MECHANISM OF THE SLOW INDUCTION OF APOLIPOPROTEIN-A-I SYNTHESIS BY RETINOIDS IN CYNOMOLGUS HEPATOCYTES - INVOLVEMENT OF RETINOIC ACID AND RETINOID-X-RECEPTORS

We showed previously that retinoids stimulate apolipoprotein A-I (apoA -I) synthesis in cultured cynomolgus hepatocytes only after a 24-h lag phase, Here we report on the biochemical background of the slow respo nse, the requirement for high retinoic acid concentrations, and the in volvement of different retinoid receptors. The time course of the effe ct of 10 mu M all-trans retinoic acid (at-RA) on apoA-I mRNA levels an d protein secretion were comparable, i.e., minor increases were observ ed after a 24-h incubation and mRNA levels were increased 2.2- and 3.5 -fold after 48 h and 72 h, respectively. In contrast, apoA-I gene tran scription was already increased (2.6-fold) after a 4-h incubation with 10 mu M at-RA. At-RA disappeared rapidly from die cultures: after 2 h of incubation 40% of the added amount was left and alter 24 h only 2% . RAR beta mRNA and gene expression were increased after incubation wi th 10 mu M at-RX, whereas RAR alpha and RXR alpha mRNA levels and expr ession remained uncharged. No transcriptional activity and mRNA for ot her retinoid receptors were detectable. Both RAR-selective (TTNPB) and RXR-selective (3-methyl-TTNEB) agonists induced apoA-I synthesis at 1 and 10 mu M. These results show that Ij the slow increase in apoA-I s ecretion is caused by a slow increase of its mRNA level; ii) tile apoA -I gene transcription in cynomolgus hepatocytes is induced rapidly by retinoids; iii) the added at-RA disappeared rapidly from the cultures, explaining the necessity for high initial concentrations; iv) RAR alp ha and/or RAR beta and RXR alpha are involved in the activation of apo A-I expression by retinoids.