Inhibition of Histone Deacetylases Stimulates HBV Replication Independent of Protein X

Abstract

aim: HBV expresses an accessory protein called X (HBx), which supports HBV replication by increasing transcription from episomal templates. Here, we investigate whether HBx augments HBV replication by interfering with the deacetylation of HBV DNA associated histones by histone deacetylases (HDACs). materials & methods: To study the effect of HBx on episomal transcription, we transfected HEK 293 cells with luciferaseexpressing constructs together with HBx in the presence and absence of HDAC inhibitors. We confirmed our results in the context of the full HBV replication cycle in HepG2 cells. results & conclusion: Inhibition of HDAC activity and HBx expression stimulated transcription from episomal DNA independently, showing that HBx does not affect the histone deacetylation. HDAC inhibitors also augmented HBV replication in vitro independent of HBx expression. This suggests that treatment with HDAC inhibitors can (re)activate HBV infection in patients with cleared or ongoing HBV infection. KeywordsWorldwide, chronic infections with the HBV affect more than 240,000,000 people, and more than 780,000 people die each year as a consequence of HBV infection. The virus infects hepatocytes in the human liver. After entry, the core particle containing the partially double stranded HBV genome is transported to the nucleus, where the partially double stranded DNA genome is released into the nucleus and repaired by host enzymes to form a fully double stranded episome of covalently closed circular DNA (cccDNA). This episomal cccDNA is wrapped around histones and thus forms a minichromosome that is subject to various epigenetic modifications to both the cccDNA and the associated histones [1]. It has been shown that these epigenetic modifications regulate the transcription of the viral RNAs [2,3].The HBV protein called X (HBx) is essential for HBV replication in vivo [4]. In vitro, HBx acts as a transcriptional transactivator [4][5][6][7][8] that increases transcription from episomal DNA templates independent of promoter sequences [9]. The capacity of HBx to transactivate transcription in vitro is related to its in vivo function and can therefore be used to study HBx functionality [10]. It has been shown that HBx expression critically affects the epigenetic status of the cccDNA [3,11,1,3,2]. In the absence of HBx, the cccDNA-associated histones undergo modifications that effectively abrogate HBV RNA transcription. A hallmark of this transcriptional silencing in the absence of HBx is the deacetylation of the histones around which the cccDNA is wrapped [1,11].The acetylation status of histones is regulated by dynamic processes. Preceding and during transcription, histones are acetylated by histone acetyl transferases. Well-known members are the CREB-binding protein (CBP/p300) and transcription initiation factor TFIID subunit 1 (TAF1).For reprint orders, please contact: reprints@futuremedicine.com