Mitochondria mediated cell death in diabetes

miR-146a-5p mediates inflammation-induced β cell mitochondrial dysfunction and apoptosis

We previously showed that miR-146a-5p is upregulated in pancreatic islets treated with pro-inflammatory cytokines. Others have reported that miR-146a-5p overexpression is associated with β cell apoptosis and impaired insulin secretion. However, the molecular mechanisms mediating these effects remain elusive. To investigate the role of miR-146a-5p in β cell function, we developed stable MIN6 cell lines to either overexpress or inhibit the expression of miR-146a-5p. Monoclonal cell populations were treated with pro-inflammatory cytokines (IL-1β, IFNγ, and TNFα) to model T1D in vitro. We found that overexpression of miR-146a-5p increased cell death under conditions of inflammatory stress, whereas inhibition of miR-146a-5p reversed these effects. Additionally, inhibition of miR-146a-5p increased mitochondrial DNA copy number, respiration rate, and ATP production. Further, RNA sequencing data showed enrichment of pathways related to insulin secretion, apoptosis, and mitochondrial function when the expression levels of miR-146a-5p were altered. Finally, a temporal increase in miR-146a-5p expression levels and a decrease in mitochondria function markers was observed in islets derived from NOD mice. Collectively, these data suggest that miR-146a-5p may promote β cell dysfunction and death during inflammatory stress by suppressing mitochondrial function.

Cholinergic deficiency in the cholinergic system as a pathogenetic link in the formation of various syndromes in COVID-19

According to recent data, several mechanisms of viral invasion of the central nervous system (CNS) have been proposed, one of which is both direct penetration of the virus through afferent nerve fibers and damage to the endothelium of cerebral vessels. It has been proven that the SARS-CoV-2 virus affects pathologically not only the human cardiorespiratory system but is also associated with a wide range of neurological diseases, cerebrovascular accidents, and neuromuscular pathologies. However, the observed post-COVID symptom complex in patients, manifested in the form of headache, "fog in the head," high temperature, muscle weakness, lowering blood pressure, does it make us think about the pathophysiological mechanisms that contribute to the development of this clinical picture? One possible explanation is a disruption in the signaling of the acetylcholine system (AChS) in the body. Viral invasions, and in particular COVID-19, can negatively affect the work of the AChS, disrupting its coordination activities. Therefore, the main goal of this literature review is to analyze the information and substantiate the possible mechanisms for the occurrence of post-COVID syndrome in people who have had COVID-19 from the standpoint of AChS dysfunctions.

Divergent effects of HIV reverse transcriptase inhibitors on pancreatic beta-cell function and survival: Potential role of oxidative stress and mitochondrial dysfunction

Antiretroviral therapy (ART), a life-saving treatment strategy in HIV/AIDS, has been implicated in increasing the risk of type 2 diabetes mellitus (T2DM). Direct damaging effects on beta-cell function and survival by either non-nucleoside reverse transcriptase inhibitors (NNRTIs) or nucleoside/tide reverse transcriptase inhibitors (NRTIs) may predispose individuals to developing T2DM or if already type 2 diabetic, to insulin dependency. The aim of this study was to investigate the effects of the NNRTIs efavirenz, rilpivirine and doravirine, and the NRTIs tenofovir disoproxil fumarate and emtricitabine, on beta-cell function and survival while suggesting potential cellular and molecular mechanism(s). Our results show contrasting effects within the NNRTI class as doravirine did not cause damaging effects in the rat insulinoma INS-1E cells while efavirenz and rilpivirine reduced insulin release and cell viability, and induced apoptosis in INS-1E cells. Additionally, efavirenz and rilpivirine increased ROS generation, disrupted Δψm and upregulated the mRNA and protein expression of CHOP and GRP78, key markers of endoplasmic reticulum stress. In silico docking studies predict a possible inhibition of the mitochondrial ATP synthase by rilpivirine. On the contrary, both the NRTIs tenofovir disoproxil fumarate and emtricitabine did not affect GSIS, cell viability and apoptosis/necrosis levels in INS-1E cells. The deleterious effects observed in beta-cells exposed to efavirenz or rilpivirine may be, at least partially, mediated by oxidative stress and mitochondrial toxicity. These findings provide potential mechanism(s) by which efavirenz and rilpivirine may contribute to the pathogenesis of T2DM and the progression of T2DM to insulin dependency in HIV-infected type 2 diabetics.

Protein Kinases Signaling in Pancreatic Beta-cells Death and Type 2 Diabetes

Type 2 diabetes (T2D) is a worldwide serious public health problem. Insulin resistance and β-cell failure are the two major components of T2D pathology. In addition to defective endoplasmic reticulum (ER) stress signaling due to glucolipotoxicity, β-cell dysfunction or β-cell death initiates the deleterious vicious cycle observed in T2D. Although the primary cause is still unknown, overnutrition that contributes to the induction of the state of low-grade inflammation, and the activation of various protein kinases-related metabolic pathways are main factors leading to T2D. In this chapter following subjects, which have critical checkpoints regarding β-cell fate and protein kinases pathways are discussed; hyperglycemia-induced β-cell failure, chronic accumulation of unfolded protein in β-cells, the effect of intracellular reactive oxygen species (ROS) signaling to insulin secretion, excessive saturated free fatty acid-induced β-cell apoptosis, mitophagy dysfunction, proinflammatory responses and insulin resistance, and the reprogramming of β-cell for differentiation or dedifferentiation in T2D. There is much debate about selecting proposed therapeutic strategies to maintain or enhance optimal β-cell viability for adequate insulin secretion in T2D. However, in order to achieve an effective solution in the treatment of T2D, more intensive clinical trials are required on newer therapeutic options based on protein kinases signaling pathways.

A bioluminescent probe for longitudinal monitoring of mitochondrial membrane potential

Mitochondrial membrane potential (ΔΨm) is a universal selective indicator of mitochondrial function and is known to play a central role in many human pathologies, such as diabetes mellitus, cancer and Alzheimer's and Parkinson's diseases. Here, we report the design, synthesis and several applications of mitochondria-activatable luciferin (MAL), a bioluminescent probe sensitive to ΔΨm, and partially to plasma membrane potential (ΔΨp), for non-invasive, longitudinal monitoring of ΔΨm in vitro and in vivo. We applied this new technology to evaluate the aging-related change of ΔΨm in mice and showed that nicotinamide riboside (NR) reverts aging-related mitochondrial depolarization, revealing another important aspect of the mechanism of action of this potent biomolecule. In addition, we demonstrated application of the MAL probe for studies of brown adipose tissue (BAT) activation and non-invasive in vivo assessment of ΔΨm in animal cancer models, opening exciting opportunities for understanding the underlying mechanisms and for discovery of effective treatments for many human pathologies.

The Role of Pi, Glutamine and the Essential Amino Acids in Modulating the Metabolism in Diabetes and Cancer

Purpose

Re-examine the current metabolic models.

Methods

Review of literature and gene networks.

Results

Insulin activates Pi uptake, glutamine metabolism to stabilise lipid membranes. Tissue turnover maintains the metabolic health. Current model of intermediary metabolism (IM) suggests glucose is the source of energy, and anaplerotic entry of fatty acids and amino acids into mitochondria increases the oxidative capacity of the TCA cycle to produce the energy (ATP). The reduced cofactors, NADH and FADH2, have different roles in regulating the oxidation of nutrients, membrane potentials and biosynthesis. Trans-hydrogenation of NADH to NADPH activates the biosynthesis. FADH2 sustains the membrane potential during the cell transformations. Glycolytic enzymes assume the non-canonical moonlighting functions, enter the nucleus to remodel the genetic programmes to affect the tissue turnover for efficient use of nutrients. Glycosylation of the CD98 (4F2HC) stabilises the nutrient transporters and regulates the entry of cysteine, glutamine and BCAA into the cells. A reciprocal relationship between the leucine and glutamine entry into cells regulates the cholesterol and fatty acid synthesis and homeostasis in cells. Insulin promotes the Pi transport from the blood to tissues, activates the mitochondrial respiratory activity, and glutamine metabolism, which activates the synthesis of cholesterol and the de novo fatty acids for reorganising and stabilising the lipid membranes for nutrient transport and signal transduction in response to fluctuations in the microenvironmental cues. Fatty acids provide the lipid metabolites, activate the second messengers and protein kinases. Insulin resistance suppresses the lipid raft formation and the mitotic slippage activates the fibrosis and slow death pathways.

Attenuation of high glucose induced apoptotic and inflammatory signaling pathways in RIN-m5F pancreatic β cell lines by Hibiscus rosa sinensis L. petals and its phytoconstituents

ETHNOPHARMACOLOGICAL RELEVANCE

Hibiscus rosa sinensis petals possess wide range of pharmacological properties, with remarkable nutritional values. Diabetes is one of the most devastating diseases affecting the world today. A few side effects associated with the use of insulin and oral hypoglycemic agents prompted us to search new bioactive principles from antidiabetic plants used in traditional medicine.

AIM OF THE STUDY

The anti-diabetic therapeutic potential of the flavonoids rich ethyl acetate fraction of Hibiscus rosa sinensis petals (EHRS) was evaluated.

MATERIALS AND METHODS

High glucose (25 mM) induced apoptotic model of diabetes in RIN-m5F pancreatic β-cells was used for the study.

RESULTS

EHRS elevated the release of insulin in pancreatic cells and modulated apoptotic signaling cascades. It significantly reduced NF-κB nuclear translocation, thereby down-regulated the expressions of major inflammatory cytokines and up-regulated expressions of pancreatic β-cell functional genes such as, foxO-1, Ucn-3, Pdx-1, MafA and Nkx6.1. On comparison with its constituent phytochemicals, superior protective effect shown by EHRS may be due to the additive action of these phytoconstituents.

CONCLUSIONS

Results of the present study suggest hibiscus petals as a natural source and functional food of potential therapeutics to protect pancreatic β-cells in experimental diabetes mellitus.

Identification, characterization, and expression analysis of adiponectin receptors in black carp Mylopharyngodon piceus in response to dietary carbohydrate

In the present study, three cDNAs of AdipoRs (MpAdipoR1a, MpAdipoR1b, and MpAdipoR2) were identified from juvenile black carp Mylopharyngodon piceus. There were 375, 378, and 356 amino acids in the MpAdipoR1a, MpAdipoR1b, and MpAdipoR2, respectively. BLAST analysis reveals that MpAdipoRs share high identities with other known AdipoRs from zebrafish, rainbow trout, human, etc. And there were all seven transmembrane regions in the amino acid sequences of MpAdipoR1s and MpAdipoR2, respectively. The relative expression levels of MpAdipoR1s were higher in the liver, blood, brain, and eyes in black carp (p < 0.05). Relatively higher expression of MpAdipoR2 was detected in the liver (p < 0.05) and then in the adipose tissues and blood by real-time PCR assays. The relative expression levels of AdipoR mRNA in the liver, muscle, brain, blood, and adipose tissues were detected by real-time PCR in black carp fed with four levels of dietary carbohydrate (CHO) (10.65, 19.43, 28.84, and 37.91%) for 9 weeks, respectively. The expression levels of MpAdipoR1s in the liver, muscle, brain, and blood were induced and reached to the maximum at optimal dietary CHO (19.43 or 28.84%) and then were decreased at 37.91% dietary CHO, although there were no significant differences on the expression levels of MpAdipoR1a in the liver between 19.43, 28.84, and 37.91% dietary CHO. Similarly, MpAdipoR2 were also induced to the maximal levels in the liver, muscle, brain, and blood at optimal dietary CHO (19.43 or 28.84%) compared with that at 10.65% dietary CHO, although no significant differences were observed on the expression levels of MpAdipoR2 in the liver and muscle between groups fed 19.43 and 37.91% dietary CHO (p > 0.05). However, the expression levels of MpAdipoRs in the adipose tissues were significantly downregulated at excessive dietary CHO (37.91%) compared with other groups (p < 0.05). These results indicated that MpAdipoRs were inducible proteins and might enhance the understanding of their vital roles in the regulation of glucose metabolic homeostasis in black carp.

Crosslink between calcium and sodium signalling

NEW FINDINGS

What is the topic of this review? This paper overviews the links between Ca2+ and Na+ signalling in various types of cells. What advances does it highlight? This paper highlights the general importance of ionic signalling and overviews the molecular mechanisms linking Na+ and Ca2+ dynamics. In particular, the narrative focuses on the molecular physiology of plasmalemmal and mitochondrial Na+ -Ca2+ exchangers and plasmalemmal transient receptor potential channels. Functional consequences of Ca2+ and Na+ signalling for co-ordination of neuronal activity with astroglial homeostatic pathways fundamental for synaptic transmission are discussed.

ABSTRACT

Transmembrane ionic gradients, which are an indispensable feature of life, are used for generation of cytosolic ionic signals that regulate a host of cellular functions. Intracellular signalling mediated by Ca2+ and Na+ is tightly linked through several molecular pathways that generate Ca2+ and Na+ fluxes and are in turn regulated by both ions. Transient receptor potential (TRP) channels bridge endoplasmic reticulum Ca2+ release with generation of Na+ and Ca2+ currents. The plasmalemmal Na+ -Ca2+ exchanger (NCX) flickers between forward and reverse mode to co-ordinate the influx and efflux of both ions with membrane polarization and cytosolic ion concentrations. The mitochondrial calcium uniporter channel (MCU) and mitochondrial Na+ -Ca2+ exchanger (NCLX) mediate Ca2+ entry into and release from this organelle and couple cytosolic Ca2+ and Na+ fluctuations with cellular energetics. Cellular Ca2+ and Na+ signalling controls numerous functional responses and, in the CNS, provides for fast regulation of astroglial homeostatic cascades that are crucial for maintenance of synaptic transmission.

The Involvement of Mg2+ in Regulation of Cellular and Mitochondrial Functions

Mg²⁺ is an essential mineral with pleotropic impacts on cellular physiology and functions. It acts as a cofactor of several important enzymes, as a regulator of ion channels such as voltage-dependent Ca²⁺ channels and K⁺ channels and on Ca²⁺-binding proteins. In general, Mg²⁺ is considered as the main intracellular antagonist of Ca²⁺, which is an essential secondary messenger initiating or regulating a great number of cellular functions. This review examines the effects of Mg²⁺ on mitochondrial functions with a particular focus on energy metabolism, mitochondrial Ca²⁺ handling, and apoptosis.

Observation of Parkin-Mediated Mitophagy in Pancreatic β-Cells

Mitophagy is a cellular process of autophagy-based mitochondrial degradation that eliminates dysfunctional mitochondria and ensures cellular homeostasis. In pancreatic islet β-cells, mitochondria play a pivotal role in glucose-stimulated insulin secretion through ATP production from glucose oxidation. Recent studies have shown that impaired mitophagy and the subsequent mitochondrial compromise contribute to β-cell dysfunction and glucose intolerance. In this chapter, we describe a protocol to monitor Parkin-mediated mitophagy in pancreatic MIN6 β-cells using flow cytometry and a pH-sensitive fluorophore, mKeima.

Interleukin-22 restored mitochondrial damage and impaired glucose-stimulated insulin secretion through down-regulation of uncoupling protein-2 in INS-1 cells

Defective glucose-stimulated insulin secretion (GSIS) induced by chronic exposure to fatty acids is a hallmark of type 2 diabetes (T2D). Interleukin-22 (IL-22) has been shown to exert beneficial effects on insulin secretion and to protect pancreatic β-cells from stress. Moreover, uncoupling protein-2 (UCP-2) plays a central role in the regulation of GSIS and β-cell dysfunction, whereas the role of UCP-2 in IL-22-enhanced glycemic control under conditions of lipotoxicity remains unclear. In this present study, we investigated the effects of IL-22 on rat insulin-secreting cells (INS-1 cells) and the mechanisms that underlie IL-22 and lipotoxicity-impaired GSIS in vitro. Chronic palmitate (PA) treatment impaired insulin secretion and activated UCP-2 expression in INS-1 cells. Furthermore, in INS-1 cells, both reduced mitochondrial membrane potential (ΔΨm) and impaired GSIS induced by PA treatment were effectively reversed by an inhibitor of UCP-2 (genipin). Additionally, compared with the PA-treated group, INS-1 cells treated with IL-22 down-regulated UCP-2 expression, increased mitochondrial membrane potential, and restored GSIS. Together, our findings indicate that chronic exposure to PA could activate UCP-2, resulting in mitochondrial damage and impaired GSIS in INS-1 cells. We also suggest that IL-22 plays a protective role in this process via the down-regulation of UCP-2.

Exendin-4 Promotes Survival of Mouse Pancreatic β-Cell Line in Lipotoxic Conditions, through the Extracellular Signal-Related Kinase 1/2 Pathway

Type 2 diabetes is a heterogeneous disorder that develops as a result of relatively inappropriate insulin secretion and insulin resistance. Increased levels of free fatty acids (FFAs) are one of the important factors for the pathogenesis of type 2 diabetes and contribute to defective β-cell proliferation and increased β-cell apoptosis. Recently, glucagon-like peptide-1 (GLP-1) receptor agonists have been shown to possess an antiapoptotic effect, by increasing β-cell mass and improving β-cell function. However, their effects on β-cells in vitro against lipotoxicity have not been elucidated completely. In this study, we investigated whether the GLP-1 receptor agonist exendin-4 displays prosurvival effects in pancreatic β-cells exposed to chronic elevated FFAs. Results showed that exendin-4 inhibited apoptosis induced by palmitate in MIN6 cells. After 24 h of incubation, exendin-4 caused rapid activation of extracellular signal-related kinase 1/2 (ERK1/2) under lipotoxic conditions. The ERK1/2 inhibitor PD98059 blocked the antilipotoxic effect of exendin-4 on MIN6 cells. Exendin-4 also inhibited the mitochondrial pathway of apoptosis. This inhibition is associated with upregulation of BCL-2. Our findings suggested that exendin-4 may exert cytoprotective effects through activation of ERK1/2 and inhibition of the mitochondrial apoptosis pathway.

H₂O₂-Activated Mitochondrial Phospholipase iPLA₂γ Prevents Lipotoxic Oxidative Stress in Synergy with UCP2, Amplifies Signaling via G-Protein-Coupled Receptor GPR40, and Regulates Insulin Secretion in Pancreatic β-Cells

AIMS

Pancreatic β-cell chronic lipotoxicity evolves from acute free fatty acid (FA)-mediated oxidative stress, unprotected by antioxidant mechanisms. Since mitochondrial uncoupling protein-2 (UCP2) plays antioxidant and insulin-regulating roles in pancreatic β-cells, we tested our hypothesis, that UCP2-mediated uncoupling attenuating mitochondrial superoxide production is initiated by FA release due to a direct H2O2-induced activation of mitochondrial phospholipase iPLA2γ.

RESULTS

Pro-oxidant tert-butylhydroperoxide increased respiration, decreased membrane potential and mitochondrial matrix superoxide release rates of control but not UCP2- or iPLA2γ-silenced INS-1E cells. iPLA2γ/UCP2-mediated uncoupling was alternatively activated by an H2O2 burst, resulting from palmitic acid (PA) β-oxidation, and it was prevented by antioxidants or catalase overexpression. Exclusively, nascent FAs that cleaved off phospholipids by iPLA2γ were capable of activating UCP2, indicating that the previously reported direct redox UCP2 activation is actually indirect. Glucose-stimulated insulin release was not affected by UCP2 or iPLA2γ silencing, unless pro-oxidant activation had taken place. PA augmented insulin secretion via G-protein-coupled receptor 40 (GPR40), stimulated by iPLA2γ-cleaved FAs (absent after GPR40 silencing).

INNOVATION AND CONCLUSION

The iPLA2γ/UCP2 synergy provides a feedback antioxidant mechanism preventing oxidative stress by physiological FA intake in pancreatic β-cells, regulating glucose-, FA-, and redox-stimulated insulin secretion. iPLA2γ is regulated by exogenous FA via β-oxidation causing H2O2 signaling, while FAs are cleaved off phospholipids, subsequently acting as amplifying messengers for GPR40. Hence, iPLA2γ acts in eminent physiological redox signaling, the impairment of which results in the lack of antilipotoxic defense and contributes to chronic lipotoxicity.

Rat Humanin is encoded and translated in mitochondria and is localized to the mitochondrial compartment where it regulates ROS production

Evidence for the putative mitochondrial origin of the Humanin (HN) peptide has been lacking, although its cytoprotective activity has been demonstrated in a variety of organismal and cellular systems. We sought to establish proof-of-principle for a mitochondria-derived peptide (MDP) in a rat-derived cellular system as the rat HN sequence is predicted to lack nuclear insertions of mitochondrial origin (NUMT). We found that the rat HN (Rattin; rHN) homologue is derived from the mitochondrial genome as evidenced by decreased production in Rho-0 cells, and that peptide translation occurs in the mitochondria as it is unaffected by cycloheximide. Rat HN localizes to the mitochondria in cellular subfractionation and immunohistochemical studies. Addition of a HN analogue to isolated mitochondria from rat INS-1 beta cells reduced hydrogen peroxide production by 55%. In summary, a locally bioactive peptide is derived and translated from an open reading frame (ORF) within rat mitochondrial DNA encoding 16S rRNA.

The Mitochondrial Peptidase Pitrilysin Degrades Islet Amyloid Polypeptide in Beta-Cells

Amyloid formation and mitochondrial dysfunction are characteristics of type 2 diabetes. The major peptide constituent of the amyloid deposits in type 2 diabetes is islet amyloid polypeptide (IAPP). In this study, we found that pitrilysin, a zinc metallopeptidase of the inverzincin family, degrades monomeric, but not oligomeric, islet amyloid polypeptide in vitro. In insulinoma cells when pitrilysin expression was decreased to 5% of normal levels, there was a 60% increase in islet amyloid polypeptide-induced apoptosis. In contrast, overexpression of pitrilysin protects insulinoma cells from human islet amyloid polypeptide-induced apoptosis. Since pitrilysin is a mitochondrial protein, we used immunofluorescence staining of pancreases from human IAPP transgenic mice and Western blot analysis of IAPP in isolated mitochondria from insulinoma cells to provide evidence for a putative intramitochondrial pool of IAPP. These results suggest that pitrilysin regulates islet amyloid polypeptide in beta cells and suggest the presence of an intramitochondrial pool of islet amyloid polypeptide involved in beta-cell apoptosis.

Roles of Pyruvate, NADH, and Mitochondrial Complex I in Redox Balance and Imbalance in β Cell Function and Dysfunction

Pancreatic β cells not only use glucose as an energy source, but also sense blood glucose levels for insulin secretion. While pyruvate and NADH metabolic pathways are known to be involved in regulating insulin secretion in response to glucose stimulation, the roles of many other components along the metabolic pathways remain poorly understood. Such is the case for mitochondrial complex I (NADH/ubiquinone oxidoreductase). It is known that normal complex I function is absolutely required for episodic insulin secretion after a meal, but the role of complex I in β cells in the diabetic pancreas remains to be investigated. In this paper, we review the roles of pyruvate, NADH, and complex I in insulin secretion and hypothesize that complex I plays a crucial role in the pathogenesis of β cell dysfunction in the diabetic pancreas. This hypothesis is based on the establishment that chronic hyperglycemia overloads complex I with NADH leading to enhanced complex I production of reactive oxygen species. As nearly all metabolic pathways are impaired in diabetes, understanding how complex I in the β cells copes with elevated levels of NADH in the diabetic pancreas may provide potential therapeutic strategies for diabetes.

Novel inhibitors of macrophage migration inhibitory factor prevent cytokine-induced beta cell death

Macrophage migration inhibitory factor is a multifunctional cytokine involved in the regulation of immune processes and also in apoptosis induction. Elevated MIF expression is detrimental for insulin-producing beta cells and MIF inhibition protected beta cells from several cytotoxic insults such as inflammatory cytokines, high fatty acids or high glucose concentrations. Therefore, the aim of this study was to investigate two newly synthesized small molecule MIF inhibitors (K664-1 and K647-1) and to compare them with previously established effects of the prototypical MIF inhibitor, ISO-1. Our results indicate that K664-1 and K647-1 are 160- and 40-fold more effective in inhibition of MIF׳s tautomerase activity than ISO-1. Also, new inhibitors confer beta cell protection from cytokine-triggered apoptosis at significantly lower concentrations than ISO-1. Although all three MIF inhibitors inhibit caspase 3 activity, K664-1 and K647-1 suppress pro-apoptotic BAX protein expression and up-regulate anti-apoptotic Bcl-2 mRNA. Finally, all three MIF inhibitors operate through blockade of nitric oxide production stimulated by cytokines. In conclusion, two novel MIF inhibitors are more potent than ISO-1 and operate through inhibition of the mitochondria-related apoptotic pathway. We propose that these compounds represent a unique class of anti-MIF antagonists that should be further tested for therapeutic use.

Inhibition of p53 preserves Parkin-mediated mitophagy and pancreatic β-cell function in diabetes

Mitochondrial compromise is a fundamental contributor to pancreatic β-cell failure in diabetes. Previous studies have demonstrated a broader role for tumor suppressor p53 that extends to the modulation of mitochondrial homeostasis. However, the role of islet p53 in glucose homeostasis has not yet been evaluated. Here we show that p53 deficiency protects against the development of diabetes in streptozotocin (STZ)-induced type 1 and db/db mouse models of type 2 diabetes. Glucolipotoxicity stimulates NADPH oxidase via receptor for advanced-glycation end products and Toll-like receptor 4. This oxidative stress induces the accumulation of p53 in the cytosolic compartment of pancreatic β-cells in concert with endoplasmic reticulum stress. Cytosolic p53 disturbs the process of mitophagy through an inhibitory interaction with Parkin and induces mitochondrial dysfunction. The occurrence of mitophagy is maintained in STZ-treated p53(-/-) mice that exhibit preserved glucose oxidation capacity and subsequent insulin secretion signaling, leading to better glucose tolerance. These protective effects are not observed when Parkin is deleted. Furthermore, pifithrin-α, a specific inhibitor of p53, ameliorates mitochondrial dysfunction and glucose intolerance in both STZ-treated and db/db mice. Thus, an intervention with cytosolic p53 for a mitophagy deficiency may be a therapeutic strategy for the prevention and treatment of diabetes.

The role of reactive oxygen species and proinflammatory cytokines in type 1 diabetes pathogenesis

Type 1 diabetes (T1D) is a T cell–mediated autoimmune disease characterized by the destruction of insulin-secreting pancreatic β cells. In humans with T1D and in nonobese diabetic (NOD) mice (a murine model for human T1D), autoreactive T cells cause β-cell destruction, as transfer or deletion of these cells induces or prevents disease, respectively. CD4⁺ and CD8⁺ T cells use distinct effector mechanisms and act at different stages throughout T1D to fuel pancreatic β-cell destruction and disease pathogenesis. While these adaptive immune cells employ distinct mechanisms for β-cell destruction, one central means for enhancing their autoreactivity is by the secretion of proinflammatory cytokines, such as IFN-γ, TNF-α, and IL-1. In addition to their production by diabetogenic T cells, proinflammatory cytokines are induced by reactive oxygen species (ROS) via redox-dependent signaling pathways. Highly reactive molecules, proinflammatory cytokines are produced upon lymphocyte infiltration into pancreatic islets and induce disease pathogenicity by directly killing β cells, which characteristically possess low levels of antioxidant defense enzymes. In addition to β-cell destruction, proinflammatory cytokines are necessary for efficient adaptive immune maturation, and in the context of T1D they exacerbate autoimmunity by intensifying adaptive immune responses. The first half of this review discusses the mechanisms by which autoreactive T cells induce T1D pathogenesis and the importance of ROS for efficient adaptive immune activation, which, in the context of T1D, exacerbates autoimmunity. The second half provides a comprehensive and detailed analysis of (1) the mechanisms by which cytokines such as IL-1 and IFN-γ influence islet insulin secretion and apoptosis and (2) the key free radicals and transcription factors that control these processes.

Mitochondrial dynamics and morphology in beta-cells

Mitochondrial dynamics contribute to the regulation of mitochondrial shape as well as various mitochondrial functions and quality control. This is of particular interest in the beta-cell because of the key role mitochondria play in the regulation of beta-cell insulin secretion function. Moreover, mitochondrial dysfunction has been suggested to contribute to the development of Type 2 Diabetes. Genetic tools that shift the balance of mitochondrial fusion and fission result in alterations to beta-cell function and viability. Additionally, conditions that induce beta-cell dysfunction, such as exposure to a high nutrient environment, disrupt mitochondrial morphology and dynamics. While it has been shown that mitochondria display a fragmented morphology in islets of diabetic patients and animal models, the mechanism behind this is currently unknown. Here, we review the current literature on mitochondrial morphology and dynamics in the beta-cell as well as some of the unanswered question in this field.

Role of K(ATP) channels in β-cell resistance to oxidative stress

The importance of K(ATP) channels in stimulus-secretion coupling of β-cells is well established, although they are not indispensable for the maintenance of glycaemic control. This review article depicts a new role for K(ATP) channels by showing that genetic or pharmacological ablation of these channels protects β-cells against oxidative stress. Increased production of oxidants is a crucial factor in the pathogenesis of type 2 diabetes mellitus (T2DM). T2DM develops when β-cells can no longer compensate for the high demand of insulin resulting from excess fuel intake. Instead β-cells start to secrete less insulin and β-cell mass is diminished by apoptosis. Both, reduction of insulin secretion and β-cell mass induced by oxidative stress, are prevented by deletion or inhibition of K(ATP) channels. These findings may open up new insights into the early treatment of T2DM.

Downregulation of mitochondrial connexin 43 by high glucose triggers mitochondrial shape change and cytochrome C release in retinal endothelial cells

PURPOSE

To determine connexin 43 (Cx43) localization in mitochondria and investigate the effects of high glucose (HG) on mitochondrial Cx43 (mtCx43) expression and whether altered mtCx43 channel activity is involved in promoting apoptosis in retinal endothelial cells.

METHODS

MtCx43 localization was determined using immunostaining, green fluorescent protein (GFP)-tagged Cx43 followed by confocal imaging, and Western blot analysis using protein isolated from mitochondria of rat retinal endothelial cells (RRECs). To assess HG effects on mtCx43 expression, RRECs were grown in normal (5 mM) or HG (30 mM) medium for 7 days, and mtCx43 protein level assessed by Western blot analysis. To determine if mtCx43 channel inhibition affected mitochondrial morphology, RRECs grown sparsely were left untreated or treated with β-glycerrhetinic acid (β-GA), an inhibitor of connexin channels, and imaged using confocal microscopy. Additionally, mitochondria isolated from RRECs were treated with β-GA, and cytochrome c release assessed by Western blot.

RESULTS

Cx43 localization on the mitochondria of RRECs was confirmed with immunofluorescence staining using Cx43 antibody and GFP-tagged Cx43 imaged in live cells. Western blot analysis indicated that Cx43 was located primarily on the inner mitochondrial membrane, and mtCx43 protein level was significantly reduced in RRECs grown in HG condition. Treatment of RRECs with β-GA significantly decreased mtCx43 phosphorylation, induced mitochondrial fragmentation, and isolated mitochondria treated with β-GA showed increased cytochrome c release.

CONCLUSIONS

HG-induced downregulation of mtCx43 protein resulting in decreased channel activity may promote mitochondrial morphology changes and cytochrome c release, suggesting a novel mechanism for hyperglycemia-induced apoptosis in diabetic retinopathy.

Mitochondrial function in vascular endothelial cell in diabetes

Micro- and macrovascular complications are commonly seen in diabetic patients and endothelial dysfunction contributes to the development and progression of the complications. Abnormal functions in endothelial cells lead to the increase in vascular tension and atherosclerosis, followed by systemic hypertension as well as increased incidence of ischemia and stroke in diabetic patients. Mitochondria are organelles serving as a source of energy production and as regulators of cell survival (e.g., apoptosis and cell development) and ion homeostasis (e.g., H(+), Ca(2+)). Endothelial mitochondria are mainly responsible for generation of reactive oxygen species (ROS) and maintaining the Ca(2+) concentration in the cytosol. There is increasing evidence that mitochondrial morphological and functional changes are implicated in vascular endothelial dysfunction. Enhanced mitochondrial fission and/or attenuated fusion lead to mitochondrial fragmentation and disrupt the endothelial physiological function. Abnormal mitochondrial biogenesis and disturbance of mitochondrial autophagy increase the accumulation of damaged mitochondria, such as irreversibly depolarized or leaky mitochondria, and facilitate cell death. Augmented mitochondrial ROS production and Ca(2+) overload in mitochondria not only cause the maladaptive effect on the endothelial function, but also are potentially detrimental to cell survival. In this article, we review the physiological and pathophysiological role of mitochondria in endothelial function with special focus on diabetes.

Myricetin protects against cytokine-induced cell death in RIN-m5f β cells

Cytokine-induced cell death is recognized as a major cause of progressive β-cell loss. Tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and interferon γ (IFN-γ) in combination trigger a series of events that lead to β-cell death. In the past few decades, the use of myricetin as an anti-inflammatory and cytoprotective agent has gained much attention. The present study focused on the protective roles of myricetin against cytokine-induced cell death in insulin-secreting RIN-m5f β cells. The results showed that myricetin (especially at concentrations of 10 μM and 20 μM) increased cell viability and decreased cell apoptosis induced by the cytokine mixture of TNF-α (10 ng/mL), IL-1β (5 ng/mL), and IFN-γ (1000 IU/mL) for 3 days. Moreover, the cytokines increased the total and p65 subunit levels of nuclear factor κB, decreased inhibitor κB α levels, stimulated the accumulation of nitric oxide, increased cytochrome c release from mitochondria, and induced reactive oxygen species generation; myricetin (especially at the concentration of 20 μM) abolished all of these parameters. These results suggest that myricetin might have therapeutic value for preventing β-cell death.

The protective effect of peroxiredoxin II on oxidative stress induced apoptosis in pancreatic β-cells

Excessive loss of pancreatic β-cells, mainly through apoptosis, contributes to the development of diabetic hyperglycemia. Oxidative stress plays a major role in the process of β-cell apoptosis due to low expression level of endogenous antioxidants in the β-cells. Peroxiredoxins (PRDX) are a family of peroxide reductases which uses thioredoxin to clear peroxides. Several members of PRDX have been found in β-cells and recent studies suggested that these antioxidant enzymes possess protective effects in β-cells against oxidative stress mediated apoptosis. In this study, we aimed to investigate the role of PRDX2 in modulating β-cell functions. We detected the expression of PRDX2 both at the transcript and protein levels in the clonal β-cells INS-1 and MIN6 as well as rodent islets. Western blot showed that treatment of MIN6 β-cell line with proinflammatory cytokines, palmitic acid or streptozotocin dose- or time-dependently increased apoptosis, which was associated with reduced endogenous expression levels of PRDX2. To examine the role for PRDX2 in the apoptotic stimuli-induced β-cell apoptosis, we used plasmid overexpression and siRNA knockdown strategies to investigate whether the elevation or knockdown of PRDX2 affects stimuli-induced apoptosis in the β-cells. Remarkably, overexpression of PRDX2 in MIN6 cells significantly attenuated the oxidative stresses mediated apoptosis, as evaluated by cleaved caspase 3 expression, nuclear condensation and fragmentation, as well as FACS analysis. Conversely, attenuation of PRDX2 protein expression using siRNA knockdown exaggerated the cell death induced by proinflammatory cytokines and palmitic acid in the MIN6 cells. These results suggest that PRDX2 may play a protective role in pancreatic β-cells under oxidative stress.

A micropreparation of mitochondria from cells using magnetic beads with immunoaffinity

Mitochondrial preparation is a key technique in the study of mitochondria. Growing evidence has demonstrated that mitochondrial proteins are tissue or cell type dependent. Locating the proteins in the global presence of mitochondrial membranes is a primary consideration in adopting antibodies for affinity enrichment of mitochondria on a micro scale. Two proteins located on the outer membrane of mitochondria, cytochrome b5 type B (CYB5B) and synaptojanin-2-binding protein (SYNJ2BP), were selected as candidates based on a survey of databases and the literature. The polyclonal antibodies against the truncated CYB5B and SYNJ2BP exhibited specific recognition to mitochondria and wider sensitivity to several tested mouse tissues and cell lines, whereas the antibody 22-kDa translocase of the outer mitochondrial membrane (TOM22) nearly missed detection of mitochondria in the liver and responded minimally to mitochondria from H9C2 and L-02 cells. Through the affinity enrichment for cellular mitochondria using magnetic beads coated with anti-CYB5B or anti-SYNJ2BP, we found that the anti-CYB5B beads could enrich mitochondria more efficiently even on a scale of 10,000 cultured cells. For the integrity and protein components, the enriched mitochondria on anti-CYB5B were carefully examined and were accepted in further functional study. We propose that an anti-CYB5B immunomagnetic approach is feasible in the micropreparation of mitochondria from cultured cells.

The central role of calcium in the effects of cytokines on beta-cell function: implications for type 1 and type 2 diabetes

The appropriate regulation of intracellular calcium is a requirement for proper cell function and survival. This review focuses on the effects of proinflammatory cytokines on calcium regulation in the insulin-producing pancreatic beta-cell and how normal stimulus-secretion coupling, organelle function, and overall beta-cell viability are impacted. Proinflammatory cytokines are increasingly thought to contribute to beta-cell dysfunction not only in type 1 diabetes (T1D), but also in the progression of type 2 diabetes (T2D). Cytokine-induced disruptions in calcium handling result in reduced insulin release in response to glucose stimulation. Cytokines can alter intracellular calcium levels by depleting calcium from the endoplasmic reticulum (ER) and by increasing calcium influx from the extracellular space. Depleting ER calcium leads to protein misfolding and activation of the ER stress response. Disrupting intracellular calcium may also affect organelles, including the mitochondria and the nucleus. As a chronic condition, cytokine-induced calcium disruptions may lead to beta-cell death in T1D and T2D, although possible protective effects are also discussed. Calcium is thus central to both normal and pathological cell processes. Because the tight regulation of intracellular calcium is crucial to homeostasis, measuring the dynamics of calcium may serve as a good indicator of overall beta-cell function.

Exercise training prevents endometrial hyperplasia and biomarkers for endometrial cancer in rat model of type 1 diabetes

Background

Endometrial cancer is one of the most common types of gynecologic cancers. The ability of exercise to reduce the risk of endometrial cancer in women with type 2 diabetes has been established, but no studies have examined this link in type 1 diabetes.A randomized, controlled animal study was designed using a standard rat model of type 1 diabetes. The goal of this study was to investigate the ability of exercise to prevent increased levels of endometrial cancer biomarkers, estrogen receptor (ERα) and p16, and endometrial hyperplasia associated with diabetes.

Methods

Forty female rats were randomized into four groups: sedentary control, exercise control, sedentary or exercised diabetic. Diabetes was induced by alloxan injection. A 4-week treadmill training program was initiated with the development of diabetes. Endometrial tissues were evaluated for hyperplasia and ERα and p16 levels and subcellular localization using microscopy.

Results

Severe diabetes lead to hyperplasia in the endometrial tissue in 70% of sedentary diabetic rats. Exercise-trained diabetic rats and the non-diabetic rats displayed no hyperplasia. The expression of ERα increased significantly (p < 0.02) while the expression level of p16 decreased significantly (p < 0.04) in the diabetic sedentary group compared to the non-diabetic groups. Exercise training led to a reversal in the percentage of p16 and ERα positive cells in diabetic rats.

Conclusions

Severe diabetes leads to hyperplasia of the endometrial tissue and increased ERα levels and decreased p16 levels in rats, which can be prevented with aerobic exercise.

Keywords

Diabetes; Estrogen receptor alpha; P16; Endometrial hyperplasia; Endometrial cancer; Exercise

Methods to assess beta cell death mediated by cytotoxic T lymphocytes

Type 1 diabetes (T1D) is a T cell mediated autoimmune disease. During the pathogenesis, patients become progressively more insulinopenic as insulin production is lost, presumably this results from the destruction of pancreatic beta cells by T cells. Understanding the mechanisms of beta cell death during the development of T1D will provide insights to generate an effective cure for this disease. Cell-mediated lymphocytotoxicity (CML) assays have historically used the radionuclide Chromium 51 ((51)Cr) to label target cells. These targets are then exposed to effector cells and the release of (51)Cr from target cells is read as an indication of lymphocyte-mediated cell death. Inhibitors of cell death result in decreased release of (51)Cr. As effector cells, we used an activated autoreactive clonal population of CD8(+) Cytotoxic T lymphocytes (CTL) isolated from a mouse stock transgenic for both the alpha and beta chains of the AI4 T cell receptor (TCR). Activated AI4 T cells were co-cultured with (51)Cr labeled target NIT cells for 16 hours, release of (51)Cr was recorded to calculate specific lysis Mitochondria participate in many important physiological events, such as energy production, regulation of signaling transduction, and apoptosis. The study of beta cell mitochondrial functional changes during the development of T1D is a novel area of research. Using the mitochondrial membrane potential dye Tetramethyl Rhodamine Methyl Ester (TMRM) and confocal microscopic live cell imaging, we monitored mitochondrial membrane potential over time in the beta cell line NIT-1. For imaging studies, effector AI4 T cells were labeled with the fluorescent nuclear staining dye Picogreen. NIT-1 cells and T cells were co-cultured in chambered coverglass and mounted on the microscope stage equipped with a live cell chamber, controlled at 37°C, with 5% CO(2;), and humidified. During these experiments images were taken of each cluster every 3 minutes for 400 minutes. Over a course of 400 minutes, we observed the dissipation of mitochondrial membrane potential in NIT-1 cell clusters where AI4 T cells were attached. In the simultaneous control experiment where NIT-1 cells were co-cultured with MHC mis-matched human lymphocyte Jurkat cells, mitochondrial membrane potential remained intact. This technique can be used to observe real-time changes in mitochondrial membrane potential in cells under attack of cytotoxic lymphocytes, cytokines, or other cytotoxic reagents.

Converging evidence of mitochondrial dysfunction in a yeast model of homocysteine metabolism imbalance

An elevated level of homocysteine, a thiol amino acid, is associated with various complex disorders. The cellular effects of homocysteine and its precursors S-adenosylhomocysteine (AdoHcy) and S-adenosylmethionine (AdoMet) are, however, poorly understood. We used Saccharomyces cerevisiae as a model to understand the basis of pathogenicity induced by homocysteine and its precursors. Both homocysteine and AdoHcy but not AdoMet inhibited the growth of the str4Δ strain (which lacks the enzyme that converts homocysteine to cystathionine-mimicking vascular cells). Addition of AdoMet abrogated the inhibitory effect of AdoHcy but not that of homocysteine indicating that an increase in the AdoMet/AdoHcy ratio is sufficient to overcome the AdoHcy-mediated growth defect but not that of homocysteine. Also, the transcriptomic profile of AdoHcy and homocysteine showed gross dissimilarity based on gene enrichment analysis. Furthermore, compared with homocysteine, AdoHcy treatment caused a higher level of oxidative stress in the cells. However, unlike a previously reported response in wild type (Kumar, A., John, L., Alam, M. M., Gupta, A., Sharma, G., Pillai, B., and Sengupta, S. (2006) Biochem. J. 396, 61-69), the str4Δ strain did not exhibit an endoplasmic reticulum stress response. This suggests that homocysteine induces varied response depending on the flux of homocysteine metabolism. We also observed altered expression of mitochondrial genes, defective membrane potential, and fragmentation of the mitochondrial network together with the increased expression of fission genes indicating that the imbalance in homocysteine metabolism has a major effect on mitochondrial functions. Furthermore, treatment of cells with homocysteine or AdoHcy resulted in apoptosis as revealed by annexin V staining and TUNEL assay. Cumulatively, our results suggest that elevated levels of homocysteine lead to mitochondrial dysfunction, which could potentially initiate pro-apoptotic pathways, and this could be one of the mechanisms underlying homocysteine-induced pathogenicity.

Long-term exposure of INS-1 rat insulinoma cells to linoleic acid and glucose in vitro affects cell viability and function through mitochondrial-mediated pathways

Obesity with excessive levels of circulating free fatty acids (FFAs) is tightly linked to the incidence of type 2 diabetes. Insulin resistance of peripheral tissues and pancreatic β-cell dysfunction are two major pathological changes in diabetes and both are facilitated by excessive levels of FFAs and/or glucose. To gain insight into the mitochondrial-mediated mechanisms by which long-term exposure of INS-1 cells to excess FFAs causes β-cell dysfunction, the effects of the unsaturated FFA linoleic acid (C 18:2, n-6) on rat insulinoma INS-1 β cells was investigated. INS-1 cells were incubated with 0, 50, 250 or 500 μM linoleic acid/0.5% (w/v) BSA for 48 h under culture conditions of normal (11.1 mM) or high (25 mM) glucose in serum-free RPMI-1640 medium. Cell viability, apoptosis, glucose-stimulated insulin secretion, Bcl-2, and Bax gene expression levels, mitochondrial membrane potential and cytochrome c release were examined. Linoleic acid 500 μM significantly suppressed cell viability and induced apoptosis when administered in 11.1 and 25 mM glucose culture medium. Compared with control, linoleic acid 500 μM significantly increased Bax expression in 25 mM glucose culture medium but not in 11.1 mM glucose culture medium. Linoleic acid also dose-dependently reduced mitochondrial membrane potential (ΔΨm) and significantly promoted cytochrome c release from mitochondria in both 11.1 mM glucose and 25 mM glucose culture medium, further reducing glucose-stimulated insulin secretion, which is dependent on normal mitochondrial function. With the increase in glucose levels in culture medium, INS-1 β-cell insulin secretion function was deteriorated further. The results of this study indicate that chronic exposure to linoleic acid-induced β-cell dysfunction and apoptosis, which involved a mitochondrial-mediated signal pathway, and increased glucose levels enhanced linoleic acid-induced β-cell dysfunction.

Roles of mitochondria in human disease

The chapters throughout this volume illustrate the many contributions of mitochondria to the maintenance of normal cell and tissue function, experienced as the health of the individual. Mitochondria are essential for maintaining aspects of physiology as fundamental as cellular energy balance, the modulation of calcium signalling, in defining cellular redox balance, and they house significant biosynthetic pathways. Mitochondrial numbers and volume within cells are regulated and have an impact on their functional roles, while, especially in the CNS (central nervous system), mitochondrial trafficking is critical to ensure the cellular distribution and strategic localization of mitochondria, presumably driven by local energy demand. Maintenance of a healthy mitochondrial population involves a complex system of quality control, involving degrading misfolded proteins, while damaged mitochondria are renewed by fusion or removed by autophagy. It seems evident that mechanisms that impair any of these processes will impair mitochondrial function and cell signalling pathways, leading to disordered cell function which manifests as disease. As gatekeepers of cell life and cell death, mitochondria regulate both apoptotic and necrotic cell death, and so at its most extreme, disturbances involving these pathways may trigger untimely cell death. Conversely, the lack of appropriate cell death can lead to inappropriate tissue growth and development of tumours, which are also characterized by altered mitochondrial metabolism. The centrality of mitochondrial dysfunction to a surprisingly wide range of major human diseases is slowly becoming recognized, bringing with it the prospect of novel therapeutic approaches to treat a multitude of unpleasant and pervasive diseases.

Abeta and human amylin share a common toxicity pathway via mitochondrial dysfunction

Alzheimer's disease (AD) and type 2 diabetes mellitus (T2DM) are leading causes of morbidity and mortality in the elderly. Both diseases are characterized by amyloid deposition in target tissues: aggregation of amylin in T2DM is associated with loss of insulin-secreting beta-cells, while amyloid beta (A beta) aggregation in AD brain is associated with neuronal loss. Here, we used quantitative iTRAQ proteomics as a discovery tool to show that both A beta and human amylin (HA) deregulate identical proteins, a quarter of which are mitochondrial, supporting the notion that mitochondrial dysfunction is a common target in these two amyloidoses. A functional validation revealed that mitochondrial complex IV activity was significantly reduced after treatment with either HA or A beta, as was mitochondrial respiration. In comparison, complex I activity was reduced only after treatment with HA. A beta and HA, but not the non-amyloidogenic rat amylin, induced significant increases in the generation of ROS. Co-incubation of HA and A beta did not produce an augmented effect in ROS production, again suggesting common toxicity mechanisms. In conclusion, our data suggest that A beta and HA both exert toxicity, at least in part, via mitochondrial dysfunction, thus restoring their function may be beneficial for both AD and T2DM.

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.

Mixed lineage kinase-3 stabilizes and functionally cooperates with TRIBBLES-3 to compromise mitochondrial integrity in cytokine-induced death of pancreatic beta cells

Mixed lineage kinases (MLKs) have been implicated in cytokine signaling as well as in cell death pathways. Our studies show that MLK3 is activated in leukocyte-infiltrated islets of non-obese diabetic mice and that MLK3 activation compromises mitochondrial integrity and induces apoptosis of beta cells. Using an ex vivo model of islet-splenocyte co-culture, we show that MLK3 mediates its effects via the pseudokinase TRB3, a mammalian homolog of Drosophila Tribbles. TRB3 expression strongly coincided with conformational change and mitochondrial translocation of BAX. Mechanistically, MLK3 directly interacted with and stabilized TRB3, resulting in inhibition of Akt, a strong suppressor of BAX translocation and mitochondrial membrane permeabilization. Accordingly, attenuation of MLK3 or TRB3 expression each prevented cytokine-induced BAX conformational change and attenuated the progression to apoptosis. We conclude that MLKs compromise mitochondrial integrity and suppress cellular survival mechanisms via TRB3-dependent inhibition of Akt.

Cell signaling and cancer-possible targets for therapy

Tumor progression involves the acquisition of properties which include growth-factor independent cell proliferation, failure of inhibition by growth-inhibitory signals, ability to invade surrounding tissues, and to evade apoptosis, etc. Characterization of the profile or molecular signature of the tumor may permit the development of rational therapies that target the aberrant pathways. Rapidly growing tumor cells are usually associated with a high rates of glycolysis and in these cells, it may be advantageous to exploit this pathway which most likely is required for optimal synthetic needs. Combinatorial therapeutic agents which target the growth factor signal transduction pathways as well as apoptotic signaling pathways provide an opportunity for maximal therapeutic benefit.