BACKGROUND & AIMS The gut microbiota affects intestinal permeability and mucosal mast cells (MMCs) responses. Activation of MMCs has been associated with absorption of dietary fat. We investigated whether the gut microbiota contributes to the fat-induced activation of MMCs in rats, and how antibiotics might affect this process. METHODS Adult male Sprague-Dawley rats were given streptomycin and penicillin for 4 days (n = 6–8) to reduce the abundance of their gut flora, or normal drinking water (controls, n = 6–8). They underwent lymph fistula surgery and after an overnight recovery were given an intraduodenal bolus of intralipid. We collected intestinal tissues and lymph fluid and assessed activation of MMCs, intestinal permeability, and fat transport parameters. RESULTS Compared with controls, intestinal lymph from rats given antibiotics had reduced levels of mucosal mast cell protease II (produced by MMCs) and decreased activity of diamine oxidase (produced by enterocytes) (P < .05). Rats given antibiotics had reduced intestinal permeability in response to dietary lipid compared with controls (P < .01). Unexpectedly, antibiotics also reduced lymphatic transport of triacylglycerol and phospholipid (P < .01), concomitant with decreased levels of mucosal apolipoproteins B, A-I, and A-IV (P < .01). No differences were found in intestinal motility or luminal pancreatic lipase activity between rats given antibiotics and controls. These effects were not seen with an acute dose of antibiotics or 4 weeks after the antibiotic regimen ended. CONCLUSIONS The intestinal microbiota appears to activate MMCs after the ingestion of fat in rats; this contributes to fat-induced intestinal permeability. We found that the gut microbiome promotes absorption of lipid, probably by intestinal production of apolipoproteins and secretion of chylomicrons.

The adenosine triphosphate-binding cassette (ABC) transporter G5/G8 is critical in protecting the body from accumulating dietary plant sterols. Expressed in the liver and small intestine, it transports plant sterols into the biliary and intestinal lumens, thus promoting their excretion. The extent to which G5/G8 regulates cholesterol absorption remains unclear. G5/G8 is also implicated in reducing the absorption of dietary triacylglycerols (TAG) by unknown mechanisms. We hypothesized that G5/G8 suppresses the production of chylomicrons, and its deficiency would enhance the absorption of both dietary TAG and cholesterol. The aim of this study was to investigate the effects of G5/G8 deficiency on lipid uptake and secretion into the lymph under steady-state conditions. Surprisingly, compared with wild-type mice (WT) (n = 9), G5/G8 KO (n = 13) lymph fistula mice given a continuous intraduodenal infusion of [3H]-TAG and [14C]-cholesterol showed a significant (P < 0.05) reduction in lymphatic transport of both [3H]-TAG and [14C]-cholesterol, concomitant with a significant (P < 0.05) increase of [3H]-TAG and [14C]-cholesterol accumulated in the intestinal lumen. There was no difference in the total amount of radiolabeled lipids retained in the intestinal mucosa between the two groups. G5/G8 KO mice given a bolus of TAG showed reduced intestinal TAG secretion compared with WT, suggesting an independent role for G5/G8 in facilitating intestinal TAG transport. Our data demonstrate that G5/G8 deficiency reduces the uptake and secretion of both dietary TAG and cholesterol by the intestine, suggesting a novel role for the sterol transporter in the formation and secretion of chylomicrons.

Apolipoprotein (apo) A-V is a protein synthesized only in the liver that dramatically modulates plasma triglyceride levels. Recent studies suggest a novel role for hepatic apoA-V in regulating the absorption of dietary triglycerides, but its mode of action on the gut remains unknown. The aim of this study was to test for apoA-V in bile and to determine whether its secretion is regulated by dietary lipids. After an overnight recovery, adult male Sprague-Dawley bile fistula rats indeed secreted apoA-V into bile at a constant rate under fasting conditions. An intraduodenal bolus of intralipid (n = 12) increased the biliary secretion of apoA-V but not of other apolipoproteins, such as A-I, A-IV, B, and E. The lipid-induced increase of biliary apoA-V was abolished under conditions of poor lymphatic lipid transport, suggesting that the stimulation is regulated by the magnitude of lipids associated with chylomicrons transported into lymph. We also studied the secretion of apoA-V into bile immediately following bile duct cannulation. Biliary apoA-V increased over time (∼6-fold increase at hour 16, n = 8) but the secretions of other apolipoproteins remained constant. Replenishing luminal phosphatidylcholine and taurocholate (n = 9) only enhanced apoA-V secretion in bile, suggesting that the increase was not due to depletion of phospholipids or bile salts. This is the first study to demonstrate that apoA-V is secreted into bile, introducing a potential route of delivery of hepatic apoA-V to the gut lumen. Our study also reveals the uniqueness of apoA-V secretion into bile that is regulated by mechanisms different from other apolipoproteins.

Apolipoprotein A-V (apoA-V), a liver-synthesized apolipoprotein discovered in 2001, strongly modulates fasting plasma triglycerides (TG). Little is reported on the effect of apoA-V on postprandial plasma TG, an independent predictor for atherosclerosis. Overexpressing apoA-V in mice suppresses postprandial TG, but mechanisms focus on increased lipolysis or clearance of remnant particles. Unknown is whether apoA-V suppresses the absorption of dietary lipids by the gut. This study examines how apoA-V deficiency affects the steady-state absorption and lymphatic transport of dietary lipids in chow-fed mice. Using apoA-V knockout (KO, n ϭ 8) and wild-type (WT, n ϭ 8) lymph fistula mice, we analyzed the uptake and lymphatic transport of lipids during a continuous infusion of an emulsion containing [ 3 H]triolein and [14 C]cholesterol. ApoA-V KO mice showed a twofold increase in 3 H (P Ͻ 0.001) and a threefold increase in 14 C (P Ͻ 0.001) transport into the lymph compared with WT. The increased lymphatic transport was accompanied by a twofold reduction (P Ͻ 0.05) in mucosal 3 H, suggesting that apoA-V KO mice more rapidly secreted [3 H]TG out of the mucosa into the lymph. ApoA-V KO mice also produced chylomicrons more rapidly than WT (P Ͻ 0.05), as measured by the transit time of [ 14 C]oleic acid from the intestinal lumen to lymph. Interestingly, apoA-V KO mice produced a steadily increasing number of chylomicron particles over time, as measured by lymphatic apoB output. The data suggest that apoA-V suppresses the production of chylomicrons, playing a previously unknown role in lipid metabolism that may contribute to the postprandial hypertriglyceridemia associated with apoA-V deficiency. absorption; intestine AFTER DIETARY FAT INTAKE, the increase of plasma triglycerides (TG) is a normal metabolic consequence characterized by a transient accumulation of TG-rich lipoproteins comprised of intestinally derived chylomicrons and liver-derived very low density lipoprotein (VLDL). Hydrolysis of their TG cores releases free fatty acids to be taken up by muscle and adipose tissues, generating remnant particles that are efficiently removed from the circulation by the liver. Aberrations in lipoprotein metabolism result in elevated plasma TG and a prolonged accumulation of remnant particles that are atherogenic (16,76,24) and constitute a major risk factor for cardiovascular disease (51,43).Apolipoproteins play crucial roles in the regulation of lipoprotein metabolism by acting as structural proteins, as cofactors for enzymes involved in lipolysis, or as ligands for receptor-mediated clearance of lipoproteins (35). Apolipoprotein A-V (apoA-V) was discovered in 2001 to be expressed exclusively in the liver and located ϳ30 kb proximal to the APOAI/ CIII/AIV gene cluster (49, 69), a region well-known for regulating lipid metabolism (19). Early studies using genetically engineered mouse models revealed apoA-V to be a potent modulator of plasma TG. Knocking out apoA-V in mice resulted in a fourfold increase in plasma TG while overexp...