Regulation of two JunD isoforms by Jun N-terminal kinases

JNK Signaling in Stem Cell Self-Renewal and Differentiation

C-JUN N-terminal kinases (JNKs), which belong to the mitogen-activated protein kinase (MAPK) family, are evolutionarily conserved kinases that mediate cell responses to various types of extracellular stress insults. They regulate physiological processes such as embryonic development and tissue regeneration, playing roles in cell proliferation and programmed cell death. JNK signaling is also involved in tumorigenesis and progression of several types of malignancies. Recent studies have shown that JNK signaling has crucial roles in regulating the traits of cancer stem cells (CSCs). Here we describe the functions of the JNK signaling pathway in self-renewal and differentiation, which are essential features of various types of stem cells, such as embryonic, induced pluripotent, and adult tissue-specific stem cells. We also review current knowledge of JNK signaling in CSCs and discuss its role in maintaining the CSC phenotype. A better understanding of JNK signaling as an essential regulator of stemness may provide a basis for the development of regenerative medicine and new therapeutic strategies against malignant tumors.

A Library of Phosphoproteomic and Chromatin Signatures for Characterizing Cellular Responses to Drug Perturbations

Although the value of proteomics has been demonstrated, cost and scale are typically prohibitive, and gene expression profiling remains dominant for characterizing cellular responses to perturbations. However, high-throughput sentinel assays provide an opportunity for proteomics to contribute at a meaningful scale. We present a systematic library resource (90 drugs × 6 cell lines) of proteomic signatures that measure changes in the reduced-representation phosphoproteome (P100) and changes in epigenetic marks on histones (GCP). A majority of these drugs elicited reproducible signatures, but notable cell line- and assay-specific differences were observed. Using the "connectivity" framework, we compared signatures across cell types and integrated data across assays, including a transcriptional assay (L1000). Consistent connectivity among cell types revealed cellular responses that transcended lineage, and consistent connectivity among assays revealed unexpected associations between drugs. We further leveraged the resource against public data to formulate hypotheses for treatment of multiple myeloma and acute lymphocytic leukemia. This resource is publicly available at https://clue.io/proteomics.

HTLV-1 Alters T Cells for Viral Persistence and Transmission

Human T-cell leukemia virus type 1 (HTLV-1) was the first retrovirus to be discovered as a causative agent of adult T-cell leukemia-lymphoma (ATL) and chronic inflammatory diseases. Two viral factors, Tax and HTLV-1 bZIP factor (HBZ), are thought to be involved in the leukemogenesis of ATL. Tax expression is frequently lost due to DNA methylation in the promoter region, genetic changes to the tax gene, and deletion of the 5′ long terminal repeat (LTR) in approximately half of all ATL cases. On the other hand, HBZ is expressed in all ATL cases. HBZ is known to function in both protein form and mRNA form, and both forms play an important role in the oncogenic process of HTLV-1. HBZ protein has a variety of functions, including the suppression of apoptosis, the promotion of proliferation, and the impairment of anti-viral activity, through the interaction with several host cellular proteins including p300/CBP, Foxp3, and Foxo3a. These functions dramatically modify the transcriptional profiling of host T cells. HBZ mRNA also promotes T cell proliferation and viability. HBZ changes infected T cells to CCR4⁺TIGIT⁺CD4⁺ effector/memory T cells. This unique immunophenotype enables T cells to migrate into various organs and tissues and to survive in vivo. In this review, we summarize how HBZ hijacks the transcriptional networks and immune systems of host T cells to contribute to HTLV-1 pathogenesis on the basis of recent new findings about HBZ and tax.

Lrp6 is a target of the PTH-activated αNAC transcriptional coregulator

In the nucleus of differentiated osteoblasts, the alpha chain of nascent polypeptide associated complex (αNAC) interacts with cJUN transcription factors to regulate the expression of target genes, including Osteocalcin (Bglap2). PTH induces the phosphorylation of αNAC on serine 99 through a Gαs-PKA dependent pathway. This leads to activation of αNAC and expression of Bglap2. To identify additional target genes regulated by PTH-activated αNAC, we performed ChIP-Seq against αNAC in PTH-treated MC3T3-E1 cells. This identified Low density lipoprotein receptor-Related Protein 6 (Lrp6) as a potential αNAC target. LRP6 acts as a co-receptor for the PTH receptor to allow optimal activation of PTH signaling. PTH increased Lrp6 mRNA levels in primary osteoblasts. Conventional quantitative ChIP confirmed the ChIP-Seq results. To assess whether αNAC plays a critical role in PTH-stimulated Lrp6 expression, we knocked-down Naca expression in MC3T3-E1 cells. Reduction of αNAC levels decreased basal expression of Lrp6 by 30% and blocked the stimulation of Lrp6 expression by PTH. We cloned the proximal mouse Lrp6 promoter (-2523/+120 bp) upstream of the luciferase reporter. Deletion and point mutations analysis in electrophoretic mobility shift assays and transient transfections identified a functional αNAC binding site centered around -343 bp. ChIP and ChIP-reChIP against JUND and αNAC showed that they cohabit on the proximal Lrp6 promoter. Luciferase assays confirmed that PTH-activated αNAC potentiated JUND-mediated Lrp6 transcription and Jund knockdown abolished this response. This study identified a novel αNAC target gene induced downstream of PTH signaling and represents the first characterization of the regulation of Lrp6 transcription in osteoblasts.

Hijacking of the AP-1 Signaling Pathway during Development of ATL

Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of a fatal malignancy known as adult T-cell leukemia (ATL). One way to address the pathology of the disease lies on conducting research with a molecular approach. In addition to the analysis of ATL-relevant signaling pathways, understanding the regulation of important and relevant transcription factors allows researchers to reach this fundamental objective. HTLV-1 encodes for two oncoproteins, Tax and HTLV-1 basic leucine-zipper factor, which play significant roles in the cellular transformation and the activation of the host's immune responses. Activating protein-1 (AP-1) transcription factor has been linked to cancer and neoplastic transformation ever since the first representative members of the Jun and Fos gene family were cloned and shown to be cellular homologs of viral oncogenes. AP-1 is a dimeric transcription factor composed of proteins belonging to the Jun (c-Jun, JunB, and JunD), Fos (c-Fos, FosB, Fra1, and Fra2), and activating transcription factor protein families. Activation of AP-1 transcription factor family by different stimuli, such as inflammatory cytokines, stress inducers, or pathogens, results in innate and adaptive immunity. AP-1 is also involved in various cellular events including differentiation, proliferation, survival, and apoptosis. Deregulated expression of AP-1 transcription factors is implicated in various lymphomas such as classical Hodgkin lymphomas, anaplastic large cell lymphomas, diffuse large B-cell lymphomas, and adult T-cell leukemia. Here, we review the current thinking behind deregulation of the AP-1 pathway and its contribution to HTLV-induced cellular transformation.

TGF-β Effects on Prostate Cancer Cell Migration and Invasion Require FosB

BACKGROUND

Activator Protein-1 (AP-1) family (cJun, JunB, JunD, cFos, FosB, Fra1, and Fra2) plays a central role in the transcriptional regulation of many genes that are associated with cell proliferation, differentiation, migration, metastasis, and survival. Many oncogenic signaling pathways converge at the AP-1 transcription complex. Transforming growth factor beta (TGF-β) is a multifunctional regulatory cytokine that regulates many aspects of cellular function, including cellular proliferation, differentiation, migration, apoptosis, adhesion, angiogenesis, immune surveillance, and survival.

METHODS

This study investigated, the role of FOS proteins in TGF-β signaling in prostate cancer cell proliferation, migration, and invasion. Steady state expression levels of FOS mRNA and proteins were determined using RT-PCR and western blotting analyses. DU145 and PC3 prostate cancer cells were exposed to TGF-β1 at varying time and dosage, RT-PCR, western blot, and immunofluorescence analyses were used to determine TGF-β1 effect on FOS mRNA and protein expression levels as well as FosB subcellular localization. Transient silencing of FosB protein was used to determine its role in cell proliferation, migration, and invasion.

RESULTS

Our data show that FOS mRNA and proteins were differentially expressed in human prostate epithelial (RWPE-1) and prostate cancer cell lines (LNCaP, DU145, and PC3). TGF-β1 induced the expression of FosB at both the mRNA and protein levels in DU145 and PC3 cells, whereas cFos and Fra1 were unaffected. Immunofluorescence analysis showed an increase in the accumulation of FosB protein in the nucleus of PC3 cells after treatment with exogenous TGF-β1. Selective knockdown of endogenous FosB by specific siRNA did not have any effect on cell proliferation in PC3 and DU145 cells. However, basal and TGF-β1- and EGF-induced cell migration was significantly reduced in DU145 and PC3 cells lacking endogenous FosB. TGF-β1- and EGF-induced cell invasion were also significantly decreased after FosB knockdown in PC3 cells.

CONCLUSION

Our data suggest that FosB is required for migration and invasion in prostate cancer cells. We also conclude that TGF-β1 effect on prostate cancer cell migration and invasion may be mediated through the induction of FosB. Prostate 77:72-81, 2017. © 2016 Wiley Periodicals, Inc.

JNK Signaling: Regulation and Functions Based on Complex Protein-Protein Partnerships

The c-Jun N-terminal kinases (JNKs), as members of the mitogen-activated protein kinase (MAPK) family, mediate eukaryotic cell responses to a wide range of abiotic and biotic stress insults. JNKs also regulate important physiological processes, including neuronal functions, immunological actions, and embryonic development, via their impact on gene expression, cytoskeletal protein dynamics, and cell death/survival pathways. Although the JNK pathway has been under study for >20 years, its complexity is still perplexing, with multiple protein partners of JNKs underlying the diversity of actions. Here we review the current knowledge of JNK structure and isoforms as well as the partnerships of JNKs with a range of intracellular proteins. Many of these proteins are direct substrates of the JNKs. We analyzed almost 100 of these target proteins in detail within a framework of their classification based on their regulation by JNKs. Examples of these JNK substrates include a diverse assortment of nuclear transcription factors (Jun, ATF2, Myc, Elk1), cytoplasmic proteins involved in cytoskeleton regulation (DCX, Tau, WDR62) or vesicular transport (JIP1, JIP3), cell membrane receptors (BMPR2), and mitochondrial proteins (Mcl1, Bim). In addition, because upstream signaling components impact JNK activity, we critically assessed the involvement of signaling scaffolds and the roles of feedback mechanisms in the JNK pathway. Despite a clarification of many regulatory events in JNK-dependent signaling during the past decade, many other structural and mechanistic insights are just beginning to be revealed. These advances open new opportunities to understand the role of JNK signaling in diverse physiological and pathophysiological states.

Gαi3-Dependent Inhibition of JNK Activity on Intracellular Membranes

Heterotrimeric G-protein signaling has been shown to modulate a wide variety of intracellular signaling pathways, including the mitogen-activated protein kinase (MAPK) family. The activity of one MAPK family class, c-Jun N-terminal kinases (JNKs), has been traditionally linked to the activation of G-protein coupled receptors (GPCRs) at the plasma membrane. Using a unique set of G-protein signaling tools developed in our laboratory, we show that subcellular domain-specific JNK activity is inhibited by the activation of Gαi3, the Gαi isoform found predominantly within intracellular membranes, such as the endoplasmic reticulum (ER)–Golgi interface, and their associated vesicle pools. Regulators of intracellular Gαi3, including activator of G-protein signaling 3 (AGS3) and the regulator of G-protein signaling protein 4 (RGS4), have a marked impact on the regulation of JNK activity. Together, these data support the existence of unique intracellular signaling complexes that control JNK activity deep within the cell. This work highlights some of the cellular pathways that are regulated by these intracellular complexes and identifies potential strategies for their regulation in mammalian cells.

MicroRNAs Enhance Keratinocyte Proliferative Capacity in a Stem Cell-Enriched Epithelium

MicroRNAs are critical regulators of stem cell behavior. The miR-103/107 family is preferentially expressed in the stem cell-enriched corneal limbal epithelium and plays an important role in coordinating several intrinsic characteristics of limbal epithelial stem cells. To elucidate further the mechanisms by which miRs-103/107 function in regulating limbal epithelial stem cells, we investigate the global effects of miRs-103/107 on gene expression in an unbiased manner. Using antagomirs-103/107, we knocked down endogenous miRs-103/107 in keratinocytes and conducted an mRNA profiling study. We show that miRs-103/107 target mitogen-activated protein kinase kinase kinase 7 (MAP3K7) and thereby negatively regulate the p38/AP-1 pathway, which directs epithelial cells towards a differentiated state. Pharmacological inhibition of p38 increases holoclone colony formation, a measure of proliferative capacity. This suggests that the negative regulation of p38 by miRs-103/107 contributes to enhanced proliferative capacity, which is a hallmark of stem cells. Since miRs-103/107 also promote increased holoclone colony formation by regulating JNK activation through non-canonical Wnt signaling, we believe that this microRNA family preserves “stemness” by mediating the crosstalk between the Wnt/JNK and MAP3K7/p38/AP-1 pathways.

Regulation of human CYP2C9 expression by electrophilic stress involves activator protein 1 activation and DNA looping

Cytochrome P450 (CYP)2C9 and CYP2C19 are important human enzymes that metabolize therapeutic drugs, environmental chemicals, and physiologically important endogenous compounds. Initial studies using primary human hepatocytes showed induction of both the CYP2C9 and CYP2C19 genes by tert-butylhydroquinone (tBHQ). As a pro-oxidant, tBHQ regulates the expression of cytoprotective genes by activation of redox-sensing transcription factors, such as the nuclear factor E2-related factor 2 (Nrf2) and members of the activator protein 1 (AP-1) family of proteins. The promoter region of CYP2C9 contains two putative AP-1 sites (TGAGTCA) at positions -2201 and -1930, which are also highly conserved in CYP2C19. The CYP2C9 promoter is activated by ectopic expression of cFos and JunD, whereas Nrf2 had no effect. Using specific kinase inhibitors for mitogen-activated protein kinase, we showed that extracellular signal-regulated kinase and Jun N-terminal kinase are essential for tBHQ-induced expression of CYP2C9. Electrophoretic mobility shift assays demonstrate that cFos distinctly interacts with the distal AP-1 site and JunD with the proximal site. Because cFos regulates target genes as heterodimers with Jun proteins, we hypothesized that DNA looping might be required to bring the distal and proximal AP-1 sites together to activate the CYP2C9 promoter. Chromosome conformation capture analyses confirmed the formation of a DNA loop in the CYP2C9 promoter, possibly allowing interaction between cFos at the distal site and JunD at the proximal site to activate CYP2C9 transcription in response to electrophiles. These results indicate that oxidative stress generated by exposure to electrophilic xenobiotics and metabolites induces the expression of CYP2C9 and CYP2C19 in human hepatocytes.

Specificity of linear motifs that bind to a common mitogen-activated protein kinase docking groove

Mitogen-activated protein kinases (MAPKs) have a docking groove that interacts with linear "docking" motifs in binding partners. To determine the structural basis of binding specificity between MAPKs and docking motifs, we quantitatively analyzed the ability of 15 docking motifs from diverse MAPK partners to bind to c-Jun amino-terminal kinase 1 (JNK1), p38α, and extracellular signal-regulated kinase 2 (ERK2). Classical docking motifs mediated highly specific binding only to JNK1, and only those motifs with a sequence pattern distinct from the classical MAPK binding docking motif consensus differentiated between the topographically similar docking grooves of ERK and p38α. Crystal structures of four complexes of MAPKs with docking peptides, representing JNK-specific, ERK-specific, or ERK- and p38-selective binding modes, revealed that the regions located between consensus positions in the docking motifs showed conformational diversity. Although the consensus positions in the docking motifs served as anchor points that bound to common MAPK surface features and mostly contributed to docking in a nondiscriminatory fashion, the conformation of the intervening region between the anchor points mostly determined specificity. We designed peptides with tailored MAPK binding profiles by rationally changing the length and amino acid composition of intervening regions located between anchor points. These results suggest a coherent structural model for MAPK docking specificity that reveals how short linear motifs binding to a common kinase docking groove can mediate diverse interaction patterns and contribute to correct MAPK partner selection in signaling networks.

The role of heterodimeric AP-1 protein comprised of JunD and c-Fos proteins in hematopoiesis

Activator protein-1 (AP-1) regulates a wide range of cellular processes including proliferation, differentiation, and apoptosis. As a transcription factor, AP-1 is commonly found as a heterodimer comprised of c-Jun and c-Fos proteins. However, other heterodimers may also be formed. The function of these dimers, specifically the heterodimeric AP-1 comprised of JunD and c-Fos (AP-1(JunD/c-Fos)), has not been elucidated. Here, we identified a function of AP-1(JunD/c-Fos) in Xenopus hematopoiesis. A gain-of-function study performed by overexpressing junD and c-fos and a loss-of-function study using morpholino junD demonstrate a critical role for AP-1(JunD/c-Fos) in hematopoiesis during Xenopus embryogenesis. Additionally, we confirmed that JunD of AP-1(JunD/c-Fos) is required for BMP-4-induced hematopoiesis. We also demonstrated that BMP-4 regulated JunD activity at the transcriptional regulation and post-translational modification levels. Collectively, our findings identify AP-1(JunD/c-Fos) as a novel hematopoietic transcription factor and the requirement of AP-1(JunD/c-Fos) in BMP-4-induced hematopoiesis during Xenopus hematopoiesis.

JunD/AP-1-mediated gene expression promotes lymphocyte growth dependent on interleukin-7 signal transduction

Interleukin-7 (IL-7) is an essential cytokine for lymphocyte growth that has the potential for promoting immune reconstitution. This feature makes IL-7 an ideal candidate for therapeutic development. As with other cytokines, signaling through the IL-7 receptor induces the JAK/STAT pathway. However, the broad scope of IL-7 regulatory targets likely necessitates the use of other signaling components whose identities remain poorly defined. To this end, we used an IL-7 dependent T-cell line to examine how expression of the glycolytic enzyme, Hexokinase II (HXKII) was regulated by IL-7 in a STAT5-independent manner. Our studies revealed that IL-7 promoted the activity of JNK (Jun N-terminal Kinase), and that JNK, in turn, drove the expression of JunD, a component of the Activating Protein 1 (AP-1) transcription factors. Gel shifts showed that the AP-1 complex induced by IL-7 contained JunD but not c-Fos or c-Jun. Inhibition of JNK/JunD blocked glucose uptake and HXKII gene expression, indicating that this pathway was responsible for promoting HXKII expression. Because others had shown that JunD was a negative regulator of cell growth, we performed a bioinformatics analysis to uncover possible JunD-regulated gene targets. Our search revealed that JunD could control the expression of proteins involved in signal transduction, cell survival and metabolism. One of these growth promoters was the oncogene, Pim-1. Pim-1 is an IL-7-induced protein that was inhibited when the activities of JNK or JunD were blocked, showing that in IL-7 dependent T-cells JunD can promote positive signals transduced through Pim-1. This was confirmed when the IL-7-induced proliferation of CD8 T-cells was impaired upon JunD inhibition. These results show that engagement of the IL-7 receptor drives a signal that is more complex than the JAK/STAT pathway, activating JNK and JunD to induce rapid growth stimulation through the expression of metabolic and signaling factors like HXKII and Pim-1.

The same pocket in menin binds both MLL and JUND but has opposite effects on transcription

Menin is a tumour suppressor protein whose loss or inactivation causes multiple endocrine neoplasia 1 (MEN1), a hereditary autosomal dominant tumour syndrome that is characterized by tumorigenesis in multiple endocrine organs. Menin interacts with many proteins and is involved in a variety of cellular processes. Menin binds the JUN family transcription factor JUND and inhibits its transcriptional activity. Several MEN1 missense mutations disrupt the menin-JUND interaction, suggesting a correlation between the tumour-suppressor function of menin and its suppression of JUND-activated transcription. Menin also interacts with mixed lineage leukaemia protein 1 (MLL1), a histone H3 lysine 4 methyltransferase, and functions as an oncogenic cofactor to upregulate gene transcription and promote MLL1-fusion-protein-induced leukaemogenesis. A recent report on the tethering of MLL1 to chromatin binding factor lens epithelium-derived growth factor (LEDGF) by menin indicates that menin is a molecular adaptor coordinating the functions of multiple proteins. Despite its importance, how menin interacts with many distinct partners and regulates their functions remains poorly understood. Here we present the crystal structures of human menin in its free form and in complexes with MLL1 or with JUND, or with an MLL1-LEDGF heterodimer. These structures show that menin contains a deep pocket that binds short peptides of MLL1 or JUND in the same manner, but that it can have opposite effects on transcription. The menin-JUND interaction blocks JUN N-terminal kinase (JNK)-mediated JUND phosphorylation and suppresses JUND-induced transcription. In contrast, menin promotes gene transcription by binding the transcription activator MLL1 through the peptide pocket while still interacting with the chromatin-anchoring protein LEDGF at a distinct surface formed by both menin and MLL1.

JunD-mediated repression of GADD45α and γ regulates escape from cell death in prostate cancer

The AP-1 transcription factor complex has been implicated in a variety of biological processes including cell differentiation, proliferation, apoptosis and oncogenic transformation. We previously established that activation of the AP-1 family member JunD contributes to deregulated expression of the anti-apoptotic IL-6 gene in prostate cancer cells. We now show that inhibition of JunD in prostate cancer cells results in GADD45α- and γ-dependent induction of cell death and inhibition of tumor growth that is mediated at least partially via c-Jun N-terminal kinase (JNK) and p38 kinase activation. Apoptosis induction by dominant negative JunD and JNK and p38 kinase activation are impeded upon knock down of GADD45α and γ expression by small interfering RNA, most vividly demonstrating the central role of GADD45α and γ in JunD-mediated escape of prostate cancer cells from programmed cell death.

JunD and JunB integrate prostaglandin E2 activation of breast cancer-associated proximal aromatase promoters

Aromatase is the key enzyme in estrogen biosynthesis. Normal breast adipose tissue expresses low levels of aromatase via the distal promoter I.4. Breast adipose tissue surrounding a tumor exhibits excessive aromatase expression controlled by proximal aromatase promoters I.3/II, leading to high local levels of estrogen and breast cancer progression. Prostaglandin E(2) (PGE(2)) secreted by malignant breast epithelial cells activates breast cancer-associated aromatase promoters I.3/II, but silences promoter I.4, in cultured human breast adipose fibroblasts (BAF). The c-Jun N-terminal kinase 1 and p38α mitogen activated protein kinases are necessary for PGE(2) activation of aromatase promoters I.3/II; thus, we examined the roles of downstream targets, c-Jun, JunB, JunD, and activating transcription factor 2, in PGE(2)-mediated regulation of aromatase expression in BAF. PGE(2) induced JunB and JunD protein expression through protein kinase A and protein kinase C, respectively. JunB or JunD knockdown by small interfering RNA markedly reduced PGE(2)-induced total aromatase mRNA level and enzyme activity via promoters I.3/II. JunB knockdown also abrogated JunD expression. JunB stimulated, whereas JunD inhibited, aromatase promoter I.4 activity. Activating transcription factor 2 knockdown did not affect promoter-specific or total aromatase mRNA levels. c-Jun knockdown increased promoter I.4-specific and PGE(2)-induced promoters I.3/II-specific aromatase mRNA levels, leading to enhanced PGE(2)-induced total aromatase mRNA level and enzyme activity. JunD, c-Jun, and JunB bound to a CRE(-211/-199) essential for PGE(2) induction of aromatase promoters I.3/II. Taken together, JunD and c-Jun repress aromatase promoter I.4. JunD mediates, whereas c-Jun modulates, PGE(2) activation of aromatase promoters I.3/II via CRE(-211/-199). JunB also activates aromatase promoters I.3/II by maintaining JunD expression. Targeting JunD may abolish aromatase expression selectively in breast cancer tissue.

Computational prediction and experimental verification of new MAP kinase docking sites and substrates including Gli transcription factors

In order to fully understand protein kinase networks, new methods are needed to identify regulators and substrates of kinases, especially for weakly expressed proteins. Here we have developed a hybrid computational search algorithm that combines machine learning and expert knowledge to identify kinase docking sites, and used this algorithm to search the human genome for novel MAP kinase substrates and regulators focused on the JNK family of MAP kinases. Predictions were tested by peptide array followed by rigorous biochemical verification with in vitro binding and kinase assays on wild-type and mutant proteins. Using this procedure, we found new ‘D-site’ class docking sites in previously known JNK substrates (hnRNP-K, PPM1J/PP2Czeta), as well as new JNK-interacting proteins (MLL4, NEIL1). Finally, we identified new D-site-dependent MAPK substrates, including the hedgehog-regulated transcription factors Gli1 and Gli3, suggesting that a direct connection between MAP kinase and hedgehog signaling may occur at the level of these key regulators. These results demonstrate that a genome-wide search for MAP kinase docking sites can be used to find new docking sites and substrates.

Protein kinases are enzymes that regulate key cellular processes by covalently attaching a phosphate group to substrate proteins; they are crucial components of signaling pathways involved in cancer, diabetes, and many other diseases. Identifying the substrates of particular protein kinases is challenging, and many existing biochemical methods are biased against weakly expressed proteins like transcription factors. Here we exploited the observation that mitogen-activated protein kinases (MAPKs) briefly attach to many of their substrates before phosphorylating them, docking onto a sequence known as the ‘D-site’. We developed D-finder, a computational tool that uses a combination of expert knowledge and machine learning to search genome databases for D-sites. We then verified several of D-finder's predictions using rigorous and well-established biochemical assays. The most intriguing predicted and verified substrates were the Gli1 and Gli3 transcription factors of the ‘hedgehog’ signaling pathway. Gli transcription factors are involved in embryonic development and stem cell differentiation, and have also been found to be hyperactive in several types of cancer. There is emerging evidence that crosstalk with MAPK pathways is important in Gli-mediated regulation. Our study, however, is the first to show that MAPKs directly phosphorylate Gli transcription factors.

Signal transduction pathways associated with ATP-induced proliferation of colon adenocarcinoma cells

BACKGROUND

In previous work, we have demonstrated that extracellular adenosine 5'-triphosphate (ATP) acts on intestinal Caco-2 cell P2Y receptors promoting a rapid increase in the phosphorylation of ERK1/2, p46 JNK and p38 MAP kinases (MAPKs).

METHODS AND RESULTS

In this study, we investigated whether the extracellular ATP-P2Y receptor signalling pathways were required for the proliferation of Caco-2 cells. Confocal microscopy and immunobloting studies showed that ERK1/2 and JNK translocate into the nucleus of the cells stimulated by ATP, where they participate, together with p38 MAPK, in the phosphorylation of JunD, ATF-1 and ATF-2 transcription factors. In addition, ATP through the activation of MAPKs induces the expression of the immediate early genes products of the Jun family, c-Fos and MAP kinase phosphatase-1 (MKP-1). Moreover, ERK1/2 and p38 MAPK are involved in the phosphorylation of MKP-1 in Caco-2 cells. Of physiological significance, in agreement with the mitogenic role of the MAPK cascade, ATP increased Caco-2 cell proliferation, and this effect was blocked by UO126, SB203580 and SP600125, the specific inhibitors of ERK1/2, p38 MAPK and JNK1/2, respectively.

CONCLUSION

Extracellular ATP induces proliferation of Caco-2 human colonic cancer cells by activating MAPK cascades and modulation of transcription factors.

GENERAL SIGNIFICANCE

These findings and identification of the specific P2Y subtype receptors involved in the mitogenic effect of ATP on Caco-2 cells might be relevant for understanding tumor cell development, resistance to treatment regimens and the design of new therapeutic strategies.

Inferring genetic interactions via a nonlinear model and an optimization algorithm

Background

Biochemical pathways are gradually becoming recognized as central to complex human diseases and recently genetic/transcriptional interactions have been shown to be able to predict partial pathways. With the abundant information made available by microarray gene expression data (MGED), nonlinear modeling of these interactions is now feasible. Two of the latest advances in nonlinear modeling used sigmoid models to depict transcriptional interaction of a transcription factor (TF) for a target gene, but do not model cooperative or competitive interactions of several TFs for a target.

Results

An S-shape model and an optimization algorithm (GASA) were developed to infer genetic interactions/transcriptional regulation of several genes simultaneously using MGED. GASA consists of a genetic algorithm (GA) and a simulated annealing (SA) algorithm, which is enhanced by a steepest gradient descent algorithm to avoid being trapped in local minimum. Using simulated data with various degrees of noise, we studied how GASA with two model selection criteria and two search spaces performed. Furthermore, GASA was shown to outperform network component analysis, the time series network inference algorithm (TSNI), GA with regular GA (GAGA) and GA with regular SA. Two applications are demonstrated. First, GASA is applied to infer a subnetwork of human T-cell apoptosis. Several of the predicted interactions are supported by the literature. Second, GASA was applied to infer the transcriptional factors of 34 cell cycle regulated targets in S. cerevisiae, and GASA performed better than one of the latest advances in nonlinear modeling, GAGA and TSNI. Moreover, GASA is able to predict multiple transcription factors for certain targets, and these results coincide with experiments confirmed data in YEASTRACT.

Conclusions

GASA is shown to infer both genetic interactions and transcriptional regulatory interactions well. In particular, GASA seems able to characterize the nonlinear mechanism of transcriptional regulatory interactions (TIs) in yeast, and may be applied to infer TIs in other organisms. The predicted genetic interactions of a subnetwork of human T-cell apoptosis coincide with existing partial pathways, suggesting the potential of GASA on inferring biochemical pathways.

Dissection of a complex transcriptional response using genome-wide transcriptional modelling

Modern genomics technologies generate huge data sets creating a demand for systems level, experimentally verified, analysis techniques. We examined the transcriptional response to DNA damage in a human T cell line (MOLT4) using microarrays. By measuring both mRNA accumulation and degradation over a short time course, we were able to construct a mechanistic model of the transcriptional response. The model predicted three dominant transcriptional activity profiles—an early response controlled by NFκB and c-Jun, a delayed response controlled by p53, and a late response related to cell cycle re-entry. The method also identified, with defined confidence limits, the transcriptional targets associated with each activity. Experimental inhibition of NFκB, c-Jun and p53 confirmed that target predictions were accurate. Model predictions directly explained 70% of the 200 most significantly upregulated genes in the DNA-damage response. Genome-wide transcriptional modelling (GWTM) requires no prior knowledge of either transcription factors or their targets. GWTM is an economical and effective method for identifying the main transcriptional activators in a complex response and confidently predicting their targets.

CARMA1-mediated NF-kappaB and JNK activation in lymphocytes

Activation of transcription factor nuclear factor-kappaB (NF-kappaB) and Jun N-terminal kinase (JNK) play the pivotal roles in regulation of lymphocyte activation and proliferation. Deregulation of these signaling pathways leads to inappropriate immune response and contributes to the development of leukemia/lymphoma. The scaffold protein CARMA1 [caspase-recruitment domain (CARD) membrane-associated guanylate kinase (MAGUK) protein 1] has a central role in regulation of NF-kappaB and the JNK2/c-Jun complex in both B and T lymphocytes. During last several years, tremendous work has been done to reveal the mechanism by which CARMA1 and its signaling partners, B cell CLL-lymphoma 10 and mucosa-associated lymphoid tissue 1, are activated and mediate NF-kappaB and JNK activation. In this review, we summarize our findings in revealing the roles of CARMA1 in the NF-kappaB and JNK signaling pathways in the context of recent advances in this field.

JunD and HIF-1alpha mediate transcriptional activation of angiotensinogen by TGF-beta1 in human lung fibroblasts

Earlier work showed that TGF-beta1 potently increases angiotensinogen (AGT) gene mRNA in primary human lung fibroblasts. Here the mechanism of TGF-beta1-induced AGT expression was studied in the IMR90 human lung fibroblast cell line. The increase in AGT mRNA induced by TGF-beta1 was completely blocked by actinomycin-D. TGF-beta1 increased the activity of a full-length human AGT promoter-luciferase reporter (AGT-LUC) but did not alter AGT mRNA half-life. Serial deletion analyses revealed that 67% of TGF-beta-inducible AGT-LUC activity resides in a small domain of the AGT core promoter; this domain contains binding sites for hypoxia-inducible factor (HIF)-1 and activation protein-1 (AP-1) transcription factors. TGF-beta1 increased HIF-1alpha protein abundance and the activity of a hypoxia-responsive element reporter; overexpression of HIF-1 increased basal AGT-LUC activity. Both oligonucleotide pulldown and chromatin immunoprecipitation assays revealed increased binding of JunD and HIF-1alpha to the AGT core promoter in response to TGF-beta1. TGF-beta1-inducible AGT-LUC was reduced by an AP-1 dominant negative or by mutation of the AP-1 site. Knockdown of either JunD or HIF-1alpha individually by siRNA partially reduced AGT-LUC. In contrast, simultaneous knockdown of both JunD and HIF-1alpha completely eliminated TGF-beta1-inducible AGT-LUC activity. These data suggest that TGF-beta1 up-regulates AGT transcription in human lung fibroblasts through a mechanism that requires both JunD and HIF-1alpha binding to the AGT core promoter. They also suggest a molecular mechanism linking hypoxia signaling and fibrogenic stimuli in the lungs.

Multiple facets of junD gene expression are atypical among AP-1 family members

JunD is a versatile AP-1 transcription factor that can activate or repress a diverse collection of target genes. Precise control of junD expression and JunD protein-protein interactions modulate tumor angiogenesis, cellular differentiation, proliferation and apoptosis. Molecular and clinical knowledge of two decades has revealed that precise JunD activity is elaborated by interrelated layers of constitutive transcriptional control, complex post-transcriptional regulation and a collection of post-translational modifications and protein-protein interactions. The stakes are high, as inappropriate JunD activity contributes to neoplastic, metabolic and viral diseases. This article deconvolutes multiple layers of control that safeguard junD gene expression and functional activity. The activity of JunD in transcriptional activation and repression is integrated into a regulatory network by which JunD exerts a pivotal role in cellular growth control. Our discussion of the JunD regulatory network integrates important open issues and posits new therapeutic targets for the neoplastic, metabolic and viral diseases associated with JunD/AP-1 expression.

JunD protects the liver from ischemia/reperfusion injury by dampening AP-1 transcriptional activation

The AP-1 transcription factor modulates a wide range of cellular processes, including cellular proliferation, programmed cell death, and survival. JunD is a major component of the AP-1 complex following liver ischemia/reperfusion (I/R) injury; however, its precise function in this setting remains unclear. We investigated the functional significance of JunD in regulating AP-1 transcription following partial lobar I/R injury to the liver, as well as the downstream consequences for hepatocellular remodeling. Our findings demonstrate that JunD plays a protective role, reducing I/R injury to the liver by suppressing acute transcriptional activation of AP-1. In the absence of JunD, c-Jun phosphorylation and AP-1 activation in response to I/R injury were elevated, and this correlated with increased caspase activation, injury, and alterations in hepatocyte proliferation. The expression of dominant negative JNK1 inhibited c-Jun phosphorylation, AP-1 activation, and hepatic injury following I/R in JunD-/- mice but, paradoxically, led to an enhancement of AP-1 activation and liver injury in JunD+/- littermates. Enhanced JunD/JNK1-dependent liver injury correlated with the acute induction of diphenylene iodonium-sensitive NADPH-dependent superoxide production by the liver following I/R. In this context, dominant negative JNK1 expression elevated both Nox2 and Nox4 mRNA levels in the liver in a JunD-dependent manner. These findings suggest that JunD counterbalances JNK1 activation and the downstream redox-dependent hepatic injury that results from I/R, and may do so by regulating NADPH oxidases.

Induction of deltaFosB in the periaqueductal gray by stress promotes active coping responses

We analyzed the influence of the transcription factor DeltaFosB on learned helplessness, an animal model of affective disorder wherein a subset of mice exposed to inescapable stress (IS) develop a deficit in escape behavior. Repeated IS induces DeltaFosB in the ventrolateral periaqueductal gray (vlPAG), and levels of the protein are highly predictive of an individual's subsequent behavorial deficit-with the strongest DeltaFosB induction observed in the most resilient animals. Induction of DeltaFosB by IS predominates in substance P-positive neurons in the vlPAG, and the substance P gene, a direct target for DeltaFosB, is downregulated upon DeltaFosB induction. Local overexpression of DeltaFosB in the vlPAG using viral-mediated gene transfer dramatically reduces depression-like behaviors and inhibits stress-induced release of substance P. These results indicate that IS-induced accumulation of DeltaFosB in the vlPAG desensitizes substance P neurons enriched in this area and opposes behavioral despair by promoting active defense responses.

JunD is a profibrogenic transcription factor regulated by Jun N-terminal kinase-independent phosphorylation

JunD is implicated in the regulation of hepatic stellate cell (HSC) activation and liver fibrosis via its transcriptional regulation of the tissue inhibitor of metalloproteinases-1 (TIMP-1) gene. In the present study we found in vivo evidence of a role for JunD in fibrogenesis. Expression of JunD was demonstrated in alpha-SMA-positive activated HSCs of fibrotic rodents and human livers. The junD-/- mice were protected from carbon tetrachloride-induced fibrosis. The livers of injured junD-/- mice displayed significantly reduced formation of fibrotic crosslinked collagen and a smaller number of alpha-SMA-positive HSCs compared with those of wild-type (wt) mice. Hepatic TIMP-1 mRNA expression in injured junD-/- mice was 78% lower and in culture activated junD-/- HSCs was 50%-80% lower than that in wt mice. In examining the signal transduction mechanisms that regulate JunD-dependent TIMP-1 expression, we found a role for phosphorylation of the Ser100 residue of JunD but ruled out JNK as a mediator of this event, suggesting ERK1/2 is utilized. In conclusion, a signaling pathway for the development of fibrosis involves the regulation of TIMP-1 expression by phosphorylated JunD.

Uses for JNK: the many and varied substrates of the c-Jun N-terminal kinases

The c-Jun N-terminal kinases (JNKs) are members of a larger group of serine/threonine (Ser/Thr) protein kinases from the mitogen-activated protein kinase family. JNKs were originally identified as stress-activated protein kinases in the livers of cycloheximide-challenged rats. Their subsequent purification, cloning, and naming as JNKs have emphasized their ability to phosphorylate and activate the transcription factor c-Jun. Studies of c-Jun and related transcription factor substrates have provided clues about both the preferred substrate phosphorylation sequences and additional docking domains recognized by JNK. There are now more than 50 proteins shown to be substrates for JNK. These include a range of nuclear substrates, including transcription factors and nuclear hormone receptors, heterogeneous nuclear ribonucleoprotein K, and the Pol I-specific transcription factor TIF-IA, which regulates ribosome synthesis. Many nonnuclear substrates have also been characterized, and these are involved in protein degradation (e.g., the E3 ligase Itch), signal transduction (e.g., adaptor and scaffold proteins and protein kinases), apoptotic cell death (e.g., mitochondrial Bcl2 family members), and cell movement (e.g., paxillin, DCX, microtubule-associated proteins, the stathmin family member SCG10, and the intermediate filament protein keratin 8). The range of JNK actions in the cell is therefore likely to be complex. Further characterization of the substrates of JNK should provide clearer explanations of the intracellular actions of the JNKs and may allow new avenues for targeting the JNK pathways with therapeutic agents downstream of JNK itself.

JunD activates transcription of the human ferritin H gene through an antioxidant response element during oxidative stress

Ferritin is the major intracellular iron storage protein that sequesters excess free iron to minimize generation of iron-catalysed reactive oxygen species. We previously demonstrated that expression of ferritin heavy chain (ferritin H) was induced by pro-oxidants, which is a part of cellular antioxidant response to protect cells from oxidative damage. In this study, we have identified that the antioxidant/electrophile response element (ARE) located 4.5 kb upstream to the human ferritin H transcription initiation site is responsible for the oxidant response. The human ferritin H ARE comprises two copies of bidirectional AP1 motifs. Mutations in each AP1 motif significantly impaired protein binding and the function of the ARE, indicating that both of the AP1 motifs are required for pro-oxidant-mediated activation of the ferritin H gene. We identified that JunD, an AP1 family basic-leucine zipper (bZip) transcription factor, is one of the ferritin H ARE binding proteins and activates ferritin H transcription in HepG2 hepatocarcinoma cells. Gel retardation assay demonstrated that H2O2 (hydrogen peroxide) or t-BHQ (tert-butylhydroquinone) treatment increased total protein binding as well as JunD binding to the ferritin H ARE. Chromatin immunoprecipitation assay showed that H2O2 treatment induced JunD binding to the ferritin H ARE. Both H2O2 and t-BHQ induced phosphorylation of JunD at Ser-100, an activated form of JunD. Furthermore, overexpression of JunD induced endogenous ferritin H protein synthesis. Since JunD has recently been demonstrated to protect cells from several stress stimuli including oxidative stress, these results suggest that, in addition to NFE2-related factor 2 (Nrf2) as a major ARE regulatory protein, JunD is another ARE regulatory protein for transcriptional activation of the human ferritin H gene and probably other antioxidant genes containing the conserved ARE sequences by which JunD may confer cytoprotection during oxidative stress.

A novel role for GADD45beta as a mediator of MMP-13 gene expression during chondrocyte terminal differentiation

The growth arrest and DNA damage-inducible 45beta (GADD45beta) gene product has been implicated in the stress response, cell cycle arrest, and apoptosis. Here we demonstrated the unexpected expression of GADD45beta in the embryonic growth plate and uncovered its novel role as an essential mediator of matrix metalloproteinase-13 (MMP-13) expression during terminal chondrocyte differentiation. We identified GADD45beta as a prominent early response gene induced by bone morphogenetic protein-2 (BMP-2) through a Smad1/Runx2-dependent pathway. Because this pathway is involved in skeletal development, we examined mouse embryonic growth plates, and we observed expression of Gadd45beta mRNA coincident with Runx2 protein in pre-hypertrophic chondrocytes, whereas GADD45beta protein was localized prominently in the nucleus in late stage hypertrophic chondrocytes where Mmp-13 mRNA was expressed. In Gadd45beta(-/-) mouse embryos, defective mineralization and decreased bone growth accompanied deficient Mmp-13 and Col10a1 gene expression in the hypertrophic zone. Transduction of small interfering RNA-GADD45beta in epiphyseal chondrocytes in vitro blocked terminal differentiation and the associated expression of Mmp-13 and Col10a1 mRNA in vitro. Finally, GADD45beta stimulated MMP-13 promoter activity in chondrocytes through the JNK-mediated phosphorylation of JunD, partnered with Fra2, in synergy with Runx2. These observations indicated that GADD45beta plays an essential role during chondrocyte terminal differentiation.

Induction of AP-1 activity by androgen activation of the androgen receptor in LNCaP human prostate carcinoma cells

BACKGROUND

The androgen receptor and activator protein-1 (AP-1) transcription factors affect growth regulation in normal and cancerous prostate cells. Effects of androgen-activated androgen receptor on AP-1 activity were determined in the LNCaP human prostate carcinoma cell model.

METHODS

Cells were exposed to 1 nM androgen +/- antiandrogen bicalutamide. Cellular growth and cell cycle effects were determined by DNA, viability, and bromodeoxyuridine (BrdU) fluorescence activated cell sorter (FACS) assays. AP-1 effects were determined by an AP-1-luciferase enzyme reporter vector for transcriptional activity, electrophoretic mobility shift assay (EMSA)/antibody supershift for DNA-binding, quantitative RT-PCR for mRNA, and immunoblot for protein.

RESULTS

Androgen induced G(1) growth arrest. This growth arrest was abrogated by treatment with bicalutamide, demonstrating that growth arrest by androgen was due to androgen receptor activation. Concurrently, AP-1 DNA-binding and transcriptional activity was induced over 96 hr androgen exposure, which was also inhibited by bicalutamide. Interestingly, although no change in AP-1 transcriptional activity was observed 24 hr after androgen exposure, there was an increase in Fra-2 expression and AP-1 DNA-binding. Paradoxically, while Fra-2 mRNA and protein levels continued to increase, binding of Fra-2 to the AP-1 site decreased over 96 hr, with a concomitant increase in JunD AP-1-binding and a marked increase in expression of the 35 kDa form of JunD. Enhanced expression of this short form of JunD is a novel effect of androgen exposure that occurred during the 24-96 hr time period, as growth effects emerged.

CONCLUSION

Activation of androgen receptor by androgen induces changes in AP-1 activity and AP-1 factor DNA-binding that may contribute significantly to androgen-induced changes in prostate cancer cell growth.

Identification of promoters bound by c-Jun/ATF2 during rapid large-scale gene activation following genotoxic stress

The NH2-terminal Jun kinases (JNKs) function in diverse roles through phosphorylation and activation of AP-1 components including ATF2 and c-Jun. However, the genes that mediate these processes are poorly understood. A model phenotype characterized by rapid activation of Jun kinase and enhanced DNA repair following cisplatin treatment was examined using chromatin immunoprecipitation with antibodies against ATF2 and c-Jun or their phosphorylated forms and hybridization to promoter arrays. Following genotoxic stress, we identified 269 genes whose promoters are bound upon phosphorylation of ATF2 and c-Jun. Binding did not occur following treatment with transplatin or the JNK inhibitor SP600125 or JNK-specific siRNA. Of 89 known DNA repair genes represented on the array, 23 are specifically activated by cisplatin treatment within 3-6 hr. Thus, the genotoxic stress response occurs at least partly via activation of ATF2 and c-Jun, leading to large-scale coordinate gene expression dominated by genes of DNA repair.

Tobacco smoke control of mucin production in lung cells requires oxygen radicals AP-1 and JNK

In smokers' lungs, excessive mucus clogs small airways, impairing respiration and promoting recurrent infection. A breakthrough in understanding this pathology was the realization that smoke could directly stimulate mucin synthesis in lung epithelial cells and that this phenomenon was dependent on the cell surface receptor for epidermal growth factor, EGFR. Distal steps in the smoke-triggered pathway have not yet been determined. We report here that the predominant airway mucin (MUC5AC) undergoes transcriptional up-regulation in response to tobacco smoke; this is mediated by an AP-1-containing response element, which binds JunD and Fra-2. These transcription factors require phosphorylation by upstream kinases JNK and ERK, respectively. Whereas ERK activation results from the upstream activation of EGFR, JNK activation is chiefly EGFR-independent. Our experiments demonstrated that smoke activates JNK via a Src-dependent, EGFR-independent signaling cascade initiated by smoke-induced reactive oxygen species. Taken together with our earlier results, these data indicate that the induction of mucin by smoke is the combined effect of mutually independent, reactive oxygen species activation of both EGFR and JNK.

c-Jun N-terminal kinase regulates lamellipodial protrusion and cell sheet migration during epithelial wound closure by a gene expression-independent mechanism

c-Jun N-terminal kinase (JNK) is emerging as an important regulator of cell migration. Perturbing the JNK signaling pathway with three structurally and mechanistically distinct inhibitors that selectively target either JNKs themselves or the upstream mixed-lineage kinases, we found dramatic inhibition of membrane protrusion and cell sheet migration during wound closure in Madin-Darby canine kidney (MDCK) epithelial cell monolayers. Extension of lamellipodia is blocked from the earliest times after wounding in the presence of JNK pathway inhibitors, whereas assembly of non-protrusive actin bundles at the wound margin is unaffected. Inhibitors of the other mitogen-activated protein kinase (MAPK) pathways, the extracellular signal-regulated kinase and p38 MAPK pathways, only have comparatively weak or marginal inhibitory effects on wound closure. Multiple splice variants of both JNK1 and JNK2 are expressed in MDCK cells, and JNK1 and JNK2 are rapidly and transiently activated upon wounding. Phosphorylation of c-Jun does not appear relevant to MDCK wound closure, and membrane protrusion directly after wounding is not affected by inhibitors of RNA or protein synthesis. While most known substrates of JNK are transcription factors or proteins regulating apoptosis, our data indicate that JNK regulates protrusion and migration in a gene expression-independent manner and suggest an important cytoplasmic role for JNK in the control of cell motility.

Differential Phosphorylation of c-Jun and JunD in Response to the Epidermal Growth Factor Is Determined by the Structure of MAPK Targeting Sequences

MAPK phosphorylation of various substrates is mediated by the presence of docking sites, including the D domain and the DEF motif. Depending on the number and sequences of these domains, substrates are phosphorylated by specific subsets of MAPKs. For example, a D domain targets JNK to c-Jun, whereas a DEF motif is required for ERK phosphorylation of c-Fos. JunD, in contrast, contains both D and DEF domains. Here we show that these motifs mediate JunD phosphorylation in response to either ERK or JNK activation. An intact D domain is required for phosphorylation and activation of JunD by both subtypes of MAPK. The DEF motif acts together with the D domain to elicit efficient phosphorylation of JunD in response to the epidermal growth factor (EGF) but has no function on JunD phosphorylation and activation by JNK signaling. Furthermore, we show that conversion of a c-Jun sequence to a canonical DEF domain, as it is present in JunD, elicits c-Jun activation in response to EGF. Our results suggest that evolution of a particular modular system of MAPK targeting sequences has determined a differential response of JunD and c-Jun to ERK activation.

Inflammatory cytokine expression is independent of the c-Jun N-terminal kinase/AP-1 signaling cascade in human neutrophils

In the last decade, the ability of neutrophils to generate proinflammatory cytokines has become firmly established. Because neutrophils typically infiltrate inflammatory sites in large numbers, they could significantly contribute to the cytokine environment and even represent a substantial source of cytokines in chronic inflammatory disorders in which they predominate over other cell types. To date, however, most studies have focused on identifying which mediators are produced by neutrophils, as opposed to elucidating the molecular bases underlying this process. We previously showed that most stimuli of cytokine production in neutrophils also activate NF-kappaB in these cells. In this report, we turned our attention to another transcription factor that plays a central role in inflammation, AP-1. Among Jun/Fos proteins, only JunD and c-Fos are abundantly expressed in neutrophils, and they are mainly cytoplasmic. Both the cellular levels and distribution of the Jun/Fos proteins remain unaffected by various neutrophil stimuli, including those that are known to increase the corresponding mRNA transcripts. Similarly, c-Jun N-terminal kinase (JNK) 1 is overwhelmingly cytoplasmic in neutrophils and does not translocate to the nucleus upon cell activation. Although JNK is not activatable under most circumstances, specific conditions do allow its phosphorylation in response to TNF. However, no experimental condition (even those leading to JNK activation) resulted in the induction of genuine AP-1 complexes in neutrophils. Accordingly, the potent JNK inhibitor, SP 600125, failed to inhibit inflammatory cytokine gene expression in neutrophils. Collectively, our findings strongly suggest that the JNK/AP-1 signaling pathway has little or no impact on the generation of inflammatory mediators in neutrophils.

c-Jun N-terminal kinase activation of activator protein-1 underlies homologous regulation of the gonadotropin-releasing hormone receptor gene in alpha T3-1 cells

Reproductive function is dependent on the interaction between GnRH and its cognate receptor found on gonadotrope cells of the anterior pituitary gland. GnRH activation of the GnRH receptor (GnRHR) is a potent stimulus for increased expression of multiple genes including the gene encoding the GnRHR itself. Thus, homologous regulation of the GnRHR is an important mechanism underlying gonadotrope sensitivity to GnRH. Previously, we have found that GnRH induction of GnRHR gene expression in alpha T3-1 cells is partially mediated by protein kinase C activation of a canonical activator protein-1 (AP-1) element. In contrast, protein kinase A and a cAMP response element-like element have been implicated in mediating the GnRH response of the GnRHR gene using a heterologous cell model (GGH(3)). Herein we find that selective removal of the canonical AP-1 site leads to a loss of GnRH regulation of the GnRHR promoter in transgenic mice. Thus, an intact AP-1 element is necessary for GnRH responsiveness of the GnRHR gene both in vitro and in vivo. Based on in vitro analyses, GnRH appeared to enhance the interaction of JunD, FosB, and c-Fos at the GnRHR AP-1 element. Although enhanced binding of cFos reflected an increase in gene expression, GnRH appeared to regulate both FosB and JunD at a posttranslational level. Neither overexpression of a constitutively active Raf-kinase nor pharmacological blockade of GnRH-induced ERK activation eliminated the GnRH response of the GnRHR promoter. GnRH responsiveness was, however, lost in alpha T3-1 cells that stably express a dominant-negative c-Jun N-terminal kinase (JNK) kinase, suggesting a critical role for JNK in mediating GnRH regulation of the GnRHR gene. Consistent with this possibility, we find that the ability of forskolin and membrane-permeable forms of cAMP to inhibit the GnRH response of the GnRHR promoter is associated with a loss of both JNK activation and GnRH-mediated recruitment of the primary AP-1-binding components.

Differential targeting of the stress mitogen-activated protein kinases to the c-Jun dimerization protein 2

The mitogen-activated kinases are structurally related proline-directed serine/threonine kinases that phosphorylate similar phosphoacceptor sites and yet, in vivo, they exhibit stringent substrate specificity. Specific targeting domains (kinase docking domains) facilitate kinase-substrate interaction and play a major role in substrate specificity determination. The c-Jun N-terminal kinase (JNK) consensus docking domain comprises of a KXXK/RXXXXLXL motif located in the delta-domain of the c-Jun N-terminal to the phosphoacceptor site. The c-Jun dimerization protein 2 is phosphorylated by JNK on Thr-148. Activating transcription factor 3 (ATF3) is a basic leucine zipper protein which is highly homologous to c-Jun dimerization protein 2 (JDP2), especially within the threonine/proline phosphoacceptor site, Thr-148. Nevertheless, ATF3 does not serve as a JNK substrate in vitro or in vivo. Using ATF3 and JDP2 protein chimaeras, we mapped the JNK-docking domain within JDP2. Although a JNK consensus putative docking site is located within the JDP2 leucine zipper motif, this domain does not function to recruit JNK to JDP2. A novel putative docking domain located C-terminally to the JDP2 phosphoacceptor site was identified. This domain, when fused to the ATF3 heterologous phosphoacceptor site, can direct its phosphorylation by JNK. In addition, although the novel JNK-docking domain was found to be necessary for p38 phosphorylation of JDP2 on Thr-148, it was not sufficient to confer JDP2 phosphorylation by the p38 kinase.

Specific activation of human interleukin-5 depends on de novo synthesis of an AP-1 complex

It is clear from the biology of eosinophilia that a specific regulatory mechanism must exist. Because interleukin-5 (IL5) is the key regulatory cytokine, it follows that a gene-specific control of IL5 expression must exist that differs even from closely related cytokines such as IL4. Two features of IL5 induction make it unique compared with other cytokines; first, induction by cyclic adenosine monophosphate (cAMP), which inhibits other T-cell-derived cytokines, and second, sensitivity to protein synthesis inhibitors, which have no effect on other cytokines. This study has utilized the activation of different transcription factors by different stimuli in a human T-cell line to study the role of conserved lymphokine element 0 (CLE0) in the specific induction of IL5. In unstimulated cells the ubiquitous Oct-1 binds to CLE0. Stimulation induces de novo synthesis of the AP-1 members JunD and Fra-2, which bind to CLE0. The amount of IL5 produced correlates with the production of the AP-1 complex, suggesting a key role in IL5 expression. The formation of the AP-1 complex is essential, but the rate-limiting step is the synthesis of AP-1, especially Fra-2. This provides an explanation for the sensitivity of IL5 to protein synthesis inhibitors and a mechanism for the specific induction of IL5 compared with other cytokines.