Concomitant suppression of hyperlipidemia and intestinal polyp formation by increasing lipoprotein lipase activity in Apc-deficient mice

Epidemiologically, a high-fat diet is associated with the risk of colon cancer. In addition, serum levels of triglycerides (TGs) and cholesterol have been demonstrated to be positively associated with colon carcinogenesis. We recently found that an age-dependent hyperlipidemic state (high serum TG levels) exists in Apc-deficient mice, an animal model for human familial adenomatous polyposis. The mRNA levels of lipoprotein lipase (LPL), which catalyzes TG hydrolysis, were shown to be downregulated in the liver and intestines of mice. Moreover, treatment with a peroxisome proliferator-activated receptor (PPAR) alpha agonist, bezafibrate, or a PPARgamma agonist, pioglitazone, suppressed both hyperlipidemia and intestinal polyp formation in the mice, with induction of LPL mRNA. PPARalpha and PPARgamma agonists are reported to exert anti-proliferative and pro-apoptotic effects in cancer cells. One compound that also increases LPL expression levels but does not possess PPAR agnostic activity is NO-1886. When given at 400 or 800 ppm in the diet, it suppresses both hyperlipidemia and intestinal polyp formation in Apc-deficient mice, with elevation of LPL mRNA. In conclusion, a decrease in serum lipid levels by increasing LPL activity may contribute to a reduction in intestinal polyp formation with Apc deficiency. PPARalpha and PPARgamma agonists, as well as NO-1886, could be useful as chemopreventive agents for colon cancer.     

Decreased hepatic triglyceride accumulation and altered fatty acid uptake in mice with deletion of the liver fatty acid-binding protein gene

Liver fatty acid-binding protein (L-Fabp) is an abundant cytosolic lipid-binding protein with broad substrate specificity, expressed in mammalian enterocytes and hepatocytes. We have generated mice with a targeted deletion of the endogenous L-Fabp gene and have characterized their response to alterations in hepatic fatty acid flux following prolonged fasting. Chow-fed L-Fabp-/- mice were indistinguishable from wild-type littermates with regard to growth, serum and tissue lipid profiles, and fatty acid distribution within hepatic complex lipid species. In response to 48-h fasting, however, wild-type mice demonstrated a approximately 10-fold increase in hepatic triglyceride content while L-Fabp-/- mice demonstrated only a 2-fold increase. Hepatic VLDL secretion was decreased in L-Fabp-/- mice suggesting that the decreased accumulation of hepatic triglyceride was not the result of increased secretion. Fatty acid oxidation, as inferred from serum beta-hydroxybutyrate levels, was increased in response to fasting, although the increase in L-Fabp-/- mice was significantly reduced in comparison to wild-type controls, despite comparable induction of PPAR alpha target genes. Studies in primary hepatocytes revealed indistinguishable initial rates of oleate uptake, but longer intervals revealed reduced rates of uptake in fasted L-Fabp-/- mice. Oleate incorporation into cellular triglyceride and diacylglycerol was reduced in L-Fabp-/- mice although incorporation into phospholipid and cholesterol ester was no different than wild-type controls. These data point to an inducible defect in fatty acid utilization in fasted L-Fabp-/- mice that involves targeting of substrate for use in triglyceride metabolism.     

Appraisal of hepatic lipase and lipoprotein lipase activities in mice

A variety of methods are currently used to analyze HL and LPL activities in mice. In search of a simple methodology, we analyzed mouse preheparin and postheparin plasma LPL and HL activities using specific polyclonal antibodies raised in rabbit against rat HL (anti-HL) and in goat against rat LPL (anti-LPL). As an alternative, we analyzed HL activity in the presence of 1 M NaCl, a condition known to inhibit LPL activity in humans. The assays were validated using plasma samples from wild-type and HL-deficient C57BL/6 mice. We now show that the use of 1 M NaCl for the inhibition of plasma LPL activity in mice may generate incorrect measurements of both LPL and HL activities. Our data indicate that HL can be measured directly, without heparin injection, in preheparin plasma, because virtually all HL is present in an unbound form circulating in plasma. In contrast, measurable LPL activity is present only in postheparin plasma. Both HL and LPL can be measured using the same assay conditions (low salt and the presence of apolipoprotein C-II as an LPL activator). Total lipase activity in postheparin plasma minus preheparin HL activity reflects LPL activity. Specific antibodies are not required.     

Heparin-binding apoproteins. Effects on lipoprotein lipase and hepatic uptake of remnants.

Apoprotein-free heparin-binding and non-binding chylomicrons were used as substrates to test the effects on lipoprotein lipase activity of (a) chylomicron protein I; (b) the mixture of proteins I, II and apoprotein E and (c) human beta 2-glycoprotein I. No activation of the enzyme was observed with any of those apoproteins. When rats were injected simultaneously with [3H]cholesterol-labelled heparin-binding chylomicrons (containing proteins I and II) and [14C]cholesterol-labelled non-binding chylomicrons, no differences were detected between the rates of removal from circulation of those two types of particles. Clearance of chylomicrons from circulation was accompanied by the incorporation of 3H and 14C labels into the livers at similar rates. It is concluded that proteins I, II and apoprotein E have no effect on the degradation of chylomicrons by lipoprotein lipase and that the hepatic recognition of remnants does not appear to be affected by proteins I and II.     

CD36 mediates both cellular uptake of very long chain fatty acids and their intestinal absorption in mice

The intestine has an extraordinary capacity for fatty acid (FA) absorption. Numerous candidates for a protein-mediated mechanism of dietary FA absorption have been proposed, but firm evidence for this process has remained elusive. Here we show that the scavenger receptor CD36 is required both for the uptake of very long chain FAs (VLCFAs) in cultured cells and the absorption of dietary VLCFAs in mice. We found that the fraction of CD36-dependent saturated fatty acid association/absorption in these model systems is proportional to the FA chain length and specific for fatty acids and fatty alcohols containing very long saturated acyl chains. Moreover, intestinal VLCFA absorption is completely abolished in CD36-null mice fed a high fat diet, illustrating that the predominant mechanism for VLCFA absorption is CD36-dependent. Together, these findings represent the first direct evidence for protein-facilitated FA absorption in the intestine and identify a novel therapeutic target for the treatment of diseases characterized by elevated VLCFA levels.     

Role of reduced folate carrier in intestinal folate uptake

Studies from our laboratory and others have characterized different aspects of the intestinal folate uptake process and have shown that the reduced folate carrier (RFC) is expressed in the gut and plays a role in the uptake process. Little, however, is known about the actual contribution of the RFC system toward total folate uptake by the enterocytes. Addressing this issue in RFC knockout mice is not possible due to the embryonic lethality of the model. In this study, we describe the use of the new approach of lentivirus-mediated short hairpin RNA (shRNA) to selectively silence the endogenous RFC of the rat-derived intestinal epithelial cells (IEC-6), an established in vitro model for folate uptake, and examined the effect of such silencing on folate uptake. First we confirmed that the initial rate of [(3)H]folic acid uptake by IEC-6 cells was pH dependent with a markedly higher uptake at acidic compared with alkaline pH. We also showed that the addition of unlabeled folic acid to the incubation buffer leads to a severe inhibition ( approximately 95%) in [(3)H]folic acid (16 nM) uptake at buffer pH 5.5 but not at buffer pH 7.4. We then examined the effect of treating (for 72 h) IEC-6 cells with RFC-specific shRNA on the levels of RFC protein and mRNA and observed substantial reduction in the levels of both parameters ( approximately 80 and 78%, respectively). Such a treatment was also found to lead to a severe inhibition ( approximately 90%) in initial rate of folate uptake at buffer pH 5.5 (but not at pH 7.4); uptake of the unrelated vitamin, biotin, on the other hand, was not affected by such a treatment. These results demonstrate that the RFC system is the major (if not the only) folate uptake system that is functional in intestinal epithelial cells.     

Caveolin-1 and caveolae in atherosclerosis: differential roles in fatty streak formation and neointimal hyperplasia

PURPOSE OF REVIEW: Caveolae are 50-100 nm cell surface plasma membrane invaginations observed in terminally differentiated cells. They are characterized by the presence of the protein marker caveolin-1. Caveolae and caveolin-1 are present in almost every cell type that has been implicated in the development of an atheroma. These include endothelial cells, macrophages, and smooth muscle cells. Caveolae and caveolin-1 are involved in regulating several signal transduction pathways and processes that play an important role in atherosclerosis. RECENT FINDINGS: Several recent studies using genetically engineered mice (Cav-1 (-/-) null animals) have now clearly demonstrated a role for caveolin-1 and caveolae in the development of atherosclerosis. In fact, they suggest a rather complex one, either proatherogenic or antiatherogenic, depending on the cell type examined. For example, in endothelial cells, caveolin-1 and caveolae may play a proatherogenic role by promoting the transcytosis of LDL-cholesterol particles from the blood to the sub-endothelial space. In contrast, in smooth muscle cells, the ability of caveolin-1 to negatively regulate cell proliferation (neointimal hyperplasia) may have an antiatherogenic effect. SUMMARY: Caveolin-1 and caveolae play an important role in several steps involved in the initiation of an atheroma. Development of new drugs that regulate caveolin-1 expression may be important in the prevention or treatment of atherosclerotic vascular disease.     

Lack of the intestinal Muc1 mucin impairs cholesterol uptake and absorption but not fatty acid uptake in Muc1-/- mice

Before cholesterol and fatty acid molecules in the small intestinal lumen can interact with their possible transporters for uptake and absorption, they must pass through a diffusion barrier, which may modify the kinetics of nutrient assimilation. This barrier includes an unstirred water layer and a surface mucous coat, which is located at the intestinal lumen-membrane interface. In the present study, we investigated whether disruption of the mucin gene (Muc)1 may influence intestinal uptake and absorption of cholesterol and fatty acid in male Muc1(-/-) mice. The wild-type mice displayed relatively high levels of Muc1, Muc2, Muc3, and Muc4 mRNAs and relatively low levels of Muc5ac and Muc5b mRNAs in the small intestine. The absence of Muc1 mRNA and protein in the small intestines of Muc1(-/-) mice confirmed complete knockout of the Muc1 gene, but the mRNA expression for other mucin genes remained unchanged. Intestinal uptake and absorption of cholesterol but not palmitic acid were significantly reduced in Muc1(-/-) mice compared with the wild-type mice. However, knockout of the Muc1 gene did not impair either expression levels of the genes that encode intestinal sterol efflux transporters Abcg5 and Abcg8 and fatty acid transporter Fatp4 or small intestinal transit rates. We conclude that physiological levels of the epithelial mucin produced by the Muc1 gene are necessary for normal intestinal uptake and absorption of cholesterol in mice. Our study implies that because cholesterol absorption efficiency is reduced by approximately 50% in Muc1-deficient mice, there may be one or more additional pathways for cholesterol absorption.     

Role of an intramolecular contact on lipoprotein uptake by the LDL receptor

The LDL receptor (LDLR) is an endocytic receptor that plays a major role in the clearance of atherogenic lipoproteins from the circulation. During the endocytic process, the LDLR first binds lipoprotein at the cell surface and then traffics to endosomes, where the receptor releases bound lipoprotein. Release is acid-dependent and correlates with the formation of an intramolecular contact within the receptor. Human mutations at residues that form the contact are associated with familial hypercholesterolemia (FH) and the goal of the present study was to determine the role of contact residues on LDLR function. We show that mutations at nine contact residues reduce the ability of the LDLR to support lipoprotein uptake. Unexpectedly, only four of the mutations (W515A, W541A, H562Y and H586Y) impaired acid-dependent lipoprotein release. The remaining mutations decreased the lipoprotein-binding capacity of the LDLR through either reduction in the number of surface receptors (H190Y, K560W, H562Y and K582W) or reduction in the fraction of surface receptors that were competent to bind lipoprotein (W144A and W193A). We also examined three residues, distal to the contact, which were predicted to be necessary for the LDLR to adopt the acidic conformation. Of the three mutations we tested (G293S, F362A and G375S), one mutation (F362A) reduced lipoprotein uptake. Together, these data suggest that the intramolecular interface plays multiple roles in LDLR function.     

Function and regulation of hepatic and intestinal fatty acid binding proteins

Two structurally different fatty acid binding proteins (FABP) have been isolated from rat liver and small intestinal epithelium. hFABP is a 14 184 Da protein found in abundance in both liver and small intestine, whereas gFABP (15 063 Da) is abundantly present only in small intestine. This review discusses studies which have provided insight into the physiological functions of these proteins. These include analyses of endogenous and exogenous ligand binding to FABP in vitro; examination of the modulating effect of FABP preparations on enzyme activities in vitro; exploration of relationships between alterations in cytosolic FABP content in response to hormonal, pharmacological, and dietary manipulations and changes in the rates of cellular fatty acid uptake and utilization; and studies of hFABP turnover and the mechanisms of FABP regulation. These experiments provide compelling evidence for a broad role of the FABPs in the transport, utilization and cellular economy of free fatty acids in the liver and small intestine, and also in protecting several aspects of cellular function against the modulatory effects of fatty acids, fatty acyl-CoA esters, and other ligands. Studies of FABP regulation also suggest a role in long-term rather than short-term modulation of hepatic fatty acid metabolism and indicate that hFABP and gFABP may perform different functions in the small intestine.     

Effects of dietary cholesterol and fatty acids on plasma cholesterol level and hepatic lipoprotein metabolism

The effects of dietary cholesterol and fatty acids on the plasma cholesterol level and rates of very low density lipoprotein (VLDL) cholesterol secretion and low density lipoprotein (LDL) transport through LDL receptors in the liver of the hamster were investigated. Increases of plasma VLDL- and LDL-cholesterol levels and VLDL-cholesterol secretion from hepatocytes were observed in animals fed a diet enriched with 0.1% cholesterol for 2 weeks in comparison with animals fed a control diet. The addition of dietary palmitic acid accelerated the effect of dietary cholesterol on plasma VLDL- and LDL-cholesterol levels and VLDL-cholesterol secretion from hepatocytes. Dietary linoleic acid accelerated the effect of dietary cholesterol on VLDL-cholesterol secretion from hepatocytes and diminished the effect on the plasma LDL-cholesterol level. Hepatic LDL receptor activity was considerably suppressed by a control diet containing 0.05% cholesterol and a further small suppression was induced by a diet enriched with 0.1% cholesterol with or without 5% palmitic acid. However, dietary linoleic acid diminished the effect of dietary cholesterol on the suppression of hepatic LDL receptor activity. These results suggest that dietary palmitic acid augments the effect of dietary cholesterol in elevating the plasma LDL-cholesterol level through acceleration of VLDL-cholesterol secretion from the liver, and that dietary linoleic acid diminishes the effect of dietary cholesterol in elevating the plasma LDL-cholesterol level by preventing the suppression of hepatic LDL receptor activity induced by cholesterol.     

Primitive streak formation in mice is preceded by localized activation of Brachyury and Wnt3

The prevalent model for the generation of axial polarity in mouse embryos proposes that a radial to a linear transition in the expression of primitive streak markers precedes the formation of the primitive streak on one side of the epiblast. This model contrasts with the models of mesoderm formation in other vertebrates as it suggests that the primitive streak is initially established in a radial pattern rather than a localized region of the epiblast. Here, we examine the proposed correlation between the expression of Brachyury and Wnt3, two genes reported as expressed radially in the proximal epiblast, with the movements of proximal anterior epiblast cells at stages leading to the formation of the primitive streak. Our results reveal that neither Brachyury nor Wnt3 forms a ring of expression in the proximal epiblast as previously thought. In embryos dissected between 5.5 and 6.5 dpc, Brachyury is first expressed in the distal extra-embryonic ectoderm and subsequently on one side of the epiblast. Wnt3 expression is evident first in the posterior visceral endoderm of 5.5 dpc embryos and later in the posterior epiblast. Lineage analysis shows that the movements of the proximal epiblast do not restrict Brachyury expression to the posterior epiblast. Our data suggest a model whereby the localized expression of these genes in the posterior epiblast, and hence the formation of the primitive streak, is the result of local cell-cell interactions in the future posterior portion of the egg cylinder rather than regionalization of a radial pattern of expression in proximal epiblast cells.     

Modification of low density lipoprotein by lipoprotein lipase or hepatic lipase induces enhanced uptake and cholesterol accumulation in cells

Incubation of low density lipoprotein(s) (LDL) with either lipoprotein lipase or hepatic lipase led to modification of the core lipid composition of LDL. Both lipases modified LDL by substantially reducing core triglyceride content without producing marked differences in size, charge, or lipid peroxide content in comparison to native LDL. The triglyceride-depleted forms of LDL that result from treatment with these two enzymes were degraded at approximately twice the rate of native LDL by human monocyte-derived macrophages (HMDM). Lipase-modified LDL degradation was inhibited by chloroquine, suggesting lysosomal involvement in LDL cellular processing. The increased degradation by macrophages of the LDL modified by these lipases was accompanied by enhanced cholesterol esterification rates, as well as by an increase in cellular free and esterified cholesterol content. In a patient with hepatic triglyceride lipase deficiency, degradation of the triglyceride-rich LDL by HMDM was approximately half that of normal LDL. Following in vitro incubation of LDL from this patient with either lipoprotein or hepatic lipase, lipoprotein degradation increased to normal. Several lines of evidence indicate that LDL modified by both lipases were taken up by the LDL receptor and not by the scavenger receptor. 1) The degradation of lipase-modified LDL in nonphagocytic cells (human skin fibroblast and arterial smooth muscle cells) as well as in phagocytic cells (HMDM, J-774, HL-60, and U-937 cell lines) could be dissociated from that of acetylated LDL and was always higher than that of native LDL. A similar pattern was found for cellular cholesterol esterification and cholesterol mass. 2) LDL receptor-negative fibroblasts did not degrade lipase-modified LDL. 3) A monoclonal antibody to the LDL receptor inhibited macrophage degradation of the lipase-modified LDL. 4) Excess amounts of unlabeled LDL competed substantially with 125I-labeled lipase-modified LDL for degradation by both macrophages and fibroblasts. Thus, lipase-modified LDL can cause significant cholesterol accumulation in macrophages even though it is taken up by LDL and not by the scavenger receptor. This effect could possibly be related to the reduced triglyceride content in the core of LDL, which may alter presentation of the LDL receptor-binding domain of apolipoprotein B on the particle surface, thereby leading to increased recognition and cellular uptake via the LDL receptor pathway.     

Macrophage lipoprotein lipase promotes foam cell formation and atherosclerosis in low density lipoprotein receptor-deficient mice

The role of macrophage lipoprotein lipase (LPL) expression in atherosclerotic lesion formation was examined in low density lipoprotein receptor (LDLR(-/-)) mice using dietary conditions designed to induce either fatty streak lesions or complex atherosclerotic lesions. First, LDLR(-/-) mice chimeric for macrophage LPL expression were created by transplantation of lethally irradiated female LDLR(-/-) mice with LPL(-/-) (n = 12) or LPL(+/+) (n = 14) fetal liver cells as a source of hematopoietic cells. To induce fatty streak lesions, these mice were fed a Western diet for 8 weeks, resulting in severe hypercholesterolemia. There were no differences in plasma post-heparin LPL activity, serum lipid levels, or lipoprotein distribution between these two groups. The mean lesion area in the proximal aorta in LPL(-/-) --> LDLR(-/-) mice was significantly reduced by 33% compared with LPL(+/+) --> LDLR(-/-) mice, and a similar reduction (38%) in lesion area was found by en face analysis of the aortae. To induce complex atherosclerotic lesions, female LDLR(-/-) mice were lethally irradiated, transplanted with LPL(-/-) (n = 14), LPL(+/-) (n = 13), or LPL(+/+) (n = 14) fetal liver cells, and fed the Western diet for 19 weeks. Serum cholesterol and triglyceride levels did not differ between the three groups. After 19 weeks of diet, the lesions in the proximal aorta were complex with relatively few macrophages expressing LPL protein and mRNA in LPL(+/+) --> LDLR(-/-) mice. Analysis of cross-sections of the proximal aorta demonstrated no differences in the extent of lesion area between the groups, whereas en face analysis of the aortae revealed a dose-dependent effect of macrophage LPL on mean aortic lesion area in LPL(-/-) --> LDLR(-/-), LPL(-/+) --> LDLR(-/-), and LPL(+/+) --> LDLR(-/-) mice (1.8 +/- 0. 2%, 3.5 +/- 0.5% and 5.9 +/- 0.8%, respectively). Taken together, these data indicate that macrophage LPL expression in the artery wall promotes atherogenesis during foam cell lesion formation, but this impact may be limited to macrophage-rich lesions.     

Heart fatty acid uptake is decreased in heart fatty acid-binding protein gene-ablated mice

Cell culture systems have demonstrated a role for cytoplasmic fatty acid-binding proteins (FABP) in lipid metabolism, although a similar function in intact animals is unknown. We addressed this issue using heart fatty acid-binding protein (H-FABP) gene-ablated mice. H-FABP gene ablation reduced total heart fatty acid uptake 40 and 52% for [1-(14)C]16:0 and [1-(14)C]20:4n-6 compared with controls, respectively. Similarly, the amount of fatty acid found in the aqueous fraction was reduced 40 and 52% for [1-(14)C]16:0 and [1-(14)C]20:4n-6, respectively. Less [1-(14)C]16:0 entered the triacylglycerol pool, with significant redistribution of fatty acid between the triacylglycerol pool and the total phospholipid pool. Less [1-(14)C]20:4n-6 entered each lipid pool measured, but these changes did not alter the distribution of tracer among these pools. In gene-ablated mice, significantly more [1-(14)C]16:0 was targeted to choline and ethanolamine glycerophospholipids, whereas more [1-(14)C]20:4n-6 was targeted to the phosphatidylinositol (PtdIns) pool. H-FABP gene ablation significantly increased PtdIns mass 1.4-fold but reduced PtdIns 20:4n-6 mass 30%. Consistent with a reported effect of FABP on plasmalogen mass, ethanolamine plasmalogen mass was reduced 30% in gene-ablated mice. Further, 20:4n-6 mass was reduced in each of the three other major phospholipid classes, suggesting H-FABP has a role in maintaining steady-state 20:4n-6 mass in heart. In summary, H-FABP was important for heart fatty acid uptake and targeting of fatty acids to specific heart lipid pools as well as for maintenance of phospholipid pool mass and acyl chain composition.     

Hydrolysis of chylomicron polyenoic fatty acid esters with lipoprotein lipase and hepatic lipase

The lipolysis of rat chylomicron polyenoic fatty acid esters with bovine milk lipoprotein lipase and human hepatic lipase was examined in vitro. Chylomicrons obtained after feeding fish oil or soy bean oil emulsions were used as substrates. The lipolysis was followed by gas chromatography or by using chylomicrons containing radioactive fatty acids. Lipoprotein lipase hydrolyzed eicosapentaenoic (20:5) and arachidonic acid (20:4) esters at a slower rate than the C14-C18 acid esters. More 20:5 and 20:4 thus accumulated in remaining tri- and diacylglycerols. Eicosatrienoic, docosatrienoic and docosahexanoic acids exhibited an intermediate lipolysis pattern. When added together with lipoprotein lipase, hepatic lipase increased the rate of lipolysis of 20:5 and 20:4 esters of both tri- and diacylglycerols. Addition of NaCl (final concentration 1 M) during the course of lipolysis inhibited lipoprotein lipase as well as the enhancing effect of hepatic lipase on triacylglycerol lipolysis. Hepatic lipase however, hydrolyzed diacylglycerol that had already been formed. Chylomicron 20:4 and 20:5 esters thus exhibit a relative resistance to lipoprotein lipase. It is suggested that the tri- and diacylglycerol species containing these fatty acids may accumulate at the surface of the remnant particles and act as substrate for hepatic lipase during a concerted action of this enzyme and lipoprotein lipase.     

Determinants of intestinal mucosal uptake of short- and medium-chain fatty acids and alcohols

Uptake rates across the jejunal brush border have been measured for water-soluble fatty acids and alcohols and analyzed to determine the relative roles of the unstirred water layer and the lipid cell membrane as determinants of the intestinal absorptive process. Initial studies involving measurement of time courses of electrical transients developed across the intestine exposed to poorly permeant solute molecules showed no anomalous discrimination of probe molecules of different size or charge. This finding suggests that the diffusion barrier in the intestine can be considered as an unstirred water layer. Next, uptake rates of fatty acid were found to be linear with respect to concentration of the test solute, demonstrated no competitive inhibition or contralateral stimulation, had low temperature dependency, and were insensitive to metabolic inhibition, indicating that uptake proceeds by passive diffusion. Passive permeability coefficients, *P, varied from 22 +/- 1.4 to 395 +/- 9.2 nmoles.min(-1).100 mg(-1).mm(-1) for the saturated fatty acids 2:0 through 12:0 and from 119 +/- 3.3 to 581 +/- 45.2 for the saturated alcohols 6:0 through 10:0. Vigorous stirring of the bulk buffer solution enhanced *P values in direct proportion to chain length while the presence of bile acid micelles depressed apparent permeability coefficients in proportion to fatty acid chain length. These results demonstrate that uptake of short-chain fatty acid monomers is rate limited by the lipid cell membrane but diffusion through the unstirred water layer becomes increasingly rate limiting as the chain length increases. It is also possible to conclude from these data that diffusion through the unstirred water layer becomes totally rate limiting for uptake of long-chain fatty acid monomers of physiological importance.     

Inhibitory effect of monoacylglycerol on fatty acid uptake into rat intestinal epithelial cells

We investigated the influence of glycerol esters on oleic acid uptake into IEC-6 cells. Monoolein, especially 2-monoacylglycerol, significantly inhibited the cellular uptake. Although diolein slightly inhibited the oleic acid uptake, triolein, glycerol and monooctanoate had no effect. These results suggest that after lipid digestion in the intestine, long-chain fatty acid uptake may be influenced by another digestive product, 2-monoacylglycerol.