Physiology, structure, and regulation of the cloned organic anion transporters

Renal organic anion transporter 1: clinical relevance and the underlying mechanisms in chronic kidney disease

Organic anion transporter 1 (OAT1), primarily found in the renal proximal tubule, is essential for the excretion of various uremic toxins that contribute to the onset and progression of chronic kidney disease (CKD). OAT1 also plays a vital role in the remote sensing and signaling network, facilitating the removal of metabolites through the kidneys. The function of OAT1 is impaired under conditions such as renal ischemia/reperfusion injury, oxidative stress, and fibrosis. Several transcription factors, post-translational modifications, and endocrine hormones control the activity and expression of OAT1. This review explores the unique contribution of OAT1 to the excretion of CKD-related UTs and the mechanisms involved.

Cladophora glomerata Kützing extract exhibits antioxidant, anti-inflammation, and anti-nitrosative stress against impairment of renal organic anion transport in an in vivo study

BACKGROUND/OBJECTIVES

Cladophora glomerata extract (CGE), rich in polyphenols, was reported to exhibit antidiabetic and renoprotective effects by modulating the functions of protein kinases-mediated organic anion transporter 1 (Oat1) and 3 (Oat3) in rats with type 2 diabetes mellitus (T2DM). Nevertheless, the antioxidant effects of CGE on such renoprotection have not been investigated. This study examined the mechanisms involved in the antioxidant effects of CGE on renal organic anion transport function in an in vivo study.

MATERIALS/METHODS

Diabetes was induced in the rats through a high-fat diet combined with a single dose of 40 mg/kg body weight (BW) streptozotocin. Subsequently, normal-diet rats were supplemented with a vehicle or 1,000 mg/kg BW of CGE, while T2DM rats were supplemented with a vehicle, CGE, or 200 mg/kg BW of vitamin C for 12 weeks. The study evaluated the general characteristics of T2DM and renal oxidative stress markers. The renal organic transport function was assessed by measuring the para-aminohippurate (PAH) uptake using renal cortical slices and renal inflammatory cytokine expression in the normal diet (ND) and ND + CGE treated groups.

RESULTS

CGE supplementation significantly reduced hyperglycemia, hypertriglyceridemia, insulin resistance, and renal lipid peroxidation in T2DM rats. This was accompanied by the normalization of high expressions of renal glutathione peroxidase and nuclear factor kappa B by CGE and vitamin C. The renal anti-inflammation of CGE was evidenced by the reduction of tumor necrosis factor-1α and interleukin-1β. CGE directly blunted sodium nitroprusside-induced renal oxidative/nitrosative stresses and mediated the PAH uptake in the normally treated CGE in rats was particularly noteworthy. These data also correlated with reduced nitric oxide production, highlighting the potential of CGE as a therapeutic agent for managing T2DM-related renal complications.

CONCLUSION

These findings suggest that CGE has antidiabetic effects and directly prevents diabetic nephropathy through oxidative/nitrosative stress pathways.

Examination of the emerging role of transporters in the assessment of nephrotoxicity

INTRODUCTION

The kidney is vulnerable to various injuries based on its function in the elimination of many xenobiotics, endogenous substances and metabolites. Since transporters are critical for the renal elimination of those substances, it is urgent to understand the emerging role of transporters in nephrotoxicity.

AREAS COVERED

This review summarizes the contribution of major renal transporters to nephrotoxicity induced by some drugs or toxins; addresses the role of transporter-mediated endogenous metabolic disturbances in nephrotoxicity; and discusses the advantages and disadvantages of in vitro models based on transporter expression and function.

EXPERT OPINION

Due to the crucial role of transporters in the renal disposition of xenobiotics and endogenous substances, it is necessary to further elucidate their renal transport mechanisms and pay more attention to the underlying relationship between the transport of endogenous substances and nephrotoxicity. Considering the species differences in the expression and function of transporters, and the low expression of transporters in general cell models, in vitro humanized models, such as humanized 3D organoids, shows significant promise in nephrotoxicity prediction and mechanism study.

Using a Machine Learning Approach to Identify Key Biomarkers for Renal Clear Cell Carcinoma

Background

The most common and deadly subtype of renal carcinoma is kidney renal clear cell carcinoma (KIRC), which accounts for approximately 75% of renal carcinoma. However, the main cause of death in KIRC patients is tumor metastasis. There are no obvious clinical features in the early stage of kidney cancer, and 25–30% of patients have already metastasized when they are first diagnosed. Moreover, KIRC patients whose local tumors have been removed by nephrectomy are still at high risk of metastasis and recurrence and are not sensitive to chemotherapy and radiotherapy, leading to poor prognosis. Therefore, early diagnosis and treatment of this disease are very important.

Methods

KIRC-related patient datasets were downloaded from the GEO database and TCGA database. DEG screening and GO, KEGG and GSEA enrichment analysis was firstly conducted and then the LASSO and support vector machine (SVM) RFE algorithms were adopted to identify KIRC-associated key genes in training sets and validate them in the test set. The clinical prognostic analysis including the association between the expression of key genes and the overall survival, stage, grade across KIRC, the immune infiltration difference between normal samples and cancer samples, the correlation between the key genes and immune cells, immunomodulator, immune subtypes of KIRC were investigated in this research.

Results

We finally screened out 4 key genes, including ACPP, ANGPTL4, SCNN1G, SLC22A7. The expression of key genes show difference among normal samples and tumor samples, SCNN1G and SLC22A7 could be predictor of prognosis of patients. The expression of key genes was related with the abundance of tumor infiltration immune cells and the gene expression of immune checkpoint.

Conclusion

This study screened the 4 key genes, which contributed to early diagnosis, prognosis assessment and immune target treatment of patients with KIRC.

Endogenous Plasma Kynurenic Acid in Human: A Newly Discovered Biomarker for Drug-Drug Interactions Involving Organic Anion Transporter 1 and 3 Inhibition

As an expansion investigation of drug-drug interaction (DDI) from previous clinical trials, additional plasma endogenous metabolites were quantitated in the same subjects to further identify the potential biomarkers of organic anion transporter (OAT) 1/3 inhibition. In the single dose, open label, three-phase with fixed order of treatments study, 14 healthy human volunteers orally received 1000 mg probenecid alone, or 40 mg furosemide alone, or 40 mg furosemide at 1 hour after receiving 1000 mg probenecid on days 1, 8, and 15, respectively. Endogenous metabolites including kynurenic acid, xanthurenic acid, indo-3-acetic acid, pantothenic acid, p-cresol sulfate, and bile acids in the plasma were measured by liquid chromatography-tandem mass spectrometry. The Cmax of kynurenic acids was significantly increased about 3.3- and 3.7-fold over the baseline values at predose followed by the treatment of probenecid alone or in combination with furosemide respectively. In comparison with the furosemide-alone group, the Cmax and area under the plasma concentration-time curve (AUC) up to 12 hours of kynurenic acid were significantly increased about 2.4- and 2.5-fold by probenecid alone, and 2.7- and 2.9-fold by probenecid plus furosemide, respectively. The increases in Cmax and AUC of plasma kynurenic acid by probenecid are comparable to the increases of furosemide Cmax and AUC reported previously. Additionally, the plasma concentrations of xanthurenic acid, indo-3-acetic acid, pantothenic acid, and p-cresol sulfate, but not bile acids, were also significantly elevated by probenecid treatments. The magnitude of effect size analysis for known potential endogenous biomarkers demonstrated that kynurenic acid in the plasma offers promise as a superior addition for early DDI assessment involving OAT1/3 inhibition. SIGNIFICANCE STATEMENT: This article reports that probenecid, an organic anion transporter (OAT) 1 and OAT3 inhibitor, significantly increased the plasma concentrations of kynurenic acid and several uremic acids in human subjects. Of those, the increases of plasma kynurenic acid exposure are comparable to the increases of furosemide by OAT1/3 inhibition. Effect size analysis for known potential endogenous biomarkers revealed that plasma kynurenic acid is a superior addition for early drug-drug interaction assessment involving OAT1/3 inhibition.

Membrane Carriers and Transporters in Kidney Physiology and Disease

The growing information suggests that chronic kidney disease may affect expression and function of membrane carriers and transporters in the kidney. The dysfunction of carriers and transporters entails deficient elimination of uremic solutes as well as xenobiotics (drugs and toxins) with subsequent clinical consequences. The renal carriers and transporters are also targets of drugs used in clinical practice, and intentional drug-drug interactions in the kidney are produced to increase therapeutic efficacy. The understanding of membrane carriers and transporters function in chronic kidney disease is important not only to better characterize drug pharmacokinetics, drug actions in the kidney, or drug-drug interactions but also to define the organ pathophysiology.

Peptide Hormone Insulin Regulates Function, Expression, and SUMOylation of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays an important role in the disposition of various anionic drugs which impacts the pharmacokinetics and pharmacodynamics of the therapeutics, thus influencing the pharmacological effects and toxicity of the drugs. In this study, we investigated the effect of insulin on the regulation of OAT3 function, expression, and SUMOylation. We demonstrated that insulin induced an increase in OAT3 transport activity through a dose- and time-dependent manner in COS-7 cells. The insulin-induced elevation in OAT3 function was blocked by PKA inhibitor H89, which correlated well with OAT3 protein expression. Moreover, both PKA activator Bt2-cAMP-induced increase and insulin-induced increase in OAT3 function were blocked by PKB inhibitor AKTi1/2. To further investigate the involvement of SUMOylation, we treated OAT3-expressing cells with insulin in presence or absence of H89 or AKTi1/2 followed by examining OAT3 SUMOylation. We showed that insulin enhanced OAT3 SUMOylation, and such enhancement was abrogated by H89 and AKTi1/2. Lastly, insulin increased OAT3 function and SUMOylation in rat kidney slice. In conclusion, our investigations demonstrated that insulin regulated OAT3 function, expression, and SUMOylation through PKA/PKB signaling pathway. Graphical abstract.

Post-translational regulation of the major drug transporters in the families of organic anion transporters and organic anion-transporting polypeptides

The organic anion transporters (OATs) and organic anion-transporting polypeptides (OATPs) belong to the solute carrier (SLC) transporter superfamily and play important roles in handling various endogenous and exogenous compounds of anionic charge. The OATs and OATPs are often implicated in drug therapy by impacting the pharmacokinetics of clinically important drugs and, thereby, drug exposure in the target organs or cells. Various mechanisms (e.g. genetic, environmental, and disease-related factors, drug-drug interactions, and food-drug interactions) can lead to variations in the expression and activity of the anion drug-transporting proteins of OATs and OATPs, possibly impacting the therapeutic outcomes. Previous investigations mainly focused on the regulation at the transcriptional level and drug-drug interactions as competing substrates or inhibitors. Recently, evidence has accumulated that cellular trafficking, post-translational modification, and degradation mechanisms serve as another important layer for the mechanisms underlying the variations in the OATs and OATPs. This review will provide a brief overview of the major OATs and OATPs implicated in drug therapy and summarize recent progress in our understanding of the post-translational modifications, in particular ubiquitination and degradation pathways of the individual OATs and OATPs implicated in drug therapy.

Ubiquitin-specific peptidase 8 regulates the trafficking and stability of the human organic anion transporter 1

Background Organic anion transporter 1 (OAT1) plays a vital role in avoiding the potential toxicity of various anionic drugs through the involvement of kidney elimination. We previously demonstrated that ubiquitin conjugation to OAT1 led to OAT1 internalization from cell surface, followed by degradation. Ubiquitination is a dynamic process, where deubiquitination is catalyzed by a class of ubiquitin-specific peptidases. Methods The role of ubiquitin-specific peptidase 8 (USP8) in hOAT1 function, expression and ubiquitination was assessed by conducting transporter uptake assay, biotinylation assay and ubiquitination assay. Results We demonstrated that USP8 overexpression in hOAT1-expressing cells led to an increased hOAT1 transporter activity and expression, which correlated well with a reduced hOAT1 ubiquitination. Such phenomenon was not observed in inactive USP8 mutant-transfected cells. In addition, the knockdown of endogenous USP8 by USP8-specific siRNA resulted in an increased hOAT1 ubiquitination, which correlated well with a decrease in hOAT1 expression and transport activity. Biotinylation experiments demonstrated that USP8-induced increase in hOAT1 expression and transport activity occurred through a deceleration of the rates of hOAT1 internalization and degradation. Conclusions These results indicated the regulatory role of USP8 in OAT1 function, expression, trafficking, and stability. General significance USP8 could be a new target for modulating OAT1-mediated drug transport.

Regulation of organic anion transporters: Role in physiology, pathophysiology, and drug elimination

The members of the organic anion transporter (OAT) family are mainly expressed in kidney, liver, placenta, intestine, and brain. These transporters play important roles in the disposition of clinical drugs, pesticides, signaling molecules, heavy metal conjugates, components of phytomedicines, and toxins, and therefore critical for maintaining systemic homeostasis. Alterations in the expression and function of OATs contribute to the intra- and inter-individual variability of the therapeutic efficacy and the toxicity of many drugs, and to many pathophysiological conditions. Consequently, the activity of these transporters must be highly regulated to carry out their normal functions. This review will present an update on the recent advance in understanding the cellular and molecular mechanisms underlying the regulation of renal OATs, emphasizing on the post-translational modification (PTM), the crosstalk among these PTMs, and the remote sensing and signaling network of OATs. Such knowledge will provide significant insights into the roles of these transporters in health and disease.

Spirogyra neglecta Aqueous Extract Attenuates LPS-Induced Renal Inflammation

Spirogyra neglecta (SN), commonly named "Tao" in Thai, is a genus of filamentous green macroalgae. SN contains polyphenols such as isoquercetin, catechin, hydroquinone and kaempferol. These constituents exhibit beneficial effects including anti-oxidant, anti-gastric ulcer, anti-hyperglycaemia and anti-hyperlipidaemia in both in vitro and in vivo models. Whether SN extract (SNE) has an anti-inflammatory effect in vivo remains unclear. This study examined the effect of SNE on renal function and renal organic transport in lipopolysaccharide (LPS)-induced renal inflammation in rats. Rats were randomised and divided into normal saline (NS), NS supplemented with 1000 mg/kg body weight (BW) of SNE (NS + SNE), intraperitoneally injected with 12 mg/kg BW of LPS and LPS treated with 1000 mg/kg BW of SNE (LPS + SNE). Biochemical parameters in serum and urine, lipid peroxidation concentration, kidney function and renal organic anion and cation transports were determined. LPS-injected rats developed renal injury and inflammation by increasing urine microalbumin, total malondialdehyde (MDA) and inflammatory cytokines, tumor necrosis factor (TNF)-α and interleukin (IL)-1β protein expression, respectively. In addition, uptake of renal organic anion, [³H]-oestrone sulphate (ES), was reduced in LPS-injected rats together with increased expression of organic anion transporter 3 (Oat3). However, the renal injury and inflammation, as well as impaired Oat3 function and protein expression, were restored in LPS + SNE rats. Accordingly, SNE could be developed as nutraceutical product to prevent inflammation-induced nephrotoxicity.

The SLC Family Are Candidate Diagnostic and Prognostic Biomarkers in Clear Cell Renal Cell Carcinoma

Clear cell renal cell carcinoma (ccRCC) is the most common lethal subtype of renal cancer, and changes in tumor metabolism play a key role in its development. Solute carriers (SLCs) are important in the transport of small molecules in humans, and defects in SLC transporters can lead to serious diseases. The expression patterns and prognostic values of SLC family transporters in the development of ccRCC are still unclear. The current study analyzed the expression levels of SLC family members and their correlation with prognosis in ccRCC patients with data from Oncomine, Gene Expression Profiling Interactive Analysis (GEPIA), The Cancer Genome Atlas (TCGA), cBioPortal, the Human Protein Atlas (HPA), the International Cancer Genome Consortium (ICGC), and the Gene Expression Omnibus (GEO). We found that the mRNA expression levels of SLC22A6, SLC22A7, SLC22A13, SLC25A4, SLC34A1, and SLC44A4 were significantly lower in ccRCC tissues than in normal tissues and the protein expression levels of SLC22A6, SLC22A7, SLC22A13, and SLC34A1 were also significantly lower. Except for SLC22A7, the expression levels of SLC22A6, SLC22A13, SLC25A4, SLC34A1, and SLC44A4 were correlated with the clinical stage of ccRCC patients. The lower the expression levels of SLC22A6, SLC22A13, SLC25A4, SLC34A1, and SLC44A4 were, the later the clinical stage of ccRCC patients was. Further experiments revealed that the expression levels of SLC22A6, SLC22A7, SLC22A13, SLC25A4, SLC34A1, and SLC44A4 were significantly associated with overall survival (OS) and disease-free survival (DFS) in ccRCC patients. High SLC22A6, SLC22A7, SLC22A13, SLC25A4, SLC34A1, and SLC44A4 expression predicted improved OS and DFS. Finally, GSE53757 and ICGC were used to revalidate the differential expression and clinical prognostic value. This study suggests that SLC22A6, SLC22A7, SLC22A13, SLC25A4, SLC34A1, and SLC44A4 may be potential targets for the clinical diagnosis, prognosis, and treatment of ccRCC patients.

Environmental contaminants modulate transport activity of zebrafish organic anion transporters Oat1 and Oat3

Organic anion transporters (OATs) are transmembrane proteins which belong to SLC22 subfamily. They are responsible for the uptake of various endo- and xenobiotics into the cells of different organs and tissues. Following our previous work on characterization of zebrafish Oat1 and Oat3, in this study we analyzed interaction of various classes of environmental contaminants with these membrane transporters using the transport activity assay with HEK293 Flp-In cell line stably overexpressing zebrafish Oat1 and Oat3, respectively. Based on the initial screening of a series of 36 environmental contaminants on their ability to interact with zebrafish Oat1 and Oat3, the most potent interactors were selected, their IC50 values calculated and type of interaction determined. Finally, to further confirm the type of interaction and initially evaluate their toxic potential, the cytotoxicity assays were performed. Broad ligand selectivity and similarity of zebrafish Oat1 and Oat3 with mammalian orthologs was confirmed and potent interactors among environmental contaminants identified.

Insulin-like growth factor 1 modulates the phosphorylation, expression, and activity of organic anion transporter 3 through protein kinase A signaling pathway

Organic anion transporter 3 (OAT3) plays a vital role in removing a broad variety of anionic drugs from kidney, thus avoiding their possible toxicity in the body. In the current study, we investigated the role of insulin-like growth factor 1 (IGF-1) in the regulation of OAT3. We showed that IGF-1 induced a dose- and time-dependent increase in OAT3 transport activity, which correlated well with an increase in OAT3 expression. The IGF-1-induced increase in OAT3 expression was blocked by protein kinase A (PKA) inhibitor H89. Moreover, IGF-1 induced an increase in OAT3 phosphorylation, which was also blocked by H89. These data suggest that the IGF-1 modulation of OAT3 occurred through PKA signaling pathway. To further confirm the involvement of PKA, we treated OAT3-expressing cells with PKA activator Bt2-cAMP, followed by examining OAT activity and phosphorylation. We showed that OAT3 activity and phosphorylation were much enhanced in Bt2-cAMP-treated cells as compared to that in control cells. Finally, linsitinib, an anticancer drug that blocks the IGF-1 receptor, abrogated IGF-1-stimulated OAT3 transport activity. In conclusion, our study demonstrated that IGF-1 regulates OAT3 expression and transport activity through PKA signaling pathway, possibly by phosphorylating the transporter.

Leflunomide increased the renal exposure of acyclovir by inhibiting OAT1/3 and MRP2

Rheumatoid arthritis patients can be prescribed a combination of immunosuppressive drug leflunomide (LEF) and the antiviral drug acyclovir to reduce the high risk of infection. Acyclovir is a substrate of organic anion transporter (OAT) 1/3 and multidrug resistance-associated protein (MRP) 2. Considering the extraordinarily long half-life of LEF's active metabolite teriflunomide (TER) and the kidney injury risk of acyclovir, it is necessary to elucidate the potential impact of LEF on the disposition of acyclovir. Here we used a specific MRP inhibitor MK571 and probenecid (OAT1/3 and MRP2 inhibitor) to assess the effects of MRP2 and OAT1/3 on the pharmacokinetics and tissue distribution of acyclovir in rats. We showed that LEF and probenecid, but not MK571 significantly increased the plasma concentration of acyclovir. However, kidney and liver exposures of acyclovir were increased when coadministered with LEF, probenecid or MK571. The kidney/plasma ratio of acyclovir was increased to approximately 2-fold by LEF or probenecid, whereas it was increased to as much as 14.5-fold by MK571. Consistently, these drugs markedly decreased the urinary excretion of acyclovir. TER (0.5-100 μmol/L) dose-dependently increased the accumulation of acyclovir in MRP2-MDCK cells with an IC₅₀ value of 4.91 μmol/L. TER (5 μmol/L) significantly inhibited the uptake of acyclovir in hOAT1/3-HEK293 cells. These results suggest that LEF/TER increased the kidney accumulation of acyclovir by inhibiting the efflux transporter MRP2, which increased its kidney/plasma ratio and renal injury risk. However, the inhibitory effects of LEF/TER on OAT1/3 reduced the tubular cells' uptake of acyclovir and increased the plasma concentration.

The SUMO-Specific Protease Senp2 Regulates SUMOylation, Expression and Function of Human Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays a vital role in removing a broad array of anionic drugs from kidney, thereby avoiding their possibly toxic side effects in the body. We earlier demonstrated that OAT3 is subjected to a specific type of post-translational modification called SUMOylation. SUMOylation is a dynamic event, where de-SUMOylation is catalyzed by a class of SUMO-specific proteases. In the present investigation, we assessed the role of SUMO-specific protease Senp2 in OAT3 SUMOylation, expression and function. We report here that overexpression of Senp2 in COS-7 cells led to a reduced OAT3 SUMOylation, which correlated well with a decreased OAT3 expression and transport activity. Such phenomenon was not observed in cells overexpressing an inactive mutant of Senp2. Furthermore, transfection of cells with Senp2-specific siRNA to knockdown the endogenous Senp2 resulted in an increased OAT3 SUMOylation, which correlated well with an enhanced OAT3 expression and transport activity. Coimmunoprecipitation experiments showed that Senp2 directly interacted with OAT3 in the kidneys of rats. Together these results provided first demonstration that Senp2 is a significant regulator for OAT3-mediated organic anion/drug transport.

Interaction of Anticancer Drugs with Human Organic Anion Transporter hOAT4

Human organic anion transporter 4 (hOAT4) belongs to a family of multispecific organic anion transporters that play critical roles in the disposition of numerous drugs and therefore are the major sites for drug-drug interaction. Drug-drug interactions contribute significantly to the individual variation in drug response. hOAT4 is expressed in the kidney and placenta. In the current study, we examined the interaction of 36 anticancer drugs with hOAT4 in kidney COS-7 cells and placenta BeWo cells. Among the drugs tested, only epirubicin hydrochloride and dabrafenib mesylate exhibited > 50% cis-inhibitory effect, in COS-7 cells, on hOAT4-mediated uptake of estrone sulfate, a prototypical substrate for the transporter. The IC₅₀ values for epirubicin hydrochloride and dabrafenib mesylate were 5.24±0.95 μM and 8.30±3.30 μM, respectively. Dixon plot analysis revealed that inhibition by epirubicin hydrochloride was noncompetitive with a K_i = 3 μM whereas inhibition by dabrafenib mesylate was competitive with a K_i = 4.26 μM. Our results established that epirubicin hydrochloride and dabrafenib mesylate are inhibitors of hOAT4. Furthermore, by comparing our data with clinically relevant exposures of these drugs, we conclude that although the tendency for dabrafenib mesylate to cause drug-drug interaction through hOAT4 is insignificant in the kidney, the propensity for epirubicin hydrochloride to cause drug-drug interaction is high.

Activation of Protein Kinase A Stimulates SUMOylation, Expression, and Transport Activity of Organic Anion Transporter 3

Organic anion transporter 3 (OAT3) plays a vital role in removing a broad variety of anionic drugs from kidney, thus avoiding their possible toxicity in the body. We earlier established that activation of protein kinase C (PKC) enhances OAT3 ubiquitination, which promotes OAT3 internalization from the cell plasma membrane to intracellular endosomes and consequent degradation. As a result, OAT3 expression and transport activity are reduced. In the current study, we discovered that protein kinase A (PKA) had an opposite effect to PKC on the regulation of OAT3. We showed that activation of PKA by Bt2-cAMP stimulated OAT3 transport activity, which was largely caused by an enhanced plasma membrane expression of the transporter, kinetically reflected as an augmented maximal transport velocity V_max without notable alteration in substrate-binding affinity K_m. Additionally, we showed that PKA activation accelerated the rate of OAT3 recycling from intracellular compartments to the plasma membrane and decelerated the rate of OAT3 degradation. We further showed that OAT3 is subjected to post-translational modification by SUMO-2 and SUMO-3 not by SUMO-1. PKA activation enhanced OAT3 SUMOylation, which was accompanied by a reduced OAT3 ubiquitination. Finally, insulin-like growth factor 1 significantly stimulated OAT3 transport activity and SUMOylation through PKA signaling pathway. In conclusion, this is the first demonstration that PKA stimulated OAT3 expression and transport activity by altering the trafficking kinetics of OAT3 possibly through the crosstalk between SUMOylation and ubiquitination. Our studies are consistent with a remote sensing and signaling model for transporters (Wu et al. in Mol Pharmacol. 79(5):795-805, 2011).

The mechanistic links between insulin and human organic anion transporter 4

Human organic anion transporter 4 (hOAT4) belongs to a class of organic anion transporters that exert critical function in the secretion, absorption, and distribution of numerous drugs in the body, such as anti-viral drugs, anti-cancer therapeutics, antibiotics, antihypertensive medicine, and anti-inflammatory drugs. hOAT4 is richly existent in the kidney and placenta. We previously established that serum- and glucocorticoid-inducible kinases (sgk) stimulate hOAT4 expression and transport activity by abrogating the inhibitory effect of a ubiquitin ligase Nedd4-2. Insulin is one of the upstream signaling molecules for sgk. We therefore investigated the effect of insulin on hOAT4 function. We showed that insulin stimulated hOAT4 expression and transport activity, and the action of insulin was abolished in cells overexpressing Nedd4-2-specific siRNA to knockdown the endogenous Nedd4-2. We further showed that insulin phosphorylated serine 327 on Nedd4-2 and weakened the interaction between hOAT4 and Nedd4-2. Interestingly, in cells overexpressing sgk2, the stimulatory effect of insulin on hOAT4 was diminished. In addition, the stimulatory effect of insulin on hOAT4 was blocked by wortmannin and buparlisib, two PI3K inhibitors. In conclusion, our study demonstrated that insulin stimulates hOAT4 expression and transport activity by abrogating the inhibition effect of Nedd4-2 on the transporter. Moreover, insulin regulates hOAT4 by competing with sgk2 rather than through sgk2.

Investigation of the SLC22A23 gene in laryngeal squamous cell carcinoma

Background

Laryngeal squamous cell carcinoma (LSCC) is the second most common cancer of the head and neck. In order to identify differentially expressed genes which may have a role in LSCC carcinogenesis, we performed GeneFishing Assay. One of the differentially expressed genes was the SLC22A23 (solute carrier family 22, member 23) gene.

SLC22A23 belongs to a family of organic ion transporters that are responsible for the absorption or excretion of many drugs, xenobiotics and endogenous compounds in a variety of tissues. SLC22A23 is expressed in a various tissues but no substrates or functions have been identified for it. Although the exact function is unknown, single nucleotide polymorphisms (SNPs) which are located in SLC22A23 gene were associated with inflammatory bowel disease (IBD), endometriosis-related infertility and the clearance of antipsychotic drugs. On the other hand SLC22A23 is identified as a prognostic gene to predict the recurrence of triple-negative breast cancer.

Methods

To understand the role of the SLC22A23 gene in laryngeal carcinogenesis, we investigated its mRNA expression level in laryngeal tumor tissue and adjacent non-cancerous tissue samples obtained from 83 patients by quantitative real-time PCR. To understand the association between SNPs in SLC22A23 and LSCC, selected genetic variations (rs4959235, rs6923667, rs9503518) were genotyped.

Results

We found that SLC22A23 expression was increased in 46 of 83 tumor tissues (55.4%) and was decreased in 30 of 83 (36.1%) tumor tissues compared to normal tissues. 77.2% of patients were homozygote for genotype rs9503518-AA and they most frequently had histological grade 2 and 3 tumors. We also found that rs9503518-AA genotype is associated with increased SLC22A23 expression.

Conclusions

Our results indicate that SLC22A23 may play a role in the development of laryngeal cancer.

Genetic Heterogeneity of SLC22 Family of Transporters in Drug Disposition

An important aspect of modern medicine is its orientation to achieve more personalized pharmacological treatments. In this context, transporters involved in drug disposition have gained well-justified attention. Owing to its broad spectrum of substrate specificity, including endogenous compounds and xenobiotics, and its strategical expression in organs accounting for drug disposition, such as intestine, liver and kidney, the SLC22 family of transporters plays an important role in physiology, pharmacology and toxicology. Among these carriers are plasma membrane transporters for organic cations (OCTs) and anions (OATs) with a marked overlap in substrate specificity. These two major clades of SLC22 proteins share a similar membrane topology but differ in their degree of genetic variability. Members of the OCT subfamily are highly polymorphic, whereas OATs have a lower number of genetic variants. Regarding drug disposition, changes in the activity of these variants affect intestinal absorption and target tissue uptake, but more frequently they modify plasma levels due to enhanced or reduced clearance by the liver and secretion by the kidney. The consequences of these changes in transport-associated function markedly affect the effectiveness and toxicity of the treatment in patients carrying the mutation. In solid tumors, changes in the expression of these transporters and the existence of genetic variants substantially determine the response to anticancer drugs. Moreover, chemoresistance usually evolves in response to pharmacological and radiological treatment. Future personalized medicine will require monitoring these changes in a dynamic way to adapt the treatment to the weaknesses shown by each tumor at each stage in each patient.

AG490, a JAK2-specific inhibitor, downregulates the expression and activity of organic anion transporter-3

Human organic anion transporter-3 (hOAT3) is richly expressed in the kidney, where it plays critical roles in the secretion of clinically important drugs, including anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. In the current study, we examined the role of AG490, a specific inhibitor of the Janus tyrosine kinase 2 (JAK2), in hOAT3 transport activity in the kidney COS-7 cells. AG490 induced a time- and concentration-dependent inhibition of hOAT3-mediated uptake of estrone sulfate, a prototypical substrate for the transporter. The inhibitory effect of AG490 correlated with a reduced expression of hOAT3 at the cell surface. Our lab previously demonstrated that Nedd4-2, a ubiquitin ligase, down regulates OAT expression and transport activity by enhancing OAT ubiquitination, which leads to an internalization of OAT from cell surface to intracellular compartments and subsequent degradation. In the current study, we showed that treatment of hOAT3-expressing cells with AG490 resulted in an enhanced hOAT3 ubiquitination and degradation, which was accompanied by a strengthened association of Nedd4-2 with hOAT3 and a reduction in Nedd4-2 phosphorylation. SiRNA knockdown of endogenous Nedd4-2 abrogated the effects of AG490 on hOAT3. In summary, our study demonstrated that AG490 regulates hOAT3 expression and transport activity through the modulation of Nedd4-2.

The activity of organic anion transporter-3: Role of dexamethasone

Human organic anion transporter-3 (hOAT3) is richly expressed in the kidney, where it plays critical roles in the secretion, from the blood to urine, of clinically important drugs, such as anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. In the current study, we examined the role of dexamethasone in hOAT3 transport activity in the kidney HEK293 cells. Cis-inhibition study showed that dexamethasone exhibited a concentration-dependent inhibition of hOAT3-mediated uptake of estrone sulfate, a prototypical substrate for the transporter, with IC₅₀ value of 49.91 μM. Dixon plot analysis revealed that inhibition by dexamethasone was competitive with a Ki = 47.08 μM. In contrast to the cis-inhibition effect of dexamethasone, prolonged incubation (6 h) of hOAT3-expressing cells with dexamethasone resulted in an upregulation of hOAT3 expression and transport activity, kinetically revealed as an increase in the maximum transport velocity V_max without meaningful alteration in substrate-binding affinity K_m. Such upregulation was abrogated by GSK650394, a specific inhibitor for serum- and glucocorticoid-inducible kinases (sgk). Dexamethasone also enhanced sgk1 phosphorylation. Our study demonstrated that dexamethasone exhibits dual effects on hOAT3: it is a competitive inhibitor for hOAT3-mediated transport, and interestingly, when entering the cells, it stimulates hOAT3 expression and transport activity through sgk1.

SGK1/Nedd4-2 signaling pathway regulates the activity of human organic anion transporters 3

Human organic anion transporter 3 (hOAT3) is localized at the basolateral membrane of renal proximal tubule cells and facilitates the renal secretion of numerous clinical drugs, including anti-HIV therapeutics, anti-tumor drugs, antibiotics, antihypertension drugs and anti-inflammatories. The present study explored the role of serum and glucocorticoid-inducible kinase 1 (sgk1) in the regulation of hOAT3. It was shown that over-expression of sgk1 in hOAT3-expressing cells stimulated hOAT3 transport activity by enhancing the transporter expression at the plasma membrane, kinetically reflected as an increased maximal transport velocity V_max without substantial change in the substrate-binding affinity K_m . In contrast, treatment of cells with the sgk-specific inhibitor GSK650394 resulted in a dose-dependent inhibition of hOAT3 transport activity. Evidence was further provided that sgk1 regulation of hOAT3 activity was mediated by ubiquitin ligase Nedd4-2, an enzyme previously shown to have an inhibitory effect on hOAT3. It was shown that sgk1 phosphorylated Nedd4-2, weakened the association between Nedd4-2 and hOAT3, and decreased hOAT3 ubiquitination. Functionally, the sgk1-stimulated hOAT3 transport activity was attenuated in the presence of a ligase-dead mutant of Nedd4-2. In summary, the investigation established for the first time that sgk1 stimulates hOAT3 transport activity by interfering with the inhibitory effect of Nedd4-2 on the transporter.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

Organic Anion Transporter 2: An Enigmatic Human Solute Carrier

As a member of the solute carrier 22A (SLC22A) family, organic anion transporter 2 (OAT2; SLC22A7) is emerging as an important drug transporter because of its expression in both the liver and kidney, two major eliminating organs, and its ability to transport not only a wide variety of xenobiotics but also numerous physiologically important endogenous compounds, like creatinine and cGMP. However, OAT2 has received relatively little attention compared with other OATs and solute carriers (SLCs), like organic cation transporters, sodium-dependent taurocholate cotransporting polypeptide, multidrug and toxin extrusion proteins, and organic anion-transporting polypeptides. Overall, the literature describing OAT2 is rapidly evolving, with numerous publications contradicting each other regarding the transport mechanism, tissue distribution, and transport of creatinine and cGMP, two important endogenous OAT2 substrates. Despite its status as a liver and kidney SLC, tools for assessing its activity and inhibition are lacking, and its role in drug disposition and elimination remains to be defined. The current review focuses on the available and emerging literature describing OAT2. We envision that OAT2 will gain more prominence as its expression, substrate, and inhibitor profile is investigated further and compared with other SLCs.

Renal drug transporters and their significance in drug-drug interactions

The kidney is a vital organ for the elimination of therapeutic drugs and their metabolites. Renal drug transporters, which are primarily located in the renal proximal tubules, play an important role in tubular secretion and reabsorption of drug molecules in the kidney. Tubular secretion is characterized by high clearance capacities, broad substrate specificities, and distinct charge selectivity for organic cations and anions. In the past two decades, substantial progress has been made in understanding the roles of transporters in drug disposition, efficacy, toxicity and drug-drug interactions (DDIs). In the kidney, several transporters are involved in renal handling of organic cation (OC) and organic anion (OA) drugs. These transporters are increasingly recognized as the target for clinically significant DDIs. This review focuses on the functional characteristics of major human renal drug transporters and their involvement in clinically significant DDIs.

Posttranslational Regulation of Organic Anion Transporters by Ubiquitination: Known and Novel

Organic anion transporters (OATs) encoded by solute carrier 22 family are localized in the epithelia of multiple organs, where they mediate the absorption, distribution, and excretion of a diverse array of negatively charged environmental toxins and clinically important drugs. Alterations in the expression and function of OATs play important roles in intra- and interindividual variability of the therapeutic efficacy and the toxicity of many drugs. As a result, the activity of OATs must be under tight regulation so as to carry out their normal functions. The regulation of OAT transport activity in response to various stimuli can occur at several levels such as transcription, translation, and posttranslational modification. Posttranslational regulation is of particular interest, because it usually happens within a very short period of time (minutes to hours) when the body has to deal with rapidly changing amounts of substances as a consequence of variable intake of drugs, fluids, or meals as well as metabolic activity. This review article highlights the recent advances from our laboratory in uncovering several posttranslational mechanisms underlying OAT regulation. These advances offer the promise of identifying targets for novel strategies that will maximize therapeutic efficacy in drug development.

Serum- and glucocorticoid-inducible kinase sgk2 stimulates the transport activity of human organic anion transporters 1 by enhancing the stability of the transporter

Human organic anion transporter 1 (hOAT1) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs, including anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT1 is abundantly expressed in the kidney and brain. In the current study, we examined the regulation of hOAT1 by serum- and glucocorticoid-inducible kinase 2 (sgk2) in the kidney COS-7 cells. We showed that sgk2 stimulated hOAT1 transport activity. Such stimulation mainly resulted from an increased cell surface expression of the transporter, kinetically revealed as an increased maximal transport velocity V max without significant change in substrate-binding affinity K m. We further showed that stimulation of hOAT1 activity by sgk2 was achieved by preventing hOAT1 degradation. Our co-immunoprecipitation experiment revealed that the effect of sgk2 on hOAT1 was through a direct interaction between these two proteins. In conclusion, our study demonstrated that sgk2 stimulates hOAT1 transport activity by enhancing the stability of the transporter. This study provides the insights into sgk2 regulation of hOAT1-mediated transport in normal physiology and disease.

Evaluation of renal in vitro models used in ochratoxin research

Ochratoxin A (OTA) induces renal carcinomas in rodents with a specific localisation in the S3 segment of proximal tubules and distinct early severe tissue alterations, which have been observed also in other species. Pronounced species- and sex-specific differences in toxicity occur and similar effects cannot be excluded in humans, however precise mechanism(s) remain elusive until today. In such cases, the use of in vitro models for mechanistic investigations can be very useful; in particular if a non-genotoxic mechanism of cancer formation is assumed which include cytotoxic effects. However, potential genotoxic mechanisms can also be investigated in vitro. A crucial issue of in vitro research is the choice of the appropriate cell model. Apparently, the cellular target of OTA is the renal proximal tubular cell; therefore cells from this tissue area are the most reasonable model. Furthermore, cells from affected species should be used and can be compared to cells of human origin. Another important parameter is whether to use primary cultures or to choose a cell line from the huge variety of cell lines available. In any case, important characteristics and quality controls need to be verified beforehand. Therefore, this review discusses the renal in vitro models that have been used for the investigation of renal ochratoxin toxicity. In particular, we discuss the choice of the models and the essential parameters making them suitable models for ochratoxin research together with exemplary results from this research. Furthermore, new promising models such as hTERT-immortalised cells and 3D-cultures are briefly discussed.

Inhibitory effects of chemotherapeutics on human organic anion transporter hOAT4

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters which play critical roles in the body disposition of clinically important drugs. hOAT4 is expressed in the kidney and placenta. In the current study, we examined the inhibitory effects of 101 anticancer drugs from a clinical drug library on hOAT4 transport activity. The studies were carried out in hOAT4-expressing human kidney HEK-293 cells and human placenta BeWo cells. Among these drugs, only chlorambucil and cabazitaxel demonstrated more than 50% cis-inhibitory effect on hOAT4-mediated uptake of (3)H-labeled estrone sulfate, a prototypical substrate for the transporter. The IC50 values for chlorambucil and cabazitaxel were 44.28 and 3.5 µM respectively. Dixon plot analysis revealed that inhibition by chlorambucil was competitive with a Ki = 55.73 µM whereas inhibition by cabazitaxel was non-competitive with a Ki = 1.78 µM. Our results demonstrated that chlorambucil and cabazitaxel were inhibitors of hOAT4. Furthermore, by comparing our data with clinically relevant exposures of these drugs, we conclude that the propensity for chlorambucil and cabazitaxel to cause drug-drug interaction through inhibition of hOAT4 is low.

Serum- and glucocorticoid-inducible kinase SGK2 regulates human organic anion transporters 4 via ubiquitin ligase Nedd4-2

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs, including anti-viral therapeutics, anti-cancer drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT4 is abundantly expressed in the kidney and placenta. In the current study, we examined the regulation of hOAT4 by serum- and glucocorticoid-inducible kinase 2 (sgk2) in the kidney COS-7 cells. We showed that sgk2 stimulated hOAT4 transport activity. Such stimulation mainly resulted from an increased cell surface expression of the transporter, kinetically revealed as an increased maximal transport velocity Vmax without significant change in substrate-binding affinity Km. We further showed that regulation of hOAT4 activity by sgk2 was mediated by ubiquitin ligase Nedd4-2. Overexpression of Nedd4-2 enhanced hOAT4 ubiquitination, and inhibited hOAT4 transport activity, whereas overexpression of ubiquitin ligase-dead mutant Nedd4-2/C821A or siRNA knockdown of endogenous Nedd4-2 had opposite effects on hOAT4. Our co-immunoprecipitation experiment revealed that sgk2 weakened the association between hOAT4 and Nedd4-2. In conclusion, our study demonstrated for the first time that sgk2 stimulated hOAT4 transport activity by abrogating the inhibitory effect of Nedd4-2 on the transporter.

An Essential Role of Nedd4-2 in the Ubiquitination, Expression, and Function of Organic Anion Transporter-3

Organic anion transporter-3 (OAT3) is a member of the organic anion transporter family that mediates the body disposition of a diverse array of clinically important drugs. We previously demonstrated that activation of protein kinase C (PKC) inhibits OAT3 transport activity by accelerating OAT3 internalization from cell surface into intracellular compartments. In the current study, we established that PKC-induced inhibition of OAT3 transport activity occurred through an enhanced OAT3 ubiquitination, a process catalyzed by an E3 ubiquitin-protein ligase Nedd4-2 (neural precursor cell expressed, developmentally downregulated 4-2). Overexpression of Nedd4-2 enhanced OAT3 ubiquitination, decreased OAT3 expression at the cell surface, and inhibited OAT3 transport activity. In contrast, overexpression of the ubiquitin ligase-dead mutant Nedd4-2/C821A or siRNA knockdown of endogenous Nedd4-2 had opposite effects on OAT3. Furthermore, immunoprecipitation experiments conducted both in culture cells and with rat kidney slices showed that there was a physical interaction between OAT3 and Nedd4-2. In conclusion, our results provided the first evidence that Nedd4-2 is an important regulator for OAT3 ubiquitination, expression, and transport activity.

The in vivo disposition and in vitro transmembrane transport of two model radiometabolites of DOTA-conjugated receptor-specific peptides labelled with (177) Lu

In vivo metabolism of the radiolabelled receptor-specific peptides has been described; however, information regarding the pharmacokinetic behaviour of the degradation products within the body is very scarce. The present study was designed to obtain new knowledge on the disposition and elimination of low-molecular radiometabolites of receptor-specific peptides in the organism and to reveal the potential involvement of selected membrane transport mechanisms in the cellular uptake of radiometabolites, especially in the kidney. The study compared pharmacokinetics of two radiometabolites: a final metabolite of somatostatin analogues, (177)Lu-DOTA-DPhe, and a tripeptide metabolite of (177)Lu-DOTA-minigastrin 11, (177)Lu-DOTA-DGlu-Ala-Tyr. Their pharmacokinetics was compared with that of respective parent (177)Lu-radiopeptide. Both radiometabolites exhibited relative rapid clearing from most body tissues in rats in vivo along with predominant renal excretion. The long-term renal retention of the smaller radiometabolite (177)Lu-DOTA-DPhe was lower than that of (177)Lu-DOTA-DGlu-Ala-Tyr. An uptake of (177)Lu-DOTA-DPhe by human renal influx transporter organic cation transporter 2 was found in vitro in a cellular model. The study brings the first experimental data on the in vivo pharmacokinetics of radiometabolites of receptor-specific somatostatin and gastrin analogues. The found results may indicate a negative correlation between the degree of decomposition of the parent peptide chain and the renal retention of the metabolite.

Evolutionary Analysis and Classification of OATs, OCTs, OCTNs, and Other SLC22 Transporters: Structure-Function Implications and Analysis of Sequence Motifs

The SLC22 family includes organic anion transporters (OATs), organic cation transporters (OCTs) and organic carnitine and zwitterion transporters (OCTNs). These are often referred to as drug transporters even though they interact with many endogenous metabolites and signaling molecules (Nigam, S.K., Nature Reviews Drug Discovery, 14:29-44, 2015). Phylogenetic analysis of SLC22 supports the view that these transporters may have evolved over 450 million years ago. Many OAT members were found to appear after a major expansion of the SLC22 family in mammals, suggesting a physiological and/or toxicological role during the mammalian radiation. Putative SLC22 orthologs exist in worms, sea urchins, flies, and ciona. At least six groups of SLC22 exist. OATs and OCTs form two Major clades of SLC22, within which (apart from Oat and Oct subclades), there are also clear Oat-like, Octn, and Oct-related subclades, as well as a distantly related group we term "Oat-related" (which may have different functions). Based on available data, it is arguable whether SLC22A18, which is related to bacterial drug-proton antiporters, should be assigned to SLC22. Disease-causing mutations, single nucleotide polymorphisms (SNPs) and other functionally analyzed mutations in OAT1, OAT3, URAT1, OCT1, OCT2, OCTN1, and OCTN2 map to the first extracellular domain, the large central intracellular domain, and transmembrane domains 9 and 10. These regions are highly conserved within subclades, but not between subclades, and may be necessary for SLC22 transporter function and functional diversification. Our results not only link function to evolutionarily conserved motifs but indicate the need for a revised sub-classification of SLC22.

Synthesis and Characterization of 8-O-Carboxymethylpyranine (CM-Pyranine) as a Bright, Violet-Emitting, Fluid-Phase Fluorescent Marker in Cell Biology

To avoid spectral interference with common fluorophores in multicolor fluorescence microscopy, a fluid-phase tracer with excitation and emission in the violet end of the visible spectrum is desirable. CM-pyranine is easily synthesized and purified. Its excitation and emission maxima at 401.5 nm and 428.5 nm, respectively, are well suited for excitation by 405-nm diode lasers now commonly available on laser-scanning microscopes. High fluorescence quantum efficiency (Q = 0.96) and strong light absorption (ε405 > 25,000 M-1cm-1) together make CM-pyranine the brightest violet aqueous tracer. The fluorescence spectrum of CM-pyranine is invariant above pH 4, which makes it a good fluid-phase marker in all cellular compartments. CM-pyranine is very photostable, is retained for long periods by cells, does not self-quench, and has negligible excimer emission. The sum of its properties make CM-pyranine an ideal fluorescent tracer. The use of CM-pyranine as a fluid-phase marker is demonstrated by multicolor confocal microscopy of cells that are also labeled with lipid and nuclear markers that have green and red fluorescence emission, respectively.

How drugs get into cells: tested and testable predictions to help discriminate between transporter-mediated uptake and lipoidal bilayer diffusion

One approach to experimental science involves creating hypotheses, then testing them by varying one or more independent variables, and assessing the effects of this variation on the processes of interest. We use this strategy to compare the intellectual status and available evidence for two models or views of mechanisms of transmembrane drug transport into intact biological cells. One (BDII) asserts that lipoidal phospholipid Bilayer Diffusion Is Important, while a second (PBIN) proposes that in normal intact cells Phospholipid Bilayer diffusion Is Negligible (i.e., may be neglected quantitatively), because evolution selected against it, and with transmembrane drug transport being effected by genetically encoded proteinaceous carriers or pores, whose “natural” biological roles, and substrates are based in intermediary metabolism. Despite a recent review elsewhere, we can find no evidence able to support BDII as we can find no experiments in intact cells in which phospholipid bilayer diffusion was either varied independently or measured directly (although there are many papers where it was inferred by seeing a covariation of other dependent variables). By contrast, we find an abundance of evidence showing cases in which changes in the activities of named and genetically identified transporters led to measurable changes in the rate or extent of drug uptake. PBIN also has considerable predictive power, and accounts readily for the large differences in drug uptake between tissues, cells and species, in accounting for the metabolite-likeness of marketed drugs, in pharmacogenomics, and in providing a straightforward explanation for the late-stage appearance of toxicity and of lack of efficacy during drug discovery programmes despite macroscopically adequate pharmacokinetics. Consequently, the view that Phospholipid Bilayer diffusion Is Negligible (PBIN) provides a starting hypothesis for assessing cellular drug uptake that is much better supported by the available evidence, and is both more productive and more predictive.

Renal handling of amphotericin B and amphotericin B-deoxycholate and potential renal drug-drug interactions with selected antivirals

Amphotericin B (AmB) is excreted via the renal excretion route. This excretion process may result in nephrotoxicity. However, relevant information on the precise renal excretion mechanisms is not available. The aim of the study was to analyze the possible interaction of AmB or its prodrug AmB deoxycholate (AmB-DOC) with the typical renal organic anion transporters (OATs) and organic cation transporters (OCTs), using cellular and organ models. The relevant transport systems were then investigated in terms of the drug-drug interactions of AmB-DOC with antivirals that might potentially be used concomitantly. To analyze the renal excretion mechanisms of [(3)H]AmB, perfused rat kidney was employed. HeLa and MDCK II cells transiently transfected with human OAT1 (hOAT1) or hOCT2 were used as the cellular models. A significant tubular secretion of AmB was demonstrated in the perfused rat kidney. The cellular studies performed confirmed the active transport of AmB into cells. AmB did not interact with hOAT1 but strongly inhibited hOCT2. In contrast, AmB-DOC inhibited both hOAT1 and hOCT2. However, [(3)H]AmB cellular uptake by hOAT1 and hOCT2 was not found. AmB-DOC interacted significantly with adefovir, tenofovir, and cidofovir in hOAT1-transfected cells at supratherapeutic concentrations. In conclusion, the significant potency of AmB and AmB-DOC for inhibiting the transporters was demonstrated in this study. The secretion of AmB in the renal tubules is likely not related to the transporters here, since the drug was not proven to be a substrate for them. Drug-drug interactions of AmB and the antivirals used in this study on the investigated transporters are not probable.

Comparative in vivo evaluations of curcumin and its analog difluorinated curcumin against cisplatin-induced nephrotoxicity

Curcumin, a polyphenol, has pharmacological effects including antioxidant, anti-inflammatory and anti-cancer features. In this study, we have performed comparative in vivo evaluations of CDF (curcumin difluorinated) and curcumin in cisplatin-induced nephrotoxicity in rats. Male Wistar rats were divided into four groups: (1) Control; (2) Cisplatin (7 mg/kg body wt, intraperitoneal as a single dose); (3) Cisplatin and CDF (50 mg/rat/day; for 12 days); (4) Cisplatin and curcumin (50 mg/rat/day), for 12 days). Cisplatin treated rats exhibited kidney injury manifested by increased serum N-urea and creatinine (P < 0.001). Kidney from cisplatin treated rats also exhibited significant increase in malondialdehyde (MDA) and 8-isoprostane levels (P < 0.001). Treatment with CDF and curcumin prevented the rise in serum N-urea, creatinine, MDA and 8-isoprostane as compared to experimental control group in kidney (P < 0.05). Compared to curcumin, CDF had greater potential in suppressing cisplatin-induced pro-inflammatory factors NF-κB and COX-2 as well as downstream markers Nrf2 and HO-1 (P < 0.05) in kidney. The analysis on anion transport markers (OAT1 and OAT3) showed a similar trend (CDF > curcumin). CDF could reduce the expression of multi-drug resistance markers OCT1, OCT2, MRP2 and MRP4 to a much greater extent than curcumin (P < 0.05). We also demonstrate that CDF influenced the expression of p-mTOR, p-p70S6K1, p-4E-BP1 and p-Akt. These data suggest that CDF can potentially be used to reduce the chemotherapy induced nephrotoxicity thereby enhancing the therapeutic window of cisplatin. The results also proved that compared to curcumin, CDF has superior protective effect in nephrotoxicity.

SLC22, SLC44, and SLC47 transporters--organic anion and cation transporters: molecular and cellular properties

Transporters within the SLC22, SLC44, and SLC47 families of solute carriers mediate transport of a structurally diverse array of organic electrolytes, that is, molecules that are generally charged (cationic, anionic, or zwitterionic) at physiological pH. Transporters in the SLC22 family--all of which are members of the major facilitator superfamily (MFS) of transporters--represent a mechanistically diverse set of processes, including the organic anion transporters (OATs and URAT1) that physiologically operate as organic anion (OA) exchangers, the organic cation transporters (OCTs) that operate as electrogenic uniporters of organic cations (OCs), and the so-called "novel" organic cation transporters (OCTNs) that support Na-cotransport of selected zwitterions. Whereas the OCTNs display a high degree of substrate selectivity, the physiological hallmark of the OATs and OCTs is their multiselectivity--consistent with a principal role in renal and hepatic clearance of a wide array of both endogenous and xenobiotic compounds. SLC47 consists of members of the multidrug and toxin extruder (MATE) family, which are carriers that are obligatory exchangers and that physiologically support electroneutral H⁺ exchange. The MATEs also display a characteristic multiselectivity and are frequently paired with OCTs to mediate transepithelial OC secretion, with the OCTs typically supporting basolateral OC entry and the MATEs supporting apical OC efflux. The SLC44 family contains the choline transporter-like (CTL) transporters. Largely restricted to choline and a limited set of structural congeners, the CTLs appear to support the Na-independent, electrogenic uniport of choline, thereby providing choline for membrane biogenesis. The solution of X-ray crystal structures of representative prokaryotic MFS and MATE transporters has led to the development of homology models of mammalian OAT, OCT, and MATE transporters that, in turn, have supplemented studies of the molecular basis of the complex interactions of ligands with these multiselective proteins.

Renal transport of organic anions and cations

Organic anions and cations (OAs and OCs, respectively) comprise an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. The kidney, primarily the renal proximal tubule, plays a critical role in regulating the plasma concentrations of these organic electrolytes and in clearing the body of potentially toxic xenobiotics agents, a process that involves active, transepithelial secretion. This transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. Basolateral and luminal OA and OC transport reflects the concerted activity of a suite of separate proteins arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney, now allows the development of models describing the molecular basis of the renal secretion of OAs and OCs. New information on naturally occurring genetic variation of many of these processes provides insight into the basis of observed variability of drug efficacy and unwanted drug-drug interactions in human populations. The present review examines recent work on these issues.

Protein expression of kidney and liver bilitranslocase in rats exposed to mercuric chloride--a potential tissular biomarker of toxicity

Bilitranslocase (BTL) is a plasma membrane carrier that transports organic anions of physiological and pharmacological interest. It is expressed in basolateral plasma membrane of kidney and liver. BTL has been recently described as a marker of transition from normal tissue to its neoplastic transformation in human kidney. Inorganic mercury is a major environmental contaminant that produces many toxic effects. Previous reports have described an interaction between BTL and mercuric ions. This study was designed to evaluate the renal and hepatic expression of BTL in rats exposed to a nephrotoxic and hepatotoxic dose of HgCl2. Male rats were treated with a single injection of HgCl2 at a dose of 4mg/kg body wt, i.p. (HgCl2 group). Control rats received the vehicle alone (Control group). Studies were carried out 18h after injection. Afterwards, the kidneys and livers were excised and processed for histopathological studies or immunoblot (homogenates and crude membranes) techniques. In rats treated with HgCl2, immunoblotting showed a significant decrease in the abundance of BTL in homogenates and plasma membranes from kidney and liver. BTL decrease of expression might reflect the grade of damage in renal tubule cells and in hepatocytes. Thus, BTL might be postulated as a new biomarker of tissue toxicity induced by mercury.

Primary porcine proximal tubular cells as an alternative to human primary renal cells in vitro: an initial characterization

Background

A good in vitro model should approximate an in vivo-like behavior as closely as possible in order to reflect most likely the in vivo situation. Regarding renal physiology of different species, humans are more closely related to pigs than to rodents, therefore primary porcine kidney cells (PKC) and their subsequent cell strain could be a valid alternative to primary human cells for renal in vitro toxicology. For this PKC must display inherent characteristics (e.g. structural organization) and functions (e.g. transepithelial transport) as observed under in vivo conditions within the respective part of the kidney.

Results

We carried out a comprehensive characterization of PKC and their subsequent cell strain, including morphology and growth as well as transporter expression and functionality. The data presented here demonstrate that PKC express various transporters including pMrp1 (abcc1), pMrp2 (abcc2), pOat1 (slc22a6) and pOat3 (slc22a8), whereas pMdr1 (abcb1) and pOatp1a2 (slco1a2) mRNA could not be detected in either the PKCs or in the porcine cortical tissue. Functionality of the transporters was demonstrated by determining the specific PAH transport kinetics.

Conclusions

On the basis of the presented results it can be concluded that PKC and to some extent their subsequent cell strain represent a valuable model for in vitro toxicology, which might be used as an alternative to human primary cells.

Organic anion transporter 5 renal expression and urinary excretion in rats with vascular calcification

It has been described renal damage in rats with vascular calcification. The organic anion transporter 5 (Oat5) is only expressed in kidney, and its urinary excretion was proposed as potential early biomarker of renal injury. The aim of this study was to evaluate the Oat5 renal expression and its urinary excretion in an experimental model of vascular calcification in comparison with traditional markers of renal injury. Vascular calcification was obtained by the administration of an overdose of vitamin D₃ (300,000 IU/kg, b.w., i.m.) to male Wistar rats. Oat5 urinary abundance was evaluated by Western blotting. Traditional markers of renal injury, such as creatinine and urea plasma levels, urinary protein levels, and urinary alkaline phosphatase (AP) activity, were determined using commercial kits. Histology was assessed by hematoxylin/eosin staining. Oat5 renal expression was evaluated by Western blotting and by immunohistochemistry. An increased expression of Oat5 in renal homogenates, in apical membranes, and in its urinary excretion was observed in rats with vascular calcification. The traditional parameters used to evaluate renal function were not modified, with the exception of histology. It is possible to postulate the urinary excretion of Oat5 as a potential noninvasive biomarker of renal injury associated with vascular calcification.

The SLC22 family with transporters of organic cations, anions and zwitterions

The SLC22 family contains 13 functionally characterized human plasma membrane proteins each with 12 predicted α-helical transmembrane domains. The family comprises organic cation transporters (OCTs), organic zwitterion/cation transporters (OCTNs), and organic anion transporters (OATs). The transporters operate as (1) uniporters which mediate facilitated diffusion (OCTs, OCTNs), (2) anion exchangers (OATs), and (3) Na(+)/zwitterion cotransporters (OCTNs). They participate in small intestinal absorption and hepatic and renal excretion of drugs, xenobiotics and endogenous compounds and perform homeostatic functions in brain and heart. Important endogeneous substrates include monoamine neurotransmitters, l-carnitine, α-ketoglutarate, cAMP, cGMP, prostaglandins, and urate. It has been shown that mutations of the SLC22 genes encoding these transporters cause specific diseases like primary systemic carnitine deficiency and idiopathic renal hypouricemia and are correlated with diseases such as Crohn's disease and gout. Drug-drug interactions at individual transporters may change pharmacokinetics and toxicities of drugs.

P2' benzene carboxylic acid moiety is associated with decrease in cellular uptake: evaluation of novel nonpeptidic HIV-1 protease inhibitors containing P2 bis-tetrahydrofuran moiety

GRL007 and GRL008, two structurally related nonpeptidic human immunodeficiency virus type 1 (HIV-1) protease inhibitors (PIs) containing 3(R),3a(S),6a(R)-bis-tetrahydrofuranylurethane (bis-THF) as the P2 moiety and a sulfonamide isostere consisting of benzene carboxylic acid and benzene carboxamide as the P2' moiety, respectively, were evaluated for their antiviral activity and interactions with wild-type protease (PR(WT)). Both GRL007 (Ki of 12.7 pM with PR(WT)) and GRL008 (Ki of 8.9 pM) inhibited PR(WT) with high potency in vitro. X-ray crystallographic analysis of PR(WT) in complex with GRL007 or GRL008 showed that the bis-THF moiety of both compounds has three direct polar contacts with the backbone amide nitrogen atoms of Asp29 and Asp30 of PR(WT). The P2' moiety of both compounds showed one direct contact with the backbone of Asp30' and a bridging polar contact with Gly48' through a water molecule. Cell-based antiviral assays showed that GRL007 was inactive (50% effective concentration [EC50] of >1 μM) while GRL008 was highly active (EC50 of 0.04 μM) against wild-type HIV-1. High-performance liquid chromatography (HPLC)/mass spectrometry-based cellular uptake assays showed 8.1- and 84-fold higher intracellular concentrations of GRL008 than GRL007 in human MT-2 and MT-4 cell extracts, respectively. Thus, GRL007, in spite of its favorable enzyme-inhibitory activity and protease binding profile, exhibited a lack of antiviral activity in cell-based assays, most likely due to its compromised cellular uptake associated with its P2' benzene carboxylic acid moiety. The anti-HIV-1 potency, favorable toxicity, and binding profile of GRL008 suggest that further optimization of the P2' moiety may improve its antiretroviral features.

Three ubiquitination sites of organic anion transporter-1 synergistically mediate protein kinase C-dependent endocytosis of the transporter

Organic anion transporter-1 (OAT1) mediates the body disposition of a diverse array of clinically important drugs, including anti-HIV therapeutics, antitumor drugs, antibiotics, antihypertensives, and anti-inflammatories. Therefore, understanding the regulation of OAT1 has profound clinical significance. We previously established that OAT1 constitutively internalizes from and recycles back to cell surface and that activation of protein kinase C (PKC) inhibits OAT1 activity by promoting ubiquitination of the transporter, which then leads to an accelerated internalization of the transporter from cell surface to intracellular compartments. In the current study, we showed that PKC isoform PKCα was responsible for OAT1 ubiquitination. To directly address the role of OAT1 ubiquitination, we then generated two OAT1 mutants, each having multiple lysines (K) simultaneously mutated to arginine (R). One mutant K163/297/303/315/321R lost sensitivities to PKC-induced inhibition of transport activity, to PKC-induced ubiquitination, and to PKC-induced acceleration of transporter internalization. Further dissecting each lysine in this mutant, we identified Lys297, Lys303, and Lys315 as being the ubiquitin conjugation sites. Of interest, mutating any one of the three lysines prevented the ubiquitin conjugation to the other two lysines, suggesting that Lys297, Lys303, and Lys315 may form an optimal structure to interact with ubiquitination machineries. This is the first demonstration that Lys297, Lys303, and Lys315 play a synergistic role in PKC-regulated OAT1 ubiquitination, trafficking, and transport activity.