Inflammasomes and Fibrosis

Abstract

Fibrosis is the final common pathway of inflammatory diseases in various organs. The inflammasomes play an important role in the progression of fibrosis as innate immune receptors. There are four main members of the inflammasomes, such as NOD-like receptor protein 1 (NLRP1), NOD-like receptor protein 3 (NLRP3), NOD-like receptor C4 (NLRC4), and absent in melanoma 2 (AIM2), among which NLRP3 inflammasome is the most studied. NLRP3 inflammasome is typically composed of NLRP3, ASC and pro-caspase-1. The activation of inflammasome involves both "classical" and "non-classical" pathways and the former pathway is better understood. The "classical" activation pathway of inflammasome is that the backbone protein is activated by endogenous/exogenous stimulation, leading to inflammasome assembly. After the formation of "classic" inflammasome, pro-caspase-1 could self-activate. Caspase-1 cleaves cytokine precursors into mature cytokines, which are secreted extracellularly. At present, the "non-classical" activation pathway of inflammasome has not formed a unified model for activation process. This article reviews the role of NLRP1, NLRP3, NLRC4, AIM2 inflammasome, Caspase-1, IL-1β, IL-18 and IL-33 in the fibrogenesis.

Keywords: IL-1β; NLRP3; caspase-1; fibrosis; inflammasome.

Figure 1

The Composition of the NOD-like receptors (NLRs). NOD-like receptors (NLRs) are mainly composed of a carboxyl (C) terminal, a central terminal, and an amino acid (N) terminal. The C-terminus includes a leucin rich repeat (LRR), the center terminus includes NACHT, and the N-terminus includes a pyrin domain (PYD). The C-terminal LRR recognizes the pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs), the central-terminal NACHT encodes by Nlrp3 (CIAS1) gen. The N-terminal CARD interacts with the adaptor protein through PYD-PYD. Apoptosis-associated speck-like protein containing a CARD (ASC) recruits pro-cysteinyl aspartate specific proteinase-1 (pro-caspase-1) through CARD domain to activate downstream signals.

Figure 2

The activation of the NLRP3 inflammasome. ATP recognizes P2X7 purinergic receptor (P2X7R) on the cell membrane, opens the ion channel, and leads to K⁺ outflow, recruiting ubiquitinated connexin to punch holes in the cell membrane. The PAMPs enters the cells and promotes the binding of the catalytic domain of NIMA-related kinase 7 (NEK7) to NLRP3 and activates NLRP3 inflammasome. Streptozotocin (STZ), bleomycin (BLM) and statins first damage mitochondria, and then activate NADPH oxidase 4 (NOX4), leading to ROS activation. ROS induces dissociation of thioredoxin and thioredoxin interacting protein (TXNIP). TXNIP directly activates NLRP3 inflammasome. In addition, ROS also induces the conversion of mitochondrial DNA (mtDNA) into oxidized form (ox-mtDNA), which, as the ligand of NLRP3, directly binds and activates NLRP3, activating NLRP3 inflammasome. The crystals/macromolecules, such as monosodium urate (MSU) activates NADPH oxidase through chemical reaponse, which damages the lysosome, releasing cathepsin B, activating NLRP3 inflammasome. Epithelial sodium channels (ENaC) on the surface of the cell membrane are opened to allow Na⁺ inflow, leading to K⁺ outflow, and activating the NLRP3 inflammasome. Chloride intracellular channels (CLIC) act as the downstream of the K⁺ outflow-mitochondrial ROS axis. ROS induces the transfer of CLIC to the cell membrane, leading to Cl^- outflow. The outflow of Cl^- enables NEK7 to bind to NLRP3 and promotes the assembly and activation of NLRP3 inflammasome. Phospholipase C hydrolyzes phosphatidylinositol-4,5-diphosphate to form diacyl glycerol (DAG) and inositol trisphosphate (InsP3). InsP3 binds to the InsP3 receptor on the endoplasmic reticulum membrane, causing the endoplasmic reticulum to release Ca²⁺, resulting in an increase in intracellular Ca²⁺. Ca²⁺ is recognized by calcium-sensing receptor (CASR) and activates NLRP3 inflammasome. STZ cooperates with thioacetamide (TAA) through adenosine monophosphate-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) pathway inhibition autophagy effect, leading to activation of NLRP3 inflammasome. CASR leads to a decrease in intracellular cAMP, weakens the binding capacity of cAMP and NLRP3, and activates NLRP3 inflammasome.

Figure 3

The NLRP3 inflammasome in the liver fibrosis. After the PAMPs act on the livers, they destroy the hepatocytes, and activate the NLRP3 inflammasomes in hepatocytes, leading to hepatocyte necrosis. The necrotic liver cells release DAMPs, which can activate Kupffer cells (KCs). The KCs recognize PAMPs through TLRs on the one hand, and induce the expressions of NLRP3 inflammasome-related pathway components such as NLRP3, pro-caspase-1, and pro-IL-1β through TLRs-NF-κB pathways. On the other hand, the KCs recognize DAMPs, which can directly damage mitochondria and cause them to release ROS. As the upstream signal of NLRP3, ROS activates NLRP3 through the ROS-TXNIP pathway. ROS also promotes the transfer of high mobility group box 1 (HMGB1) from the nucleus to the cytoplasm. The HMGB1 can also activate NLRP3 through the TLR4-NF-κB pathways. After NLRP3 is activated, NLRP3 forms NLRP3 inflammasome together with ASC and pro-caspase-1. Nuclear factor erythroid 2-related factor 2 (Nrf2) is an important transcription factor that regulates cellular anti-oxidative stress. Under physiological conditions, the cytoplasmic protein chaperone molecule Kelch-like ECH-associated protein 1 (Keap1) in KCs binds to Nrf2 and makes it appear to be inhibited. When mitochondria release ROS, Nrf2 dissociates from Keap1 and moves into the nucleus, and combines with the antioxidant response element (ARE) to activate the antioxidant enzyme heme oxygenase-1 (HO-1) expression to inhibit the activation of ROS/NLRP3 inflammasome pathways. The antioxidant response cannot resist the oxidation response, which leads to the KCs activation. Activated KCs activate HSCs by releasing TGF-β and IL-1β, which activate the NLRP3 inflammasome in HSCs. The activated HSCs express α-smooth muscle actin (α-SMA) and Collagen I, leading to ECM deposition and eventually progressing into liver fibrosis.

Figure 4

The crosstalk between KCs and HSCs. The PAMPs in the leakage of chronic liver disease can activate NF-κB through TLRs on the surface of KCs, promote the activation of NLRP3 inflammasomes, and induce the generation of pro-inflammatory signals. These pro-inflammatory signals activate HSCs through cytokine receptors (CKRs)/myeloid differentiation factor 88 (MyD88)/NF-κB, leading to liver fibrosis-related molecules matrix metalloproteinases (MMP) and tissue inhibitor of metalloproteinases 1 (TIMP) imbalance, promote ECM deposition and form liver fibrogenesis.