cAMP-dependent transcription of steroidogenic genes in the human adrenal cortex requires a dual-specificity phosphatase in addition to protein kinase A.

Steroid hormone biosynthesis in the adrenal cortex is controlled by the peptide hormone adrenocorticotropin (ACTH), which acts to increase intracellular cAMP and results in the activation of cAMP-dependent protein kinase A (PKA) and subsequent increase in steroidogenic gene transcription. Protein phosphorylation by PKA activates transcription of genes encoding steroidogenic enzymes; however the precise proteins which are phosphorylated remain to be determined. We have recently shown that phosphoprotein phosphatase (PP) activity is essential for cAMP-dependent transcription of the human CYP17 (hCYP17) gene in H295R adrenocortical cells. The aim of our current studies was to determine if inhibition of PP activity attenuates cAMP-dependent mRNA expression of other steroidogenic genes in H295R cells. Using various inhibitors of serine/threonine and tyrosine PPs, we examined the role of phosphatase activity on cAMP-dependent transcription of steroidogenic genes in the adrenal cortex. CYP11A, CYP11B1/2, CYP21, and adrenodoxin also require PP activity for cAMP-stimulated gene expression. Inhibition of both serine/threonine and tyrosine PP activities suppresses the cAMP-dependent mRNA expression of several steroidogenic genes, suggesting that a dual-specificity PP is essential for conveying ACTH/cAMP-stimulated transcription. We propose that PKA phosphorylates and activates a dual-specificity phosphatase, which mediates steroidogenic gene transcription in response to ACTH/cAMP.