Cytotoxic effects of dexamethasone restricted to noncycling, early G1‐phase cells of L1210 leukemia

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Abstract
The cytostatic and cytolytic effects of dexamethasone were studied as functions of cell cycle position in mouse L1210 leukemia cells. To this end, the cells were separated according to size by sedimentation at unit gravity in a specially designed sedimentation chamber. The fractions were analyzed by radioautography and flow cytophotometry. The size‐distributions obtained by 1g sedimentation coincided with cell‐cycle age distribution. With increasing fraction number, samples highly enriched in G1, S, and G2/M cells, respectively were obtained: the smallest cells being in early G1 and the largest in mitosis. In the presence of dexamethasone (10−6‐10−5 M), growth slowed down after a few cell cycles and the cells accumulated in early G1 phase. Lytic cell kill by continued exposure to the drug was confined to the fractions containing the small, early G1‐phase cells. These fractions were also enriched in noncycling cells that were not labeled by prolonged exposure to 3H‐thymidine. After removal of dexamethasone, the cells in S and G2/M phase completed cell cycle traverse but were retarded again in the G1 and early S phase of the next division cycle. The data suggest a memory effect for previous drug exposure. It is concluded that the cytostatic and cytolytic effects of dexamethasone are separate, though not unrelated events. Cytolysis is confined to the noncycling cells that in untreated populations can exit from the dividing compartment during a transitional phase of about 60 minutes subsequent to mitotic division. The cytostatic effects potentiate cytolysis by accumulating the cells in the early G1 phase and thus increasing the probability of their transit to the G0 compartment, sensitive for drug‐mediated cytolysis.