Effects of thyroid hormone on proteoglycan degradation in various regions of cartilage were investigated. In propylthiouracil-treated rats with hypothyroidism, proteoglycan degradation in epiphyseal cartilage during endochondral ossification was markedly suppressed. However, injections of T4 reversed this effect of propylthiouracil on proteoglycan degradation. In pig growth plate explants, T3 also induced breakdown of proteoglycan. T3 increased the release of aggrecan monomer and core protein from the explants into the medium. Accordingly, the level of aggrecan monomer remaining in the tissue decreased after T3 treatment, and the monomer lost hyaluronic acid-binding capacity, suggesting that the cleavage site is in the interglobular domain. The aggrecan fragment released from the T3-exposed explants underwent cleavage at Glu373-Ala374, the major aggrecanase-cleavage site. The stimulation of proteoglycan degradation by T3 was less prominent in resting cartilage explants than in growth plate explants and was barely detectable in articular cartilage explants. Using rabbit growth plate chondrocyte cultures, we explored proteases that may be involved in T3-induced aggrecan degradation and found that T3 enhanced the expression of aggrecanase-2/ADAM-TS5 (a disintegrin and a metalloproteinase domain with thrombospondin type I domains) mRNA, whereas we could not detect any enhancement of stromelysin, gelatinase, or collagenase activities or any aggrecanase-1/ADAM-TS4 mRNA expression. We also found that the aggrecanse-2 mRNA level, but not aggrecanase-1, increased at the hypertrophic stage during endochondral ossification. These findings suggest that aggrecanse-2/ADAM-TS5 is involved in aggrecan breakdown during endochondral ossification, and that thyroid hormone stimulates the aggrecan breakdown partly via the enhancement of aggrecanase-2/ADAM-TS5.
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