The activation of p38 alpha, and not p38 beta, mitogen-activated protein kinase is required for ischemic preconditioning

Mitochondrial Kinase Signaling for Cardioprotection

Myocardial ischemia/reperfusion injury is reduced by cardioprotective adaptations such as local or remote ischemic conditioning. The cardioprotective stimuli activate signaling cascades, which converge on mitochondria and maintain the function of the organelles, which is critical for cell survival. The signaling cascades include not only extracellular molecules that activate sarcolemmal receptor-dependent or -independent protein kinases that signal at the plasma membrane or in the cytosol, but also involve kinases, which are located to or within mitochondria, phosphorylate mitochondrial target proteins, and thereby modify, e.g., respiration, the generation of reactive oxygen species, calcium handling, mitochondrial dynamics, mitophagy, or apoptosis. In the present review, we give a personal and opinionated overview of selected protein kinases, localized to/within myocardial mitochondria, and summarize the available data on their role in myocardial ischemia/reperfusion injury and protection from it. We highlight the regulation of mitochondrial function by these mitochondrial protein kinases.

ErbB2/Her2-dependent downregulation of a cell death-promoting protein BLNK in breast cancer cells is required for 3D breast tumor growth

A significant proportion of breast cancers are driven by ErbB2/Her2 oncoprotein that they overexpress. These malignancies are typically treated with various ErbB2-targeted drugs, but many such cancers develop resistance to these agents and become incurable. Conceivably, treatment of ErbB2-positive cancers could be facilitated by use of agents blocking oncogenic signaling mechanisms downstream of ErbB2. However, current understanding of these mechanisms is limited. The ability of solid tumor cells to resist anoikis, cell death triggered by cell detachment from the extracellular matrix (ECM), is thought to be critical for 3D tumor growth. In an effort to understand the mechanisms of ErbB2-driven breast cancer cell anoikis resistance we found that detachment of non-malignant breast epithelial cells from the ECM upregulates a cell death-promoting tumor suppressor adapter protein BLNK and that ErbB2 blocks this upregulation by reducing tumor cell levels of transcription factor IRF6. We further observed that trastuzumab, a therapeutic anti-ErbB2 antibody, upregulates BLNK in human trastuzumab-sensitive but not trastuzumab-resistant ErbB2-positive breast cancer cells. Moreover, we established that BLNK promotes anoikis by activating p38 MAP kinase and that ErbB2-dependent BLNK downregulation blocks breast cancer cell anoikis. In search for pharmacological approaches allowing to upregulate BLNK in tumor cells we found that clinically approved proteasome inhibitor bortezomib upregulates IRF6 and BLNK in human breast cancer cells and inhibits their 3D growth in a BLNK-dependent manner. In addition, we found that BLNK upregulation in human ErbB2-positive breast cancer cells blocks their ability to form tumors in mice. Furthermore, we used publicly available data on mRNA levels in multiple breast cancers to demonstrate that increased BLNK mRNA levels correlate with increased relapse-free survival in a cohort of approximately 400 patients with ErbB2-positive breast cancer. In summary, we discovered a novel mechanism of ErbB2-driven 3D breast tumor growth mediated by ErbB2-dependent BLNK downregulation.

Cancer-Induced Muscle Wasting Requires p38β MAPK Activation of p300

Cancer-associated cachexia, characterized by muscle wasting, is a lethal metabolic syndrome without defined etiology or established treatment. We previously found that p300 mediates cancer-induced muscle wasting by activating C/EBPβ, which then upregulates key catabolic genes. However, the signaling mechanism that activates p300 in response to cancer is unknown. Here, we show that upon cancer-induced activation of Toll-like receptor 4 in skeletal muscle, p38β MAPK phosphorylates Ser-12 on p300 to stimulate C/EBPβ acetylation, which is necessary and sufficient to cause muscle wasting. Thus, p38β MAPK is a central mediator and therapeutic target of cancer-induced muscle wasting. In addition, nilotinib, an FDA-approved kinase inhibitor that preferentially binds p38β MAPK, inhibited p300 activation 20-fold more potently than the p38α/β MAPK inhibitor, SB202190, and abrogated cancer cell-induced muscle protein loss in C2C12 myotubes without suppressing p38α MAPK-dependent myogenesis. Systemic administration of nilotinib at a low dose (0.5 mg/kg/day, i.p.) in tumor-bearing mice not only alleviated muscle wasting, but also prolonged survival. Therefore, nilotinib appears to be a promising treatment for human cancer cachexia due to its selective inhibition of p38β MAPK. SIGNIFICANCE: These findings demonstrate that prevention of p38β MAPK-mediated activation of p300 by the FDA-approved kinase inhibitor, nilotinib, ameliorates cancer cachexia, representing a potential therapeutic strategy against this syndrome.

p38β MAPK mediates ULK1-dependent induction of autophagy in skeletal muscle of tumor-bearing mice

Muscle wasting is the key manifestation of cancer-associated cachexia, a lethal metabolic disorder seen in over 50% of cancer patients. Autophagy is activated in cachectic muscle of cancer hosts along with the ubiquitin-proteasome pathway (UPP), contributing to accelerated protein degradation and muscle wasting. However, established signaling mechanism that activates autophagy in response to fasting or denervation does not seem to mediate cancer-provoked autophagy in skeletal myocytes. Here, we show that p38β MAPK mediates autophagy activation in cachectic muscle of tumor-bearing mice via novel mechanisms. Complementary genetic and pharmacological manipulations reveal that activation of p38β MAPK, but not p38α MAPK, is necessary and sufficient for Lewis lung carcinoma (LLC)-induced autophagy activation in skeletal muscle cells. Particularly, muscle-specific knockout of p38β MAPK abrogates LLC tumor-induced activation of autophagy and UPP, sparing tumor-bearing mice from muscle wasting. Mechanistically, p38β MAPK-mediated activation of transcription factor C/EBPβ is required for LLC-induced autophagy activation, and upregulation of autophagy-related genes LC3b and Gabarapl1. Surprisingly, ULK1 activation (phosphorylation at S555) by cancer requires p38β MAPK, rather than AMPK. Activated ULK1 forms a complex with p38β MAPK in myocytes, which is markedly increased by a tumor burden. Overexpression of a constitutively active p38Tbeta; MAPK in HEK293 cells increases phosphorylation at S555 and other amino acid residues of ULK1, but not several of AMPK-mediated sites. Finally, ULK1 activation is abrogated in tumor-bearing mice with muscle-specific knockout of p38β MAPK. Thus, p38β MAPK appears a key mediator of cancer-provoked autophagy activation, and a therapeutic target of cancer-induced muscle wasting.

Sulbactam Protects Hippocampal Neurons Against Oxygen-Glucose Deprivation by Up-Regulating Astrocytic GLT-1 via p38 MAPK Signal Pathway

Sulbactam is an atypical β-lactam medication and reported to be neuroprotective by up-regulating glial glutamate transporter-1 (GLT-1) in rats. The present study was undertaken to study the role of p38 MAPK signal pathway in sulbactam induced up-regulation of GLT-1 expression in astrocytes and anti-ischemic effect. Neuron-astrocyte co-cultures and astrocyte cultures from neonatal Wistar rats were used. Cerebral ischemia was mimicked by oxygen-glucose deprivation (OGD). Hoechst (HO)/propidium iodide (PI) double fluorescence staining and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide assay were used to evaluate neuronal death and cell viability, respectively. Immunocytochemistry and Western blot were used to detect protein expressions. Sulbactam pre-incubation significantly and dose-dependently prevented neuronal death and decline in cell viability induced by OGD in neuron-astrocyte co-cultures, and upregulated GLT-1 expression in astrocyte cultures endured OGD, which suggested that sulbactam might protect neurons against OGD by up-regulating astrocytic GLT-1 expression. It was further shown that the phosphorylated-p38 MAPK expression in astrocytes was up-regulated after the sulbactam pre-incubation and this up-regulation was moderate in amplitude. Especially, the time course of the up-regulation of phosphorylated-p38 MAPK was obviously earlier than that of GLT-1, which suggested possibility that p38 MAPK might be an upstream signal for GLT-1 up-regulation induced by sulbactam. We further found that SB203580, the specific inhibitor of p38 MAPK, dose-dependently inhibited the GLT-1 up-regulation induced by sulbactam either in non- or OGD-treated astrocytes and the protective effect of sulbactam on co-cultured neurons against OGD. Taken together, it might be concluded that sulbactam protects cerebral neurons against OGD by up-regulating astrocytic GLT-1 expression via p38 MAPK signal pathway.

TAB1-Induced Autoactivation of p38α Mitogen-Activated Protein Kinase Is Crucially Dependent on Threonine 185

p38α mitogen-activated protein kinase is essential to cellular homeostasis. Two principal mechanisms to activate p38α exist. The first relies on dedicated dual-specificity kinases such as mitogen-activated protein kinase kinase (MAP2K) 3 (MKK3) or 6 (MKK6), which activate p38α by phosphorylating Thr180 and Tyr182 within the activation segment. The second is by autophosphorylation of Thr180 and Tyr182 in cis, mediated by p38α binding the scaffold protein TAB1. The second mechanism occurs during myocardial ischemia, where it aggravates myocardial infarction. Based on the crystal structure of the p38α-TAB1 complex we replaced threonine 185 of p38α with glycine (T185G) to prevent an intramolecular hydrogen bond with Asp150 from being formed. This mutation did not interfere with TAB1 binding to p38α. However, it disrupted the consequent long-range effect of this binding event on the distal activation segment, releasing the constraint on Thr180 that oriented its hydroxyl for phosphotransfer. Based on assays performed in vitro and in vivo, the autoactivation of p38α(T185G) was disabled, while its ability to be activated by upstream MAP2Ks and to phosphorylate downstream substrates remained intact. Furthermore, myocardial cells expressing p38α(T185G) were resistant to injury. These findings reveal a mechanism to selectively disable p38α autoactivation and its consequences, which may ultimately circumvent the toxicity associated with strategies that inhibit p38α kinase activity under all circumstances, such as with ATP-competitive inhibitors.

Differential effects of p38 MAP kinase inhibitors SB203580 and SB202190 on growth and migration of human MDA-MB-231 cancer cell line

p38 mitogen-activated protein kinase (MAPK) belongs to the MAPK superfamily, phosphorylating serine and/or threonine residues of the target proteins. The activation of p38 MAPK leads to cell growth, differentiation, inflammation, survival or apoptosis. In this study, we tested the effect of two highly specific and potent inhibitors of p38 MAPK (namely, SB203580 and SB202190) on human breast cancer cell line MDA-MB-231 to elucidate the controversial role of p38 MAPK on cell proliferation and/or cell migration/metastasis further. It was determined that the IC₅₀ value of SB203580 was 85.1 µM, while that of SB202190 was 46.6 µM, suggesting that SB202190 is slightly more effective than SB203580. To verify the effect of each inhibitor on cell proliferation and cytotoxicity, the cells were treated with various doses of SB203580 and SB202190 and examined using iCELLigence system. No significant effect of 1 and 5 µM of both inhibitors were seen on cell proliferation as compared to the DMSO-treated control cells for up to 96 h. On the other hand, both SB203580 and SB202190 significantly prevented cell proliferation at a concentration of 50 µM. SB202190 was again more effective than SB203580. Afterwards, we tested the effect of each inhibitor on cell migration using wound assay. Both SB203580 and SB202190 significantly reduced cell migration in a time-dependent manner at a concentration of 50 µM. However, interestingly it was observed that a low and noncytotoxic dose of 5 µM of SB203580 and SB202190 also did cause significant cell migration inhibition at 48 h of the treatment, corroborating the fact that p38 MAPK pathway has a critical role in cell migration/metastasis. Then, we tested whether each p38 MAPK inhibitor has any effect on cell adhesion during a treatment period of 3 h using iCELLigence system. A concentration of only 50 µM of SB202190 reduced cell adhesion for about 1.5 h (p < 0.001); after that period of time, cell adhesion in 50 µM SB202190-treated cells returned to the level of the control cells. To determine the mechanism of growth and cell migration inhibitory effects of p38 MAPK inhibitors, the activation/inactivation of various proteins and enzymes was subsequently analyzed by PathScan^® Intracellular Signaling Array kit. The ERK1/2 phosphorylation level was not modified by low concentrations (1 or 5 µM) of SB202190 and SB203580; while a high concentration (50 µM) of both inhibitors caused significant reductions in the ERK1/2 phosphorylation. In addition, it was determined that both p38 MAPK inhibitors caused significant increases on the Ser15 phosphorylation of mutant p53 in MDA-MB-231 under these experimental conditions; while SB202190 was more potent than SB203580.

Activin A induces skeletal muscle catabolism via p38β mitogen-activated protein kinase

BACKGROUND

Activation of type IIB activin receptor (ActRIIB) in skeletal muscle leads to muscle atrophy because of increased muscle protein degradation. However, the intracellular signalling mechanism that mediates ActRIIB-activated muscle catabolism is poorly defined.

METHODS

We investigated the role of p38β mitogen-activated protein kinases (MAPK) in mediating ActRIIB ligand activin A-activated muscle catabolic pathways in C2C12 myotubes and in mice with perturbation of this kinase pharmacologically and genetically.

RESULTS

Treatment of C2C12 myotubes with activin A or myostatin rapidly activated p38 MAPK and its effector C/EBPβ within 1 h. Paradoxically, Akt was activated at the same time through a p38 MAPK-independent mechanism. These events were followed by up-regulation of ubiquitin ligases atrogin1 (MAFbx) and UBR2 (E3α-II), as well as increase in LC3-II, a marker of autophagosome formation, leading to myofibrillar protein loss and myotube atrophy. The catabolic effects of activin A were abolished by p38α/β MAPK inhibitor SB202190. Using small interfering RNA-mediated gene knockdown, we found that the catabolic activity of activin A was dependent on p38β MAPK specifically. Importantly, systemic administration of activin A to mice similarly activated the catabolic pathways in vivo, and this effect was blocked by SB202190. Further, activin A failed to activate the catabolic pathways in mice with muscle-specific knockout of p38β MAPK. Interestingly, activin A up-regulated MuRF1 in a p38 MAPK-independent manner, and MuRF1 did not appear responsible for activin A-induced myosin heavy chain loss and muscle atrophy.

CONCLUSIONS

ActRIIB-mediated activation of muscle catabolism is dependent on p38β MAPK-activated signalling.

Analyzing the anti-ischemia–reperfusion injury effects of ginsenoside Rb1 mediated through the inhibition of p38α MAPK

Recent studies have demonstrated that ginsenoside Rb1 protects the myocardium from ischemia–reperfusion (I/R) injury. However, the precise mechanisms for this protection have not been determined. This study aimed to determine whether the attenuation of I/R-induced myocardial injury by ginsenoside Rb1 (GS Rb1) is due to inhibition of p38α mitogen-activated protein kinase (MAPK). Sprague–Dawley rats were distributed among 6 treatment groups: sham group; I/R group; p38 MAPK inhibitor SB203580 group (SB + I/R); GS Rb1 group (GS + I/R); p38 MAPK agonist anisomycin group (Ani + I/R); and the GS Rb1 + Ani group (GS + Ani + I/R). All of the anaesthetized rats, except those in the sham group, underwent an open-chest procedure that involved 30 min of myocardial ischemia followed by 2 h of reperfusion. Myocardial infarction size (MIS), caspase-3 activity, and levels of the cytokine tumor necrosis factor alpha (TNF-α) in the myocardium were monitored. The expressions of p38α MAPK, caspase-3, and TNF-α in the myocardium were assayed. GS Rb1 reduced MIS and attenuated caspase-3 activity and the levels of TNF-α in the myocardium. Protein expression of total p38α MAPK was not significantly altered. In the Ani + I/R and I/R groups, the levels of phospho-p38α MAPK were significantly increased compared with the sham group, and these increased levels were reduced with GS Rb1. Hemodynamic parameters were not significantly different between the GS + I/R and SB + I/R groups. GS Rb1 exerts an anti-apoptotic effect that protects against I/R injury by inhibiting p38α MAPK phosphorylation, suggesting that GS Rb1-mediated protection requires the inhibition of p38α MAPK.

Gadd45γ regulates cardiomyocyte death and post-myocardial infarction left ventricular remodelling

AIMS

Post-infarction remodelling is accompanied and influenced by perturbations in mitogen-activated protein kinase (MAPK) signalling. The growth arrest and DNA-damage-inducible 45 (Gadd45) proteins are small acidic proteins involved in DNA repair and modulation of MAPK activity. Little is known about the role of Gadd45 in the heart. Here, we explored the potential contribution of Gadd45 gamma (γ) isoform to the acute and late phase of heart failure (HF) after myocardial infarction (MI) and determined the mechanisms underlying Gadd45γ actions.

METHODS AND RESULTS

The Gadd45γ isoform is up-regulated in murine cardiomyocytes subjected to simulated ischaemia and in the mouse heart during MI. To mimic the situation observed during MI, we enhanced Gadd45γ content in cardiomyocytes with a single injection of an adeno-associated viral (AAV9) vector encoding Gadd45γ under the cTNT promoter. Gadd45γ overexpression induces cardiomyocyte apoptosis, fibrosis, left ventricular dysfunction, and HF. On the other hand, genetic deletion of Gadd45γ in knockout mice confers resistance to ischaemic injury, at least in part by limiting cardiomyocyte apoptosis. Mechanistically, Gadd45γ activates receptor-interacting protein 1 (RIP1) and caspase-8 in a p38 MAPK-dependent manner to promote cardiomyocyte death.

CONCLUSION

This work is the first to demonstrate that Gadd45γ accumulation during MI promotes the development and persistence of HF by inducing cardiomyocyte apoptosis in a p38 MAPK-dependent manner. We clearly identify Gadd45γ as a therapeutic target in the development of HF.

Oxygen surrounding the heart during ischemic conservation determines the myocardial injury during reperfusion

There is discrepancy regarding the duration of reperfusion required using 2,3,5-triphenyl-2H-tetrazolium chloride (TTC) staining to assess myocardial infarction in an isolated, perfused heart model. Several investigators prefer long-term reperfusion (120 minutes) to determine myocardial injury, while others have used a shorter duration (30-40 minutes). We investigated whether oxygen surrounding the myocardium during ischemia plays a critical role in the installation of myocardial infarction during reperfusion. Mice hearts were perfused with a Langendorff apparatus using Krebs Henseleit (KH) buffer oxygenated with 95% O2 plus 5% CO2 at 37°C. Hearts were either immersed in KH or suspended in air during 18 minutes of global ischemia in a normothermic, water-jacketed chamber. Hearts then were reperfused for 40, 60, or 90 minutes. We found that hearts immersed in KH had decreased recovery of function and increased myocardial infarct size, reaching a steady-state level after 40 minutes of reperfusion. In contrast, hearts suspended in air approached steady-state after 90 minutes of reperfusion. Thus, mitochondrial reactive oxygen species (ROS) production was much lower in air-maintained hearts than in KH-immersed hearts. To investigate whether an increase in oxygen surrounding the myocardium during ischemia might cause further damage, we bubbled the KH solution with nitrogen (KH+N2) rather than oxygen (KH+O2). With this alteration, recovery of cardiac function was improved and myocardial infarct size and mitochondrial ROS production were reduced compared with hearts immersed in KH+O2. In conclusion, short-term (40 minutes) reperfusion is sufficient to reach steady-state myocardial infarct size when hearts are immersed in physiologic solution during ischemia; however, a longer duration of reperfusion (90 minutes) is required if hearts are suspended in air. Thus, oxygen surrounding the heart during ischemia determines the extent of myocardium injury during reperfusion.

Molecular basis of cardioprotection: signal transduction in ischemic pre-, post-, and remote conditioning

Reperfusion is mandatory to salvage ischemic myocardium from infarction, but reperfusion per se contributes to injury and ultimate infarct size. Therefore, cardioprotection beyond that by timely reperfusion is needed to reduce infarct size and improve the prognosis of patients with acute myocardial infarction. The conditioning phenomena provide such cardioprotection, insofar as brief episodes of coronary occlusion/reperfusion preceding (ischemic preconditioning) or following (ischemic postconditioning) sustained myocardial ischemia with reperfusion reduce infarct size. Even ischemia/reperfusion in organs remote from the heart provides cardioprotection (remote ischemic conditioning). The present review characterizes the signal transduction underlying the conditioning phenomena, including their physical and chemical triggers, intracellular signal transduction, and effector mechanisms, notably in the mitochondria. Cardioprotective signal transduction appears as a highly concerted spatiotemporal program. Although the translation of ischemic postconditioning and remote ischemic conditioning protocols to patients with acute myocardial infarction has been fairly successful, the pharmacological recruitment of cardioprotective signaling has been largely disappointing to date.

p38 MAPK in cardioprotection - are we there yet?

PKs transfer a phosphate from ATP to the side-chain hydroxyl group of a serine, threonine or tyrosine residue of a substrate protein. This in turn can alter that protein's function; modulating fundamental cellular processes including, metabolism, transcription, growth, division, differentiation, motility and survival. PKs are subdivided into families based on homology. One such group are the stress-activated kinases, which as the name suggests, are activated in response to cellular stresses such as toxins, cytokines, mechanical deformation and osmotic stress. Members include the p38 MAPK family, which is composed of α, β, γ and δ, isoforms which are encoded by separate genes. These kinases transduce extracellular signals and coordinate the cellular responses needed for adaptation and survival. However, in cardiovascular and other disease states, these same systems can trigger maladaptive responses that aggravate, rather than alleviate, the disease. This situation is analogous to adrenergic, angiotensin and aldosterone signalling in heart failure, where inhibition is beneficial despite the importance of these hormones to homeostasis. The question is whether similar benefits could accrue from p38 inhibition? In this review, we will discuss the structure and function of p38, the history of p38 inhibitors and their use in preclinical studies. Finally, we will summarize the results of recent cardiovascular clinical trials with p38 inhibitors.

Profiling of cell stress protein expression in cardiac tissue of cardiosurgical patients undergoing remote ischemic preconditioning: implications for thioredoxin in cardioprotection

Background

Transient episodes of ischemia in a remote organ (remote ischemic preconditioning, RIPC) can attenuate myocardial ischemia/reperfusion injury but the underlying mechanisms of RIPC in the target organ are still poorly understood. Recent animal studies suggested that the small redox protein thioredoxin may be a potential candidate for preconditioning-induced organprotection. Here we employed a human proteome profiler array to investigate the RIPC regulated expression of cell stress proteins and particularly of thioredoxin in heart tissue of cardiosurgical patients with cardiopulmonary bypass (CPB).

Methods

RIPC was induced by four 5 minute cycles of transient upper limb ischemia/reperfusion using a blood pressure cuff. Right atrial tissue was obtained from patients receiving RIPC (N = 19) and control patients (N = 19) before and after CPB. Cell stress proteome profiler arrays as well as Westernblotting and ELISA experiments for thioredoxin (Thio-1) were performed employing the respective tissue samples.

Results

Protein arrays revealed an up-regulation of 26.9% (7/26; CA IX, Cyt C, HSP-60, HSP-70, pJNK, SOD2, Thio-1) of cell stress associated proteins in RIPC tissue obtained before CPB, while 3.8% (1/26; SIRT2) of the proteins were down-regulated. Array results for thioredoxin were verified by semi-quantitative Westernblotting studies which showed a significant up-regulation of thioredoxin protein levels in cardiac tissue samples of RIPC patients taken before CPB (RIPC: 5.36 ± 0.85 a.u.; control: 3.23 ± 0.39 a.u.; P < 0.05). Quantification of thioredoxin levels in tissue of RIPC and control patients by ELISA experiments further confirmed the Westernblotting results (RIPC: 0.30 ± 0.02 ng/mg protein; control: 0.24 ± 0.02 ng/mg protein; P < 0.05).

Conclusion

We provide evidence for thioredoxin as a RIPC-induced factor in heart tissue of cardiosurgical patients and identified several cell stress associated proteins that are regulated by RIPC and may play a role in RIPC-mediated cardioprotection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-015-0403-6) contains supplementary material, which is available to authorized users.

Mechanism and consequence of the autoactivation of p38α mitogen-activated protein kinase promoted by TAB1

p38α mitogen-activated protein kinase (p38α) is activated by a variety of mechanisms, including autophosphorylation initiated by TGFβ-activated kinase 1 binding protein 1 (TAB1) during myocardial ischemia and other stresses. Chemical-genetic approaches and coexpression in mammalian, bacterial and cell-free systems revealed that mouse p38α autophosphorylation occurs in cis by direct interaction with TAB1(371-416). In isolated rat cardiac myocytes and perfused mouse hearts, TAT-TAB1(371-416) rapidly activates p38 and profoundly perturbs function. Crystal structures and characterization in solution revealed a bipartite docking site for TAB1 in the p38α C-terminal kinase lobe. TAB1 binding stabilizes active p38α and induces rearrangements within the activation segment by helical extension of the Thr-Gly-Tyr motif, allowing autophosphorylation in cis. Interference with p38α recognition by TAB1 abolishes its cardiac toxicity. Such intervention could potentially circumvent the drawbacks of clinical pharmacological inhibitors of p38 catalytic activity.

Role of PTEN in modulation of ADP-dependent signaling pathways in vascular endothelial cells

ADP plays critical signaling roles in the vascular endothelium. ADP receptors are targeted by several cardiovascular drugs, yet the intracellular pathways modulated by ADP are incompletely understood. These studies have identified important roles for the phosphatase PTEN in ADP-dependent modulation of the endothelial isoform of nitric oxide synthase (eNOS) as well as of lipid and protein kinase pathways in endothelial cells. We find that ADP-promoted eNOS activation as well as phosphorylation of p38 MAPK are enhanced by siRNA-mediated PTEN knockdown. However, the increase in ADP-dependent eNOS activation promoted by PTEN knockdown is abrogated by siRNA-mediated knockdown of p38 MAPK. These findings indicate that PTEN tonically suppresses both p38 phosphorylation as well as ADP-stimulated eNOS activity. A key enzymatic activity of PTEN is its role as a lipid phosphatase, catalyzing the dephosphorylation of phosphoinositol-3,4,5-trisphosphate (PIP3) to phosphoinositol-4,5-bisphosphate (PIP2). We performed biochemical analyses of cellular phospholipids in endothelial cells to show that siRNA-mediated PTEN knockdown leads to a marked increase in PIP3. Because these complex lipids activate the small GTPase Rac1, we explored the role of PTEN in ADP-modulated Rac1 activation. We used a FRET biosensor for Rac1 to show that ADP-dependent Rac1 activation is blocked by siRNA-mediated PTEN knockdown. We then exploited a FRET biosensor for PIP3 to show that the striking ADP-dependent increase in intracellular PIP3 is entirely blocked by PTEN knockdown. These studies identify a key role for PTEN in the modulation of lipid mediators involved in ADP receptor-regulated endothelial signaling pathways involving eNOS activation in vascular endothelial cells.

p38β MAPK upregulates atrogin1/MAFbx by specific phosphorylation of C/EBPβ

Background

The p38 mitogen-activated protein kinases (MAPK) family plays pivotal roles in skeletal muscle metabolism. Recent evidence revealed that p38α and p38β exert paradoxical effects on muscle protein homeostasis. However, it is unknown why p38β, but not p38α, is capable of mediating muscle catabolism via selective activation of the C/EBPβ that upregulates atrogin1/MAFbx.

Methods

Tryptic phosphopeptide mapping was carried out to identify p38α- and p38β-mediated phosphorylation sites in C/EBPβ. Chromosome immunoprecipitation (ChIP) assay was used to evaluate p38α and p38β effect on C/EBPβ binding to the atrogin1/MAFbx promoter. Overexpression or siRNA-mediated gene knockdown of p38α and p38β, and site-directed mutagenesis or knockout of C/EBPβ, were used to analyze the roles of these kinases in muscle catabolism in C2C12 myotubes and mice.

Results

Cellular expression of constitutively active p38α or p38β resulted in phosphorylation of C/EBPβ at multiple serine and threonine residues; however, only p38β phosphorylated Thr-188, which had been known to be critical to the DNA-binding activity of C/EBPβ. Only p38β, but not p38α, activated C/EBPβ-binding to the atrogin1/MAFbx promoter. A C/EBPβ mutant in which Thr-188 was replaced by alanine acted as a dominant-negative inhibitor of atrogin1/MAFbx upregulation induced by either p38β or Lewis lung carcinoma (LLC) cell-conditioned medium (LCM). In addition, knockdown of p38β specifically inhibited C/EBPβ activation and atrogin1/MAFbx upregulation induced by LCM. Finally, expression of active p38β in mouse tibialis anterior specifically induced C/EBPβ phosphorylation at Thr-188, atrogin1/MAFbx upregulation and muscle mass loss, which were blocked in C/EBPβ-null mice.

Conclusions

The α and β isoforms of p38 MAPK are capable of recognizing distinct phosphorylation sites in a substrate. The unique capacity of p38β in mediating muscle catabolism is due to its capability in phosphorylating Thr-188 of C/EBPβ.

TGF-β1 induces endothelial cell apoptosis by shifting VEGF activation of p38(MAPK) from the prosurvival p38β to proapoptotic p38α

TGF-β1 and VEGF, both angiogenesis inducers, have opposing effects on vascular endothelial cells. TGF-β1 induces apoptosis; VEGF induces survival. We have previously shown that TGF-β1 induces endothelial cell expression of VEGF, which mediates TGF-β1 induction of apoptosis through activation of p38 mitogen-activated protein kinase (MAPK). Because VEGF activates p38(MAPK) but protects the cells from apoptosis, this finding suggested that TGF-β1 converts p38(MAPK) signaling from prosurvival to proapoptotic. Four isoforms of p38(MAPK) -α, β, γ, and δ-have been identified. Therefore, we hypothesized that different p38(MAPK) isoforms control endothelial cell apoptosis or survival, and that TGF-β1 directs VEGF activation of p38(MAPK) from a prosurvival to a proapoptotic isoform. Here, we report that cultured endothelial cells express p38α, β, and γ. VEGF activates p38β, whereas TGF-β1 activates p38α. TGF-β1 treatment rapidly induces p38α activation and apoptosis. Subsequently, p38α activation is downregulated, p38β is activated, and the surviving cells become refractory to TGF-β1 induction of apoptosis and proliferate. Gene silencing of p38α blocks TGF-β1 induction of apoptosis, whereas downregulation of p38β or p38γ expression results in massive apoptosis. Thus, in endothelial cells p38α mediates apoptotic signaling, whereas p38β and p38γ transduce survival signaling. TGF-β1 activation of p38α is mediated by VEGF, which in the absence of TGF-β1 activates p38β. Therefore, these results show that TGF-β1 induces endothelial cell apoptosis by shifting VEGF signaling from the prosurvival p38β to the proapoptotic p38α.

Role of Mitogen-Activated Protein Kinases in Myocardial Ischemia-Reperfusion Injury during Heart Transplantation

In solid organ transplantation, ischemia/reperfusion (IR) injury during organ procurement, storage and reperfusion is an unavoidable detrimental event for the graft, as it amplifies graft inflammation and rejection. Intracellular mitogen-activated protein kinase (MAPK) signaling pathways regulate inflammation and cell survival during IR injury. The four best-characterized MAPK subfamilies are the c-Jun NH2-terminal kinase (JNK), extracellular signal- regulated kinase-1/2 (ERK1/2), p38 MAPK, and big MAPK-1 (BMK1/ERK5). Here, we review the role of MAPK activation during myocardial IR injury as it occurs during heart transplantation. Most of our current knowledge regarding MAPK activation and cardioprotection comes from studies of preconditioning and postconditioning in nontransplanted hearts. JNK and p38 MAPK activation contributes to myocardial IR injury after prolonged hypothermic storage. p38 MAPK inhibition improves cardiac function after cold storage, rewarming and reperfusion. Small-molecule p38 MAPK inhibitors have been tested clinically in patients with chronic inflammatory diseases, but not in transplanted patients, so far. Organ transplantation offers the opportunity of starting a preconditioning treatment before organ procurement or during cold storage, thus modulating early events in IR injury. Future studies will need to evaluate combined strategies including p38 MAPK and/or JNK inhibition, ERK1/2 activation, pre- or postconditioning protocols, new storage solutions, and gentle reperfusion.

Role of p38 inhibition in cardiac ischemia/reperfusion injury

The p38 mitogen-activated protein kinases (p38s) are Ser/Thr kinases that are activated as a result of cellular stresses and various pathological conditions, including myocardial ischemia/reperfusion. p38 activation has been shown to accentuate myocardial injury and impair cardiac function. Inhibition of p38 activation and its activity has been proposed to be cardioprotective by slowing the rate of myocardial damage and improving cardiac function. The growing body of evidence on the use of p38 inhibitors as therapeutic means for responding to heart problems is controversial, since both beneficial as well as a lack of protective effects on the heart have been reported. In this review, the outcomes from studies investigating the effect of p38 inhibitors on the heart in a wide range of study models, including in vitro, ex vivo, and in vivo models, are discussed. The correlations of experimental models with practical clinical usefulness, as well as the need for future studies regarding the use of p38 inhibitors, are also addressed.

C/EBPβ mediates tumour-induced ubiquitin ligase atrogin1/MAFbx upregulation and muscle wasting

Upregulation of ubiquitin ligase atrogin1/MAFbx and muscle wasting are hallmarks of cancer cachexia; however, the underlying mechanism is undefined. Here, we describe a novel signalling pathway through which Lewis lung carcinoma (LLC) induces atrogin1/MAFbx upregulation and muscle wasting. C2C12 myotubes treated with LLC-conditioned medium (LCM) rapidly activates p38 MAPK and AKT while inactivating FoxO1/3, resulting in atrogin1/MAFbx upregulation, myosin heavy chain loss, and myotube atrophy. The p38α/β MAPK inhibitor SB202190 blocks the catabolic effects. Upon activation, p38 associates with C/EBPβ resulting in its phosphorylation and binding to a C/EBPβ-responsive cis-element in the atrogin1/MAFbx gene promoter. The promoter activity is stimulated by LCM via p38β-mediated activation of the C/EBPβ-responsive cis-element, independent of the adjacent FoxO1/3-responsive cis-elements in the promoter. In addition, p38 activation is observed in the muscle of LLC tumour-bearing mice, and SB202190 administration blocks atrogin1/MAFbx upregulation and muscle protein loss. Furthermore, C/EBPβ(-/-) mice are resistant to LLC tumour-induced atrogin1/MAFbx upregulation and muscle wasting. Therefore, activation of the p38β MAPK-C/EBPβ signalling pathway appears a key component of the pathogenesis of LLC tumour-induced cachexia.

Targeting of mannan-binding lectin-associated serine protease-2 confers protection from myocardial and gastrointestinal ischemia/reperfusion injury

Complement research experienced a renaissance with the discovery of a third activation route, the lectin pathway. We developed a unique model of total lectin pathway deficiency, a mouse strain lacking mannan-binding lectin-associated serine protease-2 (MASP-2), and analyzed the role of MASP-2 in two models of postischemic reperfusion injury (IRI). In a model of transient myocardial IRI, MASP-2-deficient mice had significantly smaller infarct volumes than their wild-type littermates. Mice deficient in the downstream complement component C4 were not protected, suggesting the existence of a previously undescribed lectin pathway-dependent C4-bypass. Lectin pathway-mediated activation of C3 in the absence of C4 was demonstrated in vitro and shown to require MASP-2, C2, and MASP-1/3. MASP-2 deficiency also protects mice from gastrointestinal IRI, as do mAb-based inhibitors of MASP-2. The therapeutic effects of MASP-2 inhibition in this experimental model suggest the utility of anti-MASP-2 antibody therapy in reperfusion injury and other lectin pathway-mediated disorders.

The p38 mitogen-activated protein kinase pathway--a potential target for intervention in infarction, hypertrophy, and heart failure

The p38 mitogen-activated protein kinases (p38s) are stress-activated Ser/Thr kinases. Their activation has been associated with various pathological stressors in the heart. Activated p38 is implicated in a wide spectrum of cardiac pathologies, including hypertrophy, myocardial infarction, as well as systolic and diastolic heart failure. In this review, the specific contribution of different isoforms of p38 kinases to cardiac diseases as well as TAB-1-mediated non-canonical activation pathway are discussed as a rationale for inhibiting p38 activity to treat cardiac hypertrophy, ischemic injury, and heart failure. Finally, a summary of current clinical trials targeting p38 kinases in cardiovascular diseases is provided to highlight the potential promise as well as existing challenges of this therapeutic approach. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Developing small molecules to inhibit kinases unkind to the heart: p38 MAPK as a case in point

Over the last 40 years targeting G protein-coupled receptors and their ligands has had a major impact on the treatment of cardiovascular disease. However, the last decade has seen little progress and focus has shifted, particularly in the field of cancer biology, to downstream kinases. This review focuses on the kinases within the heart that become active during myocardial infarction and heart failure and contribute to cardiac dysfunction, with a special emphasis on p38 mitogen-activated protein kinase (MAPK).