Tetrahydrobiopterin in cardiovascular health and disease

Abstract

Tetrahydrobiopterin (BH4) functions as a cofactor for several important enzyme systems, and considerable evidence implicates BH4 as a key regulator of endothelial nitric oxide synthase (eNOS) in the setting of cardiovascular health and disease. BH4 bioavailability is determined by a balance of enzymatic de novo synthesis and recycling, versus degradation in the setting of oxidative stress. Augmenting vascular BH4 levels by pharmacological supplementation has been shown in experimental studies to enhance NO bioavailability. However, it has become more apparent that the role of BH4 in other enzymatic pathways, including other NOS isoforms and the aromatic amino acid hydroxylases, may have a bearing on important aspects of vascular homeostasis, inflammation, and cardiac function. This article reviews the role of BH4 in cardiovascular development and homeostasis, as well as in pathophysiological processes such as endothelial and vascular dysfunction, atherosclerosis, inflammation, and cardiac hypertrophy. We discuss the therapeutic potential of BH4 in cardiovascular disease states and attempt to address how this modulator of intracellular NO-redox balance may ultimately provide a powerful new treatment for many cardiovascular diseases.

FIG. 1.

5,6,7,8-tetrahydrobiopterin (BH4) biosynthesis proceeds from guanosine triphosphate (GTP) via 7,8-dyhydroneopterin triphosphate and 6-pyruvoyl-5,6,7,8-tetrahydropterin. The first and rate-limiting step in the pathway is GTP cyclohydrolase I (GTPCH). The following steps are catalyzed by the enzymes 6-pyruvoyl tetrahydropterin synthase (PTPS) and sepiapterin reductase (SR). An alternative pathway for BH4 synthesis has been documented by which 6-pyruvoyl-5,6,7,8-tetrahydrobiopterin is converted into sepiapterin by an enzyme termed “sepiapterin synthase”. Exogenous sepiapterin can be reduced in all cells by SR to BH2, and further by dihydrofolate reductase (DHFR) to form BH4, the so-called “salvage pathway.” The principle oxidant species leading to BH4 oxidation to BH2 is peroxynitrite. As a cofactor for the aromatic amino acid hydroxylases (AAAH) in the liver and neurons, but not as a cofactor for NOS, BH4 is converted into tetrahydrobiopterin-4a-carbinolamine, which is recycled to BH4 by the actions of pterin-4a-carbinolamine dehydratase (PCD) and DHPR. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 2.

Enzyme cofactor activity of BH4. BH4 is required for NO production by all three nitric oxide synthase isoforms. Under normal conditions, these enzymes couple oxidation of the amino acid substrate L-arginine with the reduction of molecular oxygen to form NO and L-citrulline. BH4 also has cofactor activity for the AAAH, resulting in neurotransmitter synthesis and preventing the accumulation of phenylalanine. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 3.

Antiatherogenic and antithrombotic properties of endothelial NO. NO is produced in the endothelium by endothelial nitric oxide synthase (eNOS). It diffuses into the vessel wall, leads to the relaxation of vascular smooth muscle cells, and also inhibits the proliferation and migration of smooth muscle cells. On the luminal surface of the blood vessel, NO inhibits leucocyte adhesion to endothelial cells and migration into the vascular wall as well as platelet aggregation and adhesion. NO further mediates VEGF-induced endothelial cell proliferation and plays a critical role in the prevention of endothelial cell senescence and apoptosis. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 4.

NOS catalysis. All three isoforms of NOS function as homodimers. Each monomer consists of a reductase domain with binding sites for NADPH, FAD, FMN, and calmodulin (CaM) and an oxygenase domain containing an iron haem group and binding sites for L-arginine (L-Arg) and BH4. Starting from NADPH, electrons flow to the flavins FAD and FMN of the reductase domain, to the iron of the haem in the oxygenase domain. CaM regulates electron flow between the reductase and oxygenase domain. BH4 seems to be essential to donate electrons to the haem group in the oxygenase domain in order to oxidize L-arginine. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 5.

Normal and dysfunctional roles of eNOS coupling. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 6.

The role of folic acid in 1-carbon metabolism and its potential mechanisms for cardiac protection. The active metabolites of folic acid serve functional roles in purine synthesis and methylation reactions. Besides converting homocysteine to methionine, 5-methyl THF improves vascular endothelial function by enhancing nitric oxide bioavailability and protects against oxidative injury. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 7.

The Hph-1 mouse model of BH4 deficiency. The Hph-1 mouse model of BH4 deficiency has been used in a number of studies to illustrate the importance of BH4 in various vascular disease states. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 8.

The GCH transgenic mouse. Endothelial-specific GCH overexpression in the GCH transgenic mouse has been shown to be sufficient to restore eNOS coupling and NO bioavailability in various vascular disease states. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 9.

Cell specific roles of BH4 and NOS isoforms in atherosclerosis. BH4 has distinct roles in different cell types that are implicated in atherosclerosis, in particular in endothelial cells (blue), and monocyte/macrophages (orange), which are recruited to atherosclerotic plaque and migrate into the vascular wall. The contrasting expression of eNOS in endothelial cells and inducible NOS (iNOS) in macrophages may lead to the opposing effects of BH4 deficiency in leukocytes and endothelial cells. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 10.

A comparison of three key findings demonstrating eNOS uncoupling and the ability of the GCH-Tg mouse to retard plaque progression. (i) Overexpression of eNOS accelerates atherosclerotic lesion formation in apoE-deficient mice—atherosclerotic lesions in aortas are markedly worsened in ApoE-KO/eNOS-Tg vs. ApoE-KO mice at 16 weeks of age. (a and b) Sudan III–stained, longitudinally opened aortas from apoE-KO (a) and apoE-KO/eNOS-Tg mice (b) at the age of 16 weeks. (c) Quantitative analysis of atherosclerotic lesion size in 16-week-old mice. After 12 weeks on a high-cholesterol diet, the lesion size in apoE-KO/eNOS-Tg mice was significantly greater than that in apoE-KO mice. Reproduced with permission from the Journal of Clinical Investigation, Ozaki et al. (197). (ii) Increased endothelial tetrahydrobiopterin synthesis by targeted transgenic GTP-cyclohydrolase I overexpression reduces endothelial dysfunction and atherosclerosis in ApoE-knockout mice—GCH overexpression reduces aortic root plaque area in ApoE-KO mice. Aortic root plaque area in ApoE-KO/GCH-Tg (white squares) and ApoE-KO mice (black squares), with representative aortic root sections from each group, stained with Elastic and Masson's trichrome stain (elastic laminae stain black, collagen green, and cardiac myocytes red). (*P<0.05) Taken with permission from the American Heart Association, Alp et al. (5). (iii) Augmenting BH4 levels in the endothelium by GCH overexpression reduced the accelerated atherosclerotic lesion formation in ApoE-KO/eNOS-Tg mice—Overexpression of GTPCH atherosclerotic lesion formation at the aortic sinus. (a), Representative figures (×40) of Sudan III–stained atherosclerotic lesion at the aortic sinus (a, ApoE-KO mice; b, ApoE-KO/eNOS-Tg mice; c, ApoE-KO/eNOS-Tg/GCH-Tg mice; d, vitamin C–treated ApoE-KO/eNOS-Tg mice). A black bar indicates 500 μm. Reproduced with permission from the American Heart Association, Takaya et al. (268). To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 11.

BH₄ levels are depleted in ischemic hearts. The decline of BH₄ is paralleled by an increase in XPH₂, an irreversibly oxidized product of BH4. (n=5/point). Taken with permission from the Proceedings of the National Academy of Sciences, from Dumitrescu et al. (62).

FIG. 12.

BH4 treatment reverses advanced hypertrophy caused by sustained pressure overload. Above: TAC-(thoracic aortic constriction) stimulated increases in heart weight at 4 weeks were reversed by the subsequent addition of oral BH4, whereas untreated hearts continued to enlarge. Below: Example M-mode echocardiograms showing increased dilation, wall thickening, and reduced fractional shortening after 4 weeks and 9 weeks of TAC. All parameters were significantly improved by BH4 treatment. Used with permission from the American Heart Association, from Moens et al. (182). To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars