Abstract
The passage of mammalian cells through the restriction point into the S phase of the cell cycle is regulated by the activities of Cdk4 and Cdk6 complexed with the D-type cyclins and by cyclin E/Cdk2. The activities of these holoenzymes are constrained by CDK inhibitory proteins. The importance of the restriction point is illustrated by its deregulation in many tumour cells and upon infection with DNA tumour viruses. Here we describe the properties of cyclins encoded by two herpesviruses, herpesvirus saimiri (HVS) which can transform blood lymphocytes and induce malignancies of lymphoid origin in New World primates, and human herpesvirus 8 (HHV8) implicated as a causative agent of Kaposi's sarcoma and body cavity lymphomas. Both viral cyclins form active kinase complexes with Cdk6 that are resistant to inhibition by the CDK inhibitors p16(Ink4a), p21Cip1 and p27Kip1. Furthermore, ectopic expression of a viral cyclin prevents G1 arrest imposed by each inhibitor and stimulates cell-cycle progression in quiescent fibroblasts. These results suggest a new mechanism for deregulation of the cell cycle and indicate that the viral cyclins may contribute to the oncogenic nature of these viruses.
Publication types
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Research Support, Non-U.S. Gov't
MeSH terms
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3T3 Cells
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Animals
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Cell Cycle
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Cell Cycle Proteins*
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Cell Line
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Cyclin-Dependent Kinase 6
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Cyclin-Dependent Kinase Inhibitor p16 / metabolism
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Cyclin-Dependent Kinase Inhibitor p21
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Cyclin-Dependent Kinase Inhibitor p27
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Cyclin-Dependent Kinases*
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Cyclins / antagonists & inhibitors
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Cyclins / genetics
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Cyclins / metabolism*
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Enzyme Inhibitors / metabolism
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Herpesvirus 2, Saimiriine / genetics
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Herpesvirus 2, Saimiriine / metabolism*
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Herpesvirus 8, Human / genetics
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Herpesvirus 8, Human / metabolism*
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Mice
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Microtubule-Associated Proteins / metabolism
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Protein Serine-Threonine Kinases / antagonists & inhibitors
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Protein Serine-Threonine Kinases / genetics
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Protein Serine-Threonine Kinases / metabolism*
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Tumor Suppressor Proteins*
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Viral Proteins / genetics
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Viral Proteins / metabolism*
Substances
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Cdkn1a protein, mouse
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Cdkn1b protein, mouse
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Cell Cycle Proteins
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Cyclin-Dependent Kinase Inhibitor p16
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Cyclin-Dependent Kinase Inhibitor p21
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Cyclins
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Enzyme Inhibitors
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Microtubule-Associated Proteins
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Tumor Suppressor Proteins
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Viral Proteins
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Cyclin-Dependent Kinase Inhibitor p27
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Protein Serine-Threonine Kinases
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Cdk6 protein, mouse
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Cyclin-Dependent Kinase 6
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Cyclin-Dependent Kinases