Transcriptional Induction of Rat Liver Apolipoprotein A‐I Gene Expression by Glucocorticoids Requires the Glucocorticoid Receptor and a Labile Cell‐Specific Protein

• Published in: European journal of biochemistry Vol. 239; no. 2; pp. 451 - 459
• Main Authors: Saladin, Régis, Vu‐Dac, Ngoc, Fruchart, Jean‐Charles, Auwerx, Johan, Staels, Bart
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 15.07.1996
• Subjects: Animals; apolipoprotein; Apolipoprotein A-I - biosynthesis; Apolipoprotein A-II - biosynthesis; Base Sequence; Carcinoma, Hepatocellular; Cell Line; Cell Nucleus - metabolism; Cells, Cultured; Cercopithecus aethiops; Chloramphenicol O-Acetyltransferase - biosynthesis; Cloning, Molecular; corticosteroid hormone; Dactinomycin - pharmacology; Dexamethasone - pharmacology; DNA Primers; gene expression; Glucocorticoids - pharmacology; Kinetics; liver; Liver - drug effects; Liver - metabolism; Male; Mifepristone - pharmacology; Molecular Sequence Data; Polymerase Chain Reaction; Rats; Rats, Sprague-Dawley; Receptors, Glucocorticoid - drug effects; Receptors, Glucocorticoid - physiology; Recombinant Proteins - biosynthesis; RNA, Messenger - biosynthesis; Transcription, Genetic - drug effects; Transfection
• ISSN: 0014-2956; 1432-1033
• DOI: 10.1111/j.1432-1033.1996.0451u.x

Treatment with glucocorticoids increases the concentration of plasma high‐density lipoprotein (HDL), which is inversely correlated to the development of atherosclerosis. Previously, we demonstrated that repeated administration of glucocorticoids increases apolipoprotein (apo) A‐I gene expression and decreases apoA‐II gene expression in rat liver. In the present study, the mechanism of glucocorticoid action on hepatic apoA‐I and apoA‐II expression was studied. A single injection of rats with dexamethasone increased hepatic apoA‐I mRNA levels within 6 h and further increases were observed after 12 h and 24 h. In contrast, liver apoA‐II mRNA levels gradually decreased after dexamethasone treatment to less than 25% control levels after 24 h. In rat primary hepatocytes and McARH8994 hepatoma cells, addition of dexamethasone increased apoA‐I mRNA levels in a time‐dependent and dose‐dependent manner, whereas apoA‐II mRNA levels were unchanged. Simultaneous addition of the glucocorticoid antagonist RU486 prevented the increase in apoA‐I mRNA levels after dexamethasone treatment, which suggests that the effects of dexamethasone are mediated through the glucocorticoid receptor. Inhibition of transcription by actinomycin D and nuclear‐run‐on experiments in McARH8994 cells and primary hepatocytes showed that dexamethasone induced apoA‐I, but not apoA‐II, gene transcription. Transient‐transfection assays in McARH8994 cells with a chloramphenicol acetyl transferase vector driven by the rat‐apoA‐I‐gene promoter demonstrated that the proximal apoA‐I promoter could be induced by dexamethasone, and this effect could be abolished by simultaneous treatment with RU486. However, in COS‐1 cells, apoA‐I promoter transcription was not induced by dexamethasone or cotransfected glucocorticoid receptor. In addition, the induction of apoA‐I gene transcription by dexamethasone was blocked by the protein‐synthesis inhibitor cycloheximide, which suggests the presence of a labile protein involved in apoA‐I gene activation by dexamethasone. In conclusion, our results demonstrate that dexamethasone regulates rat apoA‐I, but not apoA‐II, gene expression through direct action on the hepatocyte. The induction of apoA‐I gene transcription by dexamethasone requires the glucocorticoid receptor and a labile cell‐specific protein.