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Molecular and Cellular Biology, April 2000, p. 2529-2542, Vol. 20, No. 7
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

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

W. Clark, E. J. Black, A. MacLaren, U. Kruse,dagger N. LaThangue,Dagger P. K. Vogt,§ and D. A. F. Gillespie*

Beatson Institute for Cancer Research, Cancer Research Campaign Beatson Laboratories, Garscube Estate, Bearsden, Glasgow G61 1BD, Scotland, United Kingdom

Received 2 September 1999/Returned for modification 22 October 1999/Accepted 21 December 1999

v-Jun accelerates G1 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 G1/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 p16INK4A 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.

* Corresponding author. Mailing address: Beatson Institute for Cancer Research, Cancer Research Campaign Beatson Laboratories, Garscube Estate, Switchback Rd., Bearsden, Glasgow G61 1BD, Scotland, United Kingdom. Phone: 0141-942-9631. Fax: 0141-942-6521. E-mail: d.gillespie{at}beatson.gla.ac.uk.

dagger Present address: Institut für Biologie III, University Freiburg, D-79104 Freiburg, Germany.

Dagger Present address: Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom.

§ Present address: Department of Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, CA 92037.

Molecular and Cellular Biology, April 2000, p. 2529-2542, Vol. 20, No. 7
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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