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Organic anion transporting polypeptide (OATP) 1B3–1B7 (LST-3TM12) is a member of the OATP1B [solute carrier organic anion transporter ( SLCO) 1B] family. This transporter is not only functional but also expressed in the membrane of the smooth endoplasmic reticulum of hepatocytes and enterocytes. OATP1B3–1B7 is a splice variant of SLCO1B3 in which the initial part is encoded by SLCO1B3, whereas the rest of the mRNA originates from the gene locus of SLCO1B7. In this study, we not only showed that SLCO1B3 and the mRNA encoding for OATP1B3–1B7 share the 5′ untranslated region but also that silencing of an initial SLCO1B3 exon lowered the amount of SLCO1B3 and of SLCO1B7 mRNA in Huh-7 cells. To validate the assumption that both transcripts are regulated by the same promoter we tested the influence of the bile acid sensor farnesoid X receptor (FXR) on their transcription. Treatment of Huh-7 and HepaRG cells with activators of this known regulator of OATP1B3 not only increased SLCO1B3 but also OATP1B3–1B7 mRNA transcription. Applying a heterologous expression system, we showed that several bile acids interact with OATP1B3–1B7 and that taurocholic acid and lithocholic acid are OATP1B3–1B7 substrates. As OATP1B3–1B7 is located in the smooth endoplasmic reticulum, it may grant access to metabolizing enzymes. In accordance are our findings showing that the OATP1B3–1B7 inhibitor bromsulphthalein significantly reduced uptake of bile acids into human liver microsomes. Taken together, we report that OATP1B3–1B7 transcription can be modulated with FXR agonists and antagonists and that OATP1B3–1B7 transports bile acids. NEW & NOTEWORTHY Our study on the transcriptional regulation of the novel organic anion transporting polypeptide (OATP) 1B3–1B7 concludes that the promoter of solute carrier organic anion transporter ( SLCO) 1B3 governs SLCO1B3–1B7 transcription. Moreover, the transcription of OATP1B3–1B7 can be modulated by farnesoid X receptor (FXR) agonists and antagonists. FXR is a major regulator in bile acid homeostasis that links OATP1B3–1B7 to this physiological function. Findings in transport studies with OATP1B3–1B7 suggest that this transporter interacts with the herein tested bile acids.

Recent studies have focused on coproporphyrin (CP)-I and CP-III (CPs) as endogenous biomarkers for organic anion transporting polypeptides (OATPs). Previous data showed that CPs are also substrates of multidrug resistance-associated protein (MRP/Mrp) 2 and 3. This study was designed to examine the impact of loss of Mrp2 function on the routes of excretion of endogenous CPs in wild-type (WT) Wistar compared to Mrp2-deficient TR(−) rats. To exclude possible confounding effects of rat Oatps, the transport of CPs was investigated in Oatp-overexpressing HeLa cells. Results indicated that CPs are substrates of rodent Oatp1b2, and that CP-III is a substrate of Oatp2b1. Quantitative targeted absolute proteomic (QTAP) analysis revealed no differences in Oatps, but an expected significant increase in Mrp3 protein levels in TR(−) compared to WT rat livers. CP-I and CP-III concentrations measured by LC-MS/MS were elevated in TR(−) compared to WT rat liver, while CP-I and CP-III estimated biliary clearance was decreased 75- and 840-fold in TR(−) compared to WT rats, respectively. CP-III concentrations were decreased 14-fold in the feces of TR(−) compared to WT rats, but differences in CP-I were not significant. In summary, the disposition of CPs was markedly altered by loss of Mrp2 and increased Mrp3 function as measured in TR(−) rats.

The protein family of Organic Anion Transporting Polypeptides (OATPs) summarizes various transporters known to facilitate cellular uptake of xenobiotics. One member of this family is OATP2B1. This transporter is ubiquitously expressed and possesses a PDZ-binding motif at the C-terminus. PDZK1 (PDZ domain-containing 1) is a scaffold protein that influences function of different membrane proteins by sorting/stabilization of their membrane localization. It was aim of the herein reported study to investigate whether there is an interaction between OATP2B1 and PDZK1, and to further characterize its impact on transport function. At first expression of both OATP2B1 and PDZK1 was evaluated in liver, kidney and intestine. Based on the existence of a C-terminal PDZ-class I binding motif in OATP2B1 and the co-expression in all tested tissues an interaction was likely. Testing the influence of PDZK1 on OATP2B1 transport function revealed enhanced transport capacity for estrone 3-sulfate, thereby suggesting a change in OATP2B1 amount in the membrane. This assumption was validated by Western blot analysis. Finally, deletion of the C-terminal PDZ-binding motif in OATP2B1 lowered the impact of PDZK1 on transport function. Taken together, we report an interaction of PDZK1 with OATP2B1, which influences localization and function of the transporter. Changes in PDZK1 expression may therefore be one factor contributing to interindividual differences in OATP2B1 mediated pharmacokinetic processes.

Organic anion transporting polypeptides (OATPs) and particularly the two members of the OATP1B family are known for their role in pharmacokinetics. Both SLCO1B3 and SLCO1B1 are located on chromosome 12 encompassing the gene locus SLCO1B7. Hitherto, this particular gene has been assumed to be a pseudogene, even though there are published mRNA sequences linked to this chromosomal area. It was aim of this study to further investigate SLCO1B7 and the associated mRNA LST-3TM12. In a first step, we aligned all mRNAs linked to the chromosomal region of SLCO1B-transporters. This in silico analysis revealed that LST-3TM12 is a product of splicing of SLCO1B3 and SLCO1B7, and encodes for a protein with twelve transmembrane domains. The existence of LST-3TM12 mRNA was verified by polymerase chain reaction showing liver enriched expression. In addition, immunohistological staining showed that LST-3TM12 protein was expressed in the endoplasmic reticulum (ER) of hepatocytes. Localization in the ER was further verified by immunoblot analysis showing high amounts of LST-3TM12 in liver microsomes. Function of LST-3TM12 was assessed by transport studies after heterologous expression in HeLa cells, where the transporter was shown to be expressed not only in the ER but also in the plasma membrane. Overexpression of LST-3TM12 was associated with enhanced cellular accumulation of dehydroepiandrosterone sulfate (Vmax 300.2 pmol mg−1 min−1; Km 34.2 µm) and estradiol 17β-glucuronide (Vmax 29.9 mol mg−1 min−1 and Km 32.8 µM). In conclusion, LST-3TM12 is a functional splice variant of SLCO1B3 and SLCO1B7 expressed in the ER of human liver.

The human drug transporter Organic Anion Transporting Polypeptide (hOATP)2B1 facilitates cellular uptake of its substrates. Various studies suggest that hOATP2B1 is involved in intestinal absorption, but preclinical evaluations performed in rodents do not support this. Thus, our study aimed to compare the expression and function of hOATP2B1 with its orthologue in rats (rOatp2b1). Even if the general expression pattern was comparable, the transporters exhibited substantial differences on functional level. While bromosulfophthalein and atorvastatin were substrates of both transporters, the steroid sulfate conjugates estrone 3-sulfate (E1S), progesterone sulfate and dehydroepiandrosterone sulfate were only transported by hOATP2B1. To further elucidate these functional differences, experiments searching for the E1S substrate recognition site were conducted generating human-rat chimera as well as partly humanized variants of rOatp2b1. The rOatp2b1-329-hOATP2B1 chimera led to a significant increase in E1S uptake suggesting the C-terminal part of the human transporter is involved. However, humanization of various regions within this part, namely of the transmembrane domain (TMD)-9, TMD-10 or the extracellular loop-5 did not significantly change E1S transport function. Replacement of the intracellular loop-3, slightly enhanced cellular accumulation of sulfated steroids. Taken together, we report that OATP2B1 exhibited differences in recognition of steroid sulfate conjugates comparing the rat and human orthologues.

Active targeting and specific drug delivery to parenchymal liver cells is a promising strategy to treat various liver disorders. Here, we modified synthetic lipid-based nanoparticles with targeting peptides derived from the hepatitis B virus large envelope protein (HBVpreS) to specifically target the sodium-taurocholate cotransporting polypeptide (NTCP; SLC10A1) on the sinusoidal membrane of hepatocytes. Physicochemical properties of targeted nanoparticles were optimized and NTCP-specific, ligand-dependent binding and internalization was confirmed in vitro. The pharmacokinetics and targeting capacity of selected lead formulations was investigated in vivo using the emerging zebrafish screening model. Liposomal nanoparticles modified with 0.25 mol% of a short myristoylated HBV derived peptide, that is Myr-HBVpreS2-31, showed an optimal balance between systemic circulation, avoidance of blood clearance, and targeting capacity. Pronounced liver enrichment, active NTCP-mediated targeting of hepatocytes and efficient cellular internalization were confirmed in mice by 111In gamma scintigraphy and fluorescence microscopy demonstrating the potential use of our hepatotropic, ligand-modified nanoparticles.

Background: It was our intention to develop cathepsin B-sensitive nanoparticles for tumor-site-directed release. These nanoparticles should be able to release their payload as close to the tumor site with a decrease of off-target effects in mind. Cathepsin B, a lysosomal cysteine protease, is associated with premalignant lesions and invasive stages of cancer. Previous studies have shown cathepsin B in lysosomes and in the extracellular matrix. Therefore, this enzyme qualifies as a trigger for such an approach. Methods: Poly(dimethylsiloxane)-b-poly(methyloxazoline) (PDMS-PMOXA) nanoparticles loaded with paclitaxel were formed by a thin-film technique and standard coupling reactions were used for surface modifications. Despite the controlled release mechanism, the physical properties of the herein created nanoparticles were described. To characterize potential in vitro model systems, quantitative polymerase chain reaction and common bioanalytical methods were employed. Conclusions: Stable paclitaxel-loaded nanoparticles with cathepsin B digestible peptide were formed and tested on the ovarian cancer cell line OVCAR-3. These nanoparticles exerted a pharmacological effect on the tumor cells suggesting a release of the payload.

Cardiovascular diseases are the leading causes of death in industrialized countries. Atherosclerotic coronary arteries are commonly treated with percutaneous transluminal coronary intervention followed by stent deployment. This treatment has significantly improved the clinical outcome. However, triggered vascular smooth muscle cell (SMC) proliferation leads to in-stent restenosis in bare metal stents. In addition, stent thrombosis is a severe side effect of drug eluting stents due to inhibition of endothelialization. The aim of this study was to develop and test a stent surface polymer, where cytotoxic drugs are covalently conjugated to the surface and released by proteases selectively secreted by proliferating smooth muscle cells. Resting and proliferating human coronary artery smooth muscle cells (HCASMC) and endothelial cells (HCAEC) were screened to identify an enzyme exclusively released by proliferating HCASMC. Expression analyses and enzyme activity assays verified selective and exclusive activity of the matrix metalloproteinase-9 (MMP-9) in proliferating HCASMC. The principle of drug release exclusively triggered by proliferating HCASMC was tested using the biodegradable stent surface polymer poly-l-lactic acid (PLLA) and the MMP-9 cleavable peptide linkers named SRL and AVR. The specific peptide cleavage by MMP-9 was verified by attachment of the model compound fluorescein. Fluorescein release was observed in the presence of MMP-9 secreting HCASMC but not of proliferating HCAEC. Our findings suggest that cytotoxic drug conjugated polymers can be designed to selectively release the attached compound triggered by MMP-9 secreting smooth muscle cells. This novel concept may be beneficial for stent endothelialization thereby reducing the risk of restenosis and thrombosis.

The implantation of drug-eluting stents (DES) is a common therapy for atherosclerotic occlusions whereby the loaded compounds often exhibit unspecific cytotoxicity. In detail, clinical success is hampered by simultaneous reduction of smooth muscle (SMC) and endothelial cell proliferation, the latter being associated with reduced endothelialization resulting in prothrombotic surfaces. It was aim of this study to investigate statins as DES candidates to identify mechanisms contributing to a previously reported cell-selectivity. In vitro proliferation assays confirmed the SMC-specific activity of atorvastatin. Due to similar expression levels of the drug target HMG-CoA reductase in both cell types, cellular accumulation of atorvastatin was assessed, revealing enhanced uptake in SMCs most likely driven by significant expression of OATP2B1, a transporter known to be involved in pharmacokinetics. The higher accumulation of atorvastatin and its associated specific effect in SMCs was further confirmed by data dem...

Enhanced proliferation of human coronary artery smooth muscle cells (HCASMCs) and thereby formation of neointima is one of the factors contributing to failure of coronary stents. Even if the use of drug eluting stents (DES) and thereby the local delivery of cytotoxic compounds has significantly improved the clinical outcome, unselective cytotoxic effects are assumed to hamper clinical success. Novel pharmacological approaches are required to enhance cellular selectivity of locally delivered drugs. Cell specific overexpression of a drug transporter could be used to enhance cellular accumulation and therefore cell specificity. In the herein reported study we tested the possibility of cell specific transporter expression to enhance drug effects in HCASMCs. We generated adenoviral constructs to overexpress the organic cation transporter 1 (OCT1) under control of the promoter of SM22α, which had been previously reported as muscle cell specific gene. First the activity of the SM22α-promoter was assessed in various cell types supporting the notion of muscle cell specificity. Subsequently, the activity of the transporter was compared in infected HCAECs and HCASMCs revealing enhanced accumulation of substrate drugs in HCASMCs in presence of the SM22α-promoter. Testing the hypothesis that this kind of targeting might serve as a mechanism for cell-specific drug effects, we investigated the impact on paclitaxel treatment in HCASMC and HCAECs, showing significantly increased antiproliferative activity of this substrate drug on muscle cells. Taken together, our findings suggest that cell-specific expression of transport proteins serves as mechanism governing the uptake of cytotoxic compounds for a selective impact on targeted cells.