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 h igh-density lipoprotein (HDL), which is inversely correlated to the de velopment of atherosclerosis. Previously, we demonstrated that repeate d administration of glucocorticoids increases apolipoprotein (apo) A-I gene expression and decreases apoA-II gene expression in rat liver. I n 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 an d further increases were observed after 12 h and 24 h. In contrast, li ver apoA-II mRNA levels gradually decreased after dexamethasone treatm ent to less than 25% control levels after 24 h. In rat primary hepatoc ytes and McARH8994 hepatoma cells, addition of dexamethasone increased apoA-I mRNA levels in a time-dependent and dose-dependent manner, whe reas 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. Inh ibition of transcription by actinomycin D and nuclear-run-on experimen ts in McARH8994 cells and primary hepatocytes showed that dexamethason e induced apoA-I, but not apoA-II, gene transcription. Transient-trans fection assays in McARH8994 cells with a chloramphenicol acetyl transf erase vector driven by the rat-apoA-I-gene promoter demonstrated that the proximal apoA-I promoter could be induced by dexamethasone, and th is effect could be abolished by simultaneous treatment with RU486. How ever, 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 blocke d by the protein-synthesis inhibitor cycloheximide, which suggests the presence of a labile protein involved in apoA-I gene activation by de xamethasone. 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 cel l-specific protein.