Vol. 123, No. 3, 1984 September 28, 1984 BIOCHEMICAL AND BIOPHYSICALRESEARCH COMMUNICATIONS Pages 1122-1129 T 3 STIMULATES THE SYNTHESIS OF A SPECIFIC mRNA IN PRIMARY HEPATOCYTE CULTURE. Cary N. Marlash, Donald B. Jump and Jack H. Oppenheimer Department of Medicine, Division of Endocrinology and Metabol Ism, and Department of Genetics and Cell Biology, University of Minnesota, Minneapolis, MN 55455 Received August 3, 1984 The mechanism of trIIodothyronlne (T 3) Induction of a thyroid hormone respons!ve mRNA (mRNAs14) was studied In primary cultures of adult rat hepatocytes. T3 Induced mRNAs14 in less than one hour after addltlon to the cultures. After 24 hours exposure to T3, the level of mRNAs14 was 2.5 to 6 times above the untreated controls. Addition of Actinomycln-D to both Induced and control cultures led to slmllar mRNAs14 disappearance curves, Implying that T3 augments the synthesis of mRNAs14 rather than stablllzlng pre-formed mature mRNA. Glucagon Inhibits the T 3 induction of mRNAs14. When added to both induced and control cultures, glucagon leads to slmllar fractional decay curves for mRNAs14, confirming the Actlnomycln-D studies. These findings demonstrate T3 Induces mRNAs14 dlrectly In the hepatocyte by Increasing the synthesis of the mature mRNA. © 1984Academic Press, Inc. We have recently Identified a mRNA In rat Ilver which Is rapidly responsive to the in vivo admlnlstratlon of T3, mRNAsI 4 (1-3). The mRNA codes for a proteln of M r 17,500 and pl of 4.9 located in hepatic cytosol (3)° Because Induction of thls mRNA occurs within 20 minutes following T3 admlnlstratlon, It may well represent a primary response to thyroid hormone. However, additlonal data Indicate that factors other than thyroid hormone regulate the level of mRNAs1 4. Thus, carbohydrate feeding also Induces a response (4), an effect which appears to be as prompt as that noted with T 3 (unpublished observations). The level of mRNAs1 4 also undergoes a marked clrcadlan varlatlon (3) due to as yet undefined factors. Moreover, approximately one-third of the T 3 responsive hepatic mRNAts are not altered by T3 directly, but rather, by pituitary growth hormone secretion stimulated by T 3 (4). Therefore, It Is especial ly important to establ ish that T3 stimulates the formation of mRNAs14 directly at the hepatocellular level. An Isolated hepatocyte culture model Is Ideally suited to answer this question, lye have previously used such a model to Investigate the synergism 0006-291X/84 $1.50 Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved. 1122