Eukaryotic cells employ multiple strategies of checkpoint signaling and DNA repair mechanisms to monitor and repair damaged DNA.
There are two branches of the checkpoint response pathway, ataxia telangiectasia-mutated (ATM) pathway and ATM-Rad3-related (ATR) pathway. Virus replication presents the host cells with large amounts of exogenous genetic material, including DNA ends and unusual structures. Therefore, infected cells recognize viral replication as a DNA damage stress and elicit DNA damage signal transduction, which ultimately induces apoptosis as part of host immune surveillance. There was no evidence so far that the ATM/ATR kinases or their downstream pathways are triggered by HBV infection.
A research article to be published on October 28, 2008 in the World Journal of Gastroenterology addresses this question. This research group was leaded by Dr Hui Zhong from Beijing Institute of Biotechnology, China.
Using HL7702 hepatocytes with HBV-positive serum as material, they evaluate protein expression levels in HBV infected cells or in non-infected cells; immunofluorescence to show ATR foci ands Chk1 phosphorylation foci formation; flow cytometry to analyze the cell cycle and apoptosis; ultraviolet radiation (UV) and ionizing radiation (IR) treated cells to mimic DNA damage; Trypan blue staining to count the viable cells.
They found that HBV infection induced increased steady state of ATR protein and increased phosphorylation of multiple downstream targets including Chk1, p53 and H2AX. In contrast to ATR and its target, the phosphorylated form of ATM at Ser-1981 and its downstream substrate Chk2 phosphorylation at Thr-68 did not visibly increase upon infection. However, the level of Mre11 and p21 were reduced beginning at 0.5 h after HBV-positive serum addition. Also, HBV infection led to transient cell cycle arrest in the S and the G2 phases without accompanying increased apoptosis. Research on analyzing cell survival change upon radiation followed HBV infection showed that survival of UV treated host cells was greatly increased by HBV infection, owing to the reduced apoptosis. Meanwhile, survival of IR treated host cells was reduced by HBV infection.
Their result indicated that HBV induces cellular DNA damage response dependent on ATR but escapes the consequences of activation of the DNA damage checkpoint by degradation of checkpoint proteins on different levels. The implication of this is that with time, persistent HBV infection may lead to the accumulation of a variety of mutations which would ultimately give rise to HCC.