Distinct Modes of Regulation of Transcription of Hepatitis B Virus by the Nuclear Receptors HNF4α and COUP-TF1

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Abstract
To study the effects of the nuclear receptors (NRs) HNF4α and COUP-TF1 on the life cycle of hepatitis B virus (HBV), the human hepatoma cell line Huh7 was transiently cotransfected with plasmids containing the HBV genome and encoding these two NRs. Overexpression of HNF4α and COUP-TF1 led to a 9-fold increase and a 7- to 10-fold decrease, respectively, in viral DNA synthesis. These two NRs also exhibited distinct modes of regulation of viral transcription. Overexpression of HNF4α led to a more-than-10-fold increase in synthesis of the pregenomic RNA but to only a 2- to 3-fold increase in synthesis of the pre-C and S RNAs. Moreover, the NR response element within the pre-C promoter, NRRE preC, played the major role in activation of pregenomic RNA synthesis by HNF4α. On the other hand, overexpression of COUP-TF1 led to an over-10-fold repression of synthesis of both pre-C and pregenomic RNAs mediated through either NRRE preC or NRRE enhI . HNF4α and COUP-TF1 antagonized each other's effects on synthesis of pregenomic RNA and viral DNA when they were co-overexpressed. A naturally occurring HBV variant which allows for binding by HNF4α but not COUP-TF1 in its NRRE preC exhibited significantly higher levels of synthesis of pregenomic RNA and viral DNA than wild-type HBV in coexpression experiments. Last, deletion analysis revealed that non-NRRE sequences located within both the C and pre-S1 regions are also essential for maximum activation of the pregenomic promoter by HNF4α but not for repression by COUP-TF1. Thus, HNF4α and COUP-TF1 function through different mechanisms to regulate expression of the HBV genes.