Up-regulation of transporters and enzymes by the vitamin D receptor ligands, 1alpha,25-dihydroxyvitamin D3 and vitamin D analogs, in the Caco-2 cell monolayer

Interplay between vitamin D and the drug metabolizing enzyme CYP3A4

Cytochrome P450 3A4 (CYP3A4) is a multifunctional enzyme involved in both xenobiotic and endobiotic metabolism. This review focuses on two aspects: regulation of CYP3A4 expression by vitamin D and metabolism of vitamin D by CYP3A4. Enterohepatic circulation of vitamin D metabolites and their conjugates will be also discussed. The interplay between vitamin D and CYP3A4 provides new insights into our understanding of how enzyme induction can contribute to vitamin D deficiency. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Role of nuclear receptors in the regulation of drug transporters in the brain

ATP-binding cassette membrane-associated drug efflux transporters and solute carrier influx transporters, expressed at the blood-brain barrier, blood-cerebrospinal fluid barrier, and in brain parenchyma, are important determinants of drug disposition in the central nervous system. Targeting the regulatory pathways that govern the expression of these transporters could provide novel approaches to selectively alter drug permeability into the brain. Nuclear receptors are ligand-activated transcription factors which regulate the gene expression of several metabolic enzymes and drug efflux/influx transporters. Although efforts have primarily been focused on investigating these regulatory pathways in peripheral organs (i.e., liver and intestine), recent findings demonstrate their significance in the brain. This review addresses the role of nuclear receptors in the regulation of drug transporter functional expression in the brain. An in-depth understanding of these pathways could guide the development of novel pharmacotherapy with either enhanced efficacy in the central nervous system or minimal associated neurotoxicity.

Temporal changes in tissue 1α,25-dihydroxyvitamin D3, vitamin D receptor target genes, and calcium and PTH levels after 1,25(OH)2D3 treatment in mice

The vitamin D receptor (VDR) maintains a balance of plasma calcium and 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3], its natural active ligand, by directly regulating the calcium ion channel (TRPV6) and degradation enzyme (CYP24A1), and indirectly regulating the parathyroid hormone (PTH) for feedback regulation of the synthetic enzyme CYP27B1. Studies that examined the intricate relationships between plasma and tissue 1,25(OH)2D3 levels and changes in VDR target genes and plasma calcium and PTH are virtually nonexistent. In this study, we investigated temporal correlations between tissue 1,25(OH)2D3 concentrations and VDR target genes in ileum and kidney and plasma calcium and PTH concentrations in response to 1,25(OH)2D3 treatment in mice (2.5 μg/kg ip, singly or q2d × 4). After a single ip dose, plasma 1,25(OH)2D3 peaked at ∼0.5 h and then decayed biexponentially, falling below basal levels after 24 h and then returning to baseline after 8 days. Upon repetitive ip dosing, plasma, ileal, renal, and bone 1,25(OH)2D3 concentrations rose and decayed in unison. Temporal profiles showed increased expressions of ileal Cyp24a1 and renal Cyp24a1, Mdr1/P-gp, and VDR but decreased renal Cyp27b1 mRNA after a time delay in VDR activation. Increased plasma calcium and attenuated PTH levels and increased ileal and renal Trpv6 expression paralleled the changes in tissue 1,25(OH)2D3 concentrations. Gene changes in the kidney were more sustained than those in intestine, but the magnitudes of change for Cyp24a1 and Trpv6 were lower than those in intestine. The data revealed that 1,25(OH)2D3 equilibrates with tissues rapidly, and VDR target genes respond quickly to exogenously administered 1,25(OH)2D3.

Vitamin D and drug-metabolising enzymes

Experimental studies on the molecular regulation of human drug metabolism have revealed that vitamin D up-regulates transcription of several key enzymes, such as CYP3A4, through the vitamin D receptor pathway in intestinal and hepatic cells. Recent data suggest that this results in seasonal changes with higher clearance of orally administered drugs during periods with high UV-B radiation and vitamin D levels. Taken together, vitamin D status might contribute to inter- and intraindividual differences in drug metabolism, but the therapeutic impact of these findings remains to be established.

CYP3A4 and seasonal variation in vitamin D status in addition to CYP2D6 contribute to therapeutic endoxifen level during tamoxifen therapy

Tamoxifen is a widely utilized adjuvant anti-estrogen agent for hormone receptor-positive breast cancer, known to undergo CYP2D6-mediated bioactivation to endoxifen. However, little is known regarding additional genetic and non-genetic determinants of optimal endoxifen plasma concentration. Therefore, 196 breast cancer patients on tamoxifen were enrolled in this prospective study over a 24-month period. Blood samples were collected for pharmacogenetic and drug-level analysis of tamoxifen and metabolites. Regression analysis indicated that besides CYP2D6, the recently described CYP3A4*22 genotype, seasonal variation, and concomitant use of CYP2D6-inhibiting antidepressants were significant predictors of endoxifen concentration. Of note, genetic variation explained 33 % of the variability while non-genetic variables accounted for 13 %. Given the proposed notion of a sub-therapeutic endoxifen concentration for predicting breast cancer recurrence, we set the therapeutic threshold at 18 nM, the 20th percentile for endoxifen level among enrolled patients in this cohort. Nearly 70 % of CYP2D6 poor metabolizers as well as extensive metabolizers on potent CYP2D6-inhibiting antidepressants exhibited endoxifen levels below 18 nM, while carriers of CYP3A4*22 were twofold less likely to be in sub-therapeutic range. Unexpectedly, endoxifen levels were 20 % lower during winter months than mean levels across seasons, which was also associated with lower vitamin D levels. CYP3A4*22 genotype along with sunshine exposure and vitamin D status may be unappreciated contributors of tamoxifen efficacy. The identified covariates along with demographic variables were integrated to create an endoxifen concentration prediction algorithm to pre-emptively evaluate the likelihood of individual patients falling below the optimal endoxifen concentration.

Regulation of the cytosolic sulfotransferases by nuclear receptors

The cytosolic sulfotransferases (SULTs) are a multigene family of enzymes that catalyze the transfer of a sulfonate group from the physiologic sulfate donor, 3'-phosphoadenosine-5'-phosphosulfate, to a nucleophilic substrate to generate a polar product that is more amenable to elimination from the body. As catalysts of both xenobiotic and endogenous metabolism, the SULTs are major points of contact between the external and physiological environments, and modulation of SULT-catalyzed metabolism can not only affect xenobiotic disposition, but it can also alter endogenous metabolic processes. Therefore, it is not surprising that SULT expression is regulated by numerous members of the nuclear receptor (NR) superfamily that function as sensors of xenobiotics as well as endogenous molecules, such as fatty acids, bile acids, and oxysterols. These NRs include the peroxisome proliferator-activated receptors, pregnane X receptor, constitutive androstane receptor, vitamin D receptor, liver X receptors, farnesoid X receptor, retinoid-related orphan receptors, and estrogen-related receptors. This review summarizes current information about NR regulation of SULT expression. Because species differences in SULT subfamily composition and tissue-, sex-, development-, and inducer-dependent regulation are prominent, these differences will be emphasized throughout the review. In addition, because of the central role of the SULTs in cellular physiology, the effect of NR-mediated SULT regulation on physiological and pathophysiological processes will be discussed. Gaps in current knowledge that require further investigation are also highlighted.

Drug-vitamin D interactions: a systematic review of the literature

Extensive media coverage of the potential health benefits of vitamin D supplementation has translated into substantial increases in supplement sales over recent years. Yet, the potential for drug-vitamin D interactions is rarely considered. This systematic review of the literature was conducted to evaluate the extent to which drugs affect vitamin D status or supplementation alters drug effectiveness or toxicity in humans. Electronic databases were used to identify eligible peer-reviewed studies published through September 1, 2010. Study characteristics and findings were abstracted, and quality was assessed for each study. A total of 109 unique reports met the inclusion criteria. The majority of eligible studies were classified as class C (nonrandomized trials, case-control studies, or time series) or D (cross-sectional, trend, case report/series, or before-and-after studies). Only 2 class C and 3 class D studies were of positive quality. Insufficient evidence was available to determine whether lipase inhibitors, antimicrobial agents, antiepileptic drugs, highly active antiretroviral agents, or H2 receptor antagonists alter serum 25(OH)D concentrations. Atorvastatin appears to increase 25(OH)D concentrations, whereas concurrent vitamin D supplementation decreases concentrations of atorvastatin. Use of thiazide diuretics in combination with calcium and vitamin D supplements may cause hypercalcemia in the elderly or those with compromised renal function or hyperparathyroidism. Larger studies with stronger study designs are needed to clarify potential drug-vitamin D interactions, especially for drugs metabolized by cytochrome P450 3A4 (CYP3A4). Healthcare providers should be aware of the potential for drug-vitamin D interactions.

3D structures and ligand specificities of nuclear xenobiotic receptors CAR, PXR and VDR

The nuclear receptors constitutive androstane receptor (CAR), pregnane X receptor (PXR) and vitamin D receptor (VDR) control a large array of genes that code for important proteins in humans including metabolic enzymes and transporters. 3D structures for the ligand-binding domain (LBD) of these receptors are abundantly available, providing valuable insights into the ligand-binding specificity as well as the activation mechanisms. The ligand-binding site of PXR is large and flexible, whereas those of CAR and VDR are compact and rigid, respectively. In general, the ligand profiles of the receptors are in agreement with the LBD structures. The crystal structures have greatly helped us to understand the promiscuity and/or specificity of CAR, PXR and VDR.

Impairment of bilirubin clearance and intestinal interleukin-6 expression in bile duct-ligated vitamin D receptor null mice

The vitamin D receptor (VDR) mediates the physiological and pharmacological actions of 1α,25-dihydroxyvitamin D₃ in bone and calcium metabolism, cellular growth and differentiation, and immunity. VDR also responds to secondary bile acids and belongs to the NR1I subfamily of the nuclear receptor superfamily, which regulates expression of xenobiotic metabolism genes. When compared to knockout mouse investigations of the other NR1I nuclear receptors, pregnane X receptor and constitutive androstane receptor, an understanding of the role of VDR in xenobiotic metabolism remains limited. We examined the effect of VDR deletion in a mouse model of cholestasis. We performed bile duct ligation (BDL) on VDR-null mice and compared blood biochemistry, mRNA expression of genes involved in bile acid and bilirubin metabolism, cytokine production, and expression of inflammatory regulators with those of wild-type mice. VDR-null mice had elevated plasma conjugated bilirubin levels three days after BDL compared with wild-type mice. Urine bilirubin levels and renal mRNA and/or protein expression of multidrug resistance-associated proteins 2 and 4 were decreased in VDR-null mice, suggesting impaired excretion of conjugated bilirubin into urine. While VDR-null kidney showed mRNA expression of interleukin-6 (IL-6) after BDL and VDR-null macrophages had higher IL-6 protein levels after lipopolysaccharide stimulation, the induction of intestinal Il6 mRNA expression and plasma IL-6 protein levels after BDL was impaired in VDR-null mice. Immunoblotting analysis showed that expression of an immune regulator, IκBα, was elevated in the jejunum of VDR-null mice, a possible mechanism for the attenuated induction of Il6 expression in the intestine after BDL. Increased expression of IκBα may be a consequence of compensatory mechanisms for VDR deletion. These results reveal a role of VDR in bilirubin clearance during cholestasis. VDR is also suggested to contribute to tissue-selective immune regulation.

1α,25-Dihydroxyvitamin D3-liganded vitamin D receptor increases expression and transport activity of P-glycoprotein in isolated rat brain capillaries and human and rat brain microvessel endothelial cells

Induction of the multidrug resistance protein 1 (MDR1)/P-glycoprotein (P-gp) by the vitamin D receptor (VDR) was investigated in isolated rat brain capillaries and rat (RBE4) and human (hCMEC/D3) brain microvessel endothelial cell lines. Incubation of isolated rat brain capillaries with 10 nM of the VDR ligand, 1α,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] for 4 h increased P-gp protein expression fourfold. Incubation with 1,25(OH)(2)D(3) for 4 or 24 h increased P-gp transport activity (specific luminal accumulation of NBD-CSA, the fluorescent P-gp substrate) by 25-30%. In RBE4 cells, Mdr1b mRNA was induced in a concentration-dependent manner by exposure to 1,25(OH)(2)D(3). Concomitantly, P-gp protein expression increased 2.5-fold and was accompanied by a 20-35% reduction in cellular accumulation of the P-gp substrates, rhodamine 6G (R6G), and HiLyte Fluor 488-labeled human amyloid beta 1-42 (hAβ(42)). In hCMEC/D3 cells, a 3 day exposure to 100 nM 1,25(OH)(2)D(3) increased MDR1 mRNA expression (40%) and P-gp protein (threefold); cellular accumulation of R6G and hAβ(42) was reduced by 30%. Thus, VDR activation up-regulates Mdr1/MDR1 and P-gp protein in isolated rat brain capillaries and rodent and human brain microvascular endothelia, implicating a role for VDR in increasing the brain clearance of P-gp substrates, including hAβ(42), a plaque-forming precursor in Alzheimer's disease.

Vitamin D and colon cancer

The most active vitamin D metabolite, 1α,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), is a pleiotropic hormone with wide regulatory actions. Classically, vitamin D deficiency was known to alter calcium and phosphate metabolism and bone biology. In addition, recent epidemiological and experimental studies support the association of vitamin D deficiency with a large variety of human diseases, and particularly with the high risk of colorectal cancer. By regulating the expression of many genes via several mechanisms, 1,25(OH)(2)D(3) induces differentiation, controls the detoxification metabolism and cell phenotype, sensitises cells to apoptosis and inhibits the proliferation of cultured human colon carcinoma cells. Consistently, 1,25(OH)(2)D(3) and several of its analogues decrease intestinal tumourigenesis in animal models. Molecular, genetic and clinical data in humans are scarce but they suggest that vitamin D is protective against colon cancer. Clearly, the available evidence warrants new, well-designed, large-scale trials to clarify the role of vitamin D in the prevention and/or therapy of this important neoplasia.

The SLCO1A2 gene, encoding human organic anion-transporting polypeptide 1A2, is transactivated by the vitamin D receptor

Organic anion-transporting polypeptide 1A2 (OATP1A2) (gene symbol, SLCO1A2) mediates cellular uptake of a wide range of endogenous substrates, as well as drugs and xenobiotics. OATP1A2 is expressed in several tissues, including apical membranes of small intestinal epithelial cells. Given its role in intestinal drug absorption, a detailed analysis of the mechanisms that regulate SLCO1A2 gene expression is potentially of great pharmacological relevance. We show here that treatment of human intestine-derived Caco-2 cells with vitamin D(3) markedly increased endogenous OATP1A2 mRNA and protein levels. Suppression of endogenous vitamin D receptor (VDR) expression with siRNAs significantly reduced this induction. Two alternative promoter regions exist in genomic databases for the SLCO1A2 gene. One putative VDR response element (VDRE) that was predicted to interact efficiently with VDR-retinoid X receptor α (RXRα) was identified in silico within SLCO1A2 promoter variant 1. This VDRE served as a strong binding site for the recombinant VDR-RXRα heterodimers in vitro and was potently activated by VDR in the presence of vitamin D(3) in heterologous promoter assays. In reporter assays using native promoter constructs, SLCO1A2 promoter variant 1 was strongly induced by VDR, and site-directed mutagenesis of a single VDRE within this region abolished this activation. Native VDR-RXRα also interacted with this element both in vitro and in living cells. We showed that expression of the SLCO1A2 gene is induced by vitamin D(3) at the transcriptional level through the VDR. Our results suggest that pharmacological administration of vitamin D(3) may allow modulation of intestinal absorption of OATP1A2 transport substrates.

Effects of 1α,25-dihydroxyvitamin D3 on transport and metabolism of adefovir dipivoxil and its metabolites in Caco-2 cells

Effects of 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3), natural ligand of the VDR, on the fates of adefovir dipivoxil (P-gp substrate) and its metabolites, mono(POM)-PMEA and adefovir (MRP4 substrate), were investigated in Caco-2 cells. After 1,25(OH)2D3-treatment, higher apical efflux of adefovir was observed after a 60 min incubation of adefovir divipoxil. Changes in these washout studies were predicted by a catenary model for the Caco-2 monolayer that described a higher MRP4 activity with 1,25(OH)2D3 treatment, as confirmed by Western blotting. Moreover, 1,25(OH)2D3 treatment (100 nM for 3 days) resulted in increased basolateral (B) to apical (A) (B-to-A) transport of adefovir dipivoxil but an unchanged A-to-B flux, rendering an elevated efflux ratio (EfR) (from 1.97 to 3.19). The EfR values in control and 1,25(OH)2D3-treated groups in these transport studies were reduced to 1.32 and 1.57, respectively, in the presence of verapamil (50 μM), the P-gp inhibitor. The B-to-A transport of the metabolite, adefovir, was increased in 1,25(OH)2D3-treated cells in the presence of verapamil, whereas the A-to-B and B-to-A transport of mono(POM)-PMEA remained unchanged. But the verapamil and 1,25(OH)2D3 treatments failed to alter rates of sequential metabolism of adefovir dipivoxil in cell lysate. The composite data established that 1,25(OH)2D3 treatment increased both P-gp and MRP4 transport activities without affecting the metabolism of adefovir dipivoxil by esterases. Moreover, an asymmetric appearance of metabolites, being higher with apical application, was observed. According to the catenary model, the asymmetry is suggestive that esterases are predominantly localized on the apical membrane and within the cell.

Predicting binding to p-glycoprotein by flexible receptor docking

P-glycoprotein (P-gp) is an ATP-dependent transport protein that is selectively expressed at entry points of xenobiotics where, acting as an efflux pump, it prevents their entering sensitive organs. The protein also plays a key role in the absorption and blood-brain barrier penetration of many drugs, while its overexpression in cancer cells has been linked to multidrug resistance in tumors. The recent publication of the mouse P-gp crystal structure revealed a large and hydrophobic binding cavity with no clearly defined sub-sites that supports an “induced-fit” ligand binding model. We employed flexible receptor docking to develop a new prediction algorithm for P-gp binding specificity. We tested the ability of this method to differentiate between binders and nonbinders of P-gp using consistently measured experimental data from P-gp efflux and calcein-inhibition assays. We also subjected the model to a blind test on a series of peptidic cysteine protease inhibitors, confirming the ability to predict compounds more likely to be P-gp substrates. Finally, we used the method to predict cellular metabolites that may be P-gp substrates. Overall, our results suggest that many P-gp substrates bind deeper in the cavity than the cyclic peptide in the crystal structure and that specificity in P-gp is better understood in terms of physicochemical properties of the ligands (and the binding site), rather than being defined by specific sub-sites.

With many drugs failing in the preclinical stages of drug discovery due to undesirable ADMETox (absorption, distribution, metabolism, excretion and toxicity) properties, improvement of these properties early on in the process, alongside the optimization of the compound activity, is emerging as a new focus in the pharmaceutical field. One of the key players affecting pharmacokinetic profiles of many clinically relevant compounds is an active efflux transporter, P-glycoprotein. Expressed predominantly at various physiological barriers, it can influence drug absorption (intestinal epithelium, colon), drug elimination (kidney proximal tubules) and drug penetration of the blood-brain barrier (endothelial brain cells). Moreover, its increased expression in cancer cells has been linked to resistance to multiple drugs in tumors. In this study we describe a computational approach that allows prediction of which compounds are more likely to interact with P-gp. We have tested the ability of this method to differentiate between binders and nonbinders of P-gp by using consistently measured in vitro experimental data. We also implemented a blind test on a series of peptidic cysteine protease inhibitors with encouraging outcome. Overall, our results suggest that this method provides a qualitative, quick, and inexpensive way of evaluating potential drug efflux problem at the early stages of drug development.

The effect of vitamin D3 and ketoconazole combination on VDR-mediated P-gp expression and function in human colon adenocarcinoma cells: implications in drug disposition and resistance

The vitamin D3 metabolite 1,25-dihydroxycholecalciferol (DHC) and analogues derived from it are being investigated as potential agents for the treatment of cancer. Combining ketoconazole (KTZ) with DHC has been recommended to enhance the anticancer activity of DHC. DHC exerts its biological activities through the vitamin D receptor (VDR). VDR is recognized to be a regulator of P-glycoprotein (P-gp), a member of the ABC transporter family well known for its role in multidrug resistance in cancer chemotherapy. We have investigated the effect of DHC and adding KTZ together with DHC on P-gp and VDR expression and the functional consequences of P-gp induction in intestinal human colonic adenocarcinoma cells LS174T cells. DHC increased P-gp expression by two times, and the addition of KTZ further increased the expression to four times. The combination of DHC + KTZ also significantly increased VDR expression, consistent with the enhanced increase in P-gp expression by this combination. The increase in P-gp expression was accompanied by increased P-gp function, as measured by decreased Rh123 accumulation in the LS174T cells. In addition, DHC significantly decreased colchicine cytotoxicity in a dose-sensitive manner, and the addition of KTZ further decreased the colchicine cytotoxicity, indicating the chemo-protective effect of DHC is enhanced by KTZ, consistent with the enhanced expression of P-gp. The results of this study raise the possibility that DHC and the addition of KTZ to DHC treatment may decrease the effectiveness of cancer chemotherapy by promoting P-gp-mediated drug resistance.

Seasonal variation in blood drug concentrations and a potential relationship to vitamin D

The most important enzyme in hepatic drug metabolism is cytochrome P450 3A4. Published in vitro data indicate that vitamin D may up-regulate the expression of the CYP3A4 gene. Individual vitamin D levels are highly dependent on sunlight exposure and show great seasonal variability in northern countries. The aim of the present study was to investigate whether plasma concentrations of CYP3A4 drug substrates exhibit seasonal changes compatible with a stimulatory effect of vitamin D on drug metabolism. Three immunosuppressants (tacrolimus, sirolimus, and cyclosporine) were analyzed, because these CYP3A4 drug substrates are subject to long-term use and repeated concentration determinations. In addition, mycophenolic acid was included in the analysis as a control drug independent of CYP3A4 metabolism. Concentration-to-dose ratios were extracted from the Karolinska Therapeutic Drug Monitoring database and compared between the 3-month periods of lowest and highest vitamin D levels. Sirolimus and tacrolimus levels showed seasonal variability that was highly consistent with changes in vitamin D; for example, significantly lower drug concentrations in July to September than in January to March. As expected, no significant difference was evident for mycophenolic acid, but this result was also the case with cyclosporine, possibly due to cross-reactivity of CYP3A4-mediated metabolites with the immunoassay used for quantification. In conclusion, there is cyclic variation in blood levels of important immunosuppressants throughout the year that correlates with UV light-dependent changes in vitamin D levels. Even though a causal relationship remains to be established, it is suggested that individual differences in vitamin D may contribute to variability in drug metabolism and disposition.

1α,25-dihydroxyvitamin D3 on intestinal transporter function: studies with the rat everted intestinal sac

Previous studies have shown that 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3) treatment (2.56 nmol/kg i.p. daily×4) increased PepT1, Mrp2, Mrp4, Asbt, but not Mdr1/P-gp in the rat small intestine. In this study, the intestinal everted sac technique, together with various select probes: mannitol (paracellular transport), glycylsarcosine (PepT1), 5(and 6)-carboxy-2',7'-dichlorofluorescein (CDF) diacetate (precursor of CDF for Mrp2), adefovir dipivoxil (precursor of adefovir for Mrp4) and digoxin (P-gp) was used to examine the functional changes of these transporters. After establishing identical permeabilities (Papp) of mannitol for the apical-to-basolateral (A-to-B) and basolateral-to-apical (B-to-A) directions at 20 min in 1,25(OH)2D3-treated vs. vehicle-treated duodenal, jejunal and ileal everted sacs, a significant enhancement of net A-to-B transport of glycylsarcosine in the duodenum, increased B-to-A transport of CDF and A-to-B and B-to-A transport of adefovir in the jejunum were observed with 1,25(OH)2 D3 treatment. However, the A-to-B and B-to-A transport of digoxin in the ileum was unchanged. These changes in transporter function in the rat intestinal everted sac corresponded well to changes in proteins that were observed previously. This study confirms that the rat intestinal PepT1, Mrp2 and Mrp4, but not P-gp are functionally induced by 1,25(OH)2D3 treatment via the vitamin D receptor (VDR).

Comparative effects of doxercalciferol (1α-hydroxyvitamin D₂) versus calcitriol (1α,25-dihydroxyvitamin D₃) on the expression of transporters and enzymes in the rat in vivo

Effects of 1.28 nmol/kg doxercalciferol [1α(OH)D₂], a synthetic vitamin D₂ analog that undergoes metabolic activation to 1α,25-dihydroxyvitamin D₂, the naturally occurring, biologically active form of vitamin D₂, on rat transporters and enzymes were compared with those of 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃, active form of vitamin D₃; 4.8 and 6.4 nmol/kg] given on alternate days intraperitoneally for 8 days. Changes were mostly confined to the intestine and kidney where the vitamin D receptor (VDR) was highly expressed: increased intestinal Cyp24 and Cyp3a1 messenger RNA (mRNA) and a modest elevation of apical sodium-dependent bile salt transporter (Asbt) and P-glycoprotein (P-gp) protein; increased renal VDR, Cyp24, Cyp3a9, Mdr1a, and Asbt mRNA, as well as Asbt and P-gp protein expression; and decreased renal PepT1 and Oat1 mRNA expression. In comparison, 1α(OH)D₂ treatment exerted a greater effect than 1,25(OH)₂D₃ on Cyp3a and Cyp24 mRNA. However, the farnesoid X receptor -related repressive effects on liver Cyp7a1 were absent because intestinal Asbt, FGF15 and portal bile acid concentrations were unchanged. Rats on the alternate day regimen showed milder changes and lessened signs of hypercalcemia and weight loss compared with rats receiving daily injections (similar or greater amounts of 0.64-2.56 nmol/kg daily ×4) described in previous reports, showing that the protracted pretreatment regimen was associated with milder inductive and lesser toxic effects in vivo.

The role of lithocholic acid in the regulation of bile acid detoxication, synthesis, and transport proteins in rat and human intestine and liver slices

The effects of the secondary bile acid, lithocholic acid (LCA), a VDR, FXR and PXR ligand, on the regulation of bile acid metabolism (CYP3A isozymes), synthesis (CYP7A1), and transporter proteins (MRP3, MRP2, BSEP, NTCP) as well as nuclear receptors (FXR, PXR, LXRα, HNF1α, HNF4α and SHP) were studied in rat and human precision-cut intestine and liver slices at the mRNA level. Changes due to 5 to 10 μM of LCA were compared to those of other prototype ligands for VDR, FXR, PXR and GR. LCA induced rCYP3A1 and rCYP3A9 in the rat jejunum, ileum and colon, rCYP3A2 only in the ileum, rCYP3A9 expression in the liver, and CYP3A4 in the human ileum but not in liver. LCA induced the expression of rMRP2 in the colon but not in the jejunum and ileum but did not affect rMRP3 expression along the length of the rat intestine. In human ileum slices, LCA induced hMRP3 and hMRP2 expression. In rat liver slices, LCA decreased rCYP7A1, rLXRα and rHNF4α expression, induced rSHP expression, but did not affect rBSEP or rNTCP expression; whereas in the human liver, a small but significant decrease was found for hHNF1α expression. These data suggests profound species differences in the effects of LCA on bile acid transport, synthesis and detoxification. An examination of the effects of prototype VDR, PXR, GR and FXR ligands showed that these pathways are all intact in precision cut slices and that LCA exerted VDR, PXR and FXR effects. The LCA-induced altered enzymes and transporter expressions in the intestine and liver would affect the disposition of drugs.

Effects of 1alpha,25-dihydroxyvitamin D3 on transporters and enzymes of the rat intestine and kidney in vivo

1alpha,25-Dihydroxyvitamin D3 (1,25(OH)2D3), the natural ligand of the vitamin D receptor (VDR), was found to regulate bile acid related transporters and enzymes directly and indirectly in the rat intestine and liver in vivo. The kidney is another VDR-rich target organ in which VDR regulation on xenobiotic transporters and enzymes is ill-defined. Hence, changes in protein and mRNA expression of nuclear receptors, transporters and enzymes of the rat intestine and kidney in response to 1,25(OH)2D3 treatment (0 to 2.56 nmol/kg/day intraperitoneally in corn oil for 4 days) were studied. In the intestine, protein and not mRNA levels of Mrp2, Mrp3, Mrp4 and PepT1 in the duodenum and proximal jejunum were induced, whereas Oat1 and Oat3 mRNA were decreased in the ileum after 1,25(OH)2D3 treatment. In the kidney, VDR, Cyp24, Asbt and Mdr1a mRNA and protein expression increased significantly (2- to 20-fold) in 1,25(OH)2D3-treated rats, and a 28-fold increase of Cyp3a9 mRNA but not of total Cy3a protein nor Cyp3a1 and Cyp3a2 mRNA was observed, implicating that VDR played a significant, renal-specific role in Cyp3a9 induction. Additionally, renal mRNA levels of PepT1, Oat1, Oat3, Ostalpha, and Mrp4, and protein levels of PepT1 and Oat1 were decreased in a dose-dependent manner, and the approximately 50% concomitant reduction in FXR, SHP, HNF-1alpha and HNF-4alpha mRNA expression suggests the possibility of cross-talk among the nuclear receptors. It is concluded that the effects of 1,25(OH)2D3 changes are tissue-specific, differing between the intestine and kidney which are VDR-rich organs.

1alpha,25-Dihydroxyvitamin D(3) triggered vitamin D receptor and farnesoid X receptor-like effects in rat intestine and liver in vivo

1alpha,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), a natural ligand of the vitamin D receptor (VDR), was found to increase the rat ileal Asbt and bile acid absorption. The effects of VDR, whose expression is low in liver, on hepatic transporters and enzymes are unknown. Protein and mRNA levels of target genes in the small intestine, colon and liver after intraperitoneal dosing of 1,25(OH)(2)D(3) (0-2.56 nmol/kg/day for 4 days) to the rat were determined by Western blotting and qPCR, respectively. The 1,25(OH)(2)D(3) treatment increased total Cyp3a protein and Cyp3a1 mRNA expressions in the proximal small intestine, and the short heterodimer partner (SHP), the fibroblast growth factor 15 (FGF15), organic solute transporter (Ostalpha and Ostbeta) mRNA and Asbt protein expressions in the ileum. About 50% higher portal bile acid concentration (65.1+/-14.9 vs 41.9+/-7.8 microm, p<0.05) and elevated expressions of the hepatic farnesoid X receptor (FXR) and SHP mRNA resulted with 1,25(OH)(2)D(3) treatment. Increased Bsep and Ostalpha mRNA expressions in liver and a>50% reduction in the Cyp7a1 protein level (p<0.05) and cholesterol metabolism in rat liver microsomes (p=0.002), likely consequences of the bile acid-FXR-SHP cascade and activation of the signaling pathway for Cyp7a1 inhibition by FGF15, were found. Increased hepatic multidrug resistance-associated protein (Mrp3) and multidrug resistance protein 1a (Mdr1a) mRNA and P-gp protein were also observed. It was concluded that the changes in hepatic transporters and enzymes in the rat were indirect, secondary effects of the liver FXR-SHP cascade due to increased intestinal absorption of bile acids and elevated levels of FGF15, events that led to the activation of FXR.

Vitamin D3 and its nuclear receptor increase the expression and activity of the human proton-coupled folate transporter

Folates are essential for nucleic acid synthesis and are particularly required in rapidly proliferating tissues, such as intestinal epithelium and hemopoietic cells. Availability of dietary folates is determined by their absorption across the intestinal epithelium, mediated by the proton-coupled folate transporter (PCFT) at the apical enterocyte membranes. Whereas transport properties of PCFT are well characterized, regulation of PCFT gene expression remains less elucidated. We have studied the mechanisms that regulate PCFT promoter activity and expression in intestine-derived cells. PCFT mRNA levels are increased in Caco-2 cells treated with 1,25-dihydroxyvitamin D(3) (vitamin D(3)) in a dose-dependent fashion, and the duodenal rat Pcft mRNA expression is induced by vitamin D(3) ex vivo. The PCFT promoter region is transactivated by the vitamin D receptor (VDR) and its heterodimeric partner retinoid X receptor-alpha (RXRalpha) in the presence of vitamin D(3). In silico analyses predicted a VDR response element (VDRE) in the PCFT promoter region -1694/-1680. DNA binding assays showed direct and specific binding of the VDR:RXRalpha heterodimer to the PCFT(-1694/-1680), and chromatin immunoprecipitations verified that this interaction occurs within living cells. Mutational promoter analyses confirmed that the PCFT(-1694/-1680) motif mediates a transcriptional response to vitamin D(3). In functional support of this regulatory mechanism, treatment with vitamin D(3) significantly increased the uptake of [(3)H]folic acid into Caco-2 cells at pH 5.5. In conclusion, vitamin D(3) and VDR increase intestinal PCFT expression, resulting in enhanced cellular folate uptake. Pharmacological treatment of patients with vitamin D(3) may have the added therapeutic benefit of enhancing the intestinal absorption of folates.