Intracellular Ca²⁺ signaling and Na⁺ homeostasis are inextricably linked via ion channels and co-transporters, with alterations in the concentration of one ion having profound effects on the other. Evidence indicates that intracellular Na⁺ concentration ([Na⁺ ]_i ) is elevated in breast tumors, and that aberrant Ca²⁺ signaling regulates numerous key cancer hallmark processes. The present study therefore aimed to determine the effects of Na⁺ depletion on intracellular Ca²⁺ handling in metastatic breast cancer cell lines. The relationship between Na⁺ and Ca²⁺ was probed using fura-2 and SBFI fluorescence imaging and replacement of extracellular Na⁺ with equimolar N-methyl-D-glucamine (0Na⁺ /NMDG) or choline chloride (0Na⁺ /ChoCl). In triple-negative MDA-MB-231 and MDA-MB-468 cells and Her2+ SKBR3 cells, but not ER+ MCF-7 cells, 0Na⁺ /NMDG and 0Na⁺ /ChoCl resulted in a slow, sustained depletion in [Na⁺ ]_i that was accompanied by a rapid and sustained increase in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ). Application of La³⁺ in nominal Ca²⁺ -free conditions had no effect on this response, ruling out reverse-mode NCX activity and Ca²⁺ entry channels. Moreover, the Na⁺ -linked [Ca²⁺ ]_i increase was independent of membrane potential hyperpolarization (NS-1619), but was inhibited by pharmacological blockade of IP₃ receptors (2-APB), phospholipase C (PLC, U73122) or following depletion of endoplasmic reticulum Ca²⁺ stores (cyclopiazonic acid). Thus, Na⁺ is linked to PLC/IP₃ -mediated activation of endoplasmic reticulum Ca²⁺ release in metastatic breast cancer cells and this may have an important role in breast tumors where [Na⁺ ]_i is perturbed.

Katherine P. Unthank

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