High-content kinetic calcium imaging in drug-sensitive and drug-resistant human breast cancer cells

Intracellular calcium (Ca2+)(i) is involved in the regulation of a variety of biological functions in cancer cells, including growth inhibition, tumor invasiveness, and drug resistance. To gain insight into the possible role played by Ca2+ in the development of drug resistance in breast cancer, we performed a comparative high-content analysis of the intracellular Ca2+ dynamics in drug-sensitive human breast cancer MCF-7 cells and five drug-resistant, MCF-7-derived clonal cell lines. Fura-2 single cell ratiometric fluorescence microscopy was used to monitor real-time quantitative changes in cytosolic-free Ca2+ concentration ([Ca2+](i)) upon addition of phosphoinositol-coupled receptor agonists. While the magnitude and the onset kinetics of the [Ca2+](i) rise were similar in drug-sensitive and drug-resistant cell lines, the decay kinetics of the [Ca2+](i) increase was found to be consistently slower in drug-resistant than drug-sensitive cells. Such a delay in reestablishing homeostatic [Ca2+](i) persisted in the absence of extracellular Ca2+ and was independent of the expression or function of specific drug efflux pumps associated with drug resistance. Moreover, intracellular Ca2+ pools releasable by phosphoinositol-coupled receptor agonists or thapsigargin appeared to be differentially shared in drug-sensitive and drug-resistant cells. In light of the clinical relevance that drug resistance has in the treatment of cancer, the molecular and biochemical relationship between alterations in Ca2+ dynamics and drug resistance demands to be further investigated and tested in a wider array of cell types. Automated microscopy will help greatly in this pursuit by facilitating both sample imaging and data analysis, thus allowing high-content as well as high-throughput screening of large sample sets. A protocol for studying [Ca2+](i) kinetics with a commercially available automated imaging platform is described.