Two discrete zinc transporter‐2 (ZnT2) variants integrate zinc secretion and efflux in mammary epithelial cells

Abstract

The lactating mammary gland secretes an extraordinary amount of zinc (Zn) (1‐2 mg Zn) into milk each day. Thus, the mammary gland has developed specialized mechanisms to regulate the transfer of such a large amount of Zn. Zinc transport is regulated by Zn‐specific transporters. We previously determined that women with an amino acid substitution in ZnT2 had significantly lower milk Zn concentration verifying that ZnT2 plays a major role in mammary gland Zn secretion. The primary objective of this study was to further characterize the role of ZnT2 in mammary gland Zn secretion. Using a cultured mammary epithelial cell model (HC11), ZnT2 was detected in intracellular vesicles and at the plasma membrane using confocal microscopy. Two ZnT2 protein variants with molecular masses of ~42 and ~32 kDa were identified. Expression of the 42 kDa variant was higher in secreting relative to non‐secreting cells while abundance of the 32 kDa variant was not affected. Over‐expression of both variants resulted in increased Zn transport relative to mock‐transfected cells indicating both proteins are functional Zn transporters. The 42 kDa variant was localized to late endosomal/secretory vesicles in HEK293 cells expressing a 42 kDa ZnT2‐HA fusion protein. In contrast, the 32 kDa variant was detected at the plasma membrane by cell surface biotinylation and confirmed in HEK293 cells expressing a 32 kDa ZnT2‐HA fusion protein. In summary, expression of two ZnT2 variants in mammary cells likely integrates hormonally stimulated Zn secretion and constitutive Zn efflux to provide optimal Zn for secretion into milk. Supported by UCD intramural grants and PSU to SLK.