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Abnormally elevated expression of tRNA is a common feature of breast tumours. Rather than a uniform increase in all tRNAs, some are deregulated more strongly than others. Elevation of particular tRNAs has been associated with poor prognosis for patients, and experimental models have demonstrated the ability of some tRNAs to promote proliferation or metastasis. Each tRNA isoacceptor is encoded redundantly by multiple genes, which are commonly dispersed across several chromosomes. An unanswered question is whether the consistently high expression of a tRNA in a cancer type reflects the consistent activation of the same members of a gene family, or whether different family members are activated from one patient to the next. To address this question, we interrogated ChIP-seq data to determine which tRNA genes were active in individual breast tumours. This revealed that distinct sets of tRNA genes become activated in individual cancers, whereas there is much less variation in the expression patterns of families. Several pathways have been described that are likely to contribute to increases in tRNA gene transcription in breast tumours, but none of these can adequately explain the observed variation in the choice of genes between tumours. Current models may therefore lack at least one level of regulation.

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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 cells and Her2+ SKBR3 cells, but not ER+ MCF7 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.