COMP does not directly modify the expression of genes involved in cartilage homeostasis in contrast to several other cartilage matrix proteins.

Objective: We investigated whether COMP may modify cartilage metabolism and play a role as an endogenous disease aggravating factor in OA. Materials and methods: Full-length and momomeric COMP was recombinantly expressed in human embryonic kidney cells and purified it via affinity chromatography. Purified COMP was used to stimulate either primary human chondrocytes or cartilage explants. Changes in the expression profiles of inflammatory genes, differentiation markers and growth factors were examined by immunoassay and by quantitative real-time reverse-transcription polymerase chain reaction. Results: Incubation of primary human chondrocytes or cartilage explants in the presence of COMP did not induce statistically significant changes in the expression of IL-6, MMP1, MMP13, collagen I, collagen II, collagen X, TGF-beta 1 and BMP-2. Conclusions: In contrast to collagen II and matrilin-3, COMP lacks the ability to trigger a proinflammatory response in chondrocytes, although it carries an RGD motif and can bind to integrins. COMP is a well-accepted biomarker for osteoarthritis but increased COMP levels do not necessarily correlate with inflammation.