Phorbol ester transiently increases topoisomerase I mRNA levels in human skin fibroblasts

Exposure of human skin fibroblasts to phorbol 12-myristate 13-acetate (PMA) results in a transient increase in the level of topoisomerase I mRNA. A maximal increase (about 10-fold) in the level of topoisomerase I mRNA was observed 3 h after adding 200 nM PMA, but a decrease to the basal level was observed within 12 h of PMA treatment. The lowest PMA concentration to give an observable induction of topoisomerase I gene expression was found to be 20 nM. In addition, the induction of topoisomerase I gene expression by 20 nM PMA pulse treatment for 10 min followed by incubation for an additional 3 h attained the same level of induction seen with 3 h of continuous exposure to 20 nM PMA. This observation suggests that once the signal for protein kinase C activation is transduced, continuous exposure to PMA is not necessary for the maximal effect. To gain insight into the mechanism by which PMA stimulates topoisomerase I gene expression, cells were treated with PMA, cycloheximide, and actinomycin D, either alone or in various combinations. The results show that actinomycin D, but not cycloheximide, specifically abolishes the stimulatory effect of PMA, suggesting that PMA affects topoisomerase I gene expression at the transcriptional level.