We have demonstrated that reducing zinc availability with the extracellular chelator diethylenetriaminepentaacetic acid (DTPA) causes rapid inhibition of cellular zinc efflux in H4IIE hepatoma cells but increases zinc efflux in primary hepatocytes. Similar differences were also observed between the rat anterior pituitary cell line GH3 and primary anterior pituitary cells. We hypothesized that the difference between the transformed and primary cells is due to differential regulation of ZnT-1 or SLC-30A-1 because this is the only zinc efflux transporter localized to the plasma membrane. The effects of DTPA (50 μM) and zinc (100 μM) treatment on messenger RNA (mRNA) and protein expression and protein localization of ZnT-1 were studied in H4IIE cells and primary hepatocytes. Although zinc tended to increase ZnT-1 mRNA in H4IIE cells, DTPA had no effect on ZnT-1 mRNA and protein expression in either hepatoma cells or hepatocytes. Although ZnT-1 is thought to be localized on the plasma membrane, this localization was not seen in these liver cells where ZnT-1 was distributed throughout the cytoplasm. Vesicular localization of ZnT-1 appeared to increase with zinc treatment. Total zinc content was reduced by DTPA in H4IIE cells. Diethylenetriaminepentaacetic acid also reduced metallothionein 1 mRNA, reflecting this reduction in cellular zinc. We conclude that the rapid homeostatic response of cells to altered zinc availability must be attributed to a transporter other than ZnT-1 or to changes in the activity of ZnT-1 by a novel mechanism.
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