STAT3 regulates the growth and immunoglobulin production of BCL(1) B cell lymphoma through control of cell cycle progression

Bioinformatic identification and expression analysis of the chicken B cell lymphoma (BCL) gene

BACKGROUND

B cell lymphoma (BCL) families play an important role in apoptosis as a growth factor, cell death programming, cytokine expression and immune-related genes expression.

OBJECTIVES

In this study, to investigate the roles of BCLs, we performed genome-wide identification, expression and functional analyses of the BCL family in chicken.

METHODS

Chicken BCLs genes were identified and analyzed by using bioinformatics approach. Expression profiles and Hierarchical cluster analysis of the BCLs genes in different chicken tissues were obtained from the genome-wide RNA-seq in the GEO, and Cluster and Java Treeview, respectively.

RESULTS

A total of 16 BCLs genes were identified from the chicken genome, which could be further classified into five distinct groups in the phylogenetic tree. On the other hand, the interaction among BCLs proteins and between BCLs proteins with NF-κB subunits are limited, indicating that the remaining the functions of BCLs protein could be investigated in chicken. Moreover, KEGG pathway analysis indicated that BCL gene family was involved in regulation of apoptotic and immune response. Finally, BCL gene family was differentially expressed in chicken tissues, pathogen infection and growth stages of early chicken early embryo.

CONCLUSION

This study provides significant insights into the potential functions of BCLs in chicken, including the regulation of apoptosis, cell death and expression of immune-related genes.

Antagonism of cannabinoid receptor 2 pathway suppresses IL-6-induced immunoglobulin IgM secretion

Background

Cannabinoid receptor 2 (CB2) is expressed predominantly in the immune system, particularly in plasma cells, raising the possibility that targeting the CB2 pathway could yield an immunomodulatory effect. Although the role of CB2 in mediating immunoglobulin class switching has been reported, the effects of targeting the CB2 pathway on immunoglobulin secretion per se remain unclear.

Methods

Human B cell line SKW 6.4, which is capable of differentiating into IgM-secreting cells once treated with human IL-6, was employed as the cell model. SKW 6.4 cells were incubated for 4 days with CB2 ligands plus IL-6 (100 U/ml). The amount of secreted IgM was determined by an ELISA. Cell proliferation was determined by the ³H-Thymidine incorporation assay. Signal molecules involved in the modulation of IgM secretion were examined by real-time RT-PCR and Western blot analyses or by using their specific inhibitors.

Results

We demonstrated that CB2 inverse agonists SR144528 and AM630, but not CB2 agonist HU308 or CB1 antagonist SR141716, effectively inhibited IL-6-induced secretion of soluble IgM without affecting cell proliferation as measured by thymidine uptake. SR144528 alone had no effects on the basal levels of IgM in the resting cells. These effects were receptor mediated, as pretreatment with CB2 agonist abrogated SR144528-mediated inhibition of IL-6 stimulated IgM secretion. Transcription factors relevant to B cell differentiation, Bcl-6 and PAX5, as well as the protein kinase STAT3 pathway were involved in the inhibition of IL-6-induced IgM by SR144528.

Conclusions

These results uncover a novel function of CB2 antagonists and suggest that CB2 ligands may be potential modulators of immunoglobulin secretion.

Antisense technology: an emerging platform for cardiovascular disease therapeutics

Antisense oligonucleotides and small interfering RNAs, which suppress the translation of specific mRNA target proteins, are emerging as important therapeutic modalities for the treatment of cardiovascular disease. Over the last 25 years, the advances in all aspects of antisense technology, as well as a detailed understanding of the mechanism of action of antisense drugs, have enabled their use as therapeutic agents. These advancements culminated in the FDA approval of the first chronically administered cardiovascular antisense therapeutic, mipomersen, which targets hepatic apolipoprotein B mRNA. This review provides a brief history of antisense technology, highlights the progression of mipomersen from preclinical studies to multiple Phase III registration trials, and gives an update on the status of other cardiovascular antisense therapeutics currently in the clinic.

Intertissue flow of glutathione (GSH) as a tumor growth-promoting mechanism: interleukin 6 induces GSH release from hepatocytes in metastatic B16 melanoma-bearing mice

B16 melanoma F10 (B16-F10) cells with high glutathione (GSH) content show high metastatic activity in vivo. An intertissue flow of GSH, where the liver is the main reservoir, can increase GSH content in metastatic cells and promote their growth. We have studied here possible tumor-derived molecular signals that could activate GSH release from hepatocytes. GSH efflux increases in hepatocytes isolated from mice bearing liver or lung metastases, thus suggesting a systemic mechanism. Fractionation of serum-free conditioned medium from cultured B16-F10 cells and monoclonal antibody-induced neutralization techniques facilitated identification of interleukin (IL)-6 as a tumor-derived molecule promoting GSH efflux in hepatocytes. IL-6 activates GSH release through a methionine-sensitive/organic anion transporter polypeptide 1- and multidrug resistance protein 1-independent channel located on the sinusoidal site of hepatocytes. Specific siRNAs were used to knock down key factors in the main signaling pathways activated by IL-6, which revealed a STAT3-dependent mechanism. Our results show that IL-6 (mainly of tumor origin in B16-F10-bearing mice) may facilitate GSH release from hepatocytes and its interorgan transport to metastatic growing foci.

SOCS3 regulates p21 expression and cell cycle arrest in response to DNA damage

Genotoxic agents such as ionizing radiation trigger cell cycle arrest at the G1/S and G2/M checkpoints, allowing cells to repair damaged DNA before entry into mitosis. DNA damage-induced G1 arrest involves p53-dependent expression of p21 (Cip1/Waf-1), which inhibits cyclin-dependent kinases and blocks S phase entry. While much of the core DNA damage response has been well-studied, other signaling proteins that intersect with and modulate this response remain uncharacterized. In this study, we identify Suppressor of Cytokine Signaling (SOCS)-3 as an important regulator of radiation-induced G1 arrest. SOCS3-deficient fibroblasts fail to undergo G1 arrest and accumulate in the G2/M phase of the cell cycle. SOCS3 knockout cells phosphorylate p53 and H2AX normally in response to radiation, but fail to upregulate p21 expression. In addition, STAT3 phosphorylation is elevated in SOCS3-deficient cells compared to WT cells. Normal G1 arrest can be restored in SOCS3 KO cells by retroviral transduction of WT SOCS3 or a dominant-negative mutant of STAT3. Our results suggest a novel function for SOCS3 in the control of genome stability by negatively regulating STAT3-dependent radioresistant DNA synthesis, and promoting p53-dependent p21 expression.

Activated Signal Transducers and Activators of Transcription 3 Signaling Induces CD46 Expression and Protects Human Cancer Cells from Complement-Dependent Cytotoxicity

CD46 is one of the complement-regulatory proteins expressed on the surface of normal and tumor cells for protection against complement-dependent cytotoxicity. Cancer cells need to access the blood circulation for continued growth and metastasis, thus exposing themselves to destruction by complement system components. Previous studies have established that the signal transducers and activators of transcription 3 (STAT3) transcription factor is persistently activated in a wide variety of human cancer cells and primary tumor tissues compared with their normal counterparts. Using microarray gene expression profiling, we identified the CD46 gene as a target for activated STAT3 signaling in human breast and prostate cancer cells. The CD46 promoter contains two binding sites for activated STAT3 and mutations introduced into the major site abolished STAT3 binding. Chromatin immunoprecipitation confirms binding of STAT3 to the CD46 promoter. CD46 promoter activity is induced by activation of STAT3 and blocked by a dominant-negative form of STAT3 in luciferase reporter assays. CD46 mRNA expression is induced by interleukin-6 and by transient transfection of normal human epithelial cells with a persistently active mutant construct of STAT3, STAT3C. Furthermore, we show that inhibition of STAT3-mediated CD46 cell surface expression sensitizes DU145 prostate cancer cells to cytotoxicity in an in vitro complement lysis assay using rabbit anti-DU145 antiserum and rabbit complement. These results show that activated STAT3 signaling induces the CD46 promoter and protects human cancer cells from complement-dependent cytotoxicity, suggesting a potential mechanism whereby oncogenic signaling contributes to tumor cell evasion of antibody-mediated immunity.

Blockade of STAT3 by antisense oligonucleotide in TNBS-induced murine colitis

BACKGROUND AND AIMS

The expression of signal transducers and activators of transcription 3 (STAT3) is increased in Crohn's disease (CD), and nuclear translocated STAT3 is also found in the disease. However, the role of STAT3 protein on the pathogenesis of CD is not clear. This study was executed to investigate the role of STAT3 protein on the pathogenesis of trinitrobenzene sulfonic acid (TNBS)-induced colitis, the pathogenesis of which is CD-like.

METHODS

TNBS-induced colitis was produced, and STAT3 antisense oligonucleotide was administrated intracolonically during the early phase of colitis. The mice were killed 7 days later, and the expressions of STAT3 and phosphorylated STAT3 were identified by Western blot and immunofluorescence. The lamina propria mononuclear cells (LPMCs) were isolated freshly, and the percent of cell death and the expressions of Bcl-2 and Bax in LPMCs were evaluated. Colonic tissue damage and the production of inflammatory cytokines were measured also.

RESULTS

Administration of STAT3 antisense oligonucleotide effectively inhibited STAT3 expression and phosphorylation in inflamed colonic mucosa of colitis. The mice that were administered STAT3 antisense oligonucleotide showed less colonic tissue damage with decreased production of inflammatory cytokines such as TNF-alpha and INF-gamma in mucosa compared with that of those TNBS-induced colitis. Administration of STAT3 antisense oligonucleotide successfully induced apoptosis of LPMCs and counteracted the unbalanced expressions of Bcl-2 and Bax in LPMCs from colitis.

CONCLUSIONS

STAT3 activation may play an important role in the inflammatory process of TNBS-induced colitis, and inhibiting STAT3 activation during the early phase of the inflammatory response may have a beneficial effect on the colitis.

A Preferential Role for STAT5, not Constitutively Active STAT3, in Promoting Survival of a Human Lymphoid Tumor

STATs are believed to play key roles in normal and abnormal cell function. In the present work, we investigated the role of STATs in an IL-2-responsive human lymphoblastic lymphoma-derived cell line, YT. Only STAT3 was found constitutively tyrosine phosphorylated, but not other STATs. Hyperactive STAT3 was not attributable to a pre-existing intermediate affinity IL-2R complex and/or hyperactive Jak activity. Depletion of STAT3 protein expression reduced tumor cell viability with protracted kinetics (72-96 h), while TUNEL assays demonstrated cell death occurred via apoptosis. Interestingly, depletion of STAT5 in this same tumor induced more pronounced cell death compared with STAT3 depletion (24 h). Although IL-2 was able to rescue STAT3-depleted cells from death, it could not compensate for the loss of STAT5. To determine the prosurvival function of STAT3 vs STAT5 within the same tumor model, genes were profiled in STAT3- or STAT5-depleted YT cells by apoptosis-specific microarrays. Several differentially expressed genes were identified. Interestingly, those genes involved in NF-kappaB regulation, such as TNFR-associated factors 2 and 5 and B cell leukemia/lymphoma 10, were readily decreased upon STAT5, but not STAT3, depletion as validated by quantitative RT-PCR. These results suggest that STAT5 and, to a lesser extent, hyperactive STAT3 provide preferential and critical cell survival signals for certain human lymphoid tumors, indicating that nonhyperactive STATs should be considered as therapeutic targets for abrogating tumorigenesis.

Persistent activation of stat3 signaling induces survivin gene expression and confers resistance to apoptosis in human breast cancer cells

PURPOSE

Signal transducer and activator of transcription 3 (Stat3) protein is persistently activated in breast cancer and promotes tumor cell survival. To gain a better understanding of the role of constitutive Stat3 signaling in breast cancer progression, we evaluated the expression profile of potential Stat3-regulated genes that may confer resistance to apoptosis.

EXPERIMENTAL DESIGN

Stat3 signaling was blocked with antisense oligonucleotides in human MDA-MB-435s breast cancer cells and Affymetrix GeneChip microarray analysis was done. The candidate Stat3 target gene Survivin was further evaluated in molecular assays using cultured breast cancer cells and immunohistochemistry of breast tumor specimens.

RESULTS

Survivin, a member of the inhibitor of apoptosis protein family, was identified as a potential Stat3-regulated gene by microarray analysis. This was confirmed in Survivin gene promoter studies and chromatin immunoprecipitation assays showing that Stat3 directly binds to and regulates the Survivin promoter. Furthermore, direct inhibition of Stat3 signaling blocked the expression of Survivin protein and induced apoptosis in breast cancer cells. Direct inhibition of Survivin expression also induced apoptosis. Increased Survivin protein expression correlates significantly (P = 0.001) with elevated Stat3 activity in primary breast tumor specimens from high-risk patients who were resistant to chemotherapy treatment.

CONCLUSIONS

We identify Survivin as a direct downstream target gene of Stat3 in human breast cancer cells that is critical for their survival in culture. Our findings suggest that activated Stat3 signaling contributes to breast cancer progression and resistance to chemotherapy by, at least in part, inducing expression of the antiapoptotic protein, Survivin.

A novel platinum compound inhibits constitutive Stat3 signaling and induces cell cycle arrest and apoptosis of malignant cells

Previous studies have established constitutive activation of Stat3 protein as one of the molecular changes required for tumorigenesis. To develop novel therapeutics for tumors harboring constitutively active Stat3, compounds from the NCI 2000 diversity set were evaluated for inhibition of Stat3 DNA-binding activity in vitro. Of these, a novel platinum (IV) compound, IS3 295, interacted with Stat3 and inhibited its binding to specific DNA-response elements. Further analysis suggested noncompetitive-type kinetics for the inhibition of Stat3 binding to DNA. In human and mouse tumor cell lines with constitutively active Stat3, IS3 295 selectively attenuated Stat3 signaling, thereby inducing cell growth arrest at G0/G1 phase and apoptosis. Moreover, in transformed cells, IS3 295 repressed expression of cyclin D1 and bcl-xL, two of the known Stat3-regulated genes that are overexpressed in malignant cells, suggesting that IS3 295 mediates anti-tumor cell activity in part by blocking Stat3-mediated sub-version of cell growth and apoptotic signals. Together, our findings provide evidence for the inhibition of Stat3 activity and biological functions by IS3 295 through interaction with Stat3 protein. This study represents a significant advance in small molecule-based approaches to target Stat3 and suggests potential new applications for platinum (IV) complexes as modulators of the Stat3 pathway for cancer therapy.

B1 lymphocytes and myeloid dendritic cells in lymphoid organs are preferential extratumoral sites of parvovirus minute virus of mice prototype strain expression

Due to their oncolytic properties and apathogenicity, autonomous parvoviruses have attracted significant interest as possible anticancer agents. Recent preclinical studies provided evidence of the therapeutic potential of minute virus of mice prototype strain (MVMp) and its recombinant derivatives. In a murine model of hemangiosarcoma, positive therapeutic outcome correlated with high intratumoral expression of MVMp-encoded genes in tumors and lymphoid organs, especially in tumor-draining lymph nodes. The source and relevance of this extratumoral expression, which came as a surprise because of the known fibrotropism of MVMp, remained unclear. In the present study, we investigated (i) whether the observed expression pattern occurs in different tumor models, (ii) which cell population is targeted by the virus, and (iii) the immunological consequences of this infection. Significant MVMp gene expression was detected in lymphoid tissues from infected tumor-free as well as melanoma-, lymphoma-, and hemangiosarcoma-bearing mice. This expression was especially marked in lymph nodes draining virus-injected tumors. Fluorescent in situ hybridization analysis, multicolor fluorescence-activated cell sorting, and quantitative reverse transcription-PCR revealed that MVMp was expressed in rare subpopulations of CD11b (Mac1)-positive cells displaying CD11c+ (myeloid dendritic cells [MDC]) or CD45B (B220+ [B1 lymphocytes]) markers. Apart from the late deletion of cytotoxic memory cells (CD8+ CD44+ CD62L-), this infection did not lead to significant alteration of the immunological profile of cells populating lymphoid organs. However, subtle changes were detected in the production of specific proinflammatory cytokines in lymph nodes from virus-treated animals. Considering the role of B1 lymphocytes and MDC in cancer and immunological surveillance, the specific ability of these cell types to sustain parvovirus-driven gene expression may be exploited in gene therapy protocols.

Novel single-stranded oligonucleotides that inhibit signal transducer and activator of transcription 3 induce apoptosis in vitro and in vivo in prostate cancer cell lines

Signal transducers and activators of transcription (STAT) were originally discovered as components of cytokine signal transduction pathways. Persistent activation of one of these transcription factors, STAT3, is a feature of many malignancies, including hormone-resistant prostate cancer. In this regard, malignant cells expressing persistently activated STAT3 become dependent on it for survival, thus rendering STAT3 a potential molecular target for therapy of hormone-resistant prostate cancer. Previously, we reported that antisense oligonucleotides specific for STAT3 were better at inducing apoptosis than inhibitors of JAK1 or JAK2, the upstream activating kinases of STAT3. Here, we report that novel single-stranded oligonucleotides, which putatively block STAT3-DNA binding, were better at inducing hormone-resistant prostate cancer apoptosis than antisense STAT3 oligonucleotides. We observed that the novel STAT3-inhibiting oligonucleotides induced apoptosis by a mitochondrial-dependent pathway involving the activation of caspase-3. Prostate cell lines not expressing persistently activated STAT3 did not become apoptotic after treatment with these same oligonucleotides. Scrambled-sequence control oligonucleotides had none of the effects of the active sequence oligonucleotides on any variable measured. Furthermore, the novel STAT3-inhibiting oligonucleotides, but not scrambled-sequence control oligonucleotide, significantly reduced the volume of s.c. DU145 tumors in vivo. Histologic examination of the tumors revealed no infiltrate of mononuclear or granulocytic cells, which would be indicative of evocation of a nonspecific immune response by the oligonucleotides. We conclude that single-stranded oligonucleotides based on the binding sequences of STAT3 are an additional strategy to design inhibitors for this molecular target and that these inhibitors should be useful as experimental therapeutics for hormone-resistant prostate cancer.

Effect of Stat3 antisense oligonucleotide on signal transduction pathway related to human stomach adenocarcinoma MKN45 cell line

AIM: To investigate the effect of antisense oligonucleotide (as-ODN) of Stat3 on the cell proliferation of human stomach adenocarcinoma cell line MKN45 and to determine the novel molecular target for treatment of gastric cancer.

METHODS: Stat3 as-ODN, a mixed backbone oligonucleic acid (MBO), was transfected into MKN45 cells mediated by liposomal reagent. The effect on cell proliferation was examined by MTT method. After transfection of Stat3 as-ODN, electrophoretic mobility shift assay (EMSA) and Western blot were used to detect the Stat3 DNA-binding activity and the expression of phospho-Stat3 protein, respectively.

RESULTS: Stat3 as-ODN could significantly inhibit the proliferation of MKN45 cells. After transfection of Stat3 as-ODN, both the constitutive activation of Stat3 and the expression of phospho-Stat3 protein were decreased 50.65% and 78.86%, respectively.

CONCLUSION: Stat3 as-ODN could remarkably inhibit the signal transduction of Stat3 in MKN45 cells. Activated Stat3 signaling in human stomach adenocarcinoma cell line provides a novel molecular target for therapeutic intervention of gastric cancer.

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.

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.

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.

Cooperative regulation of Mcl-1 by Janus kinase/stat and phosphatidylinositol 3-kinase contribute to granulocyte-macrophage colony-stimulating factor-delayed apoptosis in human neutrophils

Polymorphonuclear neutrophils (PMN) are phagocytic cells constitutively programmed for apoptotic cell death. Exposure to GM-CSF delays apoptosis as measured by annexin-V staining and cell morphological change. We found that STAT5B, STAT1, and STAT3 DNA-binding activity was induced by GM-CSF. We also detected activation of the phosphatidylinositol 3-kinase (PI 3-kinase) pathway after GM-CSF treatment which was inhibited by treatment with the PI 3-kinase inhibitors, wortmannin and LY294002. We investigated whether STAT or PI 3-kinase activity was necessary for the pro-survival response of GM-CSF in PMN. Exposure of PMN to GM-CSF in the presence of either AG-490, antisense STAT3 oligonucleotides, or wortmannin resulted in a partial inhibition of GM-CSF-mediated pro-survival activity. GM-CSF induced a time-dependent increase in the mRNA and protein expression of the anti-apoptotic Bcl-2-family protein, Mcl-1. We examined the hypothesis that Janus kinase/STAT and PI 3-kinase regulation of Mcl-1 contributed to GM-CSF-delayed apoptosis. Using either AG-490 or wortmannin alone, we observed a dose-dependent inhibition of GM-CSF-induced Mcl-1 expression. Using suboptimal doses of AG-490 and wortmannin, we found that both drugs together had an additive effect on delayed apoptosis and Mcl-1 expression. These data suggest that cooperative regulation of Mcl-1 by the Janus kinase/STAT and PI 3-kinase pathways contribute to GM-CSF-delayed apoptosis.

Inhibition of STAT3 signaling leads to apoptosis of leukemic large granular lymphocytes and decreased Mcl-1 expression

Large granular lymphocyte (LGL) leukemia is characterized by the expansion of antigen-activated cytotoxic T lymphocytes. These leukemic cells are resistant to Fas-mediated apoptosis despite expressing high levels of Fas. We found that leukemic LGL from 19 patients displayed high levels of activated STAT3. Treatment of leukemic LGL with the JAK-selective tyrosine kinase inhibitor AG-490 induced apoptosis with a corresponding decrease in STAT-DNA binding activity. Moreover, using an antisense oligonucleotide approach to diminish STAT3 expression, we found that Fas sensitivity was restored in leukemic LGL. AG-490-induced apoptosis in leukemic LGL was independent of Bcl-xL or Bcl-2 expression. However, we found that the Bcl-2-family protein Mcl-1 was significantly reduced by AG-490 treatment. Activated STAT3 was shown to bind an SIE-related element in the murine mcl-1 promoter. Using a luciferase reporter assay, we demonstrated that v-src overexpression in NIH3T3 induced STAT3-dependent transcriptional activity from the mcl-1 promoter and increased endogenous Mcl-1 protein levels. We conclude that STAT3 activation contributed to accumulation of the leukemic LGL clones. These findings suggest that investigation should focus on novel strategies targeting STAT3 in the treatment of LGL leukemia.

STAT proteins: novel molecular targets for cancer drug discovery

Signal Transducers and Activators of Transcription (STATs) are a family of cytoplasmic proteins with roles as signal messengers and transcription factors that participate in normal cellular responses to cytokines and growth factors. Frequently, however, abnormal activity of certain STAT family members, particularly Stat3 and Stat5, is associated with a wide variety of human malignancies, including hematologic, breast, head and neck, and prostate cancers. Application of molecular biology and pharmacology tools in disease-relevant models has confirmed Stat3 as having a causal role in oncogenesis, and provided validation of Stat3 as a target for cancer drug discovery and therapeutic intervention. Furthermore, a constitutively-active mutant form of Stat3 is sufficient to induce oncogenic transformation of cells, which form tumors in vivo. Constitutive activation of Stat3 signaling is accompanied by upregulation of cyclin D1, c-Myc, and Bcl-x, changes consistent with subversion of normal cellular growth and survival control mechanisms. Block of constitutive Stat3 signaling results in growth inhibition and apoptosis of Stat3-positive tumor cells in vitro and in vivo. The observed dependence of certain tumors on constitutive Stat3 signaling for growth and survival has wide implications for cancer therapy, offering the potential for preferential tumor cell killing. This review evaluates constitutive Stat3 activation as a 'cancer-causing' factor, and proposes a number of molecular strategies for targeting Stat3 signaling for therapeutic intervention.