Introduction: Cell cycle arrest and apoptosis are the two major antitumor mechanisms which inhibit tumor cell proliferation. The extent of inhibition of tumor growth by either process has not been systematically compared. We showed previously that HCT116 colon carcinoma cells treated with the chemotherapeutic agent irinotecan (CPT-11) maintain long-term arrest while the isogenic p53−/− cells execute apoptosis in vitro [Magrini et al., Int. J. Cancer, 101(1), 23-31, 2002]. Here, we analysed the response of 4 colon carcinoma cell lines or xenografts derived from these cell lines to SN-38 or CPT-11 treatment, respectively to compare the contributions of cell cycle arrest and apoptosis to growth inhibition in vitro and in vivo. Materials and methods: For analysis in vitro, HCT116, LS174T (p53-intact), HT-29, HCT116p53−/− (p53-deficient) colon carcinoma cell lines were treated with SN-38. Cell growth was followed by counting the viable cells over a period of 20 days. For the in vivo studies, the growth of non-treated and CPT-11-treated xenografts was followed over a period of 30 days. Expression of cell cycle regulatory molecules was analysed by Western blotting and cell cycle distribution by FACS. Apoptosis was analysed by the extent of the sub-G1 peak, by anti-PARP p89 and M30 antibody staining. Results: All cell lines underwent a transient G2 arrest, accompanied by an upregulation of cyclin B1 and CDK1 and downregulation of cyclin D1 protein. In the HCT116 and LS174T cell lines this was followed by a tetraploid G1 arrest, concomitant with upregulation of cyclin D1 and a downregulation of cyclin B1 and CDK1 and lasted for at least 8 days. The cell cycle arrest was associated with a senescence-like phenotype. In the p53-deficient cell lines the G2 arrest was followed by mitotic catastrophe and apoptosis; the surviving cells were found to be in the G2 cell cycle phase as seen by the upregulation of cyclin B1, CDK1 and downregulation of p27 and cyclin D1 proteins. The delay of the exponential growth phase in response to SN-38 treatment was 10 days for LS174T and HT-29 cell lines and 13 days and 18 days respectively for HCT116 and HCT116p53−/− cell lines. CPT-11 treated xenografts showed cell cycle arrest in p53-intact tumors whereas apoptosis was induced in the p53-deficient tumors. The delay of tumor doubling was 16, 13, 12 and 18 days respectively in tumor derived from HCT116, LS174T, HT-29 and HCT116p53−/− cells. Conclusions: 1. SN-38 treatment induces in the p53-intact cell lines a long-term tetraploid G1 arrest which is associated with a senescence-like phenotype. 2. The p53-deficient cell lines undergo mitotic catastrophe and apoptosis. 3. Cell cycle arrest results in a growth delay which is comparable to that induced by apoptosis in vitro as well as in vivo. 4. Knowledege of the type of biological reaction to chemotherapy is essential for rational enhancement of either reaction aimed at potentiation of clinical response.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]