Enhancement of Tumorigenicity and Invasion Capacity of Rat Mammary Adenocarcinoma Cells by Epidermal Growth Factor and Transforming Growth Factor‐β

We have studied the effects of growth factors and cytokines on the tumorigenicity and invasion capacity of tumor cells by using regressor and progressor tumor cell lines (ER‐1 and ERpP, respectively) derived from an SHR rat mammary adenocarcinoma. ER‐1 cells regress spontaneously whereas ERpP cells show invasive growth and high metastasis to lung and other organs in syngeneic SHR rats. When ER‐1 cells were pretreated with either epidermal growth factor (EGF) or transforming growth factor‐β (TGF‐β) for 24 h in vitro, and intraperitoneally transplanted into SHR rats, they grew and killed the host, whereas ER‐1 cells pretreated with tumor necrosis factor‐α did not. Tumorigenicity and invasion capacity of ERpP cells were also enhanced by treatment with EGF and TGF‐β. The ER‐1 cells pretreated with EGF, once grown in vivo, had acquired irreversible tumorigenicity and invasion capacity without requiring further EGF treatment, and the enhanced malignancy was irreversible. These findings suggest that growth factors play an important role in acquisition of malignancy of tumor cells.