Oxidative Stress, GTPCH1, and Endothelial Nitric Oxide Synthase Uncoupling in Hypertension

Abstract

Significance: Hypertension has major health consequences, which is associated with endothelial dysfunction. Endothelial nitric oxide synthase (eNOS)-produced nitric oxide (NO) signaling in the vasculature plays an important role in maintaining vascular homeostasis. Considering the importance of NO system, this review aims to provide a brief overview of the biochemistry of members of NO signaling, including GTPCH1 [guanosine 5'-triphosphate (GTP) cyclohydrolase 1], tetrahydrobiopterin (BH₄), and eNOS. Recent Advances: Being NO signaling activators and regulators of eNOS signaling, BH₄ treatment is getting widespread attention either as potential therapeutic agents or as preventive agents. Recent clinical trials also support that BH₄ treatment could be considered a promising therapeutic in hypertension. Critical Issues: Under conditions of BH₄ depletion, eNOS-generated superoxides trigger pathological events. Abnormalities in NO availability and BH₄ deficiency lead to disturbed redox regulation causing pathological events. This disturbed signaling influences the development of systemic hypertension as well as pulmonary hypertension. Future Directions: Considering the importance of BH₄ and NO to improve the translational significance, it is essential to continue research on this field to manipulate BH₄ to increase the efficacy for treating hypertension. Thus, this review also examines the current state of knowledge on the effects of eNOS activators on preclinical models and humans to utilize this information for potential therapy.

Keywords: GTPCH1; eNOS uncoupling; endothelial nitric oxide synthase; hypertension.

FIG. 1.

Mechanisms by which eNOS activity is regulated in endothelial cells. eNOS is regulated by PKC (phosphorylates at S116); PKC-α (phosphorylates at T495); PKA (phosphorylates at S635); and AMPK, PI3K/AKT (phosphorylates at S1177). eNOS is an obligate homodimer and this dimeric association is mediated by a cysteine-complexed Zn²⁺ (zinc-tetrathiolate) at the dimer interface. AMPK, 5′ adenosine monophosphate-activated protein kinase; BH₂, dihydrobiopterin; BH₄, tetrahydrobiopterin; DHFR, dihydrofolate reductase; eNOS, endothelial nitric oxide synthase; FAD, flavin adenine dinucleotide; FMN, flavin mononucleotide; NO, nitric oxide. Color images are available online.

FIG. 2.

Inhibitory factors of NO^. production. DHFR recycles the BH₄ to BH₂ and further inhibits the eNOS activity. ROS and HOCl target the GTPCH1 enzyme and release the Zn ion, and the BH₄ production is inhibited. Inflammation, oxidative stress, peroxynitrite, and HOCl all can target eNOS and reduce the NO^. production. GTPCH1, guanosine 5′-triphosphate (GTP)-cyclohydrolase 1; HOCl, hypochlorous acid; O₂·⁻, superoxide; ONOO⁻, peroxynitrite; ROS, reactive oxygen species. Color images are available online.

FIG. 3.

Mechanisms by which coupled eNOS regulates the vascular homeostasis and uncoupled eNOS determines hypertension. Increased bioavailability of BH₄ and coupled form of eNOS lead to production of NO, by utilizing the l-arginine as a substrate. The NO^. further helps to keep the vessel homeostasis and normal blood pressure. Whereas the uncoupled form of eNOS along with increased BH2 levels helps vessel constriction and leads to high blood pressure. Color images are available online.

FIG. 4.

GTPCH1 regulation and BH₄ synthesis. Cytokines, resveratrol, ARBs, and statins are known to increase the GTPCH1 expression as well as BH₄ synthesis. GTP is used as a substrate by GTPCH1 and produces dihydroneopterin-3P. Dihydroneopterin-3P is again used by 6-PTS and produces 6-pyruvoyl-tetrahydropterin and further converted into BH₄ by SPR. BH₂ is recycled by DHFR into BH_4, whereas BH₂ is produced by the oxidation of BH_4. 6-PTS, 6-pyruvoyl-tetrahydropterin synthase; ARBs, angiotensin receptor blockers; GFRP, GTP cyclohydrolase feedback regulatory protein; SPR, sepiapterin reductase. Color images are available online.

FIG. 5.

GTPCH1 structure and zinc release by oxidants. GTPCH1 is a homodecameric enzyme. Zinc ion generates a hydroxyl nucleophile for the attack of imidazole ring carbon atom eight of the substrate, GTP. Both HOCl and ONOO⁻ react fast with the positively charged zinc atom resulting in its release from zinc-containing proteins. Color images are available online.

FIG. 6.

Zinc releases from GTPCH1 by oxidants and polyubiquitination of GTPCH1. Exposure of ONOO⁻ or SIN-1, an NO^. donor, both target the GTPCH1-Zn complex and release the Zn ion. High-glucose exposure in endothelial cells triggers the GTPCH1 ubiquitination and inhibits BH₄ synthesis. Whereas Mn-SOD overexpression, L-NAME exposure, TEMPO, or uric acid exposure all can inhibit the GTPCH1 ubiquitination triggered by high d-glucose exposure. L-NAME, N-nitro-l-arginine methyl ester; SOD, superoxide dismutase. Color images are available online.