Clinical Cancer Research, Vol 3, Issue 9 1653-1660, Copyright © 1997 by American Association for Cancer Research

ARTICLES

Inactivation of p53 increases the cytotoxicity of camptothecin in human colon HCT116 and breast MCF-7 cancer cells

M Gupta, S Fan, Q Zhan, KW Kohn, PM O'Connor and Y Pommier
Laboratory of Molecular Pharmacology, Division of Basic Sciences, National Cancer Institute, NIH, Bethesda, Maryland 20892-4255, USA.

Camptothecin (CPT) derivatives are topoisomerase I (top1) inhibitors recently introduced as clinical agents. To explore the role of p53 in CPT-induced cytotoxicity, we examined CPT effects in two isogenic pairs of human cancer cell lines, MCF-7 breast carcinoma and HCT116 colon carcinoma cells, in which p53 function had been disrupted by transfection with the human papillomavirus type-16 E6 gene. Clonogenic survival assays showed that both MCF-7/E6 and HCT116/E6 cells were more sensitive to CPT. No differences in top1 protein levels and activity analyzed by a novel in vitro oligonucleotide assay were observed in the E6 transfectants. Also, CPT showed comparable top1 cleavable complex formation in vivo, as determined by DNA single-strand breaks and DNA protein cross-links. These results suggest that p53 can protect against CPT-induced cytotoxicity and that this protection is mediated downstream of CPT-induced DNA damage. Flow cytometry analyses showed that CPT can induce G1 arrest in cells with normal p53. This G1 arrest was markedly reduced in the p53-deficient cells. These results demonstrate a critical role of p53 as a G1 checkpoint regulator after CPT-induced DNA damage and suggest a rationale for the selectivity of CPT toward tumors with p53 mutations.

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