Gas6 anti-apoptotic signaling requires NF-kappa B activation

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.

AXL/Gas6 signaling mechanisms in the hypothalamic-pituitary-gonadal axis

AXL is a receptor tyrosine kinase commonly associated with a variety of human cancers. Along with its ligand Gas6 (growth arrest-specific protein 6), AXL is emerging as an important regulator of neuroendocrine development and function. AXL signaling in response to Gas6 binding impacts neuroendocrine structure and function at the level of the brain, pituitary, and gonads. During development, AXL has been identified as an upstream inhibitor of gonadotropin receptor hormone (GnRH) production and also plays a key role in the migration of GnRH neurons from the olfactory placode to the forebrain. AXL is implicated in reproductive diseases including some forms of idiopathic hypogonadotropic hypogonadism and evidence suggests that AXL is required for normal spermatogenesis. Here, we highlight research describing AXL/Gas6 signaling mechanisms with a focus on the molecular pathways related to neuroendocrine function in health and disease. In doing so, we aim to present a concise account of known AXL/Gas6 signaling mechanisms to identify current knowledge gaps and inspire future research.

Fetal Programming of the Endocrine Pancreas: Impact of a Maternal Low-Protein Diet on Gene Expression in the Perinatal Rat Pancreas

In rats, the time of birth is characterized by a transient rise in beta cell replication, as well as beta cell neogenesis and the functional maturation of the endocrine pancreas. However, the knowledge of the gene expression during this period of beta cell expansion is incomplete. The aim was to characterize the perinatal rat pancreas transcriptome and to identify regulatory pathways differentially regulated at the whole organ level in the offspring of mothers fed a regular control diet (CO) and of mothers fed a low-protein diet (LP). We performed mRNA expression profiling via the microarray analysis of total rat pancreas samples at embryonic day (E) 20 and postnatal days (P) 0 and 2. In the CO group, pancreas metabolic pathways related to sterol and lipid metabolism were highly enriched, whereas the LP diet induced changes in transcripts involved in RNA transcription and gene regulation, as well as cell migration and apoptosis. Moreover, a number of individual transcripts were markedly upregulated at P0 in the CO pancreas: growth arrest specific 6 (Gas6), legumain (Lgmn), Ets variant gene 5 (Etv5), alpha-fetoprotein (Afp), dual-specificity phosphatase 6 (Dusp6), and angiopoietin-like 4 (Angptl4). The LP diet induced the downregulation of a large number of transcripts, including neurogenin 3 (Neurog3), Etv5, Gas6, Dusp6, signaling transducer and activator of transcription 3 (Stat3), growth hormone receptor (Ghr), prolactin receptor (Prlr), and Gas6 receptor (AXL receptor tyrosine kinase; Axl), whereas upregulated transcripts were related to inflammatory responses and cell motility. We identified differentially regulated genes and transcriptional networks in the perinatal pancreas. These data revealed marked adaptations of exocrine and endocrine in the pancreas to the low-protein diet, and the data can contribute to identifying novel regulators of beta cell mass expansion and functional maturation and may provide a valuable tool in the generation of fully functional beta cells from stem cells to be used in replacement therapy.

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.

Tet2 Controls the Responses of β cells to Inflammation in Autoimmune Diabetes

β cells may participate and contribute to their own demise during Type 1 diabetes (T1D). Here we report a role of their expression of Tet2 in regulating immune killing. Tet2 is induced in murine and human β cells with inflammation but its expression is reduced in surviving β cells. Tet2-KO mice that receive WT bone marrow transplants develop insulitis but not diabetes and islet infiltrates do not eliminate β cells even though immune cells from the mice can transfer diabetes to NOD/scid recipients. Tet2-KO recipients are protected from transfer of disease by diabetogenic immune cells.Tet2-KO β cells show reduced expression of IFNγ-induced inflammatory genes that are needed to activate diabetogenic T cells. Here we show that Tet2 regulates pathologic interactions between β cells and immune cells and controls damaging inflammatory pathways. Our data suggests that eliminating TET2 in β cells may reduce activating pathologic immune cells and killing of β cells.

Targeting AXL in NSCLC

State-of-the-art cancer precision medicine approaches involve targeted inactivation of chemically and immunologically addressable vulnerabilities that often yield impressive initial anti-tumor responses in patients. Nonetheless, these responses are overshadowed by therapy resistance that follows. AXL, a receptor tyrosine kinase with bona fide oncogenic capacity, has been associated with the emergence of resistance in an array of cancers with varying pathophysiology and cellular origins, including in non-small-cell lung cancers (NSCLCs). Here in this review, we summarize AXL biology during normal homeostasis, oncogenic development and therapy resistance with a focus on NSCLC. In the context of NSCLC therapy resistance, we delineate AXL's role in mediating resistance to tyrosine kinase inhibitors (TKIs) deployed against epidermal growth factor receptor (EGFR) as well as other notable oncogenes and to chemotherapeutics. We also discuss the current understanding of AXL's role in mediating cell-biological variables that function as important modifiers of therapy resistance such as epithelial to mesenchymal transition (EMT), the tumor microenvironment and tumor heterogeneity. We also catalog and discuss a set of effective pharmacologic tools that are emerging to strategically perturb AXL mediated resistance programs in NSCLC. Finally, we enumerate ongoing and future exciting precision medicine approaches targeting AXL as well as challenges in this regard. We highlight that a holistic understanding of AXL biology in NSCLC may allow us to predict and improve targeted therapeutic strategies, such as through polytherapy approaches, potentially against a broad spectrum of NSCLC sub-types to forestall tumor evolution and drug resistance.

AXL, an Important Host Factor for DENV and ZIKV Replication

Flaviviruses, as critically important pathogens, are still major public health problems all over the world. For instance, the evolution of ZIKV led to large-scale outbreaks in the Yap island in 2007. DENV was considered by the World Health Organization (WHO) as one of the 10 threats to global health in 2019. Enveloped viruses hijack a variety of host factors to complete its replication cycle. Phosphatidylserine (PS) receptor, AXL, is considered to be a candidate receptor for flavivirus invasion. In this review, we discuss the molecular structure of ZIKV and DENV, and how they interact with AXL to successfully invade host cells. A more comprehensive understanding of the molecular mechanisms of flavivirus-AXL interaction will provide crucial insights into the virus infection process and the development of anti-flavivirus therapeutics.

TAM kinases as regulators of cell death

Receptor Tyrosine Kinases are critical regulators of signal transduction that support cell survival, proliferation, and differentiation. Dysregulation of normal Receptor Tyrosine Kinase function by mutation or other activity-altering event can be oncogenic or can impact the transformed malignant cell so it becomes particularly resistant to stress challenge, have increased proliferation, become evasive to immune surveillance, and may be more prone to metastasis of the tumor to other organ sites. The TAM family of Receptor Tyrosine Kinases (TYRO3, AXL, MERTK) is emerging as important components of malignant cell survival in many cancers. The TAM kinases are important regulators of cellular homeostasis and proper cell differentiation in normal cells as receptors for their ligands GAS6 and Protein S. They also are critical to immune and inflammatory processes. In malignant cells, the TAM kinases can act as ligand independent co-receptors to mutant Receptor Tyrosine Kinases and in some cases (e.g. FLT3-ITD mutant) are required for their function. They also have a role in immune checkpoint surveillance. At the time of this review, the Covid-19 pandemic poses a global threat to world health. TAM kinases play an important role in host response to many viruses and it is suggested the TAM kinases may be important in aspects of Covid-19 biology. This review will cover the TAM kinases and their role in these processes.

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.

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.

p5RHH nanoparticle-mediated delivery of AXL siRNA inhibits metastasis of ovarian and uterine cancer cells in mouse xenografts

Ovarian and uterine serous cancers are extremely lethal diseases that often present at an advanced stage. The late-stage diagnosis of these patients results in the metastasis of their cancers throughout the peritoneal cavity leading to death. Improving survival for these patients will require identifying therapeutic targets, strategies to target them, and means to deliver therapies to the tumors. One therapeutic target is the protein AXL, which has been shown to be involved in metastasis in both ovarian and uterine cancer. An effective way to target AXL is to silence its expression with small interfering RNA (siRNA). We investigate the ability of the novel siRNA delivery platform, p5RHH, to deliver anti-AXL siRNA (siAXL) to tumor cells both in vitro and in vivo as well as examine the phenotypic effects of this siRNA interference. First, we present in vitro assays showing p5RHH-siAXL treatment reduces invasion and migration ability of ovarian and uterine cancer cells. Second, we show p5RHH nanoparticles target to tumor cells in vivo. Finally, we demonstrate p5RHH-siAXL treatment reduces metastasis in a uterine cancer mouse xenograft model, without causing an obvious toxicity. Collectively, these findings suggest that this novel therapy shows promise in the treatment of ovarian and uterine cancer patients.

Exogenous testosterone alleviates cardiac fibrosis and apoptosis via Gas6/Axl pathway in the senescent mice

BACKGROUND

Androgen has been implicated in aging-related cardiac remodeling, but its precise role in aging heart remains controversial. We aimed to investigate the role of testosterone in the development of aging-related cardiac remodeling and the mechanisms involved.

METHODS

Wild type and Axl knockout mice (Axl^-/-) were randomized into three groups: the young group (n = 30, 3 months old), the aging group (n = 30, 18 months old), the testosterone undecanoate treatment group (TU, n = 30, 18 months old). Mice in the TU group were given testosterone undecanoate (39 mg/kg) by subcutaneous injection on the back at fifteen-months-old, once a month, a total of three times. The old group received solvent reagent (corn oil) by the same method.

RESULTS

The aging mice exhibited a decrease in serum testosterone, and Gas6 levels and an increase in apoptosis, and manifested cardiac fibrosis. Testosterone injection to wild type mice increased the levels of testosterone and Gas6 in serum and decreased cardiac apoptosis and fibrosis. Axl^-/-mice receiving testosterone injection exhibited no obvious improvement in cardiac remodeling although the levels of testosterone and Gas6 in serum elevated.

CONCLUSIONS

These data indicated that testosterone replacement therapy (TRT) alleviates cardiac fibrosis and apoptosis, at least in part by enhancing Gas6 expression. Moreover, deletion of Axl disables testosterone, which indicated that Axl is an important downstream regulator of testosterone. TRT would improve aging-related cardiac remolding via Gas6/Axl signaling pathway, implicating its therapeutic potential to treat aging-related heart disease.

Dynamics of Axl Receptor Shedding in Hepatocellular Carcinoma and Its Implication for Theranostics

Signaling of the receptor tyrosine kinase Axl and its ligand Gas6 is crucially involved in the development of liver fibrosis and hepatocellular carcinoma (HCC) by activation of hepatic stellate cells and modulation of hepatocyte differentiation. Shedding of Axl’s ectodomain leads to the release of soluble Axl (sAxl), which is increased in advanced fibrosis and in early-to-late stage HCC in the presence and absence of cirrhosis. Here, we focus on the dynamics of Axl receptor shedding and delineate possible scenarios how Axl signaling might act as driver of fibrosis progression and HCC development. Based on experimental and clinical data, we discuss the consequences of modifying Axl signaling by sAxl cleavage, as well as cellular strategies to escape from antagonizing effects of Axl shedding by the involvement of the hepatic microenvironment. We emphasize a correlation between free Gas6 and free sAxl levels favoring abundant Gas6/Axl signaling in advanced fibrosis and HCC. The raised scenario provides a solid basis for theranostics allowing the use of sAxl as an accurate diagnostic biomarker of liver cirrhosis and HCC, as well as Axl receptor signaling for therapeutic intervention in stratified HCC patients.

EMT: A mechanism for escape from EGFR-targeted therapy in lung cancer

Epithelial mesenchymal transition (EMT) is a reversible developmental genetic programme of transdifferentiation of polarised epithelial cells to mesenchymal cells. In cancer, EMT is an important factor of tumour cell plasticity and has received increasing attention for its role in the resistance to conventional and targeted therapies. In this paper we provide an overview of EMT in human malignancies, and discuss contribution of EMT to the development of the resistance to Epidermal Growth Factor Receptor (EGFR)-targeted therapies in non-small cell lung cancer (NSCLC). Patients with the tumours bearing specific mutations in EGFR have a good clinical response to selective EGFR inhibitors, but the resistance inevitably develops. Several mechanisms responsible for the resistance include secondary mutations in the EGFR gene, genetic or non-mutational activation of alternative survival pathways, transdifferentiation of NSCLC to the small cell lung cancer histotype, or formation of resistant tumours with mesenchymal characteristics. Mechanistically, application of an EGFR inhibitor does not kill all cancer cells; some cells survive the exposure to a drug, and undergo genetic evolution towards resistance. Here, we present a theory that these quiescent or slow-proliferating drug-tolerant cell populations, or so-called "persisters", are generated via EMT pathways. We review the EMT-activated mechanisms of cell survival in NSCLC, which include activation of ABC transporters and EMT-associated receptor tyrosine kinase AXL, immune evasion, and epigenetic reprogramming. We propose that therapeutic inhibition of these pathways would eliminate pools of persister cells and prevent or delay cancer recurrence when applied in combination with the agents targeting EGFR.

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.

A novel AXL chimeric antigen receptor endows T cells with anti-tumor effects against triple negative breast cancers

Identifying targets for chimeric antigen receptor-modulated T lymphocyte (CAR-T) therapy against solid tumors is an urgent problem to solve. In this study, we showed for the first time that the receptor tyrosine kinase, AXL, is overexpressed in various tumor cell lines and patient tumor tissues including triple negative breast cancer (TNBC) cell lines and patient samples, making AXL a potent novel target for cancer therapy, specifically for TNBC treatment. We also engineered T cells with a CAR consisting of a novel single-chain variable fragment against AXL and revealed its antigen-specific cytotoxicity and ability to release cytokines in a TNBC cell line and other AXL-positive tumors in vitro. Furthermore, AXL-CAR-T cells displayed a significant anti-tumor effect and in vivo persistence in a TNBC xenograft model. Taken together, our findings indicate that AXL-CAR-T cells can represent a promising therapeutic strategy against TNBC.

The role of TAM family receptors and ligands in the nervous system: From development to pathobiology

Tyro3, Axl, and Mertk, referred to as the TAM family of receptor tyrosine kinases, are instrumental in maintaining cell survival and homeostasis in mammals. TAM receptors interact with multiple signaling molecules to regulate cell migration, survival, phagocytosis and clearance of metabolic products and cell debris called efferocytosis. The TAMs also function as rheostats to reduce the expression of proinflammatory molecules and prevent autoimmunity. All three TAM receptors are activated in a concentration-dependent manner by the vitamin K-dependent growth arrest-specific protein 6 (Gas6). Gas6 and the TAMs are abundantly expressed in the nervous system. Gas6, secreted by neurons and endothelial cells, is the sole ligand for Axl. ProteinS1 (ProS1), another vitamin K-dependent protein functions mainly as an anti-coagulant, and independent of this function can activate Tyro3 and Mertk, but not Axl. This review will focus on the role of the TAM receptors and their ligands in the nervous system. We highlight studies that explore the function of TAM signaling in myelination, the visual cortex, neural cancers, and multiple sclerosis (MS) using Gas6^-/- and TAM mutant mice models.

Axl inhibitors as novel cancer therapeutic agents

Overexpression and activation of Axl receptor tyrosine kinase have been widely accepted to promote cell proliferation, chemotherapy resistance, invasion, and metastasis in several human cancers, such as lung, breast, and pancreatic cancers. Axl, a member of the TAM (Tyro3, Axl, Mer) family, and its inhibitors can specifically break the kinase signaling nodes, allowing advanced patients to regain drug sensitivity with improved therapeutic efficacy. Therefore, the research on Axl is promising and it is worthy of further investigations. In this review, we present an update on the Axl inhibitors and provide new insights into their latent application.

AXL modulates extracellular matrix protein expression and is essential for invasion and metastasis in endometrial cancer

The receptor tyrosine kinase AXL promotes migration, invasion, and metastasis. Here, we evaluated the role of AXL in endometrial cancer. High immunohistochemical expression of AXL was found in 76% (63/83) of advanced-stage, and 77% (82/107) of high-grade specimens and correlated with worse survival in uterine serous cancer patients. In vitro, genetic silencing of AXL inhibited migration and invasion but had no effect on proliferation of ARK1 endometrial cancer cells. AXL-deficient cells showed significantly decreased expression of phospho-AKT as well as uPA, MMP-1, MMP-2, MMP-3, and MMP-9. In a xenograft model of human uterine serous carcinoma with AXL-deficient ARK1 cells, there was significantly less tumor burden than xenografts with control ARK1 cells. Together, these findings underscore the therapeutic potentials of AXL as a candidate target for treatment of metastatic endometrial cancer.

Autophagy induced by AXL receptor tyrosine kinase alleviates acute liver injury via inhibition of NLRP3 inflammasome activation in mice

Severe hepatic inflammation is a common cause of acute or chronic liver disease. Macrophages are one of the key mediators which regulate the progress of hepatic inflammation. Increasing evidence shows that the TAM (TYRO3, AXL and MERTK) family of RTKs (receptor tyrosine kinases), which is expressed in macrophages, alleviates inflammatory responses through a negative feedback loop. However, the functional contribution of each TAM family member to the progression of hepatic inflammation remains elusive. In this study, we explore the role of individual TAM family proteins during autophagy induction and evaluate their contribution to hepatic inflammation. Among the TAM family of RTKs, AXL (AXL receptor tyrosine kinase) only induces autophagy in macrophages after interaction with its ligand, GAS6 (growth arrest specific 6). Based on our results, autophosphorylation of 2 tyrosine residues (Tyr815 and Tyr860) in the cytoplasmic domain of AXL in mice is required for autophagy induction and AXL-mediated autophagy induction is dependent on MAPK (mitogen-activated protein kinase)14 activity. Furthermore, induction of AXL-mediated autophagy prevents CASP1 (caspase 1)-dependent IL1B (interleukin 1, β) and IL18 (interleukin 18) maturation by inhibiting NLRP3 (NLR family, pyrin domain containing 3) inflammasome activation. In agreement with these observations, axl^-/- mice show more severe symptoms than do wild-type (Axl^+/+) mice following acute hepatic injury induced by administration of lipopolysaccharide (LPS) or carbon tetrachloride (CCl₄). Hence, GAS6-AXL signaling-mediated autophagy induction in murine macrophages ameliorates hepatic inflammatory responses by inhibiting NLRP3 inflammasome activation.

Key Roles of AXL and MER Receptor Tyrosine Kinases in Resistance to Multiple Anticancer Therapies

A major challenge in anticancer treatment is the pre-existence or emergence of resistance to therapy. AXL and MER are two members of the TAM (TYRO3-AXL-MER) family of receptor tyrosine kinases, which, when activated, can regulate tumor cell survival, proliferation, migration and invasion, angiogenesis, and tumor-host interactions. An increasing body of evidence strongly suggests that these receptors play major roles in resistance to targeted therapies and conventional cytotoxic agents. Multiple resistance mechanisms exist, including the direct and indirect crosstalk of AXL and MER with other receptors and the activation of feedback loops regulating AXL and MER expression and activity. These mechanisms may be innate, adaptive, or acquired. A principal role of AXL appears to be in sustaining a mesenchymal phenotype, itself a major mechanism of resistance to diverse anticancer therapies. Both AXL and MER play a role in the repression of the innate immune response which may also limit response to treatment. Small molecule and antibody inhibitors of AXL and MER have recently been described, and some of these have already entered clinical trials. The optimal design of treatment strategies to maximize the clinical benefit of these AXL and MER targeting agents are discussed in relation to the different cancer types and the types of resistance encountered. One of the major challenges to successful development of these therapies will be the application of robust predictive biomarkers for clear-cut patient stratification.

AXL-Driven EMT State as a Targetable Conduit in Cancer

The receptor tyrosine kinase (RTK) AXL has been intrinsically linked to epithelial-mesenchymal transition (EMT) and promoting cell survival, anoikis resistance, invasion, and metastasis in several cancers. AXL signaling has been shown to directly affect the mesenchymal state and confer it with aggressive phenotype and drug resistance. Recently, the EMT gradient has also been shown to rewire the kinase signaling nodes that facilitate AXL-RTK cross-talk, protracted signaling, converging on ERK, and PI3K axes. The molecular mechanisms underplaying the regulation between the kinome and EMT require further elucidation to define targetable conduits. Therapeutically, as AXL inhibition has shown EMT reversal and resensitization to other tyrosine kinase inhibitors, mitotic inhibitors, and platinum-based therapy, there is a need to stratify patients based on AXL dependence. This review elucidates the role of AXL in EMT-mediated oncogenesis and highlights the reciprocal control between AXL signaling and the EMT state. In addition, we review the potential in inhibiting AXL for the development of different therapeutic strategies and inhibitors. Cancer Res; 77(14); 3725-32. ©2017 AACR.

TWIST1 drives cisplatin resistance and cell survival in an ovarian cancer model, via upregulation of GAS6, L1CAM, and Akt signalling

Epithelial ovarian cancer (EOC) is the most deadly gynaecologic malignancy due to late onset of symptoms and propensity towards drug resistance. Epithelial-mesenchymal transition (EMT) has been linked to the development of chemoresistance in other cancers, yet little is known regarding its role in EOC. In this study, we sought to determine the role of the transcription factor TWIST1, a master regulator of EMT, on cisplatin resistance in an EOC model. We created two Ovcar8-derived cell lines that differed only in their TWIST1 expression. TWIST1 expression led to increased tumour engraftment in mice, as well as cisplatin resistance in vitro. RNA sequencing analysis revealed that TWIST1 expression resulted in upregulation of GAS6 and L1CAM and downregulation of HMGA2. Knockdown studies of these genes demonstrated that loss of GAS6 or L1CAM sensitized cells to cisplatin, but that loss of HMGA2 did not give rise to chemoresistance. TWIST1, in part via GAS6 and L1CAM, led to higher expression and activation of Akt upon cisplatin treatment, and inhibition of Akt activation sensitized cells to cisplatin. These results suggest TWIST1- and EMT-driven increase in Akt activation, and thus tumour cell proliferation, as a potential mechanism of drug resistance in EOC.

Role of AXL expression in non-small cell lung cancer

The present study aimed to investigate the expression profile of AXL in non-small cell lung cancer (NSCLC) and its clinical significance. The current study included 257 NSCLC patients, tyrosine-protein kinase receptor UFO (AXL) expression in paired lung cancer and adjacent normal lung tissues of NSCLC patients were compared by immunohistochemistry, western blot analysis and quantitative polymerase chain reaction (qPCR). These methods were used to detect the expression of the AXL gene and protein in fresh tissues from 35 patients. Small interfering RNA (siRNA) was transfected into the H1299 lung cancer cell line to knock down AXL expression; the effects of AXL-siRNA on cell proliferation and migration were examined by MTT and Transwell migration assay, respectively. It was found that AXL staining density in lung cancer tissues was significantly increased compared with adjacent normal lung tissues (55.25 vs. 26.85%; P<0.01); and the expression level of AXL in NSCLC patients was significantly associated with the degree of tumor differentiation (P<0.01) and the clinical stage of disease (P<0.01). Western blotting and qPCR showed that AXL expression was significantly higher in cancer tissues compared with that in adjacent lung tissue (P<0.05). Additionally, the current study also showed that AXL-siRNA inhibited H1299 cell proliferation and migration in vitro. The present study demonstrates the association between increased expression of AXL in NSCLC and the low differentiation phenotype, and its effects on cell proliferation and migration, suggesting its potential clinical values for the prognosis of NSCLC.

Axl-EGFR receptor tyrosine kinase hetero-interaction provides EGFR with access to pro-invasive signalling in cancer cells

Acquired resistance to conventional and targeted therapies is becoming a major hindrance in cancer management. It is increasingly clear that cancer cells are able to evolve and rewire canonical signalling pathways to their advantage, thus evading cell death and promoting cell invasion. The Axl receptor tyrosine kinase (RTK) has been shown to modulate acquired resistance to EGFR-targeted therapies in both breast and lung cancers. Glioblastoma multiforme (GBM) is a highly infiltrative and invasive form of brain tumour with little response to therapy. Both Axl and EGFR have been identified as major players in gliomagenesis and invasiveness. However, the mechanisms underlying a potential signalling crosstalk between EGFR and Axl RTKs are unknown. The purpose of this study was to investigate this novel and unconventional interaction among RTKs of different families in human GBM cells. With the use of western blotting, in vitro kinase activity, co-immunoprecipitation and bimolecular fluorescence complementation assays, we show that EGF stimulates activation of Axl kinase and that there is a hetero-interaction between the two RTKs. Through small interfering RNA knockdown and quantitative PCR screening, we identified distinct gene expression patterns in GBM cells that were specifically regulated by signalling from EGFR-EGFR, Axl–Axl and EGFR-Axl RTK parings. These included genes that promote invasion, which were activated only via the EGFR-Axl axis (MMP9), while EGFR-EGFR distinctly regulated the cell cycle and Axl–Axl regulated invasion. Our findings provide critical insights into the role of EGFR-Axl hetero-dimerisation in cancer cells and reveal regulation of cell invasion via Axl as a novel function of EGFR signalling.

Gene of the month: Axl

The interaction between Axl receptor tyrosine kinase and its main ligand Gas6 has been implicated in the progression of a wide number of malignancies. More recently, overexpression of Axl has emerged as a key molecular determinant underlying the development of acquired resistance to targeted anticancer agents. The activation of Axl is overexpression-dependent and controls a number of hallmarks of cancer progression including proliferation, migration, resistance to apoptosis and survival through a complex network of intracellular second messengers. Axl has been noted to influence clinically meaningful end points including metastatic recurrence and survival in the vast majority of tumour types. With Axl inhibitors having gained momentum as novel anticancer therapies, we provide an overview of the biological and clinical relevance of this molecular pathway, outlining the main directions of research.

Negative feedback regulation of AXL by miR-34a modulates apoptosis in lung cancer cells

The AXL receptor tyrosine kinase is frequently overexpressed in cancers and is important in cancer invasion/metastasis and chemoresistance. Here, we demonstrate a regulatory feedback loop between AXL and microRNA (miRNA) at the post-transcriptional level. Both the GAS6-binding domain and the kinase domain of AXL, particularly the Y779 tyrosine phosphorylation site, are shown to be crucial for this autoregulation. To clarify the role of miRNAs in this regulation loop, approaches using bioinformatics and molecular techniques were applied, revealing that miR-34a may target the 3' UTR of AXL mRNA to inhibit AXL expression. Interestingly and importantly, AXL overexpression may induce miR-34a expression by activating the transcription factor ELK1 via the JNK signaling pathway. In addition, ectopic overexpression of ELK1 promotes apoptosis through, in part, down-regulation of AXL. Therefore, we propose that AXL is autoregulated by miR-34a in a feedback loop; this may provide a novel opportunity for developing AXL-targeted anticancer therapies.

Axl as a mediator of cellular growth and survival

The control of cellular growth and proliferation is key to the maintenance of homeostasis. Survival, proliferation, and arrest are regulated, in part, by Growth Arrest Specific 6 (Gas6) through binding to members of the TAM receptor tyrosine kinase family. Activation of the TAM receptors leads to downstream signaling through common kinases, but the exact mechanism within each cellular context varies and remains to be completely elucidated. Deregulation of the TAM family, due to its central role in mediating cellular proliferation, has been implicated in multiple diseases. Axl was cloned as the first TAM receptor in a search for genes involved in the progression of chronic to acute-phase leukemia, and has since been established as playing a critical role in the progression of cancer. The oncogenic nature of Axl is demonstrated through its activation of signaling pathways involved in proliferation, migration, inhibition of apoptosis, and therapeutic resistance. Despite its recent discovery, significant progress has been made in the development of effective clinical therapeutics targeting Axl. In order to accurately define the role of Axl in normal and diseased processes, it must be analyzed in a cell type-specific context.

Tumor-associated macrophages promote oral cancer progression through activation of the Axl signaling pathway

BACKGROUND

Recent studies suggest that tumor-associated macrophages (TAMs) promote tumor growth and metastasis. Our previous report demonstrated that Axl signaling promotes carcinogenesis and progression of oral squamous cell carcinoma (OSCC). This study aims to test the potential involvement of growth arrest-specific gene 6 (Gas6)/Axl signaling in the protumoral effect of TAMs.

METHODS

Co-culture experiments by incubation of OSCC cells (YD38 and OE) and macrophages (THP-1) were performed. The expression of Gas6/Axl and epithelial-mesenchymal transition (EMT) genes were examined in YD38 and OE cells. The effect of Gas6/Axl signaling on co-cultured cancer cells was further investigated by knocking down Axl expression and neutralizing Gas6. Axl and TAM distribution were analyzed by immunohistochemistry in OSCC tissues.

RESULTS

Activation of Axl signaling and increased expression of mesenchymal markers, along with increased invasion/migration ability of OSCC cells, was noted upon co-culture with THP-1. Neutralization of Gas6 in the co-culture system or knockdown of Axl in YD38 caused the co-culture effects to be diminished. Co-culture with THP-1 increased nuclear factor (NF)-κB nuclear translocation and transcription activity in YD38 cells. A significant association between the TAM count and expression of phosphorylated Axl (P = 0.004) was found in vivo cancer tissues.

CONCLUSIONS

TAMs play a protumor role in OSCC and likely promote tumor progression through activation of the Gas6/Axl-NF-κB signaling pathway. Therefore, Gas6/Axl and NF-κB signaling in OSCC cells may be a putative target for therapeutic intervention.

Dual involvement of growth arrest-specific gene 6 in the early phase of human IgA nephropathy

Background

Gas6 is a growth factor that causes proliferation of mesangial cells in the development of glomerulonephritis. Gas6 can bind to three kinds of receptors; Axl, Dtk, and Mer. However, their expression and functions are not entirely clear in the different glomerular cell types. Meanwhile, representative cell cycle regulatory protein p27 has been reported to be expressed in podocytes in normal glomeruli with decreased expression in proliferating glomeruli, which inversely correlated with mesangial proliferation in human IgA nephropathy (IgAN).

Methods

The aim of this study is to clarify Gas6 involvement in the progression of IgAN. Expression of Gas6/Axl/Dtk was examined in 31 biopsy proven IgAN cases. We compared the expression levels with histological severity or clinical data. Moreover, we investigated the expression of Gas6 and its receptors in cultured podocytes.

Results

In 28 of 31 cases, Gas6 was upregulated mainly in podocytes. In the other 3 cases, Gas6 expression was induced in endothelial and mesangial cells, which was similar to animal nephritis models. Among 28 podocyte type cases, the expression level of Gas6 correlated with the mesangial hypercellularity score of IgAN Oxford classification and urine protein excretion. It also inversely correlated with p27 expression in glomeruli. As for the receptors, Axl was mainly expressed in endothelial and mesangial cells, while Dtk was expressed in podocytes. In vitro, Dtk was expressed in cultured murine podocytes, and the expression of p27 was decreased by Gas6 stimulation.

Conclusions

Gas6 was uniquely upregulated in either endothelial/mesangial cells or podocytes in IgAN. The expression pattern can be used as a marker to classify IgAN. Gas6 has a possibility to be involved in not only mesangial proliferation via Axl, but also podocyte injury via Dtk in IgAN.

Inhibition of MerTK increases chemosensitivity and decreases oncogenic potential in T-cell acute lymphoblastic leukemia

Pediatric leukemia survival rates have improved dramatically over the past decades. However, current treatment protocols are still largely ineffective in cases of relapsed leukemia and are associated with a significant rate of chronic health conditions. Thus, there is a continued need for new therapeutic options. Here, we show that mer receptor tyrosine kinase (MerTK) was abnormally expressed in approximately one half of pediatric T-cell leukemia patient samples and T-cell acute lymphoblastic leukemia (T-ALL) cell lines. Stimulation of MerTK by the ligand Gas6 led to activation of the prosurvival proteins Erk 1/2 and Stat5, and MerTK-dependent activation of the STAT pathway in leukemia represents a novel finding. Furthermore, inhibition of MerTK expression increased the sensitivity of T-ALL cells to treatment with chemotherapeutic agents and decreased the oncogenic potential of the Jurkat T-ALL cell line in a methylcellulose colony-forming assay. Lastly, inhibition of MerTK expression significantly increased median survival in a xenograft mouse model of leukemia (30.5 days vs 60 days, P<0.0001). These results suggest that inhibition of MerTK is a promising therapeutic strategy for the treatment of leukemia and may allow for dose reduction of currently used chemotherapeutics resulting in decreased rates of therapy-associated toxicities.

Axl/Gas6/NFκB signalling in schwannoma pathological proliferation, adhesion and survival

TAM family receptor tyrosine kinases comprising Tyro3 (Sky), Axl, and Mer are overexpressed in some cancers, correlate with multidrug resistance and contribute to tumourigenesis by regulating invasion, angiogenesis, cell survival and tumour growth. Mutations in the gene coding for a tumour suppressor merlin cause development of multiple tumours of the nervous system such as schwannomas, meningiomas and ependymomas occurring spontaneously or as part of a hereditary disease neurofibromatosis type 2. The benign character of merlin-deficient tumours makes them less responsive to chemotherapy. We previously showed that, amongst other growth factor receptors, TAM family receptors (Tyro3, Axl and Mer) are significantly overexpressed in schwannoma tissues. As Axl is negatively regulated by merlin and positively regulated by E3 ubiquitin ligase CRL4DCAF1, previously shown to be a key regulator in schwannoma growth we hypothesized that Axl is a good target to study in merlin-deficient tumours. Moreover, Axl positively regulates the oncogene Yes-associated protein, which is known to be under merlin regulation in schwannoma and is involved in increased proliferation of merlin-deficient meningioma and mesothelioma. Here, we demonstrated strong overexpression and activation of Axl receptor as well as its ligand Gas6 in human schwannoma primary cells compared to normal Schwann cells. We show that Gas6 is mitogenic and increases schwannoma cell-matrix adhesion and survival acting via Axl in schwannoma cells. Stimulation of the Gas6/Axl signalling pathway recruits Src, focal adhesion kinase (FAK) and NFκB. We showed that NFκB mediates Gas6/Axl-mediated overexpression of survivin, cyclin D1 and FAK, leading to enhanced survival, cell-matrix adhesion and proliferation of schwannoma. We conclude that Axl/FAK/Src/NFκB pathway is relevant in merlin-deficient tumours and is a potential therapeutic target for schwannoma and other merlin-deficient tumours.

Profiling phospho-signaling networks in breast cancer using reverse-phase protein arrays

Measuring the states of cell signaling pathways in tumor samples promises to advance the understanding of oncogenesis and identify response biomarkers. Here, we describe the use of Reverse Phase Protein Arrays (RPPAs or RPLAs) to profile signaling proteins in 56 breast cancers and matched normal tissue. In RPPAs, hundreds to thousands of lysates are arrayed in dense regular grids and each grid is probed with a different antibody (100 in the current work, of which 71 yielded strong signals with breast tissue). Although RPPA technology is quite widely used, measuring changes in phosphorylation reflective of protein activation remains challenging. Using repeat deposition and well-validated antibodies, we show that diverse patterns of phosphorylation can be monitored in tumor samples and changes mapped onto signaling networks in a coherent fashion. The patterns are consistent with biomarker-based classification of breast cancers and known mechanisms of oncogenesis. We explore in detail one tumor-associated pattern that involves changes in the abundance of the Axl receptor tyrosine kinase (RTK) and phosphorylation of the cMet RTK. Both cMet and Axl have been implicated in breast cancer, or in resistance to anticancer drugs, but the two RTKs are not known to be linked functionally. Protein depletion and overexpression studies in a 'triple-negative' breast cell line reveal cross talk between Axl and cMet involving Axl-mediated modification of cMet, a requirement for cMet in efficient and timely signal transduction by the Axl ligand Gas6 and the potential for the two receptors to interact physically. These findings have potential therapeutic implications, as they imply that bi-specific receptor inhibitors (for example, ATP-competitive small-kinase inhibitors such as GSK1363089, BMS-777607 or MP470) may be more efficacious than the mono-specific therapeutic antibodies currently in development (for example, Onartuzumab).

Mucins and toll-like receptors: kith and kin in infection and cancer

Inflammation is underlying biological phenomenon common in infection and cancer. Mucins are glycoproteins which establish a physical barrier for undesirable entry of foreign materials through epithelial surfaces. A deregulated expression and an anomalous glycosylation pattern of mucins are known in large number of cancers. TLRs are class of receptors which recognize the molecular patterns of invading pathogens and activate complex inflammatory pathways to clear them. Aberrant expression of TLRs is observed in many cancers. A highly orchestrated action of mucins and TLRs is well evolved host defence mechanism; however, a link between the two in other non-infectious conditions has received less attention. Here we present an overview as to how mucins and TLRs give protection to the host and are deregulated during carcinogenesis. Further, we propose the possible mechanisms of cross-regulation between them in pathogenesis of cancer. As both mucins and TLRs are therapeutically important class of molecules, an understanding of the underlying molecular mechanisms connecting the two will open new avenues for the therapeutic targeting of cancer.

Mer or Axl receptor tyrosine kinase inhibition promotes apoptosis, blocks growth and enhances chemosensitivity of human non-small cell lung cancer

Non-small cell lung cancer (NSCLC) is a prevalent and devastating disease that claims more lives than breast, prostate, colon, and pancreatic cancers combined. Current research suggests that standard chemotherapy regimens have been optimized to maximal efficiency. Promising new treatment strategies involve novel agents targeting molecular aberrations present in subsets of NSCLC. We evaluated 88 human NSCLC tumors of diverse histology and identified Mer and Axl as receptor tyrosine kinases (RTKs) overexpressed in 69% and 93%, respectively, of tumors relative to surrounding normal lung tissue. Mer and Axl were also frequently overexpressed and activated in NSCLC cell lines. Ligand-dependent Mer or Axl activation stimulated MAPK, AKT, and FAK signaling pathways indicating roles for these RTKs in multiple oncogenic processes. In addition, we identified a novel pro-survival pathway—involving AKT, CREB, Bcl-xL, survivin, and Bcl-2—downstream of Mer, which is differentially modulated by Axl signaling. We demonstrated that shRNA knockdown of Mer or Axl significantly reduced NSCLC colony formation and growth of subcutaneous xenografts in nude mice. Mer or Axl knockdown also improved in vitro NSCLC sensitivity to chemotherapeutic agents by promoting apoptosis. When comparing the effects of Mer and Axl knockdown, Mer inhibition exhibited more complete blockade of tumor growth while Axl knockdown more robustly improved chemosensitivity. These results indicate that Mer and Axl play complementary and overlapping roles in NSCLC and suggest that treatment strategies targeting both RTKs may be more effective than singly-targeted agents. Our findings validate Mer and Axl as potential therapeutic targets in NSCLC and provide justification for development of novel therapeutic compounds that selectively inhibit Mer and/or Axl.

The receptor tyrosine kinase Axl is an essential regulator of prostate cancer proliferation and tumor growth and represents a new therapeutic target

Deregulation of the receptor tyrosine kinase Axl has been implicated in the progression of several human cancers. However, the role of Axl in prostate cancer remains poorly understood, and the therapeutic efficacy of Axl targeting remains untested. In this report we identified Axl as a new therapeutic target for prostate cancer. Axl is consistently overexpressed in prostate cancer cell lines and human prostate tumors. Interestingly, the blockage of Axl gene expression strongly inhibits proliferation, migration, invasion and tumor growth. Furthermore, inhibition of Axl expression by small interfering RNA regulates a transcriptional program of genes involved in cell survival, strikingly all connected to the nuclear factor-κB pathway. Additionally, blockage of Axl expression leads to inhibition of Akt, IKKα and IκBα phosphorylation, increasing IκBα expression and stability. Furthermore, induction of Akt phosphorylation by insulin-like growth factor 1 in Axl knockdown cells restores Akt activity and proliferation. Taken together, our results establish an unambiguous role for Axl in prostate cancer tumorigenesis with implications for prostate cancer treatment.

Growth arrest-specific gene 6 (gas6) and vascular hemostasis

Gas6 (growth arrest-specific 6) belongs structurally to the family of plasma vitamin K-dependent proteins. Gas6 has a high structural homology with the natural anticoagulant protein S, sharing the same modular composition. Interestingly, despite the presence of a γ-carboxyglutamic acid domain in its structure, no role in the coagulation cascade has been identified for gas6. Gas6 has been shown to be involved in vascular homeostasis and more precisely is involved in proliferation, apoptosis, efferocytosis, leukocyte migration, and sequestration and platelet aggregation. It is also involved in the activation of different cell types, from platelets to endothelial and vascular smooth muscle cells. Thus, it has been shown to play a role in several pathophysiological processes such as atherosclerosis, cancer, and thrombosis. Interestingly, studies using gas6 null mice highlighted that gas6 may represent a novel potential target for anticoagulant therapy, because these animals are protected from lethal venous thromboembolism without excessive bleeding. However, the mechanism in thrombus occurrence remains to be further explored. In the present review, we will focus on the role of gas6 in innate immunity, atherosclerosis, thrombosis, and cancer-related events.

Design, Synthesis and Biological Evaluation of a Series of Novel Axl Kinase Inhibitors

The receptor tyrosine kinase AXL has emerged in recent years as an potential oncology target due to its over expression in several types of cancers coupled with its ability to promote tumor growth and metastasis. In order to identify small molecule inhibitors of AXL, we built a homology model of its catalytic domain to virtually screen and identify scaffolds displaying an affinity for AXL. Further computational and structure-based design resulted in the synthesis of a series of 2,4,5-trisubstitued pyrimidines which demonstrated potent inhibition of AXL in vitro (IC(50) 19 nM) and strongly inhibited the growth of several pancreatic cell lines.

The protein-tyrosine kinase Syk interacts with the C-terminal region of tensin2

Syk is a 72-kDa protein-tyrosine kinase that regulates signaling through multiple cell surface receptors including those for antigens, immunoglobulins and proteins of the extracellular matrix. As part of its function, Syk binds a variety of downstream effectors through interactions that are often mediated by motifs that recognize phosphotyrosines. In a search for novel Syk-interacting proteins by yeast two-hybrid analysis, we identified tensin2 as a Syk-binding protein. Syk interacts with a fragment of tensin2 located near the C-terminus that contains SH2 and PTB domains. In epithelial cells, tensin2 localizes both to focal adhesions and to large cytoplasmic puncta. It is within these punctuate structures that Syk and tensin2 are co-localized. The clustering of Syk within these structures leads to its phosphorylation on tyrosine.

Axl is a prognostic marker in oral squamous cell carcinoma

BACKGROUND

Overexpression of the receptor tyrosine kinase Axl is implicated in several diseases. The present study was conducted to determine the biologic and clinical significance of Axl in oral squamous cell carcinoma (OSCC).

METHODS

The expression of Axl was examined in a panel of OSCC cell lines. Activation of Axl by Gas6 treatment and silencing of Axl via Axl shRNA were used to examine the effect of Axl on OSCC cell line. Expression of Axl in cancer tissues were examined by immunohistochemical staining. The associations between Axl expression and clinicopathologic features and prognosis were analyzed.

RESULTS

Varied Axl expression was noted in OSCC cell lines. Compared with control cells, modulated Axl signal affected epithelial-mesenchymal gene expression and cell invasion and migration. The immunoreactivity of Axl was low in normal epithelium, and a progressively increased positive percentage was noted, from normal/hyperplastic epithelium (10.9%) to dysplasia (30.8%) to cancer tissue (54.5%). Axl expression correlated with lymph node status (P = .001) and clinical stage (P = .014) of OSCC. Patients with high expression of Axl showed poor prognosis compared with those with low Axl expression patients (P < .001). In multivariate prognostic analysis according to the Cox proportional hazard regression model, Axl expression remained as an independent prognostic factor (P = .037; CI, 1.042-3.839).

CONCLUSIONS

Our data indicated that Axl signal promotes OSCC carcinogenesis and progression. The expression of Axl is a valuable marker for OSCC aggressiveness and clinical outcome.

Tyrosine kinase receptor Axl enhances entry of Zaire ebolavirus without direct interactions with the viral glycoprotein

In a bioinformatics-based screen for cellular genes that enhance Zaire ebolavirus (ZEBOV) transduction, AXL mRNA expression strongly correlated with ZEBOV infection. A series of cell lines and primary cells were identified that require Axl for optimal ZEBOV entry. Using one of these cell lines, we identified ZEBOV entry events that are Axl-dependent. Interactions between ZEBOV-GP and the Axl ectodomain were not detected in immunoprecipitations and reduction of surface-expressed Axl by RNAi did not alter ZEBOV-GP binding, providing evidence that Axl does not serve as a receptor for the virus. However, RNAi knock down of Axl reduced ZEBOV pseudovirion internalization and α-Axl antisera inhibited pseudovirion fusion with cellular membranes. Consistent with the importance of Axl for ZEBOV transduction, Axl transiently co-localized on the surface of cells with ZEBOV virus particles and was internalized during virion transduction. In total, these findings indicate that endosomal uptake of filoviruses is facilitated by Axl.

PMA up-regulates the transcription of Axl by AP-1 transcription factor binding to TRE sequences via the MAPK cascade in leukaemia cells

BACKGROUND

Axl is a receptor tyrosine kinase promoting anti-apoptosis, invasion and mitogenesis, and is highly expressed in different solid cancers. Axl basal transcriptional activity is driven by Sp1/Sp3, and overexpression of MZF-1 (myeloid zinc-finger 1) induces Axl transcription and gene expression. Furthermore, Axl expression is epigenetically controlled by CpG hypermethylation; however, little is known about inducible Axl gene expression and Axl regulation in haematopoetic malignancies.

RESULTS

In the present study, we studied Axl transcriptional regulation under PMA-stimulated conditions in leukaemia cells. Luciferase analysis with sequential 5'-deletion constructs revealed that the -660/-580 region of the Axl promoter is indispensable for induced promoter activity under PMA stimulation. This region includes AP-1 (activator protein 1)/CREB [CRE (cAMP-response-element)-binding protein] motifs, five times partially overlapping TGCGTG repeats and multiple GT repeats. Mutational, supershift and ChIP (chromatin immunoprecipitation) analysis determined that AP-1 family members bind to AP-1 motifs and to the 5 × TGCGTG overlapping repeats, thus transactivating Axl promoter activity. Furthermore, specific inhibitors of PKC (protein kinase C), ERK1/2 (extracellular-signal-regulated kinase 1/2) and p38 reduced Axl expression. Additionally, mithramycin treatment abolished constitutive and PMA-induced Axl expression.

CONCLUSIONS

Taken together the results of the present study suggest that PMA-induced Axl gene expression in leukaemia cells is mediated by AP-1 motifs and 5 × TGCGTG repeats within the promoter region -660/-580, and through the PKC/ERK1/2/AP-1 or PKC/p-38/AP-1 signalling axis.

AXL receptor kinase is a mediator of YAP-dependent oncogenic functions in hepatocellular carcinoma

Yes-associated protein (YAP) is a downstream effector of the Hippo signaling pathway, which controls organ expansion and tissue development. We have recently defined the tumorigenic potential and clinical significance of the YAP1 oncogene in human hepatocellular carcinoma (HCC). The present study aims to define the tumorigenic properties of YAP in HCC and elucidate the related downstream signaling mechanism. In a gain-of-function study, we demonstrated that ectopic increased expression of YAP in the immortalized non-tumorigenic hepatocyte cell line MIHA confers tumorigenic and metastatic potentials, as evidenced by (1) enhanced aptitudes in cell viability, anchorage-independent growth, migration and invasion; (2) tumor formation in a xenograft mouse model; and (3) induction of HCC biomarker α-fetoprotein and activation of mitogen-activated protein kinase. Furthermore, we have identified AXL, a receptor tyrosine kinase, as a key downstream target that drives YAP-dependent oncogenic functions. RNAi-mediated knockdown of AXL expression decreased the ability of YAP-expressing MIHA cells and of the primary HCC cell line to proliferate and invade. These results indicate that AXL is a mediator of YAP-dependent oncogenic activities and implicates it as a potential therapeutic target for HCC.

Myeloid zinc finger 1 induces migration, invasion, and in vivo metastasis through Axl gene expression in solid cancer

Myeloid zinc finger 1 (MZF1) is a member of the SCAN domain family transcription factors that form dimers through their highly conserved SCAN motifs. Silencing of MZF1 inhibits cell proliferation, and abnormal expression of MZF1 results in cancer development. However, a potential role of MZF1 in metastasis remains unclear. Axl is a receptor tyrosine kinase and was first identified as a transforming gene in chronic myeloid leukemia. Axl overexpression induces proliferation, migration, and invasion and is highly expressed in different human cancers. In this study, we show that overexpression of MZF1 induces migration and invasion in colorectal (Rko, SW480) and cervical (HeLa) cancer cells. In addition, we show that MZF1 binds to the Axl promoter, transactivates promoter activity, and enhances Axl-mRNA and protein expression in a dose-dependent manner. In vitro, sh-RNA knockdown of Axl reduced MZF1-induced migration and invasion in HeLa and Rko cells (P = 0.05). Additionally, Rko cells overexpressing MZF1 showed increased tumor formation and liver metastasis in the chicken-embryo-metastasis assay in vivo. Furthermore, the expression of MZF1 and Axl was significantly higher in resected colorectal tumors compared with corresponding normal tissues (P = 0.02; P = 0.05), and MZF1 expression was positively correlated with Axl gene expression in tumor tissues (P < 0.01). Taken together, this is the first study to show that MZF1 induces invasion and in vivo metastasis in colorectal and cervical cancer, at least in part by regulating Axl gene expression.

Cytoplasmic ACK1 interaction with multiple receptor tyrosine kinases is mediated by Grb2: an analysis of ACK1 effects on Axl signaling

ACK1 (activated Cdc42-associated kinase 1), a cytoplsmic tyrosine kinase, is implicated in metastatic behavior, cell spreading and migration, and epidermal growth factor receptor (EGFR) signaling. The function of ACK1 in the regulation of receptor tyrosine kinases requires a C-terminal region that demonstrates a significant homology to the EGFR binding domain of MIG6. In this study, we have identified additional receptor tyrosine kinases, including Axl, leukocyte tyrosine kinase, and anaplastic lymphoma kinase, that can bind to the ACK1/MIG6 homology region. Unlike the interaction between MIG6 and EGFR, our data suggest that these receptor tyrosine kinases require the adaptor protein Grb2 for efficient binding, which interacts with highly conserved proline-rich regions that are conserved between ACK1 and MIG6. We have focused on Axl and compared how ACK1/Axl differs from the ACK1/EGFR axis by investigating effects of knockdown of endogenous ACK1. Although EGFR activation promotes ACK1 turnover, Axl activation by GAS6 does not; interestingly, the reciprocal down-regulation of GAS6-stimulated Axl is blocked by removing ACK1. Thus, ACK1 functions in part to control Axl receptor levels. Silencing of ACK1 also leads to diminished ruffling and migration in DU145 and COS7 cells upon GAS6-Axl signaling. The ability of ACK1 to modulate Axl and perhaps anaplastic lymphoma kinase (altered in anaplastic large cell lymphomas) might explain why ACK1 can promote metastatic and transformed behavior in a number of cancers.

Induction of Akt activity by chemotherapy confers acquired resistance

Resistance to chemotherapy is a major cause of treatment failure in human cancer. Accumulating evidence has indicated that the acquisition of resistance to chemotherapeutic drugs involves the activation of the PI3K/Akt pathway. Modulating Akt activity in response to chemotherapy has been observed often in chemoresistant cancers. The potential molecular mechanisms by which chemotherapeutic agents activate the PI3K/Akt pathway are emerging. Activation of this pathway evades the cytotoxic effects of chemotherapeutic agents via regulation of essential cellular functions such as protein synthesis, antiapoptosis, survival and proliferation in cancer. How chemotherapeutic agents induce Akt activation and how activated Akt confers chemoresistance through regulation of signaling networks are discussed in this review. Combining PI3K/Akt inhibitors with standard chemotherapy has been successful in increasing the efficacy of chemotherapeutic agents both in vivo and in vitro. Several small molecules have been developed to specifically target PI3K/Akt and other components of this pathway, which in combination with chemotherapy may be a valid approach to overcome therapeutic resistance. We propose several feedback and feedforward regulatory mechanisms of signaling networks for maintenance of the Akt activity for cell survival. These regulatory mechanisms may limit the efficacy of PI3K/Akt-targeted therapy; therefore, disruption of these mechanisms may be an effective strategy for development of novel anti-cancer therapies.

TRAIL and Taurolidine induce apoptosis and decrease proliferation in human fibrosarcoma

BACKGROUND

Disseminated soft tissue sarcoma still represents a therapeutic dilemma because effective cytostatics are missing. Therefore we tested TRAIL and Tarolidine (TRD), two substances with apoptogenic properties on human fibrosarcoma (HT1080).

METHODS

Viability, apoptosis and necrosis were visualized by TUNEL-Assay and quantitated by FACS analysis (Propidiumiodide/AnnexinV staining). Gene expression was analysed by RNA-Microarray and the results validated for selected genes by rtPCR. Protein level changes were documented by Western Blot analysis. NFKB activity was analysed by ELISA and proliferation assays (BrdU) were performed.

RESULTS AND DISCUSSION

The single substances TRAIL and TRD induced apoptotic cell death and decreased proliferation in HT1080 cells significantly. Gene expression of several genes related to apoptotic pathways (TRAIL: ARHGDIA, NFKBIA, TNFAIP3; TRD: HSPA1A/B, NFKBIA, GADD45A, SGK, JUN, MAP3K14) was changed. The combination of TRD and TRAIL significantly increased apoptotic cell death compared to the single substances and lead to expression changes in a variety of genes (HSPA1A/B, NFKBIA, PPP1R15A, GADD45A, AXL, SGK, DUSP1, JUN, IRF1, MYC, BAG5, BIRC3). NFKB activity assay revealed an antipodal regulation of the several subunits of NFKB by TRD and TRD+TRAIL compared to TRAIL alone.

CONCLUSION

TRD and TRAIL are effective to induce apoptosis and decrease proliferation in human fibrosarcoma. A variety of genes seems to be involved, pointing to the NFKB pathway as key regulator in TRD/TRAIL-mediated apoptosis.

Isothiocyanate NB7M causes selective cytotoxicity, pro-apoptotic signalling and cell-cycle regression in ovarian cancer cells

The present report identifies indole-3-ethyl isothiocyanate NB7M as a potent cytotoxic agent with selective activity against cell lines derived from various tumour types. Ovarian cancer cell lines showed sensitivity to NB7M (60–70% cytotoxicity at 2.5 μM), in contrast to control cells (TCL-1 and HTR-8; IC₅₀ ∼15 μM). In a screen performed by the National Cancer Institute (NCI) (NCI₆₀ cancer cell-line assay) NB7M (NSC746077) reduced growth up to 100% with an IC₅₀ between 0.1 and 10 μM depending on the cell line studied. Using SKOV-3 ovarian cancer cells as a model, mechanisms of cytotoxicity were analysed. NB7M caused hallmarks of apoptosis such as PARP-1 deactivation, chromatin condensation, DNA nicks, activation of caspases-9, -8, -3, loss of mitochondrial transmembrane depolarisation potential and upregulation of pro-apoptotic mitogen activated protein kinases (p38, SAP/JNK). NB7M downregulated phosphorylation of prosurvival kinases (PI-3K, AKT, IKKα), transcription factor NF-κB, and expression of DNA-Pk and AXL receptor tyrosine kinase. Subcytotoxic doses of NB7M inhibited DNA synthesis, caused G1-phase cell-cycle arrest and upregulated p27 expression. The present report suggests that NB7M is a selective cytotoxic agent in vitro for cell lines derived from ovarian and certain other tumours. In addition, NB7M acts as a growth/cell-cycle-suppressing agent and may be developed as a potential therapeutic drug to treat ovarian cancer.