TLR2 signaling in chondrocytes drives calcium pyrophosphate dihydrate and monosodium urate crystal-induced nitric oxide generation.
Microcrystals of calcium pyrophosphate dihydrate (CPPD) and monosodium urate (MSU) deposited in synovium and articular cartilage initiate joint inflammation and cartilage degradation in large part by binding and directly activating resident cells. TLRs trigger innate host defense responses to infectious pathogens, and the expression of certain TLRs by synovial fibroblasts has revealed the potential for innate immune responses to be triggered by mesenchymally derived resident cells in the joint. In this study we tested the hypothesis that chondrocytes also express TLRs and that one or more TLRs centrally mediate chondrocyte responsiveness to CPPD and MSU crystals in vitro. We detected TLR2 expression in normal articular chondrocytes and up-regulation of TLR2 in osteoarthritic cartilage chondrocytes in situ. We demonstrated that transient transfection of TLR2 signaling-negative regulator Toll-interacting protein or treatment with TLR2-blocking Ab suppressed CPPD and MSU crystal-induced chondrocyte release of NO, an inflammatory mediator that promotes cartilage degeneration. Conversely, gain-of-function of TLR2 in normal chondrocytes via transfection was associated with increased CPPD and MSU crystal-induced NO release. Canonical TLR signaling by parallel pathways involving MyD88, IL-1R-associated kinase 1, TNF receptor-associated factor 6, and IkappaB kinase and Rac1, PI3K, and Akt critically mediated NO release in chondrocytes stimulated by both CPPD and MSU crystals. We conclude that CPPD and MSU crystals critically use TLR2-mediated signaling in chondrocytes to trigger NO generation. Our results indicate the potential for innate immunity at the level of the articular chondrocyte to directly contribute to inflammatory and degenerative tissue reactions associated with both gout and pseudogout.
Liu-Bryan
Ru
R
Veterans Affairs Medical Center, San Diego, CA 92161, USA.
Pritzker
Kenneth
K
Firestein
Gary S
GS
Terkeltaub
Robert
R
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D000943:Antigens, Differentiation / Q000378:metabolism; D001483:Base Sequence; D002131:Calcium Pyrophosphate / Q000378:metabolism / Q000633:toxicity*; D002417:Cattle; D002478:Cells, Cultured; D002805:Chondrocalcinosis / Q000209:etiology / Q000276:immunology / Q000378:metabolism; D019902:Chondrocytes / Q000187:drug effects* / Q000276:immunology / Q000378:metabolism*; D003460:Crystallization; D018076:DNA, Complementary / Q000235:genetics; D015870:Gene Expression; D006073:Gout / Q000209:etiology / Q000276:immunology / Q000378:metabolism; D006801:Humans; D051550:I-kappa B Kinase; D007113:Immunity, Innate; D053592:Interleukin-1 Receptor-Associated Kinases; D008562:Membrane Glycoproteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D053594:Myeloid Differentiation Factor 88; D009569:Nitric Oxide / Q000096:biosynthesis*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D011971:Receptors, Immunologic / Q000378:metabolism; D015398:Signal Transduction; D048029:TNF Receptor-Associated Factor 6 / Q000378:metabolism; D051195:Toll-Like Receptor 2; D051193:Toll-Like Receptors; D014527:Uric Acid / Q000378:metabolism / Q000633:toxicity*; D020830:rac1 GTP-Binding Protein / Q000378:metabolism
D048868:Adaptor Proteins, Signal Transducing; D000943:Antigens, Differentiation; D002131:Calcium Pyrophosphate; D018076:DNA, Complementary; D051550:I-kappa B Kinase; D053592:Interleukin-1 Receptor-Associated Kinases; D008562:Membrane Glycoproteins; D053594:Myeloid Differentiation Factor 88; D009569:Nitric Oxide; D019869:Phosphatidylinositol 3-Kinases; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D011971:Receptors, Immunologic; D048029:TNF Receptor-Associated Factor 6; D051195:Toll-Like Receptor 2; D051193:Toll-Like Receptors; D014527:Uric Acid; D020830:rac1 GTP-Binding Protein; C507779:MYD88 protein, human; C495342:TLR2 protein, human; C494918:AKT1 protein, human; C496559:CHUK protein, human; C496560:IKBKB protein, human; C496565:IKBKE protein, human