© 1996 American Heart Association, Inc.
Articles |
Effects of Colestipol Alone and in Combination With Simvastatin on Apolipoprotein B Metabolism
From the Institute of Biochemistry (A.G., C.J.P., G.M.L., E.F.M., B.A.G., M.J.C., J.S.), Department of Cardiac Surgery (I.C., D.J.W.), and the Department of Medical Cardiology (A.R.L.), Glasgow Royal Infirmary, Glasgow, UK.
Correspondence to Dr A. Gaw, Department of Pathological Biochemistry, Royal Infirmary University/NHS Trust, Glasgow G31 2ER, UK.
 |
Abstract |
|---|
 Top Abstract IntroductionMethods Results Discussion References |
|---|
Abstract The effects of colestipol therapy alone (20 g/d) or combined with simvastatin (20 mg/d) were examined in a group of eight male patients with primary moderate hypercholesterolemia (total cholesterol
6.5 mmol/L [250 mg/dL]) who had undergone
coronary artery bypass grafting more than 3 months previously.
Colestipol therapy decreased total cholesterol by 14%
(P<.001) and LDL cholesterol (LDL-C) by 23%
(P<.001), while dual therapy decreased total
cholesterol by 38% and LDL-C by 52% (both
P<.001 versus baseline). No significant changes were
observed in plasma triglyceride, VLDL
cholesterol, or HDL cholesterol levels. VLDL
subfraction turnovers were conducted at baseline and again on each
regimen. ApoB kinetic parameters derived from a
multicompartmental model suggested that colestipol therapy resulted in
an expansion of the total VLDL apoB pool (36%, P<.05) that
was largely due to a fall in the clearance rate of VLDL1
apoB (49%), while the LDL apoB pool decreased 23% as a result of
diminished direct LDL input. The model used also revealed that addition
of simvastatin to the resin therapy caused increases in the
fractional transfer rates of VLDL2 to IDL and IDL to
LDL together with a 37% increment in the LDL apoB fractional catabolic
rate. Compared with baseline, combined therapy generated falls in both
IDL (35%, P=.01) and LDL (37%, P<.04) apoB
pools due to enhanced clearance of IDL (214%, P<.03) and
reduced total input of LDL (39%, P<.003).
Key Words: VLDL turnover • bile acid sequestrant resin • 3-hydroxy-3-methylglutaryl coenzyme A–reductase inhibitor
|
Introduction |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
Interruption of the enterohepatic circulation and consequent hepatic depletion of the bile acid pool has been demonstrated by pharmacological and surgical means to be both effective as a lipid-lowering strategy and as a preventive measure for coronary heart disease.1 2 3 4 Resin therapy can reduce plasma TC by 18% to 25%,1 5 6 7 with average LDL-C reductions of 25% to 35%.1 6 7 Most studies also report small increases in plasma TG levels.1 6 7 This change is usually not considered clinically relevant except in those patients with baseline hypertriglyceridemia. Furthermore, Witztum et al8 have shown that the TG rise is transient, persisting for only 2 to 10 weeks. Small increments in HDL-C have been reported,1 7 as have small but favorable increases in the HDL2-to-HDL3 ratio.7 9
The resins primarily exert their effects on the lipoprotein profile by
preventing both the passive and active reabsorption of bile acids. As
noted by Angelin et al,10 the metabolic
consequences of this are of considerable importance, affecting
cholesterol and TG synthesis and the activities of three
key hepatic enzymes: cholesterol 7
hydroxylase,
phosphatidic acid phosphatase, and HMG CoA reductase.
Combined drug therapy for hyperlipidemia, especially with a statin and a resin, has been used in FH,11 12 in those who do not respond adequately to monotherapy,13 and in patients with established coronary heart disease, in whom inhibition of the progression of atherosclerosis and perhaps the induction of its regression are possible with profound lipid lowering.14 15 HMG CoA reductase inhibitor therapy alone has also been used with great effect to reduce overall mortality in the secondary prevention of coronary heart disease.16
The first investigation into the effects of such a combination of drugs on lipoprotein metabolism was reported by Bilheimer et al,17 who studied the apolipoprotein-LDL kinetics in a male FH heterozygote before and during lovastatin and colestipol therapy. The marked fall in LDL-C that they observed was solely attributed to an increase in apolipoprotein-LDL clearance. Grundy et al18 went on to investigate the same combination of drugs in a larger group of FH heterozygotes. They again showed the regimen to be highly effective in reducing LDL-C levels (52%), but on this occasion they attributed the fall to a combination of increased clearance and decreased production of apolipoprotein-LDL. A more recent study19 of FH heterozygotes and the same combination of lovastatin and colestipol reports that the fall in LDL-C level could be attributed only to enhanced LDL apoB clearance.
Further information has been provided by studies with lovastatin and cholestyramine in miniature pigs20 21 . The model used by Huff and Telford21 to explain their data suggests that the combined drug regimen causes a marked reduction in LDL direct input, while the LDL derived from VLDL is unaffected. Furthermore, they report21 an increase in VLDL clearance, suggesting that although no obvious increase in the LDL apoB FCR was observed, these drugs may still enhance LDL receptor activity, thus resulting in the removal of LDL precursor lipoproteins from the circulation. The only published study designed to examine the effects of this drug combination in the same type of patients studied here (ie, with primary moderate hypercholesterolemia) is reported by Vega and Grundy,22 who studied a group of 10 subjects with an initial plasma TC >6.5 mmol/L (>250 mg/dL) and normal plasma TG levels. They concluded that the significant reduction they observed in LDL-C (48%) was due to a combination of three factors: a 27% fall in the production rate of apolipoprotein-LDL, a 20% rise in the apolipoprotein-LDL FCR, and a 15% depletion in the cholesterol content of LDL particles. Thus, these workers drew attention to the possibility of mixed mechanisms of LDL-C lowering in those subjects with primary moderate hypercholesterolemia.
The aim of the present study was to test the hypothesis that colestipol alone or in combination with simvastatin, when administered to a group of moderately hypercholesterolemic individuals, affects apoB-containing lipoproteins (VLDL1 [Svedberg flotation unit 60 to 400], VLDL2 [Sf 20 to 60], IDL [Sf 12 to 20], and LDL [Sf 0 to 12]) by increasing their catabolic rate.
|
Methods |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
Protocol
During the 5-week preliminary screening period, each patient was screened for cardiological, hematologic, hepatic, endocrine, renal, ophthalmological, and metabolic disease by routine clinical and laboratory testing to exclude secondary causes of hyperlipidemia and any illness thought to influence the outcome of the study. Baseline studies included a VLDL turnover study on each patient to serve as a control, thus allowing apoB kinetic parameters to be derived from a multicompartmental model. Baseline lipid, lipoprotein, and lipoprotein subfraction concentrations were measured as described below. During the first part of the active treatment period, immediately after the baseline assessment of apoB metabolism, the patients began colestipol therapy, rising to a dose of 20 g/d over 7 to 10 days. Patients remained on this dose for 10 weeks. During the final 2 weeks each patient underwent a second turnover investigation while on colestipol therapy, and lipid, lipoprotein, and lipoprotein subfraction analyses were repeated. During the second part of the active treatment period, immediately after completing the second VLDL turnover study, the patients began a combined regimen of simvastatin 20 mg nocte and colestipol 20 g/d. Patients remained on these doses for 10 weeks. During the final 2 weeks each patient underwent a third and final VLDL turnover investigation while on combined colestipol and simvastatin therapy, and lipid, lipoprotein, and lipoprotein subfraction analyses were again repeated.
Subjects
Eight male study patients were recruited from the
cardiac
surgery database at Glasgow Royal Infirmary. All patients had an
initial elevated TC level (6.5 mmol/L [250 mg/dL]) despite
adherence to a standard lipid-lowering diet23 and a TG
level <3.0 mmol/L (<265 mg/dL) but without clinical evidence of FH.
All subjects had undergone coronary artery bypass grafting
between 3 and 12 months previously. The characteristics of each patient
are summarized in Table 1
. All eight subjects were
receiving prescribed medications as part of their postoperative
management, and four were receiving drugs for other clinical
conditions. The fact that the subjects were taking other drugs may have
influenced their baseline and subsequent kinetic studies. This should
be noted when comparing the present group of subjects with others
not on medication. All prescribed medications were continued unchanged
throughout the course of the study; thus, intraindividual comparisons
are valid.
|
All subjects participating in the study gave informed, written consent. The study met the requirements of the ethics committee of Glasgow Royal Infirmary.
Lipid and Lipoprotein Analyses
Plasma lipid and lipoprotein
levels (Table 2)
were measured on three occasions throughout each 13-day turnover period
according to the protocol of the Lipid Research Clinics
Program.24 Plasma LDL subfraction profiles were determined
by nonequilibrium density-gradient centrifugation
as described by Griffin et al.25 This separation technique
generated LDL subfraction profiles in which it was possible to resolve
three subfractions that corresponded in size and density to those
described by Krauss,26 ie, LDL-I (d=1.025 to
1.034 g/mL), LDL-II (d=1.034 to 1.044 g/mL), and LDL-III
(d=1.044 to 1.060 g/mL). The individual subfraction areas
beneath the LDL profile were quantified by using Data Leader software
(Beckman). The integrated areas, after being adjusted by specific
extinction coefficients previously calculated for LDL-I through
LDL-III, were expressed as fractions of total LDL mass by proportioning
the lipid and protein mass of total LDL (d=1.019 to 1.063
g/mL). This provided concentration values for each LDL subfraction in
milligrams of lipoprotein per 100 mL of plasma.25 ApoE
phenotypes were determined by an adaptation of the
Western blotting technique of Havekes et al.27
|
Lipoprotein Isolation and Labeling
The methods for
preparation of tracer VLDL subfractions
(VLDL1 [Sf 60 to 400] and VLDL2 [Sf
20 to 60]) are available.28 Briefly, 250 mL fasting
plasma was collected by plasmapheresis, and from this, total VLDL
(d<1.006 g/mL, Sf 20 to 400) was isolated by
centrifugation for 22 hours at 39 000 rpm and 10°C
in a Beckman Ti60 rotor. The supernatant VLDL was harvested and used
for the preparation of the subfractions. The VLDL solution was adjusted
to d=1.118 g/mL by the addition of solid NaCl (0.34 g/2 mL
solution) and layered in a Beckman SW40 rotor tube that had been
precoated with polyvinyl alcohol to reduce internal surface tension and
permit improved layering. A six-step gradient from
d=1.099 through d=1.059 g/mL was constructed
above the sample, and centrifugation was performed at
23°C to separate sequentially VLDL1 (1 hour and 38
minutes at 39 000 rpm) and VLDL2 (15 hours and 41
minutes at 36 000 rpm). The subfractions were labeled with
131I-Na and 125I-Na, respectively, by a
modification of the iodine monochloride method29 and were
sterilized immediately before reinjection by filtration through a
0.45-µm filter (Acrodisc, Gelman Sciences).
Turnover Protocol
On the third day after plasmapheresis each
subject was admitted
at 8 AM after an overnight fast. An indwelling cannula was
placed in a peripheral vein to facilitate repeated venous
blood sampling. This was flushed with sterile 0.15 mol/L NaCl to
maintain patency. Autologous 131I-VLDL1 and
125I-VLDL2 were injected in rapid
sequence into a peripheral vein in the opposite arm. These
injectates were administered separately and chased with boluses of
sterile 0.15 mol/L NaCl. Venous blood samples (10 mL) were collected
via the cannula at the following times after injection: 10 and 30
minutes and 1, 1.5, 2, 3, 4, 6, 8, 10, and 14 hours; thereafter 10-mL
fasting venous samples were obtained each morning for the next 12 days.
All samples were collected in tubes containing potassium-EDTA as
anticoagulant to give a final concentration of 1 mg/mL. To minimize
chylomicron production the subject remained fasting for the
first 10 hours of the study but was allowed unlimited noncaloric
fluids. Between the 10- and 14-hour samples the patient was allowed a
small meal. It is implicit in our analysis that at this late
time point after the 22-hour fast, the consumption of a small meal had
no impact on LDL apoB kinetics. At 14 hours less than 
10% of
injected radioactivity remained in the VLDL fractions.
Plasma was obtained from each of the blood samples by low-speed centrifugation at 1000g at 4°C. From 2.0-mL aliquots of plasma, apoB-containing lipoproteins (VLDL1 [Sf 60 to 400], VLDL2 [Sf 20 to 60], IDL [Sf 12 to 20], and LDL [Sf 0 to 12]) were isolated by a modification28 of the cumulative-gradient ultracentrifugation procedure of Lindgren et al.30 ApoB was precipitated by adding an equal volume of freshly redistilled 1,1,3,3-tetramethylurea at 37°C to each lipoprotein fraction.31 Specific activities were determined by radioactivity counting and protein assay.32
ApoB concentrations in the lipoprotein classes were determined by replicate analyses of lipoproteins (Sf 0 to 400) collected at four times during the study. Correction for centrifugal losses was made by comparing the recovered VLDL1+VLDL2+IDL+LDL cholesterol with the "non-HDL" cholesterol in plasma.32 Pool sizes for apoB in the four lipoprotein fractions were calculated from the product of plasma volume (taken as 4% of body weight) and the plasma concentration of apoB in each fraction. The constancies of the VLDL1, VLDL2, IDL, and LDL apoB pool sizes measured over the course of the study were taken as an indication that the individuals were in steady state.
The composition of each lipoprotein fraction was determined by assaying TC, esterified cholesterol, TG, phospholipid, and protein concentrations.33 The concentrations of TC, TGs, free cholesterol, and phospholipids in the lipoprotein fractions were determined by using enzymatic, colorimetric assays and commercially available reagents (BCL; catalogue Nos. 816302 [cholesterol], 816370 [TGs], 310328 [free cholesterol], and 691844 [phospholipids]). Protein measurements were performed according to the method of Lowry et al.34
In all studies it was essential to ensure that thyroidal uptake of radioiodide had been blocked by the oral administration of potassium iodate (170 mg/d BID). This regimen began 3 days before the injection of radiolabeled lipoproteins and was continued for the next 28 days. The turnover studies were conducted on an outpatient basis, and all subjects were instructed to adhere strictly to their established lipid-lowering diet and lifestyle. This was done to ensure free-living steady-state conditions for the investigation of lipoprotein metabolism.
Kinetic Analysis
The radioactivity associated with the apoB
present in each
of the four lipoprotein fractions was calculated as the product of
apoB-specific activities and the individual pool sizes. These were then
expressed as a percentage of the total apoB radioactivity present
in the subjects' plasma 10 minutes after injection, and the resulting
values were used to construct decay curves. These were analyzed
by the SAAM 30 multicompartmental modeling program35 on a
VAX system. The metabolic model used (Fig 1)
is essentially that used
previously,28 32 36 37 and a
description of the development and validation of the model and the
modeling strategy is available.38 The model has five main
features. First, apoB direct input may occur at the level of
VLDL1, VLDL2, or LDL. IDL
direct input was not required in any subject at baseline or on therapy,
since the calculated IDL apoB pool size consistently matched
the observed value. Second, this kinetic model involves a direct input
component for LDL apoB. Third, VLDL is delipidated in a stepwise manner
according to the concept of Berman et al.39 Fourth, slowly
catabolized remnant pools are present in VLDL2
and IDL. Fifth, two delipidation pathways from VLDL2
through IDL to LDL are present in parallel.
|
The rate constants, fluxes, and apoB masses were compared in each subject before and after each treatment phase by one-way ANOVA and Fisher's pairwise comparisons by using the software package Minitab Release 10 for Windows (Minitab Inc).
|
Results |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
Changes in Lipid and Lipoprotein Levels
Baseline data from the eight subjects are available40 ; their plasma lipid and lipoprotein levels are shown in Table 2
. Colestipol treatment reduced TC by 14%
(P<.001). This decrement was due to a 23% fall in LDL-C
(P<.001), while VLDL-C and HDL-C were unchanged. There was
also, as expected, a nonsignificant rise in TGs of 21%. Combined
treatment reduced TC by 38% (P<.001 versus baseline); this
decrement was due to a 52% fall in LDL-C (P<.001 versus
baseline). VLDL-C remained unchanged, and TG and HDL-C levels were not
significantly affected by combined therapy.
There was some
interindividual variation in the LDL subfraction profile
pattern observed, but in most subjects three distinct populations of
particles could be distinguished both before and during therapy. One
data set (for subject COL 08 on colestipol) was lost during
analysis, and comparison of means was performed on the
remaining seven pairs of data. The decrement in the major LDL-II
species was the most significant change in these subjects (Table
2).
Quantitative analysis revealed that its concentration fell by
38% (P<.001). LDL-I, the least dense fraction, was also
reduced by 38% (P<.001). The colestipol-induced
response in LDL-III was variable (Table 2). With combined
colestipol and simvastatin therapy there was a marked fall
in total LDL mass (54%, P<.001) that could be explained by
profound decreases in both of the larger, more buoyant species, LDL-I
and LDL-II (72%, P<.001, and 61%, P<.001,
respectively). Despite a 31% reduction in the mean value of the
smaller, denser LDL-III (Table 2), the overall effect was not
significant.
Colestipol therapy resulted in nonsignificant decreases in the percentage of free cholesterol of VLDL2 and IDL and a significant fall in that of LDL (11.5 to 8.1 g/100 g, P<.001). Further decreases were observed in all apoB-containing lipoproteins when simvastatin was added to the regimen, but only LDL achieved significance (11.5 to 5.3 g/100 g, P<.001). The relative cholesteryl ester content of all species was relatively unchanged by colestipol monotherapy or combined colestipol and simvastatin therapy. Further relative TG enrichment with combined therapy was observed in all species except LDL. No changes were observed in the relative phospholipid or total protein content of the apoB-containing particles in response to colestipol monotherapy or combined therapy. Overall, the apoB-containing lipoproteins became relatively cholesterol depleted and TG enriched by both colestipol monotherapy and the combined regimen.
Effects of Colestipol on ApoB Metabolism
The influence of
colestipol therapy on apoB metabolism
was examined before and during drug treatment
(representative decay curves for one subject, COL 01,
are shown in Figs 2 and 3). ApoB
disappeared rapidly from the VLDL1 flotation interval and
appeared in VLDL2 6 to 18 hours after injection. The
decay of radioactive apoB in VLDL2 was slower than
in VLDL1, taking almost 2 days to reduce to 1% of
the injected dose. Colestipol therapy appeared to increase the
clearance rate of both VLDL1 and VLDL2
apoB (Fig 3). The flux of apoB into IDL was comparable before
and
during therapy for both tracers. However, the decay rate of IDL apoB
appeared faster during drug treatment, with the peak value attained in
the LDL interval being consistently less than at baseline.
Before therapy, LDL apoB radioactivity reached a maximum of 25% of the
injected dose about 24 hours after VLDL2 injection
(Fig 2), whereas during therapy (Fig 3) the LDL
apoB maximum was only
15%. These findings suggest that the metabolic fate of IDL
apoB is markedly affected by therapy, with more material being lost
directly from the circulation rather than being converted to LDL. In
this subject (COL 01) the clearance rate of LDL apoB radioactivity
appeared unchanged by colestipol therapy.
|
|
Kinetic rate constants and
apoB fluxes were derived from the
compartmental model shown in Fig 1 by using the approach
described
above. The results for all eight subjects are summarized in Table
3a through 3d, and
individual kinetic constants and masses are shown in the
"Appendix" (Tables 4 through
7).
|
|
|
|
|
|
|
|
VLDL1 apoB input, pool size, and fractional transfer rate
to VLDL2 were largely unchanged by therapy, but the
mean direct FCR of VLDL1 apoB was reduced by 49%, from 2.5
to 1.3 pools/d. The VLDL2 apoB pool size increased
by 36% on drug therapy. This alteration in pool size was associated
with increases in VLDL2 apoB direct input in six of
the eight subjects and increased input from VLDL1 in seven.
IDL formation from VLDL2 was unaffected by resin
therapy, but there was a 13% fall in the mean IDL apoB plasma pool
size, from 781 to 678 mg. The IDL-to-LDL fractional transfer rate was
unaffected by drug treatment, but the amount of LDL apoB derived from
VLDL2 plus IDL fell significantly
(P<.02). There was also a fall in the direct input of LDL
apoB in every subject that resulted in a mean decrement of 61%
(P<.005). The calculated VLDL-derived LDL apoB plasma pool
was unaffected by therapy, but the observed total LDL apoB mass, which
includes apoB unaccounted for by that generated through the
delipidation cascade, was reduced from 3470 to 2657 mg. The mean FCR of
LDL apoB, which may have been expected to rise on therapy, was
unchanged, as was the mean calculated total apoB synthetic rate. The
observed fall in LDL in this group of eight subjects may be accounted
for by the reduction in total LDL apoB input (flux from
VLDL2 and IDL plus direct input) of 32%
(P<.003) rather than any change in catabolism. Fig
4 summarizes the flux of apoB through the delipidation
cascade. Following therapy the direct catabolism from the
VLDL2 and IDL density ranges was increased
significantly (P<.05), while there was reduced clearance of
VLDL1 apoB and direct input of LDL apoB.
|
P<.05 vs colestipol monotherapy.
Effects of Colestipol and Simvastatin on ApoB
Metabolism
The influence of colestipol and simvastatin therapy on
apoB metabolism was examined before and on the combined
drug regimen (representative plasma disappearance
curves for one subject, COL 01, are shown in Figs 2 and
5). Combination therapy with colestipol and
simvastatin did not appear to increase the clearance rate
of VLDL1 apoB, but in this individual the slope of the
VLDL2-apoB curve and therefore its clearance rate
were increased. The flux of apoB into IDL was comparable before and
during combined therapy for both tracers. However, the decay rate of
IDL apoB was much faster on combined drug therapy, with the peak value
attained in the LDL interval being consistently less than at
baseline. Before therapy, LDL apoB radioactivity reached a maximum of
15% of the injected dose about 48 hours after VLDL1
injection (Fig 2), whereas during therapy (Fig
5) the LDL apoB maximum
was only 5%. These findings suggest that the metabolic
fate of IDL apoB is markedly affected by therapy, with more material
being lost directly from the circulation rather than being converted to
LDL. In this subject (COL 01) the clearance rate of LDL apoB
radioactivity was also increased by combination therapy. Kinetic rate
constants and apoB fluxes for all eight subjects were again derived by
compartmental modeling (Table 3a through 3d).
Individual kinetic
constants and masses are shown in the "Appendix" (Tables 4 through
7).
|
VLDL1 apoB input, pool size, and fractional transfer
rate to VLDL2 were unchanged from baseline by
combination therapy. The VLDL2 apoB pool size
returned to pretreatment levels with the addition of
simvastatin to the regimen, while the amount of material
derived from VLDL2 or by direct input was unaltered.
IDL formation from VLDL2, presumably by
delipidation, was the same before and during combination therapy. The
IDL apoB plasma pool size fell significantly, from 781 to 505 mg (35%,
P=.01) due to an increase in direct catabolism (214%,
P<.03) and fractional transfer rate into LDL (27%) of IDL
apoB. The total LDL apoB plasma pool showed a highly significant fall
in response to colestipol and simvastatin therapy, from
3470 to 1853 mg (P<.003). The calculated LDL apoB pool, ie,
that derived from VLDL, was also reduced by therapy, from 2408 to 1528
mg (P<.04), due to a combination of decreased
production and increased catabolic rates. The FCR of LDL apoB
rose in six subjects, but because of the scatter of responses this
increase did not achieve overall significance. In this relatively small
group of subjects it was the reduction in LDL apoB input rather than
the change in catabolism that appeared to be the predominant factor in
reducing the plasma LDL-C level. Total LDL apoB input fell by 39%
(P<.003), and direct input of LDL apoB fell by 64%
(P<.005). It is also of note that the mean calculated total
apoB input rate was unchanged with combination therapy (Table
3a through 3d). Fig 4
summarizes the flux of apoB through the delipidation
cascade at baseline and on combination therapy.
A number of kinetic
parameters were significantly
different on combination versus colestipol therapy. The
VLDL2 apoB mass fell 30% due to an increased
VLDL2-to-IDL transfer rate (100%,
P=.005). However, these two changes were balanced, and
overall there was no change in the flux of apoB from
VLDL2 to IDL (Table 3c). The IDL apoB pool fell 26%
(P=.01) as a result of an enhanced IDL-to-LDL transfer rate.
Compared with colestipol treatment, combination therapy generated a
38% increase in the LDL apoB FCR (P<.04), and this change
was responsible for the further decrement of 30% in the LDL apoB pool
(P<.04). Neither direct LDL input nor flux from
VLDL2 plus IDL were affected by the addition of
simvastatin.
|
Discussion |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
Bile acid sequestrants have their primary action in the gut, which results in alterations in plasma lipoproteins through their effects on hepatic lipid metabolism. Interruption of the enterohepatic circulation of bile acids leads to a depletion of intrahepatic cholesterol and subsequent activation of HMG CoA reductase and LDL receptor activity. Concomitantly, TG production is increased due to a release of feedback inhibition by bile acids on phosphatidic acid phosphatase, a rate-limiting enzyme in TG production.10 In the plasma of our subjects, concentrations of VLDL1 and VLDL2 were increased by colestipol therapy, and VLDL2, in agreement with earlier studies,41 became relatively enriched in TGs. These alterations were largely reversed with the addition of simvastatin. As expected, the average cholesterol content of IDL and LDL fell on colestipol, and this compositional change was accentuated by combination therapy. Associated with these perturbations in overall composition were reductions in the mean plasma concentrations of LDL-I and LDL-II, which fell on colestipol alone and showed a further decrement when simvastatin was added. These changes are consistent with our earlier observations on simvastatin,32 with the view that these more buoyant subfractions are more rapidly removed by the LDL receptor than denser species,42 and with the earlier work with resins, which indicates that the total LDL is smaller and denser on therapy.21 Although there were varying responses to therapy, the mean percentage of LDL-III within total LDL increased on both colestipol and combined therapy (from 28.6% to 30% and 43.2%, respectively). Since it is known that larger LDL species are better ligands for the LDL receptor,43 44 upregulation of this receptor would cause a fall in these cholesteryl ester–rich subfractions. Thus, from the lipoprotein composition and concentration data it appears that colestipol has two effects. On the one hand, it caused the generation of TG-rich VLDL and, on the other, the depletion of cholesterol-rich LDL.
The metabolic basis for the hypocholesterolemic action of bile acid sequestrant resins was originally reported as enhanced hepatic receptor–mediated uptake of LDL.45 46 Subjects with heterozygous FH were able to upregulate their one normal allele in response to the need for cholesterol in the liver. However, later studies performed in animal models and humans with other more moderate forms of hypercholesterolemia indicated that resin therapy caused a fall in LDL production rather than a stimulation of catabolism. The resolution of this apparent paradox lies in the recognition that "LDL" receptors play a role in the catabolism of VLDL and IDL as well as LDL. In the present study and that of Gaw et al,32 drugs known to enhance LDL receptor activity increased VLDL2 and IDL direct catabolism. This receptor-mediated removal of LDL precursors leads to diminished LDL production and is a possible explanation for the reduced LDL levels. In the present group of patients, our model suggests a mean fall of 18% in apoB flux from VLDL to LDL during colestipol therapy. However, this change did not result in an automatic fall in circulating LDL apoB; the pool derived from VLDL was not significantly reduced. Rather, the amount of LDL apoB calculated to be derived from direct LDL input fell from a mean of 834 to 448 mg, leading to the 23% drop in total LDL apoB. The nature of direct LDL input is a controversial topic. Some workers have evidence that LDL itself is released from the liver, while others report that a rapidly catabolized VLDL precursor pool is responsible for the phenomenon. This pathway contributed significantly to LDL production in the present group of subjects and appeared to be suppressed specifically during colestipol administration. It is not clear why direct LDL input should be reduced by colestipol administration, although recent studies from our laboratory suggest a potential mechanism. In a larger group of untreated moderate hypercholesterolemics (to which the present eight subjects contributed), we found that the percentage of direct LDL input was inversely related to plasma TG levels.40 The TG levels are, in turn, principally a function of hepatic TG production rates in normotriglyceridemic subjects.47 48 Thus, by stimulating TG production, colestipol may cause fewer particles to be released into the LDL density range and thus favor the assembly of VLDL. However, because it is difficult to detect any increased apoB release in VLDL1 or VLDL2, this explanation must remain tentative. The effects of the drug on the LDL apoB FCR were unremarkable. In fact, in agreement with earlier findings in normolipemic animals, the majority of subjects showed no change or a fall in this kinetic parameter.20 49
The effects of the sequestrant resin on the LDL apoB FCR in the present study differed from those observed in a group of heterozygous FH subjects on the drug cholestyramine.45 This we attribute to the fact that LDL receptor activity is subnormal in FH and responded to the resin, whereas in the present group of subjects there was no evidence to suggest a primary defect in LDL clearance. Rather, the basis of the hypercholesterolemia in this and other similar groups of patients studied in our laboratory has proved to be the overproduction of small VLDL.40
Addition of simvastatin to the colestipol regimen
produced perturbations in apoB metabolism that were similar
to those seen with the drug as monotherapy in a comparable group of
patients.32 VLDL1 and VLDL2
apoB circulating mass and turnover were unaffected, but IDL apoB mass
fell 26%, due in part to increased direct catabolism of this fraction
and also to enhanced transfer to LDL. The amount of apoB removed at the
level of VLDL2 and IDL increased on combined therapy
(Fig 4), but the principal cause of the further fall in
circulating LDL
apoB mass was a 38% increase in the LDL FCR (P<.04,
combined therapy versus monotherapy). Overall, the addition of
simvastatin did not alter the amount of direct LDL input or
the quantity of LDL derived from outside the VLDL delipidation pathway
either in this or the previous study.32 Thus, colestipol
and simvastatin, which were thought to have similar
mechanisms of action, perturb LDL production in different ways,
possibly, as mentioned above, as a result of the specific effect of the
former on TG metabolism. Comparison of the data from this
study and the one on simvastatin alone32
revealed that the effects of the drug were similar in both situations,
although the action of simvastatin in accelerating
delipidation of VLDL2, IDL, and LDL was more
pronounced in the present group. The major effect of the drug is to
increase the FCR of IDL and LDL. The latter is, as initially documented
by Vega and Grundy,22 a variable feature that occurred
in eight of the 10 patients they studied, five of the seven in Gaw et
al32 and seven of the eight in the present group. We
hypothesize that the response to statin therapy reveals an as yet
unknown variation in the populations of moderately
hypercholesterolemic subjects. The effect on IDL
catabolism in both the present and previous studies is dramatic and
perhaps not unexpected since this lipoprotein, which contains apoB and
apoE, is a ligand superior to LDL for the LDL receptor.
In conclusion, the combination of colestipol and simvastatin is highly effective in lowering plasma LDL-C levels in subjects with primary moderate hypercholesterolemia. The marked changes in plasma LDL-C are due to increased IDL and LDL clearance and reduced VLDL-dependent and direct LDL input rates. Thus, our original hypothesis that decreased plasma LDL was simply due to enhanced LDL receptor–mediated removal within the VLDL-IDL-LDL delipidation chain was not supported. The changes observed in the kinetics of apoB-containing lipoproteins can be understood in terms of the influence of colestipol on apoB production and the effect of simvastatin on apoB catabolism.
|
Selected Abbreviations and Acronyms |
|---|
|
|
Acknowledgments |
|---|
We acknowledge financial support from Scottish Home and Health Department grant No. K/MRS/41/10/1/F3 and British Heart Foundation grant Nos. 87/101 and 89/107. Dr A. Gaw is the recipient of an Intermediate Fellowship from the British Heart Foundation FS 94001. Colestipol and simvastatin for use in this study were generously provided by Upjohn Ltd, Crawley, UK, and Merck, Sharp and Dohme Ltd, Hoddesdon, UK, respectively.
Received March 7, 1995; accepted October 19, 1995.
|
References |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
1. Lipid Research Clinics Program. The Lipid Research Clinics Coronary Primary Prevention Trial results, I: reduction in incidence of coronary heart disease. JAMA. 1984;251:351-364.
2.
Lipid Research Clinics Program. The Lipid Research
Clinics Coronary Primary Prevention Trial results, II: the
relationship of reduction in incidence of coronary heart
disease to cholesterol lowering. JAMA. 1984;251:365-374.
3. Watts GF, Lewis B, Brunt JNH, Lewis ES, Colthart DJ, Smith LDR, Mann JI, Swan AV. Effects on coronary artery disease of lipid-lowering diet, or diet plus cholestyramine, in the St. Thomas' Atherosclerosis Regression Study (STARS). Lancet. 1992;339:563-569. [Medline] [Order article via Infotrieve]
4. Buchwald H, Varco RL, Matts JP, Long JM, Fitch LL, Campbell GS, Pearce MB, Yellin AE, Edmiston WA, Smink RD Jr, Sawin HS Jr, Campos CT, Hansen BJ, Tuna N, Karnegis JN, Sanmarco ME, Amplatz K, Castaneda-Zuniga WR, Hunter DW, Bissett JK, Weber FJ, Stevenson JW, Leon AS, Chalmers TC. Effect of partial ileal bypass surgery on mortality and morbidity from coronary heart disease in patients with hypercholesterolemia: report of the program on the surgical control of the hyperlipidemias (POSCH). N Engl J Med. 1990;323:946-955. [Abstract]
5.
Grundy SM. Treatment of
hypercholesterolemia by interference with bile
acid metabolism. Arch Intern Med. 1972;130:638-648.
6. Glueck CJ. Colestipol and probucol: treatment of primary and familial hypercholesterolemia and amelioration of atherosclerosis. Ann Intern Med. 1982;96:475-482.
7.
Levy RI, Brensike JF, Epstein SF, Kelsey SF, Passamani
ER, Richardson JM, Loh IK, Stone NJ, Battaglini JW, Moriarty DJ, Fisher
ML, Fiedman L, Friedwald W, Detre KM. The influence of changes
in lipid values induced by cholestyramine and diet on progression of
coronary artery disease: results of the NHLBI type II
coronary intervention study.
Circulation. 1984;69:325-337.
8. Witztum JL, Schonfeld G, Weidman SW, Giese WE, Dillingham MA. Bile sequestrant therapy alters the compositions of low-density and high-density lipoproteins. Metabolism. 1979;28:221-229. [Medline] [Order article via Infotrieve]
9. Shepherd J, Packard CJ, Morgan HG, Third JLHC, Stewart JM, Lawrie TDV. The effects of cholestyramine on high density lipoprotein metabolism. Atherosclerosis. 1979;33:433-444. [Medline] [Order article via Infotrieve]
10.
Angelin B, Bjorkhem I, Einarsson K.
Cholesterol 7 hydroxylase and bile acid synthesis in
relation to triglyceride and lipoprotein
metabolism. In: Fears R, Sabine JR, eds.
Cholesterol 7 Hydroxylase. Boca Raton, Fla:
CRC Press; 1986:167-177.
11. Weisweiler P. Simvastatin plus low-dose colestipol in the treatment of severe familial hypercholesterolemia. Curr Ther Res Clin Exp. 1988;44:802-806.
12. Leren TP, Hjermann X, Berg K, Leren P, Foss OP, Viksmoen L. Effects of lovastatin alone and in combination with cholestyramine on serum lipids and apolipoproteins in heterozygotes for familial hypercholesterolemia. Atherosclerosis. 1988;73:135-141. [Medline] [Order article via Infotrieve]
13. Miettinen TA. Combined drug treatment of hyperlipidemia. Curr Opin Lipidol. 1990;1:39-42.
14.
Blankenhorn DH, Nessim SA, Johnson RL, Sanmarco ME,
Azen SP, Cashin-Hemphill L. Beneficial effects of combined
colestipol-niacin therapy on coronary
atherosclerosis and coronary venous bypass
grafts. JAMA. 1987;257:3233-3240.
15. Brown G, Albers JJ, Fisher LD, Schaefer SM, Lin J-T, Kaplan C, Zhao X-Q, Bisson BD, Fitzpatrick VF, Dodge HT. Regression of coronary artery disease as a result of intensive lipid-lowering therapy in men with high levels of apolipoprotein B. N Engl J Med. 1990;323:1289-1298. [Abstract]
16. Scandinavian Simvastatin Survival Study Group. Randomised trial of cholesterol lowering in 4444 patients with coronary heart disease: the Scandinavian Simvastatin Survival Study [4S]. Lancet. 1994;344:1383-1389. [Medline] [Order article via Infotrieve]
17.
Bilheimer DW, Grundy SM, Brown MS, Goldstein JL.
Mevinolin and colestipol stimulate receptor-mediated clearance of
low density lipoprotein from plasma in familial
hypercholesterolemia heterozygotes.
Proc Natl Acad Sci U S A. 1983;80:4124-4128.
18. Grundy SM, Vega GL, Bilheimer DW. Influence of combined therapy with mevinolin and interruption of bile acid reabsorption on low density lipoproteins in heterozygous familial hypercholesterolemia. Ann Intern Med. 1985;103:339-343.
19. Vega GL, East C, Grundy SM. Effects of combined therapy with lovastatin and colestipol in heterozygous familial hypercholesterolemia: effects on kinetics of apolipoprotein B. Arteriosclerosis. 1989;9(suppl I):I-135-I-144.
20. Huff MW, Telford DE, Woodcroft K, Strong WLP. Mevinolin and cholestyramine inhibit the direct synthesis of low density lipoprotein apolipoprotein B in miniature pigs. J Lipid Res. 1985;26:1175-1186. [Abstract]
21. Huff MW, Telford DE. Regulation of low density lipoprotein apoprotein B metabolism by lovastatin and cholestyramine in miniature pigs: effects on LDL composition and synthesis of LDL subfractions. Metabolism. 1989;38:256-264. [Medline] [Order article via Infotrieve]
22.
Vega GL, Grundy SM. Treatment of primary
moderate hypercholesterolemia with
lovastatin (mevinolin) and colestipol.
JAMA. 1987;257:33-38.
23.
Study Group, European Atherosclerosis
Society. The recognition and management of hyperlipidaemia in adults: a
policy statement of the European Atherosclerosis
Society. Eur Heart J. 1988;9:571-600.
24. Lipid Research Clinics Program. Lipid and Lipoprotein Analysis: Manual of Laboratory Operations. Washington, DC: US Dept of Health, Education and Welfare; 1982. NIH publication 75-628.
25. Griffin BA, Caslake MJ, Yip B, Tait GW, Packard CJ, Shepherd J. Rapid isolation of low density lipoprotein (LDL) subfractions from plasma by density gradient ultracentrifugation. Atherosclerosis. 1990;83:59-67. [Medline] [Order article via Infotrieve]
26. Krauss RM. Physical heterogeneity of apolipoprotein B-containing lipoproteins. In: Lippel K, ed. Proceedings of Workshop on Lipoprotein Heterogeneity. Washington, DC: US Government Printing Office; 1987:15-21. NIH publication 87-2646.
27. Havekes LM, Knijff P, Beisiegel U, Habing J, Smit M, Klasen E. A rapid micromethod for apolipoprotein E phenotyping directly in serum. J Lipid Res. 1987;28:455-463. [Abstract]
28. Packard CJ, Munro A, Lorimer AR, Gotto AM Jr, Shepherd J. Metabolism of apolipoprotein B in large triglyceride-rich very low density lipoproteins of normal and hypertriglyceridemic subjects. J Clin Invest. 1984;74:2178-2192.
29. Bilheimer DW, Eisenberg S, Levy RI. The metabolism of very low density lipoprotein proteins, I: preliminary in vitro and in vivo observations. Biochim Biophys Acta. 1972;260:212-221. [Medline] [Order article via Infotrieve]
30. Lindgren FT, Jensen CL, Hatch FT. The isolation and quantitative analysis of serum lipoproteins. In: Nelson GK, ed. Blood Lipids and Lipoproteins: Quantitation, Composition and Metabolism. New York, NY: Wiley-Interscience; 1972:221-245.
31. Kane JP, Sata T, Hamilton RL, Havel RJ. Apoprotein composition of very low density lipoproteins of human serum. J Clin Invest. 1975;56:1622-1634.
32.
Gaw A, Packard CJ, Murray EF, Lindsay GM, Griffin BA,
Caslake MJ, Vallance BD, Lorimer AR, Shepherd J. Effects of
simvastatin on apoB metabolism and LDL
subfraction distribution. Arterioscler
Thromb. 1993;13:170-189.
33. Shepherd J, Packard CJ, Grundy SM, Yeshurun D, Gotto AM, Taunton OD. Effects of saturated and polyunsaturated fat diets on the chemical composition and metabolism of low density lipoproteins in man. J Lipid Res. 1980;21:91-99. [Abstract]
34.
Lowry OH, Rosebrough NJ, Farr AL, Randall RJ.
Protein measurement with the Folin phenol reagent.
J Biol Chem. 1951;193:265-275.
35. Berman M, Weiss MF. SAAM Manual. Washington, DC: US Government Printing Office; 1974. US Dept of Public Health Services publication 1703.
36. Demant T, Bedford D, Packard CJ, Shepherd J. Influence of apolipoprotein E polymorphism on apolipoprotein B-100 metabolism in normolipemic subjects. J Clin Invest. 1991;88:1490-1501.
37. Packard CJ, Shepherd J, Lindsay GM, Gaw A, Taskinen M-R. Thyroid replacement therapy and its influence on postheparin plasma lipases and apolipoprotein-B metabolism in hypothyroidism. J Clin Endocrinol Metab. 1993;76:1209-1216. [Abstract]
38. Packard CJ, Gaw A, Demant T, Shepherd J. Development and application of a multicompartmental model to study very low density lipoprotein subfraction metabolism. J Lipid Res. 1995;36:172-187. [Abstract]
39. Berman M, Hall M, Levy RI, Eisenberg S, Bilheimer DW, Phair RD, Goebel RM. Metabolism of apoB and apoC lipoproteins in man: kinetic studies in normal and hyperlipoproteinemic subjects. J Lipid Res. 1978;19:38-56. [Abstract]
40. Gaw A, Packard CJ, Lindsay GM, Griffin BA, Caslake MJ, Lorimer AR, Shepherd J. Overproduction of small very low density lipoproteins (Sf 20-60) in primary moderate hypercholesterolemia: relationships between apolipoprotein B kinetics and plasma lipoproteins. J Lipid Res. 1995;36:158-171. [Abstract]
41. Witztum JL, Schonfeld G, Weidman SW. The effects of colestipol on the metabolism of very-low-density lipoproteins in man. J Lab Clin Med. 1976;88:1008-1018. [Medline] [Order article via Infotrieve]
42. Thompson GR, Teng B, Sniderman AD. Kinetics of LDL subfractions. Am Heart J. 1987;113:514-517. [Medline] [Order article via Infotrieve]
43. Nigon F, Lesnik P, Rouis M, Chapman MJ. Discrete subspecies of human low density lipoproteins are heterogeneous in their interaction with the cellular LDL receptor. J Lipid Res. 1991;32:1741-1753. [Abstract]
44.
Chen GC, Liu W, Duchateau P, Allaart J, Hamilton RL,
Mendel CM, Lau K, Hardman DA, Frost PH, Malloy MJ, Kane JP.
Conformational differences in human apolipoprotein B-100 among
subspecies of low density lipoproteins (LDL). J
Biol Chem. 1994;269:29121-29128.
45. Shepherd J, Packard CJ, Bicker S, Lawrie TDV, Morgan HG. Cholestyramine promotes receptor-mediated LDL catabolism. N Engl J Med. 1980;302:1219-1222. [Abstract]
46.
Slater HR, Packard CJ, Bicker S, Shepherd J.
Effects of cholestyramine on receptor-mediated plasma clearance and
tissue uptake of human low density lipoproteins in the rabbit.
J Biol Chem. 1980;255:10210-10213.
47. Kissebah AH, Alfari S, Adams PW. Integrated regulation of very low density lipoprotein triglyceride and apolipoprotein B kinetics in man: normolipemic subjects, familial hypertriglyceridemia and familial combined hyperlipidemia. Metabolism. 1981;30:856-858. [Medline] [Order article via Infotrieve]
48.
Stalenhoef AFH, Demacker PNM, Lutterman JA, van't Laar
A. Plasma lipoproteins, apolipoproteins and
triglyceride metabolism in familial
hypertriglyceridemia.
Arteriosclerosis. 1986;6:387-394.
49. Witztum JL, Young SG, Elam RL, Carew TE, Fisher M. Cholestyramine-induced changes in low density lipoprotein composition and metabolism, I: studies in the guinea pig. J Lipid Res. 1985;26:92-103.[Abstract]
This article has been cited by other articles:
 |
|
 |
|
  J. Chapman Beyond LDL-cholesterol reduction: the way ahead in managing dyslipidaemia Eur. Heart J. Suppl., July 1, 2005; 7(suppl_F): F56 - F62. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. E. Telford, J. Y. Edwards, S. M. Lipson, B. Sutherland, P. H. R. Barrett, J. R. Burnett, E. S. Krul, B. T. Keller, and M. W. Huff Inhibition of both the apical sodium-dependent bile acid transporter and HMG-CoA reductase markedly enhances the clearance of LDL apoB J. Lipid Res., May 1, 2003; 44(5): 943 - 952. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. W. Huff, D. E. Telford, J. Y. Edwards, J. R. Burnett, P. H. R. Barrett, S. R. Rapp, N. Napawan, and B. T. Keller Inhibition of the Apical Sodium-Dependent Bile Acid Transporter Reduces LDL Cholesterol and ApoB by Enhanced Plasma Clearance of LDL ApoB Arterioscler. Thromb. Vasc. Biol., November 1, 2002; 22(11): 1884 - 1891. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. K. Spady, J. A. Cuthbert, M. N. Willard, and R. S. Meidell Overexpression of Cholesterol 7alpha -Hydroxylase (CYP7A) in Mice Lacking the Low Density Lipoprotein (LDL) Receptor Gene. LDL TRANSPORT AND PLASMA LDL CONCENTRATIONS ARE REDUCED J. Biol. Chem., January 2, 1998; 273(1): 126 - 132. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. R. Burnett, L. J. Wilcox, D. E. Telford, S. J. Kleinstiver, P. H. R. Barrett, R. S. Newton, and M. W. Huff Inhibition of HMG-CoA Reductase by Atorvastatin Decreases Both VLDL and LDL Apolipoprotein B Production in Miniature Pigs Arterioscler. Thromb. Vasc. Biol., November 1, 1997; 17(11): 2589 - 2600. [Abstract] [Full Text]
|
|
|
|
 |
|
 B. D. Miller, E. L. Alderman, W. L. Haskell, J. M. Fair, and R. M. Krauss Predominance of Dense Low-Density Lipoprotein Particles Predicts Angiographic Benefit of Therapy in the Stanford Coronary Risk Intervention Project Circulation, November 1, 1996; 94(9): 2146 - 2153. [Abstract] [Full Text]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||