Targeting endothelial and myocardial dysfunction with tetrahydrobiopterin.

Tetrahydrobiopterin (BH(4)) is an essential cofactor for aromatic amino acid hydroxylases and for all three nitric oxide synthase (NOS) isoforms. It also has a protective role in the cell as an antioxidant and scavenger of reactive nitrogen and oxygen species. Experimental studies in humans and animals demonstrate that decreased BH(4)-bioavailability, with subsequent uncoupling of endothelial NOS (eNOS) plays an important role in the pathogenesis of endothelial dysfunction, hypertension, ischemia-reperfusion injury, and pathologic cardiac remodeling. Synthetic BH(4) is clinically approved for the treatment of phenylketonuria, and experimental studies support its capacity for ameliorating cardiovascular pathophysiologies. To date, however, the translation of these studies to human patients remains limited, and early results have been mixed. In this review, we discuss the pathophysiologic role of decreased BH(4) bioavailability, molecular mechanisms regulating its metabolism, and its potential therapeutic use as well as pitfalls as an NOS-modulating drug. This article is part of a special issue entitled ''Key Signaling Molecules in Hypertrophy and Heart Failure.''

Ligand binding and subtype selectivity of the human A(2A) adenosine receptor: identification and characterization of essential amino acid residues.

The crystal structure of the human A(2A) adenosine receptor bound to the A(2A) receptor-specific antagonist, ZM241385, was recently determined at 2.6-A resolution. Surprisingly, the antagonist binds in an extended conformation, perpendicular to the plane of the membrane, and indicates a number of interactions unidentified before in ZM241385 recognition. To further understand the selectivity of ZM241385 for the human A(2A) adenosine receptor, we examined the effect of mutating amino acid residues within the binding cavity likely to have key interactions and that have not been previously examined. Mutation of Phe-168 to Ala abolishes both agonist and antagonist binding as well as receptor activity, whereas mutation of this residue to Trp or Tyr had only moderate effects. The Met-177 --> Ala mutation impeded antagonist but not agonist binding. Finally, the Leu-249 --> Ala mutant showed neither agonist nor antagonist binding affinity. From our results and previously published mutagenesis data, we conclude that conserved residues Phe-168(5.29), Glu-169(5.30), Asn-253(6.55), and Leu-249(6.51) play a central role in coordinating the bicyclic core present in both agonists and antagonists. By combining the analysis of the mutagenesis data with a comparison of the sequences of different adenosine receptor subtypes from different species, we predict that the interactions that determine subtype selectivity reside in the more divergent "upper" region of the binding cavity while the "lower" part of the binding cavity is conserved across adenosine receptor subtypes.

Lung ischemia-reperfusion injury: a molecular and clinical view on a complex pathophysiological process.

Lung ischemia-reperfusion injury remains one of the major complications after cardiac bypass surgery and lung transplantation. Due to its dual blood supply system and the availability of oxygen from alveolar ventilation, the pathogenetic mechanisms of ischemia-reperfusion injury in the lungs are more complicated than in other organs, where loss of blood flow automatically leads to hypoxia. In this review, an extensive overview is given of the molecular and cellular mechanisms that are involved in the pathogenesis of lung ischemia-reperfusion injury and the possible therapeutic strategies to reduce or prevent it. In addition, the roles of neutrophils, alveolar macrophages, cytokines, and chemokines, as well as the alterations in the cell-death related pathways, are described in detail.

Pathogenetic role of eNOS uncoupling in cardiopulmonary disorders.

The homodimeric flavohemeprotein endothelial nitric oxide synthase (eNOS) oxidizes l-arginine to l-citrulline and nitric oxide (NO), which acutely vasodilates blood vessels and inhibits platelet aggregation. Chronically, eNOS has a major role in the regulation of blood pressure and prevention of atherosclerosis by decreasing leukocyte adhesion and smooth muscle proliferation. However, a disturbed vascular redox balance results in eNOS damage and uncoupling of oxygen activation from l-arginine conversion. Uncoupled eNOS monomerizes and generates reactive oxygen species (ROS) rather than NO. Indeed, eNOS uncoupling has been suggested as one of the main pathomechanisms in a broad range of cardiovascular and pulmonary disorders such as atherosclerosis, ventricular remodeling, and pulmonary hypertension. Therefore, modulating uncoupled eNOS, in particular eNOS-dependent ROS generation, is an attractive therapeutic approach to preventing and/or treating cardiopulmonary disorders, including protective effects during cardiothoracic surgery. This review provides a comprehensive overview of the pathogenetic role of uncoupled eNOS in both cardiovascular and pulmonary disorders. In addition, the related therapeutic possibilities such as supplementation with the eNOS substrate l-arginine, volatile NO, and direct NO donors as well as eNOS modulators such as the eNOS cofactor tetrahydrobiopterin and folic acid are discussed in detail.