Distinct regulation of cytoplasmic calcium signals and cell death pathways by different plasma membrane calcium ATPase isoforms in MDA-MB-231 breast cancer cells

Plasma membrane calcium ATPases (PMCAs) actively extrude Ca ²⁺ from the cell and are essential components in maintaining intracellular Ca ²⁺ homeostasis. There are four PMCA isoforms (PMCA1-4), and alternative splicing of the PMCAgenes creates a suite of calcium efflux pumps. The role of these different PMCA isoforms in the control of calcium-regulated cell death pathways and the significance of the expression of multiple isoforms of PMCA in the same cell type are not well understood. In these studies, we assessed the impact of PMCA1 and PMCA4 silencing on cytoplasmic free Ca ²⁺ signals and cell viability in MDA-MB-231 breast cancer cells. The PMCA1 isoform was the predominant regulator of global Ca ²⁺ signals in MDA-MB-231 cells. PMCA4 played only a minor role in the regulation of bulk cytosolic Ca ²⁺, which was more evident at higher Ca ²⁺ loads. Although PMCA1 or PMCA4 knockdown alone had no effect on MDA-MB-231 cell viability, silencing of these isoforms had distinct consequences on caspase-independent (ionomycin) and -dependent (ABT-263) cell death. PMCA1 knockdown augmented necrosis mediated by the Ca ²⁺ ionophore ionomycin, whereas apoptosis mediated by the Bcl-2 inhibitor ABT-263 was enhanced by PMCA4 silencing. PMCA4 silencing was also associated with an inhibition of NFκB nuclear translocation, and an NFκB inhibitor phenocopied the effects of PMCA4 silencing in promoting ABT-263-induced cell death. This study demonstrates distinct roles for PMCA1 and PMCA4 in the regulation of calcium signaling and cell death pathways despite the widespread distribution of these two isoforms. The targeting of some PMCA isoforms may enhance the effectiveness of therapies that act through the promotion of cell death pathways in cancer cells.