The critical role of toll-like receptor (TLR) 4 in alcoholic liver disease is independent of the common TLR adapter MyD88.
The Toll-like receptor 4 (TLR4) that recognizes endotoxin, a trigger of inflammation in alcoholic liver disease (ALD), activates two signaling pathways utilizing different adapter molecules: the common TLR adapter, myeloid differentiation factor 88 (MyD88), or Toll/interleukin immune-response-domain-containing adaptor inducing interferon (IFN)-beta. The MyD88 pathway induces proinflammatory cytokine activation, a critical mediator of ALD. Here we evaluated the role of MyD88 in alcohol-induced liver injury in wild-type, TLR2-deficient, TLR4-deficient, or MyD88-deficient (knockout [KO]) mice after administration of the Lieber-De-Carli diet (4.5% volume/volume ethanol) or an isocaloric liquid control diet for 5 weeks. Alcohol feeding resulted in a significant increase in serum alanine aminotransferase levels, liver steatosis and triglyceride levels suggesting liver damage in WT, TLR2-KO, and MyD88-KO but not in TLR4-KO mice. Expression of inflammatory mediators (tumor necrosis factor-alpha and interleukin-6) and TLR4 coreceptors (CD14 and MD2) was significantly higher in livers of alcohol-fed WT, TLR2-KO, or MyD88-KO, but not in TLR4-KO mice, compared to controls. Reactive oxygen radicals produced by cytochrome P450 and the nicotinamide adenine dinucleotide phosphate complexes contribute to alcoholic liver damage. Alcohol feeding-induced expression and activation of cytochrome P450 and the nicotinamide adenine dinucleotide phosphate complex were prevented by TLR4-deficiency but not by MyD88-deficiency. Liver expression of interferon regulatory factor 3 (IRF3), a MyD88-independent signaling molecule, was not affected by chronic alcohol treatment in whole livers of WT mice or in any of the KO mice. However, the induction of IRF7, an IRF3-inducible gene, was found in Kupffer cells of alcohol-fed WT mice. Alcohol feeding also induced nuclear factor-kappaB activation in a TLR4-dependent MyD88-independent manner.


CONCLUSIONS
While TLR4 deficiency was protective, MyD88 deficiency failed to prevent alcohol-induced liver damage and inflammation. These results suggest that the common TLR adapter, MyD88, is dispensable in TLR4-mediated liver injury in ALD.
Hritz
Istvan
I
Department of Medicine, University of Massachusetts Medical School, University of Massachusetts, Worcester, MA 01605, USA.
Mandrekar
Pranoti
P
Velayudham
Arumugam
A
Catalano
Donna
D
Dolganiuc
Angela
A
Kodys
Karen
K
Kurt-Jones
Evelyn
E
Szabo
Gyongyi
G
D000818:Animals; D019392:Cytochrome P-450 CYP2E1 / Q000378:metabolism; D004195:Disease Models, Animal; D000431:Ethanol; D005234:Fatty Liver / Q000139:chemically induced / Q000378:metabolism / Q000517:prevention & control; D022781:Hepatocytes / Q000378:metabolism / Q000473:pathology; D015850:Interleukin-6 / Q000378:metabolism; D018950:Lipopolysaccharide Receptors / Q000378:metabolism; D008099:Liver / Q000378:metabolism / Q000473:pathology; D008108:Liver Diseases, Alcoholic / Q000378:metabolism* / Q000473:pathology / Q000517:prevention & control; D051218:Lymphocyte Antigen 96 / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D053594:Myeloid Differentiation Factor 88 / Q000235:genetics / Q000378:metabolism*; D009249:NADP / Q000378:metabolism; D017382:Reactive Oxygen Species / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D051195:Toll-Like Receptor 2 / Q000378:metabolism; D051197:Toll-Like Receptor 4 / Q000235:genetics / Q000378:metabolism*; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism
D015850:Interleukin-6; D018950:Lipopolysaccharide Receptors; C411728:Ly96 protein, mouse; D051218:Lymphocyte Antigen 96; C507780:Myd88 protein, mouse; D053594:Myeloid Differentiation Factor 88; D017382:Reactive Oxygen Species; C493486:Tlr2 protein, mouse; C493487:Tlr4 protein, mouse; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D014409:Tumor Necrosis Factor-alpha; D000431:Ethanol; D009249:NADP; D019392:Cytochrome P-450 CYP2E1
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