Identification and characterization of basal and cyclic AMP response elements in the promoter of the rat GTP cyclohydrolase I gene

Increased Reactive Oxygen Species Generation Contributes to the Atherogenic Activity of the B2 Bradykinin Receptor

Atherosclerosis and ensuing cardiovascular disease are major causes of death with insufficient treatment options. In search for pathomechanisms of atherosclerosis, we investigated the impact of the B2 bradykinin receptor, Bdkrb2, on atherosclerotic lesion formation, because to date it is not clear whether the B2 bradykinin receptor is atheroprotective or atherogenic. As a model of atherosclerosis, we used hypercholesterolemic ApoE-deficient (apolipoprotein E-deficient) mice, which develop atherosclerotic lesions in the aorta with increasing age. The role of Bdkrb2 in atherosclerosis was studied in ApoE-deficient mice, which were either Bdkrb2-deficient, or had moderately increased aortic B2 bradykinin receptor protein levels induced by transgenic BDKRB2 expression under control of the ubiquitous CMV promoter. We found that Bdkrb2 deficiency led to a significantly decreased atherosclerotic plaque area whereas transgenic BDKRB2 expression enhanced atherosclerotic lesion formation in the aorta of ApoE-deficient mice at an age of 8 months. Concomitantly, the aortic content of reactive oxygen species (ROS) was higher in BDKRB2-expressing mice whereas Bdkrb2 deficiency decreased aortic ROS levels of ApoE-deficient mice. In addition, aortic nitrate as a marker of nitric oxide activity and the endothelial nitric oxide synthase (eNOS) co-factor, tetrahydrobiopterin (BH4) were reduced in BDKRB2-expressing ApoE-deficient mice. The decreased aortic BH4 content could be a consequence of increased ROS generation and down-regulated aortic expression of the BH4-synthesizing enzyme, Gch1 (GTP cyclohydrolase 1). In agreement with a causal involvement of decreased BH4 levels in the atherogenic function of BDKRB2, we found that treatment with the BH4 analog, sapropterin, significantly retarded atherosclerotic plaque formation in BDKRB2-expressing ApoE-deficient mice. Together our data show that the B2 bradykinin receptor is atherogenic, and the atherosclerosis-promoting function of BDKRB2 is partially caused by decreased aortic BH4 levels, which could account for eNOS uncoupling and further enhancement of ROS generation.

Promoter haplotypes of the corticotropin-releasing hormone encoding gene modulate the physiological stress response in vitro and in vivo

The corticotropin-releasing hormone (CRH) is a neuropeptide mediating stress responses. CRH exerts effects via the hypothalamus pituitary adrenal axis as well as immediate effects on the sympathetic-adrenal-medullary system. Genetic variants of the CRH promoter were previously found to be associated with altered CRH promoter activity and physiological reactions. Functional characterization of three CRH promoter haplotypes have been performed in vitro using a reporter gene assay under different stimulation conditions. Furthermore, 232 healthy subjects were genotyped and the influence of CRH haplotypes on basal parameters such as post-awakening cortisol and blood pressure as well as on stress reactivity measured after socially evaluated cold pressor test (SeCPT) was investigated. In vitro, CRH haplotype 2 showed the highest promoter activity under baseline conditions and after forskolin stimulation compared with other haplotypes. Forskolin treatment resulted in a two fold increase of haplotype 2 promoter activity compared with the baseline condition. Cell line-dependent promoter activation was found after hydrocortisone treatment. In vivo, CRH haplotype 2 carriers showed significant higher baseline blood pressure (p = .002) and blood pressure after SeCPT (p < .001), but did not differ in cortisol levels. This study provides converging evidence for the importance of CRH promoter variants on physiological stress response parameters.

Shear stress-mediated upregulation of GTP cyclohydrolase/tetrahydrobiopterin pathway ameliorates hypertension-related decline in reendothelialization capacity of endothelial progenitor cells

OBJECTIVES

Guanosine triphosphate cyclohydrolase/tetrahydrobiopterin (GTPCH)/(BH4) pathway has been proved to regulate the function of endothelial progenitor cells (EPCs) in deoxycorticosterone acetate-salt hypertensive mice, indicating that GTPCH/BH4 pathway may be an important repair target for hypertension-related endothelial injury. Shear stress is an important nonpharmacologic strategy to modulate the function of EPCs. Here, we investigated the effects of laminar shear stress on the GTPCH/BH4 pathway and endothelial repair capacity of circulating EPCs in hypertension.

METHOD

Laminar shear stress was loaded on the human EPCs from hypertensive patients and normotensive patients. The in-vitro function, in-vivo reendothelialization capacity and GTPCH/BH4 pathway of human EPCs were evaluated.

RESULTS

Both in-vitro function and reendothelialization capacity of EPCs were lower in hypertensive patients than that in normotensive patients. The GTPCH/BH4 pathway of EPCs was downregulated in hypertensive patients. Shear stress increased in-vitro function and reendothelialization capacity of EPCs from hypertensive patients and normotensive patients. Furthermore, shear stress upregulated the expression of GTPCH I and levels of BH4, nitric oxide, and cGMP of EPCs, and reduced thrombospondin-1 expression. With treatment of GTPCH knockdown or nitroarginine methyl ester inhibition, shear stress-induced increased levels of BH4, nitric oxide and cGMP of EPCs was suppressed. When GTPCH/BH4 pathway of EPCs was blocked, the effects of shear stress on in-vitro function and reendothelialization capacity of EPCs were inhibited.

CONCLUSION

The study demonstrates for the first time that shear stress-induced upregulation of the GTPCH/BH4 pathway ameliorates hypertension-related decline in endothelial repair capacity of EPCs. These findings provide novel nonpharmacologic therapeutic approach for hypertension-related endothelial repair.

Complexity of dopamine metabolism

: Parkinson's disease (PD) coincides with a dramatic loss of dopaminergic neurons within the substantia nigra. A key player in the loss of dopaminergic neurons is oxidative stress. Dopamine (DA) metabolism itself is strongly linked to oxidative stress as its degradation generates reactive oxygen species (ROS) and DA oxidation can lead to endogenous neurotoxins whereas some DA derivatives show antioxidative effects. Therefore, DA metabolism is of special importance for neuronal redox-homeostasis and viability.In this review we highlight different aspects of dopamine metabolism in the context of PD and neurodegeneration. Since most reviews focus only on single aspects of the DA system, we will give a broader overview by looking at DA biosynthesis, sequestration, degradation and oxidation chemistry at the metabolic level, as well as at the transcriptional, translational and posttranslational regulation of all enzymes involved. This is followed by a short overview of cellular models currently used in PD research. Finally, we will address the topic from a medical point of view which directly aims to encounter PD.

The neurobiology of tetrahydrobiopterin biosynthesis: a model for regulation of GTP cyclohydrolase I gene transcription within nigrostriatal dopamine neurons

Within the brain, the reduced pteridine cofactor 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) is absolutely required for the synthesis of the monoamine (MA) neurotransmitters dopamine (DA), norepinephrine, epinephrine (E), and serotonin (5-HT), the novel gaseous neurotransmitter nitric oxide and the production of yet to be identified 1-O-alkylglycerol-derived lipids. GTP cyclohydrolase I (GTPCH) catalyzes the first and limiting step in the BH4 biosynthetic pathway, which is now thought to involve up to eight different proteins supporting six alternate de novo and two alternate salvage pathways. Gene expression analysis across different regions of the human brain shows the abundance of transcripts coding for all eight of these proteins to be highly correlated with each other and to be enriched within human MA neurons. The potential for multiple routes for BH4 synthesis therefore exists within the human brain. GTPCH expression is particularly heterogeneous across different populations of human and rodent MA-containing neurons, with low expression levels and therefore BH4 being a characteristic of nigrostriatal DA (NSDA) neurons. Basic knowledge of how GCH1 gene transcription is controlled within NSDA neurons may explain the distinctive susceptibility of these neurons to human genetic mutations that result in BH4 deficiency. A model for cyclic adenosine monophosphate-dependent GCH1 transcription is described that involves a unique combination of DNA regulatory sequences and transcription factors. This model proposes that low levels of GCH1 transcription within NSDA neurons are driven by their distinctive physiology, suggesting that pharmacological manipulation of GCH1 gene transcription can be used to modify BH4 levels and therefore DA synthesis in the basal ganglia.

Endothelial nitric oxide synthase impairment is restored by clofibrate treatment in an animal model of hypertension

Adequate production of nitric oxide (NO) by endothelial nitric oxide synthase (eNOS) requires eNOS coupling promoted by tetrahydrobiopterin (BH(4)). Under pathological conditions such as hypertension, BH(4) is diminished, avoiding eNOS coupling. When eNOS is "uncoupled", it yields a superoxide anion instead of NO. Peroxisome proliferator activated receptors (NR1C) are a family of nuclear receptors activated by ligand. Clofibrate, a member of a hypolipidemic class of drugs, acts by activating the alpha isoform of NR1C. To determine the participation of NR1C1 activation in BH(4) and dihydrobiopterin (BH(2)) metabolism and its implications on eNOS coupling in hypertension, we performed aortic coarctation (AoCo) at inter-renal level on male Wistar rats in order to have a hypertensive model. Rats were divided into the following groups: Sham+vehicle (Sham-V); AoCo+vehicle (AoCo-V); Sham+clofibrate (Sham-C), and AoCo+clofibrate (AoCo-C). Clofibrate (7 days) increased eNOS coupling in the AoCo-C group compared with AoCo-V. Clofibrate also recovered the BH(4):BH(2) ratio in control values and prevented the rise in superoxide anion production, lipoperoxidation, and reactive oxygen species production. In addition, clofibrate increased GTP cyclohydrolase-1 (GTPCH-1) protein expression, which is related with BH(4) recovered production. NR1C1 stimulation re-establishes eNOS coupling, apparently through recovering the BH(4):BH(2) equilibrium and diminishing oxidative stress. Both can contribute to high blood pressure attenuation in hypertension secondary to AoCo.

NF-Y is essential for expression of the proapoptotic bim gene in sympathetic neurons

Neuronal apoptosis has a major role during development and aberrant apoptosis contributes to the pathology of certain neurological conditions. Studies with nerve growth factor (NGF)-dependent sympathetic neurons have provided important insights into the molecular mechanisms of neuronal apoptosis and the signalling pathways that regulate the cell death programme in neurons. The BH3-only protein Bim is a critical mediator of apoptosis in many cell types and in sympathetic neurons is required for NGF withdrawal-induced death. However, regulation of bim expression is complex and remains incompletely understood. We report that a conserved inverted CCAAT box (ICB) in the rat bim promoter is bound by the heterotrimeric transcription factor NF-Y. Interestingly, NF-Y is required for bim promoter activity and its induction following NGF withdrawal. We demonstrate that NF-Y activity is essential for endogenous Bim expression and contributes to NGF withdrawal-induced death. Furthermore, we find that the transcriptional coactivators CBP and p300 interact with NF-Y and FOXO3a and bind to this region of the bim promoter. The amount of CBP/p300 bound to bim increases after NGF deprivation and inhibition of CBP/p300 activity reduces bim induction. Our results indicate that NF-Y cooperates with FOXO3a to recruit CBP/p300 to the bim promoter to form a stable multi-protein/DNA complex that activates bim transcription after survival factor withdrawal.

GCH1 in early-onset Parkinson's disease

Mutations in GTP-cyclohydrolase 1 (GCH1) cause autosomal dominant dopa-responsive dystonia (DRD), characterized by childhood-onset foot dystonia that later generalizes. DRD patients frequently present with associated Parkinsonism. Conversely, early-onset Parkinson's disease (EOPD) patients commonly display dystonia. Herein, we investigated the frequency of GCH1 mutations in a series of 53 familial EOPD patients (21 with dystonia) and screened them for mutations in PRKN, PINK1, and DJ-1. In addition, we examined a matched EOPD patient-control series for association of common variability at the GCH1 locus and EOPD susceptibility. No GCH1 coding change or copy-number abnormality was identified in familial EOPD patients. A novel 18-bp deletion was found in the proximal promoter (two patients, one control), which is expected to knock out two regulatory elements previously shown to regulate GCH1 transcription. No association was found between GCH1 variability and risk of EOPD. Fourteen (26.4%) familial EOPD patients had homozygous or compound heterozygous PRKN mutations. PRKN-positive patients were 10 years younger than PRKN-negative patients and had a twofold higher prevalence of dystonia. This study does not support a significant role for genetic variation at the GCH1 locus in EOPD. However, our results further highlight the relevance of PRKN screening in familial EOPD.

Estradiol increases guanosine 5'-triphosphate cyclohydrolase expression via the nitric oxide-mediated activation of cyclic adenosine 5'-monophosphate response element binding protein

A number of studies have demonstrated that estradiol can stimulate endothelial nitric oxide synthase expression and activity, resulting in enhanced nitric oxide (NO) generation. However, its effect on the NO synthase cofactor, tetrahydrobiopterin are less clear. Cellular tetrahydrobiopterin levels are regulated, at least in part, by GTP cyclohydrolase 1 (GCH1). Thus, the purpose of this study was to determine the effect of estradiol on GCH1 expression and the regulatory mechanisms in pulmonary arterial endothelial cells. Our data indicate that 17beta-estradiol (E2) increases GCH1 transcription in a dose- and time-dependent manner, whereas estrogen receptor antagonism or NO synthase inhibition attenuated E2-stimulated GCH1 expression. Analysis of the GCH1 promoter fragment responsive to E2 revealed the presence of a cAMP response element, and we found that E2 triggers a rapid but transient elevation of phospho-cAMP response element-binding protein (CREB; <1 h) followed by a second sustained rise after 6 h. EMSA analysis revealed an increase in the binding of CREB during E2 treatment and mutation of the cAMP response element in the GCH1 promoter attenuated the E2-mediated increase in transcription. Furthermore, inhibition of the cAMP-dependent kinase, protein kinase A (PKA) completely abolished the E2-stimulated GCH1 promoter activity, whereas the stimulation of cAMP levels with forskolin increased GCH1 promoter activity, indicating the key role of cAMP in regulating GCH1 promoter activity. In conclusion, our results demonstrate that estradiol can modulate GCH1 expression via NO-mediated activation of CREB in pulmonary arterial endothelial cells. These findings provide new insight into the vascular protective effect of estradiol.

GTP cyclohydrolase I expression is regulated by nitric oxide: role of cyclic AMP

Our previous studies have demonstrated that nitric oxide (NO) leads to nitric oxide synthase (NOS) uncoupling and an increase in NOS-derived superoxide. However, the cause of this uncoupling has not been adequately resolved. The pteridine cofactor tetrahydrobiopterin (BH(4)) is a critical determinant of endothelial NOS (eNOS) activity and coupling, and GTP cyclohydrolase I (GCH1) is the rate-limiting enzyme in its generation. Thus the initial purpose of this study was to determine whether decreases in BH(4) could underlie, at least in part, the NO-mediated uncoupling of eNOS we have observed both in vitro and in vivo. Initially we evaluated the effect of inhaled NO levels on GCH1 expression and BH(4) levels in the intact lamb. Contrary to our hypothesis, we found that there was a significant increase in both plasma BH4 levels and peripheral lung GCH1 protein levels. Furthermore, in vitro, we found that exposure to the NO donor spermine NONOate (SPNONO) led to an increase in GCH1 protein and BH(4) levels in both COS-7 and pulmonary arterial endothelial cells. However, SPNONO treatment also caused a significant increase in phospho-cAMP response element binding protein (CREB) levels, as detected by Western blot analysis, and significantly increased cAMP levels, as detected by enzyme immunoassay. Furthermore, utilizing GCH1 promoter fragments fused to a luciferase reporter gene, we found that GCH1 promoter activity was enhanced by SPNONO in a CREB-dependent manner, and electromobility shift assays revealed an NO-dependent increase in the nuclear binding of CREB. These data suggest that NO increases BH(4) levels through a cAMP/CREB-mediated increase in GCH1 transcription and that the eNOS uncoupling associated with exogenous NO does not involved reduced BH(4) levels.

Identification of neuronal target genes for CCAAT/enhancer binding proteins

CCAAT/Enhancer Binding Proteins (C/EBPs) play pivotal roles in the development and plasticity of the nervous system. Identification of the physiological targets of C/EBPs (C/EBP target genes) should therefore provide insight into the underlying biology of these processes. We used unbiased genome-wide mapping to identify 115 C/EBPbeta target genes in PC12 cells that include transcription factors, neurotransmitter receptors, ion channels, protein kinases and synaptic vesicle proteins. C/EBPbeta binding sites were located primarily within introns, suggesting novel regulatory functions, and were associated with binding sites for other developmentally important transcription factors. Experiments using dominant negatives showed C/EBPbeta to repress transcription of a subset of target genes. Target genes in rat brain were subsequently found to preferentially bind C/EBPalpha, beta and delta. Analysis of the hippocampal transcriptome of C/EBPbeta knockout mice revealed dysregulation of a high percentage of transcripts identified as C/EBP target genes. These results support the hypothesis that C/EBPs play non-redundant roles in the brain.

Cilostazol activates AMP-activated protein kinase and restores endothelial function in diabetes

BACKGROUND

Endothelial dysfunction plays a key role in atherogenesis. We investigated whether AMP-activated protein kinase (AMPK) activity is a downstream mediator of the beneficial effects of cilostazol on vascular endothelial cells and whether cilostazol might reverse endothelial dysfunction in diabetic rats.

METHODS AND RESULTS

Treatment of human umbilical vein endothelial cells (HUVECs) with cilostazol resulted in time-dependent activation of AMPK, as monitored by phosphorylation of AMPK and its down-stream target, acetyl-CoA carboxylase (ACC). Activation of AMPK by cilostazol was through signaling pathway independent of cyclic AMP and caused phosphorylation of endothelial nitric oxide synthase (eNOS), leading to increased production of nitric oxide (NO), while inhibiting cytokine-induced nuclear factor-kappaB (NF-kappaB) activation, leading to suppression of VCAM-1 gene expression. Significantly reduced eNOS activity and NO production in response to cilostazol and attenuation of cilostazol-induced inhibition of NF-kappaB activation were observed in cells treated with AMPK siRNA. We also demonstrated that administration of cilostazol to diabetic rats significantly restored endothelium-dependent vasodilation. Furthermore, treatment of diabetic rats with cilostazol increased tetrahydrobiopterin (BH4) levels in the aorta. Thus, recovery of BH4 following administration of cilostazol might also contribute to restoration of endothelial function in diabetic rats.

CONCLUSIONS

Our findings suggest that the beneficial effects of cilostazol on endothelial function may be due to AMPK activation. Restoration of endothelial dysfunction in diabetic rats by cilostazol is at least partly attributed to amelioration of biopterin metabolism in the aorta.

Specificity proteins Sp1 and Sp3 interact with the rat GTP cyclohydrolase I proximal promoter to regulate transcription

The role of the proximal promoter GC-box in regulating basal and cAMP-dependent GTP Cyclohydrolase I gene transcription was investigated using a variety of cell lines and techniques. These studies show that the GC-box is composed of a triad of cis-elements that in vitro bind specificity proteins Sp1 and Sp3. Sp1 and Sp3 were found associated with the native proximal promoter in PC12 cells but were not recruited to the promoter during cAMP-dependent transcription. Studies using Drosophila SL2 cells showed that Sp3 occupies two sites within the GC-box and enhances transcription when acting alone and synergistically when combined with nuclear factor-Y (NF-Y) and CCAAT/Enhancer-Binding Protein (C/EBP)beta, cognate binding proteins for the adjacent cAMP response element (CRE) and CCAAT-box cAMP response elements. In contrast, Sp1 bound only one site within the GC-box and did not enhance transcription unless combined with NF-Y and C/EBPbeta. Studies in SL2 cells also showed that Sp1 and Sp3 do not co-occupy the GC-box, and accordingly Sp1 competes for Sp3 binding to repress Sp3-dependent transcription. In PC12 cells, complete mutation of the GC-box reduced basal but not cAMP-dependent transcription, resulting in an overall increase in the cAMP response and demonstrating that formation of this enhanceosome does not require Sp1 or Sp3. Experiments in which the GC-box was replaced with a Gal4 element and the promoter challenged with Gal4 fusion proteins support this conclusion and a role for Sp3 in maintaining high levels of basal transcription in PC12 cells. Equivalent amounts of Sp1 and Sp3 were found associated with the native proximal promoter in PC12 and Rat2 cells, which differ 10-fold in basal transcription. Similar levels of methylation of CpG dinucleotides located within the GC-box were also observed in these two cells lines. These results suggest that Sp1 and Sp3 bound to the GC-box might help to preserve an open chromatin configuration at the proximal promoter in cells which constitutively express low levels of GTP Cyclohydrolase I.

Regulation of GTP cyclohydrolase I expression by orphan receptor Nurr1 in cell culture and in vivo

Nurr1 is an orphan nuclear transcription factor essential for the terminal differentiation of dopamine (DA) neurons in the ventral midbrain (VM). To identify the Nurr1-target genes, we carried out microarray and quantitative real-time PCR analyses of Nurr1 null and wild-type mice in VM at embryonic day (E) 12.5 and shortly after birth (P0). In addition to the absence of mRNAs of DA synthesizing enzymes, the guanosine 5'-triphosphate (GTP) cyclohydrolase I (GTPCH) was also substantially reduced in the VM of Nurr1-null mice. GTPCH is the first enzyme in the synthesis pathway of tetrahydrobiopterin (BH4), an essential cofactor for tyrosine hydroxylase in DA synthesis. In the mouse, Nurr1 and GTPCH mRNA were first detected at E10.5, and GTPCH transcription paralleled that of Nurr1. Small interfering RNA targeted against Nurr1 decreases GTPCH expression in MC3T3-E1 osteoblasts in cell culture. Cotransfection of Nurr1 and the GTPCH-luciferase (luc) reporter increased the luc activity by about threefold in N2A cells. Additional analysis using 5'-deletions and mutants revealed that Nurr1 activates GTPCH transcription indirectly through the proximal promoter region, in the absence of the nerve growth factor-induced clone B (NGFI-B) responsive element-like sites, similarly, as recently reported for DA transporter regulation by Nurr1.

Protein kinase A-dependent recruitment of RNA polymerase II, C/EBP beta and NF-Y to the rat GTP cyclohydrolase I proximal promoter occurs without alterations in histone acetylation

Cyclic-AMP stimulation of GTP cyclohydrolase I (GCH1) gene transcription was investigated in PC12 cells, the protein kinase A-deficient PC12 cell line 126-1B2 and C6 cells using transient transfection assays of proximal promoter reporter constructs and wild type or dominant negative proteins, chromatin immunoprecipitation and real-time quantitative PCR. These studies show that protein kinase A is necessary and sufficient for cAMP-dependent transcription conferred by both the cAMP regulatory element and the adjacent CCAAT-box. In intact cells these cis-elements were shown to bind cAMP response element binding protein, CCAAT-enhancer binding protein beta and nuclear factor-Y, with each protein controlling a different aspect of the cAMP response. Cyclic-AMP acting through protein kinase A stimulated promoter recruitment of CCAAT-enhancer binding protein beta, nuclear factor-Y and RNA polymerase II while depleting the promoter of cyclic-AMP response element binding protein. Stimulation of transcription by cAMP was not associated with increased acetylation of histones H3 and H4 at proximal promoter nucleosomes, indicating that histone acetyltransferases are not involved in this response. Nonetheless, pharmacological inhibition of histone deacetylase activity did increase histone H4 acetylation and the recruitment of RNA polymerase II, indicating that histone acetyltransferases are normally associated with the proximal promoter. Only in C6 cells, however, did inhibition of histone deacetylases stimulate transcription and synergize with cAMP. These experiments provide the first glimpse of the GCH1 gene promoter functioning within intact cells and supply evidence for the involvement of histone acetyltransferase-containing complexes in GCH1 gene transcription.

Multiple genes and factors associated with bipolar disorder converge on growth factor and stress activated kinase pathways controlling translation initiation: implications for oligodendrocyte viability

Famine and viral infection, as well as interferon therapy have been reported to increase the risk of developing bipolar disorder. In addition, almost 100 polymorphic genes have been associated with this disease. Several form most of the components of a phosphatidyl-inositol signalling/AKT1 survival pathway (PIK3C3, PIP5K2A, PLCG1, SYNJ1, IMPA2, AKT1, GSK3B, TCF4) which is activated by growth factors (BDNF, NRG1) and also by NMDA receptors (GRIN1, GRIN2A, GRIN2B). Various other protein products of genes associated with bipolar disorder either bind to or are affected by phosphatidyl-inositol phosphate products of this pathway (ADBRK2, HIP1R, KCNQ2, RGS4, WFS1), are associated with its constituent elements (BCR, DUSP6, FAT, GNAZ) or are downstream targets of this signalling cascade (DPYSL2, DRD3, GAD1, G6PD, GCH1, KCNQ2, NOS3, SLC6A3, SLC6A4, SST, TH, TIMELESS). A further pathway relates to endoplasmic reticulum-stress (HSPA5, XBP1), caused by problems in protein glycosylation (ALG9), growth factor receptor sorting (PIK3C3, HIP1R, SYBL1), or aberrant calcium homoeostasis (WFS1). Key processes relating to these pathways appear to be under circadian control (ARNTL, CLOCK, PER3, TIMELESS). DISC1 can also be linked to many of these pathways. The growth factor pathway promotes protein synthesis, while the endoplasmic reticulum stress pathway, and other stress pathways activated by viruses and cytokines (IL1B, TNF, Interferons), oxidative stress or starvation, all factors associated with bipolar disorder risk, shuts down protein synthesis via control of the EIF2 alpha and beta translation initiation complex. For unknown reasons, oligodendrocytes appear to be particularly prone to defects in the translation initiation complex (EIF2B) and the convergence of these environmental and genomic signalling pathways on this area might well explain their vulnerability in bipolar disorder.

GTP cyclohydrolase and tetrahydrobiopterin regulate pain sensitivity and persistence

We report that GTP cyclohydrolase (GCH1), the rate-limiting enzyme for tetrahydrobiopterin (BH4) synthesis, is a key modulator of peripheral neuropathic and inflammatory pain. BH4 is an essential cofactor for catecholamine, serotonin and nitric oxide production. After axonal injury, concentrations of BH4 rose in primary sensory neurons, owing to upregulation of GCH1. After peripheral inflammation, BH4 also increased in dorsal root ganglia (DRGs), owing to enhanced GCH1 enzyme activity. Inhibiting this de novo BH4 synthesis in rats attenuated neuropathic and inflammatory pain and prevented nerve injury-evoked excess nitric oxide production in the DRG, whereas administering BH4 intrathecally exacerbated pain. In humans, a haplotype of the GCH1 gene (population frequency 15.4%) was significantly associated with less pain following diskectomy for persistent radicular low back pain. Healthy individuals homozygous for this haplotype exhibited reduced experimental pain sensitivity, and forskolin-stimulated immortalized leukocytes from haplotype carriers upregulated GCH1 less than did controls. BH4 is therefore an intrinsic regulator of pain sensitivity and chronicity, and the GTP cyclohydrolase haplotype is a marker for these traits.

Sequence variants of the CRH 5'-flanking region: effects on DNA-protein interactions studied by EMSA in PC12 cells

Recently, studies in adult rheumatoid arthritis patients have shown an association with four single-nucleotide polymorphisms (SNPs) in the 3.7-kb regulatory region of human corticotropin-releasing hormone (hCRH) gene located at positions -3531, -3371, -2353, and -684 bp. Three of these novel polymorphisms are in absolute linkage disequilibrium, resulting in three combined alleles, named A1B1, A2B1, and A2B2. To study whether the described polymorphic nucleotide sequences in the 5' region of the hCRH gene interfere with binding of nuclear proteins, an electric mobility shift assay (EMSA) was performed. At position -2353 bp, a specific DNA protein complex was detected for the wild-type sequence only, possibly interfering with a binding site for the activating transcription factor 6 (ATF6). In contrast, no difference could be detected for the other SNPs. However, at position -684, a quantitative difference in protein binding due to cAMP incubation could be observed. To further investigate whether these SNPs in the CRH promoter are associated with an altered regulation of the CRH gene, we performed a luciferase reporter gene assay with transiently transfected rat pheochromocytoma cells PC12. Incubation with 8-Br-cAMP alone or in combination with cytokines enhanced significantly the promoter activity in PC12 cells. The promoter haplotypes studied exhibited a differential capacity to modulate CRH gene expression. In all our experiments, haplotype A1B1 showed the most pronounced influence on promoter activity. Taken together, our results demonstrate a differential binding capacity of nuclear proteins of the promoter polymorphisms resulting in a different gene regulation. Most probably the SNP at position -2,353 plays a major role in mediating these differences.

Estrogen-triggered activation of GTP cyclohydrolase 1 gene expression: role of estrogen receptor subtypes and interaction with cyclic AMP

Guanosinetriphosphate cyclohydrolase I (GTPCH) catalyzes the initial step in the de novo biosynthesis of (6R)-5,6,7,8-tetrahydrobiopterin, an important determinant of the rate of catecholamine and nitric oxide biosynthesis. Administration of estrogen in vivo was found to elevate GTPCH mRNA levels in several catecholaminergic locations. To examine the mechanism, PC12 cells were co-transfected with a reporter construct containing 2988 bp of rat GTPCH promoter fused to luciferase gene, and expression vectors for estrogen receptors. Addition of 2.5-20 nM of 17 beta-estradiol increased GTPCH promoter-driven luciferase activity in the presence of either estrogen receptor alpha or estrogen receptor beta indicating, for the first time, that 17 beta-estradiol can regulate GTPCH gene expression via transcriptional mechanisms. However, there were differences in dose dependence and time course with estrogen receptor alpha or estrogen receptor beta. With estrogen receptor alpha, the effect was greater with lower doses of 17 beta-estradiol. At the same dose, the response with estrogen receptor beta was observed somewhat earlier than with estrogen receptor alpha and with 20 nM 17 beta-estradiol was effective even after 6 h. These responses to 17 beta-estradiol required estrogen receptors and specific agonists for estrogen receptor alpha and estrogen receptor beta, 4,4,4,-(4-propil-[1H-pyrazole-1,3,5-triyl)tris-phenol and 2,3-bis[4-hydroxyphenyl]propionitrile respectively, triggered increased GTPCH promoter activity. In addition, neither estradiol, nor the selective agonists activated GTPCH promoter without transfection of appropriate estrogen receptor expression vectors. Addition of 17 beta-estradiol, or the selective agonists, also elevated endogenous GTPCH mRNA levels. The results demonstrate that estrogen can have a direct effect on GTPCH gene expression. Although estradiol increased GTPCH promoter activity in the presence of estrogen receptors, it attenuated the response of the promoter and endogenous gene to cyclic AMP, suggesting the crosstalk between estrogen and cyclic AMP pathways in the regulation of GTPCH gene expression. These findings reveal the significance of estrogen in modulating regulation of rate limiting enzyme in the (6R)-5,6,7,8-tetrahydrobiopterin biosynthesis, which may have implications for sex-related differences in vulnerability in related disorders.

Regulation of the preprotachykinin-I gene promoter through a protein kinase A-dependent, cyclic AMP response element-binding protein-independent mechanism

Preprotachykinin-I (PPT) gene expression is regulated by a number of stimuli that signal through cyclic AMP (cAMP)-mediated pathways. In the present study, forskolin, an adenylyl cyclase stimulator, significantly increased PPT mRNA levels in PPT-expressing RINm5F cells, an effect paralleled by an increase in PPT promoter-luciferase reporter construct activity. The forskolin-induced stimulation of PPT transcription was protein kinase A dependent (PKA), as shown by blockade with the PKA inhibitor N-[2-(p-bromocinnamylamino) ethyl]-5-isoquinolinesulfonamide. We found that the activation protein 1/cAMP response element (AP1/CRE) site centered at -196 relative to the transcription start site was important for basal and forskolin-induced PPT promoter activity. Because of the involvement of PKA and the similarity of the AP1/CRE element to consensus CRE sequences, we investigated the role of CRE-binding protein (CREB) in the regulation of the PPT promoter. Surprisingly, overexpression of a dominant-negative CREB (i.e. CREB-A) did not affect basal or forskolin-induced PPT promoter activity. Furthermore, binding of CREB to the PPT promoter AP1/CRE site was not demonstrable in electrophoretic mobility shift assays. Rather, our experiments suggested that c-Jun is a member of the complex that binds to this site. We conclude that, at least in RINm5F cells, cAMP-mediated up-regulation of PPT gene expression does not involve CREB or CREB-related transcription factor recruitment to the AP1/CRE site.

Increased blood flow causes coordinated upregulation of arterial eNOS and biosynthesis of tetrahydrobiopterin

Shear stress, imposed on the vascular endothelium by circulating blood, critically sustains vascular synthesis of nitric oxide (NO). Endothelial NO synthase (eNOS) activity is determined by heat shock protein 90 (HSP90), caveolin-1, and the cofactor tetrahydrobiopterin (BH4). To determine whether increased blood flow concomitantly upregulates eNOS and GTP cyclohydrolase I (GTPCH I, the rate-limiting enzyme in BH4 biosynthesis), an aortocaval fistula model in the rat was employed wherein aortic blood flow is enhanced proximal but decreased distal to the fistula. Eight weeks after the creation of the aortocaval fistula, the proximal and distal aortic segments were harvested; sham-operated rats served as controls. Vasomotor function was assessed by isometric force recording. Expression of eNOS, HSP90, caveolin-1, Akt, phosphorylated eNOS (eNOS-Ser1177), and GTPCH I were determined by Western blot analysis. Biosynthesis of BH4 and GTPCH-I activity was examined by HPLC. In the aortic segments exposed to increased flow, contractions to KCl and phenylephrine were reduced, whereas endothelium-dependent relaxations were not affected compared with sham-operated or aortic segments with reduced blood flow. Expression of eNOS, caveolin-1, phosphorylated Akt, and eNOS-Ser1177 was enhanced in aortas exposed to increased blood flow. High flow augmented levels of cGMP and BH4 and increased expression of GTPCH I. In aggregate, these findings provide the first demonstration in vivo that coordinated vascular upregulation of eNOS, and GTPCH I accompanies increased blood flow. This induction of GTPCH I increases BH4 production, thereby optimizing the generation of NO by eNOS and thus the adaptive, vasorelaxant response required in sustaining increased blood flow.

Nerve growth factor-induced expression of the GTP cyclohydrolase I gene via Ras/MEK pathway in PC12D cells

Neurotrophins are essential for the development and survival of the catecholaminergic neurons. GTP cyclohydrolase I (GCH) is the first and rate-limiting enzyme in the biosynthesis of 5,6,7,8-tertahydrobiopterin (BH4), the required cofactor for tyrosine hydroxylase. Previously, we reported that TH requires the Ras/mitogen-activated protein kinase kinase (MEK) pathway for its induction by nerve growth factor (NGF). Here, we examined intracellular signals required for NGF-induced expression of the GCH gene in PC12D cells. The activity of GCH was increased up to 5-fold after the NGF treatment, and the increase was repressed by pretreatment with U0126, an MEK1/2 inhibitor, but not with protein kinase A (PKA), phosphoinositide 3-kinase (PI3K), p38 mitogen-activated protein kinase (MAPK), and c-Jun NH2-terminal kinase (JNK) inhibitors. Induction of GCH mRNA by NGF was also abolished by pretreatment with U0126. The human GCH promoter activity was significantly enhanced by NGF treatment. Deletion analysis showed that the 465-bp 5'-flanking region is responsible for NGF-enhanced promoter activity. These data suggest that the Ras-MEK pathway is required for coordinate expression of the GCH and TH genes induced by neurotrophins.

Sp1 and Sp3 activate transcription of the human dopamine transporter gene

The dopamine transporter is a plasma membrane protein that controls extracellular concentrations of the neurotransmitter dopamine. The physiological importance of the DAT provides the impetus for studies aimed at understanding the molecular mechanisms underlying regulation of the DAT gene. In this study, we identified a DAT-expressing neuroblastoma cell line (SK-N-AS) and employed it to investigate the transcriptional regulation of the human DAT gene. Two GC boxes (located at -130 and -60, respectively, relative to the transcriptional start site) were identified as important cis-acting elements mediating DAT promoter activity in dopaminergic SK-N-AS cells. Utilizing Sp-deficient Drosophila Schneider line (SL-2) cells, we showed that both Sp1 and Sp3 are strong activators of DAT transcriptional activity. Differential binding of Sp1 and Sp3 to the two GC boxes was demonstrated by electrophoretic mobility shift assays and super-shift assays. Our results indicate that the Sp1 family of proteins plays an important role in controlling the expression of the dopamine transporter gene within dopaminergic neurons.

Role of basic region leucine zipper transcription factors cyclic AMP response element binding protein (CREB), CREB2, activating transcription factor 2 and CAAT/enhancer binding protein alpha in cyclic AMP response element-mediated transcription

The transcription factor cAMP response element binding protein (CREB), a member of the basic region leucine zipper (bZIP) family of proteins, is the major cAMP response element (CRE) binding. Other bZIP proteins, including CREB2, activating transcription factor 2 (ATF2), or CAAT/enhancer binding protein (C/EBP) have been reported to transactivate CRE-containing genes or to interfere with transactivation by CREB. We have designed a simple transactivation assay using expression of either a constitutively active CREB mutant or a nuclear targeted mutant of the catalytic subunit of cAMP-dependent protein kinase. In both cases, a striking stimulation of transcription of CRE-containing reporter genes was observed in noradrenergic locus coeruleus-like CATH.a cells. In addition, a constitutively active mutant of ATF2 specifically transactivated a secretogranin II promoter/luciferase reporter gene, but had no effect on the tyrosine hydroxylase promoter. In contrast, CREB2 and C/EBPalpha did not transactivate CRE-containing reporter genes, indicating that these bZIP proteins target distinct genetic elements. Experiments involving dominant-negative bZIP mutants revealed that CREB does not heterodimerize with CREB2, ATF2, c-Jun or C/EBP. Rather, CREB and ATF2 compete for binding to the CRE, and are independently able to up-regulate transcription of genes containing CRE motifs in their regulatory regions.

Regulation of GTP cyclohydrolase I gene transcription by basic region leucine zipper transcription factors

Tetrahydrobiopterin is an essential cofactor for the phenylalanine, tyrosine and tryptophan hydroxylases, and the family of nitric oxide synthases. The initial and rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin is GTP cyclohydrolase I. The proximal promoter of the human GTP cyclohydrolase I gene contains the sequence motif 5'-TGACGCGA-3', resembling a cAMP response element (CRE). The objective of this study was to analyze the regulation of GTP cyclohydrolase I gene transcription by basic region leucine zipper (bZIP) transcription factors. A constitutively active mutant of the cAMP response element binding (CREB) protein strongly stimulated GTP cyclohydrolase I promoter activity, indicating that the CRE in the context of the GTP cyclohydrolase I gene is functional. Likewise, GTP cyclohydrolase I promoter/luciferase gene transcription was stimulated following nuclear expression of the catalytic subunit of cAMP-dependent protein kinase. Constitutively active mutants of activating transcription factor 2 (ATF2) and c-Jun additionally stimulated GTP cyclohydrolase I promoter activity, but to a lesser extent than the constitutively active CREB mutant. The fact that stress-activated protein kinases target the GTP cyclohydrolase I gene was corroborated by expression experiments involving p38 and MEKK1 protein kinases. We conclude that signaling pathways involving either the cAMP-dependent protein kinase or stress-activated protein kinases converge to the GTP cyclohydrolase I gene. Hence, enzymatic reactions that require tetrahydrobiopterin as cofactor are therefore indirectly controlled by signaling cascades involving the signal-responsive transcription factors CREB, c-Jun, and ATF2.

AVP-induced VIT32 gene expression in collecting duct cells occurs via trans-activation of a CRE in the 5'-flanking region of the VIT32 gene

VIT32, a vasopressin-induced transcript, inhibits Na(+) transport when coexpressed with the epithelial sodium channel in Xenopus laevis oocytes (EMBO J 21: 5109-5117, 2002). To understand the mechanism of VIT32 gene regulation, we examined the effect of DDAVP and cAMP stimulation on VIT32 expression in M-1 mouse collecting duct cells and in H441 human airway epithelial cells. Elevation of cAMP with forskolin and IBMX increased VIT32 gene expression with a peak effect at 2 h. The increase in gene expression was abolished by H89 and by actinomycin D, suggesting that cAMP stimulates VIT32 mRNA expression by a PKA-mediated increase in gene transcription. An approximately 1.5-kb fragment of the 5'-flanking region of VIT32 was cloned and was able to confer cAMP-stimulated reporter gene activity when transfected into M-1 and H441 cells. By deletion analysis and site-directed mutagenesis, a cAMP response element (CRE) was identified within the proximal promoter region that was sufficient to account for the increase in VIT32 gene expression seen with DDAVP and elevation of cAMP. Furthermore, DDAVP-stimulated VIT32 promoter-reporter activity was inhibited by H89 and by a dominant negative CREB construct. Finally, we were able to identify CREB as a nuclear protein that bound to the VIT32 CRE in gel mobility shift assays. In summary, DDAVP stimulates transcription of VIT32 via a CRE within the proximal promoter region of the VIT32 gene.

Molecular chaperones affect GTP cyclohydrolase I mutations in dopa-responsive dystonia

Unstable GTP cyclohydrolase I (GCH) mutations in dopa-responsive dystonia (DRD) can exert a dominant-negative effect in the HeLa cell model, but in a batch of cells this effect could not be shown. Through differential display, we found a higher Hsc70 expression in the non-dominant-negative cells. We further demonstrated that ectopic expression of Hsp40/Hsp70 stabilized the GCH mutant G201E. Moreover, Hsp90 inhibitor geldanamycin destroyed the wild-type GCH level, and heat shock increased the synthesis of GCH protein. Therefore, the dominant-negative effect produced by unstable proteins would be susceptible to the status of molecular chaperones, which could be the modifying genes and therapeutic targets for DRD and other genetic diseases.

GTP cyclohydrolase I: purification, characterization, and effects of inhibition on nitric oxide synthase in nocardia species

GTP cyclohydrolase I (GTPCH) catalyzes the first step in pteridine biosynthesis in Nocardia sp. strain NRRL 5646. This enzyme is important in the biosynthesis of tetrahydrobiopterin (BH4), a reducing cofactor required for nitric oxide synthase (NOS) and other enzyme systems in this organism. GTPCH was purified more than 5,000-fold to apparent homogeneity by a combination of ammonium sulfate fractionation, GTP-agarose, DEAE Sepharose, and Ultragel AcA 34 chromatography. The purified enzyme gave a single band for a protein estimated to be 32 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular mass of the native enzyme was estimated to be 253 kDa by gel filtration, indicating that the active enzyme is a homo-octamer. The enzyme follows Michaelis-Menten kinetics, with a Km for GTP of 6.5 micromoles. Nocardia GTPCH possessed a unique N-terminal amino acid sequence. The pH and temperature optima for the enzyme were 7.8 and 56 degrees C, respectively. The enzyme was heat stable and slightly activated by potassium ion but was inhibited by calcium, copper, zinc, and mercury, but not magnesium. BH4 inhibited enzyme activity by 25% at a concentration of 100 micromoles. 2,4-Diamino-6-hydroxypyrimidine (DAHP) appeared to competitively inhibit the enzyme, with a Ki of 0.23 mM. With Nocardia cultures, DAHP decreased medium levels of NO2- plus NO3-. Results suggest that in Nocardia cells, NOS synthesis of nitric oxide is indirectly decreased by reducing the biosynthesis of an essential reducing cofactor, BH4.

Regulation of GTP cyclohydrolase I by alternative splicing in mononuclear cells

GTP cyclohydrolase I (GCH, EC 3.5.4.16) regulates the level of tetrahydrobiopterin and in turn the activities of nitric oxide synthase and aromatic amino acid hydroxylases. Type II GCH mRNA, an alternatively spliced species abundant in blood cells, encodes a truncated and nonfunctional protein. When we stimulate peripheral blood mononuclear cells by PHA, the transcription of full-length GCH mRNA increased, but that of type II mRNA decreased transiently. We further demonstrated that the type II cDNA exerted a dominant-negative effect on the wild-type cDNA, similar to the effect of some GCH mutants. Therefore, type II mRNA may regulate GCH and then contribute to the regulation of NO production by BH4-dependent iNOS in mononuclear cells. Selection of the splicing sites may be coupled with transcriptional activation of the GCH gene.

The pteridine pathway in zebrafish: regulation and specification during the determination of neural crest cell-fate

This review describes pteridine biosynthesis and its relation to the differentiation of neural crest derivatives in zebrafish. During the embryonic development of these fish, neural crest precursor cells segregate into neural elements, ectomesenchymal cells and pigment cells; the latter then diversifying into melanophores, iridophores and xanthophores. The differentiation of neural cells, melanophores, and xanthophores is coupled closely with the onset of pteridine synthesis which starts from GTP and is regulated through the control of GTP cyclohydrolase I activity. De novo pteridine synthesis in embryos of this species increases during the first 72-h postfertilization, producing H4biopterin, which serves as a cofactor for neurotransmitter synthesis in neural cells and for tyrosine production in melanophores. Thereafter, sepiapterin (6-lactoyl-7,8-dihydropterin) accumulates as yellow pigment in xanthophores, together with 7-oxobiopterin, isoxanthopterin and 2,4,7-trioxopteridine. Sepiapterin is the key intermediate in the formation of 7-oxopteridines, which depends on the availability of enzymes belonging to the xanthine oxidoreductase family. Expression of the GTP cyclohydrolase I gene (gch) is found in neural cells, in melanoblasts and in early xanthophores (xanthoblasts) of early zebrafish embryos but steeply declines in xanthophores by 42-h postfertilization. The mechanism(s) whereby sepiapterin branches off from the GTP-H4biopterin pathway is currently unknown and will require further study. The surge of interest in zebrafish as a model for vertebrate development and its amenability to genetic manipulation provide powerful tools for analysing the functional commitment of neural crest-derived cells and the regulation of pteridine synthesis in mammals.

Identification of ATF-2 as a transcriptional regulator for the tyrosine hydroxylase gene

Transcriptional regulation of catecholamine-synthesizing genes is important for the determination of neurotransmitters during brain development. We found that three catecholamine-synthesizing genes were transcriptionally up-regulated in cloned PC12D cells overexpressing V-1, a protein that is highly expressed during postnatal brain development (1). To reveal the molecular mechanism to regulate the expression of tyrosine hydroxylase (TH), which is the rate-limiting enzyme for catecholamine biosynthesis, we analyzed the transcription factors responsible for TH induction in the V-1 clonal cells. First, by using reporter constructs, we found that the transcription mediated by cAMP-responsive element (CRE) was selectively enhanced in the V-1 cells, and TH promoter activity was totally dependent on the CRE in the promoter region of the TH gene. Next, immunoblot analyses and a transactivation assay using a GAL4 reporter system revealed that ATF-2, but not cAMP-responsive element-binding protein (CREB), was highly phosphorylated and activated in the V-1 cells, while both CREB and ATF-2 were bound to the TH-CRE. Finally, the enhanced TH promoter activity was competitively attenuated by expression of a plasmid containing the ATF-2 transactivation domain. These data demonstrated that activation of ATF-2 resulted in the increased transcription of the TH gene and suggest that ATF-2 may be deeply involved in the transcriptional regulation of catecholamine-synthesizing genes during neural development.

Predicting transcription factor synergism

Transcriptional regulation is mediated by a battery of transcription factor (TF) proteins, that form complexes involving protein-protein and protein-DNA interactions. Individual TFs bind to their cognate cis-elements or transcription factor-binding sites (TFBS). TFBS are organized on the DNA proximal to the gene in groups confined to a few hundred base pair regions. These groups are referred to as modules. Various modules work together to provide the combinatorial regulation of gene transcription in response to various developmental and environmental conditions. The sets of modules constitute a promoter model. Determining the TFs that preferentially work in concert as part of a module is an essential component of understanding transcriptional regulation. The TFs that act synergistically in such a fashion are likely to have their cis-elements co-localized on the genome at specific distances apart. We exploit this notion to predict TF pairs that are likely to be part of a transcriptional module on the human genome sequence. The computational method is validated statistically, using known interacting pairs extracted from the literature. There are 251 TFBS pairs up to 50 bp apart and 70 TFBS pairs up to 200 bp apart that score higher than any of the known synergistic pairs. Further investigation of 50 pairs randomly selected from each of these two sets using PubMed queries provided additional supporting evidence from the existing biological literature suggesting TF synergism for these novel pairs.

Enhanced expression of GTP cyclohydrolase I in V-1-overexpressing PC12D cells

Three of the catecholamine-synthesizing enzymes, i.e., tyrosine hydroxylase (TH), aromatic l-amino acid decarboxylase, and dopamine beta-hydroxylase, were earlier shown to be up-regulated in cloned PC12D cells overexpressing V-1, a cdc10/SWI6 motif-containing protein. GTP cyclohydrolase I (GCH) is the rate-limiting enzyme for the biosynthesis of tetrahydrobiopterin (BH(4)), known as an essential cofactor for TH; and here we found the increased expression of GCH in V-1-overexpressing clones. Both GCH activity and total biopterin content were highly increased in the V-1 clones; whereas the activity of sepiapterin reductase, enzyme in the final step of the BH(4) biosynthesis, was not altered. Biochemical analyses revealed increased levels of GCH protein, mRNA, and transcription in the V-1 clones. Promoter analysis showed increased reporter activity in the construct with 150 bp of the promoter region of the human GCH gene, suggesting the involvement of cAMP-responsive element-mediated transcriptional regulation.

Involvement of sphingosine kinase in TNF-alpha-stimulated tetrahydrobiopterin biosynthesis in C6 glioma cells

In C6 glioma cells, the sphingolipid second messenger ceramide potentiates expression of inducible nitric-oxide synthase (iNOS) induced by tumor necrosis factor alpha (TNF-alpha) without affecting GTP cyclohydrolase I (GTPCH), the rate-limiting enzyme in the biosynthesis of 6(R)-5,6,7,8-tetrahydrobiopterin (BH(4)), a cofactor required for iNOS activity. TNF-alpha also stimulates sphingosine kinase, the enzyme that phosphorylates sphingosine to form sphingosine-1-phosphate (SPP), a further metabolite of ceramide. Several clones of C6 cells, expressing widely varying levels of sphingosine kinase, were used to examine the role of SPP in regulation of GTPCH and BH(4) biosynthesis. Overexpression of sphingosine kinase, with concomitant increased endogenous SPP levels, potentiated the effect of TNF-alpha on GTPCH expression and activity and BH(4) biosynthesis. In contrast, enforced expression of sphingosine kinase had no effect on iNOS expression or NO formation. Furthermore, N,N-dimethylsphingosine, a potent sphingosine kinase inhibitor, completely eliminated the increased GTPCH activity and expression induced by TNF-alpha. Surprisingly, we found that, although C6 cells can secrete SPP, which is enhanced by TNF-alpha, treatment of C6 cells with exogenous SPP or dihydro-SPP had no affect on BH(4) biosynthesis. However, both SPP and dihydro-SPP markedly stimulated ERK 1/2 in C6 cells, which express cell surface SPP receptors. Interestingly, although this ERK activation was blocked by PD98059, which also reduced cellular proliferation induced by enforced expression of sphingosine kinase, PD98059 had no effect on GTPCH activity. Collectively, these results suggest that only intracellularly generated SPP plays a role in regulation of GTPCH and BH(4) levels.

cAMP inhibits cytokine-induced biosynthesis of tetrahydrobiopterin in human umbilical vein endothelial cells

We studied the effects of cAMP on cytokine (interferon-gamma plus tumor necrosis factor-alpha)-induced stimulation of tetrahydrobiopterin (BH4) synthesis in human umbilical vein endothelial cells (HUVEC). The cytokine mixture caused a marked increase in the biosynthesis and release of BH4 by HUVEC. Dibutyryl-cAMP produced a dose-dependent inhibition of this cytokine-induced stimulation of synthesis and release of BH4 by these cells. 8-Bromo-cAMP also caused a significant inhibition, although the effects were less marked than those of dibutyryl-cAMP. Both forskolin and the stable analog of prostacyclin, iloprost, caused cAMP accumulation and a concomitant diminution of the cytokine-induced BH4 synthesis in HUVEC. Dibutyryl-cAMP and iloprost also significantly inhibited the cytokine-induced stimulation of GTP cyclohydrolase I (GCHI) activity and mRNA production. We concluded that the suppression by the cAMP messenger system of cytokine-induced stimulation of synthesis and release of BH4 by HUVEC can be attributed to the inhibition of the activity of GCHI, the rate-limiting enzyme in BH4 biosynthetic pathway, in HUVEC. The data also suggest that the cAMP-mediated reduction in the GCHI mRNA level may at least partially explain the decline in GCHI activity. It is reasoned that under inflammatory conditions, cAMP-elevating agents such as prostacyclin exert regulatory effects on circulation by inhibiting cytokine-induced synthesis and release of BH4 by HUVEC.

CCAAT/enhancer binding proteins: do they possess intrinsic cAMP-inducible activity?

CCAAT/enhancer binding proteins (C/EBPs) are transcription factors that are enriched in tissues which play a central role in energy metabolism, such as adipose and liver. Structure/function analyses of these proteins have identified several transactivation domains, some of which can physically interact with general transcription factors present in the preinitiation complex. C/EBPs are generally considered to be constitutively-acting factors, unlike other transcription factors whose activities can be regulated by covalent modification, binding of a specific ligand, etc. However, studies of the regulatory property of the phosphoenolpyruvate carboxykinase gene promoter have uncovered a role for C/EBPs in mediating cAMP responsiveness, and identified specific domains within the proteins, which mediate this effect. Interestingly, a number of other gene promoters that are activated in response to cAMP also contain binding sites for C/EBP, and these binding sites are often located within the region of the promoter that is responsible for mediating the acute responsiveness to cAMP. The evidence presented in this review provides compelling support for the hypothesis that C/EBPs have both constitutive and cAMP-inducible activities, and should be considered as a cAMP-responsive nuclear regulator.

Regulation of the cellular prion protein gene expression depends on chromatin conformation

Conversion of the normal cellular prion protein (PrPc), whose physiological function is still under investigation, to an infectious form called prion is the cause of some neurodegenerative diseases. Therefore, the elucidation of PrPc gene regulation is important both to define a strategy to control the infection and to better understand PrPc function. We cloned the rat PrPc gene promoter region into a luciferase reporter vector, transfected C6 and PC-12 cells, and isolated clones with stable enzyme expression. The dependence of chromatin conformation on PrPc promoter activity was evaluated using the histone deacetylase inhibitor, trichostatin A, which was able to highly increase not only promoter activity but also PrPc mRNA and protein levels. The phorbol ester (12-O-tetradecanoylphorbol-13-acetate) and cAMP poorly induced promoter activity; retinoic acid decreased it by 50%, whereas nerve growth factor and dexamethasone had no effect. When 12-O-tetradecanoylphorbol-13-acetate or cAMP but not retinoic acid was associated with trichostatin A, a potentiation of the primary effects was observed. These new data indicate that PrPc gene regulation is highly dependent on disruption of chromatin fiber assembly, which allows some ubiquitous transcription factors accession to specific DNA elements.

Characterization of GTP cyclohydrolase I gene expression in the human neuroblastoma SKN-BE(2)M17: enhanced transcription in response to cAMP is conferred by the proximal promoter

GTP cyclohydrolase I (GTPCH) gene expression was investigated in the human monoamine-containing neuroblastoma cell line SK-N-BE(2)M17. Northern blot analysis revealed a single GTPCH mRNA transcript that was confirmed by RNase protection assay to encode for Type 1 GTPCH; no alternatively spliced forms of GTPCH mRNA were detected with this assay. Incubation with 8Br-cAMP, but not nerve growth factor or leukemia inhibitory factor, produced a rapid increase in GTPCH mRNA and protein levels; protein levels remained elevated during the entire treatment period while mRNA content declined rapidly between 10 and 24 h. Treatment with 8Br-cAMP did not significantly modify the stability of GTPCH mRNA but did increase GTPCH transcription as determined by transient transfection assays of a luciferase reporter construct containing 1171 bp of human GTPCH 5'-flanking sequence. Cis-acting elements required for maximal basal and cAMP-dependent transcription were localized by deletion analysis to the 146 bp proximal promoter. DNase I footprint analysis of the proximal promoter using SK-N-BE(2)M17 nuclear extracts identified two protein binding domains: one an upstream Sp1-like site and the other a combined CRE-Sp1-CCAAT-box element. EMSA and supershift assays demonstrated that the combined CRE-Sp1-CCAAT-box element recruits ATF-2 and NF-Y but not Sp1-4 or Egr-1-3. NF-Y binding was confirmed using pure recombinant human NF-Y protein. Transcription of the human GTPCH gene in human SK-N-BE(2)M17 cells is thus enhanced by cAMP acting through regulatory elements located in the proximal promoter and may involve the transcription factors NF-Y and ATF-2.

ATF-7, a novel bZIP protein, interacts with the PRL-1 protein-tyrosine phosphatase

We have identified a novel basic leucine zipper (bZIP) protein, designated ATF-7, that physically interacts with the PRL-1 protein-tyrosine phosphatase (PTPase). PRL-1 is a predominantly nuclear, farnesylated PTPase that has been linked to the control of cellular growth and differentiation. This interaction was initially found using the yeast two-hybrid system. ATF-7 is most closely related to members of the ATF/CREB family of bZIP proteins, with highest homology to ATF-4. ATF-7 homodimers can bind specifically to CRE elements. ATF-7 is expressed in a number of different tissues and is expressed in association with differentiation in the Caco-2 cell model of intestinal differentiation. We have confirmed the PRL-1.ATF-7 interaction and mapped the regions of ATF-7 and PRL-1 important for interaction to ATF-7's bZIP region and PRL-1's phosphatase domain. Finally, we have determined that PRL-1 is able to dephosphorylate ATF-7 in vitro. Further insight into ATF-7's precise cellular roles, transcriptional function, and downstream targets are likely be of importance in understanding the mechanisms underlying the complex processes of maintenance, differentiation, and turnover of epithelial tissues.

GTP cyclohydrolase I mRNA: novel splice variants in the slime mould Physarum polycephalum and in human monocytes (THP-1) indicate conservation of mRNA processing

GTP cyclohydrolase I (EC 3.5.4.16) is the first enzyme in the biosynthesis of tetrahydrobiopterin [(6R)-5,6,7,8-tetrahydro-L-biopterin, H(4)-biopterin] in mammals and of folic acid in bacteria. Here we have characterized the GTP cyclohydrolase I gene structure and two mRNA species from Physarum polycephalum, an acellular slime mould that synthesizes H(4)-biopterin and metabolites of the folic acid biosynthetic pathway. Its GTP cyclohydrolase I gene consists of seven exons, and the two GTP cyclohydrolase I cDNA species isolated from Physarum encode for proteins with 228 (25.7 kDa) and 195 (22.1 kDa) amino acids. Furthermore, we identified two previously undescribed mRNA species in interferon-gamma-treated human myelomonocytoma cells (THP-1) in addition to the cDNA coding for the fully functional 250-residue (27.9 kDa) protein, which is identical with that in human phaeochromocytoma cells. One of the new splice variants codes for a 233-residue (25.7 kDa) protein, whereas the other codes for the full-length protein but is alternatively spliced within the 3'-untranslated region. In heterologous expression, the shorter proteins of Physarum as well as of THP-1 cells identified here are degraded by proteolysis. Accordingly, only the 27.9 kDa protein was detectable in Western blots from THP-1 cell extracts. Quantification of GTP cyclohydrolase I mRNA species in different human cell types with and without cytokine treatment showed that in addition to the correct mRNA the two splice variants isolated here, as well as the two splice variants known from human liver, are strongly induced by cytokines in cell types with inducible GTP cyclohydrolase I (THP-1, dermal fibroblasts), but not in cell types with constitutive GTP cyclohydrolase I expression (SK-N-SH, Hep-G2). As in human liver, splicing of the new mRNA variant found in THP-1 cells occurs at the boundary of exons 5 and 6. Strikingly, the 195-residue protein from Physarum is alternatively spliced at a homologous position, i.e. at the boundary of exons 6 and 7. Thus alternative splicing of GTP cyclohydrolase I at this position occurs in two species highly distant from each other in terms of evolution. It remains to be seen whether variant proteins encoded by alternatively spliced GTP cyclohydrolase I mRNA transcripts do occur in vivo and whether they participate in regulation of enzyme activity.

Divergence in regulation of nitric-oxide synthase and its cofactor tetrahydrobiopterin by tumor necrosis factor-alpha. Ceramide potentiates nitric oxide synthesis without affecting GTP cyclohydrolase I activity

Synthesis of 6(R)-5,6,7,8-tetrahydrobiopterin (BH(4)), a required cofactor for inducible nitric-oxide synthase (iNOS) activity, is usually coordinately regulated with iNOS expression. In C6 glioma cells, tumor necrosis factor-alpha (TNF-alpha) concomitantly potentiated the stimulation of nitric oxide (NO) and BH(4) production induced by IFN-gamma and interleukin-1beta. Expression of both iNOS and GTP cyclohydrolase I (GTPCH), the rate-limiting enzyme in the BH(4) biosynthetic pathway, was also markedly increased, as were their activities and protein levels. Ceramide, a sphingolipid metabolite, may mediate some of the actions of TNF-alpha. Indeed, we found that bacterial sphingomyelinase, which hydrolyzes sphingomyelin and increases endogenous ceramide, or the cell permeable ceramide analogue, C(2)-ceramide, but not C(2)-dihydroceramide (N-acetylsphinganine), significantly mimicked the effects of TNF-alpha on NO production and iNOS expression and activity in C6 cells. Surprisingly, although TNF-alpha increased BH(4) synthesis and GTPCH activity, neither BH(4) nor GTPCH expression was affected by C(2)-ceramide or sphingomyelinase in IFN-gamma- and interleukin-1beta-stimulated cells. It is likely that increased BH(4) levels results from increased GTPCH protein and activity in vivo rather than from reduced turnover of BH(4), because the GTPCH inhibitor, 2,4-diamino-6-hydroxypyrimidine, blocked cytokine-stimulated BH(4) accumulation. Moreover, expression of the GTPCH feedback regulatory protein, which if decreased might increase GTPCH activity, was not affected by TNF-alpha or ceramide. Treatment with the antioxidant pyrrolidine dithiocarbamate, which is known to inhibit NF-kappaB and sphingomyelinase in C6 cells, or with the peptide SN-50, which blocks translocation of NF-kappaB to the nucleus, inhibited TNF-alpha-dependent iNOS mRNA expression without affecting GTPCH mRNA levels. This is the first demonstration that cytokine-stimulated iNOS and GTPCH expression, and therefore NO and BH(4) biosynthesis, may be regulated by discrete pathways. As BH(4) is also a cofactor for the aromatic amino acid hydroxylases, discovery of distinct mechanisms for regulation of BH(4) and NO has important implications for its specific functions.