Cell stimulation with viral double-stranded (ds) RNA and bacterial lipopolysaccharide (LPS) activate Toll-like receptors 3 (TLR3) and TLR4, respectively, triggering the activation the activation of two IKK-related serine/threonine kinases, TANK-binding kinase 1 (TBK1) and IκB kinase ε (IKKε, IKBKE) which directly phosphorylate interferon regulatory factor 3 (IRF3) and IRF7 promoting their dimerization and translocation into the nucleus. Although both kinases show structural and functional similarities, it seems that TBK1 and IKBKE differ in their regulation of downstream signaling events of TLR3/TLR4.
IRF3 activation and interferon β (IFNβ) production by poly(I:C), a synthetic analog of dsRNA, are decreased in TBK1-deficient mouse fibroblasts, whereas normal activation was observed in the IKBKE-deficient fibroblasts. However, in double-deficient mouse fibroblasts, the activation of IRF3 is completely abolished, suggesting a partially redundant functions of TBK1 and IKKε (IKBKE) (Hemmi H et al., 2004).
The poly(I:C)-induced phosphorylation of TBK1 and IRF3 was abolished in TRIF (TICAM1)-knockout human keratinocyte HACAT cells (Bakshi S et al., 2017). TICAM1 is utilized as an adaptor protein by TLR3 and TLR4 (Yamamoto M et al., 2003).
TLR3 recruits and activates PI3 kinase (PI3K), which activates the downstream kinase, Akt, leading to full phosphorylation and activation of IRF3 (Sarkar SN et al., 2004). When PI3K is not recruited to TLR3 or its activity is blocked, IRF3 is only partially phosphorylated and fails to bind the promoter of the target gene (Sarkar SN et al., 2004).
