The possible role of one-electron reduction of aminochrome in the neurodegenerative process of the dopaminergic system

Effects of platinum-coexisting dopamine with X-ray irradiation upon human glioblastoma cell proliferation

In brain tumors, neurotransmitters and platinum drugs may have some interaction, but their role in radiation therapy remains unclear. We investigated the effects of dopamine in combination with platinum on human glioblastoma U-251MG cells upon X-ray irradiation, comparing with L-DOPA, 2-phenylethylamine and temozolomide. Cell proliferation of U-251MG cells was prominently decreased by dopamine in combination with 10 μM platinum upon 4 Gy of X-ray irradiation, accompanied with intracellular reactive oxygen species generation and mitotic catastrophe. Platinum alone did not increase intracellular reactive oxygen species. On the other hand, L-DOPA in combination with platinum did not decrease cell proliferation regardless of X-ray irradiation. It was clearly shown that 2-phenylethylamine did not suppress cell proliferation as compared to dopamine. Temozolomide decreased cell proliferation in a dose-dependent manner upon X-ray irradiation. However, the combined administration of temozolomide and platinum did not further decrease cell proliferation. The platinum nanoparticles were gradually taken up by cells after administration as determined by ICP analysis. Our results suggest that platinum-coexisting dopamine led cells to mitotic catastrophe due to increased production of intracellular reactive oxygen species which was boosted by X-ray and platinum-catalyzed auto-oxidation of dopamine, and thereby cell proliferation was suppressed. In addition, normal human fibroblast OUMS-36T-1 cells were subjected to experiments. Regarding the effect of the combined administration of dopamine and platinum on each cell which was exposed to X-ray, cell proliferation was decreased in U-251MG cells by the combined administration of platinum, whereas that was not decreased in OUMS-36T-1 cells. This provides one basic insight into the effects of dopamine in combined with platinum on radiation therapy for glioblastoma.

Association of copper levels in the hair with gray matter volume, mean diffusivity, and cognitive functions

Although copper plays a critical role in normal brain functions and development, it is known that excess copper causes toxicity. Here we investigated the associations of copper levels in the hair with regional gray matter volume (rGMV), mean diffusivity (MD), and cognitive differences in a study cohort of 924 healthy young adults. Our findings showed that high copper levels were associated mostly with low cognitive abilities (low scores on the intelligence test consisting of complex speed tasks, involving reasoning task, a complex arithmetic task, and a reading comprehension task) as well as lower reverse Stroop interference, high rGMV over widespread areas of the brain [mainly including the bilateral lateral and medial parietal cortices, medial temporal structures (amygdala, hippocampus, and parahippocampal gyrus), middle cingulate cortex, orbitofrontal cortex, insula, perisylvian areas, inferior temporal lobe, temporal pole, occipital lobes, and supplementary motor area], as well as high MD of the right substantia nigra and bilateral hippocampus, which are indicative of low density in brain tissues. These results suggest that copper levels are associated with mostly aberrant cognitive functions, greater rGMV in extensive areas, greater MD (which are indicative of low density in brain tissues) in subcortical structures in the healthy young adults, possibly reflecting copper's complex roles in neural mechanisms.

In cellulo monitoring of quinone reductase activity and reactive oxygen species production during the redox cycling of 1,2 and 1,4 quinones

Quinones are highly reactive molecules that readily undergo either one- or two-electron reduction. One-electron reduction of quinones or their derivatives by enzymes such as cytochrome P450 reductase or other flavoproteins generates unstable semiquinones, which undergo redox cycling in the presence of molecular oxygen leading to the formation of highly reactive oxygen species. Quinone reductases 1 and 2 (QR1 and QR2) catalyze the two-electron reduction of quinones to form hydroquinones, which can be removed from the cell by conjugation of the hydroxyl with glucuronide or sulfate thus avoiding its autoxidation and the formation of free radicals and highly reactive oxygen species. This characteristic confers a detoxifying enzyme role to QR1 and QR2, even if this character is strongly linked to the excretion capacity of the cell. Using EPR spectroscopy and confocal microscopy we demonstrated that the amount of reactive oxygen species (ROS) produced by Chinese hamster ovary (CHO) cells overexpressing QR1 or QR2 compared to naive CHO cells was determined by the quinone structural type. Indeed, whereas the amount of ROS produced in the cell was strongly decreased with para-quinones such as menadione in the presence of quinone reductase 1 or 2, a strong increase in ROS was recorded with ortho-quinones such as adrenochrome, aminochrome, dopachrome, or 3,5-di-tert-butyl-o-benzoquinone in cells overexpressing QR, especially QR2. These differences could originate from the excretion process, which is different for para- and ortho-quinones. These results are of particular interest in the case of dopamine considering the association of QR2 with various neurological disorders such as Parkinson disease.

Norepinephrine and its metabolites are involved in the synthesis of neuromelanin derived from the locus coeruleus

In order to elucidate the chemical structure of black to brown pigments, neuromelanins (NMs), in the substantia nigra (SN) and the locus coeruleus (LC) in the central nervous system of humans and other mammalian species during aging, chemical degradative methods are powerful tools. HPLC analysis after hydroiodic acid hydrolysis detected aminohydroxyphenylethylamines, aminohydroxyphenylacetic acids, and aminohydroxyethylbenzenes, which confirmed that SN-NM and LC-NM contain melanin derived not only from dopamine and norepinephrine (NE) but also from several other catecholic metabolites, such as 3,4-dihydroxyphenylalanine, 3,4-dihydroxyphenylacetic acid, 3,4-dihydroxymandelic acid, 3,4-dihydroxyphenylethanol, and 3,4-dihydroxyphenylethylene glycol, in addition to the corresponding Cys-derivatives in varying degrees. However, hydroiodic acid hydrolysis showed that LC-NM produced the same degradation products as were detected in SN-NM. Thus, we needed to develop a new chemical detection method to validate the existence of NE in LC-NM. In the present study, we report that HCl hydrolysis of LC-NM in the presence of thioglycolic acid yields new products arising from substitution of the hydroxyl group by thioglycolic acid at the benzyl position of NE and cysteinyl-NE. This is the first chemical evidence showing that NE and cysteinyl-NE are incorporated into LC-NM. Using the chemical degradation methods for the determination of catechols in neuromelanin (NM), we have shown that dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), 3,4-dihydroxyphenylethanol (DOPE), and 3,4-dihydroxyphenylalanine (DOPA) are mainly responsible for the structure of NM from substantia nigra (SN), while norepinephrine (NE), 3,4-dihydroxymandelic acid (DOMA), and 3,4-dihydroxyphenylethylene glycol (DOPEG) are additionally responsible for the structure of NM from locus coeruleus (LC).

The heart as a target for xenobiotic toxicity: the cardiac susceptibility to oxidative stress

The heart is a target organ for oxidative stress-related injuries. Because of its very high energetic metabolic demand, the heart has the highest rate of production of reactive oxygen species, namely, hydrogen peroxide (H2O2), per gram of tissue. Additionally, the heart has lower levels of antioxidants and total activity of antioxidant enzymes when compared to other organs. Furthermore, drugs that have relevant antioxidant activity and that are used in the treatment of oxidative stress related cardiac diseases demonstrate better clinical cardiac outcomes than other drugs with similar receptor affinity but with no antioxidant activity. Several xenobiotics particularly target the heart and promote toxicity. Anticancer drugs, like anthracyclines, cyclophosphamide, mitoxantrone, and more recently tyrosine kinase targeting drugs, are well-known cardiac toxicants whose therapeutic application has been associated to a high prevalence of heart failure. High levels of catecholamines or drugs of abuse, namely, amphetamines, cocaine, and even the consumption of alcohol for long periods of time, are linked to cardiovascular abnormalities. Oxidative stress may be one common link for the cardiac toxicity elicited by these compounds. We aim to revise the mechanisms involved in cardiac lesions caused by the above-mentioned substances specially focusing in oxidative stress related pathways. Oxidative stress biomarkers can be useful in the early recognition of cardiotoxicity in patients treated with these drugs and aid to minimize the setting of cardiac irreversible events.

Biosynthetic pathway to neuromelanin and its aging process

Using model compounds of the melanic component of neuromelanin (NM) prepared by tyrosinase oxidation at various ratios of dopamine (DA) and cysteine (Cys) under physiological conditions, we examined a biosynthetic pathway to NM and its aging process by following the time course of oxidation to NM and the subsequent structural modification of NM under various heating conditions. Chemical degradation methods were applied to the synthetic NM. 4-Amino-3-hydroxyphenylethylamine (4-AHPEA) and thiazole-2,4,5-tricarboxylic acid (TTCA) were used as markers of benzothiazine and benzothiazole units, respectively. By following the time course of the biosynthetic pathway of synthetic NM, we found that neurotoxic molecules are trapped in NM. An aging simulation of synthetic NM showed that benzothiazine units in NM are gradually converted to benzothiazole during the aging process. Thus, natural NM was found to be similar to aged (heated) NM prepared from a 2:1 molar ratio of DA and Cys.

[A new approach to studying the autoxidation of adrenaline: possibility of the determination of superoxide dismutase activity and the antioxidant properties of various preparations by polarography]

The reaction of adrenaline autoxidation in an alkaline buffer with the formation of superoxide radicals and the product of its oxidation, adrenochrome, which models the quinoid pathway of adrenaline conversion in the body, is accompanied by oxygen consumption. This reaction is applicable for polarographic determination of the activity of superoxide dismutase and the antioxidant properties of biological and chemical compounds, it is based on evaluation of the latent period and the rate of oxygen consumption, which are measured in the presence of the compounds examined. It was assumed that the neuro- and cardiotoxicity of quinone products of adrenaline oxidation is related not only to their "own" properties and reactive oxygen species formed but also the hypoxia of those regions of the cell and tissue where the quinoid oxidation of adrenaline occurs.

Cytotoxicity of dopaminochrome in the mesencephalic cell line, MN9D, is dependent upon oxidative stress

Parkinson disease is a specific form of neurodegeneration characterized by a loss of nigra-striatal dopaminergic neurons in the midbrain of humans. The disease is also characterized by an increase in oxidative stress and a loss of glutathione in the midbrain region. A potential link between all these factors is the oxidation of dopamine to dopaminochrome (DAC). Using the murine mesencephalic cell line MN9D, we have shown that DAC [50-250 microM] leads to cell death in a concentration-dependent manner, whereas oxidized l-dopa, dopachrome [50-250 microM], is only toxic at the highest concentration used. Furthermore, chronic exposure of MN9D cells to low concentrations of DAC [50-100 microM] is cytotoxic between 48 and 96 h. DAC also increases superoxide production within MN9D cells as indicated by dihydroethidium fluorescence, that can be prevented by co-administration with the antioxidant, N-acetylcysteine [5 mM]. Moreover, the cytotoxicity induced by DAC can also be prevented by administration of N-acetylcysteine [1-5mM]. Finally, depletion of reduced glutathione in MN9D cells by buthionine sulfoximine [50-100 microM] administration significantly enhances the cytotoxic effect of low concentrations of DAC [50-100 microM] and DAC [175 microM] itself reduces the proportion of oxidized glutathione in total glutathione within 30 min of administration in MN9D cells. Overall, we have shown that DAC causes MN9D cell death in an oxidatively dependent manner that appears closely linked with a rapid loss of reduced glutathione. These findings have implications for understanding the pathogenesis of neurodegenerative pathways in Parkinson disease.

Molecular and neurochemical mechanisms in PD pathogenesis

Oxidation of dopamine to aminochrome seems to be a normal process leading to aminochrome polymerization to form neuromelanin, since normal individuals have this pigment in their dopaminergic neurons in the substantia nigra. The neurons lost in individuals with Parkinson's disease are dopaminergic neurons containing neuromelanin. This raises two questions. First, why are those cells containing neuromelanin lost in this disease? Second, what is the identity of the neurotoxin that induces this cell death? We propose that aminochrome is the agent responsible for the death of dopaminergic neurons containing neuromelanin in individuals with Parkinson's disease. The normal oxidative pathway of dopamine, in which aminochrome polymerizes to form neuromelanin, can be neurotoxic if DT-diaphorase is inhibited under certain conditions. Inhibition of DT-diaphorase allows two neurotoxic reactions to proceed: (i) the formation of aminochrome adducts with alpha-synuclein, which induce and stabilize the formation of neurotoxic protofibrils; and (ii) the one electron reduction of aminochrome to the neurotoxic leukoaminochrome o-semiquinone radical. Therefore, we propose that DT-diaphorase is an important neuroprotective enzyme in dopaminergic neurons containing neuromelanin.

Monoamine transporter inhibitors and norepinephrine reduce dopamine-dependent iron toxicity in cells derived from the substantia nigra

The role of dopamine in iron uptake into catecholaminergic neurons, and dopamine oxidation to aminochrome and its one-electron reduction in iron-mediated neurotoxicity, was studied in RCSN-3 cells, which express both tyrosine hydroxylase and monoamine transporters. The mean +/- SD uptake of 100 microm 59FeCl3 in RCSN-3 cells was 25 +/- 4 pmol per min per mg, which increased to 28 +/- 8 pmol per min per mg when complexed with dopamine (Fe(III)-dopamine). This uptake was inhibited by 2 microm nomifensine (43%p < 0.05), 100 microm imipramine (62%p < 0.01), 30 microm reboxetine (71%p < 0.01) and 2 mm dopamine (84%p < 0.01). The uptake of 59Fe-dopamine complex was Na+, Cl- and temperature dependent. No toxic effects in RCSN-3 cells were observed when the cells were incubated with 100 microm FeCl3 alone or complexed with dopamine. However, 100 microm Fe(III)-dopamine in the presence of 100 microm dicoumarol, an inhibitor of DT-diaphorase, induced toxicity (44% cell death; p < 0.001), which was inhibited by 2 microm nomifensine, 30 microm reboxetine and 2 mm norepinephrine. The neuroprotective action of norepinephrine can be explained by (1) its ability to form complexes with Fe3+, (2) the uptake of Fe-norepinephrine complex via the norepinephrine transporter and (3) lack of toxicity of the Fe-norepinephrine complex even when DT-diaphorase is inhibited. These results support the proposed neuroprotective role of DT-diaphorase and norepinephrine.

On the neurotoxicity mechanism of leukoaminochrome o-semiquinone radical derived from dopamine oxidation: mitochondria damage, necrosis, and hydroxyl radical formation

Leukoaminochrome o-semiquinone radical is generated during one-electron reduction of dopamine oxidation product aminochrome when DT-diaphorase is inhibited. Incubation of 100 microM aminochrome with 100 microM dicoumarol, an inhibitor of DT-diaphorase during 2 h, induces 56% cell death (P < 0.001) with concomitant formation of (i) intracellular hydroperoxides (4.2-fold increase compared to control; P < 0.001); (ii) hydroxyl radicals, detected with ESR and spin trapping agents (2.4-fold increase when cells were incubated with aminochrome in the presence of dicoumarol compared to aminochrome alone); (iii) intracellular edema, and cell membrane deterioration determined by transmission electron microscopy; (iv) absence of apoptosis, supported by using anexin-V with flow cytometry; (v) a strong decrease of mitochondrial membrane potential determined by the fluorescent dye 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolylcarbocyanineiodide (P < 0.01); (vi) swelling and disruption of outer and inner mitochondrial membranes determined by transmission electron microscopy. These results support the proposed role of leukoaminochrome o-semiquinone radical as neurotoxin in Parkinson's disease neurodegeneration and DT-diaphorase as neuroprotective enzyme.

Effects of the DT-diaphorase inhibitor dicumarol on striatal monoamine levels in L-DOPA and L-deprenyl pre-treated rats

It has been proposed that DT-diaphorase plays a strategic role as a neuroprotective enzyme for monoamine neurons, perhaps together with monoamine oxidase (MAO). Thus, we investigated the long-term effects produced by DT-diaphorase inhibition with dicumarol injected unilaterally into the medial forebrain bundle (MFB) on monoamine and metabolite levels, alone, or following dopamine loading with 3,4-dihydroxyphenyl-L-alanine (L-DOPA) or MAO inhibition with L-deprenyl. Monoamine levels were assayed in aliquots from tissue samples from right and left striatum, including both dorsal and ventral regions. Dicumarol alone produced increases in 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), but not in dopamine and metabolite levels when assayed two weeks later. However, following preloading with L-DOPA (3 x 25 mg/kg s.c. 7, 4 and 1 h before surgery), a long-lasting bilateral increase in dopamine and metabolite levels was observed after dicumarol. No effect was observed on dopamine, 5-HT and metabolite levels after L-deprenyl (3 x 10 mg/kg, s.c.) alone, but the levels were unilaterally increased when L-deprenyl was followed by dicumarol. The same result was produced when both L-deprenyl and dicumarol were injected simultaneously into the same brain region. In conclusion, the present study shows that intracerebral inhibition of DT-diaphorase produces long-term changes in 5-HT, but also in dopamine metabolism when DT-diaphorase inhibition is combined with MAO inhibition by systemic or intracerebral treatment with L-deprenyl. It is suggested that both MAO and DT-diaphorase have to be inhibited for inducing long-term changes in monoamine metabolism. Thus, DT-diaphorase is an enzyme to be taken into account when L-DOPA is used to treat Parkinson's disease, or when an MAO-inhibitor is used to treat depression.

MPP+-induced degeneration is potentiated by dicoumarol in cultures of the RCSN-3 dopaminergic cell line. Implications of neuromelanin in oxidative metabolism of dopamine neurotoxicity

We have tested the idea that oxidative metabolism of dopamine may be involved in MPTP toxicity using the RCSN-3 cell line derived from the substantia nigra of an adult rat. Treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (10 microM), MPTP combined with 40 microM dicoumarol (an inhibitor of DT-diaphorase) and dicoumarol alone, did not induce toxicity in RCSN-3 cells after 72 h incubation. The lack of toxicity in MPTP-treated RCSN-3 cells may be explained by the fact that they are unable to metabolize MPTP to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridinium ion (MPP+ as determined by HPLC. Incubation for 72 h with 100 microM MPP+ induced a 6.6 +/- 1.4% cell death of RCSN-3 cells compared to 3.5 +/- 0.4 observed in control cells. However, when the cells were treated with 100 microM MPP+ and 40 microM dicoumarol, cell death increased 4-fold compared to that of cells treated solely with MPP+ (27 +/- 2%; P<0.001). Under these conditions, a significant increase in DNA fragmentation (3-fold compared to MPP+ alone; P<0.01) and in calpain activation (P<0.05 compared to control) was evident. The inhibition of DT-diaphorase by dicoumarol supports the idea that oxidative metabolism of dopamine is involved in MPP+ toxicity in RCSN-3 cells.

Effect of single and repeated methamphetamine treatment on neurotransmitter release in substantia nigra and neostriatum of the rat

The main purpose of this study was to characterize the initial neurotransmission cascade elicited by methamphetamine, analysing simultaneously with in vivo microdialysis monoamine, amino acid and neuropeptide release in substantia nigra and neostriatum of the rat. The main effect of a single systemic dose of methamphetamine (15 mg/kg, subcutaneously) was an increase in dopamine levels, both in substantia nigra ( approximately 10-fold) and neostriatum ( approximately 40-fold), accompanied by a significant, but lesser, increase in dynorphin B ( approximately two-fold, in both regions), and a decrease in monoamine metabolites. A similar effect was also observed after local administration of methamphetamine (100 microm) via the microdialysis probes, but restricted to the treated region. In other experiments, rats were repeatedly treated with methamphetamine or saline, with the last dose administered 12 h before microdialysis. Dopamine K+-stimulated release was decreased following repeated methamphetamine administration compared with that following saline, both in the substantia nigra (by approximately 65%) and neostriatum (by approximately 20%). In contrast, the effect of K+-depolarization on glutamate, aspartate and GABA levels was increased following repeated administration of methamphetamine. In conclusion, apart from an impairment of monoamine neurotransmission, repeated methamphetamine produces changes in amino acid homeostasis, probably leading to NMDA-receptor overstimulation.

Possible role of salsolinol quinone methide in the decrease of RCSN-3 cell survival

The endogenous dopamine-derived neurotoxin salsolinol was found to decrease survival in the dopaminergic neuronal cell line RCSN-3, derived from adult rat substantia nigra in a concentration-dependent manner (208 microM salsolinol induced a 50% survival decrease). Incubation of RCSN-3 cells with 100 micro;M dicoumarol and salsolinol significantly decreased cell survival by 2.5-fold (P < 0.001), contrasting with a negligible effect on RCHT cells, which exhibited nearly a 5-fold lower nomifensine-insensitive dopamine uptake. The levels of catalase and glutathione peroxidase mRNA were decreased when RCSN-3 cells were treated with 100 microM salsolinol alone or in the presence of 100 microM dicoumarol. In vitro oxidation of salsolinol to o-quinone catalyzed by lactoperoxidase gave the quinone methide and 1,2-dihydro-1-methyl-6,7-isoquinoline diol as final products of salsolinol oxidation as determined by NMR analysis. Evidence of the formation of salsolinol o-semiquinone radical has been provided by ESR studies during one-electron oxidation of salsolinol catalyzed by lactoperoxidase.