ROLE OF THE CCAAT ENHANCER BINDING PROTEIN-ALPHA TRANSCRIPTION FACTORIN THE GLUCOCORTICOID STIMULATION OF P21(WAF1/CIP1) GENE PROMOTER ACTIVITY IN GROWTH-ARRESTED RAT HEPATOMA-CELLS/

The preceding paper (Cha, H. H., Cram, E. J., Wang, E. C., Huang, A. J ., Kasler, R. G., and Firestone, G. L. (1998) J. Biol. Chem. 273, 0000 -0000(478563) defined a glucocorticoid responsive region within the pr omoter of the p21 CDK inhibitor gene that contains a putative DNA-bind ing site for the transcription factor CCAAT/enhancer binding protein-a lpha (C/EBP alpha). Wild type rat BDS1 hepatoma cells as well as as4 h epatoma cells, which: express antisense sequences to C/EBP alpha and a blate its protein production, were utilized to investigate the role of this transcription factor in the glucocorticoid regulation of p21 gen e expression, The stimulation of p21 protein levels and promoter activ ity, as well as inhibition of CDR2-mediated retinoblastoma protein pho sphorylation, by the synthetic glucocorticoid, dexamethasone, required the expression of C/EBP alpha. Overexpression of C/EBP alpha in as4 c ells rescued the dexamethasone responsiveness of the p21 promoter, Sit e-directed mutagenesis of the p21 promoter revealed that dexamethasone stimulation of p21 promoter activity required the C/EBP consensus DNA -binding site, Furthermore, in glucocorticoid receptor-defective EDR1 hepatoma cells, dexamethasone failed to stimulate C/EBP alpha and p21 protein expression and promoter activities, Our results have establish ed a functional link between the glucocorticoid receptor signaling pat hway that mediates a G(1) cell cycle arrest of rat hepatoma cells and the transcriptional control of p21 by a cascade that requires the ster oid induction of C/EBP alpha gene expression.