Organic anion transporter 1B1: an important factor in hepatic thyroid hormone and estrogen transport and metabolism

Metabolism of endogenous and exogenous estrogens in women

Estrogens regulate important processes in reproductive, skeletal, cardiovascular, and central nervous systems that impact women's overall health. Understanding endogenous and exogenously administered estrogen metabolism is vital to determining therapeutic estrogen levels. The present review provides an overview of estrogen metabolites formed in non-pregnant and pregnant women, and those resulting from exogenous estrogen administration. There are four principal endogenous estrogens: estrone (E1), estradiol (E2), estriol (E3), and estetrol (E4). E4, which is produced only in pregnancy, has emerged recently as an estrogen with significant therapeutic potential. E1, E2, and E3 undergo extensive metabolism primarily through phase I (hydroxylation, oxidation, reduction) and phase II (primarily conjugation) reactions, whereas E4 undergoes only phase II reactions. Exogenous estrogens commonly used for menopausal treatment and/or contraception, including micronized E2, conjugated equine estrogens, and ethinyl estradiol, also undergo phase I and phase II reactions, but differ widely in the types of metabolites formed. The mechanisms by which estrogen metabolites are formed and their excretion in urine, bile, and feces, are still poorly understood. We highlight areas that require further research to foster a better understanding of how estrogen metabolism impacts dosing of oral estrogens for therapeutic use, as well as the physiological regulation of endogenous estrogens.

3,3',5-Triiodothyroacetic Acid Transporters

Introduction: Thyroid hormone transporters are essential for thyroid hormones to enter target cells. Monocarboxylate transporter (MCT) 8 is a key transporter and is expressed at the blood-brain barrier (BBB), in neural cells and many other tissues. Patients with MCT8 deficiency have severe neurodevelopmental delays because of cerebral hypothyroidism and chronic sequelae of peripheral thyrotoxicosis. The T3 analog 3,3',5-triiodothyroacetic acid (TRIAC) rescued neurodevelopmental features in animal models mimicking MCT8 deficiency and improved key metabolic features in patients with MCT8 deficiency. However, the identity of the transporter(s) that facilitate TRIAC transport are unknown. Here, we screened candidate transporters that are expressed at the human BBB and/or brain-cerebrospinal fluid barrier and known thyroid hormone transporters for TRIAC transport. Materials and Methods: Plasma membrane expression was determined by cell surface biotinylation assays. Intracellular accumulation of 1 nM TRIAC was assessed in COS-1 cells expressing candidate transporters in Dulbecco's phosphate-buffered saline (DPBS)/0.1% glucose or Dulbecco's modified Eagle's medium (DMEM) with or without 0.1% bovine serum albumin (BSA). Expression of Slc22a8 was determined by fluorescent in situ hybridization in brain sections from wild-type and Mct8/Oatp1c1 knockout mice at postnatal days 12, 21, and 120. Results: In total, 59 plasma membrane transporters were selected for screening of TRIAC accumulation (n = 40 based on expression at the human BBB and/or brain-cerebrospinal fluid barrier and having small organic molecules as substrates; n = 19 known thyroid hormone transporters). Screening of the selected transporter panel showed that 18 transporters facilitated significant intracellular accumulation of TRIAC in DPBS/0.1% glucose or DMEM in the absence of BSA. In the presence of BSA, substantial transport was noted for SLCO1B1 and SLC22A8 (in DPBS/0.1% glucose and DMEM) and SLC10A1, SLC22A6, and SLC22A24 (in DMEM). The zebrafish and mouse orthologs of these transporters similarly facilitated intracellular accumulation of TRIAC. Highest Slc22a8 mRNA expression was detected in mouse brain capillary endothelial cells and choroid plexus epithelial cells at early postnatal time points, but was reduced at P120. Conclusions: Human SLC10A1, SLCO1B1, SLC22A6, SLC22A8, and SLC22A24 as well as their mouse and zebrafish orthologs are efficient TRIAC transporters. These findings contribute to the understanding of TRIAC treatment in patients with MCT8 deficiency and animal models thereof.

Gene polymorphisms and thyroid hormone signaling: implication for the treatment of hypothyroidism

INTRODUCTION

Mutations and single nucleotide polymorphisms (SNPs) in the genes encoding the network of proteins involved in thyroid hormone signaling (TH) may have implications for the effectiveness of the treatment of hypothyroidism with LT4. It is conceivable that loss-of-function mutations or SNPs impair the ability of LT4 to be activated to T3, reach its targets, and ultimately resolve symptoms of hypothyroidism. Some of these patients do benefit from therapy containing LT4 and LT3.

METHODS

Here, we reviewed the PubMed and examined gene mutations and SNPs in the TH cellular transporters, deiodinases, and TH receptors, along with their impact on TH signaling, and potential clinical implications.

RESULTS

In some mechanisms, such as the Thr92Ala-DIO2 SNP, there is a compelling rationale for reduced T4 to T3 activation that limits the effectiveness of LT4 to restore euthyroidism. In other mechanisms, a potential case can be made but more studies with a larger number of individuals are needed.

DISCUSSION/CONCLUSION

Understanding the clinical impact of the genetic makeup of LT4-treated patients may help in the preemptive identification of those individuals that would benefit from therapy containing LT3.

Developing and evaluating a predictive model for neonatal hyperbilirubinemia based on UGT1A1 gene polymorphism and clinical risk factors

Background

Neonatal hyperbilirubinemia (NHB) is one of the most common diseases in the neonatal period. Without timely diagnosis and treatment, it can lead to long-term complications. In severe cases, it may even result in fatality. The UGT1A1 gene and clinical risk factors play important roles in the development and progression of NHB.

Methods

In this study, we conducted a cohort study and analyzed 3258 newborns from the Jilin Women And Children Health Hospital in northern China, including 372 children with hyperbilirubinemia. We established a predictive model using a logistic regression model based on clinical risk factors and the polymorphism of the G211A locus in the UGT1A1 gene of newborns. Furthermore, the performance of the prediction model was evaluated using the ROC curve.

Results

The logistic regression model indicates that the following factors are associated with an increased risk of NHB: the time when stool turns yellow [P ≤ 0.001, OR 1.266 (95% CI: 1.125-1.425)]; neonatal cephalohematoma [P ≤ 0.001, OR 33.642 (95% CI: 21.823-51.861)]; hemolytic disease of newborn [P ≤ 0.001, OR 33.849 (95% CI: 18.589-61.636)]; neonatal weight loss [P ≤ 0.001, OR 11.275 (95% CI: 7.842-16.209)]; neonatal premature rupture of membranes (PROM) history [P = 0.021, OR 1.422 (95% CI: 1.056-1.917)]; genetic polymorphism at the UGT1A1 gene G211A locus. Gestational age is a protective factor [P ≤ 0.001, OR 0.766 (95% CI: 0.686-0.855)]. Compared to natural labor, cesarean section is a protective factor [P = 0.011, OR 0.711 (95% CI: 0.546-0.926)], while assisted delivery is a risk factor [P = 0.022, OR 2.207 (95% CI: 1.121-4.346)]. The area under the curve (AUC) of this prediction model is 0.804 (95% CI: 0.777-0.831), indicating good discrimination ability and value for predicting the risk of NHB after birth.

Conclusion

We have developed and evaluated a predictive model that combines UGT1A1 gene polymorphism and clinical risk factors for the first time. By using this nomogram and taking into account the results of serum total bilirubin measurement or transcutaneous bilirubin measurement, early prediction of the risk of neonatal hyperbilirubinemia can be achieved.

Thyroid Hormone Transporters in Pregnancy and Fetal Development

Thyroid hormone is essential for fetal (brain) development. Plasma membrane transporters control the intracellular bioavailability of thyroid hormone. In the past few decades, 15 human thyroid hormone transporters have been identified, and among them, mutations in monocarboxylate transporter (MCT)8 and organic anion transporting peptide (OATP)1C1 are associated with clinical phenotypes. Different animal and human models have been employed to unravel the (patho)-physiological role of thyroid hormone transporters. However, most studies on thyroid hormone transporters focus on postnatal development. This review summarizes the research on the thyroid hormone transporters in pregnancy and fetal development, including their substrate preference, expression and tissue distribution, and physiological and pathophysiological role in thyroid homeostasis and clinical disorders. As the fetus depends on the maternal thyroid hormone supply, especially during the first half of pregnancy, the review also elaborates on thyroid hormone transport across the human placental barrier. Future studies may reveal how the different transporters contribute to thyroid hormone homeostasis in fetal tissues to properly facilitate development. Employing state-of-the-art human models will enable a better understanding of their roles in thyroid hormone homeostasis.

Cell-Specific Transport and Thyroid Hormone Receptor Isoform Selectivity Account for Hepatocyte-Targeted Thyromimetic Action of MGL-3196

Thyroid hormones (THs) and TH receptor-beta (TRβ) reduce hepatic triglycerides, indicating a therapeutic potential for TH analogs in liver steatosis. To avoid adverse extrahepatic, especially TRα-mediated effects such as tachycardia and bone loss, TH analogs with combined TRβ and hepatocyte specificity are desired. MGL-3196 is a new TH analog that supposedly meets these criteria. Here, we characterize the thyromimetic potential of MGL-3196 in cell-based assays and address its cellular uptake requirements. We studied the contribution of liver-specific organic anion transporters (OATP)1B1 and 1B3 to MGL-3196 action. The TR isoform-specific efficacy of MGL-3196 compared with 3,5,3'-triiodothyronine (T₃) was determined with luciferase assays and gene expression analysis in OATP1B1 and OATP1B3 and TRα- or TRβ-expressing cells and in primary murine hepatocytes (PMHs) from wild-type and TRβ knockout mice. We measured the oxygen consumption rate to compare the effects of MGL-3196 and T₃ on mitochondrial respiration. We identified OATP1B1 as the primary transporter for MGL-3196. MGL-3196 had a high efficacy (90% that of T₃) in activating TRβ, while the activation of TRα was only 25%. The treatment of PMHs with T₃ and MGL-3196 at EC₅₀ resulted in a similar induction of Dio1 and repression of Serpina7. In HEK293 cells stably expressing OATP1B1, MGL-3196 had comparable effects on mitochondrial respiration as T₃. These data indicate that MGL-3196's hepatic thyromimetic action, the basis for its therapeutic use, results from a combination of hepatocyte-specific transport by OATP1B1 and the selective activation of TRβ over TRα.

The Effects of Common Genetic Variation in 96 Genes Involved in Thyroid Hormone Regulation on TSH and FT4 Concentrations

OBJECTIVE

While most of the variation in thyroid function is determined by genetic factors, single nucleotide polymorphisms (SNPs) identified via genome-wide association analyses have only explained ~5% to 9% of this variance so far. Most SNPs were in or nearby genes with no known role in thyroid hormone (TH) regulation. Therefore, we performed a large-scale candidate gene study investigating the effect of common genetic variation in established TH regulating genes on serum thyrotropin [thyroid-stimulating hormone (TSH)] and thyroxine (FT4) concentrations.

METHODS

SNPs in or within 10 kb of 96 TH regulating genes were included (30 031 TSH SNPs, and 29 962 FT4 SNPs). Associations were studied in 54 288 individuals from the ThyroidOmics Consortium. Linkage disequilibrium-based clumping was used to identify independently associated SNPs. SNP-based explained variances were calculated using SumHer software.

RESULTS

We identified 23 novel TSH-associated SNPs in predominantly hypothalamic-pituitary-thyroid axis genes and 25 novel FT4-associated SNPs in mainly peripheral metabolism and transport genes. Genome-wide SNP variation explained ~21% (SD 1.7) of the total variation in both TSH and FT4 concentrations, whereas SNPs in the 96 TH regulating genes explained 1.9% to 2.6% (SD 0.4).

CONCLUSION

Here we report the largest candidate gene analysis on thyroid function, resulting in a substantial increase in the number of genetic variants determining TSH and FT4 concentrations. Interestingly, these candidate gene SNPs explain only a minor part of the variation in TSH and FT4 concentrations, which substantiates the need for large genetic studies including common and rare variants to unravel novel, yet unknown, pathways in TH regulation.

Functional Characterization of the Novel and Specific Thyroid Hormone Transporter SLC17A4

Background: A recent genome-wide association study identified the SLC17A4 locus associated with circulating free thyroxine (T4) concentrations. Human SLC17A4, being widely expressed in the gastrointestinal tract, was characterized as a novel triiodothyronine (T3) and T4 transporter. However, apart from the cellular uptake of T3 and T4, transporter characteristics are currently unknown. In this study, we delineated basic transporter characteristics of this novel thyroid hormone (TH) transporter. Methods: We performed a broad range of well-established TH transport studies in COS-1 cells transiently overexpressing SLC17A4. We studied cellular TH uptake in various incubation buffers, TH efflux, and the inhibitory effects of different TH metabolites and known inhibitors of other TH transporters on SLC17A4-mediated TH transport. Finally, we determined the effect of tunicamycin, a pharmacological inhibitor of N-linked glycosylation, and targeted mutations in Asn residues on SLC17A4 function. Results: SLC17A4 induced the cellular uptake of T3 and T4 by ∼4 times, and of reverse (r)T3 by 1.5 times over control cells. The uptake of T4 by SLC17A4 was Na⁺ and Cl^- independent, stimulated by low extracellular pH, and reduced by various iodothyronines and metabolites thereof, particularly those that contain at least three iodine moieties irrespective of the presence of modification at the alanine side chain. None of the classical TH transporter inhibitors studied attenuated SLC17A4-mediated TH transport. SLC17A4 also facilitates the efflux of T3 and T4, and to a lesser extent of 3,3'-diiodothyronine (T2). Immunoblot studies on lysates of transfected cells cultured in absence or presence of tunicamycin indicated that SLC17A4 is subject to N-linked glycosylation. Complementary mutational studies identified Asn66, Asn75, and Asn90, which are located in extracellular loop 1, as primary targets. Conclusions: Our studies show that SLC17A4 facilitates the transport of T3 and T4, and less efficiently rT3 and 3,3'-T2. Further studies should reveal the physiological role of SLC17A4 in TH regulation.

The Role of Uptake and Efflux Transporters in the Disposition of Glucuronide and Sulfate Conjugates

Glucuronidation and sulfation are the most typical phase II metabolic reactions of drugs. The resulting glucuronide and sulfate conjugates are generally considered inactive and safe. They may, however, be the most prominent drug-related material in the circulation and excreta of humans. The glucuronide and sulfate metabolites of drugs typically have limited cell membrane permeability and subsequently, their distribution and excretion from the human body requires transport proteins. Uptake transporters, such as organic anion transporters (OATs and OATPs), mediate the uptake of conjugates into the liver and kidney, while efflux transporters, such as multidrug resistance proteins (MRPs) and breast cancer resistance protein (BCRP), mediate expulsion of conjugates into bile, urine and the intestinal lumen. Understanding the active transport of conjugated drug metabolites is important for predicting the fate of a drug in the body and its safety and efficacy. The aim of this review is to compile the understanding of transporter-mediated disposition of phase II conjugates. We review the literature on hepatic, intestinal and renal uptake transporters participating in the transport of glucuronide and sulfate metabolites of drugs, other xenobiotics and endobiotics. In addition, we provide an update on the involvement of efflux transporters in the disposition of glucuronide and sulfate metabolites. Finally, we discuss the interplay between uptake and efflux transport in the intestine, liver and kidneys as well as the role of transporters in glucuronide and sulfate conjugate toxicity, drug interactions, pharmacogenetics and species differences.

Organic Anion Transporting Polypeptide 2B1 (OATP2B1) Genetic Variants: In Vitro Functional Characterization and Association With Circulating Concentrations of Endogenous Substrates

Organic anion transporting polypeptide 2B1 (OATP2B1, gene SLCO2B1) is an uptake transporter that is thought to determine drug disposition and in particular, the oral absorption of medications. At present, the clinical relevance of SLCO2B1 genetic variation on pharmacokinetics is poorly understood. We sought to determine the functional activity of 5 of the most common missense OATP2B1 variants (c.76_84del, c.601G>A, c.917G>A, c.935G>A, and c.1457C>T) and a predicted dysfunctional variant (c.332G>A) in vitro. Furthermore, we measured the basal plasma concentrations of endogenous OATP2B1 substrates, namely estrone sulfate, dehydroepiandrosterone sulfate (DHEAS), pregnenolone sulfate, coproporphyrin I (CPI), and CPIII, and assessed their relationships with SLCO2B1 genotypes in 93 healthy participants. Compared to reference OATP2B1, the transport activities of the c.332G>A, c.601G>A and c.1457C>T variants were reduced among the substrates examined (estrone sulfate, DHEAS, CPI, CPIII and rosuvastatin), although there were substrate-dependent effects. Lower transport function of OATP2B1 variants could be explained by diminished cell surface expression. Other OATP2B1 variants (c.76-84del, c.917G>A and c.935G>A) had similar activity to the reference transporter. In the clinical cohort, the SLCO2B1 c.935G>A allele was associated with both higher plasma CPI (42%) and CPIII (31%) concentrations, while SLCO2B1 c.917G>A was linked to lower plasma CPIII by 28% after accounting for the effects of age, sex, and SLCO1B1 genotypes. No association was observed between SLCO2B1 variant alleles and estrone sulfate or DHEAS plasma concentrations, however 45% higher plasma pregnenolone sulfate level was associated with SLCO2B1 c.1457C>T. Taken together, we found that the impacts of OATP2B1 variants on transport activities in vitro were not fully aligned with their associations to plasma concentrations of endogenous substrates in vivo. Additional studies are required to determine whether circulating endogenous substrates reflect OATP2B1 activity.

The Genetic Basis of Thyroid Function: Novel Findings and New Approaches

CONTEXT

Genetic factors are major determinants of thyroid function. Over the last two decades, multiple genetic variants have been associated with variations in normal range thyroid function tests. Most recently, a large-scale genome-wide association study (GWAS) doubled the number of known variants associated with normal range thyrotropin (TSH) and free thyroxine (FT4) levels.

EVIDENCE ACQUISITION

This review summarizes the results of genetic association studies on normal range thyroid function and explores how these genetic variants can be used in future studies to improve our understanding of thyroid hormone regulation and disease.

EVIDENCE SYNTHESIS

Serum TSH and FT4 levels are determined by multiple genetic variants on virtually all levels of the hypothalamus-pituitary-thyroid (HPT) axis. Functional follow-up studies on top of GWAS hits has the potential to discover new key players in thyroid hormone regulation, as exemplified by the identification of the thyroid hormone transporter SLC17A4 and the metabolizing enzyme AADAT. Translational studies may use these genetic variants to investigate causal associations between thyroid function and various outcomes in Mendelian Randomization (MR) studies, to identify individuals with an increased risk of thyroid dysfunction, and to predict the individual HPT axis setpoint.

CONCLUSIONS

Recent genetic studies have greatly improved our understanding of the genetic basis of thyroid function, and have revealed novel pathways involved in its regulation. In addition, these findings have paved the way for various lines of research that can improve our understanding of thyroid hormone regulation and thyroid diseases, as well as the potential use of these markers in future clinical practice.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

SLCO1B1 genetic variation and hormone therapy in menopausal women

Objective:

Response to menopausal hormone therapy (MHT) shows individual variation. SLCO1B1 encodes the OATP1B1 transporter expressed in the liver that transports many endogenous substances, including estrone sulfate, from the blood into hepatocytes. This study evaluated the relationship between genetic variation in SLCO1B1 and response to MHT in women enrolled in the Kronos Early Estrogen Prevention Study (KEEPS) at Mayo Clinic, Rochester, MN.

Methods:

KEEPS participants were randomized to oral conjugated equine estrogen (n = 33, oCEE), transdermal 17β-estradiol (n = 33, tE₂), or placebo (n = 34) for 48 months. Menopausal symptoms (hot flashes, night sweats, insomnia, palpitations) were self-reported before treatment and at 48 months. Estrone (E₁), E₂, and sulfated conjugates (E₁S, E₂S) were measured using high-performance liquid chromatography-tandem mass spectrometry. SLCO1B1 rs4149056 (c.521T>C, p.Val174Ala) was genotyped using a TaqMan assay.

Results:

After adjusting for treatment, there was a significant association between the SLCO1B1 rs4149056 TT genotype (encoding normal function transporter) and lower E₁S, E₁S/E₁, and E₂S (P = 0.032, 0.010, and 0.008, respectively) compared with women who were heterozygous (TC) or homozygous (CC) for the reduced function allele. The interactions between genotype, treatment, and E₂S concentration were stronger in women assigned to tE₂ (P = 0.013) than the women taking oCEE (P = 0.056). Among women assigned to active treatment, women with the CT genotype showed a significantly greater decrease in night sweats (P = 0.041) than those with the TT genotype.

Conclusions:

Individual variation in sulfated estrogens is explained, in part, by genetic variation in SLCO1B1. Bioavailability of sulfated estrogens may contribute to relief of night sweats.

A New Genetic Risk Score to Predict the Outcome of Locally Advanced or Metastatic Breast Cancer Patients Treated With First-Line Exemestane: Results From a Prospective Study

INTRODUCTION

Approximately 50% of locally advanced or metastatic breast cancer (MBC) patients treated with first-line exemestane do not show objective response and currently there are no reliable biomarkers to predict the outcome of patients using this therapy. The constitutive genetic background might be responsible for differences in the outcome of exemestane-treated patients. We designed a prospective study to investigate the role of germ line polymorphisms as biomarkers of survival.

PATIENTS AND METHODS

Three hundred two locally advanced or MBC patients treated with first-line exemestane were genotyped for 74 germ line polymorphisms in 39 candidate genes involved in drug activity, hormone balance, DNA replication and repair, and cell signaling pathways. Associations with progression-free survival (PFS) and overall survival (OS) were tested with multivariate Cox regression. Bootstrap resampling was used as an internal assessment of results reproducibility.

RESULTS

Cytochrome P450 19A1-rs10046TC/CC, solute carrier organic anion transporter 1B1-rs4149056TT, adenosine triphosphate binding cassette subfamily G member 2-rs2046134GG, fibroblast growth factor receptor-4-rs351855TT, and X-ray repair cross complementing 3-rs861539TT were significantly associated with PFS and then combined into a risk score (0-1, 2, 3, or 4-6 risk points). Patients with the highest risk score (4-6 risk points) compared with ones with the lowest score (0-1 risk points) had a median PFS of 10 months versus 26.3 months (adjusted hazard ratio [AdjHR], 3.12 [95% confidence interval (CI), 2.18-4.48]; P < .001) and a median OS of 38.9 months versus 63.0 months (AdjHR, 2.41 [95% CI, 1.22-4.79], P = .012), respectively.

CONCLUSION

In this study we defined a score including 5 polymorphisms to stratify patients for PFS and OS. This score, if validated, might be translated to personalize locally advanced or MBC patient treatment and management.

Genome-wide association meta-analysis for total thyroid hormone levels in Croatian population

Thyroid hormones (THs) are key regulators of cellular growth, development, and metabolism. The thyroid gland secretes two THs, thyroxine (T4) and triiodothyronine (T3), into the plasma where they are almost all bound reversibly to plasma proteins. Free forms of THs are metabolically active, however, they represent a very small fraction of total TH levels. No genome-wide studies have been performed to date on total TH levels, comprising of protein-bound and free forms of THs. To detect genetic variants associated with total TH levels, we carried out the first GWAS meta-analysis of total T4 levels in 1121 individuals from two Croatian cohorts (Split and Korcula). We also performed GWAS analyses of total T3 levels in 577 individuals and T3/T4 ratio in 571 individuals from the Split cohort. The top association in GWAS meta-analysis of total T4 was detected for an intronic variant within SLC22A9 gene (rs12282281, P = 4.00 × 10^-7). Within the same region, a genome-wide significant variant (rs11822642, P = 2.50 × 10^-8) for the T3/T4 ratio was identified. SLC22A9 encodes for an organic anion transporter protein expressed predominantly in the liver and belongs to the superfamily of solute carriers (SLC), a large group of transport membrane proteins. The transport of THs across the plasma membrane in peripheral tissues is facilitated by the membrane proteins, and all TH transport proteins known to date belong to the same SLC superfamily as SLC22A9. These results suggest a potential role for SLC22A9 as a novel transporter protein of THs.

Associations of the SLCO1B1 Polymorphisms With Hepatic Function, Baseline Lipid Levels, and Lipid-lowering Response to Simvastatin in Patients With Hyperlipidemia

Our goal was to examine the associations of the 388A>G and 521T>C polymorphisms in the solute carrier organic anion transporter 1B1 (SLCO1B1) gene with hepatic function, baseline lipid levels, and the lipid-lowering efficiency of simvastatin. We recruited 542 patients with hyperlipidemia. The 388A>G and 521T>C polymorphisms were genotyped. Serum alanine aminotransferase (ALT) and aspartate transaminase (AST), Serum triglyceride (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C) levels were measured before and after an oral 20-mg dose of simvastatin. Individuals with the 388AA genotype had higher ALT and AST levels than those with the 388AG or 388GG genotypes (P = .037 and P = .002, respectively). Individuals with both the 388AA and the 521TT genotypes had the highest levels of ALT and AST (P = .001 and P = .001, respectively). Moreover, we divided all patients into normal and abnormal subgroups based on elevated ALT and AST values (≥ 40 U/L), participants in the abnormal subgroup had a higher frequency of the 388A/521T haplotype and a lower frequency of the 388G/521T haplotype compared to those in the normal subgroup.  In addition, compared to 388G allele and 521C allele carriers, individuals with the 388G allele and 521TT genotype carriers had greater TC and LDL-C reduction in response to simvastatin after 4 weeks of treatment. Our conclusion suggests that the interaction between the SLCO1B1 388A>G and 521T>C polymorphisms could be an important genetic determinant of hepatic function and the therapeutic efficiency of simvastatin in Chinese patients with hyperlipidemia.

CYP2C9 and OATP1B1 genetic polymorphisms affect the metabolism and transport of glimepiride and gliclazide

The therapeutic use of glimepiride and gliclazide shows substantial inter-individual variation in pharmacokinetics and pharmacodynamics in human populations, which might be caused by genetic differences among individuals. The aim of this study was to assess the effect of CYP2C9 and OATP1B1 genetic polymorphisms on the metabolism and transport of glimepiride and gliclazide. The uptake of glimepiride and gliclazide was measured in OATP1B1*1a, *5 and *15-HEK293T cells, and their metabolism was measured using CYP2C9*1, *2 and *3 recombinase by LC-MS. Glimepiride in OATP1B1*1a, *5 and *15-HEK293T cells had V_max values of 155 ± 18.7, 80 ± 9.6, and 84.5 ± 8.2 pmol/min/mg, while gliclazide had V_max values of 15.7 ± 4.6, 7.2 ± 2.5, and 8.7 ± 2.4 pmol/min/mg, respectively. The clearance of glimepiride and gliclazide in OATP1B1*5 and *15 was significantly reduced compared to the wild-type. Glimepiride in the presence of CYP2C9*1, *2 and *3 recombinase had V_max values of 21.58 ± 7.78, 15.69 ± 5.59, and 9.17 ± 3.03 nmol/min/mg protein, while gliclazide had V_max values of 15.73 ± 3.11, 10.53 ± 4.06, and 6.21 ± 2.94 nmol/min/mg protein, respectively. The clearance of glimepiride and gliclazide in CYP2C9*2 and *3 was significantly reduced compared to the wild-type. These findings collectively indicate that OATP1B1*5 and *15 and CYP2C9*2 and *3 have a significant effect on the transport and metabolism of glimepiride and gliclazide.

Pharmacokinetic Changes during Pregnancy According to Genetic Variants: a Prospective Study in HIV-Infected Patients Receiving Atazanavir-Ritonavir

Atazanavir-ritonavir concentrations change over time during pregnancy in HIV-positive patients; the impact of genetic variants is unknown. Twenty patients were enrolled in this study; plasma and intracellular concentrations of antiretrovirals were measured, in addition to single-nucleotide polymorphisms in transport-related genes. Linear logistic regression showed that genetic variants in organic-anion-transporter-1B1- and pregnane-X-receptor-encoding genes affected third-trimester atazanavir exposure. In this prospective study, genetic variants partially explained the observed interpatient variability in third-trimester exposure to antiretrovirals.

SLCO1B1 Polymorphism Is a Drug Response Predictive Marker for Advanced Pancreatic Cancer Patients Treated With Gemcitabine, S-1, or Gemcitabine Plus S-1

OBJECTIVES

The aim of this study was to evaluate the effects of single-nucleotide polymorphisms (SNPs) on advanced pancreatic cancer risk and overall survival (OS) in a candidate-gene approach.

METHODS

Overall, 5438 SNPs in 219 candidate genes encoding several drug-metabolizing enzymes or transporters were analyzed. In the screening study, 3 SNPs were found associated with OS (P ≤ 0.0005). We validated these SNPs as part of the randomized phase 3 study (GEST study). The associations between OS and SNPs were investigated using log-rank test and Cox proportional hazards model.

RESULTS

From the GEST study, the SNP rs4149086 in the 3' UTR of the solute carrier organic anion transporter family member 1B1 (SLCO1B1) gene showed significant interaction with treatment (P = 0.02). In the gemcitabine group, the SNP was associated with short OS (hazard ratio [HR], 3.75; 95% confidence interval [CI], 1.30-10.8; P = 0.008) even after multiple-comparisons adjustment. In contrast, the SNP was not associated with OS in S-1 (HR, 0.77; 95% CI, 0.33-1.81; P = 0.55) or gemcitabine plus S-1 groups (HR, 1.18; 95% CI, 0.46-3.00; P = 0.72).

CONCLUSIONS

Patients with advanced pancreatic cancer with the rs4149086 AG or GG genotype may obtain good clinical results when treated with S-1-containing regimens.

Attitudes and perceptions of health professionals towards management of hypothyroidism in general practice: a qualitative interview study

OBJECTIVE

To explore the attitudes and perceptions of health professionals towards management of hypothyroidism that contributes to the suboptimal treatment of hypothyroidism in general practice.

DESIGN

A qualitative interview study using semistructured interviews.

PARTICIPANTS

Sixteen participants were interviewed between March and August 2016 comprising nine general practitioners (GPs), four pharmacists, two practice nurses and one nurse practitioner.

SETTING

General practice and community pharmacies in the counties of Northumberland, Tyne and Wear, Stockton-on-Tees and North Cumbria, North of England, UK.

METHOD

A grounded-theory approach was used to generate themes from interviews, which were underpinned by the theory of planned behaviour to give explanation to the data.

RESULTS

Although health professionals felt that hypothyroidism was easy to manage, GPs and nurses generally revealed inadequate knowledge of medication interactions and levothyroxine pharmacokinetics. Pharmacists felt limited in the advice that they provide to patients due to lack of access to patient records. Most GPs and nurses followed local guidelines, and relied on blood tests over clinical symptoms to adjust levothyroxine dose. The information exchanged between professional and patient was usually restricted by time and often centred on symptoms rather than patient education. Health professionals felt that incorrect levothyroxine adherence was the main reason behind suboptimal treatment, although other factors such as comorbidity and concomitant medication were mentioned. Enablers perceived by health professionals to improve the management of hypothyroidism included continuity of care, blood test reminders, system alerts for interfering medications and prescription renewal, and accessible blood tests and levothyroxine prescriptions for patients.

CONCLUSION

There is a significant health professional behavioural component to the management of hypothyroidism. Addressing the differences in patient and professional knowledge and perceptions could reduce the barriers to optimal treatment, while continuity of care and increased involvement of pharmacists and practice nurses would help to promote optimal thyroid replacement.

Endogenous Metabolites-Mediated Communication Between OAT1/OAT3 and OATP1B1 May Explain the Association Between SLCO1B1 SNPs and Methotrexate Toxicity

Although OATP1B1 is not expressed in the kidney, polymorphisms in SLCO1B1 have been associated with methotrexate clearance or toxicity. This unexpected pharmacogenetic association may reflect remote communication between liver and kidney transporters. This study confirms the pharmacogenetic association with methotrexate toxicity in adult patients with hematological malignancies. Using a targeted urinary metabolomics approach, we identified 38 and 34 metabolites which were differentially excreted between wildtype and carriers of the c.388A>G or c.521T>C variant alleles, respectively, half of them being associated with methotrexate toxicity. These metabolites mainly consisted of fatty acid derivatives and microbiota catabolites, including glycine conjugates and other uremic toxins, all known OATs substrates. These results suggest that dysfunction of a transporter affects the excretion profile of endogenous or exogenous substrates, possibly through metabolite-mediated interactions involving other transport systems, even in distant organs. This opens the way for better comprehension of complex pharmacokinetics and transporter-mediated drug-drug or nutrient-drug interactions.

Role of the Bile Acid Transporter SLC10A1 in Liver Targeting of the Lipid-Lowering Thyroid Hormone Analog Eprotirome

The thyroid hormone (TH) analog eprotirome (KB2115) was developed to lower cholesterol through selective activation of the TH receptor (TR) β1 in the liver. Interestingly, eprotirome shows low uptake in nonhepatic tissues, explaining its lipid-lowering action without adverse extrahepatic thyromimetic effects. Clinical trials have shown marked decreases in serum cholesterol levels. We explored the transport of eprotirome across the plasma membrane by members of three TH transporter families: monocarboxylate transporters MCT8 and MCT10; Na-independent organic anion transporters 1A2, 1B1, 1B3, 1C1, 2A1, and 2B1; and Na-dependent organic anion transporters SLC10A1 to SLC10A7. Cellular transport was studied in transfected COS1 cells using [14C]eprotirome and [125I]TH analogs. Of the 15 transporters tested initially, the liver-specific bile acid transporter SLC10A1 showed the highest eprotirome uptake (greater than a sevenfold induction after 60 minutes) as well as TRβ1-mediated transcriptional activity. Uptake of eprotirome by SLC10A1 was Na+ dependent and saturable with a Michaelis constant of 8 μM. Eprotirome transport was inhibited by known substrates for SLC10A1 (e.g., cholate and taurocholate), and by TH analogs such as triiodothyropropionic acid and triiodothyroacetic acid. However, no significant SLC10A1-mediated transport was observed of these [125I]TH analogs. We also studied the plasma disappearance and biliary excretion of [14C]eprotirome injected in control and Slc10a1 knockout mice. Although eprotirome is also transported by mouse Slc10a1, the pharmacokinetics of eprotirome were not affected by Slc10a1 deficiency. In conclusion, we have demonstrated that the liver-specific bile acid transporter SLC10A1 effectively transports eprotirome. However, Slc10a1 does not appear to be critical for the liver targeting of this TH analog in mice. Therefore, the importance of SLC10A1 for liver uptake of eprotirome in humans remains to be elucidated.

Patients' attitudes and perceptions towards treatment of hypothyroidism in general practice: an in-depth qualitative interview study

Background

Suboptimal thyroid hormone replacement is common in patients with hypothyroidism and the behavioural factors underlying this are poorly understood.

Aim

To explore the attitudes and perceptions of patients to thyroid hormone replacement therapy.

Design & setting

An in-depth qualitative interview study with patients with hypothyroidism residing in Northumberland, and Tyne and Wear, UK.

Method

Twenty-seven patients participated, of which 15 patients had thyroid stimulating hormone (TSH) levels within the reference range (0.4–4.0 mU/L) and 12 patients had TSH levels outside the reference range. A grounded theory approach was used to explore and develop emerging themes, which were mapped to the health belief model (HBM).

Results

Patients generally had a low understanding of their condition or of the consequences of suboptimal thyroid hormone replacement. Patients that had experienced hypothyroid symptoms at initial diagnosis had a better perception of disease susceptibility, and this was reflected in excellent adherence to levothyroxine in this group of patients. The main benefits of optimal thyroid replacement were improved wellbeing and performance. However, patients who remained unwell despite a normal serum TSH level felt that their normal result presented a barrier to further evaluation of their symptoms by their GP.

Conclusion

Educating patients with hypothyroidism regarding the consequences of inadequate thyroid hormone replacement may reduce barriers and improve treatment outcomes. An over-reliance on TSH as a sole marker of wellbeing reduced opportunities for clinicians to address patient symptoms. Evaluating symptoms in combination with biochemical indices, may lead to better patient outcomes than relying on laboratory tests alone.

Genetics of thyroid function

Recent studies show that subtle variations in thyroid function, including subclinical thyroid dysfunction, and even variation in thyroid function within the normal range, are associated with morbidity and mortality. It is estimated that 40-65% of the inter-individual variation in serum TSH and FT4 levels is determined by genetic factors. To identify these factors, various linkage and candidate gene studies have been performed in the past, which have identified only a few genes. In the last decade, genome-wide association studies identified many new genes, while recent whole-genome sequencing efforts have also been proven to be effective. In the current review, we provide a systematic overview of these studies, including strengths and limitations. We discuss new techniques which will further clarify the genetic basis of thyroid function in the near future, as well as the potential use of these genetic markers in personalizing the management of thyroid disease patients.

Emerging role of thyroid hormone metabolites

Thyroid hormones (THs) are essential for the regulation of development and metabolism in key organs. THs produce biological effects both by directly affecting gene expression through the interaction with nuclear receptors (genomic effects) and by activating protein kinases and/or ion channels (short-term effects). Such activations can be either direct, in the case of ion channels, or mediated by membrane or cytoplasmic receptors. Short-term-activated signalling pathways often play a role in the regulation of genomic effects. Several TH intermediate metabolites, which were previously considered without biological activity, have now been associated with a broad range of actions, mostly attributable to short-term effects. Here, we give an overview of the physiological roles and mechanisms of action of THs, focusing on the emerging position that TH metabolites are acquiring as important regulators of physiology and metabolism.

SLCO1B1*5 polymorphism (rs4149056) is associated with chemotherapy-induced amenorrhea in premenopausal women with breast cancer: a prospective cohort study

Background

Because inheritance is recognized as playing a role in age at menarche and natural menopause, the development of chemotherapy-induced amenorrhea (CIA) might depend on inherited genetic factors; however, studies that explore such a correlation are few and have received scant attention. Given the importance of this topic we conducted a comprehensive genotype study in young women (≤45 years) with early-stage breast cancer.

Methods

Our approach tested the effect of variant polymorphisms in drug metabolism enzymes (DMEs) using a predesigned pharmacogenomics panel (TaqMan® OpenArray®, Life Technologies GmbH, Darmstadt, Germany) in premenopausal women (n = 50). Patients received contemporary chemotherapy; in all cases a cyclophosphamide-based regimen with a dose of at least 500 mg/m² for six cycles. CIA was considered to be present in women with no resumption of menstrual bleeding within 12 months after completion of chemotherapy or goserelin.

Results

Twenty-six patients (52 %) showed CIA during follow-up whereas 24 women (48 %) remained premenopausal. Of all the DMEs studied, only the SLCO1B1*5 (rs4149056) genotype was associated with the development of CIA (P = 0.017). Of the 26 patients who were homozygous for the T/T allele SLCO1B1*5, 18 (69.2 %) developed CIA compared with 8 (30.8 %) of the 22 patients who were heterozygous (C/T allele). The association of heterozygous SLCO1B1*5 allele (OR 0.038; 95%CI: 0.05–0.92) with a lower risk of developing CIA remained significant in a binary logistic regression analysis that include age, SLCO1B1*5 allele variants, and goserelin therapy.

Conclusions

Patient age and SLCO1B1*5 allele variants predict the likelihood of young women with breast cancer developing CIA.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-016-2373-3) contains supplementary material, which is available to authorized users.

Genetic determination of the hypothalamic-pituitary-thyroid axis: where do we stand?

For a long time it has been known that both hypo- and hyperthyroidism are associated with an increased risk of morbidity and mortality. In recent years, it has also become clear that minor variations in thyroid function, including subclinical dysfunction and variation in thyroid function within the reference range, can have important effects on clinical endpoints, such as bone mineral density, depression, metabolic syndrome, and cardiovascular mortality. Serum thyroid parameters show substantial interindividual variability, whereas the intraindividual variability lies within a narrow range. This suggests that every individual has a unique hypothalamus-pituitary-thyroid axis setpoint that is mainly determined by genetic factors, and this heritability has been estimated to be 40-60%. Various mutations in thyroid hormone pathway genes have been identified in persons with thyroid dysfunction or altered thyroid function tests. Because these causes are rare, many candidate gene and linkage studies have been performed over the years to identify more common variants (polymorphisms) associated with thyroid (dys)function, but only a limited number of consistent associations have been found. However, in the past 5 years, advances in genetic research have led to the identification of a large number of new candidate genes. In this review, we provide an overview of the current knowledge about the polygenic basis of thyroid (dys)function. This includes new candidate genes identified by genome-wide approaches, what insights these genes provide into the genetic basis of thyroid (dys)function, and which new techniques will help to further decipher the genetic basis of thyroid (dys)function in the near future.

Hormone metabolism pathway genes and mammographic density change after quitting estrogen and progestin combined hormone therapy in the California Teachers Study

INTRODUCTION

Mammographic density (MD) is a strong biomarker of breast cancer risk. MD increases after women start estrogen plus progestin therapy (EPT) and decreases after women quit EPT. A large interindividual variation in EPT-associated MD change has been observed, but few studies have investigated genetic predictors of the EPT-associated MD change. Here, we evaluate the association between polymorphisms in hormone metabolism pathway genes and MD changes when women quit EPT.

METHODS

We collected mammograms before and after women quit EPT and genotyped 405 tagging single nucleotide polymorphisms (SNPs) in 30 hormone metabolism pathway genes in 284 non-Hispanic white participants of the California Teachers Study (CTS). Participants were ages 49 to 71 years at time of mammography taken after quitting EPT. We assessed percent MD using a computer-assisted method. MD change was calculated by subtracting MD of an 'off-EPT' mammogram from MD of an 'on-EPT' (that is baseline) mammogram. Linear regression analysis was used to investigate the SNP-MD change association, adjusting for the baseline 'on-EPT' MD, age and BMI at time of baseline mammogram, and time interval and BMI change between the two mammograms. An overall pathway and gene-level summary was obtained using the adaptive rank truncated product (ARTP) test. We calculated 'P values adjusted for correlated tests (P(ACT))' to account for multiple testing within a gene.

RESULTS

The strongest associations were observed for rs7489119 in SLCO1B1, and rs5933863 in ARSC. SLCO1B1 and ARSC are involved in excretion and activation of estrogen metabolites of EPT, respectively. MD change after quitting was 4.2% smaller per minor allele of rs7489119 (P = 0.0008; P(ACT) = 0.018) and 1.9% larger per minor allele of rs5933863 (P = 0.013; P(ACT) = 0.025). These individual SNP associations did not reach statistical significance when we further used Bonferroni correction to consider the number of tested genes. The pathway level summary ARTP P value was not statistically significant.

CONCLUSIONS

Data from this longitudinal study of EPT quitters suggest that genetic variation in two hormone metabolism pathway genes, SLCO1B1 and ARSC, may be associated with change in MD after women stop using EPT. Larger longitudinal studies are needed to confirm our findings.

Intracellular accumulation of atazanavir/ritonavir according to plasma concentrations and OATP1B1, ABCB1 and PXR genetic polymorphisms

OBJECTIVES

The rate of accumulation of atazanavir and ritonavir within cells is still debated due to methodological limitations. Our aim was to measure peripheral blood mononuclear cell (PBMC) concentrations of atazanavir and ritonavir and investigate whether single-nucleotide polymorphisms of OATP, ABCB1, CYP3A4 and PXR genes are involved in intracellular drug penetration.

METHODS

HIV-positive patients administered 300 mg of atazanavir/100 mg of ritonavir were enrolled. Blood sampling was performed at the end of the dosing interval (Ctrough). PBMC-associated and plasma atazanavir and ritonavir concentrations were measured by validated HPLC coupled with a single mass detector (HPLC-MS) and HPLC-photodiode array (PDA) methods, respectively. Cell count and mean cellular volume were determined using a Coulter counter. Genotyping was conducted using real-time PCR.

RESULTS

Thirty-five patients were enrolled. Median atazanavir and ritonavir intracellular concentrations were 1844 and 716 ng/mL, respectively. Median plasma concentrations were 645 ng/mL for atazanavir and 75 ng/mL for ritonavir, while median intracellular/plasma concentration ratios were 2.4 and 9.2, respectively. Median ritonavir intracellular concentrations were higher for OATP1B1 521 T→C TC or CC carriers and for PXR 44477 A→G AG or GG carriers. Atazanavir intracellular/plasma concentration ratios were higher in patients GG for the ABCB1 2677 G→T single-nucleotide polymorphism (SNP) compared with GT and TT groups.

CONCLUSIONS

Our study showed a higher intracellular ritonavir accumulation than previously reported. Ritonavir intracellular concentrations were associated with OATP1B1 521 and PXR 44477 SNPs while intracellular atazanavir exposure was associated with the ABCB1 2677 SNP. Further clinical studies are necessary in order to confirm these data.

The SLCO (former SLC21) superfamily of transporters

The members of the organic anion transporting polypeptide superfamily (OATPs) are classified within the SLCO solute carrier family. All functionally well characterized members are predicted to have 12 transmembrane domains and are sodium-independent transport systems that mediate the transport of a broad range of endo- as well as xenobiotics. Substrates are mainly amphipathic organic anions with a molecular weight of more than 300Da, but some of the known transported substrates are also neutral or even positively charged. Among the well characterized substrates are numerous drugs including statins, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, antibiotics, antihistaminics, antihypertensives and anticancer drugs. Based on their amino acid sequence identities, the different OATPs cluster into families (in general with more than 40% amino acid sequence identity) and subfamilies (more than 60% amino acid identity). With the sequencing of genomes from different species and the computerized prediction of encoded proteins more than 300 OATPs can be found in the databases, however only a fraction of them have been identified in humans, rodents, and some additional species important for pharmaceutical research like the rhesus monkey (Macaca mulatta), the dog (Canis lupus familiaris) and the pig (Sus scrofa). These OATPs form 6 families (OATP1-OATP6) and 13 subfamilies. In this review we try to summarize what is currently known about OATPs with respect to endogenous substrates, tissue distribution, transport mechanisms, regulation of expression, structure-function relationship and mutations and polymorphisms.

Polymorphic variants of SLCO1B1 in neonatal hyperbilirubinemia in China

Background

To evaluate the association between the genetic polymorphism of the solute carrier organic anion transporter family member 1B1 (SLCO1B1, also known as organic anion transport polypeptide C) and hyperbilirubinemia in Chinese neonates.

Methods

183 infants with hyperbilirubinemia and 192 control subjects from the Fifth People’s Hospital of Shenzhen were recruited. Polymerase chain reaction, restriction fragment length polymorphisms and agarose gel electrophoresis techniques were used to detect genetic variants of SLCO1B1.

Results

The study revealed that SLCO1B1 388 G > A occurred significantly more frequently in neonates with hyperbilirubinemia than in controls (RR = 1.50; 95% CI: 1.13–2.00). There were no significant differences in SLCO1B1 521 T > C between the hyperbilirubinemia and the control group (RR, 1.00; 95% CI, 0.72–1.40). No carriage of the C to A substitution at nucleotide 463 was detected.

Conclusion

The SLCO1B1 388 G > A variant is associated with neonatal hyperbilirubinemia in Chinese neonates.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Catalysis leads to posttranslational inactivation of the type 1 deiodinase and alters its conformation

Previously, it was shown that the type 1 deiodinase (D1) is subject to substrate-dependent inactivation that is blocked by pretreatment with the inhibitor of D1 catalysis, propylthiouracil (PTU). Using HepG2 cells with endogenous D1 activity, we found that while considerable D1-mediated catalysis of reverse tri-iodothyronine (rT(3)) is observed in intact cells, there was a significant loss of D1 activity in sonicates assayed from the same cells in parallel. This rT(3)-mediated loss of D1 activity occurs despite no change in D1 mRNA levels and is blocked by PTU treatment, suggesting a requirement for catalysis. Endogenous D1 activity in sonicates was inactivated in a dose-dependent manner in HepG2 cells, with a ∼50% decrease after 10 nM rT(3) treatment. Inactivation of D1 was rapid, occurring after only half an hour of rT(3) treatment. D1 expressed in HEK293 cells was inactivated by rT(3) in a similar manner. (75)Se labeling of the D1 selenoprotein indicated that after 4 h rT(3)-mediated inactivation of D1 occurs without a corresponding decrease in D1 protein levels, though rT(3) treatment causes a loss of D1 protein after 8-24 h. Bioluminescence resonance energy transfer studies indicate that rT(3) exposure increases energy transfer between the D1 homodimer subunits, and this was lost when the active site of D1 was mutated to alanine, suggesting that a post-catalytic structural change in the D1 homodimer could cause enzyme inactivation. Thus, both D1 and type 2 deiodinase are subject to catalysis-induced loss of activity although their inactivation occurs via very different mechanisms.

Genetic polymorphisms of OATP transporters and their impact on intestinal absorption and hepatic disposition of drugs

There is convincing evidence that many organic anion transporting polypeptide (OATP) transporters influence the pharmacokinetics and pharmacological efficacy of their substrate drugs. Each OATP family member has a unique combination of tissue distribution, substrate specificity and mechanisms of gene expression. Among them, OATP1B1, OATP1B3 and OATP2B1 have been considered as critical molecular determinants of the pharmacokinetics of a variety of clinically important drugs. Liver-specific expression of OATP1B1 and OATP1B3 contributes to the hepatic uptake of drugs from the portal vein, and OATP2B1 may alter their intestinal absorption as well as hepatic extraction. Accordingly, changes in function and expression of these three OATPs owing to genetic polymorphisms may lead to altered pharmacological effects, including decreased drug efficacy and increased risk of adverse effects. Association of genetic polymorphisms in OATP genes with alterations in the pharmacokinetic properties of their substrate drugs has been reported; however, there still exists a degree of discordance between the reported outcomes in different clinical settings. For better understanding of the clinical relevance of genetic polymorphisms of OATP1B1, OATP1B3 and OATP2B1, the present review focuses on the association of the genotypes of these OATPs with in vitro activity changes and in vivo clinical outcomes of substrate drugs.

Hepatic organic anion transporting polypeptide transporter and thyroid hormone receptor interplay determines cholesterol and glucose homeostasis

The role of organic anion transporting polypeptides (OATPs), particularly the members of OATP1B subfamily, in hepatocellular handling of endogenous and exogenous compounds is an important and emerging area of research. Using a mouse model lacking Slco1b2, the murine ortholog of the OATP1B subfamily, we have demonstrated previously that genetic ablation causes reduced hepatic clearance capacity for substrates. In this study, we focused on the physiological function of the hepatic OATP1B transporters. First, we studied the influence of the Oatp1b2 deletion on bile acid (BA) metabolism, showing that lack of the transporter results in a significantly reduced expression of Cyp7a1, the key enzyme of BA synthesis, resulting in elevated cholesterol levels after high dietary fat challenge. Furthermore, Slco1b2-/- mice exhibited delayed clearance after oral glucose challenge resulting from reduced hepatic glucose uptake. In addition to increased hepatic glycogen content, Slco1b2-/- mice exhibited reduced glucose output after pyruvate challenge. This is in accordance with reduced hepatic expression of phosphoenolpyruvate carboxykinase (PEPCK) in knockout mice. We show that this phenotype is due to the loss of liver-specific Oatp1b2-mediated hepatocellular thyroid hormone entry, which then leads to reduced transcriptional activation of target genes of hepatic thyroid hormone receptor (TR), including Cyp7a1 and Pepck but also Dio1 and Glut2. Importantly, we assessed human relevance using a cohort of archived human livers in which OATP1B1 expression was noted to be highly associated with TR target genes, especially for glucose facilitating transporter 2 (GLUT2). Furthermore, GLUT2 expression was significantly decreased in livers harboring a common genetic polymorphism in SLCO1B1.

CONCLUSION

Our findings reveal that OATP1B-mediated hepatic thyroid hormone entry is a key determinant of cholesterol and glucose homeostasis.

A large-scale association analysis of 68 thyroid hormone pathway genes with serum TSH and FT4 levels

OBJECTIVE

Minor variation in serum thyroid hormone (TH) levels can have important effects on various clinical endpoints. Although 45-65% of the inter-individual variation in serum TH levels is due to genetic factors, the causative genes are not well established. We therefore studied the effects of genetic variation in 68 TH pathway genes on serum TSH and free thyroxine (FT(4)) levels.

DESIGN AND METHODS

Sixty-eight genes (1512 polymorphisms) were studied in relation to serum TSH and FT(4) levels in 1121 Caucasian subjects. Promising hits (P<0.01) were studied in three independent Caucasian populations (2656 subjects) for confirmation. A meta-analysis of all four studies was performed.

RESULTS

For TSH, eight PDE8B polymorphisms (P=4×10(-17)) remained significant in the meta-analysis. For FT(4), two DIO1 (P=8×10(-12)) and one FOXE1 (P=0.0003) polymorphisms remained significant in the meta-analysis. Suggestive associations were detected for one FOXE1 (P=0.0028) and three THRB (P=0.0045) polymorphisms with TSH, and one SLC16A10 polymorphism (P=0.0110) with FT(4), but failed to reach the significant multiple-testing corrected P value (P<0.0022 and P<0.0033 respectively).

CONCLUSIONS

Using a large-scale association analysis, we replicated previously reported associations with genetic variation in PDE8B, THRB, and DIO1. We demonstrate effects of genetic variation in FOXE1 on serum FT(4) levels, and borderline significant effects on serum TSH levels. A suggestive association of genetic variation in SLC16A10 with serum FT(4) levels was found. These data provide insight into the molecular basis of inter-individual variation in TH serum levels.

The association of polymorphisms in hormone metabolism pathway genes, menopausal hormone therapy, and breast cancer risk: a nested case-control study in the California Teachers Study cohort

Introduction

The female sex steroids estrogen and progesterone are important in breast cancer etiology. It therefore seems plausible that variation in genes involved in metabolism of these hormones may affect breast cancer risk, and that these associations may vary depending on menopausal status and use of hormone therapy.

Methods

We conducted a nested case-control study of breast cancer in the California Teachers Study cohort. We analyzed 317 tagging single nucleotide polymorphisms (SNPs) in 24 hormone pathway genes in 2746 non-Hispanic white women: 1351 cases and 1395 controls. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated by fitting conditional logistic regression models using all women or subgroups of women defined by menopausal status and hormone therapy use. P values were adjusted for multiple correlated tests (P_ACT).

Results

The strongest associations were observed for SNPs in SLCO1B1, a solute carrier organic anion transporter gene, which transports estradiol-17β-glucuronide and estrone-3-sulfate from the blood into hepatocytes. Ten of 38 tagging SNPs of SLCO1B1 showed significant associations with postmenopausal breast cancer risk; 5 SNPs (rs11045777, rs11045773, rs16923519, rs4149057, rs11045884) remained statistically significant after adjusting for multiple testing within this gene (P_ACT = 0.019-0.046). In postmenopausal women who were using combined estrogen-progestin therapy (EPT) at cohort enrollment, the OR of breast cancer was 2.31 (95% CI = 1.47-3.62) per minor allele of rs4149013 in SLCO1B1 (P = 0.0003; within-gene P_ACT = 0.002; overall P_ACT = 0.023). SNPs in other hormone pathway genes evaluated in this study were not associated with breast cancer risk in premenopausal or postmenopausal women.

Conclusions

We found evidence that genetic variation in SLCO1B1 is associated with breast cancer risk in postmenopausal women, particularly among those using EPT.

Organic anion transporting polypeptide 1B1: a genetically polymorphic transporter of major importance for hepatic drug uptake

The importance of membrane transporters for drug pharmacokinetics has been increasingly recognized during the last decade. Organic anion transporting polypeptide 1B1 (OATP1B1) is a genetically polymorphic influx transporter expressed on the sinusoidal membrane of human hepatocytes, and it mediates the hepatic uptake of many endogenous compounds and xenobiotics. Recent studies have demonstrated that OATP1B1 plays a major, clinically important role in the hepatic uptake of many drugs. A common single-nucleotide variation (coding DNA c.521T>C, protein p.V174A, rs4149056) in the SLCO1B1 gene encoding OATP1B1 decreases the transporting activity of OATP1B1, resulting in markedly increased plasma concentrations of, for example, many statins, particularly of active simvastatin acid. The variant thereby enhances the risk of statin-induced myopathy and decreases the therapeutic indexes of statins. However, the effect of the SLCO1B1 c.521T>C variant is different on different statins. The same variant also markedly affects the pharmacokinetics of several other drugs. Furthermore, certain SLCO1B1 variants associated with an enhanced clearance of methotrexate increase the risk of gastrointestinal toxicity by methotrexate in the treatment of children with acute lymphoblastic leukemia. Certain drugs (e.g., cyclosporine) potently inhibit OATP1B1, causing clinically significant drug interactions. Thus, OATP1B1 plays a major role in the hepatic uptake of drugs, and genetic variants and drug interactions affecting OATP1B1 activity are important determinants of individual drug responses. In this article, we review the current knowledge about the expression, function, substrate characteristics, and pharmacogenetics of OATP1B1 as well as its role in drug interactions, in parts comparing with those of other hepatocyte-expressed organic anion transporting polypeptides, OATP1B3 and OATP2B1.

Organic anion transporter 1B1 (SLCO1B1) polymorphism and gallstone formation: High incidence of Exon4 CA genotype in female patients in North India

AIM

  Gallstone disease is an important cause of abdominal morbidity Organic anion transport protein 1B1 (OATP1B1) (encoded by SLCO1B1) is a major transporter protein for bile salt uptake in enterohepatic circulation of bile salts. Disturbance in this pathway can decrease relative concentration of bile salts in gallbladder and may lead to formation of gallstones. We investigated role of SLCO1B1 polymorphisms [(Exon4 C > A (Pro155Thr; rs11045819) and Ex6 + 40T > C (Val174Ala; rs4149056)] in conferring interindividual susceptibility to gallstone disease.

METHODS

  A total of 173 healthy controls and 226 gallstone patients (USG positive) were recruited. Genotyping was done by using standard polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) method.

RESULTS

  The observed control frequencies of both polymorphisms of SLCO1B1 gene [(Exon4 C > A (Pro155Thr; rs11045819) and Ex6 + 40T > C (Val174Ala; rs4149056)] were in agreement with Hardy-Weinberg equilibrium. The frequency CA genotype and A allele of Exon4 C > A polymorphism was higher in gallstones patients (12.4% and 6.2%) as compared to controls (5.2% and 2.6%) which was statistically significant [(P = 0.029; OR = 2.31; 95% CI = 1.1-5.0); (P = 0.034; OR = 2.22; 95% CI = 1.1-4.8)], respectively). However, distribution of genotypes and alleles of Ex6 + 40T > C polymorphism was almost similar between gallstone patients and controls. Haplotype analysis showed frequency of A,T haplotype consisting of was significantly higher in gallstone patients as compared to controls and was imposing risk for the disease (P = 0.036; OR = 2.34; 95% CI = 1.0-5.1).

CONCLUSION

  These results suggest that SLCO1B1 Exon4 C > A polymorphism confers increased risk for gallstone disease in North Indian population.

Several conserved positively charged amino acids in OATP1B1 are involved in binding or translocation of different substrates

OATP1B1 and 1B3 are related transporters mediating uptake of numerous compounds into hepatocytes. A putative model of OATP1B3 with a "positive binding pocket" containing conserved positively charged amino acids was predicted (Meier-Abt et al. J Membr Biol 208:213-227, 2005). Based on this model, we tested the hypothesis that these positive amino acids are important for OATP1B1 function. We made mutants and measured surface expression and uptake of estradiol-17β-glucuronide, estrone-3-sulfate and bromosulfophthalein in HEK293 cells. Two of the mutants had low surface expression levels: R181K at 10% and R580A at 30% of wild-type OATP1B1. A lysine at position 580 (R580K) rescued the expression of R580A. Mutations of several amino acids resulted in substrate-dependent effects. The largest changes were seen for estradiol-17β-glucuronide, while estrone-3-sulfate and bromosulfophthalein transport were less affected. The wild-type OATP1B1 K (m) value for estradiol-17β-glucuronide of 5.35 ± 0.54 μM was increased by R57A to 30.5 ± 3.64 μM and decreased by R580K to 0.52 ± 0.18 μM. For estrone-3-sulfate the wild-type high-affinity K (m) value of 0.55 ± 0.12 μM was increased by K361R to 1.8 ± 0.47 μM and decreased by R580K to 0.1 ± 0.04 μM. In addition, R580K reduced the V (max) values for all three substrates to <25% of wild-type OATP1B1. Mutations at intracellular K90, H92 and R93 mainly affected V (max) values for estradiol-17β-glucuronide uptake. In conclusion, the conserved amino acids R57, K361 and R580 seem to be part of the substrate binding sites and/or translocation pathways in OATP1B1.

Minireview: thyroid hormone transporters: the knowns and the unknowns

The effects of thyroid hormone (TH) on development and metabolism are exerted at the cellular level. Metabolism and action of TH take place intracellularly, which require transport of the hormone across the plasma membrane. This process is mediated by TH transporter proteins. Many TH transporters have been identified at the molecular level, although a few are classified as specific TH transporters, including monocarboxylate transporter (MCT)8, MCT10, and organic anion-transporting polypeptide 1C1. The importance of TH transporters for physiology has been illustrated dramatically by the causative role of MCT8 mutations in males with psychomotor retardation and abnormal serum TH concentrations. Although Mct8 knockout animals have provided insight in the mechanisms underlying parts of the endocrine phenotype, they lack obvious neurological abnormalities. Thus, the pathogenesis of the neurological abnormalities in males with MCT8 mutations is not fully understood. The prospects of identifying other transporters and transporter-based syndromes promise an exciting future in the TH transporter field.

Lack of effect of genetic polymorphisms of SLCO1B1 on the lipid-lowering response to pitavastatin in Chinese patients

AIM

To investigate the SLCO1B1 388A>G and 521T>C polymorphisms in hyperlipidemia patients and evaluate the effect of the two polymorphisms on the lipid-lowering efficacy of pitavastatin.

METHODS

The functional polymorphisms of SLCO1B1 (388A>G and 521T>C) were genotyped in 140 Chinese patients with essential hyperlipidemia using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and one-step tetra-primers ARMS-PCR. Eighty-five patients were enrolled in the clinical trial and given 2 mg of pitavastatin daily for 8 weeks. Total cholesterol (TC), triglyceride (TG), high-density lipoprotein (HDL), and low-density lipoprotein (LDL) serum levels were measured at baseline, after 4 weeks and after 8 weeks of treatment.

RESULTS

The allele frequencies of SLCO1B1 388A>G and 521T>C in essential hyperlipidemia patients were 71.1% and 11.1%, respectively. The 4- and 8-week treatment with pitavastatin significantly reduced TC, TG, and LDL levels, but there was no statistical difference among patients with wild type, SLCO1B1 388A>G or SLCO1B1 521T>C in the lipid-lowering efficacy of pitavastatin.

CONCLUSION

The present study found that the allele frequencies of SLCO1B1 388A>G and 521T>C in Chinese patients with essential hyperlipidemia are comparable to those in healthy Chinese population. SLCO1B1 388A>G and 521T>C do not affect the lipid-lowering efficacy of pitavastatin.

Study of the transport of thyroid hormone by transporters of the SLC10 family

Transport of (sulfated) iodothyronines across the plasma membrane is required for their intracellular metabolism. Rat Na(+)/taurocholate cotransporting polypeptide (Ntcp; Slc10a1) has been identified as an important transporter protein. We demonstrate that among the 7 members of the solute carrier family SLC10, only human SLC10A1 mediates sodium-dependent transport of the iodothyronine T4 and iodothyronine sulfates T3S and T4S. In contrast to SLC10A2-7, cells co-expressing SLC10A1 and the deiodinase D1 demonstrate a dramatic increase in T3S and T4S metabolism. The SLC10A1 substrates taurocholate, DHEAS and E3S inhibit T3S and T4S transport. Furthermore, co-transfection of SLC10A1 with CRYM, a well-known intracellular iodothyronine-binding protein, results in an enhanced intracellular accumulation of T3S and T4S, indicating that CRYM binds iodothyronine sulfates. The present findings indicate that the liver-specific transporter SLC10A1 transports (sulfated) iodothyronines, thereby increasing their intracellular availability. Therefore, SLC10A1 may fulfill a critical step in providing liver D1 with iodothyronine sulfates for rapid degradation.

Tryptophan and iodothyronine transport interactions in HepG2 human hepatoma cells

This study identifies interactions between transport of the aromatic amino acid L: -tryptophan (Trp) and thyroid hormones (TH) in HepG2 human hepatoma cells. The major portion of Trp uptake in HepG2 cells occurs via the NEM-sensitive amino acid transport System L2 (consistent with hepatic LAT3 expression), with a smaller aromatic-AA selective System T (MCT10) component. LAT3 and MCT10 mRNA were both detected in HepG2 cells. Uptake of TH does not involve System L2, but a significant portion of T(3) uptake is mediated by System T, alongside a taurocholate-sensitive organic anion transporter. T(4) uptake into HepG2 cells appears to be mediated principally by organic anion/monocarboxylate transporters, with smaller contributions by System T and receptor-mediated endocytosis. TH-Trp transport interactions in liver cells centre on System T which, due to a perivenous localisation alongside deiodinase 1, may impact on hepatic T(3) generation and release.

Effect of SLCO1B1 polymorphism on the plasma concentrations of bile acids and bile acid synthesis marker in humans

BACKGROUND AND OBJECTIVE

Organic anion transporting polypeptide 1B1 (OATP1B1, encoded by SLCO1B1) is a sinusoidal influx transporter of human hepatocytes. Our aim was to characterize the role of OATP1B1 in the hepatic uptake of bile acids in vivo.

METHODS

Fasting blood samples were drawn from 24 healthy volunteers with SLCO1B1 c.388AA-c.521TT (*1A/*1A) genotype, eight with c.388GG-c.521TT (*1B/*1B) genotype, 24 with c.521TC genotype, and nine with c.521CC genotype. Plasma concentrations of 15 endogenous bile acids, their synthesis marker, and cholesterol were determined by liquid chromatography-tandem mass spectrometry.

RESULTS

The concentrations of ursodeoxycholic acid, glycoursodeoxycholic acid, chenodeoxycholic acid, and glycochenodeoxycholic acid were approximately 50-240% higher in individuals with the SLCO1B1 c.521CC, c.521TC, or c.388AA-c.521TT genotype than in those with the c.388GG-c.521TT genotype (P<0.05), with the largest differences seen between the c.521CC and c.388GG-c.521TT individuals. The concentration of tauroursodeoxycholic acid was approximately 120% higher in individuals with the c.521TC genotype and that of taurochenodeoxycholic acid 110% higher in individuals with the c.521CC or c.521TC genotype than in those with the c.388GG-c.521TT genotype (P<0.05). The cholic acid concentration was approximately 30% higher in individuals with the c.521CC or c.388AA-c.521TT genotype than in those with the c.388GG-c.521TT genotype (P<0.05), but its conjugates remained unaffected by the genotype. The bile acid synthesis marker 7alpha-hydroxy-4-cholesten-3-one/cholesterol concentration ratio was 62 or 45% higher in the c.388AA-c.521TT participants than in the c.388GG-c.521TT or c.521TC participants, respectively (P<0.05).

CONCLUSION

SLCO1B1 polymorphism considerably affects the disposition of several endogenous bile acids and bile acid synthesis marker, indicating that OATP1B1 plays an important role in the hepatic uptake of bile acids in vivo in humans.