Salmonella Effector AvrA Regulation of Colonic Epithelial Cell Inflammation by Deubiquitination

Abstract

AvrA is a newly described bacterial effector existing in Salmonella. Here, we test the hypothesis that AvrA is a deubiquitinase that removes ubiquitin from two inhibitors of the nuclear factor-B (NF-B) pathway, IB␣ and ␤-catenin, thereby inhibiting the inflammatory responses of the host. The role of AvrA was assessed in intestinal epithelial cell models and in mouse models infected with AvrA-deficient and -sufficient Salmonella strains. We also purified AvrA and AvrA mutant proteins and characterized their deubiquitinase activity in a cellfree system. We investigated target gene and inflammatory cytokine expression, as well as effects on epithelial cell proliferation and apoptosis induced by AvrA-deficient and -sufficient bacterial strains in vivo. Our results show that AvrA blocks degradation of IB␣ and ␤-catenin in epithelial cells. AvrA deubiquitinates IB␣, which blocks its degradation and leads to the inhibition of NF-B activation. Target genes of the NF-B pathway, such as interleukin-6, were correspondingly down-regulated during bacterial infection with Salmonella expressing AvrA. AvrA also deubiquitinates and thus blocks degradation of ␤-catenin. Target genes of the ␤-catenin pathway, such as c-myc and cyclinD1, were correspondingly up-regulated with AvrA expression. Increased ␤-catenin further negatively regulates the NF-B pathway. Our findings suggest an important role for AvrA in regulating host inflammatory responses through NF-B and ␤-catenin pathways. (Am J Pathol

The Listeria monocytogenes InlC protein interferes with innate immune responses by targeting the IκB kinase subunit IKKα

“…The HIV type I Vpu protein competitively inhibits the β-TrCP/ubiquitin ligase-dependent degradation of IκB, thereby keeping NF-κB in the cytosol and resulting in inhibition of NF-κB activity in T cells (30,31). The Salmonella SseL effector protein deubiquitinates IκBα and prevents its degradation, thereby impairing NF-κB activation (32,33); another Salmonella effector, AvrA, also has an IκBα deubiquitinase activity (34). Similarly, ChlaDub1 of Chlamydia deubiquitinates IκB (35).…”

Salmonella Secreted Factor L Deubiquitinase of Salmonella typhimurium Inhibits NF-κB, Suppresses IκBα Ubiquitination and Modulates Innate Immune Responses

“…Salmonicida), were also shown to inhibit activation of NF-B downstream of IKK␤ (27,43). AvrA has recently been characterized as a DUB that removes ubiquitin from I B␣ and ␤-catenin, thereby preventing their degradation and subsequent NF-B activation (13).…”

“…This "stealth" capability would also favor establishment of chronic carriers, providing a reservoir from which to seed new infections. Similar to SseL, the AvrA protein, also impairs NF-B activation by impairing deubiquitination and subsequent degradation of I B␣ (13), but studies of AvrA Salmonella in mice have not focused on NF-B-driven or inflammatory events. OspG, a kinase produced by S. flexneri also prevents phospho-I B␣ degradation and subsequent NF-B activation.…”

“…Some types of pathogenic bacteria encode virulence factors that interfere with NF-B activation and ubiquitination, thus blunting signaling events involved in host defense. For example, AvrA, the bacterial effector from Salmonella, deubiquitinates I B␣ inhibiting NF-B activation (13). The OspG protein kinase of Shigella flexneri has also been shown to bind various ubiquitin-conjugating enzymes belonging to the SCF ␤ -TrCP complex, and prevent phospho-I B␣ degradation (14).…”

The ubiquitin system: a critical regulator of innate immunity and pathogen–host interactions

“…typhimurium is AvrA, which inhibits host inflammatory responses by deubiquitinating IκBα and β-catenin. 112 Similar to the IpaH family effectors in S. flexneri, Sa. typhimurium encodes three NELs: SlrP, SspH1 and SspH2.…”