FIG. 3.

CPT induces transient transcription inhibition. (A) Transcription inhibition and recovery in V79 cells treated with CPT. V79 cells were treated with CPT (25 μM) in six-well tissue culture plates. Transcription rates were measured by 15 min of pulse-labeling with [³H]uridine at different times following drug treatment. The transcription rate of untreated cells was taken as 100%. (B) Transcription recovery in BSC cells treated with CPT. BSC cells were treated with 50 μM CPT in 24-well tissue culture plates. Transcription rates were measured at different times following drug treatment as for panel A. (C). Recovery of transcription is not due to CPT inactivation. Rates of transcription were measured by 30 min of pulse-labeling with [³H]uridine. Sample 1, control (no drug treatment); sample 2, BSC cells treated with CPT (50 μM) for 4 h; sample 3, fresh CPT (50 μM) was added to BSC cells that had been treated with CPT (50 μM) for 4 h (sample 2). Cells were incubated for another 15 min prior to lysis. Sample 4, BSC cells treated with CPT (50 μM) for 15 min; sample 5, incubated culture medium (50 μM CPT for 4 h) from sample 2 was transferred to fresh untreated cells for 15 min; sample 6, the incubated culture medium (50 μM CPT for 15 min) from sample 4 was transferred to fresh untreated cells for 15 min.