title: Phosphorylation of the cyclin-dependent kinase inhibitor p21Cip1 on serine 130 is essential for viral cyclin-mediated bypass of a p21Cip1-imposed G1 arrest.
abstract: K cyclin encoded by Kaposi's sarcoma-associated herpesvirus confers resistance to the cyclin-dependent kinase (cdk) inhibitors p16Ink4A, p21Cip1, and p27Kip1 on the associated cdk6. We have previously shown that K cyclin expression enforces S-phase entry on cells overexpressing p27Kip1 by promoting phosphorylation of p27Kip1 on threonine 187, triggering p27Kip1 down-regulation. Since p21Cip1 acts in a manner similar to that of p27Kip1, we have investigated the subversion of a p21Cip1-induced G1 arrest by K cyclin. Here, we show that p21Cip1 is associated with K cyclin both in overexpression models and in primary effusion lymphoma cells and is a substrate of the K cyclin/cdk6 complex, resulting in phosphorylation of p21Cip1 on serine 130. This phosphoform of p21Cip1 appeared unable to associate with cdk2 in vivo. We further demonstrate that phosphorylation on serine 130 is essential for K cyclin-mediated release of a p21Cip1-imposed G1 arrest. Moreover, we show that under physiological conditions of cell cycle arrest due to elevated levels of p21Cip1 resulting from oxidative stress, K cyclin expression enabled S-phase entry and was associated with p21Cip1 phosphorylation and partial restoration of cdk2 kinase activity. Thus, expression of the viral cyclin enables cells to subvert the cell cycle inhibitory function of p21Cip1 by promoting cdk6-dependent phosphorylation of this antiproliferative protein.
authors.affiliation: Molecular Cancer Biology Program, Institute of Biomedicine, Biomedicum Helsinki, P.O. Box 63, FIN-00014 University of Helsinki, Finland.
authors.forename: Annika
authors.initials: A
authors.lastname: Järviluoma
authors.forename: Emma S
authors.initials: ES
authors.lastname: Child
authors.forename: Grzegorz
authors.initials: G
authors.lastname: Sarek
authors.forename: Papinya
authors.initials: P
authors.lastname: Sirimongkolkasem
authors.forename: Gordon
authors.initials: G
authors.lastname: Peters
authors.forename: Päivi M
authors.initials: PM
authors.lastname: Ojala
authors.forename: David J
authors.initials: DJ
authors.lastname: Mann
mesh_terms: D016475:3T3 Cells / Q000378:metabolism; D000818:Animals; D051357:Cyclin-Dependent Kinase 2 / Q000235:genetics / Q000378:metabolism; D051361:Cyclin-Dependent Kinase 6 / Q000235:genetics / Q000378:metabolism; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics / Q000378:metabolism*; D016213:Cyclins / Q000235:genetics / Q000378:metabolism*; D016193:G1 Phase / Q000502:physiology*; D051379:Mice; D018384:Oxidative Stress; D010766:Phosphorylation; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D012694:Serine / Q000378:metabolism*; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: C493957:Cdkn1a protein, mouse; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; C483104:K cyclin protein, Herpesvirus 8, Human; D011994:Recombinant Proteins; D014764:Viral Proteins; D012694:Serine; C495893:Cdk2 protein, mouse; C495912:Cdk6 protein, mouse; D051357:Cyclin-Dependent Kinase 2; D051361:Cyclin-Dependent Kinase 6