title: Thyroxine stimulates transglutaminase activity in articular chondrocytes.
abstract: OBJECTIVE
  Thyroid hormones induce features of the hypertrophic phenotype in mature articular chondrocytes as well as in growth plate chondrocytes. Hypertrophic chondrocytes are responsible for extracellular matrix mineralization, with formation of bone mineral in growth plate cartilage and pathologic calcium crystals in aging articular cartilage. Elevated activity levels of the two transglutaminase (Tgase) enzymes (type II Tgase and Factor XIIIA (FXIIIA)) have recently been described as additional features of hypertrophic growth plate chondrocytes. Because Tgases may participate in pathologic mineralization in aging cartilage, we explored the effects of thyroid hormones on Tgase activity in articular chondrocytes.
  
  
  METHODS
  Adult porcine articular chondrocytes were incubated with or without 250-750nM L-thyroxine (T4) or 10-100 nM 3,3',5-tri-iodothyronine (T3). Tgase activity was measured with a standard radiometric assay. The effects of thyroid hormones on protein and mRNA levels of type II Tgase and FXIIIA were determined. As Tgase activity can be stimulated by proteases, endoproteinase levels were also measured. The mechanisms of these effects were explored.
  
  
  RESULTS
  T4 (750 nM) or T3 (100 nM) stimulated Tgase activity by twofold in articular chondrocytes at 4h and increased the percentage of Tgase activity in the extracellular matrix. Chondrocytes rapidly converted T4 to T3, but the time course suggests similar mechanisms for T4 and T3. T4-induced Tgase activity was suppressed with cycloheximide and protein kinase C inhibitors. The effects of T4 on type II Tgase and FXIIIA levels were modest, but T4 strongly induced endoproteinase activity in chondrocytes.
  
  
  CONCLUSIONS
  We report in this study that thyroid hormones increase Tgase activity in articular chondrocytes via a non-genomic mechanism, which may involve increased endoproteinase secretion.
authors.affiliation: Zablocki VA Medical Center and Medical College of Wisconsin, Division of Rheumatology, Department of Medicine, Milwaukee, WI, USA. akrose@mcw.edu
authors.forename: A K
authors.initials: AK
authors.lastname: Rosenthal
authors.forename: D
authors.initials: D
authors.lastname: Heinkel
authors.forename: C M
authors.initials: CM
authors.lastname: Gohr
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002358:Cartilage, Articular / Q000201:enzymology*; D002478:Cells, Cultured; D019902:Chondrocytes / Q000201:enzymology*; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods; D020133:Reverse Transcriptase Polymerase Chain Reaction / Q000379:methods; D013552:Swine; D013974:Thyroxine / Q000378:metabolism*; D011503:Transglutaminases / Q000378:metabolism*
chemical_list: D011503:Transglutaminases; D013974:Thyroxine