Origin of corticosteroid-binding globulin in fetal rat. Comparative dynamics of corticosteroid-binding globulin, alpha-fetoprotein, and albumin secretion in primary cultures of fetal rat hepatocytes

The effect of dexamethasone on the adrenal gland in fetal and neonatal rats

The development and regeneration of the adrenal glands were examined by stereological and morphological methods in 20-day-old fetal, as well as 3-day- and 14-day-old neonatal male rats born to dams treated with dexamethasone (Dx) on day 16 of gestation. In the fetuses and 3-day-old rats, zona fasciculata (ZF) and zona reticularis (ZR) were analyzed as one inner zone (IZ), while in 14-day-old animals they were analyzed separately. Single Dx treatment (1.5 mg/kg b.w.) of the dams led to atrophic changes in the adrenal cortex of the fetuses. These changes were visible to a certain degree up to the 14th neonatal day. Administration of Dx to pregnant rats induced a significant decrease of both adrenal weight and volume, as well as the volume of zona glomerulosa + capsule (ZG + C) and IZ, both in fetuses and 3-day-old rats. This was due to a decrease in the number but not the volume of cortical cells. Also, necrotic cortical cells, infiltrations and resorption zones accompanied by the presence of macrophages, giant cells and lymphocytes were observed. In 14-day-old animals, the degree of atrophic changes in the adrenal cortex was reduced. Changes were observed only in ZR which was decreased in volume resulting from both a significant decrease of the volume and number of cortical cells. Then number of macrophages was somewhat increased, while giant cells were not present. However, the total number of parenchyma cells in ZG was increased, pointing to the possibility of renewal of cortical cells within this zone. The results of the present study demonstrate that even a single Dx dose given to pregnant rat during the period critical for the development of the hypothalamo-pituitary-adrenal system in the fetuses leads to marked changes in the structure and function of the fetal adrenal glands which are partially maintained up to the 14th day of postnatal life.

Antagonistic effects of retinoic acid and triiodothyronine in the expression of corticoid-binding globulin (CBG) by cultured fetal hepatocytes

Cultures of rat fetal hepatocytes were used to investigate the effects and interplay of triiodothyronine (T3) and retinoic acid (RA) in the regulation of gene expression of CBG, compared to that of alpha-fetoprotein (AFP). The cultured cells showed cytological features typical to hepatocytes and actually synthesized CBG and AFP, as evidenced from in situ hybridization with specific radioactive probes. Time course studies indicated that CBG (but not AFP) binding capacity in culture medium and cell mRNA levels disappeared with a half-life of about 2 days, thereby reflecting the decrease previously seen in hepatic CBG mRNA contents during embryonic life. The Kd values for CBG binding were unchanged under these conditions. Culturing of hepatocytes in the presence of T3 resulted in dose-dependent stimulations of both medium CBG and cell mRNA levels, with an EC50 concentration of about 10(-9) M. In sharp contrast, RA caused a reduction in CBG biosynthesis (IC50 = 1.7 x 10(-7) M) and, in addition, antagonized the stimulatory influence of T3. Under the same experimental conditions, AFP synthesis failed to be affected in a similar fashion. We conclude that thyroid hormones and RA directly act on hepatocytes to specifically regulate the expression of CBG in an antagonistic way.

Molecular studies of corticosteroid binding globulin structure, biosynthesis and function

Phylogenetic comparisons of the primary structure of corticosteroid binding globulin (CBG) have revealed several conserved domains that include sites for N-glycosylation and a region which probably represents a portion of the steroid binding site. The major site of CBG biosynthesis in adults is clearly the liver, and the human CBG gene promoter contains sequence elements that interact with liver-specific transcription factors. Low levels of CBG gene expression have been detected in other tissues, and these may be important for fetal development during late gestation when hepatic CBG mRNA levels are low. Studies of the ontogeny of CBG biosynthesis in the rat have also indicated that plasma CBG levels may be influenced by a more rapid clearance of the protein during pubertal development. Analyses of the structural organization and chromosomal location of the human CBG gene have further confirmed its close relationship with the serine proteinase inhibitors, and suggests that CBG, alpha 1-proteinase inhibitor and alpha 1-antichymotrypsin evolved relatively recently by gene duplication. The functional significance of this relationship has been examined and our studies suggest that a specific interaction between CBG and elastase on the surface of neutrophils may represent a physiologically important event that promotes the delivery of glucocorticoids to these cells at sites of inflammation.