The protein kinase C-eta isoform induces proliferation in glioblastoma cell lines through an ERK/Elk-1 pathway

Glioblastoma multiforme (GBM) is the highest grade of astrocytoma. GBM pathogenesis has been linked to receptor tyrosine kinases and kinases further down signal-transduction pathways - in particular, members of the protein kinase C (PKC) family. The expression and activity of various PKC isoforms are increased in malignant astrocytomas, but not in non-neoplastic astrocytes. This suggests that PKC activity contributes to tumor progression. The level of PKC-eta expressed correlates with the degree of phorbol-12-myristate-13-acetate (PMA)-induced proliferation of two glioblastoma cell lines, U-1242 MG and U-251 MG. Normally, U-1242 cells do not express PKC-eta, and PMA inhibits their proliferation. Conversely, PMA increases proliferation of U-1242 cells that are stably transfected with PKC-eta (U-1242-PKC-eta). PMA treatment also stimulates proliferation of U-251 cells, which express PKC-eta. Here, we determined that extracellular signal-regulated kinase (ERK) and Elk-1 are downstream targets of PKC-eta. Elk-1-mediated transcriptional activity correlates with the PKC-eta-mediated mitogenic response. Pretreatment of U-1242-PKC-eta cells with inhibitors of PKC or MAPK/ERK kinase (MEK) (bisindolyl maleimide (BIM) or U0126, respectively) blocked both PMA-induced Elk-1 transcriptional activity and PMA-stimulated proliferation. An overexpressed dominant-negative PKC-eta reduced the mitogenic response in U-251 cells, as did reduction of Elk-1 by small interfering RNA. Taken together, these results strongly suggest that PKC-eta-mediated glioblastoma proliferation involves MEK/mitogen-activated protein (MAP) kinase phosphorylation, activation of ERK and subsequently of Elk-1. Elk-1 target genes involved in GBM proliferative responses have yet to be identified.

ERK signaling leads to mitochondrial dysfunction in extracellular zinc-induced neurotoxicity

A zinc-induced signaling pathway leading to extracellular signal-regulated kinase 1/2 (ERK1/2) activation and subsequent neuronal death has been investigated. We find that an extracellular zinc application stimulates biphasic phosphorylation of ERK1/2 and p38 MAPK in rat cultured neurons. The activation of ERK1/2, but not p38, is responsible for zinc neurotoxicity as only U0126, a MEK inhibitor that blocks ERK1/2 phosphorylation, significantly protects cortical neurons from zinc exposure. Over-expression of a dominant negative Ras mutant blocks zinc-induced Elk1-dependent gene expression in neurons, indicating the involvement of Ras activation in the zinc pathway leading to ERK phosphorylation and Elk1 signaling. We also find that zinc treatment results in neuronal mitochondrial hyperpolarization. Importantly, both U0126 and bongkrekic acid, an inhibitor of the mitochondrial adenine nucleotide translocase, effectively reduce zinc-triggered mitochondrial changes. As bongkrekic acid also prevents zinc-triggered neuronal death but not ERK1/2 phosphorylation, activation of MAPK signaling precedes and is required for mitochondrial dysfunction and cell death. These results provide new insight on the mechanism of extracellular zinc-induced toxicity in which the regulation of mitochondrial function by the Ras/MEK/ERK pathway is closely associated with neuronal viability.

APC inhibits ERK pathway activation and cellular proliferation induced by RAS

Inactivating mutations in the adenomatous polyposis coli gene (APC), and activating mutations in RAS, occur in a majority of colorectal carcinomas. However, the relationship between these changes and tumorigenesis is poorly understood. RAS-induced activation of the ERK pathway was reduced by overexpressing APC in DLD-1 colorectal cancer cells. ERK activity was increased by Cre-virus-induced Apc knockout in primary Apc(flox/flox) mouse embryonic fibroblasts, indicating that APC inhibits ERK activity. ERK activity was increased by overexpression and decreased by knock down of beta-catenin. The activation of Raf1, MEK and ERK kinases by beta-catenin was reduced by co-expression of APC. These results indicate that APC inhibits the ERK pathway by an action on beta-catenin. RAS-induced activation of the ERK pathway was reduced by the dominant negative form of TCF4, indicating that the ERK pathway regulation by APC/beta-catenin signaling is, at least, partly caused by effects on beta-catenin/TCF4-mediated gene expression. The GTP loading and the protein level of mutated RAS were decreased in cells with reduced ERK activity as a result of APC overexpression, indicating that APC regulates RAS-induced ERK activation at least partly by reduction of the RAS protein level. APC regulates cellular proliferation and transformation induced by activation of both RAS and beta-catenin signaling.

Regulation of MDMX expression by mitogenic signaling

MDMX is an important regulator of p53 transcriptional activity and stress response. MDMX overexpression and gene amplification are implicated in p53 inactivation and tumor development. Unlike MDM2, MDMX is not inducible by p53, and little is known about its regulation at the transcriptional level. We found that MDMX levels in tumor cell lines closely correlate with promoter activity and mRNA level. Activated K-Ras and insulin-like growth factor 1 induce MDMX expression at the transcriptional level through mechanisms that involve the mitogen-activated protein kinase and c-Ets-1 transcription factors. Pharmacological inhibition of MEK results in down-regulation of MDMX in tumor cell lines. MDMX overexpression was detected in approximately 50% of human colon tumors and showed strong correlation with increased extracellular signal-regulated kinase phosphorylation. Therefore, MDMX expression is regulated by mitogenic signaling pathways. This mechanism may protect normal proliferating cells from p53 but also hamper p53 response during tumor development.

4-Hydroxy-2-nonenal adduction of extracellular signal-regulated kinase (Erk) and the inhibition of hepatocyte Erk-Est-like protein-1-activating protein-1 signal transduction

4-Hydroxy-2-nonenal (4-HNE) is a major lipid peroxidation (LPO) product formed during oxidative stress. 4-HNE is highly reactive toward cellular nucleophiles and is implicated in the evolution of numerous pathologies associated with oxidative stress and LPO. Recent evidence suggests that chronic prooxidant exposure results in the loss of extracellular signal-regulated kinase (Erk)-1/2 phosphorylation in vivo, a signaling pathway associated with cellular proliferation, survival, and homeostasis. Immunodetection and molecular analysis were used in this study to evaluate the hypothesis that 4-HNE modification of Erk-1/2 inhibits constitutive Erk-Est-like protein (Elk)-1-activating protein (AP)-1 signaling. Primary rat hepatocytes treated with subcytotoxic, pathologically relevant concentrations of 4-HNE demonstrated a concentration-dependent loss of constitutive Erk-1/2 phosphorylation, activity, and nuclear localization. These findings were consistent with iron-induced intracellular LPO, which also resulted in a concentration-dependent decrease in hepatocyte Erk-1/2 phosphorylation and activity. 4-HNE and iron-induced inhibition of Erk-1/2 was inversely correlated with the accumulation of 4-HNE-Erk-1/2 monomer adducts. 4-HNE treatment of hepatocytes decreased nuclear total and phosphorylated Erk-1/2, Elk-1, and AP-1 phosphorylation as well as cFos and cJun activities. The cytosolic modification of unphosphorylated Erk-1/2 was evaluated in vitro using molar ratios of inactive Erk-2 to 4-HNE consistent with increasing oxidative stress in vivo. Liquid chromatography combined with tandem mass spectrometry confirmed monomer adduct formation and identified the major adduct species at the histidine 178 residue within the kinase phosphorylation lip. These novel results show that the formation of 4-HNE-Erk-1/2 monomer-adducts results in the inhibition of Erk-Elk-AP-1 signaling in hepatocytes and implicates the His 178 residue with the mechanism of inhibition.

HOXA1-stimulated oncogenicity is mediated by selective upregulation of components of the p44/42 MAP kinase pathway in human mammary carcinoma cells

Expression of homeobox A1 (HOXA1) results in oncogenic transformation of immortalized human mammary epithelial cells with aggressive tumor formation in vivo. However, the mechanisms by which HOXA1 mediates oncogenic transformation is not well defined. To identify molecules that could potentially be involved in HOXA1-mediated oncogenic transformation, microarray analysis was utilized to characterize and compare the gene expression pattern in response to forced expression or depletion of HOXA1 in human mammary carcinoma cells. Gene expression profiling identified that genes involved in the p44/42 mitogen-activated protein (MAP) kinase activation pathway (GRB2, MAP kinase kinase (MEK1) and SDFR1) or p44/42 MAP kinase-regulated genes (IER3, EPAS1, PCNA and catalase) are downstream expression targets of HOXA1. Forced expression of HOXA1 increased GRB2 and MEK1 mRNA and protein expression and increased p44/42 MAP kinase phosphorylation, activity and Elk-1-mediated transcription. Use of a MEK1 inhibitor demonstrated that increased p44/42 MAP kinase activity is required for the HOXA1-mediated increase in cell proliferation, survival, oncogenicity and oncogenic transformation. Thus, modulation of the p44/42 MAP kinase pathway is one mechanism by which HOXA1 mediates oncogenic transformation of the human mammary epithelial cell.

TBP is differentially regulated by c-Jun N-terminal kinase 1 (JNK1) and JNK2 through Elk-1, controlling c-Jun expression and cell proliferation

Emerging evidence supports the idea that the c-Jun N-terminal kinases (JNKs) possess overlapping but distinct functions. The potential roles of the ubiquitously expressed JNK1 and JNK2 in regulating expression of the central transcription initiation factor, TATA-binding protein (TBP), were examined. Relative to wild-type fibroblasts, TBP was decreased in Jnk1(-/-) cells and increased in Jnk2(-/-) cells. Similarly, reduction of JNK1 in human hepatoma cells decreased TBP expression, whereas reduction of JNK2 enhanced it. JNK-mediated regulation of TBP expression occurs at the transcriptional level through their ability to target Elk-1, which directly regulates the TBP promoter in response to epidermal growth factor stimulation. JNK1 increases, whereas JNK2 decreases, the phosphorylation state of Elk-1, which differentially affects Elk-1 occupancy at a defined site within the TBP promoter. These JNK-mediated alterations in TBP expression, alone, serve to regulate c-Jun expression and fibroblast proliferation rates. These studies uncovered several new molecular events that distinguish the functions of JNK1 and JNK2 that are critical for their regulation of cellular proliferation.

hypoxia-inducible factors activate CD133 promoter through ETS family transcription factors

CD133 is a cellular surface protein that has been reported to be a cancer stem cell marker, and thus it is considered to be a potential target for cancer treatment. However, the mechanism regulating CD133 expression is not yet understood. In this study, we analyzed the activity of five putative promoters (P1–P5) of CD133 in human embryonic kidney (HEK) 293 cells and colon cancer cell line WiDr, and found that the activity of promoters, particularly of P5, is elevated by overexpression of hypoxia-inducible factors (HIF-1α and HIF-2α). Deletion and mutation analysis identified one of the two E-twenty six (ETS) binding sites (EBSs) in the P5 region as being essential for its promoter activity induced by HIF-1α and HIF-2α. In addition, a chromatin imunoprecipitation assay demonstrated that HIF-1α and HIF-2α bind to the proximal P5 promoter at the EBSs. The immunoprecipitation assay showed that HIF-1α physically interacts with Elk1; however, HIF-2α did not bind to Elk1 or ETS1. Furthermore, knockdown of both HIF-1α and HIF-2α resulted in a reduction of CD133 expression in WiDr. Taken together, our results revealed that HIF-1α and HIF-2α activate CD133 promoter through ETS proteins.

Several transcription factors regulate COX-2 gene expression in pancreatic beta-cells

Cyclooxygenase-2 (COX-2) expression is associated with many aspects of physiological and pathological conditions, including pancreatic beta-cell dysfunction. Prostaglandin E2 (PGE2) production, as a consequence of COX-2 gene induction, has been reported to impair beta-cell function. The molecular mechanisms involved in the regulation of COX-2 gene expression are not fully understood. In this report, we used pancreatic beta-cells (RINm5F) to explore the potential transcription factors regulating COX-2 promoter activity. Using promoter screening method, we selected several transcription factors in our study. Through luciferase reporter studies, we found that these factors can regulate COX-2 promoter activity in RINm5F cells. Among these factors, cyclic AMP response-element binding protein (CREB), Ets family members Ets-1 and Elk-1 can positively regulate COX-2 promoter activity. On the contrary, signal transducer and activator of transcription 1 (STAT1) plays a negative role on COX-2 promoter. Our findings will be helpful for better understanding the transcriptional regulation of COX-2 in pancreatic beta-cells. Moreover, these transcriptional regulators of COX-2 expression will be potential targets for the prevention of beta-cell damage mediated by PGE2.

Downstream of FGF during mesoderm formation in Xenopus: the roles of Elk-1 and Egr-1

Signalling by members of the FGF family is required for induction and maintenance of the mesoderm during amphibian development. One of the downstream effectors of FGF is the SRF-interacting Ets family member Elk-1, which, after phosphorylation by MAP kinase, activates the expression of immediate-early genes. Here, we show that Xenopus Elk-1 is phosphorylated in response to FGF signalling in a dynamic pattern throughout the embryo. Loss of XElk-1 function causes reduced expression of Xbra at neurula stages, followed by a failure to form notochord and muscle and then the partial loss of trunk structures. One of the genes regulated by XElk-1 is XEgr-1, which encodes a zinc finger transcription factor: we show that phosphorylated XElk-1 forms a complex with XSRF that binds to the XEgr-1 promoter. Superficially, Xenopus tropicalis embryos with reduced levels of XEgr-1 resemble those lacking XElk-1, but to our surprise, levels of Xbra are elevated at late gastrula stages in such embryos, and over-expression of XEgr-1 causes the down-regulation of Xbra both in whole embryos and in animal pole regions treated with activin or FGF. In contrast, the myogenic regulatory factor XMyoD is activated by XEgr-1 in a direct manner. We discuss these counterintuitive results in terms of the genetic regulatory network to which XEgr-1 contributes.

Neurokinin B induces c-fos transcription via protein kinase C and activation of serum response factor and Elk-1 in immortalized GnRH neurons

Mutations in neurokinin B (NKB) and its receptor, NK3R, were identified in human patients with hypogonadotropic hypogonadism, a disorder characterized by lack of puberty and infertility. Further studies have suggested that NKB acts at the level of the hypothalamus to control GnRH neuron activity, either directly or indirectly. We recently reported that treatment with senktide, a NK3R agonist, induced GnRH secretion and expression of c-fos mRNA in GT1-7 cells. Here, we map the responsive region in the murine c-fos promoter to between -400 and -200 bp, identify the signal transducer and activator of transcription (STAT) (-345) and serum response element (-310) sites as required for induction, a modulatory role for the Ets site (-318), and show that induction is protein kinase C dependent. Using gel shift and Gal4 assays, we further show that phosphorylation of Elk-1 leads to binding to DNA in complex with serum response factor at serum response element and Ets sites within the c-fos promoter. Thus, we determine molecular mechanisms involved in NKB regulation of c-fos induction, which may play a role in modulation of GnRH neuron activation.

Amitriptyline induces early growth response-1 gene expression via ERK and JNK mitogen-activated protein kinase pathways in rat C6 glial cells

Astrocytes play important roles in guiding the construction of the nervous system, controlling extracellular ions and neurotransmitters, and regulating CNS synaptogenesis. Egr-1 is a transcription factor involved in neuronal differentiation and astrocyte cell proliferation. In this study, we investigated whether the tricyclic antidepressant (TCA) amitriptyline induces Egr-1 expression in astrocytes using rat C6 glioma cells as a model. We found that amitriptyline increased the expression of Egr-1 in a dose- and time-dependent manner. The amitriptyline-induced Egr-1 expression was mediated through serum response elements (SREs) in the Egr-1 promoter. SREs were activated by the Ets-domain transcription factor Elk-1 through the ERK and JNK mitogen-activated protein (MAP) kinase pathways. The inhibition of the ERK and JNK MAP kinase signals attenuated amitriptyline-induced transactivation of Gal4-Elk-1 and Egr-1 promoter activity. Our findings suggest that the induction of Egr-1 expression in astrocytes may be required to attain the therapeutic effects of antidepressant drugs.

Critical Protein-Protein Interactions Determine the Biological Activity of Elk-1, a Master Regulator of Stimulus-Induced Gene Transcription

Elk-1 is a transcription factor that binds together with a dimer of the serum response factor (SRF) to the serum-response element (SRE), a genetic element that connects cellular stimulation with gene transcription. Elk-1 plays an important role in the regulation of cellular proliferation and apoptosis, thymocyte development, glucose homeostasis and brain function. The biological function of Elk-1 relies essentially on the interaction with other proteins. Elk-1 binds to SRF and generates a functional ternary complex that is required to activate SRE-mediated gene transcription. Elk-1 is kept in an inactive state under basal conditions via binding of a SUMO-histone deacetylase complex. Phosphorylation by extracellular signal-regulated protein kinase, c-Jun N-terminal protein kinase or p38 upregulates the transcriptional activity of Elk-1, mediated by binding to the mediator of RNA polymerase II transcription (Mediator) and the transcriptional coactivator p300. Strong and extended phosphorylation of Elk-1 attenuates Mediator and p300 recruitment and allows the binding of the mSin3A-histone deacetylase corepressor complex. The subsequent dephosphorylation of Elk-1, catalyzed by the protein phosphatase calcineurin, facilitates the re-SUMOylation of Elk-1, transforming Elk-1 back to a transcriptionally inactive state. Thus, numerous protein–protein interactions control the activation cycle of Elk-1 and are essential for its biological function.

[Activation of ERK1/2 and Elk1 in A549 cells induced by crocidolite]

OBJECTIVE

To investigate the role of extracellular signal-regulated kinase (ERK) 1/2 and Elk1 in pulmonary disease induced by crocidolite asbestos fiber.

METHOD

Western blotting and Immunoprecipitation were used to detect the expression of phosphorylated ERK1/2 and Elk1 in human bronchial airway A549 cell line stimulated by crocidolite.

RESULTS

The expression of phosphorylated ERK1/2 and Elk1 were striking higher than those of the control, the differences were significant (P < 0.05).

CONCLUSION

Phosphorylated ERK1/2 and Elk1 probably involved in the process of the diseases induced by crocidolite.