Adaptive secretion of granulocyte-macrophage colony-stimulating factor (GM-CSF) mediates imatinib and nilotinib resistance in BCR/ABL+ progenitors via JAK-2/STAT-5 pathway activation

A conserved ZFX/WNT3 axis modulates the growth and imatinib response of chronic myeloid leukemia stem/progenitor cells

BACKGROUND

Zinc finger protein X-linked (ZFX) has been shown to promote the growth of tumor cells, including leukemic cells. However, the role of ZFX in the growth and drug response of chronic myeloid leukemia (CML) stem/progenitor cells remains unclear.

METHODS

Real-time quantitative PCR (RT-qPCR) and immunofluorescence were used to analyze the expression of ZFX and WNT3 in CML CD34+ cells compared with normal control cells. Short hairpin RNAs (shRNAs) and clustered regularly interspaced short palindromic repeats/dead CRISPR-associated protein 9 (CRISPR/dCas9) technologies were used to study the role of ZFX in growth and drug response of CML cells. Microarray data were generated to compare ZFX-silenced CML CD34+ cells with their controls. Chromatin immunoprecipitation (ChIP) and luciferase reporter assays were performed to study the molecular mechanisms of ZFX to regulate WNT3 expression. RT-qPCR and western blotting were used to study the effect of ZFX on β-catenin signaling.

RESULTS

We showed that ZFX expression was significantly higher in CML CD34+ cells than in control cells. Overexpression and gene silencing experiments indicated that ZFX promoted the in vitro growth of CML cells, conferred imatinib mesylate (IM) resistance to these cells, and enhanced BCR/ABL-induced malignant transformation. Microarray data and subsequent validation revealed that WNT3 transcription was conservatively regulated by ZFX. WNT3 was highly expressed in CML CD34+ cells, and WNT3 regulated the growth and IM response of these cells similarly to ZFX. Moreover, WNT3 overexpression partially rescued ZFX silencing-induced growth inhibition and IM hypersensitivity. ZFX silencing decreased WNT3/β-catenin signaling, including c-MYC and CCND1 expression.

CONCLUSION

The present study identified a novel ZFX/WNT3 axis that modulates the growth and IM response of CML stem/progenitor cells.

Clonal interactions in cancer: integrating quantitative models with experimental and clinical data

Tumors consist of different genotypically distinct subpopulations-or subclones-of cells. These subclones can influence neighboring clones in a process called "clonal interaction." Conventionally, research on driver mutations in cancer has focused on their cell-autonomous effects that lead to an increase in fitness of the cells containing the driver. Recently, with the advent of improved experimental and computational technologies for investigating tumor heterogeneity and clonal dynamics, new studies have shown the importance of clonal interactions in cancer initiation, progression, and metastasis. In this review we provide an overview of clonal interactions in cancer, discussing key discoveries from a diverse range of approaches to cancer biology research. We discuss common types of clonal interactions, such as cooperation and competition, its mechanisms, and the overall effect on tumorigenesis, with important implications for tumor heterogeneity, resistance to treatment, and tumor suppression. Quantitative models-in coordination with cell culture and animal model experiments-have played a vital role in investigating the nature of clonal interactions and the complex clonal dynamics they generate. We present mathematical and computational models that can be used to represent clonal interactions and provide examples of the roles they have played in identifying and quantifying the strength of clonal interactions in experimental systems. Clonal interactions have proved difficult to observe in clinical data; however, several very recent quantitative approaches enable their detection. We conclude by discussing ways in which researchers can further integrate quantitative methods with experimental and clinical data to elucidate the critical-and often surprising-roles of clonal interactions in human cancers.

BCR/ABL1ΔE7-8-9 isoform contributes to tyrosine kinase inhibitor resistance in chronic myeloid leukemia

In chronic myeloid leukemia (CML) patients, the involvement of the BCR/ABL1 isoform in tyrosine kinase inhibitors (TKIs) resistance has attracted lots of attention. In this work, a novel isoform that encoded truncated protein due to the deletion of ABL1 exon7, 8, and 9 was reported and named BCR/ABL1ΔE7-8-9 here. This isoform was detected only in 10.2% of CML patients with inadequate responses to TKIs. BCR/ABL1Δexon7-8-9 isoform promoted S phase cell proliferation and reduced the expression of fusion gene and ABL1 phosphorylation level more slowly than that of control cells after TKIs treatment. The novel isoform has the qualities of a functional tyrosine kinase, localized in the cytoplasm, and could not be imported into the nucleus by TKIs. These results indicated that BCR/ABL1Δexon7-8-9 showed poorer sensitivity to imatinib and nilotinib than wild-type BCR/ABL1. According to molecular docking studies, nilotinib and imatinib present different binding sites and have a lower binding capacity with BCR/ABL1ΔE7-8-9 protein than the wild type. Our findings suggested that the novel isoform BCR/ABL1ΔE7-8-9 may contribute to TKIs resistance in CML due to its weakened TKIs binding ability. It enriched the mechanism of spliceosome involved in TKIs resistance. Monitoring the expression of BCR/ABL1ΔE7-8-9 helps guide the treatment of CML patients in the clinic.

Properties of Leukemic Stem Cells in Regulating Drug Resistance in Acute and Chronic Myeloid Leukemias

Notoriously known for their capacity to reconstitute hematological malignancies in vivo, leukemic stem cells (LSCs) represent key drivers of therapeutic resistance and disease relapse, posing as a major medical dilemma. Despite having low abundance in the bulk leukemic population, LSCs have developed unique molecular dependencies and intricate signaling networks to enable self-renewal, quiescence, and drug resistance. To illustrate the multi-dimensional landscape of LSC-mediated leukemogenesis, in this review, we present phenotypical characteristics of LSCs, address the LSC-associated leukemic stromal microenvironment, highlight molecular aberrations that occur in the transcriptome, epigenome, proteome, and metabolome of LSCs, and showcase promising novel therapeutic strategies that potentially target the molecular vulnerabilities of LSCs.

BCR-ABL1 Tyrosine Kinase Complex Signaling Transduction: Challenges to Overcome Resistance in Chronic Myeloid Leukemia

The constitutively active BCR-ABL1 tyrosine kinase, found in t(9;22)(q34;q11) chromosomal translocation-derived leukemia, initiates an extremely complex signaling transduction cascade that induces a strong state of resistance to chemotherapy. Targeted therapies based on tyrosine kinase inhibitors (TKIs), such as imatinib, dasatinib, nilotinib, bosutinib, and ponatinib, have revolutionized the treatment of BCR-ABL1-driven leukemia, particularly chronic myeloid leukemia (CML). However, TKIs do not cure CML patients, as some develop TKI resistance and the majority relapse upon withdrawal from treatment. Importantly, although BCR-ABL1 tyrosine kinase is necessary to initiate and establish the malignant phenotype of Ph-related leukemia, in the later advanced phase of the disease, BCR-ABL1-independent mechanisms are also in place. Here, we present an overview of the signaling pathways initiated by BCR-ABL1 and discuss the major challenges regarding immunologic/pharmacologic combined therapies.

Simvastatin potentiates the cell-killing activity of imatinib in imatinib-resistant chronic myeloid leukemia cells mainly through PI3K/AKT pathway attenuation and Myc downregulation

Constitutively activated BCR-ABL kinase is considered the driver event responsible in the initiation and development of chronic myeloid leukemia (CML). The advent of the first BCR-ABL inhibitor imatinib has significantly improved the clinical outcome of CML cases. However, resistance to imatinib occurs in 25-30% of CML patients. Due to the lack of effective therapeutic strategies, novel treatment approaches are urgently required for imatinib-resistant CML. Simvastatin, a well-known HMG-CoA reductase inhibitor that confers tremendous clinical benefits in cardiovascular diseases, has attracted mounting attentions for its potent antitumor effects on multiple tumor types. In this study, we demonstrated that simvastatin monotherapy was effective in diminishing cell viability in both imatinib-sensitive and imatinib-resistant CML cells, including T351I mutated cells, with the latter being less vulnerable to the simvastatin than the former. Notably, we found that simvastatin acted as a robust cytotoxic sensitizer of imatinib to kill imatinib-resistant and T315I mutated CML cells in vitro and in vivo. Mechanistically, the cooperative interaction of simvastatin and imatinib was associated with the inactivation of the PI3K/Akt signaling pathway, which was a classical downstream pro-survival cascade of the BCR-ABL kinase. In addition, this drug combination obviously decreased Myc expression through attenuation of canonical Wnt/β-catenin signaling and increased H3K27 trimethylation. Taken together, we provide attractive preclinical results for the combinatorial regimen of simvastatin and imatinib against imatinib-resistant and T315I mutated CML cells. This combined regimens warrants further clinical investigations in patients with imatinib-resistant CML.

Colony Stimulating Factor-1 and its Receptor in Gastrointestinal Malignant Tumors

Gastrointestinal malignant tumor is the fourth most common cancer in the world and the second cause of cancer death. Due to the susceptibility to lymphatic metastasis and liver metastasis, the prognosis of advanced tumor patients is still poor till now. With the development of tumor molecular biology, the tumor microenvironment and the cytokines, which are closely related to the proliferation, infiltration and metastasis, have become a research hotspot in life sciences. Colony stimulating factor-1 (CSF-1), a polypeptide chain cytokine, and its receptor CSF-1R are reported to play important roles in regulating tumor-associated macrophages in tumor microenvironment and participating in the occurrence and development in diversities of cancers. Targeted inhibition of the CSF-1/CSF-1R signal axis has broad application prospects in cancer immunotherapy. Here, we reviewed the biological characters of CSF-1/CSF-1R and their relationship with gastrointestinal malignancies.

GM-CSF mediates immune evasion via upregulation of PD-L1 expression in extranodal natural killer/T cell lymphoma

Background

Granulocyte-macrophage colony stimulating factor (GM-CSF) is a cytokine that is used as an immunopotentiator for anti-tumor therapies in recent years. We found that some of the extranodal natural killer/T cell lymphoma (ENKTL) patients with the treatment of hGM-CSF rapidly experienced disease progression, but the underlying mechanisms remain to be elucidated. Here, we aimed to explore the mechanisms of disease progression triggered by GM-CSF in ENKTL.

Methods

The mouse models bearing EL4 cell tumors were established to investigate the effects of GM-CSF on tumor growth and T cell infiltration and function. Human ENKTL cell lines including NK-YS, SNK-6, and SNT-8 were used to explore the expression of programmed death-ligand 1 (PD-L1) induced by GM-CSF. To further study the mechanisms of disease progression of ENKTL in detail, the mutations and gene expression profile were examined by next-generation sequence (NGS) in the ENKTL patient’s tumor tissue samples.

Results

The mouse-bearing EL4 cell tumor exhibited a faster tumor growth rate and poorer survival in the treatment with GM-CSF alone than in treatment with IgG or the combination of GM-CSF and PD-1 antibody. The PD-L1 expression at mRNA and protein levels was significantly increased in ENKTL cells treated with GM-CSF. STAT5A high-frequency mutation including p.R131G, p.D475N, p.F706fs, p.V707E, and p.S710F was found in 12 ENKTL cases with baseline tissue samples. Importantly, STAT5A-V706fs mutation tumor cells exhibited increased activation of STAT5A pathway and PD-L1 overexpression in the presence of GM-CSF.

Conclusions

These findings demonstrate that GM-CSF potentially triggers the loss of tumor immune surveillance in ENKTL patients and promotes disease progression, which is associated with STAT5 mutations and JAK2 hyperphosphorylation and then upregulates the expression of PD-L1. These may provide new concepts for GM-CSF application and new strategies for the treatment of ENKTL.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12943-021-01374-y.

Signal-transducing adapter protein-1 is required for maintenance of leukemic stem cells in CML

The family of signal-transducing adapter proteins (STAPs) has been reported to be involved in a variety of intracellular signaling pathways and implicated as transcriptional factors. We previously cloned STAP-2 as a c-Fms interacting protein and explored its effects on chronic myeloid leukemia (CML) leukemogenesis. STAP-2 binds to BCR-ABL, upregulates BCR-ABL phosphorylation, and activates its downstream molecules. In this study, we evaluated the role of STAP-1, another member of the STAP family, in CML pathogenesis. We found that the expression of STAP-1 is aberrantly upregulated in CML stem cells (LSCs) in patients’ bone marrow. Using experimental model mice, deletion of STAP-1 prolonged the survival of CML mice with inducing apoptosis of LSCs. The impaired phosphorylation status of STAT5 by STAP-1 ablation leads to downregulation of antiapoptotic genes, Bcl-2 and Bcl-xL. Interestingly, transcriptome analyses indicated that STAP-1 affects several signaling pathways related to BCR-ABL, JAK2, and PPARγ. This adapter protein directly binds to not only BCR-ABL, but also STAT5 proteins, showing synergistic effects of STAP-1 inhibition and BCR-ABL or JAK2 tyrosine kinase inhibition. Our results identified STAP-1 as a regulator of CML LSCs and suggested it to be a potential therapeutic target for CML.

Response and Resistance to BCR-ABL1-Targeted Therapies

Chronic myeloid leukemia (CML), caused by constitutively active BCR-ABL1 fusion tyrosine kinase, has served as a paradigm for successful application of molecularly targeted cancer therapy. The development of the tyrosine kinase inhibitor (TKI) imatinib allows patients with CML to experience near-normal life expectancy. Specific point mutations that decrease drug binding affinity can produce TKI resistance, and second- and third-generation TKIs largely mitigate this problem. Some patients develop TKI resistance without known resistance mutations, with significant heterogeneity in the underlying mechanism, but this is relatively uncommon, with the majority of patients with chronic phase CML achieving long-term disease control. In contrast, responses to TKI treatment are short lived in advanced phases of the disease or in BCR-ABL1-positive acute lymphoblastic leukemia, with relapse driven by both BCR-ABL1 kinase-dependent and -independent mechanisms. Additionally, the frontline CML treatment with second-generation TKIs produces deeper molecular responses, driving disease burden below the detection limit for a greater number of patients. For patients with deep molecular responses, up to half have been able to discontinue therapy. Current efforts are focused on identifying therapeutic strategies to drive deeper molecular responses, enabling more patients to attempt TKI discontinuation.

SRSF1 mediates cytokine-induced impaired imatinib sensitivity in chronic myeloid leukemia

Patients with chronic myeloid leukemia (CML) who are treated with tyrosine kinase inhibitors (TKIs) experience significant heterogeneity regarding depth and speed of responses. Factors intrinsic and extrinsic to CML cells contribute to response heterogeneity and TKI resistance. Among extrinsic factors, cytokine-mediated TKI resistance has been demonstrated in CML progenitors, but the underlying mechanisms remain obscure. Using RNA-sequencing, we identified differentially expressed splicing factors in primary CD34⁺ chronic phase (CP) CML progenitors and controls. We found SRSF1 expression to be increased as a result of both BCR-ABL1- and cytokine-mediated signaling. SRSF1 overexpression conferred cytokine independence to untransformed hematopoietic cells and impaired imatinib sensitivity in CML cells, while SRSF1 depletion in CD34⁺ CP CML cells prevented the ability of extrinsic cytokines to decrease imatinib sensitivity. Mechanistically, PRKCH and PLCH1 were upregulated by elevated SRSF1 levels, and contributed to impaired imatinib sensitivity. Importantly, very high SRSF1 levels in the bone marrow of CML patients at presentation correlated with poorer clinical TKI responses. In summary, we find SRSF1 levels to be maintained in CD34⁺ CP CML progenitors by cytokines despite effective BCR-ABL1 inhibition, and that elevated levels promote impaired imatinib responses. Together, our data support an SRSF1/PRKCH/PLCH1 axis in contributing to cytokine-induced impaired imatinib sensitivity in CML.

The emergence of drug resistance to targeted cancer therapies: Clinical evidence

For many decades classical anti-tumor therapies included chemotherapy, radiation and surgery; however, in the last two decades, following the identification of the genomic drivers and main hallmarks of cancer, the introduction of therapies that target specific tumor-promoting oncogenic or non-oncogenic pathways, has revolutionized cancer therapeutics. Despite the significant progress in cancer therapy, clinical oncologists are often facing the primary impediment of anticancer drug resistance, as many cancer patients display either intrinsic chemoresistance from the very beginning of the therapy or after initial responses and upon repeated drug treatment cycles, acquired drug resistance develops and thus relapse emerges, resulting in increased mortality. Our attempts to understand the molecular basis underlying these drug resistance phenotypes in pre-clinical models and patient specimens revealed the extreme plasticity and adaptive pathways employed by tumor cells, being under sustained stress and extensive genomic/proteomic instability due to the applied therapeutic regimens. Subsequent efforts have yielded more effective inhibitors and combinatorial approaches (e.g. the use of specific pharmacologic inhibitors with immunotherapy) that exhibit synergistic effects against tumor cells, hence enhancing therapeutic indices. Furthermore, new advanced methodologies that allow for the early detection of genetic/epigenetic alterations that lead to drug chemoresistance and prospective validation of biomarkers which identify patients that will benefit from certain drug classes, have started to improve the clinical outcome. This review discusses emerging principles of drug resistance to cancer therapies targeting a wide array of oncogenic kinases, along with hedgehog pathway and the proteasome and apoptotic inducers, as well as epigenetic and metabolic modulators. We further discuss mechanisms of resistance to monoclonal antibodies, immunomodulators and immune checkpoint inhibitors, potential biomarkers of drug response/drug resistance, along with possible new therapeutic avenues for the clinicians to combat devastating drug resistant malignancies. It is foreseen that these topics will be major areas of focused multidisciplinary translational research in the years to come.

Bone marrow-derived mesenchymal stromal cells promote resistance to tyrosine kinase inhibitors in chronic myeloid leukemia via the IL-7/JAK1/STAT5 pathway

Chronic myeloid leukemia (CML) is caused by the fusion of the BCR activator of RhoGEF and GTPase activating protein (BCR) and ABL proto-oncogene, the nonreceptor tyrosine kinase (ABL) genes. Although the tyrosine kinase inhibitors (TKIs) imatinib (IM) and nilotinib (NI) have remarkable efficacy in managing CML, the malignancies in some patients become TKI-resistant. Here, we isolated bone marrow (BM)-derived mesenchymal stem cells (MSCs) from several CML patients by Ficoll-Hypaque density-gradient centrifugation for coculture with K562 and BV173 cells with or without TKIs. We used real-time quantitative PCR to assess the level of interleukin 7 (IL-7) expression in the MSCs and employed immunoblotting to monitor protein expression in the BCR/ABL, phosphatidylinositol 3-kinase (PI3K)/AKT, and JAK/STAT signaling pathways. We also used a xenograft tumor model to examine the in vivo effect of different MSCs on CML cells. MSCs from patients with IM-resistant CML protected K562 and BV173 cells against IM- or NI-induced cell death, and this protection was due to increased IL-7 secretion from the MSCs. Moreover, IL-7 levels in the BM of patients with IM-resistant CML were significantly higher than in healthy donors or IM-sensitive CML patients. IL-7 elicited IM and NI resistance via BCR/ABL-independent activation of JAK1/STAT5 signaling, but not of JAK3/STAT5 or PI3K/AKT signaling. IL-7 or JAK1 gene knockdown abrogated IL-7-mediated STAT5 phosphorylation and IM resistance in vitro and in vivo Because high IL-7 levels in the BM mediate TKI resistance via BCR/ABL-independent activation of JAK1/STAT5 signaling, combining TKIs with IL-7/JAK1/STAT5 inhibition may have significant utility for managing CML.

Current outlook on drug resistance in chronic myeloid leukemia (CML) and potential therapeutic options

Chronic myeloid leukemia cells are armed with several resistance mechanisms that can make current drugs ineffective. A better understanding of resistance mechanisms is yielding new approaches to management of the disease. Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm the hallmark of which, the breakpoint cluster region-Abelson (BCR-ABL) oncogene, has been the target of tyrosine kinase inhibitors (TKIs), which have significantly improved the survival of patients with CML. However, because of an increase in TKI resistance, it is becoming imperative to identify resistance mechanisms so that drug therapies can be better prescribed and new agents developed. In this review, we discuss the various BCR-ABL-dependent and -independent mechanisms of resistance observed in CML, and the range of therapeutic solutions available to overcome such resistance and to ultimately improve the survival of patients with CML.

The novel immunosuppressant prenylated quinolinecarboxylic acid-18 (PQA-18) suppresses macrophage differentiation and cytotoxicity in xenotransplantation

Innate immunity plays a major role in xenograft rejection. However, the majority of immunosuppressants focus on inhibiting acquired immunity and not innate immunity. Therefore, a novel immunosuppressant suitable for use in conjunction with xenografts continues to be needed. It has been reported that prenylated quinolinecarboxylic acid-18 (PQA-18), a p21-activated kinase 2 (PAK2) inhibitor, exerts an immunosuppressive function on T cells. Hence, the possibility exists that PQA-18 might be used in conjunction with xenografts, which prompted us to investigate the efficacy of PQA-18 on macrophages compared with Tofacitinib, a janus kinase (JAK) inhibitor. Initial experiments confirmed that PQA-18 is non-toxic to swine endothelial cells (SECs) and human monocytes. Both PQA-18 and Tofacitinib suppressed macrophage-mediated cytotoxicity in both the differentiation and effector phases. Both PQA-18 and tofacitinib suppressed the expression of HLA-ABC by macrophages. However, contrary to Tofacitinib, PQA-18 also significantly suppressed the expression of CD11b, HLA-DR and CD40 on macrophages. PQA-18 significantly suppressed CCR7 expression on day 3 and on day 6, but Tofacitinib-induced suppression only on day 6. In a mixed lymphocyte reaction (MLR) assay, PQA-18 was found to suppress Interleukin-2 (IL-2)-stimulated T cell proliferation to a lesser extent than Tofacitinib. However, PQA-18 suppressed xenogeneic-induced T cell proliferation more strongly than Tofacitinib on day 3 and the suppression was similar on day 7. In conclusion, PQA-18 has the potential to function as an immunosuppressant for xenotransplantation.

Mechanisms of Resistance to Targeted Therapies in Chronic Myeloid Leukemia

Patients with newly diagnosed chronic myeloid leukemia (CML) usually received as first-line treatment a first- or second-generation tyrosine kinase inhibitor (TKI). Although initial responses are high, therapy fails in up to 40% of patients and initial response is lost within 2 years in approximately 25% of patients. In the last few years, intensive efforts have been spent to explain treatment failure, and different mechanisms of resistance have been identified, ranging from BCR-ABL1 kinase domain mutations to lack of adherence to therapy. In this review, we briefly summarize the clinical efficacy of approved TKIs and describe the main mechanisms of TKI resistance.

BM microenvironmental protection of CML cells from imatinib through Stat5/NF-κB signaling and reversal by Wogonin

Constitutive Stat5 activation enhanced cell survival and resistance to imatinib (IM) in chronic myelogenous leukemia (CML) cells. However, the mechanism of Stat5 activation in mediating resistance to IM in bone marrow (BM) microenvironment has not been evaluated precisely. In this study, we reported HS-5-derived conditioned medium (CM) significantly enhanced IM resistance in K562 and KU812. Interestingly, upregulation of the proportion of CD34+ subpopulation was found in CML cells. Subsequently, the BCR/ABL-independent activation of Stat5 increased P-glycoprotein (P-gp) activity in CM-mediated protection of CML stem cells (LSCs) from IM. Further research revealed Stat5 activation increased the DNA binding activity of NF-κB though binding of p-Stat5 and p-RelA in nucleus. Moreover, highly acetylated RelA was required for Stat5-mediated RelA nuclear binding. The study further confirmed that Wogonin potentiated the inhibitory effects of IM on leukemia development by suppressing Stat5 pathway both in CM model and the K562 xenograft model. In summary, results clearly demonstrated BCR/ABL-independent Stat5 survival pathway could contribute to resistance of CML LSCs to IM in BM microenvironment and suggested that natural durgs effectively inhibiting Stat5 may be an attractive approach to overcome resistance to BCR/ABL kinase inhibitors.

Persistent STAT5-mediated ROS production and involvement of aberrant p53 apoptotic signaling in the resistance of chronic myeloid leukemia to imatinib

The persistent activation of signal transducer and activator of transcription 5 (STAT5) may principally be attributed to breakpoint cluster region (BCR)-Abelson murine leukemia viral oncogene homolog 1 (ABL1), and have multi-faceted effects in the development of chronic myeloid leukemia (CML). The p53 protein network regulates important mechanisms in DNA damage repair, cell cycle regulation/checkpoints, and cell senescence and apoptosis, as demonstrated by its ability to positively regulate the expression of various pro-apoptotic genes, including B-cell lymphoma-2 (Bcl-2) and Bcl-2-associated X protein (Bax). In the present study, it was observed that the mRNA levels of STAT5A and STAT5B were upregulated in patients with imatinib-resistant CML and in the imatinib-resistant K562/G CML cell line. In addition, increased expression of STAT5 was observed in the BCR-ABL1 mutation group, compared with that in the non-BCR-ABL1 mutation group, regardless of patient imatinib resistance state. Elevated levels of reactive oxygen species (ROS) and DNA double-strand breaks were identified in K562/G cells using flow cytometric and phosphorylated H2AX (γ-H2AX) foci immunofluorescence assays, respectively, compared with the imatinib-sensitive K562 cells. The levels of intracellular ROS and γ-H2AX were decreased by the ROS scavenger (N-acetylcysteine), and ROS levels were also markedly reduced by STAT5 inhibitor (SH-4-54). In addition, imatinib significantly inhibited the proliferation of K562 and K562/G cells, with half maximal inhibitory concentration values of 0.17±0.07 and 14.78±0.43 µM, respectively, and the levels of apoptosis were significantly different between K562 and K562/G cells following treatment with imatinib. The mRNA and protein levels of STAT5 and mouse double minute 2 homolog (MDM2) were upregulated, whereas those of Bax were downregulated in K562/G cells, as determined using western blot analysis. Additionally, although the two cell lines exhibited relatively low protein expression levels of p53, lower levels of p53 and TPp53BP1 transcripts were detected in the K562/G cells. Taken together, these findings suggest that the resistance of CML to the tyrosine kinase inhibitor, imatinib, may be associated with persistent STAT5-mediated ROS production, and the abnormality of the p53 pathway.

Chronic Myeloid Leukemia in the Era of Tyrosine Kinase Inhibitors: An Evolving Paradigm of Molecularly Targeted Therapy

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm, characterized by the unrestrained expansion of pluripotent hematopoietic stem cells. CML was the first malignancy in which a unique chromosomal abnormality was identified and a pathophysiologic association was suggested. The hallmark of CML is a reciprocal chromosomal translocation between the long arms of chromosomes 9 and 22, t(9; 22)(q34; q11), creating a derivative 9q+ and a shortened 22q-. The latter, known as the Philadelphia (Ph) chromosome, harbors the breakpoint cluster region-abelson (BCR-ABL) fusion gene, encoding the constitutively active BCR-ABL tyrosine kinase that is necessary and sufficient for initiating CML. The successful implementation of tyrosine kinase inhibitors (TKIs) for the treatment of CML remains a flagship for molecularly targeted therapy in cancer. TKIs have changed the clinical course of CML; however, some patients nonetheless demonstrate primary or secondary resistance to such therapy and require an alternative therapeutic strategy. Therefore, the assessment of early response to treatment with TKIs has become an important tool in the clinical monitoring of CML patients. Although mutations in the BCR-ABL have proven to be the most prominent mechanism of resistance to TKIs, other mechanisms-either rendering the leukemic cells still dependent on BCR-ABL activity or supporting oncogenic properties of the leukemic cells independent of BCR-ABL signaling-have been identified. This article provides an overview of the current understanding of CML pathogenesis; recommendations for diagnostic tools, treatment strategies, and management guidelines; and highlights the BCR-ABL-dependent and -independent mechanisms that contribute to the development of resistance to TKIs.

Wogonin reversed resistant human myelogenous leukemia cells via inhibiting Nrf2 signaling by Stat3/NF-κB inactivation

Constitutive NF-E2-related factor 2 (Nrf2, NFE2L2) activation has been recently reported to play a pivotal role in enhancing cell survival and resistance to anticancer drugs in many tumors. Wogonin had strong reversal potency via reduction of Nrf2 mRNA in Adriamycin (ADR)-induced resistant human chronic myelogenous leukemia (CML) K562/A02, but the mechanism of reduction of Nrf2 mRNA was still unclear. In this study, we aimed to delineate the mechanism by which Wogonin suppressed transcription of Nrf2 in resistant CML cells and further evaluate the reversal effects of Wogonin on the established animal models. Data indicated that Wogonin suppressed transcription of Nrf2 by NF-κB inactivation. Wogonin inhibited the binding of p65 to Nrf2 by suppression of the κB-binding activity. Further research revealed the κB2 site was responsible for the decreased Nrf2 by Wogonin in resistant K562 cells. Furthermore, reduction of pY705-Stat3 was involved in inhibition of the binding of p65 to Nrf2 by Wogonin. In vivo, Wogonin potentiated the inhibitory effect of ADR on leukemia development by suppressing pY705-Stat3 and Nrf2 signaling. In summary, these results demonstrated Wogonin could combat chemoresistance effectively through inhibiting Nrf2 via Stat3/NF-κB signaling, and supported that Wogonin can be developed into an efficient natural sensitizer for resistant human myelogenous leukemia.

CSF2 Overexpression Is Associated with STAT5 Phosphorylation and Poor Prognosis in Patients with Urothelial Carcinoma

Background: Urothelial carcinoma (UC) commonly occurs in the urinary bladder (UB) and rarely in upper the upper urinary tract (UT). Its molecular pathogenesis, however, remains obscure. Though the constitutive phosphorylation of Signal Transducer and Activator of Transcription 5 (STAT5) is an important part of carcinogenesis generally, researchers have not systematically investigated this process specifically in relation to UC. The present study addresses this gap. Through data mining a published transcriptomic database of UBUCs (GSE32894), it identified Colony Stimulating Factor 2 (CSF2) as the stepwise upregulated gene of much significance among those related to the positive regulation of tyrosine phosphorylation of STAT5 (GO:0042523). Since the phosphorylation of STAT5, a key process in the development of UC, is closely associated with CSF2, we then examine CSF2 transcript and protein expression, justifying their association with clinicopathological features and survival in our well-established cohort of patients with UC.

Design: Laser capture microdissection in conjunction with real-time qRT-PCR are used to detect CSF2 transcript levels in 24 UBUCs and 6 non-tumor urothelium samples. We then used the H-score method to evaluate the immunohistochemistry in order to determine CSF2 protein expression in 296 UBUCs and 340 UTUCs, respectively. After correlating protein expression status with key clinicopathological features, the prognostic significance of CSF2 protein expression was determined for disease-specific survival (DSS) and metastasis-free survival (MeFS).

Results: We exclusively detected the CSF2 transcript, which was stepwise upregulated in tumor lesions (p=0.010). In both groups of UC we found overexpression of CSF2 significantly related to incremental pT status (UTUC, p=0.011; UBUC, p<0.001), as well as with perineural invasion (UTUC, p=0.002; UBUC, p=0.001). Univariate analysis found a close correlation between CSF2 overexpression and unfavorable prognosis for both DSS (UTUC, p=0.0001; UBUC, p<0.0001) and MeFS (UTUC, p=0.0001; UBUC, p=0.0002). High expression of CSF2 still remained prognostically for DSS (UTUC, p=0.015; UBUC, p=0.004) and MeFS (UTUC, p=0.008; UBUC, p=0.027) in multivariate comparison.

Conclusion: Our data showed that overexpression of CSF2 was inferred in advanced disease status and poor clinical outcomes for both UTUC and UBUC patients, suggesting that CSF2 may serve as an important prognosticator and a potential therapeutic target of UC.

Metabolic characterization of imatinib-resistant BCR-ABL T315I chronic myeloid leukemia cells indicates down-regulation of glycolytic pathway and low ROS production

Long-term imatinib treatment induces drug-resistant chronic myeloid leukemia (CML) cells harboring T315I gate keeper mutation of breakpoint cluster region (BCR)-ABL oncogenic kinase. However, although cell proliferation is coupled with cellular energy status in CML carcinogenesis, the metabolic characteristics of T315I-mutant CML cells have never been investigated. Here, we analyzed cell proliferation activities and metabolic phenotypes, including cell proliferation, oxygen consumption, lactate production, and redox state in the KBM5 (imatinib-sensitive) and KBM5-T315I (imatinib-resistant) CML cell lines. Interestingly, KBM5-T315I cells showed decreased cell proliferation, lactate production, fatty acid synthesis, ROS production, and down regulation of mRNA expression related to ROS scavengers, such as SOD2, catalase, GCLm, and GPx1. Taken together, our data demonstrate that the lower growth ability of KBM5-T315I CML cells might be related to the decreased expression of glycolysis-related genes and ROS levels, and this will be used to identify therapeutic targets for imatinib resistance in CML.

Knockout Serum Replacement Promotes Cell Survival by Preventing BIM from Inducing Mitochondrial Cytochrome C Release

Knockout serum replacement (KOSR) is a nutrient supplement commonly used to replace serum for culturing stem cells. We show here that KOSR has pro-survival activity in chronic myelogenous leukemia (CML) cells transformed by the BCR-ABL oncogene. Inhibitors of BCR-ABL tyrosine kinase kill CML cells by stimulating pro-apoptotic BIM and inhibiting anti-apoptotic BCL2, BCLxL and MCL1. We found that KOSR protects CML cells from killing by BCR-ABL inhibitors—imatinib, dasatinib and nilotinib. The protective effect of KOSR is reversible and not due to the selective outgrowth of drug-resistant clones. In KOSR-protected CML cells, imatinib still inhibited the BCR-ABL tyrosine kinase, reduced the phosphorylation of STAT, ERK and AKT, down-regulated BCL2, BCLxL, MCL1 and up-regulated BIM. However, these pro-apoptotic alterations failed to cause cytochrome c release from the mitochondria. With mitochondria isolated from KOSR-cultured CML cells, we showed that addition of recombinant BIM protein also failed to cause cytochrome c release. Besides the kinase inhibitors, KOSR could protect cells from menadione, an inducer of oxidative stress, but it did not protect cells from DNA damaging agents. Switching from serum to KOSR caused a transient increase in reactive oxygen species and AKT phosphorylation in CML cells that were protected by KOSR but not in those that were not protected by this nutrient supplement. Treatment of KOSR-cultured cells with the PH-domain inhibitor MK2206 blocked AKT phosphorylation, abrogated the formation of BIM-resistant mitochondria and stimulated cell death. These results show that KOSR has cell-context dependent pro-survival activity that is linked to AKT activation and the inhibition of BIM-induced cytochrome c release from the mitochondria.

Cytokine Regulation of Microenvironmental Cells in Myeloproliferative Neoplasms

The term myeloproliferative neoplasms (MPN) refers to a heterogeneous group of diseases including not only polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), but also chronic myeloid leukemia (CML), and systemic mastocytosis (SM). Despite the clinical and biological differences between these diseases, common pathophysiological mechanisms have been identified in MPN. First, aberrant tyrosine kinase signaling due to somatic mutations in certain driver genes is common to these MPN. Second, alterations of the bone marrow microenvironment are found in all MPN types and have been implicated in the pathogenesis of the diseases. Finally, elevated levels of proinflammatory and microenvironment-regulating cytokines are commonly found in all MPN-variants. In this paper, we review the effects of MPN-related oncogenes on cytokine expression and release and describe common as well as distinct pathogenetic mechanisms underlying microenvironmental changes in various MPN. Furthermore, targeting of the microenvironment in MPN is discussed. Such novel therapies may enhance the efficacy and may overcome resistance to established tyrosine kinase inhibitor treatment in these patients. Nevertheless, additional basic studies on the complex interplay of neoplastic and stromal cells are required in order to optimize targeting strategies and to translate these concepts into clinical application.

Interferon gamma modulates sensitivity of CML cells to tyrosine kinase inhibitors

Immune effector cells such as T and NK cells can efficiently eliminate tumor cells. However, when activating oncogenic signaling pathways or protective mechanisms against cell death are active, immune cells can also confer therapy resistance. Here, we analyzed the role of activated T and NK cells and released cytokines on tyrosine kinase inhibitors imatinib and nilotinib - mediated apoptosis induction and proliferation of chronic myelogenous leukemia (CML) cells. Incubation of CML cells with activated, but not with resting CD3⁺ T cells or with activated NK cells significantly inhibited TKI-induced apoptosis induction in CML cells as quantified by nuclear fragmentation assays. Transwell experiments revealed a critical role for T or NK cell-derived cytokines for CML cell protection. Accordingly, CML cells treated with IFNγ also showed a clearly reduced sensitivity to TKI-mediated cell death induction and inhibition of proliferation. In contrast, IFNα or other pro-inflammatory mediators and cytokines, such as TNFα and GM-CSF did not impair TKI-induced apoptosis in CML cells. On a molecular level, IFNγ-exposed CML cells showed a significantly reduced caspase-3 activation and PARP-1 cleavage as well as an increased expression of anti-apoptotic molecule xIAP. Finally, IFNγ diminished TKI-induced downregulation of Jak-2 and STAT-5 phosphorylation and increased nuclear expression of RUNX-1, which may at least in part contribute to the reduced sensitivity to TKI effects. Our results demonstrate that IFNγ released by activated T or NK cells may interfere with the therapeutic effects of TKI in CML. Our findings may have important implications for the understanding of inflammation-mediated BCR-ABL independent resistance mechanisms.

Selective JAK2/ABL dual inhibition therapy effectively eliminates TKI-insensitive CML stem/progenitor cells

Imatinib Mesylate (IM) and other tyrosine kinase inhibitor (TKI) therapies have had a major impact on the treatment of chronic myeloid leukemia (CML). However, TKI monotherapy is not curative, with relapse and persistence of leukemic stem cells (LSCs) remaining a challenge. We have recently identified an AHI-1-BCR-ABL-JAK2 protein complex that contributes to the transforming activity of BCR-ABL and IM-resistance in CML stem/progenitor cells. JAK2 thus emerges as an attractive target for improved therapies, but off-target effects of newly developed JAK2 inhibitors on normal hematopoietic cells remain a concern. We have examined the biological effects of a highly selective, orally bioavailable JAK2 inhibitor, BMS-911543, in combination with TKIs on CD34⁺ treatment-naïve IM-nonresponder cells. Combination therapy reduces JAK2/STAT5 and CRKL activities, induces apoptosis, inhibits proliferation and colony growth, and eliminates CML LSCs in vitro. Importantly, BMS-911543 selectively targets CML stem/progenitor cells while sparing healthy stem/progenitor cells. Oral BMS-911543 combined with the potent TKI dasatinib more effectively eliminates infiltrated leukemic cells in hematopoietic tissues than TKI monotherapy and enhances survival of leukemic mice. Dual targeting BCR-ABL and JAK2 activities in CML stem/progenitor cells may consequently lead to more effective disease eradication, especially in patients at high risk of TKI resistance and disease progression.

Chronic myeloid leukemia: advances in understanding disease biology and mechanisms of resistance to tyrosine kinase inhibitors

The successful implementation of tyrosine kinase inhibitors (TKIs) for the treatment of chronic myeloid leukemia (CML) remains a flagship for molecularly targeted therapy in cancer. This focused review highlights critical elements of the underlying biology of CML and provides a summary of the molecular mechanisms that lead to TKI resistance: BCR-ABL1 mutation-based resistance and therapy escape through alternative pathway activation despite inhibition of BCR-ABL1 tyrosine kinase activity. We direct attention to the most current manifestations of these issues, including emergence of pan-TKI-resistant BCR-ABL1 compound mutants, new strategies for identification and therapeutic targeting of alternative pathways, and the exciting, controversial topic of cessation of TKI therapy leading to durable treatment-free remissions for a subset of patients. Further gains in our understanding of the biology of Philadelphia chromosome-positive (Ph-positive) leukemia and mechanisms of resistance to BCR-ABL1 TKIs will benefit patients and also provide a blueprint for similar discovery in other cancers.

NFATc1 as a therapeutic target in FLT3-ITD-positive AML

Internal tandem duplications (ITD) in the Fms-related tyrosine kinase 3 receptor (FLT3) are associated with a dismal prognosis in acute myeloid leukemia (AML). FLT3 inhibitors such as sorafenib may improve outcome, but only few patients display long-term responses, prompting the search for underlying resistance mechanisms and therapeutic strategies to overcome them. Here we identified that the nuclear factor of activated T cells, NFATc1, is frequently overexpressed in FLT3-ITD-positive (FLT3-ITD+) AML. NFATc1 knockdown using inducible short hairpin RNA or pharmacological NFAT inhibition with cyclosporine A (CsA) or VIVIT significantly augmented sorafenib-induced apoptosis of FLT3-ITD+ cells. CsA also potently overcame sorafenib resistance in FLT3-ITD+ cell lines and primary AML. Vice versa, de novo expression of a constitutively nuclear NFATc1-mutant mediated instant and robust sorafenib resistance in vitro. Intriguingly, FLT3-ITD+ AML patients (n=26) who received CsA as part of their rescue chemotherapy displayed a superior outcome when compared with wild-type FLT3 (FLT3-WT) AML patients. Our data unveil NFATc1 as a novel mediator of sorafenib resistance in FLT3-ITD+ AML. CsA counteracts sorafenib resistance and may improve treatment outcome in AML by means of inhibiting NFAT.

HSP90 inhibitor AUY922 induces cell death by disruption of the Bcr-Abl, Jak2 and HSP90 signaling network complex in leukemia cells

The Bcr-Abl protein is an important client protein of heat shock protein 90 (HSP90). We evaluated the inhibitory effects of the HSP90 ATPase inhibitor AUY922 on 32D mouse hematopoietic cells expressing wild-type Bcr-Abl (b3a2, 32Dp210) and mutant Bcr-Abl imatinib (IM)-resistant cell lines. Western blotting results of fractions from gel filtration column chromatography of 32Dp210 cells showed that HSP90 together with Bcr-Abl, Jak2 Stat3 and several other proteins co-eluted in peak column fractions of a high molecular weight network complex (HMWNC). Co-IP results showed that HSP90 directly bound to Bcr-Abl, Jak2, Stat 3 and Akt. The associations between HSP90 and Bcr-Abl or Bcr-Abl kinase domain mutants (T315I and E255K) were interrupted by AUY922 treatment. Tyrosine phosphorylation of Bcr-Abl showed a dose-dependent decrease in 32Dp210T315I following AUY922 treatment for 16h. AUY922 also markedly inhibited cell proliferation of both IM-sensitive 32Dp210 (IC50 =6 nM) and IM-resistant 32Dp210T315I cells (IC50 ≈6 nM) and human KBM-5R/KBM-7R cell lines (IC50 =50 nM). AUY922 caused significant G1 arrest in 32Dp210 cells but not in T315I or E255K cells. AUY922 efficiently induced apoptosis in 32Dp210 (IC50 =10 nM) and T315I or E255K lines with IC50 around 20 to 50 nM. Our results showed that Bcr-Abl and Jak2 form HMWNC with HSP90 in CML cells. Inhibition of HSP90 by AUY922 disrupted the structure of HMWNC, leading to Bcr-Abl degradation, nhibiting cell proliferation and inducing apoptosis. Thus, inhibition of HSP90 is a powerful way to inhibit not only IM-sensitive CML cells but also IM-resistant CML cells.

Leukemia stem cells: the root of chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder characterized by a chromosome translocation that generates the Bcr-Abl oncogene encoding a constitutive kinase activity. Despite remarkable success in controlling CML at chronic phase by Bcr-Abl tyrosine kinase inhibitors (TKIs), a significant proportion of CML patients treated with TKIs develop drug resistance due to the inability of TKIs to kill leukemia stem cells (LSCs) that are responsible for initiation, drug resistance, and relapse of CML. Therefore, there is an urgent need for more potent and safer therapies against leukemia stem cells for curing CML. A number of LSC-associated targets and corresponding signaling pathways, including CaMKII-γ, a critical molecular switch for co-activating multiple LSC-associated signaling pathways, have been identified over the past decades and various small inhibitors targeting LSC are also under development. Increasing evidence shows that leukemia stem cells are the root of CML and targeting LSC may offer a curable treatment option for CML patients. This review summarizes the molecular biology of LSC and its-associated targets, and the potential clinical application in chronic myeloid leukemia.

Leukemia stem cells: the root of chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder characterized by a chromosome translocation that generates the Bcr-Abl oncogene encoding a constitutive kinase activity. Despite remarkable success in controlling CML at chronic phase by Bcr-Abl tyrosine kinase inhibitors (TKIs), a significant proportion of CML patients treated with TKIs develop drug resistance due to the inability of TKIs to kill leukemia stem cells (LSCs) that are responsible for initiation, drug resistance, and relapse of CML. Therefore, there is an urgent need for more potent and safer therapies against leukemia stem cells for curing CML. A number of LSC-associated targets and corresponding signaling pathways, including CaMKII-γ, a critical molecular switch for co-activating multiple LSC-associated signaling pathways, have been identified over the past decades and various small inhibitors targeting LSC are also under development. Increasing evidence shows that leukemia stem cells are the root of CML and targeting LSC may offer a curable treatment option for CML patients. This review summarizes the molecular biology of LSC and its-associated targets, and the potential clinical application in chronic myeloid leukemia.

Diagnosing and managing advanced chronic myeloid leukemia

Clinical staging of chronic myeloid leukemia (CML) distinguishes between chronic phase (CP-CML), accelerated phase (AP-CML), and blastic phase (BP-CML), reflecting its natural history in the absence of effective therapy. Morphologically, transformation from CP-CML to AP/BP-CML is characterized by a progressive or sudden loss of differentiation. Multiple different somatic mutations have been implicated in transformation from CP-CML to AP/BC-CML, but no characteristic mutation or combination of mutations have emerged. Gene expression profiles of AP-CML and BP-CML are similar, consistent with biphasic evolution at the molecular level. Gene expression of tyrosine kinase inhibitor (TKI)-resistant CP-CML and second CP-CML resemble AP/BP-CML, suggesting that morphology alone is a poor predictor of biologic behavior. At the clinical level, progression to AP/BP-CML or resistance to first-line TKI therapy distinguishes a good risk condition with survival close to the general population from a disease likely to reduce survival. Progression while receiving TKI therapy is frequently caused by mutations in the target kinase BCR-ABL1, but progression may occur in the absence of explanatory BCR-ABL1 mutations, suggesting involvement of alternative pathways. Identifying patients in whom milestones of TKI response fail to occur or whose disease progress while receiving therapy requires appropriate molecular monitoring. Selection of salvage TKI depends on prior TKI history, comorbidities, and BCR-ABL1 mutation status. Despite the introduction of novel TKIs, therapy of AP/BP-CML remains challenging and requires accepting modalities with substantial toxicity, such as hematopoietic stem cell transplantation (HSCT).

Chaetocin antileukemia activity against chronic myelogenous leukemia cells is potentiated by bone marrow stromal factors and overcomes innate imatinib resistance

Chronic myelogenous leukemia (CML) is maintained by a minor population of leukemic stem cells (LSCs) that exhibit innate resistance to tyrosine kinase inhibitors (TKIs) targeting BCR-ABL. Innate resistance can be induced by secreted bone marrow stromal cytokines and growth factors (BMSFs) that protect CML-LSCs from TKIs, resulting in minimal residual disease. Developing strategies to eradicate innate TKI resistance in LSCs is critical for preventing disease relapse. Cancer cells balance reactive oxygen species (ROS) at higher than normal levels, promoting their proliferation and survival, but also making them susceptible to damage by ROS-generating agents. Bcr-Abl increases cellular ROS levels, which can be reduced with TKI inhibitors, whereas, BMSFs increase ROS levels. We hypothesized that BMSF-mediated increases in ROS would trigger ROS damage in TKI-treated CML-LSCs when exposed to chaetocin, a mycotoxin that imposes oxidative stress by inhibiting thioredoxin reductase-1. Here, we showed that chaetocin suppressed viability and colony formation, and induced apoptosis of the murine hematopoietic cell line TonB210 with and without Bcr-Abl expression, and these effects were potentiated by BMSFs. In contrast, imatinib activities in Bcr-Abl-positive TonB210 cells were inhibited by BMSFs. Further, BMSFs did not inhibit imatinib activities when TonB210 cells expressing Bcr-Abl were cotreated with chaetocin. Chaetocin showed similar activities against LSC-enriched CML cell populations isolated from a murine transplant model of CML blast crisis that were phenotypically negative for lineage markers and positive for Sca-1 and c-Kit (CML-LSK). BMSFs and chaetocin increased ROS in CML-LSK cells and addition of BMSFs and chaetocin resulted in higher levels compared with chaetocin or BMSF treatment alone. Pretreatment of CML-LSKs with the antioxidant N-acetylcysteine blocked chaetocin cytotoxicity, even in the presence of BMSFs, demonstrating the importance ROS for chaetocin activities. Chaetocin effects on self-renewal of CML-LSKs were assessed by transplanting CML-LSKs into secondary recipients following ex vivo exposure to chaetocin, in the presence or absence of BMSFs. Disease latency in mice transplanted with CML-LSKs following chaetocin treatment more than doubled compared with untreated CML-LSKs or BMSFs-treated CML-LSKs. Mice transplanted with CML-LSKs following chaetocin treatment in the presence of BMSFs had significantly extended survival time compared with mice transplanted with CML-LSKs treated with chaetocin alone. Our findings indicate that chaetocin activity against CML-LSKs is significantly enhanced in the presence of BMSFs and suggest that chaetocin may be effective as a codrug to complement TKIs in CML treatment by disrupting the innate resistance of CML-LSKs through an ROS dependent mechanism.

The Hepatocyte Growth Factor (HGF)/Met Axis: A Neglected Target in the Treatment of Chronic Myeloproliferative Neoplasms?

Met is the receptor of hepatocyte growth factor (HGF), a cytoprotective cytokine. Disturbing the equilibrium between Met and its ligand may lead to inappropriate cell survival, accumulation of genetic abnormalities and eventually, malignancy. Abnormal activation of the HGF/Met axis is established in solid tumours and in chronic haematological malignancies, including myeloma, acute myeloid leukaemia, chronic myelogenous leukaemia (CML), and myeloproliferative neoplasms (MPNs). The molecular mechanisms potentially responsible for the abnormal activation of HGF/Met pathways are described and discussed. Importantly, inCML and in MPNs, the production of HGF is independent of Bcr-Abl and JAK2V617F, the main molecular markers of these diseases. In vitro studies showed that blocking HGF/Met function with neutralizing antibodies or Met inhibitors significantly impairs the growth of JAK2V617F-mutated cells. With personalised medicine and curative treatment in view, blocking activation of HGF/Met could be a useful addition in the treatment of CML and MPNs for those patients with high HGF/MET expression not controlled by current treatments (Bcr-Abl inhibitors in CML; phlebotomy, hydroxurea, JAK inhibitors in MPNs).

Dual-inhibitors of STAT5 and STAT3: studies from molecular docking and molecular dynamics simulations

Although molecularly targeted therapy with imatinib has improved treatments of chronic myeloid leukemia (CML), clinical resistance gradually develops in patients with accelerated or blast phase CML. The inability of imatinib to cure CML suggests that inactivation of BCR-ABL kinase activity alone is not sufficient to control the disease. Aberrant STAT signaling and constitutive STAT5 or STAT3 activation are frequently found in both acute and chronic leukemia. Constitutive activation of STAT5 and STAT3 are associated with imatinib resistance on leukemia cells. Development of drugs targeting SH2 domains of STAT5 and STAT3 provides a novel strategy for the treatment of the imatinib-resistant CML. Here, molecular docking and molecular dynamics simulations were used to investigate the interactions of the drugs targeting STAT3 and STAT5 receptors at molecular level. The calculated binding free energies are consistent with the ranking of the experimental affinities and our simulations also explained their differences in binding energy. Then virtual screening based on molecular docking and molecular dynamics was applied to screen a set of ~1500 compounds for dual inhibitors of the SH2 domains of STAT5 and STAT3. Three top score compounds obtained in virtual screening were compound 660, 304, and 561. Results show that the three predicted dual-inhibitors are well fitted within the two binding domains and are predicted to present improved STAT5 and STAT3 SH2 inhibitory activity.

STAT signaling in the pathogenesis and treatment of myeloid malignancies

STAT transcription factors play a critical role in mediating the effects of cytokines on myeloid cells. As STAT target genes control key processes such as survival, proliferation and self-renewal, it is not surprising that constitutive activation of STATs, particularly STAT3 and STAT5, are common events in many myeloid tumors. STATs are activated both by mutant tyrosine kinases as well as other pathogenic events, and continued activation of STATs is common in the setting of resistance to kinase inhibitors. Thus, the targeting of STATs, alone or in combination with other drugs, will likely have increasing importance for cancer therapy.

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.

Physiologic hypoxia promotes maintenance of CML stem cells despite effective BCR-ABL1 inhibition

Hypoxia mediates TKI resistance. Hypoxia enhances CML stem cell maintenance.

FTY720 induces apoptosis of chronic myelogenous leukemia cells via dual activation of BIM and BID and overcomes various types of resistance to tyrosine kinase inhibitors

PP2A activator FTY720 has been shown to possess the anti-leukemic activity for chronic myelogenous leukemia (CML), however, the cell killing mechanism underlying its anti-leukemic activity has remained to be verified. We investigated the precise mechanisms underlying the apoptosis induction by FTY720, especially focusing on the roles of BH3-only proteins, and the therapeutic potency of FTY720 for CML. Enforced expression of either BCL2 or the dominant-negative protein of FADD (FADD.DN) partly protected CML cells from apoptosis by FTY720, indicating the involvement of both cell extrinsic and intrinsic apoptosis pathways. FTY720 activates pro-apoptotic BH3-only proteins: BIM, which is essential for apoptosis by BCR-ABL1 tyrosine kinase inhibitors (TKIs), and BID, which accelerates the extrinsic apoptosis pathway. Gene knockdown of either BIM or BID partly protected K562 cells from apoptosis by FTY720, but the extent of cell protection was not as much as that by overexpression of either BCL2 or FADD.DN. Moreover, knockdown of both BIM and BID did not provide additional protection compared with knockdown of only BIM or BID, indicating that BIM and BID complement each other in apoptosis by FTY720, especially when either is functionally impaired. FTY720 can overcome TKI resistance caused by ABL kinase domain mutations, dysfunction of BIM resulting from gene deletion polymorphism, and galectin-3 overexpression. In addition, ABT-263, a BH3-mimetic, significantly augmented cell death induction by FTY720 both in TKI-sensitive and -resistant leukemic cells. These results provide the rationale that FTY720, with its unique effects on BIM and BID, could lead to new therapeutic strategies for CML.

Combination of the ABL kinase inhibitor imatinib with the Janus kinase 2 inhibitor TG101348 for targeting residual BCR-ABL-positive cells

Background

The ABL kinase inhibitor imatinib is highly effective in treating most, but not all, patients with chronic myeloid leukemia (CML). This is because residual CML cells are generally present in the bone marrow microenvironment and are refractory to imatinib. Hematopoietic cytokine receptor signaling is mediated by Janus kinases (JAKs) and their downstream transcription factor, signal transducer and activator of transcription (STAT). TG101348 (SAR302503) is an oral inhibitor of JAK2.

Methods

We investigated the efficacy of imatinib and TG101348 using the break point cluster region-c-Abelson (BCR-ABL)-positive cell line and primary CML samples wherein leukemia cells were protected by a feeder cell line (HS-5).

Results

Imatinib treatment resulted in partial inhibition of cell growth in HS-5-conditioned medium. Furthermore, combined treatment with imatinib and TG101348 abrogated the protective effects of HS-5-conditioned medium on K562 cells. Phosphorylation of Crk-L, a BCR-ABL substrate, decreased considerably, while apoptosis increased. In addition, the combined treatment of CD34-positive primary samples resulted in considerably increased cytotoxicity, decreased Crk-L phosphorylation, and increased apoptosis. We also investigated TG101348 activity against feeder cells and observed that STAT5 phosphorylation, granulocyte macrophage colony-stimulating factor, and interleukin 6 levels decreased, indicating reduced cytokine production in HS-5 cells treated with TG101348.

Conclusions

These results showed that JAK inhibitors may enhance the cytotoxic effect of imatinib against residual CML cells and that a combined approach may be a powerful strategy against the stroma-associated drug resistance of Philadelphia chromosome-positive cells.

BCR-ABL-induced deregulation of the IL-33/ST2 pathway in CD34+ progenitors from chronic myeloid leukemia patients

Although it is generally acknowledged that cytokines regulate normal hematopoiesis in an autocrine/paracrine fashion, their possible role in chronic myelogenous leukemia (CML) and resistance to imatinib mesylate treatment remain poorly investigated. Here, we report that CD34(+) progenitors from patients with CML at diagnosis are selectively targeted by the cytokine/alarmin interleukin (IL)-33. Indeed, CML CD34(+) progenitors upregulate their cell surface expression of the IL-33-specific receptor chain ST2, proliferate and produce cytokines in response to IL-33, conversely to CD34(+) cells from healthy individuals. Moreover, ST2 overexpression is normalized following imatinib mesylate therapy, whereas IL-33 counteracts in vitro imatinib mesylate-induced growth arrest in CML CD34(+) progenitors via reactivation of the STAT5 pathway, thus supporting the notion that IL-33 may impede the antiproliferative effects of imatinib mesylate on CD34(+) progenitors in CML. Clinically, the levels of circulating soluble ST2, commonly considered a functional signature of IL-33 signaling in vivo, correlate with disease burden. Indeed, these elevated peripheral concentrations associated with a high Sokal score predictive of therapeutic outcome are normalized in patients in molecular remission. Finally, we evidenced a facilitating effect of IL-33 on in vivo maintenance of CD34(+) progenitors from patients with CML by using xenotransplant experiments in immunodeficient NOG mice, and we showed that engraftment of mouse BCR-ABL-transfected bone marrow progenitors was less efficient in IL-33-deficient mice compared with wild-type recipients. Taken together, our results provide evidence that IL-33/ST2 signaling may represent a novel cytokine-mediated mechanism contributing to CML progenitor growth and support a role for this pathway in CML maintenance and imatinib mesylate resistance.

BEX1 promotes imatinib-induced apoptosis by binding to and antagonizing BCL-2

An enhanced anti-apoptotic capacity of tumor cells plays an important role in the process of breakpoint cluster region/Abelson tyrosine kinase gene (BCR/ABL)-independent imatinib resistance. We have previously demonstrated that brain expressed X-linked 1 (BEX1) was silenced in secondary imatinib-resistant K562 cells and that re-expression of BEX1 can restore imatinib sensitivity resulting in the induction of apoptosis. However, the mechanism by which BEX1 executes its pro-apoptotic function remains unknown. We identified B-cell lymphoma 2 (BCL-2) as a BEX1-interacting protein using a yeast two-hybrid screen. The interaction between BEX1 and BCL-2 was subsequently confirmed by co-immunoprecipitation assays. Like BCL-2, BEX1 was localized to the mitochondria. The region between 33K and 64Q on BEX1 is important for its localization to the mitochondria and its ability to interact with BCL-2. Additionally, we found that this region is essential for BEX1-regulated imatinib-induced apoptosis. Furthermore, we demonstrated that the interaction between BCL-2 and BEX1 promotes imatinib-induced apoptosis by suppressing the formation of anti-apoptotic BCL-2/BCL-2-associated X protein (BAX) heterodimers. Our results revealed an interaction between BEX1 and BCL-2 and a novel mechanism of imatinib resistance mediated by the BEX1/BCL-2 pathway.