Lipoprotein-bound endotoxin exerts an immunomodulatory effect on hepatocytes through the lipid A domain of LPS.
BACKGROUND
We have previously shown that chylomicron (CM)-bound lipopolysaccharide (LPS) inhibits the host innate immune response by rendering hepatocytes tolerant to pro-inflammatory cytokine stimulation. However, LPS is a complex macromolecule containing both lipid and carbohydrate domains. We hypothesized that just as lipid A confers the toxicity of LPS, it is also responsible for the immunoregulatory effect on hepatocytes.


METHODS
We pretreated primary rat hepatocytes for 2 h with a series of CM-LPS complexes in which the endotoxin moiety varied in its structure and/or toxicity. Subsequently, the cells were stimulated with a mixture of pro-inflammatory cytokines. Nitric oxide production was measured as an indicator of hepatocellular activation.


RESULTS
All pretreatments wherein the CM-bound complex contained the lipid A moiety readily inhibited the hepatocellular cytokine response, including CM bound to lipid A alone. In contrast, CM-LPS complexes containing detoxified LPS, which lacks the lipid A domain, had no effect on the hepatocellular response to cytokines.


CONCLUSIONS
The lipid A domain of the LPS macromolecule is both sufficient and essential for the CM-mediated induction of cytokine tolerance in hepatocytes. However, this process is independent of the specific endotoxic activity of the lipid A moiety.
Kasravi
F Behzad
FB
UCSF Surgical Research Laboratory at San Francisco General Hospital, San Francisco, California 94110, USA.
Lee
Diana H
DH
Weisgraber
Karl
K
Harris
Hobart W
HW
D000818:Animals; D018929:Cell Culture Techniques; D002478:Cells, Cultured; D002914:Chylomicrons / Q000378:metabolism; D016207:Cytokines / Q000378:metabolism; D004731:Endotoxins / Q000494:pharmacology*; D022781:Hepatocytes / Q000187:drug effects* / Q000378:metabolism; D008050:Lipid A / Q000737:chemistry; D008070:Lipopolysaccharides / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology*; D008264:Macrophages / Q000187:drug effects / Q000201:enzymology; D008297:Male; D009569:Nitric Oxide / Q000096:biosynthesis; D017434:Protein Structure, Tertiary; D051381:Rats; D017207:Rats, Sprague-Dawley; D013997:Time Factors
D002914:Chylomicrons; D016207:Cytokines; D004731:Endotoxins; D008050:Lipid A; D008070:Lipopolysaccharides; D009569:Nitric Oxide