Background: Inflammatory response mediated by oxidative stress is considered as an important pathogenesis of spinal cord injury (SCI). Advanced oxidation protein products (AOPPs) are novel markers of oxidative stress and their role in inflammatory response after SCI remained unclear. This study aimed to investigate the role of AOPPs in SCI pathogenesis and explore the possible underlying mechanisms. Methods: A C5 hemi-contusion injury was induced in Sprague-Dawley rats to confirm the involvement of AOPPs after SCI. For in vivo study, apocynin, the NADPH oxidase inhibitor was used to study the neuroprotective effects after SCI. For in vitro study, the BV2 microglia cell lines were pretreated with or without the inhibitor or transfected with or without small interference RNA (siRNA) and then stimulated with AOPPs. A combination of molecular and histological methods was used to clarify the mechanism and explore the signaling pathway both in vivo and in vitro. One-way analysis of variance (ANOVA) was conducted with Bonferroni post hoc tests to examine the differences between groups. Results: The levels of AOPPs in plasma and cerebrospinal fluid as well as the contents in the spinal cord showed significant increase after SCI. Meanwhile, apocynin ameliorated tissue damage in the spinal cord after SCI, improving the functional recovery. Immunofluorescence staining and western blot analysis showed activation of microglia after SCI, which was in turn inhibited by apocynin. Pretreated BV2 cells with AOPPs triggered excessive generation of reactive oxygen species (ROS) by activating NADPH oxidase. Increased ROS induced p38 MAPK and JNK phosphorylation, subsequently triggering nuclear translocation of NF-κB p65 to express pro-inflammatory cytokines. Also, treatment of BV2 cells with AOPPs induced NLRP3 inflammasome activation and cleavage of Gasdermin-d (GSDMD), causing pyroptosis. This was confirmed by cleavage of caspase-1, production of downstream mature interleukin (IL)-1β and IL-18 as well as rupture of rapid cell membrane.

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Interleukin (IL)-1β and tumor necrosis factor (TNF)-α, in particular, control the degeneration of articular cartilage, making them prime targets for osteoarthritis (OA) therapeutic strategies. Advanced oxidation protein products (AOPPs) are prevalent in numerous diseases. Our previous work demonstrates that intra-articular injections of AOPPs accelerate regression of cartilage in OA models. Whether AOPPs exist in the course of OA and their effects on TNF-α and IL-1β expression in chondrocytes are still unclear. This study confirmed that AOPPs levels in human synovial fluid were positively associated with severity of OA. We also found AOPPs deposition in articular cartilage in anterior cruciate ligament transection (ACLT) induced rodent OA models. AOPPs increased expression of TNF-α and IL-1β in chondrocytes in vitro, which was inhibited by pre-treatment with SB202190 (p38-MAPK inhibitor) or apocynin (NADPH oxidase inhibitor) or NOX4 knockdown by siRNAs. Subsequently, we further verified in vivo that exogenous injection of AOPPs in OA mice up-regulated expression of TNF-α and IL-1β in cartilage, which was blocked by treatment with apocynin. In parallel, apocynin attenuated articular cartilage degeneration resulting in substantially lower OARSI scores. Specifically, apocynin reduced NOX4, p-P38, TNF-α and IL-1β and increased collagen II and glycosaminoglycan (GAG). This study demonstrated that AOPPs increased expression of TNF-α and IL-1β in chondrocytes via the NADPH oxidase4-dependent and p38-MAPK mediated pathway, and accelerated cartilage degeneration in OA progression. These findings suggest an endogenous pathogenic role of AOPPs in OA progression. Targeting AOPPs-triggered cellular mechanisms might be a promising therapeutic option for patients with OA.