Upon intracellular bacterial exposure, the Crohn's disease and sarcoidosis susceptibility protein NOD2 (nucleotide oligomerization domain protein 2) binds to the protein kinase RIP2 (receptor-interacting protein 2) to coordinate NF-kB (nuclear factor k B)-mediated cytokine responses. While RIP2 clearly has kinase activity, the function of its kinase domain has been enigmatic. Although originally classified as a serine-threonine kinase based on homology scans, we find that RIP2 also has tyrosine kinase activity. RIP2 undergoes autophosphorylation on Tyr 474 (Y474). This phosphorylation event is necessary for effective NOD2 signaling and does not occur in the presence of the most common Crohn's disease-associated NOD2 allele. Given this tyrosine kinase activity, a small-molecule inhibitor screen designed to identify pharmacologic agents that inhibit RIP2's tyrosine kinase activity was performed. At nanomolar concentrations, the EGFR (epidermal growth factor receptor) tyrosine kinase inhibitors gefitinib (Iressa) and erlotinib (Tarceva) were found to inhibit both RIP2 tyrosine phosphorylation and MDP (muramyl dipeptide)-induced cytokine release in a variety of NOD2 hyperactivation states. This effect is specific for RIP2 and does not depend on EGFR. The finding that RIP2 has tyrosine kinase activity and the finding that gefitinib and erlotinib, two agents already used clinically for cancer chemotherapy, can inhibit this activity suggest that RIP2's tyrosine kinase activity could be targeted specifically in the treatment of inflammatory diseases.[Keywords: NOD2; RIP2; ITCH; innate immunity; tyrosine kinase] Supplemental material is available at http://www.genesdev.org.

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House dust mites (HDMs) are one of the most significant environmental allergens in the establishment of the so-called "Atopic March." It is known that the immune response to HDM is Th2 dominant, but the innate mechanisms leading to HDM-induced type 2 responses are still not completely understood. A number of innate immune receptors have been implicated in the response to HDM including toll-like receptors, C-type lectin receptors, and protease activated receptors. NOD2 is a member of the NOD-like receptor family, which has been reported to be involved in the establishment of type 2 immunity and in blocking respiratory tolerance. NOD2 mediates its effects through its downstream effector kinase, receptor interacting protein (RIP2). It has not been shown if RIP2 is involved in the innate response to HDM and in the resulting generation of type 2 immunity. Furthermore, the role of RIP2 in modulating allergic airway inflammation has been controversial. In this study, we show that RIP2 is activated in airway epithelial cells in response to HDM and is important for the production of CCL2. Using a murine HDM asthma model, we demonstrate that lung pathology, local airway inflammation, inflammatory cytokines, HDM-specific IgG antibody production, and HDM-specific Th2 responses are all reduced in RIP2 knockout mice compared to WT animals. These data illustrate that RIP2 can be activated by a relevant allergic stimulus and that such activation can contribute to allergic airway inflammation. These findings also suggest that RIP2 inhibitors might have some efficacy in down-regulating the inflammatory response in type 2 dominated diseases.

The immune system is exquisitely balanced. It has the ability to effectively respond to and control infections while at the same time preventing inappropriate responses to self and environmental antigens. When this response goes awry, either through a failure to activate the immune response, or failure to terminate it, inflammatory pathology results. Posttranslational modifications (PTMs) such as ubiquitination and phosphorylation help ensure that the delicate balance underlying immune signal transduction is maintained. Ubiquitination and phosphorylation affect localization, activity, stability, and interactions of various components of the immune signal transduction machinery. Moreover, ubiquitination and phosphorylation are tightly linked, with one PTM affecting the other. Therefore, in order to find potential therapies for many immune-related pathologies, it is necessary to understand not only how the immune response is activated by ubiquitination and phosphorylation, but also how it is regulated by these PTMs at different stages of the response. An excellent system to study such activation and regulation is the NOD2 pathway. Dysregulation of NOD2 signaling is involved in the pathogenesis of a variety of inflammatory disorders including Crohn’s Disease, Early Onset Sarcoidosis, and Blau Syndrome. More recently NOD2 has been implicated in the development of autoimmune disease, allergy and asthma. This review will focus on what is currently known about how ubiquitination and phosphorylation regulate NOD2 signaling with particular emphasis on novel in vitro substrates which may serve as potential in vivo therapeutic targets for hyperactive NOD2 states.

bDysregulation of NOD2 signaling is implicated in the pathology of various inflammatory diseases, including Crohn's disease, asthma, and sarcoidosis, making signaling proteins downstream of NOD2 potential therapeutic targets. Inhibitor-of-apoptosis (IAP) proteins, particularly cIAP1, are essential mediators of NOD2 signaling, and in this work, we describe a molecular mechanism for cIAP1's regulation in the NOD2 signaling pathway. While cIAP1 promotes RIP2's tyrosine phosphorylation and subsequent NOD2 signaling, this positive regulation is countered by another E3 ubiquitin ligase, ITCH, through direct ubiquitination of cIAP1. This ITCH-mediated ubiquitination leads to cIAP1's lysosomal degradation. Pharmacologic inhibition of cIAP1 expression in ITCH ؊/؊ macrophages attenuates heightened ITCH ؊/؊ macrophage muramyl dipeptide-induced responses. Transcriptome analysis, combined with pharmacologic inhibition of cIAP1, further defines specific pathways within the NOD2 signaling pathway that are targeted by cIAP1. This information provides genetic signatures that may be useful in repurposing cIAP1-targeted therapies to correct NOD2-hyperactive states and identifies a ubiquitin-regulated signaling network centered on ITCH and cIAP1 that controls the strength of NOD2 signaling. R ecognition of microorganisms is mediated through various extracellular and intracellular pattern recognition receptors, mainly the Toll-like receptors (1), NOD-like receptors, and RIG-I-like receptors. However, this recognition and response are a double-edged sword. Too weak a response causes immunodeficiency, while too strong a response causes autoimmune or inflammatory syndromes. In no protein is this dichotomy more evident than in the NOD-like receptor NOD2. The NOD2 protein responds to muramyl dipeptide (MDP), a breakdown product of peptidoglycan found in both Gram-positive and Gram-negative bacteria (1, 2). Signaling through the NOD2 pathway culminates in the activation of inflammatory signaling cascades, stimulation of autophagy and antigen presentation, and release of antimicrobial peptides (3-9). These functions not only help in the direct control of invading microorganisms but also promote the appropriate cytokine and inflammatory milieu for directing the adaptive immune response to eradicate the offending pathogen. NOD2 has a critical role in immunologic homeostasis, as both loss-offunction polymorphisms and gain-of-function mutations in NOD2 precipitate inflammatory disease (2, 10-17). Though most notably recognized for its involvement in the pathogenesis of Crohn's disease through a mechanism involving a loss of downstream NOD2 signaling, it is becoming more and more appreciated that overactivation of NOD2 signaling is an underlying theme in a number of disorders such as early-onset sarcoidosis, Blau syndrome, allergy and asthma, and autoimmunity (13,14,(16)(17)(18)(19)(20)(21)(22)(23). Therefore, deeper insights into the molecular mechanisms underlying not only NOD2 activation but also NOD2 downregulation are critical for the unde...