REGULATED ECTOPIC EXPRESSION OF CYCLIN D1 INDUCES TRANSCRIPTIONAL ACTIVATION OF THE CDK INHIBITOR P21 GENE WITHOUT ALTERING CELL-CYCLE PROGRESSION

Cyclin D1 plays a key regulatory role during the G(1) phase of the cel l cycle and its gene is amplified and overexpressed in many cancers. T o address the relationship between cyclin D1 and other cell cycle regu latory proteins, we established human glioma and rodent fibroblast cel l lines in which cyclin D1 expression could be regulated ectopically w ith tetracycline. In both of these cell lines, we found that ectopic e xpression of cyclin D1 in asynchronously growing cells was accompanied by increased levels of the p53 tumor suppressor protein and the cycli n/cdk inhibitor p21. Despite the induction of these cell cycle inhibit ory proteins, cyclin D1-associated cdk kinase remained activated and t he cells grew essentially like that of the parent cells. Although grow th parameters were unchanged in these cells, morphological changes wer e clearly identifiable and anchorage independent growth was observed i n NIH3T3 cells. In a first step toward elaborating the mechanism for c yclin D1-mediated induction of p21 gene expression we show that co-exp ression of E2F-1 and DP-1 can specifically transactivate the p21 promo ter. In support of these findings and a direct effect of E2F on induct ion of p21 gene expression a putative E2F binding site was identified within the p21 promoter. In summary, our results demonstrate that ecto pic expression of cyclin D1 can induce gene expression of the cdk inhi bitor p21 through an E2F mechanism the consequences of which are not t o growth arrest tells but possibly to stabilize cyclin D1/cdk function .