Role of Gas6 in erythropoiesis and anemia in mice

Recent advances in TAM mechanisms in lung diseases

TYRO3, MERTK, and AXL receptor tyrosine kinases, collectively known as TAM receptors, play a vital role in maintaining lung tissue homeostasis by regulating integrity and self-renewal. These receptors activate signalling pathways that inhibit apoptosis, promote cell proliferation and differentiation, mediate cell adhesion and migration, and perform other essential biological functions. Additionally, TAM receptors are implicated in mechanisms that suppress anti-tumor immunity and confer resistance to immune checkpoint inhibitors. Disruption of the homeostatic balances can lead to pathological conditions such as lung inflammation, fibrosis, or tumors. Recent studies highlight their significant role in COVID-19-induced lung injury. However, the exact mechanisms by which TAM receptors contribute to lung diseases remain unclear. This article reviews the potential mechanisms of TAM receptor involvement in disease progression, focusing on lung inflammation, fibrosis, cancer, and COVID-19-induced lung injury. It also explores future research aspects and the therapeutic potentials of targeting TAM receptors, providing a theoretical foundation for understanding lung disease mechanisms and identifying treatment targets.

AXL signaling in cancer: from molecular insights to targeted therapies

AXL, a member of the TAM receptor family, has emerged as a potential target for advanced-stage human malignancies. It is frequently overexpressed in different cancers and plays a significant role in various tumor-promoting pathways, including cancer cell proliferation, invasion, metastasis, epithelial-mesenchymal transition (EMT), angiogenesis, stemness, DNA damage response, acquired therapeutic resistance, immunosuppression, and inflammatory responses. Beyond oncology, AXL also facilitates viral infections, including SARS-CoV-2 and Zika highlighting its importance in both cancer and virology. In preclinical models, small-molecule kinase inhibitors targeting AXL have shown promising anti-tumorigenic potential. This review primarily focuses on the induction, regulation and biological functions of AXL in mediating these tumor-promoting pathways. We discuss a range of therapeutic strategies, including recently developed small-molecule tyrosine kinase inhibitors (TKIs), monoclonal antibodies, and antibody-drug conjugates (ADCs), anti-AXL-CAR, and combination therapies. These interventions are being examined in both preclinical and clinical studies, offering the potential for improved drug sensitivity and therapeutic efficacy. We further discuss the mechanisms of acquired therapeutic resistance, particularly the crosstalk between AXL and other critical receptor tyrosine kinases (RTKs) such as c-MET, EGFR, HER2/HER3, VEGFR, PDGFR, and FLT3. Finally, we highlight key research areas that require further exploration to enhance AXL-mediated therapeutic approaches for improved clinical outcomes.

Therapeutic targeting of the functionally elusive TAM receptor family

The TAM receptor family of TYRO3, AXL and MERTK regulates tissue and immune homeostasis. Aberrant TAM receptor signalling has been linked to a range of diseases, including cancer, fibrosis and viral infections. Specifically, the dysregulation of TAM receptors can enhance tumour growth and metastasis due to their involvement in multiple oncogenic pathways. For example, TAM receptors have been implicated in the epithelial-mesenchymal transition, maintaining the stem cell phenotype, immune modulation, proliferation, angiogenesis and resistance to conventional and targeted therapies. Therapeutically, multiple TAM receptor inhibitors are in preclinical and clinical development for cancers and other indications, with those targeting AXL being the most clinically advanced. Although there has been notable clinical advancement in recent years, challenges persist. This Review aims to provide both biological and clinical insights into the current therapeutic landscape of TAM receptor inhibitors, and evaluates their potential for the treatment of cancer and non-malignant diseases.

Synergistic immunotherapy targeting cancer-associated anemia: prospects of a combination strategy

Cancer-associated anemia promotes tumor progression, leads to poor quality of life in patients with cancer, and even obstructs the efficacy of immune checkpoint inhibitors therapy. However, the precise mechanism for cancer-associated anemia remains unknown and the feasible strategy to target cancer-associated anemia synergizing immunotherapy needs to be clarified. Here, we review the possible mechanisms of cancer-induced anemia regarding decreased erythropoiesis and increased erythrocyte destruction, and cancer treatment-induced anemia. Moreover, we summarize the current paradigm for cancer-associated anemia treatment. Finally, we propose some prospective paradigms to slow down cancer-associated anemia and synergistic the efficacy of immunotherapy. Video Abstract.

The role of phosphatidylserine on the membrane in immunity and blood coagulation

The negatively charged aminophospholipid, phosphatidylserine (PtdSer), is located in the inner leaflet of the plasma membrane in normal cells, and may be exposed to the outer leaflet under some immune and blood coagulation processes. Meanwhile, Ptdser exposed to apoptotic cells can be recognized and eliminated by various immune cells, whereas on the surface of activated platelets Ptdser interacts with coagulation factors prompting enhanced production of thrombin which significantly facilitates blood coagulation. In the case where PtdSer fails in exposure or mistakenly occurs, there are occurrences of certain immunological and haematological diseases, such as the Scott syndrome and Systemic lupus erythematosus. Besides, viruses (e.g., Human Immunodeficiency Virus (HIV), Ebola virus (EBOV)) can invade host cells through binding the exposed PtdSer. Most recently, the Corona Virus Disease 2019 (COVID-19) has been similarly linked to PtdSer or its receptors. Therefore, it is essential to comprehensively understand PtdSer and its functional characteristics. Therefore, this review summarizes Ptdser, its eversion mechanism; interaction mechanism, particularly with its immune receptors and coagulation factors; recognition sites; and its function in immune and blood processes. This review illustrates the potential aspects for the underlying pathogenic mechanism of PtdSer-related diseases, and the discovery of new therapeutic strategies as well.

AXL Inhibition Represents a Novel Therapeutic Approach in BCR-ABL Negative Myeloproliferative Neoplasms

BCR-ABL negative myeloproliferative neoplasms (MPNs) consist of essential thrombocythemia, polycythemia vera, and myelofibrosis. The majority of patients harbor the JAK2-activating mutation V617F. JAK2 inhibitors were shown to reduce symptom burden and splenomegaly in MPN patients. However, treatment options are limited after failure of JAK2 inhibitors. AXL, a member of the TAM family of receptor tyrosine kinases, mediates survival and therapy resistance of different myeloid cancers including acute myeloid leukemia and chronic myeloid leukemia. We studied the relevance of AXL as a target in MPN using primary patient cells and preclinical disease models. We found that AXL is abundantly activated in MPN cells and that its ligand growth arrest-specific gene 6 is upregulated in MPN patients. Pharmacologic and genetic blockade of AXL impaired viability, decreased proliferation and increased apoptosis of MPN cells. Interestingly, ruxolitinib treatment induced increased phosphorylation of AXL indicating that activation of AXL might mediate resistance to ruxolitinib. Consistently, the AXL inhibitor bemcentinib exerted additive effects with ruxolitinib via impaired STAT3, STAT5, and AKT signaling. Both agents had activity when employed alone and exerted an additive effect on survival and splenomegaly in vivo. Moreover, bemcentinib treatment normalized red blood cell count and hemoglobin levels in vivo. Thus, our data indicate that AXL inhibition represents a novel treatment option in MPN warranting clinical investigation.

An induced pluripotent stem cell model of Fanconi anemia reveals mechanisms of p53-driven progenitor cell differentiation

Fanconi anemia (FA) is a disorder of DNA repair that manifests as bone marrow (BM) failure. The lack of accurate murine models of FA has refocused efforts toward differentiation of patient-derived induced pluripotent stem cells (IPSCs) to hematopoietic progenitor cells (HPCs). However, an intact FA DNA repair pathway is required for efficient IPSC derivation, hindering these efforts. To overcome this barrier, we used inducible complementation of FANCA-deficient IPSCs, which permitted robust maintenance of IPSCs. Modulation of FANCA during directed differentiation to HPCs enabled the production of FANCA-deficient human HPCs that recapitulated FA genotoxicity and hematopoietic phenotypes relative to isogenic FANCA-expressing HPCs. FANCA-deficient human HPCs underwent accelerated terminal differentiation driven by activation of p53/p21. We identified growth arrest specific 6 (GAS6) as a novel target of activated p53 in FANCA-deficient HPCs and modulate GAS6 signaling to rescue hematopoiesis in FANCA-deficient cells. This study validates our strategy to derive a sustainable, highly faithful human model of FA, uncovers a mechanism of HPC exhaustion in FA, and advances toward future cell therapy in FA.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

Vitamin C Inhibits Metastasis of Peritoneal Tumors By Preventing Spheroid Formation in ID8 Murine Epithelial Peritoneal Cancer Model

High mortality is associated with exclusively metastasis within the peritoneal cavity among patients with epithelial ovarian cancer that is the most lethal gynecologic cancer. There is an unmet need to develop more effective therapies to prevent metastasis of peritoneal cancer. Multicellular spheroid formation, during which cancer cells migrate and adhere to tumor-associated macrophages, is a critical step of ovarian cancer metastasis. Here, we showed that vitamin C inhibited spheroid formation and metastasis in ID8 ovarian cancer-bearing mice. We further found that vitamin C treatment decreased the levels of M2 macrophages in tumor nodules and suppressed the epithelial-mesenchymal transition (EMT). In vitro studies revealed that vitamin C inhibited proliferation, arrested cell cycle, attenuated migration, and prevented the spheroid formation of ID8 ovarian cancer cells. Vitamin C induced apoptosis of ID8 cells, which was confirmed by membrane potential collapse, cytosolic calcium overload, ATP depletion, and caspase-3 activation in vitamin C-treated cells. Intriguingly, vitamin C treatment caused striking morphological change and apoptosis of macrophages. The presented proof of concept study strategically identifies new anticancer mechanisms of vitamin C.

Reappraising the role of α5 integrin and the microenvironmental support in stress erythropoiesis

We previously studied the role of β1 integrin and some of its different α partners relevant to erythropoiesis. Although clear and consistent answers regarding the role of α4β1 (VLA-4) were evident, the role of its companion integrin α5β1 (VLA-5) was clouded by inconsistent outcomes in all prior publications. Furthermore, the functional consequences of integrin deficiencies only in microenvironmental (ME) cells supporting erythroid cell expansion and maturation post stress have never been explored. In the study described here, we created several additional mouse models in the aim of addressing unanswered questions regarding functional consequences of single or combined integrin deficiencies in erythroid cells or only in ME supporting cells. Our novel and expansive data solidified the intrinsic requirement of both α4 and α5 integrins in erythroid cells for their proliferative expansion and maturation in response to stress; α5 integrin alone, deleted either early in all hematopoietic cells or only in erythroid cell, has only a redundant role in proliferative expansion and is dispensable for erythroid maturation. By contrast, α4 integrin, on its own, exerts a dominant effect on timely and optimal erythroid maturation. Deficiency of both α4 and α5 integrins in ME cells, including macrophages, does not negatively influence stress response by normal erythroid cells, in great contrast to the effect of ME cells deficient in all β1 integrins. Collectively the present data offer deeper insight into the coordination of different β1 integrin functional activities in erythroid cells or in ME cells for optimal erythroid stress response.

Targeting of TAM Receptors Ameliorates Fibrotic Mechanisms in Idiopathic Pulmonary Fibrosis

RATIONALE

Idiopathic pulmonary fibrosis (IPF) is characterized by aberrant lung remodeling, which progressively abolishes lung function in an RTK (receptor tyrosine kinase)-dependent manner. Gas6 (growth arrest-specific 6) ligand, Tyro3 (TYRO3 protein tyrosine kinase 3), and Axl (anexelekto) RTK expression and activity are increased in IPF.

OBJECTIVES

To determine if targeting these RTK pathways would inhibit fibroblast activation and the development of pulmonary fibrosis.

METHODS

Quantitative genomic, proteomic, and functional analyses were used to determine Gas6/TAM (Tyro3, Axl, and Mertk [MER proto-oncogene, tyrosine kinase]) RTK expression and activation in tissues and fibroblasts from normal and IPF lungs. The profibrotic impact of these RTK pathways were also examined in bleomycin-induced pulmonary fibrosis and in SCID/Bg mice that developed pulmonary fibrosis after the intravenous administration of primary IPF fibroblasts.

MEASUREMENTS AND MAIN RESULTS

Gas6, Axl, and Tyro3 were increased in both rapidly and slowly progressive IPF compared with normal lung samples and fibroblasts. Targeting these pathways with either specific antibodies directed at Gas6 or Axl, or with small-molecule TAM inhibitors indicated that the small molecule-mediated targeting approach was more efficacious in both in vitro and in vivo studies. Specifically, the TAM receptor inhibitor R428 (also known as BGB324) significantly inhibited the synthetic, migratory, and proliferative properties of IPF fibroblasts compared with the other Gas6/TAM receptor targeting agents. Finally, loss of Gas6 expression decreased lung fibrotic responses to bleomycin and treatment with R428 inhibited pulmonary fibrosis in humanized SCID/Bg mice.

CONCLUSIONS

Gas6/TAM receptor activity contributes to the activation of pulmonary fibroblasts in IPF, suggesting that targeting this RTK pathway might be an effective antifibrotic strategy in this disease.

AXL Inhibition Extinguishes Primitive JAK2 Mutated Myeloproliferative Neoplasm Progenitor Cells

Myeloproliferative neoplasms (MPN) are clonal stem cell associated disorders inclusive of chronic myeloid leukemia (CML), Polycythaemia vera (PV), myelofibrosis (MF), and essential thrombocythemia (ET). They are characterized by increased production of myeloid cells with minimal effects on terminal differentiation but can undergo transformation to acute leukemias. PV is the most common chronic myeloproliferative neoplasm and in the majority of cases is characterized by a V617F point mutation in JAK2. This JAK2 activating mutation is also found in about half the patients with MF and ET. Such aberrant proteins offer great potential for the treatment of these diseases however inhibitors to JAK2 have had limited success in the clinic in terms of curing the disease. We have previously used advanced proteomic techniques to identify drug targets and thus develop novel treatment strategies to distinguish the leukemic clone in both CML and PV. Here, we build on our proteomic data sets to characterize a new target, the receptor tyrosine kinase AXL. AXL is overexpressed in acute myeloid leukemia and importantly small molecule inhibitors have been developed which are currently in clinical trial hence offer the opportunity to repurpose this drug for the treatment of MPNs. We demonstrate that AXL is upregulated and activated in JAK2 associated MPNs. Further we show that inhibition of AXL preferentially kills early hemopoietic stem cells from PV patients and as such represents a promising therapeutic approach for JAK2 driven MPNs.

The Shape Shifting Story of Reticulocyte Maturation

The final steps of erythropoiesis involve unique cellular processes including enucleation and reorganization of membrane proteins and the cytoskeleton to produce biconcave erythrocytes. Surprisingly this process is still poorly understood. In vitro erythropoiesis protocols currently produce reticulocytes rather than biconcave erythrocytes. In addition, immortalized lines and iPSC-derived erythroid cell suffer from low enucleation and suboptimal final maturation potential. In light of the increasing prospect to use in vitro produced erythrocytes as (personalized) transfusion products or as therapeutic delivery agents, the mechanisms driving this last step of erythropoiesis are in dire need of resolving. Here we review the elusive last steps of reticulocyte maturation with an emphasis on protein sorting during the defining steps of reticulocyte formation during enucleation and maturation.

GAS6/TAM Pathway Signaling in Hemostasis and Thrombosis

The GAS6/TYRO3-AXL-MERTK (TAM) signaling pathway is essential for full and sustained platelet activation, as well as thrombus stabilization. Inhibition of this pathway decreases platelet aggregation, shape change, clot retraction, aggregate formation under flow conditions, and surface expression of activation markers. Transgenic mice deficient in GAS6, or any of the TAM family of receptors that engage this ligand, exhibit in vivo protection against arterial and venous thrombosis but do not demonstrate either spontaneous or prolonged bleeding compared to their wild-type counterparts. Comparable results are observed in wild-type mice treated with pharmacological inhibitors of the GAS6-TAM pathway. Thus, GAS6/TAM inhibition offers an attractive novel therapeutic option that may allow for a moderate reduction in platelet activation and decreased thrombosis while still permitting the primary hemostatic function of platelet plug formation.

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

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

TAM receptors Tyro3 and Mer as novel targets in colorectal cancer

Purpose

CRC remains the third most common cancer worldwide with a high 5-year mortality rate in advanced cases. Combined with chemotherapy, targeted therapy is an additional treatment option. However as CRC still escapes targeted therapy the vigorous search for new targets is warranted to increase patients' overall survival.

Results

In this study we describe a new role for Gas6/protein S-TAM receptor interaction in CRC. Gas6, expressed by tumor-infiltrating M2-like macrophages, enhances malignant properties of tumor cells including proliferation, invasion and colony formation. Upon chemotherapy macrophages increase Gas6 synthesis, which significantly attenuates the cytotoxic effect of 5-FU chemotherapy on tumor cells. The anti-coagulant protein S has similar effects as Gas6.

In CRC patient samples Tyro3 was overexpressed within the tumor. In-vitro inhibition of Tyro3 and Mer reduces tumor cell proliferation and sensitizes tumor cells to chemotherapy. Moreover high expression of Tyro3 and Mer in tumor tissue significantly shortens CRC patients' survival.

Experimental design

Various in vitro models were used to investigate the role of Gas6 and its TAM receptors in human CRC cells, by stimulation (rhGas6) and knockdown (siRNA) of Axl, Tyro3 and Mer. In terms of a translational research, we additionally performed an expression analysis in human CRC tissue and analyzed the medical record of these patients.

Conclusions

Tyro3 and Mer represent novel therapeutic targets in CRC and warrant further preclinical and clinical investigation in the future.

NiaoDuQing granules relieve chronic kidney disease symptoms by decreasing renal fibrosis and anemia

NiaoDuQing (NDQ) granules, a traditional Chinese medicine, has been clinically used in China for over fourteen years to treat chronic kidney disease (CKD). To elucidate the mechanisms underlying the therapeutic benefits of NDQ, we designed an approach incorporating chemoinformatics, bioinformatics, network biology methods, and cellular and molecular biology experiments. A total of 182 active compounds were identified in NDQ granules, and 397 putative targets associated with different diseases were derived through ADME modelling and target prediction tools. Protein-protein interaction networks of CKD-related and putative NDQ targets were constructed, and 219 candidate targets were identified based on topological features. Pathway enrichment analysis showed that the candidate targets were mostly related to the TGF-β, the p38MAPK, and the erythropoietin (EPO) receptor signaling pathways, which are known contributors to renal fibrosis and/or renal anemia. A rat model of CKD was established to validate the drug-target mechanisms predicted by the systems pharmacology analysis. Experimental results confirmed that NDQ granules exerted therapeutic effects on CKD and its comorbidities, including renal anemia, mainly by modulating the TGF-β and EPO signaling pathways. Thus, the pharmacological actions of NDQ on CKD symptoms correlated well with in silico predictions.

From the Cradle to the Grave: The Role of Macrophages in Erythropoiesis and Erythrophagocytosis

Erythropoiesis is a highly regulated process where sequential events ensure the proper differentiation of hematopoietic stem cells into, ultimately, red blood cells (RBCs). Macrophages in the bone marrow play an important role in hematopoiesis by providing signals that induce differentiation and proliferation of the earliest committed erythroid progenitors. Subsequent differentiation toward the erythroblast stage is accompanied by the formation of so-called erythroblastic islands where a central macrophage provides further cues to induce erythroblast differentiation, expansion, and hemoglobinization. Finally, erythroblasts extrude their nuclei that are phagocytosed by macrophages whereas the reticulocytes are released into the circulation. While in circulation, RBCs slowly accumulate damage that is repaired by macrophages of the spleen. Finally, after 120 days of circulation, senescent RBCs are removed from the circulation by splenic and liver macrophages. Macrophages are thus important for RBCs throughout their lifespan. Finally, in a range of diseases, the delicate interplay between macrophages and both developing and mature RBCs is disturbed. Here, we review the current knowledge on the contribution of macrophages to erythropoiesis and erythrophagocytosis in health and disease.

Targeting the TAM Receptors in Leukemia

Targeted inhibition of members of the TAM (TYRO-3, AXL, MERTK) family of receptor tyrosine kinases has recently been investigated as a novel strategy for treatment of hematologic malignancies. The physiologic functions of the TAM receptors in innate immune control, natural killer (NK) cell differentiation, efferocytosis, clearance of apoptotic debris, and hemostasis have previously been described and more recent data implicate TAM kinases as important regulators of erythropoiesis and megakaryopoiesis. The TAM receptors are aberrantly or ectopically expressed in many hematologic malignancies including acute myeloid leukemia, B- and T-cell acute lymphoblastic leukemia, chronic lymphocytic leukemia, and multiple myeloma. TAM receptors contribute to leukemic phenotypes through activation of pro-survival signaling pathways and interplay with other oncogenic proteins such as FLT3, LYN, and FGFR3. The TAM receptors also contribute to resistance to both cytotoxic chemotherapeutics and targeted agents, making them attractive therapeutic targets. A number of translational strategies for TAM inhibition are in development, including small molecule inhibitors, ligand traps, and monoclonal antibodies. Emerging areas of research include modulation of TAM receptors to enhance anti-tumor immunity, potential roles for TYRO-3 in leukemogenesis, and the function of the bone marrow microenvironment in mediating resistance to TAM inhibition.

The Role of TAM Family Receptors in Immune Cell Function: Implications for Cancer Therapy

The TAM receptor protein tyrosine kinases-Tyro3, Axl, and Mer-are essential regulators of immune homeostasis. Guided by their cognate ligands Growth arrest-specific gene 6 (Gas6) and Protein S (Pros1), these receptors ensure the resolution of inflammation by dampening the activation of innate cells as well as by restoring tissue function through promotion of tissue repair and clearance of apoptotic cells. Their central role as negative immune regulators is highlighted by the fact that deregulation of TAM signaling has been linked to the pathogenesis of autoimmune, inflammatory, and infectious diseases. Importantly, TAM receptors have also been associated with cancer development and progression. In a cancer setting, TAM receptors have a dual regulatory role, controlling the initiation and progression of tumor development and, at the same time, the associated anti-tumor responses of diverse immune cells. Thus, modulation of TAM receptors has emerged as a potential novel strategy for cancer treatment. In this review, we discuss our current understanding of how TAM receptors control immunity, with a particular focus on the regulation of anti-tumor responses and its implications for cancer immunotherapy.

The erythroblastic island as an emerging paradigm in the anemia of inflammation

Terminal erythroid differentiation occurs in the bone marrow, within specialized niches termed erythroblastic islands. These functional units consist of a macrophage surrounded by differentiating erythroblasts and have been described more than five decades ago, but their function in the pathophysiology of erythropoiesis has remained unclear until recently. Here we propose that the central macrophage in the erythroblastic island contributes to the pathophysiology of anemia of inflammation. After introducing erythropoiesis and the interactions between the erythroblasts and the central macrophage within the erythroblastic islands, we will discuss the immunophenotypic characterization of this specific subpopulation of macrophages. We will then integrate these concepts into the currently known pathophysiological drivers of anemia of inflammation and address the role of the central macrophage in this disorder. Finally, as a means of furthering our understanding of the various concepts, we will discuss the differences between murine and rat models with regard to developmental and stress erythropoiesis in an attempt to define a model system representative of human pathophysiology.

The macrophage contribution to stress erythropoiesis: when less is enough

Although the importance of native bone marrow and spleen macrophages in enhancing baseline and stress erythropoiesis has been emphasized over several decades, their kinetic and phenotypic changes during a variety of stress responses have been unclear. Furthermore, whether monocyte-derived recruited macrophages can functionally substitute for inadequate or functionally impaired native macrophages has been controversial and seem to be not only tissue- but also stress-type dependent. To provide further insight into these issues, we made detailed observations at baseline and post-erythroid stress (E-stress) in 2 mouse models with genetically depressed macrophage numbers and compared them to their controls. We documented that, irrespective of the stress-induced (hemolytic or post-erythropoietin [Epo]) treatment, only native CD11b(lo) splenic macrophages expand dramatically post-stress in normal mice without significant changes in the monocyte-derived CD11b(hi) subset. The latter remained a minority and did not change post-stress in 2 genetic models lacking either Spi-C or VCAM-1 with impaired native macrophage proliferative expansion. Although CD11b(lo) macrophages in these mice were one-fifth of normal at their peak response, surprisingly, their erythroid response was not compromised and was similar to controls. Thus, despite the prior emphasis on numerical macrophage reliance to provide functional rescue from E-stress, our data highlight the importance of previously described non-macrophage-dependent pathways activated under certain stress conditions to compensate for low macrophage numbers.

Gas6 protein: its role in cardiovascular calcification

BACKGROUND

Cardiovascular calcifications can be prevented by vitamin K and are accelerated by vitamin K antagonists. These effects are believed to be mainly mediated by the vitamin K-dependent matrix Gla protein. Another vitamin K-dependent protein, Gas6, is also expressed in vascular smooth muscle cells (VSMC). In vitro Gas6 expression was shown to be regulated in VSMC calcification and apoptotic processes.

METHODS

We investigated the role of Gas6 in vitro using VSMC cultures and in vivo in young and old Gas6-deficient (Gas6(-/-)) and wildtype (WT) mice. In addition, Gas6(-/-) and WT mice were challenged by (a) warfarin administration, (b) uninephrectomy (UniNX) plus high phosphate diet, or (c) UniNX plus high phosphate plus electrocautery of the residual kidney.

RESULTS

In vitro VSMC from WT and Gas6(-/-) mice exposed to warfarin showed increased apoptosis and calcified similarly. In vivo, aortic, cardiac and renal calcium content in all groups was similar, except for a lower cardiac calcium content in Gas6(-/-) mice (group a). Von Kossa staining revealed small vascular calcifications in both WT and Gas6(-/-) mice (groups a-c). In aging, non-manipulated mice, no significant differences in vascular calcification were identified between Gas6(-/-) and WT mice. Gas6(-/-) mice exhibited no upregulation of matrix Gla protein in any group. Cardiac output was similar in all treatment groups.

CONCLUSIONS

Taken together, in our study Gas6 fails to aggravate calcification against the previous assumption.

Circulating growth arrest-specific protein 6 levels are associated with erythropoietin resistance in hemodialysis patients

Growth arrest-specific protein 6 (Gas6) works synergistically with erythropoietin (EPO) to increase the proliferation and maturation of erythroblasts. However, the role of Gas 6 levels on EPO resistance in hemodialysis (HD) patients remains unclear. Therefore, the objective of this study was the first to examine the correlation between plasma Gas6 levels and EPO resistance in HD patients. We enrolled 134 HD patients and 85 healthy individuals. The HD patients were divided into 2 groups: 98 non-EPO-resistant patients and 36 EPO-resistant patients. Plasma levels of Gas6, interleukin 6 (IL-6), high-sensitivity C-reactive protein (hs-CRP), and albumin were quantified. Compared with non-EPO-resistant patients, EPO-resistant patients had elevated plasma concentrations of Gas6 (15.4 ± 3.3 vs. 13.7 ± 3.2 ng/mL, P = 0.006), IL-6 (3.1 ± 3.1 vs. 2.1 ± 1.5 pg/mL, P = 0.009), and hs-CRP (12.7 ± 25.2 vs. 4.5 ± 5.5 mg/L, P = 0.002). In EPO-resistant HD patients, plasma Gas6 levels were negatively correlated with albumin levels (r = −0.388, P < 0.021). Elevated Gas6 levels are associated with EPO resistance in HD patients. Also, EPO resistance is related to inflammation and malnutrition. Thus, circulating Gas6 levels could be used as the potential marker in HD patients with EPO resistance.

Phylogenetic and Ontogenetic View of Erythroblastic Islands

Erythroblastic islands are a hallmark of mammalian erythropoiesis consisting of a central macrophage surrounded by and interacting closely with the maturing erythroblasts. The macrophages are thought to serve many functions such as supporting erythroblast proliferation, supplying iron for hemoglobin, promoting enucleation, and clearing the nuclear debris; moreover, inhibition of erythroblastic island formation is often detrimental to erythropoiesis. There is still much not understood about the role that macrophages and microenvironment play in erythropoiesis and insights may be gleaned from a comparative analysis with erythropoietic niches in nonmammalian vertebrates which, unlike mammals, have erythrocytes that retain their nucleus. The phylogenetic development of erythroblastic islands in mammals in which the erythrocytes are anucleate underlines the importance of the macrophage in erythroblast enucleation.

Elevated Serum Gas6 Is a Novel Prognostic Biomarker in Patients with Oral Squamous Cell Carcinoma

Objective

This study explored the level and clinical significance of serum Gas6 in patients with oral squamous cell carcinoma (OSCC).

Methods

A total of 128 OSCC patients and 145 normal controls were selected. Enzyme-linked immunosorbent assay was used to detect Gas6 concentration in sera from the OSCC patients and controls. The correlations of serum Gas6 concentration and clinicopathological characteristics of OSCC patients were assessed, and the prognostic significance of serum Gas6 was evaluated with a Kaplan–Meier curve and log-rank test.

Results

The results showed that serum Gas6 concentration was significantly higher in OSCC patients than in controls (P < 0.05). OSCC patients with late TNM stage (III, IV) had a relatively high serum Gas6 concentration compared with those with early stage (I, II) (P < 0.01) and patients with poorly differentiated tumors had a higher level of serum Gas6 than those with well-differentiated tumors (P < 0.01). Multivariate logistic regression analysis demonstrated that high serum Gas6 was an independent risk factor for lymph nodal metastases in OSCC patients (OR = 2.79, 95% CI: 1.72–4.48). For predicting OSCC development, ROC curve analysis showed a sensitivity of 0.63 with a specificity of 0.92 (AUC = 0.79, 95% CI: 0.74–0.85). Cox analysis revealed that high serum Gas6 was an independent biomarker for predicting poor overall survival in OSCC patients (HR = 2.07, 95% CI: 1.79–3.62). In addition, we found that Gas6 expression was increased in OSCC tissues and it may significantly decrease E-cadherin expression, and increase P-cadherin and N-cadherin expression, in OSCC cells. Further, Gas6 could promote the migratory and invasive ability of OSCC cells in vitro.

Conclusion

Taken together, these results suggest that Gas6 increases the metastatic capacity of OSCC cells and serum Gas6 could be a candidate biomarker for diagnostic and prognostic use in OSCC patients.

Mer receptor tyrosine kinase mediates both tethering and phagocytosis of apoptotic cells

Billions of inflammatory leukocytes die and are phagocytically cleared each day. This regular renewal facilitates the normal termination of inflammatory responses, suppressing pro-inflammatory mediators and inducing their anti-inflammatory counterparts. Here we investigate the role of the receptor tyrosine kinase (RTK) Mer and its ligands Protein S and Gas6 in the initial recognition and capture of apoptotic cells (ACs) by macrophages. We demonstrate extremely rapid binding kinetics of both ligands to phosphatidylserine (PtdSer)-displaying ACs, and show that ACs can be co-opsonized with multiple PtdSer opsonins. We further show that macrophage phagocytosis of ACs opsonized with Mer ligands can occur independently of a requirement for αV integrins. Finally, we demonstrate a novel role for Mer in the tethering of ACs to the macrophage surface, and show that Mer-mediated tethering and subsequent AC engulfment can be distinguished by their requirement for Mer kinase activity. Our results identify Mer as a receptor uniquely capable of both tethering ACs to the macrophage surface and driving their subsequent internalization.

The TAM family: phosphatidylserine sensing receptor tyrosine kinases gone awry in cancer

The TYRO3, AXL (also known as UFO) and MERTK (TAM) family of receptor tyrosine kinases (RTKs) are aberrantly expressed in multiple haematological and epithelial malignancies. Rather than functioning as oncogenic drivers, their induction in tumour cells predominately promotes survival, chemoresistance and motility. The unique mode of maximal activation of this RTK family requires an extracellular lipid–protein complex. For example, the protein ligand, growth arrest-specific protein 6 (GAS6), binds to phosphatidylserine (PtdSer) that is externalized on apoptotic cell membranes, which activates MERTK on macrophages. This triggers engulfment of apoptotic material and subsequent anti-inflammatory macrophage polarization. In tumours, autocrine and paracrine ligands and apoptotic cells are abundant, which provide a survival signal to the tumour cell and favour an anti-inflammatory, immunosuppressive microenvironment. Thus, TAM kinase inhibition could stimulate antitumour immunity, reduce tumour cell survival, enhance chemosensitivity and diminish metastatic potential.

MIF contributes to Trypanosoma brucei associated immunopathogenicity development

African trypanosomiasis is a chronic debilitating disease affecting the health and economic well-being of many people in developing countries. The pathogenicity associated with this disease involves a persistent inflammatory response, whereby M1-type myeloid cells, including Ly6C^high inflammatory monocytes, are centrally implicated. A comparative gene analysis between trypanosusceptible and trypanotolerant animals identified MIF (macrophage migrating inhibitory factor) as an important pathogenic candidate molecule. Using MIF-deficient mice and anti-MIF antibody treated mice, we show that MIF mediates the pathogenic inflammatory immune response and increases the recruitment of inflammatory monocytes and neutrophils to contribute to liver injury in Trypanosoma brucei infected mice. Moreover, neutrophil-derived MIF contributed more significantly than monocyte-derived MIF to increased pathogenic liver TNF production and liver injury during trypanosome infection. MIF deficient animals also featured limited anemia, coinciding with increased iron bio-availability, improved erythropoiesis and reduced RBC clearance during the chronic phase of infection. Our data suggest that MIF promotes the most prominent pathological features of experimental trypanosome infections (i.e. anemia and liver injury), and prompt considering MIF as a novel target for treatment of trypanosomiasis-associated immunopathogenicity.

Uncontrolled inflammation is a major contributor to pathogenicity development during many chronic parasitic infections, including African trypanosome infections. Hence, therapies should aim at re-establishing the balance between pro- and anti-inflammatory responses to reduce tissue damage. Our experiments uncovered that macrophage migration inhibitory factor (MIF) plays a pivotal role in trypanosomiasis-associated pathogenicity development. Hereby, MIF-deficient and neutralizing anti-MIF antibody-treated wild type (WT) T. brucei-infected mice exhibited decreased inflammatory responses, reduced liver damage and anemia (i.e. the most prominent pathogenicity features) compared to WT control mice. The reduced tissue damage coincided with reduced infiltration of pathogenic monocytic cells and neutrophils, whereby neutrophil-derived MIF contributed more significantly than monocyte-derived MIF to tissue damage. MIF also promoted anemia development by suppressing red blood cell production and enhancing their clearance. The clinical significance of these findings follows from human genetic data indicating that low-expression (protective) MIF alleles are enriched in Africans. The current findings therefore offer promise for human translation and open the possibility of assessing MIF levels or MIF genotype as an indication of an individual's risk for severe trypanosomiasis. Furthermore, given the unmet medical need of African trypanosomiasis affecting millions of people, these findings highlight MIF as a potential new therapeutic target for treatment of trypanosomiasis-associated pathogenicity.

Management of anemia with erythropoietic-stimulating agents in children with chronic kidney disease

Anemia management is an important component of the care provided to children with chronic kidney disease (CKD) and influences both morbidity and mortality risk. The introduction of recombinant human erythropoietin to the treatment regimen three decades ago revolutionized the therapy and significantly decreased the need for repeated blood transfusions and exposure to associated risks. Recent data on the efficacy and complications associated with erythropoietic-stimulating agent (ESA) usage has, however, prompted a reassessment of treatment-related recommendations. This review will address these recommendations, in addition to describing pediatric outcomes associated with current ESAs and presenting information on alternative ESAs, many of which will likely soon be incorporated into clinical practice.

Role of Growth arrest-specific gene 6-Mer axis in multiple myeloma

Multiple myeloma is a mostly incurable malignancy characterized by the expansion of a malignant plasma cell (PC) clone in the human bone marrow (BM). Myeloma cells closely interact with the BM stroma, which secretes soluble factors that foster myeloma progression and therapy resistance. Growth arrest-specific gene 6 (Gas6) is produced by BM-derived stroma cells and can promote malignancy. However, the role of Gas6 and its receptors Axl, Tyro3 and Mer (TAM receptors) in myeloma is unknown. We therefore investigated their expression in myeloma cell lines and in the BM of myeloma patients and healthy donors. Gas6 showed increased expression in sorted BMPCs of myeloma patients compared with healthy controls. The fraction of Mer(+) BMPCs was increased in myeloma patients in comparison with healthy controls whereas Axl and Tyro3 were not expressed by BMPCs in the majority of patients. Downregulation of Gas6 and Mer inhibited the proliferation of different myeloma cell lines, whereas knocking down Axl or Tyro3 had no effect. Inhibition of the Gas6 receptor Mer or therapeutic targeting of Gas6 by warfarin reduced myeloma burden and improved survival in a systemic model of myeloma. Thus, the Gas6-Mer axis represents a novel candidate for therapeutic intervention in this incurable malignancy.

Of macrophages and red blood cells; a complex love story

Macrophages tightly control the production and clearance of red blood cells (RBC). During steady state hematopoiesis, approximately 10¹⁰ RBC are produced per hour within erythroblastic islands in humans. In these erythroblastic islands, resident bone marrow macrophages provide erythroblasts with interactions that are essential for erythroid development. New evidence suggests that not only under homeostasis but also under stress conditions, macrophages play an important role in promoting erythropoiesis. Once RBC have matured, these cells remain in circulation for about 120 days. At the end of their life span, RBC are cleared by macrophages residing in the spleen and the liver. Current theories about the removal of senescent RBC and the essential role of macrophages will be discussed as well as the role of macrophages in facilitating the removal of damaged cellular content from the RBC. In this review we will provide an overview on the role of macrophages in the regulation of RBC production, maintenance and clearance. In addition, we will discuss the interactions between these two cell types during transfer of immune complexes and pathogens from RBC to macrophages.

Comparative analysis of gene transcripts for cell signaling receptors in bone marrow-derived hematopoietic stem/progenitor cell and mesenchymal stromal cell populations

Introduction

Knowing the repertoire of cell signaling receptors would provide pivotal insight into the developmental and regenerative capabilities of bone marrow cell (BMC)-derived hematopoietic stem/progenitor cells (HSPCs) and bone marrow mesenchymal stromal cells (BMMSCs).

Methods

Murine HSPCs were enriched from fluorescence-activated cell sorting (FACS)-sorted Lin^–c-Kit⁺Sca-1⁺ BMCs isolated from the tibia and femoral marrow compartments. Purified BMMSCs (CD73⁺, CD90⁺, CD105⁺, and CD45^–, CD34^–, CD31^–, c-Kit^–) with extensive self-renewal potential and multilineage differentiation capacity (into different mesodermal cell lineages including osteocytes, chrondrocytes, adipocytes) were derived from adherent BMC cultures after CD45⁺ cell depletion. Adherent colony-forming cells were passaged two to three times and FACS analysis was used to assess cell purity and validate cell-specific surface marker phenotype prior to experimentation. Gene transcripts for a number of cell signaling molecules were assessed using a custom quantitative real-time RT-PCR low-density microarray (94 genes; TaqMan^® technology).

Results

We identified 16 mRNA transcripts that were specifically expressed in BMC-derived HSPC (including Ptprc, c-Kit, Csf3r, Csf2rb2, Ccr4, Cxcr3 and Tie-1), and 14 transcripts specifically expressed in BMMSCs (including Pdgfra, Ddr2, Ngfr, Mst1r, Fgfr2, Epha3, and Ephb3). We also identified 27 transcripts that were specifically upregulated (≥2-fold expression) in BMMSCs relative to HSPCs (Axl, Bmpr1a, Met, Pdgfrb, Fgfr1, Mertk, Cmkor1, Egfr, Epha7, and Ephb4), and 19 transcripts that were specifically upregulated in HSPCs relative to BMMSCs (Ccr1, Csf1r, Csf2ra, Epor, IL6ra, and IL7r). Eleven transcripts were equally expressed (<2-fold upregulation) in HSPCs and BMMSCs (Flt1, Insr, Kdr, Jak1, Agtrl1, Ccr3, Ednrb, Il3ra, Hoxb4, Tnfrsf1a, and Abcb1b), whilst another seven transcripts (Epha6, Epha8, Musk, Ntrk2, Ros1, Srms, and Tnk1) were not expressed in either cell population.

Conclusions

We demonstrate that besides their unique immunophenotype and functional differences, BMC-derived HSPCs and BMMSCs have different molecular receptor signaling transcript profiles linked to cell survival, growth, cell differentiation status, growth factor/cytokine production and genes involved in cell migration/trafficking/adhesion that may be critical to maintain their pluripotency, plasticity, and stem cell function.

Erythroid cells generated in the absence of specific β1-integrin heterodimers accumulate reactive oxygen species at homeostasis and are unable to mount effective antioxidant defenses

We have previously reported that β1(Δ/Δ) mice have a markedly impaired response to hemolytic stress, but the mechanisms of this were unclear. In the present study we explored in detail quantitative, phenotypic and functional aspects of erythropoiesis at homeostasis in a large number of animals for each of 3 murine models with specific β1 heterodimer integrin deficiencies. We found that, at homeostasis, β1-deficient mice have a modest uncompensated anemia with ineffective erythropoiesis and decreased red blood cell survival. Mice lacking only α4 integrins (α4β1/α4β7) do not share this phenotype. There is an increased tendency for reactive oxygen species accumulation in β1(Δ/Δ) erythroid cells with decreased anti-oxidant defenses at homeostasis which are exaggerated after stress. Furthermore, expansion of erythroid cells in spleen post-stress is dependent on α5β1, likely through mechanisms activating focal adhesion kinase complexes that are distinct from α4β1-mediated responses. In vivo inhibition of focal adhesion kinase activation partially recapitulates the β1(Δ/Δ) stress response. Mice lacking all α4 and β1 integrins (double knockouts) had, at homeostasis, the most severe phenotype with selective impairment of erythroid responses. The fact that integrins participate in mitigating stress in erythroid cells through redox activation of distinct signaling pathways by specific integrin heterodimers is a link that has not been appreciated until now.

Macrophages support pathological erythropoiesis in polycythemia vera and β-thalassemia

Regulation of erythropoiesis is achieved by the integration of distinct signals. Among them, macrophages are emerging as erythropoietin-complementary regulators of erythroid development, particularly under stress conditions. We investigated the contribution of macrophages to physiological and pathological conditions of enhanced erythropoiesis. We used mouse models of induced anemia, polycythemia vera and β-thalassemia in which macrophages were chemically depleted. Our data indicate that macrophages contribute decisively to recovery from induced anemia, as well as the pathological progression of polycythemia vera and β-thalassemia, by modulating erythroid proliferation and differentiation. We validated these observations in primary human cultures, showing a direct impact of macrophages on the proliferation and enucleation of erythroblasts from healthy individuals and patients with polycythemia vera or β-thalassemia. The contribution of macrophages to stress and pathological erythropoiesis, which we have termed stress erythropoiesis macrophage-supporting activity, may have therapeutic implications.

Macrophage-tumor crosstalk: role of TAMR tyrosine kinase receptors and of their ligands

Ample clinical and preclinical evidence indicates that macrophages interact with tumor cells as well as with virtually all populations of host cells present in the tumor microenvironment. This crosstalk can strongly promote malignancy, but also has in principle the potential to inhibit tumor growth. Thus, it is of the utmost importance to improve our understanding of the mechanisms driving the pro- and antimalignant behavior of tumor-associated macrophages (TAMs) in order to develop better anticancer therapies. In this review, we discuss the biological consequences of reciprocal interactions between TAMs, cancer cells, endothelial cells, fibroblasts and other leukocyte subfractions within tumors. It was recently elucidated that tumors specifically educate macrophages to secrete growth arrest-specific gene 6 (Gas6), the common ligand of the Tyro3, Axl, Mer receptor (TAMR) family. In turn, Gas6 fosters tumor growth by promoting cancer cell proliferation. Therefore, the Gas6-TAMR axis might represent a novel target for disrupting tumor-macrophage crosstalk. We summarize here what is known about TAMR and their ligands in (human) cancer biology. In order to shed more light on the role of macrophages in human cancer, we additionally provide an overview of what is currently known about the prognostic impact of TAMs in human cancer.

Lipopolysaccharide inhibits macrophage phagocytosis of apoptotic neutrophils by regulating the production of tumour necrosis factor α and growth arrest-specific gene 6

Removal of apoptotic cells from inflammatory sites by macrophages is an important step in the resolution of inflammation. However, the effect of inflammatory modulators on phagocytic clearance of apoptotic cells remains to be clarified. In this paper, we demonstrate that lipopolysaccharide (LPS), a potent inflammatory agent, inhibits the phagocytosis of apoptotic neutrophils by mouse peritoneal macrophages. This inhibition can be attributed to both LPS-mediated induction of tumour necrosis factor (TNF-α) and suppression of growth arrest-specific gene 6 (Gas6) in macrophages. We found that LPS-induced TNF-α production inhibited phagocytic ability of macrophages in an autocrine manner. In contrast, Gas6 expression in macrophages was blocked by LPS, which also contributes to the inhibition of macrophage phagocytosis by LPS. Our data suggest that phagocytic clearance of apoptotic neutrophils by macrophages can be regulated by local pro- and anti-inflammatory factors in two opposite states.

Combinatorial and distinct roles of α₅ and α₄ integrins in stress erythropoiesis in mice

To delineate the role of specific members of β₁ integrins in stress erythropoiesis in the adult, we compared the response to phenylhydrazine stress in 3 genetically deficient models. The survival of β₁-conditionally deficient mice after phenylhydrazine is severely compromised because of their inability to mount a successful life saving splenic erythroid response, a phenotype reproduced in β₁(Δ/Δ) reconstituted animals. The response of bone marrow to phenylhydrazine-induced stress was, unlike that of spleen, appropriate in terms of progenitor cell expansion and mobilization to peripheral blood although late differentiation defects qualitatively similar to those in spleen were present in bone marrow. In contrast to β₁-deficient mice, α₄(Δ/Δ) mice showed only a kinetic delay in recovery and similar to β₁(Δ/Δ), terminal maturation defects in both bone marrow and spleen, which were not present in VCAM-1(Δ/Δ) mice. Convergence of information from these comparative studies lends new insight to the distinct in vivo roles of α₄ and α₅ integrins in erythroid stress, suggesting that the presence of mainly α₅β₁ integrin in all hematopoietic progenitor cells interacting with splenic microenvironmental ligands/cells is instrumental for their survival and accumulation during hemolytic stress, whereas presence of α₄ or of both α₅ and α₄, is important for completion of terminal maturation steps.

Taking aim at Mer and Axl receptor tyrosine kinases as novel therapeutic targets in solid tumors

IMPORTANCE OF THE FIELD

Axl and/or Mer expression correlates with poor prognosis in several cancers. Until recently, the role of these receptor tyrosine kinases (RTKs) in development and progression of cancer remained unexplained. Studies demonstrating that Axl and Mer contribute to cell survival, migration, invasion, metastasis and chemosensitivity justify further investigation of Axl and Mer as novel therapeutic targets in cancer.

AREAS COVERED IN THIS REVIEW

Axl and Mer signaling pathways in cancer cells are summarized and evidence validating these RTKs as therapeutic targets in glioblastoma multiforme, NSCLC, and breast cancer is examined. A discussion of Axl and/or Mer inhibitors in development is provided.

WHAT THE READER WILL GAIN

Potential toxicities associated with Axl or Mer inhibition are addressed. We propose that the probable action of Mer and Axl inhibitors on cells within the tumor microenvironment will provide a therapeutic opportunity to target both tumor cells and the stromal components that facilitate disease progression.

TAKE HOME MESSAGE

Axl and Mer mediate multiple oncogenic phenotypes and activation of these RTKs constitutes a mechanism of chemoresistance in a variety of solid tumors. Targeted inhibition of these RTKs may be effective as anti-tumor and/or anti-metastatic therapy, particularly if combined with standard cytotoxic therapies.

AXL is an essential factor and therapeutic target for metastatic ovarian cancer

The receptor tyrosine kinase AXL is thought to play a role in metastasis; however, the therapeutic efficacy of an AXL-targeting agent remains largely untested in metastatic disease. In this study, we defined AXL as a therapeutic target for metastatic ovarian cancer. AXL is primarily expressed in metastases and advanced-stage human ovarian tumors but not in normal ovarian epithelium. Genetic inhibition of AXL in human metastatic ovarian tumor cells is sufficient to prevent the initiation of metastatic disease in vivo. Mechanistically, inhibition of AXL signaling in animals with metastatic disease results in decreased invasion and matrix metalloproteinase activity. Most importantly, soluble human AXL receptors that imposed a specific blockade of the GAS6/AXL pathway had a profound inhibitory effect on progression of established metastatic ovarian cancer without normal tissue toxicity. These results offer the first genetic validation of GAS6/AXL targeting as an effective strategy for inhibition of metastatic tumor progression in vivo. Furthermore, this study defines the soluble AXL receptor as a therapeutic candidate agent for treatment of metastatic ovarian cancer, for which current therapies are ineffective.

During EPO or anemia challenge, erythroid progenitor cells transit through a selectively expandable proerythroblast pool

Investigations of bone marrow (BM) erythroblast development are important for clinical concerns but are hindered by progenitor cell and tissue availability. We therefore sought to more specifically define dynamics, and key regulators, of the formation of developing BM erythroid cell cohorts. A unique Kit(-)CD71(high)Ter119(-) "stage E2" proerythroblast pool first is described, which (unlike its Kit(+) "stage E1" progenitors, or maturing Ter119(+) "stage E3" progeny) proved to selectively expand ∼ 7-fold on erythropoietin challenge. During short-term BM transplantation, stage E2 proerythroblasts additionally proved to be a predominantly expanded progenitor pool within spleen. This E1→E2→E3 erythroid series reproducibly formed ex vivo, enabling further characterizations. Expansion, in part, involved E1 cell hyperproliferation together with rapid E2 conversion plus E2 stage restricted BCL2 expression. Possible erythropoietin/erythropoietin receptor proerythroblast stage specific events were further investigated in mice expressing minimal erythropoietin receptor alleles. For a hypomorphic erythropoietin receptor-HM allele, major defects in erythroblast development occurred selectively at stage E2. In addition, stage E2 cells proved to interact productively with primary BM stromal cells in ways that enhanced both survival and late-stage development. Overall, findings reveal a novel transitional proerythroblast compartment that deploys unique expansion devices.

Gas6 deficiency in recipient mice of allogeneic transplantation alleviates hepatic graft-versus-host disease

Growth arrest-specific gene 6 (Gas6) is expressed in antigen-presenting cells and endothelial cells (ECs) but not in T cells. When wild-type (WT) or Gas6(-/-) mice received allogeneic non-T cell-depleted bone marrow cells, hepatic graft-versus-host disease (GVHD) was alleviated in Gas6(-/-) recipients regardless of donor genotype, but not in WT recipients. T-cell infiltration was more prominent and diffuse in WT than in Gas6(-/-) recipients' liver. When mice received 0.5 x 10(6) allogeneic T cells with T cell-depleted allogeneic bone marrow, clinical signs indicated that GVHD was less severe in Gas6(-/-) than in WT recipients, as shown by a significant improvement of the survival and reduced liver GVHD. These data demonstrate that donor cells were not involved in the protection mechanism. In addition, lack of Gas6 in antigen-presenting cells did not affect WT or Gas6(-/-) T-cell proliferation. We therefore assessed the response of WT or Gas6(-/-) ECs to tumor necrosis factor-alpha. Lymphocyte transmigration was less extensive through Gas6(-/-) than WT ECs and was not accompanied by increases in adhesion molecule levels. Thus, the lack of Gas6 in ECs impaired donor T-cell transmigration into the liver, providing a rationale for considering Gas6 pathway as a potential nonimmunosuppressive target to minimize GVHD in patients receiving allogeneic hematopoietic stem cell transplantation.

TAM receptors and the regulation of erythropoiesis in mice

BACKGROUND

TAM receptors (Tyro3, Axl and Mer) are expressed in hematopoietic tissues. The roles of the three receptors in hematopoiesis are, however, largely unknown. We investigated the role of TAM receptors in regulating erythropoiesis.

DESIGN AND METHODS

Single and double mutant mice for Axl and Mer were used in the study. Cellularity of bone marrow and spleen, hematologic parameters, flow cytometry analysis of erythroid cell maturation, erythropoietic response to acute hemolytic anemia, bone marrow transplantation and the expression of erythropoisis were analyzed to evaluate the function of Axl and Mer in erythropoiesis.

RESULTS

Axl and Mer, but not Tyro3, were constitutively expressed in developing erythroid cells. Mice lacking Axl and Mer (Axl(-/-)Me(-/-)) had impaired erythropoiesis in bone marrow and expanded splenic erythropoiesis. We found an inhibition of differentiation at the transition from erythroid progenitors to proerythroblasts in Axl(-/-)Mer(-/-) mice. These mice exhibited a low rate of erythropoietic response to acute anemia induced by phenylhydrazine. Bone marrow transplantation studies showed that the impaired erythropoiesis in Axl(-/-)Mer(-/-) mice is erythroid cell-autonomous. TAM receptors may influence erythropoiesis through the regulation of GATA-1 erythropoietin receptor and EpoR expression in erythroid progenitors. Notably, mice lacking single Axl or Mer exhibited normal erythropoiesis in steady-state conditions.

CONCLUSIONS

Axl and Mer play an important role in regulating erythropoiesis. This finding provides a novel insight into the mechanism of erythropoiesis.

Erythroblastic islands: niches for erythropoiesis

Erythroblastic islands, the specialized niches in which erythroid precursors proliferate, differentiate, and enucleate, were first described 50 years ago by analysis of transmission electron micrographs of bone marrow. These hematopoietic subcompartments are composed of erythroblasts surrounding a central macrophage. A hiatus of several decades followed, during which the importance of erythroblastic islands remained unrecognized as erythroid progenitors were shown to possess an autonomous differentiation program with a capacity to complete terminal differentiation in vitro in the presence of erythropoietin but without macrophages. However, as the extent of proliferation, differentiation, and enucleation efficiency documented in vivo could not be recapitulated in vitro, a resurgence of interest in erythroid niches has emerged. We now have an increased molecular understanding of processes operating within erythroid niches, including cell-cell and cell-extracellular matrix adhesion, positive and negative regulatory feedback, and central macrophage function. These features of erythroblast islands represent important contributors to normal erythroid development, as well as altered erythropoiesis found in such diverse diseases as anemia of inflammation and chronic disease, myelodysplasia, thalassemia, and malarial anemia. Coupling of historical, current, and future insights will be essential to understand the tightly regulated production of red cells both in steady state and stress erythropoiesis.

Immunobiology of the TAM receptors

Recent studies have revealed that the TAM receptor protein tyrosine kinases--TYRO3, AXL and MER--have pivotal roles in innate immunity. They inhibit inflammation in dendritic cells and macrophages, promote the phagocytosis of apoptotic cells and membranous organelles, and stimulate the maturation of natural killer cells. Each of these phenomena may depend on a cooperative interaction between TAM receptor and cytokine receptor signalling systems. Although its importance was previously unrecognized, TAM signalling promises to have an increasingly prominent role in studies of innate immune regulation.