CHARACTERIZATION OF TRANSPORT PROTEIN EXPRESSION IN MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN (MRP) 2-DEFICIENT RATS

Influence of Slco2b1-knockout and SLCO2B1-humanization on coproporphyrin I and III levels in rats

BACKGROUND AND PURPOSE

Coproporphyrin (CP) I and III are byproducts of haem synthesis currently investigated as biomarkers for drug-drug interactions involving hepatic organic anion transporting polypeptide (OATP) 1B transporters. Another hepatically expressed OATP-member is OATP2B1. The aim of this study was to test the impact of OATP2B1, which specifically transports CPIII, on CP serum levels, applying novel rat models.

EXPERIMENTAL APPROACH

CPIII transport kinetics and the interplay between OATP2B1 and multidrug resistance-associated proteins (MRPs) were determined in vitro using the vTF7 expression system. Novel rSlco2b1-/- and SLCO2B1+/+ rat models were characterized for physiological parameters and for CP serum levels. Hepatic and renal expression of transporters involved in CP disposition were determined by real-time qPCR, Western blot analysis, and immunohistochemistry.

KEY RESULTS

In vitro experiments revealed differences in transport kinetics comparing human and rat OATP2B1 and showed a consistent, species-specific interplay with hMRP3/rMRP3. Deletion of rOATP2B1 was associated with a trend towards lower CPI serum levels compared with wildtype rats, while CPIII remained unchanged. Comparing SLCO2B1+/+ with knockout rats revealed an effect of sex: only in females the genetic modification influenced CP serum levels. Analysis of hepatic and renal transporters revealed marginal, but in part, statistically significant differences in rMRP2 abundance, which may contribute to the observed changes in CP serum levels.

CONCLUSION AND IMPLICATIONS

Our findings support that factors other than OATP1B transporters are of relevance for basal CP levels. Only in female rats, humanization of SLCO2B1 affects basal CPI and CPIII serum levels, despite isomer selectivity of OATP2B1.

Brain-Penetrating Peptide Shuttles across the Blood-Brain Barrier and Extracellular-like Space

Systemic delivery of therapeutics into the brain is greatly impaired by multiple biological barriers─the blood-brain barrier (BBB) and the extracellular matrix (ECM) of the extracellular space. To address this problem, we developed a combinatorial approach to identify peptides that can shuttle and transport across both barriers. A cysteine-constrained heptapeptide M13 phage display library was iteratively panned against an established BBB model for three rounds to select for peptides that can transport across the barrier. Using next-generation DNA sequencing and in silico analysis, we identified peptides that were selectively enriched from successive rounds of panning for functional validation in vitro and in vivo. Select peptide-presenting phages exhibited efficient shuttling across the in vitro BBB model. Two clones, Pep-3 and Pep-9, exhibited higher specificity and efficiency of transcytosis than controls. We confirmed that peptides Pep-3 and Pep-9 demonstrated better diffusive transport through the extracellular matrix than gold standard nona-arginine and clinically trialed angiopep-2 peptides. In in vivo studies, we demonstrated that systemically administered Pep-3 and Pep-9 peptide-presenting phages penetrate the BBB and distribute into the brain parenchyma. In addition, free peptides Pep-3 and Pep-9 achieved higher accumulation in the brain than free angiopep-2 and may exhibit brain targeting. In summary, these in vitro and in vivo studies highlight that combinatorial phage display with a designed selection strategy can identify peptides as promising carriers, which are able to overcome the multiple biological barriers of the brain and shuttle different-sized molecules from small fluorophores to large macromolecules for improved delivery into the brain.

Prandial state and biological sex modulate clinically relevant efflux transporters to different extents in Wistar and Sprague Dawley rats

P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and multidrug resistance-associated protein 2 (MRP2) are clinically relevant efflux transporters implicated in the oral absorption of many food and drug substrates. Here, we hypothesised that food intake could influence protein and mRNA intestinal expression of P-gp/abcb1a, BCRP/abcg2, and MRP2/abcc2 differently in male and female Wistar and Sprague Dawley rats. To test this hypothesis, we used enzyme-linked immunosorbent assay (ELISA) and real-time polymerase chain reaction (PCR) to quantify the protein and mRNA intestinal expression of these transporters, respectively. Our study found food and sex differences in P-gp expression, whereby in the fed state P-gp expression decreased in male Wistar rats, but P-gp expression increased in females. In the fed state, BCRP expression increased in both male and female Wistar rats, compared with the fasted state. In contrast, no sex differences or food effect differences were seen in Sprague Dawley rats for P-gp and BCRP expression. On the other hand, in the fed state, MRP2 expression was higher in male and female Wistar and Sprague Dawley rats when compared with the fasted state. Sex differences were also observed in the fasted state. Overall, significant strain differences were reported for P-gp, BCRP and MRP2 expression. Strong to moderate positive linear correlations were found between ELISA and PCR quantification methods. ELISA may be more useful than PCR as it reports protein expression as opposed to transcript expression. Researchers must consider the influence of sex, strain and feeding status in preclinical studies of P-gp, BCRP and MRP2 drug substrates.

The Ramp Atlas: facilitating tissue and cell-specific ramp sequence analyses through an intuitive web interface

Ramp sequences occur when the average translational efficiency of codons near the 5′ end of highly expressed genes is significantly lower than the rest of the gene sequence, which counterintuitively increases translational efficiency by decreasing downstream ribosomal collisions. Here, we show that the relative codon adaptiveness within different tissues changes the existence of a ramp sequence without altering the underlying genetic code. We present the first comprehensive analysis of tissue and cell type-specific ramp sequences and report 3108 genes with ramp sequences that change between tissues and cell types, which corresponds with increased gene expression within those tissues and cells. The Ramp Atlas (https://ramps.byu.edu/) allows researchers to query precomputed ramp sequences in 18 388 genes across 62 tissues and 66 cell types and calculate tissue-specific ramp sequences from user-uploaded FASTA files through an intuitive web interface. We used The Ramp Atlas to identify seven SARS-CoV-2 genes and seven human SARS-CoV-2 entry factor genes with tissue-specific ramp sequences that may help explain viral proliferation within those tissues. We anticipate that The Ramp Atlas will facilitate personalized and creative tissue-specific ramp sequence analyses for both human and viral genes that will increase our ability to utilize this often-overlooked regulatory region.

Evaluation of a Clinically Relevant Drug-Drug Interaction Between Rosuvastatin and Clopidogrel and the Risk of Hepatotoxicity

Purpose: The combination therapy of rosuvastatin (RSV) and the platelet inhibitor clopidogrel (CP) is widely accepted in the management of cardiovascular diseases. The objective of the present study was to identify the mechanism of RSV–CP DDI and evaluate the risk of hepatotoxicity associated with the concomitant use of CP.

Methods: We first studied the effect of CP and its major circulating metabolite, carboxylic acid metabolite (CPC), on RSV transport by overexpressing cells and membrane vesicles. Second, we investigated whether a rat model could replicate this DDI and then be used to conduct mechanistic studies and assess the risk of hepatotoxicity. Then, cytotoxicity assay in hepatocytes, biochemical examination, and histopathology were performed to measure the magnitude of liver injury in the presence and absence of DDI.

Results: CP inhibited OATP1B1-mediated transport of RSV with an IC₅₀ value of 27.39 μM. CP and CPC inhibited BCRP-mediated RSV transport with IC₅₀ values of <0.001 and 5.96 μM, respectively. The CP cocktail (0.001 μM CP plus 2 μM CPC) significantly inhibited BCRP-mediated transport of RSV by 26.28%. Multiple p.o. doses of CP significantly increased intravenous RSV plasma AUC_0-infinity by 76.29% and decreased intravenous RSV CL by 42.62%. Similarly, multiple p.o. doses of CP significantly increased p.o. RSV plasma AUC_0-infinity by 87.48% and decreased p.o. RSV CL by 43.27%. CP had no effect on cell viability, while RSV exhibited dose-dependent cytotoxicity after 96 h incubation. Co-incubation of 100 μM CP and RSV for 96 h significantly increased intracellular concentrations and cell-to-medium concentration ratios of RSV and reduced hepatocyte viability. Histological evaluation of liver specimens showed patterns of drug-induced liver injury. Cholestasis was found in rats in the presence of DDI.

Conclusion: CP is not a clinically relevant inhibitor for OATP1B1 and OATP1B3. The primary mechanism of RSV–CP DDI can be attributed to the inhibition of intestinal BCRP by CP combined with the inhibition of hepatic BCRP by CPC. The latter is likely to be more clinically relevant and be a contributing factor for increased hepatotoxicity in the presence of DDI.

Endogenous Coproporphyrin I and III are Altered in Multidrug Resistance-Associated Protein 2-Deficient (TR-) Rats

Recent studies have focused on coproporphyrin (CP)-I and CP-III (CPs) as endogenous biomarkers for organic anion transporting polypeptides (OATPs). Previous data showed that CPs are also substrates of multidrug resistance-associated protein (MRP/Mrp) 2 and 3. This study was designed to examine the impact of loss of Mrp2 function on the routes of excretion of endogenous CPs in wild-type (WT) Wistar compared to Mrp2-deficient TR- rats. To exclude possible confounding effects of rat Oatps, the transport of CPs was investigated in Oatp-overexpressing HeLa cells. Results indicated that CPs are substrates of rodent Oatp1b2, and that CP-III is a substrate of Oatp2b1. Quantitative targeted absolute proteomic (QTAP) analysis revealed no differences in Oatps, but an expected significant increase in Mrp3 protein levels in TR- compared to WT rat livers. CP-I and CP-III concentrations measured by LC-MS/MS were elevated in TR- compared to WT rat liver, while CP-I and CP-III estimated biliary clearance was decreased 75- and 840-fold in TR- compared to WT rats, respectively. CP-III concentrations were decreased 14-fold in the feces of TR- compared to WT rats, but differences in CP-I were not significant. In summary, the disposition of CPs was markedly altered by loss of Mrp2 and increased Mrp3 function as measured in TR- rats.

ATP-Binding Cassette Transporter C4 is a Prostaglandin D2 Exporter in HMC-1 cells

ATP-binding cassette transporter C4 (ABCC4) is associated with multidrug resistance and the regulation of cell signalling. Some prostaglandins (PGs), including: PGE₂, PGF_2α, PGE₃, and PGF_3α are known substrates of ABCC4, and are released from some types of cells to exert their biological effects. In the present study, we demonstrate that PGD₂ is a novel substrate of ABCC4 using a transport assay based on inside-out membrane vesicles prepared from ABCC4-overexpressing cells. Then, we used two types of cell lines with confirmed ABCC4 mRNA and PGD₂ release capacity (human mast cell lines HMC-1 cells and human rhabdomyosarcoma cell lines TE671 cells) to evaluate the contribution of ABCC4. The extracellular levels of PGD₂ were unchanged following addition of a selective ABCC4 inhibitor in TE671 cells. Pharmacological inhibition and knockdown of ABCC4 significantly reduced the extracellular levels of PGD₂ by at least 53% in HMC-1 cells. Moreover, the extracellular levels of PGD₂ decreased by at least 20% using the selective ABCC4 inhibitor in the other mast cell line RBL-2H3 cells. Therefore, our results suggest that ABCC4 functions as a PGD₂ exporter in HMC-1 cells.

Fetal androgen exposure is a determinant of adult male metabolic health

Androgen signalling is a critical driver of male development. Fetal steroid signalling can be dysregulated by a range of environmental insults and clinical conditions. We hypothesised that poor adult male health was partially attributable to aberrant androgen exposure during development. Testosterone was directly administered to developing male ovine fetuses to model excess prenatal androgenic overexposure associated with conditions such as polycystic ovary syndrome (PCOS). Such in utero androgen excess recreated the dyslipidaemia and hormonal profile observed in sons of PCOS patients. 1,084 of 15,134 and 408 of 2,766 quantifiable genes and proteins respectively, were altered in the liver during adolescence, attributable to fetal androgen excess. Furthermore, prenatal androgen excess predisposed to adolescent development of an intrahepatic cholestasis-like condition with attendant hypercholesterolaemia and an emergent pro-fibrotic, pro-oxidative stress gene and protein expression profile evident in both liver and circulation. We conclude that prenatal androgen excess is a previously unrecognised determinant of lifelong male metabolic health.

Mechanistic Modeling of the Hepatic Disposition of Estradiol-17β-Glucuronide in Sandwich-Cultured Human Hepatocytes

Estradiol-17β-glucuronide (E₂17G) is an estrogen metabolite that has cholestatic properties. In humans, circulating E₂17G is transported into hepatocytes by organic anion transporting polypeptides (OATPs) and is excreted into bile by multidrug-resistance associated protein 2 (MRP2). E₂17G is also a substrate of the basolateral efflux transporters MRP3 and MRP4, which translocate E₂17G from hepatocytes to blood. However, the contribution of basolateral efflux to hepatocyte disposition of E₂17G has not been evaluated previously. To address this question, E₂17G disposition was studied in sandwich-cultured human hepatocytes and mechanistic modeling was applied to calculate clearance values (mean ± S.D.) for uptake, intrinsic biliary excretion (CL_int,bile) and intrinsic basolateral efflux (CL_int,BL). The biliary excretion index of E₂17G was 45% ± 6%. The CL_int,BL of E₂17G [0.18 ± 0.03 (ml/min)/g liver) was 1.6-fold higher than CL_int,bile [0.11 ± 0.06 (ml/min)/g liver]. Simulations were performed to study the effects of increased CL_int,BL and a concomitant decrease in CL_int,bile on hepatic E₂17G exposure. Results demonstrated that increased CL_int,BL can effectively reduce hepatocellular and biliary exposure to this potent cholestatic agent. Simulations also revealed that basolateral efflux can compensate for impaired biliary excretion and, vice versa, to avoid accumulation of E₂17G in hepatocytes. However, when both clearance processes are impaired by 90%, hepatocyte E₂17G exposure increases up to 10-fold. These data highlight the contribution of basolateral efflux transport, in addition to MRP2-mediated biliary excretion, to E₂17G disposition in human hepatocytes. This elimination route could be important, especially in cases where basolateral efflux is induced, such as cholestasis. SIGNIFICANCE STATEMENT: The disposition of the cholestatic estrogen metabolite estradiol-17β-glucuronide (E₂17G) was characterized in sandwich-cultured human hepatocytes. The intrinsic basolateral efflux clearance was estimated to be 1.6-fold higher than the intrinsic biliary excretion clearance, emphasizing the contribution of basolateral elimination in addition to biliary excretion. Simulations highlight how hepatocytes can effectively cope with increased E₂17G through the regulation of both basolateral and biliary transporters.

Modulation of Hepatic MRP3/ABCC3 by Xenobiotics and Pathophysiological Conditions: Role in Drug Pharmacokinetics

Liver transporters play an important role in the pharmacokinetics and disposition of pharmaceuticals, environmental contaminants, and endogenous compounds. Among them, the family of ATP-Binding Cassette (ABC) transporters is the most important due to its role in the transport of endo- and xenobiotics. The ABCC sub-family is the largest one, consisting of 13 members that include the cystic fibrosis conductance regulator (CFTR/ABCC7); the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) and the multidrug resistanceassociated proteins (MRPs). The MRP-related proteins can collectively confer resistance to natural, synthetic drugs and their conjugated metabolites, including platinum-containing compounds, folate anti-metabolites, nucleoside and nucleotide analogs, among others. MRPs can be also catalogued into "long" (MRP1/ABCC1, -2/C2, -3/C3, -6/C6, and -7/C10) and "short" (MRP4/C4, -5/C5, -8/C11, -9/C12, and -10/C13) categories. While MRP2/ABCC2 is expressed in the canalicular pole of hepatocytes, all others are located in the basolateral membrane. In this review, we summarize information from studies examining the changes in expression and regulation of the basolateral hepatic transporter MPR3/ABCC3 by xenobiotics and during various pathophysiological conditions. We also focus, primarily, on the consequences of such changes in the pharmacokinetic, pharmacodynamic and/or toxicity of different drugs of clinical use transported by MRP3.

MDR1 inhibition increases sensitivity to doxorubicin and etoposide in adrenocortical cancer

Chemotherapy for adrenocortical carcinoma (ACC) has limited efficacy and is accompanied by severe toxicity. This lack of effectiveness has been associated with high tumoral levels of the multidrug resistance (MDR) pump P-glycoprotein (P-gp), encoded by the MDR1 gene. In this study, effects of P-gp inhibition on the sensitivity of ACC cells to cytotoxic drugs were evaluated. MDR1 mRNA and P-gp expression were determined in human adrenal tissues and cell lines. H295R, HAC15 and SW13 cells were treated with mitotane, doxorubicin, etoposide, cisplatin and streptozotocin, with or without the P-gp inhibitors verapamil and tariquidar. Cell growth and surviving fraction of colonies were assessed. MDR1 mRNA and P-gp protein expression were lower in ACCs than in adrenocortical adenomas (P < 0.0001; P < 0.01, respectively). MDR1 and P-gp expression were positively correlated in ACC (P < 0.0001, ρ = 0.723). Mitotane, doxorubicin, cisplatin and etoposide dose dependently inhibited cell growth in H295R, HAC15 and SW13. Tariquidar, and in H295R also verapamil, increased the response of HAC15 and H295R to doxorubicin (6.3- and 7.5-fold EC50 decrease in H295R, respectively; all P < 0.0001). Sensitivity to etoposide was increased in H295R and HAC15 by verapamil and tariquidar (all P < 0.0001). Findings were confirmed when assessing colony formation. We show that cytotoxic drugs, except streptozotocin, used for ACC treatment, inhibit ACC cell growth and colony formation at clinically achievable concentrations. P-gp inhibition increases sensitivity to doxorubicin and etoposide, suggesting that MDR1 is involved in sensitivity to these drugs and could be a potential target for cytotoxic treatment improvement in ACC.

MRP2 and the Transport Kinetics of Cysteine Conjugates of Inorganic Mercury

Human exposure to mercuric species occurs regularly throughout the world. Mercuric ions may accumulate in target cells and subsequently lead to cellular intoxication and death. Therefore, it is important to have a thorough understanding of how transportable species of mercury are handled by specific membrane transporters. The purpose of the current study was to characterize the transport kinetics of cysteine (Cys)-S-conjugates of inorganic mercury (Cys-S-Hg-S-Cys) at the site of the multidrug resistance-associated transporter 2 (MRP2). In order to estimate the maximum velocity (V _max) and Michaelis constant (K _m) for the uptake of Cys-S-Hg-S-Cys mediated by MRP2, in vitro studies were carried out using radioactive Cys-S-Hg-S-Cys (5 μM) and inside-out membrane vesicles made from Sf9 cells transfected with MRP2. The V _max was estimated to be 74.3 ± 10.1 nmol mg protein^-1 30 s^-1 while the K _m was calculated to be 63.4 ± 27.3 μM. In addition, in vivo studies were utilized to measure the disposition of inorganic mercury (administered dose 0.5 μmol kg^-1 in 2 mL normal saline) over time in Wistar and TR¯ (Mrp2-deficient) rats. These studies measured the disposition of mercuric ions in the kidney, liver, and blood. In general, the data suggest that the initial uptake of mercuric conjugates into select target cells is rapid followed by a period of slower uptake and accumulation. Overall, the data indicate that MRP2 transports Cys-S-Hg-S-Cys in a manner that is similar to that of other MRP2 substrates.

Glucuronidation: driving factors and their impact on glucuronide disposition

Glucuronidation is a well-recognized phase II metabolic pathway for a variety of chemicals including drugs and endogenous substances. Although it is usually the secondary metabolic pathway for a compound preceded by phase I hydroxylation, glucuronidation alone could serve as the dominant metabolic pathway for many compounds, including some with high aqueous solubility. Glucuronidation involves the metabolism of parent compound by UDP-glucuronosyltransferases (UGTs) into hydrophilic and negatively charged glucuronides that cannot exit the cell without the aid of efflux transporters. Therefore, elimination of parent compound via glucuronidation in a metabolic active cell is controlled by two driving forces: the formation of glucuronides by UGT enzymes and the (polarized) excretion of these glucuronides by efflux transporters located on the cell surfaces in various drug disposition organs. Contrary to the common assumption that the glucuronides reaching the systemic circulation were destined for urinary excretion, recent evidences suggest that hepatocytes are capable of highly efficient biliary clearance of the gut-generated glucuronides. Furthermore, the biliary- and enteric-eliminated glucuronides participate into recycling schemes involving intestinal microbes, which often prolong their local and systemic exposure, albeit at low systemic concentrations. Taken together, these recent research advances indicate that although UGT determines the rate and extent of glucuronide generation, the efflux and uptake transporters determine the distribution of these glucuronides into blood and then to various organs for elimination. Recycling schemes impact the apparent plasma half-life of parent compounds and their glucuronides that reach intestinal lumen, in addition to prolonging their gut and colon exposure.

Pharmacokinetics of Boldine in Control and Mrp2-Deficient Rats

The aim of the present study was to describe the currently poorly understood pharmacokinetics (PK) of boldine in control rats (LW, Lewis rats), and Mrp2 transporter-deficient rats (TR-). Animals from the LW and TR- groups underwent a bolus dose study with 10 mg/kg of boldine applied either orally or intravenously in order to evaluate the major PK parameters. The TR- rats demonstrated significantly reduced total clearance with prolonged biological half-life (LW 12±4.6 versus TR- 20±4.4 min), decreased volume of distribution (LW 3.2±0.4 l/kg versus TR- 2.4±0.4 l/kg) and reduced bioavailability (LW 7 % versus TR- 4.5 %). Another set of LW and TR- rats were used for a clearance study with continuous intravenous administration of boldine. The LW rats showed that biliary and renal clearance formed less than 2 % of the total clearance of boldine. The treatment of samples with β glucuronidase showed at least a 38 % contribution of conjugation reactions to the overall clearance of boldine. The TR- rats demonstrated reduced biliary clearance of boldine and its conjugates, which was partly compensated by their increased renal clearance. In conclusion, this study presents the PK parameters of boldine and shows the importance of the Mrp2 transporter and conjugation reactions in the elimination of the compound.

Consequences of Mrp2 deficiency for diclofenac toxicity in the rat intestine ex vivo

The non-steroidal anti-inflammatory drug diclofenac (DCF) has a high prevalence of intestinal side effects in humans and rats. It has been reported that Mrp2 transporter deficient rats (Mrp2) are more resistant to DCF induced intestinal toxicity. This was explained in vivo by impaired Mrp2-dependent biliary transport of DCF-acylglucuronide (DAG), leading to decreased intestinal exposure to DAG and DCF. However, it is not known to what extent adaptive changes in the Mrp2 intestine itself influence its sensitivity to DCF toxicity without the influence of liver metabolites. To investigate this, DCF toxicity and disposition were studied ex vivo by precision-cut intestinal slices and Ussing chamber using intestines from wild type(WT) and Mrp2 rats. The results show that adaptive changes due to Mrp2 deficiency concerning Mrp2, Mrp3 and BCRP gene expression, GSH content and DAG formation were different between liver and intestine. Furthermore, Mrp2 intestine was intrinsically more resistant to DCF toxicity than its WT counterpart ex vivo. This can at least partly be explained by a reduced DCF uptake by the Mrp2 intestine, but isnot related to the other adaptive changes in the intestine. The extrapolation of this data to humans with MRP2 deficiency is uncertain due to species differences in activity and regulation of transporters.

Boldine enhances bile production in rats via osmotic and farnesoid X receptor dependent mechanisms

Boldine, the major alkaloid from the Chilean Boldo tree, is used in traditional medicine to support bile production, but evidence to support this function is controversial. We analyzed the choleretic potential of boldine, including its molecular background. The acute- and long-term effects of boldine were evaluated in rats either during intravenous infusion or after 28-day oral treatment. Infusion of boldine instantly increased the bile flow 1.4-fold in healthy rats as well as in animals with Mrp2 deficiency or ethinylestradiol induced cholestasis. This effect was not associated with a corresponding increase in bile acid or glutathione biliary excretion, indicating that the effect is not related to stimulation of either bile acid dependent or independent mechanisms of bile formation and points to the osmotic activity of boldine itself. We subsequently analyzed bile production under conditions of changing biliary excretion of boldine after bolus intravenous administration and found strong correlations between both parameters. HPLC analysis showed that bile concentrations of boldine above 10 μM were required for induction of choleresis. Importantly, long-term pretreatment, when the bile collection study was performed 24-h after the last administration of boldine, also accelerated bile formation despite undetectable levels of the compound in bile. The effect paralleled upregulation of the Bsep transporter and increased biliary clearance of its substrates, bile acids. We consequently confirmed the ability of boldine to stimulate the Bsep transcriptional regulator, FXR receptor. In conclusion, our study clarified the mechanisms and circumstances surrounding the choleretic activity of boldine.

Hepatocellular exposure of troglitazone metabolites in rat sandwich-cultured hepatocytes lacking Bcrp and Mrp2: interplay between formation and excretion

Inhibition of bile acid transport by troglitazone (TGZ) and its major metabolite, TGZ sulfate (TS), may lead to hepatocellular accumulation of toxic bile acids; TS accumulation and hepatotoxicity may be associated with impaired TS biliary excretion. This study evaluated the impact of impaired transport of breast cancer resistance protein (Bcrp) and multidrug resistance-associated protein 2 (Mrp2) on the hepatobiliary disposition of generated metabolites, TS and TGZ glucuronide (TG). Sandwich-cultured hepatocytes (SCH) from Mrp2-deficient (TR(-)) rats in combination with Bcrp knockdown using RNA interference were employed. The biliary excretion index (BEI) of generated TS was not significantly altered by impaired Bcrp (20.9 to 21.1%) and/or Mrp2 function (24.4% and 17.5% in WT and TR(-) rat SCH, respectively). Thus, loss-of-function of Mrp2 and/or Bcrp do not appear to be risk factors for increased hepatocellular TS accumulation in rats, potentially because of a compensatory transporter(s) that excretes TS into bile. Further investigations revealed that the compensatory TS biliary transporter was not the bile salt export pump (Bsep) or P-glycoprotein (P-gp). Interestingly, TGZ sulfation was significantly decreased in TR(-) compared with WT rat SCH (total recovery: 2.8 versus 5.0% of TGZ dose), resulting in decreased hepatocellular TS accumulation, even though sulfotransferase activity in TR(-) rat hepatocyte S9 fraction was similar. Hepatocellular TG accumulation was significantly increased in TR(-) compared with WT rat SCH due to increased glucuronidation and negligible TG biliary excretion. These data emphasize that the interplay between metabolite formation and excretion determines hepatocellular exposure to generated metabolites such as TS and TG.

Quantification of biliary excretion and sinusoidal excretion of 5(6)-carboxy-2',7'-dichlorofluorescein (CDF) in cultured hepatocytes isolated from Sprague Dawley, Wistar and Mrp2-deficient Wistar (TR(-)) rats

Hepatic efflux of drug candidates is an important issue in pre-clinical drug development. Here we utilise a method which quantifies and distinguishes efflux of drugs at the canalicular and sinusoidal membranes in rat hepatocyte cultures. Bi-phasic kinetics of transport of 5(6)-carboxydichlorofluorescein (CDF) at the canalicular membrane was demonstrated in Sprague Dawley (SD) and Wistar (W) rat hepatocytes. The high affinity component (Km=3.2±0.8μM (SD), 9.0±3.1μM (W)) was attributed to Mrp2-mediated transport, the low affinity component (Km=192.1±291.5μM (SD), 69.2±36.2μM (W)) may be attributed to transport involving a separate Mrp2 binding site. Data from membranes (Hill coefficient (h)=2.0±0.5) and vesicles (h=1.6±0.2) expressing Mrp2 and from SD (h=1.6±0.4) and Wistar (h=4.0±0.6) hepatocytes suggests transport involves more than one binding site. In TR(-) hepatocytes, CDF efflux was predominantly over the sinusoidal membrane (Km=100.7±36.0μM), consistent with low abcc2 (Mrp2) expression and compensatory increase in abcc3 (Mrp3) expression. This report shows the potential of using this in vitro method to model changes in biliary excretion due to alterations in transporter expression.

Species differences in sinusoidal and canalicular efflux transport of mycophenolic acid 7-O-glucuronide in sandwich-cultured hepatocytes

Metabolism and sinusoidal/canalicular efflux of mycophenolic acid (MPA) was investigated using sandwich-cultured hepatocytes (SCHs). After applying MPA to SCHs from humans, wild-type rats, and multidrug resistance-associated protein (Mrp) 2-deficient rats, the MPA metabolites 7-O-glucuronide (MPAG) and acyl glucuronide (AcMPAG) were detected in the intracellular compartment of the SCHs. Sinusoidal efflux of MPAG was detected in all SCH preparations including Mrp2-deficient rat SCHs, whereas canalicular efflux of MPAG was observed in wild-type rat and human SCHs but not in Mrp2-deficient rat SCHs. The ratio of canalicular efflux to net (canalicular plus sinusoidal) efflux was 37 ± 8% in wild-type rat SCHs, while the ratio in human SCHs was significantly lower (20 ± 2%, P < 0.05), indicating species differences in the direction of hepatic MPAG transport. This 20% ratio in human SCHs corresponds to a high sinusoidal MPAG efflux (80%) that can in part account for the urine-dominated recovery of MPAG in humans. Both sinusoidal and canalicular MPAG efflux in rat SCHs shows a good correspondence to urinary and biliary recovery of MPAG after MPA dosing. The sinusoidal efflux of AcMPAG in human SCHs was detected from one out of three donors, suggesting donor-to-donor variation. In conclusion, this study demonstrates the predictive value of SCHs for elucidating the interplay of metabolism and efflux transport, in addition to demonstrating a species difference between rat and human in sinusoidal and canalicular efflux of MPAG.

The role of canalicular ABC transporters in cholestasis

Cholestasis, a hallmark feature of hepatobiliary disease, is characterized by the retention of biliary constituents. Some of these constituents, such as bile acids, inflict damage to hepatocytes and bile duct cells. This damage may lead to inflammation, fibrosis, cirrhosis, and eventually carcinogenesis, sequelae that aggravate the underlying disease and deteriorate clinical outcome. Canalicular ATP-binding cassette (ABC) transporters, which mediate the excretion of individual bile constituents, play a key role in bile formation and cholestasis. The study of these transporters and their regulatory nuclear receptors has revolutionized our understanding of cholestatic disease. This knowledge has served as a template to develop novel treatment strategies, some of which are currently already undergoing phase III clinical trials. In this review we aim to provide an overview of the structure, function, and regulation of canalicular ABC transporters. In addition, we will focus on the role of these transporters in the pathogenesis and treatment of cholestatic bile duct and liver diseases.

Hepatic basolateral efflux contributes significantly to rosuvastatin disposition II: characterization of hepatic elimination by basolateral, biliary, and metabolic clearance pathways in rat isolated perfused liver

Basolateral efflux clearance (CLBL) contributes significantly to rosuvastatin (RSV) elimination in sandwich-cultured hepatocytes (SCH). The contribution of CLBL to RSV hepatic elimination was determined in single-pass isolated perfused livers (IPLs) from wild-type (WT) and multidrug resistance-associated protein 2 (Mrp2)-deficient (TR(-)) rats in the absence and presence of the P-glycoprotein and breast cancer resistance protein (Bcrp) inhibitor, elacridar (GF120918); clearance values were compared with SCH. RSV biliary clearance (CLBile) was ablated almost completely by GF120918 in TR(-) IPLs, confirming that Mrp2 and Bcrp primarily are responsible for RSV CLBile. RSV appearance in outflow perfusate was attributed primarily to CLBL, which was impaired in TR(-) IPLs. CLBL was ≈ 6-fold greater than CLBile in the linear range in WT IPLs in the absence of GF120918. Recovery of unchanged RSV in liver tissue increased in TR(-) compared with WT (≈ 25 versus 6% of the administered dose) due to impaired CLBL and CLBile. RSV pentanoic acid, identified by high-resolution liquid chromatography-tandem mass spectroscopy, comprised ≈ 40% of total liver content and ≈ 16% of the administered dose in TR(-) livers at the end of perfusion, compared with ≈ 30 and 3% in WT livers, consistent with impaired RSV excretion and "shunting" to the metabolic pathway. In vitro-ex vivo extrapolation between WT SCH and IPLs (without GF120918) revealed that uptake clearance and CLBL were 4.2- and 6.4-fold lower, respectively, in rat SCH compared with IPLs; CLBile translated almost directly (1.1-fold). The present IPL data confirmed the significant role of CLBL in RSV hepatic elimination, and demonstrated that both CLBL and CLBile influence RSV hepatic and systemic exposure.

Hepatic basolateral efflux contributes significantly to rosuvastatin disposition I: characterization of basolateral versus biliary clearance using a novel protocol in sandwich-cultured hepatocytes

Transporters responsible for hepatic uptake and biliary clearance (CLBile) of rosuvastatin (RSV) have been well characterized. However, the contribution of basolateral efflux clearance (CLBL) to hepatic and systemic exposure of RSV is unknown. Additionally, the appropriate design of in vitro hepatocyte efflux experiments to estimate CLBile versus CLBL remains to be established. A novel uptake and efflux protocol was developed in sandwich-cultured hepatocytes (SCH) to achieve desired tight junction modulation while maintaining cell viability. Subsequently, studies were conducted to determine the role of CLBL in the hepatic disposition of RSV using SCH from wild-type (WT) and multidrug resistance-associated protein 2 (Mrp2)-deficient (TR(-)) rats in the absence and presence of the P-glycoprotein and breast cancer resistance protein (Bcrp) inhibitor elacridar (GF120918). RSV CLBile was nearly ablated by GF120918 in TR(-) SCH, confirming that Mrp2 and Bcrp are responsible for the majority of RSV CLBile. Pharmacokinetic modeling revealed that CLBL and CLBile represent alternative elimination routes with quantitatively similar contributions to the overall hepatocellular excretion of RSV in rat SCH under baseline conditions (WT SCH in the absence of GF120918) and also in human SCH. Membrane vesicle experiments revealed that RSV is a substrate of MRP4 (Km = 21 ± 7 µM, Vmax = 1140 ± 210 pmol/min per milligram of protein). Alterations in MRP4-mediated RSV CLBL due to drug-drug interactions, genetic polymorphisms, or disease states may lead to changes in hepatic and systemic exposure of RSV, with implications for the safety and efficacy of this commonly used medication.

Induction of hepatic multidrug resistance-associated protein 3 by ethynylestradiol is independent of cholestasis and mediated by estrogen receptor

Multidrug resistance-associated protein 3 (Mrp3; Abcc3) expression and activity are up-regulated in rat liver after in vivo repeated administration of ethynylestradiol (EE), a cholestatic synthetic estrogen, whereas multidrug resistance-associated protein 2 (Mrp2) is down-regulated. This study was undertaken to determine whether Mrp3 induction results from a direct effect of EE, independent of accumulation of any endogenous common Mrp2/Mrp3 substrates resulting from cholestasis and the potential mediation of estrogen receptor (ER). In in vivo studies, male rats were given a single, noncholestatic dose of EE (5 mg/kg s.c.), and basal bile flow and the biliary excretion rate of bile salts and glutathione were measured 5 hours later. This treatment increased Mrp3 mRNA by 4-fold, detected by real-time polymerase chain reaction, despite the absence of cholestasis. Primary culture of rat hepatocytes incubated with EE (1-10 µM) for 5 hours exhibited a 3-fold increase in Mrp3 mRNA (10 µM), consistent with in vivo findings. The increase in Mrp3 mRNA by EE was prevented by actinomycin D, indicating transcriptional regulation. When hepatocytes were incubated with an ER antagonist [7α,17β-[9-[(4,4,5,5,5-Pentafluoropentyl)sulfinyl]nonyl]estra-1,3,5(10)-triene-3,17-diol (ICI182/780), 1 µM], in addition to EE, induction of Mrp3 mRNA was abolished, implicating ER as a key mediator. EE induced an increase in ER-α phosphorylation at 30 minutes and expression of c-Jun, a well-known ER target gene, at 60 minutes, as detected by Western blotting of nuclear extracts. These increases were prevented by ICI182/780. In summary, EE increased the expression of hepatic Mrp3 transcriptionally and independently of any cholestatic manifestation and required participation of an ER, most likely ER-α, through its phosphorylation.

Placental and fetal disposition of mercuric ions in rats exposed to methylmercury: role of Mrp2

Methylmercury is a prevalent environmental toxicant that can have deleterious effects on a developing fetus. Previous studies indicate that the multidrug resistance-associated protein 2 (Mrp2) is involved in renal and hepatic export of mercuric ions. Therefore, we hypothesize that Mrp2 is also involved in export of mercuric ions from placental trophoblasts and fetal tissues. To test this hypothesis, we assessed the disposition of mercuric ions in pregnant Wistar and TR(-) (Mrp2-deficient) rats exposed to a single dose of methylmercury. The amount of mercury in renal tissues (cortex and outer stripe of outer medulla), liver, blood, amniotic fluid, uterus, placentas and fetuses was significantly greater in TR(-) rats than in Wistar rats. Urinary and fecal elimination of mercury was greater in Wistar dams than in TR(-) dams. Thus, our findings suggest that Mrp2 may be involved in the export of mercuric ions from maternal and fetal organs following exposure to methylmercury.

Involvement of Multiple Transporters-mediated Transports in Mizoribine and Methotrexate Pharmacokinetics

Mizoribine is administered orally and excreted into urine without being metabolized. Many research groups have reported a linear relationship between the dose and peak serum concentration, between the dose and AUC, and between AUC and cumulative urinary excretion of mizoribine. In contrast, a significant interindividual variability, with a small intraindividual variability, in oral bioavailability of mizoribine is also reported. The interindividual variability is mostly considered to be due to the polymophisms of transporter genes. Methotrexate (MTX) is administered orally and/or by parenteral routes, depending on the dose. Metabolic enzymes and multiple transporters are involved in the pharmacokinetics of MTX. The oral bioavailability of MTX exhibits a marked interindividual variability and saturation with increase in the dose of MTX, with a small intraindividual variability, where the contribution of gene polymophisms of transporters and enzymes is suggested. Therapeutic drug monitoring of both mizoribine and MTX is expected to improve their clinical efficacy in the treatment of rheumatoid arthritis.

LOSS of Mrp1 alters detoxification enzyme expression in a tissue- and hormonal-status-specific manner

The multidrug resistance-associated protein1 (MRP1/ABCC1) is a member of the ABCC transporter subfamily that mediates the efflux of pharmaceuticals, xenobiotics and steroid hormones, typically as glutathione, glucuronide or sulfate conjugates. Since loss of one transporter can be compensated by increasing the expression of other transporters and conjugation enzymes, we sought to examine compensatory changes in phase I, II and III enzyme expression in extrahepatic tissues, including the kidney, lungs and small intestine of intact or castrated Mrp1(-/-) male mice. In the kidney, the expression of several P450s, sulfotransferase 1a1 (Sult), glucuronosyltransferases (Ugt) and Mrps2-4, were significantly changed owing to castration alone. The only time genotype mattered was between the castrated FVB and Mrp1 knockout mice. In contrast, expression of the Ugts, Sult 1a1 and Mrp3 in the lungs was significantly downregulated in the Mrp1 knockout mice, so based exclusively on genotype. In the small intestine, there were interactions between steroid hormone levels and genotype, as the expression differences were only found in mice lacking Mrp1, and were changed between intact and castrated animals. The mechanism behind this pattern of expression may be to due to Nrf2 regulation, as its expression mirrors that of the phase II and phase III enzymes. These results indicate that compensatory responses owing to the loss of Mrp1 vary dramatically, depending on the particular tissue. This information will aid in the understanding of how drug uptake, disposition and elimination can be influenced by both hormone status and the presence and magnitude of transporter expression.

Membrane transporters in drug development

Membrane transporters have wide, but specific tissue distributions. They can impact on multiple endogenous and xenobiotic processes. Knowledge and awareness within the pharmaceutical industry of their impact on drug absorption, distribution, metabolism and elimination (ADME) and drug safety is growing rapidly. Clinically important transporter-mediated drug-drug interactions (DDIs) have been observed. Up to nine diverse transporters are implicated in the DDIs of a number of widely prescribed drugs, posing a significant challenge to the pharmaceutical industry. There is a complex interplay between multiple transporters and/or enzymes in the ADME and pharmacogenomics of drugs. Integrating these different mechanisms to understand their relative contributions to ADME is a key challenge. Many different factors complicate the study of membrane transporters in drug development. These include a lack of specific substrates and inhibitors, non-standard in vitro tools, and competing/complementary mechanisms (e.g. passive permeability and metabolism). Discovering and contextualizing the contribution of membrane transporters to drug toxicity is a significant new challenge. Drug interactions with key membrane transporters are routinely assessed for central nervous system (CNS) drug discovery therapies, but are not generally considered across the wider drug discovery. But, there is interest in utilizing membrane transporters as drug delivery agents. Computational modeling approaches, notably physiology-based/pharmacokinetic (PB/PK) modeling are increasingly applied to transporter interactions, and permit integration of multiple ADME mechanisms. Because of the range of tissues and transporters of interest, robust transporter, in vitro to in vivo, scaling factors are required. Empirical factors have been applied, but absolute protein quantitation will probably be required.

Understanding the interplay of drug transporters involved in the disposition of rosuvastatin in the isolated perfused rat liver using a physiologically-based pharmacokinetic model

The role of hepatic uptake (Oatp1a1 and Oatp1b4) and efflux (Bcrp and Mrp2) transporters in the disposition of rosuvastatin were investigated using the isolated perfused rat liver (IPRL). A simple physiologically-based pharmacokinetic model was developed to quantitatively determine the interplay between the individual transporters. Uptake and elimination of rosuvastatin in the IPRL was rapid and extensive. In the presence of rifamycin (an equipotent inhibitor of both Oatp1a1 and Oatp1a4) the perfusate clearance of rosuvastatin was reduced, but rifampicin (a potent inhibitor of Oatp1a4) had no effect upon the perfusate clearance. This might indicate a limited role for Oatp1a4, but it is possible that Oatp1a1 (or other uptake transporters) may have redundancy in their affinity for rosuvastatin. In the presence of GF120918 (a potent inhibitor of Bcrp) and in the Wistar TR- rat (a naturally occurring mutant not expressing Mrp2) the biliary clearance was reduced and virtually abolished in the TR- pre-incubated GF120918. Bcrp and Mrp2 appear to represent the primary efflux mechanisms for rosuvastatin in the rat. Rosuvastatin disposition in the IPRL is mediated in part by Oatp1a1 and efflux is almost entirely by Mrp2 and Bcrp. Other uptake processes may be involved.

Characterization of organic anion-transporting polypeptide (Oatp) 1a1 and 1a4 null mice reveals altered transport function and urinary metabolomic profiles

Organic anion-transporting polypeptides (Oatp) 1a1 and 1a4 were deleted by homologous recombination, and mice were characterized for Oatp expression in liver and kidney, transport in isolated hepatocytes, in vivo disposition of substrates, and urinary metabolomic profiles. Oatp1a1 and Oatp1a4 proteins were undetected in liver, and both lines were viable and fertile. Hepatic constitutive messenger RNAs (mRNAs) for Oatp1a4, 1b2, or 2b1 were unchanged in Oatp1a1⁻/⁻ mice, whereas renal Oatp1a4 mRNA decreased approximately 50% (both sexes). In Oatp1a4⁻/⁻ mice, no changes in constitutive mRNAs for other Oatps were observed. Uptake of estradiol-17β-D-glucuronide and estrone-3-sulfate in primary hepatocytes decreased 95 and 75%, respectively, in Oatp1a1⁻/⁻ mice and by 60 and 30%, respectively, in Oatp1a4⁻/⁻ mice. Taurocholate uptake decreased by 20 and 50% in Oatp1a1⁻/⁻ and Oatp1a4⁻/⁻ mice, respectively, whereas digoxin was unaffected. Plasma area under the curve (AUC) for estradiol-17β-D-glucuronide increased 35 and 55% in male and female Oatp1a1⁻/⁻ mice, respectively, with a concurrent 50% reduction in liver-to-plasma ratios. In contrast, plasma AUC or tissue concentrations of estradiol-17β-D-glucuronide were unchanged in Oatp1a4⁻/⁻ mice. Plasma AUCs for dibromosulfophthalein increased nearly threefold in male Oatp1a1⁻/⁻ and Oatp1a4⁻/⁻ mice, increased by 40% in female Oatp1a4⁻/⁻ mice, and were unchanged in female Oatp1a1⁻/⁻ mice. In both lines, no changes in serum ALT, bilirubin, and cholesterol were noted. NMR analyses showed no generalized increase in urinary excretion of organic anions. However, urinary excretion of taurine decreased by 30-40% and was accompanied by increased excretion of isethionic acid, a taurine metabolite generated by intestinal bacteria, suggesting some perturbations in intestinal bacteria distribution.

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).

In vivo probes of drug transport: commonly used probe drugs to assess function of intestinal P-glycoprotein (ABCB1) in humans

Intestinal P-glycoprotein (P-gp, ABCB1) may significantly influence drug absorption and elimination. Its expression and function is highly variable, regio-selective and influenced by genetic polymorphisms, drug interactions and intestinal diseases. An in vivo probe drug for intestinal P-gp should a registered, safe and well tolerated nonmetabolized selective substrate with low protein binding for which P-gp is rate-limiting during absorption. Other P-gp dependent processes should be of minor influence. The mechanism(s) and kinetics of intestinal uptake must be identified and quantified. Moreover, the release properties of the dosage form should be known. So far, the cardiac glycoside digoxin and the ß₁-selective blocker talinolol have been used in mechanistic clinical studies, because they meet most of these criteria. Digoxin and talinolol are suitable in vivo probe drugs for intestinal P-gp under the precondition, that they are used as tools in carefully designed pharmacokinetic studies with adequate biometrically planning of the sample size and that several limitations are considered in interpreting and discussion of the study results.

Decreased hepatic breast cancer resistance protein expression and function in multidrug resistance-associated protein 2-deficient (TR⁻) rats

Multidrug resistance-associated protein (Mrp) 2-deficient (TR(-)) Wistar rats have been used to elucidate the role of Mrp2 in drug disposition. Decreased breast cancer resistance protein (Bcrp) levels were reported in sandwich-cultured hepatocytes (SCH) from TR(-) rats compared with those from wild-type (WT) rats. This study was designed to characterize hepatic Bcrp expression and function in TR(-) rats, using nitrofurantoin and pitavastatin as substrates. Bcrp was knocked down by RNA interference in rat SCH. Antibody BXP53, but not BXP21, specifically detected Bcrp knockdown in SCH. Bcrp protein levels were decreased markedly in TR(-) but not Mrp2-deficient Sprague-Dawley [Eisai hyperbilirubinemic rats (EHBR)] rats. Bcrp mRNA levels were decreased significantly in TR(-) livers as determined by TaqMan real-time reverse transcriptase-polymerase chain reaction. Biliary excretion of nitrofurantoin, a specific Bcrp substrate, was decreased significantly in SCH and isolated perfused livers from TR(-) rats compared with those from WT controls, indicating that hepatic Bcrp function is decreased in TR(-) rats. In Bcrp knockdown SCH, the biliary excretion index and in vitro biliary clearance of pitavastatin were decreased significantly to ∼ 58 and ∼ 52% of control, respectively, indicating that Bcrp plays a role in pitavastatin biliary excretion. Pitavastatin biliary excretion was decreased significantly in perfused livers from TR(-) compared with those from WT rats. In conclusion, expression and function of hepatic Bcrp are decreased significantly in TR(-) rats. The potential role of both Bcrp and Mrp2 should be considered when data generated in TR(-) rats are interpreted. TR(-) and EHBR rats in combination may be useful in differentiating the role of Mrp2 and Bcrp in drug/metabolite disposition.

Synergistic influence of Abcb1 and Abcc2 on disposition and sterol lowering effects of ezetimibe in rats

Pharmacokinetics of the sterol-lowering drug ezetimibe (EZ) is influenced by intestinal ABCB1 and ABCC2. This study in Lew.1W rats with "chemical" and genetic Abcb1 and Abcc2 deficiency was initiated to evaluate the individual contribution of both efflux carriers to the overall disposition and sterol-lowering effects of EZ. Disposition and sterol-lowering effects of EZ (5 mg/kg, 14 days) were measured in wild-type (WT) and Abcc2-deficient (Abcc2-) rats (N = 8 per group) and in animals treated with PSC833 (20 mg/kg) to generate "chemical" Abcb1-deficiency (Abcb1-, Abcb1-/Abcc2-). EZ serum levels decreased in the order WT (3.11 +/- 1.09 ng/mL), Abcb1- (1.94 +/- 1.10 ng/mL), Abcc2- (1.42 +/- 0.42 ng/mL, p = 0.003 vs. WT), Abcb1-/Abcc2- (1.17 +/- 0.53 ng/mL, p = 0.002 vs. WT) whereas the serum EZ glucuronide levels increased as follows: WT (23.2 +/- 24.6 ng/mL), Abcb1- (119 +/- 74.5 ng/mL, p = 0.002 vs. WT), Abcc2- (195+/-76.5 ng/mL, p < 0.001 vs. WT), Abcb1-/Abcc2- (676 +/- 207 ng/mL, p < 0.001 vs. WT, Abcb1- and Abcc2-). Abcb1 and Abcc2 protein deficiency resulted synergistically in lower fecal but increased renal excretion of total EZ although to a much lower extent. The sterol-lowering effects of EZ were significantly correlated to serum levels of EZ. In conclusion, Abcb1 and Abcc2 deficiency leads to lower levels of the active EZ and in turn to decreased sterol-lowering effects.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

ABC transporters and drug efflux at the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic physical and biological barrier between blood circulation and the central nervous system (CNS). This unique feature of the BBB lies in the structure of the neurovascular unit and its cerebral micro-vascular endothelial cells. The BBB restricts the passage of blood-borne drugs, neurotoxic substances and peripheral immune cells from entering the brain, while selectively facilitating the transport of nutrients across the BBB into the brain. Thus, the integrity and proper function of the BBB is crucial to homeostasis and physiological function of the CNS. A number of transport and carrier systems are expressed and polarized on the luminal or abluminal surface of the BBB to realize these discrete functions. Among these systems, ABC transporters play a critical role in keeping drugs and neurotoxic substances from entering the brain and in transporting toxic metabolites out of the brain. A number of studies have demonstrated that ABCB1 and ABCG2 are critical to drug efflux at the BBB and that ABCC1 is essential for the blood-cerebral spinal fluid (CSF) barrier. The presence of these efflux ABC transporters also creates a major obstacle for drug delivery into the brain. We have comprehensively reviewed the literature on ABC transporters and drug efflux at the BBB. Understanding the molecular mechanisms of these transporters is important in the development of new drugs and new strategies for drug delivery into the brain.

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.

Role of intestinal efflux transporters in the intestinal absorption of methotrexate in rats

The role of intestinal efflux transporters such as P-glycoprotein (P-gp), breast cancer resistance protein (BCRP) and multidrug resistance-associated proteins (MRPs) in intestinal absorption of methotrexate was examined in rats. In everted intestine, the mucosal efflux of methotrexate after application to serosal side was higher in jejunum than ileum, and the efflux in jejunum was suppressed by pantoprazole, a BCRP inhibitor, and probenecid, an MRP inhibitor, but not by verapamil, a P-gp inhibitor. The mucosal methotrexate efflux in ileum was suppressed by pantoprazole, but not by other inhibitors. On the other hand, the serosal efflux of methotrexate after application to mucosal side was greater in ileum than jejunum, and was suppressed by probenecid. In in-vivo rat studies, the intestinal absorption of methotrexate was significantly higher when methotrexate was administered to ileum than jejunum. Pantoprazole increased methotrexate absorption from jejunum and ileum. Probenecid increased the absorption of methotrexate from jejunum but decreased the absorption from ileum, as evaluated by peak plasma methotrexate levels. In conclusion, BCRP and MRPs are involved in the regional difference in absorption of methotrexate along the intestine, depending on their expression sites.

Site-specific bidirectional efflux of 2,4-dinitrophenyl-S-glutathione, a substrate of multidrug resistance-associated proteins, in rat intestine and Caco-2 cells

The site-specific function of multidrug-resistance-associated proteins (MRPs), especially MRP2 and MRP3, was examined in rat intestine and human colon adenocarcinoma (Caco-2) cells. The MRP function was evaluated pharmacokinetically by measuring the efflux transport of 2,4-dinitrophenyl-S-glutathione (DNP-SG), an MRP substrate, after application of 1-chloro-2,4-dinitrobenzene (CDNB), a precursor of DNP-SG. The expression of rat and human MRP2 and MRP3 was analysed by Western blotting. The rat jejunum exhibited a higher apical MRP2 and a lower basolateral MRP3 expression than ileum. In accordance with the expression level, DNP-SG efflux to the mucosal surface was significantly greater in jejunum, while serosal efflux was greater in ileum. Site-specific bidirectional efflux of DNP-SG was also observed in in-vivo studies, in which portal and femoral plasma levels and biliary excretion rate of DNP-SG were significantly higher when CDNB was administered to ileum. Caco-2 cells also showed a bidirectional efflux of DNP-SG. Probenecid, an MRP inhibitor, significantly suppressed the mucosal efflux in jejunum and serosal efflux in ileum. In contrast, probenecid significantly suppressed both apical and basolateral efflux of DNP-SG in Caco-2 cells, though the inhibition was of small magnitude. In conclusion, the efflux of DNP-SG from enterocytes mediated by MRPs exhibited a significant regional difference in rat intestine, indicating possible variability in intestinal bioavailabilities of MRP substrates, depending on their absorption sites along the intestine.

Interplay of phase II enzymes and transporters in futile cycling: influence of multidrug resistance-associated protein 2-mediated excretion of estradiol 17beta-D-glucuronide and its 3-sulfate metabolite on net sulfation in perfused TR(-) and Wistar rat liver preparations

The hepatic disposition of estradiol 17beta-D-glucuronide (E(2)17G), a substrate of the organic anion-transporting polypeptides Oatp1a1, Oatp1a4, and Oatp1b2, was investigated in Wistar and TR(-) [multidrug resistance-associated protein (Mrp) 2-mutant] rats to elucidate how absence of Mrp2, the major excretory transporter for both E(2)17G and its 3-sulfate metabolite (E(2)3S17G), affected the net sulfation. With absence of Mrp2, lower microsomal desulfation activity and higher Mrp3 but unchanged immunoreactive protein expression of other transporters (Oatps and Mrp4) and estrogen sulfotransferase were found in TR(-) rats. In recirculating, perfused liver preparations, the rapid decay of E(2)17G and sluggish appearance of low levels of E(2)3S17G in perfusate for Wistar livers were replaced by a protracted, biexponential decay of E(2)17G and greater accumulation of E(2)3S17G, whose levels reached plateaus upon the almost complete obliteration of biliary excretion of E(2)17G and E(2)3S17G in the TR(-) liver. Much higher amounts of E(2)17G (28x) and E(2)3S17G (11x) in liver and reduced net sulfation (40 +/- 6 from 77 +/- 6% dose, P < 0.05) were observed at 2 h for the TR(-) versus the Wistar rats. With use of a physiologically based pharmacokinetic model, analytical solutions for the areas under the curve for the precursor and metabolite were obtained to reveal how enzyme- and transporter-mediated processes affected the hepatic disposition of the precursor and metabolite in futile cycling. The analytical solutions were useful to explain transporter-enzyme interplay in futile cycling and predicted that a shutdown of Mrp2 function led to decreased net sulfation of E(2)17G by raising the intracellular concentration of the metabolite, E(2)3S17G, which readily refurnished E(2)17G via desulfation.

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.

Liver X receptor regulates expression of MRP2 but not that of MDR1 and BCRP in the liver

Liver X receptors (LXRs) belong to the nuclear hormone receptor superfamily. Multidrug resistance-associated protein 2 (MRP2), multidrug resistance 1 (MDR1) and breast cancer resistance protein (BCRP) play an important role in the efflux of a broad range of endogenous and xenobiotic compounds from hepatocytes. Since the effects of LXR activation on there transporters have been obscure, we investigated the effects of LXR agonists, TO901317 and 25-hydroxycholesterol, on MRP2, MDR1, BCRP expression in HepG2 cells and the rat liver. In an in vitro study, TO901317 increased ABCA1, an LXR target gene, and MRP2 mRNA and protein levels. On the other hand, TO901317 had little effect on MDR1 and BCRP mRNA levels. In an in vivo study, Abca1 and Mrp2 mRNA and protein levels were increased by TO901317, but TO901317 had no effect on Mdr1a and Bcrp mRNA levels in the rat liver. Moreover, TO901317-induced MRP2 mRNA expression was blocked by LXRalpha knockdown. In this study, we demonstrated that LXR activation induced expression of MRP2 but not that of MDR1 and BCRP in hepatocytes. The results suggest that agonists for LXR activate transcription of the MRP2 gene in order to promote excretion of endogenous and xenobiotic compounds from hepatocytes into bile.