Signaling by most TLRs activates the transcription factor NFκB (Fig. 3.15), which is related to DIF, the factor activated by Drosophila Toll. Mammalian TLRs also activate several members of the interferon regulatory factor (IRF) transcription factor family through a second pathway, and they activate members of the activator protein 1 (AP-1) family, such as c-Jun, through yet another signaling pathway involving mitogen-activated protein kinases (MAPKs). NFκB and AP-1 act primarily to induce the expression of pro-inflammatory cytokines and chemotactic factors. The IRF factors IRF3 and IRF7 are particularly important for inducing antiviral type I interferons, whereas a related factor, IRF5, is involved in the production of pro-inflammatory cytokines. Here we will describe how TLR signaling induces the transcription of various cytokine genes; later in the chapter, we will explain how those cytokines exert their various actions.
We consider first the signaling pathway triggered by TLRs that use MyD88. Two protein domains of MyD88 are responsible for its function as an adaptor. MyD88 has a TIR domain at its carboxy terminus that associates with the TIR domains in the TLR cytoplasmic tails. At its amino terminus, MyD88 has a death domain, so named because it was first identified in signaling proteins involved in apoptosis, a type of programmed cell death. The MyD88 death domain associates with a similar death domain present in other intracellular signaling proteins. Both MyD88 domains are required for signaling, since rare mutations in either domain are associated with immunodeficiency characterized by recurrent bacterial infections in humans. The MyD88 death domain recruits and activates two serine–threonine protein kinases—IRAK4 (IL-1receptor associated kinase 4) and IRAK1—via their death domains. This IRAK complex performs two functions: it recruits enzymes that produce a signaling scaffold, and uses this scaffold to recruit other molecules that are then phosphorylated by the IRAKs.
To form a signaling scaffold, the IRAK complex recruits the enzyme TRAF6 (tumor necrosis factor receptor-associated factor 6), which is an E3 ubiquitin ligase that acts in cooperation with UBC13, an E2 ubiquitin ligase, and its cofactor Uve1A (together called TRIKA1) (see Fig. 3.15). The combined activity of TRAF-6 and UBC13 is to ligate (unite with a chemical bond) one ubiquitin molecule to another protein, which can be another ubiquitin molecule, and thereby generate protein polymers. The polyubiquitin involved in signaling contains linkages between the lysine 63 on one ubiquitin and the carboxy terminus of the next, forming K63 linkages. This polyubiquitin polymer can be initiated on other proteins, including TRAF-6 itself, or produced as free linear ubiquitin polymers, and can be extended to produce polyubiquitin chains that act as a platform—or scaffold—that bind to other signaling molecules. Next, the scaffold recruits a signaling complex consisting of the polyubiquitin-binding adaptor proteins TAB1, TAB2, and the serine–threonine kinase TAK1 (see Fig. 3.15). By being brought onto the scaffold, TAK1 is phosphorylated by the IRAK complex, and activated TAK1 propagates signaling by activating certain MAPKs, such as c-Jun terminal kinase (JNK) and MAPK14 (p38 MAPK). These then activate AP-1-family transcription factors that transcribe cytokine genes.