Pharmacological inhibition of Axl affects smooth muscle cell functions under oxidative stress

Functions of TAM Receptors and Ligands Protein S and Gas6 in Atherosclerosis and Cardiovascular Disease

Atherosclerosis and cardiovascular disease are associated with high morbidity and mortality in industrialized nations. The Tyro3, Axl, and Mer (TAM) family of receptor tyrosine kinases is involved in the amplification or resolution of atherosclerosis pathology and other cardiovascular pathology. The ligands of these receptors, Protein S (PS) and growth arrest specific protein 6 (Gas6), are essential for TAM receptor functions in the amplification and resolution of atherosclerosis. The Axl-Gas6 interaction has various effects on cardiovascular disease. Mer and PS dampen inflammation, thereby protecting against atherosclerosis progression. Tyro3, the least studied TAM receptor in cardiovascular disease, appears to protect against fibrosis in post-myocardial infarction injury. Ultimately, PS, Gas6, and TAM receptors present an exciting avenue of potential therapeutic targets against inflammation associated with atherosclerosis and cardiovascular disease.

SIRT3 overexpression and epigenetic silencing of catalase regulate ROS accumulation in CLL cells activating AXL signaling axis

Mitochondrial metabolism is the key source for abundant ROS in chronic lymphocytic leukemia (CLL) cells. Here, we detected significantly lower superoxide anion (O₂⁻) levels with increased accumulation of hydrogen peroxide (H₂O₂) in CLL cells vs. normal B-cells. Further analysis indicated that mitochondrial superoxide dismutase (SOD)2, which converts O₂⁻ into H₂O₂ remained deacetylated in CLL cells due to SIRT3 overexpression resulting its constitutive activation. In addition, catalase expression was also reduced in CLL cells suggesting impairment of H₂O₂-conversion into water and O₂ which may cause H₂O₂-accumulation. Importantly, we identified two CpG-islands in the catalase promoter and discovered that while the distal CpG-island (−3619 to −3765) remained methylated in both normal B-cells and CLL cells, variable degrees of methylation were discernible in the proximal CpG-island (−174 to −332) only in CLL cells. Finally, treatment of CLL cells with a demethylating agent increased catalase mRNA levels. Functionally, ROS accumulation in CLL cells activated the AXL survival axis while upregulated SIRT3, suggesting that CLL cells rapidly remove highly reactive O₂⁻ to avoid its cytotoxic effect but maintain increased H₂O₂-level to promote cell survival. Therefore, abrogation of aberrantly activated cell survival pathways using antioxidants can be an effective intervention in CLL therapy in combination with conventional agents.

DS-1205b, a novel selective inhibitor of AXL kinase, blocks resistance to EGFR-tyrosine kinase inhibitors in a non-small cell lung cancer xenograft model

The AXL receptor tyrosine kinase is involved in signal transduction in malignant cells. Recent studies have shown that the AXL upregulation underlies epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitor (TKI) resistance in EGFR-mutant non-small cell lung cancer (NSCLC). In this study, we investigated the effect of DS-1205b, a novel and selective inhibitor of AXL, on tumor growth and resistance to EGFR TKIs. In AXL-overexpressing NIH3T3 cells, DS-1205b potently inhibited hGAS6 ligand-induced migration in vitro and exerted significant antitumor activity in vivo. AXL was upregulated by long-term erlotinib or osimertinib treatment in HCC827 EGFR-mutant NSCLC cells, and DS-1205b treatment in combination with osimertinib or erlotinib effectively inhibited signaling downstream of EGFR in a cell-based assay. In an HCC827 EGFR-mutant NSCLC xenograft mouse model, combination treatment with DS-1205b and erlotinib significantly delayed the onset of tumor resistance compared to erlotinib monotherapy, and DS-1205b restored the antitumor activity of erlotinib in erlotinib-resistant tumors. DS-1205b also delayed the onset of resistance when used in combination with osimertinib in the model. These findings strongly suggest that DS-1205b can prolong the therapeutic benefit of EGFR TKIs in nonclinical as well as clinical settings.

Innate Immune Cells Are Regulated by Axl in Hypertensive Kidney

The balance between adaptive and innate immunity in kidney damage in salt-dependent hypertension is unclear. We investigated early renal dysfunction and the influence of Axl, a receptor tyrosine kinase, on innate immune response in hypertensive kidney in mice with lymphocyte deficiency (Rag1^-/-). The data suggest that increased presence of CD11b⁺ myeloid cells in the medulla might explain intensified salt and water retention as well as initial hypertensive response in Rag1^-/- mice. Global deletion of Axl on Rag1^-/- background reversed kidney dysfunction and accumulation of myeloid cells in the kidney medulla. Chimeric mice that lack Axl in innate immune cells (in the absence of lymphocytes) significantly improved kidney function and abolished early hypertensive response. The bioinformatics analyses of Axl-related gene-gene interaction networks established tissue-specific variation in regulatory pathways. It was confirmed that complement C3 is important for Axl-mediated interactions between myeloid and vascular cells in hypertensive kidney. In summary, innate immunity is crucial for renal dysfunction in early hypertension, and is highly influenced by the presence of Axl.

TAM receptors in cardiovascular disease

The TAM receptors are a distinct family of three receptor tyrosine kinases, namely Tyro3, Axl, and MerTK. Since their discovery in the early 1990s, they have been studied for their ability to influence numerous diseases, including cancer, chronic inflammatory and autoimmune disorders, and cardiovascular diseases. The TAM receptors demonstrate an ability to influence multiple aspects of cardiovascular pathology via their diverse effects on cells of both the vasculature and the immune system. In this review, we will explore the various functions of the TAM receptors and how they influence cardiovascular disease through regulation of vascular remodelling, efferocytosis and inflammation. Based on this information, we will suggest areas in which further research is required and identify potential targets for therapeutic intervention.

Cisplatin and Pemetrexed Activate AXL and AXL Inhibitor BGB324 Enhances Mesothelioma Cell Death from Chemotherapy

Reactive oxygen species (ROS) can promote or inhibit tumorigenesis. In mesothelioma, asbestos exposure to serous membranes induces ROS through iron content and chronic inflammation, and ROS promote cell survival signaling in mesothelioma. Moreover, a current chemotherapy regimen for mesothelioma consisting of a platinum and antifolate agent combination also induce ROS. Mesothelioma is notoriously chemotherapy-resistant, and we propose that ROS induced by cisplatin and pemetrexed may promote cell survival signaling pathways, which ultimately may contribute to chemotherapy resistance. In The Cancer Genome Atlas datasets, we found AXL kinase expression is relatively high in mesothelioma compared to other cancer samples. We showed that ROS induce the phosphorylation of AXL, which was blocked by the selective inhibitor BGB324 in VMC40 and P31 mesothelioma cells. We also showed that cisplatin and pemetrexed induce the phosphorylation of AXL and Akt, which was also blocked by BGB324 as well as by N-acetylcysteine antioxidant. AXL knockdown in these cells enhances sensitivity to cisplatin and pemetrexed. Similarly, AXL inhibitor BGB324 also enhances sensitivity to cisplatin and pemetrexed. Finally, higher synergy was observed when cells were pretreated with BGB324 before adding chemotherapy. These results demonstrate cisplatin and pemetrexed induce ROS that activate AXL, and blocking AXL activation enhances the efficacy of cisplatin and pemetrexed. These results suggest AXL inhibition combined with the current chemotherapy regimen may represent an effective strategy to enhance the efficacy of chemotherapy in mesothelioma. This is the first study, to our knowledge, on chemotherapy-induced activation of AXL and cell survival pathways associated with ROS signaling.

Identification of the soluble form of tyrosine kinase receptor Axl as a potential biomarker for intracranial aneurysm rupture

Background

Subarachnoid hemorrhage caused by a ruptured intracranial aneurysm (RIA) is a devastating condition with significant morbidity and mortality. Despite the fact that RIAs can be prevented by microsurgical clipping or endovascular coiling, there are no reliable means of effectively predicting IA patients at risk for rupture. The purpose of our study was to discover differentially-expressed glycoproteins in IAs with or without rupture as potential biomarkers to predict rupture.

Methods

Forty age/gender-matched patients with RIA, unruptured IA (UIA), healthy controls (HCs) and disease controls (DCs) (discovery cohort, n = 10 per group) were recruited and a multiplex quantitative proteomic method, iTRAQ (isobaric Tagging for Relative and Absolute protein Quantification), was used to quantify relative changes in the lectin-purified glycoproteins in CSF from RIAs and UIAs compared to HCs and DCs. Then we verified the proteomic results in an independent set of samples (validation cohort, n = 20 per group) by enzyme-linked immunosorbent assay. Finally, we evaluated the specificity and sensitivity of the candidate marker with receiver operating characteristic (ROC) curve methods.

Results

The proteomic findings identified 294 proteins, 40 of which displayed quantitative changes unique to RIA, 13 to UIA, and 20 to IA. One of these proteins, receptor tyrosine kinase Axl, was significantly increased in RIA, as confirmed in CSF from the discovery cohort as well as in CSF and plasma from the validation cohort (p <0.05). Spearman’s correlation analysis revealed that the CSF and plasma Axl levels were strongly correlated (r = 0.93, p <0.0001). The ROC curve indicated an optimal CSF Axl threshold of 0.12 nM for discriminating RIA from UIA with corresponding sensitivity/specificity of 73.33%/90% and an area under the curve (AUC) of 0.89 (95% CI: 0.80-0.97, p < 0.0001). The optimal threshold for plasma Axl was 1.7 nM with corresponding sensitivity/specificity of 50%/80% and an AUC of 0.71 (95% CI: 0.54-0.87, p = 0.027).

Conclusions

Both CSF and plasma Axl levels are significantly elevated in RIA patients. Axl might serve as a promising biomarker to predict the rupture of IA.

Electronic supplementary material

The online version of this article (doi:10.1186/s12883-015-0282-8) contains supplementary material, which is available to authorized users.

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.

Immune modulation of vascular resident cells by Axl orchestrates carotid intima-media thickening

Cellular mechanisms of carotid intima-media thickening (IMT) are largely unknown. The receptor tyrosine kinase Axl is essential for function of both bone marrow (BM) and non-BM cells. We studied the mechanisms by which Axl expression in BM-derived cells (compared with non-BM-derived cells) mediates carotid IMT. Partial ligation of the left carotid artery resulted in a similar carotid blood flow reduction in Axl chimeras. Neither irradiation nor bone marrow transplantation had any effect on the 40% difference in carotid IMT between Axl genotypes. Axl-dependent survival is very important for intimal leukocytes; however, Axl expression in BM cells contributes to <30% of carotid IMT. Axl in non-BM cells has a greater effect on carotid remodeling. Expression of Axl in non-BM cells is crucial for the up-regulation of several key proinflammatory signals (eg, IL-1) in the carotid. We found that Axl is involved in immune activation of cultured smooth muscle cells and in immune heterogeneity of medial cells (measured by major histocompatibility complex class II) after carotid injury. Finally, a lack of Axl in non-BM cells increased collagen Iα expression, which may play a critical role in carotid remodeling. Our data suggest that Axl contributes to carotid remodeling not only by inhibition of apoptosis but also via regulation of immune heterogeneity of vascular cells, cytokine/chemokine expression, and extracellular matrix remodeling.

Novel tyrosine kinase signaling pathways: implications in vascular remodeling

PURPOSE OF REVIEW

This review summarizes the recent advances in molecular mechanisms by which five classes of receptor tyrosine kinases (RTKs) contribute to vascular remodeling.

RECENT FINDINGS

Recent findings have expanded our knowledge regarding RTK regulation. In particular, G-protein-coupled receptors, mineralocorticoid receptors, mechanical and oxidative stresses transactivate RTKs. These receptors are highly interactive with many downstream targets (including tyrosine kinases and other RTKs) and function as key regulatory nodes in a dynamic signaling network. Interactions between vascular and nonvascular (immune and neuronal) cells are controlled by RTKs in vascular remodeling. Inhibition of RTKs could be an advantageous therapeutic strategy for vascular disorders.

SUMMARY

RTK-dependent signaling is important for regulation of key functions during vascular remodeling. However, current challenges are related to integration of the data on multiple RTKs in vascular pathology.

Axl-dependent signalling: a clinical update

Axl is a receptor tyrosine kinase that was originally cloned from cancer cells. Axl belongs to the TAM (Tyro3, Axl and Mertk) family of receptor tyrosine kinases. Gas6 (growth-arrest-specific protein 6) is a ligand for Axl. Activation of Axl protects cells from apoptosis, and increases migration, aggregation and growth through multiple downstream pathways. Up-regulation of the Gas6/Axl pathway is more evident in pathological conditions compared with normal physiology. Recent advances in Axl receptor biology are summarized in the present review. The emphasis is given to translational aspects of Axl-dependent signalling under pathological conditions. In particular, inhibition of Axl reduces tumorigenesis and prevents metastasis as well. Axl-dependent signals are important for the progression of cardiovascular diseases. In contrast, deficiency of Axl in innate immune cells contributes to the pathogenesis of autoimmune disorders. Current challenges in Axl biology are related to the functional interactions of Axl with other members of the TAM family or other tyrosine kinases, mechanisms of ligand-independent activation, inactivation of the receptor and cell-cell interactions (with respect to immune cells) in chronic diseases.

The novel receptor tyrosine kinase Axl is constitutively active in B-cell chronic lymphocytic leukemia and acts as a docking site of nonreceptor kinases: implications for therapy

Recently, we detected that chronic lymphocytic leukemia (CLL) B-cell-derived microvesicles in CLL plasma carry a constitutively phosphorylated novel receptor tyrosine kinase (RTK), Axl, indicating that Axl was acquired from the leukemic B cells. To examine Axl status in CLL, we determined the expression of phosphorylated-Axl (P-Axl) in freshly isolated CLL B cells by Western blot analysis. We detected differential levels of P-Axl in CLL B cells, and further analysis showed that expression of P-Axl was correlated with the other constitutively phosphorylated kinases, including Lyn, phosphoinositide-3 kinase, SyK/ζ-associated protein of 70 kDa, phospholipase C γ2 in CLL B cells. We found that these intracellular signaling molecules were complexed with P-Axl in primary CLL B cells. When Axl and Src kinases were targeted by a Src/Abl kinase inhibitor, bosutinib (SKI-606), or a specific-inhibitor of Axl (R428), robust induction of CLL B-cell apoptosis was observed in both a dose- and time-dependent manner. Therefore, we have identified a novel RTK in CLL B cells which appears to work as a docking site for multiple non-RTKs and drives leukemic cell survival signals. These findings highlight a unique target for CLL treatment.