Adrenocorticotropic hormone regulates the activities of the orphan nuclear receptor Nur77 through modulation of phosphorylation

ACTH treatment of Y1 adrenocortical cells induces the synthesis of Nur77, an orphan nuclear receptor that can act as a potent trans-activator for such genes as 21-hydroxylase (CYP21). Nur77 has thus been proposed to be a mediator of ACTH action in activating the expression of genes that encode steroidogenic enzymes. Here we show that ACTH regulates the activity of Nur77 at the level of phosphorylation. ACTH induces the synthesis of transcriptionally active, DNA-binding Nur77 that is unphosphorylated at Ser(354), which resides within the DNA-binding domain. By contrast, the Nur77 population that is constitutively present in Y1 cells is phosphorylated at Ser(354) and does not bind DNA. Substitutions of Ser(354) with negatively charged amino acids, such as Asp or Glu, dramatically decreased Nur77 DNA-binding and trans-activation activities, whereas mutation to the neutral Ala had no effect. Aside from phosphorylation within the DNA-binding domain, ACTH treatment does not induce modifications in the N- and C-terminal domains of Nur77 that significantly affect activity. Although the specific kinases that phosphorylate Nur77 in vivo are not known, the mitogen-activated protein kinase/pp90(RSK) pathway is not critical to Nur77 regulation. We propose that ACTH treatment of Y1 cells results in modulation of the activities of both kinases and phosphatases, which, in turn, regulate the activities of such transcription factors as Nur77.