Differential requirement for TANK-binding kinase-1 in type I interferon responses to toll-like receptor activation and viral infection

Abstract

TANK-binding kinase-1 (TBK1) and the inducible IkappaB kinase (IKK-i) have been shown recently to activate interferon (IFN) regulatory factor-3 (IRF3), the primary transcription factor regulating induction of type I IFNs. Here, we have compared the role and specificity of TBK1 in the type I IFN response to lipopolysaccharide (LPS), polyI:C, and viral challenge by examining IRF3 nuclear translocation, signal transducer and activator of transcription 1 phosphorylation, and induction of IFN-regulated genes. The LPS and polyI:C-induced IFN responses were abolished and delayed, respectively, in macrophages from mice with a targeted disruption of the TBK1 gene. When challenged with Sendai virus, the IFN response was normal in TBK1(-/-) macrophages, but defective in TBK1(-/-) embryonic fibroblasts. Although both TBK1 and IKK-i are expressed in macrophages, only TBK1 but not IKK-i was detected in embryonic fibroblasts by Northern blotting analysis. Furthermore, the IFN response in TBK1(-/-) embryonic fibroblasts can be restored by reconstitution with wild-type IKK-i but not a mutant IKK-i lacking kinase activity. Thus, our studies suggest that TBK1 plays an important role in the Toll-like receptor-mediated IFN response and is redundant with IKK-i in the response of certain cell types to viral infection.

Figure 1.

TBK1 is required for LPS-mediated activation of type I IFN responses. TBK1^+/+TNFR1^+/− and TBK1^−/−TNFR1^−/− BMMs were stimulated with 10 ng/ml LPS for the indicated time points. (A) Nuclear fractions were probed for IRF3 and USF2 as a loading control for nuclear proteins. (B) Total cell extracts were probed for phospho-STAT1 and total STAT1. (C) Total RNA was extracted and analyzed by Q-PCR for expression of IFNβ, IP-10, IFNα5, IRF7, IL-15, and Mx1.

Figure 2.

TBK1-deficient BMMs have defective type I IFN responses to polyI:C. TBK1^+/+TNFR1^+/− and TBK1^−/−TNFR1^−/− BMMs were stimulated with 1 μg/ml polyI:C for the indicated time points. (A) Nuclear fractions were probed for IRF3 and USF2 as a loading control. (B) Total cell extracts were probed for phospho-STAT1 and total STAT1. (C) Total RNA was extracted and analyzed by Q-PCR for expression of IFNβ, IP-10, IFNα5, IRF7, IL-15, and Mx1.

Figure 3.

TBK1-deficient BMMs induce normal NF-κB responses to LPS and polyI:C. TBK1^+/+TNFR1^+/− and TBK1^−/−TNFR1^−/− BMMs were stimulated with 10 ng/ml LPS or 1 μg/ml polyI:C for the indicated time points. (A) Nuclear fractions were probed for p65 and USF2 as a loading control. (B) EMSA was performed by incubating nuclear extracts were with an NF-κB-specific oligonucleotide. (C) Total RNA was extracted and analyzed by Q-PCR for expression of ICAM1 and IκBα.

Figure 4.

TBK1-deficient BMMs have normal IFN responses to SeV. TBK1^+/+TNFR1^+/− and TBK1^−/−TNFR1^−/− BMMs were infected with SeV for the indicated time points. (A) Nuclear fractions were probed for IRF3 and USF2 as a loading control. (B) Total cell extracts were probed for phospho-STAT1 and total STAT1. (C) Total RNA was extracted and analyzed by Q-PCR for expression of IFNβ, IP-10, IFNα5, IRF7, IL-15, and Mx1.

Figure 5.

TBK1-deficient MEF cells have impaired IFN responses to SeV. Wild-type and TBK1^−/− MEF cells were infected with SeV for the indicated time points. (A) Nuclear fractions were probed for IRF3 and USF2 as a loading control. Total cell extracts were probed for phospho-STAT1 and total STAT1. (B) Total RNA was extracted and analyzed by Q-PCR for expression of IFNβ, IP-10, IRF7, and IL-15. (C) EMSA was performed by incubating nuclear extracts were with an NF-κB-specific oligonucleotide. (D) Nuclear fractions were probed for p65 and USF2. (E) Total RNA was extracted and analyzed by Q-PCR for expression of ICAM1 and IκBα.

Figure 6.

IKK-i is differentially expressed in BMMs and MEFs, and can rescue a TBK1 deficiency during SeV infection. (A) Total RNA extracted from BMMs, wild-type MEFs, and TBK1^−/− MEFs were probed by Northern blot for expression of TBK1 and IKK-i. 28 and 18 sRNA bands are shown as loading controls. (B) Wild-type MEFs, TBK1^−/− MEFs, and TBK1^−/− MEFs reconstituted with wild-type TBK1 or IKK-i, or their kinase-inactive (KA) mutants were infected with SeV for the time points indicated. Nuclear fractions were probed for IRF3, p65, and USF2 as a loading control. Total cell extracts were probed for phospho-STAT1 and total STAT1. (C) Cells were infected with SeV as in B and total RNA was extracted and analyzed by Q-PCR for expression of IFNβ.