Molecular and Cellular Mechanisms Responsible for Low-grade Stress and Inflammation Triggered By Super-low Dose Endotoxin

Abstract

The gram-negative endotoxin, lipopolysaccharide (LPS), has been extensively researched in high doses (10-200ng/ml) and is well-documented in the literature for its ability to result in devastating effects such as multi-organ failure, sepsis, and septic shock. In high doses, LPS signals through Toll-like-receptor 4 (TLR4) and triggers a cascade of events culminating in the release of pro- and anti-inflammatory cytokines and the activation of NF-κB. In contrast, super-low doses of LPS (1-100pg/ml) are able to trigger the persistent release of pro-inflammatory mediators while evading the compensatory activation of NF-κB. This mild yet persistent induction of inflammation may lie at the heart of numerous inflammatory diseases and disorders and warrants studies such as this to elucidate the novel mechanisms. In this study, we explored the novel mechanisms utilized by super-low dose LPS in cellular stress and low-grade inflammation. In the first study, the molecular mechanisms governing the role of super-low dose LPS on cellular stress and necroptosis were examined. We show that in the presence of super-low dose LPS (50pg/ml), the key regulators of mitochondrial fission and fusion, Drp1 and Mfn1 respectively, are inversely regulated. An increase in mitochondrial fragmentation and cell death which was not dependent on caspase activation was observed. In addition, super-low dose LPS was able to activate RIP3, a kinase responsible for inducing the inflammatory cell death, necroptosis. These mechanisms were regulated in an Interleukin-1 receptor-associated kinase 1 (IRAK-1) dependent manner. In the second study, the molecular mechanisms governing the role of super-low dose LPS on cellular stress and endosome/lysosome fusion were examined. In the presence of low-dose LPS (50pg/ml), endosomal-lysosomal fusion is inhibited and a loss of endosomal acidification required for the successful clearance of cellular debris and resolution of inflammation was observed. Additionally, super-low dose LPS induced the accumulation of p62 indicative of the suppression of autophagy. Tollip and Interleukin-1 receptor-associated kinase 3 (IRAK-M) appear to be critical regulators in this process. Collectively, these studies show that low-dose endotoxemia is capable of causing persistent cellular stress, not observed in the presence of high-dose LPS (10-200ng/ml), and that it promotes necroptotic cell death while suppressing mechanisms necessary for the resolution of inflammation such as endosome-lysosome fusion. This research reveals novel mechanisms utilized by low-dose endotoxemia which could aid future efforts to develop prevention and treatment for various debilitating inflammatory diseases.