Sodium regulates PLC and IP(3)R‐mediated calcium signaling in invasive breast cancer cells

Intracellular Ca(2+) 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 Ca(2+) signaling regulates numerous key cancer hallmark processes. The present study therefore aimed to determine the effects of Na(+) depletion on intracellular Ca(2+) handling in metastatic breast cancer cell lines. The relationship between Na(+) and Ca(2+) 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 Ca(2+) concentration ([Ca(2+)](i)). Application of La(3+) in nominal Ca(2+)‐free conditions had no effect on this response, ruling out reverse‐mode NCX activity and Ca(2+) entry channels. Moreover, the Na(+)‐linked [Ca(2+)](i) increase was independent of membrane potential hyperpolarization (NS‐1619), but was inhibited by pharmacological blockade of IP(3) receptors (2‐APB), phospholipase C (PLC, U73122) or following depletion of endoplasmic reticulum Ca(2+) stores (cyclopiazonic acid). Thus, Na(+) is linked to PLC/IP(3)‐mediated activation of endoplasmic reticulum Ca(2+) release in metastatic breast cancer cells and this may have an important role in breast tumors where [Na(+)](i) is perturbed.