Mechanisms of linoleic acid uptake by rabbit small intestinal brush border membrane vesicles

Phospholipid Remodeling in Physiology and Disease

Phospholipids are major constituents of biological membranes. The fatty acyl chain composition of phospholipids determines the biophysical properties of membranes and thereby affects their impact on biological processes. The composition of fatty acyl chains is also actively regulated through a deacylation and reacylation pathway called Lands' cycle. Recent studies of mouse genetic models have demonstrated that lysophosphatidylcholine acyltransferases (LPCATs), which catalyze the incorporation of fatty acyl chains into the sn-2 site of phosphatidylcholine, play important roles in pathophysiology. Two LPCAT family members, LPCAT1 and LPCAT3, have been particularly well studied. LPCAT1 is crucial for proper lung function due to its role in pulmonary surfactant biosynthesis. LPCAT3 maintains systemic lipid homeostasis by regulating lipid absorption in intestine, lipoprotein secretion, and de novo lipogenesis in liver. Mounting evidence also suggests that changes in LPCAT activity may be potentially involved in pathological conditions, including nonalcoholic fatty liver disease, atherosclerosis, viral infections, and cancer. Pharmacological manipulation of LPCAT activity and membrane phospholipid composition may provide new therapeutic options for these conditions.

Intestinal Phospholipid Remodeling Is Required for Dietary-Lipid Uptake and Survival on a High-Fat Diet

Phospholipids are important determinants of membrane biophysical properties, but the impact of membrane acyl chain composition on dietary-lipid absorption is unknown. Here we demonstrate that the LXR-responsive phospholipid-remodeling enzyme Lpcat3 modulates intestinal fatty acid and cholesterol absorption and is required for survival on a high-fat diet. Mice lacking Lpcat3 in the intestine thrive on carbohydrate-based chow but lose body weight rapidly and become moribund on a triglyceride-rich diet. Lpcat3-dependent incorporation of polyunsaturated fatty acids into phospholipids is required for the efficient transport of dietary lipids into enterocytes. Furthermore, loss of Lpcat3 amplifies the production of gut hormones, including GLP-1 and oleoylethanolamide, in response to high-fat feeding, contributing to the paradoxical cessation of food intake in the setting of starvation. These results reveal that membrane phospholipid composition is a gating factor in passive lipid absorption and implicate LXR-Lpcat3 signaling in a gut-brain feedback loop that couples absorption to food intake.

Pathways of polyunsaturated fatty acid utilization: implications for brain function in neuropsychiatric health and disease

Essential polyunsaturated fatty acids (PUFAs) have profound effects on brain development and function. Abnormalities of PUFA status have been implicated in neuropsychiatric diseases such as major depression, bipolar disorder, schizophrenia, Alzheimer's disease, and attention deficit hyperactivity disorder. Pathophysiologic mechanisms could involve not only suboptimal PUFA intake, but also metabolic and genetic abnormalities, defective hepatic metabolism, and problems with diffusion and transport. This article provides an overview of physiologic factors regulating PUFA utilization, highlighting their relevance to neuropsychiatric disease.

Electrical stimulation of the isolated rat intestine in the presence of nutrient stimulus enhances glucagon-like peptide-1 release

The release of small intestinal hormones by constituents of ingested food, such as fatty acids, is integral to post-prandial responses that reduce food intake. Recent evidence suggests that small intestinal electrical stimulation reduces food intake, although the mechanism of action is debated. To test the hypothesis that intestinal stimulation directly alters hormone release locally we used isolated rat distal ileum and measured glucagon-like peptide-1 (GLP-1) released in the presence or absence of linoleic acid (LA) and electrical field stimulation (EFS). Intact segments were oriented longitudinally between bipolar stimulating electrodes in organ bath chambers containing modified Krebs-Ringers bicarbonate (KRB) buffer including protease inhibitors. Incubation in LA (3 mg ml(-1)) for 45 min increased GLP-1 concentration (21.9 +/- 2.6 pM versus KRB buffer alone 3.6 +/- 0.1 pM). Eleven electrical stimulation conditions were tested. In the presence of LA none of the stimulation conditions inhibited LA-evoked GLP-1 release, whereas two high frequency short pulse widths (14 V, 20 Hz, 5 ms and 14 V, 40 Hz, 5 ms) and one low frequency long pulse width (14 V, 0.4 Hz, 300 ms) EFS conditions enhanced LA-evoked GLP-1 release by >250%. These results are consistent with a local effect of intestinal electrical stimulation to enhance GLP-1 release in response to luminal nutrients in the intestines. Enhancing hormone release could improve the efficacy of intestinal electrical stimulation and provide a potential treatment for obesity and metabolic conditions.

Identification of the major intestinal fatty acid transport protein

While intestinal transport systems for metabolites such as carbohydrates have been well characterized, the molecular mechanisms of fatty acid (FA) transport across the apical plasmalemma of enterocytes have remained largely unclear. Here, we show that FATP4, a member of a large family of FA transport proteins (FATPs), is expressed at high levels on the apical side of mature enterocytes in the small intestine. Further, overexpression of FATP4 in 293 cells facilitates uptake of long chain FAs with the same specificity as enterocytes, while reduction of FATP4 expression in primary enterocytes by antisense oligonucleotides inhibits FA uptake by 50%. This suggests that FATP4 is the principal fatty acid transporter in enterocytes and may constitute a novel target for antiobesity therapy.

Molecular mechanisms for the hepatic uptake of magnetic resonance imaging contrast agents

The mechanisms were investigated for the hepatic transport of 4 different gadolinium complexes used as contrast agents for magnetic resonance imaging (MRI). In basolateral rat hepatocyte plasma membrane vesicles, Gd-DTPA uptake was indistinguishable from non-specific binding to vesicles; Gd-BOPTA and Gd-EOB-DTPA entered plasma membrane vesicles following a linear, concentration-dependent mechanism up to 1.5 mM of substrate. By contrast, Gd-B 20790 uptake followed a saturative kinetic with an apparent Km of 92 +/- 15 microM and a Vmax of 143 +/- 42 pmol/mg prot/15 sec, and it occurred into an osmotic-sensitive space. Sulfobromophthalein ant taurocholate, but not unconjugated bilirubin inhibited the uptake rate of Gd-B 20790 but not that of the other three compounds. Injection into Xenopus laevis oocytes of 5 ng of human OATP cRNA resulted, after 3 days, in a >/=2-fold stimulation (p < 0.001) of transport of Gd-B 20790 but not of Gd-BOPTA or Gd-EOB-DTPA. Collectively, these data indicate that the hepatic uptake of the MRI contrast agent Gd-B 20790 is a carrier-mediated mechanism operated by OATP while MRI compounds with other chemical structures enter the hepatocyte by other mechanisms.

Cholestyramine alters the lipid and energy metabolism of chicks fed dietary medium- or long-chain triacylglycerol

The effects of cholestyramine, a bile acid binding polymer, on the lipid and energy metabolism of chicks given dietary medium-chain triacylglycerol (MCT) or long-chain triacylglycerol (LCT) were investigated. Chicks (from 8 to 17 days of age) were fed diets containing MCT or LCT at 200 g oil/kg diet with or without 2% cholestyramine under equalized feeding conditions. An adjusted LCT diet was formulated in order to supply another group with daily nutrients and dietary metabolizable energy (ME) equal to MCT groups, except for corn starch. ME intakes of chicks given MCT or LCT diets were reduced by cholestyramine; consequently, fat and energy retention was reduced, though the reduction was more drastic in chicks fed LCT. This was caused by a change in amounts of the fecal excretion of fat and bile acids. Cholestyramine enhanced the excretion of octanoic acid (8:0) in the feces, which suggests that bile acids are needed for 8:0 absorption. Cholestyramine affects the utilization of dietary MCT and LCT by lowering fat and energy retention in chicks. However, the effect of cholestyramine on MCT utilization was smaller than its effect on utilization of LCT.

Linoleic acid uptake by isolated enterocytes: influence of alpha-linolenic acid on absorption

In a previous study we showed that intestinal uptake of alpha-linolenic acid (18:3n-3) was carrier-mediated and we suggested that a plasma membrane fatty acid protein was involved in the transport of long-chain fatty acids. To further test this hypothesis, the mechanism of linoleic acid (18:2n-6) uptake by isolated intestinal cells was examined using a rapid filtration method and 20 mM sodium taurocholate as solubilizing agent. Under these experimental conditions transport of [1-14C]linoleic acid monomers in the concentration range of 2 to 2220 nM was saturable with a Vm of 5.1 +/- 0.6 nmol/mg protein/min and a Km of 183 +/- 7 nM. Experiments carried out in the presence of metabolic inhibitors, such as 2,4-dinitrophenol and antimycin A, suggested that an active, carrier-mediated mechanism was involved in the intestinal uptake of this essential fatty acid. The addition of excess unlabeled linoleic acid to the incubation medium led to a 89% decrease in the uptake of [1-14C]linoleic acid, while D-glucose did not compete for transport into the cell. Other long-chain polyunsaturated fatty acids added to the incubation mixture inhibited linoleic acid uptake by more than 80%. The presence of alpha-linolenic acid (18:3n-3) in the incubation medium caused the competitive inhibition (Ki = 353 nM) of linoleic acid uptake. The data are compatible with the hypothesis that intestinal uptake of both linoleic, and alpha-linolenic acid is mediated by a membrane carrier common to long-chain fatty acids.

In vivo effect of calcitriol on calcium transport and calcium binding proteins in the spontaneously hypertensive rat

The abnormal intestinal Ca2+ transport reported in spontaneously hypertensive rats (SHR) has been attributed to decreased responsiveness to calcitriol. We reexamined this hypothesis by studying the calcitriol regulation of SHR duodenal calbindin-D9K and calmodulin and the relation of calcitriol to Ca2+ uptake by isolated enterocytes. SHR and normotensive Wistar-Kyoto (WKY) rats were injected with either 50 ng/d calcitriol (vit-D) or vehicle alone (control) for 3 days. Decreased calbindin-D9K (P < .001) and cellular Ca2+ flux (P < .001) were observed in control SHR. Calcitriol increased total cell and brush border calbindin-D9K (P < .0001); this variation paralleled plasma calcitriol levels in both strains. In contrast, Ca2+ flux, which increased in vit-D animals, remained lower in SHR for plasma calcitriol levels similar to those in WKY rats. Immunoreactive calmodulin was similar in both strains whether assayed in total cell or brush border membranes. In contrast, when measured by ligand blotting (45Ca), calmodulin was lower in SHR than in WKY rats (P < .01), suggesting the existence of a calmodulin pool with reduced Ca2+ binding capacity in the hypertensive strain. Calcitriol had no effect on calmodulin in either strain. In conclusion, Ca2+ binding protein regulation by calcitriol is normal in the SHR, and decreased hormone responsiveness cannot account for the defective duodenal calcium transport of this experimental model of hypertension.

Interactions between medium-chain and long-chain triacylglycerols in lipid and energy metabolism in growing chicks

The combined effects of dietary medium-chain triacylglycerols (MCT) and long-chain triacylglycerols (LCT) on lipid and energy utilization in chicks were investigated. Corn oil was used as the LCT source, and trioctanoin (8:0) was used as the MCT source. The efficiency of dietary energy utilization (such as metabolizable energy values and fat and energy retention) decreased linearly as the level of MCT increased, but the efficiency of dietary protein utilization (protein retained per protein consumed) was not affected in a consistent manner. Fecal saturated and monounsaturated fatty acid composition was dependent on the dietary fatty acid composition, whereas fecal linoleic acid content was not. It is concluded that dietary MCT and LCT influence each other to some degree, with respect to protein and lipid metabolism in chicks. Moreover, in most cases, the nutritional characteristics of each triacylglycerol, including food efficiency and fat and energy retention, are independent of each other in growing chicks.

Gradient for D-glucose and linoleic acid uptake along the crypt-villus axis of rabbit jejunal brush border membrane vesicles

Glucose uptake into jejunal brush border membrane (BBM) varies along the crypt-villus axis (CVA). In the present study, the question was addressed whether uptake of the essential long-chain fatty acid linoleic acid also varies along the CVA. Using agitation techniques, five jejunal enterocyte fractions were sequentially isolated from female New Zealand white rabbits. A sixth and final fraction of lower-villus/crypt cells was obtained by the scraping of the remaining jejunal mucosa. Cell fraction along the CVA was proven histologically, by noting decreasing alkaline phosphatase activities in sequentially isolated fractions, and by demonstrating [3H-methyl]thymidine uptake mainly in the final fraction of the lower villus/crypt cells. BBM vesicles were prepared from the upper, mid- and lower-villus/crypt enterocyte fractions, using differential centrifugation and divalent ion precipitation. D-Glucose uptake into each fraction showed an Na(+)-gradient dependent time-course "overshoot" with linear uptake to 15 s and a subsequent decline to a steady-state plateau. Varying D-glucose concentrations from 50-1000 microM demonstrated saturation kinetics of uptake, with maximal transport rates (Vmax) and Michaelis affinity constants (Km) varying between fractions; the Km and Vmax were both lowest in the upper-villus fraction. A linear relationship existed between linoleic acid concentration (25-200 microM) and uptake in each fraction. Linoleic acid uptake was equivalent in all fractions when expressed per mg protein, but when expressed in terms of the estimated minimal BBM, vesicle surface area uptake was greater in the upper- than in the lower-villus/crypt fractions. Thus, BBM vesicle uptake of both linoleic acid and glucose vary along the crypt-villus axis of the rabbit jejunum.

Different mechanisms of uptake of stearic acid and cholesterol into rabbit jejunal brush border membrane vesicles

The rate of uptake of stearic acid and cholesterol solubilized in taurocholic acid (TC) was examined in rabbit jejunal brush border membrane vesicles (BBMV). For stearic acid (18:0) or cholesterol there was an initial rapid rate of uptake, which reached a plateau within approximately 1 min and remained stable thereafter. At low concentrations of 18:0 and 20 mM, but not 2 mM, TC, there was a curvilinear relationship between the concentration of 18:0 and uptake, whereas the relationship between cholesterol uptake and concentration was linear over a wide range of values. When the concentration of TC was held constant at increasing concentrations of 18:0 or cholesterol, there was a linear increase in the rate of uptake. When the concentration of 18:0 or cholesterol was held constant and the concentration of TC was increased from 2 to 20 mM, the uptake of 18:0 declined, but the rate of uptake of cholesterol increased. When the concentrations of 18:0 plus TC, or cholesterol plus TC, were both increased in unison and their ratio was held constant, their rate of uptake increased. Thus, (i) BBMV may be used to assess the rate of uptake of lipids; (ii) the partitioning of cholesterol from bile acid micelles into the BBMV appears to be by way of "collision" of the cholesterol with the membrane. In contrast, the uptake of 18:0 from the micelle into the membrane vesicles may be by both the collision and the aqueous/dissociation models; and (iii) 18:0 uptake may be mediated by both a concentration-dependent and a concentration-independent component.(ABSTRACT TRUNCATED AT 250 WORDS)