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APPETITE, OBESITY, DIGESTION, AND METABOLISM

Carnitine/xenobiotics transporters in the human mammary gland epithelia, MCF12A

¹Division of Clinical Pharmacology and Toxicology, Department of Pediatrics, Research Institute, Hospital for Sick Children, ²University of Toronto, Toronto, Ontario, Canada; ³Department of Plastic Surgery, Sunnybrook and Women's College Health Sciences Centre, Toronto, Ontario, Canada; ⁴Faculty of Pharmaceutical Sciences, Kanazawa University, Kanazawa, and ⁵Chugai Pharmaceutical Company Limited, Ibaragi, Japan

Submitted 7 February 2005 ; accepted in final form 23 September 2005

The barrier function of the human mammary gland collapses if challenged with cationic drugs, causing their accumulation in milk. However, underlying molecular mechanisms are not well understood. To gain insight into the mechanism, we characterized transport of organic cations in the MCF12A human mammary gland epithelial cells, using carnitine and tetraethylammonium (TEA) as representative nutrient and xenobiotics probes, respectively. Our results show that the mammary gland cells express mRNA and proteins of human (h) novel organic cation transporters (OCTN) 1 and hOCTN2 (a Na⁺-dependent carnitine carrier with Na⁺-independent xenobiotics transport function), which belong to the solute carrier superfamily (SLC) of transporters. Other SLC OCTs such as hOCT1 and extraneuronal monoamine transporter (EMT)/hOCT3 are also expressed at mRNA levels, but hOCT2 was undetectable. We further showed mRNA expression of ATB⁰⁺ (an amino acid transporter with a Na⁺/Cl^–-dependent carnitine transport activity), and Fly-like putative transporter 2/OCT6 (a splice variant of carnitine transporter 2: a testis-specific Na⁺-dependent carnitine transporter). TEA uptake was pH dependent. Carnitine uptake was dependent on Na⁺, and partly on Cl^–, compatible with hOCTN2 and ATB⁰⁺ function. Modeling analyses predicted multiplicity of the uptake mechanisms with the high-affinity systems characterized by K_m of 5.1 µM for carnitine and 1.6 mM for TEA, apparently similar to the reported hOCTN2 parameter for carnitine, and that of EMT/hOCT3 for TEA. Verapamil, cimetidine, carbamazepine, quinidine, and desipramine inhibited the carnitine uptake but required supratherapeutic concentrations, suggesting robustness of the carnitine uptake systems against xenobiotic challenge. Our findings suggest functional roles of a network of multiple SLC organic cation/nutrient transporters in human mammary gland drug transfer.

organic cation transporter; milk; human; novel organic cation transporter 2; amino acid transporter B⁰⁺

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