Vascular smooth muscle cell-derived, Gla-containing growth-potentiating factor for Ca(2+)-mobilizing growth factors

Gas6 induces AIM to suppress acute lung injury in mice by inhibiting NLRP3 inflammasome activation and inducing autophagy

Introduction

Growth arrest-specific 6 (Gas6) protein signaling plays a critical role in maintaining immune homeostasis and regulating inflammation. However, novel mechanisms for modulating macrophage activity through the Gas6 axis are being identified. Gas6 enhances the production of apoptosis inhibitor of macrophages (AIM), a protein with potent anti-inflammatory properties. This study investigates whether Gas6-induced AIM suppresses acute lung injury (ALI) in mice by modulating key inflammatory pathways, including inflammasome activation, autophagy, reactive oxygen species (ROS) generation, and efferocytosis.

Methods

ALI was induced in wild-type (WT) and AIM-/- mice via intratracheal administration of LPS. To evaluate the effects of the Gas6-AIM axis on lung inflammation, recombinant Gas6 (rGas6) was treated intraperitoneally. Inflammatory responses were evaluated using enzyme-linked immunosorbent assay, a cell-sizing analyzer, and Bicinchoninic acid assays. Lung pathology was assessed using hematoxylin-eosin staining. NLRP3 inflammasome activation and autophagy were evaluated using western blot, quantitative real-time PCR, and immunofluorescence. Reactive oxygen species (ROS) levels in alveolar macrophages were measured via fluorescence microscopy, while efferocytosis was assessed in cytospin-stained BAL cells and cultured alveolar macrophages co-cultured with apoptotic Jurkat cells. Additionally, rGas6-mediated effects on NLRP3 inflammasome activation and autophagy were validated in mouse bone marrow-derived macrophages (BMDMs) using siRNAs targeting AIM, Axl, LXRα, or LXRβ.

Results

Proinflammatory cytokine production, neutrophil infiltration, and protein levels in BALF were significantly reduced by rGas6 administration in WT mice but not in AIM-/- mice. Specifically, rGas6 reduced IL-1β and IL-18 levels, caspase-1 activity, and the production of apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC) in alveolar macrophages. Additionally, rGas6 promoted autophagy and efferocytosis in alveolar macrophages while reducing ROS levels through AIM production. These protective effects were absent in AIM-/- mice. Furthermore, siRNA-mediated silencing of Axl, LXRα, LXRβ, or AIM reversed the inhibitory effects of rGas6 on NLRP3 inflammasome activation in BMDMs, and AIM was essential for rGas6-induced autophagy.

Conclusion

Gas6-induced AIM production protects against LPS-induced ALI by inhibiting NLRP3 inflammasome activation, enhancing autophagy and efferocytosis, and reducing oxidative stress. These findings highlight the Gas6-AIM axis as a potential therapeutic target for mitigating inflammatory lung diseases.

Immunological role of Gas6/TAM signaling in hemostasis and thrombosis

The immune system is an emerging regulator of hemostasis and thrombosis. The concept of immunothrombosis redefines the relationship between coagulation and immunomodulation, and the Gas6/Tyro3-Axl-MerTK (TAM) signaling pathway builds the bridge across them. During coagulation, Gas6/TAM signaling pathway not only activates platelets, but also promotes thrombosis through endothelial cells and vascular smooth muscle cells involved in inflammatory responses. Thrombosis appears to be a common result of a Gas6/TAM signaling pathway-mediated immune dysregulation. TAM TK and its ligands have been found to be involved in coagulation through the PI3K/AKT or JAK/STAT pathway in various systemic diseases, providing new perspectives in the understanding of immunothrombosis. Gas6/TAM signaling pathway serves as a breakthrough target for novel therapeutic strategies to improve disease management. Many preclinical and clinical studies of TAM receptor inhibitors are in process, confirming the pivotal role of Gas6/TAM signaling pathway in immunothrombosis. Therapeutics targeting the TAM receptor show potential both in anticoagulation management and immunotherapy. Here, we review the immunological functions of the Gas6/TAM signaling pathway in coagulation and its multiple mechanisms in diseases identified to date, and discuss the new clinical strategies that may generated by these roles.

Modification of Gas6 Protein in the Brain by a Functional Endogenous Tissue Vitamin K Cycle

The TAM receptor ligand Gas6 is known for regulating inflammatory and immune pathways in various organs including the brain. Gas6 becomes fully functional through the post-translational modification of multiple glutamic acid residues into γ-carboxyglutamic in a vitamin K-dependent manner. However, the significance of this mechanism in the brain is not known. We report here the endogenous expression of multiple components of the vitamin K cycle within the mouse brain at various ages as well as in distinct brain glial cells. The brain expression of all genes was increased in the postnatal ages, mirroring their profiles in the liver. In microglia, the proinflammatory agent lipopolysaccharide caused the downregulation of all key vitamin K cycle genes. A secreted Gas6 protein was detected in the medium of both mouse cerebellar slices and brain glial cell cultures. Furthermore, the endogenous Gas6 γ-carboxylation level was abolished through incubation with the vitamin K antagonist warfarin and could be restored through co-incubation with vitamin K1. Finally, the γ-carboxylation level of the Gas6 protein within the brains of warfarin-treated rats was found to be significantly reduced ex vivo compared to the control brains. In conclusion, we demonstrated for the first time the existence of a functional vitamin K cycle within rodent brains, which regulates the functional modification of endogenous brain Gas6. These results indicate that vitamin K is an important nutrient for the brain. Furthermore, the measurement of vitamin K-dependent Gas6 functionality could be an indicator of homeostatic or disease mechanisms in the brain, such as in neurological disorders where Gas6/TAM signalling is impaired.

Circulating small extracellular vesicles promote proliferation and migration of vascular smooth muscle cells via AXL and MerTK activation

The proliferation and migration of vascular smooth muscle cells (VSMCs) after vascular injury lead to neointimal hyperplasia, thus aggravating vascular diseases. However, the molecular mechanisms underlying neointima formation are not fully elucidated. Extracellular vesicles (EVs) are mediators of various intercellular communications. The potential of EVs as regulators in cardiovascular diseases has raised significant interest. In the current study we investigated the role of circulating small extracellular vesicles (csEVs), the most abundant EVs (1010 EVs/mL serum) in VSMC functions. csEVs were prepared from bovine, porcine or rat serum. We showed that incubation with csEVs (0.5 × 1010-2 × 1010) dose-dependently enhanced the proliferation and migration of VSMCs via the membrane phosphatidylserine (PS). In rats with ligation of right carotid artery, we demonstrated that application of csEVs in the ligated vessels aggravated neointima formation via interaction of membrane PS with injury. Furthermore, incubation with csEVs markedly enhanced the phosphorylation of AXL and MerTK in VSMCs. Pretreatment with BSM777607 (pan-TAM inhibitor), bemcentinib (AXL inhibitor) or UNC2250 (MerTK inhibitor) blocked csEV-induced proliferation and migration of VSMCs. We revealed that csEV-activated AXL and MerTK shared the downstream signaling pathways of Akt, extracellular signal-regulated kinase (ERK) and focal adhesion kinase (FAK) that mediated the effects of csEVs. We also found that csEVs increased the expression of AXL through activation of transcription factor YAP, which might constitute an AXL-positive feedback loop to amplify the signals. Finally, we demonstrated that dual inhibition of AXL/MerTK by ONO-7475 (0.1 µM) effectively hindered csEV-mediated proliferation and migration of VSMCs in ex vivo mouse aorta injury model. Based on these results, we propose an essential role for csEVs in proliferation and migration of VSMCs and highlight the feasibility of dual AXL/MerTK inhibitors in the treatment of vascular diseases.

Gas6 Ameliorates Inflammatory Response and Apoptosis in Bleomycin-Induced Acute Lung Injury

Acute lung injury (ALI) is characterized by alveolar damage, lung edema, and exacerbated inflammatory response. Growth arrest-specific protein 6 (Gas6) mediates many different functions, including cell survival, proliferation, inflammatory signaling, and apoptotic cell clearance (efferocytosis). The role of Gas6 in bleomycin (BLM)-induced ALI is unknown. We investigated whether exogenous administration of mouse recombinant Gas6 (rGas6) has anti-inflammatory and anti-apoptotic effects on BLM-induced ALI. Compared to mice treated with only BLM, the administration of rGas6 reduced the secretion of proinflammatory cytokines, including tumor necrosis factor-α, interleukin-1β, and macrophage inflammatory protein-2, and increased the secretion of hepatocyte growth factor in bronchoalveolar lavage (BAL) fluid. rGas6 administration also reduced BLM-induced inflammation and apoptosis as evidenced by reduced neutrophil recruitment into the lungs, total protein levels in BAL fluid, caspase-3 activity, and TUNEL-positive lung cells in lung tissue. Apoptotic cell clearance by alveolar macrophages was also enhanced in mice treated with both BLM and rGas6 compared with mice treated with only BLM. rGas6 also had pro-resolving and anti-apoptotic effects in mouse bone marrow-derived macrophages and alveolar epithelial cell lines stimulated with BLM in vitro. These findings indicate that rGas6 may play a protective role in BLM-induced ALI.

Phosphatidylethanolamine and Phosphatidylserine Synergize To Enhance GAS6/AXL-Mediated Virus Infection and Efferocytosis

Phosphatidylserine (PS) receptors mediate clearance of apoptotic cells-efferocytosis-by recognizing the PS exposed on those cells. They also mediate the entry of enveloped viruses by binding PS in the virion membrane. Here, we show that phosphatidylethanolamine (PE) synergizes with PS to enhance PS receptor-mediated efferocytosis and virus entry. The presence of PE on the same surface as PS dramatically enhances recognition of PS by PS-binding proteins such as GAS6, PROS, and TIM1. Liposomes containing both PE and PS bound to GAS6 and were engulfed by AXL-expressing cells much more efficiently than those containing PS alone. Further, infection of AXL-expressing cells by infectious Zika virus or Ebola, Chikungunya, or eastern equine encephalitis pseudoviruses was inhibited with greater efficiency by the liposomes containing both PS and PE compared to a mixture of liposomes separately composed of PS and PE. These data demonstrate that simultaneous recognition of PE and PS maximizes PS receptor-mediated virus entry and efferocytosis and underscore the important contribution of PE in these major biological processes.IMPORTANCE Phosphatidylserine (PS) and phosphatidylethanolamine (PE) are usually sequestered to the inner leaflet of the plasma membrane of the healthy eukaryotic cells. During apoptosis, these phospholipids move to the cell's outer leaflet where they are recognized by so-called PS receptors on surveilling phagocytes. Several pathogenic families of enveloped viruses hijack these PS receptors to gain entry into their target cells. Here, we show that efficiency of these processes is enhanced, namely, PE synergizes with PS to promote PS receptor-mediated virus infection and clearance of apoptotic cells. These findings deepen our understanding of how these fundamental biological processes are executed.

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

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

Regulation of Energy Metabolism by Receptor Tyrosine Kinase Ligands

Metabolic diseases, such as diabetes, obesity, and fatty liver disease, have now reached epidemic proportions. Receptor tyrosine kinases (RTKs) are a family of cell surface receptors responding to growth factors, hormones, and cytokines to mediate a diverse set of fundamental cellular and metabolic signaling pathways. These ligands signal by endocrine, paracrine, or autocrine means in peripheral organs and in the central nervous system to control cellular and tissue-specific metabolic processes. Interestingly, the expression of many RTKs and their ligands are controlled by changes in metabolic demand, for example, during starvation, feeding, or obesity. In addition, studies of RTKs and their ligands in regulating energy homeostasis have revealed unexpected diversity in the mechanisms of action and their specific metabolic functions. Our current understanding of the molecular, biochemical and genetic control of energy homeostasis by the endocrine RTK ligands insulin, FGF21 and FGF19 are now relatively well understood. In addition to these classical endocrine signals, non-endocrine ligands can govern local energy regulation, and the intriguing crosstalk between the RTK family and the TGFβ receptor family demonstrates a signaling network that diversifies metabolic process between tissues. Thus, there is a need to increase our molecular and mechanistic understanding of signal diversification of RTK actions in metabolic disease. Here we review the known and emerging molecular mechanisms of RTK signaling that regulate systemic glucose and lipid metabolism, as well as highlighting unexpected roles of non-classical RTK ligands that crosstalk with other receptor pathways.

AXL Receptor Tyrosine Kinase as a Therapeutic Target in Hematological Malignancies: Focus on Multiple Myeloma

AXL belongs to the TAM (TYRO3, AXL, and MERTK) receptor family, a unique subfamily of the receptor tyrosine kinases. Their common ligand is growth arrest-specific protein 6 (GAS6). The GAS6/TAM signaling pathway regulates many important cell processes and plays an essential role in immunity, hemostasis, and erythropoiesis. In cancer, AXL overexpression and activation has been associated with cell proliferation, chemotherapy resistance, tumor angiogenesis, invasion, and metastasis; and has been correlated with a poor prognosis. In hematological malignancies, the expression and function of AXL is highly diverse, not only between the different tumor types but also in the surrounding tumor microenvironment. Most research and clinical evidence has been provided for AXL inhibitors in acute myeloid leukemia. However, recent studies also revealed an important role of AXL in lymphoid leukemia, lymphoma, and multiple myeloma. In this review, we summarize the basic functions of AXL in various cell types and the role of AXL in different hematological cancers, with a focus on AXL in the dormancy of multiple myeloma. In addition, we provide an update on the most promising AXL inhibitors currently in preclinical/clinical evaluation and discuss future perspectives in this emerging field.

Gas6/TAM System: A Key Modulator of the Interplay between Inflammation and Fibrosis

Fibrosis is the result of an overly abundant deposition of extracellular matrix (ECM) due to the fact of repetitive tissue injuries and/or dysregulation of the repair process. Fibrogenesis is a pathogenetic phenomenon which is involved in different chronic human diseases, accounting for a high burden of morbidity and mortality. Despite being triggered by different causative factors, fibrogenesis follows common pathways, the knowledge of which is, however, still unsatisfactory. This represents a significant limit for the development of effective antifibrotic drugs. In the present paper, we aimed to review the current evidence regarding the potential role played in fibrogenesis by growth arrest-specific 6 (Gas6) and its receptors Tyro3 protein tyrosine kinase (Tyro3), Axl receptor tyrosine kinase (Axl), and Mer tyrosine kinase protooncogene (MerTK) (TAM). Moreover, we aimed to review data about the pathogenetic role of this system in the development of different human diseases characterized by fibrosis. Finally, we aimed to explore the potential implications of these findings in diagnosis and treatment.

TAM Receptor Pathways at the Crossroads of Neuroinflammation and Neurodegeneration

Increasing evidence suggests that pathogenic mechanisms underlying neurodegeneration are strongly linked with neuroinflammatory responses. Tyro3, Axl, and Mertk (TAM receptors) constitute a subgroup of the receptor tyrosine kinase family, cell surface receptors which transmit signals from the extracellular space to the cytoplasm and nucleus. TAM receptors and the corresponding ligands, Growth Arrest Specific 6 and Protein S, are expressed in different tissues, including the nervous system, playing complex roles in tissue repair, inflammation and cell survival, proliferation, and migration. In the nervous system, TAM receptor signalling modulates neurogenesis and neuronal migration, synaptic plasticity, microglial activation, phagocytosis, myelination, and peripheral nerve repair, resulting in potential interest in neuroinflammatory and neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and Multiple Sclerosis. In Alzheimer and Parkinson diseases, a role of TAM receptors in neuronal survival and pathological protein aggregate clearance has been suggested, while in Multiple Sclerosis TAM receptors are involved in myelination and demyelination processes. To better clarify roles and pathways involving TAM receptors may have important therapeutic implications, given the fine modulation of multiple molecular processes which could be reached. In this review, we summarise the roles of TAM receptors in the central nervous system, focusing on the regulation of immune responses and microglial activities and analysing in vitro and in vivo studies regarding TAM signalling involvement in neurodegeneration.

Gas6/TAM Signaling Components as Novel Biomarkers of Liver Fibrosis

Liver fibrosis consists in the accumulation of extracellular matrix components mainly derived from activated hepatic stellate cells. This is commonly the result of chronic liver injury repair and represents an important health concern. As liver biopsy is burdened with many drawbacks, not surprisingly there is great interest to find new reliable noninvasive methods. Among the many are new potential fibrosis biomarkers under study, some of the most promising represented by the growth arrest-specific gene 6 (Gas6) serum protein and its family of tyrosine kinase receptors, namely, Tyro3, Axl, and MERTK (TAM). Gas6/TAM system (mainly, Axl and MERTK) has in fact recently emerged as an important player in the progression of liver fibrosis. This review is aimed at giving an overall perspective of the roles played by these molecules in major chronic liver diseases. The most promising findings up to date acknowledge that both Gas6 and its receptor serum levels (such as sAxl and, probably, sMERTK) have been shown to potentially allow for easy and accurate measurement of hepatic fibrosis progression, also providing indicative parameters of hepatic dysfunction. Although most of the current scientific evidence is still preliminary and there are no in vivo validation studies on large patient series, it still looks very promising to imagine a possible future prognostic role for these biomarkers in the multidimensional assessment of a liver patient. One may also speculate on a potential role for this system targeting (e.g., with small molecule inhibitors against Axl) as a therapeutic strategy for liver fibrosis management, always bearing in mind that any such therapeutic approach might face toxicity.

TAM receptors in cardiovascular disease

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

Gas6 Prevents Epithelial-Mesenchymal Transition in Alveolar Epithelial Cells via Production of PGE2, PGD2 and Their Receptors

The epithelial-mesenchymal transition (EMT) is important in organ fibrosis. We hypothesized that growth arrest-specific protein 6 (Gas6) and its underlying mechanisms play roles in the prevention of EMT in alveolar epithelial cells (ECs). In this study, to determine whether Gas6 prevents TGF-β1-induced EMT in LA-4 and primary alveolar type II ECs, real-time PCR and immunoblotting in cell lysates and ELISA in culture supernatants were performed. Migration and invasion assays were performed using Transwell chambers. Pretreatment of ECs with Gas6 inhibited TGF-β1-induced EMT based on cell morphology, changes in EMT marker expression, and induction of EMT-activating transcription factors. Gas6 enhanced the levels of cyclooxygenase-2 (COX-2)-derived prostaglandin E₂ (PGE₂) and PGD₂ as well as of their receptors. COX-2 inhibitors and antagonists of PGE₂ and PGD₂ receptors reversed the inhibition of TGF-β1-induced EMT, migration, and invasion by Gas6. Moreover, knockdown of Axl or Mer reversed the enhancement of PGE₂ and PGD₂ and suppression of EMT, migration and invasion by Gas6. Our data suggest Gas6-Axl or -Mer signalling events may reprogram ECs to resist EMT via the production of PGE₂, PGD₂, and their receptors.

Role of Vascular Smooth Muscle Cell Phenotypic Switching and Calcification in Aortic Aneurysm Formation

Aortic aneurysm is a vascular disease whereby the ECM (extracellular matrix) of a blood vessel degenerates, leading to dilation and eventually vessel wall rupture. Recently, it was shown that calcification of the vessel wall is involved in both the initiation and progression of aneurysms. Changes in aortic wall structure that lead to aneurysm formation and vascular calcification are actively mediated by vascular smooth muscle cells. Vascular smooth muscle cells in a healthy vessel wall are termed contractile as they maintain vascular tone and remain quiescent. However, in pathological conditions they can dedifferentiate into a synthetic phenotype, whereby they secrete extracellular vesicles, proliferate, and migrate to repair injury. This process is called phenotypic switching and is often the first step in vascular pathology. Additionally, healthy vascular smooth muscle cells synthesize VKDPs (vitamin K-dependent proteins), which are involved in inhibition of vascular calcification. The metabolism of these proteins is known to be disrupted in vascular pathologies. In this review, we summarize the current literature on vascular smooth muscle cell phenotypic switching and vascular calcification in relation to aneurysm. Moreover, we address the role of vitamin K and VKDPs that are involved in vascular calcification and aneurysm. Visual Overview- An online visual overview is available for this article.

Vitamin K Dependent Proteins in Kidney Disease

Patients with chronic kidney disease (CKD) have an increased risk of developing vascular calcifications, as well as bone dynamics impairment, leading to a poor quality of life and increased mortality. Certain vitamin K dependent proteins (VKDPs) act mainly as calcification inhibitors, but their involvement in the onset and progression of CKD are not completely elucidated. This review is an update of the current state of knowledge about the relationship between CKD and four extrahepatic VKDPs: matrix Gla protein, osteocalcin, growth-arrest specific protein 6 and Gla-rich protein. Based on published literature in the last ten years, the purpose of this review is to address fundamental aspects about the link between CKD and circulating VKDPs levels as well as to raise new topics about how the interplay between molecular weight and charge could influence the modifications of circulating VKDPs at the glomerular level, or whether distinct renal etiologies have effect on VKDPs. This review is the output of a systematic literature search and may open future research avenues in this niche domain.

Tyro3, Axl, and Mertk receptors differentially participate in platelet activation and thrombus formation

Background

Previously, several studies have shown that Tyro3, Axl, and Mertk (TAM) receptors participate in platelet activation and thrombosis. However, the role of individual receptors is not fully understood.

Methods

Using single receptor-deficient platelets from TAM knockout mice in the C57BL/6 J strain, we performed a knockout study using single TAM-deficient mice. We treated platelets isolated from TAM knockout mice with the Glycoprotein VI (GPVI) agonists convulxin, poly(PHG), and collagen-related triple-helical peptide (CRP), as well as thrombin for in-vitro experiments. We used a laser-induced cremaster arterial injury model for thrombosis experiments in vivo.

Results

Deficiency of the tyrosine kinase receptors, Axl or Tyro3, but not Mertk, inhibited aggregation, spreading, JON/A binding, and P-selectin expression of platelets in vitro. In vivo, platelet thrombus formation was significantly decreased in Axl^−/− and Tyro3^−/− mice, but not in Mertk^−/− mice. Upon stimulation with glycoprotein VI (GPVI) agonists, tyrosine phosphorylation of signaling molecules, including spleen tyrosine kinase (Syk) and phospholipase C-γ2 (PLCγ2), was decreased in Axl^−/− and Tyro3^−/− platelets, but not in Mertk^−/− platelets. While platelet aggregation induced by agonists did not differ in the presence or absence of the Gas6 neutralizing antibody, the platelet aggregation was inhibited by anti-Axl or anti-Tyro3 neutralizing antibodies antibody, but not the anti-Mertk antibody. Additionally, the recombinant extracellular domain of Axl or Tyro3, but not that of Mertk, also inhibited platelet aggregation.

Conclusions

These data suggest that Axl and Tyro3, but not Mertk, have an important role in platelet activation and thrombus formation, and mechanistically may do so by a pathway that regulates inside to outside signaling and heterotypic interactions via the extracellular domains of TAMs.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0308-0) contains supplementary material, which is available to authorized users.

The Effects of Parenteral K1 Administration in Pseudoxanthoma Elasticum Patients Versus Controls. A Pilot Study

Introduction

Pseudoxanthoma elasticum (PXE) is a rare disease caused by mutations in the ABCC6 gene. Vitamin K1 is involved in the posttranslational carboxylation of some proteins related to inhibition of the calcification process. Our aim was to investigate, in patients affected by PXE, baseline levels of vitamin K₁-dependent proteins and -metabolites and whether parenteral administration of phytomenadione was effective in modulating their levels.

Methods

We included eight PXE patients with typical clinical symptoms (skin, retina, and vascular calcification) and two ABCC6 causative mutations; 13 clinically unaffected first-degree patients’ relatives (9 carrying one ABCC6 mutation and 4 non-carriers). We assessed urinary vitamin K1 metabolites and serum Glu- and Gla-OC, Gas6 and undercaboxylated prothrombin (PIVKA-II), at baseline and after 1 and 6 weeks after a single intramuscular injection of 10 mg vitamin K1.

Results

Comparison of PXE patients, heterozygous, and non-carriers revealed differences in baseline levels of serum MK-4 and of urinary vitamin K metabolites. The response to phytomenadione administration on vitamin K-dependent proteins was similar in all groups.

Conclusion

The physiological axis between vitamin K₁ and vitamin K-dependent proteins is preserved; however, differences in the concentration of vitamin K metabolites and of MK-4 suggest that vitamin K1 metabolism/catabolism could be altered in PXE patients.

Molecular insights of Gas6/TAM in cancer development and therapy

Since growth arrest-specific gene 6 (Gas6) was discovered in 1988, numerous studies have highlighted the role of the Gas6 protein and its receptors Tyro3, Axl and Mer (collectively referred to as TAM), in proliferation, apoptosis, efferocytosis, leukocyte migration, sequestration and platelet aggregation. Gas6 has a critical role in the development of multiple types of cancers, including pancreatic, prostate, oral, ovarian and renal cancers. Acute myelocytic leukaemia (AML) is a Gas6-dependent cancer, and Gas6 expression predicts poor prognosis in AML. Interestingly, Gas6 also has a role in establishing tumour dormancy in the bone marrow microenvironment and in suppressing intestinal tumorigenesis. Numerous studies regarding cancer therapy have targeted Gas6 and TAM receptors with good results. However, some findings have suggested that Gas6 is associated with the development of resistance to cancer therapies. Concerning these significant effects of Gas6 in numerous cancers, we discuss the roles of Gas6 in cancer development in this review. First, we introduce basic knowledge on Gas6 and TAM receptors. Next, we describe and discuss the involvement of Gas6 and TAM receptors in cancers from different organ systems. Finally, we highlight the progress in therapies targeting Gas6 and TAM receptors. This review presents the significant roles of Gas6 in cancers from different systems and may contribute to the continued promotion of Gas6 as a therapeutic target.

Ligand Activation of TAM Family Receptors-Implications for Tumor Biology and Therapeutic Response

The TAM family of receptors (i.e., Tyro3, Axl, and Mertk), and their ligands Growth arrest specific factor 6 (Gas6) and Protein S (Pros1) contribute to several oncogenic processes, such as cell survival, invasion, migration, chemo-resistance, and metastasis, whereby expression often correlates with poor clinical outcomes. In recent years, there has been great interest in the study of TAM receptors in cancer, stemming both from their roles as oncogenic signaling receptors, as well as their roles in tumor immunology. As a result, several classes of TAM inhibitors that include small molecule tyrosine kinase inhibitors, monoclonal antibodies, decoy receptors, as well as novel strategies to target TAM ligands are being developed. This paper will review the biology of TAM receptors and their ligands with a focus on cancer, as well as evidence-based data for the continued pursuit of TAM/Gas6 inhibitors in clinical practice.

Study of Early Elevated Gas6 Plasma Level as a Predictor of Mortality in a Prospective Cohort of Patients with Sepsis

BACKGROUND

Growth arrest-specific gene 6 (Gas6), a vitamin K-dependent protein interacting with anionic phospholipids and TAM tyrosine kinase receptors, is elevated in plasma of septic patients. Previous studies did not find different levels between survivors and non-survivors at admission because either they included a low number of patients (<50) or a low number of non-survivors (5%).

OBJECTIVES

To determine, in a larger cohort of septic patients comprising an expected number of non-survivors, the performance of the plasma level of Gas6 and its soluble receptor Axl (sAxl) within 24 hours of admission to predict in-ICU mortality.

PATIENTS

Septic adults with or without shock.

METHODS

Gas6 and sAxl were prospectively measured by ELISA at day 0, 3, 7, and then weekly until discharge or death.

RESULTS

We evaluated 129 septic patients, including 82 with and 47 without shock, with in-ICU mortality rate of 19.4% and in-hospital mortality rate of 26%. Gas6 level was higher in non-survivors than in survivors (238 vs. 167%, P = 0.003); this difference remained constant during the ICU stay. The area under the ROC curve for Gas6 (0.695 [95% CI: 0.58-0.81]) was higher than for sAxl, procalcitonin, CRP, IL-1beta, IL-6 and-alpha, and slightly higher than for IL-8, IL-10, SOFA and APACHEII scores in predicting in-ICU mortality. Considering 249% as a cut-off value, Gas6 measurement had a negative predictive value for mortality of 87%.

CONCLUSION

It seems that Gas6 plasma level within 24 hours of ICU admission may predicts in-ICU mortality in patients with sepsis. If our result are confirmed in external validation, Gas6 plasma level measurement could contribute to the identification of patients who may benefit most from more aggressive management.

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.

The Gas6-Axl Protein Interaction Mediates Endothelial Uptake of Platelet Microparticles

Upon activation, platelets release plasma membrane-derived microparticles (PMPs) exposing phosphatidylserine on their surface. The functions and clearance mechanism of these microparticles are incompletely understood. As they are pro-coagulant and potentially pro-inflammatory, rapid clearance from the circulation is essential for prevention of thrombotic diseases. The tyrosine kinase receptors Tyro3, Axl, and Mer (TAMs) and their ligands protein S and Gas6 are involved in the uptake of phosphatidylserine-exposing apoptotic cells in macrophages and dendritic cells. Both TAMs and their ligands are expressed in the vasculature, the functional significance of which is poorly understood. In this study, we investigated how vascular TAMs and their ligands may mediate endothelial uptake of PMPs. PMPs, generated from purified human platelets, were isolated by ultracentrifugation and labeled with biotin or PKH67. The uptake of labeled microparticles in the presence of protein S and Gas6 in human aortic endothelial cells and human umbilical vein endothelial cells was monitored by flow cytometry, Western blotting, and confocal/electron microscopy. We found that both endothelial cell types can phagocytose PMPs, and by using TAM-blocking antibodies or siRNA knockdown of individual TAMs, we show that the uptake is mediated by endothelial Axl and Gas6. As circulating PMP levels were not altered in Gas6(-/-) mice compared with Gas6(+/+) mice, we hypothesize that the Gas6-mediated uptake is not a means to clear the bulk of circulating PMPs but may serve to locally phagocytose PMPs generated at sites of platelet activation and as a way to effect endothelial responses.

Growth Arrest-Specific 6 (Gas6) and TAM Receptors in Mouse Platelets

OBJECTIVE

Growth arrest-specific 6 (Gas6) is a newly discovered vitamin K-dependent protein, which is a ligand for TAM receptors [Tyro3 (Sky), Axl, and Mer] from the tyrosine kinase family. Gas6 knockout mice were resistant to venous and arterial thrombosis. There are contradictory reports on the presence of Gas6 and its receptors in mouse platelets. The objective of this study was to investigate whether Gas6 and its receptors were present in mouse platelets or not.

MATERIALS AND METHODS

Specific pathogen-free BALB/c male and female mice of 8-10 weeks old and 25-30 g in weight were anesthetized under light ether anesthesia and blood samples were taken from their hearts. RNAs were isolated from isolated platelets, and then mRNAs encoding Gas6 and TAM receptors were detected by reverse transcription-polymerase chain reaction (RT-PCR). Protein concentrations of Gas6 and TAM receptors in platelets were measured by ELISA, but not those of Mer, because of the absence of any commercial ELISA kit for mouse specimens.

RESULTS

RT-PCR results indicated the presence of mRNAs encoding Gas6 and Mer in mouse platelets. However, although RT-PCR reactions were performed at various temperatures and cycles, we could not detect the presence of mRNAs encoding Axl and Tyro3 (Sky). Receptor protein levels of Axl and Tyro3 were below the detection limits of the ELISA method.

CONCLUSION

We found the presence of mRNAs encoding Gas6 and the receptor Mer in mouse platelets, but not Axl and Tyro3. Gas6, Axl, and Tyro3 protein levels were below the detection limits of the ELISA. The presence of mRNA is not obvious evidence of protein expression in platelets that have no nucleus or DNA. Further studies are required to clarify the presence of Gas6/TAM receptors in platelets using real-time PCR and more sensitive immunological methods, and future studies on mechanisms will indicate whether the Gas6/TAM pathway is a strategy for treatment of disorders.

TAM receptor signaling in immune homeostasis

The TAM receptor tyrosine kinases (RTKs)-TYRO3, AXL, and MERTK-together with their cognate agonists GAS6 and PROS1 play an essential role in the resolution of inflammation. Deficiencies in TAM signaling have been associated with chronic inflammatory and autoimmune diseases. Three processes regulated by TAM signaling may contribute, either independently or collectively, to immune homeostasis: the negative regulation of the innate immune response, the phagocytosis of apoptotic cells, and the restoration of vascular integrity. Recent studies have also revealed the function of TAMs in infectious diseases and cancer. Here, we review the important milestones in the discovery of these RTKs and their ligands and the studies that underscore the functional importance of this signaling pathway in physiological immune settings and disease.

The Axl receptor tyrosine kinase is a discriminator of macrophage function in the inflamed lung

Much of the biology surrounding macrophage functional specificity has arisen through examining inflammation-induced polarizing signals, but this also occurs in homeostasis, requiring tissue-specific environmental triggers that influence macrophage phenotype and function. The TAM receptor family of receptor tyrosine kinases (Tyro3, Axl and MerTK) mediates the non-inflammatory removal of apoptotic cells by phagocytes through the bridging phosphatidylserine-binding molecules growth arrest-specific 6 (Gas6) or Protein S. We show that one such TAM receptor (Axl) is exclusively expressed on mouse airway macrophages, but not interstitial macrophages and other lung leukocytes, under homeostatic conditions and is constitutively ligated to Gas6. Axl expression is potently induced by granulocyte-macrophage colony-stimulating factor expressed in the healthy and inflamed airway, and by type I interferon or Toll-like receptor-3 stimulation on human and mouse macrophages, indicating potential involvement of Axl in apoptotic cell removal under inflammatory conditions. Indeed, an absence of Axl does not cause sterile inflammation in health, but leads to exaggerated lung inflammatory disease upon influenza infection. These data imply that Axl allows specific identification of airway macrophages, and that its expression is critical for macrophage functional compartmentalization in the airspaces or lung interstitium. We propose that this may be a critical feature to prevent excessive inflammation because of secondary necrosis of apoptotic cells that have not been cleared by efferocytosis.

RhoA/phosphatidylinositol 3-kinase/protein kinase B/mitogen-activated protein kinase signaling after growth arrest-specific protein 6/mer receptor tyrosine kinase engagement promotes epithelial cell growth and wound repair via upregulation of hepatocyte growth factor in macrophages

Growth arrest-specific protein 6 (Gas6)/Mer receptor tyrosine kinase (Mer) signaling modulates cytokine secretion and helps to regulate the immune response and apoptotic cell clearance. Signaling pathways that activate an epithelial growth program in macrophages are still poorly defined. We report that Gas6/Mer/RhoA signaling can induce the production of epithelial growth factor hepatic growth factor (HGF) in macrophages, which ultimately promotes epithelial cell proliferation and wound repair. The RhoA/protein kinase B (Akt)/mitogen-activated protein (MAP) kinases, including p38 MAP kinase, extracellular signal-regulated protein kinase, and Jun NH2-terminal kinase axis in RAW 264.7 cells, was identified as Gas6/Mer downstream signaling pathway for the upregulation of HGF mRNA and protein. Conditioned medium from RAW 264.7 cells that had been exposed to Gas6 or apoptotic cells enhanced epithelial cell proliferation of the epithelial cell line LA-4 and wound closure. Cotreatment with an HGF receptor-blocking antibody or c-Met antagonist downregulated this enhancement. Inhibition of Mer with small interfering RNA (siRNA) or the RhoA/Rho kinase pathway by RhoA siRNA or Rho kinase pharmacologic inhibitor suppressed Gas6-induced HGF mRNA and protein expression in macrophages and blocked epithelial cell proliferation and wound closure induced by the conditioned medium. Our data provide evidence that macrophages can be reprogrammed by Gas6 to promote epithelial proliferation and wound repair via HGF, which is induced by the Mer/RhoA/Akt/MAP kinase pathway. Thus, defects in Gas6/Mer/RhoA signaling in macrophages may delay tissue repair after injury to the alveolar epithelium.

Association of plasma growth arrest-specific protein 6 (Gas6) concentrations with albuminuria in patients with type 2 diabetes

AIMS

New biomarkers are required to detect diabetic nephropathy earlier in persons with type 2 diabetes mellitus. Recent experimental studies indicate that growth arrest-specific protein 6 (Gas6) may have a role in pathogenesis of complications associated with diabetes. The objective of the current study is to examine whether plasma Gas6 concentrations are associated with albuminuria in persons with type 2 diabetes mellitus.

METHODS

About 32 patients with diabetes which have micro or macroalbuminuria, 37 patients with diabetes and normoalbuminuria, and 30 healthy volunteers were recruited. Plasma Gas6 levels were measured by ELISA. Hemoglobin A1c (HbA1c), serum C reactive protein, fibrinogen and 24-h urine samples for microalbuminuria were analyzed by Primus PDQ, Beckman Coulter Immage 800, STA Compact and Roche Cobas Integra 800 analyzer, respectively. Statistical analysis was performed using SPSS (Statistical Package for Social Sciences) for Windows 11.5.

RESULTS

There was a noteworthy difference among the three groups for Gas6 according to the Kruskal-Wallis test (p < 0.01). Plasma Gas6 concentrations were higher in patients with micro or macroalbuminuria [20.9 ng/mL (16.7-27.0); median (25-75% percentile)] compared to patients with normoalbuminuria [16.5 ng/mL (13.1-22.9)], and healthy controls [15.3 ng/mL (8.3-33.6)].

CONCLUSIONS

In conclusion, this is the first study indicating that plasma Gas6 levels are associated with albuminuria in patients with type 2 diabetes. This study could be considered a starting point to focus on the association between Gas6 and diabetic nephropathy.

Expression of growth arrest-specific protein 6 and Axl molecules in the left internal mammary artery of patients undergoing coronary artery bypass grafting

AIMS

Studies have demonstrated that using a left internal mammary artery (LIMA) graft yields excellent long-term results in coronary artery bypass grafting (CABG). The growth arrest-specific 6 (Gas6) gene and its receptor, Axl, are crucial in vascular haemostasis and atherosclerosis. The objective of this study was to determine the expression of Gas6 and Axl molecules in the aorta and LIMA in patients undergoing CABG.

METHODS

Plasma and tissue specimens were collected from 19 patients undergoing elective CABG. The expression of the Gas6 and Axl in the injured aorta and LIMA was examined using reverse transcription PCR (RT-PCR), real-time RT-PCR, western blot and immunohistochemical staining.

RESULTS

In CABG patients, the mRNA, immunoreactivity and protein expressions of the Gas6 and Axl were considerably higher in the LIMA than those in the aorta. Further analysis revealed that the expression of the Gas6 positively correlated with that of Axl in the LIMA and aorta. The plasma Gas6 level was considerably and positively correlated with the expression of Gas6 protein in the LIMA and aorta.

CONCLUSIONS

The present study discovered that the higher expression of Gas6/Axl pathway components in the LIMA compared with that in the aorta may partly explain the less frequent atherosclerotic events involving the LIMA compared with other arteries. Moreover, Gas6 may play a critical and protective role in human vascular biology.

Growth arrest-specific gene 6 protein promotes the proliferation and migration of endothelial progenitor cells through the PI3K/AKT signaling pathway

Endothelial progenitor cells (EPCs) play an important role in endothelial repair and vascular regeneration. Growth arrest-specific gene 6 (Gas6) is a novel key regulator of the vascular system, which is linked to a number of cardiovascular diseases. However, the effects of Gas6 on EPCs have not been elucidated to date. The present study was designed to determine the biological function of EPCs treated with Gas6 and to eludicate the underlying mechanisms. EPCs were isolated from umbilical cord blood and treated with various concentrations (25, 50, 100 and 200 ng/ml) of Gas6. The proliferation, migration and angiogenesis of the Gas6-treated EPCs were evaluated by MTT assay, Transwell assay and in vitro tube formation assay, respectively. The phosphorylation status of AKT and ERK was evaluated by western blot analysis. The results demonstrated that treatment with Gas6 enhanced the proliferation and migration of the EPCs in a dose-dependent manner. However, Gas6 did not promote the differentiation of EPCs on Matrigel. Gas6 induced the phosphorylation of AKT, but not that of ERK. The enhanced proliferation and migration induced by Gas6 was markedly suppressed by the inhibitor of PI3K but not by that of ERK. These results suggest that Gas6 activates the AKT signaling pathway, which, in turn, promotes the proliferation and migration of EPCs.

TAM receptors, Gas6, and protein S: roles in inflammation and hemostasis

TAM receptors (Tyro3, Axl, and Mer) belong to a family of receptor tyrosine kinases that have important effects on hemostasis and inflammation. Also, they affect cell proliferation, survival, adhesion, and migration. TAM receptors can be activated by the vitamin K–dependent proteins Gas6 and protein S. Protein S is more commonly known as an important cofactor for protein C as well as a direct inhibitor of multiple coagulation factors. To our knowledge, the functions of Gas6 are limited to TAM receptor activation. When activated, the TAM receptors have effects on primary hemostasis and coagulation and display an anti-inflammatory or a proinflammatory effect, depending on cell type. To comprehend the effects that the TAM receptors and their ligands have on hemostasis and inflammation, we compare studies that report the different phenotypes displayed by mice with deficiencies in the genes of this receptor family and its ligands (protein S+/−, Gas6−/−, TAM−/−, and variations of these). In this manner, we aim to display which features are attributable to the different ligands. Because of the effects TAM receptors have on hemostasis, inflammation, and cancer growth, their modulation could make interesting therapeutic targets in thromboembolic disease, atherosclerosis, sepsis, autoimmune disease, and cancer.

Thrombin-mediated activation of Akt signaling contributes to pulmonary vascular remodeling in pulmonary hypertension

Chronic thromboembolic pulmonary hypertension (CTEPH) has been increasingly recognized as a common source of elevated pulmonary vascular resistance and pulmonary hypertension. It is clear that development of pulmonary thromboemboli is the inciting event for this process, yet it remains unclear why some patients have persistent pulmonary artery occlusion leading to distal pulmonary vascular remodeling and CTEPH. Thrombin, a serine protease, is an integral part of the common coagulation cascade, yet thrombin also has direct cellular effects through interaction with the family of PAR membrane receptors. This study is designed to determine the effects of thrombin on Akt signaling in pulmonary artery smooth muscle cells (PASMC) from normal humans and pulmonary hypertension patients. Thrombin treatment of PASMC resulted in a transient increase in Akt phosphorylation and had similar effects on the downstream targets of the Akt/mTOR pathway. Ca²⁺ is shown to be required for Akt phosphorylation as well as serum starvation, a distinct effect compared to platelet‐derived growth factor. Thrombin treatment was associated with a rise in intracellular [Ca²⁺] and enhanced store‐operated calcium entry (SOCE). These effects lead to enhanced proliferation, which is more dramatic in both IPAH and CTEPH PASMC. Enhanced proliferation is also shown to be attenuated by inhibition of Akt/mTOR in CTEPH PASMC. Thrombin has direct effects on PASMC increasing intracellular [Ca²⁺] and PASMC proliferation, an effect attributed to Akt phosphorylation. The current results implicate the effects of thrombin in the pathogenesis of idiopathic pulmonary arterial hypertension (IPAH) and CTEPH, which may potentially be a novel therapeutic target.

Thrombin is known to play an important role in thrombotic events including pulmonary embolism. In this manuscript, we show a direct effect of thrombin on pulmonary artery smooth muscle cells in both normal and diseased states through Akt signaling, which leads to increased store‐operated calcium entry and cellular proliferation. These direct effects of thrombin may play a role in the development and progression of chronic thromboembolic pulmonary hypertension.

Plasma concentrations of sAxl are associated with severe preeclampsia

OBJECTIVE

Preeclampsia, a multisystem disorder unique to pregnancy, is a major cause of maternal and perinatal mortality and morbidity. Inadequate trophoblast invasion and vascular dysfunction are believed to be involved in preeclampsia. Axl, which interacts with its ligand Gas6, known to regulate cell migration, adhesion, and vascular angiogenesis or homeostasis and vascular network formation, may be implicated in preeclampsia, as preeclampsia is a specific vascular disease. sAxl, a soluble form of Axl, is bound to Gas6 in plasma, which inhibits the activation of the Axl-Gas6 pathway. The aim of this study was to determine the changes and significance of plasma concentrations of sAxl in severe preeclampsia, as well as its correlation with the clinical parameters of severe preeclampsia.

DESIGN AND METHODS

Fifty-eight women with severe preeclampsia and 31 healthy pregnant women were included in the study, from April 2012 to October 2012. Plasma sAxl concentrations were detected with an immunosorbent assay (ELISA) kit.

RESULTS

Plasma sAxl concentrations were significantly higher in the preeclampsia group (61.52±17.57ng/mL) than in the normal pregnancy group (45.29±15.44ng/mL) (P<0.05). Plasma sAxl concentrations in the severe preeclampsia patients correlated positively with systolic and diastolic blood pressures (r=0.628, P<0.05 vs. r=0.394, P<0.05, respectively) and proteinuria (r=0.583, P<0.05), but inversely with plasma albumin (r=-0.444, P<0.05), gestation of delivery (r=-0.554, P<0.05), and birth weight (r=-0.476, P<0.05). Multiple linear regression analysis indicated that systolic blood pressure and proteinuria were influence factors of plasma sAxl levels (β1=0.520, P<0.05; β2=0.461, P<0.05).

CONCLUSIONS

Plasma sAxl concentrations were higher in the preeclampsia patients, and plasma sAxl levels were correlated with the clinical parameters of severe preeclampsia. Furthermore, systolic blood pressure and proteinuria might be influence factors of plasma sAxl level.

Meta-analysis of microarray data identifies GAS6 expression as an independent predictor of poor survival in ovarian cancer

Seeking new biomarkers for epithelial ovarian cancer, the fifth most common cause of death from all cancers in women and the leading cause of death from gynaecological malignancies, we performed a meta-analysis of three independent studies and compared the results in regard to clinicopathological parameters. This analysis revealed that GAS6 was highly expressed in ovarian cancer and therefore was selected as our candidate of choice. GAS6 encodes a secreted protein involved in physiological processes including cell proliferation, chemotaxis, and cell survival. We performed immunohistochemistry on various ovarian cancer tissues and found that GAS6 expression was elevated in tumour tissue samples compared to healthy control samples (P < 0.0001). In addition, GAS6 expression was also higher in tumours from patients with residual disease compared to those without. Our data propose GAS6 as an independent predictor of poor survival, suggesting GAS6, both on the mRNA and on the protein level, as a potential biomarker for ovarian cancer. In clinical practice, the staining of a tumour biopsy for GAS6 may be useful to assess cancer prognosis and/or to monitor disease progression.

Inhibitory role of Gas6 in intestinal tumorigenesis

Growth arrest-specific gene (Gas) 6 is a γ-carboxyglutamic acid domain-containing protein, which shares 43% amino acid identity with protein S. Gas6 has been shown to enhance cancer cell proliferation in vitro. On the other hand, recent studies have demonstrated that Gas6 inhibits toll-like receptor-mediated immune reactions. Immune reactions are known to affect intestinal tumorigenesis. In this study, we investigated how Gas6 contributes to tumorigenesis in the intestine. Administration of recombinant Gas6 weakly, but significantly, enhanced proliferation of intestinal cancer cells (SW480 and HT29), whereas it suppressed the inflammatory responses of Lipopolysaccharide (LPS)-stimulated monocytes (THP-1). Compared with Gas6(+/+) mice, Gas6(-/-) mice exhibited enhanced azoxymethane/dextran sulfate sodium (DSS)-induced tumorigenesis and had a shorter survival. Gas6(-/-) mice also exhibited more severe DSS-induced colitis. DSS-treated Gas6(-/-) mice showed attenuated Socs1/3 messenger RNA expression and enhanced nuclear factor-kappaB activation in the colonic stroma, suggesting that the target of Gas6 is stromal cells. Bone marrow transplantation experiments indicated that both epithelial cells and bone marrow-derived cells are Gas6 sources. Furthermore, the number of intestinal tumors in Apc(Min) Gas6(-/-) mice was higher than that in Apc(Min) Gas6(+/+) mice, resulting in shorter survival. In a group of 62 patients with advanced colorectal cancer, Gas6 immunoreactivity in cancer tissues was positively correlated with prognosis. Thus, we revealed a unique in vivo inhibitory role of Gas6 during the progression of intestinal tumors associated with suppression of stromal immune reactions. These results suggest a novel therapeutic approach for colorectal cancer patients by regulation of stromal immune responses.

Vascular Gas6 contributes to thrombogenesis and promotes tissue factor up-regulation after vessel injury in mice

Gas6 (growth-arrest specific gene 6) plays a role in thrombus stabilization. Gas6 null (-/-) mice are protected from lethal venous and arterial thromboembolism through platelet signaling defects induced only by 5 μM ADP and 10 μM of the thromboxane analog, U46619. This subtle platelet defect, despite a dramatic clinical phenotype, raises the possibility that Gas6 from a source other than platelets contributes to thrombus formation. Thus, we hypothesize that Gas6 derived from the vascular wall plays a role in venous thrombus formation. Bone marrow transplantation and platelet depletion/reconstitution experiments generating mice with selective ablations of Gas6 from either the hematopoietic or nonhematopoietic compartments demonstrate an approximately equal contribution by Gas6 from both compartments to thrombus formation. Tissue factor expression was significantly reduced in the vascular wall of Gas6(-/-) mice compared with WT. In vitro, thrombin-induced tissue factor expression was reduced in Gas6(-/-) endothelial cells compared with wild-type endothelium. Taken together, these results demonstrate that vascular Gas6 contributes to thrombus formation in vivo and can be explained by the ability of Gas6 to promote tissue factor expression and activity. These findings support the notion that vascular wall-derived Gas6 may play a pathophysiologic role in venous thromboembolism.

Cross-phosphorylation, signaling and proliferative functions of the Tyro3 and Axl receptors in Rat2 cells

The dysregulation of receptor protein tyrosine kinase (RPTK) function can result in changes in cell proliferation, cell growth and metastasis leading to malignant transformation. Among RPTKs, the TAM receptor family composed of three members Tyro3, Axl, and Mer has been recognized to have a prominent role in cell transformation. In this study we analyzed the consequences of Tyro3 overexpression on cell proliferation, activation of signaling pathways and its functional interactions with Axl. Overexpression of Tyro3 in the Rat2 cell line that expresses Axl, but not Mer or Tyro3, resulted in a 5 fold increase in cell proliferation. This increase was partially blocked by inhibitors of the mitogen-activated protein kinase (MAPK) signaling pathway but not by inhibitors of the phosphatidylinositol 3-kinase (PI(3)K) signaling pathway. Consistent with these findings, an increase in ERK1/2 phosphorylation was detected with Tyro3 but not with Axl overexpression. In contrast, activation of Axl stimulated the PI(3)K pathway, which was mitigated by co-expression of Tyro3. The overexpression of Tyro3 enhanced Gas6-mediated Axl phosphorylation, which was not detected upon overexpression of a “kinase dead” form of Tyro3 (kdTyro3). In addition, the overexpression of Axl induced kdTyro3 phosphorylation. Co-immunoprecipitation experiments confirmed that the Axl and Tyro3 receptors are closely associated. These findings show that overexpression of Tyro3 in the presence of Axl promotes cell proliferation, and that co-expression of Axl and Tyro3 can affect the outcome of Gas6-initiated signaling. Furthermore, they demonstrate a functional interaction between the members of the TAM receptor family which can shed light on the molecular mechanisms underlying the functional consequences of TAM receptor activation in cell transformation, neural function, immune function, and reproductive function among others.

Vascular calcification: the price to pay for anticoagulation therapy with vitamin K-antagonists

Vitamin K-antagonists (VKA) are the most widely used anti-thrombotic drugs with substantial efficacy in reducing risk of arterial and venous thrombosis. Several lines of evidence indicate, however, that VKA inhibit not only post-translational activation of vitamin K-dependent coagulation factors but also synthesis of functional extra-hepatic vitamin K-dependent proteins thereby eliciting undesired side-effects. Vascular calcification is one of the recently revealed side-effects of VKA. Vascular calcification is an actively regulated process involving vascular cells and a number of vitamin K-dependent proteins. Mechanistic understanding of vascular calcification is essential to improve VKA-based treatments of both thrombotic disorders and atherosclerosis. This review addresses vitamin K-cycle and vitamin K-dependent processes of vascular calcification that are affected by VKA. We conclude that there is a growing need for better understanding of the effects of anticoagulants on vascular calcification and atherosclerosis.

Growth arrest-specific gene 6 (gas6) and vascular hemostasis

Gas6 (growth arrest-specific 6) belongs structurally to the family of plasma vitamin K-dependent proteins. Gas6 has a high structural homology with the natural anticoagulant protein S, sharing the same modular composition. Interestingly, despite the presence of a γ-carboxyglutamic acid domain in its structure, no role in the coagulation cascade has been identified for gas6. Gas6 has been shown to be involved in vascular homeostasis and more precisely is involved in proliferation, apoptosis, efferocytosis, leukocyte migration, and sequestration and platelet aggregation. It is also involved in the activation of different cell types, from platelets to endothelial and vascular smooth muscle cells. Thus, it has been shown to play a role in several pathophysiological processes such as atherosclerosis, cancer, and thrombosis. Interestingly, studies using gas6 null mice highlighted that gas6 may represent a novel potential target for anticoagulant therapy, because these animals are protected from lethal venous thromboembolism without excessive bleeding. However, the mechanism in thrombus occurrence remains to be further explored. In the present review, we will focus on the role of gas6 in innate immunity, atherosclerosis, thrombosis, and cancer-related events.

Increased secretion of Gas6 by smooth muscle cells in human atherosclerotic carotid plaques

Vitamin K-dependent protein Gas6 (growth-arrest specific gene 6) plays a role in vascular smooth muscle cell (VSMC) survival and migration, as well as in endothelium and leukocyte activation, and could therefore be involved in atherosclerosis. However, the study of mouse models has led to contradictory results regarding the pro- or anti-atherogenic properties of Gas6, and relatively few data are available in human pathophysiology. To better understand the implication of Gas6 in human atherosclerosis, we studied Gas6 expression and secretion in vitro in human VSMC, and analysed the effect of Gas6 on inflammatory gene expression in these cells. We show that Gas6 secretion in VSMC is strongly induced by the anti-inflammatory cytokine transforming growth factor (TGF)β, and that VSMC stimulation by recombinant Gas6 decreases the expression of inflammatory genes tumour necrosis factor (TNF)α and intracellular adhesion molecule (ICAM)-1. The study of Gas6 expression in human carotid endarterectomy samples revealed that Gas6 is mainly expressed by VSMC at all stages of human atherosclerosis, but is not detected in normal vessel wall. Analysis of plaque secretomes showed that Gas6 secretion is markedly higher in non-complicated plaques than in complicated plaques, and that TGFβ secretion pattern mirrors that of Gas6. We conclude that Gas6 is secreted in human atherosclerotic plaques by VSMC following stimulation by TGFβ, and that Gas6 secretion decreases with plaque complication. Therefore, we propose that Gas6 acts as a protective factor, in part by reducing the pro-inflammatory phenotype of VSMC.

Gas6 downregulation impaired cytoplasmic maturation and pronuclear formation independent to the MPF activity

Previously, we found that the growth arrest-specific gene 6 (Gas6) is more highly expressed in germinal vesicle (GV) oocytes than in metaphase II (MII) oocytes using annealing control primer (ACP)-PCR technology. The current study was undertaken to investigate the role of Gas6 in oocyte maturation and fertilization using RNA interference (RNAi). Interestingly, despite the specific and marked decrease in Gas6 mRNA and protein expression in GVs after Gas6 RNAi, nuclear maturation including spindle structures and chromosome segregation was not affected. The only discernible effect induced by Gas6 RNAi was a change in maturation promoting factor (MPF) activity. After parthenogenetic activation, Gas6 RNAi-treated oocytes at the MII stage had not developed further and arrested at MII (90.0%). After stimulation with Sr(2+), Gas6-silenced MII oocytes had markedly reduced Ca(2+) oscillation and exhibited no exocytosis of cortical granules. In these oocytes, sperm penetration occurred during fertilization but not pronucleus (PN) formation. By roscovitine and colcemid treatment, we found that the Gas6 knockdown affected cytoplasmic maturation directly, independent to the changed MPF activity. These results strongly suggest that 1) the Gas6 signaling itself is important to the cytoplasmic maturation, but not nuclear maturation, and 2) the decreased Gas6 expression and decreased MPF activity separately or mutually influence sperm head decondensation and PN formation.

Differential expression of Axl in hepatocellular carcinoma and correlation with tumor lymphatic metastasis

Protein kinases play important roles in tumor development and progression. A variety of members of the signal transduction enzymes serve as targets for therapeutic intervention in cancer. The dysregulation of Axl receptor and its ligand growth arrest-specific 6 (Gas6) is implicated in the pathogenesis of several cancers. In this study, the differential expressions of Axl were investigated in mouse hepatocarcinoma cell lines Hca-F and Hca-P, which have high- and low-metastatic potential to lymph nodes. Experimental inhibition of Axl by siRNA assessed further the metastatic potential of Axl. The results showed that down-regulation of Axl expression attenuated Hca-F cells proliferation, migration, and invasion in vitro, as well as inhibited metastasis to peripheral lymph nodes in vivo. Further analysis demonstrated that the addition of exogenous Gas6 mediated the migration and invasion of Hca-F cells both in vitro and in vivo through Axl. Furthermore, Gas6 stimulation of Axl in Hca-F cells resulted primarily in the down-regulation of Cyr61, a member of the CCN protein family involved in tumor progression. These data suggest that Axl acts as a tumor lymphatic metastasis-associated gene, and may function partly through the regulation of Cyr61.

Mechanisms underlying the biphasic effect of vitamin K1 (phylloquinone) on arterial blood pressure

Phylloquinone (vitamin K1, VK1) is widely used therapeutically and intravenous administration of this quinone can induce hypotension. We aimed to investigate the mechanisms underlying the effects induced by VK1 on arterial blood pressure. With this purpose a catheter was inserted into the abdominal aorta of male Wistar rats for blood pressure and heart rate recording. Bolus intravenous injection of VK1 (0.5–20 mgkg−1) produced a transient increase in blood pressure followed by a fall. Both the pressor and depressor response induced by VK1 were dose-dependent. On the other hand, intravenous injection of VK1 did not alter heart rate. The nitric oxide synthase (NOS) inhibitor NG-nitro-l-arginine methyl ester (L-NAME, 10 and 20 mgkg−1) reduced both the increase and decrease in blood pressure induced by VK1 (5 mgkg−1). On the other hand, indometacin (10 mgkg−1), a non-selective cyclooxygenase inhibitor, did not alter the increase in mean arterial pressure (MAP) induced by VK1. However, VK1-induced fall in MAP was significantly attenuated by indometacin. We concluded that VK1 induces a dose-dependent effect on blood pressure that consists of an acute increase followed by a more sustained decrease in MAP. The hypotension induced by VK1 involves the activation of the nitric oxide (NO) pathway and the release of vasodilator prostanoid(s).

Lack of protein S in mice causes embryonic lethal coagulopathy and vascular dysgenesis

Protein S (ProS) is a blood anticoagulant encoded by the Pros1 gene, and ProS deficiencies are associated with venous thrombosis, stroke, and autoimmunity. These associations notwithstanding, the relative risk that reduced ProS expression confers in different disease settings has been difficult to assess without an animal model. We have now described a mouse model of ProS deficiency and shown that all Pros1-/- mice die in utero,from a fulminant coagulopathy and associated hemorrhages. Although ProS is known to act as a cofactor for activated Protein C (aPC), plasma from Pros1+/- heterozygous mice exhibited accelerated thrombin generation independent of aPC, and Pros1 mutants displayed defects in vessel development and function not seen in mice lacking protein C. Similar vascular defects appeared in mice in which Pros1 was conditionally deleted in vascular smooth muscle cells. Mutants in which Pros1 was deleted specifically in hepatocytes, which are thought to be the major source of ProS in the blood, were viable as adults and displayed less-severe coagulopathy without vascular dysgenesis. Finally, analysis of mutants in which Pros1 was deleted in endothelial cells indicated that these cells make a substantial contribution to circulating ProS. These results demonstrate that ProS is a pleiotropic anticoagulant with aPC-independent activities and highlight new roles for ProS in vascular development and homeostasis.

Macrophage death and defective inflammation resolution in atherosclerosis

A key event in atherosclerosis is a maladaptive inflammatory response to subendothelial lipoproteins. A crucial aspect of this response is a failure to resolve inflammation, which normally involves the suppression of inflammatory cell influx, effective clearance of apoptotic cells and promotion of inflammatory cell egress. Defects in these processes promote the progression of atherosclerotic lesions into dangerous plaques, which can trigger atherothrombotic vascular disease, the leading cause of death in industrialized societies. In this Review I provide an overview of these concepts, with a focus on macrophage death and defective apoptotic cell clearance, and discuss new therapeutic strategies designed to boost inflammation resolution in atherosclerosis.

Malignant cells fuel tumor growth by educating infiltrating leukocytes to produce the mitogen Gas6

The transforming and tumor growth-promoting properties of Axl, a member of the Tyro3, Axl, and Mer (TAM) family of receptor tyrosine kinases (TAMRs), are well recognized. In contrast, little is known about the role of the TAMR ligand growth arrest-specific gene 6 (Gas6) in tumor biology. By using Gas6-deficient (Gas6(-/-)) mice, we show that bone marrow-derived Gas6 promotes growth and metastasis in different experimental cancer models, including one resistant to vascular endothelial growth factor inhibitors. Mechanistic studies reveal that circulating leukocytes produce minimal Gas6. However, once infiltrated in the tumor, leukocytes up-regulate Gas6, which is mitogenic for tumor cells. Consistent herewith, impaired tumor growth in Gas6(-/-) mice is rescued by transplantation of wild-type bone marrow and, conversely, mimicked by transplantation of Gas6(-/-) bone marrow into wild-type hosts. These findings highlight a novel role for Gas6 in a positive amplification loop, whereby tumors promote their growth by educating infiltrating leukocytes to up-regulate the production of the mitogen Gas6. Hence, inhibition of Gas6 might offer novel opportunities for the treatment of cancer.

Genetic loss of Gas6 induces plaque stability in experimental atherosclerosis

The growth arrest-specific gene 6 (Gas6) plays a role in pro-atherogenic processes such as endothelial and leukocyte activation, smooth muscle cell migration and thrombosis, but its role in atherosclerosis remains uninvestigated. Here, we report that Gas6 is expressed in all stages of human and mouse atherosclerosis, in plaque endothelial cells, smooth muscle cells and macrophages. Gas6 expression is most abundant in lesions containing high amounts of macrophages, ie thin fibrous cap atheroma and ruptured plaque. Genetic loss of Gas6 does not affect the number and size of initial and advanced plaques in ApoE(-/-) mice, but alters its plaque composition. Compared to Gas6(+/+): ApoE(-/-) mice, initial and advanced plaques of Gas6(-/-): ApoE(-/-) mice contained more smooth muscle cells and more collagen and developed smaller lipid cores, while the expression of TGFbeta was increased. In addition, fewer macrophages were found in advanced plaques of Gas6(-/-): ApoE(-/-) mice. Hence, loss of Gas6 promotes the formation of more stable atherosclerotic lesions by increasing plaque fibrosis and by attenuating plaque inflammation. These findings identify a role for Gas6 in plaque composition and stability.