Transcriptional control of ornithine aminotransferase synthesis in rat kidney by estrogen and thyroid hormone.

Ornithine aminotransferase, a mitochondrial matrix enzyme, is expressed constitutively in many mammalian tissues and is subject to complex hormonal and dietary regulation in rat liver and kidney. The kidney enzyme is induced by estrogen and thyroid hormone (3,3',5-triiodo-L-thyronine, T3). We demonstrated previously that the hormonal induction in kidney is the result of an increase in ornithine aminotransferase mRNA levels (Mueckler, M. M., and Pitot, H. C. (1983) J. Biol. Chem. 258, 1781-1784). In order to define further the level at which the enzyme is induced in the kidney, the transcriptional activity of the ornithine aminotransferase gene was measured after hormonal induction. This was accomplished by incubating purified kidney nuclei in an in vitro transcription reaction in the presence of [32P] UTP and hybridizing the nascent, 32P-labeled, nuclear RNA to cloned ornithine aminotransferase cDNA immobilized on nitrocellulose filters. Administration of estrogen resulted in an increase in the rate of ornithine aminotransferase gene transcription by 1 h, and the rate remained elevated until at least 20 h after administration. The activity of the gene was elevated 12 h after T3 administration. A lag time of at least 4 h was observed for the effect of T3 on ornithine aminotransferase gene transcription. When administered in combination, estrogen and T3 exerted a synergistic effect on the activity of the gene. The administration of estrogen or both estrogen and T3 also resulted in the increased steady state level of two putative ornithine aminotransferase nuclear precursor RNAs, 3190 and 4260 nucleotides in length.