Transcriptional regulation by extracellular signals: mechanisms and specificity

Loss of sustained Fus3p kinase activity and the G1 arrest response in cells expressing an inappropriate pheromone receptor

The yeast pheromone response pathway is mediated by two G protein-linked receptors, each of which is expressed only in its specific cell type. The STE3DAF mutation results in inappropriate expression of the a-factor receptor in MATa cells. Expression of this receptor in the inappropriate cell type confers resistance to pheromone-induced G1 arrest, a phenomenon that we have termed receptor inhibition. The ability of STE3DAF cells to cycle in the presence of pheromone was found to correlate with reduced phosphorylation of the cyclin-dependent kinase inhibitor Far1p. Measurement of Fus3p mitogen-activated protein (MAP) kinase activity in wild-type and STE3DAF cells showed that induction of Fus3p activity was the same in both strains at times of up to 1 h after pheromone treatment. However, after 2 or more hours, Fus3p activity declined in STE3DAF cells but remained high in wild-type cells. The level of inducible FUS1 RNA paralleled the changes seen in Fus3p activity. Short-term activation of the Fus3p MAP kinase is therefore sufficient for the early transcriptional induction response to pheromone, but sustained activation is required for cell cycle arrest. Escape from the cell cycle arrest response was not seen in wild-type cells treated with low doses of pheromone, indicating that receptor inhibition is not simply a result of weak signaling but rather acts selectively at late times during the response. STE3DAF was found to inhibit the pheromone response pathway at a step between the G beta subunit and Ste5p, the scaffolding protein that binds the components of the MAP kinase phosphorylation cascade. Overexpression of Ste20p, a kinase thought to act between the G protein and the MAP kinase cascade, suppressed the STE3DAF phenotype. These findings are consistent with a model in which receptor inhibition acts by blocking the signaling pathway downstream of G protein dissociation and upstream of MAP kinase cascade activation, at a step that could directly involve Ste20p.

Multiple steps in the regulation of transcription-factor level and activity

This review focuses on the regulation of transcription factors, many of which are DNA-binding proteins that recognize cis-regulatory elements of target genes and are the most direct regulators of gene transcription. Transcription factors serve as integration centres of the different signal-transduction pathways affecting a given gene. It is obvious that the regulation of these regulators themselves is of crucial importance for differential gene expression during development and in terminally differentiated cells. Transcription factors can be regulated at two, principally different, levels, namely concentration and activity, each of which can be modulated in a variety of ways. The concentrations of transcription factors, as of intracellular proteins in general, may be regulated at any of the steps leading from DNA to protein, including transcription, RNA processing, mRNA degradation and translation. The activity of a transcription factor is often regulated by (de) phosphorylation, which may affect different functions, e.g. nuclear localization DNA binding and trans-activation. Ligand binding is another mode of transcription-factor activation. It is typical for the large super-family of nuclear hormone receptors. Heterodimerization between transcription factors adds another dimension to the regulatory diversity and signal integration. Finally, non-DNA-binding (accessory) factors may mediate a diverse range of functions, e.g. serving as a bridge between the transcription factor and the basal transcription machinery, stabilizing the DNA-binding complex or changing the specificity of the target sequence recognition. The present review presents an overview of different modes of transcription-factor regulation, each illustrated by typical examples.

Inhibition of growth factor-induced protein synthesis by a selective MEK inhibitor in aortic smooth muscle cells

A common response of cells to mitogenic and hypertrophic factors is the activation of high rates of protein synthesis. To investigate the molecular basis of this action, we have used the recently developed MAP kinase/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitor PD 98059 to examine the involvement of the ERK pathway in the regulation of global protein synthesis by growth factors in rat aortic smooth muscle cells (SMC). Incubation with PD 98059 blocked angiotensin II (AII)-dependent phosphorylation and enzymatic activity of both MEK1 and MEK2 isoforms, leading to inhibition of the phosphorylation and activation of p44(mapk) and p42(mapk). The compound was found to selectively inhibit activation of the ERK pathway by AII, but not the stimulation of p70 S6 kinase, phospholipase C, or tyrosine phosphorylation. Most importantly, treatment of aortic SMC with PD 98059 potently inhibited AII-stimulated protein synthesis with a half-maximal inhibitory concentration of 4.3 microM. The effect of PD 98059 was not restricted to AII, since the compound also blocked to various extent the induction of protein synthesis by growth factors acting through tyrosine kinase receptors, G protein-coupled receptors, or protein kinase C. These results provide strong evidence that activation of ERK isoforms is an obligatory step for growth factor-induced protein synthesis in aortic SMC.

Signaling in the yeast pheromone response pathway: specific and high-affinity interaction of the mitogen-activated protein (MAP) kinases Kss1 and Fus3 with the upstream MAP kinase kinase Ste7

Kss1 and Fus3 are mitogen-activated protein kinases (MAPKs or ERKs), and Ste7 is their activating MAPK/ERK kinase (MEK), in the pheromone response pathway of Saccharomyces cerevisiae. To investigate the potential role of specific interactions between these enzymes during signaling, their ability to associate with each other was examined both in solution and in vivo. When synthesized by in vitro translation, Kss1 and Fus3 could each form a tight complex (Kd of approximately 5 nM) with Ste7 in the absence of any additional yeast proteins. These complexes were specific because neither Hog1 nor Mpk1 (two other yeast MAPKs), nor mammalian Erk2, was able to associate detectably with Ste7. Neither the kinase catalytic core of Ste7 nor the phosphoacceptor regions of Ste7 and Kss1 were necessary for complex formation. Ste7-Kss1 (and Ste7-Fus3) complexes were present in yeast cell extracts and were undiminished in extracts prepared from a ste5delta-ste11delta double mutant strain. In Ste7-Kss1 (or Ste7-Fus3) complexes isolated from naive or pheromone-treated cells, Ste7 phosphorylated Kss1 (or Fus3), and Kss1 (or Fus3) phosphorylated Ste7, in a pheromone-stimulated manner; dissociation of the high-affinity complex was shown to be required for either phosphorylation event. Deletions of Ste7 in the region required for its stable association with Kss1 and Fus3 in vitro significantly decreased (but did not eliminate) signaling in vivo. These findings suggest that the high-affinity and active site-independent binding observed in vitro facilitates signal transduction in vivo and suggest further that MEK-MAPK interactions may utilize a double-selection mechanism to ensure fidelity in signal transmission and to insulate one signaling pathway from another.

Schizosaccharomyces pombe map1+ encodes a MADS-box-family protein required for cell-type-specific gene expression

We cloned the Schizosaccharomyces pombe map1 gene by virtue of its ability to stimulate transcription of the sxa2 gene, which encodes a carboxypeptidase expressed specifically in h- cells in response to mating-pheromone signaling. The cloned gene had a coding capacity of 398 amino acids split by two introns, and the deduced product was a protein of the MADS box family. This gene was most similar to Saccharomyces cerevisiae MCM1, which regulates cell-type-specific gene expression in budding yeast cells. Disruption of the S. pombe gene did not affect vegetative cell growth but conferred sterility. It blocked the mating ability of h+ cells completely and that of h- cells partially. Genetic and sequencing analysis indicated that the cloned gene is map1], which was originally defined by a mutation that caused h+-speciftic sterility. Northern (RNA) blot analysis showed that the function of map1 is absolutely essential for the expression of h+-specific genes and is required for the full activation of h--specific gene expression. Overexpression of map1 resulted in enhanced transcription of cell-type-specilic genes, but the range of genes affected by Map1 was restricted by the mating type of the cell. Results of yeast two-hybrid analysis suggested that Map1 may physically interact with Mat1-Pc, the product of the h(+)-specific mating-type gene mat1-Pc. On the basis of these observations, we speculate that Map1 may be a transcriptional regulator of cell-type-specific genes similar to S. cerevisiae MCM1, whose activity is modulated by the oil and alpha2 mating-type gene products.

The extracellular signal-regulated kinase pathway phosphorylates AML1, an acute myeloid leukemia gene product, and potentially regulates its transactivation ability

AML1 (also called PEBP2alphaB, CBFA2, or CBFalpha2) is one of the most frequently disrupted genes in chromosome abnormalities seen in human leukemias. It has been reported that AML1 plays several pivotal roles in myeloid hematopoietic differentiation and other biological phenomena, probably through the transcriptional regulation of various relevant genes. Here, we investigated the mechanism of regulation of AML1 functions through signal transduction pathways. The results showed that AML1 is phosphorylated in vivo on two serine residues within the proline-, serine-, and threonine-rich region, with dependence on the activation of extracellular signal-regulated kinase (ERK) and with interleukin-3 stimulation in a hematopoietic cell line. These in vivo phosphorylation sites of AML1 were phosphorylated directly in vitro by ERK. Although differences between wild-type AML1 and phosphorylation site mutants in DNA-binding affinity were not observed, we have shown that ERK-dependent phosphorylation potentiates the transactivation ability of AML1. Furthermore the phosphorylation site mutations reduced the transforming capacity of AML1 in fibroblast cells. These data indicate that AML1 functions are potentially regulated by ERK, which is activated by cytokine and growth factor stimuli. This study provides some important clues for clarifying unidentified facets of the regulatory mechanism of AML1 function.

Oncogenic Ras activation of Raf/mitogen-activated protein kinase-independent pathways is sufficient to cause tumorigenic transformation

Substantial evidence supports a critical role for the activation of the Raf-1/MEK/mitogen-activated protein kinase pathway in oncogenic Ras-mediated transformation. For example, dominant negative mutants of Raf-1, MEK, and mitogen-activated protein kinase all inhibit Ras transformation. Furthermore, the observation that plasma membrane-localized Raf-1 exhibits the same transforming potency as oncogenic Ras suggests that Raf-1 activation alone is sufficient to mediate full Ras transforming activity. However, the recent identification of other candidate Ras effectors (e.g., RalGDS and phosphatidylinositol-3 kinase) suggests that activation of other downstream effector-mediated signaling pathways may also mediate Ras transforming activity. In support of this, two H-Ras effector domain mutants, H-Ras(12V, 37G) and H-Ras(12V, 40C), which are defective for Raf binding and activation, induced potent tumorigenic transformation of some strains of NIH 3T3 fibroblasts. These Raf-binding defective mutants of H-Ras induced a transformed morphology that was indistinguishable from that induced by activated members of Rho family proteins. Furthermore, the transforming activities of both of these mutants were synergistically enhanced by activated Raf-1 and inhibited by the dominant negative RhoA(19N) mutant, indicating that Ras may cause transformation that occurs via coordinate activation of Raf-dependent and -independent pathways that involves Rho family proteins. Finally, cotransfection of H-Ras(12V, 37G) and H-Ras(12V, 40C) resulted in synergistic cooperation of their focus-forming activities, indicating that Ras activates at least two Raf-independent, Ras effector-mediated signaling events.

The Schizosaccharomyces pombe pyp1 protein tyrosine phosphatase negatively regulates nutrient monitoring pathways

The Schizosaccharomyces pombe pyp1+ gene, encoding a protein tyrosine phosphatase (pyp1), was isolated as a high copy number suppressor of a mutation that results in reduced cAMP-dependent protein kinase (PKA) activity. Overexpression of pyp1+ inhibits both transcription of the fbp1 gene, which is negatively regulated by a glucose-induced activation of PKA, and sexual development, which is negatively regulated by PKA through a nitrogen- and glucose-monitoring mechanism. Overexpression of a catalytically inactive form of pyp1 has little effect on either process. Previous studies suggest that overexpression of pyp1+ results in a mitotic delay by positively regulating wee1 activity. We show that pyp1 repression of fbp1 transcription is independent of wee1. The direct role of the pyp1 protein is to dephosphorylate and inactivate the sty1/spc1 mitogen-activated protein kinase (MAPK) that is activated by the wis1 MAPK kinase. As overexpression of pyp1+ has no further effect upon the mitotic delay observed in a wis1 deletion strain, the role of pyp1 appears to be restricted to negative regulation of the sty1/spc1 MAPK. This study indicates that pyp1 negatively regulates fbp1 transcription, sexual development and mitosis by inactivation of the sty1/spc1 MAPK, but that bifurcations downstream of the MAPK separate these processes as seen by the differential role for the wee1 gene.

Integrin-dependent signal transduction

Integrins are receptor molecules for extracellular matrix molecules (e.g., the beta(1) family), serum components (alpha(v) family) and immunoglobulin family adhesion molecules (beta(2) family). Integrin-dependent adhesion has also been shown to have metabolic consequences. Adhesion to a variety of extracellular matrix proteins, such as fibronectin, collagen, and laminin, is a potent regulator of cell growth, differentiation, and gene expression. Ligand binding or aggregation of integrin receptors initiates a number of metabolic changes including activation of serine/threonine and tyrosine kinases, increased Ca2+ influx, increased cytoplasmic alkalinization, and altered inositol lipid metabolism. In some instances activation of transcription factors and induction of gene expression have also been demonstrated. Components of key signaling pathways involving integrins are beginning to be identified. Some studies have shown that integrins form multi-component complexes with signal transduction molecules. Elucidating the interactions of the signal transduction molecules with each other and with the integrin cytoplasmic domains will be key to understanding the initial events of signal transduction through the integrins.

Modulation of AP-1 activity by the human progesterone receptor in endometrial adenocarcinoma cells

The composite transcription factor activating protein 1 (AP-1) integrates various mitogenic signals in a large number of cell types, and is therefore a major regulator of cell proliferation. In the normal human endometrium, proliferation and differentiation alternate in a cyclic fashion, with progesterone being largely implicated in the latter process. However, the effects of progesterone and the progesterone receptor (hPR) on AP-1 activity in the human endometrium are not known. To address this issue, HEC-1-B endometrial adenocarcinoma cells, which are devoid of hPR, were transfected with luciferase reporter constructs driven by two different AP-1-dependent promoters. Unexpectedly, cotransfection of hPR caused a marked induction of luciferase activity in the absence of ligand on both promoters. The magnitude of this induction was similar to that observed in response to the phorbol ester TPA. Addition of ligand reversed the stimulating effect of the unliganded hPR on AM activity in these cells. These effects were specific for hPR, and were not observed with either human estrogen receptor or human glucocorticoid receptor. Furthermore, they strictly depended on the presence of AP-1-responsive sequences within target promoters. Finally, the described effects of hPR on AP-1 activity were shown to be cell-type specific, because they could not be demonstrated in SKUT-1-B, JEG-3, and COS-7 cells. To our knowledge this is the first report of an unliganded steroid receptor stimulating AP-1 activity. This effect and its reversal in the presence of ligand suggest a novel mechanism, through which hPR can act as a key regulator of both proliferation and differentiation in the human endometrium.

Casein kinase II associates with Egr-1 and acts as a negative modulator of its DNA binding and transcription activities in NIH 3T3 cells

Although the activation domains within early growth response gene protein 1 (Egr-1) have been mapped, little is known of the kinases which phosphorylate Egr-1 and how phosphorylation correlates with the transcriptional activity of Egr-1. In this study we report that casein kinase II (CKII) co-immunoprecipitates with Egr-1 from NIH 3T3 cell lysates. The association of Egr-1 and CKII requires the C terminus of Egr-1 and CKII phosphorylates Egr-1 in vitro. The in vitro phosphorylation of Egr-1 by CKII and that induced by serum in vivo was compared by examining the CNBr-digested fragments of the phosphorylated Egr-1. CKII strongly phosphorylates fragments 7 and 10 which cover part of the activation/nuclear localization and DNA binding domains of Egr-1. CKII also phosphorylates, albeit weakly, fragments 5 and 8 which cover part of activation domain and the entire repression domain of Egr-1, respectively. Strong phosphorylation on fragment 10 as well as fragment 5 was also observed in Egr-1 immunoprecipitated from serum-induced, 32P-labeled cells. CKII phosphorylation of Egr-1 resulted in a decrease of its DNA binding as well as its transcriptional activities.

Activation of the mitogen-activated protein kinase pathway induces transcription of the PAC-1 phosphatase gene

PAC-1, an early-response gene originally identified in activated T cells, encodes a dual-specificity mitogen-activated protein kinase phosphatase. Here we report on the regulation of PAC-1 expression in murine hemopoietic cells. PAC-1 mRNA levels rapidly increase in mitogen-stimulated lymphocytes, with the induced expression being transient in B cells but sustained in activated T cells. Transfection analysis of murine PAC-1 promoter-reporter constructs established that in T cells, sequences necessary for basal and induced transcription reside within a 200-bp region located immediately upstream of the transcription initiation sites. Basal transcription is regulated in part by an E-box element that binds a 53-kDa protein. PAC-1 transcription induced by phorbol myristate acetate stimulation and the expression of the v-ras or v-raf oncogene is mediated via the E-box motif and an AP-2-related site and coincides with increased binding activity of the constitutive 53-kDa E-box-binding protein and induced binding of AP-2. The ability of an interfering ERK-2 mutant to block phorbol myristate acetate and v-ras-dependent PAC-1 transcription indicates that mitogen-activated protein kinase activation is necessary for these stimuli to induce transcription of the PAC-1 gene in T cells.

Regulation of stress-induced transcriptional changes in the hypothalamic neurosecretory neurons

Transcriptional changes in corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) gene expression were studied by in situ hybridization histochemistry using cRNA probes directed against intronic sequences. Acute ether stress resulted in a rapid induction of CRF and a delayed activation of vasopressin heteronuclear (hn)RNA in the parvocellular neurosecretory neurons within the paraventricular nucleus (PVN) of the hypothalamus. To explore possible molecular mechanisms regulating stress-related neuropeptide expression in vivo, the time-courses of stress-induced activation of different transcription factor classes were compared to that of changes in neuropeptide transcription. The peak of CRF transcription was parallel to that of cAMP response-element binding protein (CREB) phosphorylation but preceded the induction of c-fos and NGFI-B mRNAs and Fos protein. In contrast, AVP expression occurred in step with immediate-early gene (IEG) responses, suggesting involvement of different mechanisms underlying stress-induced neuropeptide responses. The interference of glucocorticoid hormones with stress-induced neuropeptide and transcription-factor responses has also been revealed in rats acutely or chronically pretreated with glucocorticoids. Acute dexamethasone injection did not prevent neuropeptide and transcription factor responses to either inhalation, whereas chronic corticosterone administration completely blocked IEG and neuropeptide induction in the stress-related neurosecretory neurons.

Mitogen-activated protein kinase cascades and regulation of gene expression

Mitogen-activated protein kinases (MAPKs) are a group of serine/threonine specific, proline directed, protein kinases which are activated by a wide spectrum of extracellular stimuli. MAPK activation is achieved through kinase cascades, which include a MAPK kinase (MAPKK or MEK) and a MAPKK/MEK kinase (MAPKKK/MEKK). These cascades serve as information relays, connecting cell-surface receptors to specific transcription factors and other regulatory proteins, thus allowing extracellular signals to regulate the expression of specific genes. Genetic and biochemical analyses have revealed many tiers in the regulation of the activities of MAPKs, as well as different routes that lead to the activation of an individual MAPK. An emerging topic of great interest is the basis for specificity in the activation of individual MAPKs and their ability to recognize their substrates.

Nuclear accumulation of multiple protein kinases during prolactin-induced proliferation of Nb2 rat lymphoma cells

Intracellular kinases play important roles in signal transduction and are involved in the surface receptor-mediated regulation of cellular functions, including mitogenesis. In the present study, we examined the possible involvement of various protein kinases in the passage of a mitogenic signal from the cell surface to the nucleus of Nb2 cells, a rat nodal lymphoma cell line in which prolactin is a mitogen. Following a prolactin challenge, various kinase activities were monitored at short intervals in different cellular fractions over a 60 min period. Protein kinase C (PKC) activity in the cytosolic fraction rapidly declined to 50% of its original activity within the first 30 min, while PKC activity in the nuclear fractions increased sharply, reaching its highest level by 30 min following a prolactin challenge. There were also increases in both casein kinase and protein tyrosine kinase (PTK) activities in the nuclear fractions during the first 30 min following a prolactin challenge that paralleled PKC activity. The activities of all three kinases declined thereafter, reaching levels close to their respective basal values by 60 min following initiation of prolactin treatment. These observations suggest the possibility that multiple protein kinases may be involved in mitogenic signal transduction for prolactin in Nb2 cells.

Ste50p sustains mating pheromone-induced signal transduction in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the heterotrimeric G protein transduces the mating pheromone signal from a cell-surface receptor. Free G beta gamma then activates a mitogen-activated protein (MAP) kinase cascade. STE50 has been shown to be involved in this pheromone signal-transduction pathway. In this study, we present a functional characterization of Ste50p, a protein that is required to sustain the pheromone-induced signal which leads cells to hormone-induced differentiation. Inactivation of STE50 leads to the attenuation of mating pheromone-induced signal transduction, and overexpression of STE50 intensifies the pheromone-induced signalling. By genetic analysis we have positioned the action of Ste50p downstream of the alpha-pheromone receptor (STE2), at the level of the heterotrimeric G protein, and upstream of STE5 and the kinase cascade of STE11 and STE7. In a two-hybrid assay Ste50p interacts weakly with the G protein and strongly with the MAPKKK Ste11p. The latter interaction is absent in the constitutive mutant Ste11pP279S. These data show that a new component, Ste50p, determines the extent and the duration of signal transduction by acting between the G protein and the MAP kinase complex in S. cerevisiae.

Mechanisms of hormone and growth factor action in the bovine corpus luteum

The binding of hormones and growth factors to their cell surface receptors leads to an orderly cascade of events leading to activation of cytoplasmic effector molecules. The mechanism of action of luteinizing hormone involves the stimulation of multiple signal transduction effector systems including adenylyl cyclase and inositol phospholipid-specific phospholipase C (PLC). This results in the formation of second messengers that activate cAMP-dependent, Ca(2+)-dependent and lipid-dependent protein kinases. Prostaglandin F(2alpha) activates PLC which increases intracellular calcium and activates protein kinase C. This results in the activation of a series of protein kinases in the mitogen-activated protein (MAP) kinase cascade, leading to the activation of nuclear transcription factors c-fos and c-jun. Hormone responsive effector systems, therefore, operate by activating families of protein kinases which regulate cell metabolism, secretion, and gene transcription. Growth factors activate specific receptor protein tyrosine kinases which recruit additional signaling molecules (phospholipase Cgamma, phosphatidylinositol 3-kinase, Shc, Grb2, etc.) initiating a cascade of events mediated via MAP kinases. The signaling pathways activated by hormones interact or cross talk with the signaling pathways activated by growth factors. The diversity of cellular signaling mechanisms elicited by hormones and the potential for interactions with signals generated by growth factor receptor tyrosine kinases, may allow fine tuning of cellular responses during the life span of the corpus luteum.

Review: regulation of liver regeneration by pro-inflammatory cytokines

The liver has tremendous regenerative capacity. This distinguishes it from other vital organs (e.g. the brain, heart and lungs) that cannot replace functional tissue once it has been destroyed. Although hepatocytes rarely proliferate in the healthy adult liver, virtually all surviving hepatocytes replicate at least once after 70% partial hepatectomy. Therefore, partial liver resection has been used to characterize mechanisms that regulate liver regeneration. Residual hepatocytes up-regulate both proliferative and liver-specific gene expression in order to preserve tissue specific function. In addition, hepatocyte proliferation is tightly co-ordinated to complement regenerative responses in hepatic non-parenchymal cells (e.g. endothelia, biliary epithelia, stellate and Kupffer cells), so that the entire organ can be reconstituted within days. Studies with neutralizing antibodies to tumour necrosis factor-alpha (TNF) clearly demonstrate that, after partial hepatectomy, TNF promotes liver cell proliferation. The present review focuses on the regulation of the hepatocyte proliferative response by pro-inflammatory cytokines.

Post-natal lethality and neurological and gastrointestinal defects in mice with targeted disruption of the A-Raf protein kinase gene

The Ras/Raf/MEK/MAP kinase cascade transmits signals from activated cell-surface receptors to transcription factors in the nucleus and is an essential component of metazoan intracellular signaling pathways (see, for example, [1-6]). In the mouse, the Raf protein kinase family is comprised of three homologous genes, Raf-1, A-Raf and B-Raf [5] which are ubiquitously expressed in the developing embryo [7]. We have introduced into the mouse germ line a loss-of-function mutation in the X-chromosomal A-Raf gene, by homologous recombination in embryonic stem cells. On a predominantly C57 Bl/6 genetic background, A-Raf-deficient mice displayed neurological and intestinal abnormalities and died between 7 and 21 days post-partum. When the mutated allele was maintained on a predominantly 129/OLA background, by contrast, A-Raf-deficient animals survived to adulthood, did not display obvious intestinal abnormalities, were fertile, but did have a subset of the neurological defects.

Integration of growth factor signals at the c-fos serum response element

A transcription factor ternary complex composed of serum response factor (SRF) and a second factor, ternary complex factor (TCF), mediates the response of the c-fos Serum Response Element to growth factors and mitogens. In NIH3T3 fibroblasts, TCF binding is required for transcriptional activation by the SRE in response to activation of the Ras-Raf-ERK pathway. We compared the properties of three members of the TCF family, Elk-1, SAP-1 and SAP-2 (ERP/NET). Although all the proteins contain sequences required for ternary complex formation with SRF, only Elk-1 and SAP-1 appear to interact with the c-fos SRE efficiently in vivo. Each TCF contains a C-terminal activation domain capable of transcriptional activation in response to activation of the Ras-Raf-ERK pathway, and this is dependent on the integrity of S/T-P motifs conserved between all the TCF family members. In contrast, activation of the SRE by whole serum and the mitogenic phospholipid LPA requires SRF binding alone. Constitutively activated members of the Rho subfamily of Ras-like GTPases are also capable of inducing activation of the SRE in the absence of TCF; unlike activated Ras itself, these proteins do not activate the TCFs in NIH3T3 cells. At the SRE, SRF- and TCF-linked signalling pathways act synergistically to potentiate transcription.

STAT3 participates in transcriptional activation of the C-reactive protein gene by interleukin-6

Interleukin-6 (IL-6) is the major cytokine inducing transcription of human C-reactive protein (CRP) during the acute phase response. STAT (signal transducers and activators of transcription) family members, recently shown to be important mediators of the effects of many cytokines including IL-6, generally induce their effects by binding to palindromic sequences with TT(N)5AA motifs. We report an IL-6 responsive element in the proximal region of the human CRP 5'-flanking region that bears a TT(N)4AA motif, which we have termed CRP acute phase response element (CRP-APRE). In Hep3B cells, IL-6 but not interferon-gamma was capable of activating CAT constructs driven by the CRP promoter containing CRP-APRE. Overexpressed STAT3 was able to transactivate CRP-chloramphenicol acetyltransferase constructs through the CRP-APRE and was able to enhance endogenous CRP mRNA accumulation in response to IL-6. STAT3 (or an antigenically related molecule) bound to the CRP-APRE in response to IL-6. Overexpression of STAT3 in the presence of IL-6 was capable of inducing expression of a construct consisting of the CRP-APRE and a minimal thymidine kinase promoter lacking a C/EBP site. Taken together, these findings indicate that STAT3 participates in the transcriptional activation of CRP in response to IL-6.

Pituitary adenylate cyclase-activating polypeptide induces expression of corticosteroid-binding globulin in cultured fetal hepatocytes: synergy with tri-iodothyronine

The purpose of the present study was to determine whether functional receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) are expressed in cultured rat fetal hepatocytes and eventually play a role in regulating gene expression of corticosteroid-binding globulin (CBG). We found PACAP38 and PACAP27 to elevate cAMP levels in hepatocytes in a dose-dependent manner, with a plateau being achieved at 10 nM and EC50 values of about 0.5-1 nM. PACAP failed to alter the turnover of inositol phosphates, whereas PACAP and VIP stimulated cAMP accumulation in an equipotent manner, suggesting the presence in these cells of type II receptor isoforms. As revealed by measurements of both CBG mRNA levels and concentrations of binding sites, long-term treatment of fetal cells with 10 nM PACAP, although resulting in partial desensitization of peptide-induced cAMP accumulation, caused a significant 3-fold elevation in CBG synthesis. This stimulatory influence of PACAP was mimicked by the cell permeant N6,2'-O-dibutyryladenosine 3',5'-phosphate (dbcAMP). Treatment of hepatocytes with tri-iodothyronine (T3) enhanced CBG expression and, most interestingly, appeared to synergize with PACAP to elicit a 2-3-fold amplification of CBG synthesis. This study thus provides first evidence for the up-regulation by PACAP and cAMP of CBG expression in fetal hepatocytes and for T3's playing a synergistic role in enhancing PACAP-induced synthesis of the binder.

Delta-9-tetrahydrocannabinol: an inhibitor of STAT1 alpha protein tyrosine phosphorylation

Tyrosine-phosphorylated signal transducer and activator of transcription 1 alpha (STAT1 alpha) is a 91-kDa protein responsible for interferon-gamma (IFN-gamma)-dependent transcription. The present study demonstrates that activation by IFN-gamma of murine macrophages resulted in tyrosine phosphorylation of STAT1 alpha identified by immunoprecipitation. The tyrosine phosphorylation of STAT1 alpha was found highly sensitive to treatment by delta-9 tetrahydrocannabinol (THC), a major marijuana component. Subsequently, the isoform formation of p91 due to tyrosine phosphorylation was reduced in THC-treated macrophages. Although inhibition by THC of the tyrosine phosphorylation of STAT1 alpha induced by IFN-gamma was in a THC concentration-related manner, the tyrosine phosphorylation of other proteins induced by lipopolysaccharide/IFN-gamma treatment of macrophages appeared insensitive to THC treatment. Our data suggest that blockade by THC of tyrosine phosphorylation of STAT1 alpha may be an important mechanism involved in the broad immunosuppressive effects of THC.

Effects of prostaglandins and cyclic AMP on cytokine production in rat leukocytes

Prostaglandins E1, prostaglandin E2, 3-oxa-methano-prostaglandin I1 (SM-10906), a stable prostaglandin I2 analog, and dibutyryl cyclic AMP suppressed the production of tumor necrosis factor and interleukin-1 in lipopolysaccharide-stimulated rat pleural resident monocytic cells, whereas they enhanced the production of interleukin-6 and cytokine-induced neutrophil chemoattractant (CINC), a rat interleukin-8-like chemokine, in these cells. SM-10906 also inhibited the in vivo production of tumor necrosis factor and interleukin-1 in pleural exudates, when injected into the rat pleural cavity concomitantly with carrageenin. The cyclic AMP (cAMP) level in the lipopolysaccharide-stimulated resident cells was increased when the cells were incubated in the presence of prostaglandin E1, prostaglandin E2 or SM-10906. Prostaglandin I2 showed only slight effects. The addition of pentoxifylline, a phosphodiesterase inhibitor, to the incubation mixture increased the cAMP level and also enhanced the effect of prostaglandins, indicating that these regulating actions of prostaglandins may be exerted partly through a mechanism involving an increased intracellular cAMP level.

Cloning and characterization of HuR, a ubiquitously expressed Elav-like protein

The neuronal-specific Elav-like proteins (HuD, Hel-N, and HuC) contain three RNP-type concensus motifs and bind to AU-rich elements. We have identified and cloned a fourth member of this family (HuR) that is expressed in a wide variety of cell types. The purified recombinant protein binds avidly to the AU-rich element in c-fos and interleukin-3 mRNAs. In the case of the c-fos AU-rich element, HuR binds to a core element of 27 nucleotides that contain AUUUA, AUUUUA, and AUUUUUA motifs. Mutational analysis has shown that all three AU motifs are required for maximal binding.

Raf-1 kinase targets GA-binding protein in transcriptional regulation of the human immunodeficiency virus type 1 promoter

The serine/threonine protein kinase Raf-1 is a component of a conserved intracellular signaling cascade that controls responses to various extracellular stimuli. Transcription from several promoters, including the oncogene-responsive element in the polyomavirus enhancer, the c-fos promoter, as well as other AP-1- and Ets-dependent promoters, can be induced by Raf-1 kinase. Previously, we have shown that activated Raf-1 kinase transactivates the human immunodeficiency virus type 1 (HIV-1) long terminal repeat and have identified the NF-kappaB binding motif as a Raf-1-responsive element (RafRE). We now report that Raf-1 kinase-induced transactivation from the HIV RafRE involves the purine-rich-repeat-binding protein (GABP), which is composed of two distinct subunits (alpha and beta). GABP alpha is an Ets oncogene-related DNA-binding protein, and GABP beta contains four ankyrin-like repeats that have been shown to be essential in protein-protein interactions. In electrophoretic mobility shift assays using nuclear extracts from human Jurkat T cells, a protein-DNA complex which was supershifted with antiserum against GABP alpha and GABP beta was observed. Purified recombinant GABP alpha and beta interact with the HIV RafRE as judged from DNA binding assays. Cotransfection experiments with GABP alpha and beta and Raf-1 kinase demonstrate synergistic transactivation of the HIV-1 promoter. Point mutations in the HIV RafRE abolished the Raf-1 kinase as well as GABP alpha- and beta-induced transactivation. The observed Raf-1-GABP synergism presumably involves phosphorylation of GABP subunits, as treatment of cells with Raf-1 kinase activators serum and 12-O-tetradecanoylphorbol-13-acetate increases phosphorylation of GABP in vivo. However, GABP is not a target of Raf-1 kinase; instead, it is a substrate of mitogen-activated protein kinase (MAPK/ERK), since in vitro phosphorylation of GABP alpha and beta was achieved by the reconstituted protein kinase cascade but not with purified Raf-1 or MEK. These results suggest that Raf-1 kinase- induced activation of the HIV-1 promoter is mediated by the classical cytoplasmic cascade resulting in MAPK/ERK-mediated phosphorylation of GABP alpha and beta. Because the HIV RafRE corresponds to a region within the promoter which is essential for regulation of HIV-1 expression, the data indicate that in addition to NK-kappaB, GABP transcription factors are important for induced expression of HIV.

Multiple growth factors are associated with lesions of atherosclerosis: specificity or redundancy?

Within the last five years, a number of specific growth factors have been localized in developing lesions of atherosclerosis. This localization of growth factors that is not observed in normal vessels, together with the pleotrophic activities of growth factors, have suggested a role for growth factors in atherosclerotic lesion progression. However, based on in vitro studies, many of the growth factors identified in lesions have overlapping target cells and are derived from the same cellular sources. What is the relative role of the specific growth factors identified? How is the their activity altered by the local conditions in the vessel wall? How do different risk factors for atherosclerosis alter the balance between growth factors and their natural regulators? Evidence for the involvement of specific growth factors in the progression of lesions of atherosclerosis is discussed, as well as the multiple levels at which the activities of these growth factors may be regulated by the vessel wall.

Regulation of transcription by MAP kinase cascades

Kinases belonging to the mitogen-activated protein kinase (MAPK) family are used throughout evolution to control the cellular responses to external signals such as growth factors, nutrient status, stress or inductive signals. Many important substrates for MAPKs are transcription factors, and both the genetic and the biochemical links between MAPKs and transcription factors are becoming increasingly well understood.

Dynamic production of tumour necrosis factor-alpha (TNF-alpha) messenger RNA, intracellular and extracellular TNF-alpha by murine macrophages and possible association with protein tyrosine phosphorylation of STAT1 alpha and ERK2 as an early signal

Tumour necrosis factor-alpha (TNF-alpha), an important mediator in both immune and inflammation responses, is one of the major cytokines released by activated macrophages. The present study shows that, during macrophage activation, protein tyrosine phosphorylation of STAT1 alpha and ERK2 occurred as an immediate early signal, whereas maximum TNF-alpha mRNA transcription appeared at 3 hr, precursor TNF-alpha formation at 3 to 4 hr, and TNF-alpha release at 5 to 6 hr after stimulation of an RPMI-1640-based induction medium containing lipopolysaccharide (100 ng/ml), interferon-gamma (100 U/ml), and 0.5% bovine serum albumin. Herbimycin A, a tyrosine kinase inhibitor, suppresses protein tyrosine phosphorylation of STAT1 alpha and ERK2 and also blocks TNF-alpha production by resident peritoneal macrophages from BALB/c mice, suggesting a possible association between protein tyrosine phosphorylation of STAT1 alpha and ERK2 and macrophage activation resulting in TNF-alpha production.

Mechanisms of transcriptional synergism of eukaryotic genes. The interferon-beta paradigm

The virus-inducible enhancer of the human interferon-beta gene has served as an excellent example for the mechanisms controlling the activation and repression of transcription. This enhancer is activated by three different transcription factors that, with the help of the high mobility group protein HMG I(Y), assemble in a unique nucleoprotein complex that interacts as a unit with the basal transcriptional machinery. The assembly of unique enhancer complexes from similar sets of transcription factors may provide the specificity required for regulation of complex patterns of gene expression in higher eukaryotes.

The mitogen-activated protein kinase phosphatases PAC1, MKP-1, and MKP-2 have unique substrate specificities and reduced activity in vivo toward the ERK2 sevenmaker mutation

Mitogen-activated protein (MAP) kinases can be grouped into three structural families, ERK, JNK, and p38, which are thought to carry out unique functions within cells. We demonstrate that ERK, JNK, and p38 are activated by distinct combinations of stimuli in T cells that simulate full or partial activation through the T cell receptor. These kinases are regulated by reversible phosphorylation on Tyr and Thr, and the dual specific phosphatases PAC1 and MKP-1 previously have been implicated in the in vivo inactivation of ERK or of ERK and JNK, respectively. Here we characterize a new MAP kinase phosphatase, MKP-2, that is induced in human peripheral blood T cells with phorbol 12-myristate 13-acetate and is expressed in a variety of nonhematopoietic tissues as well. We show that the in vivo substrate specificities of individual phosphatases are unique. PAC1, MKP-2, and MKP-1 recognize ERK and p38, ERK and JNK, and ERK, p38, and JNK, respectively. Thus, individual MAP kinase phosphatases can differentially regulate the potential for cross-talk between the various MAP kinase pathways. A hyperactive allele of ERK2 (D319N), analogous to the Drosophila sevenmaker gain-of-function mutation, has significantly reduced sensitivity to all three MAP kinase phosphatases in vivo.

Participation of JAK and STAT proteins in growth hormone-induced signaling

The binding of growth hormone leads to dimerization of its receptor, accompanied by phosphorylation and activation of intracellular tyrosine kinases (JAKs) and the latent cytoplasmic transcriptions factors STAT1, STAT3, and STAT5. Both JAK1 and JAK2 are phosphorylated in response to growth hormone in mouse 3T3 F442A and human HT1080 cells. The roles of JAKs in growth hormone signal transduction were examined by using mutant HT1080 cells missing either JAK1 or JAK2. JAK2 is absolutely required for growth hormone-dependent phosphorylation of the receptor, STAT1 and STAT3, JAK1, and the SH2-containing adaptor molecule Shc. In contrast, JAK1 is not required for any of the above functions. These data indicate that JAK2 is both necessary and sufficient for the growth hormone-dependent phosphorylation events required to couple the receptor both to STAT-dependent signaling pathways and to pathways involving Shc. Furthermore, STAT5 is activated by growth hormone in 3T3 F442A cells, but not in HT1080 cells, revealing that the set of STATs activated by growth hormone can vary, possibly contributing to the specificity of the growth hormone response in different cell types.

Trans-repressor BEF-1 phosphorylation. A potential control mechanism for human ApoE gene regulation

Human apolipoprotein E is a plasma lipoprotein that appears to play an important protective role in the development of atherosclerosis. While little is known about the regulation of apoE, recent studies have shown that cytokines repress apoE synthesis both in vivo and in vitro. Furthermore, we have recently shown that the endogenous apoE gene is negatively regulated by the nuclear trans-repressor BEF-1 in the human HepG2 cell line. In this study we demonstrate that treatment of HepG2 cells with the cytokine interleukin-1 and interleukin-6 resulted in the induction of an isoform of BEF-1, designated B1. The induction of the B1 isoform could be blocked by the protein kinase inhibitor staurosporine, suggesting that B1 is a phosphorylated form of BEF-1. As further support, the B1 isoform could also be induced by phorbol ester, and subsequently inhibited by staurosporine, implicating a role for protein kinase C-mediated phosphorylation. Quantitation of the levels of the BEF-1 isoforms, and studies in the presence of cyclohexamide, provided evidence for the phosphorylation of an existing intracellular pool of BEF-1, with no change in the total intracellular level. Under conditions that generated increased levels of the B1 isoform, there was a concomitant and proportional decrease in the level of apoE mRNA. The effect did not appear to be the result of improved binding to the apoE regulatory region as the DNA binding affinity of B1 was identical to native BEF-1. Our data suggest that the regulation of apoE by BEF-1 is modulated by differential phosphorylation, possibly through the protein kinase C pathway.

Analysis of ATF3, a transcription factor induced by physiological stresses and modulated by gadd153/Chop10

We demonstrate that ATF3, a member of the ATF/CREB family of transcription factors, is induced in a variety of stressed tissues: mechanically injured liver, toxin-injured liver, blood-deprived heart, and postseizure brain. We also demonstrate that an ATF3-interacting protein, gadd153/Chop10, forms a nonfunctional heterodimer with ATF3: the heterodimer, in contrast to the ATF3 homodimer, does not bind to the ATF/cyclic AMP response element consensus site and does not repress transcription. Interestingly, ATF3 and gadd153/Chop10 are expressed in inverse but overlapping manners during the liver's response to carbon tetrachloride (CCl4): the level of gadd153/Chop10 mRNA is high in the normal liver and greatly decreases upon CCl4 treatment; the level of ATF3 mRNA, on the other hand, is low in the normal liver and greatly increases upon CCl4 treatment. We hypothesize that in nonstressed liver, gadd153/Chop10 inhibits the limited amount of ATF3 by forming an inactive heterodimer with it, whereas in CCl4-injured liver, the synthesis of gadd153/Chop10 is repressed, allowing the induced ATF3 to function.

Multiple requirements for SHPTP2 in epidermal growth factor-mediated cell cycle progression

Using transient overexpression and microinjection approaches, we examined SHPTP2's function in growth factor signaling. Overexpression of catalytically inactive SHPTP2 (PTP2CS) but not catalytically inactive SHPTP1, inhibited mitogen-activated protein (MAP) kinase activation and Elk-1 transactivation following epidermal growth factor (EGF) stimulation of 293 cells. An SHPTP2 mutant with both C-terminal tyrosyl phosphorylation sites converted to phenylalanine (PTP2YF) was also without effect; moreover, PTP2YF rescued PTP2CS-induced inhibition of EGF-induced Elk-1 transactivation. PTP2CS did not inhibit transactivation by activated Ras, suggesting that SHPTP2 acts upstream of or parallel to Ras. Neither PTP2CS nor PTP2YF inhibited platelet-derived growth factor (PDGF)-induced Elk-1 transactivation. Thus, protein-tyrosine phosphatase activity, but not tyrosyl phosphorylation of SHPTP2, is required for the immediate-early responses to EGF but not to PDGF. To determine whether SHPTP2 is required later in the cell cycle, we assessed S-phase entry in NIH 3T3 cells microinjected with anti-SHPTP2 antibodies or with a glutathione S-transferase (GST) fusion protein encoding both SH2 domains (GST-SH2). Microinjection of anti-SHPTP2 antibodies prior to stimulation inhibited EGF- but no PDGF- or serum-induced S-phase entry. Anti-SHPTP2 antibodies or GST-SH2 fusion protein could inhibit EGF-induced S-phase entry for up to 8 h after EGF addition. Although MAP kinase activation was detected shortly after EGF stimulation, no MAP kinase activation was detected around the restriction point. Therefore, SHPTP2 is absolutely required for immediate-early and late events induced by some, but not all, growth factors, and the immediate-early and late signal transduction pathways regulated by SHPTP2 are distinguishable.

The regulation of AP-1 activity by mitogen-activated protein kinases

AP-1 is a collection of dimeric sequence specific, DNA binding, transcriptional activators composed of Jun and Fos subunits. The composition, the level and the activity of AP-1 complexes are regulated in response to extracellular stimuli. An important role in this regulation is played by mitogen-activated protein kinases (MAPKs). The specific roles of three MAPKs, namely ERK, JNK and FRK, in modulation of both the level and activity of AP-1, are discussed.

Reconstitution of novel signalling cascades responding to cellular stresses

Mammalian cells respond to their immediate environment by inducing signal transduction cascades that regulate metabolism, secretion and gene expression. Several of these signalling pathways are structurally and organizationally related insofar as they require activation of a protein-serine kinase via it's phosphorylation on tyrosine and threonine; the archetype being mitogen-activated protein kinase (MAPK) which responds primarily to mitogenic stimuli via Ras. In contrast, two more recently identified cascades are responsive to cellular stresses such as heat, inflammatory cytokines, ischaemia and metabolic poisons. The recent identification of the components of these pathways has allowed manipulation of the stress-responsive pathways and evaluation of their physiological roles. These studies reveal a high degree of independence between the pathways not apparent from in vitro studies. Manipulation of the pathways in vivo will likely result in novel therapies for inflammatory disease and reperfusion injury.

Signaling from G protein-coupled receptors to c-Jun kinase involves beta gamma subunits of heterotrimeric G proteins acting on a Ras and Rac1-dependent pathway

Stimulation of a variety of cell surface receptors enhances the enzymatic activity of mitogen-activated protein kinases (MAPKs). MAPKs have been classified in three subfamilies: extracellular signal-regulated kinases (ERKs), stress-activated protein kinases or c-Jun NH2-terminal kinases (SAPKs/JNKs), and p38 kinase. Whereas the pathway linking cell surface receptors to ERKs has been partially elucidated, the mechanism of activation of JNKs is still poorly understood. Recently, we have shown that stimulation of G protein-coupled receptors can effectively induce JNK in NIH 3T3 cells (Coso, O. A., Chiariello, M., Kalinec, G., Kyriakis, J. M., Woodgett, J., and Gutkind, J. S. (1995) J. Biol. Chem. 270, 5620-5624). In the present study, we have used the transient expression in COS-7 cells of m1 and m2 muscarinic receptors (mAChRs) as a model system to study the signaling pathway linking G protein-coupled receptors to JNK. We show that stimulation of either muscarinic receptor subtype leads to JNK activation; however, this effect was not mimicked by expression of activated forms of alphas, alphai2, alphaq, or alpha13 G protein alpha subunits. In contrast, overexpression of Gbetagamma subunits potently induced JNK activity. Furthermore, we show that signaling from m1 and m2 mAChRs to JNK involves betagamma subunits of heterotrimeric G proteins, acting on a Ras and Rac1-dependent pathway.

MKP-3, a novel cytosolic protein-tyrosine phosphatase that exemplifies a new class of mitogen-activated protein kinase phosphatase

MKP-1 (also known as CL100, 3CH134, Erp, and hVH-1) exemplifies a class of dual-specificity phosphatase able to reverse the activation of mitogen-activated protein (MAP) kinase family members by dephosphorylating critical tyrosine and threonine residues. We now report the cloning of MKP-3, a novel protein phosphatase that also suppresses MAP kinase activation state. The deduced amino acid sequence of MKP-3 is 36% identical to MKP-1 and contains the characteristic extended active-site sequence motif VXVHCXXGXSRSXTXXXAYLM (where X is any amino acid) as well as two N-terminal CH2 domains displaying homology to the cell cycle regulator Cdc25 phosphatase. When expressed in COS-7 cells, MKP-3 blocks both the phosphorylation and enzymatic activation of ERK2 by mitogens. Northern analysis reveals a single mRNA species of 2.7 kilobases with an expression pattern distinct from other dual-specificity phosphatases. MKP-3 is expressed in lung, heart, brain, and kidney, but not significantly in skeletal muscle or testis. In situ hybridization studies of MKP-3 in brain reveal enrichment within the CA1, CA3, and CA4 layers of the hippocampus. Metrazole-stimulated seizure activity triggers rapid (<1 h) but transient up-regulation of MKP-3 mRNA in the cortex, piriform cortex, and some amygdala nuclei. Metrazole stimulated similar regional up-regulation of MKP-1, although this was additionally induced within the thalamus. MKP-3 mRNA also undergoes powerful induction in PC12 cells after 3 h of nerve growth factor treatment. This response appears specific insofar as epidermal growth factor and dibutyryl cyclic AMP fail to induce significant MKP-3 expression. Subcellular localization of epitope-tagged MKP-3 in sympathetic neurons reveals expression in the cytosol with exclusion from the nucleus. Together, these observations indicate that MKP-3 is a novel dual-specificity phosphatase that displays a distinct tissue distribution, subcellular localization, and regulated expression, suggesting a unique function in controlling MAP kinase family members. Identification of a second partial cDNA clone (MKP-X) encoding the C-terminal 280 amino acids of an additional phosphatase that is 76% identical to MKP-3 suggests the existence of a distinct structurally homologous subfamily of MAP kinase phosphatases.

Dyrk, a dual specificity protein kinase with unique structural features whose activity is dependent on tyrosine residues between subdomains VII and VIII

The cDNA of a novel, ubiquitously expressed protein kinase (Dyrk) was cloned from a rat brain cDNA library. The deduced amino acid sequence (763 amino acids) contains a catalytic domain that is only distantly related to that of other mammalian protein kinases. Its closest relative is the protein kinase Mnb of Drosophila, which is presumably involved in postembryonic neurogenesis (85% identical amino acids within the catalytic domain). Outside the catalytic domain, the sequence comprises several striking structural features: a bipartite nuclear translocation signal, a tyrosine-rich hydrophilic motif flanking the nuclear localization signal, a PEST region, a repeat of 13 histidines, a repeat of 17 serine/threonine residues, and an alternatively spliced insertion of nine codons. A recombinant glutathione S-transferase-Dyrk fusion protein catalyzed autophosphorylation and histone phosphorylation on tyrosine and serine/threonine residues with an apparent Km of approximately 3.4 microM. Exchange of two tyrosine residues in the "activation loop" between subdomains VII and VIII for phenylalanine almost completely suppressed the activity and tyrosine autophosphorylation of Dyrk. Tyrosine autophosphorylation was also reduced by exchange of the tyrosine (Tyr-219) in a tyrosine phosphorylation consensus motif. The data suggest that Dyrk is a dual specificity protein kinase that is regulated by tyrosine phosphorylation in the activation loop and might be a component of a signaling pathway regulating nuclear functions.

Activation of mammalian gene expression by the UV component of sunlight--from models to reality

Ultraviolet radiation activates the expression of a wide variety of genes, by pathways which differ between the short non-solar ultraviolet C (UVC) wavelengths, which are strongly absorbed by nucleic acids, and the long solar ultraviolet A (UVA, 320-380 nm) wavelengths, which generate active oxygen intermediates. Intermediate solar ultraviolet (UV) wavelengths in the UVB (290-320 nm) range also contain an oxidative component, but more closely resemble UVC in their gene activating properties. Short wavelength UV, in common with other extracellular stimuli including growth factors, activates signal transduction events that involve both stress- and mitogen-activated protein kinase cascades. The extrapolation of the complex modulation of gene expression that ensues to the consequences of natural UV exposure requires careful attention to the details of doses and wavelength employed in the model experiments. Nevertheless, there is evidence that UVB irradiation of skin can activate the expression of proteins including immunomodulating cytokines, ornithine decarboxylase and, to a limited extent, nuclear oncogene products, as well as lead to stabilisation of p53. Non-cytotoxic doses of UVA radiation also lead to the strong activation of several genes which would be expected to have functional relevance in vivo.

Ras-mediated phosphorylation of a conserved threonine residue enhances the transactivation activities of c-Ets1 and c-Ets2

The Ras oncogene products regulate the expression of genes in transformed cells, and members of the Ets family of transcription factors have been implicated in this process. To determine which Ets factors are the targets of Ras signaling pathways, the abilities of several Ets factors to activate Ras-responsive enhancer (RRE) reporters in the presence of oncogenic Ras were examined. In transient transfection assay, reporters containing RREs composed of Ets-AP-1 binding sites could be activated 30-fold in NIH 3T3 fibroblasts and 80-fold in the macrophage-like line RAW264 by the combination of Ets1 or Ets2 and Ras but not by several other Ets factors that were tested in the assay. Ets2 and Ras also superactivated an RRE composed of Ets-Ets binding sites, but the Ets-responsive promoter of the c-fms gene was not superactivated. Mutation of a threonine residue to alanine in the conserved amino-terminal regions of Ets1 and Ets2 (threonine 38 and threonine 72, respectively) abrogated the ability of each of these proteins to superactivate reporter gene expression. Phosphoamino acid analysis of radiolabeled Ets2 revealed that Ras induced normally absent threonine-specific phosphorylation of the protein. The Ras-dependent increase in threonine phosphorylation was not observed in Ets2 proteins that had the conserved threonine 72 residue mutated to alanine or serine. These data indicate that Ets1 and Ets2 are specific nuclear targets of Ras signaling events and that phosphorylation of a conserved threonine residue is a necessary molecular component of Ras-mediated activation of these transcription factors.

Insulin stimulation of a MEK-dependent but ERK-independent SOS protein kinase

The Ras guanylnucleotide exchange protein SOS undergoes feedback phosphorylation and dissociation from Grb2 following insulin receptor kinase activation of Ras. To determine the serine/threonine kinase(s) responsible for SOS phosphorylation in vivo, we assessed the role of mitogen-activated, extracellular-signal-regulated protein kinase kinase (MEK), extracellular-signal-regulated protein kinase (ERK), and the c-JUN protein kinase (JNK) in this phosphorylation event. Expression of a dominant-interfering MEK mutant, in which lysine 97 was replaced with arginine (MEK/K97R), resulted in an inhibition of insulin-stimulated SOS and ERK phosphorylation, whereas expression of a constitutively active MEK mutant, in which serines 218 and 222 were replaced with glutamic acid (MEK/EE), induced basal phosphorylation of both SOS and ERK. Although expression of the mitogen-activated protein kinase-specific phosphatase (MKP-1) completely inhibited the insulin stimulation of ERK activity both in vitro and in vivo, SOS phosphorylation and the dissociation of the Grb2-SOS complex were unaffected. In addition, insulin did not activate the related protein kinase JNK, demonstrating the specificity of insulin for the ERK pathway. The insulin-stimulated and MKP-1-insensitive SOS-phosphorylating activity was reconstituted in whole-cell extracts and did not bind to a MonoQ anion-exchange column. In contrast, ERK1/2 protein was retained by the MonoQ column, eluted with approximately 200 mM NaCl, and was MKP-1 sensitive. Although MEK also does not bind to MonoQ, immunodepletion analysis demonstrated that MEK is not the insulin-stimulated SOS-phosphorylating activity. Together, these data demonstrate that at least one of the kinases responsible for SOS phosphorylation and functional dissociation of the Grb2-SOS complex is an ERK-independent but MEK-dependent insulin-stimulated protein kinase.

Insulin regulation of phosphoenolpyruvate carboxykinase gene expression does not require activation of the Ras/mitogen-activated protein kinase signaling pathway

Expression of phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting step in hepatic gluconeogenesis, is primarily regulated at the level of gene transcription. Insulin and phorbol esters inhibit basal PEPCK transcription and antagonize the induction of PEPCK gene expression by glucocorticoids and glucagon (or its second messenger cAMP). Insulin activates a signaling cascade involving Ras --> Raf --> p42/p44 mitogen-activated protein (MAP) kinase kinase (MEK) --> p42/p44 MAP kinase (ERK 1 and 2). Recent reports suggest that activation of this Ras/MAP kinase pathway is critical for the effects of insulin on mitogenesis and c-fos transcription but is not required for insulin action on metabolic processes such as glycogen synthesis, lipogenesis, and Glut-4-mediated glucose transport. We have used three distinct approaches to examine the role of the Ras/MAP kinase pathway in the regulation of PEPCK transcription by insulin in H4IIE-derived liver cells: (i) chemical inhibition of Ras farnesylation, (ii) infection of cells with an adenovirus vector encoding a dominant-negative mutant of Ras, and (iii) use of a chemical inhibitor of MEK. Although each of these methods blocks insulin activation of MAP kinase, none alters insulin antagonism of cAMP- and glucocorticoid-stimulated PEPCK transcription. Although phorbol esters activate MAP kinase and mimic the effects of insulin on PEPCK gene transcription, inhibition of MEK has no effect on phorbol ester inhibition of PEPCK gene transcription. Using the structurally and mechanistically distinct phosphatidylinositol 3-kinase (PI 3-kinase) inhibitors, wortmannin and LY 294002, we provide further evidence supporting a role for PI 3-kinase activation in the regulation of PEPCK gene transcription by insulin. We conclude that neither insulin nor phorbol ester regulation of PEPCK gene transcription requires activation of the Ras/MAP kinase pathway and that insulin signaling to the PEPCK promoter is dependent on PI 3-kinase activation.

ATF3 gene. Genomic organization, promoter, and regulation

ATF3 gene, which encodes a member of the activating transcription factor/cAMP responsive element binding protein (ATF/CREB) family of transcription factors, is induced by many physiological stresses. As a step toward understanding the induction mechanisms, we isolated the human ATF3 gene and analyzed its genome organization and 5'-flanking region. We found that the human ATF3 mRNA is derived from four exons distributed over 15 kilobases. Sequence analysis of the 5'-flanking region revealed a consensus TATA box and a number of transcription factor binding sites including the AP-1, ATF/CRE, NF-kappa B, E2F, and Myc/Max binding sites. As another approach to understanding the mechanisms by which the ATF3 gene is induced by stress signals, we studied the regulation of the ATF3 gene in tissue culture cells by anisomycin, an approach that has been used to study the stress responses in tissue culture cells. We showed that anisomycin at a low concentration activates the ATF3 promoter and stabilizes the ATF3 mRNA. Significantly, co-transfection of DNAs expressing ATF2 and c-Jun activates the ATF3 promoter. A possible mechanism implicating the C-Jun NH2-terminal kinase/stress-activated protein kinase (JNK/SAPK) stress-inducible signaling pathway in the induction of the ATF3 gene is discussed.