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 coexpressed 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.

Demonstration and Characterization of the Heterodimerization of ZnT5 and ZnT6 in the Early Secretory Pathway

“…Human ZnT5 has one splice variant, ZnT5B (hZTL1), that has quite different characteristics from hZnT5 (38,39). ZnT5B (hZTL1) is localized to the plasma membrane and functions to enable zinc uptake from the extracellular environment -the opposite direction of zinc transport by hZnT5/hZnT6 heterodimers.…”

Identification of the Zn2+ Binding Site and Mode of Operation of a Mammalian Zn2+ Transporter

“…Among the most striking are decreased bone density, reduced weight, and fatal, male-specific, cardiac bradyarrhythmia (9). ZnT5 has been also shown to mediate Zn 2ϩ uptake (10). It has also been suggested that ZnT5 subunits assemble with those of ZnT6 and that this hetero-oligomerization is required for the activation of the Zn 2ϩ -dependent alkaline phosphatases (11).…”

Does promoter methylation of the SLC30A5 (ZnT5) zinc transporter gene contribute to the ageing-related decline in zinc status?

“…Mammalian Zn transporters play a key role in the tight regulation of Zn homeostasis (18) and are classified into two metal-transporter families, ZRT or IRT-like proteins (ZIP transporters; classified as solute carrier family SLC39) and the cation diffusion facilitators (ZnT transporters; classified as solute carrier family SLC30). Members of the SLC39 family have been found to promote influx of Zn into the cytosol of the cell, whereas those of the SLC30 family reduce cytosolic Zn concentration by either Zn efflux from the cell or the sequestration of Zn into intracellular compartments (25) . To date, a number of human Zn transporters have been characterised.…”

“…The human splice variant, ZnT5 variant B, acts as a bidirectional Zn transporter when expressed in Xenopus laevis oocytes, i.e. it is active in both the uptake and efflux of Zn (25) . The bidirectional function of ZnT5 suggests a possible role in dietary Zn absorption.…”