Figure 4.

Mechanisms of NLRP3 activation in OA chondrocytes

Chondrocytes have become active players driving NLRP3 inflammasome-dependent mechanisms. Stimulation of these cells with LPS or calcium-containing crystals, which are critical activators of the NF-κB signaling pathway favors the transcription of NLRP3, ASC, CASP1, IL1B, and IL18. Moreover, IL-1β can also activate several pathogenic processes, for instance, NF-κB activation and overexpression of miRNAs that function as activators of NLRP3. The miR-30b-3p directly induces the activation of NLRP3 and binds to SIRT1 mRNA, which is involved in chondrocyte apoptosis induction. The miR-244-5p act by blocking the inhibitory effect of the long-non-coding RNA SNHG7, an inhibitor of NLRP3. Adenosine is another NLRP3 inductor after binding with A2AR, A2BR, and A3R. Similarly, ATP can activate the NLRP3 inflammasome by interacting with P2X7R and P2X4R. Complex mechanisms have also been described for NLRP3 activation, i.e., the stimulation of chondrocytes with H₂O₂ caused upregulation of USP7. USP7 is a ubiquitin protease capable of deubiquitinating NOX4, thus, preventing NOX4 proteasomal degradation. NOX4 promotes the release of ROS, a process that leads to oxidative stress and converges in NLRP3 activation. An additional mechanism for NLRP3 activation is mediated by the upregulation of Ctbp in cartilage from OA patients, where Ctbp proteins act as transcriptional co-activators of the AP1 transcription factor. The active complex Ctbp-p300-AP1 specifically binds to the promoter region of the NLRP3 gene promoting its transcription in OA chondrocytes. On the other hand, NRF2 functions as a transcriptional repressor of NLRP3 and NF-κB in an HO-1-dependent mechanism; however, in OA several molecules such as AGEs inhibit the effect of NRF2 and contribute to the promotion and maintenance of the inflammatory process. Abbreviations: IL-1β, Interleukin-1 beta; IL-1R, Interleukin-1 receptor; LPS, Lipopolysaccharide; TLR, Toll-like receptors; CPPD, Calcium pyrophosphatase dihydrate; ATP, Adenosine triphosphate; NLRP3, NLR family pyrin domain containing 3; A2AR, Adenosine A2A receptor; A2BR, Adenosine A2B receptor; A3R, Adenosine A3 receptor; P2X7R, P2X purinoceptor 7; P2X4R, P2X purinoceptor 4; USP7, Ubiquitin specific protease 7; NOX4, NADPH oxidase 4; MyD88, Myeloid differentiation primary response 88; IRAK1/4, Interleukin 1 receptor-associated kinase 1/4; TRAF6, TNF receptor-associated factor 6; SIRT, Sirtuin 1; FoxO3a, Forkhead box class O 3a; NF-κB, Nuclear factor kappa-light-chain-enhancer of activated B cells; CASP1, Caspase-1; ASC, Apoptosis-associated speck-like protein containing a CARD; GSDMD, Gasdermin D; ROS, Reactive oxygen species; AGEs, Advanced glycation end products; Ctbp, C-terminal-binding proteins; p300, E1A binding protein p300; AP1, Activator protein 1 complex; NRF2, Nuclear factor erythroid 2–related factor 2; HO-1, Heme oxygenase-1.