The Role of ZnR/GPR39 in Regulation of Na⁺/K⁺ ATPase in Salivary Gland Duct Cells.

Human tissues and fluids contain large concentrations of zinc, which fulfills crucial roles in cell functionality, growth, proliferation and survival. Zinc may also be found as a free ion, Zn²⁺, in organelles. Particularly, Zn²⁺ is selectively concentrated in secretory granules of the salivary glands. In the salivary duct cell line, HSY, extracellular Zn²⁺ activates a Zn²⁺ sensing receptor, ZnR/GPR39. Based on the role of Zn²⁺ in regulating salivary contents, we hypothesized that ZnR/GPR39 may regulate ion transport in these cells. Cellular pH imaging studies, using NH₄⁺ as a surrogate to K⁺, suggest that ZnR/GPR39 signaling leads to enhanced K⁺ transport. We therefore studied the role of the Na⁺/K⁺ ATPase pump, which is present on basolateral plasma membrane and regulates the osmolarity of the cytosol by controlling intracellular solute concentration. The ZnR/GPR39-dependent ion transporter activity is reduced by using a specific inhibitor of the pump Ouabain. Silencing the α1 subunit of Na⁺/K⁺ ATPase using siRNA constructs abolished the enhanced ion transporter activity. To study the link between the ZnR/GPR39 and Na⁺/K⁺ ATPase pump activity we used cyclosporine A, an inhibitor of calcineurin. Zn²⁺-dependent upregulation of the ion transporter activity was partially attenuated suggesting that FXYD-dependent activation may regulate the pump activity. Thus we conclude that ZnR/GPR39 activates the Na⁺/K⁺ ATPase, and may regulate duct secretory activity, that is important for regulation of salivary contents.