THYROID-HORMONE INFLUENCES THE MATURATION OF APOLIPOPROTEIN-A-I MESSENGER-RNA IN RAT-LIVER

Chronic administration of thyroid hormone (T-3) increases apolipoprote in (ape) A-I gene expression in rat liver, That transcriptional activi ty of the apoA-I gene is reduced to 50% of control, whereas abundance levels of nuclear and total cellular apoA-I mRNA are increased 3-fold, implies more effective apoA-I mRNA maturation. To study hormonal effe cts on apoA-I RNA processing, we quantified mRNA precursors in control and T-3-treated rats (50 mu g/100 g body weight for 7 days). Northern blotting, amplification of reverse-transcribed RNA, and ribonuclease protection assays showed that the splicing pathway is branched, in tha t either intron 1 or intron 2 is removed first from the primary transc ript, whereas intron 3 is removed last. In T-3-treated rats, abundance levels of the primary transcript, the intron 1-containing precursor d evoid of intron 2, the intron 2 containing precursor devoid of intron 1, the intron S-containing precursor lacking both introns 1 and 2, and nuclear mRNA were 65, 183, 78, 195, and 268% of controls, Compared wi th control rats, the half-life of the intron 1-containing precursor, m easured after injection of actinomycin D, was increased 2-fold in T-3- treated rats. In contrast, half-lives of the primary transcript and th e intron 2-containing precursor were similar in control and T-3-treate d rats. Ribonuclease protection assays revealed an RNA species extendi ng from the transcription start site close to the 3' end of intron 1. The abundance of this RNA fragment, probably representing a degradatio n product, was 2.5-fold higher in control than in T-3-treated animals (p < 0.001). Sequences of apoA-I mRNA precursors were identical in con trol and T-3-treated rats which excluded hormonal effects on splice-si te selection or post-transcriptional editing of apoA-I transcripts. Co mpartmental modeling of apoA-I mRNA processing suggested that chronic thyroid hormone administration enhances apoA-I mRNA maturation more th an 7-fold by protecting the intron 1-containing precursor devoid of in tron 2 from degradation and by facilitating the splicing of intron 1 f rom this precursor.