Pulse-chase studies of the synthesis and intracellular transport of apolipoprotein B-100 in Hep G2 cells

The synthesis and secretion of apolipoprotein B-100 (apoB-100) have been studied in a human hepatoma cell line, the Hep G2 cells. The time needed for the synthesis of apoB-100 was estimated to be 14 min, which corresponds to a translation rate of approximately 6 amino acids/s. ApoB-100 was compared with albumin and alpha 2-macroglobulin as to the distribution between the membrane and the luminal content in the endoplasmic reticulum (ER) and the Golgi apparatus. The results suggested that apoB-100 approximately followed the distribution of these secretory proteins in the Golgi, while the ratios between the percent membrane-bound apoB-100 and percent membrane-bound albumin or alpha 2-macroglobulin were 3-4:1 in the ER. This may suggest that apoB-100 occurs in a membrane-associated form in ER prior to the integration in the lipoproteins. Pulse-chase studies combined with subcellular fractionation was used to investigate the kinetics for the intracellular transfer of apoB-100. A 3-min pulse of [35S]methionine was followed by an increase in apoB-100 radioactivity in the ER during the first 10-15 min of chase. The following 10-15 min of chase were characterized by linear decrease in apoB-100 radioactivity with a decay rate of approximately 6%/min. The residence kinetics for apoB-100 in the ER differed from that of transferrin and probably also from that of albumin. By comparing the time for the pulse maximum in ER with that in the denser Golgi fractions the time needed for the transfer between ER and Golgi could be estimated to be 10 min. The time needed for the secretion of newly synthesized apoB-100 was estimated to be 30 min. This indicates that the transfer of the protein through the Golgi apparatus to the extracellular space requires 20 min.

RFLPs for the human apolipoprotein B gene: HincII and PvuII

Images

Molecular cloning of human apolipoprotein B cDNA

In this paper we describe the isolation of cDNA clones which code for parts of apolipoprotein B (apoB). The clones were obtained by immunoscreening of an expression library (lambda gt 11) derived from a human hepatoma cell line (Hep G2). The relationship between positive clones and apoB was established with immunochemical techniques using polyclonal as well as monoclonal antibodies. Recombinants, expressing nonoverlapping regions of apoB are described, all hybridizing with a very large mRNA (approximately 20,000 bases long). The nucleotide sequence obtained predicts a primary protein structure with a composition suitable for the formation of stretches of an amphipatic alpha-helix.

Stoichiometric binding of apolipoprotein B-specific monoclonal antibodies to low density lipoproteins

The structure of apolipoprotein B and its stoichiometry on plasma lipoproteins has been a major issue and one refractory to a variety of analyses. Immunochemical analyses represent an independent approach. Examinations of apolipoprotein B (apo-B) epitopes on human plasma low density lipoproteins (LDL) using monoclonal antibodies have consistently revealed the existence of extensive apo-B heterogeneity. In the present study, we have addressed the solution of the stoichiometry problem using quantitative analysis of the maximum number of identical antibodies that can be bound per LDL particle in which we take into account this ligand heterogeneity. We have estimated the molecular weight of apo-B by quantifying the number of times a given apo-B epitope is expressed on the surface of LDL. The quantitative binding of eight previously characterized monoclonal antibodies was measured in a fluid phase radioimmunoassay. The results were analyzed by Scatchard analysis and expressed on the basis of independent measurements of the maximum amount of LDL that could be bound by each antibody. Affinity constants for each of the eight antibodies varied between 8.5 X 10(7) and 80 X 10(7) M-1. For these same antibodies, the concentration of maximally bound antibody at a normalized LDL concentration of 1000 ng/ml was estimated to be 0.9-1.8 nM with a mean of 1.23 nM. Adopting a molecular mass from physicochemical analysis for LDL apo-B of 550,000 daltons, the molar ratio between bound antibody and LDL varied between 0.5 and 1.2 (mean 0.75 +/- 0.15). The results supported the hypothesis that apo-B is present as a single large molecular weight polypeptide in LDL.

Pulse-chase studies of the synthesis of apolipoprotein B in a human hepatoma cell line, Hep G2

We have used pulse-chase methodology to study the synthesis of apolipoprotein B in a human hepatoma-derived cell line, the Hep G2 cells. A 2-min pulse with [35S]methionine was followed by a chase period varying from 5-90 min. A protein of large molecular mass (estimated molecular mass: 312 +/- 41 kDa, mean +/- SD, n = 8) could be immunoprecipitated from the cells at all chase periods between 5 min and 60 min with both monoclonal antibodies to a narrow density cut of the low density lipoprotein LDL-2 (density: 1.030-1.055 g/ml) and polyclonal antibodies to the apolipoprotein B apo B 100 or to a narrow density cut of LDL-2 (density: 1.030-1.055 g/ml). In addition to this large molecular mass protein, nascent polypeptides could be precipitated after 5, 10 and 15 min of chase. The apolipoprotein B molecules that had been labelled during the pulse disappeared from the cells after 60-90 min of chase, while they started to appear in the medium after 30-35 min of chase. The results obtained indicate (a) that apolipoprotein B is synthesized as one polypeptide with a large molecular mass, (b) that newly synthesized apolipoprotein B molecules are secreted after a delay of 30-35 min, (c) that no intracellular accumulation of apolipoprotein B occurs, and (d) that apolipoprotein B is recovered in the density fraction less than 1.21 g/ml of the medium suggesting that it is secreted in lipoprotein form.

Role of the low density lipoprotein receptor in penetration of low density lipoprotein into rabbit aortic wall

The present study was designed to determine whether binding of low density lipoprotein (LDL) to endothelial LDL receptors contributes significantly to the penetration of LDL into the normal rabbit aorta. Initial flux rate was used as a measure of uptake of LDL. Reductive methylation of LDL is known to block its recognition by the LDL receptor. Therefore, the difference in flux rates of native LDL and reductively methylated LDL (methyl-LDL) was assumed to represent the receptor-dependent uptake. Native LDL and methyl-LDL were labeled with different isotopes (125I or 131I) and both were injected simultaneously into the same rabbit. After 30 to 60 minutes, trichloroacetic acid-precipitable counts were determined in aortic specimens. The initial flux rates, expressed as plasma clearance (nl/g/hr), were 1787 for native LDL and 1924 for methyl-LDL. The difference was not significant, which suggests that the flux of LDL into the aorta is not significantly dependent upon, or regulated by, endothelial LDL receptors, but is mediated by other mechanisms.

Evidence for a structural relationship between apoB75kDa and human plasma apolipoprotein B 100, from translation of human liver mRNA in vitro and immunochemical studies with monoclonal and polyclonal antibodies

We have investigated the relation between an 80-kDa protein synthesized in vitro in protein-synthesizing system programmed with human liver mRNA [Olofsson, S.-O., Elias, P., Boström, K., Lundholm, K., Norfeldt, P.-I., Wiklund, O., Fager, G., and Bondjers, G. (1983) FEBS Lett. 156, 63-66] and a 70-80-kDa protein, apoB75kDa, isolated from the low-density lipoproteins-2 (LDL-2) [Olofson, S.-O., Boström, K., Svanberg, U., and Bondjers, G. (1980) Biochemistry 19, 1059-1064]. Five monoclonal antibodies directed against LDL-2 as well as polyclonal antibodies against a narrow density cut of LDL-2 (d = 1.030 - 1.055) were used to precipitate apoB-related proteins synthesized in vitro in a protein-synthesizing system programmed with human liver mRNA (or total RNA fraction). With all monoclonal antibodies as well as the polyclonal antibodies, a protein with an estimated molecular mass of 80 +/- 1.3 kDa (mean +/- SD, n = 12) could be precipitated. The observation that all monoclonal antibodies used reacted with apoB75kDa indicates a close immunological relation between this 80-kDa protein and apoB75kDa. Limited proteolysis of the 80-kDa protein (synthesized in the presence of [35S]-methionine) with Staphylococcus aureus V8 protease generated six [35S]-methionine-containing bands that could be separated on a polyacrylamide gradient gel (12-20%). All these radioactive bands corresponded to major protein-stained bands obtained after limited proteolysis of apoB75kDa. This observation suggests a structural relation between the two proteins. Taken together, our results indicate that a protein corresponding to apoB75kDa is synthesized in vitro in a protein synthesizing system programmed with human liver mRNA (or total RNA fraction). We have also compared apoB75kDa and the major component of apoLDL-2, apoB100 [Kane, J. P., Hardman, D.A., and Paulus, H.E. (1980) Proc. Natl Acad. Sci USA 77, 2465-2469] by immunochemical methods. We could demonstrate that six monoclonal antibodies directed against four to six different epitopes on LDL-2, as well as polyclonal antibodies to apoB100 and apoB75kDa, all reacted with apoB75kDa and apoB100. These observations indicate a close immunological relation between the two proteins. Taken together our results support the hypothesis that apoB100 has a subunit structure. We therefore suggest that apoB75kDa is a subunit of apoB100 synthesized in human liver.

Human liver RNA-programmed in vitro synthesis of a polypeptide related to human apolipoprotein B

In an in vitro synthesizing system programmed with RNA from human liver a polypeptide with an estimated Mr of 80 000 (80 kDa) +/- 1400 (mean +/- SD, n = 5) was synthesized. This polypeptide could be precipitated with antiserum to a narrow density cut of LDL (d = 1.030-1.055) or antiserum against the high-Mr form of apoB (apoB 100 [4]). The synthesized protein is immunologically related to a 75 kDa protein isolated from LDL. We suggest that the 80 kDa protein represents a primary translation product of apoB synthesized in human liver.

Effects of anti-hypertensive therapy on serum lipoproteins. Treatment with metoprolol, propranolol and hydrochlorothiazide

The effects of metoprolol, propranolol and hydrochlorothiazide on lipoprotein metabolism were studied in three different but comparable groups of middle-aged men with previously untreated hypertension (n=10, n=10, and n=11, respectively). All three treatments were associated with an increase in serum triglyceride and VLDL-cholesterol levels. Propranolol treatment was associated with consistent and significant decreases in HDL-cholesterol, apoA-I and A-II levels, whereas these changes during the other treatments were neither significant nor consistent. An increase in adipose tissue LPL-activity and a decrease in serum free fatty acids were seen in the propranolol treatment group. Significant changes were not observed in glucose tolerance or catecholamine excretion. The blood pressure reduction was similar in the three groups. The design of the present study was in some important respects different from that of others. This may help to explain why we found a difference between the two beta-blockers in our study.

Arterial cholesterol and DNA contents in relation to serum lipids and apolipoproteins. Studies on cystic artery biopsies

Though various relationships between serum lipoprotein levels and risk for atherosclerotic disease have been shown there are only a few studies on the relationships between serum lipoprotein levels and the lipid contents of the arterial wall. This study presents cholesterol and DNA contents of arterial tissue from biopsies of the cystic artery in 23 patients with uncomplicated cholecystolithiasis. Serum levels of cholesterol, triglycerides, alphalipoprotein cholesterol, apoA-I, A-II and B were determined, and the relationships between artery and serum variables were calculated. There was a positive correlation between serum apoB and the arterial cholesterol, normalized to the DNA contents (r = 0.43, P less than 0.05). There was a tendency towards an inverse correlation between the alpha-lipoprotein cholesterol levels and the arterial cholesterol (r = 0.39, P less than 0.10). There were no significant correlations between serum apoA-I or A-II and the arterial cholesterol contents. These data indicate that deposition of cholesterol in the arterial wall is related to the serum level of apoB, with higher levels of arterial cholesterol at higher serum levels of apoB. Earlier observations of an inverse correlation between alphalipoprotein cholesterol and arterial cholesterol could, however, not be conclusively confirmed.U

Seasonal variations in serum lipid and apolipoprotein levels evaluated by periodic regression analyses

Periodic regression analyses have been employed in a study of the variations in serum lipoprotein variables in 12 men during 12 months. The monthly means in the A-II polypeptide (A-II) and apolipoprotein B (apoB) varied significantly with time according to sine curves. Maxima of +5 and +8% from the annual averages were observed for A-II and apoB, respectively, during the winter months, whereas minima of -5 and -8%, respectively, occurred during summer. There were considerable individual differences in amplitude and phase in serum cholesterol, alpha-lipoprotein cholesterol and apolipoprotein A-I (apoA-I), whereas the variations in monthly means of the whole group were not significant. Storage of sera at -85 degrees C for 1 yr seems not to have influenced the results of determinations of serum cholesterol and triglycerides, apoA-I, and A-II, but may have caused differences observed in apoB between the two months of May 1978 and 1979.

Multivariate analyses of serum apolipoproteins and risk factors in relation to acute myocardial infarction

In 25 middle-aged infarction survivors and 76 corresponding controls, representative for a well-defined community, multivariate analysis was used to evaluate whether serum apolipoproteins were better discriminators of infarction survivors than serum lipids and other risk factors. Levels of serum cholesterol and triglycerides, alphalipoprotein cholesterol, apolipoproteins A-I, A-II, B, and D, as well as tobacco smoking and other risk factors, were included. In descending order, serum apo A-II levels (t(b) = -3.12, p = 0.002), tobacco consumption (t(b) = 2.64, p = 0.010), and serum triglycerides (t(b) = 2.06, p = 0.042) contributed significantly to the multiple regression on myocardial infarction (R = 0.53, p = 0.00001). When entered into ka discriminant function, these three variables gave a good separation between survivors and controls. Of the survivors, 50% were above the 90th percentile inthe control group. The relative prevalence of infarction increased continuously with increasing values of the function from zero to more than 6 times the average. Serum apo A-II levels alone were almost as good in separating cases and controls. From this study, we concluded that, among apolipoproteins, apo A-II seems to be a more sensitive discriminator of infarction survivors than other risk factors.

Serum apolipoprotein levels in relation to acute myocardial infarction and its risk factors--determination of apolipoprotein D

Apolipoprotein A-I, A-II and apoD are all primarily found in the density region d > 1.063 g/ml. In the present study the serum apoD level was determined by electroimmunoassay in a random population sample of middle-aged men (n = 76). The mean level was 0.075 g/l with a standard deviation of 0.017. The apoD level was also determined in a group of patients, in the same age range, with sustained acute myocardial infarction (n = 25). The patients were compared with the random population sample and with a control group matched to the patients with regard to age, serum cholesterol level and body weight index. There was no difference in apoD level between patients and either control group. This is in contrast with the earlier reported low apoA-I, A-II as well as alphalipoprotein cholesterol levels in the same patient group.

Quantitation of human serum apolipoprotein A-I by electroimmunoassay. Studies on some techniques for standardization of the assay and determination of serum apolipoprotein A-I levels in a random population sample of middle-aged men

Serum apolipoprotein A-I levels have been determined in a population sample of seventy-six randomly selected men aged 41-48 years. Electroimmunoassay with a monospecific rabbit antiserum to human apoA-I was used. Different routes to standardization of the electroimmunoassay were exploited. Standardization was finally carried out with purified apoA-I and confirmed with LpA-I. The mean apoA-I level was 1.75 +/- 0.20 g/1 in the populaton sample, with a normal distribution of the values. A highly significant, positive correlation to alphalipoprotein cholesterol was observed. ApoA-I levels were negatively correlated to tobacco consumption. Significant correlations were also observed to the levels of serum cholesterol and physical activity during leisure time, but no significant correlations to serum triglycerides or body weight index were found.

Alphalipoprotein cholesterol levels in relation to acute myocardial infarction and its risk factors

During 1975-1977 twenty-nine males surviving acute myocardial infarction at an age between 40-44 years were registered in Gothenburg, Sweden. Twenty-five of these were studied and compared with two control groups. One group, the reference group (RG, n = 76), was randomly selected from the male population from which the acute myocardial infarction (AMI) group was derived. A second group, the matched control group (MC, n = 47), consisted of men with no history of coronary heart disease, matched with patients for age, serum cholesterol and body weight index. Serum triglyceride levels were higher and alphalipoprotein cholesterol lower in the AMI group than in RG. Prior to infarction, patients had a higher degree of physical activity at work and a higher tobacco consumption than RG. When AMI cases were compared with MC subjects lower alphalipoprotein cholesterol levels were found in AMI, and they also had a higher tobacco consumption prior to infarction. There was a negative correlation between alphalipoprotein cholesterol levels and tobacco consumption in the RG. The differences in alphalipoprotein cholesterol levels between AMI cases and controls could not attributed to smoking habits, but smoking may at least to some extent exert its effect as a risk factor through influence on alphalipoprotein cholesterol levels.

Serum apolipoprotein levels in relation to acute myocardial infarction and its risk factors. Apolipoprotein A-I levels in male survivors of myocardial infarction

The significance of high density lipoproteins in the etiology of clinical complications to atherosclerosis has recently received increased attention. The levels of the major apolipoprotein in high density lipoproteins, apoA-I, have been determined in patients who had had an acute myocardial infarction, and compared with a cholesterol-matched and a randomly selected control group. ApoA-I levels were lower in the patients than in the control groups. ApoA-I levels were also lower in smokers than in non-smokers. The difference between patients and control groups persisted even when the groups were stratified according to smoking habits. This suggests that low levels of apo-A-I as well as alphalipoprotein cholesterol are additional characteristics of the infarction patients, even when the established risk factors, smoking and hyperlipidemia are taken into account.

Lecithin : cholesterol acyl transfer (LACT) and fatty acid composition of lecithin and cholesterol esters in young male myocardial infarction survivors

Lecithin : cholesterol acyl transfer (LCAT) and relative fatty acid composition of serum lecithin and cholesterol esters were studied in 20 young male survivors of myocardial infarction (MI). Comparisons were made with controls matched for serum cholesterol. There was no difference in LCAT rate between MI patients and controls. The relative content of arachidonic acid in cholesterol esters was higher in MI patients. The fatty acid composition of lecithin and cholesterol esters suggests an equal transfer of linoleic and oleic acids from lecithin to cholesterol. Furthermore negative correlations were found between LCAT and linoleic acid content of lecithin (r = --0.43, P less than 0.01) and cholesterol esters (r = --0.45, P less than 0.01). This inverse relationship does not seem to be linked to substrate specificity, but rather to be mediated by influences in common on serum lipid content and turnover.

Treatment of oral estramustine phosphate (Estracyt) in prostatic carcinoma: influences on lipid and carbohydrate metabolism

Estramustine phosphate (Estracyt), a combination of estradiol and nitrogen mustard given to males with prostatic carcinoma, had the same effect on serum lipids, lipoproteins, and serum phosphoglyceride fatty acid composition as ethynyl estradiol (Etivex). The characteristic effects on serum lipids caused by both drugs, i.e., a reduction in serum cholesterol and an increase in serum phospholipids, were apparently expressions for reduced low density lipoproteins and increased alpha-lipoproteins. Serum lecithin fatty acid composition revealed during the administration of both drugs a characteristic increase in palmitic acid (16:0) and a decrease in stearic acid (18:0), interpreted as evidence for a cholestatic, although subclinical, liver involvement. Similar changes have earlier been revealed in women given ethynyl estradiol; however, the increase in serum triglycerides and very low density lipoprotein cholesterol in young women was not duplicated in aged males with prostatic carcinoma. Furthermore, in aged males, the administration of these estrogens did not change carbohydrate metabolism but did produce an increase in adipose tissue lipoprotein lipase.

Biochemical typing of hyperlipoproteinemia. The development of a nomogram

The accurate biochemical typing of hyperlipoproteinemia would require quantification of lipoprotein fractions. At present, typing is frequently based on the lipoprotein electrophoresis pattern together with serum lipid analyses. Lack of facilities for lipoprotein electrophoresis has however focused the clinical interest for classification of hyperlipoproteinemia into other means of a simple biochemical typing. In the present study a nomogram was developed to allow biochemical typing of hyperlipoproteinemia from serum cholesterol and triglyceride values. Serum cholesterol was determined according to a Liebermann-Burchard reaction by the method of Cramer and Isaksson [1], serum triglycerides by the determination of glyceride glycerol according to Carlson [2], and serum lipoprotein electrophoresis was performed on agarose gel [3]. Cholesterol content [4] of alpha-lipoproteins ("alpha-LP cholesterol") was obtained in serum after the precipitation of very-low-density lipoproteins(VLDL) and low-density lipoproteins(LDL) by manganese chloride and heparin [5]. Preparative ultracentrifugation was performed in a one-step, gradient procedure [6] isolating VLDL, d less than 1.006, LDL, d 1.006--1.063 g/ml and high-density lipoprotein (HDL) including very-high-density lipoproteins, d greater than 1.063 g/ml, or alternatively at d 1.006 g/ml according to the procedure by Gustafson et al. [7].