Modulation of the far-upstream enhancer of the rat alpha-fetoprotein gene by members of the ROR alpha, Rev-erb alpha, and Rev-erb beta groups of monomeric orphan nuclear receptors

Expression of the oncodevelopmental alpha -fetoprotein (AFP) gene is tightl y regulated and occurs in the yolk sac, fetal liver and intestine, and canc erous liver cells. Transcription of the AFP gene is under the control of th ree enhancers that are very tissue specific. We have shown that the most up stream of these enhancers, located at -6 kb, works through the combined act ion of liver-enriched factors and nuclear receptors that bind to three regi ons of this DNA regulatory element. This study showed that orphan nuclear r eceptors of the ROR alpha, Rev-erb alpha, and Rev-erb beta groups can bind as monomers with high affinity and specificity to an evolutionarily conserv ed AGGTCA motif in the functionally important region 1 of this AFP enhancer . Transient transfection experiments performed with human HepG2 hepatoma ce lls showed that overproduction of ROR alpha4 stimulated the activity of the AFP enhancer in a dose-dependent manner, while that of Rev-erb alpha and R ev-erb beta had the opposite effect. These effects were highly specific and required the integrity of the AGGTCA motif. The action of these nuclear re ceptors also occurred in the context of the entire 7-kb regulatory region o f the rat AFP gene. These results suggest that altering the amounts or acti vities of these orphan receptors in cells of hepatic or endodermal origin c ould modulate AFP gene expression in response to a variety of developmental or carcinogenic stimuli.