Transcriptional regulation by extracellular signals: mechanisms and specificity

Negative regulation of transactivation function but not DNA binding of NF-kappaB and AP-1 by IkappaBbeta1 in breast cancer cells

The transcription factor NF-kappaB regulates the expression of genes involved in cancer cell invasion, metastasis, angiogenesis, and resistance to chemotherapy. In normal cells NF-kappaB is maintained in the cytoplasm by protein-protein interaction with inhibitor IkappaBs. In contrast, in cancer cells a substantial amount of NF-kappaB is in the nucleus and constitutively activates target genes. To understand the mechanisms of constitutive NF-kappaB activation, we have analyzed the function of IkappaBalpha and IkappaBbeta in breast cancer cells. In most cases, constitutive NF-kappaB DNA binding correlated with reduced levels of either IkappaBalpha or IkappaBbeta isoforms. Overexpression of IkappaBalpha but not IkappaBbeta1 resulted in reduced constitutive DNA binding of NF-kappaB in MDA-MB-231 cells. Unexpectedly, IkappaBbeta1 overexpression moderately increased 12-O-tetradecanoylphorbol-13-acetate- and interleukin-1-inducible NF-kappaB DNA binding. 12-O-Tetradecanoylphorbol-13-acetate- and interleukin-1-induced transactivation by NF-kappaB, however, was lower in IkappaBbeta1-overexpressing cells. Mutants of IkappaBbeta1 lacking the C-terminal casein kinase II phosphorylation sites, which form a stable complex with DNA bound NF-kappaB without inhibiting its transactivation in other cell types, repressed the transactivation by NF-kappaB in MDA-MB-231 cells. Consistent with the results of transient transfections, the expression of urokinase plasminogen activator, an NF-kappaB target gene, was reduced in IkappaBbeta1-overexpressing cells. These results suggest that depending on the cell type, IkappaBbeta1 represses the expression of NF-kappaB-regulated genes by inhibiting either DNA binding or transactivation function of NF-kappaB.

Extracellular matrix and integrin signalling: the shape of things to come

The extracellular matrix (ECM) and integrins collaborate to regulate gene expression associated with cell growth, differentiation and survival. Biochemical and molecular analyses of integrin signalling pathways have uncovered several critical cytoplasmic proteins that link the ECM and integrins to intracellular pathways that may contribute to anchorage-dependent growth. A large body of evidence now indicates that the non-receptor protein kinases focal adhesion kinase (FAK) and specific members of the mitogen-activated protein kinases (MAPKs), including the extracellular-signal-regulated kinases (ERKs), mediate these ECM- and integrin-derived signalling events. However, little is known about how FAK and MAPKs contribute to biological processes other than cell proliferation or migration. In addition, remarkably little is known concerning the signalling events that occur in cells that adhere to complex multivalent extracellular matrices via multiple integrin receptors. Given the stringent requirement for attaining a proper morphology in ECM/integrin-directed cell behaviour, it is still not clear how cell shape and tissue architecture impact upon intracellular signalling programmes involving FAK and MAPKs. However, the recent discovery that members of the Rho family of small GTPases are able to regulate ECM/integrin pathways that modulate both cell shape and intracellular signalling provides new insights into how cell morphology and signal transduction become integrated, especially within three-dimensional differentiated tissues.

Extracellular matrix and integrin signalling: the shape of things to come

The extracellular matrix (ECM) and integrins collaborate to regulate gene expression associated with cell growth, differentiation and survival. Biochemical and molecular analyses of integrin signalling pathways have uncovered several critical cytoplasmic proteins that link the ECM and integrins to intracellular pathways that may contribute to anchorage-dependent growth. A large body of evidence now indicates that the non-receptor protein kinases focal adhesion kinase (FAK) and specific members of the mitogen-activated protein kinases (MAPKs), including the extracellular-signal-regulated kinases (ERKs), mediate these ECM- and integrin-derived signalling events. However, little is known about how FAK and MAPKs contribute to biological processes other than cell proliferation or migration. In addition, remarkably little is known concerning the signalling events that occur in cells that adhere to complex multivalent extracellular matrices via multiple integrin receptors. Given the stringent requirement for attaining a proper morphology in ECM/integrin-directed cell behaviour, it is still not clear how cell shape and tissue architecture impact upon intracellular signalling programmes involving FAK and MAPKs. However, the recent discovery that members of the Rho family of small GTPases are able to regulate ECM/integrin pathways that modulate both cell shape and intracellular signalling provides new insights into how cell morphology and signal transduction become integrated, especially within three-dimensional differentiated tissues.

Ca2+/calmodulin-dependent kinase II phosphorylates the epidermal growth factor receptor on multiple sites in the cytoplasmic tail and serine 744 within the kinase domain to regulate signal generation

Down-regulation of receptor tyrosine kinase activity plays an essential role in coordinating and controlling cellular growth/differentiation. Ca2+/calmodulin-dependent kinase II (CaM kinase II)-mediated phosphorylation of threonine 1172 in the cytoplasmic tail of HER2/c-erbB2 can modulate tyrosine kinase activity and consensus phosphorylation sites are also found at serines 1046/1047 in the structurally related epidermal growth factor receptor (EGFR). We show that serines 1046/1047 are sites for CaM kinase II phosphorylation, although there is a preference for serine 1047, which resides within the consensus -R-X-X-S-. In addition, we have identified major phosphorylation sites at serine 1142 and serine 1057, which lie within a novel -S-X-D- consensus. Mutation of serines 1046/1047 in full-length EGFR enhanced both fibroblast transformation and tyrosine autokinase activity that was significantly potentiated by additional mutation of serines 1057 and 1142. A single CaM kinase II site was also identified at serine 744 within sub-kinase domain III, and autokinase activity was significantly affected by mutation of this serine to an aspartic acid making this site appear constitutively phosphorylated. We have addressed the mechanism by which CaM kinase II phosphorylation of the EGFR might regulate receptor autokinase activity and show that this modification can hinder association of the cytoplasmic tail with the kinase domain to prevent an enzyme-substrate interaction. We postulate that the location and greater number of CaM kinase II phosphorylation sites in the EGFR compared with HER-2/c-erbB2, leading to differential regulation of autokinase activity, contributes to differences in the strength of downstream signaling events and may explain the higher relative transforming potential of HER-2/cerbB2.

Constitutive Activation of Extracellular Signal-Regulated Kinase in Human Acute Leukemias: Combined Role of Activation of MEK, Hyperexpression of Extracellular Signal-Regulated Kinase, and Downregulation of a Phosphatase, PAC1

Extracellular signal-regulated kinase (ERK) is an important intermediate in signal transduction pathways that are initiated by many types of cell surface receptors. It is thought to play a pivotal role in integrating and transmitting transmembrane signals required for growth and differentiation. Constitutive activation of ERK in fibroblasts elicits oncogenic transformation, and recently, constitutive activation of ERK has been observed in some human malignancies, including acute leukemia. However, mechanisms underlying constitutive activation of ERK have not been well characterized. In this study, we examined the activation of ERK in 79 human acute leukemia samples and attempted to find factors contributing to constitutive ERK activation. First, we showed that ERK and MEK were constitutively activated in acute leukemias by in vitro kinase assay and immunoblot analysis. However, in only one half of the studied samples, the pattern of ERK activation was similar to that of MEK activation. Next, by semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and immunoblot analysis, we showed hyperexpression of ERK in a majority of acute leukemias. In 17 of 26 cases (65.4%) analyzed by immunoblot, the pattern of ERK expression was similar to that of ERK activation. The fact of constitutive activation of ERK in acute leukemias suggested to us the possibility of an abnormal downregulation mechanism of ERK. Therefore, we examined PAC1, a specific ERK phosphatase predominantly expressed in hematopoietic tissue and known to be upregulated at the transcription level in response to ERK activation. Interestingly, in our study, PAC1 gene expression in acute leukemias showing constitutive ERK activation was significantly lower than that in unstimulated, normal bone marrow (BM) samples showing minimal or no ERK activation (P = .002). Also, a significant correlation was observed between PAC1 downregulation and phosphorylation of ERK in acute leukemias (P= .002). Finally, by further analysis of 26 cases, we showed that a complementary role of MEK activation, ERK hyperexpression, and PAC1 downregulation could contribute to determining the constitutive activation of ERK in acute leukemia. Our results suggest that ERK is constitutively activated in a majority of acute leukemias, and in addition to the activation of MEK, the hyperexpression of ERK and downregulation of PAC1 also contribute to constitutive ERK activation in acute leukemias.

Constitutive Activation of Extracellular Signal-Regulated Kinase in Human Acute Leukemias: Combined Role of Activation of MEK, Hyperexpression of Extracellular Signal-Regulated Kinase, and Downregulation of a Phosphatase, PAC1

Extracellular signal-regulated kinase (ERK) is an important intermediate in signal transduction pathways that are initiated by many types of cell surface receptors. It is thought to play a pivotal role in integrating and transmitting transmembrane signals required for growth and differentiation. Constitutive activation of ERK in fibroblasts elicits oncogenic transformation, and recently, constitutive activation of ERK has been observed in some human malignancies, including acute leukemia. However, mechanisms underlying constitutive activation of ERK have not been well characterized. In this study, we examined the activation of ERK in 79 human acute leukemia samples and attempted to find factors contributing to constitutive ERK activation. First, we showed that ERK and MEK were constitutively activated in acute leukemias by in vitro kinase assay and immunoblot analysis. However, in only one half of the studied samples, the pattern of ERK activation was similar to that of MEK activation. Next, by semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and immunoblot analysis, we showed hyperexpression of ERK in a majority of acute leukemias. In 17 of 26 cases (65.4%) analyzed by immunoblot, the pattern of ERK expression was similar to that of ERK activation. The fact of constitutive activation of ERK in acute leukemias suggested to us the possibility of an abnormal downregulation mechanism of ERK. Therefore, we examined PAC1, a specific ERK phosphatase predominantly expressed in hematopoietic tissue and known to be upregulated at the transcription level in response to ERK activation. Interestingly, in our study, PAC1 gene expression in acute leukemias showing constitutive ERK activation was significantly lower than that in unstimulated, normal bone marrow (BM) samples showing minimal or no ERK activation (P = .002). Also, a significant correlation was observed between PAC1 downregulation and phosphorylation of ERK in acute leukemias (P= .002). Finally, by further analysis of 26 cases, we showed that a complementary role of MEK activation, ERK hyperexpression, and PAC1 downregulation could contribute to determining the constitutive activation of ERK in acute leukemia. Our results suggest that ERK is constitutively activated in a majority of acute leukemias, and in addition to the activation of MEK, the hyperexpression of ERK and downregulation of PAC1 also contribute to constitutive ERK activation in acute leukemias.

Intracellular oxidation/reduction status in the regulation of transcription factors NF-kappaB and AP-1

The eukaryotic cell contains a multitude of pathways coupling environmental stimuli to the specific regulation of gene expression. Two early response transcriptional complexes, NF-kappaB and AP-1, appear to respond to environmental stressors by inducing the expression of response specific downstream genes. Both are well-characterized transcriptional regulatory factors that are induced by a wide variety of seemingly unrelated exogenous and endogenous agents and serve important roles in cell growth and differentiation, immunity, inflammation, and other preprogrammed cellular genetic processes. The activities of NF-kappaB and AP-1 are also affected following exposure to chemicals, drugs, or other agents that appear to alter the cellular oxidation/reduction (redox) status. From these observations, it has been suggested that changes in cellular oxidation/reduction status, communicated via a series of cellular redox-sensitive signaling circuitry employing metal- and thiol-containing proteins, serve as common mechanisms linking environmental stressors to adaptive cellular responses. As such, these transcription factors are ideal paradigms to study the mechanism and possible physiological significance of early response genes in the cellular response to changes in cellular redox status. In this article we summarize the evidence suggesting that cellular redox regulates these transcription factors.

Cell type-specific requirement of the MAPK pathway for the growth factor action of gastrin

Gastrin (G17) has a CCKB receptor-mediated growth-promoting effect on the AR42J rat acinar cell line that is linked to induction of both mitogen-activated protein kinase (MAPK) and c-fos gene expression. We investigated the mechanisms that regulate the growth factor action of G17 on the rat pituitary adenoma cell line GH3. Both AR42J and GH3 cells displayed equal levels of CCKB receptor expression and similar binding kinetics of 125I-labeled G17. G17 stimulation of cell proliferation was identical in both cell lines. G17 stimulation of GH3 cell proliferation was completely blocked by the CCKB receptor antagonist D2 but not by the MEK inhibitor PD-98059 or the protein kinase C inhibitor GF-109203X, which completely inhibited G17 induction of AR42J cell proliferation. G17 induced a c-fos SRE-luciferase reporter gene plasmid more than fourfold in the AR42J cells, whereas it had no effect in the GH3 cells. In contrast to what we observed in the AR42J cells, G17 failed to stimulate MAPK activation and Shc tyrosyl phosphorylation and association with the adapter protein Grb2. Epidermal growth factor induced the MAPK pathway in the GH3 cells, demonstrating the integrity of this signaling system. G17 induced Ca2+ mobilization in both the GH3 and AR42J cells. The calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide inhibited AR42J cell proliferation by 20%, whereas it completely blocked G17 induction of GH3 cell growth. The Ca2+ ionophore ionomycin stimulated GH3 cell proliferation to a level similar to that observed in response to G17, but it had no effect on AR42J cell proliferation. Thus there are cell type specific differences in the requirement of the MAPK pathway for the growth factor action of G17. Whereas in the AR42J cells G17 stimulates cell growth through activation of MAPK and c-fos gene expression, in the GH3 cells, G17 fails to activate MAPK, and it induces cell proliferation through Ca2+-dependent signaling pathways. Furthermore, induction of Ca2+ mobilization in the AR42J cells appears not to be sufficient to sustain cell proliferation.

Sustained MAP kinase activation is required for the expression of cyclin D1, p21Cip1 and a subset of AP-1 proteins in CCL39 cells

In CCL39 cells thrombin is a potent growth factor which requires sustained activation of mitogen activated protein kinases (MAPKs) to promote DNA synthesis. Some of the effects of thrombin can be mimicked by peptides based on the new amino terminus of the cleaved receptor; however, these thrombin receptor peptides (TRPs) fail to induce sustained activation of MAPK or DNA synthesis. We have used thrombin, TRP-7 and other agonists which elicit sustained or transient MAPK activation to identify immediate-early and delayed-early genes which are only expressed under conditions of sustained MAPK activation focusing on cyclin D1, p21CiP1 and the AP-1 transcription factor. Of the stimuli tested only FBS and thrombin were able to stimulate a sustained activation of MAPK, expression of cyclin D1, p21Cip1 and cell cycle re-entry. The expression of cyclin D1 was strongly, though not completely, inhibited by the MEK1 inhibitor PD098059. Thrombin stimulated a rapid but transient accumulation of c-Fos whereas the expression of Fra-1, Fra-2, c-Jun and JunB was sustained throughout the G1 phase of the cell cycle. We focussed our analysis on c-Fos (typical of AP-1 genes which are expressed rapidly and transiently) and Fra-1 and JunB (typical of AP-1 genes expressed after a delay but in a sustained manner). The expression of c-Fos, Fra-1 and JunB was dependent upon the activation of MAPK since these responses were inhibited by PD098059. However, a comparison of responses to FBS, thrombin, TRPs, LPA and EGF revealed that Fra-1 and JunB expression required sustained activation of MAPK whereas c-Fos expression was strongly induced even by non-mitogenic stimuli which elicited only transient MAPK activation. The expression of c-Fos (in response to thrombin, TRP or LPA) or Fra-1, JunB and cyclin D1 (thrombin only) was also inhibited by pertussis toxin. This suggests that both early and late AP-1 gene expression is regulated by the same Gi-mediated, MEK-dependent MAPK signalling pathway but that expression of late AP-1 genes and cyclin D1 requires that this pathway be persistently activated. The results suggest that the duration of receptor signalling and therefore MAPK activation is a key determinant of qualitative changes in gene expression during cell cycle re-entry.

Regulation of gene expression by reactive oxygen

Reactive oxygen intermediates are produced in all aerobic organisms during respiration and exist in the cell in a balance with biochemical antioxidants. Excess reactive oxygen resulting from exposure to environmental oxidants, toxicants, and heavy metals perturbs cellular redox balance and disrupts normal biological functions. The resulting imbalance may be detrimental to the organism and contribute to the pathogenesis of disease and aging. To counteract the oxidant effects and to restore a state of redox balance, cells must reset critical homeostatic parameters. Changes associated with oxidative damage and with restoration of cellular homeostasis often lead to activation or silencing of genes encoding regulatory transcription factors, antioxidant defense enzymes, and structural proteins. In this review, we examine the sources and generation of free radicals and oxidative stress in biological systems and the mechanisms used by reactive oxygen to modulate signal transduction cascades and redirect gene expression.

Signaling in human osteoblasts by extracellular nucleotides. Their weak induction of the c-fos proto-oncogene via Ca2+ mobilization is strongly potentiated by a parathyroid hormone/cAMP-dependent protein kinase pathway independently of mitogen-activated protein kinase

Extracellular nucleotides acting through specific P2 receptors activate intracellular signaling cascades. Consistent with the expression of G protein-coupled P2Y receptors in skeletal tissue, the human osteosarcoma cell line SaOS-2 and primary osteoblasts express P2Y1 and P2Y2 receptors, respectively. Their activation by nucleotide agonists (ADP and ATP for P2Y1; ATP and UTP for P2Y2) elevates [Ca2+]i and moderately induces expression of the c-fos proto-oncogene. A synergistic effect on c-fos induction is observed by combining ATP and parathyroid hormone, a key bone cell regulator. Parathyroid hormone elevates intracellular cAMP levels and correspondingly activates a stably integrated reporter gene driven by the Ca2+/cAMP-responsive element of the human c-fos promoter. Nucleotides have little effect on either cAMP levels or this reporter, instead activating luciferase controlled by the full c-fos promoter. This induction is reproduced by a stably integrated serum response element reporter independently of mitogen-activated protein kinase activation and ternary complex factor phosphorylation. This novel example of synergy between the cAMP-dependent protein kinase/CaCRE signaling module and a non-mitogen-activated protein kinase/ternary complex factor pathway that targets the serum response element shows that extracellular ATP, via P2Y receptors, can potentiate strong responses to ubiquitous growth and differentiative factors.

Diminished G1 checkpoint after gamma-irradiation and altered cell cycle regulation by insulin-like growth factor II overexpression

High levels of insulin-like growth factor II (IGFII) mRNA expression are detected in many human tumors of different origins including rhabdomyosarcoma, a tumor of skeletal muscle origin. To investigate the role of IGFII in tumorigenesis, we have compared the mouse myoblast cell line C2C12-2.7, which was stably transfected with human IGFII cDNA and expressed high and constant amounts of IGFII, to a control cell line C2C12-1.1. A rhabdomyosarcoma cell line, RH30, which expresses high levels of IGFII and contains mutated p53, was also used in these studies. IGFII overexpression in mouse myoblast C2C12 cells causes a reduced cycling time and higher growth rate. After gamma-irradiation treatment, C2C12-1.1 cells were arrested mainly in G0/G1 phase. However, C2C12-2.7 and RH30 cells went through a very short G1 phase and then were arrested in an extended G2/M phase. To verify further the effect of IGFII on the cell cycle, we developed a Chinese hamster ovary (CHO) cell line with tetracycline-controlled IGFII expression. We found that CHO cells with high expression of IGFII have a shortened cycling time and a diminished G1 checkpoint after treatment with methylmethane sulfonate (MMS), a DNA base-damaging agent, when compared with CHO cells with very low IGFII expression. It was also found that IGFII overexpression in C2C12 cells was associated with increases in cyclin D1, p21, and p53 protein levels, as well as mitogen-activated protein kinase activity. These studies suggest that IGFII overexpression shortens cell cycling time and diminishes the G1 checkpoint after DNA damage despite an intact p53/p21 induction. In addition, IGFII overexpression is also associated with multiple changes in the levels and activities of cell cycle regulatory components following gamma-irradiation. Taken together, these changes may contribute to the high growth rate and genetic alterations that occur during tumorigenesis.

Extracellular regulated kinases (ERK) 1 and ERK2 are authentic substrates for the dual-specificity protein-tyrosine phosphatase VHR. A novel role in down-regulating the ERK pathway

The mammalian dual-specificity protein-tyrosine phosphatase VHR (for VH1-related) has been identified as a novel regulator of extracellular regulated kinases (ERKs). To identify potential cellular substrates of VHR, covalently immobilized mutant VHR protein was employed as an affinity trap. A tyrosine-phosphorylated protein(s) of approximately 42 kDa was specifically adsorbed by the affinity column and identified as ERK1 and ERK2. Subsequent kinetic analyses and transfection studies demonstrated that VHR specifically dephosphorylates and inactivates ERK1 and ERK2 in vitro and in vivo. Only the native structure of phosphorylated ERK was recognized by VHR and was inactivated with a second-order rate constant of 40,000 M-1 s-1. VHR was found to dephosphorylate endogenous ERK, but not p38 and JNK. Immunodepletion of endogenous VHR eliminated the dephosphorylation of cellular ERK. Transfection studies in COS-1 cells demonstrated that in vivo phosphorylation of epidermal growth factor-stimulated ERK depended on VHR protein levels. Overexpression above endogenous levels of VHR led to accelerated ERK inactivation, but did not alter the normal activation of ERK. Unique among reported mitogen activated protein kinase phosphatases, VHR is constitutively expressed, localized to the nucleus, and tyrosine-specific. This study is the first to report the identification of authentic substrates of dual-specificity phosphatases utilizing affinity absorbents and is the first to identify a nuclear, constitutively expressed, and tyrosine-specific ERK phosphatase. The data strongly suggest that VHR is responsible for the rapid inactivation of ERK following stimulation and for its repression in quiescent cells.

Hepatocellular carcinoma cell lines from diethylnitrosamine phenobarbital-treated rats. Characterization and sensitivity to endothall, a protein serine/threonine phosphatase-2A inhibitor

Primary hepatocellular carcinoma (HCC) is probably one of the most common fatal forms of liver cancer. We have established permanent cell lines from diethylnitrosamine/phenobarbital induced primary rat liver carcinomas to study new anticancer therapies. The rat hepatocellular carcinoma cell lines (HR-2, HR-3, and HR-4) have been maintained in culture for over 3 years. They form tumors when transplanted sc or im into young syngeneic rats. Immunocytology (alpha-fetoprotein, albumin), biochemical (gamma-glutamyl transferase), and histochemical (glycogen) marker studies and electron microscopy (biliary canaliculi) showed unique, stable differentiation patterns in these tumor lines. They overproduced the c-met protooncogene product and formed colonies spontaneously in semisolid culture with high cloning efficiency (HR-2: 50%-80%, HR-3: 35%-50% and HR-4: 50%-65%). The sensitivity of these cell lines to inhibitors of protein ser/thr phosphatase-2A (PP2A), a key enzyme in the control of G1/S and G2/M cell cycle phase transitions in eukaryotes, was studied in vitro. The specific, weak inhibitor of PP2A, endothall, caused dose- and time-dependent cytostasis specifically in G2/M. The cells died later by apoptosis, which was confirmed by cytology (annexin V-FITC labeling, propidium iodide painting of apoptotic bodies) and by fluorescent activated cell sorter (FACS) DNA measurements. The HR-2, HR-3, HR-4, and Zajdela hepatocellular carcinomas were most sensitive to endothall (IC50 of 1.7, 1.2, 0.9, and 1.7 microg/mL), whereas newborn rat hepatocytes growing exponentially in primary culture (IC50 = 6.2 microg/mL), rat DHD/K12 colon carcinoma cells (IC50 = 3.6 microg/mL), or human HT-29 colon carcinoma cells (IC50 = 4.9 microg/mL) were less sensitive. Thus, endothall inhibits preferentially HCC growth and these new rat hepatocellular carcinoma lines may be useful for further biochemical and pharmacological studies on PP2A inhibitors, and for testing new forms of treatment of hepatic cell carcinomas.

Urea-associated oxidative stress and Gadd153/CHOP induction

Urea treatment (100-300 mM) increased expression of the oxidative stress-responsive transcription factor, Gadd153/CHOP, at the mRNA and protein levels (at >/=4 h) in renal medullary mIMCD3 cells in culture, whereas other solutes did not. Expression of the related protein, CCAAT/enhancer-binding protein (C/EBP-beta), was not affected, nor was expression of the sensor of endoplasmic reticulum stress, grp78. Urea modestly increased Gadd153 transcription by reporter gene analysis but failed to influence Gadd153 mRNA stability. Importantly, upregulation of Gadd153 mRNA and protein expression by urea was antioxidant sensitive. Accordingly, urea treatment was associated with oxidative stress, as quantitated by intracellular reduced glutathione content in mIMCD3 cells. In addition, antioxidant treatment partially inhibited the ability of urea to activate transcription of an Egr-1 luciferase reporter gene. Therefore oxidative stress represents a novel solute-signaling pathway in the kidney medulla and, potentially, in other tissues.

TrkA glycosylation regulates receptor localization and activity

The human nerve growth factor receptor (TrkA) contains four potential N-glycosylation sites that are highly conserved within the Trk family of neurotrophin receptors, and nine additional sites that are less well conserved. Using a microscale deglycosylation assay, we show here that both conserved and variable N-glycosylation sites are used during maturation of TrkA. Glycosylation at these sites serves two distinct functions. First, glycosylation is necessary to prevent ligand-independent activation of TrkA. Unglycosylated TrkA core protein is phosphorylated even in the absence of ligand stimulation and displays constitutive kinase activity as well as constitutive interaction with the signaling molecules Shc and PLC-gamma. Second, glycosylation is required to localize TrkA to the cell surface, where it can trigger the Ras/Raf/MAP kinase cascade. Using confocal microscopy, we show that unglycosylated active Trk receptors are trapped intracellularly. Furthermore, the unglycosylated active TrkA receptors are unable to activate kinases in the Ras-MAP kinase pathway, MEK and Erk. Consistent with these biochemical observations, unglycosylated TrkA core protein does not promote neuronal differentiation in Trk PC12 cells even at high levels of constitutive catalytic activity.

Age-dependent decline in mitogenic stimulation of hepatocytes. Reduced association between Shc and the epidermal growth factor receptor is coupled to decreased activation of Raf and extracellular signal-regulated kinases

The proliferative potential of the liver has been well documented to decline with age. However, the molecular mechanism of this phenomenon is not well understood. Cellular proliferation is the result of growth factor-receptor binding and activation of cellular signaling pathways to regulate specific gene transcription. To determine the mechanism of the age-related difference in proliferation, we evaluated extracellular signal-regulated kinase-mitogen-activated protein kinase activation and events upstream in the signaling pathway in epidermal growth factor (EGF)-stimulated hepatocytes isolated from young and old rats. We confirm the age-associated decrease in extracellular signal-regulated kinase-mitogen-activated protein kinase activation in response to EGF that has been previously reported. We also find that the activity of the upstream kinase, Raf kinase, is decreased in hepatocytes from old compared with young rats. An early age-related difference in the EGF-stimulated pathway is shown to be the decreased ability of the adapter protein, Shc, to associate with the EGF receptor through the Shc phosphotyrosine binding domain. To address the mechanism of decreased Shc/EGF receptor interaction, we examined the phosphorylation of the EGF receptor at tyrosine 1173, a site recognized by the Shc phosphotyrosine binding domain. Tyrosine 1173 of the EGF receptor is underphosphorylated in the hepatocytes from old animals compared with young in a Western blot analysis using a phosphospecific antibody that recognizes phosphotyrosine 1173 of the EGF receptor. These data suggest that a molecular mechanism underlying the age-associated decrease in hepatocyte proliferation involves an age-dependent regulation of site-specific tyrosine residue phosphorylation on the EGF receptor.

TCR Engagement Regulates Differential Responsiveness of Human Memory T Cells to Fas (CD95)-Mediated Apoptosis

In this work, we have tried to establish whether human memory T cells may be protected from Fas (CD95)-induced apoptosis when correctly activated by Ag, and not protected when nonspecifically or incorrectly activated. In particular, we wanted to investigate the molecular mechanisms that regulate the fate of memory T cells following an antigenic challenge. To address this issue, we chose an experimental system that closely mimics physiological T cell activation such as human T cell lines and clones specific for viral peptides or alloantigens. We demonstrate that memory T cells acquire an activation-induced cell death (AICD)-resistant phenotype when TCRs are properly engaged by specific Ag bound to MHC molecules. Ag concentration and costimulation are critical parameters in regulating the protective effect. The analysis of the mechanisms involved in the block of CD95 signal transduction pathways revealed that the crucial events are the inhibition of CD95-associated IL-1β-converting enzyme (ICE)-like protease (FLICE) activation and poly(ADP)-ribose polymerase cleavage, and the mRNA expression of FLICE-like inhibitory protein. Furthermore, we have observed that TCR-mediated neosynthesis of FLICE-like inhibitory protein mRNA is suppressed either by protein tyrosine kinase inhibitors or cyclosporin A. In conclusion, the present analysis of the effects of TCR triggering on the regulation of AICD suggests that AICD could be inhibited in human memory T cells activated in vivo by a foreign Ag, but may become operative when the Ag has been cleared.

Confluence of vascular endothelial cells induces cell cycle exit by inhibiting p42/p44 mitogen-activated protein kinase activity

Like other cellular models, endothelial cells in cultures stop growing when they reach confluence, even in the presence of growth factors. In this work, we have studied the effect of cellular contact on the activation of p42/p44 mitogen-activated protein kinase (MAPK) by growth factors in mouse vascular endothelial cells. p42/p44 MAPK activation by fetal calf serum or fibroblast growth factor was restrained in confluent cells in comparison with the activity found in sparse cells. Consequently, the induction of c-fos, MAPK phosphatases 1 and 2 (MKP1/2), and cyclin D1 was also restrained in confluent cells. In contrast, the activation of Ras and MEK-1, two upstream activators of the p42/p44 MAPK cascade, was not impaired when cells attained confluence. Sodium orthovanadate, but not okadaic acid, restored p42/p44 MAPK activity in confluent cells. Moreover, lysates from confluent 1G11 cells more effectively inactivated a dually phosphorylated active p42 MAPK than lysates from sparse cells. These results, together with the fact that vanadate-sensitive phosphatase activity was higher in confluent cells, suggest that phosphatases play a role in the down-regulation of p42/p44 MAPK activity. Enforced long-term activation of p42/p44 MAPK by expression of the chimera DeltaRaf-1:ER, which activates the p42/p44 MAPK cascade at the level of Raf, enhanced the expression of MKP1/2 and cyclin D1 and, more importantly, restored the reentry of confluent cells into the cell cycle. Therefore, inhibition of p42/p44 MAPK activation by cell-cell contact is a critical step initiating cell cycle exit in vascular endothelial cells.

NF-kappaB function in growth control: regulation of cyclin D1 expression and G0/G1-to-S-phase transition

Nuclear factor kappa B (NF-kappaB) has been implicated in the regulation of cell proliferation, transformation, and tumor development. We provide evidence for a direct link between NF-kappaB activity and cell cycle regulation. NF-kappaB was found to stimulate transcription of cyclin D1, a key regulator of G1 checkpoint control. Two NF-kappaB binding sites in the human cyclin D1 promoter conferred activation by NF-kappaB as well as by growth factors. Both levels and kinetics of cyclin D1 expression during G1 phase were controlled by NF-kappaB. Moreover, inhibition of NF-kappaB caused a pronounced reduction of serum-induced cyclin D1-associated kinase activity and resulted in delayed phosphorylation of the retinoblastoma protein. Furthermore, NF-kappaB promotes G1-to-S-phase transition in mouse embryonal fibroblasts and in T47D mammary carcinoma cells. Impaired cell cycle progression of T47D cells expressing an NF-kappaB superrepressor (IkappaBalphaDeltaN) could be rescued by ectopic expression of cyclin D1. Thus, NF-kappaB contributes to cell cycle progression, and one of its targets might be cyclin D1.

Extracellular Signal-Regulated Kinases Regulate Leukotriene C4 Generation, But Not Histamine Release or IL-4 Production from Human Basophils

Human basophils secrete histamine and leukotriene C4 (LTC4) in response to various stimuli, such as Ag and the bacterial product, FMLP. IgE-mediated stimulation also results in IL-4 secretion. However, the mechanisms of these three classes of secretion are unknown in human basophils. The activation of extracellular signal-regulated kinases (ERKs; ERK-1 and ERK-2) during IgE- and FMLP-mediated stimulation of human basophils was examined. Following FMLP stimulation, histamine release preceded phosphorylation of ERKs, whereas phosphorylation of cytosolic phospholipase A2 (cPLA2), and arachidonic acid (AA) and LTC4 release followed phosphorylation of ERKs. The phosphorylation of ERKs was transient, decreasing to baseline levels after 15 min. PD98059 (MEK inhibitor) inhibited the phosphorylation of ERKs and cPLA2 without inhibition of several other tyrosine phosphorylation events, including phosphorylation of p38 MAPK. PD98059 also inhibited LTC4 generation (IC50 = ∼2 μM), but not histamine release. Stimulation with anti-IgE Ab resulted in the phosphorylation of ERKs, which was kinetically similar to both histamine and LTC4 release and decreased toward resting levels by 30 min. Similar to FMLP, PD98059 inhibited anti-IgE-mediated LTC4 release (IC50, ∼2 μM), with only a modest effect on histamine release and IL-4 production at higher concentrations. Taken together, these results suggest that ERKs might selectively regulate the pathway leading to LTC4 generation by phosphorylating cPLA2, but not histamine release or IL-4 production, in human basophils.

A differential requirement for the COOH-terminal region of the epidermal growth factor (EGF) receptor in amphiregulin and EGF mitogenic signaling

The epidermal growth factor receptor (EGFR) mediates the actions of a family of bioactive peptides that include epidermal growth factor (EGF) and amphiregulin (AR). Here we have studied AR and EGF mitogenic signaling in EGFR-devoid NR6 fibroblasts that ectopically express either wild type EGFR (WT) or a truncated EGFR that lacks the three major sites of autophosphorylation (c'1000). COOH-terminal truncation of the EGFR significantly impairs the ability of AR to (i) stimulate DNA synthesis, (ii) elicit Elk-1 transactivation, and (iii) generate sustained enzymatic activation of mitogen-activated protein kinase. EGFR truncation had no significant effect on AR binding to receptor but did result in defective GRB2 adaptor function. In contrast, EGFR truncation did not impair EGF mitogenic signaling, and in c'1000 cells EGF was able to stimulate the association of ErbB2 with GRB2 and SHC. Elk-1 transactivation was monitored when either ErbB2 or a truncated dominant-negative ErbB2 mutant (ErbB2-(1-813)) was overexpressed in cells. Overexpression of full-length ErbB2 resulted in a strong constitutive transactivation of Elk-1 in c'1000 but only slightly stimulated Elk-1 in WT or parental NR6 cells. Conversely, overexpression of ErbB2-(1-813) inhibited EGF-stimulated Elk-1 transactivation in c'1000 but not in WT cells. Thus, the cytoplasmic tail of the EGFR plays a critical role in AR mitogenic signaling but is dispensable for EGF, since EGF-activated truncated EGFRs can signal through ErbB2.

TC21 and Ras share indistinguishable transforming and differentiating activities

Constitutively activated mutants of the Ras-related protein TC21/R-Ras2 cause tumorigenic transformation of NIH3T3 cells. However, unlike Ras, TC21 fails to bind to and activate the Raf-1 serine-threonine kinase. Thus, whereas Ras transformation is critically dependent on Raf-1 TC21 activity is promoted by activation of Raf-independent signaling pathways. In the present study, we have further compared the functions of Ras and TC21. First we determined the basis for the inability of TC21 to activate Raf-1. Whereas Ras can interact with the two distinct Ras-binding sequences in NH2-terminus of Raf-1, designated RBS1 and Raf-Cys, TC21 could only bind Raf-Cys. Thus, the inability of TC21 to bind to RBS1 may prevent it from promoting the translocation of Raf-1 to the plasma membrane. Second, we found that TC21 is an activator of the JNK and p38, but not ERK, mitogen-activated protein kinase cascades and that TC21 transforming activity was dependent on Rac function. Thus, like Ras, TC21 may activate a Rac/JNK pathway. Third, we determined if TC21 could cause the same biological consequences as Ras in three distinct cell types. Like Ras, activated TC21 caused transformation of RIE-1 rat intestinal epithelial cells and terminal differentiation of PC12 pheochromocytoma cells. Finally, activated TC21 blocked serum starvation-induced differentiation of C2 myoblasts, whereas dominant negative TC21 greatly accelerated this differentiation process. Therefore, TC21 and Ras share indistinguishable biological activities in all cell types that we have evaluated. These results support the importance of Raf-independent pathways in mediating the actions of Ras and TC21.

Interferon-alpha induction of STATs1, -3 DNA binding and growth arrest is independent of Lck and active mitogen-activated kinase in T cells

Type I interferons (IFNs) are a family of cytokines that have antiviral and antiproliferative effects. Data regarding the processes by which these cytokines transduce signals from the cell membrane to the nucleus are becoming increasingly complex. The most characterized pathway is via JAK-STAT signaling. Previous studies established a potential role for the Src-family kinase Lck in JAK-STAT signaling. Therefore, this study was designed to analyze the role of Lck in IFN-alpha signaling by using the Jurkat, JCam (an Lck-defective cell line derived from Jurkat), and JCam/Lck (JCam cells with Lck restored). The results show that IFN-alpha can induce MAPK activity, but only in cells containing Lck. Furthermore, STATs1 and -3 are effectively phosphorylated and activated to bind DNA in the absence of Lck expression in IFN-alpha-treated cells. Finally, the results demonstrate that IFN-alpha exerts an antiproliferative effect in all three cell lines. These data indicate that Lck and active MAPK do not affect IFN-alpha-induced growth arrest or induction of STAT1s1 and -3 DNA binding ability.

Expression pattern of the plasminogen activator-plasmin system in human cholesteatoma

The plasminogen activator-plasmin system plays a pivotal role in the delicately regulated process of extracellular matrix remodeling. Recent studies have shown that an imbalance of proteolytic enzymes over specific inhibitors in this system may lead to an aggressive, expanding, and infiltrating cellular phenotype. As cholesteatoma resembles a tumor in many ways, we investigated the pattern of expression for members of the plasminogen activator-plasmin system in 12 human cholesteatomas, using immunohistochemistry. As controls, 3 tympanic membranes and 4 ear canal skin specimens were used. In contrast to the tympanic membranes, all cholesteatoma specimens showed a strong expression of plasminogen at the basal epithelial cell layers. In ear canal skin, only the basal surface of the most basal epithelia stained discretely positive. The urokinase-type plasminogen activator (uPA) could be detected in the basal stratum of the cholesteatoma matrix and in the surrounding granulation tissue, while tissue-type plasminogen activator (tPA) was not detectable at all. Plasminogen activator inhibitor-1 (PAI-1) was expressed in both the granulation tissue and the granular cell layer of the matrix, but not in the basal epithelial cells; PAI-2 showed a pericellular expression pattern in the subbasal and granular cell layers. Neither uPA, tPA, nor the PAIs could be detected in tympanic membrane controls; ear canal skin showed the same staining pattern as cholesteatoma only for PAI-2. Our data demonstrate that there is a clear imbalance in favor of proteolytic activity in the basal epithelial layers of the cholesteatoma matrix, which might at least partly account for the aggressive behavior of this tumorlike lesion. Further, the pattern of expression resembles the pattern described for several epithelial malignancies.

Tyrosine phosphorylated proteins in synovial cells of rheumatoid arthritis

Two of the key events in the pathogenesis of rheumatoid arthritis are the synovial cell proliferation and lymphocyte infiltration into the synovium. The resulting synovitis is longlasting and leads to destructive arthritis, which is a hallmark of rheumatoid arthritis. Accumulating evidence suggests that one of the key biochemical events in the altered cell function of RA is phosphorylation of the tyrosine residues of proteins. In this paper we review the cellular components participating in the chronic inflammation of RA joints. We present the results of analyzing tyrosine phosphorylated proteins of synovial cells from RA patients and discuss a possible pathogenic role of non-receptor tyrosine kinase in RA.

Induction of vascular smooth muscle cell tenascin-C gene expression by denatured type I collagen is dependent upon a beta3 integrin-mediated mitogen-activated protein kinase pathway and a 122-base pair promoter element

Tenascin-C is an extracellular matrix glycoprotein, the expression of which is upregulated in remodeling arteries. In previous studies we showed that the presence of tenascin-C alters vascular smooth muscle cell shape and amplifies their proliferative response by promoting growth factor receptor clustering and phosphorylation. Moreover, we demonstrated that denatured type I collagen induces smooth muscle cell tenascin-C protein production via beta3 integrins. In the present study, we examine the pathway by which beta3 integrins stimulate expression of tenascin-C, and define a promoter sequence that is critical for its induction. On native collagen, A10 smooth muscle cells adopt a stellate morphology and produce low levels of tenascin-C mRNA and protein, whereas on denatured collagen they spread extensively and produce high levels of tenascin-C mRNA and protein, which is incorporated into an elaborate extracellular matrix. Increased tenascin-C synthesis on denatured collagen is associated with elevated protein tyrosine phosphorylation, including activation of extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2). beta3 integrin function-blocking antibodies attenuate ERK1/2 activation and tenascin-C protein synthesis. Consistent with these findings, treatment with the specific MEK inhibitor, PD 98059, results in suppression of tenascin-C protein synthesis. To investigate whether beta3 integrin-dependent activation of ERK1/2 regulates the tenascin-C promoter, we transfected A10 cells with a full-length (approx. 4 kb) mouse tenascin-C gene promoter-chloramphenicol acetyltransferse reporter construct and showed that, relative to native collagen, its activity is increased on denatured collagen. Next, to identify regions of the promoter involved, we examined a series of tenascin-C promoter constructs with 5′ deletions and showed that denatured collagen-dependent promoter activity was retained by a 122-base pair element, located -43 to -165 bp upstream of the RNA start site. Activation of this element was suppressed either by blocking beta3 integrins, or by preventing ERK1/2 activation. These observations demonstrate that smooth muscle cell binding to beta3 integrins activates the mitogen activated protein kinase pathway, which is required for the induction of tenascin-C gene expression via a potential extracellular matrix response element in the tenascin-C gene promoter. Our data suggest a mechanism by which remodeling of type I collagen modulates tenascin-C gene expression via a beta3 integrin-mediated signaling pathway, and as such represents a paradigm for vascular development and disease whereby smooth muscle cells respond to perturbations in extracellular matrix composition by altering their phenotype and patterns of gene expression.

Long-range chromatin reorganization of the human serpin gene cluster at 14q32.1 accompanies gene activation and extinction in microcell hybrids

The genes encoding alpha1-antitrypsin (alpha1AT, gene symbol PI) and corticosteroid-binding globulin (CBG) are part of a cluster of six serine protease inhibitor (serpin) genes located on human chromosome 14q32.1. Both genes are actively transcribed in the liver and in human hepatoma cells, but they are not expressed in most other cell types. In this study we mapped DNase I-hypersensitive sites (DHSs) in an approximately 130-kb region of 14q32.1 that includes both genes. The distributions of DHSs in expressing (HepG2) vs nonexpressing (HeLa S3) cells were very different: HepG2 cells displayed 29 DHSs in this interval, but only 7 of those sites were present in HeLa cells. To determine the chromatin organization of activated or extinguished serpin alleles, we transferred human chromosome 14 into rat hepatoma cells or fibroblasts, respectively. Human alpha1AT and CBG gene expression was activated in rat hepatoma microcell hybrids containing human chromosome 14, but extinguished in rat fibroblast hybrids with the same genotype. DHS mapping in these microcell hybrids demonstrated that the chromatin structure of the entire 130-kb region was reorganized in microcell hybrids, and the distributions of DHSs in activated and extinguished alleles recapitulated those of expressing and nonexpressing cells, respectively. Thus, microcell hybrids provide a system in which reproducible changes in gene activity and long-range chromatin organization can be induced experimentally. This provides a basis for studying the effects of targeted modifications of the alpha1AT and CBG loci on the regulation of gene activity and chromatin structure.

Stress-activated protein kinase-2/p38 and a rapamycin-sensitive pathway are required for C2C12 myogenesis

The differentiation of C2C12 myoblasts to myotubes was found to be accompanied by a strong activation of p70 S6 kinase and the mitogen-activated protein kinase (MAPK) family member SAPK2/p38, without significant activation of p42 MAPK and only slight activation of SAPK1/JNK and protein kinase Balpha. Consistent with these findings, SB 203580 (a specific inhibitor of SAPK2/p38) or rapamycin (which blocks the activation of p70 S6 kinase) prevented the formation of multinucleated myotubes, as well as the expression of muscle-specific proteins that included SAPK3 (another MAPK family member). PD 098059 (which prevents the activation of p42 MAPK) had no effect on myotube formation. Surprisingly, the slow activation of p70 S6 kinase during differentiation was not only prevented by rapamycin but also by SB 203580, and the activation of MAPKAP kinase-2 (an in vivo substrate of SAPK2/p38) was not only prevented by SB 203580 but also by rapamycin. In contrast, the acute activation of p70 S6 kinase in C2C12 myoblasts induced by phorbol esters was unaffected by SB 203580 and the acute activation of MAPKAP kinase-2 induced by anisomycin was unaffected by rapamycin. These results show for the first time that SAPK2/p38 plays an essential role in C2C12 cell differentiation.

MAP kinases, phosphatidylinositol 3-kinase, and p70 S6 kinase mediate the mitogenic response of human endothelial cells to vascular endothelial growth factor

Although the significance of vascular endothelial growth factor (VEGF) and its receptors in angiogenesis is well established, the signal transduction cascades activated by VEGF and their involvement in mediating the mitogenic response of endothelial cells to VEGF are incompletely characterized. Here we demonstrate that VEGF activates mitogen-activated protein (MAP) kinases, including the extracellular signal-regulated protein kinase (ERK) and p38 MAP kinase, phosphatidylinositol 3-kinase (PI 3-kinase), and p70 S6 kinase in human umbilical vein endothelial cells (HUVEC). The activation of these enzymes was assayed by kinase phosphorylation and by kinase activity towards substrates. Studies with PI 3-kinase inhibitors revealed that activation of p70 S6 kinase was mediated by PI 3-kinase. Selective inhibition of ERK, PI 3-kinase, and p70 S6 kinase with the inhibitors PD098059, LY294002, and rapamycin, respectively, inhibited VEGF-stimulated HUVEC proliferation. In marked contrast, the p38 MAP kinase inhibitor SB203580 not only failed to inhibit but actually enhanced HUVEC proliferation; this effect was associated with the phosphorylation of Rb protein. Rb phosphorylation resulted from a decrease in the level of the cdk inhibitor p27KiP1. These results indicate that the activities of ERK, PI 3-kinase, and p70 S6 kinase are essential for VEGF-induced HUVEC proliferation. p38 MAP kinase suppresses endothelial cell proliferation by regulating cell-cycle progression.

The immunohistochemical localization of the interferon-gamma and granulocyte colony-stimulating factor receptors during early amelogenesis in rat molars

Previous studies, in which the known janus kinase and signal transducer and activator of transcription (STAT) isoforms were immunohistochemically mapped in developing rat molars, implicated a sizeable list of cytokine superfamily receptor (CSR)/signal-transduction pathway (STP) linkages in the cells of the enamel organ involved in the events leading directly to early amelogenesis. Various combinations of upregulated janus kinases and STATs are known to be linked to single or small groups of CSRs. On the basis of the previous observations it was hypothesized that the interferon-gamma receptor (IFNgamma r) and the granulocyte colony-stimulating factor receptor (G-CSF receptor) would be localized in specific sites in the cells of the enamel organ during early amelogenesis. To verify this, whole-head, freeze-dried sections were here obtained at the level of the mandibular first and second molar from newborn and 5-day-old rats. These sections were not demineralized or fixed, reducing the possibility of false-negative results. Antibodies to the IFNgamma r and the G-CSF receptor were localized using a modification of the avidin-biotin complex method. In the newborn rats, IFNgamma r was localized in the preameloblasts in the cervical loop, the proximal and distal ends of presecretory ameloblasts, the outer enamel epithelium, the dental lamina, and in bone. In 5-day-old rats, it was confined to the proximal ends of the presecretory and secretory ameloblasts. The G-CSF receptor was observed in the molars of newborn rats in the preameloblasts, the proximal and distal ends of the presecretory ameloblasts, outer enamel epithelium, and in bone. In 5-day-old rats, G-CSF receptor was localized in the preameloblasts, the proximal ends of presecretory and secretory ameloblasts, the stellate reticulum, the outer enamel epithelium, and in bone. These findings indicate that the IFNgamma r and the G-CSF receptor, and their downstream STP linkages, are upregulated in the cells of the enamel organ and may be involved in the events leading directly to early enamel formation.

PolyADP-ribose polymerase is a coactivator for AP-2-mediated transcriptional activation

Overexpression of transcription factor AP-2 has been implicated in the tumorigenicity of the human teratocarcinoma cell lines PA-1 that contain an activated ras oncogene. Here we show evidence that overexpression of AP-2 sequesters transcriptional coactivators which results in self-inhibition. We identified AP-2-interacting proteins and determined whether these proteins were coactivators for AP-2-mediated transcription. One such interacting protein is polyADP-ribose polymerase (PARP). PARP suppresses AP-2 self-inhibition and enhances AP-2 activity in PA-1 cells indicating that it is a coactivator for AP-2-transcription. PARP significantly restores AP-2 transcriptional activity in ras oncogene-transformed cells suggesting that it might suppress transformation in these cells. Another AP-2-interacting protein, RAP74, a subunit of transcription factor TFIIF, does not affect AP-2-mediated transcriptional activation alone or in the presence of RAP30, the other subunit of TFIIF. RAP74 also fails to relieve AP-2-mediated transcriptional self-interference and cross-interference. These studies suggest that the interaction between AP-2 and RAP74 may have functions other than activation of AP-2-mediated transcription.

Gastrin induces c-fos gene transcription via multiple signaling pathways

We previously observed that the trophic actions of gastrin (G17) on the AR42J rat acinar cell line are mediated by mitogen-activated protein kinase (MAPK)-induced c-fos gene transcription via protein kinase C (PKC)-dependent and -independent pathways. In this study, we further investigated the signaling pathways that target c-fos in response to G17. G17 led to a sixfold induction in luciferase activity in cells transfected with plasmids containing the -356+109 sequence of the murine c-fos promoter, which includes the Sis-inducible element (SIE), serum response element (SRE), and the Ca2+/cAMP response element (CRE) regulatory elements. Addition of either the selective PKC inhibitor GF-109203X or the MAPK/extracellular signal-regulated kinase inhibitor PD-98059 resulted in an 80% reduction in luciferase activity. G17 induced the transcriptional activity of both Elk-1 and Sap-1a, transcription factors that bind to the E26 transformation specific (Ets) DNA sequence of the SRE, and this effect was inhibited by both GF-109203X and PD-98059. Point mutations in the Ets sequence led to a 4-fold induction of c-fos transcription stimulated by G17 and to a 1.3-fold induction in response to epidermal growth factor (EGF). In contrast, mutations in the CA rich G (CArG) sequence of the SRE prevented transcriptional activation by both G17 and EGF. G17 induction of the Ets mutant construct was unaffected by either GF-109203X or PD-98059. Because activation of the SRE involves the small GTP-binding protein Rho A, we examined the role of Rho A in G17 induction of c-fos transcription. Inactivation of Rho A by either the specific inhibitor C3 or by expression of a dominant negative Rho A gene inhibited G17 induction of both the wild-type and the Ets mutant constructs by 60%. C3 also inhibited G17-stimulated AR42J cell proliferation. Thus G17 targets the c-fos promoter CArG sequence via Rho A-dependent pathways, and Rho A appears to play an important role in the regulation of the trophic action of G17.

Cell membrane and nuclear estrogen receptors (ERs) originate from a single transcript: studies of ERalpha and ERbeta expressed in Chinese hamster ovary cells

The existence of a putative membrane estrogen receptor (ER) has been supported by studies accomplished over the past 20 yr. However, the origin and functions of this receptor are not well defined. To study the membrane receptor, we transiently transfected cDNAs for ERalpha or ERbeta into Chinese hamster ovary (CHO) cells. Transfection of ERalpha resulted in a single transcript by Northern blot, specific binding of labeled 17beta-estradiol (E2), and expression of ER in both nuclear and membrane cell fractions. Competitive binding studies in both compartments revealed near identical dissociation constants (K(d)S) of 0.283 and 0.287 nM, respectively, but the membrane receptor number was only 3% as great as the nuclear receptor density. Transfection of ERbeta3 also yielded a single transcript and nuclear and membrane receptors with respective Kd values of 1.23 and 1.14 nM; the membrane receptor number was only 2% compared with expressed nuclear receptors. Estradiol binding to CHO-ERalpha or CHO-ERbeta activated Galphaq and G(alpha)s proteins in the membrane and rapidly stimulated corresponding inositol phosphate production and adenylate cyclase activity. Binding by 17-beta-E2 to either expressed receptor comparably enhanced the nuclear incorporation of thymidine, critically dependent upon the activation of the mitogen-activated protein kinase, ERK (extracellular regulated kinase). In contrast, c-Jun N-terminal kinase activity was stimulated by 17-beta-E2 in ERbeta-expressing CHO, but was inhibited in CHO-ERalpha cells. In summary, membrane and nuclear ER can be derived from a single transcript and have near-identical affinities for 17-beta-E2, but there are considerably more nuclear than membrane receptors. This is also the first report that cells can express a membrane ERbeta. Both membrane ERs activate G proteins, ERK, and cell proliferation, but there is novel differential regulation of c-Jun kinase activity by ERbeta and ERalpha.

Insulin inhibition of glucocorticoid-stimulated gene transcription: requirement for an insulin response element?

The glucocorticoid hormone receptor binds to regulatory elements of target genes and activates transcription through interactions with coactivators. For a subset of genes, glucocorticoid receptor activity is inhibited by insulin. The present paper analyzes recent data on the molecular mechanisms whereby insulin exerts this antiglucocorticoid effect. Two models are proposed. In the first model insulin controls the activity of an insulin-responsive factor bound to an insulin-responsive DNA element. In a second model, insulin targets a non-DNA bound coactivator of the glucocorticoid receptor. Here, the gene-specificity of the effect of insulin is conferred by the combined action of the glucocorticoid receptor, of DNA-bound transcription factors and of coactivators, which form a higher order structure that binds to a DNA sequence called glucocorticoid/insulin responsive unit.

Sp1 phosphorylation by Erk 2 stimulates DNA binding

EGF stimulates gene expression through a variety of signal transduction pathways that include the ras-Erk pathway. We have shown previously that EGF receptor activation stimulates gastrin gene expression through a GC-rich element called gERE. This element binds Sp1 family members and raises the possibility that the ras-Erk signal transduction cascade may target this novel EGF responsive element. Moreover, it is known that Erk 2 is capable of phosphorylating other mitogen-inducible transcription factors, e.g., Elk, Sap suggesting that Erk may also inducibly phosphorylate Sp1. To test this hypothesis directly using cotransfection experiments, we show that ras and Erk 2 activation indeed target the gERE element. The Mek 1 kinase inhibitor, PD98059, blocks 50% of EGF-inducible gastrin promoter activity. Pretreatment of the extracts with recombinant Erk2 stimulated Sp1 binding; whereas dephosphorylation reduced but did not eliminate Sp1 binding. Together, these studies demonstrate the novel finding that inducible binding of Sp1 is regulated by its state of phosphorylation. Further, gastrin promoter activation is mediated in part by the ras-Erk signaling cascade that targets Sp1.

Regulation of MCL1 through a serum response factor/Elk-1-mediated mechanism links expression of a viability-promoting member of the BCL2 family to the induction of hematopoietic cell differentiation

Proliferation, differentiation, and apoptosis are tightly regulated during hematopoiesis, allowing amplification along specific lineages while preventing excessive proliferation of immature cells. The MCL1 member of the BCL2 family is up-regulated during the induction of monocytic differentiation (approximately 10-fold with 12-O-tetradecanoylphorbol 13-acetate (TPA)). MCL1 has effects similar to those of BCL2, up-regulation promoting viability, but differs from BCL2 in its rapid inducibility and its pattern of expression. Nuclear factors that regulate MCL1 transcription have now been identified, extending the previous demonstration of signal transduction through mitogen-activated protein kinase. A 162-base pair segment of the human MCL1 5'-flank was found to direct luciferase reporter activity, allowing approximately 10-fold induction with TPA that was suppressible upon inhibition of the extracellular signal-regulated kinase (ERK) pathway. Serum response factor (SRF), Elk-1, and Sp1 bound to cognate sites within this segment, SRF and Elk-1 acting coordinately to affect both basal activity and TPA inducibility, whereas Sp1 affected basal activity only. Thus, the mechanism of the TPA-induced increase in MCL1 expression seen in myelomonocytic cells at early stages of differentiation involves signal transduction through ERKs and transcriptional activation through SRF/Elk-1. This finding provides a parallel to early response genes (e.g. c-FOS and EGR1) that affect maturation commitment in these cells and therefore suggests a means through which enhancement of cell viability may be linked to the induction of differentiation.

Mitogen-activated protein kinase phosphatase: a negative regulator of the mitogen-activated protein kinase cascade

Mitogen-activated protein kinases (MAPKs) are activated by various stimuli, such as growth factors, cytokines, or stress, and are considered to be important mediators in intracellular signal transduction networks. The dual-specificity kinases, MAPK kinases (MKKs), which phosphorylate the TXY motif in the catalytic domain of MAPKs, can cause the activation of MAPKs. Recently, a family of dual-specificity phosphatases has been identified, members of which are able to dephosphorylate and inactivate MAPKs. The studies cited in this review have revealed that these MAPK phosphatases might play an important role in various cellular functions by downregulating the MAPK cascade.

Low-density lipoprotein activates Jun N-terminal kinase (JNK) in human endothelial cells

We have reported previously that native low-density lipoprotein (LDL) activates c-Jun and transcription factor AP-1 in human umbilical vein endothelial cells (HUVEC). The aim of this study was to elucidate the upstream signaling mechanisms mediating LDL activation of c-Jun/AP-1. Using a c-Jun NH2-terminal kinase (JNK) activity assay, we have detected an increase in JNK activity in LDL-exposed HUVEC, which started at 15 min and reached maximum activity after 1-2 h. This JNK activity, increased by LDL, occurred in a dose-dependent fashion starting at a concentration of 80 mg/dl of LDL and reaching maximum activation at a concentration of 160-240 mg/dl. Following cotransfection, the increase of AP-1 driven luciferase activity by LDL was attenuated 54% by a kinase-deficient JNK1. Furthermore, a specific trans-reporting system was utilized to confirm c-Jun activation by upstream signal mechanisms. The results show c-Jun activity increased by 3-fold after LDL exposure when compared with respective controls. In contrast, LDL exposure did not affect the activation of extracellular signal regulated kinase 1 and 2 (ERK1/2), even though phorbol 12-myristate 13-acetate treatment remarkably increased the activity of these kinases. Thus, this study demonstrates, for the first time, that JNK mediates LDL-induced endothelial cell activation.

Estrogen promotes the initial migration and inception of NgCAM-dependent calcium-signaling by new neurons of the adult songbird brain

The adult avian forebrain continues to generate neurons from ventricular zone (VZ) precursor cells, whose neuronal progeny then migrate into the brain parenchyma. Migrating neurons respond to the Ig-family adhesion molecule NgCAM with increments in cytosolic calcium, and migration is disrupted by anti-NgCAM Ig. The calcium response to NgCAM is developmentally restricted to bipolar migrants during a period spanning 6 to 9 DIV. This period corresponds to the postmitotic age at which new neurons leave the adult VZ to traverse a subjacent layer of estrogen-receptive "gatekeeper" neurons. Since neuronal passage through this layer occurs concurrently with the onset of NgCAM-dependent calcium signaling, we asked whether acquisition of the calcium response to NgCAM required estrogen exposure. Among neurons arising from explants of the adult finch neostriatal VZ, only those supplemented with estrogen developed calcium responses to NgCAM; neither explants raised in the absence of estrogen, nor those supplemented with testosterone, did so. Neurons in all three groups expressed NgCAM, had equivalent baseline calcium levels, and responded identically to K+-depolarization. Nonetheless, many more neurons migrated from explants of both finch and canary VZ raised in estrogen-supplemented media than from their estrogen-deprived counterparts, even though no effect of estrogen on neuronal survival per se was noted. These findings suggest that estrogen encourages the initial departure and assumption of signal competence by neurons arising from the adult avian VZ, thereby promoting their parenchymal recruitment and migration success.

Growth hormone stimulates the tyrosine kinase activity of JAK2 and induces tyrosine phosphorylation of insulin receptor substrates and Shc in rat tissues

GH stimulates the tyrosine phosphorylation of various cellular polypeptides, including the GH receptor itself, in an early part of the intracellular response. Some of these phosphorylations are catalyzed by a GH receptor-associated kinase identified as JAK2, a member of the Janus family of tyrosine kinases. In cultured cells, GH stimulates the tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1), IRS-2, and Shc. This study investigated whether GH could cause the tyrosine phosphorylation of IRSs and Shc proteins in fasted rat tissues in vivo. GH was administered to fasted Wistar rats via a portal vein, and extracts of different tissues were immunoprecipitated with specific antibodies. GH increased the tyrosine phosphorylation of IRS-1, IRS-2, JAK2, and Shc proteins in the liver, heart, kidney, muscle, and adipose tissue of rats. The roles of these substrates as signaling molecules for GH were further demonstrated by the finding that GH stimulated the association of IRS-1/2 with phosphatidylinositol 3-kinase, Grb2, and phosphotyrosine phosphatase and of Shc with Grb2. The correlation between JAK2 tyrosyl phosphorylation and IRS-1 tyrosyl phosphorylation in response to GH together with the results of the in vitro tyrosine kinase assay are consistent with the hypothesis that JAK2 may mediate GH-induced phosphorylation of IRS-1.

Withdrawal of survival factors results in activation of the JNK pathway in neuronal cells leading to Fas ligand induction and cell death

The JNK pathway modulates AP-1 activity. While in some cells it may have proliferative and protective roles, in neuronal cells it is involved in apoptosis in response to stress or withdrawal of survival signals. To understand how JNK activation leads to apoptosis, we used PC12 cells and primary neuronal cultures. In PC12 cells, deliberate JNK activation is followed by induction of Fas ligand (FasL) expression and apoptosis. JNK activation detected by c-Jun phosphorylation and FasL induction are also observed after removal of either nerve growth factor from differentiated PC12 cells or KCl from primary cerebellar granule neurons (CGCs). Sequestation of FasL by incubation with a Fas-Fc decoy inhibits apoptosis in all three cases. CGCs derived from gld mice (defective in FasL) are less sensitive to apoptosis caused by KCl removal than wild-type neurons. In PC12 cells, protection is also conferred by a c-Jun mutant lacking JNK phosphoacceptor sites and a small molecule inhibitor of p38 mitogen-activated protein kinase and JNK, which inhibits FasL induction. Hence, the JNK-to-c-Jun-to-FasL pathway is an important mediator of stress-induced neuronal apoptosis.

Activated MEK stimulates expression of AP-1 components independently of phosphatidylinositol 3-kinase (PI3-kinase) but requires a PI3-kinase signal To stimulate DNA synthesis

To investigate the contribution that ERK/mitogen-activated protein kinase signalling makes to cell cycle progression and gene expression, we have constructed cell lines to express an inducible version of activated MEK1. Using these cells, we show that activation of MEK leads to the expression of Fra-1 and Fra-2 but not c-Fos. Treatment of Ras-transformed cells with the MEK inhibitor PD098059 blocks expression of Fra-1 and Fra-2, showing that in Ras transformation ERK signalling is responsible for Fra-1 and Fra-2 expression. Activation of MEK1 in growth-arrested cells leads to DNA synthesis; however, ERK activation alone is insufficient because the induction of DNA synthesis is blocked by inhibition of phosphatidylinositol 3-kinase (PI3-kinase). Activation of PI3-kinase is indirect, perhaps through autocrine growth factors, and is required for the induction of cyclin D1.

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

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

TP5 triggers signal transduction involving mitogen activated protein kinases in monocytes

The pentapetide thymopentin (TP5) corresponding to the aminoacids RKDVY represents the residues 32-36 of thymopoietin (TP), which was originally isolated from bovine thymus. Both were observed to induce T-cell differentiation and maturation. Recently however it was shown, that TP represents the N-terminal 49 aa of the human thymopoietin (TMPO) isoforms TMPO alpha, beta and gamma, which are localized in the nucleus. TP5 was investigated in a variety of diseases and showed efficacy by improving the immune balance, whereby different cells increased in cell number or activity. Findings which support the assumption of multifunctional efficacy and a description of TP and TP5 modulating T cells lack any interpretation on molecular level. In the present study we investigated the binding of TP5 on white blood cells. We identified monocytes and neutrophils as TP5-binding cells by displacing fluorescein-labelled TP5 with an excess of unlabelled TP5 in competition assays. Binding of TP5 on cell surface proteins resulted in cellular signalling and we report here that TP5 triggers signal transduction involving mitogen activated protein kinases p42/p44 (MAPKs) in monocytes.

Neurotrophin activation of catecholamine storage vesicle protein gene expression: signaling to chromogranin a biosynthesis

Nerve growth factor differentiates precursor cells into sympathetic neurons. Does acquisition of a "neuronal" phenotype after nerve growth factor involve biosynthesis of chromogranin A, the major soluble protein in chromaffin granule cores? Nerve growth factor activated chromogranin A gene expression 7.6-fold in PC12 pheochromocytoma cells, and similarly activated PC12-transfected mouse, rat or human chromogranin A promoter/reporter constructs. Chromogranin A promoter 5'-deletions narrowed the nerve growth factor response element to a region from - 77 to - 61 bp upstream of the cap site, a region containing the chromogranin A cyclic AMP response element (TGACGTAA). Three different site-directed mutations of the cyclic AMP response element each reduced the nerve growth factor effect by >90%. Transfer of the cyclic AMP response element to a heterologous (thymidine kinase) promoter activated that promoter approximately 5-fold after nerve growth factor, while transfer of a cyclic AMP response element point-gap mutant (TGA-GTAA) to a heterologous promoter abolished the nerve growth factor effect. These findings indicate that the cyclic AMP response element in cis is, at least in part, both necessary and sufficient to activate the chromogranin A gene. Chemical blockade of the nerve growth factor receptor TrkA or the mitogen-activated protein kinase pathway component MEK substantially diminished nerve growth factor-induced expression of chromogranin A. By contrast, the response of chromogranin A to nerve growth factor was not impaired after blockade of phospholipase C-gamma or phosphoinositide-3 kinase. Chemical blockade of TrkA, Ras, MEK or mitogen-activated protein kinase similarly inhibited nerve growth factor activation of chromogranin A. Expression of constitutively activated Ras, Raf or MEK mutants increased chromogranin A promoter activity. Expression of dominant negative (inhibitory) mutants of Sos, Ha-Ras, Rafl, mitogen-activated protein kinase, ribosomal protein S6 serine kinase II (CREB kinase) or CREB (KCREB) each inhibited the nerve growth factor-induced increase in chromogranin A promoter activity. Thus, each component of the mitogen-activated protein kinase pathway is crucially involved in relaying the nerve growth factor signal in trans to the chromogranin A gene, in the following proposed sequence: nerve growth factor --> TrkA --> Shc/Grb2/Sos --> Ras --> Raf --> MEK --> mitogen-activated protein kinase --> ribosomal protein S6 serine kinase II --> CREB cyclic AMP response element.