title: Inhibition of tyrosinase reduces cell viability in catecholaminergic neuronal cells.
abstract: The biosynthesis of dopamine (DA) in catecholaminergic neurons is regulated by tyrosine hydroxylase, which converts tyrosine into 3, 4-dihydroxyphenylalanine (L-DOPA). In melanocytes, tyrosinase catalyzes both the hydroxylation of tyrosine and the consequent oxidation of L-DOPA to form melanin. Although it has been demonstrated that tyrosinase is also expressed in the brain, the physiological role of tyrosinase in the brain is still obscure. In this study, to investigate the role of tyrosinase in catecholaminergic neuronal cells, we examined the effects of tyrosinase inhibition on the viability of CATH.a and SH-SY5Y cells using tyrosinase inhibitors-specifically, phenylthiourea (PTU) and 5-hydroxyindole (5-HI)-and the transfection of antisense tyrosinase cDNA. Both inhibitors significantly reduced the cell viability of CATH.a cells in a dose-dependent manner. PTU also specifically enhanced DA-induced cell death, but 5-HI did not. This discrepancy in cell death is probably due to the inhibitors' different mechanism of action: 5-HI inhibits the hydroxylation of tyrosine as a competitor for the substrate to induce cell death that may be due to depletion of DA, whereas PTU mainly inhibits the enzymatic oxidation of L-DOPA and DA rather than tyrosine hydroxylation to increase consequently autooxidation of DA. Indeed, the intracellular DA content in CATH.a cells was enhanced by PTU exposure. In contrast, PTU showed no enhancing effects on DA-induced cell death of SH-SY5Y cells, which express little tyrosinase. Furthermore, transfection with antisense tyrosinase cDNA into CATH.a cells dramatically reduced cell viability and significantly enhanced DA-induced cell death. These results suggest that tyrosinase controls the intracellular DA content by biosynthesis or enzymatic oxidation of DA, and the dysfunction of this activity induces cell death by elevation of intracellular DA level and consequent gradual autooxidation of DA to generate reactive oxygen species.
authors.affiliation: Department of Neuroscience, Institute of Molecular and Cellular Medicine, Okayama University Medical School, Okayama, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Higashi
authors.forename: M
authors.initials: M
authors.lastname: Asanuma
authors.forename: I
authors.initials: I
authors.lastname: Miyazaki
authors.forename: N
authors.initials: N
authors.lastname: Ogawa
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D002395:Catecholamines / Q000378:metabolism*; D016923:Cell Death / Q000187:drug effects; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D004298:Dopamine / Q000378:metabolism / Q000494:pharmacology; D050483:Dopamine Plasma Membrane Transport Proteins; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D017930:Genes, Reporter; D006801:Humans; D007211:Indoles / Q000494:pharmacology; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D051379:Mice; D014442:Monophenol Monooxygenase / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D009419:Nerve Tissue Proteins; D009474:Neurons / Q000166:cytology / Q000187:drug effects / Q000201:enzymology*; D016376:Oligonucleotides, Antisense / Q000494:pharmacology; D016627:Oxidopamine / Q000494:pharmacology; D010670:Phenylthiourea / Q000494:pharmacology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014162:Transfection; D001616:beta-Galactosidase / Q000235:genetics
chemical_list: D002352:Carrier Proteins; D002395:Catecholamines; D050483:Dopamine Plasma Membrane Transport Proteins; D004791:Enzyme Inhibitors; D007211:Indoles; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D016376:Oligonucleotides, Antisense; C026785:5-hydroxyindole; D010670:Phenylthiourea; D016627:Oxidopamine; D014442:Monophenol Monooxygenase; D001616:beta-Galactosidase; D004298:Dopamine