Autocrine insulin action activates Akt and increases survival of isolated human islets

Exosomes derived from human mesenchymal stem cells preserve mouse islet survival and insulin secretion function

Islet cell death and loss of function after isolation and before transplantation is considered a key barrier to successful islet transplantation outcomes. Mesenchymal stem cells (MSCs) have been used to protect isolated islets owing to their paracrine potential partially through the secretion of vascular endothelial growth factor (VEGF). The paracrine functions of MSCs are also mediated, at least in part, by the release of extracellular vesicles including exosomes. In the present study, we examined (i) the effect of exosomes from human MSCs on the survival and function of isolated mouse islets and (ii) whether exosomes contain VEGF and the potential impact of exosomal VEGF on the survival of mouse islets. Isolated mouse islets were cultured for three days with MSC-derived exosomes (MSC-Exo), MSCs, or MSC-conditioned media without exosomes (MSC-CM-without-Exo). We investigated the effects of the exosomes, MSCs, and conditioned media on islet viability, apoptosis and function. Besides the expression of apoptotic and pro-survival genes, the production of human and mouse VEGF proteins was evaluated. The MSCs and MSC-Exo, but not the MSC-CM-without-Exo, significantly decreased the percentage of apoptotic cells and increased islet viability following the downregulation of pro-apoptotic genes and the upregulation of pro-survival factors, as well as the promotion of insulin secretion. Human VEGF was observed in the isolated exosomes, and the gene expression and protein production of mouse VEGF significantly increased in islets cultured with MSC-Exo. MSC-derived exosomes are as efficient as parent MSCs for mitigating cell death and improving islet survival and function. This cytoprotective effect was probably mediated by VEGF transfer, suggesting a pivotal strategy for ameliorating islet transplantation outcomes.

Amniotic Membrane Extract Protects Islets From Serum-Deprivation Induced Impairments and Improves Islet Transplantation Outcome

Islet culture prior to transplantation is a standard practice in many transplantation centers. Nevertheless, the abundant islet mass loss and function impairment during this serum-deprivation culture period restrain the success of islet transplantation. In the present study, we used a natural biomaterial derived product, amniotic membrane extract (AME), as medium supplementation of islet pretransplant cultivation to investigate its protective effect on islet survival and function and its underlying mechanisms, as well as the engraftment outcome of islets following AME treatment. Results showed that AME supplementation improved islet viability and function, and decreased islet apoptosis and islet loss during serum-deprived culture. This was associated with the increased phosphorylation of PI3K/Akt and MAPK/ERK signaling pathway. Moreover, transplantation of serum-deprivation stressed islets that were pre-treated with AME into diabetic mice revealed better blood glucose control and improved islet graft survival. In conclusion, AME could improve islet survival and function in vivo and in vitro, and was at least partially through increasing phosphorylation of PI3K/Akt and MAPK/ERK signaling pathway.

Long-term c-Kit overexpression in beta cells compromises their function in ageing mice

AIMS/HYPOTHESIS

c-Kit signalling regulates intracellular pathways that enhance beta cell proliferation, insulin secretion and islet vascularisation in mice up to 28 weeks of age and on short-term high-fat diet. However, long-term c-Kit activation in ageing mouse islets has yet to be examined. This study utilises beta cell-specific c-Kit-overexpressing transgenic (c-KitβTg) ageing mice (~60 weeks) to determine the effect of its activation on beta cell dysfunction and insulin secretion.

METHODS

Wild-type and c-KitβTg mice, aged 60 weeks, were examined using metabolic tests to determine glucose tolerance and insulin secretion. Pancreas histology and proteins in isolated islets were examined to determine the expression of beta cell transcription factors, proliferation and intracellular signalling. To determine the role of insulin receptor signalling in ageing c-KitβTg mice, we generated beta cell-specific inducible insulin receptor knockout in ageing c-KitβTg mice (c-KitβTg;βIRKO mice) and examined the ageing mice for glucose tolerance and islet histology.

RESULTS

Ageing c-KitβTg mice progressively developed glucose intolerance, compared with age-matched wild-type littermates, due to impaired insulin secretion. Increased beta cell mass, proliferation and nuclear forkhead box transcription factor O1 (FOXO1) expression and reduced exocytotic protein levels were detected in ageing c-KitβTg mouse islets. Protein analyses of isolated islets showed increased insulin receptor, phosphorylated IRS-1^Ser612 and cleaved poly(ADP-ribose) polymerase levels in ageing c-KitβTg mice. Ageing c-KitβTg mouse islets treated ex vivo with insulin demonstrated reduced Akt phosphorylation, indicating that prolonged c-Kit induced beta cell insulin insensitivity. Ageing c-KitβTg;βIRKO mice displayed improved glucose tolerance and beta cell function compared with ageing c-KitβTg mice.

CONCLUSIONS/INTERPRETATION

These findings indicate that long-term c-Kit overexpression in beta cells has a negative impact on insulin exocytosis and that temporally dependent regulation of c-Kit-insulin receptor signalling is important for optimal beta cell function.

Prolonged Exposure to Insulin Inactivates Akt and Erk1/2 and Increases Pancreatic Islet and INS1E β-Cell Apoptosis

Chronic hyperinsulinemia, in vivo, increases the resistance of peripheral tissues to insulin by desensitizing insulin signaling. Insulin, in a heterologous manner, can also cause IGF-1 resistance. The aim of the current study was to investigate whether insulin-mediated insulin and IGF-1 resistance develops in pancreatic β-cells and whether this resistance results in β-cell decompensation. Chronic exposure of rat islets or INS1E β-cells to increasing concentrations of insulin decreased Akt^S473 phosphorylation in response to subsequent acute stimulation with 10 nM insulin or IGF-1. Prolonged exposure to high insulin levels not only inhibited Akt^S473 phosphorylation, but it also resulted in a significant inhibition of the phosphorylation of P70S6 kinase and Erk_1/2 phosphorylation in response to the acute stimulation by glucose, insulin, or IGF-1. Decreased activation of Akt, P70S6K, and Erk_1/2 was associated with decreased insulin receptor substrate 2 tyrosine phosphorylation and insulin receptor β-subunit abundance; neither IGF receptor β-subunit content nor its phosphorylation were affected. These signaling impairments were associated with decreased SERCA2 expression, perturbed plasma membrane calcium current and intracellular calcium handling, increased endoplasmic reticulum stress markers such as eIF2α^S51 phosphorylation and Bip (GRP78) expression, and increased islet and β-cell apoptosis. We demonstrate that prolonged exposure to high insulin levels induces not only insulin resistance, but in a heterologous manner causes resistance to IGF-1 in rat islets and insulinoma cells resulting in decreased cell survival. These findings suggest the possibility that chronic exposure to hyperinsulinemia may negatively affect β-cell mass by increasing β-cell apoptosis.

β-Cell Receptor Tyrosine Kinases in Controlling Insulin Secretion and Exocytotic Machinery: c-Kit and Insulin Receptor

Insulin secretion from pancreatic β-cells is initiated through channel-mediated depolarization, cytoskeletal remodeling, and vesicle tethering at the cell membrane, all of which can be regulated through cell surface receptors. Receptor tyrosine kinases (RTKs) promote β-cell development and postnatal signaling to improve β-cell mass and function, yet their activation has also been shown to initiate exocytotic events in β-cells. This review examines the role of RTK signaling in insulin secretion, with a focus on RTKs c-Kit and insulin receptor (IR). Pathways that control insulin release and the potential interplay between c-Kit and IR signaling are discussed, along with clinical implications of RTK therapy on insulin secretion.

The pseudophosphatase phogrin enables glucose-stimulated insulin signaling in pancreatic β cells

Autocrine insulin signaling is critical for pancreatic β-cell growth and activity and is at least partially controlled by protein-tyrosine phosphatases (PTPs) that act on insulin receptors (IRs). The receptor-type PTP phogrin primarily localizes on insulin secretory granules in pancreatic β cells. We recently reported that phogrin knockdown decreases the protein levels of insulin receptor substrate 2 (IRS2), whereas high-glucose stimulation promotes formation of a phogrin-IR complex that stabilizes IRS2. However, the underlying molecular mechanisms by which phogrin affects IRS2 levels are unclear. Here, we found that relative to wildtype mice, IRS2 levels in phogrin-knockout mice islets decreased by 44%. When phogrin was silenced by shRNA in pancreatic β-cell lines, glucose-induced insulin signaling led to proteasomal degradation of IRS2 via a negative feedback mechanism. Phogrin overexpression in a murine hepatocyte cell line consistently prevented chronic insulin treatment-induced IRS2 degradation. In vitro, phogrin directly bound the IR without the assistance of other proteins and protected recombinant PTP1B from oxidation to potentiate its activity toward the IR. Furthermore, phogrin expression suppressed insulin-induced local generation of hydrogen peroxide and subsequent PTP1B oxidation, which allowed progression of IR dephosphorylation. Together, these results suggest that a transient interaction of phogrin with the IR enables glucose-stimulated autocrine insulin signaling through the regulation of PTP1B activity, which is essential for suppressing feedback-mediated IRS2 degradation in pancreatic β cells.

Loss of TRB3 alters dynamics of MLK3-JNK signaling and inhibits cytokine-activated pancreatic beta cell death

Disabling cellular defense mechanisms is essential for induction of apoptosis. We have previously shown that cytokine-mediated activation of the MAP3K MLK3 stabilizes TRB3 protein levels to inhibit AKT and compromise beta cell survival. Here, we show that genetic deletion of TRB3 results in basal activation of AKT, preserves mitochondrial integrity, and confers resistance against cytokine-induced pancreatic beta cell death. Mechanistically, we find that TRB3 stabilizes MLK3, most likely by suppressing AKT-directed phosphorylation, ubiquitination, and proteasomal degradation of MLK3. Accordingly, TRB3(-/-) islets show a decrease in both the amplitude and duration of cytokine-stimulated MLK3 induction and JNK activation. It is well known that JNK signaling is facilitated by a feed forward loop of sequential kinase phosphorylation and is reinforced by a mutual stabilization of the module components. The failure of TRB3(-/-) islets to mount an optimal JNK activation response, coupled with the ability of TRB3 to engage and maintain steady state levels of MLK3, recasts TRB3 as an integral functional component of the JNK module in pancreatic beta cells.

Loss of TRB3 alters dynamics of MLK3-JNK signaling and inhibits cytokine-activated pancreatic beta cell death

Disabling cellular defense mechanisms is essential for induction of apoptosis. We have previously shown that cytokine-mediated activation of the MAP3K MLK3 stabilizes TRB3 protein levels to inhibit AKT and compromise beta cell survival. Here, we show that genetic deletion of TRB3 results in basal activation of AKT, preserves mitochondrial integrity, and confers resistance against cytokine-induced pancreatic beta cell death. Mechanistically, we find that TRB3 stabilizes MLK3, most likely by suppressing AKT-directed phosphorylation, ubiquitination, and proteasomal degradation of MLK3. Accordingly, TRB3(-/-) islets show a decrease in both the amplitude and duration of cytokine-stimulated MLK3 induction and JNK activation. It is well known that JNK signaling is facilitated by a feed forward loop of sequential kinase phosphorylation and is reinforced by a mutual stabilization of the module components. The failure of TRB3(-/-) islets to mount an optimal JNK activation response, coupled with the ability of TRB3 to engage and maintain steady state levels of MLK3, recasts TRB3 as an integral functional component of the JNK module in pancreatic beta cells.

TGF-β superfamily member Nodal stimulates human β-cell proliferation while maintaining cellular viability

In an effort to expand human islets and enhance allogeneic islet transplant for the treatment of type 1 diabetes, identifying signaling pathways that stimulate human β-cell proliferation is paramount. TGF-β superfamily members, in particular activin-A, are likely involved in islet development and may contribute to β-cell proliferation. Nodal, another TGF-β member, is present in both embryonic and adult rodent islets. Nodal, along with its coreceptor, Cripto, are pro-proliferative factors in certain cell types. Although Nodal stimulates apoptosis of rat insulinoma cells (INS-1), Nodal and Cripto signaling have not been studied in the context of human islets. The current study investigated the effects of Nodal and Cripto on human β-cell proliferation, differentiation, and viability. In the human pancreas and isolated human islets, we observed Nodal mRNA and protein expression, with protein expression observed in β and α-cells. Cripto expression was absent from human islets. Furthermore, in cultured human islets, exogenous Nodal stimulated modest β-cell proliferation and inhibited α-cell proliferation with no effect on cellular viability, apoptosis, or differentiation. Nodal stimulated the phosphorylation of mothers against decapentaplegic (SMAD)-2, with no effect on AKT or MAPK signaling, suggesting phosphorylated SMAD signaling was involved in β-cell proliferation. Cripto had no effect on human islet cell proliferation, differentiation, or viability. In conclusion, Nodal stimulates human β-cell proliferation while maintaining cellular viability. Nodal signaling warrants further exploration to better understand and enhance human β-cell proliferative capacity.

Glucose regulation of a cell cycle gene module is selectively lost in mouse pancreatic islets during ageing

AIMS/HYPOTHESIS

Transcriptional networks in beta cells are modulated by extracellular signals such as glucose, thereby ensuring beta cell adaptation to systemic insulin demands. Ageing is a main risk factor for type 2 diabetes and has been associated with perturbed expression of genes essential for beta cell function. We aimed to uncover glucose-dependent gene modules in mouse pancreatic islets and investigate how this regulation is affected by ageing.

METHODS

Global gene expression was assessed in pancreatic islets from young and aged wild-type and Cdkn2a (Ink4a/Arf)-deficient mice exposed to different glucose concentrations. Gene modules were identified by gene ontology and gene set enrichment analysis.

RESULTS

Gene expression profiling revealed that variations in glucose levels have a widespread and highly dynamic impact on the islet transcriptome. Stimulatory glucose levels induced the expression of highly beta cell-selective genes and repressed the expression of ubiquitous genes involved in stress and antiproliferative responses, and in organelle biogenesis. Interestingly, a module comprising cell cycle genes was significantly induced between non-stimulatory and stimulatory glucose concentrations. Unexpectedly, glucose regulation of gene expression was broadly maintained in islets from old mice. However, glucose induction of mitotic genes was selectively lost in aged islets and was not even restored in the absence of the cell cycle inhibitors p16(INK4a) and p19(ARF), which have been implicated in the restricted proliferative capacity of beta cells with advanced age.

CONCLUSIONS/INTERPRETATION

Glucose-dependent transcriptional networks in islets are globally conserved during ageing, with the exception of the ability of stimulatory glucose levels to induce a cell cycle gene module.

Three-dimensional scaffolds reduce islet amyloid formation and enhance survival and function of cultured human islets

Islet transplantation provides a promising approach for treatment of type 1 diabetes mellitus. Amyloid formation and loss of extracellular matrix are two nonimmune factors contributing to death of isolated human islets. We tested the effects of two types of three-dimensional scaffolds, collagen matrix (CM) and fibroblast-populated collagen matrix (FPCM), on amyloid formation, viability, and function of isolated islets. Islets from cadaveric donors were cultured in FPCM, CM, or two-dimensional plate (2D) for 7 days. After 7 days, compared with the 2D culture condition, CM and FPCM markedly reduced amyloid formation of cultured islets and decreased apoptotic β-cell rate by ∼75%. IL-1β and Fas levels were also reduced in scaffold-embedded islets. Furthermore, β/α cell ratios were increased by ∼18% and ∼36% in CM- and FPCM-embedded islets, respectively. Insulin content and insulin response to elevated glucose were also enhanced by both three-dimensional scaffolds. Moreover, culture in CM and FPCM (but not 2D) preserved insulin, GLUT-2, and PDX-1 mRNA expression. FPCM-embedded islets had significantly higher insulin response and lower amyloid formation than CM-embedded islets. These findings suggest that three-dimensional scaffolds reduce amyloid formation and improve viability and function of human islets in vitro, and that CM and fibroblasts have additive effects in enhancing islet function and reducing amyloid formation. Using this strategy is likely to improve outcome in human islet transplantation.

Systematic review and meta-analysis: islet autotransplantation after pancreatectomy for minimizing diabetes

OBJECTIVE

Islet autotransplantation (IAT) may decrease the morbidity and mortality of postpancreatectomy diabetes mellitus. The current systematic review and meta-analysis examined the rate of insulin independence (II) and mortality after IAT post-total (TP) or partial pancreatectomy (PP).

METHODS

Ovid MEDLINE, EMBASE, Web of Science, SCOPUS and reference lists were searched until 31 January 2011. Eligible studies enrolled adult patients with IAT post-TP or PP, regardless of study design, sample size and language. Two investigators identified eligible studies and extracted data independently. From each study, 95% confidence intervals (CIs) were estimated and pooled using random effects meta-analysis.

RESULTS

Fifteen observational studies were eligible (11 IAT post-TP, two post-PP and two including both). The II rates for IAT post-TP at last follow-up and transiently during the study were 4·62 per 100 person-years (95% CI: 1·53-7·72) and 8·34 per 100 person-years (95% CI: 3·32-13·37), respectively. In the later group, patients achieved transient II lasting 15·57 months (95% CI: 10·35-20·79). The II rate at last follow-up for IAT post-PP was 24·28 per 100 person-years (95% CI: 0·00-48·96). Whereas the 30-day mortality for IAT post-TP and post-PP was 5% (95% CI: 2-10%) and 0, respectively, the long-term mortality was 1·38 per 100 person-years (95% CI: 0·66-2·11) and 0·70 per 100 person-years (95% CI: 0·00-1·80) respectively.

CONCLUSIONS

IAT postpancreatectomy offers some patients a chance for insulin independence. Better data reporting are essential to establish the risks and benefits of IAT after pancreatic surgery.

Epidermal growth factor induces adult human islet cell dedifferentiation

Given the inherent therapeutic potential of the morphogenetic plasticity of adult human islets, the identification of factors controlling their cellular differentiation is of interest. The epidermal growth factor (EGF) family has been identified previously in the context of pancreatic organogenesis. We examined the role of EGF in an in vitro model whereby adult human islets are embedded in a collagen gel and dedifferentiated into duct-like epithelial structures (DLS). We demonstrated that DLS formation was EGF dependent, while residual DLS formation in the absence of added EGF was abrogated by EGF receptor inhibitor treatment. With respect to signaling, EGF administration led to an increase in c-Jun NH2-terminal kinase (JNK) phosphorylation early in DLS formation and in AKT and extracellular signal-regulated kinase (ERK) phosphorylation late in the process of DLS formation, concomitant with the increased proliferation of dedifferentiated cells. In the absence of EGF, these phosphorylation changes are not seen and the typical increase in DLS epithelial cell proliferation seen after 10 days in culture is attenuated. Thus, in our model, EGF is necessary for islet cell dedifferentiation, playing an important role in both the onset of DLS formation (through JNK) and in the proliferation of these dedifferentiated cells (through AKT and ERK).

Calcium/calmodulin-dependent kinase IV controls glucose-induced Irs2 expression in mouse beta cells via activation of cAMP response element-binding protein

AIMS/HYPOTHESIS

Irs2, which is upregulated by glucose, is important for beta cell plasticity. Cyclic AMP response element-binding protein (CREB) stimulates beta cell Irs2 expression and is a major calcium/calmodulin-dependent kinase (CaMK)(IV) target in neurons. We therefore hypothesised that CaMK(IV) mediates glucose-induced Irs2 expression in beta cells via CREB activation.

METHODS

The functions of CaMK(IV) and CREB were investigated in MIN6 beta cells and mouse islets using the CaMK inhibitor KN62, the calcium chelator bapta-(AM) and the voltage-dependent calcium channel inhibitor nifedipine. Small interfering RNAs were used to silence endogenous CaMK(IV) production and expression vectors to overproduce constitutively active and dominant negative forms of CaMK(IV) and CREB. Irs1 and Irs2 expression were determined by quantitative PCR and Western blotting, and the role of CREB was also investigated by assessing its phosphorylation on serine 133.

RESULTS

Increasing the glucose concentration from 2.5 to 25 mmol/l stimulated CREB phosphorylation on serine 133 and specifically stimulated Irs2 but not Irs1 expression. Similarly, overproduction of a constitutively active form of CaMK(IV) promoted sustained CREB phosphorylation and a significant increase in Irs2 but not Irs1 expression. In contrast, these stimulatory effects of glucose were all suppressed by overproducing an inactive CaMK(IV) mutant. Inhibition of glucose-induced calcium influx with nifedipine or chelation of intracellular calcium with bapta-(AM), as well as silencing of CaMK(IV) or inhibition of its activity with KN62 resulted in similar observations. Finally, overproduction of a dominant negative form of CREB completely suppressed glucose and CaMK(IV) stimulation of Irs2 expression.

CONCLUSIONS/INTERPRETATION

Our results suggest that the Ca(2+)/CaMK(IV)/CREB cascade plays a critical role in the regulation of Irs2 expression in beta cells.

Insulin increases H2O2-induced pancreatic beta cell death

Insulin resistance results, in part, from impaired insulin signaling in insulin target tissues. Consequently, increased levels of insulin are necessary to control plasma glucose levels. The effects of elevated insulin levels on pancreatic beta (β) cell function, however, are unclear. In this study, we investigated the possibility that insulin may influence survival of pancreatic β cells. Studies were conducted on RINm, RINm5F and Min-6 pancreatic β-cells. Cell death was induced by treatment with H(2)O(2), and was estimated by measurements of LDH levels, viability assay (Cell-Titer Blue), propidium iodide staining and FACS analysis, and mitochondrial membrane potential (JC-1). In addition, levels of cleaved caspase-3 and caspase activity were determined. Treatment with H(2)O(2) increased cell death; this effect was increased by simultaneous treatment of cells with insulin. Insulin treatment alone caused a slight increase in cell death. Inhibition of caspase-3 reduced the effect of insulin to increase H(2)O(2)-induced cell death. Insulin increased ROS production by pancreatic β cells and increased the effect of H(2)O(2). These effects were increased by inhibition of IR signaling, indicative of an effect independent of the IR cascade. We conclude that elevated levels of insulin may act to exacerbate cell death induced by H(2)O(2) and, perhaps, other inducers of apoptosis.

Sex steroids effects on the endocrine pancreas

The endocrine pancreas is central in the physiopathology of diabetes mellitus. Nutrients and hormones control endocrine pancreatic function and the secretion of insulin and other pancreatic islet hormones. Although the pancreas is not usually considered as a target of steroids, increasing evidence indicates that sex steroid hormones modify pancreatic islet function. The biological effects of steroid hormones are transduced by both, classical and non-classical steroid receptors that in turn produce slow genomic and rapid non-genomic responses. In this review, we focused on the effects of sex steroid hormones on endocrine pancreatic function, with special emphasis in animal studies.

Upgrading pretransplant human islet culture technology requires human serum combined with media renewal

BACKGROUND.: The original Edmonton protocol used fresh islets, but for obvious logistic advantages most transplant centers have implemented pretransplant culture in human albumin. The aim of this study was to improve current pretransplant human islet culture techniques. METHODS.: Clinical-grade purified human islets from a total of 24 donors were directly resuspended after isolation in CMRL 1066-based media at 37 degrees C, and media additions and renewal were tested. At days 1 and 5 of culture, in vitro quality controls included islet viability, insulin content and function, apoptosis, and in vivo islet potency assay in nude mice. RESULTS.: Replacing human albumin with human AB serum improved 1- and 5-day preservation of islet function and viability which was further enhanced with antioxidant Stem Ease, leading to the iCulture medium (enriched CMRL: pyruvate, zinc sulfate, insulin, transferrin, selenium, 2.5% human AB serum and Stem Ease). Major damage occurs in the first day of culture and frequent media renewal (25% vol/hr) in this period further improved viability, apoptosis, islet recovery, and function in vitro and in vivo, compared with only changing medium after overnight culture. CONCLUSIONS.: The described human islet culture technique (iCulture medium+renewal) seems to be the best choice for clinical human islet culture when short (1 day) or long (5 days) periods are used. Media choice and dilution play a major role in the function and survival of human islets in culture.

Trophic molecules derived from human mesenchymal stem cells enhance survival, function, and angiogenesis of isolated islets after transplantation

BACKGROUND

Mesenchymal stem cells (MSCs), also known as multipotent progenitor cells, release several factors that support cell survival and enhance wound healing. We hypothesized that MSC-secreted molecules would induce a trophic effect in pancreatic islet culture conditions.

METHODS

Pancreatic islets were co-cultured with MSCs, and ADP/ATP ratios, glucose stimulated insulin secretion (GSIS), and DNA fragmentation were evaluated to measure islet quality and viability in vitro. The induction of signal molecules related to the control of survival, function, and angiogenesis was also analyzed. Cell quality assays, DNA fragmentation assays, and islet transplantation into streptozotocin-induced diabetic mice were performed using MSC-conditioned medium (CM)-cultured islets. Furthermore, we identified soluble molecules within MSC-CM.

RESULTS

Islets co-cultured with MSCs demonstrated lower ADP/ATP ratios, and higher GSIS indexes and viability. Furthermore, co-cultured islets revealed higher levels of anti-apoptotic signal molecules (X-linked inhibitor of apoptosis protein, Bcl-xL, Bcl-2, and heat shock protein-32) and demonstrated increased vascular endothelial growth factor receptor 2 and Tie-2 mRNA expression and increased levels of phosphorylated Tie-2 and focal adhesion kinase protein. Islets cultured in MSC-CM demonstrated lower ADP/ATP ratios, less apoptosis, and a higher GSIS indexes. Diabetic mice that received islet transplants (200 islet equivalent) cultured in MSC-CM for 48 hr demonstrated significantly lower blood glucose levels and enhanced blood vessel formation. In addition, interleukin-6, interleukin-8, vascular endothelial growth factor-A, hepatocyte growth factor, and transforming growth factor-beta were detected at significant levels in MSC-CM.

CONCLUSIONS

These results suggest that the trophic factors secreted by human MSCs enhance islet survival and function after transplantation.

NDRG2 is highly expressed in pancreatic beta cells and involved in protection against lipotoxicity

The N-myc downstream-regulated gene 2 (NDRG2) is involved in cell differentiation and apoptosis, but its function in the pancreas remains to be established. Herein we examine the expression and function of NDRG2 in the endocrine pancreas. NDRG2 immunoreactivity was localized mainly in the cytoplasm of pancreatic beta cells. When beta-TC3 cells were exposed chronically to high levels of free fatty acid (FFA), cell viability was impaired, and Akt and NDRG2 phosphorylation were reduced. NDRG2 is a potential substrate of protein kinase Akt. Overexpression of constitutively active Akt enhanced NDRG2 phosphorylation and abolished the apoptosis induced by FFA in beta-TC3 cells, whereas NDRG2 knock-down attenuated Akt-mediated protection of beta cells against fatty acid-triggered apoptosis. Collectively, these data indicate that NDRG2 acts as a key molecule in pancreatic beta cells and is involved in the Akt-mediated protection of beta cells against lipotoxicity.

Proinflammatory cytokines activate the intrinsic apoptotic pathway in beta-cells

OBJECTIVE

Proinflammatory cytokines are cytotoxic to beta-cells and have been implicated in the pathogenesis of type 1 diabetes and islet graft failure. The importance of the intrinsic mitochondrial apoptotic pathway in cytokine-induced beta-cell death is unclear. Here, cytokine activation of the intrinsic apoptotic pathway and the role of the two proapoptotic Bcl-2 proteins, Bad and Bax, were examined in beta-cells.

RESEARCH DESIGN AND METHODS

Human and rat islets and INS-1 cells were exposed to a combination of proinflammatory cytokines (interleukin-1beta, interferon-gamma, and/or tumor necrosis factor-alpha). Activation of Bad was determined by Ser136 dephosphorylation, mitochondrial stress by changes in mitochondrial metabolic activity and cytochrome c release, downstream apoptotic signaling by activation of caspase-9 and -3, and DNA fragmentation. The inhibitors FK506 and V5 were used to investigate the role of Bad and Bax activation, respectively.

RESULTS

We found that proinflammatory cytokines induced calcineurin-dependent dephosphorylation of Bad Ser136, mitochondrial stress, cytochrome c release, activation of caspase-9 and -3, and DNA fragmentation. Inhibition of Bad Ser136 dephosphorylation or Bax was found to inhibit cytokine-induced intrinsic proapoptotic signaling.

CONCLUSIONS

Our findings demonstrate that the intrinsic mitochondrial apoptotic pathway contributes significantly to cytokine-induced beta-cell death and suggest a functional role of calcineurin-mediated Bad Ser136 dephosphorylation and Bax activity in cytokine-induced apoptosis.

Serum albumin protects from cytokine-induced pancreatic beta cell death by a phosphoinositide 3-kinase-dependent mechanism

The present study was undertaken to investigate the biological activity of serum albumin when pancreatic beta cells were challenged by cytokines and pro-apoptotic reactive oxygen species like H(2)O(2). Culture of mouse islets or INS-1E beta cells for 24 h in the presence of H(2)O(2) (25 micromol/l) increased cell death. This demise was prevented by serum albumin, dependent on its free sulfhydryl group, emphasizing that albumin may scavenge H(2)O(2) due to its antioxidant properties. Culture for 48 h with a cytokine mixture of IL-1beta (160 pg/ml), IFN-gamma (200 ng/ml), and TNF-alpha (2 ng/ml) revealed that albumin, also protected against cytokine-induced death of both mouse islets and INS-1E beta cells. This protective effect against cytokine-induced beta cell death was, however, not dependent on albumins free sulfhydryl group, but was inhibited by the phosphoinositide 3-kinase (PI3K) inhibitors LY294002 (25 micromol/l) and wortmannin (1 micromol/l), suggesting that albumin may rescue beta cells from cytokine-induced cell death by activation of PI3K. In accordance, albumin stimulated phosphorylation of Akt, a down-stream target for PI3K. In conclusion, it is suggested that albumin may be a survival factor for pancreatic beta cells through scavenging of reactive oxygen species and by PI3K-dependent activation of Akt.

Islet auto transplantation following total pancreatectomy: a long-term assessment of graft function

Total pancreatectomy is considered the final resort in the treatment of chronic pancreatitis; however, here we show that simultaneous islet autotransplantation can abrogate the onset of diabetes.

METHODS

: In Leicester, 46 patients have now undergone total pancreatectomy with immediate islet auto transplant, and they have received a median of 2246 islet equivalent (IEQ)/kg body weight (range, 405-20,385 IEQ/kg body weight).

RESULTS

: Twelve patients have shown periods of insulin independence, for a median of 16.5 months (range, 2-63 months), and 5 remain insulin independent. Over the 10 years of follow-up, there has been a notable increase in insulin requirement per kilogram per day, and percentage of glycosylated hemoglobin levels have increased significantly (r = 0.66, P = 0.01). However, 100% of patients tested were C-peptide positive at their most recent assessment, and high fasting and stimulated C-peptide values recorded at 10 years after transplantation, 1.44 (range, 1.09-1.8 ng/mL) and 2.86 ng/mL (range, 1.19-4.53 ng/mL), respectively, suggest significant graft function in the long term. In addition, median serum creatinine has increased very little after the operation (71 nmol/L [range, 49-125 nmol/L] atpreoperation vs 76.5 nmol/L [range 72-81 nmol/L] at year 10), suggesting no diabetic nephropathy.

CONCLUSIONS

: Although there is a notable decline in islet function after islet auto transplant, there is still evidence of significant long-term insulin secretion and possible protection against diabetic complications.

FoxO proteins in pancreatic β-cells as potential therapeutic targets in diabetes

Diabetes results from complete (Type 1) or progressive (Type 2) insulin insufficiency. Resulting chronic and acute hyperglycemia are thus prevented mainly by insulin injections, a therapy that is care intensive, costly and does not abolish vascular damage, with severe consequences for the patient in the long term. In view of the epidemic spread of the disease, diabetes is considered a major threat for public healthcare systems. Thus, there is a great incentive to find therapies and drugs preserving or restoring pancreatic β-cells mass and function. In this context, this review addresses the FoxO transcription factors as direct or indirect, in vivo or ex vivo drug targets, since FoxO proteins play a central role for β-cells growth and resistance to oxidative stress. The review includes specific proposals for preclinical drug development.

Transcription factor 7-like 2 regulates beta-cell survival and function in human pancreatic islets

OBJECTIVE

Type 2 diabetes is characterized by impaired insulin secretion in response to increased metabolic demand. This defect in beta-cell compensation seems to result from the interplay between environmental factors and genetic predisposition. Genome-wide association studies reveal that common variants in transcription factor 7-like 2 (TCF7L2) are associated with increased risk of type 2 diabetes. The aim of the present study was to establish whether TCF7L2 plays a role in beta-cell function and/or survival.

RESEARCH DESIGN AND METHODS

To investigate the effects of TCFL7L2 depletion, isolated islets were exposed to TCF7L2 small interfering RNA (siRNA) versus scrambled siRNA, and beta-cell survival and function were examined. For TCF7L2 overexpression, islets were cultured in glucose concentrations of 5.5-33.3 mmol/l and the cytokine mix interleukin-1 beta/gamma-interferon with or without overexpression of TCF7L2. Subsequently, glucose-stimulated insulin secretion (GSIS), beta-cell apoptosis [by transferase-mediated dUTP nick-end labeling assay and Western blotting for poly(ADP-ribose) polymerase and Caspase-3 cleavage], and beta-cell proliferation (by Ki67 immunostaining) were analyzed.

RESULTS

Depleting TCF7L2 by siRNA resulted in a 5.1-fold increase in beta-cell apoptosis, 2.2-fold decrease in beta-cell proliferation (P < 0.001), and 2.6-fold decrease in GSIS (P < 0.01) in human islets. Similarly, loss of TCF7L2 resulted in impaired beta-cell function in mouse islets. In contrast, overexpression of TCF7L2 protected islets from glucose and cytokine-induced apoptosis and impaired function.

CONCLUSIONS

TCF7L2 is required for maintaining GSIS and beta-cell survival. Changes in the level of active TCF7L2 in beta-cells from carriers of at-risk allele may be the reason for defective insulin secretion and progression of type 2 diabetes.

Inhibition of c-jun N terminal kinase (JNK) improves functional beta cell mass in human islets and leads to AKT and glycogen synthase kinase-3 (GSK-3) phosphorylation

AIMS/HYPOTHESIS

Activation of c-jun N-terminal kinase (JNK) has been described in islet isolation and engraftment, making JNK a key target in islet transplantation. The objective of this study was to investigate if JNK inhibition with a cell-permeable TAT peptide inhibitor (L-JNKI) protects functional beta cell mass in human islets and affects AKT and its substrates in islet cells.

METHODS

The effect of L-JNKI (10 micromol/l) on islet count, mitochondrial membrane potential, glucose-stimulated insulin release and phosphorylation of both AKT and its substrates, as well as on reversal of diabetes in immunodeficient diabetic Nu/Nu mice was studied.

RESULTS

In vitro, L-JNKI reduced the islet loss in culture and protected from cell death caused by acute cytokine exposure. In vivo, treatment of freshly isolated human islets and diabetic Nu/Nu mice recipients of such islets resulted in improved functional beta cell mass. We showed that L-JNKI activates AKT and downregulates glycogen synthase kinase-3 beta (GSK-3B) in human islets exposed to cytokines, while other AKT substrates were unaffected, suggesting that a specific AKT/GSK-3B regulation by L-JNKI may represent one of its mechanisms of cytoprotection.

CONCLUSIONS/INTERPRETATION

In conclusion, we have demonstrated that targeting JNK in human pancreatic islets results in improved functional beta cell mass and in the regulation of AKT/GSK3B activity.

Inhibition of Raf-1 alters multiple downstream pathways to induce pancreatic beta-cell apoptosis

The serine threonine kinase Raf-1 plays a protective role in many cell types, but its function in pancreatic beta-cells has not been elucidated. In the present study, we examined whether primary beta-cells possess Raf-1 and tested the hypothesis that Raf-1 is critical for beta-cell survival. Using reverse transcriptase-PCR, Western blot, and immunofluorescence, we identified Raf-1 in human islets, mouse islets, and in the MIN6 beta-cell line. Blocking Raf-1 activity using a specific Raf-1 inhibitor or dominant-negative Raf-1 mutants led to a time- and dose-dependent increase in cell death, assessed by real-time imaging of propidium iodide incorporation, TUNEL, PCR-enhanced DNA laddering, and Caspase-3 cleavage. Although the rapid increase in apoptotic cell death was associated with decreased Erk phosphorylation, studies with two Mek inhibitors suggested that the classical Erk-dependent pathway could explain only part of the cell death observed after inhibition of Raf-1. An alternative Erk-independent pathway downstream of Raf-1 kinase involving the pro-apoptotic protein Bad has recently been characterized in other tissues. Inhibiting Raf-1 in beta-cells led to a striking loss of Bad phosphorylation at serine 112 and an increase in the protein levels of both Bad and Bax. Together, our data strongly suggest that Raf-1 signaling plays an important role regulating beta-cell survival, via both Erk-dependent and Bad-dependent mechanisms. Conversely, acutely inhibiting phosphatidylinositol 3-kinase Akt had more modest effects on beta-cell death. These studies identify Raf-1 as a critical anti-apoptotic kinase in pancreatic beta-cells and contribute to our understanding of survival signaling in this cell type.

Nitric oxide mediates the survival action of IGF-1 and insulin in pancreatic beta cells

Generation of low levels of nitric oxide (NO) contributes to beta cell survival in vitro. The purpose of this study was to explore the link between NO and the survival pathway triggered by insulin-like growth factor-1 (IGF-1) and insulin in insulin producing RINm5F cells and in pancreatic islets. Results show that exposure of cells to IGF-1/insulin protects against serum deprivation-induced apoptosis. This action is prevented with inhibitors of NO generation, PI3K and Akt. Moreover, transfection with the negative dominant form of the tyrosine kinase c-Src abrogates the effect of IGF-1 and insulin on DNA fragmentation. An increase in the expression level of NOS3 protein and in the enzyme activity is observed following exposure of serum-deprived RINm5F cells to IGF-1 and insulin. Phosphorylation of IRS-1, IRS-2 and to less extent IRS-3 takes place when serum-deprived RINm5F cells and rat pancreatic islets are exposed to either IGF-1, insulin, or diethylenetriamine nitric oxide adduct (DETA/NO). In human islets, IRS-1 and IRS-2 proteins are present and tyrosine phosphorylated upon exposure to IGF-1, insulin and DETA/NO. Both rat and human pancreatic islets undergo DNA fragmentation when cultured in serum-free medium and IGF-1, insulin and DETA/NO protect efficiently from this damage. We then conclude that generation of NO participates in the activation of survival pathways by IGF-1 and insulin in beta cells.

Transforming Growth Factor β Is a Critical Regulator of Adult Human Islet Plasticity

Tissue plasticity is well documented in the context of pancreatic regeneration and carcinogenesis, with recent reports implicating dedifferentiated islet cells both as endocrine progenitors and as the cell(s) of origin in pancreatic adenocarcinoma. Accordingly, it is noteworthy that accumulating evidence suggests that TGFβ signaling is essential to pancreatic endocrine development and maintenance, whereas its loss is associated with the progression to pancreatic adenocarcinoma. The aim of this study was to examine the role of TGFβ in an in vitro model of islet morphogenetic plasticity. Human islets were embedded in a collagen gel and cultured under conditions that induced transformation into duct-like epithelial structures (DLS). Addition of TGFβ caused a dose-dependent decrease in DLS formation. Although it was demonstrated that collagen-embedded islets secrete low levels of TGFβ, antibody-mediated neutralization of this endogenously released TGFβ improved DLS formation rates, suggesting local TGFβ concentrations may in fact be higher. Time course studies indicated that TGFβ signaling was associated with an increase in ERK and p38 MAPK phosphorylation, although inhibitor-based studies were consistent with an islet endocrine-stabilizing effect mediated by p38 alone. Localization of TGFβ signaling molecules suggested that the action of TGFβ is directly on the β-cell to inhibit apoptosis and thus stabilize endocrine phenotype.