Expression of the vitamin K-dependent proteins GAS6 and protein S and the TAM receptor tyrosine kinases in human atherosclerotic carotid plaques

Atherosclerotic plaque development in mice is enhanced by myeloid ZEB1 downregulation

Accumulation of lipid-laden macrophages within the arterial neointima is a critical step in atherosclerotic plaque formation. Here, we show that reduced levels of the cellular plasticity factor ZEB1 in macrophages increase atherosclerotic plaque formation and the chance of cardiovascular events. Compared to control counterparts (Zeb1WT/ApoeKO), male mice with Zeb1 ablation in their myeloid cells (Zeb1∆M/ApoeKO) have larger atherosclerotic plaques and higher lipid accumulation in their macrophages due to delayed lipid traffic and deficient cholesterol efflux. Zeb1∆M/ApoeKO mice display more pronounced systemic metabolic alterations than Zeb1WT/ApoeKO mice, with higher serum levels of low-density lipoproteins and inflammatory cytokines and larger ectopic fat deposits. Higher lipid accumulation in Zeb1∆M macrophages is reverted by the exogenous expression of Zeb1 through macrophage-targeted nanoparticles. In vivo administration of these nanoparticles reduces atherosclerotic plaque formation in Zeb1∆M/ApoeKO mice. Finally, low ZEB1 expression in human endarterectomies is associated with plaque rupture and cardiovascular events. These results set ZEB1 in macrophages as a potential target in the treatment of atherosclerosis.

Prognostic value of plasma sAXL in patients with heart failure: insights from the DRAGON-HF trial

BACKGROUND

Little is known about the predictive value of soluble AXL (sAXL) in heart failure (HF). This study aimed to describe the prognostic value of plasma sAXL in patients with symptomatic HF.

METHODS

This is a multicentre observational prospective cohort study (Registration No. NCT03727828). Plasma sAXL were measured on admission. The primary endpoint is a composite of cardiovascular mortality and HF rehospitalization. Associations between plasma sAXL levels and clinical endpoints are described using Cox regression models and Kaplan-Meier methods.

RESULTS

A total of 1030 symptomatic HF patients were enrolled in the study; the mean age (65% men) was 71 ± 12 years, with a median follow-up of 32 months (IQR: 26-41 months). The mean baseline sAXL levels were 20.03 ± 6.74 ng/mL. Plasma sAXL positively associated with NYHA classification and negatively associated with left ventricular ejection fraction (both P < 0.001). Cox regression showed that 1-SD increment of sAXL was associated with primary endpoint [HR (CI): 1.128 (1.024-1.242)], cardiovascular mortality [1.112 (1.032-1.198)], all-cause mortality [1.142 (1.057-1.234)], and HF rehospitalization [1.122 (1.030-1.224)] after adjustment for potential confounders including NT-proBNP. Kaplan-Meier curves revealed that patients with the highest sAXL levels were at the highest risk of primary endpoint events, cardiovascular mortality, and all-cause mortality (all P values < 0.001). Furthermore, both Kaplan-Meier method and Categorical analysis demonstrated that the combined use of sAXL and NT-proBNP were more likely to predict all-cause or cardiovascular mortality (both P < 0.001). Similar results were observed when separating patients with respect to left ventricular ejection fraction, namely, in HFrEF, HFmrEF, and HFpEF groups.

CONCLUSIONS

Plasma sAXL concentrations are of great importance in predicting clinical outcomes in HF patients, independent of NT-proBNP, suggesting that sAXL is a promising prognostic marker for further study.

The Impaired Mechanism and Facilitated Therapies of Efferocytosis in Atherosclerosis

ABSTRACT

Cardiovascular disease is responsible for the largest number of deaths worldwide, and atherosclerosis is the primary cause. Apoptotic cell accumulation in atherosclerotic plaques leads to necrotic core formation and plaque rupture. Emerging findings show that the progression of atherosclerosis appears to suppress the elimination of apoptotic cells. Mechanistically, the reduced edibility of apoptotic cells, insufficient phagocytic capacity of phagocytes, downregulation of bridging molecules, and dysfunction in the polarization of macrophages lead to impaired efferocytosis in atherosclerotic plaques. This review focuses on the characteristics of efferocytosis in plaques and the therapeutic strategies aimed at promoting efferocytosis in atherosclerosis, which would provide novel insights for the development of antiatherosclerotic drugs based on efferocytosis.

Association between plasma growth arrest-specific protein 6 and carotid atherosclerosis in type 2 diabetes mellitus

BACKGROUND AND AIMS

Growth arrest-specific 6 protein (Gas6) has been established to play important roles in various biological processes, but little is currently known on the role of Gas6 signaling in humans. This research explored the association between Gas6 expression and carotid atherosclerosis (AS) in type 2 diabetes mellitus (T2DM).

METHODS AND RESULTS

As many as 126 T2DM patients were recruited in this study and classified into two groups based on their carotid intima-media thickness (CIMT). Meanwhile, 50 healthy individuals were recruited for the normal control group (NC). The subgroups were compared in terms of clinical data and Gas6 expression levels. Gas6 levels were decreased in T2DM patients with or without AS compared to NC subjects (9.64 ± 1.41 ng/ml, 11.38 ± 2.08 ng/ml, and 13.64 ± 2.61 ng/ml, respectively) (p < 0.001). The interaction between Gas6 and AS in T2DM was analyzed by logistic regression model and receiver operating characteristic (ROC) curve analysis. Decreased Gas6 expression was an independent risk factor relevant to AS in T2DM (p = 0.027). The area under the ROC curve to estimate the diagnostic value of low Gas6 expression for AS in T2DM was 0.750. The correlation between Gas6 and other parameters was evaluated by Pearson correlation analysis and linear regression model. Body mass index (BMI), hemoglobin A1c (HbA1c) and tumor necrosis factor-α(TNF-α) were independently correlated with Gas6.

CONCLUSION

Low Gas6 expression is an independent risk factor for AS in T2DM. Gas6 expression is affected by BMI, HbA1c and TNF-α levels.

Vitamin K Effects on Gas6 and Soluble Axl Receptors in Intensive Care Patients: An Observational Screening Study

Growth arrest-specific gene 6 protein (Gas6) is avitamin K-dependent tissue bound protein. Gas6 has been shown to promote growth and therapy resistance among different types of cancer as well as thromboembolism. The aim of this prospective screening study: ClinicalTrials.gov; Identifier: NTC3782025, was to evaluate the effects of intravenously administered vitamin K1 on Gas6 and its soluble (s)Axl receptor plasma levels in intensive care patients. Vitamin K1 was intravenously injected in non-warfarin treated patients with prolonged Owren prothrombin time international normalized ratio (PT-INR) > 1.2 and blood samples were retrieved before and 20-28 h after injection. Citrate plasma samples from 52 intensive care patients were analysed for different vitamin K dependent proteins. There was a significant, but small increase in median Gas6. Only one patient had a large increase in sAxl, but overall, no significant changes in sAxl Gas6 did not correlate to PT-INR, thrombin generation assay, coagulation factors II, VII, IX and X, but to protein S and decarboxylated matrix Gla protein (dp-ucMGP). In conclusion, there was a small increase in Gas6 over 20-28 h. The pathophysiology and clinical importance of this remains to be investigated. To verify a true vitamin K effect, improvement of Gas6 carboxylation defects needs to be studied.

Role of Apoptotic Cell Clearance in Pneumonia and Inflammatory Lung Disease

Pneumonia and inflammatory diseases of the pulmonary system such as chronic obstructive pulmonary disease and asthma continue to cause significant morbidity and mortality globally. While the etiology of these diseases is highly different, they share a number of similarities in the underlying inflammatory processes driving disease pathology. Multiple recent studies have identified failures in efferocytosis-the phagocytic clearance of apoptotic cells-as a common driver of inflammation and tissue destruction in these diseases. Effective efferocytosis has been shown to be important for resolving inflammatory diseases of the lung and the subsequent restoration of normal lung function, while many pneumonia-causing pathogens manipulate the efferocytic system to enhance their growth and avoid immunity. Moreover, some treatments used to manage these patients, such as inhaled corticosteroids for chronic obstructive pulmonary disease and the prevalent use of statins for cardiovascular disease, have been found to beneficially alter efferocytic activity in these patients. In this review, we provide an overview of the efferocytic process and its role in the pathophysiology and resolution of pneumonia and other inflammatory diseases of the lungs, and discuss the utility of existing and emerging therapies for modulating efferocytosis as potential treatments for these diseases.

Role of Vitamin K-Dependent Factors Protein S and GAS6 and TAM Receptors in SARS-CoV-2 Infection and COVID-19-Associated Immunothrombosis

The vitamin K-dependent factors protein S (PROS1) and growth-arrest-specific gene 6 (GAS6) and their tyrosine kinase receptors TYRO3, AXL, and MERTK, the TAM subfamily of receptor tyrosine kinases (RTK), are key regulators of inflammation and vascular response to damage. TAM signaling, which has largely studied in the immune system and in cancer, has been involved in coagulation-related pathologies. Because of these established biological functions, the GAS6-PROS1/TAM system is postulated to play an important role in SARS-CoV-2 infection and progression complications. The participation of the TAM system in vascular function and pathology has been previously reported. However, in the context of COVID-19, the role of TAMs could provide new clues in virus-host interplay with important consequences in the way that we understand this pathology. From the viral mimicry used by SARS-CoV-2 to infect cells, to the immunothrombosis that is associated with respiratory failure in COVID-19 patients, TAM signaling seems to be involved at different stages of the disease. TAM targeting is becoming an interesting biomedical strategy, which is useful for COVID-19 treatment now, but also for other viral and inflammatory diseases in the future.

Testosterone ameliorates vascular aging via the Gas6/Axl signaling pathway

Low serum testosterone level is associated with aging-related vascular stiffness, but the underlying mechanism is unclear. The Growth arrest-specific protein 6 (Gas6) /Axl pathway has been proved to play important roles in cell senescence. In this study, we intend to explore whether Gas6/Axl is involved in the effect of testosterone on vascular aging amelioration. Vascular aging models of wild type and Axl^-/- mice were established by natural aging. Mice of these two gene types were randomized into young group, aging group and testosterone undecanoate (TU) treatment group. Mice were treated with TU (37.9 mg/kg) in the TU group, which treated with solvent reagent served as control. The aging mice exhibited decreases in serum testosterone, Gas6 and Axl levels and an increase in cell senescence, manifested age-related vascular remodeling. Testosterone treatment induced testosterone and Gas6 levels in serum, and ameliorated cell senescence and vascular remodeling in aging mice. Furthermore, we uncover the underlying molecular mechanism and show that testosterone treatment restored the phosphorylation of Akt and FoxO1a. Axl knockout accelerated cell senescence and vascular remodeling, and resisted the anti-aging effect of testosterone. Testosterone might exert a protective effect on vascular aging by improving cell senescence and vascular remodeling through the Gas6/Axl pathway.

Crosstalk between Akt and NF-κB pathway mediates inhibitory effect of gas6 on monocytes-endothelial cells interactions stimulated by P. gingivalis-LPS

Correlation between periodontitis and atherosclerosis is well established, and the inherent mechanisms responsible for this relationship remain unclear. The biological function of growth arrest‐specific 6 (gas6) has been discovered in both atherosclerosis and inflammation. Inhibitory effects of gas6 on the expression of inflammatory factors in human umbilical vein endothelial cells (HUVECs) stimulated by Porphyromonas gingivalis lipopolysaccharide (P. gingivalis‐LPS) were reported in our previous research. Herein, the effects of gas6 on monocytes‐endothelial cells interactions in vitro and their probable mechanisms were further investigated. Gas6 protein in HUVECs was knocked down with siRNA or overexpressed with plasmids. Transwell inserts and co‐culturing system were introduced to observe chemotaxis and adhering affinity between monocytes and endothelial cells in vitro. Expression of gas6 was decreased in inflammatory periodontal tissues and HUVECs challenged with P. gingivalis‐LPS. The inhibitory effect of gas6 on chemotaxis and adhesion affinity between monocytes and endothelial cells was observed, and gas6 promoted Akt phosphorylation and inhibited NF‐κB phosphorylation. To our best knowledge, we are first to report that gas6 inhibit monocytes‐endothelial cells interactions in vitro induced by P. gingivalis‐LPS via Akt/NF‐κB pathway. Additionally, inflammation‐mediated inhibition of gas6 expression is through LncRNA GAS6‐AS2, rather than GAS6‐AS1, which is also newly reported.

Glutathione-Allylsulfur Conjugates as Mesenchymal Stem Cells Stimulating Agents for Potential Applications in Tissue Repair

The endogenous gasotransmitter H₂S plays an important role in the central nervous, respiratory and cardiovascular systems. Accordingly, slow-releasing H₂S donors are powerful tools for basic studies and innovative pharmaco-therapeutic agents for cardiovascular and neurodegenerative diseases. Nonetheless, the effects of H₂S-releasing agents on the growth of stem cells have not been fully investigated. H₂S preconditioning can enhance mesenchymal stem cell survival after post-ischaemic myocardial implantation; therefore, stem cell therapy combined with H₂S may be relevant in cell-based therapy for regenerative medicine. Here, we studied the effects of slow-releasing H₂S agents on the cell growth and differentiation of cardiac Lin⁻ Sca1⁺ human mesenchymal stem cells (cMSC) and on normal human dermal fibroblasts (NHDF). In particular, we investigated the effects of water-soluble GSH–garlic conjugates (GSGa) on cMSC compared to other H₂S-releasing agents, such as Na₂S and GYY4137. GSGa treatment of cMSC and NHDF increased their cell proliferation and migration in a concentration dependent manner with respect to the control. GSGa treatment promoted an upregulation of the expression of proteins involved in oxidative stress protection, cell–cell adhesion and commitment to differentiation. These results highlight the effects of H₂S-natural donors as biochemical factors that promote MSC homing, increasing their safety profile and efficacy after transplantation, and the value of these donors in developing functional 3D-stem cell delivery systems for cardiac muscle tissue repair and regeneration.

Angiotensin II deteriorates advanced atherosclerosis by promoting MerTK cleavage and impairing efferocytosis through the AT1R/ROS/p38 MAPK/ADAM17 pathway

Vulnerable plaques in advanced atherosclerosis have defective efferocytosis. The role of ANG II in the progression of atherosclerosis is not fully understood. Herein, we investigated the effects and the underlying mechanisms of ANG II on macrophage efferocytosis in advanced atherosclerosis. ANG II decreased the surface expression of Mer tyrosine kinase (MerTK) in macrophages through a disintegrin and metalloproteinase17 (ADAM17)-mediated shedding of the soluble form of MerTK (sMer) in the medium, which led to efferocytosis suppression. ANG II-activated ADAM17 required reactive oxygen species (ROS) and p38 MAPK phosphorylation. Selective angiotensin II type 1 receptor (AT1R) blocker losartan suppressed ROS production, and ROS scavenger N-acetyl-l-cysteine (NAC) prevented p38 MAPK phosphorylation. In addition, mutant MERTKΔ483-488was resistant to ANG II-induced MerTK shedding and efferocytosis suppression. The advanced atherosclerosis model that is characterized by larger necrotic cores, and less collagen content was established by feeding apolipoprotein E knockout (ApoE−/−) mice with a high-fat diet for 16 wk. NAC and losartan oral administration prevented atherosclerotic lesion progression. Meanwhile, the inefficient efferocytosis represented by decreased macrophage-associated apoptotic cells and decreased MerTK+CD68+double-positive macrophages in advanced atherosclerosis were prevented by losartan and NAC. Additionally, the serum levels of sMer were increased and positively correlated with the upregulated levels of ANG II in acute coronary syndrome (ACS) patients. In conclusion, ANG II promotes MerTK shedding via AT1R/ROS/p38 MAPK/ADAM17 pathway in macrophages, which led to defective efferocytosis and atherosclerosis progression. Defining the molecular mechanisms of defective efferocytosis may provide a promising prognosis and therapy for ACS patients.

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.

Acute CD47 Blockade During Ischemic Myocardial Reperfusion Enhances Phagocytosis-Associated Cardiac Repair

Our data suggest that, after a myocardial infarction, integrin-associated protein CD47 on cardiac myocytes is elevated. In culture, increased CD47 on the surface of dying cardiomyocytes impairs phagocytic removal by immune cell macrophages. After myocardial ischemia and reperfusion, acute CD47 inhibition with blocking antibodies enhanced dead myocyte clearance by cardiac phagocytes and also improved the resolution of cardiac inflammation, reduced infarct size, and preserved cardiac contractile function. Early targeting of CD47 in the myocardium after reperfusion may be a new strategy to enhance wound repair in the ischemic heart.

Upregulation of arylsulfatase B in carotid atherosclerosis is associated with symptoms of cerebral embolization

The aim of this study was to identify genes for which the expression within carotid atherosclerosis was reproducibly associated with the symptoms of cerebral embolization. Two publically available microarray datasets E-MEXP-2257 and GSE21545 were analysed using GeneSpring 11.5. The two datasets utilized a total of 22 and 126 carotid atherosclerosis samples, obtained from patients with and without symptoms of cerebral embolization, respectively. To assess whether the findings were reproducible we analysed carotid atherosclerosis samples from another 8 patients with and 7 patients without symptoms of cerebral embolization using real-time PCR. In vitro studies using VSMC were performed to assess the functional relevance of one of the validated genes. We identified 1624 and 135 differentially expressed genes within carotid atherosclerosis samples of symptomatic compared to asymptomatic patients using the E-MEXP-2257 and GSE21545 datasets, respectively (≥1.15-absolute fold-change, P < 0.05). Only 7 differentially expressed genes or 0.4% (7/1,752) were consistent between the datasets. We validated the differential expression of ARSB which was upregulated 1.15-fold (P = 0.029) in atherosclerosis from symptomatic patients. In vitro incubation of VSMCs with the ARSB inhibitor L-ascorbic acid resulted in marked upregulation of SIRT1 and AMPK. This study suggests that ARSB may represent a novel target to limit carotid embolization.

Deficiency of AXL in Bone Marrow-Derived Cells Does Not Affect Advanced Atherosclerotic Lesion Progression

AXL, a member of the TAM (Tyro3, Axl, MerTK) family of receptors, plays important roles in cell survival, clearance of dead cells (efferocytosis), and suppression of inflammation, which are processes that critically influence atherosclerosis progression. Whereas MerTK deficiency promotes defective efferocytosis, inflammation, and plaque necrosis in advanced murine atherosclerosis, the role of Axl in advanced atherosclerosis progression is not known. Towards this end, bone marrow cells from Axl^-/- or wild-type mice were transplanted into lethally irradiated Ldlr^-/- mice. These chimeric mice were then fed the Western-type diet (WD) for 17 weeks. We demonstrate that lesional macrophages in WT mice express Axl but that Axl deficiency in bone marrow-derived cells does not affect lesion size, cellularity, necrosis, or inflammatory parameters in advanced atherosclerotic plaques. Moreover, apoptosis of lesional cells was unaffected, and we found no evidence of defective lesional efferocytosis. In contrast to previously reported findings with MerTK deficiency, hematopoietic cell-Axl deficiency in WD-fed Ldlr^-/- mice does not affect the progression of advanced atherosclerosis or lesional processes associated with TAM receptor signaling. These findings suggest a heretofore unappreciated TAM receptor hierarchy in advanced atherosclerosis.

Testosterone delays vascular smooth muscle cell senescence and inhibits collagen synthesis via the Gas6/Axl signaling pathway

Testosterone deficiency is associated with a higher incidence of cardiovascular diseases in men. However, its effect on cell senescence, which plays a causal role in vascular aging, remains unclear. Here, we tested the hypothesis that testosterone alleviated vascular smooth muscle cell (VSMC) senescence and collagen synthesis via growth arrest-specific protein 6 (Gas6)/Axl- and Akt/FoxO1a-dependent pathways. Testosterone significantly ameliorated angiotensin II-induced VSMC senescence and collagen overexpression. In addition, testosterone inhibited angiotensin II-induced matrix metalloproteinase-2 (MMP-2) activity, which played a pivotal role in facilitating age-related collagen deposition. Testosterone increased the expression of tissue inhibitor of metalloproteinase-2 but decreased the expression of MMP-2 and membrane type-1 metalloproteinase which contributed to increase MMP-2 activity. The effects on VSMCs senescence and collagen synthesis were mediated by restoration of angiotensin II-induced downregulation of Gas6 and Axl expression and a subsequent reduction of Akt and FoxO1a phosphorylation. The effects of testosterone were reversed by a Gas6 blocker, Axl-Fc, and a specific inhibitor of Axl, R428. Treatment of VSMCs with PI3K inhibitor LY294002 abrogated the downregulating effect of testosterone on MMP-2 activity. Furthermore, when FoxO1a expression was silenced by using a specific siRNA, the inhibitory effect of testosterone on MMP-2 activity was revered as well, that indicated this process was Akt/FoxO1a dependence. Taken together, Gas6/Axl and Akt/FoxO1a were involved in protective effects of testosterone on VSMCs senescence and collagen synthesis. Our results provide a novel mechanism underlying the protective effect of testosterone on vascular aging and may serve as a theoretical basis for testosterone replacement therapy.

Elevated serum levels of soluble Axl in acute coronary syndrome

BACKGROUND

Morbidities related to atherosclerosis, such as acute coronary syndrome (ACS), remain the leading cause of mortality. Axl is a receptor tyrosine kinase that is expressed in mammalian vascular and immune cells. Axl signaling is involved in the regulation of the inflammatory response. A considerable amount of evidence indicates that inflammation is responsible for the development of atherosclerosis in patients with ACS.

METHODS

To assess the relation of Axl and ACS, we recruited 64 patients with coronary heart disease: 34 with ACS, 30 with stable coronary heart disease, and 24 apparently healthy controls. Serum concentrations of soluble Axl (sAxl) were quantified by enzyme-linked immunosorbent assay. High-sensitivity C-reactive protein, tumor necrosis factor alpha, troponin I, and other routine biochemical markers were also measured.

RESULTS

The levels of sAxl were significantly higher in patients with ACS than in the controls (P=0.005). Furthermore, correlation analysis indicated that sAxl was significantly associated with serum levels of high-sensitivity C-reactive protein (r=0.283, P=0.008), tumor necrosis factor alpha (r=0.565, P<0.001), and troponin I (r=0.264, P=0.013). Logistic regression analysis (odds ratio=1.038, 95% confidence interval, 1.008-1.069, P=0.012) indicated a significant association between sAxl and ACS.

CONCLUSIONS

Serum levels of sAxl correlate to inflammatory biochemical markers. These findings demonstrate for the first time that sAxl does have a role in ACS, presumably connected to the inflammation.

Axl as a mediator of cellular growth and survival

The control of cellular growth and proliferation is key to the maintenance of homeostasis. Survival, proliferation, and arrest are regulated, in part, by Growth Arrest Specific 6 (Gas6) through binding to members of the TAM receptor tyrosine kinase family. Activation of the TAM receptors leads to downstream signaling through common kinases, but the exact mechanism within each cellular context varies and remains to be completely elucidated. Deregulation of the TAM family, due to its central role in mediating cellular proliferation, has been implicated in multiple diseases. Axl was cloned as the first TAM receptor in a search for genes involved in the progression of chronic to acute-phase leukemia, and has since been established as playing a critical role in the progression of cancer. The oncogenic nature of Axl is demonstrated through its activation of signaling pathways involved in proliferation, migration, inhibition of apoptosis, and therapeutic resistance. Despite its recent discovery, significant progress has been made in the development of effective clinical therapeutics targeting Axl. In order to accurately define the role of Axl in normal and diseased processes, it must be analyzed in a cell type-specific context.

AXL receptor tyrosine kinase is increased in patients with heart failure

BACKGROUND

AXL is a membrane receptor tyrosine kinase highly expressed in the heart and has a conspicuous role in cardiovascular physiology. The role of AXL in heart failure (HF) has not been previously addressed.

METHODS AND RESULTS

AXL protein was enhanced 6-fold in myocardial biopsies of end-stage HF patients undergoing heart transplantation compared to controls from heart donors (P<0.0001). Next, we performed a transversal study of patients with chronic HF (n=192) and a group of controls with no HF (n=67). sAXL and BNP circulating levels were quantified and clinical and demographic data were collected. sAXL levels in serum were higher in HF (86.3 ± 2.0 ng/mL) than in controls (67.8 ± 2.0 ng/mL; P<0.0001). Also, sAXL correlated with several parameters associated with worse prognosis in HF. Linear regression analysis indicated that serum creatinine, systolic blood pressure and atrial fibrillation, but not BNP levels, were predictive of sAXL levels. Cox regression analysis indicated that high sAXL values at enrollment time were related to the major HF events (all-cause mortality, heart transplantation and HF hospitalizations) at one year follow-up (P<0.001), adding predictive value to high BNP levels.

CONCLUSIONS

Myocardial expression and serum concentration of AXL is elevated in HF patients compared to controls. Furthermore, peripheral sAXL correlates with parameters associated with the progression of HF and with HF events at short term follow-up. All together these results suggest that sAXL could belong to a new molecular pathway involved in myocardial damage in HF, independent from BNP.

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.

Oxidatively modified phosphatidylserines on the surface of apoptotic cells are essential phagocytic 'eat-me' signals: cleavage and inhibition of phagocytosis by Lp-PLA2

Diversified anionic phospholipids, phosphatidylserines (PS), externalized to the surface of apoptotic cells are universal phagocytic signals. However, the role of major PS metabolites, such as peroxidized species of PS (PSox) and lyso-PS, in the clearance of apoptotic cells has not been rigorously evaluated. Here, we demonstrate that H2O2 was equally effective in inducing apoptosis and externalization of PS in naive HL60 cells and in cells enriched with oxidizable polyunsaturated species of PS (supplemented with linoleic acid (LA)). Despite this, the uptake of LA-supplemented cells by RAW264.7 and THP-1 macrophages was more than an order of magnitude more effective than that of naive cells. A similar stimulation of phagocytosis was observed with LA-enriched HL60 cells and Jurkat cells triggered to apoptosis with staurosporine. This was due to the presence of PSox on the surface of apoptotic LA-supplemented cells (but not of naive cells). This enhanced phagocytosis was dependent on activation of the intrinsic apoptotic pathway, as no stimulation of phagocytosis occurred in LA-enriched cells challenged with Fas antibody. Incubation of apoptotic cells with lipoprotein-associated phospholipase A2 (Lp-PLA2), a secreted enzyme with high specificity towards PSox, hydrolyzed peroxidized PS species in LA-supplemented cells resulting in the suppression of phagocytosis to the levels observed for naive cells. This suppression of phagocytosis by Lp-PLA2 was blocked by a selective inhibitor of Lp-PLA2, SB-435495. Screening of possible receptor candidates revealed the ability of several PS receptors and bridging proteins to recognize both PS and PSox, albeit with diverse selectivity. We conclude that PSox is an effective phagocytic 'eat-me' signal that participates in the engulfment of cells undergoing intrinsic apoptosis.

Plasma concentrations predict aortic expression of growth-arrest-specific protein 6 in patients undergoing coronary artery bypass grafting

AIMS

The tyrosine kinase receptor Axl is expressed in the vasculature, and growth arrest-specific protein 6 (Gas6) is its ligand. Plasma Gas6 levels have been shown to be associated with endothelial dysfunction markers and cardiovascular events. We set out to determine the plasma Gas6 levels in patients undergoing coronary artery bypass grafting (CABG) and investigate the expression of Gas6 and Axl in the aorta.

METHODS AND RESULTS

Immunoassays were used to investigate plasma Gas6 levels in CABG patients (n = 19) and control subjects (n = 20). The expression of Gas6 and Axl in the injured aorta were examined by reverse transcription-polymerase chain reactions, real-time reverse transcription-polymerase chain reactions, western blotting, and immunohistochemical staining. Plasma Gas6 levels were significantly lower in CABG patients than in matched control subjects. In CABG patients, plasma Gas6 levels were negatively correlated with fasting glucose, E-selectin, and vascular cell adhesion molecule-1 levels. The levels predicted the operative mortality rate and were positively correlated with plasma soluble Axl (sAxl) levels and Gas6 expression in the aorta. Moreover, Gas6 expression was positively correlated with Axl expression in the aorta.

CONCLUSION

We concluded that plasma Gas6 is associated with fasting glucose, endothelial dysfunction markers, sAxl values, and vascular Gas6 expression in CABG patients, and it predicts the operative mortality of these patients. These findings suggest that the Gas6/Axl system is crucial in vascular biology.

Biology of the TAM receptors

The TAM receptors--Tyro3, Axl, and Mer--comprise a unique family of receptor tyrosine kinases, in that as a group they play no essential role in embryonic development. Instead, they function as homeostatic regulators in adult tissues and organ systems that are subject to continuous challenge and renewal throughout life. Their regulatory roles are prominent in the mature immune, reproductive, hematopoietic, vascular, and nervous systems. The TAMs and their ligands--Gas6 and Protein S--are essential for the efficient phagocytosis of apoptotic cells and membranes in these tissues; and in the immune system, they act as pleiotropic inhibitors of the innate inflammatory response to pathogens. Deficiencies in TAM signaling are thought to contribute to chronic inflammatory and autoimmune disease in humans, and aberrantly elevated TAM signaling is strongly associated with cancer progression, metastasis, and resistance to targeted therapies.

Protein S: A multifunctional anticoagulant vitamin K-dependent protein at the crossroads of coagulation, inflammation, angiogenesis, and cancer

Since its discovery in 1970, protein S (PS) has emerged as a key vitamin K-dependent natural anticoagulant protein at the crossroads of multiple biological processes, including coagulation, apoptosis, atherosclerosis, angiogenesis/vasculogenesis, and cancer progression. Following the binding to a unique family of protein tyrosine kinase receptors referred to as Tyro-3, Axl and Mer (TAM) receptors, PS can lead to regulation of coagulation, phagocytosis of apoptotic cells, cell survival, activation of innate immunity, vessel integrity and angiogenesis, and local invasion and metastasis. Because of these dynamics and multiple functions of PS, which are largely lost following invalidation of the mouse PROS1 gene, this molecule is currently intensively studied in biomedical research. The purpose of this review is to provide a brief chronicle of the discovery and current understanding of the mechanisms of PS signaling, and how PS and their signaling partners regulate various cellular functions, with a particular focus on TAM receptors.

Activity-based kinase profiling of approved tyrosine kinase inhibitors

The specificities of nine approved tyrosine kinase inhibitors (imatinib, dasatinib, nilotinib, gefitinib, erlotinib, lapatinib, sorafenib, sunitinib, and pazopanib) were determined by activity-based kinase profiling using a large panel of human recombinant active kinases. This panel consisted of 79 tyrosine kinases, 199 serine/threonine kinases, three lipid kinases, and 29 disease-relevant mutant kinases. Many potential targets of each inhibitor were identified by kinase profiling at the K(m) for ATP. In addition, profiling at a physiological ATP concentration (1 mm) was carried out, and the IC(50) values of the inhibitors against each kinase were compared with the estimated plasma-free concentration (calculated from published pharmacokinetic parameters of plasma C(trough) and C(max) values). This analysis revealed that the approved kinase inhibitors were well optimized for their target kinases. This profiling also implicates activity at particular off-target kinases in drug side effects. Thus, large-scale kinase profiling at both K(m) and physiological ATP concentrations could be useful in characterizing the targets and off-targets of kinase inhibitors.

Contrasting Inflammation Resolution during Atherosclerosis and Post Myocardial Infarction at the Level of Monocyte/Macrophage Phagocytic Clearance

In cardiovascular disorders including advanced atherosclerosis and myocardial infarction (MI), increased cell death and tissue destabilization is associated with recruitment of inflammatory monocyte subsets that give rise to differentiated macrophages. These phagocytic cells clear necrotic and apoptotic bodies and promote inflammation resolution and tissue remodeling. The capacity of macrophages for phagocytosis of apoptotic cells (efferocytosis), clearance of necrotic cell debris, and repair of damaged tissue are challenged and modulated by local cell stressors that include increased protease activity, oxidative stress, and hypoxia. The effectiveness, or lack thereof, of phagocyte-mediated clearance, in turn is linked to active inflammation resolution signaling pathways, susceptibility to atherothrombosis and potentially, adverse post MI cardiac remodeling leading to heart failure. Previous reports indicate that in advanced atherosclerosis, defective efferocytosis is associated with atherosclerotic plaque destabilization. Post MI, the role of phagocytes and clearance in the heart is less appreciated. Herein we contrast the roles of efferocytosis in atherosclerosis and post MI and focus on how targeted modulation of clearance and accompanying resolution and reparative signaling may be a strategy to prevent heart failure post MI.

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.

Antithrombotic activity of protein S infused without activated protein C in a baboon thrombosis model

Protein S (ProS) is an essential plasma protein that enhances the anticoagulant activity of activated protein C (APC). In vitro , purified native human Zn2+-containing ProS also exerts direct anticoagulant activity by inhibiting prothrombinase and extrinsic FXase activities independently of APC. We investigated antithrombotic effects of ProS infused without APC in a baboon shunt model of thrombogenesis that employs a device consisting of arterial and venous shear flow segments. In in vitro experiments, the Zn2+-containing human ProS used for the studies displayed >10-fold higher prothrombinase inhibitory activity and anticoagulant activity in tissue factor-stimulated plasma, and four-fold higher inhibition of the intrinsic pathway than the Zn2+-deficient ProS used. In the thrombosis model, ProS (33 μg/minute for 1 hour) or saline was infused locally; platelet and fibrin deposition in the shunt were measured over 2 hours. During experiments performed at 50 ml/minute blood flow, Zn2+-containing ProS inhibited platelet deposition 73-96% in arterial-type flow segments and 90-99% in venous-type flow segments; Zn2+-deficient ProS inhibited platelet deposition 52% in arterial-type flow segments and 65-73% in venous-type flow segments. At 100 ml/min blood flow rate, Zn2+-containing ProS inhibited platelet deposition by 39% and 73% in the respective segments; Zn2+-deficient ProS inhibited platelet deposition by 5% and 0% in the respective segments. Zn2+-containing ProS suppressed fibrin deposition by 67-90%. Systemic APC-independent ProS activity was significantly increased and thrombin-antithrombin complex levels were significantly decreased after infusion of ProS. Thus, infused human Zn2+-containing ProS is antithrombotic in primates, and may have therapeutic potential even in protein C-deficient human patients.

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.

Thrombin as a multi-functional enzyme. Focus on in vitro and in vivo effects

Thrombin is the central protease in the coagulation cascade and one of the most extensively studied of all enzymes. In addition to its recognised role in the coagulation cascade and haemostasis, thrombin is known to have multiple pleiotropic effects, which mostly have been shown only in in vitro studies: it plays a role in inflammation and cellular proliferation and displays a mitogen activity on smooth muscle cells and endothelial cells, predominantly by activation of angiogenesis. In vivo , thrombin effects were examined in animal models of intravenous or intraarterial thrombin infusion. An extensive literature search regarding in vivo data showed that i) thrombin administered as a bolus causes microembolism, ii) thrombin infused slowly at steady-state conditions (up to 1.6 U/kg/min) leads to bleeds but not to intravascular clotting, iii) large quantity of thrombin infused at low rates (0.05 U/kg/min) does not have any measurable effect, and iv) thrombin increases vascular permeability leading to tissue damage. Although several decades of research on thrombin functions have provided a framework for understanding the biology of thrombin, animal and human studies with use of newer laboratory techniques are still needed to confirm the pleiotropic thrombin functions shown in in vitro studies.

The role of macrophages and dendritic cells in the clearance of apoptotic cells in advanced atherosclerosis

Accumulating evidence supports the notion that defective phagocytic clearance of dying cells, or defective "efferocytosis," is causally linked to the progression of advanced atherosclerosis. In advanced atherosclerotic lesions, defective efferocytosis leads to post-apoptotic necrosis, expansion of plaque necrotic cores, and susceptibility to atherothrombosis. Both macrophages and DC-like efferocytes are juxtaposed near expanding necrotic cores, where they engage apoptotic cells. In this Viewpoint, we discuss how reduced efferocytosis by macrophages and CD11c(HI) DC-like cells may combine to reduce overall plaque stability and therefore promote susceptibility to acute atherothrombosis.

Shedding of the Mer tyrosine kinase receptor is mediated by ADAM17 protein through a pathway involving reactive oxygen species, protein kinase Cδ, and p38 mitogen-activated protein kinase (MAPK)

Mer tyrosine kinase (MerTK) is an integral membrane protein that is preferentially expressed by phagocytic cells, where it promotes efferocytosis and inhibits inflammatory signaling. Proteolytic cleavage of MerTK at an unidentified site leads to shedding of its soluble ectodomain (soluble MER; sMER), which can inhibit thrombosis in mice and efferocytosis in vitro. Herein, we show that MerTK is cleaved at proline 485 in murine macrophages. Site-directed deletion of 6 amino acids spanning proline 485 rendered MerTK resistant to proteolysis and suppression of efferocytosis by cleavage-inducing stimuli. LPS is a known inducer of MerTK cleavage, and the intracellular signaling pathways required for this action are unknown. LPS/TLR4-mediated generation of sMER required disintegrin and metalloproteinase ADAM17 and was independent of Myd88, instead requiring TRIF adaptor signaling. LPS-induced cleavage was suppressed by deficiency of NADPH oxidase 2 (Nox2) and PKCδ. The addition of the antioxidant N-acetyl cysteine inhibited PKCδ, and silencing of PKCδ inhibited MAPK p38, which was also required. In a mouse model of endotoxemia, we discovered that LPS induced plasma sMER, and this was suppressed by Adam17 deficiency. Thus, a TRIF-mediated pattern recognition receptor signaling cascade requires NADPH oxidase to activate PKCδ and then p38, culminating in ADAM17-mediated proteolysis of MerTK. These findings link innate pattern recognition receptor signaling to proteolytic inactivation of MerTK and generation of sMER and uncover targets to test how MerTK cleavage affects efferocytosis efficiency and inflammation resolution in vivo.

Role of vitamin K-dependent proteins in the arterial vessel wall

Vitamin K was discovered early last century at the same time as the vitamin K-antagonists. For many years the role of vitamin K was solely ascribed to coagulation and coagulation was thought to be involved only at the venous blood side. This view has dramatically changed with the discovery of vitamin K-dependent proteins outside the coagulation cascade and the role of coagulation factors at the arterial side. Vitamin K-dependent proteins are involved in the regulation of vascular smooth muscle cell migration, apoptosis, and calcification. Vascular calcification has become an important independent predictor of cardiovascular disease. Vitamin K-antagonists induce inactivity of inhibitors of vascular calcification, leading to accelerated calcification. The involvement of vitamin K-dependent proteins such as MGP in vascular calcification make that calcification is amendable for intervention with high intake of vitamin K. This review focuses on the effect of vitamin K-dependent proteins in vascular disease.