NLRC3 protein inhibits inflammation by disrupting NALP3 inflammasome assembly via competition with the adaptor protein ASC for pro-caspase-1 binding.

Inflammasomes are multiprotein complexes that sense pathogen-associated and danger-associated molecular patterns and induce inflammation in cells. The NALP3 inflammasome is tightly regulated by recently discovered control mechanisms, but other modulators still remain to be characterized. NLR family CARD-containing 3 (NLRC3) protein, a caspase recruitment domain (CARD)-containing member of the nucleotide oligomerization domain-like receptor (NLR) family, was found to down-regulate the NF-κB pathway and stimulator of interferon genes (STING)-dependent cytokine secretion. However, the effect of NLRC3 on the NALP3 inflammasome or other inflammasomes is still unknown. We hypothesized that NLRC3 might inhibit NALP3 inflammasome complex assembly. Toward this end, we tested whether NLRC3 overexpression or knockdown influences NALP3 activity in human monocyte and HEK293FT cells when the complex is ectopically reconstituted. We found that NLRC3 indeed decreases NALP3-induced IL-1β maturation and secretion, pro-caspase-1 cleavage, and speck formation by apoptosis-associated speck-like protein containing a CARD (ASC) protein in response to NALP3 activators. We also show that endogenous NLRC3 interacts with both ASC and pro-caspase-1 but not with NALP3, disrupts ASC speck formation through its CARD, and impairs the ASC and pro-caspase-1 interaction. Moreover, the NLRC3 CARD alone could dampen IL-1β secretion and ASC speck formation induced by NALP3 mutants associated with autoinflammatory diseases. In conclusion, we show here that, besides its role in the inhibition of the NF-κB pathway, NLRC3 interferes with the assembly and activity of the NALP3 inflammasome complex by competing with ASC for pro-caspase-1 binding.

Eren E ，Berber M ，Özören N 《-》

Overexpressed NLRC3 acts as an anti-inflammatory cytosolic protein.

The novel nucleotide oligomerization domain (NOD)-like receptor (NLR) with a caspase activation and recruitment domain (CARD) 3 (NLRC3) protein belongs to the NLR family of cytosolic pathogen recognition receptors. NLRC3 has the characteristic NOD and leucine-rich repeat configuration with a less well defined CARD. T lymphocytes are known to have high NLRC3 expression, which may be involved in suppression of T cell activation. Here, we report that NLRC3 is a cytoplasmic protein that negatively regulates pro-IL-1β maturation. Among well-known inflammasome components, NLRC3 can interact with apoptosis-associated speck-like protein containing a CARD (ASC) and caspases 1 and 5. Transient transfection of NLRC3 into stable EGFP-ASC-expressing HEK293FT cells reduces NLR family, pyrin domain-containing 3 (NLRP3)/cryopyrin-induced formation of ASC specks in a dose- and time-dependent manner. This suggests that NLRC3 can regulate ASC speck formation, caspase-1 activation and IL-1β maturation. We show for the first time that inflammasome-like complexes assemble when caspase-1 and ASC are cotransfected together with NLRC3 in HEK293FT cells. However, overexpression of NLRC3 with NLRP3/cryopyrin inflammasome components suppresses pro-caspase-1 cleavage and IL-1β processing. Our study suggests that NLRC3 negatively regulates inflammatory responses.

Gültekin Y ，Eren E ，Özören N 《-》

Knockdown of TLR4 attenuates high glucose-induced podocyte injury via the NALP3/ASC/Caspase-1 signaling pathway.

Diabetic nephropathy (DN) is one of the most common syndromes of the diabetic mellitus, and is the leading cause of end-stage renal disease. TLR4 (Toll-like receptor 4) has been shown to be implicated in the progression of DN. However, the role of TLR4 in DN and its underlying mechanism remain elusive. In this study, we found that high glucose (HG) activated nucleotide binding and oligomerization domain-like receptor family pyrin domain-containing 3 (NALP3) inflammasome, as shown by elevated NALP3, apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (ASC) and cleaved caspase-1 protein levels, and increased caspase-1 activity in mouse podocytes in vitro. Simultaneously, HG decreased the mRNA and protein expression of Wilm's tumor-1 and synaptopodin, and increased the mRNA and protein expression levels of desmin in podocytes, in a dose-dependent manner. Administration of HG increased the number of TUNEL-positive cells, inhibited the viability of podocytes, elevated the intracellular level of ROS, and increased the levels of IL-1β, IL-18, TNF-α and TGF-β1. Moreover, the effects of HG were effectively blocked by repression of NALP3 or ASC. Furthermore, TLR4 was upregulated in HG-stimulated podocytes. TLR4 knockdown inhibited the activation of NALP3 inflammasome in HG-treated podocytes, as indicated by the decrease in NALP3, ASC and cleaved caspase-1 protein levels and the reduction in caspase-1 activity. Additionally, knockdown of TLR4 attenuated the HG-induced increase of cell apoptosis and reduction of cell viability in podocytes. TLR4 knockdown reduced the intracellular level of ROS in conjunction with reduced IL-1β, IL-18, TNF-α and TGF-β1 levels in podocytes in the presence of HG. In conclusion, knockdown of TLR4 attenuates HG-induced podocyte injury via the NALP3/ASC/Caspase-1 signaling pathway.

Liu Y ，Xu Z ，Ma F ，Jia Y ，Wang G ... -  《-》

The CARD plays a critical role in ASC foci formation and inflammasome signalling.

Proell M ，Gerlic M ，Mace PD ，Reed JC ，Riedl SJ ... -  《-》

Fluorofenidone attenuates pulmonary inflammation and fibrosis via inhibiting the activation of NALP3 inflammasome and IL-1β/IL-1R1/MyD88/NF-κB pathway.

Interleukin (IL)-1β plays an important role in the pathogenesis of idiopathic pulmonary fibrosis. The production of IL-1β is dependent upon caspase-1-containing multiprotein complexes called inflammasomes and IL-1R1/MyD88/NF-κB pathway. In this study, we explored whether a potential anti-fibrotic agent fluorofenidone (FD) exerts its anti-inflammatory and anti-fibrotic effects through suppressing activation of NACHT, LRR and PYD domains-containing protein 3 (NALP3) inflammasome and the IL-1β/IL-1R1/MyD88/NF-κB pathway in vivo and in vitro. Male C57BL/6J mice were intratracheally injected with Bleomycin (BLM) or saline. Fluorofenidone was administered throughout the course of the experiment. Lung tissue sections were stained with haemotoxylin and eosin and Masson's trichrome. Cytokines were measured by ELISA, and α-smooth muscle actin (α-SMA), fibronectin, collagen I, caspase-1, IL-1R1, MyD88 were measured by Western blot and/or RT-PCR. The human actue monocytic leukaemia cell line (THP-1) were incubated with monosodium urate (MSU), with or without FD pre-treatment. The expression of caspase-1, IL-1β, NALP3, apoptosis-associated speck-like protein containing (ASC) and pro-caspase-1 were measured by Western blot, the reactive oxygen species (ROS) generation was detected using the Flow Cytometry, and the interaction of NALP3 inflammasome-associated molecules were measured by Co-immunoprecipitation. RLE-6TN (rat lung epithelial-T-antigen negative) cells were incubated with IL-1β, with or without FD pre-treatment. The expression of nuclear protein p65 was measured by Western blot. Results showed that FD markedly reduced the expressions of IL-1β, IL-6, monocyte chemotactic protein-1 (MCP-1), myeloperoxidase (MPO), α-SMA, fibronectin, collagen I, caspase-1, IL-1R1 and MyD88 in mice lung tissues. And FD inhibited MSU-induced the accumulation of ROS, blocked the interaction of NALP3 inflammasome-associated molecules, decreased the level of caspase-1 and IL-1β in THP-1 cells. Besides, FD inhibited IL-1β-induced the expression of nuclear protein p65. This study demonstrated that FD, attenuates BLM-induced pulmonary inflammation and fibrosis in mice via inhibiting the activation of NALP3 inflammasome and the IL-1β/IL-1R1/MyD88/ NF-κB pathway.

Song C ，He L ，Zhang J ，Ma H ，Yuan X ，Hu G ，Tao L ，Zhang J ，Meng J ... -  《-》