Integrity of cartilage fails in
joint disease. The current work aimed to identify candidate active
proteinases in
joint diseases using an in vitro model for cartilage degradation induced by
interleukin-1. A critical event in the process of cartilage destruction in
joint disease is the failure of the
collagen fiber network to maintain integrity.
Proteins binding to the surface of the fibers are likely early points of failure.
Fibromodulin, a member of the
leucine-rich repeat protein family, is one predominant
protein in cartilage and is known for its roles in the formation of
collagen fibrils and sustained interaction with these formed fibers. Cleavage removes the
tyrosine sulfate-rich region in the N terminus of
fibromodulin. Whereas
fibromodulin bound to
collagen in tissue was digested, purified
fibromodulin was not cleaved. In contrast an N-terminal 10-kDa fragment, Gln19-Lys98, of the
protein generated by Lys-C digestion contains the cleavage site and was a substrate cleaved by the
enzyme in medium from stimulated cultures. In
solution, digestion of this substrate with
matrix metalloproteinase (MMP)-2, -9, -8, and -13 demonstrated that only MMP-13 was capable to efficiently cleave it. The cleavage product obtained after MMP-13 digestion was identical to that observed in cleaved
fibromodulin from cartilage explant cultures stimulated with
interleukin-1. MMP-13 treatment of fresh articular cartilage also produced the fragment under study. The elucidation of the
enzyme responsible for such cleavage may lead to treatment modalities involving its selective inhibition for patients suffering from
arthritis. The known structure of the fragments permits the generation of neo-
epitope antibodies to the cleavage site, which can be used to detect ongoing cartilage degradation in patients with arthritic disease, an important adjunct in monitoring
disease progression, active disease, and efficacy of treatment.