An enhancer element and a functional cyclic AMP-dependent protein kinase are required for expression of...

The steroid 21-hydroxylase (21-OHase) gene is selectively expressed at high levels in cells of the adrenal cortex and is transcriptionally regulated by corticotropin (ACTH). In this study, we examined the contribution of cis-acting nucleotide sequences to the regulated expression of the mouse 21-OHase gene. The 5'-flanking sequences of the mouse 21-OHase gene, extending 330 bp upstream from the transcription initiation site, were placed in front of the human growth hormone (hGH) reporter gene, and expression of the fusion gene was measured following transient transfection in Y1 mouse adrenocortical tumor cells. The 330 bp of 21-OHase flanking sequence directed both basal and ACTH-stimulated expression of hGH in Y1 adrenocortical cells but did not direct hGH expression in I-10 mouse testicular Leydig cells or in mouse fibroblast L cells. The 21-OHase/hGH fusion gene was poorly expressed in Y1 mutants defective in cAMP-dependent protein kinase activity. These results indicate that sequences necessary for adrenal cell-selective and ACTH-regulated expression of the 21-OHase gene reside within the first 330 bp of 5'-flanking DNA and that constitutive expression of the gene requires the integrity of cAMP-dependent protein kinase. The constitutive expression of hGH in Y1 cells was decreased dramatically (40-fold) when the 21-OHase flanking sequences in front of hGH were shortened to 156 bp from the transcription initiation site and was restored when the upstream sequences of the 21-OHase gene, from -330 to -150, were added back; the sequences from -330 to -150 were equally effective in either the correct or reverse orientation. From these observations, we conclude that an enhancer element is contained within the sequences from -330 to -150 bp upstream of the 21-OHase transcription initiation site.