MPP⁺ downregulates mitochondrially encoded gene transcripts and their activities in dopaminergic neuronal cells: Protective role of Bcl-2

The effects of neurotoxins on levels of mitochondrially encoded gene transcripts in a dopaminergic neuronal cell line, MN9D, were examined following treatment with 200 μM N-methyl-4-phenylpyridinium (MPP⁺) or 6-hydroxydopamine (6-OHDA). As confirmed by a Northern blot analysis, levels of cytochrome c oxidase subunit 3 (COX III) and ATPase subunit 6 (ATPase 6) transcript were decreased in a time-dependent manner following treatment with MPP⁺ but not with 6-OHDA. Accordingly, enzymatic activity of cytochrome c oxidase (COX) and the intracellular ATP content were also decreased in MPP⁺-treated cells while these remained unaltered in 6-OHDA-treated cells. In the cell death paradigm induced by MPP⁺, overexpression of Bcl-2 in MN9D cells (MN9D/ Bcl-2) significantly blocked MPP⁺-induced downregulation of COX III and ATPase 6 transcripts. In MN9D/ Bcl-2 cells, MPP⁺-induced downregulation of COX activity and the intracellular level of ATP was also blocked. Treatment with a pan-caspase inhibitor, however, neither prevented MPP⁺-induced downregulation of COX activity nor affected intracellular level of ATP in MN9D cells. Taken together, our present data suggest that Bcl-2 may play a regulatory role in energy metabolism by preventing downregulation of mitochondrially encoded gene(s) at a point distinct from its known anticaspase activity in MPP⁺-induced dopaminergic neuronal death.