Sustained low-grade inflammation mediated by non-resolving inflammatory monocytes has long been suspected in the pathogenesis of atherosclerosis; however, the molecular mechanisms responsible for the sustainment of non-resolving inflammatory monocytes during atherosclerosis are poorly understood. Here we observe that subclinical endotoxemia, often seen in humans with chronic inflammation, aggravates murine atherosclerosis through programming monocytes into a non-resolving inflammatory state with elevated Ly6C, CCR5, MCP-1 and reduced SR-B1. The sustainment of inflammatory monocytes is due to the disruption of homeostatic tolerance through the elevation of miR-24 and reduction of the key negative-feedback regulator IRAK-M. miR-24 reduces the levels of Smad4 required for the expression of IRAK-M and also downregulates key lipid-processing molecule SR-B1. IRAK-M deficiency in turn leads to elevated miR-24 levels, sustains disruption of monocyte homeostasis and aggravates atherosclerosis. Our data define an integrated feedback circuit in monocytes and its disruption may lead to non-resolving low-grade inflammation conducive to atherosclerosis.

Low-dose endotoxemia is prevalent in humans with adverse health conditions, and it correlates with the pathogenesis of chronic inflammatory diseases such as atherosclerosis, diabetes, and neurologic inflammation. However, the underlying molecular mechanisms are poorly understood. In this study, we demonstrate that subclinical low-dose LPS skews macrophages into a mild proinflammatory state, through cell surface TLR4, IL-1R–associated kinase-1, and the Toll-interacting protein. Unlike high-dose LPS, low-dose LPS does not induce robust activation of NF-κB, MAPKs, PI3K, or anti-inflammatory mediators. Instead, low-dose LPS induces activating transcription factor 2 through Toll-interacting protein–mediated generation of mitochondrial reactive oxygen species, allowing mild induction of proinflammatory mediators. Low-dose LPS also suppresses PI3K and related negative regulators of inflammatory genes. Our data reveal novel mechanisms responsible for skewed and persistent low-grade inflammation, a cardinal feature of chronic inflammatory diseases.

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Inflammatory stimulants such as bacterial endotoxin (lipopolysaccharide (LPS)) are known to induce tissue damage and injury partly through the induction of reactive oxygen species (ROS). Although it is recognized that the induction of ROS in macrophages byReactive oxygen species (ROS) 2 play a critical role in the regulation of inflammatory processes causing the oxidation of lipids and proteins and eventually leading to tissue damage and organ failure. The generation of ROS is modulated by two families of opposing enzymes, oxidative enzymes such as NADPH oxidase and antioxidative enzymes, including glutathione peroxidase, catalase, and superoxide dismutase. Bacterial products such as lipopolysaccharide (LPS) selectively induce the expression and activation of oxidative enzymes, while decreasing the expression of antioxidative enzymes (1, 2). Taken together, LPS challenge significantly contributes to the production of ROS and the pathogenesis of diverse inflammatory diseases.Most of the published studies regarding NADPH oxidase have been specifically focused on the regulation and activation of NOX-2, the enzymatic NADPH oxidase component primarily expressed in neutrophils (3, 4). NOX-2 protein is constitutively expressed and is not regulated transcriptionally (3). LPS challenge causes rapid translocation of the functional NOX-2 containing NADPH oxidase to the membrane complex, leading to its activation (3). In contrast, NOX-1, the primary NADPH oxidase in macrophages, can be both transcriptionally induced and post-transcriptionally activated by LPS. However, the molecular mechanism for LPS-induced expression and activation of NOX-1 is poorly defined. Based on studies done in other cell types (5, 6), it is conceivable that LPS may contribute to the activation of NOX-1 containing NADPH oxidase via the small GTPase Rac1 in macrophages (7). However, the detailed molecular mechanism underlying LPS-mediated activation of Rac1 in macrophages is not known.On the other hand, LPS treatment decreases the levels of nuclear receptor family transcription factors such as PPAR␣ and PGC-1, which are responsible for the sustained expression of antioxidative enzymes, including glutathione peroxidase and catalase (8 -11). Collectively, the LPS-triggered up-regulation of oxidative enzymes and concurrent down-regulation of antioxidases leads to the generation and accumulation of ROS and tissue damage.IRAK-1 is one of many intracellular signaling components downstream of the LPS receptor (TLR4) (12)(13)(14). A series of studies have revealed that IRAK-1 positively contributes to the activation of NFB, STAT1/3, and IRF5/7, while negatively regulating the activities of nuclear factor of activated T-cells and nuclear receptors (15)(16)(17)(18)(19)(20). Despite the prominent role that IRAK-1 plays within the TLR4 signaling pathway, its involve-

Subclinical levels of circulating endotoxin are associated with the pathogenesis of diverse human inflammatory diseases, by mildly inducing the expression of proinflammatory mediators. In this study, we examined the molecular mechanism responsible for the effect of low-dose LPS in macrophages. In contrast to high-dose LPS, which activates NF-κB and induces the robust expression of proinflammatory mediators, we observed that low-dose LPS failed to activate NF-κB. Instead, it selectively activated C/EBPδ and removed nuclear repressors, including peroxisome proliferator-activated receptor α and retinoic acid receptor α, enabling a mild and leaky expression of proinflammatory mediators. The effect of low-dose LPS required IRAK-1, which interacts with and acts upstream of IκB kinase ε to contribute to LPS-mediated induction of C/EBPδ and proinflammatory mediators. Additionally, mice fed a high-fat diet acquired elevated levels of endotoxin and proinflammatory mediators in an IRAK-1–dependent fashion. Taken together, these data reveal a distinct pathway preferentially used by low-dose endotoxin in initiating low-grade inflammation.

Host immune responses are finely regulated by the opposing effects of Th17 and T regulatory (Treg) cells. Treg cells help to dampen inflammatory processes and Th17 cells facilitate various aspects of immune activation. The differentiation of Th cells depends on a unique combination of stimulants and subsequent activation of diverse transcription factors. In particular, cooperative activation of NFAT and Smad3 leads to the induction of Treg cells, and cooperation among STAT3 and Smad3 switches to the induction of Th17 cells. We have previously shown that the IL-1 receptor associated kinase 1 (IRAK-1) selectively activates STAT3 and inactivates NFAT. Physiological studies have shown that IRAK-1−/− mice are protected from developing various inflammatory diseases, including experimental autoimmune encephalomyelitis and atherosclerosis with unknown mechanism. In this study, we demonstrate that IRAK-1 plays a critical modulatory role in the differentiation of Th17 and Treg cells. Following stimulation with TCR agonists and TGFβ, IRAK-1−/− CD4 Th cells display elevated nuclear NFATc2 levels and increased interaction of NFATc2 and Smad3, resulting in increased expression of Foxp3, a key marker for Treg cells. IRAK-1−/− mice have constitutively higher populations of Treg cells. In contrast, when stimulated with TCR agonists together with IL-6 and TGF-β, IRAK-1−/− CD4 Th cells exhibit attenuated STAT3 Ser727 phosphorylation and reduced expression of IL-17 and RORγt compared with wild-type cells. Correspondingly, IRAK-1 deletion results in decreased IL-17 expression and dampened inflammatory responses in acute and chronic inflammatory mice models. Our data provides mechanistic explanation for the anti-inflammatory phenotypes of IRAK-1−/− mice.

Although both inflammatory and metabolic complications occur during sepsis and endotoxemia, relatively few studies have examined the molecular mechanism underlying LPS-modulated metabolic changes during sepsis. In this report, we have demonstrated that LPS suppresses free fatty acid (FFA) oxidation, and consequently contributes to elevated plasma levels of FFA and triglyceride (TG). Furthermore, this process depends upon the interleukin-1 receptor associated kinase 1 (IRAK-1), one of the key TLR4 intracellular signaling kinases. IRAK-1 −/− mice fail to exhibit the dramatic rise in plasma FFA and TG levels compared to wild type (WT) mice following lethal LPS injection. Mechanistically, we demonstrated that LPS suppresses FFA oxidation through decreasing the expression levels of key FFA oxidative genes including CPT-1 and MCAD in both liver and kidney tissues of WT but not IRAK-1 −/− mice. The expression of CPT-1 and MCAD is controlled by nuclear receptors and co-receptors including PPARα and PGC-1α. We observed that LPS selectively suppresses the levels of PPARα and PGC-1α in tissues from WT, but not IRAK-1 −/− mice. Consequently, IRAK-1 −/− mice have a higher survival rate following a lethal dose of LPS. Our current study reveals a novel role for IRAK-1 in the metabolic alterations induced by LPS.

Parkinson’s disease (PD) is a progressive, neurodegenerative movement disorder characterized by the loss of dopaminergic (DA) neurons. Limited understanding of the early molecular pathways associated with the demise of DA neurons, including those of inflammatory exacerbation of neurodegeneration, is a major impediment to therapeutic development. Recent studies have implicated gene-environment interactions in PD susceptibility. We used transcriptomic profiling in a Drosophila PD model in response to paraquat (PQ)-induced oxidative stress to identify pre-symptomatic signatures of impending neuron dysfunction. Our RNAseq data analysis revealed extensive regulation of innate immune response genes following PQ ingestion. We found that PQ exposure leads to the activation of the NF-κB transcription factor, Relish, and the stress signaling factor JNK, encoded by the gene basket in Drosophila . Relish knockdown in the dopaminergic neurons confers PQ resistance and rescues mobility defects and DA neuron loss. Furthermore, PQ-induced toxicity is mediated through the immune deficiency signaling pathway. Surprisingly, the expression of Relish-dependent anti-microbial peptide (AMPs) genes is suppressed upon PQ exposure causing increased sensitivity to Gram-negative bacterial infection. This work provides a novel link between PQ exposure and innate immune system modulation underlying environmental toxin-induced neurodegeneration, thereby underscoring the role of the innate immune system in PD pathogenesis.