Facilitated mitochondrial import of antiviral and anticancer nucleoside drugs by human equilibrative nucleoside transporter-3

The plasma membrane monoamine transporter (PMAT): Structure, function, and role in organic cation disposition

Plasma membrane monoamine transporter (PMAT) is a new polyspecific organic cation transporter that transports a variety of biogenic amines and xenobiotic cations. Highly expressed in the brain, PMAT represents a major uptake₂ transporter for monoamine neurotransmitters. At the blood-cerebrospinal fluid (CSF) barrier, PMAT is the principal organic cation transporter for removing neurotoxins and drugs from the CSF. Here I summarize our latest understanding of PMAT and its roles in monoamine uptake and xenobiotic disposition.

Neurotoxicity in the Post-HAART Era: Caution for the Antiretroviral Therapeutics

Despite the advent of highly active antiretroviral therapy (HAART), HIV-associated neurological disorders (HAND) remain a major challenge in human immunodeficiency virus (HIV) treatment. The early implementation of HAART in the infected individuals helps suppress the viral replication in the plasma and other compartments. Several studies also report the beneficial effect of drugs that successfully penetrate central nervous system (CNS). However, recent data in both clinical setup and in in vitro studies indicate CNS toxicity of the antiretrovirals (ARVs). Although the evidence is limited, correlation between prolonged use of ARVs and neurotoxicity strongly suggests that it is essential to study the underlying mechanisms responsible for such toxicity. Furthermore, closer attention toward clinical outcomes is required to screen various ARV regimens for their association with HAND and other comorbidities. A growing body of literature also indicates a possible role of accelerated aging in the antiretroviral therapy-associated neurotoxicity. Lastly, owing to high pill burden, multiple drugs in the HIV treatment also invite a possible role of drug-drug interaction via various cytochrome P450 enzymes. The particular emphasis of this review is to highlight the need to identify alternative approaches in reducing the CNS toxicity of the ARV drugs in HIV-infected individuals.

Compound heterozygous SLC29A3 mutation causes H syndrome in a Moroccan patient: A case report

H syndrome is an autosomal recessive syndrome, which affects the skin and some vital organs, it is caused by mutations in the SLC29A3 gene, encoding the human equilibrative nucleoside transporter hENT3. This report describes a patient with typical features of H syndrome. Based on the patient's clinical features, SLC29A3 was selected for molecular investigation. Through direct sequencing, a compound heterozygous alteration in the SLC29A3 gene was found. The c.243delA frameshift mutation leading to a premature termination, resulting in a truncated protein, and a splice site mutation c.300+1G>C predicted to cause a splicing error. This contribution extends the clinical variability of compound heterozygous SLC29A3 mutations resulting in an additional multisystemic manifestation of the clinical spectrum of SLC29A3 disorders.

The Deoxynucleoside Triphosphate Triphosphohydrolase Activity of SAMHD1 Protein Contributes to the Mitochondrial DNA Depletion Associated with Genetic Deficiency of Deoxyguanosine Kinase

The dNTP triphosphohydrolase SAMHD1 is a nuclear antiviral host restriction factor limiting HIV-1 infection in macrophages and a major regulator of dNTP concentrations in human cells. In normal human fibroblasts its expression increases during quiescence, contributing to the small dNTP pool sizes of these cells. Down-regulation of SAMHD1 by siRNA expands all four dNTP pools, with dGTP undergoing the largest relative increase. The deoxyguanosine released by SAMHD1 from dGTP can be phosphorylated inside mitochondria by deoxyguanosine kinase (dGK) or degraded in the cytosol by purine nucleoside phosphorylase. Genetic mutations of dGK cause mitochondrial (mt) DNA depletion in noncycling cells and hepato-cerebral mtDNA depletion syndrome in humans. We studied if SAMHD1 and dGK interact in the regulation of the dGTP pool during quiescence employing dGK-mutated skin fibroblasts derived from three unrelated patients. In the presence of SAMHD1 quiescent mutant fibroblasts manifested mt dNTP pool imbalance and mtDNA depletion. When SAMHD1 was silenced by siRNA transfection the composition of the mt dNTP pool approached that of the controls, and mtDNA copy number increased, compensating the depletion to various degrees in the different mutant fibroblasts. Chemical inhibition of purine nucleoside phosphorylase did not improve deoxyguanosine recycling by dGK in WT cells. We conclude that the activity of SAMHD1 contributes to the pathological phenotype of dGK deficiency. Our results prove the importance of SAMHD1 in the regulation of all dNTP pools and suggest that dGK inside mitochondria has the function of recycling the deoxyguanosine derived from endogenous dGTP degraded by SAMHD1 in the nucleus.

Drug transporters in the nasal epithelium: an overview of strategies in targeted drug delivery

In this article, we discussed the expression of some ABC (e.g., P-glycoprortein, MRP and CFTR) and SLC (e.g., POT, DAT, OAT, OATP, OCT, EAAT2/GLT1 and GLUT) amino acid, metal and nucleoside transporters in the nasal mucosa. The localization and therapeutic targeting of these transporters are explored in detail. The wide array of transporters discovered so far in the nasal mucosa implies that a plethora of compounds can be delivered by targeting these transporters. The article concludes with a discussion of the potential challenges and delivery options for transporter-mediated drug targeting via the nasal route.

Genomic signatures for paclitaxel and gemcitabine resistance in breast cancer derived by machine learning

Increasingly, the effectiveness of adjuvant chemotherapy agents for breast cancer has been related to changes in the genomic profile of tumors. We investigated correspondence between growth inhibitory concentrations of paclitaxel and gemcitabine (GI50) and gene copy number, mutation, and expression first in breast cancer cell lines and then in patients. Genes encoding direct targets of these drugs, metabolizing enzymes, transporters, and those previously associated with chemoresistance to paclitaxel (n = 31 genes) or gemcitabine (n = 18) were analyzed. A multi-factorial, principal component analysis (MFA) indicated expression was the strongest indicator of sensitivity for paclitaxel, and copy number and expression were informative for gemcitabine. The factors were combined using support vector machines (SVM). Expression of 15 genes (ABCC10, BCL2, BCL2L1, BIRC5, BMF, FGF2, FN1, MAP4, MAPT, NFKB2, SLCO1B3, TLR6, TMEM243, TWIST1, and CSAG2) predicted cell line sensitivity to paclitaxel with 82% accuracy. Copy number profiles of 3 genes (ABCC10, NT5C, TYMS) together with expression of 7 genes (ABCB1, ABCC10, CMPK1, DCTD, NME1, RRM1, RRM2B), predicted gemcitabine response with 85% accuracy. Expression and copy number studies of two independent sets of patients with known responses were then analyzed with these models. These included tumor blocks from 21 patients that were treated with both paclitaxel and gemcitabine, and 319 patients on paclitaxel and anthracycline therapy. A new paclitaxel SVM was derived from an 11-gene subset since data for 4 of the original genes was unavailable. The accuracy of this SVM was similar in cell lines and tumor blocks (70-71%). The gemcitabine SVM exhibited 62% prediction accuracy for the tumor blocks due to the presence of samples with poor nucleic acid integrity. Nevertheless, the paclitaxel SVM predicted sensitivity in 84% of patients with no or minimal residual disease.

Equilibrative nucleoside transporter 3 depletion in β-cells impairs mitochondrial function and promotes apoptosis: Relationship to pigmented hypertrichotic dermatosis with insulin-dependent diabetes

Loss of function recessive mutations in the SLC29A3 gene that encodes human equilibrative nucleoside transporter 3 (ENT3) have been identified in patients with pigmented hypertrichotic dermatosis with insulin-dependent diabetes (PHID). ENT3 is a member of the equilibrative nucleoside transporter (ENT) family whose primary function is mediating transport of nucleosides and nucleobases. The aims of this study were to characterise ENT3 expression in islet β-cells and identify the effects of its depletion on β-cell mitochondrial activity and apoptosis. RT-PCR amplification identified ENT3 expression in human and mouse islets and exocrine pancreas, and in MIN6 β-cells. Immunohistochemistry using human and mouse pancreas sections exhibited extensive ENT3 immunostaining of β-cells, which was confirmed by co-staining with an anti-insulin antibody. In addition, exposure of dispersed human islet cells and MIN6 β-cells to MitoTracker and an ENT3 antibody showed co-localisation of ENT3 to β-cell mitochondria. Consistent with this, Western blot analysis confirmed enhanced ENT3 immunoreactivity in β-cell mitochondria-enriched fractions. Furthermore, ENT3 depletion in β-cells increased mitochondrial DNA content and promoted an energy crisis characterised by enhanced ATP-linked respiration and proton leak. Finally, inhibition of ENT3 activity by dypridamole and depletion of ENT3 by siRNA-induced knockdown resulted in increased caspase 3/7 activities in β-cells. These observations demonstrate that ENT3 is predominantly expressed by islet β-cells where it co-localises with mitochondria. Depletion of ENT3 causes mitochondrial dysfunction which is associated with enhanced β-cell apoptosis. Thus, apoptotic loss of islet β-cells may contribute to the occurrence of autoantibody-negative insulin-dependent diabetes in individuals with non-functional ENT3 mutations.

Novel homozygous SLC29A3 mutations among two unrelated Egyptian families with spectral features of H-syndrome

OBJECTIVES

H syndrome and pigmented hypertrichosis with insulin-dependent diabetes mellitus (PHID) had been described as two autosomal recessive disorders. We aim to screen for pathogenic SLC29A3 mutations in two unrelated Egyptian families with affected siblings of these overlapping syndromes.

METHODS

Clinical, laboratory, histopathological, and radiological characteristics of individuals probably diagnosed as H and/or PHID syndrome were reported. Mutation analysis of SLC29A3 gene was performed for all members of the two Egyptian families.

RESULTS

All affected individuals were females; proband of family-I (A1961) displayed overlapping features of H syndrome and PHID, while her younger brother (A1962) was asymptomatic. A1961 presented with previously undescribed features; absent pectoralis major muscle and a supracondylar bony spur in left humerus. In family-II, probands (A1965 and A1966) had clinical features consistent with classical H syndrome with unique early onset of cutaneous phenomena at birth. Mutation analysis of SLC29A3 revealed homozygous mutation previously reported in literature c.1279G>A [p.G427S] in A1961 and unexpectedly in the asymptomatic A1962 of family-I. Probands of family-II were homozygous for a novel mutation c.401G>A [p.R134H], in the same codon that was published in an Indian boy [p.R134C].

CONCLUSIONS

We emphasize the inter- and intra-familial genetic heterogeneity among Egyptian patients with overlapping features of SLC29A3 disorders. This suggests the presence of other factors like regulatory genes or epigenetic factors that may explain variable disease manifestations and severity.

The nucleobase adenine as a signalling molecule in the kidney

In 2002, the first receptor activated by the nucleobase adenine was discovered in rats. In the past years, two adenine receptors (AdeRs) in mice and one in Chinese hamsters, all of which belong to the family of G protein-coupled receptors (GPCRs), were cloned and pharmacologically characterized. Based on the nomenclature for other purinergic receptor families (P1 for adenosine receptors and P2 for nucleotide, e.g. ATP, receptors), AdeRs were designated P0 receptors. Pharmacological data indicate the existence of G protein-coupled AdeRs in pigs and humans as well; however, those have not been cloned so far. Current data suggest a role for adenine and AdeRs in renal proximal tubules. Furthermore, AdeRs are suggested to be functional counterplayers of vasopressin in the collecting duct system, thus exerting diuretic effects. We are only at the beginning of understanding the significance of this new class of purinergic receptors, which might become future drug targets.

Nucleoside transporter proteins as biomarkers of drug responsiveness and drug targets

Nucleoside and nucleobase analogs are currently used in the treatment of solid tumors, lymphoproliferative diseases, viral infections such as hepatitis and AIDS, and some inflammatory diseases such as Crohn. Two gene families are implicated in the uptake of nucleosides and nucleoside analogs into cells, SCL28 and SLC29. The former encodes hCNT1, hCNT2, and hCNT3 proteins. They translocate nucleosides in a Na⁺ coupled manner with high affinity and some substrate selectivity, being hCNT1 and hCNT2 pyrimidine- and purine-preferring, respectively, and hCNT3 a broad selectivity transporter. SLC29 genes encode four members, being hENT1 and hENT2 the only two which are unequivocally implicated in the translocation of nucleosides and nucleobases (the latter mostly via hENT2) at the cell plasma membrane. Some nucleoside-derived drugs can also interact with and be translocated by members of the SLC22 gene family, particularly hOCT and hOAT proteins. Inter-individual differences in transporter function and perhaps, more importantly, altered expression associated with the disease itself might modulate the transporter profile of target cells, thereby determining drug bioavailability and action. Drug transporter pharmacology has been periodically reviewed. Thus, with this contribution we aim at providing a state-of-the-art overview of the clinical evidence generated so far supporting the concept that these membrane proteins can indeed be biomarkers suitable for diagnosis and/or prognosis. Last but not least, some of these transporter proteins can also be envisaged as drug targets, as long as they can show “transceptor” functions, in some cases related to their role as modulators of extracellular adenosine levels, thereby providing a functional link between P1 receptors and transporters.

The human SLC25A33 and SLC25A36 genes of solute carrier family 25 encode two mitochondrial pyrimidine nucleotide transporters

The human genome encodes 53 members of the solute carrier family 25 (SLC25), also called the mitochondrial carrier family, many of which have been shown to transport inorganic anions, amino acids, carboxylates, nucleotides, and coenzymes across the inner mitochondrial membrane, thereby connecting cytosolic and matrix functions. Here two members of this family, SLC25A33 and SLC25A36, have been thoroughly characterized biochemically. These proteins were overexpressed in bacteria and reconstituted in phospholipid vesicles. Their transport properties and kinetic parameters demonstrate that SLC25A33 transports uracil, thymine, and cytosine (deoxy)nucleoside di- and triphosphates by an antiport mechanism and SLC25A36 cytosine and uracil (deoxy)nucleoside mono-, di-, and triphosphates by uniport and antiport. Both carriers also transported guanine but not adenine (deoxy)nucleotides. Transport catalyzed by both carriers was saturable and inhibited by mercurial compounds and other inhibitors of mitochondrial carriers to various degrees. In confirmation of their identity (i) SLC25A33 and SLC25A36 were found to be targeted to mitochondria and (ii) the phenotypes of Saccharomyces cerevisiae cells lacking RIM2, the gene encoding the well characterized yeast mitochondrial pyrimidine nucleotide carrier, were overcome by expressing SLC25A33 or SLC25A36 in these cells. The main physiological role of SLC25A33 and SLC25A36 is to import/export pyrimidine nucleotides into and from mitochondria, i.e. to accomplish transport steps essential for mitochondrial DNA and RNA synthesis and breakdown.

Many faces of monogenic diabetes

Monogenic diabetes represents a heterogeneous group of disorders resulting from defects in single genes. Defects are categorized primarily into two groups: disruption of β-cell function or a reduction in the number of β-cells. A complex network of transcription factors control pancreas formation, and a dysfunction of regulators high in the hierarchy leads to pancreatic agenesis. Dysfunction among factors further downstream might cause organ hypoplasia, absence of islets of Langerhans or a reduction in the number of β-cells. Many transcription factors have pleiotropic effects, explaining the association of diabetes with other congenital malformations, including cerebellar agenesis and pituitary agenesis. Monogenic diabetes variants are classified conventionally according to age of onset, with neonatal diabetes occurring before the age of 6 months and maturity onset diabetes of the young (MODY) manifesting before the age of 25 years. Recently, certain familial genetic defects were shown to manifest as neonatal diabetes, MODY or even adult onset diabetes. Patients with neonatal diabetes require a thorough genetic work-up in any case, and because extensive phenotypic overlap exists between monogenic, type 2, and type 1 diabetes, genetic analysis will also help improve diagnosis in these cases. Next generation sequencing will facilitate rapid screening, leading to the discovery of digenic and oligogenic diabetes variants, and helping to improve our understanding of the genetics underlying other types of diabetes. An accurate diagnosis remains important, because it might lead to a change in the treatment of affected subjects and influence long-term complications.

Genomic polymorphisms of SLC29A3 associated with overall survival in advanced non-small-cell lung cancer treated with gemcitabine

Several polymorphisms in the genes involved in drug metabolism or elimination are known to be associated with pharmacokinetic or pharmacodynamic variability, but their correlations with overall survival (OS) and response rate (RR) in lung cancer have been unknown. This prospective study was to investigate whether genetic polymorphisms could influence OS and RR in advanced or metastatic non-small-cell lung cancer (NSCLC) patients treated with third-generation cytotoxic chemotherapy. Three hundred and sixty-four patients with chemotherapy naïve stage IIIB or IV NSCLC, receiving standard first-line chemotherapy, were eligible for this study. The DNA samples were extracted from peripheral blood mononuclear cells before treatment and single nucleotide polymorphisms of solute carrier family 29 member 3 SLC29A3 (rs10999776), SULT1E1 (rs4149525), and TBXAS1 (rs2267703), a category of drug-metabolizing enzymes or transporters were analyzed. Statistical analyses were performed by the log-rank test and Cox proportional hazards model. Patients with SLC29A3 C/T+T/T genotype had longer overall survival (median OS 12.3 months, 95 % CI 11.0-13.6 months) than those with C/C genotype (median OS 11.0 months, 95 % CI 9.9-12.1 months, P = 0.030 for log-rank test). More evidently significant association was found between the SLC29A3 polymorphism and overall survival in patients treated with gemcitabine-based chemotherapy (C/T+T/T versus C/C: median OS 12.0 months, 95 % CI 10.5-13.5 months versus median OS 10.0 months, 95 % CI 9.0-11.0 months, P = 0.027 for log-rank test). No association between the other two genetic polymorphisms and OS was observed. Genes involved in the drug metabolism or elimination (SLC29A3) may be new prognostic biomarkers for patients with advanced NSCLC who receive gemcitabine as the first-line chemotherapy and may unveil an unexplored molecular pathway correlated with the drug response and further may be predictive biomarkers for these patients.

Short communication: expression of transporters and metabolizing enzymes in the female lower genital tract: implications for microbicide research

Topical vaginal microbicides have been considered a promising option for preventing the male-to-female sexual transmission of HIV; however, clinical trials to date have not clearly demonstrated robust and reproducible effectiveness results. While multiple approaches may help enhance product effectiveness observed in clinical trials, increasing the drug exposure in lower genital tract tissues is a compelling option, given the difficulty in achieving sufficient drug exposure and positive correlation between tissue exposure and microbicide efficacy. Since many microbicide drug candidates are substrates of transporters and/or metabolizing enzymes, there is emerging interest in improving microbicide exposure and efficacy through local modulation of transporters and enzymes in the female lower genital tract. However, no systematic information on transporter/enzyme expression is available for ectocervical and vaginal tissues of premenopausal women, the genital sites most relevant to microbicide drug delivery. The current study utilized reverse transcriptase polymerase chain reaction (RT-PCR) to examine the mRNA expression profile of 22 transporters and 19 metabolizing enzymes in premenopausal normal human ectocervix and vagina. Efflux and uptake transporters important for antiretroviral drugs, such as P-gp, BCRP, OCT2, and ENT1, were found to be moderately or highly expressed in the lower genital tract as compared to liver. Among the metabolizing enzymes examined, most CYP isoforms were not detected while a number of UGTs such as UGT1A1 were highly expressed. Moderate to high expression of select transporters and enzymes was also observed in mouse cervix and vagina. The implications of this information on microbicide research is also discussed, including microbicide pharmacokinetics, the utilization of the mouse model in microbicide screening, as well as the in vivo functional studies of cervicovaginal transporters and enzymes.

The human concentrative and equilibrative nucleoside transporter families, SLC28 and SLC29

Nucleoside transport in humans is mediated by members of two unrelated protein families, the SLC28 family of cation-linked concentrative nucleoside transporters (CNTs) and the SLC29 family of energy-independent, equilibrative nucleoside transporters (ENTs). These families contain three and four members, respectively, which differ both in the stoichiometry of cation coupling and in permeant selectivity. Together, they play key roles in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis. Moreover, they facilitate cellular uptake of several nucleoside and nucleobase drugs used in cancer chemotherapy and treatment of viral infections. Thus, the transporter content of target cells can represent a key determinant of the response to treatment. In addition, by regulating the concentration of adenosine available to cell surface receptors, nucleoside transporters modulate many physiological processes ranging from neurotransmission to cardiovascular activity. This review describes the molecular and functional properties of the two transporter families, with a particular focus on their physiological roles in humans and relevance to disease treatment.

An Egyptian family with H syndrome due to a novel mutation in SLC29A3 illustrating overlapping features with pigmented hypertrichotic dermatosis with insulin-dependent diabetes and Faisalabad histiocytosis

The SLC29A3 gene, encoding hENT3, a member of the equilibrative nucleoside transporter family, has recently been found mutated in Faisalabad histiocytosis, pigmented hypertrichotic dermatosis with insulin-dependent diabetes, familial sinus histiocytosis with massive lymphadenopathy (SHML), and H syndromes. We here report clinical and genetic findings of an Egyptian family with H syndrome. We describe two siblings, a 19-yr old girl and a 15-yr old boy, of consanguineous parents. From 5 yr of age, the girl developed bilateral flexion deformity of interphalengeal joints and insulin-dependent diabetes mellitus. At age 7 yr, prominent hyperpigmented patches appeared on the skin at lower limbs, genitalia, and trunk. On clinical examination, she had hepatosplenomegaly, generalized lymphadenopathy, left ventricular hypertrophy, sensorineural hearing loss, hypogonadism, short stature, and characteristic dysmorphic features. Her brother had fixed flexion contractures of the feet, profound sensorineural hearing loss, characteristic dysmorphic features, but no diabetes. DNA sequence analysis revealed a homozygous mutation (c.300+1G>C) in SLC29A3 in both siblings. The phenotype and genotype of the siblings were compatible with that of the H syndrome, although the features were overlapping with those found in pigmented hypertrichotic dermatosis with insulin-dependent diabetes, familial SHML, and Faisalabad histiocytosis, indicating that these four syndromes may be regarded as one disease with varying phenotypic features. A new, common name for these conditions is warranted.

The association between the expression of solute carrier transporters and the prognosis of pancreatic cancer

OBJECTIVES

The aim of this study was to investigate the prognostic significance of fourteen anticancer drug-relevant solute carrier transporters (SLCs) in pancreatic cancer in the context of clinical-pathological characteristics and the KRAS mutation status of tumors.

METHODS

Tumors and non-neoplastic pancreatic tissues were obtained from 32 histologically verified patients with pancreatic ductal adenocarcinoma. The transcript profile of SLCs was assessed using quantitative real-time PCR. KRAS mutations in exon 2 were assessed by high-resolution melting analysis and confirmed by sequencing.

RESULTS

SLC22A3 and SLC22A18 were upregulated and SLC22A1, SLC22A2, SLC22A11, SLC28A1, SLC28A3 and SLC29A1 were downregulated when compared with non-neoplastic pancreatic tissues. Moreover, significantly lower levels of SLC22A1, SLC22A11 and SLC29A1 were found in tumors with angioinvasion. There was also a significantly higher transcript level of SLC28A1 in tumors with regional lymph nodes affected by metastasis. The study found that a high expression of SLC28A1 was significantly associated with poor overall survival in unselected patients. In contrast, a high expression of SLC22A3 or SLC29A3 was significantly associated with longer overall survival in patients treated with nucleoside analogs. Protein expression of SLC22A1, SLC22A3 and SLC29A3 in tumor tissues of patients with pancreatic carcinoma was observed by immunoblotting for the first time. Finally, SLC levels were not found to be associated with KRAS mutation status in exon 2.

CONCLUSIONS

This study identified a number of associations of transcript levels of SLCs with prognosis of pancreatic cancer patients.

SLC29A3 mutation in a patient with syndromic diabetes with features of pigmented hypertrichotic dermatosis with insulin-dependent diabetes, H syndrome and Faisalabad histiocytosis

AIMS

Atypical forms of diabetes may be caused by monogenic mutations in key genes controlling beta-cell development, survival and function. This report describes an insulin-dependent diabetes patient with a syndromic presentation in whom a homozygous SLC29A3 mutation was identified.

METHODS

SLC29A3 was selected as the candidate gene based on the patient's clinical manifestations, and all exons and flanking regions in the patient's genomic DNA were sequenced.

RESULTS

A homozygous splice mutation (c.300+1G>C) resulting in a frameshift and truncated protein (p.N101LfsX34) was identified. The patient had insulin-dependent diabetes, congenital deafness, short stature, hyperpigmented patches on the skin, dysmorphic features, cardiomegaly, arthrogryposis, hepatosplenomegaly, anaemia with erythroblastopenia, and an inflammatory syndrome with fever and arthritis; she also presented with a fibrotic mediastinal mass. These clinical features overlapped with pigmented hypertrichosis with insulin-dependent diabetes (PHID), H syndrome, Faisalabad histiocytosis and sinus histiocytosis with massive lymphadenopathy (SHML), all of which are also caused by SLC29A3 mutations.

CONCLUSION

This is the most severe case reported of SLC29A3 mutations with cumulative features of all these syndromes. This extreme severity coincides with the most N-terminal location of the truncation mutation, thereby affecting all alternative transcripts of the gene. This case report extends the clinical variability of homozygous SLC29A3 mutations that result in a spectrum of multisystemic manifestations.

Intracellular drug concentrations and transporters: measurement, modeling, and implications for the liver

Intracellular concentrations of drugs and metabolites are often important determinants of efficacy, toxicity, and drug interactions. Hepatic drug distribution can be affected by many factors, including physicochemical properties, uptake/efflux transporters, protein binding, organelle sequestration, and metabolism. This white paper highlights determinants of hepatocyte drug/metabolite concentrations and provides an update on model systems, methods, and modeling/simulation approaches used to quantitatively assess hepatocellular concentrations of molecules. The critical scientific gaps and future research directions in this field are discussed.

Mutation in the SLC29A3 gene: a new cause of a monogenic, autoinflammatory condition

Germline mutations in the SLC29A3 gene result in a range of recessive, clinically related syndromes: H syndrome, pigmented hypertrichosis with insulin-dependent diabetes mellitus syndrome, Faisalabad histiocytosis, and sinus histiocytosis with massive lymphadenopathy. The main symptoms of these diseases are hyperpigmentation with hypertrichosis, sensorineural deafness, diabetes, short stature, uveitis, and Rosai-Dorfman like histiocytosis. Here, we report the case of an 11-month-old boy with early-onset, recurrent episodes of unprovoked fever lasting 7 to 10 days and associated with pericardial effusion, abdominal pain, diarrhea, and inflammation. Physical examination revealed hyperpigmentation with hypertrichosis, dysmorphic features, and spleen and liver enlargement. Failure to thrive, sensorineural deafness, retarded psychomotor development, and a Rosai-Dorfman like cheek lesion developed subsequently. The febrile episodes did not respond to tumor necrosis factor α antagonists and interleukin-1. Sequencing of the SLC29A3 gene revealed a homozygous missense mutation c.1088G>A (p.Arg363Gln). These observations suggest that a newly identified mutation in the SLC29A3 gene may be associated with an autoinflammatory disorder. Genetic defects in SLC29A3 should be considered in patients with autoinflammatory manifestations, recurrent febrile attacks, and 1 or more of the symptoms found in the broad spectrum of SLC29A3-related disorders (especially hyperpigmentation with hypertrichosis).

Interindividual variability in the hepatic expression of the human breast cancer resistance protein (BCRP/ABCG2): effect of age, sex, and genotype

Breast cancer resistance protein (BCRP), an efflux transporter expressed at the bile canalicular membrane, is responsible for the biliary clearance of many drugs. Data on the interindividual variability of hepatic BCRP expression are needed for in vitro to in vivo extrapolation of the biliary clearance of a BCRP substrate drug. Therefore, we measured the expression of BCRP in human livers (n = 65) by liquid chromatography coupled with tandem mass spectrometry. A calibration curve was generated using a synthetic signature peptide (SSLLDVLAAR) as the calibrator and the corresponding synthetic stable isotope-labeled peptide as the internal standard. The analytical method was accurate and precise. BCRP expression in 50 livers, where it was measurable, was 137.9 ± 42.1 atmol/µg of membrane protein (range 69.7-246.4 atmol/µg of membrane protein). BCRP expression was not associated with age (7-70 years), sex, or mRNA expression. BCRP expression in livers with the variant C421A (rs2231142) allele (14 heterozygotes, two homozygotes; among these, eight livers were below lower limit of quantification) was significantly lower than that in the wild-type livers (p < 0.002). Integration of these data with data on the hepatic expression of other transporters will allow refinement of physiologically based pharmacokinetic models to predict the pharmacokinetics, hepatic exposure, and drug-drug interactions of drugs (and/or their metabolites).

Pharmacotherapy in pregnancy; effect of ABC and SLC transporters on drug transport across the placenta and fetal drug exposure

Pharmacotherapy during pregnancy is often inevitable for medical treatment of the mother, the fetus or both. The knowledge of drug transport across placenta is, therefore, an important topic to bear in mind when deciding treatment in pregnant women. Several drug transporters of the ABC and SLC families have been discovered in the placenta, such as P-glycoprotein, breast cancer resistance protein, or organic anion/cation transporters. It is thus evident that the passage of drugs across the placenta can no longer be predicted simply on the basis of their physical-chemical properties. Functional expression of placental drug transporters in the trophoblast and the possibility of drug-drug interactions must be considered to optimize pharmacotherapy during pregnancy. In this review we summarize current knowledge on the expression and function of ABC and SLC transporters in the trophoblast. Furthermore, we put this data into context with medical conditions that require maternal and/or fetal treatment during pregnancy, such as gestational diabetes, HIV infection, fetal arrhythmias and epilepsy. Proper understanding of the role of placental transporters should be of great interest not only to clinicians but also to pharmaceutical industry for future drug design and development to control the degree of fetal exposure.

Organic cation transporters OCT1 and OCT2 determine the accumulation of lamivudine in CD4 cells of HIV-infected patients

PURPOSE

Identifying factors that determine concentrations of antiretroviral drugs in CD4 cells are important for improving therapeutic efficacy. Experimental models indicate that the nucleoside reverse transcriptase inhibitor lamivudine is transported by the organic cation transporters 1 and 2 (OCT1 and OCT2, respectively). Here, we tested whether OCT1 and OCT2 contribute to the uptake of lamivudine into native CD4 cells of human immunodeficiency virus (HIV)-infected individuals.

METHODS

CD4 cells obtained by non-activated cell sorting from 35 individuals with HIV-1 infection were incubated with lamivudine (10 μM, 30 min), and intracellular concentrations of lamivudine and its active metabolite lamivudine triphosphate were determined by liquid chromatography tandem mass spectrometry. The expression of OCT1 and OCT2 mRNA was measured by quantitative real-time polymerase chain reaction (PCR). A model of OCT2-transfected CD4 cells was established for mechanistic investigations.

RESULTS

Intracellular concentrations of lamivudine and its active metabolite lamivudine triphosphate showed strong linear correlations with each other and with the CD4 mRNA expression of OCT1 and OCT2 (r > 0.80). Coincubation with protease inhibitors (ritonavir, nelfinavir) that inhibit OCT1 and OCT2 yielded decreased intracellular concentrations of lamivudine and lamivudine triphosphate. Incubation of CD4 cells from healthy donors transfected with an OCT2 expression vector yielded increased concentrations of lamivudine and lamivudine triphosphate.

CONCLUSION

Our studies indicate a role of OCT1 and OCT2 for the cellular accumulation of lamivudine in HIV-infected individuals.

Role of human nucleoside transporters in the uptake and cytotoxicity of azacitidine and decitabine

The nucleoside analogs 5-azacytidine (azacitidine) and 5-aza-2'-deoxycytidine (decitabine) are active against acute myeloid leukemia and myelodysplastic syndromes. Cellular transport across membranes is crucial for uptake of these highly polar hydrophilic molecules. We assessed the ability of azacitidine, decitabine, and, for comparison, gemcitabine, to interact with human nucleoside transporters (hNTs) in Saccharomyces cerevisiae cells (hENT1/2, hCNT1/2/3) or Xenopus laevis oocytes (hENT3/4). All three drugs inhibited hCNT1/3 potently (K (i) values, 3-26 μM), hENT1/2 and hCNT2 weakly (K (i) values, 0.5-3.1 mM), and hENT3/4 poorly if at all. Rates of transport of [(3)H]gemcitabine, [(14)C]azacitidine, and [(3)H]decitabine observed in Xenopus oocytes expressing individual recombinant hNTs differed substantially. Cytotoxicity of azacitidine and decitabine was assessed in hNT-expressing or hNT-deficient cultured human cell lines in the absence or presence of transport inhibitors where available. The rank order of cytotoxic sensitivities (IC (50) values, μM) conferred by hNTs were hCNT1 (0.1) > hENT1 (0.3) ≫ hCNT2 (8.3), hENT2 (9.0) for azacitidine and hENT1 (0.3) > hCNT1 (0.8) ⋙ hENT2, hCNT2 (>100) for decitabine. Protection against cytotoxicity was observed for both drugs in the presence of inhibitors of nucleoside transport, thus suggesting the importance of hNTs in manifestation of toxicity. In summary, all seven hNTs transported azacitidine, with hCNT3 showing the highest rates, whereas hENT1 and hENT2 showed modest transport and hCNT1 and hCNT3 poor transport of decitabine. Our results show for the first time that azacitidine and decitabine exhibit different human nucleoside transportability profiles and their cytotoxicities are dependent on the presence of hNTs, which could serve as potential biomarkers of clinical response.

The pyrimidine nucleotide carrier PNC1 and mitochondrial trafficking of thymidine phosphates in cultured human cells

In cycling cells cytosolic de novo synthesis of deoxynucleotides is the main source of precursors for mitochondrial (mt) DNA synthesis. The transfer of deoxynucleotides across the inner mt membrane requires protein carriers. PNC1, a SLC25 family member, exchanges pyrimidine nucleoside triphosphates in liposomes and its downregulation decreases mtUTP concentration in cultured cells. By an isotope-flow protocol we confirmed transport of uridine nucleotides by PNC1 in intact cultured cells and investigated PNC1 involvement in the mt trafficking of thymidine phosphates. Key features of our approach were the manipulation of PNC1 expression by RNA interference or inducible overexpression, the employment of cells proficient or deficient for cytosolic thymidine kinase (TK1) to distinguish the direction of flow of thymidine nucleotides across the mt membrane during short pulses with [(3)H]-thymidine, the determination of mtdTTP specific radioactivity to quantitate the rate of mtdTTP export to the cytoplasm. Downregulation of PNC1 in TK1(-) cells increased labeled dTTP in mitochondria due to a reduced rate of export. Overexpression of PNC1 in TK1(+) cells increased mtdTTP pool size and radioactivity, suggesting an involvement in the import of thymidine phosphates. Thus PNC1 is a component of the network regulating the mtdTTP pool in human cells.

Residue Ile89 in human plasma membrane monoamine transporter influences its organic cation transport activity and sensitivity to inhibition by dilazep

Plasma membrane monoamine transporter (PMAT) is a polyspecific organic cation transporter belonging to the equilibrative nucleoside transporter (ENT) family. Despite its distinct substrate specificity from the classic nucleoside transporters ENT1 and 2, PMAT appears to share similar protein architecture with ENT1/2 and retains low affinity binding to classic ENT inhibitors such as nitrobenzylmercaptopurine riboside (NBMPR) and the coronary vasodilators dilazep and dipyridamole. Here we investigated the role of residue Ile89, a position known to be important for ENT interaction with dilazep, dipyridamole, and nucleoside substrates, in PMAT transport function and its interaction with classic ENT inhibitors using Madin-Darby canine kidney (MDCK) cells stably expressing human PMAT. Substitution of Ile89 in PMAT with Met, the counterpart residue in ENT1, resulted in normal plasma membrane localization and protein expression. Transport kinetic analysis revealed that I89M mutant had a 2.7-fold reduction in maximal transport velocity (V(max)) with no significant change in apparent binding affinity (K(m)) towards the prototype PMAT substrate 1-methyl-4-phenylpyridinium (MPP+), suggesting that I89 is an important determinant for the catalytic activity of PMAT. Dose-dependent inhibition studies further showed that the I89M mutation significantly increased PMAT's sensitivity to dilazep by 2.5-fold without affecting its sensitivity to dipyridamole and NBMPR. Located at the extracellular end of transmembrane domain 1 of PMAT, I89 may occupy an important position close to the substrate permeation pathway and may be involved in direct interaction with the vasodilator dilazep.

A mild form of SLC29A3 disorder: a frameshift deletion leads to the paradoxical translation of an otherwise noncoding mRNA splice variant

We investigated two siblings with granulomatous histiocytosis prominent in the nasal area, mimicking rhinoscleroma and Rosai-Dorfman syndrome. Genome-wide linkage analysis and whole-exome sequencing identified a homozygous frameshift deletion in SLC29A3, which encodes human equilibrative nucleoside transporter-3 (hENT3). Germline mutations in SLC29A3 have been reported in rare patients with a wide range of overlapping clinical features and inherited disorders including H syndrome, pigmented hypertrichosis with insulin-dependent diabetes, and Faisalabad histiocytosis. With the exception of insulin-dependent diabetes and mild finger and toe contractures in one sibling, the two patients with nasal granulomatous histiocytosis studied here displayed none of the many SLC29A3-associated phenotypes. This mild clinical phenotype probably results from a remarkable genetic mechanism. The SLC29A3 frameshift deletion prevents the expression of the normally coding transcripts. It instead leads to the translation, expression, and function of an otherwise noncoding, out-of-frame mRNA splice variant lacking exon 3 that is eliminated by nonsense-mediated mRNA decay (NMD) in healthy individuals. The mutated isoform differs from the wild-type hENT3 by the modification of 20 residues in exon 2 and the removal of another 28 amino acids in exon 3, which include the second transmembrane domain. As a result, this new isoform displays some functional activity. This mechanism probably accounts for the narrow and mild clinical phenotype of the patients. This study highlights the 'rescue' role played by a normally noncoding mRNA splice variant of SLC29A3, uncovering a new mechanism by which frameshift mutations can be hypomorphic.

Iatrogenic mitochondriopathies: a recent lesson from nucleoside/nucleotide reverse transcriptase inhibitors

The use of nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) has revolutionized the treatment of infection by human immunodeficiency virus (HIV) and hepatitis-B virus. NRTIs can suppress viral replication in the long-term, but possess significant toxicity that can seriously compromise treatment effectiveness. The major toxicity of NRTIs is mitochondrial toxicity. This manifests as serious side effects such as myopathy, peripheral neuropathy and lactic acidosis. In general, it is believed that the mitochondrial pathogenesis is closely related to the effect of NRTIs on mitochondrial DNA polymerase-γ. Depletion and mutation of mitochondrial DNA during chronic NRTI therapy may lead to cellular respiratory dysfunction and release of reactive oxidative species, resulting in cellular damage. It is now apparent that the etiology is far more complex than originally thought. It appears to involve multiple mechanisms as well as host factors such as HIV per se, inborn mitochondrial mutation, and sex. Management of mitochondrial toxicity during NRTI therapy remains a challenge. Interruption of NRTI therapy and substitution of the causative agents with alternative better-tolerated NRTIs represents the mainstay of management for mitochondrial toxicity and its clinical manifestations. A range of pharmacological approaches has been proposed as treatments and prophylaxes.

Mitochondrial interference by anti-HIV drugs: mechanisms beyond Pol-γ inhibition

The combined pharmacological approach to the treatment of HIV infection, known as highly active antiretroviral therapy (HAART), has dramatically reduced AIDS-related morbidity and mortality. However, its use has been associated with serious adverse reactions, of which those resulting from mitochondrial dysfunction are particularly widespread. Nucleos(t)ide-reverse transcriptase inhibitors (NRTIs) have long been considered the main source of HAART-related mitochondrial toxicity due to their ability to inhibit Pol-γ, the DNA polymerase responsible for the synthesis of mitochondrial DNA. Nevertheless, accumulating evidence points to a more complex relationship between these organelles and NRTIs. Also, alternative pathways by which other groups of anti-HIV drugs (non-nucleoside reverse transcriptase inhibitors and protease inhibitors) interfere with mitochondria have been suggested, although their implications, both pharmacological and clinical, are open to debate. This review aims to provide a comprehensive overview of the mechanisms and factors which influence the mitochondrial involvement in the toxicity of all three major classes of anti-HIV drugs.

Nucleobase transport by human equilibrative nucleoside transporter 1 (hENT1)

The human equilibrative nucleoside transporters hENT1 and hENT2 (each with 456 residues) are 40% identical in amino acid sequence and contain 11 putative transmembrane helices. Both transport purine and pyrimidine nucleosides and are distinguished functionally by a difference in sensitivity to inhibition by nanomolar concentrations of nitrobenzylmercaptopurine ribonucleoside (NBMPR), hENT1 being NBMPR-sensitive. Previously, we used heterologous expression in Xenopus oocytes to demonstrate that recombinant hENT2 and its rat ortholog rENT2 also transport purine and pyrimidine bases, h/rENT2 representing the first identified mammalian nucleobase transporter proteins (Yao, S. Y., Ng, A. M., Vickers, M. F., Sundaram, M., Cass, C. E., Baldwin, S. A., and Young, J. D. (2002) J. Biol. Chem. 277, 24938-24948). The same study also revealed lower, but significant, transport of hypoxanthine by h/rENT1. In the present investigation, we have used the enhanced Xenopus oocyte expression vector pGEMHE to demonstrate that hENT1 additionally transports thymine and adenine and, to a lesser extent, uracil and guanine. Fluxes of hypoxanthine, thymine, and adenine by hENT1 were saturable and inhibited by NBMPR. Ratios of V(max) (pmol/oocyte · min(-1)):K(m) (mm), a measure of transport efficiency, were 86, 177, and 120 for hypoxantine, thymine, and adenine, respectively, compared with 265 for uridine. Hypoxanthine influx was competitively inhibited by uridine, indicating common or overlapping nucleobase and nucleoside permeant binding pockets, and the anticancer nucleobase drugs 5-fluorouracil and 6-mercaptopurine were also transported. Nucleobase transport activity was absent from an engineered cysteine-less version hENT1 (hENT1C-) in which all 10 endogenous cysteine residues were mutated to serine. Site-directed mutagenesis identified Cys-414 in transmembrane helix 10 of hENT1 as the residue conferring nucleobase transport activity to the wild-type transporter.

Nucleoside/nucleobase transporters: drug targets of the future?

BACKGROUND

Nucleoside/nucleobase transporters have been investigated since the 1960s. In particular, equilibrative nucleoside transporters were thought to be valuable drug targets, since they are involved in various kinds of viral and parasitic diseases as well as cancers.

DISCUSSION

In the postgenomic era multiple transporters, including different subtypes, have been cloned and characterized on the molecular level. In this article we summarize recent advances regarding structure, function and localization of nucleoside/nucleobase transporters as well as the pharmacological profile of selected drugs.

CONCLUSION

Knowledge of the different kinetic properties and structural features of nucleoside transporters can either be used for the rational design of therapeutics directly targeting the transporter itself or for the delivery of drugs using the transporter as a port of entry into the target cell. Equilibrative nucleoside transporters are of considerable pharmacological interest as drug targets for the development of drugs tailored to each patient's need for the treatment of cardiac disease, cancer and viral infections.

Tyrosine 112 is essential for organic cation transport by the plasma membrane monoamine transporter

Plasma membrane monoamine transporter (PMAT) is a polyspecific organic cation transporter in the solute carrier 29 (SLC29) family. Previous studies suggested that the major substrate recognition sites are located within transmembrane domains (TM) 1-6, and interaction of PMAT with organic cations may involve aromatic residues. In this study, we analyzed the roles of tyrosine and tryptophan residues located within TM1-6 with a goal of identifying potential residues involved in substrate recognition and translocation. The six tyrosines and one tryptophan in this region were each mutated to alanine followed by analysis of the mutant's membrane localization and transport activity toward 1-methyl-4-phenylpyridinium (MPP(+)), serotonin (5-HT), and dopamine. Two mutants, Y85A and Y112A, exhibited normal cell surface expressions but lost their transport activities toward organic cations. At position Y85, aromatic substitution with phenylalanine or tryptophan fully restored organic cation transport activity. Interestingly, at position Y112, phenylalanine substitution was not allowed. Tryptophan substitution at Y112 partially restored transport activity toward 5-HT and dopamine but severely impaired MPP(+) transport. Detailed kinetic analyses revealed that tryptophan substitution at Y85 and Y112 affected the apparent binding affinity (K(m)) and maximal transport velocity (V(max)) in a substrate-dependent manner. Together, our data suggest that Y85 and Y112 are important molecular determinants for PMAT function, and Y112 is indispensable for optimal interaction with organic cation substrates. Our analyses also suggest the involvement of transmembrane domains 1 and 2 in forming the substrate permeation pathway of PMAT.

Deoxynucleoside salvage enzymes and tissue specific mitochondrial DNA depletion

Adequate mitochondrial DNA (mtDNA) copies are required for normal mitochondria function and reductions in mtDNA copy number due to genetic alterations cause tissue-specific mtDNA depletion syndrome (MDS). There are eight nuclear genes, directly or indirectly involved in mtDNA replication and mtDNA precursor synthesis, which have been identified as the cause of MDS. However, the tissue specific pathology of these nuclear gene mutations is not well understood. Here, mtDNA synthesis, mtDNA copy number control, and mtDNA turnover, as well as the synthesis of mtDNA precursors in relation to the levels of salvage enzymes are discussed. The question why MDS caused by TK2 and p53R2 mutations are predominantly muscle specific while dGK deficiency affected mainly liver will be addressed.

Testicular expression of Adora3i2 in Adora3 knockout mice reveals a role of mouse A3Ri2 and human A3Ri3 adenosine receptors in sperm

Adenosine is a candidate modulator of sperm motility in the female reproductive tract that increases sperm flagellar beat frequency in vitro. Past work suggested that this acceleration may involve equilibrative (ENT) and concentrative (CNT) nucleoside transporters. Here we show that Slc29a1 (ENT-1) is the predominant nucleoside transporter expressed in the mouse testis. Unexpectedly, the beat of Slc29a1-null sperm still accelerates in response to 2-chloro-2'-deoxyadenosine (Cl-dAdo). Moreover, in wild-type sperm neither blockade of CNTs by removal of external Na(+), nor inhibition of ENTs with nitrobenzylthioionosine, prevents acceleration of the sperm beat by Cl-dAdo. In contrast, pertussis toxin produces strong blockade, indicating involvement of a Gα(i/o)-coupled adenosine receptor. Although agonists selective for adenosine receptors A1R, A2aR, and A2bR are ineffective, A3R-selective agonists Cl-IB-MECA and IB-MECA do accelerate the beat. Consistent with this pharmacological profile, the predominant Adora transcripts in the testis are products of the nested Adora3i1 and Adora3i2 genes. Surprisingly, Cl-IB-MECA and Cl-dAdo still accelerate the beat of Adora3i1-null sperm indicating that the remaining Adora3i2 transcript produces an A3R that functions in sperm. When cloned Adora3i2 is heterologously expressed in tsA-201 cells, Cl-dAdo decreases forskolin-evoked accumulation of cAMP, indicating that Adora3i2 specifies a functional A3Ri2 adenosine receptor that couples through Gα(i). Database mining reveals that mouse Adora3i2 is expressed primarily in testis, almost exclusively in spermatids. Expression of the orthologous ADORA3i3 transcript also is most prominent in human testis; presumably producing an A3Ri3 receptor that is functional in sperm and that may be a target for development of male-directed contraceptives.

Multivesicular body formation requires OSBP-related proteins and cholesterol

In eukaryotes, different subcellular organelles have distinct cholesterol concentrations, which is thought to be critical for biological functions. Oxysterol-binding protein-related proteins (ORPs) have been assumed to mediate nonvesicular cholesterol trafficking in cells; however, their in vivo functions and therefore the biological significance of cholesterol in each organelle are not fully understood. Here, by generating deletion mutants of ORPs in Caenorhabditis elegans, we show that ORPs are required for the formation and function of multivesicular bodies (MVBs). In an RNAi enhancer screen using obr quadruple mutants (obr-1; -2; -3; -4), we found that MVB-related genes show strong genetic interactions with the obr genes. In obr quadruple mutants, late endosomes/lysosomes are enlarged and membrane protein degradation is retarded, although endocytosed soluble proteins are normally delivered to lysosomes and degraded. We also found that the cholesterol content of late endosomes/lysosomes is reduced in the mutants. In wild-type worms, cholesterol restriction induces the formation of enlarged late endosomes/lysosomes, as observed in obr quadruple mutants, and increases embryonic lethality upon knockdown of MVB-related genes. Finally, we show that knockdown of ORP1L, a mammalian ORP family member, induces the formation of enlarged MVBs in HeLa cells. Our in vivo findings suggest that the proper cholesterol level of late endosomes/lysosomes generated by ORPs is required for normal MVB formation and MVB-mediated membrane protein degradation.

Sandwich-cultured hepatocytes: an in vitro model to evaluate hepatobiliary transporter-based drug interactions and hepatotoxicity

Sandwich-cultured hepatocytes (SCH) are a powerful in vitro tool that can be utilized to study hepatobiliary drug transport, species differences in drug transport, transport protein regulation, drug-drug interactions, and hepatotoxicity. This review provides an up-to-date summary of the SCH model, including a brief history of, and introduction to, the use of SCH, as well as methodology to evaluate hepatobiliary drug disposition. A summary of the literature that has utilized this model to examine the interplay between drug-metabolizing enzymes and transport proteins, drug-drug interactions at the transport level, and hepatotoxicity as a result of altered hepatic transport also is provided.

Drug transporter expression and localization in rat nasal respiratory and olfactory mucosa and olfactory bulb

Uptake of drugs and other xenobiotics from the nasal cavity and into either the brain or systemic circulation can occur through several different mechanisms, including paracellular transport and movement along primary olfactory nerve axons, which extend from the nasal cavity to the olfactory bulb of the brain. The present study was conducted to expand knowledge on a third means of uptake, namely the expression of drug transporters in the rat nasal epithelium. We used branched DNA technology to compare the level of expression of nine transporters [(equilibrative nucleoside transporters (ENT)1 and ENT2; organic cation transporter (OCT)1, 2, and 3; OCTN1; organic anion-transporting polypeptide (OATP)3; and multidrug resistance (MRP)1 and MRP4] in nasal respiratory mucosa, olfactory mucosa, and olfactory bulb to the level of expression of these transporters in the liver and kidney. Transporters with high expression in the nasal respiratory mucosa or olfactory tissues were immunolocalized by immunohistochemistry. ENT1 and ENT2 expression was relatively high in nasal epithelia and olfactory bulb, which may explain the uptake of intranasally administered nucleoside derivatives observed by other investigators. OATP3 immunoreactivity was high in olfactory epithelium and olfactory nerve bundles, which suggests that substrates transported by OATP3 may be candidates for intranasal administration.

Early-onset sensorineural hearing loss is a prominent feature of H syndrome

This case report describes two patients with H syndrome, a multisystemic autosomal recessive disorder, caused by mutations in the SLC29A3 gene. It is characterized by cutaneous hyperpigmentation, camptodactyly or flexion contractures and other features, among them hearing loss. The two patients had hearing loss as their presenting symptom, and had mutations in SLC29A3, one of them a novel mutation. The aim of this paper is to increase awareness to this recently described disorder, and to emphasize that H syndrome should be included in the differential diagnosis of congenital or acquired syndromic hearing loss in children.

Human equilibrative nucleoside transporter-3 (hENT3) spectrum disorder mutations impair nucleoside transport, protein localization, and stability

Accumulating evidence reveals that sole mutations in hENT3 cause a spectrum of human genetic disorders. Among these include H syndrome, characterized by scleroderma, hyperpigmentation, hypertrichosis, hepatomegaly, cardiac abnormalities and musculoskeletal deformities, pigmented hypertrichotic dermatosis with insulin-dependent diabetes syndrome, characterized by autoantibody-negative diabetes mellitus and skin deformities, familial Rosai-Dorfman disease, characterized by short stature, familial histiocytosis and sinus histiocytosis with massive lymphadenopathy (SHML), characterized by severe tissue infiltration of immune cells and swollen lymph nodes. hENT3 spectrum disorders share a common mutation and share overlapping clinical manifestations that display many intriguing resemblances to mitochondrial and lysosomal disorders. Although earlier studies identify hENT3 as a mitochondrial and a lysosomal nucleoside transporter, the precise connections between hENT3 and the pathophysiology of these disorders remain unresolved. In this study, we performed functional and biochemical characterization of these mutations in hENT3. We report severe reductions/losses of hENT3 nucleoside transport functions of hENT3 syndrome mutants. In addition to transport alterations, we provide evidence for possible loss of hENT3 functions in all H and pigmented hypertrichotic dermatosis with insulin-dependent diabetes syndromes due to either mistrafficking or altered stability of mutant hENT3 proteins.

Adenosine transport by plasma membrane monoamine transporter: reinvestigation and comparison with organic cations

The plasma membrane monoamine transporter (PMAT) belongs to the equilibrative nucleoside transporter family (solute carrier 29) and was alternatively named equilibrative nucleoside transporter 4. Previous studies from our laboratory characterized PMAT as a polyspecific organic cation transporter that minimally interacts with nucleosides. Recently, PMAT-mediated uptake of adenosine (a purine nucleoside) was reported, and the transporter was proposed to function as a dual nucleoside/organic cation transporter. To clarify the substrate specificity of PMAT, we comprehensively analyzed the transport activity of human PMAT toward nucleosides, nucleobases, and organic cations in heterologous expression systems under well controlled conditions. Among 12 naturally occurring nucleosides and nucleobases, only adenosine was significantly transported by PMAT. PMAT-mediated adenosine transport is saturable, pH-dependent, and membrane-potential sensitive. Under both neutral (pH 7.4) and acidic (pH 6.6) conditions, adenosine is transported by PMAT at an efficiency (V(max)/K(m)) at least 10-fold lower than that of the organic cation substrates 1-methyl-4-phenylpyridinium and serotonin. PMAT-mediated adenosine uptake rate was significantly enhanced by an acidic extracellular pH. However, the effect of acidic pH was not adenosine-specific but was common to organic cation substrates as well. Our results demonstrated that although PMAT transports adenosine, the transporter kinetically prefers organic cation substrates. Functionally, PMAT should be viewed as a polyspecific organic cation transporter rather than an archetypical nucleoside transporter.

Why we should be vigilant: drug cytotoxicity observed with in vitro transporter inhibition studies

From routine in vitro drug-transporter inhibition assays, observed inhibition is typically assumed from direct interaction with the transporter. Other mechanisms that possibly reduce substrate uptake are not frequently fully examined. The objective of this study was to investigate the association of transporter inhibition with drug cytotoxicity. From a pool of drugs that were identified as known ASBT or OCTN2 inhibitors, 21 drugs were selected to screen inhibitory potency of their prototypical substrate and cytotoxicity against three human sodium-dependent solute carrier (SLC) transporters: apical sodium-dependent bile acid transporter (ASBT), organic cation/carnitine transporter (OCTN2), and the excitatory amino acid transporter 4 (EAAT4) in stable cell lines. Twenty drugs showed apparent inhibition in OCTN2-MDCK and ASBT-MDCK. Four dihydropyridine calcium channel blockers were cytotoxic to MDCK cells, and the observed cytotoxicity of three of them accounted for their apparent OCTN2 inhibition, and consequently were classified as non-OCTN2 inhibitors. Meanwhile, since their cytotoxicity only moderately contributed to ASBT inhibition, these three were still considered ASBT inhibitors. Four other drugs showed apparent inhibition in EAAT4-HEK cells, and cytotoxicity of three drugs corresponded with their inhibition of this transporter. Therefore, cytotoxicity significantly affected EAAT4 observations. Results showed the potential of cytotoxicity as a mechanism that can account for apparent in vitro transporter inhibition. Drug cytotoxicity varied in different cell lines, which could increase false positives for pharmacophore development.

Drug transporter pharmacogenetics in nucleoside-based therapies

This article focuses on the different types of transporter proteins that have been implicated in the influx and efflux of nucleoside-derived drugs currently used in the treatment of cancer, viral infections (i.e., AIDS) and other conditions, including autoimmune and inflammatory diseases. Genetic variations in nucleoside-derived drug transporter proteins encoded by the gene families SLC15, SLC22, SLC28, SLC29, ABCB, ABCC and ABCG will be specifically considered. Variants known to affect biological function are summarized, with a particular emphasis on those for which clinical correlations have already been established. Given that relatively little is known regarding the genetic variability of the players involved in determining nucleoside-derived drug bioavailability, it is anticipated that major challenges will be faced in this area of research.