Activation of p38 Mitogen-Activated Protein Kinase Is Required for Death Receptor–Independent Caspase-8 Activation and Cell Death in Response to Sphingosine

Characterization of the RAS/RAF/ERK Signal Cascade as a Novel Regulating Factor in Alpha-Amanitin-Induced Cytotoxicity in Huh-7 Cells

The well-known hepatotoxicity mechanism resulting from alpha-amanitin (α-AMA) exposure arises from RNA polymerase II (RNAP II) inhibition. RNAP Ⅱ inhibition occurs through the dysregulation of mRNA synthesis. However, the signaling pathways in hepatocytes that arise from α-AMA have not yet been fully elucidated. Here, we identified that the RAS/RAF/ERK signaling pathway was activated through quantitative phosphoproteomic and molecular biological analyses in Huh-7 cells. Bioinformatics analysis showed that α-AMA exposure increased protein phosphorylation in a time-dependent α-AMA exposure. In addition, phosphorylation increased not only the components of the ERK signaling pathway but also U2AF65 and SPF45, known splicing factors. Therefore, we propose a novel mechanism of α-AMA as follows. The RAS/RAF/ERK signaling pathway involved in aberrant splicing events is activated by α-AMA exposure followed by aberrant splicing events leading to cell death in Huh-7 cells.

Kaempferia parviflora Rhizome Extract Inhibits Glutamate-Induced Toxicity in HT-22 Mouse Hippocampal Neuronal Cells and Extends Longevity in Caenorhabditis elegans

Kaempferia parviflora Wall. ex Baker (KP) or "Kra-chai-dam" has been shown to exhibit several pharmacological effects including anti-inflammation, antimicrobial, and sexual-enhancing activity. The objectives of this study included an investigation of the effect of KP rhizome extract against glutamate-induced toxicity in mouse hippocampal HT-22 neuronal cells, determination of the underlying mechanism of neuroprotection, and an evaluation of the effect of KP extract on the longevity of Caenorhabditis elegans. HT-22 cells were co-treated with glutamate (5 mM) and KP extract (25, 50, and 75 μg/mL) for 14 h. Cell viability, intracellular reactive oxygen species (ROS) assay, fluorescence-activated cell sorting (FACS) analysis, and Western blotting were performed. The longevity effect of KP extract on C. elegans was studied by lifespan measurement. In HT-22 cells, co-treatment of glutamate with KP extract significantly inhibited glutamate-mediated cytotoxicity and decreased intracellular ROS production. Additionally, the glutamate-induced apoptosis and apoptotic-inducing factor (AIF) translocation were blocked by KP extract co-treatment. Western blot analysis also demonstrated that KP extract significantly diminished extracellular signal-regulated kinase (ERK) phosphorylation induced by glutamate, and brain-derived neurotrophic factor (BDNF) was recovered to the control. Moreover, this KP extract treatment prolonged the lifespan of C. elegans. Altogether, this study suggested that KP extract possesses both neuroprotective and longevity-inducing properties, thus serving as a promising candidate for development of innovative health products.

Regulation of docosahexaenoic acid-induced apoptosis of confluent endothelial cells: Contributions of MAPKs and caspases

Endothelial cells, which help to maintain vascular homeostasis, can be functionally modulated by polyunsaturated fatty acids. Previously, we reported that docosahexaenoic acid (DHA) reduced the viability of confluent EA.hy926 endothelial cells with caspase-3 activation. This study therefore examined the molecular mechanism by which DHA affects the viability of confluent cells, with a focus on the interaction between caspase-9, caspase-8, caspase-3, p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK) by Western blotting. Our results revealed that DHA induces apoptosis of confluent cells through both intrinsic and extrinsic pathways, which requires activation of p38 MAPK, and involves activation of JNK, caspase-9, caspase-8 and caspase-3 with the exception that cleavage of caspase-8 was incomplete and truncated BID was not detected at the maximum time (8 h) examined. Apoptosis induced by high levels of DHA in healthy endothelial cells is achieved through positive feedback loops linking these MAPKs to multiple caspases, as well as negative feedback from p38 MAPK to JNK. However, only p38 MAPK is crucial in apoptosis induction in comparison with JNK or any other caspase examined. This study has expanded the knowledge on the molecular mechanism of DHA-induced apoptosis in human endothelial cells and has also implied the differential roles of MAP kinases and caspases in apoptosis.

Optical control of MAP kinase kinase 6 (MKK6) reveals that it has divergent roles in pro-apoptotic and anti-proliferative signaling

The selective pressure imposed by extrinsic death signals and stressors adds to the challenge of isolating and interpreting the roles of proteins in stress-activated signaling networks. By expressing a kinase with activating mutations and a caged lysine blocking the active site, we can rapidly switch on catalytic activity with light and monitor the ensuing dynamics. Applying this approach to MAP kinase 6 (MKK6), which activates the p38 subfamily of MAPKs, we found that decaging active MKK6 in fibroblasts is sufficient to trigger apoptosis in a p38-dependent manner. Both in fibroblasts and in a murine melanoma cell line expressing mutant B-Raf, MKK6 activation rapidly and potently inhibited the pro-proliferative extracellular signal-regulated kinase (ERK) pathway; to our surprise, this negative cross-regulation was equally robust when all p38 isoforms were inhibited. These results position MKK6 as a new pleiotropic signal transducer that promotes both pro-apoptotic and anti-proliferative signaling, and they highlight the utility of caged, light-activated kinases for dissecting stress-activated signaling networks.

A Link Between Alzheimer's and Type II Diabetes Mellitus? Ca+2 -Mediated Signal Control and Protein Localization

We propose protein localization dependent signal activation (PLDSA) as a model to describe pre-existing protein partitioning between the cytosol, and membrane surface, as a means to modulate signal activation, specificity, and robustness. We apply PLDSA to explain possible molecular links between type II diabetes mellitus (T2DM) and Alzheimer's disease (AD) by describing Ca+2 -mediated interactions between the Src non-receptor tyrosine kinase and p52Shc adaptor protein. We suggest that these interactions may serve as a contributing factor to disease development and progression. In particular, we propose that signaling response is regulated, in part, by Ca+2 -mediated partitioning of lipid-bound and soluble forms of Src and p52shc. Thus, protein-protein interactions that drive signaling in response to extracellular ligand binding are also mediated by partitioning of signaling proteins between membrane-bound and soluble populations. We propose that PLDSA effects may explain, in part, the evolutionary basis of promiscuous protein interaction domains and their importance in cellular function.

The apoptotic mechanism of action of the sphingosine kinase 1 selective inhibitor SKI-178 in human acute myeloid leukemia cell lines

We previously developed SKI-178 (N'-[(1E)-1-(3,4-dimethoxyphenyl)ethylidene]-3-(4-methoxxyphenyl)-1H-pyrazole-5-carbohydrazide) as a novel sphingosine kinase-1 (SphK1) selective inhibitor and, herein, sought to determine the mechanism-of-action of SKI-178-induced cell death. Using human acute myeloid leukemia (AML) cell lines as a model, we present evidence that SKI-178 induces prolonged mitosis followed by apoptotic cell death through the intrinsic apoptotic cascade. Further examination of the mechanism of action of SKI-178 implicated c-Jun NH2-terminal kinase (JNK) and cyclin-dependent protein kinase 1 (CDK1) as critical factors required for SKI-178-induced apoptosis. In cell cycle synchronized human AML cell lines, we demonstrate that entry into mitosis is required for apoptotic induction by SKI-178 and that CDK1, not JNK, is required for SKI-178-induced apoptosis. We further demonstrate that the sustained activation of CDK1 during prolonged mitosis, mediated by SKI-178, leads to the simultaneous phosphorylation of the prosurvival Bcl-2 family members, Bcl-2 and Bcl-xl, as well as the phosphorylation and subsequent degradation of Mcl-1. Moreover, multidrug resistance mediated by multidrug-resistant protein1 and/or prosurvival Bcl-2 family member overexpression did not affect the sensitivity of AML cells to SKI-178. Taken together, these findings highlight the therapeutic potential of SKI-178 targeting SphK1 as a novel therapeutic agent for the treatment of AML, including multidrug-resistant/recurrent AML subtypes.

p38α mitogen-activated kinase mediates cardiomyocyte apoptosis induced by palmitate

RATIONALE

The mechanisms underlying lipotoxic/diabetic cardiomyopathy remain poorly understood. Saturated fatty acid (SFA) levels, elevated in obesity and type 2 diabetes, induce apoptosis in many cell types including cardiomyocytes. Signaling pathways, including the p38α mitogen-activated kinase (MAPK)-dependent pathway, have been implicated in apoptosis due to a diverse range of insults.

OBJECTIVE

We tested the hypothesis that SFA-induced cardiomyocyte apoptosis is dependent on p38α activation.

METHODS AND RESULTS

Human adult ventricular cardiomyocytes (AC16 cells) were exposed to high physiological levels of palmitate (PA), a SFA. The apoptotic response was measured using annexin-V by flow cytometry, and the p38α-dependent pathway was evaluated using a p38 inhibitor PD169316, and by p38α small interfering RNA (siRNA) knockdown. PA exposure for 16 h dose-dependently increased apoptosis in AC16 cardiomyocytes (control: 2.6±0.6%, 150 μM PA: 3.5±0.9%, 300 μM PA: 11.5±1.6%, n=4, p<0.01). PA did not change total p38α protein levels, but increased p38α phosphorylation dose-dependently (n=5, p<0.01). PD169316 tended to reduce PA-induced apoptosis (n=4, p=0.05). Specific p38α siRNA markedly reduced the expression of p38α but not p38β (n=3, p<0.0001), and dose-dependently attenuated PA-induced apoptosis (control siRNA: 7.7±1.0%, 300 μM PA: 34.4±5.0%, 300 μM PA+30 pmol siRNA: 23.7±4.4%, 300 μM PA+60 pmol siRNA: 19.7±2.6%, 300 μM PA+120 pmol siRNA: 17.3±2.8%, n=4, p<0.0001).

CONCLUSIONS

These results demonstrate that PA induces p38α activation, and reducing p38α expression attenuates PA-induced cardiomyocyte apoptosis. Our results support a potential mechanism by which high plasma SFA levels through p38α activation may lead to the development of lipotoxic/diabetic cardiomyopathy.

Targeting RET to induce medullary thyroid cancer cell apoptosis: an antagonistic interplay between PI3K/Akt and p38MAPK/caspase-8 pathways

Mutations in REarranged during Transfection (RET) receptor tyrosine, followed by the oncogenic activation of RET kinase is responsible for the development of medullary thyroid carcinoma (MTC) that responds poorly to conventional chemotherapy. Targeting RET, therefore, might be useful in tailoring surveillance of MTC patients. Here we showed that theaflavins, the bioactive components of black tea, successfully induced apoptosis in human MTC cell line, TT, by inversely modulating two molecular pathways: (i) stalling PI3K/Akt/Bad pathway that resulted in mitochondrial transmembrane potential (MTP) loss, cytochrome-c release and activation of the executioner caspases-9 and -3, and (ii) upholding p38MAPK/caspase-8/caspase-3 pathway via inhibition of Ras/Raf/ERK. Over-expression of either constitutively active myristoylated-Akt-cDNA (Myr-Akt-cDNA) or dominant-negative-caspase-8-cDNA (Dn-caspase-8-cDNA) partially blocked theaflavin-induced apoptosis, while co-transfection of Myr-Akt-cDNA and Dn-caspase-8-cDNA completely eradicated the effect of theaflavins thereby negating the possibility of existence of other pathways. A search for the upstream signaling revealed that theaflavin-induced disruption of lipid raft caused interference in anchorage of RET in lipid raft that in turn stalled phosphorylation of Ras and PI3Kinase. In such anti-survival cellular micro-environment, pro-apoptotic signals were triggered to culminate into programmed death of MTC cell. These findings not only unveil a hitherto unexplained mechanism underlying theaflavin-induced MTC death, but also validate RET as a promising and potential target for MTC therapy.

Glutamate excitotoxicity activates the MAPK/ERK signaling pathway and induces the survival of rat hippocampal neurons in vivo

Current knowledge concerning the molecular mechanisms of the cellular response to excitotoxic insults in neurodegenerative diseases is insufficient. Although glutamate (Glu) has been widely studied as the main excitatory neurotransmitter and principal excitotoxic agent, the neuroprotective response enacted by neurons is not yet completely understood. Some of the molecular participants have been revealed, but the signaling pathways involved in this protective response are just beginning to be identified. Here, we demonstrate in vivo that, in response to the cell damage and death induced by Glu excitotoxicity, neurons orchestrate a survival response through the extracellular signal-regulated kinase (ERK) signaling pathway by increasing ERK expression in the rat hippocampal (CA1) region, allowing increased neuronal survival. In addition, this protective response is specifically reversed by U0126, an ERK inhibitor, which promotes cell death only when it is administered together with Glu. Our findings demonstrate that the ERK signaling pathway has a neuroprotective role in the response to Glu-induced excitotoxicity in hippocampal neurons. Therefore, the ERK signaling pathway may be activated as a cellular response to excitotoxic injury to prevent damage and neural loss, representing a novel therapeutic target in the treatment of neurodegenerative diseases.

DDX3 regulates DNA damage-induced apoptosis and p53 stabilization

The DEAD box protein family member DDX3 was previously identified as an inhibitor of death receptor-mediated extrinsic apoptotic signaling. However, there had been no studies of the role of DDX3 in regulating the other major type of apoptosis, intrinsic apoptotic signaling, which was examined here. Intrinsic apoptosis was induced in MCF-7 cells by treatment with staurosporine, a general kinase inhibitor, thapsigargin, which induces endoplasmic reticulum (ER) stress, and camptothecin, which causes DNA damage. Each of these treatments caused time-dependent activation of caspase-7, the predominant executioner caspase in these cells. Depletion of DDX3 using shRNA did not alter apoptotic responses to staurosporine or thapsigargin. However, caspase-7 activation induced by camptothecin was regulated by DDX3 in a manner dependent on the functional status of p53. Depletion of DDX3 abrogated camptothecin-induced caspase-7 activation in MCF-7 cells expressing functional wild-type p53, but oppositely potentiated camptothecin-mediated caspase activation in cells expressing mutant or non-functional p53, which was accompanied by increased activation of the extrinsic apoptotic signaling initiator caspase-8. In MCF-7 cells, depletion of DDX3 reduced by more than 50% camptothecin-induced p53 accumulation, and this effect was blocked by inhibition of the proteasome with MG132, indicating that DDX3 regulates p53 not at expression level but rather its stabilization after DNA damage. Co-immunoprecipitation experiments demonstrated that DDX3 associates with p53, and overexpression of DDX3 was sufficient to double the accumulation of p53 in the nucleus after DNA damage. Thus, DDX3 associates with p53, increases p53 accumulation, and positively regulates camptothecin-induced apoptotic signaling in cells expressing functional wild-type p53, whereas in cells expressing mutant or non-functional p53 DDX3 inhibits activation of the extrinsic apoptotic pathway to reduce caspase activation. These results demonstrate that DDX3 not only regulates extrinsic apoptotic signaling, as previously reported, but also selectively regulates intrinsic apoptotic signaling following DNA damage.

Structure, regulation, and pharmacological modulation of PP2A phosphatases

Protein phosphatases of the type 2A family (PP2A) represent a major fraction of cellular Ser/Thr phosphatase activity in any given human tissue. In this review, we describe how the holoenzymic nature of PP2A and the existence of several distinct PP2A composing subunits allow for the generation of multiple structurally and functionally different PP2A complexes, explaining why PP2A is involved in the regulation of so many diverse cell biological and physiological processes. Moreover, in human disease, most notably in several cancers and Alzheimer's Disease, PP2A expression and/or activity have been found significantly decreased, underscoring its important functions as a major tumor suppressor and tau phosphatase. Hence, several recent preclinical studies have demonstrated that pharmacological restoration of PP2A activity, as well as pharmacological PP2A inhibition, under certain conditions, may be of significant future therapeutic value.

Sequential application of anticancer drugs enhances cell death by rewiring apoptotic signaling networks

Crosstalk and complexity within signaling pathways and their perturbation by oncogenes limit component-by-component approaches to understanding human disease. Network analysis of how normal and oncogenic signaling can be rewired by drugs may provide opportunities to target tumors with high specificity and efficacy. Using targeted inhibition of oncogenic signaling pathways, combined with DNA-damaging chemotherapy, we report that time-staggered EGFR inhibition, but not simultaneous coadministration, dramatically sensitizes a subset of triple-negative breast cancer cells to genotoxic drugs. Systems-level analysis-using high-density time-dependent measurements of signaling networks, gene expression profiles, and cell phenotypic responses in combination with mathematical modeling-revealed an approach for altering the intrinsic state of the cell through dynamic rewiring of oncogenic signaling pathways. This process converts these cells to a less tumorigenic state that is more susceptible to DNA damage-induced cell death by reactivation of an extrinsic apoptotic pathway whose function is suppressed in the oncogene-addicted state.

Defining a role for sphingosine kinase 1 in p53-dependent tumors

p53 is a crucial tumor suppressor that is mutated or deleted in a majority of cancers. Exactly how p53 prevents tumor progression has proved elusive for many years; however, this information is crucial to define targets for chemotherapeutic development that can effectively restore p53 function. Bioactive sphingolipids have recently emerged as important regulators of proliferative, apoptotic and senescent cellular processes. In this study, we demonstrate that the enzyme sphingosine kinase 1 (SK1), a critical enzyme in the regulation of the key bioactive sphingolipids ceramide, sphingosine and sphingosine-1-phosphate (S1P), serves as a key downstream target for p53 action. Our results show that SK1 is proteolysed in response to genotoxic stress in a p53-dependent manner. p53 null mice display elevation of SK1 levels and a tumor-promoting dysregulation of bioactive sphingolipids in which the anti-growth sphingolipid ceramide is decreased and the pro-growth sphingolipid S1P is increased. Importantly, deletion of SK1 in p53 null mice completely abrogated thymic lymphomas in these mice and prolonged their life span by ~30%. Deletion of SK1 also significantly attenuated the formation of other cancers in p53 heterozygote mice. The mechanism of p53 tumor suppression by loss of SK1 is mediated by elevations of sphingosine and ceramide, which in turn were accompanied by increased expression of cell cycle inhibitors and tumor cell senescence. Thus, targeting SK1 may restore sphingolipid homeostasis in p53-dependent tumors and provide insights into novel therapeutic approaches to cancer.

Sphingosine-sphingosine-1-phosphate pathway regulates trophoblast differentiation and syncytialization

Sphingosine and sphingosine-1-phosphate (S1P) are involved in regulating cell differentiation. This study postulated that changes in sphingolipid biosynthesis and metabolism are important in trophoblast syncytialization and therefore examined the production, metabolism and actions of sphingosine and S1P during spontaneous trophoblast differentiation and fusion in vitro. Significant declines in intracellular sphingosine concentration (P≤0.05) and sphingosine kinase 1 (SPHK1) expression (P≤0.01) were observed during trophoblast syncytialization. Secreted S1P concentrations dropped steeply after 72h, before rising to basal concentrations with syncytialization. Intracellular S1P concentrations were undetectable throughout. Treating cells with exogenous sphingosine (P≤0.01), S1P (P≤0.001) or a specific SPHK1 inhibitor (P≤0.05) for up to 72h in culture significantly inhibited trophoblast differentiation (measured as reduced human chorionic gonadotrophin production); effects on other biochemical and morphological markers of differentiation were absent or inconsistent. Phosphorylation of Akt, an established down-stream target of S1P that spontaneously declines with trophoblast differentiation, was markedly reduced by S1P (P≤0.05). In conclusion, changes in the sphingosine-S1P pathway are involved in the regulation of trophoblast differentiation in term human placenta. Dysregulation of sphingolipid homeostasis could, therefore, disrupt placental formation and function with deleterious consequences for pregnancy outcome.

ERK/MAPK is essential for endogenous neuroprotection in SCN2.2 cells

Background

Glutamate (Glu) is essential to central nervous system function; however excessive Glu release leads to neurodegenerative disease. Strategies to protect neurons are underdeveloped, in part due to a limited understanding of natural neuroprotective mechanisms, such as those present in the suprachiasmatic nucleus (SCN). This study tests the hypothesis that activation of ERK/MAPK provides essential protection to the SCN after exposure to excessive Glu using the SCN2.2 cells as a model.

Methodology

Immortalized SCN2.2 cells (derived from SCN) and GT1-7 cells (neurons from the neighboring hypothalamus) were treated with 10 mM Glu in the presence or absence of the ERK/MAPK inhibitor PD98059. Cell death was assessed by Live/Dead assay, MTS assay and TUNEL. Caspase 3 activity was also measured. Activation of MAPK family members was determined by immunoblot. Bcl2, neuritin and Bid mRNA (by quantitative-PCR) and protein levels (by immunoblot) were also measured.

Principal Findings

As expected Glu treatment increased caspase 3 activity and cell death in the GT1-7 cells, but Glu alone did not induce cell death or affect caspase 3 activity in the SCN2.2 cells. However, pretreatment with PD98059 increased caspase 3 activity and resulted in cell death after Glu treatment in SCN2.2 cells. This effect was dependent on NMDA receptor activation. Glu treatment in the SCN2.2 cells resulted in sustained activation of the anti-apoptotic pERK/MAPK, without affecting the pro-apoptotic p-p38/MAPK. In contrast, Glu exposure in GT1-7 cells caused an increase in p-p38/MAPK and a decrease in pERK/MAPK. Bcl2-protein increased in SCN2.2 cells following Glu treatment, but not in GT1-7 cells; bid mRNA and cleaved-Bid protein increased in GT1-7, but not SCN2.2 cells.

Conclusions

Facilitation of ERK activation and inhibition of caspase activation promotes resistance to Glu excitotoxicity in SCN2.2 cells.

Significance

Further research will explore ERK/MAPK as a key molecule in the prevention of neurodegenerative processes.

Protein phosphatase 1-dependent dephosphorylation of Akt is the prime signaling event in sphingosine-induced apoptosis in Jurkat cells

Sphingosine (SPH) is an important bioactive lipid involved in mediating a variety of cell functions including apoptosis. However, the signaling mechanism of SPH-induced apoptosis remains unclear. We have investigated whether SPH inhibits survival signaling in cells by inhibiting Akt kinase activity. This study demonstrates that treatment of Jurkat cells with SPH leads to Akt dephosphorylation as early as 15  min, and the cells undergo apoptosis after 6  h. This Akt dephosphorylation is not mediated through deactivation of upstream kinases, since SPH does not inhibit the upstream phosphoinositide-dependent kinase 1 (PDK1) phosphorylation. Rather, sensitivity to the Ser/Thr protein phosphatase inhibitors (calyculin A, phosphatidic acid, tautomycin, and okadaic acid) indicates an important role for protein phosphatase 1 (PP1) in this process. In vitro phosphatase assay, using Akt immunoprecipitate following treatment with SPH, reveals an increase in Akt-PP1 association as determined by immunoprecipitation analysis. Moreover, SPH-induced dephosphorylation of Akt at Ser(473) subsequently leads to the activation of GSK-3β, caspase 3, PARP cleavage, and ultimately apoptosis. Pre-treatment with caspase 3 inhibitor z-VAD-fmk and Ser/Thr phosphatase inhibitor abrogates the effect of SPH on facilitating apoptosis. Altogether, these results demonstrate that PP1-mediated inhibition of the key anti-apoptotic protein, Akt, plays an important role in SPH-mediated apoptosis in Jurkat cells.

Phosphoprotein phosphatase 2A: a novel druggable target for Alzheimer's disease

Tau hyperphosphorylation is thought to play an important role in the etiology of Alzheimer's disease by facilitating the formation of neurofibrillary tangles. Reducing phosphorylation through kinase inhibition has therefore emerged as a target for drug development, but despite considerable efforts to develop therapeutic kinase inhibitors, success has been limited. An alternative approach is to develop pharmaceuticals to enhance the activity of the principal phospho-tau phosphatase, phosphoprotein phosphatase 2A (PP2A). In this article we review evidence that this mechanism is pharmacologically achievable and has promise for delivering the next generation of Alzheimer's disease therapeutics. A number of different chemotypes have been reported to lead to enhanced PP2A activity through a range of proposed mechanisms. Some of these compounds appear to act directly as allosteric activators of PP2A, while others act indirectly by inhibiting the binding of PP2A inhibitors or by altering post-translational modifications that act in turn to regulate PP2A activity towards phospho-tau. These results indicate that PP2A may provide a useful target that can be safely, selectively and effectively modulated through pharmaceutical intervention to treat Alzheimer's disease.

Ikarugamycin induces DNA damage, intracellular calcium increase, p38 MAP kinase activation and apoptosis in HL-60 human promyelocytic leukemia cells

Ikarugamycin (IKA) is an antibiotic with strong antiprotozoal and cytotoxic activity. The purpose of our work was to provide insight into the mechanism of action characterizing the cytotoxic effect of IKA in HL-60 leukemia cells in order to evaluate its potential as an antineoplastic agent. Cell viability was reduced in response to IKA (IC(50) of 221.3nM), while the amount of HL-60 cells with a subdiploid DNA content increased significantly after 24h. Apoptotic cell death was confirmed by the cleavage of caspase-9, -8 and -3 using immunoblotting. Single cell gel electrophoresis pointed to an early genotoxic effect. Monitoring of intracellular calcium ([Ca(2+)](i)) levels by flow cytometric analysis of Fluo-3-AM fluorescence indicated an increase in cytosolic calcium that correlated with the cleavage of caspases. In addition, IKA triggered the activation of p38 MAP kinase which was partly dependent on elevated [Ca(2+)](i) concentrations and contributed to caspase activation. The data demonstrate that IKA induced apoptosis in HL-60 cells through genotoxicity and caspase activation which was in part correlated to an increase in intracellular calcium levels and activation of p38 MAP kinase.

Gene regulation mediating fiber-type transformation in skeletal muscle cells is partly glucose- and ChREBP-dependent

Adaptations in the oxidative capacity of skeletal muscle cells can occur under several physiological or pathological conditions. We investigated the effect of increasing extracellular glucose concentration on the expression of markers of energy metabolism in primary skeletal muscle cells and the C2C12 muscle cell line. Growth of myotubes in 25mM glucose (high glucose, HG) compared with 5.55mM led to increases in the expression and activity of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a marker of glycolytic energy metabolism, while oxidative markers peroxisome proliferator-activated receptor γ coactivator 1α and citrate synthase decreased. HG induced metabolic adaptations as are seen during a slow-to-fast fiber transformation. Furthermore, HG increased fast myosin heavy chain (MHC) IId/x but did not change slow MHCI/β expression. Protein phosphatase 2A (PP2A) was shown to mediate the effects of HG on GAPDH and MHCIId/x. Carbohydrate response element-binding protein (ChREBP), a glucose-dependent transcription factor downstream of PP2A, partially mediated the effects of glucose on metabolic markers. The glucose-induced increase in PP2A activity was associated with an increase in p38 mitogen-activated protein kinase activity, which presumably mediates the increase in MHCIId/x promoter activity. Liver X receptor, another possible mediator of glucose effects, induced only an incomplete metabolic shift, mainly increasing the expression of the glycolytic marker. Taken together, HG induces a partial slow-to-fast transformation comprising metabolic enzymes together with an increased expression of MHCIId/x. This work demonstrates a functional role for ChREBP in determining the metabolic type of muscle fibers and highlights the importance of glucose as a signaling molecule in muscle.

Sphingosine interaction with acidic leucine-rich nuclear phosphoprotein-32A (ANP32A) regulates PP2A activity and cyclooxygenase (COX)-2 expression in human endothelial cells

Sphingolipid metabolites regulate cell fate by acting on specific cellular targets. Although the influence of sphingolipids in cellular signaling has been well recognized, the exact molecular targets and how these targets influence cellular signaling mechanisms remain poorly understood. Toward this goal, we used affinity chromatography coupled with proteomics technology and identified acidic leucine-rich nuclear phosphoprotein-32A (ANP32A), an inhibitor of protein phosphatase 2A (PP2A) as a direct target of sphingosine, N,N'-dimethyl sphingosine (DMS) and phytosphingosine but not dihydrosphingosine or sphingosine 1-phosphate. Treatment of human umbilical vein endothelial cells (HUVEC) with DMS, which is not phosphorylated by sphingosine kinases, led to the activation of PP2A activity. Suppression of ANP32A with siRNA enhanced basal and DMS-activated PP2A activity suggesting that the sphingoid base binds to and relieves the inhibitory action of ANP32A on the PP2A complex. Indeed, DMS relieved the ANP32A-mediated inhibition of PP2A enzyme complex in vitro. Interestingly, DMS treatment induced the p38 stress-activated protein kinase (SAPK) and expression of cyclooxygenase (COX)-2 transcript and protein. Knockdown of ANP32A expression further induced p38 SAPK and COX-2. These data identify ANP32A as a novel molecular target of sphingoid bases that regulates cellular signaling events and inflammatory gene expression.

Treatment with p38 inhibitor partially prevents Calu-6 lung cancer cell death by a proteasome inhibitor, MG132

MG132 (carbobenzoxy-Leu-Leu-leucinal) as a proteasome inhibitor has been shown to induce apoptotic cell death through formation of reactive oxygen species (ROS). In this study, we investigated the effects of MEK (mitogen-activated protein [MAP] kinase or extracellular signal-regulated kinase [ERK] kinase) or p38 inhibitor on MG132-treated Calu-6 lung cancer cells in relation to cell growth, cell death, ROS, and glutathione (GSH) levels. Treatment with 10 mumol/L MG132 inhibited the growth of Calu-6 cells at 24 hours. MG132 induced apoptosis in Calu-6 cells, which was accompanied by the loss of mitochondrial membrane potential (MMP; DeltaPsi(m)). ROS were increased in MG132-treated Calu-6 cells. MG132 also induced GSH depletion in Calu-6 cells. Treatment with MEK inhibitor did not significantly affect cell growth, cell death, ROS, and GSH levels in MG132-treated Calu-6 cells. Furthermore, MG132 increased the phosphorylation of p38 in Calu-6 cells at 1 and 24 hours. Treatment with p38 inhibitor significantly prevented cell growth inhibition, MMP (DeltaPsi(m)) loss and apoptosis in MG132-treated Calu-6 cells. This inhibitor increased ROS level and decreased GSH depletion in these cells. In conclusion, p38 inhibitor partially prevented Calu-6 cell death by MG132, which might be affected by GSH level changes.