Developmental regulation of maturation and steroidogenesis in the baboon fetal adrenal gland

Abstract

In the second half of baboon pregnancy, the inner fetal adrenocortical zone of the fetal adrenal gland exhibits a decline in functional capacity to form C19-steroid precursors for estrogen production, while the outer definitive zone undergoes developmental maturation and enhanced capacity to form cortisol important for fetal lung maturation. The role of pituitary adrenocorticotropin (ACTH) upon this aspect of fetal adrenal development and function, however, has not been previously investigated. The present study used pregnant baboons to investigate in vivo the role of ACTH and its receptor upon this divergence in fetal adrenal cortical zone-specific function. ACTH receptor mRNA levels, determined by Northern analysis, increased (P<0.01, ANOVA) 13-fold between early to midgestation in the baboon fetal adrenal, then declined by 70% in the second half of pregnancy. ACTH receptor mRNA levels determined by in situ hybridization, displayed a 50% decrease in the fetal zone and a 3-fold increase in the definitive zone of the fetal adrenal cortex between mid and late gestation. The mRNAs for the P-450 cholesterol side chain cleavage and P-450 17alpha-hydroxylase 17/20-lyase enzymes were expressed at constant levels throughout gestation. In contrast, mRNA expression of the Delta\5-3beta-hydroxysteroid dehydrogenase (3beta-HSD) enzyme, which was localized specifically to definitive zone cells, was minimal in early and midgestation and increased (P<0.01) 13-fold in late gestation. To determine the role of ACTH on fetal adrenal function, baboons were administered betamethasone which decreased (P<0.05) fetal pituitary proopiomelanocortin mRNA expression at mid and late gestation. This resulted in apoptosis, cellular disorganization, loss of 3beta-HSD mRNA expression, and a decline in ACTH receptor and P-450 mRNA levels in the fetal adrenal gland. Concomitant administration of ACTH to suppressed fetuses restored adrenal gland integrity, 3beta-HSD expression, and partially restored the ACTH receptor and P-450 enzymes mRNAs. Consequently, ACTH appears essential for the development of the fetal and definitive zones, as well as the coordinated increase in ACTH receptor and P-450 enzyme components critical to steroidogenesis at mid and late gestation. Very different patterns of mRNA expression for insulin-like growth factor(IGF)-II and its principal IGF type-1 receptor and binding protein-2, and basic fibroblast growth factor were observed in the baboon fetal adrenal with advancing gestation. This differential pattern of peptide growth factor expression may reflect functionally important differences in cellular localization, and a divergence in functional development of the fetal and definitive adrenocortical zones. We conclude that the decrease in ACTH receptor mRNA expression and capacity to form C19-steroids in the second half of gestation reflect a selective decline in tropic responsivity and functional capacity of the fetal zone, while the increase in 3beta-HSD mRNA expression and de novo cortisol formation result from the ACTH receptor-mediated development and functional capacity of the definitive zone. Consequently, we propose that there is an uncoupling of ACTH receptor-mediated adrenocortical zone-specific steroidogenesis within the primate fetal adrenal gland during the second half of primate pregnancy.