The role of p38 MAP kinase in TGF-beta1-induced signal transduction in human neutrophils

Transforming growth factor-beta1 induces apoptosis in vascular endothelial cells by activation of mitogen-activated protein kinase

BACKGROUND

Vascular endothelial cell apoptosis is central in atherosclerosis and intimal hyperplasia. Transforming growth factor (TGF)-beta1 induces endothelial cell apoptosis through unidentified mechanism(s). Although TGF-beta1 signals through the Smad proteins, in some nonendothelial cell types it also activates the mitogen-activated protein kinase (MAPK) (extracellular signal-regulated kinase, c-Jun N-terminal kinase, and p38 MAPK [p38(MAPK)]). p38(MAPK) relays apoptotic signals in several cell types. We hypothesized that TGF-beta1 activates endothelial cell MAPKs and induces apoptosis through p38(MAPK) activation.

METHODS

Human umbilical vein or bovine capillary endothelial cells were incubated with TGF-beta1 for 0.5 to 12 hours. MAPK activation was characterized by Western blotting with antibodies to phosphorylated extracellular signal-regulated kinase 1/2, p38(MAPK), or c-Jun N-terminal kinases 1/2. To study apoptosis, extracts of cells incubated with TGF-beta1 for 6 hours with or without MAPK inhibitors were characterized by Western blotting analysis of poly (ADP-Ribose) polymerase degradation.

RESULTS

TGF-beta1 induced p38(MAPK), extracellular signal-regulated kinase 1/2, and c-Jun N-terminal kinase 1/2 activation and increased apoptosis. Inhibition of p38(MAPK) significantly reduced TGF-beta1-induced apoptosis. In contrast, inhibition of other signaling pathways was ineffective.

CONCLUSIONS

TGF-beta1 induces endothelial cell apoptosis through p38(MAPK) activation. Because TGF-beta1 is upregulated in vascular remodeling, p38(MAPK) is a potential target to prevent endothelial cell apoptosis during this process.

p38 mitogen-activated protein kinase is required for TGFβ-mediated fibroblastic transdifferentiation and cell migration

Transforming growth factor β (TGFβ) contributes to tumor progression by inducing an epithelial to mesenchymal transdifferentiation(EMT) and cell migration. We found that TGFβ-induced EMT was blocked by inhibiting activation of p38 mitogen-activated protein kinase (MAPK) with H-7,a protein kinase C inhibitor, and with SB202190, a direct inhibitor of p38MAPK. Inhibition of the p38MAPK pathway affected TGFβ-mediated phosphorylation of ATF2, but did not inhibit phosphorylation of Smad2. SB202190 impaired TGFβ-mediated changes in cell shape and reorganization of the actin cytoskeleton. Forced expression of dominant-negative (DN) MAPK kinase 3 (MKK3) inhibited TGFβ-mediated activation of p38MAPK and EMT. Expression of DN-p38α impaired TGFβ-induced EMT. Inhibition of p38MAPK blocked TGFβ-induced migration of non-tumor and tumor mammary epithelial cells. TGFβ induced activation of the p38MAPK pathway within 15 minutes. Expression of TGFβ type II (TβRII) and type I(TβRI/Alk5) kinase-inactive receptors blocked EMT and activation of p38MAPK, whereas expression of constitutively active Alk5-T204D resulted in EMT and phosphorylation of MKK3/6 and p38MAPK. Finally, dominant-negative Rac1N17 blocked TGFβ-induced activation of the p38MAPK pathway and EMT,suggesting that Rac1 mediates activation of the p38MAPK pathway. These studies suggest that the p38MAPK pathway is required for TGFβ-mediated EMT and cell migration.

Cross-talk between ERK and p38 MAPK mediates selective suppression of pro-inflammatory cytokines by transforming growth factor-beta

Phagocytosis of apoptotic cells by macrophages results in the production of transforming growth factor-beta (TGF-beta), which plays an important role in induction of an anti-inflammatory phenotype and resolution of inflammation. In this study, we show that TGF-beta prevents pro-inflammatory cytokine production through inhibition of p38 mitogen-activated protein kinase (MAPK) and NF-kappaB. Blockade of extracellular signal-regulated kinase (ERK) signaling by the MEK-1/2 inhibitor PD 98059 reversed the inhibitory effects of TGF-beta, suggesting that cross-talk between MAPKs is essential for this response. Further investigation indicated that TGF-beta activated ERK, which in turn up-regulated MAPK phosphatase-1, thereby inactivating p38 MAPK. On the other hand, TGF-beta maintained or slightly increased production of the CC chemokine MCP-1, which is regulated predominantly by AP-1. Although SB 203580, an inhibitor of p38 MAPK, and dominant-negative p38 MAPK both increased AP-1 transcription, lack of effect of TGF-beta on lipopolysaccharide-stimulated SAPK/JNK phosphorylation along with a demonstrated inhibition of TGF-beta-induced AP-1 activation by dominant-negative Smad3 suggest that TGF-beta-stimulated AP-1 activation was not caused by inhibition of p38 MAPK but rather through the activation of Smads. Our data provide evidence that TGF-beta selectively inhibits inflammatory cytokine production through cross-talk between MAPKs.

Transforming growth factor-beta stabilizes elastin mRNA by a pathway requiring active Smads, protein kinase C-delta, and p38

Transforming growth factors (TGFs)-beta are multipotent in their biologic activity, regulating cell growth and differentiation as well as extracellular matrix deposition and degradation. Most of these activities involve modulation of gene transcription, but TGF-beta1 has been shown previously to substantially increase the expression of elastin by stabilization of tropoelastin mRNA through a signaling pathway that likely involves a phosphatidylcholine-specific phospholipase C, a protein kinase C, prenylated and acylated protein(s), and one or more tyrosine kinases. However, there is a 4- to 6-h lag period after the addition of TGF-beta1 before significant stimulation of elastin expression is observed and the question of whether the Smads are involved has not been addressed. In the present work, using cultured human fetal lung fibroblasts, we show through the use of specific inhibitors and transfection of a Smad 7 construct that in addition to de novo protein synthesis and active Smads, the extended activity of protein kinase C (PKC)-delta and the stress-activated protein kinase, p38, is required for TGF-beta1 to achieve elastin mRNA stabilization.

MAP kinase pathways involving hsp27 regulate fibroblast-mediated wound contraction

BACKGROUND

Heat shock protein 27 (hsp27) has been shown to modulate actin arrays in a manner dependent on its phosphorylation status. Hsp27 is phosphorylated by mitogen-activated protein kinase-activated protein kinase 2/3, which is regulated by mitogen-activated protein (MAP) kinases. We hypothesize that hsp27 phosphorylation modulates wound contraction.

MATERIALS AND METHODS

In these studies, a specific p38 MAP kinase inhibitor, SB203580, and a specific MAPK/extracellular signal-regulated kinase kinase 1,2 inhibitor, PD98059, were used to inhibit kinase activity. The effect of MAP kinase inhibitors was tested using a tissue culture model, the fibroblast-populated collagen lattice (FPCL) contraction assay, and a rat full-thickness skin defect model of wound healing. Hsp27 phosphorylation status was determined by isoelectric focus and Western blot analysis.

RESULTS

We show here that hsp27 phosphorylation correlates with FPCL contraction and with contraction in vivo. In the tissue culture model, each inhibitor reduced FPCL contraction and hsp27 phosphorylation. Hsp27 phosphorylation correlated with both p38 and ERK1, 2 activation. Hsp27 was highly phosphorylated in the wound edge during wound healing in a rat in vivo model. The phosphorylation status was highest in the granulation tissue. Treatment with both kinase inhibitors significantly delayed wound contraction in vivo, which correlated with inhibition of hsp27 phosphorylation.

CONCLUSIONS

This study demonstrates that ERK and p38 kinase cascades play important roles in wound contraction. Additionally, these data implicate hsp27 as being a key molecule in modulating the effects of these kinases.

Signaling events required for transforming growth factor-beta stimulation of connective tissue growth factor expression by cultured human lung fibroblasts

It is possible that many of the fibrogenic effects of transforming growth factor-beta (TGF-beta) are mediated by connective tissue growth factor (CTGF). In the present work, we show that TGF-beta1 produces a 5- to 6-fold increase in CTGF expression by cultured human lung fibroblasts that is due mainly to increased transcription. The half-life of CTGF mRNA is 1.96 h, consistent with its role as a cytokine. In addition to requiring Smad activity, based upon the effects of specific inhibitors, the TGF-beta intracellular signaling pathway requires the activity of a phosphatidylcholine-specific phospholipase C, a protein kinase C, and one or more tyrosine kinases. It is also likely that the pathway requires a member of the Ras superfamily of small GTPases, but not trimeric G proteins. Pharmacologic inhibition of TGF-beta stimulation of CTGF expression may be an effective therapeutic approach to a variety of undesirable fibrotic reactions.

Involvement of p38 MAP kinase in TGF-beta-stimulated VEGF synthesis in aortic smooth muscle cells

Although it is known that transforming growth factor (TGF)-beta induces vascular endothelial growth factor (VEGF) synthesis in vascular smooth muscle cells, the underlying mechanisms are still poorly understood. In the present study, we examined whether the mitogen-activated protein (MAP) kinase superfamily is involved in TGF-beta-stimulated VEGF synthesis in aortic smooth muscle A10 cells. TGF-beta stimulated the phosphorylation of p42/p44 MAP kinase and p38 MAP kinase, but not that of SAPK (stress-activated protein kinase)/JNK (c-Jun N-terminal kinase). The VEGF synthesis induced by TGF-beta was not affected by PD98059 or U0126, specific inhibitors of the upstream kinase that activates p42/p44 MAP kinase. We confirmed that PD98059 or U0126 did actually suppress the phosphorylation of p42/p44 MAP kinase by TGF-beta in our preparations. PD169316 and SB203580, specific inhibitors of p38 MAP kinase, significantly reduced the TGF-beta-stimulated synthesis of VEGF (each in a dose-dependent manner). PD169316 or SB203580 attenuated the TGF-beta-induced phosphorylation of p38 MAP kinase. These results strongly suggest that p38 MAP kinase plays a part in the pathway by which TGF-beta stimulates the synthesis of VEGF in aortic smooth muscle cells.

Abnormal migration phenotype of mitogen-activated protein kinase-activated protein kinase 2-/- neutrophils in Zigmond chambers containing formyl-methionyl-leucyl-phenylalanine gradients

Time-lapsed video microscopy and confocal imaging were used to study the migration of wild-type (WT) and mitogen-activated protein kinase-activated protein kinase 2 (MK2-/-) mouse neutrophils in Zigmond chambers containing fMLP gradients. Confocal images of polarized WT neutrophils showed an intracellular gradient of phospho-MK2 from the anterior to the posterior region of the neutrophils. Compared with WT neutrophils, MK2-/- neutrophils showed a partial loss of directionality but higher migration speed. Immunoblotting experiments showed a lower protein level of p38 mitogen-activated protein kinase and a loss of fMLP-induced extracellular signal-related kinase phosphorylation in MK2-/- neutrophils. These results suggest that MK2 plays an important role in the regulation of neutrophil migration and may also affect other signaling molecules.

Stem cell factor-induced migration of mast cells requires p38 mitogen-activated protein kinase activity

Stem cell factor (SCF) can be considered a cardinal cytokine in mast cell biology as it affects mast cell differentiation, survival, and migration. The objective of this study was to investigate the role of two mitogen-activated protein (MAP) kinases, extracellular signal-regulated kinase (ERK) and p38, in SCF-induced cell migration. This was examined in mouse mast cells by using PD 098059 and SB203580, which are specific inhibitors of mitogen-induced extracellular kinase (MEK) and p38 MAP kinase, respectively. SCF induced a rapid and transient activation of ERK and p38 in a dose-dependent manner. Inhibition of p38 activity by SB203580 was paralleled with a marked reduction of migration toward SCF, whereas the effect of the MEK inhibitor was less pronounced. This is the first report of a physiological function of SCF-dependent activation of p38. Whether p38-mediated mast cell migration is a possible target for suppression of mast cell hyperplasia remains to be determined.

Smad4 mediates activation of mitogen-activated protein kinases by TGF-beta in pancreatic acinar cells

Transforming growth factor-beta (TGF-beta) inhibits pancreatic acinar cell growth. In many cell types, TGF-beta mediates its growth inhibitory effects by activation of Smad proteins. Recently, it has been reported that Smad proteins may interact with the mitogen-activated protein (MAP) kinase signaling pathways. In this study, we report on the interactions between the TGF-beta and MAP kinase signaling pathways in isolated rat pancreatic acinar cells. TGF-beta activated the MAP kinases extracellular signal-related kinases (ERKs) and p38 in pancreatic acinar cells, but had no effect on c-jun NH2-terminal kinase activity. Activation of MAP kinase by TGF-beta was maximal 4 h after treatment. The ability of TGF-beta to activate ERKs was concentration dependent and dependent on protein synthesis. TGF-beta's stimulation of ERK activation was blocked by PD-98059, an inhibitor of MAP kinase kinase 1, and by adenoviral transfer of dominant negative RasN17. Furthermore, adenoviral-mediated expression of dominant negative Smad4 blocked the ability of TGF-beta to activate acinar cell MAP kinase, demonstrating that this activation is downstream of Smads. The biological relevance of ERK activation by TGF-beta was indicated by demonstrating that inhibition of ERK signaling by PD-98059 blocked the ability of TGF-beta to activate the transcription factor activator protein-1. These studies provide new insight into the signaling mechanisms by which TGF-beta mediates biological actions in pancreatic acinar cells.

Activation of p38MAPK by TGF-beta in fetal rat hepatocytes requires radical oxygen production, but is dispensable for cell death

We have previously found that transforming growth factor-beta (TGF-beta) induces an increase in radical oxygen species (ROS) production that mediates its apoptotic effects in fetal hepatocytes. In this paper we show that TGF-beta activates p38 mitogen-activated protein kinase (p38MAPK) and ROS may be responsible for this activation. Activation of p38MAPK occurs late, coincident with the maximal production of ROS, it is inhibited by radical scavengers and it is accentuated by the presence of glutathione synthesis inhibitors. However, p38MAPK does not appear to be involved in any of the apoptotic events: loss of Bcl-x(L) levels, cytochrome c release, cleavage of caspase substrates and loss of cell viability.

Smad-mediated transcription is required for transforming growth factor-beta 1-induced p57(Kip2) proteolysis in osteoblastic cells

Cyclin-dependent kinase inhibitory proteins (CKIs) are negative regulators of the cell cycle. Of all CKIs, only p57(Kip2) plays an essential role(s) that other CKIs cannot compensate for in embryonic development. Recently, we found that p57(Kip2) is degraded through the ubiquitin-proteasome pathway in osteoblastic cells stimulated to proliferation by transforming growth factor (TGF)-beta1 (Urano, T., Yashiroda, H., Muraoka, M., Tanaka, K., Hosoi, T., Inoue, S., Ouchi, Y., and Toyoshima, H. (1999) J. Biol. Chem. 274, 12197-12200). We report here that TGF-beta1-induced p57(Kip2) proteolysis is mediated through transcription by the Smad pathway. When the constitutively active form of the TGF-beta type I receptor ALK-5(TD) was ectopically expressed in osteoblastic cells, p57(Kip2) that had been accumulated by serum starvation causing the cell-cycle arrest was rapidly degraded in a manner analogous to TGF-beta1 stimulation. Moreover, Smad2 or Smad3 with Smad4 enhanced the proteolytic pathway of p57(Kip2). The degradation of p57(Kip2) evoked by TGF-beta1 was blocked by forced expression of an inhibitory Smad called Smad7 or by the addition of actinomycin D or alpha-amanitin. These results indicate that accelerated degradation of p57(Kip2) by TGF-beta1/Smad signaling is mediated through a newly synthesized factor(s) that modifies p57(Kip2) or the ubiquitin-proteasome pathway.

Stimulation of pro-alpha(1)(I) collagen by TGF-beta(1) in mesangial cells: role of the p38 MAPK pathway

Transforming growth factor-beta(1) (TGF-beta(1)) is a potent inducer of extracellular matrix protein synthesis and a key mediator of renal fibrosis. However, the intracellular signaling mechanisms by which TGF-beta(1) stimulates this process remain incompletely understood. In this report, we examined the role of a major stress-activated intracellular signaling cascade, belonging to the mitogen-activated protein kinase (MAPK) superfamily, in mediating TGF-beta(1) responses in rat glomerular mesangial cells, using dominant-negative inhibition of TGF-beta(1) signaling receptors. We first stably transfected rat glomerular mesangial cells with a kinase-deleted mutant TGF-beta type II receptor (TbetaR-II(M)) designed to inhibit TGF-beta(1) signaling in a dominant-negative fashion. Next, expression of TbetaR-II(M) mRNA was confirmed by Northern analysis. Cell surface expression and ligand binding of TbetaR-II(M) protein were demonstrated by affinity cross-linking with (125)I-labeled-TGF-beta(1). TGF-beta(1) rapidly induced p38 MAPK phosphorylation in wild-type and empty vector (pcDNA3)-transfected control mesangial cells. Interestingly, transfection with dominant-negative TbetaR-II(M) failed to block TGF-beta(1)-induced p38 MAPK phosphorylation. Moreover, dominant-negative TbetaR-II(M) failed to block TGF-beta(1)-stimulated pro-alpha(1)(I) collagen mRNA expression and cellular protein synthesis, whereas TGF-beta(1)-induced extracellular signal-regulated kinase (ERK) 1/ERK2 activation and antiproliferative responses were blocked by TbetaR-II(M). In the presence of a specific inhibitor of p38 MAPK, SB-203580, TGF-beta(1) was unable to stimulate pro-alpha(1)(I) collagen mRNA expression in the control and TbetaR-II(M)-transfected mesangial cells. Finally, we confirmed that both p38 MAPK activation and pro-alpha(1)(I) collagen stimulation were TGF-beta(1) effects that were abrogated by dominant-negative inhibition of TGF-beta type I receptor. Thus we show first demonstration of p38 MAPK activation by TGF-beta(1) in mesangial cells, and, given the rapid kinetics, this TGF-beta(1) effect is likely a direct one. Furthermore, our findings suggest that the p38 MAPK pathway functions as a component in the signaling of pro-alpha(1)(I) collagen induction by TGF-beta(1) in mesangial cells.

The decorin high glucose response element and mechanism of its activation in human mesangial cells

The decorin gene encodes a proteoglycan with putative structural and regulatory functions whose expression is markedly increased in human mesangial cells (HMC) exposed to high concentrations of glucose (15 to 30 mM). The gene has two promoters (P1 and P2) upstream of two alternative first exons. Transcripts driven by both promoters are present in HMC maintained in 4 mM D-glucose medium. After exposure to 30 mM D-glucose for 7 to 21 d, transcripts driven by P1 are markedly increased, whereas those driven by P2 decrease. Culture in 4 mM D-glucose medium containing transforming growth factor-beta1 (TGF-beta1) (1.25 ng/ml) has the same effect. However, addition of an excess of TGF-beta neutralizing antibody to the 30 mM D-glucose cultures only partly suppressed increased decorin transcription from P1. In transformed HMC transfected with a reporter (p-SAEP) driven by P1 or P2, P1 activity increased twofold on treatment with either 30 mM D-glucose or TGF-beta1 in 4 mM medium. P2 had little activity under any conditions. 5' deletion of P1 showed that basal transcriptional activity lies within the proximal 378 bp, while the major high glucose and TGF-beta response element is located in the -683 to -583-bp region. A putative cAMP response-like sequence (TGACGTTT) lies within this region. Electrophoretic mobility shift assays revealed the same pattern of multiple complexes between oligonucleotides containing this sequence and nuclear proteins extracted from HMC maintained in either 4 or 30 mM D-glucose conditions, but the latter were more prominent. cAMP response element binding protein (CREB) was identified as one transcription factor forming these complexes but other factors remain unidentified. Increased levels of phospho-(Ser 133) CREB were found in HMC exposed to 30 mM D-glucose. High glucose also activated and led to nuclear translocation of p42/44 mitogen-activated protein kinase and p38 mitogen-activated protein kinase, both of which can activate CREB by phosphorylation of serine 133.

Role of mitogen-activated protein kinases as downstream effectors of transforming growth factor-beta in mesangial cells

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine that regulates cell proliferation, differentiation, and production of extracellular matrix proteins in various types of cells including mesangial cells. Although TGF-beta has been also known as an important player in the pathogenesis of various fibrotic diseases including glomerulosclerosis, signal-transduction cascades of TGF-beta have remained to be clarified. However, emerging evidence indicates that TGF-beta can activate various signal transduction cascades such as Smad proteins and mitogen-activated protein kinases (MAPKs) in many types of cells. Here, we examine the role of MAPKs in TGF-beta-induced gene expression of extracellular matrix proteins in mesangial cells. TGF-beta increases extracellular signal-regulated kinase (ERK) activity, one of the MAPKs, and the expression of fibronectin mRNA and protein in rat mesangial cells. Furthermore, PD98059, a specific inhibitor of MAPK/ERK kinase (MEK), can inhibit this TGF-beta-induced fibronectin expression. These data suggest that MAPKs play an important role in TGF-beta-mediated extracellular matrix production in mesangial cells.

Differential role of p38 and c-Jun N-terminal kinase 1 mitogen-activated protein kinases in NK cell cytotoxicity

The serine-threonine mitogen-activated protein kinase (MAPK) family includes extracellular signal-regulated kinases (ERK), c-Jun N-terminal kinases (JNK), and p38 kinases. In NK cells, spontaneous or Ab-mediated recognition of target cells leads to activation of an ERK-2 MAPK-dependent biochemical pathway(s) involved in the regulation of NK cell effector functions. Here we assessed the roles of p38 and JNK MAPK in NK cell-mediated cytotoxicity. Our data indicate that p38 is activated in primary human NK cells upon stimulation with immune complexes and interaction with NK-sensitive target cells. FcgammaRIIIA-induced granule exocytosis and both spontaneous and Ab-dependent cytotoxicity were reduced in a dose-dependent manner in cells pretreated with either of two specific inhibitors of this kinase. Target cell-induced IFN-gamma and FcgammaRIIIA-induced TNF-alpha mRNA accumulation was similarly affected under the same conditions. Lack of inhibition of NK cell cytotoxicity in cells overexpressing an inactive form of JNK1 indicates that this kinase, activated only upon FcgammaRIIIA ligation, does not play a significant role in cytotoxicity. These data underscore the involvement of p38, but not JNK1, in the molecular mechanisms regulating NK cell cytotoxicity.

Lysophosphatidylcholine activates p38 and p42/44 mitogen-activated protein kinases in monocytic THP-1 cells, but only p38 activation is involved in its stimulated chemotaxis

Oxidized LDLs (OxLDLs) have been shown to be involved in recruitment of blood monocytes into the arterial subendothelial space, which is the earliest step in atherogenesis, but the underlying molecular mechanisms are poorly understood. The present study demonstrated that lysophosphatidylcholine (LPC), a major phospholipid component of OxLDL, strongly evoked phosphorylation and activation of p38 and p42/44 mitogen-activated protein kinases in monocytic cells. The stimulation of p38 and p42/44 occurred in a dose- and time-dependent manner, reaching the maximal activation at 25 microg/mL LPC within 5 minutes. Interestingly, inhibition of p38 activation by OxLDL or LPC, using its selective inhibitors (SB203580 and SKF86002), completely blocked OxLDL- or LPC-stimulated chemotaxis of THP-1 cells, which was measured in a transwell chemotaxis assay. In contrast, inhibition of p42/44 activation by its potent inhibitor (PD98059) did not block OxLDL- or LPC-stimulated chemotaxis. Moreover, expression of a p38 dominant-negative mutant (p38AF) reduced cell chemotaxis significantly. In addition, activation of p38 by LPC was apparently mediated neither by scavenger receptors nor by tyrosine kinase receptors. It was, however, effectively blocked by pertussis toxin and substantially reduced by phospholipase C inhibitor (U73122) and phosphatidylinositol 3-kinase inhibitors (wortmannin and LY294002). LPC also inhibited forskolin-stimulated cAMP accumulation in a pertussis toxin-sensitive manner, indicating that Gi/Go proteins likely mediated the effects of LPC. Our results suggested that OxLDL/LPC efficiently activated both p38 and p42/44, but only the activation of p38 was functionally associated with OxLDL-/LPC-induced chemotaxis in THP-1 cells.

Transforming growth factor beta 1 stimulates expression of the Epstein-Barr virus BZLF1 immediate-early gene product ZEBRA by an indirect mechanism which requires the MAPK kinase pathway

Disruption of Epstein-Barr virus (EBV) latency is mediated by ZEBRA, the protein product of the immediate-early EBV gene, BZLF1. In vitro, phorbol 12-myristate 13-acetate (PMA), a potent activator of protein kinase C (PKC), induces reactivation of EBV. However, the physiological stimuli responsible for the disruption of viral latency are not well characterized. Transforming growth factor beta 1 (TGF-beta1) has also been shown to trigger the reactivation of EBV in Burkitt lymphoma cell lines; however, the effect of TGF-beta1 on ZEBRA expression has not been reported. To further understand this phenomenon, we have investigated the effect of TGF-beta1 on ZEBRA expression. Our results indicate that the treatment of different EBV-positive Burkitt's lymphoma cell lines with TGF-beta1 induces a time-dependent activation of BZLF1 transcription with a corresponding increase in the production of the protein ZEBRA. TGF-beta1 has been shown to exert its effects through a wide range of intracellular routes; in the present study, we have explored these pathways. Transient expression of Smad proteins on their own had no effect on ZEBRA expression. A specific inhibitor of p38 mitogen-activated protein kinase (MAPK), SB203580, did not affect TGF-beta1-induced ZEBRA expression, whereas treatment with the MAPK/ERK kinase inhibitors, PD98059 and U0126, dramatically decreased this induction. This suggests that TGF-beta1 effect on BZLF1 expression requires the MAPK pathway. However, in Raji and B95-8 cells additional routes can be used, as (i) the inhibition of ZEBRA induction by PD98059 or U0126 was incomplete, whereas these inhibitors completely abolished PMA-induced ZEBRA expression, (ii) TGF-beta1 induction of ZEBRA expression occurs in PKC-depleted cells, (iii) in Raji and in B95-8 cells, the effect of TGF-beta1 and PMA are additive. Transient transfection of the EBV-negative B-cell line DG75 with a BZLF1 promoter-fusion construct (Zp-CAT) showed that under conditions where the BZLF1 promoter is activated by PMA treatment, TGF-beta1 had no significant effect on the expression of the chloramphenicol acetyltransferase gene. Furthermore, TGF-beta1 induction of BZLF1 transcripts is dependent on de novo protein synthesis, which suggests that TGF-beta1 induces BZLF1 expression by an indirect mechanism.

L-selectin signaling of neutrophil adhesion and degranulation involves p38 mitogen-activated protein kinase

The adhesion molecules known as selectins mediate the capture of neutrophils from the bloodstream. We have previously reported that ligation and cross-linking of L-selectin on the neutrophil surface enhances the adhesive function of beta(2)-integrins in a synergistic manner with chemotactic agonists. In this work, we examined degranulation and adhesion of neutrophils in response to cross-linking of L-selectin and addition of interleukin-8. Cross-linking of L-selectin induced priming of degranulation that was similar to that observed with the alkaloid cytochalasin B. Activation mediated by L-selectin of neutrophil shape change and adhesion through CD11b/CD18 were strongly blocked by Merck C, an imidazole-based inhibitor of p38 mitogen-activated protein kinase (MAPK), but not by a structurally similar non-binding regioisomer. Priming by L-selectin of the release of secondary, tertiary, and secretory, but not primary, granules was blocked by inhibition of p38 MAPK. Peak phosphorylation of p38 MAPK was observed within 1 min of cross-linking L-selectin, whereas phosphorylation of ERK1/2 was highest at 10 min. Phosphorylation of p38 MAPK, but not ERK1/2, was inhibited by Merck C. These data suggest that signal transduction as a result of clustering L-selectin utilizes p38 MAPK to effect neutrophil shape change, integrin activation, and the release of secondary, tertiary, and secretory granules.

5-lipoxygenase is phosphorylated by p38 kinase-dependent MAPKAP kinases

5-lipoxygenase (5-LO) catalyzes the initial steps in the formation of leukotrienes, a group of inflammatory mediators derived from arachidonic acid (AA). Here we describe that activation of p38 mitogen-activated protein kinase in human polymorphonuclear leukocytes and in Mono Mac 6 cells leads to activation of downstream kinases, which can subsequently phosphorylate 5-LO in vitro. Different agents activated the 5-LO kinase activities, including stimuli for cellular leukotriene biosynthesis (A23187, thapsigargin, N-formyl-leucyl-phenylalanine), compounds that up-regulate the capacity for leukotriene biosynthesis (phorbol 12-myristate 13-acetate, tumor necrosis factor alpha, granulocyte/macrophage colony-stimulating factor), and well known p38 stimuli as sodium arsenite and sorbitol. For all stimuli, 5-LO kinase activation was counteracted by SB203580 (3 microM or less), an inhibitor of p38 kinase. At least two p38-dependent 5-LO kinase activities were found. Based on migration properties in in-gel kinase assays and immunoreactivity, one of these was identified as mitogen-activated protein kinase-activated protein kinase 2 (MAPKAP kinase 2). The other appeared to be MAPKAP kinase 3; however, it could not be excluded that also other p38-dependent kinases contributed. When polymorphonuclear leukocytes were incubated with sodium arsenite (strong activator of 5-LO kinases), platelet-activating factor and exogenous AA, there was a 4-fold increase in 5-LO activity as compared with incubations with only platelet-activating factor and AA. This indicates that 5-LO phosphorylation can be one factor determining cellular 5-LO activity.

Inhibition of p38 MAP kinase as a therapeutic strategy

Since the discovery of p38 MAP kinase in 1994, our understanding of its biology has progressed dramatically. The key advances include (1) identification of p38 MAP kinase homologs and protein kinases that act upstream and downstream from p38 MAP kinase, (2) identification of interesting and potentially important substrates, (3) elucidation of the role of p38 MAP kinase in cellular processes and (4) the establishment of the mechanism by which the pyridinylimidazole p38 MAP kinase inhibitors inhibit enzyme activity. It is now known that there are four members of the p38 MAP kinase family. They differ in their tissue distribution, regulation of kinase activation and subsequent phosphorylation of downstream substrates. They also differ in terms of their sensitivities toward the p38 MAP kinase inhibitors. The best-studied isoform is p38 alpha, whose activation has been observed in many hematopoietic and non-hematopoietic cell types upon treatment with appropriate stimuli. The pyridinylimidazole compounds, exemplified by SB 203580, were originally prepared as inflammatory cytokine synthesis inhibitors that subsequently were found to be selective inhibitors of p38 MAP kinase. SB 203580 inhibits the catalytic activity of p38 MAP kinase by competitive binding in the ATP pocket. X-ray crystallographic studies of the target enzyme complexed with inhibitor reinforce the observations made from site-directed mutagenesis studies, thereby providing a molecular basis for understanding the kinase selectivity of these inhibitors. The p38 MAP kinase inhibitors are efficacious in several disease models, including inflammation, arthritis and other joint diseases, septic shock, and myocardial injury. In all cases, p38 activation in key cell types correlated with disease initiation and progression. Treatment with p38 MAP kinase inhibitors attenuated both p38 activation and disease severity. Structurally diverse p38 MAP kinase inhibitors have been tested extensively in preclinical studies.

Vascular endothelial growth factor (VEGF)-driven actin-based motility is mediated by VEGFR2 and requires concerted activation of stress-activated protein kinase 2 (SAPK2/p38) and geldanamycin-sensitive phosphorylation of focal adhesion kinase

In endothelial cells, vascular endothelial growth factor (VEGF) induces an accumulation of stress fibers associated with new actin polymerization and rapid formation of focal adhesions at the ventral surface of the cells. This cytoskeletal reorganization results in an intense motogenic activity. Using porcine endothelial cells expressing one or the other type of the VEGF receptors, VEGFR1 or VEGFR2, or human umbilical vein endothelial cells pretreated with a VEGFR2 neutralizing antibody, we show that VEGFR2 is responsible for VEGF-induced activation of the stress-activated protein kinase-2/p38 (SAPK2/p38), phosphorylation of focal adhesion kinase (FAK), and enhanced migratory activity. Activation of SAPK2/p38 triggered actin polymerization whereas FAK, which was phosphorylated independently of SAPK2/p38, initiated assembly of focal adhesions. Both processes contributed to the formation of stress fibers. Geldanamycin, an inhibitor of HSP90 blocked tyrosine phosphorylation of FAK, assembly of focal adhesions, actin reorganization, and cell migration, all of which were reversed by overexpressing HSP90. We conclude that VEGFR2 mediates the physiological effect of VEGF on cell migration and that two independent pathways downstream of VEGFR2 regulate actin-based motility. One pathway involves SAPK2/p38 and leads to enhanced actin polymerization activity. The other involves HSP90 as a permissive signal transduction factor implicated in FAK phosphorylation and assembly of focal adhesions.

MAP kinase pathways activated by stress: the p38 MAPK pathway

A stress-activated serine/threonine protein kinase, p38 mitogen-activated protein kinase (p38 MAPK), belongs to the MAP kinase superfamily. Diverse extracellular stimuli, including ultraviolet light, irradiation, heat shock, high osmotic stress, proinflammatory cytokines and certain mitogens, trigger a stress-regulated protein kinase cascade culminating in activation of p38 MAPK through phosphorylation on a TGY motif within the kinase activation loop. p38 MAPK appears to play a major role in apoptosis, cytokine production, transcriptional regulation, and cytoskeletal reorganization, and has been causally implicated in sepsis, ischemic heart disease, arthritis, human immunodeficiency virus infection, and Alzheimer's disease. The availability of specific inhibitors helps to clarify the role that p38 MAPK plays in these processes, and may ultimately offer therapeutic benefit for certain critically ill patients.

Adult human mesenchymal stem cell differentiation to the osteogenic or adipogenic lineage is regulated by mitogen-activated protein kinase

Adult human mesenchymal stem cells are primary, multipotent cells capable of differentiating to osteocytic, chondrocytic, and adipocytic lineages when stimulated under appropriate conditions. To characterize the molecular mechanisms that regulate osteogenic differentiation, we examined the contribution of mitogen-activated protein kinase family members, ERK, JNK, and p38. Treatment of these stem cells with osteogenic supplements resulted in a sustained phase of ERK activation from day 7 to day 11 that coincided with differentiation, before decreasing to basal levels. Activation of JNK occurred much later (day 13 to day 17) in the osteogenic differentiation process. This JNK activation was associated with extracellular matrix synthesis and increased calcium deposition, the two hallmarks of bone formation. Inhibition of ERK activation by PD98059, a specific inhibitor of the ERK signaling pathway, blocked the osteogenic differentiation in a dose-dependent manner, as did transfection with a dominant negative form of MAP kinase kinase (MEK-1). Significantly, the blockage of osteogenic differentiation resulted in the adipogenic differentiation of the stem cells and the expression of adipose-specific mRNAs peroxisome proliferator-activated receptor gamma2, aP2, and lipoprotein lipase. These observations provide a potential mechanism involving MAP kinase activation in osteogenic differentiation of adult stem cells and suggest that commitment of hMSCs into osteogenic or adipogenic lineages is governed by activation or inhibition of ERK, respectively.

Cyclosporin A induces an atypical heat shock response

Cyclosporin A is a widely used immunosuppressive drug having toxic side effects, in particular on kidneys and liver, as a result of its action on different molecular targets. Here we demonstrate that low doses of CsA are able to induce the expression of the heat shock protein HSP27 and its hyperphosphorylation. It also activates the two heat shock transcription factors, HSF1 and HSF2. Since these factors have been shown to be activated by proteasome inhibition, we tested the hypothesis that the inhibitory action of CsA on the proteasome might be responsible for the activation of HSFs and the subsequent expression of HSP27. The increase in multiubiquitinated proteins as well as the stabilization of p53 following CsA addition argues in favor of this hypothesis. The kidney BSC-1 cells are highly responsive to the addition of CsA: the possible link between HSP27 induction and hyperphosphorylation and nephrotoxicity is discussed.

TGF-beta1 stimulation of fibronectin transcription in cultured human lung fibroblasts requires active geranylgeranyl transferase I, phosphatidylcholine-specific phospholipase C, protein kinase C-delta, and p38, but not erk1/erk2

The cytokine transforming growth factor-beta (TGF-beta) has multiple effects on a variety of cell types, modulating cell growth and differentiation as well as extracellular matrix deposition and degradation. In the present work, we demonstrate that TGF-beta1 produces a fourfold increase in transcription of the fibronectin gene in cultured human fetal lung fibroblasts with only a small increase in mRNA stability resulting in a significant increase in fibronectin mRNA steady state level. A corresponding increase in production of fibronectin protein accompanied the increase in mRNA. Through the use of specific inhibitors, we demonstrate that geranylgeranylated, but not farnesylated or acylated protein(s), protein kinase C-delta, phosphatidylcholine-specific phospholipse C, tyrosine kinase activity, and stress-activated protein kinase p38 are required for this TGF-beta1 effect. Trimeric G proteins and mitogen-activated protein kinases erk1 and erk2 do not appear to be involved. While these results emphasize the complexities involved in the control of extracellular matrix synthesis by TGF-beta, they also identify reaction sites that may be amenable to pharmacologic modulation. Such modulation could be of great advantage in the treatment of a wide variety of undesirable fibrotic reactions.

Roles of bone morphogenetic protein type I receptors and Smad proteins in osteoblast and chondroblast differentiation

The biological effects of type I serine/threonine kinase receptors and Smad proteins were examined using an adenovirus-based vector system. Constitutively active forms of bone morphogenetic protein (BMP) type I receptors (BMPR-IA and BMPR-IB; BMPR-I group) and those of activin receptor-like kinase (ALK)-1 and ALK-2 (ALK-1 group) induced alkaline phosphatase activity in C2C12 cells. Receptor-regulated Smads (R-Smads) that act in the BMP pathways, such as Smad1 and Smad5, also induced the alkaline phosphatase activity in C2C12 cells. BMP-6 dramatically enhanced alkaline phosphatase activity induced by Smad1 or Smad5, probably because of the nuclear translocation of R-Smads triggered by the ligand. Inhibitory Smads, i.e., Smad6 and Smad7, repressed the alkaline phosphatase activity induced by BMP-6 or the type I receptors. Chondrogenic differentiation of ATDC5 cells was induced by the receptors of the BMPR-I group but not by those of the ALK-1 group. However, kinase-inactive forms of the receptors of the ALK-1 and BMPR-I groups blocked chondrogenic differentiation. Although R-Smads failed to induce cartilage nodule formation, inhibitory Smads blocked it. Osteoblast differentiation induced by BMPs is thus mediated mainly via the Smad-signaling pathway, whereas chondrogenic differentiation may be transmitted by Smad-dependent and independent pathways.

p38 MAPK signalling cascades in inflammatory disease

Inflammatory mediators released during acute and chronic diseases activate multiple intracellular signalling cascades including the mitogen-activated protein kinase (MAPK) signal transduction pathway, which plays a significant role in the recruitment of leukocytes to sites of inflammation. Stimulation of leukocytes by pro-inflammatory cytokines is known to result in the activation of the MAPK isoform p38. However, the functional consequences of p38 MAPK activation during leukocyte recruitment, including adhesion, migration and effector functions such as oxidative burst and degranulation, are only just beginning to be elucidated. Specific p38 inhibitors aimed at reducing the production of inflammatory mediators are now being developed, and might in the future provide more effective treatment for inflammatory diseases.

C-reactive protein inhibits chemotactic peptide-induced p38 mitogen-activated protein kinase activity and human neutrophil movement

Serum levels of the acute-phase reactant, C-reactive protein (CRP), increase dramatically during acute inflammatory episodes. CRP inhibits migration of neutrophils toward the chemoattractant, f-Met-Leu-Phe (fMLP) and therefore acts as an anti-inflammatory agent. Since tyrosine kinases are involved in neutrophil migration and CRP has been shown to decrease phosphorylation of some neutrophil proteins, we hypothesized that CRP inhibits neutrophil chemotaxis via inhibition of MAP kinase activity. The importance of p38 MAP kinase in neutrophil movement was determined by use of the specific p38 MAP kinase inhibitor, SB203580. CRP and SB203580 both blocked random and fMLP-directed neutrophil movement in a concentration-dependent manner. Additionally, extracellular signal-regulated MAP kinase (ERK) was not involved in fMLP-induced neutrophil movement as determined by use of the MEK-specific inhibitor, PD98059. Blockade of ERK with PD98059 did not inhibit chemotaxis nor did it alter the ability of CRP or SB203580 to inhibit fMLP-induced chemotaxis. More importantly, CRP inhibited fMLP-induced p38 MAP kinase activity in a concentration-dependent manner as measured by an in vitro kinase assay. Impressively, CRP-mediated inhibition of p38 MAP kinase activity correlated with CRP-mediated inhibition of fMLP-induced chemotaxis (r = -0.7144). These data show that signal transduction through p38 MAP kinase is necessary for neutrophil chemotaxis and that CRP intercedes through this pathway in inhibiting neutrophil movement.

TGF-beta induces fibronectin synthesis through a c-Jun N-terminal kinase-dependent, Smad4-independent pathway

Transforming growth factor-beta (TGF-beta) exerts its effects on cell proliferation, differentiation and migration in part through its modulation of extracellular matrix components, such as fibronectin and plasminogen activator inhibitor-1 (PAI-1). Although the SMAD family of proteins recently has been shown to be a key participant in TGF-beta signaling, other signaling pathways have also been shown to be activated by TGF-beta. We report here that c-Jun N-terminal kinase (JNK), a member of the MAP kinase family, is activated in response to TGF-beta in the human fibrosarcoma HT1080-derived cell line BAHgpt. Stable expression of dominant-negative forms of JNK1 and MKK4, an upstream activator of JNK, results in loss of TGF-beta-stimulated fibronectin mRNA and protein induction, while having little effect on TGF-beta-induced levels of PAI-1. The human fibronectin promoter contains three CRE elements, one of which has been shown to bind a c-Jun-ATF-2 heterodimer. Utilizing a GAL4 fusion trans-reporting system, we demonstrate a decrease in transactivating potential of GAL4-c-Jun and GAL4-ATF-2 in dominant-negative JNK1- and MKK4-expressing cells. Finally, we show that TGF-beta-induced fibronectin synthesis is independent of Smad4. These results demonstrate that TGF-beta-mediated fibronectin induction requires activation of JNK which in turn modulates the activity of c-Jun and ATF-2 in a Smad4independent manner.