Canine Mammary Tumor Cell Lines Derived from Metastatic Foci Show Increased RAD51 Expression but Diminished Radioresistance via p21 Inhibition

SIMPLE SUMMARY: Canines have a high incidence of mammary tumors, which is a major problem in the veterinary field. Approximately half of canine mammary tumors are malignant and metastasize to nearby lymph nodes. In this study, we analyzed the expression levels of RAD51, a DNA homologous recombination repair-related molecule, in canine mammary tumor cell lines derived from the primary tumor CHMp and metastatic tumor CHMm, which were established from the same individual. The RAD51 expression level in CHMm was higher than that in CHMp. However, CHMp was more resistant than CHMm to irradiation and formed functional Rad51 foci, which form at DNA lesions for homologous recombination repair. The cell cycle regulator p21 was expressed upon radiation exposure in CHMp cells, but not in CHMm cells, indicating that CHMm did not induce cell cycle arrest after irradiation. These results suggest that the radioresistance of CHMm, with high RAD51 expression, is lower than that of CHMp. ABSTRACT: Due to the high incidence of mammary tumors in dogs, it is important to elucidate the pathogenesis of these tumors in veterinary medicine. Radiation therapy is often used to treat mammary tumors that target DNA lesions. RAD51 is a key molecule that repairs DNA damage via homologous recombination. We examined the relationship between RAD51 expression and radiosensitivity in mammary tumor cell lines. CHMp and CHMm from the same individual were selected based on the differences in RAD51 expression. The radiosensitivity of both cell lines was examined using MTT and scratch assays; CHMm, which has high RAD51 expression, showed higher sensitivity to radiation than CHMp. However, the nuclear focus of RAD51 during DNA repair was formed normally in CHMp, but not in most of CHMm. Since irradiation resulted in the suppression of cell cycle progression in CHMp, the expression of p21, a cell cycle regulatory factor, was detected in CHMp after 15 Gy irradiation but not in CHMm. These results indicate that functional expression is more important than the quantitative expression of RAD51 in canine mammary tumor cells in response to DNA damage.