The calcium pump plasma membrane Ca2+-ATPase 2 (PMCA2) regulates breast cancer cell proliferation and sensitivity to doxorubicin

Abstract

Regulation of Ca2+ transport is vital in physiological processes, including lactation, proliferation and apoptosis. The plasmalemmal Ca 2+ pump isoform 2 (PMCA2) a calcium ion efflux pump, was the first protein identified to be crucial in the transport of Ca 2+ ions into milk during lactation in mice. In these studies we show that PMCA2 is also expressed in human epithelia undergoing lactational remodeling and also report strong PMCA2 staining on apical membranes of luminal epithelia in approximately 9% of human breast cancers we assessed. Membrane protein expression was not significantly associated with grade or hormone receptor status. However, PMCA2 mRNA levels were enriched in Basal breast cancers where it was positively correlated with survival. Silencing of PMCA2 reduced MDA-MB-231 breast cancer cell proliferation, whereas silencing of the related isoforms PMCA1 and PMCA4 had no effect. PMCA2 silencing also sensitized MDA-MB-231 cells to the cytotoxic agent doxorubicin. Targeting PMCA2 alone or in combination with cytotoxic therapy may be worthy of investigation as a therapeutic strategy in breast cancer. PMCA2 mRNA levels are also a potential tool in identifying poor responders to therapy in women with Basal breast cancer.The enrichment of milk with calcium is vital to neonatal and infant development. The process by which calcium ions are transferred from the maternal blood supply into milk is highly coordinated, and involves specific calcium-permeable ion channels, and calcium pumps of both the secretory pathway and plasma membrane [1][2][3][4][5][6] . Recent studies have associated many of these specific calcium channels and pumps in processes important in breast cancer progression. Calcium signaling is a key regulator of many processes important in tumor progression including cellular proliferation, sensitivity to death stimuli, migration and invasion 7 . Indeed, specific calcium channels and pumps are identified as potential therapeutic targets in a number of cancer types including those of the prostate and breast 8,9 . Expression of the canonical store-operated Ca 2+ channel Orai1 10 is increased during lactation in mice 1 . In vitro models 11,12 and more recently Orai1-null mice studies 3 , suggest that Orai1 plays an important role in the basolateral influx of Ca 2+ across mammary epithelial cells during lactation. Orai1 is also a potential drug

PMCA2 silencing potentiates MDA-MB-231 breast cancer cell death initiated with the Bcl-2 inhibitor ABT-263

“…This result may be surprising, however, not entirely unexpected because PMCA2 expression is relatively low in MDA-MB-231 cells compared with other isoforms [4]. PMCA1, which is the predominate isoform expressed in MDA-MB-231 cells, when knocked down has a pronounced effect on the shape of global [Ca 2+ ]CYT signals generated with many stimuli [4,7].…”

“…PMCA2 levels were downregulated in MDA-MB-231 cells according to methods previously described [4,7] using ON-TARGETplus™ Human SMARTpool siRNAs (Thermo Scientific, Waltham, MA, USA). These siRNAs consist of four pooled sequences rationally designed to minimize off-target effects [13][14][15].…”

“…Aberrant overexpression of PMCA2 in some breast cancer cell lines [2] and the positive correlation between PMCA2 protein expression and tumor characteristics including tumor grade and HER2 status in clinical breast cancer cases [3] suggests that PMCA2 could facilitate disease progression [3]. More recently, PMCA2 mRNA levels were assessed in breast cancers grouped by molecular subtype [4]. PMCA2 mRNA was higher in basal breast cancers compared to HER2, luminal A and luminal B subtypes [4].…”

“…One cell line widely used as a model for basal breast cancer subtypes, which overlaps with breast cancers that are negative for hormone receptor expression and HER2 amplification is the MDA-MB-231 breast cancer cell line [6]. Studies in this cell line demonstrate PMCA2 silencing attenuates proliferation and potentiates the anti-proliferative action of low concentrations of the cytotoxic doxorubicin [4]. Since neither PMCA1 nor PMCA4…”

“…More recently, PMCA2 mRNA levels were assessed in breast cancers grouped by molecular subtype [4]. PMCA2 mRNA was higher in basal breast cancers compared to HER2, luminal A and luminal B subtypes [4]. Although a feature of some breast cancers, only a few studies have evaluated the functional significance of PMCA2 upregulation in the context of cell death in breast cancer; and detailed assessment of PMCA2 through overexpression or silencing techniques has been limited to luminal-like breast cancer cell lines that express at least one hormone receptor (estrogen or progesterone receptors) or those with HER2 amplification [3,5].…”