Sodium regulates PLC and IP3R-mediated calcium signalling in invasive breast cancer cells

Intracellular Ca 2+ signalling 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 tumours, and that aberrant Ca 2+ signalling 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 hyperpolarisation (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 tumours where [Na + ] i is perturbed.