GLUCOCORTICOIDS STIMULATE P21 GENE-EXPRESSION BY TARGETING MULTIPLE TRANSCRIPTIONAL ELEMENTS WITHIN A STEROID-RESPONSIVE REGION OF THE P21(WAF1 CIP1) PROMOTER IN RAT HEPATOMA-CELLS/

Glucocorticoids can induce a G(1) arrest in the cell cycle progression of BDS1 rat hepatoma cells. In these cells, dexamethasone, a syntheti c glucocorticoid, stimulated a rapid:and selective increase in express ion of the p21 cyclin-dependent kinase (CDR) inhibitor mRNA and protei n and virtually abolished CDK2 phosphorylation of the retinoblastoma p rotein. Expression of the p27 CDK inhibitor, and other G(1)-acting cel l cycle proteins, remained unaffected, Dexamethasone stimulated p21 pr omoter activity in a p53-independent manner that required functional g lucocorticoid receptors. Transforming growth factor-beta, which also i nduced a G, cell cycle arrest of the hepatoma cells, failed to elicit this response, Analysis of 5' deletions of the p21 promoter uncovered a glucocorticoid responsive region between nucleotides -1481 and -1184 , which does not contain a canonical glucocorticoid response element b ut which can confer dexamethasone responsiveness to a heterologous pro moter. Fine mapping of this region uncovered three distinct 50-60-base pair transcriptional elements that likely function as targets of gluc ocorticoid receptor signaling. Finally, ectopic expression of p21 had no effect oh hepatoma cell growth in the absence of glucocorticoids bu t facilitated the ability of dexamethasone to inhibit cell proliferati on. Thus, our results have established a direct transcriptional link b etween glucocorticoid receptor signaling and the regulated promoter ac tivity of a CDK inhibitor gene that is involved in the cell cycle arre st of hepatoma cells.