Jun kinase delays caspase-9 activation by interaction with the apoptosome

Extracellular Vesicle Inhibitors Enhance Cholix-Induced Cell Death via Regulation of the JNK-Dependent Pathway

Vibrio cholerae is an important foodborne pathogen. Cholix cytotoxin (Cholix), produced by V. cholerae, is a novel eukaryotic elongation factor 2 (eEF2) adenosine diphosphate ribosyltransferase that causes host cell death by inhibiting protein synthesis. However, the role of Cholix in the infectious diseases caused by V. cholerae remains unclear. Some bacterial cytotoxins are carried by host extracellular vesicles (EVs) and transferred to other cells. In this study, we investigated the effects of EV inhibitors and EV-regulating proteins on Cholix-induced hepatocyte death. We observed that Cholix-induced cell death was significantly enhanced in the presence of EV inhibitors (e.g., dimethyl amiloride, and desipramine) and Rab27a-knockdown cells, but it did not involve a sphingomyelin-dependent pathway. RNA sequencing analysis revealed that desipramine, imipramine, and EV inhibitors promoted the Cholix-activated c-Jun NH2-terminal kinase (JNK) pathway. Furthermore, JNK inhibition decreased desipramine-enhanced Cholix-induced poly (ADP-ribose) polymerase (PARP) cleavage. In addition, suppression of Apaf-1 by small interfering RNA further enhanced Cholix-induced PARP cleavage by desipramine. We identified a novel function of desipramine in which the stimulated JNK pathway promoted a mitochondria-independent cell death pathway by Cholix.

The RELT Family of Proteins: An Increasing Awareness of Their Importance for Cancer, the Immune System, and Development

This review highlights Receptor Expressed in Lymphoid Tissues (RELT), a Tumor Necrosis Factor Superfamily member, and its two paralogs, RELL1 and RELL2. Collectively, these three proteins are referred to as RELTfms and have gained much interest in recent years due to their association with cancer and other human diseases. A thorough knowledge of their physiological functions, including the ligand for RELT, is lacking, yet emerging evidence implicates RELTfms in a variety of processes including cytokine signaling and pathways that either promote cell death or survival. T cells from mice lacking RELT exhibit increased responses against tumors and increased inflammatory cytokine production, and multiple lines of evidence indicate that RELT may promote an immunosuppressive environment for tumors. The relationship of individual RELTfms in different cancers is not universal however, as evidence indicates that individual RELTfms may be risk factors in certain cancers yet appear to be protective in other cancers. RELTfms are important for a variety of additional processes related to human health including microbial pathogenesis, inflammation, behavior, reproduction, and development. All three proteins have been strongly conserved in all vertebrates, and this review aims to provide a clearer understanding of the current knowledge regarding these interesting proteins.

Identification of reactive oxygen species that induce spoptosis, a novel and distinctive mode of regulated cell death

Using some solutions activated by irradiation with non-thermal atmospheric pressure plasma (NTAPP), we had discovered that a new and distinctive mode of cell death, named spoptosis, exists in cells, the induction of which involves the action of reactive oxygen species (ROS). However, it was unknown what types of ROS and how they trigger the cell death. When cells were treated with a higher dose of Ascorbic acid (AA) generating O₂^- and H₂O₂ or Antimycin A (AM) generating O₂^-, cell death occurred along with cellular shrinkage, Pdcd4 disappearance, and vesicle formation. Only in cells treated with AA, genomic DNA was digested irregularly and membrane permeability increased aberrantly. On the other hand, cells treated with a higher dose of H₂O₂ displayed cell death and cellular shrinkage but not the other events, and those treated with a lower dose of H₂O₂ displayed cell death but not the other events. Strikingly, when cells underwent double treatment with AM and H₂O₂, the events, which had not been observed by their single treatment, became compensated. All the events were suppressed with an antioxidant, confirming that they were mediated by ROS. Thus, the mode of cell death induced by AA or combination of AM and H₂O₂ was consistent with that of cell death by NTAPP-activated solutions. These results suggested that O₂^- and H₂O₂ collaboratively trigger spoptotic cell death with the associated events, and that AA and combination of AM and H₂O₂ are functionally alternative in place of NTAPP-activated solutions.

Understanding MAPK Signaling Pathways in Apoptosis

MAPK (mitogen-activated protein kinase) signaling pathways regulate a variety of biological processes through multiple cellular mechanisms. In most of these processes, such as apoptosis, MAPKs have a dual role since they can act as activators or inhibitors, depending on the cell type and the stimulus. In this review, we present the main pro- and anti-apoptotic mechanisms regulated by MAPKs, as well as the crosstalk observed between some MAPKs. We also describe the basic signaling properties of MAPKs (ultrasensitivity, hysteresis, digital response), and the presence of different positive feedback loops in apoptosis. We provide a simple guide to predict MAPKs’ behavior, based on the intensity and duration of the stimulus. Finally, we consider the role of MAPKs in osmostress-induced apoptosis by using Xenopus oocytes as a cell model. As we will see, apoptosis is plagued with multiple positive feedback loops. We hope this review will help to understand how MAPK signaling pathways engage irreversible cellular decisions.

JNK signaling in cancer cell survival

c-Jun N-terminal kinase (JNK) is involved in cancer cell apoptosis; however, emerging evidence indicates that this Janus signaling promotes cancer cell survival. JNK acts synergistically with NF-κB, JAK/STAT, and other signaling molecules to exert a survival function. JNK positively regulates autophagy to counteract apoptosis, and its effect on autophagy is related to the development of chemotherapeutic resistance. The prosurvival effect of JNK may involve an immune evasion mechanism mediated by transforming growth factor-β, toll-like receptors, interferon-γ, and autophagy, as well as compensatory JNK-dependent cell proliferation. The present review focuses on recent advances in understanding the prosurvival function of JNK and its role in tumor development and chemoresistance, including a comprehensive analysis of the molecular mechanisms underlying JNK-mediated cancer cell survival. There is a focus on the specific "Yin and Yang" functions of JNK1 and JNK2 in the regulation of cancer cell survival. We highlight recent advances in our knowledge of the roles of JNK in cancer cell survival, which may provide insight into the distinct functions of JNK in cancer and its potential for cancer therapy.

Potential of Bacillus subtilis lipopeptides in anti-cancer I: induction of apoptosis and paraptosis and inhibition of autophagy in K562 cells

The lipopeptide iturin from Bacillus subtilis has been found to have a potential inhibitory effect on breast cancer, alveolar adenocarcinoma, renal carcinoma, and colon adenocarcinoma. In this study, the potential of B. subtilis lipopeptides (a mixture of iturin homologues, concentration of 42.75%) to inhibit chronic myelogenous leukemia was evaluated using K562 myelogenous leukemia cells. The results showed that the lipopeptides could completely inhibit the growth of K562 at 100 μM, with an IC50 value of 65.76 μM. The lipopeptides inhibited the profile of K562 via three pathways: (1) induction of paraptosis indicated by the occurrence of cytoplasmic vacuoles, and swelling of the mitochondria and endoplasmic reticulum (ER) without membrane blebbing in the presence of a caspase inhibitor; (2) inhibition of autophagy progress illustrated by the upregulated expression of LCII and P62; and (3) induction of apoptosis by causing ROS burst, and induction of the intrinsic pathway indicated by the upregulated expression of cytochrome c (Cyto-c), bax, and bad, together with downregulated expression of Bcl-2. The ROS-dependent apoptosis and caspase-independent paraptosis were verified using the ROS inhibitor and caspase inhibitor, respectively. The extrinsic apoptosis pathway was not involved in the lipopeptide’s effects on K562. Overall, the B. subtilis lipopeptides (consisting of a majority of iturin) exhibited promising potential in inhibiting chronic myelogenous leukemia in vitro via simultaneously causing paraptosis, apoptosis, and inhibition of autophagy.

Role of glutathione, ROS, and Bcl-xL in the inhibition of apoptosis of monocyte-derived dendritic cells by Leishmania mexicana promastigotes

Dendritic cells (DCs) are one of the principal host cells of the obligate intracellular parasite Leishmania that can survive and reproduce within cells due to the ability to regulate different cellular events, including apoptosis. Inhibition of host cell apoptosis is a strategy employed by multiple pathogens to ensure their survival in the infected cell. We have previously reported that Leishmania mexicana promastigotes and amastigotes inhibit camptothecin-induced apoptosis of monocyte-derived dendritic cells (moDCs) through the downregulation of p38 and JNK phosphorylation. The upregulation of glutathione (GSH), the most important regulator of reactive oxygen species (ROS) concentration, has proven to protect cells from apoptosis through the inhibition of JNK1. Another mechanism employed by cells for the protection of apoptosis is the expression of anti-apoptotic proteins of the Bcl-2 family. The aim of this study was to determine if GSH, ROS, and Bcl-xL participate in the inhibition of camptothecin-induced apoptosis of moDC by L. mexicana promastigotes. GSH quantification assays showed that camptothecin and BSO (an inhibitor of glutathione synthesis) strongly decreased intracellular GSH concentration in moDC, while infection with L. mexicana promastigotes had no effect in the level of GSH. On the other hand, infection with L. mexicana promastigotes of BSO- and camptothecin-treated moDC diminished the concentration of ROS and induced the expression of the anti-apoptotic protein Bcl-xL. Our findings suggest that inhibition of camptothecin-induced apoptosis of moDC by L. mexicana promastigotes is preferentially regulated by the expression of anti-apoptotic proteins of the Bcl-2 family rather than by the redox status of the cell.

Systems pharmacological analysis of mitochondrial cardiotoxicity induced by selected tyrosine kinase inhibitors

Tyrosine kinase inhibitors (TKIs) are targeted therapies rapidly becoming favored over conventional cytotoxic chemotherapeutics. Our study investigates two FDA approved TKIs, DASATINIB; indicated for IMATINIB-refractory chronic myeloid leukemia, and SORAFENIB; indicated for hepatocellular carcinoma and advanced renal cell carcinoma. Limited but crucial evidence suggests that these agents can have cardiotoxic side effects ranging from hypertension to heart failure. A greater understanding of the underlying mechanisms of this cardiotoxicity are needed as concerns grow and the capacity to anticipate them is lacking. The objective of this study was to explore the mitochondrial-mediated cardiotoxic mechanisms of the two selected TKIs. This was achieved experimentally using immortalized human cardiomyocytes, AC16 cells, to investigate dose- and time-dependent cell killing, along with measurements of temporal changes in key signaling proteins involved in the intrinsic apoptotic and autophagy pathways upon exposure to these agents. Quantitative systems pharmacology (QSP) models were developed to capture the toxicological response in AC16 cells using protein dynamic data. The developed QSP models captured well all the various trends in protein signaling and cellular responses with good precision on the parameter estimates, and were successfully qualified using external data sets. An interplay between the apoptotic and autophagic pathways was identified to play a major role in determining toxicity associated with the investigated TKIs. The established modeling platform showed utility in elucidating the mechanisms of cardiotoxicity of SORAFENIB and DASATINIB. It may be useful for other small molecule targeted therapies demonstrating cardiac toxicities, and may aid in informing alternate dosing strategies to alleviate cardiotoxicity associated with these therapies.

Angiotensin II-preconditioning is associated with increased PKCε/PKCδ ratio and prosurvival kinases in mitochondria

Angiotensin II-preconditioning (APC) has been shown to reproduce the cardioprotective effects of ischaemic preconditioning (IPC), however, the molecular mechanisms mediating the effects of APC remain unknown. In this study, Langendorff-perfused rat hearts were subjected to IPC, APC or both (IPC/APC) followed by ischaemia-reperfusion (IR), to determine translocation of PKCε, PKCδ, Akt, Erk1/2, JNK, p38 MAPK and GSK-3β to mitochondria as an indicator of activation of the protein kinases. In agreement with previous observations, IPC, APC and IPC/APC increased the recovery of left ventricular developed pressure (LVDP), reduced infarct size (IS) and lactate dehydrogenase (LDH) release, compared to controls. These effects were associated with increased mitochondrial PKCε/PKCδ ratio, Akt, Erk1/2, JNK, and inhibition of permeability transition pore (mPTP) opening. Chelerythrine, a pan-PKC inhibitor, abolished the enhancements of PKCε but increased PKCδ expression, and inhibited Akt, Erk1/2, and JNK protein levels. The drug had no effect on the APC- and IPC/APC-induced cardioprotection as previously reported, but enhanced the post-ischaemic LVDP in controls. Losartan, an angiotensin II type 1 receptor (AT1-R) blocker, abolished the APC-stimulated increase of LVDP and reduced PKCε, Akt, Erk1/2, JNK, and p38. Both drugs reduced ischaemic contracture and LDH release, and abolished the inhibition of mPTP by the preconditioning. Chelerythrine also prevented the reduction of IS by APC and IPC/APC. These results suggest that the cardioprotection induced by APC and IPC/APC involves an AT1-R-dependent translocation of PKCε and survival kinases to the mitochondria leading to mPTP inhibition. In chelerythrine-treated hearts, however, alternate mechanisms appear to maintain cardiac function.

Signaling Pathways in Cardiac Myocyte Apoptosis

Cardiovascular diseases, the number 1 cause of death worldwide, are frequently associated with apoptotic death of cardiac myocytes. Since cardiomyocyte apoptosis is a highly regulated process, pharmacological intervention of apoptosis pathways may represent a promising therapeutic strategy for a number of cardiovascular diseases and disorders including myocardial infarction, ischemia/reperfusion injury, chemotherapy cardiotoxicity, and end-stage heart failure. Despite rapid growth of our knowledge in apoptosis signaling pathways, a clinically applicable treatment targeting this cellular process is currently unavailable. To help identify potential innovative directions for future research, it is necessary to have a full understanding of the apoptotic pathways currently known to be functional in cardiac myocytes. Here, we summarize recent progress in the regulation of cardiomyocyte apoptosis by multiple signaling molecules and pathways, with a focus on the involvement of these pathways in the pathogenesis of heart disease. In addition, we provide an update regarding bench to bedside translation of this knowledge and discuss unanswered questions that need further investigation.

Leishmania mexicana amastigotes inhibit p38 and JNK and activate PI3K/AKT: role in the inhibition of apoptosis of dendritic cells

Leishmania mexicana is the causal agent of cutaneous leishmaniasis in Mexico. Dendritic cells (DC) are one of the host cells of Leishmania parasites. Intracellular microorganisms inhibit host cell apoptosis as a strategy to ensure their survival in infected cells. We have previously shown that Leishmania mexicana promastigotes and amastigotes inhibit camptothecin-induced apoptosis of monocyte-derived dendritic cells (moDC), but the mechanisms underlying the inhibition of apoptosis of DC by Leishmania have not been established. MAP kinases and PI3K participate in the process of apoptosis and are modulated by different species of Leishmania. As shown in this study, the infection of moDC with L. mexicana amastigotes diminished significantly the phosphorylation of the MAP kinases p38 and JNK. The inhibition of both kinases diminished significantly DNA fragmentation in moDC stimulated with camptothecin. On the other hand, L. mexicana amastigotes were able to activate the anti-apoptotic pathways PI3K and AKT. Our results indicate that L. mexicana amastigotes have the capacity to diminish MAP kinases activation and activate PI3K and AKT, which is probably one of the strategies employed by L. mexicana amastigotes to inhibit apoptosis in the infected moDC.

Leishmania mexicana promastigotes down regulate JNK and p-38 MAPK activation: Role in the inhibition of camptothecin-induced apoptosis of monocyte-derived dendritic cells

Dendritic cells (DC) are one of the principal host cells of the obligate intracellular parasite Leishmania. Inhibition of host cell apoptosis is a strategy employed by multiple pathogens to ensure their survival in the infected cell. We have previously shown that the infection of monocyte-derived dendritic cells (moDC) with Leishmania mexicana inhibits campthotecin-induced apoptosis. Nevertheless, the mechanisms involved in the inhibition of apoptosis of dendritic cells by Leishmania have not been established. Mitogen-activated protein kinases (MAPK) are key participants in the process of apoptosis and different species of Leishmania have been shown to regulate these kinases. In the present study, we analyzed the effect of L. mexicana promastigotes in the activation of JNK and p38 MAP kinase and their participation in the inhibition of apoptosis. The infection of moDC with L. mexicana promastigotes diminished significantly the phosphorylation of the MAP kinases JNK and p38. The inhibition of both kinases diminished DNA fragmentation, but in a major extent was the reduction of DNA fragmentation when JNK was inhibited. The capacity of L. mexicana promastigotes to diminish MAP kinases activation is probably one of the strategies employed to delay apoptosis induction in the infected moDC and may have implications for Leishmania pathogenesis by favoring the invasion of its host and the persistence of the parasite in the infected cells.

Alpha B-crystallin induction in skeletal muscle cells under redox imbalance is mediated by a JNK-dependent regulatory mechanism

The small heat shock protein α-B-crystallin (CRYAB) is critically involved in stress-related cellular processes such as differentiation, apoptosis, and redox homeostasis. The up-regulation of CRYAB plays a key role in the cytoprotective and antioxidant response, but the molecular pathway driving its expression in muscle cells during oxidative stress still remains unknown. Here we show that noncytotoxic exposure to sodium meta-arsenite (NaAsO2) inducing redox imbalance is able to increase the CRYAB content of C2C12 myoblasts in a transcription-dependent manner. Our in silico analysis revealed a genomic region upstream of the Cryab promoter containing two putative antioxidant-responsive elements motifs and one AP-1-like binding site. The redox-sensitive transcription factors Nrf2 and the AP-1 component c-Jun were found to be up-regulated in NaAsO2-treated cells, and we demonstrated a specific NaAsO2-mediated increase of c-Jun and Nrf2 binding activity to the genomic region identified, supporting their putative involvement in CRYAB regulation following a shift in redox balance. These changes also correlated with a specific phosphorylation of JNK and p38 MAPK kinases, the well-known molecular mediators of signaling pathways leading to the activation of these transcription factors. Pretreatment of C2C12 cells with the JNK inhibitor SP600125 induced a decrease in c-Jun and Nrf2 content and was able to counteract the NaAsO2-mediated increase in CRYAB expression. Thus these data show a direct role of JNK in CRYAB regulation under redox imbalance and also point to a previously unrecognized link between c-Jun and Nrf2 transcription factors and redox-induced CRYAB expression in muscle cells.

Regulation of the intrinsic apoptosis pathway by reactive oxygen species

SIGNIFICANCE

The intrinsic apoptosis pathway is conserved from worms to humans and plays a critical role in the normal development and homeostatic control of adult tissues. As a result, numerous diseases from cancer to neurodegeneration are associated with either too little or too much apoptosis.

RECENT ADVANCES

B cell lymphoma-2 (BCL-2) family members regulate cell death, primarily via their effects on mitochondria. In stressed cells, proapoptotic BCL-2 family members promote mitochondrial outer membrane permeabilization (MOMP) and cytochrome c (cyt c) release into the cytoplasm, where it stimulates formation of the "apoptosome." This large, multimeric complex is composed of the adapter protein, apoptotic protease-activating factor-1, and the cysteine protease, caspase-9. Recent studies suggest that proteins involved in the processes leading up to (and including) formation of the apoptosome are subject to various forms of post-translational modification, including proteolysis, phosphorylation, and in some cases, direct oxidative modification.

CRITICAL ISSUES

Despite intense investigation of the intrinsic pathway, significant questions remain regarding how cyt c is released from mitochondria, how the apoptosome is formed and regulated, and how caspase-9 is activated within the complex.

FUTURE DIRECTIONS

Further studies on the biochemistry of MOMP and apoptosome formation are needed to understand the mechanisms that underpin these critical processes, and novel animal models will be necessary in the future to ascertain the importance of the many posttranslational modifications reported for BCL-2 family members and components of the apoptosome.

Hydrogen peroxide-induced necrotic cell death in cardiomyocytes is independent of matrix metalloproteinase-2

Matrix metalloproteinase-2 (MMP-2) is well known to proteolyse both extracellular and intracellular proteins. Reactive oxygen species activate MMP-2 at both transcriptional and post-translational levels, thus MMP-2 activation is considered an early event in oxidative stress injury. Although hydrogen peroxide is widely used to trigger oxidative stress-induced cell death, the type of cell death (apoptosis vs. necrosis) in cardiomyocytes is still controversial depending on the concentration used and the exposure time. We carefully investigated the mode of cell death in neonatal rat cardiomyocytes induced by different concentrations (50-500 μM) of hydrogen peroxide at various time intervals after exposure and determined whether MMP-2 is implicated in hydrogen peroxide-induced cardiomyocyte death. Treating cardiomyocytes with hydrogen peroxide led to elevated MMP-2 level/activity with maximal effects seen at 200 μM. Hydrogen peroxide caused necrotic cell death by disrupting the plasmalemma as evidenced by the release of lactate dehydrogenase in a concentration- and time-dependent manner as well as the necrotic cleavage of PARP-1. The absence of both caspase-3 cleavage/activation and apoptotic cleavage of PARP-1 illustrated the weak contribution of apoptosis. Pre-treatment with selective MMP inhibitors did not protect against hydrogen peroxide-induced necrosis. In conclusion hydrogen peroxide increases MMP-2 level/activity in cardiomyocytes and induces necrotic cell death, however, the later effect is MMP-2 independent.

Repression of miR-142 by p300 and MAPK is required for survival signalling via gp130 during adaptive hypertrophy

An increase in cardiac workload, ultimately resulting in hypertrophy, generates oxidative stress and therefore requires the activation of both survival and growth signal pathways. Here, we wanted to characterize the regulators, targets and mechanistic roles of miR-142, a microRNA (miRNA) negatively regulated during hypertrophy. We show that both miRNA-142-3p and -5p are repressed by serum-derived growth factors in cultured cardiac myocytes, in models of cardiac hypertrophy in vivo and in human cardiomyopathic hearts. Levels of miR-142 are inversely related to levels of acetyltransferase p300 and MAPK activity. When present, miR-142 inhibits both survival and growth pathways by directly targeting nodal regulators p300 and gp130. MiR-142 also potently represses multiple components of the NF-κB pathway, preventing cytokine-mediated NO production and blocks translation of α-actinin. Forced expression of miR-142 during hypertrophic growth induced extensive apoptosis and cardiac dysfunction; conversely, loss of miR-142 fully rescued cardiac function in a murine heart failure model. Downregulation of miR-142 is required to enable cytokine-mediated survival signalling during cardiac growth in response to haemodynamic stress and is a critical element of adaptive hypertrophy.

Daunorubicin induces cell death via activation of apoptotic signalling pathway and inactivation of survival pathway in muscle-derived stem cells

Daunorubicin (as well as other anthracyclines) is known to be toxic to heart cells and other cells in organism thus limiting its applicability in human cancer therapy. To investigate possible mechanisms of daunorubicin cytotoxicity, we used stem cell lines derived from adult rabbit skeletal muscle. Recently, we have shown that daunorubicin induces apoptotic cell death in our cell model system and distinctly influences the activity of MAP kinases. Here, we demonstrate that two widely accepted antagonistic signalling pathways namely proapoptotic JNK and prosurvival PI3K/AKT participate in apoptosis. Using the Western blot method, we observed the activation of JNK and phosphorylation of its direct target c-Jun along with inactivation of AKT and its direct target GSK in the course of programmed cell death. By means of small-molecule kinase inhibitors and transfection of cells with the genes of the components of these pathways, c-Jun and AKT, we confirm that JNK signalling pathway is proapoptotic, whereas AKT is antiapoptotic in daunorubicin-induced muscle cells. These findings could contribute to new approaches which will result in less toxicity and fewer side effects that are currently associated with the use of daunorubicin in cancer therapies.

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.

Mitogen-activated protein kinase signaling in the heart: angels versus demons in a heart-breaking tale

Among the myriad of intracellular signaling networks that govern the cardiac development and pathogenesis, mitogen-activated protein kinases (MAPKs) are prominent players that have been the focus of extensive investigations in the past decades. The four best characterized MAPK subfamilies, ERK1/2, JNK, p38, and ERK5, are the targets of pharmacological and genetic manipulations to uncover their roles in cardiac development, function, and diseases. However, information reported in the literature from these efforts has not yet resulted in a clear view about the roles of specific MAPK pathways in heart. Rather, controversies from contradictive results have led to a perception that MAPKs are ambiguous characters in heart with both protective and detrimental effects. The primary object of this review is to provide a comprehensive overview of the current progress, in an effort to highlight the areas where consensus is established verses the ones where controversy remains. MAPKs in cardiac development, cardiac hypertrophy, ischemia/reperfusion injury, and pathological remodeling are the main focuses of this review as these represent the most critical issues for evaluating MAPKs as viable targets of therapeutic development. The studies presented in this review will help to reveal the major challenges in the field and the limitations of current approaches and point to a critical need in future studies to gain better understanding of the fundamental mechanisms of MAPK function and regulation in the heart.

c-Jun N-terminal kinase phosphorylation of stathmin confers protection against cellular stress

The cell stress response encompasses the range of intracellular events required for adaptation to stimuli detrimental to cell survival. Although the c-Jun N-terminal kinase (JNK) is a stress-activated kinase that can promote either cell survival or death in response to detrimental stimuli, the JNK-regulated mechanisms involved in survival are not fully characterized. Here we show that in response to hyperosmotic stress, JNK phosphorylates a key cytoplasmic microtubule regulatory protein, stathmin (STMN), on conserved Ser-25 and Ser-38 residues. In in vitro biochemical studies, we identified STMN Ser-38 as the critical residue required for efficient phosphorylation by JNK and identified a novel kinase interaction domain in STMN required for recognition by JNK. We revealed that JNK was required for microtubule stabilization in response to hyperosmotic stress. Importantly, we also demonstrated a novel cytoprotective function for STMN, as the knockdown of STMN levels by siRNA was sufficient to augment viability in response to hyperosmotic stress. Our findings show that JNK targeting of STMN represents a novel stress-activated cytoprotective mechanism involving microtubule network changes.

Molecules that modulate Apaf-1 activity

Programmed cell death, apoptosis, is a highly regulated cellular pathway, responsible for the elimination of cells in the organism that are no longer needed or extensively damaged. Defects in the regulation of apoptosis could be at the molecular basis of different diseases, either when it is insufficient or excessive. The formation of the macromolecular complex, apoptosome, is a key event in this pathway, which has also been defined as the intrinsic apoptosis pathway. The apoptosome is a holoenzyme multiprotein complex formed by cytochrome c-activated apoptotic protease-activating factor (Apaf-1), dATP, and procaspase-9. Recent studies have produced a wealth of information about the regulation and functions of Apaf-1, but additional studies aimed at elucidating its role as a signaling device at the crosstalk between different signaling pathways are needed to take advantage for the development of modulators of apoptosis pathways and possible therapeutic applications.

Tribbles-1 and -2 are tumour suppressors, down-regulated in human acute myeloid leukaemia

Constitutive MAPK signalling is observed in approximately 50% of acute myeloid leukaemia (AML) cases. JNK activation in particular is associated with treatment failure in AML. Tribbles proteins (trb-1, trb-2 and trb-3) are potent negative regulators of MAPK pathways influencing apoptosis, differentiation and cell-cycle progression. Here we aimed to examine tribbles gene expression in AML and to characterise their role in leukaemic cells. A microarray dataset was interrogated for tribbles expression levels in AML cases and healthy controls. Myeloid cell proliferation and apoptosis were assayed in response to trb-1/trb-2 gene knockdown and overexpression, as well as a physical and functional interaction between trb and C/EBPalpha. Trb-2 expression was reduced in AML compared to healthy controls (correlating with nucleophosmin (NPM1) mutations), while low trb-1 expression was associated with inactive C/EBPalpha. In vitro assays indicated that trb-1/trb-2 are growth restrictive and pro-apoptotic in Me-1 cells, each capable of inhibiting JNK activation. JNK inactivation was itself associated with reduced Bcl-2 Ser70 phosphorylation, a residue which, when phosphorylated, maintains the anti-apoptotic activity of Bcl-2. Consistent with this, tribbles-mediated dephosphorylation of Bcl-2 Ser70 was associated with subsequent apoptosis. Trb-1/trb-2 transcription appeared to be moderately C/EBPalpha-responsive, and physical interaction between C/EBPalpha and trb-1/trb-2 was observed, suggesting a potential for auto-regulation of trb-1 and trb-2 transcription. In conclusion, we propose that trb-1 and trb-2 tumour suppressor activity may be abrogated in a proportion of AML patients. This may lead to enhanced cell survival, and therefore contribute to pathogenesis of the disease. Trb-1/trb-2 may, therefore, represent useful therapeutic targets for the treatment of AML in patients with dys-regulated trb activity.

Generation of digital responses in stress sensors

Ultrasensitivity, hysteresis (a form of biochemical memory), and all-or-none (digital) responses are important signaling properties for the control of irreversible processes and are well characterized in the c-Jun N-terminal kinase (JNK) system using Xenopus oocytes. Our aim was to study these properties in the AMP-activated protein kinase (AMPK) signaling system under stress conditions that could engage a cell death program, and compare them to the JNK responses. After characterization of Xenopus AMPK, we show here that the response to antimycin (nonapoptotic) was slightly cooperative and graded (analog) in individual oocytes, whereas the response to sorbitol (which induced cytochrome c release and caspase activation) was ultrasensitive, digital in single cells, and without hysteresis, hallmarks of a monostable system. Moreover, initial graded responses of AMPK and JNK turned into digital during a critical period for the execution of the cell death program, although single cell analysis did not show complete correlation between AMPK or JNK activation and cytochrome c release. We propose a model where the life or death decision in the cell is made by integration of multiple digital signals from stress sensors.

Inhibition of JNK enhances TGF-beta1-activated Smad2 signaling in mouse embryonic lung

The Smad2/3 pathway plays a key role in mediating TGF-beta1 inhibition of branching morphogenesis and induction of connective tissue growth factor (CTGF) expression in embryonic lungs. Because a number of cell-specific interactions have been described between TGF-beta1-driven Smad signaling and the c-Jun N-terminal kinase (JNK) pathway, we have investigated the effects of JNK inhibition on TGF-beta1 activation of Smad2, inhibition of branching, induction of CTGF expression, and apoptosis in mouse embryonic lung explants. Mouse embryonic day 12.5 (E12.5) lung explants were treated with TGF-beta1 in the presence or absence of a specific pharmacologic JNK inhibitor (SP600125) and a specific JNK peptide inhibitor (JNKI). We found that TGF-beta1 activated the JNK pathway by stimulating c-Jun phosphorylation, which was blocked by JNK inhibitors. Treatment with SP600125 stimulated Smad2 phosphorylation and enhanced TGF-beta1-induced Smad2 phosphorylation. Treatment with JNK inhibitors also decreased normal branching morphogenesis and induced CTGF expression as well as augmented TGF-beta1 inhibition of branching and induction of CTGF expression. Furthermore, JNK inhibition-induced apoptosis. Our results demonstrate that inhibition of the JNK pathway promotes TGF-beta1-driven Smad2 responses in lung branching morphogenesis. These data suggest that the JNK pathway may antagonize TGF-beta1 dependent Smad2 signaling during mouse embryonic lung development.

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.