Proteinases involved in matrix turnover during cartilage and bone breakdown |
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Authors: | Tim E Cawston David A Young |
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Institution: | (1) Musculoskeletal Research Group, Institute of Cellular Medicine, The Medical School, University of Newcastle upon Tyne, 4th Floor Cookson Building, Newcastle upon Tyne, NE2 4HH, UK |
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Abstract: | The joint is a discrete unit that consists of cartilage, bone, tendon and ligaments. These tissues are all composed of an
extracellular matrix made of collagens, proteoglycans and specialised glycoproteins that are actively synthesised, precisely
assembled and subsequently degraded by the resident connective tissue cells. A balance is maintained between matrix synthesis
and degradation in healthy adult tissues. Different classes of proteinases play a part in connective tissue turnover in which
active proteinases can cleave matrix protein during resorption, although the proteinase that predominates varies between different
tissues and diseases. The metalloproteinases are potent enzymes that, once activated, degrade connective tissue and are inhibited
by tissue inhibitors of metalloproteinases (TIMPs); the balance between active matrix metalloproteinases and TIMPs determines,
in many tissues, the extent of extracellular matrix degradation. The serine proteinases are involved in the initiation of
activation cascades and some, such as elastase, can directly degrade the matrix. Cysteine proteinases are responsible for
the breakdown of collagen in bone following the removal of the osteoid layer and the attachment of osteoclasts to the exposed
bone surface. Various growth factors increase the synthesis of matrix and proteinase inhibitors, whereas cytokines (alone
or in combination) can inhibit matrix synthesis and stimulate proteinase production and matrix destruction. |
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