Glycine attenuates endotoxin-induced liver injury by downregulating TLR4 signaling in Kupffer cells.
BACKGROUND
Several experimental studies have observed better outcomes after glycine treatment in patients with endotoxin-induced liver injuries, but its molecular mechanism is not yet fully understood. The purpose of this study was to evaluate the hypothesis that glycine attenuates endotoxin-induced liver injury by affecting endotoxin signal transduction in liver macrophages.


METHODS
An animal model of endotoxin-induced liver injury was established by intraperitoneally injecting mice with 10 mg/kg body weight endotoxin fed a pretreatment diet with or without 5% (w/w) glycine. Blood and liver samples were obtained for analysis of liver morphology and to determine concentrations of alanine aminotransferase, endotoxin receptor Toll-like receptor 4 (TLR4), tumor necrosis factor-alpha (TNF-alpha), and interleukin (IL)-10 at various time points after injection. To investigate the effect of glycine on liver macrophages, Kupffer cells (KCs) were isolated and challenged by LPS (100 ng/mL), with or without glycine (4 mmol/l) pretreatment, and the expressions of TLR4, IL-10, and TNF-alpha were assayed at mRNA and protein levels. DNA-binding activity of nuclear factor-kappa B (NF-kappaB) was also analyzed using enzyme-linked immunosorbent assay.


RESULTS
Dietary glycine significantly improved the survival rate of endotoxemic mice (P < .05), whereas serum alanine aminotransferase and TNF-alpha levels were significantly decreased at different time points (P < .05); IL-10 levels were increased (P < .05). Concurrently, LPS-induced hepatic tissue injury was attenuated as indicated by morphologic analysis; secretion of IL-10 in liver tissue (P < .05) was enhanced; and expression of TLR4 and TNF-alpha in liver tissue was downregulated (P < .05). Consistent with these in vivo experiments, enhanced secretion of IL-10 and inhibited expression of TLR4 and TNF-alpha caused by glycine pretreatment were also observed in LPS-stimulated KCs. NF-kappaB DNA-binding activity was also significantly inhibited by glycine (P < .05, respectively).


CONCLUSIONS
Dietary glycine improved survival rates and liver function in endotoxemic mice by regulating the production of proinflammatory or anti-inflammatory cytokines in liver. It attenuated liver injury by deactivating KCs through inhibiting TNF-alpha secretion and increasing IL-10 production. The downregulative effect of glycine on the endotoxin signaling pathway and TLR4/NF-kappaB/TNF-alpha may be a novel potential mechanism by which glycine inhibits KC activity.
Chongqing Medical University, Chongqing, PRC.
Fa Liang
FL
Xu
Hai Bo
HB
You
Xu Hong
XH
Li
Xian Feng
XF
Chen
Zuo Jin
ZJ
Liu
Jian Ping
JP
Gong
D000284:Administration, Oral; D000818:Animals; D000893:Anti-Inflammatory Agents / Q000008:administration & dosage*; D004195:Disease Models, Animal; D015536:Down-Regulation; D005260:Female; D005998:Glycine / Q000008:administration & dosage*; D007728:Kupffer Cells / Q000378:metabolism*; D008070:Lipopolysaccharides / Q000009:adverse effects; D008107:Liver Diseases / Q000209:etiology / Q000382:microbiology / Q000517:prevention & control*; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D015398:Signal Transduction; D051197:Toll-Like Receptor 4 / Q000378:metabolism*
D000893:Anti-Inflammatory Agents; D008070:Lipopolysaccharides; C493487:Tlr4 protein, mouse; D051197:Toll-Like Receptor 4; D005998:Glycine