Induction of tyrosine aminotransferase mRNA by glucocorticoids and cAMP in fetal rat liver.

Abstract

Tyrosine aminotransferase (L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) enzyme and mRNA activity were not detectable in day 20 fetal rat liver. Precocious induction of catalytic activity by in utero injection of dibutyryl cAMP was a direct consequence ofthe de novo appearance oftranslatable tyrosine aminotransferase mRNA. In contrast, in utero injection ofhydrocortisone acetate failed to elicit fetal liver enzyme activity. This failure was due to the inability of the steroid hormone to induce the appearance of tyrosine aminotransferase mRNA activity. In fetal rat liver explants, either compound was capable of stimulating the synthesis of adult levels of enzyme and mRNA. However, catalytic and mRNA activity comparable with that seen in vivo 24 hr after birth required the concerted action of both inducers.One of the central problems in the study of development is the determination of the factors responsible for differential gene expression and the appearance of tissue-specific proteins. The enzymatic differentiation of developing rat liver has proved to be a useful model for these studies. During development, most rat liver enzymes accumulate rapidly and in discrete groups or clusters: the late fetal cluster (16-22 days ofgestation), the neonatal cluster, and the late suckling cluster (1).Tyrosine aminotransferase (L-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) is one of the enzymes appearing in the neonatal cluster (1). Enzyme activity, virtually absent in fetal rat liver, starts to increase 2 hr after birth, reaching a maximum of at least twice the adult level by 12 hr. The catalytic activity then decreases to adult level by 2 days after birth (2, 3). Premature appearance of tyrosine aminotransferase can be elicited in fetal rats by in utero injection of glucagon (4, 5) or dibutyryl cAMP (6); glucocorticoid injection does not precociously induce catalytic activity (4, 6). However, all three compounds induce enzyme activity in vitro (7-9).As part of our studies on the regulation of tyrosine aminotransferase gene expression, we are investigating the process responsible for the appearance of tyrosine aminotransferase in newborn rats. We present evidence that the appearance of enzyme activity after precocious induction in utero is a direct consequence of the de novo accumulation of mRNA coding for tyrosine aminotransferase. In addition, we have used fetal rat liver explants to correlate tyrosine aminotransferase mRNA and enzyme activity during induction by dibutyryl cAMP and hydrocortisone acetate.MATERIALS AND METHODS Chemicals. Cortef acetate (hydrocortisone acetate, 50 mg/ ml) was provided by Upjohn. BGJb medium, Fitton-Jackson modification, and penicillin/streptomycin (penicillin, 10,000 units/ml; streptomycin, 10,000 Ag/ml) were from GIBCO.Translation grade [3S]methionine (specific activity, 1350-1450 Ci/mmol; 1 Ci = 3.7 X 101°becquerels) was from Amersham. Cordycepin, protein A-Sepharose, and other biochemicals were from Sigma.Animals. Sprague-Dawley (CD) rats from Charles River Br...

Development of tyrosine aminotransferase in perinatal rat liver: Changes in functional messenger RNA and the role of inducing hormones

“…Earlier work at the enzyme level has indicated that synthesis of the enzyme in fetal liver is not responsive to glucocorticoids [ Figure 5 show that the aminotransferase mRNA activity of unfractionated poly(A)-RNA from fetal liver remains undectable after glucocorticoid treatment, in accord with the failure of enzyme levels to change. Similar results have recently been reported by Ruiz-Bravo and Ernest [26]. By 24 hr after birth, after the differentiation event has occurred, the steroid-elicited response is readily observed in the translation assay.…”

The tissue-specific extinguisher locus TSE1 encodes a regulatory subunit of cAMP-Dependent protein kinase

“…This is accompanied by an increase in the newborn of hormones acting via the CAMP pathway, owing to postnatal hypoglycemia. These hormones seem to play a role in triggering the developmentally programmed onset of expression, as premature activation of the TAT gene can be elicited by administration of CAMP in utero (Greengard, 1970;Ruiz-Bravo and Ernest, 1982). TSEl activity, as determined by the fusion assay and Rla expression (data not shown) are high in mouse hepatoma ceils expressing fetal liver markers but low in adult hepatocytes (Gourdeau et al, 1989).…”

Isolation of cDNA clones coding for rat tyrosine aminotransferase.

“…Premature induction of TyrATase activity can be elicited in fetal rats by in utero injection of Bt2cAMP but not by hydrocortisone (10). In contrast, both compounds induce TyrATase activity prematurely in fetal rat liver explants (11).…”