Abnormalities of very low density lipoprotein apolipoprotein B-100 metabolism contribute to the dyslipidaemia of adult growth hormone deficiency

Increased cardiovascular mortality in adult growth hormone deficiency (GHD) may be, in part, explained by the dyslipidaemia associated with this condition. It is possible that abnormalities of very low density lipoprotein apolipoprotein B-100 (VLDL apoB) metabolism contribute to this dyslipidaemia. To test this hypothesis, we measured VLDL apoB kinetics in adult GH deficient patients (4 females, 3 males; age 50.1 +/- 4.7 yr (mean +/- SEM); BMI 28.2 +/- 1.1 kg/m2; total cholesterol (TC) 6.6 +/- 0.3 mmol/l; triglyceride (TG) 2.8 +/- 0.6 mmol/l; HDL cholesterol 1.1 +/- 0.1 mmol/l) and in control subjects (4 females, 3 male; age 47.0 +/- 4.7 yr; BMI 27.0 +/- 2.6 kg/m2; TC 5.0 +/- 0.4 mmol/l; TG 0.9 +/- 0.2 mmol/l; HDL cholesterol 1.4 +/- 0.1 mmol/l). [1-(13)C] leucine was administered by a primed (1 mg/kg), constant intravenous infusion (1 mg/kg/hr) and VLDL apoB enrichment with 13C leucine was determined using gas-chromatography mass-spectrometry. The GHD patients had a significantly higher hepatic secretion rate of VLDL apoB (15.5 +/- 1.8 mg/kg/day vs 9.4 +/- 0.6 mg/kg/day p = 0.007) and reduced catabolism ofVLDL apoB (metabolic clearance rate; 12.3 +/- 1.7 ml/min vs 24.3 +/- 4.8 ml/min p < 0.05) compared with control subjects. These findings suggest that GH is integrally involved in the regulation of VLDL apoB metabolism.

Dusty punch cards and an eternal enigma: high-density lipoproteins and atherosclerosis

Epidemiological, clinical, and experimental evidence has accumulated during the last decades suggesting that high-density lipoproteins (HDLs) may protect from atherosclerosis and its clinical consequences. However, more than 55 years after the first description of the link between HDL and heart attacks, many facets of the biochemistry, function, and clinical significance of HDL remain enigmatic. This applies particularly to the completely unexpected results that became available from some recent clinical trials of nicotinic acid and of inhibitors of cholesteryl ester transfer protein (CETP). The concept that raising HDL cholesterol by pharmacological means would decrease the risk of vascular disease has therefore been challenged.

Untainted puncturing for irregular low-density parity-check codes

Puncturing is a well-known coding technique widely used for constructing rate-compatible codes. In this paper, we consider the problem of puncturing low-density parity-check codes and propose a new algorithm for intentional puncturing. The algorithm is based on the puncturing of untainted symbols, i.e. nodes with no punctured symbols within their neighboring set. It is shown that the algorithm proposed here performs better than previous proposals for a range of coding rates and short proportions of punctured symbols.

Low-density InAs QDs with subcritical coverage obtained by conversion of In nanocrystals

We report growth of InAs/GaAs quantum dots (QDs) by molecular beam epitaxy with low density of 2 μm−2 by conversion of In nanocrystals deposited at low temperatures. The total amount of InAs used is about one monolayer, which is less than the critical thickness for conventional Stranski–Krastanov QDs. We also demonstrate the importance of the starting surface reconstruction for obtaining uniform QDs. The QD emission wavelength is easily tunable upon post-growth annealing with no wetting layer signal visible for short anneals. Microphotoluminescence measurements reveal well separated and sharp emission lines of individual QDs.

Efficient information reconciliation for Continuous-Variable QKD using Non-Binary Low-Density Parity-Check Codes

We present here an information reconciliation method and demonstrate for the first time that it can achieve efficiencies close to 0.98. This method is based on the belief propagation decoding of non-binary LDPC codes over finite (Galois) fields. In particular, for convenience and faster decoding we only consider power-of-two Galois fields.

Low density lipoprotein receptor-negative mice expressing human apolipoprotein B-100 develop complex atherosclerotic lesions on a chow diet: No accentuation by apolipoprotein(a)

We have generated mice with markedly elevated plasma levels of human low density lipoprotein (LDL) and reduced plasma levels of high density lipoprotein. These mice have no functional LDL receptors [LDLR−/−] and express a human apolipoprotein B-100 (apoB) transgene [Tg(apoB+/+)] with or without an apo(a) transgene [Tg(apoa+/−)]. Twenty animals (10 males and 10 females) of each of the following four genotypes were maintained on a chow diet: (i) LDLR−/−, (ii) LDLR−/−;Tg(apoa+/−), (iii) LDLR−/−;Tg(apoB+/+), and (iv)LDLR−/−;Tg(apoB+/+);Tg(apo+/−). The mice were killed at 6 mo, and the percent area of the aortic intimal surface that stained positive for neutral lipid was quantified. Mean percent areas of lipid staining were not significantly different between the LDLR−/− and LDLR−/−;Tg(apoa+/−) mice (1.0 ± 0.2% vs. 1.4 ± 0.3%). However, the LDLR−/−;Tg(apoB+/+) mice had ≈15-fold greater mean lesion area than the LDLR−/− mice. No significant difference was found in percent lesion area in the LDLR−/−;Tg(apoB+/+) mice whether or not they expressed apo(a) [18.5 ± 2.5%, without lipoprotein(a), Lp(a), vs. 16.0 ± 1.7%, with Lp(a)]. Histochemical analyses of the sections from the proximal aorta of LDLR−/−;Tg(apoB+/+) mice revealed large, complex, lipid-laden atherosclerotic lesions that stained intensely with human apoB-100 antibodies. In mice expressing Lp(a), large amounts of apo(a) protein colocalized with apoB-100 in the lesions. We conclude that LDLR−/−; Tg(apoB+/+) mice exhibit accelerated atherosclerosis on a chow diet and thus provide an excellent animal model in which to study atherosclerosis. We found no evidence that apo(a) increased atherosclerosis in this animal model.

Hepatitis C virus and other Flaviviridae viruses enter cells via low density lipoprotein receptor

Endocytosis of the Flaviviridae viruses, hepatitis C virus, GB virus C/hepatitis G virus, and bovine viral diarrheal virus (BVDV) was shown to be mediated by low density lipoprotein (LDL) receptors on cultured cells by several lines of evidence: by the demonstration that endocytosis of these virus correlated with LDL receptor activity, by complete inhibition of detectable endocytosis by anti-LDL receptor antibody, by inhibition with anti-apolipoprotein E and -apolipoprotein B antibodies, by chemical methods abrogating lipoprotein/LDL receptor interactions, and by inhibition with the endocytosis inhibitor phenylarsine oxide. Confirmatory evidence was provided by the lack of detectable LDL receptor on cells known to be resistant to BVDV infection. Endocytosis via the LDL receptor was shown to be mediated by complexing of the virus to very low density lipoprotein or LDL but not high density lipoprotein. Studies using LDL receptor-deficient cells or a cytolytic BVDV system indicated that the LDL receptor may be the main but not exclusive means of cell entry of these viruses. Studies on other types of viruses indicated that this mechanism may not be exclusive to Flaviviridae but may be used by viruses that associate with lipoprotein in the blood. These findings provide evidence that the family of LDL receptors may serve as viral receptors.

Two sterol regulatory element-like sequences mediate up-regulation of caveolin gene transcription in response to low density lipoprotein free cholesterol

Caveolae form the terminus for a major pathway of intracellular free cholesterol (FC) transport. Caveolin mRNA levels in confluent human skin fibroblasts were up-regulated following increased uptake of low density lipoprotein (LDL) FC. The increase induced by FC was not associated with detectable change in mRNA stability, indicating that caveolin mRNA levels were mediated at the level of gene transcription. A total of 924 bp of 5′ flanking region of the caveolin gene were cloned and sequenced. The promoter sequence included three G+C-rich potential sterol regulatory elements (SREs), a CAAT sequence and a Sp1 consensus sequence. Deletional mutagenesis of individual SRE-like sequences indicated that of these two (at −646 and −395 bp) were essential for the increased transcription rates mediated by LDL-FC, whereas the third was inconsequential. Gel shift analysis of protein binding from nuclear extracts to these caveolin promoter DNA sequences, together with DNase I footprinting, confirmed nucleoprotein binding to the SRE-like elements as part of the transcriptional response to LDL-FC. A supershift obtained with antibody to SRE-binding protein 1 (SPEBP-1) indicated that this protein binds at −395 bp. There was no reaction at −395 bp with anti-Sp1 antibody nor with either antibody at −646 bp. The cysteine protease inhibitor N-acetyl-leu-leu-norleucinal (ALLN), which inhibits SREBP catabolism, superinhibited caveolin mRNA levels regardless of LDL-FC. This finding suggests that SREBP inhibits caveolin gene transcription in contrast to its stimulating effect on other promoters. The findings of this study are consistent with the postulated role for caveolin as a regulator of cellular FC homeostasis in quiescent peripheral cells, and the coordinate regulation by SREBP of FC influx and efflux.

Acceleration of atherogenesis by COX-1-dependent prostanoid formation in low density lipoprotein receptor knockout mice

The cyclooxygenase (COX) product, prostacyclin (PGI2), inhibits platelet activation and vascular smooth-muscle cell migration and proliferation. Biochemically selective inhibition of COX-2 reduces PGI2 biosynthesis substantially in humans. Because deletion of the PGI2 receptor accelerates atherogenesis in the fat-fed low density lipoprotein receptor knockout mouse, we wished to determine whether selective inhibition of COX-2 would accelerate atherogenesis in this model. To address this hypothesis, we used dosing with nimesulide, which inhibited COX-2 ex vivo, depressed urinary 2,3 dinor 6-keto PGF1α by approximately 60% but had no effect on thromboxane formation by platelets, which only express COX-1. By contrast, the isoform nonspecific inhibitor, indomethacin, suppressed platelet function and thromboxane formation ex vivo and in vivo, coincident with effects on PGI2 biosynthesis indistinguishable from nimesulide. Indomethacin reduced the extent of atherosclerosis by 55 ± 4%, whereas nimesulide failed to increase the rate of atherogenesis. Despite their divergent effects on atherogenesis, both drugs depressed two indices of systemic inflammation, soluble intracellular adhesion molecule-1, and monocyte chemoattractant protein-1 to a similar but incomplete degree. Neither drug altered serum lipids and the marked increase in vascular expression of COX-2 during atherogenesis. Accelerated progression of atherosclerosis is unlikely during chronic intake of specific COX-2 inhibitors. Furthermore, evidence that COX-1-derived prostanoids contribute to atherogenesis suggests that controlled evaluation of the effects of nonsteroidal anti-inflammatory drugs and/or aspirin on plaque progression in humans is timely.

Myofibroblasts differentiate from fibroblasts when plated at low density.

Myofibroblasts, defined by their expression of smooth muscle alpha-actin, appear at corneal and dermal incisions and promote wound contraction. We report here that cultured fibroblasts differentiate into myofibroblasts by a cell density-dependent mechanism. Fibroblasts seeded at low density (5 cells per mm2) produced a cell culture population consisting of 70-80% myofibroblasts, 5-7 days after seeding. In contrast, fibroblasts seeded at high density (500 cells per mm2) produced cultures with only 5-10% myofibroblasts. When the myofibroblast-enriched cultures were subsequently passaged at high density, the smooth muscle alpha-actin phenotype was lost within 3 days. Furthermore, initially 60% of the low density-cultured cells incorporated BrdUrd compared to 30% of cells passaged at high density. Media from myofibroblast-enriched cultures had more latent and active transforming growth factor beta (TGF-beta) than did media from fibroblast-enriched cultures. Although there was a trend towards increased numbers of myofibroblasts after addition of exogenous TGF-beta, the results did not reach statistical significance. We conclude that myofibroblast differentiation can be induced in fibroblasts by plating at low density. We propose a cell density-dependent model of myofibroblast differentiation during wounding and healing in which at least two factors interact: loss of cell contact and the presence of TGF-beta.

Mutant oocytic low density lipoprotein receptor gene family member causes atherosclerosis and female sterility.

The so-called very low density lipoprotein receptors (VLDLRs) are related to the LDLR gene family. So far, naturally occurring mutations have only been described for the prototype LDLR; in humans, they cause familial hypercholesterolemia. Here we describe a naturally occurring mutation in a VLDLR that causes a dramatic abnormal phenotype. Hens of the mutant restricted-ovulator chicken strain carry a single mutation, lack functional oocyte receptors, are sterile, and display severe hyperlipidemia with associated premature atherosclerosis. The mutation converts a cysteine residue into a serine, resulting in an unpaired cysteine and greatly reduced expression of the mutant avian VLDLR on the oocyte surface. Extraoocytic cells in the mutant produce higher than normal amounts of a differentially spliced form of the receptor that is characteristic for somatic cells but absent from germ cells.

Older plasma lipoproteins are more susceptible to oxidation: a linking mechanism for the lipid and oxidation theories of atherosclerotic cardiovascular disease.

Increases in plasma cholesterol are associated with progressive increases in the risk of atherosclerotic cardiovascular disease. In humans plasma cholesterol is contained primarily in apolipoprotein B-based low density lipoprotein (LDL). Cells stop making the high-affinity receptor responsible for LDL removal as they become cholesterol replete; this slows removal of LDL from plasma and elevates plasma LDL. As a result of this delayed uptake, hypercholesterolemic individuals not only have more LDL but have significantly older LDL. Oxidative modification of LDL enhances their atherogenicity. This study sought to determine whether increased time spent in circulation, or aging, by lipoprotein particles altered their susceptibility to oxidative modification. Controlled synchronous production of distinctive apolipoprotein B lipoproteins (yolk-specific very low density lipoproteins; VLDLy) with a single estrogen injection into young turkeys was used to model LDL aging in vivo. VLDLy remained in circulation for at least 10 days. Susceptibility to oxidation in vitro was highly dependent on lipoprotein age in vivo. Oxidation, measured as hexanal release from n-6 fatty acids in VLDLy, increased from 13.3 +/- 5.5 nmol of 2-day-old VLDLy per ml, to 108 +/- 17 nmol of 7-day-old VLDLy per ml. Oxidative instability was not due to tocopherol depletion or conversion to a more unsaturated fatty acid composition. These findings establish mathematically describable linkages between the variables of LDL concentration and LDL oxidation. The proposed mathematical models suggest a unified investigative approach to determine the mechanisms for acceleration of atherosclerotic cardiovascular disease risk as plasma cholesterol rises.

Domains of transcription factor Sp1 required for synergistic activation with sterol regulatory element binding protein 1 of low density lipoprotein receptor promoter.

Feedback regulation of transcription from the low density lipoprotein (LDL) receptor gene is fundamentally important in the maintenance of intracellular sterol balance. The region of the LDL receptor promoter responsible for normal sterol regulation contains adjacent binding sites for the ubiquitous transcription factor Sp1 and the cholesterol-sensitive sterol regulatory element-binding proteins (SREBPs). Interestingly, both are essential for normal sterolmediated regulation of the promoter. The cooperation by Sp1 and SREBP-1 occurs at two steps in the activation process. SREBP-1 stimulates the binding of Sp1 to its adjacent recognition site in the promoter followed by enhanced stimulation of transcription after both proteins are bound to DNA. In the present report, we have defined the protein domains of Sp1 that are required for both synergistic DNA binding and transcriptional activation. The major activation domains of Sp1 that have previously been shown to be essential to activation of promoters containing multiple Sp1 sites are required for activation of the LDL receptor promoter. Additionally, the C domain is also crucial. This slightly acidic approximately 120-amino acid region is not required for efficient synergistic activation by multiple Sp1 sites or in combination with other recently characterized transcriptional regulators. We also show that Sp1 domain C is essential for full, enhanced DNA binding by SREBP-1. Taken together with other recent studies on the role of Sp1 in promoter activation, the current experiments suggest a unique combinatorial mechanism for promoter activation by two distinct transcription factors that are both essential to intracellular cholesterol homeostasis.