Oxidized phospholipid inhibition of LPS-signaling: a good side to the bad guys?

An overview of the non-canonical inflammasome

Inflammasomes are large cytosolic multiprotein complexes assembled in response to infection and cellular stress, and are crucial for the activation of inflammatory caspases and the subsequent processing and release of pro-inflammatory mediators. While caspase-1 is activated within the canonical inflammasome, the related caspase-4 (also known as caspase-11 in mice) and caspase-5 are activated within the non-canonical inflammasome upon sensing of cytosolic lipopolysaccharide (LPS) from Gram-negative bacteria. However, the consequences of canonical and non-canonical inflammasome activation are similar. Caspase-1 promotes the processing and release of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 and the release of danger signals, as well as a lytic form of cell death called pyroptosis, whereas caspase-4, caspase-5 and caspase-11 directly promote pyroptosis through cleavage of the pore-forming protein gasdermin D (GSDMD), and trigger a secondary activation of the canonical NLRP3 inflammasome for cytokine release. Since the presence of the non-canonical inflammasome activator LPS leads to endotoxemia and sepsis, non-canonical inflammasome activation and regulation has important clinical ramifications. Here we discuss the mechanism of non-canonical inflammasome activation, mechanisms regulating its activity and its contribution to health and disease.

Lipid and Non-lipid Factors Affecting Macrophage Dysfunction and Inflammation in Atherosclerosis

Atherosclerosis is a chronic inflammatory disease and a leading cause of human mortality. The lesional microenvironment contains a complex accumulation of variably oxidized lipids and cytokines. Infiltrating monocytes become polarized in response to these stimuli, resulting in a broad spectrum of macrophage phenotypes. The extent of lipid loading in macrophages influences their phenotype and consequently their inflammatory status. In response to excess atherogenic ligands, many normal cell processes become aberrant following a loss of homeostasis. This can have a direct impact upon the inflammatory response, and conversely inflammation can lead to cell dysfunction. Clear evidence for this exists in the lysosomes, endoplasmic reticulum and mitochondria of atherosclerotic macrophages, the principal lesional cell type. Furthermore, several intrinsic cell processes become dysregulated under lipidotic conditions. Therapeutic strategies aimed at restoring cell function under disease conditions are an ongoing coveted aim. Macrophages play a central role in promoting lesional inflammation, with plaque progression and stability being directly proportional to macrophage abundance. Understanding how mixtures or individual lipid species regulate macrophage biology is therefore a major area of atherosclerosis research. In this review, we will discuss how the myriad of lipid and lipoprotein classes and products used to model atherogenic, proinflammatory immune responses has facilitated a greater understanding of some of the intricacies of chronic inflammation and cell function. Despite this, lipid oxidation produces a complex mixture of products and with no single or standard method of derivatization, there exists some variation in the reported effects of certain oxidized lipids. Likewise, differences in the methods used to generate macrophages in vitro may also lead to variable responses when apparently identical lipid ligands are used. Consequently, the complexity of reported macrophage phenotypes has implications for our understanding of the metabolic pathways, processes and shifts underpinning their activation and inflammatory status. Using oxidized low density lipoproteins and its oxidized cholesteryl esters and phospholipid constituents to stimulate macrophage has been hugely valuable, however there is now an argument that only working with low complexity lipid species can deliver the most useful information to guide therapies aimed at controlling atherosclerosis and cardiovascular complications.

Directly interact with Keap1 and LPS is involved in the anti-inflammatory mechanisms of (-)-epicatechin-3-gallate in LPS-induced macrophages and endotoxemia

Disruption of the Kelch-like ECH-associated protein 1 (Keap1)-Nuclear factor erythroid-derived factor 2-related factor 2 (Nrf2) interaction has emerged as a promising strategy to reduce oxidative stress-induced inflammation. However, its roles in regulating downstream events, including the cross talk between Nrf2 and nuclear factor-kappa B (NF-κB), are not well defined. The objective of this study was to elucidate the mechanistic connection between Keap1-Nrf2 signaling and the transcription factor NF-κB and to investigate the function of (-)-epicatechin-3-gallate (ECG) in the repression of multiple inflammatory mediators. ECG attenuated lipopolysaccharide (LPS)-induced inflammatory mediator expression and intracellular reactive oxygen species (ROS) generation through the induction of Nrf2/antioxidant response element (ARE)-driven glutathione (GSH) and hemeoxygenase-1 (HO-1) levels, interference with NF-κB and Nfr2/ARE transcriptional activities, and suppression of the MAPKs (JNK1/2 and p38) and PI3K/Akt signaling pathways. Importantly, anti-inflammatory effects of ECG partly require activation of ERK1/2 signaling to mediate HO-1 expression and Nrf2/ARE signaling activation. Furthermore, ECG may directly interact intracellularly with the Kelch repeat domains of Keap1 and bind to extracellular LPS, thereby promoting the nuclear accumulation of the Nrf2 protein and blockading the activation of LPS-induced downstream target signaling pathways. Consistent with in vitro studies, ECG attenuates pathological syndromes of LPS-induced sepsis and systemic inflammation. Our results identified ECG as a novel Keap1-Nrf2 interaction disruptor and LPS-induced TLR4 activation inhibitor, thereby providing an innovative strategy to prevent or treat immune, oxidative stress and inflammatory-related diseases.

Apolipoprotein(a) inhibits lipopolysaccharide-induced IL-6 secretion in human astrocytoma cell line by interfering with lipopolysaccharide signaling

OBJECTIVE

To examine the role of lipoprotein(a) [Lp(a)] on the inflammatory response of cells in the nervous system by investigating its effect on lipopolysaccharide (LPS)-induced interleukin-6 (IL-6) secretion.

MATERIALS AND METHODS

Human astrocytoma U373 cells were treated with recombinant apolipoprotein(a) [r-apo(a)] A10K (175-11 nM), alone or in combination with LPS (100 and 10 ng/ml). IL-6 levels were evaluated by immunoblotting. Statistical analysis was performed by one-way ANOVA.

RESULTS

r-apo(a) caused dose-dependent inhibition of LPS-induced IL-6 secretion (100 ng/ml LPS, p = 0.0205; 10 ng/ml LPS, p = 0.0005). Pre-treatment of cells with 88 nM r-apo(a), rinsing, and activation with 10 ng/ml LPS did not reverse the inhibition (p = 0.0048), which could be reversed by supplementation with excess serum (5-20%) (p = 0.0454) or recombinant CD14 (2.0-0.05 μg/ml) (p = 0.0230).

CONCLUSIONS

Our data indicate that apo(a) plays a natural anti-endotoxin role which relies on its interference with cell-associated and serum components of LPS signaling.