On Trk--the TrkB signal transduction pathway is an increasingly important target in cancer biology

Identification of a Novel Series of Potent TrkA Receptor Tyrosine Kinase Inhibitors

A novel series of N-(3-(6-substituted-aminopyridin-3-yloxy)phenyl)-2-oxo-3-phenylimidazolidine-1-carboxamides targeting TrkA receptor tyrosine kinase was identified. SAR study of the series allowed us to design and synthesize compounds possessing inhibitory activity of TrkA kinase enzyme in the low nanomolar range with low residual activity against c-Met and with no significant activity against VEGFR2.

Characterization of critical domains within the tumor suppressor CASZ1 required for transcriptional regulation and growth suppression

CASZ1 is a zinc finger (ZF) transcription factor that is critical for controlling the normal differentiation of subtypes of neural and cardiac muscle cells. In neuroblastoma tumors, loss of CASZ1 is associated with poor prognosis and restoration of CASZ1 function suppresses neuroblastoma tumorigenicity. However, the key domains by which CASZ1 transcription controls developmental processes and neuroblastoma tumorigenicity have yet to be elucidated. In this study, we show that loss of any one of ZF1 to ZF4 resulted in a 58 to 79% loss in transcriptional activity, as measured by induction of tyrosine hydroxylase promoter-luciferase activity, compared to that of wild-type (WT) CASZ1b. Mutation of ZF5 or deletion of the C-terminal sequence of amino acids (aa) 728 to 1166 (a truncation of 38% of the protein) does not significantly alter transcriptional function. A series of N-terminal truncations reveals a critical transcriptional activation domain at aa 31 to 185 and a nuclear localization signal at aa 23 to 29. Soft agar colony formation assays and xenograft studies show that WT CASZ1b is more active in suppressing neuroblastoma growth than CASZ1b with a ZF4 mutation or a deletion of aa 31 to 185. This study identifies key domains needed for CASZ1b to regulate gene transcription. Furthermore, we establish a link between loss of CASZ1b transcriptional activity and attenuation of CASZ1b-mediated inhibition of neuroblastoma growth and tumorigenicity.

Discovery of GNF-5837, a Selective TRK Inhibitor with Efficacy in Rodent Cancer Tumor Models

Neurotrophins and their receptors (TRKs) play key roles in the development of the nervous system and the maintenance of the neural network. Accumulating evidence points to their role in malignant transformations, chemotaxis, metastasis, and survival signaling and may contribute to the pathogenesis of a variety of tumors of both neural and non-neural origin. By screening the GNF kinase collection, a series of novel oxindole inhibitors of TRKs were identified. Optimization led to the identification of GNF-5837 (22), a potent, selective, and orally bioavailable pan-TRK inhibitor that inhibited tumor growth in a mouse xenograft model derived from RIE cells expressing both TRKA and NGF. The properties of 22 make it a good tool for the elucidation of TRK biology in cancer and other nononcology indications.

A quantitative proteomic approach of the different stages of colorectal cancer establishes OLFM4 as a new nonmetastatic tumor marker

Expression profiles represent new molecular tools that are useful to characterize the successive steps of tumor progression and the prediction of recurrence or chemotherapy response. In this study, we have used quantitative proteomic analysis to compare different stages of colorectal cancer. A combination of laser microdissection, OFFGEL separation, iTRAQ labeling, and MALDI-TOF/TOF MS was used to explore the proteome of 28 colorectal cancer tissues. Two software packages were used for identification and quantification of differentially expressed proteins: Protein Pilot and iQuantitator. Based on ∼1,190,702 MS/MS spectra, a total of 3138 proteins were identified, which represents the largest database of colorectal cancer realized to date and demonstrates the value of our quantitative proteomic approach. In this way, individual protein expression and variation have been identified for each patient and for each colorectal dysplasia and cancer stage (stages I-IV). A total of 555 proteins presenting a significant fold change were quantified in the different stages, and this differential expression correlated with immunohistochemistry results reported in the Human Protein Atlas database. To identify a candidate biomarker of the early stages of colorectal cancer, we focused our study on secreted proteins. In this way, we identified olfactomedin-4, which was overexpressed in adenomas and in early stages of colorectal tumors. This early stage overexpression was confirmed by immunohistochemistry in 126 paraffin-embedded tissues. Our results also indicate that OLFM4 is regulated by the Ras-NF-κB2 pathway, one of the main oncogenic pathways deregulated in colorectal tumors.

Fine-tuning roles of endogenous brain-derived neurotrophic factor, TrkB and sortilin in colorectal cancer cell survival

Background

Neurotrophin receptors were initially identified in neural cells. They were recently detected in some cancers in association with invasiveness, but the function of these tyrosine kinase receptors was not previously investigated in colorectal cancer (CRC) cells.

Methods and Findings

We report herein that human CRC cell lines synthesize the neural growth factor Brain-derived neurotrophic factor (BDNF) under stress conditions (serum starvation). In parallel, CRC cells expressed high- (TrkB) and low-affinity (p75^NTR) receptors at the plasma membrane, whereas TrkA and TrkC, two other high affinity receptors for NGF and NT-3, respectively, were undetectable. We demonstrate that BDNF induced cell proliferation and had an anti-apoptotic effect mediated through TrkB, as assessed by K252a, a Trk pharmacologic inhibitor. It suppressed both cell proliferation and survival of CRC cells that do not express TrkA nor TrkC. In parallel to the increase of BDNF secretion, sortilin, a protein acting as a neurotrophin transporter as well as a co-receptor for p75^NTR, was increased in the cytoplasm of primary and metastatic CRC cells, which suggests that sortilin could regulate neurotrophin transport in these cells. However, pro-BDNF, also detected in CRC cells, was co-expressed with p75^NTR at the cell membrane and co-localized with sortilin. In contrast to BDNF, exogenous pro-BDNF induced CRC apoptosis, which suggests that a counterbalance mechanism is involved in the control of CRC cell survival, through sortilin as the co-receptor for p75^NTR, the high affinity receptor for pro-neurotrophins. Likewise, we show that BDNF and TrkB transcripts (and not p75^NTR) are overexpressed in the patients' tumors by comparison with their adjacent normal tissues, notably in advanced stages of CRC.

Conclusion

Taken together, these results highlight that BDNF and TrkB are essential for CRC cell growth and survival in vitro and in tumors. This autocrine loop could be of major importance to define new targeted therapies.

Brain-derived neurotrophic factor expression predicts adverse pathological & clinical outcomes in human breast cancer

Introduction

Brain-derived neurotrophic factor (BDNF) has established physiological roles in the development and function of the vertebrate nervous system. BDNF has also been implicated in several human malignancies, including breast cancer (BC). However, the precise biological role of BDNF and its utility as a novel biomarker have yet to be determined. The objective of this study was to determine the mRNA and protein expression of BDNF in a cohort of women with BC. Expression levels were compared with normal background tissues and evaluated against established pathological parameters and clinical outcome over a 10 year follow-up period.

Methods

BC tissues (n = 127) and normal tissues (n = 33) underwent RNA extraction and reverse transcription, BDNF transcript levels were determined using real-time quantitative PCR. BDNF protein expression in mammary tissues was assessed with standard immuno-histochemical methodology. Expression levels were analyzed against tumour size, grade, nodal involvement, TNM stage, Nottingham Prognostic Index (NPI) and clinical outcome over a 10 year follow-up period.

Results

Immuno-histochemical staining revealed substantially greater BDNF expression within neoplastic cells, compared to normal mammary epithelial cells. Significantly higher mRNA transcript levels were found in the BC specimens compared to background tissues (p = 0.007). The expression of BDNF mRNA was demonstrated to increase with increasing NPI; NPI-1 vs. NPI-2 (p = 0.009). Increased BDNF transcript levels were found to be significantly associated with nodal positivity (p = 0.047). Compared to patients who remained disease free, higher BDNF expression was significantly associated with local recurrence (LR) (p = 0.0014), death from BC (p = 0.018) and poor prognosis overall (p = 0.013). After a median follow up of 10 years, higher BDNF expression levels were significantly associated with reduced overall survival (OS) (106 vs. 136 months, p = 0.006). BDNF emerged as an independent prognostic variable in multivariate analysis for disease free survival (DFS) (p = 0.026) and approached significance for OS (p = 0.055).

Conclusion

BDNF expression was found to be significantly higher in BC specimens compared to normal tissue. Higher transcript levels were significantly associated with unfavourable pathological parameters including nodal positivity and increasing NPI; and adverse clinical outcomes including LR, death from BC, poor prognosis, reduced DFS and OS. BDNF offers utility as a prognostic marker and potential for targeted therapeutic strategies.

Targeted therapy in pediatric and adolescent oncology

Cancers in children and adolescents are fortunately infrequent. Overall, cure rates are good, approximately 80%, although this varies by histology and stage. Targeted therapies aim to improve efficacy and decrease toxicity by more specifically affecting malignant cells or their supporting stroma. Cancers of early life are often of different histology than those seen in adults. Sometimes, the same pathway is affected, even if the histology is different. Toxicities may also be different, particularly in younger children. These factors render drug development in young people challenging. This article reviews some successes and challenges to that development, including brief discussions of imatinib, lestaurtinib, antiangiogenesis, and anti-GD2 therapies.

Targets of miR-200c mediate suppression of cell motility and anoikis resistance

INTRODUCTION

miR-200c and other members of the miR-200 family promote epithelial identity by directly targeting ZEB1 and ZEB2, which repress E-cadherin and other genes involved in polarity. Loss of miR-200c is often observed in carcinoma cells that have undergone epithelial to mesenchymal transition (EMT). Restoration of miR-200c to such cells leads to a reduction in stem cell-like characteristics, reduced migration and invasion, and increased sensitivity to taxanes. Here we investigate the functional role of novel targets of miR-200c in the aggressive behavior of breast and endometrial cancer cells.

METHODS

Putative target genes of miR-200c identified by microarray profiling were validated as direct targets using dual luciferase reporter assays. Following restoration of miR-200c to triple negative breast cancer and type 2 endometrial cancer cell lines that had undergone EMT, levels of endogenous target mRNA and respective protein products were measured. Migration and sensitivity to anoikis were determined using wound healing assays or cell-death ELISAs and viability assays respectively.

RESULTS

We found that restoration of miR-200c suppresses anoikis resistance, a novel function for this influential miRNA. We identified novel targets of miR-200c, including genes encoding fibronectin 1 (FN1), moesin (MSN), neurotrophic tyrosine receptor kinase type 2 (NTRK2 or TrkB), leptin receptor (LEPR), and Rho GTPase activating protein 19 (ARHGAP19). These targets all encode proteins normally expressed in cells of mesenchymal or neuronal origin; however, in carcinoma cells that lack miR-200c they become aberrantly expressed and contribute to the EMT phenotype and aggressive behavior. We showed that these targets are inhibited upon restoration of miR-200c to aggressive breast and endometrial cancer cells. We demonstrated that inhibition of MSN and/or FN1 is sufficient to mediate the ability of miR-200c to suppress cell migration. Lastly, we showed that targeting of TrkB mediates the ability of miR-200c to restore anoikis sensitivity.

CONCLUSIONS

miR-200c maintains the epithelial phenotype not only by targeting ZEB1/2, which usually facilitates restoration of E-cadherin expression, but also by actively repressing a program of mesenchymal and neuronal genes involved in cell motility and anoikis resistance.

Zeb1 is required for TrkB-induced epithelial-mesenchymal transition, anoikis resistance and metastasis

Anoikis (detachment-induced apoptosis) prevents the survival of cells at inappropriate sites of the body and can therefore act as a barrier to metastasis. In a function-based genome-wide screen, we have previously identified the neurotrophic tyrosine kinase receptor TrkB as a potent suppressor of anoikis. Consistently, activated TrkB oncogenically transforms non-malignant epithelial cells and causes them to invade and produce metastatic tumors in vivo. Overexpression of activated TrkB also results in morphological transformation, resembling epithelial-mesenchymal transition (EMT). E-cadherin, an important EMT regulator, and two E-cadherin repressors, Twist and Snail, are critical for these TrkB functions. As Snail has been shown to induce Zeb1, another E-cadherin repressor, we hypothesized that Zeb1 could be a TrkB target, too. We show here that Zeb1 is required for TrkB-induced EMT in epithelial cells, as RNAi-mediated knockdown of Zeb1 reverted the morphological changes induced by TrkB. Furthermore, Zeb1 is involved in TrkB-induced anoikis resistance, migration and invasion. In vivo, knockdown of Zeb1 strongly reduced TrkB-induced metastasis. Finally, epistasis experiments showed that Zeb1 acts downstream of Twist and Snail. We conclude that Zeb1 is required for several TrkB-induced effects in vitro and in vivo, including metastasis.

BDNF/TrkB content and interaction with gastrin-releasing peptide receptor blockade in colorectal cancer

OBJECTIVE

Neurotrophin and neuropeptide pathways are emerging targets in cancer. Here we show that brain-derived neurotrophic factor (BDNF) and its receptor, TrkB, are present in colorectal cancer and that BDNF levels are increased in tumors compared to nontumor tissue. In addition, we investigate the role of BDNF in influencing the response of colorectal cancer cells to inhibition of gastrin-releasing peptide receptors (GRPR).

METHODS

Fresh-frozen sporadic colorectal adenocarcinoma specimens and adjacent nonneoplastic tissue from 30 patients, as well as paraffin-embedded colorectal cancer samples from 21 patients, were used in this study. Cell proliferation and mRNA and protein levels were examined in HT-29 or SW620 cells treated with a GRPR antagonist, human recombinant BDNF (hrBDNF), a Trk antagonist K252a, or cetuximab.

RESULTS

Expression of BDNF and TrkB was detected in tumor samples and cell lines. BDNF levels were higher in tumor samples compared to nonneoplastic tissue. BDNF expression and secretion were increased by GRPR blockade in HT-29 cells through a mechanism dependent on epidermal growth factor receptors. Treatment with hrBDNF prevented the effect of GRPR blockade on cell proliferation, whereas a Trk inhibitor reduced proliferation.

CONCLUSIONS

BDNF and TrkB are present in colorectal cancer and might contribute to resistance to GRPR antagonists.

Comparison of the specificity of Trk inhibitors in recombinant and neuronal assays

Neurotrophins are important mediators of neuronal development, survival and plasticity. They act via binding to Trk receptors, which results in the stimulation of the intracellular tyrosine kinase domain of the receptor leading to autophosphorylation of this domain. This in turn creates a scaffold that recruits various adapter proteins allowing the activation of intracellular signaling cascades including the PLCγ, MAPK and PI3K pathways. Compounds that specifically block the activity of the tyrosine kinase domain of Trk receptors would provide a powerful tool to study the role of these receptors in cells. K252a has previously been used for this purpose, however we show here that it can inhibit many tyrosine and serine/threonine kinases in vitro. Profiling of 3 newer inhibitors, referred to here as SHN-753, SHN-722 and GSK-Trk, demonstrate that they have significantly improved specificity for the kinase activity of TrkA in vitro compared to K252a. In addition these compounds were found to block the TrkB mediated activation of ERK1/2 by BDNF, but did not affect NMDA induced ERK1/2 activation. These compounds, while still not completely specific for Trk receptor kinase activity, do represent a considerable improvement over K252a and should prove valuable in the study of neurotrophin-mediated actions in the nervous system.

TrkB Isoforms Differentially Affect AICD Production through Their Intracellular Functional Domains

We report that NTRK2, the gene encoding for the TrkB receptor, can regulate APP metabolism, specifically AICD levels. Using the human neuroblastoma cell line SH-SY5Y, we characterized the effect of three TrkB isoforms (FL, SHC, T) on APP metabolism by knockdown and overexpression. We found that TrkB FL increases AICD-mediated transcription and APP levels while it decreases sAPP levels. These effects were mainly mediated by the tyrosine kinase activity of the receptor and partially by the PLC-γ- and SHC-binding sites. The TrkB T truncated isoform did not have significant effects on APP metabolism when transfected by itself, while the TrkB SHC decreased AICD-mediated transcription. TrkB T abolished TrkB FL effects on APP metabolism when cotransfected with it while TrkB SHC cotransfected with TrkB FL still showed increased APP levels. In conclusion, we demonstrated that TrkB isoforms have differential effects on APP metabolism.

Brain-derived neurotrophic factor and neurotrophin-4/5 are expressed in breast cancer and can be targeted to inhibit tumor cell survival

PURPOSE

Given that nerve growth factor has previously been shown to be involved in breast cancer progression, we have tested here the hypothesis that the other neurotrophins (NT) are expressed and have an influence in breast tumor growth.

EXPERIMENTAL DESIGN

The expression of brain-derived neurotrophic factor (BDNF), NT-3 and NT-4/5, as well as the neurotrophin receptor p75(NTR), TrkB, and TrkC, was studied by RT-PCR, Western blotting, and immunohistochemistry in cell lines and tumor biopsies. The biological impacts of neurotrophins, and associated mechanisms, were analyzed in cell cultures and xenografted mice.

RESULTS

BDNF and NT-4/5 were expressed and secreted by breast cancer cells, and the use of blocking antibodies suggested an autocrine loop mediating cell resistance to apoptosis. The corresponding tyrosine kinase receptor TrkB was only rarely observed at full length, whereas the expression of TrkB-T1, lacking the kinase domain, as well as p75(NTR), were detected in all tested breast cancer cell lines and tumor biopsies. In contrast, NT-3 and TrkC were not detected. SiRNA against p75(NTR) and TrkB-T1 abolished the antiapoptotic effect of BDNF and NT-4/5, whereas the pharmacological inhibitors K252a and PD98059 had no effect, suggesting the involvement of p75(NTR) and TrkB-T1, but not kinase activities from Trks and MAPK. In xenografted mice, anti-BDNF, anti-NT-4/5, anti-p75(NTR), or anti-TrkB-T1 treatments resulted in tumor growth inhibition, characterized by an increase in cell apoptosis, but with no change in proliferation.

CONCLUSION

BDNF and NT-4/5 contribute to breast cancer cell survival and can serve as prospective targets in attempts to inhibit tumor growth.

Phase I trial of lestaurtinib for children with refractory neuroblastoma: a new approaches to neuroblastoma therapy consortium study

PURPOSE

TrkB acts as an oncogenic kinase in a subset of human neuroblastomas. Lestaurtinib, a multi-kinase inhibitor with potent activity against Trk kinases, has demonstrated activity in preclinical models of neuroblastoma.

METHODS

Patients with refractory high-risk neuroblastoma received lestaurtinib twice daily for 5 days out of seven in 28-day cycles, starting at 70% of the adult recommended Phase 2 dose. Lestaurtinib dose was escalated using a 3 + 3 design. Pharmacokinetics and plasma phospho-TrkB inhibitory activity were evaluated in the first cycle.

RESULTS

Forty-seven subjects were enrolled, and 10 dose levels explored starting at 25 mg/M(2)/dose BID. Forty-six subjects were evaluable for response, and 42 subjects were fully evaluable for determination of dose escalation. Asymptomatic and reversible grade 3-4 transaminase elevation was dose limiting in 4 subjects. Reversible pancreatitis (grade 2) was observed in 3 subjects after prolonged treatment at higher dose levels. Other toxicities were mild and reversible. Pharmacokinetic analyses revealed rapid drug absorption, however inter-patient variability was large. Plasma inhibition of phospho-TrkB activity was observed 1 h post-dosing at 85 mg/M(2) with uniform inhibition at 120 mg/M(2). There were two partial responses and nine subjects had prolonged stable disease at dose levels ≥ 5, (median: 6 cycles). A biologically effective and recommended phase 2 dose of 120 mg/M(2)/dose BID was established.

CONCLUSIONS

Lestaurtinib was well tolerated in patients with refractory neuroblastoma, and a dose level sufficient to inhibit TrkB activity was established. Safety and signs of activity at the higher dose levels warrant further evaluation in neuroblastoma.

Neurotrophic tyrosine kinase receptor B/neurotrophin 4 signaling axis is perturbed in clinical and experimental pulmonary fibrosis

The neurotrophins (NTs) are emerging as exciting new participants in normal lung physiology, as well as in several pathological processes in diseased lungs. In this study, the increased expression of NT4/5 and of its cognate receptor, the neurotrophic tyrosine kinase receptor Type 2 (TrkB), was observed in human lungs explanted from patients with idiopathic pulmonary fibrosis (IPF), and in lungs from mice with bleomycin-induced pulmonary fibrosis. The expression of NT4/5 and TrkB localized to hyperplastic alveolar Type II cells (ATII) and fibroblastic foci in affected lungs. Increased concentrations of NT4/5 and TrkB were evident in ATII isolated from the lungs of bleomycin-treated mice. Primary ATII were shown to secrete NT4/5 into the cell culture medium. The profibrotic cytokine transforming growth factor-β1, stimulated TrkB, but not NT4/5 gene expression, suggesting that perturbed profibrotic growth factor signaling in affected lungs may drive the expression of TrkB. NT4/5 enhanced the proliferation of ATII through a TrkB/extracellular-regulated kinase/protein kinase B pathway, and could also drive the proliferation of primary human and murine lung fibroblasts, through TrkB-dependent and protein kinase B-dependent pathways. Taken together, these data suggest that a dysregulated TrkB/NT4/5 axis may contribute to several of the pathological lesions associated with pulmonary fibrosis, including ATII hyperplasia and the proliferation of fibroblasts, and we would add IPF to the list of disorders, such as pain and cancer, for which therapeutic targeting of the TrkB/neurotrophin axis has been proposed for further investigation.

Small molecule kinase inhibitor screen identifies polo-like kinase 1 as a target for neuroblastoma tumor-initiating cells

Neuroblastoma (NB) is an often fatal pediatric tumor of neural crest origin. We previously isolated NB tumor-initiating cells (NB TIC) from bone marrow metastases that resemble cancer stem cells and form metastatic NB in immunodeficient animals with as few as ten cells. To identify signaling pathways important for the survival and self-renewal of NB TICs and potential therapeutic targets, we screened a small molecule library of 143 protein kinase inhibitors, including 33 in clinical trials. Cytostatic or cytotoxic drugs were identified that targeted PI3K (phosphoinositide 3-kinase)/Akt, PKC (protein kinase C), Aurora, ErbB2, Trk, and Polo-like kinase 1 (PLK1). Treatment with PLK1 siRNA or low nanomolar concentrations of BI 2536 or BI 6727, PLK1 inhibitors in clinical trials for adult malignancies, were cytotoxic to TICs whereas only micromolar concentrations of the inhibitors were cytotoxic for normal pediatric neural stem cells. Furthermore, BI 2536 significantly inhibited TIC tumor growth in a therapeutic xenograft model, both as a single agent and in combination with irinotecan, an active agent for relapsed NB. Our findings identify candidate kinases that regulate TIC growth and survival and suggest that PLK1 inhibitors are an attractive candidate therapy for metastatic NB.

Neurotrophins in lung health and disease

Neurotrophins (NTs) are a family of growth factors that are well-known in the nervous system. There is increasing recognition that NTs (nerve growth factor, brain-derived neurotrophic factor and NT3) and their receptors (high-affinity TrkA, TrkB and TrkC, and low-affinity p75NTR) are expressed in lung components including the nasal and bronchial epithelium, smooth muscle, nerves and immune cells. NT signaling may be important in normal lung development, developmental lung disease, allergy and inflammation (e.g., rhinitis, asthma), lung fibrosis and even lung cancer. In this review, we describe the current status of our understanding of NT signaling in the lung, with hopes of using aspects of the NT signaling pathway in the diagnosis and therapy of lung diseases.

Heat shock protein 90 inhibition depletes TrkA levels and signaling in human acute leukemia cells

Nerve growth factor (NGF) induces autophosphorylation and downstream progrowth and prosurvival signaling from the receptor tyrosine kinase TrkA. Overexpression or activating mutation of TrkA has been described in human acute myeloid leukemia cells. In the present study, we show the chaperone association of TrkA with heat shock protein 90 (hsp90) and the inhibitory effect of the hsp90 inhibitor, 17-DMAG, on TrkA levels and signaling in cultured and primary myeloid leukemia cells. Treatment with 17-DMAG disrupted the binding of TrkA with hsp90 and the cochaperone cdc37, resulting in polyubiquitylation, proteasomal degradation, and depletion of TrkA. Exposure to 17-DMAG inhibited NGF-induced p-TrkA, p-AKT, and p-ERK1/2 levels, as well as induced apoptosis of K562, 32D cells with ectopic expression of wild-type TrkA or the constitutively active mutant Delta TrkA, and of primary myeloid leukemia cells. Additionally, 17-DMAG treatment inhibited NGF-induced neurite formation in the rat pheochromocytoma PC-12 cells. Cotreatment with 17-DMAG and K-252a, an inhibitor of TrkA-mediated signaling, induced synergistic loss of viability of cultured and primary myeloid leukemia cells. These findings show that TrkA is an hsp90 client protein, and inhibition of hsp90 depletes TrkA and its progrowth and prosurvival signaling in myeloid leukemia cells. These findings also support further evaluation of the combined activity of an hsp90 inhibitor and TrkA antagonist against myeloid leukemia cells.