Synthesis and recycling of tetrahydrobiopterin in endothelial function and vascular disease.
ABSTRACT Nitric oxide, generated by the nitric oxide synthase (NOS) enzymes, plays pivotal roles in cardiovascular homeostasis and in the pathogenesis of cardiovascular disease. The NOS cofactor, tetrahydrobiopterin (BH4), is an important regulator of NOS function, since BH4 is required to maintain enzymatic coupling of L-arginine oxidation, to produce NO. Loss or oxidation of BH4 to 7,8-dihydrobiopterin (BH2) is associated with NOS uncoupling, resulting in the production of superoxide rather than NO. In addition to key roles in folate metabolism, dihydrofolate reductase (DHFR) can 'recycle' BH2, and thus regenerate BH4. It is therefore likely that net BH4 cellular bioavailability reflects the balance between de novo BH4 synthesis, loss of BH4 by oxidation to BH2, and the regeneration of BH4 by DHFR. Recent studies have implicated BH4 recycling in the direct regulation of eNOS uncoupling, showing that inhibition of BH4 recycling using DHFR-specific siRNA and methotrexate treatment leads to eNOS uncoupling in endothelial cells and the hph-1 mouse model of BH4 deficiency, even in the absence of oxidative stress. These studies indicate that not only BH4 level, but the recycling pathways regulating BH4 bioavailability represent potential therapeutic targets and will be discussed in this review.
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ABSTRACT: Tetrahydrobiopterin (BH4), which fosters the formation of and stabilizes endothelial NO synthase (eNOS) as an active dimer, tightly regulates eNOS coupling / uncoupling. Moreover, studies conducted in genetically-modified models demonstrate that BH4 pulmonary deficiency is a key determinant in the pathogenesis of pulmonary hypertension. The present study thus investigates biopterin metabolism and eNOS expression, as well as the effect of sepiapterin (a precursor of BH4) and eNOS gene deletion, in a mice model of hypoxic pulmonary hypertension. In lungs, chronic hypoxia increased BH4 levels and eNOS expression, without modifying dihydrobiopterin (BH2, the oxidation product of BH4) levels, GTP cyclohydrolase-1 or dihydrofolate reductase expression (two key enzymes regulating BH4 availability). In intrapulmonary arteries, chronic hypoxia also increased expression of eNOS, but did not induce destabilisation of eNOS dimers into monomers. In hypoxic mice, sepiapterin prevented increase in right ventricular systolic pressure and right ventricular hypertrophy, whereas it modified neither remodelling nor alteration in vasomotor responses (hyper-responsiveness to phenylephrine, decrease in endothelium-dependent relaxation to acetylcholine) in intrapulmonary arteries. Finally, deletion of eNOS gene partially prevented hypoxia-induced increase in right ventricular systolic pressure, right ventricular hypertrophy and remodelling of intrapulmonary arteries. Collectively, these data demonstrate the absence of BH4/BH2 changes and eNOS dimer destabilisation, which may induce eNOS uncoupling during hypoxia-induced pulmonary hypertension. Thus, even though eNOS gene deletion and sepiapterin treatment exert protective effects on hypoxia-induced pulmonary vascular remodelling, increase on right ventricular pressure and / or right ventricular hypertrophy, these effects appear unrelated to biopterin-dependent eNOS uncoupling within pulmonary vasculature of hypoxic wild-type mice.PLoS ONE 01/2013; 8(11):e82594. · 3.73 Impact Factor
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ABSTRACT: The intense systemic inflammatory response characterizing septic shock is associated with an increased generation of free radicals by multiple cell types in cardiovascular and non cardiovascular tissues. The oxygen-centered radical superoxide anion (O2 .-) rapidly reacts with the nitrogen-centered radical nitric oxide (NO.) to form the potent oxidant species peroxynitrite. Peroxynitrite oxidizes multiple targets molecules, either directly or via the secondary generation of highly reactive radicals, resulting in significant alterations in lipids, proteins and nucleic acids, with significant cytotoxic consequences. The formation of peroxynitrite is a key pathophysiological mechanism contributing to the cardiovascular collapse of septic shock, promoting vascular contractile failure, endothelial and myocardial dysfunction, and is also implicated in the occurrence of multiple organ dysfunction in this setting. The recent development of various porphyrin-based pharmacological compounds accelerating the degradation of peroxynitrite has allowed to specifically address these pathophysiological roles of peroxynitrite in experimental septic shock. Such agents, including 5,10,15,20-tetrakis(4- sulfonatophenyl)porphyrinato iron III chloride (FeTTPs), manganese tetrakis(4-N-methylpyridyl)porphyrin (MnTMPyP), Fe(III) tetrakis-2-(N-triethylene glycol monomethyl ether)pyridyl porphyrin) (FP-15) and WW-85, have been shown to improve the cardiovascular and multiple organ failure in small and large animal models of septic shock. Therefore, these findings support the development of peroxynitrite decomposition catalysts as potentially useful novel therapeutic agents to restore cardiovascular function in sepsis.Current Vascular Pharmacology 03/2013; 11(2):196-207. · 2.82 Impact Factor
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ABSTRACT: Oxidative stress and inflammation in the vascular wall are essential mechanisms of atherosclerosis and vascular dysfunctions associated with risk factors such as metabolic diseases, aging, hypertension, etc. Evidence has been provided that activation of the vascular endothelial cells in the presence of the risk factors promotes oxidative stress and vascular inflammatory responses, leading to acceleration of atherosclerotic vascular disease. Increasing number of studies from recent years demonstrates that uncoupling of endothelial nitric oxide synthase (eNOS), whereby the enzyme eNOS produces detrimental amount of superoxide anion [Formula: see text] instead the vasoprotective nitric oxide (NO(⋅)), plays a critical role in vascular dysfunction under various pathophysiological conditions and in aging. The mechanisms of eNOS-uncoupling seem multiple and complex. Recent research provides emerging evidence supporting an essential role of increased activity of arginases including arginase-I and arginase-II in causing eNOS-uncoupling, which results in vascular oxidative stress and inflammatory responses, and ultimately leading to vascular diseases. This review article will summarize the most recent findings on the functional roles of arginases in vascular diseases and/or dysfunctions and the underlying mechanisms in relation to oxidative stress and inflammations. Moreover, regulatory mechanisms of arginases in the vasculature are reviewed and the future perspectives of targeting arginases as therapeutic options in vascular diseases are discussed.Frontiers in Immunology 01/2013; 4:149.