Modulation of the Far-Upstream Enhancer of the Rat α-Fetoprotein Gene by Members of the RORα, Rev-erbα, and Rev-erbβ Groups of Monomeric Orphan Nuclear Receptors

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Abstract
Expression of the oncodevelopmental α-fetoprotein (AFP) gene is tightly regulated and occurs in the yolk sac, fetal liver and intestine, and cancerous liver cells. Transcription of the AFP gene is under the control of three enhancers that are very tissue specific. We have shown that the most upstream of these enhancers, located at - 6 kb, works through the combined action of liver-enriched factors and nuclear receptors that bind to three regions of this DNA regulatory element. This study showed that orphan nuclear receptors of the RORα, Reverbα, and Rev-erbβ groups can bind as monomers with high affinity and specificity to an evolutionarily conserved AGGTCA motif in the functionally important region 1 of this AFP enhancer. Transient transfection experiments performed with human HepG2 hepatoma cells showed that overproduction of RORα4 stimulated the activity of the AFP enhancer in a dose-dependent manner, while that of Rev-erbα and Rev-erbβ had the opposite effect. These effects were highly specific and required the integrity of the AGGTCA motif. The action of these nuclear receptors also occurred in the context of the entire 7-kb regulatory region of the rat AFP gene. These results suggest that altering the amounts or activities of these orphan receptors in cells of hepatic or endodermal origin could modulate AFP gene expression in response to a variety of developmental or carcinogenic stimuli.