S‐adenosyl‐methionine‐induced apoptosis in PC12 cells

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Abstract
Our previous studies showed that S‐adenosyl‐methionine (SAM) induced Parkinson's disease‐like changes in rat. It caused death to dopamine neurons in the substantia nigra, which appeared shrunken and fragmented, indicative of apoptosis‐like changes (Charlton and Crowell [1995] Mol. Chem. Neuropathol. 26:269–284; Charlton [1997] Life Sci. 61:495–502). In this study, we investigated whether SAM causes apoptosis in both undifferentiated PC12 (PC12) cells and nerve growth factor (NGF)‐differentiated PC12 (D‐PC12) cells. S‐adenosyl‐homocysteine (SAH), the nonmethyl analog of SAM, was also tested. SAM and SAH (1.0 nM to 10.0 μM) caused lactate dehydrogenase (LDH) release from the PC12 cells and D‐PC12 cells; cells with morphological changes and fluorescent DNA fragmentation staining were detected among both PC12 cell and D‐PC12 cell. Compared with the PC12 cell, the D‐PC12 cell, a postmitotic cell, was more sensitive to the toxic effects of SAM or SAH and presented much greater LDH release, suggesting a lethal effect; surprisingly, the amounts of apoptotic cells did not differ significantly between the two kinds of cells. In medium deprived of exogenous methionine, a decline in LDH release was observed in PC12 and D‐PC12 cells. Also, lower levels of intracellular SAM and SAH were observed in the methionine‐deleted media, which were reversed by the addition of either SAM or SAH. An antivitamin B12 monoclonal antibody was added to methionine‐depleted medium, resulting in deficiency of both endogenous and exogenous methionine, which caused further decreases in LDH release and reduction in the levels of intracellular SAM and SAH. The preliminary data showed different sensitivities to SAM or SAH between PC12 cell and D‐PC12 cells, which suggests that PC12 cell may be more stable as a metabolic model. Apoptosis of PC12 cells was also assessed by PARP cleavage detection, Western blot analysis of Bax and Bcl‐2 proteins, and DNA laddering on agarose gel electrophoresis. The proapoptoic protein Bax was dominantly expressed, whereas Bcl‐2 was slightly down‐regulated by SAM. SAH weakly induced the expression of Bax and slightly decreased Bcl‐2 levels. The effects of SAM and its analog, SAH, were demonstrated conclusively to induce apoptosis in PC12 cells.