Consistent with its ability to

bind PARP1 for transcriptional activation, the “ACTTCAAA” HBVCP PARP1 binding motif could also interfere with histone H1 ADP-ribosylation (Fig. 5A). This raises the possibility that HBVCP-PARP1 Tamoxifen manufacturer interaction not only supports HBV replication, but also impairs PARP1 enzyme-dependent functions, such as DNA repair in vivo. If this were true, the ability of cells to effectively carry out DNA strand-break repair when challenged by DNA-damaging agents would be compromised. To verify this, a construct bearing the HBV-PARP1 binding motif in three tandem copies (Fig. 5B) was tested for its capacity to inhibit cellular PARP1 enzymatic activity by determining the degree of DNA damage induced with etoposide (DNA single- and double-strand

break inducer) or bleomycin (DNA double-strand break inducer). Alkaline comet assays revealed that HepG2 cells transfected with the PARP1 motif had significantly more DNA in comet tails than cells treated with dimethyl sulfoxide (DMSO) or the control vector (Fig. 5C), ICG-001 concentration reflecting enhanced DNA damage. This suggests that the ability of PARP1 to ADP-ribosylate protein targets required in DNA damage-repair pathways was reduced, supporting the inhibitory role of HBVCP-PARP1 motif expression on nuclear PARP1 enzymatic activity. The effect of the PARP1 motif was further assessed for its ability to sensitize cells to induced cytotoxicity caused by DNA-damaging agents. Consistent with accumulation

of damaged DNA, etoposide or bleomycin treated HepG2 cells transfected with the PARP1 motif had a significantly larger population of Annexin V–positive cells (Fig. 5D). In contrast, DMSO treatment or vector control did not show significant changes in Annexin V staining. The enhanced cytotoxicity toward sublethal amounts of etoposide Leukotriene-A4 hydrolase and bleomycin in cells transfected with the motif is reminiscent of the hypersensitivity of PARP1 knockout and haploinsufficient mice toward DNA-damaging agents,18, 20 reflecting compromised DNA repair with the loss of PARP1 enzymatic function. The ability of the PARP1 motif to specifically disrupt cellular PARP1 function was also demonstrated by diminished HBs expression in HepG2 cells cotransfected with HBV-RFP (red fluorescent protein) (Supporting Fig. 7). To confirm that the effects of the HBVCP-PARP1 motif are specific to PARP1, rescue experiments were performed, in which PARP1 was overexpressed to compensate for the loss of DNA repair. Excess PARP1 cannot avert the accumulation of cytotoxic DNA lesions if alternative DNA repair pathways were instead compromised. Using apoptotic cell death as the end-point of extensive irreparable DNA damage, the effect of etoposide or bleomycin on HepG2 cells cotransfected with the HBVCP-PARP1 binding motif and PARP1 or RFP expression vectors was determined by apoptotic caspase-dependent cleavage of luminogenic substrates.