CLA-enriched diet containing t10,c12-CLA alters bile acid homeostasis and increases the risk of cholelithiasis in mice.

Mice fed a mixture of CLA containing t10,c12-CLA lose fat mass and develop hyperinsulinemia and hepatic steatosis due to an accumulation of TG and cholesterol. Because cholesterol is the precursor in bile acid (BA) synthesis, we investigated whether t10,c12-CLA alters BA metabolism. In Expt. 1, female C57Bl/6J mice were fed a standard diet for 28 d supplemented with a CLA mixture (1 g/100 g) or not (controls). In Expt. 2, the feeding period was reduced to 4, 6, and 10 d. In Expt. 3, mice were fed a diet supplemented with linoleic acid, c9,t11-CLA, or t10,c12-CLA (0.4 g/100 g) for 28 d. In Expt. 1, the BA pool size was greater in CLA-fed mice than in controls and the entero-hepatic circulation of BA was altered due to greater BA synthesis and ileal reclamation. This resulted from higher hepatic cholesterol 7α-hydroxylase (CYP7A1) and ileal apical sodium BA transporter expressions in CLA-fed mice. Furthermore, hepatic Na(+)/taurocholate co-transporting polypeptide (NTCP) (-52%) and bile salt export pump (BSEP) (-77%) protein levels were lower in CLA-fed mice than in controls, leading to a greater accumulation of BA in the plasma (+500%); also, the cholesterol saturation index and the concentration of hydrophobic BA in the bile were greater in CLA-fed mice, changes associated with the presence of cholesterol crystals. Expt. 2 suggests that CLA-mediated changes were caused by hyperinsulinemia, which occurred after 6 d of the CLA diet before NTCP and BSEP mRNA downregulation (10 d). Expt. 3 demonstrated that only t10,c12-CLA altered NTCP and BSEP mRNA levels. In conclusion, t10,c12-CLA alters BA homeostasis and increases the risk of cholelithiasis in mice.

Luminal lipid regulates CD36 levels and downstream signaling to stimulate chylomicron synthesis.

The membrane glycoprotein CD36 binds nanomolar concentrations of long chain fatty acids (LCFA) and is highly expressed on the luminal surface of enterocytes. CD36 deficiency reduces chylomicron production through unknown mechanisms. In this report, we provide novel insights into some of the underlying mechanisms. Our in vivo data demonstrate that CD36 gene deletion in mice does not affect LCFA uptake and subsequent esterification into triglycerides by the intestinal mucosa exposed to the micellar LCFA concentrations prevailing in the intestine. In rodents, the CD36 protein disappears early from the luminal side of intestinal villi during the postprandial period, but only when the diet contains lipids. This drop is significant 1 h after a lipid supply and associates with ubiquitination of CD36. Using CHO cells expressing CD36, it is shown that the digestion products LCFA and diglycerides trigger CD36 ubiquitination. In vivo treatment with the proteasome inhibitor MG132 prevents the lipid-mediated degradation of CD36. In vivo and ex vivo, CD36 is shown to be required for lipid activation of ERK1/2, which associates with an increase of the key chylomicron synthesis proteins, apolipoprotein B48 and microsomal triglyceride transfer protein. Therefore, intestinal CD36, possibly through ERK1/2-mediated signaling, is involved in the adaptation of enterocyte metabolism to the postprandial lipid challenge by promoting the production of large triglyceride-rich lipoproteins that are rapidly cleared in the blood. This suggests that CD36 may be a therapeutic target for reducing the postprandial hypertriglyceridemia and associated cardiovascular risks.

Chronic high-fat diet affects intestinal fat absorption and postprandial triglyceride levels in the mouse.

The effects of chronic fat overconsumption on intestinal physiology and lipid metabolism remain elusive. It is unknown whether a fat-mediated adaptation to lipid absorption takes place. To address this issue, mice fed a high-fat diet (40%, w/w) were refed or not a control diet (3%, w/w) for 3 additive weeks. Despite daily lipid intake 7.7-fold higher than in controls, fecal lipid output remained unchanged in mice fed the triglyceride (TG)-rich diet. In situ isolated jejunal loops revealed greater [1-(14)C]linoleic acid uptake without TG accumulation in mucosa, suggesting an increase in lipid absorption capacity. Induction both in intestinal mitotic index and in the expression of genes involved in fatty acid uptake, trafficking, and lipoprotein synthesis was found in high-fat diet mice. These changes were lipid-mediated, in that they were fully abolished in mice refed the control diet. A lipid load test performed in the presence or absence of the LPL inhibitor tyloxapol showed a sustained blood TG clearance in fat-fed mice likely attributable to intestinal modulation of LPL regulators (apolipoproteins C-II and C-III). These data demonstrate that a chronic high-fat diet greatly affects intestinal physiology and body lipid use in the mouse.

Sterol regulatory element-binding protein-1c is responsible for cholesterol regulation of ileal bile acid-binding protein gene in vivo. Possible involvement of liver-X-receptor.

Ileal bile acid-binding protein (I-BABP) is a cytosolic protein that binds bile acid (BA) specifically. In the ileum, it is thought to be implied in their enterohepatic circulation. Because the fecal excretion of BA represents the main physiological way of elimination for cholesterol (CS), the I-BABP gene could have a major function in CS homeostasis. Therefore, the I-BABP gene expression might be controlled by CS. I-BABP mRNA levels were significatively increased when the human enterocyte-like CaCo-2 cells were CS-deprived and repressed when CS were added to the medium. A highly conserved sterol regularory element-like sequence (SRE) and a putative GC box were found in human I-BABP gene promoter. Different constructs of human I-BABP promoter, cloned upstream of a chloramphenicol acetyltransferase (CAT) reporter gene, have been used in transfections studies. CAT activity of the wild type promoter was increased in presence of CS-deprived medium, and conversely, decreased by a CS-supplemented medium. The inductive effect of CS depletion was fully abolished when the putative SRE sequence and/or GC box were mutated or deleted. Co-transfections experiments with the mature isoforms of human sterol responsive element-binding proteins (SREBPs) and Sp1 demonstrate that the CS-mediated regulation of I-BABP gene was dependent of these transcriptional factors. Paradoxically, mice subjected to a standard chow supplemented with 2% CS for 14 days exhibited a significant rise in both I-BABP and SREBP1c mRNA levels. We show that in vivo, this up-regulation could be explained by a recently described regulatory pathway involving a positive regulation of SREBP1c by liver-X-receptor following a high CS diet.