Bone Abstracts

Background: Cartilage matrix breakdown in osteoarthritis (OA) is due to mechanical stress and inflammation leading to increased metalloproteinases (MMPs) production. Currently, IL1β is thought to have a major role in this process. IL1β is synthesized as an inactive precursor, which is cleaved into the secreted active form. This maturation process mainly occurs in the inflammasome complex. Inflammasome is constituted by initiators (including NALP3) and adaptor molecules (ASC) which oligomerize to recruit and activate caspase-1, which in turn processes IL1β precursor. We aimed to clarify the role of both inflammasome and IL1β in cartilage breakdown.

Methods: IL1β release from cartilage explants of OA patients were assessed (ELISA). LPS, IL1α and TNFα treatments were used to induce MMP (−3, −8, −13) gene expression (real-time PCR) and protein release (ELISA, zymography, and western blot) in primary mouse articular chondrocytes cultures. Effects of NALP3 deficiency (using NALP3^−/− mice), caspase-1 inhibition (using Z-YVAD-FMK) and IL1 blockade (using IL1RA) were investigated. Finally, excessive dynamic compression (0.5 Hz and 1 MPa for 6 h) leading to increased MMP activity was applied on mouse cartilage explants from WT, NLRP3−/− or IL1R1^−/− mice and load-induced GAG release were assessed.

Results: Despite NLRP3, ASC, and caspase-1 expression in OA chondrocytes, no IL1β production was found. In mouse articular chondrocytes, LPS, IL1α, and TNFα dose-dependently increased MMP-3, MMP-9 and MMP-13 both at gene and protein levels. This response was similar in NALP3^−/− chondrocytes and was unchanged by caspase-1 inhibition. Furthermore, this response was unchanged after IL1RA treatment. In cartilage explants, excessive load induced an increase in GAG release (threefold) and MMP activity (3.7-fold). This response was similar in NALP3^−/− and IL1R1^−/−-derived cartilage. Likewise, the process leading to MMP production was independent of both NLRP3-inflammasome and IL1.

Conclusion: This study suggests that OA cartilage can be degraded independently of NLRP3-inflammasome activity.