Expression, Localization, and Function of the Carnitine Transporter Octn2 (Slc22a5) in Human Placenta

Abstract

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 sodiumand pH-dependent with an apparent K m value of 21 M 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.Carnitine plays an important physiological role, in particular, in ␤-oxidation because it facilitates long-chain fatty acid transport across the inner mitochondrial membrane. Moreover, carnitine is involved in intracellular coenzyme A homeostasis and functions as an antioxidant (Bremer, 1983;Arduini et al., 1992;Pons and De Vivo, 1995). Only a few organs like brain, liver, and kidney have the ability to biosynthesize carnitine (Bremer, 1983), whereas other tissues like skeletal and heart muscles, where ␤-oxidation plays a major role in energy metabolism, are highly dependent on active carnitine uptake from blood to maintain their carnitine steady-state concentration (Siliprandi et al., 1989).Recent studies describe the organic cation transporter novel type II (OCTN2) as a high affinity uptake system for carnitine. The OCTN2 cDNA codes for 557 amino acids consisting of 12 putative transmembrane domains with a predicted molecular mass of 63 kDa . The transport of carnitine is sodium-dependent (Tamai et al., 1998), whereas other compounds such as tetraethylammonium are transported by OCTN2 in a sodium-independent way (Ohashi et al., 2001).Besides its physiological function, OCTN2 is of pharmacological relevance. Drugs like verapamil, pyrilamine, and ␤-lactam antibiotics have been characte...

Effect of Gestational Diabetes on Circulating Levels of Maternal and Neonatal Carnitine

“…This finding is suggestive of an active, carrier-mediated transplacental transfer of FC to the fetus. The active transport of carnitine through the placenta is supported by experimental studies documenting the presence of the carnitine transporter organic cation transporter novel type II (OCTN2) in the placenta [29]. Levels of triglycerides and FAs were decreased in the neonates compared to the respective maternal levels.…”

Determination of Carnitine Ester Patterns During the Second Half of Pregnancy, at Delivery, and in Neonatal Cord Blood by Tandem Mass Spectrometry: Complex and Dynamic Involvement of Carnitine in the Intermediary Metabolism

“…In general, the placental transport of carnitine and carnitine esters is carrier mediated. The involvement of the OCTN2 Na ϩ -dependent, high-affinity carnitine transporter and expression of the ATB (0,ϩ) amino acid transporter with broad substrate specificity are documented (16,32).…”

“…Fetal carnitine is derived from the mother via transplacental transfer of carnitine (13). The Na ϩ -dependent high-affinity carnitine transporter OCTN2 (14,15) is expressed in human placenta (16), and placental OCTN2 is necessary for the accumulation of carnitine in placenta and fetus, and, consequently, for the oxidation of long-chain fatty acids in fetal-placental unit (17,18). Studies on OCTN2 -/-mice suggest that the placental carnitine synthesis might depend on the carnitine status of the fetus and/or the mother (13).…”

Excretion of biliary compounds during intrauterine life

“…Examples of OCT3 substrates include cimetidine, MPP + , agmatine, tetraethylammonium, and prazosin [131] . The sodium-dependent carnitine transporter (OCTN2) also belongs to the SLC22A family and is expressed in human placenta [132] . OCTN2 transports a variety of organic cations including tetraethylammonium, nicotine, MPP + , pyrilamine, cimetidine, clonidine, procainamide, quinidine, quinine, and verapamil [133] and certain β-lactam antibiotics of zwitterionic nature [134] .…”