Increase in ALK1/ALK5 ratio as a cause for elevated MMP-13 expression in osteoarthritis in humans and mice

J Immunol. 2009 Jun 15;182(12):7937-45. doi: 10.4049/jimmunol.0803991.


During osteoarthritis (OA) chondrocytes show deviant behavior resembling terminal differentiation of growth-plate chondrocytes, characterized by elevated MMP-13 expression. The latter is also a hallmark for OA. TGF-beta is generally thought to be a protective factor for cartilage, but it has also displayed deleterious effects in some studies. Recently, it was shown that besides signaling via the ALK5 (activin-like kinase 5) receptor, TGF-beta can also signal via ALK1, thereby activating Smad1/5/8 instead of Smad2/3. The Smad1/5/8 route can induce chondrocyte terminal differentiation. Murine chondrocytes stimulated with TGF-beta activated the ALK5 receptor/Smad2/3 route as well as the ALK1/Smad1/5/8 route. In cartilage of mouse models for aging and OA, ALK5 expression decreased much more than ALK1. Thus, the ALK1/ALK5 ratio increased, which was associated with changes in the respective downstream markers: an increased Id-1 (inhibitor of DNA binding-1)/PAI-1 (plasminogen activator inhibitor-1) ratio. Transfection of chondrocytes with adenovirus overexpressing constitutive active ALK1 increased MMP-13 expression, while small interfering RNA against ALK1 decreased MMP-13 expression to nondetectable levels. Adenovirus overexpressing constitutive active ALK5 transfection increased aggrecan expression, whereas small interfering RNA against ALK5 resulted in increased MMP-13 expression. Moreover, in human OA cartilage ALK1 was highly correlated with MMP-13 expression, whereas ALK5 correlated with aggrecan and collagen type II expression, important for healthy cartilage. Collectively, we show an age-related shift in ALK1/ALK5 ratio in murine cartilage and a strong correlation between ALK1 and MMP-13 expression in human cartilage. A change in balance between ALK5 and ALK1 receptors in chondrocytes caused changes in MMP-13 expression, thereby causing an OA-like phenotype. Our data suggest that dominant ALK1 signaling results in deviant chondrocyte behavior, thereby contributing to age-related cartilage destruction and OA.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Activin Receptors, Type I / genetics
  • Activin Receptors, Type I / metabolism*
  • Activin Receptors, Type II / genetics
  • Activin Receptors, Type II / metabolism*
  • Aging / physiology
  • Animals
  • Cartilage / enzymology
  • Cells, Cultured
  • Chondrocytes / enzymology
  • Gene Expression Regulation, Enzymologic
  • Humans
  • Inhibitor of Differentiation Protein 1 / metabolism
  • Male
  • Matrix Metalloproteinase 13 / genetics
  • Matrix Metalloproteinase 13 / metabolism*
  • Mice
  • Mice, Inbred C57BL
  • Osteoarthritis / enzymology*
  • Osteoarthritis / genetics
  • Plasminogen Activator Inhibitor 1 / metabolism
  • Protein-Serine-Threonine Kinases / genetics
  • Protein-Serine-Threonine Kinases / metabolism*
  • RNA Interference
  • RNA, Messenger / genetics
  • Receptor, Transforming Growth Factor-beta Type I
  • Receptors, Transforming Growth Factor beta / genetics
  • Receptors, Transforming Growth Factor beta / metabolism*
  • Signal Transduction


  • Idb1 protein, mouse
  • Inhibitor of Differentiation Protein 1
  • Plasminogen Activator Inhibitor 1
  • RNA, Messenger
  • Receptors, Transforming Growth Factor beta
  • Protein-Serine-Threonine Kinases
  • ACVRL1 protein, human
  • Activin Receptors, Type I
  • Activin Receptors, Type II
  • Acvrl1 protein, mouse
  • Receptor, Transforming Growth Factor-beta Type I
  • TGFBR1 protein, human
  • Tgfbr1 protein, mouse
  • MMP13 protein, human
  • Matrix Metalloproteinase 13
  • Mmp13 protein, mouse