MALIGNANT TRANSFORMATION BY CYCLIN-E AND HA-RAS CORRELATES WITH LOWERSENSITIVITY TOWARDS INDUCTION OF CELL-DEATH BUT REQUIRES FUNCTIONAL MYC AND CDK4

We demonstrate in this paper that the G1 phase specific cell cycle reg ulator cyclin E is able to provoke focus formation when cotransfected with activated Ha-ras into primary rat embryo fibroblasts (REFs). Cycl in E/Ha-ras transformed cells are highly tumorigenic in synergeneic ra ts, are able to form colonies in soft agar and show protection towards apoptosis upon serum starvation or DNA damage compared to cells trans formed by the combination of Myc, cyclin D1 or SV40 large T-antigen an d Ha-ras. Lines that were established after cyclin El Ha-ras or cyclin D1/Ha-ras transformation contain a large percentage of polyploid cell s, This was not observed in cells transformed with other oncoproteins and Ha-ras pointing to an involvement of D-and E type cyclins in genom ic instability. The cyclin dependent kinase inhibitors p21 and p27 but also p16 completely abrogate focus formation by cyclin E and Ha-ras s uggesting that the oncogenic activity of cyclin E still requires funct ional G1 specific cyclin/CDK complexes. Moreover, inhibition of Myc fu nction also blocks the oncogenic activity of cyclin E indicating a req uirement of Myc for cyclin E function, The findings presented here dem onstrate that cyclin E can act as an oncoprotein with a potential invo lvement in genomic instability and the prevention of cell death, Our d ata also present more evidence for a strict functional interdependency between G1 cyclin/CDK complexes and c-Myc.