Natural killer (NK) cells are a major component of the host innate immune defense against various pathogens. Several viruses, including hepatitis C virus (HCV), have developed strategies to evade the NK-cell response. In our study, we found HCV infection could trigger DNA damage response by both ataxia telangiectasia mutated (ATM) and ATM-and Rad3-related (ATR) pathways. Recent reports had revealed that NKG2D ligands (NK cellactivating ligands) were upregulated when a major DNA damage checkpoint pathway was activated. However, here we found that DNA damage response was activated but NKG2D ligands were downregulated upon HCV infection. Further studies showed that the protease NS3/4A of HCV which had been shown relation with immune invasion contributed to the reduced expression of NKG2D ligands. These findings provide a novel insight into the mechanisms evolved by HCV to escape from the NK cell response. Cellular & Molecular Immunology. 2008;5(6): 475-478.

AIM:To investigate whether hepatitis B virus (HBV) infection activates DNA damage response and DNA repair cofactors inhibit HBV infection and replication. METHODS: Human hepatocyte cell line HL7702 was studied. Immunoblotting was performed to test the expression of ataxia telangiectasia-mutated (ATM)-Rad3-related protein (ATR), p21 and the level of phosphorylation of Chk1, p53, H2AX, ATM in HBVinfected or non-infected-cells. Special short RNAi oligos was transfected to induce transient ATR knockdown in HL7702. ATR-ATM chemical inhibitors caffeine (CF) and theophylline (TP), or Chk1 inhibitor 7-hydroxystaurosporine (UCN01) was studied to determine whether they suppress cellular DNA damage response and MG132 inhibits proteasome. RESULTS: The ATR checkpoint pathway, responding to single-strand breaks in DNA, was activated in response to HBV infection. ATR knockdown cells decreased the HBV DNA yields, implying that HBV infection and replication could activate and exploit the activated DNA damage response. CF/TP or UCN01 reduced the HBV DNA yield by 70% and 80%, respectively. HBV abrogated the ATR-dependent DNA damage signaling pathway by degrading p21, and introduction of the p21 protein before HBV infection reduced the HBV DNA yield. Consistent with this result, p21 accumulation after MG132 treatment also sharply decreased the HBV DNA yield. CONCLUSION: HBV infection can be treated with therapeutic approaches targeting host cell proteins by inhibiting a cellular gene required for HBV replication or by restoring a response abrogated by HBV, thus providing a potential approach to the prevention and treatment of HBV infection.

apoptosis. Research on cell survival changes upon radiation following HBV infection showed that survival of UV-treated host cells was greatly increased by HBV infection, owing to the reduced apoptosis. INTRODUCTIONEukar yotic cells employ multiple strategies of checkpoint signaling and DNA repair mechanisms to monitor and repair damaged DNA [1][2][3][4][5] . There are two branches of the checkpoint response pathway, ataxia telangiectasia-mutated (ATM) pathway and ATM-Rad3-related (ATR) pathway. The major difference between ATM and ATR is the type of DNA damage to which each responds. For example, ATM responds to ionizing radiation (IR) and other agents that cause double-strand breaks (DSBs) in DNA. ATR responds to ultraviolet radiation (UV) radiation and other agents that induce the accumulation of stalled replication forks and subsequent single-stranded breaks (SSBs) in DNA. The DSBs are recognized by the Mre11-Rad50-Nbs1 complex, which recruits and activates ATM kinase [6] . The SSBs are coated METHODS:We incubated HL7702 hepatocytes with HBV-positive serum, mimicking a natural HBV infection process. We used immunoblotting to 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 (UV) radiation and ionizing radiation (IR)-treated cells to mimic DNA damage; and Trypan blue staining to count the viable cells. RESULTS: We found that HBV infection induced an 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