Biomedical subjects

C M Horvath

Publications and source records attributed to C M Horvath.

At least 19 recordsLinked to original sources

PubMed2001

TYK2 and JAK2 are substrates of protein-tyrosine phosphatase 1B.

The reversible tyrosine phosphorylation of proteins, modulated by the coordinated actions of protein-tyrosine kinases and protein-tyrosine phosphatases (PTPs), regulates the cellular response to a wide variety of stimuli. It is established that protein kinases possess discrete sets of substrates and that substrate recognition is often dictated by the presence of consensus phosphorylation sites. Here, we have extended this concept to the PTPs and demonstrated that (E/D)-pY-pY-(R/K) is a consensus substrate recognition motif for PTP1B. We have shown that JAK2 and TYK2 are substrates of PTP1B and that the substrate recognition site within theses kinases is similar to the site of dephosphorylation previously identified within the insulin receptor. A substrate-trapping mutant of PTP1B formed a stable interaction with JAK2 and TYK2 in response to interferon stimulation. Expression of wild type or substrate-trapping mutant PTP1B inhibited interferon-dependent transcriptional activation. Finally, mouse embryo fibroblasts deficient in PTP1B displayed subtle changes in tyrosine phosphorylation, including hyperphosphorylation of JAK2. The closely related JAK family member, JAK1, which does not match the consensus dephosphorylation site, was not recognized as a substrate. These data illustrate that PTP1B may be an important physiological regulator of cytokine signaling and that it may be possible to derive consensus substrate recognition motifs for other members of the PTP family, which may then be used to predict novel physiological substrates.

Amino Acid Sequence↗

PubMed2001

The V protein of human parainfluenza virus 2 antagonizes type I interferon responses by destabilizing signal transducer and activator of transcription 2.

Type I interferon (IFN) induces antiviral responses through the activation of the ISGF3 transcription factor complex that contains the subunit proteins STAT1, STAT2, and p48/ISGF3 gamma/IRF9. The ability of some human paramyxoviruses to overcome IFN actions by specific proteolysis of STAT proteins has been examined. Infection of cells with type 2, but not type 1 or type 3 human parainfluenza virus (HPIV) leads to a loss of cellular STAT2 protein. Expression of a single HPIV2 protein derived from the V open reading frame blocks IFN-dependent transcriptional responses in the absence of other viral proteins. The loss of IFN response is due to V-protein-induced proteolytic degradation of STAT2. Expression of HPIV2 V causes the normally stable STAT2 protein to be rapidly degraded, and this proteolytic activity can be partially alleviated by proteasome inhibition. No V-protein-specific effects on STAT2 mRNA levels were observed. The results indicate that the V protein of HPIV2 is sufficient to recognize and target a specific cellular transcription factor for destruction by cellular machinery.

Cell Line↗

PubMed2001

Cooperation between STAT3 and c-jun suppresses Fas transcription.

Decreased Fas expression during tumor progression often results in a loss of Fas-ligand (FasL)-mediated apoptosis. Human and mouse melanoma exhibit an inverse correlation between the degree of Fas cell surface expression, tumorigenicity, and metastatic capacity. The expression of dominant negative Stat3 or c-Jun in melanoma cells efficiently increased Fas expression and sensitized cells to FasL-induced apoptosis. Stat3+/- as well as c-Jun-/- cells exhibited increased Fas cell surface expression and higher sensitivity to FasL-mediated apoptosis. Suppression of Fas expression by Stat3 and c-Jun is uncoupled from Stat3-mediated transcriptional activation. Our findings indicate that Stat3 oncogenic activities could also be mediated through its cooperation with c-Jun, resulting in downregulation of Fas surface expression, which is implicated in the tumor's ability to resist therapy and metastasize.

Animals↗

PubMed2000

DNA binding specificity of different STAT proteins. Comparison of in vitro specificity with natural target sites.

STAT transcription factors are expressed in many cell types and bind to similar sequences. However, different STAT gene knock-outs show very distinct phenotypes. To determine whether differences between the binding specificities of STAT proteins account for these effects, we compared the sequences bound by STAT1, STAT5A, STAT5B, and STAT6. One sequence set was selected from random oligonucleotides by recombinant STAT1, STAT5A, or STAT6. For another set including many weak binding sites, we quantified the relative affinities to STAT1, STAT5A, STAT5B, and STAT6. We compared the results to the binding sites in natural STAT target genes identified by others. The experiments confirmed the similar specificity of different STAT proteins. Detailed analysis indicated that STAT5A specificity is more similar to that of STAT6 than that of STAT1, as expected from the evolutionary relationships. The preference of STAT6 for sites in which the half-palindromes (TTC) are separated by four nucleotides (N(4)) was confirmed, but analysis of weak binding sites showed that STAT6 binds fairly well to N(3) sites. As previously reported, STAT1 and STAT5 prefer N(3) sites; however, STAT5A, but not STAT1, weakly binds N(4) sites. None of the STATs bound to half-palindromes. There were no specificity differences between STAT5A and STAT5B.

Base Sequence↗

PubMed2000

Interferon regulatory factor subcellular localization is determined by a bipartite nuclear localization signal in the DNA-binding domain and interaction with cytoplasmic retention factors.

The transduction of type I interferon signals to the nucleus relies on activation of a protein complex, ISGF3, involving two signal transducers and activators of transcription (STAT) proteins, STAT1 and STAT2, and the interferon (IFN) regulatory factor (IRF) protein, p48/ISGF3gamma. The STAT subunits are cytoplasmically localized in unstimulated cells and rapidly translocate to the nucleus of IFN-stimulated cells, but the p48/ISGF3gamma protein is found in both the nucleus and the cytoplasm, regardless of IFN stimulation. Here, we demonstrate that p48 is efficiently and constitutively targeted to the nucleus. Analysis of the subcellular distribution of green fluorescent protein-p48 fragments indicates that p48 contains a bipartite nuclear retention signal within its amino-terminal DNA-binding domain. This signal is preserved in two other IRF proteins involved in immune responses, ICSBP and IRF4. Mutations to clustered basic residues within amino acids 50-100 of p48 or IRF4 disrupt their nuclear accumulation, and DNA-binding ability is not required for nuclear targeting. This is the only example of a nuclear localization signal for any ISGF3 component and assigns a second function to the IRF DNA-binding domain. We also demonstrate that the nuclear distribution of p48 is dramatically altered by coexpression of the STAT2 protein, indicating that STAT2 forms a cytoplasmic complex with p48, overriding the intrinsic p48 nuclear targeting. Retention by STAT2 may serve to regulate the activity of free p48 and/or guarantee that cytoplasmic pools of preassociated STAT2:p48 are available for rapid activation of the IFN response. These findings suggest that analogous mechanisms may exist for regulating the distribution of other IRF proteins.

3T3 Cells↗

PubMed2000

Regrowth of 5-fluorouracil-treated human colon cancer cells is prevented by the combination of interferon gamma, indomethacin, and phenylbutyrate.

We previously reported that phenylbutyrate (PB), a differentiation agent, retarded the regrowth of fluoropyrimidine-treated HT29 cells to a greater extent in a well-differentiated subclone as compared with a poorly differentiated subclone (Y. Huang and S. Waxman, Clin. Cancer Res., 4: 2503-2509, 1998). To extend these results and to overcome the known heterogeneity of human colon carcinoma (HCC) cells, the effect of cytostatic agents reported to inhibit HCC growth [IFN-alpha and IFN-gamma, indomethacin, and PB alone or in combination] on clonogenicity and HCCs recovery from 5-fluorouracil (FUra) treatment was studied in eight different HCCs. IFN-alpha proved to be ineffective in all eight HCCs, whereas IFN-gamma induced marked growth inhibition in four HCCs that expressed wild-type K-ras. Despite large differences in HCC response to the other individual agents, strong growth inhibition was observed when PB was added in combination with indomethacin. The inhibition was even more pronounced when IFN-gamma was included in the regimen. Most importantly, after treatment with the combination of three agents, the clonogenic potential was severely inhibited (92-100%) in the IFN-gamma-sensitive cell lines, whereas in the IFN-gamma-insensitive cell lines, comparable loss of clonogenecity was obtained when the cells were pretreated with FUra. As known and described in detail, the three cytostatic agents inhibit different processes necessary for cell growth, thus requiring the cells to repair multiple pathways to restore growth. The induction of STAT1 DNA binding activity by IFN-gamma and p21WAF1 by PB, alone or in combination, correlated with growth inhibition and loss of clonogenicity. The finding that the readily reversible growth inhibition and decrease in clonogenicity of FUra-treated HCC are prolonged by subsequent treatment with the three cytostatic agents in all HCCs may be of clinical importance because FUra continues to be the most widely used cytotoxic agent in the treatment of colon carcinoma.

Antimetabolites, Antineoplastic↗

PubMed2000

Collaboration of signal transducer and activator of transcription 1 (STAT1) and BRCA1 in differential regulation of IFN-gamma target genes.

Most of the activities of IFN-gamma are the result of STAT1-mediated transcriptional responses. In this study, we show that the BRCA1 tumor suppressor acts in concert with STAT1 to differentially activate transcription of a subset of IFN-gamma target genes and mediates growth inhibition by this cytokine. After IFN-gamma treatment, induction of the cyclin-dependent kinase inhibitor, p21WAF1, was synergistically activated by BRCA1, whereas the IRF-1 gene was unaffected. Importantly, the differential induction of p21WAF1 was impaired in breast cancer cells homozygous for the mutant BRCA1 5382C allele. Biochemical analysis illustrated that the mechanism of this transcriptional synergy involves interaction between BRCA1 aa 502-802 and the C-terminal transcriptional activation domain of STAT1 including Ser-727 whose phosphorylation is crucial for transcriptional activation. Significantly, STAT1 proteins mutated at Ser-727 bind poorly to BRCA1, reinforcing the importance of Ser-727 in the recruitment of transcriptional coactivators by STAT proteins. These findings reveal a novel mechanism for BRCA1 function in the IFN-gamma-dependent tumor surveillance system.

Animals↗

PubMed2000

Stat3-mediated transformation of NIH-3T3 cells by the constitutively active Q205L Galphao protein.

Expression of Q205L Galphao (Galphao*), an alpha subunit of heterotrimeric guanine nucleotide-binding proteins (G proteins) that lacks guanosine triphosphatase (GTPase) activity in NIH-3T3 cells, results in transformation. Expression of Galphao* in NIH-3T3 cells activated signal transducer and activator of transcription 3 (Stat3) but not mitogen-activated protein (MAP) kinases 1 or 2. Coexpression of dominant negative Stat3 inhibited Galphao*-induced transformation of NIH-3T3 cells and activation of endogenous Stat3. Furthermore, Galphao* expression increased activity of the tyrosine kinase c-Src, and the Galphao*-induced activation of Stat3 was blocked by expression of Csk (carboxyl-terminal Src kinase), which inactivates c-Src. The results indicate that Stat3 can function as a downstream effector for Galphao* and mediate its biological effects.

3T3 Cells↗

PubMed2000

STAT proteins and transcriptional responses to extracellular signals.

Signal transducer and activator of transcription (STAT) transcription factors are implicated in programming gene expression in biological events as diverse as embryonic development, programmed cell death, organogenesis, innate immunity, adaptive immunity and cell growth regulation in organisms ranging from slime molds to insects to man. Rapid progress has unearthed much about the activation of STATs by Janus kinases (JAKs) and other tyrosine kinases and their ability to interface with other signaling systems. Once inside the nucleus, the STATs bind to promoters and join other transcriptional activators in the regulation of gene expression.

Animals↗

PubMed2000

Activation of interferon regulatory factor 3 is inhibited by the influenza A virus NS1 protein.

We present a novel mechanism by which viruses may inhibit the alpha/beta interferon (IFN-alpha/beta) cascade. The double-stranded RNA (dsRNA) binding protein NS1 of influenza virus is shown to prevent the potent antiviral interferon response by inhibiting the activation of interferon regulatory factor 3 (IRF-3), a key regulator of IFN-alpha/beta gene expression. IRF-3 activation and, as a consequence, IFN-beta mRNA induction are inhibited in wild-type (PR8) influenza virus-infected cells but not in cells infected with an isogenic virus lacking the NS1 gene (delNS1 virus). Furthermore, NS1 is shown to be a general inhibitor of the interferon signaling pathway. Inhibition of IRF-3 activation can be achieved by the expression of wild-type NS1 in trans, not only in delNS1 virus-infected cells but also in cells infected with a heterologous RNA virus (Newcastle disease virus). We propose that inhibition of IRF-3 activation by a dsRNA binding protein significantly contributes to the virulence of influenza A viruses and possibly to that of other viruses.

Animals↗

PubMed1999

Interacting regions in Stat3 and c-Jun that participate in cooperative transcriptional activation.

Independent but closely spaced DNA binding sites for Stat3 and c-Jun are required for maximal enhancer function in a number of genes, including the gene encoding the interleukin-6 (IL-6)-induced acute-phase response protein, alpha(2)-macroglobulin. In addition, a physical interaction of Stat3 with c-Jun, based on yeast two-hybrid interaction experiments, has been reported. Here we confirm the existence of an interaction between Stat3 and c-Jun both in vitro, with recombinant proteins, and in vivo, during transient transfection. Using fragments of both proteins, we mapped the interactive sites to the C-terminal region of c-Jun and to two regions in Stat3, within the coiled-coil domain and in a portion of the DNA binding domain distant from DNA contact sites. In transient-transfection experiments with the alpha(2)-macroglobulin enhancer, Stat3 and c-Jun cooperated to yield maximal enhancer function. Point mutations of Stat3 within the interacting domains blocked both physical interaction of Stat3 with c-Jun and their cooperation in IL-6-induced transcription directed by the alpha(2)-macroglobulin enhancer. While the amino acid sequences and the three-dimensional structures of Stat3 and Stat1 cores are very similar, fragments of Stat1 failed to bind c-Jun in vitro. Although Stat1 binds in vitro to the gamma interferon gene response (GAS) element in the alpha(2)-macroglobulin enhancer, Stat1 did not stimulate transcription, nor did Stat1 and c-Jun cooperate in driving transcription controlled by the alpha(2)-macroglobulin enhancer.

Amino Acid Sequence↗

PubMed1998

Stat3 activation is required for cellular transformation by v-src.

Stat3 activation has been associated with cytokine-induced proliferation, anti-apoptosis, and transformation. Constitutively activated Stat3 has been found in many human tumors as well as v-abl- and v-src-transformed cell lines. Because of these correlations, we examined directly the relationship of activated Stat3 to cellular transformation and found that wild-type Stat3 enhances the transforming potential of v-src while three dominant negative Stat3 mutants inhibit v-src transformation. Stat3 wild-type or mutant proteins did not affect v-ras transformation. We conclude that Stat3 has a necessary role in v-src transformation.

3T3 Cells↗

PubMed1997

Defective TNF-alpha-induced apoptosis in STAT1-null cells due to low constitutive levels of caspases.

Signal transducers and activators of transcription (STATs) enhance transcription of specific genes in response to cytokines and growth factors. STAT1 is also required for efficient constitutive expression of the caspases Ice, Cpp32, and Ich-1 in human fibroblasts. As a consequence, STAT1-null cells are resistant to apoptosis by tumor necrosis factor alpha (TNF-alpha). Reintroduction of STAT1alpha restored both TNF-alpha-induced apoptosis and the expression of Ice, Cpp32, and Ich-1. Variant STAT1 proteins carrying point mutations that inactivate domains required for STAT dimer formation nevertheless restored protease expression and sensitivity to apoptosis, indicating that the functions of STAT1 required for these activities are different from those that mediate induced gene expression.

Apoptosis↗

PubMed1997

The state of the STATs: recent developments in the study of signal transduction to the nucleus.

The STATs (signal transducers and activators of transcription) are latent cytoplasmic proteins that, upon activation by cell surface bound polypeptide ligands, move to the nucleus to direct transcription. A variety of protein-protein interactions that affect the function of STATs has been recently recognized. It has become clear that the STATs are functional mosaics, or mixtures of signal transduction and transcription modules.

Animals↗

PubMed1997

Signal transducer and activator of transcription-3 serine phosphorylation by insulin is mediated by a Ras/Raf/MEK-dependent pathway.

We recently reported that insulin stimulation results in the serine phosphorylation of STAT3 (signal transducer and activator of transcription-3). In the present study, we identified serine 727 as the site of insulin-stimulated STAT3 serine phosphorylation. This phosphorylation event occurs independent of tyrosine phosphorylation. Furthermore, interleukin-6-induced tyrosine phosphorylation can occur independent of serine phosphorylation, demonstrating that these two phosphorylation pathways are mechanistically unrelated. Selective activation of the JNK and p38 family of mitogen-activated protein (MAP) kinases by anisomycin treatment did not result in the phosphorylation of STAT3. In contrast, activation of the ERK MAP kinase pathway with both insulin and osmotic shock resulted in the serine phosphorylation of STAT3. In addition, expression of a dominant-interfering Ras mutant (N17Ras) or treatment with the specific MEK inhibitor (PD98059) prevented the insulin stimulation of STAT3 serine phosphorylation. Blockade of ERK activation by expression of the MAP kinase phosphatase (MKP-1) had no effect on insulin-stimulated STAT3 serine phosphorylation. Together, these data demonstrate that the insulin-stimulated serine phosphorylation of STAT3 occurs by a MEK-dependent pathway that is independent of ERK activation.

Animals↗

PubMed1996

Two contact regions between Stat1 and CBP/p300 in interferon gamma signaling.

Interferon gamma (IFN-gamma) induces rapid tyrosine phosphorylation of the latent cytoplasmic transcription factor, Stat1, which then forms homodimers, translocates to the nucleus and participates in IFN-gamma-induced transcription. However, little is known of the interactions between Stat1 and the general transcription machinery during transcriptional activation. We show here that Stat1 can directly interact with the CREB-binding protein (CBP)/p300 family of transcriptional coactivators. Specifically, two interaction regions were identified: the amino-terminal region of Stat1 interacts with the CREB-binding domain of CBP/p300 and the carboxyl-terminal region of Stat1 interacts with the domain of CBP/p300 that binds adenovirus E1A protein. Transfection experiments suggest a role for these interactions in IFN-gamma-induced transcription. Because CBP/p300-binding is required for the adenovirus E1A protein to regulate transcription of many genes during viral replication and cellular transformation, it is possible that the anti-viral effect of IFN-gamma is based at least in part on direct competition by nuclear Stat1 with E1A for CBP/p300 binding.

Adenovirus E1A Proteins↗

PubMed1996

Transcriptionally active Stat1 is required for the antiproliferative effects of both interferon alpha and interferon gamma.

Type I (alpha, beta) and type II (gamma) interferons (IFNs) can restrict the growth of many cell types. INF-stimulated gene transcription, a key early event in IFN response, acts through the Janus kinase-signal transducers and activators of transcription pathway, in which both IFN-alpha and IFN-gamma activate the transcription factor Stat1. A cell line lacking Stat1 (U3A) was not growth-arrested by IFN-alpha or IFN-gamma, and experiments were carried out with U3A cells permanently expressing normal or various mutant forms of Stat1 protein. Only cells in which complete Stat1 activity was available (Stat1alpha) were growth-inhibited by IFN-gamma. A mutant that supports 20-30% normal transcription did not cause growth restraint. In contrast, IFN-alpha growth restraint was imposed by cells producing Stat1beta, which lacks transcriptional activation potential. This parallels earlier results showing the truncated Stat1 can function in IFN-alpha gene activation. In addition to experiments on long-term cultured cells, we also found that wild-type primary mouse embryonic fibroblasts were inhibited by IFNs, but fibroblasts from Stat1-deficient mouse embryos were not inhibited by IFNs.

Animals↗

PubMed1996

Leptin activation of Stat3 in the hypothalamus of wild-type and ob/ob mice but not db/db mice.

Leptin, a hormone secreted by adipocytes, regulates the size of the adipose tissue mass through effects on satiety and energy metabolism. Leptin's precise sites of action are not known. The leptin receptor (Ob-R) is found in many tissues in several alternatively spliced forms raising the possibility that leptin exerts effects on many tissues including the hypothalamus. Ob-R is a member of the gp130 family of cytokine receptors which are known to stimulate gene transcription via activation of cytosolic STAT proteins. In order to identify the sites of leptin action in vivo, we assayed for activation of STAT proteins in mice treated with leptin. The STAT proteins bind to phosphotyrosine residues in the cytoplasmic domain of the ligand-activated receptor where they are phosphorylated. The activated STAT proteins dimerize and translocate to the nucleus where they bind DNA and activate transcription. The activation of STAT proteins in response to leptin was assayed in a variety of mouse tissues known to express Ob-R. Leptin injection activated Stat3 but no other STAT protein in the hypothalamus of ob/ob and wild-type mice but not db/db mice, mutants that lack an isoform of the leptin receptor. Leptin did not induce STAT activation in any of the other tissues tested. Activation of Stat3 by leptin was dose dependent and first observed after 15 minutes and maximal at 30 minutes. Our data indicate the hypothalamus is a direct target of leptin action and that this activation is critically dependent on the gp-130-like leptin receptor isoform missing in C57BLKS/J db/db mice. This is the first in vivo demonstration of leptin signal transduction.

Animals↗