Thymocytes Proliferating Hepatoma Cells and Nonproliferating Immature Effects of Cisplatin on the Induction of Apoptosis in

A 2-h exposure of JB1 rat hepatoma cells in late log phase of growth to 50 UMci.s-diamminedichloroplatinum (II) (cisplatin) resulted in the asyn chronous detachment of cells from the monolayer over 4 days. Detached but not monolayer cells exhibited condensed chromatin and DNA frag mentation, which is indicative of endonuclease activation, the hallmarks of apoptosis in epithelial cells. The number of cisplatin-treated cells identi fied as apoptotic at any one time was never >1% of the total cell number present on addition of drug. Two days after drug addition there was a decrease from 85% to 29% cells in GÌphase of the cell cycle, cells in S phase increased from 9% to 18%, and cells in (¡,/Mphase increased from 6% to 51% with respect to untreated cells. Previous studies by Eastman and colleagues demonstrated that cisplatin-induced apoptosis of Chinese hamster ovary cells occurred in the G2 phase of the cell cycle [A. Eastman, Cancer Cells (Cold Spring Harbor), 2: 275-280, 1990]. Continuous expo sure of JB1 cells to cycloheximide (1 UM)during and after exposure to cisplatin prevented both drug-induced changes in cell cycle distribution and the engagement of apoptosis. Freshly isolated immature rat thymocytes are known to be exquisitely sensitive to the induction of apopto sis by multiple stimuli including dexamethasone, etoposide, and irradia tion. However, no significant increase in the amount of apoptosis above control levels was observed up to 36 h after a 2-h exposure to 50 UMcis platin. JB1 cells have a doubling time of 24 h, whereas >90% of imma ture rat thymocytes are noncycling. The data presented here provide in direct evidence that initiation of cisplatin-induced apoptosis may need to be coupled to a cell cycle-mediated event.