Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:2.7.12.2 (MEK)
 18,161 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Activation of alpha1B adrenergic receptors (alpha(1B)AR) promotes DNA synthesis in primary cultures of hepatocytes, yet expression of alpha(1B)AR in hepatocytes rapidly declines during proliferative events. HepG2 human hepatoma cells, which do not express alpha(1B)AR, were stably transfected with a rat alpha1B(AR) cDNA (TFG2 cells), in order to study the effects of maintained alpha(1B)AR expression on hepatoma cell proliferation. TFG2 cells had a decreased rate of growth compared to mock transfected HepG2 cells as revealed by a decrease in [3H]thymidine incorporation into DNA. Stimulation of alpha(1B)AR with phenylephrine caused a further large reduction in TFG2 cell growth, whereas no effect on growth was observed in mock transfected cells. Reduced cell growth correlated with increased percentages of cells found in G0/G1 and G2/M phases of the cell cycle. In TFG2 cells, phenylephrine increased p42MAPkinase activity by 1.5- to 2.0-fold for up to 24 h and increased expression of the cyclin dependent kinase inhibitor protein p21Cip1/WAF1. Treatment of TFG2 cells with the specific MEKI inhibitor PD98059, or infection with a -/- MEK1 recombinant adenovirus permitted phenylephrine to increase rather than decrease [3H]thymidine incorporation. In addition, inhibition of MAP kinase signaling by PD98059 or MEK1 -/- blunted the ability of phenylephrine to increase p21Cip1/WAF1 expression. In agreement with a role for increased p21Cip1/WAF1 expression in causing growth arrest, infection of TFG2 cells with a recombinant adenovirus to express antisense p21Cip1/WAF1 mRNA blocked the ability of phenylephrine to increase p21Cip1/WAF1 expression and to inhibit DNA synthesis. Antisense p21Cip1/WAF1 permitted phenylephrine to stimulate DNA synthesis in TFG2 cells, and abrogated growth arrest. These results suggest that transformed hepatocytes may turn off the expression of alpha1B(ARs) in order to prevent the activation of a growth inhibitory pathway. Activation of this inhibitory pathway via alpha1B(AR) appears to be p42MAPkinase and p21Cip1/WAF1 dependent.
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PMID:Alpha-adrenergic inhibition of proliferation in HepG2 cells stably transfected with the alpha1B-adrenergic receptor through a p42MAPkinase/p21Cip1/WAF1-dependent pathway. 977 8

Ras mutations are common in lung adenocarcinomas and squamous-cell cancers, which are non-small-cell lung cancers (NSCLCs). However, small-cell lung cancers (SCLCs) rarely have ras mutations, suggesting that ras activation may not confer a growth advantage in these cells. In one SCLC cell line DMS53, activated ras expression induced increased neuroendocrine differentiation and decreased cell proliferation. We show here that DMS53 cells undergo differentiation and G1-specific growth arrest in response to ras/raf/ mitogen-activated protein kinase kinase (MEK)/mitogen-activated protein kinase (MAPK) pathway activation. To assess the consequences of activating the raf/MEK/MAPK pathway downstream of ras, we transfected a DMS53 cell line with DeltaRaf-1:ER, an activatable form of c-raf-1. DeltaRaf-1:ER activation suppressed cell proliferation and cloning on soft agar by 90% without evidence of apoptosis. Cell cycle analysis showed a reduced proportion of cells in S phase, and was associated with induction of the cyclin-dependent kinase (cdk) inhibitor p16(INK4). Expression of the cell cycle-specific proteins pRb, Rb2/p130, p107, cyclin A, cdc-2, and E2F-1 was decreased after DeltaRaf-1:ER activation in DMS53 cells. The activity cdk4 and cdk2 was also reduced, as consistent with cell cycle arrest in cells with activated DeltaRaf-1:ER cells. In addition, DeltaRaf-1:ER reduced the expression of neuroendocrine markers, gastrin releasing peptide, and ret gene in DMS53:DeltaRaf-1:ER cells. These results provide further evidence that activation of the raf/MEK/ MAPK signaling pathway, which is associated with transformation in many circumstances, can reduce the growth of SCLC cells, and suggest that activation of this pathway might be clinically efficacious in some settings.
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PMID:Raf-1 causes growth suppression and alteration of neuroendocrine markers in DMS53 human small-cell lung cancer cells. 1010 Sep 84

Aberrancies of growth and proliferation-regulating mechanisms might be critically involved in the processes of neurodegeneration in Alzheimer's disease (AD). Expression of p21ras and further downstream signalling elements involved in regulation of proliferation and differentiation as, for example, MEK, ERK1/2, cyclins, cyclin-dependent kinases and their inhibitors such as those of the p16INK4a family, are elevated early during the course of neurodegeneration. Activation of p21ras can also directly be triggered by nitric oxide (NO), synthesized in the brain by various isoforms of nitric oxide synthase (NOS) that might be differentially involved into the pathomechanism of AD. To study the potential link of NO and critical regulators of cellular proliferation and differentiation in the process of neurofibrillary degeneration, we analyzed the expression pattern of NOS-isoforms, p21ras and p16INK4a compared to neurofibrillary degeneration in AD. Additionally to its expression in a subtype of cortical interneurons that contain the nNOS-isoform also in normal brain, nNOS was detected in pyramidal neurons containing neurofibrillary tangles or were even unaffected by neurofibrillary degeneration. Expression of nNOS in these neurons was highly co-localized with p21ras and p16INK4a. Because endogenous NO can activate p21ras in the same cell which in turn leads to cellular activation and stimulation of NOS expression [H.M. Lander, J.S. Ogiste, S.F.A. Pearce, R. Levi, A. Novogrodsky, Nitric oxide-stimulated guanine nucleotide exchange on p21 ras, J. Biol. Chem. 270 (1995) 7017-7020], the high level of co-expression of NOS and p21ras in neurons vulnerable to neurofibrillary degeneration early in the course of AD thus provides the basis for an autocrine feedback mechanism that might exacerbate the progression of neurodegeneration in a self-propagating manner.
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PMID:Aberrant expression of nNOS in pyramidal neurons in Alzheimer's disease is highly co-localized with p21ras and p16INK4a. 1066 94

The p16INK4a cyclin-dependent kinase inhibitor is implicated in replicative senescence, the state of permanent growth arrest provoked by cumulative cell divisions or as a response to constitutive Ras-Raf-MEK signalling in somatic cells. Some contribution to senescence presumably underlies the importance of p16INK4a as a tumour suppressor but the mechanisms regulating its expression in these different contexts remain unknown. Here we demonstrate a role for the Ets1 and Ets2 transcription factors based on their ability to activate the p16INK4a promoter through an ETS-binding site and their patterns of expression during the lifespan of human diploid fibroblasts. The induction of p16INK4a by Ets2, which is abundant in young human diploid fibroblasts, is potentiated by signalling through the Ras-Raf-MEK kinase cascade and inhibited by a direct interaction with the helix-loop-helix protein Id1 (ref. 11). In senescent cells, where the Ets2 levels and MEK signalling decline, the marked increase in p16INK4a expression is consistent with the reciprocal reduction of Id1 and accumulation of Ets1.
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PMID:Opposing effects of Ets and Id proteins on p16INK4a expression during cellular senescence. 1123 19

The dual specificity kinases mitogen-activated protein kinase (MAPK) kinase (MKK)7 and MKK4 are the only molecules known to directly activate the stress kinases stress-activated protein kinases (SAPKs)/c-Jun N-terminal kinases (JNKs) in response to environmental or mitogenic stimuli. To examine the physiological role of MKK7 in hematopoietic cells, we used a gene targeting strategy to mutate MKK7 in murine T and B cells and non-lymphoid mast cells. Loss of MKK7 in thymocytes and mature B cells results in hyperproliferation in response to growth factor and antigen receptor stimulation and increased thymic cellularity. Mutation of mkk7 in mast cells resulted in hyperproliferation in response to the cytokines interleukin (IL)-3 and stem cell factor (SCF). SAPK/JNK activation was completely abolished in the absence of MKK7, even though expression of MKK4 was strongly upregulated in mkk7(-/-) mast cell lines, and phosphorylation of MKK4 occurred normally in response to multiple stress stimuli. Loss of MKK7 did not affect activation of extracellular signal-regulated kinase (ERK)1/2 or p38 MAPK. mkk7(-/-) mast cells display reduced expression of JunB and the cell cycle inhibitor p16INK4a and upregulation of cyclinD1. Reexpression of p16INK4a in mkk7(-/-) mast cells abrogates the hyperproliferative response. Apoptotic responses to a variety of stimuli were not affected. Thus, MKK7 is an essential and specific regulator of stress-induced SAPK/JNK activation in mast cells and MKK7 negatively regulates growth factor and antigen receptor-driven proliferation in hematopoietic cells. These results indicate that the MKK7-regulated stress signaling pathway can function as negative regulator of cell growth in multiple hematopoietic lineages.
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PMID:The stress kinase mitogen-activated protein kinase kinase (MKK)7 is a negative regulator of antigen receptor and growth factor receptor-induced proliferation in hematopoietic cells. 1156 Sep 92

It is often assumed that MAPK pathways drive proliferation of normal uroepithelial (UEC) and urothelial carcinoma (TCC) cells. To check this assumption, activities and inducibilities of promoters containing serum-response elements (SRE) or AP-1 binding sites were investigated in cultured UEC and seven TCC lines. Reporter plasmids dependent on SRE or AP-1 sites were highly active in UEC, but significantly less so in TCC lines. Reporter activity in TCC lines could be induced by constitutively active MEKK4 or TPA. Accordingly, phosphorylation of the MAPK pathway components MEK, ERK, and ELK1 was most pronounced in UEC and lower in TCC lines. MAPK-dependent promoter activities and bromodeoxyuridine incorporation decreased in UEC upon withdrawal of growth factors, but less so in TCC lines, in which serum diminution increased apoptosis. Likewise, E2F-dependent promoters responded to growth factors in UEC, but were more serum-independent in the TCC lines, which lack either RB1 or p16(INK4A). MEK inhibitors inhibited BrdU incorporation in UEC more strongly than in TCC lines. Thus, proliferation of normal uroepithelial cells is indeed associated with activation of MAPK pathways. However, autonomous proliferation of TCC lines--unexpectedly--appears much less dependent on MAPK activation and may rather be promoted by defects in cell cycle regulation.
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PMID:Activities of MAP-kinase pathways in normal uroepithelial cells and urothelial carcinoma cell lines. 1249 Jan 93

Although oncogenic ras plays a pivotal role in neoplastic transformation, it triggers an anti-oncogenic defense mechanism known as premature senescence in normal cells. In this study, we investigated the induction of cellular responses by different expression levels of oncogenic ras in primary human fibroblasts. We found that a moderate, severalfold increase in ras expression promoted cell growth. Further elevation of ras expression initially enhanced proliferation but eventually induced p16INK4A expression and senescence. The induction of these opposing cellular responses by ras signals of different intensity was achieved through differential activation of the MAPK pathways that mediated these responses. Whereas moderate ras activities only stimulated the mitogenic MEK-ERK pathway, high intensity ras signals induced MEK and ERK to higher levels, leading to stimulation of the MKK3/6-p38 pathway, which had been shown previously to act downstream of Ras-MEK to trigger the senescence response. Thus, these studies have revealed a mechanism for the differential effects of ras on cell proliferation. Furthermore, moderate ras activity mediated transformation in cooperation with E6E7 and hTERT, suggesting that a moderate intensity ras signal can provide sufficient oncogenic activities for tumorigenesis. This result also implies that the ability of ras to promote proliferation and oncogenic transformation can be uncoupled with that to induce senescence in cell culture and that the development of tumors with relatively low ras activities may not need to acquire genetic alterations that bypass premature senescence.
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PMID:High intensity ras signaling induces premature senescence by activating p38 pathway in primary human fibroblasts. 1459 17

The mitogen-activated protein kinase cascade operates downstream of Ras to convey cell-surface signals to the nucleus via nuclear translocation of ERK1 and ERK2. We and others have recently demonstrated that activation of ERK1/2 by growth factors is required for proliferation of intestinal epithelial crypt cells. However, it remained to be established whether ERK1/2 activation alone was sufficient to trigger intestinal epithelial cell (IEC) proliferation. To this aim, retrovirus encoding the hemagglutinin-tagged MAPK/ERK kinase (MEK)1 wild type (wtMEK), the upstream activator of ERK1/2, or a constitutively active mutant of MEK1 (MEK1-S218D/S222D; caMEK) were used to infect nonimmortalized human normal intestinal epithelial crypt cell cultures [human intestinal epithelial cells (HIEC)] and rodent immortalized intestinal crypt cells (IEC-6). Stable expression of caMEK but not wtMEK in HIEC led to the irreversible arrest of cellular proliferation (premature senescence). Concomitant with the onset of cell-cycle arrest was the induction of the cyclin-dependent kinase inhibitors p21(Cip), p53, and p16(INK4A). By contrast, overexpression of caMEK in IEC-6 cells induced growth factor relaxation for DNA synthesis, promoted morphological transformation and growth in soft agar, and did not affect expression of p21(Cip), p53, and p16(INK4A). We provided evidences that ERK1b, an alternatively spliced isoform of ERK1, is activated and may contribute to the deregulation of contact inhibition cell growth and transformation of these cells. Constitutive activation of MEK in IECs can produce either premature senescence or forced mitogenesis depending on the integrity of a senescence program controlled by the cell cycle inhibitors p53, p16(INK4A), and p21(CIP).
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PMID:Dual role of MEK/ERK signaling in senescence and transformation of intestinal epithelial cells. 1470 21

Even though RAS usually acts as a dominant transforming oncogene, in primary fibroblasts and some established cell lines Ras inhibits proliferation. This can explain the virtual absence of RAS mutations in some types of tumors, such as chronic myeloid leukemia (CML). We report that in the CML cell line K562 Ras induces p21Cip1 expression through the Raf-MEK-ERK pathway. Because K562 cells are deficient for p15INK4b, p16INK4a, p14ARF, and p53, this would be the main mechanism whereby Ras up-regulates p21 expression in these cells. Accordingly, we also found that Ras suppresses K562 growth by signaling through the Raf-ERK pathway. Because c-Myc and Ras cooperate in cell transformation and c-Myc is up-regulated in CML, we investigated the effect of c-Myc on Ras activity in K562 cells. c-Myc antagonized the induction of p21Cip1 mediated by oncogenic H-, K-, and N-Ras and by constitutively activated Raf and ERK2. Activation of the p21Cip1 promoter by Ras was dependent on Sp1/3 binding sites in K562. However, mutational analysis of the p21 promoter and the use of a Gal4-Sp1 chimeric protein strongly suggest that c-Myc affects Sp1 transcriptional activity but not the binding of Sp1 to the p21 promoter. c-Myc-mediated impairment of Ras activity on p21 expression required a transactivation domain, a DNA binding region, and a Max binding region. Moreover, the effect was independent of Miz1 binding to c-Myc. Consistent with its effect on p21Cip1 expression, c-Myc rescued cell growth inhibition induced by Ras. The data suggest that in particular tumor types, such as those associated with CML, c-Myc contributes to tumorigenesis by inhibiting Ras antiproliferative activity.
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PMID:Myc antagonizes Ras-mediated growth arrest in leukemia cells through the inhibition of the Ras-ERK-p21Cip1 pathway. 1552 12

CD4+CD25+ regulatory T cells (Tregs) are essential negative regulators of immune responses. Here, we examined the signaling properties of human Tregs, using CD4+CD25+ Treg and CD4+CD25- control (Tcont) cell lines generated from cord blood. Treg cell lines were markedly hyporesponsive to stimulation with dendritic cells and with anti-CD3/CD28-coated beads. Hyporesponsiveness was reversed by exogenous interleukin-2 (IL-2). T-cell receptor (TCR)-CD3/CD28-mediated activation of Rap1 and Akt was retained in Tregs, but activation of Ras, mitogenactivated protein kinase 1/2 (MEK1/2), and extracellular signal-regulated kinase 1/2 (Erk1/2) was impaired. Tregs were blocked from cell cycle progression due to decrease of cyclin E and cyclin A and increase of p27kip1 (p27kip cyclin dependent kinase inhibitor). IL-2 induced sustained increase of cyclin E and cyclin A and prevented up-regulation of p27kip1. Tregs had high susceptibility to apoptosis that was reversed by IL-2, which correlated with activation of Erk1/2, up-regulation of Bcl-x(L) (B-cell CLL/lymphoma 2-like nuclear gene encoding mitochondrial protein, transcript variant 2), and phosphorylation of Bad (Bcl2 antagonist of cell death) at Ser112. Thus, Tregs share biochemical characteristics of anergy, including abortive activation of Ras-MEK-Erk, increased activation of Rap1, and increased expression of p27kip1. In addition, our results indicate that TCR-CD3/CD28-mediated and IL-2 receptor-mediated signals converge at the level of MEK-Erk kinases to regulate Treg survival and expansion and suggest that manipulation of the MEK-Erk axis may represent a novel strategy for Treg expansion for immunotherapy.
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PMID:CD4+CD25+ regulatory T-cell lines from human cord blood have functional and molecular properties of T-cell anergy. 1602 May 8


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