title: ZnT5 variant B is a bidirectional zinc transporter and mediates zinc uptake in human intestinal Caco-2 cells.
abstract: Zinc is an essential micronutrient, so it is important to elucidate the molecular mechanisms of zinc homeostasis, including the functional properties of zinc transporters. Mammalian zinc transporters are classified in two major families: the SLC30 (ZnT) family and the SLC39 family. The prevailing view is that SLC30 family transporters function to reduce cytosolic zinc concentration, either through efflux across the plasma membrane or through sequestration in intracellular compartments, and that SLC39 family transporters function in the opposite direction to increase cytosolic zinc concentration. We demonstrated that human ZnT5 variant B (ZnT5B (hZTL1)), an isoform expressed at the plasma membrane, operates in both the uptake and the efflux directions when expressed in Xenopus laevis oocytes. We measured increased activity of the zinc-responsive metallothionein 2a (MT2a) promoter when ZnT5b was co-expressed with an MT2a promoter-reporter plasmid construct in human intestinal Caco-2 cells, indicating increased total intracellular zinc concentration. Increased cytoplasmic zinc concentration mediated by ZnT5B, in the absence of effects on intracellular zinc sequestration by the Golgi apparatus or endoplasmic reticulum, was also confirmed by a dramatically enhanced signal from the zinc fluorophore Rhodzin-3 throughout the cytoplasm of Caco-2 cells overexpressing ZnT5B at the plasma membrane when compared with control cells. Our findings demonstrate clearly that, in addition to mediating zinc efflux, ZnT5B at the plasma membrane can function to increase cytoplasmic zinc concentration, thus indicating a need to reevaluate the current paradigm that SLC30 family zinc transporters operate exclusively to decrease cytosolic zinc concentration.
authors.affiliation: The Human Nutrition Research Centre, School of Dental Sciences, Institute for Cell and Molecular Biosciences, School of Clinical Medical Sciences, Newcastle University, Newcastle upon Tyne, UK. r.a.valentine@ncl.ac.uk
authors.forename: Ruth A
authors.initials: RA
authors.lastname: Valentine
authors.forename: Kelly A
authors.initials: KA
authors.lastname: Jackson
authors.forename: Graham R
authors.initials: GR
authors.lastname: Christie
authors.forename: John C
authors.initials: JC
authors.lastname: Mathers
authors.forename: Peter M
authors.initials: PM
authors.lastname: Taylor
authors.forename: Dianne
authors.initials: D
authors.lastname: Ford
mesh_terms: D000818:Animals; D001692:Biological Transport; D018938:Caco-2 Cells / Q000378:metabolism; D027682:Cation Transport Proteins / Q000494:pharmacology*; D002462:Cell Membrane / Q000378:metabolism; D003600:Cytosol; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D008668:Metallothionein / Q000235:genetics; D009865:Oocytes / Q000378:metabolism; D011401:Promoter Regions, Genetic; D014982:Xenopus laevis / Q000378:metabolism; D015032:Zinc / Q000008:administration & dosage / Q000378:metabolism*; D001616:beta-Galactosidase / Q000378:metabolism
chemical_list: D027682:Cation Transport Proteins; C491053:MT2A protein, human; C457225:SLC30A5 protein, human; D008668:Metallothionein; D001616:beta-Galactosidase; D015032:Zinc