APOPTOSIS INDUCED BY SERUM DEPRIVATION OF PC12 CELLS IS NOT PRECEDED BY GROWTH ARREST AND CAN OCCUR AT EACH PHASE OF THE CELL-CYCLE

Previous studies have shown that PC12 cells undergo apoptosis (program med cell death) when deprived of serum. In the present study, we exami ned the relationship of this death process to the cell cycle. PC12 cel l populations synchronized at different, specific phases of the cell c ycle exhibit similar kinetics of cell death following deprivation of s erum. Flow cytometry analysis was used to examine the levels of apopto tic death in these cell populations in relationship to their progressi on in the cell cycle during the course of serum deprivation. Such anal ysis revealed that the cells die during the G(0)-G(1), S, and perhaps G(2)-M phases and at the G(2) to G(1) transition. These results, there fore, suggest that the death of synchronized, serum-deprived PC12 cell s occurs throughout the cell cycle and is not dependent on growth arre st. Flow cytometry methodology (acridine orange staining), which deter mines the RNA content of cells in relationship to their position in th e cell cycle, was used to address these questions in nonsynchronized c ells. These experiments revealed that, upon serum deprivation, an imme diate loss of RNA occurred from cells in G(1), S, and G(2)-M phases. T his loss is accompanied by a slower appearance of cells with degraded DNA content. These results show that cells from all phases of the cell cycle are damaged upon serum deprivation and thus suggest that the ap optotic cell death of nonsynchronized PC12 cells may occur from each p hase of the cell cycle.