A confocal immunofluorescence study on the expression and localisation of zinc homeostasis- related proteins in breast and prostate cancer cells

Zinc transport proteins (ZIP/ZnT), metallothioneins and protein kinase CK2 are involved in maintaining intracellular zinc balance for the cell. Dysregulation of cellular zinc homeostasis is the key feature of breast and prostate cancer cells. Here, we attempted to follow up our previous finding that ZIP12, ZnT1 and metallothionein (MT2A) were differentially expressed at the gene level in breast and prostate cancer cells in response to extracellular zinc exposure. By immunofluorescence confocal microscopy, we determined the protein expression and subcellular localisation of ZIP12, ZnT1, MT2A, as well as the catalytic subunits CK2α/α' and the regulatory subunit CK2β of CK2 in breast and prostate cancer cells. The findings demonstrated that without extracellular zinc exposure, ZIP12 was up-regulated in normal breast epithelial cells but down-regulated in breast cancer cells, in contrast to its overexpression in prostate cancer cells but a reduction in normal prostate cells. Upon extracellular zinc exposure, ZIP12 was conspicuously localised in the plasma membrane of breast cancer cells but not in normal breast, nor in cancerous or noncancerous prostate cells. ZnT1 is only localised in the plasma membrane of breast cancer cells but not in the other cells. MT2A is distinctively seen close to the plasma membrane in breast cancer cells. Intriguingly, protein kinase CK2 was demonstrated to be an ecto-kinase by the localisation of its subunits CK2α/α' and CK2β in proximity to the plasma membrane of breast cancer cells. Taken together, these novel findings provide molecular details for our understanding of zinc homeostasis in breast and prostate cancer cells.