title: Chondrocyte innate immune myeloid differentiation factor 88-dependent signaling drives procatabolic effects of the endogenous Toll-like receptor 2/Toll-like receptor 4 ligands low molecular weight hyaluronan and high mobility group box chromosomal protein 1 in mice.
abstract: OBJECTIVE
  Toll-like receptor 2 (TLR-2)/TLR-4-mediated innate immunity serves as a frontline antimicrobial host defense, but also modulates tissue remodeling and repair responses to endogenous ligands released during low-grade inflammation. We undertook the present study to assess whether the endogenous TLR-2/TLR-4 ligands low molecular weight hyaluronan (LMW-HA) and high mobility group box chromosomal protein 1 (HMGB-1), which are increased in osteoarthritic (OA) joints, drive procatabolic chondrocyte responses dependent on TLR-2 and TLR-4 signaling through the cytosolic adaptor myeloid differentiation factor 88 (MyD88).
  
  
  METHODS
  We studied mature femoral head cap cartilage explants and immature primary knee articular chondrocytes from TLR-2/TLR-4-double-knockout, MyD88-knockout, and congenic wild-type mice. Generation of nitric oxide (NO), degradation of hyaluronan, release of HMGB-1, matrix metalloproteinase 3 (MMP-3), and MMP-13, and protein expression of type X collagen were assessed by Griess reaction and Western blotting analyses. Expression of messenger RNA for type II and type X collagen, MMP-13, and RUNX-2 was examined by real-time quantitative reverse transcription-polymerase chain reaction.
  
  
  RESULTS
  Interleukin-1beta and TLR-2 and TLR-4 ligands induced both HMGB-1 release from chondrocytes and extracellular LMW-HA generation in normal chondrocytes. TLR-2/TLR-4(-/-) and MyD88(-/-) mouse cartilage explants and chondrocytes lost the capacity to mount procatabolic responses to both LMW-HA and HMGB-1, demonstrated by >95% suppression of NO production (P < 0.01), and attenuated induction of MMP-3 and MMP-13. Combined deficiency of TLR-2/TLR-4, or of MyD88 alone, also attenuated release of NO and blunted induction of MMP-3 and MMP-13 release. MyD88 was necessary for HMGB-1 and hyaluronidase 2 (which generates LMW-HA) to induce chondrocyte hypertrophy, which is implicated in OA progression.
  
  
  CONCLUSIONS
  MyD88-dependent TLR-2/TLR-4 signaling is essential for procatabolic responses to LMW-HA and HMGB-1, and MyD88 drives chondrocyte hypertrophy. Therefore, LMW-HA and HMGB-1 act as innate immune cytokine-like signals with the potential to modulate chondrocyte differentiation and function in OA progression.
authors.affiliation: VA Medical Center, San Diego, California 92161, USA. rliu@vapop.ucsd.edu
authors.forename: Ru
authors.initials: R
authors.lastname: Liu-Bryan
authors.forename: Robert
authors.initials: R
authors.lastname: Terkeltaub
mesh_terms: D000818:Animals; D015415:Biomarkers / Q000378:metabolism; D002358:Cartilage, Articular / Q000166:cytology / Q000378:metabolism; D049108:Cell Enlargement; D019902:Chondrocytes / Q000166:cytology / Q000378:metabolism*; D005260:Female; D015870:Gene Expression; D005786:Gene Expression Regulation; D024261:HMGB2 Protein / Q000378:metabolism*; D006820:Hyaluronic Acid / Q000378:metabolism*; D008297:Male; D053509:Matrix Metalloproteinase 13 / Q000235:genetics / Q000378:metabolism; D019278:Matrix Metalloproteinase 3 / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D008970:Molecular Weight; D053594:Myeloid Differentiation Factor 88; D009569:Nitric Oxide / Q000378:metabolism; D015398:Signal Transduction; D051195:Toll-Like Receptor 2 / Q000378:metabolism*; D051197:Toll-Like Receptor 4 / Q000378:metabolism*
chemical_list: D015415:Biomarkers; D024261:HMGB2 Protein; D053594:Myeloid Differentiation Factor 88; C493486:Tlr2 protein, mouse; C493487:Tlr4 protein, mouse; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D009569:Nitric Oxide; D006820:Hyaluronic Acid; D053509:Matrix Metalloproteinase 13; D019278:Matrix Metalloproteinase 3