A NOVEL FUNCTION OF ADENOVIRUS E1A IS REQUIRED TO OVERCOME GROWTH ARREST BY THE CDK2 INHIBITOR P27(KIP1)

We show here that the adenovirus E1A oncoprotein prevents growth arres t by the CDK2 inhibitor p27(Kip1) (p27) in rodent fibroblasts. However , E1A neither binds p27 nor prevents inhibition of CDK2, complexes in vivo. In contrast, the amount of free p27 available to inhibit cyclin E/CDK2 is increased in E1A-expressing cells, owing to reduced expressi on of cyclins D1 and D3. Moreover, E1A allows cell proliferation in th e presence of supraphysiological p27 levels, while c-Myc, known to ind uce a cellular p27-inhibitory activity, is only effective against phys iological p27 concentrations. E1A also bypasses G(1) arrest by roscovi tine, a chemical inhibitor of CDK2, Altogether, these findings imply t hat E1A can act downstream of p27 and CDK2, Retinoblastoma (pRb)-famil y proteins are known CDK substrates; as expected, association of E1A w ith these proteins (but not with p300/CBP) is required for E1A to prev ent growth arrest by either p27 or the CDK4/6 inhibitor p16(INK4a). By passing CDK2 inhibition requires an additional function of E1A: the mu tant E1A Delta 26-35 does not overcome p27-induced arrest, while it bi nds pRb-family proteins, prevents p16-induced arrest, and alleviates p Rb-mediated repression of E2F-1 transcriptional activity (although E1A Delta 26-35 fails to restore expression of E2F-regulated genes in p27 -arrested cells). We propose that besides the pRb family, E1A targets specific effector(s) of CDK2 in G(1-)S control.