TRANSCRIPTIONAL REGULATION OF THE BOVINE CYP17 GENE BY CAMP

The transcription of steroid hydroxylase genes is controlled by ACTH a nd cAMP in the adrenal cortex. In most instances the regulation appear s to rely on transcription factors traditionally not associated with c AMP-dependent gene expression. For the non-traditional factors it rema ins necessary to elucidate the coupling of increases in intracellular cAMP and cAMP-dependent protein kinase (PKA) activity to the function of these proteins. The bovine CYP17 gene, which encodes the steroid 17 alpha-hydroxylase, contains two discrete DNA elements within its prom oter and upstream region (CRS1 and CRS2) that individually can confer cAMP responsiveness. The CRS1 element is a target for PKA signalling a nd for negative regulation via the protein kinase C signal transductio n pathway. The homeodomain protein Pbx1 enhances CRS1-dependent transc ription, but additional CRS1-binding proteins remain to be identified. Furthermore it is not known how PKA regulates the activity of Pbx1 or its possible binding partners. Closer to the promoter, the nuclear or phan receptors SF-I and COUP-TF have overlapping binding sires in CRS2 and they bind in a mutually exclusive manner with very similar affini ties; 8 and 10 nM, respectively. SF-1 stimulates whereas COUP-TF inhib its transcription from the bovine CYP17 promoter. Together, the data s uggest that cAMP-dependent control of the amounts of the activator SF- I vs. the repressor COUP-TF could influence CRS2-dependent transcripti on. In addition, PKA may influence the phosphorylation of SF-I, thus i no-easing its activity. In vitro, PKA will elicit phosphorylation of S F-1. However, although SF-1 can be immunoprecipitated from adrenocorti cal cells as a phosphroprotein, we have not been able to show cAMP-dep endent increase in net phosphorylation in intact cells. More careful e xamination of individual phosphorylation sites in SF-1 may still revea l hormone- and cAMP-induced phosphorylation of SF-1. (C) 1997 by Elsev ier Science Inc.