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

Dual role of inositol-requiring enzyme 1α (IRE-1α) in Cd-induced apoptosis in human renal tubular epithelial cells: Endoplasmic reticulum stress and STAT3 signaling activation

Cadmium (Cd) is a nephrotoxicant that primarily damages renal proximal tubular cells. Endoplasmic reticulum (ER) stress is mechanistically linked to Cd-induced renal injury. Inositol-requiring enzyme 1 (IRE-1α) is the most conserved ER stress transducer protein, which has both kinase and endonuclease activities. This study aimed to investigate whether the two enzymatic activities of IRE-1α have different effects in its regulation of Cd-induced apoptosis. Human proximal tubular (HK-2) cells were treated with 20 μM CdCl₂ for 0-24 h, and mice were fed with Cd-containing drinking water (100-400 mg/L) for 24 weeks. We found that Cd increased cell apoptosis in HK-2 cells and mouse kidneys in a time-dependent manner. Such cytotoxicity was correlated with activation of ER stress, evidenced by upregulation of IRE-1α and its target protein spliced X-box binding protein-1 (XBP-1 s). Interestingly, inhibition of IRE-1α kinase activity by KIRA6 was more protective against Cd-induced apoptosis than inhibition of its RNase activity by STF-083010. Mechanistically, Cd promoted the binding of IRE-1α with signal transducer and activator of transcription-3 (STAT3) leading to elevated phosphorylation of STAT3 at Ser727 and thus inactivation of STAT3 signaling, which resulted in aggravation of Cd-induced apoptosis in HK-2 cells. Collectively, our findings indicate that IRE-1α coordinate ER stress and STAT3 signaling in mediating Cd-induced renal toxicity, suggesting that targeting IRE-1α might be a potential therapeutic approach for Cd-induced renal dysfunction and disease.

Signal transduction pathways and resistance to targeted therapies in glioma

Although surgical techniques and adjuvant therapies have undergone progressive development for decades, the therapeutic outcomes for treating glioblastoma (GBM) remain poor. The main reasons for the poor prognosis of gliomas are that limited tumor tissue that can be resected (to preserve brain functions) and that residual tumors are often resistant to irradiation and chemotherapy. Therefore, overcoming the resistance of residual tumors against adjuvant therapy is urgently needed for glioma treatment. Recent large cohort studies of genetic alterations in GBM demonstrated that both genetic information and intracellular molecular signaling are networked in gliomas and that such information may help clarify which molecules or signals serve essential roles in resistance against radiation or chemotherapy, highlighting them as potential novel therapeutic targets against refractory gliomas. In this review, we summarize the current understanding of molecular networks that govern glioma biology, mainly based on cohort studies or recent evidence, with a focus on how intracellular signaling molecules in gliomas associate with each other and regulate refractoriness against current therapy.

Transcription Factor Activities Enhance Markers of Drug Sensitivity in Cancer

Transcriptional dysregulation induced by aberrant transcription factors (TF) is a key feature of cancer, but its global influence on drug sensitivity has not been examined. Here, we infer the transcriptional activity of 127 TFs through analysis of RNA-seq gene expression data newly generated for 448 cancer cell lines, combined with publicly available datasets to survey a total of 1,056 cancer cell lines and 9,250 primary tumors. Predicted TF activities are supported by their agreement with independent shRNA essentiality profiles and homozygous gene deletions, and recapitulate mutant-specific mechanisms of transcriptional dysregulation in cancer. By analyzing cell line responses to 265 compounds, we uncovered numerous TFs whose activity interacts with anticancer drugs. Importantly, combining existing pharmacogenomic markers with TF activities often improves the stratification of cell lines in response to drug treatment. Our results, which can be queried freely at dorothea.opentargets.io, offer a broad foundation for discovering opportunities to refine personalized cancer therapies.Significance: Systematic analysis of transcriptional dysregulation in cancer cell lines and patient tumor specimens offers a publicly searchable foundation to discover new opportunities to refine personalized cancer therapies. Cancer Res; 78(3); 769-80. ©2017 AACR.

Effect of hyperprolactinemia on PRL-receptor expression and activation of Stat and Mapk cell signaling in the prostate of long-term sexually-active rats

The abnormal elevation of serum PRL, referred to as hyperprolactinemia (HyperPRL), produces alterations in several reproductive parameters of male rats such as penile erection or decreased tendency to reach ejaculation. Additionally, this situation produces a significant modification of prostate histology, as observed in the epithelial structure and alveolar area, which could reach a level of hyperplasia in the long-term. In this tissue, HyperPRL produces an increase in expression of PRL receptors and activation of the Stat3 signaling pathway that is correlated with the evolution of prostate pathologies. However, the impact of HyperPRL in long-term sexually active male rats is unknown. In this work, using constantly copulating Wistar male rats with induced HyperPRL, we analyzed the level of serum PRL, the effect on prostate PRL receptors, and activation of pStat3, pStat5 and Mapk signaling pathways. Two procedures to induce HyperPRL were employed, comprising daily IP administration or adenohypophysis transplant, and although neither affected the execution of sexual behavior, the serum PRL profile following successive ejaculations was affected. Messenger RNA expression of the short and long isoforms of the PRL receptor at the ventral prostate was affected in different ways depending on the procedure to induce HyperPRL. The ventral prostate did not show any modification in terms of activation of the pStat5 signaling pathway in subjects with daily administration of PRL, although this was significantly increased in ADH transplanted subjects in the second and fourth consecutive ejaculation. A similar profile was found for the pStat3 pathway which additionally showed a significant increase in the third and fourth ejaculation of daily-injected subjects. The Mapk signaling pathway did not show any modifications in subjects with daily administration of PRL, but showed a significant increase in the second and third ejaculations of subjects with ADH transplants. Thus, although sexual behavior was not modified, HyperPRL modified the expression of PRL receptors and the activation of signal pathways in the prostate tissue. Hence, it is probable that prostatic alterations precede the sexual behavioral deficits observed in subjects with HyperPRL.

The effectiveness of an anti-human IL-6 receptor monoclonal antibody combined with chemotherapy to target colon cancer stem-like cells

Recent studies have demonstrated that cancer stem cells (CSCs) can initiate and sustain tumor growth and exhibit resistance to clinical cytotoxic therapies. Therefore, CSCs represent the main target of anticancer therapy. Interleukin-6 (IL-6) promotes cellular proliferation and drug resistance in colorectal cancer, and its serum levels correlate with patient survival. Therefore, IL-6 and its downstream signaling molecule the signal transducer and activator of transcription-3 (STAT3) represent potential molecular targets. In the present study, we investigated the effects of IL-6 and its downstream signaling components on stem cell biology, particularly the chemoresistance of CSCs, to explore potential molecular targets for cancer therapy. The colon cancer cell line WiDr was cultured in serum-free, non-adherent, and three-dimensional spheroid-forming conditions to enrich the stem cell-like population. Spheroid-forming cells slowly proliferated and expressed high levels of Oct-4, Klf4, Bmi-1, Lgr5, IL-6, and Notch 3 compared with adherent cells. Treatment with an anti-human IL-6 receptor monoclonal antibody reduced spheroid formation, stem cell-related gene expression, and 5-fluorouracil (5-FU) resistance. In addition, IL-6 treatment enhanced the levels of p-STAT3 (Tyr705), the expression of Oct-4, Klf4, Lgr5, and Notch 3, and chemoresistance to 5-FU. siRNA targeting Notch 3 suppressed spheroid formation, Oct-4 and Lgr5 expression, and 5-FU chemoresistance, whereas STAT3 inhibition enhanced Oct-4, Klf4, Lgr5, and Notch 3 expression and 5-FU chemoresistance along with reduced spheroid growth. Taken together, these results indicate that IL-6 functions in dichotomous pathways involving Notch 3 induction and STAT3 activation. The former pathway is involved in cancer stem-like cell biology and enhanced chemoresistance, and the latter pathway leads to accelerated proliferation and reduced chemoresistance. Thus, an anti-human IL-6 receptor monoclonal antibody or Notch 3 inhibition may be superior to STAT3 inhibition for CSC-targeting therapies concomitant with anticancer drugs.

Serine-727 phosphorylation activates hypothalamic STAT-3 independently from tyrosine-705 phosphorylation

Transcriptional activity of signal transducer and activator of transcription-3 (STAT-3) is a key element in the central regulation of appetite and energy homeostasis. Activation of hypothalamic STAT-3 has been attributed to cytokine-promoted phosphorylation at tyrosine-705 (Tyr-705). In nonhypothalamic cells, STAT-3 is also phosphorylated at serine-727 (Ser-727), but the functional significance of Ser-727 in the regulation of hypothalamic STAT-3 is not known. We used 2 hypothalamic cell lines and analyzed the effects of various hormones on STAT-3-dependent reporter gene activity and observed that IFN-γ, epidermal growth factor (EGF), and bradykinin (BK) induce similar STAT-3 reporter activation. EGF and BK solely increased Ser-727 and IFN-γ increased Tyr-705 phosphorylation of STAT-3. Specific inhibition of ERK-1/2 activity blocked EGF- and BK-induced STAT-3 activation and Ser-727 phosphorylation. BK-induced ERK-1/2 activation occurred via EGF receptor transactivation. Consequently, the BK-mediated effects on STAT-3 were blocked by a specific EGF receptor antagonist. Next, we analyzed the effects of IFN-γ and EGF on the expression of the STAT-3-dependent genes thyroliberin-releasing hormone and suppressors of cytokine signaling-3. EGF but not IFN-γ enhanced thyroliberin-releasing hormone expression via STAT-3. With regard to suppressors of cytokine signaling-3, we observed prolonged expression induced by IFN-γ and a transient effect of EGF that required coactivation of the activator protein-1. Thus, EGF-promoted Ser-727 phosphorylation by ERK-1/2 is not only sufficient to fully activate hypothalamic STAT-3, but, in terms of targeted genes and required cofactors, entails distinct modes of STAT-3 actions compared with IFN-γ-induced Tyr-705 phosphorylation.

Extracellular UDP-glucose activates P2Y14 Receptor and Induces Signal Transducer and Activator of Transcription 3 (STAT3) Tyr705 phosphorylation and binding to hyaluronan synthase 2 (HAS2) promoter, stimulating hyaluronan synthesis of keratinocytes

Hyaluronan, a major matrix molecule in epidermis, is often increased by stimuli that enhance keratinocyte proliferation and migration. We found that small amounts of UDP-sugars were released from keratinocytes and that UDP-glucose (UDP-Glc) added into keratinocyte cultures induced a specific, rapid induction of hyaluronan synthase 2 (HAS2), and an increase of hyaluronan synthesis. The up-regulation of HAS2 was associated with JAK2 and ERK1/2 activation, and specific Tyr(705) phosphorylation of transcription factor STAT3. Inhibition of JAK2, STAT3, or Gi-coupled receptors blocked the induction of HAS2 expression by UDP-Glc, the latter inhibitor suggesting that the signaling was triggered by the UDP-sugar receptor P2Y14. Chromatin immunoprecipitations demonstrated increased promoter binding of Tyr(P)(705)-STAT3 at the time of HAS2 induction. Interestingly, at the same time Ser(P)(727)-STAT3 binding to its response element regions in the HAS2 promoter was unchanged or decreased. UDP-Glc also stimulated keratinocyte migration, proliferation, and IL-8 expression, supporting a notion that UDP-Glc signals for epidermal inflammation, enhanced hyaluronan synthesis as an integral part of it.

Transcription Factor STAT3 as a Novel Molecular Target for Cancer Prevention

Signal Transducers and Activators of Transcription (STATs) are a family of transcription factors that regulate cell proliferation, differentiation, apoptosis, immune and inflammatory responses, and angiogenesis. Cumulative evidence has established that STAT3 has a critical role in the development of multiple cancer types. Because it is constitutively activated during disease progression and metastasis in a variety of cancers, STAT3 has promise as a drug target for cancer therapeutics. Recently, STAT3 was found to have an important role in maintaining cancer stem cells in vitro and in mouse tumor models, suggesting STAT3 is integrally involved in tumor initiation, progression and maintenance. STAT3 has been traditionally considered as nontargetable or undruggable, and the lag in developing effective STAT3 inhibitors contributes to the current lack of FDA-approved STAT3 inhibitors. Recent advances in cancer biology and drug discovery efforts have shed light on targeting STAT3 globally and/or specifically for cancer therapy. In this review, we summarize current literature and discuss the potential importance of STAT3 as a novel target for cancer prevention and of STAT3 inhibitors as effective chemopreventive agents.

A cross-platform comparison of affymetrix and Agilent microarrays reveals discordant miRNA expression in lung tumors of c-Raf transgenic mice

Non-coding RNAs play major roles in the translational control of gene expression. In order to identify disease-associated miRNAs in precursor lesions of lung cancer, RNA extracts from lungs of either c-Raf transgenic or wild-type (WT) mice were hybridized to the Agilent and Affymetrix miRNA microarray platforms, respectively. This resulted in the detection of a range of miRNAs varying between 111 and 267, depending on the presence or absence of the transgene, on the gender, and on the platform used. Importantly, when the two platforms were compared, only 11–16% of the 586 overlapping genes were commonly detected. With the Agilent microarray, seven miRNAs were identified as significantly regulated, of which three were selectively up-regulated in male transgenic mice. Much to our surprise, when the same samples were analyzed with the Affymetrix platform, only two miRNAs were identified as significantly regulated. Quantitative PCR performed with lung RNA extracts from WT and transgenic mice confirmed only partially the differential expression of significant regulated miRNAs and established that the Agilent platform failed to detect miR-433. Finally, bioinformatic analyses predicted a total of 152 mouse genes as targets of the regulated miRNAs of which 4 and 11 genes were significantly regulated at the mRNA level, respectively in laser micro-dissected lung dysplasia and lung adenocarcinomas of c-Raf transgenic mice. Furthermore, for many of the predicted mouse target genes expression of the coded protein was also repressed in human lung cancer when the publically available database of the Human Protein Atlas was analyzed, thus supporting the clinical significance of our findings. In conclusion, a significant difference in a cross-platform comparison was observed that will have important implications for research into miRNAs.

Negative regulation of STAT3 protein-mediated cellular respiration by SIRT1 protein

In mammals, the transcriptional activity of signal transducer and activator of transcription 3 (STAT3) is regulated by the deacetylase SIRT1. However, whether the newly described nongenomic actions of STAT3 toward mitochondrial oxidative phosphorylation are dependent on SIRT1 is unclear. In this study, Sirt1 gene knock-out murine embryonic fibroblast (MEF) cells were used to delineate the role of SIRT1 in the regulation of STAT3 mitochondrial function. Here, we show that STAT3 mRNA and protein levels and the accumulation of serine-phosphorylated STAT3 in mitochondria were increased significantly in Sirt1-KO cells as compared with wild-type MEFs. Various mitochondrial bioenergetic parameters, such as the oxygen consumption rate in cell cultures, enzyme activities of the electron transport chain complexes in isolated mitochondria, and production of ATP and lactate, indicated that Sirt1-KO cells exhibited higher mitochondrial respiration as compared with wild-type MEFs. Two independent approaches, including ectopic expression of SIRT1 and siRNA-mediated knockdown of STAT3, led to reduction in intracellular ATP levels and increased lactate production in Sirt1-KO cells that were approaching those of wild-type controls. Comparison of profiles of phospho-antibody array data indicated that the deletion of SirT1 was accompanied by constitutive activation of the pro-inflammatory NF-κB pathway, which is key for STAT3 induction and increased cellular respiration in Sirt1-KO cells. Thus, SIRT1 appears to be a functional regulator of NF-κB-dependent STAT3 expression that induces mitochondrial biogenesis. These results have implications for understanding the interplay between STAT3 and SIRT1 in pro-inflammatory conditions.

Suppressor of cytokine signaling-2 gene disruption promotes Apc(Min/+) tumorigenesis and activator protein-1 activation

Epigenetic in vitro and in vivo studies suggest that suppressor of cytokine signaling-2 (SOCS2) may normally limit tumorigenesis in the intestine; however, this theory has not been directly tested. We hypothesized that SOCS2 deficiency promotes spontaneous intestinal tumorigenesis in Apc(Min/+) mice. Therefore, we quantified tumor number, size, and load in the small intestine and colon using SOCS2(+/+)/Apc(Min/+), SOCS2(+/-)/Apc(Min/+), and SOCS2(-/-)/Apc(Min/+) mice and assayed hematocrit as an indirect marker of disease severity. Biochemical and histological assays were used to assess mechanisms. Heterozygous and homozygous disruption of SOCS2 alleles promoted 166 and 441% increases in tumor load in the small intestine, respectively, accelerated development of colon tumors, and caused severe anemia. SOCS2 deletion promoted significant increases in intestinal insulin-like growth factor-I mRNA but did not affect plasma insulin-like growth factor-I. Western blots and immunohistochemical analysis demonstrated that tumor and nontumor intestinal tissue of SOCS2(-/-)/Apc(Min/+) mice had increased serine 727 phosphorylation of signal transducer and activator of transcription 3 compared with SOCS2(+/+)/Apc(Min/+) mice. Moreover, electromobility shift assays showed that SOCS2 deletion did not alter signal transducer and activator of transcription 3 DNA binding. However, tumors and small intestine from SOCS2(-/-)/Apc(Min/+) showed dramatic increases in activator protein-1 (AP-1) DNA binding, and SOCS2 overexpression in vitro reduced levels of AP-1. These studies indicate that SOCS2 deletion promotes the spontaneous development of intestinal tumors driven by mutations in the adenomatous polyposis coli/beta-catenin pathway and activates AP-1. Therefore, reduced expression or epigenetic silencing of SOCS2 may serve as a useful biomarker for colorectal cancer risk.

Purvalanol A induces apoptosis and downregulation of antiapoptotic proteins through abrogation of phosphorylation of JAK2/STAT3 and RNA polymerase II

To clarify the mechanisms of purvalanol A in the induction of apoptosis, we investigated whether purvalanol A influenced the RNA synthesis and expression of RNA polymerase II and signal transducer and activator of transcription 3 (STAT3). When MKN45 cells were treated with 30 micromol/l purvalanol A, mitochondrial dysfunction occurred before the induction of the apoptosis and the expression of antiapoptotic proteins survivin, Bcl-XL, and Bcl-2 was reduced. The treatment with parvalanol A was also shown to reduce not only mRNA for these proteins but also global RNA synthesis. The phosphorylation of the carboxy-terminal domain of RNA polymerase II, which was involved in transcriptional regulation, was strongly inhibited by purvalanol A, followed by the partial inhibition of the expression of RNA polymerase II. Furthermore, the phosphorylation at Tyr705 of STAT3, which is known to be a phosphorylation site for Janus kinase 2 (JAK2), was completely inhibited by purvalanol A early (3 h) after drug treatment, although the phosphorylation of STAT3 at Ser727, which is a phosphorylation site for Ras/Raf/MEK and extracellular signal-regulated protein kinase 1/2, was still detectable until late (12 h) after treatment. In addition, the tyrosine phosphorylation of JAK2 was efficiently inhibited by purvalanol A. These results suggest that the inhibition of JAK2/STAT3 and RNA polymerase II is crucial in the downregulation of antiapoptotic proteins leading to the apoptotic cell death induced by parvalanol A.

STAT3 sensitizes insulin signaling by negatively regulating glycogen synthase kinase-3 beta

OBJECTIVE

Glucose homeostasis is achieved by triggering regulation of glycogen synthesis genes in response to insulin when mammals feed, but the underlying molecular mechanism remains largely unknown. The aim of our study was to examine the role of the signal transducers and activators of transcription 3 (STAT3) in insulin signaling.

RESEARCH DESIGN AND METHODS

We generated a strain of mice carrying a targeted disruption of Stat3 gene in the liver (L-Stat3(-/-) mice). Hepatocytes of the L-Stat3(-/-) mice were isolated to establish cell lines for mechanistic studies. Nuclear translocation and DNA-protein interaction of STAT3 was analyzed with immunofluorescent and chromatin immunoprecipitation methods, respectively. Levels of glucose, insulin, leptin, and glucagon were profiled, and putative downstream molecules of STAT3 were examined in the presence of various stimuli in L-Stat3(-/-) and control mice.

RESULTS

STAT3 was found to sensitize the insulin signaling through suppression of GSK-3beta, a negative regulator of insulin signaling pathway. During feeding, both mRNA and protein levels of GSK-3beta decreased in Stat3(f/+) mice, which reflected the need of hepatocytes for insulin to induce glycogen synthesis. In contrast, the L-Stat3(-/-) mice lost this control and showed a monophasic increase in the GSK-3beta level in response to insulin. Administration of GSK-3beta inhibitors lithium chloride and L803-mts restored glucose homeostasis and rescued the glucose intolerance and impaired insulin response in L-Stat3(-/-) mice.

CONCLUSIONS

These data indicate that STAT3 sensitizes insulin signaling by negatively regulating GSK-3beta. Inactivation of STAT3 in the liver contributes significantly to the pathogenesis of insulin resistance.

Characterization of the SOCS3 promoter response to prostaglandin E2 in T47D cells

Suppressor of cytokine signaling 3 (SOCS3), a negative regulator of cytokine signaling, is expressed in breast cancer cells where it can modify sensitivity and responsiveness to cytokine signaling through the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathways. Although it is widely accepted that SOCS3 expression is in itself regulated by STATs, we and others have shown that prostaglandins can also up-regulate SOCS3 expression. Here we used T47D breast cancer cells treated with prostaglandin E2 (PGE2) to examine this pathway. T47D cells responded to PGE2 stimulation with a significant increase in SOCS3 mRNA that was independent of de novo protein synthesis. PGE2 stimulation resulted in STAT3 serine and tyrosine phosphorylation, although mutation of either of the two previously characterized STAT response elements on the SOCS3 promoter did not affect SOCS3 promoter activation by PGE2. In addition, overexpression of STAT3 wild-type, constitutively active or dominant-negative constructs did not affect PGE2-induced SOCS3 promoter activation, indicating that STATs are unlikely mediators of this pathway in these cells. PGE2 is a known activator of the cAMP/protein kinase A (PKA) pathway, and in T47D cells, up-regulation of SOCS3 mRNA by PGE2 was abolished by pretreatment with H89, a PKA inhibitor and increased by cAMP and forskolin treatment. Consistent with this, PGE2 treatment increased cAMP response element (CRE)-binding protein serine phosphorylation. However, mutation of the activator protein 1/CRE on the promoter did not affect basal or PGE2-stimulated activation, suggesting a role for cAMP/PKA that is independent of CRE-binding protein binding. Mutation of the GC-rich region of the SOCS3 promoter, a putative Sp1/Sp3 binding site, abolished both basal and PGE2-stimulated activation. Gel-shift assays showed increased complex formation after treatment, and this was inhibited by the addition of an Sp1 antibody or pretreatment with PKA inhibitor. Chromatin immunoprecipitation assay verified Sp1 binding to the promoter in response to PGE2. Sp1 overexpression increased SOCS3 promoter activation, and both basal and PGE2-induced SOCS3 mRNA expression was prevented by mithramycin, an inhibitor of Sp1 DNA binding. Finally, a physiological role for PGE2 was demonstrated with PGE2 pretreatment reducing lipopolysaccharide-induced STAT3 activation. Collectively, this study details a novel mechanism of SOCS3 up-regulation by PGE2 in breast cancer cells that appears to be STAT independent and involve Sp1 binding to the promoter. This process has possible implications for cytokine responsiveness and tumor progression.

Promotion of cancer cell migration: an insulin-like growth factor (IGF)-independent action of IGF-binding protein-6

A family of six high affinity IGF-binding proteins (IGFBPs 1-6) plays an important role in modulating IGF activities. Recent studies suggest that some IGFBPs may have IGF-independent effects, including induction of apoptosis and modulation of cell migration. However, very little is known about possible IGF-independent actions of IGFBP-6. We have generated a non-IGF-binding IGFBP-6 mutant by substituting Ala for four amino acid residues (Pro(93)/Leu(94)/Leu(97)/Leu(98)) in its N-domain IGF-binding site. A >10,000-fold loss of binding affinity for IGF-I and IGF-II was observed using charcoal solution binding assay, BIAcore biosensor, and ligand blotting. Wild-type and mutant IGFBP-6, as well as IGF-II, induced cell migration in RD rhabdomyosarcoma and LIM 1215 colon cancer cells. Cell migration was mediated by the C-domain of IGFBP-6. Transient p38 phosphorylation was observed in RD cells after treatment with IGFBP-6, whereas no change was seen in phospho-ERK1/2 levels. Phospho-JNK was not detected. IGFBP-6-induced cell migration was inhibited by SB203580, an inhibitor of p38 MAPK, and PD98059, an inhibitor of ERK1/2 MAPK activation. In contrast, SP600125, a JNK MAPK inhibitor, had no effect on migration. Knockdown of p38 MAPK using short interfering RNA blocked IGFBP-6-induced migration of RD cells. These results indicate that p38 MAPK is involved in IGFBP-6-induced IGF-independent RD cell migration.

Insulin Antagonizes Interleukin-6 Signaling and Is Anti-inflammatory in 3T3-L1 Adipocytes

Adipose tissue secretes different adipokines, including interleukin-6 (IL-6), that have been implicated in the insulin resistance and inflammatory state characterizing obesity. We examined the putative cross-talk between insulin and IL-6 in adipose cells and found that insulin exerts an inhibitory effect on the IL-6 signaling pathway by altering the post-translational modifications of the signal transducer and activator of transcription 3 (STAT3). Insulin reduces the tyrosine phosphorylation and increases the serine phosphorylation of STAT3, thereby reducing its nuclear localization and transcriptional activity. Signaling through the MEK/MAPK pathway plays an important role as treatment with the MEK inhibitor PD98059 reduces the effects of insulin on IL-6 signaling. We also show that the protein tyrosine phosphatase SHP2 is activated upon insulin signaling and is required for the dephosphorylation of STAT3 and that insulin exerts a synergistic effect with IL-6 on suppressor of cytokine signaling 3 expression. As a consequence, the IL-6-induced expression of the inflammatory markers serum amyloid A 3 and haptoglobin are significantly decreased in cells incubated with both IL-6 and insulin. Thus, insulin exerts an important anti-inflammatory effect in adipose cells by impairing the IL-6 signal at several levels.

Inhibition of signal transducer and activator of transcription 3 activity results in down-regulation of Survivin following irradiation

Signal transducer and activator of transcription 3 (Stat3) and Survivin are constitutively up-regulated in various human tumor cells. We previously found Survivin to be significantly reduced in response to radiation in human umbilical vein endothelial cells (HUVEC) but not in tumor cell lines. In this study, we examined the effect of Stat3 on Survivin expression in irradiated HUVECs and breast cancer cells. We also studied how inhibition of Stat3 and Survivin activity affects cell survival and angiogenesis following irradiation. We determined that Survivin was significantly increased by overexpression of an active Stat3 (Stat3-C). Following irradiation, the level of phospho-Stat3 Tyr(705), but not phospho-Stat3 Ser(727), was reduced in HUVECs, whereas it remained unchanged in irradiated breast cancer cells. Correspondingly, Stat3 DNA-binding activity following irradiation was specifically down-regulated in HUVECs but not in breast cancer cells. Mutation of Tyr(705) abolished radiation-induced down-regulation of Survivin. Clonogenic and endothelial cell morphogenesis assays suggested that DN-Stat3 and DN-Survivin together resulted in the greatest radiosensitization of MDA-MB-231, decreasing angiogenesis and cell survival. In summary, Stat3 modulates Survivin, and both are potential therapeutic targets for radiation sensitization in breast cancer.

Chronic nicotine exposure enhances insulin-induced mitogenic signaling via up-regulation of alpha7 nicotinic receptors in isolated rat aortic smooth muscle cells

Insulin resistance and smoking are significant risk factors for cardiac and cerebral vascular diseases. Because vascular smooth muscle cells play a key role in the development and progression of atherosclerosis, we investigated the effect of nicotine on insulin-induced mitogenic signaling in aortic vascular smooth muscle cells isolated from Sprague Dawley rats. RT-PCR revealed the expression of alpha2-7, alpha10, beta1-3, delta, and epsilon subunits of the nicotinic acetylcholine receptor (nAChR) in the cells. Short-term nicotine treatment stimulated phosphorylation of p44/42-MAPK, p38-MAPK, and signal transducer and activator of transcription 3. However, an additive effect of nicotine pretreatment on insulin stimulation was only observed on p44/42-MAPK. The nicotine-induced phosphorylation of p44/42-MAPK and [methyl-(3)H]thymidine incorporation were effectively suppressed by a alpha7-nAChR-selective antagonist, methyllycaconitine, and the phosphorylation of p44/42-MAPK was stimulated by a alpha7-nAChR-specific agonist, GTS21. Furthermore, the phosphorylation was mediated via calmodulin kinase II, Src, and Shc. Interestingly, long-term (48-h) pretreatment with nicotine increased the amount of alpha7-AChR in the plasma membrane and insulin-induced phosphorylation of p44/42-MAPK. These results provide the first evidence that acute exposure to nicotine enhances insulin-induced mitogenesis predominantly by affecting the phosphorylation of p44/42-MAPK and that chronic exposure further augments the insulin signal via up-regulation of alpha7-nAChR, which may be crucial for the development and progression of atherosclerosis in large vessels.

Ras/ERK1/2-mediated STAT3 Ser727 phosphorylation by familial medullary thyroid carcinoma-associated RET mutants induces full activation of STAT3 and is required for c-fos promoter activation, cell mitogenicity, and transformation

The precise role of STAT3 Ser(727) phosphorylation in RET-mediated cell transformation and oncogenesis is not well understood. In this study, we have shown that familial medullary thyroid carcinoma (FMTC) mutants RET(Y791F) and RET(S891A) induced, in addition to Tyr(705) phosphorylation, constitutive STAT3 Ser(727) phosphorylation. Using inhibitors and dominant negative constructs, we have demonstrated that RET(Y791F) and RET(S891A) induce STAT3 Ser(727) phosphorylation via a canonical Ras/ERK1/2 pathway and that integration of the Ras/ERK1/2/ELK-1 and STAT3 pathways was required for up-regulation of the c-fos promoter by FMTC-RET. Moreover, inhibition of ERK1/2 had a more severe effect on cell proliferation and cell phenotype in HEK293 cells expressing RET(S891A) compared with control and RET(WT)-transfected cells. The transforming activity of RET(Y791F) and RET(S891A) in NIH-3T3 cells was also inhibited by U0126, indicating a role of the ERK1/2 pathway in RET-mediated transformation. To investigate the biological significance of Ras/ERK1/2-induced STAT3 Ser(727) phosphorylation for cell proliferation and transformation, N-Ras-transformed NIH-3T3 cells were employed. These cells displayed elevated levels of activated ERK1/2 and Ser(727)-phosphorylated STAT3, which were inhibited by treatment with U0126. Importantly, overexpression of STAT3, in which the Ser(727) was mutated into Ala (STAT3(S727A)), rescued the transformed phenotype of N-Ras-transformed cells. Immunohistochemistry in tumor samples from FMTC patients showed strong nuclear staining of phosphorylated ERK1/2 and Ser(727) STAT3. These data show that FMTC-RET mutants activate a Ras/ERK1/2/STAT3 Ser(727) pathway, which plays an important role in cell mitogenicity and transformation.

Intracellular pathways triggered by the selective FLT-1-agonist placental growth factor in vascular smooth muscle cells exposed to hypoxia

We have previously shown that hypoxia makes vascular smooth muscle cells (VSMCs) responsive to placental growth factor (PlGF) through the induction of functional fms-like tyrosine kinase (Flt-1) receptors. The aim of this study was to investigate the molecular mechanisms involved in the PlGF effects on proliferation and contraction of VSMCs previously exposed to hypoxia (3% O2). In cultured rat VSMCs exposed to hypoxia, PlGF increased the phosphorylation of protein kinase B (Akt), p38 and STAT3; activation of STAT3 was higher than that of other kinases. In agreement with this finding, the proliferation of hypoxia-treated VSMCs in response to PlGF was significantly impaired by the p38 and the phosphatidylinositol 3-kinase inhibitors SB202190 and LY294002, respectively, and was almost completely prevented by AG490, a janus tyrosine kinase (JAK)/signal transducer and activator of transcription (STAT) inhibitor. Since hypoxia was able to reverse the vasorelaxant effect of PlGF into a vasoconstrictor response, the mechanism of this latter effect was also investigated. Significant Flt-1 activity was measured in isolated preparations from rat aorta exposed to hypoxia. Inhibitors of mitogen-activated protein kinase kinase, Akt and STAT3 induced a modest inhibition of the vasoconstrictor response to PlGF, while the p38 inhibitor SB202190 markedly impaired the PlGF-induced contractile response. These effects were selectively mediated by Flt-1 without any involvement of foetal liver kinase-1 receptors. These data are the first evidence that different intracellular pathways activated by Flt-1 receptor in VSMCs are involved in diverse biological effects of PlGF: while mitogen activated protein kinase kinase/extracellular signal regulated kinase(1/2) and JAK/STAT play a role in VSMC proliferation, p38 is involved in VSMC contraction. These findings may highlight the role of PlGF in vascular pathology.

Influence of the BRAF V600E mutation on expression of vascular endothelial growth factor in papillary thyroid cancer

CONTEXT

The BRAF mutation may influence the expression patterns of molecular markers that are related to the development and progression of thyroid cancer.

OBJECTIVE

The objective of the study was to investigate the effects of the BRAF V600E mutation on expression of galectin-3, cyclooxygenase-2, cyclin D1, p53, and vascular endothelial growth factor (VEGF) in papillary thyroid cancer (PTC).

DESIGN, SETTING, AND SUBJECTS

One hundred sixty-three PTC and 28 nodular hyperplasia patients were selected retrospectively. The presence of the BRAF V600E mutation and the level of expression of the molecular markers were determined.

RESULTS

Of 161 PTC patients, 102 patients (63.4%) were BRAF V600E(+), and these cases had significantly larger tumor sizes (P = 0.01), compared with V600E(-) cases (n = 59, 36.6%). Although PTC tissues had higher expression levels of the selected molecular markers than nodular hyperplasia tissues, expression levels of several molecular markers in BRAF V600E(+) PTC were not significantly different from those of BRAF V600E(-) PTC. But VEGF was significantly up-regulated in BRAF V600E(+) PTC, compared with BRAF V600E(-) PTC. VEGF expression levels were strongly positively correlated to tumor size (P < 0.001), extrathyroidal invasion (P = 0.02), and tumor stage (P = 0.04). Multivariate analysis clearly showed that VEGF expression was up-regulated in BRAF V600E(+) PTC (odds ratio 2.5, confidence interval 1.1-5.6; P = 0.03).

CONCLUSIONS

BRAF V600E(+) PTC tended to have larger tumor volumes and higher expression of VEGF. The level of VEGF expression was closely correlated with tumor size, extrathyroidal invasion, and stage. The relatively high levels of VEGF expression may be related to poorer clinical outcomes and recurrences in BRAF V600E(+) PTC.

MEK hyperphosphorylation coincides with cell cycle shut down of cultured smooth muscle cells

Smooth muscle cells (SMCs) form the backbone of arteries and their proliferation hallmarks collateral vessel growth, a process termed arteriogenesis, as well as pathogenic responses such as restenosis. Since signaling pathways in SMCs are the main targets for therapeutic interventions, we aimed to determine how and to what extent the activation of the ubiquitous MEK-ERK signaling pathway correlates with important in vivo phenomena such as dedifferentiation, nuclear activation and proliferation of SMCs. Specificity of this pathway was monitored using MEK inhibitors UO126 and PD98059 in platelet derived growth factor-AB (PDGF-AB)- and fibroblast growth factor-2 (FGF-2)-stimulated SMCs. PDGF-AB induced a rapid MEK activation followed by phosphorylation of the MEK substrates ERK1/2 while FGF-2 showed a less pronounced and delayed activation. Both growth factors triggered a marked phosphorylation of c-Myc and expression of Egr1. Pretreatment with MEK inhibitors suppressed the activation of the ERK cascade, abolished the down-regulation of desmin and led to cell cycle arrest. However, the reversibility of p27Kip1 down-regulation by UO126 was mainly observed after PDGF-AB stimulation, indicating MEK independent p27Kip1 down-regulation by FGF-2. Surprisingly, treatment of SMCs with UO126 or PD98059 increased the level of MEK phosphorylation in a dose dependent manner at serine residues 217/221 in the presence as well as in the absence of both growth factors. Our results strongly imply that depending on the environmental context phosphorylation of serines 217/221 serves as an "on" as well as an "off " switch.

Leptin and insulin stimulation of signalling pathways in arcuate nucleus neurones: PI3K dependent actin reorganization and KATP channel activation

Background

Leptin and insulin are long-term regulators of body weight. They act in hypothalamic centres to modulate the function of specific neuronal subtypes, by altering transcriptional control of releasable peptides and by modifying neuronal electrical activity. A key cellular signalling intermediate, implicated in control of food intake by these hormones, is the enzyme phosphoinositide 3-kinase. In this study we have explored further the linkage between this enzyme and other cellular mediators of leptin and insulin action on rat arcuate nucleus neurones and the mouse hypothalamic cell line, GT1-7.

Results

Leptin and insulin increased the levels of various phosphorylated signalling intermediates, associated with the JAK2-STAT3, MAPK and PI3K cascades in the arcuate nucleus. Inhibitors of PI3K were shown to reduce the hormone driven phosphorylation through the PI3K and MAPK pathways. Using isolated arcuate neurones, leptin and insulin were demonstrated to increase the activity of K_ATP channels in a PI3K dependent manner, and to increase levels of PtdIns(3,4,5)P₃. K_ATP activation by these hormones in arcuate neurones was also sensitive to the presence of the actin filament stabilising toxin, jasplakinolide. Using confocal imaging of fluorescently labelled actin and direct analysis of G- and F-actin concentration in GT1-7 cells, leptin was demonstrated directly to induce a re-organization of cellular actin, by increasing levels of globular actin at the expense of filamentous actin in a PI3-kinase dependent manner. Leptin stimulated PI3-kinase activity in GT1-7 cells and an increase in PtdIns(3,4,5)P₃ could be detected, which was prevented by PI3K inhibitors.

Conclusions

Leptin and insulin mediated phosphorylation of cellular signalling intermediates and of K_ATP channel activation in arcuate neurones is sensitive to PI3K inhibition, thus strengthening further the likely importance of this enzyme in leptin and insulin mediated energy homeostasis control. The sensitivity of leptin and insulin stimulation of K_ATP channel opening in arcuate neurones to jasplakinolide indicates that cytoskeletal remodelling may be an important contributor to the cellular signalling mechanisms of these hormones in hypothalamic neurones. This hypothesis is reinforced by the finding that leptin induces actin filament depolymerization, in a PI3K dependent manner in a mouse hypothalamic cell line.

EphA2: expression in the renal medulla and regulation by hypertonicity and urea stress in vitro and in vivo

EphA2, a member of the large family of Eph receptor tyrosine kinases, is highly expressed in epithelial tissue and has been implicated in cell-cell and cell-matrix interactions, as well as cell growth and survival. Expression of EphA2 mRNA and protein was markedly upregulated by both hypertonic stress and by elevated urea concentrations in cells derived from the murine inner medullary collecting duct. This upregulation likely required transactivation of the epidermal growth factor (EGF) receptor tyrosine kinase and metalloproteinase-dependent ectodomain cleavage of an EGF receptor ligand, based on pharmacological inhibitor studies. A human EphA2 promoter fragment spanning nucleotides -4030 to +21 relative to the putative EphA2 transcriptional start site was responsive to tonicity but insensitive to urea. A promoter fragment spanning -1890 to +128 recapitulated both tonicity- and urea-dependent upregulation of expression, consistent with transcriptional activation. Neither the bona fide p53 response element at approximately -1.5 kb nor a pair of putative TonE elements at approximately -3 kb conferred the tonicity responsiveness. EphA2 mRNA and protein were expressed at low levels in rat renal cortex but at high levels in the collecting ducts of the renal medulla and papilla. Water deprivation in rats increased EphA2 expression in renal papilla, whereas dietary supplementation with 20% urea increased EphA2 expression in outer medulla. These data indicate that transcription and expression of the EphA2 receptor tyrosine kinase are regulated by tonicity and urea in vitro and suggest that this phenomenon is also operative in vivo. Renal medullary EphA2 expression may represent an adaptive response to medullary hypertonicity or urea exposure.

Glucose induces de novo lipogenesis in rat muscle satellite cells through a sterol-regulatory-element-binding-protein-1c-dependent pathway

We previously reported that sterol-regulatory-element-binding-protein-1c (SREBP-1c) mediates insulin upregulation of genes encoding glycolytic and lipogenic enzymes in rat skeletal muscle. Here, we assessed whether glucose could regulate gene expression in contracting myotubes deriving from cultured muscle satellite cells. Glucose uptake increased twofold after a 30 minute treatment with a high glucose concentration, suggesting an acute glucose-stimulated glucose uptake. Time-course experiments showed that, within 3 hours, glucose stimulated the expression of hexokinase II, fatty acid synthase and acetyl-CoA-carboxylase-2 proteins, leading to an increased lipogenic flux and intracellular lipid accumulation in contracting myotubes. Furthermore, kinetic experiments indicated that glucose upregulated SREBP-1c precursor and nuclear proteins within 30 minutes, SREBP-1c nuclear translocation being confirmed using immunocytochemistry. In addition, the knockdown of SREBP-1 mRNA using a RNA-interference technique totally abrogated the glucose-induced upregulation of lipogenic enzymes, indicating that SREBP-1c mediates the action of glucose on these genes in rat skeletal muscle. Finally, we found that glucose rapidly stimulated SREBP-1c maturation through a Jak/STAT dependent pathway. We propose that increased intramuscular lipid accumulation associated with muscle insulin resistance in obesity or type-2 diabetes could arise partly from de novo fatty acid synthesis in skeletal muscle.

sst2 Somatostatin receptor inhibits cell proliferation through Ras-, Rap1-, and B-Raf-dependent ERK2 activation

The G protein-coupled sst2 somatostatin receptor is a critical negative regulator of cell proliferation. sstII prevents growth factor-induced cell proliferation through activation of the tyrosine phosphatase SHP-1 leading to induction of the cyclin-dependent kinase inhibitor p27Kip1. Here, we investigate the signaling molecules linking sst2 to p27Kip1. In Chinese hamster ovary-DG-44 cells stably expressing sst2 (CHO/sst2), the somatostatin analogue RC-160 transiently stimulates ERK2 activity and potentiates insulin-stimulated ERK2 activity. RC-160 also stimulates ERK2 activity in pancreatic acini isolated from normal mice, which endogenously express sst2, but has no effect in pancreatic acini derived from sst2 knock-out mice. RC-160-induced p27Kip1 up-regulation and inhibition of insulin-dependent cell proliferation are both prevented by pretreatment of CHO/sst2 cells with the MEK1/2 inhibitor PD98059. In addition, using dominant negative mutants, we show that sst2-mediated ERK2 stimulation is dependent on the pertussis toxin-sensitive Gi/o protein, the tyrosine kinase Src, both small G proteins Ras and Rap1, and the MEK kinase B-Raf but is independent of Raf-1. Phosphatidylinositol 3-kinase (PI3K) and both tyrosine phosphatases, SHP-1 and SHP-2, are required upstream of Ras and Rap1. Taken together, our results identify a novel mechanism whereby a Gi/o protein-coupled receptor inhibits cell proliferation by stimulating ERK signaling via a SHP-1-SHP-2-PI3K/Ras-Rap1/B-Raf/MEK pathway.

Serine phosphorylation of STAT3 is essential for Mcl-1 expression and macrophage survival

The Bcl-2 family member Mcl-1 is essential for macrophage survival. However, the mechanisms that contribute to the expression of Mcl-1 in these cells have not been fully characterized. The present study focused on the role of signal transducer and activator of transcription 3 (STAT3) in regulation of Mcl-1 in macrophages. Sodium salicylate (NaSal) treatment induced apoptotic cell death in primary human macrophages in a dose- and time-dependent fashion. Incubation with NaSal resulted in the loss of mitochondrial transmembrane potential, the release of cytochrome c and second mitochondria-derived activator of caspase/direct IAP binding protein with low pH of isoelectric point (pI) from the mitochondria, and the activation of caspases 9 and 3. Western blot analysis and reverse transcription-polymerase chain reaction demonstrated that NaSal down-regulated the expression of Mcl-1. Electrophoretic mobility shift assay and Western blot analysis for phosphorylated STAT3 demonstrated that STAT3 was constitutively activated in macrophages and that this STAT3 activation was suppressed by NaSal. The activation of STAT3 in macrophages was dependent on Ser727 phosphorylation, in the absence of detectable Tyr705 phosphorylation. Ectopic expression of STAT3 in murine RAW264.7 macrophages rescued the inhibition of Mcl-1 promoter-reporter gene activation and the cell death induced by NaSal treatment, while a dominant-negative STAT3 resulted in cell death. To confirm its role in primary macrophages, STAT3 antisense (AS) oligodeoxynucleotides (ODNs) were employed. STAT3 AS, but not control, ODNs decreased STAT3 and Mcl-1 expression and resulted in macrophage apoptosis. These observations demonstrate that the STAT3-mediated expression of Mcl-1 is essential for the survival of primary human in vitro differentiated macrophages.

Ataxia telangiectasia mutated proteins, MAPKs, and RSK2 are involved in the phosphorylation of STAT3

Phosphorylation at Ser(727) is known to be required for complete activation of STAT3 by diverse stimuli including UV irradiation, but the kinase(s) responsible for phosphorylating STAT3 (Ser(727)) is still not well discerned. In the present study, we observed that activation of ATM is required for a UVA-stimulated increase in Ser(727) phosphorylation of STAT3 as well as in activation and phosphorylation of p90 ribosomal protein S6 kinases (RSKs). Moreover, UVA-stimulated activation of upstream kinases, such as c-Jun N-terminal kinases (JNKs) and ERKs, involved in mediating phosphorylation of RSKs and STAT3 was defective or delayed in ATM-deficient cells. Furthermore, we provide evidence that RSK2-deficient cells were defective for UV-induced Ser(727) phosphorylation of STAT3, and the defect was restored after ectopic expression of transfected full-length RSK2. In vitro experiments showed that active RSK2 and JNK1 induce the phosphorylation of STAT3 precipitates from immunoprecipitation but not from glutathione S-transferase (GST) pull-down. Interestingly, the GST fusion STAT3 proteins mixed together with STAT3 immunoprecipitates can be phosphorylated by JNK. However, the in vitro phosphorylation of STAT3 was reduced by the GST-STAT3 beta protein, a dominant negative form of STAT3. Taken together, our results demonstrate that the STAT3 phosphorylation at Ser(727) is triggered by active RSK2 or JNK1 in the presence of a downstream kinase or a cofactor, and thereby the intracellular phosphorylation process is stimulated through a signaling pathway involving ATM, MAPKs, RSK2, and an as yet unidentified kinase or cofactor. Additionally, RSK2-mediated phosphorylation of STAT3 (Ser(727)) was further determined to be required for basal and UVA-stimulated STAT3 transcriptional activities.

Epidermal growth factor receptor dependence of radiation-induced transcription factor activation in human breast carcinoma cells

Ionizing radiation (1-5 Gy) activates the epidermal growth factor receptor (EGFR), a major effector of the p42/44 mitogen-activated protein kinase (MAPK) pathway. MAPK and its downstream effector, p90 ribosomal S6 kinase (p90RSK), phosphorylate transcription factors involved in cell proliferation. To establish the role of the EGFR/MAPK pathway in radiation-induced transcription factor activation, MDA-MB-231 human breast carcinoma cells were examined using specific inhibitors of signaling pathways. Gel-shift analysis revealed three different profile groups: 1) transcription factors that responded to both radiation (2 Gy) and epidermal growth factor (EGF) (CREB, Egr, Ets, and Stat3); 2) factors that responded to radiation, but not EGF (C/EBP and Stat1); and 3) those that did not respond significantly to either radiation or EGF (AP-1 and Myc). Within groups 1 and 2, a two- to fivefold maximum stimulation of binding activity was observed at 30-60 min after irradiation. Interestingly, only transcription factors that responded to EGF had radiation responses significantly inhibited by the EGFR tyrosine kinase inhibitor, AG1478; these responses were also abrogated by farnesyltransferase inhibitor (FTI) or PD98059, inhibitors of Ras and MEK1/2, respectively. Moreover, radiation-induced increases in CREB and p90RSK phosphorylation and activation of Stat3 and Egr-1 reporter constructs by radiation were all abolished by AG1478. These data demonstrate a distinct radiation response profile at the transcriptional level that is dependent on enhanced EGFR/Ras/MAPK signaling.

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

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

Simultaneous tyrosine and serine phosphorylation of STAT3 transcription factor is involved in Rho A GTPase oncogenic transformation

Stats (signal transducers and activators of transcription) are latent cytoplasmic transcription factors that on a specific stimulus migrate to the nucleus and exert their transcriptional activity. Here we report a novel signaling pathway whereby RhoA can efficiently modulate Stat3 transcriptional activity by inducing its simultaneous tyrosine and serine phosphorylation. Tyrosine phosphorylation is exerted via a member of the Src family of kinases (SrcFK) and JAK2, whereas the JNK pathway mediates serine phosphorylation. Furthermore, cooperation of both tyrosine as well as serine phosphorylation is necessary for full activation of Stat3. Induction of Stat3 activity depends on the effector domain of RhoA and correlates with induction of both Src Kinase-related and JNK activities. Activation of Stat3 has biological implications. Coexpression of an oncogenic version of RhoA along with the wild-type, nontransforming Stat3 gene, significantly enhances its oncogenic activity on human HEK cells, suggesting that Stat3 is an essential component of RhoA-mediated transformation. In keeping with this, dominant negative Stat3 mutants or inhibition of its tyrosine or serine phosphorylation completely abrogate RhoA oncogenic potential. Taken together, these results indicate that Stat3 is an important player in RhoA-mediated oncogenic transformation, which requires simultaneous phosphorylation at both tyrosine and serine residues by specific signaling events triggered by RhoA effectors.

Interleukin-6-resistant melanoma cells exhibit reduced activation of STAT3 and lack of inhibition of cyclin E-associated kinase activity

Development of cytokine resistance is an important feature of melanoma cells during tumor progression. To study the mechanisms of interleukin-6 resistance, we examined an interleukin-6 sensitive (WM35) and an interleukin-6 unresponsive cell line (WM9). Interleukin-6 treatment resulted in rapid inhibition of cyclin-dependent kinase 2/cyclin E activity and accumulation of the hypophosphorylated retinoblastoma protein in WM35 but not in WM9 cells. In contrast to previous reports, no differences in the expression of the cyclin-dependent kinase 2 inhibitor p21Cip1/WAF1 upon interleukin-6 treatment were found in both cell lines. Interleukin-6-induced inhibition of cyclin-dependent kinase 2 was also not due to changes in protein expression of cyclin-dependent kinase 2, cyclin E, p27Kip1 and cdc25A, a phosphatase positively regulating cyclin-dependent kinase 2 activity. As it is established that interleukin-6 resistance of WM9 cells is not caused by differential interleukin-6 receptor expression, we studied whether this is due to defective interleukin-6 signaling in which activation of signal transducer and activator of transcription 3 is a critical step. WM9 cells showed reduced tyrosine phosphorylation, DNA binding, and delayed nuclear translocation of signal transducer and activator of transcription 3 as compared with WM35 cells. The kinase upstream of signal transducer and activator of transcription 3, Janus kinase 1, was constitutively tyrosine-phosphorylated in WM9 cells and did not respond to interleukin-6 with increased phosphorylation. As compared with WM35 cells, interleukin-6 treatment of WM9 cells was not paralleled by reduced activity of the mitogen-activated protein kinase kinase-1, which suppresses activation of signal transducer and activator of transcription 3. Our data suggest that resistance of advanced melanoma cells to interleukin-6 is associated with reduced inhibition of cyclin-dependent kinase 2, which appears to be a consequence of a complex alteration in interleukin-6 signal transduction.

G protein coupled receptor signaling through the Src and Stat3 pathway: role in proliferation and transformation

Extracellular signals when routed through signaling pathways that use heterotrimeric G proteins can engage multiple signaling pathways leading to diverse biological consequences. One locus at which signal sorting occurs is at the level of G proteins. G protein alpha-subunits appear to be capable of interacting with different effectors leading to engagement of distinct signaling pathways. Regulation of different pathways in turn leads to different biological outcomes. The process of neoplastic transformation is controlled to a large extent through the activation and inhibition of signaling pathways. Signaling pathways such as the Ras-MAPK, v-Src-Stat3 pathways are activated in the process of transformation. Expression of activated Galpha subunits have been shown to cause transformation of cells. While activation of the MAPK 1,2 pathway by various Galpha subunits has been reported for several years, recent studies show the activation and involvement of Src and Stat3 pathways in Galphao and Galphai mediated transformation of cells. Recent studies also suggest that both Galphai and Galphas may be able to interact with and activate Src. The activation of Src and Stat3 by G proteins has also been demonstrated by ligand-induced activation of G protein receptors. So increasingly it is becoming clear that the Src and Stat3 pathways are potential effectors for G proteins and that they may play a role in G protein function.

Thyroxine promotes association of mitogen-activated protein kinase and nuclear thyroid hormone receptor (TR) and causes serine phosphorylation of TR

Activated nongenomically by l-thyroxine (T(4)), mitogen-activated protein kinase (MAPK) complexed in 10-20 min with endogenous nuclear thyroid hormone receptor (TRbeta1 or TR) in nuclear fractions of 293T cells, resulting in serine phosphorylation of TR. Treatment of cells with the MAPK kinase inhibitor, PD 98059, prevented both T(4)-induced nuclear MAPK-TR co-immunoprecipitation and serine phosphorylation of TR. T(4) treatment caused dissociation of TR and SMRT (silencing mediator of retinoid and thyroid hormone receptor), an effect also inhibited by PD 98059 and presumptively a result of association of nuclear MAPK with TR. Transfection into CV-1 cells of TR gene constructs in which one or both zinc fingers in the TR DNA-binding domain were replaced with those from the glucocorticoid receptor localized the site of TR phosphorylation by T(4)-activated MAPK to a serine in the second zinc finger of the TR DNA-binding domain. In an in vitro cell- and hormone-free system, purified activated MAPK phosphorylated recombinant human TRbeta1 (). Thus, T(4) activates MAPK and causes MAPK-mediated serine phosphorylation of TRbeta1 and dissociation of TR and the co-repressor SMRT.

TNF down-regulation of receptor tyrosine kinase-dependent mitogenic signal pathways as an important step in cytostasis induction and commitment to apoptosis of Kym-1 rhabdomyosarcoma cells

Growth of Kym-1 rhabdomyosarcoma cells depends on endogenous receptor tyrosine kinase signals activated by insulin and insulin-like growth factors (IGF), as revealed from enhancement of proliferation by insulin and IGF-1 and cytostatic action of inhibitors of IR/IGFR kinases. Depending on the presence or absence of the caspase inhibitor z-VAD-fmk, TNF induced full growth arrest or apoptosis, respectively, indicating dominance of TNF over mitogenic signal pathways in Kym-1 cells. In accordance with a caspase-independent cytostatic action, TNF downregulated IR kinase activity and caused a profound inhibition of downstream mitogenic signals including the MAPK cascade and STAT5, key pathways of proliferation and cell survival. Removal of z-VAD-fmk after 24 h induced rapid cell death in the absence of TNF. The inhibition of survival signals concomitant with persisting proapoptotic signals may tip the balance towards an irreversible commitment of the cell to apoptosis that becomes apparent upon relief of suppression of effector caspases.

STATs in oncogenesis

Since their discovery as key mediators of cytokine signaling, considerable progress has been made in defining the structure-function relationships of Signal Transducers and Activators of Transcription (STATs). In addition to their central roles in normal cell signaling, recent studies have demonstrated that diverse oncoproteins can activate specific STATs (particularly Stat3 and Stat5) and that constitutively-activated STAT signaling directly contributes to oncogenesis. Furthermore, extensive surveys of primary tumors and cell lines derived from tumors indicate that inappropriate activation of specific STATs occurs with surprisingly high frequency in a wide variety of human cancers. Together, these findings provide compelling evidence that aberrant STAT activation associated with oncogenesis is not merely adventitious but instead contributes to the process of malignant transformation. These studies are beginning to reveal the molecular mechanisms leading to STAT activation in the context of oncogenesis, and candidate genes regulated by STATs that may contribute to oncogenesis are being identified. Recent studies suggest that activated STAT signaling participates in oncogenesis by stimulating cell proliferation and preventing apoptosis. This review presents the evidence for critical roles of STATs in oncogenesis and discusses the potential for development of novel cancer therapies based on mechanistic understanding of STAT signaling. Oncogene (2000).

Characterization of insulin-like growth factor-1-induced activation of the JAK/STAT pathway in rat cardiomyocytes

This study was designed to investigate whether insulin-like growth factor-1 (IGF-1) transduces signaling through the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway in cardiomyocytes and to assess the upstream signals of serine and tyrosine phosphorylation of STAT family proteins. Primary cultured neonatal rat cardiomyocytes were stimulated with IGF-1 (10(-8) mol/L). JAK1, but not JAK2 or Tyk2, was phosphorylated by IGF-1 as early as 2 minutes and peaked at 5 minutes. IGF-1 induced both tyrosine and serine phosphorylation of STAT1 and STAT3. Tyrosine phosphorylation of STAT1 peaked at 15 minutes and correlated with that of JAK1, whereas that of STAT3 was sustained up to 120 minutes and was dissociated from the activation of JAK1. Tyrosine phosphorylation of STAT3 was unaffected by the preincubation with CV11974 (AT(1) blocker), TAK044 (endothelin-1 receptor blocker), RX435 (anti-gp130 blocking antibody), PD98058, wortmannin, EDTA, or KN62 but was significantly attenuated by BAPTA-AM and chelerythrine. The time course of a gel mobility shift of SIE (sis-inducing element) coincided with the phosphorylation of STAT3. Serine phosphorylation of STAT1 peaked at 30 minutes and that of STAT3 was observed from 5 to 60 minutes. These results indicated that (1) IGF-1 activated JAK1 but not JAK2 or Tyk2 in rat cardiomyocytes; (2) IGF-1 induced both tyrosine and serine phosphorylation of STAT1 and STAT3; and (3) the tyrosine phosphorylation of STAT3 was not caused by JAK1 alone, and protein kinase C and intracellular Ca(2+) were required for phosphorylation.

Requirement for Ras/Rac1-mediated p38 and c-Jun N-terminal kinase signaling in Stat3 transcriptional activity induced by the Src oncoprotein

Signal transducers and activators of transcription (STATs) are transcription factors that mediate normal biologic responses to cytokines and growth factors. However, abnormal activation of certain STAT family members, including Stat3, is increasingly associated with oncogenesis. In fibroblasts expressing the Src oncoprotein, activation of Stat3 induces specific gene expression and is required for cell transformation. Although the Src tyrosine kinase induces constitutive Stat3 phosphorylation on tyrosine, activation of Stat3-mediated gene regulation requires both tyrosine and serine phosphorylation of Stat3. We investigated the signaling pathways underlying the constitutive Stat3 activation in Src oncogenesis. Expression of Ras or Rac1 dominant negative protein blocks Stat3-mediated gene regulation induced by Src in a manner consistent with dependence on p38 and c-Jun N-terminal kinase (JNK). Both of these serine/threonine kinases and Stat3 serine phosphorylation are constitutively induced in Src-transformed fibroblasts. Furthermore, inhibition of p38 and JNK activities suppresses constitutive Stat3 serine phosphorylation and Stat3-mediated gene regulation. In vitro kinase assays with purified full-length Stat3 as the substrate show that both JNK and p38 can phosphorylate Stat3 on serine. Moreover, inhibition of p38 activity and thus of Stat3 serine phosphorylation results in suppression of transformation by v-Src but not v-Ras, consistent with a requirement for Stat3 serine phosphorylation in Src transformation. Our results demonstrate that Ras- and Rac1-mediated p38 and JNK signals are required for Stat3 transcriptional activity induced by the Src oncoprotein. These findings delineate a network of tyrosine and serine/threonine kinase signaling pathways that converge on Stat3 in the context of oncogenesis.

Inhibition of protein phosphatase 2A induces serine/threonine phosphorylation, subcellular redistribution, and functional inhibition of STAT3

Signal transducers and activators of transcription (STATs) are rapidly phosphorylated on tyrosine residues in response to cytokine and growth factor stimulation of cell surface receptors. STATs hereafter are translocated to the nucleus where they act as transcription factors. Recent reports suggest that serine phosphorylation of STATs also is involved in the regulation of STAT-mediated gene transcription. Here, we studied the role of serine/threonine phosphatases in STAT3 signaling in human antigen-specific CD4(+) T cell lines and cutaneous T cell lymphoma lines, expressing a constitutively activated STAT3. We show that an inhibitor of protein phosphatases (PPs) PP1/PP2A, calyculin A, induces (i) phosphorylation of STAT3 on serine and threonine residues, (ii) inhibition of STAT3 tyrosine phosphorylation and DNA binding activity, and (iii) relocation of STAT3 from the nucleus to the cytoplasm. Similar results were obtained with other PP2A inhibitors (okadaic acid, endothall thioanhydride) but not with inhibitors of PP1 (tautomycin) or PP2B (cyclosporine A). Pretreatment with the broad serine/threonine kinase inhibitor staurosporine partly blocked the calyculin A-induced STAT3 phosphorylation, whereas inhibitors of serine/threonine kinases, such as mitogen-activated protein kinase-1 extracellular-regulated kinase-kinase, mitogen-activated protein p38 kinase, and phosphatidylinositol 3-kinase, did not. In conclusion, we provide evidence that PP2A plays a crucial role in the regulation of STAT3 phosphorylation and subcellular distribution in T cells. Moreover, our findings suggest that the level of STAT3 phosphorylation is balanced between a staurosporine-sensitive kinase(s) and PP2A.

Thyroid hormone induces activation of mitogen-activated protein kinase in cultured cells

Thyroid hormone [L-thyroxine (T4)] rapidly induced phosphorylation and nuclear translocation (activation) of mitogen-activated protein kinase (MAPK) in HeLa and CV-1 cells in the absence of cytokine or growth factor. A pertussis toxin-sensitive and guanosine 5'-O-(3-thiotriphosphate)-sensitive cell surface mechanism responsive to T4 and agarose-T4, suggesting a G protein-coupled receptor, was implicated. Cells depleted of MAPK or treated with MAPK pathway inhibitors showed reduced activation of MAPK and of the signal transducer and activator of transcription STAT1alpha by T4; they also showed reduced T4 potentiation of the antiviral action of interferon-gamma (IFN-gamma). T4 treatment caused tyrosine-phosphorylated MAPK-STAT1alpha nuclear complex formation and enhanced Ser-727 phosphorylation of STAT1alpha, in the presence or absence of IFN-gamma. STAT1alpha-deficient cells transfected with STAT1alpha containing an alanine-for-serine substitution at residue 727 (STAT1alphaA727) showed minimal T4-stimulated STAT1alpha activation. IFN-gamma induced the antiviral state in cells containing wild-type STAT1alpha (STAT1alphawt) or STAT1alphaA727; T4 potentiated IFN-gamma action in STAT1alphawt cells but not in STAT1alphaA727 cells. T4-directed STAT1alpha Ser-727 phosphorylation is MAPK mediated and results in potentiated STAT1alpha activation and enhanced IFN-gamma activity.

MEK is a negative regulator of Stat5b in PDGF-stimulated cells

In this study we show that platelet-derived growth factor (PDGF)-induced DNA binding as well as transcriptional activation of Stat5b are markedly increased by inhibition of the MAP (mitogen-activated protein) kinase kinase MEK. In addition to the previously demonstrated tyrosine phosphorylation, we show that serine and threonine phosphorylation of Stat5b is increased in response to PDGF stimulation. However, inhibition of MEK had no effect on the phosphorylation level of Stat5b or on the nuclear translocation of Stat5b. These observations indicate that MEK is a negative modulator of PDGF-induced Stat5b activation through a mechanism not involving direct phosphorylation of Stat5b.

Extracellular signal-regulated kinase (ERK) interacts with signal transducer and activator of transcription (STAT) 5a

Serine phosphorylation of signal transducers and activators of transcription (STAT) 1 and 3 modulates their DNA-binding capacity and/or transcriptional activity. Earlier we suggested that STAT5a functional capacity could be influenced by the mitogen-activated protein kinase (MAPK) pathway. In the present study, we have analyzed the interactions between STAT5a and the MAPKs, extracellular signal-regulated kinases ERK1 and ERK2. GH treatment of Chinese hamster ovary cells stably transfected with the GH receptor (CHOA cells) led to rapid and transient activation of both STAT5a and ERK1 and ERK2. Pretreatment of cells with colchicine, which inhibits tubulin polymerization, did not inhibit STAT5a translocation to the nucleus and ERK1/2 activation. In vitro precipitation with a glutathione-S-transferase-fusion protein containing the C-terminal transactivation domain of STAT5a showed GH-regulated association of ERK1/2 with the fusion protein, while this was not seen when serine 780 in STAT5a was changed to alanine. In vitro phosphorylation of the glutathione-S-transferase-fusion proteins using active ERK only worked when the fusion protein contained wild-type STAT5a sequence. The same experiment, performed with full-length wild-type STAT5a and the corresponding S780A mutant, showed that serine 780 is the only substrate in full-length STAT5a for active ERK. In coimmunoprecipitation experiments, larger amounts of STAT5a-ERK1/2 complexes were detected in cytosol from untreated CHOA cells than in cytosol from GH-treated cells, suggesting the presence of preformed STAT5a-ERK1/2 complexes in unstimulated cells. Transfection experiments with COS cells showed that kinase-inactive ERK1 decreased GH stimulation of STAT5-regulated reporter gene expression. These observations show, for the first time, direct physical interaction between ERK and STAT5a and also clearly identify serine 780 as a target for ERK. Furthermore, it is also established that serine phosphorylation of STAT5a transactivation domain, via the MAPK pathway, is a means of modifying GH-induced transcriptional activation.

Differential control of the phosphorylation state of proline-juxtaposed serine residues Ser725 of Stat5a and Ser730 of Stat5b in prolactin-sensitive cells

Transcription factors of the Stat family are controlled by protein kinases. Phosphorylation of a positionally conserved tyrosine residue is obligatory for Stat dimerization, nuclear translocation, and specific DNA binding. Studies of Stat1 and Stat3 have suggested that serine phosphorylation may also regulate function. We now identify serine residues located in a conserved PSP motif of Stat5a (Ser725) and Stat5b (Ser730) as major phosphorylation sites, using mutagenesis, phosphoamino acid analysis, and site-specific anti-Stat5-phosphoserine antibodies. Unexpectedly, phosphorylation control of this PSP motif differed between the highly homologous Stat5a and Stat5b proteins. Whereas Ser725 of Stat5a was constitutively phosphorylated both in COS-7 cells and Nb2 lymphocytes, phosphorylation of Ser730 of Stat5b was markedly stimulated by prolactin. The data also suggested the existence of a second major serine phosphorylation site in Stat5a. Interestingly, constitutive phosphorylation of the PSP motif was suppressed by PD98059 but not by staurosporine under conditions in which both agents inhibited mitogen-activated protein kinases. Furthermore, pretreatment of cells with staurosporine, PD98059, H7, or wortmannin did not prevent either Stat5a or Stat5b from becoming maximally serine-phosphorylated after prolactin exposure. We propose that two pathways regulate Stat5 serine phosphorylation, one that is prolactin-activated and PD98059-resistant and one that is constitutively active and PD98059-sensitive and preferentially targets Stat5a. Finally, phosphorylation of the PSP motif of Stat5a or Stat5b was not essential for DNA binding or transcriptional activation of a beta-casein reporter gene in COS-7 cells, suggesting that serine kinase control of Stat5 activity differs from that of Stat1 and Stat3.

Rapid inhibition of interleukin-6 signaling and Stat3 activation mediated by mitogen-activated protein kinases

Gene activation and cellular differentiation induced by interleukin-6 (IL-6) and transcription factor Stat3 are suppressed by several factors, including ionomycin, granulocyte/macrophage-colony-stimulating factor, and phorbol 12-myristate 13-acetate (PMA), that block IL-6-induced Stat3 activation. These inhibitory agents activate mitogen activated protein kinases (MAPKs), and thus the role of MAPKs in the mechanism of inhibition of Stat3 activation was investigated. Inhibition of IL-6-induced Stat3 activation by PMA and ionomycin was rapid (within 5 min) and did not require new RNA or protein synthesis. Inhibition of Stat3 DNA-binding activity and tyrosine phosphorylation by PMA, ionomycin, and granulocyte/macrophage-colony-stimulating factor was reversed when activation of the extracellular signal-regulated kinase (ERK) group of MAPKs was blocked by using specific kinase inhibitors. Expression of constitutively active MEK1, the kinase that activates ERKs, or overexpression of ERK2, but not JNK1, inhibited Stat3 activation. Inhibition of Stat3 correlated with suppression of IL-6-induction of a signal transducer and activator of transcription (STAT)-dependent reporter gene. In contrast to IL-6, activation of Stat3 by interferon-alpha was not inhibited. MEKs and ERKs inhibited IL-6 activation of Stat3 harboring a mutation at serine-727, the major site for serine phosphorylation, similar to inhibition of wild-type Stat3, and inhibited Janus kinases Jak1 and Jak2 upstream of Stat3 in the Jak-STAT-signaling pathway. These results demonstrate an ERK-mediated mechanism for inhibiting IL-6-induced Jak-STAT signaling that is rapid and inducible, and thus differs from previously described mechanisms for downmodulation of the Jak-STAT pathway. This inhibitory pathway provides a molecular mechanism for the antagonism of Stat3-mediated IL-6 activity by factors that activate ERKs.

Insulin induction of protein kinase C alpha expression is independent of insulin receptor Tyr1162/1163 residues and involves mitogen-activated protein kinase kinase 1 and sustained activation of nuclear p44MAPK

We examined the effect of insulin on protein kinase C alpha (PKCalpha) expression and the implication of the mitogen-activated protein kinase kinase 1 mitogen-activated protein kinase (MAPK) pathway in this effect. PKCalpha expression was measured by quantitative RT-PCR and Western blotting using Chinese hamster ovary (CHO) cells overexpressing human insulin receptors of the wild type (CHO-R) or insulin receptors mutated at Tyr1162/1163 autophosphorylation sites (CHO-Y2). In CHO-R cells, insulin caused a time- and concentration-dependent increase in PKCalpha messenger RNA, with a maximum at 6 h and 10-(8)M insulin. This increase involved a transcriptional mechanism, as it was not due to stabilization of PKCalpha messenger RNA and was associated with a similar increase in the immunoreactive PKCalpha level. Insulin induction of PKCalpha expression involved the MEK1MAPK pathway, as it was 1) almost completely suppressed by the potent MEK1 inhibitor PD98059, 2) mimicked by the dominant-active MEK1 (S218D/S222D) mutant, and 3) associated with sustained MAPK activation. In CHO-Y2 cells in which the early phase of MAPK activation by insulin was lost and only the late and sustained phase of activation was observed, insulin signaling of PKCalpha expression was preserved and again involved the MEK1-MAPK pathway. Moreover, we show that in both CHO-R and CHO-Y2 cells, insulin stimulation of PKCalpha gene expression was associated with prolonged activation of nuclear p44MAPK. These results indicate that induction of PKCalpha gene expression by insulin is independent of Tyr1162/1163 autophosphorylation sites and correlates with sustained activation of p44MAPK at the nuclear level.