CELLS EN-ROUTE TO APOPTOSIS ARE CHARACTERIZED BY THE UP-REGULATION OFC-FOS, C-MYC, C-JUN, CDC2 AND RB PHOSPHORYLATION, RESEMBLING EVENTS OF EARLY CELL-CYCLE TRAVERSE

Density-arrested quiescent murine Balb/c-3T3 cells are dependent upon growth factors for their survival. Withdrawal of serum from their medi um induces rapid cell death, the mechanism of which is not yet fully u nderstood. We have studied the effect of serum deprivation on density- inhibited quiescent Swiss 3T3 cells and found that they undergo rapid cell death upon total withdrawal of serum. The nature of this cell dea th is similar to apoptosis, as shown by cellular and nuclear morpholog y and DNA fragmentation into oligonucleosomal fragments. Investigating the regulation of early cell-cycle genes during this process, we foun d that c-myc, c-jun, c-fos, and cdc2 protein presence is induced after serum deprivation, when the phosphorylated form of the RE protein als o appears. The upregulation of these genes' protein products is couple d with the appearance of PCNA, a proliferation-specific nuclear antige n, as well as significant incorporation of BrdU, which may reflect DNA repair activity; in situ analysis shows that BrdU-positive cells are also positive for DNA fragmentation. These results suggest that en rou te to apoptosis, cells undergo events typical of early cell-cycle trav erse by expressing early G(1) genes and may even experience the late G (1)/S phase boundary, as shown by the presence of PCNA. However, the d emonstrated ability of these cells to traverse the G(1) phase of the c ell cycle seems to be an abortive event, since they die shortly afterw ards. (C) 1995 Wiley-Liss, Inc.