Objective: To determine whether an aggrecan 32-mer fragment derived from dual ADAMTS and matrix metalloproteinase (MMP) cleavage in the aggrecan interglobular domain was bioactive and, if so, to elucidate its mechanism of action.
Methods: Mouse primary chondrocytes, synovial fibroblasts, or peritoneal macrophages, human primary chondrocytes, and cells or cell lines from myeloid differentiation factor 88 (MyD88)-deficient and Toll-like receptor 2 (TLR-2)-deficient mice were stimulated with synthetic mouse 32-mer peptide, human 32-mer peptide, a 32-mer scrambled peptide, or native, glycosylated 32-mer peptide. Cells stimulated with 32-mer peptide were analyzed for changes in messenger RNA (mRNA) expression by quantitative polymerase chain reaction. Conditioned medium was analyzed for levels of interleukin-6 protein by an AlphaLISA or for levels of MMP-3 and MMP-13 protein by Western blotting. NF-κB activation was measured in a luciferase reporter assay.
Results: Treatment of mouse cells or cartilage explants with 32-mer peptide or scrambled peptide revealed that the 32-mer peptide, but not the scrambled peptide, had antianabolic, procatabolic, and proinflammatory bioactivity in vitro. Chondrocytes, synovial fibroblasts, and macrophages from MyD88-deficient mice failed to respond to 32-mer peptide stimulation. A macrophage cell line derived from TLR-2-deficient mice also failed to respond to 32-mer peptide stimulation. Stimulation of human chondrocytes with human 32-mer peptide increased the expression of catabolic markers at the mRNA and protein levels. Mouse and human 32-mer peptide stimulated NF-κB activation in a TLR-2-dependent reporter assay, and the response of chondrocytes from both species to native, glycosylated 32-mer peptide was similar to the response to synthetic peptides.
Conclusion: The aggrecan 32-mer fragment is a novel endogenous ligand of TLR-2 with the potential to accelerate cartilage destruction in vivo.
© 2015, American College of Rheumatology.