Requirement of serine phosphorylation for formation of STAT-promoter complexes

S179D prolactin: antagonistic agony!

The aims of this review are three-fold: first, to collate what is known about the production and activities of phosphorylated prolactin (PRL), the latter largely, but not exclusively, as illustrated through the use of the molecular mimic, S179D PRL; second, to apply this and related knowledge to produce an updated model of prolactin-receptor interactions that may apply to other members of this cytokine super-family; and third, to promote a shift in the current paradigm for the development of clinically important growth antagonists. This third aim explains the title since, based on results with S179D PRL, it is proposed that agents which signal to antagonistic ends may be better therapeutics than pure antagonists-hence antagonistic agony. Since S179D PRL is not a pure antagonist, we have proposed the term selective prolactin receptor modulator (SPeRM) for this and like molecules.

The role of p38 mitogen-activated protein kinase in regulating interleukin-10 gene expression in Burkitt's lymphoma cell lines

In malignant B lymphoma cells interleukin-10 (IL-10) expression is frequently upregulated. This effect is thought to support to the malignant transformation of these cells and to be a potential target for pharmacotherapy. To define better the mechanism for upregulation of the IL-10 gene, we tested the association between IL-10 and p38 mitogen-activated protein kinase (MAPK) in several Epstein-Barr virus (EBV) infected and non-infected Burkitt's lymphoma (BL) cell lines. The all BL cell lines expressed IL-10 and IL-10 receptor mRNAs, and produced IL-10. p38 MAPK was constitutively phosphorylated in the cytoplasm of the BL cell lines. We further analyzed molecular effects of p38 MAPK on IL-10 expression in Akata cells. Exogenous IL-10 lead rapidly to phosphorylation of Jak1 and Tyk2 as transducers of signals of IL-10, and promoted growth of Akata cells in a dose-dependent manner. The phosphorylation of cytoplasmic p38 MAPK in Akata cells was reduced by the serine/threonine kinase inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7). A specific inhibitor of p38 MAPK, SB203580, blocked simultaneously STAT3 DNA-binding activity, and IL-10 mRNA expression, IL-10 production, and then the cell growth was inhibited. These results indicate that the p38 MAPK pathway is functionally linked to IL-10 gene expression and supports the view that the constitutive activation of cytoplasmic p38 MAPK in BL cells is a step in the upregulation of IL-10 gene expression and lymphomagenesis.

Acute alcohol activates STAT3, AP-1, and Sp-1 transcription factors via the family of Src kinases to promote IL-10 production in human monocytes

Alcohol consumption is associated with an imbalance in pro- and anti-inflammatory cytokines and immunosuppression, partially as a result of enhanced IL-10 production. The mechanisms of IL-10 induction by alcohol remain poorly understood. We identified that increased IL-10 production in human monocytes after acute in vivo alcohol consumption or in vitro alcohol treatment was associated with increased STAT3 activation. Alcohol alone induced and in combination with LPS augmented STAT3 phosphorylation at tyrosine 705 (tyr705) and serine 727 (ser727) residues and increased STAT3 binding to DNA. Upstream, alcohol activated the Src kinases, as indicated by an increase in phosphorylated and a decrease in nonphosphorylated Src proteins. STAT3 activation by Src kinases occurred directly at the tyr705 residue and indirectly at the ser727 residue via JNK MAPKs. Using specific Src (PP2), JNK1/2 (SB600125), or p38 (SB203580) inhibitors, we determined that alcohol treatment alone induced and together with LPS, augmented the DNA-binding capacity of the specificity protein-1 (Sp-1) and AP-1 transcription factors involved in IL-10 production via Src-mediated activation of p38 MAPK and JNK, respectively. Our data suggest that acute alcohol activates Src/STAT3 and Src/MAPK/STAT3, AP-1, and Sp-1 pathways as important mechanisms for IL-10-mediated immunomodulation after acute alcohol use.

STAT1 activation causes translocation of Bax to the endoplasmic reticulum during the resolution of airway mucous cell hyperplasia by IFN-gamma

Disruption of the normal resolution process of inflammation-induced mucous cell hyperplasia may lead to sustained mucous hypersecretion in chronic diseases. During prolonged exposure of mice to allergen, IFN-gamma reduces mucous cell hyperplasia, but the signaling responsible for the cell death is largely unknown. A brief phosphorylation of STAT1 by IFN-gamma was required for cell death in airway epithelial cells (AEC), and during prolonged exposure to allergen, mucous cell hyperplasia remained elevated in STAT1(-/-) but was resolved in STAT1(+/+) mice. Although IFN-gamma treatment of primary human AECs and other airway cell lines left Bax protein levels unchanged, it caused translocation of Bax from the cytosol to the endoplasmic reticulum (ER) but not to the mitochondria. Localization of Bax to the ER was observed in IFN-gamma-treated primary AECs isolated from STAT1(+/+) mice but not in cells from STAT1(-/-) mice. In addition, ER Bax was detected in mucous cells of STAT1(+/+) but not STAT1(-/-) airways of mice exposed to allergen for prolonged periods. IFN-gamma did not release cytochrome c from mitochondria but reduced ER calcium stores and dilated the ER, confirming that the IFN-gamma-induced cell death is mediated through changes localized in the ER. Collectively, these observations suggest that STAT1-dependent translocation of Bax to the ER is crucial for IFN-gamma-induced cell death of AECs and the resolution of allergen-induced mucous cell hyperplasia.

Signal transducer and activator of transcription signals in allergic disease

Signal transducer and activator of transcription (STAT) proteins are a group of transcription factors that transmit signals from the extracellular milieu of cells to the nucleus. They are crucial for the signaling of many cytokines that are mediators of allergic inflammation and impact various cell types critical to allergy including epithelial cells, mast cells, lymphocytes, dendritic cells, and eosinophils. Dysregulation of STAT signaling has been implicated in allergic disease, highlighting the importance of these ubiquitous molecules in allergic inflammation and the potential of these pathways as a target for therapeutic intervention. This review will summarize the current understanding of the roles of STAT signaling in allergic disease and the potential of targeting STATs for the treatment of allergic disorders, emphasizing recent observations.

Constitutive and induced activation of JAK/Stat pathway in leukemogenic and asymptomatic human T-cell lymphoptropic virus type 1 (HTLV-1) transformed rabbit cell lines

We have shown that Vav and C-cbl are activated in the leukemogenic HTLV-I transformed rabbit T cell line RH/K34 but not in the asymptomatic one RH/K30. We extended these observations and investigated the activation of JAKs (Janus Kinase) and the STATs (signal transducers and activators of transcription) pathway in these cell lines. We found that Tyk2 and Stat3 are constitutively tyrosine phosphorylated in the leukemogenic cell line. Phosphorylation of Tyk2 can be induced in RH/K30 by treatment with IL-10, interferon alpha (INFalpha) and by the supernatant of RH/K34 which contain both these cytokines. Stat3 tyrosine phosphorylation can be induced in RH/K30 by treatment with IL-10. Transfection of RL-5, a rabbit T-cell line, with the RH/K34 viral clone transiently increased the expression of serine/threonine phosphorylated Stat3. Differences were also observed on induced Stat5 phosphorylation. These results highlight the relation between the virulence of HTLV-I and the activation of the Jak/Stat pathway.

Role of ERK 1/2 signaling in neuronal differentiation of cultured embryonic stem cells

Embryonic stem (ES) cells represent an ideal source for cell engraftment in the damaged central nervous system (CNS). Understanding key signals that control ES cell differentiation may improve cell type-specific differentiation that is suitable for transplantation therapy. We tested the hypothesis that extracellular signal-regulated kinase (ERK) 1/2 phosphorylation is an early signaling event required for the neuronal differentiation of ES cells. Cultured mouse ES cells were treated with an all-trans-retinoic-acid (RA) protocol to generate neurally induced progenitor cells. Western blot analysis showed a dramatic increase in ERK 1/2 phosphorylation (p-ERK 1/2) 1-5 days after RA induction, which was attenuated in the presence of the p-ERK 1/2-specific inhibitor UO126. Phospho-ERK 1/2 inhibition significantly reduced the number of NeuN-positive cells and the expression of associated cytoskeletal proteins. In differentiating ES cells, there was increased nuclear translocation of STAT3 and decreased protein expression levels of GDNF, BDNF and NGF. STAT3 translocation was attenuated by UO126. Finally, caspase-3 activation was observed in the presence of UO126, suggesting that the ERK pathway also contributes to the survival of differentiating ES cells. These data indicate that ERK 1/2 phosphorylation is a key event required for early neuronal differentiation and survival of ES cells.

STAT3 as a central mediator of neoplastic cellular transformation

Much of the focus in understanding the molecular pathogenesis of tumors has centered on kinases that are activated in cancer. However, cancers driven by a diversity of activated kinases may have very similar pathological and clinical properties. This likely relates to the fact that the biological characteristics of a tumor are driven by the pattern of gene expression in that tumor, and that a wide spectrum of activating events at the cell surface and in the cytoplasm converge on a relatively small number of transcription factors that regulate the expression of key target genes. One transcription factor that has been found to be activated inappropriately in a wide range of human cancers is STAT3. STAT3 target genes are involved in fundamental events of tumor development including proliferation, survival, self-renewal, invasion, and angiogenesis. Furthermore, there is strong evidence that STAT3 is critical for these processes, in that inhibition of STAT3 by a variety of means can exert an anti-cancer effect. Since normal cells are relatively tolerant of interruption in STAT3 signaling, these findings suggest that STAT3 may also be an excellent target for the molecular therapy of cancer.

Dual ligand stimulation of RAW 264.7 cells uncovers feedback mechanisms that regulate TLR-mediated gene expression

To characterize how signaling by TLR ligands can be modulated by non-TLR ligands, murine RAW 264.7 cells were treated with LPS, IFN-gamma, 2-methyl-thio-ATP (2MA), PGE(2), and isoproterenol (ISO). Ligands were applied individually and in combination with LPS, for 1, 2, and 4 h, and transcriptional changes were measured using customized oligo arrays. We used nonadditive transcriptional responses to dual ligands (responses that were reproducibly greater or less than the expected additive responses) as a measure of pathway interaction. Our analysis suggests that cross-talk is limited; <24% of the features with significant responses to the single ligands responded nonadditively to a dual ligand pair. PGE(2) and ISO mainly attenuated, while 2MA enhanced, LPS-induced transcriptional changes. IFN-gamma and LPS cross-regulated the transcriptional response induced by each other: while LPS preferentially enhanced IFN-gamma-induced changes in gene expression at 1 h, IFN-gamma signaling primarily attenuated LPS-induced changes at 4 h. Our data suggest specific cross-talk mechanisms: 1) LPS enhances the expression of IFN-gamma-response genes by augmenting STAT1 activity and by activating NF-kappaB, which synergizes with IFN-gamma-induced transcriptional factors; 2) IFN-gamma attenuates the late LPS transcriptional response by increasing the expression of suppressor of cytokine signaling 1 and cytokine-inducible SH2-containing protein expression; 3) 2MA modulates LPS secondary transcriptional response by increasing IFN-beta and inhibiting IL-10 gene expression; 4) PGE(2) and ISO similarly regulate the LPS transcriptional response. They increase IL-10 transcription, resulting in attenuated expression of known IL-10-suppressed genes.

The interaction of hepatitis B virus X protein and protein phosphatase type 2 Calpha and its effect on IL-6

HBx has been suggested as an important determinant mediating the pathological effects of HBV via interacting with various cellular proteins. To identify new HBx-interacting proteins and elucidate a possible mechanism associated with HBx and HBx-interacting proteins in hepatocellular carcinoma, yeast two-hybrid screening was performed. We identified a novel HBx-interacting protein, serine/threonine protein phosphatase PP2Calpha, and investigated the effects of PP2Calpha on HBx-mediated IL-6 regulation. The interaction between endogenous PP2Calpha, and HBx was confirmed by co-immunoprecipitation. Recombinant HBx dose-dependently reduced enzyme activity of recombinant PP2Calphain vitro. While ectopically expressed PP2Calpha in Cos-7 and Huh-7 cells reduced the expression of IL-6, overexpressed HBx with recombinant HBx-expressing adenovirus overcame PP2Calpha-mediated IL-6 downregulation. In the response of IL-6, HBx phosphorylated STAT3 and recovered PP2Calpha-mediated dephosphorylation of STAT3. These results supported that HBx might play a crucial role in HBV-associated hepatocarcinogenesis even in cases where cells express a negative regulator, PP2Calpha.

Oncostatin M decreases adiponectin expression and induces dedifferentiation of adipocytes by JAK3- and MEK-dependent pathways

Adiponectin, an adipokine secreted from adipocytes, plays a crucial role in the regulation of glucose and lipid metabolism. In the present study, we examine the role of the IL-6 family of cytokines in the expression of adiponectin in human adipocytes derived from human adipose tissue-derived stromal cells. Oncostatin M (OSM), but not IL-6, attenuated the expression level of adiponectin dose- and time-dependently, and the inhibitory effect of OSM on adiponectin expression was as potent as that of TNF-alpha. The OSM-induced down-regulation of adiponectin expression was correlated with the down-regulation of PPARgamma2 and lipoprotein lipase, markers for adipogenic differentiation, and depletion of intracellular lipid droplets, suggesting dedifferentiation of adipocytes in response to OSM. OSM induced phosphorylation of STAT1, and treatment of adipocytes with JAK3 inhibitor WHI-P131 or MEK inhibitor U0126, but not with JAK2 inhibitor AG490, prevented the activation of STAT1. Furthermore, the OSM-induced suppression of adiponectin expression and dedifferentiation of adipocytes were ameliorated by WHI-P131 or U0126, but not by AG490. These results suggest that OSM inhibits adiponectin expression by inducing dedifferentiation of adipocytes through signaling pathways involving JAK3 and MEK, but not JAK2.

Structure, function, and regulation of STAT proteins

The Signal Transducer and Activator of Transcription (STAT) family of proteins was first discovered in the 1990's as key proteins in cytokine signaling. Since then, the field has greatly advanced in the past 15 years, providing significant insight into the structure, function, and regulation of STATs. STATs are latent cytoplasmic transcription factors consisting of seven mammalian members. They are Tyr phosphorylated upon activation, a post-translational modification critical for dimerization, nuclear import, DNA binding, and transcriptional activation. In recent years, unphosphorylated STATs have also been observed to dimerize and drive transcription, albeit by yet an obscure mechanism. In addition, the function of cytoplasmic STATs is beginning to emerge. Here, we describe the structure, function, and regulation of both unphosphorylated and phosphorylated STATs. STAT isoforms from alternative splicing or proteolytic processing, and post-translational modifications affecting STAT activities are also discussed.

Janus tyrosine kinases and signal transducers and activators of transcription regulate critical functions of T cells in allograft rejection and transplantation tolerance

Full activation of T cells requires three sequential signals. Engagement by antigen presenting cells (APC) delivers signals 1/2, whereas signal 3 is delivered by multiple cytokines to regulate the immune homeostasis by influencing proliferation, differentiation, and survival/death. Signaling by cytokines acting through their receptors is delivered by two major molecular families, namely Janus tyrosine kinases (Jaks) and signal transducers and activators of transcription (Stats). Findings obtained from mice genetically deficient in Jaks and Stats suggest that these molecules may serve as therapeutic targets to prevent allograft rejection, induce transplantation tolerance, and inhibit autoimmune disease and lymphoid-derived tumors. This review describes the role of Jak tyrosine kinases and Stat transcription factors and their putative function in regulating T and B cell activity.

Monocyte 15-Lipoxygenase Expression Is Regulated by a Novel Cytosolic Signaling Complex with Protein Kinase C δ and Tyrosine-Phosphorylated Stat3

Our previous studies demonstrated that the IL-13-induced 15-lipoxygenase expression in primary human monocytes is regulated by the activation of both Stat1 and Stat3 and by protein kinase C (PKC)delta. IL-13 stimulated the phosphorylation of Stat3 on both Tyr705 and Ser727. In this study we show that IL-13 induces the association of PKCdelta with Stat3, not with Stat1, and is required for Stat3 Ser727 phosphorylation. We found a novel IL-13-dependent cytosolic signaling complex of PKCdelta and tyrosine-phosphorylated Stat3. A tyrosine kinase inhibitor blocked PKCdelta association with Stat3 as well as Stat3 Ser727 phosphorylation. We therefore hypothesized that tyrosine phosphorylation was required for Stat3 interaction with PKCdelta and subsequent PKCdelta-dependent phosphorylation of Stat3 Ser727. We developed an efficient transfection protocol for human monocytes. Expression of Stat3 containing a mutation in Tyr705 inhibited the association of PKCdelta with Stat3 and blocked Stat3 Ser727 phosphorylation, whereas transfection with wild-type Stat3 did not. Furthermore, by transfecting monocytes with Stat3 containing mutations in Tyr705 or Ser727 or with wild-type Stat3, we demonstrated that both Stat3 tyrosine and serine phosphorylations are required for optimal binding of Stat3 with DNA and maximal expression of 15-lipoxygenase, an important regulator of inflammation and apoptosis.

BCR-ABL activates STAT3 via JAK and MEK pathways in human cells

Chronic myeloid leukaemia (CML) is characterised by a progression from a chronic towards an acute phase. We previously reported that signal transducer and activator of transcription 3 (STAT3), a major oncogenic signalling protein, is the target of p210-BCR-ABL in a murine embryonic stem (ES) cell model and in primary CD34+ CML cells. This activation was associated with inhibition of differentiation in ES cells. The present study found that BCR-ABL greatly phosphorylated STAT3 Ser727 residue and, to a lesser extent, Tyr705 residue in BCR-ABL-expressing cell lines (UT7-p210, MO7E-p210, and K562) and in primary CD34+ CML cells. Using BCR-ABL mutants, it was shown that BCR-ABL tyrosine kinase activity and its Tyr177 residue were necessary for STAT3 Ser727 phosphorylation. Constitutive STAT3 Tyr705 phosphorylation was associated with constitutive phosphorylation of Janus kinase (JAK)1 and JAK2, and was inhibited by the JAK inhibitor AG490, suggesting the involvement of JAK proteins in this process. Specific MEK [mitogen-activated protein (MAP) kinase/extracellular signal-regulated kinase (ERK) kinase] inhibitors PD98056 and UO126, as well as the use of a dominant-negative form of MEK1 abrogated STAT3 Ser727 phosphorylation, suggesting involvement of MAP-Kinase/Erk pathway. Inhibition of BCR-ABL with imatinib mesylate led to a dose-dependent downregulation of total STAT3 protein and mRNA, suggesting that BCR-ABL is involved in the transcriptional regulation of STAT3. Targeting JAK, MEK and STAT3 pathways could therefore be of therapeutic value, especially in advanced stage CML.

Signal transducers and activators of transcription 1 and 3 in prostate: Effect of sexual activity

The signal transducers and activators of transcription (Stat) are effector molecules downstream of cytokine receptors. Ligand occupancy of these receptors results in the tyrosine phosphorylation, dimerization and nuclear translocation of the Stat family of transcription factors and by these means regulate gene expression. Prolactin receptors as members of the cytokine-hematopoietin receptor superfamily, are linked to Stat activation. Sexual stimulation leads to an increase in prolactin secretion that might be involved in long-term changes in the protein repertoire associated to prostate hyperplasia. In order to gain insight into this phenomenon, we analyzed the tyrosine phosphorylation and DNA binding activity of two members of the Stat family in the prostate of sexual experienced rats after different number of ejaculations. A significant increase in Stat-1 and Stat-3 tyrosine phosphorylation was found after three ejaculations. Concomitantly an increase in Stat-1 and Stat-3 DNA-binding activity is detected after two and three ejaculation series. These results, favor the notion that ejaculation-induced prolactin secretion activates its prostate receptors resulting in Stat-1 and Stat-3 nuclear translocation, event likely to be associated to the so-called benign prostate hyperplasia.

Expression changes of ERK1/2, STAT3 and SHP-2 in bone marrow cells from gamma-ray induced leukemia mice

The aim is to clarify expression changes of ERK1/2, STAT3 and SHP-2 in bone marrow cells from gamma-ray induced leukemia mice. A mouse model of gamma-ray induced leukemia was produced, and by means of quantitative real-time PCR, immunoprecipitation, Western blotting and electrophoretic mobility shift assays (EMSA), the expression of mRNA and protein, phosphorylation level, and protein activity of ERK1/2, STAT3 and SHP-2 in bone marrow cells were investigated in these mice. The results indicated that mRNA and protein expressions of ERK1/2 were upregulated, with significant increase of phosphorylation level and protein activity, but with insignificant differences in mRNA and protein expressions, phosphorylation level and protein activity of STAT3 and SHP-2 in bone marrow cells from gamma-ray induced leukemia mice compared to the radiation/tumor-free or control mice. It is concluded that in the pathogenesis of gamma-ray induced leukemia in Balb/C mice, activated ERK1/2 pathway may play a role, without involving STAT3 pathway; meanwhile, SHP-2 exerts no regulative effect on pathways of Ras-ERK1/2 and JAK-STAT.

Heparin synergistically enhances interleukin-11 signaling through up-regulation of the MAPK pathway

Using an animal model of heparin-induced osteoporosis we previously demonstrated that heparin causes bone loss, in part, by increasing osteoclast number and activity. Furthermore, we found that, although heparin alone has no effect, it is able to synergistically enhance Interleukin-11 (IL-11)-induced signal transducer and activator of transcription 3 (STAT3) activation and thus increase osteoclast formation in vitro. In the present study, we examine the effect of various serine kinase inhibitors on the ability of heparin to act synergistically with IL-11. Inhibition of the c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (MAPK), or the phosphatidylinositol 3-kinase pathways had no effect on the ability of heparin to promote either IL-11-induced STAT3.DNA complex formation or osteoclast formation in vitro. In contrast, PD098059, a MAPK kinase inhibitor, completely abolished the synergy between heparin and IL-11. In an attempt to resolve the mechanism by which this was occurring, we examined the effect of heparin on STAT3 Ser-727 phosphorylation and extracellular signal-regulated kinases 1 and 2 (Erk1/2) activation, either in the presence or absence of IL-11. Heparin alone was found to have no effect on Ser-727 phosphorylation, nor did heparin alter the phosphorylation status of Ser-727 in the presence of IL-11. Heparin was, however, found to increase Erk1/2 activation in both a time- and dose-dependent manner. When taken together, these findings suggest that heparin enhances IL-11-induced STAT3 activation and thus osteoclast formation, by a mechanism that is independent of STAT3 Ser-727 phosphorylation but that involves up-regulation of the MAPK pathway.

Overexpression of STAT-1 by adenoviral gene transfer does not inhibit hepatitis B virus replication

OBJECTIVES

Interferons are known to inhibit the replication of hepatitis B viruses (HBV) in several animal models in vitro and in vivo as well in humans. The STAT-1 protein plays a central role in the biological activity of both type I and type II interferons. The lack of functional STAT-1 renders cells and organisms susceptible to bacterial and viral infectious agents. We analysed whether the overexpression of STAT-1 protein enhances the biological interferon response and whether it elicits antiviral activity against HBV in vitro.

METHODS

To achieve an efficient STAT-1 overexpression in primary liver cells and hepatoma cells, we generated a recombinant, replication-deficient adenovirus expressing human STAT-1 (Adv-STAT-1). We analysed whether the overexpression of STAT-1 inhibits the replication of duck HBV and human HBV in vitro using Western blot analysis, the immunofluorescence of viral proteins and quantification of HBV-DNA copies, respectively.

RESULTS

In the duck model of HBV infection the overexpression of STAT-1 neither inhibited an established infection nor prevented the establishment of duck HBV replication when administered simultaneously with Adv-STAT-1. These observations were confirmed in an in-vitro model of human HBV infection using the human hepatoma cell line HepG2.2.15, which continuously replicates HBV.

CONCLUSION

These data demonstrate that the over-expression of STAT-1 alone is not sufficient to strengthen the biological response of interferon as an antiviral agent.

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.

Accelerated proliferation of myeloma cells by interleukin-6 cooperating with fibroblast growth factor receptor 3-mediated signals

Interleukin-6 (IL-6) is a cytokine that regulates the proliferation of some tumor cells including multiple myeloma (MM). Ectopic expression of fibroblast growth factor receptor 3 (FGFR 3) associated with the chromosomal translocation, t(4;14)(p16.3;q32), is frequently found in MM, and therefore, has been implicated in the neoplastic transformation of this disease. Here, we show that IL-6 together with FGF enhanced proliferation of a myeloma cell line, KMS-11 carrying t(4;14)(p16.3;q32) and the FGFR 3-transfected U 266 myeloma cell line which ectopically expressed FGFR 3 but responded to neither IL-6 nor FGF alone. In KMS-11, IL-6 activated signal transducer and activator of transcription 3 (STAT 3) while FGF activated extracellular signal-regulated kinase 1/2 (ERK 1/2) and phosphatidylinositol (PI)-3 kinase. As both MEK inhibitors and a PI 3-kinase inhibitor abolished the effect of IL-6 and FGF, the activation of both the ERK 1/2 and PI 3-kinase signaling cascades is essential for the proliferation of KMS-11 enhanced by IL-6 and FGF. Furthermore, the FGF-induced activation of ERK 1/2 contributed to the serine phosphorylation of STAT 3, suggesting that the signaling crosstalk between the cytokine receptor, IL-6 receptor alpha/gp 130 and the growth factor receptor tyrosine kinase, FGFR 3. These results indicate that FGFR 3 plays a crucial role in the accelerated proliferation of MM carrying t(4;14)(p16.3;q32).

Cytokine-induced myeloid differentiation is dependent on activation of the MEK/ERK pathway

The intracellular signaling pathways that mediate cytokine-induced granulocytic and monocytic differentiation are incompletely understood. In this study, we examined the importance of the MEK/ERK signal transduction pathway in granulocyte-colony stimulating factor (G-CSF)-induced granulocytic differentiation of murine 32 Dc l3 cells, and in interleukin-6 (IL-6)-induced monocytic differentiation of murine M1 cells. Induction of granulocytic differentiation with G-CSF, or monocytic differentiation with IL-6, led to rapid and sustained activation of the MEK-1/-2 and ERK-1/-2 enzymes. Inhibition of the MEK/ERK pathway by pretreatment with the MEK inhibitor U 0126 dramatically attenuated G-CSF-induced granulocytic differentiation and IL-6-induced monocytic differentiation. Inhibition of MEK/ERK signaling also significantly reduced cytokine-induced DNA binding activities of STAT 3 and PU.1, transcription factors that have been implicated in myeloid differentiation. Additionally, interleukin-3, which inhibits G-CSF-induced differentiation of 32 Dc l3 cells, also inhibited the ability of G-CSF to stimulate prolonged MEK/ERK activation. Thus, the opposing actions of different hematopoietic cytokines on myeloid progenitors may be mediated at the level of MEK/ERK activation. Taken together, these studies demonstrate an important requirement for MEK/ERK activation during cytokine-induced granulocytic and monocytic differentiation.

The molecular pathogenesis of acute myeloid leukemia

The description of the molecular pathogenesis of acute myeloid leukemias (AML) has seen dramatic progress over the last years. Two major types of genetic events have been described that are crucial for leukemic transformation: alterations in myeloid transcription factors governing hematopoietic differentiation and activating mutations of signal transduction intermediates. These processes are highly interdependent, since the molecular events changing the transcriptional control in hematopoietic progenitor cells modify the composition of signal transduction molecules available for growth factor receptors, while the activating mutations in signal transduction molecules induce alterations in the activity and expression of several transcription factors that are crucial for normal myeloid differentiation. The purpose of this article is to review the current literature describing these genetic events, their biological consequences and their clinical implications. As the article will show, the recent description of several critical transforming mutations in AML may soon give rise to more efficient and less toxic molecularly targeted therapies of this deadly disease.

Central leptin acutely reverses diet-induced hepatic insulin resistance

Voluntary overfeeding rapidly induces resistance to the effects of systemic insulin and leptin on liver glucose metabolism. To examine whether central administration of recombinant leptin can restore leptin and insulin action on liver glucose fluxes, we infused leptin in the third cerebral ventricle of conscious overfed rats during pancreatic-insulin clamp studies. The effect of leptin on the phosphorylation of the signal transducer and activator of transcription-3 in the arcuate nuclei of the hypothalamus was similar in animals fed a regular diet or a high-fat diet for 3 days. The infusion of leptin in the third cerebral ventricle markedly inhibited glucose production in rats fed a high-fat diet mainly by decreasing glycogenolysis. The inhibition of glycogenolysis was sufficient to normalize glucose production and was accompanied by leptin-induced decreases in the hepatic expression of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. Thus central administration of leptin rescues the hepatic insulin resistance induced by short-term hyperphagia.

Stimulation of inducible nitric oxide synthase by monosodium urate crystals in macrophages and expression of iNOS in gouty arthritis

From the studies on the involvement of iNOS in arthritis, it is clear that attention has focused primarily on rheumatoid arthritis (RA) and osteoarthritis (OA). To date, little is known about the role of iNOS in the pathophysiology of gouty arthritis (GA). Here, we investigated the significance of iNOS expression in cell culture system as well as in GA patients. Gouty crystals monosodium urate (MSU) appeared to up-regulate inducible nitric oxide synthase (iNOS) mRNA and protein expression in a concentration- and time-dependent manner in RAW264.7 macrophages. This increase of iNOS expression is attributable to the activation of multiple signaling pathways. Evidence for this was initially established by inhibitor treatment of cells in the presence of MSU. While the JAK inhibitor AG490, the PI3K inhibitor LY294002, and the NFkappaB inhibitor PDTC abrogated almost completely the expression of iNOS induced by MSU, the ERK1/2 inhibitor PD98059 was only partially effective. Furthermore, the effect of MSU on the activation of PI3K/Akt, JAK/STAT, ERK1/2, and NFkappaB signaling molecules was carefully examined. Moreover, it was shown that GAS and NFkappaB motifs are required for iNOS expression mediated by MSU. In addition, synovial tissues obtained from GA patients displayed enhanced expression of iNOS when compared with normal synovium. Taken together, these findings provide the first evidence for the potential importance of iNOS in the pathogenesis of GA as well as RA and OA, and in turn raise the possibility that iNOS may be an ideal target for preventive therapy in human arthritis.

Janus kinase 3: a novel target for selective transplant immunosupression

Existing immunosuppressants inhibit lymphocyte activation and T cell cytokine signal transduction pathways, reducing the rate of acute rejection episodes to < 10%. However, the widespread tissue distribution of their molecular targets engenders pleiotropic toxicities. One strategy to address this problem seeks to identify compounds that selectively inhibit a target restricted in distribution to the lymphoid system. Janus kinase (Jak) 3 is such a molecule; it mediates signal transduction via the gamma common chain of lymphokine surface receptors. Disruption of this lymphoid-restricted enzyme would not be predicted to produce collateral damage in other organ systems. Development of selective Jak3 inhibitors has been difficult due to crossreactivity with its homologue, Jak2. In contrast to all other putative antagonists, which are discussed in detail herein, one Jak3 inhibitor, NC1153, shows at least 40-fold greater selective inhibition for Jak3 than for Jak2, is robustly synergistic with calcineurin antagonists, and, either alone or in combination with cyclosporin, produces no adverse effects in rodents preconditioned to be at heightened risk for nephrotoxicity, bone marrow suppression, or altered lipid metabolism.

Proline-, glutamic acid-, and leucine-rich protein-1 is essential in growth factor regulation of signal transducers and activators of transcription 3 activation

Proline-, glutamic acid-, and leucine-rich protein-1 (PELP1) is a novel estrogen receptor coactivator that plays an important role in the genomic and nongenomic actions of estrogen receptor by interacting with histones and src-mitogen-activated protein kinase pathway, respectively. A great deal of information has emerged in recent years about the possible role of PELP1 in estrogen receptor signaling. However, the participation and significance of PELP1 in other cellular signaling pathways remains unknown. Using a yeast two-hybrid screen, we identified PELP1 as a novel interacting protein of signal transducers and activators of transcription 3 (STAT3) and found evidence of physiologic interaction between PELP1 and STAT3. We also found that these interactions played a mechanistic role in the positive regulation of STAT3 transcription from synthetic promoters and endogenous target genes such as cyclin D1, c-myc, and c-fos. Overexpression of PELP1 enhanced phosphorylation of STAT3 at Ser727 in a src-mitogen-activated protein kinase-sensitive manner and, conversely, down-regulation of PELP1 compromised growth factor-mediated induction of STAT3 target genes. We also discovered that PELP1 interacts with STAT3 in the nuclear compartment and down-regulation of PELP1 interfered with the recruitment of STAT3 to its target gene promoters. In summary, our results highlight a novel role for PELP1 in growth factor signaling and indicate that PELP1-mediated genomic and nongenomic functions play a role in the growth factor-mediated STAT3 transactivation functions. Such regulatory interactions of PELP1 may have important functional implications in the cross-talk of estrogen receptor and growth factor signaling.

Inhibition of Stat3 activation in the endometrium prevents implantation: a nonsteroidal approach to contraception

Activation of the receptors for leukemia inhibitory factor (LIF) and IL-11 is essential for embryo attachment and decidualization in mice. Both receptors induce activation of the Stat family of signal transducers via the Jak/Stat pathway. Here, we aimed to establish whether activation of Stat3 in maternal endometrium is essential for successful implantation. Functional blockade of Stat3 before implantation, by injection into the uterine lumen of a cell-permeable Stat3 peptide inhibitor, reduced embryo implantation specifically by 70% (P < 0.001). Stat3 is phosphorylated in the luminal epithelium (LE) in response to LIF, and this phosphorylation was significantly reduced both in vitro and in vivo by the Stat3 inhibitor. The inhibitor also blocked induction by LIF of several LIF-regulated genes in the LE including Irg1, which has been shown previously to be essential for implantation. Successful implantation is therefore dependent on phosphorylation and activation of Stat3 in the endometrium before implantation. This finding provides a target for contraceptive development, based on selective blockade of signal transduction pathways essential for implantation. This study demonstrates that cell-permeable peptide inhibitors can be used effectively to target intracellular signaling pathways in the uterine LE.

Farnesyl transferase inhibitor (R115777)-induced inhibition of STAT3(Tyr705) phosphorylation in human pancreatic cancer cell lines require extracellular signal-regulated kinases

In this study, we report that R115777, a nonpeptidomimetic farnesyl transferase inhibitor, suppresses the growth of human pancreatic adenocarcinoma cell lines and that this growth inhibition is associated with modulation in the phosphorylation levels of signal transducers and activators of transcription 3 (STAT3) and extracellular signal-regulated kinases (ERK). Treatment of cells with R115777 inhibited the tyrosine phosphorylation of STAT3((Tyr705)), while increasing the serine phosphorylation of STAT3((Ser727)). We found the differential phosphorylation of STAT3 was due to an increased and prolonged activation of ERKs. The biological significance of ERK-mediated inhibition of STAT3((Tyr705)) phosphorylation was further assessed by treating the cells with an inhibitor (PD98059) of mitogen-activated protein kinase kinase (MEK) or by transfecting the cells with a vector that expresses constitutively active MEK-1. Expression of constitutively active MEK-1 caused an increase of ERK activity and inhibited STAT3((Tyr705)) phosphorylation. Conversely, inhibition of ERK activity by PD98059 reversed the R115777-induced inhibition of STAT3((Tyr705)) phosphorylation. R115777 also caused the inhibition of the binding of STAT3 to its consensus binding element. An increase in the activation of ERKs either by overexpressing MEK-1 or treatment of cells with R115777 caused an up-regulation in the levels of a cyclin-dependent kinase (cdk) inhibitor, p21(cip1/waf1). These observations suggest that R115777-induced growth inhibition is partly due to the prolonged activation of ERKs that mediates an inhibition of STAT3((Tyr705)) phosphorylation and an increase in the levels of p21(cip1/waf1) in human pancreatic adenocarcinoma cell lines.

STAT module can function as a biphasic amplitude filter

Signal transducer and actuator of transcription (STATs) are a family of transcription factors activated by various cytokines, growth factors and hormones. They are important mediators of immune responses and growth and differentiation of various cell types. The STAT signalling system represents a defined functional module with a pattern of signalling that is conserved from flies to mammals. In order to probe and gain insights into the signalling properties of the STAT module by computational means, we developed a simple non-linear ordinary differential equations model within the 'Virtual Cell' framework. Our results demonstrate that the STAT module can operate as a 'biphasic amplitude filter' with an ability to amplify input signals within a specific intermediate range. We show that dimerisation of phosphorylated STAT is crucial for signal amplification and the amplitude filtering function. We also demonstrate that maximal amplification at intermediate levels of STAT activation is a moderately robust property of STAT module. We propose that these observations can be extrapolated to the analogous SMAD signalling module.

Alpha-interferon and its effects on signal transduction pathways

Interferon-alpha (IFNalpha) is a recombinant protein widely used in the therapy of several neoplasms such as myeloma, renal cell carcinoma, epidermoid cervical and head and neck tumors, and melanoma. IFNalpha, the first cytokine to be produced by recombinant DNA technology, has emerged as an important regulator of cancer cell growth and differentiation, affecting cellular communication and signal transduction pathways. However, the way by which tumor cell growth is directly suppressed by IFNalpha is not well known. Wide evidence exists on the possibility that cancer cells undergo apoptosis after the exposure to the cytokine. Here we will review the consolidate signal transducer and activator of transcription (STAT)-dependent mechanism of action of IFNalpha. We will discuss data obtained by us and others on the triggering of the stress-dependent kinase pathway induced by IFNalpha and its correlations with the apoptotic process. The regulation of the expression of proteins involved in apoptosis occurrence will be also described. In this regard, IFNalpha is emerging as a post-translational controller of the intracellular levels of the apoptosis-related protein tissue transglutaminase (tTG). This new way of regulation of tTG occurs through the modulation of their proteasome-dependent degradation induced by the cytokine. Until today, inconsistent data have been obtained regarding the clinical effectiveness of IFNalpha in the therapy of solid tumors. In fact, the benefit of IFNalpha treatment is limited to some neoplasms while others are completely or partially resistant. The mechanisms of tumor resistance to IFNalpha have been studied in vitro. The alteration of JAK-STAT components of the IFNalpha-induced signaling, can be indeed a mechanism of resistance to IFN. However, we have recently described a reactive mechanism of protection of tumor cells from the apoptosis induced by IFNalpha dependent on the epidermal growth factor (EGF)-mediated Ras/extracellular signal regulated kinase (Erk) signaling. The involvement of the Ras-->Erk pathway in the protection of tumor cells from the apoptosis induced by IFNalpha is further demonstrated by both Ras inactivation by RASN17 transfection and mitogen extracellular signal regulated kinase 1 (Mek-1) inhibition by exposure to PD098059. These data strongly suggest that the specific disruption of the latter could be a useful approach to potentiate the antitumour activity of IFNalpha against human tumors based on the new mechanistic insights achieved in the last years.

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.

HIV-1 Nef promotes survival of TF-1 macrophages by inducing Bcl-XL expression in an extracellular signal-regulated kinase-dependent manner

The Nef protein of human immunodeficiency virus-1 (HIV-1) is essential for the progression from human and simian immunodeficiency virus infection to full-blown AIDS. Recent studies indicate that Nef generates anti-apoptotic signals in HIV-infected T cells, suppressing cell death early in infection to allow productive viral replication. Previous work from our laboratory has shown that Nef also promotes proliferation of myeloid cells through a signal transducer and activator of transcription 3-dependent pathway. Here we demonstrate that Nef suppresses cell death induced by cytokine deprivation in the human macrophage precursor cell line, TF-1. Nef selectively induced up-regulation of Bcl-XL, an anti-apoptotic gene that is also regulated by granulocyte/macrophage-colony stimulating factor in this cell line. Activation of the extracellular signal-regulated kinase (Erk) mitogen-activated protein kinase pathway also correlated with the survival of TF-1/Nef cells. Using the selective mitogen-activated protein kinase kinase inhibitor PD98059, we found that Nef-induced Erk signaling is essential for Bcl-XL up-regulation and cell survival. In contrast, expression of Bcl-XL and TF-1 survival was not affected by dominant-negative signal transducer and activator of transcription 3. These data suggest that Nef produces survival signals in myeloid cells through Erk-mediated Bcl-XL induction, a pathway distinct from Nef survival pathways recently reported in T lymphocytes.

Activation of formyl peptide receptor-like 1 by WKYMVm induces serine phosphorylation of STAT3, which inhibits its tyrosine phosphorylation and nuclear translocation induced by hydrogen peroxide

Signal transducer and activator of transcription 3 (STAT3) is an important transcription factor that modulates the expression of several genes. The activation of STAT3 accompanies tyrosine phosphorylation and its translocation to the nucleus. Formyl peptide receptor like 1 (FPRL1) is an important classical chemoattractant receptor. In this study, we observed that the stimulation of FPRL1 by Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm) caused serine phosphorylation but not tyrosine phosphorylation of STAT3 in a pertussis toxin-sensitive manner. Moreover, downstream of FPRL1 stimulation, phospholipase D (PLD) activity was dramatically increased. n-butanol, a well-known phosphatidic acid (PA) acceptor, completely inhibited WKYMVm-induced STAT3 serine phosphorylation. Moreover, the exogenous addition of PA mimicked STAT3 phosphorylation by WKYMVm. We also found that WKYMVm stimulated extracellular signal regulated kinase (ERK), and that ERK activity is required for STAT3 serine phosphorylation. This WKYMVm-induced ERK activation was inhibited by n-butanol, whereas ERK activation was also induced by the addition of exogenous PA. In terms of the functional aspects of the WKYMVm-induced serine phosphorylation of STAT3, we found that hydrogen peroxide-stimulated STAT3 activation was blocked by pretreating WKYMVm. Taken together, we found that WKYMVm stimulated FPRL1, and that this resulted in STAT3 serine phosphorylation via PLD-mediated ERK activation, and that the serine phosphorylation of STAT3 blocked hydrogen peroxide-induced STAT3 activity.

Interleukin 4 Regulates Phosphorylation of Serine 756 in the Transactivation Domain of Stat6

Lymphokines interleukin-4 (IL4) and IL13 exert overlapping biological activities via the shared use of the IL4 receptor alpha-chain and signal transducer and activator of transcription 6 (Stat6). Stat6 is critical for T-helper 2 cell differentiation, B-cell Ig class switch, and allergic diseases; thus, understanding its regulation is of central importance. Phosphorylation is crucial for Stat activity. Whereas Stat6 is phosphorylated on Tyr(641), less is known about serine or threonine. We demonstrate in primary human T-cells (>95% CD3+) that IL4 and for the first time IL13 induce Stat6 serine but not threonine phosphorylation that closely paralleled early IL4 receptor alpha-chain activation (10 min). Stat6 uniquely fails to share a positionally conserved Stat serine phosphorylation sequence; however, known phosphoacceptor sites are proline-flanked. Alanine substitutions of these conserved residues revealed that the transactivation domain, which localized Ser(756) but not Ser(827) or Ser(176), is the IL4-regulated site based on phosphoamino acid analysis. Tyr(641) was dispensable for IL4-mediated serine phosphorylation, suggesting that dimerization is not preconditional. Only Stat6 Y641F variant showed a significant effect on IL4-inducible Cepsilon DNA-binding and reporter gene expression. Lastly, recent work has shown that protein phosphatase 2A negatively regulates Stat6. We propose this target residue(s) is distinct from Ser(756) and may be proximal to Tyr(641) at Thr(645), a residue conserved only among Stat6 members. The phosphomimic variants T645E or T645D ablated Stat6 activation, whereas polar uncharged substitutions (Gln or Asn) and additional mutants (Ala, Val, or Phe) showed no effect. These findings suggest that Stat6 has mechanisms of regulation distinct from other Stats.

Alternate interferon signaling pathways

More than a half a century ago, interferons (IFN) were identified as antiviral cytokines. Since that discovery, IFN have been in the forefront of basic and clinical cytokine research. The pleiotropic nature of these cytokines continues to engage a large number of investigators to define their actions further. IFN paved the way for discovery of Janus tyrosine kinase (JAK)-signal transducing activators of transcription (STAT) pathways. A number of important tumor suppressive pathways are controlled by IFN. Several infectious pathogens counteract IFN-induced signaling pathways. Recent studies indicate that IFN activate several new protein kinases, including the MAP kinase family, and downstream transcription factors. This review not only details the established IFN signaling paradigms but also provides insights into emerging alternate signaling pathways and mechanisms of pathogen-induced signaling interference.

Collagen-induced STAT family members activation in Entamoeba histolytica trophozoites

The interaction of Entamoeba histolytica trophozoites with collagen type I and calcium induces a membrane to nuclei signaling. The transduction pathways involved in such phenomena are still poorly understood. Using a combination of immunoprecipitation assays, Western immunoblot analysis, electrophoretic mobility shift assays and immunocytochemistry we demonstrate here the expression, tyrosine phosphorylation, nuclear translocation and DNA binding of two members of the signal transducers and activators of transcription family of inducible transcription factors in the protozoan parasite E. histolytica. These results support the notion that the interaction of the extracellular matrix components with the parasite turns on a genetic program that facilitates the invasion of the host.

Role of protein kinase C isoforms in the regulation of interleukin-13-induced 15-lipoxygenase gene expression in human monocytes

We reported previously that interleukin-13 (IL-13) induces tyrosine phosphorylation/activation of Jak2 and Tyk2 kinases and Stats 1, 3, 5, and 6 in primary human monocytes. We recently revealed that p38 MAPK-mediated serine phosphorylation of both Stat1 and Stat3 is required for the induction of 15-lipoxygenase (15-LO) expression by IL-13. In this study, we present data indicating that another serine/threonine kinase, PKCdelta, is also required for IL-13-induced 15-LO expression. PKCdelta, a member of the novel protein kinase C (PKC) subclass, was rapidly phosphorylated and activated upon exposure to IL-13. Treatment of cells with rottlerin, a PKCdelta inhibitor, blocked IL-13-induced 15-LO mRNA and protein expression, whereas Go6976, an inhibitor of the conventional PKC subclass, had no inhibitory effects. Down-regulation of cellular PKCdelta protein levels by PKCdelta-specific antisense oligodeoxyribonucleotides also inhibited 15-LO expression markedly. IL-13-induced 15-LO expression resulted in significant inhibition of synthesis of the potent chemotactic factor leukotriene B4, and that process was reversed by rottlerin, presumably through the blockage of PKCdelta-dependent 15-LO expression. Furthermore, our data demonstrate that IL-13-mediated activation of PKCdelta and p38 MAPK are independent pathways, because inhibition of one kinase activity had no effect on the other, suggesting that the two pathways act in parallel to regulate the downstream targets necessary for 15-LO expression. Inhibition of PKCdelta activation by rottlerin also markedly attenuated IL-13-induced Stat3 DNA binding activity. Our findings indicate that PKCdelta plays an important role in regulating IL-13-induced 15-LO expression in human monocytes and subsequently modulates the inflammatory responses mediated by 15-LO products.

Analysis of SOCS-3 promoter responses to interferon gamma

SOCS-3 (suppressor of cytokine signaling 3) is an intracellular protein that is selectively and rapidly induced by appropriate agonists and that modulates responses of immune cells to cytokines by interfering with the Janus kinase/signal transducer and activator of transcription (Jak/STAT) pathway. On the basis of the observations that interferon gamma (IFNgamma) up-regulates SOCS-3 gene and protein expression in primary mouse macrophages, J774 macrophage cell line and embryonal fibroblasts, we investigated which sequences of the 5' SOCS-3 gene are responsive to IFNgamma. By promoter deletion analysis we identified a functional IFNgamma-responsive element, located at nucleotides -72/-64 upstream from the transcription initiation, whose presence and integrity is necessary to ensure responsiveness to IFNgamma. This element contains a STAT consensus binding sequence (SOCS-3/STAT-binding element (SBE)) whose specific mutation totally abolished the responsiveness to IFNgamma. In contrast, discrete deletion of other 5' regions of the SOCS-3 promoter did not substantially modify the inducibility by IFNgamma. Electromobility shift assay analyses revealed that IFNgamma promotes specific DNA binding activities to an oligonucleotide probe containing the SOCS-3/SBE sequence. Even though IFNgamma triggered tyrosine phosphorylation of both STAT1 and STAT3 in macrophages and J774 cells, only STAT1 was appropriately activated and thus found to specifically bind to the SOCS-3/SBE oligonucleotide probe. Accordingly, IFNgamma-induced SOCS-3 protein expression was not impaired in STAT3-deficient embryonal fibroblasts. Taken together, these results demonstrate that the induction of SOCS-3 by IFNgamma depends upon the presence of a STAT-binding element in the SOCS-3 promoter that is specifically activated by STAT1.

The Role of Signal Transducer and Activator of Transcription Factors in Leukemogenesis

Human leukemias are frequently associated with the aberrant expression of activated fusion tyrosine kinases or activated protein tyrosine kinases carrying insertional or point mutations. The activated kinase enzymes typically phosphorylate one or more signal transducer and activator of transcription (STAT) factors, which translocate to the cell nucleus and regulate the expression of genes associated with survival and proliferation. The phosphorylation and activation of STAT family members has been described in a wide range of human leukemias. Furthermore, animal models of leukemia have demonstrated the pivotal contribution of STAT activation to leukemic pathogenesis. This review discusses evidence for the functional importance of STAT activation in the biology of leukemia and current opportunities for modulating STAT proteins in the therapy of this group of diseases.

Fibroblast growth factor-1 transcriptionally induces membrane type-1 matrix metalloproteinase expression in prostate carcinoma cell line

BACKGROUND

We and others have shown that the matrix metalloproteinases, MT1-MMP is overexpressed in human prostate PIN lesions and invasive cancers compared to normal prostate epithelium. However, the mechanism for this overexpression is not understood. Evidence from our laboratory and others has indicated that fibroblast growth factors (FGFs) can regulate the expression of certain matrix metalloproteinase. In addition, human prostate fibroblasts are known to express certain FGFs, including FGF-1. The purpose of the work in this paper was to determine the mechanism involved in FGF-1 induced MT1-MMP expression in prostate carcinoma cells.

METHODS

We tested the ability of recombinant FGF-1 to induce MT1-MMP expression in prostate carcinoma cell line, LNCaP cells. We measured the MT1-MMP message by using Northern analyses and protein levels by Western analysis after FGF-1 treatment. Downstream signaling was investigated using dominant negative constructs for FGFR-1 and signal transducer and activator of transcription-3 (STAT3). Transient transfection was performed using reporter plasmids of the MT1-MMP gene promoter region (7.2 kb) linked to the firefly luciferase gene in the pGL3-Basic vector. For dominant negative studies FGFR-1 dominant negative plasmid in PCEP4 vector or STAT3 dominant negative plasmid in pCMV-1 vector was co-transfected with the MT1-MMP reporter plasmid.

RESULTS

Recombinant FGF-1 significantly induced MT1-MMP expression in LNCaP prostate carcinoma cells. MT1-MMP message increased with FGF-1 treatment compared to that of untreated control LNCaP cells. Quantitation by digital image analysis revealed that this increase was twofold over untreated LNCaP cells. Treatment of pGL3-MT1-MMP-luciferase transfected cells with FGF-1 resulted in a twofold to fourfold increase in luciferase enzyme activity compared with untreated cells. Co-transfection of LNCaP with human MT1-MMP reporter construct and a dominant negative FGFR1 mutant showed that FGF-1-induced MT1-MMP expression in LNCaP cells was completely inhibited by the mutated FGFR-1, indicating that FGF receptor (FGFR) activation is necessary for induction of MT1-MMP. Further, expression of dominant negative STAT3 inhibited the FGF-1-induced transactivation of the human MT1-MMP 7.2-kb promoter.

CONCLUSIONS

From these data, we conclude that FGF-1 induces MT1-MMP expression in prostate carcinoma cells through a transcriptional mechanism mediated through the FGFR and the transcription factor, STAT3. These results confirm earlier data indicating that acidic FGF and STAT3 are involved in the signaling leading to the expression of a MMP. Our findings support the idea that paracrine and autocrine factors play an important role in the regulation of MT1-MMP in human prostate carcinoma cells.

Interferon-gamma: an overview of signals, mechanisms and functions

Interferon-gamma (IFN-gamma) coordinates a diverse array of cellular programs through transcriptional regulation of immunologically relevant genes. This article reviews the current understanding of IFN-gamma ligand, receptor, signal transduction, and cellular effects with a focus on macrophage responses and to a lesser extent, responses from other cell types that influence macrophage function during infection. The current model for IFN-gamma signal transduction is discussed, as well as signal regulation and factors conferring signal specificity. Cellular effects of IFN-gamma are described, including up-regulation of pathogen recognition, antigen processing and presentation, the antiviral state, inhibition of cellular proliferation and effects on apoptosis, activation of microbicidal effector functions, immunomodulation, and leukocyte trafficking. In addition, integration of signaling and response with other cytokines and pathogen-associated molecular patterns, such as tumor necrosis factor-alpha, interleukin-4, type I IFNs, and lipopolysaccharide are discussed.

Interleukin-11 and interleukin-6 protect cultured human endothelial cells from H2O2-induced cell death

Acute lung injury is a frequent and treatment-limiting consequence of therapy with 100% oxygen. Previous studies have determined that both interleukin (IL)-6 and IL-11 are protective in oxygen toxicity. This protection was associated with markedly diminished alveolar-capillary protein leak, endothelial and epithelial membrane injury, lipid peroxidation, and pulmonary neutrophil recruitment. Hyperoxia also caused cell death with DNA fragmentation in the lungs of transgene (-) animals, and both IL-6 and IL-11 markedly diminished this cell death response. However, the mechanism(s) by which these cytokines protect cells from death is unclear. In the present study, we characterized the effects of H2O2 on subconfluent human umbilical vein endothelial cell (HUVEC) and human pulmonary microvascular endothelial cell (HPMEC) cultures. We found that preincubation of HUVEC cultures with either IL-6 or IL-11 diminished H2O2 (1.0 mM)-induced cell death. Similar effects were noted with HPMEC showing that this effect is not HUVEC-specific. The protective effects of both IL-6 and IL-11 were not associated with any changes in antioxidants and were decreased by approximately 80% in the presence of U0126, a specific inhibitor of MEK-1-dependent pathways. The cytoprotective effects of IL-11 and IL-6 were also completely eliminated in STAT3 dominant-negative transduced HUVEC cultures. These studies demonstrate that IL-6 and IL-11 both confer cytoprotective effects that diminish oxidant-mediated endothelial cell injury. They also demonstrate that this protection is mediated, at least in part, by a STAT3 and MEK-1-dependent specific signal transduction pathway(s).

Induction of an interferon-γ Stat3 response in nerve cells by pre-treatment with gp130 cytokines

Many cytokines mediate their effects through Jak/STAT signaling pathways providing many opportunities for cross-talk between different cytokines. We examined the interaction between two cytokine families, gp130-related cytokines and interferon-gamma (IFN-gamma), which are coexpressed in the nervous system during acute trauma and pathological conditions. Typical nerve cells show an IFN-gamma response that is restricted to activating STAT1, with minor activation of STAT3. IFN-gamma elicited a pronounced STAT3 response in cells pre-treated for 5-7 h with ciliary neurotrophic factor (CNTF), leukemia inhibitory factor or interleukin-6. CNTF or interleukin-6 induced an IFN-gamma STAT3 response in a variety of cells including SH-SY5Y human neuroblastoma, HMN-1 murine motor neuron hybrid cells, rat sympathetic neurons and human hepatoma HepG2 cells. The enhancement was measured as an increase in tyrosine phosphorylated STAT3, in STAT3-DNA binding and in STAT-luciferase reporter gene activity. The enhanced STAT3 response was not due to an increase in overall STAT3 levels but was dependent upon ongoing protein synthesis. The induction by CNTF was inhibited by the protein kinase C inhibitor, BIM, and the MAPK-kinase inhibitor, U0126. Further, H-35 hepatoma cells expressing gp130 receptor chimeras lacking either the SHP-2 docking site or the Box 3 STAT binding sites failed to enhance the IFN-gamma STAT3 response. These results provide evidence for an interaction between gp130 and IFN-gamma cytokines that can significantly alter the final cellular response to IFN-gamma.

STATs dimerize in the absence of phosphorylation

Upon activation by tyrosine kinases, members of the STAT family of transcription factors form stable dimers that are able to rapidly translocate to the nucleus and bind DNA. Although crystal structures of activated, near full-length, Stat1 and Stat3 illustrate how STATs bind to DNA, they provide little insight into the dynamic regulation of STAT activity. To explore the unique structural changes Stat1 and Stat3 undergo when they become activated, full-length inactive recombinant proteins were prepared. To our surprise, even though these proteins are unable to bind DNA, our studies demonstrate that they exist as stable homodimers. Similarly, the Stat1 and Stat3 found in the cytoplasm of unstimulated cells also exhibit a dimeric structure. These observations indicate that Stat1 and Stat3 exist as stable homodimers prior to activation.

Tudor and nuclease-like domains containing protein p100 function as coactivators for signal transducer and activator of transcription 5

Signal transducer and activator of transcription 5 (Stat5) plays a critical role in prolactin (PRL)-induced transcription of several milk protein genes. Stat5-mediated gene regulation is modulated by cooperation of Stat5 with cell type- and promoter-specific transcription factors as well as by interaction with transcriptional coregulators. Recently, the expression of a tudor and staphylococcal nuclease-like domains containing protein p100 was found to be increased in mammary epithelial cells during lactation in response to lactogenic hormones. p100 was initially identified as a transcriptional coactivator of the Epstein-Barr virus nuclear antigen 2. In this study we investigated the potential role of p100 in PRL-induced Stat5-mediated transcriptional activation. PRL stimulation increased the p100 protein levels in HC11 mouse mammary epithelial cells. p100 did not affect the early activation events of Stat5, but p100 enhanced the Stat5-dependent transcriptional activation in HC11 cells. p100 associated with Stat5 both in vivo and in vitro, and the interaction was mediated by both the tudor and staphylococcal nuclease-like domains of p100. Together these results suggest that p100 functions as a transcriptional coactivator for Stat5-dependent gene regulation and the existence of a positive regulatory loop in PRL-induced transcription, in which PRL stabilizes p100 protein, which in turn can cooperate with Stat5 in transcriptional activation.

The cell death regulator GRIM-19 is an inhibitor of signal transducer and activator of transcription 3

GRIM-19 (gene associated with retinoid-IFN-induced mortality 19), isolated as a cell death activator in a genetic screen used to define mechanisms involved in IFN-beta- and retinoic acid-induced cell death, codes for a approximately 16-kDa protein that induces apoptosis in a number of cell lines. Antisense ablation of GRIM-19 caused resistance to cell death induced by IFN plus retinoic acid and conferred a growth advantage to cells. To understand the molecular bases for its cell death regulatory activity, we used a yeast two-hybrid screen and identified that the transcription factor STAT3 (signal transducer and activator of transcription 3) binds to GRIM-19. GRIM-19 inhibits transcription driven by activation of STAT3, but not STAT1. It neither inhibits the ligand-induced activation of STAT3 nor blocks its ability to bind to DNA. Mutational analysis indicates that the transactivation domain of STAT3, especially residue S727, is required for GRIM-19 binding. Because GRIM-19 does not bind significantly to other STATs, our studies identify a specific inhibitor of STAT3. Because constitutively active STAT3 up-regulates antiapoptotic genes to promote tumor survival, its inhibition by GRIM-19 also demonstrates an antioncogenic effect exerted by biological therapeutics.

Constitutive and specific activation of STAT3 by BCR-ABL in embryonic stem cells

BCR-ABL oncogene, the molecular hallmark of chronic myelogenous leukemia (CML) arises in a primitive hematopoietic stem cell with both differentiation and self-renewal ability. To study the phenotypic effects of BCR-ABL in a clonal in vitro self-renewal and differentiation model, we have introduced BCR-ABL in the ES cell line CCE. The major effect of BCR-ABL expression was the persistence of primitive morphology of ES cells despite LIF deprivation, correlated with a constitutive activation of STAT3, the major self-renewal factor of ES cells, but no evidence of activation of STAT5. The enforced expression of BCR-ABL in an ES cell line, engineered to express a tetracycline-inducible dominant-negative form of a STAT3, triggered ES cell differentiation with an increased generation of hematopoietic cells expressing erythroid and megakaryocytic phenotypes. RT-PCR analysis for Oct4, Brachyury and beta-globin expression confirmed a delay of differentiation in BCR-ABL expressing clones, which could be entirely reversed upon activation of the dominant-negative form of STAT3. To study the possible relevance of STAT3 activation by BCR-ABL in human CML, Western blot analyses performed on the CD34+ cells, purified from CML patients at different stages of their disease, also demonstrated increased levels of STAT3 proteins phosphorylated both on tyrosine and serine residues. These results represent to our knowledge the first functional link between BCR-ABL oncogene and a self-renewal in the context of ES cells through constitutive activation of STAT3. Thus, the BCR-ABL embryonic stem cell model that we developed as well as the results obtained in human CML samples suggests a role for STAT3 in the pathogenesis of human CML.

BMPs signal alternately through a SMAD or FRAP-STAT pathway to regulate fate choice in CNS stem cells

The ability of stem cells to generate distinct fates is critical for the generation of cellular diversity during development. Central nervous system (CNS) stem cells respond to bone morphogenetic protein (BMP) 4 by differentiating into a wide variety of dorsal CNS and neural crest cell types. We show that distinct mechanisms are responsible for the generation of two of these cell types, smooth muscle and glia. Smooth muscle differentiation requires BMP-mediated Smad1/5/8 activation and predominates where local cell density is low. In contrast, glial differentiation predominates at high local densities in response to BMP4 and is specifically blocked by a dominant-negative mutant Stat3. Upon BMP4 treatment, the serine-threonine kinase FKBP12/rapamycin-associated protein (FRAP), mammalian target of rapamycin (mTOR), associates with Stat3 and facilitates STAT activation. Inhibition of FRAP prevents STAT activation and glial differentiation. Thus, glial differentiation by BMP4 occurs by a novel pathway mediated by FRAP and STAT proteins. These results suggest that a single ligand can regulate cell fate by activating distinct cytoplasmic signals.