Cystathionine γ Lyase Sulfhydrates the RNA Binding Protein Human Antigen R to Preserve Endothelial Cell Function and Delay Atherogenesis.

BACKGROUND: Hydrogen sulfide (H2S), generated by cystathionine γ lyase (CSE), is an important endogenous regulator of vascular function. The aim of the present study was to investigate the control and consequences of CSE activity in endothelial cells under physiological and proatherogenic conditions. METHODS: Endothelial cell CSE knockout mice were generated, and lung endothelial cells were studied in vitro (gene expression, protein sulfhydration, and monocyte adhesion). Mice were crossed onto the apolipoprotein E-deficient background, and atherogenesis (partial carotid artery ligation) was monitored over 21 days. CSE expression, H2S bioavailability, and amino acid profiling were also performed with human material. RESULTS: The endothelial cell-specific deletion of CSE selectively increased the expression of CD62E and elevated monocyte adherence in the absence of an inflammatory stimulus. Mechanistically, CD62E mRNA was more stable in endothelial cells from CSE-deficient mice, an effect attributed to the attenuated sulfhydration and dimerization of the RNA-binding protein human antigen R. CSE expression was upregulated in mice after partial carotid artery ligation and in atheromas from human subjects. Despite the increase in CSE protein, circulating and intraplaque H2S levels were reduced, a phenomenon that could be attributed to the serine phosphorylation (on Ser377) and inhibition of the enzyme, most likely resulting from increased interleukin-1ß. Consistent with the loss of H2S, human antigen R sulfhydration was attenuated in atherosclerosis and resulted in the stabilization of human antigen R-target mRNAs, for example, CD62E and cathepsin S, both of which are linked to endothelial cell activation and atherosclerosis. The deletion of CSE from endothelial cells was associated with the accelerated development of endothelial dysfunction and atherosclerosis, effects that were reversed on treatment with a polysulfide donor. Finally, in mice and humans, plasma levels of the CSE substrate l-cystathionine negatively correlated with vascular reactivity and H2S levels, indicating its potential use as a biomarker for vascular disease. CONCLUSIONS: The constitutive S-sulfhydration of human antigen R (on Cys13) by CSE-derived H2S prevents its homodimerization and activity, which attenuates the expression of target proteins such as CD62E and cathepsin S. However, as a consequence of vascular inflammation, the beneficial actions of CSE-derived H2S are lost owing to the phosphorylation and inhibition of the enzyme.

AMP-Activated Protein Kinase α2 in Neutrophils Regulates Vascular Repair via Hypoxia-Inducible Factor-1α and a Network of Proteins Affecting Metabolism and Apoptosis.

RATIONALE: The AMP-activated protein kinase (AMPK) is stimulated by hypoxia, and although the AMPKα1 catalytic subunit has been implicated in angiogenesis, little is known about the role played by the AMPKα2 subunit in vascular repair. OBJECTIVE: To determine the role of the AMPKα2 subunit in vascular repair. METHODS AND RESULTS: Recovery of blood flow after femoral artery ligation was impaired (>80%) in AMPKα2-/- versus wild-type mice, a phenotype reproduced in mice lacking AMPKα2 in myeloid cells (AMPKα2ΔMC). Three days after ligation, neutrophil infiltration into ischemic limbs of AMPKα2ΔMC mice was lower than that in wild-type mice despite being higher after 24 hours. Neutrophil survival in ischemic tissue is required to attract monocytes that contribute to the angiogenic response. Indeed, apoptosis was increased in hypoxic neutrophils from AMPKα2ΔMC mice, fewer monocytes were recruited, and gene array analysis revealed attenuated expression of proangiogenic proteins in ischemic AMPKα2ΔMC hindlimbs. Many angiogenic growth factors are regulated by hypoxia-inducible factor, and hypoxia-inducible factor-1α induction was attenuated in AMPKα2-deficient cells and accompanied by its enhanced hydroxylation. Also, fewer proteins were regulated by hypoxia in neutrophils from AMPKα2ΔMC mice. Mechanistically, isocitrate dehydrogenase expression and the production of α-ketoglutarate, which negatively regulate hypoxia-inducible factor-1α stability, were attenuated in neutrophils from wild-type mice but remained elevated in cells from AMPKα2ΔMC mice. CONCLUSIONS: AMPKα2 regulates α-ketoglutarate generation, hypoxia-inducible factor-1α stability, and neutrophil survival, which in turn determine further myeloid cell recruitment and repair potential. The activation of AMPKα2 in neutrophils is a decisive event in the initiation of vascular repair after ischemia.

Myeloid-Specific Deletion of the AMPKα2 Subunit Alters Monocyte Protein Expression and Atherogenesis.

The AMP-activated protein kinase (AMPK) is an energy sensing kinase that is activated by a drop in cellular ATP levels. Although several studies have addressed the role of the AMPKα1 subunit in monocytes and macrophages, little is known about the α2 subunit. The aim of this study was to assess the consequences of AMPKα2 deletion on protein expression in monocytes/macrophages, as well as on atherogenesis. A proteomics approach was applied to bone marrow derived monocytes from wild-type mice versus mice specifically lacking AMPKα2 in myeloid cells (AMPKα2∆MC mice). This revealed differentially expressed proteins, including methyltransferases. Indeed, AMPKα2 deletion in macrophages increased the ratio of S-adenosyl methionine to S-adenosyl homocysteine and increased global DNA cytosine methylation. Also, methylation of the vascular endothelial growth factor and matrix metalloproteinase-9 (MMP9) genes was increased in macrophages from AMPKα2∆MC mice, and correlated with their decreased expression. To link these findings with an in vivo phenotype, AMPKα2∆MC mice were crossed onto the ApoE-/- background and fed a western diet. ApoExAMPKα2∆MC mice developed smaller atherosclerotic plaques than their ApoExα2fl/fl littermates, that contained fewer macrophages and less MMP9 than plaques from ApoExα2fl/fl littermates. These results indicate that the AMPKα2 subunit in myeloid cells influences DNA methylation and thus protein expression and contributes to the development of atherosclerotic plaques.

VASP regulates leukocyte infiltration, polarization, and vascular repair after ischemia.

In ischemic vascular diseases, leukocyte recruitment and polarization are crucial for revascularization and tissue repair. We investigated the role of vasodilator-stimulated phosphoprotein (VASP) in vascular repair. After hindlimb ischemia induction, blood flow recovery, angiogenesis, arteriogenesis, and leukocyte infiltration into ischemic muscles in VASP-/- mice were accelerated. VASP deficiency also elevated the polarization of the macrophages through increased signal transducer and activator of transcription (STAT) signaling, which augmented the release of chemokines, cytokines, and growth factors to promote leukocyte recruitment and vascular repair. Importantly, VASP deletion in bone marrow-derived cells was sufficient to mimic the increased blood flow recovery of global VASP-/- mice. In chemotaxis experiments, VASP-/- neutrophils/monocytes were significantly more responsive to M1-related chemokines than wild-type controls. Mechanistically, VASP formed complexes with the chemokine receptor CCR2 and ß-arrestin-2, and CCR2 receptor internalization was significantly reduced in VASP-/- leukocytes. Our data indicate that VASP is a major regulator of leukocyte recruitment and polarization in postischemic revascularization and support a novel role of VASP in chemokine receptor trafficking.

Role of secreted modular calcium-binding protein 1 (SMOC1) in transforming growth factor ß signalling and angiogenesis.

AIMS: Secreted modular calcium-binding protein 1 (SMOC1) is a matricellular protein that potentially interferes with growth factor receptor signalling. The aim of this study was to determine how its expression is regulated in endothelial cells and its role in the regulation of endothelial cell function. METHODS AND RESULTS: SMOC1 was expressed by native murine endothelial cells as well as by cultured human, porcine, and murine endothelial cells. SMOC1 expression in cultured cells was increased by hypoxia via the down-regulation of miR-223, and SMOC1 expression was increased in lungs from miR-223-deficient mice. Silencing SMOC1 (small interfering RNA) attenuated endothelial cell proliferation, migration, and sprouting in in vitro angiogenesis assays. Similarly endothelial cell sprouting from aortic rings ex vivo as well as postnatal retinal angiogenesis in vivo was attenuated in SMOC1(+/-) mice. In endothelial cells, transforming growth factor (TGF)-ß signalling via activin-like kinase (ALK) 5 leads to quiescence, whereas TGF-ß signalling via ALK1 results in endothelial cell activation. SMOC1 acted as a negative regulator of ALK5/SMAD2 signalling, resulting in altered α2 integrin levels. Mechanistically, SMOC1 associated (immunohistochemistry, proximity ligation assay, and co-immunoprecipitation) with endoglin; an endothelium-specific type III auxiliary receptor for the TGF-ß super family and the effects of SMOC1 down-regulation on SMAD2 phosphorylation were abolished by the down-regulation of endoglin. CONCLUSION: These results indicate that SMOC1 is an ALK5 antagonist produced by endothelial cells that tips TGF-ß signalling towards ALK1 activation, thus promoting endothelial cell proliferation and angiogenesis.