Decreased Production and Increased Catabolism of Apolipoprotein B-100 in Apolipoprotein B-67/B-100 Heterozygotes
Abstract Apolipoprotein (apo) B-67 is a truncated form of apoB-100 due to deletion of an adenine at cDNA 9327. Heterozygotes have one allele making apoB-100; therefore, plasma apoB levels would be predicted to be at least 50% of normal. However, apoB-67 heterozygotes have total plasma apoB levels that are 24% of normal. To determine the mechanisms responsible for the lower-than-expected levels of apoB, in vivo kinetics of apoB-100 were performed in three apoB-67/apoB-100 heterozygotes and compared with those of six control subjects by using a primed-constant infusion of [5,5,5-2H3]leucine in the fed state. Kinetic parameters were calculated by multicompartmental modeling of the data. The mean total apoB plasma concentration of the apoB-67 subjects was 21.8±6.1 mg/dL, or 24% of that of control subjects (89.6±24.1 mg/dL, P=.002). ApoB-67 subjects had lower mean VLDL apoB-100 production rates (3.6±1.2 versus 13.9±3.5 mg·kg−1·d−1, P=.002) and lower mean transport rates of apoB-100 into LDL (3.5±1.4 versus 12.6±4.1 mg·kg−1·d−1, P=.008) compared with control subjects. The transport rate into IDL was not significantly different (1.2±0.5 versus 6.2±4.0 mg·kg−1·d−1, P=.07). The fractional catabolic rate of VLDL apoB-100 was significantly higher in apoB-67 subjects than in control subjects (18.1±8.6 versus 7.6±1.6 mg·kg−1·d−1, P=.017). ApoB-100 IDL and LDL fractional catabolic rates were not significantly different. VLDL apoB-100 pool size in apoB-67 subjects was 11% of that of control subjects (15.8±7.7 versus 141.6±33.7 mg, P=.0004) due to a 74% lower production rate (26% of control values) and a 2.4-fold higher fractional catabolic rate. LDL apoB-100 pool size in apoB-67 subjects was 22% of that of control subjects (665.3±192.4 versus 2968.3±765.2 mg, P=.002) due primarily to a lower production rate (27% of control values). Thus, both decreased production of VLDL and LDL apoB-100 and increased catabolism of VLDL apoB-100 are responsible for the low levels of apoB-100 in apoB-67 subjects.
- Received May 31, 1996.
- Accepted August 9, 1996.
Familial hypobetalipoproteinemia is an autosomal codominant disorder characterized by low plasma concentrations of apoB and LDL-C. ApoB normally exists in two isoforms in plasma, apoB-100 and apoB-48,1 both of which are products of the same structural gene on chromosome 2.2 ApoB-100 is synthesized by the liver and secreted in the form of VLDL, which is metabolized in plasma to form LDL. ApoB-100 contains the LDL receptor–binding domain; therefore, VLDL remnants (IDL) and LDL are removed from the circulation by binding to hepatic LDL receptors.1 Chylomicrons contain apoB-48, which is synthesized in the intestine and produced as a result of a premature stop codon at the apoB-100 codon 2153 by tissue-specific mRNA processing.3 Both chylomicrons and VLDL are the major TG carriers in plasma, and the TG therein is hydrolyzed by lipoprotein lipase to form chylomicron remnants and VLDL remnants, respectively. ApoB-48 probably does not contain an LDL receptor–binding domain; therefore, the chylomicron remnants are most likely taken up by the liver by receptors that recognize apoE.4 5
Some cases of hypobetalipoproteinemia have been shown to be due to a truncated form of apoB-100.6 ApoB-67 is a truncated form of apoB in which affected family members have low levels of TC and LDL-C, low TG levels, elevated HDL-C levels, and the absence of clinical coronary heart disease.7 DNA sequencing in this apoB-67 kindred revealed a deletion of an adenine at cDNA 9327.7 This frameshift mutation converts a lysine to an arginine, followed by a premature stop codon (TAG), producing the apoB-67 molecule. Since apoB-67 subjects are heterozygous for the apoB-67 mutation, they have one normal allele making apoB-100; therefore, plasma apoB levels would be predicted to be 50% of normal. However, apoB levels are ≈24% of normal.
In this study, we investigated the mechanism for the lower-than-expected plasma levels of apoB in three individuals of the apoB-67 kindred by using a primed-constant infusion of deuterated leucine and performing multicompartmental modeling to determine apolipoprotein kinetic parameters.
Three subjects of the apoB-67 kindred (one premenopausal female and two males) and six normolipidemic control subjects (all male) underwent a medical history and physical examination. They had no evidence of any chronic illness including endocrine, hepatic, renal, thyroid, or cardiac dysfunction. They did not smoke and were not taking any medications known to affect lipid levels. The experimental protocol was approved by the Human Investigation Review Committee of the New England Medical Center and Tufts University.
Experimental Protocol for In Vivo Stable Isotope Kinetics
To determine the kinetics of VLDL, IDL, and LDL apoB-100, the subjects underwent a primed-constant infusion of deuterated leucine while they were in the fed state, as previously described.8 9 10 Starting at 6 am, the subjects received 20 identical small hourly meals, each equivalent to 1/20 of their daily food intake, with 15% of calories as protein, 49% carbohydrate, 36% fat (15% saturated, 15% monounsaturated, 6% polyunsaturated), and 180 mg cholesterol per 1000 kcal. At 11 am, with two intravenous lines in place, one for the infusate and one for blood sampling, [5,5,5-2H3]-l-leucine (10 μmol/kg body weight) was injected as a bolus intravenously over 1 minute and then by continuous infusion (10 μmol/kg body weight per hour) over a 15-hour period. Blood samples (20 mL) were collected at hours 0, 1, 2, 3, 4, 6, 8, 10, 12, and 15.
Plasma Lipid and Lipoprotein Characterization
Blood was collected in sterile tubes containing EDTA (0.1% final concentration). Plasma was separated from red cells in a refrigerated centrifuge at 3000 rpm for 30 minutes at 4°C. Plasma and lipoprotein fractions were assayed for TC and TG with an Abbott Diagnostics ABA-200 bichromatic analyzer with enzymatic reagents.11 12 HDL-C was measured as previously described.13 Lipid assays were standardized through the Centers for Disease Control Lipid Standardization Program.
The VLDL (d<1.006 g/mL), IDL (d=1.006 to 1.019 g/mL), and LDL (d=1.019 to 1.063 g/mL) fractions were isolated from fresh plasma by ultracentrifugation.14 ApoB was assayed in plasma and lipoprotein fractions with a noncompetitive, enzyme-linked immunosorbent assay using immunopurified polyclonal antibodies.15 The coefficient of variation for the apoB assay was <5% within runs and <10% between runs.15
Quantitation and Isolation of the Apolipoproteins
ApoB-100 was isolated from lipoproteins by preparative SDS–polyacrylamide gradient gel electrophoresis by using a Tris-glycine buffer system as previously described.16 17 ApoB concentration within individual apoB species was assessed by scanning each gel with laser densitometry as previously described.17 18 We scanned VLDL, IDL, and LDL fractions from each time point and averaged all 10 to calculate ratios and estimate concentrations of apoB-100.
Isotopic Enrichment Determinations
ApoB-100 bands were excised from polyacrylamide gels. Plasma (0.3 mL) and the excised apoB-100 bands were hydrolyzed in 12N HCl at 100°C for 24 hours. The plasma was subjected to cation exchange chromatography, and both were converted to the n-propyl ester, N-heptafluorobutyramide derivatives before analysis on a Hewlett Packard 5890/5988A gas chromatograph/mass spectrometer.8 9 10 Isotope enrichment (percent) and tracer/tracee ratio (percent) were calculated from the observed ion current ratios using the method of Cobelli et al.19 Data in this format are analogous to specific radioactivity in radiotracer experiments. The isotopic enrichment of leucine in the apolipoproteins was expressed as tracer/tracee ratio (percent).19
The kinetics of apoB-100 in VLDL, IDL, and LDL fractions were described by a modification of a multicompartmental model previously used to describe the kinetics of apoB in subjects heterozygous for apoB-75,20 apoB-89,21 apoB-54.8, apoB-52, and apoB-31,22 in normolipidemic subjects who had plasma cholesterol levels between the 5th and 50th percentiles of age-matched control subjects23 and subjects with cholesteryl ester transfer protein deficiency.24 The SAAM II program was used to fit the model to the observed tracer data, using a weighted least-squares approach to find the best fit.25 The model (Fig 1⇓) consists of a precursor compartment (compartment 1), which is the plasma leucine pool. Compartment 2 is an intracellular delay compartment accounting for the synthesis of apoB and the assembly of lipoproteins. Compartments 4, 5, and 6 are used to account for the kinetics of the VLDL apoB-100 fraction and represent a classical delipidation chain, a concept originally proposed by Phair et al26 and supported by others.27 28 29 In the previous models,20 21 22 23 24 two VLDL compartments were used; in the current case, however, three compartments in the delipidation chain provided a better fit for the appearance of tracer in VLDL and IDL pools. The delipidation chain represents VLDL particles that turn over more slowly, whereas compartment 3 represents more rapidly turning–over VLDL particles. The rate constants between compartments 4, 5, and 6 are set as being equal, as previously described.26 The rate constants for direct removal from compartments 4, 5, 6, and 7 are also set as being equal. IDL apoB-100 (compartment 7) can be derived from either VLDL compartment 3 or the delipidation chain (compartments 4 through 6). LDL apoB-100 (compartment 8) is derived from the IDL fraction (compartment 7) or directly from VLDL compartment 3 through a rapidly turning–over VLDL compartment via a shunt pathway as previously described.20 21 22 23 24
It is assumed that plasma leucine (compartment 1) is the source of the leucine that is incorporated into apoB-100 and that all apoB-100 enters the plasma via compartment 3. Therefore, transport rates into compartment 3 correspond to total apoB-100 production. We have shown that plasma leucine enrichments remain constant during the course of the infusion.8 10 We therefore assumed a constant enrichment of the precursor pool and used a constant precursor enrichment in calculating the kinetic parameters. We assumed that the kinetics of leucine in plasma reflect the kinetics of leucine before its incorporation into apoB-100. Consequently, the plasma leucine tracer/tracee ratio data were used as a forcing function to drive the appearance of tracer into apoB-100.
After fitting the model to the observed data, FCRs and PRs (also referred to as transport rates for IDL and LDL) for apoB-100 were calculated. The FCR of VLDL apoB-100 is the weighted average (relative to mass distribution) of the FCRs of pools 3, 4, 5, and 6. The FCR of each VLDL pool is the sum of individual rate constants, including conversion to IDL or LDL, as well as removal from plasma: for compartment 3, rate constants relating to the following metabolic pathways were k(4,3), k(7,3), and k(8,3); for compartments 4, 5, and 6, rate constants relating to k(5,4) and k(0,4), k(6,5) and k(0,5), and k(7,6) and k(0,6), respectively. The FCR of IDL apoB-100 corresponds to the sum of individual rate constants of compartment 7, k(8,7) and k(0,7). The FCR of LDL apoB-100 corresponds to the rate of irreversible loss from compartment 8, k(0,8). The term “rate constant” refers to the fractional turnover rate of a pool related to a specific pathway (eg, the fraction of compartment 3 that is converted to compartment 4 per day).
Due to the uncertainty related to low concentration of apoB-100 in VLDL and IDL pools, we fixed the calculated value to the measured value for LDL apoB-100 and left VLDL and IDL masses adjustable. The measured VLDL and IDL apoB-100 masses were added to the analysis as weighted data.
It is assumed that each subject remains in steady state with respect to apoB-100 metabolism during the course of the study. Under this condition, the FCR is equivalent to the fractional synthetic rate. ApoB PRs were determined by the formula: PR (milligrams per kilogram per day)=[FCR (pools per day)×apoB concentration (milligrams per deciliter)×plasma volume (liters)]/body weight (kilograms).9 10 Plasma volume was estimated as 4.5% of body weight.
Data were analyzed using the SigmaStat program and presented as mean±SD and percent fractional SD. Unpaired t tests were performed. Values of P≤.05 were considered to be significant.
Characteristics of the Subjects
The characteristics of the apoB-67 subjects and control subjects are shown in Table 1⇓. Each male apoB-67 subject was matched to one male control subject of the same race and apoE genotype within 1 year of age and two additional male control subjects within 2 to 4 years of age. We did not have a female control subject for the premenopausal female apoB-67 subject; therefore, we performed statistical analyses with and without her. The nonfasting plasma apoB concentrations in the VLDL, IDL, and LDL lipoprotein fractions are shown in Table 2⇓. These values represent means of measures at all 10 time points during the study period. There was no significant difference in age, weight, or body mass index between the subjects and control subjects. The mean TC, LDL-C, TG, and apoB levels were all significantly lower and HDL-C levels were significantly higher in apoB-67 subjects than in control subjects. The mean total apoB plasma concentration of the apoB-67 subjects was 21.8±6.1 mg/dL or 24% of that of the control subjects (89.6±24.1 mg/dL).
In Vivo Kinetics of ApoB-100
To determine the mechanism for the low levels of apoB-100–containing lipoproteins in the apoB-67 kindred, kinetic studies were performed in three apoB-67/apoB-100 heterozygotes and six control subjects. During the kinetic studies, plasma apoB and lipid concentrations did not change significantly throughout the infusion period, as has been shown previously,8 10 indicating steady state conditions. Plasma leucine tracer/tracee ratios ranged from 7% to 9% for all the subjects and remained constant during the course of the infusion, as has been previously shown.10 The VLDL, IDL, and LDL apoB-100 leucine tracer/tracee ratios and model predicted values are shown for the apoB-67 subjects in Fig 2⇓ and the six control subjects in Fig 3⇓. There is good agreement between the model-derived fits and the observed tracer data.
Table 3⇓ shows the VLDL, IDL, and LDL pool sizes, FCRs, and PRs. ApoB-67 subjects had lower VLDL apoB-100 PRs (3.6±1.2 versus 13.9±3.5 mg·kg−1·d−1, P=.002) and lower transport rates of apoB-100 into LDL (3.5±1.4 versus 12.6±4.1 mg·kg−1·d−1, P=.008) compared with control subjects. The transport rate into IDL was not significantly different (1.2±0.5 versus 6.2±4.0 mg·kg−1·d−1, P=.070). The FCR of VLDL apoB-100 was significantly higher in apoB-67 subjects than in control subjects (18.1±8.6 versus 7.6±1.6 mg·kg−1·d−1, P=.017). ApoB-100 IDL and LDL FCRs were not significantly different. VLDL apoB-100 pool size in apoB-67 subjects was 11% of that of control subjects (15.8±7.7 versus 141.6±33.7 mg, P=.0004) due to a 74% lower PR (26% of control value) and a 2.4-fold higher FCR. LDL apoB-100 pool size in apoB-67 subjects was 22% of that of control subjects (665.3±192.4 versus 2968.3±765.2 mg, P=.002) due primarily to a lower PR (27% of control value). We also performed the analyses without the apoB-67 female subject. After exclusion of her data, the FCR of VLDL apoB-100 was still significantly higher in apoB-67 subjects than in control subjects (13.2±2.4 versus 7.6±1.5 pools per day, P=.008). It is of note that the probability value is more significant after exclusion of the female subject due to the lower SD. None of the other analyses changed significantly after exclusion of the female subject. Thus, both decreased production of VLDL and LDL apoB-100 and increased catabolism of VLDL apoB-100 are responsible for the low levels of apoB-100 in apoB-67 subjects.
Table 4⇓ shows the metabolic channeling of apoB-100. Fractional conversion rates of VLDL to LDL were not significantly different between apoB-67 subjects and control subjects. There were no significant differences in the proportion of VLDL apoB-100 converted to LDL via the shunt pathway in apoB-67 subjects compared with control subjects (64.9±4.2% versus 47.5±20.0%, respectively; P=.193), via the delipidation cascade (22.8±5.7% versus 41.2±23.9%, respectively; P=.245), and via the VLDL-IDL-LDL pathway (12.3±7.5% versus 11.4±7.0%, respectively; P=.855).
Table 5⇓ shows the rate constants of individual lipoprotein pools and delay. These are the rate constants for each individual compartment in each subject; they correspond to the arrows in the model depicted in Fig 1⇑. They are used to derive the FCRs, which are listed for each individual subject in Table 3⇑. The fractional SDs provide a measure of the error for each rate constant. In four subjects, the delay time was fixed at 30 minutes due to inability to estimate d(3,2) with certainty. Varying the delay from 12 to 80 minutes in these four subjects changed the FCRs and PRs from 2% to 4%, which did not significantly affect the results. Table 6⇓ shows the calculated and measured masses of VLDL and IDL pools.
The apoB-67 subjects are heterozygous for the apoB-67 mutation and have one normal apoB-100 allele; therefore, one would predict the level of apoB-100 to be at least 50% of normal. However, the levels are 24% of normal. The kinetic analysis in the present study demonstrates that apoB-100 levels are lower than expected in the apoB-67 subjects due to decreased production of VLDL and LDL apoB-100 and increased catabolism of VLDL apoB-100.
The mechanism for the decreased production of apoB-100 in apoB-67 heterozygotes is unknown. Perhaps the defective apoB-67 allele alters the transcription of the normal apoB-100 allele in the apoB-67 mutation or affects mRNA translation rates, thus affecting apoB-100 mRNA levels. Posttranscriptional events could also be affected. Studies of cultured hepatoma cells demonstrate that apoB-100 secretion is under posttranscriptional control and that most of the apoB is degraded before secretion.30 31 32 33 34 35 There is evidence that apoB is produced intracellularly at a constant rate but that the amount secreted depends on the amount of lipid available to prevent intracellular degradation.30 31 32 33 34 35 Addition of oleate to culture medium resulted in increased secretion of apoB-containing lipoprotein particles as a result of decreased degradation of apoB before secretion.32 33 34 Therefore, it is possible that decreased availability of hepatic lipid, whether TG, cholesterol ester, or phospholipid, may result in enhanced intracellular apoB-100 degradation before secretion in the apoB-67 subjects.
The lower-than-expected levels of apoB-100 are also due to a faster catabolic rate of VLDL apoB-100 in the apoB-67 subjects. One possible reason for this could be a different content of apoE in VLDL apoB-100 in apoB-67 subjects compared with control subjects. There are two alleles making apoE but only one making apoB-100 in apoB-67 heterozygotes; therefore, the apoB-67 subjects may have a greater proportion of apoE than apoB-100 in VLDL apoB-100 particles compared with the VLDL apoB-100 particles in the control subjects. However, one would predict that this might result in increased receptor-mediated direct removal of the VLDL apoB-100, which is not the case, since fractional conversion rates of VLDL apoB-100 to LDL apoB-100 were not significantly different in apoB-67 subjects compared with control subjects.
The metabolic behavior of VLDL lipoprotein particles has been shown to be affected by particle size; larger VLDL (Sf 60-400) is catabolized more rapidly than smaller VLDL (Sf 20-60).36 37 Perhaps the majority of VLDL lipoprotein particles in apoB-67 heterozygotes are larger than the VLDL in the control subjects, such that catabolism via lipoprotein lipase occurs faster. Alternatively, the VLDL in apoB-67 heterozygotes may have a different conformation and interact more favorably with lipoprotein lipase. VLDL particle composition may also be a factor affecting interaction with lipoprotein lipase and a factor in determining the FCR.
One prior study has examined the kinetics of apoB-100 in two apoB-89, two apoB-75, two apoB-54.8, one apoB-52, and one apoB-31 heterozygote subject using stable isotopes.22 The production of apoB-100 was decreased in all eight heterozygous subjects compared with the control subjects. The VLDL apoB-100 FCRs were variable. In two of the pairs, the apoB-54.8 and apoB-52 heterozygotes, the VLDL apoB-100 FCR was twice that of the control subject, suggesting that in these heterozygotes with truncated apoBs, an increased VLDL apoB-100 FCR may also contribute to the low apoB-100 levels.22 In the remaining heterozygous subjects, the VLDL apoB-100 FCRs were similar to those of the matched control subject, with the exception of one apoB-89 heterozygote, in whom the VLDL apoB-100 FCR was half that of the matched control subject. This latter pair was not matched for apoE genotype. Our study included three apoB-67 heterozygotes, and it is of note that the VLDL apoB-100 FCR was significantly faster in all three compared with the control subjects. In contrast to the prior study, which examined subjects in the fasting state, our subjects were studied in the fed state. There may be differences in catabolism of TG-rich VLDL apoB-100 related to feeding and the amount of TG on the particle. However, since two heterozygotes, the apoB-54.8 and apoB-52 subjects, had VLDL apoB-100 FCRs twice that of control subjects when studied in the fasting state,22 the fact that the apoB-67 subjects were studied in the fed state is less likely to be the reason for the faster VLDL apoB-100 FCRs in the present study.
Further studies will be required to determine the reasons for the decreased production and increased catabolism of apoB-100 in subjects with apoB truncations. Since elevated levels of apoB are related to the development of coronary heart disease, the mechanisms leading to naturally occurring low levels of apoB-100 are of great interest.
Selected Abbreviations and Acronyms
|FCR||=||fractional catabolic rate|
This research was supported by grants HL-02626 (Dr Welty), HL-39326 (Dr Schaefer), HL-49110 (Dr Barrett), and RR-02176 (Dr Barrett) from the National Institutes of Health. Dr Welty is the Irving and Charlotte Rabb Harvard Scholar in Medicine in memory of Dr Grete Bibring and in honor of the 50th anniversary of admission of women to Harvard Medical School. Dr Welty was a Research and Teaching Scholar of the American College of Physicians. We are grateful to the apoB-67 kindred for their participation in the kinetic studies.
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