INSULIN-LIKE GROWTH FACTOR-II (IGF-II) MESSENGER-RIBONUCLEIC-ACID IS EXPRESSED IN STEROIDOGENIC CELLS OF THE DEVELOPING OVINE ADRENAL-GLAND - EVIDENCE OF AN AUTOCRINE PARACRINE ROLE FOR IGF-II

Insulin-like growth factors (IGFs) are potent mitogenic and differentiation-promoting factors that regulate the growth and development of many fetal tissues. Their role in the development of the adrenal gland and activation of its function is not known. The latter is crucial in providing the stimulus for the maturation of various fetal organs and determines the onset of parturition in sheep. To examine the hypothesis that IGFs are important autocrine/paracrine regulators of fetal adrenal development in vivo, we localized IGF-I and IGF-II mRNAs and peptides in the adrenal glands of developing sheep fetuses and correlated the cellular distribution with localization of 3beta-hydroxy-steroid dehydrogenase, tyrosine hydroxylase, and phenylethanolamine-N-methyltransferase enzymes by immunohistochemistry. Adrenal glands from 60- to 75-day-old (n = 4), 100- to 110-day-old (n = 4), 120- to 130-day-old (n = 4), and 145- to 147-day-old (term; n = 4) fetal sheep and 1- to 4-day-old newborn lambs (n = 4) were dissected and either snap-frozen or fixed. Total RNAs were subjected to Northern analysis using ovine IGF-I and IGF-II cDNA probes. Seven IGF-II transcripts of 1.2-6.0 kilobases (kb) were identified in the adrenal glands of fetuses at all gestational ages, and in the newborn. By densitometry, the abundance of IGF-II mRNA was highest in the fetal adrenal gland at 60 days, decreased slightly between 60 and 100 days, remained relatively constant until term, and decreased significantly after birth. At all gestational ages, IGF-II mRNA was detectable in significantly greater abundance than IGF-I mRNA. IGF-I and IGF-II mRNAs were localized by in situ hybridization using S-35-labeled antisense cRNA probes, and the peptides by immunohistochemistry using specific antisera. Low levels of IGF-I mRNA were detected in the zona fasciculata, but not in the zona glomerulosa. There was strong hybridization of the IGF-II cRNA to the zona glomerulosa and fasciculata and to the capsule. The hybridization signal was greater in the zona fasciculata than in the zona glomerulosa. IGF-II mRNA was also detected in groups of cells within the medulla. Localization of IGF-II mRNA by in situ hybridization correlated well with the distribution of IGF-II immunoreactivity and with 3beta-hydroxysteroid dehydrogenase-positive cells in the cortex and in groups of cells within the medulla. IGF-II mRNA was colocalized with phenylethanolamine-N-methyl-transferase-immunoreactive (epinephrine) cells at 60 days, but not at later gestational ages; it was not colocalized with the strongly positive class of tyrosine hydroxylase-immunoreactive (norepinephrine) cells. These findings indicate the close association in the fetal adrenal gland of IGF-II mRNA and peptide with epinephrine cells within the medulla in early gestation and with steroidogenic cells within the cortex and medulla throughout pregnancy. Our results strongly support the hypothesis that IGF-II and, to a lesser extent, IGF-I play important roles in the development of fetal adrenal medullary cells in early pregnancy and in fetal adrenal cortical cells throughout pregnancy by autocrine and/or paracrine mechanisms.