Endocrine and neurogenic regulation of the orphan nuclear receptors Nur77 and Nurr-1 in the adrenal glands.

Abstract

nurr77 and nurr-1 are growth factor-inducible members of the steroid/thyroid hormone receptor gene superfamily. In order to gain insight into the potential roles of nur77 in the living organism, we used pharmacologic treatments to examine the expression of nur77 in the mouse adrenal gland. We found that nur77 and nurr-1 are induced in the adrenal gland upon treatment with pentylene tetrazole (Ptz; Metrazole). This induction is separable into distinct endocrine and neurogenic mechanisms. In situ hybridization analysis demonstrates that nur77 expression upon Ptz treatment in the adrenal cortex is localized primarily to the inner cortical region, the zona fasciculata-reticularis, with minimal induction in the zona glomerulosa. This induction is inhibitable by pretreatment with dexamethasone, indicating involvement of the hypothalamic-pituitary-adrenal axis in the activation of adrenal cortical expression. When mice were injected with adrenocorticotrophic hormone (ACTH), nur77 expression in the adrenal gland spanned all cortical layers including the zona glomerulosa, but medullary expression was not induced. Ptz also induces expression of both nur77 and nurr-1 in the adrenal medulla. Medullary induction is likely to have a neurogenic origin, as nur77 expression was not inhibitable by dexamethasone pretreatment and induction was seen after treatment with the cholinergic neurotransmitter nicotine. nur77 is also inducible by ACTH, forskolin, and the second messenger analog dibutyryl cyclic AMP in the ACTH-responsive adrenal cortical cell line Y-1. Significantly, Nur77 isolated from ACTH-stimulated Y-1 cells bound to its response element whereas Nur77 present in unstimulated cells did not. Moreover, Nur77 in ACTH-treated Y-1 cells was hypophosphorylated at serine 354 compared with that in untreated cells. These results, taken together with the previous observation that dephosphorylation of serine 354 affects DNA binding affinity in vitro, show for the first time that phosphorylation of Nur77 at serine 354 is under hormonal regulation, modulating its DNA binding affinity. Thus, ACTH regulates Nur77 in two ways: activation of its gene and posttranslational modification. A promoter analysis of nur77 induction in Y-1 cells indicates that the regulatory elements mediating ACTH induction differ from those required for induction in the adrenal medullary tumor cell line PC12 and in 3T3 fibroblasts.