Secretion of angiopoietin-like 4 protein from intestinal cells

Molecular metabolism, 2014

Dietary triglycerides are hydrolyzed in the small intestine principally by pancreatic lipase. Following uptake by enterocytes and secretion as chylomicrons, dietary lipids are cleared from the bloodstream via lipoprotein lipase. Whereas lipoprotein lipase is inhibited by several proteins including Angiopoietin-like 4 (Angptl4), no endogenous regulator of pancreatic lipase has yet been identified. Here we present evidence that Angptl4 is an endogenous inhibitor of dietary lipid digestion. Angptl4-/- mice were heavier compared to their wild-type counterparts without any difference in food intake, energy expenditure or locomotor activity. However, Angptl4-/- mice showed decreased lipid content in the stools and increased accumulation of dietary triglycerides in the small intestine, which coincided with elevated luminal lipase activity in Angptl4-/- mice. Furthermore, recombinant Angptl4 reduced the activity of pancreatic lipase as well as the lipase activity in human ileostomy output. ...

Frontiers in Physiology, 2015

Intestinal lipoprotein production is a multistep process, essential for the absorption of dietary fats and fat-soluble vitamins. Chylomicron assembly begins in the endoplasmic reticulum with the formation of primordial, phospholipids-rich particles that are then transported to the Golgi for secretion. Several classes of transporters play a role in the selective uptake and/or export of lipids through the villus enterocytes. Once secreted in the lymph stream, triglyceride-rich lipoproteins (TRLs) are metabolized by Lipoprotein lipase (LPL), which catalyzes the hydrolysis of triacylglycerols of very low density lipoproteins (VLDLs) and chylomicrons, thereby delivering free fatty acids to various tissues. Genetic mutations in the genes codifying for these proteins are responsible of different inherited disorders affecting chylomicron metabolism. This review focuses on the molecular pathways that modulate the uptake and the transport of lipoproteins of intestinal origin and it will highlight recent findings on TRLs assembly.

Atherosclerosis, 2014

Overproduction of hepatic apoB100-containing VLDL particles has been well documented in animal models and in humans with insulin resistance such as the metabolic syndrome and type 2 diabetes, and contributes to the typical dyslipidemia of these conditions. In addition, postprandial hyperlipidemia and elevated plasma concentrations of intestinal apoB48-containing chylomicron and chylomicron remnant particles have been demonstrated in insulin resistant states. Intestinal lipoprotein production is primarily determined by the amount of fat ingested and absorbed. Until approximately 10 years ago, however, relatively little attention was paid to the role of the intestine itself in regulating the production of triglyceride-rich lipoproteins (TRL) and its dysregulation in pathological states such as insulin resistance. We and others have shown that insulin resistant animal models and humans are characterized by overproduction of intestinal apoB48-containing lipoproteins. Whereas various fac...

Journal of Biological …, 2002

Bile salt-stimulated carboxyl ester lipase (CEL), also called cholesterol esterase, is one of the major proteins secreted by the pancreas. The physiological role of CEL was originally thought to be its mediation of dietary cholesterol absorption. However, recent studies showed no ...

Diabetes, 2015

Individuals with metabolic syndrome and frank type 2 diabetes are at increased risk of atherosclerotic cardiovascular disease, partially due to the presence of lipid and lipoprotein abnormalities. In these conditions, the liver and intestine overproduce lipoprotein particles, exacerbating the hyperlipidemia of fasting and postprandial states. Incretin-based, antidiabetes therapies (i.e., glucagon-like peptide [GLP]-1 receptor agonists and dipeptidyl peptidase-4 inhibitors) have proven efficacy for the treatment of hyperglycemia. Evidence is accumulating that these agents also improve fasting and postprandial lipemia, the latter more significantly than the former. In contrast, the gut-derived peptide GLP-2, cosecreted from intestinal L cells with GLP-1, has recently been demonstrated to enhance intestinal lipoprotein release. Understanding the roles of these emerging regulators of intestinal lipoprotein secretion may offer new insights into the regulation of intestinal lipoprotein as...

American Journal of Physiology-gastrointestinal and Liver Physiology, 1999

Antitrypsin (AAT) is secreted by the enterocyte, but its regulation of expression, intramucosal distribution, and functional status are unclear. After corn oil gavage (plus Pluronic L-81 to block chylomicron release), rat intestine was examined for mRNA encoding AAT, immunoreactivity by light and electron microscopy, and protein content by Western blot. Species-specific antisera used were raised against both AAT and surfactant-like particle (SLP), a membrane secreted by the enterocyte in response to fat feeding. Purified luminal SLP was fractionated by Bio-Gel P-200 chromatography to assess its interaction with AAT. Triacylglycerol feeding maximally increased mucosal mRNAencoding AAT and AAT intracellular protein content by 3 and 5 h, respectively. Immunocytochemistry revealed predominance of AAT in basolateral spaces around enterocytes and Pluronic-blocked extracellular accumulation of AAT, patterns nearly identical to those of secreted SLP. About 10% of AAT was reversibly associated with SLP. Luminal AAT was smaller (51 kDa) than mature AAT (55 kDa) and did not form a complex with pancreatic elastase. When the common bile duct was tied, excluding pancreatic proteases from the lumen, mature AAT that was cleaved by pancreatic elastase was secreted. The luminal secretion of AAT and its reversible association with SLP suggest an intracellular association and a possible role for AAT during lipid digestion and absorption.

Biochemical and Biophysical Research Communications, 2013

RNAi Heparin-releasable 3T3-L1 Angiopoietin-like protein 4 Lipoprotein lipase a b s t r a c t Lipoprotein lipase (LPL) and angiopoietin-like protein 4 (Angptl4) were studied in 3T3-L1 adipocytes. Transfections of the adipocytes with Angptl4 esiRNA caused reduction of the expression of Angptl4 to about one fourth of that in cells treated with vehicle only. This resulted in higher levels of LPL activity both on cell surfaces (heparin-releasable) and in the medium, while LPL activity within the cells remained unaffected. This demonstrated that even though both proteins are made in the same cell, Angptl4 does not inactivate LPL during intracellular transport. Most of the Angptl4 protein was present as covalent dimers and tetramers on cell surfaces, while within the cells there were only monomers. LPL gradually lost activity when incubated in medium, but there was no marked difference between conditioned medium from normal cells (rich in Angptl4) and medium after knockdown of Angptl4. Hence Angptl4 did not markedly accelerate inactivation of LPL in the medium. Experiments with combinations of different cells and media indicated that inactivation of LPL occurred on the surfaces of cells producing Angptl4.

Atherosclerosis Supplements, 2008

Various dietary factors affect postprandial metabolism yet precise mechanisms have not necessarily been pinpointed. The effects of various meal components on postprandial lipemia lead to the following question: do we need a standardized oral lipid tolerance test? A number of transporters, enzymes, receptors and hormones directly influence and act as "gatekeepers" of these processes. Each protein appears to have specific and individual functional roles in the overall process and selected developments in these areas will be reviewed.

2007

Lipoprotein lipase (LPL) is the main enzyme hydrolyzing triglyceride-rich lipoproteins in plasma. Proteoglycan-bound LPL on the vascular endothelium represent the functional pool of active enzyme. LPL is regulated in a tissue specific manner according to metabolic demands. Rapid regulation of LPL activity is necessary to provide free fatty acids for storage or energy production. This regulatory mechanism appears to be post-translational and requires synthesis of other protein/proteins. Recently it was demonstrated that angiopoietin-like protein 4 (ANGPTL4) is involved in the metabolism of plasma triglycerides and that it is able to inhibit LPL activity in vitro. These properties were linked to the N-terminal coiled-coil domain of ANGPTL4 (ccd-ANGPTL4), but the mechanism for the inhibition was not known. The aim of this thesis was to investigate the molecular mechanism for inhibition of LPL by ccd-ANGPTL4, to characterize regions in ccd-ANGPTL4 that are important for inactivation of LPL and to study the role of ANGPTL4 for regulation of LPL in vivo. Binding of ccd-ANGPTL4 to LPL was demonstrated by several methods, including surface plasmon resonance. The interaction was transient and resulted in conversion of the enzyme from catalytically active dimers to inactive monomers with decreased affinity for heparin. We have shown that ANGPTL4 mRNA in rat adipose tissue turns over rapidly and that changes in the ANGPTL4 mRNA abundance were inversely correlated to LPL activity, both during the fed to fasted and the fasted to fed transitions. We conclude that ANGPTL4 is a fasting-induced controller of LPL in adipose tissue, acting extracellularly on the native conformation of LPL in an unusual fashion, like an unfolding molecular chaperone. Site directed mutagenesis was used to explore regions in ccd-ANGPTL4 important for inactivation of LPL, and for binding of ANGPTL4 to heparin. Others had shown that ccd-ANGPTL4 forms higher oligomers. Structure prediction analyses demonstrated that the coiled-coil domain of ccd-ANGPTL4 probably forms three consecutive α-helices with strong hydrophobic faces, and that there are clusters of positively charged residues both on the helices and in intervening sequences. We made replacements of hydrophobic residues, positively charged residues, cysteine residues and negatively charged residues in ccd-ANGPTL4. In addition, helix-breaking proline residues were introduced in all three helices. We found that hydrophobic residues are important for oligomer formation. The higher oligomers appeared to be stabilized by disulfide bonds, but cysteines are not crucial for oligomerization. Introduction of Pro-residues in the first and second helix prevented formation of higher oligomers and reduced the ability of ccd-ANGPTL4 to inactivate LPL. We found that negatively charged residues in ccd-ANGPTL4 are important for inactivation of LPL. A heparin binding site was localized in the C-terminal end of ccd-ANGPTL4 (amino acid residues 114-140). To investigate whether LPL is differently processed in different depots of adipose tissue we measured the levels of LPL mRNA, protein and activity in omental and subcutaneous adipose tissue in human subjects undergoing elective surgery. Our results show that, although the expression level of LPL was higher in subcutaneous adipose tissue, the specific LPL activity (ratio of activity over the LPL protein mass) was higher in omental adipose tissue. Interestingly, the levels of ANGPTL4 mRNA were lower in omental compared to subcutaneous adipose tissue in most of the studied subjects. This difference can possibly explain the higher specific activity of LPL in omental adipose tissue and indicated that ANGPTL4 is involved in regulation of LPL activity also in humans. LPL produced by macrophages in the artery wall promotes local accumulation of lipids in these cells, and thereby plays an important role in development of atherosclerosis. The known association between type 2 diabetes and atherosclerosis forwarded us to study production of LPL by THP-1 macrophages under hyperglycemic conditions and under treatment with a peroxisome proliferator-activated receptor delta (PPARδ) agonist (GW501516). We found that LPL activity (but not LPL mass) produced by macrophages was decreased by GW501516. The loss of LPL activity coincided with increased level of ANGPTL4 mRNA, indicating that the agonist regulates LPL activity through expression of ANGPTL4. This effect was even more pronounced in cells grown under hyperglycemic conditions. Our data suggest that a suitable PPARδ agonist, like GW501516, may have protective effects against development of atherosclerosis in subjects with diabetes type 2.

The Journal of nutrition, 1997

Apolipoprotein (apo) A-IV, a component of intestinally secreted, triacylglycerol-rich lipoproteins, has recently been proposed as a physiological controller of gastric function and food intake. Thus, it is important to understand the mechanisms involved in the control of expression, synthesis and secretion of apo A-IV. Apo A-IV is a member of a closely linked, multigene cluster which includes apolipoproteins A-I and C-III. Expression and synthesis of apo A-IV display marked variability with regard to species, tissue, stage of development and response to hormones, but intestinal apo A-IV is consistently stimulated by dietary lipid. The precise molecular mechanisms underlying the response of apo A-IV to lipid have not been clearly defined. Most evidence supports the hypothesis that some aspect of lipid transport is necessary for the apo A-IV response, but only part of this response may be due to a direct effect of intestinal lipid: recent findings suggest a connection between intestin...