STEPWISE TRANSFORMATION OF RAT EMBRYO FIBROBLASTS - C-JUN, JUNB, OR JUND CAN COOPERATE WITH RAS FOR FOCUS FORMATION, BUT A C-JUN-CONTAININGHETERODIMER IS REQUIRED FOR IMMORTALIZATION

Among the Jun family of transcription factors, only c-Jun displays ful l transforming potential in cooperation with activated c-Ha-Ras in pri mary rat embryo fibroblasts. c-Jun in combination with Ras can both in duce foci of transformed tells from rat embryo fibroblast monolayers a nd promote the establishment of these foci as tumoral cell lines, JunB can also cooperate with Ras to induce foci but is unable to promote i mmortalization. We report here that JunD, in cooperation with Ras, ind uces foci with an efficiency similar to that of JunB, Artificial Jun/e b1 derivatives from each of the three Jun proteins were also analyzed. These constructs carry a heterologous homodimerization domain from th e viral EB1 transcription factor and are thought to form only homodime rs in the cell. We show here that these Jun/eb1 chimeras are potent tr ansactivators of AP1 sites and that they can cooperate with c-Ha-Ras t o induce foci, However, among all the Ras-Jun and Ras-Jun/eb1 combinat ions tested, only foci from Ras-c-Jun can be efficiently expanded and maintained as long-term growing cultures. Therefore, we suggest that a heterodimer containing c-Jun might be required for in vitro establish ment of these primary mammalian cells.