v-Jun overrides the mitogen dependence of S-phase entry by deregulating retinoblastoma protein phosphorylation and E2F-pocket protein interactions as a 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-Jun affects mitogen-dependent processes which control the G(1)/S transition. We show that v-Jun enables cells to express cyclin A and 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 protein 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 of cyclin A nevertheless occur normally at mitosis, indicating that v-Jun enables cells to rephosphorylate Rb and reaccumulate cyclin A without exogenous mitogenic stimulation each time the mitotic "clock" is reset. D-cyclin-cdk 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 does not stimulate D-cyclin-cdk activity but does induce a marked deregulation of cyclin E-cdk2. In particular, hormonal activation of a conditional v-Jun-estrogen receptor fusion protein in quiescent, growth factor-deprived cells stimulates cyclin E-cdk2 activity and triggers Rb phosphorylation and DNA synthesis. Thus, v-Jun overrides the mitogen dependence of S-phase entry 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 kinase target of v-Jun.
Clark
W
W
Beatson Institute for Cancer Research, Cancer Research Campaign Beatson Laboratories, Garscube Estate, Bearsden, Glasgow G61 1BD, Scotland, United Kingdom.
Black
E J
EJ
MacLaren
A
A
Kruse
U
U
LaThangue
N
N
Vogt
P K
PK
Gillespie
D A
DA
D000818:Animals; D042846:CDC2-CDC28 Kinases; D002352:Carrier Proteins / Q000235:genetics; D002453:Cell Cycle / Q000235:genetics*; D002455:Cell Division / Q000235:genetics; D002642:Chick Embryo; D051357:Cyclin-Dependent Kinase 2; D019941:Cyclin-Dependent Kinase Inhibitor p16; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000378:metabolism*; D005347:Fibroblasts; D016193:G1 Phase; D008845:Microinjections; D008934:Mitogens / Q000494:pharmacology*; D016759:Oncogene Protein p65(gag-jun) / Q000378:metabolism*; D015513:Oncogene Proteins / Q000378:metabolism*; D010766:Phosphorylation; D010957:Plasmids; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D016160:Retinoblastoma Protein / Q000378:metabolism*; D016196:S Phase; D014170:Transformation, Genetic
D002352:Carrier Proteins; D019941:Cyclin-Dependent Kinase Inhibitor p16; D016213:Cyclins; D008934:Mitogens; D016759:Oncogene Protein p65(gag-jun); D015513:Oncogene Proteins; D016160:Retinoblastoma Protein; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases