Marked differences in proliferative response to stimulation of growth hormone and insulin-like growth factor-I between thymoma- and normal thymus-derived epithelial cell lines.

Two kinds of thymus epithelial cell lines have been utilized: the TAD3 from lymphocyte-dominant rat thymoma and the IT-45R1 derived from normal rat thymus. In the preconfluent state, the percent increase of DNA synthetic activity of TAD3 cells stimulated by GH or IGF-I was significantly higher than that of IT-45R1 cells. In the confluent state, at the contrary, no any noticeable modification of the activity by GH could be detected and that in both cell lines. However, a treatment of the confluent TAD3 cells with IGF-I caused a significant increase of their DNA synthetic activity, whereas the confluent IT-45R1 cells treated with these molecules did not. It was observed that TAD3 cells in the preconfluent state proliferate markedly after stimulation by GH or IGF-I as compared with IT-45R1 cells and that the former is able to proliferate even in the confluent state, after an exposure of IGF-I, whereas the latter did not. Thymus epithelial cells treated with GH were reported to release more IGF-I molecules what had as a consequence a marked proliferative response of thymic lymphocytes. It is therefore assumed that the very particular nature of TAD3 cells treated with IGF-I, able of proliferating by crossing over the contact inhibition, might have a close relationship in the formation of the lymphocyte-dominant thymoma.