RETINOIDS INCREASE HUMAN APOLIPOPROTEIN-A-II EXPRESSION THROUGH ACTIVATION OF THE RETINOID-X RECEPTOR BUT NOT THE RETINOIC ACID RECEPTOR

Considering the link between plasma high-density lipoprotein (HDL) cho lesterol levels and a protective effect against coronary artery diseas e as well as the suggested beneficial effects of retinoids on the prod uction of the major HDL apolipoprotein (ape), apo A-I, the goal of thi s study was to analyze the influence of retinoids on the expression of apo A-II, the other major HDL protein. Retinoic acid (RA) derivatives have a direct effect on hepatic apo A-II production, since all-trans (at) RA induces apo A-II mRNA levels and apo A-II secretion in primary cultures of human hepatocytes. In the HepG2 human hepatoblastoma cell line, both at-RA and 9-cis RA as well as the retinoid X receptor (RXR )-specific agonist LGD 1069, but not the RA receptor (RAR) agonist -5, 5,8,8-tetramethyl-2-nathyl)-1-propenyl]-benzoic acid (TTNPB), induce a po A-II mRNA levels, Transient-transfection experiments with a reporte r construct driven by the human apo A-II gene promoter indicated that 9-cis RA and at-RA, as well as the RXR agonists LGD 1069 and LG 100268 , induced apo A-II gene expression at the transcriptional level. Only minimal effects of the RAR agonist TTNPB were observed on the apo A-II promoter reporter construct. Unilateral deletions and site-directed m utagenesis identified the J site of the apo A-II promoter mediating th e responsiveness to RA. This element contains two imperfect half-sites spaced by 1 oligonucleotide. Cotransfection assays in combination wit h the use of RXR or RAR agonists showed that RXR but not RAR transacti vates the apo A-II promoter through this element. By contrast, RAR inh ibits the inductive effects of RXR on the apo A-II J site in a dose-de pendent fashion. Gel retardation assays demonstrated that RXR homodime rs bind, although with a lower affinity than RAR-RXR heterodimers, to the AII-RXR response element. In conclusion, retinoids induce hepatic apo A-II production at the transcriptional level via the interaction o f RXR with an element in the J site containing two imperfect half-site s spaced by 1 oligonucleotide, thereby demonstrating an important role of RXR in controlling human lipoprotein metabolism. Since the J site also confers responsiveness of the apo A-II gene to fibrates and fatty acids via the activation of peroxisome proliferator-activated recepto r-RXR heterodimers, this site can be considered a plurimetabolic respo nse element.