Mouse neuroblastoma clone N1E-115: a suitable model for studying the action of dopamine agonists of tyrosine hydroxylase activity

Neurotoxin-induced impairment of biopterin synthesis and function: Initial stage of a Parkinson-like dopamine deficiency syndrome

Disorders of the function of the tyrosine hydroxylase play an important role in the occurrence of the Parkinson syndrome. The enzyme that catalyses the first, rate-limiting step in the biosynthesis to dopamine requires the cofactor tetrahydrobiopterin. This compound supplies the reduction equivalent for activation of molecular oxygen. Binding of the cofactor to the enzyme is affected by phosphorylation or dephosphorylation of the enzyme protein and, thereby, influences the activity. Nerve and chromaffin cells that synthesize dopamine, noradrenaline and serotonin are able to synthesize the cofactor tetrahydrobiopterin de novo from guanosine-triphosphate as a precursor. In patients suffering from Parkinson's disease a remarkable decrease in biopterin content was found in the brain. The function of the dopaminergic system was studied with an experimental Parkinson model. The antimetabolite 6-aminonicotinamide induces a dopamine deficit in the striatum with a significant slowdown in the utilization of this transmitter. The abolition of the 6-aminonicotinamide-induced muscular rigidity by l-DOPA and dopamine agonists implies that the antimetabolite produces a Parkinson-like syndrome in rats. There are reports on the molecular basis of this effect which are also important for understanding possible disturbances of the synthesis of biopterins. The effector 6-aminonicotinamide-adenine-dinucleotide-phosphate (6-ANADP), which blocks the pentose phosphate pathway, is formed by an enzymatic neurotoxic synthesis. The clonal cell line PC-12 was used to study the molecular basis of the disturbances occurring in the dopaminergic system. These cells contain all the enzymes for catecholamine synthesis, including those for the synthesis of the cofactor tetrahydrobiopterin. Addition of 6-aminonicotinamide to the culture medium resulted in the synthesis of the neurotoxic agent, 6-ANADP, by a glycohydrolase localized in the endoplasmic reticulum. The synthesis of biopterin was depressed after application of 6-aminonicotinamide. The decrease of intracellular tetrahydrobiopterin and total biopterin resulted in reduced DOPA production. The decreased content of biopterin cofactor synthesis was compensated for by the addition of the precursor sepiapterin, indicating that the NADPH-dependent reductases in biopterin synthesis were not inhibited by the antimetabolic nucleotide 6-ANADP. DOPA production was not fully normalized by sepiapterin. Addition of NADH to the medium resulted in a further increase of DOPA production, probably by activation of the recycling pathway. The first step in the synthesis of biopterin from GTP to 7,8-neopterin-triphosphate seems to be particularly sensitive to the action of exogenous neurotoxins. A further sensitive site of action in synthesis to the cofactor BH(4) concerns the function of the dihydropteridin-reductase, which recycles qBH(2) to BH(4). Neurotoxin-induced impairment of biopterin synthesis is probably a pathogenetically important disorder at the initial stage of Parkinson's disease.

The effect of nNOS inhibitors on toxin-induced cell death in dopaminergic cell lines depends on the extent of enzyme expression

Nitric oxide is linked with neurodegeneration in Parkinson's disease (PD) through the involvement of both inducible (iNOS) and neuronal nitric oxide synthase (nNOS). While non-selective NOS inhibitors are neuroprotective, the role of nNOS has not been determined using selective NOS inhibitors. The present study investigated the neuroprotective effect of selective iNOS and nNOS inhibitors on MPP(+)- and MG-132-induced cell death in cell lines with differing levels of nNOS expression. Inhibition of endogenously expressed nNOS by 7-NI and ARR17477 enhanced the toxicity of MPP(+) and MG-132 in N1E-115 cells, whereas in transfected SH-SY5Y cells overexpressing nNOS, ARR17477 and 7-NI protected against MPP(+)- and MG-132-induced cell death. In contrast, inhibition of iNOS by 1400W was ineffective in preventing MPP(+) and MG-132 toxicity in these cell lines. These results suggest a dual role for NOS in dopaminergic cell viability. nNOS is protective against toxic insult when produced endogenously. When nNOS is overexpressed, it becomes neurotoxic to cells suggesting that inhibition of nNOS may be a promising strategy to prevent cell death in PD.

Agonist selective modulation of tyrosine hydroxylase expression by cannabinoid ligands in a murine neuroblastoma cell line

Functional interactions between catecholamines and cannabinoid transmission systems could explain the influence of Delta(9)-tetrahydrocannabinol on several central activities. Hence, the presence of cannabinoid CB(1) receptors in tyrosine hydroxylase (TH) containing cells has been suggested, providing clue for a direct control of catecholamines synthesis. In the present study, we evidenced the constitutive expression of functional cannabinoid CB(1) receptors in N1E-115 neuroblastoma and reported on the use of this model to examine the influence of diverse cannabinoid ligands on TH expression. Exposure of the cells to the high-affinity agonist HU 210 (5 h) resulted in a significant decrease in TH content (pEC(50): 6.40). In contrast, no change was observed after a similar treatment with the structurally unrelated agonist CP 55,940. Besides, the use of a luciferase reporter assay revealed that these two agonists showed opposite influences on TH gene promoter activity. Thus, in cells expressing pTH-luc constructs, inhibition and induction of luciferase activity were respectively observed with HU 210 (pEC(50): 8.95) and CP 55,940 (pEC(50): 9.09). Pharmacological characterisation revealed that these reciprocal responses were both related to the specific activation of cannabinoid CB(1) receptor, suggesting an agonist-dependent modulation of distinct signalling pathways. While these data points out the possible pharmacological manipulation of TH expression by cannabinoid ligands, such approach should take into account the existence of agonist selective trafficking of cannabinoid CB(1) receptor signalling.

Neurotensin induces tyrosine hydroxylase gene activation through nitric oxide and protein kinase C signaling pathways

The regulation of tyrosine hydroxylase (TH) represents an effective means to control the level of catecholamines, because TH is the major limiting enzyme of monoamine biosynthesis. The neuropeptide neurotensin (NT) is a neuromodulator of dopaminergic systems, and a direct interaction between NT and TH expression has been demonstrated in vivo and in vitro. In the present work, the molecular mechanisms and signaling pathways responsible for TH gene activation have been explored. In N1E-115 cells, NT agonist induced a TH protein level increase, correlating with a significant increase in TH mRNA abundance. This cellular response was the result of TH promoter activation, via c-fos and Jun D binding at the AP-1 responsive element. Using selective protein kinase C and nitric oxide synthase inhibitors, we demonstrate, by quantitative reverse transcription-polymerase chain reaction, gel shift, and protein assays, that TH gene activation by NT agonist requires both protein kinase C stimulation and nitric oxide production. The two pathways exert distinct roles; whereas nitric oxide synthase inhibitors blocked c-fos expression, protein kinase C inhibitors blocked that of Jun D. The requirement for two distinct and concomitant pathways by NT demonstrates a very fine level of control of specificity on TH gene activation.

Inhibition of biopterin synthesis and DOPA production in PC-12 pheochromocytoma cells induced by 6-aminonicotinamide

Pheochromocytoma cells (clone PC-12) were treated with 6-aminonicotinamide. Tetrahydrobiopterin content and DOPA production of the cells were determined by reverse-phase HPLC and subsequent electrochemical detection. The same chromatographic system was used to determine total biopterin (tetrahydrobiopterin, dihydrobiopterin and quinoide dihydrobiopterin) by fluorescence detection. Tetrahydrobiopterin plays a decisive role as cofactor of tyrosine hydroxylase for the biosynthesis of DOPA and dopamine. Addition of 6-aminonicotinamide to the culture medium resulted in the accumulation of 6-phosphogluconate, suggesting that PC-12 cells synthesize 6-aminonicotinamide-adenine-dinucleotide-phosphate (6-ANADP) by a glycohydrolase localized in the endoplasmic reticulum. This substance is known to be a strong inhibitor of 6-phosphogluconate dehydrogenase and leads to a blockade of the pentose phosphate pathway. In our experiments, the synthesis of biopterins was depressed after application of 6-aminonicotinamide. The decrease of intracellular tetrahydrobiopterin and total biopterin by 6-aminonicotinamide at different concentrations was strongly correlated with a reduced cellular DOPA production. The decreased content of biopterin cofactor was compensated by addition of the precursor sepiapterin, indicating that the NADPH2-dependent reductases in biopterin synthesis are not inhibited by the antimetabolite. However, DOPA production remained suppressed at the same time. After application of NADH2, we observed an increased DOPA production though the decreased biopterin levels remained almost unchanged. The results imply that the first step in the synthesis of biopterin from GTP as well as the recycling pathways of the oxidized cofactor might be the site of action of the antimetabolite.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of apomorphine, alpha-methylparatyrosine, haloperidol and reserpine on DOPA production in clonal cell lines (PC-12 and N1E-115)

The effect of various drugs on DOPA production in the pheochromocytoma clone PC-12 and the neuroblastoma clone N1E-115 was studied. The N1E-115 cells contain only very low amounts of dopamine due to a lack of the aromatic L-amino acid decarboxylase, whereas the PC-12 cells are rich in dopamine. alpha-Methyl-p-tyrosine and apomorphine blocked DOPA production in both cell clones. Reserpine and haloperidol reduced the intracellular dopamine in the PC-12 cells and simultaneously induced a blockade of cellular DOPA production. The released dopamine was primarily recovered as 3,4-dihydroxyphenylacetic acid indicating a release of dopamine into the cytoplasm. This transient increase of cytoplasmic dopamine by reserpine or haloperidol brings about the inhibition of DOPA production in the PC-12 cells. Our results show that the PC-12 clone especially reacts to various drugs like other in vitro systems and may serve as an additional model for studying drug effects on catecholamine biosynthesis and metabolism.

Inhibition of striatal tyrosine hydroxylase by low concentrations of apomorphine

The inhibitory effect of apomorphine on tyrosine hydroxylase (TH) was tested using enzyme preparations from rat striatum, neuroblastoma clone N1E-115 and pheochromocytoma clone PC-12. When the striatal enzyme preparation was incubated at pH 7.2 with (6R,S)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) as cofactor (100-1,000 mumol/l), the IC50 for apomorphine was found to be in the 0.1-1 mumol/l range depending on the BH4-concentration used. Changing the incubation medium to pH 6.0 yielded an IC50 of about 2.5 mumol/l (BH4 = 100 mumol/l). Apomorphine was even less effective when 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine (100 mumol/l) was used as cofactor (IC50 approximately 10 mumol/l). Similar results were obtained with the enzyme preparations of the two cell clones. These experiments show that, even in low concentrations, apomorphine inhibits TH directly, provided more physiological test conditions are used. The relevance of these results for the autoreceptor-mediated mechanism of the apomorphine action on catecholamine synthesis is discussed.

Enkephalin degradation by enkephalinergic neuroblastoma cells. Involvement of angiotensin-converting-enzyme

Degradation of tritiated [Leu5]enkephalin was studied in cultures of neuroblastoma cells (clone N1E-115). Incubation of cells in suspension revealed Tyr as the main tritiated metabolite; however, Tyr-Gly-Gly and Tyr-Gly were detectable as well. In a crude membrane preparation of the neuroblastoma cells the level of Tyr is reduced to 13% and that of Tyr-Gly to 10% of the initial value, whereas Tyr-Gly-Gly is increased to about 5 times the initial value. Of the degraded enkephalin, 66% was accounted for by the formation of Tyr, 30% by the formation of Tyr-Gly-Gly and 4% by the formation of Tyr-Gly. The production of Tyr was inhibited by bestatin, an inhibitor of aminopeptidases, and that of Tyr-Gly-Gly by captopril, an inhibitor of angiotensin-converting-enzyme. The results prove the ability of neuroblastoma cells (N1E-115) to degrade enkephalin by aminopeptidase and the membrane-bound angiotensin-converting-enzyme.

Tetrahydrobiopterin and total biopterin content of neuroblastoma (N1E-115, N2A) and pheochromocytoma (PC-12) clones and the dependence of catecholamine synthesis on tetrahydrobiopterin concentration in PC-12 cells

Tetrahydrobiopterin content was determined in several clonal cell lines by reversed-phase HPLC and subsequent electrochemical detection. The same chromatography system was used to determine the total biopterin (tetrahydrobiopterin and 7,8-dihydrobiopterin) by fluorescence detection. The catecholamine-producing clones neuroblastoma N1E-115 and pheochromocytoma PC-12 contained 96 and 60 ng tetrahydrobiopterin/mg protein, respectively. The corresponding amount for the neuroblastoma clone N2A was 36 ng/mg protein. The tetrahydrobiopterin content in C-6 glioma cells was below the limit of detection. The total biopterin is about 20% above the tetrahydrobiopterin content. Tetrahydrobiopterin and biopterin from the cells were identified by coelution with standard solutions and by potential-current relationship or emission and excitation spectra, respectively. Addition of 2,4-diamino-6-hydroxypyrimidine, an inhibitor of biopterin synthesis from GTP, to the culture medium of PC-12 cells resulted in a dose-dependent decrease of tetrahydrobiopterin and total biopterin content within 4 h, suggesting that the cells are capable of synthesising the biopterin which was found. A decrease in intracellular tetrahydrobiopterin levels by different concentrations of 2,4-diamino-6-hydroxypyrimidine reduces the cellular production of dihydroxyphenylalanine after inhibition of aromatic L-amino acid decarboxylase, indicating that the concentration of tetrahydrobiopterin might be a limiting factor for catecholamine synthesis in catecholamine-producing cells.

[Nerve cell clonal lines in culture--models for studying the molecular basis of neuropharmacological actions]

Nerve cell lines with stable properties are isolated from neuroblastomas, glioblastomas or pheochromocytomas by periodic cloning using defined culture media. After the action of different drugs, these cells show all morphological and biochemical signs of differentiation and maturation. Depending on the origin of the clone, the cell lines synthesise typical neurotransmitters, which are stored in vesicles. It is demonstrated on cell lines which synthesise catecholamines that noradrenergic and dopaminergic clones are particularly suitable test objects for basic research in neuropharmacology.

No evidence for enkephalinase A on neuronal cells

It is generally assumed that enkephalinase A, a highly thiorphan-sensitive dipeptidylcarboxypeptidase cleaving the Gly-Phe bond during enkephalin degradation, is bound to the neuronal membrane. To clarify the localization of the enzyme, we used three neuron-like models (neuroblastoma cells N1E-115, bulk prepared neurons and neurons in primary culture) and three glial models (astrocytes from rat brain and from chick embryo brain in primary culture and bulk prepared glial cells). After incubating membranes and cells with (Leu5)-enkephalin, we found that, contrary to the present opinion, the enkephalinase A is located on the glial cells, whereas the neuronal cells possess angiotensin-converting enzyme.

Dopa-release from mouse neuroblastoma clone N 1 E-115 into the culture medium. A test for tyrosine hydroxylase activity

Due to a lack of L-Dopa-decarboxylase, the mouse neuroblastoma clone N 1 E-115 contains a high intracellular Dopa-content compared to a low noradrenaline- and dopamine-content. Because of this decarboxylase deficiency, the N 1 E-115 clone releases more than 95% of the produced Dopa into the culture medium. After renewal of the culture medium, Dopa production of the cells can be measured by the increase of Dopa in the medium. Dopa production was linear during 2 h and varied from 50-180 micrograms/mg prot-1 x h-1 between different subcultures. Dopa release into the medium was used as an indirect measure for the tyrosine-hydroxylase activity. Several dopaminergic agonist and antagonists were tested. Dopa production could be blocked dose-dependent by apomorphine (1 X 10(-7)-1 x 10(-6) M), but not by lisuride hydrogen maleate and bromocryptine. Several dopaminergic and adrenergic antagonists failed to reverse the apomorphine induced blockade of the tyrosine-hydroxylase activity.