Preclinical validation of AXL receptor as a target for antibody-based pancreatic cancer immunotherapy

Hallmarks of pancreatic cancer: spotlight on TAM receptors

Pancreatic ductal adenocarcinoma (PDAC) represents the most prevalent type of pancreatic cancer and ranks among the most aggressive tumours, with a 5-year survival rate of less than 11%. Projections indicate that by 2030, it will become the second leading cause of cancer-related deaths. PDAC presents distinctive hallmarks contributing to its dismal prognosis: (i) late diagnosis, (ii) heterogenous and complex mutational landscape, (iii) high metastatic potential, (iv) dense fibrotic stroma, (v) immunosuppressive microenvironment, and (vi) high resistance to therapy. Mounting evidence has shown a role for TAM (Tyro3, AXL, MerTK) family of tyrosine kinase receptors in PDAC initiation and progression. This review aims to describe the impact of TAM receptors on the defining hallmarks of PDAC and discuss potential future directions using these proteins as novel biomarkers for early diagnosis and targets for precision therapy in PDAC, an urgent unmet clinical need.

Antibody dependent cellular cytotoxicity-inducing anti-EGFR antibodies as effective therapeutic option for cutaneous melanoma resistant to BRAF inhibitors

Introduction

About 50% of cutaneous melanoma (CM) patients present activating BRAF mutations that can be effectively targeted by BRAF inhibitors (BRAFi). However, 20% of CM patients exhibit intrinsic drug resistance to BRAFi, while most of the others develop adaptive resistance over time. The mechanisms involved in BRAFi resistance are disparate and globally seem to rewire the cellular signaling profile by up-regulating different receptor tyrosine kinases (RTKs), such as the epidermal growth factor receptor (EGFR). RTKs inhibitors have not clearly demonstrated anti-tumor activity in BRAFi resistant models. To overcome this issue, we wondered whether the shared up-regulated RTK phenotype associated with BRAFi resistance could be exploited by using immune weapons as the antibody-dependent cell cytotoxicity (ADCC)-mediated effect of anti-RTKs antibodies, and kill tumor cells independently from the mechanistic roots.

Methods and results

By using an in vitro model of BRAFi resistance, we detected increased membrane expression of EGFR, both at mRNA and protein level in 4 out of 9 BRAFi-resistant (VR) CM cultures as compared to their parental sensitive cells. Increased EGFR phosphorylation and AKT activation were observed in the VR CM cultures. EGFR signaling appeared dispensable for maintaining resistance, since small molecule-, antibody- and CRISPR-targeting of EGFR did not restore sensitivity of VR cells to BRAFi. Importantly, immune-targeting of EGFR by the anti-EGFR antibody cetuximab efficiently and specifically killed EGFR-expressing VR CM cells, both in vitro and in humanized mouse models in vivo, triggering ADCC by healthy donors' and patients' peripheral blood cells.

Conclusion

Our data demonstrate the efficacy of immune targeting of RTKs expressed by CM relapsing on BRAFi, providing the proof-of-concept supporting the assessment of anti-RTK antibodies in combination therapies in this setting. This strategy might be expected to concomitantly trigger the crosstalk of adaptive immune response leading to a complementing T cell immune rejection of tumors.

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.

Inhibition of a new AXL isoform, AXL3, induces apoptosis of mantle cell lymphoma cells

Mantle cell lymphoma (MCL) is an aggressive B-cell non-Hodgkin lymphoma having a poor overall survival that is in need for the development of new therapeutics. In this study, we report the identification and expression of a new isoform splice variant of the tyrosine kinase receptor AXL in MCL cells. This new AXL isoform, called AXL3, lacks the ligand-binding domain of the commonly described AXL splice variants and is constitutively activated in MCL cells. Interestingly, functional characterization of AXL3, using CRISPR inhibition, revealed that only the knock down of this isoform leads to apoptosis of MCL cells. Importantly, pharmacological inhibition of AXL activity resulted in a significant decrease in the activation of well-known proproliferative and survival pathways activated in MCL cells (ie, β-catenin, Ak strain transforming, and NF-κB). Therapeutically, preclinical studies using a xenograft mouse model of MCL indicated that bemcentinib is more effective than ibrutinib in reducing the tumor burden and to increase the overall survival. Our study highlights the importance of a previously unidentified AXL splice variant in cancer and the potential of bemcentinib as a targeted therapy for MCL.

Fc-engineered monoclonal antibodies to reduce off-target liver uptake

BACKGROUND

Radiolabeled-antibodies usually display non-specific liver accumulation that may impair image analysis and antibody biodistribution. Here, we investigated whether Fc silencing influenced antibody biodistribution. We compared recombinant 89Zr-labeled antibodies (human IgG1 against different targets) with wild-type Fc and with mutated Fc (LALAPG triple mutation to prevent binding to Fc gamma receptors; FcγR). After antibody injection in mice harboring xenografts of different tumor cell lines or of immortalized human myoblasts, we analyzed antibody biodistribution by PET-CT and conventional biodistribution analysis.

RESULTS

Accumulation in liver was strongly reduced and tumor-specific targeting was increased for the antibodies with mutated Fc compared with wild-type Fc.

CONCLUSION

Antibodies with reduced binding to FcγR display lower liver accumulation and better tumor-to-liver ratios. These findings need to be taken into account to improve antibody-based theragnostic approaches.

GAS6-based CAR-T cells exhibit potent antitumor activity against pancreatic cancer

BACKGROUND

The receptor tyrosine kinases TAM family (TYRO3, AXL, and MERTK) are highly expressed in multiple forms of cancer cells and tumor-associated macrophages and promote the development of cancers including pancreatic tumor. Targeting TAM receptors could be a promising therapeutic option.

METHODS

We designed a novel CAR based on the extracellular domain of growth arrest-specific protein 6 (GAS6), a natural ligand for all TAM members. The ability of CAR-T to kill pancreatic cancer cells is tested in vitro and in vivo, and the safety is evaluated in mice and nonhuman primate.

RESULTS

GAS6-CAR-T cells efficiently kill TAM-positive pancreatic cancer cell lines, gemcitabine-resistant cancer cells, and cancer stem-like cells in vitro. GAS6-CAR-T cells also significantly suppressed the growth of PANC1 xenografts and patient-derived xenografts in mice. Furthermore, these CAR-T cells did not induce obvious side effects in nonhuman primate or mice although the CAR was demonstrated to recognize mouse TAM.

CONCLUSIONS

Our findings indicate that GAS6-CAR-T-cell therapy may be effective for pancreatic cancers with low toxicity.

AXL Inhibitors: Status of Clinical Development

PURPOSE OF REVIEW

The AXL signaling pathway is associated with tumor growth as well as poor prognosis in cancer. Here, we highlight recent strategies for targeting AXL in the treatment of solid and hematological malignancies.

RECENT FINDINGS

AXL is a key player in survival, metastasis, and therapeutic resistance in many cancers. A range of AXL-targeted therapies, including tyrosine kinase inhibitors, monoclonal antibodies, antibody-drug conjugates, and soluble receptors, have entered clinical development. Notably, AXL inhibitors in combination with immune checkpoint inhibitors demonstrate early promise; however, further understanding of predictive biomarkers and treatment sequencing is necessary. Based on its role in tumor growth and drug resistance, AXL represents a promising therapeutic target in oncology. Results from ongoing clinical trials will provide valuable insights into the role of AXL inhibitors, both as single agents and in combination with other therapies.

Antitumor effect of 3-(quinolin-2-ylmethylene)-4,6-dimethyl-5-hydroxy-7-azaoxindole down-regulating the Gas6-Axl axis

In this study, a new series of 3-arylidene-4,6-dimethyl-5-hydroxy-7-azaoxindole compounds with a wide range of functional groups were designed, synthesized, and evaluated for their antitumor activity. Among the 35 compounds, compound 6-15, with a quinoline moiety, showed cytotoxic IC₅₀ values superior to those of sunitinib against the seven cancer cell lines (MCF-7, MDA-MB-231, HT-29, DU145, U937, A549, and PANC-1). However, its inhibitory activity against receptor tyrosine kinases (VEGFR2, PDGFRβ, c-KIT, FGFR1, FLT3, CSF1R, EGFR, Axl, and Axl mutant) was 100 -3000-fold weaker than that of sunitinib. Interestingly, compound 6-15 exerted a 3.6-fold stronger cytotoxicity than sunitinib in the gemcitabine-resistant PANC-1 cell line and significantly inhibited Axl, which was in contrast with the effect of sunitinib. Nonetheless, both compounds suppressed the expression of growth arrest-specific 6 (Gas6), the ligand of Axl. The inhibitory effect of compound 6-15 on the Gas6-Axl axis was similar to that of Gas6 knockdown by siRNA in PANC-1 cells in terms of apoptosis induction, increase in Bax/Bcl-2 ratio, Axl down-regulation, and PI3K/Akt inhibition. The inhibitory effect of compound 6-15 on tumor growth in mouse tumor models with A549 and PANC-1 xenografts was much greater than that of cisplatin or gemcitabine. Taken together, the current findings demonstrate that compound 6-15 is a promising anticancer drug candidate that acts by inhibiting the Gas6-Axl axis.

C/EBPδ Suppresses Motility-Associated Gene Signatures and Reduces PDAC Cell Migration

Pancreatic Ductal Adenocarcinoma (PDAC) is among the most aggressive human cancers and occurs globally at an increasing incidence. Metastases are the primary cause of cancer-related death and, in the majority of cases, PDAC is accompanied by metastatic disease at the time of diagnosis, making it a particularly lethal cancer. Regrettably, to date, no curative treatment has been developed for patients with metastatic disease, resulting in a 5-year survival rate of only 11%. We previously found that the protein expression of the transcription factor CCAAT/Enhancer-Binding Protein Delta (C/EBPδ) negatively correlates with lymph node involvement in PDAC patients. To better comprehend the etiology of metastatic PDAC, we explored the role of C/EBPδ at different steps of the metastatic cascade, using established in vitro models. We found that C/EBPδ has a major impact on cell motility, an important prerequisite for tumor cells to leave the primary tumor and to reach distant sites. Our data suggest that C/EBPδ induces downstream pathways that modulate actin cytoskeleton dynamics to reduce cell migration and to induce a more epithelial-like cellular phenotype. Understanding the mechanisms dictating epithelial and mesenchymal features holds great promise for improving the treatment of PDAC.

Remodelling of tumour microenvironment by microwave ablation potentiates immunotherapy of AXL-specific CAR T cells against non-small cell lung cancer

The complex immunosuppressive tumour microenvironment (TME) and lack of tumour-specific targets hinder the application of chimeric antigen receptor (CAR) T cells in the treatment of solid tumours. Combining local treatment with CAR T cell immunotherapy may regulate the TME and enhance the killing potency of CAR T cells in solid tumours. Here, we show that AXL, which is highly expressed in non-small cell lung cancer (NSCLC) but not in normal tissues, might be a target for CAR T cell therapy. AXL-CAR T cells alone cause moderate tumour regression in subcutaneous and pulmonary metastatic lung cancer cell-derived xenograft models. Combination of microwave ablation (MWA) and AXL-CAR T cells have superior antitumour efficacy. MWA enhances the activation, infiltration, persistence and tumour suppressive properties of AXL-CAR T cells in AXL-positive NSCLC patient-derived xenograft tumours via TME remodelling. The combination therapy increases the mitochondrial oxidative metabolism of tumour-infiltrating CAR T cells. Combination treatment induces significant tumour suppression without observed toxicities in humanized immunocompetent mice. The synergistic therapeutic effect of MWA and AXL-CAR T cells may be valuable for NSCLC treatment.

Two-Front War on Cancer-Targeting TAM Receptors in Solid Tumour Therapy

Simple Summary

In recent years, many studies have shown the importance of TAM kinases in both normal and neoplastic cells. In this review, we present and discuss the role of the TAM family (AXL, MERTK, TYRO3) of receptor tyrosine kinases (RTKs) as a dual target in cancer, due to their intrinsic roles in tumour cell survival, migration, chemoresistance, and their immunosuppressive roles in the tumour microenvironment. This review presents the potential of TAMs as emerging therapeutic targets in cancer treatment, focusing on the distinct structures of TAM receptor tyrosine kinases. We analyse and compare different strategies of TAM inhibition, for a full perspective of current and future battlefields in the war with cancer.

Abstract

Receptor tyrosine kinases (RTKs) are transmembrane receptors that bind growth factors and cytokines and contain a regulated kinase activity within their cytoplasmic domain. RTKs play an important role in signal transduction in both normal and malignant cells, and their encoding genes belong to the most frequently affected genes in cancer cells. The TAM family proteins (TYRO3, AXL, and MERTK) are involved in diverse biological processes: immune regulation, clearance of apoptotic cells, platelet aggregation, cell proliferation, survival, and migration. Recent studies show that TAMs share overlapping functions in tumorigenesis and suppression of antitumour immunity. MERTK and AXL operate in innate immune cells to suppress inflammatory responses and promote an immunosuppressive tumour microenvironment, while AXL expression correlates with epithelial-to-mesenchymal transition, metastasis, and motility in tumours. Therefore, TAM RTKs represent a dual target in cancer due to their intrinsic roles in tumour cell survival, migration, chemoresistance, and their immunosuppressive roles in the tumour microenvironment (TME). In this review, we discuss the potential of TAMs as emerging therapeutic targets in cancer treatment. We critically assess and compare current approaches to target TAM RTKs in solid tumours and the development of new inhibitors for both extra- and intracellular domains of TAM receptor kinases.

Upregulated flotillins and sphingosine kinase 2 derail AXL vesicular traffic to promote epithelial-mesenchymal transition

Altered endocytosis and vesicular trafficking are major players during tumorigenesis. Flotillin overexpression, a feature observed in many invasive tumors and identified as a marker of poor prognosis, induces a deregulated endocytic and trafficking pathway called upregulated flotillin-induced trafficking (UFIT). Here, we found that in non-tumoral mammary epithelial cells, induction of the UFIT pathway promotes epithelial-to-mesenchymal transition (EMT) and accelerates the endocytosis of several transmembrane receptors, including AXL, in flotillin-positive late endosomes. AXL overexpression, frequently observed in cancer cells, is linked to EMT and metastasis formation. In flotillin-overexpressing non-tumoral mammary epithelial cells and in invasive breast carcinoma cells, we found that the UFIT pathway-mediated AXL endocytosis allows its stabilization and depends on sphingosine kinase 2, a lipid kinase recruited in flotillin-rich plasma membrane domains and endosomes. Thus, the deregulation of vesicular trafficking following flotillin upregulation, and through sphingosine kinase 2, emerges as a new mechanism of AXL overexpression and EMT-inducing signaling pathway activation.

Small-Molecule RAS Inhibitors as Anticancer Agents: Discovery, Development, and Mechanistic Studies

Mutations of RAS oncogenes are responsible for about 30% of all human cancer types, including pancreatic, lung, and colorectal cancers. While KRAS1 is a pseudogene, mutation of KRAS2 (commonly known as KRAS oncogene) is directly or indirectly associated with human cancers. Among the RAS family, KRAS is the most abundant oncogene related to uncontrolled cellular proliferation to generate solid tumors in many types of cancer such as pancreatic carcinoma (over 80%), colon carcinoma (40-50%), lung carcinoma (30-50%), and other types of cancer. Once described as 'undruggable', RAS proteins have become 'druggable', at least to a certain extent, due to the continuous efforts made during the past four decades. In this account, we discuss the chemistry and biology (wherever available) of the small-molecule inhibitors (synthetic, semi-synthetic, and natural) of KRAS proteins that were published in the past decades. Commercial drugs, as well as investigational molecules from preliminary stages to clinical trials, are categorized and discussed in this study. In summary, this study presents an in-depth discussion of RAS proteins, classifies the RAS superfamily, and describes the molecular mechanism of small-molecule RAS inhibitors.

The Development of AXL Inhibitors in Lung Cancer: Recent Progress and Challenges

AXL, along with MER and TYRO3, is a receptor tyrosine kinase from the TAM family. Although AXL itself is not thought to be a potent oncogenic driver, overexpression of AXL is known to trigger tumor cell growth, survival, invasion, metastasis, angiogenesis, epithelial to mesenchymal transition, and immune suppression. Overexpression of AXL is associated with therapy resistance and poor prognosis. Therefore, it is being studied as a marker of prognosis in cancer treatment or as a target in various cancer types. Recently, many preclinical and clinical studies on agents with various mechanisms targeting AXL have been actively conducted. They include small molecule inhibitors, monoclonal antibodies, and antibody-drug conjugates. This article reviewed the fundamental role of AXL in solid tumors, and the development in research of AXL inhibitors in recent years. Emphasis was placed on the function of AXL in acquired therapy resistance in patients with non-small cell lung cancer (NSCLC). Since clinical needs increase in NSCLC patients with acquired resistance after initial therapy, recent research efforts have focused on a combination treatment with AXL inhibitors and tyrosine kinase inhibitors or immunotherapy to overcome resistance. Lastly, we deal with challenges and limitations encountered in the development of AXL inhibitors.

Soluble AXL is a novel blood marker for early detection of pancreatic ductal adenocarcinoma and differential diagnosis from chronic pancreatitis

Background

Early diagnosis is crucial for patients with pancreatic ductal adenocarcinoma (PDAC). The AXL receptor tyrosine kinase is proteolytically processed releasing a soluble form (sAXL) into the blood stream. Here we explore the use of sAXL as a biomarker for PDAC.

Methods

AXL was analysed by immunohistochemistry in human pancreatic tissue samples. RNA expression analysis was performed using TCGA/GTEx databases. The plasma concentrations of sAXL, its ligand GAS6, and CA19-9 were studied in two independent cohorts, the HMar cohort (n = 59) and the HClinic cohort (n = 142), including healthy controls, chronic pancreatitis (CP) or PDAC patients, and in a familial PDAC cohort (n = 68). AXL expression and sAXL release were studied in PDAC cell lines and murine models.

Findings

AXL is increased in PDAC and precursor lesions as compared to CP or controls. sAXL determined in plasma from two independent cohorts was significantly increased in the PDAC group as compared to healthy controls or CP patients. Patients with high levels of AXL have a lower overall survival. ROC analysis of the plasma levels of sAXL, GAS6, or CA19-9 in our cohorts revealed that sAXL outperformed CA19-9 for discriminating between CP and PDAC. Using both sAXL and CA19-9 increased the diagnostic value. These results were validated in murine models, showing increased sAXL specifically in animals developing PDAC but not those with precursor lesions or acinar tumours.

Interpretation

sAXL appears as a biomarker for early detection of PDAC and PDAC–CP discrimination that could accelerate treatment and improve its dismal prognosis.

Funding

This work was supported by grants PI20/00625 (PN), RTI2018-095672-B-I00 (AM and PGF), PI20/01696 (MG) and PI18/01034 (AC) from MICINN-FEDER and grant 2017/SGR/225 (PN) from Generalitat de Catalunya.

Overcoming biological barriers to improve solid tumor immunotherapy

Cancer immunotherapy has been at the forefront of therapeutic interventions for many different tumor types over the last decade. While the discovery of immunotherapeutics continues to occur at an accelerated rate, their translation is often hindered by a lack of strategies to deliver them specifically into solid tumors. Accordingly, significant scientific efforts have been dedicated to understanding the underlying mechanisms that govern their delivery into tumors and the subsequent immune modulation. In this review, we aim to summarize the efforts focused on overcoming tumor-associated biological barriers and enhancing the potency of immunotherapy. We summarize the current understanding of biological barriers that limit the entry of intravascularly administered immunotherapies into the tumors, in vitro techniques developed to investigate the underlying transport processes, and delivery strategies developed to overcome the barriers. Overall, we aim to provide the reader with a framework that guides the rational development of technologies for improved solid tumor immunotherapy.

Immune Evasion Mechanism and AXL

Extensive interest in cancer immunotherapy is reported according to the clinical importance of CTLA-4 and (PD-1/PD-L1) [programmed death (PD) and programmed death-ligand (PD-L1)] in immune checkpoint therapies. AXL is a receptor tyrosine kinase expressed in different types of cancer and in relation to resistance against various anticancer therapeutics due to poor clinical prognosis. AXL and its ligand, i.e., growth arrest-specific 6 (GAS6) proteins, are expressed on many cancer cells, and the GAS6/AXL pathway is reported to promote cancer cell proliferation, survival, migration, invasion, angiogenesis, and immune evasion. AXL is an attractive and novel therapeutic target for impairing tumor progression from immune cell contracts in the tumor microenvironment. The GAS6/AXL pathway is also of interest immunologically because it targets fewer antitumor immune responses. In effect, several targeted therapies are selective and nonselective for AXL, which are in preclinical and clinical development in multiple cancer types. Therefore, this review focuses on the role of the GAS6/AXL signaling pathway in triggering the immunosuppressive tumor microenvironment as immune evasion. This includes regulating its composition and activating T-cell exclusion with the immune-suppressive activity of regulatory T cells, which is related to one of the hallmarks of cancer survival. Finally, this article discusses the GAS6/AXL signaling pathway in the context of several immune responses such as NK cell activation, apoptosis, and tumor-specific immunity, especially PD-1/PDL-1 signaling.

Targeting MERTK and AXL in EGFR Mutant Non-Small Cell Lung Cancer

MERTK and AXL are members of the TAM family of receptor tyrosine kinases and are abnormally expressed in 69% and 93% of non-small cell lung cancers (NSCLCs), respectively. Expression of MERTK and/or AXL provides a survival advantage for NSCLC cells and correlates with lymph node metastasis, drug resistance, and disease progression in patients with NSCLC. The TAM receptors on host tumor infiltrating cells also play important roles in the immunosuppressive tumor microenvironment. Thus, MERTK and AXL are attractive biologic targets for NSCLC treatment. Here, we will review physiologic and oncologic roles for MERTK and AXL with an emphasis on the potential to target these kinases in NSCLCs with activating EGFR mutations.

Introducing Fluorescence-Guided Surgery for Pediatric Ewing, Osteo-, and Rhabdomyosarcomas: A Literature Review

Sarcomas are a rare heterogeneous group of malignant neoplasms of mesenchymal origin which represent approximately 13% of all cancers in pediatric patients. The most prevalent pediatric bone sarcomas are osteosarcoma (OS) and Ewing sarcoma (ES). Rhabdomyosarcoma (RMS) is the most frequently occurring pediatric soft tissue sarcoma. The median age of OS and ES is approximately 17 years, so this disease is also commonly seen in adults while non-pleiomorphic RMS is rare in the adult population. The mainstay of all treatment regimens is multimodal treatment containing chemotherapy, surgical resection, and sometimes (neo)adjuvant radiotherapy. A clear resection margin improves both local control and overall survival and should be the goal during surgery with a curative intent. Real-time intraoperative fluorescence-guided imaging could facilitate complete resections by visualizing tumor tissue during surgery. This review evaluates whether non-targeted and targeted fluorescence-guided surgery (FGS) could be beneficial for pediatric OS, ES, and RMS patients. Necessities for clinical implementation, current literature, and the positive as well as negative aspects of non-targeted FGS using the NIR dye Indocyanine Green (ICG) were evaluated. In addition, we provide an overview of targets that could potentially be used for FGS in OS, ES, and RMS. Then, due to the time- and cost-efficient translational perspective, we elaborate on the use of antibody-based tracers as well as their disadvantages and alternatives. Finally, we conclude with recommendations for the experiments needed before FGS can be implemented for pediatric OS, ES, and RMS patients.

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.

The complex roles of efferocytosis in cancer development, metastasis, and treatment

When tumor cells are killed by targeted therapy, radiotherapy, or chemotherapy, they trigger their primary tumor by releasing pro-inflammatory cytokines. Microenvironmental interactions can also promote tumor heterogeneity and development. In this line, several immune cells within the tumor microenvironment, including macrophages, dendritic cells, regulatory T-cells, and CD8⁺ and CD4⁺ T cells, are involved in the clearance of apoptotic tumor cells through a process called efferocytosis. Although the efficiency of apoptotic tumor cell efferocytosis is positive under physiological conditions, there are controversies regarding its usefulness in treatment-induced apoptotic tumor cells (ATCs). Efferocytosis can show the limitation of cytotoxic treatments, such as chemotherapy and radiotherapy. Since cytotoxic treatments lead to extensive cell mortality, efferocytosis, and macrophage polarization toward an M2 phenotype, the immune response may get involved in tumor recurrence and metastasis. Tumor cells can use the anti-inflammatory effect of apoptotic tumor cell efferocytosis to induce an immunosuppressive condition that is tumor-tolerant. Since M2 polarization and efferocytosis are tumor-promoting processes, the receptors on macrophages act as potential targets for cancer therapy. Moreover, researchers have shown that efferocytosis-related molecules/pathways are potential targets for cancer therapy. These include phosphatidylserine and calreticulin, Tyro3, Axl, and Mer tyrosine kinase (MerTK), receptors of tyrosine kinase, indoleamine-2,3-dioxygenase 1, annexin V, CD47, TGF-β, IL-10, and macrophage phenotype switch are combined with conventional therapy, which can be more effective in cancer treatment. Thus, we set out to investigate the advantages and disadvantages of efferocytosis in treatment-induced apoptotic tumor cells.

Biparatopic single-domain antibodies against Axl achieve ultra-high affinity through intramolecular engagement

Overexpression of Axl, a TAM-family receptor tyrosine kinase, plays key roles in the formation, growth, and spread of tumors as well as resistance to targeted therapies and chemotherapies. We identified novel llama V_HHs against human Axl using multiple complementary phage display selection strategies and characterized a subset of high-affinity V_HHs. The V_HHs targeted multiple sites in Ig-like domains 1 and 2 of the Axl extracellular domain, including an immunodominant epitope overlapping the site of Gas6 interaction and two additional non-Gas6 competitive epitopes recognized by murine monoclonal antibodies. Only a subset of V_HHs cross-reacted with cynomolgus monkey Axl and none recognized mouse Axl. As fusions to human IgG1 Fc, V_HH-Fcs bound Axl⁺ tumor cell lines and mertansine-loaded V_HH-Fcs were cytotoxic in vitro against Axl⁺ cells in proportion to their binding affinities. Engineered biparatopic V_HH-V_HH heterodimers bound Axl avidly, and a subset of molecules showed dramatically enhanced association rates indicative of intramolecular binding. These V_HHs may have applications as modular elements of biologic drugs such as antibody-drug conjugates.

Inhibition of the Axl pathway impairs breast and prostate cancer metastasis to the bones and bone remodeling

Approximately 90% of cancer-related deaths result from cancer metastasis. In prostate and breast cancers, bone is the most common site of cancer cell dissemination. Key steps in the metastatic cascade are promoted through upregulation of critical cell signaling pathways in neoplastic cells. The present study assessed the role of the receptor tyrosine kinase Axl in prostate and breast cancer cell metastasis to bones using (i) Axl knockdown neoplastic cells and osteoclast progenitor cells in vitro, (ii) intracardiac injection of Axl knockdown tumor cells in vivo, and (iii) selective Axl inhibitor BGB324. Axl inhibition in neoplastic cells significantly decreased their metastatic potential, and suppression of Axl signaling in osteoclast precursor cells also reduced the formation of mature osteoclasts. In vivo, Axl knockdown in prostate and breast cancer cells significantly suppressed the formation and progression of bone metastases. Hence, therapeutic targeting of Axl may impair tumor metastasis to the bones through neoplastic and host cell signaling axes.

Bufalin down-regulates Axl expression to inhibit cell proliferation and induce apoptosis in non-small-cell lung cancer cells

Axl, a member of the TAM (Tyro3, AXL, Mer) receptor tyrosine kinase family, plays critical roles in cell growth, proliferation, apoptosis, and migration. In the present study, we demonstrated that the anti-cancer activity of bufalin, a major bioactive component of the Chinese traditional medicine Chan Su, is mediated by the down-regulation of Axl in non-small-cell lung cancer (NSCLC) cells. We observed the inhibitory effect of bufalin on the proliferation of A549 and H460 NSCLC cells and the clonogenicity of these cells was reduced by bufalin treatment in a dose-dependent manner. Next, we found that the protein level of Axl was decreased in proportion to the concentration of bufalin in both A549 and H460 cells. Moreover, the promoter activity of the Axl gene was decreased by bufalin in a dose- and time-dependent manner, indicating that bufalin down-regulates Axl gene expression at the transcriptional level. We further examined if the anti-proliferative property of bufalin is influenced by Axl at the protein level. Axl overexpression attenuated the effect of bufalin in inhibiting cell proliferation and colony formation and inducing apoptosis in H460 cells, while knockdown of Axl gene expression induced the opposite effect. Taken together, our data indicate that the anti-proliferative and pro-apoptotic effects of bufalin were associated with the protein level of Axl, suggesting that Axl is a potent therapeutic target of bufalin in suppressing proliferation and inducing apoptosis in NSCLC cells.

Lotus (Nelumbo nucifera) seedpod extract inhibits cell proliferation and induces apoptosis in non-small cell lung cancer cells via downregulation of Axl

Non-small-cell lung cancer (NSCLC) is the most frequent cause of cancer-related death. In this study, we found the anticancer activity of lotus seedpod extract (LSPE) in NSCLC cells, since LSPE treatment inhibited cell proliferation of A549 and H460 cells in a dose-dependent manner and the clonogenic activities of LSPE-treated cells were also reduced. In LSPE-treated cells, the cleavage of poly (ADP-ribose) polymerase (PARP) and phosphorylation of H2X, were also observed, indicating the pro-apoptotic effect of LSPE. Next, we found that LPSE treatment diminished the levels of protein and mRNA of Axl, a receptor tyrosine kinase (RTK) that transduces critical signals for cell proliferation and inhibition of apoptosis. The promoter activity of Axl was found to be dose-dependently decreased in response to LSPE treatment, implying that LSPE inhibited Axl gene expression at transcriptional level. In addition, Axl overexpression was found to decrease the effects of LSPE on inhibition of cell proliferation and colony formation as well as induction of PARP cleavage and phosphorylation of H2AX, while the same activities of LPSE were increased by knockdown of Axl gene expression, indicating that the antiproliferative and pro-apoptotic effect of LSPE is inversely proportional to the protein level of Axl. Taken together, we found that the LSPE suppressed cell proliferation and induced apoptosis of NSCLC cells, which is attenuated or augmented by overexpression or RNA interference of Axl expression, respectively. Our data suggest that Axl is a novel therapeutic target of LSPE to inhibit cell proliferation and promote apoptosis in NSCLC cells. PRACTICAL APPLICATIONS: In this study, lotus seedpod extract (LSPE) was found to have the cytotoxic and apoptosis-inducing potentials in non-small-cell lung cancer (NSCLC) cells. LSPE downregulated the Axl expression at transcriptional level and the effects of LSPE on cell proliferation as well as apoptosis were affected by Axl protein level. Therefore, the inference of Axl-mediated intracellular signals by LSPE must be a novel approach to control NSCLC. Since our data imply that LSPE contains bioactive compounds targeting Axl, further studies to elucidate these compounds might discover a potent therapeutic agent.

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.

Therapeutic aspects of the Axl/Gas6 molecular system

Axl receptor tyrosine kinase (RTK) and its ligand, growth arrest-specific protein 6 (Gas6), are involved in several biological functions and participate in the development and progression of a range of malignancies and autoimmune disorders. In this review, we present this molecular system from a drug discovery perspective, highlighting its therapeutic implications and challenges that need to be addressed. We provide an update on Axl/Gas6 axis biology, exploring its role in fields ranging from angiogenesis, cancer development and metastasis, immune response and inflammation to viral infection. Finally, we summarize the molecules that have been developed to date to target the Axl/Gas6 molecular system for therapeutic and diagnostic applications.

Diverse Macrophage Populations Contribute to the Inflammatory Microenvironment in Premalignant Lesions During Localized Invasion

Myeloid cell heterogeneity remains poorly studied in breast cancer, and particularly in premalignancy. Here, we used single cell RNA sequencing to characterize macrophage diversity in mouse pre-invasive lesions as compared to lesions undergoing localized invasion. Several subpopulations of macrophages with transcriptionally distinct profiles were identified, two of which resembled macrophages in the steady state. While all subpopulations expressed tumor-promoting genes, many of the populations expressed pro-inflammatory genes, differing from reports in tumor-associated macrophages. Gene profiles of the myeloid cells were similar between early and late stages of premalignancy, although expansion of some subpopulations occurred. These results unravel macrophage heterogeneity in early progression and may provide insight into early intervention strategies that target macrophages.

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.

Enhancing the Efficacy of CAR T Cells in the Tumor Microenvironment of Pancreatic Cancer

Pancreatic cancer has the worst prognosis and lowest survival rate among all types of cancers and thus, there exists a strong need for novel therapeutic strategies. Chimeric antigen receptor (CAR)-modified T cells present a new potential option after successful FDA-approval in hematologic malignancies, however, current CAR T cell clinical trials in pancreatic cancer failed to improve survival and were unable to demonstrate any significant response. The physical and environmental barriers created by the distinct tumor microenvironment (TME) as a result of the desmoplastic reaction in pancreatic cancer present major hurdles for CAR T cells as a viable therapeutic option in this tumor entity. Cancer cells and cancer-associated fibroblasts express extracellular matrix molecules, enzymes, and growth factors, which can attenuate CAR T cell infiltration and efficacy. Recent efforts demonstrate a niche shift where targeting the TME along CAR T cell therapy is believed or hoped to provide a substantial clinical added value to improve overall survival. This review summarizes therapeutic approaches targeting the TME and their effect on CAR T cells as well as their outcome in preclinical and clinical trials in pancreatic cancer.

Regulation of efferocytosis as a novel cancer therapy

Efferocytosis is a physiologic phagocytic clearance of apoptotic cells, which modulates inflammatory responses and the immune environment and subsequently facilitates immune escape of cancer cells, thus promoting tumor development and progression. Efferocytosis is an equilibrium formed by perfect coordination among “find-me”, “eat-me” and “don’t-eat-me” signals. These signaling pathways not only affect the proliferation, invasion, metastasis, and angiogenesis of tumor cells but also regulate adaptive responses and drug resistance to antitumor therapies. Therefore, efferocytosis-related molecules and pathways are potential targets for antitumor therapy. Besides, supplementing conventional chemotherapy, radiotherapy and other immunotherapies with efferocytosis-targeted therapy could enhance the therapeutic efficacy, reduce off-target toxicity, and promote patient outcome.

Gas6 expression is reduced in advanced breast cancers

Growth arrest-specific gene 6 (Gas6) is a cytokine that binds to receptor tyrosine kinases Tyro3, Axl, and Mer. Numerous studies have suggested that macrophage-derived Gas6 interacts with Axl to promote cancer progression, and Axl has been associated with poor clinical outcome. However, the expression and relevance of Gas6 in human breast cancer patients has not been studied. Analysis of tissue microarrays showed that Gas6 was highly expressed in ductal carcinoma in situ (DCIS) but markedly decreased in invasive breast cancer. Gas6 and Axl were weakly correlated, suggesting that their functions may not exclusively rely on each other. Analyses of publicly available databases showed significantly improved overall and relapse-free survival in patients with high Gas6 mRNA, particularly in luminal A breast cancers. These findings indicate that tumor-derived Gas6 is not overexpressed in invasive breast cancer, and may not be a negative prognostic factor in human breast cancer.

MYC Instructs and Maintains Pancreatic Adenocarcinoma Phenotype

The signature features of pancreatic ductal adenocarcinoma (PDAC) are its fibroinflammatory stroma, poor immune activity, and dismal prognosis. We show that acute activation of Myc in indolent pancreatic intraepithelial neoplasm (PanIN) epithelial cells in vivo is, alone, sufficient to trigger immediate release of instructive signals that together coordinate changes in multiple stromal and immune-cell types and drive transition to pancreatic adenocarcinomas that share all the characteristic stromal features of their spontaneous human counterpart. We also demonstrate that this Myc-driven PDAC switch is completely and immediately reversible: Myc deactivation/inhibition triggers meticulous disassembly of advanced PDAC tumor and stroma and concomitant death of tumor cells. Hence, both the formation and deconstruction of the complex PDAC phenotype are continuously dependent on a single, reversible Myc switch. SIGNIFICANCE: We show that Myc activation in indolent Kras G12D-induced PanIN epithelium acts as an immediate pleiotropic switch, triggering tissue-specific signals that instruct all the diverse signature stromal features of spontaneous human PDAC. Subsequent Myc deactivation or inhibition immediately triggers a program that coordinately disassembles PDAC back to PanIN.See related commentary by English and Sears, p. 495.

Hematopoietic progenitor kinase 1 down-regulates the oncogenic receptor tyrosine kinase AXL in pancreatic cancer

The oncogenic receptor tyrosine kinase AXL is overexpressed in cancer and plays an important role in carcinomas of multiple organs. However, the mechanisms of AXL overexpression in cancer remain unclear. In this study, using HEK293T, Panc-1, and Panc-28 cells and samples of human pancreatic intraepithelial neoplasia (PanIN), along with several biochemical approaches and immunofluorescence microscopy analyses, we sought to investigate the mechanisms that regulate AXL over-expression in pancreatic ductal adenocarcinoma (PDAC). We found that AXL interacts with hematopoietic progenitor kinase 1 (HPK1) and demonstrate that HPK1 down-regulates AXL and decreases its half-life. The HPK1-mediated AXL degradation was inhibited by the endocytic pathway inhibitors leupeptin, bafilomycin A1, and monensin. HPK1 accelerated the movement of AXL from the plasma membrane to endosomes in pancreatic cancer cells treated with the AXL ligand growth arrest-specific 6 (GAS6). Moreover, HPK1 increased the binding of AXL to the Cbl proto-oncogene (c-Cbl); promoted AXL ubiquitination; decreased AXL-mediated signaling, including phospho-AKT and phospho-ERK signaling; and decreased the invasion capability of PDAC cells. Importantly, we show that AXL expression inversely correlates with HPK1 expression in human PanINs and that patients whose tumors have low HPK1 and high AXL expression levels have shorter survival than those with low AXL or high HPK1 expression (p < 0.001). Our results suggest that HPK1 is a tumor suppressor that targets AXL for degradation via the endocytic pathway. HPK1 loss of function may contribute to AXL overexpression and thereby enhance AXL-dependent downstream signaling and tumor invasion in PDAC.

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.

AXL receptor tyrosine kinase as a promising anti-cancer approach: functions, molecular mechanisms and clinical applications

Molecular targeted therapy for cancer has been a research hotspot for decades. AXL is a member of the TAM family with the high-affinity ligand growth arrest-specific protein 6 (GAS6). The Gas6/AXL signalling pathway is associated with tumour cell growth, metastasis, invasion, epithelial-mesenchymal transition (EMT), angiogenesis, drug resistance, immune regulation and stem cell maintenance. Different therapeutic agents targeting AXL have been developed, typically including small molecule inhibitors, monoclonal antibodies (mAbs), nucleotide aptamers, soluble receptors, and several natural compounds. In this review, we first provide a comprehensive discussion of the structure, function, regulation, and signalling pathways of AXL. Then, we highlight recent strategies for targeting AXL in the treatment of cancer.AXL-targeted drugs, either as single agents or in combination with conventional chemotherapy or other small molecule inhibitors, are likely to improve the survival of many patients. However, future investigations into AXL molecular signalling networks and robust predictive biomarkers are warranted to select patients who could receive clinical benefit and to avoid potential toxicities.

Behind the Wheel of Epithelial Plasticity in KRAS-Driven Cancers

Cellular plasticity, a feature associated with epithelial-to-mesenchymal transition (EMT), contributes to tumor cell survival, migration, invasion, and therapy resistance. Phenotypic plasticity of the epithelium is a critical feature in multiple phases of human cancer in an oncogene- and tissue-specific context. Many factors can drive epithelial plasticity, including activating mutations in KRAS, which are found in an estimated 30% of all cancers. In this review, we will introduce cellular plasticity and its effect on cancer progression and therapy resistance and then summarize the drivers of EMT with an emphasis on KRAS effector signaling. Lastly, we will discuss the contribution of cellular plasticity to metastasis and its potential clinical implications. Understanding oncogenic KRAS cellular reprogramming has the potential to reveal novel strategies to control metastasis in KRAS-driven cancers.

Tissue-Specific Monoallelic Expression of Bovine AXL is Associated with DNA Methylation of Promoter DMR

The AXL protein is a receptor tyrosine kinase and is often implicated in proliferation, migration and therapy resistance in various cancers. The AXL gene in humans is maternally expressed and paternally imprinted with differentially methylated regions (DMR) surrounding the promoter region. However, the imprinting status and epigenetic regulation of AXL gene in cattle remain unclear. Therefore, we explored the molecular structure along with the patterns of allelic expression and DNA methylation of the bovine AXL gene. First, the complete cDNA sequence of bovine AXL was gathered by Sanger method, from transcripts obtained from RT-PCR, 5' and 3' -RACE. In silico BLAST alignments showed that the longest mRNA sequence of bovine AXL consists of 19 exons and encodes a protein of 887 amino acids. We further analyzed the allelic expression of bovine AXL by employing single-nucleotide polymorphism (SNP)-based sequencing method. A SNP site (GenBank Accession no: rs210020651) found in exon 7 allowed us to distinguish the two parental alleles. Monoallelic expression of AXL was observed in four adult bovine tissues (heart, liver, spleen and fat), while biallelic expression was found in the other adult tissues such as the lung, kidney, muscle, brain and placenta. To determine whether the DNA methylation played a role in the tissue-specific imprinting of bovine AXL, we performed bisulfite sequencing of two regions: region 1 was a CpG island (CGI) in AXL promoter, mapping to 643 bp upstream of the transcription start site of AXL 5'-v1 transcripts, while region two was homologous to the region of human AXL DMR, with 10 CpG sites overlapping the first translation start site (TSS1) of bovine AXL. In region 2, DNA from both monoallelic and biallelic expressed tissues were mostly found to be completely unmethylated. However, tissue-specific differential methylation patterns were found in monoallelic expressed tissues such as the heart and liver while hypomethylation was noted in the promoter CpG island in biallelic expressed tissues such as the lung. These observations demonstrated that the tissue-specific monoallelic expression of bovine AXL is dependent on the DNA methylation of its promoter region.

Engineered AXL-ECD-Fc variants that abolish the AXL/Gas6 interaction suppress tumor cell migration

AXL receptor tyrosine kinase ligand (AXL), a tyrosine kinase receptor that is commonly overexpressed in numerous types of cancer, significantly promotes drug resistance and metastasis in tumor cells. Inhibition of the AXL/growth arrest-specific 6 (Gas6) signaling pathway is emerging as a potential anticancer therapeutic strategy. In the present study, on the basis of the three-dimensional complex structure of AXL/Gas6, the critical residues (E⁵⁶, E⁵⁹ and T⁷⁷) in AXL binding to Gas6 were determined using computer graphics analysis and the distance geometry method. Subsequently, four-variant AXL^-ECD-Fc-M1 (G³²S, D⁸⁷G, V⁹²A and G¹²⁷R) and AXL^-ECD-Fc-M2 (G³²A, D⁸⁷A, V⁹²A and G¹²⁷A) were predicted as high-affinity mutants; AXL^-ECD-Fc-M3 (E⁵⁶R and T⁷⁷R) and AXL^-ECD-Fc-M4 (E⁵⁹R and T⁷⁷R) were predicted as low-affinity mutants. The results of the present study revealed that the half-maximal effect concentrations of AXL^-ECD-Fc-M1 and AXL^-ECD-Fc-M2 were ~0.141 and 0.375 µg/ml, respectively, whereas that of the wild-type protein (AXL^-ECD-Fc-WT) was 0.514 µg/ml. Furthermore, adding the high-affinity mutants into culture medium to capture free Gas6 significantly inhibited AXL/Gas6 binding and thus blocked the downstream signaling pathway. In addition, the high-affinity mutants effectively suppressed the migration and metastasis of SKOV3 and A549 cells. Conversely, compared with AXL^-ECD-Fc-WT, the low-affinity AXL mutants AXL^-ECD-Fc-M3 and AXL^-ECD-Fc-M4 lost all inhibitory activities. These findings highlight AXL as a potential therapeutic target and demonstrated that the key residues E⁵⁶, E⁵⁹ and T⁷⁷ may be crucial sites for abolishing the activity of the AXL/Gas6 pathway in cancer therapy.

An auristatin-based antibody-drug conjugate targeting HER3 enhances the radiation response in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive cancer characterized by poor response to chemotherapy and radiotherapy due to the lack of efficient therapeutic tools and early diagnostic markers. We previously generated the nonligand competing anti-HER3 antibody 9F7-F11 that binds to pancreatic tumor cells and induces tumor regression in vivo in experimental models. Here, we asked whether coupling 9F7-F11 with a radiosensitizer, such as monomethylauristatin E (MMAE), by using the antibody-drug conjugate (ADC) technology could improve radiation therapy efficacy in PDAC. We found that the MMAE-based HER3 antibody-drug conjugate (HER3-ADC) was efficiently internalized in tumor cells, increased the fraction of cells arrested in G2/M, which is the most radiosensitive phase of the cell cycle, and promoted programmed cell death of irradiated HER3-positive pancreatic cancer cells (BxPC3 and HPAC cell lines). HER3-ADC decreased the clonogenic survival of irradiated cells by increasing DNA double-strand break formation (based on γH2AX level), and by modulating DNA damage repair. Tumor radiosensitization with HER3-ADC favored the inhibition of the AKT-induced survival pathway, together with more efficient caspase 3/PARP-mediated apoptosis. Incubation with HER3-ADC before irradiation synergistically reduced the phosphorylation of STAT3, which is involved in chemoradiation resistance. In vivo, the combination of HER3-ADC with radiation therapy increased the overall survival of mice harboring BxPC3, HPAC cell xenografts or patient-derived xenografts, and reduced proliferation (KI67-positive cells). Combining auristatin radiosensitizer delivery via an HER3-ADC with radiotherapy is a new promising therapeutic strategy in PDAC.

TYRO3 as a molecular target for growth inhibition and apoptosis induction in bladder cancer

Background

Muscle-invasive bladder cancer (MIBC) is an aggressive neoplasm with poor prognosis, lacking effective therapeutic targets. Oncogenic dependency on members of the TAM tyrosine kinase receptor family (TYRO3, AXL, MERTK) has been reported in several cancer types, but their role in bladder cancer has never been explored.

Methods

TAM receptor expression was evaluated in two series of human bladder tumours by gene expression (TCGA and CIT series), immunohistochemistry and western blotting analyses (CIT series). The role of the different TAM receptors was assessed by loss-of-function experiments and pharmaceutical inhibition in vitro and in vivo.

Results

We reported a significantly higher expression of TYRO3, but not AXL or MERTK, in both non-MIBCs and MIBCs, compared to normal urothelium. Loss-of-function experiments identified a TYRO3-dependency of bladder carcinoma-derived cells both in vitro and in a mouse xenograft model, whereas AXL and MERTK depletion had only a minor impact on cell viability. Accordingly, TYRO3-dependent bladder tumour cells were sensitive to pharmacological treatment with two pan-TAM inhibitors. Finally, growth inhibition upon TYRO3 depletion relies on cell cycle inhibition and apoptosis associated with induction of tumour-suppressive signals.

Conclusions

Our results provide a preclinical proof of concept for TYRO3 as a potential therapeutic target in bladder cancer.

TYRO3: A potential therapeutic target in cancer

IMPACT STATEMENT

Cancer is among the leading causes of death worldwide. In 2016, 8.9 million people are estimated to have died from various forms of cancer. The current treatments, including surgery with chemotherapy and/or radiation therapy, are not effective enough to provide full protection from cancer, which highlights the need for developing novel therapy strategies. In this review, we summarize the molecular biology of a unique member of a subfamily of receptor tyrosine kinase, TYRO3 and discuss the new insights in TYRO3-targeted treatment for cancer therapy.

AXL phosphorylates and up-regulates TNS2 and its implications in IRS-1-associated metabolism in cancer cells

Background

TNS2 is a focal adhesions protein and a binding partner for many proteins, including the receptor tyrosine kinase Axl. Although TNS2 can bind with Axl, the details of their interactions have not been elucidated. TNS2 is involved in IRS-1 signaling pathway. In this study, we confirmed the relationship between TNS2 expression and the expression of Axl, IRS-1, PDK1 and Glut4 in pancreatic cancer patients.

Methods

The expression levels of TNS2, Axl, IRS-1, PDK1 and Glut4 in human cancer cells were measured by Western blot and/or IP-Western blot assays. Paired samples of pancreatic cancer and non-cancer tissues were obtained from 33 patients and were used to construct tissue microarrays. The expression levels of these markers in the tissue microarrays were measured by enzyme-linked Immunohistochemistry assay, and the relationships were analyzed by Pearson’s chi-square test and two-tailed t-test analysis.

Results

We demonstrated for the first time that TNS2 is a phosphorylation substrate of Axl. Moreover, we found a positive relationship between TNS2 expression and the expression of Axl, IRS-1, PDK1 and Glut4 in pancreatic cancer patients. Based on these results, we suggest that Axl modulates glucose metabolism potentially through TNS2 and IRS-1. We hypothesize that there exists a novel mechanism whereby Axl binds to and phosphorylates TNS2, releasing TNS2 from interaction with IRS-1 and resulting in increased stability of IRS-1. The two key enzymes of aerobic glycolysis (Glut4 and PDK1) were found to be up-regulated by Axl/TNS2/IRS-1 cross-talk and may play a critical role in glucose metabolism of cancer cells.

Conclusions

Our results revealed for the first time that Axl binds to and phosphorylates TNS2 and that Axl/TNS2/IRS-1 cross-talk may potentially play a critical role in glucose metabolism of cancer cells.

Electronic supplementary material

The online version of this article (10.1186/s12929-018-0465-x) contains supplementary material, which is available to authorized users.

Does Axl have potential as a therapeutic target in pancreatic cancer?

INTRODUCTION

Pancreatic cancer is a leading cause of cancer-related death. Metastasis, therapy resistance, and immunosuppression are dominant characteristics of pancreatic tumors. Strategies that enhance the efficacy of standard of care and/or immune therapy are likely the most efficient route to improve overall survival in this disease. Areas covered: Axl, a member of the TAM (Tyro3, Axl, MerTK) family of receptor tyrosine kinases, is involved in cell plasticity, chemoresistance, immune suppression, and metastasis in various cancers, including pancreatic cancer. This review provides an overview of Axl and its function in normal conditions, summarizes the regulation and function of Axl in cancer, and highlights the contribution of Axl to pancreatic cancer as well as its potential as a therapeutic target. Expert opinion: Axl is an attractive therapeutic target in pancreatic cancer because it contributes to many of the roadblocks that hamper therapeutic efficacy. Clinical evidence supporting Axl inhibition in pancreatic cancer is currently limited; however, multiple clinical trials have been initiated or are in the planning phase to test the effect of inhibiting Axl in conjunction with standard therapy in pancreatic cancer patients. We anticipate that these studies will provide robust validation of Axl as a therapeutic target in pancreatic cancer.

Targeting the NRG1/HER3 pathway in tumor cells and cancer-associated fibroblasts with an anti-neuregulin 1 antibody inhibits tumor growth in pre-clinical models of pancreatic cancer

Neuregulin 1 (NRG1), a ligand for HER3 and HER4 receptors, is secreted by both pancreatic tumor cells (PC) and cancer-associated fibroblasts (CAFs), the latter representing the most abundant compound of pancreatic stroma. This desmoplastic stroma contributes to Pancreatic Ductal Adenocarcinoma (PDAC) aggressiveness and therapeutic failure by promoting tumor progression, invasion and resistance to chemotherapies. In the present work, we aimed at disrupting the complex crosstalk between PC and CAF in order to prevent tumor cell proliferation. To do so, we demonstrated the promising tumor growth inhibitory effect of the 7E3, an original antibody directed to NRG1. This antibody promotes antibody dependent cellular cytotoxicity in NRG1-positive PC and CAFs and inhibits NRG1-associated signaling pathway induction, by blocking NRG1-mediated HER3 activation. Moreover, 7E3 inhibits migration and growth of pancreatic cancer cells co-cultured with CAFs, both in vitro and in vivo using orthotopic pancreatic tumor xenografts. Our preclinical results demonstrate that the anti-NRG1 antibody 7E3 could represent a promising approach to target pancreatic stroma and cancer cells, thereby providing novel therapeutic options for PDAC.

Targeting Gas6/TAM in cancer cells and tumor microenvironment

Growth arrest-specific 6, also known as Gas6, is a human gene encoding the Gas6 protein, which was originally found to be upregulated in growth-arrested fibroblasts. Gas6 is a member of the vitamin K-dependent family of proteins expressed in many human tissues and regulates several biological processes in cells, including proliferation, survival and migration, by binding to its receptors Tyro3, Axl and Mer (TAM). In recent years, the roles of Gas6/TAM signalling in cancer cells and the tumour microenvironment have been studied, and some progress has made in targeted therapy, providing new potential directions for future investigations of cancer treatment. In this review, we introduce the Gas6 and TAM receptors and describe their involvement in different cancers and discuss the roles of Gas6 in cancer cells, the tumour microenvironment and metastasis. Finally, we introduce recent studies on Gas6/TAM targeting in cancer therapy, which will assist in the experimental design of future analyses and increase the potential use of Gas6 as a therapeutic target for cancer.

Carcinogenesis-relevant biological events in the pathophysiology of the efferocytosis phenomenon

The effective removal of cells undergoing programmed cell death, which is referred to as efferocytosis, prevents the leakage of intracellular contents into the surrounding tissue, which could lead to tissue damage and inflammation. Efferocytosis involves a coordinated orchestration of multiple steps that lead to a swift, coherent and immunologically silent removal of dying cells. The release of wound healing cytokines, which resolve inflammation and enhance tissue repair, is an important feature of efferocytosis. However, in addition to the healing cytokines released during efferocytosis, the immunosuppressive action of cytokines promotes the tumor microenvironment, enhances the motility of cancer cells and promotes the evasion of antitumor immunity. The aim of the present review was to comprehensively discuss the efferocytosis phenomenon, the important players associated with this process and their role in cancer-related biological events.

Small-Molecule Inhibition of Axl Targets Tumor Immune Suppression and Enhances Chemotherapy in Pancreatic Cancer

Activation of the receptor tyrosine kinase Axl is associated with poor outcomes in pancreatic cancer (PDAC), where it coordinately mediates immune evasion and drug resistance. Here, we demonstrate that the selective Axl kinase inhibitor BGB324 targets the tumor-immune interface to blunt the aggressive traits of PDAC cells in vitro and enhance gemcitibine efficacy in vivo Axl signaling stimulates the TBK1-NFκB pathway and innate immune suppression in the tumor microenvironment. In tumor cells, BGB324 treatment drove epithelial differentiation, expression of nucleoside transporters affecting gemcitabine response, and an immune stimulatory microenvironment. Our results establish a preclinical mechanistic rationale for the clinical development of Axl inhibitors to improve the treatment of PDAC patients.Significance: These results establish a preclinical mechanistic rationale for the clinical development of AXL inhibitors to improve the treatment of PDAC patients. Cancer Res; 78(1); 246-55. ©2017 AACR.