The coiled-coil domain of Stat3 is essential for its SH2 domain-mediated receptor binding and subsequent activation induced by epidermal growth factor and interleukin-6

Preassociation of nonactivated STAT3 molecules demonstrated in living cells using bioluminescence resonance energy transfer: a new model of STAT activation?

Signal transducers and activators of transcription (STATs) are crucial molecules in cytokine signaling. In the conventional model of STAT activation, STAT molecules are recruited from a latent pool of cytoplasmic monomers to the activated cytokine receptor. After binding to the receptor, they get tyrosine-phosphorylated, dissociate from the receptor, and translocate to the nucleus as activation-induced dimers. Recently, several publications questioned this model of STAT activation and showed the existence of preassociated STAT molecules before activation. We were able to demonstrate the existence of these preassociated STAT3 molecules in living mammalian cells using bioluminescence resonance energy transfer. Our results support the new hypothesis that STAT molecules exist in the cytoplasm as dimers or multimers and point to an activation-induced change in STAT3 conformation. Therefore, we propose a new model of STAT activation and discuss a hypothetical structure of "cytoplasmic" STAT dimers as opposed to the known "activation-induced" dimer.

Grb2 regulates Stat3 activation negatively in epidermal growth factor signalling

EGF (epidermal growth factor) binding to its receptor (EGFR) induces dimerization and autophosphorylation of the receptor at multiple tyrosine residues, which serve as docking sites for recruitment of proteins with SH2 (Src homology 2) domains that activate multiple downstream signalling pathways. The adaptor protein Grb2 (growth factor receptor-binding protein 2) binds to EGFR, which leads to activation of Ras-MAPK (mitogen-activated protein kinase) cascade. The latent transcription factors, STAT (signal transduction and activator of transcription), can also be activated by EGF in certain cell types. Since Ras-MAPK and STAT pathways are simultaneously stimulated by EGF, and Tyr-1086 and Tyr-1068 of EGFR are reported to be the binding sites for both Grb2 and Stat3, we investigated the possible regulatory role of Grb2 in STAT activation. In the present study, we report that transient expression of Grb2 specifically down-regulates EGF-stimulated tyrosine phosphorylation of Stat3, which leads to a repression of Stat3 transcriptional activity. In contrast, depletion of Grb2 by RNA interference substantially increases Stat3 tyrosine phosphorylation induced by EGF. The inhibition is neither mediated by a direct interaction between Grb2 and Stat3 nor via activation of tyrosine phosphatases. However, the repression was abolished by a mutation in the SH2 domain, but not the SH3 domains of Grb2, suggesting that inhibition involves binding of the receptor. Indeed, Grb2 inhibits the interaction between Stat3 and EGFR by competitive binding to the EGFR. On the other hand, Grb2 does not interact with the same sites as Stat3 on the interleukin-6 receptor and, therefore, has no effect on interleukin-6-induced tyrosine phosphorylation of Stat3. Taken together, our results demonstrate that, in EGF signalling, Grb2 regulates Stat3 activation negatively at the receptor level.

STAT4 requires the N-terminal domain for efficient phosphorylation

STAT4 (signal transducer and activator of transcription-4) mediates biological effects in response to interleukin-12 (IL-12). STAT4 has multiple domains that have distinct functions in signaling and gene activation. To characterize the role of the STAT4 N-terminal domain in mediating STAT4 biological function, we have generated STAT4-deficient transgenic mice that express human full-length STAT4 or an N-terminal deletion mutant (Delta N-STAT4) lacking the N-terminal 51 amino acids. Whereas full-length STAT4 rescued IL-12 responsiveness, T lymphocytes expressing the STAT4 N-terminal mutant failed to proliferate in response to IL-12 and had limited Th1 cell development as evidenced by minimal interferon-gamma production. Deletion of the N-terminal domain resulted in failure of STAT4 to be phosphorylated following IL-12 stimulation despite similar phosphorylation of JAK2 and TYK2 in full-length STAT4 and Delta N-STAT4 transgenic T cells. We demonstrate that the lack of phosphorylation in cultured cells is due to reduced efficiency of phosphorylation of Delta N-STAT4 by Janus kinases. These data support a new model wherein the N-terminal domain is required to mediate the phosphorylation of STAT4 in addition to the previously documented role in gene transactivation.

The role of Kaposi's sarcoma-associated herpesvirus/human herpesvirus-8 regulator of transcription activation (RTA) in control of gene expression

The mechanisms that control the replication state, latency versus lytic, of human herpesviruses have been under intense investigations. Here we summarize some of the recent findings that help define such mechanisms for Kaposi's sarcoma-associated herpesvirus/human herpesvirus type 8 (KSHV/HHV-8). For HHV-8, the viral regulator of transcription activation (RTA) is a key mediator of the switch from latency to lytic gene expression in infected cells. RTA is necessary and sufficient to drive HHV-8 lytic replication and the production of viral progeny. The RTA is an immediate-early gene product, it is the initial activator of expression of a multitude of viral and cellular genes that have been implicated in the replication of HHV-8 and pathogenesis of KS. Interactions of RTA with a number of viral promoters, and with a number of transcription factors or transcriptional co-activators are highlighted. Modulation of transactivation, through alternate RTA-protein, or RTA-promoter interactions, is hypothesized to participate in the selective tissue tropism and differential pathogenesis observed in KS.

A novel sequence in the coiled-coil domain of Stat3 essential for its nuclear translocation

Stat3 is activated by cytokines and growth factors via specific tyrosine phosphorylation, dimerization, and nuclear translocation. However, the mechanism involved in its nuclear translocation is unclear. In this study, by systematic deletion and site-directed mutagenesis we identified Arg-214/215 in the alpha-helix 2 region of the coiled-coil domain of Stat3 as a novel sequence element essential for its nuclear translocation, stimulated by epidermal growth factor as well as by interleukin-6. Furthermore, we identified Arg-414/417 in the DNA binding domain as also required for the nuclear localization of Stat3. This sequence element corresponds to Lys-410/413 of Stat1, a reported sequence for Stat1 nuclear translocation. On the other hand, Leu-411 of Stat3, corresponding to Leu-407 of Stat1, a necessary residue for Stat1 nuclear transport, is not essential for Stat3 nuclear import. The mutant of Arg-214/215 or Arg-414/417 was shown to be tyrosyl-phosphorylated normally but failed to enter the nucleus in response to epidermal growth factor or interleukin-6. The defect, however, can be rescued by the wild-type Stat3 but cannot be compensated by these two mutants. Mutations on Arg-414/417, but not Arg-214/215, destroy the DNA binding activity of Stat3. Our data for the first time identified a sequence element located in the coiled-coil domain that is involved in the ligand-induced nuclear translocation of Stat3. This novel sequence together with a conserved sequence element in the DNA binding domain coordinates to mediate the nuclear translocation of Stat3.

Suppressor of cytokine signaling 1/JAB and suppressor of cytokine signaling 3/cytokine-inducible SH2 containing protein 3 negatively regulate the signal transducers and activators of transcription signaling pathway in normal human epidermal keratinocytes

The suppressor of cytokine signaling/cytokine-inducible SH2 containing proteins are cytokine inducible and are negative regulators of the signal transducers and activators of the transcription signaling pathway. We investigated the mechanism regulating signal transducers and activators of transcription and the suppressor of cytokine signaling/cytokine-inducible SH2 containing protein family in keratinocytes, one of the major target cells for cytokines. Suppressor of cytokine signaling 1 mRNA was upregulated 3 h post-interferon gamma, and a 8.1-fold increase in the suppressor of cytokine signaling 1 mRNA occurred 48 h post-interferon gamma. The suppressor of cytokine signaling 3 mRNA was also upregulated from 1 h post-interferon gamma, and a 6.7-fold increase in the suppressor of cytokine signaling 3/cytokine-inducible SH2 containing protein 3 mRNA occurred between 6 and 12 h post-interferon gamma. Interleukin-6 exposure for 1 h enhanced the expression of the suppressor of cytokine signaling 3/cytokine-inducible SH2 containing protein 3 mRNA, but the suppressor of cytokine signaling 1/JAB mRNA was not induced by interleukin-6. Interleukin-4 upregulated the suppressor of cytokine signaling 1/JAB and cytokine-inducible SH2 containing protein 1 mRNA, with 3.4-fold and 5.1-fold increases in mRNA observed at 1 h post-interleukin-4, respectively. In contrast, epidermal growth factor, which phosphorylates signal transducers and activators of transcription 3, did not influence the level of the suppressor of cytokine signaling/cytokine-inducible SH2 containing protein family mRNA expression. Transfection of an adenovirus vector expressing the suppressor of cytokine signaling 1/JAB completely inhibited interferon gamma-dependent signal transducers and activators of transcription 1 phosphorylation and interleukin-4-dependent signal transducers and activators of transcription 6 phosphorylation. Transfection of adenovirus vector expressing the suppressor of cytokine signaling 1/JAB did not inhibit interleukin-6-dependent signal transducers and activators of transcription 3 phosphorylation-several reports show that the suppressor of cytokine signaling 1/JAB is a potent inhibitor of signal transducers and activators of transcription 3 signaling in the myeloid leukemia M1 cell. Transfection of the adenovirus vector expressing suppressor of cytokine signaling 3/cytokine-inducible SH2 containing protein 3 completely inhibited interleukin-6-dependent signal transducers and activators of transcription 3 phosphorylation and partially inhibited interferon gamma-dependent signal transducers and activators of transcription 1 phosphorylation. Transfection of the adenovirus vector expressing suppressor of cytokine signaling 3/cytokine-inducible SH2 containing protein 3, however, did not inhibit interleukin-4-dependent signal transducers and activators of transcription 6 phosphorylation. Transfection of the adenovirus vector expressing cytokine-inducible SH2 containing protein 1 had no effect on signal transducers and activators of transcription 1, 3, and 6 signaling in normal keratinocytes. Therefore, the relationship between signal transducers and activators of transcription and suppressor of cytokine signaling is unique in the keratinocytes, and the suppressor of cytokine signaling regulates cytokine signals in these cells.

GRIM-19, a death-regulatory gene product, suppresses Stat3 activity via functional interaction

Signal transducer and activator of transcription 3 (Stat3) is a latent cytoplasmic transcription factor that can be activated by cytokines and growth factors. Stat3 plays important roles in cell growth, anti-apoptosis and cell transformation, and is constitutively active in various cancers. We examined its potential regulators by yeast two-hybrid screening. GRIM-19, a gene product related to interferon-beta- and retinoic acid-induced cancer cell death, was identified and demonstrated to interact with Stat3 in various cell types. The interaction is specific for Stat3, but not for Stat1 and Stat5a. The interaction regions in both proteins were mapped, and the cellular localization of the interaction was examined. GRIM-19 itself co-localizes with mitochondrial markers, and forms aggregates at the perinulear region with co-expressed Stat3, which inhibits Stat3 nuclear translocation stimulated by epidermal growth factor (EGF). GRIM-19 represses Stat3 transcriptional activity and its target gene expression, and also suppresses cell growth in Src-transformed cells and a Stat3-expressing cell line. Our data suggest that GRIM-19 is a novel negative regulator of Stat3.

Protein kinase C delta associates with the interleukin-6 receptor subunit glycoprotein (gp) 130 via Stat3 and enhances Stat3-gp130 interaction

The transcriptional regulation of Stat proteins is controlled through their C-terminal domains, which harbor both a tyrosine phosphorylation site, required for dimerization and subsequent nuclear translocation, and a serine phosphorylation site, required for maximum transcriptional activity. Previously, we reported that protein kinase Cdelta (PKCdelta) phosphorylates and interacts with Stat3 in an interleukin (IL)-6-dependent manner. In this study, we further characterized this interaction, and investigated the potential role of such an interaction. We show here that the catalytic domain of PKCdelta interacts with the Src homology 2 domain and part of the adjacent C-terminal transactivation domain of Stat3. This interaction, which does not seem to involve a classical phosphotyrosine SH2-mediated binding, however, significantly enhances the interaction of Stat3 and the IL-6 receptor subunit glycoprotein (gp) 130, which is the initial step for Stat3 activation by IL-6. Expression of a dominant negative PKCdelta or depletion of the endogenous PKCdelta by phorbol 12-myristate 3-acetate treatment abrogates the association of Stat3 with gp130. At the same time, PKCdelta is recruited to gp130 via association with Stat3, which may facilitate its phosphorylation on the gp130 receptor. Finally, we identified Thr-890, a putative PKC phosphorylation site on gp130, to be critical for the effect of PKCdelta. Our data indicate that PKCdelta plays important regulatory roles in IL-6 signaling.

Interdomain interaction of Stat3 regulates its Src homology 2 domain-mediated receptor binding activity

Activation of Stat proteins by cytokines is initiated by their Src homology 2 (SH2) domain-mediated association with the cytokine receptors. Previously, we identified an essential role of the coiled-coil domain of Stat3 in binding of the receptor peptides derived from the interleukin-6 receptor subunit, gp130. In this study, we further investigated the molecular basis of this regulation. We found that the C-terminal domain of Stat3 negatively regulates its receptor binding activity only in the absence of the first alpha-helix of the coiled-coil domain, which leads to a hypothesis of intramolecular interaction. Physical interactions between the coiled-coil domain and the C-terminal domain, as well as the SH2 domain, were indeed detected. Furthermore, a sub-region of the C-terminal domain (amino acids 720-740), which is also involved in the interaction with the coiled-coil domain, was demonstrated to be critical for the regulation of the receptor binding. Correspondingly, phosphorylation on Ser-727 within this region inhibits this interaction. In agreement with the peptide binding results, both the coiled-coil domain and the C-terminal sub-region are necessary for the functional recruitment of Stat3 to the cellular gp130 in response to interleukin-6, suggesting that the interdomain interaction is a prerequisite for the SH2-mediated receptor binding in interleukin-6 signaling.

The Stat5-RARalpha fusion protein represses transcription and differentiation through interaction with a corepressor complex

The transcription factor Stat5 mediates the cellular response to activation of multiple cytokine receptors involved in the regulation of proliferation and differentiation of hematopoietic cells. Recently, the human Stat5 gene was found to be translocated to the RARalpha gene in a patient with acute promyelocytic leukemia indicating that Stat5 might also play a role in cellular transformation. We investigated the mechanism by which Stat5 might exert this function and studied the biochemical and cellular functions of fusion proteins comprising Stat5 and RARalpha. The expression of Stat5-RARalpha causes the transcriptional repression of gene transcription, a process that requires the coiled-coil domain of Stat5 (amino acid positions 133-333). Oligomerization of this domain in the Stat5-RARalpha fusion protein leads to stable binding of the corepressor SMRT independent of all-trans retinoic acid (ATRA) stimulation and is accompanied by an impaired response to differentiation signals in hematopoietic cells. This inhibitory effect on myeloid differentiation cannot be overcome by simultaneous coexpression of RARalpha. We conclude that Stat5 is capable of interacting with a corepressor complex that alters the pattern of corepressor binding to RARalpha and its dissociation in response to ATRA stimulation, leading to enhanced repressor activity and a block of hematopoietic differentiation.

Signaling through the JAK/STAT pathway, recent advances and future challenges

Investigation into the mechanism of cytokine signaling led to the discovery of the JAK/STAT pathway. Following the binding of cytokines to their cognate receptor, signal transducers and activators of transcription (STATs) are activated by members of the janus activated kinase (JAK) family of tyrosine kinases. Once activated, they dimerize and translocate to the nucleus and modulate the expression of target genes. During the past several years significant progress has been made in the characterization of the JAK/STAT signaling cascade, including the identification of multiple STATs and regulatory proteins. Seven STATs have been identified in mammals. The vital role these STATs play in the biological response to cytokines has been demonstrated through the generation of murine 'knockout' models. These mice will be invaluable in carefully elucidating the role STATs play in regulating the host response to various stresses. Similarly, the solution of the crystal structure of two STATs has and will continue to facilitate our understanding of how STATs function. This review will highlight these exciting developments in JAK/STAT signaling.

MSK1 and JNKs mediate phosphorylation of STAT3 in UVA-irradiated mouse epidermal JB6 cells

Phosphorylation of Tyr(705) and Ser(727) of signal transducer and activator of transcription 3 (STAT3) are known to be required for maximal activation by diverse stimuli. Tyr(705) phosphorylation is generally accepted to be mediated by the Janus kinase family. But the mechanism for STAT3 (Ser(727)) phosphorylation is not well understood. Here, we provide evidence that UVA-induced phosphorylation of STAT3 at Ser(727) is inhibited by pretreatment of JB6 cells with PD98059 or SB202190. Phosphorylation of STAT3 (Ser(727)) is also markedly prevented by a dominant negative mutant of ERK2, c-Jun N-terminal kinase 1 (JNK1), or p38 kinase and in knockout Jnk1(-/-) or Jnk2(-/-) cells. Furthermore, STAT3 (Ser(727)) phosphorylation is suppressed by C- or N-terminal "kinase-dead" mutants of mitogen- and stress-activated protein kinase 1 (MSK1), a downstream kinase of ERKs and p38 kinase, and H89, a potential MSK1 inhibitor. In vitro experiments showed that active MSK1 and JNKs, but not ERKs or p38 kinase, phosphorylate STAT3 (Ser(727)). Additionally, the role of MAPKs in mediating UVA-stimulated DNA binding activity of STAT3 was investigated. Overall, these results suggest that UVA-induced Ser(727) phosphorylation of STAT3 may occur through MSK1 and JNKs.