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: (6R)-5,6,7,8-Tetrahydro-L-biopterin (BH4) availability regulates nitric oxide and superoxide formation by endothelial nitric oxide synthase (eNOS). At low BH4 or low BH4 to 7,8-dihydrobiopterin (BH2) ratios the enzyme becomes uncoupled and generates superoxide at the expense of NO. We studied the effects of exogenously added BH2 on intracellular BH4/BH2 ratios and eNOS activity in different types of endothelial cells. Incubation of porcine aortic endothelial cells with BH2 increased BH4/BH2 ratios from 8.4 (controls) and 0.5 (BH4-depleted cells) up to ∼20, demonstrating efficient reduction of BH2. Uncoupled eNOS activity observed in BH4-depleted cells was prevented by preincubation with BH2. Recycling of BH4 was much less efficient in human endothelial cells isolated from umbilical veins or derived from dermal microvessels (HMEC-1 cells), which exhibited eNOS uncoupling and low BH4/BH2 ratios under basal conditions and responded to exogenous BH2 with only moderate increases in BH4/BH2 ratios. The kinetics of dihydrofolate reductase-catalyzed BH4 recycling in endothelial cytosols showed that the apparent BH2 affinity of the enzyme was 50- to 300-fold higher in porcine than in human cell preparations. Thus, the differential regulation of eNOS uncoupling in different types of endothelial cells may be explained by striking differences in the apparent BH2 affinity of dihydrofolate reductase.Biochemical pharmacology. 05/2014;
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ABSTRACT: Obesity is a risk factor for developing type 2 diabetes and cardiovascular disease and has quickly become a world-wide pandemic with few tangible and safe treatment options. While it is generally accepted that the primary cause of obesity is energy imbalance, i.e., the calories consumed are greater than are utilized, understanding how caloric balance is regulated has proven a challenge. Many "distal" causes of obesity, such as the structural environment, occupation, and social influences, are exceedingly difficult to change or manipulate. Hence, molecular processes and pathways more proximal to the origins of obesity-those that directly regulate energy metabolism or caloric intake-appear to be more feasible targets for therapy. In particular, nitric oxide (NO) is emerging as a central regulator of energy metabolism and body composition. NO bioavailability is decreased in animal models of diet-induced obesity and in obese and insulin resistant patients, and increasing NO output has remarkable effects on obesity and insulin resistance. This review discusses the role of NO in regulating adiposity and insulin sensitivity and places its modes of action into context with the known causes and consequences of metabolic disease.Free radical biology & medicine. 05/2014;
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ABSTRACT: The aim of the present study was to conduct an updated meta-analysis of relevant randomized controlled trials (RCTs) in order to estimate the effect of folic acid supplementation on endothelial function and the concentration of plasma homocysteine in patients with coronary artery disease (CAD). An extensive search of PubMed was conducted to identify RCTs that compared folic acid with placebo therapy. The mean difference (MD) and 95% confidence interval (CI) were used as a measure of the correlation between folic acid supplementation and endothelial function/plasma homocysteine concentration. Of the 377 patients included in this analysis, 191 patients underwent folic acid supplementation and 186 individuals underwent placebo treatment. Compared with the use of a placebo, folic acid supplementation alone exhibited significant efficacy on increasing flow-mediated dilation (FMD; MD, 57.72 μm; 95% CI, 50.14-65.31; P<0.05) and lowering the concentration of plasma homocysteine (MD, -3.66 μmol/l; 95% CI, -5.44--1.87; P<0.05; I(2), 87%). There was no significant change in the response to end diastolic diameter, glyceryl-trinitrate diameter, heart rate, baseline and peak hyperemic flow and systolic and diastolic blood pressure between the folic acid and placebo groups (P>0.05). Therefore, the meta-analysis indicated that 5 mg folic acid daily supplementation for >4 weeks significantly improved FMD and lowered the concentration of plasma homocysteine in patients with CAD. However, more RCTs are required in order to confirm these observations.Experimental and therapeutic medicine 05/2014; 7(5):1100-1110. · 0.34 Impact Factor