Immune modulation of vascular resident cells by Axl orchestrates carotid intima-media thickening

Growth Arrest Specific-6 and Axl Coordinate Inflammation and Hypertension

[Figure: see text].

Deficiency of Axl aggravates pulmonary arterial hypertension via BMPR2

Pulmonary arterial hypertension (PAH), is a fatal disease characterized by a pseudo-malignant phenotype. We investigated the expression and the role of the receptor tyrosine kinase Axl in experimental (i.e., monocrotaline and Su5416/hypoxia treated rats) and clinical PAH. In vitro Axl inhibition by R428 and Axl knock-down inhibited growth factor-driven proliferation and migration of non-PAH and PAH PASMCs. Conversely, Axl overexpression conferred a growth advantage. Axl declined in PAECs of PAH patients. Axl blockage inhibited BMP9 signaling and increased PAEC apoptosis, while BMP9 induced Axl phosphorylation. Gas6 induced SMAD1/5/8 phosphorylation and ID1/ID2 increase were blunted by BMP signaling obstruction. Axl association with BMPR2 was facilitated by Gas6/BMP9 stimulation and diminished by R428. In vivo R428 aggravated right ventricular hypertrophy and dysfunction, abrogated BMPR2 signaling, elevated pulmonary endothelial cell apoptosis and loss. Together, Axl is a key regulator of endothelial BMPR2 signaling and potential determinant of PAH.

Bone marrow-derived AXL tyrosine kinase promotes mitogenic crosstalk and cardiac allograft vasculopathy

Cardiac Allograft Vasculopathy (CAV) is a leading contributor to late transplant rejection. Although implicated, the mechanisms by which bone marrow-derived cells promote CAV remain unclear. Emerging evidence implicates the cell surface receptor tyrosine kinase AXL to be elevated in rejecting human allografts. AXL protein is found on multiple cell types, including bone marrow-derived myeloid cells. The causal role of AXL from this compartment and during transplant is largely unknown. This is important because AXL is a key regulator of myeloid inflammation. Utilizing experimental chimeras deficient in the bone marrow-derived Axl gene, we report that Axl antagonizes cardiac allograft survival and promotes CAV. Flow cytometric and histologic analyses of Axl-deficient transplant recipients revealed reductions in both allograft immune cell accumulation and vascular intimal thickness. Co-culture experiments designed to identify cell-intrinsic functions of Axl uncovered complementary cell-proliferative pathways by which Axl promotes CAV-associated inflammation. Specifically, Axl-deficient myeloid cells were less efficient at increasing the replication of both antigen-specific T cells and vascular smooth muscle cells (VSMCs), the latter a key hallmark of CAV. For the latter, we discovered that Axl-was required to amass the VSMC mitogen Platelet-Derived Growth Factor. Taken together, our studies reveal a new role for myeloid Axl in the progression of CAV and mitogenic crosstalk. Inhibition of AXL-protein, in combination with current standards of care, is a candidate strategy to prolong cardiac allograft survival.

Natriuretic Peptide Receptor 2 Locus Contributes to Carotid Remodeling

Background Carotid artery intima/media thickness (IMT) is a hallmark trait associated with future cardiovascular events. The goal of this study was to map new genes that regulate carotid IMT by genome-wide association. Methods and Results We induced IMT by ligation procedure of the left carotid artery in 30 inbred mouse strains. Histologic reconstruction revealed significant variation in left carotid artery intima, media, adventitia, external elastic lamina volumes, intima-to-media ratio, and (intima+media)/external elastic lamina percent ratio in inbred mice. The carotid remodeling trait was regulated by distinct genomic signatures with a dozen common single-nucleotide polymorphisms associated with left carotid artery intima volume, intima-to-media ratio, and (intima+media)/external elastic lamina percent ratio. Among genetic loci on mouse chromosomes 1, 4, and 12, there was natriuretic peptide receptor 2 (Npr2), a strong candidate gene. We observed that only male, not female, mice heterozygous for a targeted Npr2 deletion (Npr2^+/-) exhibited defective carotid artery remodeling compared with Npr2 wild-type (Npr2^+/+) littermates. Fibrosis in carotid IMT was significantly increased in Npr2^+/- males compared with Npr2^+/- females or Npr2^+/+ mice. We also detected decreased Npr2 expression in human atherosclerotic plaques, similar to that seen in studies in Npr2^+/- mice. Conclusions We found that components of carotid IMT were regulated by distinct genetic factors. We also showed a critical role for Npr2 in genetic regulation of vascular fibrosis associated with defective carotid remodeling.

The Protective Role of Natriuretic Peptide Receptor 2 against High Salt Injury in the Renal Papilla

Mutations in natriuretic peptide receptor 2 (Npr2) gene cause a rare form of short-limbed dwarfism, but its physiological effects have not been well studied. Human and mouse genetic data suggest that Npr2 in the kidney plays a role in salt homeostasis. Herein, we described anatomic changes within renal papilla of Npr2 knockout (Npr2-/-) mice. Dramatic reduction was found in diuresis, and albuminuria was evident after administration of 1% NaCl in drinking water in Npr2-/- and heterozygous (Npr2+/-) mice compared with their wild-type (Npr2+/+) littermates. There was indication of renal epithelial damage accompanied by high numbers of red blood cells and inflammatory cells (macrophage surface glycoproteins binding to galectin-3) and an increase of renal epithelial damage marker (T-cell Ig and mucin domain 1) in Npr2-/- mice. Addition of 1% NaCl tended to increase apoptotic cells (cleaved caspase 3) in the renal papilla of Npr2-/- mice. In vitro, genetic silencing of the Npr2 abolished protective effects of C-type natriuretic peptide, a ligand for Npr2, against death of M-1 kidney epithelial cells exposed to 360 mmol/L NaCl. Finally, significantly lower levels of expression of the NPR2 protein were detected in renal samples of hypertensive compared with normotensive human subjects. Taken together, these findings suggest that Npr2 is essential to protect renal epithelial cells from high concentrations of salt and prevent kidney injury.

Targeting of TAM Receptors Ameliorates Fibrotic Mechanisms in Idiopathic Pulmonary Fibrosis

RATIONALE

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

OBJECTIVES

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

METHODS

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

MEASUREMENTS AND MAIN RESULTS

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

CONCLUSIONS

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

Innate Immune Cells Are Regulated by Axl in Hypertensive Kidney

The balance between adaptive and innate immunity in kidney damage in salt-dependent hypertension is unclear. We investigated early renal dysfunction and the influence of Axl, a receptor tyrosine kinase, on innate immune response in hypertensive kidney in mice with lymphocyte deficiency (Rag1^-/-). The data suggest that increased presence of CD11b⁺ myeloid cells in the medulla might explain intensified salt and water retention as well as initial hypertensive response in Rag1^-/- mice. Global deletion of Axl on Rag1^-/- background reversed kidney dysfunction and accumulation of myeloid cells in the kidney medulla. Chimeric mice that lack Axl in innate immune cells (in the absence of lymphocytes) significantly improved kidney function and abolished early hypertensive response. The bioinformatics analyses of Axl-related gene-gene interaction networks established tissue-specific variation in regulatory pathways. It was confirmed that complement C3 is important for Axl-mediated interactions between myeloid and vascular cells in hypertensive kidney. In summary, innate immunity is crucial for renal dysfunction in early hypertension, and is highly influenced by the presence of Axl.

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.

Strain-selective efficacy of sacubitril/valsartan on carotid fibrosis in response to injury in two inbred mouse strains

BACKGROUND AND PURPOSE

Sacubitril/valsartan (Sac/val) is more effective than valsartan in lowering BP and mortality in patients with heart failure. Here, we proposed that Sac/val treatment would be more effective in preventing pathological vascular remodelling in 129X1/SvJ (129X1), than in C57BL/6J (B6) inbred mice.

EXPERIMENTAL APPROACH

Sac/val (60 mg·kg^-1 ·day^-1 ) and valsartan (27 mg·kg^-1 ·day^-1 ) were given as prophylactic or therapeutic treatments, to 129X1 or B6 mice with carotid artery ligation for 14 days. Blood flow was measured by ultrasound. Ex vivo, carotid tissue was analysed with histological and morphometric techniques, together with RNA sequencing and gene ontology.

KEY RESULTS

Sac/val was more effective than valsartan in lowering BP in 129X1 compared with B6 mice. Liver expression of CYP2C9 and plasma cGMP levels were similar across treatments. A reduction in carotid thickening after prophylactic treatment with valsartan or Sac/val also resulted in significant arterial shrinkage in B6 mice. In 129X1 mice, Sac/val and prophylactic treatment with valsartan had no effect on carotid thickening but preserved carotid size. BP lowering significantly correlated with a decline in carotid stiffness (R²  = .37, P = .0096) in 129X1 but not in B6 mice. The gene expression signature associated with hyalurononglucosaminidase activity was down-regulated in injured arteries after both regimens of Sac/val only in 129X1 mice. Administration of Sac/val but not valsartan significantly reduced deposition of hyaluronic acid and carotid fibrosis in 129X1 mice.

CONCLUSION AND IMPLICATIONS

These results underscore the importance of the genetic background in the efficacy of the Sac/val on vascular fibrosis.

Axl expression is increased in early stages of left ventricular remodeling in an animal model with pressure-overload

Background

AXL is a receptor tyrosine kinase that has been related to kidney and vascular disorders. Heart failure patients with reduced ejection fraction have higher AXL in serum than controls. No information about Axl expression with HF progression is available.

Methods

Thoracic transverse aortic constriction (TAC) was successfully performed on male Wistar rats (n = 25) with different constriction levels. Controls underwent sham surgery (n = 12). Echocardiography measurements were performed 4–8 weeks after surgery. Collagen deposition was measured with picrosirius red staining. Axl mRNA levels in left ventricle (LV), left kidney (LK) and ascending aorta (aAo) and the LV expression of cardiac remodeling and fibrogenic factors were quantified with real-time PCR. AXL LV protein levels were quantified with western blot and localization was analyzed by immunohistochemistry. Soluble AXL levels in plasma were assayed with ELISA.

Results

Successful TAC rats were classified into LV hypertrophy (LVH) or heart failure (HF), modeling the progressive cardiac changes after pressure overload. Collagen deposition was increased only in the HF group. LV Axl mRNA levels were higher in LVH and HF than in Sham rats, and correlated with LVHi, and hypertrophic and fibrogenic mediators. However, no association was found with LV systolic function. AXL was expressed in LV myocytes and other cell types. Concentration of circulating sAXL in plasma was increased in the LVH group compared to Sham and HF rats. Axl mRNA levels were similar in all groups in the LK and aAo.

Conclusions

Axl expression pattern suggests a role in the early progression of LV remodeling in HF but not in the later systolic dysfunction. The higher levels of circulating AXL found in HF patients most probably shed from the heart.

Role of Axl in T-Lymphocyte Survival in Salt-Dependent Hypertension

OBJECTIVE

Survival of immune and nonimmune cells relies on Axl, a receptor tyrosine kinase, which is implicated in hypertension. Activated T lymphocytes are involved in regulation of high blood pressure. The goal of the study was to investigate the role of Axl in T-lymphocyte functions and its contribution to salt-dependent hypertension.

APPROACH AND RESULTS

We report increased apoptosis in peripheral blood from Axl(-/-) mice because of lower numbers of white blood cells mostly lymphocytes. In vitro studies showed modest reduction in interferon gamma production in Axl(-/-) type 1 T helper cells. Axl did not affect basic proliferation capacity or production of interleukin 4 in Axl(-/-) type 2 T helper cells. However, competitive repopulation of Axl(-/-) bone marrow or adoptive transfer of Axl(-/-) CD4(+) T cells to Rag1(-/-) mice showed robust effect of Axl on T lymphocyte expansion in vivo. Adoptive transfer of Axl(-/-) CD4(+) T cells was protective in a later phase of deoxycorticosterone-acetate and salt hypertension. Reduced numbers of CD4(+) T cells in circulation and in perivascular adventitia decreased vascular remodeling and increased vascular apoptosis in the late phase of hypertension.

CONCLUSIONS

These findings suggest that Axl is critical for survival of T lymphocytes, especially during vascular remodeling in hypertension.

Axl modulates immune activation of smooth muscle cells in vein graft remodeling

The pathophysiological mechanisms of the immune activation of smooth muscle cells are not well understood. Increased expression of Axl, a receptor tyrosine kinase, was recently found in arteries from patients after coronary bypass grafts. In the present study, we hypothesized that Axl-dependent immune activation of smooth muscle cells regulates vein graft remodeling. We observed a twofold decrease in intimal thickening after vascular and systemic depletion of Axl in vein grafts. Local depletion of Axl had the greatest effect on immune activation, whereas systemic deletion of Axl reduced intima due to an increase in apoptosis in vein grafts. Primary smooth muscle cells isolated from Axl knockout mice had reduced proinflammatory responses by prevention of the STAT1 pathway. The absence of Axl increased suppressor of cytokine signaling (SOCS)1 expression in smooth muscle cells, a major inhibitory protein for STAT1. Ultrasound imaging suggested that vascular depletion of Axl reduced vein graft stiffness. Axl expression determined the STAT1-SOCS1 balance in vein graft intima and progression of the remodeling. The results of this investigation demonstrate that Axl promotes STAT1 signaling via inhibition of SOCS1 in activated smooth muscle cells in vein graft remodeling.

Autonomic dysfunction determines stress-induced cardiovascular and immune complications in mice

BACKGROUND

Clinical studies suggest that acute inflammation in patients with elevated heart rate (HR) increases morbidity and mortality. The SJL/J (SJL) inbred mouse strain is a unique genetic model that has higher HR and systemic and vascular inflammation compared with C3HeB/FeJ (C3HeB) mice. The goal of this study was to investigate the role of stress on cardiac and vascular complications between 2 strains.

METHODS AND RESULTS

Radiotelemetry was used for continuous recordings of HR and blood pressure in mice. Hemodynamic differences between mouse strains were very small without stress; however, tail-cuff training generated mild stress and significantly increased HR (≈2-fold) in SJL compared with C3HeB mice. Circulating proinflammatory monocytes (CD11b(+)Ly6C(H) (i)) significantly increased in SJL mice but not in C3HeB mice after stress. Presence of Ly6C(+) cells in injured carotids was elevated only in SJL mice after stress; however, a transfer of bone marrow cells from SJL/C3HeB to C3HeB/SJL chimeras had no effect on HR or vascular inflammation following stress. Arterial inflammation (VCAM-1(+)) was greater in SJL inbred mice or SJL recipient chimeras, even without stress or injury. HR variability was reduced in SJL mice compared with C3HeB mice.

CONCLUSIONS

We found that impaired parasympathetic activity is central for stress-induced elevation of HR and systemic and vascular inflammation; however, immune cells from stress-susceptible mice had no effect on HR or vascular inflammation in stress-protected mice.

Disturbed flow-activated p90RSK kinase accelerates atherosclerosis by inhibiting SENP2 function

Disturbed blood flow (d-flow) causes endothelial cell (EC) dysfunction, leading to atherosclerotic plaque formation. We have previously shown that d-flow increases SUMOylation of p53 and ERK5 through downregulation of sentrin/SUMO-specific protease 2 (SENP2) function; however, it is not known how SENP2 itself is regulated by d-flow. Here, we determined that d-flow activated the serine/threonine kinase p90RSK, which subsequently phosphorylated threonine 368 (T368) of SENP2. T368 phosphorylation promoted nuclear export of SENP2, leading to downregulation of eNOS expression and upregulation of proinflammatory adhesion molecule expression and apoptosis. In an LDLR-deficient murine model of atherosclerosis, EC-specific overexpression of p90RSK increased EC dysfunction and lipid accumulation in the aorta compared with control animals; however, these pathologic changes were not observed in atherosclerotic mice overexpressing dominant negative p90RSK (DN-p90RSK). Moreover, depletion of SENP2 in these mice abolished the protective effect of DN-p90RSK overexpression. We propose that p90RSK-mediated SENP2-T368 phosphorylation is a master switch in d-flow-induced signaling, leading to EC dysfunction and atherosclerosis.

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.

Role of Axl in early kidney inflammation and progression of salt-dependent hypertension

The Gas6/Axl pathway regulates many cell functions and is implicated in hypertension. In this study, we aimed to investigate the role of Axl in immune cells on initiation and progression of salt-dependent hypertension. Deoxycorticosterone acetate (75 mg/60 days release)-salt hypertension was induced for 1 week or 6 weeks in Axl chimeras generated by bone marrow transplant to restrict Axl deficiency to hematopoietic or nonhematopoietic compartments. Depletion of Axl in hematopoietic cells (Axl(-/-) →Axl(+/+)) reduced (133 ± 2 mm Hg) increase in systolic blood pressure compared with other Axl chimeras (≈150 mm Hg) 1 week after deoxycorticosterone acetate-salt. Urine protein and renal oxidative stress were lowest in Axl(-/-) →Axl(+/+) at 1 week after deoxycorticosterone acetate-salt. Compensatory increase in Gas6 in kidneys of recipient Axl(-/-) may affect kidney function and blood pressure in early phase of hypertension. Flow cytometry on kidneys from Axl(-/-) →Axl(+/+) showed increase in total leukocytes, B, and dendritic cells and decrease in macrophages compared with Axl(+/+) →Axl(+/+). These immune changes were associated with decrease in proinflammatory gene expression, in particular interferon γ. Systolic blood pressure returned to baseline in Axl(-/-) →Axl(+/+) and Axl(-/-) →Axl(-/-) but remained increased in Axl(+/+) →Axl(+/+) and Axl(+/+) →Axl(-/-) chimeras after 6 weeks of deoxycorticosterone acetate-salt. Vascular apoptosis was increased in the global Axl(-/-) chimeras in the late phase of hypertension. In summary, we found that expression of Axl in hematopoietic cells is critical for kidney pathology in early phase of salt-dependent hypertension. However, Axl in both hematopoietic and nonhematopoietic lineages contributes to the late phase of hypertension.

De-SUMOylation enzyme of sentrin/SUMO-specific protease 2 regulates disturbed flow-induced SUMOylation of ERK5 and p53 that leads to endothelial dysfunction and atherosclerosis

RATIONALE

Disturbed flow induces proinflammatory and apoptotic responses in endothelial cells, causing them to become dysfunctional and subsequently proatherogenic.

OBJECTIVE

Although a possible link between SUMOylation of p53 and ERK5 detected during endothelial apoptosis and inflammation has been suggested, the mechanistic insights, especially under the proatherogenic flow condition, remain largely unknown.

METHODS AND RESULTS

SUMOylation of p53 and ERK5 was induced by disturbed flow but not by steady laminar flow. To examine the role of the disturbed flow-induced p53 and ERK5 SUMOylation, we used de-SUMOylation enzyme of sentrin/Small Ubiquitin-like MOdifier (SUMO)-specific protease 2 deficiency (Senp2(+/-)) mice and observed a significant increase in endothelial apoptosis and adhesion molecule expression both in vitro and in vivo. These increases, however, were significantly inhibited in endothelial cells overexpressing p53 and ERK5 SUMOylation site mutants. Senp2(+/-) mice exhibited increased leukocyte rolling along the endothelium, and accelerated formation of atherosclerotic lesions was observed in Senp2(+/-)/Ldlr(-/-), but not in Senp2(+/+)/Ldlr(-/-), mice fed a high-cholesterol diet. Notably, the extent of lesion size in the aortic arch of Senp2(+/-)/Ldlr(-/-) mice was much larger than that in the descending aorta, also suggesting a crucial role of the disturbed flow-induced SUMOylation of proteins, including p53 and ERK5 in atherosclerosis formation.

CONCLUSIONS

These data show the unique role of sentrin/SUMO-specific protease 2 on endothelial function under disturbed flow and suggest that SUMOylation of p53 and ERK5 by disturbed flow contributes to the atherosclerotic plaque formation. Molecules involved in this newly discovered signaling will be useful targets for controlling endothelial cells dysfunction and consequently atherosclerosis formation.