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-ß1 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.

Discoidin domain receptor 2 mediates the collagen II-dependent release of interleukin-6 in primary human chondrocytes.

We deciphered constituent parts of a signal transduction cascade that is initiated by collagen II and results in the release of various pro-inflammatory cytokines, including interleukin-6 (IL-6), in primary human chondrocytes. This cascade represents a feed-forward mechanism whereby cartilage matrix degradation is exacerbated by the mutually inducing effect of released collagen II fragments and pro-inflammatory cytokines. We previously proposed discoidin domain receptor 2 as a central mediator in this event. Since this cascade plays a prominent role in the pathogenesis of osteoarthritis, our study further investigates the hypothesis that discoidin domain receptor 2 is a candidate receptor for collagen II, and that transcription factor NFkappaB, lipid kinase PI3K, and the MAP kinases are constituent parts of this very signal transduction cascade. To accomplish this, we selectively knocked down the molecules of interest in primary human chondrocytes, induced the specified cascade by incubating primary human chondrocytes with collagen II, and observed the outcome, specifically the changes in interleukin-6 release. Knockdown was performed by siRNA-mediated gene silencing in the case of discoidin domain receptor 2 (DDR2) or by using specific inhibitors for the remainder of the molecules. Results indicated that discoidin domain receptor 2 mediates the collagen II-dependent release of interleukin-6 in primary human chondrocytes and that MAP kinases p38, JNK and ERK, as well as transcription factor NFkappaB, are integral components of intracellular collagen II signalling. Given the detrimental role of these molecules in osteoarthritis, our findings provide new targets for more specific therapeutics, which may have fewer side effects than those currently applied.

Matrilin-3 activates the expression of osteoarthritis-associated genes in primary human chondrocytes.

Here, we tested the matrilin-3-dependent induction of osteoarthritis-associated genes in primary human chondrocytes. Matrilin stimulation leads to the induction of MMP1, MMP3, MMP13, COX-2, iNOS, IL-1beta, TNFalpha, IL-6 and IL-8. Furthermore, we show the participation of ADAMTS4 and ADAMTS5 in the in vitro degradation of matrilin-3. We provide evidence for a matrilin-3-dependent feed-forward mechanism of matrix degradation, whereby proteolytically-released matrilin-3 induces pro-inflammatory cytokines as well as ADAMTS4 and -5 indirectly via IL-1beta. ADAMTS4 and ADAMTS5, in turn, cleave matrilin-3 and may release more matrilin-3 from the matrix, which could lead to further release of pro-inflammatory cytokines and proteases in cartilage.