Impact of genetic variability in the ABCG2 gene on ABCG2 expression, function, and interaction with AT1 receptor antagonist telmisartan

Deletion of vascular thromboxane A2 receptors and its impact on angiotensin II-induced hypertension and atherosclerotic lesion formation in the aorta of Ldlr-deficient mice

The thromboxane A2 receptor (TP) has been shown to play a role in angiotensin II (Ang II)-mediated hypertension and pathological vascular remodeling. To assess the impact of vascular TP on Ang II-induced hypertension, atherogenesis, and pathological aortic alterations, i.e. aneurysms, we analysed Western-type diet-fed and Ang II-infused TPVSMC KO/Ldlr KO, TPEC KO/Ldlr KO mice and their respective wild-type littermates (TPWT/Ldlr KO). These analyses showed that neither EC- nor VSMC-specific deletion of the TP significantly affected basal or Ang II-induced blood pressure or aortic atherosclerotic lesion area. In contrast, VSMC-specific TP deletion abolished and EC-specific TP deletion surprisingly reduced the ex vivo reactivity of aortic rings to the TP agonist U-46619, whereas VSMC-specific TP knockout also diminished the ex vivo response of aortic rings to Ang II. Furthermore, despite similar systemic blood pressure, there was a trend towards less atherogenesis in the aortic arch and a trend towards fewer pathological aortic alterations in Ang II-treated female TPVSMC KO/Ldlr KO mice. Survival was impaired in male mice after Ang II infusion and tended to be higher in TPVSMC KO/Ldlr KO mice than in TPWT/Ldlr KO littermates. Thus, our data may suggest a deleterious role of the TP expressed in VSMC in the pathogenesis of Ang II-induced aortic atherosclerosis in female mice, and a surprising role of the endothelial TP in TP-mediated aortic contraction. However, future studies are needed to substantiate and further elucidate the role of the vascular TP in the pathogenesis of Ang II-induced hypertension, aortic atherosclerosis and aneurysm formation.

Active RhoA Exerts an Inhibitory Effect on the Homeostasis and Angiogenic Capacity of Human Endothelial Cells

Background

The small GTPase RhoA (Ras homolog gene family, member A) regulates a variety of cellular processes, including cell motility, proliferation, survival, and permeability. In addition, there are reports indicating that RhoA‐ROCK (rho associated coiled‐coil containing protein kinase) activation is essential for VEGF (vascular endothelial growth factor)‐mediated angiogenesis, whereas other work suggests VEGF‐antagonistic effects of the RhoA‐ROCK axis.

Methods and Results

To elucidate this issue, we examined human umbilical vein endothelial cells and human coronary artery endothelial cells after stable overexpression (lentiviral transduction) of constitutively active (G14V/Q63L), dominant‐negative (T19N), or wild‐type RhoA using a series of in vitro angiogenesis assays (proliferation, migration, tube formation, angiogenic sprouting, endothelial cell viability) and a human umbilical vein endothelial cells xenograft assay in immune‐incompetent NOD scid gamma mice in vivo. Here, we report that expression of active and wild‐type RhoA but not dominant‐negative RhoA significantly inhibited endothelial cell proliferation, migration, tube formation, and angiogenic sprouting in vitro. Moreover, active RhoA increased endothelial cell death in vitro and decreased human umbilical vein endothelial cell‐related angiogenesis in vivo. Inhibition of RhoA by C3 transferase antagonized the inhibitory effects of RhoA and strongly enhanced VEGF‐induced angiogenic sprouting in control‐treated cells. In contrast, inhibition of RhoA effectors ROCK1/2 and LIMK1/2 (LIM domain kinase 1/2) did not significantly affect RhoA‐related effects, but increased angiogenic sprouting and migration of control‐treated cells. In agreement with these data, VEGF did not activate RhoA in human umbilical vein endothelial cells as measured by a Förster resonance energy transfer–based biosensor. Furthermore, global transcriptome and subsequent bioinformatic gene ontology enrichment analyses revealed that constitutively active RhoA induced a differentially expressed gene pattern that was enriched for gene ontology biological process terms associated with mitotic nuclear division, cell proliferation, cell motility, and cell adhesion, which included a significant decrease in VEGFR‐2 (vascular endothelial growth factor receptor 2) and NOS3 (nitric oxide synthase 3) expression.

Conclusions

Our data demonstrate that increased RhoA activity has the potential to trigger endothelial dysfunction and antiangiogenic effects independently of its well‐characterized downstream effectors ROCK and LIMK.

Thromboxane A2 receptor activation via Gα13-RhoA/C-ROCK-LIMK2-dependent signal transduction inhibits angiogenic sprouting of human endothelial cells

We could previously show that thromboxane A₂ receptor (TP) activation inhibits the angiogenic capacity of human endothelial cells, but the underlying mechanisms remained unclear. Therefore, the aim of this study was to elucidate TP signal transduction pathways relevant to angiogenic sprouting of human endothelial cells. To clarify this matter, we used RNAi-mediated gene silencing as well as pharmacological inhibition of potential TP downstream targets in human umbilical vein endothelial cells (HUVEC) and VEGF-induced angiogenic sprouting of HUVEC spheroids in vitro as a functional read-out. In this experimental set-up, the TP agonist U-46619 completely blocked VEGF-induced angiogenic sprouting of HUVEC spheroids. Moreover, in live-cell analyses TP activation induced endothelial cell contraction, sprout retraction as well as endothelial cell tension and focal adhesion dysregulation of HUVEC. These effects were reversed by pharmacological TP inhibition or TP knockdown. Moreover, we identified a TP-G_α13-RhoA/C-ROCK-LIMK2-dependent signal transduction pathway to be relevant for U-46619-induced inhibition of VEGF-mediated HUVEC sprouting. In line with these results, U-46619-mediated TP activation potently induced RhoA and RhoC activity in live HUVEC as measured by FRET biosensors. Interestingly, pharmacological inhibition of ROCK and LIMK2 also normalized U-46619-induced endothelial cell tension and focal adhesion dysregulation of HUVEC. In summary, our work reveals mechanisms by which the TP may disturb angiogenic endothelial function in disease states associated with sustained endothelial TP activation.

A Thromboxane A2 Receptor-Driven COX-2-Dependent Feedback Loop That Affects Endothelial Homeostasis and Angiogenesis

BACKGROUND

TP (thromboxane A₂ receptor) plays an eminent role in the pathophysiology of endothelial dysfunction and cardiovascular disease. Moreover, its expression is reported to increase in the intimal layer of blood vessels of cardiovascular high-risk individuals. Yet it is unknown, whether TP upregulation per se has the potential to affect the homeostasis of the vascular endothelium.

METHODS

We combined global transcriptome analysis, lipid mediator profiling, functional cell analyses, and in vivo angiogenesis assays to study the effects of endothelial TP overexpression or knockdown/knockout on the angiogenic capacity of endothelial cells in vitro and in vivo.

RESULTS

Here we report that endothelial TP expression induces COX-2 (cyclooxygenase-2) in a G_i/o- and G_q/11-dependent manner, thereby promoting its own activation via the auto/paracrine release of TP agonists, such as PGH₂ (prostaglandin H₂) or prostaglandin F₂ but not TxA₂ (thromboxane A₂). TP overexpression induces endothelial cell tension and aberrant cell morphology, affects focal adhesion dynamics, and inhibits the angiogenic capacity of human endothelial cells in vitro and in vivo, whereas TP knockdown or endothelial-specific TP knockout exerts opposing effects. Consequently, this TP-dependent feedback loop is disrupted by pharmacological TP or COX-2 inhibition and by genetic reconstitution of PGH₂-metabolizing prostacyclin synthase even in the absence of functional prostacyclin receptor expression.

CONCLUSIONS

Our work uncovers a TP-driven COX-2-dependent feedback loop and important effector mechanisms that directly link TP upregulation to angiostatic TP signaling in endothelial cells. By these previously unrecognized mechanisms, pathological endothelial upregulation of the TP could directly foster endothelial dysfunction, microvascular rarefaction, and systemic hypertension even in the absence of exogenous sources of TP agonists.

Multidrug efflux transporter ABCG2: expression and regulation

The adenosine triphosphate (ATP)-binding cassette efflux transporter G2 (ABCG2) was originally discovered in a multidrug-resistant breast cancer cell line. Studies in the past have expanded the understanding of its role in physiology, disease pathology and drug resistance. With a widely distributed expression across different cell types, ABCG2 plays a central role in ATP-dependent efflux of a vast range of endogenous and exogenous molecules, thereby maintaining cellular homeostasis and providing tissue protection against xenobiotic insults. However, ABCG2 expression is subjected to alterations under various pathophysiological conditions such as inflammation, infection, tissue injury, disease pathology and in response to xenobiotics and endobiotics. These changes may interfere with the bioavailability of therapeutic substrate drugs conferring drug resistance and in certain cases worsen the pathophysiological state aggravating its severity. Considering the crucial role of ABCG2 in normal physiology, therapeutic interventions directly targeting the transporter function may produce serious side effects. Therefore, modulation of transporter regulation instead of inhibiting the transporter itself will allow subtle changes in ABCG2 activity. This requires a thorough comprehension of diverse factors and complex signaling pathways (Kinases, Wnt/β-catenin, Sonic hedgehog) operating at multiple regulatory levels dictating ABCG2 expression and activity. This review features a background on the physiological role of transporter, factors that modulate ABCG2 levels and highlights various signaling pathways, molecular mechanisms and genetic polymorphisms in ABCG2 regulation. This understanding will aid in identifying potential molecular targets for therapeutic interventions to overcome ABCG2-mediated multidrug resistance (MDR) and to manage ABCG2-related pathophysiology.

Impact of DICER1 and DROSHA on the Angiogenic Capacity of Human Endothelial Cells

RNAi-mediated knockdown of DICER1 and DROSHA, enzymes critically involved in miRNA biogenesis, has been postulated to affect the homeostasis and the angiogenic capacity of human endothelial cells. To re-evaluate this issue, we reduced the expression of DICER1 or DROSHA by RNAi-mediated knockdown and subsequently investigated the effect of these interventions on the angiogenic capacity of human umbilical vein endothelial cells (HUVEC) in vitro (proliferation, migration, tube formation, endothelial cell spheroid sprouting) and in a HUVEC xenograft assay in immune incompetent NSG^TM mice in vivo. In contrast to previous reports, neither knockdown of DICER1 nor knockdown of DROSHA profoundly affected migration or tube formation of HUVEC or the angiogenic capacity of HUVEC in vivo. Furthermore, knockdown of DICER1 and the combined knockdown of DICER1 and DROSHA tended to increase VEGF-induced BrdU incorporation and induced angiogenic sprouting from HUVEC spheroids. Consistent with these observations, global proteomic analyses showed that knockdown of DICER1 or DROSHA only moderately altered HUVEC protein expression profiles but additively reduced, for example, expression of the angiogenesis inhibitor thrombospondin-1. In conclusion, global reduction of miRNA biogenesis by knockdown of DICER1 or DROSHA does not inhibit the angiogenic capacity of HUVEC. Further studies are therefore needed to elucidate the influence of these enzymes in the context of human endothelial cell-related angiogenesis.

The Role of ABCG2 in the Pathogenesis of Primary Hyperuricemia and Gout-An Update

Urate homeostasis in humans is a complex and highly heritable process that involves i.e., metabolic urate biosynthesis, renal urate reabsorption, as well as renal and extrarenal urate excretion. Importantly, disturbances in urate excretion are a common cause of hyperuricemia and gout. The majority of urate is eliminated by glomerular filtration in the kidney followed by an, as yet, not fully elucidated interplay of multiple transporters involved in the reabsorption or excretion of urate in the succeeding segments of the nephron. In this context, genome-wide association studies and subsequent functional analyses have identified the ATP-binding cassette (ABC) transporter ABCG2 as an important urate transporter and have highlighted the role of single nucleotide polymorphisms (SNPs) in the pathogenesis of reduced cellular urate efflux, hyperuricemia, and early-onset gout. Recent publications also suggest that ABCG2 is particularly involved in intestinal urate elimination and thus may represent an interesting new target for pharmacotherapeutic intervention in hyperuricemia and gout. In this review, we specifically address the involvement of ABCG2 in renal and extrarenal urate elimination. In addition, we will shed light on newly identified polymorphisms in ABCG2 associated with early-onset gout.

Analysis of Sequence Divergence in Mammalian ABCGs Predicts a Structural Network of Residues That Underlies Functional Divergence

The five members of the mammalian G subfamily of ATP-binding cassette transporters differ greatly in their substrate specificity. Four members of the subfamily are important in lipid transport and the wide substrate specificity of one of the members, ABCG2, is of significance due to its role in multidrug resistance. To explore the origin of substrate selectivity in members 1, 2, 4, 5 and 8 of this subfamily, we have analysed the differences in conservation between members in a multiple sequence alignment of ABCG sequences from mammals. Mapping sets of residues with similar patterns of conservation onto the resolved 3D structure of ABCG2 reveals possible explanations for differences in function, via a connected network of residues from the cytoplasmic to transmembrane domains. In ABCG2, this network of residues may confer extra conformational flexibility, enabling it to transport a wider array of substrates.

Medically Important Alterations in Transport Function and Trafficking of ABCG2

Several polymorphisms and mutations in the human ABCG2 multidrug transporter result in reduced plasma membrane expression and/or diminished transport function. Since ABCG2 plays a pivotal role in uric acid clearance, its malfunction may lead to hyperuricemia and gout. On the other hand, ABCG2 residing in various barrier tissues is involved in the innate defense mechanisms of the body; thus, genetic alterations in ABCG2 may modify the absorption, distribution, excretion of potentially toxic endo- and exogenous substances. In turn, this can lead either to altered therapy responses or to drug-related toxic reactions. This paper reviews the various types of mutations and polymorphisms in ABCG2, as well as the ways how altered cellular processing, trafficking, and transport activity of the protein can contribute to phenotypic manifestations. In addition, the various methods used for the identification of the impairments in ABCG2 variants and the different approaches to correct these defects are overviewed.

Picky ABCG5/G8 and promiscuous ABCG2 - a tale of fatty diets and drug toxicity

Structural data on ABCG5/G8 and ABCG2 reveal a unique molecular architecture for subfamily G ATP‐binding cassette (ABCG) transporters and disclose putative substrate‐binding sites. ABCG5/G8 and ABCG2 appear to use several unique structural motifs to execute transport, including the triple helical bundles, the membrane‐embedded polar relay, the re‐entry helices, and a hydrophobic valve. Interestingly, ABCG2 shows extreme substrate promiscuity, whereas ABCG5/G8 transports only sterol molecules. ABCG2 structures suggest a large internal cavity, serving as a binding region for substrates and inhibitors, while mutational and pharmacological analyses support the notion of multiple binding sites. By contrast, ABCG5/G8 shows a collapsed cavity of insufficient size to hold substrates. Indeed, mutational analyses indicate a sterol‐binding site at the hydrophobic interface between the transporter and the lipid bilayer. In this review, we highlight key differences and similarities between ABCG2 and ABCG5/G8 structures. We further discuss the relevance of distinct and shared structural features in the context of their physiological functions. Finally, we elaborate on how ABCG2 and ABCG5/G8 could pave the way for studies on other ABCG transporters.

Effects of UGT1A1 Polymorphism, Gender and Triglyceride on the Pharmacokinetics of Telmisartan in Chinese Patients with Hypertension: A Population Pharmacokinetic Analysis

BACKGROUND AND OBJECTIVE

Telmisartan is an angiotensin receptor blocker used for the treatment of hypertension. The effects of gender and uridine diphosphate-glycosytransferase 1A1 (UGT1A1) genetic polymorphisms (rs4124874, rs4148323, and rs6742078) on telmisartan plasma concentration and blood pressure in Chinese patients with hypertension have been reported previously. In this study, we aimed to develop a population pharmacokinetic (PopPK) model to quantify the effects of gender and UGT1A1 polymorphisms on the pharmacokinetics of telmisartan.

METHODS

Population pharmacokinetic analyses were performed using data collected prospectively from 58 Chinese patients with mild to moderate essential hypertension (aged 45-72 years; 36 men, 22 women) receiving 80 mg/day telmisartan orally for 4 weeks. Blood samples were collected in heparinized tubes at 0, 0.5, 1, and 6 h on day 28 after telmisartan administration. The plasma concentrations and UGT1A1 genetic variants were determined by high-performance liquid chromatography-mass spectrometry and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, respectively.

RESULTS

A two-compartment pharmacokinetic structural model with first-order elimination and absorption best described the pharmacokinetic characteristics of telmisartan. Gender and triglyceride influenced the apparent oral clearance (CL) of telmisartan. UGT1A1 (rs4124874) affected the bioavailability (F1) of telmisartan. Lower CL and bioavailability resulted in higher plasma concentrations being observed in female subjects with UGT1A1 CC or CA genotype and high triglyceride.

CONCLUSION

A PopPK model of telmisartan was established to confirm that UGT1A1 genotype, gender and triglyceride can affect the pharmacokinetics of telmisartan in Chinese patients with hypertension. Our findings can provide relevant pharmacokinetic parameters for further study of telmisartan.

Polymorphisms of the Multidrug Pump ABCG2: A Systematic Review of Their Effect on Protein Expression, Function, and Drug Pharmacokinetics

The widespread expression and polyspecificity of the multidrug ABCG2 efflux transporter make it an important determinant of the pharmacokinetics of a variety of substrate drugs. Null ABCG2 expression has been linked to the Junior blood group. Polymorphisms affecting the expression or function of ABCG2 may have clinically important roles in drug disposition and efficacy. The most well-studied single nucleotide polymorphism (SNP), Q141K (421C>A), is shown to decrease ABCG2 expression and activity, resulting in increased total drug exposure and decreased resistance to various substrates. The effect of Q141K can be rationalized by inspection of the ABCG2 structure, and the effects of this SNP on protein processing may make it a target for pharmacological intervention. The V12M SNP (34G>A) appears to improve outcomes in cancer patients treated with tyrosine kinase inhibitors, but the reasons for this are yet to be established, and this residue's role in the mechanism of the protein is unexplored by current biochemical and structural approaches. Research into the less-common polymorphisms is confined to in vitro studies, with several polymorphisms shown to decrease resistance to anticancer agents such as SN-38 and mitoxantrone. In this review, we present a systematic analysis of the effects of ABCG2 polymorphisms on ABCG2 function and drug pharmacokinetics. Where possible, we use recent structural advances to present a molecular interpretation of the effects of SNPs and indicate where we need further in vitro experiments to fully resolve how SNPs impact ABCG2 function.

Pharmacogenetic Aspects of the Interaction of AT1 Receptor Antagonists With ATP-Binding Cassette Transporter ABCG2

The ATP-binding cassette transporter ABCG2 (BCRP and MXR) is involved in the absorption, distribution, and elimination of numerous drugs. Thus, drugs that are able to reduce the activity of ABCG2, e.g., antihypertensive AT1 receptor antagonists (ARBs), may cause drug-drug interactions and compromise drug safety and efficacy. In addition, genetic variability within the ABCG2 gene may influence the ability of the transporter to interact with ARBs. Thus, the aim of this study was to characterize the ARB-ABCG2 interaction in the light of naturally occurring variations (F489L, R482G) or amino acid substitutions with in silico-predicted relevance for the ARB-ABCG2 interaction (Y469A; M483F; Y570A). For this purpose, ABCG2 variants were expressed in HEK293 cells and the impact of ARBs on ABCG2 activity was studied in vitro using the pheophorbide A (PhA) efflux assay. First, we demonstrated that both the F489L and the Y469A substitution, respectively, reduced ABCG2 protein levels in these cells. Moreover, both substitutions enhanced the inhibitory effect of candesartan cilexetil, irbesartan, losartan, and telmisartan on ABCG2-mediated PhA efflux, whereas the R482G substitution blunted the inhibitory effect of candesartan cilexetil and telmisartan in this regard. In contrast, the ARB-ABCG2 interaction was not altered in cells expressing either the M483F or the Y570A variant, respectively. In conclusion, our data indicate that the third transmembrane helix and adjacent regions of ABCG2 may be of major importance for the interaction of ARBs with the ABC transporter. Moreover, we conclude from our data that individuals carrying the F489L polymorphism may be at increased risk of developing ABCG2-related drug-drug interactions in multi-drug regimens involving ARBs.

Physiologically-based pharmacokinetic predictions of intestinal BCRP-mediated drug interactions of rosuvastatin in Koreans

It was recently reported that the C_max and AUC of rosuvastatin increases when it is coadministered with telmisartan and cyclosporine. Rosuvastatin is known to be a substrate of OATP1B1, OATP1B3, NTCP, and BCRP transporters. The aim of this study was to explore the mechanism of the interactions between rosuvastatin and two perpetrators, telmisartan and cyclosporine. Published (cyclosporine) or newly developed (telmisartan) PBPK models were used to this end. The rosuvastatin model in Simcyp (version 15)'s drug library was modified to reflect racial differences in rosuvastatin exposure. In the telmisartan–rosuvastatin case, simulated rosuvastatin C_maxI/C_max and AUC_I/AUC (with/without telmisartan) ratios were 1.92 and 1.14, respectively, and the T_max changed from 3.35 h to 1.40 h with coadministration of telmisartan, which were consistent with the aforementioned report (C_maxI/C_max: 2.01, AUC_I/AUC:1.18, T_max: 5 h → 0.75 h). In the next case of cyclosporine–rosuvastatin, the simulated rosuvastatin C_maxI/C_max and AUC_I/AUC (with/without cyclosporine) ratios were 3.29 and 1.30, respectively. The decrease in the CL_{int,BCRP,intestine} of rosuvastatin by telmisartan and cyclosporine in the PBPK model was pivotal to reproducing this finding in Simcyp. Our PBPK model demonstrated that the major causes of increase in rosuvastatin exposure are mediated by intestinal BCRP (rosuvastatin–telmisartan interaction) or by both of BCRP and OATP1B1/3 (rosuvastatin–cyclosporine interaction).

ABCG2 polymorphisms in gout: insights into disease susceptibility and treatment approaches

As a result of the association of a common polymorphism (rs2231142, Q141K) in the ATP-binding cassette G2 (ABCG2) transporter with serum urate concentration in a genome-wide association study, it was revealed that ABCG2 is an important uric acid transporter. This review discusses the relevance of ABCG2 polymorphisms in gout, possible etiological mechanisms, and treatment approaches. The 141K ABCG2 urate-increasing variant causes instability in the nucleotide-binding domain, leading to decreased surface expression and function. Trafficking of the protein to the cell membrane is altered, and instead, there is an increased ubiquitin-mediated proteasomal degradation of the variant protein as well as sequestration into aggresomes. In humans, this leads to decreased uric acid excretion through both the kidney and the gut with the potential for a subsequent compensatory increase in renal urinary excretion. Not only does the 141K polymorphism in ABCG2 lead to hyperuricemia through renal overload and renal underexcretion, but emerging evidence indicates that it also increases the risk of acute gout in the presence of hyperuricemia, early onset of gout, tophi formation, and a poor response to allopurinol. In addition, there is some evidence that ABCG2 dysfunction may promote renal dysfunction in chronic kidney disease patients, increase systemic inflammatory responses, and decrease cellular autophagic responses to stress. These results suggest multiple benefits in restoring ABCG2 function. It has been shown that decreased ABCG2 141K surface expression and function can be restored with colchicine and other small molecule correctors. However, caution should be exercised in any application of these approaches given the role of surface ABCG2 in drug resistance.

ABC Transport Proteins in Cardiovascular Disease-A Brief Summary

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters may play an important role in the pathogenesis of atherosclerotic vascular diseases due to their involvement in cholesterol homeostasis, blood pressure regulation, endothelial function, vascular inflammation, as well as platelet production and aggregation. In this regard, ABC transporters, such as ABCA1, ABCG5 and ABCG8, were initially found to be responsible for genetically-inherited syndromes like Tangier diseases and sitosterolemia. These findings led to the understanding of those transporter’s function in cellular cholesterol efflux and thereby also linked them to atherosclerosis and cardiovascular diseases (CVD). Subsequently, further ABC transporters, i.e., ABCG1, ABCG4, ABCB6, ABCC1, ABCC6 or ABCC9, have been shown to directly or indirectly affect cellular cholesterol efflux, the inflammatory response in macrophages, megakaryocyte proliferation and thrombus formation, as well as vascular function and blood pressure, and may thereby contribute to the pathogenesis of CVD and its complications. Furthermore, ABC transporters, such as ABCB1, ABCC2 or ABCG2, may affect the safety and efficacy of several drug classes currently in use for CVD treatment. This review will give a brief overview of ABC transporters involved in the process of atherogenesis and CVD pathology. It also aims to briefly summarize the role of ABC transporters in the pharmacokinetics and disposition of drugs frequently used to treat CVD and CVD-related complications.

Transmembrane Domain Single-Nucleotide Polymorphisms Impair Expression and Transport Activity of ABC Transporter ABCG2

Purpose

To study the function and expression of nine naturally occurring single-nucleotide polymorphisms (G406R, F431L, S441N, P480L, F489L, M515R, L525R, A528T and T542A) that are predicted to reside in the transmembrane regions of the ABC transporter ABCG2.

Methods

The transport activity of the variants was tested in inside-out membrane vesicles from Sf9 insect and human derived HEK293 cells overexpressing ABCG2. Lucifer Yellow and estrone sulfate were used as probe substrates of activity. The expression levels and cellular localization of the variants was compared to the wild-type ABCG2 by western blotting and immunofluorescence microscopy.

Results

All studied variants of ABCG2 displayed markedly decreased transport in both Sf9-ABCG2 and HEK293-ABCG2 vesicles. Impaired transport could be explained for some variants by altered expression levels and cellular localization. Moreover, the destructive effect on transport activity of variants G406R, P480L, M515R and T542A is, to our knowledge, reported for the first time.

Conclusions

These results indicate that the transmembrane region of ABCG2 is sensitive to amino acid substitution and that patients harboring these ABCG2 variant forms could suffer from unexpected pharmacokinetic events of ABCG2 substrate drugs or have an increased risk for diseases such as gout where ABCG2 is implicated.

Electronic supplementary material

The online version of this article (doi:10.1007/s11095-017-2127-1) contains supplementary material, which is available to authorized users.

Alterations in ACE and ABCG2 expression levels in the testes of rats subjected to atropine-induced toxicity

Atropine-induced damage is associated with enzyme and protein alterations. The aim of the present study was to investigate atropine‑induced alterations in testicular expression levels of angiotensin‑converting enzyme (ACE) and adenosine 5'-triphosphate binding cassette sub‑family G member 2 (ABCG2) following atropine treatment. Male Wistar rats received 15 mg/kg/day atropine for 7 days; control rats received an identical volume of saline, Following treatment, the testes were harvested for immunohistochemistry and in situ hybridization to examine the protein and gene expression levels of ACE and ABCG2 by digital image analysis. ACE gene and protein expression levels were significantly reduced in the testes of atropine‑treated rats, compared with control rats (P=0.0001 and P<0.001, respectively). In addition, ABCG2 gene and protein expression levels were significantly increased in the testes of atropine‑treated rats, compared with control rats (P=0.0017 and P<0.001, respectively). Thus, the results of the present study demonstrate that testicular protein and gene expression levels of ACE and ABCG2 were altered as a result of atropine‑induced toxicity in the rats. These alterations may result in abnormal testicular function, and the proteins and genes identified in the present study may be useful to elucidate the mechanisms underlying atropine‑induced toxicity and provide a direction for further studies.

The C421A (Q141K) polymorphism enhances the 3'-untranslated region (3'-UTR)-dependent regulation of ATP-binding cassette transporter ABCG2

The impact of the gout-causing C421A (Q141K) single nucleotide polymorphism (SNP) on ABC transporter ABCG2 expression and function has been extensively characterized. However, the influence of the C421A SNP on 3'-UTR-dependent ABCG2 regulation has not been analysed so far. To elucidate this matter, we generated vectors for expression of either the ABCG2 coding sequence (ORF) or the ABCG2 ORF fused to its 3'-UTR, inserted the C421A mutation via site-directed mutagenesis and expressed wild-type and C421A-mutated ABCG2 transcripts in HEK293-Tet-On cells. As shown previously, the C421A SNP significantly reduced ABCG2 protein levels in ABCG2 ORF-transfected HEK293-Tet-On cells. Interestingly, the presence of the 3'-UTR in the ABCG2 transcript dramatically reduced ABCG2 protein content in cells transfected with the C421A variant but not significantly in those transfected with ABCG2 wild-type sequence, whereas ABCG2 mRNA levels were similar. siRNA-mediated DICER1 knockdown to reduce cellular microRNA biogenesis and selective mutation of putative microRNA binding sites within the ABCG2 3'-UTR partially antagonized C421A-associated reduction of ABCG2 protein content but did not significantly affect wild-type ABCG2 protein levels. In addition, antagomir-mediated inhibition of two microRNAs (hsa-miR-519c and hsa-miR-328) again partially reversed C421A-associated ABCG2 translational repression, thereby indicating that the C421A SNP may facilitate microRNA-dependent repression of ABCG2 protein translation. We conclude from our results that the C421A SNP may lead to reduced ABCG2 protein levels not only by affecting cellular protein stability but also via enhanced microRNA-dependent ABCG2 repression. Moreover, tissue-specific variation in ABCG2 3'-UTR processing may profoundly affect ABCG2 expression levels in individuals carrying the C421A mutation.

Negligible Pharmacokinetic Interaction of Red Ginseng and Losartan, an Antihypertensive Agent, in Sprague-Dawley Rats

Red ginseng (RG) is one of the top selling herbal medicines in Korea, but is not recommended in hypertensive patients. In this study, the pharmacokinetic (PK) interaction between RG and losartan, an antihypertensive drug, was examined. RG was orally administered for 2 wk to male Sprague-Dawley (S-D) rats at either control (0), 0.5, 1, or 2 g/kg/d for 2 wk. After the last administration of RG and 30 min later, all animals were treated with 10 mg/kg losartan by oral route. In addition, some S-D rats were administered RG orally for 21 d at 2 g/kg followed by losartan intravenously (iv) at 10 mg/kg/d. Post losartan administration, plasma samples were collected at 5, 15, and 30 min and 1, 1.5, 2, 3, 6, 12, and 24 h. Plasma concentrations of losartan and E-3174, the active metabolite of losartan, were analyzed by a high-pressure liquid chromatography-tandem mass spectrometer system (LC-MS/MS). Oral losartan administration showed dose-dependent pharmacokinetics (PK) increase with time to maximum plasma, but this was not significant between different groups. There was no significant change in tmax with E-3174 PK. With iv losartan, pharmacokinetics showed elevation of area under the plasma concentration-time curve from time zero extrapolated to infinitity. There was not a significant change in AUCinf with E-3174 PK. Therefore, RG appeared to interfere with biotransformation of losartan, as RG exerted no marked effect on E-3174 PK in S-D rats. Data demonstrated that oral or iv treatment with losartan in rats pretreated with RG for 2 wk showed that losartan PK was affected but E-3174 PK remained unchanged among different dose groups. These results suggested that RG induces negligible influence on losartan and E-3174 PK in rats.

New medications in development for the treatment of hyperuricemia of gout

PURPOSE OF REVIEW

To update recent developments in medications targeting hyperuricemia, but not including medications recently labelled in the European Union and the United States.

RECENT FINDINGS

A new xanthine oxidase inhibitor, Topiloric (Fujiyakuhin Co., Ltd. Japan) Uriadec (Sanwa Kagaku Kenkyusho Co., Ltd. Japan), has been developed and labelled in Japan. An inhibitor of purine nucleoside phosphorylase, Ulodesine, is in development in combination with allopurinol. The rest of the medications in the pipeline for hyperuricemia are targeting renal transporters of uric acid, mainly URAT1 and OAT4, acting as uricosuric agents. Most of them, such as lesinurad and arhalofenate, are being tested in trials in combination with allopurinol and febuxostat. The most potent RDEA3170 is being tested in monotherapy, but also associated with febuxostat. Recently, medications showing dual activity, inhibiting both xanthine oxidoreductase and URAT1, have been communicated or started exploratory clinical trials. There is no report of medications targeting other transporters such as Glut9 or ABCG2.

SUMMARY

There are a number of medications in the pipeline targeting hyperuricemia, mostly uricosurics in combination with xanthine oxidase inhibitors, but some targeting both xanthine oxidoreductase and URAT1. Increasing the number of available medications will ensure proper control of hyperuricemia to target serum urate levels in the near future for most, if not all, patients with hyperuricemia.