TISSUE-SPECIFIC REGULATION OF 2 FUNCTIONAL MALIC ENZYME MESSENGER-RNAS BY TRIIODOTHYRONINE

Rat liver malic enzyme (ME) synthesis is known to be regulated by 3,5,3''-triiodo-L-thyronine (T3). Hybridization of 32P-labeled ME cDNA with RNA extracted from normal and T3-induced livers (15 or 50 .mu.g/100 g body weight for 10 days) showed an increase in the ME mRNA concentration by .apprx. 11-fold in T3-treated rats. ME activity and ME mass were stimulated to the same degree as ME mRNA. Northern blot analysis of either total or poly(A+) RNA revealed two distinct ME mRNAs(21 and 27 S) which were equally induced by T3 treatment. Both mRNAs were shown by in vitro translation assay to program the synthesis of the same immunoprecipitable protein corresponding to full-sized ME. From all the above, we concluded that both messages code for active enzyme. ME activity and ME mRNA were also detected in nonhepatic tissues for which different responses to T3 induction were observed without direct correlation with their respective content of T3 nuclear receptor. Increases in ME activity and level of hybridizable ME mRNA were seen 48 h after a single administration of T3 (200 .mu.g/100 g body weight) in liver, kidney, and heart (10.3- and 15.5, 1.7- and 2.6-, and 1.72- and 3.4-fold above basal values, respectively). Lower levels of induction could already be detected after 24 h, liver being the most stimulated tissue. ME was not affected in brain, lung, testis, and spleen. Northern blot analysis showed that both ME mRNAs are present in all tissues testes, although in different relative proportions. As in liver, the two mRNAs were stimulated to the same extent in T3-induced kidney and heart. These data provide direct evidence for pretranslational regulation by T3 of ME synthesis through two functional mRNAs and for tissue specificity of this hormonal control. Furthermore, both malic enzyme mRNAs are under T3 control in all responsive tissues.