New Tatu A.
Insights
Miettinen,
Into Cholesterol
Dynamics
MD
\s=b\ Kinetic aspects of cholesterol dynamics are described, with flux from the gut, to the liver, to the tissues, and back to the liver and gut, with a discussion of modifying mechanisms, synthesis, and transport. Overproduction of cholesterol, and bile acid and cholesterol malabsorption, are related to clinical problems.
(Arch Surg 113:45-49, 1978).
Cholesterol point by tion), consisting endogenous synthesis
metabolism is characterized from the quan¬ cholesterol influx (produc¬ of view of and absorption from the diet on one side and by cholesterol elimination (catabolism), primarily fecal excretion as bile acids and neutral sterols of cholesterol origin, on the other. Before the newly synthesized or absorbed cholesterol molecule is excreted into the stool, it can be released from the liver and transported by the lipoproteins in the blood, transferred into or exchanged with tissue cholesterol, and finally transported back to the liver for ultimate elimination to be reutilized for lipoprotein synthesis. Many aspects of this dynamic process are poorly understood, particularly under pathological conditions. Some of the more recent devel¬ opments in this field will be discussed in the present titative
report. KINETIC ASPECTS OF CHOLESTEROL DYNAMICS
By computer analysis of radioactivity data of serumfree, esterified, and red blood cell cholesterol during the
first hours and days after administration of labeled mevalonate, we determined that the serum-free cholesterol turnover was about 26 gm/day, and that of esterified cholesterol, in agreement with results by others,1 about 3 gm/day (Table 1). It should be stressed, however, that most of the turnover takes place by exchange of the labeled free cholesterol of lipoproteins with tissue cholesterol, and only a small fraction is caused by esterification, actual mass transfer to tissues and back, or by synthesis and catabolism of cholesterol. The data summarized in Table I are compat¬ ible with a cholesterol synthesis rate of about 1 gm/day. Esterification occurs mainly in plasma from free cholester¬ ol, while removal of the ester cholesterol takes place partly in the liver and partly by a receptor-mediated and recep¬ tor-independent uptake of lipoproteins by peripheral tissues.2 Thus, about 13 gm of the turnover outlined in Table 1 is due to esterification and by exchange with the red blood cell cholesterol. Of the remaining 13 gm, the bulk is exchanged with tissue cholesterol, primarily in the liver; a part is transferred into peripheral tissues via the recepAccepted for publication July 1, 1977. From the Second Department of Medicine, University of Helsinki. Reprint requests to Second Department of Medicine, University Helsinki, 00290 Helsinki 29, Finland (Dr Miettinen).
of
tor-mediated and receptor-independent uptake and deg¬ radation of lipoproteins, and a part of lipoprotein choles¬ terol is taken up by the liver. About 1 gm of cholesterol is catabolized as fecal bile acids and neutral sterols. Turnover measurements of low-density lipoprotein (LDL):l have indicated that, depending on the LDL concentration, 20.5% to 63.3% of the intravascular LDLcholesterol is removed daily with LDL catabolism. Since, at the present time, this catabolism is believed to take place in the peripheral tissues by the receptor and nonreceptor mechanisms,2 it can be calculated from the data in Table 1 that 1.83 gm of LDL-cholesterol is transferred daily into the tissues. In a steady state, this amount must be trans¬ ported back to plasma by lipoproteins, presumably mainly by high-density lipoprotein (HDL). With advancing age, and in hyperlipidemic states in particular, this back trans¬ port does not keep pace with the influx, resulting in the gradual accumulation of tissue cholesterol and the wellknown development of cholesterol deposits, cutaneous and tendon xanthomata, and arterial atheromata. The behavior of the specific activity-time curve of plasma cholesterol after a single dose of radioactive choles¬ terol can be interpreted by a three-pool model4: the label mixes rapidly with pool 1, which simultaneously mixes slowly with pool 2 and very slowly with pool 3. Computer analysis allows us to quantitate production rate, pool sizes, and rate constants between the pools. Pool 1 comprises cholesterol in the blood, liver, intestine, and portions of some other organs, with their exact anatomical site currently impossible to define. That the size of pool 1 is not solely determined by the blood compartment is indicated by the finding that in obesity, its size is supernormal, even after subtraction of the blood compartment. Pool 2 is very poorly defined. Portions of xanthomata cholesterol may belong to this pool.5 Pool 3 apparently includes portions of cholesterol in connective tissue (including xanthomata), skeletal muscle, arterial walls (including atheromata), and adipose tissue in particular. A positive correlation between the size of pool 3 and the serum cholesterol concentration suggests that the serum cholesterol level can be used for estimation of abnormal cholesterol accumulation in tissues and possible changes in tissue deposits caused by various hypocholesterolemic measures.
It is interesting to note, however, that, owing to a reduced rate constant for the flux of cholesterol from pool 1 to pool 2, the absolute flux from pool 1 to tissue pool 2 plus pool 3 is not significantly higher in hypercholesterolemic than in normocholesterolemic subjects; the respective values are 1.82 and 2.06 gm/day.4 The corresponding figures calculated from the LDL-turnover studies3 for the
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removal of LDL-cholesterol from the plasma pool are 1.66 and 1.43 gm/day; from our data (Table 1) it is 1.83 gm/day. This would suggest that LDL-cholesterol accounts primari¬ ly for the flux of cholesterol from pool 1 to slowly exchangeable pools and that the role of HDL and very-low-
density lipoprotein (VLDL) are negligible. If, however, LDL is catabolized also in pool 1, then an amount of the flux to slowly miscible pools must originate from HDL- and VLDL-cholesterol, as well. Entry of cholesterol into the plasma compartment as free and esterified cholesterol in chylomicrons and VLDL can be calculated to be several grams per day and, owing to enhanced turnover of VLDL, this value could be considerably higher in hypertriglyceridemic than in normal subjects.1'1 The kinetic analysis after administration of prelabeled cholesterol requires that cholesterol production and Table 1—Some Measurements of Cholesterol Kinetics* Plasma
Turnover, gm/Day
Pool, gm Cholesterol Free Ester Red blood cell Sterol balance Fecal steroids
3.14 7.32 2.49
25.9 2.9 9.8 —0.99
.
Low-density lipoprotein
cholesterol
Free Ester Total
.
1.14
.
...
0.55t 1.28t
2.33 5.38 7.71
1.83t
"'Mean turnover data computerized for plasma free, ester, and red blood cell cholesterol after Intravenous injection of tritiated mevalonate to seven normocholesterolemic and hypercholesterolemic subjects. tCalculated assuming that fractional catabolic rate of low-density lipo¬ protein (LDL)-cholesterol is the same as LDL-protein.3
removal take
place
in and from
pool
1. More recent
computer analysis revealed, however, that when small
molecular cholesterol precursors (mevalonic acid or tritiated water) are used, about one third of cholesterol is actually synthesized in the slowly exchangeable pool(s).7 The release of the newly synthesized cholesterol from the latter pool to the blood stream results in the specific activity-time curve of the mevalonate-derived cholesterol deviating from that of prelabeled cholesterol for the first two to five weeks after administration of the two labels. The initial extent and duration of this deviation appar¬ ently depend on the magnitude of pool 2 synthesis. The latter, and also the magnitude of the deviation of the specific activity curves, tended to correlate with the amount of adipose tissue, suggesting that fat tissue contributes to the pool 2 synthesis.7-8 Determination of the cholesterol precursors revealed very high concentrations of squalene in adipose tissue.a Evidence has been obtained that newly formed squalene is trapped by the large squalene pool of fat cells and slowly converted to cholesterol, which in turn equilibrates with the slowly exchangeable cholesterol pool of adipose tissue and is ultimately released (or exchanged, or both) into the blood, probably with the aid of HDL.S This mechanism can be hypothesized to contribute to pool 2 synthesis of choles¬ terol. The presence of fairly large amounts of squalene and methyl sterols in the skeletal muscle and in the intestinal mucosa, as well as the finding that in in vitro incubation of these tissues the bulk of the radioactivity utilized for sterol synthesis accumulated in squalene and methyl sterols, suggest that at least these organs, in addition to the kidney and adipose tissue,8 can contribute to pool 2 synthesis of cholesterol.
Table 2.—Measurements of Cholesterol Metabolism Under Different Conditions*
Controls
Cholestyramine Resin Therapy (N = 11) Obese
_(N 12)_(N Relative body weight_1.01 ± 0.03_1.33 ± Serum cholesterol, mmole/liter_234 ± 13_204 ± Serum triglycéride, mmoie/liter_1.73 ± 0.25_1.65 ± Liver FCH, mg/100gm_248 ± 29_267 ± Liver ECH, mg/100gm_73 ± 26_79 ± Liver TCH, mg/100gm_321 ± 51_346 ± Esterification, %_18 ± 3_20 ± =
Liver carbon 14-FCH, disintegrations per minute Carbon 14/tritium x 10- in liver FCH_6.9 Carbon 14/tritium x 10' in serum
=
,-*-,
9)_Before_During_ 0.05f_1.09 ± 0.04_1 09 ± 0.04 15_480 ± 49f_346 ± 26t 0.31_1.95 ± 0.28_1.85 ± 0.29 24_267 ± 43_191 ± 19f 26_123 ± 28_75 ± 14f 46_390 ± 71_266 ± 29f 3_30 ± 2t_26 ± 3_
xICF/gm/hr_5.5 ± 1.6_7.7 ± 1.8_3.5 ± 1.9_26.6
16.5 15_3.3 ± 1.0f FCH|_.___.„_3 ± 1_15 Serum TMS, ^g x 10;/mg of FCH_._._.„_144 ± 28_534 Fecal bile acid, mg/day_235 ± 25§_376 ± 10t§_196 ± 28_1,380 Fecal neutral sterol, mg/day_651 ± 66§_1,338 ± 118f§_582 ± 62_522
Fecal total steroids,
mg/day
889
-
±
1.3_9.3
85§
1,714
t
±
145f§
778
+
84
1,902
±
5.6f
! 3.4t ±
4t_
±
133f 248f
±
34
±
275f
±
'All values mean ± SE. Percutaneous liver biopsy specimens were obtained from hypercholesterolemic volunteers, or during routine diagnostic procedures from patients who later turned out to be clinically healthy and have normal liver histology or mild to moderate fatty liver. Biopsy specimens were incubated for three hours in Krebs-Ringer bicarbonate buffer with 1 mM of carbon 14 acetate and tritiated mevalonate. Hypercholesterolemic (familial with tendon xanthomata) subjects were sampled before and after 12 to 30 days on a regimen of cholestyramine resin therapy (32 gm /day). FCH indicates free cholesterol; ECH, esterified cholesterol; TCH, total cholesterol; TMS, serum total methylsterols. fStatistically significant difference (at least P < .05) from controls or pretreatment values. JRatio in serum free cholesterol four hours after intravenous injection of carbon 14-tritiated mevalonate mixture.
§Data taken from Mittinen.9
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OVERPRODUCTION OF CHOLESTEROL
Obesity, hypertriglyceridemia, and a high cholesterol diet are the three major conditions in which cholesterol influx (production) is increased and followed, secondarily, by an enhanced elimination. Quantitation of fecal bile acids and neutral sterols, end products of cholesterol metabolism, have indicated that cholesterol synthesis is considerably increased in obesity." In nonobese human subjects, daily cholesterol synthesis is about 12 mg/kg, while in overweight subjects, the figure is about 20 mg/kg of excess body weight. Kinetic studies have revealed similar results,10 but in some overweight subjects, the increase in synthesis is less than expected from their excess weight. The rate of cholesterol synthesis is not correlated with the serum cholesterol level. This apparently is due to effective removal of lipoproteins from plasma and effective elimination of excess cholesterol in feces by most obese subjects. Others, however, are incapable of doing this sufficiently and hypercholesterolemia develops. The hypercholesterolemia, as well as the increased synthesis, is usually normalized by successful weight reduction. When associated with a primary hypercholesterolemia, the obesi¬ ty-induced increase in cholesterol synthesis can increase the hyperlipoproteinemia, again, this can be improved, though not usually normalized, by weight reduction. The exact site of synthesis of excess cholesterol in obesity is not known. The in vitro studies by Schreibman and Dell10 indicated that the daily synthesis was less than 1 mg/kg of adipose tissue. However, considering the exten¬ sive dilution of the labeled precursors by the large endoge¬ nous intermediate pools of cholesterol during the experi¬ mental incubations, the actual synthesis was substantially higher8 than believed earlier.10 Thus, though the exact amount of synthesis is difficult to quantitate in vitro, the data indicate that adipose tissue can significantly contribute to cholesterol production.8 Another interesting organ is the liver, which can be assumed to enhance its fatty acid, cholesterol, and lipopro¬ tein production under the influence of an excessive calorie load. An increase in food consumption is actually asso¬ ciated with enhanced cholesterol synthesis." As shown in Table 2, synthesis of free fatty acids, but not of cholesterol, is increased from acetate l4C and tritiated mevalonate in liver biopsy specimens of obese subjects in vitro. However, the ratio of carbon 14/tritium in newly formed cholesterol tended to be higher in obese than in lean subjects and showed a significant correlation with the relative body weight (P < .05), as if early synthetic step(s), probably /3-hydroxyl-/?-methyl-glutaryl-coenzymeA-reductase (HMG-CoA-reductase), were activated. The increased fatty acid synthesis may be associated with enhanced hepatic release of VLDL-triglycerides. The production and catabolism of the latter actually appear to be increased in obesity; this accelerated turnover also promotes substantially the flux of cholesterol via VLDL and has been assumed to be one causal factor in enhanced cholesterol synthesis and catabolism in obesity and, partic¬ ularly, in hypertriglyceridemia." It should be borne in mind, however, that cholesterol production is not increased more in hyperglyceridemic than normoglyceridemic obese
• 0
a> o
Serum Total Cholesterol
Triglycérides
20
4?5
E
Ë
to
2?
E E
10 1.5 D Neutral Sterols
«1.0
_
E
Fecal Bile Acids
"O.ô -75
4
-76
-77
t
Op Effects of portacaval shunt on serum lipid and fecal steroid levels in 12-year-old girl homozygous for familial hypercholesterolemia. Heal bypass was performed four years earlier for treatment of hypercholesterolemia. Top, Serum methyl sterol levels expressed per 100 mg of free cholesterol (FLAN indicates free lanosterol; FDMS, free dimethyl sterol; ELAN, esterified lanosterol; and FCH, free cholesterol). Center, Serum total cholesterol and triglycéride levels and percent distribution of cholesterol in different lipopro¬ teins. Each column is adjusted to 100 (VLDL indicates very-lowdensity lipoprotein; LDL, low-density lipoprotein; and HDL, highdensity lipoprotein). Bottom, Fecal bile acid and neutral sterol levels. Decrease in total serum cholesterol level (free and ester cholesterol decreased equally), mainly caused by fall in LDL, is due to reduction in markedly high cholesterol synthesis (caused by ileal exclusion) and is indicated by fall in serum methyl sterol and fecal steroid levels. Cholesterol synthesis, measured from sterol balance studies, was 17 mg/kg/day before ileal bypass, 49 mg/kg/day after bypass and 29 to 43 mg/kg/day after portacaval shunt (Op).
subjects.9
The enhanced cholesterol synthesis in obesity and hyper¬ associated with an elevated elimination of cholesterol as both fecal neutral sterols and bile acids.911 The relative contribution of the latter tends, however, to be subnormal, particularly in females, indicating that with a high cholesterol production rate, relatively more of the excessively produced cholesterol is eliminated as neutral sterols, as compared to bile acids. The studies on biliary secretion of lipids in obesity have actually shown that biliary cholesterol output is greatly increased, while bile acid secretion is within normal limits, resulting in supersaturation of bile, a state promoting gallstone formation.12 Thus, increased flux of cholesterol into the intestinal lumen characterizes obesity. Though cholesterol loading usually enhances absolute cholesterol absorption, the absorption rate is unchanged, or decreased, and the amount of unabsorbed cholesterol is increased.13 Since, in addition, absorp¬ tion of bile acids may not be decreased, elimination of
triglyceridemia is
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Table 3.—Effects of
Cholestyramine
Resin and Ileal Bypass on Different Measurements of Cholesterol Metabolism in Patients With Familial Hypercholesterolemia* Fecal Steroids, mg/Day r-*-—>
Serum Cholesterol
Carbon 14/Tritium
DMS, /¿gx
_Treatment_mg/100 ml_Acidic_Neutral_1Q'/mg 01 FCHf_X100Í 648 ± 81 (13)_28 ± 6 (8)_4 ± 1 (5) None_546 ± 58 (13)_193 ± 31 +21 ± 66(13)_+ 40 ± 5 (8)_+11 ± 2 (5) Cholestyramine resin§_-93 ± 15(13)_+1240 ± 164 -45 ± 68 (13)_+70 ± 15 (8)_+ 25 ± 4 (5) Ileal bypass§_-178 ± 12(13)_+2221 ± 245 Ileal
bypass &
cholestyraminelj_-66
±
19
(10)_+887
±141
+177
±
45(10)_+39
+
11
(8)_+ 21
±
8
(5)
SD. Numbers in
•Data expressed parentheses indicate number of patients. fDMS indicates free dimethyl sterol; FCH, plasma free cholesterol. serum level in cholesterol tCarbon 14/tritium eight hours after injection of a carbon 14-acetate-tritiated mevalonate mixture. (¡Changes from initial values. ¡¡Changes from ileal bypass values. as mean ±
Table
4.—Follow-up
of Different Measurements of Cholesterol Metabolism in 25 Patients With Familial After Ileal Bypass Operation (Mean ± SE) Mo
Hypercholesterolemia
Postoperative
_0_1_6_12_24_36_60 373 ± 8 Serum cholesterol, mg/100 ml_558 ± 17 360 ± 12 372 ± 15 356 ± 15_372 ± 12_399 + 18 ± 20 ± 248 Fecal bile acids, 2164 ± 176_.___2330 ± 269 2429 ± 240 2400 ± 269 2320 mg/day_155 519 ± 21 Fecal neutral sterols, mg/day_572 ± 40 589 ± 38___534 ± 25_540 ± 22_452 ±23 97 ± 25 66 ± 10_89 ± 7_90 ± 5_96 ± 16 DMS, /ig X 1Q'/mg of FCH*_26 ± 5_74 ±10 Carbon 14/tritium
x
4
100 in FCH
±
1
30
±
6
31
±
3
35
±
9
22 ± 3
32
±
8
...
*DMS indicates excess
in
dimethyl sterol; FCH, plasma
cholesterol
as
fecal neutral sterols
obesity. It is obviously owing
groups that, in
some
free cholesterol.
to the
can
predominate
heterogeneity
of
patient
reports of hyperglyceridemia, endoge¬
nous cholesterol production and fecal elimination as bile acids and neutral steroids are consistently increased; in others, they are not.4-6-9-1415 Several modifying variables can be hypothesized, eg, the presence or absence of obesity, the VLDL level, the turnover rate of VLDL, and the level of LDL. A negative correlation between the sterol balance value, fecal bile acids, neutral sterols and the serum cholesterol level in hypertriglyceridemic subjects16 sug¬ gests that the higher the turnover of VLDL and the lower the LDL level, the higher is the endogenous synthesis of cholesterol in type IV hyperlipoproteinemia. The relationship between bile acid metabolism and different hyperlipoproteinemias, investigated especially by Hellström and Einarsson,14 has been summarized recently. In about two thirds of type IV and V patients, bile acid production, that of cholic acid in particular, is increased. A decrease of plasma triglycérides, or their turnover with different therapeutic measures, is frequent¬ ly associated with a reduced synthesis of bile acids, suggesting a close, as yet poorly understood relationship between the metabolism of VLDL and bile acids (or
cholesterol).
An increased influx of cholesterol from the diet is followed by (1) enhanced elimination of cholesterol as neutral sterols, (2) unchanged or slightly increased bile acid excretion, (3) reduced cholesterol synthesis, (4) an increased serum cholesterol or LDL-level, and (5) retention of cholesterol in the body. These metabolic responses show a substantial interindividual variation.1517 On a high intake, particularly if associated with a poor removal capacity, the retention may be considerable. On a lower
intake, the compensating mechanism, increased reexcretion of dietary cholesterol and the decrease of synthesis in particular, prevent the increase of serum cholesterol levels and a major retention of cholesterol in the body. As compared to enhanced cholesterol flux in obesity and hyperglyceridemia, the lack of increased bile acid synthesis during high cholesterol absorption is an interesting phenomenon. Low cholesterol synthesis and a different lipoprotein metabolism may be factors promoting choles¬ terol catabolism, primarily by way of neutral sterols. Our cholesterol feeding experiments (1 extra gram per day for four weeks) in a random population group, under normal living habits, showed an insignificant increase in serum cholesterol concentrations associated with a 40% decrease in cholesterol synthesis and a 46% ( + 134 mg/day; P < .05) increase in fecal bile acid excretion. BILE ACID AND CHOLESTEROL MALABSORPTION
Interruption of the enterohepatic circulation of bile acids by ion exchangers or ileal exclusion, or inhibition of the cholesterol absorption by plant sterols or neomycin, in the treatment of hypercholesterolemia result in an increase in fecal elimination of cholesterol, a compensatory increase in cholesterol synthesis (shown in vivo in Tables 2 through 4) by an increase of the carbon 14/tritium ratio and by plasma methyl sterols), and yet, in a decrease of the serum
cholesterol and LDL levels.11151819 The mechanism of the latter is poorly understood, since studies of LDL turnover1 and LDL metabolism by fibroblasts of hypercholesterol¬ emic subjects2 emphasize the causal role of peripheral LDL catabolism in the development of hypercholesterolemia. The fall in the serum cholesterol level (apparently also that of LDL) is usually positively correlated with an increase in fecal steroids, indicating that the higher the
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increase of cholesterol elimination as neutral sterols or as bile acids or both, the higher the decrease in the serum cholesterol level.1118 It is owing to an effective compensa¬ tory increase in cholesterol synthesis that this rule of thumb is not valid in patients with homozygous familial hypercholesterolemia in whom complete biliary diversion has no cholesterol-lowering effect.2" Portacaval shunt effectively reduces the serum cholesterol level and choles¬ terol and LDL synthesis in these patients.21 Inhibition of synthesis with this procedure in homozygous subjects with ileal exclusion (Figure) may offer an additional advantage in the serum cholesterol level lowering in these patients. In heterozygous subjects, ion exchangers, and ileal exclusion in particular, cause a substantial decrease in the serum cholesterol level.111 Though the ileal bypass is more effective than ion exchangers on both serum cholesterol and fecal bile acid levels, normal serum cholesterol values are rarely obtained except in combination with large doses of ion exchangers, which clearly potentiate the effects of the ileal
bypass (Table 3). A lack of LDL-receptors
in familial
hypercholesterol¬
emia may result in insufficient suppression of HMG-CoAreductase, and thus, despite a high plasma LDL concentra¬ tion, in enhanced cholesterol synthesis by peripheral tissues.2 Sterol balance data, however, indicate that the overall synthesis is low normal.1118 This could be due to low hepatic cholesterol synthesis (Table 2), which in turn could be a reason for the subnormal bile acid production.1 With cholestyramine resin, or after ileal exclusion, cholesterol synthesis, as indicated by incubation studies, by the acetate-mevalonate test, and also by plasma methyl sterol levels (Tables 2 through 4), increases relatively, much more than the overall production shown by the fecal steroids. This can be expected if most of the increased synthesis takes place in the liver and if the initial hepatic production is low. Assuming that the change in the carbon 14/tritium ratio of the acetate-mevalonate test is primarily caused by the activation of the hepatic cholesterol synthesis, as suggested by the findings in Table 2, the quantity of basal ' '"
hepatic
cholesterol synthesis could be calculated from the data in Tables 2 through 4. This ratio increases fourfold to tenfold, and fecal steroids by 1 to 3 gm/day. Thus, the hepatic production was about 200 to 300 mg/day. Cholesterol malabsorption caused by gluten enteropathy, plant sterols, or small doses of neomycin is not associated with an increased bile acid production, although cholesterol synthesis is apparently increased.1118 In addition, the latter is not detected with the acetate-mevalonate test or plasma methyl sterols, particularly in long-term studies, suggest¬ ing that later steps in the synthetic pathway of cholesterol are stimulated or the synthesis is increased in extrahepatic tissues or both. Table 4 shows that, after ileal exclusion, enhanced cholesterol synthesis can be detected perma¬ nently with these two procedures. It is a common clinical observation that serum choles¬ terol level lowering is associated, in the long term, with a reduction in the size of visible cholesterol deposits of xanthelasmata and cutaneous and tendon xanthomata. However, though the decrease in the serum cholesterol level may be consistently associated with a mobilization of tissue cholesterol, provided that the latter is increased, and with a decrease in the pool 1 and liver cholesterol (Table 2),
the decrease in the size of the slowly exchangeable choles¬ terol pool is difficult to demonstrate with current methods. For instance, the size of that pool is not consistently changed by anion exchange resins.22 However, the ileal bypass operation, which is associated with a substantial increase in cholesterol elimination and a clear decrease in the serum cholesterol level, seems to cause a significant reduction in the pool size of the slowly exchangeable cholesterol.11' The patients in Table 4 lost about 3.5 kg of extra cholesterol during the postoperative follow-up period of five years. It is possible that this was not totally balanced by increased synthesis, but that some of it was due to mobilization of tissue cholesterol, particularly since the average serum cholesterol level was decreased perma¬ nently by about 35%. This study was supported by grants from the Finnish State Council for Medical'Research. E. Gustafsson, I. Aaltonen, and M. Aarnio provided technical and secretarial assistance.
References 1. Kudchodkar BJ, Sodhi HS: Turnover of plasma cholesteryl esters and its relationship to other parameters of lipid metabolism in man. Eur J Clin
Invest 6:285-298, 1976. 2. Brown MS, Goldstein JL: Receptor-mediated control of cholesterol metabolism. Science 191:150-154, 1976. 3. Langer T, Strober W, Levy RI: The metabolism of low density lipoprotein in familial type II hyperlipoproteinemia. J Clin Invest 51:1528\x=req-\ 1536, 1972. 4. Smith FR, Dell RB, Noble RP, et al: Parameters of the three-pool model of the turnover of plasma cholesterol in normal and hyperlipidemic humans. J Clin Invest 57:137-148, 1976. 5. Bhattacharyya AK, Connor WE, Mausolf FA, et al: Turnover of xanthoma cholesterol in hyperlipoproteinemia patients. J Clin Lab Med 87:503-518, 1976. 6. Sodhi HS: Cholesterol metabolism in man, in Kritchevsky D (ed): Hyperlipidemic Agents: Handbook of Experimental Pharmacology. New York, Springer-Verlag, 1975, vol 41, pp 29-107. 7. Kekki M, Miettinen TA, Wahlstr\l=o"\mB: Measurement of cholesterol synthesis in kinetically defined pools using fecal steroid analysis and double labeling technique in man. J Lipid Res 18:99-114, 1977. 8. Miettinen TA: Cholesterol synthesis in slowly exchangeable cholesterol pool in man, in Proceedings of the Fourth International Symposium on Atherosclerosis. New York, Springer-Verlag, 1976. 9. Miettinen TA: Cholesterol production in obesity. Circulation 44:842\x=req-\
850,1971.
10. Schreibman PH, Dell RB: Human adipocyte cholesterol: Concentration, localization, synthesis and turnover. J Clin Invest 55:986-993, 1975. 11. Miettinen TA: Clinical implications of bile acid metabolism, in Nair PP, Kritchevsky D (eds): The Bile Acids. New York, Plenum Press Inc, 1973,
vol 2, pp 191-247. 12. Bennion LJ, Grundy SM: Effects of obesity and caloric intake on biliary lipid metabolism in man. J Clin Invest 56:996-1011, 1975. 13. Grundy SM, Mok HYI: Determination of cholesterol absorption in man by intestinal perfusion. J Lipid Res 18:263-271, 1977. 14. Hellstr\l=o"\mK, Einarsson K: Bile acid metabolism in hyperlipoproteinemia. Clin Gastroenterol 6:103-128, 1977. 15. Grundy SM: Dietary and drug regulation of cholesterol metabolism in man, in Paoletti R, Glueck CJ, (eds): Lipid Pharmacology. New York, Academic Press Inc, 1975, vol 2, pp 127-159. 16. Miettinen TA: Cholesterol metabolism in patients with coronary heart disease. Ann Clin Res 3:313-322, 1971. 17. Nestel PJ, Poyser A: Changes in cholesterol synthesis and excretion when cholesterol intake is increased. Metabolism 25:1591-1599, 1976. 18. Miettinen TA: Methods for evaluation of hypolipidemic drugs in man: Mechanisms of their action, in Paoletti R, Glueck CJ (eds): Lipid Pharmacology. New York, Academic Press Inc, 1975, vol 2, pp 83-125. 19. Buchwald H, Moore RB, Varco RL: Surgical treatment of hyperlipidemia. Circulation 49(suppl 1): I1-35, 1974. 20. Deckelbaum RJ, Lees RS, Small DM, et al: Failure of complete bile diversion and oral bile acid therapy in the treatment of homozygous familial hypercholesterolemia. N Engl J Med 296:465-470, 1977. 21. Bilheimer DW, Goldstein JL, Grundy SM, et al: Reduction in cholesterol and low density lipoprotein synthesis after portacaval shunt surgery in a patient with homozygous familial hypercholesterolemia. J Clin Invest 56:1420-1430, 1976. 22. Goodman DS, Noble RP, Dell RB: The effects of colestipol resin and of colestipol plus clofibrate on the turnover of plasma cholesterol in man. J Clin Invest 52:2646-2655, 1973.
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Insights
Miettinen,
Into Cholesterol
Dynamics
MD
\s=b\ Kinetic aspects of cholesterol dynamics are described, with flux from the gut, to the liver, to the tissues, and back to the liver and gut, with a discussion of modifying mechanisms, synthesis, and transport. Overproduction of cholesterol, and bile acid and cholesterol malabsorption, are related to clinical problems.
(Arch Surg 113:45-49, 1978).
Cholesterol point by tion), consisting endogenous synthesis
metabolism is characterized from the quan¬ cholesterol influx (produc¬ of view of and absorption from the diet on one side and by cholesterol elimination (catabolism), primarily fecal excretion as bile acids and neutral sterols of cholesterol origin, on the other. Before the newly synthesized or absorbed cholesterol molecule is excreted into the stool, it can be released from the liver and transported by the lipoproteins in the blood, transferred into or exchanged with tissue cholesterol, and finally transported back to the liver for ultimate elimination to be reutilized for lipoprotein synthesis. Many aspects of this dynamic process are poorly understood, particularly under pathological conditions. Some of the more recent devel¬ opments in this field will be discussed in the present titative
report. KINETIC ASPECTS OF CHOLESTEROL DYNAMICS
By computer analysis of radioactivity data of serumfree, esterified, and red blood cell cholesterol during the
first hours and days after administration of labeled mevalonate, we determined that the serum-free cholesterol turnover was about 26 gm/day, and that of esterified cholesterol, in agreement with results by others,1 about 3 gm/day (Table 1). It should be stressed, however, that most of the turnover takes place by exchange of the labeled free cholesterol of lipoproteins with tissue cholesterol, and only a small fraction is caused by esterification, actual mass transfer to tissues and back, or by synthesis and catabolism of cholesterol. The data summarized in Table I are compat¬ ible with a cholesterol synthesis rate of about 1 gm/day. Esterification occurs mainly in plasma from free cholester¬ ol, while removal of the ester cholesterol takes place partly in the liver and partly by a receptor-mediated and recep¬ tor-independent uptake of lipoproteins by peripheral tissues.2 Thus, about 13 gm of the turnover outlined in Table 1 is due to esterification and by exchange with the red blood cell cholesterol. Of the remaining 13 gm, the bulk is exchanged with tissue cholesterol, primarily in the liver; a part is transferred into peripheral tissues via the recepAccepted for publication July 1, 1977. From the Second Department of Medicine, University of Helsinki. Reprint requests to Second Department of Medicine, University Helsinki, 00290 Helsinki 29, Finland (Dr Miettinen).
of
tor-mediated and receptor-independent uptake and deg¬ radation of lipoproteins, and a part of lipoprotein choles¬ terol is taken up by the liver. About 1 gm of cholesterol is catabolized as fecal bile acids and neutral sterols. Turnover measurements of low-density lipoprotein (LDL):l have indicated that, depending on the LDL concentration, 20.5% to 63.3% of the intravascular LDLcholesterol is removed daily with LDL catabolism. Since, at the present time, this catabolism is believed to take place in the peripheral tissues by the receptor and nonreceptor mechanisms,2 it can be calculated from the data in Table 1 that 1.83 gm of LDL-cholesterol is transferred daily into the tissues. In a steady state, this amount must be trans¬ ported back to plasma by lipoproteins, presumably mainly by high-density lipoprotein (HDL). With advancing age, and in hyperlipidemic states in particular, this back trans¬ port does not keep pace with the influx, resulting in the gradual accumulation of tissue cholesterol and the wellknown development of cholesterol deposits, cutaneous and tendon xanthomata, and arterial atheromata. The behavior of the specific activity-time curve of plasma cholesterol after a single dose of radioactive choles¬ terol can be interpreted by a three-pool model4: the label mixes rapidly with pool 1, which simultaneously mixes slowly with pool 2 and very slowly with pool 3. Computer analysis allows us to quantitate production rate, pool sizes, and rate constants between the pools. Pool 1 comprises cholesterol in the blood, liver, intestine, and portions of some other organs, with their exact anatomical site currently impossible to define. That the size of pool 1 is not solely determined by the blood compartment is indicated by the finding that in obesity, its size is supernormal, even after subtraction of the blood compartment. Pool 2 is very poorly defined. Portions of xanthomata cholesterol may belong to this pool.5 Pool 3 apparently includes portions of cholesterol in connective tissue (including xanthomata), skeletal muscle, arterial walls (including atheromata), and adipose tissue in particular. A positive correlation between the size of pool 3 and the serum cholesterol concentration suggests that the serum cholesterol level can be used for estimation of abnormal cholesterol accumulation in tissues and possible changes in tissue deposits caused by various hypocholesterolemic measures.
It is interesting to note, however, that, owing to a reduced rate constant for the flux of cholesterol from pool 1 to pool 2, the absolute flux from pool 1 to tissue pool 2 plus pool 3 is not significantly higher in hypercholesterolemic than in normocholesterolemic subjects; the respective values are 1.82 and 2.06 gm/day.4 The corresponding figures calculated from the LDL-turnover studies3 for the
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removal of LDL-cholesterol from the plasma pool are 1.66 and 1.43 gm/day; from our data (Table 1) it is 1.83 gm/day. This would suggest that LDL-cholesterol accounts primari¬ ly for the flux of cholesterol from pool 1 to slowly exchangeable pools and that the role of HDL and very-low-
density lipoprotein (VLDL) are negligible. If, however, LDL is catabolized also in pool 1, then an amount of the flux to slowly miscible pools must originate from HDL- and VLDL-cholesterol, as well. Entry of cholesterol into the plasma compartment as free and esterified cholesterol in chylomicrons and VLDL can be calculated to be several grams per day and, owing to enhanced turnover of VLDL, this value could be considerably higher in hypertriglyceridemic than in normal subjects.1'1 The kinetic analysis after administration of prelabeled cholesterol requires that cholesterol production and Table 1—Some Measurements of Cholesterol Kinetics* Plasma
Turnover, gm/Day
Pool, gm Cholesterol Free Ester Red blood cell Sterol balance Fecal steroids
3.14 7.32 2.49
25.9 2.9 9.8 —0.99
.
Low-density lipoprotein
cholesterol
Free Ester Total
.
1.14
.
...
0.55t 1.28t
2.33 5.38 7.71
1.83t
"'Mean turnover data computerized for plasma free, ester, and red blood cell cholesterol after Intravenous injection of tritiated mevalonate to seven normocholesterolemic and hypercholesterolemic subjects. tCalculated assuming that fractional catabolic rate of low-density lipo¬ protein (LDL)-cholesterol is the same as LDL-protein.3
removal take
place
in and from
pool
1. More recent
computer analysis revealed, however, that when small
molecular cholesterol precursors (mevalonic acid or tritiated water) are used, about one third of cholesterol is actually synthesized in the slowly exchangeable pool(s).7 The release of the newly synthesized cholesterol from the latter pool to the blood stream results in the specific activity-time curve of the mevalonate-derived cholesterol deviating from that of prelabeled cholesterol for the first two to five weeks after administration of the two labels. The initial extent and duration of this deviation appar¬ ently depend on the magnitude of pool 2 synthesis. The latter, and also the magnitude of the deviation of the specific activity curves, tended to correlate with the amount of adipose tissue, suggesting that fat tissue contributes to the pool 2 synthesis.7-8 Determination of the cholesterol precursors revealed very high concentrations of squalene in adipose tissue.a Evidence has been obtained that newly formed squalene is trapped by the large squalene pool of fat cells and slowly converted to cholesterol, which in turn equilibrates with the slowly exchangeable cholesterol pool of adipose tissue and is ultimately released (or exchanged, or both) into the blood, probably with the aid of HDL.S This mechanism can be hypothesized to contribute to pool 2 synthesis of choles¬ terol. The presence of fairly large amounts of squalene and methyl sterols in the skeletal muscle and in the intestinal mucosa, as well as the finding that in in vitro incubation of these tissues the bulk of the radioactivity utilized for sterol synthesis accumulated in squalene and methyl sterols, suggest that at least these organs, in addition to the kidney and adipose tissue,8 can contribute to pool 2 synthesis of cholesterol.
Table 2.—Measurements of Cholesterol Metabolism Under Different Conditions*
Controls
Cholestyramine Resin Therapy (N = 11) Obese
_(N 12)_(N Relative body weight_1.01 ± 0.03_1.33 ± Serum cholesterol, mmole/liter_234 ± 13_204 ± Serum triglycéride, mmoie/liter_1.73 ± 0.25_1.65 ± Liver FCH, mg/100gm_248 ± 29_267 ± Liver ECH, mg/100gm_73 ± 26_79 ± Liver TCH, mg/100gm_321 ± 51_346 ± Esterification, %_18 ± 3_20 ± =
Liver carbon 14-FCH, disintegrations per minute Carbon 14/tritium x 10- in liver FCH_6.9 Carbon 14/tritium x 10' in serum
=
,-*-,
9)_Before_During_ 0.05f_1.09 ± 0.04_1 09 ± 0.04 15_480 ± 49f_346 ± 26t 0.31_1.95 ± 0.28_1.85 ± 0.29 24_267 ± 43_191 ± 19f 26_123 ± 28_75 ± 14f 46_390 ± 71_266 ± 29f 3_30 ± 2t_26 ± 3_
xICF/gm/hr_5.5 ± 1.6_7.7 ± 1.8_3.5 ± 1.9_26.6
16.5 15_3.3 ± 1.0f FCH|_.___.„_3 ± 1_15 Serum TMS, ^g x 10;/mg of FCH_._._.„_144 ± 28_534 Fecal bile acid, mg/day_235 ± 25§_376 ± 10t§_196 ± 28_1,380 Fecal neutral sterol, mg/day_651 ± 66§_1,338 ± 118f§_582 ± 62_522
Fecal total steroids,
mg/day
889
-
±
1.3_9.3
85§
1,714
t
±
145f§
778
+
84
1,902
±
5.6f
! 3.4t ±
4t_
±
133f 248f
±
34
±
275f
±
'All values mean ± SE. Percutaneous liver biopsy specimens were obtained from hypercholesterolemic volunteers, or during routine diagnostic procedures from patients who later turned out to be clinically healthy and have normal liver histology or mild to moderate fatty liver. Biopsy specimens were incubated for three hours in Krebs-Ringer bicarbonate buffer with 1 mM of carbon 14 acetate and tritiated mevalonate. Hypercholesterolemic (familial with tendon xanthomata) subjects were sampled before and after 12 to 30 days on a regimen of cholestyramine resin therapy (32 gm /day). FCH indicates free cholesterol; ECH, esterified cholesterol; TCH, total cholesterol; TMS, serum total methylsterols. fStatistically significant difference (at least P < .05) from controls or pretreatment values. JRatio in serum free cholesterol four hours after intravenous injection of carbon 14-tritiated mevalonate mixture.
§Data taken from Mittinen.9
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OVERPRODUCTION OF CHOLESTEROL
Obesity, hypertriglyceridemia, and a high cholesterol diet are the three major conditions in which cholesterol influx (production) is increased and followed, secondarily, by an enhanced elimination. Quantitation of fecal bile acids and neutral sterols, end products of cholesterol metabolism, have indicated that cholesterol synthesis is considerably increased in obesity." In nonobese human subjects, daily cholesterol synthesis is about 12 mg/kg, while in overweight subjects, the figure is about 20 mg/kg of excess body weight. Kinetic studies have revealed similar results,10 but in some overweight subjects, the increase in synthesis is less than expected from their excess weight. The rate of cholesterol synthesis is not correlated with the serum cholesterol level. This apparently is due to effective removal of lipoproteins from plasma and effective elimination of excess cholesterol in feces by most obese subjects. Others, however, are incapable of doing this sufficiently and hypercholesterolemia develops. The hypercholesterolemia, as well as the increased synthesis, is usually normalized by successful weight reduction. When associated with a primary hypercholesterolemia, the obesi¬ ty-induced increase in cholesterol synthesis can increase the hyperlipoproteinemia, again, this can be improved, though not usually normalized, by weight reduction. The exact site of synthesis of excess cholesterol in obesity is not known. The in vitro studies by Schreibman and Dell10 indicated that the daily synthesis was less than 1 mg/kg of adipose tissue. However, considering the exten¬ sive dilution of the labeled precursors by the large endoge¬ nous intermediate pools of cholesterol during the experi¬ mental incubations, the actual synthesis was substantially higher8 than believed earlier.10 Thus, though the exact amount of synthesis is difficult to quantitate in vitro, the data indicate that adipose tissue can significantly contribute to cholesterol production.8 Another interesting organ is the liver, which can be assumed to enhance its fatty acid, cholesterol, and lipopro¬ tein production under the influence of an excessive calorie load. An increase in food consumption is actually asso¬ ciated with enhanced cholesterol synthesis." As shown in Table 2, synthesis of free fatty acids, but not of cholesterol, is increased from acetate l4C and tritiated mevalonate in liver biopsy specimens of obese subjects in vitro. However, the ratio of carbon 14/tritium in newly formed cholesterol tended to be higher in obese than in lean subjects and showed a significant correlation with the relative body weight (P < .05), as if early synthetic step(s), probably /3-hydroxyl-/?-methyl-glutaryl-coenzymeA-reductase (HMG-CoA-reductase), were activated. The increased fatty acid synthesis may be associated with enhanced hepatic release of VLDL-triglycerides. The production and catabolism of the latter actually appear to be increased in obesity; this accelerated turnover also promotes substantially the flux of cholesterol via VLDL and has been assumed to be one causal factor in enhanced cholesterol synthesis and catabolism in obesity and, partic¬ ularly, in hypertriglyceridemia." It should be borne in mind, however, that cholesterol production is not increased more in hyperglyceridemic than normoglyceridemic obese
• 0
a> o
Serum Total Cholesterol
Triglycérides
20
4?5
E
Ë
to
2?
E E
10 1.5 D Neutral Sterols
«1.0
_
E
Fecal Bile Acids
"O.ô -75
4
-76
-77
t
Op Effects of portacaval shunt on serum lipid and fecal steroid levels in 12-year-old girl homozygous for familial hypercholesterolemia. Heal bypass was performed four years earlier for treatment of hypercholesterolemia. Top, Serum methyl sterol levels expressed per 100 mg of free cholesterol (FLAN indicates free lanosterol; FDMS, free dimethyl sterol; ELAN, esterified lanosterol; and FCH, free cholesterol). Center, Serum total cholesterol and triglycéride levels and percent distribution of cholesterol in different lipopro¬ teins. Each column is adjusted to 100 (VLDL indicates very-lowdensity lipoprotein; LDL, low-density lipoprotein; and HDL, highdensity lipoprotein). Bottom, Fecal bile acid and neutral sterol levels. Decrease in total serum cholesterol level (free and ester cholesterol decreased equally), mainly caused by fall in LDL, is due to reduction in markedly high cholesterol synthesis (caused by ileal exclusion) and is indicated by fall in serum methyl sterol and fecal steroid levels. Cholesterol synthesis, measured from sterol balance studies, was 17 mg/kg/day before ileal bypass, 49 mg/kg/day after bypass and 29 to 43 mg/kg/day after portacaval shunt (Op).
subjects.9
The enhanced cholesterol synthesis in obesity and hyper¬ associated with an elevated elimination of cholesterol as both fecal neutral sterols and bile acids.911 The relative contribution of the latter tends, however, to be subnormal, particularly in females, indicating that with a high cholesterol production rate, relatively more of the excessively produced cholesterol is eliminated as neutral sterols, as compared to bile acids. The studies on biliary secretion of lipids in obesity have actually shown that biliary cholesterol output is greatly increased, while bile acid secretion is within normal limits, resulting in supersaturation of bile, a state promoting gallstone formation.12 Thus, increased flux of cholesterol into the intestinal lumen characterizes obesity. Though cholesterol loading usually enhances absolute cholesterol absorption, the absorption rate is unchanged, or decreased, and the amount of unabsorbed cholesterol is increased.13 Since, in addition, absorp¬ tion of bile acids may not be decreased, elimination of
triglyceridemia is
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Table 3.—Effects of
Cholestyramine
Resin and Ileal Bypass on Different Measurements of Cholesterol Metabolism in Patients With Familial Hypercholesterolemia* Fecal Steroids, mg/Day r-*-—>
Serum Cholesterol
Carbon 14/Tritium
DMS, /¿gx
_Treatment_mg/100 ml_Acidic_Neutral_1Q'/mg 01 FCHf_X100Í 648 ± 81 (13)_28 ± 6 (8)_4 ± 1 (5) None_546 ± 58 (13)_193 ± 31 +21 ± 66(13)_+ 40 ± 5 (8)_+11 ± 2 (5) Cholestyramine resin§_-93 ± 15(13)_+1240 ± 164 -45 ± 68 (13)_+70 ± 15 (8)_+ 25 ± 4 (5) Ileal bypass§_-178 ± 12(13)_+2221 ± 245 Ileal
bypass &
cholestyraminelj_-66
±
19
(10)_+887
±141
+177
±
45(10)_+39
+
11
(8)_+ 21
±
8
(5)
SD. Numbers in
•Data expressed parentheses indicate number of patients. fDMS indicates free dimethyl sterol; FCH, plasma free cholesterol. serum level in cholesterol tCarbon 14/tritium eight hours after injection of a carbon 14-acetate-tritiated mevalonate mixture. (¡Changes from initial values. ¡¡Changes from ileal bypass values. as mean ±
Table
4.—Follow-up
of Different Measurements of Cholesterol Metabolism in 25 Patients With Familial After Ileal Bypass Operation (Mean ± SE) Mo
Hypercholesterolemia
Postoperative
_0_1_6_12_24_36_60 373 ± 8 Serum cholesterol, mg/100 ml_558 ± 17 360 ± 12 372 ± 15 356 ± 15_372 ± 12_399 + 18 ± 20 ± 248 Fecal bile acids, 2164 ± 176_.___2330 ± 269 2429 ± 240 2400 ± 269 2320 mg/day_155 519 ± 21 Fecal neutral sterols, mg/day_572 ± 40 589 ± 38___534 ± 25_540 ± 22_452 ±23 97 ± 25 66 ± 10_89 ± 7_90 ± 5_96 ± 16 DMS, /ig X 1Q'/mg of FCH*_26 ± 5_74 ±10 Carbon 14/tritium
x
4
100 in FCH
±
1
30
±
6
31
±
3
35
±
9
22 ± 3
32
±
8
...
*DMS indicates excess
in
dimethyl sterol; FCH, plasma
cholesterol
as
fecal neutral sterols
obesity. It is obviously owing
groups that, in
some
free cholesterol.
to the
can
predominate
heterogeneity
of
patient
reports of hyperglyceridemia, endoge¬
nous cholesterol production and fecal elimination as bile acids and neutral steroids are consistently increased; in others, they are not.4-6-9-1415 Several modifying variables can be hypothesized, eg, the presence or absence of obesity, the VLDL level, the turnover rate of VLDL, and the level of LDL. A negative correlation between the sterol balance value, fecal bile acids, neutral sterols and the serum cholesterol level in hypertriglyceridemic subjects16 sug¬ gests that the higher the turnover of VLDL and the lower the LDL level, the higher is the endogenous synthesis of cholesterol in type IV hyperlipoproteinemia. The relationship between bile acid metabolism and different hyperlipoproteinemias, investigated especially by Hellström and Einarsson,14 has been summarized recently. In about two thirds of type IV and V patients, bile acid production, that of cholic acid in particular, is increased. A decrease of plasma triglycérides, or their turnover with different therapeutic measures, is frequent¬ ly associated with a reduced synthesis of bile acids, suggesting a close, as yet poorly understood relationship between the metabolism of VLDL and bile acids (or
cholesterol).
An increased influx of cholesterol from the diet is followed by (1) enhanced elimination of cholesterol as neutral sterols, (2) unchanged or slightly increased bile acid excretion, (3) reduced cholesterol synthesis, (4) an increased serum cholesterol or LDL-level, and (5) retention of cholesterol in the body. These metabolic responses show a substantial interindividual variation.1517 On a high intake, particularly if associated with a poor removal capacity, the retention may be considerable. On a lower
intake, the compensating mechanism, increased reexcretion of dietary cholesterol and the decrease of synthesis in particular, prevent the increase of serum cholesterol levels and a major retention of cholesterol in the body. As compared to enhanced cholesterol flux in obesity and hyperglyceridemia, the lack of increased bile acid synthesis during high cholesterol absorption is an interesting phenomenon. Low cholesterol synthesis and a different lipoprotein metabolism may be factors promoting choles¬ terol catabolism, primarily by way of neutral sterols. Our cholesterol feeding experiments (1 extra gram per day for four weeks) in a random population group, under normal living habits, showed an insignificant increase in serum cholesterol concentrations associated with a 40% decrease in cholesterol synthesis and a 46% ( + 134 mg/day; P < .05) increase in fecal bile acid excretion. BILE ACID AND CHOLESTEROL MALABSORPTION
Interruption of the enterohepatic circulation of bile acids by ion exchangers or ileal exclusion, or inhibition of the cholesterol absorption by plant sterols or neomycin, in the treatment of hypercholesterolemia result in an increase in fecal elimination of cholesterol, a compensatory increase in cholesterol synthesis (shown in vivo in Tables 2 through 4) by an increase of the carbon 14/tritium ratio and by plasma methyl sterols), and yet, in a decrease of the serum
cholesterol and LDL levels.11151819 The mechanism of the latter is poorly understood, since studies of LDL turnover1 and LDL metabolism by fibroblasts of hypercholesterol¬ emic subjects2 emphasize the causal role of peripheral LDL catabolism in the development of hypercholesterolemia. The fall in the serum cholesterol level (apparently also that of LDL) is usually positively correlated with an increase in fecal steroids, indicating that the higher the
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increase of cholesterol elimination as neutral sterols or as bile acids or both, the higher the decrease in the serum cholesterol level.1118 It is owing to an effective compensa¬ tory increase in cholesterol synthesis that this rule of thumb is not valid in patients with homozygous familial hypercholesterolemia in whom complete biliary diversion has no cholesterol-lowering effect.2" Portacaval shunt effectively reduces the serum cholesterol level and choles¬ terol and LDL synthesis in these patients.21 Inhibition of synthesis with this procedure in homozygous subjects with ileal exclusion (Figure) may offer an additional advantage in the serum cholesterol level lowering in these patients. In heterozygous subjects, ion exchangers, and ileal exclusion in particular, cause a substantial decrease in the serum cholesterol level.111 Though the ileal bypass is more effective than ion exchangers on both serum cholesterol and fecal bile acid levels, normal serum cholesterol values are rarely obtained except in combination with large doses of ion exchangers, which clearly potentiate the effects of the ileal
bypass (Table 3). A lack of LDL-receptors
in familial
hypercholesterol¬
emia may result in insufficient suppression of HMG-CoAreductase, and thus, despite a high plasma LDL concentra¬ tion, in enhanced cholesterol synthesis by peripheral tissues.2 Sterol balance data, however, indicate that the overall synthesis is low normal.1118 This could be due to low hepatic cholesterol synthesis (Table 2), which in turn could be a reason for the subnormal bile acid production.1 With cholestyramine resin, or after ileal exclusion, cholesterol synthesis, as indicated by incubation studies, by the acetate-mevalonate test, and also by plasma methyl sterol levels (Tables 2 through 4), increases relatively, much more than the overall production shown by the fecal steroids. This can be expected if most of the increased synthesis takes place in the liver and if the initial hepatic production is low. Assuming that the change in the carbon 14/tritium ratio of the acetate-mevalonate test is primarily caused by the activation of the hepatic cholesterol synthesis, as suggested by the findings in Table 2, the quantity of basal ' '"
hepatic
cholesterol synthesis could be calculated from the data in Tables 2 through 4. This ratio increases fourfold to tenfold, and fecal steroids by 1 to 3 gm/day. Thus, the hepatic production was about 200 to 300 mg/day. Cholesterol malabsorption caused by gluten enteropathy, plant sterols, or small doses of neomycin is not associated with an increased bile acid production, although cholesterol synthesis is apparently increased.1118 In addition, the latter is not detected with the acetate-mevalonate test or plasma methyl sterols, particularly in long-term studies, suggest¬ ing that later steps in the synthetic pathway of cholesterol are stimulated or the synthesis is increased in extrahepatic tissues or both. Table 4 shows that, after ileal exclusion, enhanced cholesterol synthesis can be detected perma¬ nently with these two procedures. It is a common clinical observation that serum choles¬ terol level lowering is associated, in the long term, with a reduction in the size of visible cholesterol deposits of xanthelasmata and cutaneous and tendon xanthomata. However, though the decrease in the serum cholesterol level may be consistently associated with a mobilization of tissue cholesterol, provided that the latter is increased, and with a decrease in the pool 1 and liver cholesterol (Table 2),
the decrease in the size of the slowly exchangeable choles¬ terol pool is difficult to demonstrate with current methods. For instance, the size of that pool is not consistently changed by anion exchange resins.22 However, the ileal bypass operation, which is associated with a substantial increase in cholesterol elimination and a clear decrease in the serum cholesterol level, seems to cause a significant reduction in the pool size of the slowly exchangeable cholesterol.11' The patients in Table 4 lost about 3.5 kg of extra cholesterol during the postoperative follow-up period of five years. It is possible that this was not totally balanced by increased synthesis, but that some of it was due to mobilization of tissue cholesterol, particularly since the average serum cholesterol level was decreased perma¬ nently by about 35%. This study was supported by grants from the Finnish State Council for Medical'Research. E. Gustafsson, I. Aaltonen, and M. Aarnio provided technical and secretarial assistance.
References 1. Kudchodkar BJ, Sodhi HS: Turnover of plasma cholesteryl esters and its relationship to other parameters of lipid metabolism in man. Eur J Clin
Invest 6:285-298, 1976. 2. Brown MS, Goldstein JL: Receptor-mediated control of cholesterol metabolism. Science 191:150-154, 1976. 3. Langer T, Strober W, Levy RI: The metabolism of low density lipoprotein in familial type II hyperlipoproteinemia. J Clin Invest 51:1528\x=req-\ 1536, 1972. 4. Smith FR, Dell RB, Noble RP, et al: Parameters of the three-pool model of the turnover of plasma cholesterol in normal and hyperlipidemic humans. J Clin Invest 57:137-148, 1976. 5. Bhattacharyya AK, Connor WE, Mausolf FA, et al: Turnover of xanthoma cholesterol in hyperlipoproteinemia patients. J Clin Lab Med 87:503-518, 1976. 6. Sodhi HS: Cholesterol metabolism in man, in Kritchevsky D (ed): Hyperlipidemic Agents: Handbook of Experimental Pharmacology. New York, Springer-Verlag, 1975, vol 41, pp 29-107. 7. Kekki M, Miettinen TA, Wahlstr\l=o"\mB: Measurement of cholesterol synthesis in kinetically defined pools using fecal steroid analysis and double labeling technique in man. J Lipid Res 18:99-114, 1977. 8. Miettinen TA: Cholesterol synthesis in slowly exchangeable cholesterol pool in man, in Proceedings of the Fourth International Symposium on Atherosclerosis. New York, Springer-Verlag, 1976. 9. Miettinen TA: Cholesterol production in obesity. Circulation 44:842\x=req-\
850,1971.
10. Schreibman PH, Dell RB: Human adipocyte cholesterol: Concentration, localization, synthesis and turnover. J Clin Invest 55:986-993, 1975. 11. Miettinen TA: Clinical implications of bile acid metabolism, in Nair PP, Kritchevsky D (eds): The Bile Acids. New York, Plenum Press Inc, 1973,
vol 2, pp 191-247. 12. Bennion LJ, Grundy SM: Effects of obesity and caloric intake on biliary lipid metabolism in man. J Clin Invest 56:996-1011, 1975. 13. Grundy SM, Mok HYI: Determination of cholesterol absorption in man by intestinal perfusion. J Lipid Res 18:263-271, 1977. 14. Hellstr\l=o"\mK, Einarsson K: Bile acid metabolism in hyperlipoproteinemia. Clin Gastroenterol 6:103-128, 1977. 15. Grundy SM: Dietary and drug regulation of cholesterol metabolism in man, in Paoletti R, Glueck CJ, (eds): Lipid Pharmacology. New York, Academic Press Inc, 1975, vol 2, pp 127-159. 16. Miettinen TA: Cholesterol metabolism in patients with coronary heart disease. Ann Clin Res 3:313-322, 1971. 17. Nestel PJ, Poyser A: Changes in cholesterol synthesis and excretion when cholesterol intake is increased. Metabolism 25:1591-1599, 1976. 18. Miettinen TA: Methods for evaluation of hypolipidemic drugs in man: Mechanisms of their action, in Paoletti R, Glueck CJ (eds): Lipid Pharmacology. New York, Academic Press Inc, 1975, vol 2, pp 83-125. 19. Buchwald H, Moore RB, Varco RL: Surgical treatment of hyperlipidemia. Circulation 49(suppl 1): I1-35, 1974. 20. Deckelbaum RJ, Lees RS, Small DM, et al: Failure of complete bile diversion and oral bile acid therapy in the treatment of homozygous familial hypercholesterolemia. N Engl J Med 296:465-470, 1977. 21. Bilheimer DW, Goldstein JL, Grundy SM, et al: Reduction in cholesterol and low density lipoprotein synthesis after portacaval shunt surgery in a patient with homozygous familial hypercholesterolemia. J Clin Invest 56:1420-1430, 1976. 22. Goodman DS, Noble RP, Dell RB: The effects of colestipol resin and of colestipol plus clofibrate on the turnover of plasma cholesterol in man. J Clin Invest 52:2646-2655, 1973.
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