Immune recognition of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRR) often activates proinflammatory nuclear factor kappa B (NF-κB) signalling. Lipopolysaccharide (LPS) is a well-known PAMP produced by gram-negative bacteria. LPS is recognized by toll like receptor 4 (TLR4) and is a strong activator of NF-κB inflammatory responses (Akashi S et al. 2003). LPS is also recognized in the cytosol by mouse caspase-11 and related human caspase-4 and caspase-5, which stimulate pyroptosis, a proinflammatory form of cell death (Kayagaki N et al. 2011; Shi J et al. 2015). Key metabolic intermediates in LPS biosynthesis, d-glycero-β-d-manno-heptose 1,7-bisphosphate (HBP) and ADP L-glycero-β-d-manno-heptose (ADP-heptose) were reported to activate the NF-κB pathway and trigger the innate immune responses (Milivojevic M et al. 2017; Zimmermann S et al. 2017; Zhou P et al. 2018; García-Weber D; 2018). ADP-heptose but not HBP can enter host cells autonomously (Zhou P et al. 2018). During infection, ADP-heptose or HBP translocate into the host cytosol where their presence is sensed by alpha-protein kinase 1 (ALPK1) (Zimmermann S et al. 2017; Zhou P et al. 2018). ADP-heptose directly binds and activates ALPK1 (Garcia-Weber D et al. 2018; Zhou P et al. 2018); instead, HBP is converted by host-derived adenylyltransferases, such as nicotinamide nucleotide adenylyltransferases, to ADP-heptose 7-P, a substrate which can then activate ALPK1 (Zhou P et al. 2018). The ADP-heptose binding to ALPK1 is thought to trigger conformational changes and stimulate the kinase domain of ALPK1 (Zhou P et al. 2018). ALPK1 kinase activity in turn leads to the phosphorylation-dependent oligomerization of the tumor necrosis factor (TNF-α) receptor–associated factor (TRAF)–interacting protein with the forkhead-associated domain (TIFA) (Zimmermann S et al. 2017; Zhou P et al. 2018). This process activates TRAF6 oligomerization and ubiquitination, and the recruitment of transforming growth factor β-activated kinase 1 (TAK1)-binding protein 2 (TAB2), a component of the TAK1 (MAP3K7) complex (Ea CK et al. 2004; Gaudet RG et al. 2017). This TIFA oligomer signaling platform was given the term: TIFAsome. TIFAsome-activated TAK1 induces NF-κB nuclear translocation and proinflammatory gene expression. The ALPK1-TIFA signaling pathway has been identified in human embryonic kidney cells, intestinal epithelial cells, gastric cells and cervical cancer cells (Gaudet RG et al. 2015, 2017; Stein SC et al. 2017; Gall A et al. 2017; Zimmermann S et al. 2017; Milivojevic M et al. 2017; Zhou P et al. 2018). In vivo studies demonstrate that ADP-heptose and Burkholderia cenocepacia trigger massive inflammatory responses with increased production of several NF-κB-dependent cytokines and chemokines in wild type (WT), but not in Alpk1-/- mice (Zhou P et al. 2018).

This Reactome module describes ALPK1 as a cytosolic innate immune receptor for bacterial ADP-heptose.