In vivo antitumor efficacy of STAT3 blockade using a transcription factor decoy approach: implications for cancer therapy

The expression of p-STAT3 in stage IV melanoma: risk of CNS metastasis and survival

Purpose

The signal transducer and activator of transcription 3 (STAT3) is a key molecular hub of tumorigenesis and immune suppression. The expression of phosphorylated STAT3 (p-STAT3) has been shown to be higher in melanoma metastasis to the central nervous system (CNS) relative to distant metastasis in the rest of the body (systemic). We sought to determine whether the increased expression of p-STAT3 in non-CNS systemic melanoma metastasis is associated with an increased risk of developing CNS metastasis and is a negative prognostic factor for overall survival time.

Methods

We retrospectively identified 299 patients with stage IV melanoma. In a tissue microarray of systemic non-CNS metastasis specimens from these patients, we used immunohistochemical analysis to measure the percentage of cells with p-STAT3 expression and Kaplan–Meier survival estimates to analyze the association of p-STAT3 expression with median survival time, time to first CNS metastasis, and development of CNS metastasis.

Results

Lung metastases exhibited the highest level of p-STAT3 expression while spleen lesions had the lowest. The p-STAT3 expression was not associated with an increased risk of developing CNS metastasis or time to CNS metastasis. However, p-STAT3 expression was a negative prognostic factor for overall survival time in patients that did not develop CNS metastasis.

Conclusions

Stage IV melanoma patients without CNS metastasis treated with p-STAT3 inhibitors in efficacy studies should be stratified based on tumor expression of p-STAT3; however since p-STAT3 expression is not associated with the risk of CNS disease, increased MRI surveillance of the brain is not likely necessary.

Targeting Stat3 abrogates EGFR inhibitor resistance in cancer

PURPOSE

EGF receptor (EGFR) is upregulated in most epithelial cancers where signaling through EGFR contributes to cancer cell proliferation and survival. The limited clinical efficacy of EGFR inhibitors suggests that identification of resistance mechanisms may identify new pathways for therapeutic targeting. STAT3 is upregulated in many cancers and activated via both EGFR-dependent and -independent pathways. In the present study, we tested the consequences of STAT3 inhibition in EGFR inhibitor-resistant head and neck squamous cell carcinoma (HNSCC) and bladder cancer models to determine whether STAT3 blockade can enhance responses to EGFR targeting.

EXPERIMENTAL DESIGN

pSTAT3 expression was assessed in human HNSCC tumors that recurred following cetuximab treatment. Cetuximab-sensitive and -resistant cell lines were treated with a STAT3 decoy to determine EC(50) concentrations and the effects on STAT3 target gene expression by Western blotting. In vivo assays included evaluation of antitumor efficacy of STAT3 decoy in cetuximab-sensitive and -resistant models followed by immunoblotting for STAT3 target protein expression.

RESULTS

Targeting STAT3 with a STAT3 decoy reduced cellular viability and the expression of STAT3 target genes in EGFR inhibitor resistance models. The addition of a STAT3 inhibitor to EGFR blocking strategies significantly enhanced antitumor effects in vivo. Biopsies from HNSCC tumors that recurred following cetuximab treatment showed increased STAT3 activation compared with pretreatment biopsies.

CONCLUSIONS

These results suggest that STAT3 activation contributes to EGFR inhibitor resistance both in HNSCC and bladder cancer where concomitant targeting of STAT3 may represent an effective treatment strategy.

Targeting proliferation and survival pathways in head and neck cancer for therapeutic benefit

Head and neck squamous cell carcinomas (HNSCC) are common human malignancies with poor clinical outcomes. The 5-year survival rates for patients with advanced stage HNSCC have not changed appreciably in the past few decades, underscoring a dire need for improved therapeutic options. Recent studies have elucidated a key signaling axis, the EGFR-STAT3-Bcl-XL signaling axis, that is aberrantly activated in a majority of HNSCC and contributes to the proliferation and survival of malignant cells. Considerable effort is being placed on developing highly specific inhibitors of different components of this pathway. This review highlights the progress that is being made towards achieving potent inhibition of the EGFR-STAT3-Bcl-XL signaling axis in HNSCC and the promising therapeutic strategies that are currently under development for this disease.

First-in-human trial of a STAT3 decoy oligonucleotide in head and neck tumors: implications for cancer therapy

Despite evidence implicating transcription factors, including STAT3, in oncogenesis, these proteins have been regarded as "undruggable." We developed a decoy targeting STAT3 and conducted a phase 0 trial. Expression levels of STAT3 target genes were decreased in head and neck cancers following injection with the STAT3 decoy compared with tumors receiving saline control. Decoys have not been amenable to systemic administration due to instability. To overcome this barrier, we linked the oligonucleotide strands using hexaethylene glycol spacers. This cyclic STAT3 decoy bound with high affinity to STAT3 protein, reduced cellular viability, and suppressed STAT3 target gene expression in cancer cells. Intravenous injection of the cyclic STAT3 decoy inhibited xenograft growth and downregulated STAT3 target genes in the tumors. These results provide the first demonstration of a successful strategy to inhibit tumor STAT3 signaling via systemic administration of a selective STAT3 inhibitor, thereby paving the way for broad clinical development.

SIGNIFICANCE

This is the fi rst study of a STAT3-selective inhibitor in humans and the fi rst evidence that a transcription factor decoy can be modifi ed to enable systemic delivery. These findings have therapeutic implications beyond STAT3 to other “undruggable” targets in human cancers.

Signal transducer and activator of transcription 1 (Stat1) maintains basal mRNA expression of pro-survival stat3-target genes in glioma C6 cells

The STAT proteins (signal transducers and activators of transcription) are transcription factors mediating cytokine/growth factor signaling, which play important role in controlling cell cycle progression and apoptosis. In many cancer cell lines and tumors (including gliomas) the STAT proteins (in particular Stats 1, 3, and 5) are persistently activated. In this study, we employed DNA decoys, siRNAs, and protein overexpression, to elucidate the role of Stat1 and Stat3 in regulation of expression of endogenous Stat3-target genes (Bcl2l1, Myc, Ccnd1) and a Stat-driven reporter plasmid, in rat C6 glioma cells. The results obtained with the decoys and siRNA suggest that in proliferating C6 cells, Stat1 supports the basal expression of Bcl2l1, while the decoy and chromatin immunoprecipitation results suggest it also plays a similar role for Myc. In the Stat-driven reporter system, overexpression of Stat1 stimulated, while overexpression of Stat3 inhibited the reporter gene expression. The level of Stat1 phosphorylation observed under basal conditions in proliferating glioma C6 cells is very low. Therefore, we speculate that it is the activity of the unphosphorylated Stat1 that is inhibited by Stat1 decoy or Stat1 siRNA. Taken together, our results demonstrate that it is Stat1 not Stat3 that maintains the expression of Bcl2l1 and possibly Myc in proliferating glioma C6 cells. An established paradigm is that Stat3 exerts a pro-survival and potentially oncogenic effects, while Stat1 is mainly associated with the immune response. Our results add to a number of reports that challenge this paradigm.

Targeting the interleukin-6/Jak/stat pathway in human malignancies

The Janus kinase/signal transducer and activator of transcription (Jak/Stat) pathway was discovered 20 years ago as a mediator of cytokine signaling. Since this time, more than 2,500 articles have been published demonstrating the importance of this pathway in virtually all malignancies. Although there are dozens of cytokines and cytokine receptors, four Jaks, and seven Stats, it seems that interleukin-6-mediated activation of Stat3 is a principal pathway implicated in promoting tumorigenesis. This transcription factor regulates the expression of numerous critical mediators of tumor formation and metastatic progression. This review will examine the relative importance and function of this pathway in nonmalignant conditions as well as malignancies (including tumor intrinsic and extrinsic), the influence of other Stats, the development of inhibitors to this pathway, and the potential role of inhibitors in controlling or eradicating cancers.

The curcumin analogue hydrazinocurcumin exhibits potent suppressive activity on carcinogenicity of breast cancer cells via STAT3 inhibition

Curcumin, the active component of turmeric, has been shown to protect against carcinogenesis and prevent tumor development in cancer. In our study, we tested the efficacy of a synthetic curcumin analogue, known as hydrazinocurcumin (HC), in breast cancer cells. The results demonstrated that compared to curcumin, HC was more effective in inhibiting STAT3 phosphorylation and downregulation of an array of STAT3 downstream targets which contributed to suppression of cell proliferation, loss of colony formation, depression of cell migration and invasion as well as induction of cell apoptosis. It was concluded that HC is a potent agent in the inhibition of STAT3 with more favorable pharmacological activity than curcumin, and HC may have translational potential as an effective cancer therapeutic or preventive agent for human breast carcinoma.

IL-6/IL-6R as a potential key signaling pathway in prostate cancer development

Interleukin-6 (IL-6) is a pleiotropic cytokine involved in prostate regulation and in prostate cancer (PC) development/progression. IL-6 acts as a paracrine and autocrine growth stimulator in benign and tumor prostate cells. The levels of IL-6 and respective receptors are increased during prostate carcinogenesis and tumor progression. Several studies reported that increased serum and plasma IL-6 and soluble interleukin-6 receptor levels are associated with aggressiveness of the disease and are associated with a poor prognosis in PC patients. In PC treatment, patients diagnosed with advanced stages are frequently submitted to hormonal castration, although most patients will eventually fail this therapy and die from recurrent castration-resistant prostate cancer (CRPC). Therefore, it is important to understand the mechanisms involved in CRPC. Several pathways have been proposed to be involved in CRPC development, and their understanding will improve the way to more effective therapies. In fact, the prostate is known to be dependent, not exclusively, on androgens, but also on growth factors and cytokines. The signaling pathway mediated by IL-6 may be an alternative pathway in the CRPC phenotype acquisition and cancer progression, under androgen deprivation conditions. The principal goal of this review is to evaluate the role of IL-6 pathway signaling in human PC development and progression and discuss the interaction of this pathway with the androgen recepto pathway. Furthermore, we intend to evaluate the inclusion of IL-6 and its receptor levels as a putative new class of tumor biomarkers.The IL-6/IL-6R signaling pathway may be included as a putative molecular marker for aggressiveness in PC and it may be able to maintain tumor growth through the AR pathway under androgen-deprivation conditions. The importance of the IL-6/IL-6R pathway in regulation of PC cells makes it a good candidate for targeted therapy.

The therapeutic potential of inhibitors of the signal transducer and activator of transcription 3 for central nervous system malignancies

Background:

High-grade primary and metastatic central nervous system (CNS) tumors are common, deadly, and refractory to conventional therapy and continue to be therapeutically challenging. A key nodal transcriptional factor, the signal transducer and activator of transcription 3 (STAT3), drives the fundamental components of tumor malignancy and metastases in the CNS by enhancing proliferation, angiogenesis, invasion, metastasis, and immunosuppression. The introduction of STAT3 inhibitors in clinical trials for this patient population is imminent.

Methods:

STAT3 inhibitors have been extensively tested in a variety of preclinical murine models.

Results:

The STAT3 inhibitor, WP1066, has displayed marked efficacy with minimal toxicity against malignancy in murine models, including established intracerebral tumors. The mechanism of this in vivo efficacy of the STAT3 blockade agents is a combination of direct tumor cytotoxicity and immune cytotoxic clearance.

Conclusions:

Given their direct antitumor cytotoxic effects, STAT3 inhibitors may exert therapeutic activity in the monotherapy setting but may also have compelling use as immunotherapeutic modulators or as a salvage therapy.

New advances in molecular approaches to head and neck squamous cell carcinoma

Head and neck squamous cell cancer is the sixth most common cancer in the world. Despite advances in combined modality therapy, poor outcomes continue to be observed in the form of locoregional recurrence, metastasis, and development of second primary tumors. As tumors vary in their molecular and genetic etiology and because often there is already deregulation at the molecular level in otherwise histopathologically normal tissue, risk stratification using clinical and pathologic criteria alone has proved to be inadequate. In this article, the reader will gain an appreciation for the current advances in biomarker discovery using advanced technology and data interpretation in microarray analysis and proteomics. In addition, other molecular targets, aside from epidermal growth factor receptor, are discussed in the context of their promising role in predicting recurrence, response to therapy, survival, and overall prognosis.

Cross-talk between phospho-STAT3 and PLCγ1 plays a critical role in colorectal tumorigenesis

Hyperphosphorylation at the Y705 residue of signal transducer and activator of transcription 3 (STAT3) is implicated in tumorigenesis of leukemia and some solid tumors. However, its role in the development of colorectal cancer is not well defined. To rigorously test the impact of this phosphorylation on colorectal tumorigenesis, we engineered a STAT3 Y705F knock-in to interrupt STAT3 activity in HCT116 and RKO colorectal cancer cells. These STAT3 Y705F mutant cells fail to respond to cytokine stimulation and grow slower than parental cells. These mutant cells are also greatly diminished in their abilities to form colonies in culture, to exhibit anchorage-independent growth in soft agar, and to grow as xenografts in nude mice. These observations strongly support the premise that STAT3 Y705 phosphorylation is crucial in colorectal tumorigenesis. Although it is generally believed that STAT3 functions as a transcription factor, recent studies indicate that transcription-independent functions of STAT3 also play an important role in tumorigenesis. We show here that wild-type STAT3, but not STAT3 Y705F mutant protein, associates with phospholipase Cγ1 (PLCγ1). PLCγ1 is a central signal transducer of growth factor and cytokine signaling pathways that are involved in tumorigenesis. In STAT3 Y705F mutant colorectal cancer cells, PLCγ1 activity is reduced. Moreover, overexpression of a constitutively active form of PLCγ1 rescues the transformation defect of STAT3 Y705F mutant cells. In aggregate, our study identifies previously unknown cross-talk between STAT3 and the PLCγ signaling pathways that may play a critical role in colorectal tumorigenesis.

Deactivation of signal transducer and activator of transcription 3 reverses chemotherapeutics resistance of leukemia cells via down-regulating P-gp

Multidrug resistance (MDR) caused by overexpression of p-glycoprotein is a major obstacle in chemotherapy of malignant cancer, which usually is characterized by constitutive activation of signal transducer and activator of transcription 3 (STAT3), but their relation between MDR and STAT3 remains unclear. Here, we showed that STAT3 was overexpressed and highly activated in adriamycin-resistant K562/A02 cells compared with its parental K562 cells. Blockade of activation of STAT3 by STAT3 decoy oligodeoxynucleotide (ODN) promoted the accumulation and increased their sensitivity to adriamycin by down-regulating transcription of mdr1 and expression of P-gp, which were further confirmed by using STAT3-specific inhibitor JSI-124. Inhibition of STAT3 could also decrease mdr1 promoter mediated luciferase expression by using mdr1 promoter luciferase reporter construct. Otherwise, activation of STAT3 by STAT3C improved mdr1 transcription and P-gp expression. The ChIP results demonstrated that STAT3 could bind to the potential promoter region of mdr1, and STAT3 decoy depressed the binding. Further mutation assay show +64∼+72 region could be the STAT3 binding site. Our data demonstrate a role of STAT3 in regulation of mdr1 gene expression in myeloid leukemia and suggest that STAT3 may be a promising therapeutic target for overcoming MDR resistance in myeloid leukemia.

A STAT3-decoy oligonucleotide induces cell death in a human colorectal carcinoma cell line by blocking nuclear transfer of STAT3 and STAT3-bound NF-κB

Background

The transcription factor STAT3 (signal transducer and activator of transcription 3) is frequently activated in tumor cells. Activated STAT3 forms homodimers, or heterodimers with other TFs such as NF-κB, which becomes activated. Cytoplasmic STAT3 dimers are activated by tyrosine phosphorylation; they interact with importins via a nuclear localization signal (NLS) one of which is located within the DNA-binding domain formed by the dimer. In the nucleus, STAT3 regulates target gene expression by binding a consensus sequence within the promoter. STAT3-specific decoy oligonucleotides (STAT3-decoy ODN) that contain this consensus sequence inhibit the transcriptional activity of STAT3, leading to cell death; however, their mechanism of action is unclear.

Results

The mechanism of action of a STAT3-decoy ODN was analyzed in the colon carcinoma cell line SW 480. These cells' dependence on activated STAT3 was verified by showing that cell death is induced by STAT3-specific siRNAs or Stattic. STAT3-decoy ODN was shown to bind activated STAT3 within the cytoplasm, and to prevent its translocation to the nucleus, as well as that of STAT3-associated NF-κB, but it did not prevent the nuclear transfer of STAT3 with mutations in its DNA-binding domain. The complex formed by STAT3 and the STAT3-decoy ODN did not associate with importin, while STAT3 alone was found to co-immunoprecipitate with importin. Leptomycin B and vanadate both trap STAT3 in the nucleus. They were found here to oppose the cytoplasmic trapping of STAT3 by the STAT3-decoy ODN. Control decoys consisting of either a mutated STAT3-decoy ODN or a NF-κB-specific decoy ODN had no effect on STAT3 nuclear translocation. Finally, blockage of STAT3 nuclear transfer correlated with the induction of SW 480 cell death.

Conclusions

The inhibition of STAT3 by a STAT3-decoy ODN, leading to cell death, involves the entrapment of activated STAT3 dimers in the cytoplasm. A mechanism is suggested whereby this entrapment is due to STAT3-decoy ODN's inhibition of active STAT3/importin interaction. These observations point to the high potential of STAT3-decoy ODN as a reagent and to STAT3 nucleo-cytoplasmic shuttling in tumor cells as a potential target for effective anti-cancer compounds.

Fluacrypyrim, a novel STAT3 activation inhibitor, induces cell cycle arrest and apoptosis in cancer cells harboring constitutively-active STAT3

STAT3 protein has an important role in oncogenesis and is a promising anticancer target. Herein, we demonstrate that a novel small molecule fluacrypyrim (FAPM) inhibits the growth of leukemia cells by a predominant G1 arrest with significant decrease of the protein and mRNA levels of cyclin D1. As cyclin D1 is transcriptionally regulated by STAT3, FAPM is then shown to markedly inhibit the STAT3 phosphorylation with marginal effect on the other signal transducers and activators of transcription, and without effect on phosphoinositide-3-kinase and mitogen-activated protein kinase pathways. Further analysis shows that FAPM significantly increases the protein tyrosine phosphatases (PTPs) activity in a dose-dependent manner, and the inhibition of PTP activation by sodium pervanadate reverses FAPM-induced suppression of STAT3 tyrosine phosphorylation, indicating an important role of PTP in the action of FAPM. Finally, FAPM treatment results in selective suppression of STAT3-mediated transcriptional activity and its downstream effectors, and subsequent induction of growth arrest and apoptosis in STAT3-dependent cancer cell lines. This study therefore identifies FAPM as a potent STAT3 activation inhibitor with possible therapeutic potential against malignancies with constitutive STAT3 activation.

Immunologic consequences of signal transducers and activators of transcription 3 activation in human squamous cell carcinoma

Paracrine cross-talk between tumor cells and immune cells within the tumor microenvironment underlies local mechanisms of immune evasion. Signal transducer and activator of transcription 3 (STAT3), which is constitutively activated in diverse cancer types, is a key regulator of cytokine and chemokine expression in murine tumors, resulting in suppression of both innate and adaptive antitumor immunity. However, the immunologic effects of STAT3 activation in human cancers have not been studied in detail. To investigate how STAT3 activity in human head and neck squamous cell carcinoma (HNSCC) might alter the tumor microenvironment to enable immune escape, we used small interfering RNA and small-molecule inhibitors to suppress STAT3 activity. STAT3 inhibition in multiple primary and established human squamous carcinoma lines resulted in enhanced expression and secretion of both proinflammatory cytokines and chemokines. Although conditioned medium containing supernatants from human HNSCC inhibited lipopolysaccharide-induced dendritic cell activation in vitro, supernatants from STAT3-silenced tumor cells reversed this immune evasion mechanism. Moreover, supernatants from STAT3-silenced tumor cells were able to stimulate the migratory behavior of lymphocytes from human peripheral blood in vitro. These results show the importance of STAT3 activation in regulating the immunomodulatory mediators by human tumors and further validate STAT3 as a promising target for therapeutic intervention.

Epidermal growth factor receptor variant III mediates head and neck cancer cell invasion via STAT3 activation

Epidermal Growth Factor Receptor (EGFR) is frequently over-expressed in head and neck squamous cell carcinoma (HNSCC) where aberrant signaling downstream of this receptor contributes to tumor growth. EGFR variant III (EGFRvIII) is the most commonly altered form of EGFR and contains a truncated ligand-binding domain. We previously reported that EGFRvIII is expressed in up to 40% of HNSCC tumors where it is associated with increased proliferation, tumor growth and chemoresistance to anti-tumor drugs including the EGFR targeting monoclonal antibody cetuximab. Cetuximab was FDA-approved in 2006 for HNSCC but has not been shown to prevent invasion or metastasis. The present study was undertaken to evaluate the mechanisms of EGFRvIII-mediated cell motility and invasion in HNSCC. We found that EGFRvIII induced HNSCC cell migration and invasion in conjunction with increased STAT3 activation, which was not abrogated by cetuximab treatment. Further investigation demonstrated that EGF-induced expression of the STAT3 target gene HIF1-α, was abolished by cetuximab in HNSCC cells expressing wild-type EGFR under hypoxic conditions, but not in EGFRvIII-expressing HNSCC cells. These results suggest that EGFRvIII mediates HNSCC cell migration and invasion via increased STAT3 activation and induction of HIF1-α, which contribute to cetuximab resistance in EGFRvIII-expressing HNSCC tumors.

EGFR-targeted therapy in malignant glioma: novel aspects and mechanisms of drug resistance

Glioblastoma, GBM, is the most frequent brain malignancy in adults. Patients with these tumors survive only, approximately, one year after diagnosis and rarely survive beyond two years. This poor prognosis is, in part, due to our insufficient understanding of the complex aggressive nature of these tumors and the lack of effective therapy. In GBM, over-expression of EGFR and/or its constitutively activated variant EGFRvIII is a major characteristic and is associated with tumorigenesis and more aggressive phenotypes, such as, invasiveness and therapeutic resistance. Consequently, both have been major targets for GBM therapy, however, clinical trials of EGFR- and EGFRvIII-targeted therapies have yielded unsatisfactory results and the molecular basis for the poor results is still unclear. Thus, in this review, we will summarize results of recent clinical trials and recent advances made in the understanding of the EGFR/EGFRvIII pathways with a key focus on those associated with intrinsic resistance of GBM to EGFR-targeted therapy. For example, emerging evidence indicates an important role that PTEN plays in predicting GBM response to EGFR-targeted therapy. Aberrant Akt/mTOR pathway has been shown to contribute to the resistant phenotype. Also, several studies have reported that EGFR/EGFRvIII's cross-talk with the oncogenic transcription factorSTAT3 and receptor tyrosine kinases, (c-Met and PDGFR) potentially lead to GBM resistance to anti-EGFR therapy. Other emerging mechanisms, including one involving HMG-CoA reductase, will also be discussed in this mini-review. These recent findings have provided new insight into the highly complex and interactive nature of the EGFR pathway and generated rationales for novel combinational targeted therapies for these tumors.

Defining the role of the JAK-STAT pathway in head and neck and thoracic malignancies: implications for future therapeutic approaches

Although the role of the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway has been most extensively studied in hematopoietic cells and hematologic malignancies, it is also activated in epithelial tumors, including those originating in the lungs and head and neck. The canonical pathway involves the activation of JAK following ligand binding to cytokine receptors. The activated JAKs then phosphorylate STAT proteins, leading to their dimerization and translocation into the nucleus. In the nucleus, STATs act as transcription factors with pleiotropic downstream effects. STATs can be activated independently of JAKs, most notably by c-Src kinases. In cancer cells, STAT3 and STAT5 activation leads to the increased expression of downstream target genes, leading to increased cell proliferation, cell survival, angiogenesis, and immune system evasion. STAT3 and STAT5 are expressed and activated in head and neck squamous cell carcinoma (HNSCC) where they contribute to cell survival and proliferation. In HNSCC, STATs can be activated by a number of signal transduction pathways, including the epidermal growth factor receptor (EGFR), alpha7 nicotinic receptor, interleukin (IL) receptor, and erythropoietin receptor pathways. Activated STATs are also expressed in lung cancer, but the biological effects of JAK/STAT inhibition in this cancer are variable. In lung cancer, STAT3 can be activated by multiple pathways, including EGFR. Several approaches have been used to inhibit STAT3 in the hopes of developing an antitumor agent. Although several STAT3-specific agents are promising, none are in clinical development, mostly because of drug delivery and stability issues. In contrast, several JAK inhibitors are in clinical development. These orally available, ATP-competitive, small-molecule kinase inhibitors are being tested in myeloproliferative disorders. Future studies will determine whether JAK inhibitors are useful in the treatment of HNSCC or lung cancer.

12-O-tetradecanoylphorbol-13-acetate inhibits melanoma growth by inactivation of STAT3 through protein kinase C-activated tyrosine phosphatase(s)

The growth of most melanoma cells in vitro is inhibited by the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). In this study, the involvement of the signal transducer and activator of transcription 3 (STAT3) in the TPA-induced growth inhibition of melanoma cells was examined. The in vitro growth and DNA synthesis of five melanoma cell lines, whose STAT3 was activated (phosphorylated), was inhibited by TPA, whereas that of WM35 and WM39 cells, whose STAT3 activity was at negligible levels, was considerably slow and not affected by TPA. Blockade of STAT3 activity by small interfering RNAs suppressed the growth of WM1205Lu cells containing constitutively activated STAT3. Treatment of WM1205Lu cells with TPA decreased both the phosphorylated STAT3 and the DNA-binding activity of STAT3. Pretreatment of WM1205Lu cells with either a protein-tyrosine phosphatase inhibitor or a protein kinase C (PKC) inhibitor prevented the inhibitory effects of TPA on the level of phosphorylated STAT3. The five melanoma cell lines containing phosphorylated STAT3 commonly expressed PKCalpha, PKCdelta, and PKCepsilon. Introduction of the dominant negative mutant of one of these PKC isoforms into WM1205Lu cells inhibited the TPA-induced dephosphorylation of STAT3. A Src inhibitor attenuated the STAT3 phosphorylation in WM1205Lu cells. These results indicate that constitutively activated STAT3 is positively regulated by c-Src and negatively regulated by a PKC-activated tyrosine phosphatase(s) in melanoma cells. Because TPA did not affect c-Src activity, we conclude that the growth inhibitory effect of TPA on melanoma cells is mediated through inactivation of STAT3 by a PKC-activated tyrosine phosphatase(s).

Bortezomib up-regulates activated signal transducer and activator of transcription-3 and synergizes with inhibitors of signal transducer and activator of transcription-3 to promote head and neck squamous cell carcinoma cell death

Head and neck squamous cell carcinomas (HNSCC) are commonly resistant to conventional chemotherapy drugs and exhibit overexpression of signal transducer and activator of transcription 3 (STAT3). STAT3 promotes both the proliferation and survival of HNSCC cells. Recent studies have shown that the proteasome inhibitor bortezomib shows cytotoxic activity against HNSCC in vitro and in vivo. We report that treatment of HNSCC cells with bortezomib led to up-regulation of total STAT3 protein and the phosphorylated/activated form of STAT3, as well as an increase in cellular STAT3 activity. This suggested that the ability of bortezomib to kill HNSCC cells may be blunted due to induction of STAT3, and inhibition of STAT3 may be a useful means for improving bortezomib efficacy. Indeed, forced expression of dominant-active STAT3 inhibited bortezomib-induced cell death, whereas expression of dominant-negative STAT3 served to enhance killing by this compound. In addition, specific inhibition of STAT3 with the use of a STAT3 decoy oligonucleotide resulted in enhancement of bortezomib-induced apoptosis signaling and loss of clonogenic survival. Cotreatment of HNSCC cells with bortezomib and guggulsterone, a naturally occurring compound known to inhibit STAT3 activation, led to synergistic activation of cell death and loss of clonogenic survival. In summary, these studies show that bortezomib induces the expression of active STAT3, a key growth- promoting protein in HNSCC cells. Furthermore, our findings suggest that the therapeutic activity of bortezomib against HNSCC may be markedly improved by cotreatment with molecular targeting agents against STAT3.

Expression of activated signal transducer and activator of transcription-3 predicts poor prognosis in cervical squamous-cell carcinoma

BACKGROUND

Stat3 is a member of the Janus-activated kinase/STAT signalling pathway. It normally resides in the cytoplasm and can be activated through phosphorylation. Activated Stat3 (p-Stat3) translocates to the nucleus to activate the transcription of several molecules involved in cell survival and proliferation. The constitutive activation of Stat3 has been shown in various types of malignancies, and its expression has been reported to indicate a poor prognosis. However, the correlation between the constitutive activation of Stat3 and the prognosis of cervical cancer patients has not been reported.

METHODS

The immunohistochemical analysis of p-Stat3 expression was performed on tissues from 125 cervical squamous-cell carcinoma patients who underwent extended hysterectomy and pelvic lymphadenectomy, and the association of p-Stat3 expression with several clinicopathological factors and survival was investigated.

RESULTS

Positive p-Stat3 expression was observed in 71 of 125 (56.8%) cases and was significantly correlated with lymph node metastasis, lymph vascular space invasion, and large tumour diameter (>4 cm) by Fisher's exact test. Kaplan-Meier survival analysis showed that p-Stat3 expression was statistically indicative of a poor prognosis for overall survival (P=0.006) and disease-free survival (P=0.010) by log-rank test.

CONCLUSION

These data showed that p-Stat3 expression in cervical cancer acts as a predictor of poor prognosis.

IL-6, but not IFN-gamma, triggers apoptosis and inhibits in vivo growth of human malignant T cells on STAT3 silencing

STAT1 and STAT3 are the main mediators of the signaling of interferons (IFNs) and of gp130 cytokines, respectively. Neoplastic T lymphocytes frequently become resistant to the IFN-gamma/STAT1 apoptotic pathway, often because of the downregulation of the IFN-gammaR2 receptor chain. Many studies suggest that cross-regulation between different STATs, in particular between STAT1 and STAT3, may profoundly affect cytokine/growth factor signaling. Here, the function of STAT3 in the negative regulation of STAT1 apoptotic pathway was investigated by RNA interference-mediated STAT3 silencing in human malignant T lymphocytes. In STAT3-depleted cells, interleukin (IL)-6 acquired the capacity to induce apoptosis, correlating with prolonged STAT1 activation and the induction of major histocompatibility complex (MHC) class I expression. In contrast, in the absence of STAT3, IFN-gamma could slightly enhance apoptosis but its ability to induce MHC class I expression was unchanged. Accordingly, IL-6, but not IFN-gamma, could significantly impair the in vivo growth of STAT3-depleted human neoplastic T lymphocytes transplanted into severe combined immunodeficient mice. Therefore, treatment with IL-6 and simultaneous STAT3 silencing may represent a potential therapeutic approach to control the expansion of IFN-gamma-unresponsive neoplastic T cells.

Targeting STAT3 in cancer: how successful are we?

BACKGROUND

Aberrant activation of the signal transducer and activator of transcription (STAT)3 occurs in many human tumors. Moreover, studies utilizing genetic and pharmacological approaches to modulate constitutive STAT3 activity have provided compelling evidence for the critical role of aberrant STAT3 activity in malignant transformation and tumor progression, and thereby validated STAT3 as a novel cancer drug target.

OBJECTIVE

This review is intended to be a full coverage of the efforts to develop direct STAT3 inhibitors and will provide a discussion on the inhibitory modalities developed to date.

METHODS

Review of the literature focused on the modalities and mechanisms that directly target and inhibit the STAT protein or its functions.

RESULTS/CONCLUSION

While a variety of STAT3 inhibitors have been identified that induce antitumor cell effects in vitro and in vivo, the landscape remains murky. With a few exceptions, most of the STAT3 inhibitors reported to date have not undergone an in vivo efficacy, pharmacology or toxicity testing. Also, there is no evidence, per the published literature of an impending clinical development for the few agents that were reported to exhibit in vivo efficacy. Overall, there is the need for a reassessment of the ongoing strategies to target STAT3 intended not only for refinement, but also for incorporating some new technologies to strengthen our efforts and ensure the success - sooner, rather than later - of identifying suitable anti-STAT3 agents for development into clinically useful anticancer therapeutics.

Conformationally constrained peptidomimetic inhibitors of signal transducer and activator of transcription. 3: Evaluation and molecular modeling

Signal transducer and activator of transcription 3 (Stat3) is involved in aberrant growth and survival signals in malignant tumor cells and is a validated target for anticancer drug design. We are targeting its SH2 domain to prevent docking to cytokine and growth factor receptors and subsequent signaling. The amino acids of our lead phosphopeptide, Ac-pTyr-Leu-Pro-Gln-Thr-Val-NH(2), were replaced with conformationally constrained mimics. Structure-affinity studies led to the peptidomimetic, pCinn-Haic-Gln-NHBn (21), which had an IC(50) of 162 nM (fluorescence polarization), compared to 290 nM for the lead phosphopeptide (pCinn = 4-phosphoryloxycinnamate, Haic = (2S,5S)-5-amino-1,2,4,5,6,7-hexahydro-4-oxo-azepino[3,2,1-hi]indole-2-carboxylic acid). pCinn-Haic-Gln-OH was docked to the SH2 domain (AUTODOCK), and the two highest populated clusters were subjected to molecular dynamics simulations. Both converged to a common peptide conformation. The complex exhibits unique hydrogen bonding between Haic and Gln and Stat3 as well as hydrophobic interactions between the protein and pCinn and Haic.

Mechanisms of tumor resistance to EGFR-targeted therapies

BACKGROUND

Much effort has been devoted to development of cancer therapies targeting EGFR, based on its role in regulating cell growth. Small-molecule and antibody EGFR inhibitors have clinical roles based on their efficacy in a subset of cancers, generally as components of combination therapies. Many cancers are either initially resistant to EGFR inhibitors or become resistant during treatment, limiting the efficacy of these reagents.

OBJECTIVE/METHODS

To review cellular resistance mechanisms to EGFR-targeted therapies.

RESULTS/CONCLUSIONS

The best validated of these mechanisms include activation of classic ATP-binding casette (ABC) multidrug transporters; activation or mutation of EGFR; and overexpression or activation of signaling proteins operating in relation to EGFR. We discuss current efforts and potential strategies to override these sources of resistance. We describe emerging systems-biology-based concepts of alternative resistance to EGFR-targeted therapies, and discuss their implications for use of EGFR-targeted and other targeted therapies.

Sustained Src inhibition results in signal transducer and activator of transcription 3 (STAT3) activation and cancer cell survival via altered Janus-activated kinase-STAT3 binding

Locoregional and distant recurrence remains common and usually fatal for patients with advanced head and neck squamous cell carcinoma (HNSCC). One promising molecular target in HNSCC is the Src family kinases (SFK). SFKs can affect cellular proliferation and survival by activating the signal transducer and activator of transcription (STAT) family of transcription factors, especially STAT3. Surprisingly, sustained SFK inhibition resulted in only transient inhibition of STAT3. We investigated the mechanism underlying STAT3 activation and its biological importance. Specific c-Src knockdown with small interfering RNA (siRNA) resulted in STAT3 activation showing specificity, which was inhibited by Janus-activated kinase (JAK; TYK2 and JAK2) depletion with siRNA. Sustained SFK inhibition also resulted in recovered JAK-STAT3 binding and JAK kinase activity after an initial reduction, although JAK phosphorylation paradoxically decreased. To determine the biological significance of STAT3 activation, we combined specific STAT3 depletion with a pharmacologic SFK inhibitor and observed increased cell cycle arrest and apoptosis. Likewise, the addition of STAT3- or JAK-specific siRNA to c-Src-depleted cells enhanced cytotoxicity relative to cells incubated with c-Src siRNA alone. These results show that reactivation of STAT3 after sustained, specific c-Src inhibition is mediated through altered JAK-STAT3 binding and JAK kinase activity and that this compensatory pathway allows for cancer cell survival and proliferation despite durable c-Src inhibition. To our knowledge, this novel feedback pathway has never been described previously. Given that pharmacologic SFK inhibitors are currently being evaluated in clinical trials, these results have potential clinical implications for cancer therapy.

Signal transducers and activators of transcription mediate fibroblast growth factor-induced vascular endothelial morphogenesis

The fibroblast growth factors (FGF) play diverse roles in development, wound healing, and angiogenesis. The intracellular signal transduction pathways, which mediate these pleiotropic activities, remain incompletely understood. We show here that the proangiogenic factors FGF2 and FGF8b can activate signal transducers and activators of transcription (STAT) in mouse microvascular endothelial cells (EC). Both FGF2 and FGF8b activate STAT5 and to a lesser extent STAT1, but not STAT3. The FGF2-dependent activation of endothelial STAT5 was confirmed in vivo with the Matrigel plug angiogenesis assay. In tissue samples of human gliomas, a tumor type wherein FGF-induced angiogenesis is important, STAT5 is detected in tumor vessel EC nuclei, consistent with STAT5 activation. By forced expression of constitutively active or dominant-negative mutant STAT5A in mouse brain ECs, we further show that STAT5 activation is both necessary and sufficient for FGF-induced cell migration, invasion, and tube formation, which are key events in vascular endothelial morphogenesis and angiogenesis. In contrast, STAT5 is not required for brain EC mitogenesis. The cytoplasmic tyrosine kinases Src and Janus kinase 2 (Jak2) both seem to be involved in the activation of STAT5, as their inhibition reduces FGF2- and FGF8b-induced STAT5 phosphorylation and EC tube formation. Constitutively active STAT5A partially restores tube formation in the presence of Src or Jak2 inhibitors. These observations show that FGFs use distinct signaling pathways to induce angiogenic phenotypes. Together, our findings implicate the FGF-Jak2/Src-STAT5 cascade as a critical angiogenic FGF signaling pathway.

The incidence, correlation with tumor-infiltrating inflammation, and prognosis of phosphorylated STAT3 expression in human gliomas

PURPOSE

The signal transducer and activator of transcription 3 (STAT3) is frequently overexpressed in most cancers, propagates tumorigenesis, and is a key regulator of immune suppression in cancer patients. We sought to determine the incidence of phosphorylated STAT3 (p-STAT3) expression in malignant gliomas of different pathologic types, whether p-STAT3 expression is a negative prognostic factor, and whether p-STAT3 expression influences the inflammatory response within gliomas.

METHODS

Using immunohistochemical analysis, we measured the incidence of p-STAT3 expression in 129 patients with gliomas of various pathologic types in a glioma tissue microarray. We categorized our results according to the total number of p-STAT3-expressing cells within the gliomas and correlated this number with the number of infiltrating T cells and T regulatory cells. We then evaluated the association between p-STAT3 expression and median survival time using univariate and multivariate analyses.

RESULTS

We did not detect p-STAT3 expression in normal brain tissues or low-grade astrocytomas. We observed significant differences in the incidence of p-STAT3 expression between the different grades of astrocytomas and different pathologic glioma types. p-STAT3 expression was associated with the population of tumor-infiltrating immune cells but not with that of T regulatory cells. On univariate analysis, we found that p-STAT3 expression within anaplastic astrocytomas was a negative prognostic factor.

CONCLUSIONS

p-STAT3 expression is common within gliomas of both the astrocytic and oligodendroglial lineages and portends poor survival in patients with anaplastic astrocytomas. p-STAT3 expression differs significantly between gliomas of different pathologic types and grades and correlated with the degree of immune infiltration.

Serpin B3/B4, activated by STAT3, promote survival of squamous carcinoma cells

Persistently activated STAT3 contributes to cell survival in many different human cancers. Cancer cell secretion of IL-6 is a frequent basis for persistent STAT3 activation; we show that antibodies against IL-6 or gp-130, the signaling unit of the IL-6 receptor, can abruptly remove persistently activated STAT3 causing prompt disappearance of cysteine proteases of serpin B3/B4 mRNAs, known as squamous cell carcinoma antigens 1 and 2. STAT3 occupies the promoter of serpin B3/B4 before removal and siRNA removal of B3/B4 mRNA caused cell death in HN13 head and neck cancer cells. Thus persistently activated STAT3 is a required part of the continuous activation of B3/B4 genes, which protects tumor cells from dying.

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.

Lack of toxicity of a STAT3 decoy oligonucleotide

BACKGROUND

STAT3 overexpression has been detected in several cancers including head and neck squamous cell carcinoma (HNSCC). Previous studies using intratumoral administration of a STAT3 decoy oligonucleotide that abrogates STAT3-mediated gene transcription in preclinical cancer models have demonstrated antitumor efficacy. This study was conducted to observe the toxicity and biologic effects of the STAT3 decoy in a non-human primate model, in anticipation of initiating a clinical trial in HNSCC patients.

METHODS

Three study groups (two monkeys/sex/group) were administered a single intramuscular injection of low dose of STAT3 decoy (0.8 mg total dose/monkey), high dose of STAT3 decoy (3.2 mg total dose/monkey) or vehicle control (PBS alone) on day 1 and necropsies were performed on days 2 and 15 (one monkey/sex/group/day). Low and high doses of the decoy were administered in the muscle in a volume of 0.9 ml. Tissue and blood were harvested for toxicology and biologic analyses.

RESULTS

Upon observation, the STAT3 decoy-treated animals exhibited behavior that was similar to the vehicle control group. Individual animal body weights remained within 1% of pretreatment weights throughout the study. Hematological parameters were not significantly different between the control and the treatment groups. Clinical chemistry fluctuations were considered within normal limits and were not attributed to the STAT3 decoy. Assessment of complement activation breakdown product (Bb) levels demonstrated no activation of the alternative pathway of complement in any animal at any dose level. At necropsy, there were no gross or microscopic findings attributed to STAT3 decoy in any organ examined. STAT3 target gene expression at the injection site revealed decreased Bcl-X(L) and cyclin D1 expression levels in the animals treated with high dose of STAT3 decoy compared to the animals injected with low dose of STAT3 decoy or the vehicle as control.

CONCLUSION

Based on these findings, the no-observable-adverse-effect-level (NOAEL) was greater than 3.2 mg/kg when administered as a single dose to male and female Cynomolgus monkeys. Plans are underway to test the safety and biologic effects of intratumoral administration of the STAT3 decoy in HNSCC patients.

Immunohistochemical analysis of phosphotyrosine signal transducer and activator of transcription 3 and epidermal growth factor receptor autocrine signaling pathways in head and neck cancers and metastatic lymph nodes

PURPOSE

To determine the effect of tyrosine-phosphorylated signal transducer and activator of transcription 3 (pSTAT3) immunoexpression on survival in two independent cohorts of patients with squamous cell carcinoma of the head and neck (SCCHN) and to evaluate pSTAT3, transforming growth factor-alpha (TGF-alpha), epidermal growth factor receptor (EGFR), and gastrin-releasing peptide receptor (GRPR) expression in matched tumor and lymph node metastases in one of these cohorts. EXPERIMENTAL TECHNIQUE: Immunostaining for pSTAT3, TGF-alpha, EGFR, and GRPR was done in two SCCHN cohorts (cohort 1, 61 tumors; cohort 2, 69 paired primary tumors and lymph node metastases). Semiquantitative scores derived from the product of staining intensity (scale 0-3) score and percentage of positive tumor cells were correlated with clinical outcome.

RESULTS

Immunoexpression of pSTAT3 did not correlate with clinical outcome in either cohort (cohort 1, P = 0.914; cohort 2, P = 0.312). In cohort 2, TGF-alpha and EGFR expression in the primary tumors showed some association with decreased disease-free survival (P = 0.0306 and P = 0.0985, respectively). Both pSTAT3 and EGFR showed a correlation of expression between tumor and matched lymph node metastasis (P < 0.0001 and P = 0.0046, respectively). In addition, the expression of EGFR and GRPR in the primary tumors correlated with TGF-alpha expression in paired nodal metastases (P = 0.0043 and P = 0.0268, respectively). In the nodal metastases, TGF-alpha expression correlated with EGFR expression (P = 0.0069). In primary tumors, GRPR expression correlated with TGF-alpha and EGFR expression (P = 0.0378 and P = 0.0026, respectively).

CONCLUSIONS

These findings support an autocrine signaling pathway involving TGF-alpha, EGFR, and pSTAT3 in metastatic SCCHN as well as transactivation of EGFR by GRPR via TGF-alpha, but fails to identify an independent prognostic role for pSTAT3 immunoexpression.

Genomic dissection of the cytokine-controlled STAT5 signaling network in liver

Growth hormone (GH) controls the physiology and pathophysiology of the liver, and its signals are conducted by two members of the family of signal transducers and activators of transcription, STAT5A and STAT5B. Mice in which the Stat5a/b locus has been inactivated specifically in hepatocytes display GH resistance, the sex-specific expression of genes associated with liver metabolism and the cytochrome P-450 system is lost, and they develop hepatosteatosis. Several groups have shown by global gene expression profiling that a cadre of STAT5A/B target genes identify genetic cascades induced by GH and other cytokines. Evidence is accumulating that in the absence of STAT5A/B GH aberrantly activates STAT1 and STAT3 and their downstream target genes and thereby offers a partial explanation of some of the physiological alterations observed in Stat5a/b-null mice and human patients. We hypothesize that phenotypic changes observed in the absence of STAT5A/B are due to two distinct molecular consequences: first, the failure of STAT5A/B target genes to be activated by GH and second, the rerouting of GH signaling to other members of the STAT family. Rerouting of GH signaling to STAT1 and STAT3 might partially compensate for the loss of STAT5A/B, but it certainly activates biological programs distinct from STAT5A/B. Here we discuss the extent to which studies on global gene expression profiling have fostered a better understanding of the biology behind cytokine-STAT5A/B networks in hepatocytes. We also explore whether this wealth of information on gene activity can be used to further understand the roles of cytokines in liver disease.

Structural basis for the binding of high affinity phosphopeptides to Stat3

Signal transducer and activator of transcription 3 (Stat3) is constitutively active in a number of cancers where it participates in aberrant transcription of prosurvival, cell cycling, and angiogenesis genes. Since Stat3 initiates its signaling activity through binding of its SH2 domain to phosphotyrosine residues on cell surface receptors, inhibitors targeting this region of the protein are potential chemotherapeutic agents. To date, no NMR or X-ray crystallographic structures of high-affinity phosphopeptides complexed with the Stat3 SH2 domain are available to aid in the development of peptidomimetic antagonists. Examination of the crystal structures of several STAT proteins and the complex of Stat1 with Ac-pTyr-Asp-Lys-Pro-His-NH(2) led to a hypothesis that the specificity determinant for Stat3, glutamine at position pY+3 in pTyr-Xxx-Xxx-Gln sequences, resides in a unique pocket on the protein surface at the juncture of the third strand of the central beta-sheet and a unique, STAT specific alpha-helix. Docking of Ac-pTyr-Leu-Pro-Gln-NHBn to the SH2 domain of Stat3 using molecular modeling showed that the Gln binds tightly in this pocket and participates in a network of hydrogen bonds. Novel interactions between the peptide main chain and the protein were also discovered. Phosphopeptide structure-affinity studies using unnatural amino acids and glutamine derivatives provide evidence for the peptide-protein interactions revealed by the model and lend support to the binding hypothesis.

STAT-mediated EGFR signaling in cancer

The epidermal growth factor receptor (EGFR) and signal transducers and activators of transcription (STATs) are commonly expressed and activated in many malignancies. EGFR is an upstream activator of several pathways involved in tumor progression, and STATs activate selected genes involved in oncogenesis. There are several different mechanisms by which STAT proteins can mediate intracellular EGFR signaling, including direct activation of STATs by EGFR binding and indirect activation of STATs through Src-mediated EGFR signaling. EGFR likely activates STAT in a manner distinctive from other mechanisms of STAT activation; STAT5 can be phosphorylated in an EGF-dependent manner at unique sites, conferring novel functions. Cumulative evidence suggests that targeting EGFR signaling pathways at several levels may demonstrate synergistic therapeutic effects compared with targeting the upstream receptor alone. Thus, methods to inhibit EGFR in conjunction with oncogenic STATs may represent a novel therapeutic strategy for cancers characterized by upregulation of EGFR signaling.

STAT3 as a target for inducing apoptosis in solid and hematological tumors

Studies in the past few years have provided compelling evidence for the critical role of aberrant Signal Transducer and Activator of Transcription 3 (STAT3) in malignant transformation and tumorigenesis. Thus, it is now generally accepted that STAT3 is one of the critical players in human cancer formation and represents a valid target for novel anticancer drug design. This review focuses on aberrant STAT3 and its role in promoting tumor cell survival and supporting the malignant phenotype. A brief evaluation of the current strategies targeting STAT3 for the development of novel anticancer agents against human tumors harboring constitutively active STAT3 will also be presented.

A signal network involving coactivated NF-kappaB and STAT3 and altered p53 modulates BAX/BCL-XL expression and promotes cell survival of head and neck squamous cell carcinomas

Abrogation of apoptosis to sustain cell survival is an essential step in development of cancer. Aberrant activation of signal transcription factors NF-kappaB or STAT3, alterations in p53 status, or BCL/BAX family expression have each been reported to affect cell survival in cancer, including head and neck squamous cell carcinomas (HNSCC). However, molecular targeting of these alterations individually has yielded disappointing results. In our study, we examined the hypothesis that alterations in a signal network involving NF-kappaB, STAT3 and p53 modulates expression of proapoptotic BAX and antiapoptotic BCL-XL proteins, and promotes cell survival of HNSCC. We found that NF-kappaB and STAT3 are coactivated together, and with cytokine stimulation or siRNA knock-down, both modulate BAX/BCL-XL. Greater modulation among HNSCC lines expressing low wt p53 than those over-expressing mt p53 protein suggested that decreased p53 expression might enhance activation of NF-kappaB, STAT3 and BCL-XL. Reexpression of wt p53 suppressed NF-kappaB and STAT3 nuclear binding activity, and BCL-XL expression, while inducing p21 and BAX. Over-expression of p53 together with inhibition of NF-kappaB or STAT3 induced greater increase in the BAX/BCL-XL ratio and apoptosis than modulation of these transcription factors individually. Conversely, NF-kappaB or STAT3 inducing cytokines decreased the BAX/BCL-XL ratio. Thus, a network involving signal coactivation of NF-kappaB and STAT3, differentially modified by p53 inactivation or mutation, promotes altered BAX/BCL-XL expression and cell survival in HNSCC. Inhibition of signal activation of both NF-kappaB and STAT3 together with reexpression of p53 could be the most effective strategy to restore BAX/BCL-XL regulation and for cytotoxic therapy of HNSCC.

Combined targeting of epidermal growth factor receptor, signal transducer and activator of transcription-3, and Bcl-X(L) enhances antitumor effects in squamous cell carcinoma of the head and neck

Squamous cell carcinoma of the head and neck (SCCHN) is a leading cause of cancer deaths worldwide. Epidermal growth factor receptor (EGFR), an upstream mediator of signal transducer and activator of transcription (STAT)-3 is overexpressed in a variety of cancers, including SCCHN. Therapies such as monoclonal antibodies and tyrosine kinase inhibitors targeting EGFR have demonstrated limited antitumor efficacy, which may be explained, in part, by persistent STAT3 activation despite EGFR inhibition. STAT3 activation induces expression of target genes in SCCHN, including Bcl-X(L), a mediator of antiapoptotic activity. Bcl-X(L) is commonly overexpressed in SCCHN where it correlates with chemoresistance, making it a potential therapeutic target. Targeting the EGFR-STAT3-Bcl-X(L) pathway at several levels, including the upstream receptor, the intracellular transcription factor, and the downstream target gene, has not been investigated previously. Using erlotinib, an EGFR-specific reversible tyrosine kinase inhibitor in combination with a STAT3 transcription factor decoy, we found enhanced antitumor effects in vitro and in vivo. The combination of the STAT3 decoy and gossypol, a small molecule targeting Bcl-X(L), also yielded enhanced inhibition of cell proliferation. The triple combination of erlotinib, STAT3 decoy, and gossypol further enhanced cell growth inhibition and apoptosis in vitro, and it down-regulated signaling molecules further downstream of the EGFR-STAT3 signaling pathway, such as cyclin D1. These results suggest that combined targeting of several components of an oncogenic signaling pathway may be an effective therapeutic strategy for SCCHN.

Lipopolysaccharide-squamous cell carcinoma-monocyte interactions induce cancer-supporting factors leading to rapid STAT3 activation

Oral and oro-pharyngeal squamous cell carcinomas (OSCC) exhibit surface breach, and recent studies have demonstrated bacterial contamination of primary and metastatic OSCC. Increasing concentrations of inflammatory products, such as interleukin (IL)-6 and vascular endothelial growth factor (VEGF), correlate with, and contribute to, cancer progression, but their regulation in OSCC is poorly understood. We hypothesized that monocyte-lineage cells and bacterial contamination may contribute important inflammatory products that can support OSCC progression. We found that relative to non-specific chronic mucositis, oral carcinoma-in-situ/superficially-invasive OSCC contained more monocyte-lineage cells. In vitro, we used lipopolysaccharide (LPS) to model bacterial contamination, and evaluated the effects of oral and oropharyngeal (O)SCC-monocyte interactions and of LPS on OSCC cells and on the production of IL-6 and VEGF. OSCC cell lines varied in constitutive cytokine and chemokine production, and OSCC-monocyte interactions in the absence of LPS stimulated IL-6 and VEGF occasionally, while LPS-OSCC-monocyte interactions were always strongly stimulatory. Importantly, LPS independently stimulated some OSCC lines to secrete monocyte-dendritic cell chemoattractants CCL2 and/or CCL20, as well as IL-6 and/or VEGF. While very little constitutive Y705-STAT3 phosphorylation (pY705-STAT3) was detectable in HNSCC lines, IL-6 rapidly induced pY705-STAT3 in OSCC lines that produced little IL-6 constitutively. Supernatants from LPS-OSCC-monocyte co-cultures always rapidly and strongly activated STAT3, which was partly due to IL-6. We conclude that monocytes and microbial contamination have the potential to contribute to OSCC progression, as STAT3 activation in OSCC cells depends on soluble factors, which are consistently available through LPS-OSCC-monocyte interactions.

Galiellalactone is a novel therapeutic candidate against hormone-refractory prostate cancer expressing activated Stat3

BACKGROUND

Signal transducer and activator of transcription 3 (Stat3) is constitutively active (phosphorylated) in several forms of cancer, including prostate cancer (PCa). Stat3 signaling may be an interesting target for cancer therapy since inhibition of this pathway mediates growth inhibition and apoptosis of these cells. In this study we investigated the in vitro and in vivo effects of the fungal metabolite galiellalactone, a direct inhibitor of Stat3, on PCa cells.

METHODS

The human PCa cell lines DU145, PC-3, and LNCaP were used. Nude mice with subcutaneous PCa cell xenografts were subjected to daily intraperitoneal injections of galiellalactone for 3 weeks. The effect of galiellalactone on the induction of apoptosis of cultured PCa cells was investigated by Western blot analysis, immunocytochemistry, and annexin V staining. Effects of galiellalactone on Stat3 signaling were investigated by a luciferase reporter gene assay. Expression of Stat3 associated proteins and mRNA was investigated by Western blot and real-time quantitative PCR analysis.

RESULTS

Galiellalactone induced apoptosis of p-Stat3 positive PCa cells (androgen-insensitive DU145 and PC-3) but not in cells lacking p-Stat3 (androgen-sensitive LNCaP). Galiellalactone inhibited Stat3-mediated luciferase activity (IC(50) approximately 5 microM) and reduced the expression of Bcl-2, Bcl-x(L), c-myc, and cyclin D1. Furthermore, galiellalactone significantly suppressed DU145 xenograft growth in vivo (42% growth reduction; P<0.002) and reduced the relative mRNA expression of Bcl-x(L) and Mcl-1.

CONCLUSIONS

Galiellalactone induced growth inhibition and apoptosis in androgen-insensitive PCa cells expressing p-Stat3. We suggest that galiellalactone is a potential anti-tumor lead against hormone-refractory PCa with constitutively active Stat3.

Role of gp130-mediated signalling pathways in the heart and its impact on potential therapeutic aspects

IL-6-type cytokines bind to plasma membrane receptor complexes containing the common signal transducing receptor chain gp130 that is ubiquitously expressed in most tissues including the heart. The two major signalling cascades activated by the gp130 receptor, SHP2/ERK and STAT pathways, have been demonstrated to play important roles in cardiac development, hypertrophy, protection and remodelling in response to physiological and pathophysiological stimuli. Experimental data, both in vivo and in vitro, imply beneficial effects of gp130 signalling on cardiomyocytes in terms of growth and survival. In contrast, it has been reported that elevated serum levels of IL-6 cytokines and gp130 proteins are strong prognostic markers for morbidity and mortality in patients with heart failure or after myocardial infarction. Moreover, it has been shown that the local gp130 receptor system is altered in failing human hearts. In the present review, we summarize the basic principles of gp130 signalling, which requires simultaneous activation of STAT and ERK pathways under the tight control of positive and negative intracellular signalling modulators to provide a balanced biological outcome. Furthermore, we highlight the key role of the gp130 receptor and its major downstream effectors in the heart in terms of development and regeneration and in response to various physiological and pathophysiological stress situations. Finally, we comment on tissue-specific diversity and challenges in targeted pharmacological interference with components of the gp130 receptor system.

Transcriptional activation of the suppressor of cytokine signaling-3 (SOCS-3) gene via STAT3 is increased in F9 REX1 (ZFP-42) knockout teratocarcinoma stem cells relative to wild-type cells

Rex1 (Zfp42), first identified as a gene that is transcriptionally repressed by retinoic acid (RA), encodes a zinc finger transcription factor expressed at high levels in F9 teratocarcinoma stem cells, embryonic stem cells, and other stem cells. Loss of both alleles of Rex1 by homologous recombination alters the RA-induced differentiation of F9 cells, a model of pluripotent embryonic stem cells. We identified Suppressor of Cytokine Signaling-3 (SOCS-3) as a gene that exhibits greatly increased transcriptional activation in RA, cAMP, and theophylline (RACT)-treated F9 Rex1(-/-) cells (approximately 25-fold) as compared to wild-type (WT) cells ( approximately 2.5-fold). By promoter deletion, mutation, and transient transfection analyses, we have shown that this transcriptional increase is mediated by the STAT3 DNA-binding elements located between -99 to -60 in the SOCS-3 promoter. Overexpression of STAT3 dominant-negative mutants greatly diminishes this SOCS-3 transcriptional increase in F9 Rex1(-/-) cells. This increase in SOCS-3 transcription is associated with a four- to fivefold higher level of tyrosine-phosphorylated STAT3 in the RACT-treated F9 Rex1(-/-) cells as compared to WT. Dominant-negative Src tyrosine kinase, Jak2, and protein kinase A partially reduce the transcriptional activation of the SOCS 3 gene in RACT-treated F9 Rex1 null cells. In contrast, parathyroid hormone peptide enhances the effect of RA in F9 Rex1(-/-) cells, but not in F9 WT. Thus, Rex1, which is highly expressed in stem cells, inhibits signaling via the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway, thereby modulating the differentiation of F9 cells.

Targeting antiapoptotic Bcl-2 family members with cell-permeable BH3 peptides induces apoptosis signaling and death in head and neck squamous cell carcinoma cells

Head and neck squamous cell carcinomas (HNSCC) are frequently characterized by chemotherapy and radiation resistance, and by overexpression of Bcl-XL, an antiapoptotic member of the Bcl-2 protein family. In this report we examined whether cell-permeable peptides derived from the BH3 domains of proapoptotic Bax, Bad, or Bak could be used to target Bcl-XL and/or Bcl-2 in HNSCC cells, and induce apoptotic death in these cells. To render the peptides cell permeable, Antennapedia (Ant) or polyarginine (R8) peptide transduction domains were fused to the amino termini. Fluorescence microscopy of peptide-treated HNSCC cells revealed that the BH3 peptides colocalized with mitochondria, the site of Bcl-XL and Bcl-2 expression. By contrast, a mutant peptide (BaxE BH3) which cannot bind Bcl-XL or Bcl-2 was diffusely localized throughout the cytoplasm. Treatment of three HNSCC cell lines (1483, UM-22A, UM-22B) with the wild-type BH3 peptides resulted in loss of viability and induction of apoptosis, as assessed by MTS assays and annexin V staining. In general, Ant-conjugated peptides were more potent than R8-conjugated peptides, and Bad BH3 peptide was typically more potent than Bax BH3 or Bak BH3. Treatment of purified HNSCC mitochondria with BH3 peptides resulted in robust release of cytochrome c. Thus, the relative apoptosis resistance of HNSCC cells is not due to a deficit in this step of the intrinsic, mitochondrial-mediated apoptosis pathway. We conclude that cell-permeable BH3 peptides can be used to target Bcl-XL and/or Bcl-2 in HNSCC, and targeting of these proteins may have therapeutic value in the treatment of this disease.

Targeting the cytoplasmic and nuclear functions of signal transducers and activators of transcription 3 for cancer therapy

Signal transducers and activators of transcription (STAT) are a highly conserved family of transcription factors that are activated by phosphorylation in the cytoplasm, after which they translocate to the nucleus to regulate gene expression. Among the seven STATs, STAT3 is of particular interest due to its constitutive phosphorylation in a large proportion of human cancers and its ability to induce neoplastic transformation. Inhibition of STAT3 can reverse tumor growth in experimental systems while having few effects in normal cells. These findings have implicated STAT3 as a potentially important target for therapeutic intervention. In addition to its well-described role as a transcription factor, STAT3 has been found recently to have important effects in the cytoplasm. Collectively, these functions of STAT3 directly contribute to tumorigenesis, invasion, and metastasis. Given the potential importance of STAT3 as a target for cancer therapy, molecules have been developed that can block STAT3 function at a variety of steps. These drugs show promise as anticancer agents in model systems of a variety of common human cancers. Thus, elucidating the functions of STAT3 and developing agents to inhibit this protein remain important scientific and clinical challenges.

STAT nuclear translocation: potential for pharmacological intervention

The signal transducer and activator of transcription (STAT) proteins are extracellular ligand-responsive transcription factors that mediate broadly diverse biological processes, including cell proliferation, transformation, apoptosis, differentiation, fetal development, inflammation and immune response. Stimulation with multiple cytokines or growth factors all result in the tyrosine phosphorylation of STAT proteins and the subsequent gene regulation via their direct binding to the promoters of responsive genes. Cytokine-regulated gene activation is dependent on the continuous nucleocytoplasmic cycling of STAT signal transducers. The STATs use intricately intertwined karyopherin-dependent and -independent translocation mechanisms to coordinate the activation step at the cell membrane and gene expression in the nucleus. In addition, STATs appear to have cytokine-independent gene regulatory functions that may also depend on their regulated nucleocytoplasmic transfer. Numerous studies have implicated aberrant STAT signalling in cancer, immune defects and inflammatory diseases. Given the central role of intracellular trafficking for the proper signal processing by STAT proteins, pharmacological targeting of STAT nucleocytoplasmic translocation appears to be an attractive strategy to interfere with dysregulated cytokine signalling. This review will discuss possible scenarios that would result from the use of novel modulators of STAT shuttling, which may both increase or decrease STAT activation and, hence, transcriptional activity.

Gene therapy using ets-1 transcription factor decoy for peritoneal dissemination of gastric cancer

The ets-1 transcription factor plays an important role in cell proliferation, differentiation, apoptosis and tissue remodeling. Aberrant ets-1 expression correlates with aggressive tumor behavior and poorer prognosis in patients with various malignancies. This study evaluated the efficacy of double-stranded decoy oligonucleotides targeting ets-1-binding cis elements for the suppression of ets-1 in treatment of a peritoneal dissemination model of gastric cancer. In vitro, MTT assay was performed to evaluate the effect of the ets-1 decoy on cell growth. Electrophoretic mobility shift assay (EMSA) was performed to determine ets-1 activity. In vivo, the effect of the ets-1 decoy was investigated in the peritoneal dissemination nude mice model. Disseminated nodules were analyzed immunohistochemically. Ets-1 decoy, but not scrambled decoy, significantly inhibited cell growth in 2 gastric cancer cell lines, which showed overexpression of ets-1 protein by inhibiting the binding activity of ets-1. In the peritoneal dissemination model, the ets-1 decoy significantly suppressed the disseminated nodules, and tended to prolong the survival rate. PCNA index, microvessel density and VEGF expression were also reduced in peritoneal tumors treated with ets-1 decoy. Intraperitoneal injection of ets-1 decoy inhibited peritoneal dissemination of gastric cancer in a nude mice model. The results indicate that the decoy strategy for ets-1 offers a promising therapy for patients with incurable peritoneal dissemination of gastric cancer, most of which show overexpression of ets-1 protein.