The receptor tyrosine kinase AXL promotes migration and invasion in colorectal cancer

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.

A Phase 1 Study of Cabozantinib and Trifluridine/Tipiracil in Metastatic Colorectal Adenocarcinoma

INTRODUCTION

This study determined the safety and recommended phase 2 dose (RP2D) of the multikinase inhibitor cabozantinib in combination with trifluridine/tipiracil (FTD/TPI) in refractory metastatic colorectal carcinoma (mCRC).

PATIENTS AND METHODS

Single institution investigator-initiated phase 1 study using 3+3 design. Eligible mCRC patients had received prior standard regimens. Cabozantinib was given orally (p.o.) at 20 mg (dose level [DL] 0) or 40 mg (DL 1) daily on days 1-28, and FTD/TPI p.o. at 35 mg/m2 on days 1-5 and 8-12 every 28 days. Prophylactic growth-factor support was allowed.

RESULTS

Fifteen patients were enrolled. Median age 56 years (31-80), male (12/15), ECOG 0/1 = 9/6. Three patients were treated at DL 0 and another nine were treated at DL 1, none exhibiting a DLT. Most common any grade (G) treatment related adverse events (TRAE) were diarrhea (50%), nausea (42%), neutropenia (42%), fatigue (33%), and rash (25%). G3-4 TRAE were neutropenia (25%) and thrombocytopenia, hypokalemia, and weight loss (each 8%). No serious TRAE or G5 were reported. The RP2D was determined to be DL 1. Median PFS was 3.8 months (95% CI 1.9-6.8) and disease control rate was 86.7%.

CONCLUSION

The combination of cabozantinib and FTD/TPI is feasible and tolerable at standard doses with the use of growth factors and showed encouraging clinical activity in refractory mCRC.

CLINICALTRIALS

GOV: NCT04868773.

Azaindole derivatives as potential kinase inhibitors and their SARs elucidation

Currently, heterocycles have occupied an important position in the fields of drug design. Among them, azaindole moiety is regarded as one privileged scaffold to develop therapeutic agents. Since two nitrogen atoms of azaindole increase the possibility to form hydrogen bonds in the adenosine triphosphate (ATP)-binding site, azaindole derivatives are important sources of kinase inhibitors. Moreover, some of them have been on the market or in clinical trials for the treatment of some kinase-related diseases (e.g., vemurafenib, pexidartinib, decernotinib). In this review, we focused on the recent development of azaindole derivatives as potential kinase inhibitors based on kinase targets, such as adaptor-associated kinase 1 (AAK1), anaplastic lymphoma kinase (ALK), AXL, cell division cycle 7 (Cdc7), cyclin-dependent kinases (CDKs), dual-specificity tyrosine (Y)-phosphorylation regulated kinase 1A (DYRK1A), fibroblast growth factor receptor 4 (FGFR4), phosphatidylinositol 3-kinase (PI3K) and proviral insertion site in moloney murine leukemia virus (PIM) kinases. Meanwhile, the structure-activity relationships (SARs) of most azaindole derivatives were also elucidated. In addition, the binding modes of some azaindoles complexed with kinases were also investigated during the SARs elucidation. This review may offer an insight for medicinal chemists to rationally design more potent kinase inhibitors bearing the azaindole scaffold.

Proteomic quantification of receptor tyrosine kinases involved in the development and progression of colorectal cancer liver metastasis

Introduction

Alterations in expression and activity of human receptor tyrosine kinases (RTKs) are associated with cancer progression and in response to therapeutic intervention.

Methods

Thus, protein abundance of 21 RTKs was assessed in 15 healthy and 18 cancerous liver samples [2 primary and 16 colorectal cancer liver metastasis (CRLM)] matched with non-tumorous (histologically normal) tissue, by a validated QconCAT-based targeted proteomic approach.

Results

It was demonstrated, for the first time, that the abundance of EGFR, INSR, VGFR3 and AXL, is lower in tumours relative to livers from healthy individuals whilst the opposite is true for IGF1R. EPHA2 was upregulated in tumour compared with histologically normal tissue surrounding it. PGFRB levels were higher in tumours relative to both histologically normal tissue surrounding tumour and tissues taken from healthy individuals. The abundances of VGFR1/2, PGFRA, KIT, CSF1R, FLT3, FGFR1/3, ERBB2, NTRK2, TIE2, RET, and MET were, however, comparable in all samples. Statistically significant, but moderate correlations were observed (Rs > 0.50, p < 0.05) for EGFR with INSR and KIT. FGFR2 correlated with PGFRA and VGFR1 with NTRK2 in healthy livers. In non-tumorous (histologically normal) tissues from cancer patients, there were correlations between TIE2 and FGFR1, EPHA2 and VGFR3, FGFR3 and PGFRA (p < 0.05). EGFR correlated with INSR, ERBB2, KIT and EGFR, and KIT with AXL and FGFR2. In tumours, CSF1R correlated with AXL, EPHA2 with PGFRA, and NTRK2 with PGFRB and AXL. Sex, liver lobe and body mass index of donors had no impact on the abundance of RTKs, although donor age showed some correlations. RET was the most abundant of these kinases in non-tumorous tissues (~35%), while PGFRB was the most abundant RTK in tumours (~47%). Several correlations were also observed between the abundance of RTKs and proteins relevant to drug pharmacokinetics (enzymes and transporters).

Discussion

DiscussionThis study quantified perturbation to the abundance of several RTKs in cancer and the value generated in this study can be used as input to systems biology models defining liver cancer metastases and biomarkers of its progression.

MERTK activation drives osimertinib resistance in EGFR-mutant non-small cell lung cancer

Acquired resistance is inevitable in non-small cell lung cancers (NSCLCs) treated with osimertinib (OSI), and the mechanisms are not well defined. The MERTK ligand GAS6 promoted downstream oncogenic signaling in EGFR-mutated (EGFRMT) NSCLC cells treated with OSI, suggesting a role for MERTK activation in OSI resistance. Indeed, treatment with MRX-2843, a first-in-class MERTK kinase inhibitor, resensitized GAS6-treated NSCLC cells to OSI. Both GAS6 and EGF stimulated downstream PI3K/AKT and MAPK/ERK signaling in parental cells, but only GAS6 activated these pathways in OSI-resistant (OSIR) derivative cell lines. Functionally, OSIR cells were more sensitive to MRX-2843 than parental cells, suggesting acquired dependence on MERTK signaling. Furthermore, MERTK and/or its ligands were dramatically upregulated in EGFRMT tumors after treatment with OSI in both xenograft models and patient samples, consistent with induction of autocrine/paracrine MERTK activation. Moreover, treatment with MRX-2843 in combination with OSI, but not OSI alone, provided durable suppression of tumor growth in vivo, even after treatment was stopped. These data identify MERTK as a driver of bypass signaling in treatment-naive and EGFRMT-OSIR NSCLC cells and predict that MRX-2843 and OSI combination therapy will provide clinical benefit in patients with EGFRMT NSCLC.

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.

Disruption of beta-catenin dependent Wnt signaling in colon cancer cells remodels the microenvironment to promote tumor invasion

The recent classification of colon cancer into molecular subtypes revealed that patients with the poorest prognosis harbor tumors with the lowest levels of Wnt signaling. This is contrary to the general understanding that overactive Wnt signaling promotes tumor progression from early initiation stages through to the later stages including invasion and metastasis. Here, we directly test this assumption by reducing the activity of ß-catenin-dependent Wnt signaling in colon cancer cell lines at either an upstream or downstream step in the pathway. We determine that Wnt-reduced cancer cells exhibit a more aggressive disease phenotype, including increased mobility in vitro and disruptive invasion into mucosa and smooth muscle in an orthotopic mouse model. RNA sequencing reveals that interference with Wnt signaling leads to an upregulation of gene programs that favor cell migration and invasion and a downregulation of inflammation signatures in the tumor microenvironment. We identify a set of upregulated genes common among the Wnt perturbations that are predictive of poor patient outcomes in early-invasive colon cancer. Our findings suggest that while targeting Wnt signaling may reduce tumor burden, an inadvertent side effect is the emergence of invasive cancer. Implications: Decreased Wnt signaling in colon tumors leads to a more aggressive disease phenotype due to an upregulation of gene programs favoring cell migration in the tumor and downregulation of inflammation programs in the tumor microenvironment; these impacts must be carefully considered in developing Wnt-targeting therapies.

Fragment-based lead discovery of indazole-based compounds as AXL kinase inhibitors

AXL is a member of the TAM (TYRO3, AXL, MER) subfamily of receptor tyrosine kinases. It is upregulated in a variety of cancers and its overexpression is associated with poor disease prognosis and acquired drug resistance. Utilizing a fragment-based lead discovery approach, a new indazole-based AXL inhibitor was obtained. The indazole fragment hit 11, identified through a high concentration biochemical screen, was expeditiously improved to fragment 24 by screening our in-house expanded library of fragments (ELF) collection. Subsequent fragment optimization guided by docking studies provided potent inhibitor 54 with moderate exposure levels in mice. X-ray crystal structure of analog 50 complexed with the I650M mutated kinase domain of Mer revealed the key binding interactions for the scaffold. The good potency coupled with reasonable kinase selectivity, moderate in vivo exposure levels, and availability of structural information for the series makes it a suitable starting point for further optimization efforts.

ARE STEM CELL MARKER EXPRESSION AND CD133 ANALYSIS RELEVANT TO DIFFERENTIATE COLORECTAL CANCER?

Background:

CD133 and AXL have been described as cancer stem cell markers, and c-MYC as a key regulatory cellular mechanism in colorectal cancer (CRC).

Aim:

Evaluate the prognostic role of the biomarkers CD133, AXL and c-MYC and their association with clinicopathologic characteristics in colorectal adenocarcinomas and adenomas.

Methods:

A total of 156 patients with UICC stage I-IV adenocarcinomas (n=122) and adenomas (n=34) were analyzed. Tissue microarrays (TMA) from primary tumors and polyps for CD133, c-MYC and AXL expression were performed and analyzed for their significance with clinicopathologic characteristics.

Results:

Poorly differentiated adenocarcinomas and disease progression were independent risk factors for poor overall survival. The median overall survival time was 30 months. Positive CD133 expression (35.9% of all cases), particularly of right-sided CRCs (44.8% of the CD133+ cases), was negatively correlated with death in the univariate analysis, which did not reach significance in the multivariate analysis. c-MYC (15.4% of all cases) was predominantly expressed in advanced-stage patients with distant (non-pulmonary/non-hepatic) metastasis. AXL expression was found only occasionally, and predominantly dominated in adenomas, with less penetrance in high-grade dysplasia.

Conclusions:

CD133 expression was not associated with inferior overall survival in CRC. While AXL showed inconclusive results, c-MYC expression in primary CRCs was associated with distant metastasis.

G-749 Promotes Receptor Tyrosine Kinase TYRO3 Degradation and Induces Apoptosis in Both Colon Cancer Cell Lines and Xenograft Mouse Models

G-749 is an FLT3 kinase inhibitor that was originally developed as a treatment for acute myeloid leukemia. Some FLT3 kinase inhibitors are dual kinase inhibitors that inhibit the TAM (Tyro3, Axl, Mer) receptor tyrosine kinase family and are used to treat solid cancers such as non-small cell lung cancer (NSCLC) and triple-negative breast cancer (TNBC). AXL promotes metastasis, suppression of immune response, and drug resistance in NSCLC and TNBC. G-749, a potential TAM receptor tyrosine kinase inhibitor, and its derivative SKI-G-801, effectively inhibits the phosphorylation of AXL at nanomolar concentration (IC50 = 20 nM). This study aimed to investigate the anticancer effects of G-749 targeting the TAM receptor tyrosine kinase in colon cancer. Here, we demonstrate the potential of G-749 to effectively inhibit tumorigenesis by degrading TYRO3 via regulated intramembrane proteolysis both in vitro and in vivo. In addition, we demonstrated that G-749 inhibits the signaling pathway associated with cell proliferation in colon cancer cell lines HCT15 and SW620, as well as tumor xenograft mouse models. We propose G-749 as a new therapeutic agent for the treatment of colon cancer caused by abnormal TYRO3 expression or activity.

20 (S)-ginsenoside Rh2 inhibits colorectal cancer cell growth by suppressing the Axl signaling pathway in vitro and in vivo

Background

Colorectal cancer (CRC) has a high morbidity and mortality worldwide. 20 (S)-ginsenoside Rh2 (G-Rh2) is a natural compound extracted from ginseng, which exhibits anticancer effects in many cancer types. In this study, we demonstrated the effect and underlying molecular mechanism of G-Rh2 in CRC cells in vitro and in vivo.

Methods

Cell proliferation, migration, invasion, apoptosis, cell cycle, and western blot assays were performed to evaluate the effect of G-Rh2 on CRC cells. In vitro pull-down assay was used to verify the interaction between G-Rh2 and Axl. Transfection and infection experiments were used to explore the function of Axl in CRC cells. CRC xenograft models were used to further investigate the effect of Axl knockdown and G-Rh2 on tumor growth in vivo.

Results

G-Rh2 significantly inhibited proliferation, migration, and invasion, and induced apoptosis and G₀/G₁ phase cell cycle arrest in CRC cell lines. G-Rh2 directly binds to Axl and inhibits the Axl signaling pathway in CRC cells. Knockdown of Axl suppressed the growth, migration and invasion ability of CRC cells in vitro and xenograft tumor growth in vivo, whereas overexpression of Axl promoted the growth, migration, and invasion ability of CRC cells. Moreover, G-Rh2 significantly suppressed CRC xenograft tumor growth by inhibiting Axl signaling with no obvious toxicity to nude mice.

Conclusion

Our results indicate that G-Rh2 exerts anticancer activity in vitro and in vivo by suppressing the Axl signaling pathway. G-Rh2 is a promising candidate for CRC prevention and treatment.

Conversion of RNA Aptamer into Modified DNA Aptamers Provides for Prolonged Stability and Enhanced Antitumor Activity

Aptamers, synthetic single-strand oligonucleotides that are similar in function to antibodies, are promising as therapeutics because of their minimal side effects. However, the stability and bioavailability of the aptamers pose a challenge. We developed aptamers converted from RNA aptamer to modified DNA aptamers that target phospho-AXL with improved stability and bioavailability. On the basis of the comparative analysis of a library of 17 converted modified DNA aptamers, we selected aptamer candidates, GLB-G25 and GLB-A04, that exhibited the highest bioavailability, stability, and robust antitumor effect in in vitro experiments. Backbone modifications such as thiophosphate or dithiophosphate and a covalent modification of the 5'-end of the aptamer with polyethylene glycol optimized the pharmacokinetic properties, improved the stability of the aptamers in vivo by reducing nuclease hydrolysis and renal clearance, and achieved high and sustained inhibition of AXL at a very low dose. Treatment with these modified aptamers in ovarian cancer orthotopic mouse models significantly reduced tumor growth and the number of metastases. This effective silencing of the phospho-AXL target thus demonstrated that aptamer specificity and bioavailability can be improved by the chemical modification of existing aptamers for phospho-AXL. These results lay the foundation for the translation of these aptamer candidates and companion biomarkers to the clinic.

Tyro3, Axl, Mertk receptor-mediated efferocytosis and immune regulation in the tumor environment

Three structurally related tyrosine receptor cell surface kinases, Tyro3, Axl, and Mertk (TAM) have been recognized to modulate immune function, tissue homeostasis, cardiovasculature, and cancer. The TAM receptor family appears to operate in adult mammals across multiple cell types, suggesting both widespread and specific regulation of cell functions and immune niches. TAM family members regulate tissue homeostasis by monitoring the presence of phosphatidylserine expressed on stressed or apoptotic cells. The detection of phosphatidylserine on apoptotic cells requires intermediary molecules that opsonize the dying cells and tether them to TAM receptors on phagocytes. This complex promotes the engulfment of apoptotic cells, also known as efferocytosis, that leads to the resolution of inflammation and tissue healing. The immune mechanisms dictating these processes appear to fall upon specific family members or may involve a complex of different receptors acting cooperatively to resolve and repair damaged tissues. Here, we focus on the role of TAM receptors in triggering efferocytosis and its consequences in the regulation of immune responses in the context of inflammation and cancer.

lncRNA GSEC Promotes the Progression of Triple Negative Breast Cancer (TNBC) by Targeting the miR-202-5p/AXL Axis

Background

This study aimed to explore the biological functions of G-quadruplex-forming sequence containing lncRNA (GSEC) in triple negative breast cancer (TNBC).

Methods

The expression of GSEC in TNBC tissues was evaluated by qRT-PCR. Cell viability was evaluated by Cell Counting Kit-8 assay. Cell proliferation was evaluated by 5-ethynyl-20-deoxyuridine (EdU) staining assay. Cell invasion and migration were evaluated by Transwell assay. Gain- and loss-function assays were performed to assess the biological functions of GSEC in TNBC. The interactions between GSEC, miR-202-5p and AXL were determined by luciferase report assay and RNA immunoprecipitation (RIP) assay. In addition, a nude mouse xenograft model was used to confirm the oncogenic role of GSEC in TNBC.

Results

GSEC was significantly upregulated in TNBC tissues and cancer cell lines, and high level of GSEC was associated with advanced tumor stage, positive lymph-node metastasis and the poor prognosis of TNBC patients. Knockdown of GSEC effectively inhibited TNBC cell proliferation, invasion and migration in vitro. GSEC regulated the expression of AXL by directly sponging miR-202-5p. Downregulation of miR-202-5p attenuated GSEC knockdown-induced inhibition on TNBC cell proliferation, invasion and migration in vitro. Meanwhile, overexpression of AXL obviously reversed the inhibitory effects of miR-202-5p mimics in TNBC progression in vitro.

Conclusion

GSEC functioned as a potential oncogene and promoted AXL-mediated TNBC progression by sponging miR-202-5p, which might be a novel diagnostic and therapeutic target for TNBC.

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.

ARE STEM CELL MARKER EXPRESSION AND CD133 ANALYSIS RELEVANT TO DIFFERENTIATE COLORECTAL CANCER?

Background:

CD133 and AXL have been described as cancer stem cell markers, and c-MYC as a key regulatory cellular mechanism in colorectal cancer (CRC).

Aim:

Evaluate the prognostic role of the biomarkers CD133, AXL and c-MYC and their association with clinicopathologic characteristics in colorectal adenocarcinomas and adenomas.

Methods:

A total of 156 patients with UICC stage I-IV adenocarcinomas (n=122) and adenomas (n=34) were analyzed. Tissue microarrays (TMA) from primary tumors and polyps for CD133, c-MYC and AXL expression were performed and analyzed for their significance with clinicopathologic characteristics.

Results:

Poorly differentiated adenocarcinomas and disease progression were independent risk factors for poor overall survival. The median overall survival time was 30 months. Positive CD133 expression (35.9% of all cases), particularly of right-sided CRCs (44.8% of the CD133+ cases), was negatively correlated with death in the univariate analysis, which did not reach significance in the multivariate analysis. c-MYC (15.4% of all cases) was predominantly expressed in advanced-stage patients with distant (non-pulmonary/non-hepatic) metastasis. AXL expression was found only occasionally, and predominantly dominated in adenomas, with less penetrance in high-grade dysplasia.

Conclusions:

CD133 expression was not associated with inferior overall survival in CRC. While AXL showed inconclusive results, c-MYC expression in primary CRCs was associated with distant metastasis.

Immuno-oncology: are TAM receptors in glioblastoma friends or foes?

Tyro3, Axl, and Mertk (TAM) receptors are a subfamily of receptor tyrosine kinases. TAM receptors have been implicated in mediating efferocytosis, regulation of immune cells, secretion of inflammatory factors, and epithelial-to-mesenchymal transition in the tumor microenvironment, thereby serving as a critical player in tumor development and progression. The pro-carcinogenic role of TAM receptors has been widely confirmed, overexpression of TAM receptors is tied to tumor cells growth, metastasis, invasion and treatment resistance. Nonetheless, it is surprising to detect that inhibiting TAM signaling is not all beneficial in the tumor immune microenvironment. The absence of TAM receptors also affects anti-tumor immunity under certain conditions by modulating different immune cells, as the functional diversification of TAM signaling is closely related to tumor immunotherapy. Glioblastoma is the most prevalent and lethal primary brain tumor in adults. Although research regarding the crosstalk between TAM receptors and glioblastoma remains scarce, it appears likely that TAM receptors possess potential anti-tumor effects rather than portraying a total cancer-driving role in the context of glioblastoma. Accordingly, we doubt whether TAM receptors play a double-sided role in glioblastoma, and propose the Janus-faced TAM Hypothesis as a conceptual framework for comprehending the precise underlying mechanisms of TAMs. In this study, we aim to cast a spotlight on the potential multidirectional effects of TAM receptors in glioblastoma and provide a better understanding for TAM receptor-related targeted intervention.

Regulation of ARL2 in colorectal cancer cell proliferation and tumorigenicity, and its negative association with AXL

Colorectal cancer (CRC) is the third most common malignant disease in adults. ADP ribosylation factor-like GTPase 2 (ARL2) is crucial for controlling the dynamics of microtubules and mitochondrial functions. However, the biological function of ARL2 in CRC remains unclear. The present study was performed to identify the expression level and functional role of ARL2 in CRC. A total of 19 CRC and 3 normal healthy colorectal tissues were collected. Furthermore, ARL2 expression was analyzed in healthy colorectal and CRC tissues by immunohistochemistry (IHC). ARL2 overexpression and knockdown was achieved using lentiviral vectors and plasmid transfection in HCT8 and HCT116 cells. The protein and mRNA expression levels of ARL2 and AXL were analyzed using western blot and reverse transcription-quantitative PCR in ARL2 knockdown and ARL2 overexpressing HCT8 and HCT116 cells. Cell Counting Kit-8, colony formation, wound healing, and Matrigel assays were used to investigate the biological functions of ARL2. Taken together, ARL2 protein expression level was upregulated in CRC tissues. Furthermore, ARL2 overexpression decreased proliferation and weakened the colony-formation abilities of the CRC cells, as well as their migratory and invasive abilities. ARL2 interference enhanced proliferation and colony-formation rates of the CRC cells, as well as their migratory and invasive abilities. ARL2 regulated CRC proliferation and tumorigenicity and was negatively associated with AXL. The results of the present study suggested that the proliferation, migration and tumorigenicity of the CRC cells could be inhibited by ARL2 overexpression. The latter may be used as a predicted and potential therapeutic target for CRC.

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.

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.

The Multifaceted Roles of TAM Receptors during Viral Infection

Tyro3, Axl, and Mertk (TAM) receptors play multiple roles in a myriad of physiological and pathological processes, varying from promoting the phagocytic clearance of apoptotic cells, sustaining the immune and inflammatory homeostasis, maintaining the blood-brain barrier (BBB) integrity and central nervous system (CNS) homeostasis, to mediating cancer malignancy and chemoresistance. Growth arrest-specific protein 6 (Gas6) and protein S (Pros1) are the two ligands that activate TAM receptors. Recently, TAM receptors have been reported to mediate cell entry and infection of multitudinous enveloped viruses in a manner called apoptotic mimicry. Moreover, TAM receptors are revitalized during viral entry and infection, which sequesters innate immune and inflammatory responses, facilitating viral replication and immune evasion. However, accumulating evidence have now proposed that TAM receptors are not required for the infection of these viruses in vivo. In addition, TAM receptors protect mice against the CNS infection of neuroinvasive viruses and relieve the brain lesions during encephalitis. These protective effects are achieved through maintaining BBB integrity, attenuating proinflammatory cytokine production, and promoting neural cell survival. TAM receptors also regulate the programmed cell death modes of virus-infected cells, which have profound impacts on the pathogenesis and outcome of infection. Here, we systematically review the functionalities and underlying mechanisms of TAM receptors and propose the potential application of TAM agonists to prevent severe viral encephalitis.

Epidermal growth factor receptor conjugated fucoidan/alginates loaded hydrogel for activating EGFR/AKT signaling pathways in colon cancer cells during targeted photodynamic therapy

In the present study, we developed epidermal growth factor receptor conjugated fucoidan/alginate loaded hydrogels for targeting the delivery of hydrogel through the signaling pathway of the epidermal growth factor receptor (EGFR) to treat colon cancer. We aim to develop a drug delivery system of chlorin e6 encapsulated in hydrogel and tag it with EFGR to target cancer cells with low toxicity and limited side effects by using photodynamic therapy (PDT). The characterization and in vitro studies were conducted to evaluate the efficiency of the EGFR-hydrogel in colon cancer cells. Also, western blot analysis was used to assess protein expression levels. The in vitro results confirmed significant cell viability, proliferation, and migration of hydrogel in colon cancer. The cellular effects of the EFGR/AKT pathway were cell proliferation, inhibition of apoptosis, cell cycle progression, and cell survival and migration of colon cancer because of significant protein expression levels. The data suggested that hydrogel appears to be a promising targeting approach-PDT for treating colon cancer. Further in vivo studies are needed to conclude the overexpression level of EGFR on cancer cells. The study concluded that EGFR-H improved the targeting efficiency of hydrogel in colon cancer.

Skipping of exon 10 in Axl pre-mRNA regulated by PTBP1 mediates invasion and metastasis process of liver cancer cells

The Axl gene is known to encode for a receptor tyrosine kinase involved in the metastasis process of cancer. In this study, we investigated the underlying molecular mechanism of Axl alternative splicing. Methods: The expression levels of PTBP1 in hepatocellular carcinoma (HCC) tissues were obtained from TCGA samples and cell lines. The effect of Axl-L, Axl-S, and PTBP1 on cell growth, migration, invasion tumor formation, and metastasis of liver cancer cells were measured by cell proliferation, wound-healing, invasion, xenograft tumor formation, and metastasis. Interaction between PTBP1 and Axl was explored using cross-link immunoprecipitation, RNA pull-down assays and RNA immunoprecipitation assays. Results: Knockdown of the PTBP1 and exon 10 skipping isoform of Axl (Axl-S), led to impaired invasion and metastasis in hepatoma cells. Immunoprecipitation results indicated that Axl-S protein binds more robustly with Gas6 ligand than Axl-L (exon 10 including) and is more capable of promoting phosphorylation of ERK and AKT proteins. Furthermore, cross-link immunoprecipitation and RNA-pulldown assays revealed that PTBP1 binds to the polypyrimidine sequence(TCCTCTCTGTCCTTTCTTC) on Axl-Intron 9. MS2-GFP-IP experiments demonstrated that PTBP1 competes with U2AF2 for binding to the aforementioned polypyrimidine sequence, thereby inhibiting alternative splicing and ultimately promoting Axl-S production. Conclusion: Our results highlight the biological significance of Axl-S and PTBP1 in tumor metastasis, and show that PTBP1 affects the invasion and metastasis of hepatoma cells by modulating the alternative splicing of Axl exon 10.

Current Advances in the Treatment of BRAF-Mutant Melanoma

Melanoma is the most lethal form of skin cancer. Melanoma is usually curable with surgery if detected early, however, treatment options for patients with metastatic melanoma are limited and the five-year survival rate for metastatic melanoma had been 15-20% before the advent of immunotherapy. Treatment with immune checkpoint inhibitors has increased long-term survival outcomes in patients with advanced melanoma to as high as 50% although individual response can vary greatly. A mutation within the MAPK pathway leads to uncontrollable growth and ultimately develops into cancer. The most common driver mutation that leads to this characteristic overactivation in the MAPK pathway is the B-RAF mutation. Current combinations of BRAF and MEK inhibitors that have demonstrated improved patient outcomes include dabrafenib with trametinib, vemurafenib with cobimetinib or encorafenib with binimetinib. Treatment with BRAF and MEK inhibitors has met challenges as patient responses began to drop due to the development of resistance to these inhibitors which paved the way for development of immunotherapies and other small molecule inhibitor approaches to address this. Resistance to these inhibitors continues to push the need to expand our understanding of novel mechanisms of resistance associated with treatment therapies. This review focuses on the current landscape of how resistance occurs with the chronic use of BRAF and MEK inhibitors in BRAF-mutant melanoma and progress made in the fields of immunotherapies and other small molecules when used alone or in combination with BRAF and MEK inhibitors to delay or circumvent the onset of resistance for patients with stage III/IV BRAF mutant melanoma.

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.

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.

Cross-Talk between Receptor Tyrosine Kinases AXL and ERBB3 Regulates Invadopodia Formation in Melanoma Cells

The invasive phenotype of metastatic cancer cells is accompanied by the formation of actin-rich invadopodia, which adhere to the extracellular matrix and degrade it. In this study, we explored the role of the tyrosine kinome in the formation of invadopodia in metastatic melanoma cells. Using a microscopy-based siRNA screen, we identified a series of regulators, the knockdown of which either suppresses (e.g., TYK2, IGFR1, ERBB3, TYRO3, FES, ALK, PTK7) or enhances (e.g., ABL2, AXL, CSK) invadopodia formation and function. Notably, the receptor tyrosine kinase AXL displayed a dual regulatory function, where both depletion or overexpression enhanced invadopodia formation and activity. This apparent contradiction was attributed to the capacity of AXL to directly stimulate invadopodia, yet its suppression upregulates the ERBB3 signaling pathway, which can also activate core invadopodia regulators and enhance invadopodia function. Bioinformatic analysis of multiple melanoma cell lines points to an inverse expression pattern of AXL and ERBB3. High expression of AXL in melanoma cells is associated with high expression of invadopodia components and an invasive phenotype. These results provide new insights into the complexity of metastasis-promoting mechanisms and suggest that targeting of multiple invadopodia signaling networks may serve as a potential anti-invasion therapy in melanoma. SIGNIFICANCE: These findings uncover a unique interplay between AXL and ERBB3 in invadopodia regulation that points to the need for combined therapy in order to prevent invadopodia-mediated metastasis in melanoma.

Targeted Quantitative Kinome Analysis Identifies PRPS2 as a Promoter for Colorectal Cancer Metastasis

Kinases are among the most important families of enzymes involved in cell signaling. In this study, we employed a recently developed parallel-reaction monitoring (PRM)-based targeted proteomic method to examine the reprogramming of the human kinome during colorectal cancer (CRC) metastasis. We were able to quantify the relative expression of 299 kinase proteins in a pair of matched primary/metastatic CRC cell lines. We also found that, among the differentially expressed kinases, phosphoribosyl pyrophosphate synthetase 2 (PRPS2) promotes the migration and invasion of cultured CRC cells through regulating the activity of matrix metalloproteinase 9 (MMP-9) and the expression of E-cadherin. Moreover, we found that the up-regulation of PRPS2 in metastatic CRC cells could be induced by the MYC proto-oncogene. Together, our unbiased kinome profiling approach led to the identification, for the first time, of PRPS2 as a promoter for CRC metastasis.

Targeting Receptor Kinases in Colorectal Cancer

Colorectal cancer is the third most common malignancy in men and the second most common cancer in women. Despite the success of screening programs and the development of adjuvant therapies, the global burden of colorectal cancer is expected to increase by 60% to more than 2.2 million new cases and 1.1 million deaths by 2030. In recent years, a great effort has been made to demonstrate the utility of protein kinase inhibitors for cancer treatment. Considering this heterogeneous disease is defined by mutations that activate different Receptor Tyrosine Kinases (RTKs) and affect downstream components of RTK-activated transduction pathways, in this review we analyze the potential utility of different kinase inhibitors for colorectal cancer treatment.

Role of the receptor tyrosine kinase Axl in hepatocellular carcinoma and its clinical relevance

The receptor tyrosine kinase Axl and its ligand Gas6 regulate fundamental biological processes, including cell proliferation, survival and motility, through multiple downstream signaling pathways. Evidence to date suggests that aberrant Axl expression frequently occurs in many malignancies, including hepatocellular carcinoma, and that this is critical for promoting cell proliferation, migration, angiogenesis and metastasis. Moreover, deregulated Axl expression or activation is reportedly associated with resistance to cancer drugs and targeted cancer therapies. Thus, Axl inhibitors may represent a novel therapeutic approach for cancer treatment. This Review summarizes the latest advances concerning the biological role of Axl in hepatocellular carcinoma and its potential clinical relevance.

The prognostic and therapeutic role of hormones in colorectal cancer: a review

Colorectal cancer (CRC) is one of the commonest cancers in Western society with a poor prognosis in patients with advanced disease. Targeted therapy is of increasing interest and already, targeted hormone treatment for breast and prostate cancer has improved survival. The aim of this literature review is to summarise the role of hormones in CRC prognosis and treatment. A literature review of all human and animal in vivo and in vitro studies in the last 20 years, which assessed the role of hormones in CRC treatment or prognosis, was carried out. The hormones described in this review have been subdivided according to their secretion origin. Most of the studies are based on in vitro or animal models. The main findings point to adipokines, insulin and the insulin growth factor axis as key players in the link between obesity, type 2 diabetes mellitus and a subset of CRC. Gut-derived hormones, especially uroguanylin and guanylin are being increasingly investigated as therapeutic targets, with promising results. Using hormones as prognostic and therapeutic markers in CRC is still in the preliminary stages for only a fraction of the hormones affecting the GIT. In light of the increasing interest in tailoring treatment strategies, hormones are an important area of focus in the future of CRC management.

Discovery of N-[4-(Quinolin-4-yloxy)phenyl]benzenesulfonamides as Novel AXL Kinase Inhibitors

The overexpression of AXL kinase has been described in many types of cancer. Due to its role in proliferation, survival, migration, and resistance, AXL represents a promising target in the treatment of the disease. In this study we present a novel compound family that successfully targets the AXL kinase. Through optimization and detailed SAR studies we developed low nanomolar inhibitors, and after further biological characterization we identified a potent AXL kinase inhibitor with favorable pharmacokinetic profile. The antitumor activity was determined in xenograft models, and the lead compounds reduced the tumor size by 40% with no observed toxicity as well as lung metastasis formation by 66% when compared to vehicle control.

Expression, regulation and targeting of receptor tyrosine kinases in esophageal squamous cell carcinoma

Esophageal cancer is one of the most common types of cancer, which is a leading cause of cancer-related death worldwide. Based on histological behavior, it is mainly of two types (i) Esophageal squamous cell carcinoma (ESCC), and (ii) esophageal adenocarcinoma (EAD or EAC). In astronomically immense majority of malignancies, receptor tyrosine kinases (RTKs) have been kenned to play a consequential role in cellular proliferation, migration, and metastasis of the cells. The post-translational modifications (PTMs) including phosphorylation of tyrosine (pY) residue of the tyrosine kinase (TK) domain have been exploited for treatment in different malignancies. Lung cancer where pY residues of EGFR have been exploited for treatment purpose in lung adenocarcinoma patients, but we do not have such kind of felicitously studied and catalogued data in ESCC patients. Thus, the goal of this review is to summarize the studies carried out on ESCC to explore the role of RTKs, tyrosine kinase inhibitors, and their pertinence and consequentiality for the treatment of ESCC patients.

Expression, regulation and targeting of receptor tyrosine kinases in esophageal squamous cell carcinoma

Esophageal cancer is one of the most common types of cancer, which is a leading cause of cancer-related death worldwide. Based on histological behavior, it is mainly of two types (i) Esophageal squamous cell carcinoma (ESCC), and (ii) esophageal adenocarcinoma (EAD or EAC). In astronomically immense majority of malignancies, receptor tyrosine kinases (RTKs) have been kenned to play a consequential role in cellular proliferation, migration, and metastasis of the cells. The post-translational modifications (PTMs) including phosphorylation of tyrosine (pY) residue of the tyrosine kinase (TK) domain have been exploited for treatment in different malignancies. Lung cancer where pY residues of EGFR have been exploited for treatment purpose in lung adenocarcinoma patients, but we do not have such kind of felicitously studied and catalogued data in ESCC patients. Thus, the goal of this review is to summarize the studies carried out on ESCC to explore the role of RTKs, tyrosine kinase inhibitors, and their pertinence and consequentiality for the treatment of ESCC patients.