© 1996 American Heart Association, Inc.
Articles |
Importance of a Novel Oxidative Mechanism for Elimination of Intracellular Cholesterol in Humans
From the Department of Medical Laboratory Sciences and Technology (E.L., A.B., G.A., U.D., I.B.), Department of Lung Medicine (O.A.), and Department of Internal Medicine (K.E.), Karolinska Institute, Huddinge Hospital (Sweden), and the Department of Medical Biochemistry and Biophysics (J.Z., J.S.), Karolinska Institute, Stockholm, Sweden.
Correspondence to Dr Ingemar Björkhem, Department of Medical Laboratory Sciences and Technology, Division of Clinical Chemistry, Huddinge University Hospital, S-141 86 Huddinge, Sweden.
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Abstract |
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Abstract We have recently demonstrated that cultured human alveolar macrophages efficiently convert cholesterol into excretable 27-oxygenated products. We show here that increasing the intracellular concentration of cholesterol by a factor of 10 leads to about a twofold increase in the excretion of 27-oxygenated products from cultured macrophages. Inhibition of the sterol 27-hydroxylase caused a significant intracellular accumulation of cholesterol. A direct comparison was made between flux of cholesterol and 27-oxygenated products from macrophages preloaded with [4-14C]cholesterol. Under the specific conditions employed with fetal calf serum in the culture medium, the flux of 27-oxygenated products was about 10% of that of cholesterol. Since the sterol 27-hydroxylase, which converts cholesterol to 27-oxygenated products, is present in many cell types, we suggest that 27-oxygenation is a general mechanism for removal of intracellular cholesterol. To evaluate this hypothesis, we measured the net uptake by the human liver of circulating 27-oxygenated products, which was found to be about 20 mg/24 h. This uptake corresponds to
4% of the bile acid
production, assuming quantitative conversion into bile acids.
It is concluded that the 27-hydroxylase pathway is of significance for
elimination of extrahepatic cholesterol.
Key Words: sterol 27-hydroxylase • atherosclerosis • macrophages • bile acid biosynthesis • cholesterol degradation
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Introduction |
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TopAbstractIntroductionMethods Results and Discussion References |
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Cholesterol homeostasis is achieved through a highly sophisticated regulation of the uptake, synthesis, and esterification of cholesterol in the body. Excess intracellular cholesterol may be stored as cholesteryl esters or may be transported out from the cell. The liver has the unique capacity to degrade cholesterol into bile acids that can ultimately be removed from the body. The complicated flux of lipoprotein-bound cholesterol to and from the liver, as well as the secretion of cholesterol and bile acids from the liver into the bile, are dependent upon the overall intake and synthesis of cholesterol.1
At the cellular level, the most important mechanism for removal of excess cholesterol is believed to be reverse cholesterol transport involving HDL. Part of the HDL cholesterol reaching the liver may be degraded into bile acids and thereby finally removed from the body.2
In some cells, however, an alternative mechanism seems to be of
importance for the elimination of cholesterol. Recently, we
demonstrated that human cultured macrophages have a very high
capacity to convert cholesterol into the more polar
metabolites 27-hydroxycholesterol and
3ß-hydroxy-5-cholestenoic acid and excrete them into the
medium.3 Sterol 27-hydroxylase is likely to be responsible
for the formation of the two products, as shown by use of
immunoblotting, a specific inhibitor, and
an oxygen-18 technique. Since the two products of the reaction are
present in plasma4 5 and since they are efficiently
converted into bile acids in the liver,6 we suggested that
conversion of cholesterol into 27-oxygenated
products may constitute a defense mechanism for macrophages
and possibly also other peripheral cells exposed to
cholesterol. We7 8 and others9
have shown that sterol 27-hydroxylase is present in several tissues
and cell types, including fibroblasts, endothelial
cells, brain, kidney, and lung. The hypothesis is further supported by
the demonstration of 7
-hydroxylation of oxysterols in human
diploid fibroblasts.10
The aim of the present work was to evaluate the quantitative importance of this novel mechanism for cholesterol removal.
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Methods |
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TopAbstractIntroductionMethods Results and Discussion References |
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Isolation and Culture of Alveolar Macrophages
Human alveolar macrophages were isolated from bronchoalveolar lavage fluid as described.3 11 The macrophages were harvested from patients with pulmonary malignancies undergoing bronchoscopy for diagnostic purposes. The bronchoalveolar lavage was performed on the contralateral side of the tumor. From a clinical point of view, the patients represented a homogenous population. The cells were allowed to adhere to plastic culture flasks and cultured in minimum essential medium supplemented with 10% fetal calf serum, benzylpenicillin (400 U/mL), and streptomycin (0.2 mg/mL) as described in Reference 3. In some experiments, as shown in Table 1
, 20% fetal calf
serum or 2% Ultroser was used (compare Reference 3). In these
experiments, [4-14C]cholesterol (55 mCi/mmol,
Radiochemical Centre, Amersham) was added to the medium, as described
in Table 1.
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Synthesis of Cholestenoic and Norcholestenoic Acids
Cholestenoic acid was synthesized via standard Wittig-Horner
reaction of 3ß-tert-butyldimethylsilyloxycholen-24-al
(synthesis described in Reference 12) with triethyl
2-phosphonopropionate and base. For synthesis of 27-norcholestenoic
acid, triethyl 2-phosphonopropionate was replaced by triethyl
phosphonoacetate.
Sodium hydride (25 mg) was suspended in
2,2-dimethoxypropane (2 mL).
Triethyl 2-phosphonopropionate (100 µL, or the same amount of
triethyl phosphonoacetate) was added and the mixture was stirred for 20
minutes. 3ß-tert-Butyldimethylsilyloxycholen-24-al, 100 mg
in 2 mL of tetrahydrofuran/2,2-dimethoxypropane, 1:1 (vol/vol) was
added dropwise during 10 minutes and the reaction mixture stirred for 1
hour at room temperature. The reaction was terminated with addition of
moist acidic ether, and the product was extracted with ether,
washed with water, and the solvent removed under reduced pressure. The
resulting
ethyl-3ß-tert-butyldimethylsilyloxy-
24-(nor-)cholestenoate
was saturated in the 24 double bond by bubbling hydrogen
gas through a solution of this compound dissolved in ethanol. This
procedure was readily accomplished since the 24 double
bond is more easily saturated than the 5 double bond.
The reaction was monitored by gas chromatography. The
resulting product,
ethyl-3ß-tert-butyldimethylsilyloxy-(nor-)cholestenoate
was stirred in tetrabutylammonium fluoride–containing
tetrahydrofuran overnight to remove the
tert-butyldimethylsilyloxy group. The product,
ethyl-(nor-)cholestenoate, was purified on an aluminum oxide column
deactivated with 3% water and eluted with toluene/ethyl
acetate, 8:2 (vol/vol). Finally, the ethyl ester group was removed via
alkaline hydrolysis. The product was >95% pure, as determined by
gas chromatography of the trimethylsilyl (TMS)
ether/methyl ester. Mass spectra of cholestenoic acid/TMS ether/methyl
ester were consistent with those of an authentic compound with
prominent peaks at m/z 502, 412, 473, and 129, and mass spectra of
norcholestenoic acid showed m/z peaks 14 mass units lower than
corresponding ions of cholestenoic acid containing the steroid side
chain. The presence of an intense ion in the mass spectrum of both
compounds at m/z 129 demonstrated that the 5 double bond
was intact.
Determination of Cholesterol in Cells
Free and esterified
cholesterol were
determined by using gas chromatography/mass
spectrometry with
[26,26,26,27,27,27-2H6]cholesterol
as the internal standard, essentially as described.13
Determination of 27-Oxygenated Cholesterol
in Plasma
7-Hydroxycholesterol and
27-hydroxycholesterol in plasma were determined by
isotope dilution/mass spectrometry using deuterium-labeled internal
standards.14 3ß,7-Dihydroxy-5-cholestenoic acid
and 7-hydroxy-3-oxo-4-cholestenoic acid were determined as
described.15
Western Blot Analysis
Western blot analysis using human
sterol 27-hydroxylase
antibodies was performed as described in Reference 3. Antibodies to
human sterol 27-hydroxylase were a kind gift of Dr D. Russell,
University of Texas Southwestern Medical Center, Dallas.
All experiments involving human volunteers and patient materials were reviewed and approved by the ethics committee at the Huddinge Hospital.
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Results and Discussion |
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TopAbstractIntroductionMethods Results and Discussion References |
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Cultured macrophages have a high capacity to convert cholesterol into 27-oxygenated products and secrete them into the medium. As shown earlier,3 human alveolar macrophages cultured in the presence of 10% fetal calf serum had a high capacity to secrete 27-oxygenated cholesterol into the medium (Table 2
). There was no further
metabolism of 27-hydroxycholesterol into
7-hydroxylated products, as found in diploid
fibroblasts.10 The 27-oxygenated products
recovered from the cell medium were always unesterified. The ratio
between 27-hydroxycholesterol and
3ß-hydroxy-5-cholestenoic acid varied between 0.1 and 0.5
in the experiments. The medium was found to contain 82±13% of the
27-hydroxycholesterol and 99±1% of the
3ß-hydroxy-5-cholestenoic acid (mean±SEM, n=4). It is
evident
that 3ß-hydroxy-5-cholestenoic acid is secreted from the
macrophages more efficiently than the less polar
27-hydroxycholesterol.
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Macrophages isolated from eight patients secreted about 4 fmol/cell of the 27-oxygenated products during 24 hours of incubation. The cholesterol content in these macrophages was about 10 fmol/cell. Thus, the oxidative mechanism has the potential to eliminate about 40% of the cell content of cholesterol in the cell in 24 hours.
We have previously shown that a major portion of the
27-oxygenated products secreted from cultured
macrophages was originally derived from extracellular
cholesterol.3 Accumulation of
27-oxygenated metabolites increased with increasing
concentration of cholesterol in the medium up to 0.1
mmol/L. Further addition of free cholesterol in ethanol did
not increase the flux of 27-oxygenated metabolites into the
medium. As shown in Table 2
, addition of
cholesterol-containing calf serum to the
macrophages gave about the same flux of
27-oxygenated products into the medium as did the
addition of free cholesterol in ethanol in the previous
work.
For unknown reasons, macrophages isolated from one of the
patients (Experiment 9 in Table 2) had a cholesterol
content about five times higher (53 fmol/cell) than the other cells,
and 70% of this cholesterol was esterified. However, the
secretion of 27-oxygenated products from these specific
macrophages was only slightly higher than above, 4.9 fmol/cell
per 24 hours. To further study the effect of a high intracellular
concentration of cholesterol on the flux of
27-oxygenated metabolites, macrophages isolated
from another patient were exposed to acetylated
LDL16 (Experiment 10 in Table 2). The total
cholesterol content increased to 103 fmol/cell, 57% of
which was esterified. The excretion of 27-oxygenated
products from these macrophages was higher than above, 7.6
fmol/cell per 24 hours (about twice as high as in control
macrophages). Thus, a higher cholesterol content in
the macrophages did not lead to a marked upregulation of the
sterol 27-hydroxylase within the time period studied.
In one experiment, macrophages with a high content of intracellular cholesterol were homogenized and analyzed by Western blotting, using antibodies specific for the human sterol 27-hydroxylase. The intensity of the band was similar to that obtained in a parallel analysis of control macrophages with a normal content of cholesterol. Thus, the concentration of sterol 27-hydroxylase protein also does not seem to increase as a consequence of a higher cholesterol content in the macrophages.
Inhibition of the sterol 27-hydroxylase in cultured
macrophages leads to accumulation of intracellular
cholesterol. If 27-hydroxylation is important for
cholesterol removal, inhibition of this mechanism might
increase the size of the intracellular pool of
cholesterol. Cyclosporin is an efficient
inhibitor of 27-hydroxylase.17 As shown in the
Figure, 20 µmol/L cyclosporin in the culture medium
reduced the secretion of 27-oxygenated metabolites from
macrophages during 24 hours by more than 90%, with a
concomitant increase in the intracellular total cholesterol
content by about 40%. This increase of intracellular
cholesterol was of the same magnitude as the secretion of
27-oxygenated cholesterol metabolites from
control macrophages during the same time. In both
macrophages exposed to cyclosporin and control
macrophages, the amount of esterified cholesterol
was <8% (mean levels, 4.5% and 5.7%, respectively). Theoretically,
the effect of cyclosporin may be the result of an effect on the flux of
unmetabolized cholesterol from the cells to the medium. In
a separate experiment, macrophages were preloaded with
[4-14C]cholesterol, as described in the
experiments below (Table 1). The flux of
[4-14C]cholesterol from such preloaded
macrophages into the culture medium was not significantly
affected by 20 µmol/L cyclosporin in the medium. The difference in
flux between the macrophages exposed to cyclosporin and the
control macrophages was thus only about 4% (mean of two
experiments).
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Comparison between flux of cholesterol and
27-oxygenated metabolites from cultured
macrophages. Under the conditions employed, there is a
flux of cholesterol both from the medium into the cultured
macrophages3 and from the cultured
macrophages into the medium. To compare the flux of
cholesterol from the cultured macrophages with the
corresponding flux of 27-oxygenated products,
macrophages were preloaded with [4-14C]-labeled
cholesterol (Table 1). When such macrophages were
cultured in medium containing fetal calf serum, there was a
considerable flux of [4-14C]cholesterol from
the cells into the medium. As shown in Table 1, about 50% of
the
labeled intracellular cholesterol was recovered in the
medium after 24 hours of culture. In the two experiments shown in Table
1, the amount of 27-oxygenated products in the medium
was about 10% of that of cholesterol (12% in Experiment I
and 8% in Experiment II).
It should be emphasized that a 10-fold higher flux of cholesterol than of 27-oxygenated products from the macrophages does not mean that the former flux is 10-fold more important than the latter under in vivo conditions. The 27-oxygenated products are rapidly transported to the liver and rapidly eliminated as bile acids. In contrast, cholesterol may recirculate or be taken up by other cells before it is ultimately eliminated as bile acids.
There is a significant net flux of 27-oxygenated metabolites of cholesterol from extrahepatic cells to the liver. The present mechanism for removal of intracellular cholesterol is not restricted to macrophages. It was recently shown that cultured arterial endothelial cells have a high capacity to secrete 27-oxygenated products into the medium,18 and the mechanism is also active in human umbilical vein endothelial cells.3 In view of the broad tissue distribution of the sterol 27-hydroxylase, it seems likely that other types of cells may also utilize this mechanism for cholesterol elimination.
If the mechanism is of general importance, one would expect the
presence of 27-oxygenated cholesterol
metabolites in the circulation and a net flux of these compounds
to the liver, where they are known to be converted into bile
acids.6 In addition to
27-hydroxycholesterol and
3ß-hydroxy-5-cholestenoic acid, there are significant amounts of
3ß,7-dihydroxy-5-cholestenoic acid and
7-hydroxy-3-oxo-4-cholestenoic acid in the
peripheral circulation.5 The latter two
metabolites may also be formed in nonhepatic cells.10
Table 3 summarizes the results of measurements of
27-hydroxycholesterol, 3ß-hydroxy-5-cholestenoic
acid, 3ß,7-dihydroxy-5-cholestenoic acid, and
7-hydroxy-3-oxo-4-cholestenoic acid in the hepatic vein and a
peripheral artery in six healthy volunteers (compare
Reference 19). The lowest concentrations of the four products were
found in the hepatic vein, indicating an uptake of the compounds in the
splanchnic area. The total uptake of the four 27-oxygenated
compounds was 26±4 mg/24 h.
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For reasons of comparison we also measured the concentration of
7-hydroxycholesterol in the two vessels. This
steroid is mainly produced in the liver and is also taken up by that
organ and converted into bile acids.20 The concentration
of 7-hydroxycholesterol was practically
identical in the two vessels (about 40 µg/L), and the difference in
concentration was 2±2 µg/L in the six patients (mean±SEM).
Theoretically, elimination of the 27-oxygenated metabolites in the splanchnic region may occur in the liver and/or the intestine. If the intestine is involved, there should be lower levels in the portal vein than in a peripheral artery or vein. In a separate experiment, we collected blood from the portal vein and a peripheral vein from five patients subjected to cholecystectomy (compare Reference 21). The concentration of the 27-oxygenated metabolites was 274±26 µg/L in the portal vein and 280±28 µg/L in the peripheral vein. The difference was 6±3 µg/L (mean±SEM). With an assumed flux of plasma through the portal vein of about 400 mL/min, this corresponds to an uptake of <4 mg of 27-oxygenated products per 24 hours in the intestine.
The present results demonstrate that there is a significant net
flux of 27-oxygenated products from extrahepatic
sources to the liver. In the liver, these 27-oxygenated
compounds are known to be efficiently converted into bile acids (for a
review see Reference 6). If 20 mg of the 27-oxygenated
products taken up by the liver is converted into bile acids, it
would correspond to 4% of the total bile acid formation. This
percentage could conceivably be higher in patients with liver disease
and a downregulated cholesterol 7-hydroxylase.
In the above calculations, it is assumed that the levels of the
different 27-oxygenated products are relatively stable
and that there is little or no diurnal variation. In previous work by
one of us,22 it was shown that there is little diurnal
variation in the case of 7-hydroxy-3-oxo-4-cholestenoic acid and
3ß,7-dihydroxy-5-cholestenoic acid. In a study of three
subjects in which the two acids were analyzed repeatedly over a
24-hour period, the coefficients of variation varied between 6% and
27%. In the present study, blood was collected from two healthy
subjects at 4-hour intervals during 24 hours. The coefficients of
variation for the level of 27-hydroxycholesterol in the
circulation of the two subjects were 8% and 16%, respectively. The
corresponding figures for 3ß-hydroxy-5-cholestenoic acid were
14% and 13%, respectively. It is evident that the small variations in
the levels of the different 27-oxygenated products do
not significantly affect the above calculations of the total flux of
these compounds during a 24-hour period.
Importance of the present mechanism. We have previously shown that patients with the rare inborn disease cerebrotendinous xanthomatosis (CTX) lack sterol 27-hydroxylase activity, and mutations in the sterol 27-hydroxylase gene have been defined (reviewed in Reference 23). Despite normal or low levels of circulating cholesterol, patients with CTX are predisposed to develop premature atherosclerosis.23 This correlation suggests a protective role of sterol 27-hydroxylase in the development of atherosclerosis. It is of interest that we3 and others24 have shown that 27-hydroxycholesterol is present in relatively high amounts in human atheromas. This accumulation may reflect a failure of macrophages to remove excess cholesterol at an early stage of development of an atheroma. In view of this, it would be of interest to investigate whether patients with atherosclerosis have higher or lower levels than normal of 27-oxygenated products in the circulation. According to preliminary experiments on patients with well-defined atherosclerosis in our laboratory, however, atherosclerosis in itself does not seem to affect the circulating levels of 27-oxygenated products.
To summarize, it is evident that the present pathway for elimination of cholesterol in extrahepatic cells is of quantitative importance. Further work is needed, however, to evaluate the exact role of this mechanism in relation to reverse cholesterol transport.
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Acknowledgments |
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This work was supported by grants from the Swedish Medical Research Council, Marianne and Marcus Wallenberg Foundation, Hjärt-Lungfonden, Stiftelsen Ragnhild och Einar Lunströms Minne, Stiftelsen Sigurd och Elsa Goljes Minne, and the European Community (No. PL 931790).
Received May 12, 1995; accepted November 3, 1995.
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References |
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 G. L. Vega, M. F. Weiner, A. M. Lipton, K. von Bergmann, D. Lutjohann, C. Moore, and D. Svetlik Reduction in Levels of 24S-Hydroxycholesterol by Statin Treatment in Patients With Alzheimer Disease Arch Neurol, April 1, 2003; 60(4): 510 - 515. [Abstract] [Full Text] [PDF]
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G. Escher, Z. Krozowski, K. D. Croft, and D. Sviridov Expression of Sterol 27-Hydroxylase (CYP27A1) Enhances Cholesterol Efflux J. Biol. Chem., March 21, 2003; 278(13): 11015 - 11019. [Abstract] [Full Text] [PDF]
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 S. Jabara, L. K. Christenson, C. Y. Wang, J. M. McAllister, N. B. Javitt, A. Dunaif, and J. F. Strauss III Stromal Cells of the Human Postmenopausal Ovary Display a Distinctive Biochemical and Molecular Phenotype J. Clin. Endocrinol. Metab., January 1, 2003; 88(1): 484 - 492. [Abstract] [Full Text] [PDF]
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W. M. Pandak, S. Ren, D. Marques, E. Hall, K. Redford, D. Mallonee, P. Bohdan, D. Heuman, G. Gil, and P. Hylemon Transport of Cholesterol into Mitochondria Is Rate-limiting for Bile Acid Synthesis via the Alternative Pathway in Primary Rat Hepatocytes J. Biol. Chem., December 6, 2002; 277(50): 48158 - 48164. [Abstract] [Full Text] [PDF]
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D. Murtazina, A. V. Puchkaev, C. H. Schein, N. Oezguen, W. Braun, A. Nanavati, and I. A. Pikuleva Membrane-Protein Interactions Contribute to Efficient 27-Hydroxylation of Cholesterol by Mitochondrial Cytochrome P450 27A1 J. Biol. Chem., September 27, 2002; 277(40): 37582 - 37589. [Abstract] [Full Text] [PDF]
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K. Meir, D. Kitsberg, I. Alkalay, F. Szafer, H. Rosen, S. Shpitzen, L. B. Avi, B. Staels, C. Fievet, V. Meiner, et al. Human Sterol 27-Hydroxylase (CYP27) Overexpressor Transgenic Mouse Model. EVIDENCE AGAINST 27-HYDROXYCHOLESTEROL AS A CRITICAL REGULATOR OF CHOLESTEROL HOMEOSTASIS J. Biol. Chem., September 6, 2002; 277(37): 34036 - 34041. [Abstract] [Full Text] [PDF]
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 S. von Bahr, T. Movin, N. Papadogiannakis, I. Pikuleva, P. Ronnow, U. Diczfalusy, and I. Bjorkhem Mechanism of Accumulation of Cholesterol and Cholestanol in Tendons and the Role of Sterol 27-hydroxylase (CYP27A1) Arterioscler Thromb Vasc Biol, July 1, 2002; 22(7): 1129 - 1135. [Abstract] [Full Text] [PDF]
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I. Bjorkhem and U. Diczfalusy Oxysterols: Friends, Foes, or Just Fellow Passengers? Arterioscler Thromb Vasc Biol, May 1, 2002; 22(5): 734 - 742. [Abstract] [Full Text] [PDF]
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G. L. Moore and R. A. Davis Expression of cholesterol-7{alpha}-hydroxylase in murine macrophages prevents cholesterol loading by acetyl-LDL J. Lipid Res., April 1, 2002; 43(4): 629 - 635. [Abstract] [Full Text] [PDF]
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 J. M. Upston, X. Niu, A. J. Brown, R. Mashima, H. Wang, R. Senthilmohan, A. J. Kettle, R. T. Dean, and R. Stocker Disease Stage-Dependent Accumulation of Lipid and Protein Oxidation Products in Human Atherosclerosis Am. J. Pathol., February 1, 2002; 160(2): 701 - 710. [Abstract] [Full Text] [PDF]
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A. B. Reiss, N. W. Awadallah, S. Malhotra, M. C. Montesinos, E. S. L. Chan, N. B. Javitt, and B. N. Cronstein Immune complexes and IFN-{gamma} decrease cholesterol 27-hydroxylase in human arterial endothelium and macrophages J. Lipid Res., November 1, 2001; 42(11): 1913 - 1922. [Abstract] [Full Text] [PDF]
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X. Fu, J. G. Menke, Y. Chen, G. Zhou, K. L. MacNaul, S. D. Wright, C. P. Sparrow, and E. G. Lund 27-Hydroxycholesterol Is an Endogenous Ligand for Liver X Receptor in Cholesterol-loaded Cells J. Biol. Chem., October 12, 2001; 276(42): 38378 - 38387. [Abstract] [Full Text] [PDF]
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E. Hall, P. Hylemon, Z. Vlahcevic, D. Mallonee, K. Valerie, N. Avadhani, and W. Pandak Overexpression of CYP27 in hepatic and extrahepatic cells: role in the regulation of cholesterol homeostasis Am J Physiol Gastrointest Liver Physiol, July 1, 2001; 281(1): G293 - G301. [Abstract] [Full Text] [PDF]
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I. Björkhem, L. Starck, U. Andersson, D. Lütjohann, S. von Bahr, I. Pikuleva, A. Babiker, and U. Diczfalusy Oxysterols in the circulation of patients with the Smith-Lemli-Opitz syndrome: abnormal levels of 24S- and 27-hydroxycholesterol J. Lipid Res., March 1, 2001; 42(3): 366 - 371. [Abstract] [Full Text]
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S. Meaney, M. Hassan, A. Sakinis, D. Lütjohann, K. von Bergmann, A. Wennmalm, U. Diczfalusy, and I. Björkhem Evidence that the major oxysterols in human circulation originate from distinct pools of cholesterol: a stable isotope study J. Lipid Res., January 1, 2001; 42(1): 70 - 78. [Abstract] [Full Text]
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 G. J. Schroepfer Jr. Oxysterols: Modulators of Cholesterol Metabolism and Other Processes Physiol Rev, January 1, 2000; 80(1): 361 - 554. [Abstract] [Full Text] [PDF]
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M. A. Lyons, S. Samman, L. Gatto, and A. J. Brown Rapid hepatic metabolism of 7-ketocholesterol in vivo: implications for dietary oxysterols J. Lipid Res., October 1, 1999; 40(10): 1846 - 1857. [Abstract] [Full Text]
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A. Babiker, O. Andersson, D. Lindblom, J. van der Linden, B. Wiklund, D. Lütjohann, U. Diczfalusy, and I. Björkhem Elimination of cholesterol as cholestenoic acid in human lung by sterol 27-hydroxylase: evidence that most of this steroid in the circulation is of pulmonary origin J. Lipid Res., August 1, 1999; 40(8): 1417 - 1425. [Abstract] [Full Text]
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W. C. Duane and N. B. Javitt 27-Hydroxycholesterol: production rates in normal human subjects J. Lipid Res., July 1, 1999; 40(7): 1194 - 1199. [Abstract] [Full Text]
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I. Björkhem, D. Lütjohann, U. Diczfalusy, L. Ståhle, G. Ahlborg, and J. Wahren Cholesterol homeostasis in human brain: turnover of 24S-hydroxycholesterol and evidence for a cerebral origin of most of this oxysterol in the circulation J. Lipid Res., August 1, 1998; 39(8): 1594 - 1600. [Abstract] [Full Text]
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I. A. Pikuleva, A. Babiker, M. R. Waterman, and I. Bjorkhem Activities of Recombinant Human Cytochrome P450c27 (CYP27) Which Produce Intermediates of Alternative Bile Acid Biosynthetic Pathways J. Biol. Chem., July 17, 1998; 273(29): 18153 - 18160. [Abstract] [Full Text] [PDF]
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H. Rosen, A. Reshef, N. Maeda, A. Lippoldt, S. Shpizen, L. Triger, G. Eggertsen, I. Bjorkhem, and E. Leitersdorf Markedly Reduced Bile Acid Synthesis but Maintained Levels of Cholesterol and Vitamin D Metabolites in Mice with Disrupted Sterol 27-Hydroxylase Gene J. Biol. Chem., June 12, 1998; 273(24): 14805 - 14812. [Abstract] [Full Text] [PDF]
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J. Westman, B. Kallin, I. Bjorkhem, J. Nilsson, and U. Diczfalusy Sterol 27-Hydroxylase– and ApoAI/Phospholipid–Mediated Efflux of Cholesterol From Cholesterol-Laden Macrophages : Evidence for an Inverse Relation Between the Two Mechanisms Arterioscler Thromb Vasc Biol, April 1, 1998; 18(4): 554 - 561. [Abstract] [Full Text] [PDF]
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I. Bjorkhem, D. Lutjohann, O. Breuer, A. Sakinis, and A. Wennmalm Importance of a Novel Oxidative Mechanism for Elimination of Brain Cholesterol. TURNOVER OF CHOLESTEROL AND 24(S)-HYDROXYCHOLESTEROL IN RAT BRAIN AS MEASURED WITH 18O2 TECHNIQUES IN VIVO AND IN VITRO J. Biol. Chem., November 28, 1997; 272(48): 30178 - 30184. [Abstract] [Full Text] [PDF]
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A. Babiker, O. Andersson, E. Lund, R.-J. Xiu, S. Deeb, A. Reshef, E. Leitersdorf, U. Diczfalusy, and I. Bjorkhem Elimination of Cholesterol in Macrophages and Endothelial Cells by the Sterol 27-Hydroxylase Mechanism. COMPARISON WITH HIGH DENSITY LIPOPROTEIN-MEDIATED REVERSE CHOLESTEROL TRANSPORT J. Biol. Chem., October 17, 1997; 272(42): 26253 - 26261. [Abstract] [Full Text] [PDF]
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A. J. Brown, G. F. Watts, J. R. Burnett, R. T. Dean, and W. Jessup Sterol 27-Hydroxylase Acts on 7-Ketocholesterol in Human Atherosclerotic Lesions and Macrophages in Culture J. Biol. Chem., September 1, 2000; 275(36): 27627 - 27633. [Abstract] [Full Text] [PDF]
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