Transcriptional induction of rat liver apolipoprotein A-I gene expression by glucocorticoids requires the glucocorticoid receptor and a labile cell-specific protein.
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.
Saladin
R
R
U325 INSERM, Département d'Athérosclérose, Institut Pasteur, Lille, France.
Vu-Dac
N
N
Fruchart
J C
JC
Auwerx
J
J
Staels
B
B
D000818:Animals; D016632:Apolipoprotein A-I / Q000096:biosynthesis*; D016633:Apolipoprotein A-II / Q000096:biosynthesis*; D001483:Base Sequence; D006528:Carcinoma, Hepatocellular; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D002478:Cells, Cultured; D015500:Chloramphenicol O-Acetyltransferase / Q000096:biosynthesis; D002522:Chlorocebus aethiops; D003001:Cloning, Molecular; D017931:DNA Primers; D003609:Dactinomycin / Q000494:pharmacology; D003907:Dexamethasone / Q000494:pharmacology*; D005938:Glucocorticoids / Q000494:pharmacology*; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D015735:Mifepristone / Q000494:pharmacology; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D012333:RNA, Messenger / Q000096:biosynthesis; D051381:Rats; D017207:Rats, Sprague-Dawley; D011965:Receptors, Glucocorticoid / Q000187:drug effects / Q000502:physiology*; D011994:Recombinant Proteins / Q000096:biosynthesis; D014158:Transcription, Genetic / Q000187:drug effects; D014162:Transfection
D016632:Apolipoprotein A-I; D016633:Apolipoprotein A-II; D017931:DNA Primers; D005938:Glucocorticoids; D012333:RNA, Messenger; D011965:Receptors, Glucocorticoid; D011994:Recombinant Proteins; D003609:Dactinomycin; D015735:Mifepristone; D003907:Dexamethasone; D015500:Chloramphenicol O-Acetyltransferase