Rapid LPS transport during lipid absorption in rat small intestine

抄録

Lipopolysaccharide (LPS; endotoxin) is a lipophilic pathogenic factor derived from gut Gram-negative bacteria. Increased serum LPS concentrations are associated with the metabolic syndrome, termed metabolic endotoxemia. Since the origin of circulating LPS is the gut lumen, we studied the mechanisms of LPS transport across the gut mucosa, hypothesizing that transport occurs via known lipid uptake pathways. Since glucagon-like peptide-2 (GLP-2) reduces intestinal solute permeability but enhances fat absorption, we also hypothesized that GLP-2 affects LPS transport during fat absorption. We thus examined LPS transport mechanisms and the effect of exogenous GLP-2 on LPS transport in intact intestinal tissues in vitro and in vivo.

FITC-LPS absorption in vivo was quantitated by simultaneously measuring the appearance of FITC-LPS into the portal vein (PV) and the mesenteric lymph, with or without luminal oleic acid (30 mM) with taurocholate (OA/TCA). We also examined LPS transport from mucosal to serosal solution (m-to-s) in Ussing chambered muscle-stripped intestinal tissues, the latter measured by the Limulus amebocyte lysate test.

Bolus intraduodenal infusion of FITC-LPS (50 μg/ml) with OA/TCA rapidly increased FITC-LPS appearance into PV, followed by a gradual increase of FITC into the lymph, whereas FITC-LPS appearance into PV or lymph was not observed without OA/TCA. In Ussing chambered jejunum, luminally-applied LPS (10 μg/ml) rapidly appeared in the serosal solution with luminal OA/TCA, whereas LPS alone or LPS+TCA was not transported. OA/TCA-induced LPS m-to-s transport in the jejunum was inhibited by luminal pretreatment with the lipid raft inhibitor methyl-β-cyclodextrin and the CD36 inhibitor sulfosuccinimidyl oleate, but was enhanced by pretreatment with serosal carbachol and luminal application of the competitive alkaline phosphatase inhibitor glycerol phosphate. Pretreatment with serosal GLP-2 (100 nM) with a dipeptidyl peptidase-4 inhibitor NVP728 (10 μM) reduced OA/TCA-induced LPS transport, whereas carbachol-induced LPS transport was not affected.

These results suggest that luminal LPS crosses the gut barrier physiologically during fat absorption via lipid raft- and CD36-mediated transport mechanisms, similar to chylomicron transport, and possibly through emptied goblet cells. By reducing lipid raft-, CD36-, and chylomicron-mediated LPS transport, GLP-2 treatment may be a novel therapy for metabolic endotoxemia.