p53 protein expression and radiation-induced apoptosis in human tumors transplanted to nude mice

Relationships between p53 protein expression and radiation-induced apoptosis in five human tumors transplanted to nude mice were studied immunohistochemically. They were irradiated with 200 kV X-rays. Six hours after 2, 5, 10, or 20 Gy irradiation, tumors were excised and fixed. Tumors were also examined 1-48 hours after 10 Gy. Immunohistochemical studies were performed for analysis of p53 protein expression. TUNEL and electron microscopic studies were performed to identify apoptosis. In non-irradiated control groups, the incidence of apoptosis was very low in all of the five tumors. Most tumor cells of the ependymoblastoma were p53 protein negative, but the other four tumors were p53 positive. Following irradiation, most tumor cells of the ependymoblastoma became p53 positive, and the incidence of apoptosis increased. However, we could not find significant changes in the percentage of p53 positive cells in the other tumors, and radiation-induced apoptosis in them was low or negligible. These results suggest that radiation-induced apoptosis in a radiosensitive human tumor is related to wild-type p53 protein expression. In contrast, the p53 protein expression of the other four human tumors should be mutant type, and it does not lead to radiation-induced apoptosis.