The interplay between hypoxia signalling and hepatitis B virus replication

Viral replication is influenced by the host cell microenvironment, and local oxygen tension is a major factor that shapes the intracellular landscape. Hepatitis B virus (HBV) replicates in the liver which harbours a naturally low oxygen environment. However, in vitro culture systems apply atmospheric oxygen conditions, and so our understanding of the role of oxygen sensitive factors in the viral life cycle is limited. We uncover evidence of hypoxic signalling in the HBV infected liver that associates with the inflammatory status of this chronic disease. We develop simple PCR assays to determine the source of HBV RNAs and ascribe patterns of viral transcription. Investigating the interplay between the hepatic inflammasome and viral transcription identified 24 host factors that positively associate with HBV transcriptional activity. Culturing HBV infected hepatoma cells under low oxygen uncovered a new role for hypoxia inducible factors (HIFs) in the viral life cycle. Our studies demonstrate that HIFs bind an evolutionarily conserved hypoxic response element in the basal core promoter in vitro and in vivo, which drives the synthesis of pre-genomic RNA and potentiates particle genesis. HIFs are not the only regulators of the cellular response to low oxygen, and we expand our observation to show that hypoxic inhibition of histone lysine demethylase 4 (KDM4) also promotes transcription from episomal and integrated HBV genomes. We show that KDM4 is a major orchestrator of host gene expression and this defines host susceptibility to HBV infection. Identifying roles for oxygen sensors in the HBV life cycle suggests that this pathogen has evolved to exploit the hypoxic signalling pathways in the liver. This study highlights the importance of understanding the pathways which are active in physiological oxygen tensions when considering virus-host interactions, and provides innovative routes to therapeutically target HBV.