title: HIF-1-Dependent IL-6 Activation in Articular Chondrocytes Initiating Synovitis in Femoral Head Ischemic Osteonecrosis.
abstract: BACKGROUND
  Ischemic osteonecrosis of the femoral head in children is associated with chronic hip synovitis and increased levels of the pro-inflammatory cytokine interleukin-6 (IL-6) in the synovial fluid due to unknown mechanisms. The purpose of this study was to investigate hypoxia-inducible factor-1 (HIF-1) activation as a molecular mechanism linking the induction of ischemic osteonecrosis to IL-6 production and the initiation of hip synovitis.
  
  
  METHODS
  Ischemic osteonecrosis was surgically induced in the right femoral head of 6 piglets. A histologic score, synovial fluid volume, and IL-6 level were used to assess hip synovitis. IL-6 immunostaining of articular cartilage and synovial tissue was performed as well. To study the role of HIF-1 in IL-6 activation, in vitro experiments using an HIF-1α activator (deferoxamine) and inhibitor (HIF-1 small interfering-RNA [siRNA]) were carried out. Synovial cell responses to hypoxic chondrocyte-conditioned media with and without an IL-6 receptor blocker (tocilizumab) were assessed on the basis of IL-1β and tumor necrosis factor-alpha (TNF-α) gene expressions and with a synovial cell-proliferation assay.
  
  
  RESULTS
  Induction of ischemic osteonecrosis produced hip synovitis and increased IL-6 levels in the synovial fluid. Immunostaining and protein analysis demonstrated articular chondrocytes as a source of increased IL-6 production. When articular chondrocytes were cultured under hypoxic conditions, significantly increased HIF-1α and IL-6 expressions were observed. Under hypoxic culture conditions, IL-6 gene expression was significantly increased by HIF-1α activation using deferoxamine and inhibited by HIF-1α inhibition using HIF-1 siRNA. Synovial cells exposed to hypoxic chondrocyte-conditioned medium showed significant increases in IL-1β and TNF-α gene expressions and cell proliferation, which were inhibited by the IL-6 receptor blocker tocilizumab.
  
  
  CONCLUSIONS
  Induction of ischemic osteonecrosis results in IL-6 production in the articular cartilage through an HIF-1-dependent pathway. IL-6 produced by hypoxic articular chondrocytes stimulates inflammatory cytokine responses in synovial cells, which were significantly decreased by tocilizumab.
  
  
  CONCLUSIONS
  This study provides new insight into the inherent relationship between the induction of ischemia and the initiation of hip synovitis following ischemic osteonecrosis and suggests a potential therapeutic target in the treatment of the synovitis.
authors.affiliation: Center for Excellence in Hip Disorders, Texas Scottish Rite Hospital for Children, Dallas, Texas Department of Orthopaedic Surgery, University of Texas Southwestern Medical Center, Dallas, Texas.
authors.forename: Ryosuke
authors.initials: R
authors.lastname: Yamaguchi
authors.affiliation: Faculty of Budo and Sport Studies, Tenri University, Tenri, Nara, Japan.
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Kamiya
authors.affiliation: Center for Excellence in Hip Disorders, Texas Scottish Rite Hospital for Children, Dallas, Texas Department of Orthopaedic Surgery, University of Texas Southwestern Medical Center, Dallas, Texas.
authors.forename: Naga Suresh
authors.initials: NS
authors.lastname: Adapala
authors.affiliation: Department of Orthopaedic Surgery, University of Connecticut Health Center, Farmington, Connecticut.
authors.forename: Hicham
authors.initials: H
authors.lastname: Drissi
authors.affiliation: Center for Excellence in Hip Disorders, Texas Scottish Rite Hospital for Children, Dallas, Texas Department of Orthopaedic Surgery, University of Texas Southwestern Medical Center, Dallas, Texas Harry.Kim@tsrh.org.
authors.forename: Harry K W
authors.initials: HK
authors.lastname: Kim
mesh_terms: D000818:Animals; D061067:Antibodies, Monoclonal, Humanized / Q000494:pharmacology; D002358:Cartilage, Articular / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D049109:Cell Proliferation / Q000187:drug effects / Q000502:physiology; D019902:Chondrocytes / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D004195:Disease Models, Animal; D005271:Femur Head Necrosis / Q000378:metabolism* / Q000473:pathology; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000378:metabolism*; D053583:Interleukin-1beta / Q000378:metabolism; D015850:Interleukin-6 / Q000378:metabolism*; D019947:Receptors, Interleukin-6 / Q000037:antagonists & inhibitors; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D013552:Swine; D013582:Synovial Fluid / Q000378:metabolism*; D013583:Synovial Membrane / Q000187:drug effects / Q000378:metabolism / Q000473:pathology; D013585:Synovitis / Q000378:metabolism* / Q000473:pathology; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism
chemical_list: D061067:Antibodies, Monoclonal, Humanized; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D053583:Interleukin-1beta; D015850:Interleukin-6; D019947:Receptors, Interleukin-6; D014409:Tumor Necrosis Factor-alpha; C502936:tocilizumab