A New Perspective on Cholesterol Metabolism in Man H. S. Sodhi Perspectives in Biology and Medicine, Volume 18, Number 4, Summer 1975, pp. 477-485 (Article) Published by Johns Hopkins University Press DOI: https://doi.org/10.1353/pbm.1975.0064
For additional information about this article https://muse.jhu.edu/article/406502/summary
Access provided at 18 Nov 2019 05:31 GMT from University of Toledo
A NEW PERSPECTIVE ON CHOLESTEROL METABOLISM IN MAN H. S. SODHI*
Ever since cholesterol was suspected to play a role in the development
of atherosclerotic heart disease, there has been a surge of interest in the
study of cholesterol metabolism in man. Although plasma cholesterol constitutes only about 7 percent of the total cholesterol in the body, it is the only pool of cholesterol which has been incriminated in this disease. Plasma cholesterol has therefore been the main focus of attention. Even
studies on tissue metabolism of cholesterol are generally viewed in relationship to their bearing directly or indirectly on plasma cholesterol.
Although free cholesterol has been recognized to be a structural component of tissue membranes and of plasma lipoproteins, the thinking in this field has been dominated by the "results" rather than the "function"
of cholesterol.
Traditionally plasma cholesterol is grouped with triglycerides and other plasma lipids, and the teaching has been that the lipids are combined with specific proteins in order to make them water soluble for their transport in plasma. These "apoproteins" serve as solubilizers of plasma lipids, and under most conditions their availability is assumed to
be adjusted to the needs for transport of plasma lipids. Although often
not stated explicitly, it is generally assumed that there is homeostatic
control of plasma cholesterol—that is, that there are mechanisms specifi-
cally sensitive to changes in the plasma cholesterol concentration and which come into play in the form of alterations in the absorption of
dietary cholesterol or changes in tissue synthesis or catabolism of endogenous cholesterol.
In contrast to the above, I suggest that there are no homeostatic mechanisms specifically sensitive to plasma concentration of cholesterol, and that absorption of dietary cholesterol or the tissue synthesis and
catabolism of endogenous cholesterol are not directed specifically to
?Departments of Medicine, University of Saskatchewan, Saskatoon, Canada, and Stanford University, Palo Alto, California. Much of the work was done in collaboration with my colleague Dr. B. J. Kudchodkar and was supported by Medical Research Council of Canada and Canadian and Saskatchewan Heart Foundations.
Perspectives in Biology and Medicine · Summer 1975 | 477
maintain levels of plasma cholesterol. Free cholesterol in plasma lipoproteins is viewed as only one of the packaging materials necessary for the transport of the contents of lipoprotein package. Similarly, all free cholesterol in tissues is assumed to be "structural" (except, of course, in
organs having special roles in cholesterol metabolism). The tissue metabofam ofcholesterol, in general, relates more to the needs ofthe tüsues than to homeostasis ofplasma cholesterol. The tissue synthesis of free cholesterol is coupled primarily to the need for synthesis and turnover of cellular and subcellular membranes. Synthesis of free cholesterol in organs which make plasma lipoproteins is coupled to the need for synthesis of new plasma lipoproteins as well as to the need for synthesis of cellular organelles. All tissues have some degree of turnover, but only the liver and intestines synthesize plasma lipoproteins. (Chylomicra for the purposes of this discussion are considered lipoproteins.) That cholesterol synthet-
ic activity in liver and intestines constitutes more than 90 percent of the total synthetic activity in experimental animals [1] suggests that turnover of plasma lipoproteins is associated with much greater synthesis of cholesterol than the turnover of cellular membranes and intracellular
organelles. This concept is supported by the demonstration of marked differences in cholesterol synthesis associated with changes limited to
hepatic synthesis of plasma lipoproteins in man [2]. Although sufficient information is not yet available, it appears that
tissues with faster turnover rates synthesize more cholesterol than those
which have relatively slow turnover. The epidermis has a rapid turnover and synthesizes more cholesterol than the dermis, which has a rather slow turnover [3]. The mature nervous tissue and red blood cells have
perhaps the least turnover, and they also show little, if any, synthesis of
cholesterol [4]. In man the lipoproteins synthesized by the intestines contain both
cholesterol esters and triglycerides as their content or "core," but under normal conditions the newly synthesized hepatic lipoproteins are assumed to contain triglycerides as the only or the predominant core lipid [5]. When perfused in vitro, livers from cholesterol-fed rats and containing large amounts of stored cholesterol esters synthesize lipoproteins which transport the stored cholesterol esters out of the liver [6]. However, such accumulation of cholesterol esters is not seen in man, and it is
therefore unlikely that hepatic cholesterol is nearly as important as hepatic triglycerides in driving the hepatic machinery for synthesis of new lipoproteins to transport lipids out of that organ.
Most lipoproteins in fasting plasma are synthesized in the liver. The fractional turnover rate of plasma very-low-density lipoproteins (VLDL) is at least 10 times greater than those of plasma low-density lipoproteins (LDL) or high-density lipoproteins (HDL) [7, 8]. Modest to moderate elevations of plasma VLDL are generally associated with corresponding 478 J H. S. Sodhi · Cholesterol Metabolism in Man
increases in their net turnover rates even if there is some diminution in
their fractional rates of clearance [9, 10]. Since elevations of plasma
triglycerides above normal are generally due to the elevations of VLDL, most hypertriglyceridemic patients would therefore have an overall increase in hepatic synthesis of plasma lipoproteins. If hepatic choles-
terogenesis is coupled to the need for synthesis of new lipoproteins, hypertriglyceridemic patients should, according to this concept, have
increased synthesis of endogenous cholesterol; and this indeed was what
we reported recently [H]. We have since examined 60 additional subjects and noted an excellent correlation (r = .83, P < .001) between relative rates of cholesterol synthesis and plasma concentrations of triglycerides [12]. In order to test further this association of hypertriglyceridemia with increased synthesis of cholesterol, we pooled our own and the published data [13-24] on approximately 100 cholesterol balance studies. Our analysis of the results indicated that cholesterol syn-
thesis in patients with hypertriglyceridemia was significantly greater than in those with normal plasma triglycerides (fig. 1).
Lipoproteins synthesized by the intestines contain cholesterol derived from the diet, bile, and local synthesis. However, the major content of the intestinal lipoproteins is the dietary triglycerides. After their entry
into circulation, most of the triglycerides in the intestinal lipoproteins
are removed by extrahepatic tissues, and the remnants containing most
of the cholesterol are then removed by the liver [25]. Our model for hepatic metabolism of cholesterol suggests that dietary and other cholesterol in the intestinal lipoproteins, when removed by the liver, enters the same pool as that of cholesterol synthesized in the liver [26]. Cholesterol Synthesis Absorption mg/day""*"'·"""'
Cholesterol Intake ? 300 mg/day Ej 300 mg/day
1200
800
400
. E NP ¦ Il
Il Z
Il
No. of Subjects (19) (12) TypeNormal
(19) (10)
(16) (13)
(6) (4)
Hy perchel. Hy percho). Hyperfrig. and
Hypertng.
Fig. 1.—Negative feedback inhibition
Perspectives in Biology and Medicine · Summer 1975 | 479
in this "anabolic" pool provides one of the packaging materials required
for the transport of triglycerides out of the liver into plasma. This suggests that exogenous cholesterol can replace the synthesized cholesterol for making new tissue (or plasma) lipoproteins.
After a high-cholesterol diet, the suppression of synthesis in various
tissues of guinea pig was proportional to the amounts of accumulated tissue cholesterol [27]. In rats the inhibition by exogenous cholesterol is
much greater in the liver than in any organ, which may well be related to
the fact that accumulation of cholesterol in the rat liver is greater than in any other organ. Since exogenous cholesterol is transported from lumen into the intestinal lymph rapidly, there is little accumulation of exogenous cholesterol in mucosal cells. The proportion of exogenous to endogenous cholesterol in the intestinal wall at any one time is less than that in the liver, and therefore the intestinal synthesis of cholesterol is
not suppressed to the same degree as that in the liver.
In accord with these data, it is suggested that the hepatic cholesterol in "anabolic" pool, whether derived from local synthesis or from diet, is first incorporated into plasma (or tissue) lipoproteins before it is
catabolised. The following evidence supports this suggestion. During the 3 hours after the addition of radioactive precursors to the perfusing medium, most of the cholesterol newly synthesized by the perfused liver either remained in the liver (presumably as a constituent of structural lipoproteins) or was released as lipoproteins into the circulating medium. Less than 0.5 percent was excreted into the bile during this period [28]. Preliminary studies in our own laboratories [H. S. Sodhi and
M. Avigan, unpublished observations] support the preceding data from Bricker. Rats were given radioactive mevalonate, and the total radioac-
tivity in hepatic and biliary cholesterol was determined. The loss of radioactive cholesterol from the rat livers between 1 and 2 hours was
about 8 percent of the total injected radioactivity. Excretion of the isotope in biliary cholesterol and bile acids amounted to only 0.4 percent.
This indicated that 95 percent of the newly synthesized cholesterol lost from the liver was transported out into the plasma. Similar evidence with regard to dietary cholesterol is also available. Bhattacharyya, Connor, and Spector [29] noted that in normal subjects pool sizes and production rates calculated from changes in plasma cholesterol specific-activity
slopes were similar whether the radioactive cholesterol was given by
intravenous injection or by oral feeding. This would not have been the case unless all the dietary cholesterol absorbed from the intestinal tract
was first incorporated into plasma lipoproteins or at least into pool A .
Zilversmit [30] proposed a method for calculating absorption of dietary
cholesterol based on the ratios of two isotopes in plasma after intravenous injection of one label and feeding of the other label in cholesterol.
Recent validation of this method [31] also implies that the absorbed 480 I H. S. Sodhi · Cholesterol Metabolism in Man
dietary cholesterol was first incorporated quantitatively into plasma
lipoproteins before it was catabolised. This is also supported by the
studies on /3-sitosteroI. The absorption of /3-sitosterol calculated by the sterol balance method was the same as that calculated by isotope kinetics
of the sterol in plasma [32]. In a fashion somewhat similar to dietary cholesterol, particulate cholesterol injected intravenously is removed primarily by the liver and is first quantitatively incorporated into plasma
lipoproteins before it is catabolised and excreted into the bile [31, 33].
Two predictions can be made from this aspect of the proposed concept.
1.If turnover of plasma lipoproteins remains constant, greater contributions from dietary cholesterol will decrease the need for de novo
synthesis of cholesterol (for making new plasma lipoproteins).
2.The metabolism of the absorbed cholesterol, after its removal by the liver, will be identical with the cholesterol derived from endogenous synthesis.
The first prediction is nothing other than the well-known negative feedback inhibition of endogenous cholesterol synthesis by exogenous
cholesterol. Its existence in many species of animals has been well substantiated [34], although there is still some question about its existence in
man. In our analysis of pooled data from cholesterol balance studies, we divided the subjects with various forms of hyperlipemia into low- and high-cholesterol-intake groups. The synthesis of cholesterol in the lowintake group was always significantly greater than the synthesis in the
high-intake group (fig. 1). Furthermore, the amounts of daily choles-
terol turnover (absorbed + synthesized )were similar in both groups,
suggesting that the depression in endogenous synthesis by dietary cholesterol was more or less precisely made up by the amounts of cholesterol derived from the diet. This supports our hypothesis [26] that the
exogenous cholesterol and synthesized cholesterol enter the same hepatic pool, and it was supported by following further observations in the same group of patients. The amounts of fecal neutral and acidic metabolites of endogenous cholesterol, as well as their ratios in the low- and high-intake groups, were remarkably similar (fig. 2). Although there is considerable information about the sites of synthesis
of plasma lipoproteins, little is known of the sites of their catabolism. Liver is the most important organ for the catabolism of cholesterol, but it may or may not be an equally important site for the catabolism of lipoproteins present in the postabsorptive plasma. However, cholesterol released from the lipoproteins, whatever the sites of their catabolism, has to be transported to the liver, since it is the only organ capable of excreting large amounts of cholesterol from the system. Cholesterol from catabolism of tissue membranes, including red blood cells, also has to be
transported to the liver for disposal. We recently presented evidence Perspectives in Biology and Medicine · Summer 1975 | 48 1
Cholestrol Intake ? < 300 mg/day
ËËJ>300 mg/day
3.0
•2
-
400 H
1
1
1400 X O
i5 1000 '5 a
£5 600 O
0)
1
Z
200 H
No. of Subjects (19) (12) TypeNormal
(19) (10)
(16) (13)
(6) (4)
Hyperchol.
Hyperchol. |
Hypertrig.
and
Hypertrig. Fig. 2.—Effect of dietary cholesterol on fecal excretion of endogenous steroids
[35] to suggest that the cholesterol from the tissues is promptly mobilized and transported to the liver for excretion in the bile when
plasma cholesterol levels are reduced. The cholesterol mobilized from
the tissues did not appear to remain in plasma long enough to increase its concentration in plasma. Recent work of Brown and Goldstein [36, 37] suggests that connective tissue may well be an important site for
degradation of plasma LDL, in which case cholesterol from degradation of LDL will need to be transported to the liver for catabolism. The impres482 I H- S. Sodhi · Cholesterol Metabolism in Man
sion given by the published literature is that the free cholesterol derived from absorption, local synthesis, or from any other source, for all practical purposes constitutes a single (common) pool undergoing rapid fluxes into and from various pathways related to the different roles of liver in cholesterol metabolism. Our model suggests that the cholesterol returning to the liver for catabolism does not completely mix with the cholesterol in the "anabolic" pool and thus is not available for synthesis of new
lipoproteins. This model therefore postulates the existence of two sepa-
rate pools which functionally (despite isotopic exchange) remain more or less independent. Most of the biliary cholesterol and biliary bile acids are derived from the catabolic pool, and little, if any, from the anabolic
pool. Although there is no direct evidence for or against the existence of
two independent functional pools of cholesterol in the liver, the idea is quite tenable. Accepting the assumption that cholesterol synthesis is coupled to the need for synthesis of new lipoproteins, and the evidence that hypertri-
glyceridemic patients have a greater hepatic synthesis of plasma lipoproteins, continued increases in synthesis and catabolism of cholesterol in such patients favour the two-pool rather than the single-pool model. In conditions of increased turnover of plasma lipoproteins, greater amounts of cholesterol would return to the liver from increased degra-
dation of lipoproteins. In a single-pool model there would be no reason
for the continued increase either in hepatic synthesis of cholesterol or in the biliary and fecal excretion, since the cholesterol from degradation of lipoproteins is available for synthesis of new lipoproteins. Studies conducted in our laboratories over the last few years have suggested that patients with hypertriglyceridemia not only continue to make larger amounts of cholesterol [11] but also continue to extrete abnormally large amounts of neutral and acidic metabolites of endogenous cholesterol [38]. This suggests that at least some of the cholesterol derived from catabolism of lipoproteins is not available for synthesis of new lipopro-
teins and is channeled into catabolic pathways.
The situation in liver is in a sense similar to what happens in intestinal mucosa. Both have special roles in the metabolism of cholesterol. Cholesterol is brought to the intestines for absorption and to the liver for catabolism. As early as 1965, it was suggested by Sodhi and associates that exogenous cholesterol did not completely mix with the cholesterol synthesized in the intestinal mucosa [39]. More recently evidence was presented by us [40] and by Rudel, Morris, and Felts [41] indicating that the exogenous cholesterol and endogenous cholesterol in the intestinal mucosa remain in separate pools. The rate of transport of exogenous cholesterol is two to three times greater than that of cholesterol synthesized locally [40]. The dietary cholesterol in intestinal mucosa may be considered as merely "in transit," as opposed to the structural cholesterol synthesized by the intestinal mucosa. The cholesterol brought to Perspectives in Biology and Medicine · Summer 1975 | 483
the liver for catabolism is comparable to "in transit" cholesterol of intes-
tinal mucosa, since both are related to the specialized roles of respective organs. Cholesterol metabolism in adrenal glands also suggests that
cholesterol for synthesis of steroid hormones (a specialized role of adre-
nal glands) may also be present in a pool different from that of choles-
terol synthesized by the gland itself [42].
Tissue cholesterol esters have generally been regarded as storage
forms of cholesterol. Most tissues have enzymes for esterification of free cholesterol and hydrolysis of cholesterol esters. The pools of free and esterified cholesterol are assumed to be in equilibrium, implying that the free cholesterol derived from hydrolysis of cholesterol esters mixes in-
distinguishably with the pool of free cholesterol from which the choles-
terol esters were orginally derived. In accord with the model for hepatic metabolism of lipoprotein cholesterol, another possibility is suggested. Free cholesterol is incorporated into plasma lipoproteins as an essential structural constituent, and the esterification of plasma free cholesterol is a reflection of the catabolism or turnover of plasma or tissue lipopro-
teins. Therefore, in this light the esterified cholesterol may be consid-
ered to lie on the catabolic pathway of free cholesterol which is a structural component of tissue or plasma lipoproteins. Studies in our
laboratories have demonstrated that turnover of plasma cholesterol es-
ters had an excellent correlation with endogenous synthesis of cholesterol, on the one hand, and with plasma VLDL turnover, on the other [43]. Cholesterol esters in the adrenal gland, however, constitute a special case, since the cholesterol esters are stored for synthesis of steroid hormones, the need for which may arise rather acutely.
In summary, it is suggested that changes in the absorption of dietary cholesterol or alterations in tissue synthesis and catabolism of endoge-
nous cholesterol are not specifically directed to maintain levels of plasma cholesterol. Free cholesterol is viewed only as a structural component of
plasma or tissue lipoproteins, and its metabolism is coupled to the turn-
over of these lipoproteins. The esterification of free cholesterol is a reflection of this turnover process. The cholesterol released by degrada-
tion of lipoproteins is not reused for synthesis of new lipoproteins, but is
catabolised.
REFERENCES
1.J. M. DiETSCHY and J. D. Wilson. J. Clin. Invest., 47:166, 1968.
2.H. S. Sodhi and B.J. Kudchodkar. Circulation, 44:11-57, 1971. 3.J. L. Gaylor.J. Biol. Chem., 238:1643, 1963. 4.J. M. Dietschv and J. D. Wilson. N. Eng. J. Med., 282:1 128, 1179, 1241, 1970.
5.J. A. Glomset. Am. J. Clin. Nutr., 23:1129, 1970.
484 I H. S. Sodhi · Cholesterol Metabolism in Man
6.P. S. RoHEiM, D. E. Haft, L. I. Gidez, A. White, and H. A. Eder. J. Clin. Invest., 42:1277, 1963.
7.D. Gitlin, D. G. Cornwall, D. Nakasato, J. L. Oncley, W. L. Hughes, and C. A. Janeway. J. Clin. Invest., 37:172, 1958. 8.T. Langer, W. Strober, and R. I. Levy. J. Clin. Invest., 51:1528, 1972. 9.G. M. Reaven, D. B. Hill, R. C. Gross, and J. W. Farquhar. J. Clin. Invest.,
44:1826, 1965. 10.A. H. Kissebah, P. W. Adams, and V. Wynn. Clin. Sei. Mol. Med., 47:259, 1974.
11.H. S. Sodhi and B. J. Kudchodkar. Metabolism, 22:895, 1973. 12.H. S. Sodhi, J. Merriman, M. Mishkel, B. J. Kudchodkar, and G. S. Sundaram. Clin. Res., 22:753A, 1974.
13.N. Spritz, E. H. Ahrens, and S. M. Grundy. J. Clin. Invest., 44:1482, 1965. 14.S. M. Grundy and E. H. Ahrens, Jr. J. Clin. Invest., 45:1503, 1966. 15. ---------.J. Lipid Res., 10:91, 1969. 16.S. M. Grundy, E. H. Ahrens, Jr., and J. Davignon. J. Lipid Res., 10:304, 1969.
17.S. M. Grundy and E. H. Ahrens, Jr. J. Clin. Invest. 49:1135, 1970. 18.S. M. Grundy, E. H. Ahrens, Jr., and G. Salen. J. Lab. Clin. Med., 78:94, 1971.
19.S. M. Grundy, E. H. Ahrens, Jr., G. Salen, P. H. Schreibman, and P.J. Nestel. J. Lipid Res., 13:531, 1972. 20.G. Salen, E. H. Ahrens, Jr., and S. M. Grundy. J. Clin. Invest., 49:952, 1970.
21.E. Quintao, S. M. Grundy, and E. H. Ahrens. J. Lipid Res., 12:233, 1971. 22.W. E. Connor, D. T. Witiak, D. B. Stone, and M. L. Armstrong. J. Clin. Invest., 48:1363, 1969. 23.T. A. MiETTiNEN. Circulation, 44:842, 1971.
24.P.J. Nestel, N. Havenstein, H. M. Whyte, T.J. Scott, and L.J. Cook. N. Engl. J. Med., 288:379, 1973. 25.T. G. Redgrave. J. Clin. Invest., 49:465, 1970. 26.H. S. Sodhi and B.J. Kudchodkar. Lancet, 1:513, 1973. 27.A. Swann and M. D. Siperstein. J. Clin. Invest., 51:95a, 1972. 28.L. A. Bricker. J. Clin. Invest., 50:12a, 1971.
29.A. Bhattacharyya, W. E. Connor, and A. A. Spector. Circulation, 44:11-2, 1971.
30.D. B. Zilversmit. Proc. Soc. Exp. Biol. Med., 140:862, 1972. 31.D. B. Zilversmit and L. B. Hughes. J. Lipid Res., 15:465, 1974. 32.G. Salen, E. H. Ahrens, and S. M. Grundy. J. Clin. Invest., 49:952, 1972. 33.H. S. Sodhi and B.J. Kudchodkar. J. Lab. Clin. Med., 82:111, 1973. 34.M. D. Siperstein. Cell Res., 2:65, 1970.
35.H. S. Sodhi, B. J. Kudchodkar, and L. Horlick. Atherosclerosis, 17:1, 1973.
36.M. S. Brown and J. L. Goldstein. Science, 185:6, 1974. 37. ---------. Proc. Natl. Acad. Sei. USA, 71:788, 1974.
38.H. S. Sodhi and B.J. Kudchodkar. Clin. Chim. Acta, 46:161, 1973.
39.H. S. Sodhi, V. Berger, E. A. Swyryd, and R. G. Gould. Circulation, 38:11-31, 1965.
40.H. S. Sodhi, R. Orchard, N. D. Agnish, P. Verughese, and B. J. Kudchodkar. Atherosclerosis, 17:197, 1973.
41.L. L. Rudel, M. D. Morris, and J. M. Felts. J. Clin. Invest., 51:2682, 1972. 42.A. Borkowski, C. Delcroix, and S. Levin. J. Clin. Invest., 51:1664, 1972. 43.H. S. Sodhi and B.J. Kudchodkar. Proc. 3d Int. Symp. Atherosclerosis, West Berlin, October 1973, p. 516. Berlin: Springer-Verlag, 1974.
Perspectives in Biology and Medicine · Summer 1975 | 485
For additional information about this article https://muse.jhu.edu/article/406502/summary
Access provided at 18 Nov 2019 05:31 GMT from University of Toledo
A NEW PERSPECTIVE ON CHOLESTEROL METABOLISM IN MAN H. S. SODHI*
Ever since cholesterol was suspected to play a role in the development
of atherosclerotic heart disease, there has been a surge of interest in the
study of cholesterol metabolism in man. Although plasma cholesterol constitutes only about 7 percent of the total cholesterol in the body, it is the only pool of cholesterol which has been incriminated in this disease. Plasma cholesterol has therefore been the main focus of attention. Even
studies on tissue metabolism of cholesterol are generally viewed in relationship to their bearing directly or indirectly on plasma cholesterol.
Although free cholesterol has been recognized to be a structural component of tissue membranes and of plasma lipoproteins, the thinking in this field has been dominated by the "results" rather than the "function"
of cholesterol.
Traditionally plasma cholesterol is grouped with triglycerides and other plasma lipids, and the teaching has been that the lipids are combined with specific proteins in order to make them water soluble for their transport in plasma. These "apoproteins" serve as solubilizers of plasma lipids, and under most conditions their availability is assumed to
be adjusted to the needs for transport of plasma lipids. Although often
not stated explicitly, it is generally assumed that there is homeostatic
control of plasma cholesterol—that is, that there are mechanisms specifi-
cally sensitive to changes in the plasma cholesterol concentration and which come into play in the form of alterations in the absorption of
dietary cholesterol or changes in tissue synthesis or catabolism of endogenous cholesterol.
In contrast to the above, I suggest that there are no homeostatic mechanisms specifically sensitive to plasma concentration of cholesterol, and that absorption of dietary cholesterol or the tissue synthesis and
catabolism of endogenous cholesterol are not directed specifically to
?Departments of Medicine, University of Saskatchewan, Saskatoon, Canada, and Stanford University, Palo Alto, California. Much of the work was done in collaboration with my colleague Dr. B. J. Kudchodkar and was supported by Medical Research Council of Canada and Canadian and Saskatchewan Heart Foundations.
Perspectives in Biology and Medicine · Summer 1975 | 477
maintain levels of plasma cholesterol. Free cholesterol in plasma lipoproteins is viewed as only one of the packaging materials necessary for the transport of the contents of lipoprotein package. Similarly, all free cholesterol in tissues is assumed to be "structural" (except, of course, in
organs having special roles in cholesterol metabolism). The tissue metabofam ofcholesterol, in general, relates more to the needs ofthe tüsues than to homeostasis ofplasma cholesterol. The tissue synthesis of free cholesterol is coupled primarily to the need for synthesis and turnover of cellular and subcellular membranes. Synthesis of free cholesterol in organs which make plasma lipoproteins is coupled to the need for synthesis of new plasma lipoproteins as well as to the need for synthesis of cellular organelles. All tissues have some degree of turnover, but only the liver and intestines synthesize plasma lipoproteins. (Chylomicra for the purposes of this discussion are considered lipoproteins.) That cholesterol synthet-
ic activity in liver and intestines constitutes more than 90 percent of the total synthetic activity in experimental animals [1] suggests that turnover of plasma lipoproteins is associated with much greater synthesis of cholesterol than the turnover of cellular membranes and intracellular
organelles. This concept is supported by the demonstration of marked differences in cholesterol synthesis associated with changes limited to
hepatic synthesis of plasma lipoproteins in man [2]. Although sufficient information is not yet available, it appears that
tissues with faster turnover rates synthesize more cholesterol than those
which have relatively slow turnover. The epidermis has a rapid turnover and synthesizes more cholesterol than the dermis, which has a rather slow turnover [3]. The mature nervous tissue and red blood cells have
perhaps the least turnover, and they also show little, if any, synthesis of
cholesterol [4]. In man the lipoproteins synthesized by the intestines contain both
cholesterol esters and triglycerides as their content or "core," but under normal conditions the newly synthesized hepatic lipoproteins are assumed to contain triglycerides as the only or the predominant core lipid [5]. When perfused in vitro, livers from cholesterol-fed rats and containing large amounts of stored cholesterol esters synthesize lipoproteins which transport the stored cholesterol esters out of the liver [6]. However, such accumulation of cholesterol esters is not seen in man, and it is
therefore unlikely that hepatic cholesterol is nearly as important as hepatic triglycerides in driving the hepatic machinery for synthesis of new lipoproteins to transport lipids out of that organ.
Most lipoproteins in fasting plasma are synthesized in the liver. The fractional turnover rate of plasma very-low-density lipoproteins (VLDL) is at least 10 times greater than those of plasma low-density lipoproteins (LDL) or high-density lipoproteins (HDL) [7, 8]. Modest to moderate elevations of plasma VLDL are generally associated with corresponding 478 J H. S. Sodhi · Cholesterol Metabolism in Man
increases in their net turnover rates even if there is some diminution in
their fractional rates of clearance [9, 10]. Since elevations of plasma
triglycerides above normal are generally due to the elevations of VLDL, most hypertriglyceridemic patients would therefore have an overall increase in hepatic synthesis of plasma lipoproteins. If hepatic choles-
terogenesis is coupled to the need for synthesis of new lipoproteins, hypertriglyceridemic patients should, according to this concept, have
increased synthesis of endogenous cholesterol; and this indeed was what
we reported recently [H]. We have since examined 60 additional subjects and noted an excellent correlation (r = .83, P < .001) between relative rates of cholesterol synthesis and plasma concentrations of triglycerides [12]. In order to test further this association of hypertriglyceridemia with increased synthesis of cholesterol, we pooled our own and the published data [13-24] on approximately 100 cholesterol balance studies. Our analysis of the results indicated that cholesterol syn-
thesis in patients with hypertriglyceridemia was significantly greater than in those with normal plasma triglycerides (fig. 1).
Lipoproteins synthesized by the intestines contain cholesterol derived from the diet, bile, and local synthesis. However, the major content of the intestinal lipoproteins is the dietary triglycerides. After their entry
into circulation, most of the triglycerides in the intestinal lipoproteins
are removed by extrahepatic tissues, and the remnants containing most
of the cholesterol are then removed by the liver [25]. Our model for hepatic metabolism of cholesterol suggests that dietary and other cholesterol in the intestinal lipoproteins, when removed by the liver, enters the same pool as that of cholesterol synthesized in the liver [26]. Cholesterol Synthesis Absorption mg/day""*"'·"""'
Cholesterol Intake ? 300 mg/day Ej 300 mg/day
1200
800
400
. E NP ¦ Il
Il Z
Il
No. of Subjects (19) (12) TypeNormal
(19) (10)
(16) (13)
(6) (4)
Hy perchel. Hy percho). Hyperfrig. and
Hypertng.
Fig. 1.—Negative feedback inhibition
Perspectives in Biology and Medicine · Summer 1975 | 479
in this "anabolic" pool provides one of the packaging materials required
for the transport of triglycerides out of the liver into plasma. This suggests that exogenous cholesterol can replace the synthesized cholesterol for making new tissue (or plasma) lipoproteins.
After a high-cholesterol diet, the suppression of synthesis in various
tissues of guinea pig was proportional to the amounts of accumulated tissue cholesterol [27]. In rats the inhibition by exogenous cholesterol is
much greater in the liver than in any organ, which may well be related to
the fact that accumulation of cholesterol in the rat liver is greater than in any other organ. Since exogenous cholesterol is transported from lumen into the intestinal lymph rapidly, there is little accumulation of exogenous cholesterol in mucosal cells. The proportion of exogenous to endogenous cholesterol in the intestinal wall at any one time is less than that in the liver, and therefore the intestinal synthesis of cholesterol is
not suppressed to the same degree as that in the liver.
In accord with these data, it is suggested that the hepatic cholesterol in "anabolic" pool, whether derived from local synthesis or from diet, is first incorporated into plasma (or tissue) lipoproteins before it is
catabolised. The following evidence supports this suggestion. During the 3 hours after the addition of radioactive precursors to the perfusing medium, most of the cholesterol newly synthesized by the perfused liver either remained in the liver (presumably as a constituent of structural lipoproteins) or was released as lipoproteins into the circulating medium. Less than 0.5 percent was excreted into the bile during this period [28]. Preliminary studies in our own laboratories [H. S. Sodhi and
M. Avigan, unpublished observations] support the preceding data from Bricker. Rats were given radioactive mevalonate, and the total radioac-
tivity in hepatic and biliary cholesterol was determined. The loss of radioactive cholesterol from the rat livers between 1 and 2 hours was
about 8 percent of the total injected radioactivity. Excretion of the isotope in biliary cholesterol and bile acids amounted to only 0.4 percent.
This indicated that 95 percent of the newly synthesized cholesterol lost from the liver was transported out into the plasma. Similar evidence with regard to dietary cholesterol is also available. Bhattacharyya, Connor, and Spector [29] noted that in normal subjects pool sizes and production rates calculated from changes in plasma cholesterol specific-activity
slopes were similar whether the radioactive cholesterol was given by
intravenous injection or by oral feeding. This would not have been the case unless all the dietary cholesterol absorbed from the intestinal tract
was first incorporated into plasma lipoproteins or at least into pool A .
Zilversmit [30] proposed a method for calculating absorption of dietary
cholesterol based on the ratios of two isotopes in plasma after intravenous injection of one label and feeding of the other label in cholesterol.
Recent validation of this method [31] also implies that the absorbed 480 I H. S. Sodhi · Cholesterol Metabolism in Man
dietary cholesterol was first incorporated quantitatively into plasma
lipoproteins before it was catabolised. This is also supported by the
studies on /3-sitosteroI. The absorption of /3-sitosterol calculated by the sterol balance method was the same as that calculated by isotope kinetics
of the sterol in plasma [32]. In a fashion somewhat similar to dietary cholesterol, particulate cholesterol injected intravenously is removed primarily by the liver and is first quantitatively incorporated into plasma
lipoproteins before it is catabolised and excreted into the bile [31, 33].
Two predictions can be made from this aspect of the proposed concept.
1.If turnover of plasma lipoproteins remains constant, greater contributions from dietary cholesterol will decrease the need for de novo
synthesis of cholesterol (for making new plasma lipoproteins).
2.The metabolism of the absorbed cholesterol, after its removal by the liver, will be identical with the cholesterol derived from endogenous synthesis.
The first prediction is nothing other than the well-known negative feedback inhibition of endogenous cholesterol synthesis by exogenous
cholesterol. Its existence in many species of animals has been well substantiated [34], although there is still some question about its existence in
man. In our analysis of pooled data from cholesterol balance studies, we divided the subjects with various forms of hyperlipemia into low- and high-cholesterol-intake groups. The synthesis of cholesterol in the lowintake group was always significantly greater than the synthesis in the
high-intake group (fig. 1). Furthermore, the amounts of daily choles-
terol turnover (absorbed + synthesized )were similar in both groups,
suggesting that the depression in endogenous synthesis by dietary cholesterol was more or less precisely made up by the amounts of cholesterol derived from the diet. This supports our hypothesis [26] that the
exogenous cholesterol and synthesized cholesterol enter the same hepatic pool, and it was supported by following further observations in the same group of patients. The amounts of fecal neutral and acidic metabolites of endogenous cholesterol, as well as their ratios in the low- and high-intake groups, were remarkably similar (fig. 2). Although there is considerable information about the sites of synthesis
of plasma lipoproteins, little is known of the sites of their catabolism. Liver is the most important organ for the catabolism of cholesterol, but it may or may not be an equally important site for the catabolism of lipoproteins present in the postabsorptive plasma. However, cholesterol released from the lipoproteins, whatever the sites of their catabolism, has to be transported to the liver, since it is the only organ capable of excreting large amounts of cholesterol from the system. Cholesterol from catabolism of tissue membranes, including red blood cells, also has to be
transported to the liver for disposal. We recently presented evidence Perspectives in Biology and Medicine · Summer 1975 | 48 1
Cholestrol Intake ? < 300 mg/day
ËËJ>300 mg/day
3.0
•2
-
400 H
1
1
1400 X O
i5 1000 '5 a
£5 600 O
0)
1
Z
200 H
No. of Subjects (19) (12) TypeNormal
(19) (10)
(16) (13)
(6) (4)
Hyperchol.
Hyperchol. |
Hypertrig.
and
Hypertrig. Fig. 2.—Effect of dietary cholesterol on fecal excretion of endogenous steroids
[35] to suggest that the cholesterol from the tissues is promptly mobilized and transported to the liver for excretion in the bile when
plasma cholesterol levels are reduced. The cholesterol mobilized from
the tissues did not appear to remain in plasma long enough to increase its concentration in plasma. Recent work of Brown and Goldstein [36, 37] suggests that connective tissue may well be an important site for
degradation of plasma LDL, in which case cholesterol from degradation of LDL will need to be transported to the liver for catabolism. The impres482 I H- S. Sodhi · Cholesterol Metabolism in Man
sion given by the published literature is that the free cholesterol derived from absorption, local synthesis, or from any other source, for all practical purposes constitutes a single (common) pool undergoing rapid fluxes into and from various pathways related to the different roles of liver in cholesterol metabolism. Our model suggests that the cholesterol returning to the liver for catabolism does not completely mix with the cholesterol in the "anabolic" pool and thus is not available for synthesis of new
lipoproteins. This model therefore postulates the existence of two sepa-
rate pools which functionally (despite isotopic exchange) remain more or less independent. Most of the biliary cholesterol and biliary bile acids are derived from the catabolic pool, and little, if any, from the anabolic
pool. Although there is no direct evidence for or against the existence of
two independent functional pools of cholesterol in the liver, the idea is quite tenable. Accepting the assumption that cholesterol synthesis is coupled to the need for synthesis of new lipoproteins, and the evidence that hypertri-
glyceridemic patients have a greater hepatic synthesis of plasma lipoproteins, continued increases in synthesis and catabolism of cholesterol in such patients favour the two-pool rather than the single-pool model. In conditions of increased turnover of plasma lipoproteins, greater amounts of cholesterol would return to the liver from increased degra-
dation of lipoproteins. In a single-pool model there would be no reason
for the continued increase either in hepatic synthesis of cholesterol or in the biliary and fecal excretion, since the cholesterol from degradation of lipoproteins is available for synthesis of new lipoproteins. Studies conducted in our laboratories over the last few years have suggested that patients with hypertriglyceridemia not only continue to make larger amounts of cholesterol [11] but also continue to extrete abnormally large amounts of neutral and acidic metabolites of endogenous cholesterol [38]. This suggests that at least some of the cholesterol derived from catabolism of lipoproteins is not available for synthesis of new lipopro-
teins and is channeled into catabolic pathways.
The situation in liver is in a sense similar to what happens in intestinal mucosa. Both have special roles in the metabolism of cholesterol. Cholesterol is brought to the intestines for absorption and to the liver for catabolism. As early as 1965, it was suggested by Sodhi and associates that exogenous cholesterol did not completely mix with the cholesterol synthesized in the intestinal mucosa [39]. More recently evidence was presented by us [40] and by Rudel, Morris, and Felts [41] indicating that the exogenous cholesterol and endogenous cholesterol in the intestinal mucosa remain in separate pools. The rate of transport of exogenous cholesterol is two to three times greater than that of cholesterol synthesized locally [40]. The dietary cholesterol in intestinal mucosa may be considered as merely "in transit," as opposed to the structural cholesterol synthesized by the intestinal mucosa. The cholesterol brought to Perspectives in Biology and Medicine · Summer 1975 | 483
the liver for catabolism is comparable to "in transit" cholesterol of intes-
tinal mucosa, since both are related to the specialized roles of respective organs. Cholesterol metabolism in adrenal glands also suggests that
cholesterol for synthesis of steroid hormones (a specialized role of adre-
nal glands) may also be present in a pool different from that of choles-
terol synthesized by the gland itself [42].
Tissue cholesterol esters have generally been regarded as storage
forms of cholesterol. Most tissues have enzymes for esterification of free cholesterol and hydrolysis of cholesterol esters. The pools of free and esterified cholesterol are assumed to be in equilibrium, implying that the free cholesterol derived from hydrolysis of cholesterol esters mixes in-
distinguishably with the pool of free cholesterol from which the choles-
terol esters were orginally derived. In accord with the model for hepatic metabolism of lipoprotein cholesterol, another possibility is suggested. Free cholesterol is incorporated into plasma lipoproteins as an essential structural constituent, and the esterification of plasma free cholesterol is a reflection of the catabolism or turnover of plasma or tissue lipopro-
teins. Therefore, in this light the esterified cholesterol may be consid-
ered to lie on the catabolic pathway of free cholesterol which is a structural component of tissue or plasma lipoproteins. Studies in our
laboratories have demonstrated that turnover of plasma cholesterol es-
ters had an excellent correlation with endogenous synthesis of cholesterol, on the one hand, and with plasma VLDL turnover, on the other [43]. Cholesterol esters in the adrenal gland, however, constitute a special case, since the cholesterol esters are stored for synthesis of steroid hormones, the need for which may arise rather acutely.
In summary, it is suggested that changes in the absorption of dietary cholesterol or alterations in tissue synthesis and catabolism of endoge-
nous cholesterol are not specifically directed to maintain levels of plasma cholesterol. Free cholesterol is viewed only as a structural component of
plasma or tissue lipoproteins, and its metabolism is coupled to the turn-
over of these lipoproteins. The esterification of free cholesterol is a reflection of this turnover process. The cholesterol released by degrada-
tion of lipoproteins is not reused for synthesis of new lipoproteins, but is
catabolised.
REFERENCES
1.J. M. DiETSCHY and J. D. Wilson. J. Clin. Invest., 47:166, 1968.
2.H. S. Sodhi and B.J. Kudchodkar. Circulation, 44:11-57, 1971. 3.J. L. Gaylor.J. Biol. Chem., 238:1643, 1963. 4.J. M. Dietschv and J. D. Wilson. N. Eng. J. Med., 282:1 128, 1179, 1241, 1970.
5.J. A. Glomset. Am. J. Clin. Nutr., 23:1129, 1970.
484 I H. S. Sodhi · Cholesterol Metabolism in Man
6.P. S. RoHEiM, D. E. Haft, L. I. Gidez, A. White, and H. A. Eder. J. Clin. Invest., 42:1277, 1963.
7.D. Gitlin, D. G. Cornwall, D. Nakasato, J. L. Oncley, W. L. Hughes, and C. A. Janeway. J. Clin. Invest., 37:172, 1958. 8.T. Langer, W. Strober, and R. I. Levy. J. Clin. Invest., 51:1528, 1972. 9.G. M. Reaven, D. B. Hill, R. C. Gross, and J. W. Farquhar. J. Clin. Invest.,
44:1826, 1965. 10.A. H. Kissebah, P. W. Adams, and V. Wynn. Clin. Sei. Mol. Med., 47:259, 1974.
11.H. S. Sodhi and B. J. Kudchodkar. Metabolism, 22:895, 1973. 12.H. S. Sodhi, J. Merriman, M. Mishkel, B. J. Kudchodkar, and G. S. Sundaram. Clin. Res., 22:753A, 1974.
13.N. Spritz, E. H. Ahrens, and S. M. Grundy. J. Clin. Invest., 44:1482, 1965. 14.S. M. Grundy and E. H. Ahrens, Jr. J. Clin. Invest., 45:1503, 1966. 15. ---------.J. Lipid Res., 10:91, 1969. 16.S. M. Grundy, E. H. Ahrens, Jr., and J. Davignon. J. Lipid Res., 10:304, 1969.
17.S. M. Grundy and E. H. Ahrens, Jr. J. Clin. Invest. 49:1135, 1970. 18.S. M. Grundy, E. H. Ahrens, Jr., and G. Salen. J. Lab. Clin. Med., 78:94, 1971.
19.S. M. Grundy, E. H. Ahrens, Jr., G. Salen, P. H. Schreibman, and P.J. Nestel. J. Lipid Res., 13:531, 1972. 20.G. Salen, E. H. Ahrens, Jr., and S. M. Grundy. J. Clin. Invest., 49:952, 1970.
21.E. Quintao, S. M. Grundy, and E. H. Ahrens. J. Lipid Res., 12:233, 1971. 22.W. E. Connor, D. T. Witiak, D. B. Stone, and M. L. Armstrong. J. Clin. Invest., 48:1363, 1969. 23.T. A. MiETTiNEN. Circulation, 44:842, 1971.
24.P.J. Nestel, N. Havenstein, H. M. Whyte, T.J. Scott, and L.J. Cook. N. Engl. J. Med., 288:379, 1973. 25.T. G. Redgrave. J. Clin. Invest., 49:465, 1970. 26.H. S. Sodhi and B.J. Kudchodkar. Lancet, 1:513, 1973. 27.A. Swann and M. D. Siperstein. J. Clin. Invest., 51:95a, 1972. 28.L. A. Bricker. J. Clin. Invest., 50:12a, 1971.
29.A. Bhattacharyya, W. E. Connor, and A. A. Spector. Circulation, 44:11-2, 1971.
30.D. B. Zilversmit. Proc. Soc. Exp. Biol. Med., 140:862, 1972. 31.D. B. Zilversmit and L. B. Hughes. J. Lipid Res., 15:465, 1974. 32.G. Salen, E. H. Ahrens, and S. M. Grundy. J. Clin. Invest., 49:952, 1972. 33.H. S. Sodhi and B.J. Kudchodkar. J. Lab. Clin. Med., 82:111, 1973. 34.M. D. Siperstein. Cell Res., 2:65, 1970.
35.H. S. Sodhi, B. J. Kudchodkar, and L. Horlick. Atherosclerosis, 17:1, 1973.
36.M. S. Brown and J. L. Goldstein. Science, 185:6, 1974. 37. ---------. Proc. Natl. Acad. Sei. USA, 71:788, 1974.
38.H. S. Sodhi and B.J. Kudchodkar. Clin. Chim. Acta, 46:161, 1973.
39.H. S. Sodhi, V. Berger, E. A. Swyryd, and R. G. Gould. Circulation, 38:11-31, 1965.
40.H. S. Sodhi, R. Orchard, N. D. Agnish, P. Verughese, and B. J. Kudchodkar. Atherosclerosis, 17:197, 1973.
41.L. L. Rudel, M. D. Morris, and J. M. Felts. J. Clin. Invest., 51:2682, 1972. 42.A. Borkowski, C. Delcroix, and S. Levin. J. Clin. Invest., 51:1664, 1972. 43.H. S. Sodhi and B.J. Kudchodkar. Proc. 3d Int. Symp. Atherosclerosis, West Berlin, October 1973, p. 516. Berlin: Springer-Verlag, 1974.
Perspectives in Biology and Medicine · Summer 1975 | 485
