Metabolism of low density lipoproteins in rainbow trout

Scavenger endothelial cells of fish, a review

The definition of scavenger endothelial cells (SEC) is exclusively based on functional and structural characteristics. The following characteristics are common hallmarks for the vertebrate SEC: (a) All vertebrates examined are furnished with a population of special SEC that plays a role in the catabolism of physiologic and non-physiologic soluble waste macromolecules. (b) From the ligands that are endocytosed, SEC in all seven vertebrate classes appear to express the collagen α-chain receptor and the scavenger receptors. In addition, the hyaluronan and the mannose receptors are present on SEC of mammalia (several species) and osteichthyes (e.g., salmon and cod). It is likely that all four receptor types are present in all vertebrate classes. (c) Like liver endothelial cells (LEC) in mammals, SEC in all vertebrate classes are geared to endocytosis of soluble macromolecules, but phagocytic uptake of particles is taken care of mainly by macrophages. (d) The most primitive vertebrates (hagfish, lamprey and ray) carry their SEC in gill vessels, whereas phylogenetically younger fishes (salmon, carp, cod and plaice) carry their SEC in either kidney or heart and in all terrestrial vertebrates-SEC are found exclusively in the liver. (e) SEC of all vertebrates are localized in blood sinusoids or trabeculae that carry large amounts of slowly flowing and O₂ poor blood. (f) SEC differs functionally and structurally from what is normally associated with "conventional vascular endothelium."

Exploratory Factor Analysis of Rainbow Trout Serum Chemistry Variables

Clinical chemistry offers a valuable, affordable, moderately invasive, and nondisruptive way to assess animal physiological status and wellness within defined ranges and is widely used as a diagnostic clinical tool. Because of physiological differences between mammals, clinical correlates of blood chemistry variables are not known in detail in fish, in which tissue/organ function tests are inferred from mammal-derived clinical chemistry data. The aim of the present study was to apply exploratory factor analysis on a serum chemistry dataset from clinically healthy, reared rainbow trout Oncorhynchusmykiss (Walbaum, 1792) to select the most correlated variables and to test for possible underlying factors explaining the observed correlations as possible physiological status estimates in trout. The obtained factors were tested for correlation with hepatosomatic and splenosomatic indexes. Thirteen highly correlated variables were selected out of 18 original serum chemistry variables, and three underlying factors (Factors 1, 2, and 3) were identified that explained the observed correlations among variables. Moreover, Factor 1 correlated negatively with the hepatosomatic index and Factors 2 and 3 negatively with the splenosomatic index. The obtained factors were tentatively associated with: protein (liver) metabolism (Factor 1), cell turnover (Factor 2), and lipid (muscle) metabolism (Factor 3).

Tocopherols in Seafood and Aquaculture Products

Fish products contain various nutritionally beneficial components, namely, ω3-polyunsaturated fatty acids (ω3-PUFA), minerals, and vitamins. Particularly, tocopherols (α-, β-, γ-, and δ-tocopherol) can be provided by seafood and aquaculture products. Hence, this review shows the various aspects of tocopherols in seafood and aquaculture products. For tocopherol determination in these products, HPLC methods coupled with diode array detection in the UV area of the spectrum or fluorescence detection have been shown as sensitive and accurate. These newest methods have helped in understanding tocopherols fate upon ingestion by seafood organisms. Tocopherols pass through the intestinal mucosa mainly by the same passive diffusion mechanism as fats. After absorption, the transport mechanism is thought to consist of two loops. The first loop is dietary, including chylomicrons and fatty acids bound to carrier protein, transporting lipids mainly to the liver. The other is the transport from the liver to tissues and storage sites. Moreover, tocopherol levels in fish organisms correlate with diet levels, being adjusted in fish body depending on diet concentration. For farmed fish species, insufficient levels of tocopherols in the diet can lead to poor growth performance or to nutritional disease. The tocopherol quantity needed as a feed supplement depends on various factors, such as the vitamer mixture, the lipid level and source, the method of diet preparation, and the feed storage conditions. Other ingredients in diet may be of great importance, it has been proposed that α-tocopherol may behave as a prooxidant synergist at higher concentrations when prooxidants such as transition metals are present. However, the antioxidant action of tocopherols outweighs this prooxidant effect, provided that adequate conditions are used. In fact, muscle-based foods containing higher levels of tocopherol show, for instance, higher lipid stability. Besides, tocopherols are important not only from the nutritional point of view but also from the physiological one, since they are involved in many metabolic processes in the human organism. Moreover, synergistic interactions with selenium and ascorbic acid have been reported. It deserves attention that there is evidence tocopherols taken with food can prevent heart disease, while no such evidence was found for α-tocopherol as supplement. From this perspective, eating fish is advisable, since, for instance, a 100 g serving of salmon may provide nearly 14% of recommended dietary allowance.

Identification of a scavenger receptor homologue on nonspecific cytotoxic cells and evidence for binding to oligodeoxyguanosine

In mammals scavenger receptors (SR) are expressed by monocytic-macrophage lineage cells and B-cells. Studies of various teleost species have indirectly demonstrated the presence of SR receptors on phagocytic or endothelial cells by showing the uptake of SR ligands (i.e. derivatised (acetylated) lipoproteins) by these cells. In the present study, nonspecific cytotoxic cells (NCC) were examined for membrane expression of an SR-like protein. Approximately 15-25% of purified NCC expressed scavenger receptor class A (SR-A) demonstrated by binding by a monoclonal (2F8) specific for mouse SR-A (types I, II). Flow cytometric analysis determined that SR binding cells had the same size and 'side scatter' characteristics as NCC. Two colour flow analysis of NCC demonstrated that only a subset of NCC expressed the SR-A-like protein and non-NCC were SR-A negative. Membrane expression of SR on NCC was confirmed by fluorescence microscopy. Analysis of the tissue distribution of SR bearing cells demonstrated that in both catfish and tilapia, SR-A was expressed by NCC in the peripheral blood, spleen and anterior kidney. Experiments were also done to determine if the ligands known to bind mammalian SR-A had a similar specificity for the teleost receptor. Cold competition binding experiments determined that anti-SR-A antibody competed with and reduced biotinylated polyguanosine 20-mer binding to NCC by approximately 40%. Two other types of ligands known to bind (mammalian) SR-A (i.e. polyvinyl sulphate and dextran sulphate) likewise decreased anti-SR-A antibody binding to NCC by 40%. These studies for the first time demonstrated that NCC express the teleost orthologue of mammalian SR-A, suggesting that NCC may participate in physiologic regulation of lipid metabolism in addition to functions of innate immunity.

Cardiac endothelial-myocardial signaling: its role in cardiac growth, contractile performance, and rhythmicity

Experimental work during the past 15 years has demonstrated that endothelial cells in the heart play an obligatory role in regulating and maintaining cardiac function, in particular, at the endocardium and in the myocardial capillaries where endothelial cells directly interact with adjacent cardiomyocytes. The emerging field of targeted gene manipulation has led to the contention that cardiac endothelial-cardiomyocytal interaction is a prerequisite for normal cardiac development and growth. Some of the molecular mechanisms and cellular signals governing this interaction, such as neuregulin, vascular endothelial growth factor, and angiopoietin, continue to maintain phenotype and survival of cardiomyocytes in the adult heart. Cardiac endothelial cells, like vascular endothelial cells, also express and release a variety of auto- and paracrine agents, such as nitric oxide, endothelin, prostaglandin I(2), and angiotensin II, which directly influence cardiac metabolism, growth, contractile performance, and rhythmicity of the adult heart. The synthesis, secretion, and, most importantly, the activities of these endothelium-derived substances in the heart are closely linked, interrelated, and interactive. It may therefore be simplistic to try and define their properties independently from one another. Moreover, in relation specifically to the endocardial endothelium, an active transendothelial physicochemical gradient for various ions, or blood-heart barrier, has been demonstrated. Linkage of this blood-heart barrier to the various other endothelium-mediated signaling pathways or to the putative vascular endothelium-derived hyperpolarizing factors remains to be determined. At the early stages of cardiac failure, all major cardiovascular risk factors may cause cardiac endothelial activation as an adaptive response often followed by cardiac endothelial dysfunction. Because of the interdependency of all endothelial signaling pathways, activation or disturbance of any will necessarily affect the others leading to a disturbance of their normal balance, leading to further progression of cardiac failure.

Metabolism of oxidized and chemically modified low density lipoproteins in rainbow trout--clearance via scavenger receptors

Oxidative modifications of low density lipoprotein (LDL) convert LDL into a ligand recognized by a variety of scavenger receptors (SR) in mammals. This oxidized LDL (oxLDL) activate several cell types, and have been shown to induce expression of a variety of genes in mammals. Lipoproteins of poikilothermic animals like salmonid fishes contain high levels of polyunsaturated fatty acids susceptible to oxidative modifications. We have investigated, and found trout LDL to be susceptible to oxidation in the presence of Cu(2+). When oxidized or acetylated trout LDL was injected intravenously, the clearance rate was increased compared to that of native LDL. Modified LDL was taken up almost exclusively in the kidney, whereas native LDL was also taken up in the liver. Uptake of both oxLDL and acetylated LDL in the kidney was significantly inhibited by lipoteichoic acid (LTA) and formaldehyde treated BSA (fBSA), both of which are known ligands of SR.

Mannose-receptor-mediated clearance of lysosomal alpha-mannosidase in scavenger endothelium of cod endocardium

Mannose-receptor-mediated clearance of circulating glycoproteins was studied in Atlantic cod (Gadus morhua). Distribution studies with radioiodinated and fluorescently labelled ligands showed that cod liver lysosomal alpha-mannosidase and yeast invertase were rapidly eliminated from blood via a mannose specific pathway in liver parenchymal cells and endocardial endothelial cells of atrium and ventricle. Asialo-orosomucoid, a galactose-terminated glycoprotein, was cleared by liver only. In vitro studies were performed with primary cultures of atrial-endocardial endothelial cells (AEC), incubated at 12 degrees C in a serum free medium. Cod AEC endocytosed mannose-terminated glycoproteins (125I-alpha-mannosidase, 125I-invertase, 125I-mannan, 125I-ovalbumin and unlabelled lysosomal alpha-mannosidase), whereas 125I-asialo-orosomucoid was not recognised. Uptake of radiolabelled mannose-terminated ligands was inhibited 80-100% in the presence of excess amounts of mannan, invertase, D-mannose, L-fucose or EGTA. Our results suggest that the cod endocardial endothelial cells express a specific Ca(2+)-dependent mannose receptor, analogous to the mannose receptor on mammalian macrophages and liver sinusoidal endothelial cells.

Transport of alpha-tocopherol in Atlantic salmon (Salmo salar) during vitellogenesis

The transport of α-tocopherol was studied during vitellogenesis in Atlantic salmon that were fed diets with two levels of α-tocopherol. α-Tocopherol levels were measured in the flesh, liver, ovary and serum, and in the serum the α-tocopherol levels in the very low density lipoprotein (VLDL), low density lipoprotein (LDL), high density lipoprotein (HDL) and very high density lipoprotein (VHDL or vitellogenin) were also measured.Atlantic salmon store α-tocopherol mainly in their flesh because the muscle mass comprises 50% or more of live weight. During vitellogenesis the α-tocopherol content declined to about 10% of the level prior to maturation. The relative range of level of α-tocopherol in the lipoproteins was: HDL> LDL> VLDL> VHDL, irrespective of dietary levels of α-tocopherol.From the recent knowledge on lipid transport during vitellogenesis and the present data, we hypothesize that α-tocopherol is transported from peripheral tissues to liver by HDL and further transported from liver to ovary by LDL. Vitellogenin appears to play a minor role in the transportation of vitamin E to the ovary.

Circannual variation in the fatty acid composition of high-density lipoprotein phospholipids during acclimatization in trout

A circannual variation in the fatty acid composition of plasma and high-density lipoprotein (HDL) phospholipids occurs in rainbow trout (Oncorhynchus mykiss) in response to seasonal alterations in environmental water temperature. The compensatory mechanisms employed in cold adaptation include a decrease in the level of saturated fatty acids and of monoenes of the oleic acid (n-9) family and an increase in the level of unsaturated fatty acids of the linolenic acid (n-3) family, especially in docosahexaenoic acid (22:6(n-3)). The present study demonstrates that in trout, a poikilothermic vertebrate, the weight percentage of 22:6(n-3) in HDL phospholipids is inversely correlated (r = -0.88, P < 0.0001) with water temperature.

Hepatic uptake and intracellular processing of LDL in rainbow trout

The process of receptor-mediated endocytosis in poikilothermic vertebrates such as salmonid fish have not been subjected to much research, compared to the detailed studies done in mammalian systems. We have investigated the hepatic uptake and intracellular processing of low-density lipoprotein (LDL) in rainbow trout liver. After intravenous injection of the [125I]tyramine-cellobiose ([125I]TC) -labelled lipoprotein, the liver was perfused and cells isolated or fractionated by differential centrifugation and isopycnic centrifugation in Nycodenz gradients. We found that LDL was mainly endocytosed by parenchymal cells of the liver. Cell fractionation experiments showed that LDL was localized sequentially in three groups of organelles of increasing density. Initially, LDL was localized in small, slowly sedimenting endosomes before being transferred to denser endosomes (prelysosomes) and finally to dense lysosomes. The lysosomes were identified by three lysosomal marker enzymes. Degradation products formed from [125I]TC-labelled LDL could also be detected in prelysosomal vesicles. In vitro experiments with cultured trout hepatocytes demonstrated that intracellular processing of [125I]TC-LDL in these cells could be suppressed by endocytic and lysosomal inhibitors. The catabolism of LDL in rainbow trout therefore follows the endocytic-lysosomal pathway described for many macromolecules in mammalian cells.

Interaction of low density lipoproteins with liver cells in rainbow trout

Liver is the main catabolic tissue for low density lipoprotein in rainbow trout (Gjøen and Berg 1992). We have investigated the interaction of LDL with isolated trout liver cells and liver membranes. (125)I-TC labelled trout LDL bound to isolated trout liver cells in a time dependent and saturable manner with an apparant Kd of 20.1 μg/ml, suggesting the existence of a specific binding site on the surface of these cells. The binding was Ca(2+) dependent assessed by the 50% reduction obtained by 5 mM EDTA. Saturable binding to isolated trout liver membranes could also be demonstrated, but with lower affinity as compared to intact cells. Degradation of (125)I-TC-LDL in hepatocytes was also saturable as degradation could be inhibited about 60% by a 100 fold surplus of unlabelled LDL. The rate of degradation increased with temperature up to 20°C. Both cell association (binding + uptake) and degradation were reduced down to 20% of control in the presence of microtubular and lysosomal inhibitors. Hepatic catabolism of trout LDL therefore seems to depend on receptormediated endocytosis, followed by lysosomal degradation.