[Show abstract][Hide abstract] ABSTRACT: To determine whether n-3 fatty acids (n-3) influence arterial cholesterol delivery and lipoprotein lipase (LpL) levels in insulin-resistant mice.
Insulin resistance contributes to risk of cardiovascular disease. It was previously reported that saturated fat (SAT) diets increased, but n-3 diets decreased, arterial low-density lipoprotein (LDL) cholesterol deposition from LDL total and selective uptake; this was associated with increased or decreased arterial LpL, respectively. Insulin receptor transgenic knockout mice (L1) were fed a chow, SAT, or n-3 diet for 12 weeks. Double-fluorescent boron dipyrromethene (BODIPY)-cholesteryl ester (CE) and Alexa dye-labeled human LDL were injected to separately trace LDL-CE and LDL-apolipoprotein B whole particle uptake. In contrast to SAT, n-3 diets markedly reduced all plasma lipids, ameliorating progression of insulin resistance. As opposed to SAT, n-3 reduced arterial LDL uptake, CE deposition, and selective uptake. Disparate patterns of CE deposition between diets were comparable with arterial LpL distribution; SAT induced high LpL levels throughout aortic media; LpL was limited only to intima in n-3-fed mice.
n-3 diets diminish arterial LDL-cholesterol deposition in mice with insulin resistance, and this is associated with changes in arterial LpL levels and distribution.
[Show abstract][Hide abstract] ABSTRACT: Omega-3 (n-3) fatty acids are emerging as bioactive agents protective against cardiovascular disease. However, their cellular delivery pathways are poorly defined. Here we questioned whether the uptake of n-3 triglyceride-rich particles (TGRP) is mediated by cell surface proteoglycans (PG) using LDL receptor (LDLR)+/+ and LDLR-/- cell models. LDLR+/+ but not LDLR-/- cells showed higher n-6 over n-3 TGRP uptake. Removal of cell surface proteins and receptors by pronase markedly enhanced the uptake of n-3 but not n-6 TGRP. Lactoferrin blockage of apoE-mediated pathways decreased the uptake of n-6 TGRP by up to 85% (p<0.05) but had insignificant effect on n-3 TGRP uptake. PG removal by sodium chlorate in LDLR+/+ cells substantially reduced n-3 TGRP uptake but had little effect on n-6 TGRP uptake. Thus, while n-6 TGRP uptake is preferentially mediated by LDLR-dependent pathways, the uptake of n-3 TGRP depends more on PG and non-LDLR cell surface anchoring.
Biochemical and Biophysical Research Communications 02/2010; 392(2):135-9. DOI:10.1016/j.bbrc.2009.12.164 · 2.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Glucose ingestion stimulates the secretion of the incretin hormones, glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1). Despite the critical role of incretins in glucose homeostasis, the mechanism of glucose-induced incretin secretion has not been established. We investigated the underlying mechanism of glucose-induced incretin secretion in vivo in mice. Injection of glucose at 1 g/kg in the upper intestine significantly increased plasma GIP and GLP-1 levels, whereas injection of glucose in the colon did not increase GIP or GLP-1 levels. This finding indicates that the glucose sensor for glucose-induced incretin secretion is in the upper intestine. Coadministration of a sodium-glucose cotransporter-1 (SGLT1) inhibitor, phloridzin, with glucose in the upper intestine blocked glucose absorption and glucose-induced incretin secretion. alpha-methyl-d-glucopyranoside (MDG), an SGLT1 substrate that is a nonmetabolizable sugar, significantly increased plasma GIP and GLP-1 levels, whereas phloridzin blocked these increases, indicating that concomitant transport of sodium ions and glucose (substrate) via SGLT1 itself triggers incretin secretion without the need for subsequent glucose metabolism. Interestingly, oral administration of MDG significantly increased plasma GIP, GLP-1, and insulin levels and reduced blood glucose levels during an intraperitoneal glucose tolerance test. Furthermore, chronic MDG treatment in drinking water (3%) for 13 days reduced blood glucose levels after a 2-h fast and in an oral glucose tolerance test in diabetic db/db mice. Our findings indicate that SGLT1 serves as the intestinal glucose sensor for glucose-induced incretin secretion and that a noncalorigenic SGLT1 substrate ameliorates hyperglycemia by stimulating incretin secretion.
[Show abstract][Hide abstract] ABSTRACT: We previously reported that saturated fat (SAT)-enriched diets increase arterial cholesteryl ester (CE) deposition, especially from LDL-selective uptake (SU), and this was associated with increased arterial lipoprotein lipase (LpL). We now question how n-3 fatty acid rich diets influence arterial cholesterol delivery and arterial LpL levels.
C57BL/6 mice were fed chow or eucaloric high-fat diets enriched in SAT or fish oil (n-3) for 12 weeks, and then injected with double radiolabeled or fluorescent-labeled human LDL to separately trace LDL-CE and LDL-apoB uptake. SAT and n-3 diets increased plasma cholesterol levels similarly; n-3 diets lowered plasma triglyceride concentrations. SAT increased arterial LDL-SU with significantly higher CE infiltration into aortic media. In contrast, n-3 markedly reduced total LDL uptake and CE deposition and abolished SU with LDL localized only in aortic intima. Disparate patterns of CE deposition between diets were consistent with distribution of arterial LpL-SAT diets induced higher LpL levels throughout the aorta; n-3 diets decreased LpL levels and limited LpL expression to the aortic intima.
n-3 rich diets decrease arterial total LDL delivery and abrogate LDL-SU in parallel with changing arterial wall LpL expression and distribution.
[Show abstract][Hide abstract] ABSTRACT: Because the mechanisms of (n-3) fatty acid-enriched triglyceride-rich particle [(n-3)-TGRP] uptake are not well characterized, we questioned whether (n-3)-TGRP are removed via "nonclassical" pathways, e.g., pathways other than an LDL receptor and/or involving apolipoprotein E (apoE). Chylomicron-sized model (n-3)-TGRP labeled with [3H]cholesteryl ether were injected into wild-type (WT) and CD36 knockout (CD36-/-) mice at low, nonsaturating and high, saturating doses. Blood clearance of (n-3)-TGRP was determined by calculating fractional catabolic rates. At saturating doses, blood clearance of (n-3)-TGRP was slower in CD36-/- mice relative to WT mice, suggesting that in part CD36 contributes to (n-3)-TGRP uptake. To further examine the potential nonclassical clearance pathways, peritoneal-elicited macrophages from WT and CD36-/- mice were incubated with (n-3)-TGRP in the presence of apoE, lactoferrin, and/or sodium chlorate. Cellular (n-3)-TGRP uptake was measured to test the roles of apoE-mediated pathways and/or proteoglycans. ApoE-mediated pathways compensated in part for defective (n-3)-TGRP uptake in CD36-/- cells. Lactoferrin decreased (n-3)-TGRP uptake in the presence of apoE. Inhibition of cell proteoglycan synthesis by chlorate reduced (n-3)-TGRP uptake in both groups of macrophages, and chlorate effects were independent of apoE. We conclude that although CD36 is involved, it is not the primary contributor to the blood clearance of (n-3)-TGRP. The removal of (n-3)-TGRP likely relies more on nonclassical pathways, such as proteoglycan-mediated pathways.
Journal of Nutrition 03/2008; 138(2):257-61. · 4.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sterol regulatory element binding proteins (SREBPs) are key transcription proteins that bind to sterol regulatory elements (SRE) of genes essential for cellular cholesterol and fatty acid homeostasis. Polyunsaturated fatty acids (PUFA) strongly inhibit SREBP processing at post-transcriptional levels. We questioned if delivering PUFA as part of a triglyceride (TG) molecule would have similar effects and efficiency as free non-esterified PUFA. CHO cells stably transfected with an SRE-promoter linked to the luciferase reporter gene were incubated for 8-24 h with linoleic acid (LA) complexed to BSA (molar ratios 0.5-4:1), VLDL-sized trilinolein emulsions (TL, 25-200 microg/ml), and chylomicron-sized soy oil emulsions in the presence and absence of apoE. Effects of LA and TL on decreasing SRE-luciferase activity were similar and dose and time dependent. Both TL and LA significantly and rapidly (<or=2-12 h) reduced SRE-mediated gene expression by up to 75%. At equal fatty acid concentrations, SRE inhibition by TL was as effective as LA. ApoE addition increased inhibition by TL. Inhibition of gene expression was highly correlated to cell TG accumulation. We conclude that TG like fatty acids are rapid and efficient modulators of SRE-mediated gene expression.
[Show abstract][Hide abstract] ABSTRACT: Regulation of cholesterol metabolism in cultured cells and in the liver is dependent on actions of the LDL receptor. However, nonhepatic tissues have multiple pathways of cholesterol uptake. One possible pathway is mediated by LPL, an enzyme that primarily hydrolyzes plasma triglyceride into fatty acids. In this study, LDL uptake and tissue cholesterol levels in heart and skeletal muscle of wild-type and transgenic mice with alterations in LPL expression were assessed. Overexpression of a myocyte-anchored form of LPL in heart muscle led to increased uptake of LDL and greater heart cholesterol levels. Loss of LDL receptors did not alter LDL uptake into heart or skeletal muscle. To induce LDL receptors, mice were treated with simvastatin. Statin treatment increased LDL receptor expression and LDL uptake by liver and skeletal muscle but not heart muscle. Plasma creatinine phosphokinase as well as muscle mitochondria, cholesterol, and lipid droplet levels were increased in statin-treated mice overexpressing LPL in skeletal muscle. Thus, pathways affecting cholesterol balance in heart and skeletal muscle differ.
The Journal of Lipid Research 04/2007; 48(3):646-55. DOI:10.1194/jlr.M600301-JLR200 · 4.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Lipid emulsions containing long-chain triglycerides (LCT) and medium chain triglycerides (MCT) are widely used in parenteral nutrition. Recently, fish oil (FO) triglyceride (TG)-derived emulsions are considered therapeutic because of their many beneficial biological modulatory actions. We investigated in mice whether adding 10% FO to an intravenous lipid emulsion with MCT and LCT (MCT:LCT:FO -50:40:10% by wt) would affect particle blood clearance and tissue targeting in comparison to LCT (100% by wt) and MCT:LCT (50:50% by wt) emulsions. The 3 emulsions were labeled with [3H] cholesteryl oleoyl ether and administered by bolus injection (400 microg TG/mouse) to C57BL/6J mice. Contributions of LDL receptor (LDL-R) and LDL-R-related protein to emulsion catabolism were assessed using LDL-R-deficient mice and preinjection of lactoferrin, and the effects of lipoprotein lipase (LPL) were determined by preinjection of heparin and Triton WR 1339. Although fractional catabolic rates did not differ among the 3 emulsions, blood removal at each time point after injection was greater for MCT:LCT:FO particles due to their higher initial margination volume. Compared with MCT:LCT and LCT emulsions, patterns of tissue uptake of the MCT:LCT:FO emulsions were different, e.g. MCT:LCT:FO emulsion particle uptake was lower in heart, adipose tissue, and muscle, and higher in lung, and the removal of MCT:LCT:FO emulsion particles was less dependent on LPL, LDL-R, and lactoferrin-sensitive pathways. These data suggest that the addition of a low percentage of FO to MCT:LCT emulsions substantially changes their particle clearance and tissue uptake mechanisms.
Journal of Nutrition 12/2006; 136(11):2766-72. · 4.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Accumulating evidence in both humans and animal models clearly indicates that a group of very-long-chain polyunsaturated fatty acids, the n-3 fatty acids (or omega-3), have distinct and important bioactive properties compared with other groups of fatty acids. n-3 Fatty acids are known to reduce many risk factors associated with several diseases, such as cardiovascular diseases, diabetes, and cancer. The mechanisms whereby n-3 fatty acids affect gene expression are complex and involve multiple processes. As examples, n-3 fatty acids regulate 2 groups of transcription factors, such as sterol-regulatory-element binding proteins and peroxisome proliferator-activated receptors, that are critical for modulating the expression of genes controlling both systemic and tissue-specific lipid homeostasis. Modulation of specific genes by n-3 fatty acids and cross-talk between these genes are responsible for many effects of n-3 fatty acids.
American Journal of Clinical Nutrition 07/2006; 83(6 Suppl):1520S-1525S. · 6.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Plasma LDL levels and atherosclerosis both increase on a saturated fat-rich (SAT) diet. LDL cholesterol delivery to tissue may occur via uptake of the LDL particles or via selective uptake (SU), wherein cholesteryl ester (CE) enters cells without concomitant whole-particle uptake. It is not known how dietary fats might directly affect arterial LDL-CE uptake and whether SU is involved. Thus, mice that are relatively atherosclerosis resistant (C57BL/6) or susceptible to atherosclerosis (apoE) were fed a chow or SAT diet and injected with double radiolabeled or fluorescent-labeled human LDL to independently trace LDL-CE core and whole-particle uptake, respectively. Our results show that a SAT diet increased contributions of SU to total arterial LDL-CE delivery in C57BL/6 and apoE mice. The SAT diet increased plasma fatty acid and cholesterol levels; cholesterol, but not fatty acid, levels correlated with SU, as did the degree of atherosclerosis. Increased SU did not correlate with arterial scavenger receptor class B type I levels but paralleled increased lipoprotein lipase (LPL) levels and LPL distribution in the arterial wall. These studies suggest that arterial LDL-CE delivery via SU can be an important mechanism in vivo and that dietary influences on arterial LPL levels and atherogenesis modulate arterial LDL-CE delivery, cholesterol deposition, and SU.
[Show abstract][Hide abstract] ABSTRACT: This review discusses recent advances in delineating basic mechanisms underlying the beneficial effects of omega-3 fatty acids on health and on disease.
While a substantial number of studies have delineated many differences between the biological effects of saturated versus polyunsaturated fatty acids, less is known about the long-chain omega-3 fatty acids commonly present in certain fish oils. In this review, we focus on recent studies relating to basic mechanisms whereby omega-3 fatty acids modulate cellular pathways to exert beneficial effects on promoting health and decreasing risks of certain diseases. We will use, as examples, conditions of the cardiovascular, neurological, and immunological systems as well as diabetes and cancer, and then discuss basic regulatory pathways.
Omega-3 fatty acids are major regulators of multiple molecular pathways, altering many areas of cellular and organ function, metabolism and gene expression. Generally, these regulatory events lead to "positive" endpoints relating to health and disease.
Current Opinion in Lipidology 03/2005; 16(1):11-8. · 5.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sterol-regulatory element-binding proteins (SREBPs) regulate transcription of genes of lipid metabolism. Ceramide decreases transcriptionally active SREBP levels independently of intracellular cholesterol levels. Mechanisms of the ceramide-mediated decrease of SREBP levels were investigated.
Experiments were performed in Chinese hamster ovary cells. Inhibition of ceramide synthesis with myriocin, cycloserine, or fumonisin decreases levels of transcriptionally active SREBP and reduces SRE-mediated gene transcription. When ceramide synthesis is increased through exogenous sphingosine or inhibition of sphingosine kinase, SRE-mediated gene transcription is increased. The important role of ceramide synthesis in SRE-mediated gene transcription is confirmed in LY-B cells that do not synthesize ceramide de novo. LY-B cells fail to increase SRE-mediated gene transcription in sterol depletion.
Ceramide synthesis correlates with the generation of transcriptionally active SREBP and SRE-mediated gene transcription. Inhibition of ceramide synthesis decreases levels of transcriptionally active SREBP and SRE-mediated gene transcription. It is hypothesized that the process of ongoing ceramide synthesis contributes to the physiological processing of SREBP, perhaps affecting ER-to-Golgi trafficking. Taken together, modification of ceramide synthesis could be a novel target for drug development in the pharmacologic modification of SRE-dependent pathways.
[Show abstract][Hide abstract] ABSTRACT: Lipid accumulation is associated with cardiac dysfunction in diabetes and obesity. Transgenic mice expressing non-transferable lipoprotein lipase (LpL) with a glycosylated phosphatidyl-inositol (GPI) anchor in cardiomyocytes have dilated cardiomyopathy. However, the mechanisms responsible for lipid accumulation and cardiomyopathy are not clear. Hearts from 3-month-old mice expressing GPI-anchored human LpL (hLpLGPI) mice had increased fatty acid oxidation and heart failure genes and decreased glucose transporter genes. 6-month-old mice had increased mRNA expression and activation of the apoptosis marker caspase-3. Moreover, hLpLGPI hearts had significant cytochrome c release from mitochondria to cytosol. Low density lipoprotein uptake was greater in hLpLGPI hearts, and this was associated with more intracellular apolipoprotein B (apoB). To test whether lipid accumulation in the hLpLGPI heart is reduced by cardiac expression of apoB, hLpLGPI mice were bred with transgenic human apoB (HuB)-expressing mice. Hearts of HuB/hLpLGPI mice had less triglyceride (38%) and free fatty acids (19%), secreted more apoB, and expressed less atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) and more glucose transporter 4 (GLUT4). The increased mortality of the mice was abrogated by the transgenic expression of apoB. Therefore, we hypothesize that cardiac apoB expression improves cardiomyopathy by increasing lipid resecretion from the heart.
[Show abstract][Hide abstract] ABSTRACT: Lipoprotein lipase is the principal enzyme that hydrolyzes circulating triglycerides and liberates free fatty acids that can be used as energy by cardiac muscle. Although lipoprotein lipase is expressed by and is found on the surface of cardiomyocytes, its transfer to the luminal surface of endothelial cells is thought to be required for lipoprotein lipase actions. To study whether nontransferable lipoprotein lipase has physiological actions, we placed an alpha-myosin heavy-chain promoter upstream of a human lipoprotein lipase minigene construct with a glycosylphosphatidylinositol anchoring sequence on the carboxyl terminal region. Hearts of transgenic mice expressed the altered lipoprotein lipase, and the protein localized to the surface of cardiomyocytes. Hearts, but not postheparin plasma, of these mice contained human lipoprotein lipase activity. More lipid accumulated in hearts expressing the transgene; the myocytes were enlarged and exhibited abnormal architecture. Hearts of transgenic mice were dilated, and left ventricular systolic function was impaired. Thus, lipoprotein lipase expressed on the surface of cardiomyocytes can increase lipid uptake and produce cardiomyopathy.
[Show abstract][Hide abstract] ABSTRACT: Particle size of IV lipid emulsions affects the catabolism of long-chain triglyceride (LCT) emulsions, but little is known about its effect on the catabolism of medium-chain triglyceride (MCT)- and fish oil (FO)-containing emulsions.
Large (VLDL size), intermediate, and small (IDL size) emulsions with different triglyceride (TG) compositions were labeled with [3H]cholesteryl oleoyl ether: LCT (triolein 100%), MCT:LCT (trioctanoin:triolein 50%:50%), MCT:LCT:FO (trioctanoin:triolein:triDHA 50%:40%:10%), and FO (triDHA 100%). Emulsions (0.4 mg TG/mouse) were injected into C57BL/6J mice, and blood clearance and tissue uptake of emulsion particles were determined.
Large emulsion particles had 2- to 3-fold faster fractional catabolic rates (FCR) compared with small particles with the same TG content. There was 1.5- to 2.0-fold higher FCR of large FO-containing emulsions (FO and MCT:LCT:FO) compared with large LCT and MCT:LCT emulsions, whereas effects of FO on FCR in small emulsions were not observed. Large FO-containing emulsions were taken up more by adipose tissue compared with small particles with concomitant decreases in hepatic uptake. Preinjection of heparin reduced heart and adipose uptakes of FO and MCT:LCT:FO emulsions with increased uptake by liver, suggesting a role of lipoprotein lipase in catabolism of FO-containing emulsions.
In a mouse model, FO addition to large emulsions increased blood clearance and changed organ delivery. In contrast, there was no or little effect when particle size became smaller. We hypothesize that in humans, FO addition to lipid emulsions can help target emulsion delivery to certain extrahepatic tissues, a factor that may be of use for delivering specific fatty acids, or even drugs, to specific organs.
Journal of Parenteral and Enteral Nutrition 01/2003; 27(1):58-64. DOI:10.1177/014860710302700158 · 3.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We previously reported that unsaturated fatty acids stimulated low-density lipoprotein (LDL) particle uptake in J774 macrophages by increasing LDL receptor activity. Since free fatty acids (FFA) also change plasma membrane properties, a putative cholesteryl ester (CE) acceptor for selective uptake (SU), we questioned the ability of FFA to modulate SU from LDL. Using [(3)H]cholesteryl ether/(125)I-LDL to trace CE core and whole particle uptake, we found that oleic acid and eicosapentaenoic acid, but not saturated stearic acid, increased SU by 30% over control levels. An ACAT inhibitor, Dup128, abolished FFA effects on SU, indicating that increased SU by FFA was secondary to changes in cell-free cholesterol (FC). Consistent with these observations, ACAT inhibition increased cell FC and reduced LDL SU by half. The important role of plasma membrane composition was further demonstrated in that beta-cyclodextrin- (beta-CD-) mediated FC removal from the plasma membrane increased SU from LDL and was further stimulated by U18666A, a compound that inhibits FC transport between lysosomes and the plasma membrane. In contrast, cholesterol-saturated beta-CD markedly reduced LDL SU. In contrast to LDL SU, oleic acid, ACAT inhibition, U18666A, or beta-CD had no effects on HDL SU. Moreover, HDL SU was inhibited by antimouse SR-BI antibody by more than 50% but had little effect on LDL SU. In C57BL/6 mice fed a high fat diet, plasma FFA levels increased, and SU accounted for an almost 4-fold increased proportion of total cholesterol delivery to the arterial wall. Taken together, these data suggest that LDL SU is mediated by pathways independent of SR-BI and is influenced by plasma membrane FC content. Moreover, in conditions where elevated plasma FFA occur, SU from LDL can be an important mechanism for cholesterol delivery in vivo.