Characterization of the structure and function of a new mitogen-activated protein kinase (p38beta)

Kinase Signaling in Apoptosis Induced by Saturated Fatty Acids in Pancreatic β-Cells

Pancreatic β-cell failure and death is considered to be one of the main factors responsible for type 2 diabetes. It is caused by, in addition to hyperglycemia, chronic exposure to increased concentrations of fatty acids, mainly saturated fatty acids. Molecular mechanisms of apoptosis induction by saturated fatty acids in β-cells are not completely clear. It has been proposed that kinase signaling could be involved, particularly, c-Jun N-terminal kinase (JNK), protein kinase C (PKC), p38 mitogen-activated protein kinase (p38 MAPK), extracellular signal-regulated kinase (ERK), and Akt kinases and their pathways. In this review, we discuss these kinases and their signaling pathways with respect to their possible role in apoptosis induction by saturated fatty acids in pancreatic β-cells.

Comparative chemical array screening for p38γ/δ MAPK inhibitors using a single gatekeeper residue difference between p38α/β and p38γ/δ

Mammalian p38 mitogen activated protein kinases (MAPKs) are responsive to a variety of cellular stresses. The development of specific pyridinyl imidazole inhibitors has permitted the characterization of the p38 MAPK isoform p38α, which is expressed in most cell types, whereas the physiological roles of p38γ and p38δ are poorly understood. In this study, we report an approach for identifying selective inhibitors against p38γ and p38δ by focusing on the difference in gatekeeper residues between p38α/β and p38γ/δ. Using GST-fused p38α wild type and T106M mutant constructs, wherein the p38α gatekeeper residue (Thr-106) was substituted by the p38γ/δ-type (Met), we performed comparative chemical array screening to identify specific binders of the mutant and identified SU-002 bound to p38αT106M specifically. SU-002 was found to inhibit p38αT106M but not p38α kinase activity in in vitro kinase assays. SU-005, the analog of SU-002, had inhibitory effects against the kinase activity of p38γ and p38δ in vitro but not p38α. In addition, SU-005 inhibited both p38γ and p38δ auto-phosphorylation in HeLa and HEK293T cells. These results demonstrate that the comparative chemical array screening approach is a powerful technique to explore specific inhibitors for mutant proteins with even single amino-acid substitutions in a high-throughput manner.

NMR Characterization of Information Flow and Allosteric Communities in the MAP Kinase p38γ

The intramolecular network structure of a protein provides valuable insights into allosteric sites and communication pathways. However, a straightforward method to comprehensively map and characterize these pathways is not currently available. Here we present an approach to characterize intramolecular network structure using NMR chemical shift perturbations. We apply the method to the mitogen activated protein kinase (MAPK) p38γ. p38γ contains allosteric sites that are conserved among eukaryotic kinases as well as unique to the MAPK family. How these regulatory sites communicate with catalytic residues is not well understood. Using our method, we observe and characterize for the first time information flux between regulatory sites through a conserved kinase infrastructure. This network is accessed, reinforced, and broken in various states of p38γ, reflecting the functional state of the protein. We demonstrate that the approach detects critical junctions in the network corresponding to biologically significant allosteric sites and pathways.

p38 MAPK α and β isoforms differentially regulate plasma membrane localization of MRP2

In hepatocytes, cAMP both activates p38 mitogen-activated protein kinase (MAPK) and increases the amount of multidrug resistance-associated protein-2 (MRP2) in the plasma membrane (PM-MRP2). Paradoxically, taurolithocholate (TLC) activates p38 MAPK but decreases PM-MRP2 in hepatocytes. These opposing effects of cAMP and TLC could be mediated via different p38 MAPK isoforms (α and β) that are activated differentially by upstream kinases (MKK3, MKK4, and MKK6). Thus we tested the hypothesis that p38α MAPK and p38β MAPK mediate increases and decreases in PM-MRP2 by cAMP and TLC, respectively. Studies were conducted in hepatocytes isolated from C57BL/6 wild-type (WT) and MKK3-knockout (MKK3(-/-)) mice and in a hepatoma cell line (HuH7) that overexpresses sodium-taurocholate cotransporting polypeptide (NTCP) (HuH-NTCP). Cyclic AMP activated MKK3, p38 MAPK, and p38α MAPK and increased PM-MRP2 in WT hepatocytes, but failed to activate p38α MAPK or increase PM-MRP2 in MKK3(-/-) hepatocytes. In contrast to cAMP, TLC activated total p38 MAPK but decreased PM-MRP2, and did not activate MKK3 or p38α MAPK in WT hepatocytes. In MKK3(-/-) hepatocytes, TLC still decreased PM-MRP2 and activated p38 MAPK, indicating that these effects are not MKK3-dependent. Additionally, TLC activated MKK6 in MKK3(-/-) hepatocytes, and small interfering RNA knockdown of p38β MAPK abrogated TLC-mediated decreases in PM-MRP2 in HuH-NTCP cells. Taken together, these results suggest that p38α MAPK facilitates plasma membrane insertion of MRP2 by cAMP, whereas p38β MAPK mediates retrieval of PM-MRP2 by TLC.

Differential expression of p38 MAPK α, β, γ, δ isoforms in nucleus pulposus modulates macrophage polarization in intervertebral disc degeneration

P38MAPK mediates cytokine induced inflammation in nucleus pulposus (NP) cells and involves in multiple cellular processes which are related to intervertebral disc degeneration (IDD). The aim of this study was to investigate the expression, activation and function of p38 MAPK isoforms (α,β, γ and δ) in degenerative NP and the effect of p38 activation in NP cells on macrophage polarization. P38 α, β and δ isoforms are preferential expressed, whereas the p38γ isoform is absent in human NP tissue. LV-sh-p38α, sh-p38β transfection in NP cells significantly decreased the ADAMTS-4,-5, MMP-13,CCL3 expression and restored collagen-II and aggrecan expression upon IL-1β stimulation. As compared with p38α and p38β, p38δ exhibited an opposite effect on ADAMTS-4,-5, MMP-13 and aggrecan expression in NP cells. Furthermore, the production of GM-CSF and IFNγ which were trigged by p38α or p38β in NP cells induced macrophage polarization into M1 phenotype. Our finding indicates that p38 MAPK α, β and δ isoform are predominantly expressed and activated in IDD. P38 positive NP cells modulate macrophage polarization through the production of GM-CSF and IFNγ. Hence, Our study suggests that selectively targeting p38 isoforms could ameliorate the inflammation in IDD and regard IDD progression.

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.

How do kinases contribute to tonicity-dependent regulation of the transcription factor NFAT5?

NFAT5 plays a critical role in maintaining the renal functions. Its dis-regulation in the kidney leads to or is associated with certain renal diseases or disorders, most notably the urinary concentration defect. Hypertonicity, which the kidney medulla is normally exposed to, activates NFAT5 through phosphorylation of a signaling molecule or NFAT5 itself. Hypotonicity inhibits NFAT5 through a similar mechanism. More than a dozen of protein and lipid kinases have been identified to contribute to tonicity-dependent regulation of NFAT5. Hypertonicity activates NFAT5 by increasing its nuclear localization and transactivating activity in the early phase and protein abundance in the late phase. The known mechanism for inhibition of NFAT5 by hypotonicity is a decrease of nuclear NFAT5. The present article reviews the effect of each kinase on NFAT5 nuclear localization, transactivation and protein abundance, and the relationship among these kinases, if known. Cyclosporine A and tacrolimus suppress immune reactions by inhibiting the phosphatase calcineurin-dependent activation of NFAT1. It is hoped that this review would stimulate the interest to seek explanations from the NFAT5 regulatory pathways for certain clinical presentations and to explore novel therapeutic approaches based on the pathways. On the basic science front, this review raises two interesting questions. The first one is how these kinases can specifically signal to NFAT5 in the context of hypertonicity or hypotonicity, because they also regulate other cellular activities and even opposite activities in some cases. The second one is why these many kinases, some of which might have redundant functions, are needed to regulate NFAT5 activity. This review reiterates the concept of signaling through cooperation. Cells need these kinases working in a coordinated way to provide the signaling specificity that is lacking in the individual one. Redundancy in regulation of NFAT5 is a critical strategy for cells to maintain robustness against hypertonic or hypotonic stress.

Gene structure, molecular characterization and transcriptional expression of two p38 isoforms (MAPK11 and MAPK14) from rock bream (Oplegnathus fasciatus)

The p38 kinases are one of the four subgroups of mitogen-activated protein kinase (MAPK) superfamily which are involved in the innate immunity. The p38 subfamily that includes four members namely p38α (MAPK14), p38β (MAPK11), p38γ (MAPK12) and p38δ (MAPK13), regulates the activation of several transcription factors. In this study, a p38β (OfMAPK11) homolog and a p38α (OfMAPK14) homolog of Oplegnathus fasciatus were identified at genomic level. Results clearly showed that both MAPK11 and MAPK14 are well-conserved at both genomic structural- and amino acid (aa)-levels. Genomic sequences of OfMAPK11 (∼ 15.6 kb) and OfMAPK14 (∼ 13.4 kb) had 12 exons. A comparison of exon-intron structural arrangement of these genes from different vertebrate lineages indicated that all the exon lengths are highly conserved, except their terminal exons. Full-length cDNAs of OfMAPK11 (3957 bp) and OfMAPK14 (2504 bp) encoded corresponding proteins of 361 aa and 360 aa, respectively. Both OfMAPK proteins harbored a Ser/Thr protein kinases catalytic domain (S_TKc domain) which includes an activation loop with a dual phosphorylation site (TGY motif) and several specific-binding sites for ATP and substrates. Molecular modeling of the activation loop and substrate binding sites of rock bream MAPKs revealed the conservation of crucial residues and their orientation in 3D space. Transcripts of OfMAPKs were ubiquitously detected in eleven tissues examined, however at different levels. The modulation of OfMAPKs' transcription upon pathogen-associated molecular patterns (PAMPs: flagellin, lipopolysaccharide and poly I:C) and pathogens (Edwardsiella tarda, Streptococcus iniae and rock bream iridovirus) was investigated. Among the seven examined tissues, the flagellin-challenge upregulated the mRNA level of both OfMAPKs in the head kidney. Meanwhile, modulation of OfMAPK mRNA expression in the liver upon other immune-challenges varied in a time-dependent manner. Collectively, these results suggest that OfMAPKs are true members of p38 subfamily, which might be induced by different immune stimuli.

CyHV-2 ORF104 activates the p38 MAPK pathway

Cyprinid herpesvirus 2 (CyHV-2) is the pathogen responsible for herpesviral hematopoietic necrosis disease, which causes huge losses on aquaculture. So far the studies of CyHV-2 mainly focus on the identification and detection of this virus, but little is known about the role of specific CyHV-2 genes in the infection process. Based on the genomic information, CyHV-2 ORF104 encodes a kinase-like protein, which is highly conserved among the three CyHVs. Our study was initiated to investigate the role of kinase-like protein ORF104 during virus infection. Subcellular localization study showed that ORF104 was mainly expressed in the nucleus in both human HEK293T and fish EPC cells. However, deletion of the putative nuclear localization signal of ORF104 (ORF104M) resulted in the cytoplasmic distribution in HEK293T. We then examined whether MAPKs were involved in the ORF104-mediated signaling pathway by overexpressing ORF104 and ORF104M in HEK293T. Overexpression of ORF104 and ORF104M resulted in the up-regulation of p38 phosphorylation, but not JNK or ERK, indicating that ORF104 specifically activates p38 signaling pathway. In vivo study showed that CyHV-2 infection enhanced p38 phosphorylation in gibel carp (Carassius auratus gibelio). Interestingly, p38 inhibitor SB203580 strongly reduced fish death caused by CyHV-2 infection. Therefore, our study for the first time reveals the function of ORF104 during CyHV-2 infection, indicating that ORF104 is a potential vaccine candidate for CyHV-2.

p38 MAP Kinase Inhibition Reduces Propionibacterium acnes-Induced Inflammation in Vitro

Introduction

Propionibacterium acnes, a ubiquitous skin bacterium, stimulates keratinocytes to produce a number of proinflammatory cytokines and may contribute to inflammatory acne. The aim of the study was to investigate whether P. acnes-induced proinflammatory cytokine release is mediated by P. acnes-induced activation of p38 mitogen-activated protein kinase (p38 MAPK or p38) in human keratinocytes.

Methods

Immunohistochemistry was used to evaluate p38 phosphorylation in human skin samples with or without acne. Primary human keratinocytes and epidermal skin equivalents were exposed to viable P. acnes. Phosphorylation of MAPKs without or with p38 inhibitors was examined by Western blot and cytokine secretion was detected by Enzyme-Linked Immunosorbent Assay (ELISA).

Results

Increased levels of phospho-p38 were observed in human acne lesions, predominantly in follicular and perifollicular keratinocytes. Exposure of cultured human keratinocytes to viable P. acnes resulted in phosphorylation of multiple members of the MAPK family, including rapid and transient activation of p38 and extracellular signal-related kinase (ERK1/2) and relatively slow but sustained activation of c-Jun N-terminal kinases (JNK1/2). Viable P. acnes induced the secretion of interleukin-1α (IL-1α), tumor necrosis factor-α (TNF-α), and IL-8 from human keratinocytes. The phosphorylation of p38 (phospho-p38) and the secretion of cytokines induced by P. acnes in cultured keratinocytes were inhibited by SB203580, a p38α/β inhibitor. Furthermore, SCIO-469, a selective inhibitor of p38α, showed similar effects in cultured keratinocytes. Topical treatment of SCIO-469 inhibited the P. acnes-induced phospho-p38 and cytokine secretion in human epidermal equivalents.

Conclusion

The data demonstrate that P. acnes induces p38-dependent inflammatory responses in keratinocytes, and suggest that p38 may play an important role in the pathogenesis of inflammatory acne.

Funding

Johnson & Johnson.

Electronic supplementary material

The online version of this article (doi:10.1007/s13555-015-0072-7) contains supplementary material, which is available to authorized users.

MAP kinase p38α regulates type III interferon (IFN-λ1) gene expression in human monocyte-derived dendritic cells in response to RNA stimulation

Recognition of viral nucleic acids leads to type I and type III IFN gene expression and activation of host antiviral responses. At present, type III IFN genes are the least well-characterized IFN types. Here, we demonstrate that the p38 MAPK signaling pathway is involved in regulating IFN-λ1 gene expression in response to various types of RNA molecules in human moDCs. Inhibition of p38 MAPK strongly reduced IFN gene expression, and overexpression of p38α MAPK enhanced IFN-λ1 gene expression in RNA-stimulated moDCs. The regulation of IFN gene expression by p38 MAPK signaling was independent of protein synthesis and thus, a direct result of RNA stimulation. Moreover, the RIG-I/MDA5-MAVS-IRF3 pathway was required for p38α MAPK to up-regulate IFN-λ1 promoter activation, whereas the MyD88-IRF7 pathway was not needed, and the regulation was not involved directly in IRF7-dependent IFN-α1 gene expression. The stimulatory effect of p38α MAPK on IFN-λ1 mRNA expression in human moDCs did not take place directly via the activating TBK1/IKKε complex, but rather, it occurred through some other parallel pathways. Furthermore, mutations in ISRE and NF-κB binding sites in the promoter region of the IFN-λ1 gene led to a significant reduction in p38α MAPK-mediated IFN responses after RNA stimulation. Altogether, our data suggest that the p38α MAPK pathway is linked with RLR signaling pathways and regulates the expression of early IFN genes after RNA stimulation cooperatively with IRF3 and NF-κB to induce antiviral responses further.

miR-155 induced transcriptome changes in the MCF-7 breast cancer cell line leads to enhanced mitogen activated protein kinase signaling

A single microRNA (miRNA) has the potential to regulate thousands of genes and thus govern multiple signaling pathways at once. miR-155 is an oncogenic miRNA which regulates many cellular pathways, designating it as a multifaceted regulator of proliferation, chemo-resistance, and apoptosis. While many singular targeted effects of miR-155 have been defined and an oncogenic role has been attributed to miR-155 expression, the global effect of miR-155 on the cellular transcriptomes of an ER⁺ breast cancer cell line has yet to be determined. Here we demonstrate that miR-155 expression increases tumorigenesis in vivo and we determine miR-155 mediated transcriptome changes through next generation sequencing analysis. miR-155 expression alters many signaling pathways, with the chief altered pathway being the MAPK signaling cascade and miR-155 induces shortening of target mRNA 3′UTRs and alternative isoform expression of MAPK related genes. In addition there is an observed increase in protein phosphorylation of components of MAPK signaling including ERK1/2 and AP-1 complex members (Fra-1 and c-Fos) as well as elevated gene expression of MAPK regulated genes Zeb1, Snail, Plaur, and SerpinE1.

p38δ MAPK: Emerging Roles of a Neglected Isoform

p38δ mitogen activated protein kinase (MAPK) is a unique stress responsive protein kinase. While the p38 MAPK family as a whole has been implicated in a wide variety of biological processes, a specific role for p38δ MAPK in cellular signalling and its contribution to both physiological and pathological conditions are presently lacking. Recent emerging evidence, however, provides some insights into specific p38δ MAPK signalling. Importantly, these studies have helped to highlight functional similarities as well as differences between p38δ MAPK and the other members of the p38 MAPK family of kinases. In this review we discuss the current understanding of the molecular mechanisms underlying p38δ MAPK activity. We outline a role for p38δ MAPK in important cellular processes such as differentiation and apoptosis as well as pathological conditions such as neurodegenerative disorders, diabetes, and inflammatory disease. Interestingly, disparate roles for p38δ MAPK in tumour development have also recently been reported. Thus, we consider evidence which characterises p38δ MAPK as both a tumour promoter and a tumour suppressor. In summary, while our knowledge of p38δ MAPK has progressed somewhat since its identification in 1997, our understanding of this particular isoform in many cellular processes still strikingly lags behind that of its counterparts.

Identification of p38β as a therapeutic target for the treatment of Sézary syndrome

Cutaneous T-Cell Lymphomas (CTCL) represent a group of hematopoietic malignancies that home to the skin and have no known molecular basis for disease pathogenesis. Sézary syndrome (SS) is the leukemic variant of CTCL. Currently, CTCL is incurable, highlighting the need for new therapeutic modalities. We have previously observed that combined smallmolecule inhibition of protein kinase C (PKC) β and glycogen synthase kinase 3 (GSK3) causes synergistic apoptosis in CTCL cell lines and patient cells. Through microarray analysis of a SS cell line, we surveyed global gene expression following combined PKCβ-GSK3 treatment to elucidate therapeutic targets responsible for cell death. Clinically relevant targets were defined as genes differentially expressed in SS patients that were modulated by combination-drug treatment of SS cells. Gene set enrichment analysis uncovered candidate genes enriched for an immune cell signature, specifically the T-cell receptor and MAPK signaling pathways. Further analysis identified p38 as a potential therapeutic target that is over-expressed in SS patients and decreased by synergistic-inhibitor treatment. This target was verified through small-molecule inhibition of p38 leading to cell death in both SS cell lines and patient cells. These data establish p38 as a SS biomarker and potential therapeutic target for the treatment of CTCL.

GADD45γ regulates the thermogenic capacity of brown adipose tissue

The coactivator peroxisome proliferator-activated receptor-gamma coactivator 1 α (PGC-1α) is widely considered a central transcriptional regulator of adaptive thermogenesis in brown adipose tissue (BAT). However, mice lacking PGC-1α specifically in adipose tissue have only mild thermogenic defects, suggesting the presence of additional regulators. Using the activity of estrogen-related receptors (ERRs), downstream effectors of PGC-1α, as read-out in a high-throughput genome-wide cDNA screen, we identify here growth arrest and DNA-damage-inducible protein 45 γ (GADD45γ) as a cold-induced activator of uncoupling protein 1 (UCP1) and oxidative capacity in BAT. Mice lacking Gadd45γ have defects in Ucp1 induction and the thermogenic response to cold. GADD45γ works by activating MAPK p38, which is a potent activator of ERRβ and ERRγ transcriptional function. GADD45γ activates ERRγ independently of PGC-1 coactivators, yet synergizes with PGC-1α to induce the thermogenic program. Our findings elucidate a previously unidentified GADD45γ/p38/ERRγ pathway that regulates BAT thermogenesis and may enable new approaches for the stimulation of energy expenditure. Our study also implicates GADD45 proteins as general metabolic regulators.

Inhibition of neuronal p38α, but not p38β MAPK, provides neuroprotection against three different neurotoxic insults

The p38 mitogen-activated protein kinase (MAPK) pathway plays a key role in pathological glial activation and neuroinflammatory responses. Our previous studies demonstrated that microglial p38α and not the p38β isoform is an important contributor to stressor-induced proinflammatory cytokine upregulation and glia-dependent neurotoxicity. However, the contribution of neuronal p38α and p38β isoforms in responses to neurotoxic agents is less well understood. In the current study, we used cortical neurons from wild-type or p38β knockout mice, and wild-type neurons treated with two highly selective inhibitors of p38α MAPK. Neurons were treated with one of three neurotoxic insults (L-glutamate, sodium nitroprusside, and oxygen-glucose deprivation), and neurotoxicity was assessed. All three stimuli led to neuronal death and neurite degeneration, and the degree of neurotoxicity induced in wild-type and p38β knockout neurons was not significantly different. In contrast, selective inhibition of neuronal p38α was neuroprotective. Our results show that neuronal p38β is not required for neurotoxicity induced by multiple toxic insults, but that p38α in the neuron contributes quantitatively to the neuronal dysfunction responses. These data are consistent with our previous findings of the critical importance of microglia p38α compared to p38β, and continue to support selective targeting of the p38α isoform as a potential therapeutic strategy.

Role of p38 MAP Kinase Signal Transduction in Solid Tumors

Mitogen-activated protein kinases (MAPKs) mediate a wide variety of cellular behaviors in response to extracellular stimuli. One of the main subgroups, the p38 MAP kinases, has been implicated in a wide range of complex biologic processes, such as cell proliferation, cell differentiation, cell death, cell migration, and invasion. Dysregulation of p38 MAPK levels in patients are associated with advanced stages and short survival in cancer patients (e.g., prostate, breast, bladder, liver, and lung cancer). p38 MAPK plays a dual role as a regulator of cell death, and it can either mediate cell survival or cell death depending not only on the type of stimulus but also in a cell type specific manner. In addition to modulating cell survival, an essential role of p38 MAPK in modulation of cell migration and invasion offers a distinct opportunity to target this pathway with respect to tumor metastasis. The specific function of p38 MAPK appears to depend not only on the cell type but also on the stimuli and/or the isoform that is activated. p38 MAPK signaling pathway is activated in response to diverse stimuli and mediates its function by components downstream of p38. Extrapolation of the knowledge gained from laboratory findings is essential to address the clinical significance of p38 MAPK signaling pathways. The goal of this review is to provide an overview on recent progress made in defining the functions of p38 MAPK pathways with respect to solid tumor biology and generate testable hypothesis with respect to the role of p38 MAPK as an attractive target for intervention of solid tumors.

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.

Arachidonic acid induces brain endothelial cell apoptosis via p38-MAPK and intracellular calcium signaling

Arachidonic acid (AA), a bioactive fatty acid whose levels increase during neuroinflammation, contributes to cerebral vascular damage and dysfunction. However, the mode of injury and underlying signaling mechanisms remain unknown. Challenge of primary human brain endothelial cells (HBECs) with AA activated a stress response resulting in caspase-3 activation, poly(ADP-ribose) polymerase cleavage, and disruption of monolayer integrity. AA also induced loss of mitochondrial membrane potential and cytochrome c release consistent with activation of intrinsic apoptosis. HBEC stimulation with AA resulted in sustained p38-MAPK activation and subsequent phosphorylation of mitogen-activated protein kinase activated protein-2 (MAPKAP-2) kinase and heat shock protein-27 (Hsp27). Conversely, other unsaturated and saturated fatty acids had no effect. Pharmacological and RNA interference-mediated p38α or p38β suppression abrogated AA signaling to caspase-3 and Hsp27, suggesting involvement of both p38 isoforms in AA-induced HBEC apoptosis. Hsp27 silencing also blocked caspase-3 activation. AA stimulated intracellular calcium release, which was attenuated by inositol 1,4,5-trisphosphate (IP3) receptor antagonists. Blockade of intracellular calcium release decreased caspase-3 activation, but had no effect on AA-induced p38-MAPK activation. However, inhibition of p38-MAPK or blockade of intracellular calcium mobilization abrogated AA-induced cytochrome c release. AA-induced caspase-3 activation was abrogated by pharmacological inhibition of lipooxygenases. These findings support a previously unrecognized signaling cooperation between p38-MAPK/MAPKAP-2/Hsp27 and intracellular calcium release in AA-induced HBEC apoptosis and suggest its relevance to neurological disorders associated with vascular inflammation.

Functional roles of p38 mitogen-activated protein kinase in macrophage-mediated inflammatory responses

Inflammation is a natural host defensive process that is largely regulated by macrophages during the innate immune response. Mitogen-activated protein kinases (MAPKs) are proline-directed serine and threonine protein kinases that regulate many physiological and pathophysiological cell responses. p38 MAPKs are key MAPKs involved in the production of inflammatory mediators, including tumor necrosis factor-α (TNF-α) and cyclooxygenase-2 (COX-2). p38 MAPK signaling plays an essential role in regulating cellular processes, especially inflammation. In this paper, we summarize the characteristics of p38 signaling in macrophage-mediated inflammation. In addition, we discuss the potential of using inhibitors targeting p38 expression in macrophages to treat inflammatory diseases.

Mitochondrial p38β and manganese superoxide dismutase interaction mediated by estrogen in cardiomyocytes

Aims

While etiology behind the observed acceleration of ischemic heart disease in postmenopausal women is poorly understood, collective scientific data suggest cardioprotective effects of the endogenous female sex hormone, estrogen. We have previously shown that 17β-estradiol (E2) protects cardiomyocytes exposed to hypoxia-reoxygenation (H/R) by inhibiting p38α - p53 signaling in apoptosis and activating pro-survival p38β mitogen activated protein kinase (p38β MAPK), leading to suppression of reactive oxygen species (ROS) post H/R. However, little is known about the mechanism behind the antioxidant actions of E2-dependent p38β. The aim of this study is to determine whether the cytoprotection by estrogen involves regulation of manganese superoxide dismutase (MnSOD), a major mitochondrial ROS scavenging enzyme, via cardiac p38β.

Methods and Results

We identified mitochondrial p38β by immunocytochemistry and by immunoblotting in mitochondria isolated from neonatal cardiomyocytes of Sprague-Dawley rats. E2 facilitated the mitochondrial localization of the active form of the kinase, phosphorylated p38β (p-p38β). E2 also reduced the H/R-induced mitochondrial membrane potential decline, augmented the MnSOD activity and suppressed anion superoxide generation, while the dismutase protein expression remained unaltered. Co-immunoprecipitation studies showed physical association between MnSOD and p38β. p38β phosphorylated MnSOD in an E2-dependent manner in in-vitro kinase assays.

Conclusion

This work demonstrates for the first time a mitochondrial pool of active p38β and E2-mediated phosphorylation of MnSOD by the kinase. The results shed light on the mechanism behind the cytoprotective actions of E2 in cardiomyocytes under oxidative stress.

EGF receptor-dependent mechanism may be involved in the Tamm-Horsfall glycoprotein-enhanced PMN phagocytosis via activating Rho family and MAPK signaling pathway

Our previous studies showed that urinary Tamm–Horsfall glycoprotein (THP) potently enhanced polymorphonuclear neutrophil (PMN) phagocytosis. However, the domain structure(s), signaling pathway and the intracellular events responsible for THP-enhanced PMN phagocytosis remain to be elucidated. THP was purified from normal human urine. The human promyelocytic leukemia cell line HL-60 was induced to differentiate into PMNs by all-trans retinoid acid. Pretreatment with different MAPK and PI3K inhibitors was used to delineate signaling pathways in THP-enhanced PMN phagocytosis. Phosphorylation of molecules responsible for PMN phagocytosis induced by bacterial lipopolysaccharide (LPS), THP, or human recombinant epidermal growth factor (EGF) was evaluated by western blot. A p38 MAPK inhibitor, SB203580, effectively inhibited both spontaneous and LPS- and THP-induced PMN phagocytosis. Both THP and LPS enhanced the expression of the Rho family proteins Cdc42 and Rac that may lead to F-actin re-arrangement. Further studies suggested that THP and EGF enhance PMN and differentiated HL-60 cell phagocytosis in a similar pattern. Furthermore, the EGF receptor inhibitor GW2974 significantly suppressed THP- and EGF-enhanced PMN phagocytosis and p38 and ERK1/2 phosphorylation in differentiated HL-60 cells. We conclude that EGF receptor-dependent signaling may be involved in THP-enhanced PMN phagocytosis by activating Rho family and MAP kinase.

Sodium-dependent bile salt transporters of the SLC10A transporter family: more than solute transporters

The SLC10A transporter gene family consists of seven members and substrates transported by three members (SLC10A1, SLC10A2 and SLC10A6) are Na(+)-dependent. SLC10A1 (sodium taurocholate cotransporting polypeptide [NTCP]) and SLC10A2 (apical sodium-dependent bile salt transporter [ASBT]) transport bile salts and play an important role in maintaining enterohepatic circulation of bile salts. Solutes other than bile salts are also transported by NTCP. However, ASBT has not been shown to be a transporter for non-bile salt substrates. While the transport function of NTCP can potentially be used as liver function test, interpretation of such a test may be complicated by altered expression of NTCP in diseases and presence of drugs that may inhibit NTCP function. Transport of bile salts by NTCP and ASBT is inhibited by a number of drugs and it appears that ASBT is more permissive to drug inhibition than NTCP. The clinical significance of this inhibition in drug disposition and drug-drug interaction remains to be determined. Both NCTP and ASBT undergo post-translational regulations that involve phosphorylation/dephosphorylation, translocation to and retrieval from the plasma membrane and degradation by the ubiquitin-proteasome system. These posttranslational regulations are mediated via signaling pathways involving cAMP, calcium, nitric oxide, phosphoinositide-3-kinase (PI3K), protein kinase C (PKC) and protein phosphatases. There appears to be species difference in the substrate specificity and the regulation of plasma membrane localization of human and rodent NTCP. These differences should be taken into account when extrapolating rodent data for human clinical relevance and developing novel therapies. NTCP has recently been shown to play an important role in HBV and HDV infection by serving as a receptor for entry of these viruses into hepatocytes.

Inhibition of p38 mitogen-activated protein kinase impairs influenza virus-induced primary and secondary host gene responses and protects mice from lethal H5N1 infection

Highly pathogenic avian influenza viruses (HPAIV) induce severe inflammation in poultry and men. One characteristic of HPAIV infections is the induction of a cytokine burst that strongly contributes to viral pathogenicity. This cell-intrinsic hypercytokinemia seems to involve hyperinduction of p38 mitogen-activated protein kinase. Here we investigate the role of p38 MAPK signaling in the antiviral response against HPAIV in mice as well as in human endothelial cells, the latter being a primary source of cytokines during systemic infections. Global gene expression profiling of HPAIV-infected endothelial cells in the presence of the p38-specific inhibitor SB 202190 revealed that inhibition of p38 MAPK leads to reduced expression of IFNβ and other cytokines after H5N1 and H7N7 infection. More than 90% of all virus-induced genes were either partially or fully dependent on p38 signaling. Moreover, promoter analysis confirmed a direct impact of p38 on the IFNβ promoter activity. Furthermore, upon treatment with IFN or conditioned media from HPAIV-infected cells, p38 controls interferon-stimulated gene expression by coregulating STAT1 by phosphorylation at serine 727. In vivo inhibition of p38 MAPK greatly diminishes virus-induced cytokine expression concomitant with reduced viral titers, thereby protecting mice from lethal infection. These observations show that p38 MAPK acts on two levels of the antiviral IFN response. Initially the kinase regulates IFN induction and, at a later stage, p38 controls IFN signaling and thereby expression of IFN-stimulated genes. Thus, inhibition of MAP kinase p38 may be an antiviral strategy that protects mice from lethal influenza by suppressing excessive cytokine expression.

Thymoquinone induces apoptosis in oral cancer cells through p38β inhibition

Oral cancer is a common malignancy associated with high morbidity and mortality. While p38 MAPK is reported to be involved in different cellular activities such as proliferation and differentiation, reports rarely define the roles of the individual members of the p38 MAPK family in cancer. We used two unique cell lines developed by our lab representing chemically induced oral cancer cells (T28) and non-tumor cells (N28) obtained from tissues surrounding the induced cancer as a model to screen out whether p38 MAPK is involved in the malignant transformation processes. The results suggest an association between p38β not p38α and oral cancer development. Additionally, the anti-cancer activity of thymoquinone (TQ) was screened out and we found evidences suggesting that the anti-tumor activity of TQ may be attributed to the downregulation of p38β MAPK.

Sodium-dependent bile salt transporters of the SLC10A transporter family: more than solute transporters

The SLC10A transporter gene family consists of seven members and substrates transported by three members (SLC10A1, SLC10A2 and SLC10A6) are Na(+)-dependent. SLC10A1 (sodium taurocholate cotransporting polypeptide [NTCP]) and SLC10A2 (apical sodium-dependent bile salt transporter [ASBT]) transport bile salts and play an important role in maintaining enterohepatic circulation of bile salts. Solutes other than bile salts are also transported by NTCP. However, ASBT has not been shown to be a transporter for non-bile salt substrates. While the transport function of NTCP can potentially be used as liver function test, interpretation of such a test may be complicated by altered expression of NTCP in diseases and presence of drugs that may inhibit NTCP function. Transport of bile salts by NTCP and ASBT is inhibited by a number of drugs and it appears that ASBT is more permissive to drug inhibition than NTCP. The clinical significance of this inhibition in drug disposition and drug-drug interaction remains to be determined. Both NCTP and ASBT undergo post-translational regulations that involve phosphorylation/dephosphorylation, translocation to and retrieval from the plasma membrane and degradation by the ubiquitin-proteasome system. These posttranslational regulations are mediated via signaling pathways involving cAMP, calcium, nitric oxide, phosphoinositide-3-kinase (PI3K), protein kinase C (PKC) and protein phosphatases. There appears to be species difference in the substrate specificity and the regulation of plasma membrane localization of human and rodent NTCP. These differences should be taken into account when extrapolating rodent data for human clinical relevance and developing novel therapies. NTCP has recently been shown to play an important role in HBV and HDV infection by serving as a receptor for entry of these viruses into hepatocytes.

Induction of p38δ expression plays an essential role in oncogenic ras-induced senescence

Oncogene-induced senescence is a stable proliferative arrest that serves as a tumor-suppressing defense mechanism. p38 mitogen-activated protein kinase (MAPK) has been implicated in oncogene-induced senescence and tumor suppression. However, the specific role of each of the four p38 isoforms in oncogene-induced senescence is not fully understood. Here, we demonstrate that p38δ mediates oncogene-induced senescence through a p53- and p16(INK4A)-independent mechanism. Instead, evidence suggests a link between p38δ and the DNA damage pathways. Moreover, we have discovered a novel mechanism that enhances the expression of p38δ during senescence. In this mechanism, oncogenic ras induces the Raf-1-MEK-extracellular signal-regulated kinase (ERK) pathway, which, in turn, activates the AP-1 and Ets transcription factors that are bound to the p38δ promoter, leading to increased transcription of p38δ. These findings indicate that induction of the prosenescent function of p38δ by oncogenic ras is achieved through 2 mechanisms, transcriptional activation by the Raf-1-MEK-ERK-AP-1/Ets pathway, which increases the cellular concentration of the p38δ protein, and posttranslational modification by MKK3/6, which stimulates the enzymatic activity of p38δ. In addition, these studies identify the AP-1 and Ets transcription factors as novel signaling components in the senescence-inducing pathway.

Molecular modeling of p38α mitogen-activated protein kinase inhibitors through 3D-QSAR and molecular dynamics simulations

The p38 mitogen-activated protein kinase (MAPK) signaling pathway plays an essential role in inflammation and other physiological processes. Because specific inhibitors of p38α and p38β MAPK block the production of the major inflammatory cytokines and other proteins, p38α and p38β MAPK represent promising targets for the treatment of inflammation. In this work, a series of p38α inhibitors based on the structural scaffold of 4-benzoyl-5-aminopyrazole were analyzed using a combination of molecular modeling techniques. We generated three-dimensional quantitative structure-activity relationship (3D-QSAR) models for both comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA) to highlight the structural requirements for p38 MAPK inhibition. Furthermore, we employed molecular dynamics (MD) simulations and the MM/GBSA method to compare the binding modes and binding free energies of a potent and selective compound interacting with p38α, p38β, p38γ, and p38δ MAPK in detail. Contour maps generated via 3D-QSAR analysis identified several key interactions that were also indicated through MD simulations. The binding free energies calculated via the MM/GBSA method were strongly correlated with experimentally observed biological activities and explained the selective inhibition of p38α and p38β, but not p38γ and p38δ detected here. On the basis of the obtained results, we provide insights regarding the development of novel potent p38α MAPK inhibitors.

p38β, A novel regulatory target of Pokemon in hepatic cells

Pokemon is an important proto-oncogene involved in various biological processes and cancer development, such as cell differentiation, tumorigenesis and metastasis. Pokemon is recognized as a transcription factor localized upstream of several oncogenes, regulating their expression. p38MAPKs act as key regulatory factors in cellular signaling pathways associated with inflammatory responses, cell proliferation, differentiation and survival. p38β, a member of p38MAPK family, is closely correlated with tumorigenesis, but the mechanism of activation remains unclear. In this study, we found overexpression of Pokemon promoted the growth, migration and invasion of HepG2 cells. However, a p38 inhibitor SB202190 efficiently attenuated the promoting effect of Pokemon in the HepG2 cells. Targeted expression or silencing of Pokemon changed cellular p38β protein level and phosphorylation of downstream ATF2 in the p38 signaling pathway. Both dual luciferase report assay and ChIP assay suggested that p38β is a novel regulatory target of the transcription factor Pokemon and positively regulated by Pokemon in hepatic cells.

MEK and the inhibitors: from bench to bedside

Four distinct MAP kinase signaling pathways involving 7 MEK enzymes have been identified. MEK1 and MEK2 are the prototype members of MEK family proteins. Several MEK inhibitors are in clinical trials. Trametinib is being evaluated by FDA for the treatment of metastatic melanoma with BRAF V600 mutation. Selumetinib has been studied in combination with docetaxel in phase II randomized trial in previously treated patients with advanced lung cancer. Selumetinib group had better response rate and progression-free survival. This review also summarized new MEK inhibitors in clinical development, including pimasertib, refametinib, PD-0325901, TAK733, MEK162 (ARRY 438162), RO5126766, WX-554, RO4987655 (CH4987655), GDC-0973 (XL518), and AZD8330.

The mitogen-activated protein kinase p38α regulates tubular damage in murine anti-glomerular basement membrane nephritis

p38 mitogen-activated protein kinase (MAPK) is thought to play a central role in acute and chronic inflammatory responses. Whether p38MAPK plays a pathogenic role in crescentic GN (GN) and which of its four isoforms is preferentially involved in kidney inflammation is not definitely known. We thus examined expression and activation of p38MAPK isoforms during anti-glomerular basement membrane (GBM) nephritis. Therefore, p38α conditional knockout mice (MxCre-p38α^Δ/Δ) were used to examine the role of p38α in anti-GBM induced nephritis. Both wild type and MxCre-p38α^Δ/Δ mice developed acute renal failure over time. Histological examinations revealed a reduced monocyte influx and less tubular damage in MxCre-p38α^Δ/Δ mice, whereas glomerular crescent formation and renal fibrosis was similar. Likewise, the levels of pro- and anti-inflammatory cytokines such as TNF, IL-1 and IL-10 were similar, but IL-8 was even up-regulated in MxCre-p38α^Δ/Δ mice. In contrast, we could detect strong down-regulation of chemotactic cytokines such as CCL-2, -5 and -7, in the kidneys of MxCre-p38α^Δ/Δ mice. In conclusion, p38α is the primary p38MAPK isoform expressed in anti-GBM nephritis and selectively affects inflammatory cell influx and tubular damage. Full protection from nephritis is however not achieved as renal failure and structural damage still occurs.

Molecular pathways: mitogen-activated protein kinase pathway mutations and drug resistance

Receptor tyrosine kinases are a diverse family of transmembrane proteins that can activate multiple pathways upon ligation of the receptor, one of which is the series of mitogen-activated protein kinase (MAPK) signaling cascades. The MAPK pathways play critical roles in a wide variety of cancer types, from hematologic malignancies to solid tumors. Aberrations include altered expression levels and activation states of pathway components, which can sometimes be attributable to mutations in individual members. The V600E mutation of BRAF was initially described in 2002 and has been found at particularly high frequency in melanoma and certain subtypes of colorectal cancer. In the relatively short time since this discovery, a family of drugs has been developed that specifically target this mutated BRAF isoform, which, after results from phase I/II and III clinical trials, was granted U.S. Food and Drug Administration approval in August 2011. Although these drugs produce clinically meaningful increases in progression-free and overall survival, due to acquired resistance they have not improved mortality rates. New drugs targeting other members of the MAPK pathways are in clinical trials or advanced stages of development. It is hoped that combination therapies of these new drugs in conjunction with BRAF inhibitors will counteract the mechanisms of resistance and provide cures. The clinical implementation of next-generation sequencing is leading to a greater understanding of the genetic architecture of tumors, along with acquired mechanisms of drug resistance, which will guide the development of tumor-specific inhibitors and combination therapies in the future.

Mechanisms of resistance to endocrine therapy in breast cancer: focus on signaling pathways, miRNAs and genetically based resistance

Breast cancer is the most frequent malignancy diagnosed in women. Approximately 70% of breast tumors express the estrogen receptor (ER). Tamoxifen and aromatase inhibitors (AIs) are the most common and effective therapies for patients with ERα-positive breast cancer. Alone or combined with chemotherapy, tamoxifen significantly reduces disease progression and is associated with more favorable impact on survival in patients. Unfortunately, endocrine resistance occurs, either de novo or acquired during the course of the treatment. The mechanisms that contribute to hormonal resistance include loss or modification in the ERα expression, regulation of signal transduction pathways, altered expression of specific microRNAs, balance of co-regulatory proteins, and genetic polymorphisms involved in tamoxifen metabolic activity. Because of the clinical consequences of endocrine resistance, new treatment strategies are arising to make the cells sensitive to tamoxifen. Here, we will review the current knowledge on mechanisms of endocrine resistance in breast cancer cells. In addition, we will discuss novel therapeutic strategies to overcome such resistance. Undoubtedly, circumventing endocrine resistance should help to improve therapy for the benefit of breast cancer patients.

Involvement of the p38 MAPK signaling pathway in S-phase cell-cycle arrest induced by Furazolidone in human hepatoma G2 cells

Given the previously described essential role for the p38 mitogen-activation protein kinase (p38 MAPK) signaling pathway in human hepatoma G2 cells (HepG2), we undertook the present study to investigate the role of the p38 MAPK signaling pathway in cell-cycle arrest induced by Furazolidone (FZD). The aim of this study was to determine the effects of FZD on HepG2 cells by activating and inhibiting the p38 MAPK signaling pathway. The cell cycle and proliferation of HepG2 cells treated with FZD were detected by flow cytometry and MTT assay in the presence or absence of p38 MAPK inhibitors (SB203580), respectively. Cyclin D1, cyclin D3 and CDK6 were detected by quantitative real-time PCR and western blot analysis. Our data showed that p38 MAPK became phosphorylated after stimulation with FZD. Activation of p38 MAPK could arise S-phase cell-cycle arrest and suppress cell proliferation. Simultaneously, inhibition of the p38 MAPK signaling pathway significantly prevented S-phase cell-cycle arrest, increased the percentage of cell viability and decreased the expression of cyclin D1, cyclin D3 and CDK6. These results demonstrated that FZD arose S-phase cell-cycle arrest via activating the p38 MAPK signaling pathway in HepG2 cells. Cyclin D1, cyclin D3 and CDK6 are target genes functioning at the downstream of p38 MAPK in HepG2 cells induced by FZD.

p38α mitogen-activated protein kinase depletion and repression of signal transduction to translation machinery by miR-124 and -128 in neurons

The p38α to p38δ mitogen-activated protein kinases (MAPKs) are central regulatory nodes coordinating acute stress and inflammatory responses. Their activation leads to rapid adjustment of protein synthesis, for instance translational induction of proinflammatory cytokines. The only known direct link of p38 to translation machinery is the MAPK signal-integrating kinase Mnk. Only p38α and p38β transcripts are ubiquitously expressed. These mRNAs encode highly conserved proteins that equally phosphorylate recombinant Mnk1 in vitro. We discovered that expression of the p38α protein, but not the p38β isoform, is suppressed in the brain. This is due to p38α depletion by two neuron-selective microRNAs (miRNAs), miR-124 and -128. Suppression of p38α protein was reversed by miR-124/-128 antisense oligonucleotides in primary explant neuronal cultures. Targeted p38α depletion reduced Mnk1 activation, which cannot be compensated by p38β. Our research shows that p38α alone controls acute stress and cytokine signaling from p38 MAPK to translation machinery. This regulatory axis is greatly diminished in neurons, which may insulate brain physiology and function from p38α-Mnk1-mediated signaling.

Control of late apoptotic events by the p38 stress kinase in L-glutamine-deprived mouse hybridoma cells

L-Glutamine (Gln) starvation rapidly triggers apoptosis in Sp2/0-Ag14 (Sp2/0) murine hybridoma cells. Here, we report on the role played by the stress-activated kinase p38 mitogen-activated protein kinase (MAPK) in this process. p38 activation was detected 2 h after Gln withdrawal and, although treatment with the p38 inhibitor SB203580 did not prevent caspase activation in Gln-starved cells, it reduced the occurrence of both nuclear condensation/fragmentation and apoptotic body formation. Similarly, transfection of Sp2/0 cells with a dominant negative p38 MAPK reduced the incidence of nuclear pyknosis and apoptotic body formation following 2 h of Gln starvation. Gln withdrawal-induced apoptosis was blocked by the overexpression of the anti-apoptotic protein Bcl-xL or by the caspase inhibitor Z-VAD-fmk. Interestingly, Bcl-xL expression inhibited p38 activation, but Z-VAD-fmk treatment did not, indicating that activation of this MAPK occurs downstream of mitochondrial dysfunction and is independent of caspases. Moreover, the anti-oxidant N-acetyl-l-cysteine prevented p38 phosphorylation, showing that p38 activation is triggered by an oxidative stress. Altogether, our findings indicate that p38 MAPK does not contribute to the induction of apoptosis in Gln-starved Sp2/0 cells. Rather, Gln withdrawal leads to mitochondrial dysfunction, causing an oxidative stress and p38 activation, the latter contributing to the formation of late morphological features of apoptotic Sp2/0 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.