TYRO3, AXL, And MERTK Research Articles

The protective role of Mertk in JEV-induced encephalitis by maintaining the integrity of blood–brain barrier

Japanese encephalitis is an acute infectious disease of the central nervous system caused by neurotropic Japanese encephalitis virus (JEV). As a member of TAM (Tyro3, Axl and Mertk) family, Mertk has involved in multiple biological processes by engaging with its bridging ligands Gas6 and Protein S, including invasion of pathogens, phagocytosis of apoptotic cells, inflammatory response regulation, and the maintenance of blood brain barrier (BBB) integrity. However, its role in encephalitis caused by JEV infection has not been studied in detail. Here, we found that Mertk−/− mice exhibited higher mortality and more rapid disease progression than wild-type mice after JEV challenge. There were no significant differences in viral load and cytokines expression level in peripheral tissues between Wild type and Mertk−/− mice. Furthermore, the absence of Mertk had little effect on the inflammatory response and immunopathological damage while it can cause an increased viral load in the brain. For the in vitro model of BBB, Mertk was shown to maintain the integrity of the BBB. In vivo, Mertk−/− mice exhibited higher BBB permeability and lower BBB integrity. Taken together, our findings demonstrate that Mertk acts as a protective factor in the development of encephalitis induced by JEV infection, which is mainly associated with its beneficial effect on BBB integrity, rather than its regulation of inflammatory response.

Development of New N-{4-[(7-Chloro-5-methylpyrrolo[2,1-f] [1,2,4]triazin-4-yl)oxy]-3-fluorophenyl}benzenesulfonamide Analogues: Exploring Anticancer Potential through MerTK Inhibition

AbstractMer proto-oncogene tyrosine-protein kinase (MerTK), a part of the TAM (TYRO3, AXL, and MerTK) family, is directly correlated with metastasis and various types of cancers. The inhibition of this receptor is a promising strategy for more-effective chemotherapy. Considering the pharmacophoric features of the active domain of MerTK and the structural characteristics of the investigational drug BMS794833, we designed five new N-{4-[(7-chloro-5-methylpyrrolo[2,1-f][1,2,4]triazin-4-yl)oxy]-3-fluorophenyl}benzenesulfonamide analogues. In cytotoxicity studies, one of the analogues displayed a significantly higher cytotoxicity than cisplatin. It showed IC50 values of 2.09, 1.96, and 3.08 μM against A549, MCF-7, and MDA-MB-231 cancer cell lines, respectively. In drug metabolism and pharmacokinetic studies, it was the most stable analogue and displayed a moderate MerTK inhibitory potential. Molecular-docking studies were performed to corroborate the MerTK inhibition, and the same analogue achieved the most significant docking score (–12.33 kcal/mol). Docking interactions demonstrated that the imine and amine group of the 3-chloropyridine moiety of BMS794833 formed hydrogen bonds with the main chain of the ATP pocket residue Met674, while the oxygen atoms of the 4-oxo-1,4-dihydropyridine-3-carboxamide moiety established hydrogen bonds with the Lys619 and Asp741 amino acid residues of the allosteric pocket of MerTK protein. These promising results provide evidence that the N-{4-[(7-chloro-5-methylpyrrolo[2,1-f][1,2,4]triazin-4-yl)oxy]-3-fluorophenyl}benzenesulfonamide pharmacophore can give potential insights into the development of new MerTK inhibitors.

Targeting Tyro3, Axl, and MerTK Receptor Tyrosine Kinases Significantly Sensitizes Triple-Negative Breast Cancer to CDK4/6 Inhibition.

Triple-negative breast cancer (TNBC) is the most aggressive subtype with high metastasis and mortality rates. Given the lack of actionable targets such as ER and HER2, TNBC still remains an unmet therapeutic challenge. Despite harboring high CDK4/6 expression levels, the efficacy of CDK4/6 inhibition in TNBC has been limited due to the emergence of resistance. The resistance to CDK4/6 inhibition is mainly mediated by RB1 inactivation. Since our aim is to overcome resistance to CDK4/6 inhibition, in this study, we primarily used the cell lines that do not express RB1. Following a screening for activated receptor tyrosine kinases (RTKs) upon CDK4/6 inhibition, we identified the TAM (Tyro3, Axl, and MerTK) RTKs as a crucial therapeutic vulnerability in TNBC. We show that targeting the TAM receptors with a novel inhibitor, sitravatinib, significantly sensitizes TNBC to CDK4/6 inhibitors. Upon prolonged HER2 inhibitor treatment, HER2+ breast cancers suppress HER2 expression, physiologically transforming into TNBC-like cells. We further show that the combined treatment is highly effective against drug-resistant HER2+ breast cancer as well. Following quantitative proteomics and RNA-seq data analysis, we extended our study into the immunophenotyping of TNBC. Given the roles of the TAM receptors in promoting the creation of an immunosuppressive tumor microenvironment (TME), we further demonstrate that the combination of CDK4/6 inhibitor abemaciclib and sitravatinib modifies the immune landscape of TNBC to favor immune checkpoint blockade. Overall, our study offers a novel and highly effective combination therapy against TNBC and potentially treatment-resistant HER2+ breast cancer that can be rapidly moved to the clinic.

The therapeutic effect of GAS6 in remyelination is dependent upon Tyro3.

Multiple sclerosis is an autoimmune disease of the central nervous system (CNS) characterized by demyelination, axonal damage and, for the majority of people, a decline in neurological function in the long-term. Remyelination could assist in the protection of axons and their functional recovery, but such therapies are not, as yet, available. The TAM (Tyro3, Axl, and MERTK) receptor ligand GAS6 potentiates myelination in vitro and promotes recovery in pre-clinical models of MS. However, it has remained unclear which TAM receptor is responsible for transducing this effect and whether post-translational modification of GAS6 is required. In this study, we show that the promotion of myelination requires post-translational modification of the GLA domain of GAS6 via vitamin K-dependent γ-carboxylation. We also confirmed that the intracerebroventricular provision of GAS6 for 2 weeks to demyelinated wild-type (WT) mice challenged with cuprizone increased the density of myelinated axons in the corpus callosum by over 2-fold compared with vehicle control. Conversely, the provision of GAS6 to Tyro3 KO mice did not significantly improve the density of myelinated axons. The improvement in remyelination following the provision of GAS6 to WT mice was also accompanied by an increased density of CC1+ve mature oligodendrocytes compared with vehicle control, whereas this improvement was not observed in the absence of Tyro3. This effect occurs independent of any influence on microglial activation. This work therefore establishes that the remyelinative activity of GAS6 is dependent on Tyro3 and includes potentiation of oligodendrocyte numbers.

GAS6/TAM Axis as Therapeutic Target in Liver Diseases.

TAM (TYRO3, AXL, and MERTK) protein tyrosine kinase membrane receptors and their vitamin K-dependent ligands GAS6 and protein S (PROS) are well-known players in tumor biology and autoimmune diseases. In contrast, TAM regulation of fibrogenesis and the inflammation mechanisms underlying metabolic dysfunction-associated steatohepatitis (MASH), cirrhosis, and, ultimately, liver cancer has recently been revealed. GAS6 and PROS binding to phosphatidylserine exposed in outer membranes of apoptotic cells links TAMs, particularly MERTK, with hepatocellular damage. In addition, AXL and MERTK regulate the development of liver fibrosis and inflammation in chronic liver diseases. Acute hepatic injury is also mediated by the TAM system, as recent data regarding acetaminophen toxicity and acute-on-chronic liver failure have uncovered. Soluble TAM-related proteins, mainly released from activated macrophages and hepatic stellate cells after hepatic deterioration, are proposed as early serum markers for disease progression. In conclusion, the TAM system is becoming an interesting pharmacological target in liver pathology and a focus of future biomedical research in this field.

From MASH to HCC: the role of Gas6/TAM receptors.

Metabolic dysfunction-associated steatohepatitis (MASH) is the replacement term for what used to be called nonalcoholic steatohepatitis (NASH). It is characterized by inflammation and injury of the liver in the presence of cardiometabolic risk factors and may eventually result in the development of hepatocellular carcinoma (HCC), the most common form of primary liver cancer. Several pathogenic mechanisms are involved in the transition from MASH to HCC, encompassing metabolic injury, inflammation, immune dysregulation and fibrosis. In this context, Gas6 (Growth Arrest-Specific 6) and TAM (Tyro3, Axl, and MerTK) receptors may play important roles. The Gas6/TAM family is involved in the modulation of inflammation, lipid metabolism, fibrosis, tumor progression and metastasis, processes which play an important role in the pathophysiology of acute and chronic liver diseases. In this review, we discuss MASH-associated HCC and the potential involvement of the Gas6/TAM system in disease development and progression. In addition, since therapeutic strategies for MASH and HCC are limited, we also speculate regarding possible future treatments involving the targeting of Gas6 or TAM receptors.

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

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

Repurposing Axl Kinase Inhibitors for the Treatment of Respiratory Syncytial Virus Infection.

Respiratory syncytial virus (RSV) infection persists as a common pathogen of pulmonary infection in infants and in the elderly with high morbidity and mortality. However, no specific therapeutics are available. Axl, a member of the TAM (Tyro3, Axl, and Mertk) family receptor kinases, is a pleiotropic inhibitor of the innate immune response and functions as a negative regulator of interferon pathway activation. In this report, we investigated Axl inhibitors for their effects against RSV infection. Axl inhibition with kinase inhibitors, including BMS-777607, R428, and TP-0903, or Axl ablation resulted in a significant reduction of RSV infection in cell-based assays. In an animal model of pulmonary RSV infection, treatment with BMS-777607, R428, or TP-0903 ameliorated pulmonary pathology with a significant reduction of RSV titers in the lung tissues and, consequently, decreased the expression of proinflammatory genes. The host promotes ISG expression for the antiviral response and for viral clearance. We found that Axl inhibition led to more robust IFN-β expression and antiviral gene induction. Thus, the results of this study imply that Axl kinase inhibitors may possess a broad spectrum of antiviral effects by promoting ISG expression.

Gas6 Promotes Microglia Efferocytosis and Suppresses Inflammation Through Activating Axl/Rac1 Signaling in Subarachnoid Hemorrhage Mice.

Early brain injury (EBI) following subarachnoid hemorrhage (SAH) is characterized byrapid development of neuron apoptosis and dysregulated inflammatory response. Microglia efferocytosis plays a critical role in the clearance of apoptotic cells, attenuation of inflammation, and minimizing brain injury in various pathological conditions. Here, using a mouse SAH model, we aim to investigate whether microglia efferocytosis is involved in post-SAH inflammation and to determine the underlying signaling pathway. We hypothesized that TAM receptors and their ligands regulate this process. To prove our hypothesis, the expression and cellular location of TAM (Tyro3, Axl, and Mertk) receptors and their ligands growth arrest-specific 6 (Gas6) and Protein S (ProS1) were examined by PCR, western blots, and fluorescence immunostaining. Thirty minutes after SAH, mice received an intraventricular injection of recombinant Gas6 (rGas6) or recombinant ProS1 (rPros1) and underwent evaluations of inflammatory mediator expression, neurological deficits, and blood-brain barrier integrity at 24h. Microglia efferocytosis of apoptotic neurons was analyzed in vivo and in vitro. The potential mechanism was determined by inhibiting or knocking down TAM receptors and Rac1 by specific inhibitors or siRNA. SAH induced upregulation of Axl and its ligand Gas6. The administration of rGas6 but not rPros1 promoted microglia efferocytosis, alleviated inflammation, and ameliorated SAH-induced BBB breakdown and neurological deficits. The beneficial effects of rGas6 were arrogated by inhibiting or knocking down Axl and Rac1. We concluded that rGas6 attenuated the development of early brain injury in mice after SAH by facilitating microglia efferocytosis and preventing inflammatory response, which is partly dependent on activation of Axl and Rac1.

A cathelicidin antimicrobial peptide from Hydrophis cyanocinctus inhibits Zika virus infection by downregulating expression of a viral entry factor

Zika virus (ZIKV) is a re-emerging flavivirus that causes conditions such as microcephaly and testis damage. The spread of ZIKV has become a major public health concern. Recent studies indicated that antimicrobial peptides are an ideal source for screening antiviral candidates with broad-spectrum antiviral activities, including against ZIKV. We herein found that Hc-CATH, a cathelicidin antimicrobial peptide identified from the sea snake Hydrophis cyanocinctus in our previous work, conferred protection against ZIKV infection in host cells and showed preventative efficacy and therapeutic efficacy in C57BL/6J mice, Ifnar1−/− mice, and pregnant mice. Intriguingly, we revealed that Hc-CATH decreased the susceptibility of host cells to ZIKV by downregulating expression of AXL, a TAM (TYRO3, AXL and MERTK) family kinase receptor that mediates ZIKV infection, and subsequently reversed the negative regulation of AXL on host’s type I interferon response. Furthermore, we showed that the cyclo-oxygenase-2/prostaglandin E2/adenylyl cyclase/protein kinase A pathway was involved in Hc-CATH-mediated AXL downregulation, and Hc-CATH in addition directly inactivated ZIKV particles by disrupting viral membrane. Finally, while we found Hc-CATH did not act on the late stage of ZIKV infection, structure–function relationship studies revealed that α-helix and phenylalanine residues are key structural requirements for its protective efficacy against initial ZIKV infection. In summary, we demonstrate that Hc-CATH provides prophylactic and therapeutic efficacy against ZIKV infection via downregulation of AXL, as well as inactivating the virion. Our findings reveal a novel mechanism of cathelicidin against viral infection and highlight the potential of Hc-CATH to prevent and treat ZIKV infection.

Anti-inflammatory clearance of amyloid-β by a chimeric Gas6 fusion protein.

Clearing amyloid-β (Aβ) through immunotherapy is one of the most promising therapeutic approaches to Alzheimer's disease (AD). Although several monoclonal antibodies against Aβ have been shown to substantially reduce Aβ burden in patients with AD, their effects on improving cognitive function remain marginal. In addition, a significant portion of patients treated with Aβ-targeting antibodies experience brain edema and microhemorrhage associated with antibody-mediated Fc receptor activation in the brain. Here, we develop a phagocytosis inducer for Aβ consisting of a single-chain variable fragment of an Aβ-targeting monoclonal antibody fused with a truncated receptor binding domain of growth arrest-specific 6 (Gas6), a bridging molecule for the clearance of dead cells via TAM (TYRO3, AXL, and MERTK) receptors. This chimeric fusion protein (αAβ-Gas6) selectively eliminates Aβ plaques through TAM receptor-dependent phagocytosis without inducing NF-kB-mediated inflammatory responses or reactive gliosis. Furthermore, αAβ-Gas6 can induce synergistic clearance of Aβ by activating both microglial and astrocytic phagocytosis, resulting in better behavioral outcomes with substantially reduced synapse elimination and microhemorrhage in AD and cerebral amyloid angiopathy model mice compared with Aβ antibody treatment. Our results suggest that αAβ-Gas6 could be a novel immunotherapeutic agent for AD that overcomes the side effects of conventional antibody therapy.

Neuroprotective Effects of Omentin-1 Against Cerebral Hypoxia/Reoxygenation Injury via Activating GAS6/Axl Signaling Pathway in Neuroblastoma Cells.

Ischemic stroke is characterized by insufficient blood supply to brain tissue and is associated with increased morbidity and mortality in adults worldwide. Growth arrest-specific protein 6 (GAS6) is a vitamin K-dependent protein and is widely expressed in the central nervous system. The biological functions of GAS6 are mediated by the interaction with TAM (Tyro3, Axl and Mertk) receptors, including cell survival and proliferation, immune regulation and apoptosis. Omentin-1, also known as intelectin-1 (ITLN-1), is a novel adipocytokine that is involved in a variety of biological events, such as insulin resistance, endothelial dysfunction, programmed cell death and metabolic disorders. Our previous study has found that omentin-1 act as a novel regulator of vascular and anti-apoptotic response in cerebral ischemia. However, the specific molecular mechanism of omentin-1’s protective effect on cerebral ischemia-reperfusion injury (IRI) is still unclear. First, the toxicity of recombinant human omentin-1 (rh-omentin) was assessed and a safe concentration was chosen for the next experiments. Then, rh-omentin exerted neuroprotection against hypoxia/reoxygenation (H/R) injury in N2a cells, indicated by increased cell viability, decreased LDH, ROS generation, and cell apoptotic rate. Furthermore, the similar protective effect was observed in omentin-1 overexpression cells constructed by lentivirus transfection. Rh-omentin could also inhibit H/R-induced apoptotic molecules, oxidative stress molecules, and GAS6/Axl signaling molecules which as evidence by increased omentin-1, GAS6, Axl, p-Axl, NQO1, HO-1, Nrf2, Bcl2 and decreased Bax expressions. However, GAS6 siRNA could reverse rh-omentin-induced neuroprotection and the levels of these molecules mentioned above. In conclusion, these findings suggest that omentin-1 treatment exerts neuroprotection against H/R injury partly via activating GAS6/Axl signaling at least. Therefore, these finding may favor omentin-1 a potential neuroprotective drug candidate to alleviate ischemia-reperfusion injury in clinic.

Mertk: An emerging target in cancer biology and immuno-oncology.

Mertk, a type I Receptor Tyrosine Kinase (RTK) and member of the TAM (Tyro3, Axl, and Mertk) family of homologous tyrosine kinases, has important roles in signal transduction both homeostatically on normal cells as well as patho-physiologically on both tumor-associated macrophages and malignant cells by its overexpression in a wide array of cancers. The main ligands of Mertk are Vitamin K-modified endogenous proteins Gas6 and Protein S (ProS1), heterobifunctional modular proteins that bind Mertk via two carboxyl-terminal laminin-like globular (LG) domains, and an N-terminal Gla domain that binds anionic phospholipids, whereby externalized phosphatidylserine (PS) on stressed viable and caspase-activated apoptotic cells is most emblematic. Recent studies indicate that Vitamin K-dependent γ-carboxylation on the N-terminal Gla domain of Gas6 and Protein S is necessary for PS binding and Mertk activation, implying that Mertk is preferentially active in tissues where there is high externalized PS, such as the tumor microenvironment (TME) and acute virally infected tissues. Once stimulated, activated Mertk can provide a survival advantage for cancer cells as well as drive compensatory proliferation. On monocytes and tumor-associated macrophages, Mertk promotes efferocytosis and acts as an inhibitory receptor that impairs host anti-tumor immunity, functioning akin to a myeloid checkpoint inhibitor. In recent years, inhibition of Mertk has been implicated in a dual role to enhance the sensitivity of cancer cells to cytotoxic agents along with improving host anti-tumor immunity with anti-PD-1/PD-L1 immunotherapy. Here, we examine the rationale of Mertk-targeted immunotherapies, the current and potential therapeutic strategies, the clinical status of Mertk-specific therapies, and potential challenges and obstacles for Mertk-focused therapies.

Accelerating AXL targeting for TNBC therapy.

The tyrosine kinase receptor AXL of the TAM (TYRO3, AXL and MERTK) family is considered as a promising therapeutic target for different hematological cancers and solid tumors. AXL is involved in multiple pro-​tumorigenic processes including cell migration, invasion, epithelial-mesenchymal transition (EMT), and stemness, and recent studies demonstrated its impact on cancer metastasis and drug resistance. Extensive studies on AXL have highlighted its unique characteristics and physiological functions and suggest that targeting of AXL could be beneficial in combination with chemotherapy, radiotherapy, immunotherapy, and targeted therapy. In this mini review, we discuss possible outcomes of AXL targeting either alone or together with other therapeutic agents and emphasize its impact on triple negative breast cancer (TNBC).

Abstract 941: TAM receptors as potential therapeutic targets in NF2-related schwannomas and meningiomas

Abstract Background: Mutations in the NF2 gene, which codes for the tumor suppressor Merlin, is responsible for the development of all Neurofibromatosis Type 2 (NF2)-related tumors including schwannomas and meningiomas. These tumors can also occur spontaneously in non-NF2 patients. The only available treatments for this group of tumors are surgery and radiosurgery/radiotherapy. Therefore, in a search of effective drug-based treatments for NF2-mutated schwannomas and meningiomas, we are investigating TYRO3, AXL and MERTK (TAM) receptors as new therapeutic targets. Methods: The expression and activation of TAM receptors were investigated using human Merlin-deficient meningioma and schwannoma tissues compared to control by Western blotting. Infiltration of tumor associated macrophages and their correlation to TAM family receptors were investigated using immunofluorescence double staining and flow cytometry. To understand the role of TAM receptors in the pathogenesis of schwannomas and meningiomas, in vitro shRNA knock-down and pharmacological inhibition analysis were conducted using patient-derived schwannoma and meningioma cells. Results: The results revealed overexpression and hyper-activation of AXL, MERTK, TYRO3 in human Merlin-deficient meningiomas and schwannomas. Meningiomas and schwannomas show a varied level of infiltration by M2 macrophages (CD163+) and TAM receptors are expressed on these CD163+ macrophages in addition to the tumor cells. The expression of MERTK correlates to the level of infiltrated CD163+ macrophages. MERTK forms a complex with TYRO3 in both meningioma and schwannoma tissues, and its expression is mandatory to maintain AXL and TYRO3 levels in both cell types. MERTK and AXL contribute to the increased proliferation and survival of schwannoma and meningioma primary cells in vitro. Pathological proliferation and survival of both cell types were successfully reversed by AXL inhibitor BGB324 (BerGenBio, FDA approved for AML) and MERTK inhibitor UNC2025 (preclinical) in vitro, with UNC2025 being more effective. TYRO3 did not affect the proliferation of either schwannoma or meningioma primary cells. Conclusion: Our findings suggest that TAM receptors are aberrantly expressed and activated in human Merlin-deficient meningiomas and schwannomas. Infiltration of M2 macrophages may contribute to an overall increase in the expression of TAM receptors in these tumors. AXL and MERTK are important in schwannoma and meningioma pathogenesis and are potentially good therapeutic targets. MERTK inhibitor UNC2025 has the potential to be used as a common candidate for the treatment of NF2-related tumors. Citation Format: Foram Dave, Sylwia Ammoun, C Oliver Hanemann. TAM receptors as potential therapeutic targets in NF2-related schwannomas and meningiomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 941.

Myeloid cell-derived PROS1 inhibits tumor metastasis by regulating inflammatory and immune responses via IL-10.

Stimulation of TAM (TYRO3, AXL, and MERTK) receptor tyrosine kinases promotes tumor progression through numerous cellular mechanisms. TAM cognate ligands GAS6 and PROS1 (for TYRO3 and MERTK) are secreted by host immune cells, an interaction which may support tumor progression. Here, we revealed an unexpected antimetastatic role for myeloid-derived PROS1: suppressing metastatic potential in lung and breast tumor models. Pros1 deletion in myeloid cells led to increased lung metastasis, independent of primary tumor infiltration. PROS1-cKO bone marrow-derived macrophages (BMDMs) led to elevated TNF-α, IL-6, Nos2, and IL-10 via modulation of the Socs3/NF-κB pathway. Conditioned medium from cKO BMDMs enhanced EMT, ERK, AKT, and STAT3 activation within tumor cells and promoted IL-10-dependent invasion and survival. Macrophages isolated from metastatic lungs modulated T cell proliferation and function, as well as expression of costimulatory molecules on DCs in a PROS1-dependent manner. Inhibition of MERTK kinase activity blocked PROS1-mediated suppression of TNF-α and IL-6 but not IL-10. Overall, using lung and breast cancer models, we identified the PROS1/MERTK axis within BMDMs as a potent regulator of adaptive immune responses with a potential to suppress metastatic seeding and revealed IL-10 regulation by PROS1 to deviate from that of TNF-α and IL-6.

TAM Receptor Inhibition–Implications for Cancer and the Immune System

Simple SummaryTAM receptors are a family of receptor tyrosine kinases, comprising Tyro3, Axl and MerTK. Their primary role is in digestion of dying cells by macrophages without alarming the immune system. TAM receptors are also expressed by cancer cells in which signaling is oncogenic, and for this reason there is growing interest and research into TAM inhibition. This approach to cancer treatment may, however, come into conflict with beneficial and costimulatory TAM receptor signaling in T cells and natural killer (NK) cells. The aim of this review is to explore in detail the effects of TAM receptor inhibition on cancer cells and immune cells, and how the ramifications of this inhibition may affect cancer treatment in humans.Tyro3, Axl and MerTK (TAM) receptors are receptor tyrosine kinases which play important roles in efferocytosis and in the balancing of immune responses and inflammation. TAM receptor activation is induced upon binding of the ligands protein S (Pros1) or growth arrest-specific protein 6 (Gas6) which act as bridging molecules for binding of phosphatidyl serine (PtdSer) exposed on apoptotic cell membranes. Upon clearance of apoptotic cell material, TAM receptor activation on innate cells suppresses proinflammatory functions, thereby ensuring the immunologically silent removal of apoptotic material in the absence of deleterious immune responses. However, in T cells, MerTK signaling is costimulatory and promotes activation and functional output of the cell. MerTK and Axl are also aberrantly expressed in a range of both hematological and solid tumor malignancies, including breast, lung, melanoma and acute myeloid leukemia, where they have a role in oncogenic signaling. Consequently, TAM receptors are being investigated as therapeutic targets using small molecule inhibitors and have already demonstrated efficacy in mouse tumor models. Thus, inhibition of TAM signaling in cancer cells could have therapeutic value but given the opposing roles of TAM signaling in innate cells and T cells, TAM inhibition could also jeopardize anticancer immune responses. This conflict is discussed in this review, describing the effects of TAM inhibition on cancer cells as well as immune cells, while also examining the intricate interplay of cancer and immune cells in the tumor microenvironment.

AXL Inhibition Induces DNA Damage and Replication Stress in Non-Small Cell Lung Cancer Cells and Promotes Sensitivity to ATR Inhibitors.

AXL, a TAM (TYRO3, AXL, and MERTK) family receptor tyrosine kinase, is increasingly being recognized as a key determinant of resistance to targeted therapies, as well as chemotherapy and radiation in non-small cell lung cancer (NSCLC) and other cancers. We further show here that high levels of AXL and epithelial-to-mesenchymal transition were frequently expressed in subsets of both treatment-naïve and treatment-relapsed NSCLC. Previously, we and others have demonstrated a role for AXL in mediating DNA damage response (DDR), as well as resistance to inhibition of WEE1, a replication stress response kinase. Here, we show that BGB324 (bemcentinib), a selective small-molecule AXL inhibitor, caused DNA damage and induced replication stress, indicated by ATR/CHK1 phosphorylation, more significantly in TP53-deficient NSCLC cell lines. Similar effects were also observed in large-cell neuroendocrine carcinoma (LCNEC) cell lines. High AXL protein levels were also associated with resistance to ATR inhibition. Combined inhibition of AXL and ATR significantly decreased cell proliferation of NSCLC and LCNEC cell lines. Mechanistically, combined inhibition of AXL and ATR significantly increased RPA32 hyperphosphorylation and DNA double-strand breaks and induced markers of mitotic catastrophe. Notably, NSCLC cell lines with low levels of SLFN11, a known predictive biomarker for platinum and PARP inhibitor sensitivity, were more sensitive to AXL/ATR cotargeting. These findings demonstrate a novel and unexpected role for AXL in replication stress tolerance, with potential therapeutic implications. IMPLICATIONS: These findings demonstrate that the combination of AXL and ATR inhibitors could be a promising therapeutic combination for NSCLC, LCNEC, and other cancers.

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

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

Recent advancements in role of TAM receptors on efferocytosis, viral infection, autoimmunity, and tissue repair.

Evolutionarily conserved highly regulated process of apoptosis has been a major physiological process throughout the entire evolutionary history of living beings that has impacted the process of evolution itself. One of the key features of this highly researched field of science is the process of phosphatidylserine (PS) externalization by the different membrane bound enzymes. The process is a result of series of biological events and is associated with various biological outcomes depending on the proper recognition of this ligand. In this review, we will briefly summarize the recent advancement in the field pertaining to the set of receptors, known as TAM (Tyro3, Axl and Mertk) receptors, for their influence in the recognition of various PS externalization events and mediation of pathological outcomes such as autoimmunity, cancer, and tissue repair.