Abstract
Innate immune signaling via TLR4 plays critical roles in pathogenesis of metabolic disorders, but the contribution of different lipid species to metabolic disorders and inflammatory diseases is less clear. GM3 ganglioside in human serum is composed of a variety of fatty acids, including long‐chain (LCFA) and very‐long‐chain (VLCFA). Analysis of circulating levels of human serum GM3 species from patients at different stages of insulin resistance and chronic inflammation reveals that levels of VLCFA‐GM3 increase significantly in metabolic disorders, while LCFA‐GM3 serum levels decrease. Specific GM3 species also correlates with disease symptoms. VLCFA‐GM3 levels increase in the adipose tissue of obese mice, and this is blocked in TLR4‐mutant mice. In cultured monocytes, GM3 by itself has no effect on TLR4 activation; however, VLCFA‐GM3 synergistically and selectively enhances TLR4 activation by LPS/HMGB1, while LCFA‐GM3 and unsaturated VLCFA‐GM3 suppresses TLR4 activation. GM3 interacts with the extracellular region of TLR4/MD2 complex to modulate dimerization/oligomerization. Ligand‐molecular docking analysis supports that VLCFA‐GM3 and LCFA‐GM3 act as agonist and antagonist of TLR4 activity, respectively, by differentially binding to the hydrophobic pocket of MD2. Our findings suggest that VLCFA‐GM3 is a risk factor for TLR4‐mediated disease progression.
Highlights
Chronic inflammation plays critical roles in pathogenesis of a variety of human diseases, including metabolic disorders [Hotamisligil, 2017; Lumeng, 2011]
very long-chain fatty acid (VLCFA)-GM3 species are involved in progression of chronic inflammation in metabolic disorders To elucidate the role of GM3 species in pathophysiology of metabolic disorders, we analyzed expression patterns of serum GM3 species in human subjects [Veillon et al, 2015] (Appendix Fig. 1A-I)
This study demonstrated that human Toll-Like Receptor 4 (TLR4)/MD-2 received positive regulation by VLCFA-/ α-hydroxyVLCFA-GM3 and negative regulation by long-chain fatty acid (LCFA)-/ unsaturated VLCFA-GM3 in the presence of LPS and high-mobility group box-1 protein (HMGB1)
Summary
Chronic inflammation plays critical roles in pathogenesis of a variety of human diseases, including metabolic disorders [Hotamisligil, 2017; Lumeng, 2011]. Various ligands activate TLR4: (i) exogenous lipopolysaccharides elevated in serum [Cani et al, 2007], (ii) endogenous damage-associated molecular patterns (DAMPs), e.g., high-mobility group box-1 protein (HMGB1) [Harris et al, 2012; Guzmán-Ruiz et al, 2014], free fatty acids (FFAs) [Shi et al, 2006], and fetuin-A protein [Pal et al, 2012], which are released from macrophages and adipose tissue. Glycosyltransferases, UGCG and B4GALT5/6, convert ceramide into glucosylceramide (GlcCer) and lactosylceramide (LacCer), precursor glycosphigolipids (GSLs) for GM3 ganglioside. ST3GAL5, a GM3 synthase (GM3S), converts LacCer into GM3 by conjugating a sialic acid (N-acetylneuraminic acid) (Fig. 1A), which is followed by biosynthesis of complex gangliosides
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