Dopamine-mediated oxidation of methionine 127 in α-synuclein causes cytotoxicity and oligomerization of α-synuclein.

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the selective loss of dopaminergic neurons and the presence of Lewy bodies. Many recent studies focused on the interaction between α-synuclein (α-syn) and dopamine in the pathogenesis of PD, and fluorescent anisotropy suggested that the C-terminal region of α-syn may be a target for modification by dopamine. However, it is not well understood why PD-related pathogenesis occurs selectively in dopaminergic neurons. We investigated the interaction between dopamine and α-syn with regard to cytotoxicity. A soluble oligomer was formed by co-incubating α-syn and dopamine in vitro. To clarify the effect of dopamine on α-syn in cells, we generated PC12 cells expressing human α-syn, as well as the α-syn mutants, M116A, Y125D, M127A, S129A, and M116A/M127A, in a tetracycline-inducible manner (PC12-TetOFF-α-syn). Overexpression of wildtype α-syn in catecholaminergic PC12 cells decreased cell viability in long-term cultures, while a competitive inhibitor of tyrosine hydroxylase blocked this vulnerability, suggesting that α-syn-related cytotoxicity is associated with dopamine metabolism. The vulnerabilities of all mutant cell lines were lower than that of wildtype α-syn-expressing cells. Moreover, α-syn containing dopamine-mediated oxidized methionine (Met(O)) was detected in PC12-TetOFF-α-syn. Met(O) was lower in methionine mutant cells, especially in the M127A or M116A/M127A mutants, but also in the Y125D and S129A mutants. Co-incubation of dopamine and the 125YEMPS129 peptide enhanced the production of H2O2, which may oxidize methionine residues and convert them to Met(O). Y125- or S129-lacking peptides did not enhance the dopamine-related production of H2O2. Our results suggest that M127 is the major target for oxidative modification by dopamine, and that Y125 and S129 may act as enhancers of this modification. These results may describe a mechanism of dopaminergic neuron-specific toxicity of α-syn in the pathogenesis of PD.

Mechanisms underlying production and externalization of oxidized phosphatidylserine in apoptosis: involvement of mitochondria.

The present study was performed by using selective inhibitors of caspase-8 and caspase-3 functioning upstream and downstream from mitochondria, respectively to determine whether mitochondria are involved in the mechanisms underlying production and externalization of oxidized phosphatidylserine (PSox) during Fas-mediated apoptosis. Treatment with anti-Fas antibody induced caspase-3 activation, chromatin condensation, release of cytochrome c (cyt c) from mitochondria into the cytosol as well as production of PSox and its exposure to the cell surface in Jurkat cells. Inhibition of caspase-8 by pretreatment with Z-IETD-FMK, a membrane permeable selective caspase-8 inhibitor reduced mitochondrial cyt c release, the amount of PSox not only within but also on the surface of Jurkat cells, caspase-3 activation, and apoptotic cell number after treatment with anti-Fas antibody. In contrast, Z-DEVD-FMK, a membrane permeable selective caspase-3 inhibitor was unable to inhibit cyt c release, and the amount of PSox both within and on the surface of the cells after anti-Fas antibody, although it suppressed caspase-3 activation and apoptosis. Thus, these results strongly suggest that mitochondria play an important role in production of PSox and subsequent its externalization during apoptosis.

Cytoprotective effect of chlorogenic acid against α-synuclein-related toxicity in catecholaminergic PC12 cells.

Parkinson's disease is a major neurodegenerative disease involving the selective degeneration of dopaminergic neurons and α-synuclein containing Lewy bodies formation in the substantia nigra. Although α-synuclein is a key molecule for both dopaminergic neuron death and the formation of inclusion bodies, the mechanism of α-synuclein induction of Parkinson's disease-related pathogenesis is not understood. In the present study, we found that the interaction between dopamine and α-synuclein requires the oxidation of dopamine. Furthermore, we examined the protective effect of chlorogenic acid, a major polyphenol contained in coffee, against α-syn and dopamine-related toxicity. Chlorogenic acid inhibits several DA/α-synuclein-related phenomenon, including the oxidation of dopamine, the interaction of oxidized dopamine with α-synuclein, and the oligomerization of α-synuclein under dopamine existing conditions in vitro. Finally, we showed that the cytoprotective effect against α-synuclein-related toxicity in PC12 cells that can be controlled by the Tet-Off system. Although the induction of α-synuclein in catecholaminergic PC12 cells causes a decrease in cell viability, chlorogenic acid rescued this cytotoxicity significantly in a dose dependent manner. These results suggest that the interaction of oxidized DA with α-synuclein may be a novel therapeutic target for Parkinson's disease, and polyphenols, including chlorogenic acid, are candidates as protective and preventive agents for Parkinson's disease onset.

Protection by Exogenously Added Coenzyme Q(9) against Free Radical-Induced Injuries in Human Liver Cells.

Reduced coenzyme Q(10) (CoQ(10)H(2)) is known as a potent antioxidant in biological systems. However, it is not yet known whether CoQ(9)H(2) could act as an antioxidant in human cells. The aim of this study is to assess whether exogenously added CoQ(9) can protect human liver cells against injuries induced by a water-soluble radical initiator, 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) and a lipid-soluble radical initiator, 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN). CoQ(9)-enriched cells were obtained by treatment of HepG2 cells with 10 microM CoQ(9) liposomes for 24 h. CoQ(9)-enriched cells were exposed to 10 mM AAPH and 500 microM AMVN over 4 h and 24 h, respectively. The loss of viability after treatment with AAPH or AMVN was much less in CoQ(9)-enriched cells than in naive HepG2 cells. The decrease in glutathione and the increase in thiobarbituric acid-reactive substance after treatment with AAPH or AMVN were also suppressed in CoQ(9)-enriched cells. The incubation of CoQ(9)-enriched cells with AAPH or AMVN led to a decrease in cellular CoQ(9)H(2) and reciprocal increase in cellular CoQ(9) resulting from its antioxidant function. Taken together, it was demonstrated for the first time that exogenously added CoQ(9) could prevent oxidative stress-mediated damage to human cells by virtue of its antioxidant activity.