LOCALIZATION AND REGULATION OF CORTICOTROPIN RECEPTOR EXPRESSION IN THE MIDGESTATION HUMAN FETAL ADRENAL-CORTEX - IMPLICATIONS FOR IN-UTEROHOMEOSTASIS

Developmental changes in the responsiveness of the fetal adrenals to c orticotropin (ACTH) play an important role in the regulation of the fe tal hypothalamic-pituitary-adrenal axis. Responsiveness of adrenal cor tical cells to ACTH is dependent on the extent of ACTH receptor expres sion. Therefore, we examined the localization and regulation of ACTH r eceptor expression in the midgestation (16-24 weeks) human fetal adren al cortex. In situ hybridization analysis was used to localize messeng er RNA (mRNA) encoding the ACTH receptor in sections of human fetal ad renal glands. Messenger RNA encoding the ACTH receptor was localized i n cells from all cortical zones; abundance was higher in definitive zo ne than in fetal zone cells and was least abundant in the more central portions of the cortex. Regulation of ACTH receptor expression was st udied using Northern blot analysis of total RNA extracted from primary cultures of fetal and definitive zone cells. Two major (1.5 and 3.5 k ilobases) and, upon stimulation with ACTH. 3 minor (4.0, 6.0 and 10.0 kb) ACTH receptor mRNA transcripts were detected in RNA from fetal and definitive zone cells. In both cell types. ACTH-(1-24) increased the abundance of mRNA encoding the ACTH receptor 10- to 20-fold compared w ith untreated cells. The effects of ACTH-(1-24) on ACTH receptor expre ssion in fetal zone cells were time- and dose-dependent. The ED,, for the stimulation of ACTH receptor expression by ACTH-(1-24) was 1-10 pM , and maximal response to 0.1 nm ACTH-(1-24) was detected after 12-16 h. Eight-bromoadenosine cAMP and forskolin also stimulated ACTH recept or expression in fetal zone cells and closely mimicked the effects of ACTH-(1-24). In contrast, stimulation of protein kinase C with 12-O-te tradccanoyl phorbol 13-acetate had no effect on ACTH receptor expressi on. Changes in ACTH receptor expression in response to ACTH-(1-24), cA MP and forskolin were paralleled by changes in expression of the P450 cholesterol side chain cleavage (P450scc) enzyme. These data demonstra te that expression of the ACTH receptor by the human fetal adrenal cor tex is up-regulated by its own ligand and that this effect is mediated by a cAMP-dependent mechanism. In addition, the coordinate stimulatio n of ACTH receptor and P450scc expression by ACTH indicates that the g ene for the ACTH receptor is one of a specific cohort of genes regulat ed by ACTH that are required to facilitate fetal adrenal cortical resp onse to ACTH. ACTH regulation of its own receptor may represent a mech anism by which fetal adrenal responsiveness to ACTH is maintained and possibly enhanced during fetal development.