Identification and characterization of novel polymorphisms in the basal promoter of the human transporter, MATE1

In Vitro and In Vivo Inhibition of MATE1 by Tyrosine Kinase Inhibitors

The membrane transport of many cationic prescription drugs depends on facilitated transport by organic cation transporters of which several members, including OCT2 (SLC22A2), are sensitive to inhibition by select tyrosine kinase inhibitors (TKIs). We hypothesized that TKIs may differentially interact with the renal transporter MATE1 (SLC47A1) and influence the elimination and toxicity of the MATE1 substrate oxaliplatin. Interactions with FDA-approved TKIs were evaluated in transfected HEK293 cells, and in vivo pharmacokinetic studies were performed in wild-type, MATE1-deficient, and OCT2/MATE1-deficient mice. Of 57 TKIs evaluated, 37 potently inhibited MATE1 function by >80% through a non-competitive, reversible, substrate-independent mechanism. The urinary excretion of oxaliplatin was reduced by about 2-fold in mice with a deficiency of MATE1 or both OCT2 and MATE1 (p < 0.05), without impacting markers of acute renal injury. In addition, genetic or pharmacological inhibition of MATE1 did not significantly alter plasma levels of oxaliplatin, suggesting that MATE1 inhibitors are unlikely to influence the safety or drug-drug interaction liability of oxaliplatin-based chemotherapy.

Organic Cation Transporters in Health and Disease

The organic cation transporters (OCTs) OCT1, OCT2, OCT3, novel OCT (OCTN)1, OCTN2, multidrug and toxin exclusion (MATE)1, and MATE kidney-specific 2 are polyspecific transporters exhibiting broadly overlapping substrate selectivities. They transport organic cations, zwitterions, and some uncharged compounds and operate as facilitated diffusion systems and/or antiporters. OCTs are critically involved in intestinal absorption, hepatic uptake, and renal excretion of hydrophilic drugs. They modulate the distribution of endogenous compounds such as thiamine, L-carnitine, and neurotransmitters. Sites of expression and functions of OCTs have important impact on energy metabolism, pharmacokinetics, and toxicity of drugs, and on drug-drug interactions. In this work, an overview about the human OCTs is presented. Functional properties of human OCTs, including identified substrates and inhibitors of the individual transporters, are described. Sites of expression are compiled, and data on regulation of OCTs are presented. In addition, genetic variations of OCTs are listed, and data on their impact on transport, drug treatment, and diseases are reported. Moreover, recent data are summarized that indicate complex drug-drug interaction at OCTs, such as allosteric high-affinity inhibition of transport and substrate dependence of inhibitor efficacies. A hypothesis about the molecular mechanism of polyspecific substrate recognition by OCTs is presented that is based on functional studies and mutagenesis experiments in OCT1 and OCT2. This hypothesis provides a framework to imagine how observed complex drug-drug interactions at OCTs arise. Finally, preclinical in vitro tests that are performed by pharmaceutical companies to identify interaction of novel drugs with OCTs are discussed. Optimized experimental procedures are proposed that allow a gapless detection of inhibitory and transported drugs.

Transcriptional activity of FIGLA, NEUROG2, and EGR1 transcription factors associated with polymorphisms in the proximal regulatory region of GPR54 gene in cattle

Activity of transcription factors affect synthesis of G-protein coupled receptor 54 (GPR54), an important factor in regulation of initiation of puberty. Expression of the GPR54 gene in cattle is associated with polymorphisms in the proximal regulatory region (PRR) of the GPR54 gene. Transcription resulting in production of GPR54 mRNA transcript occurs as a result of transcription factor (TF) interactions in the PRR. Polymorphisms in the PRR may be associated with extent of activity of these TFs. Folliculogenesis-specific BHLH TF (FIGLA), neurogenin 2 (NEUROG2), and early growth response 1 (EGR1) are important in modulation of ovarian follicle development and neurons synthesizing GnRH, thus, regulating biosynthesis of luteinizing hormone. The aim of this study, therefore, was to assess the transcription-activating potential of binding sites for FIGLA, NEUROG2, and EGR1 TFs in the GPR54 promoter of cattle. Two luciferase-based promoters, ATC and CCT, which contain three single nucleotide polymorphisms (SNPs), A/C-794, T/C-663, and C/T-601, in the GPR54 PRR, were analyzed to evaluate gene expression and activation of different promoters by FIGLA, NEUROG2, and EGR1. The FIGLA induced GPR54 transcription through the CCT, whereas NEUROG2 and EGR1 induced GPR54 transcription through the ATC promoter-binding site. The CCT-activating effects of FIGLA were greater (2.56-fold) than the ATC-activating effects (P < 0.05). The ATC-activating effects of NEUROG2 and EGR1 were markedly greater (12.91- and 8.41-fold; P < 0.01) than CCT-activating effects. The polymorphisms, CCT and ATC, of the cattle GPR54 affect the activity of transcription factors, therefore, have an important effect on production of GPR54 mRNA transcript.

Impact of Promoter Polymorphisms on the Transcriptional Regulation of the Organic Cation Transporter OCT1 (SLC22A1)

The organic cation transporter 1 (OCT1, SLC22A1) is strongly expressed in the human liver and facilitates the hepatic uptake of drugs such as morphine, metformin, tropisetron, sumatriptan and fenoterol and of endogenous substances such as thiamine. OCT1 expression is inter-individually highly variable. Here, we analyzed SNPs in the OCT1 promoter concerning their potential contribution to the variability in OCT1 expression. Using electrophoretic mobility shift and luciferase reporter gene assays in HepG2, Hep3B, and Huh7 cell lines, we identified the SNPs −1795G>A (rs6935207) and −201C>G (rs58812592) as having effects on transcription factor binding and/or promoter activity. The A-allele of the −1795G>A SNP showed allele-specific binding of the transcription factor NF-Y leading to 2.5-fold increased enhancer activity of the artificial SV40 promoter. However, the −1795G>A SNP showed no significant effects on the native OCT1 promoter activity. Furthermore, the −1795G>A SNP was not associated with the pharmacokinetics of metformin, fenoterol, sumatriptan and proguanil in healthy individuals or tropisetron efficacy in patients undergoing chemotherapy. Allele-dependent differences in USF1/2 binding and nearly total loss in OCT1 promoter activity were detected for the G-allele of −201C>G, but the SNP is apparently very rare. In conclusion, common OCT1 promoter SNPs have only minor effects on OCT1 expression.

Rare Variants in the ABCG2 Promoter Modulate In Vivo Activity

ABCG2 encodes the breast cancer resistance protein (BCRP), an efflux membrane transporter important in the detoxification of xenobiotics. In the present study, the basal activity of the ABCG2 promoter in liver, kidney, intestine, and breast cell lines was examined using luciferase reporter assays. The promoter activities of reference and variant ABCG2 sequences were compared in human hepatocellular carcinoma cell (HepG2), human embryonic kidney cell (HEK293T), human colorectal carcinoma cell (HCT116), and human breast adenocarcinoma cell (MCF-7) lines. The ABCG2 promoter activity was strongest in the kidney and intestine cell lines. Four variants in the basal ABCG2 promoter (rs76656413, rs66664036, rs139256004, and rs59370292) decreased the promoter activity by 25%-50% in at least three of the four cell lines. The activity of these four variants was also examined in vivo using the hydrodynamic tail vein assay, and two single nucleotide polymorphisms (rs76656413 and rs59370292) significantly decreased in vivo liver promoter activity by 50%-80%. Electrophoretic mobility shift assays confirmed a reduction in nuclear protein binding to the rs59370292 variant probe, whereas the rs76656413 probe had a shift in transcription factor binding specificity. Although both rs59370292 and rs76656413 are rare variants in all populations, they could contribute to patient-level variation in ABCG2 expression in the kidney, liver, and intestine.

Influence of pharmacogenetic polymorphisms and demographic variables on metformin pharmacokinetics in an admixed Brazilian cohort

AIMS

To identify pharmacogenetic and demographic variables that influence the systemic exposure to metformin in an admixed Brazilian cohort.

METHODS

The extreme discordant phenotype was used to select 106 data sets from nine metformin bioequivalence trials, comprising 256 healthy adults. Eleven single-nucleotide polymorphisms in SLC22A1, SLC22A2, SLC47A1 SLC47A2 and in transcription factor SP1 were genotyped and a validated panel of ancestry informative markers was used to estimate the individual proportions of biogeographical ancestry. Two-step (univariate followed by multivariate) regression modelling was developed to identify covariates associated with systemic exposure to metformin, accessed by the area under the plasma concentration-time curve, between 0 and 48 h (AUC_0-48h ), after single oral doses of metformin (500 or 1000 mg).

RESULTS

The individual proportions of African, Amerindian and European ancestry varied widely, as anticipated from the structure of the Brazilian population The dose-adjusted, log-transformed AUC_0-48h 's (ng h ml^-1  mg^-1 ) differed largely in the two groups at the opposite ends of the distribution histogram, namely 0.82, 0.79-0.85 and 1.08, 1.06-1.11 (mean, 95% confidence interval; P = 6.10^-26 , t test). Multivariate modelling revealed that metformin AUC_0-48h increased with age, food and carriage of rs12208357 in SLC22A1 but was inversely associated with body surface area and individual proportions of African ancestry.

CONCLUSIONS

A pharmacogenetic marker in OCT1 (SLC22A1 rs12208357), combined with demographic covariates (age, body surface area and individual proportion of African ancestry) and a food effect explained 29.7% of the variability in metformin AUC_0-48h .

Identification and Functional Characterization of ST3GAL5 and ST8SIA1 Variants in Patients with Thyroid-Associated Ophthalmopathy

PURPOSE

This study was conducted to identify and to functionally characterize genetic variants in ST3GAL5 and ST8SIA1 in Korean patients with thyroid-associated ophthalmopathy (TAO).

MATERIALS AND METHODS

Genetic analyses were conducted using DNA samples from TAO patients (n=50) and healthy subjects (n=48) to identify TAO-specific genetic variants of ST3GAL5 or ST8SIA1. The effect of each genetic variant on the transcription or expression of these genes was examined. Additionally, correlations between functional haplotypes of ST3GAL5 or ST8SIA1 and clinical characteristics of the patients were investigated.

RESULTS

Six promoter variants and one nonsynonymous variant of ST3GAL5 were identified, and four major promoter haplotypes were assembled. Additionally, three promoter variants and two major haplotypes of ST8SIA1 were identified. All ST3GAL5 and ST8SIA1 variants identified in TAO patients were also found in healthy controls. Promoter activity was significantly decreased in three promoter haplotypes of ST3GAL5 and increased in one promoter haplotype of ST8SIA1. Transcription factors activating protein-1, NKX3.1, and specificity protein 1 were revealed as having roles in transcriptional regulation of these haplotypes. The nonsynonymous variant of ST3GAL5, H104R, did not alter the expression of ST3GAL5. While no differences in clinical characteristics were detected in patients possessing the functional promoter haplotypes of ST3GAL5, exophthalmic values were significantly lower in patients with the ST8SIA1 haplotype, which showed a significant increase in promoter activity.

CONCLUSION

These results from genotype-phenotype analysis might suggest a possible link between the ST8SIA1 functional promoter haplotype and the clinical severity of TAO. However, further studies with larger sample sizes are warranted.

Relationship between DNA Methylation in the 5' CpG Island of the SLC47A1 (Multidrug and Toxin Extrusion Protein MATE1) Gene and Interindividual Variability in MATE1 Expression in the Human Liver

Multidrug and toxin extrusion protein 1 (MATE1), which is encoded by solute carrier 47A1 (SLC47A1), mediates the excretion of organic cations into bile and urine. Some genetic variants in human MATE1 altered its transport function in in vitro experiments; however, differences in the pharmacokinetics of substrate drugs cannot be explained by genetic variations in humans. In this study, we investigated whether DNA methylation was involved in interindividual variability in MATE1 expression in the human liver. Approximately 20-fold interindividual variability in MATE1 mRNA expression levels was observed in liver samples and mRNA expression levels negatively correlated with methylation levels of the CpG island in the 27 kb upstream of SLC47A1 DNA demethylation by treatment with 5-aza-2'-deoxycytidine increased MATE1 mRNA expression in MATE1-negative cell lines. The luciferase reporter assay showed that the CpG island increased the transcriptional activity of the SLC47A1 promoter. MATE1 mRNA expression levels were significantly lower in CpG island knockout HepG2 cells than in control cells. These results suggest that the 5' CpG island of SLC47A1 acts as an enhancer for SLC47A1, and DNA methylation in the CpG island plays an important role in interindividual differences in hepatic MATE1 expression.

The Effect of Famotidine, a MATE1-Selective Inhibitor, on the Pharmacokinetics and Pharmacodynamics of Metformin

INTRODUCTION

Pharmacokinetic outcomes of transporter-mediated drug-drug interactions (TMDDIs) are increasingly being evaluated clinically. The goal of our study was to determine the effects of selective inhibition of multidrug and toxin extrusion protein 1 (MATE1), using famotidine, on the pharmacokinetics and pharmacodynamics of metformin in healthy volunteers.

METHODS

Volunteers received metformin alone or with famotidine in a crossover design. As a positive control, the longitudinal effects of famotidine on the plasma levels of creatinine (an endogenous substrate of MATE1) were quantified in parallel. Famotidine unbound concentrations in plasma reached 1 µM, thus exceeding the in vitro concentrations that inhibit MATE1 [concentration of drug producing 50 % inhibition (IC50) 0.25 µM]. Based on current regulatory guidance, these concentrations are expected to inhibit MATE1 clinically [i.e. maximum unbound plasma drug concentration (C max,u)/IC50 >0.1].

RESULTS

Consistent with MATE1 inhibition, famotidine administration significantly altered creatinine plasma and urine levels in opposing directions (p < 0.005). Interestingly, famotidine increased the estimated bioavailability of metformin [cumulative amount of unchanged drug excreted in urine from time zero to infinity (A e∞)/dose; p < 0.005] without affecting its systemic exposure [area under the plasma concentration-time curve (AUC) or maximum concentration in plasma (C max)] as a result of a counteracting increase in metformin renal clearance. Moreover, metformin-famotidine co-therapy caused a transient effect on oral glucose tolerance tests [area under the glucose plasma concentration-time curve between time zero and 0.5 h (AUCglu,0.5); p < 0.005].

CONCLUSIONS

These results suggest that famotidine may improve the bioavailability and enhance the renal clearance of metformin.

Differential increments of basal glucagon-like-1 peptide concentration among SLC47A1 rs2289669 genotypes were associated with inter-individual variability in glycaemic response to metformin in Chinese people with newly diagnosed Type 2 diabetes

AIM

To elucidate the effects of rs2289669, an intron variant of the SLC47A1 gene, on glucose response to metformin in Chinese people with newly diagnosed Type 2 diabetes.

METHODS

Rs2289669 was genotyped, using Sequenom, in 291 participants receiving 48 weeks of metformin monotherapy. The changes in HbA_1c were compared among rs2289669 genotypes, and associations with rs2289669 were evaluated using linear regression analysis.

RESULTS

We found that, compared with participants with a homozygous G allele, those carrying the minor A allele had significantly greater HbA_1c reduction and greater increases in basal glucagon-like peptide-1 concentration. Regression analysis showed that there was a significant association between rs2289669 and the glucose response to metformin after adjusting for confounding factors, except for changes in basal glucagon-like peptide-1, for which an association was not observed.

CONCLUSIONS

Our findings suggest that rs2289669 might help predict the glycaemic response to metformin in Chinese people newly diagnosed with Type 2 diabetes, and that differential increases in basal glucagon-like peptide-1 concentration among rs2289669 genotypes might be associated with inter-individual response to metformin.

ABC transporter polymorphisms are associated with irinotecan pharmacokinetics and neutropenia

Neutropenia is a common dose-limiting toxicity associated with irinotecan treatment. Although UGT1A1 variants have been associated with neutropenia, a fraction of neutropenia risk remains unaccounted for. To identify additional genetic markers contributing to variability in irinotecan pharmacokinetics and neutropenia, a regression analysis was performed in 78 irinotecan-treated patients to analyze comprehensively three hepatic efflux transporter genes (ABCB1, ABCC1 and ABCG2). rs6498588 (ABCC1) and rs12720066 (ABCB1) were associated with increased SN-38 exposure, and rs17501331 (ABCC1) and rs12720066 were associated with lower absolute neutrophil count nadir. rs6498588 and a variant in high linkage disequilibrium are located in transcriptionally active regions or are predicted to alter transcription factor binding sites. While enhancer activity was not evident in vitro for genomic regions containing these single-nucleotide polymorphisms, rs6498588 was significantly associated with ABCC1 expression in human liver. These results suggest that genetic variation in ABCC1 and ABCB1 may contribute to irinotecan-induced neutropenia by altering expression of transporters involved in irinotecan metabolite disposition.

The role of variants regulating metformin transport and action in women with polycystic ovary syndrome

Aims: Variants in genes encoding metformin transport proteins and the ATM gene are associated with metformin response. We hypothesized that these gene variants contribute to variable metformin treatment response in polycystic ovary syndrome. Materials & methods: The discovery cohort (n = 38) was studied in an open-label study. Results were replicated in two additional cohorts (n = 26 and n = 131). Response was assessed after 3–6 months of treatment with metformin extended-release 1500–2000 mg/day. Results: The rs683369 variant was associated with less weight loss in the discovery cohort (p = 0.003), but these results were not replicated (p = 0.8). There were no differences in glucose parameters, testosterone levels or ovulatory frequency as a function of genotype. Conclusion: Variants in organic ion transporters do not explain the variable metformin response in polycystic ovary syndrome.

Lack of genomic diversity in the SLC47A1 gene within the indigenous Xhosa population

BACKGROUND

Multidrug and toxin extrusion 1 (MATE1) is an organic cation/H+ exchanger, localized in the apical membrane of proximal renal tubules, which mediates the cellular elimination of organic cations into the renal lumen. These organic cations include clinically important drugs such as metformin, oxaliplatin and cimetidine. Moreover, genetic polymorphisms of SLC47A1, the pharmacogenetically relevant gene encoding human MATE1, have been implicated in reduced transport or accumulation to cytotoxic levels of these drugs in vitro. However, little or no information is available on the minor allele frequency distribution of known SLC47A1 coding SNPs in the sub-Saharan African populations.

METHODS

Thus, the aim of this study was to determine the baseline minor allele frequency distribution of 20 known coding SNPs in the SLC47A1 gene of 148 Xhosa individuals residing in Cape Town, South Africa.

RESULTS

This study did not identify any of these known SLC47A1 coding SNPs in the Xhosa individuals who participated in this study.

CONCLUSIONS

It is anticipated that whole genome or exome sequencing may reveal novel SNPs in the Xhosa and other sub-Saharan African populations, which may have been missed with the current genotyping strategy.

Structure and function of multidrug and toxin extrusion proteins (MATEs) and their relevance to drug therapy and personalized medicine

Multidrug and toxin extrusion (MATE; SLC47A) proteins are membrane transporters mediating the excretion of organic cations and zwitterions into bile and urine and thereby contributing to the hepatic and renal elimination of many xenobiotics. Transported substrates include creatinine as endogenous substrate, the vitamin thiamine and a number of drug agents with in part chemically different structures such as the antidiabetic metformin, the antiviral agents acyclovir and ganciclovir as well as the antibiotics cephalexin and cephradine. This review summarizes current knowledge on the structural and molecular features of human MATE transporters including data on expression and localization in different tissues, important aspects on regulation and their functional role in drug transport. The role of genetic variation of MATE proteins for drug pharmacokinetics and drug response will be discussed with consequences for personalized medicine.

Influence of genetic polymorphisms of multidrug and toxin extrusion protein 1 on its mRNA expression in peripheral blood cells

This study aimed to determine the effect of multidrug and toxin extrusion protein 1 (MATE1) genetic variants on its transcript expression in peripheral blood cells. Consistent with previous in vitro findings, MATE1 mRNA levels were significantly higher in subjects carrying rs2453579, but not rs2252281, compared to those without either of these promoter variants. In addition, the mRNA levels did not differ between subjects with both variants and those with neither allele. Thus, this study reveals that the influence of MATE1 genetic variants on its mRNA expression can be detected in vivo using peripheral blood.

Pharmacogenetics: Implications for Modern Type 2 Diabetes Therapy

Many clinical treatment studies have reported remarkable interindividual variability in the response to pharmaceutical drugs, and uncovered the existence of inadequate treatment response, non-response, and even adverse drug reactions. Pharmacogenetics addresses the impact of genetic variants on treatment outcome including side-effects. In recent years, it has also entered the field of clinical diabetes research. In modern type 2 diabetes therapy, metformin is established as first-line drug. The latest pharmaceutical developments, including incretin mimetics, dipeptidyl peptidase 4 inhibitors (gliptins), and sodium/glucose cotransporter 2 inhibitors (gliflozins), are currently experiencing a marked increase in clinical use, while the prescriptions of α-glucosidase inhibitors, sulfonylureas, meglitinides (glinides), and thiazolidinediones (glitazones) are declining, predominantly because of reported side-effects. This review summarizes the current knowledge about gene-drug interactions observed in therapy studies with the above drugs. We report drug interactions with candidate genes involved in the pharmacokinetics (e.g., drug transporters) and pharmacodynamics (drug targets and downstream signaling steps) of the drugs, with known type 2 diabetes risk genes and previously unknown genes derived from hypothesis-free approaches such as genome-wide association studies. Moreover, some new and promising candidate genes for future pharmacogenetic assessment are highlighted. Finally, we critically appraise the current state of type 2 diabetes pharmacogenetics in the light of its impact on therapeutic decisions, and we refer to major problems, and make suggestions for future efforts in this field to help improve the clinical relevance of the results, and to establish genetically determined treatment failure.

SLC47A1 gene rs2289669 G>A variants enhance the glucose-lowering effect of metformin via delaying its excretion in Chinese type 2 diabetes patients

AIMS

The SLC47A1 gene encodes the multi-drug and toxic excretion-1(MATE1) protein, which plays a key role in the transport and excretion of metformin. This study is to clarify the influence of variants in SLC47A1 (rs2289669 G→A) on metformin pharmacokinetics and the long-term glucose-lowering effect of metformin.

METHODS

A total of 220 newly diagnosed type 2 diabetes patients were recruited, genotyped and divided into three groups by SLC47A1 genotypes (G/G, G/A, A/A). Ten patients in each group were randomly selected for metformin pharmacokinetics. All the participants received metformin oral treatment and were followed for one year.

RESULTS

After one-year follow-up, the decline of HbA1c level was significantly greater in subjects with variant genotype (AA) than other two groups (-2.32% [-25.4 mmol/mol] in AA vs. -1.16% [-12.7 mmol/mol] in GA, -1.07% [-11.7 mmol/mol] in GG, P<0.05). Then taking GG genotype as the referent, the association between AA genotype and change of HbA1c still existed after adjusted for age, sex, BMI, baseline HbA1c and diabetes duration (P<0.05). Pharmacokinetic parameters of metformin indicated that patients carrying MATE1 homozygous A had higher area under the plasma concentration versus time curve (AUC12h), but lower renal clearance (CLR) and renal clearance by secretion (CLSR) than other patients (all P<0.01). Multivariate lineal stepwise analysis further revealed that SLC47A1 genotype was an independent impact factor for urine excretion of metformin (P<0.01).

CONCLUSIONS

SLC47A1 rs2289669 G>A variants improve the glucose-lowering effect of metformin through slowing its excretion in type 2 diabetes populations.

An update on the pharmacogenomics of metformin: progress, problems and potential

The increasing prevalence of Type 2 diabetes has emphasized the need to optimize treatment regimens. Metformin, the most widely used oral agent, is recommended as first-line drug therapy by multiple professional organizations. Response to metformin varies significantly at the individual level; this heterogeneity may be explained in part by genetic factors. Understanding these underlying factors may aid with tailoring treatment for individual patients as well as with designing improved Type 2 diabetes therapies. The past 10 years have seen substantial progress in the understanding of the pharmacogenetics of metformin response. The majority of this work has focused on genes involved in the pharmacokinetics of metformin. Owing to the uncertainty surrounding its mechanism of action, studies of pharmacodynamic genetics have been relatively few; genome-wide approaches have the potential to illuminate the molecular details of metformin response. In this review we summarize current knowledge about metformin pharmacogenetics and suggest directions for future investigation.

Genetic variants in transcription factors are associated with the pharmacokinetics and pharmacodynamics of metformin

One-third of type 2 diabetes patients do not respond to metformin. Genetic variants in metformin transporters have been extensively studied as a likely contributor to this high failure rate. Here, we investigate, for the first time, the effect of genetic variants in transcription factors on metformin pharmacokinetics (PK) and response. Overall, 546 patients and healthy volunteers contributed their genome-wide, pharmacokinetic (235 subjects), and HbA1c data (440 patients) for this analysis. Five variants in specificity protein 1 (SP1), a transcription factor that modulates the expression of metformin transporters, were associated with changes in treatment HbA1c (P < 0.01) and metformin secretory clearance (P < 0.05). Population pharmacokinetic modeling further confirmed a 24% reduction in apparent clearance in homozygous carriers of one such variant, rs784888. Genetic variants in other transcription factors, peroxisome proliferator-activated receptor-α and hepatocyte nuclear factor 4-α, were significantly associated with HbA1c change only. Overall, our study highlights the importance of genetic variants in transcription factors as modulators of metformin PK and response.

Identification and functional characterization of novel nonsynonymous variants in the human multidrug and toxin extrusion 2-K

This study was performed to identify genetic polymorphisms in multidrug and toxin extrusion 2-K (MATE2-K, SLC47A2), a proton/organic cation antiporter that plays a role in the transport of organic cations across the apical membrane in kidney epithelial cells into the urine, and to demonstrate their effects on MATE2-K functions in vitro. Four of the thirty single nucleotide polymorphisms (SNPs) we identified in three ethnic groups (Caucasian, African American, and Japanese) were novel [308C>G (P103R), c.487-8C>T, 818A>G (Y273C), and c.1018+14T>C]. The transport activities of the prototypical substrates, tetraethylammonium and metformin, for four nonsynonymous SNPs (P103R, P162L, G211V, and Y273C) were significantly different from those of the wild-type. In particular, transport activity was higher in P103R than in the wild-type, which is the first time elevated transport activity was demonstrated due to these coding SNPs. Kinetic analysis revealed that P103R had a higher Vmax value, whereas Y273C had a lower value than that in the wild-type. Cell surface protein expression levels were higher for P103R than for the wild-type, whereas Y273C expression was decreased. Immunofluorescence analysis revealed that the P103R protein was localized to the plasma membrane, whereas Y273C showed cytoplasmic localization. Therefore, the difference in transport activities between P103R and Y273C variants was suggested to be responsible for the different protein expression levels observed at the plasma membrane. Four nonsynonymous SNPs in this study showed relatively low allelic frequencies (0.5 to 2.1%), but these were associated with markedly reduced or increased MATE2-K function.

Functional characterization of MATE2-K genetic variants and their effects on metformin pharmacokinetics

OBJECTIVE

Human multidrug and toxin extrusion member 2 (MATE2-K, SLC47A2) plays an important role in the renal elimination of various clinical drugs including the antidiabetic drug metformin. The goal of this study was to characterize genetic variants of MATE2-K and determine their association with the pharmacokinetics of metformin.

METHODS

We screened DNA samples from 48 healthy Koreans for variants in the promoter and coding regions of MATE2-K and examined the function of common haplotypes in the promoter region using in-vitro luciferase assays. Then, the metformin pharmacokinetic study was carried out to determine the association between MATE2-K promoter haplotypes and metformin pharmacokinetics.

RESULTS

Nine variants in the promoter region of MATE2-K and one nonsynonymous variant, p.G211V, were identified. The MATE2-K promoter haplotype 1 containing a known functional polymorphism, g.-130G>A and haplotype 2 containing two polymorphisms, g.-609G>A and g.-396G>A showed a significant increase in reporter activity. Among the 45 individuals who participated in the metformin pharmacokinetic study, 12 healthy Koreans who were homozygous for haplotype 1 or 2 showed a significant increase in renal clearance [539 ± 76 (reference group) vs. 633 ± 102 (variant group) ml/min; P=0.006] and secretion clearance [439 ± 81 (reference group) vs. 531 ± 102 (variant group) ml/min; P=0.007] of metformin compared with that shown by the reference group.

CONCLUSION

Our study suggests that common promoter haplotypes of MATE2-K are associated with the pharmacokinetics of metformin.

Emerging transporters of clinical importance: an update from the International Transporter Consortium

The International Transporter Consortium (ITC) has recently described seven transporters of particular relevance to drug development. Based on the second ITC transporter workshop in 2012, we have identified additional transporters of emerging importance in pharmacokinetics, interference of drugs with transport of endogenous compounds, and drug-drug interactions (DDIs) in humans. The multidrug and toxin extrusion proteins (MATEs, gene symbol SLC47A) mediate excretion of organic cations into bile and urine. MATEs are important in renal DDIs. Multidrug resistance proteins (MRPs or ABCCs) are drug and conjugate efflux pumps, and impaired activity of MRP2 results in conjugated hyperbilirubinemia. The bile salt export pump (BSEP or ABCB11) prevents accumulation of toxic bile salt concentrations in hepatocytes, and BSEP inhibition or deficiency may cause cholestasis and liver injury. In addition, examples are presented on the roles of nucleoside and peptide transporters in drug targeting and disposition.

The hydrodynamic tail vein assay as a tool for the study of liver promoters and enhancers

The hydrodynamic tail vein injection is a technique that is used to deliver nucleic acids into live mice. Delivery through this method results in the in vivo transfection of foreign DNA primarily in the liver. Here, we describe the use of this technique to test for regulatory activity of liver promoters and enhancers, using a dual luciferase reporter system as the readable/measureable output and how this application can be used for pharmacogenomic studies.

Metformin and cancer: from the old medicine cabinet to pharmacological pitfalls and prospects

Metformin is a biguanide derivative used in the treatment of type II diabetes (T2D) and one of the world's most widely prescribed drugs. Owing to its safety profile, it has been recently promoted for a range of other indications, particularly for its role in cancer prevention. There is evidence from studies in diabetic cohorts, as well as laboratory studies, that the action of metformin depends on a balance between the concentration and duration of exposure, which depends crucially on cell- and tissue-specific pharmacological factors. Mechanistic studies have revealed the involvement of increasingly complex pathways. Yet, there are several missing links regarding the role of drug transporters and drug-drug interactions, as well as the expression levels of transporters in normal versus tumor tissues, which may affect patient exposure and dosing when metformin is used in cancer prevention. This review highlights the current knowledge on metformin action and pharmacology, including novel insights into genomic factors, with a specific focus on cancer prevention. Furthermore, future challenges that may influence therapeutic outcome will be discussed.

The effect of novel promoter variants in MATE1 and MATE2 on the pharmacokinetics and pharmacodynamics of metformin

Interindividual variation in response to metformin, first-line therapy for type 2 diabetes, is substantial. Given that transporters are determinants of metformin pharmacokinetics, we examined the effects of promoter variants in both multidrug and toxin extrusion protein 1 (MATE1) (g.-66T → C, rs2252281) and MATE2 (g.-130G → A, rs12943590) on variation in metformin disposition and response. The pharmacokinetics and glucose-lowering effects of metformin were assessed in healthy volunteers (n = 57) receiving metformin. The renal and secretory clearances of metformin were higher (22% and 26%, respectively) in carriers of variant MATE2 who were also MATE1 reference (P < 0.05). Both MATE genotypes were associated with altered post-metformin glucose tolerance, with variant carriers of MATE1 and MATE2 having an enhanced (P < 0.01) and reduced (P < 0.05) response, respectively. Consistent with these results, patients with diabetes (n = 145) carrying the MATE1 variant showed enhanced metformin response. These findings suggest that promoter variants of MATE1 and MATE2 are important determinants of metformin disposition and response in healthy volunteers and diabetic patients.

Multidrug and toxin extrusion proteins as transporters of antimicrobial drugs

INTRODUCTION

Antimicrobial drugs are essential in the treatment of infectious diseases. A better understanding of transport processes involved in drug disposition will improve the predictability of drug-drug interactions with consequences for drug response. Multidrug And Toxin Extrusion (MATE; SLC47A) proteins are efflux transporters mediating the excretion of several antimicrobial drugs as well as other organic compounds into bile and urine, thereby contributing to drug disposition.

AREAS COVERED

This review summarizes current knowledge of the structural and molecular features of human MATE transporters including their functional role in drug transport with a specific focus on antimicrobial drugs. The PubMed database was searched using the terms "MATE1," "MATE-2K," "MATE2," "SLC47A1," "SLC47A2," and "toxin extrusion protein" (up to June 2012).

EXPERT OPINION

MATE proteins have been recognized as important transporters mediating the final excretion step of cationic drugs into bile and urine. These include the antiviral drugs acyclovir, amprenavir, and ganciclovir, the antibiotics cephalexin, cephradine and levofloxacin, as well as the antimalarial agents chloroquine and quinine. It is therefore important to enhance our understanding of the role of MATEs in drug extrusion with particular emphasis on the functional consequences of genetic variants on disposition of these antimicrobial drugs.

The pharmacogenetics of metformin and its impact on plasma metformin steady-state levels and glycosylated hemoglobin A1c

OBJECTIVE

The aim of this study was to evaluate the effect of genetic variations in OCT1, OCT2, MATE1, MATE 2, and PMAT on the trough steady-state plasma concentration of metformin and hemoglobin A1c (Hb1Ac).

METHOD

The South Danish Diabetes Study was a 2 x 2 x 2 factorial, prospective, randomized, double-blind, placebo-controlled, multicentre study. One hundred and fifty-nine patients received 1 g of metformin, twice daily continuously, and 415 repeated plasma metformin measurements were obtained after 3, 6, and 9 months of treatment.

RESULTS

The mean trough steady-state metformin plasma concentration was estimated to be 576 ng/ml (range, 54–4133 ng/ml, p = 0.55) and correlated to the number of reduced function alleles in OCT1 (none, one or two: 642, 542, 397 ng/ml; P = 0.001). The absolute decrease in Hb1Ac both initially and long term was also correlated to the number of reduced function alleles in OCT1 resulting in diminished pharmacodynamic effect of metformin after 6 and 24 months.

CONCLUSION

In a large cohort of type 2 diabetics, we either confirm or show for the first time: (a) an enormous (80-fold) variability in trough steady-state metformin plasma concentration, (b) OCT1 activity affects metformin steady-state pharmacokinetics, and (c) OCT1 genotype has a bearing on HbA1c during metformin treatment.

Effects of genetic variants in SLC22A2 organic cation transporter 2 and SLC47A1 multidrug and toxin extrusion 1 transporter on cisplatin-induced adverse events

BACKGROUND

Susceptibility to cisplatin (CDDP) nephrotoxicity is known to vary between individuals, but the basis of this variation has not been fully elucidated. In the kidney, CDDP is taken up into the renal proximal tubular cells mainly via SLC22A2 organic cation transporter 2 (OCT2) and secreted into lumen via other transporters including SLC47A1 multidrug and toxin extrusion 1 (MATE1). Here, we explore the effect of single-nucleotide polymorphisms (SNPs) at 808G>T in OCT2 and at rs2289669 G>A in MATE1 on CDDP-induced adverse events.

METHODS

Fifty-three patients who had been treated with CDDP were enrolled. The plasma concentration of CDDP was measured on days 4 and 7 after treatment. The grade of hematology and nephrotoxicity was evaluated by Common Terminology Criteria for Adverse Events.

RESULTS

In the first treatment cycle, serum creatinine (SCr) levels in the patients with OCT2 808GG and 808GT were increased by 1.43- and 1.19-fold, respectively. In the total treatment cycles, 12 patients (27 %) with 808GG experienced over grade 2 SCr elevation, whereas those with 808GT did not show any apparent nephrotoxicity. The hematological toxicity and plasma concentrations of CDDP showed no difference between patients in both groups. The rs2289669 G>A SNP in MATE1 was not associated with adverse effects and disposition of CDDP.

CONCLUSION

The 808G>T SNP in OCT2 ameliorated CDDP-induced nephrotoxicity without alteration of disposition, whereas the rs2289669 G>A SNP in MATE1 had no effect on CDDP toxicity.

Importance of the multidrug and toxin extrusion MATE/SLC47A family to pharmacokinetics, pharmacodynamics/toxicodynamics and pharmacogenomics

The renal organic cation transport system mediates the tubular secretion of cationic compounds including drugs, toxins and endogenous metabolites into urine. It consists of a membrane potential-dependent organic cation transporter at the basolateral membrane and an H(+) /organic cation antiporter at the brush-border membrane. In 2005, human multidrug and toxin extrusion MATE1/SLC47A1 was identified as a mammalian homologue of bacterial NorM. Thereafter, human MATE2-K/SLC47A2 and rodent MATE were found. Functional characterization revealed that MATE1 and MATE2-K were H(+) /organic cation antiporter, mediating the renal tubular secretion of cationic drugs in cooperation with the basolateral organic cation transporter OCT2. Recently, substrate specificity, transcription mechanisms, structure, polymorphisms, in vivo contributions and clinical outcomes on MATE have been investigated intensively. In this review, we summarize recent findings on MATE1/SLC47A1 and MATE2-K/SLC47A2 and discuss the importance of these transporters to the pharmacokinetics, pharmacodynamics/toxicodynamics and pharmacogenomics of cationic drugs.

Disposition of metformin: variability due to polymorphisms of organic cation transporters

Considerable interindividual variability in clinical efficacy is recognized in the treatment of type 2 diabetes mellitus with the biguanide metformin. Metformin is a substrate of organic cation transporters, which play important roles in gastrointestinal absorption, renal and biliary elimination, and distribution to target sites of substrate drugs. This raises the question of whether genetic variations in these transporters affect efficacy and risk of adverse events associated with metformin use. In this review, the pharmacogenetics of metformin is discussed in the light of the most recent literature. Overall, results from healthy volunteers support the notion that metformin pharmacokinetics can be affected by polymorphisms in genes encoding organic cation transporters. When considering the glycemic response to metformin in patients, however, the likely multifactorial nature of metformin response masks the effects of transporter polymorphisms observed in some clinical studies.

Genetic and epigenetic regulation of the organic cation transporter 3, SLC22A3

Human organic cation transporter 3 (OCT3 and SLC22A3) mediates the uptake of many important endogenous amines and basic drugs in a variety of tissues. OCT3 is identified as one of the important risk loci for prostate cancer, and is markedly underexpressed in aggressive prostate cancers. The goal of this study was to identify genetic and epigenetic factors in the promoter region that influence the expression level of OCT3. Haplotypes that contained the common variants, g.-81G>delGA (rs60515630) (minor allele frequency 11.5% in African American) and g.-2G>A (rs555754) (minor allele frequency>30% in all ethnic groups) showed significant increases in luciferase reporter activities and exhibited stronger transcription factor-binding affinity than the haplotypes that contained the major alleles. Consistent with the reporter assays, OCT3 messenger RNA expression levels were significantly higher in Asian (P<0.001) and Caucasian (P<0.05) liver samples from individuals who were homozygous for g.-2A/A in comparison with those homozygous for the g.-2G/G allele. Studies revealed that the methylation level in the basal promoter region of OCT3 was associated with OCT3 expression level and tumorigenesis capability in various prostate cancer cell lines. The methylation level of the OCT3 promoter was higher in 62% of prostate tumor samples compared with matched normal samples. Our studies demonstrate that genetic polymorphisms in the proximal promoter region of OCT3 alter the transcription rate of the gene and may be associated with altered expression levels of OCT3 in human liver. Aberrant methylation contributes to the reduced expression of OCT3 in prostate cancer.

A common 5'-UTR variant in MATE2-K is associated with poor response to metformin

Multidrug and toxin extrusion 2 (MATE2-K (SLC47A2)), a polyspecific organic cation exporter, facilitates the renal elimination of the antidiabetes drug metformin. In this study, we characterized genetic variants of MATE2-K, determined their association with metformin response, and elucidated their impact by means of a comparative protein structure model. Four nonsynonymous variants and four variants in the MATE2-K basal promoter region were identified from ethnically diverse populations. Two nonsynonymous variants-c.485C>T and c.1177G>A-were shown to be associated with significantly lower metformin uptake and reduction in protein expression levels. MATE2-K basal promoter haplotypes containing the most common variant, g.-130G>A (>26% allele frequency), were associated with a significant increase in luciferase activities and reduced binding to the transcriptional repressor myeloid zinc finger 1 (MZF-1). Patients with diabetes who were homozygous for g.-130A had a significantly poorer response to metformin treatment, assessed as relative change in glycated hemoglobin (HbA1c) (-0.027 (-0.076, 0.033)), as compared with carriers of the reference allele, g.-130G (-0.15 (-0.17, -0.13)) (P=0.002). Our study showed that MATE2-K plays a role in the antidiabetes response to metformin.

Drug transporters in drug efficacy and toxicity

Drug transporters are now widely acknowledged as important determinants governing drug absorption, excretion, and, in many cases, extent of drug entry into target organs. There is also a greater appreciation that altered drug transporter function, whether due to genetic polymorphisms, drug-drug interactions, or environmental factors such as dietary constituents, can result in unexpected toxicity. Such effects are in part due to the interplay between various uptake and efflux transporters with overlapping functional capabilities that can manifest as marked interindividual variability in drug disposition in vivo. Here we review transporters of the solute carrier (SLC) and ATP-binding cassette (ABC) superfamilies considered to be of major importance in drug therapy and outline how understanding the expression, function, and genetic variation in such drug transporters will result in better strategies for optimal drug design and tissue targeting as well as reduce the risk for drug-drug interactions and adverse drug responses.

Mammalian MATE (SLC47A) transport proteins: impact on efflux of endogenous substrates and xenobiotics

Multidrug and toxin extrusion proteins (MATEs; SLC47A) are mammalian transporters being predominately expressed in the brush-border membrane of proximal tubule epithelial cells in the kidney and the canalicular membrane of hepatocytes. Functionally, MATEs act as efflux transporters for organic compounds, thereby mediating the elimination process. Two isoforms, MATE1 and 2, have been identified, and, so far, only a limited number of substrates, including clinically used drugs such as metformin and cimetidine, are known. A knockout mouse model has been established, as well, and is a valuable tool for further systematic pharmacokinetic analyses. In this review, we summarize the progress in MATE research on structural, molecular, functional, and pathophysiological aspects. Consequences of genetic variants for pharmacokinetic alterations and drug therapy are discussed.

Functional characterization of liver enhancers that regulate drug-associated transporters

Little is known about how genetic variations in enhancers influence drug response. In this study, we investigated whether nucleotide variations in enhancers that regulate drug transporters can alter their expression levels. Using comparative genomics and liver-specific transcription factor binding site (TFBS) analyses, we identified evolutionary conserved regions (ECRs) surrounding nine liver membrane transporters that interact with commonly used pharmaceuticals. The top 50 ECRs were screened for enhancer activity in vivo, of which five--located around ABCB11, SLC10A1, SLCO1B1, SLCO1A2, and SLC47A1--exhibited significant enhancer activity. Common variants identified in a large ethnically diverse cohort (n = 272) were assayed for differential enhancer activity, and three variants were found to have significant effects on reporter activity as compared with the reference allele. In addition, one variant was associated with reduced SLCO1A2 mRNA expression levels in human liver tissues, and another was associated with increased methotrexate (MTX) clearance in patients. This work provides a general model for the rapid characterization of liver enhancers and identifies associations between enhancer variants and drug response.

Organic cation transporters (OCTs, MATEs), in vitro and in vivo evidence for the importance in drug therapy

Organic cation transporters (OCTs) of the solute carrier family (SLC) 22 and multidrug and toxin extrusion (MATE) transporters of the SLC47 family have been identified as uptake and efflux transporters, respectively, for xenobiotics including several clinically used drugs such as the antidiabetic agent metformin, the antiviral agent lamivudine, and the anticancer drug oxaliplatin. Expression of human OCT1 (SLC22A1) and OCT2 (SLC22A2) is highly restricted to the liver and kidney, respectively. By contrast, OCT3 (SLC22A3) is more widely distributed. MATEs (SLC47A1, SLC47A2) are predominantly expressed in human kidney. Data on in vitro studies reporting a large number of substrates and inhibitors of OCTs and MATEs are systematically summarized. Several genetic variants of human OCTs and in part of MATE1 have been reported, and some of them result in reduced in vitro transport activity corroborating data from studies with knockout mice. A comprehensive overview is given on currently known genotype-phenotype correlations for variants in OCTs and MATE1 related to protein expression, pharmacokinetics/-dynamics of transporter substrates, treatment outcome, and disease susceptibility.

Molecular mechanisms of drug transporter regulation

Interindividual differences in drug transporter expression can result in variability in drug response. This variation in gene expression is determined, in part, by the actions of nuclear hormone receptors that act as xenobiotic- and endobiotic-sensing transcription factors. Among the ligand-activated nuclear receptors, signaling through the pregnane X receptor (PXR), constitutive androstane receptor (CAR), farnesoid X receptor (FXR), and vitamin D receptor (VDR) constitute major pathways regulating drug transporter expression in tissues. Hence, these endobiotic- and xenobiotic-sensing nuclear receptors are intrinsically involved in environmental influences of drug response. Moreover, because nuclear receptor genes are polymorphic, these transcription factors are also thought to contribute to heritability of variable drug action. In this chapter, the molecular aspects of drug transporter gene regulation by ligand-activated nuclear receptors will be reviewed including their clinical relevance.

Pharmacogenomics of membrane transporters: past, present and future

Membrane transporters are major determinants of the absorption, distribution and elimination of many of the most commonly used drugs. In the past decade, the field of membrane transporter pharmacogenomics has undergone enormous growth. In particular, functional genomic and clinical studies have provided new information regarding the contribution of coding variants in transporters to drug disposition and response. With continuing advances in sequencing technologies and large-scale human variation studies, over the next decade, knowledge in the field will be transformed. In particular, functional variants in noncoding regions of transporters will be discovered, and large clinical studies will result in the identification of variants in multiple genes, including transporter genes, which contribute to variation in clinical drug response.

The pharmacogenomics of membrane transporters project: research at the interface of genomics and transporter pharmacology

Since the cloning of the first membrane transporter, our understanding of the role of transporters in clinical drug disposition and response has grown enormously. In parallel, large-scale genome-wide variation studies and the emerging field of pharmacogenomics have ushered in a new understanding of variations in drug response. At the crossroads of pharmacogenomics and transporter biology is the National Institutes of Health-funded Pharmacogenomics of Membrane Transporters (PMT) project, centered at the University of California, San Francisco.