells using short hairpin RNA (shRNA). CAPN1 knockdown induced glucose uptake, while CAPN10 knockdown diminished glucose uptake, which correlated with a significant reduction of calpain activity after the pulse with 11.1 mM of glucose. These data showed that CAPN10 was responsible for the induction of calpain activity after the challenge with 11.1 mM of glucose and that CAPN1 and CAPN10 regulate glucose uptake in lymphocytes. Altogether, our results suggest that arsenite impairs GLUT1 trafficking and function through calpain dysregulation.

Serum from Jiao-Tai-Wan treated rats increases glucose consumption by 3T3-L1 adipocytes through AMPK pathway signaling

Type 2 diabetes (T2DM) is characterized by hyperglycemia resulting from insulin resistance. Jiao-Tai-Wan (JTW), a traditional Chinese medicine consisting of a 10:1 formulation of Rhizoma Coptidis (RC) and Cortex Cinnamomi (cinnamon) was shown to have hypoglycemic efficacy in a type 2 diabetic mouse model. Here we investigated whether glucose consumption by insulin-resistant adipocytes could be modulated by serum from JTW-treated rats, and if so, through what mechanism. JTW-medicated serum was prepared from rats following oral administration of JTW decoction twice a day for 4 days. Fully differentiated 3T3-L1 adipocytes – rendered insulin resistance by dexamethasone treatment – were cultured in medium containing JTW-medicated rat serum. JTW-medicated serum treatment increased glucose uptake, up-regulated levels of phosphorylated adenosine 5′-monophoshate-activated protein kinase (p-AMPK), and stimulated expression and translocation of glucose transporter 4 (GLUT4). JTW-medicated serum induced significantly greater up-regulation of p-AMPK and GLUT4 than either RC or cinnamon-medicated serum. JTW-medicated serum induced effects on 3T3-L1 adipocytes could be partially inhibited by treatment with the AMPK inhibitor compound C. In conclusion, JTW-medicated serum increased glucose consumption by IR adipocytes partially through the activation of the AMPK pathway, and JTW was more effective on glucose consumption than either RC or cinnamon alone.

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 mTOR Complex Controls HIV Latency

A population of CD4 T lymphocytes harboring latent HIV genomes can persist in patients on antiretroviral therapy, posing a barrier to HIV eradication. To examine cellular complexes controlling HIV latency, we conducted a genome-wide screen with a pooled ultracomplex shRNA library and in vitro system modeling HIV latency and identified the mTOR complex as a modulator of HIV latency. Knockdown of mTOR complex subunits or pharmacological inhibition of mTOR activity suppresses reversal of latency in various HIV-1 latency models and HIV-infected patient cells. mTOR inhibitors suppress HIV transcription both through the viral transactivator Tat and via Tat-independent mechanisms. This inhibition occurs at least in part via blocking the phosphorylation of CDK9, a p-TEFb complex member that serves as a cofactor for Tat-mediated transcription. The control of HIV latency by mTOR signaling identifies a pathway that may have significant therapeutic opportunities.

Role of the calpain on the development of diabetes mellitus and its chronic complications

Diabetes mellitus (DM) is associated with acute and chronic complications that cause major morbidity and significant mortality. Calpains, a family of Ca(2+)-dependent cytosolic cysteine proteases, can modulate their substrates' structure and function through limited proteolytic activity. Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic function. However, alterations in calcium homeostasis lead to pathologic activation of calpain in diabetes mellitus. Since not much is known on the relationship between calpain and diabetes mellitus, this review outlines the contribution of calpain to chronic complications of diabetes mellitus, such as diabetic cardiomyopathy, diabetic nephropathy and diabetic retinopathy.

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.

Regulation of myosin IIA and filamentous actin during insulin-stimulated glucose uptake in 3T3-L1 adipocytes

Insulin stimulated glucose uptake requires the colocalization of myosin IIA (MyoIIA) and the insulin-responsive glucose transporter 4 (GLUT4) at the plasma membrane for proper GLUT4 fusion. MyoIIA facilitates filamentous actin (F-actin) reorganization in various cell types. In adipocytes F-actin reorganization is required for insulin-stimulated glucose uptake. What is not known is whether MyoIIA interacts with F-actin to regulate insulin-induced GLUT4 fusion at the plasma membrane. To elucidate the relationship between MyoIIA and F-actin, we examined the colocalization of MyoIIA and F-actin at the plasma membrane upon insulin stimulation as well as the regulation of this interaction. Our findings demonstrated that MyoIIA and F-actin colocalized at the site of GLUT4 fusion with the plasma membrane upon insulin stimulation. Furthermore, inhibition of MyoII with blebbistatin impaired F-actin localization at the plasma membrane. Next we examined the regulatory role of calcium in MyoIIA-F-actin colocalization. Reduced calcium or calmodulin levels decreased colocalization of MyoIIA and F-actin at the plasma membrane. While calcium alone can translocate MyoIIA it did not stimulate F-actin accumulation at the plasma membrane. Taken together, we established that while MyoIIA activity is required for F-actin localization at the plasma membrane, it alone is insufficient to localize F-actin to the plasma membrane.

Insulin growth factor binding proteins as therapeutic targets in type 2 diabetes

INTRODUCTION

The signaling pathways of the insulin-like growth factors (IGFs) have been implicated in the aetiology of type 2 diabetes (T2D) and a number of therapeutic modalities aiming at the IGF-axis have been considered. Administration of IGF-I has been reported to improve insulin sensitivity in healthy subjects and patients with T2D. In recent years, the IGF binding proteins (IGFBPs) have also been associated with metabolic disorders, prompting the idea that IGFBPs play important roles in the pathogenesis of T2D. Thus, by virtue of their role in the regulation of IGF effects, the IGFBPs have emerged as potential biomarkers and therapeutic targets in metabolic syndromes and T2D.

AREAS COVERED

The article provides an overview on recent findings in clinical and experimental IGFBP-research and addresses the studies that have investigated the potentials of the IGFBPs as therapeutic targets in T2D.

EXPERT OPINION

There is plenty of therapeutic promise within the IGF system, but further understanding of the IGFs in T2D is necessary to avoid off-target effects. Strong evidence supports the use of IGFBPs as therapeutic targets in the treatment of T2D, and it is not difficult to foresee the use of IGFBPs as part of a combination therapy alongside other anti-diabetic drugs.

Regulation of myosin light chain kinase during insulin-stimulated glucose uptake in 3T3-L1 adipocytes

Myosin II (MyoII) is required for insulin-responsive glucose transporter 4 (GLUT4)-mediated glucose uptake in 3T3-L1 adipocytes. Our previous studies have shown that insulin signaling stimulates phosphorylation of the regulatory light chain (RLC) of MyoIIA via myosin light chain kinase (MLCK). The experiments described here delineate upstream regulators of MLCK during insulin-stimulated glucose uptake. Since 3T3-L1 adipocytes express two MyoII isoforms, we wanted to determine which isoform was required for insulin-stimulated glucose uptake. Using a siRNA approach, we demonstrate that a 60% decrease in MyoIIA protein expression resulted in a 40% inhibition of insulin-stimulated glucose uptake. We also show that insulin signaling stimulates the phosphorylation of MLCK. We further show that MLCK can be activated by calcium as well as signaling pathways. We demonstrate that adipocytes treated with the calcium chelating agent, 1,2-b (iso-aminophenoxy) ethane-N,N,N',N'-tetra acetic acid, (BAPTA) (in the presence of insulin) impaired the insulin-induced phosphorylation of MLCK by 52% and the RLC of MyoIIA by 45% as well as impairing the recruitment of MyoIIA to the plasma membrane when compared to cells treated with insulin alone. We further show that the calcium ionophore, A23187 alone stimulated the phosphorylation of MLCK and the RLC associated with MyoIIA to the same extent as insulin. To identify signaling pathways that might regulate MLCK, we examined ERK and CaMKII. Inhibition of ERK2 impaired phosphorylation of MLCK and insulin-stimulated glucose uptake. In contrast, while inhibition of CaMKII did inhibit phosphorylation of the RLC associated with MyoIIA, inhibition of CAMKIIδ did not impair MLCK phosphorylation or translocation to the plasma membrane or glucose uptake. Collectively, our results are the first to delineate a role for calcium and ERK in the activation of MLCK and thus MyoIIA during insulin-stimulated glucose uptake in 3T3-L1 adipocytes.

Arsenic exposure and calpain-10 polymorphisms impair the function of pancreatic beta-cells in humans: a pilot study of risk factors for T2DM

The incidence of type 2 diabetes mellitus (T2DM) is increasing worldwide and diverse environmental and genetic risk factors are well recognized. Single nucleotide polymorphisms (SNPs) in the calpain-10 gene (CAPN-10), which encodes a protein involved in the secretion and action of insulin, and chronic exposure to inorganic arsenic (iAs) through drinking water have been independently associated with an increase in the risk for T2DM. In the present work we evaluated if CAPN-10 SNPs and iAs exposure jointly contribute to the outcome of T2DM. Insulin secretion (beta-cell function) and insulin sensitivity were evaluated indirectly through validated indexes (HOMA2) in subjects with and without T2DM who have been exposed to a gradient of iAs in their drinking water in northern Mexico. The results were analyzed taking into account the presence of the risk factor SNPs SNP-43 and -44 in CAPN-10. Subjects with T2DM had significantly lower beta-cell function and insulin sensitivity. An inverse association was found between beta-cell function and iAs exposure, the association being more pronounced in subjects with T2DM. Subjects without T2DM who were carriers of the at-risk genotype SNP-43 or -44, also had significantly lower beta-cell function. The association of SNP-43 with beta-cell function was dependent on iAs exposure, age, gender and BMI, whereas the association with SNP-44 was independent of all of these factors. Chronic exposure to iAs seems to be a risk factor for T2DM in humans through the reduction of beta-cell function, with an enhanced effect seen in the presence of the at-risk genotype of SNP-43 in CAPN-10. Carriers of CAPN-10 SNP-44 have also shown reduced beta-cell function.

Regulation and cellular functions of class II phosphoinositide 3-kinases

Class II isoforms of PI3K (phosphoinositide 3-kinase) are still the least investigated and characterized of all PI3Ks. In the last few years, an increased interest in these enzymes has improved our understanding of their cellular functions. However, several questions still remain unanswered on their mechanisms of activation, their specific downstream effectors and their contribution to physiological processes and pathological conditions. Emerging evidence suggests that distinct PI3Ks activate different signalling pathways, indicating that their functional roles are probably not redundant. In the present review, we discuss the recent advances in our understanding of mammalian class II PI3Ks and the evidence suggesting their involvement in human diseases.

Tau phosphorylation and μ-calpain activation mediate the dexamethasone-induced inhibition on the insulin-stimulated Akt phosphorylation

Evidence has suggested that insulin resistance (IR) or high levels of glucocorticoids (GCs) may be linked with the pathogenesis and/or progression of Alzheimer's disease (AD). Although studies have shown that a high level of GCs results in IR, little is known about the molecular details that link GCs and IR in the context of AD. Abnormal phosphorylation of tau and activation of μ-calpain are two key events in the pathology of AD. Importantly, these two events are also related with GCs and IR. We therefore speculate that tau phosphorylation and μ-calpain activation may mediate the GCs-induced IR. Akt phosphorylation at Ser-473 (pAkt) is commonly used as a marker for assessing IR. We employed two cell lines, wild-type HEK293 cells and HEK293 cells stably expressing the longest human tau isoform (tau-441; HEK293/tau441 cells). We examined whether DEX, a synthetic GCs, induces tau phosphorylation and μ-calpain activation. If so, we examined whether the DEX-induced tau phosphorylation and μ-calpain activation mediate the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation. The results showed that DEX increased tau phosphorylation and induced tau-mediated μ-calpain activation. Furthermore, pre-treatment with LiCl prevented the effects of DEX on tau phosphorylation and μ-calpain activation. Finally, both LiCl pre-treatment and calpain inhibition prevented the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation. In conclusion, our study suggests that the tau phosphorylation and μ-calpain activation mediate the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation.

Proteomic analysis reveals cellular pathways regulating carbohydrate metabolism that are modulated in primary human skeletal muscle culture due to treatment with bioactives from Artemisia dracunculus L

Insulin resistance is a major pathophysiologic abnormality that characterizes metabolic syndrome and type 2 diabetes. A well characterized ethanolic extract of Artemisia dracunculus L., termed PMI 5011, has been shown to improve insulin action in vitro and in vivo, but the cellular mechanisms remain elusive. Using differential proteomics, we have studied mechanisms by which PMI 5011 enhances insulin action in primary human skeletal muscle culture obtained by biopsy from obese, insulin-resistant individuals. Using iTRAQ™ labeling and LC-MS/MS, we have identified over 200 differentially regulated proteins due to treatment with PMI 5011 and insulin stimulation. Bioinformatics analyses determined that several metabolic pathways related to glycolysis, glucose transport and cell signaling were highly represented and differentially regulated in the presence of PMI 5011 indicating that this extract affects several pathways modulating carbohydrate metabolism, including translocation of GLUT4 to the plasma membrane. These findings provide a molecular mechanism by which a botanical extract improves insulin stimulated glucose uptake, transport and metabolism at the cellular level resulting in enhanced whole body insulin sensitivity.

Mitochondrial calpain 10 is degraded by Lon protease after oxidant injury

Calpain 10 is ubiquitously expressed and is one of four mitochondrial matrix proteases. We determined that over-expression or knock-down of mitochondrial calpain 10 results in cell death, demonstrating that mitochondrial calpain 10 is required for viability. Thus, we studied calpain 10 degradation in isolated mitochondrial matrix, mitochondria and in renal proximal tubular cells (RPTC) under control and toxic conditions. Using isolated renal cortical mitochondria and mitochondrial matrix, calpain 10 underwent rapid degradation at 37°C that was blocked with Lon inhibitors but not by calpain or proteasome inhibitors. While exogenous Ca(2+) addition, Ca(2+) chelation or exogenous ATP addition had no effect on calpain 10 degradation, the oxidants tert-butyl hydroperoxide (TBHP) or H(2)O(2) increased the rate of degradation. Using RPTC, mitochondrial and cytosolic calpain 10 increased in the presence of MG132 (Lon/proteasome inhibitor) but only cytosolic calpain 10 increased in the presence of epoxomicin (proteasome inhibitor). Furthermore, TBHP and H(2)O(2) oxidized mitochondrial calpain 10, decreased mitochondrial, but not cytosolic calpain 10, and pretreatment with MG132 blocked TBHP-induced degradation of calpain 10. In summary, mitochondrial calpain 10 is selectively degraded by Lon protease under basal conditions and is enhanced under and oxidizing conditions, while cytosolic calpain 10 is degraded by the proteasome.

Chronic high glucose downregulates mitochondrial calpain 10 and contributes to renal cell death and diabetes-induced renal injury

Whereas most calpains are cytosolic proteases, calpain 10 is resident in mitochondria and is important in mitochondrial homeostasis. Because calpain 10 has been implicated in type 2 diabetes, we studied its possible role in diabetes-induced renal dysfunction. We treated renal proximal tubular cells with high glucose (17 mmol/l) and found decreased mitochondrial calpain 10 mRNA and protein at 96 h compared with cells incubated with 0 or 5 mmol/l glucose or 17 mmol/l D-mannitol. High glucose increased mitochondrial calpain 10 substrates (NDUFB8 and ATP synthase β), decreased basal and uncoupled respiration, and initiated cell apoptosis as indicated by cleaved caspase 3 and nuclear condensation. Renal calpain 10 protein and mRNA were specifically decreased in streptozotocin-induced diabetic rats with kidney dysfunction, and in diabetic ob/ob mice. In agreement with our in vitro data, the kidneys of streptozotocin-induced diabetic rats had elevated calpain 10 substrates and cleaved caspase 3. Finally, specific siRNA-induced knockdown of calpain 10 in the proximal tubules of control rats resulted in decreased renal function as evidenced by increased serum creatinine, and increased caspase 3 cleavage compared with rats receiving scrambled siRNA. Thus, the glucose-induced loss of calpain 10 in vivo results in renal cell apoptosis and organ failure through accumulation of mitochondrial calpain 10 substrates and mitochondrial dysfunction. Whether this is a major cause of the decreased renal function in diabetic nephropathy will require further studies.

New type 2 diabetes risk genes provide new insights in insulin secretion mechanisms

Type 2 diabetes results from the inability of beta cells to increase insulin secretion sufficiently to compensate for insulin resistance. Insulin resistance is thought to result mainly from environmental factors, such as obesity. However, there is compelling evidence that the decline of both insulin sensitivity and insulin secretion have also a genetic component. Recent genome-wide association studies identified several novel risk genes for type 2 diabetes. The vast majority of these genes affect beta cell function by molecular mechanisms that remain unknown in detail. Nevertheless, we and others could show that a group of genes affect glucose-stimulated insulin secretion, a group incretin-stimulated insulin secretion (incretin sensitivity or secretion) and a group proinsulin-to-insulin conversion. The most important so far type 2 diabetes risk gene, TCF7L2, interferes with all three mechanisms. In addition to advancing knowledge in the pathophysiology of type 2 diabetes, the discovery of novel genetic determinants of diabetes susceptibility may help understanding of gene-environment, gene-therapy and gene-gene interactions. It was also hoped that it could make determination of the individual risk for type 2 diabetes feasible. However, the allelic relative risks of most genetic variants discovered so far are relatively low. Thus, at present, clinical criteria assess the risk for type 2 diabetes with greater sensitivity and specificity than the combination of all known genetic variants.

Class II phosphoinositide 3-kinase C2alpha: what we learned so far

More than fifteen years after the first identification of a class II isoform of phosphoinositide 3-kinase (PI3K) in Drosophila melanoǵaster this subfamily remains the most enigmatic among all PI3Ks. What are the functions of these enzymes? What are their mechanisms of activation? Which downstream effectors are specifically regulated by these isoforms? Are class I and class II PI3Ks redundant or do they control different intracellular processes? And, more important, do class II PI3Ks have a role in human diseases? The recent increased interest on class II PI3Ks has started providing some answers to these questions but still a lot needs to be done to completely uncover the contribution of these enzymes to physiological processes and possibly to pathological conditions. Here we will summarise the recent findings on the alpha isoform of mammalian class II PI3Ks (PI3K-C2α ) and we will discuss the potential involvement of this enzyme in human diseases.

Calpain chronicle--an enzyme family under multidisciplinary characterization

Calpain is an intracellular Ca2+-dependent cysteine protease (EC 3.4.22.17; Clan CA, family C02) discovered in 1964. It was also called CANP (Ca2+-activated neutral protease) as well as CASF, CDP, KAF, etc. until 1990. Calpains are found in almost all eukaryotes and a few bacteria, but not in archaebacteria. Calpains have a limited proteolytic activity, and function to transform or modulate their substrates' structures and activities; they are therefore called, "modulator proteases." In the human genome, 15 genes--CAPN1, CAPN2, etc.--encode a calpain-like protease domain. Their products are calpain homologs with divergent structures and various combinations of functional domains, including Ca2+-binding and microtubule-interaction domains. Genetic studies have linked calpain deficiencies to a variety of defects in many different organisms, including lethality, muscular dystrophies, gastropathy, and diabetes. This review of the study of calpains focuses especially on recent findings about their structure-function relationships. These discoveries have been greatly aided by the development of 3D structural studies and genetic models.

Class II phosphoinositide 3-kinase regulates exocytosis of insulin granules in pancreatic beta cells

Phosphoinositide 3-kinases (PI3Ks) are critical regulators of pancreatic β cell mass and survival, whereas their involvement in insulin secretion is more controversial. Furthermore, of the different PI3Ks, the class II isoforms were detected in β cells, although their role is still not well understood. Here we show that down-regulation of the class II PI3K isoform PI3K-C2α specifically impairs insulin granule exocytosis in rat insulinoma cells without affecting insulin content, the number of insulin granules at the plasma membrane, or the expression levels of key proteins involved in insulin secretion. Proteolysis of synaptosomal-associated protein of 25 kDa, a process involved in insulin granule exocytosis, is impaired in cells lacking PI3K-C2α. Finally, our data suggest that the mRNA for PI3K-C2α may be down-regulated in islets of Langerhans from type 2 diabetic compared with non-diabetic individuals. Our results reveal a critical role for PI3K-C2α in β cells and suggest that down-regulation of PI3K-C2α may be a feature of type 2 diabetes.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on adipogenic differentiation and insulin-induced glucose uptake in 3T3-L1 cells

Dioxin exposure has been positively associated with human type II diabetes. Because lipophilic dioxins accumulate mainly in adipose tissue, this study aimed to determine if dioxins induce metabolic dysfunction in fat cells. Using 3T3-L1 cells as an in vitro model, we analyzed the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a model dioxin, on adipogenic differentiation, glucose uptake, and lipolysis. TCDD inhibited adipogenic differentiation, as determined by using oil droplet formation and adipogenic marker gene expression, including PPARgamma (peroxisome proliferator-activated receptor gamma), C/EBPalpha (CCAAT/enhancer-binding protein alpha), and Glut4 (glucose transporter type 4). Effects of TCDD on glucose uptake were evaluated using fully differentiated 3T3-L1 adipocytes, revealing that TCDD significantly attenuated insulin-induced glucose uptake dose dependently. Inhibition of aryl hydrocarbon receptor (AhR) by alpha-naphthoflavone (alpha-NF), an AhR inhibitor, did not prevent the inhibitory effect of TCDD on glucose uptake, suggesting that TCDD attenuates insulin-induced glucose uptake in an AhR-independent manner. Effects of TCDD on lipolysis were determined using glycerol release assay. We found that TCDD had no marked effect on isoproterenol-induced glycerol release in fully differentiated 3T3-L1 adipocytes. These results provide in vitro evidence of TCDD's effects on fat cell metabolism, suggesting dioxin exposure in development of insulin resistance and type II diabetes.

Pathomechanisms of type 2 diabetes genes

Type 2 diabetes mellitus is a complex metabolic disease that is caused by insulin resistance and beta-cell dysfunction. Furthermore, type 2 diabetes has an evident genetic component and represents a polygenic disease. During the last decade, considerable progress was made in the identification of type 2 diabetes risk genes. This was crucially influenced by the development of affordable high-density single nucleotide polymorphism (SNP) arrays that prompted several successful genome-wide association scans in large case-control cohorts. Subsequent to the identification of type 2 diabetes risk SNPs, cohorts thoroughly phenotyped for prediabetic traits with elaborate in vivo methods allowed an initial characterization of the pathomechanisms of these SNPs. Although the underlying molecular mechanisms are still incompletely understood, a surprising result of these pathomechanistic investigations was that most of the risk SNPs affect beta-cell function. This favors a beta-cell-centric view on the genetics of type 2 diabetes. The aim of this review is to summarize the current knowledge about the type 2 diabetes risk genes and their variants' pathomechanisms.

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.

Dual role for myosin II in GLUT4-mediated glucose uptake in 3T3-L1 adipocytes

Insulin-stimulated glucose uptake requires the activation of several signaling pathways to mediate the translocation and fusion of GLUT4 vesicles to the plasma membrane. Our previous studies demonstrated that GLUT4-mediated glucose uptake is a myosin II-dependent process in adipocytes. The experiments described in this report are the first to show a dual role for the myosin IIA isoform specifically in regulating insulin-stimulated glucose uptake in adipocytes. We demonstrate that inhibition of MLCK but not RhoK results in impaired insulin-stimulated glucose uptake. Furthermore, our studies show that insulin specifically stimulates the phosphorylation of the RLC associated with the myosin IIA isoform via MLCK. In time course experiments, we determined that GLUT4 translocates to the plasma membrane prior to myosin IIA recruitment. We further show that recruitment of myosin IIA to the plasma membrane requires that myosin IIA be activated via phosphorylation of the RLC by MLCK. Our findings also reveal that myosin II is required for proper GLUT4-vesicle fusion at the plasma membrane. We show that once at the plasma membrane, myosin II is involved in regulating the intrinsic activity of GLUT4 after insulin stimulation. Collectively, our results are the first to reveal that myosin IIA plays a critical role in mediating insulin-stimulated glucose uptake in 3T3-LI adipocytes, via both GLUT4 vesicle fusion at the plasma membrane and GLUT4 activity.

Association of calpain-10 polymorphisms with type 2 diabetes in the Tunisian population

BACKGROUND

Genome-wide analyses of the genetic predisposition to type 2 diabetes mellitus (T2DM) in different isolates and populations have identified regions of interest called non insulin-dependent diabetes mellitus (NIDDM) 1, 2, 3 and 4. At the NIDDM1 locus (2q37.3), calpain-10 (CAPN10) encodes for a ubiquitously expressed protease implicated in the two fundamental pathophysiological aspects of T2DM. This is a report of the results of a study of the association of four CAPN10 polymorphisms with T2DM in the Tunisian population.

PARTICIPANTS AND METHODS

A total of 222 T2DM patients with a diabetes duration of 10 years or more and 206 healthy controls were enrolled to analyze the frequency distribution of four CAPN10 polymorphisms (UCSNP-43, UCSNP-19, UCSNP-110 and UCSNP-63) using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) in the Tunisian population. We also investigated the association of T2DM with different haplotypes and haplotype combinations.

RESULTS

Only the A allele of UCSNP-43 showed an association with T2DM (odds ratio, OR=1.86). We also identified a novel combination of haplotypes (121/221) defined by three polymorphisms (UCNSP-43, -19 and -63) that is associated with an increased risk of T2DM (OR=2.38).

CONCLUSION

In this study involving the Tunisian population, we identified genetic variants within CAPN10 that are linked with T2DM and a novel haplotype combination, 121/221, associated with an increased susceptibility to T2DM.

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.

Calpain-10 gene and protein expression in human skeletal muscle: effect of acute lipid-induced insulin resistance and type 2 diabetes

OBJECTIVE

Our objective was to investigate the effect of lipid-induced insulin resistance and type 2 diabetes on skeletal muscle calpain-10 mRNA and protein levels.

RESEARCH DESIGN AND METHODS

In the first part of this study, 10 healthy subjects underwent hyperinsulinemic euglycemic (4.5 mmol/liter) clamps for 6 h with iv infusion of either saline or a 20% Intralipid emulsion (Fresenius Kabi AG, Bad Homburg, Germany). Skeletal muscle biopsies were taken before and after 3- and 6-h insulin infusion and analyzed for calpain-10 mRNA and protein expression. In the second part of the study, muscle samples obtained after an overnight fast in 10 long-standing, sedentary type 2 diabetes patients, 10 sedentary, weight-matched, normoglycemic controls, and 10 age-matched, endurance-trained cyclists were analyzed for calpain-10 mRNA and protein content.

RESULTS

Intralipid infusion in healthy subjects reduced whole body glucose disposal by approximately 50% (P<0.001). Calpain-10 mRNA (P=0.01) but not protein content was reduced after 6-h insulin infusion in both the saline and Intralipid emulsion trials. Skeletal muscle calpain-10 mRNA and protein content did not differ between the type 2 diabetes patients and normoglycemic controls, but there was a strong trend for total calpain-10 protein to be greater in the endurance-trained athletes (P=0.06).

CONCLUSIONS

These data indicate that skeletal muscle calpain-10 expression is not modified by insulin resistance per se and suggest that hyperinsulinemia and exercise training may modulate human skeletal muscle calpain-10 expression.

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.

Coordinated control of both insulin secretion and insulin action through calpain-10-mediated regulation of exocytosis?

Calpain-10 was first identified through a genome scan seeking to identify diabetes predisposition genes. Both genetic and functional data has since indicated that calpain-10 has an important role in insulin resistance and intermediate phenotypes, including those associated with adipocytes and skeletal muscle. Evidence presented in this issue by Brown, Yeaman, and Walker utilizes siRNA technology to specifically knock down calpain-10 expression, and suggests that calpain-10 facilitates GLUT4 translocation through effects on the distal secretory pathway. Calpain-10 is also an important molecule in the pancreatic beta-cell, where it has been shown to regulate exocytosis through partial proteolysis of a member of the secretory granule fusion machinery. In addition, calpain-10 has also been implicated in reorganization of the actin cytoskeleton that accompanies both GLUT4 vesicle translocation and insulin secretion. Taken together, these findings provide fresh hope for the development of novel diabetic treatments, utilizing either pharmacological activators that specifically target calpain-10, or through targeted calpain-10 gene therapy. Therapeutic intervention in this way could simultaneously enhance both insulin secretion and insulin action.

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.

Emerging functions of the calpain superfamily of cysteine proteases in neuroendocrine secretory pathways

The first calpain protease was discovered over 40 years ago now, yet despite the vast amount of literature that has subsequently emerged detailing their involvement in the pathophysiology of a variety of human diseases, it is only in the last decade that calpain-mediated actions along the secretory pathway have begun to emerge. However, the number of secretory pathway substrates identified and their diversity of function continues to grow. This review summarizes our current knowledge of calpain-mediated mechanisms of action that are pertinent to synaptic vesicle assembly and budding, cytoskeletal organization, endosomal recycling, and exocytotic membrane fusion.

Molecular mechanisms of the diabetogenic effects of arsenic: inhibition of insulin signaling by arsenite and methylarsonous acid

BACKGROUND

Increased prevalences of diabetes mellitus have been reported among individuals chronically exposed to inorganic arsenic (iAs). However, the mechanisms underlying the diabetogenic effects of iAs have not been characterized. We have previously shown that trivalent metabolites of iAs, arsenite (iAs(III)) and methylarsonous acid (MAs(III)) inhibit insulin-stimulated glucose uptake (ISGU) in 3T3-L1 adipocytes by suppressing the insulin-dependent phosphorylation of protein kinase B (PKB/Akt).

OBJECTIVES

Our goal was to identify the molecular mechanisms responsible for the suppression of PKB/Akt phosphorylation by iAs(III) and MAs(III).

METHODS

The effects of iAs(III) and MAs(III) on components of the insulin-activated signal transduction pathway that regulate PKB/Akt phosphorylation were examined in 3T3-L1 adipocytes.

RESULTS

Subtoxic concentrations of iAs(III) or MAs(III) had little or no effect on the activity of phosphatidylinositol 3-kinase (PI-3K), which synthesizes phosphatidylinositol-3,4,5-triphosphate (PIP(3)), or on phosphorylation of PTEN (phosphatase and tensin homolog deleted on chromosome ten), a PIP(3) phosphatase. Neither iAs(III) nor MAs(III) interfered with the phosphorylation of 3-phosphoinositide-dependent kinase-1 (PDK-1) located downstream from PI-3K. However, PDK-1 activity was inhibited by both iAs(III) and MAs(III). Consistent with these findings, PDK-1-catalyzed phosphorylation of PKB/Akt(Thr308) and PKB/Akt activity were suppressed in exposed cells. In addition, PKB/Akt(Ser473) phosphorylation, which is catalyzed by a putative PDK-2, was also suppressed. Notably, expression of constitutively active PKB/Akt restored the normal ISGU pattern in adipocytes treated with either iAs(III) or MAs(III).

CONCLUSIONS

These results suggest that inhibition of the PDK-1/PKB/Akt-mediated transduction step is the key mechanism for the inhibition of ISGU in adipocytes exposed to iAs(III) or MAs(III), and possibly for impaired glucose tolerance associated with human exposures to iAs.

"Actin"g on GLUT4: membrane & cytoskeletal components of insulin action

The dissection of mechanisms that regulate glucose transport by insulin has revealed an intricate network of signaling molecules scattered from the insulin receptor to the intracellular glucose transporter GLUT4. It is also appreciated that some insulin receptor signals jaunt in different directions to regulate events essential for the efficient redistribution of GLUT4 to the plasma membrane. Moreover key assists in the process appear to be arranged by membrane lipids and cytoskeletal proteins. Following current considerations of insulin signals regulating GLUT4, this review will focus on in vitro and in vivo evidence that supports an essential role for phosphoinositides and actin filaments in the control of glucose transport. The discussion will visit recent cell culture, whole animal, and human data highlighting membrane and cytoskeletal aspects of insulin resistance.

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.

Characterization of GLUT4 and calpain expression in healthy human skeletal muscle during fasting and refeeding

AIMS

Calpain-10 and calpain-3 and the diabetes ankyrin repeat protein (DARP) have all been linked to insulin resistance and type 2 diabetes. We set out to measure the expression of these genes in human skeletal muscle and relate them to functional measurements of insulin action during fasting (which induces insulin resistance) and refeeding (which reverses it).

METHODS

Ten healthy male volunteers underwent 48 h of starvation followed by 24 h of high carbohydrate refeeding. On three occasions, before and after starvation and after refeeding, subjects underwent a 16 min insulin tolerance test to quantify insulin sensitivity. Muscle biopsies were obtained before and after fasting and after refeeding for the analysis of calpain-10 and calpain-3, GLUT4 and DARP expression by Western blotting and real-time PCR.

RESULTS

Fasting led to a marked reduction in whole body insulin sensitivity by approx. 45% (P<0.01) and skeletal muscle GLUT4 gene expression by approx. 40% (P<0.05). However, fasting had no effect on calpain-10 and calpain-3 mRNA or protein levels, or DARP mRNA expression. Refeeding only partly restored insulin sensitivity and GLUT4 gene expression to their pre-fast values, but did not effect the expression of calpain-10, calpain-3 or DARP.

CONCLUSIONS

These findings demonstrate that in healthy non-diabetic humans induction of insulin resistance by fasting and its reversal by refeeding with a high CHO diet is mirrored by changes in skeletal muscle GLUT4 but not calpain-10 and calpain-3 expression.

The Crystal Structures of Human Calpains 1 and 9 Imply Diverse Mechanisms of Action and Auto-inhibition

Calpains are calcium activated cysteine proteases found throughout the animal, plant, and fungi kingdoms; 14 isoforms have been described in the human genome. Calpains have been implicated in multiple models of human disease; for instance, calpain 1 is activated in the brains of individuals with Alzheimer's disease, and the digestive tract specific calpain 9 is down-regulated in gastric cancer cell lines. We have solved the structures of human calpain 1 and calpain 9 protease cores using crystallographic methods; both structures have clear implications for the function of non-catalytic domains of full-length calpains in the calcium-mediated activation of the enzyme. The structure of minicalpain 1 is similar to previously solved structures of the protease core. Auto-inhibition in this system is most likely through rearrangements of a central helical/loop region near the active site cysteine, which occlude the substrate binding site. However, the structure of minicalpain 9 indicates that auto-inhibition in this enzyme is mediated through large intra-domain movements that misalign the catalytic triad. This disruption is reminiscent of the full-length inactive calpain conformation. The structures of the highly conserved, ubiquitously expressed human calpain 1 and the more tissue specific human calpain 9 indicate that although there are high levels of sequence conservation throughout the calpain family, isolated structures of family members are insufficient to explain the molecular mechanism of activation for this group of proteins.

Calpain-5 gene variants are associated with diastolic blood pressure and cholesterol levels

Background

Genes implicated in common complex disorders such as obesity, type 2 diabetes mellitus (T2DM) or cardiovascular diseases are not disease specific, since clinically related disorders also share genetic components. Cysteine protease Calpain 10 (CAPN10) has been associated with T2DM, hypertension, hypercholesterolemia, increased body mass index (BMI) and polycystic ovary syndrome (PCOS), a reproductive disorder of women in which isunlin resistance seems to play a pathogenic role. The calpain 5 gene (CAPN5) encodes a protein homologue of CAPN10. CAPN5 has been previously associated with PCOS by our group. In this new study, we have analysed the association of four CAPN5 gene variants(rs948976A>G, rs4945140G>A, rs2233546C>T and rs2233549G>A) with several cardiovascular risk factors related to metabolic syndrome in general population.

Methods

Anthropometric measurements, blood pressure, insulin, glucose and lipid profiles were determined in 606 individuals randomly chosen from a cross-sectional population-based epidemiological survey in the province of Segovia in Central Spain (Castille), recruited to investigate the prevalence of anthropometric and physiological parameters related to obesity and other components of the metabolic syndrome. Genotypes at the four polymorphic loci in CAPN5 gene were detected by polymerase chain reaction (PCR).

Results

Genotype association analysis was significant for BMI (p ≤ 0.041), diastolic blood pressure (p = 0.015) and HDL-cholesterol levels (p = 0.025). Different CAPN5 haplotypes were also associated with diastolic blood pressure (DBP) (0.0005 ≤ p ≤ 0.006) and total cholesterol levels (0.001 ≤ p ≤ 0.029). In addition, the AACA haplotype, over-represented in obese individuals, is also more frequent in individuals with metabolic syndrome defined by ATPIII criteria (p = 0.029).

Conclusion

As its homologue CAPN10, CAPN5 seems to influence traits related to increased risk for cardiovascular diseases. Our results also may suggest CAPN5 as a candidate gene for metabolic syndrome.

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.

Calpain system regulates the differentiation of adult primitive mesenchymal ST-13 adipocytes

The activity of calpain, a calcium-activated protease, is required during the mitotic clonal expansion phase of 3T3-L1 embryonic preadipocyte differentiation. Here we examined the role of calpain in the adipogenesis of ST-13 preadipocytes established from adult primitive mesenchymal cells, which do not require mitotic clonal expansion. After exposure to the calpain inhibitor, N-benzyloxycarbonyl-L-leucyl-L-leucinal or overexpression of calpastatin, a specific endogenous inhibitor of calpain, ST-13 preadipocytes acquired the adipocyte phenotype. Overexpression of calpastatin in ST-13 adipocytes stimulated the expression of adipocyte-specific CCAAT/enhancer-binding protein-alpha (C/EBPalpha), peroxisome proliferator-activated receptor (PPAR)-gamma, sterol regulatory element-binding protein 1, and the insulin signaling molecules, insulin receptor alpha, insulin-receptor substrates, and GLUT4. However, insulin-stimulated glucose uptake was reduced by approximately 52%. The addition of calpain to the nuclear fraction of ST-13 adipocytes resulted in the Ca(2+)-dependent degradation of PPARgamma and C/EBPalpha but not sterol regulatory element-binding protein 1. Exposing ST-13 adipocytes to A23187 also led to losses of endogenous PPARgamma and C/EBPalpha. Under both conditions, calpain inhibitors almost completely prevented C/EBPalpha cleavage but partially blocked the decrease of PPARgamma. Two ubiquitous forms of calpain, mu- and m-calpain, localized to the cytosol and the nucleus, whereas the activated form of mu- but not m-calpain was found in the nucleus. Finally, stable dominant-negative mu-calpain transfectants showed accelerated adipogenesis and increase in the levels of PPARgamma and C/EBPalpha during adipocyte program. These results support evidence that the calpain system is involved in regulating the differentiation of adult primitive mesenchymal ST-13 preadipocytes.