9-CIS-RETINOIC ACID IS MORE EFFECTIVE THAN ALL-TRANS-RETINOIC ACID INUP-REGULATING EXPRESSION OF THE ALPHA-FETOPROTEIN GENE

In McA-RH 8994 rat hepatoma cells, all-trans-retinoic acid (t-RA) indu ces expression of the alpha-fetoprotein (AFP) and albumin genes and re sults in a phenotype similar to differentiated fetal hepatocytes. The present study elucidated the mechanism involved in AFP gene regulation mediated by retinoic acid. Northern blot analyses demonstrated that 9 -cis-retinoic acid (c-RA), a ligand for retinoid x receptors (RXRs), a lso induced expression of the AFP gene in McA-RH 8994 cells. The induc tion was time- and dose-dependent. Northern blots and transfection ass ays using the 7.3 kb full-length regulatory region of the AFP gene dem onstrated that c-RA was more effective than t-RA in regulating express ion of the AFP gene. At 10(-7) M, c-RA increased AFP mRNA 5-fold and c hloramphenicol acetyltransferase (CAT) activity 2.5-fold. In contrast, t-RA at a concentration of 10(-7) M exerted no significant effect; 10 (-6) to 10(-5) M t-RA was needed to affect AFP gene expression. These data suggested that activation of RXRs is essential for the regulation of the AFP Co-transfection experiments revealed that expression of RX R alpha in McA-RH 8994 cells further enhanced the CAT activity induced by c-RA. In addition, c-RA did not alter the half-life of AFP mRNA. T hus, RXR alpha may play a crucial role in transcriptional regulation o f the AFP gene and in controlling hepatocyte phenotype.