Hepatocyte Nuclear Factor 4 (HNF4) Plays a Controlling Role in Expression of the Retinoic Acid Receptor fi (RAR) Gene in Hepatocytes

HNF4 alpha, a member of the nuclear receptor superfamily, regulates the genes involved in lipid and glucose metabolism. The expression of the RAR beta gene in the liver of HNF4 alpha knock-out mice was higher versus wildtype controls, whereas oppositely, RARb promoter activity was 50% reduced by the overexpression of HNF4 alpha in HepG2 cells, and treatment with retinoic acid (RA), a major metabolite of vitamin A, increased RAR b promoter activity 15-fold. The human RAR beta 2 promoter contains two DR5 and one DR8 binding motifs, as RA response elements (RARE) proximal to the transcription start site. While DR5 RARE1 was previously reported to be responsive to RARs but not to other nuclear receptors, we show here that mutation in DR5 RARE2 suppresses the promoter response to HNF4 alpha and RARff/RXRff. Mutational analysis of ligand-binding pocket amino acids shown to be critical for fatty acid (FA) binding indicated that RA may interfere with interactions of FA carboxylic acid headgroups with side chains of S190 and R235, and the aliphatic group with I355. These results could explain the partial suppression of HNF4 alpha transcriptional activation toward gene promoters that lack RARE, including APOC3 and CYP2C9, while conversely, HNF4 alpha may bind to RARE sequences in the promoter of the genes such as CYP26A1 and RAR b, activating these genes in the presence of RA. Thus, RA could act as either an antagonist towards HNF4 alpha in genes lacking RAREs, or as an agonist for RARE-containing genes. Overall, RA may interfere with the function of HNF4 alpha and deregulate HNF4 alpha targets genes, including the genes important for lipid and glucose metabolism.