Zn Transporter Levels and Localization Change Throughout Lactation in Rat Mammary Gland and Are Regulated by Zn in Mammary Cells

Mechanisms regulating the decrease in milk zinc (Zn) concentration that occurs during the course of lactation are currently unknown. We demonstrated Zn transporter expression (Zip3, ZnT-1, ZnT-2 and ZnT-4) in rat mammary gland during mid-lactation and we hypothesize that changes in the levels and localization of these transporters play a role in the longitudinal decrease in milk Zn concentration. Furthermore, we suggest that cellular Zn levels can mediate these responses and determined the effects of Zn exposure on Zn transporter expression and localization in cultured mouse mammary epithelial (HC11) cells. Although the milk Zn level declined, mammary gland Zn, ZnT-1 and ZnT-2 mRNA levels increased through mid-lactation; ZnT-4 was unaltered and ZIP3 decreased. Zip3 protein decreased through lactation and localized to the basolateral membrane of rat mammary cells. Although ZnT-1 and ZnT-4 protein increased, data indicate that these proteins are members of larger complexes whose levels change throughout lactation. ZnT-2 protein decreased, whereas apical membrane staining of ZnT-1, ZnT-2 and ZnT-4 was low by the end of lactation. Zn-treated HC11 cells had lower 65Zn uptake and ZIP3 mRNA levels and higher 65Zn export, ZnT-1 and ZnT-2 mRNA levels than untreated cells. Zn treatment resulted in relocalization from the plasma membrane (Zip3) or Golgi apparatus (ZnT-4) to an intracellular compartment, from an intracellular compartment toward the plasma membrane (ZnT-2) or from a perinuclear to an intracellular compartment (ZnT-1). The results from this study indicate that the decrease in milk Zn concentration that occurs throughout lactation is in part a result of changing Zn transporter protein levels and cellular localization, possibly as a consequence of increasing mammary gland Zn concentration.