Oxysterols Modify NLRP2 in Epithelial Cells, Identifying a Mediator of Ozone-induced Inflammation.

Ozone (O3) is a prevalent air pollutant causing lung inflammation. Previous studies demonstrate that O3 oxidizes lipids, such as cholesterol, in the airway to produce oxysterols, such as secosterol A (SecoA), which are electrophiles that are capable of forming covalent linkages preferentially with lysine residues and that consequently modify protein function. The breadth of proteins modified by this oxysterol as well as the biological consequences in the lung are unknown. By using an alkynyl-tagged form of SecoA and shotgun proteomics, we identified 135 proteins as being modified in bronchial epithelial cells. Among them was NLRP2 (NLR family pyrin domain-containing protein 2), which forms an alkynyl-tagged SecoA-protein adduct at lysine residue 1019 (K1019) in the terminal leucine-rich repeat region, a known regulatory region for NLR proteins. NLRP2 expression in airway epithelial cells was characterized, and CRISPR-Cas9 knockout (KO) and shRNA knockdown of NLRP2 were used to determine its function in O3-induced inflammation. No evidence for NLPR2 inflammasome formation or an NLRP2-dependent increase in caspase-1 activity in response to O3 was observed. O3-induced proinflammatory gene expression for CXCL2 and CXCL8/IL8 was further enhanced in NLRP2-KO cells, suggesting a negative regulatory role. Reconstitution of NLRP2-KO cells with the NLRP2 K1019 mutated to arginine partially blocked SecoA adduction and enhanced O3-induced IL-8 release as compared with wild-type NLRP2. Together, our findings uncover NLRP2 as a highly abundant, key component of proinflammatory signaling pathways in airway epithelial cells and as a novel mediator of O3-induced inflammation.

Ozone-derived Oxysterols Affect Liver X Receptor (LXR) Signaling: A POTENTIAL ROLE FOR LIPID-PROTEIN ADDUCTS.

When inhaled, ozone (O3) interacts with cholesterols of airway epithelial cell membranes or the lung-lining fluid, generating chemically reactive oxysterols. The mechanism by which O3-derived oxysterols affect molecular function is unknown. Our data show that in vitro exposure of human bronchial epithelial cells to O3 results in the formation of oxysterols, epoxycholesterol-α and -ß and secosterol A and B (Seco A and Seco B), in cell lysates and apical washes. Similarly, bronchoalveolar lavage fluid obtained from human volunteers exposed to O3 contained elevated levels of these oxysterol species. As expected, O3-derived oxysterols have a pro-inflammatory effect and increase NF-κB activity. Interestingly, expression of the cholesterol efflux pump ATP-binding cassette transporter 1 (ABCA1), which is regulated by activation of the liver X receptor (LXR), was suppressed in epithelial cells exposed to O3 Additionally, exposure of LXR knock-out mice to O3 enhanced pro-inflammatory cytokine production in the lung, suggesting LXR inhibits O3-induced inflammation. Using alkynyl surrogates of O3-derived oxysterols, our data demonstrate adduction of LXR with Seco A. Similarly, supplementation of epithelial cells with alkynyl-tagged cholesterol followed by O3 exposure causes observable lipid-LXR adduct formation. Experiments using Seco A and the LXR agonist T0901317 (T09) showed reduced expression of ABCA1 as compared with stimulation with T0901317 alone, indicating that Seco A-LXR protein adduct formation inhibits LXR activation by traditional agonists. Overall, these data demonstrate that O3-derived oxysterols have pro-inflammatory functions and form lipid-protein adducts with LXR, thus leading to suppressed cholesterol regulatory gene expression and providing a biochemical mechanism mediating O3-derived formation of oxidized lipids in the airways and subsequent adverse health effects.

Inhibition of endothelial- and neuronal-type, but not inducible-type, nitric oxide synthase by the oxidized cholesterol metabolite secosterol aldehyde: Implications for vascular and neurodegenerative diseases.

The cholesterol ozonolysis products secosterol-A and its aldolization product secosterol-B were recently detected in human atherosclerotic tissues and brain specimens, and have been postulated to play pivotal roles in the pathogenesis of atherosclerosis and neurodegenerative diseases. We examined several oxidized cholesterol metabolites including secosterol-A, secosterol-B, 25-hydroxycholesterol, 5ß,6ß-epoxycholesterol and 7-ketocholesterol for their effects on the activities of three nitric oxide synthases. In contrast to other oxidized metabolites, secosterol-A was found to be a potent inhibitor against the neuronal- and endothelial-type, but not the inducible-type nitric oxide synthase, with IC(50) values of 22 ± 1 and 50 ± 5 µM, respectively. The calmodulin-binding regions of the neuronal- and endothelial-nitric oxide synthases contain lysine residues which are not present in the inducible-type nitric oxide synthase. Secosterol-A modifies proteins through the formation of a Schiff base with the lysine epsilon-amino group. It is possible that secosterol-A modifies lysine residues of constitutive nitric oxide synthases, leading to the inhibition of enzymatic activities. As nitric oxide is a critical signaling molecule in vascular function and in long-term potentiation, its reduced production through inhibition of constitutive nitric oxide synthases by secosterol-A may contribute to the development of atherosclerosis and memory impairment in particular neurodegenerative diseases.