HBV infects and preferentially replicates in quiescent hepatocytes. However, quiescent cells normally don’t have sufficient amounts of deoxynucleotides (dNTPs), the building blocks of DNA, to support productive viral replication. We found that HBV induces expression of the host R2 gene encoding the catalytic subunit of the Ribonucleotide Reductase (RNR) holoenzyme. R2 gene is selectively induced in cycling cells in increasing the cellular dNTP pool. Remarkably HBV induces R2 expression in growth-arrested cells. Here we report on the mechanism of unprecedented R2 expression in growth-arrested cells. We found that HBV induces the DNA damage response (DDR), via supporting Chk1 phosphorylation and activation. Chk1 is a key checkpoint protein of the DDR. Importantly, Chk1 inhibition or knockdown prevents R2 upregulation by HBV. An important target of Chk1 is E2F1, a major transcription factor of cell-cycle genes. E2F1 also plays a role in DDR, where it specifically activates DNA repair and apoptotic genes. R2 is a known target of E2F1. We found that E2F1 is upregulated and phosphorylated in the presence of HBV, and that it is indispensible for R2 upregulation in quiescent cells. Remarkably, our data demonstrate that while it activates DDR, HBV attenuates host DNA repair. Since the process of DNA repair consumes dNTP, its inhibition serves the virus need to maximize dNTP availability. Thus, Chk1-E2F1-R2 axis induction by HBV in growth-arrested cells is a critical step in the HBV productive life cycle.
