title: Physiological and excessive mechanical compression of articular cartilage activates Smad2/3P signaling.
abstract: OBJECTIVE
  Transforming growth factor beta (TGF-β) in articular cartilage can signal via two routes, the ALK5/Smad2/3P and the ALK1/Smad1/5/8P route, the first being protective and the latter favoring chondrocyte terminal differentiation. Since biomechanical factors are known to play an essential role in osteoarthritis (OA) initiation and progression, we investigated if excessive mechanical compression can alter TGF-β signaling in cartilage shifting it from ALK5/Smad2/3P to ALK1/Smad1/5/8P pathway, favoring terminal differentiation of chondrocytes.
  
  
  METHODS
  Articular cartilage explants were harvested from bovine metacarpophalangeal joints. After equilibration, explants were subjected to unconfined dynamic mechanical compression (1 Hz) with 3 MPa (physiological) or 12 MPa (excessive) stress. After different time intervals samples were frozen and mRNA levels of selected genes were examined using real-time polymerase chain reaction.
  
  
  RESULTS
  In articular cartilage compressed with 3 MPa and also 12 MPa stress the expression of Smad2/3P responsive genes bSerpine1, bSmad7 and bAlk5 was up-regulated, whereas the expression of Smad1/5/8P responsive gene bId1 was down-regulated. Furthermore, the expression of bTgfb1 was significantly up-regulated in both compression groups. When ALK5/Smad2/3P pathway was blocked with a selective ALK4/5/7 inhibitor, the effect of excessive mechanical compression on bSmad7 and bAlk5 expression was prevented.
  
  
  CONCLUSIONS
  Here we show that excessive mechanical compression alone is not able to shift TGF-β signaling toward the ALK1/Smad1/5/8P pathway. In contrast, we show that mechanical compression not only with physiological but also with excessive stress can activate Smad2/3P signaling, which is known to be protective for articular cartilage and to block chondrocyte terminal differentiation.
authors.affiliation: Orthopaedic Research Laboratory, Radboud University Medical Centre, Nijmegen, The Netherlands. Electronic address: Wojciech.Madej@radboudumc.nl.
authors.forename: W
authors.initials: W
authors.lastname: Madej
authors.affiliation: Experimental Rheumatology & Advanced Therapeutics, Radboud University Medical Centre, Nijmegen, The Netherlands. Electronic address: Arjan.vanCaam@radboudumc.nl.
authors.forename: A
authors.initials: A
authors.lastname: van Caam
authors.affiliation: Experimental Rheumatology & Advanced Therapeutics, Radboud University Medical Centre, Nijmegen, The Netherlands. Electronic address: Esmeralda.BlaneyDavidson@radboudumc.nl.
authors.forename: E N
authors.initials: EN
authors.lastname: Blaney Davidson
authors.affiliation: Experimental Rheumatology & Advanced Therapeutics, Radboud University Medical Centre, Nijmegen, The Netherlands. Electronic address: Peter.vanderKraan@radboudumc.nl.
authors.forename: P M
authors.initials: PM
authors.lastname: van der Kraan
authors.affiliation: Orthopaedic Research Laboratory, Radboud University Medical Centre, Nijmegen, The Netherlands. Electronic address: Pieter.Buma@radboudumc.nl.
authors.forename: P
authors.initials: P
authors.lastname: Buma
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology*; D002358:Cartilage, Articular / Q000166:cytology / Q000502:physiology*; D002417:Cattle; D002454:Cell Differentiation / Q000502:physiology; D002478:Cells, Cultured; D019902:Chondrocytes / Q000166:cytology / Q000502:physiology; D019245:Compressive Strength / Q000502:physiology*; D005260:Female; D023421:Models, Animal; D017346:Protein Serine-Threonine Kinases / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D051899:Smad2 Protein / Q000502:physiology*; D051900:Smad3 Protein / Q000502:physiology*; D016212:Transforming Growth Factor beta / Q000502:physiology
chemical_list: D051899:Smad2 Protein; D051900:Smad3 Protein; D016212:Transforming Growth Factor beta; D017346:Protein Serine-Threonine Kinases
keywords: Cartilage; Dynamic mechanical compression; Osteoarthritis; Transforming growth factor beta