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

Effect of X-ray irradiation on renal excretion of bestatin through down-regulating organic anion transporters via the vitamin D receptor in rats

Pharmacokinetic changes induced by radiation following radiotherapy ("RT-PK" phenomenon) are of great significance to the effectiveness and safety of chemotherapeutic agents in clinical settings. The aims of this study were to clarify the organic anion transporters (Oats) involved in the "RT-PK" phenomenon of bestatin in rats following X-ray irradiation and to elucidate its potential mechanism via vitamin D signalling. Pharmacokinetic studies, uptake assays using rat kidney slices and primary proximal tubule cells, and molecular biological studies were performed. Significantly increased plasma concentrations and systemic exposure to bestatin were observed at 24 and 48 h following abdominal X-ray irradiation, regardless of oral or intravenous administration of the drugs in rats. Reduced renal clearance and cumulative urinary excretion of bestatin were observed at 24 and 48 h post-irradiation in rats following intravenous administration. The uptake of the probe substrates p-aminohippuric acid and oestrone 3-sulphate sodium in vitro and the expression of Oat1 and Oat3 in vivo were reduced in the corresponding models following irradiation. Moreover, the upregulation of the vitamin D receptor (Vdr) in mRNA and protein levels negatively correlated with the expressions and functions of Oat1 and Oat3 following irradiation. Additionally, elevated plasma urea nitrogen levels and histopathological changes were observed in rats after exposure to irradiation. The "RT-PK" phenomenon of bestatin occurs in rats after exposure to irradiation, possibly resulting in the regulation of the expressions and activities of renal Oats via activation of the Vdr signalling pathway.

The Impact of Suboptimal 25-Hydroxyvitamin D Levels and Cholecalciferol Replacement on the Pharmacokinetics of Oral Midazolam in Control Subjects and Patients With Chronic Kidney Disease

The aim of this study was to investigate the impact of suboptimal 25-hydroxyvitamin D (25-VitD) and cholecalciferol (VitD3 ) supplementation on the pharmacokinetics of oral midazolam (MDZ) in control subjects and subjects with chronic kidney disease (CKD). Subjects with CKD (n = 14) and controls (n = 5) with suboptimal 25-VitD levels (<30 ng/mL) were enrolled in a 2-phase study. In phase 1 (suboptimal), subjects were administered a single oral dose of VitD3 (5000 IU) and MDZ (2 mg). In phase 2 (replete) subjects who achieved 25-VitD repletion after receiving up to 16 weeks of daily cholecalciferol were given the identical single oral doses of VitD3 and MDZ as in phase 1. Concentrations of MDZ and metabolites, 1'-hydroxymidazolam (1'-OHMDZ), and 1'-OHMDZ glucuronide (1'-OHMDZ-G) were measured by liquid chromatography-tandem mass spectrometry and pharmacokinetic analysis was performed. Under suboptimal 25-VitD, reductions in MDZ clearance and renal clearance of 47% and 87%, respectively, and a 72% reduction in renal clearance of 1'-OHMDZ-G were observed in CKD vs controls. In phase 1 versus phase 2, MDZ clearance increased in all control subjects, with a median (interquartile range) increase of 10.5 (0.62-16.7) L/h. No changes in MDZ pharmacokinetics were observed in subjects with CKD between phases 1 and 2. The effects of 25-VitD repletion on MDZ disposition was largely observed in subjects without kidney disease. Impaired MDZ metabolism and/or excretion alterations due to CKD in a suboptimal 25-VitD state may not be reversed by cholecalciferol therapy. Suboptimal 25-VitD may augment the reductions in MDZ and 1'-OHMDZ-G clearance values observed in patients with CKD.

Significance of the Vitamin D Receptor on Crosstalk with Nuclear Receptors and Regulation of Enzymes and Transporters

The vitamin D receptor (VDR), in addition to other nuclear receptors, the pregnane X receptor (PXR) and constitutive androstane receptor (CAR), is involved in the regulation of enzymes, transporters and receptors, and therefore intimately affects drug disposition, tissue health, and the handling of endogenous and exogenous compounds. This review examines the role of 1α,25-dihydroxyvitamin D₃ or calcitriol, the natural VDR ligand, on activation of the VDR and its crosstalk with other nuclear receptors towards the regulation of enzymes and transporters, notably many of the cytochrome P450s including CYP3A4 and sulfotransferase 2A1 (SULT2A1) as well as cholesterol 7α-hydroxylase (CYP7A1). Moreover, the VDR upregulates the intestinal channel, TRPV6, for calcium absorption, LDL receptor-related protein 1 (LRP1) and receptor for advanced glycation end products (RAGE) in brain for β-amyloid peptide efflux and influx, the sodium phosphate transporters (NaP_i), the apical sodium-dependent bile acid transporter (ASBT) and organic solute transporters (OSTα-OSTβ) for bile acid absorption and efflux, respectively, the renal organic anion transporter 3 (OAT3) and several of the ATP-binding cassette protein transporters-the multidrug resistance protein 1 (MDR1) and the multidrug resistance-associated proteins (MRPs). Hence, the role of the VDR is increasingly being recognized for its therapeutic potential and pharmacologic activity, giving rise to drug-drug interactions (DDI). Therapeutically, ligand-activated VDR shows anti-inflammatory effects towards the suppression of inflammatory mediators, improves cognition by upregulating amyloid-beta (Aβ) peptide clearance in brain, and maintains phosphate, calcium, and parathyroid hormone (PTH) balance and kidney function and bone health, demonstrating the crucial roles of the VDR in disease progression and treatment of diseases.

Vitamin D Deficiency Impacts Exposure and Response of Pravastatin in Male Rats by Altering Hepatic OATPs

This study aimed to determine the effect of vitamin D (VD) deficiency on the efficacy and pharmacokinetics of pravastatin and clarify whether the effects are mediated by Organic anion-transporting polypeptides (OATPs). Experiments were conducted in rats to explore the effect of VD deficiency on the pharmacodynamics and pharmacokinetics of pravastatin. In the pharmacodynamic study, rats were fed a VD-free or VD-supplement high-fat diet for 25–30 days, and plasma 25(OH)VD was dynamically monitored. The response of pravastatin (changes in blood lipids) on rats were then examined after 15 days of pravastatin treatment. In the pharmacokinetic study, rats were fed a VD-free or VD-supplement diet for 25–30 days. The pharmacokinetics of single oral dose pravastatin was then studied, and intestinal and hepatic Oatp1a1 and Oatp2b1 expression was determined using quantitative polymerase chain reaction (qPCR) and western blot. Furthermore, OATP1B1 and OATP2B1 expression in Huh7 cells with or without 1.25(OH)₂D were assessed via qPCR and western blot. For the pharmacodynamic study, the decrease of total cholesterol and increase of high-density lipoprotein cholesterol in VD-deficient rats were smaller than in VD-sufficient rats, indicating that VD deficiency reduced the response of pravastatin in rats. For the pharmacokinetic study, the plasma exposure slightly increased, and liver exposure decreased in VD-deficient rats, but not significantly. VD deficiency decreased the Oatp1a1 and Oatp2b1 expression in the liver, but not in the small intestine. Similarly, OATP1B1 and OATP2B1 protein levels in Huh7 cells were reduced when 1.25(OH)₂D was absent. In conclusion, VD deficiency can decrease the response of pravastatin in rats by reducing the liver pravastatin exposure and expression of hepatic OATPs, consistent with the extended hepatic clearance model theory.

Regulation of Solute Carriers OCT2 and OAT1/3 in the Kidney: A Phylogenetic, Ontogenetic and Cell Dynamic Perspective

Over the course of more than 500 million years, the kidneys have undergone a remarkable evolution from primitive nephric tubes to intricate filtration-reabsorption systems that maintain homeostasis and remove metabolic end products from the body. The evolutionarily conserved solute carriers Organic Cation Transporter 2 (OCT2), and Organic Anion Transporters 1 and 3 (OAT1/3) coordinate the active secretion of a broad range of endogenous and exogenous substances, many of which accumulate in the blood of patients with kidney failure despite dialysis. Harnessing OCT2 and OAT1/3 through functional preservation or regeneration could alleviate the progression of kidney disease. Additionally, it would improve current in vitro test models that lose their expression in culture. With this review, we explore OCT2 and OAT1/3 regulation using different perspectives: phylogenetic, ontogenetic and cell dynamic. Our aim is to identify possible molecular targets to both help prevent or compensate for the loss of transport activity in patients with kidney disease, and to enable endogenous OCT2 and OAT1/3 induction in vitro in order to develop better models for drug development.

Effects of 1α,25-Dihydroxyvitamin D3 on the Pharmacokinetics of Procainamide and Its Metabolite N-Acetylprocainamide, Organic Cation Transporter Substrates, in Rats with PBPK Modeling Approach

In this study, possible changes in the expression of rat organic cationic transporters (rOCTs) and rat multidrug and toxin extrusion proteins (rMATEs) following treatment with 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) were investigated. Rats received intraperitoneal administrations of 1,25(OH)₂D₃ for four consecutive days, and the tissues of interest were collected. The mRNA expression of rOCT1 in the kidneys was significantly increased in 1,25(OH)₂D₃-treated rats compared with the control rats, while the mRNA expressions of rOCT2 and rMATE1 in the kidneys, rOCT1 and N-acetyltransferase-II (NAT-II) in the liver, and rOCT3 in the heart were significantly decreased. Changes in the protein expression of hepatic rOCT1 and renal rOCT2 and rMATE1 were confirmed by western blot analysis. We further evaluated the pharmacokinetics of procainamide (PA) hydrochloride and its major metabolite N-acetyl procainamide (NAPA) in the presence of 1,25(OH)₂D₃. When PA hydrochloride was administered intravenously at a dose 10 mg/kg to 1,25(OH)₂D₃-treated rats, a significant decrease in renal and/or non-renal clearance of PA and NAPA was observed. A physiological model for the pharmacokinetics of PA and NAPA in rats was useful for linking changes in the transcriptional and translational expressions of rOCTs and rMATE1 transporters to the altered pharmacokinetics of the drugs.

The Segregated Intestinal Flow Model (SFM) for Drug Absorption and Drug Metabolism: Implications on Intestinal and Liver Metabolism and Drug-Drug Interactions

The properties of the segregated flow model (SFM), which considers split intestinal flow patterns perfusing an active enterocyte region that houses enzymes and transporters (<20% of the total intestinal blood flow) and an inactive serosal region (>80%), were compared to those of the traditional model (TM), wherein 100% of the flow perfuses the non-segregated intestine tissue. The appropriateness of the SFM model is important in terms of drug absorption and intestinal and liver drug metabolism. Model behaviors were examined with respect to intestinally (M1) versus hepatically (M2) formed metabolites and the availabilities in the intestine (F_I) and liver (F_H) and the route of drug administration. The %contribution of the intestine to total first-pass metabolism bears a reciprocal relation to that for the liver, since the intestine, a gateway tissue, regulates the flow of substrate to the liver. The SFM predicts the highest and lowest M1 formed with oral (po) and intravenous (iv) dosing, respectively, whereas the extent of M1 formation is similar for the drug administered po or iv according to the TM, and these values sit intermediate those of the SFM. The SFM is significant, as this drug metabolism model explains route-dependent intestinal metabolism, describing a higher extent of intestinal metabolism with po versus the much reduced or absence of intestinal metabolism with iv dosing. A similar pattern exists for drug–drug interactions (DDIs). The inhibitor or inducer exerts its greatest effect on victim drugs when both inhibitor/inducer and drug are given po. With po dosing, more drug or inhibitor/inducer is brought into the intestine for DDIs. The bypass of flow and drug to the enterocyte region of the intestine after intravenous administration adds complications to in vitro–in vivo extrapolations (IVIVE).

Role of vitamin D receptor in the regulation of CYP3A gene expression

Vitamin D₃ (VD₃) is a multifunctional nutrient which can be either synthesized or absorbed from the diet. It plays a pivotal role in systemic calcium and phosphate homeostasis, as well as in various physiological and pathological processes. VD₃ is converted to the active form, 1α,25-dihydroxyvitamin D₃ (1,25-D3), by cytochrome P450 2R1 (CYP2R1)/CYP27A1 and CYP27B1 sequentially, and deactivated by multiple enzymes including CYP3A4. On the other hand, 1,25-D3 is capable of activating the transcription of CYP3A genes in humans, mice and rats. The vitamin D receptor (VDR)-mediated transactivation of human CYP3A4 and CYP3A5 resembles that known for pregnane X receptor (PXR). Activated VDR forms a heterodimer with retinoid X receptor α (RXRα), recruits co-activators, translocates to the cell nucleus, binds to the specific vitamin D responsive elements (VDRE), and activates the gene transcription. In mice, intestinal Cyp3a11 mRNA levels, but not those of hepatic CYP3As, were induced by in vivo administration of VDR and PXR agonists. In rats, intestinal Cyp3a1 and Cyp3a2 mRNAs were induced by 1,25-D3 or lithocholic acid (LCA), whereas hepatic Cyp3a2, but not Cyp3a1 and Cyp3a9, was modulated to 1,25-D3 treatment. In general, the VDR-mediated regulation of CYP3A presents species and organ specificity.

Administration of Vitamin D Metabolites Affects RNA Expression of Xenobiotic Metabolising Enzymes and Function of ABC Transporters in Rats

From studies on different species and in cell culture systems, it has been suggested that vitamin D metabolites might affect the metabolism and elimination of xenobiotics. Although most studies performed on rodents and cell cultures report an upregulation of respective enzymes and transporters, data from the literature are inconsistent. Especially results obtained with sheep differ from these observations. As vitamin D metabolites are widely used as feed additives or therapeutics in livestock animals, we aimed to assess whether these differences indicate species-specific responses or occurred due to the very high dosages used in the rodent studies. Therefore, we applied treatment protocols to rats that had been used previously in sheep or cattle. Forty-eight female rats were divided into three treatment and corresponding placebo groups: (1) a single intraperitoneal injection of 1,25-(OH)2D3 or placebo 12 h before sacrifice; (2) daily supplementation with 25-OHD3 by oral gavage or placebo for 10 days; and (3) a single intramuscular injection of vitamin D3 10 days before sacrifice. In contrast to a previous study using sheep, treatment of rats with 1,25-dihydroxyvitamin D3 did not result in an upregulation of cytochrome P450 3A isoenzymes (CYP3A), but a decrease was found in hepatic and intestinal expressions. In addition, a downregulation of P-glycoprotein (P-gp) and breast cancer resistance protein was found in the brain. Taken together, the stimulating effects of vitamin D metabolites on the expression of genes involved in the metabolism and elimination of xenobiotics reported previously for rodents and sheep could not be reproduced. In contrast, we even observed a negative impact on the expression of CYP3A enzymes and their most important regulator, the pregnane X receptor. Most interestingly, we could demonstrate an effect of treatment with 25-hydroxyvitamin D3 and vitamin D3 on the functional activity of ileal P-glycoprotein (P-gp) using the Ussing chamber technique.

Upregulation of reduced folate carrier by vitamin D enhances brain folate uptake in mice lacking folate receptor alpha

Folates are critical for central nervous system function. Folate transport is mediated by 3 major pathways, reduced folate carrier (RFC), proton-coupled folate transporter (PCFT), and folate receptor alpha (FRα/Folr1), known to be regulated by ligand-activated nuclear receptors. Cerebral folate delivery primarily occurs at the choroid plexus through FRα and PCFT; inactivation of these transport systems can result in very low folate levels in the cerebrospinal fluid causing childhood neurodegenerative disorders. These disorders have devastating effects in young children, and current therapeutic approaches are not sufficiently effective. Our group has previously reported in vitro that functional expression of RFC at the blood-brain barrier (BBB) and its upregulation by the vitamin D nuclear receptor (VDR) could provide an alternative route for brain folate uptake. In this study, we further demonstrated in vivo, using Folr1 knockout (KO) mice, that loss of FRα led to a substantial decrease of folate delivery to the brain and that pretreatment of Folr1 KO mice with the VDR activating ligand, calcitriol (1,25-dihydroxyvitamin D₃), resulted in over a 6-fold increase in [¹³C₅]-5-formyltetrahydrofolate ([¹³C₅]-5-formylTHF) concentration in brain tissues, with levels comparable to wild-type animals. Brain-to-plasma concentration ratio of [¹³C₅]-5-formylTHF was also significantly higher in calcitriol-treated Folr1 KO mice (15-fold), indicating a remarkable enhancement in brain folate delivery. These findings demonstrate that augmenting RFC functional expression at the BBB could effectively compensate for the loss of Folr1-mediated folate uptake at the choroid plexus, providing a therapeutic approach for neurometabolic disorders caused by defective brain folate transport.

Differential regulation of intestinal and hepatic CYP3A by 1α,25-dihydroxyvitamin D3 : Effects on in vivo oral absorption and disposition of buspirone in rats

1α,25-Dihydroxyvitamin D₃ (also called 1,25(OH)₂ D₃ or calcitriol) is the biologically active form of vitamin D, which functions as a ligand to the vitamin D receptor (VDR). It was previously reported that intestinal cytochrome P450 3A (CYP3A) expression was altered by 1,25(OH)₂ D₃ -mediated VDR activation. However, to clarify whether the change in CYP3A subfamily expression by VDR activation can affect metabolic function, further evidence is needed to prove the effect of 1,25(OH)₂ D₃ treatment on CYP3A-mediated drug metabolism and pharmacokinetics. Here, we report the effects of 1,25(OH)₂ D₃ on CYP3A activity and in vivo pharmacokinetics of buspirone in Sprague-Dawley rats. CYP3A mRNA expression and CYP3A-mediated testosterone metabolism were enhanced in the intestine but were unaffected in the livers of rats treated with 1,25(OH)₂ D₃ . Notably, the oral pharmacokinetic profile of buspirone (CYP3A substrate drug) and 6'-hydroxybuspirone (major active metabolite of buspirone formed via CYP3A-mediated metabolism) was significantly altered, while its intravenous pharmacokinetic profile was not affected by 1,25(OH)₂ D₃ treatment. To the best of our knowledge, this study provides the first reported data regarding the effects of 1,25(OH)₂ D₃ treatment on the in vivo pharmacokinetics of intravenous and oral buspirone in rats, by the differential modulation of hepatic and intestinal CYP3A activity. Our present results could lead to further studies in clinically significant CYP3A-mediated drug-nutrient interactions with 1,25(OH)₂ D₃ , including 1,25(OH)₂ D₃ -buspirone interaction. Preclinical Research & Development.

Regulation of Reduced Folate Carrier (RFC) by Vitamin D Receptor at the Blood-Brain Barrier

Folates are essential for brain development and function. Folate transport in mammalian tissues is mediated by three major folate transport systems, i.e., reduced folate carrier (RFC), proton-coupled folate transporter (PCFT), and folate receptor alpha (FRα), known to be regulated by ligand-activated nuclear receptors, such as vitamin D receptor (VDR). Folate uptake at the choroid plexus, which requires the actions of both FRα and PCFT, is critical to cerebral folate delivery. Inactivating FRα or PCFT mutations cause severe cerebral folate deficiency resulting in early childhood neurodegeneration. The objective of this study was to investigate the role of RFC in folate uptake at the level of the blood-brain barrier (BBB) and its potential regulation by VDR. We detected robust expression of RFC in different in vitro BBB model systems, particularly in immortalized cultures of human cerebral microvascular endothelial cells (hCMEC/D3) and isolated mouse brain capillaries. [³H]-methotrexate uptake by hCMEC/D3 cells at pH 7.4 was inhibited by PT523 and pemetrexed, antifolates with high affinity for RFC. We also showed that activation of VDR through calcitriol (1,25-dihydroxyvitamin D₃) exposure up-regulates RFC mRNA and protein expression as well as function in hCMEC/D3 cells and isolated mouse brain capillaries. We further demonstrated that RFC expression could be down-regulated by VDR-targeting siRNA, further confirming the role of VDR in the direct regulation of this folate transporter. Together, these data suggest that augmenting RFC functional expression could constitute a novel strategy for enhancing brain folate delivery for the treatment of neurometabolic disorders caused by loss of FRα or PCFT function.

Effects of 1α,25-Dihydroxyvitamin D3 on Intestinal Absorption and Disposition of Adefovir Dipivoxil and Its Metabolite, Adefovir, in Rats

The aim of this study was to investigate the effect of 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3), an active form of vitamin D, on the oral absorption and disposition of adefovir dipivoxil (P-glycoprotein (P-gp) substrate) and its major active metabolite, adefovir (multidrug resistance-associated protein 4 (Mrp4) substrate), in rats. The pharmacokinetics of intravenous adefovir and oral adefovir dipivoxil was evaluated in control and 1,25(OH)2D3-treated rats. The intestinal absorption of adefovir dipivoxil was investigated through an in situ closed loop study, and the tissue distribution of adefovir after oral administration of adefovir dipivoxil was evaluated in the two groups. There was no significant difference in pharmacokinetic parameters of intravenous adefovir between the two groups. Importantly, the total area under the plasma concentration-time curve from time zero to time infinity (AUC), peak plasma concentration (Cmax) and extent of absolute oral bioavailability (F) of adefovir after oral administration of adefovir dipivoxil were significantly higher in 1,25(OH)2D3-treated rats than in control rats. In the in situ closed loop study, there was no significant difference in the remaining fraction of adefovir dipivoxil in the duodenum, jejunum and ileum loops between the two groups. In the tissue distribution study after oral administration of adefovir dipivoxil, the tissue-to-plasma partition coefficients of adefovir in the liver, brain, kidney, and intestine were significantly lower in the 1,25(OH)2D3-treated rats than in control rats. The present study indicates that 1,25(OH)2D3 treatment can enhance the oral absorption of adefovir dipivoxil, likely via the induction of basolateral Mrp4 function in rat intestine. However, the impact of 1,25(OH)2D3 treatment on the pharmacokinetics of intravenous adefovir was limited. These results could lead to further studies in clinically significant P-gp and/or MRP4-mediated 1,25(OH)2D3-drug interactions.

What do drug transporters really do?

Potential drug-drug interactions mediated by the ATP-binding cassette (ABC) transporter and solute carrier (SLC) transporter families are of clinical and regulatory concern. However, the endogenous functions of these drug transporters are not well understood. Discussed here is evidence for the roles of ABC and SLC transporters in the handling of diverse substrates, including metabolites, antioxidants, signalling molecules, hormones, nutrients and neurotransmitters. It is suggested that these transporters may be part of a larger system of remote communication ('remote sensing and signalling') between cells, organs, body fluid compartments and perhaps even separate organisms. This broader view may help to clarify disease mechanisms, drug-metabolite interactions and drug effects relevant to diabetes, chronic kidney disease, metabolic syndrome, hypertension, gout, liver disease, neuropsychiatric disorders, inflammatory syndromes and organ injury, as well as prenatal and postnatal development.

Vitamin D receptor activation induces P-glycoprotein and increases brain efflux of quinidine: an intracerebral microdialysis study in conscious rats

PURPOSE

Since the vitamin D receptor (VDR) was found to up-regulate cerebral P-glycoprotein expression in vitro and in mice, we extend our findings to rats by assessing the effect of rat Vdr activation on brain efflux of quinidine, a P-gp substrate that is eliminated primarily by cytochrome P450 3a.

METHODS

We treated rats with vehicle or the active VDR ligand, 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3] (4.8 or 6.4 nmol/kg i.p. every 2nd day × 4) and examined P-gp expression and cerebral quinidine disposition via microdialysis in control and treatment studies conducted longitudinally in the same rat.

RESULTS

The 6.4 nmol/kg 1,25(OH)2D3 dose increased cerebral P-gp expression 1.75-fold whereas hepatic Cyp3a remained unchanged. Although there was no change in systemic clearance elicited by 1,25(OH)2D3, brain extracellular fluid quinidine concentrations were lower in treated rats. We noted that insertion of indwelling catheters increased plasma protein binding of quinidine and serial sampling decreased the blood:plasma concentration ratio, factors that alter distribution ratios in microdialysis studies. After appropriate correction, KECF/P,uu and KECF/B,uu, or ratios of quinidine unbound concentrations in brain extracellular fluid to plasma or blood at steady-state, were more than halved.

CONCLUSION

We demonstrate that VDR activation increases cerebral P-gp expression and delimits brain penetration of P-gp substrates.

The solute carrier family 10 (SLC10): beyond bile acid transport

The solute carrier (SLC) family 10 (SLC10) comprises influx transporters of bile acids, steroidal hormones, various drugs, and several other substrates. Because the seminal transporters of this family, namely, sodium/taurocholate cotransporting polypeptide (NTCP; SLC10A1) and the apical sodium-dependent bile acid transporter (ASBT; SLC10A2), were primarily bile acid transporters, the term "sodium bile salt cotransporting family" was used for the SLC10 family. However, this notion became obsolete with the finding of other SLC10 members that do not transport bile acids. For example, the sodium-dependent organic anion transporter (SOAT; SLC10A6) transports primarily sulfated steroids. Moreover, NTCP was shown to also transport steroids and xenobiotics, including HMG-CoA inhibitors (statins). The SLC10 family contains four additional members, namely, P3 (SLC10A3; SLC10A3), P4 (SLC10A4; SLC10A4), P5 (SLC10A5; SLC10A5) and SLC10A7 (SLC10A7), several of which were unknown or considered hypothetical until approximately a decade ago. While their substrate specificity remains undetermined, great progress has been made towards their characterization in recent years. Explicitly, SLC10A4 may participate in vesicular storage or exocytosis of neurotransmitters or mastocyte mediators, whereas SLC10A5 and SLC10A7 may be involved in solute transport and SLC10A3 may have a role as a housekeeping protein. Finally, the newly found role of bile acids in glucose and energy homeostasis, via the TGR5 receptor, sheds new light on the clinical relevance of ASBT and NTCP. The present mini-review provides a brief summary of recent progress on members of the SLC10 family.

Nuclear-receptor-mediated regulation of drug- and bile-acid-transporter proteins in gut and liver

Adverse drug events (ADEs) are a common cause of patient morbidity and mortality and are classically thought to result, in part, from variation in expression and activity of hepatic enzymes of drug metabolism. It is now known that alterations in the expression of genes that encode drug- and bile-acid-transporter proteins in both the gut and liver play a previously unrecognized role in determining patient drug response and eventual clinical outcome. Four nuclear receptor (NR) superfamily members, including pregnane X receptor (PXR, NR1I2), constitutive androstane receptor (NR1I3), farnesoid X receptor (NR1H4), and vitamin D receptor (NR1I1), play pivotal roles in drug- and bile-acid-activated programs of gene expression to coordinately regulate drug- and bile-acid transport activity in the intestine and liver. This review focuses on the NR-mediated gene activation of drug and bile-acid transporters in these tissues as well as the possible underlying molecular mechanisms.

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.

1α,25-Dihydroxyvitamin D3 enhances cerebral clearance of human amyloid-β peptide(1-40) from mouse brain across the blood-brain barrier

Background

Cerebrovascular dysfunction has been considered to cause impairment of cerebral amyloid-β peptide (Aβ) clearance across the blood-brain barrier (BBB). Further, low levels of vitamin D are associated with increased risk of Alzheimer's disease, as well as vascular dysfunction. The purpose of the present study was to investigate the effect of 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D3), an active form of vitamin D, on cerebral Aβ clearance from mouse brain.

Methods

The elimination of [¹²⁵I]hAβ(1-40) from mouse brain was examined by using the Brain Efflux Index method to determine the remaining amount of [¹²⁵I]hAβ(1-40) radioactivity after injection into the cerebral cortex. [¹²⁵I]hAβ(1-40) internalization was analyzed using conditionally immortalized mouse brain capillary endothelial cells (TM-BBB4).

Results

Twenty-four hours after intraperitoneal injection of 1,25(OH)₂D3 (1 μg/mouse), [¹²⁵I]hAβ(1-40) elimination from mouse brain was increased 1.3-fold, and the level of endogenous Aβ(1-40) in mouse brain was reduced. These effects were observed at 24 h after i.p. injection of 1,25(OH)₂D3, while no significant effect was observed at 48 or 72 h. Vitamin D receptor (VDR) mRNA was detected in mouse brain capillaries, suggesting that 1,25(OH)₂D3 has a VDR-mediated genomic action. Furthermore, forskolin, which activates mitogen-activated protein kinase kinase (MEK), enhanced [¹²⁵I]hAβ(1-40) elimination from mouse brain. Forskolin also enhanced [¹²⁵I]hAβ(1-40) internalization in TM-BBB4 cells, and this enhancement was inhibited by a MEK inhibitor, suggesting involvement of non-genomic action.

Conclusions

The active form of vitamin D, 1,25(OH)₂D3, appears to enhance brain-to-blood Aβ(1-40) efflux transport at the BBB through both genomic and non-genomic actions. Compounds activating these pathways may be candidate agents for modulating Aβ(1-40) elimination at the BBB.

Reduced hepatic synthesis of calcidiol in uremia

Calcidiol insufficiency is highly prevalent in chronic kidney disease (CKD), but the reasons for this are incompletely understood. CKD associates with a decrease in liver cytochrome P450 (CYP450) enzymes, and specific CYP450 isoforms mediate vitamin D(3) C-25-hydroxylation, which forms calcidiol. Abnormal levels of parathyroid hormone (PTH), which also modulates liver CYP450, could also contribute to the decrease in liver CYP450 associated with CKD. Here, we evaluated the effects of PTH and uremia on liver CYP450 isoforms involved in calcidiol synthesis in rats. Uremic rats had 52% lower concentrations of serum calcidiol than control rats (P < 0.002). Compared with controls, uremic rats produced 71% less calcidiol and 48% less calcitriol after the administration of vitamin D(3) or 1alpha-hydroxyvitamin D(3), respectively, suggesting impaired C-25-hydroxylation of vitamin D(3). Furthermore, uremia associated with a reduction of liver CYP2C11, 2J3, 3A2, and 27A1. Parathyroidectomy prevented the uremia-associated decreases in calcidiol and liver CYP450 isoforms. In conclusion, these data suggest that uremia decreases calcidiol synthesis secondary to a PTH-mediated reduction in liver CYP450 isoforms.