有机阳离子转运体的基因多态性研究进展

人体的肝脏、肾脏和肠道上广泛分布有各种转运蛋白,其中有机阳离子转运体（OCTs）负责转运一些内、外源性的有机阳离子的转运。在此我们主要讨论的是OCTs家族的成员OCT1和OCT2的基因多态性以及其功能意义。同时我们也就OCT对于药物代谢动力学以及药效学的影响进行阐述,以及OCTs在药物相互作用上的意义。     

Intestinal absorption of drugs mediated by drug transporters: mechanisms and regulation

The absorption of drugs from the gastrointestinal tract is one of the important determinants for oral bioavailability. Development of in vitro experimental techniques such as isolated membrane vesicles and cell culture systems has allowed us to elucidate the transport mechanisms of various drugs across the plasma membrane. Recent introduction of molecular biological techniques resulted in the successful identification of drug transporters responsible for the intestinal absorption of a wide variety of drugs. Each transporter exhibits its own substrate specificity, though it usually shows broad substrate specificity. In this review, we first summarize the recent advances in the characterization of drug transporters in the small intestine, classified into peptide transporters, organic cation transporters and organic anion transporters. In particular, peptide transporter (PEPT1) is the best-characterized drug transporter in the small intestine, and therefore its utilization to improve the oral absorption of poorly absorbed drugs is briefly described. In addition, regulation of the activity and expression levels of drug transporters seems to be an important aspect, because alterations in the functional characteristics and/or expression levels of drug transporters in the small intestine could be responsible for the intra- and interindividual variability of oral bioavailability of drugs. As an example, regulation of the activity and expression of PEPT1 is summarized.     

有机阳离子转运体的研究进展

有机阳离子转运体(Organic cation transporters,OCTs)参与许多重要内源性物质和药物的体内转运过程,其作用愈来愈被人们所重视.本文就近年来有关OCTs与内源性物质和药物的相互作用、OCTs的表达水平调节及基因多态性的影响等方面的研究进展进行综述.     

Polyspecific Organic Cation Transporters: Structure,Function, Physiological Roles,and Biopharmaceutical Implications

The body is equipped with broad-specificity transporters for the excretion and distribution of endogeneous organic cations and for the uptake, elimination and distribution of cationic drugs, toxins and environmental waste products. This group of transporters consists of the electrogenic cation transporters OCT1-3 (SLC22A1-3), the cation and carnitine transporters OCTN1 (SLC22A4), OCTN2 (SLC22A5) and OCT6 (SLC22A16), and the proton/cation antiporters MATE1, MATE2-K and MATE2-B. The transporters show broadly overlapping sites of expression in many tissues such as small intestine, liver, kidney, heart, skeletal muscle, placenta, lung, brain, cells of the immune system, and tumors. In epithelial cells they may be located in the basolateral or luminal membranes. Transcellular cation movement in small intestine, kidney and liver is mediated by the combined action of electrogenic OCT-type uptake systems and MATE-type efflux transporters that operate as cation/proton antiporters. Recent data showed that OCT-type transporters participate in the regulation of extracellular concentrations of neurotransmitters in brain, mediate the release of acetylcholine in non-neuronal cholinergic reactions, and are critically involved in the regulation of histamine release from basophils. The recent identification of polymorphisms in human OCTs and OCTNs allows the identification of patients with an increased risk for adverse drug reactions. Transport studies with expressed OCTs will help to optimize pharmacokinetics during development of new drugs.     

The role of organic ion transporters in drug disposition: an update

Transporters for organic anions and organic cations in kidney, liver, intestine, brain, and placenta play essential roles in drug disposition. The cloning and characterization of these transporters have significantly advanced our understanding of the molecular and cellular mechanisms of the drug disposition process. This review aims at updating the recent knowledge of general properties, structure-function relationships, and regulation mechanisms of the organic anion transporters (OATs) and the organic cation transporters (OCTs). Such information will be essential for the design and development of new drugs to maximize therapeutic efficacy and minimize drug-induced toxicity as well as unwanted drug-drug interactions.     

Organic cation transporters

1.?Organic cation transporters (OCTs) translocate endogenous (e.g. dopamine) and exogenous (e.g. drugs) substances of cationic nature and, therefore, play an important role in the detoxification of exogenous compounds. This review aims to furnish essential information on OCTs, with an emphasis on pharmacological aspects.

2.?Analysis of the literature on OCTs makes clear that there is a species- and organ-specific distribution of the different isoforms, which can also be differentially regulated. OCTs are responsible for the excretion and/or distribution of many drugs and also for serious tissue-specific side-effects such as cisplatin-induced nephrotoxicity. The presence of single nucleotide polymorphisms in these transporters significantly influences the response of patients to medication, as demonstrated for the antidiabetic drug metformin.

3.?A substantial amount of research has to be undertaken to clarify further the OCT structure–function relationships specifically to define the role of oligomerization on their activity and regulation, to identify intracellular interaction partners of OCTs, and to characterize their pharmacogenetic aspects.     

The inhibitory effects of five alkaloids on the substrate transport mediated through human organic anion and cation transporters

1.?Human solute carrier transporters (SLCs) are important membrane proteins mediate the cellular transport of many endogenous and exogenous substances. Organic anion/cation transporters (OATs/OCTs) and organic anion transporting polypeptides (OATPs) are essential SLCs involved in drug influx. Drug–drug/herb interactions through competing for specific SLCs often lead to unsatisfied therapeutic outcomes and/or unwanted side effects. In this study, we comprehensively investigated the inhibitory effects of five clinically relevant alkaloids (dendrobine, matrine, oxymatrine, tryptanthrin and chelerythrine) on the substrate transport through several OATs/OCTs and OATPs.

2.?We performed transport functional assay and kinetic analysis on the HEK-293 cells over-expressing each SLC gene.

3.?Our data showed tryptanthrin significantly inhibited the transport activity of OAT3 (IC₅₀?=?0.93?±?0.22?μM, K_i?=?0.43?μM); chelerythrine acted as a potent inhibitor to the substrate transport mediated through OATP1A2 (IC₅₀?=?0.63?±?0.43?μM, K_i?=?0.60?μM), OCT1 (IC₅₀?=?13.60?±?2.81?μM) and OCT2 (IC₅₀?=10.80?±?1.16?μM).

4.?Our study suggested tryptanthrin and chelerythrine could potently impact on the drug transport via specific OATs/OCTs. Therefore, the co-administration of these alkaloids with drugs could have clinical consequences due to drug–drug/herb interactions. Precautions should be warranted in the multi-drug therapies involving these alkaloids.     

Activation of ATP-sensitive K+ channels by ADP and K+ channel openers: homology model of sulfonylurea receptor carboxyl-termini

The multispecific organic anion transporters have been indicated to be involved in the transmembrane transport of various anionic substances. The kidney and liver possess the distinct organic anion transport pathways for the elimination of potentially toxic anionic drugs and metabolites. In the kidney, proximal tubular cells actively excrete organic anions of both endogenous and exogenous origin. We have isolated the renal multispecific organic anion transporter, OAT1 (organic anion transporter 1), from the rat kidney. OAT1 is a 551-amino acid residue protein with 12 putative membrane spanning domains. OAT1 mediates sodium-independent, anion exchange for a variety of organic anions including p-aminohippurate, cyclic nucleotides, prostanoides, dicarboxylates, and anionic drugs including beta-lactams, non-steroidal antiinflammatory drugs, diuretics and antiviral drugs. So far, three other isoforms have been identified. OATs comprise a new family of multispecific organic anion transporter, i.e., the OAT family. OATs show weak structural similarity to organic cation transporters (OCTs) and OCTN/carnitine transporters. All of the members of the OAT family are commonly expressed in the kidney, suggesting its significance in the renal organic anion excretion. In addition, OAT members appear to be responsible for the distribution/elimination of water soluble anionic drugs into/from the liver, brain and fetus.     

Organic Cation Transporter OCTs (SLC22) and MATEs (SLC47) in the Human Kidney

In the kidney, human organic cation transporters (OCTs) and multidrug and toxin extrusion proteins (MATEs) are the major transporters for the secretion of cationic drugs into the urine. In the human kidney, OCT2 mediates the uptake of drugs from the blood at the basolateral membrane of tubular epithelial cells, and MATE1 and MATE2-K secrete drugs from cells into the lumen of proximal tubules. However, the expression of these transporters depends on the species of the animal. In the rodent kidney, OCT1 and OCT2 are expressed at the basolateral membrane, and MATE1 localizes at the brush-border membrane. Together, these transporters recognize various compounds and have overlapping, but somewhat different, substrate specificities. OCTs and MATEs can transport important drugs, such as metformin and cisplatin. Therefore, functional variation in OCTs and MATEs, including genetic polymorphisms or inter-individual variation, may seriously affect the pharmacokinetics and/or pharmacodynamics of cationic drugs. In this review, we summarize the recent findings and clinical importance of these transporters.     

The Complexities of Hepatic Drug Transport: Current Knowledge and Emerging Concepts

Recently, hepatic transport processes have been recognized as important determinants of drug disposition. Therefore, it is not surprising that characterization of the hepatic transport and biliary excretion properties of potential drug candidates is an important part of the drug development process. Such information also is useful in understanding alterations in the hepatobiliary disposition of compounds due to drug interactions or disease states. Basolateral transport systems are responsible for translocating molecules across the sinusoidal membrane, whereas active canalicular transport systems are responsible for the biliary excretion of drugs and metabolites. Several transport proteins involved in basolateral transport have been identified including the Na(+)-taurocholate co-transporting polypeptide [NTCP (SLC10A1)], organic anion transporting polypeptides [OATPs (SLCO family)], multidrug resistance-associated proteins [MRPs (ABCC family)], and organic anion and cation transporters [OATs, OCTs (SLC22A family)]. Canalicular transport is mediated predominantly via P-glycoprotein (ABCB1), MRP2 (ABCC2), the bile salt export pump [BSEP (ABCB11)], and the breast cancer resistance protein [BCRP (ABCG2)]. This review summarizes current knowledge regarding these hepatic basolateral and apical transport proteins in terms of substrate specificity, regulation by nuclear hormone receptors and intracellular signaling pathways, genetic differences, and role in drug interactions. Transport knockout models and other systems available for hepatobiliary transport studies also are discussed. This overview of hepatobiliary drug transport summarizes knowledge to date in this rapidly growing field and emphasizes the importance of understanding these fundamental processes in hepatic drug disposition.     

Salbutamol: how does it enter smooth muscle cells?

Polyspecific organic cation transporters (OCTs) in human cell membranes are involved in the uptake, distribution and excretion of cationic compounds. Although their relevance to drug disposition in the liver, small intestine and kidney has been investigated previously, less is known about the influence of these transporters on the pharmacokinetics and pharmacodynamics of inhaled drugs. Drugs that are commonly administered by inhalation for the treatment of respiratory diseases, such as glucocorticoids and cationic β(2)-agonists, might interact with several of these transporters, which are strongly expressed on the surfaces of airway epithelial cells. We evaluated the expression of OCT3 and measured the in vitro uptake of the short-acting β(2)-agonist salbutamol (SALB), alone or in combination with corticosterone (CS) and beclomethasone dipropionate (BDP), by bronchial smooth muscle cells. Our results showed that these cells express the OCT3 transporter and that SALB enters the cell in a transporter-independent fashion. Moreover, CS and BDP have different activities on SALB transport inside the cell. CS increases SALB transport and BDP decreases SALB transport, although neither of these effects are statistically significant. A better understanding of these mechanisms might lead to the improved treatment of airway diseases.     

Overview of SLC22A and SLCO families of drug uptake transporters in the context of cancer treatments

The effectiveness of many anticancer agents is dependent on their disposition to the intracellular space of cancerous tissue. Accumulation of anticancer drugs at their sites of action can be altered by both uptake and efflux transport proteins, however the majority of research on the disposition of anticancer drugs has focused on drug efflux transporters and their ability to confer multidrug resistance. Here we review the roles of uptake transporters of the SLC22A and SLCO families in the context of cancer therapy. The many first-line anticancer drugs that are substrates of organic cation transporters (OCTs) organic cation/carnitine transporters (OCTNs) and organic anion- transporting polypeptides (OATPs) are summarized. In addition, where data is available a comparison of the localization of drug uptake transporters in healthy and cancerous tissues is provided. Expression of drug uptake transporters increases the sensitivity of cancer cell lines to anticancer substrates. Furthermore, early observational studies have suggested a causal link between drug uptake transporter expression and positive outcome in some cancers. Quantification of drug transporters by mass spectrometry will provide an essential technique for generation of expression data during future observational clinical studies. Screening of drug uptake transporter expression in primary tumors may help differentiate between susceptible and resistant cancers prior to therapy.     

疾病状态下有机阳离子转运体对底物转运影响的研究进展

有机阳离子转运体家族是一类位于细胞膜上的功能性膜蛋白,负责转运内源性、外源性阳离子底物,在底物的吸收、分布和排泄过程中发挥着重要作用,其中肝脏、肾脏和消化道处有机阳离子转运体的作用最明显。在疾病状态下有机阳离子转运体的表达和功能会发生改变,会影响底物的体内处置过程,使底物的动力学行为及其效应发生改变。综述了肝脏疾病、肾脏疾病和消化道疾病状态下有机阳离子转运体表达和功能改变对底物体内转运的影响及其生理、药理学意义。     

Role of organic cation transporters in drug-induced toxicity

INTRODUCTION: Membrane transporters are important determinants of in vivo drug disposition, therapeutic efficacy and adverse drug reactions. Many commonly used drugs are organic cations and substrates of organic cation transporters (OCTs). These transporters have a large binding site containing partially overlapping interaction domains for different substrates and are specifically distributed around the body. Consequently, drug interactions with these transporters can result in specific toxicity. AREAS COVERED: This review describes the general properties of OCT and illustrates their importance for the development of important drug toxicities using the examples of metformin and cisplatin. Additionally, this review discusses the role of OCT polymorphisms in the modulation of these toxic effects. EXPERT OPINION: Understanding how drugs interact with membrane transporters is pivotally important in explaining the mechanisms of specific toxicities and also in designing new drugs or new therapeutic protective protocols by specific competition at the transporter. Defining the pharmacogenomics of these transporters will be essential to personalized medicine, enabling physicians to choose drugs for patients based on efficacy, availability, cost, safety, tolerability and convenience.     

Organic cation transporters

1. Organic cation transporters (OCTs) translocate endogenous (e.g. dopamine) and exogenous (e.g. drugs) substances of cationic nature and, therefore, play an important role in the detoxification of exogenous compounds. This review aims to furnish essential information on OCTs, with an emphasis on pharmacological aspects. 2. Analysis of the literature on OCTs makes clear that there is a species- and organ-specific distribution of the different isoforms, which can also be differentially regulated. OCTs are responsible for the excretion and/or distribution of many drugs and also for serious tissue-specific side-effects such as cisplatin-induced nephrotoxicity. The presence of single nucleotide polymorphisms in these transporters significantly influences the response of patients to medication, as demonstrated for the antidiabetic drug metformin. 3. A substantial amount of research has to be undertaken to clarify further the OCT structure-function relationships specifically to define the role of oligomerization on their activity and regulation, to identify intracellular interaction partners of OCTs, and to characterize their pharmacogenetic aspects.     

Interaction of organic cations with a newly identified plasma membrane monoamine transporter

Many endogenous compounds and xenobiotics are organic cations that rely on polyspecific organic cation transporters (OCTs) to traverse cell membranes. We recently cloned a novel human plasma membrane monoamine transporter (PMAT) that belongs to the equillibrative nucleoside transporter (ENT) family. We have reported previously that, unlike other ENTs, PMAT (also known as ENT4) is a Na+-independent and membrane potential-sensitive transporter that transports monoamine neurotransmitters and the neurotoxin 1-methyl-4-phenylpyridinium (MPP+). These compounds are the known substrates for OCTs, which raises the possibility that PMAT functions as a polyspecific transporter like the OCTs. In the present study, we analyzed the interaction of PMAT with a series of structurally diverse organic cations using MDCK cells stably expressing human PMAT. Our study showed that PMAT interacts with many organic cations that have heterogeneous chemical structures. PMAT transports classic OCT substrates, such as tetraethylammonium, guanidine, and histamine. Prototype OCT inhibitors, including cimetidine, and type II cations (e.g., quinidine, quinine, verapamil, and rhodamine123) are also PMAT inhibitors. An analysis of molecular structures and apparent binding affinities revealed that charge and hydrophobicity are the principal determinants for transporter-substrate/inhibitor interaction. A planar aromatic mass seems to be important for high affinity interaction. trans-Stimulation and efflux studies demonstrate that PMAT is able to mediate bidirectional transport. These functional properties of PMAT are strikingly similar to those of the OCTs. We therefore conclude that PMAT can function as a polyspecific organic cation transporter, which may play a role in organic cation transport in vivo.     

Membrane transport function in primary cultures of human proximal tubular cells

To further develop primary cultures of human proximal tubular (hPT) cells for study of drug disposition, we determined kinetics and protein expression of several key transporters for organic anions and cations, peptides, and neutral amino acids. p-Aminohippurate uptake exhibited similar kinetics as published values, was inhibited by cephaloridine, cimetidine, methotrexate, and urate, consistent with function of both organic anion transporter 1 (OAT1) and OAT3. Transport rates by organic cation transporters (OCTs) were up to three-fold higher than those of OATs. Of the OCT substrates tested, triethanolamine exhibited the highest transport rates across the basolateral membrane (BLM). OCTN1 exhibited high-affinity, low-capacity BLM transport of l-carnitine. Glycylsarcosine transport by PepT2 was rapid and comparable to that of OCTs. Amino acid System L on the BLM exhibited comparable kinetic parameters for transport of l-leucine as the OATs. Efflux of verapamil across the brush-border membrane by P-glycoprotein was very rapid. Expression of carriers was generally maintained throughout 5 days of culture. Of the four OAT proteins studied (OAT1-4), expression of OAT1 and OAT3 was the most readily detected and exhibited interindividual variation. OCTN2 was the major OCT in hPT cells. Expression was also quantified for multidrug resistance-associated proteins 2 and 5 and P-glycoprotein. These results show that primary cultures of hPT cells express a diverse array of transporters for major classes of important drugs and are suitable for study of drug transport and disposition and assessment of potential drug-drug interactions in human kidney.     

Pharmacogenetics of OATP (SLC21/SLCO), OAT and OCT (SLC22) and PEPT (SLC15) transporters in the intestine, liver and kidney

The role of carrier-mediated transport in determining the pharmacokinetics of drugs has become increasingly evident with the discovery of genetic variants that affect expression and/or function of a given drug transporter. Drug transporters are expressed at numerous epithelial barriers, such as intestinal epithelial cells, hepatocytes, renal tubular cells and at the blood-brain barrier. Several recent studies have associated alterations in substrate uptake with the presence of SNPs. Here, we summarize the current knowledge on the functional and phenotypic consequences of genetic variation in intestinally, hepatically and renally expressed members of the organic anion-transporting polypeptide family (OATPs; SLC21/SLCO family), the organic anion and organic cation transporters (OATs/OCTs; SLC22 family) and the peptide transporter-1 (PEPT1; SLC15 family).