Human hepatoma cell lines, Huh7 cells and HepG2 cells, the stable HBV-producing cell lines, HepG2.2.1.5 cells and HepAD38 cells were used. Huh7 and HepG2 cells were cultured in Eagle’s minimum essential medium (MEM; HyClone, Utah, USA) supplemented with 10% fetal bovine serum (FBS), 1% Penicillin-Streptomycin, 1% L-glutamine, 1% Non essential Amino Acid; HepG2.2.1.5 and HepAD38 cells were cultured in Dulbecco’s modified Eagle’s medium (MEM; HyClone, Utah, USA) supplemented with 10% FBS, 1% Penicillin-Streptomycin. To turn off HBV replication, 1μg/ml tetracycline was added to the culture medium of HepAD38 cultures. All the cells were incubated in a 5% CO₂ incubator at 37^oC.

Plasmid PCH9/3091 is an HBV expression vector containing a CMV-early-promoter-driven 1.1 copies of an HBV genome (genotype D) (15). pCMV-HBV-gtA is an HBV genomic expression vector containing 1.2 copies of HBV (adw2) genome (Genbank X02763.1, genotype A) downstream of the CMV promoter in pBS (Promega)(33, 34). pCMV-HBV-gtB and pCMV-HBV-gtC are analogous HBV genomic expression constructs (From Dr. Shuping Tong, Shanghai Medical University, Shanghai, China) (16). The ADAR1 p110 and p150 constructs were generated with 3×HA tag and DDK tag, respectively. ADAR1 P150E/A and p150△R plasmids were used for the functional analysis as reported previously (17). P110-HA and p150-DDK point mutants were generated by PCR-based site-directed mutagenesis, and confirmed by sequencing. The mutations within the catalytic domain, resulting in amino acid change (H910A, E912A, C966A, and C1036A) (18) that abolishes the editing activity of ADAR1 protein. The mutant primers are listed in table 1. Specific small interfering RNA targeting ADAR1 mRNA (siRNAs) were purchased from Guangzhou RiboBio (GuangZhou, China) and transfected into cells using Lipofectamine 3000 (Invitrogen, Massachusetts, USA) according to manufacturer’s instructions.

Fourth eight hours after transfection, cells were lysed in RIPA buffer (50mM Tris-HCl, pH7.4; 150 mM, NaCl; 1% NP-40) supplemented with a mixture of protease inhibitor cocktail (Roche, Berlin, Germany) and 1 mM PMSF for 30 min on ice. The lysates were clarified by centrifugation at 13,000×g for 5min at 4^oC. The clarified lysates were quantitated and denatured following the BCA manufacturer’s protocol. The total lysates were separated on a 10% SDS-PAGE and transferred to an Immobilon-P transfer PVDF membrane (Millipore). The membranes were blocked in 5% nonfat milk before incubating with specific antibodies. The antibodies used were anti-HA (1:500 dilution, Thermo, Ohio , USA), anti-DDK (1:500 dilution, OriGene, OriGene, Maryland, USA), anti-ADAR1 (1:1000 dilution, Abcam, Cambridge), anti-GAPDH (1:10000 dilution, ZSGB-Bio, Beijing, China). After incubation with the HRP-conjugated secondary antibody, the blots were visualized by ECL reagent (Solaibio Science and Tech Ltd., China). GAPDH was used as a housekeeping control.

Quantitative reverse transcription PCR (qRT-PCR) was performed to determine the mRNA levels. Total RNA was extracted with TRIzol reagent (Invitrogen, Massachusetts, USA) according to the manufacturer’s instructions. 1μg total RNA was reverse transcribed into cDNA using PrimeScript 1st Strand cDNA Synthesis Kit (Takara, Shiga, Japan), followed by detection by qPCR in BIO-RAD CFX 96 (Bio-Rad, Washington,USA) system using the FastStart Universal SYBR Green Master (Roche, Berlin, Germany). The primers used are listed in Table 1. GAPDH was used as a housekeeping control

Intracellular viral core DNA of HBV was extracted 4 days after transfection as previously reported (15). Briefly, core-associated HBV DNA was collected by lysis of the cells, DNase I and proteinase K digestion, and phenol/chloroform extraction. The DNA was precipitated with ethanol and dissolved in TE buffer (10 mM Tris–HCl (pH 8.0), 1 mM EDTA). To detect HBV DNA by Southern blot analysis, the extracted viral DNAs were separated on a 1% agarose gel, transferred onto a nylon membrane (Roche, Berlin, Germany). The membranes were hybridized with a digoxigenin-labeled full-length HBV DNA probe using The DIG DNA Labeling and Detection Kit (Roche, Berlin, Germany) according to the manufacturer’s instructions. The signal was detected by exposure on an X-ray film and scanning using the Versa Doc Imaging system (Bio-Rad, Washington,USA). Real-time PCR detection of HBV DNA was performed as previously described (16).

All the results were presented as mean±S.D. Student’s t-test for paired samples was used for data analysis. Differences analyzed by GraphPad Prime 5.0 were considered statistically significant at a value of P<0.05.

ADAR1 is endogenously expressed in human hepatoma cells. We first examined the effects of ADAR1 siRNA knockdown on HBV replication. As shown in Figure 1A, the expression level of HBV replicative intermediate (DNA) was substantially repressed with the ADAR1 knockdown. The repression was more obvious at 100 nM of ADAR1 siRNA than 50 nM (Figure 1A). Meanwhile, Levels of HBV DNA was reduced in HepG2.2.1.5 cells stably transfected by HBV by real-time PCR using primers targeting HBV core gene. The levels of HBV RNA showed no significant change (Figure 1B).

Figure 1

The knockdown of ADAR1 proteins decreased HBV replication. (A and B) Huh7 cells were transfected with 50 nM and 100 nM siADAR1 or siNC for 16h and then transfected with 2 ug of HBV plasmid pCH9/3091, and HBV replicative intermediates in the cells were evaluated at 48 h after transfection by Southern blotting (A) and the efficiency of ADAR1 siRNA knockdown was confirmed by Western blotting (B). (C, D, and E) Relative expression of HBV DNA (C) and RNA (D) in the HepG2.2.1.5 were detected by qPCR at 48h posttransfected with 100 nM ADAR1 siRNA. The levels of ADAR1 expression were detected by western blot (E). **P<0.01 from three independent experiments.

Further, an HBV expression plasmid was co-transfected with a control plasmid or ADAR1 (p110 and p150) in different human hepatoma cell lines, followed by HBV DNA and RNA detection. The level of HBV DNA replication increased significantly in both HepG2 cells and Huh7 cells (Figure 2A, B). Similarly, upon transfection of ADAR1 plasmid into two HBV stably replicative cell lines, HepG2.2.1.5 and HepAD38 cells, while HBV DNA was up-regulated, there was no significant change in the level of HBV RNA (Figure 2C.D).

Figure 2

Overexpression of ADAR1 facilitates the replication of HBV. (A, B, C, and B) 0.4ug of HBV plasmid pCH9/3091 were cotransfected with 1.2ug empty and ADAR1 constructs (P110 or P150) into Huh7 cells (A and B) and HepG2 cells (C and D), HBV DNA replicative intermediates were detected by Southern blotting (A, C). (E, F, G, H, I, and J) 1.2ug ADAR1 constructs (P110 or P150) were transfected into HepG2.2.1.5 cells and HepAD38 cells, HBV DNA and mRNA were detected by qPCR in HepG2.2.1.5 cells (E and F) and southern blot in HepAD38 cells (H and I). The levels of ADAR1 protein were detected by western blot using anti-HA or anti-DDK antibody. GAPDH was used as an internal control. *P<0.05 from three independent experiments.

4.3. ADAR1 increased the replication of HBV independent of genotypes

To examine the effect of genotypes on the promotion of ADAR1 on HBV replication, the three most common HBV genotypes (A, B and C) expression plasmids were co-transfected into Huh7 cells with ADAR1 plasmids, and then the HBV DNA were examined by Southern blotting. Results showed that both p110 and p150 isoform of ADAR1 promoted HBV DNA replication of all three genotypes that were examined (Figure 3A).

Figure 3

ADAR1 increased HBV replication of genotypes A, B and C. 0.4ug of genotype A, B or C HBV constructs were co-transfected with 1.2ug of P110 or P150 construct into Huh7 cells. HBV replicative intermediates were analyzed by Southern blot (A). ADAR1 expressions were confirmed by western blot using anti-HA or anti-DDK antibody (B). GAPDH was used as an internal control.

Human ADAR1 has three different domains (Figure 4AB) that specify distinct functions of ADAR1 (4). We constructed domain mutants of ADAR1 and further explored the function of the mutated domain of ADAR1 on HBV replication in Huh7 cells (Figure 4A). Consistent with such results, as compared to empty vector control, ADAR1 p110, to a less degree, and the p150 isoform of ADAR1 enhanced HBV replication. Over-expressed ADAR1 p150 mutants (p150EA and p150△R) abolished the promoting effects of p150 isoform of ADAR1 on HBV replication (Figure 4C). This finding was consistent with the concept that RNA binding domain and RNA editing domain, solely or collectively, regulate ADAR1 function. It is reasonable to consider that RNA binding is a prerequisite for editing function.

Figure 4

RNA editing and RNA binding domain of ADAR1 are required to promote HBV replication. (A) Schematic representation of human ADAR1 (p110 and p150) and its mutant constructs (p110H910A, p110E912A, p110C966A, p110C1036A, and p150H910A). The asterisks indicate the position of replaced amino acid in the catalytic domain of ADAR1. HBV replicative intermediates were detected by Southern blotting in Huh7 cells transfected with ADAR1 p110, ADAR1 p150 or its mutant expression plasmid (B, D, and F). Level of ADAR1 protein and its mutant constructs were confirmed by Western blotting using anti-HA or anti-DDK antibody (C, E, and G). GAPDH was used as an internal control.

To further determine the key amino acids responsible for the promoting effect of ADAR1 on HBV replication, we constructed four ADAR1 mutants harbored mutations of amino acids in the catalytic domain, and co-transfected them with HBV expressing plasmid into Huh7 cells (Figure 4A). As validated by southern blotting, both p110 and p150 isoforms of ADAR1, when over-expressed in Huh7 cells, increased HBV DNA replication (Figure 4C-D). However, over-expression of ADAR1 mutants (H910A, E912A, and C1036A) did not increase and rather decreased the HBV DNA replication (Figure 4D). C966A mutant showed a comparable or slightly increased level of HBV DNA replication as compared to the empty vector control. These data support the idea that HBV replication can be stimulated by ADAR1 through its editing activity, and conservative amino acids within the editing domain could play distinct roles during the replication.