The endothelium lines the luminal surface of every blood vessel, allowing it contact with circulating blood elements, as well as the underlying vascular smooth muscle layer. In healthy vessels, the endothelium expresses constitutive forms of nitric oxide synthase (NOSIII) and cyclo-oxygenase (COX-1), which produce the vasoactive hormones NO and prostacyclin, respectively. Both NO and prostacyclin relax blood vessels and inhibit platelet activation. The actions of prostacyclin are mediated by cell surface prostacyclin (IP) receptors and/or intracellular peroxisome proliferator-activated receptors (PPAR)β. The actions of NO are mediated predominately by activation of intracellular guanylyl cyclase, leading to the formation of cGMP. In platelets, the actions of NO and prostacyclin are synergistic, but in vessels their actions are additive. In diseased vessels, inducible forms of NOS (NOSII) and cyclo-oxygeanse (COX-2) are expressed in vascular smooth muscle, resulting in the release of large amounts of NO, prostacyclin and prostaglandin E 2 . The relative contribution of NOSII and COX-2 to vascular inflammation is still debated, but is likely to result in both protective and damaging responses. The relative contribution of constitutive forms of NOS and COX, as well as interactions between IP, PPARβ and guanylyl cyclase pathways in vessels and platelets, is discussed.

Prostacyclin is an antithrombotic hormone produced by the endothelium, whose production is dependent on cyclooxygenase (COX) enzymes of which two isoforms exist. It is widely believed that COX-2 drives prostacyclin production and that this explains the cardiovascular toxicity associated with COX-2 inhibition, yet the evidence for this relies on indirect evidence from urinary metabolites. Here we have used a range of experimental approaches to explore which isoform drives the production of prostacyclin in vitro and in vivo. Our data show unequivocally that under physiological conditions it is COX-1 and not COX-2 that drives prostacyclin production in the cardiovascular system, and that urinary metabolites do not reflect prostacyclin production in the systemic circulation. With the idea that COX-2 in endothelium drives prostacyclin production in healthy individuals removed, we must seek new answers to why COX-2 inhibitors increase the risk of cardiovascular events to move forward with drug discovery and to enable more informed prescribing advice.

Rationale RhoA and Rho kinase contribute to pulmonary vasoconstriction and vascular remodeling in pulmonary hypertension. RhoB, a protein homologous to RhoA and activated by hypoxia, regulates neoplastic growth and vasoconstriction but its role in the regulation of pulmonary vascular function is not known. Objective To determine the role of RhoB in pulmonary endothelial and smooth muscle cell responses to hypoxia and in pulmonary vascular remodeling in chronic hypoxia-induced pulmonary hypertension. Methods and Results Hypoxia increased expression and activity of RhoB in human pulmonary artery endothelial and smooth muscle cells, coincidental with activation of RhoA. Hypoxia or adenoviral overexpression of constitutively activated RhoB increased actomyosin contractility, induced endothelial permeability, and promoted cell growth; dominant negative RhoB or manumycin, a farnesyltransferase inhibitor that targets the vascular function of RhoB, inhibited the effects of hypoxia. Coordinated activation of RhoA and RhoB maximized the hypoxia-induced stress fiber formation caused by RhoB/mammalian homolog of Drosophila diaphanous-induced actin polymerization and RhoA/Rho kinase-induced phosphorylation of myosin light chain on Ser19. Notably, RhoB was specifically required for hypoxia-induced factor-1α stabilization and for hypoxia-and platelet-derived growth factor-induced cell proliferation and migration. RhoB deficiency in mice markedly attenuated development of chronic hypoxia-induced pulmonary hypertension, despite compensatory expression of RhoA in the lung. Conclusions RhoB mediates adaptational changes to acute hypoxia in the vasculature, but its continual activation by chronic hypoxia can accentuate vascular remodeling to promote development of pulmonary hypertension. RhoB is a potential target for novel approaches (eg, farnesyltransferase inhibitors) aimed at regulating pulmonary vascular tone and structure. (Circ Res. 2012;110:1423–1434.)