Receptor tyrosine kinase AXL is induced by chemotherapy drugs and overexpression of AXL confers drug resistance in acute myeloid leukemia

N6-methyladenosine RNA modifications: a potential therapeutic target for AML

N6-methyladenosine (m6A) RNA modification has recently emerged as an essential regulator of normal and malignant hematopoiesis. As a reversible epigenetic modification found in messenger RNAs and non-coding RNAs, m6A affects the fate of the modified RNA molecules. It is essential in most vital bioprocesses, contributing to cancer development. Here, we review the up-to-date knowledge of the pathological functions and underlying molecular mechanism of m6A modifications in normal hematopoiesis, leukemia pathogenesis, and drug response/resistance. At last, we discuss the critical role of m6A in immune response, the therapeutic potential of targeting m6A regulators, and the possible combination therapy for AML.

AXL receptor as an emerging molecular target in colorectal cancer

AXL receptor tyrosine kinase (AXL) is a receptor tyrosine kinase whose aberrant expression has recently been associated with colorectal cancer (CRC), contributing to tumor growth, epithelial-mesenchymal transition (EMT), increased invasiveness, metastatic spreading, and the development of drug resistance. In this review we summarize preclinical data, the majority of which are limited to recent years, convincingly linking the AXL receptor to CRC. These findings support the value of targeting AXL with molecules in drug discovery, offering novel and advanced therapeutic or diagnostic tools for CRC management.

In Situ Nanofiber Formation Blocks AXL and GAS6 Binding to Suppress Ovarian Cancer Development

Anexelekto (AXL) is an attractive molecular target for ovarian cancer therapy because of its important role in ovarian cancer initiation and progression. To date, several AXL inhibitors have entered clinical trials for the treatment of ovarian cancer. However, the disadvantages of low AXL affinity and severe off-target toxicity of these inhibitors limit their further clinical applications. Herein, by rational design of a nonapeptide derivative Nap-Phe-Phe-Glu-Ile-Arg-Leu-Arg-Phe-Lys (Nap-IR), a strategy of in situ nanofiber formation is proposed to suppress ovarian cancer growth. After administration, Nap-IR specifically targets overexpressed AXL on ovarian cancer cell membranes and undergoes a receptor-instructed nanoparticle-to-nanofiber transition. In vivo and in vitro experiments demonstrate that in situ formed Nap-IR nanofibers efficiently induce apoptosis of ovarian cancer cells by blocking AXL activation and disrupting subsequent downstream signaling events. Remarkably, Nap-IR can synergistically enhance the anticancer effect of cisplatin against HO8910 ovarian tumors. It is anticipated that the Nap-IR can be applied in clinical ovarian cancer therapy in the near future.

AXL/WRNIP1 Mediates Replication Stress Response and Promotes Therapy Resistance and Metachronous Metastasis in HER2+ Breast Cancer

Therapy resistance and metastatic progression are primary causes of cancer-related mortality. Disseminated tumor cells possess adaptive traits that enable them to reprogram their metabolism, maintain stemness, and resist cell death, facilitating their persistence to drive recurrence. The survival of disseminated tumor cells also depends on their ability to modulate replication stress in response to therapy while colonizing inhospitable microenvironments. In this study, we discovered that the nuclear translocation of AXL, a TAM receptor tyrosine kinase, and its interaction with WRNIP1, a DNA replication stress response factor, promotes the survival of HER2+ breast cancer cells that are resistant to HER2-targeted therapy and metastasize to the brain. In preclinical models, knocking down or pharmacologically inhibiting AXL or WRNIP1 attenuated protection of stalled replication forks. Furthermore, deficiency or inhibition of AXL and WRNIP1 also prolonged metastatic latency and delayed relapse. Together, these findings suggest that targeting the replication stress response, which is a shared adaptive mechanism in therapy-resistant and metastasis-initiating cells, could reduce metachronous metastasis and enhance the response to standard-of-care therapies.

SIGNIFICANCE

Nuclear AXL and WRNIP1 interact and mediate replication stress response, promote therapy resistance, and support metastatic progression, indicating that targeting the AXL/WRNIP1 axis is a potentially viable therapeutic strategy for breast cancer.

Signaling Pathways of AXL Receptor Tyrosine Kinase Contribute to the Pathogenetic Mechanisms of Glioblastoma

Brain tumors are a diverse collection of neoplasms affecting the brain with a high prevalence rate in people of all ages around the globe. In this pathological context, glioblastoma, a form of glioma that belongs to the IV-grade astrocytoma group, is the most common and most aggressive form of the primary brain tumors. Indeed, despite the best treatments available including surgery, radiotherapy or a pharmacological approach with Temozolomide, glioblastoma patients' mortality is still high, within a few months of diagnosis. Therefore, to increase the chances of these patients surviving, it is critical to keep finding novel treatment opportunities. In the past, efforts to treat glioblastoma have mostly concentrated on customized treatment plans that target specific mutations such as epidermal growth factor receptor (EGFR) mutations, Neurotrophic Tyrosine Receptor Kinase (NTRK) fusions, or multiple receptors using multi-kinase inhibitors like Sunitinib and Regorafenib, with varying degrees of success. Here, we focused on the receptor tyrosine kinase AXL that has been identified as a mediator for tumor progression and therapy resistance in various cancer types, including squamous cell tumors, small cell lung cancer, and breast cancer. Activated AXL leads to a significant increase in tumor proliferation, tumor cell migration, and angiogenesis in different in vitro and in vivo models of cancer since this receptor regulates interplay with apoptotic, angiogenic and inflammatory pathways. Based on these premises, in this review we mainly focused on the role of AXL in the course of glioblastoma, considering its primary biological mechanisms and as a possible target for the application of the most recent treatments.

Structure-activity relationship study of 1,6-naphthyridinone derivatives as selective type II AXL inhibitors with potent antitumor efficacy

The role of AXL in various oncogenic processes has made it an attractive target for cancer therapy. Currently, kinase selectivity profiles, especially circumventing MET inhibition, remain a scientific issue of great interest in the discovery of selective type II AXL inhibitors. Starting from a dual MET/AXL-targeted lead structure from our previous work, we optimized a 1,6-naphthyridinone series using molecular modeling-assisted compound design to improve AXL potency and selectivity over MET, resulting in the potent and selective type II AXL-targeted compound 25c. This showed excellent AXL inhibitory activity (IC50 = 1.1 nM) and 343-fold selectivity over the highly homologous kinase MET in biochemical assays. Moreover, compound 25c significantly inhibited AXL-driven cell proliferation, dose-dependently suppressed 4T1 cell migration and invasion, and induced apoptosis. Compound 25c also showed noticeable antitumor efficacy in a BaF3/TEL-AXL xenograft model at well-tolerated doses. Overall, this study presented a potent and selective type II AXL-targeted lead compound for further drug discovery.

Development of Resistance to Type II JAK2 Inhibitors in MPN Depends on AXL Kinase and Is Targetable

PURPOSE

Myeloproliferative neoplasms (MPN) dysregulate JAK2 signaling. Because clinical JAK2 inhibitors have limited disease-modifying effects, type II JAK2 inhibitors such as CHZ868 stabilizing inactive JAK2 and reducing MPN clones, gain interest. We studied whether MPN cells escape from type ll inhibition.

EXPERIMENTAL DESIGN

MPN cells were continuously exposed to CHZ868. We used phosphoproteomic analyses and ATAC/RNA sequencing to characterize acquired resistance to type II JAK2 inhibition, and targeted candidate mediators in MPN cells and mice.

RESULTS

MPN cells showed increased IC50 and reduced apoptosis upon CHZ868 reflecting acquired resistance to JAK2 inhibition. Among >2,500 differential phospho-sites, MAPK pathway activation was most prominent, while JAK2-STAT3/5 remained suppressed. Altered histone occupancy promoting AP-1/GATA binding motif exposure associated with upregulated AXL kinase and enriched RAS target gene profiles. AXL knockdown resensitized MPN cells and combined JAK2/AXL inhibition using bemcentinib or gilteritinib reduced IC50 to levels of sensitive cells. While resistant cells induced tumor growth in NOD/SCID gamma mice despite JAK2 inhibition, JAK2/AXL inhibition largely prevented tumor progression. Because inhibitors of MAPK pathway kinases such as MEK are clinically used in other malignancies, we evaluated JAK2/MAPK inhibition with trametinib to interfere with AXL/MAPK-induced resistance. Tumor growth was halted similarly to JAK2/AXL inhibition and in a systemic cell line-derived mouse model, marrow infiltration was decreased supporting dependency on AXL/MAPK.

CONCLUSIONS

We report on a novel mechanism of AXL/MAPK-driven escape from type II JAK2 inhibition, which is targetable at different nodes. This highlights AXL as mediator of acquired resistance warranting inhibition to enhance sustainability of JAK2 inhibition in MPN.

DNMT1 determines osteosarcoma cell resistance to apoptosis by associatively modulating DNA and mRNA cytosine-5 methylation

Cellular apoptosis is a central mechanism leveraged by chemotherapy to treat human cancers. 5-Methylcytosine (m5C) modifications installed on both DNA and mRNA are documented to regulate apoptosis independently. However, the interplay or crosstalk between them in cellular apoptosis has not yet been explored. Here, we reported that promoter methylation by DNMT1 coordinated with mRNA methylation by NSun2 to regulate osteosarcoma cell apoptosis. DNMT1 was induced during osteosarcoma cell apoptosis triggered by chemotherapeutic drugs, whereas NSun2 expression was suppressed. DNMT1 was found to repress NSun2 expression by methylating the NSun2 promoter. Moreover, DNMT1 and NSun2 regulate the anti-apoptotic genes AXL, NOTCH2, and YAP1 through DNA and mRNA methylation, respectively. Upon exposure to cisplatin or doxorubicin, DNMT1 elevation drastically reduced the expression of these anti-apoptotic genes via enhanced promoter methylation coupled with NSun2 ablation-mediated attenuation of mRNA methylation, thus rendering osteosarcoma cells to apoptosis. Collectively, our findings establish crosstalk of importance between DNA and RNA cytosine methylations in determining osteosarcoma resistance to apoptosis during chemotherapy, shedding new light on future treatment of osteosarcoma, and adding additional layers to the control of gene expression at different epigenetic levels.

TAM family kinases as therapeutic targets at the interface of cancer and immunity

Novel treatment approaches are needed to overcome innate and acquired mechanisms of resistance to current anticancer therapies in cancer cells and the tumour immune microenvironment. The TAM (TYRO3, AXL and MERTK) family receptor tyrosine kinases (RTKs) are potential therapeutic targets in a wide range of cancers. In cancer cells, TAM RTKs activate signalling pathways that promote cell survival, metastasis and resistance to a variety of chemotherapeutic agents and targeted therapies. TAM RTKs also function in innate immune cells, contributing to various mechanisms that suppress antitumour immunity and promote resistance to immune-checkpoint inhibitors. Therefore, TAM antagonists provide an unprecedented opportunity for both direct and immune-mediated therapeutic activity provided by inhibition of a single target, and are likely to be particularly effective when used in combination with other cancer therapies. To exploit this potential, a variety of agents have been designed to selectively target TAM RTKs, many of which have now entered clinical testing. This Review provides an essential guide to the TAM RTKs for clinicians, including an overview of the rationale for therapeutic targeting of TAM RTKs in cancer cells and the tumour immune microenvironment, a description of the current preclinical and clinical experience with TAM inhibitors, and a perspective on strategies for continued development of TAM-targeted agents for oncology applications.

AXL is required for hypoxia-mediated hypoxia-inducible factor-1 alpha function in glioblastoma

Glioblastoma (GBM) is the most aggressive type of central nervous system tumor. Molecular targeting may be important when developing efficient GBM treatment strategies. Sequencing of GBMs revealed that the receptor tyrosine kinase (RTK)/RAS/phosphatidylinositol-3-kinase pathway was altered in 88% of samples. Interestingly, AXL, a member of RTK, was proposed as a promising target in glioma therapy. However, the molecular mechanism of AXL modulation of GBM genesis and proliferation is still unclear. In this study, we investigated the expression and localization of hypoxia-inducible factor-1 alpha (HIF-1α) by AXL in GBM. Both AXL mRNA and protein are overexpressed in GBM. Short-interfering RNA knockdown of AXL in U251-MG cells reduced viability and migration. However, serum withdrawal reduced AXL expression, abolishing the effect on viability. AXL is also involved in hypoxia regulation. In hypoxic conditions, the reduction of AXL decreased the level and nuclear localization of HIF-1α. The co-expression of HIF-1α and AXL was found in human GBM samples but not normal tissue. This finding suggests a mechanism for GBM proliferation and indicates that targeting AXL may be a potential GBM therapeutic.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43188-023-00195-z.

Gas6/AXL pathway: immunological landscape and therapeutic potential

Cancer is a disease with ecological and evolutionary unity, which seriously affects the survival and quality of human beings. Currently, many reports have suggested Gas6 plays an important role in cancer. Binding of gas6 to TAM receptors is associated with the carcinogenetic mechanisms of multiple malignancies, such as in breast cancer, chronic lymphocytic leukemia, non-small cell lung cancer, melanoma, prostate cancer, etc., and shortened overall survival. It is accepted that the Gas6/TAM pathway can promote the malignant transformation of various types of cancer cells. Gas6 has the highest affinity for Axl, an important member of the TAM receptor family. Knockdown of the TAM receptors Axl significantly affects cell cycle progression in tumor cells. Interestingly, Gas6 also has an essential function in the tumor microenvironment. The Gas6/AXL pathway regulates angiogenesis, immune-related molecular markers and the secretion of certain cytokines in the tumor microenvironment, and also modulates the functions of a variety of immune cells. In addition, evidence suggests that the Gas6/AXL pathway is involved in tumor therapy resistance. Recently, multiple studies have begun to explore in depth the importance of the Gas6/AXL pathway as a potential tumor therapeutic target as well as its broad promise in immunotherapy; therefore, a timely review of the characteristics of the Gas6/AXL pathway and its value in tumor treatment strategies is warranted. This comprehensive review assessed the roles of Gas6 and AXL receptors and their associated pathways in carcinogenesis and cancer progression, summarized the impact of Gas6/AXL on the tumor microenvironment, and highlighted the recent research progress on the relationship between Gas6/AXL and cancer drug resistance.

A Receptor Tyrosine Kinase Inhibitor Sensitivity Prediction Model Identifies AXL Dependency in Leukemia

Despite incredible progress in cancer treatment, therapy resistance remains the leading limiting factor for long-term survival. During drug treatment, several genes are transcriptionally upregulated to mediate drug tolerance. Using highly variable genes and pharmacogenomic data for acute myeloid leukemia (AML), we developed a drug sensitivity prediction model for the receptor tyrosine kinase inhibitor sorafenib and achieved more than 80% prediction accuracy. Furthermore, by using Shapley additive explanations for determining leading features, we identified AXL as an important feature for drug resistance. Drug-resistant patient samples displayed enrichment of protein kinase C (PKC) signaling, which was also identified in sorafenib-treated FLT3-ITD-dependent AML cell lines by a peptide-based kinase profiling assay. Finally, we show that pharmacological inhibition of tyrosine kinase activity enhances AXL expression, phosphorylation of the PKC-substrate cyclic AMP response element binding (CREB) protein, and displays synergy with AXL and PKC inhibitors. Collectively, our data suggest an involvement of AXL in tyrosine kinase inhibitor resistance and link PKC activation as a possible signaling mediator.

Discovery of AXL Degraders with Improved Potencies in Triple-Negative Breast Cancer (TNBC) Cells

AXL kinase is heavily involved in tumorigenesis, metastasis, and drug resistance of many cancers, and several AXL inhibitors are in clinical investigations. Recent studies demonstrated that the N-terminal distal region of AXL plays more important roles in cell invasiveness than its C-terminal kinase domain. Therefore, degradation of AXL may present a novel superior therapeutic approach than the kinase inhibitor therapy. Herein, we report the discovery of a series of new AXL PROTAC degraders. One representative compound 6n potently depletes AXL with a DC₅₀ value of 5 nM in MDA-MB-231 TNBC cells. It also demonstrates significantly improved potencies against the AXL signaling activation, cell proliferation, migration and invasion of TNBC cells comparing with the corresponding kinase inhibitor. Moreover, the compound exhibits promising therapeutic potential both in patient-derived organoids and a xenograft mouse model of MDA-MB-231 cells.

ER-851, a Novel Selective Inhibitor of AXL, Overcomes Resistance to Antimitotic Drugs

Innate and adaptive resistance to cancer therapies, such as chemotherapies, molecularly targeted therapies, and immune-modulating therapies, is a major issue in clinical practice. Subpopulations of tumor cells expressing the receptor tyrosine kinase AXL become enriched after treatment with antimitotic drugs, causing tumor relapse. Elevated AXL expression is closely associated with drug resistance in clinical samples, suggesting that AXL plays a pivotal role in drug resistance. Although several molecules with AXL inhibitory activity have been developed, none have sufficient activity and selectivity to be clinically effective when administered in combination with a cancer therapy. Here, we report a novel small molecule, ER-851, which is a potent and highly selective AXL inhibitor. To investigate resistance mechanisms and identify driving molecules, we conducted a comprehensive gene expression analysis of chemoresistant tumor cells in mouse xenograft models of genetically engineered human lung cancer and human triple-negative breast cancer. Consistent with the effect of AXL knockdown, cotreatment of ER-851 and antimitotic drugs produced an antitumor effect and prolonged relapse-free survival in the mouse xenograft model of human triple-negative breast cancer. Importantly, when orally administered to BALB/c mice, this compound did not induce retinal toxicity, a known side effect of chronic MER inhibition. Together, these data strongly suggest that AXL is a therapeutic target for overcoming drug resistance and that ER-851 is a promising candidate therapeutic agent for use against AXL-expressing antimitotic-resistant tumors.

Enapotamab Vedotin, an AXL-Specific Antibody-Drug Conjugate, Demonstrates Antitumor Efficacy in Patient-Derived Xenograft Models of Soft Tissue Sarcoma

Doxorubicin (doxo) remains the standard of care for patients with advanced soft tissue sarcoma (STS), even though response rates to doxo are only around 14% to 18%. We evaluated enapotamab vedotin (EnaV), an AXL-specific antibody-drug conjugate (ADC), in a panel of STS patient-derived xenografts (PDX). Eight models representing multiple STS subtypes were selected from our STS PDX platform (n = 45) by AXL immunostaining on archived passages. Models were expanded by unilateral transplantation of tumor tissue into the left flank of 20 NMRI nu/nu mice. Once tumors were established, mice were randomized into an EnaV treatment group, or a group treated with isotype control ADC. Treatment efficacy was assessed by tumor volume evaluation, survival analysis, and histological evaluation of tumors, and associated with AXL expression. EnaV demonstrated significant tumor growth delay, regression, and/or prolonged survival compared to isotype control ADC in 5/8 STS PDX models investigated. Experimental passages of responding models were all found positive for AXL at varying levels, but no linear relationship could be identified between the level of expression and level of response to EnaV. One model was found negative for AXL on experimental passage and did not respond to EnaV. This study provides a preclinical rationale for the evaluation of AXL-targeting ADCs in the treatment of AXL-expressing sarcomas.

Comprehensive Analysis of N6-Methyladenosine-Related Long Noncoding RNA Prognosis of Acute Myeloid Leukemia and Immune Cell Infiltration

N6-Methyladenosine-related long noncoding RNAs play an essential role in many cancers’ development. However, the relationship between m6A-related lncRNAs and acute myelogenous leukemia (AML) prognosis remains unclear. We systematically analyzed the association of m6A-related lncRNAs with the prognosis and tumor immune microenvironment (TME) features using the therapeutically applicable research to generate effective treatment (TARGET) database. We screened 315 lncRNAs associated with AML prognosis and identified nine key lncRNAs associated with m6A by the LASSO Cox analysis. A model was established based on these nine lncRNAs and the predictive power was explored in The Cancer Genome Atlas (TCGA) database. The areas under the ROC curve of TARGET and TCGA databases for ROC at 1, 3, and 5 years are 0.701, 0.704, and 0.696, and 0.587, 0.639, and 0.685, respectively. The nomogram and decision curve analysis (DCA) showed that the risk score was more accurate than other clinical indicators in evaluating patients’ prognoses. The clusters with a better prognosis enrich the AML pathways and immune-related pathways. We also found a close correlation between prognostic m6A-related lncRNAs and tumor immune cell infiltration. LAG3 expression at the immune checkpoint was lower in the worse prognostic cluster. In conclusion, m6A-related lncRNAs partly affected AML prognosis by remodeling the TME and affecting the anticarcinogenic ability of immune checkpoints, especially LAG3 inhibitors. The prognostic model constructed with nine key m6A-related lncRNAs can provide a method to assess the prognosis of AML patients in both adults and children.

A FBXO7/EYA2-SCFFBXW7 axis promotes AXL-mediated maintenance of mesenchymal and immune evasion phenotypes of cancer cells

A mesenchymal tumor phenotype associates with immunotherapy resistance, although the mechanism is unclear. Here, we identified FBXO7 as a maintenance regulator of mesenchymal and immune evasion phenotypes of cancer cells. FBXO7 bound and stabilized SIX1 co-transcriptional regulator EYA2, stimulating mesenchymal gene expression and suppressing IFNα/β, chemokines CXCL9/10, and antigen presentation machinery, driven by AXL extracellular ligand GAS6. Ubiquitin ligase SCFFBXW7 antagonized this pathway by promoting EYA2 degradation. Targeting EYA2 Tyr phosphatase activity decreased mesenchymal phenotypes and enhanced cancer cell immunogenicity, resulting in attenuated tumor growth and metastasis, increased infiltration of cytotoxic T and NK cells, and enhanced anti-PD-1 therapy response in mouse tumor models. FBXO7 expression correlated with mesenchymal and immune-suppressive signatures in patients with cancer. An FBXO7-immune gene signature predicted immunotherapy responses. Collectively, the FBXO7/EYA2-SCFFBXW7 axis maintains mesenchymal and immune evasion phenotypes of cancer cells, providing rationale to evaluate FBXO7/EYA2 inhibitors in combination with immune-based therapies to enhance onco-immunotherapy responses.

Gab2 deficiency prevents Flt3-ITD driven acute myeloid leukemia in vivo

Internal tandem duplications (ITD) of the FMS-like tyrosine kinase 3 (FLT3) predict poor prognosis in acute myeloid leukemia (AML) and often co-exist with inactivating DNMT3A mutations. In vitro studies implicated Grb2-associated binder 2 (GAB2) as FLT3-ITD effector. Utilizing a Flt3-ITD knock-in, Dnmt3a haploinsufficient mouse model, we demonstrate that Gab2 is essential for the development of Flt3-ITD driven AML in vivo, as Gab2 deficient mice displayed prolonged survival, presented with attenuated liver and spleen pathology and reduced blast counts. Furthermore, leukemic bone marrow from Gab2 deficient mice exhibited reduced colony-forming unit capacity and increased FLT3 inhibitor sensitivity. Using transcriptomics, we identify the genes encoding for Axl and the Ret co-receptor Gfra2 as targets of the Flt3-ITD/Gab2/Stat5 axis. We propose a pathomechanism in which Gab2 increases signaling of these receptors by inducing their expression and by serving as downstream effector. Thereby, Gab2 promotes AML aggressiveness and drug resistance as it incorporates these receptor tyrosine kinases into the Flt3-ITD signaling network. Consequently, our data identify GAB2 as a promising biomarker and therapeutic target in human AML.

Dual Inhibition of FLT3 and AXL by Gilteritinib Overcomes Hematopoietic Niche-Driven Resistance Mechanisms in FLT3-ITD Acute Myeloid Leukemia

PURPOSE

AXL has been shown to play a pivotal role in the selective response of FLT3-ITD acute myeloid leukemia (AML) cells to FLT3 tyrosine kinase inhibitors (TKI), particularly within the bone marrow microenvironment.

EXPERIMENTAL DESIGN

Herein, we compared the effect of dual FLT3/AXL-TKI gilteritinib with quizartinib through in vitro models mimicking hematopoietic niche conditions, ex vivo in primary AML blasts, and in vivo with dosing regimens allowing plasma concentration close to those used in clinical trials.

RESULTS

We observed that gilteritinib maintained a stronger proapoptotic effect in hypoxia and coculture with bone marrow stromal cells compared with quizartinib, linked to a dose-dependent inhibition of AXL phosphorylation. In vivo, use of the MV4-11 cell line with hematopoietic engraftment demonstrated that gilteritinib was more effective than quizartinib at targeting leukemic cells in bone marrow. Finally, FLT3-ITD AML patient-derived xenografts revealed that this effect was particularly reproducible in FLT3-ITD AML with high allelic ratio in primary and secondary xenograft. Moreover, gilteritinib and quizartinib displayed close toxicity profile on normal murine hematopoiesis, particularly at steady state.

CONCLUSIONS

Overall, these findings suggest that gilteritinib as a single agent, compared with quizartinib, is more likely to reach leukemic cells in their protective microenvironment, particularly AML clones highly dependent on FLT3-ITD signaling.

Anti-Axl monoclonal antibodies attenuate the migration of MDA-MB-231 breast cancer cells

The receptor tyrosine kinase, anexelekto (Axl) is involved in tumor cell growth, migration and invasion, and has been associated with chemotherapy resistance, which makes it an attractive target for cancer therapy. In total, six Axl-targeted monoclonal antibodies (mAbs) and two antibody-drug conjugates have been reported in the last 10 years, which have been shown to have bioactivity in inhibiting tumor cell proliferation and migration. The Axl external cell domain (Axl^−ECD), consisting of 426 amino acids, has always been used as an antigen in the screening process for all six of these Axl-targeted mAbs. However, the Axl functional domain, which interacts with its natural ligand, growth arrest-specific protein 6 (Gas6), is only a small part of the Axl^−ECD. Antibodies targeting the Axl functional domain may efficiently block Gas6-Axl binding and attenuate its downstream signals and activities. To the best of our knowledge, no mAbs targeting the Axl functional domain have been reported. In the present study, a major Axl functional domain interacting with Gas6 was determined using bioinformatics and structural biology methods. In MDA-MB-231 breast cancer cell assays, anti-Axl mAbs targeting this relatively specific Axl functional domain almost completely neutralized the stimulation of Gas6 in both Axl phosphorylation and cell migration assays, and showed similar activity to the positive control drug R428 (a small molecular tyrosine kinase inhibitor of Axl currently in phase II clinical trials) in the cell migration assay. Given the important role of Axl in tumor development and chemotherapy resistance, Axl-targeted mAbs could be used to inhibit tumor cells directly, as well as reduce the development of chemotherapy resistance by blocking Axl activity. The application of Axl-targeted mAbs combined with chemotherapy provides a promising treatment strategy for patients with tumors, particularly those with triple-negative breast cancer, for whom no targeted therapy is currently available.

Therapeutic Targeting of the Gas6/Axl Signaling Pathway in Cancer

Many signaling pathways are dysregulated in cancer cells and the host tumor microenvironment. Aberrant receptor tyrosine kinase (RTK) pathways promote cancer development, progression, and metastasis. Hence, numerous therapeutic interventions targeting RTKs have been actively pursued. Axl is an RTK that belongs to the Tyro3, Axl, MerTK (TAM) subfamily. Axl binds to a high affinity ligand growth arrest specific 6 (Gas6) that belongs to the vitamin K-dependent family of proteins. The Gas6/Axl signaling pathway has been implicated to promote progression, metastasis, immune evasion, and therapeutic resistance in many cancer types. Therapeutic agents targeting Gas6 and Axl have been developed, and promising results have been observed in both preclinical and clinical settings when such agents are used alone or in combination therapy. This review examines the current state of therapeutics targeting the Gas6/Axl pathway in cancer and discusses Gas6- and Axl-targeting agents that have been evaluated preclinically and clinically.

Post-translational modifications of the ligands: Requirement for TAM receptor activation

The Tyro3, Axl, and MerTK (TAM) receptors are three homologous Type I Receptor Tyrosine Kinases that have important homeostatic functions in multicellular organisms by regulating the clearance of apoptotic cells (efferocytosis). Pathologically, TAM receptors are overexpressed in a wide array of human cancers, and often associated with aggressive tumor grade and poor overall survival. In addition to their expression on tumor cells, TAMs are also expressed on infiltrating myeloid-derived cells in the tumor microenvironment, where they appear to act akin to negative immune checkpoints that impair host anti-tumor immunity. The ligands for TAMs are two endogenous proteins, Growth Arrest-Specific 6 (Gas6) and Protein S (Pros1), that function as bridging molecules between externalized phosphatidylserine (PtdSer) on apoptotic cells and the TAM ectodomains. One interesting feature of TAMs biology is that their ligand proteins require specific post-translational modifications to acquire activities. This chapter summarized these important modifications and explained the molecular mechanisms behind such phenomenon. Current evidences suggest that these modifications help Gas6/Pros1 to achieve optimal PtdSer-binding capacities. In addition, this chapter included recent discovery of regulating machineries of PtdSer dynamic across the plasma membrane, as well as their potential impacts in the tumor microenvironment. Taken together, this review highlights the importance of the upstream PtdSer and Gas6 in regulating TAMs' function and hope to provide researchers with new perspectives to inspire future studies of TAM receptors in human disease models.

Discovery of 5,6,7,8-tetrahydropyrido[3,4-d]pyrimidine derivatives as novel selective Axl inhibitors

Axl and Mer are members of the TAM (Tyro3-Axl-Mer) family of receptor tyrosine kinases. Previously, we reported that enzyme-mediated inhibition of Mer by an Axl/Mer dual inhibitor led to retinal toxicity in mice, whereas selective Axl inhibition by compound 1 did not. On the other hand, compound 1 showed low membrane permeability. Here, we designed and synthesized a novel series of 5,6,7,8-tetrahydropyrido[3,4-d]pyrimidine derivatives and evaluated their Axl and Mer inhibitory activities, leading to identification of ER-001259851-000 as a potent and selective Axl inhibitor with drug-likeness and a promising pharmacokinetic profile in mice.

Inhibition of AXL enhances chemosensitivity of human ovarian cancer cells to cisplatin via decreasing glycolysis

Anexelekto (AXL), a member of the TYRO3-AXL-MER (TAM) family of receptor tyrosine kinases (RTK), is overexpressed in varieties of tumor tissues and promotes tumor development by regulating cell proliferation, migration and invasion. In this study, we investigated the role of AXL in regulating glycolysis in human ovarian cancer (OvCa) cells. We showed that the expression of AXL mRNA and protein was significantly higher in OvCa tissue than that in normal ovarian epithelial tissue. In human OvCa cell lines suppression of AXL significantly inhibited cell proliferation, and increased the sensitivity of OvCa cells to cisplatin, which also proved by nude mice tumor formation experiment. KEGG analysis showed that AXL was significantly enriched in the glycolysis pathways of cancer. Changes in AXL expression in OvCa cells affect tumor glycolysis. We demonstrated that the promotion effect of AXL on glycolysis was mediated by phosphorylating the M2 isoform of pyruvate kinase (PKM2) at Y105. AXL expression was significantly higher in cisplatin-resistant OvCa cells A2780/DDP compared with the parental A2780 cells. Inhibition of AXL decreased the level of glycolysis in A2780/DDP cells, and increased the cytotoxicity of cisplatin against A2780/DDP cells, suggesting that AXL-mediated glycolysis was associated with cisplatin resistance in OvCa. In conclusion, this study demonstrates for the first time that AXL is involved in the regulation of the Warburg effect. Our results not only highlight the clinical value of targeting AXL, but also provide theoretical basis for the combination of AXL inhibitor and cisplatin in the treatment of OvCa.

AXL receptor tyrosine kinase: a possible therapeutic target in acute promyelocytic leukemia

BACKGROUND

Acute promyelocytic leukemia (APL) is a subset of acute myeloid leukemia (AML) which is characterized by the fusion of promyelocytic leukemia PML and retinoic acid receptor- alpha (RAR-alpha) genes. All-trans retinoic acid (ATRA) and/or arsenic trioxide (ATO) have resulted in durable cytogenetic and molecular remissions in most APL patients and have altered the natural history of the disease. Most APL patients treated with ATRA and/or ATO are now anticipated to have a nearly normal life expectancy. Unfortunately, relapse and resistance to the current treatment occur in APL patients and the outcome remains dismal in these refractory patients. AXL receptor tyrosine kinase (AXL-RTK) has been shown to increase tumour burden, provide resistance to therapy and is critical to maintain cancer stem cells (CSCs) in chronic myeloid leukemia (CML) by stabilizing β-catenin in the Wnt/β-catenin signalling pathway. However, the role of AXL-RTK has not been explored in PML/RARα-positive APL. This study aimed to explore the role of AXL-RTK receptor in PML/RARα-positive APL.

METHODS AND RESULTS

By using biochemical and pharmacological approaches, here we report that targeting of AXL-RTK is related to the down-regulation of β-catenin target genes including c-myc (p < 0.001), AXIN2 (p < 0.001), and HIF1α (p < 0.01) and induction of apoptosis in PML/RARα-positive APL cell line. Resistance to all-trans retinoic acid (ATRA) was also overcomed by targeting AXL-RTK with R428 in APL (p < 0.05).

CONCLUSION

Our results provide clear evidence of the involvement of AXL-RTK in leukemogenic potential of PML/RARα-positive APL and suggest targeting of AXL-RTK in the treatment of therapy resistant APL patients.

Growth arrest-specific protein-6/AXL signaling induces preeclampsia in rats†

Preeclampsia (PE) is a complicated obstetric complication characterized by increased blood pressure, decreased trophoblast invasion, and inflammation. The growth arrest-specific 6 (Gas6) protein is known to induce dynamic cellular responses and is elevated in PE. Gas6 binds to the AXL tyrosine kinase receptor and AXL-mediated signaling is implicated in proliferation and migration observed in several tissues. Our laboratory utilized Gas6 to induce preeclamptic-like conditions in pregnant rats. Our objective was to determine the role of Gas6/AXL signaling as a possible model of PE. Briefly, pregnant rats were divided into three groups that received daily intraperitoneal injections (from gestational day 7.5 to 17.5) of phosphate buffered saline (PBS), Gas6, or Gas6 + R428 (an AXL inhibitor administered from gestational day 13.5 to 17.5). Animals dispensed Gas6 experienced elevated blood pressure, increased proteinuria, augmented caspase-3-mediated placental apoptosis, and diminished trophoblast invasion. Gas6 also enhanced expression of several PE-related genes and a number of inflammatory mediators. Gas6 further enhanced placental oxidative stress and impaired mitochondrial respiration. Each of these PE-related characteristics was ameliorated in dams and/or their placentae when AXL inhibition by R428 occurred in tandem with Gas6 treatment. We conclude that Gas6 signaling is capable of inducing PE and that inhibition of AXL prevents disease progression in pregnant rats. These results provide insight into pathways associated with PE that could be useful in the clarification of potential therapeutic approaches.

Functions of RNA N6-methyladenosine modification in acute myeloid leukemia

Acute myeloid leukemia (AML) is a hematologic malignancy with an unfavorable prognosis. A better understanding of AML pathogenesis and chemotherapy resistance at the molecular level is essential for the development of new therapeutic strategies. Apart from DNA methylation and histone modification, RNA epigenetic modification, another layer of epigenetic modification, also plays a critical role in gene expression regulation. Among the more than 150 kinds of RNA epigenetic modifications, N⁶-methyladenosine (m⁶A) is the most prevalent internal mRNA modification in eukaryotes and is involved in various biological processes, such as circadian rhythms, adipogenesis, T cell homeostasis, spermatogenesis, and the heat shock response. As a reversible and dynamic modification, m⁶A is deposited on specific target RNA molecules by methyltransferases and is removed by demethylases. Moreover, m⁶A binding proteins recognize m⁶A modifications, influencing RNA splicing, stability, translation, nuclear export, and localization at the posttranscriptional level. Emerging evidence suggests that dysregulation of m⁶A modification is involved in tumorigenesis, including that of AML. In this review, we summarize the most recent advances regarding the biological functions and molecular mechanisms of m⁶A RNA methylation in normal hematopoiesis, leukemia cell proliferation, apoptosis, differentiation, therapeutic resistance, and leukemia stem cell/leukemia initiating cell (LSC/LIC) self-renewal. In addition, we discuss how m⁶A regulators are closely correlated with the clinical features of AML patients and may serve as new biomarkers and therapeutic targets for AML.

Discovery of a potent and selective Axl inhibitor in preclinical model

Axl and Mer are a members of the TAM (Tyro3-Axl-Mer) family of receptor tyrosine kinases, which, when activated, can promote tumor cell survival, proliferation, migration, invasion, angiogenesis, and tumor-host interactions. Chronic inhibition of Mer leads to retinal toxicity in mice. Therefore, successful development of an Axl targeting agent requires ensuring that it is safe for prolonged treatment. Here, to clarify whether enzyme inhibition of Mer by a small molecule leads to retinal toxicity in mice, we designed and synthesized Axl/Mer inhibitors and Axl-selective inhibitors. We identified an Axl/Mer dual inhibitor 28a, which showed retinal toxicity at a dose of 100 mg/kg in mice. Subsequent derivatization of a pyridine derivative led to the discovery of a pyrimidine derivative, 33g, which selectively inhibited the activity of Axl over Mer without retinal toxicity at a dose of 100 mg/kg in mice. Additionally, the compound displayed in vivo anti-tumor effects without influencing body weight in a Ba/F3-Axl isogenic subcutaneous model.

The Role of the Receptor Tyrosine Kinase Axl in Carcinogenesis and Development of Therapeutic Resistance: An Overview of Molecular Mechanisms and Future Applications

Resistance to chemotherapeutic agents by cancer cells has remained a major obstacle in the successful treatment of various cancers. Numerous factors such as DNA damage repair, cell death inhibition, epithelial-mesenchymal transition, and evasion of apoptosis have all been implicated in the promotion of chemoresistance. The receptor tyrosine kinase Axl, a member of the TAM family (which includes TYRO3 and MER), plays an important role in the regulation of cellular processes such as proliferation, motility, survival, and immunologic response. The overexpression of Axl is reported in several solid and hematological malignancies, including non-small cell lung, prostate, breast, liver and gastric cancers, and acute myeloid leukaemia. The overexpression of Axl is associated with poor prognosis and the development of resistance to therapy. Reports show that Axl overexpression confers drug resistance in lung cancer and advances the emergence of tolerant cells. Axl is, therefore, an important candidate as a prognostic biomarker and target for anticancer therapies. In this review, we discuss the consequence of Axl upregulation in cancers, provide evidence for its role in cancer progression and the development of drug resistance. We will also discuss the therapeutic potential of Axl in the treatment of cancer.

Novel AXL-targeted agents overcome FLT3 inhibitor resistance in FLT3-ITD+ acute myeloid leukemia cells

AXL receptor tyrosine kinase (AXL) upregulation mediates drug resistance in several types of human cancer and has become a therapeutic target worthy of exploration. The present study investigated AXL antigen expression and the effects of novel AXL-targeted agents in acute myeloid leukemia (AML) cells. AXL antigen expression in drug-sensitive and drug-resistant human AML cell lines, and AML blast cells from 57 patients with different clinical characteristics, was analyzed by flow cytometry and compared. Furthermore, the effects of the novel AXL antibody DAXL-88, antibody-drug conjugate DAXL-88-monomethyl auristatin E (MMAE), AXL small molecule inhibitor R428 and their combination with FMS-like tyrosine kinase 3 (FLT3) inhibitor quizartinib (AC220) in AML cells were analyzed by Cell Counting Kit-8 assay, flow cytometry and western blotting. The present study revealed that AXL antigen expression was upregulated in FLT3-internal tandem duplication (ITD)/tyrosine kinase domain mutation-positive (TKD)⁺ AML blast cells compared with FLT3-ITD/TKD^- AML cells. Additionally, AXL antigen expression was markedly upregulated in the AC220-resistant FLT3-ITD⁺ MV4-11 cell line (MV4-11/AC220) and in FLT3 inhibitor-resistant blast cells from a patient with FLT3-ITD⁺ AML compared with parental sensitive cells. The AXL-targeted agents DAXL-88, DAXL-88-MMAE and R428 exhibited dose-dependent cytotoxic effects on FLT3-mutant AML cell lines (THP-1, MV4-11 and MV4-11/AC220) and blast cells from patients with FLT3-ITD⁺ AML. Combinations of AXL-targeted agents with AC220 exerted synergistic cytotoxic effects and induced apoptosis in MV4-11/AC220 cells and FLT3 inhibitor-resistant blast cells. The antileukemic effect of DAXL-88 and DAXL-88-MMAE may rely on their ability to block AXL, FLT3 and their downstream signaling pathways. The present study demonstrated the association between AXL antigen expression upregulation and drug resistance in FLT3-ITD⁺ AML, and proposed a method for overcoming FLT3 inhibitor resistance of FLT3-ITD⁺ AML using novel AXL-targeted agents.

Novel Tyrosine Kinase Targets in Urothelial Carcinoma

Urothelial carcinoma represents one of the most prevalent types of cancer worldwide, and its incidence is expected to grow. Although the treatment of the advanced disease was based on chemotherapy for decades, the developments of different therapies, such as immune checkpoint inhibitors, antibody drug conjugates and tyrosine kinase inhibitors, are revolutionizing the therapeutic landscape of this tumor. This development coincides with the increasing knowledge of the pathogenesis and genetic alterations in urothelial carcinoma, from the non-muscle invasive setting to the metastatic one. The purpose of this article is to provide a comprehensive review of the different tyrosine kinase targets and their roles in the therapeutic scene of urothelial carcinoma.

Transcriptional activators YAP/TAZ and AXL orchestrate dedifferentiation, cell fate, and metastasis in human osteosarcoma

Osteosarcoma (OS) is a molecularly heterogeneous, aggressive, poorly differentiated pediatric bone cancer that frequently spreads to the lung. Relatively little is known about phenotypic and epigenetic changes that promote lung metastases. To identify key drivers of metastasis, we studied human CCH-OS-D OS cells within a previously described rat acellular lung (ACL) model that preserves the native lung architecture, extracellular matrix, and capillary network. This system identified a subset of cells—termed derived circulating tumor cells (dCTCs)—that can migrate, intravasate, and spread within a bioreactor-perfused capillary network. Remarkably, dCTCs highly expressed epithelial-to-mesenchymal transition (EMT)-associated transcription factors (EMT-TFs), such as ZEB1, TWIST, and SOX9, which suggests that they undergo cellular reprogramming toward a less differentiated state by coopting the same epigenetic machinery used by carcinomas. Since YAP/TAZ and AXL tightly regulate the fate and plasticity of normal mesenchymal cells in response to microenvironmental cues, we explored whether these proteins contributed to OS metastatic potential using an isogenic pair of human OS cell lines that differ in AXL expression. We show that AXL inhibition significantly reduced the number of MG63.2 pulmonary metastases in murine models. Collectively, we present a laboratory-based method to detect and characterize a pure population of dCTCs, which provides a unique opportunity to study how OS cell fate and differentiation contributes to metastatic potential. Though the important step of clinical validation remains, our identification of AXL, ZEB1, and TWIST upregulation raises the tantalizing prospect that EMT-TF-directed therapies might expand the arsenal of therapies used to combat advanced-stage OS.

Dual Kinase Targeting in Leukemia

Pharmacological cancer therapy is often based on the concurrent inhibition of different survival pathways to improve treatment outcomes and to reduce the risk of relapses. While this strategy is traditionally pursued only through the co-administration of several drugs, the recent development of multi-targeting drugs (i.e., compounds intrinsically able to simultaneously target several macromolecules involved in cancer onset) has had a dramatic impact on cancer treatment. This review focuses on the most recent developments in dual-kinase inhibitors used in acute myeloid leukemia (AML), chronic myelogenous leukemia (CML), and lymphoid tumors, giving details on preclinical studies as well as ongoing clinical trials. A brief overview of dual-targeting inhibitors (kinase/histone deacetylase (HDAC) and kinase/tubulin polymerization inhibitors) applied to leukemia is also given. Finally, the very recently developed Proteolysis Targeting Chimeras (PROTAC)-based kinase inhibitors are presented.

Targeting multiple signaling pathways: the new approach to acute myeloid leukemia therapy

Acute myeloid leukemia (AML) is the most common form of acute leukemia in adults and the second most common form of acute leukemia in children. Despite this, very little improvement in survival rates has been achieved over the past few decades. This is partially due to the heterogeneity of AML and the need for more targeted therapeutics than the traditional cytotoxic chemotherapies that have been a mainstay in therapy for the past 50 years. In the past 20 years, research has been diversifying the approach to treating AML by investigating molecular pathways uniquely relevant to AML cell proliferation and survival. Here we review the development of novel therapeutics in targeting apoptosis, receptor tyrosine kinase (RTK) signaling, hedgehog (HH) pathway, mitochondrial function, DNA repair, and c-Myc signaling. There has been an impressive effort into better understanding the diversity of AML cell characteristics and here we highlight important preclinical studies that have supported therapeutic development and continue to promote new ways to target AML cells. In addition, we describe clinical investigations that have led to FDA approval of new targeted AML therapies and ongoing clinical trials of novel therapies targeting AML survival pathways. We also describe the complexity of targeting leukemia stem cells (LSCs) as an approach to addressing relapse and remission in AML and targetable pathways that are unique to LSC survival. This comprehensive review details what we currently understand about the signaling pathways that support AML cell survival and the exceptional ways in which we disrupt them.

Immunological role of TAM receptors in the cancer microenvironment

TAM receptors belong to the family of receptor tyrosine kinases, comprising of Tyro3, Axl and Mertk receptors (TAMs) and are important homeostatic regulators of inflammation in higher eukaryotes. Along with their ligands, Gas6 and ProteinS, TAMs acts as receptors to phosphatidylserine (PtdSer), an anionic phospholipid that becomes externalized on the surface of apoptotic and stressed cells. TAM receptors, specially Mertk, have been well established to play a role in the process of efferocytosis, the engulfment of dying cells. Besides being efferocytic receptors, TAMs are pleiotropic immune modulators as the lack of TAM receptors in various mouse models lead to chronic inflammation and autoimmunity. Owing to their immune modulatory role, the PtdSer-TAM receptor signaling axis has been well characterized as a global immune-suppressive signal, and in cancers, and emerging literature implicates TAM receptors in cancer immunology and anti-tumor therapeutics. In the tumor microenvironment, immune-suppressive signals, such as ones that originate from TAM receptor signaling can be detrimental to anti-tumor therapy. In this chapter, we discuss immune modulatory functions of TAM receptors in the tumor microenvironment as well role of differentially expressed TAM receptors and their interactions with immune and tumor cells. Finally, we describe current strategies being utilized for targeting TAMs in several cancers and their implications in immunotherapy.

Dual targeting of TAM receptors Tyro3, Axl, and MerTK: Role in tumors and the tumor immune microenvironment

In both normal and tumor tissues, receptor tyrosine kinases (RTKs) may be pleiotropically expressed. The RTKs not only regulate ordinary cellular processes, including proliferation, survival, adhesion, and migration, but also have a critical role in the development of many types of cancer. The Tyro3, Axl, and MerTK (TAM) family of RTKs (Tyro3, Axl, and MerTK) plays a pleiotropic role in phagocytosis, inflammation, and normal cellular processes. In this article, we highlight the cellular activities of TAM receptors and discuss their roles in cancer and immune cells. We also discuss cancer therapies that target TAM receptors. Further research is needed to elucidate the function of TAM receptors in immune cells toward the development of new targeted immunotherapies for cancer.

TAM family receptors in conjunction with MAPK signalling are involved in acquired resistance to PI3Kα inhibition in head and neck squamous cell carcinoma

Background

Aberrant activation of the phosphatidylinositol 3-kinase (PI3K) pathway is common in many malignancies, including head and neck squamous cell carcinoma (HNSCC). Despite pre-clinical and clinical studies, outcomes from targeting the PI3K pathway have been underwhelming and the development of drug resistance poses a significant barrier to patient treatment. In the present study, we examined mechanisms of acquired resistance to the PI3Kα inhibitor alpelisib (formerly BYL719) in HNSCC cell lines and patient-derived xenografts (PDXs).

Methods

Five unique PDX mouse models and three HNSCC cell lines were used. All cell lines and xenografts underwent genomic characterization prior to study. Serial drug treatment was conducted in vitro and in vivo to develop multiple, clinically-significant models of resistance to alpelisib. We then used reverse phase protein arrays (RPPAs) to profile the expression of proteins in parental and drug-resistant models. Top hits were validated by immunoblotting and immunohistochemistry. Flow cytometric analysis and RNA interference studies were then used to interrogate the molecular mechanisms underlying acquired drug resistance.

Results

Prolonged treatment with alpelisib led to upregulation of TAM family receptor tyrosine kinases TYRO3 and AXL. Importantly, a significant shift in expression of both TYRO3 and AXL to the cell surface was detected in drug-resistant cells. Targeted knockdown of TYRO3 and AXL effectively re-sensitized resistant cells to PI3Kα inhibition. In vivo, resistance to alpelisib emerged following 20–35 days of treatment in all five PDX models. Elevated TYRO3 expression was detected in drug-resistant PDX tissues. Downstream of TYRO3 and AXL, we identified activation of intracellular MAPK signalling. Inhibition of MAPK signalling also re-sensitized drug-resistant cells to alpelisib.

Conclusions

We have identified TYRO3 and AXL receptors to be key mediators of resistance to alpelisib, both in vitro and in vivo. Our findings suggest that pan-TAM inhibition is a promising avenue for combinatorial or second-line therapy alongside PI3Kα inhibition. These findings advance our understanding of the role TAM receptors play in modulating the response of HNSCC to PI3Kα inhibition and suggest a means to prevent, or at least delay, resistance to PI3Kα inhibition in order to improve outcomes for HNSCC patients.

Targeting TAM to Tame Pancreatic Cancer

Pancreatic cancer is expected to become the second leading cause of cancer-related death within the next few years. Current therapeutic strategies have limited effectiveness and therefore there is an urgency to develop novel effective therapies. The receptor tyrosine kinase subfamily TAM (Tyro3, Axl, MerTK) is directly implicated in the pathogenesis of the metastatic, chemoresistant, and immunosuppressive phenotype in pancreatic cancer. TAM inhibitors are promising investigational therapies for pancreatic cancer due to their potential to target multiple aspects of pancreatic cancer biology. Specifically, recent mechanistic investigations and therapeutic combinations in the preclinical setting suggest that TAM inhibition with chemotherapy, targeted therapy, and immunotherapy should be evaluated clinically.

AXL Mediates Cetuximab and Radiation Resistance Through Tyrosine 821 and the c-ABL Kinase Pathway in Head and Neck Cancer

PURPOSE

Radiation and cetuximab are therapeutics used in management of head and neck squamous cell carcinoma (HNSCC). Despite clinical success with these modalities, development of both intrinsic and acquired resistance is an emerging problem in the management of this disease. The purpose of this study was to investigate signaling of the receptor tyrosine kinase AXL in resistance to radiation and cetuximab treatment.

EXPERIMENTAL DESIGN

To study AXL signaling in the context of treatment-resistant HNSCC, we used patient-derived xenografts (PDXs) implanted into mice and evaluated the tumor response to AXL inhibition in combination with cetuximab or radiation treatment. To identify molecular mechanisms of how AXL signaling leads to resistance, three tyrosine residues of AXL (Y779, Y821, Y866) were mutated and examined for their sensitivity to cetuximab and/or radiation. Furthermore, reverse phase protein array (RPPA) was employed to analyze the proteomic architecture of signaling pathways in these genetically altered cell lines.

RESULTS

Treatment of cetuximab- and radiation-resistant PDXs with AXL inhibitor R428 was sufficient to overcome resistance. RPPA analysis revealed that such resistance emanates from signaling of tyrosine 821 of AXL via the tyrosine kinase c-ABL. In addition, inhibition of c-ABL signaling resensitized cells and tumors to cetuximab or radiotherapy even leading to complete tumor regression without recurrence in head and neck cancer models.

CONCLUSIONS

Collectively, the studies presented herein suggest that tyrosine 821 of AXL mediates resistance to cetuximab by activation of c-ABL kinase in HNSCC and that targeting of both EGFR and c-ABL leads to a robust antitumor response.

Calix[6]arene diminishes receptor tyrosine kinase lifespan in pancreatic cancer cells and inhibits their migration and invasion efficiency

Pancreatic cancer is a challenging malignancy, mainly due to aggressive regional involvement, early systemic dissemination, high recurrence rate, and subsequent low patient survival. Scientific advances have contributed in particular by identification of molecular targets as well as the definition of the mechanism of action of the drug candidate in the cellular microenvironment. Previously, we have reported the identification of the molecular mechanisms by which calix[6]arene (CLX6) reduces the viability and proliferation of pancreatic cancer cells. Now, we show the biochemical mechanisms by which CLX6 decreases the aggressiveness of Panc-1 cells, focusing specifically on receptor tyrosine kinases (RTK). The results show that clathrin-mediated endocytosis is involved in CLX6-induced AXL receptor tyrosine kinase degradation in Panc-1 cells. This response may be related to the interaction of CLX6 with the tyrosine kinase receptor binding site (such as AXL). As a result, RTK is internalized and degraded by endocytosis, a condition that negatively impacts events dependent on its signaling. Additionally, CLX6 inhibits migration and invasion of Panc-1 cells by downregulating FAK (downstream mediator of AXL) activity and reducing expression levels of MMP2 and MMP9, directly related to the metastatic profile of these cells. It is noteworthy that according to the mechanism proposed here, CLX6 appears as a candidate to be used in therapeutic protocols of patients that display high expression of AXL and consequently, poor diagnosis.

Metformin suppresses the growth of leukemia cells partly through downregulation of AXL receptor tyrosine kinase

Metformin is an anti-diabetic drug known to have anticancer activity by inhibiting mechanistic target of rapamycin (mTOR); however, other molecular mechanisms may also be involved. In this study, we examined the effects of metformin on the activity of receptor tyrosine kinases of the TAM (TYRO3, AXL, and MERTK) family, which have important roles in leukemia cell growth. The results indicated that metformin suppressed the in vitro growth of four leukemia cell lines, OCI/AML2, OCI/AML3, THP-1, and K562, in a dose-dependent manner, which corresponded to the downregulation of the expression and phosphorylation of AXL and inhibition of its downstream targets such as phosphorylation of STAT3. Furthermore, metformin augmented the suppressive effects of a small-molecule AXL inhibitor TP-0903 on the growth of OCI/AML3 and K562 cells and prevented doxorubicin-induced AXL activation in K562 cells, which induces chemoresistance in leukemia cells, thus potentiating doxorubicin anti-proliferative effects. Given that metformin also downregulated expression of TYRO3 and phosphorylation of MERTK, these findings indicate that anti-leukemic effects exerted by metformin could be partly due to the inhibition of TAM kinases. Thus, metformin has a clinical potential for patients with leukemia cells positive for AXL and the other TAM proteins as well as activated mTOR.

Leukemogenic Chromatin Alterations Promote AML Leukemia Stem Cells via a KDM4C-ALKBH5-AXL Signaling Axis

N⁶-methyladenosine (m⁶A) is a commonly present modification of mammalian mRNAs and plays key roles in various cellular processes. m⁶A modifiers catalyze this reversible modification. However, the underlying mechanisms by which these m⁶A modifiers are regulated remain elusive. Here we show that expression of m⁶A demethylase ALKBH5 is regulated by chromatin state alteration during leukemogenesis of human acute myeloid leukemia (AML), and ALKBH5 is required for maintaining leukemia stem cell (LSC) function but is dispensable for normal hematopoiesis. Mechanistically, KDM4C regulates ALKBH5 expression via increasing chromatin accessibility of ALKBH5 locus, by reducing H3K9me3 levels and promoting recruitment of MYB and Pol II. Moreover, ALKBH5 affects mRNA stability of receptor tyrosine kinase AXL in an m⁶A-dependent way. Thus, our findings link chromatin state dynamics with expression regulation of m⁶A modifiers and uncover a selective and critical role of ALKBH5 in AML that might act as a therapeutic target of specific targeting LSCs.

AXL as a Target in Breast Cancer Therapy

AXL is a receptor tyrosine kinase (RTK) that has been implicated in diverse tumor-promoting processes such as proliferation, migration, invasion, survival, and apoptosis. AXL therefore plays a role in cancer progression, and AXL has been implicated in a wide variety of malignancies from solid tumors to hematopoietic cancers where it is often associated with poor prognosis. In cancer, AXL has been shown to promote epithelial to mesenchymal transition (EMT), metastasis formation, drug resistance, and a role for AXL in modulation of the tumor microenvironment and immune response has been identified. In light of these activities multiple AXL inhibitors have been developed, and several of these have entered clinical trials in the U.S. In breast cancer, high levels of AXL expression have been observed. The role of AXL in cancer with a focus on therapeutic implications for breast cancer is discussed.

Rational Drug Design of Axl Tyrosine Kinase Type I Inhibitors as Promising Candidates Against Cancer

The high level of Axl tyrosine kinase expression in various cancer cell lines makes it an attractive target for the development of anti-cancer drugs. In this study, we carried out several sets of in silico screening for the ATP-competitive Axl kinase inhibitors based on different molecular docking protocols. The best drug-like candidates were identified, after parental structure modifications, by their highest affinity to the target protein. We found that our newly designed compound R5, a derivative of the R428 patented analog, is the most promising inhibitor of the Axl kinase according to the three molecular docking algorithms applied in the study. The molecular docking results are in agreement with the molecular dynamics simulations using the MM-PBSA/GBSA implicit solvation models, which confirm the high affinity of R5 toward the protein receptor. Additionally, the selectivity test against other kinases also reveals a high affinity of R5 toward ABL1 and Tyro3 kinases, emphasizing its promising potential for the treatment of malignant tumors.

Detection of AXL expression in circulating tumor cells of lung cancer patients using an automated microcavity array system

Noninvasive diagnostics using circulating tumor cells (CTCs) are expected to be useful for decision making in precision cancer therapy. AXL, a receptor tyrosine kinase is associated with tumor progression, epithelial‐to‐mesenchymal transition (EMT), and drug resistance, and is a potential therapeutic target. However, the epithelial markers generally used for CTC detection may be not enough to detect AXL‐expressing CTCs due to EMT. Here, we evaluated the detection of AXL‐expressing CTCs using the mesenchymal marker vimentin with a microcavity array system. To evaluate the recovery of cancer cells, spike‐in experiments were performed using cell lines with varying cytokeratin (CK) or vimentin (VM) expression levels. With high CK and low VM‐expressing cell lines, PC‐9 and HCC827, the recovery rate of AXL‐expressing cancer cells was 1%‐17% using either CK or VM as markers. Whereas, with low CK and high VM‐expressing cell lines, MDA‐MB231 and H1299, it was 52%‐75% using CK and 72%‐88% using VM as a marker. For clinical evaluation, peripheral blood was collected from 20 non–small cell lung cancer patients and CTCs were detected using CK or VM as markers in parallel. Significantly more AXL‐expressing single CTCs were detected in VM‐positive than CK‐positive CTCs (P < .001). Furthermore, CTC clusters were identified only among VM‐positive CTCs in 20% of patients. Patients with one or more prior treatments harbored significantly more VM‐positive AXL‐expressing CTCs, suggesting the involvement of these CTCs in drug resistance. These results indicate the necessity of integrating mesenchymal markers with CTC detection and this should be further evaluated clinically.

AXL Controls Directed Migration of Mesenchymal Triple-Negative Breast Cancer Cells

Triple-negative breast cancer (TNBC) is an aggressive form of breast cancer with high risk of relapse and metastasis. TNBC is a heterogeneous disease comprising different molecular subtypes including those with mesenchymal features. The tyrosine kinase AXL is expressed in mesenchymal cells and plays a role in drug resistance, migration and metastasis. We confirm that AXL is more expressed in mesenchymal TNBC cells compared to luminal breast cancer cells, and that its invalidation impairs cell migration while having no or little effect on cell viability. Here, we found that AXL controls directed migration. We observed that AXL displays a polarized localization at the Golgi apparatus and the leading edge of migratory mesenchymal TNBC cells. AXL co-localizes with F-actin at the front of the cells. In migratory polarized cells, the specific AXL inhibitor R428 displaces AXL and F-actin from the leading edge to a lateral area localized between the front and the rear of the cells where both are enriched in protrusions. In addition, R428 treatment disrupts the polarized localization of the Golgi apparatus towards the leading edge in migratory cells. Immunohistochemical analysis of aggressive chemo-resistant TNBC samples obtained before treatment reveals inter- and intra-tumor heterogeneity of the percentage of AXL expressing tumor cells, and a preference of these cells to be in contact with the stroma. Taken together, our study demonstrates that AXL controls directed cell migration most likely by regulating cell polarity.

Soluble AXL as a marker of disease progression and survival in melanoma

Receptor tyrosine kinase AXL is a one-pass transmembrane protein upregulated in cancers and associated with lower survival and therapy resistance. AXL can be cleaved by the A Disintegrin and Metalloproteinases (ADAM)10 and ADAM17, yielding a soluble version of the protein. Elevated soluble AXL (sAXL) has been reported to be associated with disease progression in hepatocellular carcinoma, renal cancer, neurofibromatosis type 1 and inflammatory diseases. In the present work, we analyzed sAXL levels in blood from melanoma patients and showed that sAXL increases with disease progression. Additionally, increased sAXL levels were found correlated with shorter two-year survival in stage IV patients treated with ipilimumab. Furthermore, we showed that sAXL levels were related to the percentage of cells expressing AXL in resected melanoma lymph node metastases. This finding was verified in vitro, where sAXL levels in the cell media corresponded to AXL expression in the cells. AXL inhibition using the small-molecular inhibitor BGB324 reduced sAXL levels, while the cellular expression was elevated through increased protein stability. Our findings signify that quantification of sAXL blood levels is a simple and easily assessable method to determine cellular AXL levels and should be further evaluated for its use as a biomarker of disease progression and treatment response.

Overcoming the Intrinsic Gefitinib-resistance via Downregulation of AXL in Non-small Cell Lung Cancer

Background

Resistance to epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs), such as gefitinib, is a limited factor in the treatment of non-small-cell lung cancer (NSCLC) patients. Therefore, ongoing studies are trying to identify EGFR-TKIs-resistant mechanisms and to discover novel therapeutic strategies and targets for NSCLC treatment.

Methods

In the present study, the possibility of overcoming intrinsic gefitinib-resistance was examined by regulating the expression of AXL. A natural product-derived antitumor agent, yuanhuadine (YD) was employed to modulate the expression of AXL in the cells.

Results

Treatment with YD effectively downregulated AXL expression in AXL-overexpressed gefitinib-resistant H1299 cells. The combination of gefitinib and YD exhibited a synergistic grwoth-inhibitory activity in H1299 cells by downregulation of AXL expression.

Conclusions

Based on these findings, AXL was found to be a promising therapeutic target to overcome the intrinsic resistance to gefitinib in NSCLC. Furthermore, YD is able to effectively regulate the expression of AXL and thus it may be applicable as a potential lead compound for the treatment of gefitinib-resistant NSCLC.