Decreased resistance against in vitro oxidation of LDL from patients with familial defective apolipoprotein B-100

Familial defective apolipoprotein B-100 (FDB) is caused by a mutation in the receptor-binding region of apolipoprotein B-100, the structural protein of the low-density lipoprotein (LDL) particle. We studied the effect of this mutation on the composition and susceptibility to oxidative modification of LDL in patients with FDB. Twenty Dutch carriers of the mutation identified in a family study were matched with 20 unaffected siblings of similar age and sex. The mean concentration of LDL cholesterol was 5.19 +/- 0.94 versus 2.9 +/- 0.5 mmol/L in control subjects (P < .0001). Measurement of LDL oxidizability in vitro by continuously monitoring conjugated-diene absorbance showed that LDL from FDB patients was significantly less resistant against oxidation (lag time, 90 +/- 22 minutes versus 108 +/- 21 minutes; P < .05); furthermore, the maximal rate of diene production and total diene production were also significantly increased. Analysis of the chemical composition revealed an increased relative content of cholesteryl esters and reduced content of protein in the LDL of FDB patients (cholesterol-to-protein ratio, 1.54 +/- 0.24 versus 1.25 +/- 0.23; P < .01). The relative amount of arachidonic acid in LDL was increased and that of stearic acid was decreased. The vitamin E (alpha-tocopherol) content per gram of LDL protein was similar to that in control subjects. The relative amount of cholesteryl esters and protein in LDL as well as the fatty acid composition were significantly correlated with LDL oxidizability. Thus, compositional factors in LDL resulting in increased susceptibility to oxidative modification may contribute to the increased risk of premature vascular disease in FDB.     

Characterization and quantitation of apolipoprotein B-100 by capillary electrophoresis

The interaction of low density lipoproteins (LDL) with different surfactants was studied by capillary electrophoresis (CE) and sucrose density gradient ultracentrifugation as part of developing a method for quantitation of apoB-100 in serum. A mixture of surfactants consisting of 70% sodium dodecyl sulfate (SDS), 25% sodium myristyl sulfate, and 5% sodium cetyl sulfate was found to delipidate LDL particles more effectively than pure SDS or sodium decyl sulfate. The delipidation products of LDL [apolipoprotein B-100 (apoB-100) and lipids] were resolved as two distinct peaks by CE when using a 3.5 mM 70% SDS mixture, 20% (v/v) aceto nitrile, 50 mM sodium borate, pH 9.1 buffer. This CE method was also used to characterize apoB-100 derived from samples of lipoprotein [a] and very low density lipoproteins (VLDL). A CE-based quantitation method for apoB-100 was developed utilizing the observed linear relationship between apoB-100 concentration and its corrected 214 nm absorbance peak area measured on-line by CE. Concentration values of apoB-100 in LDL and VLDL samples were determined by CE and found to be accurate when compared to values obtained by immunoturbidimetric analysis and the Lowry method. Capillary electrophoresis can be used as a precise, accurate, and specific on-line method for the qualitative and quantitative analysis of the apoB-100 component of VLDL and LDL-related lipoproteins.     

Determinants of the kinetics of very low-density lipoprotein apolipoprotein B-100 in non-obese men

1. Apolipoprotein B-100 (ApoB) is the principal structural and functional protein of the pro-atherogenic lipoproteins. Elevated plasma apoB is an independent risk factor for coronary artery disease. In the present study we aimed to assess the factors that determine the kinetics of apoB in the very low-density lipoprotein (VLDL) in healthy men. 2. We studied 17 non-obese men who were consuming an ad libitum diet and had the following characteristics: mean (+/-SD) age 45.5 +/- 9.7 years, body mass index (BMI) 25.1 +/- 1.4 kg/m2, waist:hip ratio 0.91 +/- 0.04, serum cholesterol 5.2 +/- 0.6 mmol/L, triglycerides 1.08 +/- 0.53 mmol/L and high-density lipoprotein-cholesterol 1.24 +/- 0.31 mmol/L. Daily dietary intake was as follows: total fat 76 +/- 26 g, carbohydrate 238 +/- 67 g, protein 103 +/- 33 g and alcohol 20 +/- 16 g. 3. The kinetics of VLDL ApoB were studied using a primed, constant infusion (1 mg/kg per h) of 1-[13C]-leucine over 8 h with measurement of isotopic enrichment of ApoB using gas chromatography/mass spectrometry. The fractional turnover rate of VLDL ApoB was estimated using a monoexponential function. The mean (+/-SD) absolute hepatic secretion rate (ASR) of ApoB was 8.5 +/- 4.6 mg/kg per day and the fractional catabolic rate (FCR) was 7.9 +/- 5.6 pools/day. The ASR was significantly correlated with the waist:hip ratio (r = 0.60; P = 0.04), but not with age, BMI, weight or nutrient intake. The FCR was significantly and inversely correlated with plasma triglycerides (r = -0.53; P = 0.03) and alcohol intake (r = -0.48; P = 0.05). 4. In conclusion, the hepatic secretion of VLDL ApoB in nonobese, healthy men is primarily determined by the waist:hip ratio, a measure of visceral fat. This is consistent with the hypothesis that the rate of lipid substrate supply in the liver regulates the output of ApoB. The fractional catabolism of VLDL ApoB may, however, be inversely related to alcohol intake and appears to determine the plasma concentration of triglycerides.     

In vivo evidence for increased apolipoprotein A-I catabolism in subjects with impaired glucose tolerance

The in vivo kinetics of the HDL apolipoproteins (apo) A-I and A-II were studied in six subjects with impaired glucose tolerance (IGT) and six control subjects with normal glucose tolerance (NGT), using a stable isotope approach. During a 12-h primed constant infusion of L-[ring-13C6]-phenylalanine, tracer enrichment was determined in apoA-I and apoA-II from ultracentrifugally isolated HDL. The rates of HDL apoA-I and apoA-II production and catabolism were estimated using a one-compartment model-based analysis. Triglycerides were higher in IGT subjects (1.33 +/- 0.21 vs. 0.84 +/- 0.27 mmol/l, P < 0.05), but were within the normal range. HDL cholesterol and apoA-I levels were significantly lower in subjects with IGT (1.07 +/- 0.15 vs. 1.36 +/- 0.14 mmol/l, P < 0.05; 0.94 +/- 0.10 vs. 1.34 +/- 0.07 g/l, P < 0.01). In IGT subjects, HDL composition was significantly altered, characterized by an increase in HDL triglycerides (4.9 +/- 1.9 vs. 3.2 +/- 1.0%, P < 0.05) and HDL phospholipids (34.7 +/- 2.6 vs. 27.5 +/- 5.8%, P < 0.05) and a decrease in HDL cholesteryl esters (10.1 +/- 2.0 vs. 12.7 +/- 2.9%, P < 0.05) and HDL apoA-I (31.5 +/- 4.4 vs. 43.2 +/- 2.4%, P < 0.05). The mean fractional catabolic rate (FCR) of HDL apoA-I was significantly higher in IGT subjects (0.34 +/- 0.05 vs. 0.26 +/- 0.03 day(-1), P < 0.01), while the HDL apoA-I production rate (PR), as well as the PR and FCR of HDL apoA-II, showed no differences between the two groups. There were significant correlations between HDL apoA-I FCR and the following parameters: HDL apoA-I (r = -0.902, P < 0.001), HDL cholesterol (r = -0.797, P = 0.001), plasma triglycerides (r = 0.743, P < 0.01), HDL triglycerides (r = 0.696, P < 0.01), and cholesterol ester transfer protein activity (r = 0.646, P < 0.01). We observed a strong positive association between increased apoA-I catabolism and insulin (r = 0.765, P < 0.01) and proinsulin (r = 0.797, P < 0.01) concentrations. These data support the hypothesis that the decrease in HDL cholesterol and apoA-I levels in IGT is principally the result of an enhanced apoA-I catabolism. The latter seems to be an early metabolic finding in IGT even when other lipid parameters, especially plasma triglycerides, still appear to be not or only weakly affected.     

Increased hepatic secretion of very-low-density lipoprotein apolipoprotein B-100 in cholesteryl ester storage disease

Using a stable isotope method, we measured the hepatic secretion rate of very-low-density lipoprotein apolipoprotein B-100 (VLDL apoB) in a 26-year-old women who had dyslipidemia due to cholesteryl ester storage disease (CESD) and in five normolipidemic subjects. [1-13C]Leucine was administered by a primed constant intravenous infusion and the enrichment of VLDL apoB was determined by gas chromatography-mass spectrometry. The absolute secretion rate (ASR) of VLDL apoB in the patient was more than twice the mean ASR of the normolipidemic group (17.1 vs 8.0 +/- 0.8 mg/kg body wt. per day). The plasma mevalonic acid concentration, a measure of intrahepatic cholesterol synthesis, was also greater in the patient than in the normolipidemic subjects (8.3 vs 4.4 +/- 1.8 micrograms/L). The findings are consistent with the hypothesis that in CESD increased intrahepatic synthesis of cholesterol stimulates hepatic secretion of VLDL apoB and this may partly account for the dyslipidemia.     

Characterization of six patients who are double heterozygotes for familial hypercholesterolemia and familial defective apo B-100

Familial defective apolipoprotein B-100 (FDB) and familial hypercholesterolemia (FH) are the common causes of monogenic primary hypercholesterolemia. An individual of mixed English and Afrikaner descent with both FDB and the FH Afrikaner-1 low-density lipoprotein receptor mutation was identified in our laboratory. Subsequent analysis of her extended family revealed the presence of heterozygotes for either FH Afrikaner-1, FH Afrikaner-2, or FDB as well as five additional double heterozygotes for FH Afrikaner-1 and FDB and one "complex" heterozygote with all three mutations. The hypercholesterolemic and clinical features of the pure FDB subjects were similar to those of the pure FH heterozygotes. The double heterozygotes with both FH and FDB have lipid levels and clinical features that are intermediate in severity between heterozygous and homozygous FH.     

Incidence of familial defective apolipoprotein B-100 in cases of patients diagnosed with familial hypercholesterolemia

Familial Hypercholesterolemia (FH) and Familial Defective Apolipoprotein B-100 (FDB) are monogenic, autosome, dominantly inherited diseases appearing as type II/a primary hypercholesterolemia. The frequency of the heterozygositic forms is 1:700-1:500 in European population. Both forms of hypercholesterolemia causes early onset coronary heart diseases (CHD). According to the recommendations of the international MED-PED program (Make Early Diagnoses--Prevent Early Death), we found 73 FH cases and their 377 first relatives (parents, siblings, children) were also assessed. 156 patients were diagnosed clinically FH (131 alive and 25 deceased), and 31.8% of the males and 32.4% of females suffered from early onset CHD. One family with FH consists of 5.46 members on the average and there are 2.39 FH patients in one family. In our FH cohort four patients with FDB (R3500Q mutation) were diagnosed with allelspecific PCR, and the mutation was detectable also in 9 cases out of 11 living family members. The plasma total cholesterol level of the FDB patients--especially at younger age--was very close to the normal values, which is in contrast to the findings in FH patients. Nevertheless, FDB can be one of the independent causes of the early onset CHD. Therefore, in families with high frequency of cardiovascular diseases the R3500Q mutation has to be considered.     

Decreased flow-induced dilation and increased production of cGMP in spontaneously hypertensive rats

-We investigated flow (shear stress)- and agonist-induced cGMP release in mesenteric vascular beds of spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY). The mesenteric vascular bed was perfused in situ with Tyrode's solution. Vascular relaxation and cGMP release in the perfusate were determined on stimulation by flow or by acetylcholine (0.1 micromol/L) or sodium nitroprusside (0.1 mmol/L). Flow-induced release of cGMP was significantly greater in SHR than in WKY (P<0.01), despite a lower flow-induced dilation in SHR. In both strains, NG-nitro-L-arginine methyl ester (L-NAME) completely inhibited cGMP release in response to flow (P<0.001), although flow-induced dilation was not affected by L-NAME in SHR. Moreover, the activity of the constitutive nitric oxide synthase (NOS) was significantly greater in SHR (82+/-3.5 fmol/min) than in WKY (66+/-3.5 fmol/min; P<0.05) and was associated with increased expression of endothelial NOS mRNA in SHR. Sodium nitroprusside induced larger increases in cGMP release in SHR (3593+/-304 fmol/min) than in WKY (2467+/-302 fmol/min; P<0.05). The release of cGMP in response to acetylcholine was significantly lower in SHR (292+/-80 fmol/min) than in WKY (798+/-218 fmol/min; P<0.05) in parallel with smaller acetylcholine-induced relaxation in SHR. Despite increased cGMP production in response to flow and NOS activity, flow-induced dilation was decreased in SHR, suggesting an upregulation of the NO/cGMP pathway to compensate for the increased vascular tone in SHR.     

Decreased serum apolipoprotein AII/AI ratio in systemic amyloidosis

BACKGROUND: Intrapericardial teratomas are rare and usually present early in infancy or childhood. PROCEDURE: We describe herein a rare case of an adult patient with an intrapericardial teratoma who presented with fever, cardiac arrhythmias, and oppressive substernal chest pain. Preoperative diagnosis was suggested by echocardiography and computerized tomography of the chest. The tumor weighed 530 g and its histologic features were those of a mature cystic teratoma. It was excised totally and 10 years' follow-up revealed no evidence of residual disease. DISCUSSION: Our patient is one of the very few adult patients with intrapericardial teratomas who was treated successfully with surgery. Both echocardiography and tomography of the chest suggested the diagnosis and delineated the relationship of the tumor to the great vessels. CONCLUSION: The diagnosis of Intrapericardial teratomas is suspected by echocardiography and/or tomography of the chest and confirmed by specific histologic features. These tumors should be excised whenever detected.     

Immunoreactivity of apolipoprotein B-100 and binding to LDL-receptor of phospholipase A2-treated low density lipoproteins

Lipid--protein particles were obtained by treatment of low density lipoproteins (LDL) with phospholipase A2 from bee venom. Under these conditions, half of the phosphatidylcholine (PC) of LDL was changed to lysophosphatidylcholine (LPC). At the same time, the composition of other lipids and the apoprotein structure were unaffected. Three monoclonal antibodies (MAbs) against various apo B epitopes were used to test immunoreactivity of phospholipase A2-treated LDL (pl-LDL). The apo B epitope interacting with MAb 4C11 (amino acid residues 2377-2658) showed significantly decreased immunoreactivity. Increase in MAb 4C11 binding was demonstrated to depend on oxidation degree of LDL. Thus, changing of half of PC to LPC modified apo B translocation in the lipoprotein globule in an opposite manner as compared with changes induced by oxidative modification. A minor increase in immunoreactivity of pl-LDL with 1D1 MAb against a large middle part of apo B (residues 1297-3249) may be due to the effect of the change of surface lipid composition on the extent of immersion of apo B into the hydrophobic phase. No changes in the interaction of pl-LDL with MAb 2G8 (residues 3748-4306) were observed in comparison with native LDL. This fact demonstrates that 50% phospholipolysis of LDL does not affect the expression of apo B C-terminal residues in pl-LDL. Twofold increase in pl-LDL affinity to immobilized LDL-receptor was shown in contrast to LDL. The data indicate that LPC accumulation in LDL results in better elimination of LDL from the blood stream than in case of accumulation of oxidative products.     

Sequences within apolipoprotein(a) kringle IV types 6-8 bind directly to low-density lipoprotein and mediate noncovalent association of apolipoprotein(a) with apolipoprotein B-100

Lipoprotein(a) [Lp(a)] particle formation is a two-step process in which initial noncovalent interactions between apolipoprotein(a) [apo(a)] and the apolipoprotein B-100 (apoB-100) component of low-density lipoprotein (LDL) precede disulfide bond formation. To identify kringle (K) domains in apo(a) that bind noncovalently to apoB-100, the binding of a battery of purified recombinant apo(a) [r-apo(a)] species to immobilized human LDL has been assessed. The 17K form of r-apo(a) (containing all 10 types of kringle IV sequences) as well as other truncated r-apo(a) derivatives exhibited specific binding to a single class of sites on immobilized LDL, with Kd values ranging from approximately 340 nM (12K) to approximately 7900 nM (KIV5-8). The contribution of kringle IV types 6-8 to the noncovalent interaction of r-apo(a) with LDL was demonstrated by the decrease in binding affinity observed upon sequential removal of these kringle domains (Kd approximately 700 nM for KIV6-P, Kd approximately 2000 nM for KIV7-P, Kd approximately 5100 nM for KIV8-P, and no detectable specific binding of KIV9-P). Interestingly, KIV9 also appears to participate in the noncovalent binding of apo(a) to LDL since the binding of KIV5-8 (Kd approximately 7900 nM) was considerably weaker than that of KIV5-9 (Kd approximately 2000 nM). Finally, it is demonstrated that inhibition of Lp(a) assembly by proline, lysine, and lysine analogues, as well as by arginine and phenylalanine, is due to their ability to inhibit noncovalent association of apo(a) and apoB-100 and that these compounds directly exert their effects primarily through interactions with sequences contained within apo(a) kringle IV types 6-8. On the basis of the obtained data, a model is proposed for the interaction of apo(a) and LDL in which apo(a) contacts the single high-affinity binding site on apoB-100 through multiple, discrete interactions mediated primarily by kringle IV types 6-8.     

Lovastatin decreases de novo cholesterol synthesis and LDL Apo B-100 production rates in combined-hyperlipidemic males

The effect of lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity, on the kinetics of de novo cholesterol synthesis and apolipoprotein (apo) B in very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL) was investigated in five male patients with combined hyperlipidemia. Subjects were counseled to follow a Step 2 diet and were treated with lovastatin and placebo in randomly assigned order for 6-week periods. At the end of each experimental period, subjects were given deuterium oxide orally and de novo cholesterol synthesis was assessed from deuterium incorporation into cholesterol and expressed as fractional synthesis rate (C-FSR) and production rate (C-PR). Simultaneously, the kinetics of VLDL, IDL, and LDL apo B-100 were studied in the fed state using a primed-constant infusion of deuterated leucine to measure fractional catabolic rates (FCR) and production rates (PR). Drug treatment resulted in significant decreases in total cholesterol (-29%), VLDL cholesterol (-40%), LDL cholesterol (-27%), and apo B (-16%) levels and increases in HDL cholesterol (+13%) and apolipoprotein (apo) A-I (+11%) levels. Associated with these plasma lipoprotein responses was a significant reduction in both de novo C-FSR (-40%; P = .04) and C-PR (-42%; P = .03). Treatment with lovastain in these patients had no significant effect on the FCR of apoB-100 in VLDL, IDL, or LDL, but resulted in a significant decrease in the PR of apoB-100 in IDL and LDL. Comparing the kinetic data of these patients with those of 10 normolipidemic control subjects indicates that lovastatin treatment normalized apoB-100 IDL and LDL PR. The results of these studies suggest that the declines in plasma lipid levels observed after treatment of combined hyperlipidemic patients with lovastatin are attributable to reductions in the C-FSR and C-PR of de novo cholesterol synthesis and the PR of apoB-100 containing lipoproteins. The decline in de novo cholesterol synthesis, rather than an increase in direct uptake of VLDL and IDL, may have contributed to the decline in the PR observed.     

Familial ligand-defective apolipoprotein B-100: simultaneous detection of the ARG3500-->GLN and ARG3531-->CYS mutations in a French population

BACKGROUND: Flask pulmonary edema (FPE) may be a manifestation of renovascular hypertension (RVHTN) and unresponsive to antihypertensive therapy. METHODS: Response to antihypertensive therapy and perioperative outcomes were determined in 5 consecutive patients with FPE. RESULTS: A mean of 2.3 admissions for the treatment of FPE were observed despite a mean cardiac ejection fraction of 60%. Preoperative treatment was attempted for 12 days and included ventilatory support (n = 3) and hemodialysis (n = 2). Total decreased renal perfusion was demonstrated by arteriography and radionuclide scans, no patient having a functional, contralateral kidney. Renal revascularizations were not associated with mortalities; 1 patient experienced atalectasis requiring bronchoscopy. All patients were extubated within 48 hours of surgery. A significant reduction in blood pressure (BP, 46%) and serum creatinine (Cr, 53%, P < or = 0.05) was observed. A mean of 1 antihypertensive medication was required at discharge compared with 3.4 on admission. At follow-up (mean 57 months) all patients remain cured of FPE. CONCLUSIONS: Medical management was unsuccessful in the treatment of FPE. Renal revascularization was associated with low morbidity and mortality, control of BP, restoration of renal function, and cure of FPE. These data suggest surgical intervention is the optimal mode of treatment of RVHTN associated with FPE.     

The microsomal triglyceride transfer protein catalyzes the post-translational assembly of apolipoprotein B-100 very low density lipoprotein in McA-RH7777 cells

In cells in which the lipoprotein assembly process had been inactivated by brefeldin A (BFA), membrane-associated apoB-100 disappeared without forming lipoproteins or being secreted, indicating that it was degraded. Reactivation of the assembly process by chasing the cells in the absence of BFA, gave rise to a quantitative recovery of the membrane-associated apoB-100 in the very low density lipoprotein (VLDL) fraction in the medium. These results indicate that the membrane-associated apoB-100 can be converted to VLDL. A new method was developed by which the major amount (88%) of microsomal apoB-100 but not integral membrane proteins could be extracted. The major effect of this method was to increase the recovery of apoB-100 that banded in the LDL and HDL density regions, suggesting that the membrane-associated form of apoB-100 is partially lipidated. We also investigated the role of the microsomal triglyceride transfer protein (MTP) in the assembly of apoB-100 VLDL using a photoactivatable MTP inhibitor (BMS-192951). This compound strongly inhibited the assembly and secretion of apoB-100 VLDL when present during the translation of the protein. To investigate the importance of MTP during the later stages in the assembly process, the cells were preincubated with BFA (to reversibly inhibit the assembly of apoB-100 VLDL) and pulse-labeled (+BFA) and chased (+BFA) for 30 min to obtain full-length apoB-100 associated with the microsomal membrane. Inhibition of MTP after the 30-min chase blocked assembly of VLDL. This indicates that MTP is important for the conversion of full-length apoB-100 into VLDL. Results from experiments in which a second chase (-BFA) was introduced before the inactivation of MTP indicated that only early events in this conversion of full-length apoB-100 into VLDL were blocked by the MTP inhibitor. Together these results indicate that there is a MTP-dependent "window" in the VLDL assembly process that occurs after the completion of apoB-100 but before the major amount of lipids is added to the VLDL particle. Thus the assembly of apoB-100 VLDL from membrane-associated apoB-100 involves an early MTP-dependent phase and a late MTP-independent phase, during which the major amount of lipid is added.     

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 approximately 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.     

Estrogen increases low-density lipoprotein receptor-independent catabolism of apolipoprotein B in hyperlipidemic rabbits

Estrogen has been reported to increase the catabolism of low-density lipoprotein (LDL) apolipoprotein (apo) B by increasing LDL receptor activity. To determine the effect of estrogen on LDL receptor-independent pathways, paired turnover studies of native LDL and chemically modified LDL (methyl-LDL) were performed before and during estrogen administration in female New Zealand rabbits consuming a diet containing 0.5% (wt/wt) cholesterol. Rabbits were matched by plasma cholesterol concentration and assigned randomly to receive estrogen (estradiol cypionate 0.5 mg/kg/wk) or placebo. The residence time of both the native LDL apo B tracer and the methyl-LDL apo B tracer in plasma was decreased by estrogen but not by placebo. Multicompartmental modeling of the paired, double-labeled turnover studies indicated that an increase in fractional catabolic rate (FCR) of the fast-turnover pool, a kinetically distinct LDL subpopulation in plasma, accounted for the observed decrease in residence time in plasma for both tracers. These data support the hypothesis that, in addition to any effect on the LDL receptor, estrogen promotes the activity of LDL receptor-independent pathways.     

Decreased phosphorylation of GAP-43/B-50 in striatal synaptic plasma membranes after circling motor activity

The effects of spontaneous circling motor activity on the in vitro phosphorylation of the protein kinase C substrate GAP-43/B-50 was studied on striatal membranes of developing rats (30 days of age). At this time of postnatal development, permanent plastic changes in cholinergic and dopaminergic systems are produced by physiological motor activity. Exercised animals showed a significant reduction of 31% in the level of GAP-43/B-50 endogenous phosphorylation in the contralateral striatum respect to the ipsilateral side (P < 0.01), while control animals did not show asymmetric differences. Compared to controls, the contralateral striatum of exercised animals showed a 33% reduction in the incorporation of 32P-phosphate into GAP-43/B-50 30 minutes post-exercise (P < 0.01). This change in GAP-43/B-50 phosphorylation was correlated with the running speed developed by the animals (r:0.8986, P = 0.015). GAP-43/B-50 immunoblots revealed no changes in the amount of this protein in any group. Moreover, a significant variation of 25% (P < 0.05) in the PKC activity was seen between both exercised striata. Interhemispheric differences were not found in control animals. We conclude that endogenous phosphorylation of this protein is also altered by motor activity in the same period that permanent changes in striatal neuroreceptors are triggered after motor training.     

Inhibition of N-linked glycosylation results in retention of intracellular apo[a] in hepatoma cells, although nonglycosylated and immature forms of apolipoprotein[a] are competent to associate with apolipoprotein B-100 in vitro

Apolipoprotein[a] (apo[a]) is a highly polymorphic glycoprotein that forms a covalent complex with apolipoprotein B-100 (apoB-100), producing a lipoprotein species referred to as lipoprotein[a] (Lp[a]). We have studied the effects of alterations in glycosylation of apo[a] on its intracellular processing and secretion as well as its ability to associate with low density lipoprotein (LDL) apoB-100. HepG2 cells transfected with a 6 kringle IV (6 K-IV) apo[a] minigene were treated with tunicamycin, an inhibitor of N-linked glycosylation, which eliminated apo[a]-B-100 complexes from the media. Tunicamycin treatment also reduced secretion of the 6 K-IV apo[a] protein from transfected McA-RH7777 cells by approximately 50%, but completely eliminated secretion of apo[a] species containing 9 and 17 K-IV repeats. Mixing experiments, performed with radiolabeled media (+/-tunicamycin) from transfected McA-RH7777 cells, demonstrated no alteration in the extent of association of apo[a] with human LDL. Similar mixing experiments using culture media from glycosylation-defective mutant chinese hamster ovary (CHO) cells transfected with the same apo[a] minigene showed identical results. Apo[a] secretion was demonstrated in all mutant cell lines in the absence of either N- or O-linked (or both) glycosylation. The mechanisms underlying the reduced secretion of apo[a] from transfected hepatoma cells were examined by pulse-chase radiolabeling and apo[a] immunoprecipitation. Tunicamycin treatment altered the efficiency of precursor apo[a] processing from the ER by increasing its ER retention time. The increased accumulation of precursor apo[a] in the ER was associated with alterations in the kinetics of association with two resident endoplasmic reticulum (ER) chaperone proteins, calnexin and BiP. These findings suggest that the glycosylation state and size of apo[a] appear to play a role in regulating its efficient exit from the endoplasmic reticulum. However, neither N- nor O-linked glycosylation of apo[a] exerts a major regulatory role in its covalent association with apoB-100.