Chinese hamster ovary (CHO) cells, synchronized by selective detachment at mitosis, were treated with various concentrations of actinomycin D (AMD) or cycloheximide (CHX) either immediately, or 1, 2, or 3 hr after mitosis. Since the minimum duration of G1 phase in these cultures was 3.4 hr, the addition of RNA or protein synthesis inhibitors took place at the beginning, first third, second third, or end (G1-S boundary) of G1 phase. The kinetics of exit from G1 phase, the rate and extent of traverse of S phase, and the reaccumulation of RNA were estimated under each set of growth conditions by flow cytometry of acridine orange-stained cells. A mathematical model was constructed to describe the trajectories of the cell populations with respect to their increase in RNA and DNA content in the absence or presence of the inhibitor. The chronologic synchrony imposed on the CHO cell population began to decay within 3 hr, resulting in stochastic entrance of cells into S phase in the absence of inhibitor. Addition of AMD or CHX at 0, 1, 2, or 3 hr after mitosis, regardless of the inhibitor concentration, did not provide evidence of a critical restriction point in G1 beyond which cells were committed to enter S phase and were no longer sensitive to moderate suppression of RNA or protein synthesis. The observed kinetics of cell entrance into an traverse of S phase were consistent with an inherently heterogeneous response to serum stimulation occurring at or just after cell division.
The effects of 1,4-bis(2'-chloroethyl)-1,4-diazabicyclo-[2.2.1] heptane diperchlorate (CBH; NSC 57198) on cell viability, growth, progression through the cell cycle, survival, and differentiation were investigated in suspension cultures of murine lymphocytic leukemia (L1210) and erythroleukemic (FL) cells and normal human lymphocytes stimulated with phytohemagglutinin (PHA) and in adherent cultures of Chinese hamster ovary (CHO) cells. CBH was equally cytotoxic toward stationary and exponentially growing CHO cells. Cell viability was diminished by 50% following 24 hr exposure to approximately 50 micrograms CBH per ml. Treatment of quiescent human lymphocytes for 24 hr with up to 100 micrograms CBH per ml did not appreciably diminish cell viability though the subsequent stimulation of such lymphocytes with PHA was inhibited in a dose dependent fashion. L1210, FL cells, and PHA stimulated human lymphocytes were equally sensitive to CBH, 50% inhibition of growth was obtained following 24 hr treatment with 25 micrograms CBH per ml. Incubation for up to 48 hr with CBH did not result in differentiation of FL cells to mature hemoglobin containing cells. Constant exposure of L1210 cells and PHA-stimulated human lymphocytes to 10-50 micrograms CBH per ml resulted in accumulation of cells in G2 + M phase; higher drug concentrations resulted in cell arrest in mid to late S phase and G2 phase. A short 1-hr pulse of the drug resulted in a transient accumulation of L1210 cells in S and G2 phases. However, cells recovered from a short pulse of drug and by 48 hr, both cell proliferation and the cell cycle distribution appeared normal. A detailed analysis of cell cycle progression of L1210 cells in the presence of the drug indicated that the duration of G2 phase was extended at low concentrations (10 micrograms/ml) while the transit of cells through S was retarded with subsequent accumulation in late S and G2 phase at higher (50 micrograms/ml) concentrations. Concomitant with cell arrest in S and G2 phase an increase in cellular RNA content indicating unbalanced growth was observed. This state of unbalanced growth was reversible in cultures exposed to a 1-hr pulse of up to 100 micrograms CBH per ml; cellular RNA content returned to control values by 48 hr. No effect on nuclear chromatin as assayed by acid denaturation was observed. Though the exact mechanism of drug action is not known, the data are not incompatible with the drug acting as an alkylating agent.
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