Metaplastic change in mesenchymal stem cells induced by activated ras oncogene. 3T3 T murine mesenchymal stem cells have the potential to differentiate into a variety of different cell types even though they show a predilection to undergo adipocyte differentiation in vitro. The possibility that the activated c-Ha-ras (EJras) oncogene might influence the pathway of differentiation of these stem cells is investigated in the current study. Activated ras oncogene was transfected and stably expressed in 3T3 T cells; assays then were performed to determine its effect on differentiation. The results show that all EJras-transfected cell lines lose their ability to differentiate to adipocytes and instead differentiate into cells that express many characteristics of macrophages. Such cells contain numerous cytoplasmic granules, extensive nonspecific esterase activity, and anchorage-independent growth. The modulation of differentiation pathway from an adipocyte lineage to a macrophagelike cell lineage does not result from the transforming effect of EJras, because a nontransformed cell clone that expresses p21EJras protein also exhibits this modified differentiation pathway. These data suggest that the EJras oncogene specifically modulates the differentiation pathway of 3T3 T mesenchymal stem cells. This experimental system should therefore provide an excellent model to evaluate the mechanistic role of EJras in the process of metaplasia. 