ACTH and AII differentially stimulate steroid hormone orphan receptor mRNAs in adrenal cortical cells

Abstract

NGFI-B and Ad4BP are steroid hormone receptor-like transcription factors that may control steroidogenesis, growth and differentiation in the adrenal cortex. We have studied the induction of NGFI-B and Ad4BP mRNAs by the peptide hormones, ACTH, AII, IGF, FGF, and by KCl depolarization in cultured bovine adrenocortical cells. The mRNAs for these two transcription factors were most effectively but differentially induced by ACTH and AII. mRNA for NGFI-B was typically undetectable in unstimulated cells, but rapidly (< 30 min) accumulated in response to ACTH and AII. Peak increases occurred within 2–3 h after which mRNA levels declined. At maximally effective concentrations, AII produced increases in NGFI-B mRNA 2.7-fold larger than those triggered by ACTH ( n = 7). In contrast to NGFI-B, Ad4BP mRNA was readily detectable in unstimulated cells. ACTH and AII induced smaller, slower and more sustained increases in Ad4BP mRNA. Peak values were obtained in 6–8 h and Ad4BP mRNA remained elevated for at least 18 h. ACTH produced increases in Ad4BP that were 2.6-fold larger than those stimulated by AII ( n = 8). Antagonists of major signaling pathways that couple ACTH and AII receptors to cortisol secretion, including T-type Ca 2+ antagonists Ni 2+ and penfluridol, the CaM kinase antagonist KN-62, the A-kinase antagonist H-89 and the non-selective kinase antagonist staurosporine, all failed to suppress increases in NGFI-B and Ad4BP mRNAs triggered by these two peptides. Each of these agents effectively inhibited cortisol production stimulated by the peptides. Further, arguing against their proposed role as transcription factors for steroidogenic enzymes, ACTH- and AII-stimulated increases in steroid orphan receptor mRNAs were not correlated with corresponding increases in cortisol production measured over 24 h. The results show that NGFI-B and Ad4BP mRNAs are differentially regulated by ACTH and AII. Only NGFI-B is rapidly and transiently increased with kinetics common to immediate early genes. The lack of correlation between peptide-stimulated increases in orphan receptor mRNAs and cortisol production in combination with the apparent divergence in the associated signaling pathways argue against a primary role for these transcription factors in ACTH- and AII-stimulated steroidogenesis. The dual function of these peptide hormones as mediators of development and corticosteroid synthesis could necessitate the presence of separate, parallel signaling pathways.