Background/Purpose: The cartilage matrix breakdown in osteoarthritis (OA) is due to both abnormal mechanical stress and activation of catabolic processes involving metalloproteinases (MMPs). Currently, IL-1beta is thought to have a major role in shifting the metabolic balance toward degradation. IL-1beta is first synthesized as an inactive precursor, which is cleaved into the secreted active form. This maturation process mainly occurs in the “inflammasome”, where initiators (including NLRP3) and adaptor molecules (ASC) oligomerize to recruit and activate caspase-1, which in turn processes IL-1beta precursor. We aimed to clarify the role of both inflammasome and IL-1beta in cartilage breakdown.

Methods: First, amounts of IL-1β released from cartilage explants of 18 OA patients were assessed (ELISA). Second, in primary mouse articular chondrocytes cultures, LPS, IL-1alpha and TNF-alpha treatments were used to induce a pro-degradative phenotype, characterized by an increase in gene expression (real-time PCR) and in protein release of MMP-3, MMP-9 and MMP-13 (ELISA, zymography and Western-blot). Effects of a deficiency in NLRP3 (using NLRP3-/- mice), of an inhibition of caspase-1 (using Z-YVAD-FMK) and of a blockade of IL-1 (using IL-1RA) were investigated. At last, excessive dynamic compression was applied on mouse cartilage explants to trigger degradation (0.5 Hz and 1 MPa for 6 h). Load-induced GAG release and increase in MMP enzymatic activity were assessed in WT, NLRP3-/- or IL-1R1-/- mice, the latter being deficient in IL-1 receptor type 1.

Results: Despite the expression of NLRP3, ASC and caspase-1 in OA chondrocytes, OA cartilage was not able to produce active IL-1beta. In mouse articular chondrocytes, LPS, IL-1alpha and TNF-alpha dose-dependently increased MMP-3, MMP-9 and MMP-13 both at gene and protein levels. This response was similar in NLRP3-/- chondrocytes and was unchanged by caspase-1 inhibition. These results demonstrate that the inflammatory stress-induced degradative phenotype was inflammasome-independent. Furthermore, the catabolic response to LPS and TNF-alpha was unchanged when IL-1 was inhibited by IL-1RA. In cartilage explants, excessive load induced an increase in GAG release (3-fold) and MMP activity (3.7-fold). This response was similar in NLRP3-/- and IL-1R1-/- -derived cartilage. Likewise, the load-induced degradative response was independent of NLRP3-inflammasome and of IL-1.

Conclusion: This study suggests that OA cartilage can be degraded independently of NLRP3- inflammasome activity. This result may explain, at least in part, why previous trials with IL-1beta inhibitors were all negative.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/stress-induced-cartilage-degradation-does-not-depend-on-nlrp3-inflammasome-in-osteoarthritis/