PTEN, the Achilles' heel of myocardial ischaemia/reperfusion injury?

Advancing stem cell therapy from bench to bedside: lessons from drug therapies

The inadequacy of existing therapeutic tools together with the paucity of organ donors have always led medical researchers to innovate the current treatment methods or to discover new ways to cure disease. Emergence of cell-based therapies has provided a new framework through which it has given the human world a new hope. Though relatively a new concept, the pace of advancement clearly reveals the significant role that stem cells will ultimately play in the near future. However, there are numerous uncertainties that are prevailing against the present setting of clinical trials related to stem cells: like the best route of cell administration, appropriate dosage, duration and several other applications. A better knowledge of these factors can substantially improve the effectiveness of disease cure or organ repair using this latest therapeutic tool. From a certain perspective, it could be argued that by considering certain proven clinical concepts and experience from synthetic drug system, we could improve the overall efficacy of cell-based therapies. In the past, studies on synthetic drug therapies and their clinical trials have shown that all the aforementioned factors have critical ascendancy over its therapeutic outcomes. Therefore, based on the knowledge gained from synthetic drug delivery systems, we hypothesize that by employing many of the clinical approaches from synthetic drug therapies to this new regenerative therapeutic tool, the efficacy of stem cell-based therapies can also be improved.

Cell death proteins as markers of early postmortem interval

Estimation of time since death is one of the challenges in forensic science. There are many approaches to estimate the postmortem interval, including both physical and thanatochemistry methods. Decomposition is triggered by a process called autolysis, which induces destructive changes in the cell leading to cell death. Based on the process of cell death signaling, this study analyzed the early postmortem interval (2-8 h since death) using the study of the mRNA expression of Fas Ligand (FasL) and phosphatase and tensin homologue deleted on chromosome 10 (PTEN) by Quantitative-PCR. Results of the study indicate a time-dependent increase in the mRNA levels of both proteins up until 6 h after death. Using a regression analysis in these first 6 h, a positive linear correlation was found between the mRNA expression of these proteins and the time since death. Since PTEN and FasL are implicated in signaling pathways, both proteins are potential candidates to analyze the time since death in time intervals of 6 h or less. Further research is needed to find additional cell death markers and expand the time period for time since death estimation.

Cancer affects microRNA expression, release, and function in cardiac and skeletal muscle

Circulating microRNAs (miRNA) are emerging as important biomarkers of various diseases, including cancer. Intriguingly, circulating levels of several miRNAs are lower in patients with cancer compared with healthy individuals. In this study, we tested the hypothesis that a circulating miRNA might serve as a surrogate of the effects of cancer on miRNA expression or release in distant organs. Here we report that circulating levels of the muscle-enriched miR486 is lower in patients with breast cancer compared with healthy individuals and that this difference is replicated faithfully in MMTV-PyMT and MMTV-Her2 transgenic mouse models of breast cancer. In tumor-bearing mice, levels of miR486 were relatively reduced in muscle, where there was elevated expression of the miR486 target genes PTEN and FOXO1A and dampened signaling through the PI3K/AKT pathway. Skeletal muscle expressed lower levels of the transcription factor MyoD, which controls miR486 expression. Conditioned media (CM) obtained from MMTV-PyMT and MMTV-Her2/Neu tumor cells cultured in vitro were sufficient to elicit reduced levels of miR486 and increased PTEN and FOXO1A expression in C2C12 murine myoblasts. Cytokine analysis implicated tumor necrosis factor α (TNFα) and four additional cytokines as mediators of miR486 expression in CM-treated cells. Because miR486 is a potent modulator of PI3K/AKT signaling and the muscle-enriched transcription factor network in cardiac/skeletal muscle, our findings implicated TNFα-dependent miRNA circuitry in muscle differentiation and survival pathways in cancer.

Mechanisms involved in attenuated cardio-protective role of ischemic preconditioning in metabolic disorders

Myocardial infarction is a pathological state which occurs due to severe abrogation of the blood supply (ischemia) to a part of heart, which can cause myocardial damage. The short intermittent cycles of sub-lethal ischemia and reperfusion has shown to improve the tolerance of the myocardium against subsequent prolonged ischemia/reperfusion (I/R)-induced injury, which is known as ischemic preconditioning (IPC). Although, IPC-induced cardioprotection is well demonstrated in various species, including human beings, accumulated evidence clearly suggests critical abrogation of the beneficial effects of IPC in diabetes mellitus, hyperlipidemia and hyperhomocysteinemia. Various factors are involved in the attenuation of the cardioprotective effect of preconditioning, such as the reduced release of calcitonin gene-related peptide (CGRP), the over-expression of glycogen synthase kinase-3β (GSK-3β) and phosphatase and tensin homolog (PTEN), impairment of mito-KATP channels, the consequent opening of mitochondrial permeability transition pore (MPTP), etc. In this review, we have critically discussed the various signaling pathways involved in abrogated preconditioning in chronic diabetes mellitus, hyperlipidemia and hyperhomocysteinemia. We have also focused on the involvement of PTEN in abrogated preconditioning and the significance of PTEN inhibitors.

Dual-specificity phosphatases as molecular targets for inhibition in human disease

SIGNIFICANCE

The dual-specificity phosphatases (DUSPs) constitute a heterogeneous group of cysteine-based protein tyrosine phosphatases, whose members exert a pivotal role in cell physiology by dephosphorylation of phosphoserine, phosphothreonine, and phosphotyrosine residues from proteins, as well as other non-proteinaceous substrates.

RECENT ADVANCES

A picture is emerging in which a selected group of DUSP enzymes display overexpression or hyperactivity that is associated with human disease, especially human cancer, making feasible targeted therapy approaches based on their inhibition. A panoply of molecular and functional studies on DUSPs have been performed in the previous years, and drug-discovery efforts are ongoing to develop specific and efficient DUSP enzyme inhibitors. This review summarizes the current status on inhibitory compounds targeting DUSPs that belong to the MAP kinase phosphatases-, small-sized atypical-, and phosphatases of regenerating liver subfamilies, whose inhibition could be beneficial for the prevention or mitigation of human disease.

CRITICAL ISSUES

Achieving specificity, potency, and bioavailability are the major challenges in the discovery of DUSP inhibitors for the clinics. Clinical validation of compounds or alternative inhibitory strategies of DUSP inhibition has yet to come.

FUTURE DIRECTIONS

Further work is required to understand the dual role of many DUSPs in human cancer, their function-structure properties, and to identify their physiologic substrates. This will help in the implementation of therapies based on DUSPs inhibition.

TAT-protein blockade during ischemia/reperfusion reveals critical role for p85 PI3K-PTEN interaction in cardiomyocyte injury

Recent work shows that cooling protection after mouse cardiac arrest and cardiomyocyte ischemia is mediated by Akt activation. The PI3K p85 subunit can either augment or inhibit Akt activation depending on its binding to p110 or PTEN respectively. To further clarify the role of PI3K p85 in cardioprotection, we studied novel TAT-p85 fusion proteins that selectively inhibit PI3K p85 binding. We hypothesized that TAT fused p85 lacking the PTEN binding site (TAT-ΔPTEN p85) would enhance Akt phosphorylation to afford cardioprotection. Conversely, TAT fused p85 lacking the p110 binding site (TAT-Δp110p85) would decrease Akt phosphorylation and abrogate cardioprotection. Microscopy and Western blot analysis demonstrated that TAT fusion protein was transduced into cardiomyocytes within 5 min and remained more than 2 h. Inhibition of PI3K/Akt by TAT-Δp110 p85 significantly increased cell death from 44.6±2.7% to 92.5±3.4% after simulated ischemia and reperfusion. By contrast, PTEN inhibition using TAT-ΔPTEN p85 decreased cell death to 11.9±5.3%, a similar level of cardioprotection seen with past cooling studies. Additional studies with the small molecule PTEN inhibitor VO-OHpic confirmed that PTEN inhibition was highly protective against cell death induced by ischemia and reperfusion. We conclude that blockade of p85-PTEN interaction and PTEN inhibition may be promising strategies for rescuing the heart from ischemia and reperfusion injury.

MicroRNA-214 protects cardiac myocytes against H2O2-induced injury

Reactive oxygen species (ROS)-induced cardiac myocyte injury resulting from changes in the expression levels of multiple genes plays a critical role in the pathogenesis of numerous heart diseases. The purpose of this study was to determine the potential roles of microRNA-214 (miR-214) in hydrogen peroxide (H2O2)-mediated gene regulation in cardiac myocytes. In this study, we used quantitative real-time RT-PCR (qRT-PCR) to demonstrate that miR-214 was upregulated in cardiac myocytes after treatment with H2O2. We transfected cells with pre-miR-214 to upregulate miR-214 expression and transfected cells with a miR-214 inhibitor (anti-miR-214) to downregulate miR-214 expression. H2O2-induced cardiac cell apoptosis was detected by flow cytometry. The level of apoptosis was increased by the miR-214 inhibitor and decreased by pre-miR-214. Therefore, we believe that miR-214 plays a positive role in H2O2-induced cardiac cell apoptosis. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is constitutively active and is considered to be the primary downregulator of the pro-oncogenic PI3K/Akt pathway. Western blot analysis revealed that the expression of the PTEN protein in cardiac myocytes decreased after H2O2 induction. Anti-miR-214 increased PTEN protein expression level, in contrast, pre-miR-214 decreased the PTEN protein expression level in cultured cardiac myocytes. These results indicate that PTEN is regulated by miR-214 and serves as an important target of miR-214 in cardiac myocytes. In conclusion, miR-214 is sensitive to H2O2 stimulation, and miR-214 protects cardiac myocytes against H2O2-induced injury via one of its targets, PTEN.

Signals regulating necrosis of cardiomyoblast by BTG2(/TIS21/PC3) via activation of GSK3β and opening of mitochondrial permeability transition pore in response to H2O2

To investigate signal transduction pathway of cell death regulated by a tumor suppressor after oxidative stress, cardiomyoblasts were virally transfected with BTG2(/TIS21/PC3) (BTG2) and subsequently treated with H2O2. Heart muscle rarely expresses BTG2 unless oxidative stress occurs, however, ischemia induced BTG2 expression and necrosis, not apoptosis, of cardiomyoblasts. BTG2-expressioning cardiomyblasts showed impaired recoveries of survival kinases, Akt and Erk, thus sustaining GSK-3β activity in 30 min of H2O2 exposure, in contrast to their rapid recoveries in LacZ control. The phenomenon was accompanied by the failure of ATP regeneration and the sustained activation of AMPK in the BTG2 expresser. Furthermore, H2O2 treatment markedly induced BTG2 translocation from nuclei to mitochondria along with cell death by cyclophilin D activation and mPTP opening. Exogenous and endogenous effect of BTG2 was confirmed by chemical inhibitors and BTG2-KO-MEF, respectively. Here, we suggest tumor suppressor, BTG2, as one of the regulators of necrosis in myocardium via inhibiting Akt/Erk, but activating GSK3β and cyclophilin D, which resulted in mPTP opening in response to H2O2.

PTEN plasticity: how the taming of a lethal gene can go too far

PTEN loss drives many cancers and recent genetic studies reveal that often PTEN is antagonised at the protein level without alteration of DNA or RNA expression. This scenario can already cause malignancy, because PTEN is haploinsufficient. We here review normally occurring mechanisms of PTEN protein regulation and discuss three processes where PTEN plasticity is needed: ischaemia, development, and wound healing. These situations demand transient PTEN suppression, whereas cancer exploits them for continuous proliferation and survival advantages. Therefore, increased understanding of PTEN plasticity may help us better interpret tumour development and ultimately lead to drug targets for PTEN supporting cancer therapy.

MicroRNAs in ischemia-reperfusion injury

Ischemia-reperfusion injury (IRI) is a major causal factor of tissue injury in various clinical settings including myocardial infarction, stroke, and free microsurgical tissue transfer. MicroRNAs (miRNAs, miRs) are short, non-coding RNA molecules involved in post-transcriptional regulation of gene expression. During the last years they have emerged as regulators of IRI as well as ischemic preconditioning and ischemic postconditioning. Here we give an overview of the current literature and describe the potential use of miRNA-based therapeutics for the treatment and prevention of ischemia-reperfusion injury in the future.

Remote ischemic preconditioning confers late protection against myocardial ischemia-reperfusion injury in mice by upregulating interleukin-10

Remote ischemic preconditioning (RIPC) induces a prolonged late phase of multi-organ protection against ischemia-reperfusion (IR) injury. In the present study, we tested the hypothesis that RIPC confers late protection against myocardial IR injury by upregulating expression of interleukin (IL)-10. Mice were exposed to lower limb RIPC or sham ischemia. After 24 h, mice with RIPC demonstrated decreased myocardial infarct size and improved cardiac contractility following 30-min ischemia and 120-min reperfusion (I-30/R-120). These effects of RIPC were completely blocked by anti-IL-10 receptor antibodies. In IL-10 knockout mice, RIPC cardioprotection was lost, but it was mimicked by exogenous IL-10. Administration of IL-10 to isolated perfused hearts increased phosphorylation of the protein kinase Akt and limited infarct size after I-30/R-120. In wild-type mice, RIPC increased plasma and cardiac IL-10 protein levels and caused activation of Akt and endothelial nitric oxide synthase in the heart at 24 h, which was also blocked by anti-IL-10 receptor antibodies. In the gastrocnemius muscle, RIPC resulted in immediate inactivation of the phosphatase PTEN and activation of Stat3, with increased IL-10 expression 24 h later. Myocyte-specific PTEN inactivation led to increased Stat3 phosphorylation and IL-10 protein expression in the gastrocnemius muscle. Taken together, these results suggest that RIPC induces late protection against myocardial IR injury by increasing expression of IL-10 in the remote muscle, followed by release of IL-10 into the circulation, and activation of protective signaling pathways in the heart. This study provides a scientific basis for the use of RIPC to confer systemic protection against IR injury.

Regulation of phosphatase and tensin homolog on chromosome 10 in response to hypoxia

Phosphatase and tensin homolog on chromosome 10 (PTEN) is downregulated during hypertrophic and cancerous cell growth, leading to activation of the prosurvival Akt pathway. However, PTEN regulation in cardiac myocytes upon exposure to hypoxia remains unclear. We explored the role of PTEN in response to hypoxia/ischemia in the myocardium. We validated that PTEN is a transcriptional target of activating transcription factor 2 (ATF-2) and is positively regulated via a p38/ATF-2 signaling pathway. Accordingly, hypoxia-induced upregulation of phosphorylation of ATF-2 and PTEN were reversed by a dominant negative mutant p38. Inhibition of PTEN in cardiomyocytes attenuated hypoxia-induced cell death and apoptosis. Cardiac-specific knockout of PTEN resulted in increased phosphorylation of Akt and forkhead box O 1 (forkhead transcription factors), limited infarct size in animals exposed to ischemia-reperfusion injury, and ameliorated deterioration of left ventricular function and remodeling following permanent coronary artery occlusion. In addition, the activation of Bim, FASL, and caspase was coupled with PTEN activation, all of which were attenuated by PTEN inhibition. In conclusion, cardiomyocyte-specific conditional PTEN deletion limited myocardial infarct size in an in vivo model of ischemia-reperfusion injury and attenuated adverse remodeling in a model of chronic permanent coronary artery ligation.

Phosphatase PTEN is critically involved in post-myocardial infarction remodeling through the Akt/interleukin-10 signaling pathway

The inflammatory cytokines interleukin (IL)-10 and tumor necrosis factor (TNF)-α play an important role in left ventricular (LV) remodeling after myocardial infarction (MI). Phosphatase and tensin homolog deleted on chromosome ten (PTEN) inactivates protein kinase Akt and promotes cell death in the heart. However, it is not known whether PTEN promotes post-MI remodeling by regulating IL-10 and TNF-α. MI was induced in wild-type (WT) mice and Pten heterozygous mutant (HET) mice. Pten adenoviruses (adPten) or empty vectors (adNull) were injected into the peri-infarct area of WT mice. LV dilation was attenuated and fractional shortening was increased in HET mice compared to WT mice. Survival rate and fractional shortening were decreased in adPten mice compared to adNull mice. Leukocyte infiltration into the peri-infarct area was attenuated in HET mice and worsened in adPten mice. PTEN expression was upregulated in the infarcted heart of WT mice. Partial inactivation of PTEN increased the production of IL-10 and decreased the expression of TNF-α and matrix metalloproteinase (MMP)-2 and -9 after MI in HET mice. PTEN overexpression caused opposite effects in the infarcted heart. Moreover in the infarcted heart of HET mice, Akt inhibition decreased Stat3 phosphorylation and IL-10 expression, and blockade of the IL-10 receptor increased TNF-α and MMP-2 expression. Both Akt inhibition and IL-10 receptor blockade abolished the attenuation of post-MI remodeling in HET mice. In conclusion, PTEN is critically involved in post-MI remodeling through the Akt/IL-10 signaling pathway. Therefore, targeting PTEN may be an effective approach to post-MI remodeling.

Cardiac-specific expression of the tetracycline transactivator confers increased heart function and survival following ischemia reperfusion injury

Mice expressing the tetracycline transactivator (tTA) transcription factor driven by the rat α-myosin heavy chain promoter (α-MHC-tTA) are widely used to dissect the molecular mechanisms involved in cardiac development and disease. However, these α-MHC-tTA mice exhibit a gain-of-function phenotype consisting of robust protection against ischemia/reperfusion injury in both in vitro and in vivo models in the absence of associated cardiac hypertrophy or remodeling. Cardiac function, as assessed by echocardiography, did not differ between α-MHC-tTA and control animals, and there were no noticeable differences observed between the two groups in HW/TL ratio or LV end-diastolic and end-systolic dimensions. Protection against ischemia/reperfusion injury was assessed using isolated perfused hearts where α-MHC-tTA mice had robust protection against ischemia/reperfusion injury which was not blocked by pharmacological inhibition of PI3Ks with LY294002. Furthermore, α-MHC-tTA mice subjected to coronary artery ligation exhibited significantly reduced infarct size compared to control animals. Our findings reveal that α-MHC-tTA transgenic mice exhibit a gain-of-function phenotype consisting of robust protection against ischemia/reperfusion injury similar to cardiac pre- and post-conditioning effects. However, in contrast to classical pre- and post-conditioning, the α-MHC-tTA phenotype is not inhibited by the classic preconditioning inhibitor LY294002 suggesting involvement of a non-PI3K-AKT signaling pathway in this phenotype. Thus, further study of the α-MHC-tTA model may reveal novel molecular targets for therapeutic intervention during ischemic injury.

Loss of the preconditioning effect of rosuvastatin during sustained therapy: a human in vivo study

Studies have demonstrated that the acute administration of 3-hydroxy-3 methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors has protective effects in the setting of ischemia-reperfusion (IR). Previously, we demonstrated that a single dose of rosuvastatin prevented IR-induced endothelial dysfunction in humans through a cyclooxygenase-2-dependent mechanism. Whether the chronic administration of HMG-CoA reductase inhibitors provides similar protection remains controversial and is unknown in humans. Eighteen male volunteers were randomized to receive a single dose of rosuvastatin (20 mg) or placebo. Twenty-four hours later, endothelium-dependent, radial artery flow-mediated dilation (FMD) was measured before and after IR (15 min of upper arm ischemia followed by 15 min of reperfusion). In a separate protocol, 30 healthy volunteers were randomized in a double-blind fashion to receive oral rosuvastatin (20 mg/day) and placebo, rosuvastatin, and celecoxib (100 mg bid) or placebo alone, all for 21 days. Twenty-four hours after the final administration of study medication, FMD was measured before and after IR. Pre-IR FMD was similar between groups in both protocols. In the acute administration protocol, rosuvastatin significantly prevented the blunting of FMD associated with IR (FMD pre-IR: 8.4 ± 1.3%; post-IR: 6.2 ± 1.3%; P = 0.01 ANOVA, treatment group interaction). In the daily administration protocol, IR significantly blunted FMD in the placebo group (FMD pre-IR: 7.5 ± 0.9%; post-IR: 3.3 ± 0.7%; P < 0.001). Chronic treatment with rosuvastatin did not modify this ischemic injury (FMD pre-IR: 6.9 ± 0.4%; post-IR: 1.6 ± 1.0%; P < 0.001; P = NS ANOVA, treatment group interaction). Similarly, FMD responses post-IR in volunteers receiving rosuvastatin and celecoxib did not significantly differ from placebo (FMD pre-IR: 8.3 ± 0.9%; post-IR: 2.1 ± 0.8%; P < 0.001; P = NS ANOVA, treatment group interaction). In contrast to acute administration, chronic rosuvastatin does not prevent the development of IR-induced endothelial dysfunction in normal humans.

PTEN inhibition improves wound healing in lung epithelia through changes in cellular mechanics that enhance migration

The phosphoinositide-3 kinase/Akt pathway is a vital survival axis in lung epithelia. We previously reported that inhibition of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a major suppressor of this pathway, results in enhanced wound repair following injury. However, the precise cellular and biomechanical mechanisms responsible for increased wound repair during PTEN inhibition are not yet well established. Using primary human lung epithelia and a related lung epithelial cell line, we first determined whether changes in migration or proliferation account for wound closure. Strikingly, we observed that cell migration accounts for the majority of wound recovery following PTEN inhibition in conjunction with activation of the Akt and ERK signaling pathways. We then used fluorescence and atomic force microscopy to investigate how PTEN inhibition alters the cytoskeletal and mechanical properties of the epithelial cell. PTEN inhibition did not significantly alter cytoskeletal structure but did result in large spatial variations in cell stiffness and in particular a decrease in cell stiffness near the wound edge. Biomechanical changes, as well as migration rates, were mediated by both the Akt and ERK pathways. Our results indicate that PTEN inhibition rapidly alters biochemical signaling events that in turn provoke alterations in biomechanical properties that enhance cell migration. Specifically, the reduced stiffness of PTEN-inhibited cells promotes larger deformations, resulting in a more migratory phenotype. We therefore conclude that increased wound closure consequent to PTEN inhibition occurs through enhancement of cell migration that is due to specific changes in the biomechanical properties of the cell.

Relationship of electrophilic stress to aging

This review begins with the premise that an organism's life span is determined by the balance between two countervailing forces: (i) the sum of destabilizing effects and (ii) the sum of protective longevity-assurance processes. Against this backdrop, the role of electrophiles is discussed, both as destabilizing factors and as signals that induce protective responses. Because most biological macromolecules contain nucleophilic centers, electrophiles are particularly reactive and toxic in a biological context. The majority of cellular electrophiles are generated from polyunsaturated fatty acids by a peroxidation chain reaction that is readily triggered by oxygen-centered radicals, but propagates without further input of reactive oxygen species (ROS). Thus, the formation of lipid-derived electrophiles such as 4-hydroxynon-2-enal (4-HNE) is proposed to be relatively insensitive to the level of initiating ROS, but to depend mainly on the availability of peroxidation-susceptible fatty acids. This is consistent with numerous observations that life span is inversely correlated to membrane peroxidizability, and with the hypothesis that 4-HNE may constitute the mechanistic link between high susceptibility of membrane lipids to peroxidation and shortened life span. Experimental interventions that directly alter membrane composition (and thus their peroxidizability) or modulate 4-HNE levels have the expected effects on life span, establishing that the connection is not only correlative but causal. Specific molecular mechanisms are considered, by which 4-HNE could (i) destabilize biological systems via nontargeted reactions with cellular macromolecules and (ii) modulate signaling pathways that control longevity-assurance mechanisms.

Diabetic inhibition of preconditioning- and postconditioning-mediated myocardial protection against ischemia/reperfusion injury

Ischemic preconditioning (IPC) or postconditioning (Ipost) is proved to efficiently prevent ischemia/reperfusion injuries. Mortality of diabetic patients with acute myocardial infarction was found to be 2-6 folds higher than that of non-diabetic patients with same myocardial infarction, which may be in part due to diabetic inhibition of IPC- and Ipost-mediated protective mechanisms. Both IPC- and Ipost-mediated myocardial protection is predominantly mediated by stimulating PI3K/Akt and associated GSK-3β pathway while diabetes-mediated pathogenic effects are found to be mediated by inhibiting PI3K/Akt and associated GSK-3β pathway. Therefore, this review briefly introduced the general features of IPC- and Ipost-mediated myocardial protection and the general pathogenic effects of diabetes on the myocardium. We have collected experimental evidence that indicates the diabetic inhibition of IPC- and Ipost-mediated myocardial protection. Increasing evidence implies that diabetic inhibition of IPC- and Ipost-mediated myocardial protection may be mediated by inhibiting PI3K/Akt and associated GSK-3β pathway. Therefore any strategy to activate PI3K/Akt and associated GSK-3β pathway to release the diabetic inhibition of both IPC and Ipost-mediated myocardial protection may provide the protective effect against ischemia/reperfusion injuries.

c-Jun N-terminal kinase (JNK-1) confers protection against brief but not extended ischemia during acute myocardial infarction

Brief periods of ischemia do not damage the heart and can actually protect against reperfusion injury caused by extended ischemia. It is not known what causes the transition from protection to irreversible damage as ischemia progresses. c-Jun N-terminal kinase-1 (JNK-1) is a stress-regulated kinase that is activated by reactive oxygen and thought to promote injury during severe acute myocardial infarction. Because some reports suggest that JNK-1 can also be protective, we hypothesized that the function of JNK-1 depends on the metabolic state of the heart at the time of reperfusion, a condition that changes progressively with duration of ischemia. Mice treated with JNK-1 inhibitors or transgenic mice wherein the JNK-1 gene was ablated were subjected to 5 or 20 min of ischemia followed by reperfusion. When JNK-1 was inactive, ischemia of only 5 min duration caused massive apoptosis, infarction, and negative remodeling that was equivalent to or greater than extended ischemia. Conversely, when ischemia was extended JNK-1 inactivation was protective. Mechanisms of the JNK-1 switch in function were investigated in vivo and in cultured cardiac myocytes. In vitro there was a comparable switch in the function of JNK-1 from protective when ATP levels were maintained during hypoxia to injurious when reoxygenation followed glucose and ATP depletion. Both apoptotic and necrotic death pathways were affected and responded reciprocally to JNK-1 inhibitors. JNK-1 differentially regulated Akt phosphorylation of the regulatory sites Ser-473 and Thr-450 and the catalytic Thr-308 site in vivo. The studies define a novel role for JNK-1 as a conditional survival kinase that protects the heart against brief but not protracted ischemia.

NO signaling and S-nitrosylation regulate PTEN inhibition in neurodegeneration

BACKGROUND

The phosphatase PTEN governs the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway which is arguably the most important pro-survival pathway in neurons. Recently, PTEN has also been implicated in multiple important CNS functions such as neuronal differentiation, plasticity, injury and drug addiction. It has been reported that loss of PTEN protein, accompanied by Akt activation, occurs under excitotoxic conditions (stroke) as well as in Alzheimer's (AD) brains. However the molecular signals and mechanism underlying PTEN loss are unknown.

RESULTS

In this study, we investigated redox regulation of PTEN, namely S-nitrosylation, a covalent modification of cysteine residues by nitric oxide (NO), and H2O2-mediated oxidation. We found that S-nitrosylation of PTEN was markedly elevated in brains in the early stages of AD (MCI). Surprisingly, there was no increase in the H2O2-mediated oxidation of PTEN, a modification common in cancer cell types, in the MCI/AD brains as compared to normal aged control. Using several cultured neuronal models, we further demonstrate that S-nitrosylation, in conjunction with NO-mediated enhanced ubiquitination, regulates both the lipid phosphatase activity and protein stability of PTEN. S-nitrosylation and oxidation occur on overlapping and distinct Cys residues of PTEN. The NO signal induces PTEN protein degradation via the ubiquitin-proteasome system (UPS) through NEDD4-1-mediated ubiquitination.

CONCLUSION

This study demonstrates for the first time that NO-mediated redox regulation is the mechanism of PTEN protein degradation, which is distinguished from the H2O2-mediated PTEN oxidation, known to only inactivate the enzyme. This novel regulatory mechanism likely accounts for the PTEN loss observed in neurodegeneration such as in AD, in which NO plays a critical pathophysiological role.

The role of microRNA in modulating myocardial ischemia-reperfusion injury

MicroRNAs (miRNAs) are small (∼22 nt) noncoding single-stranded RNA molecules that downregulate gene expression. Studies have shown that miRNAs control diverse aspects of heart disease, including hypertrophy, remodeling, heart failure, and arrhythmia. Recently, several studies have suggested that miRNAs contribute to ischemia-reperfusion injury by altering key signaling elements, thus making them potential therapeutic targets. By altering the expression of various key elements in cell survival and apoptosis [such as phosphoinositide 3-kinase (PI3K), phosphatase and tensin homolog deleted on chromosome 10 (PTEN), Bcl-2, Mcl-1, heat shock protein (HSP)60, HSP70, HSP20, programmed cell death 4 (Pdcd4), LRRFIP1, Fas ligand (FasL), Sirt-1, etc.], miRNAs alter the response to ischemia-reperfusion injury. Studies using various in vivo, ex vivo, and in vitro models have suggested the possible involvement of miR-1, miR-21, miR-29, miR-92a, miR-133, miR-199a, and miR-320 in ischemia-reperfusion injury and/or remodeling after myocardial infarction. Thus miRNAs could be potential therapeutic targets for the treatment of heart disease. Inhibiting miRNAs by antisense strategies or pharmacological approaches is likely to emerge as an alternative and safe method for conferring short- and intermediate-term protection against ischemia-reperfusion injury.

Aldehyde dehydrogenase 2 (ALDH2) rescues myocardial ischaemia/reperfusion injury: role of autophagy paradox and toxic aldehyde

AIMS

The present study was designed to examine the mechanism involved in mitochondrial aldehyde dehydrogenase (ALDH2)-induced cardioprotection against ischaemia/reperfusion (I/R) injury with a focus on autophagy.

METHODS

Wild-type (WT), ALDH2 overexpression, and knockout (KO) mice (n = 4-6 for each index measured) were subjected to I/R, and myocardial function was assessed using echocardiographic, Langendroff, and edge-detection systems. Western blotting was used to evaluate AMP-dependent protein kinase (AMPK), Akt, autophagy, and the AMPK/Akt upstream signalling LKB1 and PTEN.

RESULTS

ALDH2 overexpression and KO significantly attenuated and accentuated, respectively, infarct size, factional shortening, and recovery of post-ischaemic left ventricular function following I/R as well as hypoxia/reoxygenation-induced cardiomyocyte contractile dysfunction. Autophagy was induced during ischaemia and remained elevated during reperfusion. ALDH2 significantly promoted autophagy during ischaemia, which was accompanied by AMPK activation and mammalian target of rapamycin (mTOR) inhibition. On the contrary, ALDH2 overtly inhibited autophagy during reperfusion accompanied by the activation of Akt and mTOR. Inhibition and induction of autophagy mitigated ALDH2-induced protection against cell death in hypoxia and reoxygenation, respectively. In addition, levels of the endogenous toxic aldehyde 4-hydroxy-2-nonenal (4-HNE) were elevated by ischaemia and reperfusion, which was abrogated by ALDH2. Furthermore, ALDH2 ablated 4-HNE-induced cardiomyocyte dysfunction and protein damage, whereas 4-HNE directly decreased pan and phosphorylated LKB1 and PTEN expression.

CONCLUSION

Our data suggest a myocardial protective effect of ALDH2 against I/R injury possibly through detoxification of toxic aldehyde and a differential regulation of autophagy through AMPK- and Akt-mTOR signalling during ischaemia and reperfusion, respectively.

Mitochondrial reprogramming through cardiac oxygen sensors in ischaemic heart disease

Under hypoxic conditions, mitochondria can represent a threat to the cell because of their capacity to generate toxic reactive oxygen species (ROS). However, cardiomyocytes are equipped with an oxygen-sensing pathway that involves prolyl hydroxylase oxygen sensors and hypoxia-inducible factors (HIFs), which induces a tightly regulated programme to keep ischaemic mitochondrial activity under control. The aim of this review is to provide an update on the pathways leading to mitochondrial reprogramming, which occurs in the myocardium during ischaemia, with particular emphasis on those induced by HIF activation. We start by studying the mechanisms of mitochondrial damage during ischaemia and upon reperfusion, highlighting the importance of the formation of the mitochondrial permeability transition pore during reperfusion and its consequences for cardiomyocyte survival. Next, we analyse hypoxia-induced metabolic reprogramming through HIF and its important consequences for mitochondrial bioenergetics, as well as the phenomenon known as the hibernating myocardium. Subsequently, we examine the mechanisms underlying ischaemic preconditioning, focusing, in particular, on those that involve the HIF pathway, such as adenosine signalling, sub-lethal ROS generation, and nitric oxide production. Finally, the role of the mitochondrial uncoupling proteins in ischaemia tolerance is discussed.

MicroRNA-21 is a downstream effector of AKT that mediates its antiapoptotic effects via suppression of Fas ligand

MicroRNA-21 (miR-21) is highly up-regulated during hypertrophic and cancerous cell growth. In contrast, we found that it declines in cardiac myocytes upon exposure to hypoxia. Thus, the objective was to explore its role during hypoxia. We show that miR-21 not only regulates phosphatase and tensin homologue deleted on chromosome 10 (PTEN), but also targets Fas ligand (FasL). During prolonged hypoxia, down-regulation of miR-21 proved necessary and sufficient for enhancing expression of both proteins. We demonstrate here for the first time that miR-21 is positively regulated via an AKT-dependent pathway, which is depressed during prolonged hypoxia. Accordingly, hypoxia-induced down-regulation of miR-21 and up-regulation of FasL and PTEN were reversed by activated AKT and reproduced by a dominant negative mutant, wortmannin, or PTEN. Moreover, the antiapoptotic function of AKT partly required miR-21, which was sufficient for inhibition of caspase-8 activity and mitochondrial damage. In consensus, overexpression of miR-21 in a transgenic mouse heart resulted in suppression of ischemia-induced up-regulation of PTEN and FasL expression, an increase in phospho-AKT, a smaller infarct size, and ameliorated heart failure. Thus, we have identified a unique aspect of the function of AKT by which it inhibits apoptosis through miR-21-dependent suppression of FasL.

Protein S-nitrosylation and cardioprotection

Nitric oxide (NO) plays an important role in the regulation of cardiovascular function. In addition to the classic NO activation of the cGMP-dependent pathway, NO can also regulate cell function through protein S-nitrosylation, a redox dependent, thiol-based, reversible posttranslational protein modification that involves attachment of an NO moiety to a nucleophilic protein sulfhydryl group. There are emerging data suggesting that S-nitrosylation of proteins plays an important role in cardioprotection. Protein S-nitrosylation not only leads to changes in protein structure and function but also prevents these thiol(s) from further irreversible oxidative/nitrosative modification. A better understanding of the mechanism regulating protein S-nitrosylation and its role in cardioprotection will provide us new therapeutic opportunities and targets for interventions in cardiovascular diseases.

Pharmacological inhibition of PTEN limits myocardial infarct size and improves left ventricular function postinfarction

Phosphoinositide 3-kinase (PI3K) mediates myocardium protective signaling through phosphorylation of phosphatidylinositol (Ptdins) to produce Ptdins(3,4,5)P(3). Lipid phosphatase and tensin homolog on chromosome 10 (PTEN) antagonizes PI3K activity by dephosphorylating Ptdins(3,4,5)P(3); therefore, the inhibition of PTEN enhances PI3K/Akt signaling and could prevent myocardium from ischemia-reperfusion (I/R) injury. Here we studied 1) whether the pharmacological inhibition of PTEN by bisperoxovanadium molecules [BpV(HOpic)] attenuates simulated I/R (SIR) injury in vitro and 2) whether the administration of BpV(HOpic) either before or after ischemia limits myocardial infarct size (IS) and ameliorates cardiodysfunction caused by infarction. First, adult rat cardiomyocytes were treated with or without BpV(HOpic) and then exposure to SIR. Second, anesthetized rats received BpV(HOpic) either before or after ischemia. IS was assessed at 4 h reperfusion, and left ventricular function was evaluated by echocardiography at 28 days postreperfusion. As a result, BpV(HOpic) decreased cell death, improved 3-[4,5-yl]-2,5-diphenyltetrazolium bromide (MTT) viability, and reduced apoptosis in cells exposed to SIR. These protective effects of BpV(HOpic) are associated with increased phospho-Akt and the repression of caspase-3 activity. Second, the administration of BpV(HOpic) significantly reduced IS and suppressed caspase-3 activity following I/R injury and consequentially improved cardiac function at 28 day postinfarction. These beneficial effects of BpV(HOpic) are attributed to increases in myocardial levels of phosphorylation of Akt/endothelial nitric oxide synthase (eNOS), ERK-1/2, and calcium-dependent nitric oxide synthase activity. In conclusion, the pharmacological inhibition of PTEN protects against I/R injury through the upregulation of the PI3K/Akt/eNOS/ERK prosurvival pathway, suggesting a new therapeutic strategy to combat I/R injury.

Vascular endothelial growth factor inhibition by dRK6 causes endothelial apoptosis, fibrosis, and inflammation in the heart via the Akt/eNOS axis in db/db mice

OBJECTIVE

Vascular endothelial growth factor (VEGF), which is associated with the stimulation of angiogenesis and collateral vessel synthase, is one of the crucial factors involved in cardiac remodeling in type 2 diabetes.

RESEARCH DESIGN AND METHODS

We investigated VEGF inhibition by dRK6 on the heart in an animal model of type 2 diabetes. Male db/db and db/m mice either were treated with dRK6 starting at 7 weeks of age for 12 weeks (db/db-dRK6 and db/m-dRK6) or were untreated.

RESULTS

Cardiac dysfunction and hypertrophy were noted by echocardiogram and molecular markers in the db/db-dRK6 mice. The presence of diabetes significantly suppressed the expression of VEGF receptor (VEGFR)-1 and VEGFR-2, phospho-Akt, and phospho-endothelial nitric oxide synthase (eNOS) in the heart. In db/db-dRK6 mice, dRK6 completely inhibited VEGFR-2, phospho-Akt, and phospho-eNOS expression, whereas no effect on VEGFR-1 was observed. Cardiac fibrosis, microvascular scarcity associated with an increase in apoptotic endothelial cells, and inflammation were prominent, as well as increase in antiangiogenic growth factors. Cardiac 8-hydroxy-deoxyguanine and hypoxia-inducible factor-1alpha expression were significantly increased. No such changes were found in the other groups, including the db/m-dRK6 mice. The number of apoptotic human umbilical vein endothelial cells was increased by dRK6 in a dose-dependent manner only at high glucose concentrations, and this was associated with a decrease in phospho-Akt and phospho-eNOS related to oxidative stress.

CONCLUSIONS

Our results demonstrated that systemic blockade of VEGF by dRK6 had deleterious effects on the heart in an animal model of type 2 diabetes; dRK6 induced downregulation of the VEGFR-2 and Akt-eNOS axis and enhancement of oxidative stress.

Ischemia activates the ATF6 branch of the endoplasmic reticulum stress response

Stresses that perturb the folding of nascent endoplasmic reticulum (ER) proteins activate the ER stress response. Upon ER stress, ER-associated ATF6 is cleaved; the resulting active cytosolic fragment of ATF6 translocates to the nucleus, binds to ER stress response elements (ERSEs), and induces genes, including the ER-targeted chaperone, GRP78. Recent studies showed that nutrient and oxygen starvation during tissue ischemia induce certain ER stress response genes, including GRP78; however, the role of ATF6 in mediating this induction has not been examined. In the current study, simulating ischemia (sI) in a primary cardiac myocyte model system caused a reduction in the level of ER-associated ATF6 with a coordinate increase of ATF6 in nuclear fractions. An ERSE in the GRP78 gene not previously shown to be required for induction by other ER stresses was found to bind ATF6 and to be critical for maximal ischemia-mediated GRP78 promoter induction. Activation of ATF6 and the GRP78 promoter, as well as grp78 mRNA accumulation during sI, were reversed upon simulated reperfusion (sI/R). Moreover, dominant-negative ATF6, or ATF6-targeted miRNA blocked sI-mediated grp78 induction, and the latter increased cardiac myocyte death upon simulated reperfusion, demonstrating critical roles for endogenous ATF6 in ischemia-mediated ER stress activation and cell survival. This is the first study to show that ATF6 is activated by ischemia but inactivated upon reperfusion, suggesting that it may play a role in the induction of ER stress response genes during ischemia that could have a preconditioning effect on cell survival during reperfusion.

Occult Cardiotoxicity—Toxic Effects on Cardiac Ischemic Tolerance

The outcome of cardiac ischemic events depends not only on the extent and duration of the ischemic stimulus but also on the myocardial intrinsic tolerance to ischemic injury. Cardiac ischemic tolerance reflects myocardial functional reserves that are not always used when the tissue is appropriately oxygenated. Ischemic tolerance is modulated by ubiquitous signal transduction pathways, transcription factors and cellular enzymes, converging on the mitochondria as the main end effector. Therefore, drugs and toxins affecting these pathways may impair cardiac ischemic tolerance without affecting myocardial integrity or function in oxygenated conditions. Such effect would not be detected by current toxicological studies but would considerably influence the outcome of ischemic events. The authors refer to such effect as “occult cardiotoxicity.” In this review, the authors summarize current knowledge about main mechanisms that determine cardiac ischemic tolerance, methods to assess it, and the effects of drugs and toxins on it. The authors offer a view that low cardiac ischemic tolerance is a premorbid status and, therefore, that occult cardiotoxicity is a significant potential source of cardiac morbidity. The authors propose that toxicologic assessment of compounds would include the assessment of their effect on cardiac ischemic tolerance.

Molecular mechanism underlying Akt activation in zinc-induced cardioprotection

Our previous study demonstrated that zinc prevents cardiac reperfusion injury by targeting the mitochondrial permeability transition pore (mPTP) via Akt and glycogen synthetase kinase 3beta (GSK-3beta). We aimed to address the mechanism by which zinc activates Akt. Treatment of H9c2 cells with ZnCl(2) (10 microM) in the presence of the zinc ionophore pyrithione (4 microM) for 20 min enhanced Akt phosphorylation (Ser(473)), indicating that zinc can rapidly activate Akt. Zinc did not alter either phosphatase and tensin homolog deleted on chromosome 10 (PTEN) phosphorylation and total PTEN protein levels or PTEN oxidation, implying that PTEN may not play a role in the action of zinc. However, zinc-induced Akt phosphorylation was blocked by both the nonselective receptor tyrosine kinase (RTK) inhibitor genistein and the selective insulin-like growth factor-1 RTK (IGF-1RTK) inhibitor AG1024, indicating that zinc activates Akt via IGF-1RTK. Zinc-induced phosphorylation of protein tyrosine and Ser/Thr was also abolished by AG1024. In addition, zinc markedly enhanced phosphorylation of IGF-1 receptor (IGF-1R), which was again reversed by genistein and AG1024. A confocal imaging study revealed that AG1024 abolished the preventive effect of zinc on oxidant-induced mPTP opening, confirming that IGF-1RTK plays a role in zinc-induced cardioprotection. Furthermore, zinc decreased the activity of protein phosphatase 2A (PP2A), a major protein Ser/Thr phosphatase, implying that protein Ser/Thr phosphatases may also play a role in the action of zinc on Akt activity. Taken together, these findings demonstrate that exogenous zinc activates Akt via IGF-1RTK and prevents the mPTP opening in cardiac cells. Inactivation of Ser/Thr protein phosphatases may also contribute to zinc-induced Akt activation.

Regulation of autophagy by reactive oxygen species (ROS): implications for cancer progression and treatment

Reactive oxygen species (ROS) have been identified as signaling molecules in various pathways regulating both cell survival and cell death. Autophagy, a self-digestion process that degrades intracellular structures in response to stress, such as nutrient starvation, is also involved in both cell survival and cell death. Alterations in both ROS and autophagy regulation contribute to cancer initiation and progression, and both are targets for developing therapies to induce cell death selectively in cancer cells. Many stimuli that induce ROS generation also induce autophagy, including nutrient starvation, mitochondrial toxins, hypoxia, and oxidative stress. Some of these stimuli are under clinical investigation as cancer treatments, such as 2-methoxyestrodial and arsenic trioxide. Recently, it was demonstrated that ROS can induce autophagy through several distinct mechanisms involving Atg4, catalase, and the mitochondrial electron transport chain (mETC). This leads to both cell-survival and cell-death responses and could be selective toward cancer cells. In this review, we give an overview of the roles ROS and autophagy play in cell survival and cell death, and their importance to cancer. Furthermore, we describe how autophagy is mediated by ROS and the implications of this regulation to cancer treatments.

Inhibition of oxygen sensors as a therapeutic strategy for ischaemic and inflammatory disease

Cells in the human body need oxygen to function and survive, and severe deprivation of oxygen, as occurs in ischaemic heart disease and stroke, is a major cause of mortality. Nevertheless, other organisms, such as the fossorial mole rat or diving seals, have acquired the ability to survive in conditions of limited oxygen supply. Hypoxia tolerance also allows the heart to survive chronic oxygen shortage, and ischaemic preconditioning protects tissues against lethal hypoxia. The recent discovery of a new family of oxygen sensors--including prolyl hydroxylase domain-containing proteins 1-3 (PHD1-3)--has yielded exciting novel insights into how cells sense oxygen and keep oxygen supply and consumption in balance. Advances in understanding of the role of these oxygen sensors in hypoxia tolerance, ischaemic preconditioning and inflammation are creating new opportunities for pharmacological interventions for ischaemic and inflammatory diseases.

MicroRNA expression in response to murine myocardial infarction: miR-21 regulates fibroblast metalloprotease-2 via phosphatase and tensin homologue

AIMS

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the post-transcriptional level by either degradation or translational repression of a target mRNA. Encoded in the genome of most eukaryotes, miRNAs have been proposed to regulate specifically up to 90% of human genes through a process known as miRNA-guided RNA silencing. For the first time, we sought to test how myocardial ischaemia-reperfusion (IR) changes miR expression.

METHODS AND RESULTS

Following 2 and 7 h of IR or sham operation, myocardial tissue was collected and subjected to miRNA expression profiling and quantification using a Bioarray system that screens for human-, mice-, rat-, and Ambi-miR. Data mining and differential analyses resulted in 13 miRs that were up-regulated on day 2, 9 miRs that were up-regulated on day 7, and 6 miRs that were down-regulated on day 7 post-IR. Results randomly selected from expression profiling were validated using real-time PCR. Tissue elements laser-captured from the infarct site showed marked induction of miR-21. In situ hybridization studies using locked nucleic acid miR-21-specific probe identified that IR-inducible miR-21 was specifically localized in the infarct region of the IR heart. Immunohistochemistry data show that cardiac fibroblasts (CFs) are the major cell type in the infarct zone. Studies with isolated CFs demonstrated that phosphatase and tensin homologue (PTEN) is a direct target of miR-21. Modulation of miR-21 regulated expression of matrix metalloprotease-2 (MMP-2) via a PTEN pathway. Finally, we noted a marked decrease in PTEN expression in the infarct zone. This decrease was associated with increased MMP-2 expression in the infarct area.

CONCLUSION

This work constitutes the first report describing changes in miR expression in response to IR in the mouse heart, showing that miR-21 regulates MMP-2 expression in CFs of the infarct zone via a PTEN pathway.

Inducible and cardiac specific PTEN inactivation protects ischemia/reperfusion injury

PTEN is a dual lipid and protein phosphatase that antagonizes PI3K as well as other signaling pathways and regulates cellular survival and growth. However, its potential role in cardiac ischemia/reperfusion injury remains unknown. We established a transgenic mouse model with inducible and cardiac specific deletion of Pten gene (Pten(CKO)) in adult heart via tamoxifen dependent Cre-loxP mediated DNA recombination. 3 weeks after tamoxifen induced PTEN inactivation, elevated PI3K activity was observed in the Pten(CKO) hearts as determined from downstream AKT signaling. No significant differences in cardiac function as well as chamber size were observed between Pten(CKO) and Control animals based on echocardiography. In response to 30 min ischemia followed by 120 min reperfusion in Langendorff preparations, Pten(CKO) hearts developed significantly better function recovery than Control animals. At 60 min post reperfusion, the recovery of LVDP reached 77.9% of pre-ischemia basal in Pten(CKO) hearts vs 44.2% of Control (p<0.01). Consistent with the observed functional improvement, TTC staining revealed a significant reduction in infarct size in Pten(CKO) hearts compared with Control (24.2% vs 39.7%, p<0.05). Pten(CKO) hearts had significantly fewer apoptosis positive cardiomyocytes after I/R injury as identified by TUNEL staining. Furthermore, ERK activity and BCL-2 expression were not affected at basal but became significantly higher after ischemia/reperfusion in Pten(CKO) hearts. These data indicate that PTEN may play a role in ischemia/reperfusion injury by inhibiting anti-apoptotic survival signals. Inhibiting PTEN may serve as a potential approach to exert cardiac protection against ischemia reperfusion injury.

Post-translational regulation of PTEN

PTEN (phosphatase and tensin homolog deleted on chromosome 10) tumor suppressor regulates a variety of cellular processes including cell proliferation, growth, migration and death. This master regulator itself is also under deliberative regulation. Although the evidence for PTEN regulation and its significance in normal biology and disease is overwhelming, the mechanisms and exact functional consequences of PTEN regulation are far from clear. In this review, we discuss recent advances concerning post-translational regulation of PTEN in general, and in more detail about its regulation by ubiquitination. We also discuss some unsolved questions in the field and how they might be addressed in the future. We propose that the complex regulatory mechanisms of PTEN dictate how this tumor suppressor executes its distinct biological functions.

Ischemic preconditioning-induced cardioprotection is lost in mice with immunoproteasome subunit low molecular mass polypeptide-2 deficiency

The ubiquitin-proteasome system plays an important role in many cellular processes through degradation of specific proteins. Low molecular mass polypeptide 2 (LMP-2 or beta(1i)) is one important subunit of the immunoproteasome. Ischemic preconditioning (IPC) activates cell signaling pathways and generates cardioprotection but has not been linked to LMP-2 function previously. LMP-2 knockout mice (C57BL6 background) and wild-type C57BL6 mice were subjected to 30 min of ischemia (I-30) and 120 min of reperfusion (R-120) with or without preceding IPC (10 min of infusion and 5 min of reperfusion). IPC significantly increased left ventricular developed pressure and decreased infarct size in wild-type mice, but this protective effect of IPC was lost in LMP-2 knockout mice. IPC-mediated degradation of phosphatase and tensin homologue deleted on chromosome 10 (PTEN) and activation of the downstream protein kinase Akt were impaired in LMP-2 knockout hearts. The impairment of PTEN degradation was associated with defective immunoproteasomes and decreased proteolytic activities. When LMP-2 knockout mice were pretreated with the PTEN inhibitor bpV(HOpic), cardiac function was significantly improved, and myocardial infarct size was significantly reduced after I-30/R-120. In conclusion, LMP-2 is required for normal proteasomal function and IPC induction in the heart. Its action may be related to PTEN protein degradation.

Ischemia-reperfusion injury and cardioprotection: investigating PTEN, the phosphatase that negatively regulates PI3K, using a congenital model of PTEN haploinsufficiency

Activation of the PI3K/Akt pathway protects the heart from ischemia-reperfusion injury (IRI). The phosphatase PTEN is the main negative regulator of this pathway. We hypothesized that reduced PTEN levels could protect against IRI. Isolated perfused mouse hearts from PTEN(+/-) and their littermates PTEN(+/+) (WT), were subjected to 35 min global ischemia and 30 min reperfusion, with and without 2, 4 or 6 cycles ischemic preconditioning (IPC). The end point was infarct size, expressed as a percentage of the myocardium at risk (I/R%). PTEN and Akt levels were determined using Western blot analysis. Unexpectedly, there were no significant differences in infarction between PTEN(+/-) and WT (42.1 +/- 5.0% Vs. 45.6 +/- 3.3%). However, the preconditioning threshold was significantly reduced in the PTEN(+/-) Vs. WT, with 4 cycles of IPC being sufficient to reduce I/R%, compared to 6 cycles in the WT (4 cycles IPC: 29.8. +/- 3.69% in PTEN(+/-) Vs. 45.5. +/- 5.08% in WT, P < 0.01). In addition, the ratio between the phospho/total Akt (Ser473 and Thr308) was slightly but significantly increased in the PTEN(+/-) indicating an upregulation of PI3K/Akt pathway. Interestingly, the levels of the other phosphatases that may negatively regulate the PI3K/Akt pathway (PP2A, SHIP2 and PHLPP) were not significantly different between littermates and PTEN(+/-). In conclusion, PTEN haploinsufficiency alone does not induce cardioprotection in this model; however, it reduces the threshold of protection induced by IPC.

Mechanisms underlying acute protection from cardiac ischemia-reperfusion injury

Mitochondria play an important role in cell death and cardioprotection. During ischemia, when ATP is progressively depleted, ion pumps cannot function resulting in a rise in calcium (Ca(2+)), which further accelerates ATP depletion. The rise in Ca(2+) during ischemia and reperfusion leads to mitochondrial Ca(2+) accumulation, particularly during reperfusion when oxygen is reintroduced. Reintroduction of oxygen allows generation of ATP; however, damage to the electron transport chain results in increased mitochondrial generation of reactive oxygen species (ROS). Mitochondrial Ca(2+) overload and increased ROS can result in opening of the mitochondrial permeability transition pore, which further compromises cellular energetics. The resultant low ATP and altered ion homeostasis result in rupture of the plasma membrane and cell death. Mitochondria have long been proposed as central players in cell death, since the mitochondria are central to synthesis of both ATP and ROS and since mitochondrial and cytosolic Ca(2+) overload are key components of cell death. Many cardioprotective mechanisms converge on the mitochondria to reduce cell death. Reducing Ca(2+) overload and reducing ROS have both been reported to reduce ischemic injury. Preconditioning activates a number of signaling pathways that reduce Ca(2+) overload and reduce activation of the mitochondrial permeability transition pore. The mitochondrial targets of cardioprotective signals are discussed in detail.

The cardioprotective effect of a statin and cilostazol combination: relationship to Akt and endothelial nitric oxide synthase activation

BACKGROUND

Atorvastatin (ATV) protects against ischemia-reperfusion by upregulating Akt and subsequently, endothelial nitric oxide synthase (eNOS) phosphorylation at Ser-1177. However, when given orally, high doses of ATV (10 mg/kg/d) are needed to achieve maximal protective effect in the rat. Protein kinase A (PKA) also phosphorylates eNOS at Ser-1177. As PKA activity depends on cAMP, cilostazol (CIL), a phosphodiesterase type III inhibitor, may stimulate NO production by activating PKA.

HYPOTHESIS

CIL and ATV may have synergistic effects on eNOS phosphorylation and myocardial infarct size (IS) reduction.

METHODS

Sprague-Dawley rats received 3-day oral pretreatment with: (1) water; (2) low dose ATV (2 mg/kg/d); (3) CIL (20 mg/kg/d): (4) ATV+CIL. Rats underwent 30 min coronary artery occlusion and 4 h reperfusion, or hearts explanted for immunoblotting without being subjected to ischemia. Area at risk (AR) was assessed by blue dye and IS by triphenyl-tetrazolium-chloride.

RESULTS

Body weight and the size of AR were comparable among groups. There were no significant differences among groups in mean blood pressure and heart rate. CIL, but not ATV, reduced IS. IS in the ATV+CIL group was significantly smaller than the other three groups (P < 0.001 for each comparison). ATV, CIL and their combination did not affect total eNOS expression. ATV at 2 mg/kg/d did not affect Ser-1177 P-eNOS levels, whereas CIL increased it (258 +/- 15%). The level of myocardial P-eNOS levels was highest in the ATV+CIL group (406 +/- 7%).

CONCLUSIONS

ATV and CIL have synergistic effect on eNOS phosphorylation and IS reduction. By increased activation of eNOS, CIL may augment the pleiotropic effects of statins.

Interaction of cardiovascular risk factors with myocardial ischemia/reperfusion injury, preconditioning, and postconditioning

Therapeutic strategies to protect the ischemic myocardium have been studied extensively. Reperfusion is the definitive treatment for acute coronary syndromes, especially acute myocardial infarction; however, reperfusion has the potential to exacerbate lethal tissue injury, a process termed "reperfusion injury." Ischemia/reperfusion injury may lead to myocardial infarction, cardiac arrhythmias, and contractile dysfunction. Ischemic preconditioning of myocardium is a well described adaptive response in which brief exposure to ischemia/reperfusion before sustained ischemia markedly enhances the ability of the heart to withstand a subsequent ischemic insult. Additionally, the application of brief repetitive episodes of ischemia/reperfusion at the immediate onset of reperfusion, which has been termed "postconditioning," reduces the extent of reperfusion injury. Ischemic pre- and postconditioning share some but not all parts of the proposed signal transduction cascade, including the activation of survival protein kinase pathways. Most experimental studies on cardioprotection have been undertaken in animal models, in which ischemia/reperfusion is imposed in the absence of other disease processes. However, ischemic heart disease in humans is a complex disorder caused by or associated with known cardiovascular risk factors including hypertension, hyperlipidemia, diabetes, insulin resistance, atherosclerosis, and heart failure; additionally, aging is an important modifying condition. In these diseases and aging, the pathological processes are associated with fundamental molecular alterations that can potentially affect the development of ischemia/reperfusion injury per se and responses to cardioprotective interventions. Among many other possible mechanisms, for example, in hyperlipidemia and diabetes, the pathological increase in reactive oxygen and nitrogen species and the use of the ATP-sensitive potassium channel inhibitor insulin secretagogue antidiabetic drugs and, in aging, the reduced expression of connexin-43 and signal transducer and activator of transcription 3 may disrupt major cytoprotective signaling pathways thereby significantly interfering with the cardioprotective effect of pre- and postconditioning. The aim of this review is to show the potential for developing cardioprotective drugs on the basis of endogenous cardioprotection by pre- and postconditioning (i.e., drug applied as trigger or to activate signaling pathways associated with endogenous cardioprotection) and to review the evidence that comorbidities and aging accompanying coronary disease modify responses to ischemia/reperfusion and the cardioprotection conferred by preconditioning and postconditioning. We emphasize the critical need for more detailed and mechanistic preclinical studies that examine car-dioprotection specifically in relation to complicating disease states. These are now essential to maximize the likelihood of successful development of rational approaches to therapeutic protection for the majority of patients with ischemic heart disease who are aged and/or have modifying comorbid conditions.

PTEN: a promising pharmacological target to enhance epithelial wound healing

PI3Ks (phosphoinositide-3 kinases) produce PIP3 (phosphatidylinositol(3,4,5)-trisphosphate) which mediates signals for cell survival and proliferation. The tumour suppressor PTEN (phosphatase and tensin homologue) dephosphorylates PIP3 and is a key negative regulator of PI3K signalling. Recent research highlighted important roles for PI3K/PTEN in cell polarization and directional cell migration, pointing to a significant role for PTEN in wound healing where spatially organized tissue growth is essential. Lai et al. (in this issue of British Journal of Pharmacology) have moved a step closer in utilizing PTEN for wound healing through pharmacological inhibition. Two vanadium derivative inhibitors targeting PTEN significantly elevated the level of phosphorylated Akt (protein kinase B) and nearly doubled the wound healing rate in monolayer cultures of lung and airway epithelial cells. Damage to airway and lung epithelia underlies a wide spectrum of significant clinical conditions. With further experiments, this promising approach may find potential clinical use in situations where enhanced wound healing of pulmonary and other epithelia is important.