Protective mechanism of glucose against alloxan-induced pancreatic beta-cell damage

In-silico, in-vitro and in-vivo Biological Activities of Flavonoids for the Management of Type 2 Diabetes

In spite of the fact that many medicinal plants have been truly utilized for the management of diabetes all through the world, very few of them have been reported scientifically. Recently, a diverse variety of animal models have been established to better understand the pathophysiology of diabetes mellitus, and new medications to treat the condition have been introduced in the market. Flavonoids are naturally occurring substances that can be found in plants and various foods and may have health benefits in the treatment of neuropathic pain. Flavonoids have also been shown to have an anti-inflammatory impact that is significant to neuropathic pain, as indicated by a decrease in several pro-inflammatory mediators such TNF-, NF-B IL-6, and IL-1. Flavonoids appear to be a viable novel therapy option for macrovasular complications in preclinical models; however, human clinical data is still inadequate. Recently, several in silico, in-vitro and in-vivo aproaches were made to evaluate mechanisms associated with the pathogenesis of diabetes in a better way. Screening of natural antidiabetic agents from plant sources can be analysed by utilizing advanced in-vitro techniques and animal models. Natural compounds, mostly derived from plants, have been studied in diabetes models generated by chemical agents in the majority of research. The aim of this work was to review the available in silico, in-vitro and animal models of diabetes for screening of natural antidiabetic agents. This review contributes to the scientist's design of new methodologies for the development of novel therapeutic agents having potential antihyperglycemic activity.

Effect of chronic exposure to ketohexoses on pancreatic β-cell function in INS-1 rat insulinoma cells

Glucotoxicity, impaired insulin secretion, suppression of insulin gene expression, and apoptosis, in pancreatic β-cells caused by chronic hyperglycemia is a key component of the pathogenesis of type 2 diabetes. Recently, it has been reported that rare sugar d-allulose has antihyperglycemic and antihyperlipidemic effects in diabetic rats. However, the direct effects of rare sugars including d-allulose on pancreatic β-cell function are unclear. In this study, we investigated whether chronic exposure to ketohexoses causes glucotoxicity, suppression of insulin gene expression, and apoptosis, in INS-1 rat pancreatic insulinoma cells. d-Fructose, d-tagatose, l-allulose, and l-sorbose treatment for 1-week reduced insulin gene expression, whereas d-allulose, d-sorbose, l-fructose, and l-tagatose did not. All ketohexoses were transported into INS-1 cells, but were not metabolized. In addition, the ketohexoses did not induce apoptosis and did not affect glucose metabolism. These results suggest that long-term administration of d-allulose, d-sorbose, l-fructose, and l-tagatose does not affect pancreatic β-cell function.

Early ultra- and microstructural alterations in rat pancreas in alloxan-induced diabetes mellitus

An adequate experimental model is important to understand pathophysiological processes ongoing in the pancreas with diabetes mellitus. Our study was aimed to describe early ultra- and microstructural changes in the rat pancreas in 12-48 h after alloxan administration in a dose of 180 mg/kg. A histopathological examination of the endocrine pancreas revealed the loss of borders between endocrine cells, granular dystrophy and degranulation, sings of necrosis in central cells of the Langerhans islets and apoptosis of their peripheral ones manifested as DNA fragmentation and an increased expression of apoptosis markers. There was a gradual increase of a Langerhans islet area, a decreased percentage of insulin+ cells and an increased one of glucagon+ cells, as well as the presence of proliferating islet cells were found. Structural changes of the exocrine pancreas included fatty degeneration, signs of exocrine cell mitochondrial damage, increased acini, which are located mainly around the Langerhans islets, as well as perivascular edema and leukocytic infiltration. Described ultra- and microstructural alterations suggest a significant contribution of apoptosis to death of endocrine cells exposed to alloxan. Coexisting damage of the exocrine pancreas with its stroma involvement is for the first time described.

Environmental Factors Contribute to β Cell Endoplasmic Reticulum Stress and Neo-Antigen Formation in Type 1 Diabetes

Type 1 diabetes (T1D) is an autoimmune disease in which immune-mediated targeting and destruction of insulin-producing pancreatic islet β cells leads to chronic hyperglycemia. There are many β cell proteins that are targeted by autoreactive T cells in their native state. However, recent studies have demonstrated that many β cell proteins are recognized as neo-antigens following posttranslational modification (PTM). Although modified neo-antigens are well-established targets of pathology in other autoimmune diseases, the effects of neo-antigens in T1D progression and the mechanisms by which they are generated are not well understood. We have demonstrated that PTM occurs during endoplasmic reticulum (ER) stress, a process to which β cells are uniquely susceptible due to the high rate of insulin production in response to dynamic glucose sensing. In the context of genetic susceptibility to autoimmunity, presentation of these modified neo-antigens may activate autoreactive T cells and cause pathology. However, inherent β cell ER stress and protein PTM do not cause T1D in every genetically susceptible individual, suggesting the contribution of additional factors. Indeed, many environmental factors, such as viral infection, chemicals, or inflammatory cytokines, are associated with T1D onset, but the mechanisms by which these factors lead to disease onset remain unknown. Since these environmental factors also cause ER stress, exposure to these factors may enhance production of neo-antigens, therefore boosting β cell recognition by autoreactive T cells and exacerbating T1D pathogenesis. Therefore, the combined effects of physiological ER stress and the stress that is induced by environmental factors may lead to breaks in peripheral tolerance, contribute to antigen spread, and hasten disease onset. This Hypothesis and Theory article summarizes what is currently known about ER stress and protein PTM in autoimmune diseases including T1D and proposes a role for environmental factors in breaking immune tolerance to β cell antigens through neo-antigen formation.

T cell epitopes and post-translationally modified epitopes in type 1 diabetes

Type 1 diabetes (T1D) is an autoimmune disease in which progressive loss of self-tolerance, evidenced by accumulation of auto-antibodies and auto-reactive T cells that recognize diverse self-proteins, leads to immune-mediated destruction of pancreatic beta cells and loss of insulin secretion. In this review, we discuss antigens and epitopes in T1D and the role that post-translational modifications play in circumventing tolerance mechanisms and increasing antigenic diversity. Emerging data suggest that, analogous to other autoimmune diseases such as rheumatoid arthritis and celiac disease, enzymatically modified epitopes are preferentially recognized in T1D. Modifying enzymes such as peptidyl deiminases and tissue transglutaminase are activated in response to beta cell stress, providing a mechanistic link between post-translational modification and interactions with the environment. Although studies of such responses in the at-risk population have been limited, current data suggests that breakdown in tolerance through post-translational modification represents an important checkpoint in the development of T1D.

β cell ER stress and the implications for immunogenicity in type 1 diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disease characterized by hyperglycemia due to progressive immune-mediated destruction of insulin-producing pancreatic islet β cells. Although many elegant studies have identified β cell autoantigens that are targeted by the autoimmune response, the mechanisms by which these autoantigens are generated remain poorly understood. Normal β cell physiology includes a high demand for insulin production and secretion in response to dynamic glucose sensing. This secretory function predisposes β cells to significantly higher levels of endoplasmic reticulum (ER) stress compared to nonsecretory cells. In addition, many environmental triggers associated with T1D onset further augment this inherent ER stress in β cells. ER stress may increase abnormal post-translational modification (PTM) of endogenous β cell proteins. Indeed, in other autoimmune disorders such as celiac disease, systemic lupus erythematosus, multiple sclerosis, and rheumatoid arthritis, abnormally modified neo-antigens are presented by antigen presenting cells (APCs) in draining lymph nodes. In the context of genetic susceptibility to autoimmunity, presentation of neo-antigens activates auto-reactive T cells and pathology ensues. Therefore, the ER stress induced by normal β cell secretory physiology and environmental triggers may be sufficient to generate neo-antigens for the autoimmune response in T1D. This review summarizes what is currently known about ER stress and protein PTM in target organs of other autoimmune disease models, as well as the data supporting a role for ER stress-induced neo-antigen formation in β cells in T1D.

Neuropeptides, trophic factors, and other substances providing morphofunctional and metabolic protection in experimental models of diabetic retinopathy

Vision is the most important sensory modality for many species, including humans. Damage to the retina results in vision loss or even blindness. One of the most serious complications of diabetes, a disease that has seen a worldwide increase in prevalence, is diabetic retinopathy. This condition stems from consequences of pathological metabolism and develops in 75% of patients with type 1 and 50% with type 2 diabetes. The development of novel protective drugs is essential. In this review we provide a description of the disease and conclude that type 1 diabetes and type 2 diabetes lead to the same retinopathy. We evaluate existing experimental models and recent developments in finding effective compounds against this disorder. In our opinion, the best models are the long-term streptozotocin-induced diabetes and Otsuka Long-Evans Tokushima Fatty and spontaneously diabetic Torii rats, while the most promising substances are topically administered somatostatin and pigment epithelium-derived factor analogs, antivasculogenic substances, and systemic antioxidants. Future drug development should focus on these.

Cdk5 inhibitory peptide (CIP) inhibits Cdk5/p25 activity induced by high glucose in pancreatic beta cells and recovers insulin secretion from p25 damage

Cdk5/p25 hyperactivity has been demonstrated to lead to neuron apoptosis and degenerations. Chronic exposure to high glucose (HG) results in hyperactivity of Cdk5 and reduced insulin secretion. Here, we set out to determine whether abnormal upregulation of Cdk5/p25 activity may be induced in a pancreatic beta cell line, Min6 cells. We first confirmed that p25 were induced in overexpressed p35 cells treated with HG and increased time course dependence. Next, we showed that no p25 was detected under short time HG stimulation (4–12 hrs), however was detectable in the long exposure in HG cells (24 hrs and 48 hrs). Cdk5 activity in the above cells was much higher than low glucose treated cells and resulted in more than 50% inhibition of insulin secretion. We confirmed these results by overexpression of p25 in Min6 cells. As in cortical neurons, CIP, a small peptide, inhibited Cdk5/p25 activity and restored insulin secretion. The same results were detected in co-infection of dominant negative Cdk5 (DNCdk5) with p25. CIP also reduced beta cells apoptosis induced by Cdk5/p25. These studies indicate that Cdk5/p25 hyperactivation deregulates insulin secretion and induces cell death in pancreatic beta cells and suggests that CIP may serve as a therapeutic agent for type 2 diabetes.

Thiazolidinediones protect mouse pancreatic β-cells directly from cytokine-induced cytotoxicity through PPARγ-dependent mechanisms

Since most of the current studies of thiazolidinediones (TZDs) are only focused on improving glycemic control, increasing insulin sensitivity, and regulating inflammatory states in Type 2 Diabetes, it is still controversial whether TZDs have direct, protective effects on pancreatic β-cells in autoimmune diabetes. Here, we show the protective effects of TZDs on mouse pancreatic β-cell line cells (NIT-1) impaired by exposure to inflammatory cytokines (IL-1β and IFN-γ) and explore the potential mechanisms for this. The apoptosis rate and caspase-3 activity were remarkably increased, and insulin secretion response to glucose was impaired severely by exposure to IL-1β/IFN-γ for 48 h compared to control cells, whereas apoptosis rate and caspase-3 activity were significantly decreased in cells with treatment of rosiglitazone (RGZ) or pioglitazone (PIG), and the capacity for insulin secretion response to glucose was recovered. TZDs protect pancreatic β-cells from cytokine-induced cytotoxicity through PPARγ activation. The protective effects of the TZDs on NIT-1 cells disappeared when PPARγ was blocked with PPARγ-siRNA interference or treatment with GW9662, the PPARγ antagonist. Additionally, the enhancement of PPARγ expression by treatment with TZDs inhibited the expression of caspase 3 in IL-1β/IFN-γ-induced NIT-cells. Also, the inhibition of caspase 3 expression by TZDs was blocked by co-treatment with GW9662 or infection with PPARγ-siRNA. Taken together, our data suggest that TZDs might serve to protect pancreatic β-cells directly from cytokine-induced cytotoxicity through a PPARγ-dependent pathway, and caspase-3 may play an important role in the mechanisms involved.

Protective mechanism of reduced water against alloxan-induced pancreatic beta-cell damage: Scavenging effect against reactive oxygen species

Reactive oxygen species (ROS) cause irreversible damage to biological macromolecules, resulting in many diseases. Reduced water (RW) such as hydrogen-rich electrolyzed reduced water and natural reduced waters like Hita Tenryosui water in Japan and Nordenau water in Germany that are known to improve various diseases, could protect a hamster pancreatic beta cell line, HIT-T15 from alloxan-induced cell damage. Alloxan, a diabetogenic compound, is used to induce type 1 diabetes mellitus in animals. Its diabetogenic effect is exerted via the production of ROS. Alloxan-treated HIT-T15 cells exhibited lowered viability, increased intracellular ROS levels, elevated cytosolic free Ca(2+) concentration, DNA fragmentation, decreased intracellular ATP levels and lowering of glucose-stimulated release of insulin. RW completely prevented the generation of alloxan-induced ROS, increase of cytosolic Ca(2+) concentration, decrease of intracellular ATP level, and lowering of glucose-stimulated insulin release, and strongly blocked DNA fragmentation, partially suppressing the lowering of viability of alloxan-treated cells. Intracellular ATP levels and glucose-stimulated insulin secretion were increased by RW to 2-3.5 times and 2-4 times, respectively, suggesting that RW enhances the glucose-sensitivity and glucose response of beta-cells. The protective activity of RW was stable at 4 degrees C for over a month, but was lost by autoclaving. These results suggest that RW protects pancreatic beta-cells from alloxan-induced cell damage by preventing alloxan-derived ROS generation. RW may be useful in preventing alloxan-induced type 1-diabetes mellitus.

Butein from Rhus verniciflua protects pancreatic β cells against cytokine-induced toxicity mediated by inhibition of nitric oxide formation

Butein (3,4,2',4'-tetrahydroxychalcone), a plant polyphenol, is a major component in isolate of Rhus verniciflua STOKES (Anacardiaceae). It is shown to exert various potent effects such as antioxidant, antiinflammatory induction of apoptosis among many properties. In this study, we investigated the effect of butein on cytokine-induced β-cell damage. Pre-treatment with butein is shown to increase the viability of cytokine-treated INS-1 cells at concentrations of 15-30 µM. Butein prevented cytokine-mediated cell death, as well as nitric oxide (NO) production, and these effects correlated well with reduced levels of protein expression of the inducible nitric oxide synthase (iNOS). Furthermore, the molecular mechanisms by which butein inhibits iNOS gene expression appeared to be through the inhibition of nuclear factor-κB (NF-κB) translocation. In a second set of experiments, rat islets were used to demonstrate the protective effects of butein and the results were essentially the same as those observed in Beutin pretreated INS-1 cells. Butein prevented cytokine-induced NO production, iNOS expression, and NF-κB translocation and inhibition of glucose-stimulated insulin secretion (GSIS). In conclusion, these results suggest that butein can be used for the prevention of functional β-cell damage and preventing the progression of Type 1 diabetes mellitus (T1DM).

Calcium/calmodulin-dependent protein kinase IV involvement in the pathophysiology of glucotoxicity in rat pancreatic β-cells

Glucotoxicity is a critical component of the pathophysiology of type 2 diabetes mellitus; however, the molecular mechanisms of glucotoxicity are still not fully understood. We have attempted to determine the protein kinases involved in glucotoxicity in pancreatic β-cells by the use of a new technique. Using Multi-PK antibodies, which are capable of detecting a wide variety of protein kinases, we analyzed the protein kinase that correlated with insulin synthesis in INS-1 cells under glucotoxic conditions. When expression patterns of protein kinases in INS-1 cells were analyzed by Western blotting with Multi-PK antibodies, a kinase of 63 kd was significantly reduced concomitant with the decrease of insulin secretion under glucotoxic conditions. To identify the 63-kd kinase, we used a unique 2-dimensional gel electrophoretic technique and MicroRotofor (Bio-Rad Laboratories, Tokyo, Japan) electrophoresis. From the molecular size of a native kinase/cyanogen bromide fragment and pI value, the 63-kd protein kinase was deduced to be CaMKIV. This was confirmed by Western blotting analysis using anti-CaMKIV antibodies. The decreased CaMKIV levels under glucotoxic conditions recovered to original levels after changing the medium to a normal glucose concentration. Recombinant CaMKIV was degraded in a Ca²+-dependent manner by incubation with cell lysates from INS-1 cells under glucotoxic conditions, and degradation was protected by calpain inhibitor. Furthermore, CaMKIV was reduced in the pancreatic islets of diabetic Otsuka Long-Evans Tokushima fatty rats, whereas that of nondiabetic Long-Evans Tokushima Otsuka rats was not. This study suggests that the abnormal regulation of CaMKIV is a component of β-cell dysfunction caused by high glucose.

Oxidative stress and beta-cell dysfunction

Diabetes mellitus type 1 and 2 (T1DM and T2DM) are complex multifactorial diseases. Loss of beta-cell function caused by reduced secretory capacity and enhanced apoptosis is a key event in the pathogenesis of both diabetes types. Oxidative stress induced by reactive oxygen and nitrogen species is critically involved in the impairment of beta-cell function during the development of diabetes. Because of their low antioxidant capacity, beta-cells are extremely sensitive towards oxidative stress. In beta-cells, important targets for an oxidant insult are cell metabolism and K(ATP) channels. The oxidant-evoked alterations of K(ATP) channel activity seem to be critical for oxidant-induced dysfunction because genetic ablation of K(ATP) channels attenuates the effects of oxidative stress on beta-cell function. Besides the effects on metabolism, interference of oxidants with mitochondria induces key events in apoptosis. Consequently, increasing antioxidant defence is a promising strategy to delay beta cell failure in (pre)-diabetic patients or during islet transplantation. Knock-out of K(ATP) channels has beneficial effects on oxidant-induced inhibition of insulin secretion and cell death. Interestingly, these effects can be mimicked by sulfonylureas that have been used in the treatment of T2DM for many years. Loss of functional K(ATP) channels leads to up-regulation of antioxidant enzymes, a process that depends on cytosolic Ca(2+). These observations are of great importance for clinical intervention because they show a possibility to protect beta-cells at an early stage before dramatic changes of the secretory capacity and loss of cell mass become manifest and lead to glucose intolerance or even overt diabetes.

Mitigating effects of antioxidant properties of Artemisia campestris leaf extract on hyperlipidemia, advanced glycation end products and oxidative stress in alloxan-induced diabetic rats

Artemisia campestris is used as antivenom and anti-inflammatory Tunisian folk medicine. Recently, increased oxidative stress was shown to play an important role in the etiology and pathogenesis of diabetes mellitus and its complications. This study was designed to examine the effects of A. campestris leaf aqueous extract (Ac) on alloxan-induced diabetic rats by measuring glycemia, lipid profile, lipid peroxidation (MDA), protein carbonyl content (PCO), advanced oxidation protein products (AOPP), activities of both non-enzymatic and enzymatic antioxidants. Results of our study showed an increase in blood glucose levels, total cholesterol (TC), triglycerides (TG), low-density lipoprotein cholesterol (LDL-c), a decrease in high-density lipoprotein cholesterol (HDL-c) level and disturbed antioxidant enzyme activities (CAT, SOD, GPx) in the pancreatic tissue of diabetic rats. Furthermore, MDA, PCO and AOPP were elevated in the pancreas of the diabetic rats. The administration of Ac to diabetic rats at a dose of 200mgkg(-1)bw resulted in a significant reduction in glycemia, TC, TG, LDL-c, pancreas LPO, PCO and AOPP levels, CAT and GPx activities associated with an elevation of GSH content and SOD activity in comparison with diabetic group. We conclude that A. campestris aqueous extract may be effective for correcting hyperglycemia and preventing diabetic complications.

Sulforaphane protects against cytokine- and streptozotocin-induced beta-cell damage by suppressing the NF-kappaB pathway

Sulforaphane (SFN) is an indirect antioxidant that protects animal tissues from chemical or biological insults by stimulating the expression of several NF-E2-related factor-2 (Nrf2)-regulated phase 2 enzymes. Treatment of RINm5F insulinoma cells with SFN increases Nrf2 nuclear translocation and expression of phase 2 enzymes. In this study, we investigated whether the activation of Nrf2 by SFN treatment or ectopic overexpression of Nrf2 inhibited cytokine-induced beta-cell damage. Treatment of RIN cells with IL-1beta and IFN-gamma induced beta-cell damage through a NF-kappaB-dependent signaling pathway. Activation of Nrf2 by treatment with SFN and induction of Nrf2 overexpression by transfection with Nrf2 prevented cytokine toxicity. The mechanism by which Nrf2 activation inhibited NF-kappaB-dependent cell death signals appeared to involve the reduction of oxidative stress, as demonstrated by the inhibition of cytokine-induced H(2)O(2) production. The protective effect of SFN was further demonstrated by the restoration of normal insulin secreting responses to glucose in cytokine-treated rat pancreatic islets. Furthermore, pretreatment with SFN blocked the development of type 1 diabetes in streptozotocin-treated mice.

Guggulsterone, a plant sterol, inhibits NF-kappaB activation and protects pancreatic beta cells from cytokine toxicity

Guggulsterone has been used to treat hyperlipidemia, arthritis, and obesity. Although its anti-inflammatory and anti-hyperlipidemic effects have been well documented, the effect of guggulsterone on pancreatic beta cells is unknown. Therefore, in this study, the effect of guggulsterone on IL-1beta- and IFN-gamma-induced beta-cell damage was investigated. Treatment of RINm5F (RIN) rat insulinoma cells with IL-1beta and IFN-gamma induced cell damage, and this damage was well correlated with nitric oxide (NO) and prostaglandin E2 (PGE2) production. However, guggulsterone completely prevented cytokines-mediated cytotoxicity, as well as NO and PGE2 production, and these effects were correlated with reduced levels of the inducible form of NO synthase (iNOS) and cyclooxygenase-2 (COX-2) mRNA and protein expressions. The molecular mechanism by which guggulsterone inhibits iNOS and COX-2 gene expressions appeared to involve the inhibition of NF-kappaB activation. The cytoprotective effects of guggulsterone were also mediated through the suppression of the JAK/STAT pathway. Cells treated with the cytokines downregulated the protein level of SOCS-3, however pretreatment with guggulsterone attenuated this decrease. Additionally, in a second set of experiments in which rat islets were used, the findings regarding the beta-cell protective effects of guggulsterone were essentially the same as those observed when RIN cells were used; guggulsterone prevented cytokines-induced NO and PGE2 production, iNOS and COX-2 expressions, JAK/STAT activation, NF-kappaB activation, downregulation of SOCS-3, and impairment of glucose-stimulated insulin secretion. Collectively, these results suggest that guggulsterone may be used to preserve functional beta-cell mass.

Protective effect of Amomi semen extract on alloxan-induced pancreatic beta-cell damage

The protective effect of Amomi semen extract (ASE) on alloxan-induced pancreatic beta-cell damage was investigated in HIT T-15 cells, a Syrian hamster pancreatic beta-cell line. Alloxan caused pancreatic beta-cell damage through the generation of reactive oxygen species (ROS), the elevation of cytosolic free Ca2+, DNA fragmentation and the decrease of cellular NAD+ and ATP levels. All these effects of alloxan were significantly prevented by pretreatment with a water-soluble extract of Amomi semen. Pretreatment with ASE in pancreatic islets isolated from mice, also significantly abolished the inhibition of glucose-stimulated insulin secretion by alloxan. The results of this study provide evidence that ASE may have a protective activity on alloxan-induced beta-cell damage, and that the protective effect is primarily due to the inhibition of ROS generation by alloxan.

Genistein protects pancreatic beta cells against cytokine-mediated toxicity

In the past few decades, the use of genistein as an anti-inflammatory agent has gained much attention. Our current study focuses on the preventive effects of genistein on cytokine-induced pancreatic beta-cell damage. Treatment of RINm5F (RIN) rat insulinoma cells with interleukin (IL)-1beta and interferon (IFN)-gamma induced cell damage, which was correlated with nitric oxide (NO) production. Genistein completely prevented cytokine-mediated cytotoxicity and NO production, a finding that correlated well with reduced levels of the inducible form of NO synthase (iNOS) mRNA and protein. The molecular mechanism of genistein inhibition of iNOS gene expression appeared to involve the inhibition of NFkappaB activation. The cytokine induced increases in NFkappaB binding activity, nuclear p50 and p65 subunit levels, and IkappaBalpha degradation in cytosol compared to unstimulated cells; genistein abolished all of these parameters. The cytoprotective effects of genistein are also mediated through the suppression of ERK-1/2 and Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathways. In a second set of experiments, rat islets were used. The findings on beta-cell protective effects of genistein were essentially the same as for the RIN cell data, namely genistein prevented cytokine-induced NO production, iNOS expression, ERK-1/2 activation, JAK/STAT activation, and impairment of glucose-stimulated insulin secretion. Collectively, these results suggest that genistein might be used to preserve functional beta-cell mass.

Flavonoids protect against cytokine-induced pancreatic beta-cell damage through suppression of nuclear factor kappaB activation

OBJECTIVES

In the past few decades, the use of natural compounds, such as flavonoids, as anti-inflammatory agents has gained much attention. Our current study focuses on the preventive effects of quercetin, apigenin, and luteolin on cytokine-induced beta-cell damage.

METHODS

Pancreatic beta-cells or islets were treated with cytokine mixtures in the presence or absence of flavonoids and the inhibitory effect of flavonoids against cytokine toxicity was determined.

RESULTS

Treatment of RINm5F (RIN) rat insulinoma cells with interleukin 1beta (IL-1beta) and interferon gamma (IFN-gamma) induced cell damage. Quercetin, apigenin, and luteolin completely protected against IL-1beta- and IFN-gamma-mediated cytotoxicity in RIN cells. Incubation with quercetin, apigenin, and luteolin resulted in a significant reduction in IL-1beta- and IFN-gamma-induced nitric oxide production, a finding that correlated well with reduced levels of the inducible form of NO synthase messenger RNA and protein. The molecular mechanism by which quercetin, apigenin, and luteolin inhibited inducible NO synthase gene expression appeared to involve the inhibition of nuclear factor kappaB (NF-kappaB) activation. The IL-1beta- and IFN-gamma-stimulated RIN cells showed increases in NF-kappaB binding activity, p50 and p65 subunit levels in nucleus, and IkappaB alpha degradation in cytosol compared with unstimulated cells. Quercetin, apigenin, and luteolin also prevented IL-1beta- and IFN-gamma-mediated inhibition of insulin secretion.

CONCLUSION

Quercetin, apigenin, and luteolin inhibited cytotoxicity in RIN cells and attenuated the decrease of glucose-stimulated insulin secretion in islets by IL-1beta and IFN-gamma.

Inhibition of Cytokine-Mediated Nitric Oxide Synthase Expression in Rat Insulinoma Cells by Scoparone

Cytokines produced by immune cells infiltrating pancreatic islets are important mediators of beta-cell destruction in insulin-dependent diabetes mellitus. Scoparone (6,7-dimethoxycoumarin) is known to have a wide range of pharmacological properties in vitro. In this study, the effects of scoparone on cytokine-induced beta-cell dysfunction were examined. Presence of scoparone significantly protected interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma)-mediated cytotoxicity of RINm5F, a rat insulinoma cell line, and preserved glucose-stimulated insulin secretion in rat pancreatic islets. Scoparone also resulted in a significant reduction in IL-1beta and IFN-gamma-induced nitric oxide (NO) production, a finding that correlated well with reduced levels of the inducible form of NO synthase (iNOS) mRNA and protein. The molecular mechanism by which scoparone inhibited iNOS gene expression appeared to involve the inhibition of NF-kappaB activation. These results revealed the possible therapeutic value of scoparone for the prevention of diabetes mellitus progression.

Activation of peroxisome proliferator-activated receptor-γ protects pancreatic β-cells from cytokine-induced cytotoxicity via NFκB pathway

Diabetes mellitus is characterized by cytokine-induced insulitis and a deficit in beta-cell mass. Ligands for peroxisome proliferator-activated receptor-gamma (PPAR-gamma) have been shown to have anti-inflammatory effects in various experimental models. We questioned whether activation of endogenous PPAR-gamma by either PPAR-gamma ligands or adenoviral-directed overexpression of PPAR-gamma (Ad-PPAR-gamma) could inhibit cytokine-induced beta-cell death in RINm5F (RIN) cells, a rat insulinoma cell line. Treatment of RIN cells with interleukin-1 beta (IL-1 beta) and interferon-gamma (IFN-gamma) induced beta-cell damage through NF kappaB-dependent signaling pathways. Activation of PPAR-gamma by PPAR-gamma ligands or Ad-PPAR-gamma inhibited IL-1 beta and IFN-gamma-stimulated nuclear translocation of the p65 subunit and DNA binding activity. NF kappaB target gene expression and their product formation, namely inducible nitric oxide synthase and cyclooxygenase-2 were decreased by PPAR-gamma activation, as established by real-time PCR, Western blots and measurements of NO and PGE(2). The mechanism by which PPAR-gamma activation inhibited NF kappaB-dependent cell death signals appeared to involve the inhibition of I kappa B alpha degradation, evidenced by inhibition of cytokine-induced NF kappaB-dependent signaling events by Ad-I kappaB alpha (S32A, S36A), non-degradable I kappaB alpha mutant. I kappaB beta mutant, Ad-I kappaB beta (S19A, S23A) was not effective in preventing cytokine toxicity. Furthermore, a protective effect of PPAR-gamma ligands was proved by assaying for normal insulin secreting capacity in response to glucose in isolated rat pancreatic islets. The beta-cell protective function of PPAR-gamma ligands might serve to counteract cytokine-induced beta-cell destruction.

Inhibition of cyclin-dependent kinase 5 activity protects pancreatic beta cells from glucotoxicity

Type 2 diabetes (T2D) and Alzheimer disease are degenerative diseases that may share common pathophysiologic mechanisms. Neuronal dysfunction in Alzheimer patients has been linked to overactivity of the cyclin-dependent kinase 5 (CDK5) and its activator p35. Both of these proteins are expressed in the insulin-producing beta cells of the pancreas. Further, glucose enhances p35 gene expression, promoting the formation of active p35/CDK5 complexes that regulate the expression of the insulin gene. In T2D, chronic elevations of glucose, glucotoxicity, impair beta cell function. We therefore postulated that CDK5 and p35 may be responsible for this beta cell impairment and that inhibition of CDK5 might have a beneficial effect. To test this hypothesis, the pancreatic cell line INS-1 was selected as a known in vitro model of glucotoxicity, and roscovitine (10 mum) was used as a CDK5 inhibitor. Chronic exposure of INS-1 cells to high glucose (20-30 mm) reduced both insulin mRNA levels and the activity of an insulin promoter reporter gene. Inhibition of CDK5 prevented this decrease of insulin gene expression. We used DNA binding (gel shift) assays and Western immunoblots to demonstrate that cellular levels of the transcription factor PDX-1, normally decreased by glucotoxicity, were preserved with CDK5 inhibition, as was the binding of PDX-1 to the insulin promoter. Analyses of nuclear and cytoplasmic PDX-1 protein levels revealed that CDK5 inhibition restores nuclear PDX-1, without affecting its cytoplasmic concentration, suggesting that CDK5 regulates the nuclear/cytoplasm partitioning of PDX-1. Using a Myc-tagged PDX-1 construct, we showed that the translocation of PDX-1 from the nucleus to the cytoplasm during glucotoxic conditions was prevented when CDK5 was inhibited. These studies indicate that CDK5 plays a role in the loss of beta cell function under glucotoxic conditions and that CDK5 inhibitors could have therapeutic value for T2D.

Protective effect of Coptidis Rhizoma on S-nitroso-N-acetylpenicillamine (SNAP)-induced apoptosis and necrosis in pancreatic RINm5F cells

Coptidis rhizoma (CR) is a herb used in many traditional prescriptions against diabetes mellitus in Asia for centuries. Our purpose was to determine the protective effect and its action mechanism of CR on the cytotoxicity of pancreatic beta-cells. Nitric oxide (NO) is believed to play a key role in the process of pancreatic beta-cell destruction leading to insulin-dependent diabetes mellitus (IDDM). Exposure of RINm5F cells to chemical NO donor such as S-nitroso-N-acetylpenicillamine (SNAP) induced apoptotic events such as the disruption of mitochondrial membrane potential (Deltapsim), cytochrome c release from mitochondria, activation of caspase-3, poly (ADP-ribose) polymerase cleavage and DNA fragmentation. Also, exposure of SNAP led to LDH release into medium, one of the necrotic events. However, pretreatment of RINm5F cells with CR extract protected both apoptosis and necrosis through the inhibition of Deltapsim disruption in SNAP-treated RINm5F cells. In addition, rat islets pretreated with CR extract retained the insulin-secretion capacity even after the treatment with IL-1beta. These results suggest that CR may be a candidate for a therapeutic or preventing agent against IDDM.

Effect of prostaglandins against alloxan-induced cytotoxicity to insulin secreting insulinoma RIN cells in vitro

In the present study, we studied the effect of various prostaglandins (PGs) on alloxan-induced cytotoxicity to rat insulinoma (RIN) cells. Of all the PGs tested, PGE(1), PGE(2), PGI(2), PGF(1 alpha), and PGF(3 alpha) protected RIN cells from alloxan-induced cytotoxicity (P<0.05 compared to alloxan), whereas thromboxane B(2) and 6-keto-PGF(1 alpha) were not effective. PGE(1) induces a statistically significant increase in the activities of superoxide dismutase and glutathione peroxidase and decrease in lipid peroxides in alloxan-treated RIN cells (P<0.001). PGE(1) restored nitric oxide/lipid peroxide ratio to normalcy, suggesting that PGE(1) suppresses oxidant stress induced by alloxan in RIN cells in vitro. Furthermore, PGE(1) prevented DNA damage and apoptosis induced by alloxan. These results indicate that PGE(1) prevents alloxan-induced cytotoxicity to RIN cells in vitro.

Fructus Benincasae RecensExtract Prevents Cytokine‐Induced Nitric Oxide Formation and Cytotoxicity of RINm5F Cells

Cytokines produced by immune cells infiltrating pancreatic islets are important mediators of beta-cell destruction in insulin-dependent diabetes mellitus. In this study, the effects of Fructus Benincasae Recens (FBR) extract on cytokine-induced beta-cell dysfunction were examined. Fructus Benincasae Recens extract completely protected interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma)-mediated cytotoxicity in rat insulinoma cell line (RINm5F). Incubation with FBR extract resulted in a significant reduction of IL-1beta and IFN-gamma-induced nitric oxide (NO) production, a finding that correlated well with reduced levels of the inducible form of NO synthase (iNOS) mRNA and protein. The molecular mechanism by which FBR extract inhibited iNOS gene expression appeared to involve the inhibition of NF-kappaB activation. Our results revealed the possible therapeutic value of FBR extract for the prevention of diabetes mellitus progression.

Amomum xanthoides extract prevents cytokine-induced cell death of RINm5F cells through the inhibition of nitric oxide formation

We previously showed that Amomum xanthoides extract prevented alloxan-induced diabetes through the suppression of NF-kappaB activation. In this study, the preventive effects of A. xanthoides extract on cytokine-induced beta-cell destruction were examined. Cytokines produced by immune cells infiltrating pancreatic islets are important mediators of beta-cell destruction in insulin-dependent diabetes mellitus. A. xanthoides extract completely protected interleukin-1beta (IL-1beta) and interferon-gamma (IFN-gamma)-mediated cytotoxicity in rat insulinoma cell line (RINm5F). Incubation with A. xanthoides extract resulted in a significant reduction in IL-1beta and IFN-gamma-induced nitric oxide (NO) production, a finding that correlated well with reduced levels of the inducible form of NO synthase (iNOS) mRNA and protein. The molecular mechanism by which A. xanthoides extract inhibited iNOS gene expression appeared to involve the inhibition of NF-kappaB activation. Our results revealed the possible therapeutic value of A. xanthoides extract for the prevention of diabetes mellitus progression.

Molecular target structures in alloxan-induced diabetes in mice

Type 1 diabetes results from irreversible damage of insulin-producing beta-cells. In laboratory animals, diabetes can be induced with alloxan (ALX), a 2,4,5,6-tetraoxopyrimidine. ALX is a potent generator of reactive oxygen species (ROS), which can mediate beta-cell toxicity. However, the initial lesions on essential beta-cell structures are not known. In this study, we report that the glucose transporter 2 (GLUT2) and glucokinase (GK) are target molecules for ALX. Ex vivo, a gradual decrement of both GLUT2 and GK mRNA expression was found in islets isolated from ALX-treated C57BL/6 mice. This reduction was more pronounced for GLUT2 than for GK. The mRNA expression of beta-actin was also slightly affected with time after ALX exposure, the proinsulin mRNA, however, remained unaffected as well as the pancreatic total insulin content. Pretreatment with D-glucose (D-G) protected the mRNA expression of GLUT2 and GK against ALX toxicity and prevented diabetes. Yet, in these euglycemic mice, an impaired oral glucose tolerance persisted. Pretreatment with 5-thio-D-glucose (5-T-G) failed to prevent ALX diabetes, administration of zinc sulfate (Zn(2+))-enriched drinking water, however, reduced ALX-induced hyperglycemia. In conclusion, ALX exerted differential toxicity on beta-cell structures similar to in vitro results reported from this laboratory. Furthermore, the present results differ from those reported for the diabetogen streptozotocin (STZ). Injections of multiple low doses (MLD) of STZ reduced GLUT2 expression only, but failed to affect expression of GK and proinsulin as well as beta-actin as internal control. MLD-STZ diabetes was prevented by pretreatment with both D-G and 5-T-G and administration of Zn(2+)-enriched drinking water. Apparently, ALX and MLD-STZ exert diabetogenicity by different pathways requiring different interventional schedules for prevention.

Early manifestations of nephropathy in alloxan-treated rats

An early stage of diabetic nephropathy was studied. Rat renal function was evaluated by clearance techniques, 7 or 15 days after alloxan administration (groups A7 and A15). Significant diminutions of glomerular filtration rate (inulin clearance) and p-aminohippurate clearance were observed in alloxan-treated rats. Diabetic animals presented glucosuria and enhanced water excretion. A natriuretic response was only observed in A15-rats. Arterial pressure increased along time, and enlarged lipid deposits in glomeruli and vessels of A7-kidney sections were observed. Thus, a vascular compromise at this time was suggested. To better characterize the set up of the renal dysfunction, other studies were performed in A7-group. Urinary protein excretion remained unchanged while a higher level of glycosylation of urinary proteins was observed in A7-rats. Histological studies revealed a normal general morphology in kidneys from diabetic rats. Immunohistochemical analysis in renal sections showed enlarged deposits of fibronectin in glomeruli and interstitium of alloxan-treated rats. Higher myeloperoxidase activity was observed in renal cortex from diabetic animals indicating leukocytes infiltration. These results indicated that 7 days after hyperglycemia induction, the animals presented a renal dysfunction characterized by hemodynamic alterations associated with vascular and glomerular structural impairments, without modifications in tubular function. The higher level of protein glycosylation and the inflammatory process at this early stage could be responsible for the beginning of diabetic nephropathy.