Trypsin-sensitive modulation of intestinal epithelial MD-2 as mechanism of lipopolysaccharide tolerance.
Intestinal epithelial cells (IEC) are constantly exposed to both high concentrations of the bacterial ligand LPS and the serine protease trypsin. MD-2, which contains multiple trypsin cleavage sites, is an essential accessory glycoprotein required for LPS recognition and signaling through TLR4. The aim of this study was to characterize the expression and subcellular distribution of intestinal epithelial MD-2 and to delineate potential functional interactions with trypsin and then alteration in inflammatory bowel disease (IBD). Although MD-2 protein expression was minimal in primary IEC of normal colonic or ileal mucosa, expression was significantly increased in IEC from patients with active IBD colitis, but not in ileal areas from patients with severe Crohn's disease. Endogenous MD-2 was predominantly retained in the calnexin-calreticulin cycle of the endoplasmic reticulum; only a small fraction was exported to the Golgi. MD-2 expression correlated inversely with trypsin activity. Biochemical evidence and in vitro experiments demonstrated that trypsin exposure resulted in extensive proteolysis of endogenous and soluble MD-2 protein, but not of TLR4 in IEC, and was associated with desensitization of IEC to LPS. In conclusion, the present study suggests that endoplasmic reticulum-associated MD-2 expression in IBD may be altered by ileal protease in inflammation, leading to impaired LPS recognition and hyporesponsiveness through MD-2 proteolysis in IEC, thus implying a physiologic mechanism that helps maintain LPS tolerance in the intestine.
Division of Gastroenterology and Hepatology, University Hospital of Essen, Germany. elke.cario@uni-essen.de
Elke
E
Cario
Douglas T
DT
Golenbock
Alberto
A
Visintin
Michael
M
Rünzi
Guido
G
Gerken
Daniel K
DK
Podolsky
D002460:Cell Line; D003092:Colitis / Q000378:metabolism / Q000473:pathology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D004847:Epithelial Cells / Q000187:drug effects / Q000276:immunology / Q000378:metabolism*; D006801:Humans; D007108:Immune Tolerance / Q000187:drug effects / Q000276:immunology*; D007413:Intestinal Mucosa / Q000378:metabolism*; D007422:Intestines / Q000187:drug effects / Q000276:immunology*; D008070:Lipopolysaccharides / Q000276:immunology* / Q000494:pharmacology; D051218:Lymphocyte Antigen 96 / Q000378:metabolism*; D018832:Molecular Chaperones / Q000378:metabolism; D011485:Protein Binding; D014357:Trypsin / Q000378:metabolism*; D015854:Up-Regulation
C495377:LY96 protein, human; D008070:Lipopolysaccharides; D051218:Lymphocyte Antigen 96; D018832:Molecular Chaperones; D014357:Trypsin