Antibody to tumor necrosis factor attenuates endotoxin-stimulated amino acid transport in rat liver.
BACKGROUND
Endotoxemia stimulates amino acid consumption by the liver, but the regulation of this response is poorly understood. We studied the effect of Escherichia coli endotoxin (lipopolysaccharide) on hepatic carrier-mediated plasma membrane amino acid transport and the role of the cytokine tumor necrosis factor-alpha (TNF) in regulating this transport activity.


METHODS
We investigated the activities of the Na(+)-dependent amino acid transport systems A, ASC, and N in hepatic plasma membrane vesicles prepared from rats treated with endotoxin in vivo. Vesicle purity and functionality were evaluated by assaying marker enzymes and by the presence of classic overshoots.


RESULTS
Endotoxin treatment did not alter sodium transport but resulted in time- and dose-dependent 6-fold (system A), 3.5-fold (system N), and 3-fold (system ASC) increases in transport activity secondary to an increase in carrier maximum velocity. Lipopolysaccharide treatment did not alter transporter affinity or plasma membrane sodium transport. Transport activity increased within 2 hours of endotoxin administration, peaked at 4 hours after exposure to lipopolysaccharide, and returned to basal levels within 24 hours. Pretreatment of animals with an anti-TNF monoclonal antibody diminished the endotoxin-induced enhancement in transport activity by 50% to 75% by decreasing carrier maximum velocity. In contrast, when the antibody was given after endotoxin challenge, transport activity was not attenuated.


CONCLUSIONS
The marked acceleration in hepatic amino acid uptake that occurs during endotoxemia is secondary to an increased Na(+)-dependent hepatocyte plasma membrane transport activity and is mediated, in large part, by the cytokine TNF.
Inoue
Y
Y
Department of Surgery, Massachusetts General Hospital, Boston 02114.
Bode
B P
BP
Souba
W W
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D000596:Amino Acids / Q000378:metabolism*; D000621:Aminoisobutyric Acids / Q000378:metabolism; D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology*; D001692:Biological Transport / Q000187:drug effects; D002462:Cell Membrane / Q000378:metabolism; D066298:In Vitro Techniques; D008070:Lipopolysaccharides / Q000633:toxicity*; D008099:Liver / Q000378:metabolism* / Q000648:ultrastructure; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D014409:Tumor Necrosis Factor-alpha / Q000502:physiology*
D000596:Amino Acids; D000621:Aminoisobutyric Acids; D000911:Antibodies, Monoclonal; D008070:Lipopolysaccharides; D014409:Tumor Necrosis Factor-alpha; C017911:2-(methylamino)isobutyric acid