Abstract
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 Km value of 21 μM 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.
Footnotes
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This work was supported by the German Federal Ministry of Education and Research (NBL3 program, reference 01 ZZ 0103) and by the Karl and Lore Klein-Stiftung, Oy-Mittelberg, Germany.
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doi:10.1124/dmd.104.001560.
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ABBREVIATIONS: FCS, fetal calf serum; DMEM, Dulbecco's modified Eagle's medium; PCR, polymerase chain reaction; OATP-B, organic anion-transporting polypeptide B; PBS, phosphate-buffered saline; hCG, human choriogonadotropin; TEA, tetraethylammonium.
- Received July 23, 2004.
- Accepted October 4, 2004.
- The American Society for Pharmacology and Experimental Therapeutics
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