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The Role of the Intestine in the Control of Cholesterol Metabolism
Vol. 57, No.4 Printed in U.S.A.
GASTROENTEROLOGY
Copyright © 1969 by The Williams & Wilkins Co.
EDITORIALS THE ROLE OF THE INTESTINE IN THE CONTROL OF CHOLESTEROL METABOLISM The gastrointestinal tract plays a key role in cholesterol metabolism in the intact animal. Not only is the small bowel one of the two major sites for endogenous synthesis of circulating sterol, but, in addition, because of its central role in the absorption of cholesterol and bile acid the intestine also exerts, indirectly, significant influence on the rate of synthesis and degradation of cholesterol by the liver. Normal physiology. The relationship between cholesterol metabolism in the normal intestine and liver is shown in figure 1. Cholesterol reaches the intestinal lumen from two sources: exogenous, or dietary, cholesterol (A) and endogenous cholesterol (D). While not illustrated in the figure, endogenous cholesterol (D) is derived ultimately from biliary cholesterol, from cholesterol synthesized de novo in the intestinal mucosa and sloughed into the intestinal lumen, and from serum cholesterol.1' 2 Cholesterol esters (B), whether of exogenous or endogenous origin, are hydrolyzed to free cholesterol (C) by pancreatic cholesterol esterase present in the intestinal contents. 3 The free cholesterol is then solubilized in mixed micelles (E) of bile acids and other lipid components, from which it is absorbed across the mucosal membrane of the intestinal epithelial cell by passive diffusion. 3 • 4 Cholesterol also is synthesized from acetate (G) in the intestinal epithelium 5 so that the intramural pool of cholesterol (F) is derived from two sources: exogenous and endogenous cholesterol absorbed from the intestinal lumen ( C- E) and cholesterol Received April 3, 1969. Address requests for reprints to: Dr. John M. Dietschy, Department of Internal Medicine, University of Texas Southwestern Medical School, 5323 Harry Hines Boulevard, Dallas, Texas 75235. 461
synthesized de novo in the epithelium (G). The intramural pool of cholesterol is incorporated into lipoproteins, particularly chylomicrons, and then gains access to the general circulation exclusively via the enterolymphatic circulation. A major portion of the chylomicron cholesterol is first cleared by the liver and then returned to the circulating pool of cholesterol ([) incorporated in other classes of lipoproteins. Cholesterol also is synthesized de novo from acetate (J) by the liver and contributed to the circulating sterol pool ([). Thus, serum cholesterol is now thought to be derived predominantly, if not exclusively, from three sources: exogenous cholesterol (A) absorbed from the diet and cholesterol synthesized endogenously from acetate by the intestine (G) and liver (J). Bile acid is secreted by the liver through the common bile duct into the intestinal lumen (M). After absorption by passive mechanisms across the jejunum and by combined passive and active mechanisms across the ileum, 6 bile acids are returned to the liver exclusively via the portal circulation. Each day the liver synthesizes an amount of new bile acid (L) from cholesterol (K) equal to the amount of acidic sterols lost in the feces (N). Control of sterol synthesis. Control of the rate-limiting steps in this over-all scheme of cholesterol absorption and synthesis is exerted both by bile acid and by cholesterol through several different mechanisms. Bile acid is required for activation of cholesterol esterase (point 1, figure 1) and so is essential for effective hydrolysis of cholesterol esters. 3 Micellar solubilization of cholesterol (point 2) also is critically dependent upon the presence of adequate concentrations of bile acid in
462
EDITORIAL
Vol. 57, No. 4
@Exogenous Dietary Cholesterol
POINTS OF CONTROL MEDIATED BY . . Bile Acid •
Cholesterol
FIG. 1. Diagram showing the relationship between cholesterol metabolism in the normal intestine and the liver. For explanation see text.
the intestinal lumen. Finally, recent evidence indicates that bile acid is required for delivery of intramural cholesterol into the intestinal lymphatic circulation (point 3) incorporated in lipoproteins.' Bile acid is required, therefore, for the absorption and delivery of luminal (C) and intramural (F) cholesterol into the circulation. In addition to these effects on esterase activity and on cholesterol transport, bile acid also appears to have a direct inhibitory effect on several important biosynthetic pathways. The rate of cholesterogenesis in the intestinal wall (point 4), for example, is profoundly inhibited by the presence of bile acid in the intestinal lumen (and presumably, therefore, within the intestinal epithelial cell). 8 Furthermore, on the basis of older experimental data it has been suggested that bile acid also exerts negative feedback control in
the liver, both on the synthesis of cholesterol from acetate and of bile acid itself from cholesterol. 9 " 10 However, more recent data have seriously challenged the validity of the concept that bile acid inhibits these two latter metabolic pathways. It is now clear, for example, that bile acid does not directly influence the rate of hepatic cholesterogenesis from acetate; re-establishment of the enterohepatic circulation of bile acid in animals with biliary diversion does not prevent the rise in sterol synthetic activity that is seen following interruption of the delivery of biliary secretions into the intestine.11 Similarly, it has been found that derepression of bile acid synthesis in animals with biliary diversion cannot be prevented by infusion of exogenous bile acid. 12 Thus, even though one would reasonably expect bile acid in the entero-
October 1969
EDITORIAL
hepatic circulation to feed back and inhibit bile acid synthesis, as indicated by point 5 in figure 1, nevertheless, this control -mechanism should be viewed with some reservation until more definitive data become available. Cholesterol itself has long been known to play an important role in regulation of endogenous sterol synthesis. Feeding a diet high in cholesterol markedly suppresses hepatic cholesterogenesis (point 6); this effect is mediated by inhibition of the rate-limiting enzyme in the cholesterol biosynthetic sequence, i.e. , ,8-hydroxy-,8-methyl glutaryl reductase. l:J It has been recognized recently, however, that such inhibition is produced by both endogenous and exogenous cholesterol. In the animal receiving a sterol-free diet, endogenous cholesterol (D) is constantly circulated through the enterolymphatic circulation. The amount of endogenous sterol reaching the liver by this means is sufficient to keep hepatic cholesterogenesis partially suppressed. Interruption of the enterolymphatic outflow, either directly or by withdrawing bile acid from the intestinal lumen and wall, interrupts the enterohepatic circulation of endogenous cholesterol and leads to a 2- to 3-fold increase in the rate of cholesterol synthesis by the liver.l 4 In contrast to this effect on hepatic cholesterogenesis, it should be emphasized that cholesterol feeding has no significant effect on cholesterol synthesis in the intestine. 5 ' 8 It is apparent from these considerations that control of sterol metabolism is complex. Any disruption of these physiological relationships may lead to changes in cholesterol absorption, synthesis, or degradation through alterations at any one or more points of control. This complex interrelationship can best be illustrated by considering the changes in sterol metabolism that occur under several different physiological situations. 1. In the intact animal receiving no cholesterol in the diet, plasma cholesterol is derived solely from endogenous synthesis in the gastrointestinal tract and liver. Since the enterohepatic circulation is in-
463
tact and normal amounts of bile acid are present in the intestinal lumen, the rate of intestinal cholesterogenesis is partially suppressed and is operating at only about 10 to 20% of maximal capacity. In addition, since small amounts of endogenous cholesterol are being delivered to the liver via the enterolymphatic circulation, the rate of hepatic cholesterogenesis also is partially suppressed to about 30 to 40% of maximal capacity. Loss of neutral and acidic sterols in the feces is minimal and just balanced by the rate of endogenous synthesis. 2. In the intact animal fed a diet high in cholesterol, hepatic cholesterogenesis is nearly completely suppressed. Cholesterol synthesis in the gastrointestinal tract, in contrast, is essentially unaffected so that the small intestine becomes the major, if not the exclusive, site of endogenous sterol synthesis in the cholesterolfed animal. 3. In the animal with biliary diversion, ileal resection, or bypass, or in the animal fed cholestyramine, there is partial or complete absence of bile acid in the intestinal lumen due to interruption of the enterohepatic circulation. As a result of the diminished bile acid content in the intestinal lumen and wall, there is malabsorption of luminal cholesterol (E) and poor delivery of intramural cholesterol (F) into the intestinal lymph. The rates of endogenous sterol synthesis in the intestine and liver increase markedly, as does the synthesis of bile acid from cholesterol. 4. In the animal with biliary obstruction, bile acid also is absent from the intestinal lumen and there is interruption of the enterolymphatic circulation of cholesterol. Cholesterol synthesis in the intestine and in the liver is markedly increased, but, in contrast to the other conditions listed above that are associated with interruption of the enterohepatic circulation, in the case of biliary obstruction, there are data to suggest that conversion of cholesterol to bile acids is suppressed rather than enhanced.l 5 5. Following intestinal lymphatic diversion, feedback inhibition in the liver by endogenous or exogenous cholesterol is
464
EDITORIAL
lost, so that hepatic cholesterol synthesis is markedly enhanced. However, since the enterohepatic circulation of bile acid is intact in such animals, the rate of intestinal sterol synthesis remains at the low, partially suppressed level seen in control animals. 6. When liver cells undergo malignant degeneration, they lose their capacity to respond to feedback control from endogenous or exogenous cholesterol. 16 Thus, even though the enterohepatic circulation of bile acid and the enterolymphatic circulation of cholesterol is intact in animals with liver cell carcinoma, cholesterol synthesis in the hepatoma often occurs at a very high rate. The net effect of these various changes on serum-cholesterol levels is variable and to some extent depends upon the particular species of animal under study. However, the alterations in the rates of absorption, synthesis, and degradation of sterol brought about by these experimental manipulations serve to emphasize the overwhelming importance of the gastrointestinal tract in the control of over-all cholesterol metabolism.
4.
5.
6.
7.
8.
9.
10.
11.
12.
JOHN M. DIETSCHY, M.D.
Department of Internal Medicine The University of Texas Southwestern Medical School at Dallas 5323 Harry Hines Boulevard Dallas, Texas 75235 REFERENCES 1. Lindsey, C. A., Jr., and J. D. Wilson. 1965. Evidence for a contribution by the intestinal wall to the serum cholesterol of the rat. J. Lipid Res. 6: 173-181. :l. Wilson, J . D. 1968. Biosynthetic origin of serum cholesterol in the squirrel monkey: evidence for a contribution by the intestinal wall. J. Clin. Invest. 47: 175-187. 3. Treadwell, C. R., and G. V. Vahouny. 1968. Cholesterol absorption. In C. F. Code [ed.],
13.
14.
15.
16.
Vol. 57, No. 4 Handbook of physiology, Sect. 6: Alimentary canal, Vol. III, Intestinal absorption, p. 14071438. American Physiological Society, Washington, D. C. Simmonds, W. J., A. F. Hofmann, and E. Theodor. 1967. Absorption of cholesterol from a micellar solution: intestinal perfusion studies in man. J. Clin. Invest. 46: 874-890. Dietschy, J. M., and M. D. Siperstein. 1965. Cholesterol synthesis by the gastrointestinal tract: localization and mechanisms of control. J. Clin. Invest. 44: 1311-1327. Dietschy, J. M. 1968. Mechanisms for the intestinal absorption of bile acids. J. Lipid Res. 9: 297-309. Wilson, J. D., and R. T. Reinke. 1968. Transfer of locally synthesized cholesterol from intestinal wall to intestinal lymph. J. Lipid Res. 9: 85-92. Dietschy, J . M. 1968. The role of bile salts in controlling the rate of intestinal cholesterogenesis. J. Clin. Invest. 47: 286-300. Myant, N. B., and H. A. Eder. 1961. The effect of biliary drainage upon the synthesis of cholesterol in the liver. J. Lipid Res. 2: 363-368. Bergstrom, S., and H. Danielsson. 1958. On the regulation of bile acid formation in the rat liver. Acta Physiol. Scand. 43: 1-7. Weis, H. J., and J. M. Dietschy. 1968. Failure of bile acids to regulate hepatic cholesterogenesis (Abstr.). Clin. Res. 16: 355. Wilson, J. D., W. H. Bentley, and G. T. Crowley. The regulation of bile acid formation in intact animals. In L. Schiff, J. B. Carey, Jr., and J. M. Dietschy [eds.], Bile salt metabolism. Charles C Thomas, Spr~gfield, lll. In press. Siperstein, M. D., and V. M. Fagan . 1966. Feedback control of mevalonate synthesis by dietary cholesterol. J. Bioi. Chem. 241: 602-609. Weis, H. J., and J. M. Dietschy. 1969. Failure of bile acids to control hepatic cholesterogenesis: evidence for endogenous cholesterol feedback. J. Clin. Invest. In press. Boyd, G. S., M.A. Eastwood, and N. MacLean. 1965. Bile acids in the rat: studies in experimental occlusion of the bile duct. J. Lipid Res. 7:83-94. Siperstein, M. D., and V. M. Fagan. 1964. Deletion of the cholesterol-negative feedback system in liver tumors. Cancer Res. 24: 11081115.
GASTROENTEROLOGY
Copyright © 1969 by The Williams & Wilkins Co.
EDITORIALS THE ROLE OF THE INTESTINE IN THE CONTROL OF CHOLESTEROL METABOLISM The gastrointestinal tract plays a key role in cholesterol metabolism in the intact animal. Not only is the small bowel one of the two major sites for endogenous synthesis of circulating sterol, but, in addition, because of its central role in the absorption of cholesterol and bile acid the intestine also exerts, indirectly, significant influence on the rate of synthesis and degradation of cholesterol by the liver. Normal physiology. The relationship between cholesterol metabolism in the normal intestine and liver is shown in figure 1. Cholesterol reaches the intestinal lumen from two sources: exogenous, or dietary, cholesterol (A) and endogenous cholesterol (D). While not illustrated in the figure, endogenous cholesterol (D) is derived ultimately from biliary cholesterol, from cholesterol synthesized de novo in the intestinal mucosa and sloughed into the intestinal lumen, and from serum cholesterol.1' 2 Cholesterol esters (B), whether of exogenous or endogenous origin, are hydrolyzed to free cholesterol (C) by pancreatic cholesterol esterase present in the intestinal contents. 3 The free cholesterol is then solubilized in mixed micelles (E) of bile acids and other lipid components, from which it is absorbed across the mucosal membrane of the intestinal epithelial cell by passive diffusion. 3 • 4 Cholesterol also is synthesized from acetate (G) in the intestinal epithelium 5 so that the intramural pool of cholesterol (F) is derived from two sources: exogenous and endogenous cholesterol absorbed from the intestinal lumen ( C- E) and cholesterol Received April 3, 1969. Address requests for reprints to: Dr. John M. Dietschy, Department of Internal Medicine, University of Texas Southwestern Medical School, 5323 Harry Hines Boulevard, Dallas, Texas 75235. 461
synthesized de novo in the epithelium (G). The intramural pool of cholesterol is incorporated into lipoproteins, particularly chylomicrons, and then gains access to the general circulation exclusively via the enterolymphatic circulation. A major portion of the chylomicron cholesterol is first cleared by the liver and then returned to the circulating pool of cholesterol ([) incorporated in other classes of lipoproteins. Cholesterol also is synthesized de novo from acetate (J) by the liver and contributed to the circulating sterol pool ([). Thus, serum cholesterol is now thought to be derived predominantly, if not exclusively, from three sources: exogenous cholesterol (A) absorbed from the diet and cholesterol synthesized endogenously from acetate by the intestine (G) and liver (J). Bile acid is secreted by the liver through the common bile duct into the intestinal lumen (M). After absorption by passive mechanisms across the jejunum and by combined passive and active mechanisms across the ileum, 6 bile acids are returned to the liver exclusively via the portal circulation. Each day the liver synthesizes an amount of new bile acid (L) from cholesterol (K) equal to the amount of acidic sterols lost in the feces (N). Control of sterol synthesis. Control of the rate-limiting steps in this over-all scheme of cholesterol absorption and synthesis is exerted both by bile acid and by cholesterol through several different mechanisms. Bile acid is required for activation of cholesterol esterase (point 1, figure 1) and so is essential for effective hydrolysis of cholesterol esters. 3 Micellar solubilization of cholesterol (point 2) also is critically dependent upon the presence of adequate concentrations of bile acid in
462
EDITORIAL
Vol. 57, No. 4
@Exogenous Dietary Cholesterol
POINTS OF CONTROL MEDIATED BY . . Bile Acid •
Cholesterol
FIG. 1. Diagram showing the relationship between cholesterol metabolism in the normal intestine and the liver. For explanation see text.
the intestinal lumen. Finally, recent evidence indicates that bile acid is required for delivery of intramural cholesterol into the intestinal lymphatic circulation (point 3) incorporated in lipoproteins.' Bile acid is required, therefore, for the absorption and delivery of luminal (C) and intramural (F) cholesterol into the circulation. In addition to these effects on esterase activity and on cholesterol transport, bile acid also appears to have a direct inhibitory effect on several important biosynthetic pathways. The rate of cholesterogenesis in the intestinal wall (point 4), for example, is profoundly inhibited by the presence of bile acid in the intestinal lumen (and presumably, therefore, within the intestinal epithelial cell). 8 Furthermore, on the basis of older experimental data it has been suggested that bile acid also exerts negative feedback control in
the liver, both on the synthesis of cholesterol from acetate and of bile acid itself from cholesterol. 9 " 10 However, more recent data have seriously challenged the validity of the concept that bile acid inhibits these two latter metabolic pathways. It is now clear, for example, that bile acid does not directly influence the rate of hepatic cholesterogenesis from acetate; re-establishment of the enterohepatic circulation of bile acid in animals with biliary diversion does not prevent the rise in sterol synthetic activity that is seen following interruption of the delivery of biliary secretions into the intestine.11 Similarly, it has been found that derepression of bile acid synthesis in animals with biliary diversion cannot be prevented by infusion of exogenous bile acid. 12 Thus, even though one would reasonably expect bile acid in the entero-
October 1969
EDITORIAL
hepatic circulation to feed back and inhibit bile acid synthesis, as indicated by point 5 in figure 1, nevertheless, this control -mechanism should be viewed with some reservation until more definitive data become available. Cholesterol itself has long been known to play an important role in regulation of endogenous sterol synthesis. Feeding a diet high in cholesterol markedly suppresses hepatic cholesterogenesis (point 6); this effect is mediated by inhibition of the rate-limiting enzyme in the cholesterol biosynthetic sequence, i.e. , ,8-hydroxy-,8-methyl glutaryl reductase. l:J It has been recognized recently, however, that such inhibition is produced by both endogenous and exogenous cholesterol. In the animal receiving a sterol-free diet, endogenous cholesterol (D) is constantly circulated through the enterolymphatic circulation. The amount of endogenous sterol reaching the liver by this means is sufficient to keep hepatic cholesterogenesis partially suppressed. Interruption of the enterolymphatic outflow, either directly or by withdrawing bile acid from the intestinal lumen and wall, interrupts the enterohepatic circulation of endogenous cholesterol and leads to a 2- to 3-fold increase in the rate of cholesterol synthesis by the liver.l 4 In contrast to this effect on hepatic cholesterogenesis, it should be emphasized that cholesterol feeding has no significant effect on cholesterol synthesis in the intestine. 5 ' 8 It is apparent from these considerations that control of sterol metabolism is complex. Any disruption of these physiological relationships may lead to changes in cholesterol absorption, synthesis, or degradation through alterations at any one or more points of control. This complex interrelationship can best be illustrated by considering the changes in sterol metabolism that occur under several different physiological situations. 1. In the intact animal receiving no cholesterol in the diet, plasma cholesterol is derived solely from endogenous synthesis in the gastrointestinal tract and liver. Since the enterohepatic circulation is in-
463
tact and normal amounts of bile acid are present in the intestinal lumen, the rate of intestinal cholesterogenesis is partially suppressed and is operating at only about 10 to 20% of maximal capacity. In addition, since small amounts of endogenous cholesterol are being delivered to the liver via the enterolymphatic circulation, the rate of hepatic cholesterogenesis also is partially suppressed to about 30 to 40% of maximal capacity. Loss of neutral and acidic sterols in the feces is minimal and just balanced by the rate of endogenous synthesis. 2. In the intact animal fed a diet high in cholesterol, hepatic cholesterogenesis is nearly completely suppressed. Cholesterol synthesis in the gastrointestinal tract, in contrast, is essentially unaffected so that the small intestine becomes the major, if not the exclusive, site of endogenous sterol synthesis in the cholesterolfed animal. 3. In the animal with biliary diversion, ileal resection, or bypass, or in the animal fed cholestyramine, there is partial or complete absence of bile acid in the intestinal lumen due to interruption of the enterohepatic circulation. As a result of the diminished bile acid content in the intestinal lumen and wall, there is malabsorption of luminal cholesterol (E) and poor delivery of intramural cholesterol (F) into the intestinal lymph. The rates of endogenous sterol synthesis in the intestine and liver increase markedly, as does the synthesis of bile acid from cholesterol. 4. In the animal with biliary obstruction, bile acid also is absent from the intestinal lumen and there is interruption of the enterolymphatic circulation of cholesterol. Cholesterol synthesis in the intestine and in the liver is markedly increased, but, in contrast to the other conditions listed above that are associated with interruption of the enterohepatic circulation, in the case of biliary obstruction, there are data to suggest that conversion of cholesterol to bile acids is suppressed rather than enhanced.l 5 5. Following intestinal lymphatic diversion, feedback inhibition in the liver by endogenous or exogenous cholesterol is
464
EDITORIAL
lost, so that hepatic cholesterol synthesis is markedly enhanced. However, since the enterohepatic circulation of bile acid is intact in such animals, the rate of intestinal sterol synthesis remains at the low, partially suppressed level seen in control animals. 6. When liver cells undergo malignant degeneration, they lose their capacity to respond to feedback control from endogenous or exogenous cholesterol. 16 Thus, even though the enterohepatic circulation of bile acid and the enterolymphatic circulation of cholesterol is intact in animals with liver cell carcinoma, cholesterol synthesis in the hepatoma often occurs at a very high rate. The net effect of these various changes on serum-cholesterol levels is variable and to some extent depends upon the particular species of animal under study. However, the alterations in the rates of absorption, synthesis, and degradation of sterol brought about by these experimental manipulations serve to emphasize the overwhelming importance of the gastrointestinal tract in the control of over-all cholesterol metabolism.
4.
5.
6.
7.
8.
9.
10.
11.
12.
JOHN M. DIETSCHY, M.D.
Department of Internal Medicine The University of Texas Southwestern Medical School at Dallas 5323 Harry Hines Boulevard Dallas, Texas 75235 REFERENCES 1. Lindsey, C. A., Jr., and J. D. Wilson. 1965. Evidence for a contribution by the intestinal wall to the serum cholesterol of the rat. J. Lipid Res. 6: 173-181. :l. Wilson, J . D. 1968. Biosynthetic origin of serum cholesterol in the squirrel monkey: evidence for a contribution by the intestinal wall. J. Clin. Invest. 47: 175-187. 3. Treadwell, C. R., and G. V. Vahouny. 1968. Cholesterol absorption. In C. F. Code [ed.],
13.
14.
15.
16.
Vol. 57, No. 4 Handbook of physiology, Sect. 6: Alimentary canal, Vol. III, Intestinal absorption, p. 14071438. American Physiological Society, Washington, D. C. Simmonds, W. J., A. F. Hofmann, and E. Theodor. 1967. Absorption of cholesterol from a micellar solution: intestinal perfusion studies in man. J. Clin. Invest. 46: 874-890. Dietschy, J. M., and M. D. Siperstein. 1965. Cholesterol synthesis by the gastrointestinal tract: localization and mechanisms of control. J. Clin. Invest. 44: 1311-1327. Dietschy, J. M. 1968. Mechanisms for the intestinal absorption of bile acids. J. Lipid Res. 9: 297-309. Wilson, J. D., and R. T. Reinke. 1968. Transfer of locally synthesized cholesterol from intestinal wall to intestinal lymph. J. Lipid Res. 9: 85-92. Dietschy, J . M. 1968. The role of bile salts in controlling the rate of intestinal cholesterogenesis. J. Clin. Invest. 47: 286-300. Myant, N. B., and H. A. Eder. 1961. The effect of biliary drainage upon the synthesis of cholesterol in the liver. J. Lipid Res. 2: 363-368. Bergstrom, S., and H. Danielsson. 1958. On the regulation of bile acid formation in the rat liver. Acta Physiol. Scand. 43: 1-7. Weis, H. J., and J. M. Dietschy. 1968. Failure of bile acids to regulate hepatic cholesterogenesis (Abstr.). Clin. Res. 16: 355. Wilson, J. D., W. H. Bentley, and G. T. Crowley. The regulation of bile acid formation in intact animals. In L. Schiff, J. B. Carey, Jr., and J. M. Dietschy [eds.], Bile salt metabolism. Charles C Thomas, Spr~gfield, lll. In press. Siperstein, M. D., and V. M. Fagan . 1966. Feedback control of mevalonate synthesis by dietary cholesterol. J. Bioi. Chem. 241: 602-609. Weis, H. J., and J. M. Dietschy. 1969. Failure of bile acids to control hepatic cholesterogenesis: evidence for endogenous cholesterol feedback. J. Clin. Invest. In press. Boyd, G. S., M.A. Eastwood, and N. MacLean. 1965. Bile acids in the rat: studies in experimental occlusion of the bile duct. J. Lipid Res. 7:83-94. Siperstein, M. D., and V. M. Fagan. 1964. Deletion of the cholesterol-negative feedback system in liver tumors. Cancer Res. 24: 11081115.