title: Distinct roles for Nod2 protein and autocrine interleukin-1beta in muramyl dipeptide-induced mitogen-activated protein kinase activation and cytokine secretion in human macrophages.
abstract: Elucidating factors regulating Crohn's disease-associated nucleotide-binding oligomerization domain 2 (Nod2) responses is critical to understanding the mechanisms of intestinal immune homeostasis. Stimulation of primary monocyte-derived macrophages by muramyl dipeptide (MDP), a component of bacterial peptidoglycan and specific Nod2 ligand, produces cytokines, including IL-1β. We found that IL-1β blockade profoundly inhibits MDP-induced cytokine production in human monocyte-derived macrophages, demonstrating a key role for IL-1β autocrine secretion in Nod2-mediated responses. Importantly, although MAPK activation has previously been attributed directly to Nod2 signaling, we determined that the IL-1β autocrine loop is responsible for the majority of MDP-induced MAPK activation. Because the critical effects of IL-1β autocrine secretion on MAPK activation are observed as early as 10 min after Nod2 stimulation, we hypothesized that secretion of IL-1β from preexisting intracellular pro-IL-1β stores is necessary for optimal MDP-mediated cytokine induction. Consistently, we detected IL-1β secretion within 10 min of MDP treatment. Moreover, caspase-1 inhibition significantly attenuates MDP-mediated early MAPK activation. Importantly, selective JNK/p38 activation is sufficient to rescue the decreased cytokine secretion during Nod2 stimulation in the absence of autocrine IL-1β. Finally, we found that the IL-1β autocrine loop significantly enhances responses by a broad range of pattern recognition receptors. Taken together, MDP stimulation activates Nod2 to process and release preexisting pro-IL-1β stores in a caspase-1-dependent fashion; this secreted IL-1β, in turn, contributes to the majority of MDP-initiated MAPK activation and leads to subsequent cytokine secretion. Our findings clarify mechanisms of IL-1β contributions to Nod2 responses and elucidate the dominant role of IL-1β in MDP-initiated MAPK and cytokine secretion.
authors.affiliation: Department of Internal Medicine, Yale University, New Haven, Connecticut 06510, USA.
authors.forename: Matija
authors.initials: M
authors.lastname: Hedl
authors.forename: Clara
authors.initials: C
authors.lastname: Abraham
mesh_terms: D000119:Acetylmuramyl-Alanyl-Isoglutamine / Q000494:pharmacology*; D000276:Adjuvants, Immunologic / Q000494:pharmacology*; D019898:Autocrine Communication / Q000187:drug effects / Q000502:physiology*; D020170:Caspase 1 / Q000235:genetics / Q000378:metabolism; D061945:Caspase Inhibitors; D002460:Cell Line; D004789:Enzyme Activation / Q000187:drug effects / Q000502:physiology; D006801:Humans; D053583:Interleukin-1beta / Q000187:drug effects / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D048670:MAP Kinase Kinase 4 / Q000235:genetics / Q000378:metabolism*; D008264:Macrophages / Q000378:metabolism*; D053473:Nod2 Signaling Adaptor Protein / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D048051:p38 Mitogen-Activated Protein Kinases / Q000235:genetics / Q000378:metabolism*
chemical_list: D000276:Adjuvants, Immunologic; D061945:Caspase Inhibitors; D053583:Interleukin-1beta; C429018:NOD2 protein, human; D053473:Nod2 Signaling Adaptor Protein; D000119:Acetylmuramyl-Alanyl-Isoglutamine; D048051:p38 Mitogen-Activated Protein Kinases; D048670:MAP Kinase Kinase 4; D020170:Caspase 1