Distinct Mitogen-activated Protein Kinase Pathways Research Articles

Binding of anti-dsDNA antibodies to proximal tubular epithelial cells contributes to renal tubulointerstitial inflammation.

Immune deposits are often observed along the tubular basement membrane in patients with lupus nephritis, but the role of anti-dsDNA antibody (Ab) deposition on tubulointerstitial inflammation remains to be investigated. We examined the effect of human polyclonal anti-dsDNA Abs on inflammatory processes in cultured proximal renal tubular epithelial cells (PTEC, HK-2 cells) and their association with serum levels of interleukin (IL)-6, IL-8 and monocyte chemoattractant protein-1 (MCP-1) in patients. Binding of anti-dsDNA Abs to HK-2 cells was investigated by cellular ELISA, flow cytometry and immunohistochemistry. IL-6, IL-8 and MCP-1 secretion, mitogen-activated protein kinase (MAPK) activation and the effect of mycophenolic acid (MPA) were investigated by ELISAs and Western blot analysis. NZBWF1/J mice with active nephritis were randomized to receive either mycophenolate mofetil (MMF) (100mg/kg per day) or vehicle for up to 12weeks to study renal histopathology focusing on tubulointerstitial changes. Our results demonstrated that anti-dsDNA Abs bound to HK-2 cell surface and induced IL-6, IL-8 and MCP-1 secretion through distinct MAPK pathways. MPA inhibited anti-dsDNA Ab binding to HK-2 cells and suppressed apical and basolateral IL-6 and IL-8, but not MCP-1, secretion. Anti-dsDNA Ab level correlated with serum and tubulointerstitial expression of IL-6, IL-8 and MCP-1. MMF treatment in NZBWF1/J mice reduced anti-dsDNA Ab production and MAPK activation in the renal tubulointerstitium, together with decreased IL-6 and MCP-1 expression. Our data demonstrate that anti-dsDNA Abs contribute to inflammatory processes in the tubulointerstitium in lupus nephritis through their binding to proximal renal tubular epithelial cells and induction of pro-inflammatory mediators, and MPA ameliorates anti-dsDNA Ab induced IL-6 and IL-8 secretion in these cells.

Dual‐specificity MAP kinase phosphatases (MKPs)

Dual-specificity MAP kinase phosphatases (MKPs) provide a complex negative regulatory network that acts to shape the duration, magnitude and spatiotemporal profile of MAP kinase activities in response to both physiological and pathological stimuli. Individual MKPs may exhibit either exquisite specificity towards a single mitogen-activated protein kinase (MAPK) isoform or be able to regulate multiple MAPK pathways in a single cell or tissue. They can act as negative feedback regulators of MAPK activity, but can also provide mechanisms of crosstalk between distinct MAPK pathways and between MAPK signalling and other intracellular signalling modules. In this review, we explore the current state of knowledge with respect to the regulation of MKP expression levels and activities, the mechanisms by which individual MKPs recognize and interact with different MAPK isoforms and their role in the spatiotemporal regulation of MAPK signalling.

Distinct MAPK pathways are involved in IL-23 production in dendritic cells cocultured with NK cells in the absence or presence of angiotensin II

Accumulated evidence suggests that the crosstalk between dendritic cells (DCs) and natural killer (NK) cells enhances each other's capacity, and results in the production of a variety of soluble factors. However, little is known about the effect of DC–NK crosstalk in interleukin-23 (IL-23) production. In the present study we show that DC–NK coculture caused a high expression of IL-23, angiotensin II (Ang II) alone moderately increased IL-23 production in DCs, but decreased IL-23 secretion in the DC–NK coculture system. We found that Ang II does not influence DC maturation in DC–NK crosstalk. We next investigated the mitogen-activated protein kinase (MAPK) pathway in DCs. We found that Ang II increased IL-23 production through the extracellular signal-related kinase (ERK) pathway. All three MAPK members c-Jun N-terminal kinase (JNK), ERK and p38 were involved in IL-23 production in the DC–NK coculture system. In the presence of Ang II, only the JNK pathway seems to play a role in IL-23 production in DCs cocultured with NK. These data suggest that distinct MAPK pathways are involved in IL-23 production in DCs in response to different stimuli. This work demonstrates for the first time that IL-23 is produced in the DC–NK coculture system, and that Ang II is involved in DC–NK crosstalk. This data will act as a resource that allows further exploitation of role of immune response in atherosclerosis.

Crosstalk Between Estrogen Receptor and Mitogen-Activated Protein Kinase Signaling in the Development and Progression of Endometrial Cancer

The objectives of the study were to evaluate the role of mitogen-activated protein kinase (MAPK) signaling in normal, hyperplastic, and neoplastic endometrium in relation to estrogen receptor (ER) status and to investigate whether 17β-estradiol (E2) and tamoxifen (TAM) mediate the proliferation and apoptosis of endometrial cancer cells through the MAPK pathway. The expressions of phosphorylated and total extracellular signal-regulated kinases 1/2 (phosphorylated extracellular signal-regulated kinase 1/2 [p-ERK1/2] and total ERK1/2 [t-ERK1/2]) were analyzed with immunohistochemistry in normal, hyperplastic, and neoplastic endometrium. The expression levels of p-ERK1/2 and t-ERK1/2 in RL95-2 and KLE after stimulation by E2, progesterone (P), and TAM were detected by Western blotting. The effects of E2 and TAM in combination with MAPK pathway inhibitors on the growth and apoptosis of endometrial cancer cells were examined by the MTS assay and flow cytometry analysis. The expression level of p-ERK1/2 was significantly associated with the International Federation of Gynecology and Obstetrics stage (P = 0.0072). The ratio of phosphorylated/total ERK1/2 was higher in ER-positive endometrial cancer tissues and cells (P < 0.05). 17β-Estradiol increased ERK1/2 phosphorylation, and TAM decreased ERK1/2 phosphorylation in endometrial cancer cell lines within 30 minutes (P < 0.05). The MEK1/2 inhibitor, U0126, and the stress-activated protein kinase/c-Jun NH2-terminal kinase inhibitor, SP600125, significantly suppressed the proliferation of human endometrial cancer cell lines RL95-2 and KLE induced by E2 (P < 0.05). The level of TAM-induced apoptosis was greater in KLE than in RL95-2 cells, and the p38 cascade was involved in the TAM-induced apoptosis of both cell lines (P < 0.05). The cross-talk between MAPK signaling and ER status might exert a key role in progression of endometrial cancer. Furthermore, the effects of E2 or TAM on the proliferation or apoptosis of ER-positive and ER-negative endometrial cancer cells were mediated through distinct MAPK pathways. These mechanisms might contribute to ER-specific differences in MAPK activation for molecular-target therapies in endometrial carcinoma.

Cells Prioritize Responses When Faced With a Decision Between an Environmental Stress and a Developmental Cue

When faced with competing signals cells must decide how to prioritize and coordinate their responses. Many signals are transduced via a mitogen activated protein kinase (MAPK) cascade. Yeast, Saccharomyces cerevisiae, respond to a developmental signal (mating pheromone) and an environmental stress (hyperosmotic stress) by two distinct MAPK pathways. It has been previously suggested that the two pathways are mutually exclusive. Here we find that cells respond to each stimulus and effectively prioritize the responses. The mating response was initially measured by the formation of a mating projection (shmoo). We found that shmoo formation is delayed by the presence of hyperosmotic stress and increasing stress concentration increases the delay period. We also measured mating transcription induction and mating MAPK activation and found both to be delayed by the presence of stress. The delay in shmoo formation correlates with induction and stabilization of a shmoo landmark protein Far1. Finally we establish that the stress MAPK is necessary for the delay in mating MAPK activation. We conclude that hyperosmotic stress delays the mating response by delaying MAPK activation. This delay allows cells to restore their osmotic balance before Far1 is induced, stabilized, and promotes shmoo formation. These findings illustrate how cells can prioritize multiple competing signals at the level of the MAPK activation.

Rho GTPase CDC42 regulates directionality and random movement via distinct MAPK pathways in neutrophils

AbstractNeutrophil transmigration into tissue is a multiple-step process that results from a coordinated rearrangement of the cytoskeleton and adhesion complexes. Assembly and disassembly of actin and adhesion structures dictate motility behavior, while polarity and gradient sensing provide directionality to the cell movement. Here, using mice deficient in the CDC42 regulator CDC42 GTPase-activating protein (CDC42GAP), we demonstrate that CDC42 activity separately regulates neutrophil motility and directionality. CDC42GAP–/– neutrophils showed increased motility, while directed migration was defective. Podosome-like structures present at the leading edge in wild-type neutrophils were significantly reduced in CDC42GAP–/– cells. CDC42GAP–/– neutrophils also showed increased lateral and tail filopodia-like formation, and excess membrane protrusions. We further suggest that CDC42GAP-mediated extracellular signal–regulated kinase (ERK) activity regulates motility associated with podosome-like structures at the cell leading edge, while CDC42GAP-induced p38MAPK phosphorylation regulates directed migration by antagonizing filopodia assembly. Overall, this study reveals that CDC42 activity regulates both motility and directionality in neutrophils, but via distinct mitogen-activated protein kinase (MAPK) pathways.

Distinct Roles for Ras-Guanine Nucleotide-Releasing Factor 1 (Ras-GRF1) and Ras-GRF2 in the Induction of Long-Term Potentiation and Long-Term Depression

NMDA-type glutamate receptors (NMDARs) contribute to many forms of long-term potentiation (LTP) and long-term depression (LTD). NMDARs are heteromers containing calcium-permeating neuronal receptor 1 (NR1) subunits and a variety of NR2 subunits. Evidence suggests that, in the CA1 region of the hippocampus, NR2A-containing NMDARs promote LTP whereas NR2B-containing receptors promote LTD. However, the calcium sensors that distinguish between these signals to promote the appropriate form of synaptic plasticity are not known. Ras-guanine nucleotide-releasing factor 1 (Ras-GRF1) and Ras-GRF2 are highly similar calcium-stimulated exchange factors that activate Ras and Rac GTPases. Here, using a set of Ras-GRF knock-out mice, we show that Ras-GRF2 contributes predominantly to the induction of NMDAR-dependent LTP, whereas Ras-GRF1 contributes predominantly to the induction of NMDAR-dependent LTD in the CA1 region of the hippocampus of postpubescent mice (postnatal days 25-36). In contrast, neither Ras-GRF protein influences synaptic plasticity in prepubescent mice (postnatal days 14-18). Ras-GRF2 mediates signaling from (R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl-phosphonic acid-sensitive (NVP-AAM077-sensitive) (NR2A-containing) NMDARs to the Ras effector extracellular signal-related protein kinase 1/2 (Erk1/2) mitogen-activated protein (MAP) kinase, a promoter of NMDAR-induced LTP at this site. In contrast, Ras-GRF1 mediates signaling from ifenprodil-sensitive (NR2B-containing) NMDARs to the Rac effector p38 MAP kinase, a promoter of LTD. These findings show that, despite their similar functional domain organization, Ras-GRF1 and Ras-GRF2 mediate opposing forms of synaptic plasticity by coupling different classes of NMDARs to distinct MAP kinase pathways. Moreover, the postnatal appearance of Ras-GRF-dependent LTP and LTD coincides with the emergence of hippocampal-dependent behavior, implying that Ras-GRF proteins contribute to forms of synaptic plasticity that are required specifically for mature hippocampal function.

Integration of Caenorhabditis elegans MAPK pathways mediating immunity and stress resistance by MEK-1 MAPK kinase and VHP-1 MAPK phosphatase.

The p38 and JNK classes of mitogen-activated protein kinases (MAPKs) have evolutionarily conserved roles in the control of cellular responses to microbial and abiotic stresses. The mechanisms by which crosstalk between distinct p38 and c-Jun N-terminal kinase (JNK) MAPK pathways occurs with resultant integration of signaling information have been difficult to establish, particularly in the context of whole organism physiology. In Caenorhabditis elegans a PMK-1 p38 MAPK pathway is required for resistance to bacterial infection, and a KGB-1 JNK-like MAPK pathway has recently been shown to mediate resistance to heavy metal stress. Here, we show that two components of the KGB-1 pathway, MEK-1 MAPK kinase (MAPKK), a homolog of mammalian MKK7, and VHP-1 MAPK phosphatase (MKP), a homolog of mammalian MKP7, also regulate pathogen resistance through the modulation of PMK-1 activity. The regulation of p38 and JNK-like MAPK pathways mediating immunity and heavy metal stress by common MAPKK and MKP signaling components suggests pivotal roles for MEK-1 and VHP-1 in the integration of diverse stress signals contributing to pathogen resistance in C. elegans. In addition, these data point to mechanisms in multicellular organisms by which signals transduced by distinct MAPK pathways may be subject to physiological integration at the level of regulation of MAPK activity by MAPKKs and MKPs.

Roles of mitogen-activated protein kinase and phosphoinositide 3′-kinase in ErbB2/ErbB3 coreceptor-mediated heregulin signaling☆

ErbB2/HER2 and ErbB3/HER3, two members of the ErbB/HER family, together constitute a heregulin coreceptor complex that elicits a potent mitogenic and transforming signal. Among known intracellular effectors of the ErbB2/ErbB3 heregulin coreceptor are mitogen-activated protein kinase (MAPK) and phosphoinositide (PI) 3-kinase. Activation of the distinct MAPK and PI 3-kinase signaling pathways by the ErbB2/ErbB3 coreceptor in response to heregulin and their relative contributions to the mitogenic and transformation potentials of the activated coreceptor were investigated here. To this end, cDNAs encoding the wild-type ErbB3 protein (ErbB3-WT) and ErbB3 proteins with amino acid substitutions in either the Shc-binding site (ErbB3-Y1325F), the six putative PI 3-kinase-binding sites (ErbB3-6F), or both (ErbB3-7F) were generated and expressed in NIH-3T3 cells to form functional ErbB2/ErbB3 heregulin coreceptors. While the coreceptor incorporating ErbB3-WT activated both the MAPK and the PI 3-kinase signaling pathways, those incorporating ErbB3-Y1325F or ErbB3-6F activated either PI 3-kinase or MAPK, respectively. The ErbB2/ErbB3-7F coreceptor activated neither. Elimination of either signaling pathway lowered basal and eliminated heregulin-dependent expression of cyclin D1, which was in each case accompanied by an attenuated mitogenic response. Selective elimination of the PI 3-kinase pathway severely impaired the ability of heregulin to transform cells expressing the coreceptor, whereas attenuation of the MAPK pathway had a lesser effect. Thus, while both pathways contributed in a roughly additive manner to the mitogenic response elicited by the activated ErbB2/ErbB3 coreceptor, the PI 3-kinase pathway predominated in the induction of cellular transformation.

Growth factors can activate ATF2 via a two-step mechanism: phosphorylation of Thr71 through the Ras-MEK-ERK pathway and of Thr69 through RalGDS-Src-p38.

Transcription factor ATF2 regulates gene expression in response to environmental changes. Upon exposure to cellular stresses, the mitogen-activated proteinkinase (MAPK) cascades including SAPK/JNK and p38 can enhance ATF2's transactivating function through phosphorylation of Thr69 and Thr71. How ever, the mechanism of ATF2 activation by growth factors that are poor activators of JNK and p38 is still elusive. Here, we show that in fibroblasts, insulin, epidermal growth factor (EGF) and serum activate ATF2 via a so far unknown two-step mechanism involving two distinct Ras effector pathways: the Raf-MEK-ERK pathway induces phosphorylation of ATF2 Thr71, whereas subsequent ATF2 Thr69 phosphorylation requires the Ral-RalGDS-Src-p38 pathway. Cooperation between ERK and p38 was found to be essential for ATF2 activation by these mitogens; the activity of p38 and JNK/SAPK in growth factor-stimulated fibroblasts is insufficient to phosphorylate ATF2 Thr71 or Thr69 + 71 significantly by themselves, while ERK cannot dual phosphorylate ATF2 Thr69 + 71 efficiently. These results reveal a so far unknown mechanism by which distinct MAPK pathways and Ras effector pathways cooperate to activate a transcription factor.

Evidence for a role of p38 MAP kinase in expression of alkaline phosphatase during osteoblastic cell differentiation

In the present study, we investigate the implication of the mitogen-activated protein kinases (MAPKs) Erk, p38, and JNK in mediating the effect of fetal calf serum (FCS) on the differentiation of MC3T3-E1 osteoblast-like cells. Erk is stimulated by FCS in proliferating, early-differentiating, as well as in mature cells. Activation of p38 by FCS is not detected in proliferating cells but is observed as the cells differentiate. JNK is activated in response to FCS throughout the entire differentiation process, but a maximal stimulation is observed in early differentiating cells. The roles of Erk and p38 pathways in mediating MC3T3-E1 cell differentiation was determined using specific inhibitors such as U0126 and SB203580, respectively. These experiments confirmed that the Erk pathway is essential for mediating cell proliferation in response to FCS, but indicated that this MAP kinase has little effect in regulating the differentiation of MC3T3-E1 cells. In contrast, p38 only marginally influenced proliferation, but appeared to be critical for the control of alkaline phosphatase (ALP) expression in differentiating cells. Finally, results obtained with high doses of SB203580, which also affected JNK activity, suggest that p38 and/or JNK are probably also involved in the control of type 1 collagen and osteocalcin expression in differentiating cells. The data indicate that MAPKs regulate different stages of MC3T3-E1 cell development in response to FCS. Distinct MAPK pathways seem to independently modulate osteoblastic cell proliferation and differentiation, with Erk playing an essential role in cell replication, whereas p38 is involved in the regulation of ALP expression during osteoblastic cell differentiation. JNK is also probably involved in the regulation of osteoblastic cell differentiation, but its precise role requires further investigation.

Untangling the Web

See related article, pages 882–890 The understanding of intracellular signaling pathways and their physiological effects has been confounded by the existence of numerous isoforms of the various signaling components. Thus, many families of protein kinases comprise several subfamilies, each of which may contain multiple isoforms deriving from distinct genetic loci. Furthermore, each locus may produce multiple products through alternative splicing. A rational argument maintains that each isoform has a distinct role in cellular regulation, but evidence of this is sparse. The alternative explanation, that there is redundancy, has led to the generation of web-like diagrams of interconnecting signaling pathways as investigators attempt to decipher the wiring patterns of the cell. In the heart, two superfamilies of protein kinases, the protein kinase Cs (PKCs) and the mitogen-activated protein kinases (MAPKs), are particularly implicated in the development of cardiac pathologies.1 Both superfamilies contain numerous isoforms, but little is known about the roles of individual isoforms. In this issue of Circulation Research , Heidkamp et al2 provide some of the first data showing that specific PKC isoforms couple to distinct MAPK pathways to regulate cardiac myocyte function. The PKC superfamily comprises the “classical” cPKCs (α, β1, β2, γ), “novel” nPKCs (δ, e, η, θ), “atypical” aPKCs (ζ, λ/ι), and PKC-related kinases (PRKs).3 In cardiac myocytes, cPKCα, nPKCδ, nPKCe, aPKCζ, and aPKCλ/ι are readily detectable, although cPKCβ isoforms may also be significant.4 Hypertrophic agonists such as endothelin-1 (ET-1) or the α-adrenergic agonist phenylephrine (PE) activate nPKCδ and nPKCe in cardiac myocytes, as shown by translocation from the soluble to the particulate fraction of the cell,1 and such studies implicate these isoforms in the hypertrophic response. Although the identities of individual PKC isoforms have been known for some …

Neurotrophins use the Erk5 pathway to mediate a retrograde survival response.

Growth factors synthesized and released by target tissues promote survival and differentiation of innervating neurons. This retrograde signal begins when growth factors bind receptors at nerve terminals. Activated receptors are then endocytosed and transported through the axon to the cell body. Here we show that the mitogen-activated protein kinase (MAPK) signaling pathways used by neurotrophins during retrograde signaling differ from those used following direct stimulation of the cell soma. During retrograde signaling, endocytosed neurotrophin receptors (Trks) activate the extracellular signal-related protein kinase 5 (Erk5) pathway, leading to nuclear translocation of Erk5, phosphorylation of CREB, and enhanced neuronal survival. In contrast, Erk1/2, which mediates nuclear responses following direct cell body stimulation, does not transmit a retrograde signal. Thus, the Erk5 pathway has a unique function in retrograde signaling. Differential activation of distinct MAPK pathways may enable an individual growth factor to relay information that specifies the location and the nature of stimulation.

MAP kinases in plant signal transduction: how many, and what for?

Mitogen-activated protein kinase (MAPK) pathways are protein kinase cascades that have a function in the transduction of extracellular signals to intracellular targets in all eukaryotes. Distinct MAPK pathways are regulated by different signals and have a role in a wide variety of physiological processes. In plants there is evidence for a role of MAPKs in the signaling of pathogens, abiotic stresses, plant hormones, and cell cycle cues. A large number of distinct MAPKs in plants have been identified that are all most similar to the animal ERK MAPKs. By sequence alignment all available full length plant MAPKs can be grouped into five subfamilies. Functional data exist for members of four subfamilies and show that different subfamilies encode MAPKs for specific functions. Analysis of partial MAPK sequences from full length, truncated cDNAs and expressed sequence tags (ESTs) revealed the presence of two new subfamilies in the plant MAPK superfamily. Signature sequences valid for the superfamily of plant MAPKs and each subfamily were derived and should help in future classification of novel MAPKs. The future challenge is to unambiguously assign functions to each MAPK and decipher the other partners of their signaling pathways.

MAP kinases in plant signal transduction.

Mitogen-activated protein kinase (MAPK) pathways are modules involved in the transduction of extracellular signals to intracellular targets in all eukaryotes. Distinct MAPK pathways are regulated by different extracellular stimuli and are implicated in a wide variety of biological processes. In plants there is evidence for MAPKs playing a role in the signaling of abiotic stresses, pathogens, plant hormones, and cell cycle cues. The large number and divergence of plant MAPKs indicates that this ancient mechanism of bioinformatics is extensively used in plants and their study promises to give molecular answers to old questions.

Role of distinct mitogen-activated protein kinase pathways and cooperation between Ets-2, ATF-2, and Jun family members in human urokinase-type plasminogen activator gene induction by interleukin-1 and tetradecanoyl phorbol acetate.

We have investigated the in vivo and in vitro regulation of the human urokinase-type plasminogen activator (uPA) gene by interleukin-1 (IL-1) and analyzed the transcription factors and signalling pathways involved in the response of the -2.0-kb uPA enhancer to IL-1 induction and to tetradecanoyl phorbol acetate (TPA) induction. Mutational analysis showed the cooperative activity of the Ets-binding site (EBS) and the two AP-1 elements of the enhancer. The results reveal that the EBS is required for the response to both inducers mediated by Ets-2, which is regulated at a level subsequent to DNA binding, by an IL-1- and phorbol ester-inducible transactivation domain. Both the IL-1 and the TPA-mediated induction result in a drastic increase of AP-1 binding to the downstream site of the enhancer (uPA 3' TPA-responsive element), while a mostly qualitative change, resulting from the interplay between ATF-2 homodimers and c-Jun-ATF-2 heterodimers, takes place at the upstream AP-1 element. The analysis of two distinct mitogen-activated protein kinase pathways shows that stress-activated protein kinase-Jun N-terminal kinase activation, resulting in the phosphorylation of ATF-2, c-Jun, and JunD, is required not only for the IL-1- but also for the TPA-dependent induction, while the extracellular signal-related kinase 1 (ERK-1) and ERK-2 activation is involved in the TPA- but not in the IL-1-dependent stimulation of the uPA enhancer.

UVB Activates ERK1/2 and p38 Signaling Pathways via Reactive Oxygen Species in Cultured Keratinocytes

We have previously shown that hydrogen peroxide is an important mediator of ultraviolet B induced phosphorylation of the epidermal growth factor receptor in human keratinocytes. Here we demonstrate that physiologic doses of ultraviolet B and hydrogen peroxide stimulate activation of two related but distinct mitogen-activated protein kinase pathways: extracellular regulated kinase 1 and 2 (ERK1/2), as well as p38, the mammalian homolog of HOG1 in yeast which is a major kinase for a recently identified stress-induced signaling pathway. The time-dependent activation of ERK1/2 and p38 are distinct, and ultraviolet B-induced ERK1/2 activation is downregulated more rapidly than p38. Using dihydrorhodamine or Amplex as specific fluorescent dye probes, we show that ultraviolet B-induced peroxides can be inhibited by ascorbic acid. Ascorbic acid strongly blocks ERK1/2 and p38 activation by ultraviolet B and hydrogen peroxide whereas pyrrolidine dithiocarbamate and butyl hydroxyanisole are less effective. Pyrrolidine dithiocarbamate was unable to inhibit ultraviolet B-induced p38 activation. Cell death was increased after ultraviolet B when ERK1/2 activation was attenuated by the specific inhibitor PD098059. The distinct time courses and extents of activation and inhibition of ERK1/2 and p38 indicate that these pathways are separate and regulated independently in keratinocytes. Specific types of reactive oxygen species induced by ultraviolet B as well as selective activation or inhibition of specific phosphatases may mediate these responses in keratinocytes. These findings demonstrate that reactive oxygen species are important multifunctional mediators of ultraviolet B-induced ERK1/2 and p38 signaling transduction pathways and suggest that ERK1/2 may play an important part in protecting keratinocytes from cell death following oxidative stress.

Adaptor Proteins Grb2 and Crk Couple Pyk2 with Activation of Specific Mitogen-activated Protein Kinase Cascades

The protein tyrosine kinase Pyk2 acts as an upstream regulator of mitogen-activated protein (MAP) kinase cascades in response to numerous extracellular signals. The precise molecular mechanisms by which Pyk2 activates distinct MAP kinase pathways are not yet fully understood. In this report, we provide evidence that the protein tyrosine kinase Src and adaptor proteins Grb2, Crk, and p130Cas act as downstream mediators of Pyk2 leading to the activation of extracellular signal-regulated kinase (ERK) and c-Jun amino-terminal kinase (JNK). Pyk2-induced activation of Src is necessary for phosphorylation of Shc and p130Cas and their association with Grb2 and Crk, respectively, and for the activation of ERK and JNK cascades. Expression of a Grb2 mutant with a deletion of the amino-terminal Src homology 3 domain or the carboxyl-terminal tail of Sos strongly reduced Pyk2-induced ERK activation, with no apparent effect on JNK activity. Grb2 with a deleted carboxyl-terminal Src homology 3 domain partially blocked Pyk2-induced ERK and JNK pathways, whereas expression of dominant interfering mutants of p130Cas or Crk specifically inhibited JNK but not ERK activation by Pyk2. Taken together, our data reveal specific pathways that couple Pyk2 with MAP kinases: the Grb2/Sos complex connects Pyk2 to the activation of ERK, whereas adaptor proteins p130Cas and Crk link Pyk2 with the JNK pathway.

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.

MAP kinases in plant signal transduction.

Mitogen-activated protein kinase (MAPK) pathways are modules involved in the transduction of extracellular signals to intracellular targets in all eukaryotes. Distinct MAPK pathways are regulated by different extracellular stimuli and are implicated in a wide variety of biological processes. In plants there is evidence for MAPKs playing a role in the signaling of abiotic stresses, pathogens and plant hormones. The large number and divergence of plant MAPKs indicates that this ancient mechanism of bioinformatics is extensively used in plants and may provide a new molecular handle on old questions.