Background: Mutations predisposing to Crohn’s disease (CD) have been mapped to the CARD15/Nod2 locus,which encodes a cytoplasmic receptor hereafter referred to as Nod2, a member of the NACHT-LRR (NLR) familyof pattern recognition receptors. The binding of bacterial muramyl dipeptide (MDP) to Nod2 triggers theactivation of the nuclear factor-κB (NFκB) pathway, thereby inducing a number of pro-inflammatory genes. Themost common variant of Nod2 associated with CD is the frame shift mutation L1007fs, which results in atruncated form of the protein unable to respond to MDP. Despite active research, little is known about howNod2 is regulated. The aim of this study was to investigate if the cellular Nod2 protein level is regulated by theubiquitin-proteasome pathway.
Material and Methods/Results: Nod2 was shown to be subjected to rapid turnover in the colorectal cancer cellline SW480 as measured by immunoprecipitation following inhibition of protein synthesis with cyklohexamide.Immunoprecipitation of Nod2 also revealed co-precipitation of ubiquitin, suggesting that Nod2 wasubiquitinated. In line with this observation, inhibition of the proteasome using MG-132 or lactacystin, resultedin increased levels of Nod2 in the cells. UBE2G2, an E2 enzyme, thus conferring specificity of ubiquitin binding,was identified to have affinity for the CARD domain of Nod2. Activation of Nod2 with MDP enhanced itsubiquitination, and increasing amounts of UBE2G2 in the cells abrogated NF-κB-activation, suggesting thatubiquitination of Nod2 may be important for the resolution of the inflammatory signal.
Conclusion: Taken together, our results show that the cellular level of Nod2 protein is regulated via theubiquitin-proteasome pathway, suggesting that Nod2-driven inflammation may be resolved through rapiddegradation of Nod2. Consequently, not only polymorphisms in Nod2 directly, but also in genes regulatingNod2 protein levels may contribute to the susceptibility to Crohn’s disease.