Functional analysis of pharmacogenetic variants of human organic cation/carnitine transporter 2 (hOCTN2) identified in Singaporean populations

Expression analysis of human solute carrier (SLC) family transporters in nasal mucosa and RPMI 2650 cells

With nearly 400 members, the solute-linked carrier (SLC) superfamily is one of the most important gene classes concerning the disposition of drugs and the transport of physiological substrates in the human body. The mapping of related transport proteins is already well advanced for the intestines, kidneys and liver, but it has recently been brought into focus for various respiratory epithelia. The aim of this study was to evaluate the expression of several SLC transporters in differently cultured RPMI 2650 cells, as well as in specimens of the human nasal mucosa. The expression profiles of PEPT2, OATP1A2, OATP4C1, OCT2, OCTN1 and OCTN2 were investigated at the gene and protein levels by performing RT-PCR, western blot analysis and immunohistological staining. Uptake assays using appropriate substrates and inhibitory substances were performed to compare the activity of peptide, organic anion and organic cation transporters, respectively, among the three models. Expression of the six SLC transporters under investigation was confirmed at the mRNA and protein levels in human nasal mucosa ex vivo as well as in RPMI 2650 cells grown under different culture conditions. The functionality was almost equal among all of the models for the PEPT and OCT(N) transporters, while the functional activity of the OATP transporters was more pronounced for both in vitro models than for excised nasal tissue. Despite negligible variations in transporter capacities, the RPMI 2650 cell cultures and freshly isolated human nasal epithelium showed nearly comparable expression patterns for the examined SLC proteins. Therefore, in vitro models based on the RPMI 2650 cell line could provide helpful data during the preclinical investigation of intranasally administered drug formulations and in the development of strategies to target nasal drug transporters for either local or systemic drug delivery.

Recent advance in the pharmacogenomics of human Solute Carrier Transporters (SLCs) in drug disposition

Drug pharmacokinetics is influenced by the function of metabolising enzymes and influx/efflux transporters. Genetic variability of these genes is known to impact on clinical therapies. Solute Carrier Transporters (SLCs) are the primary influx transporters responsible for the cellular uptake of drug molecules, which consequently, impact on drug efficacy and toxicity. The Organic Anion Transporting Polypeptides (OATPs), Organic Anion Transporters (OATs) and Organic Cation Transporters (OCTs/OCTNs) are the most important SLCs involved in drug disposition. The information regarding the influence of SLC polymorphisms on drug pharmacokinetics is limited and remains a hot topic of pharmaceutical research. This review summarises the recent advance in the pharmacogenomics of SLCs with an emphasis on human OATPs, OATs and OCTs/OCTNs. Our current appreciation of the degree of variability in these transporters may contribute to better understanding the inter-patient variation of therapies and thus, guide the optimisation of clinical treatments.

Association study between OCTN1 functional haplotypes and Crohn's disease in a Korean population

Crohn's disease (CD) is a chronic inflammatory bowel disease with multifactorial causes including environmental and genetic factors. Several studies have demonstrated that the organic cation/carnitine transporter 1 (OCTN1) non-synonymous variant L503F is associated with susceptibility to CD. However, it was reported that L503F is absent in Asian populations. Previously, we identified and functionally characterized genetic variants of the OCTN1 promoter region in Koreans. In that study, four variants demonstrated significant changes in promoter activity. In the present study, we determined whether four functional variants of the OCTN1 promoter play a role in the susceptibility to or clinical course of CD in Koreans. To examine it, the frequencies of the four variants of the OCTN1 promoter were determined by genotyping using DNA samples from 194 patients with CD and 287 healthy controls. Then, associations between genetic variants and the susceptibility to CD or clinical course of CD were evaluated. We found that susceptibility to CD was not associated with OCTN1 functional promoter variants or haplotypes showing altered promoter activities in in vitro assays. However, OCTN1 functional promoter haplotypes showing decreased promoter activities were significantly associated with a penetrating behavior in CD patients (HR=2.428, p=0.009). Our results suggest that the OCTN1 functional promoter haplotypes can influence the CD phenotype, although these might not be associated with susceptibility to this disease.

Human oligopeptide transporter 2 (PEPT2) mediates cellular uptake of polymyxins

OBJECTIVES

Polymyxins are a last-line therapy to treat MDR Gram-negative bacterial infections. Nephrotoxicity is the dose-limiting factor for polymyxins and recent studies demonstrated significant accumulation of polymyxins in renal tubular cells. However, little is known about the mechanism of polymyxin uptake into these cells. Oligopeptide transporter 2 (PEPT2) is a solute carrier transporter (SLC) expressed at the apical membrane of renal proximal tubular cells and facilitates drug reabsorption in the kidney. In this study, we examined the role of PEPT2 in polymyxin uptake into renal tubular cells.

METHODS

We investigated the inhibitory effects of colistin and polymyxin B on the substrate uptake mediated through 15 essential SLCs in overexpressing HEK293 cells. The inhibitory potency of both polymyxins on PEPT2-mediated substrate uptake was measured. Fluorescence imaging was employed to investigate PEPT2-mediated uptake of the polymyxin fluorescent probe MIPS-9541 and a transport assay was conducted with MIPS-9541 and [(3)H]polymyxin B1.

RESULTS

Colistin and polymyxin B potently inhibited PEPT2-mediated [(3)H]glycyl-sarcosine uptake (IC50 11.4 ± 3.1 and 18.3 ± 4.2 μM, respectively). In contrast, they had no or only mild inhibitory effects on the transport activity of the other 14 SLCs evaluated. MIPS-9541 potently inhibited PEPT2-mediated [(3)H]glycyl-sarcosine uptake (IC50 15.9 μM) and is also a substrate of PEPT2 (Km 74.9 μM). [(3)H]polymyxin B1 was also significantly taken up by PEPT2-expressing cells (Km 87.3 μM).

CONCLUSIONS

Our study provides the first evidence of PEPT2-mediated uptake of polymyxins and contributes to a better understanding of the accumulation of polymyxins in renal tubular cells.

The altered renal and hepatic expression of solute carrier transporters (SLCs) in type 1 diabetic mice

Diabetes mellitus is a chronic metabolic disorder that significantly affects human health and well-being. The Solute carrier transporters (SLCs), particularly the Organic anion/cation transporters (Oats/Octs/Octns), Organic anion transporting polypeptides (Oatps) and Oligopeptide transporters (Pepts) are essential membrane proteins responsible for cellular uptake of many endogenous and exogenous substances such as clinically important drugs. They are widely expressed in mammalian key organs especially the kidney and liver, in which they facilitate the influx of various drug molecules, thereby determining their distribution and elimination in body. The altered expression of SLCs in diabetes mellitus could have a profound and clinically significant influence on drug therapies. In this study, we extensively investigated the renal and hepatic expression of twenty essential SLCs in the type 1 diabetic Ins2Akita murine model that develops both hyperglycemia and diabetes-related complications using real-time PCR and immunoblotting analysis. We found that the renal expression of mOatp1a1, mOatp1a6, mOat1, mOat3, mOat5, mOct2 and mPept2 was decreased; while that of mPept1 was increased at the mRNA level in the diabetic mice compared with non-diabetic controls. We found up-regulated mRNA expression of mOatp1a4, mOatp1c1, mOctn2, mOct3 and mPept1 as well as down-regulation of mOatp1a1 in the livers of diabetic mice. We confirmed the altered protein expression of several SLCs in diabetic mice, especially the decreased renal and hepatic expression of mOatp1a1. We also found down-regulated protein expression of mOat3 and mOctn1 in the kidneys as well as increased protein expression of mOatp1a4 and mOct3 in the livers of diabetic mice. Our findings contribute to better understanding the modulation of SLC transporters in type 1 diabetes mellitus, which is likely to affect the pharmacokinetic performance of drugs that are transported by these transporters and therefore, forms the basis of future therapeutic optimization of regimens in patients with type 1 diabetes mellitus.

Interactions of the active components of Punica granatum (pomegranate) with the essential renal and hepatic human Solute Carrier transporters

Abstract Context: Solute carrier transporters (SLCs) are membrane proteins responsible for cellular influx of various substances including many pharmaceutical agents; therefore, they largely impact on drug disposition and elimination in body. Punica granatum Linnaeus (Lythraceae), pomegranate, is a fruit with antidiabetic potential. Oleanolic acid (OA), ursolic acid (UA), and gallic acid (GA) are the major bioactive components of pomegranate. Co-administration of these compounds with other drugs could result in altered drug pharmacokinetics, possibly due to competing for transporter proteins.

OBJECTIVE

We investigated the interactions of these three compounds with the essential hepatic and renal SLC transporters.

MATERIALS AND METHODS

Uptake of radiolabeled transporter model substrates was assessed in HEK293 cells over-expressing SLC transporters including the organic anion transporters (OATs), organic anion transporting polypeptides (OATPs) and organic cation transporters (OCTs), in the presence or absence of 10.0 µM UA, OA, or GA. Their IC50 values on specific SLC transporters were also evaluated using varying concentrations of the particular compound (ranging from 0.10 nM to 80.0 µM).

RESULTS

Our results demonstrated UA could significantly inhibit OAT3 and OATP2B1 uptake (IC50: 18.9 ± 8.20 µM and 11.0 ± 5.00 µM, respectively) and GA has a pronounced inhibitory effect on OATP1B3 uptake (IC50: 1.60 ± 0.60 μM).

DISCUSSION AND CONCLUSION

Our study reports the interactions of OA, UA, and GA with the essential SLC transporters. This information may contribute to elucidating the drug-drug/herb interactions involved with these three compounds and form the basis of therapeutic optimization when drugs are co-administered.

PDZK1 and NHERF1 regulate the function of human organic anion transporting polypeptide 1A2 (OATP1A2) by modulating its subcellular trafficking and stability

The human organic anion transporting polypeptide 1A2 (OATP1A2) is an important membrane protein that mediates the cellular influx of various substances including drugs. Previous studies have shown that PDZ-domain containing proteins, especially PDZK1 and NHERF1, regulate the function of related membrane transporters in other mammalian species. This study investigated the role of PDZK1 and NHERF1 in the regulation of OATP1A2 in an in vitro cell model. Transporter function and protein expression were assessed in OATP1A2-transfected HEK-293 cells that co-expressed PDZK1 or NHERF1. Substrate (estrone-3-sulfate) uptake by OATP1A2 was significantly increased to ∼1.6- (PDZK1) and ∼1.8- (NHERF1) fold of control; this was dependent on the putative PDZ-binding domain within the C-terminus of OATP1A2. The functional increase of OATP1A2 following PDZK1 or NHERF1 over-expression was associated with increased transporter expression at the plasma membrane and in the whole cell, and was reflected by an increase in the apparent maximal velocity of estrone-3-sulfate uptake (V_max: 138.9±4.1 (PDZK1) and 181.4±16.7 (NHERF1) versus 55.5±3.2 pmol*(µg*4 min)⁻¹ in control; P<0.01). Co-immunoprecipitation analysis indicated that the regulatory actions of PDZK1 and NHERF1 were mediated by direct interaction with OATP1A2 protein. In further experiments PDZK1 and NHERF1 modulated OATP1A2 expression by decreasing its internalization in a clathrin-dependent (but caveolin-independent) manner. Additionally, PDZK1 and NHERF1 enhanced the stability of OATP1A2 protein in HEK-293 cells. The present findings indicated that PDZK1 and NHERF1 regulate the transport function of OATP1A2 by modulating protein internalization via a clathrin-dependent pathway and by enhancing protein stability.

The inhibitory effects of the bioactive components isolated from Scutellaria baicalensis on the cellular uptake mediated by the essential solute carrier transporters

Solute carrier transporters (SLCs), in particular the organic anion transporters (OATs), OAT polypeptides (OATPs), and organic cation transporters (OCTs/OCTNs), are the important membrane proteins responsible for the cellular influx of various drugs. Baicalein (BA), baicalin (BG), and wogonin (WG) are the three major bioactive components of Scutellaria baicalensis. In this study, we evaluated the inhibitory effects of BA, BG, and WG on the cellular uptake of specific substrates mediated by the essential SLCs in human embryonic kidney-293 cells. Our data demonstrated that BA and WG significantly inhibit the OAT1-, OAT3-, and OATP1B3-mediated uptake; BG effectively reduces the influx of substrates of OAT3, OAT4, OATP1B3, and OATP2B1; WG is a potent inhibitor of OCT3. Our further kinetic analysis derived the IC50 values of these compounds with pronounced inhibitory effects on SLCs, particularly the inhibitions of WG on OAT1 and OCT3 and that of BA and WG on OAT3. Our study comprehensively evaluated the inhibitory effects of three bioactive components of Scutellaria baicalensis on the uptake of specific substrates mediated by the essential SLC transporters, which suggested that precautions will be needed when coadministrating drugs with Scutellaria baicalensis so as to prevent the unfavorable drug-drug/herb interactions in human.