P2X7 Receptor Induces Pyroptotic Inflammation and Cartilage Degradation in Osteoarthritis via NF-κB/NLRP3 Crosstalk

Methods for monitoring cancer cell pyroptosis

Pyroptosis is a form of proinflammatory cell death that depends on the gasdermin family of proteins. The main features of pyroptosis are altered membrane permeability, cell swelling, membrane rupture, and the ability to mobilize a strong immune response. The relationship between pyroptosis and cancer has become a popular topic in immunological research. Multiple strategies for inducing pyroptosis in cancer cells have been developed for cancer therapy, including chemotherapy, small molecule drugs, and nanomedicines. In this review, we systematically discuss recent advances in research on the mechanisms of pyroptosis, and compare pyroptosis with apoptosis and necroptosis from several aspects. The development of various experimental systems has accompanied rapid progress in this field, but little consensus on monitoring pyroptosis is currently available. We focus on techniques commonly used to monitor pyroptosis, and describe future techniques that may be used to increase our knowledge in this field. Overall, the advancement of pyroptosis detection methods will help researchers to better investigate the relationships between pyroptosis and various cancers, and should provide insights into the use of these promising tools for cancer treatments.

MicroRNA-140-5p represses chondrocyte pyroptosis and relieves cartilage injury in osteoarthritis by inhibiting cathepsin B/Nod-like receptor protein 3

Osteoarthritis (OA) is a degenerative joint disease. Dysregulated microRNA (miRNA) expressions are implicated in OA progression. Consequently, the current study set out to investigate the mechanism of miR-140-5p in OA cartilage injury. Firstly, the murine and cell models of OA were established, and cartilage tissues of OA mice were observed using hematoxylin and eosin staining and safranin O staining. Chondrocyte pyroptosis was further assessed using immunohistochemical and Calcein-AM/PI staining. The levels of gasdermin-D (GSDMD)-N, cleaved caspase-1, interleukin (IL)-1β, and IL-18 in cartilage tissues and cells were determined using Western blot and enzyme-linked immunosorbent assay kits. The targeting relationship between miR-140-5p and cathepsin B (CTSB) was verified using a dual-luciferase assay. Moreover, the binding of CTSB and Nod-like receptor protein 3 (NLRP3) was detected using co-immunoprecipitation assay. Lastly, the effects of NLRP3 activation and CTSB overexpression on chondrocyte pyroptosis were documented. It was found that OA induction aggravated cartilage tissue injury and enhanced chondrocyte pyroptosis. miR-140-5p was poorly-expressed in OA models, and miR-140-5p over-expression alleviated chondrocyte pyroptosis, as evidenced by decreased GSDMD-N, cleaved caspase-1, IL-1β, and IL-18 levels. miR-140-5p targeted the CTSB gene, whereas CTSB further bound to NLRP3 and activated the NLRP3 inflammasome. Additionally, CTSB over-expression or NLRP3 activation reversed the inhibitory effect of miR-140-5p on chondrocyte pyroptosis. Collectively, our findings revealed that miR-140-5p repressed chondrocyte pyroptosis and alleviated OA cartilage injury via inhibition of the CTSB/NLRP3. This study may confer a theoretical basis for the treatment of OA cartilage injury.

Moderate-intensity exercise alleviates pyroptosis by promoting autophagy in osteoarthritis via the P2X7/AMPK/mTOR axis

Instability and excessive use of the knee joint can cause osteoarthritis (OA). Reasonable exercise can enhance the stability of the knee joint and prevent and relieve the occurrence and development of OA. As a key switch for inflammation, P2X purinoceptor 7 (P2X7) has attracted much attention in studies of OA. Exercise can regulate P2X7 expression and activation. However, the role of P2X7 in exercise-based prevention and treatment of OA is unknown. We previously showed that moderate-intensity exercise can significantly alleviate OA symptoms. Accordingly, in this study, we evaluated the effects of exercise on P2X7 expression and activation in chondrocytes. Micro-computed tomography, hematoxylin, and eosin staining, Toluidine Blue O staining, immunohistochemistry, and terminal deoxynucleotidyl transferase dUTP nick-end labeling experiments showed that P2X7 expression was lower in the moderate-intensity exercise group than in the inflammation and low- and high-intensity exercise groups. Additionally, chondrocyte death, cartilage destruction, and the degree and severity of pyroptosis were significantly reduced, whereas autophagy levels were significantly increased in the moderate-intensity exercise group. Cell Counting Kit-8 assay, lactate dehydrogenase release, flow cytometry, enzyme-linked immunosorbent assay, cell fluorescence, western blot, reverse transcription-quantitative polymerase chain reaction, and transmission electron microscopy experiments showed that moderate activation of P2X7 promoted autophagy through the AMP-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) signaling pathway and promoted autolysosome targeting for degradation of the inflammasome component NLRP3, thereby inhibiting pyroptosis. Additionally, the use of AMPK and mTOR activators and inhibitors indicated that the AMPK-mTOR signaling pathway, as the downstream of P2X7, played a key role in delaying the occurrence and development of OA. We propose that moderate-intensity exercise promoted chondrocyte autophagy through the P2X7/AMPK/mTOR signal axis to alleviate pyroptosis. Our findings provide novel insights into the positive and preventative effects of exercise on OA.

A novel function of NLRP3 independent of inflammasome as a key transcription factor of IL-33 in epithelial cells of atopic dermatitis

Atopic dermatitis (AD) is a common chronic pruritic inflammatory skin disorder characterized by recurrent eczematous lesions. Interleukin (IL)-33, a cytokine of the IL-1 family, was found to play an important role in the pathogenesis of AD. As a key component of the inflammasome, NLRP3 has been mostly described in myeloid cells that to mediate inflammasome activation conducted proinflammatory cytokine production of the IL-1 family. However, the role of NLRP3 inflammasome in the pathogenesis of AD, as well as IL-33 processing are highly controversial. Whether NLRP3 can mediate IL-33 expression and secretion independently of the inflammasome in the epithelium of AD has remained unclear. In this article, we found the mRNA expression of Il33 and Nlrp3 were notably increased in the lesional skin of AD patients compared to healthy controls. We then found a significant positive correlation between the expression of Nlrp3 and Il33 in the epithelium of MC903-mediated AD mice model, but no changes were observed for Il36α, Il36γ, Il1β, or Il18 mRNA expression, as well as IL-1β or IL-18 production. Overexpression of NLRP3 in human immortalized epithelial cells increased IL-33 expression, whereas siRNA targeting NLRP3 abolished IL-33 expression. In addition, inhibition of NLRP3 inflammasome activation or caspase-1 activity with MCC950 or VX-765 showed no effect on the expression and secretion of IL-33 in AD mice. Unlike myeloid cells, NLRP3 predominantly located in the nucleus of epithelial cells, which could directly bind to Il33 specific-promoters and transactivate it through an interaction with transcription factor IRF4. Furthermore, NLRP3 deficient mice exhibited a significant alleviated epidermis inflammation and decreased mRNA expression and secretion of IL-33 in MC903-mediated AD mice without interfering with TSLP and IL-1β production. Our results demonstrate a novel ability of NLRP3 to function as a crucial transcription factor of IL-33 in epithelium independently of inflammasome that to mediate the pathological process of AD.

IRE1-mTOR-PERK Axis Coordinates Autophagy and ER Stress-Apoptosis Induced by P2X7-Mediated Ca2+ Influx in Osteoarthritis

Moderate-intensity exercise can help delay the development of osteoarthritis (OA). Previous studies have shown that the purinergic receptor P2X ligand gated ion channel 7 (P2X7) is involved in OA development and progression. To investigate the effect of exercise on P2X7 activation and downstream signaling in OA, we used the anterior cruciate ligament transection (ACLT)-induced OA rat model and primary chondrocyte culture system. Our in vivo experiments confirmed that treadmill exercise increased P2X7 expression and that this effect was more pronounced at the later time points. Furthermore, P2X7 activation induced endoplasmic reticulum (ER) stress and increased the expression levels of ER stress markers, such as 78 kDa glucose-regulated protein (GRP78), protein kinase R-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme-1 (IRE1), and activating transcription factor 6 (ATF6). At the early time points, IRE1 and PERK were activated, and mTOR was inhibited. At the later time points, mTOR was activated, mediating PERK to promote ER stress-apoptosis, whereas IRE1 and autophagy were inhibited. To confirm our observations in vitro, we treated primary chondrocytes with the P2X7 agonist benzoylbenzoyl-ATP (Bz-ATP). Our results confirmed that P2X7-mediated Ca²⁺ influx activated IRE1-mediated autophagic flux and induced PERK-mediated ER stress-apoptosis. To further investigate the role of P2X7 in OA, we injected mTOR antagonist rapamycin or P2X7 antagonist A740003 into the knee joints of ACLT rats. Our results demonstrated that mTOR inhibition induced autophagy, decreased apoptosis, and reduced cartilage loss. However, injection of mTOR agonist MHY1485 or Bz-ATP had the opposite effect. In summary, our results indicated that during the early stages of moderate-intensity exercise, P2X7 was activated and autophagic flux was increased, delaying OA development. At the later stages, P2X7 became over-activated, and the number of apoptotic cells increased, promoting OA development. We propose that the IRE1-mTOR-PERK signaling axis was involved in the regulation of autophagy inhibition and the induction of apoptosis. Our findings provide novel insights into the positive and preventative effects of exercise on OA, suggesting that the intensity and duration of exercise play a critical role. We also demonstrated that on a molecular level, P2X7 and its downstream pathways could be potential therapeutic targets for OA.

CY-09 attenuates the progression of osteoarthritis via inhibiting NLRP3 inflammasome-mediated pyroptosis

Excessive activation of inflammation in chondrocyte has been considered to be a major reason cause of cellular death and degeneration in osteoarthritis (OA) development. The NLRP3 inflammasome-mediated pyroptosis pathway is closely related to inflammation regulation. This research was conducted to confirm whether NLRP3 expression and activity are impacted in the development of OA and to detect the role of CY-09, a selective and direct inhibitor of NLRP3 in the in vitro and in vivo models of OA. Our findings corroborated that the expression of NLRP3 is stimulated in OA cartilage. CY-09 can maintain extracellular matrix (ECM) homeostasis and regulate inflammation in TNF-α treated chondrocytes via inhibition of NLRP3 inflammasome-mediated pyroptosis. Moreover, the chondrocyte protective effects of CY-09 were further confirmed in vivo in a DMM-induced OA model. In conclusion, our research indicates that experimental OA activated the NLRP3 activity, and pharmacological inhibition of NLRP3 inflammasome activation by CY-09 protects chondrocytes against inflammation and attenuates OA development.

NLRP3 Inflammasome: A New Target for Prevention and Control of Osteoporosis?

Osteoporosis is a systemic bone metabolism disease that often causes complications, such as fractures, and increases the risk of death. The nucleotide-binding oligomerization domain-like-receptor family pyrin domain-containing 3 (NLRP3) inflammasome is an intracellular multiprotein complex that regulates the maturation and secretion of Caspase-1 dependent proinflammatory cytokines interleukin (IL)-1β and IL-18, mediates inflammation, and induces pyroptosis. The chronic inflammatory microenvironment induced by aging or estrogen deficiency activates the NLRP3 inflammasome, promotes inflammatory factor production, and enhances the inflammatory response. We summarize the related research and demonstrate that the NLRP3 inflammasome plays a vital role in the pathogenesis of osteoporosis by affecting the differentiation of osteoblasts and osteoclasts. IL-1β and IL-18 can accelerate osteoclast differentiation by expanding inflammatory response, and can also inhibit the expression of osteogenic related proteins or transcription factors. In vivo and in vitro experiments showed that the overexpression of NLRP3 protein was closely related to aggravated bone resorption and osteogenesis deficiency. In addition, abnormal activation of NLRP3 inflammasome can not only produce inflammation, but also lead to pyroptosis and dysfunction of osteoblasts by upregulating the expression of Caspase-1 and gasdermin D (GSDMD). In conclusion, NLRP3 inflammasome overall not only accelerates bone resorption, but also inhibits bone formation, thus increasing the risk of osteoporosis. Thus, this review highlights the recent studies on the function of NLRP3 inflammasome in osteoporosis, provides information on new strategies for managing osteoporosis, and investigates the ideal therapeutic target to treat osteoporosis.