Limits...
Identification of caspase-mediated decay of interferon regulatory factor-3, exploited by a Kaposi sarcoma-associated herpesvirus immunoregulatory protein.

Aresté C, Mutocheluh M, Blackbourn DJ - J. Biol. Chem. (2009)

Bottom Line: Here, we show that vIRF-2 mediates IRF-3 inactivation by a mechanism involving caspase-3, although vIRF-2 itself is not pro-apoptotic.Importantly, we also show that caspase-3 participates in normal IRF-3 turnover in the absence of vIRF-2, during the antiviral response induced by poly(I:C) transfection.These data provide unprecedented insight into negative regulation of IRF-3 following activation of the type I IFN antiviral response and the mechanism by which KSHV vIRF-2 inhibits this innate response.

View Article: PubMed Central - PubMed

Affiliation: Cancer Research UK Cancer Centre, School of Cancer Sciences, Vincent Drive, College of Medical and Dental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.

ABSTRACT
Upon virus infection, the cell mounts an innate type I interferon (IFN) response to limit the spread. This response is orchestrated by the constitutively expressed IFN regulatory factor (IRF)-3 protein, which becomes post-translationally activated. Although the activation events are understood in detail, the negative regulation of this innate response is less well understood. Many viruses, including Kaposi sarcoma-associated herpesvirus (KSHV), have evolved defense strategies against this IFN response. Thus, KSHV encodes a viral IRF (vIRF)-2 protein, sharing homology with cellular IRFs and is a known inhibitor of the innate IFN response. Here, we show that vIRF-2 mediates IRF-3 inactivation by a mechanism involving caspase-3, although vIRF-2 itself is not pro-apoptotic. Importantly, we also show that caspase-3 participates in normal IRF-3 turnover in the absence of vIRF-2, during the antiviral response induced by poly(I:C) transfection. These data provide unprecedented insight into negative regulation of IRF-3 following activation of the type I IFN antiviral response and the mechanism by which KSHV vIRF-2 inhibits this innate response.

Show MeSH

Related in: MedlinePlus

The accelerated decay of activated wild-type IRF-3 by vIRF-2 depends on caspase-3 activity. A, siRNA knockdown of caspase-3. Functional levels of caspase-3 protein were analyzed with the Caspase-Glo®-3/7 assay kit (Promega). HEK293 cells were transfected with either Lit28i Polylinker ShortCut® siRNA mix (Control siRNA) or caspase-3 ShortCut® siRNA (CAS-3 siRNA) for 24 h. They were then transfected with poly(I:C) for 18 h before harvesting for caspase activity assay. Activity assays were repeated at least twice, and average values (± S.D.) are presented from one representative experiment (*, p < 0.05, Student's t test). B, knockdown of caspase-3 elevates IFN-β promoter-driven reporter gene activity. HEK293 cells were transiently co-transfected with the full-length IFN-β promoter reporter plasmid (p125-luc) and an expression vector of FLAG-tagged IRF-3 wild-type (IRF-3WT), or Xpress-tagged vIRF-2-expression vector (vIRF-2), as indicated. The pRLSV40 plasmid constitutively expressing Renilla luciferase was added as an internal control to which firefly luciferase levels were normalized. Co-transfection with control (Control siRNA) and caspase-3 (CAS-3) siRNA was performed as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml), and luciferase activity was measured 18 h later. Average values (± S.D.) are presented from one representative experiment of at least three performed independently (*, #, and + indicate p < 0.05, Student's t test). Gray bars, cells expressing ectopic IRF-3; black bars, cells co-transfected to express ectopic vIRF-2. C, knockdown of caspase-3 elevates phospho-IRF-3 levels. HEK293 cells were transiently co-transfected with an expression vector for FLAG-tagged IRF-3 wild-type (IRF-3WT), and Xpress-tagged vIRF-2 (vIRF-2) or empty vector parental plasmid (pcDNA4), as indicated. Co-transfection with control (Control siRNA) and caspase-3 (CAS-3) siRNA was performed as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml). Protein extracts were prepared at the indicated times thereafter and analyzed by immunoblot with the following antibodies: anti-PARP, anti-Xpress (vIRF-2), anti-phospho-IRF-3 (α-p-IRF-3; Ser-396) and anti-β-actin. The anti-PARP antibody recognizes uncleaved PARP (Un-Cl, 116 kDa) and one cleaved fragment (Cleaved, 83 kDa). This experiment is representative of more than three performed independently. D, expression of vIRF-2 influences caspase-3/-7 activity. HEK293 cells were transiently transfected with an expression vector for FLAG-tagged IRF-3 wild type (IRF-3WT), and/or Xpress-tagged vIRF-2 (vIRF-2), as indicated. Twenty-four hours after plasmid transfection, the cells were treated with etoposide (50 μm) or DMSO and 30 min later transfected with poly(I:C) (10 μg/ml). Caspase-3/-7 activities were measured 18 h later. Average values (± S.D.) are presented from one representative experiment of three performed independently (*, p < 0.05, Student's t test). Black bars, cells co-transfected to express ectopic vIRF-2. E, pro-caspase-3 interacts with IRF-3 and vIRF-2. Upper panel: co-immunoprecipitation studies were performed with lysates of HEK293 cells that had been transfected with an expression vector for FLAG-tagged IRF-3 wild-type (IRF-3WT), or Xpress-tagged vIRF-2 (vIRF-2), as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml), and immunoprecipitation assays were performed 18 h later. Wild-type IRF-3 was immunoprecipitated with anti-FLAG polyclonal antibody and vIRF-2 with anti-Xpress monoclonal antibody. Immunoprecipitates and input extracts (lower panel) were separated by SDS-PAGE before immunoblotting with anti-Pro-caspase-3 antibody (α-Pro-cas-3). PE, protein extract; NT, non-transfected cells. Plasmid pCEP-4 expresses a negative control FLAG-tagged protein that did not immunoprecipitate pro-caspase-3. Note that all seven lanes were separated on the same gel and transferred to the same membrane, but because the vIRF-2 lanes were not contiguous with the others, intervening, irrelevant lanes were removed from the image. IP, antibody used for immunoprecipitation; IB, antibody used for immunoblot. Right panel, Input: these lanes demonstrate the expression levels for vIRF-2, IRF-3 and pro-caspase-3 in the lysates before immunoprecipitation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC2749101&req=5

Figure 3: The accelerated decay of activated wild-type IRF-3 by vIRF-2 depends on caspase-3 activity. A, siRNA knockdown of caspase-3. Functional levels of caspase-3 protein were analyzed with the Caspase-Glo®-3/7 assay kit (Promega). HEK293 cells were transfected with either Lit28i Polylinker ShortCut® siRNA mix (Control siRNA) or caspase-3 ShortCut® siRNA (CAS-3 siRNA) for 24 h. They were then transfected with poly(I:C) for 18 h before harvesting for caspase activity assay. Activity assays were repeated at least twice, and average values (± S.D.) are presented from one representative experiment (*, p < 0.05, Student's t test). B, knockdown of caspase-3 elevates IFN-β promoter-driven reporter gene activity. HEK293 cells were transiently co-transfected with the full-length IFN-β promoter reporter plasmid (p125-luc) and an expression vector of FLAG-tagged IRF-3 wild-type (IRF-3WT), or Xpress-tagged vIRF-2-expression vector (vIRF-2), as indicated. The pRLSV40 plasmid constitutively expressing Renilla luciferase was added as an internal control to which firefly luciferase levels were normalized. Co-transfection with control (Control siRNA) and caspase-3 (CAS-3) siRNA was performed as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml), and luciferase activity was measured 18 h later. Average values (± S.D.) are presented from one representative experiment of at least three performed independently (*, #, and + indicate p < 0.05, Student's t test). Gray bars, cells expressing ectopic IRF-3; black bars, cells co-transfected to express ectopic vIRF-2. C, knockdown of caspase-3 elevates phospho-IRF-3 levels. HEK293 cells were transiently co-transfected with an expression vector for FLAG-tagged IRF-3 wild-type (IRF-3WT), and Xpress-tagged vIRF-2 (vIRF-2) or empty vector parental plasmid (pcDNA4), as indicated. Co-transfection with control (Control siRNA) and caspase-3 (CAS-3) siRNA was performed as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml). Protein extracts were prepared at the indicated times thereafter and analyzed by immunoblot with the following antibodies: anti-PARP, anti-Xpress (vIRF-2), anti-phospho-IRF-3 (α-p-IRF-3; Ser-396) and anti-β-actin. The anti-PARP antibody recognizes uncleaved PARP (Un-Cl, 116 kDa) and one cleaved fragment (Cleaved, 83 kDa). This experiment is representative of more than three performed independently. D, expression of vIRF-2 influences caspase-3/-7 activity. HEK293 cells were transiently transfected with an expression vector for FLAG-tagged IRF-3 wild type (IRF-3WT), and/or Xpress-tagged vIRF-2 (vIRF-2), as indicated. Twenty-four hours after plasmid transfection, the cells were treated with etoposide (50 μm) or DMSO and 30 min later transfected with poly(I:C) (10 μg/ml). Caspase-3/-7 activities were measured 18 h later. Average values (± S.D.) are presented from one representative experiment of three performed independently (*, p < 0.05, Student's t test). Black bars, cells co-transfected to express ectopic vIRF-2. E, pro-caspase-3 interacts with IRF-3 and vIRF-2. Upper panel: co-immunoprecipitation studies were performed with lysates of HEK293 cells that had been transfected with an expression vector for FLAG-tagged IRF-3 wild-type (IRF-3WT), or Xpress-tagged vIRF-2 (vIRF-2), as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml), and immunoprecipitation assays were performed 18 h later. Wild-type IRF-3 was immunoprecipitated with anti-FLAG polyclonal antibody and vIRF-2 with anti-Xpress monoclonal antibody. Immunoprecipitates and input extracts (lower panel) were separated by SDS-PAGE before immunoblotting with anti-Pro-caspase-3 antibody (α-Pro-cas-3). PE, protein extract; NT, non-transfected cells. Plasmid pCEP-4 expresses a negative control FLAG-tagged protein that did not immunoprecipitate pro-caspase-3. Note that all seven lanes were separated on the same gel and transferred to the same membrane, but because the vIRF-2 lanes were not contiguous with the others, intervening, irrelevant lanes were removed from the image. IP, antibody used for immunoprecipitation; IB, antibody used for immunoblot. Right panel, Input: these lanes demonstrate the expression levels for vIRF-2, IRF-3 and pro-caspase-3 in the lysates before immunoprecipitation.

Mentions: Because Z-VAD-FMK is a pan-caspase inhibitor, the identity of the caspase most likely involved in IRF-3 decay was sought. Studies with caspase-specific inhibitors indicated that IRF-3 decay was reduced when caspase-3 activity was inhibited. Indeed, when functional levels of caspase-3 were knocked down by >50% with specific siRNA (Fig. 3A), IRF-3WT-mediated transactivation of the IFN-β promoter in reporter gene assays was increased significantly in the presence of vIRF-2. Thus, under these circumstances, inhibition of IRF-3 transactivation by vIRF-2 was significantly mitigated (Fig. 3B, compare the normalized IFN-β promoter activity in bars #7 and #8). However, mitigation was not complete, as compared with the level of promoter activity achieved with caspase-3 siRNA alone (Fig. 3B, compare the normalized IFN-β promoter activity in bars #8 and #5), suggesting vIRF-2 inhibits IRF-3 activity by another, caspase-3-independent, mechanism. Furthermore, IRF-3 transactivation of the IFN-β promoter, even in the absence of vIRF-2, was increased significantly in the presence of caspase-3 siRNA compared with control siRNA (Fig. 3B, compare bars #5 and #4, respectively). These data confirm those of Fig. 2B: that IRF-3WT turnover was reduced in the presence of Z-VAD-FMK, even in the absence of vIRF-2 (Fig. 2B, top panel, compare the level of IRF-3 in the absence of Z-VAD-FMK in lanes 1–3 with the level in its presence in lanes 4–6; see also supplemental Fig. S2B(i)). They therefore provide evidence in support of a cellular mechanism in which caspase-3 participates to turnover IRF-3 following activation of the antiviral response by poly(I:C) transfection. vIRF-2 accelerates this process.


Identification of caspase-mediated decay of interferon regulatory factor-3, exploited by a Kaposi sarcoma-associated herpesvirus immunoregulatory protein.

Aresté C, Mutocheluh M, Blackbourn DJ - J. Biol. Chem. (2009)

The accelerated decay of activated wild-type IRF-3 by vIRF-2 depends on caspase-3 activity. A, siRNA knockdown of caspase-3. Functional levels of caspase-3 protein were analyzed with the Caspase-Glo®-3/7 assay kit (Promega). HEK293 cells were transfected with either Lit28i Polylinker ShortCut® siRNA mix (Control siRNA) or caspase-3 ShortCut® siRNA (CAS-3 siRNA) for 24 h. They were then transfected with poly(I:C) for 18 h before harvesting for caspase activity assay. Activity assays were repeated at least twice, and average values (± S.D.) are presented from one representative experiment (*, p < 0.05, Student's t test). B, knockdown of caspase-3 elevates IFN-β promoter-driven reporter gene activity. HEK293 cells were transiently co-transfected with the full-length IFN-β promoter reporter plasmid (p125-luc) and an expression vector of FLAG-tagged IRF-3 wild-type (IRF-3WT), or Xpress-tagged vIRF-2-expression vector (vIRF-2), as indicated. The pRLSV40 plasmid constitutively expressing Renilla luciferase was added as an internal control to which firefly luciferase levels were normalized. Co-transfection with control (Control siRNA) and caspase-3 (CAS-3) siRNA was performed as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml), and luciferase activity was measured 18 h later. Average values (± S.D.) are presented from one representative experiment of at least three performed independently (*, #, and + indicate p < 0.05, Student's t test). Gray bars, cells expressing ectopic IRF-3; black bars, cells co-transfected to express ectopic vIRF-2. C, knockdown of caspase-3 elevates phospho-IRF-3 levels. HEK293 cells were transiently co-transfected with an expression vector for FLAG-tagged IRF-3 wild-type (IRF-3WT), and Xpress-tagged vIRF-2 (vIRF-2) or empty vector parental plasmid (pcDNA4), as indicated. Co-transfection with control (Control siRNA) and caspase-3 (CAS-3) siRNA was performed as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml). Protein extracts were prepared at the indicated times thereafter and analyzed by immunoblot with the following antibodies: anti-PARP, anti-Xpress (vIRF-2), anti-phospho-IRF-3 (α-p-IRF-3; Ser-396) and anti-β-actin. The anti-PARP antibody recognizes uncleaved PARP (Un-Cl, 116 kDa) and one cleaved fragment (Cleaved, 83 kDa). This experiment is representative of more than three performed independently. D, expression of vIRF-2 influences caspase-3/-7 activity. HEK293 cells were transiently transfected with an expression vector for FLAG-tagged IRF-3 wild type (IRF-3WT), and/or Xpress-tagged vIRF-2 (vIRF-2), as indicated. Twenty-four hours after plasmid transfection, the cells were treated with etoposide (50 μm) or DMSO and 30 min later transfected with poly(I:C) (10 μg/ml). Caspase-3/-7 activities were measured 18 h later. Average values (± S.D.) are presented from one representative experiment of three performed independently (*, p < 0.05, Student's t test). Black bars, cells co-transfected to express ectopic vIRF-2. E, pro-caspase-3 interacts with IRF-3 and vIRF-2. Upper panel: co-immunoprecipitation studies were performed with lysates of HEK293 cells that had been transfected with an expression vector for FLAG-tagged IRF-3 wild-type (IRF-3WT), or Xpress-tagged vIRF-2 (vIRF-2), as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml), and immunoprecipitation assays were performed 18 h later. Wild-type IRF-3 was immunoprecipitated with anti-FLAG polyclonal antibody and vIRF-2 with anti-Xpress monoclonal antibody. Immunoprecipitates and input extracts (lower panel) were separated by SDS-PAGE before immunoblotting with anti-Pro-caspase-3 antibody (α-Pro-cas-3). PE, protein extract; NT, non-transfected cells. Plasmid pCEP-4 expresses a negative control FLAG-tagged protein that did not immunoprecipitate pro-caspase-3. Note that all seven lanes were separated on the same gel and transferred to the same membrane, but because the vIRF-2 lanes were not contiguous with the others, intervening, irrelevant lanes were removed from the image. IP, antibody used for immunoprecipitation; IB, antibody used for immunoblot. Right panel, Input: these lanes demonstrate the expression levels for vIRF-2, IRF-3 and pro-caspase-3 in the lysates before immunoprecipitation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2749101&req=5

Figure 3: The accelerated decay of activated wild-type IRF-3 by vIRF-2 depends on caspase-3 activity. A, siRNA knockdown of caspase-3. Functional levels of caspase-3 protein were analyzed with the Caspase-Glo®-3/7 assay kit (Promega). HEK293 cells were transfected with either Lit28i Polylinker ShortCut® siRNA mix (Control siRNA) or caspase-3 ShortCut® siRNA (CAS-3 siRNA) for 24 h. They were then transfected with poly(I:C) for 18 h before harvesting for caspase activity assay. Activity assays were repeated at least twice, and average values (± S.D.) are presented from one representative experiment (*, p < 0.05, Student's t test). B, knockdown of caspase-3 elevates IFN-β promoter-driven reporter gene activity. HEK293 cells were transiently co-transfected with the full-length IFN-β promoter reporter plasmid (p125-luc) and an expression vector of FLAG-tagged IRF-3 wild-type (IRF-3WT), or Xpress-tagged vIRF-2-expression vector (vIRF-2), as indicated. The pRLSV40 plasmid constitutively expressing Renilla luciferase was added as an internal control to which firefly luciferase levels were normalized. Co-transfection with control (Control siRNA) and caspase-3 (CAS-3) siRNA was performed as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml), and luciferase activity was measured 18 h later. Average values (± S.D.) are presented from one representative experiment of at least three performed independently (*, #, and + indicate p < 0.05, Student's t test). Gray bars, cells expressing ectopic IRF-3; black bars, cells co-transfected to express ectopic vIRF-2. C, knockdown of caspase-3 elevates phospho-IRF-3 levels. HEK293 cells were transiently co-transfected with an expression vector for FLAG-tagged IRF-3 wild-type (IRF-3WT), and Xpress-tagged vIRF-2 (vIRF-2) or empty vector parental plasmid (pcDNA4), as indicated. Co-transfection with control (Control siRNA) and caspase-3 (CAS-3) siRNA was performed as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml). Protein extracts were prepared at the indicated times thereafter and analyzed by immunoblot with the following antibodies: anti-PARP, anti-Xpress (vIRF-2), anti-phospho-IRF-3 (α-p-IRF-3; Ser-396) and anti-β-actin. The anti-PARP antibody recognizes uncleaved PARP (Un-Cl, 116 kDa) and one cleaved fragment (Cleaved, 83 kDa). This experiment is representative of more than three performed independently. D, expression of vIRF-2 influences caspase-3/-7 activity. HEK293 cells were transiently transfected with an expression vector for FLAG-tagged IRF-3 wild type (IRF-3WT), and/or Xpress-tagged vIRF-2 (vIRF-2), as indicated. Twenty-four hours after plasmid transfection, the cells were treated with etoposide (50 μm) or DMSO and 30 min later transfected with poly(I:C) (10 μg/ml). Caspase-3/-7 activities were measured 18 h later. Average values (± S.D.) are presented from one representative experiment of three performed independently (*, p < 0.05, Student's t test). Black bars, cells co-transfected to express ectopic vIRF-2. E, pro-caspase-3 interacts with IRF-3 and vIRF-2. Upper panel: co-immunoprecipitation studies were performed with lysates of HEK293 cells that had been transfected with an expression vector for FLAG-tagged IRF-3 wild-type (IRF-3WT), or Xpress-tagged vIRF-2 (vIRF-2), as indicated. Twenty-four hours after plasmid transfection, the cells were transfected with poly(I:C) (10 μg/ml), and immunoprecipitation assays were performed 18 h later. Wild-type IRF-3 was immunoprecipitated with anti-FLAG polyclonal antibody and vIRF-2 with anti-Xpress monoclonal antibody. Immunoprecipitates and input extracts (lower panel) were separated by SDS-PAGE before immunoblotting with anti-Pro-caspase-3 antibody (α-Pro-cas-3). PE, protein extract; NT, non-transfected cells. Plasmid pCEP-4 expresses a negative control FLAG-tagged protein that did not immunoprecipitate pro-caspase-3. Note that all seven lanes were separated on the same gel and transferred to the same membrane, but because the vIRF-2 lanes were not contiguous with the others, intervening, irrelevant lanes were removed from the image. IP, antibody used for immunoprecipitation; IB, antibody used for immunoblot. Right panel, Input: these lanes demonstrate the expression levels for vIRF-2, IRF-3 and pro-caspase-3 in the lysates before immunoprecipitation.
Mentions: Because Z-VAD-FMK is a pan-caspase inhibitor, the identity of the caspase most likely involved in IRF-3 decay was sought. Studies with caspase-specific inhibitors indicated that IRF-3 decay was reduced when caspase-3 activity was inhibited. Indeed, when functional levels of caspase-3 were knocked down by >50% with specific siRNA (Fig. 3A), IRF-3WT-mediated transactivation of the IFN-β promoter in reporter gene assays was increased significantly in the presence of vIRF-2. Thus, under these circumstances, inhibition of IRF-3 transactivation by vIRF-2 was significantly mitigated (Fig. 3B, compare the normalized IFN-β promoter activity in bars #7 and #8). However, mitigation was not complete, as compared with the level of promoter activity achieved with caspase-3 siRNA alone (Fig. 3B, compare the normalized IFN-β promoter activity in bars #8 and #5), suggesting vIRF-2 inhibits IRF-3 activity by another, caspase-3-independent, mechanism. Furthermore, IRF-3 transactivation of the IFN-β promoter, even in the absence of vIRF-2, was increased significantly in the presence of caspase-3 siRNA compared with control siRNA (Fig. 3B, compare bars #5 and #4, respectively). These data confirm those of Fig. 2B: that IRF-3WT turnover was reduced in the presence of Z-VAD-FMK, even in the absence of vIRF-2 (Fig. 2B, top panel, compare the level of IRF-3 in the absence of Z-VAD-FMK in lanes 1–3 with the level in its presence in lanes 4–6; see also supplemental Fig. S2B(i)). They therefore provide evidence in support of a cellular mechanism in which caspase-3 participates to turnover IRF-3 following activation of the antiviral response by poly(I:C) transfection. vIRF-2 accelerates this process.

Bottom Line: Here, we show that vIRF-2 mediates IRF-3 inactivation by a mechanism involving caspase-3, although vIRF-2 itself is not pro-apoptotic.Importantly, we also show that caspase-3 participates in normal IRF-3 turnover in the absence of vIRF-2, during the antiviral response induced by poly(I:C) transfection.These data provide unprecedented insight into negative regulation of IRF-3 following activation of the type I IFN antiviral response and the mechanism by which KSHV vIRF-2 inhibits this innate response.

View Article: PubMed Central - PubMed

Affiliation: Cancer Research UK Cancer Centre, School of Cancer Sciences, Vincent Drive, College of Medical and Dental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.

ABSTRACT
Upon virus infection, the cell mounts an innate type I interferon (IFN) response to limit the spread. This response is orchestrated by the constitutively expressed IFN regulatory factor (IRF)-3 protein, which becomes post-translationally activated. Although the activation events are understood in detail, the negative regulation of this innate response is less well understood. Many viruses, including Kaposi sarcoma-associated herpesvirus (KSHV), have evolved defense strategies against this IFN response. Thus, KSHV encodes a viral IRF (vIRF)-2 protein, sharing homology with cellular IRFs and is a known inhibitor of the innate IFN response. Here, we show that vIRF-2 mediates IRF-3 inactivation by a mechanism involving caspase-3, although vIRF-2 itself is not pro-apoptotic. Importantly, we also show that caspase-3 participates in normal IRF-3 turnover in the absence of vIRF-2, during the antiviral response induced by poly(I:C) transfection. These data provide unprecedented insight into negative regulation of IRF-3 following activation of the type I IFN antiviral response and the mechanism by which KSHV vIRF-2 inhibits this innate response.

Show MeSH
Related in: MedlinePlus