v-Jun overrides the mitogen dependence of S-phase entry by deregulating retinoblastoma protein phosphorylation and E2F-pocket protein interactions asa consequence of enhanced cyclin E-cdk2 catalytic activity

v-Jun accelerates G(1) progression and shares the capacity of the Myc, E2F, and E1A oncoproteins to sustain S-phase entry in the absence of mitogens; however, how it does so is unknown. To gain insight into the mechanism, we investigated how v-dun affects mitogen-dependent processes which control th e G(1)/S transition. We show that v-Jun enables cells to express cyclin A a nd cyclin A-cdk2 kinase activity in the absence of growth factors and that deregulation of cdk2 is required for S-phase entry, Cyclin A expression is repressed in quiescent cells by E2F acting in conjunction with its pocket p rotein partners Rb, p107, and p130; however, v-Jun overrides this control, causing phosphorylated Rb and proliferation-specific E2F-p107 complexes to persist after mitogen withdrawal, Dephosphorylation of Rb and destruction o f cyclin A nevertheless occur normally at mitosis, indicating that v-Jun en ables cells to rephosphorylate Rb and reaccumulate cyclin A without exogeno us mitogenic stimulation each time the mitotic "clock" is reset. D-cyclin-c dk activity is required for Rb phosphorylation in v-Jun-transformed cells, since ectopic expression of the cdk4- and cdk6-specific inhibitor p16(INK4A ) inhibits both DNA synthesis and cell proliferation. Despite this, v-Jun d oes not stimulate D-cyclin-cdk activity but does induce a marked deregulati on of cyclin E-cdk2. In particular, hormonal activation of a conditional v- Jun-estrogen receptor fusion protein in quiescent, growth factor-deprived c ells stimulates cyclin E-cdk2 activity and triggers Rb phosphorylation and DNA synthesis, Thus, v-Jun overrides the mitogen dependence of S-phase entr y by deregulating Rb phosphorylation, E2F-pocket protein interactions, and ultimately cyclin A-cdk2 activity. This is the first report, however, that cyclin E-cdk2, rather than D-cyclin-cdk, is likely to be the critical Rb ki nase target of v-Jun.