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Important antimalarial drugs, including quinolines, act against blood schizonts by interfering with hemoglobin metabolism. To reach their site of action, these compounds have to cross the plasma membrane of red blood cells (RBCs). Organic cation transporters (OCTs) and organic anion transporting polypeptides (OATPs) are important uptake transporters and interesting candidates for local drug transport. We therefore studied their interaction with antimalarial compounds (quinine, chloroquine, mefloquine, pyrimethamine, artemisinin, and artesunate) and characterized the expression of OATP1A2 and OATP2B1 in RBCs. Competition assays using transporter-overexpressing Madin-Darby canine kidney (MDCKII) cells and the model substrate estrone-3-sulfate identified quinine and chloroquine as potent inhibitors of OATP1A2 function (IC50quinine: 0.7 ± 1.2 µM; chloroquine: 1.0 ± 1.5 µM), but no or only moderate effects were observed for OATP2B1. Subsequently, quinine was identified as a substrate of OATP1A2 (Km23.4 µM). The OATP1A2-mediated uptake was sensitive to the OATP1A2-specific inhibitor naringin. Both OATPs were expressed in human RBCs, and ex vivo transport studies demonstrated naringin-sensitive accumulation of quinine in these cells (60 pmol versus 38 pmol/5 × 105RBCs). Additional transport studies using OCT1–3 and organic cation transporter novel type 1 (OCTN1) indicated only significant quinine uptake by OCT1, which was not detected in RBCs. In conclusion, our data demonstrate expression of OATP2B1 and OATP1A2 in RBCs as well as OATP1A2-mediated uptake of quinine. Therefore, modulation of OATP1A2 function may affect quinine uptake into erythrocytes.

Pancreatic adenocarcinoma is one of the malignancies that is highly resistant to therapy and among the leading causes of cancer-related death. Several factors may influence pancreatic cancer resistance, and expression of ATP-binding cassette transport proteins is one of the major mechanisms of drug resistance. Members of this family's C-branch, also referred to as multidrug resistance-associated proteins (MRPs), might be of particular interest because they are able to efflux nucleoside analogs used in the treatment of pancreatic cancer. Expression of MRP1, MRP3, MRP4, and MRP5 in human pancreas and pancreatic carcinoma has been reported. However, contributions of MRPs to chemoresistance of pancreatic cancer are not fully understood. MRP5 mRNA expression in pancreatic adenocarcinoma cell lines correlated significantly with cellular sensitivity to 5-fluorouracil (5-FU) (r = 0.738, p < 0.05). Long-term treatment with 5-FU increased expression of MRP5 by 2.4-fold and was associated with significant drug resistance [IC50values for control and 5-fluorouracil (5-FU)-resistant Patu-T cell lines were 11.3 ± 5.3 and 33.2 ± 6.9 µM, respectively (p < 0.05)]. Consequently, overexpression of MRP5 in Colo-357 cells resulted in significantly reduced accumulation of 5-FU related radioactivity and 5-FU cytotoxicity. Knockdown of MRP5 significantly increased cellular cytotoxicity of 5-FU to Patu-02 cells and enhanced accumulation of radioactivity related to 5-FU and its metabolites. Our results suggest that MRP5 is expressed and functionally active and contributes to variable sensitivities of pancreatic adenocarcinoma cell lines to 5-FU. Further investigations using models that resemble human pancreas tumors are necessary to prove a causative relation between expression and activity of MRP5 and tumor resistance to 5-FU.

The human placenta has both protective and nurturing functions for the fetal organism. Uptake and elimination of xenobiotics and endogenous substances are facilitated by various transport proteins from the solute carrier (SLC) and ABC families, respectively. A functional interaction of uptake and elimination, which is a prerequisite for vectorial transport across cellular barriers, has not been described for placenta. In this study, we examined expression of organic anion transporter (OAT) 4 (SLC22A11), organic anion transporting polypeptide (OATP) 2B1 (SLCO2B1, OATP-B), and breast cancer resistance protein (BCRP) (ABCG2) in human placenta (n = 71) because all three proteins are involved in transmembranal transfer of estrone 3 sulfate (E3S; metabolic product) and dehydroepiandrosterone sulfate (DHEAS; precursor molecule). On the mRNA level, we found a significant correlation of OATP2B1 and BCRP (R2= 0.534; p < 0.01) but not between OAT4 and BCRP (R2= –0.104; p > 0.05). Localization studies confirmed basal expression of OATP2B1 and apical expression of BCRP. To study functional interactions between OATP2B1 and BCRP, we developed a Madin-Darby canine kidney cell model expressing both transport proteins simultaneously (OATP2B1 and BCRP in the basal and apical membrane, respectively). Using this cell model in a transwell system resulted in a significantly increased basal to apical transport of both E3S and DHEAS, when both transporters were expressed with no change of transfer in the apical to basal direction. Taken together, these data show the potential for a functional interaction of OATP2B1 and BCRP in transepithelial transport of steroid sulfates in human placenta.

L-carnitine is assumed to play an important role in fetal development, and there is evidence that carnitine is transported across the placenta. The protein involved in this transfer, however, has not been identified on a molecular level. We therefore characterized localization and function of the carnitine transporter OCTN2 in human placenta. Significant expression of OCTN2 mRNA was detected in human placenta applying real-time polymerase chain reaction technology. Confocal immunofluorescence microscopy using an antibody directed against the carboxy terminus of OCTN2 protein revealed that it is predominantly expressed in the apical membrane of syncytiotrophoblasts. This was confirmed by the costaining of organic anion-transporting polypeptide B and MRP2, which are known to be expressed mainly in the basal and apical syncytiotrophoblasts membrane, respectively. To further support this finding, we performed transport studies using basal and apical placenta membrane vesicles. We could demonstrate that the carnitine uptake into the apical vesicles was about eight times higher compared with the basal ones. Moreover, this uptake was sodium- and pH-dependent with an apparent K(m) value of 21 muM and inhibited by verapamil, which is in line with published data for recombinant OCTN2. Finally, experiments using trophoblasts in cell culture revealed that expression of OCTN2 paralleled human choriogonadotropin production and thus is modulated by cellular differentiation. In summary, we show expression and function of OCTN2 in human placenta. Moreover, several lines of evidence indicate that OCTN2 is localized in the apical membrane of syncytiotrophoblasts, thereby suggesting a major role in the uptake of carnitine during fetal development.