Thy-1 as a Negative Growth Regulator in ras-transformed Mouse Fibroblasts1

To identify molecules on the cell surface involved in negative growth regulation, we assumed that their amounts would be reduced after malig nant transformation. We analyzed several proteins by fluorescence-acti vated cell sorter in mouse NIH 3T3 and its transformed cell lines. Surprisingly, the amount of Thy-1, a cell surface glycoprotein anchored in the cell membrane by a glycophosphatidyl inositol linkage, was signif icantly decreased in the transformed NIH 3T3 lines, especially in rastransformed NIH 3T3 lines. The malignant properties of clones of NIH 3T3 transformed by Kirsten murine sarcoma virus have a good correlation not only with the high amount of RAS proteins but also inversely with the amount of Thy-1. NIH 3T3 subpopulations lacking Thy-1 exhibit more susceptibility to the induction of colony-forming ability in soft agar by Kirsten murine sarcoma virus than the Thy-1-positive populations. Finally the transfection of Thy-\ complementary DNA to the rai-transformed NIH 3T3 significantly inhibits the colony formation in soft agar as well as the tumor formation in nude mice. Our results suggest that Thy-1 has negative effects on the anchorage-independent growth of rastransformed NIH 3T3 cells.