Constitutive activation of Stat3 signaling abrogates apoptosis in squamous cell carcinogenesis in vivo

Effects of Acyclic Retinoid on Growth, Cell Cycle Control, Epidermal Growth Factor Receptor Signaling, and Gene Expression in Human Squamous Cell Carcinoma Cells

We described recently the growth inhibitory effects of the novel compound acyclic retinoid (ACR) in human hepatoma cell lines (M. Suzui et al., Cancer Res., 62: 3997-4006, 2002). In this study we examined the cellular and molecular effects of ACR on human squamous cell carcinoma (SCC) cells. ACR inhibited growth of the esophageal SCC cell line HCE7, and the head and neck SCC cell lines YCU-N861 and YCU-H891, with IC(50) values of approximately 10, 25, and 40 microM, respectively. Detailed studies were then done with HCE7 cells. Treatment of these cells with 10 microM ACR caused an increase of cells in G(0)-G(1) and induced apoptosis. This was associated with two phases of molecular events. During phase 1, which occurred within 6-12 h, there was an increase in the retinoic acid receptor beta (RARbeta) and p21(CIP1) proteins, and their corresponding mRNAs, and a decrease in the hyperphosphorylated form of the retinoblastoma protein. During phase 2, which occurred at approximately 24 h, there was a decrease in the cellular level of transforming growth factor alpha, and the phosphorylated (i.e., activated) forms of the epidermal growth factor receptor, Stat3, and extracellular signal-regulated kinase proteins, and a decrease in both cyclin D1 protein and mRNA. Reporter assays indicated that ACR inhibited the transcriptional activity of the cyclin D1, c-fos, and activator protein promoters. On the other hand, ACR markedly stimulated the activity of a retinoic acid response element-CAT reporter when the cells were cotransfected with a RARbeta expression vector. A hypothetical model explaining these two phases is presented. The diverse effects that we obtained with ACR suggest that this agent might be useful in the chemoprevention and/or therapy of human SCCs.

Inhibition of human non-small cell lung tumors by a c-Met antisense/U6 expression plasmid strategy

c-Met is a receptor tyrosine kinase whose activation by hepatocyte growth factor (HGF) can lead to transformation and tumorigenicity in a variety of tumors. We investigated the effects of suppressing c-Met protein expression in human non-small cell lung tumors. Expression plasmids containing either sense or antisense sequences of the human c-met gene were constructed under control of the U6 snRNA promoter. A U6 control plasmid was also constructed that did not contain any c-met sequence. These constructs have been examined both in vitro and in an in vivo tumor xenograft model. The c-Met protein was downregulated by 50-60% in two lung cancer cell lines that were transiently transfected with the c-Met antisense versus U6 control. Tumor cells treated with the c-Met antisense construct also show decreased phosphorylation of c-Met and MAP kinase when exposed to exogenous HGF. Lung cancer cells were grown as xenografts in mice and treated by intratumoral liposome-mediated transfer of the c-Met sense, antisense or U6 control plasmids. The treatment of lung tumors with c-Met antisense versus U6 control plasmid resulted in the downregulation of the c-Met protein expression, a 50% decrease in tumor growth over a 5-week treatment period and an increased rate of apoptosis. These results suggest that targeting the HGF/c-Met pathway may be an effective novel strategy to treat lung cancer patients.

Targeting Stat3 with G-quartet oligodeoxynucleotides in human cancer cells

Stat3 is an oncogene that is activated in many human cancer cells. Genetic approaches that disrupt Stat3 activity result in inhibition of cancer cell growth and enhanced cell apoptosis supporting the development of novel drugs targeting Stat3 for cancer therapy. G-quartet oligodeoxynucleotides (ODNs) were demonstrated to be potent inhibitors of Stat3 DNA binding activity in vitro with the G-quartet ODN, T40214, having an IC(50) of 7 microM. Computer-simulated docking studies indicated that G-quartet ODNs mainly interacted with the SH2 domain of Stat3 and were capable of inserting between the SH2 domains of Stat3 dimers bound to DNA. We demonstrated that the G-rich ODN T40214, which forms a G-quartet structure at intracellular but not extracellular K+ ion concentrations, is delivered efficiently into the cytoplasm and nucleus of cancer cells where it inhibited IL-6-stimulated Stat3 activation and suppressed Stat3-mediated upregulation of bcl-x and mcl-1 gene expression. Thus, G-quartet represents a new class of drug for targeting of Stat3 within cancer cells.

Signal transducer and activator of transcription 3 (STAT3) activation in prostate cancer: Direct STAT3 inhibition induces apoptosis in prostate cancer lines

Signal transducers and activators of transcription (STAT) were originally discovered as components of cytokine signal transduction pathways. Persistent activation of one STAT, STAT3, is a common feature of prostate cancer. Activated STAT3 was found in pathology specimens obtained from prostatectomy in the cancerous areas but not in the normal margins. Because the activation of STAT3 is mediated by the action of an upstream Janus kinase (JAK) kinase, usually JAK1 or JAK2, the activation step for STAT3 might itself be a target for therapy in prostate cancer. However, the redundancy of upstream kinases may make this strategy unreliable for therapy. To develop molecular targets for prostate cancer treatment, JAK kinase and STAT3 inhibition of two prostate cancer lines were compared. DU145 and NRP-154 cells were treated with JAK kinase inhibitors, analyzed for onset of apoptosis, and measured by annexin V binding and propidium iodide uptake. Activation of caspases in the cells was determined by measuring cleaved caspase-3 following treatment. For determining the effect on mitochondrial membrane depolarization that accompanies apoptosis, the fluorescent dye JC-1 was used. STAT3 was specifically inhibited by transfecting either a dominant-negative (DN) STAT3 plasmid or antisense STAT3 oligonucleotides into the cells. To look for reduction in STAT3 levels within cells, fixed and permeabilized prostate cancer cells were stained with antibody to STAT3. We found that more than one JAK kinase is involved in STAT3 activation in prostate cancer lines. AG490 (JAK2 specific) induced apoptosis in DU145 but not in NRP-154 prostate cancer lines, whereas piceatannol (JAK1 specific) induced apoptosis in NRP-154 but not in DU145 cells. Next, we demonstrated efficacy of specific STAT3 inhibitors in prostate cancer lines. Both induction of apoptosis and reduction in intracellular STAT3 protein were observed following treatment with antisense STAT3 oligonucleotides, while transfection of a DN-STAT3 plasmid into both prostate cancer cell lines resulted in loss of viability and onset of apoptosis. We conclude that STAT3-specific inhibitors, rather than JAK kinase-specific inhibitors, should be more useful therapeutically in treating androgen-resistant prostate cancer and that STAT3 is an appropriate target in the treatment of prostate cancer.

SOCS-3 is frequently silenced by hypermethylation and suppresses cell growth in human lung cancer

Lung cancer is the leading cause of cancer death in the world, but the molecular mechanisms for its development have not been well characterized. The suppressors of cytokine signaling (SOCS) are inhibitors of cytokine signaling that function via the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway. Eight SOCS proteins with similar structures have been identified so far. SOCS family members, however, have distinct mechanisms of inhibition of JAK/STAT signaling. Abnormalities of the JAK/STAT pathway are associated with cancer. Inhibition of signaling results in growth suppression in various cell types. Recently, the involvement of SOCS-1 in carcinogenesis has been reported. Here, we report identification of frequent hypermethylation in CpG islands of the functional SOCS-3 promoter that correlates with its transcription silencing in cell lines (lung cancer, breast cancer, and mesothelioma) and primary lung cancer tissue samples. Restoration of SOCS-3 in lung cancer cells where SOCS-3 was methylation-silenced resulted in the down-regulation of active STAT3, induction of apoptosis, and growth suppression. Our results suggest that methylation silencing of SOCS-3 is one of the important mechanisms of constitutive activation of the JAK/STAT pathway in cancer pathogenesis. The data also suggest that SOCS-3 therapy may be useful in the treatment of cancer.

SOCS-3 is frequently silenced by hypermethylation and suppresses cell growth in human lung cancer

Lung cancer is the leading cause of cancer death in the world, but the molecular mechanisms for its development have not been well characterized. The suppressors of cytokine signaling (SOCS) are inhibitors of cytokine signaling that function via the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway. Eight SOCS proteins with similar structures have been identified so far. SOCS family members, however, have distinct mechanisms of inhibition of JAK/STAT signaling. Abnormalities of the JAK/STAT pathway are associated with cancer. Inhibition of signaling results in growth suppression in various cell types. Recently, the involvement of SOCS-1 in carcinogenesis has been reported. Here, we report identification of frequent hypermethylation in CpG islands of the functional SOCS-3 promoter that correlates with its transcription silencing in cell lines (lung cancer, breast cancer, and mesothelioma) and primary lung cancer tissue samples. Restoration of SOCS-3 in lung cancer cells where SOCS-3 was methylation-silenced resulted in the down-regulation of active STAT3, induction of apoptosis, and growth suppression. Our results suggest that methylation silencing of SOCS-3 is one of the important mechanisms of constitutive activation of the JAK/STAT pathway in cancer pathogenesis. The data also suggest that SOCS-3 therapy may be useful in the treatment of cancer.

Stat3 protects against Fas-induced liver injury by redox-dependent and -independent mechanisms

Signal transducer and activator of transcription-3 (Stat3) is one of the most important molecules involved in the initiation of liver development and regeneration. In order to investigate the hepatoprotective effects of Stat3, we examined whether Stat3 protects against Fas-mediated liver injury in the mouse. A constitutively activated form of Stat3 (Stat3-C) was adenovirally overexpressed in mouse liver by intravenous injection, and then a nonlethal dose of Fas agonist (Jo2) was injected intraperitoneally into the mouse (0.3 microg/g body wt). Stat3-C dramatically suppressed both apoptosis and necrosis induced by Jo2. In contrast, liver-specific Stat3-knockout mice failed to survive following Jo2 injection. Stat3-C upregulated expression of FLICE inhibitor protein (FLIP), Bcl-xL, and Bcl-2, and accordingly downregulated activities of FLICE and caspase-3 that were redox-independent. Interestingly, Stat3-C also upregulated the redox-associated protein redox factor-1 (Ref-1) and reduced apoptosis in liver following Jo2 injection by suppressing oxidative stress and redox-sensitive caspase-3 activity. These findings indicate that Stat3 activation protects against Fas-mediated liver injury by inhibiting caspase activities in redox-dependent and -independent mechanisms.

Knockdown of STAT3 expression by RNAi induces apoptosis in astrocytoma cells

Background

Astrocytomas are the most common type of primary central nervous system tumors. They are frequently associated with genetic mutations that deregulate cell cycle and render these tumors resistant to apoptosis. STAT3, signal transducer and activator of transcription 3, participates in several human cancers by inducing cell proliferation and inhibiting apoptosis and is frequently activated in astrocytomas.

Methods

RNA interference was used to knockdown STAT3 expression in human astrocytes and astrocytoma cell lines. The effect of STAT3 knockdown on apoptosis, cell proliferation, and gene expression was then assessed by standard methods.

Results

We have found that STAT3 is constitutively activated in several human astrocytoma cell lines. Knockdown of STAT3 expression by siRNA induces morphologic and biochemical changes consistent with apoptosis in several astrocytoma cell lines, but not in primary human astrocytes. Moreover, STAT3 is required for the expression of the antiapoptotic genes survivin and Bcl-xL in the A172 glioblastoma cell line.

Conclusion

These results show that STAT3 is required for the survival of some astrocytomas. These studies suggest STAT3 siRNA could be a useful therapeutic agent for the treatment of astrocytomas.

Differentiation-dependent expression of signal transducers and activators of transcription (STATs) might modify responses to growth factors in the cancers of the head and neck

Overexpression of the epidermal growth factor receptor (EGFR) in the cancers of the head and neck is well demonstrated. In addition, copy numbers of the EGFR mRNA were significantly higher in poorly differentiated tumors than in tumors that had a differentiated phenotype. Studies by others also showed that the constitutively activated signal transducer and activator of transcription-3 (STAT3), but not STAT1, is required for EGFR-mediated cell growth. Our aim was to reveal if STAT expression is differentiation-dependent and thus, might respond to exogenous stimuli in a differentiation-dependent manner. Both reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry revealed that expression of STAT1 was high in well/moderately differentiated tumors in vivo. In contrast, STAT3 was expressed in poorly differentiated tumors. In vitro experiments showed that differentiated primary oral keratinocytes expressed higher levels of STAT1, but lower levels of STAT3 than did their undifferentiated counterparts. Epidermal growth factor treatment of oral keratinocytes with various degrees of differentiation showed the maximal induction of cyclin D1 in undifferentiated cells. Our findings suggest that the level of differentiation might modulate the outcome of EGFR signaling (i.e. cyclin D1 transcription), due to the differentiation-associated intracellular balance of transcriptional regulators (STAT1 versus STAT3).

Activated signal transducer and activator of transcription 3 (STAT3) supports the malignant phenotype of human pancreatic cancer

BACKGROUND & AIMS

Constitutive activation of signal transducer and activator of transcription 3 (STAT3) has been implicated in regulation of growth and malignant transformation. We therefore analyzed the expression and biologic significance of STAT3 in human pancreatic cancer cells.

METHODS

Expression and activation of STAT3 were investigated by immunohistochemistry and immunoblotting. Functional inactivation of STAT3 was achieved by stable transfection of dominant-negative STAT3 constructs in 2 pancreatic cancer cell lines and confirmed by electrophoretic mobility shift assay and immunoblotting. Cell proliferation and tumorigenicity were evaluated by cell counting, colony formation in soft agar, and xenotransplantation in nude mice. STAT3-dependent cell cycle distribution was monitored by flow cytometry, immunoprecipitation, immunoblotting, and histone H1 and GST-Rb kinase assays.

RESULTS

Compared with nontransformed human pancreas, activated STAT3 is overexpressed in ductal carcinoma cells but not in ducts from chronic pancreatitis. Constitutive activation was also observed in all human pancreatic cancer cell lines examined. Functional inactivation of STAT3 resulted in significant inhibition of anchorage-dependent and -independent proliferation in vitro and reduced tumor growth in vivo. Cell cycle analysis showed a delay of G(1)/S-phase progression due to inhibition of cyclin-dependent kinase 2 activity based on increased expression of p21(WAF1) in vitro and in vivo. Blocking of the STAT3 upstream activator Janus kinase 2 by tyrphostin also resulted in growth arrest because of delayed G(1)/S-phase progression and increased expression of p21(WAF1).

CONCLUSIONS

On malignant transformation, activated STAT3 promotes cellular proliferation by acceleration of G(1)/S-phase progression and thereby contributes to the malignant phenotype of human pancreatic cancer.

Src kinases mediate STAT growth pathways in squamous cell carcinoma of the head and neck

Signal transducer and activator of transcription (STAT) proteins are constitutively activated in many malignancies, including squamous cell carcinoma of the head and neck (SCCHN). Previously, we reported that phosphorylation of the epidermal growth factor receptor (EGFR) is linked to activation of STATs 3 and 5 in SCCHN cells. The present study was undertaken to determine the role of Src family kinases in STAT activation and SCCHN growth. The Src family kinases c-Src, c-Yes, Fyn, and Lyn were expressed and activated by transforming growth factor-alpha stimulation in all four SCCHN cell lines examined but not in corresponding normal epithelial cells. In nine SCCHN cell lines tested, Src phosphotyrosine expression levels were highly correlated with activation levels of STATs 3 and 5. Co-immunoprecipitation analysis demonstrated interaction between c-Src and STATs 3 or 5 and EGFR in SCCHN cells, but no heterodimerization was detected between STAT3 and STAT5. SCCHN cells treated with either of two Src-specific inhibitors or transfected with a dominant-negative c-Src construct demonstrated decreased activation of STATs 3 and 5 and reduced growth rates in vitro. These results demonstrate a role for Src kinases in mediating activation of STATs 3 and 5 in concert with the EGFR in SCCHN cells. Strategies to target Src activation may contribute to the treatment of cancers that demonstrate increased levels of EGFR and STATs, including SCCHN.

Activation of Stat3 by receptor tyrosine kinases and cytokines regulates survival in human non-small cell carcinoma cells

Overexpression of receptor tyrosine kinases including the epidermal growth factor receptor (EGF-R) as well as nonreceptor tyrosine kinases, such as Src, have been implicated in the formation of human lung cancers. In addition, cytokines like interleukin-6 (IL-6) have been demonstrated to modulate lung cancer cell growth and elevated levels of IL-6 have been shown to be an adverse prognostic factor for patients with lung cancer. Despite a large body of evidence pointing to their potential importance, few direct studies into the role of signal transducers and activators of transcription (STAT) pathways in human lung cancer have been undertaken. Here we demonstrate that multiple nonsmall cell lung cancer cell lines demonstrate constitutive Stat3 DNA-binding activity. Stat3 DNA-binding activity is specifically upregulated by the addition of epidermal growth factor (EGF), IL-6, and hepatocyte-derived growth factor (HGF). Furthermore, the stimulation of Stat3 DNA-binding activity by EGF requires the activity of EGF-R tyrosine kinase as well as Src-kinase, while the upregulation of Stat3 activity by IL-6 or HGF requires only Src-kinase activity. Treatment of A549 lung cancer cells with PD180970 or SU6656, both pharmacological inhibitors of Src-kinase, resulted in reduced Src and Stat3 activity, cell cycle arrest in G2, and reduced viability of cells accompanied by induction of apoptosis. Treatment of Stat3-positive A549 and H358 cells with antisense Stat3 oligonucleotides results in complete loss of Stat3 DNA-binding activity and apoptosis, while Stat3-positive H1299 cells remained healthy. Finally, an adenoviral vector expressing a dominant-negative Stat3 isoform results in loss of Stat3 DNA-binding activity, apoptosis, and reduced cellular viability. These results demonstrate a role of Stat3 in transducing survival signals downstream of tyrosine kinases such as Src, EGF-R, and c-Met, as well as cytokines such as IL-6, in human nonsmall cell lung cancers.

ERK and PI3K negatively regulate STAT-transcriptional activities in human melanoma cells: implications towards sensitization to apoptosis

Signal transducers and activators of transcription (STAT) proteins nuclear translocation and transcriptional activity are regulated by diverse protein kinases in response to extracellular stimuli by cytokines, growth factors and stress. Using two melanoma-derived cell lines that exhibit marked differences in basal activities of MAPKs and PI3K-AKT, we studied changes both in STAT activities and in their sensitization to apoptosis. Activating mutations of B-RAF (T1796A) and impaired expression of PTEN are detected in LU1205, but not in FEMX melanoma cells, and are reflected in high basal levels of expression and activities of MAPKs and PI3K-AKT. Treatment with either PD98059 (PD) or LY294002 (LY), the pharmacological inhibitors of MEK-ERK and PI3K, respectively, markedly increased GAS-Luc activity in LU1205, but not in FEMX cells. Tyrosine phosphorylation of STAT3/5 and of JAK2 also increased upon treatment of LU1205 cells with either PD or LY, suggesting that constitutive active MAPK and PI3K signals inhibit tyrosine phosphorylation of JAK/STATs. Treatment of FEMX and LU1205 with PD sensitized the cells to apoptosis, albeit by TNFalpha and TRAIL death cascades, respectively, indicating that additional yet distinct targets are affected by each signaling pathway. Indeed, the combination of LY and PD treatment synergistically increased the apoptosis of LU1205 and FEMX cells. Overall, whereas PI3K and MAPK downregulate JAK-STAT signaling, additional targets are affected by these kinases and sensitizes melanoma to apoptosis via distinct death cascades.

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

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

Stat3 enhances transactivation of steroid hormone receptors

BACKGROUND: Steroid hormone receptors (SHRs) are members of the superfamily of ligand-activated transcription factors that regulate many biological processes. Co-regulators act as bridging molecules between the SHR and general transcription factors to enhance transactivation of target genes. Previous studies demonstrated that Stat3 is constitutively activated in prostate cancer and can enhance prostate specific antigen (PSA) expression and promote androgen independent growth. In this study, we investigate whether Stat3 can enhance steroid hormone receptors activation. METHODS: CV-1 cells in which plasmids expressing androgen receptor (AR), glucocorticoid receptor (GR), progesterone receptor (PR) or estrogen receptor (ER) were cotransfected with a constitutively active STAT3 mutant. RESULTS: Stat3 stimulates the transcriptional activity of all four SHR tested, AR, GR, PR and ER, in a hormone-dependent manner. Stat3 acts in a synergistic fashion with other coactivators such as SRC-1, pCAF, CBP, and TIF-2 on the transcriptional activity of these SHR. In addition, Stat3 significantly enhanced the sensitivity of androgen receptor in response to androgen. STAT3 did not affect the specificity of AR for other steroid hormones other than androgen or binding of AR to other hormone responsive elements. CONCLUSIONS: These findings suggest that Stat3 can enhance the transactivation of AR, GR, PR and ER, and activated Stat3 could have a role in the development or progression of a hypersensitive AR.

Activation of Stat3 by cell confluence reveals negative regulation of Stat3 by cdk2

The signal transducing protein Stat3 activates gene transcription in cells in response to multiple cytokines. Constitutive activation of Stat3 has been observed in solid tumors including head and neck squamous cell carcinoma. Stat3 activation in cancer has been associated with autocrine stimulatory loops and is believed to convey a growth advantage to cells. We now demonstrate ligand-independent activation of Stat3 by high cell density in multiple cancer cell lines. Activation of Stat3 is associated with antiproliferative rather than proliferative conditions. Interference with cdk2 activity upregulates Stat3 phosphorylation and Stat3-directed DNA-binding activity. Our data supports a model in which Stat3 activity is partially suppressed by cdk2 in growing cells and derepressed upon cell confluence.

STAT3 ubiquitylation and degradation by mumps virus suppress cytokine and oncogene signaling

Mumps virus is a common infectious agent of humans, causing parotitis, meningitis, encephalitis, and orchitis. Like other paramyxoviruses in the genus Rubulavirus, mumps virus catalyzes the proteasomal degradation of cellular STAT1 protein, a means for escaping antiviral responses initiated by alpha/beta and gamma interferons. We demonstrate that mumps virus also eliminates cellular STAT3, a protein that mediates transcriptional responses to cytokines, growth factors, nonreceptor tyrosine kinases, and a variety of oncogenic stimuli. STAT1 and STAT3 are independently targeted by a single mumps virus protein, called V, that assembles STAT-directed ubiquitylation complexes from cellular components, including STAT1, STAT2, STAT3, DDB1, and Cullin4A. Consequently, mumps virus V protein prevents responses to interleukin-6 and v-Src signals and can induce apoptosis in STAT3-dependent multiple myeloma cells and transformed murine fibroblasts. These findings demonstrate a unique cytokine and oncogene evasion property of mumps virus that provides a molecular basis for its observed oncolytic properties.

Activation of signal transducer and activator of transcription 3 by oncogenic RET/PTC (rearranged in transformation/papillary thyroid carcinoma) tyrosine kinase: roles in specific gene regulation and cellular transformation

Thyroid papillary carcinomas are characterized by RET/PTC (rearranged in transformation/papillary thyroid carcinoma) rearrangements that result in fusion of the tyrosine kinase domain of the RET receptor to the N-terminal sequences encoded by heterologous genes. This thyroid-specific rearrangement causes aberrant expression of RET/PTC and results in constitutive ligand-independent activation of RET kinase. However, it is unclear how RET/PTC activates the specific signaling pathways for cellular transformation. In this study, we show that RET/PTC associates with signal transducer and activator of transcription 3 (STAT3) and activates it by the specific phosphorylation of the tyrosine 705 residue. Activation of STAT3 requires the intrinsic kinase activity of RET/PTC; Janus tyrosine kinase and c-Src kinase are not involved in the RET/PTC-mediated activation of STAT3. RET/PTC-induced activation of STAT3 induces the STAT3-responsive genes, vascular endothelial growth factor, cyclin D1, and intercellular adhesion molecule 1. In addition, RET/PTC-mediated cellular transformation and proliferation of transformed cells require tyrosine 705 phosphorylation of STAT3 in NIH3T3 cells. We conclude that STAT3 activation by the RET/PTC tyrosine kinase is one of the critical signaling pathways for the regulation of specific genes, such as cyclin D1, vascular endothelial growth factor, and intercellular adhesion molecule 1, and for cellular transformation.

Signal transducer and activator of transcription proteins in leukemias

Signal transducer and activator of transcription (STAT) proteins are a 7-member family of cytoplasmic transcription factors that contribute to signal transduction by cytokines, hormones, and growth factors. STAT proteins control fundamental cellular processes, including survival, proliferation, and differentiation. Given the critical roles of STAT proteins, it was hypothesized that inappropriate or aberrant activation of STATs might contribute to cellular transformation and, in particular, leukemogenesis. Constitutive activation of mutated STAT3 has in fact been demonstrated to result in transformation. STAT activation has been extensively studied in leukemias, and mechanisms of STAT activation and the potential role of STAT signaling in leukemogenesis are the focus of this review. A better understanding of mechanisms of dysregulation of STAT signaling pathways may serve as a basis for designing novel therapeutic strategies that target these pathways in leukemia cells.

In vivo cooperation between Bcl-xL and the phosphoinositide 3-kinase-Akt signaling pathway for the protection of epidermal keratinocytes from apoptosis

To investigate the function of Bcl-xL in the skin, we established keratinocyte-specific Bcl-x gene-targeted mice under the keratin 5 promoter (K5). K5.Bcl-xL-/- mice were viable, devoid of alteration in the development of skin or appendages. However, they harbored spontaneous apoptotic keratinocytes in the epidermis. Bcl-xL-deficient keratinocytes cultured in vitro readily underwent apoptosis in the absence of growth factors, but the addition of HGF or EGF resulted in restoration of cell survival, which was reversed by treatment with wortmannin, an inhibitor of phosphoinositide-3 kinase (PI3K). Topical treatment of K5.Bcl-xL-/- mice with wortmannin sensitized the skin for apoptosis induced by UV (UV) B, although wild-type epidermis was only marginally affected by this treatment, suggesting that the resistance to UVB largely depended on PI3K-Akt signaling in Bcl-xL-deficient mice but not in wild-type mice. Furthermore, UVB irradiation resulted in redistribution of phosphorylated Akt from the basal layer to the suprabasal layer, indicating that Akt could spatially cooperate with Bcl-xL upon UVB exposure in the upper epidermis where Bcl-xL is normally localized. These results suggest that Bcl-xL and the PI3K-Akt pathway form a cooperative, intercompensatory axis for the protection of epidermal keratinocytes from apoptosis in vivo.

Oral cancer: reviewing the present understanding of its molecular mechanism and exploring the future directions for its effective management

The present review aims to analyze the information available regarding the molecular mechanisms of Oral Carcinogenesis and explore the future directions where the field of Cancer Biology is venturing. Oncologists have excellently followed the proverb "Necessity is the mother of Invention". The desire to be more precise and comprehensive in their studies has led to the invention of some of the most innovative techniques like laser capture microdissection, comparative genomic hybridization, microarrays, and protein chips etc. Various Biotech companies and Cancer Institutes are on a hunt for anti-cancer drugs and molecular markers for cancers. These revolutionary approaches and the new breed of Oncologists have made the field very exciting and have generated the hope that finally the war against cancer would be won. In the end it is urged that the lead taken in other cancers like colon, breast, leukemia will be emulated in oral cancer. This is expected to provide a molecular blueprint for HNSCC, thus helping to identify suitable markers for the early detection of pre-neoplastic lesions, as well as novel targets for its pharmacological intervention.

Targeted inhibition of Stat3 with a decoy oligonucleotide abrogates head and neck cancer cell growth

The transcription factor signal transducer and activator of transcription 3 (Stat3) is constitutively activated in a variety of cancers including squamous cell carcinoma of the head and neck (SCCHN). Previous investigations have demonstrated that activated Stat3 contributes to a loss of growth control and transformation. To investigate the therapeutic potential of blocking Stat3 in cancer cells, we developed a transcription factor decoy to selectively abrogate activated Stat3. The Stat3 decoy was composed of a 15-mer double-stranded oligonucleotide, which corresponded closely to the Stat3 response element within the c-fos promoter. The Stat3 decoy bound specifically to activated Stat3 and blocked binding of Stat3 to a radiolabeled Stat3 binding element. By contrast, a mutated version of the decoy that differed by only a single base pair did not bind the activated Stat3 protein. Treatment of head and neck cancer cells with the Stat3 decoy inhibited proliferation and Stat3-mediated gene expression, but did not decrease the proliferation of normal oral keratinocytes. Thus, disruption of activated Stat3 by using a transcription factor decoy approach may serve as a novel therapeutic strategy for cancers characterized by constitutive Stat3 activation.

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

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

Stat3 activation regulates the expression of vascular endothelial growth factor and human pancreatic cancer angiogenesis and metastasis

Expression of vascular endothelial growth factor (VEGF), a key angiogenic protein, has been linked with pancreatic cancer progression. However, the molecular basis for VEGF overexpression remains unclear. Immunohistochemical studies have indicated that VEGF overexpression coincides with elevated Stat3 activation in human pancreatic cancer specimens. In our study, more than 80% of the human pancreatic cancer cell lines used exhibited constitutively activated Stat3, with Stat3 activation correlated with the VEGF expression level. Blockade of activated Stat3 via ectopic expression of dominant-negative Stat3 significantly suppressed VEGF expression, angiogenesis, tumor growth, and metastasis in vivo. Furthermore, constitutively activated Stat3 directly activated the VEGF promoter, whereas dominant-negative Stat3 inhibited the VEGF promoter. A putative Stat3-responsive element on the VEGF promoter was identified using a protein-DNA binding assay and confirmed using a promoter mutagenesis assay. These results indicate that Stat3 directly regulates VEGF expression and hence angiogenesis, growth, and metastasis of human pancreatic cancer, suggesting that Stat3 signaling may be targeted for treatment of pancreatic cancer.

Targeting epidermal growth factor receptor in head and neck cancer

It is well known that growth factors play an important role in normal cell proliferation by means of stimulation of growth factor receptors located on the surface of cells. Tumor cells express high levels of growth factor receptors that can theoretically serve as therapeutic targets in cancer treatment. Tyrosine kinase (type 1) growth factor receptors include the family of erbB receptors. The most extensively studied receptor in the erbB family is the human epidermal growth factor receptor (EGFR), also known as erbB1. Studies have shown that overexpression of EGFR is involved in the development and progression of head and neck squamous cell carcinoma (HNSCC). Blocking this receptor in HNSCC cell lines and animal models inhibits tumor growth. Strategies have been developed to target EGFR, including monoclonal antibodies, tyrosine kinase-specific inhibitors, ligand-linked immunotoxins, and antisense approaches. Laboratory studies and clinical trials are under way to explore the safety and efficacy of these various approaches in a variety of cancers, including HNSCC. Preliminary results from early phase clinical trials are encouraging and may lead to the incorporation of these EGFR targeting strategies into the management of HNSCC.

Expression of STAT3 and its phosphorylated forms in mantle cell lymphoma cell lines and tumours

The pathogenesis of mantle cell lymphoma (MCL) is incompletely understood, although cyclin D1 overexpression leading to deregulated cell proliferation is probably important. Recent data suggest that interleukin (IL)-10 can increase the proliferative activity of MCL cells. STAT3 (signal transducer and activator of transcription 3) is the signal transducer of IL-10, and STAT3 is activated by phosphorylation. The hypothesis of this study is that STAT3 is activated in MCL. The expression of the two phosphorylated (i.e. active) forms of STAT3, pSTAT3-tyr (phosphorylated at the tyrosine(705) residue) and pSTAT3-ser (phosphorylated at the serine(727) residue), was assessed in four MCL cell lines and 12 MCL tumours using western blots and/or immunofluorescence staining techniques. All MCL cell lines expressed STAT3, but only one had detectable pSTAT3-tyr and none had pSTAT3-ser. Addition of IL-10 rapidly resulted in expression of pSTAT3-tyr but not pSTAT3-ser. All eight cases of frozen MCL tumours examined had detectable pSTAT3-tyr and pSTAT3-ser. Immunofluorescence studies using four formalin-fixed, paraffin wax-embedded MCL tumours demonstrated cytoplasmic localization of STAT3, as opposed to the nuclear localization of the pSTAT3 species. In conclusion, these findings provide evidence that STAT3 is constitutively activated in MCL, supporting the concept that STAT3 signalling may be important in the pathogenesis of these tumours.

Inhibition of constitutively active Stat3 suppresses proliferation and induces apoptosis in glioblastoma multiforme cells

Glioblastoma multiforme (GBM), the most common and malignant central nervous system tumor in humans, is highly proliferative and resistant to apoptosis. Stat3, a latent transcription factor being activated by aberrant cytokine or growth factor signaling, acts as a suppressor of apoptosis in a number of cancer cells. Here we report that GBM tumors and cell lines contain high levels of constitutively activated Stat3 when compared with normal human astrocytes, white matter, and normal tissue adjacent to tumor. The persistent activation of Stat3 is in part, attributable to an autocrine action of interleukin-6 in the GBM cell line U251. Janus kinase inhibitor AG490 inhibits Stat3 activation with a concomitant reduction in steady-state levels of Bcl-X(L), Bcl-2 and Mcl-1 proteins and induces apoptosis in U251 cells as revealed by Poly (ADP-ribose) polymerase cleavage and Annexin-V staining. Expression of a dominant negative mutant Stat3 protein or treatment with AG490 markedly reduces the proliferation of U251 cells by inhibiting the constitutive activation of Stat3. These results provide evidence that constitutive activation of Stat3 contributes to the pathogenesis of glioblastoma by promoting both proliferation and survival of GBM cells. Therefore, targeting Stat3 signaling may provide a potential therapeutic intervention for GBM.

15-PGJ2, but not thiazolidinediones, inhibits cell growth, induces apoptosis, and causes downregulation of Stat3 in human oral SCCa cells

Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) has been linked to induction of differentiation, cell growth inhibition and apoptosis in several types of human cancer. However, the possible effects of PPARgamma agonists on human oral squamous cell carcinoma have not yet been reported. In this study, treatment with 15-deoxy-Delta(12,14)-PGJ(2) (15-PGJ(2)), a natural PPARgamma ligand, induced a significant reduction of oral squamous cell carcinoma cell growth, which was mainly attributed to upregulation of apoptosis. Interestingly, rosiglitazone and ciglitazone, two members of the thiazolidinedione family of PPARgamma activators, did not exert a growth inhibitory effect. Given the critical role that the oncogene signal transducer and activator of transcription 3 (Stat3) plays in head and neck carcinogenesis, its potential regulation by PPARgamma ligands was also examined. Treatment of oral squamous cell carcinoma cells with 15-PGJ(2) induced an initial reduction and eventual elimination of both phosphorylated and unphosphorylated Stat3 protein levels. In contrast, other PPARgamma did not induce similar effects. Our results provide the first evidence of significant antineoplastic effects of 15-PGJ(2) on human oral squamous cell carcinoma cells, which may be related to downmodulation of Stat3 and are at least partly mediated through PPARgamma-independent events.

Rheumatoid arthritis synoviocyte survival is dependent on Stat3

Rheumatoid arthritis (RA) synovial fibroblasts (SFs) are relatively resistant to apoptosis and exhibit dysregulated growth secondary to production of autocrine-acting growth factors and the accumulation of cell-autonomous defects. Many of the cytokines and growth factors expressed during RA synovitis, including IL-6, epidermal growth factor (EGF), and platelet-derived growth factor, activate the transcription factor Stat3 that has been implicated in promoting cell growth and survival. We analyzed the role of Stat3 in mediating the abnormal growth and survival properties of RA synoviocytes using retroviral-mediated gene transfer of a dominant negative mutant of Stat3, termed Stat3-YF. Approximately 3- to 5-fold overexpression of Stat3-YF effectively blocked endogenous Stat3 activation and Stat3-dependent gene expression, including expression of the socs3 and myc genes. Stat3-YF-transduced RA synoviocytes failed to grow in culture, exhibited markedly diminished [(3)H]thymidine incorporation (>90% decreased), and died spontaneously. Cell death occurred by apoptosis, as confirmed by annexin V staining, propidium iodide exclusion, and identification of cells with subdiploid levels of DNA. In marked contrast to control cells, EGF accelerated death of Stat3-YF-transduced SFs, such that >90% of cells were dead within 24-48 h of transduction. These results indicate that ablation of Stat3 function converts EGF from a growth/survival factor for RA synoviocytes to a death factor. Stat3-YF also induced apoptosis in osteoarthritis synoviocytes, and levels of apoptosis were increased by exogenous EGF. Apoptosis in Stat3-YF-transduced osteoarthritis synoviocytes was suppressed when Stat1 activity was blocked using a dominant negative Stat1 mutant. Our results identify Stat3 as an important molecule for RA SF survival, and suggest that Stat3 may represent a good target for gene therapy.

ErbB-2 activates Stat3 alpha in a Src- and JAK2-dependent manner

Stats (signal transducer and activator of transcription) are latent transcription factors that translocate from the cytoplasm to the nucleus. Constitutive activation of Stat3 alpha by upstream oncoproteins and receptor tyrosine kinases has been found in many human tumors and tumor-derived cell lines. Constitutively activated Stat3 alpha is often correlated with the activation of ErbB-2, a member of the EGFR family. To explore the involvement of ErbB-2 in the activation of Stat3 and the mechanism underlying this event, an ErbB-2 point mutant was used as a model of constitutively activated receptor. Phenylalanine mutations (Y-->F) were made in the autophosphorylation sites of the receptor, and their ability to activate Stat3 alpha was evaluated. Our results suggest that Stat3 alpha and JAK2 associates with ErbB-2 prior to phosphorylation of the receptor and that full activation of Stat3 alpha by ErbB-2 requires the participation of other non-receptor tyrosine kinases. Both Src and Jak2 kinases contribute to the activation of Stat3 alpha but Src binds to ErbB-2 only when the receptor is phosphorylated. Our results also suggest that tyrosine 1139 may be important for Src homology 2 domain association because a mutant lacking this tyrosine reduces the ability of the Src homology 2 domain to bind to ErbB-2 and significantly decreases its ability to activate Stat3 alpha.

Roles of activated Src and Stat3 signaling in melanoma tumor cell growth

Activation of protein tyrosine kinases is prevalent in human cancers and previous studies have demonstrated that Stat3 signaling is a point of convergence for many of these tyrosine kinases. Moreover, a critical role for constitutive activation of Stat3 in tumor cell proliferation and survival has been established in diverse cancers. However, the oncogenic signaling pathways in melanoma cells remain to be fully defined. In this study, we demonstrate that Stat3 is constitutively activated in a majority of human melanoma cell lines and tumor specimens examined. Blocking Src tyrosine kinase activity, but not EGF receptor or JAK family kinases, leads to inhibition of Stat3 signaling in melanoma cell lines. Consistent with a role of Src in the pathogenesis of melanoma, we show that c-Src tyrosine kinase is activated in melanoma cell lines. Significantly, melanoma cells undergo apoptosis when either Src kinase activity or Stat3 signaling is inhibited. Blockade of Src or Stat3 is also accompanied by down-regulation of expression of the anti-apoptotic genes, Bcl-x(L) and Mcl-1. These findings demonstrate that Src-activated Stat3 signaling is important for the growth and survival of melanoma tumor cells.

The molecular biology of laryngeal cancer

Novel techniques have led to the discovery of many genes and gene products important in the development of HNSC and laryngeal cancer. Tumor suppressive genes and oncogenes have been identified, and many of their roles have been elucidated in a genetic progression model. As these molecular pathways become better understood, the information obtained will increasingly be used to guide patient therapy. Specifically, advances will probably be made in (1) molecular characterization of steps leading to laryngeal cancer; (2) molecular screening, staging, and surveillance; (3) molecularly based therapy, including gene transfer and small molecule therapy directed at specific molecular pathways involved in neoplasia; and (4) characterization of patients who are at high risk for laryngeal cancer. In the final analysis, however, smoking cessation for those at risk for head and neck cancer would have greater effect than all these efforts combined.

Epithelial growth factor receptor interacting agents

The data reviewed here have further established the promise of anti-EGFR-targeted therapies. This statement is supported by the evidence of antitumor activity of the TK inhibitors ZD1839 and OSI-774 against several tumor types and by the ability of the monoclonal antibody IMC-C225 to reverse clinical chemotherapy resistance. These results are further supported by an emerging number of compounds, monoclonal antibodies, and TK inhibitors directed at the EGFR that are in clinical development (see Fig. 2, Table 1). Among the TK inhibitors, these compounds can be further categorized by their receptor specificity and reversibility of binding. In the case of anti-EGFR monoclonal antibodies, compounds in clinical development include chimeric, humanized, and bispecific antibodies. The fundamental observation is that these compounds have shown activity in several tumor types, including NSCL cancer, prostate carcinoma, colorectal carcinoma, ovarian carcinoma, renal cell carcinoma, and head and neck cancers. These findings observed with different agents and in different tumor types validate EGFR as a target for cancer therapy. The results of ongoing studies with these agents in diverse indications and tumor types may establish the role of these promising therapies to our current cancer treatments.

Molecular genetics of head and neck cancer

BACKGROUND

Head and neck cancers have multiple genetic abnormalities that influence tumor behavior and may be useful in developing new treatments.

METHODS

Genetic alterations implicated in head and neck cancer oncogenesis and behavior are reviewed, and molecular techniques for detection and treatment are evaluated.

RESULTS

The large number of genetic changes present in head and neck cancer cells precludes meaningful use of simple molecular tests and treatments. Detection of abnormalities in multiple genes provides better prognostic information than the detection and assessment of single mutations. Screening tests that rely on amplification of genetic material present in bodily fluids are hindered by the genomic complexity of head and neck cancer. Introduction of genetic material into head and neck cancer cells for gene therapy has shown some efficacy.

CONCLUSIONS

Head and neck cancers comprise a complex genetic disease. Although much has been learned about the molecular genetics of head and neck cancers, continued study of multiple genes is critical for further progress. Gene therapy, although promising, must also overcome this complexity.

Neuregulin-1 activates the JAK-STAT pathway and regulates lung epithelial cell proliferation

Neuregulin-1 (NRG-1) is part of a family of proteins whose members are structurally related to epidermal growth factor. NRG-1 induces cell proliferation through a high-affinity receptor complex composed of a heterodimer of human epidermal growth factor-like receptor (HER) 2 and 3. In this study, we show that NRG-1 activates the Janus kinases (JAK) and signal transducer and activator of transcription proteins (STAT). NRG-1 induced a rapid and transient increase in tyrosine phosphorylation of TYK2 and JAK3, but not JAK1 or JAK2, and induced STAT3 and STAT5 tyrosine phosphorylation. Upon phosphorylation, STAT3 translocated to the nucleus within 1 h. Activation of the JAK-STAT pathway was dependent on HER2/HER3 heterodimerization and was necessary for NRG-1-induced proliferation. Inhibition of HER2's ability to dimerize using the HER2-specific antibody 2C4 completely blocked NRG-1-induced JAK3, TYK2, STAT3, and STAT5 tyrosine phosphorylation. Blocking the JAK-STAT pathway with a specific JAK-STAT pathway inhibitor, AG490, inhibited NRG-1-induced JAK and STAT phosphorylation and cell proliferation. These data suggest that NRG-1 activates the JAK-STAT signal transduction pathway through its high-affinity receptor, the HER2/HER3 heterodimer. This pathway plays an important role in NRG-1-stimulated proliferation of pulmonary epithelial cells.

Interleukin-10 induces transcription of the early promoter of human papillomavirus type 16 (HPV16) through the 5'-segment of the upstream regulatory region (URR)

The effects of various proinflammatory cytokines on the transcription of human papillomaviruses (HPVs) have been demonstrated. On the other hand, the role of anti-inflammatory cytokines has not been elaborated, despite the fact that levels of interleukin-10 (IL-10) have been found significantly elevated in cervical dysplasias or carcinomas as well as in the cervix of HIV-positive individuals. These conditions are also associated with elevated viral transcription. Thus, the impact of IL-10 on HPV transcription might be important in pathogenesis of cervical lesions in both immunocompetent or immunosuppressed individuals. In this paper we describe the effects of IL-10 on the transcription of HPV type 16. We found that treatment of HPV 16-positive cervical carcinoma cells with IL-10 increased mRNA levels of the E7 early gene at the level of transcription. Similarly, IL-10 significantly and dose-dependently induced the transcription from the HPV early promoter in a reporter system. Employing deletion mutants we determined that this induction is mapped to the 5' segment of the URR. Transient transfection of an antisense-STAT3-expression vector abolished IL-10-induced reporter activity as well as HPV 16 E7 expression. This suggests that STAT3 either directly binds to the URR and stimulates transcription or affects expression and/or binding of transcription factors that bind to the 5'-region. Our findings suggest a mechanism by which--in addition to its immunosuppressive effects--IL-10 might enhance persistence and progression of HPV-related lesions under conditions (e.g. dysplastic progression, HIV infection) when the cytokine expression in the cervical microenvironment changes.

Interleukin-4-mediated protection of primary B cells from apoptosis through Stat6-dependent up-regulation of Bcl-xL

Apoptosis is an integral aspect of B lymphocyte development and homeostasis and is regulated by the engagement of antigen costimulatory and cytokine receptors. Although it is well established that interleukin 4 (IL-4) is a potent anti-apoptotic cytokine for B lymphocytes, little is known about the IL-4-induced molecular events regulating cell survival. Stat6 is rapidly activated after IL-4 stimulation, but its role in B lymphocyte apoptosis has not been explored. In this report we demonstrate that Stat6 is a critical signaling molecule for IL-4 in protecting primary B cells from passive and Fas-induced cell death. We show that expression of the Bcl-2 family member, Bcl-xL, is induced maximally by IL-4 and anti-IgM/IL-4 in a Stat6-dependent manner. Additionally, we demonstrate that bcl-xL transcription is likely to be directly activated through a Stat6 binding site in the bcl-xL-flanking region. Finally, reconstitution of Stat6-deficient splenic B cells with Bcl-xL was able to protect those cells from Fas-induced cell death. These results suggest that the anti-apoptotic activity of IL-4 in B cells is mediated through the activation of Stat6 and subsequent transcription of Bcl-xL.

Stat3 enhances the growth of LNCaP human prostate cancer cells in intact and castrated male nude mice

BACKGROUND

Prostate cancer frequently progresses from an initial androgen dependence to androgen independence, rendering the only effective androgen ablation therapy useless. The mechanism underlying the androgen-independent progression is unknown. Stat3, a member of the family of signal transducers and activators of transcription, is activated in numerous cancers, including prostate. This study is to investigate the role of Stat3 activation in the growth of prostate cancer cells.

METHODS

A constitutively active Stat3 was ectopically expressed in androgen-sensitive LNCaP prostate cancer cells and resulting stable clones expressing activated Stat3 were isolated. The effect of Stat3 activation on LNCaP cell growth in response to androgen in vitro and in vivo was examined.

RESULTS

We show that the levels of activated Stat3 are associated with the progression of androgen-independent prostate cancer. Activation of Stat3 in androgen-sensitive LNCaP prostate cancer cells results in enhancement of tumor growth in both intact and castrated male nude mice and enhances androgen receptor-mediated prostate specific antigen expression.

CONCLUSIONS

These findings demonstrate that intracellular signaling mediated by Stat3 can enhance the growth of androgen-sensitive human LNCaP prostate cancer cells in both intact and castrated male nude mice.

PTEN is a negative regulator of STAT3 activation in human papillomavirus-infected cells

Laryngeal papillomas are caused by infection of the laryngeal epithelium by human papillomavirus type 6 or type 11 (HPV-6/-11). Previous studies in our laboratory have demonstrated an increase in PI3 kinase levels in papilloma tissue. However, activation of the downstream effector of PI3 kinase, protein kinase B (PKB/Akt), was reduced. This observation was explained by the elevated expression of the phosphatase and tensin homologue (PTEN), a recently characterized tumour suppressor, in papilloma tissue. Recent investigation of the possible functional roles of PTEN during papilloma development has now indicated that the level of tyrosine(705)-phosphorylated signal transducer and activator of transcription 3 [PTyr(705)STAT3] could be inversely correlated to that of PTEN as well. In vitro phosphatase assays suggested the presence of an increased level of a PTyr(705)STAT3 phosphatase in papilloma extract. Immunodepletion of PTEN from papilloma extracts resulted in a reduction of the PTyr(705)STAT3 phosphatase activity. Transfection of PTEN cDNA into HeLa cells attenuated STAT3 phosphorylation at Tyr(705) in a dose-dependent manner. This attenuation of STAT3 phosphorylation was independent of the STAT3 kinase. Interestingly, introduction of a lipid phosphatase mutant of PTEN (G129E) resulted in heightened PTyr(705)STAT3 phosphatase activity, relative to that obtained from wild-type PTEN transfection. These data indicate that PTEN negatively regulates STAT3 activation in HPV-infected papilloma cells. Induction of PTEN and reduction of activated STAT3 might be a result of a host defence mechanism or a virus-directed strategy to alter normal epithelial differentiation programming.

Stat3 activation in prostatic carcinomas

BACKGROUND

Activated Stat3 is found in various types of immortal cell lines and cancers. We and others have previously demonstrated that Stat3 is constitutively activated in rat and human prostate cancer cell lines, and that Stat3 activation is involved in IL-6-mediated signaling transduction in prostate cancer cells. The aim of this study is to examine quantitative Stat3 activity in benign and malignant human prostate tissues and analyze the association between Stat3 activity levels and the clinical and pathologic parameters.

METHODS

Stat3 activity levels were analyzed in a total of 104 human primary prostate tissues using electromobility shift assay and immunohistochemical staining for phosphorylated Stat3. The tissue samples used were 42 prostate carcinomas, 42 matched normal prostate tissues from patients with prostatic adenocarcinoma (normal adjacent to tumor), and 20 normal prostate tissues from organ donors.

RESULTS

Significantly higher levels of constitutive Stat3 activity were detected in both prostate carcinomas and the matched normal prostate tissues adjacent to tumors compared to the normal prostates from donors without prostate cancer. There was no significant difference of Stat3 activity in foci of tumor and normal prostate tissue adjacent to tumor. No correlation was seen between Stat3 activity and Gleason grade or serum PSA levels in samples from prostate carcinomas.

CONCLUSIONS

These results indicate that Stat3 is constitutively activated in prostate cancer. The high level of Stat3 activity in both the prostate carcinomas and the normal prostate tissues adjacent to tumors suggests that Stat3 activation may occur before detectable histological alterations of the prostate.

Effects of pharmacologic antagonists of epidermal growth factor receptor, PI3K and MEK signal kinases on NF-kappaB and AP-1 activation and IL-8 and VEGF expression in human head and neck squamous cell carcinoma lines

We previously reported that expression of angiogenesis factors interleukin-8 (IL-8) and vascular endothelial growth factor (VEGF) is promoted by coactivation of transcription factors nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1) by interleukin-1alpha in human head and neck squamous cell carcinomas (HNSCC). However, expression of IL-1 receptor antagonist incompletely blocked reporter gene activity and cytokine expression, suggesting that other upstream signals may contribute to activation. Overexpression and autocrine activation of epidermal growth factor receptor (EGFR) is detected in 90% of HNSCC, and EGFR inhibitors have been reported to inhibit IL-8 and VEGF expression, but the intermediary signal pathways and transcription factors by which EGFR modulates proangiogenic factors is unknown. EGFR can activate the phosphotidylinositol-3 kinase (PI3K) and mitogen-activated/extracellular signal-regulated kinase (MEK) pathways, which can potentially modulate activation of NF-kappaB and AP-1, respectively. In our study, we examined the effect of EGF and antagonists of EGFR, PI3K and MEK on NF-kappaB and AP-1 activation and IL-8 and VEGF expression in HNSCC cell lines UM-SCC-9 and 11B in which EGFR is overexpressed and activated. Recombinant EGF induced EGFR phosphorylation, activation of NF-kappaB and AP-1 reporter genes and IL-8 and VEGF expression, indicating that EGFR can mediate coactivation of both transcription factors and cytokine genes in HNSCC. EGFR antagonist PD153035 and anti-EGFR antibody C225 completely inhibited EGF-induced reporter activity and cytokine expression, but only partially inhibited constitutive activity. MEK inhibitor U0126 preferentially blocked AP-1 activity and expression of both IL-8 and VEGF, while PI3K inhibitor LY-294002 or a dominant negative inhibitor-kappaB preferentially blocked NF-kappaB activation and expression of IL-8 but not VEGF. EGFR, PI3K and MEK antagonists inhibited growth of HNSCC. We conclude that antagonists of EGFR, PI3K and MEK signal pathways have inhibitory activity against EGFR-induced NF-kappaB and AP-1 activation, IL-8 and VEGF expression and growth by HNSCC. Published 2002 Wiley-Liss, Inc.

Selective activation of members of the signal transducers and activators of transcription family in prostate carcinoma

PURPOSE

Cytokines, hormones and growth factors use signal transducers and activators of transcription (STAT) signaling pathways to control various biological responses, including development, differentiation, cell proliferation and survival. Constitutive activation of STATs has been found in a wide variety of human tumors. In this study we examined the activity of STATs in primary human prostate tissues.

MATERIALS AND METHODS

STAT activity was determined in 104 human primary prostate tissues, including 42 tumors, 42 matched normal prostates adjacent to tumors and 20 normal prostates from donors without cancer by electromobility shift assay.

RESULTS

Significant levels of activated Stat4 and Stat6 were detected in primary prostate tissues. However, little or no expression of active Stat1, Stat2 or Stat5 was detected in primary prostate tissues. Significantly higher levels of constitutive Stat6 activity were found in prostate carcinomas compared with levels in normal tissue adjacent to tumors and normal prostates from donors without prostate cancer. There was no significant difference in Stat6 activity in normal prostate tissues adjacent to tumors and normal prostates from donors without prostate cancer. The levels of Stat4 activity varied but failed to yield statistically significant differences among tumors, matched normal prostates adjacent to tumors and normal prostates from donors without cancer.

CONCLUSIONS

We have previously shown that Stat3 is activated in prostate cancer. The results of the current study demonstrate that in addition to Stat3, Stat6 is selectively activated in prostate cancer.

Activation of Stat-3 as one of the early events in tobacco chewing-mediated oral carcinogenesis

BACKGROUND

The Jak/Stat signaling pathway transmits signals from many cytokines and growth factor receptors to target genes in the nucleus. Constitutive activation of Stat-3 recently has been observed in many tumor cells, and dysregulation of the Stat signaling pathway has been proposed to be implicated in malignant transformation. In the current study for the first time to the authors's knowledge, the expression of STAT-3 was analyzed in various stages and sites of squamous cell carcinoma of the head and neck (HNSCC).

METHODS

Tissue samples from 90 patients of tobacco chewing-mediated HNSCC representing various stages, sites, and differentiation states were selected for studying STAT-3 protein and RNA expression. In vivo localization of STAT-3 was studied by immunohistochemistry of paraffin embedded sections. The presence of STAT-3 and its phophorylated and activated form pSTAT-3 was checked by Western blotting. mRNA expression was analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR). Apoptosis analysis was conducted by in situ ENA nick end labeling assay and hematoxylin and eosin staining.

RESULTS

Overall, 58.9% of HNSCC tumors showed very high Stat-3 protein accumulation, and 23.3% showed intermediate accumulation whereas 17.8% of HNSCC tumors were negative for Stat-3. No Stat-3 was detected in normal samples, and only one of eight premalignant lesions showed intermediate Stat-3 accumulation. On immunoblotting, very high protein accumulation was detected in T1 and T2 classification, moderate in T3 and T4 (P = 0.033, chi-square test), whereas no Stat-3 was detected in normal samples. Similar trend also was found in Stat-3 mRNA expression by RT-PCR analysis which was high in T1 and T2 (early stages), moderate in T3 and T4 (late stages), and no expression in normal samples. The mean apoptotic indices were 1.75, 1.88, and 1.66 for normal, premalignant lesions, and HNSCC cases, respectively.

CONCLUSIONS

Stat-3 activation is an early event in head and neck carcinogenesis though its role in blocking the apoptosis in vivo in solid tumors was not observed.

Differential function of STAT5 isoforms in head and neck cancer growth control

Up-regulation of the epidermal growth factor receptor (EGFR) is critical for the loss of growth control in a variety of human cancers, including squamous cell carcinoma of the head and neck (SCCHN). Stimulation of EGFR results in activation of mitogenic signaling pathways including Signal Transducers and Activators of Transcription (STATs). Stat5 activation has been primarily demonstrated in hematopoietic malignancies. Gene disruption studies suggest potentially distinct functions of the Stat5 isoforms, Stat5a and Stat5b, which are encoded by two genes closely linked on human chromosome 17. To determine the function of Stat5 in SCCHN growth control, we studied the expression and constitutive activation of Stat5a and Stat5b in normal and transformed human squamous epithelial cells. Increased constitutive activation of Stat5 was detected in transformed compared with normal squamous cells. Blockade of TGF-alpha or EGFR, abrogated Stat5 activation. Targeting of Stat5b using antisense oligonucleotides inhibited SCCHN growth. In addition, SCCHN cells stably transfected with dominant negative mutant Stat5b failed to proliferate in vitro. In contrast, targeting of Stat5a using either antisense or dominant negative strategies had no effect on cell growth. These results suggest that TGF-alpha/EGFR-mediated autocrine growth of transformed epithelial cells is dependent on activation of Stat5b but not Stat5a.

Constitutive Stat3 activity up-regulates VEGF expression and tumor angiogenesis

Non-receptor and receptor tyrosine kinases, such as Src and EGF receptor (EGFR), are major inducers of vascular endothelial growth factor (VEGF), one of the most potent mediators of angiogenesis. While tyrosine kinases signal through multiple pathways, signal transducer and activation of transcription 3 (Stat3) is a point of convergence for many of these and is constitutively activated with high frequency in a wide range of cancer cells. Here, we show that VEGF expression correlates with Stat3 activity in diverse human cancer cell lines. An activated Stat3 mutant (Stat3C) up-regulates VEGF expression and stimulates tumor angiogenesis. Stat3C-induced VEGF up-regulation is abrogated when a Stat3-binding site in the VEGF promoter is mutated. Furthermore, interrupting Stat3 signaling with dominant-negative Stat3 protein or Stat3 antisense oligonucleotide in tumor cells down-regulates VEGF expression. Consistent with an important role of Stat3 in VEGF up-regulation induced by various oncogenic tyrosine kinases, v-Src-mediated VEGF expression is inhibited when Stat3 signaling is blocked. Moreover, chromatin immunoprecipitation assays indicate that Stat3 protein binds to the VEGF promoter in vivo and mutation of a Stat3-binding site in the VEGF promoter abrogates v-Src-induced VEGF promoter activity. These studies provide evidence that the VEGF gene is regulated directly by Stat3 protein, and indicate that Stat3 represents a common molecular target for blocking angiogenesis induced by multiple signaling pathways in human cancers.

Constitutive activation of Stat3alpha in brain tumors: localization to tumor endothelial cells and activation by the endothelial tyrosine kinase receptor (VEGFR-2)

Members of the normally latent family of transcription factors signal/inducers and activators of transcription (Stat) are activated in a number of human tumors and tumor-derived cell lines. In the case of Stat3, it is believed that this activation leads to the induction of survival signals as well as increased proliferation. In this study, we demonstrate that Stat3 is constitutively activated in glioma and medulloblastoma tumors and that the activated protein localizes predominantly to the tumor endothelial cells in the highly vascularized glioma tumors. Our efforts to elucidate potential mechanism(s) for this activated protein have shown that coexpression of Stat3alpha and the vascular endothelial growth factor receptor-2 (VEGFR-2) result in ligand-independent activation of Stat3alpha tyrosine phosphorylation and subsequent transcriptional activation in non-endothelial cells. We also show that activated Stat3alpha can increase transcription from the vascular endothelial growth factor (VEGF) gene. Taken together, these results suggest that the activated Stat3alpha found in brain tumors may be due to the endothelial tyrosine kinase VEGFR-2 and that Stat3alpha may play a central role in autocrine VEGF activation.