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

Abstract

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-I1 gene expression in rat liver. In the present study, the mechanism of glucocorticoid action on hepatic apoA-I and apoA-I1 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-I1 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-I1 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-11, gene transcription. Transient-transfection assays in McARH8994 cells with a chloramphenicol acetyl transferase vector driven by the rat-apoA-1-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-11, 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.Keywords: apolipoprotein ; liver; gene expression; corticosteroid hormone.High plasma concentrations of high-density lipoprotein (HDL) cholesterol are inversely related to the incidence of atherosclerosis [I, 21. The major protein constituents of HDL are apolipoprotein (apo) A-I and apoA-11. Epidemiological studies suggested that the protective effect of HDL on atherosclerosis development is correlated to specific particles within HDL that contain apoA-I but not apoA-11, rather than particles that contain apoA-I and apoA-11 [3]. Furthermore, studies in transgenic animals indicated that overexpression of human apoA-I confers resistance to early atherogenesis [4], whereas overexpression of apoA-I1 results in less protection and m...

Transcriptional Regulation of Apolipoprotein A-I Gene Expression by the Nuclear Receptor RORα

“…Cloning and Construction of Recombinant Plasmids-PCR amplification and cloning of the rat apoA-I gene promoter fragments into the pBLCAT5 promoterless expression vector were described previously (24). Site-directed mutagenesis of the RORE was accomplished using the oligonucleotide 5Ј-CAC ACA TAT ATA GGC AGG GAA GAA GA-3Ј as a mutagenic primer on single-stranded DNA templates according to Kunkel (25).…”

Glucocorticoids, metabolism and metabolic diseases

“…However, a direct involvement of the GR could not be demonstrated in this setting [154]. In addition, GCs have been shown to increase the rate of synthesis and secretion of apolipoprotein AI (ApoAI), the effect of which depends on direct ApoAI promoter regulation through the GR [155,156]. Patients receiving GC treatment have elevated levels of ApoAI and its major associated lipoprotein complex, high density lipoprotein (HDL).…”

“…Patients receiving GC treatment have elevated levels of ApoAI and its major associated lipoprotein complex, high density lipoprotein (HDL). This potentially beneficial effect of short-term GC exposure is counteracted, however, upon long-term GC therapy, in which also VLDL levels increase (see above) [155,[157][158][159].…”

Hormonal regulation of apolipoprotein AI

“…However, dexamethasone clearly induced site B binding activities, but these activities were not attributed to the GR. Nonetheless, the receptor has an important role in the light of recent results showing that the GR antagonist, RU486, added to rat hepatoma cells (McARH8994 cells) completely blocks the increase in apo AI mRNA that follows treatment with dexamethasone (Saladin et al 1996). In addition, the presence of the receptor alone is not sufficient, as treatment of COS-1 cells, which lack endogenous GR, with dexamethasone did not increase apo AI promoter activity in the presence of transfected GR, suggesting that the receptor acts via an indirect mechanism.…”