Enzyme Lipoprotein Lipase Research Articles

Inherited hyperchylomicronaemia in the cat

ABSTRACTCats with inherited hyperchylomicronaemia show fasting hyperlipidaemia, lipaemia retinalis, xanthomata in a variety of tissues (skin, liver, kidney, etc), peripheral neuropathies and, in some kittens, anaemia. The peripheral nerve lesions result from compression by xanthomata. The fasting hyperlipidaemia is characterised by hypertriglyceridaemia with elevation of chylomicrons and often mild very low density lipoprotein elevation. Lipoprotein lipase (LPL) activity measured after heparin injection, is absent or reduced. In one family, a high concentration of enzyme mass is present and the cats produce an abnormal LPL enzyme protein which fails to bind to the vascular endothelium. The defect is inherited as an autosomal recessive trait. Molecular analyses have not identified any major structural rearrangement in the LPL gene, but it has been suggested that a point mutation is present in the heparin binding domain of the protein (of one affected family).

Glycoprotein 330, a member of the low density lipoprotein receptor family, binds lipoprotein lipase in vitro

Glycoprotein 330 (gp330), a cell-surface protein that is localized in clathrin-coated pits, is structurally related to both the low density lipoprotein receptor (LDLR) and the LDLR-related protein/alpha 2-macroglobulin receptor (LRP). We recently demonstrated that gp330 and LRP may be functionally related as well; both bind the 39-kDa polypeptide referred to as receptor-associated protein (Kounnas, M. Z., Argraves, W. S., and Strickland, D. K. (1992) J. Biol. Chem. 267, 21162-21166). In this report, we tested several other LRP ligands for their ability to interact with human and rat gp330 in vitro. Gp330 did not exhibit detectable binding to the LRP ligands, alpha 2-macroglobulin protease complex or Pseudomonas aeruginosa exotoxin A. However, we found that gp330 (purified from human or rat) bound the lipolytic enzyme lipoprotein lipase (LPL) with high affinity (Kd = 6.1 and 2.7 nM, respectively). The binding was saturable, divalent cation dependent, and inhibited by heparin or receptor-associated protein. Because LRP has also been shown to bind LPL, the present findings further extend the functional similarities between gp330 and LRP. By analogy to the postulated role of the LRP-LPL interaction in facilitating hepatic clearance of LPL-associated lipoproteins from the blood (Beisiegel, U., Weber, W., and Bengtsson-Olivercrona, G. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 8342-8346; Chappell, D. A., Fry, G. L., Waknitz, M. A., Iverius, P. H., Williams, S. E., and Strickland, D. K. (1992) J. Biol. Chem. 267, 25764-25767), we speculate that the gp330-LPL interaction described herein may contribute to the uptake of LPL-associated lipoproteins in tissues expressing gp330. Consistent with this possibility, we found that LPL promoted in vitro binding of 125I-lipoproteins to gp330.

The novel compound NO-1886 increases lipoprotein lipase activity with resulting elevation of high density lipoprotein cholesterol, and long-term administration inhibits atherogenesis in the coronary arteries of rats with experimental atherosclerosis.

We have discovered a novel compound, NO-1886, which possesses a powerful lipoprotein lipase (LPL) activity-increasing action. Administration of NO-1886 increased LPL activity in the postheparin plasma, adipose tissue, and myocardium of rats, and produced a reduction in plasma triglyceride levels with concomitant elevation of HDL cholesterol levels. Administration of NO-1886 increased LPL enzyme mass in postheparin plasma and mRNA activity in epididymal adipose tissue, and it was concluded that the mode of action of this compound is stimulation of tissue LPL synthesis. We also conducted long-term studies to assess the impact of increases in LPL activity and HDL levels on the development of atherosclerotic lesions in rats. Administration of NO-1886 for as long as 90 d significantly decreased the degree of atherosclerotic changes in the coronary arteries of vitamin D2-treated, cholesterol-fed rats. Statistical analysis indicated that increased concentration of HDL is the factor contributing mostly to the prevention of coronary artery sclerosis. In summary, the results of our study indicate that compound NO-1886 increases LPL activity, causing an elevation in HDL levels, and that long-term administration of NO-1886 to rats with experimental atherosclerosis provides significant protection against the development of coronary artery lesions.

Prevention of raised low-density lipoprotein cholesterol in a patient with familial hypercholesterolaemia and lipoprotein lipase deficiency

Raised low-density lipoprotein (LDL) cholesterol is believed to predispose to development of atherosclerosis and coronary heart disease. Increased plasma LDL concentrations and premature coronary heart disease are present in familial hypercholesterolemia (FH), and the enzyme lipoprotein lipase (LPL) seems to have a key role in production of LDL. We describe a unique French Canadian individual who is both heterozygous for FH and homozygous for LPL deficiency (FH/LPL). In this patient, LDL cholesterol was strikingly low compared with both his FH (0·65 vs 5·84 mmol/L) and normolipidaemic (2·77 mmol/L) age-matched relatives despite the defect of LDL-receptormediated removal. No LDL peak was present in the cholesterol profile of the FH/LPL-deficient subject, as determined by density-gradient ultracentrifugation. Our results suggest that most LDL particles, in vivo, originate from triglyceride-rich lipoproteins, that LPL plays a vital part in this process, and that absence of LPL activity protects FH subjects against the increase in LDL cholesterol.

Metabolic abnormalities in hyperlipidemias

The role of hyperlipidemias as risk factors for coronary heart disease is widely acknowledged. After the first classification by Fredrickson based on plama lipid concentrations, a more comprehensive evaluation of the genetic background and the biochemical mechanism underlying these diseases is internatinally adopted nowadays. In familial hyperchylomicronemia, a tipically pediatric disease transmitted as autosomical recessive trait, postprandial chylomicrons accumulate in fasting plasma due to lack or impaired activation of lipolytic enzyme lipoprotein lipase (LPL). In polygenic hypercholesterolemia a moderate increase of low density lipoprotein (LDL) cholesterol is due to a combination of environmental and genetically transmitted factors, like, for instance, some isoforms of apolipoprotein (apo) E. In familial hypercholesterolemia, transmitted as autosomical dominant trait, the basic defect is the lack or defective activity of LDL receptors, particularly in the liver. This disease, particularly in its homozygous form, is a pediatric disease. In the broad beta disease (former type III) cholesterol enriched, small very low density lipoprotein (VLDL), also called intermediate density lipoprotein (IDL), build up in plasma. These patients show a reduced VLDL catabolism, due to a particular isoform af apo E, coupled with an increased VLDL production. An enhanced liver synthesis of Apo B is the abnormality underlying familial combined hyperlipidemia. Accumulation in plasma of VLDL, LDL or both depends on the coexistence of defects in their respective catabolism. Familial hypertriglyceridemia, characterized by massive accymulation of VLDL in plasma, is poorly understood, although an increased production of VLDL particles is usually detectable. Growing attention as risk factor for CVD is also being paid to elevated plasma concentrations of lipoprotein a Lp(a), a LDL-like particle whose structure resembles that of plasminogen and might therefore interfere with antithrombotic mechanism. The mechanisms underlying some secondary form of hyperlipidemias that are of pediatric interest are also examined. Hypertriglyceridemia is the common feature of obesity, insulin dependent diabetes mellitus and glycogen storage disease, while hypercholesterolemia is the most prevalent lipid abnormality in nephrotic syndrome.

Metabolic responses to exercise. Effects of endurance training and implications for diabetes.

In this study, some important metabolic responses to exercise will be discussed, and aspects of particular interest for patients with diabetes mellitus will be emphasized. Alterations in the metabolic responses to exercise induced by physical endurance training and consequences of training for metabolism of plasma lipids and lipoproteins will be discussed. Glucoregulation during exercise is not perfect in normal subjects and is less so in patients with diabetes mellitus. For instance, during intense exercise, large increases in the plasma glucose concentration occur and a state of insulin resistance exists for a few hours after intense exercise. Even so, increased sensitivity to insulin is found the day after intense exercise and also shortly after more moderate intensity exercise, both in healthy subjects and in patients with diabetes mellitus. Increased sensitivity to insulin is also found after endurance training, whereas insulin sensitivity is decreased after inactivity. Exercise training increases the ability of muscle to take up and oxidize free fatty acids during exercise and also increases the activity of the enzyme lipoprotein lipase in muscle. The activity of lipoprotein lipase in muscle correlates with muscle insulin sensitivity. This might explain why insulin resistance is often associated with hypercholesterolemia, hypertriglyceridemia, and low high-density lipoprotein cholesterol.

Developmental regulation of lipoprotein lipase in rats.

To evaluate changes in lipoprotein lipase (LPL) expression during development, levels of LPL mRNA, protein, and enzyme activity were measured in heart, epididymal fat, kidney, and brain of rats, from late gestation through 24 mo. LPL mRNA, protein, and enzyme activity were low in fetal and neonatal hearts. LPL mRNA increased 11-fold by 60 days and remained at this level thereafter; LPL protein and enzyme activity increased 10-fold by weaning, before declining to low values by 3 mo. LPL mRNA levels, protein, and enzyme activity did not change in epididymal fat from 3 wk to 21 mo. In the kidney, LPL mRNA levels were high at the end of gestation but fluctuated during the first month. LPL protein and activity were low at day 1 and rose eightfold to peak values by day 7 before decreasing to low levels by weaning. LPL mRNA levels were relatively high in fetal brains and then fell 60% during the neonatal period. LPL protein peaked at day 7 before falling 95% by weaning. Thus LPL is under complex tissue-specific regulation involving transcriptional and posttranscriptional mechanisms.

Effects of substitutions of glycine and asparagine for serine 132 on activity and binding of human lipoprotein lipase to very low density lipoproteins

For studying the role of Ser 132 in the putative catalytic site of human lipoprotein lipase (LPL), mutant LPL cDNAs expressing LPLs with amino acid substitutions of Gly or Asn for Ser 132 were obtained by site-directed mutagenesis, and were expressed in COS-1 cells. Considerable amounts of LPL enzyme protein mass were detected in the culture medium of COS-1 cells transfected with wild-type LPL, LPL-Gly 132, or LPL-Asn 132. LPL-Gly 132 hydrolyzed Triton X-100-triolein and tributyrin as effectively as wild-type LPL, whereas LPL-Asn 132 showed no activity. LPL-Asn 132 bound to very low density lipoproteins as effectively as wild-type LPL.

Gene expression of lipid storage-related enzymes in adipose tissue of the genetically obese Zucker rat. Co-ordinated increase in transcriptional activity and potentiation by hyperinsulinaemia.

The genetically obese Zucker rat displays excessive fat storage capacity which is due to a tissue-specific increase in the activities of a number of lipid storage-related enzymes in adipose tissue. The aim of this study was to investigate the molecular mechanism responsible for this phenomenon. Lean (Fa/fa) and obese (fa/fa) Zucker rats were studied during the early stages of adipose tissue overdevelopment, both before (at 16 days of age) and after (at 30 days of age) the emergence of hyperinsulinaemia, in order to delineate the effects of the fatty genotype independently of those of hyperinsulinaemia. Lipoprotein lipase (LPL), glycerophosphate dehydrogenase (GPDH), glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and malic enzyme (ME) mRNA levels in the adipose tissue of lean and obese rats were assessed by Northern blot analysis, and the relative transcription rates of the corresponding genes were compared in the two genotypes by a nuclear run-on assay. In normoinsulinaemic 16-day-old pre-obese rats, mRNA levels were increased over control values (LPL, 5-fold; ME, 2-fold; GAPDH, 3-fold), in close correlation with genotype-mediated differences in enzyme activities. Stimulation of the transcription rates of the ME and GAPDH genes was observed in obese rats, which could fully account for differences in steady-state mRNA levels. At this age, GPDH activity, mRNA level and transcription rate were similar in the two genotypes. In hyperinsulinaemic 30-day-old obese rats, a 6-7-fold increase in both mRNA and the transcription rate of GPDH emerged, together with an amplification of the genotype-mediated differences observed in younger animals (GAPDH, 6-fold; ME, 7.9-fold; LPL, 10-fold). These results demonstrate that the obese genotype exerts a co-ordinated control on the expression of these genes in adipose tissue, mainly at the transcriptional level. This genotype effect is greatly amplified by the development of hyperinsulinaemia.

Epinephrine inhibits lipoprotein lipase gene expression in rat adipocytes through multiple steps in posttranscriptional processing.

Previous studies have demonstrated that in vitro treatment of adipocytes with catecholamines results in a decrease in the activity of the enzyme lipoprotein lipase (LPL). To examine the mechanism of this effect, primary cultures of rat adipocytes were cultured in the presence of various concentrations of epinephrine (10(-9)-10(-5) M). Epinephrine yielded a dose-dependent decrease in LPL activity; heparin-releasable LPL activity was reduced to 66% of control values after exposure to 10(-5) M epinephrine for 2 h. However, there was no effect of epinephrine on LPL immunoreactive mass, as measured by enzyme-linked immunosorbent assay. When cells were pulse labeled with [35S]methionine, there was a rapid and dose-dependent decrease in immunoprecipitable LPL. In spite of the decrease in LPL translation, neither epinephrine nor other catecholamines altered the level of LPL mRNA or the rate of LPL transcription. To further examine LPL posttranslational processing, cells were pulse labeled with [35S]methionine in the absence of epinephrine and then chased with unlabeled methionine in the presence of epinephrine. Cells exposed to epinephrine during the chase demonstrated a decrease in LPL secretion into the medium as well as a decrease in LPL degradation. The addition of epinephrine during LPL posttranslational processing did not alter the sensitivity of the newly synthesized LPL protein to endo-beta-N-acetylglucosaminidase-H. Thus, epinephrine had multiple effects on adipocyte LPL. Although there was a rapid decrease in LPL synthesis that was not due to changes in LPL mRNA, the level of LPL protein was unchanged under these conditions due to a decrease in LPL degradation and secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of gene expression by insulin and tumor necrosis factor alpha in 3T3-L1 cells. Modulation of the transcription of genes encoding acyl-CoA synthetase and stearoyl-CoA desaturase-1.

Insulin and tumor necrosis factor alpha (TNF alpha) produce potent and opposing physiological signals in adipocytes. However, genes that are co-regulated by the hormone and cytokine during and after adipocyte differentiation have not been characterized. Using 3T3-L1 cells, we have studied the regulation of the expression of genes encoding acyl-CoA synthetase (ACS), and stearoyl CoA desaturase-1 (SCD-1), two enzymes that play key roles in the metabolism of long chain fatty acids. Insulin is required for triggering the transcriptional activation of the ACS and SCD-1 genes at an early stage in adipocyte differentiation. In mature adipocytes insulin elicits a 4-fold increase in the rates of transcription of the two genes. However, when 3T3-L1 adipocytes are treated with TNF alpha the cytokine causes a 75-90% decrease in the levels of ACS and SCD-1 mRNAs. The decline in mRNA content is associated with similar decrements in the rates of transcription of the ACS and SCD-1 genes. Thus, the ACS and SCD-1 genes are subject to stimulation and counter-regulation (at the transcriptional level) by insulin and TNF alpha, respectively. The opposing effects of insulin and TNF alpha are observed in developing and terminally differentiated adipocytes. Unlike the ACS and SCD-1 genes, the genes that encode the lipogenic enzymes lipoprotein lipase and malic enzyme are not subject to counter-regulation by insulin and TNF alpha at the transcriptional level in 3T3-L1 adipocytes. These observations on the control of ACS and SCD-1 expression suggest possible mechanisms by which adipocytes can markedly adjust their capacity for long chain fatty acid metabolism in response to external stimuli.

Cloning and characterization of the promoter of the murine lipoprotein lipase-encoding gene: structural and functional analysis

The enzyme lipoprotein lipase (LPL) is responsible for the hydrolysis of triglycerides into free fatty acids and glycerol. Its synthesis is induced as the murine bone marrow stromal cell clone, BMS2, undergoes adipocyte differentiation. The murine genomic LPL promoter has been cloned, sequenced, and characterized by functional and structural assays. The transcriptional start points have been mapped by S1 nuclease and primer extension techniques. Comparison of the 1.7-kb of LPL 5'-flanking sequence between mouse and man reveals 65% identity or greater with conservation of many potential protein-recognition motifs. Using constructs linking this region to the luciferase-encoding reporter gene, transient transfection experiments have documented the promoter function of this sequence in a number of cell lines. Based on a battery of restriction endonucleases, at least 260 bp immediately adjacent to and including the 5'-untranslated region of the first exon are hypersensitive to exogenous nuclease digestion, consistent with an altered chromatin structure. Protein-DNA interactions are detected within this area at the octamer binding protein 1 site and immediately 5' to the translation initiation site based on ExoIII footprinting and gel retention assays.

Lipoprotein lipase enhances the binding of chylomicrons to low density lipoprotein receptor-related protein.

Chylomicron catabolism is known to be initiated by the enzyme lipoprotein lipase (triacylglycero-protein acylhydrolase, EC 3.1.1.34). Chylomicron remnants, produced by lipolysis, are rapidly taken up by the liver via an apolipoprotein E (apoE)-mediated, receptor-dependent process. The low density lipoprotein (LDL) receptor-related protein (LRP) has been suggested as the potential apoE receptor. We have analyzed the binding of human chylomicrons to HepG2 cells in the absence and presence of lipoprotein lipase. Bovine and human lipoprotein lipases were able to increase the specific binding of the chylomicrons by up to 30-fold. This effect was not dependent on lipolysis but appeared to be due to the lipase protein itself. It was not found when a structurally unrelated, bacterial lipase was used. Using beta-migrating very low density lipoproteins (beta-VLDLs), known as a good ligand for LRP, binding studies were performed on LDL receptor-negative human fibroblasts. The binding was increased 40-fold by addition of lipoprotein lipase. Crosslinking experiments on cells with 125I-labeled apoE liposomes or lipoprotein lipase showed that both proteins were able to bind to LRP on the cell surface. The binding of apoE to LRP was highly increased by the addition of lipase. We conclude that lipoprotein lipase strongly enhances the binding of apoE-containing lipoproteins to LRP and therefore might play an important role in chylomicron catabolism not only because of its lipolytic activity but also because of its structural properties.

The structure of the mouse lipoprotein lipase gene: A B1 repetitive element is inserted into the 3′ untranslated region of the mRNA

The catabolism of triglycerides-rich lipoproteins and the subsequent uptake of free fatty acids by muscle and adipose tissue is dependent on the enzyme lipoprotein lipase (LPL). To better understand the regulation of this enzyme, we have isolated and characterized the mouse LPL gene. The gene is 28 kb in length and comprises 10 exons which encode a 4.0-kb mRNA. In this report, almost 6 kb of DNA sequence is presented, including 1251 bp 5′ to the gene, over 4 kb of exon and exon-intron junctions, and 583 bp 3′ to the gene. RNA from differentiated 3T3-L1 adipocytes was used in primer extension and RNase protection assays to show that the 5′ untranslated region is not interrupted by an intron and the start site of transcription is 199 bp 5′ to the ATG codon that begins translation. The first exon codes for the 5′ untranslated region and the signal peptide of 27 amino acids and 2 amino acids of the mature protein, exons 2–9 code for 445 amino acids of the mature protein. These exons are short and vary in length from 102 to 287 bp. The 10th exon codes for the 3′ untranslated region and is 2346 bp long. This exon contains a single copy of a B1 repetitive element of 152 bp followed by a 169-bp homopurine stretch. These elements are flanked by a pair of 16-bp direct repeats. The mouse gene is similar in size to the human, which also contains 10 exons in similar locations. There is a high degree of sequence homology between the two genes, 5′ region (700 bp), 75%; 5′ untranslated region, 74%; coding region, 88%; 3′ untranslated region, 75%. The most striking difference is the absence of the B1 repetitive element and homopurine region in the human 3′ untranslated region. This information about the mouse LPL gene may lead to a better understanding of its regulation and role in plasma lipoprotein metabolism.

Catalytic triad residue mutation (Asp156—-Gly) causing familial lipoprotein lipase deficiency. Co-inheritance with a nonsense mutation (Ser447—-Ter) in a Turkish family

We studied the molecular basis of familial Type I hyperlipoproteinemia in two brothers of Turkish descent who had normal plasma apolipoprotein C-II levels and undetectable plasma post-heparin lipoprotein lipase (LPL) activity. We cloned the cDNAs of LPL mRNA from adipose tissue biopsies obtained from these individuals by the polymerase chain reaction and directional cloning into M13 vectors. Direct sequencing of pools of greater than 2000 cDNA clones indicates that their LPL mRNA contains two mutations: a missense mutation changing codon 156 from GAU to GGU predicting an Asp156----Gly substitution and a nonsense mutation changing the codon for Ser447 from UCA to UGA, a stop codon, predicting a truncated LPL protein that contains 446 instead of 448 amino acid residues. Both patients were homozygous for both mutations. Analysis of genomic DNAs of the patients and their family members by the polymerase chain reaction, restriction enzyme digestion (the GAT----GGT mutation abolishes a TaqI restriction site), and allele-specific oligonucleotide hybridization confirms that the patients were homozygous for these mutations at the chromosomal level, and the clinically unaffected parents and sibling were true obligate heterozygotes for both mutations. In order to examine the functional significance of the mutations in this family, we expressed wild type and mutant LPLs in vitro using a eukaryotic expression vector. Five types of LPL proteins were produced in COS cells by transient transfection: (i) wild type LPL, (ii) Asp156----Gly mutant, (iii) Ser447----Ter mutant, (iv) Gly448----Ter mutant, and (v) Asp156----Gly/Ser447----Ter double mutant. Both LPL immunoreactive mass and enzyme activity were determined in the culture media and intracellularly. Immunoreactive LPLs were produced in all cases. The mutant LPLs, Asp156----Gly and Asp156----Gly/Ser447----Ter, were devoid of enzyme activity, indicating that the Asp156----Gly mutation is the underlying defect for the LPL deficiency in the two patients. The two mutant LPLs missing a single residue (Gly448) or a dipeptide (Ser447-Gly448) from its carboxyl terminus had normal enzyme activity. Thus, despite its conservation among all mammalian LPLs examined to date, the carboxyl terminus of LPL is not essential for enzyme activity. We further screened 224 unrelated normal Caucasians for the Ser447----Ter mutation and found 36 individuals who were heterozygous and one individual who was homozygous for this mutation, indicating that it is a sequence polymorphism of no functional significance. Human LPL shows high homology to hepatic triglyceride lipase and pancreatic lipase.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of lipoprotein lipase in muscle and adipose tissue during exercise

Lipoprotein lipase (LPL) is regulated in a tissue-specific manner; exercise increases LPL activity in muscle at the same time it is reduced in adipose tissue. The purpose of this study was to determine the relationship between LPL activity and LPL mRNA in muscle and adipose tissue in rats exposed to one bout of exercise. Immediately after a 2-h swim, LPL activity [pmol free fatty acids (FFA).min-1.mg tissue-1] in the exercised animals was reduced 43% in adipose tissue (110 +/- 26 to 63 +/- 17) and increased almost twofold in the soleus muscle (203 +/- 26 to 383 +/- 59) compared with sedentary control animals. At the same time, LPL mRNA was reduced 42% in adipose tissue and increased 50 and 100% in the red vastus and white vastus muscles, respectively. Twenty-four hours after the swim, LPL activity had returned to control levels in adipose tissue and the soleus muscle. At hour 24 of recovery, LPL mRNA was still reduced 23% in the adipose tissue of exercised animals but was not significantly different between exercised and control animals in any of the muscle tissues analyzed. Changes in total RNA concentration could not account for the changes in relative LPL mRNA expression. The relationship between LPL enzyme activity and LPL mRNA in muscle and adipose tissue was +0.86 and +0.93 at 0 and 24 h postexercise, respectively. Thus the tissue-specific changes in enzyme activity induced by exercise could be mediated, in part, through pretranslational control.

Expression of lipoprotein lipase in different human subcutaneous adipose tissue regions

Steady state expression of lipoprotein lipase was compared in abdominal and gluteal subcutaneous adipose tissue of nonobese men and women. In both regions enzyme activity and lipoprotein lipase mRNA levels were significantly higher in women than in men. In men the enzyme activity was higher in abdominal than in gluteal adipose tissue (P less than 0.01) whereas the opposite was observed in women (P less than 0.05). In both sexes, however, lipoprotein lipase mRNA levels were threefold higher in the abdominal as compared to the gluteal site, whether they were determined in isolated fat cells or in fat segments (P less than 0.01). This regional difference persisted when the mRNA values were expressed as a function of the mRNA concentration for beta-actin. There was a correlation between the two adipose tissue regions as regards the values for enzyme activity and mRNA level (r = 0.6-0.8). Northern blot analysis revealed two mRNA species of 3.5 and 3.7 kilobases, respectively. It is concluded that there are regional variations in the steady state expression of lipoprotein lipase in human subcutaneous adipose tissue. This involves site variations in gene expression as well as posttranslational modification of lipoprotein lipase enzyme activity and may contribute to the characteristic variations in adipose tissue mass and distribution between men and women.

Characterization of serum-stimulated lipoprotein lipase from bovine heart

1. 1. A triglyceride (TG) lipase is present in whole homogenate and tissue extracts of beef myocardium with chareteristics of lipoprotein lipase (LPL); i.e. activity is stimulated by serum, inhibited by NaCI and protamine sulfate, the protein binds to heparin-Sepharose, and the enzyme has an alkaline pH optimum. 2. 2. This TG lipase, eluted from heparin-Sepharose at 0.9–1.0 M NaCI, has an apparent mol. wt of 64 K daltons. Its primary mRNA is 3.7 kb. 3. 3. Expression of LPL mRNA and enzyme activities are in the ratio of approximately 20:8:1 for hearts of mouse, rat and beef, respectively and correlate with r = +0.99.

Interrelationship of triglycerides with lipoproteins and high-density lipoproteins

Triglycerides are transported by the largest and most lipid-rich of the lipoprotein particles, namely, chylomicrons and very low density lipoproteins (VLDL). These particles are buoyant because of the high triglyceride content, which makes up approximately 90% by weight of the chylomicron and 70% by weight of the VLDL. The chylomicron transports exogenous or dietary fat and cholesterol, whereas VLDL transports endogenous triglyceride and cholesterol in lipoproteins synthesized and secreted by the liver. Both chylomicrons and VLDL are hydrolyzed at the capillary surface by the enzyme lipoprotein lipase. Lipoprotein lipase catalyzes the hydrolysis of triglyceride in the lipid core of these particles, producing smaller particles known as remnants. We currently believe the remnants are atherogenic and that this is one reason why hypertriglyceridemia may predispose to coronary artery disease. Chylomicron remnants are recognized and removed by hepatic receptors that contain apolipoprotein (apo) E. The rate of clearance of remnant particles depends on which subfraction of apo E is present. Particles containing apo EII are removed more slowly than those with apo EIII and EIV. The dietary cholesterol from the chylomicron remnant particles is thought to down-regulate the hepatic low-density lipoprotein (LDL) receptors. VLDL remnants, also called intermediate-density lipoprotein (IDL), contain apo E and may be removed by the liver through the LDL or B/E receptor. The decrease in activity of these receptors results in apparent oversynthesis of LDL, the end-product of VLDL and IDL metabolism. LDL is the major cholesterol carrier, followed by high-density lipoprotein (HDL). LDL carries approximately up to three-fourths of the cholesterol in the Mood, and about two-thirds of LDL is removed by specific LDL receptors as it circulates. The other portion is removed by low-affinity pathways. HDL contains approximately 50% cholesterol and phospholipid. HDL is synthesized in the liver, the intestine and also originates from the surface of chylomicrons and VLDL during lipolysis. HDL acquires cholesterol from peripheral tissues and is esterified by the lecithin cholesterol acyltransferase or LCAT to cholesteryl ester. The cholesteryl ester may then be transferred from HDL to LDL, IDL or VLDL by the cholesteryl ester transfer protein. Thus, cholesterol can be transported to the liver indirectly by HDL, and HDL fractions containing apo E may directly transport HDL to the liver. HDL exists in a nascent form before being converted into spherical particles. There is some evidence that apo AI may play a particularly important role in this reverse cholesterol transport process. HDL may be subdivided in several ways: one based on its apoprotein content and one based on its density (e.g., HDL 2 and HDL 3). In hypertriglyceridemia, the formation of HDL from the hydrolysis of triglyceride-rich particles may be diminished. In addition, when postprandial lipemia occurs, or when there is a prolonged elevation of triglyceride-rich lipoproteins, an increased transfer of triglyceride from triglyceride-rich lipoprotein particles into LDL and HDL may occur in exchange for cholesteryl ester. The metabolic changes associated with hypertriglyceridemia may decrease the concentration of HDL and result in small, dense particles of LDL and HDL.

Corticosterone, foraging behavior, and metabolism in dark-eyed juncos, Junco hyemalis

Male, dark-eyed juncos, Junco hyemalis, were held in captivity under conditions simulating winter temperature and photoperiod. Birds were administered corticosterone (B), metapyrone (MET), or control (empty) implants in silastic tubing. B implants resulted in higher plasma levels of B, especially on the first day following implantation. After 4 days, B-implanted birds had significant atrophy of the flight muscles and increased storage of fat in the furcular and abdominal adipose depots. Despite the increased fat deposition, no differences in adipose tissue lipoprotein lipase (LPL) activity or plasma levels of glycerol or free fatty acid were observed after 4 days of treatment. There were no differences in muscle LPL activity when enzyme activity was expressed per tissue. However, when enzyme activity was expressed per gram of tissue, muscle LPL was significantly greater in the B-treated birds. Therefore, despite the atrophy of muscle tissue, the LPL enzyme had been preserved. No differences were observed in either foraging behavior or food consumption, even in light of the dramatic changes in fat and muscle masses in B-treated birds. These data are discussed in light of the chronic effects of B which may be applicable to the advanced stage of stress in birds.