Assessment of ORAI1-mediated basal calcium influx in mammary epithelial cells

Differential functional role of Orai1 variants in constitutive Ca2+ entry and calcification in luminal breast cancer cells

Resting cytosolic Ca2+ concentration is tightly regulated to fine-tune Ca2+-dependent cellular functions. Luminal breast cancer cells exhibit constitutive Ca2+ entry mediated by Orai1 and the secretory pathway Ca2+-ATPase, SPCA2, which result in mammary microcalcifications that constitute a prognostic marker of mammary lesions. Two Orai1 isoforms have been identified, the full-length Orai1α, consisting of 301 amino acids, and the short variant, Orai1β, lacking the 63 or 70 N-terminal amino acids comprising residues involved in channel inactivation and binding sites with Orai1 partners. We show that only the mammalian-specific Orai1α rescues SPCA2-dependent constitutive Ca2+ entry in Orai1-knockout MCF7 cells, a widely used luminal breast cancer cell line. FRET analysis and immunoprecipitation revealed that Orai1α shows a greater ability to interact with SPCA2 than Orai1β. Deletion of the first 38 amino acids in Orai1α reduced the interaction with SPCA2 to a similar extent as Orai1β, thus suggesting that the N-terminal 38 amino acids play a relevant role in Orai1α-SPCA2 interaction. Finally, Orai1α, but not Orai1β, rescue the ability of Orai1-deficient cells to form in vitro microcalcifications. These findings provide compelling evidence for a functional role of Orai1α in constitutive Ca2+ entry in MCF7 cells, which might be a target to prevent the development of mammary microcalcifications in luminal breast cancer.

ORAI1-Regulated Gene Expression in Breast Cancer Cells: Roles for STIM1 Binding, Calcium Influx and Transcription Factor Translocation

A remodeling of calcium homeostasis, including calcium influx via store-operated calcium entry (SOCE), is a feature of breast cancers. SOCE is critical to maintain calcium balance in the endoplasmic reticulum calcium store and is an important mechanism for calcium signaling in a variety of cell types, including breast cancer cells. The canonical mechanism of SOCE is stromal interacting molecule 1 (STIM1)-mediated activation of ORAI. Elevated ORAI1 expression is a feature of basal breast cancer cells. However, the role of ORAI1 in the regulation of transcription in breast cancer cells of the basal molecular subtype is still unclear. Using CRISPR-Cas9 gene editing, ORAI1 protein expression was disrupted in MDA-MB-231 and MDA-MB-468 basal breast cancer cells. The ORAI1 wild-type and mutants were reintroduced into ORAI1 knockout cells to study the role of ORAI1 in gene transcriptional regulation. In the absence of calcium store depletion, ORAI1 regulated PTGS2 in MDA-MB-231 cells, and this was dependent on ORAI1 pore function and STIM1 binding. The activation of SOCE by thapsigargin resulted in ORAI1-dependent increases in IL6 transcription in MDA-MB-468 cells; this was also dependent on ORAI1 pore function and STIM1 binding and was associated with the translocation of NFAT1. Given the upregulation of ORAI1 in basal breast cancer cells, our results provide further evidence that ORAI1 may contribute to cancer progression through regulation of gene expression.

Quantification of FAM20A in human milk and identification of calcium metabolism proteins

BACKGROUND

FAM20A, a recently discovered protein, is thought to have a fundamental role in inhibiting ectopic calcification. Several studies have demonstrated that variants of FAM20A are causative for the rare autosomal recessive disorder, enamel-renal syndrome (ERS). ERS is characterized by defective mineralization of dental enamel and nephrocalcinosis suggesting that FAM20A is an extracellular matrix protein, dysfunction of which causes calcification of the secretory epithelial tissues. FAM20A is a low-abundant protein that is difficult to detect in biofluids such as blood, saliva, and urine. Thus, we speculated the abundance of FAM20A to be high in human milk, since the secretory epithelium of lactating mammary tissue is involved in the secretion of highly concentrated calcium. Therefore, the primary aim of this research is to describe the processes/methodology taken to quantify FAM20A in human milk and identify other proteins involved in calcium metabolism.

METHOD

This study used mass spectrometry-driven quantitative proteomics: (1) to quantify FAM20A in human milk of three women and (2) to identify proteins associated with calcium regulation by bioinformatic analyses on whole and milk fat globule membrane fractions.

RESULTS

Shotgun MS/MS driven proteomics identified FAM20A in whole milk, and subsequent analysis using targeted proteomics also successfully quantified FAM20A in all samples. Combination of sample preparation, fractionation, and LC-MS/MS proteomics analysis generated 136 proteins previously undiscovered in human milk; 21 of these appear to be associated with calcium metabolism.

CONCLUSION

Using mass spectrometry-driven proteomics, we successfully quantified FAM20A from transitional to mature milk and obtained a list of proteins involved in calcium metabolism. Furthermore, we show the value of using a combination of both shotgun and targeted driven proteomics for the identification of this low abundant protein in human milk.

Serotonin regulates maternal calcium homeostasis during the perinatal period of sheep

The goal of this experiment was to demonstrate the ability of an infusion of serotonin (5-HT; 5-hydroxytryptamine) precursors to increase 5-HT production during the transition from pregnancy to lactation and its effects on gene expression related to calcium (Ca) transporters in the mammary gland and bone resorption markers in the femur. Thirty pregnant Bamei mutton sheep were randomly assigned to 3 experimental groups. All groups received a daily intravenous infusion of saline (control group; n = 10), saline containing 0.178 mg of L-tryptophan/kg body weight (BW) (TRP group, n = 10) or 0.178 mg of 5-hydroxytryptophan/kg BW (5-HTP group, n = 10), beginning on day 7 of prepartum and continuing until delivery. Serum (pre- and postpartum), milk (postpartum), and femur and mammary gland tissue (day 9) were collected. Sheep infused with 5-HTP had a larger total serum Ca concentration on days 3, 6, 15, and 30 of lactation and total milk Ca concentration on days 3, 6, 12, and 15 of lactation compared with that of the control group. Sheep infused with 5-HTP and TRP increased blood and milk concentrations of 5-HT on days 3, 6, 9, and 30 of lactation and parathyroid hormone-related protein (PTHrP) on day 3 of prepartum and on days 3, 6, and 15 of lactation (P < 0.05). In addition, compared to that of the control group, the TRP or 5-HTP infusion upregulated PTHrP, a sodium/calcium exchanger, plasma membrane Ca2+ ATPase 2, secretory pathway Ca2+ ATPase 1, and calcium sensing receptor mRNA expression in mammary gland and receptor-activated nuclear factor kappa-B ligand mRNA expression in the femur, but had no effect on receptor-activated nuclear factor kappa-B and osteoprotegerin mRNA expression in the femur (P < 0.05). This suggests that 5-HT and PTHrP may be involved in regulating maternal Ca homeostasis during the transition from pregnancy to lactation in the sheep.

NCS-1 expression is higher in basal breast cancers and regulates calcium influx and cytotoxic responses to doxorubicin

Neuronal calcium sensor‐1 (NCS‐1) is a positive modulator of IP₃ receptors and was recently associated with poorer survival in breast cancers. However, the association between NCS‐1 and breast cancer molecular subtypes and the effects of NCS‐1 silencing on calcium (Ca²⁺) signaling in breast cancer cells remain unexplored. Herein, we report for the first time an increased expression of NCS‐1 in breast cancers of the basal molecular subtype, a subtype associated with poor prognosis. Using MDA‐MB‐231 basal breast cancer cells expressing the GCaMP6m Ca²⁺ indicator, we showed that NCS‐1 silencing did not result in major changes in cytosolic free Ca²⁺ increases as a result of endoplasmic reticulum Ca²⁺ store mobilization. However, NCS‐1 silencing suppressed unstimulated basal Ca²⁺ influx. NCS‐1 silencing in MDA‐MB‐231 cells also promoted necrotic cell death induced by the chemotherapeutic drug doxorubicin (1 µm). The effect of NCS‐1 silencing on cell death was phenocopied by silencing of ORAI1, a Ca²⁺ store‐operated Ca²⁺ channel that maintains Ca²⁺ levels in the endoplasmic reticulum Ca²⁺ store and whose expression was significantly positively correlated with NCS‐1 in clinical breast cancer samples. This newly identified association between NCS‐1 and basal breast cancers, together with the identification of the role of NCS‐1 in the regulation of the effects of doxorubicin in MDA‐MB‐231 breast cancer cells, suggests that NCS‐1 and/or pathways regulated by NCS‐1 may be important in the treatment of basal breast cancers in women.

Forms and functions of store-operated calcium entry mediators, STIM and Orai

Calcium signals arise by multiple mechanisms, including mechanisms of release of intracellular stored Ca²⁺, and the influx of Ca²⁺ through channels in the plasma membrane. One mechanism that links these two sources of Ca²⁺ is store-operated Ca²⁺ entry, the most commonly encountered version of which involves the extensively studied calcium-release-activated Ca²⁺ (CRAC) channel. The minimal and essential molecular components of the CRAC channel are the STIM proteins that function as Ca²⁺ sensors in the endoplasmic reticulum, and the Orai proteins that comprise the pore forming subunits of the CRAC channel. CRAC channels are known to play significant roles in a wide variety of physiological functions. This review discusses the multiple forms of STIM and Orai proteins encountered in mammalian cells, and discusses some specific examples of how these proteins modulate or mediate important physiological processes.

Constitutive calcium entry and cancer: updated views and insights

Tight control of basal cytosolic Ca²⁺ concentration is essential for cell survival and to fine-tune Ca²⁺-dependent cell functions. A way to control this basal cytosolic Ca²⁺ concentration is to regulate membrane Ca²⁺ channels including store-operated Ca²⁺ channels and secondary messenger-operated channels linked to G-protein-coupled or tyrosine kinase receptor activation. Orai, with or without its reticular STIM partner and Transient Receptor Potential (TRP) proteins, were considered to be the main Ca²⁺ channels involved. It is well accepted that, in response to cell stimulation, opening of these Ca²⁺ channels contributes to Ca²⁺ entry and the transient increase in cytosolic Ca²⁺ concentration involved in intracellular signaling. However, in various experimental conditions, Ca²⁺ entry and/or Ca²⁺ currents can be recorded at rest, without application of any experimental stimulation. This led to the proposition that some plasma membrane Ca²⁺ channels are already open/activated in basal condition, contributing therefore to constitutive Ca²⁺ entry. This article focuses on direct and indirect observations supporting constitutive activity of channels belonging to the Orai and TRP families and on the mechanisms underlying their basal/constitutive activities.

Calcium-Sensing Receptor in Breast Physiology and Cancer

The calcium-sensing receptor (CaSR) is expressed in normal breast epithelial cells and in breast cancer cells. During lactation, activation of the CaSR in mammary epithelial cells increases calcium transport into milk and inhibits parathyroid hormone-related protein (PTHrP) secretion into milk and into the circulation. The ability to sense changes in extracellular calcium allows the lactating breast to actively participate in the regulation of systemic calcium and bone metabolism, and to coordinate calcium usage with calcium availability during milk production. Interestingly, as compared to normal breast cells, in breast cancer cells, the regulation of PTHrP secretion by the CaSR becomes rewired due to a switch in its G-protein usage such that activation of the CaSR increases instead of decreases PTHrP production. In normal cells the CaSR couples to Gα_i to inhibit cAMP and PTHrP production, whereas in breast cancer cells, it couples to Gα_s to stimulate cAMP and PTHrP production. Activation of the CaSR on breast cancer cells regulates breast cancer cell proliferation, death and migration, in part, by stimulating PTHrP production. In this article, we discuss the biology of the CaSR in the normal breast and in breast cancer, and review recent findings suggesting that the CaSR activates a nuclear pathway of PTHrP action that stimulates cellular proliferation and inhibits cell death, helping cancer cells adapt to elevated extracellular calcium levels. Understanding the diverse actions mediated by the CaSR may help us better understand lactation physiology, breast cancer progression and osteolytic bone metastases.

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

Regulation of Ca(2+) 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.

Essential role of Orai1 store-operated calcium channels in lactation

The nourishment of neonates by nursing is the defining characteristic of mammals. However, despite considerable research into the neural control of lactation, an understanding of the signaling mechanisms underlying the production and expulsion of milk by mammary epithelial cells during lactation remains largely unknown. Here we demonstrate that a store-operated Ca(2+) channel subunit, Orai1, is required for both optimal Ca(2+) transport into milk and for milk ejection. Using a novel, 3D imaging strategy, we visualized live oxytocin-induced alveolar unit contractions in the mammary gland, and we demonstrated that in this model milk is ejected by way of pulsatile contractions of these alveolar units. In mammary glands of Orai1 knockout mice, these contractions are infrequent and poorly coordinated. We reveal that oxytocin also induces a large transient release of stored Ca(2+) in mammary myoepithelial cells followed by slow, irregular Ca(2+) oscillations. These oscillations, and not the initial Ca(2+) transient, are mediated exclusively by Orai1 and are absolutely required for milk ejection and pup survival, an observation that redefines the signaling processes responsible for milk ejection. These findings clearly demonstrate that Ca(2+) is not just a substrate for nutritional enrichment in mammals but is also a master regulator of the spatiotemporal signaling events underpinning mammary alveolar unit contraction. Orai1-dependent Ca(2+) oscillations may represent a conserved language in myoepithelial cells of other secretory epithelia, such as sweat glands, potentially shedding light on other Orai1 channelopathies, including anhidrosis (an inability to sweat).

Fructus Corni extract-induced neuritogenesis in PC12 cells is associated with the suppression of stromal interaction molecule 1 expression and inhibition of Ca2+ influx

Fructus Corni (Cornus officinalis Sieb. et Zucc.) is commonly prescribed as a traditional Chinese herbal medicine that possesses pharmacological actions against inflammation, diabetic nephropathy, tumors, oxidation and aging. However, its function and mode of action within the nervous system remain largely unclear. In this study, the effects of Fructus Corni extract (FCE) on neuronal differentiation were investigated. It was found that FCE significantly increased the percentage of PC12 cells bearing neurites (P<0.001). Following the generation of neurite outgrowth, FCE treatment decreased the mRNA expression of stromal interaction molecule 1 (STIM1; P<0.05) and suppressed the expression of STIM1 protein (P<0.001). In addition, extracellular calcium (Ca²⁺) influx was inhibited resulting in a reduction in the intracellular Ca²⁺ level, suggesting that the inhibition of Ca²⁺ influx may be involved in the FCE-promoted neurite outgrowth of PC12 cells. These results demonstrate that FCE induces neurite outgrowth in PC12 cells and that this is associated with the suppression of STIM1 expression and the inhibition of Ca²⁺ influx, which may partially explain the FCE-induced neuritogenesis.

Caffeine-induced Ca(2+) oscillations in type I horizontal cell of carp retina: a mathematical model

Oscillations in intracellular free Ca(2+) concentration ([Ca(2+)]i) have been observed in a variety of cell types. In the present study, we constructed a mathematical model to simulate the caffeine-induced [Ca(2+)]i oscillations based on experimental data obtained from isolated type I horizontal cell of carp retina. The results of model analysis confirm the notion that the caffeine-induced [Ca(2+)]i oscillations involve a number of cytoplasmic and endoplasmic Ca(2+) processes that interact with each other. Using this model, we evaluated the importance of store-operated channel (SOC) in caffeine-induced [Ca(2+)]i oscillations. The model suggests that store-operated Ca(2+) entry (SOCE) is elicited upon depletion of the endoplasmic reticulum (ER). When the SOC conductance is set to 0, caffeine-induced [Ca(2+)]i oscillations are abolished, which agrees with the experimental observation that [Ca(2+)]i oscillations were abolished when SOC was blocked pharmacologically, verifying that SOC is necessary for sustained [Ca(2+)]i oscillations.

Peripheral serotonin regulates maternal calcium trafficking in mammary epithelial cells during lactation in mice

Lactation is characterized by massive transcellular flux of calcium, from the basolateral side of the mammary alveolar epithelium (blood) into the ductal lumen (milk). Regulation of calcium transport during lactation is critical for maternal and neonatal health. The monoamine serotonin (5-HT) is synthesized by the mammary gland and functions as a homeostatic regulation of lactation. Genetic ablation of tryptophan hydroxylase 1 (Tph1), which encodes the rate-limiting enzyme in non-neuronal serotonin synthesis, causes a deficiency in circulating serotonin. As a consequence maternal calcium concentrations decrease, mammary epithelial cell morphology is altered, and cell proliferation is decreased during lactation. Here we demonstrate that serotonin deficiency decreases the expression and disrupts the normal localization of calcium transporters located in the apical (PMCA2) and basolateral (CaSR, ORAI-1) membranes of the lactating mammary gland. In addition, serotonin deficiency decreases the mRNA expression of calcium transporters located in intracellular compartments (SERCA2, SPCA1 and 2). Mammary expression of serotonin receptor isoform 2b and its downstream pathways (PLCβ3, PKC and MAP-ERK_1/2) are also decreased by serotonin deficiency, which might explain the numerous phenotypic alterations described above. In most cases, addition of exogenous 5-hydroxy-L-tryptophan to the Tph1 deficient mice rescued the phenotype. Our data supports the hypothesis that serotonin is necessary for proper mammary gland structure and function, to regulate blood and mammary epithelial cell transport of calcium during lactation. These findings can be applicable to the treatment of lactation-induced hypocalcemia in dairy cows and can have profound implications in humans, given the wide-spread use of selective serotonin reuptake inhibitors as antidepressants during pregnancy and lactation.