The characterization of lipoprotein lipase isolated from the post-heparin plasma of the rainbow trout, Salmo gairdneri Richardson

The evolution of plasma cholesterol: direct utility or a "spandrel" of hepatic lipid metabolism?

Fats provide a concentrated source of energy for multicellular organisms. The efficient transport of fats through aqueous biological environments raises issues concerning effective delivery to target tissues. Furthermore, the utilization of fatty acids presents a high risk of cytotoxicity. Improving the efficiency of fat transport while simultaneously minimizing the cytotoxic risk confers distinct selective advantages. In humans, most of the plasma cholesterol is associated with low-density lipoprotein (LDL), a metabolic by-product of very-low-density lipoprotein (VLDL), which originates in the liver. However, the functions of VLDL are not clear. This paper reviews the evidence that LDL arose as a by-product during the natural selection of VLDL. The latter, in turn, evolved as a means of improving the efficiency of diet-derived fatty acid storage and utilization, as well as neutralizing the potential cytotoxicity of fatty acids while conserving their advantages as a concentrated energy source. The evolutionary biology of lipid transport processes has provided a fascinating insight into how and why these VLDL functions emerged during animal evolution. As causes of historical origin must be separated from current utilities, our spandrel-LDL theory proposes that LDL is a spandrel of VLDL selection, which appeared non-adaptively and may later have become crucial for vertebrate fitness.

Endurance swimming activates trout lipoprotein lipase: plasma lipids as a fuel for muscle

Fish endurance swimming is primarily powered by lipids supplied to red muscle by the circulation, but the mechanism of delivery remains unknown. By analogy to mammals, previous studies have focused on non-esterified fatty acids (NEFA bound to albumin), but lipoproteins have not been considered as an energy shuttle to working muscles. The effects of exercise on fish lipoprotein lipase (LPL) have never been investigated. We hypothesized that LPL and circulating lipoproteins would be modified by prolonged swimming. Because LPL is naturally bound to the endothelium, we have used heparin to release the enzyme in the circulation and to characterize reserve capacity for lipoprotein catabolism. The effects of exercise (4 days at 1.5 body lengths s–1 in a swim tunnel) were measured for red muscle LPL,post-heparin plasma LPL, and lipoprotein concentration/composition. Red muscle LPL activity increased from 18±5 (rest) to 49± 9 nmol fatty acids min–1 g–1 (swimming). In resting fish,heparin administration caused a 27-fold increase in plasma LPL activity that reached a maximum of 1.32± 0.67 μmol fatty acids min–1 ml–1 plasma. This heparin-induced response of plasma LPL was not different between resting controls and exercised fish. Heparin or prolonged swimming had no effect on the concentration/composition of lipoproteins that contain 92% of the energy in total plasma lipids. We conclude that (1) red muscle LPL is strongly activated by endurance swimming, (2) rainbow trout have a high reserve capacity for hydrolyzing lipoproteins, and (3) future studies should aim to measure lipoprotein flux because their concentration does not reflect changes in flux. These novel characteristics of fish LPL imply that lipoproteins are used as a metabolic shuttle between fat reserves and working muscles, a strategy exploiting an abundant source of energy in rainbow trout.

Regulation of lipoprotein lipase activity in rainbow trout (Oncorhynchus mykiss) tissues

Lipoprotein lipase (LPL) is considered as a key enzyme in the lipid deposition and metabolism of many tissues. Information on LPL activity and its regulation in fish remains very scarce. In the present study, we have examined the nutritional regulation of LPL activity by conducting post-feeding and fasting experiments in rainbow trout (Oncorhynchus mykiss). As insulin plays an important role in the nutritional regulation of LPL activity in mammals, the effects of this hormone were tested in vivo by intraperitoneal administration. Moreover, we conducted in vitro studies using fat pads of rainbow trout to better clarify the direct role of insulin and tumor necrosis factor-alpha (TNFalpha) as possible regulators of LPL activity in rainbow trout. LPL activity in adipose tissue increased in response to feeding, 4h after ingestion of food, then decreasing to basal levels at 6h. No clear response was found in either red or white muscles, where LPL values were lower. Moreover, fasting produced a down-regulation of LPL activity in adipose tissue, concomitant with low levels of plasma insulin. While insulin administration stimulated LPL activity of adipose tissue 3h after injection, no response was observed in red or white muscles. Finally, in vitro studies using fat pads revealed that insulin significantly stimulated the proportion of LPL in active conformation at the extracellular level. On the other hand, TNFalpha did not greatly affect LPL activity using this in vitro model. These data indicate that LPL activity is regulated in a tissue-specific manner following food intake, and suggest that insulin is an important regulator of LPL activity in the adipose tissue of rainbow trout.

Isolation of low density lipoprotein subfraction containing apolipoprotein B-like protein from Japanese eel Anguilla japonica plasma using dextran sulfate cellulose

Japanese eel (Anguilla japonica) possessed a unique lipoprotein profile in their plasma, reflecting high utilization of lipids. Low density lipoprotein (LDL) fraction isolated at the densities from 1.006 to 1.085 g/ml comprised the heterogeneous components with molecular weight (Mr) 1200 K, 470 K, and 250 K. LDL subfraction with Mr 1200 K was completely adsorbed to dextran sulfate cellulose (DSC) column which had been developed for LDL apheresis treatment of the patients with familial hypercholesterolemia, while LDL subfractions with Mr 470 K and 250 K had no affinity for the DSC column. LDL subfractions with Mr 470 K and 250 K were floated and settled, respectively, by centrifuging the unbound fraction of DSC column at a density of 1.063 g/ml. LDL subfraction with Mr 1200 K possessed apolipoprotein (apo) B-like protein of Mr 230 K, while apo A-I- and A-II-like proteins of Mr 25 K and 14 K were the main components in LDL subfractions with Mr 470 K and 250 K. The presence of apo B-like protein seemed to be responsible for the adsorption of LDL subfraction with Mr 1200 K for the DSC column. LDL subfractions with Mr 470 K and 250 K seemed to belong to high density lipoprotein (HDL) with respect to molecular weights and apolipoprotein features. To our knowledge, this is the first report of the separation of LDL and HDL from the plasma of Japanese eels using the DSC column.

A unique lipoprotein profile found in the plasma of cultured Japanese eelAnguilla japonica: very low density lipoprotein, but not high density lipoprotein, is the main component of plasma

A unique lipoprotein profile was found in the plasma of cultured Japanese eel,Anguilla japonica which accumulated more lipid in its muscle than in its liver. The plasma lipoprotein level of Japanese eels was in excess of 54 mg ml(1), a concentration considered to be hyperlipoproteinemic in relation to other teleosts. The plasma lipoproteins consisted of very low density lipoprotein (VLDL, density (d)<1.006 g ml(1)) low density lipoprotein (LDL, 1.006<d<1.085 g ml(1)), high density lipoprotein 2 (HDL2, 1.085<d<1.100 g ml(1)), and HDL3 (1.100<d<1.210 g ml(1)). VLDL, but not HDL, was the main component in the plasma of Japanese eels, unlike most teleosts where HDL is the main component. An additional lipoprotein, vitellogenin (1.210<d<1.280 g ml(1)), was induced by the injection of estradiol-17ß (E2), but VLDL was the main plasma component even in the E2-treated eels. VLDL was a triacylglycerol (TG)-rich lipoprotein and possessed two apolipoprotein (apo) B-like proteins of molecular weights (Mr) 260K and 230K as main components.LDL, HDL2, and HDL3 were revealed to consist of heterogeneous components by a density gradient ultracentrifugation. LDL was separated into three subclasses of LDL1 (1.030<d<1.038 g ml(1)), LDL2 (1.043<d<1.063 g ml(1)), and LDL3 (1.067<d<1.094 g ml(1)). LDL1 with apo B-like protein of Mr 230K was a TG-rich lipoprotein, while both LDL2 and LDL3 were cholesterol ester-rich lipoproteins with apo A-I-and A-II-like proteins of Mr 25K and 14K. The particle sizes of HDL2 and HDL3 subclasses differed, although all of HDL2 and HDL3 subclasses possessed apo A-I-and A-II-like proteins of Mr 25K and 14K as main components. To our knowledge, this is the first report of detailed plasma lipoprotein profile in Japanese eels.

Human lipoprotein lipase last exon is not translated, in contrast to lower vertebrates

We have sequenced the first fish (zebrafish, Brachydanio rerio) lipoprotein lipase (LPL) cDNA clone. Similarities were found in mammalian LPL cDNA, but the codon spanning the last two exons (which is thus split by the last intron) is AGA (Arg) as opposed to TGA in mammals. Exon 10 is thus partially translated. These results were confirmed with rainbow trout (Oncorhynchus mykiss). We also investigated whether mammal TGA coded for selenocystein (SeCys), the 21st amino acid, but found that this was not the case: TGA does not encode SeCys but is a stop codon. It thus appears that the sense codon AGA (fish) has been transformed into a stop codon TGA (human) during the course of evolution. It remains to be determined if the "loss" of the C-terminal end of mammalian LPL protein has conferred an advantage in terms of LPL activity or, on the contrary, a disadvantage (e.g., susceptibility to diabetes or atherosclerosis).

Identification of four ovarian receptor proteins that bind vitellogenin but not other homologous plasma lipoproteins in the rainbow trout, Oncorhynchus mykiss

Membrane proteins from ovarian follicles, testis and somatic tissues of rainbow trout, Oncorhynchus mykiss, were extracted by ultracentrifugation, separated on sodium dodecyl sulphate gels and isolated on polyvinyl difluoride membranes. Vitellogenin receptor proteins were visualized using protein staining and hybridisation with 125I-vitellogenin. Four follicle-membrane proteins, with molecular masses of 220, 210, 110 and 100 kDa, showed a strong affinity for vitellogenin and were specific to the ovary. Other homologous lipoproteins (very low density lipoprotein, low density lipoprotein and high density lipoprotein) had a very limited ability to displace 125I-vitellogenin from its receptor, indicating that the ovarian receptor proteins were fairly specific for vitellogenin. Proteins with an affinity for very low density lipoprotein and low density lipoprotein were visualised in liver, spleen and muscle, eluting on sodium dodecyl sulphate gels with molecular masses of about 150 kDa. Peptides generated from trypsin digests of the receptor proteins with a high affinity for vitellogenin showed sequence homology with receptors in the lipoprotein family, including a sequence that is believed to act as the internalisation signal [Phe-Asp-Phe-Tyr-] and a sequence identity with the recently characterised chicken vitellogenin/very low density lipoprotein receptor [Ser-Glu-Leu-Tyr-Glu-Pro-Ala-]. Together, the ligand blotting and peptide sequence data support the contention that the four ovarian membrane proteins isolated are receptor proteins specific for vitellogenin and they do not bind other plasma lipoproteins to any significant degree.

Metabolism of low density lipoproteins in rainbow trout

Plasma kinetics and tissue sites of degradation for native and chemically modified low density lipoproteins have been investigated in (Oncorhynchus mykiss). Native and modified LDL labelled with(125)I-tyramine-cellobiose, (a residualizing adduct), were injected intravenously, and plasma and organ samples analyzed. Native LDL were cleared with a half life of about 30 hours, and mainly catabolized in the liver. Acetylation of LDL resulted in accelerated clearance (t1/2=2 h) and catabolism in the kidneys. Methylation of LDL had only minor effects on catabolism. The cellular localization of lipoprotein uptake was visualized in kidney by fluorescence microscopy. Native LDL were endocytozed by spheroid, parenchymal cells, supposedly steroid-producing cells. Acetylated, fluorescent LDL were found in vacuoles of flattened, sinusoidal lining endothelial cells. Our data show that catabolism of native low density lipoproteins in salmonids takes place mainly via hepatic receptors. A scavenger receptor pathway, for modified lipoproteins (mainly localized in the kidney) is also operative in trout.

Structural features of lipoprotein lipase. Lipase family relationships, binding interactions, non-equivalence of lipase cofactors, vitellogenin similarities and functional subdivision of lipoprotein lipase

A structural homology between lipoprotein lipase, pancreatic lipase and hepatic lipase is known and indicates that all three lipases are members of a common protein family. Lipoprotein lipase and pancreatic lipase utilize small protein co-factors, apolipoprotein C-II and co-lipase, respectively, but comparisons reveal no homology between the co-factor molecules. Hence, they do not show the same relationship as their target enzymes. Neither do screenings detect any extensive similarities between lipoprotein lipase, serine hydrolases, or apolipoproteins. Scannings against data bank proteins show that a 105-residue segment of lipoprotein lipases exhibits a 35-40% residue identity with a sub-region of Drosophila vitellogenins. One fifth of the conserved amino acid residues (8 of 40) are glycine, a pattern which is typical of distantly related forms of protein families. This supports a true relationship between large segments of Drosophila vitellogenins and lipases. Physiological and functional aspects of the vitellogenin/lipoprotein lipase similarities are given. The region concerned is entirely within the N-terminal domain of lipoprotein lipase and constitutes the segment where the similarity to hepatic and pancreatic lipases is most pronounced. Within this lipase region a 10-residue putative lipid-binding site exists for which further similarities have been found to the otherwise not closely related lingual/gastric lipases, prokaryotic lipases and lecithin-cholesterol acyltransferase. Another segment in lipoprotein lipase, where the heparin-binding site has been mapped, exhibits a correlation between strength of heparin binding and extent of basic residues among members of the lipase family. It further exhibits weak similarities with the 'Zn-finger' DNA-binding segment of steroid hormone receptors and may indicate convergence in a binding interaction. Thus, a functional subdivision of lipoprotein lipase into different segments can be distinguished.

Lipid dynamics in fish: aspects of absorption, transportation, deposition and mobilization

1. Aspects of lipid metabolism, including absorption and depositional processes, appear quite different in fish as compared to homeothermic vertebrates. 2. Dietary lipids in fish are absorbed as fatty acids and as triacylglycerols aggregated into chylomicra particles. 3. Interorgan transport of lipids, like that of mammals, consists of an exogenous (dietary) loop and an endogenous loop. 4. Fish store lipids among several depot organs, including mesenteric membranes, liver and muscle. 5. Several fast-acting and slow-acting agents modulate depot lipid mobilization. 6. Mobilized lipids may be transported in the serum as free fatty acids bound to specific carrier proteins.

Plasma lipoprotein and apolipoprotein distribution as a function of density in the rainbow trout (Salmo gairdneri)

I have previously described [Babin (1987) J. Biol. Chem. 262, 4290-4296] the apolipoprotein composition of the major classes of trout plasma lipoproteins. The present work describes the use of an isopycnic density gradient centrifugation procedure and sequential flotation ultracentrifugation to show: (1) the presence of intermediate density lipoproteins (IDL) in the plasma, between 1.015 and 1.040 g/ml; (2) the existence of a single type of Mr 240,000 apoB-like in the low density lipoproteins (LDL, 1.040 less than p less than 1.085 g/ml); (3) the presence of apoA-I-like (Mr 25,000) in the densest LDL; (4) the adequacy of 1.085 g/ml as a cutoff between the LDL and high density lipoproteins (HDL); (5) the accumulation of Mr 55,000 and 76,000 apolipoproteins and apoA-like apolipoproteins in the 1.21 g/ml infranatant. The fractionation of trout lipoprotein spectrum thus furnishes the distribution of the different lipoprotein classes and leads to the description of the constituent apolipoproteins, which account for about 36% of circulating plasma proteins in this species.

Changes in plasma lipoproteins and tissue lipoprotein lipase and salt-resistant lipase activities during spawning in the rainbow trout (Salmo gairdnerii R.)

1. Lipoprotein lipase and salt-resistant lipase activities increased in the ovaries but decreased in the adipose tissue of female trout in the months leading up to spawning. 2. The activity of the plasma cholesterol esterifying enzyme increased significantly immediately prior to spawning. 3. Plasma lipoprotein concentrations decreased during the approach to spawning. 4. These studies suggest that the developing ovaries in the trout receive their nutrients by lipolysis of plasma lipoproteins as well as by vitellogenin uptake; differentiation of the roles of the lipid stores in different tissues is proposed.