Identification and functional characterization of phosphorylation sites on GTP cyclohydrolase I

Tetrahydrobiopterin metabolism attenuates ROS generation and radiosensitivity through LDHA S-nitrosylation: novel insight into radiogenic lung injury

Genotoxic therapy triggers reactive oxygen species (ROS) production and oxidative tissue injury. S-nitrosylation is a selective and reversible posttranslational modification of protein thiols by nitric oxide (NO), and 5,6,7,8-tetrahydrobiopterin (BH4) is an essential cofactor for NO synthesis. However, the mechanism by which BH4 affects protein S-nitrosylation and ROS generation has not been determined. Here, we showed that ionizing radiation disrupted the structural integrity of BH4 and downregulated GTP cyclohydrolase I (GCH1), which is the rate-limiting enzyme in BH4 biosynthesis, resulting in deficiency in overall protein S-nitrosylation. GCH1-mediated BH4 synthesis significantly reduced radiation-induced ROS production and fueled the global protein S-nitrosylation that was disrupted by radiation. Likewise, GCH1 overexpression or the administration of exogenous BH4 protected against radiation-induced oxidative injury in vitro and in vivo. Conditional pulmonary Gch1 knockout in mice (Gch1fl/fl; Sftpa1-Cre+/- mice) aggravated lung injury following irradiation, whereas Gch1 knock-in mice (Gch1lsl/lsl; Sftpa1-Cre+/- mice) exhibited attenuated radiation-induced pulmonary toxicity. Mechanistically, lactate dehydrogenase (LDHA) mediated ROS generation downstream of the BH4/NO axis, as determined by iodoacetyl tandem mass tag (iodoTMT)-based protein quantification. Notably, S-nitrosylation of LDHA at Cys163 and Cys293 was regulated by BH4 availability and could restrict ROS generation. The loss of S-nitrosylation in LDHA after irradiation increased radiosensitivity. Overall, the results of the present study showed that GCH1-mediated BH4 biosynthesis played a key role in the ROS cascade and radiosensitivity through LDHA S-nitrosylation, identifying novel therapeutic strategies for the treatment of radiation-induced lung injury.

GTP cyclohydroxylase1 (GCH1): Role in neurodegenerative diseases

GCH1 gene provides directions for the synthesis of GTP cyclohydrolase 1 which regulates the formation of Tetrahydrobiopterin (BH4). BH4 is a crucial cofactor for essential neurotransmitters synthesis such as dopamine, serotonin and nitric oxide synthases. Deficiency of GCH1 limits the synthesis of BH4 which is responsible for neuropsychiatric diseases such as dopa-responsive dystonia, hyperalaninemia, Parkinson's disease and depression. Few single nucleotide polymorphisms of GCH1 gene are also responsible for pain in sickle cell disease. Furthermore, GCH1 regulates NO activity which controls the blood pressure, vasodilatory functions and oxidative stress. Understanding the therapeutic implications of targeting GCH1 which holds promise for treating various diseases. Novel therapeutic strategies could involve small molecule drugs or gene therapy techniques that enhance GCH1 expression or activity.

Epigenetic silencing of GCH1promotes hepatocellular carcinoma growth by activating superoxide anion-mediated ASK1/p38 signaling via inhibiting tetrahydrobiopterin de novo biosynthesis

Metabolic reprogramming is a hallmark of cancer, including hepatocellular carcinoma (HCC). However, its role in HCC remains to be elucidated. Herein, we identified GTP cyclohydrolase 1 (GCH1), the first rate-limiting enzyme in tetrahydrobiopterin (BH4) de novo biosynthesis, as a novel metabolic regulator of HCC. GCH1 was frequently down-regulated in HCC tissues and cell lines by promoter methylation. Low GCH1 expression was associated with larger tumor size, increased tumor number, and worse prognosis in two independent cohorts of HCC patients. Functionally, GCH1 silencing promoted HCC growth in vitro and in vivo, while GCH1 overexpression exerted an opposite effect. The metabolite BH4 inhibited HCC growth in vitro and in vivo. GCH1 silencing exerted its growth-promoting effect through directly inhibiting BH4 de novo biosynthesis. Mechanistically, GCH1 silencing activated ASK1/p38 signaling; pharmacological or genetic inhibition of ASK1 or p38 abolished GCH1 silencing-induced growth-promoting effect. Further mechanistic studies found that GCH1 silencing-induced BH4 reduction resulted in an increase of intracellular superoxide anion levels in a dose-dependent manner, which mediated the activation of ASK1/p38 signaling. Collectively, our study reveals that epigenetic silencing of GCH1 promotes HCC growth by activating superoxide anion-mediated ASK1/p38 signaling via inhibiting BH4 de novo biosynthesis, suggesting that targeting GCH1/BH4 pathway may be a promising therapeutic strategy to combat HCC.

Caffeine ameliorates hyperoxia-induced lung injury by protecting GCH1 function in neonatal rat pups

BackgroundBronchopulmonary dysplasia (BPD) is a major morbidity in premature infants, and impaired angiogenesis is considered a major contributor to BPD. Early caffeine treatment decreases the incidence of BPD; the mechanism remains incompletely understood.MethodsSprague-Dawley rat pups exposed to normoxia or hyperoxia since birth were treated daily with either 20 mg/kg caffeine or normal saline by an intraperitoneal injection from day 2 of life. The lungs were obtained for studies at days 10 and 21.ResultsHyperoxia impaired somatic growth and lung growth in the rat pups. The impaired lung growth during hyperoxia was associated with decreased levels of cyclic AMP (cAMP) and tetrahydrobiopterin (BH4) in the lungs. Early caffeine treatment increased cAMP levels in the lungs of hyperoxia-exposed pups. Caffeine also increased the levels of phosphorylated endothelial nitric oxide synthase (eNOS) at serine1177, total and serine51 phosphorylated GTP cyclohydrolase 1 (GCH1), and BH4 levels, with improved alveolar structure and angiogenesis in hyperoxia-exposed lungs. Reduced GCH1 levels in hyperoxia were due, in part, to increased degradation by the ubiquitin-proteasome system.ConclusionOur data support the notion that early caffeine treatment can protect immature lungs from hyperoxia-induced damage by improving eNOS activity through increased BH4 bioavailability.

Recoupling the cardiac nitric oxide synthases: tetrahydrobiopterin synthesis and recycling

Nitric oxide (NO), a key regulator of cardiovascular function, is synthesized from L-arginine and oxygen by the enzyme nitric oxide synthase (NOS). This reaction requires tetrahydrobiopterin (BH4) as a cofactor. BH4 is synthesized from guanosine triphosphate (GTP) by GTP cyclohydrolase I (GTPCH) and recycled from 7,8-dihydrobiopterin (BH2) by dihydrofolate reductase. Under conditions of low BH4 bioavailability relative to NOS or BH2, oxygen activation is "uncoupled" from L-arginine oxidation, and NOS produces superoxide (O (2) (-) ) instead of NO. NOS-derived superoxide reacts with NO to produce peroxynitrite (ONOO(-)), a highly reactive anion that rapidly oxidizes BH4 and propagates NOS uncoupling. BH4 depletion and NOS uncoupling contribute to overload-induced heart failure, hypertension, ischemia/reperfusion injury, and atrial fibrillation. L-arginine depletion, methylarginine accumulation, and S-glutathionylation of NOS also promote uncoupling. Recoupling NOS is a promising approach to treating myocardial and vascular dysfunction associated with heart failure.

Cardiomyocyte GTP cyclohydrolase 1 and tetrahydrobiopterin increase NOS1 activity and accelerate myocardial relaxation

RATIONALE

Tetrahydrobiopterin (BH4) is an essential cofactor of nitric oxide synthases (NOS). Oral BH4 supplementation preserves cardiac function in animal models of cardiac disease; however, the mechanisms underlying these findings are not completely understood.

OBJECTIVE

To study the effect of myocardial transgenic overexpression of the rate-limiting enzyme in BH4 biosynthesis, GTP cyclohydrolase 1 (GCH1), on NOS activity, myocardial function, and Ca2+ handling.

METHODS AND RESULTS

GCH1overexpression significantly increased the biopterins level in left ventricular (LV) myocytes but not in the nonmyocyte component of the LV myocardium or in plasma. The ratio between BH4 and its oxidized products was lower in mGCH1-Tg, indicating that a large proportion of the myocardial biopterin pool was oxidized; nevertheless, myocardial NOS1 activity was increased in mGCH1-Tg, and superoxide release was significantly reduced. Isolated hearts and field-stimulated LV myocytes (3 Hz, 35°C) overexpressing GCH1 showed a faster relaxation and a PKA-mediated increase in the PLB Ser16 phosphorylated fraction and in the rate of decay of the [Ca2+]i transient. RyR2 S-nitrosylation and diastolic Ca2+ leak were larger in mGCH1-Tg and ICa density was lower; nevertheless the amplitude of the [Ca2+]i transient and contraction did not differ between genotypes, because of an increase in the SR fractional release of Ca2+ in mGCH1-Tg myocytes. Xanthine oxidoreductase inhibition abolished the difference in superoxide production but did not affect myocardial function in either group. By contrast, NOS1 inhibition abolished the differences in ICa density, Ser16 PLB phosphorylation, [Ca2+]i decay, and myocardial relaxation between genotypes.

CONCLUSIONS

Myocardial GCH1 activity and intracellular BH4 are a limiting factor for constitutive NOS1 and SERCA2A activity in the healthy myocardium. Our findings suggest that GCH1 may be a valuable target for the treatment of LV diastolic dysfunction.

The protein partners of GTP cyclohydrolase I in rat organs

OBJECTIVE

GTP cyclohydrolase I (GCH1) is the rate-limiting enzyme for tetrahydrobiopterin biosynthesis and has been shown to be a promising therapeutic target in ischemic heart disease, hypertension, atherosclerosis and diabetes. The endogenous GCH1-interacting partners have not been identified. Here, we determined endogenous GCH1-interacting proteins in rat.

METHODS AND RESULTS

A pulldown and proteomics approach were used to identify GCH1 interacting proteins in rat liver, brain, heart and kidney. We demonstrated that GCH1 interacts with at least 17 proteins including GTP cyclohydrolase I feedback regulatory protein (GFRP) in rat liver by affinity purification followed by proteomics and validated six protein partners in liver, brain, heart and kidney by immunoblotting. GCH1 interacts with GFRP and very long-chain specific acyl-CoA dehydrogenase in the liver, tubulin beta-2A chain in the liver and brain, DnaJ homolog subfamily A member 1 and fatty aldehyde dehydrogenase in the liver, heart and kidney and eukaryotic translation initiation factor 3 subunit I (EIF3I) in all organs tested. Furthermore, GCH1 associates with mitochondrial proteins and GCH1 itself locates in mitochondria.

CONCLUSION

GCH1 interacts with proteins in an organ dependant manner and EIF3I might be a general regulator of GCH1. Our finding indicates GCH1 might have broader functions beyond tetrahydrobiopterin biosynthesis.

Role of autophagy in angiogenesis in aortic endothelial cells

Angiogenesis plays critical roles in the recovery phase of ischemic heart disease and peripheral vascular disease. An increase in autophagy is protective under hypoxic and chronic ischemic conditions. In the present study we determined the role of autophagy in angiogenesis. 3-Methyladenine (3-MA) and small interfering RNA (siRNA) against ATG5 were used to inhibit autophagy induced by nutrient deprivation of cultured bovine aortic endothelial cells (BAECs). Assays of BAECs tube formation and cell migration revealed that inhibition of autophagy by 3-MA or siRNA against ATG5 reduced angiogenesis. In contrast, induction of autophagy by overexpression of ATG5 increased BAECs tube formation and migration. Additionally, inhibiting autophagy impaired vascular endothelial growth factor (VEGF)-induced angiogenesis. However, inhibition of autophagy did not alter the expression of pro-angiogenesis factors such as VEGF, platelet-derived growth factor, or integrin αV. Furthermore, autophagy increased reactive oxygen species (ROS) formation and activated AKT phosphorylation. Inhibition of autophagy significantly decreased the production of ROS and activation of AKT but not of extracellular regulated kinase, whereas overexpression of ATG5 increased cellular ROS production and AKT activation in BAECs. Inhibition of AKT activation or ROS production significantly decreased the tube formation induced by ATG5 overexpression. Here we report a novel observation that autophagy plays an important role in angiogenesis in BAECs. Induction of autophagy promotes angiogenesis while inhibition of autophagy suppresses angiogenesis, including VEGF-induced angiogenesis. ROS production and AKT activation might be important mechanisms for mediating angiogenesis induced by autophagy. Our findings indicate that targeting autophagy may provide an important new tool for treating cardiovascular disease.

Biochemical characterization of the tetrahydrobiopterin synthesis pathway in the oleaginous fungus Mortierella alpina

We characterized the de novo biosynthetic pathway of tetrahydrobiopterin (BH₄) in the lipid-producing fungus Mortierella alpina. The BH₄ cofactor is essential for various cell processes, and is probably present in every cell or tissue of higher organisms. Genes encoding two copies of GTP cyclohydrolase I (GTPCH-1 and GTPCH-2) for the conversion of GTP to dihydroneopterin triphosphate (H₂-NTP), 6-pyruvoyltetrahydropterin synthase (PTPS) for the conversion of H₂-NTP to 6-pyruvoyltetrahydropterin (PPH₄), and sepiapterin reductase (SR) for the conversion of PPH₄ to BH₄, were expressed heterologously in Escherichia coli. The recombinant enzymes were produced as His-tagged fusion proteins and were purified to homogeneity to investigate their enzymic activities. Enzyme products were analysed by HPLC and electrospray ionization-MS. Kinetic parameters and other properties of GTPCH, PTPS and SR were investigated. Physiological roles of BH₄ in M. alpina are discussed, and comparative analyses between GTPCH, PTPS and SR proteins and other homologous proteins were performed. The presence of two functional GTPCH enzymes has, as far as we are aware, not been reported previously, reflecting the unique ability of this fungus to synthesize both BH₄ and folate, using the GTPCH product as a common substrate. To our knowledge, this study is the first to report the comprehensive characterization of a BH₄ biosynthesis pathway in a fungus.

The N-terminal peptide of mammalian GTP cyclohydrolase I is an autoinhibitory control element and contributes to binding the allosteric regulatory protein GFRP

GTP cyclohydrolase I (GTPCH) is the rate-limiting enzyme for biosynthesis of tetrahydrobiopterin (BH4), an obligate cofactor for NO synthases and aromatic amino acid hydroxylases. BH4 can limit its own synthesis by triggering decameric GTPCH to assemble in an inhibitory complex with two GTPCH feedback regulatory protein (GFRP) pentamers. Subsequent phenylalanine binding to the GTPCH·GFRP inhibitory complex converts it to a stimulatory complex. An N-terminal inhibitory peptide in GTPCH may also contribute to autoregulation of GTPCH activity, but mechanisms are undefined. To characterize potential regulatory actions of the N-terminal peptide in rat GTPCH, we expressed, purified, and characterized a truncation mutant, devoid of 45 N-terminal amino acids (Δ45-GTPCH) and contrasted its catalytic and GFRP binding properties to wild type GTPCH (wt-GTPCH). Contrary to prior reports, we show that GFRP binds wt-GTPCH in the absence of any small molecule effector, resulting in allosteric stimulation of GTPCH activity: a 20% increase in Vmax, 50% decrease in KmGTP, and increase in Hill coefficient to 1.6, from 1.0. These features of GFRP-stimulated wt-GTPCH activity were phenocopied by Δ45-GTPCH in the absence of bound GFRP. Addition of GFRP to Δ45-GTPCH failed to elicit complex formation or a substantial further increase in GTPCH catalytic activity. Expression of Δ45-GTPCH in HEK-293 cells elicited 3-fold greater BH4 accumulation than an equivalent of wt-GTPCH. Together, results indicate that the N-terminal peptide exerts autoinhibitory control over rat GTPCH and is required for GFRP binding on its own. Displacement of the autoinhibitory peptide provides a molecular mechanism for physiological up-regulation of GTPCH activity.

Maintenance of cellular tetrahydrobiopterin homeostasis

Tetrahydrobiopterin (BH4) is a multifunctional cofactor of aromatic amino acid hydroxylases and nitric oxide synthase (NOS) as well as an intracellular antioxidant in animals. Through regulation of NOS activity BH4 plays a pivotal role not only in a variety of normal cellular functions but also in the pathogenesis of cardiovascular and neurodegenerative diseases, which develop under oxidative stress conditions. It appears that a balanced interplay between BH4 and NOS is crucial for cellular fate. If cellular BH4 homeostasis maintained by BH4 synthesis and regeneration fails to cope with increased oxidative stress, NOS is uncoupled to generate superoxide rather than NO and, in turn, exacerbates impaired BH4 homeostasis, thereby leading to cell death. The fundamental biochemical events involved in the BH4-NOS interplay are essentially the same, as revealed in mammalian endothelial, cardiac, and neuronal cells. This review summarizes information on the cellular BH4 homeostasis in mammals, focusing on its regulation under normal and oxidative stress conditions.