The Role of Signal Transducer and Activator of Transcription Factors in Leukemogenesis

STAT signaling in the pathogenesis and therapy of acute myeloid leukemia and myelodysplastic syndromes

Acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS) represent complex hematopoietic malignancies characterized by ineffective hematopoiesis and dysregulated myeloid differentiation. Recent research has underscored the critical role of aberrant STAT signaling pathways, particularly involving STAT3 and STAT5, in the pathogenesis of these disorders. Aberrant activation of STAT proteins has been implicated as a mediator of oncogenesis in several malignancies. In this review, we discuss the role of STAT proteins in both regulated and dysregulated hematopoiesis, the consequences of dysregulation in acute myeloid leukemia and myelodysplastic syndromes, therapeutic strategies, and recent advancements in STAT-targeted therapy. By integrating findings from recent preclinical and clinical studies, this review provides insights into the evolving landscape of STAT-targeted therapies, highlighting the promise of these approaches in enhancing treatment efficacy and improving patient outcomes in high-risk hematologic malignancies.

Overexpressed fibroblast growth factor receptors increase 1,25-dihydroxyvitamin D-dependent differentiation of acute myeloid leukemia cells

Many malignancies are driven by mutations within the gene for fibroblast growth factor receptor 1 (FGFR1). Previously, we have shown that signal transduction from the FOP2-FGFR1 fusion protein in acute myeloid leukemia KG1 cells is responsible for a low level of expression of the vitamin D receptor gene. In this paper, we address whether other fibroblast growth factor receptors regulate the vitamin D receptor (VDR) gene. We used the human myeloid leukemia U937 and HL60 cells, the bone cancer cell line U2OS, and cell transfection methods to answer the question. For myeloid leukemia cells, overexpression of FGFRs 1-3 genes caused a shift towards monocytic differentiation; this was extracellular regulated kinase (Erk) 1,2-dependent. Overexpression of FGFRs 1-3 genes also upregulated expression of the VDR gene, further sensitizing these cells to 1,25-dihydroxyvitamin D-induced monocyte differentiation. When we increased expression in bone cells, fibroblast growth factor receptors did not upregulate VDR gene expression, nor influence the activity of VDR. Fibroblast growth factor receptors are overexpressed in many neoplasms. Therefore, it may be reasonable to use vitamin D analogs to treat these cancers, to activate VDR and drive cell differentiation.

Overview of the STAT-3 signaling pathway in cancer and the development of specific inhibitors

Signal transducer and activator of transcription (STAT) proteins represent novel therapeutic targets for the treatment of cancer. In particular, STAT-3 serves critical roles in several cellular processes, including the cell cycle, cell proliferation, cellular apoptosis and tumorigenesis. Persistent activation of STAT-3 has been reported in a variety of cancer types, and a poor prognosis of cancer may be associated with the phosphorylation level of STAT-3. Furthermore, elevated STAT-3 activity has been demonstrated in a variety of mammalian cancers, both in vitro and in vivo. This indicates that STAT-3 serves an important role in the progression of numerous cancer types. A significant obstacle in developing STAT-3 inhibitors is the demonstration of the antitumor efficacy in in vivo systems and the lack of animal models for human tumors. Therefore, it is crucial to determine whether available STAT-3 inhibitors are suitable for clinical trials. Moreover, further preclinical studies are necessary to focus on the impact of STAT-3 inhibitors on tumor cells. When considering STAT-3 hyper-activation in human cancer, selective targeting to these proteins holds promise for significant advancement in cancer treatment. In the present study, advances in our knowledge of the structure of STAT-3 protein and its regulatory mechanisms are summarized. Moreover, the STAT-3 signaling pathway and its critical role in malignancy are discussed, in addition to the development of STAT-3 inhibitors in various cancer types.

The Role of Signal Transducer and Activator of Transcription 3 (STAT3) and Its Targeted Inhibition in Hematological Malignancies

Signal transducer and activator of transcription 3 (STAT3), a member of the STAT protein family, can be phosphorylated by receptor-associated Janus kinases (JAKs) in response to stimulation by cytokines and growth factors. It forms homo- or heterodimers that can translocate to the cell nucleus where they act as transcription activators. Constitutive activation of STAT3 has been found to be associated with initiation and progression of various cancers. It can exert proliferative as well as anti-apoptotic effects. This review focuses on the role of STAT3 in pathogenesis i.e., proliferation, differentiation, migration, and apoptosis of hematological malignancies viz. leukemia, lymphoma and myeloma, and briefly highlights the potential therapeutic approaches developed against STAT3 activation pathway.

GLI1 inhibitor GANT61 exhibits antitumor efficacy in T-cell lymphoma cells through down-regulation of p-STAT3 and SOCS3

T-cell lymphomas are lymphoid malignancies with aggressive clinical course and poor prognosis. Increasing evidences suggest that deregulation of signal transducer and activator of transcription-3 (STAT3) and suppressor of cytokine signaling 3 (SOCS3) is associated with the pathogenesis of T-cell lymphomas. The hedgehog (Hh)/glioma-associated oncogene-1 (GLI1) pathway, aberrantly activated in a number of tumors, has also been extensively studied. We found that protein expressions of GL11, p-STAT3, STAT3, and SOCS3 were up-regulated in T-cell lymphoma tissues and cell lines. Moreover, the protein expressions of p-STAT3 and SOCS3 were positively correlated with GLI1 in T-cell lymphomas. GLI1 inhibitor GANT61 and lentivirus-mediated siGLI1 exhibited inhibitory effects in the three T-cell lines (Jurkat, Karpass299 and Myla3676 cells). The protein expressions of p-STAT3 and SOCS3 were decreased accompanied with the inhibition of GLI1. These findings indicated that GANT61 is a promising agent against T-cell lymphoma and the antitumor activity might be partly mediated by down-regulating p-STAT3 and SOCS3.

SEL120-34A is a novel CDK8 inhibitor active in AML cells with high levels of serine phosphorylation of STAT1 and STAT5 transactivation domains

Inhibition of oncogenic transcriptional programs is a promising therapeutic strategy. A substituted tricyclic benzimidazole, SEL120-34A, is a novel inhibitor of Cyclin-dependent kinase 8 (CDK8), which regulates transcription by associating with the Mediator complex. X-ray crystallography has shown SEL120-34A to be a type I inhibitor forming halogen bonds with the protein's hinge region and hydrophobic complementarities within its front pocket. SEL120-34A inhibits phosphorylation of STAT1 S727 and STAT5 S726 in cancer cells in vitro. Consistently, regulation of STATs- and NUP98-HOXA9- dependent transcription has been observed as a dominant mechanism of action in vivo. Treatment with the compound resulted in a differential efficacy on AML cells with elevated STAT5 S726 levels and stem cell characteristics. In contrast, resistant cells were negative for activated STAT5 and revealed lineage commitment. In vivo efficacy in xenotransplanted AML models correlated with significant repression of STAT5 S726. Favorable pharmacokinetics, confirmed safety and in vivo efficacy provide a rationale for the further clinical development of SEL120-34A as a personalized therapeutic approach in AML.

Stat5 is critical for the development and maintenance of myeloproliferative neoplasm initiated by Nf1 deficiency

Juvenile myelomonocytic leukemia is a rare myeloproliferative neoplasm characterized by hyperactive RAS signaling. Neurofibromin1 (encoded by the NF1 gene) is a negative regulator of RAS activation. Patients with neurofibromatosis type 1 harbor loss-of-function mutations in NF1 and have a 200- to 500-fold increased risk of juvenile myelomonocytic leukemia. Leukemia cells from patients with juvenile myelomonocytic leukemia display hypersensitivity to certain cytokines, such as granulocyte-macrophage colony-stimulating factor. The granulocyte-macrophage colony-stimulating factor receptor utilizes pre-associated JAK2 to initiate signals after ligand binding. JAK2 subsequently activates STAT5, among other downstream effectors. Although STAT5 is gaining recognition as an important mediator of growth factor signaling in myeloid leukemias, the contribution of STAT5 to the development of hyperactive RAS-initiated myeloproliferative disease has not been well described. In this study, we investigated the consequence of STAT5 attenuation via genetic and pharmacological approaches in Nf1-deficient murine models of juvenile myelomonocytic leukemia. We found that homozygous Stat5 deficiency extended the lifespan of Nf1-deficient mice and eliminated the development of myeloproliferative neoplasm associated with Nf1 gene loss. Likewise, we found that JAK inhibition with ruxolitinib attenuated myeloproliferative neoplasm in Nf1-deficient mice. Finally, we found that primary cells from a patient with KRAS-mutant juvenile myelomonocytic leukemia displayed reduced colony formation in response to JAK2 inhibition. Our findings establish a central role for STAT5 activation in the pathogenesis of juvenile myelomonocytic leukemia and suggest that targeting this pathway may be of clinical utility in these patients.

Hsp90 Inhibitors for the Treatment of Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a hematological malignancy that arises due to reciprocal translocation of 3' sequences from c-Abelson (ABL) protooncogene of chromosome 9 with 5' sequence of truncated break point cluster region (BCR) on chromosome 22. BCR-ABL is a functional oncoprotein p210 that exhibits constitutively activated tyrosine kinase causing genomic alteration of hematopoietic stem cells. BCR-ABL specific tyrosine kinase inhibitors (TKIs) successfully block CML progression. However, drug resistance owing to BCR-ABL mutations and overexpression is still an issue. Heat-shock proteins (Hsps) function as molecular chaperones facilitating proper folding of nascent polypeptides. Their increased expression under stressful conditions protects cells by stabilizing unfolded or misfolded peptides. Hsp90 is the major mammalian protein and is required by BCR-ABL for stabilization and maturation. Hsp90 inhibitors destabilize the binding of BCR-ABL protein thus leading to the formation of heteroprotein complex that is eventually degraded by the ubiquitin-proteasome pathway. Results of many novel Hsp90 inhibitors that have entered into various clinical trials are encouraging. The present review targets the current development in the CML treatment by availing Hsp90 specific inhibitors.

Phase 1 and pharmacological trial of OPB-31121, a signal transducer and activator of transcription-3 inhibitor, in patients with advanced hepatocellular carcinoma

AIM

To evaluate the safety, pharmacokinetics and antitumor activity of OPB-31121, a signal transducer and activator of transcription-3 inhibitor, in patients with advanced hepatocellular carcinoma (HCC).

METHODS

HCC patients of Child-Pugh A or B who progressed on, or were intolerant to, sorafenib were eligible for this phase 1 trial. We used a standard 3 + 3 dose-escalation design with a 28-day cycle at dose levels of 50, 100, 200 and 400 mg/day. Tumor responses were assessed using the modified Response Evaluation Criteria in Solid Tumors.

RESULTS

Twenty-four patients were enrolled, of whom 23 received OPB-31121 (20 males; median age, 65 years). The most common adverse drug reactions were nausea (87.0%), vomiting (82.6%), diarrhea (69.6%), fatigue/malaise (52.2%), anorexia (47.8%) and peripheral sensory neuropathy (26.1%). The recommended dose for OPB-31121 was determined to be 200 mg. Six patients had stable disease for 8 weeks or more, resulting in disease control rates of 25.0-42.9%. In the 200-mg dose cohort, three of seven patients had stable disease and a median time to progression of 61.0 days. The maximum concentration and area under the plasma concentration-time curve of OPB-31121 were dose proportional.

CONCLUSION

OPB-31121 demonstrated insufficient antitumor activity for HCC. Furthermore, peripheral nervous system-related toxicities may negatively affect long-term administration of OPB-31121. Therefore, it was deemed difficult to continue the clinical development of OPB-31121 for treating advanced HCC and further investigation is expected in the agent with favorable profile in this category.

Phase I study of OPB-51602, an oral inhibitor of signal transducer and activator of transcription 3, in patients with relapsed/refractory hematological malignancies

We carried out a multicenter dose-escalation phase I study of oral OPB-51602, a signal transducer and activator of transcription 3 phosphorylation inhibitor, in patients with relapsed or refractory hematological malignancies to evaluate the safety, maximum tolerated dose (MTD), pharmacokinetics, and preliminary antitumor activity. Twenty patients were treated with OPB-51602 at doses of 1, 2, 3, 4, and 6 mg in the “3 + 3” dose escalation design. The most common treatment-related adverse events included nausea (55%), peripheral sensory neuropathy (45%), and diarrhea (40%). The most frequently observed grade 3 or 4 drug-related adverse events were neutropenia (20%), leukopenia (15%), lymphopenia (10%), and thrombocytopenia (10%). The MTD was 6 mg, with dose-limiting toxicities of grade 3 lactic acidosis and increased blood lactic acid levels observed in one of three patients and grade 1–2 peripheral neuropathy in three of three patients. The recommended dose was determined to be 4 mg. OPB-51602 was rapidly absorbed, and exposure tended to increase in a dose-dependent manner. Accumulation of OPB-51602 was seen with 4 weeks of multiple treatments. No clear therapeutic response was observed. Durable stable disease was observed in two patients with acute myeloid leukemia and one with myeloma. In conclusion, the MTD of OPB-51602 was 6 mg. OPB-51602 was safe and well tolerated in a dose range of 1–4 mg. However, long-term administration at higher doses was difficult with the daily dosing schedule, and no response was seen. Therefore, further clinical development of OPB-51602 for hematological malignancies with a daily dosing schedule was terminated.

Stat5 Exerts Distinct, Vital Functions in the Cytoplasm and Nucleus of Bcr-Abl+ K562 and Jak2(V617F)+ HEL Leukemia Cells

Signal transducers and activators of transcription (Stats) play central roles in the conversion of extracellular signals, e.g., cytokines, hormones and growth factors, into tissue and cell type specific gene expression patterns. In normal cells, their signaling potential is strictly limited in extent and duration. The persistent activation of Stat3 or Stat5 is found in many human tumor cells and contributes to their growth and survival. Stat5 activation plays a pivotal role in nearly all hematological malignancies and occurs downstream of oncogenic kinases, e.g., Bcr-Abl in chronic myeloid leukemias (CML) and Jak2(V617F) in other myeloproliferative diseases (MPD). We defined the mechanisms through which Stat5 affects growth and survival of K562 cells, representative of Bcr-Abl positive CML, and HEL cells, representative for Jak2(V617F) positive acute erythroid leukemia. In our experiments we suppressed the protein expression levels of Stat5a and Stat5b through shRNA mediated downregulation and demonstrated the dependence of cell survival on the presence of Stat5. Alternatively, we interfered with the functional capacities of the Stat5 protein through the interaction with a Stat5 specific peptide ligand. This ligand is a Stat5 specific peptide aptamer construct which comprises a 12mer peptide integrated into a modified thioredoxin scaffold, S5-DBD-PA. The peptide sequence specifically recognizes the DNA binding domain (DBD) of Stat5. Complex formation of S5-DBD-PA with Stat5 causes a strong reduction of P-Stat5 in the nuclear fraction of Bcr-Abl-transformed K562 cells and a suppression of Stat5 target genes. Distinct Stat5 mediated survival mechanisms were detected in K562 and Jak2(V617F)-transformed HEL cells. Stat5 is activated in the nuclear and cytosolic compartments of K562 cells and the S5-DBD-PA inhibitor most likely affects the viability of Bcr-Abl⁺ K562 cells through the inhibition of canonical Stat5 induced target gene transcription. In HEL cells, Stat5 is predominantly present in the cytoplasm and the survival of the Jak2(V617F)⁺ HEL cells is impeded through the inhibition of the cytoplasmic functions of Stat5.

Caspase-3 and caspase-6 cleave STAT1 in leukemic cells

Signal Transducer and Activator of Transcription-1 (STAT1) is phosphorylated upon interferon (IFN) stimulation, which can restrict cell proliferation and survival. Nevertheless, in some cancers STAT1 can act in an anti-apoptotic manner. Moreover, certain malignancies are characterized by the overexpression and constitutive activation of STAT1. Here, we demonstrate that the treatment of transformed hematopoietic cells with epigenetic drugs belonging to the class of histone deacetylase inhibitors (HDACi) leads to the cleavage of STAT1 at multiple sites by caspase-3 and caspase-6. This process does not occur in solid tumor cells, normal hematopoietic cells, and leukemic cells that underwent granulocytic or monocytic differentiation. STAT1 cleavage was studied under cell free conditions with purified STAT1 and a set of candidate caspases as well as with mass spectrometry. These assays indicate that unmodified STAT1 is cleaved at multiple sites by caspase-3 and caspase-6. Our study shows that STAT1 is targeted by caspases in malignant undifferentiated hematopoietic cells. This observation may provide an explanation for the selective toxicity of HDACi against rapidly proliferating leukemic cells.

STAT transcription factors in normal and cancer stem cells

Signal transducer and activator of transcription proteins (STATs) play vital roles in the regulation of cellular proliferation and survival in normal hematopoietic cells, including hematopoietic stem cells. However, aberrant activation of STATs is commonly observed in a number of hematologic malignancies, and recent studies indicate that targeting of STATs may have therapeutic benefit in these diseases. Additional studies have provided greater understanding of the cells responsible for leukemia initiation, referred to as leukemia stem cells. Emerging evidence indicates that STATs are important in maintaining leukemia stem cells and represent a promising target for eradication of this dangerous cell population. Here we summarize what is known about normal hematopoietic stem cells and the origin of leukemic stem cells. We further describe the roles of STAT proteins in these cell populations, as well as current progress toward the development of novel agents and strategies for targeting the STAT proteins.

STAT3 inhibitors: finding a home in lymphoma and leukemia

The Janus kinase (JAK) and signal transducer and activator of transcription (STAT) pathway is an active mediator of cytokine signaling in the pathogenesis of solid and hematologic malignancies. The seven-member STAT family is composed of latent cytoplasmic transcription factors that are activated by phosphorylation intertwined in a network with activation that ultimately leads to cell proliferation. An activated kinase enzyme phosphorylates one STAT factor or more, which shuttle to the nucleus to regulate gene expression, promoting cell survival. Somatic STAT3 mutations have been recently reported in large granular lymphocytic leukemia, aplastic anemia, and myelodysplastic syndrome. Furthermore, the relationship between BCL6 and STAT3 in diffuse large B-cell lymphomas, particularly on the activated B-cell subtype, needs to be further explored. The search for therapeutic STAT3 inhibitors that abrogate the JAK/STAT pathway is currently under way. Targeting the STAT pathway, which seems to be critical in tumorigenesis, is promising for multiple malignancies including lymphoma and leukemia. In this paper, we review mechanisms of action, failures, and successes of STAT3 inhibitors.

Targeting STAT5 in hematologic malignancies through inhibition of the bromodomain and extra-terminal (BET) bromodomain protein BRD2

The transcription factor signal STAT5 is constitutively activated in a wide range of leukemias and lymphomas, and drives the expression of genes necessary for proliferation, survival, and self-renewal. Thus, targeting STAT5 is an appealing therapeutic strategy for hematologic malignancies. Given the importance of bromodomain-containing proteins in transcriptional regulation, we considered the hypothesis that a pharmacologic bromodomain inhibitor could inhibit STAT5-dependent gene expression. We found that the small-molecule bromodomain and extra-terminal (BET) bromodomain inhibitor JQ1 decreases STAT5-dependent (but not STAT3-dependent) transcription of both heterologous reporter genes and endogenous STAT5 target genes. JQ1 reduces STAT5 function in leukemia and lymphoma cells with constitutive STAT5 activation, or inducibly activated by cytokine stimulation. Among the BET bromodomain subfamily of proteins, it seems that BRD2 is the critical mediator for STAT5 activity. In experimental models of acute T-cell lymphoblastic leukemias, where activated STAT5 contributes to leukemia cell survival, Brd2 knockdown or JQ1 treatment shows strong synergy with tyrosine kinase inhibitors (TKI) in inducing apoptosis in leukemia cells. In contrast, mononuclear cells isolated form umbilical cord blood, which is enriched in normal hematopoietic precursor cells, were unaffected by these combinations. These findings indicate a unique functional association between BRD2 and STAT5, and suggest that combinations of JQ1 and TKIs may be an important rational strategy for treating leukemias and lymphomas driven by constitutive STAT5 activation.

Overexpression of miR-708 and its targets in the childhood common precursor B-cell ALL

BACKGROUND

The critical function of microRNAs in the pathogenesis and prognosis of hematopoietic cancer has become increasingly apparent. However, only a few miRNAs have been reported to be altered in acute lymphocytic leukemia (ALL).

PROCEDURES

To uncover aberrantly expressed miRNAs in pediatric B-cell ALL, our study employed genome-wide miRNA microarray analysis and stem-loop real-time quantitative polymerase chain reaction (qRT-PCR) to examine common precursor B-cell ALL samples. The target genes of miRNA-708 were then identified and verified by bioinformatics, dual-luciferase reporter assay, qRT-PCR, and Western blot.

RESULTS

Significant upregulation of miR-708, miR-210, and miR-181b, and downregulation of miR-345 and miR-27a were observed in common precursor B-cell ALL (common-ALL) samples (P < 0.05). In addition, elevated expression of miR-708 and miR-181b were found in high-risk common-ALL compared to standard and intermediate ones. miR-708 inhibited luciferase reporter activity by binding to the 3'-untranslated regions (3'-UTRs) of CNTFR, NNAT, and GNG12 mRNA in HEK-293 cell line and suppressed the protein levels of CNTFR, NNAT, and GNG12 in Jurkat cells. In addition, mRNA levels of CNTFR and NNAT, but not of GNG12, were found to be downregulated in high risk common-ALL samples. Mutational analysis revealed that miR-708 binds to the 394-400 bp sequence region of the 3'-UTR of CNTFR mRNA.

CONCLUSION

The expression level of miR-708 reflects differences among the clinical types of common-ALL, and CNTFR, NNAT, and GNG12 were identified as targets of miR-708.

STAT transcription factors in hematopoiesis and leukemogenesis: opportunities for therapeutic intervention

Signal transducer and activator of transcription (STAT) proteins comprise a family of transcription factors that are activated by cytokines, hormones and growth factors. The activation of STAT proteins plays a key role in the production of mature hematopoietic cells via effects on cellular proliferation, survival and lineage-specific differentiation. Emerging evidence also demonstrates frequent, constitutive activation of STATs in primary leukemia specimens. Moreover, roles for STATs in promoting leukemia development have been delineated in numerous preclinical studies. This review summarizes our current understanding of STAT protein involvement in normal hematopoiesis and leukemogenesis, as well as recent advances in the development and testing of novel STAT inhibitors.

Unphosphorylated STAT5A stabilizes heterochromatin and suppresses tumor growth

Tumor suppressors known to date impede cancer growth by arresting the cell cycle or promoting apoptosis. Here we show that unphosphorylated human STAT5A functions as a tumor suppressor capable of repressing multiple oncogenes via heterochromatin formation. Unphosphorylated STAT5A binds to heterochromatin protein 1α (HP1α) and stabilizes heterochromatin. Expressing unphosphorylated STAT5A or HP1α inhibits colon cancer growth in mouse xenograft models. Transcriptome profiling shows that expressing an unphosphorylatable STAT5A has similar effects to overexpressing HP1α in global gene expression. Notably, the majority of the genes commonly repressed by unphosphorylated STAT5A and HP1α have been implicated in cancer development. Finally, down-regulation, somatic mutations, and deletions of STAT5 genes are found in certain human cancers. These results suggest that unphosphorylated STAT5A may epigenetically suppress tumor growth by promoting heterochromatin formation.

Signal transducer and activator of transcription 3 (STAT3) gene polymorphisms are associated with treatment outcomes in acute myeloid leukemia

INTRODUCTION

In the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway, STAT3 is one of the most prominent prognosis factors for cancer and leukemia. STAT3 activation might promote cellular transformation and therefore have an important role in human tumors. This study aimed to investigate the relationship between STAT3 polymorphisms and treatment response of acute myeloid leukemia (AML) in the Chinese population.

METHODS

Three single-nucleotide polymorphisms (SNPs) were tested in 130 patients with AML. Genomic DNA was isolated from peripheral blood and assayed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS).

RESULTS

The results of response to chemotherapy showed that there were strong relationships between unfavorable cytogenetics, partial remission (and even no remission), and GG genotype frequency in rs9909659 (P = 0.01 and 0.03). Patients younger than 45 years were associated significantly with GA/AA genotype (P = 0.01).

CONCLUSIONS

The results of this study suggest that the GG genotype in rs9909659 might confer increased resistance to standard chemotherapy.

Transforming and tumorigenic activity of JAK2 by fusion to BCR: molecular mechanisms of action of a novel BCR-JAK2 tyrosine-kinase

Chromosomal translocations in tumors frequently produce fusion genes coding for chimeric proteins with a key role in oncogenesis. Recent reports described a BCR-JAK2 fusion gene in fatal chronic and acute myeloid leukemia, but the functional behavior of the chimeric protein remains uncharacterized. We used fluorescence in situ hybridization and reverse transcription polymerase chain reaction (RT-PCR) assays to describe a BCR-JAK2 fusion gene from a patient with acute lymphoblastic leukemia. The patient has been in complete remission for six years following treatment and autologous transplantation, and minimal residual disease was monitored by real-time RT-PCR. BCR-JAK2 codes for a protein containing the BCR oligomerization domain fused to the JAK2 tyrosine-kinase domain. In vitro analysis of transfected cells showed that BCR-JAK2 is located in the cytoplasm. Transduction of hematopoietic Ba/F3 cells with retroviral vectors carrying BCR-JAK2 induced IL-3-independent cell growth, constitutive activation of the chimeric protein as well as STAT5 phosphorylation and translocation to the nuclei, where Bcl-xL gene expression was elicited. Primary mouse progenitor cells transduced with BCR-JAK2 also showed increased proliferation and survival. Treatment with the JAK2 inhibitor TG101209 abrogated BCR-JAK2 and STAT5 phosphorylation, decreased Bcl-xL expression and triggered apoptosis of transformed Ba/F3 cells. Therefore, BCR-JAK2 is a novel tyrosine-kinase with transforming activity. It deregulates growth factor-dependent proliferation and cell survival, which can be abrogated by the TG101209 inhibitor. Moreover, transformed Ba/F3 cells developed tumors when injected subcutaneously into nude mice, thus proving the tumorigenic capacity of BCR-JAK2 in vivo. Together these findings suggest that adult and pediatric patients with BCR-ABL-negative leukemia and JAK2 overexpression may benefit from targeted therapies.

The erythropoietin receptor: molecular structure and hematopoietic signaling pathways

The process of erythropoiesis in the fetal liver and adult bone marrow is regulated by the hormone erythropoietin (Epo), which is produced in the kidney at low levels under homeostatic conditions. Defects in Epo production result in severe anemia; use of recombinant hormone has improved the lives of patients with renal failure or anemia because of bone marrow suppression. Deletion of the Epo gene in mice leads to embryonic lethality at days 13 to 15, coincident with the establishment of definitive (adult-type) erythropoiesis and underscoring the absolute necessity of Epo function in vivo. Epo has proven to be a successful pharmaceutical agent, one of the early triumphs of recombinant protein technology. Because of its clinical importance, a great deal of attention has focused on the molecular mechanisms of Epo-regulated erythropoiesis. This review highlights the basic concepts of Epo signal transduction within the hematopoietic system, the major site of Epo action in vivo.

Constitutive STAT5 activation correlates with better survival in cervical cancer patients treated with radiation therapy

PURPOSE

Constitutively activated signal transducers and activators of transcription (STAT) factors, in particular STAT1, STAT3, and STAT5, have been detected in a wide variety of human primary tumors and have been demonstrated to directly contribute to oncogenesis. However, the expression pattern of these STATs in cervical carcinoma is still unknown, as is whether or not they have prognostic significance. This study investigated the expression patterns of STAT1, STAT3, and STAT5 in cervical cancer and their associations with clinical outcomes in patients treated with radical radiation therapy.

METHODS AND MATERIALS

A total of 165 consecutive patients with International Federation of Gynecology and Obstetrics (FIGO) Stages IB to IVA cervical cancer underwent radical radiation therapy, including external beam and/or high-dose-rate brachytherapy between 1989 and 2002. Immunohistochemical studies of their formalin-fixed, paraffin-embedded tissues were performed. Univariate and multivariate analyses were performed to identify and to evaluate the effects of these factors affecting patient survival.

RESULTS

Constitutive activations of STAT1, STAT3, and STAT5 were observed in 11%, 22%, and 61% of the participants, respectively. While STAT5 activation was associated with significantly better metastasis-free survival (p < 0.01) and overall survival (p = 0.04), STAT1 and STAT3 activation were not. Multivariate analyses showed that STAT5 activation, bulky tumor (≥ 4 cm), advanced stage (FIGO Stages III and IV), and brachytherapy (yes vs. no) were independent prognostic factors for cause-specific overall survival. None of the STATs was associated with local relapse. STAT5 activation (odds ratio = 0.29, 95% confidence interval = 0.13-0.63) and advanced stage (odds ratio = 2.54; 95% confidence interval = 1.03-6.26) were independent predictors of distant metastasis.

CONCLUSIONS

This is the first report to provide the overall expression patterns and prognostic significance of specific STATs in cervical carcinoma. Our results indicate that constitutive STAT5 activation correlates with better metastasis-free survival and overall survival in cervical cancer patients who have received radiation therapy.

RA-inducible gene-I induction augments STAT1 activation to inhibit leukemia cell proliferation

RA-inducible gene I (RIG-I/DDX58) has been shown to activate IFN-β promoter stimulator 1 (IPS-1) on recognizing cytoplasmic viral RNAs. It is unclear how RIG-I functions within the IFN and/or RA signaling process in acute myeloid leukemia (AML) cells, however, where obvious RIG-I induction is observed. Here, we show that the RIG-I induction functionally contributes to IFN-α plus RA-triggered growth inhibition of AML cells. Interestingly, although RIG-I induction itself is under the regulation of STAT1, a major IFN intracellular signal mediator, under circumstances in which it does not stimulate IPS-1, it conversely augments STAT1 activation to induce IFN-stimulatory gene expression and inhibit leukemia cell proliferation. Thus, our results unveil a previously undescribed RIG-I activity in regulating the cellular proliferation of leukemia cells via STAT1, which is independent of its classic role of sensing viral invasion to trigger type I IFN transcription.

PTP1B suppresses prolactin activation of Stat5 in breast cancer cells

Basal levels of nuclear localized, tyrosine phosphorylated Stat5 are present in healthy human breast epithelia. In contrast, Stat5 phosphorylation is frequently lost during breast cancer progression, a finding that correlates with loss of histological differentiation and poor patient prognosis. Identifying the mechanisms underlying loss of Stat5 phosphorylation could provide novel targets for breast cancer therapy. Pervanadate, a general tyrosine phosphatase inhibitor, revealed marked phosphatase regulation of Stat5 activity in breast cancer cells. Lentiviral-mediated shRNA allowed specific examination of the regulatory role of five tyrosine phosphatases (PTP1B, TC-PTP, SHP1, SHP2, and VHR), previously implicated in Stat5 regulation in various systems. Enhanced and sustained prolactin-induced Stat5 tyrosine phosphorylation was observed in T47D and MCF7 breast cancer cells selectively in response to PTP1B depletion. Conversely, PTP1B overexpression suppressed prolactin-induced Stat5 tyrosine phosphorylation. Furthermore, PTP1B knockdown increased Stat5 reporter gene activity. Mechanistically, PTP1B suppression of Stat5 phosphorylation was mediated, at least in part, through inhibitory dephosphorylation of the Stat5 tyrosine kinase, Jak2. PTP1B knockdown enhanced sensitivity of T47D cells to prolactin phosphorylation of Stat5 by reducing the EC(50) from 7.2 nmol/L to 2.5 nmol/L. Immunohistochemical analyses of two independent clinical breast cancer materials revealed significant negative correlations between levels of active Stat5 and PTP1B, but not TC-PTP. Collectively, our data implicate PTP1B as an important negative regulator of Stat5 phosphorylation in invasive breast cancer.

Comparative analyses of differentially induced T-cell receptor-mediated phosphorylation pathways in T lymphoma cells

Activation of T lymphoma cells expressing Syk, but not ZAP-70 tyrosine kinase, has been shown to negatively regulate cell activation and activation-induced cell death (AICD), perhaps due to differential induction of tyrosine phosphorylation modified proteins. To better understand the role of these proteins and their associated molecules/pathways, we studied a previously described model of T lymphoma cells expressing either a kinase-activated chimeric Syk or ZAP-70 genetically linked to T-cell receptor (TCR) ζ chain (Z/Syk or Z/ZAP cells, respectively). To help identify molecules and pathways linked to cell activation or AICD, a comparative semi-quantitative proteomics-based approach was utilized to analyze tyrosine-phosphorylated protein immunoprecipitates from two-minute short-term activated Z/Syk or Z/ZAP cells. Using the resulting bioinformatics data-sets, we identified several differentially immunoprecipitated proteins that could be validated biochemically. More tyrosine-phosphorylated and phosphotyrosine-associated proteins were found in Z/Syk than in Z/ZAP cells. Proteins involved in different unique functional pathways were induced in these cells and showed altered intermolecular interactions in varied pathways. Remarkably, 41% of differentially identified proteins in Z/Syk cells belonged to cell cycle or vesicle/trafficking pathways. In contrast, 21% of such proteins in Z/ZAP cells belonged to metabolism pathways. Therefore, molecular pathways involved in post-translational modifications linked to distinct cellular/physiological functions are differentially activated, which may contribute to varied activation and AICD responses of these cells. In summary, we identified proteins belonging to novel differentially activated pathways involved in TCR-mediated signaling, which may be targets for regulating activation and AICD of T lymphoma cells and for potential cancer therapy.

Innate immune signaling pathways in animals: beyond reductionism

The immune system plays a crucial role in the maintenance of the stability and equilibrium of the internal environment in living organisms. The field of animal innate immunity has been the global focus of immunological research for decades. It is now known that the functions of innate immunity inevitably rely on the action of the molecular machines of the cascades or network of immune signaling pathways. Up to date, many researches on the immune signaling pathways in animals were focused on identifying the component functions or cascade molecules in details, which essentially followed a reductionist paradigm without paying high attention to the integrated features. The main purpose of this article was dedicated to accentuating the shift of this field from a reductionist to a systemic view. First, the former part of this article made efforts to summarize the main aspects of the signaling pathways of animal innate immunity including the web resources, the recapitulation of highlighted pathways, the cross-talks, and the evolutionary considerations, which heavily emphasized the integrated characteristics of the immune signaling pathways. Subsequently, the later part of this article was based on the holistic feature of the immune signaling pathways, mainly dedicated to propose a novel hypothesis. From a whole perspective, the oscillating balance hypothesis was deliberately formulated to characterize the holistic pattern of the signaling transduction network of animal innate immune system, which might help to understand some immunological phenomena through the integral principle of the immune network.

Global reduction of the epigenetic H3K79 methylation mark and increased chromosomal instability in CALM-AF10-positive leukemias

Chromosomal translocations generating fusion proteins are frequently found in human leukemias. The fusion proteins play an important role in leukemogenesis by subverting the function of one or both partner proteins. The leukemogenic CALM-AF10 fusion protein is capable of interacting with the histone H3 lysine 79 (H3K79)-specific methyltransferase hDOT1L through the fused AF10 moiety. This interaction leads to local H3K79 hypermethylation on Hoxa5 loci, which up-regulates the expression of Hoxa5 and contributes to leukemogenesis. However, the long latency of leukemogenesis of CALM-AF10 transgenic mice suggests that the direct effects of fusion oncogene are not sufficient for the induction of leukemia. In this study, we show that the CALM-AF10 fusion protein can also greatly reduce global H3K79 methylation in both human and murine leukemic cells by disrupting the AF10-mediated association of hDOT1L with chromatin. Cells with reduced H3K79 methylation are more sensitive to gamma-irradiation and display increased chromosomal instability. Consistently, leukemia patients harboring CALM-AF10 fusion have more secondary chromosomal aberrations. These findings suggest that chromosomal instability associated with global epigenetic alteration contributes to malignant transformation in certain leukemias, and that leukemias with this type of epigenetic alteration might benefit from treatment regimens containing DNA-damaging agents. This study is registered with www.clinicaltrials.gov as NCT00266136.

Canonical and non-canonical JAK-STAT signaling

Aberrant activation of the JAK-STAT pathway has been implicated in many human cancers. It has widely been assumed that the effects of STAT activation are mediated by direct transcriptional induction of STAT target genes. However, recent findings in Drosophila have identified a non-canonical mode of JAK-STAT signaling, which directly controls heterochromatin stability. This indicates that the JAK-STAT pathway also controls cellular epigenetic status, which affects expression of genes beyond those under direct STAT transcriptional control. Given the evolutionary conservation of the canonical pathway among different species, the non-canonical mode of JAK-STAT signaling might also operate in vertebrates. In this review, canonical versus non-canonical JAK-STAT signaling and the implications for gene regulation and cancer formation are discussed.

FoxO tumor suppressors and BCR-ABL-induced leukemia: a matter of evasion of apoptosis

Numerous studies have revealed that the BCR-ABL oncoprotein abnormally engages a multitude of signaling pathways, some of which may be important for its leukemogenic properties. Central to this has been the determination that the tyrosine kinase function of BCR-ABL is mainly responsible for its transforming potential, and can be targeted with small molecule inhibitors, such as imatinib mesylate (Gleevec, STI-571). Despite this apparent success, the development of clinical resistance to imatinib therapy, and the inability of imatinib to eradicate BCR-ABL-positive malignant hematopoietic progenitors demand detailed investigations of additional effector pathways that can be targeted for CML treatment. The promotion of cellular survival via the suppression of apoptotic pathways is a fundamental characteristic of tumor cells that enables resistance to anti-cancer therapies. As substrates of survival kinases such as Akt, the FoxO family of transcription factors, particularly FoxO3a, has emerged as playing an important role in the cell cycle arrest and apoptosis of hematopoietic cells. This review will discuss our current understanding of BCR-ABL signaling with a focus on apoptotic suppressive mechanisms and alternative approaches to CML therapy, as well as the potential for FoxO transcription factors as novel therapeutic targets.

Reducing the genotoxic potential of retroviral vectors

The recent development of leukemia in gene therapy patients with X-linked severe combined immunodeficiency disease because of retroviral vector insertional mutagenesis has prompted reassessment of the genotoxic potential of integrating vector systems. In this chapter, various strategies are described to reduce the associated risks of retroviral genomic integration. These include deletion of strong transcriptional enhancer-promoter elements in the retroviral long terminal repeats, flanking the retroviral transcriptional unit with enhancer blocking sequences and designing vectors with improved RNA 3' end processing. Protocols are provided to evaluate the relative biosafety of the modified vectors based on their ability to immortalize hematopoietic progenitor cells and propensity to trigger clonal hematopoiesis or leukemogenesis following hematopoietic stem cell transplantation.

Transformation of hematopoietic cells and activation of JAK2-V617F by IL-27R, a component of a heterodimeric type I cytokine receptor

From a patient with acute myeloid leukemia (AML), we have identified IL-27Ra (also known as TCCR and WSX1) as a gene whose expression can induce the transformation of hematopoietic cells. IL-27Ra (IL-27R) is a type I cytokine receptor that functions as the ligand binding component of the receptor for IL-27 and functions with the glycoprotein 130 (gp130) coreceptor to induce signal transduction in response to IL-27. We show that IL-27R is expressed on the cell surface of the leukemic cells of AML patients. 32D myeloid cells transformed by IL-27R contain elevated levels of activated forms of various signaling proteins, including JAK1, JAK2, STAT1, STAT3, STAT5, and ERK1/2. Inhibition of JAK family proteins induces cell cycle arrest and apoptosis in these cells, suggesting the transforming properties of IL-27R depend on the activity of JAK family members. IL-27R also transforms BaF3 cells to cytokine independence. Because BaF3 cells lack expression of gp130, this finding suggests that IL-27R-mediated transformation of hematopoietic cells is gp130-independent. Finally, we show that IL-27R can functionally replace a homodimeric type I cytokine receptor in the activation of JAK2-V617F, a critical JAK2 mutation in various myeloproliferative disorders (MPDs). Our data demonstrate that IL-27R possesses hematopoietic cell-transforming properties and suggest that, analogous to homodimeric type I cytokine receptors, single-chain components of heterodimeric receptors can also enhance the activation of JAK2-V617F. Therefore, such receptors may play unappreciated roles in MPDs.

Structural analysis identifies imidazo[1,2-b]pyridazines as PIM kinase inhibitors with in vitro antileukemic activity

Much attention has recently been focused on PIM kinases as potential targets for the treatment of hematopoietic malignancies and some solid cancers. Using protein stability shift assays, we identified a family of imidazo[1,2-b]pyridazines to specifically interact with and inhibit PIM kinases with low nanomolar potency. The high-resolution crystal structure of a PIM1 inhibitor complex revealed that imidazo[1,2-b]pyridazines surprisingly interact with the NH(2)-terminal lobe helix alphaC rather than with the kinase hinge region. Thus, the identified inhibitors are ATP competitive but not ATP mimetic compounds, explaining their enhanced selectivity with respect to conventional type I kinase inhibitors. One of the identified imidazo[1,2-b]pyridazines (K00135) was further tested in several hematopoietic cellular systems. First, K00135 dose-dependently impaired survival of murine Ba/F3 cells that have been rendered cytokine independent by overexpression of human PIMs. Second, K00135 impaired survival and clonogenic growth of a panel of human acute leukemia cells. Third, exposure of K00135 significantly suppressed in vitro growth of leukemic blasts from five acute myelogenous leukemia patients but not of normal umbilical cord blood mononuclear cells. In vitro kinase assays and immunoblotting using lysates from human MV4;11 leukemic cells showed inhibition of phosphorylation of known PIM downstream targets, such as BAD and eukaryotic translation initiation factor 4E-binding protein 1, by K00135. Taken together, we report a family of small molecules that selectively interact and block PIM kinases and could serve as a lead to develop new targeted antileukemic therapeutics.

Silibinin inhibits constitutive activation of Stat3, and causes caspase activation and apoptotic death of human prostate carcinoma DU145 cells

Transcription factor signal transducer and activator of transcription (Stat)-3 is activated constitutively in prostate cancer (PCA) suggesting that its disruption could be an effective approach to control this malignancy. Here we assessed whether silibinin, a flavanone from Silybum marianum with proven anticancer efficacy in various cancer models, inhibits Stat3 activation in DU145 cells, and if it does, what is the biological fate of the cells? At 50 muM or higher concentrations for 24 or 48 h, silibinin concentration dependently reduced constitutive Stat3 phosphorylation at Tyr705 and Ser727 residues under both serum and serum-starved conditions. Constitutively active Stat3-DNA binding was also inhibited concentration dependently by silibinin; however, apoptotic death together with caspase and poly(ADP-ribose) polymerase (PARP) cleavage was observed by silibinin only under serum-starved conditions suggesting that additional survival pathways are active under serum conditions. In other studies, cells were treated with various specific pharmacological inhibitors where phosphorylation of Stat3 was not reduced by epidermal growth factor receptor and Mitogen activated protein/extracellular signal regulate kinase kinase (MEK1/2) inhibitors, suggesting lack of significant roles of these in Stat3 activation in DU145 cells. Janus kinase (JAK)-1 and JAK2 inhibitors strongly reduced Stat3 phosphorylation but did not result in apoptotic cell death. Interestingly, JAK1 inhibitor only in combination with silibinin resulted in a complete reduction in Stat3 phosphorylation at Tyr705, activated caspase-9 and caspase-3, and caused strong PARP cleavage and apoptotic death of DU145 cells. Given a critical role of Stat3 activation in PCA, our results showed that silibinin inhibits constitutively active Stat3 and induces apoptosis in DU145 cells, and thus might have potential significance in therapeutic intervention of this deadly malignancy.

STAT3 mRNA and protein expression in colorectal cancer: effects on STAT3-inducible targets linked to cell survival and proliferation

AIMS

To evaluate mRNA and protein expression of signal transducers and activators of transcription (STAT)3 in colorectal carcinomas (CRCs) and to define the association of STAT3 activity with the STAT3-inducible targets cyclin D1, survivin, Bcl-xl and Mcl-1.

MATERIALS AND METHODS

Matching serial sections of normal colonic epithelium and invasive CRCs (n = 32) were subjected to quantitative reverse transcriptase polymerase chain reaction specific to STAT3, cyclin D1, survivin, Bcl-xl and Mcl-1, as well as immunohistochemistry. For STAT3 immunohistochemistry, two antibodies, recognising unphosphorylated (UP-) and phosphorylated (tyr705, P-) STAT3 were used. Ki-67 (MIB-1) staining was included as a proliferation marker.

RESULTS

Compared with normal colonic epithelium, UP-STAT3 and P-STAT3 (p = 0.023 and 0.006) protein expression and expression of its associated targets cyclin D1, survivin and Bcl-xl were significantly (all p<0.001) increased in carcinoma. In carcinomas, STAT3 (p = 0.019) and Bcl-xl (p = 0.001) mRNAs were correlated with lymph node status. Moreover, nuclear P-STAT3 protein expression (active state) was associated with the expression of its target genes Bcl-xl (p = 0.038) and survivin (p = 0.01) as well as with Ki-67 (p = 0.017). By contrast, cytoplasmic UP-STAT was significantly linked to Bcl-xl mRNA (p = 0.024) and protein (p = 0.001) as well as to cytoplasmic survivin protein expression (p = 0.019).

CONCLUSION

Both inactive (UP-STAT3) and active (P-STAT3) STAT3 proteins are markedly increased in invasive CRCs. This is associated with Bcl-xl and survivin induction, increased proliferation and lymph node metastasis. This study therefore provides the basis for further examination of the prognostic or predictive value of these molecular markers in CRC.

Phosphotyrosine profiling identifies the KG-1 cell line as a model for the study of FGFR1 fusions in acute myeloid leukemia

The 8p11 myeloproliferative syndrome (EMS) is associated with translocations that disrupt the FGFR1 gene. To date, 8 fusion partners of FGFR1 have been identified. However, no primary leukemia cell lines were identified that contain any of these fusions. Here, we screened more than 40 acute myeloid leukemia cell lines for constitutive phosphorylation of STAT5 and applied an immunoaffinity profiling strategy to identify tyrosine-phosphorylated proteins in the KG-1 cell line. Mass spectrometry analysis of KG-1 cells revealed aberrant tyrosine phosphorylation of FGFR1. Subsequent analysis led to the identification of a fusion of the FGFR1OP2 gene to the FGFR1 gene. Small interfering RNA (siRNA) against FGFR1 specifically inhibited the growth and induced apoptosis of KG-1 cells. Thus, the KG-1 cell line provides an in vitro model for the study of FGFR1 fusions associated with leukemia and for the analysis of small molecule inhibitors against FGFR1 fusions.

Engagement of the B-cell antigen receptor activates STAT through Lyn in a Jak-independent pathway

Engagement of the B-cell antigen receptor (BCR) initiated by the Src kinase Lyn triggers rapid signaling cascades, leading to proliferation, differentiation or growth arrest of B cells. The Janus kinase (JAK)-STAT (signal transducer and activator of transcription) pathway, activated through cytokine receptors, mediates similar responses. Hypothesizing that Src and JAK pathways engage in crosstalk in B-cell signaling, we studied wild-type and Lyn-null B-cell lines, which express BCR. We found that activated BCR results in tyrosine phosphorylation of JAK-STAT, which required Lyn. To confirm that STAT activation is not due to JAK, we cloned the chicken homologs of JAK1 and JAK2 and made their antisense constructs. In cells expressing antisense JAK1 and JAK2, tyrosine phosphorylation of STAT was not inhibited following BCR stimulation. Using activation loop-specific phosphotyrosine antibodies, we did not detect phospho-JAK1 and phospho-JAK2 after BCR stimulation. The JAK inhibitor AG490 did not inhibit the tyrosine phosphorylation of Lyn or STAT after BCR simulation. An in vitro phosphorylation assay showed that Lyn directly phosphorylates STAT3. In an electrophoretic mobility shift assay, BCR stimulation led to enhanced DNA binding of the STAT3 in DT40, but not in the Lyn-null cells. We conclude that BCR engagement activates the STAT pathway via Lyn, independent of JAK.

Identification of human STAT5-dependent gene regulatory elements based on interspecies homology

STAT5 is a transcription factor essential for hematopoietic physiology. STAT5 functions to transduce signals from cytokines to the nucleus where it regulates gene expression. Although several important transcriptional targets of STAT5 are known, most remain unidentified. To identify novel STAT5 targets, we searched chromosomes 21 and 22 for clusters of STAT5 binding sites contained within regions of interspecies homology. We identified four such regions, including one with tandem STAT5 binding sites in the first intron of the NCAM2 gene. Unlike known STAT5 binding sites, this site is found within a very large intron and resides approximately 200 kb from the first coding exon of NCAM2. We demonstrate that this region confers STAT5-dependent transcriptional activity. We show that STAT5 binds in vivo to the NCAM2 intron in the NKL natural killer cell line and that this binding is induced by cytokines that activate STAT5. Neither STAT1 nor STAT3 bind to this region, despite sharing a consensus binding sequence with STAT5. Activation of STAT4 and STAT5 causes the accumulation of both of these STATs to the NCAM2 regulatory region. Therefore, using an informatics based approach to identify STAT5 targets, we have identified NCAM2 as both a STAT4- and STAT5-regulated gene, and we show that its expression is regulated by cytokines essential for natural killer cell survival and differentiation. This strategy may be an effective way to identify functional binding regions for transcription factors with known cognate binding sites anywhere in the genome.

Gö6976 is a potent inhibitor of the JAK 2 and FLT3 tyrosine kinases with significant activity in primary acute myeloid leukaemia cells

Aberrant activation of Janus kinase/signal transducers and activators of transcription (JAK/STAT) signalling is implicated in a number of haematological malignancies and effective JAK inhibitors may be therapeutically useful. We found that Gö6976, an indolocarbazole inhibitor of the calcium-dependent isozymes of protein kinase C (PKC), inhibited interleukin 3/granulocyte-macrophage colony-stimulating factor-induced signalling, proliferation and survival whereas Gö6983, a broad spectrum PKC inhibitor, had no such effects. Gö6976 was found to be a direct and potent inhibitor of JAK2 in vitro. Gö6976 also inhibited signalling, survival and proliferation in cells expressing the leukaemia-associated TEL-JAK2 fusion protein and the myeloproliferative disorder (MPD)-associated JAK2 V617F mutant. In primary acute myeloid leukaemia (AML) cells, incubation with Gö6976 reduced constitutive STAT activity in all cases studied. In addition, Akt and mitogen-activated protein kinase phosphorylation were reduced in 4/5 FLT3-internal tandem duplication (ITD) positive AML cases and 7/13 FLT3-wild-type (WT) cases. Expression of FLT3-WT, ITD and D835Y in 32D cells showed that Gö6976 is also a potent inhibitor of WT and mutant FLT3. In AML cells, Gö6976 reduced the survival to 55 +/- 5% of control in FLT3-ITD cases and to 69 +/- 5% in FLT3-WT samples. These data may help identify clinically useful compounds based on the structure of Gö6976, which can be employed for the treatment of MPDs as well as AML.

Targeting PIM kinases impairs survival of hematopoietic cells transformed by kinase inhibitor-sensitive and kinase inhibitor-resistant forms of Fms-like tyrosine kinase 3 and BCR/ABL

Previous studies have shown that activation of the signal transducer and activator of transcription 5 (STAT5) plays an essential role in leukemogenesis mediated through constitutive activated protein tyrosine kinases (PTK). Because PIM-1 is a STAT5 target gene, we analyzed the role of the family of PIM serine/threonine kinases (PIM-1 to PIM-3) in PTK-mediated transformation of hematopoietic cells. Ba/F3 cells transformed to growth factor independence by various oncogenic PTKs (TEL/JAK2, TEL/TRKC, TEL/ABL, BCR/ABL, FLT3-ITD, and H4/PDGFbetaR) show abundant expression of PIM-1 and PIM-2. Suppression of PIM-1 activity had a negligible effect on transformation. In contrast, expression of kinase-dead PIM-2 mutant (PIM-2KD) led to a rapid decline of survival in Ba/F3 cells transformed by FLT3-ITD but not by other oncogenic PTKs tested. Coexpression of PIM-1KD and PIM-2KD abrogated growth factor-independent growth of Ba/F3 transformed by several PTKs, including BCR/ABL. Targeted down-regulation of PIM-2 by RNA interference (RNAi) selectively abrogated survival of Ba/F3 cells transformed by various Fms-like tyrosine kinase 3 (FLT3)-activating mutants [internal tandem duplication (ITD) and kinase domain] and attenuated growth of human cell lines containing FLT3 mutations. Interestingly, cells transformed by FLT3 and BCR/ABL mutations that confer resistance to small-molecule tyrosine kinase inhibitors were still sensitive to knockdown of PIM-2, or PIM-1 and PIM-2 by RNAi. Our observations indicate that combined inactivation of PIM-1 and PIM-2 interferes with oncogenic PTKs and suggest that PIMs are alternative therapeutic targets in PTK-mediated leukemia. Targeting the PIM kinase family could provide a new avenue to overcome resistance against small-molecule tyrosine kinase inhibitors.

RNA interference as a potential tool in the treatment of leukaemia

Leukaemias are often characterised by nonrandom chromosomal translocations that, at the molecular level, induce the activation of specific oncogenes or create novel chimeric genes. They have frequently been regarded as optimal targets for gene silencing approaches, as these single abnormalities may directly initiate or maintain the malignant process. Since the ground-breaking discovery that double-stranded RNA molecules 21 - 23 nucleotides in length, named small interfering RNAs (siRNAs), are able to elicit gene-specific inhibition also in mammalian cells, the interest of the scientific community has rapidly been drawn to the potential of these siRNAs for targeting oncogenic fusion genes in leukaemic cells. There has been a flurry of reports describing overexpressed or mutated genes that may also serve as attractive targets for therapeutic intervention by RNA silencing methods. Although this approach seems to be relatively straightforward, many problems remain to be solved before siRNAs may become clinically implemented as 'leukaemia drugs'. Difficulties in delivering siRNAs into the leukaemic cell, inefficient target mRNA cleavage, prolonged protein half-life in cancer cells, nonspecific side effects caused by targeting other genes than those originally thought, immunological reactions of the host organism against the siRNAs, such as interferon responses, or even acquired resistance mechanisms, such as escape mutants, should be overcome. This paper reviews the current knowledge regarding the use of siRNAs, either chemically synthesised or intracellular-generated via specialised expression constructs, in order to suppress the falsely activated oncogenes in haematopoietic malignancies.

Constitutive STAT5 activation specifically cooperates with the loss of p53 function in B-cell lymphomagenesis

Signal transducers and activator of transcription 5 (STAT5) A and B are transcriptional regulators that play a central role in cytokine signaling in the hematopoietic lineage and which are frequently activated in a persistent manner in human leukemia/lymphoma, as assessed by their constitutive tyrosine phosphorylation and DNA-binding activity. To study the intrinsic oncogenic properties of persistent STAT5 activation, we generated transgenic mice in which a constitutively activated point mutant of STAT5A, STAT5A(S711F), was expressed at physiological level in their lymphoid compartment. In this model, persistent STAT5 activation is weakly oncogenic, leading to the late emergence of clonal B-cell lymphoma/leukemia at a low incidence. In contrast, STAT5(S711F) was found to cooperate with the loss of function of the p53 tumor suppressor gene to both accelerate disease onset and to skew the large tumor spectrum that normally characterize p53-deficient mice to strongly favor B-cell lymphoma/leukemia. The emergence of STAT5A(S711F)-induced B-cell tumors is associated with the activation of STAT5 tyrosine phosphorylation and DNA-binding activity, indicating that activation of STAT5 oncogenic properties in transgenic STAT5A (TgSTAT5A) (S711F) mice involves the deregulation of STAT5 phosphorylation dynamics.

Interleukin-21 receptor (IL-21R) is up-regulated by CD40 triggering and mediates proapoptotic signals in chronic lymphocytic leukemia B cells

Interleukin-21 (IL-21) is a member of the IL-2 cytokine family, which mediates proliferation or growth arrest and apoptosis of normal B cells, depending on their activation state. Here we demonstrate that surface IL-21 receptor (R) is expressed at variable levels by chronic lymphocytic leukemia (CLL) B cells freshly isolated from 33 different patients. IL-21R expression was up-regulated following cell stimulation via surface CD40. Therefore, IL-21 effects were more evident in CD40-activated CLL B cells. IL-21 induced an early signaling cascade in CLL B cells, which included JAK-1 and JAK-3 autophosphorylation and tyrosine phosphorylation of STAT-1, STAT-3, and STAT-5. IL-21 signaling failed to stimulate CLL B-cell proliferation, but induced their apoptosis. In addition, IL-21 counteracted the proliferative and antiapoptotic signals delivered by IL-15 to CLL B cells. IL-21-mediated apoptosis involved activation of caspase-8 and caspase-3, cleavage of Bid to its active form t-Bid, and cleavage of PARP and of p27Kip-1. Recent data indicate that CLL B cells require interaction with the microenvironment for their survival and expansion. The present findings thus provide a set of new mechanisms involved in the balance between cell-survival and apoptotic signals in CLL B cells.

The biological functions of the versatile transcription factors STAT3 and STAT5 and new strategies for their targeted inhibition

Signal transducers and activators of transcription (STATs) comprise a unique family of transcription factors, which transmit the interactions of cytokines, hormones and growth factors with their cell surface receptors into transcriptional programs. The mechanism of STAT activation has been well-established and comprises tyrosine phosphorylation, dimerization, nuclear translocation, binding to specific DNA response elements, recruitment of co-activators or co-repressors and transcriptional induction or repression of target genes. Gene deletion, microarrays, proteomics and chromatin immunoprecipitation experiments have revealed target genes with a broad range of functions regulated by STAT3 and STAT5. In the mammary gland, STAT5-induced genes contribute mainly to the prolactin dependent lobulo-alveolar development, whereas STAT3 induced genes control apoptosis during involution. Crucial effects have also been observed in other tissues. The germ line deletion of STAT3 or STAT5 causes early embryonal or perinatal lethality in mice. STAT5 is also required for proliferation of T- and B-cells and hematopoietic stem cell self-renewal. Deregulated STAT activity is often found associated with tumorigenesis and activated STATs seem to be limiting components in tumor cells. This review summarizes the functions of STAT3 and STAT5 in different cell types and the strategies that are used to counteract their action in tumor cells.

Identification of Drosophila genes modulating Janus kinase/signal transducer and activator of transcription signal transduction

The JAK/STAT pathway was first identified in mammals as a signaling mechanism central to hematopoiesis and has since been shown to exert a wide range of pleiotropic effects on multiple developmental processes. Its inappropriate activation is also implicated in the development of numerous human malignancies, especially those derived from hematopoietic lineages. The JAK/STAT signaling cascade has been conserved through evolution and although the pathway identified in Drosophila has been closely examined, the full complement of genes required to correctly transduce signaling in vivo remains to be identified. We have used a dosage-sensitive dominant eye overgrowth phenotype caused by ectopic activation of the JAK/STAT pathway to screen 2267 independent, newly generated mutagenic P-element insertions. After multiple rounds of retesting, 23 interacting loci that represent genes not previously known to interact with JAK/STAT signaling have been identified. Analysis of these genes has identified three signal transduction pathways, seven potential components of the pathway itself, and six putative downstream pathway target genes. The use of forward genetics to identify loci and reverse genetic approaches to characterize them has allowed us to assemble a collection of genes whose products represent novel components and regulators of this important signal transduction cascade.

Hematopoietic malignancies associated with increased Stat5 and Bcl-xL expressions in Ink4a/Arf-deficient mice

The INK4a/ARF locus, which encodes the two distinct proteins p16(INK4a) and p14(ARF), is frequently altered in various hematological malignancies as well as in other types of cancers in humans. In this study, we surveyed tumors that had spontaneously developed in Ink4a/Arf-deficient mice with an inbred FVB/NJ genetic background. We found that an Ink4a/Arf-deficiency exerted more severe effects on the induction of hematopoietic malignancies in mice with an inbred FVB/NJ genetic background than in mice with a mixed genetic background. We also provided the evidence that this prevalence of hematopoietic malignancies in Ink4a/Arf-deficient mice is associated with the upregulated expressions of Stat5 and its transcriptional target, Bcl-x(L), both of which are involved in the regulation of hematopoiesis. These results suggest a possible implication of the Ink4a/Arf locus in the control of hematopoietic pathways by negatively regulating the Stat5-signalling pathways.