Regulation of tyrosine hydroxylase and tetrahydrobiopterin biosynthetic enzymes in PC12 cells by NGF, EGF and IFN-gamma

Adipocytes and macrophages secretomes coregulate catecholamine-synthesizing enzymes

Obesity associates with macrophage accumulation in adipose tissue where these infiltrating cells interact with adipocytes and contribute to the systemic chronic metabolic inflammation present in immunometabolic diseases. Tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) are two of the main enzymes of catecholamines (CA) synthesis. Adipocytes and macrophages produce, secrete and respond to CA, but the regulation of their synthesis in the interplay between immune and metabolic systems remains unknown. A model of indirect cell coculture with conditioned medium (CM) from RAW 264.7 macrophages with or without LPS-activation and 3T3-L1 adipocytes and preadipocytes was established to study the effect of cellular secretomes on the expression of the above enzymes. During the adipocyte differentiation process, we found a decrease of TH and PNMT expression. The secretome from LPS-activated macrophages downregulated TH and PNMT expression in preadipocytes, but not in mature adipocytes. Mature adipocytes CM induced a decrease of PNMT levels in RAW 264.7 macrophages. Pre and mature adipocytes showed a similar pattern of TH, PNMT and peroxisome proliferator-activated receptor gamma expression after exposure to pro and anti-inflammatory cytokines. We evidenced macrophages and adipocytes coregulate the expression of CA synthesis enzymes through secretome, with non-inflammatory signaling networks possibly being involved. Mediators released by macrophages seem to equally affect CA production by adipocytes, while adipocytes secretome preferentially affect AD production by macrophages. CA synthesis seems to be more determinant in early stages of adipogenic differentiation. Our results suggest that CA are key signaling molecules in the regulation of immune-metabolic crosstalk within the adipose tissue.

Changes in the Metabolism of Sphingoid Bases in the Brain and Spinal Cord of Transgenic FUS(1-359) Mice, a Model of Amyotrophic Lateral Sclerosis

The aim of this study was to evaluate changes in the content of sphingoid bases - sphingosine (SPH), sphinganine, and sphingosine-1-phosphate (SPH-1-P) - and in expression of genes encoding enzymes involved in their metabolism in the brain structures (hippocampus, cortex, and cerebellum) and spinal cord of transgenic FUS(1-359) mice. FUS(1-359) mice are characterized by motor impairments and can be used as a model of amyotrophic lateral sclerosis (ALS). Lipids from the mouse brain structures and spinal cord after 2, 3, and 4 months of disease development were analyzed by chromatography/mass spectrometry, while changes in the expression of the SPHK1, SPHK2, SGPP2, SGPL1, ASAH1, and ASAH2 genes were assayed using RNA sequencing. The levels of SPH and sphinganine (i.e., sphingoid bases with pronounced pro-apoptotic properties) were dramatically increased in the spinal cord at the terminal stage of the disease. The ratio of the anti-apoptotic SPH-1-P to SPH and sphinganine sharply reduced, indicating massive apoptosis of spinal cord cells. Significant changes in the content of SPH and SPH-1-P and in the expression of genes related to their metabolism were found at the terminal ALS stage in the spinal cord. Expression of the SGPL gene (SPH-1-P lyase) was strongly activated, while expression of the SGPP2 (SPH-1-P phosphatase) gene was reduced. Elucidation of mechanisms for the regulation of sphingolipid metabolism in ALS will help to identify molecular targets for the new-generation drugs.

The Many Facets of Sphingolipids in the Specific Phases of Acute Inflammatory Response

This review provides an overview on components of the sphingolipid superfamily, on their localization and metabolism. Information about the sphingolipid biological activity in cell physiopathology is given. Recent studies highlight the role of sphingolipids in inflammatory process. We summarize the emerging data that support the different roles of the sphingolipid members in specific phases of inflammation: (1) migration of immune cells, (2) recognition of exogenous agents, and (3) activation/differentiation of immune cells.

Characterization of cerebral microvasculature in transgenic mice with endothelium targeted over-expression of GTP-cyclohydrolase I

Tetrahydrobiopterin (BH4) is a critical determinant of nitric oxide (NO) production by nitric oxide synthase (NOS) in the vascular endothelium and its biosynthesis is regulated by the enzymatic activity of GTP-cyclohydrolase I (GTPCH I). The present study was designed to determine the effects of endothelium-targeted overexpression of GTPCH I (eGCH-Tg) on murine cerebral vascular function. Endothelium targeted over-expression of GTPCH I was associated with a significant increase in levels of BH4, as well as its oxidized product, 7,8-dihydrobiopterin (7,8-BH2) in cerebral microvessels. Importantly, ratio of BH4 to 7,8-BH2, indicative of BH4 available for eNOS activation, was significantly increased in eGCH-Tg mice. However, expression of endothelial NOS, levels of nitrate/nitrite--indicative of NO production--remained unchanged between cerebral microvessels of wild-type and eGCH-Tg mice. Furthermore, increased BH4 biosynthesis neither affected production of superoxide anion nor expression of antioxidant proteins. Moreover, endothelium-specific GTPCH I overexpression did not alter intracellular levels of cGMP, reflective of NO signaling in cerebral microvessels. The obtained results suggest that, despite a significant increase in BH4 bioavailability, generation of endothelial NO in cerebral microvessels remained unchanged in eGCH-Tg mice. We conclude that under physiological conditions the levels of BH4 in the cerebral microvessels are optimal for activation of endothelial NOS and NO/cGMP signaling.

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.

Analgesia by inhibiting tetrahydrobiopterin synthesis

Physiological control of the co-factor tetrahydrobiopterin (BH4) is tight in normal circumstances but levels increase pathologically in the injured somatosensory system. BH4 is an essential co-factor in the production of serotonin, dopamine, epinephrine, norepinephrine and nitric oxide. Excess BH4 levels cause pain, likely through excess production of one or more of these neurotransmitters or signaling molecules. The rate limiting step for BH4 production is GTP Cyclohydrolase 1 (GCH1). A human GCH1 gene haplotype exists that leads to less GCH1 transcription, translation, and therefore enzyme activity, following cellular stress. Carriers of this haplotype produce less BH4 and therefore feel less pain, especially following nerve injury where BH4 production is pathologically augmented. Sulfasalazine (SSZ) an FDA approved anti-inflammatory agent of unknown mechanism of action, has recently been shown to be a sepiapterin reductase (SPR) inhibitor. SPR is part of the BH4 synthesis cascade and is also upregulated by nerve injury. Inhibiting SPR will reduce BH4 levels and therefore should act as an analgesic. We propose SSZ as a novel anti-neuropathic pain medicine.

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.

Immobilization stress causes increases in tetrahydrobiopterin, dopamine, and neuromelanin and oxidative damage in the nigrostriatal system

Oxidative stress is believed to contribute to the pathophysiology of Parkinson's disease, in which nigrostriatal dopaminergic (DA) neurons undergo degeneration. Identification of endogenous molecules that contribute to generation of oxidative stress and vulnerability of these cells is critical in understanding the etiology of this disease. Exposure to tetrahydrobiopterin (BH4), the obligatory cofactor for DA synthesis, was observed previously to cause oxidative damage in DA cells. To demonstrate the physiological relevance of this observation, we investigated whether an overproduction of BH4 and DA might actually occur in vivo, and, if it did, whether this might lead to oxidative damage to the nigrostriatal system. Immobilization stress (IMO) elevated BH4 and DA and their synthesizing enzymes, tyrosine hydroxylase and GTP cyclohydrolase I. This was accompanied by elevation of lipid peroxidation and protein-bound quinone, and activities of antioxidant enzymes. These increases in the indices of oxidative stress appeared to be due to increased BH4 synthesis because they were abolished following administration of the BH4 synthesis inhibitor, 2,4-diamino-6-hydroxy-pyrimidine. IMO also caused accumulation of neuromelanin and degeneration of the nigrostriatal system. These results demonstrate that a severe stress can increase BH4 and DA and cause oxidative damages to the DA neurons in vivo, suggesting relevance to Parkinson's disease.

Interferon-gamma and interferon-beta affect endogenous catecholamines in human peripheral blood mononuclear cells: implications for multiple sclerosis

Interferon (IFN)-gamma plays a pivotal role in the pathogenesis of multiple sclerosis (MS), while IFN-beta may be able to modify the clinical course of the disease, eventually also by counterbalancing IFN-gamma-mediated effects. Catecholamines (CA) exert important effects on the immune response, both as transmitters between the nervous and the immune system, as well as autocrine/paracrine mediators in immune cells, and several lines of evidence support their involvement in MS. In particular, dysregulated production of CA seems to occur in peripheral blood mononuclear cells (PBMCs) of MS patients. We assessed the effects of IFN-beta and IFN-gamma on endogenous CA in PBMCs. In cultured PBMCs stimulated with phytohaemagglutinin (PHA), IFN-beta increased CA production and induced CA release in the culture medium, while IFN-gamma decreased both CA production and the expression of mRNA for the CA-synthesizing enzyme tyrosine hydroxylase. Coincubation with both IFNs prevented the inhibitory effect of IFN-gamma, as well as the stimulatory effect of IFN-beta. IFNs are the first physiological compounds shown to affect endogenous CA in PBMCs: in view of the role of CA-dependent mechanisms in the immune response, these findings may help to better understand the mechanisms of action of IFN-beta as an immunomodulatory drug in MS.

Tetrahydrobiopterin precursor sepiapterin provides protection against neurotoxicity of 1-methyl-4-phenylpyridinium in nigral slice cultures

Complex-I inhibition and oxidative processes have been implicated in the loss of nigral dopamine neurones in Parkinson's disease and the toxicity of MPTP and its metabolite MPP+. Tetrahydrobiopterin, an essential cofactor for tyrosine hydroxylase, may act as an antioxidant in dopaminergic neurones and protects against the toxic consequences of glutathione depletion. Here we studied the effects of manipulating tetrahydrobiopterin levels on MPP+ toxicity in organotypic, rat ventral mesencephalic slice cultures. In cultures exposed to 30 micro m MPP+ for 2 days, followed by 8 days 'recovery' in control medium, we measured dopamine and its metabolites in the tissue and culture medium by HPLC, lactate dehydrogenase release to the culture medium, cellular uptake of propidium iodide and counted the tyrosine hydroxylase-immunoreactive neurones. Inhibition of tetrahydrobiopterin synthesis by 2,4-diamino-6-hydroxypyrimidine had no significant synergistic effect on MPP+ toxicity. In contrast, the tetrahydrobiopterin precursor l-sepiapterin attenuated the MPP+-induced dopamine depletion and loss of tyrosine hydroxylase-positive cells in a dose-dependent manner with 40 micro m l-sepiapterin providing maximal protection. Accordingly, increasing intracellular tetrahydrobiopterin levels may protect against oxidative stress by complex-I inhibition.

Developmental regulation of neurotransmitter phenotype through tetrahydrobiopterin

During development, sympathetic neurons innervating rodent sweat glands undergo a target-induced change in neurotransmitter phenotype from noradrenergic to cholinergic. Although the sweat gland innervation in the adult mouse is cholinergic and catecholamines are absent, these neurons continue to express tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis. The developmental suppression of noradrenergic function in these mouse sympathetic neurons is not well understood. We investigated whether the downregulation of the enzyme aromatic l-amino acid decarboxylase (AADC) or the TH cofactor tetrahydrobiopterin (BH4) could account for the loss of catecholamines in these neurons. AADC levels did not decrease during development, and adult cholinergic sympathetic neurons were strongly immunoreactive for AADC. In contrast, BH4 levels dropped significantly in murine sweat gland-containing footpads during the time period when the gland innervation was switching from making norepinephrine to acetylcholine. Immunoreactivity for the rate-limiting BH4 synthetic enzyme GTP cyclohydrolase (GCH) became undetectable in the sweat gland neurons during this phenotypic conversion, suggesting that sweat glands reduce BH4 levels by suppressing GCH expression during development. Furthermore, extracts from sweat gland-containing footpads suppressed BH4 in cultured mouse sympathetic neurons, and addition of the BH4 precursor sepiapterin rescued catecholamine production in neurons treated with footpad extracts. Together, these results suggest that the mouse sweat gland-derived cholinergic differentiation factor functionally suppresses the noradrenergic phenotype during development by inhibiting production of the TH cofactor, BH4. These data also indicate that GCH expression, which is often coordinately regulated with TH expression, can be controlled independently of TH during development.

GTP cyclohydrolase I induction in striatal astrocytes following intrastriatal kainic acid lesion

The cause of premature death of dopamine neurons in patients with Parkinson's disease remains unknown. It is speculated that damaging reactive species resulting from the metabolism of dopamine, nitric oxide, and tetrahydrobiopterin (BH(4)) may be involved. GTP cyclohydrolase I (GCH1) is the first and rate-limiting enzyme in the synthesis of BH(4), an essential cofactor for tyrosine hydroxylase and nitric oxide synthase in dopamine and nitric oxide production, respectively. Our studies have explored BH(4) metabolism in the nigrostriatal system following intrastriatal kainic acid lesion. We have demonstrated that 1 week following kainic acid there was an increase in striatal GCH1 mRNA, protein, and activity. There was also an elevation of BH(4) levels in the striatum. Part of the induction of GCH1 was localized in situ to astrocytes. Further, the striatal lesion caused death of both neurons and astrocytes in striatum, as shown by in situ end labeling. These novel observations suggest that the induction of GTP cyclohydrolase and BH(4) in striatal astrocytes may be mediating death of striatal neuronal and non-neuronal cells. This work supports existing and emerging reports that demonstrate the importance of dopamine metabolism in neuronal death of the nigrostriatal system.

Possible involvement of tetrahydrobiopterin in the trophic effect of insulin-like growth factor-1 on rat pheochromocytoma-12 (PC12) cells

Tetrahydrobiopterin (BH(4)) has a trophic effect on pheochromocytoma-12 (PC12) cells such as insulin-like growth factor-1 (IGF-1). We investigated involvement of BH(4) in the trophic effect of IGF-1 on PC12 cells. IGF-1 (10-300 ng/ml) increased cellular BH(4) content in a dose-related manner. Cellular BH(4) content increased after 6-36 h incubation with IGF-1. IGF-1-induced increase in the cellular BH(4) content was blunted by 0.3 mM 2,4-diamino-6-hydroxypyrimidine (DAHP), an inhibitor for BH(4) synthesis. IGF-1 protected PC12 cells from the cell death induced by depletion of serum and nerve growth factor, which was attenuated by DAHP. The effects of IGF-1 on the cellular BH(4) content and cell viability were eliminated by 0.2 microM wortmannin. These results suggest that BH(4) is involved in the trophic effect of IGF-1 on PC12 cells and that the effect of IGF-1 on BH(4) synthesis is mediated by phosphatidylinositol 3-kinase.

Glutathione depletion in nigrostriatal slice cultures: GABA loss, dopamine resistance and protection by the tetrahydrobiopterin precursor sepiapterin

Dopaminergic neurons in culture are preferentially resistant to the toxicity of glutathione (GSH) depletion. This effect may be due to high intrinsic levels of tetrahydrobiopterin (BH(4)). Here we studied the effects of manipulating GSH and/or BH(4) levels on selective neurotoxicity in organotypic nigrostriatal slice cultures. Following treatments with L-buthionine sulfoximine (BSO, 10-100 microM, 2 days exposure, 2 days recovery), either alone or in combination with the BH(4) precursor L-sepiapterin (SEP, 20 microM), or the BH(4) synthesis inhibitor 2,4-diamino-6-hydroxypyrimidine (DAHP, 5 mM), toxic effects were assessed by HPLC analysis of medium and tissues, cellular propidium iodide (PI) uptake, lactate dehydrogenase (LDH) efflux, as well as stereological counting of tyrosine-hydroxylase (TH) positive cells. Thirty micromolar BSO produced 91% GSH and 81% GABA depletion and general cell death, but no significant effect on medium homovanillic acid (HVA) or tissue dopamine (DA) levels. SEP prevented or delayed GABA depletion, PI uptake and LDH efflux by BSO, whereas DAHP in combination with BSO caused (almost) complete loss of medium HVA, tissue DA and TH positive cells. We suggest that under pathological conditions with reduced GSH, impaired synthesis of BH(4) may accelerate nigral cell loss, whereas increasing intracellular BH(4) may provide protection to both DA and GABA neurons.

Involvement of tetrahydrobiopterin in trophic effect of erythropoietin on PC12 cells

Tetrahydrobiopterin (BH(4)) synthesis is reported to be stimulated by nerve growth factor (NGF) in PC12 cells, suggesting involvement of BH(4) in the trophic effect of NGF. We have recently reported that erythropoietin (EPO) and BH(4) enhance survival of PC12 cells. In the present study, we investigated involvement of BH(4) in the trophic effect of EPO on PC12 cells. Cellular BH(4) content was increased by EPO (10(-10) to 10(-8) M) in a dose- and time-related manner. EPO (10(-10) to 10(-8) M) increased the viable cell number of PC12 cells. In addition to EPO, BH(4) (1, 3, and 10 microM) increased the viable cell number of PC12 cells. Administration of 0.3 mM 2,4-diamino-6-hydroxypyrimidine, an inhibitor of BH(4) synthesis, blunted EPO-induced increases in BH(4) content and the viable cell number of PC12 cells. These results taken together suggest that BH(4) is involved in the trophic effects of EPO on PC12 cells.

Nerve growth factor down-regulates the expression of norepinephrine transporter in rat pheochromocytoma (PC12) cells

Functional expression of norepinephrine transporter (NET) and its regulation were examined in rat pheochromocytoma cell line, PC12. Nerve growth factor (NGF) decreased [3H]-norepinephrine (NE) uptake in association with a decrease in NET mRNA levels. On the other hand, levels of tyrosine hydroxylase mRNA increased in PC12 cells treated with NGF for 4-24 h, while Oct-2 mRNA levels decreased at 4 h with NGF then recovered for 8-24 h in the presence of NGF. Both bFGF and EGF reduced [3H]NE uptake, although they failed to affect NET mRNA levels. To examine the NET transcriptional regulation, we identified the 5'-noncoding region of rat NET mRNA by the rapid amplification of cDNA end (RACE) method and cloned the 5'-flanking region of NET gene. The newly identified exon encodes the untranslated region of rat NET mRNA upstream of the known 5'-region including ATG start codon. Constructs having green fluorescent protein (GFP) as reporter were made with the cloned NET gene, and promoter activity was examined in CHO and SK-N-SH cells transiently transfected and in PC12 cells stably transfected with NET-GFP constructs. The results indicate that the 2.1 kb NET flanking region displays promoter activity and is responsible for the NGF-induced down-regulation of NET expression.

Hair cycle-dependent changes in adrenergic skin innervation, and hair growth modulation by adrenergic drugs

Skin nerves may exert "trophic" functions during hair follicle development, growth, and/or cycling. Here, we demonstrate hair cycle-related plasticity in the sympathetic innervation of skin and hair follicle in C57BL/6 mice. Compared with telogen skin, the number of nerve fibers containing norepinephrine or immunoreactive for tyrosine hydroxylase increased during the early growth phase of the hair cycle (anagen) in dermis and subcutis. The number of these fibers declined again during late anagen. beta2-adrenoreceptor-positive keratinocytes were transiently detectable in the noncycling hair follicle epithelium, especially in the isthmus and bulge region, but only during early anagen. In early anagen skin organ culture, the beta2-adrenoreceptor agonist isoproterenol promoted hair cycle progression from anagen III to anagen IV. The observed hair cycle-dependent changes in adrenergic skin innervation on the one hand, and hair growth modulation by isoproterenol, accompanied by changes in beta2-adrenoreceptor expression of selected regions of the hair follicle epithelium on the other, further support the concept that bi-directional interactions between the hair follicle and its innervation play a part in hair growth control. This invites one to systematically explore the neuropharmacologic manipulation of follicular neuroepithelial interactions as a novel therapeutic strategy for managing hair growth disorders.

Progress in direct striatal delivery of L-dopa via gene therapy for treatment of Parkinson's disease using recombinant adeno-associated viral vectors

Viral vectors have recently been used successfully to transfer genes and express different proteins in the brain. This review discusses the requirements to consider human clinical trials in which recombinant adeno-associated virus vectors are used to transfer the genes necessary to produce l-dihydroxyphenylalanine (l-dopa) directly into the striatum of Parkinson's patients. Preclinical data that apply to the criteria defined as prerequisite for clinical trials are discussed. Thus, in animal models using recombinant adeno-associated virus vectors it has been demonstrated that l-dopa can be synthesized in the striatum after in vivo transduction. In addition, these l-dopa levels are sufficient to affect behavior in a dopamine-deficient animal model, the expression is extremely long-lasting, and the ability to transcriptionally regulate tyrosine hydroxylase has been demonstrated but not fully characterized. However, while immune responses to recombinant adeno-associated virus infection in the periphery have been studied, direct assessment of the potential immune response in the brain has not been sufficiently defined. Therefore, the rationale for delivering l-dopa directly to the striatum to treat Parkinson's disease is sound and the preclinical data are promising but all the issues surrounding this strategy are not resolved.

Vasoactive intestinal peptide induces both tyrosine hydroxylase activity and tetrahydrobiopterin biosynthesis in PC12 cells

Vasoactive intestinal peptide plays an important role in the trans-synaptic activation of tyrosine hydroxylase in sympathoadrenal tissues in response to physiological stress. Since tyrosine hydroxylase is thought to be subsaturated with its cofactor, tetrahydrobiopterin, we tested the hypothesis that up-regulation of tyrosine hydroxylase gene expression following vasoactive intestinal peptide treatment is accompanied by a concomitant elevation of intracellular tetrahydrobiopterin biosynthesis. We also investigated the second messenger systems involved in vasoactive intestinal peptide's effects on tetrahydrobiopterin metabolism. Our results demonstrate that treatment of PC12 cells for 24 h with vasoactive intestinal peptide induced intracellular tetrahydrobiopterin levels 3.5-fold. This increase was due to increased expression of the gene encoding GTP cyclohydrolase, the initial and rate-limiting enzyme in tetrahydrobiopterin biosynthesis, which was blocked by the transcriptional inhibitor, actinomycin D. Activation of tyrosine hydroxylase and GTP cyclohydrolase by vasoactive intestinal peptide was mediated by cyclic-AMP. Furthermore, stimulation of cyclic-AMP-mediated responses or protein kinase C activity induced the maximal in vitro activities of both tyrosine hydroxylase and GTP cyclohydrolase; the responses were additive when both treatments were combined. Induction of sphingolipid metabolism had no effect on the activation of tyrosine hydroxylase, while it induced GTP cyclohydrolase in a protein kinase C-independent manner. Our results support the hypothesis that intracellular tetrahydrobiopterin levels are tightly linked to tyrosine hydroxylation and that tetrahydrobiopterin bioavailability modulates catecholamine synthesis.

Tetrahydrobiopterin as a mediator of PC12 cell proliferation induced by EGF and NGF

Epidermal growth factor and nerve growth factor increased the proliferation of rat phaeochromocytoma PC12 cells through obligatory elevation of intracellular (6R)-tetrahydrobiopterin (BH4). Epidermal growth factor and nerve growth factor increased intracellular BH4 by inducing GTP-cyclohydrolase, the rate-limiting enzyme in BH4 biosynthesis. Specific inhibitors of BH4 biosynthesis prevented growth factor-induced increases in BH4 levels and proliferation. The induction of GTP cyclohydrolase, BH4 and cellular proliferation by nerve growth factor was mediated by cAMP. Elevation of BH4 biosynthesis occurred downstream from cAMP in the cascade used by nerve growth factor to increase proliferation. Thus, intracellular BH4 is an essential mediator of the proliferative effects of epidermal growth factor and nerve growth factor in PC12 cells.