Intestinal CYP3A4 protects against lithocholic acid-induced hepatotoxicity in intestine-specific VDR-deficient mice.

Vitamin D receptor (VDR) mediates vitamin D signaling involved in bone metabolism, cellular growth and differentiation, cardiovascular function, and bile acid regulation. Mice with an intestine-specific disruption of VDR (VdrΔIEpC) have abnormal body size, colon structure, and imbalance of bile acid metabolism. Lithocholic acid (LCA), a secondary bile acid that activates VDR, is among the most toxic of the bile acids that when overaccumulated in the liver causes hepatotoxicity. Because cytochrome P450 3A4 (CYP3A4) is a target gene of VDR-involved bile acid metabolism, the role of CYP3A4 in VDR biology and bile acid metabolism was investigated. The CYP3A4 gene was inserted into VdrΔIEpC mice to produce the VdrΔIEpC/3A4 line. LCA was administered to control, transgenic-CYP3A4, VdrΔIEpC, and VdrΔIEpC/3A4 mice, and hepatic toxicity and bile acid levels in the liver, intestine, bile, and urine were measured. VDR deficiency in the intestine of the VdrΔIEpC mice exacerbates LCA-induced hepatotoxicity manifested by increased necrosis and inflammation, due in part to over-accumulation of hepatic bile acids including taurocholic acid and taurodeoxycholic acid. Intestinal expression of CYP3A4 in the VdrΔIEpC/3A4 mouse line reduces LCA-induced hepatotoxicity through elevation of LCA metabolism and detoxification, and suppression of bile acid transporter expression in the small intestine. This study reveals that intestinal CYP3A4 protects against LCA hepatotoxicity.

Serum samples were collected and mice were euthanized by CO 2 asphyxiation. Urine, liver, duodenum, jejunum, ileum, colon, and other tissue samples were harvested and stored at Ϫ 80°C for analysis.

Assessment of liver injury
For assessment of macroscopic liver damage, liver tissue was fl ushed with PBS and fi xed in 10% buffered formalin. Liver injury was scored by double-blinded analysis on a routine hematoxylin and eosin-stained section according to the morphological criteria previously described ( 18 ). Drug-induced liver injury was further evaluated by measuring alanine aminotransferase (ALT) and alkaline phosphatase (ALP) in serum. Briefl y, 2 l of serum was mixed with 200 l of ALT or ALP assay buffer (Catachem, Bridgeport, CT) in a 96-well microplate, and the oxidation of NADH to NAD + was monitored at 340 nm for 5 min.

Cholesterol and bile acid uptake measurements
Plasma levels of total cholesterol in serum and liver homogenate were measured in overnight-fasted mice using assay kits from Wako Diagnostics (Richmond, VA). Liver homogenate was extracted as follows: 20 mg liver was homogenized with 200 l extraction buffer [50 mM Tris/HCL, 1% Triton X-100 (v/v)]. One microliter of supernatant was used for the measurements as described for serum analysis according to the manufacturer's protocol (Wako USA, Richmond, VA). To measure the hepatic and enteric bile acid levels, 20 mg of frozen liver or mucosa cells scraped from the entire intestine were homogenized in 400 l of 75% ethanol, incubated at 50°C for 2 h, and then centrifuged . The supernatant (aqueous fraction) was retained, evaporated, and resuspended in 200 l of 0.9% saline. Twenty microliters were used for bile acid quantifi cation using the VetSpec bile acids kit (Catachem, Oxford, CT).

RNA analysis
Hepatic and intestinal RNA was extracted using TRIzol reagent (Invitrogen, Carlsbad, CA) and quantitative (q)PCR performed using cDNA generated from 1 g of total RNA with SuperScript II reverse transcriptase (Invitrogen). Primers for qPCR were designed using the Primer Express software (Applied Biosystems, Foster City, CA) and sequences are available upon request. qPCR reactions were carried out using SYBR Green PCR master mix (SuperArray, Frederick, MD) by using an ABI Prism 7900HT sequence detection system (Applied Biosystems). Values were quantitated using the comparative cycle threshold method, and results were normalized to mouse ␤ -actin.

Metabolomics analysis of urine, bile, and tissue homogenates
Urine samples were processed by mixing 40 l of urine with 160 l of 50% aqueous acetonitrile and centrifuging at 18,000 g for 10 min to remove protein and particulates. Liver tissues (100 mg) were homogenized on the ice, and 1 ml acetonitrile:water (50% v:v) was added for denaturation of protein. After multiple centrifugations, supernatants were collected. Bile samples were denatured by a 100-fold dilution with acetonitrile:water (66% v:v) and supernatants processed for liquid chromatography analysis. The homogenates of small intestine tissues (duodenum, jejunum, and ileum) were prepared and diluted with 1:20 acetonitrile:water (50% v:v). Supernatants were injected into an ultra-performance liquid chromatography (UPLC) system (Waters Corporation, Milford, MA) and metabolites separated by a gradient ranging from water to 95% aqueous acetonitrile containing 0.1% formic acid over a 10 min run. An Acquity UPLC TM BEH C18 column (Waters Corporation) was used to separate chemical components at 35°C. expression ( 11 ), which might be through regulation of sulfotransferase 2A1 to control bile acid-disrupted sterol homeostasis ( 12 ). Therefore, VDR might play a crucial role in regulation of bile acid metabolism ( 13 ).
Lithocholic acid (LCA), a monohydroxy secondary bile acid, is formed by bacterial 7-dehydroxylation of the primary bile acid, chenodeoxycholic acid, and of the secondary bile acid, ursodeoxycholic acid. The consensus emerged that the acute and chronic toxicity of LCA resulted from the administration of pharmacological loads of LCA ( 14 ). LCA also appears to induce its own detoxification in the liver. In addition, LCA is a potent activator of VDR ( 15 ). VDR controls transcription of the gene encoding sulfotransferase, an enzyme responsible for detoxifi cation of LCA in humans. It was suggested that LCA enters the colonic enterocyte, activates VDR, and thereby induces its own detoxifi cation ( 15 ). LCA was also shown to induce hepatic CYP3A4, a target gene regulated by VDR that is involved in bile acid metabolism, notably the oxidation of bile acids ( 16 ).
Due to the critical function of VDR in bile acid regulation and the unknown mechanism of bile acid accumulation in Vdr ⌬ IEpC mice, Vdr ⌬ IEpC mice and Vdr ⌬ IEpC mice expressing CYP3A4 were used to investigate the effect of VDR and CYP3A4 on LCA-induced hepatotoxicity and bile acid metabolism. The CYP3A4 transgene was inserted into Vdr ⌬ IEpC mice through crossing transgenic-CYP3A4 (Tg-3A4) mice to Vdr ⌬ IEpC mice to establish a new line designated Vdr ⌬ IEpC / 3A4. LCA was administered to these mice and hepatotoxicity and bile acid metabolism and transport evaluated; while metabolomics, a high-throughput technology platform to measure metabolic fl uctuations in exogenous and endogenous compounds ( 17 ), was employed to investigate bile fl ux changes in the gastrointestinal system. The results revealed that VDR defi ciency in the intestine increases LCAinduced liver necrosis due to over-accumulation of hepatic bile acids. CYP3A4 alleviates LCA-induced hepatotoxicity through the suppression of bile acid transporters and escalation of LCA detoxifi cation.

Generation of Vdr
⌬ IEpC /3A4 mice Vdr ⌬ IEpC and Tg-3A4 mice were housed in temperature-and light-controlled rooms and were given water and pelleted chow ad libitum. Vdr ⌬ IEpC founders were mated to Tg-3A4 mice and the litters were genotyped and divided into four groups: WT ( Vdr fl /fl ,cre Ϫ / 3A4 Ϫ ), Tg-3A4 ( Vdr fl /fl ,cre Ϫ /3A4 +) , Vdr ⌬ IEpC ( Vdr fl /fl ,cre+ /3A4 Ϫ ), and Vdr ⌬ IEpC /3A4 ( Vdr fl /fl ,cre+ /3A4 + ). All groups of mice used in this study were obtained from these same littermates. All animal experiments were carried out in accordance with the Institute of Laboratory Animal Resources guidelines and approved by the National Cancer Institute Animal Care and Use Committee.
were set at 120°C and 350°C, respectively. The capillary voltage and cone voltage were 3,000 and 20 V, respectively.

Western blotting
Mouse ileums were lysed with RIPA lysis buffer [150 mM NaCl, 0.5% Triton X-100, 50 mM Tris-HCl (pH 7.4), 25 mM NaF, 20 mM EGTA, 1 mM DTT, 1 mM Na 3 VO 4 , and protease inhibitor cocktail] for 30 min on ice, followed by centrifugation at 14,800 g for 15 min. Protein concentrations were measured with bicinchoninic acid (BCA) reagent. Protein (30-60 g) was electrophoresed on a 4-15% gradient Tris-HCl gel (Bio-Rad, Hercules, CA) and transferred onto a polyvinylidene difl uoride membrane in Tris-glycine buffer (pH 8.4) containing 20% methanol. The membrane was blocked with 5% fat-free dry milk in phosphatebuffered saline containing 0.1% Tween-20 (PBST) for 1 h. The membranes were probed with primary antibodies and horseradish peroxidase-conjugated secondary antibodies using standard Western blotting procedures. Proteins were visualized using the Femto signal chemiluminescent substrate (Pierce) under the image analyzer (Alpha Innotech Corp., San Leandro, CA). The antibody to mouse apical sodium-dependent bile acid transporter (ASBT) was generously supplied by Paul A. Dawson, Wake Forest University School of Medicine.

Statistics
Experimental values are expressed as mean ± SD. Statistical analysis was performed with two-tailed Student's t -tests with P < 0.05 considered statistically signifi cant.

LCA-induced hepatotoxicity is alleviated in Vdr ⌬ IEpC mice with CYP3A4 insertion
While the body weights of the Vdr ⌬ IEpC mice are low, the body weights of the Vdr ⌬ IEpC /3A4 mice are similar to the WT controls. The body weights of Vdr ⌬ IEpC mice ranged from 16 to 20 g at 8 weeks of age, while the body weights of Vdr ⌬ IEpC /3A4 mice ranged from 22 to 29 g, which is similar to the WT C57BL/6 mice on the same genetic background, thus suggesting that CYP3A4 rescues some of the abnormal phenotype of the Vdr ⌬ IEpC mice ( 8 ). To investigate bile acid metabolism in Vdr ⌬ IEpC /3A4 mice, LCA was administered for 4 days. LCA causes intrahepatic cholestasis and damage to the bile canaliculi in rodents ( 19 ). Histological results revealed that LCA caused severe hepatotoxicity with areas of necrosis of irregular distribution with some dilatation of centrolobular venules with infl ammatory and neutrophil infi ltration in WT, Tg-3A4, Vdr ⌬ IEpC , and Vdr ⌬ IEpC /3A4 mice ( Fig. 1A ). Liver injury was lower in Tg-3A4 mice compared with WT mice, and markedly reduced in Vdr ⌬ IEpC /3A4 mice compared with Vdr ⌬ IEpC mice as demonstrated by the reduced severity of lesion score ( P < 0.05), coagulation necrosis ( P < 0.05), biliary proliferation, inflammation cell infiltration, and portal inflammation (supplementary Fig. I) Hepatotoxicity was most severe in Vdr ⌬ IEpC mice compared with WT mice, and Vdr ⌬ IEpC /3A4 mice compared with Tg-3A4 mice. Liver and kidney weights relative to body weights were high in Vdr ⌬ IEpC mice, and these levels were lower in the Vdr ⌬ IEpC /3A4 mice. Tg-3A4 mice alone showed a slight but signifi cant decrease in liver The mobile phase fl ow rate was 0.5 ml/min with an aqueous acetonitrile gradient containing 0.1% formic acid over a 10 min run (0% acetonitrile for 0.5 min to 20% aceto nitrile by 5 min to 95% acetonitrile by 9 min, then equilibration at 100% water for 1 min before the next injection). The QTOF Premier TM mass spectrometer was operated in the positive electrospray ionization mode. Capillary voltage and cone voltage were maintained at 3 KV and 20 V, respectively. Source temperature and desolvation temperature were set at 120°C and 350°C, respectively. Nitrogen was used as both cone gas (50 l/h) and desolvation gas (600 l/h), and argon was used as collision gas. For accurate mass measurements, the TOFMS was calibrated with sodium formate solution (range m/z 100-1,000) and monitored by the intermittent injection of the lock mass sulfadimethoxine ([M+H] + = m/z 311.0814) in real-time. Mass chromatograms and mass spectral data were acquired and processed by MassLynx software (Waters Corporation) in centroid format.

Quantitation of metabolites
LCA, LCA metabolites, and bile acid standards were purchased from Sigma-Aldrich. Quantitation of bile acid metabolites was performed using an ACQUITY UPLC system coupled with a XEVO triple-quadrupole tandem mass spectrometer (Waters Corporation). The detection and quantitation of biomarkers were accomplished by multiple reaction monitoring mass spectrometry.

Metabolomic data analysis
Chromatographic and spectral data were deconvoluted by MarkerLynx software (Waters Corporation). A multivariate data matrix containing information on sample identity, ion identity (retention time and m/z ), and ion abundance was generated through centroiding, deisotoping, fi ltering, peak recognition, and integration. The intensity of each ion was calculated by normalizing the single ion counts versus the total ion counts in the whole chromatogram. The data matrix was further exported into SIMCA-P TM software (Umetrics, Kinnelon, NJ) and transformed by mean-centering and Pareto scaling, a technique that increases the importance of low abundance ions without signifi cant amplifi cation of noise. Principal components of serum were generated by principal components analysis (PCA) to represent the major latent variables in the data matrix and were described in a scores scatter plot.

In vitro incubation system for investigation of intestinal LCA phase I metabolism
Mouse intestinal microsomes obtained from WT and Tg-3A4 mice were prepared. The phase I incubation system (200 l) contained 50 mM Tris-HCl buffer solution (pH 7.4), 0.5 mg/ml mouse intestinal microsomes, 5 mM MgCl 2 , 100 M LCA, and 1 mM freshly prepared NADPH. After 0.5 h incubation at 37°C, the reaction was stopped using 200 l cold 50% aqueous methanol containing 5 M chlorpropamide. After centrifuging at 14,000 g for 20 min, a 5 l aliquot of the supernatant was injected into a UPLC-ESI-QTOFMS. An Acquity C18 BEH UPLC column (Waters Corporation) was employed to separate components in serum, urine, feces, and microsomal incubation samples. The mobile phase consisted of water containing 0.1% formic acid (A) and acetonitrile containing 0.1% formic acid (B). The following gradient condition was used: 100% A for 0.5 min, increased to 100% B over the next 7.5 min, and returned to 100% A in the last 2 min. The fl ow rate of the mobile phase was set at 0.5 ml/min. Data were collected in negative ion mode on a Waters QTOF Premier mass spectrometer, which was operated in full-scan mode at m/z 50-850. Nitrogen was used as both cone gas (50 l/h) and desolvation gas (600 l/h). Source desolvation temperatures bile acid pool in the Vdr ⌬ IEpC /3A4 mice is not known; it may be due to increased metabolism of bile acid intermediates by CYP3A4. Thus, LCA-induced hepatotoxicity might be aggravated in Vdr ⌬ IEpC mice through induction of hepatic cholesterol and bile acid synthesis, and this is alleviated by intestinal detoxifi cation by CYP3A4.

Gene and urinary metabolomic profi les reveal activation of CYP3A by LCA administration
Real-time qPCR analysis of LCA-treated mice showed that CYP3A4 mRNA is highly expressed in the small intestine but not in the liver of Tg-3A4 and Vdr ⌬ IEpC /3A4 mice ( Fig. 3 ). VDR and pregnane X receptor (PXR), known inducers of CYP3A4 ( 20 ), were also examined. While CYP3A4 was expressed at lower levels in the duodenum of Vdr ⌬ IEpC /3A4 mice as compared with Tg-3A4 mice, there was not much difference in the expression in other regions of the intestine. Vdr mRNA was similarly expressed at slightly higher levels in Tg-3A4 mice compared with Vdr ⌬ IEpC /3A4 mice only in the duodenum; there were no signifi cant differences in Pxr and farnesoid X receptor ( Fxr ) mRNA expression between the four treated groups, although they had higher levels in the liver compared with the intestine ( Fig. 3 ). The role of CYP3A4 in LCA metabolism was then investigated with microsomes prepared from the intestine. Upon treatment with LCA, Vdr ⌬ IEpC mice with or without the CYP3A4 gene had increased ALT and ALP values compared with WT and Tg-3A4 mice. The Vdr ⌬ IEpC /3A4 mice had decreased ALT and ALP compared with Vdr ⌬ IEpC mice, although there was a tendency toward lower without signifi cant differences of ALT and ALP between control and Tg-3A4 mice ( Fig. 2 ). These data suggested that intestinal VDR defi ciency results in greater LCA hepatoxicity and that CYP3A4 alleviates the hepatotoxicity induced by LCA in the intestinal Vdr -null background . Due to the potential effect of LCA hepatic metabolism, cholesterol and bile acid levels in liver and serum were measured. There were no signifi cant differences in serum cholesterol levels between the four groups. In contrast, hepatic cholesterol was signifi cantly increased in Vdr ⌬ IEpC mice, and decreased in Vdr ⌬ IEpC /3A4 mice ( Fig. 2 ). Similarly, hepatic bile acids were increased in Vdr ⌬ IEpC mice and decreased in Vdr ⌬ IEpC /3A4 mice. However, enteric bile acids extracted from epithelial cells scraped from the entire intestine showed no signifi cant difference between these mice , despite the slight increase of bile acids in Vdr ⌬ IEpC mice compared with the other groups ( Fig. 2 ). Moreover, the bile acid pool size was higher in Vdr ⌬ IEpC mice compared with Vdr ⌬ IEpC /3A4 mice, and the bile acid pool size in both groups of mice was higher than control and Tg-3A4 mice, while there was no signifi cant difference between control and Tg-3A4 mice ( Fig. 2 ). The mechanism for the lower  showed contribution ions I, II, III, and IV in positive mode were determined to be LCA, lithocholate-O-glucuronide, LCA glycine conjugate, and hydroxylated LCA, respectively, and ions V and VI in negative mode (supplementary Fig. IIIB) were lithocholyltaurine and taurine, respectively, as revealed through MS/MS fragmentation and structure identifi cation. These data indicate that the production of LCA metabolites through oxidation and conjugation is increased in Vdr ⌬ IEpC /3A4 mice compared with Vdr ⌬ IEpC mice, likely due to the involvement of CYP3A4. It was reported that increased LCA and LCA metabolites are correlated with elevated levels of bile acids (20)(21)(22). To investigate the bile acid constituents, metabolomic analysis of liver and gallbladder bile was carried out. PCA of the hepatic metabolomes revealed a clear separation between control and Tg-3A4 mice, and Vdr ⌬ IEpC and Vdr ⌬ IEpC /3A4 mice ( Fig. 4A ). Moreover, loading plots from the PCA revealed that TDCA, taurocholic acid (TCA), tauro-␣muricholic acid (T-␣ -MCA), and tauro-chenodeoxycholic M-2) were identifi ed, and a signifi cant increase ( P < 0.001) was detected for the formation M-1 in Tg-3A4 mice as compared to WT mice. An elevation trend was also observed for M-2 in Tg-3A4 mice (supplementary Fig. IIB). These data indicate that increased expression of CYP3A4 in Tg-3A4 stimulated metabolism of LCA in the intestine.

Metabolomic analysis of liver homogenate, small intestine homogenate, gallbladder, and urine
It was reported that increased LCA and LCA metabolites are correlated with elevated levels of bile acids (20)(21)(22). To investigate bile acid constituents, metabolomics was carried out on the four mouse groups treated with LCA. PCA revealed clear separation between urine metabolites in the control and treated group in positive or negative ionization modes in Vdr ⌬ IEpC and Vdr  /3A4 mice, as well as bile acid pool size in these four treated mice. * P < 0.05 compared with control in same group; ** P < 0.01 compared with control in same group; # P < 0.05 compared with Vdr ⌬ IEpC in same group.

Expression of genes responsible for bile acid synthesis and transport
Although intestinal CYP3A4 detoxifi ed LCA and reduced bile acid over-accumulation, bile acid accumulation might also infl uence bile acid synthesis and transport. Thus, expression from multiple transport genes, including Asbt , organic solute transporters ␣ and ␤ ( Ost ␣ and Ost ␤ ), multidrug resistance-associated proteins ( Mrp1 , Mrp2 , and Mrp3 ), and organic anion transporting polypeptides ( Oatp1 , Oatp2 , and Oatp4 ), in three regions of the intestine were evaluated by qPCR measurement of mRNAs ( Fig. 5 ( Fig. 4C ).
Tissue metabolomic analysis was carried out on intestine homogenates from WT and Tg-3A4 mice, and a similar trend, as noted with liver homogenates, was found in intestine homogenates, including a PCA separation trend between the two mouse lines (supplementary Fig. IV) and increased TCA levels in the duodenum and jejunum with a trend toward increase in the ileum (supplementary Fig. V).
These results indicate that the high abundance of TCA and TDCA might be correlated with increased bile acid synthesis and metabolism and resulted in accumulation in Vdr ⌬ IEpC mice and increased LCA-induced hepatotoxicity. Detoxifi cation of LCA in Vdr ⌬ IEpC /3A4 mice might be through LCA metabolism, and thus decreased the effect of LCA on specifi c bile acid accumulation in the liver and the circulation. Furthermore, PCA of the gallbladder bile revealed a clear separation between control and Tg-3A4 mice and Vdr ⌬ IEpC and Vdr ⌬ IEpC /3A4 mice ( Fig. 4B ). Moreover, loading plots from PCA revealed that TDCA, TCA, T-␣ -MCA, and T-CDCA are the four major ions contributing to the distinctive clustering of Vdr ⌬ IEpC from Vdr ⌬ IEpC /3A4 mice in negative mode, and the results were correlated with liver metabolomic analysis. Oatp2 demonstrated higher expression in Vdr ⌬ IEpC /3A4 mice compared with Vdr ⌬ IEpC mice. Because Bsep is responsible for bile acid effl ux from the liver to the gallbladder, Ntcp and Oatp2 are involved in bile acid infl ux from the circulation to the liver; these data suggest that the decreased synthesis of bile acids by CYP7A1 in Vdr ⌬ IEpC /3A4 liver, including hepatic Cyp8b1 and Cyp7a1 , bile salt export pump ( Bsep ), and Na + -taurocholate cotransporting polypeptide ( Ntcp ), were also examined. The results demonstrated higher expression of Bsep and Ntcp mRNAs in Vdr ⌬ IEpC /3A4 mice and a low expression level of Cyp7a1 in Vdr ⌬ IEpC /3A4 mice compared with Vdr ⌬ IEpC mice. Hepatic increase bile acid uptake by the intestine and lead to hepatic over-accumulation of bile acids in Vdr ⌬ IEpC mice as a result of LCA administration. CYP3A4 can metabolize LCA, and perhaps other bile acids, and reduce the accumulation of toxic bile acids in the liver. CYP3A4 is not signifi cantly expressed in the livers of Tg-3A4 and Vdr ⌬ IEpC /3A4 mice, and highly expressed in the intestine, in agreement with the low expression in the liver and high expression in the intestine system reported earlier for the CYP3A4-humanized mouse line ( 27 ). Therefore, any LCA metabolism by CYP3A4 is largely carried out in the intestine of Vdr ⌬ IEpC /3A4 mice and not in the liver. LCA is largely excreted in the bile and feces as a sulfate conjugate ( 28 ). LCA can also be conjugated with taurine or glycine and excreted in feces or urine. Thus, LCA-induced toxicity could be decreased through increased conjugation with taurine and sulfate. It is likely that the conjugates are metabolized by bacterial bile acid hydrolase in the intestine leading to free LCA that is recirculated to the liver. However, intestinal CYP3A4 could convert LCA into hydroxylated LCA, which could further facilitate conjugation and sulfation. Oxidation of LCA by intestinal CYP3A4 could reduce circulating bile acids and alleviate LCA-induced hepatotoxicity in the Vdr ⌬ IEpC /3A4 mice.
Bile and liver metabolomics revealed that taurocholic acid and TDCA, two major bile acids, were increased in the liver of Vdr ⌬ IEpC mice. Urinary metabolomics demonstrated increased lithocholate-O-glucuronide, LCA acid mice might be responsible for the decreased hepatic bile acids in Vdr ⌬ IEpC /3A4 mice compared with Vdr ⌬ IEpC mice. There were no signifi cant differences in expression of Fxr and small heterodimer partner ( Shp ) between these four groups ( Fig. 6 ). Results of gene expression analysis thus suggested that the lower bile acid accumulation in Vdr ⌬ IEpC /3A4 mice might be due to increased bile acid effl ux and reduced bile acid synthesis in the liver. This change might be correlated to CYP3A4 over-metabolism of LCA and LCA detoxifi cation in Vdr ⌬ IEpC /3A4 mice.

DISCUSSION
The results of this study are summarized and the potential role of intestinal CYP3A4 in bile acid metabolism and protection against bile acid toxicity is illustrated in Fig. 7 . In the intestine-specifi c VDR-defi cient mouse, LCA-induced hepatotoxicity is more severe than in WT mice due to overaccumulation of bile acids in the liver. It was reported that reduced VDR can lead to calcium malabsorption ( 6 ), and that decreased calcium could then damage intestinal epithelia (23)(24)(25). Intestinal permeability is a key factor regulating bile acid transport and expression of target proteins ( 26 ). Defi ciency of VDR in the intestine of Vdr ⌬ IEpC mice leads to increased LCA toxicity in the liver after its reuptake from the intestine. Changes in intestinal structure could Bile acid transport in the enterocytes basically consists of three components: 1 ) apical uptake of bile acids in the enterocytes largely facilitated by the uptake of conjugated bile acids in the terminal ileum via a Na + -dependent mechanism, such as ASBT; 2 ) intracellular bile acid transport in the enterocytes is mediated by cytosolic IBABP; and 3 ) an anion exchange mechanism for the basolateral effl ux of bile acids from enterocytes, such as OST ␣ and OST ␤ . The ileal bile acid transport system offers enormous potential as a valuable target for cholesterol/bile acid lowering glycine conjugate, and hydroxylated LCA, and reduced taurine conjugates in Vdr ⌬ IEpC /3A4 mice, which indicated increased metabolism and detoxifi cation of LCA in Vdr ⌬ IEpC /3A4 mice. In chronic cholestatic liver disease, hydrophobic and potentially cytotoxic bile acids accumulate in the liver ( 29 ). Thus, hepatic accumulation of taurocholic acid and TDCA and impaired metabolism of LCA in Vdr ⌬ IEpC mice might contribute to the tissue degeneration in cholestatic liver disease due to the detergent effects of these bile acids.  therapy. Specifi c transporters expressed in the liver and the intestine play a critical role in driving the enterohepatic circulation of bile acids (Fig. 7) . Enterohepatic circulation of bile acids is fundamentally composed of two major processes, secretion from the liver and absorption from the intestine ( 30 ). In hepatocytes, the vectorial transport of bile acids from blood to bile is carried out by NTCPs and OATPs. Bile acids are then transported into the canaliculus via BSEPs and MRPs and delivered to the intestinal lumen where they are reabsorbed into enterocytes. Intestinal epithelial cells reabsorb the majority of the secreted bile acids through ASBTs and OATPs ( 31 ). Increased portal bile acid absorption as a result of elevated ASBTs in the rat in vivo by 1,25(OH) 2 D 3 treatment could trigger indirect or secondary changes in hepatic transporters and enzymes due to the activation or inhibition of other nuclear receptors ( 32 ). The present results revealed decreased expression of Oatps , Asbt , and Ost ␣ in the intestine of Vdr ⌬ IEpC /3A4 mice compared with Vdr ⌬ IEpC mice. These data indicate that the absorption of bile acids in the intestine of Vdr ⌬ IEpC /3A4 mice is lower than in Vdr ⌬ IEpC mice. However, in the liver, there was an elevation of Bsep mRNA in Vdr ⌬ IEpC /3A4 mice indicating increased canalicular membrane effl ux, and higher expression of Oatp2 and Ntcp mRNAs indicating increased hepatic bile acid uptake and effl ux. Therefore, lowered hepatic bile acid accumulation in Vdr ⌬ IEpC /3A4 mice might not be due to increased hepatic bile acid uptake leading to less bile acid accumulation, but due to lower enterohepatic bile acid circulation in these mice as a result of lower expression of Oatps , Asbt , and Ost ␣ . The mechanisms for the changes in expression of these transporters are not known and require further investigation.
In summary, the working model is that loss of intestinal VDR decreases intestinal CYP3A expression. Decreased intestinal CYP3A4 expression reduces LCA metabolism and increases the expression of intestinal transporters, which is likely not direct but due to altered bile acid metabolism and increased intestinal bile acid transport. This results in decreased bile acid loss, increased bile acid absorption into the enterohepatic circulation, and increased hepatic bile acid exposure which contributes to the increased hepatic injury. Intestine-specifi c VDR defi ciency exacerbates LCA-induced liver cholestasis due to over-accumulation of hepatic bile acids while CYP3A4 alleviates LCA-induced hepatotoxicity through increased LCA detoxifi cation and increased effl ux of bile acids. Because intestinal VDR may contribute to the maintenance of intestinal barrier function, intestinal overexpression of CYP3A4 would be protective by detoxifying LCA and protecting intestinal barrier function. The Vdr ⌬ IEpC and Vdr ⌬ IEpC /3A4 mouse models could be valuable tools to investigate VDR regulation of bile acids, as well as coordination of VDR and CYP3A4 on detoxifi cation of bile acid-induced toxicity in the gastrointestinal system.
The authors thank Shigeaki Kato, Tokyo University, for the Vdrfl oxed mice, and Paul A. Dawson, Wake Forest University School of Medicine, for the generous gift of the antibody to mouse ASBT.