Mer Receptor Tyrosine Kinase Research Articles

The expression and clinical significance of different forms of Mer receptor tyrosine kinase in systemic lupus erythematosus.

Objective. To investigate the expression and clinical significance of trans-membrane MerTK (mMer) on circulating CD14+ monocytes/macrophages and soluble MerTK (sMer) levels in plasma in systemic lupus erythematosus (SLE). Method. 108 SLE patients and 42 healthy controls were recruited in this study. The expression of mMer on the surfaces of CD14+ monocytes/macrophages was evaluated by flow cytometry (FCM). The sMer levels were measured by ELISA. Real-time quantitative PCR was applied to evaluate the mRNA levels of MerTK and ADAM17. Results. Both mMer expression on CD14+ monocytes/macrophages and sMer levels in plasma significantly increased in SLE patients compared to healthy subjects. The frequency of anti-inflammatory MerTK expressing CD14+CD16+ monocytes decreased in SLE. mMer expression was positively correlated with CD163 expression on CD14+ cells. Both the mMer expression on CD14+ monocytes/macrophages and sMer levels in plasma were positively correlated with SLEDAI. Furthermore, more elevated mMer and sMer levels were found in patients with higher SLEDAI, presence of anti-SSA, anti-Sm autoantibodies, and lupus nephritis. Conclusion. Both mMer and sMer levels significantly increased in SLE and positively correlated with disease activity and severity. The upregulation of MerTK expression may serve as a biomarker of the disease activity and severity of SLE.

Pseudo-cyclization through intramolecular hydrogen bond enables discovery of pyridine substituted pyrimidines as new Mer kinase inhibitors.

Abnormal activation or overexpression of Mer receptor tyrosine kinase has been implicated in survival signaling and chemoresistance in many human cancers. Consequently, Mer is a promising novel cancer therapeutic target. A structure-based drug design approach using a pseudo-ring replacement strategy was developed and validated to discover a new family of pyridinepyrimidine analogues as potent Mer inhibitors. Through SAR studies, 10 (UNC2250) was identified as the lead compound for further investigation based on high selectivity against other kinases and good pharmacokinetic properties. When applied to live cells, 10 inhibited steady-state phosphorylation of endogenous Mer with an IC50 of 9.8 nM and blocked ligand-stimulated activation of a chimeric EGFR-Mer protein. Treatment with 10 also resulted in decreased colony-forming potential in rhabdoid and NSCLC tumor cells, thereby demonstrating functional antitumor activity. The results provide a rationale for further investigation of this compound for therapeutic application in patients with cancer.

Mer Receptor Tyrosine Kinase Is a Novel Therapeutic Target In Multiple Myeloma

IntroductionAlthough many novel therapeutic agents have been explored, multiple myeloma (MM) remains an incurable hematopoietic malignancy. Mer receptor tyrosine kinase (Mer TK) is a member of the TAM (Tyro3, Axl and Mer) family and is involved in the progression of several human malignancies. Inhibition of Mer expression reduces pro-survival signaling, increases chemosensitivity in vitro, and delays tumor progression in vivo. These observations make Mer TK inhibitors to excellent candidates for targeted therapies. We show here for the first time that Mer TK is ectopically expressed in MM cells and test the response of MM cell lines to our previously described small molecule Mer inhibitor, UNC1062, a substituted pyrazolopyrimidine. Here we demonstrate the biochemical and biologic effects of treatment with UNC1062, which suggests efficacy of Mer inhibitors as a novel strategy for the treatment of MM. Methods: Western blot analysis was used to determine expression of the TAM receptors and to evaluate inhibition of Mer TK activation and downstream signaling by UNC1062 in MM cell lines. UNC1062-mediated anti-myeloma activity with or without bortezomib was determined using short-term (MTT) and long-term (colony-formation) assays. Cleaved PARP and caspase 3 were detected by western blot analysis as indicators of apoptosis. Results: Western blot analysis of protein extract from MM cell lines showed that four (RPMI8226, U226, AMO-1 and KMS-12BM) out of six lines (66.6%) express Mer TK protein at varying levels, with AMO-1 expressing the least and U226 expressing the most. The MM cell lines OPM-2 and NCI-H929 did not express Mer TK. UNC1062 potently inhibits Mer TK activity in vitro (Mer IC50= 1.1 nM, Morrison Ki = 0.33 nM). In cell-based assays, treatment with UNC1062 inhibited accumulation of phospho-Mer and downstream signaling through ERK and Stat5 in U226 cells. UNC1062 also reduced proliferation and/or survival in Mer TK-expressing RPMI8226 cells (IC50 = 0.98 ± 0.14 μM, n=7) and U226 (IC50 = 0.28 ± 0.09 μM, n=3) cells. Treatment with UNC1062 did not lead to a meaningful reduction of metabolically active cells in Mer-negative NCI-H929 and OPM-2 cells. In contrast, UNC1062-treated Mer-positive MM cells exhibited increased levels of cleaved caspase 3 and cleaved PARP confirming apoptosis induction. Treatment with UNC1062 (100 nM) resulted in a statistically significant, dose-dependent decrease in colony-formation in methylcellulose compared to control cultures in Mer-positive RPMI8226 cells (418 ± 23 vs. 291 ± 37 colonies, p = 0.04, n = 3). No significant reduction of colony formation was observed in Mer-negative OPM-2 cells. Treatment of U226 cells with UNC1062 (800 nM) in combination with bortezomib resulted in a statistically significant reduction in relative cell number compared with bortezomib alone (0.61 ± 0.06 vs 0.41 ± 0.06, p = 0.04, n =3). Conclusion: Mer TK is ectopically expressed in the majority of investigated MM cell lines. UNC1062 treatment of Mer TK positive MM cells inhibits Mer TK activity and downstream signaling, mediates anti-neoplastic activity in liquid culture, and decreases colony-formation in methylcellulose. In addition, UNC1062 treatment sensitizes MM cells to bortezomib. Taken together, these data support further development of Mer TK inhibitors as novel MM therapy alone and in combination with bortezomib. Disclosures:No relevant conflicts of interest to declare.

Axl Represents a Therapeutic Target In T315I-Mutated and WT Chronic Myeloid Leukemia

Abstract BCR-ABL1 inhibitors have revolutionized treatment of CML patients. However several drawbacks remain, including therapy resistance of T315I-mutated CML and incapability of current drugs to eliminate quiescent CML stem cells warranting development of novel therapies. In addition, drugs with the potential to enhance efficacy of BCR-ABL1 targeting agents could improve treatment of CML patients. Members of the Tyro3, Axl, Mer receptor (TAMR) tyrosine kinase family are abundantly expressed in physiological and malignant hematopoiesis and their ligand Gas6 can support hematopoietic (progenitor) cells. Evidence in the literature indicates that Axl is upregulated upon treatment with imatinib (IM). In this study, we investigated the relevance of the Gas6-Axl axis in CML patients and the therapeutic potential of the clinically applicable small molecule Axl inhibitor BGB324 (former designation R428) in primary CML (stem cell) samples, cell lines and preclinical models. In a first step we quantified Axl-expressing cells by flow cytometry in chronic phase (CP) CML bone marrow at primary diagnosis and healthy bone marrow donors. Here, we found higher numbers of Axl-positive cells in CML bone marrow compared to controls (11.42±0.42% (n=5) vs. 0.65±0.10% (n=6), respectively; p=0.0015). In addition, we determined Gas6 plasma levels by ELISA in healthy controls and CML patients in CP and blast crisis (BC). These analyses revealed that Gas6 plasma levels were upregulated in a stage specific manner (plasma levels of Gas6: healthy controls 1290±684 pg/ml (n=14), CP 3465±405 pg/ml (n=50), BC 10940±3868 pg/ml (n=7); p=0.0001). Thus, the Gas6-Axl axis represents a potential therapeutic target in CML patients. Based on this finding we analyzed efficacy of BGB324 in Axl-expressing BV173, KCL22, K562, BaF3_BCR-ABL1-wt and BaF3_BCR-ABL1-T315I cell lines in vitro. We found inhibition of proliferation in all analyzed cell lines with IC50 values ranging from 500-3000 nM. Combination experiments with the IC50 dose of BGB324 and IM revealed additive effects of both treatments in all cell lines. Dose finding experiments with BGB324 in sorted CD34+ primary CML cells grown in the presence of physiological growth factors yielded a mean IC50 of 1.1 ± 0.3 mM (n=3), which was similar to nilotinib. BGB324 did not accumulate CD34+CFSEmax (undivided) cells any more than nilotinib but enhanced apoptosis of CD34+ cells in combination with nilotinib. Notably, there was a consistent inhibitory effect of 3 mM BGB324 alone or in combination with nilotinib against colony forming cells (CFC) with the more primitive BFU-E and GEMM being most sensitive to inhibition (n=4). Interestingly, the Ph- lymphocytes (confirmed by FISH) sorted simultaneously with Ph+ CD34+ cells from the same CML patient were unaffected in terms of viability when treated with Axl inhibitor; thus the BGB324’s activity is cell context specific. Encouraged by these data we analyzed efficacy of BGB324 in an aggressive preclinical CML model in which bone marrow cells were retrovirally transduced with constructs containing T315I-mutated BCR-ABL1. Transduced cells were subsequently i.v. transplanted into sublethally irradiated recipient mice, who rapidly developed blast crisis CML. Mice were treated with 25 mg BGB324 or vehicle BID by oral gavage starting from day 3 after transplantation when homing of transduced cells to the bone marrow is completed. In this model we found a significant prolongation of survival upon treatment with BGB324 (Figure 1). Analyses of the leukemia phenotype by differential blood counts and flow cytometry revealed that BGB324 reduced leukemia cell burden (WBC 138±20x103/µl (n=14) vs. 60x103±21 k/µl WBC in the BGB324 treated group (n=13); p=0.0078). Taken together, these data suggest that the Gas6-Axl axis represents a therapeutic target and that BGB324 is a potent molecule effective against T315I-mutated and wt CML alone and in combination with TKI. Furthermore, BGB324 induces apoptosis of quiescent Ph+ CML stem/progenitor cells. Thus BGB324 might open up novel therapeutic avenues in CML patients. Disclosures: Loges: BerGenBio: research support Other.

UNC569, a novel small-molecule mer inhibitor with efficacy against acute lymphoblastic leukemia in vitro and in vivo.

Acute lymphoblastic leukemia (ALL) is the most common malignancy in children. Although survival rates have improved, patients with certain biologic subtypes still have suboptimal outcomes. Current chemotherapeutic regimens are associated with short- and long-term toxicities and novel, less toxic therapeutic strategies are needed. Mer receptor tyrosine kinase is ectopically expressed in ALL patient samples and cell lines. Inhibition of Mer expression reduces prosurvival signaling, increases chemosensitivity, and delays development of leukemia in vivo, suggesting that Mer tyrosine kinase inhibitors are excellent candidates for targeted therapies. Brain and spinal tumors are the second most common malignancies in childhood. Multiple chemotherapy approaches and radiotherapies have been attempted, yet overall survival remains dismal. Mer is also abnormally expressed in atypical teratoid/rhabdoid tumors (AT/RT), providing a rationale for targeting Mer as a therapeutic strategy. We have previously described UNC569, the first small-molecule Mer inhibitor. This article describes the biochemical and biologic effects of UNC569 in ALL and AT/RT. UNC569 inhibited Mer activation and downstream signaling through ERK1/2 and AKT, determined by Western blot analysis. Treatment with UNC569 reduced proliferation/survival in liquid culture, decreased colony formation in methylcellulose/soft agar, and increased sensitivity to cytotoxic chemotherapies. MYC transgenic zebrafish with T-ALL were treated with UNC569 (4 μmol/L for two weeks). Fluorescence was quantified as indicator of the distribution of lymphoblasts, which express Mer and enhanced GFP. UNC569 induced more than 50% reduction in tumor burden compared with vehicle- and mock-treated fish. These data support further development of Mer inhibitors as effective therapies in ALL and AT/RT.

Axl, a prognostic and therapeutic target in acute myeloid leukemia mediates paracrine crosstalk of leukemia cells with bone marrow stroma

AbstractAcute myeloid leukemia (AML) represents a clonal disease of hematopoietic progenitors characterized by acquired heterogenous genetic changes that alter normal mechanisms of proliferation, self-renewal, and differentiation.1 Although 40% to 45% of patients younger than 65 years of age can be cured with current therapies, only 10% of older patients reach long-term survival.1 Because only very few novel AML drugs were approved in the past 2 decades, there is an urgent need to identify novel targets and therapeutic strategies to treat underserved AML patients. We report here that Axl, a member of the Tyro3, Axl, Mer receptor tyrosine kinase family,2-4 represents an independent prognostic marker and therapeutic target in AML. AML cells induce expression and secretion of the Axl ligand growth arrest–specific gene 6 (Gas6) by bone marrow–derived stromal cells (BMDSCs). Gas6 in turn mediates proliferation, survival, and chemoresistance of Axl-expressing AML cells. This Gas6-Axl paracrine axis between AML cells and BMDSCs establishes a chemoprotective tumor cell niche that can be abrogated by Axl-targeting approaches. Axl inhibition is active in FLT3-mutated and FLT3 wild-type AML, improves clinically relevant end points, and its efficacy depends on presence of Gas6 and Axl. Axl inhibition alone or in combination with chemotherapy might represent a novel therapeutic avenue for AML.

Mer receptor tyrosine kinase is a therapeutic target in pre–B-cell acute lymphoblastic leukemia

AbstractAcute lymphoblastic leukemia (ALL) is currently treated with an intense regimen of chemotherapy yielding cure rates near 85%. However, alterations to treatment strategies using available drugs are unlikely to provide significant improvement in survival or decrease therapy-associated toxicities. Here, we report ectopic expression of the Mer receptor tyrosine kinase in pre–B-cell ALL (B-ALL) cell lines and pediatric patient samples. Inhibition of Mer in B-ALL cell lines decreased activation of AKT and MAPKs and led to transcriptional changes, including decreased expression of antiapoptotic PRKCB gene and increase in proapoptotic BAX and BBC3 genes. Further, Mer inhibition promoted chemosensitization, decreased colony-forming potential in clonogenic assays, and delayed disease onset in a mouse xenograft model of leukemia. Our results identify Mer as a potential therapeutic target in B-ALL and suggest that inhibitors of Mer may potentiate lymphoblast killing when used in combination with chemotherapy. This strategy could reduce minimal residual disease and/or allow for chemotherapy dose reduction, thereby leading to improved event-free survival and reduced therapy-associated toxicity for patients with B-ALL. Additionally, Mer is aberrantly expressed in numerous other malignancies suggesting that this approach may have broad applications.

Paradoxical role of the proto-oncogene Axl and Mer receptor tyrosine kinases in colon cancer

The receptor tyrosine kinases Axl and Mer, belonging to the Tyro3, Axl and Mer (TAM) receptor family, are expressed in a number of tumor cells and have well-characterized oncogenic roles. The therapeutic targeting of these kinases is considered an anticancer strategy, and various inhibitors are currently under development. At the same time, Axl and Mer are expressed in dendritic cells and macrophages and have an essential function in limiting inflammation. Inflammation is an enabling characteristic of multiple cancer hallmarks. These contrasting oncogenic and anti-inflammatory functions of Axl and Mer posit a potential paradox in terms of anticancer therapy. Here we demonstrate that azoxymethane (AOM) and dextran sulfate sodium (DSS)-induced inflammation-associated cancer is exacerbated in mice lacking Axl and Mer. Ablation of Axl and Mer signaling is associated with increased production of proinflammatory cytokines and failure to clear apoptotic neutrophils in the intestinal lamina propria, thereby favoring a tumor-promoting environment. Interestingly, loss of these genes in the hematopoietic compartment is not associated with increased colitis. Axl and Mer are expressed in radioresistant intestinal macrophages, and the loss of these genes is associated with an increased inflammatory signature in this compartment. Our results raise the possibility of potential adverse effects of systemic anticancer therapies with Axl and Mer inhibitors, and underscore the importance of understanding their tissue and cell type-specific functions in cancer.

Development of a Spontaneous Liver Disease Resembling Autoimmune Hepatitis in Mice Lacking Tyro3, Axl and Mer Receptor Tyrosine Kinases

Autoimmune hepatitis (AIH) is a severe type of chronic liver disease. The lack of appropriate animal models has resulted in a limited understanding regarding the etiology of AIH. Here, we demonstrated that mice deficient in Tyro3, Axl and Mer (TAM) receptor tyrosine kinases (RTKs) developed persistent inflammatory liver damage resembling AIH. Tyro3−/−Axl−/−Mer−/− triple mutant (TAM−/−) mice exhibited chronic hepatitis, manifested by progressive appearance of interface hepatitis, immune cell infiltrations and elevated inflammatory cytokine levels in the liver. Accordingly, increased levels of transaminases were observed. Moreover, characteristic autoantibodies and high levels of plasma immunoglobulin G for AIH were detected as TAM−/− mice aged. Finally, we provided evidence that the liver damage in TAM−/− mice mainly result from bone marrow-derived cells and could be rescued by transplantation of WT bone marrow cells. Results suggest that TAM RTKs play an important role in maintaining immune tolerance of the liver.

Breakdown of immune homeostasis in the testis of mice lacking Tyro3, Axl and Mer receptor tyrosine kinases

Tyro3, Axl and Mer (TAM) receptor tyrosine kinases triple knockout (TAM(-/-)) mice are male infertile due to impaired spermatogenesis. However, the mechanism by which TAM receptors regulate spermatogenesis remains unclear. In this study, we demonstrate that the testicular immune homeostasis was impaired in TAM(-/-) mice. As development after the onset of sexual maturity, germ cells were progressively degenerated. Macrophages and lymphocytes infiltrated into the testis as TAM(-/-) mice aged. Moreover, the integrity of blood-testis barrier was impaired, and the autoantibodies against germ cell antigens were produced. Major inflammatory cytokines, including tumor necrosis factor-α, interleukin-6 and monocyte chemotactic protein 1 were upregulated in the testis of TAM(-/-) mice, and predominantly located in Sertoli cells (SCs). In vitro assays showed that TAM(-/-) SCs secrete significantly high levels of inflammatory cytokines compared with wild-type SCs after coculture with apoptotic germ cells. These results suggest that TAM receptors are important in the maintenance of the immune homeostasis in the testis.

Apoptotic cell recognition receptors, Tyro3, Axl and Mer, demonstrate distinct patterns of ligand-inducible and ligand-independent receptor activation (P2016)

Abstract Proper recognition and removal of apoptotic cells by phagocytes lead to the active suppression of inflammatory responses and the induction of tolerance. Tyro3, Axl and Mer (TAM) receptor tyrosine kinases recognize apoptotic cells through their ligands, Protein S (ProS) and Growth-Arrest-Specific Gene 6 (GAS6), which in turn interact with phosphatidylserine (PS) exposed on the surface of apoptotic cells. Although TAMs share significant similarity, very little is known about the specificity of interaction between TAMs and their ligands, in the context of apoptotic cells, and about downstream signaling cascades triggered through TAMs. To study ligand-receptor interaction, we generated a series of reporter cell lines expressing chimeric TAM receptors that allowed us to demonstrate that each TAM has a unique pattern of interaction with GAS6 and ProS, which is also differentially affected by the presence of apoptotic cells. We also developed another set of chimeric receptors that triggered ligand-independent activation of the TAM intracellular domains, leading to distinct patterns of phosphorylated signaling molecules, as well as different levels of cytokine induction. Overall, these studies suggest that despite their similarity, Tyro3, Axl and Mer are functionally unique in the recognition of apoptotic cells, activation of downstream signaling cascades and gene expression, and therefore, contribute differently to cell survival and the induction of tolerance during tumorigenesis.

Abstract 3037: Mer receptor tyrosine kinase is a novel therapeutic target in melanoma.

Abstract Metastatic melanoma is one of the most aggressive forms of cutaneous cancers. While recent therapeutic advances have prolonged patient survival, prognosis remains dismal. Mer is a receptor tyrosine kinase with oncogenic properties that is often overexpressed or activated in various malignancies. Using both protein immunohistochemistry and microarray analyses, we demonstrate that Mer expression correlates with disease progression. Mer expression was highest in metastatic melanomas followed by primary melanomas whereas the lowest expression was observed in nevi. In addition, over 50% of melanoma cell lines overexpressed Mer compared to normal human melanocytes, however overexpression did not correlate with mutations in BRAF or RAS. Stimulation of melanoma cells with the Mer ligand Gas6 resulted in activation of several downstream signaling pathways including MAPK/ERK, PI3K/Akt, and Jak/STAT. Mer inhibition via shRNA reduced Mer-mediated downstream signaling, reduced colony formation by up to 59% (p<0.05) and diminished tumor volume by 60% (p<0.05) in a human melanoma xenograft murine model. Treatment of melanoma cells with UNC1062, a novel Mer-selective small molecule tyrosine kinase inhibitor, reduced activation of Mer-mediated downstream signaling, induced apoptosis in culture, reduced colony formation in soft agar and inhibited invasion of melanoma cells. In addition, Mer inhibition synergized with mutant BRAF inhibition in signaling and apoptosis assays. This work establishes Mer as a therapeutic target in melanoma and provides rationale for the continued development of Mer-targeted therapies. Citation Format: Jennifer Schlegel, Maria Sambade, Susan Sather, Stergios Moschos, Aik-Choon Tan, Amanda Winges, Deborah DeRyckere, Craig C. Carson, Dimitri G. Trembath, John J. Tentler, Gail Eckhardt, Pei-Fen Kuan, Ronald L. Hamilton, Lyn M. Duncan, C. Ryan Miller, Nana Nikolaishvili-Feinberg, Bentley R. Midkiff, Xiaodong Wang, Jing Liu, Weihe Zhang, Chao Yang, Stephen V. Frye, H. Shelton Earp, Janiel Shields, Douglas K. Graham. Mer receptor tyrosine kinase is a novel therapeutic target in melanoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3037. doi:10.1158/1538-7445.AM2013-3037

α(1,3)-Fucosyltransferases FUT4 and FUT7 Control Murine Susceptibility to Thrombosis

The α(1,3)-fucosyltransferases, types IV and VII (FUT4 and FUT7, respectively), are required for the synthesis of functional selectin-type leukocyte adhesion molecule ligands. The selectins and their ligands modulate leukocyte trafficking, and P-selectin and its ligand, P-selectin glycoprotein ligand-1, can modulate hemostasis and thrombosis. Regulation of thrombosis by FUT4 and/or FUT7 activity was examined in mouse models of carotid artery thrombosis and collagen/epinephrine-induced thromboembolism. Mice lacking both FUT4 and FUT7 (Fut(-/-) mice) had a shorter time to occlusive thrombus formation in the injured carotid artery and a higher mortality due to collagen/epinephrine-induced pulmonary thromboemboli. Mice lacking P-selectin or P-selectin glycoprotein ligand-1 did not have a prothrombotic phenotype. Whole blood platelet aggregation was enhanced, and plasma fibrinogen content, clot weight, and clot strength were increased in Fut(-/-) mice, and invitro clot lysis was reduced compared with wild type. Fut4(-/-), but not Fut7(-/-), mice had increased pulmonary thromboembolism-induced mortality and decreased thromboemboli dissolution invivo. These data show that FUT4 and FUT7 activity regulates thrombosis in a P-selectin- and P-selectin glycoprotein ligand-1-independent manner and suggest that FUT4 activity is important for thrombolysis.

Photosensitized Oxidative Stress to ARPE-19 Cells Decreases Protein Receptors that Mediate Photoreceptor Outer Segment Phagocytosis

To determine whether previously shown photodynamic (PD)-induced inhibition of specific photoreceptor outer segment (POS) phagocytosis by ARPE-19 cells is associated with reductions in receptor proteins mediating POS phagocytosis, and if PD treatment with merocyanine-540 (MC-540) produces additional effects leading to its inhibition of nonspecific phagocytosis. ARPE-19 cells preloaded with MC-540 or rose bengal (RB) were sublethally irradiated with green light. Phagocytosis of POS was measured by flow cytometry and POS receptor proteins (Mer tyrosine kinase receptor [MerTK] and integrin subunits αv and β5) and β-actin were quantified by Western blotting at 0.5 and 24 hours after irradiation, with comparison to samples from nonsensitized control cultures. The intact integrin heterodimer αvβ5 was quantified by immunoprecipitation followed by blotting. The distribution of N-cadherin, ZO-1, and F-actin was visualized by fluorescence microscopy. Mild PD stress mediated by both photosensitizers that elicits no significant morphologic changes produces transient and recoverable reductions in MerTK. The individual αv and β5 integrin subunits are also reduced but only partially recover. However, there is sufficient recovery to support full recovery of the functional heterodimer. Light stress mediated by MC-540 also reduced levels of actin, which is known to participate in the internalization of particles regardless of type. After PD treatment POS receptor protein abundance and phagocytosis show a coincident in time reduction then recovery suggesting that diminution in receptor proteins contributes to the phagocytic defect. The additional inhibition of nonspecific phagocytosis by MC-540-mediated stress may result from more widespread effects on cytosolic proteins. The data imply that phagocytosis receptors in RPE cells are sensitive to oxidative modification, raising the possibility that chronic oxidative stress in situ may reduce the efficiency of the RPE's role in photoreceptor turnover, thereby contributing to retinal degenerations.

Aberrant Mer receptor tyrosine kinase expression contributes to leukemogenesis in acute myeloid leukemia

Acute myeloid leukemia (AML) continues to be extremely difficult to treat successfully, and the unacceptably low overall survival rates mandate that we assess new potential therapies to ameliorate poor clinical response to conventional therapy. Abnormal tyrosine kinase activation in AML has been associated with poor prognosis and provides strategic targets for novel therapy development. We found that Mer receptor tyrosine kinase was over-expressed in a majority of pediatric (29/36, 80%) and adult (10/10, 100%) primary AML patient blasts at the time of diagnosis, and 100% of patient samples at the time of relapse. Mer was also found to be expressed in 12 of 14 AML cell lines (86%). In contrast, normal bone marrow myeloid precursors expressed little to no Mer. Following AML cell line stimulation with Gas6, a Mer ligand, we observed activation of prosurvival and proliferative signaling pathways, including phosphorylation of ERK1/2, p38, MSK1, CREB, ATF1, AKT and STAT6. To assess the phenotypic role of Mer in AML, two independent short-hairpin RNA (shRNA) constructs were used to decrease Mer expression in the AML cell lines Nomo-1 and Kasumi-1. Reduction of Mer protein levels significantly increased rates of myeloblast apoptosis two to threefold in response to serum starvation. Furthermore, myeloblasts with knocked-down Mer demonstrated decreased colony formation by 67–87%, relative to control cell lines (P<0.01). NOD-SCID-gamma mice transplanted with Nomo-1 myeloblasts with reduced levels of Mer had a significant prolongation in survival compared with mice transplanted with the parental or control cell lines (median survival 17 days in parental and control cell lines, versus 32–36 days in Mer knockdown cell lines, P<0.0001). These data suggest a role for Mer in acute myeloid leukemogenesis and indicate that targeted inhibition of Mer may be an effective therapeutic strategy in pediatric and adult AML.

Mucosal Tolerance to a Combination of ApoB and HSP60 Peptides Controls Plaque Progression and Stabilizes Vulnerable Plaque in Apobtm2SgyLdlrtm1Her/J Mice

Oral tolerance to auto antigens reduces the development of atherosclerosis in mouse models. However, the effect of immune tolerance to multiple self antigenic peptides in plaque progression and stabilization is not known. We studied the protective effect of mucosal tolerance to peptides from apolipoprotein B (ApoB; 661–680) and heat shock protein 60 (HSP60; 153–163), in combination with diet, in the prevention of atherosclerotic lesion progression and plaque stabilization in ApoBtm25gyLDLrtm1Her mice. We found that oral administration of five doses of a combination of ApoB and HSP60 peptides (20 µg/mice/dose) induced tolerance to both the peptides and reduced early plaque development by 39.9% better than the individual peptides (ApoB = 28.7%;HSP60 = 26.8%)(P<0.001). Oral tolerance to combination of peptides along with diet modification arrested plaque progression by 37.6% which was associated with increases in T-regulatory cell and transforming growth factor-β expression in the plaque and peripheral circulation. Reduced macrophage infiltration and tumor necrosis factor-α expression in the plaque was also observed. Tolerance with continued hypercholesterolemia resulted in 60.8% reduction in necrotic core area suggesting plaque stabilization, which was supported by reduction in apoptosis and increased efferocytosis demonstrated by greater expression of receptor tyrosine kinase Mer (MerTK) in the plaque. Tolerance to the two peptides also reduced the expression of matrix metalloproteinase 9, tissue factor, calprotectin, and increased its collagen content. Our study suggests that oral tolerance to ApoB and HSP60 peptide combination induces CD4+ CTLA4+ Tregs and CD4+CD25+Foxp3+ Tregs secreting TGF-β, which inhibit pathogenic T cell response to both peptides thus reducing the development and progression of atherosclerosis and provides evidence for plaque stabilization in ApoBtm25gyLDLrtm1Her mice.

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.

Circulating levels of soluble MER in lupus reflect M2c activation of monocytes/macrophages, autoantibody specificities and disease activity.

IntroductionSystemic lupus erythematosus (SLE) is characterized by impaired efferocytosis and aberrant activation of innate immunity. We asked if shedding of MER receptor tyrosine kinase (MerTK) and AXL into soluble (s) ectodomains was related to immunological and clinical aspects of SLE.MethodsLevels of sMER and sAXL in the plasma of 107 SLE patients and 45 matched controls were measured by ELISA. In 40 consecutive SLE patients, we examined potential correlations between either sMER or sAXL and plasma levels of sCD163, a marker of M2 activation. All three soluble receptors were measured in supernatants of monocytes/macrophages cultured in various immunological conditions. Membrane expression of MerTK, AXL and CD163 was assessed by flow cytometry.ResultsBoth sMER and sAXL were associated with anti-chromatin and anti-phospholipid autoantibodies, and with hematological and renal involvement. However, sMER and sAXL did not significantly correlate with each other; sAXL correlated with growth arrest-specific 6 (Gas6), whereas sMER correlated with reduced free protein S (PROS) levels. Only sMER showed significant associations with lupus-specific anti-dsDNA, anti-Sm, anti-ribonucleoprotein (anti-RNP) and anti-Ro60 autoantibodies. Strong correlations with disease activity indices (Systemic Lupus Erythematosus Disease Activity Index (SLEDAI), complement reduction, titer of circulating anti-dsDNA) were found for sMER, not for sAXL. Patients with active SLEDAI, nephritis, anti-dsDNA and anti-Ro60 positivity showed higher levels of sMER compared to controls. Levels of sMER, not sAXL, correlated with sCD163 levels, and these correlated with SLEDAI. Production of sMER and sCD163 occurred under “M2c” polarizing conditions, whereas sAXL was released upon type-I IFN exposure.ConclusionsAlterations in homeostasis of anti-inflammatory and efferocytic “M2c” monocytes/macrophages may have a role in immunopathogenesis of SLE.

Aberrant Mer receptor tyrosine kinase expression contributes to leukemogenesis in acute myeloid leukemia

Aberrant Mer receptor tyrosine kinase expression contributes to leukemogenesis in acute myeloid leukemia

TAMing Colitis and Colitis-Associated Colon Cancer

Inflammatory Bowel Diseases (IBD) are chronic, relapsing, inflammatory disorders characterized by a robust cytokine-driven inflammation of the gut. This chronic inflammation leads to significant morbidity and mortality, including an increased risk for colorectal cancer. Using a combination of mouse models and analyses of human samples, we show that the receptor tyrosine kinases (RTK) Axl and Mer, members of the TAM subfamily, and their ligand Protein S (Pros1) mediate an anti-inflammatory response that is fundamental for preventing colitis. Furthermore, in marked contrast to the reported oncogenic function of Axl and Mer in tumors, genetic ablation of Axl and Mer in mouse models promotes colitis-associated colon cancer. Axl−/−Mer−/− mice develop an exacerbated inflammatory response to Dextran Sulfate Sodium (DSS)-induced colitis, as determined by colonoscopy score, histopathological analysis and inflammatory profile of lamina propria leukocytes. Axl−/−Mer−/− mice develop an increase number and larger polyps than wild type (WT) mice upon azoxymethane (AOM)-DSS treatment. Axl and Mer are expressed in intestinal macrophages upon inflammation and Axl−/−Mer−/− macrophages produce higher levels of pro-inflammatory cytokines in response to intestinal injury than WT controls. Furthermore, the TAM agonist Pros1 is essential for limiting the pathological activation of macrophages and induced colitis. Pros1 is expressed in activated T cells and functions to dampen the inflammatory response in macrophages through Axl and Mer. Transfer of Pros1−/− naïve T cells into Rag1−/− mice leads to acceleration of colitis in comparison to WT naïve T cells. Remarkably, IBD also shows a significant association with PROS1 deficiency. Patients with active IBD more frequently showed lower levels of plasma PROS1 in comparison to healthy controls. Therefore, our results identify the TAM RTKs and their ligand Pros1 as a hitherto unknown mechanism for the prevention of pathological inflammation in the intestine. While TAM inhibitors have been identified as prospective candidates for molecular anti-cancer therapeutics, the beneficial anti-inflammatory effects of the TAM pathway present a paradox at least in the context of colitis-associated colon cancer. Future efforts in TAM-based therapeutics would benefit from comprehensive understanding of TAM function both in cancer cells and in the tumor microenvironment.