FUNCTIONAL INTERACTION OF NUCLEAR FACTORS EF-C, HNF-4, AND RXR-ALPHA WITH HEPATITIS-B VIRUS ENHANCER-I

Hepatitis B virus (HBV) enhancer I contains cis-acting elements that a re both sufficient and essential for liver-specific enhancer function. The EF-C binding site was previously shown to be a key element in enh ancer I. EF-C binding activity is evident in hepatic and nonhepatic ce lls. Although the EF-C binding site is required for efficient HBV enha ncer I function, the EF-C site does not possess intrinsic enhancer act ivity when assayed in the absence of flanking elements. We have define d a novel region in HBV enhancer I, termed the GB element, that is adj acent to and functions in conjunction with the EF-C binding site. The GB element and EF-C site confer interdependent liver-specific enhancer activity in the absence of flanking HBV enhancer sequences. The nucle otide sequence of the GB element is similar to sequences of the DNA bi nding sites for members of the steroid receptor superfamily. Among the se proteins, we demonstrate that HNF-4, RXR (retinoid X receptor), and COUP-TF bind to the GB element in vitro. HNF-4 transactivates a promo ter linked to a multimerized GB/EF-C domain via the GB element in vivo in a manner that is dependent on the integrity of the adjacent EF-C b inding site. RXRalpha also transactivates promoter expression via the GB element in vivo in response to retinoic acid but in a largely EF-C- independent manner. Finally, we show that COUP-TF antagonizes the acti vity of the GB element in human liver cells.