UCN-01 and camptothecin induce DNA double-strand breaks in p53 mutant tumor cells, but not in normal or p53 negative epithelial cells

Previous research has shown synergistic growth inhibition between UCN-01 an d camptothecin (CPT) in tumor cells with mutant p53 versus tumor cells with wildtype p53. To determine the possible role of p53 in this drug combinati on, we tested the hypothesis that the synergistic growth inhibition is due to the absence of p53, and can result from the induction of DNA double-stra nd breaks (DSBs). Experiments were performed with the use of normal human m ammary epithelial cells (HMEC); HMEC transfected with HPV16 EG protein whic h inactivates p53 (HE6), or p53-mutant MDA-MB-231 tumor cells. CPT, UCN-01, or a 1:1 combination of both, in either HMEC or HE6 cells did not induce D SBs. In contrast, simultaneous treatment of MDA-MB-231 cells with both UCN- 01 and CPT induced significant levels of DSBs while treatment with either d rug alone did not. While UCN-01 was surprisingly potent against HMEC, the g rowth inhibition was only additive between UCN-01 and CPT against these cel ls. HE6 cells were much less sensitive than HMEC to UCN-01 and slightly les s sensitive to the combined treatment with UCN-01 and CPT. The drug combina tion was synergistic against HE6 cells, due to their lower sensitivity to U CN-01. Unlike what was observed previously in MDA-MB-231 cells, UCN-01 did not abrogate CPT-induced inhibition of DNA synthesis in either HMEC or HE6 cells. These data indicate that synergistic growth inhibition by UCN-01 and CPT against p53 mutant MDA-MB-231 tumor cells may be due to induction of D SBS however the loss of p53 function alone does not sensitize normal cells to the combination of both drugs.