Trypsin treatment may impair the interfacial activation action of lipoprotein lipase.

Lipoprotein lipase was expressed in Chinese hamster ovary (CHO) cells transfected with human lipoprotein lipase cDNA. The lipoprotein lipase retained tributyrin, water-soluble substrate, hydrolyzing activity (esterase activity). The catalytic action of this enzyme was studied by monitoring the esterase activity. The esterase activity was enhanced 4.5-fold by the addition of triolein emulsified with Triton X-100. This process was named interfacial activation. Treatment of LPL with trypsin (100 micrograms/ml, 37 degrees C for 10 min) caused the loss of the triolein hydrolyzing activity without that of the esterase activity. The esterase activity of trypsin-treated LPL was not enhanced by the addition of the triolein emulsion. The trypsin-treated LPL retained the ability to bind to very low density lipoproteins (VLDL). These results are consistent with the idea that LPL has a catalytic site and a lipid interface recognition site, and that the enzyme undergoes interfacial activation, in which the concealed catalytic site is revealed after the enzyme binds to the surface. Based on this hypothesis, the results obtained suggest that trypsin nicking may impair the interfacial activation process and cause the loss of the lipase activity.     

Role of lipoprotein lipase in plasma triglyceride removal.

Intravenous injections of anti-lipoprotein lipase serunis quantitatively block the catabolism of very low density lipoprotein (VLDL) and portomicron triglyceride and specifically inhibit triglyceride transport into ovarian follicles. The immunological studies presented provide information on the site of action of lipoprotein lipase (LPL). In the anti-LPL serum-treated animals initial plasma triglyceride accumulation occurs at the time of antiserum injection. This instantaneous inhibition of triglyceride removal provides direct evidence that the functional LPL responsible for VLDL and portomicron triglyceride hydrolysis is located in sites within the plasma compartment readily accessible to immunoglobulins. In vitro immunological studies show that the adipose, heart, ovarian, and liver LPL share common immunological determinants. Biochemical studies on highly purified heart and adipose LPL suggest that these enzymes have identical protein moieties.     

Adrenal and liver in normal and cld/cld mice synthesize and secrete hepatic lipase, but the lipase is inactive in cld/cld mice

Combined lipase deficiency (cld) is a recessive mutation in mice that causes a severe lack of lipoprotein lipase (LPL) and hepatic lipase (HL) activities, hyperlipemia, and death within 3 days after birth. Earlier studies showed that inactive LPL and HL were synthesized by cld/cld tissues and that LPL synthesized by cld/cld brown adipocytes was retained in their ER. We report here a study of HL in liver, adrenal, and plasma of normal newborn and cld/cld mice. Immunofluorescence studies showed HL was present in extracellular space, but not in cells, in liver and adrenal of both normal and cld/cld mice. When protein secretion was blocked with monensin, HL was retained intracellularly in liver cell cultures and in incubated adrenal tissues of both groups of mice. These findings demonstrated that HL was synthesized and secreted by liver and adrenal cells in normal newborn and cld/cld mice. HL activities in liver, adrenal, and plasma in cld/cld mice were very low, <8% of that in normal newborn mice, indicating that HL synthesized and secreted by cld/cld cells was inactive. Livers of both normal newborn and cld/cld mice synthesized LPL, but the level of LPL activity in cld/cld liver was very low, <9% of that in normal liver. Immunofluorescence studies showed that LPL was present intracellularly in liver of cld/cld mice, indicating that LPL was synthesized but not secreted by cld/cld liver cells. Immunofluorescent LPL was not found in normal newborn liver cells unless the cells were treated with monensin, thus demonstrating that normal liver cells synthesized and secreted LPL. Livers of both groups of mice contained an unidentified alkaline lipase activity which accounted for 34-54% of alkaline lipase activity in normal and 65% of that in cld/cld livers. Our findings indicate that liver and adrenal cells synthesized and secreted HL in both normal newborn and cld/cld mice, but the lipase was inactive in cld/cld mice. That cld/cld liver cells secreted inactive HL while retaining inactive LPL indicates that these closely related lipases were processed differently.     

Increased intracellular triglyceride in C(2)C(12) muscle cells transfected with human lipoprotein lipase

Much of the knowledge about the cell biology of lipoprotein lipase (LPL) in vitro has been gained from adipose tissue model systems. However, the importance of skeletal muscle lipoprotein lipase (SMLPL) to both lipoprotein and muscle metabolism remains unclear. Although the production of LPL in cultured myocytes has been documented, the amount of enzyme activity produced is small. To develop a more suitable tissue culture model for SMLPL, mouse C(2)C(12) myoblasts were stably transduced with a retroviral vector encoding the full-length human LPL (hLPL) cDNA. Control cells were transduced with a vector encoding beta-galactosidase. LPL expression was assayed as a function of cell growth by measuring LPL activity on days 3, 7, 9, 11, and 14 after subculture. The hLPL-transduced myoblasts increasingly overexpressed both heparin-releasable (HR) and intracellular (IN) LPL activity compared to nontransduced myoblasts (P < 0.001 at Day 11) and myoblasts transduced with the control vector (P < 0.001 at Day 11). This increase occurred while LPL mRNA levels remained stable between days 3 and 14. As expected, IN LPL activity was also increased in the transduced cells. High levels of LPL activity were also obtained after differentiating the C(2)C(12) cells into myotubes by serum deprivation. Additionally, throughout the time course, C(2)/LPL cells had greater amounts of intracellular triglyceride than both the C(2)C(12) and the C(2)/beta-GEO cells (P = 0.005 and P < 0.001, respectively) with the largest differences seen on day 14 of the time course (P = 0.001, C(2)/LPL vs C(2)C(12) (r) or C(2)/beta-GEO cells). Thus, C(2)C(12) myoblasts stably transduced with hLPL markedly overexpressed both HR and IN LPL activity compared to control cells which, in turn, was associated with increases in intracellular triglyceride content. Because LPL regulation in tissues is mostly posttranslational, this new in vitro model will permit the in-depth study of the posttranslational regulation of SMLPL and provide new insights into the fate of lipoprotein-derived fatty acids in muscle.     

Characterization of the lipolytic activity of endothelial lipase

Endothelial lipase (EL) is a new member of the triglyceride lipase gene family previously reported to have phospholipase activity. Using radiolabeled lipid substrates, we characterized the lipolytic activity of this enzyme in comparison to lipoprotein lipase (LPL) and hepatic lipase (HL) using conditioned medium from cells infected with recombinant adenoviruses encoding each of the enzymes. In the absence of serum, EL had clearly detectable triglyceride lipase activity. Both the triglyceride lipase and phospholipase activities of EL were inhibited in a dose-dependent fashion by the addition of serum. The ratio of triglyceride lipase to phospholipase activity of EL was 0.65, compared with ratios of 24.1 for HL and 139.9 for LPL, placing EL at the opposite end of the lipolytic spectrum from LPL. Neither lipase activity of EL was influenced by the addition of apolipoprotein C-II (apoC-II), indicating that EL, like HL, does not require apoC-II for activation. Like LPL but not HL, both lipase activities of EL were inhibited by 1 M NaCl. The relative ability of EL, versus HL and LPL, to hydrolyze lipids in isolated lipoprotein fractions was also examined using generation of FFAs as an end point. As expected, based on the relative triglyceride lipase activities of the three enzymes, the triglyceride-rich lipoproteins, chylomicrons, VLDL, and IDL, were efficiently hydrolyzed by LPL and HL. EL hydrolyzed HDL more efficiently than the other lipoprotein fractions, and LDL was a poor substrate for all of the enzymes.     

Transcytosis of lipoprotein lipase across cultured endothelial cells requires both heparan sulfate proteoglycans and the very low density lipoprotein receptor

Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of circulating lipoprotein triglyceride molecules, is synthesized in myocytes and adipocytes but functions while bound to heparan sulfate proteoglycans (HSPGs) on the luminal surface of vascular endothelial cells. This requires transfer of LPL from the abluminal side to the luminal side of endothelial cells. Studies were performed to investigate the mechanisms of LPL transcytosis using cultured monolayers of bovine aortic endothelial cells. We tested whether HSPGs and members of the low density lipoprotein (LDL) receptor superfamily were involved in transfer of LPL from the basolateral to the apical side of cultured endothelial cells. Heparinase/heparinitase treatment of the basolateral cell surface or addition of heparin to the basolateral medium decreased the movement of LPL. This suggested a requirement for HSPGs. To assess the role of receptors, we used either receptor-associated protein, the 39-kDa inhibitor of ligand binding to the LDL receptor-related protein and the very low density lipoprotein (VLDL) receptor, or specific receptor antibodies. Receptor-associated protein reduced (125)I-LPL and LPL activity transfer across the monolayers. When the basolateral surface of the cells was treated with antibodies, only anti-VLDL receptor antibodies inhibited transcytosis. Moreover, overexpression of the VLDL receptor using adenoviral-mediated gene transfer increased LPL transcytosis. Thus, movement of active LPL across endothelial cells involves both HSPGs and VLDL receptor.     

Lipoprotein lipase inhibits hepatitis C virus (HCV) infection by blocking virus cell entry

A distinctive feature of HCV is that its life cycle depends on lipoprotein metabolism. Viral morphogenesis and secretion follow the very low-density lipoprotein (VLDL) biogenesis pathway and, consequently, infectious HCV in the serum is associated with triglyceride-rich lipoproteins (TRL). Lipoprotein lipase (LPL) hydrolyzes TRL within chylomicrons and VLDL but, independently of its catalytic activity, it has a bridging activity, mediating the hepatic uptake of chylomicrons and VLDL remnants. We previously showed that exogenously added LPL increases HCV binding to hepatoma cells by acting as a bridge between virus-associated lipoproteins and cell surface heparan sulfate, while simultaneously decreasing infection levels. We show here that LPL efficiently inhibits cell infection with two HCV strains produced in hepatoma cells or in primary human hepatocytes transplanted into uPA-SCID mice with fully functional human ApoB-lipoprotein profiles. Viruses produced in vitro or in vivo were separated on iodixanol gradients into low and higher density populations, and the infection of Huh 7.5 cells by both virus populations was inhibited by LPL. The effect of LPL depended on its enzymatic activity. However, the lipase inhibitor tetrahydrolipstatin restored only a minor part of HCV infectivity, suggesting an important role of the LPL bridging function in the inhibition of infection. We followed HCV cell entry by immunoelectron microscopy with anti-envelope and anti-core antibodies. These analyses demonstrated the internalization of virus particles into hepatoma cells and their presence in intracellular vesicles and associated with lipid droplets. In the presence of LPL, HCV was retained at the cell surface. We conclude that LPL efficiently inhibits HCV infection by acting on TRL associated with HCV particles through mechanisms involving its lipolytic function, but mostly its bridging function. These mechanisms lead to immobilization of the virus at the cell surface. HCV-associated lipoproteins may therefore be a promising target for the development of new therapeutic approaches.     

Angiopoietin-like Protein 3 Inhibits Lipoprotein Lipase Activity through Enhancing Its Cleavage by Proprotein Convertases

Lipoprotein lipase (LPL)-mediated lipolysis of triglycerides is the first and rate-limiting step in chylomicron/very low density lipoprotein clearance at the luminal surface of the capillaries. Angiopoietin-like protein 3 (ANGPTL3) is shown to inhibit LPL activity and plays important roles in modulating lipoprotein metabolism in vivo. However, the mechanism by which it inhibits LPL activity remains poorly understood. Using cell-based analysis of the interaction between ANGPTL3, furin, proprotein convertase subtilisin/kexin type 5 (PCSK5), paired amino acid converting enzyme-4 (PACE4), and LPL, we demonstrated that the cleavage of LPL by proprotein convertases is an inactivation process, similar to that seen for endothelial lipase cleavage. At physiological concentrations and in the presence of cells, ANGPTL3 is a potent inhibitor of LPL. This action is due to the fact that ANGPTL3 can enhance LPL cleavage by endogenous furin and PACE4 but not by PCSK5. This effect is specific to LPL but not endothelial lipase. Both N- and C-terminal domains of LPL are required for ANGPTL3-enhanced cleavage, and the N-terminal domain of ANGPTL3 is sufficient to exert its effect on LPL cleavage. Moreover, ANGPTL3 enhances LPL cleavage in the presence of either heparan sulfate proteoglycans or glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1). By enhancing LPL cleavage, ANGPTL3 dissociates LPL from the cell surface, inhibiting both the catalytic and noncatalytic functions of LPL. Taken together, our data provide a molecular connection between ANGPTL3, LPL, and proprotein convertases, which may represent a rapid signal communication among different metabolically active tissues to maintain energy homeostasis. These novel findings provide a new paradigm of specific protease-substrate interaction and further improve our knowledge of LPL biology.     

Interaction of lipoprotein lipase and receptor-associated protein

Receptor-associated protein (RAP) is a recognized chaperone/escort protein for members of the low density lipoprotein receptor family. In this report, we show that RAP binds to lipoprotein lipase (LPL) and may play a role in the maturation of LPL. Binding of highly purified RAP to LPL was demonstrated in vitro by solid phase assays, surface plasmon resonance, and rate zonal centrifugation. The dissociation constant for this interaction measured by the first two techniques ranged between 2.4 and 13 nM, values similar to those reported for the binding of RAP to LRP or gp330. The specificity of the interaction was demonstrated by competition with a panel of LPL monoclonal antibodies. Rate zonal centrifugation demonstrated the presence of a stable complex with an apparent Mr consistent with the formation of a complex between monomeric LPL and RAP. RAP x LPL complexes were co-immunoprecipitated in adipocyte lysates or from solutions of purified LPL and RAP. The interaction was also demonstrated in whole cells by cross-linking experiments. RAP-deficient adipocytes secreted LPL with a specific activity 2.5-fold lower than the lipase secreted by control cells. Heparin addition to cultured RAP-deficient adipocytes failed to stimulate LPL secretion in the medium, suggesting defective binding of the lipase to the plasma membrane. These studies demonstrate that RAP binds to LPL with high affinity both in purified systems and cell extracts and that RAP-deficient adipocytes secrete poorly assembled LPL. A function of RAP may be to prevent premature interaction of LPL with binding partners in the secretory pathway, namely LRP and heparan sulfate proteoglycan.     

Effect of diabetes on acid and neutral triacylglycerol lipase and on lipoprotein lipase activities in isolated myocardial cells from rat heart.

A neutral triacylglycerol lipase activity that is separate and distinct from lipoprotein lipase (LPL) could be measured in homogenates of myocardial cells if protamine sulphate and high concentrations of albumin were included in the assay. This neutral lipase was predominantly particulate, with the highest relative specific activity in microsomal subcellular fractions. The induction of diabetes by the administration of streptozotocin to rats resulted in a decrease in LPL activity in myocyte homogenates and in particulate subcellular fractions, but the percentage of cellular LPL activity that was released during incubation of myocytes with heparin was normal. In contrast, neutral lipase activity was increased in diabetic myocyte homogenates and microsomal fractions. Acid triacylglycerol lipase activity was not changed in diabetic myocytes. The decrease in LPL in myocytes owing to diabetes may result in the decreased functional LPL activity at the capillary endothelium of the diabetic heart.     

脂蛋白酯酶与动脉粥样硬化

脂蛋白酯酶(1ipopmtein lipase,LPL)是调节脂蛋白代谢的一种关键酶,如具有水解血浆脂蛋白中三酰甘油的作用等．体内LPL减少会导致血三酰甘油升高和高密度脂蛋白胆固醇降低,增加患动脉粥样硬化的危险．通过提高LPL的活性可以抑制动脉粥样硬化的发生发展．已有的研究说明NO-1886促进心肌和脂肪组织LPL mRNA表达,提高心肌、脂肪组织、骨骼肌和血液中LPL活性,因而改善脂蛋白代谢,抑制动脉粥样硬化．     

Distribution and source of lipoprotein lipase in mouse mammary gland

During lactation lipoprotein lipase (LPL) is elevated in mammary tissue and depressed in adipose tissue to redirect lipids for incorporation into milk fat. The cellular origin of lipoprotein lipase in mammary tissue is thought to be the mammary epithelial cell which is the predominant cell type noticeable in the lactating gland; however, mammary adipocytes are also present. If lipoprotein lipase is produced by adipocytes in other sites of the body, then the question remains as to why mammary adipocytes have not been shown to produce lipoprotein lipase. In this study we present several lines of evidence that indicate that the mammary adipocyte is a source of LPL in the lactating mammary gland of mice. This evidence includes the absence of extracellular and intracellular lipoprotein lipase activity in two types of primary mammary epithelial cell cultures and a similarity in the changes of lipoprotein lipase activity in genital adipose tissue from nonpregnant mice and lactating mammary tissue to the nutritional state of the animal. Other evidence presented here includes strong localization of lipoprotein lipase protein and messenger RNA by fluorescence immunohistochemistry and in situ hybridization, respectively, to interstitial cells located between epithelial structures. We postulate that these interstitial cells are regressed, lipid-deleted mammary adipocytes.     

Serum lipoprotein composition, lecithin cholesterol acyltransferase and tissue lipase activities in pregnant diabetic rats and their offspring receiving enriched n-3 PUFA diet

The effects of dietary n-3 polyunsaturated fatty acids on lipoprotein concentrations and on lipoprotein lipase (LPL), hepatic triglyceride lipase (HTGL) and lecithin cholesterol acyltransferase (LCAT) activities were studied in streptozotocin-induced diabetic rats during pregnancy and in their macrosomic offspring from birth to adulthood. Pregnant diabetic and control rats were fed Isio-4 diet (vegetable oil) or EPAX diet (concentrated marine omega-3 EPA/DHA oil), the same diets were consumed by pups at weaning. Compared with control rats, diabetic rats showed, during pregnancy, a significant elevation in very low density lipoprotein (VLDL) and low and high density lipoprotein (LDL-HDL(1))-triglyceride, cholesterol and apoprotein B100 concentrations and a reduction in apoprotein A-I levels. HTGL activity was high while LPL and LCAT activities were low in these rats. The macrosomic pups of Isio-4-fed diabetic rats showed a significant enhancement in triglyceride and cholesterol levels at birth and during adulthood with a concomitant increase in lipase and LCAT activities. EPAX diet induces a significant diminution of VLDL and LDL-HDL(1) in mothers and in their macrosomic pups, accompanied by an increase in cholesterol and apoprotein A-I levels in HDL(2-3) fraction. It also restores LPL, HTGL and LCAT activities to normal range. EPAX diet ameliorates considerably lipoprotein disorders in diabetic mothers and in their macrosomic offspring.     

Coexistence of abnormalities of hepatic lipase and lipoprotein lipase in a large family.

A large family is reported with familial hepatic triglyceride lipase (HTGL) deficiency and with the coexistence of reduced lipoprotein lipase (LPL) similar to the heterozygote state of LPL deficiency. The proband was initially detected because of hypertriglyceridemia and chylomicronemia. He was later demonstrated to have beta-VLDL despite an apo E3/E3 phenotype and the lack of stigmata of type III hyperlipoproteinemia. The proband had no HTGL activity in postheparin plasma. Two of his half-sisters had very low HTGL activity (39 and 31 nmol free fatty acids/min/ml; normal adult female greater than 44). His son and daughters had decreased HTGL activity (normal male and preadolescent female greater than 102), which would be expected in obligate heterozygotes for HTGL deficiency. Low HTGL activity was associated with LDL particles which were larger and more buoyant. Several family members, including the proband, had reduced LPL activity and mass less than that circumscribed by the 95% confidence-interval ellipse for normal subjects and had hyperlipidemia similar to that described in heterozygote relatives of patients with LPL deficiency. All the sibs with hyperlipidemia had a reduced LPL activity and mass, while subjects with isolated reduced HTGL (with normal LPL activity) had normal lipid phenotypes. Analysis of genomic DNA from these subjects by restriction-enzyme digestion revealed no major abnormalities in the structure of either the HTGL or the LPL gene. Compound heterozygotes for HTGL and LPL deficiency show lipoprotein physiological characteristics typical for HTGL deficiency, while their variable lipid phenotype is typical for LPL deficiency.     

Membrane-bound lipoprotein lipase on human monocyte-derived macrophages: localization by immunocolloidal gold technique

Macrophages from both rodent and human sources have been shown to produce lipoprotein lipase (LPL), the enzyme activity of which can be measured in culture media and in cellular homogenates. The studies reported here show the presence of LPL on the surface of human monocyte-derived macrophages. An inhibitory monoclonal antibody to human LPL was used for cellular and immunoelectron microscopy studies. This antibody is a competitive inhibitor of LPL hydrolysis of triacylglycerol but does not inhibit LPL hydrolysis of a water-soluble substrate, p-nitrophenyl acetate. Furthermore, when postheparin plasma was mixed with monoclonal antibody prior to gel filtration on 6% agarose, the LPL activity eluted with the lipoproteins and was not inhibited by the antibody. These studies suggest that the antibody recognized the lipid/lipoprotein binding site of the LPL molecule. Membrane-bound LPL was demonstrated on human monocyte-derived macrophages using colloidal gold-protein A to detect the monoclonal antibody to LPL. The surface colloidal gold was randomly distributed with a surface density of 56,700 gold particles per cell. Control cells cultured in heparin-containing media (10 units/ml) or cells reacted with anti-hepatic triacylglycerol lipase monoclonal IgG or nonimmune mouse IgG did not exhibit membrane binding of protein A-gold. Macrophages were incubated with control and monoclonal anti-LPL IgGs and 125I-labeled anti-mouse IgG F(ab')2. Heparin-releasable membrane-bound anti-LPL antibody was demonstrated. These studies demonstrate the presence of LPL on the surface of human monocyte-derived macrophages, such that the LPL is oriented with its lipid-binding portion (recognized by this antibody) exposed. Membrane-associated LPL may be important in the interaction and subsequent uptake of lipid and lipoproteins by macrophages and in the generation of atherosclerotic foam cells.     

Separation of molecular species of lipoprotein lipase from adipose tissue

When NH(4)OH-NH(4)Cl extracts of adipose acetone powder were applied to agarose gel chromatography columns, two peaks of lipoprotein lipase were eluted. The first activity peak (LPL(a)) was eluted with an elution volume of a protein of molecular weight approximately five times that of the second (LPL(b)). Addition of heparin to the eluted fractions markedly stimulated activity of LPL(a), but suppressed that of LPL(b). Both lipases had the characteristics that distinguish lipoprotein lipase from other tissue lipases: a requirement for serum for substrate activation, inhibition by 1 m NaCl, and an alkaline pH optimum (pH 8.0). It is concluded that these fractions represent two species of lipoprotein lipase.     

Treatment of cardiac myocytes with 8-(4-chlorophenylthio)-adenosine 3'',5''-cyclic monophosphate, forskolin or cholera toxin does not stimulate cellular or heparin-releasable lipoprotein lipase activities.

Incubation of isolated cardiac myocytes with 500 microM-8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP) or 100 microM-forskolin for 2 1/2 h did not increase the heparin-induced release of lipoprotein lipase (LPL) into the medium. When LPL activity in cardiac myocytes was depleted by treatment of rats with cycloheximide (2 mg/kg; 2.5 h) and inclusion of the protein-synthesis inhibitor in the isolation solutions, incubation with CPT-cAMP or forskolin did not influence the rate of repletion of LPL activity in cells or the recovery of heparin-releasable LPL activity. Although the administration of cholera toxin (0.5 mg/kg; 16-17 h) to rats increased LPL activity in a low-speed supernatant fraction from heparin-perfused hearts, LPL activity was not increased in cardiac myocytes from cholera-toxin-treated rat hearts, and the heparin-induced release of LPL was unchanged. Incubation of cultured ventricular myocytes with 1 microgram of cholera toxin/ml or 500 microM-CPT-cAMP for 24 h did not increase cellular LPL activity or LPL released into the culture medium after a 40 min incubation with heparin. Therefore interventions that stimulate adenylate cyclase activity (forskolin, cholera toxin) or incubation with CPT-cAMP do not increase cellular LPL activity or promote the translocation of LPL to a heparin-releasable fraction in cardiac myocytes.     

Oral nicotine induces an atherogenic lipoprotein profile

Male squirrel monkeys were used to evaluate the effect of chronic oral nicotine intake on lipoprotein composition and metabolism. Eighteen yearling monkeys were divided into two groups: 1) Controls fed isocaloric liquid diet; and 2) Nicotine primates given liquid diet supplemented with nicotine at 6 mg/kg body wt/day. Animals were weighed biweekly, plasma lipid, glucose, and lipoprotein parameters were measured monthly, and detailed lipoprotein composition, along with postheparin plasma lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) activity, was assessed after 24 months of treatment. Although nicotine had no effect on plasma triglyceride or high density lipoproteins (HDL), the alkaloid caused a significant increase in plasma glucose, cholesterol, and low density lipoprotein (LDL) cholesterol plus protein while simultaneously reducing the HDL cholesterol/plasma cholesterol ratio and animal body weight. Levels of LDL precursors, very low density (VLDL) and intermediate density (IDL) lipoproteins, were also lower in nicotine-treated primates while total postheparin lipase (LPL + HTGL) activity was significantly elevated. Our data indicate that long-term consumption of oral nicotine induces an atherogenic lipoprotein profile (increases LDL, decreases HDL/total cholesterol ratio) by enhancing lipolytic conversion of VLDL to LDL. These results have important health implications for humans who use smokeless tobacco products or chew nicotine gum for prolonged periods.     

Overexpression of human apolipoprotein A-II in mice induces hypertriglyceridemia due to defective very low density lipoprotein hydrolysis

Two lines of transgenic mice, hAIItg-delta and hAIItg-lambda, expressing human apolipoprotein (apo)A-II at 2 and 4 times the normal concentration, respectively, displayed on standard chow postprandial chylomicronemia, large quantities of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) but greatly reduced high density lipoprotein (HDL). Hypertriglyceridemia may result from increased VLDL production, decreased VLDL catabolism, or both. Post-Triton VLDL production was comparable in transgenic and control mice. Postheparin lipoprotein lipase (LPL) and hepatic lipase activities decreased at most by 30% in transgenic mice, whereas adipose tissue and muscle LPL activities were unaffected, indicating normal LPL synthesis. However, VLDL-triglyceride hydrolysis by exogenous LPL was considerably slower in transgenic compared with control mice, with the apparent Vmax of the reaction decreasing proportionately to human apoA-II expression. Human apoA-II was present in appreciable amounts in the VLDL of transgenic mice, which also carried apoC-II. The addition of purified apoA-II in postheparin plasma from control mice induced a dose-dependent decrease in LPL and hepatic lipase activities. In conclusion, overexpression of human apoA-II in transgenic mice induced the proatherogenic lipoprotein profile of low plasma HDL and postprandial hypertriglyceridemia because of decreased VLDL catabolism by LPL.     

Apolipoprotein VLDL-II inhibits lipolysis of triglyceride-rich lipoproteins in the laying hen

In the laying hen, very low density lipoprotein (VLDL) particles contain large amounts of apolipoprotein (apo)-VLDL-II in addition to apoB. These triglyceride-rich lipoproteins are transported from the liver primarily to growing oocytes. Since no appreciable hydrolysis of triglyceride occurs during this transport, we have investigated the possibility that apoVLDL-II functions as an inhibitor of lipoprotein lipase (LPL). The presence of LPL in chicken follicular granulosa cells was demonstrated by immunoblotting, and LPL activity with the usual in vitro characteristics could be measured in cultured granulosa cell extracts. ApoVLDL-II inhibited LPL activity in these extracts as well as in the post-heparin medium of rat cardiac myocytes. Half-maximal inhibition in both systems occurred at 40 micrograms/ml, a concentration that is one-tenth of the circulating apoVLDL-II in the laying hen. Much less inhibition was observed with reduced and alkylated apoVLDL-II and with apoA-I. We conclude that the presence of apoVLDL-II on laying hen VLDL ensures efficient delivery of triglyceride to the oocyte for subsequent use as energy source by the embryo.