Protective effect of arachidonic acid and linoleic acid on 1-methyl-4-phenylpyridinium-induced toxicity in PC12 cells.

BACKGROUND: Parkinson's disease is a neurodegenerative disorder that is being characterized by the progressive loss of dopaminergic neurons of the nigrostriatal pathway in the brain. The protective effect of omega-6 fatty acids is unclear. There are lots of contradictions in the literature with regard to the cytoprotective role of arachidonic acid. To date, there is no solid evidence that shows the protective role of omega-6 fatty acids in Parkinson's disease. In the current study, the potential of two omega-6 fatty acids (i.e. arachidonic acid and linoleic acid) in alleviating 1-methyl-4-phenylpyridinium (MPP+)-induced cytotoxicity in PC12 cells was examined. METHODS: Cultured PC12 cells were either treated with MPP+ alone or co-treated with one of the omega-6 fatty acids for 1 day. Cell viability was then assessed by using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. RESULTS: Cells treated with 500 µM MPP+ for a day reduced cell viability to ~70% as compared to control group. Linoleic acid (50 and 100 µM) significantly reduced MPP+-induced cell death back to ~85-90% of the control value. The protective effect could be mimicked by arachidonic acid, but not by ciglitazone. CONCLUSIONS: Both linoleic acid and arachidonic acid are able to inhibit MPP+-induced toxicity in PC12 cells. The protection is not mediated via peroxisome proliferator-activated receptor gamma (PPAR-γ). Overall, the results suggest the potential role of omega-6 fatty acids in the treatment of Parkinson's disease.

Astrocytic poly(ADP-ribose) polymerase-1 activation leads to bioenergetic depletion and inhibition of glutamate uptake capacity.

Poly(ADP-ribose) polymerase-1 (PARP-1) is a ubiquitous nuclear enzyme involved in genomic stability. Excessive oxidative DNA strand breaks lead to PARP-1-induced depletion of cellular NAD(+), glycolytic rate, ATP levels, and eventual cell death. Glutamate neurotransmission is tightly controlled by ATP-dependent astrocytic glutamate transporters, and thus we hypothesized that astrocytic PARP-1 activation by DNA damage leads to bioenergetic depletion and compromised glutamate uptake. PARP-1 activation by the DNA alkylating agent, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), caused a significant reduction of cultured cortical astrocyte survival (EC(50) = 78.2 +/- 2.7 microM). HPLC revealed MNNG-induced time-dependent reductions in NAD(+) (98%, 4 h), ATP (71%, 4 h), ADP (63%, 4 h), and AMP (66%, 4 h). The maximal [(3)H]glutamate uptake rate (V(max)) also declined in a manner that corresponded temporally with ATP depletion, falling from 19.3 +/- 2.8 in control cells to 2.1 +/- 0.8 nmol/min/mg protein 4 h post-MNNG. Both bioenergetic depletion and loss of glutamate uptake capacity were attenuated by genetic deletion of PARP-1, directly indicating PARP-1 involvement, and by adding exogenous NAD(+) (10 mM). In mixed neurons/astrocyte cultures, MNNG neurotoxicity was partially mediated by extracellular glutamate and was reduced by co-culture with PARP-1(-/-) astrocytes, suggesting that impairment of astrocytic glutamate uptake by PARP-1 can raise glutamate levels sufficiently to have receptor-mediated effects at neighboring neurons. Taken together, these experiments showed that PARP-1 activation leads to depletion of the total adenine nucleotide pool in astrocytes and severe reduction in neuroprotective glutamate uptake capacity.

Maintenance of quantal size and immediately releasable granules in rat chromaffin cells by glucocorticoid.

Glucocorticoid is reported to regulate catecholamine synthesis and storage. However, it is not clear whether the actual amount of catecholamine released from individual granules (quantal size, Q) in mature chromaffin cells is affected by glucocorticoid. Using carbon fiber amperometry, we found that dexamethasone did not affect mean cellular Q or the proportional release from different populations of granules in rat chromaffin cells cultured for 1 day in a serum-free defined medium. After two extra days of culture in the defined medium, there was a rundown in mean cellular Q, and it was associated with a shift in the proportional release from the different granule populations. This phenomenon could not be rescued by serum supplementation but could be prevented by dexamethasone via an action that was independent of changes in voltage-gated Ca(2+) channel (VGCC) density. Using simultaneous measurements of membrane capacitance and cytosolic Ca(2+) concentration, we found that for cells cultured in defined medium dexamethasone enhanced the exocytotic response triggered by a brief depolarization (50 ms) without affecting the VGCC density or the fast exocytotic response triggered via flash photolysis of caged Ca(2+). Thus glucocorticoid may regulate the number of immediately releasable granules that are in close proximity to a subset of VGCC. Because chromaffin cells in vivo are exposed to high concentrations of glucocorticoid, our findings suggest that the paracrine actions of glucocorticoid maintain the mean catecholamine content in chromaffin cell granules as well as the colocalization of releasable granules with VGCCs.