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Lower Cyclin H and Cyclin-dependent Kinase-activating Kinase Activity in Cell Cycle Arrest Induced by Lack of Adhesion to Substratum¹

IVIC, Centre of Microbiology and Cell Biology, Caracas 1020 A, Venezuela

Knowledge about adhesion checkpoints is important to counteract dissemination of cells from solid tumors. Lack of anchorage in adherent cells is associated with growth arrest and inhibition of cyclin-dependent kinases (cdks) required to drive cell cycle progression. Because cyclin-cdk complex activation requires CDK-activating kinase comprising cdk7 and cyclin H, we now investigated their relationship to decreased proliferation by lack of cell spreading. This report shows that either UV irradiation on an adhesive substrate or culture on a nonadhesive substrate produced K1735 melanoma growth arrest. Inhibition of proliferation by UV primarily induced the cdk inhibitor p21WAF1 without a significant effect on cyclin H and cdk7. In contrast, lack of adhesion to substratum decreased cyclin H but not cdk7 with accumulation of a slower migrating, presumably unphosphorylated cdk4 isoform. These results were paralleled by decreased cdk7-mediated phosphorylation of GST-cdk2 and lower activation of a baculovirus-derived cdc2-cyclin B kinase complex. This is the first report showing that cyclin H-mediated down-regulation of cdk-activating kinase activity is involved in growth arrest induced by lack of anchorage.