Reduced STAT3 Activation Research Articles

Aged regulatory T cells protect from autoimmune inflammation despite reduced STAT3 activation and decreased constraint of IL‐17 producing T cells

Regulatory T cells (Tregs) are specialized CD4(+) T lymphocytes helping defend against autoimmunity and inflammation. Although age is associated with increased inflammation and autoimmunity, few reports address age effects of immune regulation or auto-aggressive T cells. We show here that young and aged naïve CD4(+) T cells are equivalently auto-aggressive in vivo in T cell-driven autoimmune colitis. Young and aged CD4(+) Tregs equally suppressed age-matched T cell proliferation in vitro and controlled clinical and pathologic T cell-driven autoimmune colitis, suggesting equivalent regulatory function. However, whereas young and aged CD4(+) Tregs suppressed interferon (IFN)-γ(+) T cells equivalently in this model, aged CD4(+) Tregs unexpectedly failed to restrain interleukin (IL)-17(+) T cells. Nonetheless, young and aged CD4(+) Tregs equally restrained IL-17(+) T cells in vivo during acute inflammation, suggesting a chronic inflammation-related defect in aged CD4(+) Tregs. In support, aged Tregs expressed reduced STAT3 activation, a defect associated with poor IL-17-producing T cell restraint. Aged naïve mice had markedly increased programmed death (PD)-1(+) T cells, but these exhibited no significant auto-aggressive or regulatory functions in T cell-driven colitis. Young CD8(+) CD122(-) T cells induce autoimmune bone marrow failure, but we show that aged CD8(+) CD122(-) T cells do not. These data demonstrate no apparent age-related increase in auto-aggressive T cell behavior, but disclose previously unrecognized functional defects in aged CD4(+) Tregs during chronic inflammation. IL-17 can be inflammatory and contributes to certain autoimmune disorders. Reduced aged Treg function during chronic inflammation and reduced IL-17 restraint could contribute to age-related inflammation or autoimmunity.

The JAK2/STAT3 signaling pathway is required for growth of CD44+CD24– stem cell–like breast cancer cells in human tumors

Intratumor heterogeneity is a major clinical problem because tumor cell subtypes display variable sensitivity to therapeutics and may play different roles in progression. We previously characterized 2 cell populations in human breast tumors with distinct properties: CD44+CD24- cells that have stem cell-like characteristics, and CD44-CD24+ cells that resemble more differentiated breast cancer cells. Here we identified 15 genes required for cell growth or proliferation in CD44+CD24- human breast cancer cells in a large-scale loss-of-function screen and found that inhibition of several of these (IL6, PTGIS, HAS1, CXCL3, and PFKFB3) reduced Stat3 activation. We found that the IL-6/JAK2/Stat3 pathway was preferentially active in CD44+CD24- breast cancer cells compared with other tumor cell types, and inhibition of JAK2 decreased their number and blocked growth of xenografts. Our results highlight the differences between distinct breast cancer cell types and identify targets such as JAK2 and Stat3 that may lead to more specific and effective breast cancer therapies.

CK2 Inhibitors Enhance the Radiosensitivity of Human Non-Small Cell Lung Cancer Cells Through Inhibition of Stat3 Activation

CK2 interacts and phosphorylates >300 proteins, including Stat3, and is linked to a number of human cancers. Constitutively activated Stat3 has been reported in 50% of human lung cancers. Inhibition of CK2 activity can induce apoptosis and suppression of Stat3 activation in cancer cells. This study examined the effects of CK2 inhibitors on growth inhibition of lung cancer cells and the therapeutic potential on lung cancer. The CK2 inhibitor and radiation both suppressed cancer cell growth in a dose-dependent manner. Besides, the cytotoxic effect of irradiation could be augmented by CK2 inhibitors (p<0.05, two-way analysis of variance and Tukey's Honestly Significant Difference). Moreover, the growth inhibition of CK2 inhibitor and irradiation was both associated with suppression of Stat3 activation. Taken together, inhibition of CK2 activity appears to be a promising treatment strategy for non-small cell lung cancer and CK2 inhibition results in reduced Stat3 activation. Our data warrant further effort to develop CK2-targeted radiosensitizer for lung cancer treatment.

Abstract 3311: IL6-Stat3 signaling promotes glioma stem cell (GSC) survival under hypoxia environment through inhibition of cell-death inducing gene Bnip3

Abstract Glioblastomas (GBMs) are the most common and lethal brain tumor in adults. In rapid-growing tumors like malignant glioma, hypoxia is an unavoidable condition even with aggressive angiogenesis. Recent studies implicate an important role for a restricted population of neoplastic cells (glioma stem cells, GSCs) in glioma maintenance and recurrence. However, how these glioma stem cells survive and escape from harsh conditions like hypoxia is barely known. Our previous study and others’ have shown that GSCs preferentially express the interleukin 6 (IL6) receptors and Stat3 signaling is preferentially activated in GSCs. We further found that perturbation of IL6 signaling in GSCs reduced Stat3 activation, and small molecule inhibitors of Stat3 potently induced GSC cell death partially through induction of a BCL2 family member Bnip3. Despite its controversial role in cell survival, massive expression of Bnip3 is a well-studied mediator of cell death in hypoxic tumor cells. Furthermore, we found that enhanced IL6-Stat3 signaling in GSCs could counter the induction of Bnip3 under hypoxia, which provided an extra survival advantage for these glioma stem cells under severe hypoxic condition. These data indicated that Stat3 is a downstream mediator of pro-survival IL6 signals in GSCs partially through suppressing Bnip3 expression, especially under hypoxia condition. Together, our data indicate that IL6-Stat3 signaling contributes to glioma malignancy through the promotion of GSC survival under harsh conditions like hypoxia, and that targeting IL6-Stat3 signaling pathway may offer benefit for glioma patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3311. doi:10.1158/1538-7445.AM2011-3311

Abstract 3136: Activated Src increases total Rac levels and requires Rac and IL6 for full neoplastic transformation

Abstract Activated Src (vSrc) was previously shown to activate Rac through the exchange factors Vav2 and Tiam, and to require Rac for neoplastic transformation, but the exact mechanism remains unclear. Since Src requires the function of the Signal transducer and activator of transcription-3 (Stat3), a Rac target for transformation, we explored the potential role of the Rac/Stat3 axis in transformation by vSrc. In fact, we recently demonstrated a dramatic increase in the activity of Stat3 in breast carcinoma as well as normal epithelial cells and fibroblasts, as a consequence of cell to cell adhesion (Oncogene 23:2600). In light of these findings, we revisited the question of the role of Rac and Stat3 in vSrc transformation. The results showed that, as previously demonstrated, vSrc expressing mouse fibroblasts had higher Rac activity, at all densities examined. Unexpectedly, our results also demonstrate for the first time a dramatic surge in total Rac upon Src expression, which was at least in part responsible for the Stat3 activity increase. Moreover, Rac activation caused an increase in levels of cytokines of the IL6 family, which was required for Stat3 activation, as shown by the fact that knockdown of gp130, the common receptor subunit of this family, reduced Stat3 activation and neoplastic transformation. Besides transformation, the Rac/IL6/Stat3 axis was required for apoptosis inhibition upon vSrc expression. (supported by CIHR, CBCF-Ontario chapter, US Army breast cancer program, NSERC and Breast Cancer Action Kingston). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3136.

Abstract 991: Cadherin-cadherin engagement promotes cell survival via Rac/Cdc42 and Stat3

Abstract Stat3 (signal transducer and activator of transcription-3) is activated by a number of receptor and non-receptor tyrosine kinases, while a constitutively active form of Stat3 alone is sufficient to induce neoplastic transformation. We recently demonstrated a dramatic increase in the activity of Stat3 in breast carcinoma as well as normal epithelial cells and fibroblasts, as a consequence of cell to cell adhesion (Oncogene 23:2600). Given the generally accepted, positive role of Stat3 in proliferation, the Stat3 activity increase observed in confluent cells, that is when cells do not divide, was an unexpected observation. Interestingly, by plating cells onto surfaces coated with fragments encompassing the two outermost domains of E-cadherin and cadherin-11, two members of the classical type I and II cadherin family of surface receptors, responsible for the formation of cell to cell junctions, we demonstrated that cadherin engagement per se can directly activate Stat3, in the absence of cell to cell contact. Examination of the mechanism of the cadherin-mediated, Stat3 activation unexpectedly revealed for the first time a dramatic surge in total Rac1 and Cdc42 protein levels by cadherin engagement, and a proportional increase in Rac1 and Cdc42 activity. Therefore, to examine the potential role of Rac/Cdc42 in the density-dependent, Stat3 activation, the ability of mutationally activated RacV12 to activate Stat3 at high cell densities was examined. The results revealed a dramatic increase in protein levels and activity of both the endogenous Rac and RacV12 with cell density, which was due to inhibition of proteasomal degradation in both cases. In addition, RacV12-expressing cells had higher Stat3, tyrosine-705 phosphorylation and activity levels at all densities, indicating that RacV12 is, in fact, able to activate Stat3. Further examination of the mechanism of Stat3 activation showed that both cadherin engagement and RacV12 expression caused a surge in mRNA of Interleukin-6 (IL6) family cytokines, known potent Stat3 activators. Knockdown of gp130, the common subunit of this family reduced Stat3 activity in densely growing normal, as well as in RacV12-transformed cells, indicating that the IL6 family may be responsible for the Stat3 activation both by cadherin engagement and Rac mutational activation. Indeed, Rac knockdown reduced the density-mediated, Stat3 activation, indicating that Rac is responsible for the Stat3 stimulation observed upon cadherin ligation. Inhibition of cadherin interactions using a peptide, a soluble cadherin fragment or genetic ablation induced apoptosis, pointing to a significant role of this pathway in cell survival signalling, a finding which could also have important therapeutic implications. (supported by CIHR, CBCF-Ontario chapter, US Army breast cancer program, NSERC and Breast Cancer Action Kingston). Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 991.

Activated Rac1 requires gp130 for Stat3 activation, cell proliferation and migration

Rac1 (Rac) is a member of the Rho family of small GTPases which controls cell migration by regulating the organization of actin filaments. Previous results suggested that mutationally activated forms of the Rho GTPases can activate the Signal Transducer and Activator of Transcription-3 (Stat3), but the exact mechanism is a matter of controversy. We recently demonstrated that Stat3 activity of cultured cells increases dramatically following E-cadherin engagement. To better understand this pathway, we now compared Stat3 activity levels in mouse HC11 cells before and after expression of the mutationally activated Rac1 (Rac V12), at different cell densities. The results revealed for the first time a dramatic increase in protein levels and activity of both the endogenous Rac and Rac V12 with cell density, which was due to inhibition of proteasomal degradation. In addition, Rac V12-expressing cells had higher Stat3, tyrosine-705 phosphorylation and activity levels at all densities, indicating that Rac V12 is able to activate Stat3. Further examination of the mechanism of Stat3 activation showed that Rac V12 expression caused a surge in mRNA of Interleukin-6 (IL6) family cytokines, known potent Stat3 activators. Knockdown of gp130, the common subunit of this family reduced Stat3 activity, indicating that these cytokines may be responsible for the Stat3 activation by Rac V12. The upregulation of IL6 family cytokines was required for cell migration and proliferation induced by Rac V12, as shown by gp130 knockdown experiments, thus demonstrating that the gp130/Stat3 axis represents an essential effector of activated Rac for the regulation of key cellular functions.

Jnk signaling pathway-mediated regulation of Stat3 activation is linked to the development of doxorubicin resistance in cancer cell lines

We sought to identify altered transcription factors (Stat, AP1, and NF-kB) or signal proteins (Erk1/2, p38, Akt, Jnk, Jak, and c-Src) in cancer cell lines whose growth was arrested by doxorubicin (DOX) treatment. Jnk1 was the only signal protein to be activated. DOX increased Stat3 phosphorylation, nuclear localization, and transcriptional activity. Jnk1 activation appeared to be required for Stat3 activity. Stat3 activity via the Jnk pathway was conserved in other cell lines originating from other organs. Transcriptional activity of Stat3 was increased in cells surviving DOX treatment suggesting that Stat3 activation contributed to the resistance to cytotoxicity. To better understand the role of Stat3 in Jnk1 activation, we investigated its effect on the viability of DOX-treated cells. Co-treatment with DOX and Jnk inhibitor negatively correlated with the viability of cancer cells and reduced Stat3 activity. Taken together, these results indicate that Stat3 activation via the Jnk pathway promotes the resistance of cancer cells to DOX.

Cholinergic agonists regulate JAK2/STAT3 signaling to suppress endothelial cell activation

The cholinergic anti-inflammatory pathway is a physiological mechanism that inhibits cytokine production and minimizes tissue injury during inflammation. Previous investigations revealed that cholinergic stimulation (via cholinergic agonists and vagus nerve stimulation) suppresses endothelial cell activation and leukocyte recruitment. The purpose of this study was to investigate the mechanisms by which cholinergic agonists (e.g., nicotine and GTS-21) regulate endothelial cell activation. Specifically, we examined the effects of cholinergic agonists on IL-6-mediated endothelial cell activation through the JAK2/STAT3 signaling pathway. Treatment of macrovascular human umbilical vein endothelial cells (HUVECs) and microvascular endothelial cells (MVECs) with the cholinergic agonists nicotine and GTS-21 significantly reduced IL-6-mediated monocyte chemoattractant protein-1 (MCP-1) production and ICAM-1 expression which are regulated through the JAK2/STAT3 pathway. We found that treatment of endothelial cells with cholinergic agonists significantly reduced STAT3 activation by phosphorylation and DNA binding. The inhibition of STAT3 phosphorylation was reversed by sodium orthovanadate, an inhibitor of tyrosine phosphatases, as well as by NSC-87877 suggesting a SHP1/2-dependent mechanism. Further investigations showed that cholinergic agonists reduced the phosphorylation of JAK2, an upstream component of the JAK2/STAT3 pathway. Finally, we observed that nicotine and GTS-21 treatment decreased levels of SOCS3 (suppressor of cytokine signaling; a regulator of the inflammatory activity of IL-6) in activated endothelial cells. These data demonstrate that cholinergic agonists suppress IL-6-mediated endothelial cell activation through the JAK2/STAT3 pathway. Our results have significant implications for better understanding the therapeutic potential of cholinergic agonists for treating IL-6 mediated inflammatory conditions.

NADPH oxidase and ERK1/2 are involved in cadmium induced-STAT3 activation in HepG2 cells

The molecular mechanism of Cd-induced signal transduction is not well understood. The aims of this study were to determine the system that generates reactive oxygen species in response to Cd that contribute to intracellular signaling on the activation of the STAT3 pathway in HepG2 cells and to address the participation of STAT3 in the production of Hsp70. Cadmium induced a significant increase in STAT3 DNA-binding after 1h treatment. Serine phosphorylation of STAT3 was observed as a result of cadmium treatment while no tyrosine phosphorylation was detected. Cells were pretreated with inhibitors for several ROS generating systems, only diphenylen iodonium, an inhibitor of NADPH oxidase, decreased STAT3 activation. Cd induced 2.6-fold NADPH oxidase activity. Antioxidant treatment with pegylated-catalase reduced STAT3 activation. Cells were pretreated with different MAPK's inhibitors. ERK contributes in approximately 60%, and JNK in a small proportion, while p38 does not contribute in STAT3 activation. Cells were pretreated with a specific STAT3 peptide inhibitor that decreased the Cd-induced Hsp70 expression. Data suggest that STAT3 is phosphorylated at serine 727 by a Cd stress-activated signaling pathway inducing NADPH oxidase activity which produced ROS, leading ERK activation. MAPK promotes STAT3 phosphorylation that could induce a protective mechanism against Cd toxicity.

Simultaneous inhibition of the constitutively activated nuclear factor κB and of the Interleukin-6 pathways is necessary and sufficient to completely overcome apoptosis resistance of human U266 myeloma cells

Elevated Nuclear Factor κB (NF-κB) levels have been reported in multiple myeloma cells derived from patients relapsing after chemotherapy. In the search of an in vitro a model with molecular features similar to relapsing lesions, we focused our attention on an IL-6 autocrine human myeloma cell line (U266), characterized by apoptosis resistance due to up-regulation of two constitutive signaling pathways: NF-κB and STAT-3. NF-κB activity was inhibited with proteasome inhibitory agents, such as PS-341 and Withaferin A, with an IKK inhibitor (Wedelolactone) or with the adenoviral vector HD IκBαmut-IRES-EGFP encoding a mutant IκBα protein, resistant to proteasomal degradation. We observed that the NF-κB intracellular dislocation at the beginning of the treatment affected therapeutic effectiveness of PS-341, Withaferin A and Wedelolactone; interestingly, the adenoviral vector was highly effective in inducing apopotosis even with NF-κB being predominantly nuclear at the time of infection. We also observed that U266 treated with the Interleukin-6 antagonist Sant7 exhibited reduced STAT3 activity and preferential cytoplasmic NF-κB location; moreover they became capable of undergoing apoptosis mainly from the G1 phase. Adenoviral vector treated U266 have NF-κB localized completely in the cytoplasm and also showed down-regulation of nuclear phospho STAT-3. Finally, combined targeting of NF-κB and STAT3 signaling pathways was the most effective treatment in inducing apoptosis. These findings suggest that combined NF-κB κB and STAT3 targeting warrants further investigations in other apoptosis resistant MM cell lines as well as in suitable MM animal models.

IL-6 Regulates Neutrophil Trafficking during Acute Inflammation via STAT3

The successful resolution of inflammation is dependent upon the coordinated transition from the initial recruitment of neutrophils to a more sustained population of mononuclear cells. IL-6, which signals via the common receptor subunit gp130, represents a crucial checkpoint regulator of neutrophil trafficking during the inflammatory response by orchestrating chemokine production and leukocyte apoptosis. However, the relative contribution of specific IL-6-dependent signaling pathways to these processes remains unresolved. To define the receptor-mediated signaling events responsible for IL-6-driven neutrophil trafficking, we used a series of gp130 knockin mutant mice displaying altered IL-6-signaling capacities in an experimental model of acute peritoneal inflammation. Hyperactivation of STAT1 and STAT3 in gp130(Y757F/Y757F) mice led to a more rapid clearance of neutrophils, and this coincided with a pronounced down-modulation in production of the neutrophil-attracting chemokine CXCL1/KC. By contrast, the proportion of apoptotic neutrophils in the inflammatory infiltrate remained unaffected. In gp130(Y757F/Y757F) mice lacking IL-6, neutrophil trafficking and CXCL1/KC levels were normal, and this corresponded with a reduction in the level of STAT1/3 activity. Furthermore, monoallelic ablation of Stat3 in gp130(Y757F/Y757F) mice specifically reduced STAT3 activity and corrected both the rapid clearance of neutrophils and impaired CXCL1/KC production. Conversely, genetic deletion of Stat1 in gp130(Y757F/Y757F) mice failed to rescue the altered responses observed in gp130(Y757F/Y757F) mice. Collectively, these data genetically define that IL-6-driven signaling via STAT3, but not STAT1, limits the inflammatory recruitment of neutrophils, and therefore represents a critical event for the termination of the innate immune response.

IL‐23‐mediated regulation of IL‐17 production in Helicobacter pylori‐infected gastric mucosa

Helicobacter pylori (Hp) infection is associated with a marked infiltration of the gastric mucosa by inflammatory cells. The molecular pathways that control Hp-associated inflammatory reaction are complex, but locally induced cytokines seem to contribute to maintaining the ongoing inflammation. We have previously shown that IL-17 is over-produced in Hp-infected gastric mucosa, and that IL-17 stimulates the synthesis of IL-8, the major neutrophil chemoattractant. Factors/mechanisms that regulate IL-17 expression remain, however, unknown. In this study, we initially expanded our previous data, showing that CD4(+) and CD8(+) T cells are a source of IL-17 in Hp-infected samples. Since IL-23 enhances T cell-derived IL-17 during bacterial infections, we then assessed the role of IL-23 in controlling IL-17 expression in Hp-colonized stomach. Using real-time PCR and ELISA, IL-23 was detected in all gastric biopsies, but its expression was more pronounced in Hp-infected samples in comparison to controls. Treatment of normal gastric lamina propria mononuclear cells (LPMC) with IL-23 enhanced Stat3 activation and IL-17 secretion, and pharmacological inhibition of Stat3 prevented IL-23-driven IL-17 synthesis. Consistently, blockade of IL-23 in cultures of LPMC from Hp-infected patients reduced Stat3 activation and IL-17 production. Data show that IL-23 is overexpressed in Hp-infected gastric mucosa where it could contribute to sustaining IL-17 production.

Arsenic Trioxide Affects Signal Transducer and Activator of Transcription Proteins through Alteration of Protein Tyrosine Kinase Phosphorylation

Arsenic trioxide decreases proliferation of acute myeloid leukemia (AML) cells, but its precise mechanism of action is unknown. We studied the effect of arsenic trioxide on patient samples and the AML cell line HEL, which, like leukemic blasts from 50% of AML cases, has constitutively activated signal transducer and activator of transcription (STAT) proteins. Arsenic trioxide induced mitotic arrest starting at 24 hours and significant cell death at 48 hours. These events were preceded by an arsenic trioxide dose-dependent down-regulation of activated STAT proteins starting at 6 hours. We hypothesized that arsenic trioxide inhibits protein tyrosine kinases (PTK), which, among others, phosphorylate and activate STATs. We therefore studied arsenic trioxide effects on Janus kinases and on three oncogenic PTKs that are known to activate STATs [FLT3, ZNF198/fibroblast growth factor receptor 1 (FGFR1), and BCR/ABL]. Arsenic trioxide reduced STAT3 activation by Janus kinases, altered phosphorylation and electrophoretic mobility of ZNF198/fibroblast growth factor receptor 1, reduced kinase protein level, and decreased STAT3 protein phosphorylation. Arsenic trioxide also reduced the phosphorylation of BCR/ABL and FLT3 with corresponding decreased STAT5 phosphorylation. These results suggest a selective activity of arsenic trioxide on PTKs and will assist in developing clinical trials in AML.

STAT3 Activation Regulates Growth, Inflammation, and Vascularization in a Mouse Model of Gastric Tumorigenesis

The gp130(757F/F) mouse is a well-characterized and robust model of distal gastric tumorigenesis displaying many of the characteristics of human intestinal type gastric cancer. Key to the development of tumors in this model, and in many examples of human tumor development, is hyperactivation of the transcription factor STAT3. This study addressed the requirement for STAT3 activation in tumor initiation and characterized some of the genes downstream of STAT3 required for tumor development. Furthermore, the interaction among STAT3, the microbial environment, and tumorigenesis was evaluated. The role of STAT3 in gastric tumor development was assessed in detail in gp130(757F/Y757F):STAT3(+/-) mice displaying reduced STAT3 activity. Tumor size was quantified morphologically, and the effects on endocrine cell populations, neovascularization, and inflammatory cell infiltration as well as the outcome of STAT3 activation on transcription of a number of genes relevant in growth and inflammation were quantified. Loss of one STAT3 allele in gp130(757F/F) mice reduced the frequency and rate of tumor development because of inhibition of proliferation-induced glandular hyperplasia. There was also a concomitant reduction in the degree of inflammatory infiltration and cytokine and chemokine expression, angiogenesis, and expression of metalloproteinases and growth factors. Antimicrobial treatment of gp130(757F/F) mice slowed tumor growth coincident with reduced macrophage and neutrophil infiltration. Activation of STAT3 and the microbial environment are pivotal for gastric tumor initiation and development in the gp130(757F/F) mouse, thus supporting the notion that STAT3 activation may play a role in human gastric cancer development.

Hyperactivation of Stat3 in gp130 mutant mice promotes gastric hyperproliferation and desensitizes TGF-β signaling

The latent transcription factor Stat3 is activated by gp130, the common receptor for the interleukin (IL)-6 cytokine family and other growth factor and cytokine receptors. Ligand-induced dimerization of gp130 leads to activation of the Stat1, Stat3 and Shp2-Ras-Erk signaling pathways. Here we assess genetically the contribution of exaggerated Stat3 activation to the phenotype of gp130 (Y757F/Y757F) mice, in which a knock-in mutation disrupts the negative feedback mechanism on gp130-dependent Stat signaling. Compared to gp130 (Y757F/Y757F) mice, reduced Stat3 activation in gp130 (Y757F/Y757F) Stat3(+/-) mice increased their lifespan, prevented splenomegaly, normalized exaggerated hepatic acute-phase response and lymphocyte trafficking, and suppressed the growth of spontaneously arising gastric adenomas in young mice. These lesions share histological features of gastric polyps in aging mice with monoallelic null mutations in Smad4, which encodes the common transducer for transforming growth factor (TGF)-beta signaling. Indeed, hyperactivation of Stat3 desensitizes gp130 (Y757F/Y757F) cells to the cytostatic effect of TGF-beta through transcriptional induction of inhibitory Smad7, thereby providing a novel link for cross-talk between Stat and Smad signaling in gastric homeostasis.

Growth Hormone Inhibits Signal Transducer and Activator of Transcription 3 Activation and Reduces Disease Activity in Murine Colitis

Constitutive signal transducer and activator of transcription (STAT) 3 activation promotes chronic inflammation and epithelial proliferation in murine colitis and human inflammatory bowel disease. SHP-2, through binding to the glycoprotein 130 signaling receptor, negatively regulates STAT3 activation. Growth hormone reduces disease activity and promotes mucosal healing in colitis and can activate SHP-2. We hypothesized that growth hormone administration would reduce disease activity in experimental colitis and that this would involve modulation of SHP-2/glycoprotein 130 association and STAT3 activation. Growth hormone administration improved weight gain and colon histology in interleukin 10-null mice with colitis. Growth hormone reduced apoptosis and increased proliferation of crypt epithelial cells while increasing apoptosis of lamina propria mononuclear cells. Growth hormone increased SHP-2/glycoprotein 130 association and reduced colonic STAT3 activation in interleukin 10-null mice and in biopsy samples from patients with Crohn's colitis. Expression of the antiapoptotic protein bcl-2 was increased in crypt epithelial cells after growth hormone treatment. Growth hormone increased SHP-2/glycoprotein 130 binding and reduced interleukin 6-dependent STAT3 activation in the T84 human colon carcinoma and Jurkat human T-cell leukemia lines. Growth hormone administration improves weight gain and reduces disease activity in interleukin 10-null mice with colitis. The improvement in disease activity is associated with increased SHP-2/glycoprotein 130 binding and reduced STAT3 activation in both murine and Crohn's colitis. Growth hormone may be a useful therapy in inflammatory bowel disease, in terms of both improving anabolic metabolism and enhancing mucosal healing.

Reduced STAT3 activity in mice mimics clinical disease syndromes

Phosphorylation on Y705 is obligatory for STAT3 activation, but full transcriptional activity of this widely expressed protein also requires phosphorylation on S727. We described earlier the STAT3 SA/− mice (SA, S727A allele) on a Black 6 (Bl6) background that showed 75% perinatal lethality and early growth retardation presumably due to the decreased transcription supported by STAT3 S727A. We now report additional analyses of long-term surviving SA/− animals which show no important tissue abnormalities. However, we have found a much greater susceptibility to doxorubicin-induced heart failure in the SA/− mice. Also we introduced the SA allele into strain 129 and found the SA/− mice showed greater susceptibility to LPS-induced toxicity. These results suggest a continued need for normal STAT3 transcriptional activity to resist two different noxious challenges that mimic the conditions necessary to induce adult diseases.