Induction of drug metabolism enzymes and transporters by oltipraz in rats

Regulation of the ATP-binding cassette transporters ABCB1, ABCG2 and ABCC5 by nuclear receptors in porcine blood-brain barrier endothelial cells

BACKGROUND AND PURPOSE

Blood-brain barrier (BBB) ABCB1, ABCG2 and ABCC5 transporters influence central therapeutic drug distribution. Transporter expression is regulated by the NR3C1, NR1I3 and NR1I2 nuclear receptors, but their precise roles in brain are poorly understood. We investigated the effects of selective ligand-based activation of NR3C1, NR1I3, NR1I2 and NR2B1 in porcine brain endothelial cells (PBECs).

EXPERIMENTAL APPROACH

Primary cultures of PBECs were exposed to NR3C1, NR1I3 and NR1I2 ligands and ABCB1, ABCG2 and ABCC5 transporter activities determined by measuring intracellular accumulation of fluorescent probes. Western blotting was used to determine the effects of receptor ligands on expression of ABCB1, ABCG2, ABCC5, NR1I2, NR1I3, NR3C1 and NR2B1. Fluorescent immunocytochemistry was employed to assess the effects of receptor ligands on the cellular localisation of NR1I2 and NR1I3.

KEY RESULTS

The NR1I2 agonist rifampicin significantly up-regulated ABCG2 activity, which is counteracted by co-treatment with NR1I2 antagonist l-sulforaphane. The NR1I3 agonist 6-(4-chlorophenyl)-imidazo[2,1-b]thiazole-5-carbaldehyde and inverse agonist meclizine significantly down-regulated ABCB1, ABCG2 and ABCC5 activity. NR3C1 agonist dexamethasone significantly increased ABCB1, ABCG2 and ABCC5 activity and ABCG2 and ABCC5 protein expression, which was counteracted by co-treatment with the NR3C1 antagonist mifepristone. This first study demonstrates that NR1I3 and NR3C1 regulate ABCC5 activity and protein expression in BBB endothelial cells.

CONCLUSIONS AND IMPLICATIONS

In PBECs, expression of key ATP-binding cassette (ABC) transporters and nuclear receptors is differentially regulated by NR1I3, NR1I2, NR3C1 and NR2B1. This will help to better understand the response of the BBB to physiological and pharmacological activation of nuclear receptors.

Identification of trichlormethiazide as a Mdr1a/b gene expression enhancer via a dual secretion-based promoter assay

Transporters of the ATP-binding cassette (ABC) family such as MDR1 play a pivotal role in persistence of brain homeostasis by contributing to the strict permeability properties of the blood–brain barrier. This barrier on one hand compromises treatment of central nervous system diseases by restricting access of drugs; on the other hand, an impaired or altered function of barrier building cells has been described in neurological disorders. The latter might contribute to increased vulnerability of the brain under pathological conditions or even enforce pathogenesis. Here, we present a novel approach for a systematic examination of drug impact on Mdr1 gene expression by establishing a dual reporter gene assay for the murine upstream core promoters of Mdr1a and b. We validated the time-resolved assay in comparison with single reporter gene constructs and applied it to analyze effects of a Food and Drug Administration (FDA)-approved drug library consisting of 627 substances. The chemo-preventive synthetic dithiolethione oltipraz was reidentified with our assay as an already known inducer of Mdr1 gene expression. Together with two newly characterized modifiers – gemcitabine and trichlormethiazide – we prove our findings in a blood–brain barrier culture model as well as in wild-type and Mdr1 knockout mice. In sum, we could demonstrate that our dual reporter gene assay delivers results, which also persist in the living animal and consequently is applicable for further analysis and prediction of Mdr1 regulation in vivo.

Identification of a functional antioxidant response element within the eighth intron of the human ABCC3 gene

The ATP-binding cassette (ABC) family of transporters, including ABCC3, is a large family of efflux pumps that plays a pivotal role in the elimination of xenobiotics from the body. ABCC3 has been reported to be induced during hepatic stress conditions and through the progression of some forms of cancer. Several lines of evidence have implicated the transcription factor nuclear factor (erythroid-derived 2)-like 2 (Nrf2) in this induction. However, although rodent models have been investigated, a functional antioxidant response element (ARE) in the human ABCC3 gene has not been identified. The purpose of this study was to identify and characterize the ARE(s) responsible for mediating the Nrf2-dependent induction of the human ABCC3 gene. A high-throughput chromatin immunoprecipitation-sequencing analysis performed in A549 cells revealed a specific interaction between Nrf2 and the eighth intron of the human ABCC3 gene rather than the more prototypical flanking region of the gene. Subsequent in silico analysis of the intron identified two putative ARE elements that contained the core consensus ARE sequence commonly found in several Nrf2-responsive genes. Functional characterization of these two AREs using luciferase-reporter constructs with ARE mutant constructs revealed that one of these putative AREs is functionally active. Finally, DNA pull-down assays confirmed specific binding of these intronic AREs by Nrf2 in vitro. Our findings identify a functional Nrf2 response element within the eighth intron of the ABCC3 gene, which may provide mechanistic insight into the induction of ABCC3 during antioxidant response stimuli.

Organic and inorganic transporters of the testis: A review

Transporters have a huge impact on the toxicology and pharmacological effects of xenobiotics in addition to being implicated in several diseases. While these important proteins have been well studied in organs such as the kidney or liver, characterization of transporters in the testis is still in the early stages. Knowledge of transporter function may greatly advance the field's understanding of the physiological and toxicological processes that occur in the testis. Several foundational studies involving both organic and inorganic transporters have been critical in furthering our understanding of how the testis interacts with endogenous and xenobiotic compounds. This review provides an overview of how transporters function, their clinical significance, and highlights what is known for many of the important transporters in the testis.

Regulation of drug-metabolizing enzymes by xenobiotic receptors: PXR and CAR

Drug-metabolizing enzymes (DMEs) and transporters play pivotal roles in the disposition and detoxification of numerous foreign and endogenous chemicals. To accommodate chemical challenges, the expression of many DMEs and transporters is up-regulated by a group of ligand-activated transcription factors namely nuclear receptors (NRs). The importance of NRs in xenobiotic metabolism and clearance is best exemplified by the most promiscuous xenobiotic receptors: pregnane X receptor (PXR, NR1I2) and constitutive androstane/activated receptor (CAR, NR1I3). Together, these two receptors govern the inductive expression of a largely overlapping array of target genes encoding phase I and II DMEs, and drug transporters. Moreover, PXR and CAR also represent two distinctive mechanisms of NR activation, whereby CAR demonstrates both constitutive and ligand-independent activation. In this review, recent advances in our understanding of PXR and CAR as xenosensors are discussed with emphasis placed on the differences rather than similarities of these two xenobiotic receptors in ligand recognition and target gene regulation.

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.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Therapeutic potential of dithiolethiones for hepatic diseases

Comprehensive studies support the notion that oltipraz [4-methyl-5-(2-pyrazynyl)-1,2-dithiole-3-thione] and its congeners exert cancer chemopreventive effects by the prevention, inhibition or reversal of carcinogenic processes. Recently, it was found that dithiolethione compounds had the activities to prevent or treat fibrosis, insulin resistance, and mitochondrial protective effects in the liver by a mechanism involving AMP-activated protein kinase (AMPK) and/or 70-kDa ribosomal protein S6 kinase 1 (S6K1). Moreover, chemical regulation of the AMPK-S6K1 pathway was found to affect Liver X receptor (LXR) activity and lipogenesis, leading to the identification of AMPK and S6K1 as targets for treating hepatic steatosis. These biological activities of dithiolethiones may offer a novel approach to pharmaceutical intervention. This review focuses on the interaction between oltipraz and the AMPK-mTOR-S6K1 pathway, which regulates genes that confer hepatocyte protection from intoxication, disrupted energy metabolism, and inflammation. In terms of therapeutic potential, the findings reviewed here demonstrate a new therapeutic potential for dithiolethiones, which function in a unique manner, and offer the possibility of new treatments for hepatic diseases.

Mammalian epoxide hydrolases in xenobiotic metabolism and signalling

Epoxide hydrolases catalyse the hydrolysis of electrophilic--and therefore potentially genotoxic--epoxides to the corresponding less reactive vicinal diols, which explains the classification of epoxide hydrolases as typical detoxifying enzymes. The best example is mammalian microsomal epoxide hydrolase (mEH)-an enzyme prone to detoxification-due to a high expression level in the liver, a broad substrate selectivity, as well as inducibility by foreign compounds. The mEH is capable of inactivating a large number of structurally different, highly reactive epoxides and hence is an important part of the enzymatic defence of our organism against adverse effects of foreign compounds. Furthermore, evidence is accumulating that mammalian epoxide hydrolases play physiological roles other than detoxification, particularly through involvement in signalling processes. This certainly holds true for soluble epoxide hydrolase (sEH) whose main function seems to be the turnover of lipid derived epoxides, which are signalling lipids with diverse functions in regulatory processes, such as control of blood pressure, inflammatory processes, cell proliferation and nociception. In recent years, the sEH has attracted attention as a promising target for pharmacological inhibition to treat hypertension and possibly other diseases. Recently, new hitherto uncharacterised epoxide hydrolases could be identified in mammals by genome analysis. The expression pattern and substrate selectivity of these new epoxide hydrolases suggests their participation in signalling processes rather than a role in detoxification. Taken together, epoxide hydrolases (1) play a central role in the detoxification of genotoxic epoxides and (2) have an important function in the regulation of physiological processes by the control of signalling molecules with an epoxide structure.

The Nrf2 activator oltipraz also activates the constitutive androstane receptor

Oltipraz (OPZ) is a well known inducer of NAD(P)H:quinone oxidoreductase (NQO1) along with other enzymes that comprise the nuclear factor E2-related factor 2 (Nrf2) battery of detoxification genes. However, OPZ treatment also induces expression of CYP2B, a gene regulated by the constitutive androstane receptor (CAR). Therefore, this study was designed to determine whether OPZ induces gene expression in the mouse liver through activation of CAR in addition to Nrf2. OPZ increased the mRNA expression of both Cyp2b10 and Nqo1 in C57BL/6 mouse livers. As expected, in livers from Nrf2-/- mice, OPZ induction of Nqo1 was reduced, indicating Nqo1 induction is dependent on Nrf2 activation, whereas Cyp2b10 induction was unchanged. The robust induction of Cyp2b10 by OPZ in wild-type mice was completely absent in CAR-/- mice, revealing a CAR-dependent induction by OPZ. OPZ also induced transcription of the human CYP2B6 promoter-reporter containing the phenobarbital (PB) responsive element in mouse liver using an in vivo transcription assay. Additionally, OPZ induced in vivo nuclear accumulation of CAR at 3 h but, as with PB, was unable to reverse androstanol repression of mouse CAR constitutive activity in transiently transfected HepG2 cells. In summary, OPZ induces expression of Cyp2b10 and Nqo1 via the activation of CAR and Nrf2, respectively.