Receptor-dependent and receptor-independent degradation of low density lipoprotein in normal rabbits and in receptor-deficient mutant rabbits

Low density lipoprotein (LDL) catabolism was studied using WHHL rabbits, an inbred strain deficient in LDL receptor activity and, thus, an animal model for homozygous familial hypercholesterolemia. WHHL and normal rabbits were injected with [14C]sucrose-LDL and the tissue sites of LDL degradation were determined 24 h later. On degradation of [14C]sucrose-LDL, the [14C]sucrose ligand remains trapped within tissues as a cumulative measure of degradation. The fractional catabolic rate of [14C]sucrose-LDL in Watanabe heritable hyperlipidemic (WHHL) rabbits was reduced (0.024 +/- 0.010 versus 0.063 +/- 0.026 h-1) but, by virtue of the increased plasma pool, total LDL flux was increased (33.5 +/- 9.6 versus 10.6 +/- 4.4 mg of LDL protein/kg/day). Liver was the predominant site of catabolism in both WHHL and normal rabbits (52.7 +/- 6.9 and 56.6 +/- 6.2% of total degradation). About 90% of hepatic catabolism was attributable to parenchymal cells in both cases. Thus, Kupffer cells, a major component of the reticuloendothelial system, do not play a major role in LDL catabolism in WHHL rabbits. Despite receptor deficiency, the relative contribution of various tissues to overall LDL degradation was not greatly altered and the absolute rate of delivery of LDL to all tissues was increased with the exception of the adrenal. Thus, there was no evidence that the increased degradation occurred in any special subset of "scavenger" cells. Nevertheless, local scavenger cell uptake may be critically important, especially in atherogenesis. If it is assumed that receptor-independent degradation occurs at the same rate in the tissues of WHHL and normal rabbits and that catabolism in the absence of receptors is a linear function of concentration, then one can estimate the fraction of uptake in normal tissues mediated by receptors. The difference in the fraction of the plasma LDL pool cleared per unit of time in normal and WHHL rabbits would reflect the contribution of receptors to fractional clearance. By this calculation, receptor-mediated degradation in normal rabbits was 62% overall, 63% in liver, 92% in adrenal, and 83% in gut.     

Turnover of low density lipoproteins in normal and atherosclerotic rabbits

The turnover and composition of normal and hyperlipemic (h.l.) low density lipoproteins (LDL) of rabbits, were studied. They were obtained by ultracentrifugation and labeled by Bolton and Hunter method. Normal and h.l. LDL labeled with 125I were injected directly and crossed to both groups of rabbits. Normal and h.l. LDL had a different protein/lipid ratio. The analysis of fractional catabolic rate of LDL and the half-life of the phases of rapid and slow decay, show that h.l. LDL had a fractional catabolic rate that is the half of normal LDL and an increased half life of the phases of rapid and slow decay. Apparently, two factors: a) defective LDL receptor in the h.l. rabbit and b) different physico-chemical properties between normal and h.l. LDL, would be the reason for this difference. Besides, when normal and h.l. 125I LDL were injected into h.l. and normal rabbit, respectively, LDL changed according to the injected rabbit, as can be deduced from the analysis of the half life of the phase of slow decay.     

The sites of degradation of purified rat low density lipoprotein and high density lipoprotein in the rat

Low density lipoprotein and high density lipoprotein were isolated from rat serum by sequential ultracentrifugation in the density intervals 1.025-1.050 g/ml and 1.125-1.21 g/ml, respectively. The isolated lipoproteins were radioiodinated using ICl. Low density lipoprotein was further purified by concanavalin A affinity chromatography and concentrated by ultracentrifugation. 95% of the purified low density lipoprotein radioactivity was precipitable by tetramethylurea, while only 4% was associated with lipids. The radioiodinated high density lipoprotein was incubated for 1 h at 4 degrees C with unlabelled very low density lipoprotein, followed by reisolation by sequential ultracentrifugation. Only 3% of the radioactivity was associated with lipids and 90% was present on apolipoprotein A-I. The serum decay curves of labelled and subsequently purified rat low and high density lipoprotein, measured over a period of 28 h, clearly exhibited more than one component, in contrast to the monoexponential decay curves of iodinated human low density lipoprotein. The decay curves were not affected by the methods used to purify the LDL and HDL preparations. The catabolic sites of the labelled rat lipoproteins were analyzed in vivo using leupeptin-treated rats. In vivo treatment of rats with leupeptin did not affect the rate of disappearance from serum of intravenously injected labelled rat low density lipoprotein and high density lipoprotein. Leupeptin-dependent accumulation of radioiodine occurred almost exclusively in the liver after intravenous injection of iodinated low density lipoprotein, while both the liver and the kidneys showed leupeptin-dependent accumulation of radioactivity after injection of iodinated high density lipoprotein.     

Turnover and tissue distribution of 125-I-labeled low density lipoprotein in swine and dogs.

Swine plasma low density lipoprotein (LDL) isolated ultracentrifugally (d 1.019-1.063) was labeled with 125-I, dialyzed, and reisolated by centrifugation at d 1.063. Over 96% of the radioactivity was shown to be associated with the apoprotein. After reinjection into the donor animal, disapperance of 125-I was followed for up to 122 hr. At all time intervals examined, over 95% of the total plasma 125-I was recovered in LDL (D 1.006-1.063), i.e., there was apparently no transfer of radioactivity to high density or very low density lipoproteins. The disappearance curve was biexponential, with half-lives of 0.83 plus or minus 0.06 and 22.5 plus or minus 1.7 hr for the first and second phases, respectively (13 studies). The mean calculated fractional catabolic rate was 0.041 plus or minus 0.003 hr-minus 1. Similar results were obtained in three dogs using autologous LDL of density 1.020-1.050; fractional catabolic rates were 0.031, 0.031, and 0.029 hr-minus 1. Tissue distribution of 125-I was determined in swine killed at various time intervals after [125-I]LDL injection with corrections for radioactivity in trapped plasma. Of the tissues examined, the liver showed by far the highest concentration. Total hepatic radioactivity, expressed as a percentage of total plasma radioactivity, was rather constant and independent of the time of killing from 3 to 122 hr (15.8 plus or minus 1.9%). The total extravascular LDL pool calculated from analysis of the plasma disappearance curves was about 20-30% of the size of the plasma LDL pool. These data are consistent with the conclusion that the liver accounts for a very large fraction of the total extravascular LDL pool. These data are consistent with the conclusion that the liver accounts for a very large fraction of the total extravascular LDL pool and that it is infairly rapid equilibrium with the plasma pool. To what extent the liver is involved in irreversible degradation cannot be inferred from these findings.     

Comparison of plasma clearance of low density lipoprotein with beta-very low density lipoprotein or acetoacetylated low density lipoprotein in cholesterol-fed rabbits

The plasma clearance and tissue distribution of radioiodinated low-density lipoprotein (LDL), beta-very low density lipoprotein (beta-VLDL), and acetoacetylated LDL were studied in cholesterol-fed rabbits. Radioiodinated LDL ([125I]LDL) was cleared more slowly than either [125I]beta-VLDL or acetoacetylated-[125I]LDL and its fractional catabolic rate was one-half that of [125I]beta-VLDL and one-ninth that of acetoacetylated-[125I]LDL. Forty-eight hours after the injection of the labeled lipoproteins, the hepatic uptake was the greatest among the organs evaluated with the uptake of [125I]LDL being one-third that of either [125I]beta-VLDL or acetoacetylated-[125I]LDL. The reduction in the hepatic uptake of LDL due to a down-regulation of the receptors would account for this retarded plasma clearance.     

Is hypertriglyceridemic very low density lipoprotein a precursor of normal low density lipoprotein?

The precursor-product relationship of very low density (VLDL) and low density lipoproteins (LDL) was studied. VLDL obtained from normal (NTG) and hypertriglyceridemic (HTG) subjects was fractionated by zonal ultracentrifugation and subjected to in vitro lipolysis. The individual subfractions and their isolated lipolysis products, as well as IDL and LDL, were rigorously characterized. A striking difference in the contribution of cholesteryl ester to VLDL is noted. In NTG subfractions, the cholesteryl ester to protein ratio increases with decreasing density (VLDL-I----VLDL-III). This is the expected result of triglyceride loss through lipolysis and cholesteryl ester gain through core-lipid transfer protein action. In HTG subfractions there is an abnormal enrichment of cholesteryl esters that is most marked in VLDL-I and nearly absent in VLDL-III. Thus, the trend of the cholesteryl ester to protein ratios is reversed, being highest in HTG-VLDL-I and lowest in VLDL-III. This is incompatible with the precursor-product relationship described by the VLDL----IDL----LDL cascade. In vitro lipolysis studies support the conclusion that not all HTG-VLDL can be metabolized to LDL. While all NTG subfractions yield products that are LDL-like in size, density, and composition, only HTG-VLDL-III, whose composition is most similar to normal, does so. HTG VLDL-I and VLDL-II products are large and light populations that are highly enriched in cholesteryl ester. We suggest that this abnormal enrichment of HTG-VLDL with cholesteryl ester results from the prolonged action of core-lipid transfer protein on the slowly metabolized VLDL mass. This excess cholesteryl ester load, unaffected by the process of VLDL catabolism, remains entrapped within the abnormal particle. Therefore, lipolysis yields an abnormal, cholesteryl ester-rich product that can never become LDL.     

Increased low density lipoprotein degradation in aorta of irradiated mice is inhibited by preenrichment of low density lipoprotein with alpha-tocopherol

We previously reported that upper thoracic exposure to ionizing radiation (IR) accelerates fatty streak formation in C57BL/6 mice and that such effects are inhibited by overexpression of the antioxidant enzyme CuZn-superoxide dismutase (SOD). Notably, IR-accelerated lesion formation is strictly dependent on a high fat diet (i.e., atherogenic lipoproteins) but does not involve alterations in circulating lipid or lipoprotein levels. We thus proposed that IR promotes changes in the artery wall that enhance the deposition of lipoprotein lipids. To address this hypothesis, we examined the effects of IR on aortic accumulation and degradation of low density lipoproteins (LDL). Ten-week-old C57BL/6 mice were exposed to a single (8-Gy) dose of (60)Co radiation to the upper thoracic area or were sham irradiated (controls) and were then placed on the high fat diet. Five days postexposure, the mice received either (125)I-labeled LDL ((125)I-LDL) (which was used to measure intact LDL) or (125)I-labeled tyramine cellobiose ((125)I-TC)-LDL (which was used to measure both intact and cell-degraded LDL) via tail vein injection. On the basis of trichloroacetic acid (TCA)-precipitable counts in retroorbital blood samples, > or =95% of donor LDL was cleared within 24 h and there were no differences in time-averaged plasma concentrations of the two forms of LDL among irradiated and control mice. Aortic values increased markedly within the first hour and thereafter exhibited a slow increase up to 24 h. There were no differences between irradiated and control mice at 1 h, when values primarily reflected LDL entry, but a divergence was observed thereafter. At 24 h, (125)I-TC-associated counts were 1.8-fold higher in irradiated mice (P = 0.10). In contrast, (125)I-LDL-associated counts were 30% lower in irradiated mice (P< 0.05), suggesting that most of the retained (125)I-TC was associated with LDL degradation products. Consistent with the proposed involvement of oxidative or redox-regulated events, IR-induced LDL degradation was lower in SOD-transgenic than wild-type mice (P<0.05). The importance of LDL oxidation was suggested by observations that IR-induced LDL degradation was significantly reduced by preenriching LDL with alpha-tocopherol. On the basis of these results, we propose that IR elicits SOD-inhibitable changes in the artery wall that enhance LDL oxidation and degradation leading to the deposition of LDL-borne lipids. These studies provide additional support for the role of oxidation in lipoprotein lipid deposition and atherogenesis and suggest that IR promotes an arterial environment that stimulates this process in vivo.     

Effect of insulin deficiency on low density lipoprotein metabolism in rabbits

These studies have been carried out in rabbits with alloxan-induced diabetes in order to see if insulin deficiency affects low density lipoprotein (LDL) catabolism. The results showed that plasma LDL-cholesterol was lower in diabetic rabbits, associated with a fall in the cholesterol to protein ratio of LDL particles. In addition, 125I-LDL disappeared more slowly from plasma of diabetic rabbits, leading to a significant reduction in fractional catabolic rate and a decrease in residence time of 125I-LDL. These data demonstrated that LDL composition and catabolism are greatly altered as a consequence of insulin deficiency.     

Characterization of very low density lipoprotein from Watanabe heritable hyperlipidemic rabbits

We investigated the properties of very low density lipoprotein (VLDL) from two types of Watanabe heritable hyperlipidemic (WHHL) rabbits: one with a high incidence of coronary atherosclerosis (type 1), and the other with a low incidence (type 2). When incubated with mouse peritoneal macrophages, VLDL from type 1 WHHL rabbit (type 1-VLDL) stimulated cholesteryl ester synthesis 10.5-fold more than VLDL from the type 2 WHHL rabbit (type 2-VLDL) did. Moreover, a similar difference was seen in the stimulation of cholesteryl ester synthesis in peritoneal macrophages isolated from the WHHL rabbits. The mass ratios of cholesterol to protein in type 1- and type 2-VLDL were 5.69 and 2.05, respectively. Agarose gel electrophoresis of type 1-VLDL showed beta mobility, and that of type 2-VLDL showed pre-beta mobility. No difference was seen between the sizes of VLDL particles of the two types. The amount of apolipoprotein E in type 1-VLDL was greater than that in type 2-VLDL. In conclusion, the difference between type 1 and type 2 WHHL rabbits is at least partly due to the presence in type 1 animals of VLDL particles rich in cholesteryl esters and apolipoprotein E, particles which are very similar to beta-VLDL in conformation.     

High density lipoprotein metabolism in low density lipoprotein receptor-deficient mice

The LDL receptor (LDLR) and scavenger receptor class B type I (SR-BI) play physiological roles in LDL and HDL metabolism in vivo. In this study, we explored HDL metabolism in LDLR-deficient mice in comparison with WT littermates. Murine HDL was radiolabeled in the protein (¹²⁵I) and in the cholesteryl ester (CE) moiety ([³H]). The metabolism of ¹²⁵I-/[³H]HDL was investigated in plasma and in tissues of mice and in murine hepatocytes. In WT mice, liver and adrenals selectively take up HDL-associated CE ([³H]). In contrast, in LDLR^−/− mice, selective HDL CE uptake is significantly reduced in liver and adrenals. In hepatocytes isolated from LDLR^−/− mice, selective HDL CE uptake is substantially diminished compared with WT liver cells. Hepatic and adrenal protein expression of lipoprotein receptors SR-BI, cluster of differentiation 36 (CD36), and LDL receptor-related protein 1 (LRP1) was analyzed by immunoblots. The respective protein levels were identical both in hepatic and adrenal membranes prepared from WT or from LDLR^−/− mice. In summary, an LDLR deficiency substantially decreases selective HDL CE uptake by liver and adrenals. This decrease is independent from regulation of receptor proteins like SR-BI, CD36, and LRP1. Thus, LDLR expression has a substantial impact on both HDL and LDL metabolism in mice.     

Role of receptor-independent low density lipoprotein transport in the maintenance of tissue cholesterol balance in the normal and WHHL rabbit

These studies were undertaken to determine the role of receptor-independent low density lipoprotein (LDL) transport in cholesterol balance across individual tissues and the whole animal. Homologous LDL, which measures total LDL transport, and methylated heterologous LDL, which measures receptor-independent LDL uptake, were cleared from the plasma at very different rates in the NZ control rabbit (3,900 and 1,010 microliter/hr per kg, respectively) whereas in the WHHL rabbit both preparations were cleared at essentially the same rate (approximately 1,070 microliter/hr per kg). Receptor-independent LDL clearance was detected in all tissues of the NZ control rabbit and these varied from 32 (spleen) to less than 0.5 (skeletal muscle) microliter/hr per g. In contrast, receptor-dependent LDL uptake was found in only about half of these same organs. In the WHHL rabbit, the rates of receptor-independent LDL transport were the same as in the NZ control rabbit, but no receptor-dependent uptake was detected. Using these clearance values it was calculated that in the control rabbit nearly 70% of LDL-cholesterol was removed from the plasma by the liver and 89% of this was receptor-mediated. With loss of receptor activity, however, the burden of LDL degradation was shifted away from the liver so that approximately 70% of LDL-cholesterol uptake took place in the extra-hepatic tissues of the WHHL rabbit. Thus, in the normal animal, the primary function of receptor-dependent LDL transport is to promote the rapid uptake and disposal of plasma LDL by the liver. In the absence of such receptor activity, cholesterol balance across most individual organs and the whole animal remains essentially normal and is mediated by the receptor-independent process. Because of the much lower absolute clearance rates manifested by this transport mechanism, however, substantial and predictable elevations in the circulating plasma LDL-cholesterol levels are required to maintain this balance.     

PAF-acether decreases low density lipoprotein degradation and alters lipid metabolism in cultured human fibroblasts

A 24 h pretreatment of human cultured fibroblasts with PAF-acether (PAF) induced a decrease in LDL degradation and a correlative accumulation of undegraded LDL. LDL binding was not significantly affected. Sterol and triacylglycerol synthesis from sodium acetate was enhanced whereas phospholipid synthesis decreased. Oleic acid incorporation into cholesteryl ester was markedly inhibited, whereas incorporation into triacylglycerols was increased. A decrease in the percentage of phosphatidylcholine and an increase in the percentage of phosphatidylethanolamine were found using sodium [32P]orthophosphate as precursor. These effects of PAF on LDL and lipid metabolism could be related to perturbations in membrane structure characteristics, leading to a delay in LDL delivery to lysosomes, and to modification of the activity of some key enzymes of lipid metabolism.     

Regulation of the uptake and degradation of beta-very low density lipoprotein in human monocyte macrophages

In normal human monocyte macrophages 125I-labeled beta-migrating very low density lipoproteins (125I-beta-VLDL), isolated from the plasma of cholesterol-fed rabbits, and 125I-human low density lipoprotein (LDL) were degraded at similar rates at protein concentrations up to 50 micrograms/ml. The high affinity degradation of 125I-labeled human LDL saturated at approximately 50 micrograms/ml; however, 125I-labeled rabbit beta-VLDL high affinity degradation saturated at 100-120 micrograms/ml. The activity of the beta-VLDL receptor was 3-fold higher than LDL receptor activity on freshly isolated normal monocyte macrophages, but with time-in-culture both receptor activities decreased and were similar after several days. The degradations of both beta-VLDL and LDL were Ca2+ sensitive, were markedly down regulated by sterols, and were up regulated by preincubation of the cells in a lipoprotein-free medium. The beta-VLDL receptor is genetically distinct from the LDL receptor as indicated by its presence on monocyte macrophages from a familial hypercholesterolemic homozygote. Human thoracic duct lymph chylomicrons as well as lipoproteins of Sf 20-5000 from fat-fed normal subjects inhibited the degradation of 125I-labeled rabbit beta-VLDL as effectively as nonradioactive rabbit beta-VLDL. We conclude: 1) the beta-VLDL receptor is genetically distinct from the LDL receptor, and 2) intestinally derived human lipoproteins are recognized by the beta-VLDL receptor on macrophages.     

Comparative binding and degradation of lipoprotein(a) and low density lipoprotein by human monocyte-derived macrophages.

The binding and degradation of equimolar concentrations of lipoprotein(a) (Lp(a)) and low density lipoprotein (LDL) isolated from the same individual were studied in primary cultures of human monocyte-derived macrophages (HMDM). At 4 degrees C, LDL receptor-mediated binding of both Lp(a) and LDL was of low affinity, being 0.8 and 0.23 microM, respectively. Competitive binding studies indicated that the binding of Lp(a) to HMDM was competed 63% by excess LDL. In contrast to the 4 degrees C binding data, the degradation of Lp(a) at 37 degrees C was mainly nonspecific because the amount of Lp(a) processed by the LDL receptor pathway in 5 h was 17% that of LDL. According to pulse-chase experiments, this phenomenon may be accounted for by the facts that less Lp(a) is bound to HMDM at 37 degrees C and that Lp(a) has a lower intrinsic degradation rate and was not due to increased intracellular accumulation or retroendocytosis of the lipoprotein. Degradation of both lipoproteins was primarily lysosomal and only modestly affected by up- or down-regulation of the LDL receptor. The rate of retroendocytosis in HMDM was approximately equal to the degradation rate and appeared to be independent of the type of lipoprotein used, up- or down-regulation of the LDL receptor, or the presence of the lysosomotropic agent chloroquine. Overall, the results indicate that HMDM degrade Lp(a) mainly via a nonspecific pathway with only 25% of total Lp(a) degradation occurring through the LDL receptor pathway. As both 37 degrees C degradation and 4 degrees C binding of LDL are mainly LDL receptor specific, the different metabolic behavior observed at 37 degrees C suggests that Lp(a) undergoes temperature-induced conformational changes on cooling to 4 degrees C that allows better recognition of Lp(a) by the LDL receptor at a temperature lower than the physiological temperature of 37 degrees C. How apo(a) affects these structural changes remains to be established.     

Intracellular trafficking of pigeon beta-very low density lipoprotein and low density lipoprotein at low and high concentrations in pigeon macrophages

Foam cell formation occurs in vitro at lipoprotein concentrations above 50 microgram/ml in pigeon macrophages. Hypothetically, intracellular trafficking of lipoproteins at higher concentrations may differ from uptake of lipoproteins associated with low concentrations, revealing a separate atherogenic endocytic pathway. Macrophage intracellular trafficking of pigeon beta-very low density lipoprotein (beta-VLDL) and low density lipoprotein (LDL) at low concentrations (12 microgram/ml) near the saturation of high affinity binding sites and high lipoprotein concentrations (50-150 microgram/ml) used to induce foam cell formation were examined. Pigeon beta-VLDL and LDL, differentially labeled with colloidal gold, were added simultaneously to contrast trafficking of beta-VLDL, which causes in vitro foam cell formation, with LDL, which does not. The binding of lipoproteins to cell surface structures, distribution of lipoproteins in endocytic organelles, and the extent of colabeling in the endocytic organelles were determined by thin-section transmission electron microscopy.At low concentrations, the intracellular trafficking of pigeon LDL and beta-VLDL was identical. At high concentrations, LDL was removed more rapidly from the plasma membrane and reached lysosomes more quickly than beta-VLDL. No separate endocytic route was present at high concentrations of beta-VLDL; rather, an increased residence on the plasma membrane, association with nonmicrovillar portions of the plasma membrane, and slower trafficking in organelles of coated-pit endocytosis reflected a more atherogenic trafficking pattern.     

Serology and tissue lesions in rabbits immunized with Streptococcus mutans

Rabbits were immunized i.v. or i.d. with sterile suspensions of disrupted Streptococcus mutans strain MT703 or K1R. Indirect immunofluorescence assays indicated that sera from four of 10 rabbits immunized i.d. contained antibodies reactive with monkey and human heart and kidney components; 19 of 24 rabbits immunized i.v. had antibodies reactive with these tissues. Heart-reactive antibodies were also detected by immunoelectrophoresis and indirect radioimmunoassay. These antibodies were absorbed well by cytoplasmic membranes, a whole cell extract, and an alkali extract of S. mutans but only weakly by intact bacteria. Between 6 and 8 weeks after the first i.v. administration of S. mutans vaccines, rabbits developed proteinuria and hematuria with subsequent weight loss and lethargy. Approximately 25% of the animals died from illness between the fifth and sixth month of immunization. In 13 of 15 rabbits, immune deposits of C3 and IgG, IgM, or IgA and fibrinogen were seen in kidneys within the glomeruli, basement membranes of the peritubular capillaries, and in the interstitium. In the heart, deposits were seen along the capillaries of the myocardium. In 8 of 14 rabbits, focal deposits of S. mutans antigen were detected in glomeruli and in the kidney interstitium. The kidneys showed gross pathologic and histopathologic changes. Most kidneys were pale and enlarged. Microscopic examination revealed hypercellularity of the glomeruli, presence of neutrophils, thickening of glomerular and tubular basement membranes, tubular atrophy, edema, and fibrosis of the interstitium. The kidney disease presented features of poststreptococcal glomerulonephritis. Microscopic examination of heart sections revealed mild perivascular infiltration by polymorphonuclear leukocytes and plasma cells in some of the rabbits.     

Phospholipid removal during degradation of rat plasma very low density lipoprotein in vitro.

The hydrolysis of glycerophospholipids in very low density lipoprotein by enzyme(s) released into circulation after the injection of heparin to rats was studied. [32P]Lysolecithin was formed rapidly from [32P]lecithin when very low density lipoprotein, labeled biosynthetically with 32P, was incubated with postheparin plasma. The [32P]lysolecithin was associated with the plasma protein fraction of density greater than 1.21 g/ml, whereas [32P]lecithin exchanged between very low and high density lipoproteins. Inhibition of the plasma lecithin: cholesterol acyl transferase activity did not change the excess [32P]lysolecithin formation in postheparin plasma, and only a negligible amount of radioactivity was associated with blood cells when the incubation was repeated in whole blood. Analysis of the results has demonstrated that phospholipids are removed from VLDL by two pathways: hydrolysis of glycerophospholipids by the heparin-releasable phospholipase activity (greater than50%) and transfer to high density lipoproteins (less than50%). The tissue origin of the postheparin phospholipase was studied in plasma obtained from intact rats and supradiaphragmatic rats using specific inhibitors of the extrahepatic lipase system (protamine sulfate and 0.5 M NaCl). The phospholipase activity could be ascribed to both the hepatic and extrahepatic lipase systems. It is concluded that hydrolysis of glycerophospholipids is the major mechanism responsible for the removal of phospholipids from very low density lipoprotein during the degradation of the lipoprotein. It is suggested that phospholipid hydrolysis occurs concomitantly with triglyceride hydrolysis, predominantly in extrahepatic tissues.     

Turnover products of the apo very low density lipoprotein II messenger RNA from chicken liver.

The mature apo Very Low Density Lipoprotein II (apo VLDLII) mRNA appears in chicken liver within a few hours after estrogen administration. Apart from this mRNA species, shorter RNA molecules hybridizing to apo VLDLII sequences have been detected in rooster liver upon estrogen stimulation. These molecules are present in the non-polyadenylated fraction of the total cellular- and polysomal RNA. Northern blotting and electron microscopy of R-loops were employed to show that these shorter RNA molecules are truncated at their 3'-end. The 3'-termini were further characterized by nuclease S1 analyses, and are located predominantly in the 3' untranslated region of the mRNA. Using a secondary structure model (Shelness and Williams, J. Biol. Chem. 260, 8637-8646, 1985), we show that the 3' termini map mainly in unpaired regions of the structure.     

氧化性低密度脂蛋白与细胞自噬

氧化性低密度脂蛋白(oxygenized low density lipoprotein,oxLDL)水平的升高不仅引发动脉粥样硬化,还与癌症等疾病的发生有密切关系。研究发现,高水平的oxLDL在引发细胞凋亡的同时,也诱导多种细胞自噬。本文归纳了oxLDL与血管内皮细胞、乳腺上皮细胞、结肠癌细胞、颗粒细胞和神经细胞自噬关系的最新研究进展。