Hepatitis B Virus X Gene Activates κ B-Like Enhancer Sequences in the Long Terminal Repeat of Human Immunodeficiency Virus 1

The role of the hepatitis B virus (HBV) X gene during virus infection has not been defined. We previously showed that expression of the HBV X gene in the human hepatocellular carcinoma cell line HepG2 trans-activates chloramphenicol acetyltransferase gene expression under control of the human immunodeficiency virus 1 (HIV-1) long terminal repeat and we have now identified a specific sequence in the HIV-1 long terminal repeat that is responsive to the HBV X gene. Plasmid constructs with the chloramphenicol acetyltransferase gene regulated by an isolated and twice-repeated 12-base-pair HIV-1 enhancer sequence homologous to the nucleotide sequence that binds the nuclear transcription factor NF-κ B (the HIV-1 κ B-like sequence) were trans-activated by the HBV X gene in HepG2 cells, indicating that the κ B-like enhancer sequence in the HIV-1 long terminal repeat is responsive to the X gene. When eight copies of the HIV-1 κ B-like sequence were used to regulate β -globin gene expression, transcription of this gene was activated by the HBV X gene in HepG2 cells and no β -globin gene transcription was detected in the absence of the HBV X gene. β -globin gene expression regulated by the activator protein 2 (AP-2) binding sequence was not activated by the HBV X gene. Treatment of HepG2 cells with phorbol ester resulted in modest activation of the HIV-1 κ B-like enhancer sequence suggesting that an NF-κ B-like factor was induced in these cells as it is in T lymphocytes by phorbol ester; however, phorbol ester did not demonstrably enhance the activation of the HIV-1 enhancer observed with the HBV X gene. These experiments indicate that the HIV-1 κ B-like transcriptional enhancer sequence is activated by the HBV X gene and suggest that the HBV X gene might play a role in regulating transcription of a gene under control of a κ B-like enhancer during HBV infection. Since such a sequence has not been found in the HBV genome and HBV gene expression appears not to be regulated by the HBV X gene, a cellular gene that plays a role in HBV replication could be the target of the X gene during HBV infection.