BIPHASIC DEVELOPMENTAL EXPRESSION OF ADRENOCORTICOTROPIN RECEPTOR MESSENGER-RIBONUCLEIC-ACID LEVELS IN THE BABOON FETAL ADRENAL-GLAND

We have previously shown an estrogen-dependent developmental regulatio n of placental oxidation of cortisol to cortisone that results in enha nced fetal pituitary ACTH production and the induction of steroidogeni c enzymes in and de novo cortisol production by the fetal adrenal in t he second half of baboon pregnancy. However, it is not known whether t he receptor for ACTH is simultaneously generated at this time in devel opment to provide a mechanism for mediating the tropic action of ACTH on steroidogenesis in the primate fetal adrenal gland. Therefore, in t he present study we determined the levels of ACTH receptor messenger R NA (mRNA) and correlated ACTH receptor expression with appearance of t he mRNA for Delta 5-3 beta-hydroxysteroid dehydrogenase/isomerase (3 b eta HSD), the enzyme protein that signals functional maturation of the definitive cortical zone in the baboon fetal adrenal. A baboon ACTH r eceptor complementary DNA was cloned and hybridized with polyadenylate d RNA isolated from baboon (Papio anubis) fetal adrenals obtained in e arly (days 58-64; RNA from seven baboon fetuses pooled to yield three samples), mid(days 99-103; RNA from five baboons pooled to yield four samples), and late (days 165-168; RNA of four individual baboon fetuse s) gestation (term = 184 days). Expression of the primary 3.4-kilobase ACTH receptor mRNA transcript, determined by Northern blot and expres sed as a ratio of beta-actin mRNA, was minimal early in gestation (mea n +/- SE, 0.11 +/- 0.05 arbitrary densitometric units). However, fetal adrenal ACTH receptor mRNA levels increased (P < 0.001, by ANOVA) app roximately 13-fold to 1.41 +/- 0.16 at midgestation, then declined by 70% (P < 0.001) to 0.41 +/- 0.10 in late gestation. To determine wheth er the decrease in ACTH receptor expression by the fetal adrenal in th e second half of pregnancy reflected programmed cell death, the integr ity of genomic DNA was assessed by P-32-labeled DNA gel electrophoresi s and in situ DNA end labeling. Because DNA oligonucleosomes and apopt otic DNA strand breaks characteristic of apoptosis were absent in the adrenal glands of fetal baboons, the decline in ACTH receptor mRNA lev els in the fetal adrenal did not seem to reflect programmed cell death . Expression of the single 2.0-kilobase mRNA transcript for 3 beta HSD , an enzyme localized specifically in the definitive zone of the fetal adrenal, was minimal in early (0.01 +/- 0.00 arbitrary units) and mid - (0.10 +/- 0.01) gestation. However, 3 beta HSD mRNA levels were mark edly increased late in gestation to a value (1.38 +/- 0.34) approximat ely 13-fold greater (P < 0.001)than that in midgestation. These findin gs indicate that there was a biphasic monomodal developmental expressi on of the ACTH receptor in the baboon fetal adrenal, which contrasted with the progressive increase in adrenal weight, 3 beta HSD expression , and de novo cortisol production previously determined. Because the f etal adrenal is comprised mainly of the fetal cortical zone throughout gestation, the decrease in ACTH receptor expression between mid- and late gestation seems to occur primarily in the latter zone and may sig nal a selective decline in tropic responsivity of and Delta(5)-C-19-st eroid, e.g. dehydroepiandrosterone, biosynthesis within the baboon fet al adrenal gland.