Activation of abl family kinases in solid tumors

Single-Cell Plasmonic Immunosandwich Assay Reveals the Modulation of Nucleocytoplasmic Localization Fluctuation of ABL1 on Cell Migration

Cell migration is an essential process of cancer metastasis. The spatiotemporal dynamics of signaling molecules influences cellular phenotypic outcomes. It has been increasingly documented that the Abelson (ABL) family kinases play critical roles in solid tumors. However, ABL1's shuttling dynamics in cell migration still remains unexplored. This is mainly because tools permitting the investigation of translocation dynamics of proteins in single living cells are lacking. Herein, to bridge this gap, we developed a unique multifunctional integrated single-cell analysis method that enables long-term observation of cell migration behavior and monitoring of signaling proteins and complexes at the subcellular level. We found that the shuttling of ABL1's to the cytoplasm results in a higher migration speed, while its trafficking back to the nucleus leads to a lower one. Furthermore, our results indicated that fluctuant protein-protein interactions between 14-3-3 and ABL1 modulate ABL1's nucleocytoplasmic fluctuation and eventually affect the cell speed. Importantly, based on these new insights, we demonstrated that disturbing ABL1's nuclear export traffic and 14-3-3-ABL1 complexes formation can effectively suppress cell migration. Thus, our method opens up a new possibility for simultaneous tracking of internal molecular mechanisms and cell behavior, providing a promising tool for the in-depth study of cancer.

Recent advances of benzimidazole as anticancer agents

Cancer is the second leading cause of death globally, with 9.6 million deaths yearly. As a life-threatening disease, it necessitates the emergence of new therapies. Resistance to current chemotherapies drives scientists to develop new medications that will eventually be accessible. Because heterocycles are so common in biological substances, compounds play a big part in the variety of medications that have been developed. The "Master Key" is the benzimidazole nucleus, which consists of a six-membered benzene ring fused with a five-membered imidazole/imidazoline ring, which is an azapyrrole. One of the five-membered aromatic nitrogen heterocycles identified in American therapies that have been approved by the Food and Drug Administration (FDA). Our results show that benzimidazole's broad therapeutic spectrum is due to its structural isosteres with purine, which improves hydrogen bonding, electrostatic interactions with topoisomerase complexes, intercalation with DNA, and other functions. It also enhances protein and nucleic acid inhibition, tubulin microtubule degeneration, apoptosis, DNA fragmentation, and other functions. Additionally, readers for designing the more recent benzimidazole analogues as prospective cancer treatments.

Inhibition of ABL1 by tyrosine kinase inhibitors leads to a downregulation of MLH1 by Hsp70-mediated lysosomal protein degradation

The DNA mismatch repair (MMR) pathway and its regulation are critical for genomic stability. Mismatch repair (MMR) follows replication and repairs misincorporated bases and small insertions or deletions that are not recognized and removed by the proofreading polymerase. Cells deficient in MMR exhibit an increased overall mutation rate and increased expansion and contraction of short repeat sequences in the genome termed microsatellite instability (MSI). MSI is often a clinical measure of genome stability in tumors and is used to determine the course of treatment. MMR is also critical for inducing apoptosis after alkylation damage from environmental agents or DNA-damaging chemotherapy. MLH1 is essential for MMR, and loss or mutation of MLH1 leads to defective MMR, increased mutation frequency, and MSI. In this study, we report that tyrosine kinase inhibitors, imatinib and nilotinib, lead to decreased MLH1 protein expression but not decreased MLH1 mRNA levels. Of the seven cellular targets of Imatinib and nilotinib, we show that silencing of ABL1 also reduces MLH1 protein expression. Treatment with tyrosine kinase inhibitors or silencing of ABL1 results in decreased apoptosis after treatment with alkylating agents, suggesting the level of MLH1 reduction is sufficient to disrupt MMR function. We also report MLH1 is tyrosine phosphorylated by ABL1. We demonstrate that MLH1 downregulation by ABL1 knockdown or inhibition requires chaperone protein Hsp70 and that MLH1 degradation can be abolished with the lysosomal inhibitor bafilomycin. Taken together, we propose that ABL1 prevents MLH1 from being targeted for degradation by the chaperone Hsp70 and that in the absence of ABL1 activity at least a portion of MLH1 is degraded through the lysosome. This study represents an advance in understanding MMR pathway regulation and has important clinical implications as MMR status is used in the clinic to inform patient treatment, including the use of immunotherapy.

Peptide-based targeted cancer therapeutics: design, synthesis and biological evaluation

Cancer is the leading cause for human mortality together with cardiovascular diseases. Abl (Abelson) tyrosine kinases play a fundamental role in transducing various signals that control proliferation, survival, migration and invasion in several cancers such as Chronic Myeloid Leukemia (CML), breast cancer and brain cancer. For these reasons Abl tyrosine kinases are considered important biological targets in drug discovery. In this study a series of lysine-based oligopeptides with expected Abl inhibitory activity were designed resembling the binding of FDA-approved drugs (i.e. of Imatinib and Nilotinib), synthesized, purified by High Performance Liquid Chromatography (HPLC), analyzed by mass spectrometry (MS) and biologically tested in vitro in CML (AR-230 and K-562), breast cancers (MDA-MB 231 and MDA-MB 468) and glioblastoma cell lines (U87 and U118). The solid-phase peptide synthesis (SPPS) by Fmoc (9-fluorenylmethoxycarbonyl) chemistry was used to synthesize target compounds. AutoDock Vina was applied for simulation binding to Abl. The biological activities were measured evaluating cytotoxic effect, induction of apoptosis and inhibition of cancer cells migration. The new peptides exhibited different concentration-dependent antiproliferative effect against the tumor cell lines after 72 h treatment. The most promising results were obtained with the U87 glioblastoma cell line where a significant reduction of the migration ability was detected with one compound (H-Lys¹-Lys²-Lys³-NH₂).

[Aberrant Activation Mechanism of TGF-β Signaling in Epithelial-mesenchymal Transition]

Epithelial-mesenchymal transition (EMT) is an important program in epithelial cancer cells to acquire the motility and invasion, which promotes cancer metastasis to remote organs. EMT is induced by various secreted factors, such as transforming growth factor-β (TGF-β) and epidermal growth factor (EGF). TGF-β ligand activates Smad-dependent and -independent pathways by binding to TGF-β receptors. In Smad-dependent pathway, the activated TGF-β receptor phosphorylates Smad2/3 and accelerates its association with Smad4, leading to their nuclear translocation. Smad2/3-4 complex promotes the expression of EMT-inducing transcription factors, such as Snail and Slug. In Smad-independent pathway, mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K)/AKT pathways are activated and required for TGF-β-induced EMT. Smad-independent pathway is similar to downstream of receptor tyrosine kinases, and therefore EGFR signaling is known to induce EMT synergize with TGF-β signaling. We explored a new mechanism of EGFR-mediated activation of TGF-β signaling and found that c-Abl kinase activates TGF-β signaling. Based on our proteomic analysis, we identified several TGF-β signaling molecules as nuclear c-Abl substrates, including transcriptional intermediary factor 1-γ (TIF1γ/TRIM33/Ectodermin), a suppressor of TGF-β signaling. c-Abl-mediated phosphorylation of TIF1γ inhibits its binding to Smad3, thereby increasing Smad3's transcriptional activity and promoting EMT. TIF1γ phosphorylation is also involved in the EGFR-caused aberrant activation of TGF-β signaling, suggesting that EGFR/c-Abl pathway activates TGF-β signaling through phosphorylation of nuclear substrates and promotes EMT. Our findings provide new insights into the activation machinery of TGF-β signaling, and further studies are required to clarify the clinical significance of the EGFR/c-Abl pathway in cancer metastasis.

Platelet-derived growth factor receptor beta activates Abl2 via direct binding and phosphorylation

Abl family kinases are nonreceptor tyrosine kinases activated by diverse cellular stimuli that regulate cytoskeleton organization, morphogenesis, and adhesion. The catalytic activity of Abl family kinases is tightly regulated in cells by a complex set of intramolecular and intermolecular interactions and post-translational modifications. For example, the platelet-derived growth factor receptor beta (PDGFRβ), important for cell proliferation and chemotaxis, is a potent activator of Abl family kinases. However, the molecular mechanism by which PDGFRβ engages and activates Abl family kinases is not known. We show here that the Abl2 Src homology 2 domain directly binds to phosphotyrosine Y771 in the PDGFRβ cytoplasmic domain. PDGFRβ directly phosphorylates multiple novel sites on the N-terminal half of Abl2, including Y116, Y139, and Y161 within the Src homology 3 domain, and Y299, Y303, and Y310 on the kinase domain. Y116, Y161, Y272, and Y310 are all located at or near the Src homology 3/Src homology 2-kinase linker interface, which helps maintain Abl family kinases in an autoinhibited conformation. We also found that PDGFRβ-mediated phosphorylation of Abl2 in vitro activates Abl2 kinase activity, but mutation of these four tyrosines (Y116, Y161, Y272, and Y310) to phenylalanine abrogated PDGFRβ-mediated activation of Abl2. These findings reveal how PDGFRβ engages and phosphorylates Abl2 leading to activation of the kinase, providing a framework to understand how growth factor receptors engage and activate Abl family kinases.

GAB1-ABL1 fusions in tumors that have histologic overlap with NTRK-rearranged spindle cell tumors

Fibroblastic spindle cell tumors are a heterogeneous group of rare soft tissue tumors that are increasingly recognized as associated with a variety of kinase gene fusions. We report two cases of GAB1-ABL1 fusions in spindle cell tumors that histologically overlap with neurotrophic tyrosine receptor kinase (NTRK)-rearranged spindle cell tumors. The first case occurred in a 76-year-old female who had a large deep-seated spindle cell tumor composed of monotonous ovoid to spindle cells in a background of thick stromal collagen bands with prominent hyalinized vessels and inconspicuous mitoses (<1/10 HPF). Immunohistochemical stains showed co-expression of S100 and CD34. A GAB1-ABL1 fusion was detected by whole transcriptome RNA sequencing. The patient had a partial response to imatinib. The second case was previously described as a solitary fibrous tumor, occurring in a 9-year-old female with a cellular spindle cell tumor with patchy CD34 immunoexpression but no expression of S100. Upon clinicopathologic re-review, including anchored multiplex next-generation sequencing, a GAB1-ABL1 fusion was identified. In summary, we report the first two cases of spindle cell tumors with variable expression of CD34 and/or S100, driven by GAB1-ABL1 gene fusions with histologic overlap with NTRK-rearranged spindle cell tumors, suggesting that ABL-fusions may also be oncogenic drivers within this spectrum of tumors. These cases highlight the evolving understanding of fibroblastic spindle cell tumor biology and the utility of sequencing in identifying a targetable alteration.

Recurrent Fusion of the GRB2 Associated Binding Protein 1 (GAB1) Gene With ABL Proto-oncogene 1 (ABL1) in Benign Pediatric Soft Tissue Tumors

BACKGROUND/AIM

Fusions of the ABL proto-oncogene 1 gene (ABL1 in 9q34) are common in leukemias but rare in solid tumors. The most notable is the t(9;22)(q34;q11)/BCR-ABL1 coding for a chimeric tyrosine kinase. We herein report an ABL1-fusion in a pediatric tumor.

MATERIALS AND METHODS

G-banding, fluorescence in situ hybridization, reverse transcription polymerase chain reaction and Sanger sequencing were performed on a soft tissue perineurioma found in the left musculus erector spinae of a child.

RESULTS

A der(4)t(4;9)(q31;q34) and a fusion of the GRB2 associated binding protein 1 (GAB1 in 4q31) gene with ABL1 were found. A literature search revealed 3 more cases with similar genetic and clinicopathological characteristics: a soft tissue perineurioma with t(2;9;4)(p23;q34;q31) and ABL1 rearrangement, a soft tissue angiofibroma with a GAB1-ABL1 chimeric gene, and a solitary fibrous tumor carrying a der(4)t(4;9)(q31.1;q34).

CONCLUSION

GAB1-ABL1 is a recurrent fusion gene in benign pediatric tumors.

Benzimidazoles in Drug Discovery: A Patent Review

Benzimidazole is a heterocyclic ring system that has been widely studied in the pharmaceutical field. For the past decade, numerous benzimidazole derivatives have been synthesized and evaluated for their wide range of pharmacological activities, which are beneficial for drug development. This article presents the biological effects of benzimidazole derivatives in each invention from 2015 to 2020. Two patent databases, Google Patents and Lens, were used to locate relevant granted patent applications. Specifically, this review delineates the role of patented benzimidazoles from a disease-centric perspective and examines the mechanisms of action of these compounds in related diseases. Most of the benzimidazoles have shown good activities against various target proteins. Whilst several of them have progressed into clinical trials, most patents presented novel therapeutic approaches for respective target diseases. Hence, their potential in being developed into clinical drugs are also discussed.

Multiplexable fluorescence lifetime imaging (FLIM) probes for Abl and Src-family kinases

Many commonly employed strategies to map kinase activities in live cells require expression of genetically encoded proteins (e.g. FRET sensors). In this work, we describe the development and preliminary application of a set of cell-penetrating, fluorophore labelled peptide substrates for fluorescence lifetime imaging (FLIM) of Abl and Src-family kinase activities. These probes do not rely on FRET pairs or genetically-encoded protein expression. We further demonstrate probe multiplexing and pixel-by-pixel quantification to estimate the relative proportion of modified probe, suggesting that this strategy will be useful for detailed mapping of single cell and subcellular dynamics of multiple kinases concurrently in live cells.

c-Abl Tyrosine Kinase Is Regulated Downstream of the Cytoskeletal Protein Synemin in Head and Neck Squamous Cell Carcinoma Radioresistance and DNA Repair

The intermediate filament synemin has been previously identified as novel regulator of cancer cell therapy resistance and DNA double strand break (DSB) repair. c-Abl tyrosine kinase is involved in both of these processes. Using PamGene technology, we performed a broad-spectrum kinase activity profiling in three-dimensionally, extracellular matrix grown head and neck cancer cell cultures. Upon synemin silencing, we identified 86 deactivated tyrosine kinases, including c-Abl, in irradiated HNSCC cells. Upon irradiation and synemin inhibition, c-Abl hyperphosphorylation on tyrosine (Y) 412 and threonine (T) 735 was significantly reduced, prompting us to hypothesize that c-Abl tyrosine kinase is an important signaling component of the synemin-mediated radioresistance pathway. Simultaneous targeting of synemin and c-Abl resulted in similar radiosensitization and DSB repair compared with single synemin depletion, suggesting synemin as an upstream regulator of c-Abl. Immunoprecipitation assays revealed a protein complex formation between synemin and c-Abl pre- and post-irradiation. Upon pharmacological inhibition of ATM, synemin/c-Abl protein-protein interactions were disrupted implying synemin function to depend on ATM kinase activity. Moreover, deletion of the SH2 domain of c-Abl demonstrated a decrease in interaction, indicating the dependency of the protein-protein interaction on this domain. Mechanistically, radiosensitization upon synemin knockdown seems to be associated with an impairment of DNA repair via regulation of non-homologous end joining independent of c-Abl function. Our data generated in more physiological 3D cancer cell culture models suggest c-Abl as further key determinant of radioresistance downstream of synemin.

Enhanced in vivo targeting of estrogen receptor alpha signaling in murine mammary adenocarcinoma by nilotinib/rosuvastatin novel combination

The development of resistance to endocrine therapy of estrogen receptor alpha (ERα)-positive breast cancer is inevitable, necessitating the introduction of alternative treatment strategies. Therefore, the current study was carried out to investigate the in vivo efficacy and tolerability of nilotinib/rosuvastatin novel combination against ERα-positive breast carcinoma. Results showed that treatment of tumor-bearing mice with nilotinib/rosuvastatin exerted a significant antitumor activity. Mechanistically, the combination treatment efficiently inhibited the in vivo ERα protein expression, whereas ERα mRNA levels were unaffected suggesting a posttranslational regulation. In addition, the combination treatment markedly downregulated the expression of two ERα downstream target genes: C3 and pS2 confirming the inhibition of ERα signaling in vivo. Further, nilotinib/rosuvastatin combination strongly induced apoptosis evidenced by a marked caspase-3 cleavage and downregulation of tumor nitric oxide levels. Moreover, histopathology showed significant declines in mitotic figures and tumor giant cells implying the in vivo capability of the combination treatment to interfere with cancer cell proliferation and persistence. Of note, the combination treatment abrogated nilotinib-induced hypercholesterolemia and did not adversely affect the liver function or body weight. Overall, the present study provided evidences that warrant further assessment of nilotinib/rosuvastatin combination as an alternative therapeutic modality for ERα-positive breast cancer.

The N-terminal domain of the non-receptor tyrosine kinase ABL confers protein instability and suppresses tumorigenesis

Chromosome translocation can lead to chimeric proteins that may become oncogenic drivers. A classic example is the fusion of the BCR activator of RhoGEF and GTPase and the ABL proto-oncogene nonreceptor tyrosine kinase, a result of a chromosome abnormality (Philadelphia chromosome) that causes leukemia. To unravel the mechanism underlying BCR-ABL-mediated tumorigenesis, here we compared the stability of ABL and the BCR-ABL fusion. Using protein degradation, cell proliferation, 5-ethynyl-2-deoxyuridine, and apoptosis assays, along with xenograft tumor analysis, we found that the N-terminal segment of ABL, which is lost in the BCR-ABL fusion, confers degradation capacity that is promoted by SMAD-specific E3 ubiquitin protein ligase 1. We further demonstrate that the N-terminal deletion renders ABL more stable and stimulates cell growth and tumorigenesis. The findings of our study suggest that altered protein stability may contribute to chromosome translocation-induced cancer development.

Physiological and Pathological Roles of RAD52 at DNA Replication Forks

Understanding basic molecular mechanisms underlying the biology of cancer cells is of outmost importance for identification of novel therapeutic targets and biomarkers for patient stratification and better therapy selection. One of these mechanisms, the response to replication stress, fuels cancer genomic instability. It is also an Achille’s heel of cancer. Thus, identification of pathways used by the cancer cells to respond to replication-stress may assist in the identification of new biomarkers and discovery of new therapeutic targets. Alternative mechanisms that act at perturbed DNA replication forks and involve fork degradation by nucleases emerged as crucial for sensitivity of cancer cells to chemotherapeutics agents inducing replication stress. Despite its important role in homologous recombination and recombinational repair of DNA double strand breaks in lower eukaryotes, RAD52 protein has been considered dispensable in human cells and the full range of its cellular functions remained unclear. Very recently, however, human RAD52 emerged as an important player in multiple aspects of replication fork metabolism under physiological and pathological conditions. In this review, we describe recent advances on RAD52’s key functions at stalled or collapsed DNA replication forks, in particular, the unexpected role of RAD52 as a gatekeeper, which prevents unscheduled processing of DNA. Last, we will discuss how these functions can be exploited using specific inhibitors in targeted therapy or for an informed therapy selection.

Inhibition of Abl or Src tyrosine kinase decreased porcine circovirus type 2 production in PK15 cells

Porcine circovirus type 2 (PCV2) causes huge economic losses in the global swine industry and has a complex and poorly understood virus-host interaction mechanism. We reported that the C-terminal of the capsid protein of all PCV2 isolates shared a strictly conserved PXXP motif that may interact with SH3 domain-containing tyrosine kinases; however, its roles in PCV2 cell entry and replication remain unknown. In this study, we determined that mRNA levels of two SH3 domain-containing tyrosine kinases family (Abl and Src) had distinct profiles (wild-type and PXXP-mutated) during PCV2 infections of PK15 cells. Therefore, we hypothesized that activities of tyrosine kinases (Abl and Fyn) in PK15 cells may be hijacked by PCV2 via its PXXP motif of the Cap, to favor virus replication. Specific inhibitors PP2 of Lck/Fyn and STI-571 of Abl family kinases decreased viral production through suppression of DNA and Cap synthesis at the replication stage. However, based on indirect immunofluorescence assay (IFA), entry of PCV2 virus-like particles (VLPs) into PK15 cells was not altered. Elucidating mechanisms of PCV2-host interactions should provide new insights for development of new compounds to prevent or reduce PCV2 infections.

Potential protein markers for breast cancer recurrence: a retrospective cohort study

BACKGROUND

We evaluated five key proteins involved in various cancer-related pathways and assessed their relation to breast cancer recurrence.

METHODS

We used the Kentucky Cancer Registry to retrospectively identify primary invasive breast cancer cases (n = 475) that were diagnosed and treated at University of Kentucky Medical Center between 2000 and 2007. Breast cancer recurrence was observed in 62 cases during the 5-year follow-up after diagnosis. Protein expression or activity level was analyzed from surgery tissue using immuno-histochemical assays.

RESULTS

Compared to ER+/PR+/HER2- patients without recurrence, those with recurrence had higher TWIST expression (p = 0.049) but lower ABL1/ABL2 activity (p = 0.003) in primary tumors. We also found that triple-negative breast cancer patients with recurrence had higher SNAI1 expression compared to those without recurrence (p = 0.03). After adjusting for potential confounders, the higher ABL1/ABL2 activity in primary tumors was associated with a decreased risk of recurrence (OR 0.72, 95% CI 0.85-0.90) among ER+/PR+/HER2- patients. In addition, among patients with recurrence we observed that the activity level of ABL1/ABL2 was significantly increased in recurrent tumors compared to the matched primary tumors regardless of the subtype (p = 0.013).

CONCLUSIONS

These findings provide evidence that the expression/activity level of various proteins may be differentially associated with risk of recurrence of breast tumor subtypes.

EnABLing Cathepsin-Driven Melanoma Metastasis

Metastatic melanoma remains incurable for many due to its aggressive nature. Secreted cathepsins promote metastasis by cleaving matrix and activating pro-invasive proteases. We reported that ABL kinases induce cathepsin secretion and subsequent metastasis by activating ETS1, SP1, and RELA pathways, indicating that ABL inhibitors may serve as novel anti-cathepsin agents.

Interaction of Abl Tyrosine Kinases with SOCS3 Impairs Its Suppressor Function in Tumorigenesis

Suppressor of cytokine signaling 3 (SOCS3) is involved in Bcr-Abl–induced tumorigenesis. However, how SOCS3 interacts with Bcr-Abl and is regulated by Abl kinases remains largely unknown. Since c-Abl plays a critical role in tumorigenesis, we asked whether SOCS3 is regulated by c-Abl–dependent phosphorylation. Here, we found that SOCS3 interacted with all three Abl kinases (Bcr-Abl, v-Abl, and c-Abl), and SH1 domain of the Abl kinases was critically required for such interaction. Furthermore, the SH2 domain of SOCS3 was sufficient to pull down the SH1 domain but not the full length of Bcr-Abl. Importantly, SOCS3 was highly tyrosine phosphorylated by c-Abl, leading to impairment of its ability to suppress JAK8+72 activity. In addition, disrupting the tyrosine phosphorylation of SOCS3 promoted apoptosis of c-Abl–expressing cells and impeded xenograft growth of these tumor cells in nude mice. The results demonstrate that SOCS3 is highly tyrosine phosphorylated by c-Abl and that tyrosine phosphorylation of SOCS3 is required for the survival and tumorigenesis of certain cells. Our findings provide novel insights into complicated mechanisms underlying the oncogenic function of Abl kinases.

EnABLing Tumor Growth and Progression: Recent progress in unraveling the functions of ABL kinases in solid tumor cells

Purpose of Review

The goal of this review is to summarize our current knowledge regarding how ABL family kinases are activated in solid tumors and impact on solid tumor development/progression, with a focus on recent advances in the field.

Recent Findings

Although ABL kinases are known drivers of human leukemia, emerging data also implicates the kinases in a large number of solid tumor types where they promote diverse processes such as proliferation, survival, cytoskeletal reorganization, cellular polarity, EMT (epithelial-mesenchymal-transition), metabolic reprogramming, migration, invasion and metastasis via unique signaling pathways. ABL1 and ABL2 appear to have overlapping but also unique roles in driving these processes. In some tumor types, the kinases may act to integrate pro- and anti-proliferative and -invasive signals, and also may serve as a switch during EMT/MET (mesenchymal-epithelial) transitions.

Conclusions

Most data indicate that targeting ABL kinases may be effective for reducing tumor growth and preventing metastasis; however, ABL kinases also may have a tumor suppressive role in some tumor types and in some cellular contexts. Understanding the functions of ABL kinases in solid tumors is critical for developing successful clinical trials aimed at targeting ABL kinases for the treatment of solid tumors.

Coordination of signalling networks and tumorigenic properties by ABL in glioblastoma cells

The cytoplasmic tyrosine kinase ABL exerts positive or negative effects in solid tumours according to the cellular context, thus functioning as a “switch modulator”. The therapeutic effects of drugs targeting a set of signals encompassing ABL have been explored in several solid tumours. However, the net contribution of ABL inhibition by these agents remains elusive as these drugs also act on other signalling components. Here, using glioblastoma (GBM) as a cellular paradigm, we report that ABL inhibition exacerbates mesenchymal features as highlighted by down-regulation of epithelial markers and up-regulation of mesenchymal markers. Cells with permanent ABL inhibition exhibit enhanced motility and invasive capabilities, while proliferation and tumorigenic properties are reduced. Intriguingly, permanent ABL inhibition also interferes with GBM neurosphere formation and with expression of stemness markers in sphere-cultured GBM cells. Furthermore, we show that the molecular and biological characteristics of GBM cells with impaired ABL are reversible by restoring ABL levels, thus uncovering a remarkable plasticity of GBM cells to ABL threshold. A phospho-signalling screen revealed that loss of tumorigenic and self-renewal properties in GBM cells under permanent ABL inhibition coincide with drastic changes in the expression and/or phosphorylation levels of multiple signalling components. Our findings identify ABL as a crucial player for migration, invasion, proliferation, tumorigenic, and stem-cell like properties of GBM cells. Taken together, this work supports the notion that the oncogenic role of ABL in GBM cells is associated with its capability to coordinate a signalling setting that determines tumorigenic and stem-cell like properties.

Abl and Arg mediate cysteine cathepsin secretion to facilitate melanoma invasion and metastasis

The incidence of melanoma is increasing, particularly in young women, and the disease remains incurable for many because of its aggressive, metastatic nature and its high rate of resistance to conventional, targeted, and immunological agents. Cathepsins are proteases that are critical for melanoma progression and therapeutic resistance. Intracellular cathepsins cleave or degrade proteins that restrict cancer progression, whereas extracellular cathepsins directly cleave the extracellular matrix and activate proinvasive proteases in the tumor microenvironment. Cathepsin secretion is markedly increased in cancer cells. We investigated the signaling pathways leading to increased cathepsin secretion in melanoma cells. We found that the nonreceptor tyrosine kinases Abl and Arg (Abl/Arg) promoted the secretion of cathepsin B and cathepsin L by activating transcription factors (namely, Ets1, Sp1, and NF-κB/p65) that have key roles in the epithelial-mesenchymal transition (EMT), invasion, and therapeutic resistance. In some melanoma cell lines, Abl/Arg promoted the Ets1/p65-induced secretion of cathepsin B and cathepsin L in a kinase-independent manner, whereas in other melanoma lines, Abl/Arg promoted the kinase-dependent, Sp1/Ets1/p65-mediated induction of cathepsin L secretion and the Sp1/p65-mediated induction of cathepsin B secretion. As an indication of clinical relevance, the abundance of mRNAs encoding Abl/Arg, Sp1, Ets1, and cathepsins was positively correlated in primary melanomas, and Abl/Arg-driven invasion in culture and metastasis in vivo required cathepsin secretion. These data suggest that drugs targeting Abl kinases, many of which are FDA-approved, might inhibit cathepsin secretion in some melanomas and potentially other aggressive cancers harboring activated Abl kinases.

Structure-guided design of a potent peptide inhibitor targeting the interaction between CRK and ABL kinase

CT-10 regulator of kinase (CRK) proteins play important roles in human cancer metastasis and invasion. Moreover, CRK proteins are the major phosphorylation substrates of ABL kinase and its oncogenic mutant BCR-ABL kinase. The interaction between CRK and BCR-ABL plays important roles in chronic myeloid leukemia. Hence, inhibiting the interaction of CRK with BCR-ABL is an attractive way to attenuate cancer metastasis. Herein, we report the development of a peptide inhibitor, PRM-3, targeting the interaction between CRK-II and ABL kinase. PRM-3 binds to the N-terminal SH3 (nSH3) domain in CRK-II with a 10 nM affinity and prevents the interaction between CRK-II and ABL kinase. An in vitro biochemical assay demonstrated that PRM-3 inhibits the ABL-dependent phosphorylation of CRK-II more effectively than imatinib. Remarkably, PRM-3 also inhibited the CRK phosphorylation by T315I-ABL kinase, which is resistant to all first- and second-generation tyrosine kinase inhibitors. Our study provides a promising alternative approach to overcome the drug resistance of ABL kinase.

The oleocanthal-based homovanillyl sinapate as a novel c-Met inhibitor

The hepatocyte growth factor (HGF)/mesenchymal-epithelial transition factor (c-Met) signaling axis has gained considerable attention as an attractive molecular target for therapeutic blockade of cancer. Inspired by the chemical structure of S (-)-oleocanthal, a natural secoiridoid from extra-virgin olive oil with documented anticancer activity against c-Met-dependent malignancies, the research presented herein reports on the discovery of the novel olive-derived homovanillyl sinapate (HVS) as a promising c-Met inhibitor. HVS was distinguished for its remarkable potency against wild-type c-Met and its oncogenic variant in cell-free assays and confirmed by in silico docking studies. Furthermore, HVS substantially impaired the c-Met-mediated growth across a broad spectrum of breast cancer cells, while similar treatment doses had no effect on the non-tumorigenic mammary epithelial cell growth. In addition, HVS caused a dose-dependent inhibition of HGF-induced, but not epidermal growth factor (EGF)-induced, cell scattering in addition to HGF-mediated migration, invasion, and 3-dimensional (3D) proliferation of tumor cell spheroids. HVS treatment effects were mediated via inhibition of ligand-mediated c-Met activation and its downstream mitogenic signaling and blocking molecular mediators involved in cellular motility across different cellular contexts. An interesting feature of HVS is its good selectivity for c-Met and Abelson murine leukemia viral oncogene homolog 1 (ABL1) when profiled against a panel of kinases. Docking studies revealed interactions likely to impart high dual affinity for both ABL1 and c-Met kinases. HVS markedly reduced tumor growth, showed excellent pharmacodynamics, and suppressed cell proliferation and microvessel density in an orthotopic model of triple negative breast cancer. Collectively, the present findings suggested that the oleocanthal-based HVS is a promising c-Met inhibitor lead entity with excellent therapeutic potential to control malignancies with aberrant c-Met activity.

Amplification of F-Actin Disassembly and Cellular Repulsion by Growth Factor Signaling

Extracellular cues that regulate cellular shape, motility, and navigation are generally classified as growth promoting (i.e., growth factors/chemoattractants and attractive guidance cues) or growth preventing (i.e., repellents and inhibitors). Yet, these designations are often based on complex assays and undefined signaling pathways and thus may misrepresent direct roles of specific cues. Here, we find that a recognized growth-promoting signaling pathway amplifies the F-actin disassembly and repulsive effects of a growth-preventing pathway. Focusing on Semaphorin/Plexin repulsion, we identified an interaction between the F-actin-disassembly enzyme Mical and the Abl tyrosine kinase. Biochemical assays revealed Abl phosphorylates Mical to directly amplify Mical Redox-mediated F-actin disassembly. Genetic assays revealed that Abl allows growth factors and Semaphorin/Plexin repellents to combinatorially increase Mical-mediated F-actin disassembly, cellular remodeling, and repulsive axon guidance. Similar roles for Mical in growth factor/Abl-related cancer cell behaviors further revealed contexts in which characterized positive effectors of growth/guidance stimulate such negative cellular effects as F-actin disassembly/repulsion.

Head and neck cancer cell radiosensitization upon dual targeting of c-Abl and beta1-integrin

Integrin-mediated cell adhesion to extracellular matrix (ECM) critically contributes to cancer cell therapy resistance and DNA double strand break (DSB) repair. c-Abl tyrosine kinase has been linked to both of these processes. Based on our previous findings indicating c-Abl hyperphosphorylation on tyrosine (Y) 412 and threonine (T) 735 upon beta1-integrin inhibition, we hypothesized c-Abl tyrosine kinase as an important mediator of beta1-integrin signaling for radioresistance. In a panel of 8 cell lines from different solid cancer types grown in 3D laminin-rich ECM cultures, we targeted beta1 integrin with AIIB2 (mAb) and c-Abl with Imatinib with and without X-ray irradiation and subsequently examined clonogenic survival, residual DSBs, protein expression and phosphorylation. Single or combined treatment with AIIB2 and Imatinib resulted in cell line-dependent cytotoxicity. Intriguingly, we identified a subgroup of this cell line panel that responded with a higher degree of radiosensitization to AIIB2/Imatinib relative to both single treatments. In this subgroup, we observed a non-statistically significant trend between the radioresponse and phospho-c-Abl Y412. Mechanistically, impairment of DNA repair seems to be associated with radiosensitization upon AIIB2/Imatinib and AIIB2/Imatinib-related radiosensitization could be reduced by exogenous overexpression of either wildtype or constitutively active c-Abl forms relative to controls. Our data generated in more physiological 3D cancer cell culture models suggest c-Abl as further determinant of radioresistance and DNA repair downstream of beta1-integrin. For solid cancers, c-Abl phosphorylation status might be an indicator for reasonable Imatinib application as adjuvant for conventional radio(chemo)therapy.

Abl kinase regulation by BRAF/ERK and cooperation with Akt in melanoma

The melanoma incidence continues to increase, and the disease remains incurable for many due to its metastatic nature and high rate of therapeutic resistance. In particular, melanomas harboring BRAF^V600E and PTEN mutations often are resistant to current therapies, including BRAF inhibitors (BRAFi) and immune checkpoint inhibitors. Abl kinases (Abl/Arg) are activated in melanomas and drive progression; however, their mechanism of activation has not been established. Here we elucidate a novel link between BRAF^V600E/ERK signaling and Abl kinases. We demonstrate that BRAF^V600E/ERK play a critical role in binding, phosphorylating and regulating Abl localization and Abl/Arg activation by Src family kinases. Importantly, Abl/Arg activation downstream of BRAF^V600E has functional and biological significance, driving proliferation, invasion, as well as switch in epithelial-mesenchymal-transition transcription factor expression, which is known to be critical for melanoma cells to shift between differentiated and invasive states. Finally, we describe findings of high translational significance by demonstrating that Abl/Arg cooperate with PI3K/Akt/PTEN, a parallel pathway that is associated with intrinsic resistance to BRAFi and immunotherapy, as Abl/Arg and Akt inhibitors cooperate to prevent viability, cell cycle progression and in vivo growth of melanomas harboring mutant BRAF/PTEN. Thus, these data not only provide mechanistic insight into Abl/Arg regulation during melanoma development, but also pave the way for the development of new strategies for treating patients with melanomas harboring mutant BRAF/PTEN, which often are refractory to current therapies.

Hepatitis C Virus E1 protein promotes cell migration and invasion by modulating cellular metastasis suppressor Nm23-H1

Hepatocellular carcinoma (HCC) is the most prevalent primary liver cancer and its incidence is on the rise largely attributed to Hepatitis C virus (HCV) related liver cancer. A distinct feature of HCV associated HCC is the substantially increased incidence of metastasis compared to non-viral or HBV associated HCC. Nm23-H1 is the first reported human metastasis suppressor down-regulated in many human metastatic cancers. Nm23-H1 functions are modulated in several virus associated cancers. Our study now shows that HCV E1 protein expression as well as HCV infection induces pro-metastatic effect on cancer cells which is simultaneous to Nm23-H1 transcriptional down-regulation and Nm23-H1 protein degradation. Moreover, Nm23-H1 intracellular localization is significantly altered in cells expressing HCV E1 protein. Importantly, overexpression of Nm23-H1 can rescue the cancer cells from pro-metastatic effects of HCV E1 and HCV infection. Our limited study provides evidence for role for Nm23-H1 in HCV mediated cancer metastasis.

System analysis identifies distinct and common functional networks governed by transcription factor ASCL1, in glioma and small cell lung cancer

Systematic functional network analysis of ASCL1 revealed that it regulates mitosis and cell proliferation pathways and has distinct functions in glioma and SCLC.

Extracellular Visfatin-Promoted Malignant Behavior in Breast Cancer Is Mediated Through c-Abl and STAT3 Activation

PURPOSE

Visfatin is an adipocytokine involved in cellular metabolism, inflammation, and cancer. This study investigated the roles of extracellular visfatin in breast cancer, and explored underlying mechanisms in clinical and experimental settings.

EXPERIMENTAL DESIGN

Associations of serum visfatin with clinicopathologic characteristics and patient survival were assessed with Cox regression models and Kaplan-Meier analyses. Effects of extracellular visfatin on cultured breast cancer cells were examined, followed by in vivo investigation of tumor growth and metastasis in xenograft animal models. Imatinib and Stattic were used to inhibit c-Abl and STAT3 activation, respectively.

RESULTS

Breast cancer patients with high serum visfatin levels were associated with advanced tumor stage, increased tumor size and lymph node metastasis, and poor survival. Elevated phosphorylation of c-Abl and STAT3 in breast tumor tissues were correlated with high serum visfatin levels in patients. Visfatin-promoted in vitro cell viability and metastatic capability were suppressed by imatinib (c-Abl inhibitor) and Stattic (STAT3 inhibitor). Increased in vivo cell invasiveness was observed in zebrafish xenografted with visfatin-pretreated breast cancer cells. Tumor growth and lung metastasis occurred in visfatin-administered mice xenografted with breast cancer cells. Tail vein-injected mice with visfatin-pretreated breast cancer cells showed increased lung metastasis, which was suppressed by imatinib.

CONCLUSIONS

Serum visfatin levels in breast cancer patients reveal potential prognostic values, and our findings that visfatin promoted breast cancer through activation of c-Abl and STAT3 may provide an important molecular basis for future design of targeted therapies that take into account different serum visfatin levels in breast cancer. Clin Cancer Res; 22(17); 4478-90. ©2016 AACR.

ABL kinases promote breast cancer osteolytic metastasis by modulating tumor-bone interactions through TAZ and STAT5 signaling

Bone metastases occur in up to 70% of advanced breast cancer. For most patients with breast cancer, bone metastases are predominantly osteolytic. Interactions between tumor cells and stromal cells in the bone microenvironment drive osteolytic bone metastasis, a process that requires the activation of osteoclasts, cells that break down bone. We report that ABL kinases promoted metastasis of breast cancer cells to bone by regulating the crosstalk between tumor cells and the bone microenvironment. ABL kinases protected tumor cells from apoptosis induced by TRAIL (TNF-related apoptosis-inducing ligand), activated the transcription factor STAT5, and promoted osteolysis through the STAT5-dependent expression of genes encoding the osteoclast-activating factors interleukin-6 (IL-6) and matrix metalloproteinase 1 (MMP1). Furthermore, in breast cancer cells, ABL kinases increased the abundance of the Hippo pathway mediator TAZ and the expression of TAZ-dependent target genes that promote bone metastasis. Knockdown of ABL kinases or treatment with ABL-specific allosteric inhibitor impaired osteolytic metastasis of breast cancer cells in mice. These findings revealed a role for ABL kinases in regulating tumor-bone interactions and provide a rationale for using ABL-specific inhibitors to limit breast cancer metastasis to bone.

The Emerging Role of ABL Kinases in Solid Tumors

The Abelson (ABL) tyrosine kinases were identified as drivers of leukemia in mice and humans. Emerging data has shown a role for the ABL family kinases, ABL1 and ABL2, in the progression of several solid tumors. This review will focus on recent reports of the involvement of the ABL kinases in tumor progression using mouse models as well as recent data generated from genomic and proteomic studies linking enhanced expression and hyper-activation of the ABL kinases to some human cancers. Preclinical studies on small molecule inhibitors of the ABL kinases suggest that their use may have beneficial effects for the treatment of selected solid tumors.

Invadopodia: a new therapeutic target to block cancer metastasis

Cancer cells become dangerous when they acquire the ability to invade through physical barriers in the body and disseminate to distant sites. Recent evidence has demonstrated that cancer cells utilize specialized structures called invadopodia, unique protrusions that concentrate proteases such as matrix metalloproteinases (MMPs), to escape blood vessels during the process of extravasation. Perhaps most exciting is the fact that inhibition of invadopodia through genetic or pharmacological means reduces the ability of cancer cells to extravasate and effectively blocks metastasis. This opens the door for the development of novel therapies targeting invadopodia and cancer metastasis.

ABL tyrosine kinase inhibition variable effects on the invasive properties of different triple negative breast cancer cell lines

The non-receptor tyrosine kinase ABL drives myeloid progenitor expansion in human chronic myeloid leukemia. ABL inhibition by the tyrosine kinase inhibitor nilotinib is a first-line treatment for this disease. Recently, ABL has also been implicated in the transforming properties of solid tumors, including triple negative (TN) breast cancer. TN breast cancers are highly metastatic and several cell lines derived from these tumors display high invasive activity in vitro. This feature is associated with the activation of actin-rich membrane structures called invadopodia that promote extracellular matrix degradation. Here, we investigated nilotinib effect on the invasive and migratory properties of different TN breast cancer cell lines. Nilotinib decreased both matrix degradation and invasion in the TN breast cancer cell lines MDA-MB 231 and MDA-MB 468. However, and unexpectedly, nilotinib increased by two-fold the invasive properties of the TN breast cancer cell line BT-549 and of Src-transformed fibroblasts. Both display much higher levels of ABL kinase activity compared to MDA-MB 231. Similar effects were obtained by siRNA-mediated down-regulation of ABL expression, confirming ABL central role in this process. ABL anti-tumor effect in BT-549 cells and Src-transformed fibroblasts was not dependent on EGF secretion, as recently reported in neck and squamous carcinoma cells. Rather, we identified the TRIO-RAC1 axis as an important downstream element of ABL activity in these cancer cells. In conclusion, the observation that TN breast cancer cell lines respond differently to ABL inhibitors could have implications for future therapies.

Reduced miR-3127-5p expression promotes NSCLC proliferation/invasion and contributes to dasatinib sensitivity via the c-Abl/Ras/ERK pathway

miR-3127-5p is a primate-specific miRNA which is down-regulated in recurrent NSCLC tissue vs. matched primary tumor tissue (N = 15) and in tumor tissue vs. normal lung tissue (N = 177). Reduced miR-3127-5p expression is associated with a higher Ki-67 proliferation index and unfavorable prognosis in NSCLC. Overexpression of miR-3127-5p significantly reduced NSCLC cells proliferation, migration, and motility in vitro and in vivo. The oncogene ABL1 was a direct miR-3127-5p target, and miR-3127-5p regulated the activation of the Abl/Ras/ERK pathway and transactivated downstream proliferation/metastasis-associated molecules. Overexpression of miR-3127-5p in A549 or H292 cells resulted in enhanced resistance to dasatinib, an Abl/src tyrosine kinase inhibitor. miR-3127-5p expression levels were correlated with dasatinib sensitivity in NSCLC cell lines without K-Ras G12 mutation. In conclusion, miR-3127-5p acts as a tumor suppressor gene and is a potential biomarker for dasatinib sensitivity in the non-mutated Ras subset of NSCLC.

Pathways driving the endocytosis of mutant and wild-type EGFR in cancer

EGFR (epidermal growth factor receptor) is activated through changes in expression or mutations in a number of tumors and is a driving force in cancer progression. EGFR is targeted by numerous inhibitors, including chimeric antibodies targeting the extracellular domain and small molecule kinase domain inhibitors. The kinase domain inhibitors are particularly active against mutant forms of the receptor, and subsequent mutations drive resistance to the inhibitors. Here, we review recent developments on the trafficking of wild-type and mutant EGFR, focusing on the roles of MIG6, SPRY2, ITSN, SHP2, S2R^PGRMC1 and RAK. Some classes of EGFR regulators affect wild-type and mutant EGFR equally, while others are specific for either the wild-type or mutant form of the receptor. Below we summarize multiple signaling-associated pathways that are important in trafficking wild-type and mutant EGFR with the goal being stimulation of new approaches for targeting the distinct forms of the receptor.

A phosphotyrosine switch determines the antitumor activity of ERβ

Estrogen receptors ERα and ERβ share considerable sequence homology yet exert opposite effects on breast cancer cell proliferation. While the proliferative role of ERα in breast tumors is well characterized, it is not clear whether the antitumor activity of ERβ can be mobilized in breast cancer cells. Here, we have shown that phosphorylation of a tyrosine residue (Y36) present in ERβ, but not in ERα, dictates ERβ-specific activation of transcription and is required for ERβ-dependent inhibition of cancer cell growth in culture and in murine xenografts. Additionally, the c-ABL tyrosine kinase and EYA2 phosphatase directly and diametrically controlled the phosphorylation status of Y36 and subsequent ERβ function. A nonphosphorylatable, transcriptionally active ERβ mutant retained antitumor activity but circumvented control by upstream regulators. Phosphorylation of Y36 was required for ERβ-mediated coactivator recruitment to ERβ target promoters. In human breast cancer samples, elevated phosphorylation of Y36 in ERβ correlated with high levels of c-ABL but low EYA2 levels. Furthermore, compared with total ERβ, the presence of phosphorylated Y36-specific ERβ was strongly associated with both disease-free and overall survival in patients with stage II and III disease. Together, these data identify a signaling circuitry that regulates ERβ-specific antitumor activity and has potential as both a prognostic tool and a molecular target for cancer therapy.

MicroRNA-4723 inhibits prostate cancer growth through inactivation of the Abelson family of nonreceptor protein tyrosine kinases

The Abelson (c-Abl) proto-oncogene encodes a highly conserved nonreceptor protein tyrosine kinase that plays a role in cell proliferation, differentiation, apoptosis and cell adhesion. c-Abl represents a specific anti-cancer target in prostate cancer as aberrant activity of this kinase has been implicated in the stimulation of prostate cancer growth and progression. However, the mechanism of regulation of c-Abl is not known. Here we report that Abl kinases are regulated by a novel microRNA, miR-4723, in prostate cancer. Expression profiling of miR-4723 expression in a cohort of prostate cancer clinical specimens showed that miR-4723 expression is widely attenuated in prostate cancer. Low miR-4723 expression was significantly correlated with poor survival outcome and our analyses suggest that miR-4723 has significant potential as a disease biomarker for diagnosis and prognosis in prostate cancer. To evaluate the functional significance of decreased miR-4723 expression in prostate cancer, miR-4723 was overexpressed in prostate cancer cell lines followed by functional assays. miR-4723 overexpression led to significant decreases in cell growth, clonability, invasion and migration. Importantly, miR-4723 expression led to dramatic induction of apoptosis in prostate cancer cell lines suggesting that miR-4723 is a pro-apoptotic miRNA regulating prostate carcinogenesis. Analysis of putative miR-4723 targets showed that miR-4723 targets integrin alpha 3 and Methyl CpG binding protein in addition to Abl1 and Abl2 kinases. Further, we found that the expression of Abl kinase is inversely correlated with miR-4723 expression in prostate cancer clinical specimens. Also, Abl1 knockdown partially phenocopies miR-4723 reexpression in prostate cancer cells suggesting that Abl is a functionally relevant target of miR-4723 in prostate cancer. In conclusion, we have identified a novel microRNA that mediates regulation of Abl kinases in prostate cancer. This study suggests that miR-4723 may be an attractive target for therapeutic intervention in prostate cancer.

c-Abl and Arg induce cathepsin-mediated lysosomal degradation of the NM23-H1 metastasis suppressor in invasive cancer

Metastasis suppressors comprise a growing class of genes whose downregulation triggers metastatic progression. In contrast to tumor suppressors, metastasis suppressors are rarely mutated or deleted, and little is known regarding the mechanisms by which their expression is downregulated. Here, we demonstrate that the metastasis suppressor, NM23-H1, is degraded by lysosomal cysteine cathepsins (L,B), which directly cleave NM23-H1. In addition, activation of c-Abl and Arg oncoproteins induces NM23-H1 degradation in invasive cancer cells by increasing cysteine cathepsin transcription and activation. Moreover, c-Abl activates cathepsins by promoting endosome maturation, which facilitates trafficking of NM23-H1 to the lysosome where it is degraded. Importantly, the invasion- and metastasis-promoting activity of c-Abl/Arg is dependent on their ability to induce NM23-H1 degradation, and the pathway is clinically relevant as c-Abl/Arg activity and NM23-H1 expression are inversely correlated in primary breast cancers and melanomas. Thus, we demonstrate a novel mechanism by which cathepsin expression is upregulated in cancer cells (via Abl kinases). We also identify a novel role for intracellular cathepsins in invasion and metastasis (degradation of a metastasis suppressor). Finally, we identify novel crosstalk between oncogenic and metastasis suppressor pathways, thereby providing mechanistic insight into the process of NM23-H1 loss, which may pave the way for new strategies to restore NM23-H1 expression and block metastatic progression.

Role of ABL family kinases in cancer: from leukaemia to solid tumours

The Abelson (ABL) family of nonreceptor tyrosine kinases, ABL1 and ABL2, transduces diverse extracellular signals to protein networks that control proliferation, survival, migration and invasion. ABL1 was first identified as an oncogene required for the development of leukaemias initiated by retroviruses or chromosome translocations. The demonstration that small-molecule ABL kinase inhibitors could effectively treat chronic myeloid leukaemia opened the door to the era of targeted cancer therapies. Recent reports have uncovered roles for ABL kinases in solid tumours. Enhanced ABL expression and activation in some solid tumours, together with altered cell polarity, invasion or growth induced by activated ABL kinases, suggest that drugs targeting these kinases may be useful for treating selected solid tumours.