Increases in intracellular free Ca2؉ play a major role in many cellular processes. The deregulation of Ca 2؉ signaling is a feature of a variety of diseases, and modulators of Ca 2؉ signaling are used to treat conditions as diverse as hypertension to pain. The Ca 2؉ signal also plays a role in processes important in cancer, such as proliferation and migration. Many studies in cancer have identified alterations in the expression of proteins involved in the movement of Ca 2؉ across the plasma membrane and subcellular organelles. In some cases, these Ca 2؉ channels or pumps are potential therapeutic targets for specific cancer subtypes or correlate with prognosis.

Signals from the tumor microenvironment trigger cancer cells to adopt an invasive phenotype through epithelial-mesenchymal transition (EMT). Relatively little is known regarding key signal transduction pathways that serve as cytosolic bridges between cell surface receptors and nuclear transcription factors to induce EMT. A better understanding of these early EMT events may identify potential targets for the control of metastasis. One rapid intracellular signaling pathway that has not yet been explored during EMT induction is calcium. Here we show that stimuli used to induce EMT produce a transient increase in cytosolic calcium levels in human breast cancer cells. Attenuation of the calcium signal by intracellular calcium chelation significantly reduced epidermal growth factor (EGF)- and hypoxia-induced EMT. Intracellular calcium chelation also inhibited EGF-induced activation of signal transducer and activator of transcription 3 (STAT3), while preserving other signal transduction pathways such as Akt and extracellular signal regulated kinase 1/2 (ERK1/2) phosphorylation. To identify calcium-permeable channels that may regulate EMT induction in breast cancer cells, we performed a targeted siRNA-based screen. We found that transient receptor potential-melastatin-like 7 (TRPM7) channel expression regulated EGF-induced STAT3 phosphorylation and expression of the EMT marker vimentin. While intracellular calcium chelation almost completely blocked the induction of many EMT markers, including vimentin, Twist and N-cadherin, the effect of TRPM7 silencing was specific for vimentin protein expression and STAT3 phosphorylation. These results indicate that TRPM7 is a partial regulator of EMT in breast cancer cells, and that other calcium-permeable ion channels are also involved in calcium-dependent EMT induction. In summary, this work establishes an important role for the intracellular calcium signal in the induction of EMT in human breast cancer cells. Manipulation of calcium signaling pathways controlling EMT induction in cancer cells may therefore be an important therapeutic strategy for preventing metastases.

The spread of cancer cells to distant organs represents a major clinical challenge in the treatment of cancer. Epithelial-mesenchymal transition (EMT) has emerged as a key regulator of metastasis in some cancers by conferring an invasive phenotype. As well as facilitating metastasis, EMT is thought to generate cancer stem cells and contribute to therapy resistance. Therefore, the EMT pathway is of great therapeutic interest in the treatment of cancer and could be targeted either to prevent tumor dissemination in patients at high risk of developing metastatic lesions or to eradicate existing metastatic cancer cells in patients with more advanced disease. In this review, we discuss approaches for the design of EMT-based therapies in cancer, summarize evidence for some of the proposed EMT targets, and review the potential advantages and pitfalls of each approach.

The entry of calcium into the mammary epithelial cell from the maternal plasma (i.e., calcium influx mechanisms) during lactation is poorly understood. As alterations in calcium channels and pumps are a key feature of some cancers, including breast cancer, understanding these calcium influx pathways may have significance beyond mammary biology. We show that the store-operated calcium influx protein, Orai1, is increased during lactation whereas the Orai1 activator Stim1, but not Stim2, is downregulated. Stim2 siRNA reduced basal calcium levels in a lactation model. Our results suggest that calcium influx is remodeled in mammary epithelial cells during lactation, with calcium influx increased through Orai1, activated by Stim2. Breast cancer cell lines had increased levels of ORAI1. ORAI1 siRNA in breast cancer cells reduced storeoperated calcium entry and remodeled the calcium influx associated with invasive stimuli. Analysis of microarray data from 295 breast cancers showed that the transcriptional breast cancer subtype with the poorest prognosis (basal) was associated with an altered relationship between the ORAI1 regulators STIM1 and STIM2, and that women with breast cancers with STIM1 high /STIM2 low tumors had a significantly poorer prognosis. Our studies show that during lactation there is a remodeling in the nature of calcium influx and that alteration in the ORAI1 influx pathway may be a feature of some breast cancers, particularly those with the poorest prognosis. Our studies suggest that this pathway may be a novel therapeutic target for breast cancer treatment in these women. Mol Cancer Ther; 10(3); 448-60. Ó2011 AACR.