Transcriptional regulation of the bovine CYP17 gene by cAMP

The transcription of steroid hydroxylase genes is controlled by ACTH and cAMP in the adrenal cortex. In most instances the regulation appears to rely on transcription factors traditionally not associated with cAMP-dependent gene expression. For the non-traditional factors it remains 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 promoter and upstream region (CRS1 and CRS2) that individually can confer cAMP responsiveness. The CRS1 element is a target for PKA signalling and for negative regulation via the protein kinase C signal transduction pathway. The homeodomain protein Pbx1 enhances CRS1-dependent transcription, 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 orphan receptors SF-1 and COUP-TF have overlapping binding sites in CRS2 and they bind in a mutually exclusive manner with very similar affinities; 8 and 10 nM, respectively. SF-1 stimulates whereas COUP-TF inhibits transcription from the bovine CYP17 promoter. Together, the data suggest that cAMP-dependent control of the amounts of the activator SF-1 vs. the repressor COUP-TF could influence CRS2-dependent transcription. In addition, PKA may influence the phosphorylation of SF-1, thus increasing its activity. In vitro, PKA will elicit phosphorylation of SF-1. However, although SF-1 can be immunoprecipitated from adrenocortical cells as a phosphroprotein, we have not been able to show cAMP-dependent increase in net phosphorylation in intact cells. More careful examination of individual phosphorylation sites in SF-1 may still reveal hormone- and cAMP-induced phosphorylation of SF-1.