Inhibition of endothelial nitric oxide synthase activity by proline-rich tyrosine kinase 2 in response to fluid shear stress and insulin.

In native and primary cultures of endothelial cells, fluid shear stress elicits the tyrosine phosphorylation of the endothelial NO synthase (eNOS), however, the consequences of this modification on enzyme activity are unclear. We found that fluid shear stress induces the association of eNOS with the proline-rich tyrosine kinase 2 (PYK2) in endothelial cells and that the eNOS immunoprecipitated from eNOS- and PYK2-overexpressing HEK293 cells was tyrosine-phosphorylated on Tyr657. In mouse carotid arteries, the overexpression of wild-type PYK2, but not a dominant-negative PYK2, decreased eNOS activity (approximately 50%), whereas in murine lung endothelial cells, the downregulation of PYK2 (small interfering RNA) increased ionomycin-induced NO production. Mutation of Tyr657 to the phosphomimetic residues aspartate (D) or glutamate (E) abolished enzyme activity, whereas a nonphosphorylatable mutant (phenylalanine [F]) showed activity comparable to the wild-type enzyme. Moreover, normal flow-induced vasodilatation was apparent in carotid arteries from eNOS(-/-) mice overexpressing either the wild-type eNOS or the Y657F mutant, whereas no flow-induced vasodilatation was apparent in arteries expressing the Y657E eNOS mutant. Insulin also activated PYK2 and stimulated eNOS in endothelial cells expressing the Y657F mutant but not wild-type eNOS. These data indicate that PYK2 mediates the tyrosine phosphorylation of eNOS on Tyr657 in response to fluid shear stress and insulin stimulation and that this modification attenuates the activity of the enzyme. The PYK2-dependent inhibition of NO production may serve to keep eNOS activity low and limit the detrimental consequences of maintained high NO output, ie, the generation of peroxynitrite.

Ribavirin inhibits angiogenesis by tetrahydrobiopterin depletion.

Ribavirin is a broad-spectrum antiviral drug that is used to treat hepatitis C virus (HCV)-infected patients. The virological response after ribavirin treatment appears to be insufficient to fully explain ribavirin-induced beneficial effects. Angiogenesis plays a pathogenic role in HCV-induced liver damage. Here, we investigated the influence of therapeutic ribavirin concentrations on angiogenesis. Ribavirin inhibited endothelial cell tube formation in vitro and vessel formation in the chick chorioallantoic membrane assay in vivo. Ribavirin inhibits inosine monophosphate dehydrogenase, which causes depletion of cellular GTP and in turn reduction of cellular tetrahydrobiopterin levels. The availability of tetrahydrobiopterin limits NO production by endothelial NO synthase. Ribavirin reduced levels of tetrahydrobiopterin (as revealed by HPLC), NO (as revealed by electron spin resonance spectroscopy), and cGMP (as revealed by RIA) in endothelial cells. Addition of tetrahydrobiopterin or NO prevented ribavirin-induced tube formation inhibition. In conclusion, angiogenesis inhibition by ribavirin has not been described before. This inhibition may contribute to ribavirin-induced pharmacological effects including adverse events.

Ex vivo pretreatment of bone marrow mononuclear cells with endothelial NO synthase enhancer AVE9488 enhances their functional activity for cell therapy.

Bone marrow mononuclear cells (BMC) from patients with ischemic cardiomyopathy (ICMP) show a reduced neovascularization capacity in vivo. NO plays an important role in neovascularization, and NO bioavailability is typically reduced in patients with ICMP. We investigated whether the impaired neovascularization capacity of ICMP patient-derived progenitor cells can be restored by pretreatment with the novel endothelial NO synthase (eNOS) transcription enhancer AVE9488 (AVE). Ex vivo pretreatment of BMC from patients with ICMP with AVE significantly increased eNOS mRNA expression by 2.1-fold (P < 0.05) and eNOS activity as assessed by ESR by >3-fold (P < 0.05). The increased eNOS expression was associated with an enhanced migratory capacity in vitro (P < 0.01) and improved neovascularization capacity of the infused BMC in an ischemic hind limb model in vivo (P < 0.001). The improvement in ischemic limb perfusion after infusion of AVE-pretreated BMC resulted in an increase in swimming time (P < 0.05). The enhancement of limb perfusion by AVE-treated BMC was abrogated by ex vivo pretreatment with the eNOS inhibitor N(G)-nitro-l-arginine methyl ester. Consistently, AVE showed no effect on the impaired migratory capacity of BMC derived from eNOS-deficient mice, documenting the specific involvement of NO. The reduced neovascularization capacity of BMC from patients with ICMP may limit their therapeutic potential in cell therapy studies. Here, we show that pharmacological enhancement of eNOS expression with AVE at least partially reverses the impaired functional activity of BMC from ICMP patients, highlighting the critical role of NO for progenitor cell function.

Gab1, SHP2, and protein kinase A are crucial for the activation of the endothelial NO synthase by fluid shear stress.

Fluid shear stress enhances NO production in endothelial cells by a mechanism involving the activation of the phosphatidylinositol 3-kinase and the phosphorylation of the endothelial NO synthase (eNOS). We investigated the role of the scaffolding protein Gab1 and the tyrosine phosphatase SHP2 in this signal transduction cascade in cultured and native endothelial cells. Fluid shear stress elicited the phosphorylation and activation of Akt and eNOS as well as the tyrosine phosphorylation of Gab1 and its association with the p85 subunit of phosphatidylinositol 3-kinase and SHP2. Overexpression of a Gab1 mutant lacking the pleckstrin homology domain abrogated the shear stress-induced phosphorylation of Akt but failed to affect the phosphorylation or activity of eNOS. The latter response, however, was sensitive to a protein kinase A (PKA) inhibitor. Mutation of Gab1 Tyr627 to phenylalanine (YF-Gab1) to prevent the binding of SHP2 completely prevented the shear stress-induced phosphorylation of eNOS, leaving the Akt response intact. A dominant-negative SHP2 mutant prevented the activation of PKA and phosphorylation of eNOS without affecting that of Akt. Moreover, shear stress elicited the formation of a signalosome complex including eNOS, Gab1, SHP2 and the catalytic subunit of PKA. In isolated murine carotid arteries, flow-induced vasodilatation was prevented by a PKA inhibitor as well as by overexpression of either the YF-Gab1 or the dominant-negative SHP2 mutant. Thus, the shear stress-induced activation of eNOS depends on Gab1 and SHP2, which, in turn, regulate the phosphorylation and activity of eNOS by a PKA-dependent but Akt-independent mechanism.