p38 isoforms have opposite effects on AP-1-dependent transcription through regulation of c-Jun. The determinant roles of the isoforms in the p38 MAPK signal specificity

p38gamma mitogen-activated protein kinase suppresses chondrocyte production of MMP-13 in response to catabolic stimulation

OBJECTIVE

The signaling protein p38 mitogen-activated protein kinase is required for inflammatory signaling in chondrocytes that regulates matrix metalloproteinase (MMP) production. We sought to determine the role of specific p38 isoforms in chondrocyte catabolic signaling in response to IL-1beta and fibronectin fragments (Fn-f).

METHODS

Human articular chondrocytes isolated from normal ankle cartilage from tissue donors or from osteoarthritic knee cartilage obtained during knee replacement were stimulated with IL-1beta or Fn-f, with or without pretreatment with p38 inhibitors (SB203580 or BIRB796) or growth factors (IGF-1 and OP-1). p38 isoform phosphorylation was measured by antibody array and immunoblotting. MMP-13 expression was measured by real-time polymerase chain reaction (PCR), enzyme-linked immunosorbent assay (ELISA), and immunoblotting. Chondrocytes were transfected with plasmids expressing constitutively active (CA) p38gamma or with adenovirus expressing dominant negative (DN) p38gamma.

RESULTS

Stimulation of chondrocytes with either IL-1beta or Fn-f led to enhanced phosphorylation of p38alpha and p38gamma, with little phosphorylation of p38beta or p38delta isoforms. p38alpha localized to the nucleus and p38gamma to the cytosol. Inhibition of both p38alpha and p38gamma with BIRB796 resulted in less inhibition of MMP-13 production in response to IL-1beta or FN-f than did inhibition of only p38alpha with SB203580. Transfection with CA p38gamma resulted in decreased MMP-13 production while transduction with DN p38gamma resulted in increased MMP-13 production. IGF-1 and OP-1 pretreatment inhibited p38alpha phosphorylation but not p38gamma phosphorylation.

CONCLUSIONS

p38gamma is activated by catabolic stimulation of human articular chondrocytes, but interestingly suppresses MMP-13 production. Treatments that increase p38gamma activation may be of therapeutic benefit in reducing chondrocyte production of MMP-13.

Activin A induction of erythroid differentiation through MKK6-p38alpha/p38beta pathway is inhibited by follistatin

Activin A is a member of the transforming growth factor (TGF)-beta superfamily that regulates cell proliferation and differentiation. Using the p38 inhibitor SB203580, our previous studies demonstrated that p38 was involved in activin A-mediated hemoglobin (Hb) synthesis in K562 cells. SB203580 is an inhibitor of p38alpha and p38beta isoforms. In this study, we show that p38alpha and p38beta mRNA were expressed in K562 cells and that activin A activated the kinase activities of these isoforms. To investigate the roles of p38alpha and p38beta isoforms in activin A-mediated erythroid differentiation, we generated stable clones that over-expressed the dominant negative p38 isoforms p38alpha(AF) and p38beta(AF) in K562 cells. The expressions of either p38alpha(AF) or p38beta(AF) reduced activin A-induced p38 activation, Hb synthesis, and zeta-globin promoter activity. Similarly, down-regulation of either p38alpha or p38beta by isoform-specific siRNAs also reduced activin A-induced zeta-globin promoter activity. Co-expressions of p38alpha(AF) and p38beta(AF), together, greatly inhibited the transcription activity of the zeta-globin promoter. Conversely, expression of mitogen-activated protein kinase kinase (MKK) 6b(E), a constitutive activator of p38, significantly activated zeta-globin promoter. Co-expressions of either p38alpha or p38beta with MKK6b had a similar activation of zeta-globin promoter. Activin A induction of erythroid differentiation was inhibited by follistatin. Activin A-induced phosphorylation of MKK6 and p38 was also inhibited by follistatin. Moreover, over-expression of MKK6b(E) reverted follistatin inhibition of activin A-induced zeta-globin promoter activity. These results demonstrate that activin A induces erythroid differentiation of K562 cells through activation of MKK6-p38alpha/p38beta pathway and follistatin inhibits those effects.

"Go upstream, young man": lessons learned from the p38 saga

Despite the success of biological therapies in rheumatoid arthritis (RA), orally active small-molecule drugs are desirable. Signal transduction inhibitors have been the focus of intense efforts, with some recent notable successes and failures. p38alpha is a signalling molecule that regulates proinflammatory cytokines, which makes it a logical target for RA. Unfortunately, selective p38alpha inhibitors have limited efficacy. An attempt is made here to put these studies into perspective and offer possible explanations for the failure of p38alpha blockers. Alternative strategies, such as targeting kinases higher in the signalling cascade or using less selective compounds, might be more successful as suggested by the efficacy seen with Syk and JAK inhibitors.

Silibinin induces protective superoxide generation in human breast cancer MCF-7 cells

The pharmacological activity of polyphenolic silibinin from milk thistle (Silybum marianum) is primarily due to its antioxidant property. However, this study found that silibinin promoted sustained superoxide (O(2)(.-)) production that was specifically scavenged by exogenous superoxide dismutase (SOD) in MCF-7 cells, while the activity of endogenous SOD was not changed by silibinin. Previous work proved that silibinin induced MCF-7 cell apoptosis through mitochondrial pathway and this study further proved that O(2)(.-) generation induced by silibinin was also related to mitochondria. It was found that respiratory chain complexes I, II and III were all involved in silibinin-induced O(2)(.-) generation. Moreover, it was found that silibinin-induced O(2)(.-) had protective effect, as exogenous SOD markedly enhanced silibinin-induced apoptosis.

p38gamma MAPK cooperates with c-Jun in trans-activating matrix metalloproteinase 9

Mitogen-activated protein kinases (MAPKs) regulate gene expression through transcription factors. However, the precise mechanisms in this critical signal event are largely unknown. Here, we show that the transcription factor c-Jun is activated by p38gamma MAPK, and the activated c-Jun then recruits p38gamma as a cofactor into the matrix metalloproteinase 9 (MMP9) promoter to induce its trans-activation and cell invasion. This signaling event was initiated by hyperexpressed p38gamma that led to increased c-Jun synthesis, MMP9 transcription, and MMP9-dependent invasion through p38gamma interacting with c-Jun. p38gamma requires phosphorylation and its C terminus to bind c-Jun, whereas both c-Jun and p38gamma are required for the trans-activation of MMP9. The active p38gamma/c-Jun/MMP9 pathway also exists in human colon cancer, and there is a coupling of increased p38gamma and MMP9 expression in the primary tissues. These results reveal a new paradigm in which a MAPK acts both as an activator and a cofactor of a transcription factor to regulate gene expression leading to an invasive response.

p38 MAP kinase inhibits neutrophil development through phosphorylation of C/EBPalpha on serine 21

Many extracellular stimuli regulate growth, survival, and differentiation responses through activation of the dual specificity mitogen activated protein kinase (MAPK) kinase three (MKK3) and its downstream effector p38 MAPK. Using CD34+ hematopoietic progenitor cells, here we describe a novel role for MKK3-p38MAPK in the regulation of myelopoiesis. Inhibition of p38MAPK utilizing the pharmacological inhibitor SB203580, enhanced neutrophil development ex vivo, but conversely reduced eosinophil differentiation. In contrast, constitutive activation of MKK3 dramatically inhibited neutrophil differentiation. Transplantation of beta2-microglobulin(-/-) nonobese diabetic/severe combined immune deficient (NOD/SCID) mice with CD34+ cells ectopically expressing constitutively active MKK3 resulted in reduced neutrophil differentiation in vivo, whereas eosinophil development was enhanced. Inhibitory phosphorylation of CCAAT/enhancer binding protein alpha (C/EBPalpha) on serine 21 was induced upon activation of p38MAPK. Moreover, ectopic expression of a non-phosphorylatable C/EBPalpha mutant was sufficient to abrogate MKK3-induced inhibition of neutrophil development. Furthermore, treatment of CD34+ progenitors from patients with severe congenital neutropenia with SB203580 restored neutrophil development. These results establish a novel role for MKK3-p38MAPK in the regulation of lineage choices during myelopoiesis through modulation of C/EBPalpha activity. This signaling module may thus provide an important therapeutic target in the treatment of bone marrow failure.

p38 Mitogen-activated protein kinase and hematologic malignancies

CONTEXT

p38 mitogen-activated protein kinase (MAPK) signaling has been implicated in responses ranging from apoptosis to cell cycle, induction of expression of cytokine genes, and differentiation. This plethora of activators conveys the complexity of the p38 pathway. This complexity is further complicated by the observation that the downstream effects of p38 MAPK activation may be different depending on types of stimuli, cell types, and various p38 MAPK isoforms involved.

OBJECTIVE

This review focuses on the recent advancement of the p38 MAPK isoforms as well as the roles of p38 MAPK in hematologic malignancies.

DATA SOURCES

Review of pertinent published literature and work in our laboratory.

CONCLUSIONS

In some hematologic malignancies, activation of p38 plays a key role in promoting or inhibiting proliferation and also in increasing resistance to chemotherapeutic agents. The importance of different p38 isoforms in various cellular functions has been acknowledged recently. Further understanding of these isoforms will allow the design of more specific inhibitors to target particular isoforms to maximize the treatment effect and minimize the side effects for treating hematopoietic malignancies.

Cytokines and signaling pathways regulating matrix metalloproteinase-9 (MMP-9) expression in corneal epithelial cells

Matrix metalloproteinase-9 (MMP-9) is a well-known regulator and effecter of many cellular processes including wound healing. In the cornea, either too much or too little MMP-9 can be detrimental to overall wound repair. We investigated the secreted factors as well as the intracellular signaling pathways and the promoter sequences that mediate this regulation. Primary culture rabbit corneal epithelial cells were treated with various cytokines alone or in different combinations and MMP-9 induction was assessed by gel zymography. Pharmacological inhibitors were used to determine the intracellular signaling pathways induced by the cytokines tested and deletion promoter constructs were created to determine the regions of the MMP-9 promoter involved in the cytokine regulation, thereby assessing the exact transcription factors binding the MMP-9 promoter. We found that two cytokine families, transforming growth factor beta (TGF-beta) and interleukin 1 (IL-1), act additively in an isoform non-specific manner to induce MMP-9 in this cell type. Our data suggest TGF-beta mediated MMP-9 induction may be regulated by the NF-kappaB, Smad3, and JNK pathways, whereas the IL-1beta mediated induction may be regulated by the NF-kappaB and p38 pathways. Inhibition of the p38, NF-kappaB, or JNK pathways significantly reduced, but did not abrogate, basal MMP-9 levels. Inhibition of the ERK pathway did not have an effect on MMP-9 mediated expression in either the treated or untreated co-transfected cells.

Epidermal growth factor receptor cross-talks with ligand-occupied estrogen receptor-alpha to modulate both lactotroph proliferation and prolactin gene expression

Both estrogen (E2) and EGF regulate lactotrophs, and we recently demonstrated that EGF phosphorylates S118 on estrogen receptor-alpha (ERalpha) and requires ERalpha to stimulate prolactin (PRL) release. However, the interactions between ligand-occupied ERalpha and activated ErbB1 and its impact on lactotroph function are unknown. Using rat GH3 lactotrophs, we found that both E2 and EGF independently stimulated proliferation and PRL gene expression. Furthermore, their combination resulted in an enhanced stimulatory effect on both cell proliferation and PRL gene expression. Inhibitors of ER as well as ErbB1 blocked the combined effects of E2 and EGF. Pretreatment with UO126 abolished the combined effects, demonstrating Erk1/2 requirement. Although bidirectionality in ER-ErbB1 cross-talk is a well-accepted paradigm, interestingly in lactotrophs, ErbB1 kinase inhibitor failed to block the effect of E2 on proliferation and stimulation of PRL gene expression, suggesting that ER does not require ErbB1 to mediate its effects. Furthermore, E2 did not affect the ability of EGF to induce c-Fos expression or modulate AP-1 activity. However, both E2 and EGF combine to enhance S118 phosphorylation of ERalpha, leading to enhanced E2-mediated estrogen response element transactivation. Taken together, our results suggest that, in lactotrophs, activated ErbB1 phosphorylates ERalpha to enhance the stimulatory effect of E2, thereby providing the molecular basis by which EGF amplifies the response of E2.

An autocrine VEGF/VEGFR2 and p38 signaling loop confers resistance to 4-hydroxytamoxifen in MCF-7 breast cancer cells

Tamoxifen, a partial estrogen receptor antagonist, is part of the standard treatment of both primary and advanced breast cancers. However, significant proportions of breast cancers are either de novo resistant or develop tamoxifen resistance during the course of treatment through mechanisms which have been only partly characterized. We have previously found that high vascular endothelial growth factor (VEGF) or VEGF receptor 2 (VEGFR2) expression and concomitant high p38 mitogen-activated protein kinase activity within breast cancers predict a poor outcome for tamoxifen-treated patients. Here, we have molecularly dissected how VEGF/VEGFR2 and p38 are linked, and contribute to tamoxifen resistance within breast cancer using a MCF-7 BC cell model with different 4-hydroxytamoxifen (4-OHT) responsiveness. We report that MCF-7 breast cancer cell lines with tamoxifen resistance have increased secretion of VEGF and increased signaling through VEGFR2 compared with parental MCF-7 cells. 4-OHT treatment caused the ablation of VEGF secretion in parental MCF-7 cells, whereas in the tamoxifen-resistant subline, a VEGF/VEGFR2 signaling loop was still evident upon treatment. Increased basal levels of total and phosphorylated p38 were observed in tamoxifen-resistant cells. Pharmacologic inhibition of p38 reduced the proliferation of both tamoxifen-responsive and tamoxifen-resistant cells and showed an additive growth-inhibitory effect in combination with 4-OHT. A connection between VEGF/VEGFR2 and p38 signaling was identified by VEGF and VEGFR2 knockdown, which equally reduced both the total and the active forms of p38 in tamoxifen-resistant cells. Taken together, our results suggest that decreased sensitivity to 4-OHT is caused by a death-protecting VEGF/VEGFR2 and p38 growth factor loop in breast cancer cells. Inhibition of these signaling pathways may be beneficial to overcome tamoxifen resistance.

Evidence that the bZIP domains of the Jun transcription factor bind to DNA as monomers prior to folding and homodimerization

The Jun oncoprotein belongs to the AP1 family of transcription factors that is collectively engaged in diverse cellular processes by virtue of their ability to bind to the promoters of a wide spectrum of genes in a DNA sequence-dependent manner. Here, using isothermal titration calorimetry, we report detailed thermodynamics of the binding of bZIP domain of Jun to synthetic dsDNA oligos containing the TRE and CRE consensus promoter elements. Our data suggest that binding of Jun to both sites occurs with indistinguishable affinities but with distinct thermodynamic signatures comprised of favorable enthalpic contributions accompanied by entropic penalty at physiological temperatures. Furthermore, anomalously large negative heat capacity changes observed provoke a model in which Jun loads onto DNA as unfolded monomers coupled with subsequent folding and homodimerization upon association. Taken together, our data provide novel insights into the energetics of a key protein-DNA interaction pertinent to cellular signaling and cancer. Our study underscores the notion that the folding and dimerization of transcription factors upon association with DNA may be a more general mechanism employed in protein-DNA interactions and that the conventional school of thought may need to be re-evaluated.

p38beta2-mediated phosphorylation and sumoylation of ATF7 are mutually exclusive

The ubiquitous activating transcription factor (ATF) 7 binds as a homodimer to the cAMP response element/TPA response element motifs present in the promoters of its target genes. ATF7 is homologous to ATF2 and heterodimerizes with Jun or Fos proteins, modulating their DNA-binding specificities. We previously demonstrated that TAF12, a component of the TFIID general transcription factor, mediates ATF7 transcriptional activity through direct interactions between the two proteins. By contrast, ATF7, but not ATF2, is modified in vivo by sumoylation, which restricts its subcellular localization, thereby inhibiting its transcriptional activity. In the present study, we dissect the mechanism of this functional switch. We characterized the multisite phosphorylation of the ATF7 activation domain and identified one of the involved kinase, p38beta2 mitogen-activated protein kinase. In addition, we show that epidermal growth factor treatment results in a two-step modification mechanism of ATF7 activation domain. The Thr53 residue is phosphorylated first by a presently unknown kinase, allowing p38beta2 mitogen-activated protein kinase to modify the Thr51 residue, excluding the sumoylation of ATF7 protein. The resulting activation of transcription is related to an increased association of TAF12 with this phosphorylated form of ATF7. Our data therefore conclusively establish that sumoylation and phosphorylation of ATF7 are two antagonistic posttranslational modifications.

Endothelial cell-laminin interaction: modulation of LDH expression involves alpha6beta4 integrin-FAK-p38MAPK pathway

One of the possible mechanisms of the angiogenic effect of laminin (Ln) involves modulation of the biological activity of VEGF by regulating poly ADP ribosylation (PAR). PAR modification of VEGF was found to be related with the changes in NAD(+) associated with a shift in LDH isoenzymes. Further investigations on LDH gene expression in HUVECs suggested that the effect of Ln was mediated through alpha(6)beta(4) integrin-FAK-src-p38 MAPK pathway. This was evidenced by (a) co-immunoprecipitation of beta(4) integrin with alpha(6) subunit, (b) activation by tyrosine phosphorylation of beta(4) integrin and FAK, (c) co-immunoprecipitation of FAK with beta(4) and with adapter protein, src, (d) increased phosphorylation of p38 MAPK in cells maintained on Ln and (e) blocking of effect of Ln on LDH-B gene expression by inhibition of p38 MAPK. Increase in serine phosphorylation of c-fos and c-jun and higher levels of heterodimers of AP-1 in the nucleus in cells maintained on Ln suggested activation of AP-1 transcription factor. These results provide evidence for modulation of endothelial cell function relevant to angiogenesis by Ln through alpha(6)beta(4) integrin.

TIMP-1 overexpression promotes tumorigenesis of MDA-MB-231 breast cancer cells and alters expression of a subset of cancer promoting genes in vivo distinct from those observed in vitro

TIMP-1 (Tissue inhibitor of matrix metalloproteinase-1) is typically associated with inhibition of matrix metalloproteinases (MMP) induced invasion. However, TIMP-1 is overexpressed in many malignancies and is associated with poor prognosis in breast cancer. The mechanisms by which TIMP-1 promotes tumorigenesis are unclear. Reduced levels of TIMP-1 mediated by shRNA in MDA-MB-231 breast cancer cells had no effect on cellular physiology in vitro or tumor growth in SCID mice compared to vector control MDA-MB-231 cells. However, overexpression of TIMP-1 in MDA-MB-231 cells resulted in inhibition of cell invasion and enhanced phosphorylation of p38 MAPK and AKT in vitro. Additionally, treatment of parental MDA-MB-231 cells with purified TIMP-1 protein led to activation of p38 MAPK and MKK 3/6. cDNA array analysis demonstrated that high expression of TIMP-1 in MDA-MB-231 cells resulted in alterations in expression of approximately 200 genes, 1.5 fold or greater compared to vector control cells (P < 0.1). Real-time RT-PCR confirmed changes in expression of several genes associated with cancer progression including DAPK1, FGFR4 and MAPK13. In vivo, high TIMP-1 expression induced tumor growth in SCID mice compared to vector control cells and increased tumor vessel density. Affymetrix array analysis of vector control and TIMP-1 MDA-MB-231 xenograft tumors revealed that TIMP-1 altered expression of approximately 600 genes in vivo, including MMP1, MMP13, S100A14, S100P, Rab25 and ID4. These combined observations suggest that the effects of TIMP-1 differ significantly in a 2-D environment compared to the 3-D environment and that TIMP-1 stimulates tumor growth.

Basal and IGF-I-dependent regulation of potassium channels by MAP kinases and PI3-kinase during eccentric cardiac hypertrophy

The potassium channels I(K) and I(K1), responsible for the action potential repolarization and resting potential respectively, are altered during cardiac hypertrophy. The activation of insulin-like growth factor-I (IGF-I) during hypertrophy may affect channel activity. The aim was to examine the modulatory effects of IGF-I on I(K) and I(K1) through mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways during hypertrophy. With the use of specific inhibitors for ERK1/2 (PD98059), p38 MAPK (SB203580) and PI3K/Akt (LY294002), Western blot and whole cell patch-clamp were conducted on sham and aorto-caval shunt-induced hypertrophy adult rat myocytes. Basal activation levels of MAPKs and Akt were increased during hypertrophy. Acute IGF-I (10(-8) M) enhanced basal activation levels of these kinases in normal hearts but only those of Akt in hypertrophied ones. I(K) and I(K1) activities were lowered by IGF-I. Inhibition of ERK1/2, p38 MAPK, or Akt reduced basal I(K) activity by 70, 32, or 50%, respectively, in normal cardiomyocytes vs. 53, 34, or 52% in hypertrophied ones. However, basal activity of I(K1) was reduced by 45, 48, or 45% in the former vs. 63, 43, or 24% in the latter. The inhibition of either MAPKs or Akt alleviated IGF-I effects on I(K) and I(K1). We conclude that basal I(K) and I(K1) are positively maintained by steady-state Akt and ERK activities. K+ channels seem to be regulated in a dichotomic manner by acutely stimulated MAPKs and Akt. Eccentric cardiac hypertrophy may be associated with a change in the regulation of the steady-state basal activities of K+ channels towards MAPKs, while that of the acute IGF-I-stimulated ones toward Akt.

Implication of p38 mitogen-activated protein kinase isoforms (alpha, beta, gamma and delta) in CD4+ T-cell infection with human immunodeficiency virus type I

The CD4(+) T-cell reduction characteristic of human immunodeficiency virus type 1 (HIV-1) infection is thought to result, in addition to infected T-cell death, mainly from uninfected bystander T-cell apoptosis. Nevertheless, the immunological and virological mechanisms leading to T-cell death during HIV-1 infection are not yet fully understood. In the present study, we analysed the individual implication of the p38 mitogen-activated protein kinase (MAPK) isoforms (p38alpha, p38beta, p38gamma and p38delta) during apoptosis induced by HIV-1, taking into account that HIV-1 replication is known to be blocked by p38 inhibitors. For this purpose, we used the SupT1 cell line, where death induced by HIV-1 mainly occurs by uninfected bystander cell apoptosis. A variety of SupT1-based cell lines were constructed constitutively expressing, under the control of cytomegalovirus promoter (PCMV), each dominant-negative (dn) p38 isoform and each wild-type p38 isoform as a control. An enhanced green fluorescent protein marker gene, under the control of the HIV-1 promoter, was inserted in all of them. These cell lines were infected with HIV-1 and analysed by flow cytometry. We found that survival in SupT1-based cell lines infected by HIV-1 was increased by the p38alphadn, p38gammadn and p38deltadn isoforms, but not by the p38betadn isoform. HIV-1 replication was delayed most by p38deltadn and to a lesser extent by p38alphadn and p38gammadn. Moreover, these three isoforms, p38alphadn, p38gammadn and p38deltadn, reduced apoptosis induced by HIV-1. These results suggest that, in SupT1-based cell lines, p38alpha, p38gamma and p38delta, but not p38beta, are implicated in both HIV-1 induced replication and apoptosis in infected and uninfected bystander cells.

Evidence that age-related changes in p38 MAP kinase contribute to the decreased steroid production by the adrenocortical cells from old rats

The current studies were initiated to investigate whether excessive oxidative stress exerts its antisteroidogenic action through modulation of oxidant-sensitive mitogen-activated protein kinase (MAPK) signaling pathways. Western blot analysis indicated that aging caused increased phosphorylation and activation of rat adrenal p38 MAPK, but not the ERK1/2 or JNK1/2. Lipid peroxidation measurements (an index of cellular oxidative stress) indicated that adrenal membranes from young animals contained only minimal levels of endogenous thiobarbituric acid-reactive substances (TBARS), and exposure of membranes to enzymatic and non-enzymatic pro-oxidants enhanced TBARS formation approximately 12- and 20-fold, respectively. The adrenal membranes from old animals showed much more susceptibility to lipid peroxidation and exhibited roughly 4- to 6-fold higher TBARS formation than young controls both under basal conditions and in response to pro-oxidants. Qualitatively similar results were obtained when lipid peroxide formation was measured using a sensitive FOXRS (ferrous oxidation-xylenol orange-reactive substances) technique. We next tested whether aging-induced excessive oxidative insult alters steroidogenesis through modulation of MAPK signaling pathway. Treatment of adrenocortical cells from old rats with specific p38 MAPK inhibitors restored Bt(2)cAMP-stimulated steroidogenesis approximately 60-70% of the value seen in cells of young animals. Likewise, pretreatment of cells with reactive oxygen species (ROS) scavengers MnTMPyP and N-acetyl cysteine also partially rescued age-induced loss of steroid production. In contrast, simultaneous treatment of cells with ROS scavengers and p38 MAPK inhibitor did not produce any additional effect suggesting that both types of inhibitors exert their stimulatory action through inhibition of p38 MAPK activation. Collectively, these results indicate that p38 MAPK functions as a signaling effector in oxidative stress-induced inhibition of steroidogenesis during aging.

Coupling of folding and DNA-binding in the bZIP domains of Jun-Fos heterodimeric transcription factor

In response to mitogenic stimuli, the heterodimeric transcription factor Jun-Fos binds to the promoters of a diverse array of genes involved in critical cellular responses such as cell growth and proliferation, cell cycle regulation, embryogenic development and cancer. In so doing, Jun-Fos heterodimer regulates gene expression central to physiology and pathology of the cell in a specific and timely manner. Here, using the technique of isothermal titration calorimetry (ITC), we report detailed thermodynamics of the bZIP domains of Jun-Fos heterodimer to synthetic dsDNA oligos containing the TRE and CRE consensus promoter elements. Our data suggest that binding of the bZIP domains to both TRE and CRE is under enthalpic control and accompanied by entropic penalty at physiological temperatures. Although the bZIP domains bind to both TRE and CRE with very similar affinities, the enthalpic contributions to the free energy of binding to CRE are more favorable than TRE, while the entropic penalty to the free energy of binding to TRE is smaller than CRE. Despite such differences in their thermodynamic signatures, enthalpy and entropy of binding of the bZIP domains to both TRE and CRE are highly temperature-dependent and largely compensate each other resulting in negligible effect of temperature on the free energy of binding. From the plot of enthalpy change versus temperature, the magnitude of heat capacity change determined is much larger than that expected from the direct association of bZIP domains with DNA. This observation is interpreted to suggest that the basic regions in the bZIP domains are largely unstructured in the absence of DNA and only become structured upon interaction with DNA in a coupled folding and binding manner. Our new findings are rationalized in the context of 3D structural models of bZIP domains of Jun-Fos heterodimer in complex with dsDNA oligos containing the TRE and CRE consensus sequences. Taken together, our study demonstrates that enthalpy is the major driving force for a key protein-DNA interaction pertinent to cellular signaling and that protein-DNA interactions with similar binding affinities may be accompanied by differential thermodynamic signatures. Our data corroborate the notion that the DNA-induced protein structural changes are a general feature of the bZIP family of transcription factors.

The docking interaction of caspase-9 with ERK2 provides a mechanism for the selective inhibitory phosphorylation of caspase-9 at threonine 125

Caspase-9 plays a critical role in the initiation of apoptosis by the mitochondrial pathway. Activation of caspase-9 is inhibited by phosphorylation at Thr(125) by ERK1/2 MAPKs in response to growth factors. Here, we show that phosphorylation of this site is specific for these classical MAPKs and is not strongly induced when JNK and p38alpha/beta MAPKs are activated by anisomycin. By deletion and mutagenic analysis, we identify domains in caspase-9 and ERK2 that mediate their interaction. Binding of ERK2 to caspase-9 and subsequent phosphorylation of caspase-9 requires a basic docking domain (D domain) in the N-terminal prodomain of the caspase. Mutational analysis of ERK2 reveals a (157)TTCD(160) motif required for recognition of caspase-9 that acts independently of the putative common docking domain. Molecular modeling supports the conclusion that Arg(10) in the D domain of caspase-9 interacts with Asp(160) in the TTCD motif of ERK2. Differences in the TTCD motif in other MAPK family members could account for the selective recognition of caspase-9 by ERK1/2. This selectivity may be important for the antiapoptotic role of classical MAPKs in contrast to the proapoptotic roles of stress-activated MAPKs.

p38α Antagonizes p38γ Activity through c-Jun-dependent Ubiquitin-proteasome Pathways in Regulating Ras Transformation and Stress Response

p38 MAPK family consists of four isoform proteins (alpha, beta, gamma, and delta) that are activated by the same stimuli, but the information about how these proteins act together to yield a biological response is missing. Here we show a feed-forward mechanism by which p38alpha may regulate Ras transformation and stress response through depleting its family member p38gamma protein via c-Jun-dependent ubiquitin-proteasome pathways. Analyses of MAPK kinase 6 (MKK6)-p38 fusion proteins showed that constitutively active p38alpha (MKK6-p38alpha) and p38gamma (MKK6-p38gamma) stimulates and inhibits c-Jun phosphorylation respectively, leading to a distinct AP-1 regulation. Depending on cell type and/or stimuli, p38alpha phosphorylation results in either Ras-transformation inhibition or a cell-death escalation that invariably couples with a decrease in p38gamma protein expression. p38gamma, on the other hand, increases Ras-dependent growth or inhibits stress induced cell-death independent of phosphorylation. In cells expressing both proteins, p38alpha phosphorylation decreases p38gamma protein expression, whereas its inhibition increases cellular p38gamma concentrations, indicating an active role of p38alpha phosphorylation in negatively regulating p38gamma protein expression. Mechanistic analyses show that p38alpha requires c-Jun activation to deplete p38gamma proteins by ubiquitin-proteasome pathways. These results suggest that p38alpha may, upon phosphorylation, act as a gatekeeper of the p38 MAPK family to yield a coordinative biological response through disrupting its antagonistic p38gamma family protein.

MAP kinases and the control of nuclear events

The mitogen-activated protein kinases (MAPKs) are a family of serine/threonine kinases that play an essential role in signal transduction by modulating gene transcription in the nucleus in response to changes in the cellular environment. They include the extracellular signal-regulated protein kinases (ERK1 and ERK2); c-Jun N-terminal kinases (JNK1, JNK2, JNK3); p38s (p38alpha, p38beta, p38gamma, p38delta) and ERK5. The molecular events in which MAPKs function can be separated in discrete and yet interrelated steps: activation of the MAPK by their upstream kinases, changes in the subcellular localization of MAPKs, and recognition, binding and phosphorylation of MAPK downstream targets. The resulting pattern of gene expression will ultimately depend on the integration of the combinatorial signals provided by the temporal activation of each group of MAPKs. This review will focus on how the specificity of signal transmission by MAPKs is achieved by scaffolding molecules and by the presence of structural motifs in MAPKs that are dynamically regulated by phosphorylation and protein-protein interactions. We discuss also how MAPKs recognize and phosphorylate their target nuclear proteins, including transcription factors, co-activators and repressors and chromatin-remodeling molecules, thereby affecting an intricate balance of nuclear regulatory molecules that ultimately control gene expression in response to environmental cues.

p38α and p38δ mitogen-activated protein kinase isoforms regulate invasion and growth of head and neck squamous carcinoma cells

Recent studies indicate that the specificity of p38 mitogen-activated protein kinase (MAPK)-mediated cellular stress responses is determined by the expression pattern of the distinct p38 isoforms. Here, we have analysed the function of distinct p38 isoforms in the growth and invasion of head and neck squamous cell carcinomas (HNSCCs). Activation of p38 MAPK by arsenite resulted in inactivation of the ERK1,2 signaling pathway by dephosphorylation of MEK1,2 in primary human epidermal keratinocytes (HEKs), whereas in HNSCC cells this p38-mediated inhibition of the ERK1,2 pathway was absent. Quantitation of p38 pathway component mRNA expression in HNSCC cell lines (n=42) compared to HEKs (n=8) revealed that p38alpha and p38delta isoforms are predominantly expressed in both cell types and that MKK3 is the primary upstream activator expressed. Inhibition of endogenous p38alpha or p38delta activity by adenoviral delivery of corresponding dominant-negative p38 isoforms potently reduced MMP-13 and MMP-1 expressions, and suppressed the invasion of HNSCC cells through collagen. Dominant-negative p38alpha and p38delta inhibited squamous cell carcinoma (SCC) cell proliferation and inhibition of p38alpha activity also compromised survival of SCC cells. p38alpha and p38delta were predominantly expressed in HNSCCs (n=24) and nonneoplastic epithelium in vivo (n=6), with MKK3 being the primary upstream activator. Activation and expression of p38alpha and p38delta by tumor cells was detected in HNSCCs in vivo (n=16). Adenoviral expression of dominant-negative p38alpha or p38delta in cutaneous SCC cells potently inhibited their implantation in skin of severe combined immunodeficiency mice and growth of xenografts in vivo. Our results indicate that p38alpha and p38delta specifically promote the malignant phenotype of SCC cells by regulating cell survival, proliferation and invasion, suggesting these p38 MAPK isoforms as potential therapeutic targets in HNSCCs.

Hyperactive variants of p38alpha induce, whereas hyperactive variants of p38gamma suppress, activating protein 1-mediated transcription

The p38 family of kinases is a subgroup of the mitogen-activated protein kinase family. It is composed of four isoforms and is involved in critical biological processes as well as in inflammatory diseases. The exact unique role of each p38 isoform in these processes is not understood well. To approach this question we have been developing intrinsically active variants of p38s. Recently we described a series of mutants of the human p38alpha, which were spontaneously active as recombinant proteins purified from Escherichia coli cells. We show here that some of these mutants are spontaneously active in several mammalian cells in culture. The spontaneous activity of some mutants is higher than the activity of the fully activated wild type counterpart. We further produced mutants of the other p38 isoforms and found that p38beta(D176A), p38gamma(D179A), p38delta(D176A), and p38delta(F324S) are spontaneously active in vivo. The active mutants are also spontaneously phosphorylated. To test whether the mutants actually fulfill downstream duties of p38 proteins, we tested their effect on activating protein 1(AP-1)-mediated transcription. Active mutants of p38alpha induced AP-1-driven reporter genes, as well as the c-jun and c-fos promoters. An active variant of p38gamma suppressed AP-1-mediated transcription. When active variants of p38alpha and p38gamma were co-expressed, AP-1 activity was not induced, showing that p38gamma is dominant over p38alpha with respect to AP-1 activation. Thus, intrinsically active variants that are spontaneously active in vivo have been obtained for all p38 isoforms. These variants have disclosed different effects of each isoform on AP-1 activity.

Update on the biology of the chondrocyte and new approaches to treating cartilage diseases

Osteoarthritis (OA) is a joint disease that involves degeneration of articular cartilage, limited intraarticular inflammation manifested by synovitis and changes in the subchondral bone. The aetiology of OA is largely unknown, but since it may involve multiple factors, including mechanical, biochemical and genetic factors, it has been difficult to identify unique targets for therapy. Chondrocytes, which are the unique cellular component of adult articular cartilage, are capable of responding to structural changes in the surrounding cartilage matrix. Since the initial stages of OA involve increased cell proliferation and synthesis of matrix proteins, proteinases and cytokines in the cartilage, laboratory investigations have focused on the chondrocyte as a target for therapeutic intervention. The capacity of the adult articular chondrocyte to regenerate the normal cartilage matrix architecture is limited, however, and the damage becomes irreversible unless the destructive process is interrupted. Current pharmacological interventions that address chronic pain are insufficient and no proven disease-modifying therapy is available. Identification of methods for early diagnosis is of key importance, since therapeutic interventions aimed at blocking or reversing structural damage will be more effective when there is the possibility of preserving normal homeostasis. At later stages, cartilage tissue engineering with or without gene therapy with anabolic factors will also require therapy to inhibit inflammation and block damage to newly repaired cartilage. This review will focus on experimental approaches currently under study that may lead to elucidation of effective strategies for therapy in OA, with emphasis on mediators that affect the function of chondrocytes and interactions with surrounding tissues.

p38gamma mitogen-activated protein kinase integrates signaling crosstalk between Ras and estrogen receptor to increase breast cancer invasion

Ras is believed to stimulate invasion and growth by different effector pathways, and yet, the existence of such effectors under physiologic conditions has not been shown. Estrogen receptor (ER), on the other hand, is both anti-invasive and proliferative in human breast cancer, with mechanisms for these paradoxical actions remaining largely unknown. Our previous work showed an essential role of p38gamma mitogen-activated protein kinase in Ras transformation in rat intestinal epithelial cells, and here, we show that p38gamma integrates invasive antagonism between Ras and ER to increase human breast cancer invasion without affecting their proliferative activity. Ras positively regulates p38gamma expression, and p38gamma in turn mediates Ras nonmitogenic signaling to increase invasion. Expression of the Ras/p38gamma axis, however, is trans-suppressed by ER that inhibits invasion and stimulates growth also by distinct mechanisms. Analysis of ER and its cytoplasmic localized mutant reveals that ER additionally binds to p38gamma protein, leading to its specific down-regulation in the nuclear compartment. A p38gamma-antagonistic activity of ER was further shown in a panel of breast cancer cell lines and was shown independent of estrogens by both ER depletion and ER expression. These results revealed that both Ras and ER use distinct pathways to regulate breast cancer growth and invasion, and that p38gamma specifically integrates their antagonistic activity to stimulate cell invasion. Selective targeting of p38gamma-dependent invasion pathways may be a novel strategy to control breast cancer progression.

Mechanism of action of sulforaphane: inhibition of p38 mitogen-activated protein kinase isoforms contributing to the induction of antioxidant response element-mediated heme oxygenase-1 in human hepatoma HepG2 cells

Exposure of sulforaphane to HepG2 cells increased heme oxygenase-1 (HO-1) expression by activating antioxidant response element (ARE) through induction of Nrf2 and suppression of Kelch-like ECH-associated protein 1 (Keap1). Using human HO-1 promoter reporter plasmids and ChIP assay, we have identified that sulforaphane transcriptionally activated the upstream ARE-rich enhancer region, located at -9.0 kb upstream human HO-1 promoter. Induction of HO-1 by sulforaphane was attenuated by overexpression of mutant Nrf2 plasmid in HepG2 cells and totally abolished in Nrf2 knockout mouse embryonic keratinocytes and fibroblasts. Overexpression of individual p38 mitogen-activated protein (MAP) kinase (MAPK) isoforms also suppressed constitutive as well as sulforaphane- or Nrf2-induced ARE-dependent gene expression. Among the upstream kinases, although MKK3 was not involved in suppression of ARE by any of p38 MAPK isoforms, MKK6 selectively suppressed ARE by p38 gamma or p38 delta, but not by p38 alpha or p38 beta. Importantly, sulforaphane not only activated MAP/extracellular signal-regulated kinase (ERK) kinases 1/2 and ERK1/2, but also strongly suppressed anisomycin-induced activation of p38 MAPK isoforms by blocking phosphorylation of upstream kinases, MKK3/6. Finally, we found that stimulation of p38 MAPK isoforms phosphorylated purified Nrf2 protein and caused an increase in the interaction between Nrf2 and Keap1 in vitro and the suppression of Nrf2 translocation into the nucleus. Collectively, our results indicate that transcriptional activation of Nrf2/ARE is critical in sulforaphane-mediated induction of HO-1, which can be modulated in part by the blockade of p38 MAPK signaling pathway. In addition, our study shows that p38 MAPK can phosphorylate Nrf2 and promotes the association between Nrf2 and Keap1 proteins, thereby potentially inhibiting nuclear translocation of Nrf2.

Proteasome inhibitors induce a p38 mitogen-activated protein kinase (MAPK)-dependent anti-apoptotic program involving MAPK phosphatase-1 and Akt in models of breast cancer

Proteasome inhibitors represent a novel class of anti-tumor agents that have clinical efficacy against hematologic malignancies, but single-agent activity against solid tumors such as breast cancer has been disappointing, perhaps due to activation of anti-apoptotic survival signals. To evaluate a possible role for the p38 mitogen-activated protein kinase (MAPK), A1N4-myc human mammary epithelial, and BT-474 and MDA-MB-231 breast carcinoma cells, were studied. Exposure of these lines to pharmacologic p38 blockade enhanced proteasome inhibitor-mediated apoptosis, as did overexpression of dominant negative (DN)-p38-alpha and -beta-MAPK isoforms. Inhibition of p38 resulted in suppression of induction of anti-apoptotic MAPK phosphatase (MKP)-1, in association with enhanced activation of the pro-apoptotic c-Jun-N-terminal kinase (JNK). Moreover, infection of cells treated with a proteasome inhibitor/p38 inhibitor combination with Adenovirus (Ad) inducing over-expression of MKP-1 suppressed apoptosis compared with controls. Further targets of p38 MAPK were also studied, and proteasome inhibition activated phosphorylation of MAPK-activated protein kinase-2, heat shock protein (HSP)-27, and the AKT8 virus oncogene cellular homolog (Akt). Inhibition of p38 MAPK resulted in decreased phospho-HSP-27 and phospho-Akt, while down-regulation of HSP-27 with a small interfering RNA decreased phosphorylation of Akt, directly linking activation of p38 to Akt. Finally, inhibition of Akt with phosphatidylinositol-3-kinase inhibitors increased apoptosis, as did over-expression of DN-Akt. These studies support the hypothesis that proteasome inhibitors activate an anti-apoptotic survival program through p38 MAPK that involves MKP-1 and Akt. Further, they suggest that strategies targeting MKP-1 and Akt could enhance the anti-tumor efficacy of proteasome inhibitors against breast cancer.

MAP-quest: could we produce constitutively active variants of MAP kinases?

Constitutively active mutants that acquired intrinsic activity and escaped regulation, serve as powerful tools for revealing the biochemical, biological and pathological functions of proteins. Such mutants are not available for mitogen-activated protein kinases (MAPKs). It is not known how to mimic the unusual mode of MAPK activation and to enforce, by mutations, their active conformation. In this review we describe the strategies employed in attempts to overcome this obstacle. We focus on a recent breakthrough with the p38 family that suggests that active variants of all MAPKs will soon be available.

Decreased Phosphoactive ERKs and JNKs in Malachite-Green-Transformed Syrian Hamster Embryo Fibroblasts Are Associated with Increased Phosphoactive p38 Kinase: Possible Therapeutic Importance

BACKGROUND

Malachite green (MG), consisting of green crystals with a metallic lustre, is highly soluble in water, cytotoxic to various mammalian cells and also acts as a liver tumor promoter. In view of its industrial importance and possible exposure of human beings, MG poses a potential environmental health hazard. We have earlier reported the malignant transformation of Syrian hamster embryo (SHE) cells by MG.

METHODS

Cell transformation assays were carried out as described in the literature. Western blotting and flow cytometry were carried out by standard methods.

RESULTS

In this study, we have studied the role of all three isoforms of mitogen-activated protein (MAP) kinases, i.e. extracellular regulated kinases (ERKs), Jun N-terminal kinases (JNKs) and p38 kinase in the MG-transformed SHE fibroblasts compared to controls. Our results showed that transformed cells were associated with decreased expression of ERKs and JNKs as evidenced by Western blotting studies. However, the p38 MAP kinase was found to be upregulated. Flow cytometric DNA histogram analysis indicated an increase in the expression of S phase cells in the transformed cell line as compared to their control counterparts.

CONCLUSIONS

The present studies indicate that decreased phosphoactive ERKs and JNKs and increased phosphoactive p38 kinase are associated with increased S phase cells during transformation of SHE cells by MG.

Over-expression of interleukin-6 enhances cell survival and transformed cell growth in human malignant cholangiocytes

BACKGROUND/AIMS

Over-expression of IL-6 has been implicated in cholangiocarcinoma growth but the cellular mechanisms involved are unknown. Our aims were to assess the mechanisms by which over-expression of IL-6 promotes transformed cell growth in malignant cholangiocytes.

METHODS

Stably transfected cell lines over-expressing IL-6 were derived from malignant human cholangiocytes. Transformed cell growth was assessed by anchorage independent growth in vitro and by xenograft growth in nude mice. Expression of the anti-apoptotic protein Mcl-1 was quantitated by immunoblot analysis and by real-time PCR. Gene silencing was performed using siRNA. Dominant negative upstream kinase activators and isoform-specific constructs were used to evaluate the involvement of p38 MAP kinase signaling pathways.

RESULTS

Over-expression of IL-6 increased xenograft growth, anchorage independent growth and cell survival but did not significantly alter cell proliferation. The basal expression of Mcl-1 was increased in IL-6 over-expressing cells. Selective knockdown of Mcl-1 by siRNA increased gemcitabine-induced cytotoxicity. Moreover, IL-6 increased Mcl-1 mRNA and protein expression via a p38 MAPK dependent mechanism.

CONCLUSIONS

These data demonstrate a major role of survival signaling pathways in mediating the effects of IL-6 over-expression in cholangiocarcinoma growth. Mcl-1 is identified as a mediator of IL-6-induced tumor cell survival and shown to be transcriptionally regulated by IL-6 via a p38 MAPK dependent pathway. We conclude that modulation of IL-6 mediated survival signaling pathways involving the p38 MAPK or downstream targets such as Mcl-1 may prove useful therapeutic strategies for human cholangiocarcinoma.

Over-expression of interleukin-6 enhances cell survival and transformed cell growth in human malignant cholangiocytes

BACKGROUND/AIMS

Over-expression of IL-6 has been implicated in cholangiocarcinoma growth but the cellular mechanisms involved are unknown. Our aims were to assess the mechanisms by which over-expression of IL-6 promotes transformed cell growth in malignant cholangiocytes.

METHODS

Stably transfected cell lines over-expressing IL-6 were derived from malignant human cholangiocytes. Transformed cell growth was assessed by anchorage independent growth in vitro and by xenograft growth in nude mice. Expression of the anti-apoptotic protein Mcl-1 was quantitated by immunoblot analysis and by real-time PCR. Gene silencing was performed using siRNA. Dominant negative upstream kinase activators and isoform-specific constructs were used to evaluate the involvement of p38 MAP kinase signaling pathways.

RESULTS

Over-expression of IL-6 increased xenograft growth, anchorage independent growth and cell survival but did not significantly alter cell proliferation. The basal expression of Mcl-1 was increased in IL-6 over-expressing cells. Selective knockdown of Mcl-1 by siRNA increased gemcitabine-induced cytotoxicity. Moreover, IL-6 increased Mcl-1 mRNA and protein expression via a p38 MAPK dependent mechanism.

CONCLUSIONS

These data demonstrate a major role of survival signaling pathways in mediating the effects of IL-6 over-expression in cholangiocarcinoma growth. Mcl-1 is identified as a mediator of IL-6-induced tumor cell survival and shown to be transcriptionally regulated by IL-6 via a p38 MAPK dependent pathway. We conclude that modulation of IL-6 mediated survival signaling pathways involving the p38 MAPK or downstream targets such as Mcl-1 may prove useful therapeutic strategies for human cholangiocarcinoma.

Induction of Apoptosis in Renal Cell Carcinoma by Reactive Oxygen Species: Involvement of Extracellular Signal-Regulated Kinase 1/2, p38δ/γ, Cyclooxygenase-2 Down-Regulation, and Translocation of Apoptosis-Inducing Factor

Renal cell carcinoma (RCC) is the most common malignancy of the kidney. Unfortunately, RCCs are highly refractory to conventional chemotherapy, radiation therapy, and even immunotherapy. Thus, novel therapeutic targets need to be sought for the successful treatment of RCCs. We now report that 6-anilino-5,8-quinolinequinone (LY83583), an inhibitor of cyclic GMP production, induced growth arrest and apoptosis of the RCC cell line 786-0. It did not prove deleterious to normal renal epithelial cells, an important aspect of chemotherapy. To address the cellular mechanism(s), we used both genetic and pharmacological approaches. LY83583 induced a time- and dose-dependent increase in RCC apoptosis through dephosphorylation of mitogen-activated protein kinase kinase 1/2 and its downstream extracellular signal-regulated kinases (ERK) 1 and -2. In addition, we observed a decrease in Elk-1 phosphorylation and cyclooxygenase-2 (COX-2) down-regulation. We were surprised that we failed to observe an increase in either c-Jun NH(2)-terminal kinase or p38alpha and -beta mitogen-activated protein kinase activation. In contradiction, reintroduction of p38delta by stable transfection or overexpression of p38gamma dominant negative abrogated the apoptotic effect. Cell death was associated with a decrease and increase in Bcl-x(L) and Bax expression, respectively, as well as release of cytochrome c and translocation of apoptosis-inducing factor. These events were associated with an increase in reactive oxygen species formation. The antioxidant N-acetyl l-cysteine, however, opposed LY83583-mediated mitochondrial dysfunction, ERK1/2 inactivation, COX-2 down-regulation, and apoptosis. In conclusion, our results suggest that LY83583 may represent a novel therapeutic agent for the treatment of RCC, which remains highly refractory to antineoplastic agents. Our data provide a molecular basis for the anticancer activity of LY83583.

Role of MKK3 and p38 MAPK in cytokine-induced death of insulin-producing cells

The aim of the present investigation was to elucidate further the importance of p38 MAPK (mitogen-activated protein kinase) in nitric oxide- and cytokine-induced beta-cell death. For this purpose, isolated human islets were treated with d-siRNA (diced small interfering RNA) and then exposed to the nitric oxide donor DETA/NONOate [2,2'-(hydroxynitrosohydrazono)bis-ethanamine]. We observed that cells treated with p38alpha-specific d-siRNA, but not with d-siRNA targeting GL3 (a firefly luciferase siRNA plasmid) or PKCdelta (protein kinase Cdelta), were protected against nitric oxide-induced death. This was paralleled by an increased level of Bcl-XL (B-cell leukaemia/lymphoma-X long). For an in-depth study of the mechanisms of p38 activation, MKK3 (MAPK kinase 3), MKK6 and their dominant-negative mutants were overexpressed in insulin-producing RIN-5AH cells. In transient transfections, MKK3 overexpression resulted in increased p38 phosphorylation, whereas in stable MKK3-overexpressing RIN-5AH clones, the protein levels of p38 and JNK (c-Jun N-terminal kinase) were decreased, resulting in unaffected phospho-p38 levels. In addition, a long-term MKK3 overexpression did not affect cell death rates in response to the cytokines interleukin-1beta and interferon-gamma, whereas a short-term MKK3 expression resulted in increased cytokine-induced RIN-5AH cell death. The MKK3-potentiating effect on cytokine-induced cell death was abolished by a nitric oxide synthase inhibitor, and MKK3-stimulated p38 phosphorylation was enhanced by inhibitors of phosphatases. Finally, as the dominant-negative mutant of MKK3 did not affect cytokine-induced p38 phosphorylation, and as wild-type MKK3 did not influence p38 autophosphorylation, it may be that p38 is activated by MKK3/6-independent pathways in response to cytokines and nitric oxide. In addition, it is likely that a long-term increase in p38 activity is counteracted by both a decreased expression of the p38, JNK and p42 genes as well as an increased dephosphorylation of p38.

TAK1 downregulation reduces IL-1β induced expression of MMP13, MMP1 and TNF-alpha

The paper provides evidence that transforming growth factor-beta activated kinase 1 (TAK1, MEKK7), a downstream mediator of IL-1beta signal transduction, plays an important role in the regulation of catabolic events and inflammatory processes in the context of degenerative joint diseases. We investigated the expression of TAK1 in human articular chondrocytes and in the murine growth plate by cDNA array, quantitative RT-PCR and immunohistochemistry, respectively. The human chondrosarcoma cell line SW1353 was stimulated with the proinflammatory cytokine IL-1beta. The subsequent expression of proteolytic enzymes and proinflammatory cytokines was quantified. TAK1 specific siRNA was used to study the influence of TAK1 downregulation on the expression of MMP-13, MMP1 and TNF-alpha. As a result we demonstrated the expression of TAK1 in normal and osteoarthritic human articular cartilage. Expression of TAK1 in the hypertrophic zone of the growth plate gave us a first evidence for a catabolic function of TAK1 concerning cartilage metabolism. By gene suppression with RNAi technology we could show that TAK1 downregulation leads to a 60-70% reduced release of TNF-alpha, a 40-50% reduced release of MMP13, and a 20-30% reduction of MMP1 release. As TNF-alpha is a main player in inflammatory processes, and MMP13 is one of the major proteases involved in cartilage degradation, our results suggests that TAK1 has an important regulatory role in the context of degenerative joint diseases and thus is an attractive drug target in attempts to reduce inflammation and suppress structural changes in OA induced by IL-1beta.

Streptococcus pneumoniae induced p38 MAPK- and NF-kappaB-dependent COX-2 expression in human lung epithelium

Streptococcus pneumoniae is a major cause of community-acquired pneumonia and death from infectious diseases in industrialized countries. Lung airway and alveolar epithelial cells comprise an important barrier against airborne pathogens. Cyclooxygenase (COX)-derived prostaglandins, such as PGE(2), are considered to be important regulators of lung function. Herein, we tested the hypothesis that pneumococci induced COX-2-dependent PGE(2) production in pulmonary epithelial cells. Pneumococci-infected human pulmonary epithelial BEAS-2B cells released PGE(2). Expression of COX-2 but not COX-1 was dose and time dependently increased in S. pneumoniae-infected BEAS-2B cells as well as in lungs of mice with pneumococcal pneumonia. S. pneumoniae induced degradation of IkappaBalpha and DNA binding of NF-kappaB. A specific peptide inhibitor of the IkappaBalpha kinase complex blocked pneumococci-induced PGE(2) release and COX-2 expression. In addition, we noted activation of p38 MAPK and JNK in pneumococci-infected BEAS-2B cells. PGE(2) release and COX-2 expression were reduced by p38 MAPK inhibitor SB-202190 but not by JNK inhibitor SP-600125. We analyzed interaction of kinase pathways and NF-kappaB activation: dominant-negative mutants of p38 MAPK isoforms alpha, beta(2), gamma, and delta blocked S. pneumoniae-induced NF-kappaB activation. In addition, recruitment of NF-kappaB subunit p65/RelA and RNA polymerase II to the cox2 promoter depended on p38 MAPK but not on JNK activity. In summary, p38 MAPK- and NF-kappaB-controlled COX-2 expression and subsequent PGE(2) release by lung epithelial cells may contribute significantly to the host response in pneumococcal pneumonia.

1α,25(OH)2-Vitamin D3 stimulates intestinal cell p38 MAPK activity and increases c-Fos expression

In intestinal cells, as in other target cells, the steroid hormone 1alpha,25(OH)(2)-Vitamin D(3) (1alpha,25(OH)(2)D(3)) regulates gene expression via the specific intracellular Vitamin D receptor and induces fast non-transcriptional responses involving stimulation of transmembrane signal transduction pathways. We have previously shown that the hormone activates the extracellular signal-regulated mitogen-activated protein (MAP) kinase isoforms ERK1 and ERK2 in rat intestinal cells. In the present study, we have demonstrated that 1alpha,25(OH)(2)D(3) also induces the phosphorylation and activation of p38 MAPK in these cells. The hormone effects were time and dose-dependent, with maximal stimulation at 2min (+3-fold) and 1nM. 1alpha,25(OH)(2)D(3)-dependent p38 phosphorylation was suppressed by SB 203580, a selective inhibitor of p38 MAPK. Ca(2+) chelation with EGTA, inhibition of the c-Src-tyrosine kinase family with PP1 or protein kinase A (PKA) with Rp-cAMP, attenuated hormone activation of p38 MAPK. The physiological significance of 1alpha,25(OH)(2)D(3)-dependent activation of ERK1/2 and p38 MAP kinases was addressed by monitoring c-Fos expression. Incubation of intestinal cells with the hormone was followed by a rapid induction of c-Fos expression which was blocked by SB 203580 and partially suppressed by the ERK1/2 inhibitor PD 98059. Our results suggest that 1alpha,25(OH)(2)D(3) activates p38 MAPK, involving Ca(2+), c-Src and PKA as upstream regulators, and that p38 MAPK has a central role in hormone-induction of the oncoprotein c-Fos in rat intestinal cells.

Dual roles for the phosphatase PPM1D in regulating progesterone receptor function

Although protein phosphatase magnesium-dependent 1 delta (PPM1D) was initially characterized as a p53-regulated phosphatase responsible for inactivation of p38 MAPK and consequent inactivation of p53, its overexpression and amplification in human breast cancers led us to assess its role in steroid hormone action. We found that PPM1D stimulated the activity of several nuclear receptors including the progesterone receptor (PR) and estrogen receptor. Although p38 MAPK inhibited PR activity, PPM1D stimulation of PR activity was greater than that achieved by a chemical inhibitor of p38 MAPK, SB202190. This suggests an additional novel function for PPM1D. Consistent with this, the transcriptional activity of endogenous PR in MCF-7 breast cancer cells was preferentially inhibited by small interfering RNA for PPM1D; SB202190 failed to reverse the inhibition. Although PPM1D phosphatase activity was required for stimulation of transcriptional activity, the activity of a PR phosphorylation site null mutant was enhanced by PPM1D, indicating that PR is not the direct target. Additional studies revealed that PPM1D enhanced the intrinsic activity of p160 coactivators such as steroid receptor coactivator-1 and promoted the interaction between PR and steroid receptor coactivator-1 in a mammalian two-hybrid assay. Neither activity was induced by SB202190. Although PPM1D stimulated PR activity in part through inhibition of p38 MAPK, its primary action is novel and independent of p38 MAPK. Thus, we speculate that PPM1D promotes breast tumor growth both by inhibiting p53 activity and by enhancing steroid hormone receptor action.

Mechanisms of Chlamydophila pneumoniae-mediated GM-CSF release in human bronchial epithelial cells

Chlamydophila pneumoniae is an important respiratory pathogen. In this study we characterized C. pneumoniae strain TW183-mediated activation of human small airway epithelial cells (SAEC) and the bronchial epithelial cell line BEAS-2B and demonstrated time-dependent secretion of granulocyte macrophage colony-stimulating factor (GM-CSF) upon stimulation. TW183 activated p38 mitogen-activated protein kinase (MAPK) in epithelial cells. Kinase inhibition by SB202190 blocked Chlamydia-mediated GM-CSF release on mRNA and protein levels. In addition, the chemical inhibitor as well as dominant-negative mutants of p38 MAPK isoforms p38alpha, beta2, and gamma inhibited C. pneumoniae-related NF-kappaB activation. In contrast, blocking of MAPK ERK, c-Jun kinase/JNK, or PI-3 Kinase showed no effect on Chlamydia-related epithelial cell GM-CSF release. Ultraviolet-inactivated pathogens as compared with viable bacteria induced a smaller GM-CSF release, suggesting that viable Chlamydiae were only partly required for a full effect. Presence of an antichlamydial outer membrane protein-A (OmpA) antibody reduced and addition of recombinant heat-shock protein 60 from C. pneumoniae (cHsp60, GroEL-1)-enhanced GM-CSF release, suggesting a role of these proteins in epithelial cell activation. Our data demonstrate that C. pneumoniae triggers an early proinflammatory signaling cascade involving p38 MAPK-dependent NF-kappaB activation, resulting in subsequent GM-CSF release. C. pneumoniae-induced epithelial cytokine liberation may contribute significantly to inflammatory airway diseases like chronic obstructive pulmonary disease (COPD) or bronchial asthma.

Activation of c-Jun N-terminal kinase (JNK) pathway by HSV-1 immediate early protein ICP0

The immediate early protein ICP0 encoded by herpes simplex virus 1 (HSV-1) is believed to activate transcription and consequently productive infection. The precise mechanisms of ICP0-mediated transactivation are under intensive study. Here, we demonstrate that ICP0 can strongly activate AP-1 responsive genes specifically. This activation is inhibited by c-Jun (S73A), c-Jun (S63/73A), TAK1 (K63W), but not by p38 (AF), ERK1 (K71R), ERK2 (K52R) and TRAF6 (C85A/H87A). We further investigate the relevancy of ERK, JNK and p38 MAPK pathways using their respective inhibitors PD98059, SP600125 and SB202190. Only SP600125 significantly attenuates the AP-1 responsive gene activation by ICP0. Consistent with these, the JNK is remarkably activated in response to ICP0, and this JNK activation is shown to be significantly attenuated by TAK1 (K63W). It turns out that ICP0 interacts specifically with TAK1 and stimulates its kinase activity. These findings reveal a new molecular mechanism ICP0 explores to regulate gene expression.

Activation of p38 Has Opposing Effects on the Proliferation and Migration of Endothelial Cells

Pathological conditions such as hypertension and hyperglycemia as well as abrasions following balloon angioplasty all lead to endothelial dysfunction that impacts disease morbidity. These conditions are associated with the elaboration of a variety of cytokines and increases in p38 activity in endothelial cells. However, the relationship between enhanced p38 activity and endothelial cell function remains poorly understood. To investigate the effect of enhanced p38 MAPK activity on endothelial cell function, we expressed an activated mutant of MEK6 (MEK6E), an upstream regulator of p38. Expression of MEK6E activated p38 and resulted in phosphorylation of its downstream substrate, heat shock protein 27 (Hsp27). Activation of p38 was not sufficient to induce apoptosis; however, it did induce p38-dependent cell cycle arrest. MEK6E expression was sufficient to inhibit ERK phosphorylation triggered by growth factors and integrin engagement. MAPK phosphatase-1 (MKP-1) expression was increased upon p38 activation, and expression of a "substrate-trapping" MKP-1 was sufficient to restore ERK activity. Activation of p38 was sufficient to induce cell migration, which was accompanied by alterations in actin architecture characterized by enhanced lamellipodia. Co-expression of a mutant form of Hsp27, lacking all three phosphorylation sites, reversed MEK6E-induced cell migration and altered the cytoskeletal changes induced by p38 activation. Collectively, these results suggest that cellular decisions regarding migration and proliferation are influenced by p38 activity and that prolonged activation of p38 may result in an anti-angiogenic phenotype that contributes to endothelial dysfunction.

IL-6 activates serum and glucocorticoid kinase via p38alpha mitogen-activated protein kinase pathway

Interleukin-6 (IL-6) has been implicated as an autocrine factor involved in growth of several human cancers, such as tumors arising from the biliary tract or cholangiocarcinoma. In malignant biliary tract epithelia, IL-6 activates the p38 MAPK pathway, which mediates a dominant survival signaling pathway. Serum and glucocorticoid-stimulated kinase (SGK) has been implicated as a survival kinase, but its role in survival signaling by IL-6 is unknown. After IL-6 stimulation, p38 MAPK activation preceded phosphorylation of SGK at Ser78. Pretreatment with the pharmacological inhibitors of p38 MAPK SB-203580 or SB-202190 blocked IL-6-induced SGK phosphorylation at Ser78 and SGK activation. Overexpression of p38alpha increased constitutive SGK phosphorylation at Ser78, whereas dominant negative p38alpha MAPK blocked IL-6-induced SGK phosphorylation and nuclear translocation. Interestingly, in addition to stimulating SGK phosphorylation, both IL-6 stimulation and p38alpha MAPK overexpression increased SGK mRNA and protein expression. An increase in p38 MAPK and SGK occurred following enforced expression of IL-6 in vivo. Furthermore, inhibition of SGK expression by siRNA increased toxicity due to chemotherapeutic drugs. Taken together, these data identify SGK as both a downstream kinase substrate as well as a transcriptionally regulated gene target of p38 MAPK in response to IL-6 and support a role of SGK during survival signaling by IL-6 in human cancers, such as cholangiocarcinoma.

Essential role of p38gamma in K-Ras transformation independent of phosphorylation

MAPK cascades play the critical role in regulating Ras oncogene activity by phosphorylation-dependent mechanisms. Whereas the ERK MAPK pathway is required for Ras transformation, our previous works established that the p38 activity is inhibitory to Ras signaling in both experimental and ras-mutated cancer cells (Chen, G., Hitomi, M., Han, J., and Stacey, D. W. (2000) J. Biol. Chem. 275, 38973-38980; Qi, X., Tang, J., Pramanik, R., Schultz, R. M., Shirasawa, S., Sasazuki, T., Han, J., and Chen, G. (2004) J. Biol. Chem., 279, 22138-22144). Here we report that K-Ras activated p38gamma, a p38 MAPK family member, by inducing its expression without increasing its phosphorylation and that depletion of induced p38gamma suppressed Ras transformation in rat intestinal epithelial cells. This p38gamma activity contrasts with that of its family member, p38alpha, which is activated by Ras through phosphorylation, leading to an inhibition of Ras transformation. Mechanistic analyses showed that unphosphorylated p38gamma may promote Ras transformation through an increased complex formation with ERK proteins. Significantly, functional p38gamma protein was expressed only in K-ras-mutated human colon cancer cells, and p38gamma transcripts were ubiquitously increased in a set of primary human colon cancer tissues. These studies thus demonstrate the essential role of p38gamma in K-Ras transformation independent of phosphorylation, and elevated p38gamma may serve as a novel diagnostic marker and therapeutic target for human colon cancer.

Spinal p38beta isoform mediates tissue injury-induced hyperalgesia and spinal sensitization

Antagonist studies show that spinal p38 mitogen-activated protein kinase plays a crucial role in spinal sensitization. However, there are two p38 isoforms found in spinal cord and the relative contribution of these two to hyperalgesia is not known. Here we demonstrate that the isoforms are distinctly expressed in spinal dorsal horn: p38alpha in neurons and p38beta in microglia. In lieu of isoform selective inhibitors, we examined the functional role of these two individual isoforms in nociception by using intrathecal isoform-specific antisense oligonucleotides to selectively block the expression of the respective isoform. In these rats, down-regulation of spinal p38beta, but not p38alpha, prevented nocifensive flinching evoked by intraplantar injection of formalin and hyperalgesia induced by activation of spinal neurokinin-1 receptors through intrathecal injection of substance P. Both intraplantar formalin and intrathecal substance P produced an increase in spinal p38 phosphorylation and this phosphorylation (activation) was prevented when spinal p38beta, but not p38alpha, was down-regulated. Thus, spinal p38beta, probably in microglia, plays a significant role in spinal nociceptive processing and represents a potential target for pain therapy.

The p38 SAPK pathway is required for Ha-ras induced in vitro invasion of NIH3T3 cells

Constitutive activation of the ras oncoprotein plays a critical role in cancer invasion and metastasis. Particularly, ras-related protease expression such as the serine protease urokinase plasminogen activator (u-PA) has been implicated in mediating cancer cell invasion. Previous studies have shown that ras-mediated u-PA expression is regulated through the mitogen- (MAPK) and stress-activated protein kinase (SAPK) signal transduction pathways extracellular signal-regulated kinase (ERK) and c-Jun-activating kinase (JNK). We therefore asked the question, if ras-related cell invasion might additionally require the third MAPK/SAPK signal transduction cascade, p38. Indeed, we found that ras induces invasion based on the activation of certain p38 protein kinase isoforms, in particular, p38alpha. Moreover, ras activation through transient or stable expression of a Ha-rasEJ mutant induced the expression of u-PA. This was found to be a consequence of an increase of u-PA m-RNA, which was paralleled by only a modest activation of the u-PA promoter. In conclusion, we provide evidence for the requirement of a novel ras-p38alpha-u-PA pathway for ras-dependent cellular invasion.

Breast cancer cells with inhibition of p38alpha have decreased MMP-9 activity and exhibit decreased bone metastasis in mice

p38 belongs to a family of mitogen-activated protein kinases, which transfer extracellular signals into intracellular responses. p38 is also frequently detected in clinical breast cancer specimens, but its role as a prognostic factor is not known. Of the various p38 isoforms, p38alpha has been shown to mediate the in vitro invasiveness of breast cancer cells through up-regulation of urokinase plasminogen activator (uPA). We studied the role of p38alpha in breast cancer bone metastases, using dominant negative blockade approach. Human MDA-MB-231 breast cancer clones stably expressing dominant negative p38alpha (p38/AF) exhibited decreased basal MMP-9 activity. TGF-beta1-induced MMP-9 activity was also blunted in these clones, as compared with controls in which TGF-betal up-regulated MMP-9 activity. Consistent with these findings, SB202190, a specific p38 inhibitor, also inhibited TGF-beta1-induced MMP-9 activity in parental cells. The p38/AF clones exhibited also reduced uPA production after growth on vitronectin and decreased cell motility, as compared with controls. VEGF production levels in all the studied clones were similar. The p38/AF clone, which had similar in vitro growth rate as the control pcDNA3 clone, formed significantly less bone metastases in a mouse model, as compared with the control clone. In conclusion, inhibition of the p38alpha pathway results in decreased MMP-9 activity, impaired uPA expression and decreased motility, all of which may contribute to the decreased formation of bone metastasis.

Phosphorylation of c-Fos by members of the p38 MAPK family. Role in the AP-1 response to UV light

Exposure to sources of UV radiation, such as sunlight, induces a number of cellular alterations that are highly dependent on its ability to affect gene expression. Among them, the rapid activation of genes coding for two subfamilies of proto-oncoproteins, Fos and Jun, which constitute the AP-1 transcription factor, plays a key role in the subsequent regulation of expression of genes involved in DNA repair, cell proliferation, cell cycle arrest, death by apoptosis, and tissue and extracellular matrix remodeling proteases. Besides being regulated at the transcriptional level, Jun and Fos transcriptional activities are also regulated by phosphorylation as a result of the activation of intracellular signaling cascades. In this regard, the phosphorylation of c-Jun by UV-induced JNK has been readily documented, whereas a role for Fos proteins in UV-mediated responses and the identification of Fos-activating kinases has remained elusive. Here we identify p38 MAPKs as proteins that can associate with c-Fos and phosphorylate its transactivation domain both in vitro and in vivo. This phosphorylation is transduced into changes in its transcriptional ability as p38-activated c-Fos enhances AP1-driven gene expression. Our findings indicate that as a consequence of the activation of stress pathways induced by UV light, endogenous c-Fos becomes a substrate of p38 MAPKs and, for the first time, provide evidence that support a critical role for p38 MAPKs in mediating stress-induced c-Fos phosphorylation and gene transcription activation. Using a specific pharmacological inhibitor for p38alpha and -beta, we found that most likely these two isoforms mediate UV-induced c-Fos phosphorylation in vivo. Thus, these newly described pathways act concomitantly with the activation of c-Jun by JNK/MAPKs, thereby contributing to the complexity of AP1-driven gene transcription regulation.