Selectivity of plasma membrane calcium ATPase (PMCA)-mediated extrusion of toxic divalent cations in vitro and in cultured cells

Abstract

In the recent years, the toxicity of certain divalent cations has been associated with the alteration of intracellular Ca²⁺ homeostasis. Among other mechanisms, these cations may affect the functionality of certain Ca²⁺-binding proteins and/or Ca²⁺ pumps. The plasma membrane calcium pump (PMCA) maintains Ca²⁺ homeostasis in eukaryotic cells by mediating the efflux of this cation in a process coupled to ATP hydrolysis. The aim of this work was to investigate both in vitro and in cultured cells if other divalent cations (Sr²⁺, Ba²⁺, Co²⁺, Cd²⁺, Pb²⁺ or Be²⁺) could be transported by PMCA. Current results indicate that both purified and intact cell PMCA transported Sr²⁺ with kinetic parameters close to those of Ca²⁺ transport. The transport of Pb²⁺ and Co²⁺ by purified PMCA was, respectively, 50 and 75% lower than that of Ca²⁺, but only Co²⁺ was extruded by intact cells and to a very low extent. In contrast, purified PMCA-but not intact cell PMCA-transported Ba²⁺ at low rates and only when activated by limited proteolysis or by phosphatidylserine addition. Finally, purified PMCA did not transport Cd²⁺ or Be²⁺, although minor Be²⁺ transport was measured in intact cells. Moreover, Cd²⁺ impaired the transport of Ca²⁺ through various mechanisms, suggesting that PMCA may be a potential target of Cd²⁺-mediated toxicity. The differential capacity of PMCA to transport these divalent cations may have a key role in their detoxification, limiting their noxious effects on cell homeostasis.

Keywords: Calcium homeostasis; Divalent cation toxicity; Ion transport; Plasma membrane calcium pump.