Liver X Receptor Regulates Triglyceride Absorption Through Intestinal Down-regulation of Scavenger Receptor Class B, Type 1.

BACKGROUND & AIMS: Reducing postprandial triglyceridemia may be a promising strategy to lower the risk of cardiovascular disorders associated with obesity and type 2 diabetes. In enterocytes, scavenger receptor class B, type 1 (SR-B1, encoded by SCARB1) mediates lipid-micelle sensing to promote assembly and secretion of chylomicrons. The nuclear receptor subfamily 1, group H, members 2 and 3 (also known as liver X receptors [LXRs]) regulate genes involved in cholesterol and fatty acid metabolism. We aimed to determine whether intestinal LXRs regulate triglyceride absorption. METHODS: C57BL/6J mice were either fed a cholesterol-enriched diet or given synthetic LXR agonists (GW3965 or T0901317). We measured the production of chylomicrons and localized SR-B1 by immunohistochemistry. Mechanisms of postprandial triglyceridemia and SR-B1 regulation were studied in Caco-2/TC7 cells incubated with LXR agonists. RESULTS: In mice and in the Caco-2/TC7 cell line, LXR agonists caused localization of intestinal SR-B1 from apical membranes to intracellular organelles and reduced chylomicron secretion. In Caco-2/TC7 cells, LXR agonists reduced SR-B1-dependent lipidic-micelle-induced Erk phosphorylation. LXR agonists also reduced intracellular trafficking of the apical apolipoprotein B pool toward secretory compartments. LXR reduced levels of SR-B1 in Caco-2/TC7 cells via a post-transcriptional mechanism that involves microRNAs. CONCLUSION: In Caco-2/TC7 cells and mice, intestinal activation of LXR reduces the production of chylomicrons by a mechanism dependent on the apical localization of SR-B1.

Liver X receptors inhibit proliferation of human colorectal cancer cells and growth of intestinal tumors in mice.

BACKGROUND & AIMS: Liver X receptors (LXRs) are transcriptional regulators of cholesterol metabolism, controlling cholesterol flow into cells, catabolism, and efflux. Cholesterol controls cell proliferation; disruptions in cholesterol metabolism have been associated with the development of colon cancer. We investigated whether expression of activated LXR protects against intestinal tumorigenesis in mice. METHODS: We analyzed the development of colon cancer in mice that express a constitutive active form of LXRα only in the intestinal epithelium, under the control of villin promoter (iVP16LXRα). These mice were crossed with adenomatous polyposis coli (Apc)(min/+) mice, or given azoxymethane followed by dextran sodium sulfate, to assess intestinal tumor formation. We also assessed proliferation and apoptosis of a human colorectal cancer cell line (HT29) transfected with an adenoviral vector that expressed Ad VP16hLXRα, compared with cells expressing AdVP16 (control), and their ability to form xenograft tumors in mice. HT29 cells also were incubated with the LXR ligand GW3965. RESULTS: In human colorectal cancer cells, ligand-induced activation of LXR or transfection with Ad VP16hLXRα blocked the G1 phase, increased caspase-dependent apoptosis, and slowed growth of xenograft tumors in mice. iVP16LXRα mice formed fewer, smaller tumors than VP16 (control) mice after administration of azoxymethane and dextran sodium sulfate. APC(min/+)/iVP16LXRα mice also developed fewer, smaller intestinal tumors than APC(min/+)/iVP16 mice. Gene expression analysis indicated that activation of LXRα affected lipid metabolic networks and increased cholesterol efflux in the intestine. CONCLUSIONS: Expression of activated LXRα blocks proliferation of human colorectal cancer cells and slows the growth of xenograft tumors in mice. It also reduces intestinal tumor formation after administration of chemical carcinogens, and in Apc(min/+) mice. LXR agonists therefore might be developed as therapeutic treatments for colorectal cancer.

Enterocyte superoxide dismutase 2 deletion drives obesity.

Compelling evidence support an involvement of oxidative stress and intestinal inflammation as early events in the predisposition and development of obesity and its related comorbidities. Here, we show that deficiency of the major mitochondrial antioxidant enzyme superoxide dismutase 2 (SOD2) in the gastrointestinal tract drives spontaneous obesity. Intestinal epithelium-specific Sod2 ablation in mice induced adiposity and inflammation via phospholipase A2 (PLA2) activation and increased release of omega-6 polyunsaturated fatty acid arachidonic acid. Remarkably, this obese phenotype was rescued when fed an essential fatty acid-deficient diet, which abrogates de novo biosynthesis of arachidonic acid. Data from clinical samples revealed that the negative correlation between intestinal Sod2 mRNA levels and obesity features appears to be conserved between mice and humans. Collectively, our findings suggest a role of intestinal Sod2 levels, PLA2 activity, and arachidonic acid in obesity presenting new potential targets of therapeutic interest in the context of this metabolic disorder.

Uncoupling nuclear receptor LXR and cholesterol metabolism in cancer.

Liver X receptors (LXRs) are members of the nuclear receptor superfamily of DNA-binding transcription factors and act as sensors of cholesterol homeostasis. Under normal conditions, when intracellular cholesterol concentration increases, cells synthesize oxysterols and activate the LXR transcriptional network to drive cholesterol efflux and reduce cholesterol influx and synthesis. During normal and cancer cell proliferation, there is a net uncoupling between intracellular cholesterol increase and LXR activation resulting from the reduced intracellular oxysterol concentration. This review dissects the novel mechanisms of a previously unrecognized metabolic uncoupling, supporting the activation of the LXR axis as a bona fide therapeutic approach in cancer.

Microbiota modification with probiotics induces hepatic bile acid synthesis via downregulation of the Fxr-Fgf15 axis in mice.

Gut microbiota influences host health status by providing trophic, protective, and metabolic functions, including bile acid (BA) biotransformation. Microbial imprinting on BA signature modifies pool size and hydrophobicity, thus contributing to BA enterohepatic circulation. Microbiota-targeted therapies are now emerging as effective strategies for preventing and/or treating gut-related diseases. Here, we show that gut microbiota modulation induced by VSL#3 probiotics enhances BA deconjugation and fecal excretion in mice. These events are associated with changes in ileal BA absorption, repression of the enterohepatic farnesoid X receptor-fibroblast growth factor 15 (FXR-FGF15) axis, and increased hepatic BA neosynthesis. Treatment with a FXR agonist normalized fecal BA levels in probiotic-administered mice, whereas probiotic-induced alterations in BA metabolism are abolished upon FXR and FGF15 deficiency. Our data provide clear in vivo evidence that VSL#3 probiotics promote ileal BA deconjugation with subsequent fecal BA excretion and induce hepatic BA neosynthesis via downregulation of the gut-liver FXR-FGF15 axis.

Transient GFER knockdown in vivo impairs liver regeneration after partial hepatectomy.

BACKGROUND AND AIMS: Augmenter of Liver Regeneration is a protein encoded by the Growth Factor Erv1-Like gene. Its biological properties are crucial for cell survival since knock-out mice for Growth Factor Erv1-Like gene do not survive. In this study, we injected hepatotropic adenoviral particles harboring oligonucleotide sequences against Growth Factor Erv1-Like gene into 70% partially hepatectomized rats and studied the effect of gene silencing on the progression liver regeneration. METHODS: Partially hepatectomized rats were divided into three groups of animals and, before surgery, received either phosphate buffer saline, or adenoviral particles alone or adenoviral particles harboring the oligonucleotide silencing sequence. In each group, rats were sacrificed at 12, 24 and 48 h after surgery. Liver tissues were collected to analyze the expression of Augmenter of Liver Regeneration, Bax, Bcl-2 and activated Caspase-9 and -3, as well as hepatocyte proliferation and apoptosis, polyamines levels and histological and ultrastructural features. RESULTS: Growth Factor Erv1-Like gene silencing reduced the compensatory hepatocellular proliferation triggered by surgery through (i) the reduction of polyamines synthesis, hepatocyte proliferation and anti-apoptotic gene expression and (ii) the increase of pro-apoptotic gene expression and caspase activation. CONCLUSIONS: For the first time, using a technique of gene silencing in vivo, our results demonstrate that Growth Factor Erv1-Like gene knock-down, i.e., the lack of Augmenter of Liver Regeneration, modifies the expression of genes involved in cell apoptosis and inhibits early phase of DNA synthesis. As a consequence, a promotion of cell death and a reduction of cell proliferation occurs.