Crystal structure of a tyrosine phosphorylated STAT-1 dimer bound to DNA

Tracking STAT nuclear traffic

Accurate cellular localization is crucial for the effective function of most signalling molecules and nuclear translocation is central to the function of transcription factors. The passage of large molecules between the cytoplasm and nucleus is restricted, and this restriction affords a mechanism to regulate transcription by controlling the access of transcription factors to the nucleus. In this Review, we focus on the signal transducer and activator of transcription (STAT) family of transcription factors. The regulation of the nuclear trafficking of STAT-family members is diverse. Some STAT proteins constitutively shuttle between the nucleus and cytoplasm, whereas others require tyrosine phosphorylation for nuclear localization. In either case, the regulation of nuclear trafficking can provide a target for therapeutic intervention.

Stat1 and SUMO modification

Many proteins are known to undergo small ubiquitin-related modifier (SUMO) modification by an E1-, E2-, and E3-dependent ligation process. Recognition that protein inhibitor of activated signal transducers and activators of transcription (STATs) (PIAS) proteins are SUMO E3 ligases raised the possibility that STATs may also be regulated by SUMO modification. Consistent with this possibility, a SUMO-ylation consensus site (PsiKxE; Psi indicates hydrophobic residue, and x indicates any residue) was identified in Stat1 (ie, (702)IKTE(705)), but not in other STATs. Biochemical analysis confirmed that Stat1 K(703) could be SUMO modified in vitro. Mutation of this critical lysine (ie, Stat1(K703R)) yielded a protein that, when expressed in Stat1(-/-) mouse embryonic fibroblasts (MEFs), exhibited enhanced DNA binding and nuclear retention. This was associated with modest changes in transcriptional and antiviral activity. However, mutation of the second critical residue in the SUMO consensus site, E(705) (ie, Stat1(E705A)), yielded a protein with wild-type DNA binding, nuclear retention, and transcriptional and antiviral activity. Similar observations were made when these mutants were expressed in primary Stat1(-/-) macrophages. These observations suggest that although Stat1 can uniquely be SUMO-ylated in vitro, this modification is unlikely to play an important role in regulating Stat1 activity in vivo.

Targeting Stat3 in cancer therapy

Stat3 is constitutively activated in many human cancers where it functions as a critical mediator of oncogenic signaling through transcriptional activation of genes encoding apoptosis inhibitors (e.g. Bcl-x(L), Mcl-1 and survivin), cell-cycle regulators (e.g. cyclin D1 and c-Myc) and inducers of angiogenesis (e.g. vascular endothelial growth factor). This article reviews several approaches that have been pursued for targeting Stat3 in cancer therapy including antisense strategies, tyrosine kinase inhibition, decoy phosphopeptides, decoy duplex oligonucleotides and G-quartet oligodeoxynucleotides (GQ-ODN). The GQ-ODN strategy is reviewed in somewhat greater detail than the others because it includes a novel system that effectively delivers drug into cells and tissues, addresses successfully the issue of specificity of targeting Stat3 versus Stat1, and has demonstrated efficacy in vivo.

IFN-gamma and its receptor subunit IFNGR1 are recruited to the IFN-gamma-activated sequence element at the promoter site of IFN-gamma-activated genes: evidence of transactivational activity in IFNGR1

We have shown previously that IFN-gamma and one of its receptor subunits, IFNGR1, are translocated to the nucleus, together with STAT1alpha as one macromolecular complex, via the classical importin-dependent pathway. In this study, we have identified the nuclear targets of IFN-gamma and IFNGR1. By chromatin immunoprecipitation followed by PCR, IFN-gamma, its receptor subunit IFNGR1, and STAT1alpha were found to be associated with the IFN-gamma-activated sequence (GAS) in the promoter of two of the genes stimulated by IFN-gamma. Immunoprecipitated chromatin also showed the association of the IFN-gamma, IFNGR1, and STAT1alpha on the same DNA sequence. Examination of nuclear extracts from WISH cells treated with IFN-gamma revealed the specific binding of IFN-gamma, IFNGR1, and STAT1alpha to biotinylated GAS nucleotide sequence. Association of IFN-gamma, IFNGR1, and STAT1alpha with the GAS promoter was also demonstrated by EMSA. Transfection with a GAS-luciferase gene together with the IFNGR1 and nonsecreted IFN-gamma resulted in enhanced reporter activity. In addition, IFNGR1 fused to the yeast GAL4 DNA binding domain resulted in enhanced transcription from a GAL4 response element, suggesting the presence of a trans activation domain in IFNGR1. Our observations put IFN-gamma and its receptor subunit, IFNGR1, in direct contact with the promoter region of IFN-gamma-activated genes with associated increased activity, thus suggesting a transcriptional/cotranscriptional role for IFN-gamma/IFNGR1 as well as a possible role in determining the specificity of IFN-gamma action.

Teaching resources. Cytokine receptors and Jak-STAT signaling

This Teaching Resource provides lecture notes and slides for a class covering interferons, their receptors, and Jak-STAT signaling and is part of the course "Cell Signaling Systems: A Course for Graduate Students." The lecture begins with a discussion of the different families of cytokines and their receptors and then proceeds to describe how cytokine receptor interactions lead to the activation of different members of the STAT family of transcription factors.

Systemic evaluation of total Stat3 and Stat3 tyrosine phosphorylation in normal human tissues

Stat3 plays important roles in many biological phenomena including cell survival, growth, proliferation, differentiation and cancer malignancies. As Stat3 emerges as a new therapeutic target for treatment of cancers in which the Stat3 is constitutively activated, the overall evaluation of basal expression of Stat3 and phosphorylated Stat3 at tyrosine residue 705 in human tissues would be very important and informative. We took a pilot study to examine the expression patterns of total Stat3 and phosphorylated Stat3 protein (p-Stat3) using immunohistochemistry in 47 different adult normal human tissues of 10 organ systems. Immunohistochemistry showed that total Stat3 protein was almost universally detected in all tissues except peripheral nerve. Interestingly, majorities of tissues showed to have moderate to high expression levels of total Stat3 protein. Several heart tissues displayed a unique perinuclear immunostaining for both Stat3 and p-Stat3, most likely in Golgi complexes. Based on the cell types, the p-Stat3 was also expressed in glandular, secretory, mucosal epithelial, circulatory endothelial, lymphoid, proliferative, and reabsorption-active cells.

Targeting signal transducer and activator of transcription 3 with G-quartet oligonucleotides: a potential novel therapy for head and neck cancer

Signal transducer and activator of transcription 3 (Stat3) is a critical mediator of oncogenic signaling activated frequently in many types of human cancer where it contributes to tumor cell growth and resistance to apoptosis. Stat3 has been proposed as a promising target for anticancer drug discovery. Recently, we developed a series of G-quartet oligodeoxynucleotides (GQ-ODN) as novel and potent Stat3 inhibitors, which significantly suppressed the growth of prostate and breast tumors in nude mice. In the present study, we showed that GQ-ODN specifically inhibited DNA-binding activity of Stat3 as opposed to Stat1. Computer-based docking analysis revealed that GQ-ODN predominantly interacts with the SH2 domains of Stat3 homodimers to destabilize dimer formation and disrupt DNA-binding activity. We employed five regimens in the treatment of nude mice with tumors of head and neck squamous cell carcinoma (HNSCC): placebo, paclitaxel, GQ-ODN T40214, GQ-ODN T40231, and T40214 plus paclitaxel. The mean size of HNSCC tumors over 21 days only increased by 1.7-fold in T40214-treated mice and actually decreased by 35% in T40214 plus paclitaxel-treated mice whereas the mean size of HNSCC tumors increased 9.4-fold in placebo-treated mice in the same period. These findings show that GQ-ODN has potent activity against HNSCC tumor xenografts alone and in combination with paclitaxel.

The Structure of SOCS3 Reveals the Basis of the Extended SH2 Domain Function and Identifies an Unstructured Insertion That Regulates Stability

SOCS3 is essential for regulating the extent, duration, and specificity of cellular responses to cytokines such as G-CSF and IL-6. Here we describe the solution structure of SOCS3, the first structure determined for any SOCS protein, in complex with a phosphotyrosine-containing peptide from the IL-6 receptor signaling subunit gp130. The structure of the complex shows that seven peptide residues form a predominantly hydrophobic binding motif. Regions outside the SOCS3 SH2 domain are important for ligand binding, in particular, a single 15 residue alpha helix immediately N-terminal to the SH2 domain makes direct contacts with the phosphotyrosine binding loop and, in part, determines its geometry. The SH2 domain itself is remarkable in that it contains a 35 residue unstructured PEST motif insertion that is not required for STAT inhibition. The PEST motif increases SOCS3 turnover and affects its degradation pathway, implying that it has an important regulatory role inside the cell.

Stats: multifaceted regulators of transcription

The high-affinity binding interactions between interferons (IFNs) and their cognate cell surface receptors lead to the activation of receptor-associated Janus protein tyrosine kinases (Jaks) and subsequent phosphorylation and activation of a group of transcription factors, the signal transducers and activators of transcription (Stats). Upon IFN-induced activation, these Stat proteins form homodimeric and heterodimeric complexes that translocate to the nucleus and bind specific elements within the promoters of IFN-stimulated genes (ISGs). In addition to the well-studied IFN-induced ISG factor 3 (ISGF3) and Stat1:1 complexes, IFNs induce the formation of a number of other Stat-containing complexes, including Stat3:3 and Stat5:5 homodimers, as well as Stat2:1 and Stat5:CrkL heterodimers, that also mediate gene transcription. Moreover, emerging evidence suggests that particular amino acid residues within the individual Stat proteins contribute to different aspects of Stat function. These residues modulate the transcriptional activation potential of Stat-containing complexes and thereby influence the expression of ISGs. Indeed, the Stat proteins function in a multifaceted manner to regulate the expression of proteins that mediate IFN responses.

Aberrant Folding of a Mutant Stat5b Causes Growth Hormone Insensitivity and Proteasomal Dysfunction

A predicted alanine to proline substitution in Stat5b that results in profound short stature, growth hormone insensitivity, and immunodeficiency represents the first natural mutation of this transcription factor in a human. To understand the mechanisms responsible for these pathophysiological abnormalities, we have studied the biochemical and biophysical properties of the mutant Stat5b molecule. In a cellular reconstitution model growth hormone robustly stimulated tyrosine phosphorylation and transcriptional activity of wild-type Stat5b while Stat5bA630P was minimally modified and did not promote reporter gene expression. Steady state levels of Stat5bWT were approximately 3-fold higher than Stat5bA630P in cell extracts prepared with nonionic detergents. Although initial rates of biosynthesis of both proteins were similar, pulse-chase experiments established that the apparent half-life of newly synthesized soluble Stat5bA630P was <15% of Stat5bWT (3.5 h versus >24 h). Stat5bA630P accumulated in cells primarily in cytoplasmic inclusion bodies. Structural analysis of the isolated SH2 domain containing the A630P mutation showed that it resembled the wild-type SH2 segment but that it exhibited reduced thermodynamic stability and slower folding kinetics, displayed an increased hydrophobic surface, and was prone to aggregation in solution. Our results are compatible with a model in which Stat5bA630P is an inactive transcription factor by virtue of its aberrant folding and diminished solubility triggered by a misfolded SH2 domain. The potential for aggregation and formation of cytoplasmic inclusions raises the possibility that Stat5bA630P could produce additional defects through inhibition of proteasome function.

MET meet adaptors: functional and structural implications in downstream signalling mediated by the Met receptor

The tyrosin kinase Met receptor regulates multiple cellular events, ranging from cell motility and angiogenesis to morphological differentiation and tissue regeneration. To conduce these activities, the cytoplasmic C-terminal region of this receptor acts as a docking site for multiple protein substrates, including Grb 2, Gab 1, STAT 3, Shc, SHIP-1 and Src. These substrates are characterised by the presence of multiple domains, including the PH, PTB, SH 2 and SH 3 domains, which directly interact with the multisubstrate C-terminal region of Met. How this receptor recognises and binds a specific substrate in a space-temporal mode is a central question in cell signalling. The recently solved crystal structure of the tyrosine kinase domain of the Met receptor and that of domains of diverse Met substrates provides the molecular framework to understand Met substrate specificity. This structural information also gives new insights on the plasticity of Met signalling and the implications of Met deregulation in tumorigenic processes. In the light of these advances, the present work discusses the molecular basis of Met-substrate recognition and its functional implications in signalling events mediated by this pleiotropic receptor.

Preassembly and ligand-induced restructuring of the chains of the IFN-gamma receptor complex: the roles of Jak kinases, Stat1 and the receptor chains

We previously demonstrated using noninvasive technologies that the interferon-gamma (IFN-gamma) receptor complex is preassembled (1). In this report we determined how the receptor complex is preassembled and how the ligand-mediated conformational changes occur. The interaction of Stat1 with IFN-gammaR1 results in a conformational change localized to IFN-gammaR1. Jak1 but not Jak2 is required for the two chains of the IFN-gamma receptor complex (IFN-gammaR1 and IFN-gammaR2) to interact; however, the presence of both Jak1 and Jak2 is required to see any ligand-dependant conformational change. Two IFN-gammaR2 chains interact through species-specific determinants in their extracellular domains. Finally, these determinants also participate in the interaction of IFN-gammaR2 with IFN-gammaR1. These results agree with a detailed model of the IFN-gamma receptor that requires the receptor chains to be pre-associated constitutively for the receptor to be active.

Interleukins and STAT signaling

Metazoan cells secrete small proteins termed cytokines that execute a variety of biological functions essential for the survival of organisms. Binding of cytokines that belong to the hematopoietin- or interferon-family, to their cognate receptors on the surface of target cells, induces receptor aggregation, which in turn sequentially triggers tyrosine-phosphorylation-dependent activation of receptor-associated Janus-family tyrosine kinases (JAKs), receptors, and signal transducers and activators of transcription (STATs). Phosphorylated STATs form dimers that migrate to the nucleus, bind to cognate enhancer elements and activate transcription of target genes. Each cytokine activates a specific set of genes to execute its biological functions with a certain degree of redundancy. Cytokine signals are, in general, transient in nature. Therefore, under normal physiological conditions, initiation and attenuation of cytokine signals are tightly controlled via multiple cellular and molecular mechanisms. Aberrant activation of cytokine signaling pathways is, however, found under a variety of patho-physiological conditions including cancer and immune diseases.

Investigation of the binding determinants of phosphopeptides targeted to the SRC homology 2 domain of the signal transducer and activator of transcription 3. Development of a high-affinity peptide inhibitor

Signal transducer and activator of transcription 3 (Stat3) is a cytosolic transcription factor that relates signals from the cell membrane directly to the nucleus where it, in complex with other proteins, initiates the transcription of antiapoptotic and cell cycling genes, e.g., Bcl-x(L) and cyclin D1. In normal cells Stat3 transduces signals from cytokines such as IL-6 and growth factors such as the epidermal growth factor. Stat3 is constitutively activated in a number of human tumors. Antisense and dominant negative gene delivery result in apoptosis and reduced cell growth, thus this protein is an attractive target for anticancer drug design. As part of our research on the design of Src homology 2 (SH2) directed peptidomimetic inhibitors of Stat3, in this paper we describe structure-activity relationship studies that provide information on the nature of peptide-protein interactions of a high-affinity phosphopeptide inhibitor of Stat3 dimerization and DNA binding, Ac-Tyr(PO3H2)-Leu-Pro-Gln-Thr-Val-NH2, peptide 1. There is a hydrophobic surface on the SH2 domain that can accommodate lipophilic groups on the N-terminus. Of the amino acids tested, leucine provided the highest affinity at pY+1 and its main chain NH is involved with a hydrogen bond with Stat3, presumably Ser636. cis-3,4-Methanoproline is optimal as a backbone constraint at pY+2. The side chain amide protons of Gln are required for high-affinity interactions. The C-terminal dipeptide, Thr-Val, can be replaced with groups ranging in size from methyl to benzyl. We synthesized a phosphopeptide incorporating groups that provided increases in affinity at each position. Thus, hydrocinnamoyl-Tyr(PO3H2)-Leu-cis-3,4-methanoPro-Gln-NHBn, 50, was the highest affinity peptide, exhibiting an IC50 of 125 nM versus 290 nM for peptide 1 in a fluorescence polarization assay.

Structure of the Unphosphorylated STAT5a Dimer*

STAT proteins have the function of signaling from the cell membrane into the nucleus, where they regulate gene transcription. Latent mammalian STAT proteins can form dimers in the cytoplasm even before receptor-mediated activation by specific tyrosine phosphorylation. Here we describe the 3.21-A crystal structure of an unphosphorylated STAT5a homodimer lacking the N-terminal domain as well as the C-terminal transactivation domain. The overall structure of this fragment is very similar to phosphorylated STATs. However, important differences exist in the dimerization mode. Although the interface between phosphorylated STATs is mediated by their Src-homology 2 domains, the unphosphorylated STAT5a fragment dimerizes in a completely different manner via interactions between their beta-barrel and four-helix bundle domains. The STAT4 N-terminal domain dimer can be docked onto this STAT5a core fragment dimer based on shape and charge complementarities. The separation of the dimeric arrangement, taking place upon activation and nuclear translocation of STAT5a, is demonstrated by fluorescence resonance energy transfer experiments in living cells.

Constitutive tyrosine and serine phosphorylation of STAT4 in T-cells transformed with HTLV-I

STAT4 is a critical mediator of IL-12-stimulated gene regulation in T-helper type 1 (Th1) cell. IL-12 activates the Janus family tyrosine kinases JAK2 and Tyk2, which in turn phosphorylate STAT4 on tyrosine 693. The p38 mitogen-activated protein kinase (MAPK) signaling pathway is also activated in response to IL-12, followed by phosphorylation of STAT4 on serine 721, which is required for STAT4 full transcriptional activity. In the present study, we demonstrated constitutive activation of STAT4 in HTLV-I-transformed T-cell lines MT-2, MT-4 and HUT102 by immunoprecipitation, Western blotting and electrophoretic mobility shift assay (EMSA). In HTLV-I-transformed T-cell lines, STAT4 was constitutively phosphorylated not only on tyrosine 693 but also on serine 721, and formed a heterodimer with STAT3. Constitutive phosphorylation of its upstream activators, JAK2, Tyk2 and p38 MAPK was also observed in the cells. EMSA and transient transfection studies further showed that the high-affinity sis-inducible element (hSIE) preferentially binds the STAT3/STAT4 heterodimer and is constitutively transactivated in MT-2 cells in the absence of exogenous cytokine stimulation. When STAT4 expression vector was cotransfected along with STAT3 expression vector into MT-2 cells, STAT4 significantly synergized with STAT3 to transactivate hSIE, showing the functional importance of heterodimer formation between STAT4 and STAT3.

Secondary structure assignment of mouse SOCS3 by NMR defines the domain boundaries and identifies an unstructured insertion in the SH2 domain

SOCS3 is a negative regulator of cytokine signalling that inhibits Janus kinase-signal transduction and activator of transcription (JAK-STAT) mediated signal tranduction by binding to phosphorylated tyrosine residues on intracellular subunits of various cytokine receptors, as well as possibly the JAK proteins. SOCS3 consists of a short N-terminal sequence followed by a kinase inhibitory region, an extended SH2 domain and a C-terminal suppressor of cytokine signalling (SOCS) box. SOCS3 and the related protein, cytokine-inducible SH2-containing protein, are unique among the SOCS family of proteins in containing a region of mostly low complexity sequence, between the SH2 domain and the C-terminal SOCS box. Using NMR, we assigned and determined the secondary structure of a murine SOCS3 construct. The SH2 domain, unusually, consists of 140 residues, including an unstructured insertion of 35 residues. This insertion fits the criteria for a PEST sequence and is not required for phosphotyrosine binding, as shown by isothermal titration calorimetry. Instead, we propose that the PEST sequence has a functional role unrelated to phosphotyrosine binding, possibly mediating efficient proteolytic degradation of the protein. The latter half of the kinase inhibitory region and the entire extended SH2 subdomain form a single alpha-helix. The mapping of the true SH2 domain, and the location of its C terminus more than 50 residues further downstream than predicted by sequence homology, explains a number of previously unexpected results that have shown the importance of residues close to the SOCS box for phosphotyrosine binding.

Serine phosphorylation of Stat6 negatively controls its DNA-binding function

In response to interleukin-4 (IL-4) or IL-13 stimulation of cells, Stat6 becomes phosphorylated on Tyr(641) and forms homodimers that migrate to the nucleus, bind to cognate DNA elements, and drive the transcription of target genes. Here, we show that phosphorylation of multiple serine residues ablates its DNA-binding activity in IL-4 stimulated cells. The phosphorylation sites are mapped to the transactivation domain (TAD) of Stat6. Importantly, serine phosphorylation of Stat6 TAD does not affect the phosphorylation of Tyr(641), nor does it affect the dimer formation or the ability of translocating to the nucleus in IL-4-stimulated cells. Collectively, these data suggest that phosphorylation of multiple serine residues in the TAD possibly induces conformational changes in Stat6 dimers that cause the loss of DNA binding and, thus, negatively control the expression of IL-4-responsive genes.

The conserved Leu-724 residue is required for both serine phosphorylation and co-activator recruitment for Stat1-mediated transcription activation in response to interferon-gamma

The signal transducer and activator of transcription (STAT) proteins, a family of latent cytoplasmic transcription factors, become activated in response to extracellular ligand binding to cell surface receptors through tyrosine phosphorylation. Concurrently, a serine phosphorylation event in the transcription activation domain (serine 727 for Stat1) occurs. This serine phosphorylation is essential for the maximal transcription activity of Stat1. Here we show that, in addition to the Ser-727 residue and its phosphorylation, the conserved Leu-724 residue is also essential for gene activation mediated by Stat1. When Leu-724 is mutated to Ala, phosphorylation of Stat1 Ser-727 is defective both in vivo and in vitro. Surprisingly, we found a StatL724I mutant that lacks transcription activity despite normal Ser-727 phosphorylation. Further analyses show that Leu-724, as well as the phospho-Ser-727, are essential for the recruitment of the transcription co-activator CBP/p300 to the promoters of Stat1 target genes. Our results demonstrate that the conserved Leu-724 residue is a key residue that controls the maximal transcription activities of Stat1 in IFN-gamma signaling.

Nuclear retention of STAT3 through the coiled-coil domain regulates its activity

Signal transducer and activator of transcription 3 (STAT3), which mediates biological actions in many physiological processes, is activated by cytokines and growth factors via specific tyrosine phosphorylation, dimerization, and nuclear translocation. However, the mechanism involved in its nuclear translocation remains unclear. A previous study demonstrated that STAT3 with Arg-214/215 mutations in the coiled-coil domain (R214A/R215A; STAT3 RA) failed to undergo nuclear translocation. Here, we re-examined the nature of the STAT3 RA mutant and found that it showed higher and more extensive tyrosine-phosphorylation as well as much higher STAT3 transcriptional activity in response to stimuli. Furthermore, STAT3 RA showed nuclear translocation and faster nuclear export than wild-type STAT3 after stimulation. Moreover, nuclear retention of STAT3 RA by a chromosomal region maintenance 1 (CRM1) inhibitor, leptomycin B, decreased the enhanced STAT3 activation by stimuli. These data demonstrate that Arg-214/215 are involved in CRM1-mediated STAT3 nuclear export and the regulation of STAT3 activity.

A Ras homologue member I directly inhibits signal transducers and activators of transcription 3 translocation and activity in human breast and ovarian cancer cells

A Ras homologue member I (ARHI) is a novel imprinted tumor suppressor gene whose expression is frequently lost in breast and ovarian cancers. This small GTP-binding protein is a member of the Ras superfamily with significant homology to both Ras and Rap. Unlike the Ras oncogene, however, ARHI inhibits tumor cell growth. To elucidate the mechanisms by which ARHI inhibits cancer growth, we screened a human breast epithelial cell cDNA library using a yeast two-hybrid system for ARHI-interacting proteins. ARHI was found to interact with signal transducers and activators of transcription (STAT) 3, a latent transcription factor that transduces signals from the cell surface to the nucleus and activates gene transcription. STAT3 is frequently phosphorylated and activated in breast and ovarian cancers, where cytokines and growth factors up-regulate STAT3 and stimulate proliferation. The ARHI-STAT3 interaction was confirmed by coimmunoprecipitation in mammalian cells and shown to be specific for STAT3 but not STAT1 or STAT5a. When ARHI and STAT3 were coexpressed in SKOv3 cells, ARHI formed a complex with STAT3 in the cytoplasm and prevented interleukin-6-induced STAT3 accumulation in the nucleus. ARHI markedly reduced STAT3 binding to DNA and STAT3-dependent promoter activity while only moderately affecting STAT3 phosphorylation. Deletion of the NH2 terminus of ARHI significantly compromised its inhibitory activity, suggesting that this unique NH2-terminal extension contributes to ARHI's inhibition of STAT3-mediated transcriptional activity. Thus, the physical association between STAT3 and ARHI as well as the functional inhibition of STAT3 transcriptional activity by ARHI suggests a novel mechanism through which a putative tumor suppressor gene can inhibit STAT3 activity in breast and ovarian cancers.

A Methylation-Dependent Electrostatic Switch Controls DNA Repair and Transcriptional Activation by E. coli Ada

The transcriptional activity of many sequence-specific DNA binding proteins is directly regulated by posttranslational covalent modification. Although this form of regulation was first described nearly two decades ago, it remains poorly understood at a mechanistic level. The prototype for a transcription factor controlled by posttranslational modification is E. coli Ada protein, a chemosensor that both repairs methylation damage in DNA and coordinates the resistance response to genotoxic methylating agents. Ada repairs methyl phosphotriester lesions in DNA by transferring the aberrant methyl group to one of its own cysteine residues; this site-specific methylation enhances tremendously the DNA binding activity of the protein, thereby enabling it to activate a methylation-resistance regulon. Here, we report solution and X-ray structures of the Cys-methylated chemosensor domain of Ada bound to DNA. The structures reveal that both phosphotriester repair and methylation-dependent transcriptional activation function through a zinc- and methylation-dependent electrostatic switch.

Modification of the Stat1 SH2 Domain Broadly Improves Interferon Efficacy in Proportion to p300/CREB-binding Protein Coactivator Recruitment

A normal level of interferon (IFN) responsiveness via the Stat1 transcription factor is critical to the host, since decreased Stat1 signaling causes immune compromise and increased signaling is associated with inflammatory and neoplastic disease. Here we report how this balance may be influenced by novel alterations in the efficiency of Stat1 signaling. To enable disulfide-dependent and spontaneous formation of active Stat1 homodimer (as was done previously for Stat3), we engineered Stat1-CC with double-cysteine substitutions in the Src homology 2 (SH2)-homodimerization domain (at Ala-656 and Asn-658). In this case, however, mutant and wild-type Stat1 exhibited no difference inspontaneousdimerization. Moreover, Stat1-CC still required ligand-dependent Tyr-701 phosphorylation for function and exhibited hyperresponsiveness to IFN-beta (that depends on Stat1/Stat2 heterodimerization) as well as IFN-gamma (that depends on Stat1/Stat1 homodimerization). Hyperresponsivenss of Stat1-CC was accompanied by increased capacities for Tyr-701 phosphorylation and DNA binding, but these features were also found in a similarly substituted serine mutant (Stat1-SS) that showed no hyperresponsiveness to IFN-gamma. This finding raised the possibility that SH2 domain mutations also influence downstream transcriptional efficiency. Indeed, each of these mutations also enhanced recruitment of the normally rate-limiting p300/CREB-binding Protein (CBP) coactivator to the transcriptional complex in proportion to the level of IFN-driven transactivation and gene expression. Additional modifications indicated that the mutant residues in the SH2 domain appeared to cooperate with Ser-727 in the C-terminal domain to regulate p300/CBP interaction with Stat1. The profile of IFN responsiveness translated into the same progressive increase in the level of viral clearance from Stat1- to Stat1-SS- to Stat1-CC-expressing cells. Thus, SH2 domain determinants may be modified to direct better Stat1 phosphorylation, DNA binding, and coactivator recruitment to fully improve IFN efficacy.

Cytokine and growth factor receptors in the nucleus: what's up with that?

Signaling via cell surface receptors that are anchored by a single transmembrane domain is a well-established paradigm. Ligand binding to the extracellular domain of the receptor facilitates receptor dimerization, which juxtaposes the intracellular domains, typically activating intrinsic or associated kinases. Two large families of tyrosine kinase activating receptors have been particularly well characterized: the receptor-type protein tyrosine kinases and the receptors for the alpha-helical cytokines, which activate non-covalently bound JAK family tyrosine kinases. Despite the well-established function of these receptors at the cell surface, both intact and cleaved receptors belonging to these families have been repeatedly detected in the nucleus. Furthermore, there is evidence that some of these receptors or receptor fragments function directly in modulating gene transcription. In this essay, I examine how close we are to demonstrating that direct translocation of receptors, or receptor fragments, from the cell surface to the nucleus is a physiologically relevant means of intracellular signaling that can supplant or complement canonical signaling cascades.

Transcriptional regulation of neuronal genes and its effect on neural functions: transcriptional regulation of neuropeptide Y gene by leptin and its effect on feeding

Leptin is an adipose tissue-derived secretory hormone that suppresses appetite by inhibition of neuropepeptide Y (NPY) gene expression in arcuate nucleus (ARC) in the hypothalamus. To investigate the transcriptional regulation of NPY gene by leptin, we carried out a luciferase assay using NPY gene promotor plasmid (NPY-luc) in NPY expressing cells such as N18TG2, NG108-15, and PC12 cells. In these cells, the NPY gene was transactivated by leptin through activation of leptin receptor. Leptin-induced transactivation was mediated through the 221-bp region of the NPY gene promotor, which possesses two putative STAT3 binding sites. To investigate the mechanism of in vivo suppression of NPY gene transcription in ARC by leptin, the effect of SOCS members on the leptin-induced transactivation of NPY gene was studied. In vivo SOCS2 and SOCS3 mRNAs were induced in mouse hypothalamus by leptin. Although leptin (125 ng/ml) induced significant increase in NPY gene transcriptional activity in mock-transfected cells, the leptin-induced NPY gene transcriptional activity was completely abolished in SOCS3-transfected cells. SOCS3 also suppressed the basal NPY gene transcription. These finding suggested that leptin inhibits NPY gene transcription in the hypothalamus in vivo and SOCS3 is a negative regulator of the NPY gene.

PI3K-AKT pathway negatively controls EGFR-dependent DNA-binding activity of Stat3 in glioblastoma multiforme cells

Glioblastoma multiforme (GBM) cells frequently harbor amplification and/or gain-of-function mutation of the EGFR gene leading to the activation of multiple signaling pathways. Blockade of EGFR activation inhibited the activation of both AKT and Stat3 in U87 and D54 GBM cells and induced spontaneous apoptosis, which were associated with reduction in the steady-state level of Mcl-1. Surprisingly, inhibition of PI3 kinase (PI3K) activity, which in turn inhibited AKT activation, significantly increased the DNA-binding activity of Stat3 in U87 and D54 cells. This was not due to an increase in the level of tyrosine-phosphorylated Stat3. Conversely, ectopic expression of constitutively activated AKT significantly decreased the DNA-binding activity of Stat3 in 293T cells. Interestingly, blockade of protein phosphatase 2A activity in GBM or 293T cells by calyculin A, which activated AKT, stabilized the phosphorylation of multiple Ser/Thr residues that were located in the transactivation domain (TAD) of Stat3 and this in turn completely ablated the DNA-binding activity of Stat3. Collectively, these results suggest that both Stat3 and AKT provide survival signals in U87 and D54 cells, and Ser/Thr phosphorylation of Stat3-TAD by the PI3K-AKT pathway negatively controls the DNA-binding function of Stat3.

Crystallization and X-ray crystallographic analysis of human STAT1

Unphosphorylated human STAT1 (1-683) has been crystallized in the presence of a phosphopeptide derived from the alpha-chain of human interferon gamma (IFNgamma) receptor. A complete data set has been collected from a KAu(CN)2-derivatized and dehydrated crystal. The crystal belonged to space group P6(1)22, with unit-cell parameters a = b = 102.6, c = 646.5 A, alpha = beta = 90, gamma = 120 degrees.

Extracellular signal-dependent nuclear import of STAT3 is mediated by various importin alphas

The signal transducer and activator of transcription 3 (STAT3) is a transcription factor that is involved in a variety of biological functions. STAT3 is activated by cytokines and growth factors via the phosphorylation of a tyrosine residue, dimerization, and subsequent nuclear translocation. However, the mechanism of its nuclear translocation is unclear. A study of the cytokine-stimulated import of STAT3 into the nucleus is reported herein. An oncostatin M (OSM)-dependent nuclear import assay system was first established in living cells. Using this system, we demonstrated that the microinjection of the importin alpha5/NPI-1 mutant, an anti-importin beta antibody, and the RanQ69L mutant inhibited the nuclear import of STAT3. Second, we showed that tyrosine-phosphorylated STAT3 associates, not only with importin alpha5/NPI-1 but also with other importin alphas, as a result of OSM stimulation, as evidenced by a solution binding assay. These findings suggest that the extracellular signal-dependent nuclear transport of STAT3 is mediated by various importin alphas, importin beta, and Ran.

B cell antigen receptor signaling enhances IFN-gamma-induced Stat1 target gene expression through calcium mobilization and activation of multiple serine kinase pathways

The signal transducers and activators of transcription 1 (Stat1) are essential for the majority of interferon-gamma (IFN-gamma)-regulated gene expression. Phosphorylation of serine 727 in the transcription activation domain of Stat1 is induced in response to IFN-gamma for maximal transcription activity. In this report, we show that crosslinking of B cell antigen receptor (BCR) or T cell antigen receptor (TCR) can enhance S727 phosphorylation in Stat1 and result in increased expression of Stat1 target genes. We further demonstrate that this enhancement by BCR cross-linking involves the widely used secondary messenger Ca2+ and simultaneous activation of multiple serine kinase pathways. When cells are exposed to both IFN-gamma and a Ca2+ fluxing reagent, the level of S727 phosphorylation is enhanced, resulting in increased transcription activation of Stat1 target genes. We directly demonstrate that the biochemical function of phospho-Ser-727 is to enhance the recruitment of transcription coactivator CBP/p300 to the promoters of Stat1 target genes. Furthermore, we show that both the p38 mitogen-activated protein kinase (MAPK) and the Ca(2+)/calmodulin-dependent kinase (CaMKII) are activated in response to BCR signaling to converge on Stat1 S727 for maximal gene expression. These studies demonstrate that a wide variety of noncytokine signaling pathways can modulate cytokine signaling through modulation of Stat1 serine phosphorylation.

Signal transducer and activator of transcription (STAT) signalling and T-cell lymphomas

Interaction of cytokines with their cognate receptors leads to the activation of latent transcription factors - the signal transducers and activators of transcription (STAT) proteins - whose biological activities ultimately regulate many critical aspects of cell growth, survival and differentiation. Dysregulation of the JAK-STAT pathway is frequently observed in many primary human tumours, reflecting the importance of this pathway in the maintenance of cellular integrity. Here we review the current progress in STAT structure and function, and the contribution of STAT signalling to the pathogenesis of T-cell lymphomas.

Issues in T-helper 1 development--resolved and unresolved

T-helper 1 cell (Th1) development participates in immunity to many pathogens in part by providing a source of interferon (IFN)-gamma that contributes numerous protective effects. The process of Th1 development involves signals provided by antigen-presenting cells and cytokines produced in response to pathogens, with IFN-gamma itself, interleukin (IL)-12, and IL-18 each promoting the process in some way. Despite the rapid progress into mechanisms of Th1 development in recent years, there are still a number of important unresolved issues in this area. The precise sequence of effector and cellular mechanisms represents a relatively recent avenue of research but is still the subject of current debate, as is the basis of mechanisms that may stabilize a Th1 response. Another unresolved issue is the role of type I IFNs in substituting for IL-12-mediated activation of signal transducer and activator of transcription 4 (Stat4) and induction of IFN-gamma in either murine or human T cells. It is now clear that Th1 cells acquire the property of being capable of nonantigen-dependent activation through the coordinate signaling of IL-12 and IL-18, but the precise order of intracellular signaling events and the uniqueness of this pathway's reliance on the p38 mitogen-activated protein kinase (MAPK) pathway are still issues in need of resolution. Finally, the process of verifying the effects of Stat4 mutations on functional responses has led to the recognition of an unexpected action of the STAT N-domain that may apply generally to other STAT proteins as well. None of these areas is static or resolved fully, and they likely will remain topics of rapid progress.

STATs as critical mediators of signal transduction and transcription: lessons learned from STAT5

Signal transducers and activators of transcription (Stats) comprise a family of seven transcription factors that are activated by a variety of cytokines, hormones and growth factors. Stats are activated through tyrosine phosphorylation, mainly by Jak kinases, that lead to their dimerization, nuclear translocation and regulation of target gene expression. Stat5 was originally identified as a transcription factor that regulates the beta-casein gene in response to prolactin (PRL), but Stat5 is activated also by several other cytokines and growth factors. The molecular mechanisms that underlie Stat5-mediated transcription involve interactions and cooperation with sequence specific transcription factors and transcriptional coregulators. Our studies identified p100 protein as a coactivator for Stat5, and suggest the existence of a positive regulatory loop in PRL-induced transcription, where PRL stabilizes p100 protein, which in turn can cooperate with Stat5 in transcriptional activation. Suppressors of cytokine signaling (SOCS) proteins are important negative regulators of Stats. A target gene for Stat5, the serine/threonine kinase Pim-1, was found to cooperate with SOCS-1 and SOCS-3 to inhibit Stat5 activity suggesting that Pim-1 together with SOCS-1 and SOCS-3 are components of a negative feedback mechanism that allows Stat5 to regulate its own activation.

Structural bases of unphosphorylated STAT1 association and receptor binding

The crystal structure has been determined at 3.0 A resolution for an unphosphorylated STAT1 (1-683) complexed with a phosphopeptide derived from the alpha chain of interferon gamma (IFNgamma) receptor. Two dimer interfaces are seen, one between the N domains (NDs) (amino acid residues 1-123) and the other between the core fragments (CFs) (residues 132-683). Analyses of the wild-type (wt) and mutant STAT1 proteins by static light scattering, analytical ultracentrifugation, and coimmunoprecipitation suggest that STAT1 is predominantly dimeric prior to activation, and the dimer is mediated by the ND interactions. The connecting region between the ND and the CF is flexible and allows two interconvertable orientations of the CFs, termed "antiparallel" or "parallel," as determined by SH2 domain orientations. Functional implications of these dimer conformations are discussed. Also revealed in this structure is the detailed interaction between STAT1 SH2 domain and its docking site on IFNgamma receptor.

Implications of an antiparallel dimeric structure of nonphosphorylated STAT1 for the activation-inactivation cycle

IFN-gamma treatment of cells leads to tyrosine phosphorylation of signal transducer and activator of transcription (STAT) 1 followed by dimerization through a reciprocal Src homology 2-phosphotyrosine interaction near the -COOH end of each monomer, forming a parallel structure that accumulates in the nucleus to drive transcription. Prompt dephosphorylation and return to the cytoplasm completes the activation-inactivation cycle. Nonphosphorylated STATs dimerize, and a previously described interface between N-terminal domain (ND) dimers has been implicated in this dimerization. A new crystal structure of nonphosphorylated STAT1 containing the ND dimer has two possible configurations for the body of STAT1, one of which is antiparallel. In this antiparallel structure, the Src homology 2 domains are at opposite ends of the dimer, with the coiled:coil domain of one monomer interacting reciprocally with the DNA-binding domain of its partner. Here, we find that mutations in either the coiled:coil/DNA-binding domain interface or the ND dimer interface block dimerization of nonphosphorylated molecules and cause a resistance to dephosphorylation in vivo and resistance to a tyrosine phosphatase in vitro. We conclude that a parallel STAT1 phosphodimer not bound to DNA most likely undergoes a conformational rearrangement (parallel to antiparallel) to present the phosphotyrosine efficiently for dephosphorylation.

Physical and Functional Interactions between STAP-2/BKS and STAT5

Signal-transducing adaptor protein family of proteins (STAPs), which currently contains two members, are proposed to be adaptor molecules because of their pleckstrin homology (PH) and Src-homology 2 (SH2)-like domains. STAP-1 has been shown to interact with STAT5 and the tyrosine kinase Tec. With regard to STAP-2/BKS functions, immunoprecipitation experiments and intracellular stainings revealed STAP-2/BKS binds STAT5 in several types of cells. Mutational studies revealed that the PH- and SH2-like domains of STAP-2/BKS interacted with the C-terminal region of STAT5. STAP-2/BKS and STAT5 were found to constitutively co-localize in the cytoplasm of resting cells, but STAP-2/BKS was found to dissociate upon STAT5 phosphorylation, suggesting a role in regulating signaling of STAT5. The physiological role of these interactions is not fully understood, but in studies of overexpression of STAP-2/BKS, cytokine-induced tyrosine phosphorylation and transcriptional activation of STAT5 was diminished. In addition, thymocytes from STAP-2/BKS-deficient mice showed the enhanced interleukin-2-dependent cell growth. Taken together, STAP-2/BKS is an additional modulator of STAT5-mediated signaling.

Stat3 dimerization regulated by reversible acetylation of a single lysine residue

Upon cytokine treatment, members of the signal transducers and activators of transcription (STAT) family of proteins are phosphorylated on tyrosine and serine sites within the carboxyl-terminal region in cells. We show that in response to cytokine treatment, Stat3 is also acetylated on a single lysine residue, Lys685. Histone acetyltransferase p300-mediated Stat3 acetylation on Lys685 was reversible by type I histone deacetylase (HDAC). Use of a prostate cancer cell line (PC3) that lacks Stat3 and PC3 cells expressing wild-type Stat3 or a Stat3 mutant containing a Lys685-to-Arg substitution revealed that Lys685 acetylation was critical for Stat3 to form stable dimers required for cytokine-stimulated DNA binding and transcriptional regulation, to enhance transcription of cell growth-related genes, and to promote cell cycle progression in response to treatment with oncostatin M.

Negative regulation of NF-kappaB signaling by PIAS1

The NF-kappaB family of transcription factors is activated by a wide variety of signals to regulate a spectrum of cellular processes. The proper regulation of NF-kappaB activity is critical, since abnormal NF-kappaB signaling is associated with a number of human illnesses, such as chronic inflammatory diseases and cancer. We report here that PIAS1 (protein inhibitor of activated STAT1) is an important negative regulator of NF-kappaB. Upon cytokine stimulation, the p65 subunit of NF-kappaB translocates into the nucleus, where it interacts with PIAS1. The binding of PIAS1 to p65 inhibits cytokine-induced NF-kappaB-dependent gene activation. PIAS1 blocks the DNA binding activity of p65 both in vitro and in vivo. Consistently, chromatin immunoprecipitation assays indicate that the binding of p65 to the promoters of NF-kappaB-regulated genes is significantly enhanced in Pias1-/- cells. Microarray analysis indicates that the removal of PIAS1 results in an increased expression of a subset of NF-kappaB-mediated genes in response to tumor necrosis factor alpha and lipopolysaccharide. Consistently, Pias1 null mice showed elevated proinflammatory cytokines. Our results identify PIAS1 as a novel negative regulator of NF-kappaB.

Novel Roles of Unphosphorylated STAT3 in Oncogenesis and Transcriptional Regulation

Signal transducer and activator of transcription 3 (STAT3) is phosphorylated on tyrosine residue 705 in response to growth factors or cytokines to form activated homodimers that drive gene expression. Because the stat3 promoter has a binding site for STAT3 dimers, the amount of STAT3 protein increases when STAT3 is activated (e.g., in response to interleukin 6). Unphosphorylated STAT1 is known to drive the expression of certain genes. To explore the possibility of a similar role for the induced expression of unphosphorylated STAT3, we overexpressed either Y705F STAT3, which can not be phosphorylated on residue 705, or wild-type STAT3 in normal human mammary epithelial cells or STAT3-null mouse cells. The levels of many mRNAs were affected strongly by high levels of either form of STAT3. Some genes whose expression was increased by overexpressed STAT3, but not by activated STAT3 dimers, encode well-known oncoproteins (e.g., MRAS and MET). In many tumors, STAT3 is activated constitutively, and thus the unphosphorylated form is likely to be expressed highly, driving oncogene expression by a novel mechanism. In addition, expression of the stat3 gene is increased strongly in response to interleukin 6, and the high levels of unphosphorylated STAT3 that result drive a substantial late phase of gene expression in response to this cytokine. Thus, unphosphorylated STAT3, which activates gene expression by a novel mechanism distinct from that used by STAT3 dimers, is very likely to be an important transcription factor both in cancer and in responses to cytokines.

Functional relevance of the conserved DNA-binding domain of STAT2

Several distinct type I interferon (IFN)-inducible STAT2-containing complexes have been identified. For the IFN-stimulated gene factor 3 (ISGF3), STAT1 and IRF-9 mediate IFN-stimulated response element (ISRE) binding, whereas STAT2 provides a potent transactivational domain. ISGF3-independent STAT2-containing complexes, specifically STAT2:1 and STAT2:3, bind a gamma-activated sequence (GAS)-like element, yet the contribution of each STAT to DNA binding is unknown. Moreover, the contribution of these ISGF3-independent STAT2-containing complexes to IFN-inducible responses is not defined. Accordingly, we generated mutant cDNAs, targeting the DNA-binding domain in STAT2. These cDNAs were introduced by transfection into U6A cells lacking STAT2, resulting in a panel of cell lines expressing mutant STAT2 proteins. Studies assessed the sensitivity of U6A cells reconstituted with intact STAT2 (U6A-2) and cells expressing mutant STAT2s (U6A-2E426A,E427A (EE-AA), U6A-2V453I, U6A-2V454I, U6A-2V454A, U6A-2V453I,V454I(VV-II), U6A-2N458A) to IFN-inducible responses. Our data reveal that none of the mutations in the STAT2 DNA-binding domain affected IFN-inducible ISGF3 activation, and only the VV-II mutation restricted antiviral and growth inhibitory responses to IFN. Indeed, U6A-2VV-II cells are refractory to these IFN-inducible biological activities and also exhibit impaired IFN-inducible GAS-driven transcriptional activation and subsequent gene expression. Chromatin immunoprecipitation assays revealed that the VV-II mutation in STAT2 does not abrogate, but reduces the DNA binding activity of STAT2:1 heterodimers. Taken together, these data suggest a role for the conserved DNA-binding domain of STAT2 specific to the activity of ISGF3-independent STAT2-containing complexes.

Coactivators in Gene Regulation by STAT5

Signal transducer and activator of transcription 5 (STAT5) is a member of the STAT family of transcription factors that relay the effect of diverse cytokines, hormones, and growth factors by regulating the transcription of distinct target genes. This function is emphasized by its crucial role in the development of the mammary gland and the hematopoietic system. Cytokine receptor-associated Janus kinases (JAKs) induce dimerization, nuclear translocation, and DNA binding through tyrosine phosphorylation of STAT5. STAT5 regulates the expression of cytokine target genes by binding to gamma interferon-activated sequence (GAS) motifs. Transcriptional activation requires the contact of STAT5 to coactivators and components of the transcription machinery. Another important point in transcriptional activation is the cooperation with other transcription factors that bind in close vicinity to the target gene promoters and enhancers. Their concerted action can result in an enhanced binding to the promoters or in cooperative recruitment of coactivators. In addition, cross-talk with other signaling pathways as well as secondary modifications of STAT5 have been described to affect transactivation function.

Roles and regulation of stat family transcription factors in human breast cancer

Stats (for signal transducers and activators of transcription) are a family of transcription factors that regulate cell growth and differentiation. Their activity is latent until phosphorylation by receptor-associated kinases. A sizable body of data from cell lines, mouse models, and human tissues now implicates these transcription factors in the oncogenesis of breast cancer. Because Stat activity is modulated by several posttranslational modifications and protein-protein interactions, these transcription factors are capable of integrating inputs from multiple signaling networks. Given this, the future utilization of Stats as prognostic markers and therapeutic targets in human breast cancer appears likely.

G-quartet oligonucleotides: a new class of signal transducer and activator of transcription 3 inhibitors that suppresses growth of prostate and breast tumors through induction of apoptosis

Stat3 is a signaling molecular and oncogene activated frequently in many human malignancies including the majority of prostate, breast, and head and neck cancers; yet, no current chemotherapeutic approach has been implemented clinically that specifically targets Stat3. We recently developed G-rich oligodeoxynucleotides, which form intramolecular G-quartet structures (GQ-ODN), as a new class of Stat3 inhibitor. GQ-ODN targeted Stat3 protein directly inhibiting its ability to bind DNA. When delivered into cells using polyethyleneimine as vehicle, GQ-ODN blocked ligand-induced Stat3 activation and Stat3-mediated transcription of antiapoptotic genes. To establish the effectiveness of GQ-ODN as a potential new chemotherapeutic agent, we systemically administered GQ-ODN (T40214 or T40231) plus polyethyleneimine or polyethyleneimine alone (placebo) by tail-vein injection into nude mice with prostate and breast tumor xenografts. Whereas the mean volume of breast tumor xenografts in placebo-treated mice increased >7-fold over 18 days, xenografts in the GQ-ODN-treated mice remained unchanged. Similarly, whereas the mean volume of prostate tumor xenografts in placebo-treated mice increased 9-fold over 10 days, xenografts in GQ-ODN-treated mice increased by only 2-fold. Biochemical examination of tumors from GQ-ODN-treated mice demonstrated a significant reduction in Stat3 activation, levels of the antiapoptotic proteins Bcl-2 and Bcl-xL, and an 8-fold increase in the number of apoptotic cells compared with the tumors of placebo-treated mice. Thus, GQ-ODN targeting Stat3 induces tumor cell apoptosis when delivered into tumor xenografts and represents a novel class of chemotherapeutic agents that holds promise for the systemic treatment of many forms of metastatic cancer.

Interferon induces the interaction of prothymosin-alpha with STAT3 and results in the nuclear translocation of the complex

Interferons (IFNs) play critical roles in host defense by modulating the expression of various genes via tyrosine phosphorylation of STAT transcription factors. Many cytokines including IFNs induce tyrosine phosphorylation of the STAT3 transcription factor, which regulates acute phase gene expression. Using the yeast two-hybrid interaction trap, in which a tyrosine kinase is introduced into the yeast to allow tyrosine phosphorylation of bait proteins, prothymosin-alpha (ProTalpha) was identified to interact with the amino terminal half of tyrosine-phosphorylated STAT3. ProTalpha is a small, acidic, extremely abundant, and essential protein that may play a role in chromatin remodeling, and has been implicated in regulating the growth and survival of mammalian cells. Besides the interaction of tyrosine-phosphorylated STAT3 with ProTalpha in yeast cells, IFN induced the interaction of ProTalpha with STAT3 in mammalian cells, and this interaction was dependent on the tyrosine phosphorylation of STAT3. Moreover, IFNalpha induces the translocation of STAT3 and ProTalpha from the cytoplasm to the nucleus where these proteins colocalize. Since ProTalpha has an extremely strong nuclear localization and STAT proteins apparently lack any nuclear localization signals, the association of STAT3 with ProTalpha may provide a mechanism to result in STAT localization in the nucleus.

Analysis and prediction of leucine-rich nuclear export signals

We present a thorough analysis of nuclear export signals and a prediction server, which we have made publicly available. The machine learning prediction method is a significant improvement over the generally used consensus patterns. Nuclear export signals (NESs) are extremely important regulators of the subcellular location of proteins. This regulation has an impact on transcription and other nuclear processes, which are fundamental to the viability of the cell. NESs are studied in relation to cancer, the cell cycle, cell differentiation and other important aspects of molecular biology. Our conclusion from this analysis is that the most important properties of NESs are accessibility and flexibility allowing relevant proteins to interact with the signal. Furthermore, we show that not only the known hydrophobic residues are important in defining a nuclear export signals. We employ both neural networks and hidden Markov models in the prediction algorithm and verify the method on the most recently discovered NESs. The NES predictor (NetNES) is made available for general use at http://www.cbs.dtu.dk/.

Methylation of STAT6 modulates STAT6 phosphorylation, nuclear translocation, and DNA-binding activity

Signal transducer and activator of transcription 6 is a transcription factor important for the development of Th2 cells and regulation of gene expression by IL-4 and IL-13. It has been reported that STAT1 activity is regulated by methylation of a conserved arginine residue in the N-terminal domain. Methylation of STAT6 has not yet been explored. We observed methylation of STAT6 in cells transfected with wild-type STAT6, but not in cells transfected with Arg(27)Ala mutant, confirming that STAT6 is methylated on Arg(27). Transfectants expressing mutant Arg(27)Ala STAT6 displayed markedly diminished IL-4-dependent STAT6 phosphorylation and nuclear translocation, and no STAT6 DNA-binding activity compared with wild-type STAT6 transfectants. To confirm this, the experiments were repeated using inhibitors of methylation. In the presence of methylation inhibitors, STAT6 methylation was diminished, as was phosphorylation of STAT6 and STAT6 DNA-binding activity. Thus, methylation is a critical regulator of STAT6 activity, necessary for optimal STAT6 phosphorylation, nuclear translocation, and DNA-binding activity. Furthermore, methylation of STAT6 has distinct effects from those reported with STAT1.

Characterization of the Signaling Capacities of the Novel gp130-like Cytokine Receptor

The gp130-like receptor (GPL) is a recently cloned member of the family of type I cytokine receptors. The name reflects its close relationship to gp130, the common receptor subunit of the interleukin (IL)-6-type cytokines. Indeed, the recently proposed ligand for GPL, IL-31, is closely related to the IL-6-type cytokines oncostatin M, leukemia inhibitory factor, and cardiotrophin-1. The second signal transducing receptor for IL-31 seems to be the oncostatin M receptor beta (OSMRbeta). The present study characterizes in depth the molecular mechanisms underlying GPL-mediated signal transduction. GPL is a strong activator of STAT3 and STAT5, whereas STAT1 is only marginally tyrosine-phosphorylated. We identify tyrosine residues 652 and 721 in the cytoplasmic region of the longest isoform of GPL (GPL(745)) as the major STAT5- and STAT3-activating sites, respectively. Additionally, we demonstrate Jak1 binding to GPL and its activation in heteromeric complexes with the OSMRbeta but also in a homomeric receptor complex. Most interesting, unlike OSMRbeta and gp130, GPL is insufficient to mediate ERK1/2 phosphorylation. We propose that this is due to a lack of recruitment of the tyrosine phosphatase SHP-2 or the adaptor protein Shc to the cytoplasmic domain of GPL.

Identification of the nuclear export signal and STAT-binding domains of the Nipah virus V protein reveals mechanisms underlying interferon evasion

The V proteins of Nipah virus and Hendra virus have been demonstrated to bind to cellular STAT1 and STAT2 proteins to form high-molecular-weight complexes that inhibit interferon (IFN)-induced antiviral transcription by preventing STAT nuclear accumulation. Analysis of the Nipah virus V protein has revealed a region between amino acids 174 and 192 that functions as a CRM1-dependent nuclear export signal (NES). This peptide is sufficient to complement an export-defective human immunodeficiency virus Rev protein, and deletion and substitution mutagenesis revealed that this peptide is necessary for both V protein shuttling and cytoplasmic retention of STAT1 and STAT2 proteins. However, the NES is not required for V-dependent IFN signaling inhibition. IFN signaling is blocked primarily by interaction between Nipah virus V residues 100 to 160 and STAT1 residues 509 to 712. Interaction with STAT2 requires a larger Nipah virus V segment between amino acids 100 and 300, but deletion of residues 230 to 237 greatly reduced STAT2 coprecipitation. Further, V protein interactions with cellular STAT1 is a prerequisite for STAT2 binding, and sequential immunoprecipitations demonstrate that V, STAT1, and STAT2 can form a tripartite complex. These findings characterize essential regions for Henipavirus V proteins that represent potential targets for therapeutic intervention.

Characterization of critical interactions between Ndt80 and MSE DNA defining a novel family of Ig-fold transcription factors

The Ndt80 protein of the yeast Saccharomyces cerevisiae is the founding member of a new sub-family of proteins in the Ig-fold superfamily of transcription factors. The crystal structure of Ndt80 bound to DNA shows that it makes contacts through several loops on one side of the protein that connect beta-strands which form the beta-sandwich fold common to proteins in this superfamily. However, the DNA-binding domain of Ndt80 is considerably larger than many other members of the Ig-fold superfamily and it appears to make a larger number of contacts with the DNA than these proteins. To determine the contribution of each of these contacts and to examine if the mechanism of Ndt80 DNA binding was similar to other members of the Ig-fold superfamily, amino acid substitutions were introduced at each residue that contacts the DNA and assayed for their effect on Ndt80 activity. Many of the mutations caused significant decreases in DNA-binding affinity and transcriptional activation. Several of these are in residues that are not found in other sub-families of Ig-fold proteins. These additional contacts are likely responsible for Ndt80's ability to bind DNA as a monomer while most other members require additional domains or cofactors to recognize their sites.