Differential retinoic acid radiosensitization of cervical carcinoma cell lines

The potential of retinoic acid as a radiosensitizer was investigated using SiHa and CC-1 human uterine cervical carcinoma cell lines, representative of high- and low-grade lesions, respectively. SiHa was significantly (P < 0.05) radiosensitized, whereas CC-1 was not. Although 48 h of treatment with 5 microM 13-cis-retinoic acid prior to irradiation was sufficient to induce radiosensitization, continuation of treatment after irradiation significantly increased the effect (P < 0.05). Three hypotheses were tested to explain the different responses of the two lines. One hypothesis was that SiHa is more sensitive to retinoic acid than CC-1. Measurement of growth revealed that SiHa was more sensitive to growth inhibition by retinoic acid than CC-1. The second hypothesis was that retinoic acid increases the proportion of G1-phase cells in SiHa but not in CC-1. This was found not to be true, because a retinoic acid treatment schedule that induced radiosensitization did not alter cell cycle distribution profiles in the absence of radiation. The third hypothesis was that retinoic acid alters the cell cycle response of SiHa but not CC-1 to radiation. Postirradiation cell cycle profiles revealed that retinoic acid increased G1 delay in SiHa, whereas CC-1 exhibited no significant G1 delay. Both lines exhibited G2 delays that were unaffected by retinoic acid. In conclusion, radiosensitization of SiHa but not CC-1 may be explained by different sensitivities to retinoic acid and differences in postirradiation cell cycle responses. Radiosensitization at radiation doses used clinically was observed when retinoic acid was administered both before and after irradiation.