The small-molecule MERTK inhibitor UNC2025 decreases platelet activation and prevents thrombosis.

Essentials Signaling by Gas6 through Tyro3/Axl/Mer receptors is essential for stable platelet aggregation. UNC2025 is a small molecule inhibitor of the Mer tyrosine kinase. UNC2025 decreases platelet activation in vitro and thrombus formation in vivo. UNC2025's anti-platelet effect is synergistic with inhibition of the ADP receptor, P2Y12 . SUMMARY: Background Growth arrest-specific protein 6 signals through the TAM (TYRO-3-AXL-MERTK) receptor family, mediating platelet activation and thrombus formation via activation of the aggregate-stabilizing αIIb ß3 integrin. Objective To describe the antithrombotic effects mediated by UNC2025, a small-molecule MERTK tyrosine kinase inhibitor. Methods MERTK phosphorylation and downstream signaling were assessed by immunoblotting. Light transmission aggregometry, flow cytometry and microfluidic analysis were used to evaluate the impact of MERTK inhibition on platelet activation and stability of aggregates in vitro. The effects of MERTK inhibition on arterial and venous thrombosis, platelet accumulation at microvascular injury sites and tail bleeding times were determined with murine models. The effects of combined treatment with ADP-P2Y1&12 pathway antagonists and UNC2025 were also evaluated. Results and Conclusions Treatment with UNC2025 inhibited MERTK phosphorylation and downstream activation of AKT and SRC, decreased platelet activation, and protected animals from pulmonary embolism and arterial thrombosis without increasing bleeding times. The antiplatelet effect of UNC2025 was enhanced in combination with ADP-P2Y1&12 pathway antagonists, and a greater than additive effect was observed when these two agents with different mechanisms of inhibition were coadministered. TAM kinase signaling represents a potential therapeutic target, as inhibition of this axis, especially in combination with ADP-P2Y pathway antagonism, mediates decreased platelet activation, aggregate stability, and thrombus formation, with less hemorrhagic potential than current treatment strategies. The data presented here also demonstrate antithrombotic activity mediated by UNC2025, a novel translational agent, and support the development of TAM kinase inhibitors for clinical applications.

Lymphoblastic leukemia/lymphoma in mice overexpressing the Mer (MerTK) receptor tyrosine kinase.

Mer (MerTK) is a receptor tyrosine kinase important in platelet aggregation, as well as macrophage cytokine secretion and clearance of apoptotic cells. Mer is not normally expressed in thymocytes or lymphocytes; however, ectopic Mer RNA transcript and protein expression is found in a subset of acute lymphoblastic leukemia cell lines and patient samples, suggesting a role in leukemogenesis. To investigate the oncogenic potential of Mer in vivo, we created a transgenic mouse line (Mer(Tg)) that expresses Mer in the hematopoietic lineage under control of the Vav promoter. Ectopic expression and activation of the transgenic Mer protein was demonstrated in lymphocytes and thymocytes of the Mer(Tg) mice. At 12-24 months of age, greater than 55% of the Mer(Tg) mice, compared to 12% of the wild type, developed adenopathy, hepatosplenomegaly, and circulating lymphoblasts. Histopathological analysis and flow cytometry were consistent with T-cell lymphoblastic leukemia/lymphoma. Mer may contribute to leukemogenesis by activation of Akt and ERK1/2 anti-apoptotic signals, which were upregulated in Mer(Tg) mice. Additionally, a significant survival advantage was noted in Mer(Tg) lymphocytes compared to wild-type lymphocytes after dexamethasone treatment. These data suggest that Mer plays a cooperative role in leukemogenesis and may be an effective target for biologically based leukemia/lymphoma therapy.

Consensus on a core curriculum in American training programs in pediatric hematology-oncology: a report from the ASPHO Training Committee.

The Training Committee (TC) of the American Society of Pediatric Hematology/Oncology created a foundation of common goals and objectives that could provide a structure for fellowship programs. The TC conducted a survey of program directors for input into the structure of their programs and training methods and the results are presented here. Additionally, a suggested core program is outlined, taking into account the new common requirements as stipulated by the ACGME and ABP, and additional suggestions from the program directors. This paper highlights the suggested training objectives and educational opportunities that should be afforded all fellows in this sub-specialty. The goal of this consensus statement is to provide a model curriculum to improve quality and consistency of training and achieve compliance with new requirements while simultaneously recognizing the importance of alternative approaches that emphasize each program's unique strengths and character.

Cloning and developmental expression analysis of the murine c-mer tyrosine kinase.

We have cloned the putative mouse homologue of the human c-mer receptor tyrosine kinase proto-oncogene. Comparison of the mouse and human c-mer amino acid sequences reveals an overall identity of 88%. Similar to the human, the extracellular region of the murine c-mer protein possesses two amino terminal immunoglobulin-like domains and two membrane proximal fibronectin type III domains, which places it in the Axl family of tyrosine kinases. Our analysis of the Axl family identifies eight different regions of amino acid consensus that have residues characteristic of this and no other tyrosine kinase family; six of the eight are within tyrosine kinase subdomains. The homology within the Axl family is highest between c-mer and c-eyk, the chicken proto-oncogene of the tumor virus gene product v-eyk. Northern analysis of adult tissues suggests that the mouse c-mer, although expressed in many tissues, has an expression pattern unique among Axl family members. In normal adult hematopoietic cells c-mer seems to be expressed predominantly if not exclusively in the monocytic lineage. Mouse c-mer is expressed during most, if not all, stages of embryological development beginning in the morula and blastocyst and progressing through the yolk sac and fetal liver stages. This early and consistent expression of c-mer was confirmed during in vitro differentiation of embryonic stem cells. The embryonic expression profile of c-mer suggests that this tyrosine kinase may play an important function in the developing mouse.

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.

A mechanism of resistance to gefitinib mediated by cellular reprogramming and the acquisition of an FGF2-FGFR1 autocrine growth loop.

Despite initial and often dramatic responses of epidermal growth factor receptor (EGFR)-addicted lung tumors to the EGFR-specific tyrosine kinase inhibitors (TKIs), gefitinib and erlotinib, nearly all develop resistance and relapse. To explore novel mechanisms mediating acquired resistance, we employed non-small-cell lung cancer (NSCLC) cell lines bearing activating mutations in EGFR and rendered them resistant to EGFR-specific TKIs through chronic adaptation in tissue culture. In addition to previously observed resistance mechanisms including EGFR-T790M 'gate-keeper' mutations and MET amplification, a subset of the seven chronically adapted NSCLC cell lines including HCC4006, HCC2279 and H1650 cells exhibited marked induction of fibroblast growth factor (FGF) 2 and FGF receptor 1 (FGFR1) mRNA and protein. Also, adaptation to EGFR-specific TKIs was accompanied by an epithelial to mesenchymal transition (EMT) as assessed by changes in CDH1, VIM, ZEB1 and ZEB2 expression and altered growth properties in Matrigel. In adapted cell lines exhibiting increased FGF2 and FGFR1 expression, measures of growth and signaling, but not EMT, were blocked by FGFR-specific TKIs, an FGF-ligand trap and FGFR1 silencing with RNAi. In parental HCC4006 cells, cell growth was strongly inhibited by gefitinib, although drug-resistant clones progress within 10 days. Combined treatment with gefitinib and AZD4547, an FGFR-specific TKI, prevented the outgrowth of drug-resistant clones. Thus, induction of FGF2 and FGFR1 following chronic adaptation to EGFR-specific TKIs provides a novel autocrine receptor tyrosine kinase-driven bypass pathway in a subset of lung cancer cell lines that are initially sensitive to EGFR-specific TKIs. The findings support FGFR-specific TKIs as potentially valuable additions to existing targeted therapeutic strategies with EGFR-specific TKIs to prevent or delay acquired resistance in EGFR-driven NSCLC.

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.

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 short hairpin RNA (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 have 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.

Mer receptor tyrosine kinase inhibition impedes glioblastoma multiforme migration and alters cellular morphology.

Glioblastoma multiforme (GBM) is an aggressive brain tumor, fatal within 1 year from diagnosis in most patients despite intensive multimodality therapy. The migratory and microscopically invasive nature of GBM as well as its resistance to chemotherapy renders conventional therapies inadequate in its treatment. Although Mer receptor tyrosine kinase (RTK) inhibition has been shown to decrease the long-term survival and improve the chemosensitivity of GBM in vitro, its role in malignant cellular migration has not been previously evaluated. In this study, we report for the first time a role for Mer RTK in brain tumor migration and show that Mer inhibition profoundly impedes GBM migration and alters cellular morphology. Our data demonstrate that Mer RTK inhibition results in altered signaling through focal adhesion kinase (FAK) and RhoA GTPase and a transformation of cytoskeletal organization, suggesting both molecular and structural mechanisms for the abrogation of migration. We also describe a novel and translational method of Mer RTK inhibition using a newly developed monoclonal antibody, providing proof of principle for future evaluation of Mer-targeted translational therapies in the treatment of GBM. Previous findings implicating Mer signaling in glioblastoma survival and chemotherapy resistance coupled with our discovery of the role of Mer RTK in GBM cellular migration support the development of novel Mer-targeted therapies for this devastating disease.

A kinase-deficient splice variant of the human JAK3 is expressed in hematopoietic and epithelial cancer cells.

Signal transduction of cytokine receptors is mediated by the JAK family of tyrosine kinases. Recently, the kinase partners for the interleukin (IL)-2 receptor have been identified as JAK1 and JAK3. In this study, we report the identification of splice variants that may modulate JAK3 signaling. Three splice variants were isolated from different mRNA sources: breast (B), spleen (S), and activated monocytes (M). Sequence analysis revealed that the splice variants contain identical NH2-terminal regions but diverge at the COOH termini. Analyses of expression of the JAK3 splice isoforms by reverse transcriptase-polymerase chain reaction on a panel of cell lines show splice preferences in different cell lines: the S-form is more commonly seen in hematopoietic lines, whereas the B- and M-forms are detected in cells both of hematopoietic and epithelial origins. Antibodies raised against peptides to the B-form splice variant confirmed that the 125-kDa JAK3B protein product is found abundantly in hematopoietic as well as epithelial cells, including primary breast cancers. The lack of subdomain XI in the tyrosine kinase core of the B-form JAK3 protein suggests that it is a defective kinase. This is supported by the lack of detected autokinase activity of the B-form JAK3. Intriguingly, both the S and B splice isoforms of JAK3 appear to co-immunoprecipitate with the IL-2 receptor from HUT-78 cell lysates. This and the presence of multiple COOH-terminal splice variants coexpressed in the same cells suggest that the JAK3 splice isoforms are functional in JAK3 signaling and may enrich the complexity of the intracellular responses functional in IL-2 or cytokine signaling.

Rodent L1 evolution has been driven by a single dominant lineage that has repeatedly acquired new transcriptional regulatory sequences.

All mammalian genomes contain approximately 100,000 copies of the transposable element LINES-1 (L1). Phylogenetic analysis indicates that the L1 progenitor predates the mammalian radiation; since that time, the open reading frames encoded in L1 have evolved under selection. The least conserved regions within L1 are the 5'-terminal transcriptional regulatory sequences. In rodents, four types of L1 elements (A, F, and V from mouse and R from rat) have been defined according to the type of apparently nonhomologous promoter sequence present at the 5' end. In this study, we investigate the relationships between these four types of promoters. DNA sequence was determined from approximately 1.5-kb regions from the 5' ends of seven F- and three V-type L1 elements. These sequences were aligned with 29 previously reported L1 elements. Phylogenetic analysis was then performed on the homologous regions of the alignment. The results indicate that in mouse all of the A-, F-, and V-type elements belong to a single dominant lineage but were inserted into the genome during different time periods; V-type elements are the oldest, while A-type elements are the most recently inserted. V-type elements also appear ancestral to the R-type elements found in rat and therefore were replicatively competent prior to the divergence of rat and mouse. Analysis of sequence identity indicates that the different 5' promoters did not derive from a common ancestor. Therefore, the dominant L1 lineage appears to have acquired novel promoter sequences from non-L1 sources. Transposable elements from a wide range of species show similar structural rearrangements, suggesting that acquisition of new sequences may be a common theme in their evolution.

Cloning and mRNA expression analysis of a novel human protooncogene, c-mer.

A human B-lymphoblastoid lambda gt11 expression library was screened using anti-phosphotyrosine antibodies yielding complementary DNAs encoding active tyrosine kinases. The resulting clones were used to obtain the sequence of a novel 984 amino acid transmembrane tyrosine kinase. Analysis of the complementary DNA revealed extracellular immunoglobulin and fibronectin type III domains and the unusual kinase signature sequence KWIAIES; all are characteristic of the axl family of tyrosine kinases. The novel tyrosine kinase was not expressed in normal B- and T-lymphocytes but, unlike axl, was expressed in numerous neoplastic B- and T-cell lines. Transcripts for the novel receptor-like tyrosine kinase were detected in normal peripheral blood monocytes and bone marrow. One alternatively spliced transcript was detected which contained an insert in the membrane proximal region that could encode for a truncated, soluble receptor. Sequence comparison shows that the kinase may be the human protooncogene for the recently isolated chicken retroviral oncogene v-ryk (recently renamed v-eyk), a truncated tyrosine kinase whose expression by retroviral infection produced sarcomas in chickens. The intracellular domain of the human kinase shows 83% similarity and 71% identity to v-ryk. Since the ryk designation has been used to name another tyrosine kinase and an analysis of RNA expression demonstrated that this novel human kinase is expressed in monocytes and tissues of epithelial and reproductive origin, we have designated our novel protooncogene c-mer.

Expression of axl, a transforming receptor tyrosine kinase, in normal and malignant hematopoiesis.

We previously reported the cloning, and characterization of a receptor tyrosine kinase, axl, from two patients with chronic myelogenous leukemia. Herein, we describe the expression pattern of axl in normal and malignant hematopoietic tissue axl message is detected in normal human bone marrow but not significantly in normal blood leukocytes. Cell separation experiments showed that axl is expressed in hematopoietic CD34+ progenitor and marrow stromal cells, at low levels in peripheral monocytes, but not in lymphocytes or granulocytes. Consistent with the normal pattern of axl expression, axl RNA was found predominantly in diseases of the myeloid lineage: 39 of 66 (59%) patients with myeloproliferative disorders (acute myeloid leukemia, chronic myeloid leukemia (CML) in chronic phase, CML in myeloid blast crisis, and myelodysplasia) showed significant axl transcription, as compared with 1 of 45 (2%) lymphoid leukemias (chronic lymphocytic leukemia, acute lymphocytic leukemia, and CML in lymphoid blast crisis). Treatment of K562 cells with the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), administration of interferon alpha (IFN alpha) to normal monocytes, and treatment of U937 cells with TPA and IFN tau significantly induced axl expression, supporting a role for this kinase in the intracellular signaling of myeloid cells through a variety of biochemical pathways. These results suggest that the axl kinase may be operative in normal and malignant myeloid biology.