title: System A amino acid transporter SNAT2 shows subtype-specific affinity for betaine and hyperosmotic inducibility in placental trophoblasts.
abstract: Betaine uptake is induced by hypertonic stress in a placental trophoblast cell line, and involvement of amino acid transport system A was proposed. Here, we aimed to identify the subtype(s) of system A that mediates hypertonicity-induced betaine uptake. Measurement of [(14)C]betaine uptake by HEK293 cells transiently transfected with human or rat sodium-coupled neutral amino acid transporters (SNATs), SNAT1, SNAT2 and SNAT4 revealed that only human and rat SNAT2 have betaine uptake activity. The Michaelis constants (Km) of betaine uptake by human and rat SNAT2 were estimated to be 5.3 mM and 4.6 mM, respectively. Betaine exclusively inhibited the uptake activity of SNAT2 among the rat system A subtypes. We found that rat SNAT1, SNAT2 and SNAT4 were expressed at the mRNA level under isotonic conditions, while expression of SNAT2 and SNAT4 was induced by hypertonicity in TR-TBT 18d-1 cells. Western blot analyses revealed that SNAT2 expression on plasma membrane of TR-TBT 18d-1 cells was more potently induced by hypertonicity than that in total cell lysate. Immunocytochemistry confirmed the induction of SNAT2 expression in TR-TBT 18d-1 cells exposed to hypertonic conditions and indicated that SNAT2 was localized on the plasma membrane in these cells. Our results indicate that SNAT2 transports betaine, and that tonicity-sensitive SNAT2 expression may be involved in regulation of betaine concentration in placental trophoblasts.
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Nishimura
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
authors.forename: Risa
authors.initials: R
authors.lastname: Yagi
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
authors.forename: Mariko
authors.initials: M
authors.lastname: Usuda
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Oda
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
authors.forename: Mai
authors.initials: M
authors.lastname: Yamazaki
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
authors.forename: Sayaka
authors.initials: S
authors.lastname: Suda
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
authors.forename: Yu
authors.initials: Y
authors.lastname: Takahashi
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
authors.forename: Fumiyasu
authors.initials: F
authors.lastname: Okazaki
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan; Department of Pharmacy, Kanazawa University Hospital, Takara-machi 13-1, Kanazawa 920-0841, Japan.
authors.forename: Yoshimichi
authors.initials: Y
authors.lastname: Sai
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan; School of Pharmaceutical Sciences, Teikyo University, Itabashi-ku 173-8605, Tokyo, Japan.
authors.forename: Kei
authors.initials: K
authors.lastname: Higuchi
authors.affiliation: Department of Obstetrics and Gynecology, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku-ku 160-8512, Tokyo, Japan.
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Maruyama
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan. Electronic address: tomi-ms@pha.keio.ac.jp.
authors.forename: Masatoshi
authors.initials: M
authors.lastname: Tomi
authors.affiliation: Faculty of Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
authors.forename: Emi
authors.initials: E
authors.lastname: Nakashima
mesh_terms: D026924:Amino Acid Transport System A / Q000235:genetics / Q000378:metabolism; D026905:Amino Acid Transport Systems / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001622:Betaine / Q000378:metabolism*; D001692:Biological Transport / Q000235:genetics; D002460:Cell Line; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism; D005260:Female; D057809:HEK293 Cells; D006801:Humans; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D014327:Trophoblasts / Q000378:metabolism*
chemical_list: D026924:Amino Acid Transport System A; D026905:Amino Acid Transport Systems; D012333:RNA, Messenger; C498797:SLC38A2 protein, human; D001622:Betaine
keywords: Betaine; Hypertonicity; Induction; Placenta; SNAT2; System A