Plasma membrane calcium channels in the regulation of hypoxia events in breast cancer cells

Introduction: Calcium signalling plays a crucial role in a variety of cellular processes important in tumorigenesis including migration, invasion and proliferation. Calcium influx pathways in cancer cells are often mediated by plasma membrane calcium channels including Orai proteins and transient receptor potential (TRP) channels. Hypoxia is a hallmark of the tumour microenvironment and is associated with increased metastasis and poor patient survival. The association between hypoxia and Ca²⁺ influx in breast cancer cells has not been fully explored.

Aims: To assess the role of TRP and Orai-mediated calcium entry in hypoxia mediated cellular events in basal breast cancer cells.

Methods: Basal breast cancer cell lines assessed included MDA-MB-468, HCC1569 and MDA-MB-231. Pharmacological activators or inhibitors or siRNA were used to activate or block channel activity or expression. A Fluorescence Imaging Plate Reader (FLIPRTETRA) was used to assess intracellular free Ca²⁺ ([Ca²⁺]CYT). RNA and protein levels were quantified using real-time RT-PCR and immunoblotting respectively. Real time RT-PCR and/or next-generation sequencing was used to identify targets that were sensitive to ion channel modulation during hypoxia.

Results: Specific TRP channels were identified as regulators of vimentin protein expression induced by hypoxia and epidermal growth factor (EGF) in basal breast cancer cells. Orai proteins were demonstrated to be isoform dependent regulators of cellular events associated with cell migration and inflammatory responses during hypoxia.

Discussion: The isoform specific roles of Orai and TRP proteins suggests that the hypoxic phenotype exploits different ion channels to achieve different cellular functions. These studies suggest that specific Orai and TRP isoforms may represent potential therapeutic targets for the control of processes associated with breast cancer progression.