INTRODUCTION

Two pathways of bile acid biosynthesis have been described in the mammalian liver. One pathway is initiated in the endoplasmic reticulum of the hepatocyte by the enzyme cholesterol 7-hydroxylase (referred to hereafter as 7-hydroxylase; cholesterol 7-monooxygenase (EC)), which converts cholesterol (cholest-5-en-3-ol) into 7-hydroxycholesterol (cholest-5-ene-3,7-diol). Subsequent enzymatic steps lead to the formation of the primary bile acids cholic acid and chenodeoxycholic acid (reviewed in Ref. 1). A second pathway is initiated in the mitochondria by the enzyme sterol 27-hydroxylase, which converts cholesterol into 27-hydroxycholesterol (cholest-5-ene-3,27-diol) (2). This intermediate is acted on by an oxysterol 7-hydroxylase to form 7,27-dihydroxycholesterol (cholest-5-ene-3,7,27-triol) (3, 4, 5, 6, 7), which is subsequently converted into primary bile acids.

The pathway initiated by 7-hydroxylase is thought to be the major route by which bile acids are synthesized in the liver. This assumption arises in part because the 7-hydroxylase pathway was discovered first (8) and because the level of 7-hydroxylase enzyme activity is tightly controlled by feedback regulation (1). The importance of this pathway is underscored by the finding that expression of an exogenous 7-hydroxylase gene in hamsters via infection with a recombinant adenovirus leads to a marked increase in bile acid formation (9).

To gain further insight into the role of 7-hydroxylase in bile acid metabolism, a line of mice deficient in this enzyme was created by gene targeting methods (10). Young 7-hydroxylase-deficient mice (Cyp7^/) exhibit a complex phenotype consisting of an increased rate of postnatal death, fat malabsorption, wasting, skin abnormalities, and vision problems (10). The absence of bile acids in newborn animals combined with fat-soluble vitamin deficiency is considered the most likely explanation for this phenotype. A peculiar feature of murine 7-hydroxylase deficiency is that the phenotype is only present in newborn mice: once a deficient animal reaches the age of ~3 weeks, symptoms wane to the point that adult animals are indistinguishable from wild-type mice (10).

We now explain this phenotype by showing that the mitochondrial bile acid pathway is not present at birth, but appears at day 21 in both wild-type and Cyp7^/ mice, thereby obviating the requirement for 7-hydroxylase. Induction correlates with the appearance of oxysterol 7-hydroxylase activity in the liver and results in the formation of bile with an altered composition of bile acids relative to animals in which both pathways are functioning. By studying the levels of vitamins D₃ and E, we have formulated the hypothesis that the bile acid products of the mitochondrial pathway are not as efficient as those of the endoplasmic reticulum pathway in mediating vitamin E absorption. Thus, the requirement for two pathways may reflect a need to synthesize bile acids of diverse chemical structures in order to ensure maximum solubilization of different dietary fats and vitamins.

EXPERIMENTAL PROCEDURES

Bile was drawn from the gallbladders of mice euthanized with sodium pentobarbital. Samples were stored frozen at 20 °C until analyzed. Blood for lipoprotein, cholesterol, and triglyceride analysis was drawn by cardiac puncture or exsanguination via the ascending carotid artery and clotted at room temperature for 30 min. The sample was centrifuged at 16,000 × g for 50 min, and the serum was decanted from the pelleted blood cells. The levels of cholesterol, triglyceride, and lipoprotein were determined as described previously (11).

Vitamin E levels were measured in epididymal or ovarian fat samples using high pressure liquid chromatography methods as described previously (12). Vitamin D metabolites were assayed as described previously (13, 14), except that bovine mammary gland vitamin D receptor was used in place of calf thymus gland vitamin D receptor. In brief, serum samples were supplemented with trace quantities of radioactive 25-hydroxyvitamin D₃ to quantify the yield of this metabolite during the ensuing purification. Following extraction of vitamin D metabolites from serum with acetonitrile and back-extraction with phosphate buffer, the vitamin D metabolites were purified and separated via chromatography on C₁₈ and silica Sep-Pak cartridges (Waters). After removing an aliquot of the purified fractions for determination of yield, assay of 25-hydroxyvitamin D₃ was performed via competitive protein binding using a 1:5000 dilution of human serum (vitamin D-binding protein) as the binding agent. The sensitivity of this assay is 0.2 ng or a serum value of 1 ng/ml assuming 80% yield and a single determination. The intra- and interassay coefficients of variation are 5 and 8%, respectively. Because this purification scheme does not separate the D₂ (ergocalciferol) and D₃ (cholecalciferol) forms of the metabolites, the concentration measured in serum represents total vitamin D.

The extraction, separation, derivatization, and analyses of bile acids in bile using gas chromatography-mass spectrometry and liquid secondary ionization mass spectrometry were carried out as described previously (15).

Oxysterol 7-hydroxylase activity was determined in 0.5-ml incubation mixtures containing 0.06 nmol of 25-[26,27-³H₂]hydroxycholesterol (77 Ci/mmol), 250 µg of microsomal protein, 0.75 µmol of NADPH, 50 mM Tris acetate, pH 7.4, 1 mM EDTA, 2 mM dithiothreitol, and 0.03% (v/v) Triton X-100. After incubation for 15 min at 37 °C, the reactions were terminated by the addition of 6 ml of methylene chloride. The organic phase was evaporated to dryness under nitrogen; the lipid pellet was dissolved in 40 µl of acetone and analyzed by thin-layer chromatography on Silica Gel LK5D 150-Å plates (Whatman) in a solvent system containing toluene/ethyl acetate (2:3).¹

A sample of authentic cholest-5-ene-3,7,25-triol was synthesized using a modification of a method described previously (16, 17). Briefly, cholest-5-ene-3,25-diol was photochemically converted into 5-hydroperoxycholest-6-ene-3,25-diol in the presence of oxygen and hematoporphyrin. The product was purified by silica gel chromatography with a 46% yield and incubated in chloroform to allow rearrangement to the 7-hydroperoxide derivative. This material was reduced with sodium borohydride to yield the desired product, which was purified by preparative thin-layer chromatography in a toluene/ethyl acetate (2:3) solvent system. The final yield of cholest-5-ene-3,7,25-triol was 3-5%.

Scissors were used to barber fur from the abdomens or backs of mice. A 10-mg aliquot of fur was extracted with 1.5 ml of chloroform/methanol (2:1) for 2 h at 4 °C with continuous agitation. The resulting solvent was transferred to a fresh polypropylene tube and evaporated to dryness under a nitrogen stream. The pellet was dissolved in 50 µl of chloroform/methanol (2:1) and analyzed by thin-layer chromatography on Silica Gel LK5D 150-Å plates in a solvent system containing toluene/ethyl acetate (7:3).

For stool lipid analysis, 100-mg aliquots of droppings collected from animals of the indicated ages were mixed with a small amount of [carboxyl-¹⁴C]triolein (112 mCi/mmol) and dried for 1 h in a vacuum oven at 70 °C. The solid matter was extracted with 2 ml of chloroform/methanol (2:1) for 30 min at 60 °C, passed through a Whatman No. 1 filter, and brought to a final volume of 4 ml with chloroform/methanol (2:1). The material was back-extracted with 1 ml of H₂O, and the organic phase was evaporated to dryness. The pellet was resuspended in 2 ml of chloroform/methanol (2:1) and transferred to preweighed vials. The solvent was evaporated, and the vial was taken to a constant weight by drying in a vacuum oven at 70 °C. The difference in weight between the starting empty vial and the vial containing the dried lipid was the fecal lipid amount, which was expressed as a percentage of the weight of the starting fecal sample. The percent recovery of radiolabeled triolein (85-92%) was determined by subjecting the vial to scintillation counting.

Gas chromatography-mass spectrometry of mouse fur lipids was performed before and after saponification as described previously (18). Native lipids were converted into trimethylsilyl ethers prior to analysis (19). Briefly, samples were treated with pyridine/hexamethyldisilazane/chlorotrimethylsilane (3:2:1) at 60 °C for 30 min. The solvent was evaporated under a stream of nitrogen, and the residue was dissolved in hexane for gas chromatography-mass spectrophotometry analyses. Saponified samples were converted into methyl esters by treatment with diazomethane (19), further derivatized with trimethylsilane as described above, and then subjected to chemical analysis.

The normal diet was mouse/rat diet 7001 (Harlan Teklad, Madison, WI) and contained 4% (w/w) fat, 24% (w/w) protein, and 5% (w/w) fiber. Where indicated, this diet was supplemented with 1% (w/w) cholic acid (Sigma). Vitamin supplements (CritterVites, Mardel Labs, Glendale Heights, IL) containing both water-soluble vitamins (thiamine, 180 mg/kg; riboflavin, 300 mg/kg; pantothenic acid, 600 mg/kg; niacin, 1500 mg/kg; vitamin B₁₂, 1500 µg/kg; vitamin B₆, 150 mg/kg; folic acid, 100 mg/kg; and ascorbic acid, 9000 mg/kg) and fat-soluble vitamins (vitamin A, 300,000 IU/kg; vitamin D₃, 50,000 IU/kg; vitamin E, 750 IU/kg; and menadione, 250 mg/kg) were added to water bottles at the concentration (1 g/liter) recommended by the manufacturer. Vitamin supplements were replaced on a daily basis.

RESULTS

To determine the effects of 7-hydroxylase deficiency on serum lipid levels, blood was sampled from animals of different genotypes and ages, and the serum lipids were analyzed. The data of Table I show that although there is wide interanimal variation, serum triglyceride and cholesterol contents are similar in wild-type and deficient mice, regardless of the age of the animals. In agreement with these measurements, the profiles of lipoprotein particles in the serum were similar in individual animals of different Cyp7 genotypes (Fig. 1). In experiments not shown, the concentrations of cholesterol in several tissues (liver, spleen, kidney, lung, and heart) were not significantly different between 15-day-old wild-type and mutant mice.

Table I. Serum cholesterol and triglyceride levels in wild-type and Cyp7/ mice Age Cyp7 genotype Cholesterola Triglyceridea days mg/dl mg/dl 6 +/+ 102.2  ± 16.5 177.6  ± 104.7  / 85.6  ± 22.3 140.0  ± 42.2 15 +/+ 170.9  ± 30.7 93.0  ± 42.8  / 157.5  ± 44.3 151.7  ± 63.3 23 +/+ 88.4  ± 10.0 54.2  ± 10.7  / 99.7  ± 19.6 82.3  ± 28.0 60-90 +/+ 91.5  ± 7.9 64.4  ± 7.9  / 91.7  ± 5.6 39.7  ± 7.7 150-180 +/+ 107.1  ± 15.7 76.3  ± 19.1  / 122.5  ± 20.0 82.5  ± 27.2 a  Values represent means ± S.D. derived from measurements made in the sera of 4-10 animals of the indicated age and genotype.

Lipoprotein analysis in wild-type and 7-hydroxylase-deficient mice.

Many of the phenotypic characteristics of 7-hydroxylase-deficient mice, including early death, skin abnormalities, and eye and vision problems, are reminiscent of fat-soluble vitamin deficiencies. This hypothesis is supported by the finding that these symptoms can be alleviated by supplementing the water supply of nursing mothers with a vitamin mixture (10). To determine if a fat-soluble vitamin deficiency could be directly demonstrated, the levels of vitamins D₃ and E were measured in the serum and fat, respectively, of mutant and wild-type mice. The data of Table II show that Cyp7^/ mice contain low levels of vitamins D₃ and E. These deficiencies are detected in mice of different ages and in nursing mothers. Vitamin supplementation alone partially restored levels of vitamins D₃ and E, whereas dietary supplementation with vitamins and a bile acid (cholic acid) more fully restored vitamin levels (Table II).

Table II. Serum vitamin D3 and tissue vitamin E levels in wild-type and Cyp7/ mice Agea,b Cyp7 genotype Serum vitamin D3c Tissue vitamin Ed Chow Chow + vitamins Chow + vitamins, cholic acid Chow Chow + vitamins Chow + vitamins, cholic acid days ng/ml ng/mg triglyceride 6 +/+ 20.0  e          / 2.0 5.0 12.0       16 +/+ 18.0     11.6      / 3.7 4.0 9.0 0.3 <0.1 12.0 23 +/+ 8.0     24.4      / 1.0 4.0 9.0 <0.1 <0.1 53.3 34-38 +/+ 17.7  ± 1.1     10.4  ± 2.0      / 15.7  ± 2.4     5.6  ± 3.6     120-180 +/+ 17.5  ± 2.5     90.7  ± 25.4      / 20.0  ± 3.3 11.0 ± 4.4 28.5 ± 2.5 4.4  ± 4.4 11.6 ± 2.5 146.1 ± 91.8 a  Serum or fat tissue was pooled from 3-12 male and female mice to derive the values shown for the 6-, 16-, and 23-day time points. b  Values for 34-38 and 120-180-day mice represent means ± S.E. derived from three to five animals. c  25-Hydroxyvitamin D3 levels were measured in serum as described under ``Experimental Procedures.'' d  Vitamin E levels were measured in epididymal or ovarian fat pads as described under ``Experimental Procedures.'' e  , not done.

A second characteristic feature of newborn 7-hydroxylase-deficient mice is their excretion of clay-colored stools, which are reminiscent of fat malabsorption (steatorrhea). To confirm this diagnosis, the stool fat content was monitored as a function of age in Cyp7^/ mice. The data of Fig. 2 show that newborn mice have enormously elevated levels of fat in their stools and that this elevation persists through approximately postnatal day 22, at which time the stool fat content begins to decrease and eventually (by day 28) approximates that of wild-type animals. Weaning took place on day 30 in these experiments; thus, the decline in fat content occurred while the animals were maintained on a high fat diet (mother's milk). The reduction in stool fat content on or about postnatal day 22 coincides with the time at which the survival rate of mutant mice is dramatically increased (10). Those few animals that reach this age thereafter experience a normal life span. These findings are suggestive of a major change in bile acid metabolism occurring around postnatal day 22 in the Cyp7^/ mice.

Stool fat content as function of age in wild-type and 7-hydroxylase-deficient mice.

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The chemical composition of bile from adult wild-type and Cyp7^/ animals (~3 months of age) was examined next. Animals were maintained on normal unsupplemented chow for 4 weeks; the major bile duct was cannulated; and bile was collected over a period of 30-60 min from two mice of each genotype. The withdrawn samples were analyzed by gas chromatography-mass spectrometry to identify individual bile acids. The results of these analyses are shown in Table III. The concentration of bile acids in the bile of wild-type and mutant mice was similar and ranged from 9.9 to 14.6 mM. Bile from wild-type animals contained at least 11 separable and measurable bile acid derivatives, of which cholic acid and -muricholic acid were the predominant species, accounting together for ~75% of the total bile acids. In the Cyp7^/ mice, the concentration of cholic acid, but not -muricholic acid, was decreased by more than half relative to that found in the wild-type animals (Table III). In addition, several bile acids were increased in concentration relative to levels in wild-type animals. For example, hyodeoxycholic acid (3,6-dihydroxy-5-cholanoic acid) was not detected in wild-type bile, but this compound represented 14-25% of the total bile acid in the Cyp7^/ animals. Similarly, chenodeoxycholic acid and -muricholic acid were detected only in the mutant mice (Table III).

Table III. Analysis of bile acids in bile of individual wild-type and Cyp7/ adult mice Retention timea Bile acidb Cyp7 genotype +/+ +/+  /  / MU µmol/liter 31.81 3,12-Dihydroxy bile acid 312 425 107 189 32.00 3,7,12-Trihydroxy-5-cholanoic acid (allo-cholic acid) 482 651 174 236 32.13 3,7-Dihydroxy-5-cholanoic acid (chenodeoxycholic acid) NDc ND 431 350 32.18 3,6,7-Trihydroxy-5-cholanoic acid (-muricholic acid) ND ND 766 672 32.23 3,7,12-Trihydroxy-5-cholanoic acid (cholic acid) 6710 7609 1918 2434 32.29 3,6-Dihydroxy-5-cholanoic acid (hyodeoxycholic acid) ND ND 2486 1713 32.51 3,7-Dihydroxy-5-cholanoic acid (ursodeoxycholic acid) 150 304 168 162 32.84 3,7,12-Trihydroxy-5-homocholanoic acid (homocholic acid) 155 178 ND ND 33.07 12-Oxo-3-hydroxy-5-cholanoic acid 157 217 899 746 33.21 3,6,7-Trihydroxy-5-cholanoic acid (-muricholic acid) 1778 3131 1717 3060 33.52 3,7,12-Trihydroxy-5-cholanoic acid 132 237 171 146 33.72 Oxodihydroxy bile acid 276 380 163 139 33.80 7-Oxo-3,12-Dihydroxy-5-cholanoic acid 260 290 ND 340 34.29 3,6,7-Trihydroxy-5-cholanoic acid (-muricholic acid) 555 1203 902 1714 Total bile acids 10,967 14,625 9902 11,901 a  Bile acids are listed based on retention times as methyl ester-trimethylsilyl ethers relative to a homologous series of n-alkanes, referred to as the methylene unit (MU) value. b  Chemical structures were established by electron ionization-gas chromatography-mass spectrometry. c  ND, not detected.

The data of Table IV summarize the results obtained after chemical analyses of bile acids in stool samples of adult normal and Cyp7^/ animals (~4 months of age). Feces were collected for a period of several days from five individual animals of each genotype, weighed, and then extracted and analyzed for bile acids. The average weight of stool collected from the wild-type mice was 1.32 ± 0.12 g/day (mean ± S.E.) versus 1.35 ± 0.05 g/day for the Cyp7^/ mice. The concentration of total bile acids in the droppings of wild-type mice was 1095.8 ± 88.9 µg/g of feces, whereas the droppings from Cyp7^/ mice contained 369.8 ± 18.2 µg/g of feces (Table IV). The diminished bile acid concentration in the stool samples of the mutant animals reflected a uniform decline in almost all of the individual bile acid species detected in the analyses (Table IV). For example, deoxycholic acid, which is the most abundant bile acid in mouse stool, was reduced from 316.2 ± 28.6 µg/g in the wild-type animals to 47.4 ± 12.6 µg/g in the Cyp7^/ animals (Table IV).

Table IV. Analysis of fecal bile acids in individual wild-type and Cyp7/ adult mice Retention timea Bile acidb Cyp7 genotype +/+ +/+ +/+ +/+ +/+  /  /  /  /  / MU µg/g feces 31.18 3-Hydroxy-5-cholanoic acid (lithocholic acid) 12 23 26 22 31 8 6 6 ND 4 31.61 Unknown bile acid NDc 12 23 18 18 ND ND 4 ND ND 31.76 3,12-Dihydroxy bile acid 21 27 26 34 32 3 6 5 ND 4 31.88 3-12-Dihydroxy-5-cholanoic acid (deoxycholic acid) 242 276 319 411 333 41 73 78 10 35 32.00 3,7,12-Trihydroxy-5-cholanoic acid (allo-cholic acid) 18 11 11 ND 9 ND ND 6 8 8 32.15 3,7-Dihydroxy-5-cholanoic acid (chenodeoxycholic acid) ND 8 11 ND 10 8 6 6 7 4 32.20 3,6,7-Trihydroxy-5-cholanoic acid (-muricholic acid) 45 36 56 57 69 ND ND ND ND ND 32.26 3,7,12-Trihydroxy-5-cholanoic acid (cholic acid) 127 137 158 181 132 43 62 70 188 117 32.29 3,6-Dihydroxy-5-cholanoic acid (hyodeoxycholic acid) 60 106 129 112 114 69 64 71 59 51 32.45 Unknown bile acid 13 20 24 20 20 10 7 8 7 8 32.51 3,7-Dihydroxy-5-cholanoic acid (ursodeoxycholic acid) ND ND 11 ND ND 6 ND 14 ND ND 32.60 3,12-Dihydroxy-5-cholanoic acid 12 16 17 21 20 ND ND ND ND ND 32.84 3,7,12-Trihydroxy-5-homocholanoic acid (homocholic acid) 8 ND 14 15 10 ND 4 ND ND ND 33.06 12-Oxo-3-hydroxy-5-cholanoic acid 65 44 85 75 61 26 35 30 23 26 33.22 3,6,7-Trihydroxy-5-cholanoic acid (-muricholic acid) 76 66 77 141 81 45 54 53 71 75 33.79 7-Oxo-3,12-dihydroxy-5-cholanoic acid 11 ND 9 16 6 ND 5 6 6 5 34.30 3,6,7-Trihydroxy-5-cholanoic acid (-muricholic acid) 117 140 157 194 136 39 34 29 29 35 34.55 3,6,7-Trihydroxy-5-cholanoic acid (allo--muricholic acid) 27 33 41 43 34 6 7 4 6 6 Total bile acids 854 955 1194 1360 1116 304 363 390 414 378 a  Bile acids are listed based on their retention times as methyl ester-trimethylsilyl ethers relative to a homologous series of n-alkanes, referred to as the methylene unit (MU) value. b  Chemical structures were determined by electron ionization-gas chromatography-mass spectrometry. c  ND, not detected.

Several conclusions were reached from the data summarized in Tables III and IV. First, Cyp7^/ animals that survive to adulthood when maintained on unsupplemented chow have normal concentrations of total bile acids in their bile (Table III), but reduced concentrations of bile acids in their stool (Table IV). Second, the composition of bile in terms of the individual bile acid species is different in the bile and stool of wild-type and Cyp7^/ animals. Third, adult animals deficient in 7-hydroxylase nevertheless synthesize and secrete 7-hydroxylated bile acids into their bile and excrete these compounds in their stool.

Bile acids with a 7-hydroxyl group could arise in the mutant mice as a consequence of 7-hydroxylase activity present in the intestinal flora or from the activity of another endogenous sterol 7-hydroxylase enzyme. The data of Fig. 3 (lanes 1 and 3) demonstrate the presence of an enzyme activity in the livers of both wild-type and Cyp7^/ mice that is capable of 7-hydroxylating cholest-5-ene-3,25-diol (25-hydroxycholesterol) to form cholest-5-ene-3,7,25-triol. This oxysterol 7-hydroxylase enzyme required NADPH as a cofactor (lanes 2 and 4). The specific activity of the enzyme was 6 pmol of product formed per min/mg of liver microsomal protein in adult wild-type mice and 5 pmol/min/mg of protein in Cyp7^/ mice.

Oxysterol 7-hydroxylase activity in wild-type and 7-hydroxylase-deficient mice.

The experiment shown in Fig. 4 was performed to confirm that the product of this enzyme activity was cholest-5-ene-3,7,25-triol. An authentic standard representing the predicted product was chemically synthesized as described under ``Experimental Procedures.'' This standard was chromatographed on a thin-layer plate either separately (lane 1) or together (lanes 2 and 3) with the radiolabeled products derived by incubating liver extracts with [³H]cholest-5-ene-3,25-diol. A comparison of the position of the radiolabeled product on the chromatogram (determined by autoradiography) to that of the standard (determined by phosphomolybdic acid staining) revealed that the two sterols comigrated in this solvent system. Similar results were obtained when product analysis was carried out in two other solvent systems (chloroform/methanol/H₂O/ethyl acetate (50:20:4:15) and toluene/ethyl acetate (2:3)) on silica gel plates and when the sample was analyzed by chromatography on C₁₈ plates using an acetonitrile/tetrahydrofuran (6:4) solvent system. Finally, analysis of the product by mass spectrometry confirmed the structure of the molecule as cholest-5-ene-3,7,25-triol (data not shown).

Authentication of oxysterol 7-hydroxylase product.

We next determined the ability of different sterols to inhibit the oxysterol 7-hydroxylase enzyme activity detected in the mutant mice. As shown by the data of Fig. 5, the product of the enzyme, cholest-5-ene-3,7,25-triol, specifically inhibited the reaction with an apparent IC₅₀ of 9.0 µM. The product of the cholesterol 7-hydroxylase enzyme, cholest-5-ene-3,7-diol (7-hydroxycholesterol), did not inhibit the oxysterol 7-hydroxylase activity, nor did an irrelevant triol compound, cholest-5-ene-3,17,20-triol (Fig. 5A). These data were quantitated by phosphoimage analysis and plotted as a function of oxysterol 7-hydroxylase activity versus inhibitor concentration (Fig. 5B). Interpolation predicts an IC₅₀ of 9 µM for the product of the enzyme.

Product inhibition of oxysterol 7-hydroxylase activity.

To determine whether survival of the Cyp7^/ deficient animals correlates with the appearance of the oxysterol 7-hydroxylase enzyme activity, the ontogeny of expression in the liver was determined. The data of Fig. 6A illustrate a representative experiment showing that the formation of [³H]cholest-5-ene-3,7,25-triol is low to absent in the livers of wild-type or mutant mice throughout postnatal days 1-25, but is substantially increased after this time period. A more detailed time course study reveals that the enzyme is gradually induced between postnatal days 20 and 50 (Fig. 6B). In the experiments shown in Fig. 6B, animals of both genotypes were weaned on day 24, suggesting that the induction of oxysterol 7-hydroxylase activity might correlate with the shift from a high fat diet (mother's milk) to a low fat diet (normal chow). However, in experiments not shown, we found that enzyme induction was independent of weaning in both wild-type and Cyp^/ mice.

Ontogeny of oxysterol 7-hydroxylase expression in mice.

A clearly visible and striking phenotype associated with a deficiency of 7-hydroxylase in mice is the development of an oily coat in newborn animals and nursing females (10). To determine if the unusual coat appearance was due to excess secretion of water or lipids, their concentrations in fur were examined. As determined by the Fischer method, the water content of wild-type mouse fur was 10.6 ± 0.1%, and that of nursing homozygous mothers with an oily coat was 9.9 ± 0.2% (data not shown). Qualitative thin-layer chromatography analysis of solvent-extractable lipids suggested that animals of both genotypes had near equal amounts of squalene, cholesterol, and cholesterol esters in their fur (Fig. 7). However, two lipids (designated X and Y in Fig. 7) with mobilities intermediate between cholesterol and squalene were present in excess in Cyp7^/ animals with greasy coats. A third region of the chromatogram (designated Z in Fig. 7) also appeared to contain more lipid in the homozygous animals, although the difference in the levels of this spot was not as great or as reproducible between wild-type and Cyp7^/ animals as that found for spots X and Y. Fur shaved from the abdomens of homozygous animals had a greater excess of spots X and Y relative to that obtained from the backs of the mice (Fig. 7).

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To determine the chemical structures of spots X and Y, these compounds were purified by thin-layer chromatography and subjected to mass spectrometry analyses as described under ``Experimental Procedures.'' An initial study revealed that saponification of spots X and Y yielded a similar pattern of lipids as determined by thin-layer chromatography on C₁₈ plates, indicating that the two spots contained structurally related esters. Chemical analyses indicated that spots X and Y each contained a mixture of monoglyceride esters of palmitate (16:0), stearate (18:0), and oleate (18:1). The position and stereochemistry of the double bond in the oleate moiety were not determined. Fatty acids were esterified to carbon 1 or 3 of glycerol, but not carbon 2. Analysis of saponified samples demonstrated the presence of the predicted free fatty acids. We postulate that esterification at position 1 versus 3 of glycerol may underlie the differences in the mobilities between spots X and Y. Finally, an excess of these same monoglyceride esters was detected in the stool of the knockout animals (data not shown).

DISCUSSION

The data of this paper support two surprising conclusions. First, 7-hydroxylase is not required in the mouse to maintain serum cholesterol and triglyceride levels in the normal range. Second, an alternate pathway of bile acid synthesis involving an oxysterol 7-hydroxylase is induced between the third and fourth weeks of life in this species.

An alteration in steady-state plasma cholesterol and triglyceride levels was not observed in the Cyp7^/ mice (Fig. 1). Plasma lipid levels were normal in newborn animals that lacked detectable levels of oxysterol 7-hydroxylase (i.e. animals less than 21 days old) and in older animals that contained this enzyme activity. At least two explanations may account for these findings. First, the response of the homeostatic regulatory mechanisms may be sufficient to maintain serum lipid and cholesterol levels even when key catabolic pathways are knocked out (7-hydroxylase) or not yet induced (oxysterol 7-hydroxylase). Second, alternate mechanisms that do not involve conversion of cholesterol into bile acids may exist that allow for cholesterol disposal and metabolism. Evidence to support both of these hypotheses is presented here.

In newborn Cyp7^/ animals, the synthesis of bile acids is presumed to be low to nonexistent. A direct test of this idea has not yet been possible due to technical difficulties in working with the tiny amounts of bile present in very young animals. Nevertheless, in the absence of bile acids, the solubilization of exogenous sterols and lipids should decrease, resulting in lower serum levels of these compounds. Thus, the normal levels of cholesterol and triglycerides in the young Cyp7^/ mice may reflect the absence of dietary lipids in the circulation and a subsequent compensation by the cholesterol supply pathways.

With respect to homeostasis in adult animals, although the total concentration of bile acids at ~12-16 weeks of age in the bile of older Cyp7^/ mice is similar to that in wild-type mice (Table III and Ref. 22), the types of bile acids are different, and their excretion in stool is reduced by 80% (Table IV). The altered composition of the bile acid pool may decrease the solubilization of dietary cholesterol and lipids, thus reducing the input of the exogenous (dietary) pathway to the steady state. Alternatively, the bile acid pool size may be compromised, as reflected by the reduced excretion of bile acids in stool, which in turn may decrease solubilization of dietary lipids. Since the concentration of bile acids in bile appears normal, another factor that regulates bile acid pool size such as the total volume of bile or the recycling efficiency of the enterohepatic circulation may be altered in the knockout mice. Finally, the modified composition may also lead to a more efficient excretion of cholesterol into bile, thereby maintaining cholesterol homeostasis. Future feeding studies carried out with individual bile acids together with cholesterol balance studies may shed light on these possible explanations.

With respect to alternate cholesterol and lipid disposal mechanisms, Cyp7^/ mice develop oily coats due to hypersecretion of lipids and sterols. This phenotype appears in newborn mice (10) at a time when bile acid biosynthesis is inferred to be very low based on the genetic absence of 7-hydroxylase and on the absence of induction of the oxysterol 7-hydroxylase enzyme (Fig. 6). In addition, the phenotype is reversed by dietary supplementation with bile acid (10). We postulate that in the absence of sufficient bile acids, the Cyp7^/ animals secrete lipids that then appear in their fur (Fig. 7). The secreted compounds are monoglyceride esters, which are not normally found in mouse fur (20, 21, 23). In the knockout animals, they may arise by direct excretion from the skin to the fur. Alternatively, their presence in the stool may lead to an indirect transfer to and accumulation in fur. Additional studies are currently being carried out to determine the biosynthetic origins of these lipids and how they accumulate in fur.

The spectrum of phenotypes associated with 7-hydroxylase deficiency largely disappears in animals that survive to postnatal day 21 (10). This reversal can now be explained by the induction of an alternate bile acid biosynthetic pathway beginning around the third week of life (Fig. 6). The alternate pathway most likely corresponds to the acidic or mitochondrial pathway described by Axelson and Sjövall (2). The first step in this pathway takes place in the mitochondria and is the conversion of cholesterol into cholest-5-ene-3,27-diol (27-hydroxycholesterol) by the sterol 27-hydroxylase enzyme (3). This oxysterol intermediate is then 7-hydroxylated by the oxysterol 7-hydroxylase to produce cholest-5-ene-3,7,27-triol, which is thereafter converted into a spectrum of C₂₄ bile acids (2). An oxysterol 7-hydroxylase enzyme activity has been previously characterized in pig liver (4), rat liver (5), rat ovary (6), and human fibroblasts (7). In the mouse, this enzyme activity is present in the liver (Fig. 3) and at lower levels in the kidney, brain, and ovary (data not shown). The murine enzyme is active against cholest-5-ene-3,25-diol (25-hydroxycholesterol), as are the pig and rat enzymes (4, 5), and is specifically inhibited by the product (Fig. 5).

The mouse oxysterol 7-hydroxylase activity is initially detected in the liver at 3 weeks of age, gradually increases until ~8 weeks of age, and thereafter remains constant (Fig. 6). The induction of oxysterol activity is correlated with survival of the Cyp7^/ animals (10), with a decrease in stool fat content (Fig. 2), with an increase in tissue stores of vitamins D₃ and E (Table II), and, in adult animals, with the occurrence of many species of 7-hydroxylated bile acids in gallbladder bile (Table III). These findings strongly suggest that the oxysterol 7-hydroxylase plays an active role in the biosynthesis of bile acids, which in turn participate in the metabolism of fat-soluble vitamins and lipids. The chemical reactions of the oxysterol 7-hydroxylase pathway remain to be worked out; however, analyses of the bile acids present in the bile and feces of Cyp7^/ mic