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

Intracellular Ca2+ 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 Ca2+ signalling 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 hyperpolarisation (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 tumours where [Na+]i is perturbed.