Ubiquitin-Editing Enzyme A20 Promotes Tolerance to Lipopolysaccharide in Enterocytes

Abstract

Although enterocytes are capable of innate immune responses, the intestinal epithelium is normally tolerant to commensal bacteria. To elucidate the mechanisms of tolerance, we examined the effect of preexposure to LPS on activation of p38, c-Jun, and NF-κB in enterocytes by several inflammatory and stress stimuli. Shortly after the initial LPS challenge, enterocytes become tolerant to restimulation with LPS or CpG DNA, but not with IL-17 or UV. The state of tolerance, which lasts 20–26 h, temporally coincides with LPS-induced expression of the anti-inflammatory ubiquitin-editing enzyme A20. Small interfering RNA silencing of A20 prevents tolerance, whereas ectopic expression of A20 blocks responses to LPS and CpG DNA, but not to IL-17 or UV. A20 levels in the epithelium of the small intestine are low at birth and following gut decontamination with antibiotics, but high under conditions of bacterial colonization. In the small intestine of adult rodents, A20 prominently localizes to the luminal interface of villus enterocytes. Lower parts of the crypts display relatively low levels of A20, but relatively high levels of phospho-p38. Gut decontamination with antibiotics reduces the levels of both A20 and phospho-p38. Along with the fact that A20-deficient mice develop severe intestinal inflammation, our results indicate that induction of A20 plays a key role in the tolerance of the intestinal epithelium to TLR ligands and bacteria.

Induction of Endotoxin Tolerance by Pathogenic Neisseria Is Correlated with the Inflammatory Potential of Lipooligosaccharides and Regulated by MicroRNA-146a

“…A negative regulator of TLR4, A20, is an ubiquitin-editing enzyme thought to play a key role in the development of LPS-induced tolerance through downregulation of p38, c-Jun, and NF-kB (44). When induced by LPS and enteric bacteria, A20 targets various signaling molecules for degradation, and induction of A20 coincides with the establishment of hyporesponsiveness to repeated stimulation with LPS.…”

Genome-Wide Expression Profiling Identifies an Impairment of Negative Feedback Signals in the Crohn’s Disease-Associated NOD2 Variant L1007fsinsC

“…It is tempting to speculate that this failure to induce restrictive cellular programs may also lead to a net effect of enhanced NF-kB activation via other signaling pathways (e.g., TLRs) (46). The function of A20 in tolerance to commensals and the role of A20 variants in immunopathologies may underline the importance of this negative regulation in inflammatory diseases (46,47). This aspect may also be of importance in the etiology of CD, in which an impaired host response to the resident microflora has been demonstrated (48).…”

Chronic Inflammation in CF Airways - A Persistent Issue for A20

“…A20 is also involved in the promotion of tolerance to (commensal) bacteria. In the rat intestine A20 prominently localizes to the luminal side of the villus enterocytes, while lower (more protected) parts of the crypts display relatively low levels of A20 [30]. In culture, repeated stimulation of rat enterocytes (e.g.…”

“…In culture, repeated stimulation of rat enterocytes (e.g. IEC6/18 cells) with LPS or of human monocytic THP-1 cells with Pam3CSK4, TNF-α or IL-1β results in an inhibition of subsequent LPS-induced activation of p38, c-Jun and NF-κB lasting up to 28 h in proliferating and over 72 h in stationary cultures [30] (Figure 1). Moreover, antibiotic treatment of rats led to'gut decontamination' and reduced A20 levels in the epithelium [30].…”