Interleukin-1 induces aggrecanase-mediated cleavage in human articular cartilage without up-regulating stromelysin or glycosaminoglycan release

Abstract

Normal turnover of cartilage proteoglycan as well as loss in arthritic conditions involves proteolytic cleavage of the aggrecan core protein within the interglobular domain (IGD) releasing a large GAG-containing aggrecan fragment which diffuses out of the cartilage matrix. However, the proteinases responsible for the normal turnover and pathological loss of aggrecan have not been identified. Two major sites of proteolytic cleavage have been identified within the IGD between amino acid residues Asn341-Phe342 and Glu373-Ala374. Many matrix metalloproteinases (MMP-1, -2, -3, -7, -8 and -9) have been shown to cleave in vitro at the Asn341-Phe342 site [1–3]. However, attempts to generate cleavage at the Glu373-Ala374 site with a number of purified proteinases [1–4] have been unsuccessful, indicating that this cleavage is the result of a novel, as yet unidentified, proteinase, given the name “aggrecanase” based on its ability to cleave the aggrecan core protein. Analysis of aggrecan fragments from several in vitro systems [5–9] and from human arthritic synovial fluid [10–11] have identified fragments with the Ala374 N-terminus indicating that cartilage degradation involves cleavage at the Glu373-Ala374 “aggrecanase” site. To investigate the relationship between proteoglycan degradation, matrix metallo-proteinase production and cleavage of aggrecan at the “aggrecanase” clip site, human articular cartilage was incubated in the absence or presence of interleukin -1 (IL-1).