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. 2011;6(10):e25419.
doi: 10.1371/journal.pone.0025419. Epub 2011 Oct 11.

PCBP2 enhances the antiviral activity of IFN-α against HCV by stabilizing the mRNA of STAT1 and STAT2

Zhongshuai Xin  1 , Wei Han, Zhiqiang Zhao, Qing Xia, Bin Yin, Jiangang Yuan, Xiaozhong Peng
Affiliations

PCBP2 enhances the antiviral activity of IFN-α against HCV by stabilizing the mRNA of STAT1 and STAT2

Zhongshuai Xin et al. PLoS One. 2011.

Abstract

Interferon-α (IFN-α) is a natural choice for the treatment of hepatitis C, but half of the chronically infected individuals do not achieve sustained clearance of hepatitis C virus (HCV) during treatment with IFN-α alone. The virus can impair IFN-α signaling and cellular factors that have an effect on the viral life cycles. We found that the protein PCBP2 is down-regulated in HCV-replicon containing cells (R1b). However, the effects and mechanisms of PCBP2 on HCV are unclear. To determine the effect of PCBP2 on HCV, overexpression and knockdown of PCBP2 were performed in R1b cells. Interestingly, we found that PCBP2 can facilitate the antiviral activity of IFN-α against HCV, although the RNA level of HCV was unaffected by either the overexpression or absence of PCBP2 in R1b cells. RIP-qRT-PCR and RNA half-life further revealed that PCBP2 stabilizes the mRNA of STAT1 and STAT2 through binding the 3'Untranslated Region (UTR) of these two molecules, which are pivotal for the IFN-α anti-HCV effect. RNA pull-down assay confirmed that there were binding sites located in the C-rich tracts in the 3'UTR of their mRNAs. Stabilization of mRNA by PCBP2 leads to the increased protein expression of STAT1 and STAT2 and a consistent increase of phosphorylated STAT1 and STAT2. These effects, in turn, enhance the antiviral effect of IFN-α. These findings indicate that PCBP2 may play an important role in the IFN-α response against HCV and may benefit the HCV clinical therapy.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. HCV non-structural proteins 4B and 5A mediate the down-regulation of PCBP2 in R1b cells with HCV subgenomic replicon.
(A) qRT-PCR detection of PCBP2 mRNA level in Huh7.5.1 cells and R1b cells. The fold-relative enrichment was quantified with normalization to the GAPDH level. The mRNA level of PCBP2 in Huh7.5.1 replicon cells is significantly lower compared with Huh7.5.1 cells (P<0.05). (B) Western blotting indicates the down-regulation of PCBP2 protein level in R1b cells. (C) HCV non-structural protein NS4B and NS5A inhibit the expression of PCBP2 in Huh7.5.1 cells. NS4B and NS5A lead to the down-regulated expression of PCBP2 while other NS proteins demonstrate no effect. (D) qRT-PCR detection of PCBP2 mRNA level in Huh7.5.1 cells expressed HCV NS4B or NS5A proteins. NS4B demonstrate no effect on PCBP2 RNA level (P = 0.72), while NS5A can inhibit approximately 22% of PCBP2 RNA level. Each bar represents the average of triplicate data points with the standard deviation represented as the error bar. *P<0.05 versus negative control.
Figure 2
Figure 2. IFN-α restores the expression level of PCBP2 in R1b cells after inhibiting the expression of HCV replicon.
(A) Western blotting analysis indicated prominent up-regulation of PCBP2 in R1b cells after treated with 100 IU/mL IFN-α, while no significant differences were noted in Huh7.5.1 cells. (B) The restoration of the expression of PCBP2 is IFN-α dosage dependent and due to inhibition of HCV NS proteins. IFN-α doses of 50, 100 and 200 IU/mL were used to treat R1b cells for 6 hours. Western blotting was performed to detect the expression of proteins. (C) PCBP2 protein level in other hepatocyte-derived cells after treatment of 100 IU/mL IFN-α. The PCBP2 level was unchanged, similar to that observed in Huh7.5.1 cells. (D) The protein level of PCBP2 was restored in cured R1b cells. The PCBP2 level did not return to a low level when IFN-α was removed from the media for a week.
Figure 3
Figure 3. PCBP2 enhances the antiviral activity of IFN-α against HCV.
R1b cells were transfected with pcDEF-PCBP2 or PCBP2 siRNA. 48 hours after transfection, the cells were treated with 100 IU/mL IFN-α, 200 µM ribavirin or a combination of the two, for 12 hours. Western blotting was used to check the overexpression or knockdown of PCBP2. qRT-PCR was performed to measure the HCV RNA level in the treated cells. (A) Overexpression of PCBP2 alone did not have an impact on the HCV RNA level (P = 0.13). (B) Overexpression of PCBP2 enhanced the inhibitory effects of IFN-α against HCV but did not influence the antiviral effect of ribavirin. (C) Knockdown of PCBP2 alone did not have an impact on HCV RNA level (P = 0.28). (D) Knockdown of PCBP2 impaired the inhibition effect of IFN-α against HCV but did not influence the antiviral effect of ribavirin. Each bar represents the average of triplicate data points with the standard deviation represented as the error bar. *P<0.05 versus negative control.
Figure 4
Figure 4. PCBP2 enhances the effect of IFN-α through binding STAT1 and STAT2 mRNA and up-regulates the expression of the two signal molecules in IFN-α pathway.
RIP assay and targeted qRT-PCR confirmed the enrichment of different IFN-α signal pathway factor mRNAs in the PCBP2-RNA complex. (A) Detection of the PCBP2-RNA complex precipitated with streptavidin beads using anti-PCBP2 or anti-GFP antibody and streptavidin by Western blotting. (B) Targeted qRT-PCR was carried out on RNA isolated from RNP complexes precipitated in RIP assay. Analysis of mRNAs isolated from a GFP RIP reaction was carried out in parallel as nonspecific binding. α-globin was used as positive control. The enrichment of each mRNA was compared to the GFP control. Each bar represents the average of triplicate data points with standard deviation represented as the error bar. (C) PCBP2 up-regulated the expression of STAT1 and STAT2 after the treatment of IFN-α while other factors remained intact. The R1b cells were transfected with pcDEF vector or pcDEF-PCBP2. Forty-eight hours after transfection, the cells were treated with 100 IU/mL IFN-α and paired with untrasfected cells. Next, 6 hours later, the cell lysates were assessed by Western blotting.
Figure 5
Figure 5. The C-rich tracts in the 3′UTR of mRNA of STAT1 and STAT2 are PCBP2 binding sites.
(A) Fragments of the STAT1 and STAT2 3′-UTR used in synthesis of sense RNA probe in vitro and sequences position in the 3′-UTR. (B) RNA pull-down assay using the four probes of STAT1 and STAT2. Cytoplasmic extracts from 100 IU/mL IFN-α treated R1b cells were incubated with biotin-labeled RNA probes. The mixture was pulled down by streptavidin beads. The resulting complexes were separated by SDS-PAGE and detected by Western blotting. A pGEM-3zf vector sequence was utilized as a nonspecific control (NSC), and the α-globin 3′-UTR was utilized as a positive control. The bottom panel depicts the assay results with the RNAs set of fragments shown in A. (C) 2-fold, 5-fold and 10-fold of excess unlabeled cold probes were added to compete with the labeled probes for competition analysis. The results indicate that 5-fold cold competitors of S13U2 and S23U1 probe exhibited strong competition. (D) The nucleotide sequences of S13U2 and S23U1. The CU-rich patches are underlined.
Figure 6
Figure 6. The STAT1 and STAT2 mRNA is stabilized in cells overexpressing PCBP2.
(A) qRT-PCR was carried out to measure mRNA level of STAT1 and STAT2 after 6 hours of IFN-α treatment. R1b cells were untransfected or transfected with pcDEF-PCBP2. Each bar represents the average of triplicate data points with the standard deviation represented as the error bar. **P<0.001 versus untransfection. (B) RPA and RNA half-life analysis was performed to monitor the STAT1 and STAT2 mRNA decay course. Two days after transfection, IFN-α was added and 6 hours later actinomycin D was added to inhibit transcription. Representative blots are shown in the left panel and the mRNA levels were quantified through densitometric scanning. For each set of RPAs, the band intensity at the 0 h time point was set to 100%, and the percentage of mRNA of remaining at the 2, 4 and 6 hour time point was plotted shown in the right panel.
Figure 7
Figure 7. Model of PCBP2-modulating effect of IFN-α against HCV.
PCBP2 enhances the antiviral activity of IFN-α through stabilizing the mRNA of STAT1 and STAT2. This accordingly, increases the protein level of the two molecules. The protein level increase also enhances the effect of IFN-α against HCV.
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