Transcription factor AP-1 regulation by mitogen-activated protein kinase signal transduction pathways

The concept of glomerular self-defense

The balance between local offense factors and defense machinery determines the fate of tissue injury: progression or resolution. In glomerular research, the most interest has been on the offensive side, for example, the roles of leukocytes, platelets, complement, cytokines, eicosanoids, and oxygen radical intermediates. There has been little focus on the defensive side, which is responsible for the attenuation and resolution of disease. The aim of this review is to address possible mechanisms of local defense that may be exerted during glomerular injury. Cytokine inhibitors, proteinase inhibitors, complement regulatory proteins, anti-inflammatory cytokines, anti-inflammatory eicosanoids, antithrombotic molecules, and extracellular matrix proteins can participate in the extracellular and/or cell surface defense. Heat shock proteins, antioxidants, protein phosphatases, and cyclin kinase inhibitors may contribute to the intracellular defense. This article outlines how the glomerulus, when faced with injurious cells or exposed to pathogenic mediators, defends itself via the intrinsic machinery that is brought into play in resident glomerular cells.

Hepatocyte growth factor activates the AP-1 complex: a comparison between normal and transformed rat hepatocytes

BACKGROUND/AIMS

Stimulation of activator protein-1 (AP-1), a Fos/Jun complex, is a key event in the cell response to growth factors. We have investigated whether hepatocyte growth factor (HGF) induces differential AP-1 responses in normal and transformed rat hepatocytes, the 7777 cells.

METHODS

Primocultures of isolated hepatocytes or 7777 cells were stimulated with HGF. Gene expression was evaluated by ribonuclease protection assay and Western blot analysis. AP-1 DNA binding activity was measured by electrophoretic mobility shift assay. Identification of the proteins bound to the probes was made by supershift assays with specific antibodies. Cells were electroporated with plasmids containing an AP-1-dependent chloramphenicol acetyl transferase (CAT) gene, and CAT activity was measured 24 h after treatment with medium alone or HGF.

RESULTS

In both cell types, HGF triggered the same program of jun family mRNA activation, but distinct Fos/Jun proteins accumulated in the nucleus. HGF increased DNA-binding activity to the phorbol 12-O-tetradecanoate-13-acetate responsive element (TRE) in both cell types, but distinct TRE-binding proteins were recruited in the AP-1 dimers. HGF also increased consistently binding to a cAMP responsive element (CRE) in hepatocytes only. Finally, HGF triggered TRE- and CRE-dependent gene activations in hepatocytes but TRE-dependent gene activation alone in 7777 cells.

CONCLUSIONS

HGF-induced AP-1 activation leads to the formation of distinct dimers with different functional capacities in normal and transformed hepatocytes. These data suggest the importance of qualitative abnormalities of the AP-1 complex for the establishment or maintainance of a transformed phenotype.

Enzyme-linked immunosorbent assay for the measurement of JNK activity in cell extracts

A colorimetric enzyme-linked immunosorbent assay (ELISA) for the measurement of kinase activity of c-Jun N-terminal kinases (JNKs) in cell extracts is described. The assay involves passive immobilisation of the substrate GST-cJun on the surface of a microtiter plate, selection of JNK protein kinases directly in substrate-coated wells, kinase reaction, and detection of substrate phosphorylation by a phosphoepitope-specific antibody. The ability of this assay to selectively measure JNK activity relies on the high-affinity interaction between JNKs and c-Jun. Accordingly, we found that JNKs could be captured on the microtiter plate surface through binding to the immobilised GST-cJun. Moreover, JNKs retained the specificity of their interaction with and phosphorylation of c-Jun with respect to the dependence on both intact docking domain and the dimerisation state of c-Jun. This novel procedure represents a marked improvement on conventional radioactive assays in terms of sensitivity, accuracy of evaluation, low time consumption, high throughput and amenability to automation. It is expected to be useful forthe acceleration and facilitation of JNK activity measurement in cell extracts, in particular for large-scale screening of clinical samples.

Activation pattern of mitogen-activated protein kinases elicited by peroxynitrite: attenuation by selenite supplementation

Peroxynitrite is a mediator of toxicity in pathological processes in vivo and causes damage by oxidation and nitration reactions. Here, we report a differential induction of mitogen-activated protein kinases (MAPKs) in WB-F344 rat liver epithelial cells by peroxynitrite. For the exposure of cultured cells with peroxynitrite, we employed a newly developed infusion method. At 6.5 microM steady-state concentration, the activation of p38 MAPK was immediate, while JNK1/2 and ERK1/2 were activated 60 min and 15 min subsequent to 3 min of exposure to peroxynitrite, respectively. Protein-bound 3-nitrotyrosine was detected. When cells were grown in a medium supplemented with sodium selenite (1 microM) for 48 h, complete protection was afforded against the activation of p38 and against nitration of tyrosine residues. These data suggest a new role for peroxynitrite in activating signal transduction pathways capable of modulating gene expression. Further, the abolition of the effects of peroxynitrite by selenite supplementation suggests a protective role of selenium-containing proteins.

Rapid induction and translocation of Egr-1 in response to mechanical strain in vascular smooth muscle cells

The effect of mechanical strain on transcription and expression of the immediate-early genes, early growth response gene-1 (Egr-1), c-jun, and c-fos, was investigated in neonatal rat aortic vascular smooth muscle (VSM) cells. Cells grown on silicone elastomer plates were subjected to cyclic mechanical strain (1 Hz) at various durations and magnitudes. Egr-1 mRNA increased rapidly in response to cyclic strain, reached a maximum of 10-fold after 30 minutes, and returned to baseline after 4 hours. c-jun exhibited a similar pattern, whereas c-fos mRNA expression was unaffected by strain. Cycloheximide prolonged the increase in Egr-1 and c-jun mRNA and caused superinduction of both. The threshold level of continuous cyclic strain needed to induce expression was 5% for Egr-1 and c-jun. Even a single cycle of mechanical strain that lasted 1 second was sufficient to induce Egr-1 and c-jun mRNA. Strain also increased expression of a transiently transfected Egr-1 promoter-reporter construct. The effect of varying extracellular matrices on strain-induced Egr-1 and c-jun mRNA was examined. In contrast to collagen type 1- and pronectin-coated plates, strain did not significantly alter expression of Egr-1 and c-jun was less induced on laminin-coated plates. On collagen type 1, strain increased Egr-1 protein levels by 2.1-fold at 60 minutes. Immunofluorescence microscopy revealed translocation of Egr-1 to the nucleus in response to strain. These observations indicate that Egr-1 expression and translocation are sensitive to mechanical perturbation of the cell. c-jun is also induced by strain, but c-fos is not. The signal for this induction may involve specific cell-matrix interactions.

Defective IL-12 production in mitogen-activated protein (MAP) kinase kinase 3 (Mkk3)-deficient mice

The p38 mitogen-activated protein kinase (MAPK) pathway, like the c-Jun N-terminal kinase (JNK) MAPK pathway, is activated in response to cellular stress and inflammation and is involved in many fundamental biological processes. To study the role of the p38 MAPK pathway in vivo, we have used homologous recombination in mice to inactivate the Mkk3 gene, one of the two specific MAPK kinases (MAPKKs) that activate p38 MAPK. Mkk3(-/-) mice were viable and fertile; however, they were defective in interleukin-12 (IL-12) production by macrophages and dendritic cells. Interferon-gamma production following immunization with protein antigens and in vitro differentiation of naive T cells is greatly reduced, suggesting an impaired type I cytokine immune response. The effect of the p38 MAPK pathway on IL-12 expression is at least partly transcriptional, since inhibition of this pathway blocks IL-12 p40 promoter activity in macrophage cell lines and IL-12 p40 mRNA is reduced in MKK3-deficient mice. We conclude that the p38 MAP kinase, activated through MKK3, is required for the production of inflammatory cytokines by both antigen-presenting cells and CD4(+) T cells.

GAP-43 (B-50) and C-Jun are up-regulated in axotomized neurons of Clarke's nucleus after spinal cord injury in the adult rat

The growth-associated protein GAP-43 (B-50) and the transcription factor C-Jun are involved in regeneration of the injured nervous system. In this study, we investigated the possibility of the induction of GAP-43 and C-Jun in axotomized neurons of Clarke's nucleus (CN) in adult rats, of which a large population undergoes degeneration several weeks after a low thoracic lateral funiculotomy of the spinal cord. In situ hybridization and immunohistochemistry revealed a transient up-regulation of GAP-43 mRNA, C-Jun protein, and its activated, phosphorylated form, peaking around 7 days after injury in preferentially large diameter CN-neurons ipsilateral and caudal to the lesion. Our results document that some populations of axotomized central nervous system neurons, similar to axotomized regenerating neurons of the peripheral nervous system, can up-regulate GAP-43 and C-Jun, even if they are destined to degenerate. This might reflect a transient regenerative capacity, which fails over time.

Environmental factors as regulators and effectors of multistep carcinogenesis

This review highlights current knowledge of environmental factors in carcinogenesis and their cellular targets. The hypothesis that environmental factors influence carcinogenesis is widely supported by both epidemiological and experimental studies. The fact that only a small fraction of cancers can be attributed to germline mutations in cancer-related genes further buttresses the importance of environmental factors in carcinogenesis. Furthermore, penetrance of germline mutations may be modified by either environmental or other genetic factors. Examples of environmental factors that have been associated with increased cancer risk in the human population include chemical and physical mutagens (e.g. cigarette smoke, heterocyclic amines, asbestos and UV irradiation), infection by certain viral or bacterial pathogens, and dietary non-genotoxic constituents (e.g. macro- and micronutrients). Among molecular targets of environmental influences on carcinogenesis are somatic mutation (genetic change) and aberrant DNA methylation (epigenetic change) at the genomic level and post-translational modifications at the protein level. At both levels, changes elicited affect either the stability or the activity of key regulatory proteins, including oncoproteins and tumor suppressor proteins. Together, via multiple genetic and epigenetic lesions, environmental factors modulate important changes in the pathway of cellular carcinogenesis.

Requirement of mitogen-activated protein kinase kinase 3 (MKK3) for tumor necrosis factor-induced cytokine expression

The p38 mitogen-activated protein kinase is activated by treatment of cells with cytokines and by exposure to environmental stress. The effects of these stimuli on p38 MAP kinase are mediated by the MAP kinase kinases (MKKs) MKK3, MKK4, and MKK6. We have examined the function of the p38 MAP kinase signaling pathway by investigating the effect of targeted disruption of the Mkk3 gene. Here we report that Mkk3 gene disruption caused a selective defect in the response of fibroblasts to the proinflammatory cytokine tumor necrosis factor, including reduced p38 MAP kinase activation and cytokine expression. These data demonstrate that the MKK3 protein kinase is a critical component of a tumor necrosis factor-stimulated signaling pathway that causes increased expression of inflammatory cytokines.

Lack of correlation in JNK activation and p53-dependent Fas expression induced by apoptotic stimuli

Induction of Fas expression by DNA-damaging agents is dependent on the expression of functional p53, and has been suggested to play an important role in apoptosis induction. JNK (c-Jun N-terminal kinase), which is capable of phosphorylating p53, is also involved in apoptotic signaling induced by various apoptotic stimuli. Here, we report that although Fas induction is closely linked to the expression of wild type p53, it is not correlated with JNK activation induced by apoptotic stimuli. JNK activation does not necessarily lead to Fas expression, even in cells containing wild type p53. In addition, Fas expression can be induced without significant JNK activation. Furthermore, induction of Fas expression is not sufficient for apoptosis induction; however, it may sensitize cells to Fas-ligation induced apoptosis.

Regulation of Rb and E2F by signal transduction cascades: divergent effects of JNK1 and p38 kinases

The E2F transcription factor plays a major role in cell cycle regulation, differentiation and apoptosis, but it is not clear how it is regulated by non-mitogenic signaling cascades. Here we report that two kinases involved in signal transduction have opposite effects on E2F function: the stress-induced kinase JNK1 inhibits E2F1 activity whereas the related p38 kinase reverses Rb-mediated repression of E2F1. JNK1 phosphorylates E2F1 in vitro, and co-transfection of JNK1 reduces the DNA binding activity of E2F1; treatment of cells with TNFalpha had a similar effect. Fas stimulation of Jurkat cells is known to induce p38 kinase and we find a pronounced increase in Rb phosphorylation within 30 min of Fas stimulation. Phosphorylation of Rb correlated with a dissociation of E2F and increased transcriptional activity. The inactivation of Rb by Fas was blocked by SB203580, a p38-specific inhibitor, as well as a dominant-negative p38 construct; cyclin-dependent kinase (cdk) inhibitors as well as dominant-negative cdks had no effect. These results suggest that Fas-mediated inactivation of Rb is mediated via the p38 kinase, independent of cdks. The Rb/E2F-mediated cell cycle regulatory pathway appears to be a normal target for non-mitogenic signaling cascades and could be involved in mediating the cellular effects of such signals.

Lead activates nuclear transcription factor-kappaB, activator protein-1, and amino-terminal c-Jun kinase in pheochromocytoma cells

Lead (Pb) is a ubiquitous environmental contaminant that produces variety of effects on the central and peripheral nervous system, induces inflammatory response, and modulates immune functions. Though increase in lipid peroxidation and reactive oxygen intermediates (ROI) have been observed in Pb-induced toxicity, the molecular mechanism underlying these effects is largely unknown. Since nuclear factor kappa B (NF-kappaB) and activator protein (AP-1) are known to be activated by oxidative stress, we hypothesized that Pb-induced effects may be modulated via these transcription factors. The effects of Pb on NF-kappaB, AP-1, and related kinases were studied in pheochromocytoma cells (PC-12). Our results showed that treatment of murine PC-12 cells with Pb resulted in activation of NF-kappaB and degradation of IkappaBalpha (the inhibitory subunit of NF-kappaB). Pb-induced NF-kappaB dependent gene expression was also enhanced. The binding of Pb-induced NF-kappaB to DNA was blocked by antibodies for p65 and p50 but not by c-Rel or nonspecific antibodies such as cyclin D-1 and preimmune serum, suggesting that NF-kappaB consisted of p65 and p50 subunits. Similar to its effects on NF-kappaB, Pb also activated AP-1 in a time- and dose-dependent manner. Besides activating these transcription factors, Pb was also found to upregulate the related kinases such as mitogen activated protein kinase kinase (MEK) and c-Jun N-terminal kinase (JNK) (also known as stress-activated protein kinase) in a dose- and time-dependent manner. Thus, these results suggest that NF-kappaB, AP-1, MEK, and JNK may be important mediators of Pb-induced signaling in gene expression mediating inflammatory response and immunomodulation.

pp60(v-src) induction of cyclin D1 requires collaborative interactions between the extracellular signal-regulated kinase, p38, and Jun kinase pathways. A role for cAMP response element-binding protein and activating transcription factor-2 in pp60(v-src) signaling in breast cancer cells

The cyclin D1 gene is overexpressed in breast tumors and encodes a regulatory subunit of cyclin-dependent kinases that phosphorylate the retinoblastoma protein. pp60(c-src) activity is frequently increased in breast tumors; however, the mechanisms governing pp60(c-src) regulation of the cell cycle in breast epithelium are poorly understood. In these studies, pp60(v-src) induced cyclin D1 protein levels and promoter activity (48-fold) in MCF7 cells. Cyclin D1-associated kinase activity and protein levels were increased in mammary tumors from murine mammary tumor virus-pp60(c-src527F) transgenic mice. Optimal induction of cyclin D1 by pp60(v-src) involved the extracellular signal-regulated kinase, p38, and c-Jun N-terminal kinase members of the mitogen-activated protein kinase family. Cyclin D1 promoter activation by pp60(v-src) involved a cAMP response element-binding protein (CREB)/activating transcription factor 2 (ATF-2) binding site. Dominant negative mutants of CREB and ATF-2 but not c-Jun inhibited pp60(v-src) induction of cyclin D1. pp60(v-src) induction of CREB was blocked by the p38 inhibitor SB203580 or by mutation of CREB at Ser133. pp60(v-src) induction of ATF-2 was abolished by the c-Jun N-terminal kinase inhibitor JNK-interacting protein-1 or by mutation of ATF-2 at Thr69 and Thr71. CREB and ATF-2, which bind to a common pp60(v-src) response element, are transcriptionally activated by distinct mitogen-activated protein kinases. Induction of cyclin D1 activity by pp60(v-src) may contribute to breast tumorigenesis through phosphorylation and inactivation of the retinoblastoma protein.

Tonicity-responsive enhancer binding protein, a rel-like protein that stimulates transcription in response to hypertonicity

Hypertonicity (most often present as high salinity) is stressful to the cells of virtually all organisms. Cells survive in a hypertonic environment by increasing the transcription of genes whose products catalyze cellular accumulation of compatible osmolytes. In mammals, the kidney medulla is normally hypertonic because of the urinary concentrating mechanism. Cellular accumulation of compatible osmolytes in the renal medulla is catalyzed by the sodium/myo-inositol cotransporter (SMIT), the sodium/chloride/betaine cotransporter, and aldose reductase (synthesis of sorbitol). The importance of compatible osmolytes is underscored by the necrotic injury of the renal medulla and subsequent renal failure that results from the inhibition of SMIT in vivo by administration of a specific inhibitor. Tonicity-responsive enhancers (TonE) play a key role in hypertonicity-induced transcriptional stimulation of SMIT, sodium/chloride/betaine cotransporter, and aldose reductase. We report the cDNA cloning of human TonE binding protein (TonEBP), a transcription factor that stimulates transcription through its binding to TonE sequences via a Rel-like DNA binding domain. Western blot and immunohistochemical analyses of cells cultured in hypertonic medium reveal that exposure to hypertonicity elicits slow activation of TonEBP, which is the result of an increase in TonEBP amount and translocation to the nucleus.

Mitogen-activated protein kinase (p38-, JNK-, ERK-) activation pattern induced by extracellular and intracellular singlet oxygen and UVA

Ultraviolet A (UVA; 320-400 nm) radiation in human skin fibroblasts induces a pattern of mitogen-activated protein kinase (MAPK) activation consisting of a rapid and transient induction of p38 and c-Jun-N-terminal kinase (JNK) activity but not extracellular signal-regulated kinases (ERK). UVA activation of p38 can be inhibited by the singlet oxygen (1O2) quenchers azide and imidazole, but not by the hydroxyl radical scavengers mannitol or dimethylsulfoxide, pointing to the involvement of 1O2. The same effect has been shown for JNK. Like UVA, 1O2 generated intracellularly upon photoexcitation of Rose Bengal activates p38 and JNK but not ERK. p38 and JNK activation was also elicited by chemiexcitation for the intracellular generation of 1O2 by the lipophilic 1,4-endoperoxide of N,N'-di(2,3-dihydroxypropyl)-1, 4-naphthalene dipropionamide. In contrast, extracellular generation of 1O2, by irradiation of Rose Bengal immobilized on agarose beads or by chemiexcitation employing the hydrophilic 1,4-endoperoxide of disodium 3,3'-(1,4-naphthylidene) dipropionate, was ineffective in activating p38 or JNK. These data suggest that the activation of p38 and JNK by 1O2 occurs only when the electronically excited molecule is generated intracellularly.

CD40 signaling of monocyte inflammatory cytokine synthesis through an ERK1/2-dependent pathway. A target of interleukin (il)-4 and il-10 anti-inflammatory action

Ligation of CD40 on monocytes through its interaction with CD40 ligand (CD154) present on activated T helper cells, results in activation of monocyte inflammatory cytokine synthesis and rescue of monocytes from apoptosis induced through serum deprivation. Both of these consequences of CD40 stimulation have been shown to be dependent on the induction of protein tyrosine kinase activity. CD40-mediated activation of protein tyrosine kinase activity and subsequent inflammatory cytokine production are abrogated by treatment of monocytes with the T helper type 2 cytokines interleukin 4 (IL-4) and interleukin 10 (IL-10). In the current study we demonstrate that stimulation of monocytes through CD40 resulted in the phosphorylation and activation of the extracellular signal-regulated kinases 1 and 2 (ERK1/2) mitogen-activated protein kinases, whereas phosphorylation of mitogen-activated protein kinases family members p38 and c-Jun N-terminal kinase was not observed in response to this stimuli over the time course examined. PD98059, an inhibitor of the upstream activator of ERK1/2, the MAP/ERK kinase MEK1/2, suppressed IL-1beta and tumor necrosis factor-alpha production in a dose-dependent fashion. Pretreatment of monocytes with IL-4 and IL-10 inhibited CD40-mediated activation of ERK1/2 kinase activity when used individually, and are enhanced in effectiveness when used in combination. Together, the data demonstrate that CD40-mediated induction of IL-1beta and tumor necrosis factor-alpha synthesis is dependent on a MEK/ERK pathway which is obstructed by signals generated through the action of IL-4 and IL-10.

Enterocyte-like differentiation of the Caco-2 intestinal cell line is associated with increases in AP-1 protein binding

The nuclear factor AP-1, a large family of transcription factors composed of the Jun and Fos protein families, plays a role in the differentiation of various cells; the role of the AP-1 factors in intestinal differentiation is not known. Members of the AP-1 family can be activated by the Ras pathway and, in addition, Ras appears to be important for gut differentiation. The purpose of this study was to determine whether AP-1 activity is altered in the Caco-2 cell line, which spontaneously differentiates to a small bowel phenotype after confluency, and the Caco-2-ras cell line, which exhibits differentiated properties regardless of culture conditions. AP-1 binding activity, consisting of c-Jun, JunD, c-Fos and Fra-2 proteins, was increased in Caco-2 cells at 3 days postconfluency, a time point associated with G1 block and cessation of proliferation. Steady state levels of JunD were increased at day 3 postconfluency as determined by Western blot. Furthermore, AP-1 binding was increased in preconfluent Caco-2-ras cells compared with parental Caco-2 cells, suggesting that AP-1 induction may be mediated by the Ras pathway. The early induction of AP-1 binding activity suggests a role for these proteins in the differentiation of the Caco-2 intestinal cell line.

c-Jun is a JNK-independent coactivator of the PU.1 transcription factor

The ETS domain transcription factor PU.1 is necessary for the development of monocytes and regulates, in particular, the expression of the monocyte-specific macrophage colony-stimulating factor (M-CSF) receptor, which is critical for monocytic cell survival, proliferation, and differentiation. The bZIP transcription factor c-Jun, which is part of the AP-1 transcription factor complex, is also important for monocytic differentiation, but the monocyte-specific M-CSF receptor promoter has no AP-1 consensus binding sites. We asked the question of whether c-Jun could promote the induction of the M-CSF receptor by collaborating with PU.1. We demonstrate that c-Jun enhances the ability of PU.1 to transactivate the M-CSF receptor promoter as well as a minimal thymidine kinase promoter containing only PU.1 DNA binding sites. c-Jun does not directly bind to the M-CSF receptor promoter but associates via its basic domain with the ETS domain of PU.1. Consistent with our observation that AP-1 binding does not contribute to c-Jun coactivation is the observation that the activation of PU.1 by c-Jun is blocked by overexpression of c-Fos. Phosphorylation of c-Jun by c-Jun NH2-terminal kinase on Ser-63 and -73 does not alter the ability of c-Jun to enhance PU.1 transactivation. Activated Ras enhances the transcriptional activity of PU.1 by up-regulating c-Jun expression without changing the phosphorylation pattern of PU.1. The activation of PU.1 by Ras is blocked by a mutant c-Jun protein lacking the basic domain. The expression of this mutant form of c-Jun also completely blocks 12-O-tetradecanoylphorbol-13-acetate-induced M-CSF receptor promoter activity during monocytic differentiation. We propose therefore that c-Jun acts as a c-Jun NH2-terminal kinase-independent coactivator of PU.1, resulting in M-CSF receptor expression and development of the monocytic lineage.

GCKR Links the Bcr-Abl Oncogene and Ras to the Stress-Activated Protein Kinase Pathway

The Bcr-Abl oncogene, found in Philadelphia chromosome-positive myelogenous leukemia (CML), activates Ras and triggers the stress-activated protein kinase (SAPK or Jun NH2-terminal kinase [JNK]) pathway. Interruption of Ras or SAPK activation dramatically reduces Bcr-Abl–mediated transformation. Here, we report that Bcr-Abl through a Ras-dependent pathway signals the serine/threonine protein kinase GCKR (Germinal Center Kinase Related) leading to SAPK activation. Either an oncogenic form of Ras or Bcr-Abl enhances GCKR catalytic activity and its activation of SAPK, whereas inhibition of GCKR impairs Bcr-Abl–induced SAPK activation. Bcr-Abl mutants that are impaired for GCKR activation are also unable to activate SAPK. Consistent with GCKR being a functional target in CML, GCKR is constitutively active in CML cell lines and found in association with Bcr-Abl. Our results indicate that GCKR is a downstream target of Bcr-Abl and strongly implicate GCKR as a mediator of Bcr-Abl in its transformation of cells.

GCKR Links the Bcr-Abl Oncogene and Ras to the Stress-Activated Protein Kinase Pathway

The Bcr-Abl oncogene, found in Philadelphia chromosome-positive myelogenous leukemia (CML), activates Ras and triggers the stress-activated protein kinase (SAPK or Jun NH2-terminal kinase [JNK]) pathway. Interruption of Ras or SAPK activation dramatically reduces Bcr-Abl–mediated transformation. Here, we report that Bcr-Abl through a Ras-dependent pathway signals the serine/threonine protein kinase GCKR (Germinal Center Kinase Related) leading to SAPK activation. Either an oncogenic form of Ras or Bcr-Abl enhances GCKR catalytic activity and its activation of SAPK, whereas inhibition of GCKR impairs Bcr-Abl–induced SAPK activation. Bcr-Abl mutants that are impaired for GCKR activation are also unable to activate SAPK. Consistent with GCKR being a functional target in CML, GCKR is constitutively active in CML cell lines and found in association with Bcr-Abl. Our results indicate that GCKR is a downstream target of Bcr-Abl and strongly implicate GCKR as a mediator of Bcr-Abl in its transformation of cells.

TGF-beta3, but not TGF-beta1, protects keratinocytes against 12-O-tetradecanoylphorbol-13-acetate-induced cell death in vitro and in vivo

We have examined the role that individual TGF-beta isoforms, and in particular TGF-beta3, play in control of epidermal homeostasis. Mice with a knockout mutation of the TGF-beta3 gene die a few hours after birth. A full-thickness skin grafting approach was used to investigate the postnatal development and homeostatic control of the skin of these mice. Grafted skin of mice with a disruption of the TGF-beta3 gene developed similarly to grafts of wild type and TGF-beta1 knockout animals. However, a strikingly different response was observed after acute treatment with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). When exposed to TPA, the grafted skin of wild type and TGF-beta1 knockout mice underwent a hyperplastic response similar to that of normal mouse skin. In marked contrast, TPA treatment of TGF-beta3 knockout grafts induced widespread areas of keratinocyte cell death. Analysis of cultured keratinocytes treated with purified TGF-beta isoforms revealed that TGF-beta3 plays a direct and specific function in protecting keratinocytes against TPA-induced cell death. The protective function of TGF-beta3 on TPA-induced cell death was not because of general suppression of the signaling pathways triggered by this agent, as ERK1/2 activation occurred to a similar if not greater extent in TGF-beta3-treated versus control keratinocytes. Instead, TGF-beta3 treatment led to a significant reduction in TPA-induced c-Jun N-terminal kinase activity, which was associated and possibly explained by specific counteracting effects of TGF-beta3 on TPA-induced disruption of keratinocyte focal adhesions.

Raf-1 is activated by the p38 mitogen-activated protein kinase inhibitor, SB203580

SB203580 (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imi dazole) is widely used as a specific inhibitor of p38 mitogen-activated protein kinase (MAPK). Here, we report that SB203580 activates the serine/threonine kinase Raf-1 in quiescent smooth muscle cells in a dose-dependent fashion. The concentrations of SB203580 required lie above those necessary to inhibit p38 MAPK and we were unable to detect basal levels of active p38 MAPK. SB203580 does not directly activate Raf-1 in vitro, and fails to activate Ras, MEK, and ERK in intact cells. In vitro, however, SB203580-stimulated Raf-1 activates MEK1 in a coupled assay. We conclude that activation of Raf-1 by SB203580 is not mediated by an inhibition of p38 MAPK, is Ras-independent, and is uncoupled from MEK/ERK signaling.

Angiotensin II type 1 receptor-modulated signaling pathways in neurons

Mammalian brain contains high densities of angiotensin II (Ang II) type 1 (AT1) receptors, localized mainly to specific nuclei within the hypothalamus and brainstem regions. Neuronal AT1 receptors within these areas mediate the stimulatory actions of central Ang II on blood pressure, water and sodium intake, and vasopressin secretion, effects that involve the modulation of brain noradrenergic pathways. This review focuses on the intracellular events that mediate the functional effects of Ang II in neurons, via AT1 receptors. The signaling pathways involved in short-term changes in neuronal activity, membrane ionic currents, norepinephrine (NE) release, and longer-term neuromodulatory actions of Ang II are discussed. It will be apparent from this discussion that the signaling pathways involved in these events are often distinct.

YopJ of Yersinia spp. is sufficient to cause downregulation of multiple mitogen-activated protein kinases in eukaryotic cells

Pathogenic Yersinia spp. utilize a plasmid-encoded type III secretion system to deliver a set of Yop effector proteins into eukaryotic cells. Previous studies have shown that the effector YopJ is required for Yersinia to cause downregulation of the mitogen-activated protein (MAP) kinases c-Jun N-terminal kinase (JNK), p38, and extracellular signal-regulated kinase (ERK) 1 and 2 in infected macrophages. Here we demonstrate that YopJ is sufficient to cause downregulation of multiple MAP kinases in eukaryotic cells. Cellular fractionation experiments confirmed that YopJ is delivered into the cytoplasmic fraction of macrophages by the type III system. Production of YopJ in COS-1 cells by transfection significantly reduced (5- to 10-fold) activation of JNK, p38, and ERK in response to several different stimuli, including serum and tumor necrosis factor alpha. JNK activation mediated by RacV12, an activated mutant of Rac1, was also blocked by YopJ in COS-1 cells, indicating that YopJ acts downstream of this small GTPase to downregulate MAP kinase signaling. Analysis of transfected COS-1 cells by immunofluorescence microscopy revealed that YopJ is recruited from the cytoplasmic compartment to the cell periphery in response to stimuli (e.g., serum) that induce membrane ruffling. These data indicate that YopJ functions as a "MAP kinase toxin" to selectively block nuclear responses that are triggered by Yersinia-host cell interaction.

Inhibition of ultraviolet B--induced c-fos gene expression and p38 mitogen-activated protein kinase activation by (-)-epigallocatechin gallate in a human keratinocyte cell line

(-)-Epigallocatechin gallate (EGCG), the major polyphenol isolated from green tea, is an active chemoprevention agent against cancer. However, the molecular mechanisms that underlie the inhibitory effects of EGCG are not well understood. In this study, we tested the effects of EGCG on ultraviolet (UV) B radiation-induced c-fos gene expression in a human keratinocyte cell line, HaCaT. EGCG inhibited UVB-induced steady-state message and transcriptional activation of the c-fos gene in a dose-dependent manner. Western analyses further indicated that EGCG had an inhibitory effect on UVB-induced accumulation of the c-fos protein within the same dose range. To further examine the mechanism by which EGCG inhibits UVB-induced c-fos expression, we tested the effect of EGCG on upstream activators of the c-fos gene. We found that EGCG significantly inhibited activation of p38 mitogen-activated protein kinase but not c-jun NH2-terminal kinase or extracellular signal-regulated protein kinase activation. Our previous studies have indicated that UVB-induced c-fos expression may play a key role in UVB-induced activation of the activator protein-1 transcription factor and EGCG-inhibited, UVB-induced activation of AP-1 in HaCaT cells. Because AP-1 is important for tumor promotion and c-fos is a major component of AP-1, the inhibitory effects of EGCG on c-fos expression may further explain the anti-tumor-promoting effects of EGCG.

Interaction of hematopoietic progenitor kinase 1 with adapter proteins Crk and CrkL leads to synergistic activation of c-Jun N-terminal kinase

Hematopoietic progenitor kinase 1 (HPK1), a mammalian Ste20-related protein kinase, is an upstream activator of c-Jun N-terminal kinase (JNK). In order to further characterize the HPK1-mediated JNK signaling cascade, we searched for HPK1-interacting proteins that could regulate HPK1. We found that HPK1 interacted with Crk and CrkL adaptor proteins in vitro and in vivo and that the proline-rich motifs within HPK1 were involved in the differential interaction of HPK1 with the Crk proteins and Grb2. Crk and CrkL not only activated HPK1 but also synergized with HPK1 in the activation of JNK. The HPK1 mutant (HPK1-PR), which encodes the proline-rich region alone, blocked JNK activation by Crk and CrkL. Dominant-negative mutants of HPK1 downstream effectors, including MEKK1, TAK1, and SEK1, also inhibited Crk-induced JNK activation. These results suggest that the Crk proteins serve as upstream regulators of HPK1. We further observed that the HPK1 mutant HPK1-KD(M46), which encodes the kinase domain with a point mutation at lysine-46, and HPK1-PR blocked interleukin-2 (IL-2) induction in Jurkat T cells, suggesting that HPK1 signaling plays a critical role in IL-2 induction. Interestingly, HPK1 phosphorylated Crk and CrkL, mainly on serine and threonine residues in vitro. Taken together, our findings demonstrate the functional interaction of HPK1 with Crk and CrkL, reveal the downstream pathways of Crk- and CrkL-induced JNK activation, and highlight a potential role of HPK1 in T-cell activation.

The MKK7 gene encodes a group of c-Jun NH2-terminal kinase kinases

The c-Jun NH2-terminal protein kinase (JNK) is a member of the mitogen-activated protein kinase (MAPK) group and is an essential component of a signaling cascade that is activated by exposure of cells to environmental stress. JNK activation is regulated by phosphorylation on both Thr and Tyr residues by a dual-specificity MAPK kinase (MAPKK). Two MAPKKs, MKK4 and MKK7, have been identified as JNK activators. Genetic studies demonstrate that MKK4 and MKK7 serve nonredundant functions as activators of JNK in vivo. We report here the molecular cloning of the gene that encodes MKK7 and demonstrate that six isoforms are created by alternative splicing to generate a group of protein kinases with three different NH2 termini (alpha, beta, and gamma isoforms) and two different COOH termini (1 and 2 isoforms). The MKK7alpha isoforms lack an NH2-terminal extension that is present in the other MKK7 isoforms. This NH2-terminal extension binds directly to the MKK7 substrate JNK. Comparison of the activities of the MKK7 isoforms demonstrates that the MKK7alpha isoforms exhibit lower activity, but a higher level of inducible fold activation, than the corresponding MKK7beta and MKK7gamma isoforms. Immunofluorescence analysis demonstrates that these MKK7 isoforms are detected in both cytoplasmic and nuclear compartments of cultured cells. The presence of MKK7 in the nucleus was not, however, required for JNK activation in vivo. These data establish that the MKK4 and MKK7 genes encode a group of protein kinases with different biochemical properties that mediate activation of JNK in response to extracellular stimuli.

A novel human STE20-related protein kinase, HGK, that specifically activates the c-Jun N-terminal kinase signaling pathway

The yeast serine/threonine kinase STE20 activates a signaling cascade that includes STE11 (mitogen-activated protein kinase kinase kinase), STE7 (mitogen-activated protein kinase kinase), and FUS3/KSS1 (mitogen-activated protein kinase) in response to signals from both Cdc42 and the heterotrimeric G proteins associated with transmembrane pheromone receptors. Using degenerate polymerase chain reaction, we have isolated a human cDNA encoding a protein kinase homologous to STE20. This protein kinase, designated HPK/GCK-like kinase (HGK), has nucleotide sequences that encode an open reading frame of 1165 amino acids with 11 kinase subdomains. HGK was a serine/threonine protein kinase that specifically activated the c-Jun N-terminal kinase (JNK) signaling pathway when transfected into 293T cells, but it did not stimulate either the extracellular signal-regulated kinase or p38 kinase pathway. HGK also increased AP-1-mediated transcriptional activity in vivo. HGK-induced JNK activation was inhibited by the dominant-negative MKK4 and MKK7 mutants. The dominant-negative mutant of TAK1, but not MEKK1 or MAPK upstream kinase (MUK), strongly inhibited HGK-induced JNK activation. TNF-alpha activated HGK in 293T cells, as well as the dominant-negative HGK mutants, inhibited TNF-alpha-induced JNK activation. These results indicate that HGK, a novel activator of the JNK pathway, may function through TAK1, and that the HGK --> TAK1 --> MKK4, MKK7 --> JNK kinase cascade may mediate the TNF-alpha signaling pathway.

Valproate robustly enhances AP-1 mediated gene expression

Valproic acid (VPA) is a potent broad spectrum anticonvulsant with demonstrated efficacy in the treatment of Bipolar Affective Disorder, but the biochemical basis for VPA's antimanic or mood-stabilizing actions have not been fully elucidated. It has been demonstrated that VPA, at therapeutically relevant concentrations, increases AP-1 DNA binding activity in cultured cells in vitro. These findings raise the possibility that VPA may produce its mood-stabilizing effects by regulating the expression of subsets of genes via its effects on the AP-1 family of transcription factors. To determine if VPA does, in fact, enhance AP-1 mediated gene expression, the effects of VPA on the expression of a luciferase reporter gene were studied in transiently transfected rat C6 glioma and human SH-SY5Y neuroblastoma cells using the pGL2-control vector. The luciferase gene in the vector is driven by an SV40 promoter which contains well characterized AP-1 sites. VPA produced a greater than doubling of luciferase activity in a time- and concentration-dependent manner in both cell lines. Furthermore, mutations of the AP-1 sites in the SV40 promoter markedly attenuated the VPA-induced increases in luciferase activity. These effects of VPA on AP-1 mediated gene expression are very similar to the effects observed with lithium, and suggest that the temporal regulation of AP-1 mediated gene expression in critical neuronal circuits may play a role in the long-term therapeutic efficacy of these agents.

Regulation of IL-4 expression by the transcription factor JunB during T helper cell differentiation

The molecular basis for restricted cytokine expression by T helper 1 (Th1) and T helper 2 (Th2) cells is unclear. Previous studies found that P1, an element of the interleukin 4 (IL-4) promoter that binds AP-1, is important for Th2-restricted IL-4 expression. Here we show that JunB, but not the other Jun family members, was selectively induced in Th2 cells and not in Th1 cells during differentiation. JunB has previously been considered to be a negative regulator of transcription. However, we show that JunB binds directly to the P1 site and synergizes with c-Maf to activate an IL-4 luciferase reporter gene. JunB-control of IL-4 expression is mediated by the phosphorylation of JunB at Thr102 and -104 by JNK MAP kinase. The synergy between c-Maf and JunB can be attributed to cooperative DNA binding, which is facilitated by JunB phosphorylation. In transgenic mice, elevated JunB levels caused increased expression of several Th2 cytokines in developing Th1 cells. JunB also upregulated IL-4 expression in response to immunization. Thus, the early increase of JunB protein in Th2 cells can provide the specificity for c-Maf in IL-4 expression during T cell development and directs thereby Th2 differentiation.

Angiotensin II type 2 receptor-mediated apoptosis of cultured neurons from newborn rat brain

Angiotensin II (Ang II) type 2 (AT2) receptors are highly expressed in neonate brain and may have a role in developmental processes such as apoptosis. Concurrent activation of c-Jun N-terminal kinase (JNK) and inhibition of Erk mitogen-activated protein kinase activities is important for apoptosis in many cells, and we previously demonstrated that stimulation of AT2 receptors causes decreased mitogen-activated protein kinase activity in neurons cultured from newborn rat hypothalamus and brain stem. Using such cultures we have employed terminal deoxynucleotidyl transferase-mediated deoxy-UTP nick end labeling and internucleosomal DNA fragmentation to assess the role of AT2 receptors in neuronal apoptosis. Ang II (100 nM; 4-72 h) alone produced no significant neuronal apoptosis, and AT2 receptor activation did not stimulate JNK activity. However, exposure of cultures to UV radiation (6 J/m2/sec for 4 sec) to stimulate JNK elicited neuronal apoptosis that was significantly enhanced by Ang II, an effect that was abolished by the AT2 receptor antagonist PD 123,319 (1 microM) or the serine/threonine phosphatase inhibitor okadaic acid (3 nM). Additionally, Ang II enhanced the UV radiation-induced decrease in the levels of the DNA repair enzyme poly-(ADP-ribose) polymerase. These data indicate that Ang II, via AT2 receptors and activation of a serine/threonine phosphatase, contributes to neuronal apoptosis.

Central role of the MAPK pathway in ang II-mediated DNA synthesis and migration in rat vascular smooth muscle cells

Angiotensin II (Ang II) promotes vascular smooth muscle cell (VSMC) growth and migration, but the signaling pathways mediating these VSMC behaviors critical to restenosis and atherosclerosis are not completely known. The purpose of the present investigation was to define the role of mitogen-activated protein kinase (MAPK) in Ang II-induced DNA synthesis, migration, and c-fos induction in VSMCs. PD 98059, a synthetic inhibitor of MAPK kinase, or antisense oligodeoxynucleotides (ODNs) to deplete extracellular signal-regulated kinase (ERK)1 and ERK2 MAPKs, were used to inhibit MAPK signaling. PD 98059 at 30 micromol/L reduced Ang II-induced MAPK activity by 69% (P<0.01). Under these conditions, Ang II-induced DNA synthesis was completely inhibited (P<0.01), and Ang II-directed migration was attenuated by 76% (P<0.05). In contrast, induction of c-fos by Ang II was only partially suppressed (58% inhibition, P<0.01). Antisense ODNs against the initiation site of rat ERK1 and ERK2 MAPK mRNAs reduced corresponding protein levels by 63% (P<0.01) and completely inhibited MAPK activation by either Ang II (1 micromol/L) or 10% serum. Antisense ODNs (0.4 micromol/L) completely inhibited Ang II-induced DNA synthesis (P<0.01), decreased migration by 47% (P<0.01), and reduced c-fos induction by 40% (P<0.01 versus control ODN-transfected VSMCs). The Ang II type 1 (AT1)-receptor blocker irbesartan completely blocked DNA synthesis, migration, MAPK activation, and c-fos induction by Ang II in VSMCs. These results demonstrate that activation of MAPK plays a crucial role in Ang II-directed migration and DNA synthesis through the AT1 receptor. In contrast, Ang II-mediated c-fos induction and migration were only partially inhibited by either antisense ODNs or PD 98059, suggesting that other pathways in addition to the MAPK pathway may be involved in these actions of Ang II. We conclude that MAPK is a critical regulatory factor for Ang II-mediated migration and growth in VSMCs. Ang II-induced DNA synthesis showed a stronger MAPK dependence than did Ang II-directed migration or c-fos induction.

Mitogen-activated protein kinase: conservation of a three-kinase module from yeast to human

Mitogen-activated protein kinases (MAPK) are serine-threonine protein kinases that are activated by diverse stimuli ranging from cytokines, growth factors, neurotransmitters, hormones, cellular stress, and cell adherence. Mitogen-activated protein kinases are expressed in all eukaryotic cells. The basic assembly of MAPK pathways is a three-component module conserved from yeast to humans. The MAPK module includes three kinases that establish a sequential activation pathway comprising a MAPK kinase kinase (MKKK), MAPK kinase (MKK), and MAPK. Currently, there have been 14 MKKK, 7 MKK, and 12 MAPK identified in mammalian cells. The mammalian MAPK can be subdivided into five families: MAPKerk1/2, MAPKp38, MAPKjnk, MAPKerk3/4, and MAPKerk5. Each MAPK family has distinct biological functions. In Saccharomyces cerevisiae, there are five MAPK pathways involved in mating, cell wall remodelling, nutrient deprivation, and responses to stress stimuli such as osmolarity changes. Component members of the yeast pathways have conserved counterparts in mammalian cells. The number of different MKKK in MAPK modules allows for the diversity of inputs capable of activating MAPK pathways. In this review, we define all known MAPK module kinases from yeast to humans, what is known about their regulation, defined MAPK substrates, and the function of MAPK in cell physiology.

Bombesin activates MAP kinase in non-small cell lung cancer cells

The effects of bombesin (BB) on mitogen activated protein (MAP) kinase were investigated using non-small cell lung cancer (NSCLC) cells. By Western blot, both 42 and 44 kDalton forms of MAP kinase were present in NCI-H1299 and NCI-H838 cells. Addition of BB to NCI-H1299 cells resulted in phosphorylation of the MAP kinase substrate myelin basic protein (MBP). Phosphorylation of MBP was maximal 6 min after the addition of 10 nM BB to NCI-H1299 cells. Addition of gastrin releasing peptide (GRP) or GRP14-27 but not GRP1-16 to NCI-H 1299 cells caused MBP phosphorylation. The effects of BB were inhibited by BW2258U89, a BB receptor antagonist, and PD98059, a MAP kinase kinase inhibitor. Also, PD98059 inhibited the clonal growth of NCI-H1299 cells. These data suggest that MAP kinase may be an important regulatory enzyme in NSCLC.

Effects of anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene on human small airway epithelial cells and the protective effects of myo-inositol

Benzo[a]pyrene (B[a]P), a tobacco-derived carcinogen, induces lung tumors in rodents through its carcinogenic metabolite, anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]PDE). Tumorigenesis is inhibited by dietary myo-inositol in the post-initiation phase. However, little is known about how B[a]PDE and myo-inositol affect normal human lung cells. We addressed this question using untransformed human small airway epithelial (SAE) cells. SAE cell viability decreased <50% in parallel to an increase of apoptotic cells (>20%) 2 days after the cells were treated for 1 h with B[a]PDE (>100 nM). In contrast, the cell number and viability were not altered in A549 human lung cancer cells by B[a]PDE treatment up to 10 microM with <5% apoptotic cells and <10 U/l LDH in the medium. SAE cells retain the features of basal cells in serum-free, low Ca2+ (4 nM) medium up to 4-5 passages, but in serum-supplemented or serum-free, high Ca2+ (1 mM) cultures, they differentiate into non-ciliated epithelial cells expressing Clara cell secretory protein (CCSP). A non-toxic, physiologically relevant dose of B[a]PDE (1 nM) partially inhibited serum and Ca2+-induced SAE cell differentiation. This effect was abolished by wortmannin, a phosphatidylinositol-3 kinase (PI-3K) inhibitor, and PD98059, a mitogen activated protein kinase (MAPK) kinase-1 (MEK1) inhibitor, but not by SB202190, a p38 MAPK inhibitor, or melittin, a protein kinase C inhibitor. Myo-inositol (10-100 microM) did not alter growth or differentiation of untreated SAE or A549 cells, but reversed the inhibitory effect of B[a]PDE on serum and Ca2+-induced SAE cell differentiation when supplemented to the culture after B[a]PDE treatment. This myo-inositol action was not altered by PD98059, wortmannin or melittin, but was partially suppressed by SB202190. Collectively, these results indicate that B[a]PDE inhibits serum-induced SAE cell differentiation, possibly involving activating signals through a PI-3K/MEK1 mediated MAPK pathway, whereas myo-inositol protects SAE cells against this inhibitory effect of B[a]PDE perhaps through both PI-3K/MEK1 and p38 MAPK pathways.

The repertoire of fos and jun proteins expressed during the G1 phase of the cell cycle is determined by the duration of mitogen-activated protein kinase activation

In Rat-1 fibroblasts nonmitogenic doses of lysophosphatidic acid (LPA) stimulate a transient activation of mitogen-activated protein kinase (MAPK), whereas mitogenic doses elicit a sustained response. This sustained phase of MAPK activation regulates cell fate decisions such as proliferation or differentiation, presumably by inducing a program of gene expression which is not observed in response to transient MAPK activation. We have examined the expression of members of the AP-1 transcription factor complex in response to stimulation with different doses of LPA. c-Fos, c-Jun, and JunB are induced rapidly in response to LPA stimulation, whereas Fra-1 and Fra-2 are induced after a significant lag. The expression of c-Fos is transient, whereas the expression of c-Jun, JunB, Fra-1, and Fra-2 is sustained. The early expression of c-Fos can be reconstituted with nonmitogenic doses of LPA, but the response is transient compared to that observed with mitogenic doses. In contrast, expression of Fra-1, Fra-2, and JunB and optimal expression of c-Jun are observed only with doses of LPA which induce sustained MAPK activation and DNA synthesis. LPA-stimulated expression of c-Fos, Fra-1, Fra-2, c-Jun, and JunB is inhibited by the MEK1 inhibitor PD098059, indicating that the Raf-MEK-MAPK cascade is required for their expression. In cells expressing a conditionally active form of Raf-1 (DeltaRaf-1:ER), we observed that selective, sustained activation of Raf-MEK-MAPK was sufficient to induce expression of Fra-1, Fra-2, and JunB but, interestingly, induced little or no c-Fos or c-Jun. The induction of c-Fos observed in response to LPA was strongly inhibited by buffering the intracellular [Ca2+]. Moreover, although Raf activation or calcium ionophores induced little c-Fos expression, we observed a synergistic induction in response to the combination of DeltaRaf-1:ER and ionomycin. These results suggest that kinetically distinct phases of MAPK activation serve to regulate the expression of distinct AP-1 components such that sustained MAPK activation is required for the induced expression of Fra-1, Fra-2, c-Jun, and JunB. However, in contrast to the case for Fra-1, Fra-2, and JunB, activation of the MAPK cascade alone is not sufficient to induce c-Fos expression, which rather requires cooperation with other signals such as Ca2+ mobilization. Finally, the identification of the Fra-1, Fra-2, c-Jun, and JunB genes as genes which are selectively regulated by sustained MAPK activation or in response to activated Raf suggests that they are candidates to mediate certain of the effects of Ras proteins in oncogenic transformation.

Angiotensin II signaling in vascular smooth muscle cells under high glucose conditions

The mechanisms responsible for the accelerated cardiovascular disease in diabetes, as well as the increased hypertrophic effects of angiotensin II (Ang II) under hyperglycemic conditions, are not very clear. We examined whether the culture of vascular smooth muscle cells (VSMC) under hyperglycemic conditions to simulate the diabetic state can lead to increased activation of key growth- and stress-related kinases, such as the mitogen-activated protein kinases (MAPKs), in the basal state and in response to Ang II. Treatment of porcine VSMC for short time periods (0.5 to 3 hours) with high glucose (HG; 25 mmol/L) markedly increased the activation of the extracellular signal-regulated kinase (ERK1/2) and c-Jun/N-terminal kinase (JNK) relative to cells cultured in normal glucose (NG; 5.5 mmol/L). p38 MAPK also was activated by HG, and this effect remained sustained for several hours. Ang II treatment increased the activity of all 3 families of MAPKs. Ang II-induced ERK activation was potentiated nearly 2-fold in cells treated with HG for 0.5 hour. However, Ang II-induced JNK was not altered. In VSMC cultured for 24 hours with HG, Ang II and HG displayed an additive response on p38 MAPK activity. MAPKs can lead to activation of transcription factors such as activator protein-1 (AP-1). HG alone significantly increased AP-1 DNA-binding activity. Furthermore, Ang II and HG combined had additive effects on AP-1 activity. These results suggest that increased activation of specific MAPKs and downstream transcription factors, such as AP-1, may be key mechanisms for the increased VSMC growth potential of HG alone and of Ang II under HG conditions.

Protein kinase Ctheta, a selective upstream regulator of JNK/SAPK and IL-2 promoter activation in Jurkat T cells

The predominant expression of protein kinase C (PKC) theta in T cells (J. Biol. Chem. 1993. 268: 4997-5004), its isoenzyme-specific ability to stimulate AP-1 transcriptional activity (Mol. Cell. Biol. 1996. 16: 1842-1850) and the recent discovery of its selective and antigen-dependent colocalization with the contact region between T cells and antigen-presenting cells (Nature 1997. 385: 83-89) suggest that, among the PKC family members, PKCtheta plays a specialized role in T cell activation. By investigating the downstream effectors of PKCtheta we now demonstrate a direct and isoenzyme-specific contribution of PKCtheta to c-Jun-N-terminal kinase/stress-activated protein kinase (JNK/SAPK) but not extracellular regulated kinase (ERK) activation. Expression of a constitutively active (CA) form of PKCtheta (but not CA-PKCalpha, epsilon and lambda/iota) resulted in strong activation of JNK/SAPK and expression of a dominant-negative form of PKCtheta interfered with the endogenous activation signal for JNK/SAPK. Importantly, Ca2+ ionophore and CA-PKCtheta (but not CA-PKCalpha, epsilon and lambda/iota) caused synergistic activation of the IL-2 promoter. Together, these data establish that PKCtheta is required for activation of JNK/SAPK signaling leading to IL-2 promoter transcription in T lymphocytes.

c-Jun N-terminal kinase phosphorylates peroxisome proliferator-activated receptor-gamma1 and negatively regulates its transcriptional activity

The peroxisome proliferator-activated receptor-gamma (PPARgamma) transcription factor plays a pivotal role in adipocyte differentiation and metabolic regulation. The transcriptional activity of PPARgamma is positively modulated by ligand binding and negatively regulated by phosphorylation mediated by the MEK/ERK signaling pathway. The phosphorylation of mouse PPARgamma1 at Ser82 by ERK causes a decrease in both basal and ligand-dependent transcriptional activity. In this report we examined the ability of other mitogen-activated protein kinase family members to phosphorylate PPARgamma1. We demonstrate that in vitro, PPARgamma1 is efficiently phosphorylated by JNK/SAPK (c-Jun N-terminal kinase or stress-activated protein kinase) but only weakly phosphorylated by p38. In transfected 293T cells, PPARgamma1 is phosphorylated at Ser82 in response to known JNK activators such as UV irradiation and anisomycin treatment. This phosphorylation is not blocked by either the specific MEK inhibitor PD98059 or the p38 inhibitor SB203580, indicating that it is independent of the MEK/ERK and p38 signaling pathways. Finally, in transient transfection reporter assays, activation of JNK by anisomycin or by overexpression of MKK4 (the upstream JNK kinase) decreased ligand-dependent PPARgamma1 transcriptional activity. These results suggest that the activation of the JNK/SAPK pathway by extracellular signals, perhaps by inflammatory cytokines such as tumor necrosis factor-alpha, would result in a reduction of PPARgamma transcriptional activity and reduce the effects of PPARgamma ligands.

Defective T cell differentiation in the absence of Jnk1

The c-Jun NH2-terminal kinase (JNK) signaling pathway has been implicated in the immune response that is mediated by the activation and differentiation of CD4 helper T (TH) cells into TH1 and TH2 effector cells. JNK activity observed in wild-type activated TH cells was severely reduced in TH cells from Jnk1-/- mice. The Jnk1-/- T cells hyperproliferated, exhibited decreased activation-induced cell death, and preferentially differentiated to TH2 cells. The enhanced production of TH2 cytokines by Jnk1-/- cells was associated with increased nuclear accumulation of the transcription factor NFATc. Thus, the JNK1 signaling pathway plays a key role in T cell receptor-initiated TH cell proliferation, apoptosis, and differentiation.

Sequential activation of activator protein-1-related transcription factors and JNK protein kinases may contribute to apoptotic death induced by transient hypoxia in developing brain neurons

Previous studies have demonstrated that transient hypoxia (6 h) induces apoptotic death in cultured neurons isolated from the fetal rat forebrain. Since activation of c-Jun N-terminal kinases (JNKs) and subsequent phosphorylation of c-Jun are suspected to be involved in the apoptotic pathway in several cell types, the time course of activator protein-1 (AP-1) DNA-binding, in line with induction of the AP-1 components and JNK activation, was examined during hypoxia/reoxygenation in the same model. Gel shift analysis depicted the presence of functional AP-1 transcription factors in both control and hypoxic neurons. One hour after the onset of hypoxia, all AP-1 components were markedly overexpressed. They include c-Jun, Jun B, Jun D, c-Fos and Fos-related antigens. Whereas, only c-Jun remained elevated for up to 96 h post-reoxygenation, time at which neurons were injured, other gene products showed patterned induction/repression as hypoxia progressed and then during the post-reoxygenation period, with Fos-related antigens being finally induced at 96 h. Only JNK1 was constitutively detected in cultured neurons, and its expression was inhibited during hypoxia. Nonetheless, both JNK1 and JNK3 were markedly, but transiently, induced at 48 h post-reoxygenation, when apoptosis-related morphological features became apparent. These data support the hypothesis that transient hypoxia, independently of ischemia, may trigger apoptosis through JNK signaling pathway in developing brain neurons.

Pseudomonas pyocyanin increases interleukin-8 expression by human airway epithelial cells

Pseudomonas aeruginosa, an opportunistic human pathogen, causes acute pneumonia in patients with hospital-acquired infections and is commonly associated with chronic lung disease in individuals with cystic fibrosis (CF). Evidence suggests that the pathophysiological effects of P. aeruginosa are mediated in part by virulence factors secreted by the bacterium. Among these factors is pyocyanin, a redox active compound that increases intracellular oxidant stress. We find that pyocyanin increases release of interleukin-8 (IL-8) by both normal and CF airway epithelial cell lines and by primary airway epithelial cells. Moreover, pyocyanin synergizes with the inflammatory cytokines tumor necrosis factor alpha and IL-1alpha. RNase protection assays indicate that increased IL-8 release is accompanied by increased levels of IL-8 mRNA. The antioxidant n-acetyl cysteine, general inhibitors of protein tyrosine kinases, and specific inhibitors of mitogen-activated protein kinases diminish pyocyanin-dependent increases in IL-8 release. Conversely, inhibitors of protein kinases C (PKC) and PKA have no effect. In contrast to its effects on IL-8 expression, pyocyanin inhibits cytokine-dependent expression of the monocyte/macrophage/T-cell chemokine RANTES. Increased release of IL-8, a potent neutrophil chemoattractant, in response to pyocyanin could contribute to the marked infiltration of neutrophils and subsequent neutrophil-mediated tissue damage that are observed in Pseudomonas-associated lung disease.

Structural organization of MAP-kinase signaling modules by scaffold proteins in yeast and mammals

MAP-kinase signaling pathways are activated by multiple extracellular stimuli. The specificity of activation and function of MAP-kinase signaling modules is determined, in part, by scaffold proteins that create multienzyme complexes. In Saccharomyces cerevisiae, two MAP-kinase-scaffold proteins have been identified. Recent studies of mammalian cells have also led to the identification of putative scaffold proteins. These scaffold proteins appear to facilitate MAP-kinase activation, in response to specific physiological stimuli, and to insulate the bound MAP-kinase module against activation by irrelevant stimuli. Scaffold proteins are therefore critical components of MAP-kinase modules and ensure signaling specificity.

Signaling from polyomavirus middle T and small T defines different roles for protein phosphatase 2A

Polyomavirus causes a broad spectrum of tumors as the result of the action of its early proteins. This work compares signaling from middle T antigen (MT), the major transforming protein, to that from small T antigen (ST). The abilities of MT mutants to promote cell cycle progression in serum-starved NIH 3T3 cells were compared. Transformation-defective mutants lacking association with SHC or with phosphatidylinositol 3-kinase (PI3-K) retained the ability to induce DNA synthesis as measured by bromodeoxyuridine incorporation. Only when both interactions were lost in the Y250F/Y315F double mutant was MT inactive. ST promoted cell cycle progression in a manner dependent on its binding of protein phosphatase 2A (PP2A). Since the Y250F/Y315F MT mutant was wild type for PP2A binding yet unable to promote cell cycle progression, while ST was capable of promoting cell cycle progression, these experiments revealed a functional difference in MT and ST signaling via PP2A. Assays testing the abilities of MT and ST to induce the c-fos promoter and to activate c-jun kinase led to the same conclusion. ST, but not Y250F/Y315F MT, was able to activate the c-fos promoter through its interaction with PP2A. In contrast, MT, but not ST, was able to activate c-jun kinase by virtue of its interaction with PP2A.

Downstream factors in transforming growth factor-beta family signaling

The recently identified family of Smad proteins has given insight in the understanding of how members of the transforming growth factor-beta (TGF-beta) family relay their signal to the nucleus. Besides Smad proteins, G proteins and MAPKs are also involved in the downstream signaling of TGF-beta family members. The identification of elements that function downstream in the TGF-beta signaling pathway and the fact that these downstream players can interact with the signaling cascade of other growth factors, may give insight into the diverse biological responses evoked by the TGF-beta family members.

Stimulation of the DNA binding activity of AP-1 by the peroxisome proliferator ciprofibrate and eicosanoids in cultured rat hepatocytes

Peroxisome proliferators induce hepatic peroxisome proliferation and hepatic tumors in rodents. These chemicals increase the expression of the peroxisomal beta-oxidation pathway and the cytochrome P-450 4A family, which metabolize lipids, including eicosanoids. Peroxisome proliferators also induce increased cell proliferation in vivo. However, peroxisome proliferators are only weakly mitogenic and are not comitogenic with epidermal growth factor (EGF) in cultured hepatocytes. Our earlier studies found that the peroxisome proliferator ciprofibrate is comitogenic with eicosanoids. We therefore hypothesized that the comitogenicity of the peroxisome proliferator ciprofibrate and eicosanoids may result from a synergistic increase of the DNA binding activity of AP-1. Primary rat hepatocytes were cultured on collagen gels in serum-free L-15 medium with ciprofibrate, eicosanoids, and/or growth factors. The DNA binding activity of AP-1 was determined in nuclear protein extracts by electrophoretic mobility shift assay. The DNA binding activity of AP-1 was not induced by ciprofibrate or eicosanoids alone, but the addition of eicosanoids along with ciprofibrate increased the induction of DNA binding activity of AP-1 at 30 min and 2 h after exposure. The combination of ciprofibrate and PGF2alpha blocked the inhibitory effect of transforming growth factor (TGF)-beta on the DNA binding activity of AP-1 induced by EGF. These results show that the peroxisome proliferator ciprofibrate and eicosanoids co-stimulate the DNA binding activity of AP-1 and suggest that changes in eicosanoid concentrations may modulate mitogenic signal transduction pathways by the peroxisome proliferator ciprofibrate.

Angiotensin II induces monocyte chemoattractant protein-1 gene expression in rat vascular smooth muscle cells

Monocyte infiltration into the vessel wall, a key initial step in the process of atherosclerosis, is mediated in part by monocyte chemoattractant protein-1 (MCP-1). Hypertension, particularly in the presence of an activated renin-angiotensin system, is a major risk factor for the development of atherosclerosis. To investigate a potential molecular basis for a link between hypertension and atherosclerosis, we studied the effects of angiotensin II (Ang II) on MCP-1 gene expression in rat aortic smooth muscle cells. Rat smooth muscle cells treated with Ang II exhibited a dose-dependent increase in MCP-1 mRNA accumulation that was prevented by the AT1 receptor antagonist losartan. Ang II also activated MCP-1 gene transcription. Inhibition of NADH/NADPH oxidase, which generates superoxide and H2O2, with diphenylene iodonium or apocynin decreased Ang II-induced MCP-1 mRNA accumulation. Induction of MCP-1 gene expression by Ang II was inhibited by catalase, suggesting a second messenger role for H2O2. The tyrosine kinase inhibitor genistein and the mitogen-activated protein kinase kinase inhibitor PD098059 inhibited Ang II-induced MCP-1 gene expression, consistent with a mitogen-activated protein kinase-dependent signaling mechanism. Ang II may thus promote atherogenesis by direct activation of MCP-1 gene expression in vascular smooth muscle cells.

From Vanadis to Atropos: vanadium compounds as pharmacological tools in cell death signalling

Vanadium compounds exert a variety of biological responses, the most notable being their effects as insulin mimetics. More recently, they have been used as pharmacological tools to investigate signalling pathways. Some peroxovanadium compounds act as powerful protein tyrosine phosphatase inhibitors, modulating both the extent and duration of phosphotyrosine signals at the level of the transmembrane growth factor receptors and targets in the cytoplasm and nucleus. A brief history of vanadium compounds, selected chemical properties of vanadium compounds and the ability of peroxovanadium complexes to modulate the activities of protein tyrosine phosphatases and tyrosine kinases are presented in this review by Anne Morinville, Dusica Maysinger and Alan Shaver. From the range of biological activities of these compounds, this review focuses on cytotoxic effects and possible roles of mitogen-activated protein kinases in mediating the effects exerted by vanadium compounds.