Calcium transport and signaling in the mammary gland: targets for breast cancer

Role of calcium in hormone-independent and -resistant breast cancer

Approximately one-third of estrogen receptor (ER) positive breast tumors fail to respond to or become resistant to hormonal therapy. Although the mechanisms responsible for hormone resistance are not completely understood, resistance is associated with alterations in ERα; overexpression of proteins that interact with the receptor; and hormone-independent activation of the receptor by growth factor signal transduction pathways. Our previous studies show that in estrogen dependent breast cancer cells, activation of the epidermal growth factor signaling pathway increases intracellular calcium which binds to and activates ERα through sites in the ligand-binding domain of the receptor and that treatment with extracellular calcium increases the concentration of intracellular calcium which activates ERα and induces hormone-independent cell growth. The present study asked whether overexpression of calcium channels contributes to the hormone-independent and -resistant phenotype of breast cancer cells and whether clinically used calcium channel blockers reverse hormone independence and resistance. The results show that hormone-independent and -resistant cells overexpress calcium channels, have high concentrations of intracellular calcium, overexpress estrogen responsive genes and, as expected, grow in the absence of estradiol and that treatment with calcium channel blockers decreased the concentration of intracellular calcium, the expression of estrogen responsive genes and cell growth. More importantly, in hormone-resistant cells, treatment that combined a calcium channel blocker with an antiestrogen reversed resistance to the antiestrogen.

Expression and prognostic value of TRPM7 in canine mammary tumours

Canine mammary gland tumour (CMTs) are one of the most commonly found tumours in intact female dogs. A previous study on canine mammary glands demonstrated the presence of the transient receptor potential melastatin 7 (TRPM7) ion channels in healthy canine mammary tissues. However, the significance of TRPM7 in CMT is not yet known. TRPM7 is a Ca²⁺ and Mg²⁺ permeable cation channel that contains a protein kinase domain. The aim of this study was to determine TRPM7 expression in 57 benign and malignant CMT tissues of dogs using immunohistochemistry (IHC) and evaluate its correlation with clinicopathological features and explore the potential prognostic value of TRPM7 in a prospective survival study. IHC analysis shows that TRPM7 was expressed in the cytoplasm of neoplastic epithelial cells. Moreover, TRPM7 expression was significantly associated with tumour malignancy (P = .027), Ki-67 index (P < .0001) and metastasis (P < .0001). Survival curve analysis indicates that high TRPM7 expression was significantly associated with poor disease-free (P = .035) and overall survival (P = .011) in malignant CMTs. Our results demonstrate that TRPM7 is expressed in CMTs and that its expression is positively correlated with clinicopathological parameters. Thus, TRPM7 was assumed to be a potential prognostic factor for CMTs.

From Plant to Patient: Thapsigargin, a Tool for Understanding Natural Product Chemistry, Total Syntheses, Biosynthesis, Taxonomy, ATPases, Cell Death, and Drug Development

Thapsigargin, the first representative of the hexaoxygenated guaianolides, was isolated 40 years ago in order to understand the skin-irritant principles of the resin of the umbelliferous plant Thapsia garganica. The pronounced cytotoxicity of thapsigargin is caused by highly selective inhibition of the intracellular sarco-endoplasmic Ca²⁺-ATPase (SERCA) situated on the membrane of the endo- or sarcoplasmic reticulum. Thapsigargin is selective to the SERCA pump and to a minor extent the secretory pathway Ca²⁺/Mn²⁺ ATPase (SPCA) pump. Thapsigargin has become a tool for investigation of the importance of SERCA in intracellular calcium homeostasis. In addition, complex formation of thapsigargin with SERCA has enabled crystallization and structure determination of calcium-free states by X-ray crystallography. These results led to descriptions of the mechanism of action and kinetic properties of SERCA and other ATPases. Inhibition of SERCA depletes Ca²⁺ from the sarco- and endoplasmic reticulum provoking the unfolded protein response, and thereby has enabled new studies on the mechanism of cell death. Development of protocols for selective transformation of thapsigargin disclosed the chemistry and facilitated total synthesis of the molecule. Conversion of trilobolide into thapsigargin offered an economically feasible sustainable source of thapsigargin, which enables a future drug production. Principles for prodrug development were used by conjugating a payload derived from thapsigargin with a hydrophilic peptide selectively cleaved by proteases in the tumor. Mipsagargin was developed in order to obtain a drug for treatment of cancer diseases characterized by the presence of prostate specific membrane antigen (PSMA) in the neovascular tissue of the tumors. Even though mipsagargin showed interesting clinical effects the results did not encourage funding and consequently the attempt to register the drug has been abandoned. In spite of this disappointing fact, the research performed to develop the drug has resulted in important scientific discoveries concerning the chemistry, biosynthesis and biochemistry of sesquiterpene lactones, the mechanism of action of ATPases including SERCA, mechanisms for cell death caused by the unfolded protein response, and the use of prodrugs for cancer-targeting cytotoxins. The presence of toxins in only some species belonging to Thapsia also led to a major revision of the taxonomy of the genus.

Calcium signaling: breast cancer's approach to manipulation of cellular circuitry

Calcium is a versatile element that participates in cell signaling for a wide range of cell processes such as death, cell cycle, division, migration, invasion, metabolism, differentiation, autophagy, transcription, and others. Specificity of calcium in each of these processes is achieved through modulation of intracellular calcium concentrations by changing the characteristics (amplitude/frequency modulation) or location (spatial modulation) of the signal. Breast cancer utilizes calcium signaling as an advantage for survival and progression. This review integrates evidence showing that increases in expression of calcium channels, GPCRs, pumps, effectors, and enzymes, as well as resulting intracellular calcium signals, lead to high calcium and/or an elevated calcium- mobilizing capacity necessary for malignant functions such as migratory, invasive, proliferative, tumorigenic, or metastatic capacities.

Identification of Alternatively-Activated Pathways between Primary Breast Cancer and Liver Metastatic Cancer Using Microarray Data

Alternatively-activated pathways have been observed in biological experiments in cancer studies, but the concept had not been fully explored in computational cancer system biology. Therefore, an alternatively-activated pathway identification method was proposed and applied to primary breast cancer and breast cancer liver metastasis research using microarray data. Interestingly, the results show that cytokine-cytokine receptor interaction and calcium signaling were significantly enriched under both conditions. TGF beta signaling was found to be the hub in network topology analysis. In total, three types of alternatively-activated pathways were recognized. In the cytokine-cytokine receptor interaction pathway, four active alteration patterns in gene pairs were noticed. Thirteen cytokine-cytokine receptor pairs with inverse activity changes of both genes were verified by the literature. The second type was that some sub-pathways were active under only one condition. For the third type, nodes were significantly active in both conditions, but with different active genes. In the calcium signaling and TGF beta signaling pathways, node E2F5 and E2F4 were significantly active in primary breast cancer and metastasis, respectively. Overall, our study demonstrated the first time using microarray data to identify alternatively-activated pathways in breast cancer liver metastasis. The results showed that the proposed method was valid and effective, which could be helpful for future research for understanding the mechanism of breast cancer metastasis.

Therapeutic Efficiency of an External Chinese Herbal Formula of Mammary Precancerous Lesions by BATMAN-TCM Online Bioinformatics Analysis Tool and Experimental Validation

Ruyan Neixiao Cream (RYNXC), a patented Chinese herbal formula, was reported to have the effect of treating mammary precancerous disease. In this study, we predicted the potential targets, pathways, and diseases of the ingredients contained in each herbal of RYNXC and constructed an ingredients-targets-diseases network. Then, we analyzed molecular mechanisms of this Chinese herbal formula by MCF-10AT cells and model rats of breast precancerous lesions. BATMAN-TCM prediction showed that ESR1, PGR, PTGS2, EGFR, and Src were mRNA targets of RYNXC. Our results suggested that RYNXC transdermal fluid downregulated ESR1, PGR, PTGS2, EGFR, and Src expression at gene and protein level in MCF-10AT cells. In the rat breast precancerous lesions model, high and low dose RYNXC could also significantly reduce genes and proteins expression of ESR1, PGR, PTGS2, EGFR, and Src. Taken together these data indicate that RYNXC targets multiple molecules responsible for breast precancerous lesion and is an effective Chinese herbal formula. So RYNXC may be a promising external drug for breast precancerous lesions.

Expression of calcium pumps is differentially regulated by histone deacetylase inhibitors and estrogen receptor alpha in breast cancer cells

Background

Remodeling of Ca²⁺ signaling is an important step in cancer progression, and altered expression of members of the Ca²⁺ signaling toolkit including the plasma membrane Ca²⁺ ATPases (PMCA proteins encoded by ATP2B genes) is common in tumors.

Methods

In this study PMCAs were examined in breast cancer datasets and in a variety of breast cancer cell lines representing different subtypes. We investigated how estrogen receptor alpha (ER-α) and histone deacetylase (HDAC) inhibitors regulate the expression of these pumps.

Results

Three distinct datasets displayed significantly lower ATP2B4 mRNA expression in invasive breast cancer tissue samples compared to normal breast tissue, whereas the expression of ATP2B1 and ATP2B2 was not altered. Studying the protein expression profiles of Ca²⁺ pumps in a variety of breast cancer cell lines revealed low PMCA4b expression in the ER-α positive cells, and its marked upregulation upon HDAC inhibitor treatments. PMCA4b expression was also positively regulated by the ER-α pathway in MCF-7 cells that led to enhanced Ca²⁺ extrusion capacity in response to 17β-estradiol (E2) treatment. E2-induced PMCA4b expression was further augmented by HDAC inhibitors. Surprisingly, E2 did not affect the expression of PMCA4b in other ER-α positive cells ZR-75-1, T-47D and BT-474. These findings were in good accordance with ChIP-seq data analysis that revealed an ER-α binding site in the ATP2B4 gene in MCF-7 cells but not in other ER-α positive tumor cells. In the triple negative cells PMCA4b expression was relatively high, and the effect of HDAC inhibitor treatment was less pronounced as compared to that of the ER-α positive cells. Although, the expression of PMCA4b was relatively high in the triple negative cells, a fraction of the protein was found in intracellular compartments that could interfere with the cellular function of the protein.

Conclusions

Our results suggest that the expression of Ca²⁺ pumps is highly regulated in breast cancer cells in a subtype specific manner. Our results suggest that hormonal imbalances, epigenetic modifications and impaired protein trafficking could interfere with the expression and cellular function of PMCA4b in the course of breast cancer progression.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4945-x) contains supplementary material, which is available to authorized users.

A system-based analysis of the genetic determinism of udder conformation and health phenotypes across three French dairy cattle breeds

Using GWAS to identify candidate genes associated with cattle morphology traits at a functional level is challenging. The main difficulty of identifying candidate genes and gene interactions associated with such complex traits is the long-range linkage disequilibrium (LD) phenomenon reported widely in dairy cattle. Systems biology approaches, such as combining the Association Weight Matrix (AWM) with a Partial Correlation in an Information Theory (PCIT) algorithm, can assist in overcoming this LD. Used in a multi-breed and multi-phenotype context, the AWM-PCIT could aid in identifying udder traits candidate genes and gene networks with regulatory and functional significance. This study aims to use the AWM-PCIT algorithm as a post-GWAS analysis tool with the goal of identifying candidate genes underlying udder morphology. We used data from 78,440 dairy cows from three breeds and with own phenotypes for five udder morphology traits, five production traits, somatic cell score and clinical mastitis. Cows were genotyped with medium (50k) or low-density (7 to 10k) chips and imputed to 50k. We performed a within breed and trait GWAS. The GWAS showed 9,830 significant SNP across the genome (p < 0.05). Five thousand and ten SNP did not map a gene, and 4,820 SNP were within 10-kb of a gene. After accounting for 1SNP:1gene, 3,651 SNP were within 10-kb of a gene (set1), and 2,673 significant SNP were further than 10-kb of a gene (set2). The two SNP sets formed 6,324 SNP matrix, which was fitted in an AWM-PCIT considering udder depth/ development as the key trait resulting in 1,013 genes associated with udder morphology, mastitis and production phenotypes. The AWM-PCIT detected ten potential candidate genes for udder related traits: ESR1, FGF2, FGFR2, GLI2, IQGAP3, PGR, PRLR, RREB1, BTRC, and TGFBR2.

The current status and perspectives regarding the clinical implication of intracellular calcium in breast cancer

Calcium ions (Ca²⁺ ) act as second messengers in intracellular signaling. Ca²⁺ pumps, channels, sensors, and calcium binding proteins, regulate the concentrations of intracellular Ca²⁺ as a key regulator of important cellular processes such as gene expression, proliferation, differentiation, DNA repair, apoptosis, metastasis, and hormone secretion. Intracellular Ca²⁺ also influences the functions of several organelles, that include: the endoplasmic reticulum, mitochondria, the Golgi, and cell membrane both in normal and breast cancer cells. In breast cancer, the disruption of intracellular: Ca²⁺ homeostasis may cause tumor progression by affecting key factors/pathways including phospholipase C (PLC), inositol 1,4,5-trisphosphate (IP3), calmodulin (CaM), nuclear factor of activated T-cells (NFAT), calpain, calmodulin-dependent protein kinase II (CaMKII), mitogen-activated protein kinase (MAPK), epithelial-mesenchymal transition (EMT), vascular endothelial growth factor (VEGF), poly (ADP-Ribose) polymerase-1 (PARP1), estrogen, and estrogen receptor. Because the foregoing molecules play crucial roles in breast cancer, the factors/pathways influencing intracellular Ca²⁺ concentrations are putative targets for cancer treatment, using drugs such as Mephebrindole, Tilapia piscidin 4, Nifetepimine, Paricalcitol, and Prednisolone. We have explored the factors/pathways which are related to breast cancer and Ca²⁺ homeostasis and signaling in this review, and also discussed their potential as biomarkers for breast cancer staging, prognosis, and therapy.

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.

1,25-Dihydroxyvitamin D3 modulates calcium transport in goat mammary epithelial cells in a dose- and energy-dependent manner

Background

Calcium is a vital mineral and an indispensable component of milk for ruminants. The regulation of transcellular calcium transport by 1,25-dihydroxyvitamin D₃ (1,25-(OH)₂D₃, the active form of vitamin D) has been confirmed in humans and rodents, and regulators, including vitamin D receptor (VDR), calcium binding protein D_9k (calbindin-D_9k), plasma membrane Ca²⁺-ATPase 1b (PMCA1b), PMAC2b and Orai1, are involved in this process. However, it is still unclear whether 1,25-(OH)₂D₃ could stimulate calcium transport in the ruminant mammary gland. The present trials were conducted to study the effect of 1,25-(OH)₂D₃ supplementation and energy availability on the expression of genes and proteins related to calcium secretion in goat mammary epithelial cells.

Methods

An in vitro culture method for goat secreting mammary epithelial cells was successfully established. The cells were treated with different doses of 1,25-(OH)₂D₃ (0, 0.1, 1.0, 10.0 and 100.0 nmol/L) for calcium transport research, followed by a 3-bromopyruvate (3-BrPA, an inhibitor of glucose metabolism) treatment to determine its dependence on glucose availability. Cell proliferation ratios, glucose consumption and enzyme activities were measured with commercial kits, and real-time quantitative polymerase chain reaction (RT-qPCR), and western blots were used to determine the expression of genes and proteins associated with mammary calcium transport in dairy goats, respectively.

Results

1,25-(OH)₂D₃ promoted cell proliferation and the expression of genes involved in calcium transport in a dose-dependent manner when the concentration did not exceed 10.0 nmol/L. In addition, 100.0 nmol/L 1,25-(OH)₂D₃ inhibited cell proliferation and the expression of associated genes compared with the 10.0 nmol/L treatment. The inhibition of hexokinase 2 (HK2), a rate-limiting enzyme in glucose metabolism, decreased the expression of PMCA1b and PMCA2b at the mRNA and protein levels as well as the transcription of Orai1, indicating that glucose availability was required for goat mammary calcium transport. The optimal concentration of 1,25-(OH)₂D₃ that facilitated calcium transport in this study was 10.0 nmol/L.

Conclusions

Supplementation with 1,25-(OH)₂D₃ influenced cell proliferation and regulated the expression of calcium transport modulators in a dose- and energy-dependent manner, thereby highlighting the role of 1,25-(OH)₂D₃ as an efficacious regulatory agent that produces calcium-enriched milk in ruminants when a suitable energy status was guaranteed.

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.

Loss of Panx1 Impairs Mammary Gland Development at Lactation: Implications for Breast Tumorigenesis

Pannexin1 (Panx1) subunits oligomerize to form large-pore channels between the intracellular and extracellular milieu that have been shown to regulate proliferation, differentiation and cell death mechanisms. These key cellular responses are ultimately necessary for normal tissue development and function but the role of Panx1 in development, differentiation and function in many tissues remains unexplored, including that of the breast. Panx1 was identified to be expressed in the mammary gland through western blot and immunofluorescent analysis and is dynamically upregulated during pregnancy and lactation. In order to evaluate the role of Panx1 in the context of mammary gland development and function, Panx1^-/- mice were evaluated in comparison to wild-type mice in the mammary glands of virgin, lactating and involuting mice. Our results revealed that Panx1 ablation did not affect virgin or involuting mammary glands following histological and whole mount analysis. Panx1 was necessary for timely alveolar development during early lactation based on a decreased number of alveolar lumen following histological analysis and reduced proliferation following Ki67 immunofluorescent labelling. Importantly, the loss of Panx1 in lactating mammary glands did not overtly affect epithelial or secretory differentiation of the mammary gland suggesting that Panx1 is not critical in normal mammary gland function. In addition, PANX1 mRNA expression was correlated with negative clinical outcomes in patients with breast cancer using in silico arrays. Together, our results suggest that Panx1 is necessary for timely alveolar development following the transition from pregnancy to lactation, which may have implications extending to patients with breast cancer.

5α-Dihydrotestosterone regulates the expression of L-type calcium channels and calcium-binding protein regucalcin in human breast cancer cells with suppression of cell growth

Androgens have been associated with the development of normal breast, and their role in mammary gland carcinogenesis has also been described. Several studies reported that androgens inhibit breast cancer cell growth, whereas others linked their action with the modulation of calcium (Ca(2+)) pumps, Ca(2+) channels and Ca(2+)-binding proteins. Also, it is known that deregulated Ca(2+) homeostasis has been implicated in the pathophysiology of breast. The L-type Ca(2+) channels (LTCCs) were found to be up-regulated in colon, colorectal and prostate cancer, but their presence in breast tissues remains uncharacterized. On the other hand, regucalcin (RGN) is a Ca(2+)-binding protein involved in the control of mammary gland cell proliferation, which has been identified as an androgen target gene in distinct tissues except breast. This study aimed to confirm the expression and activity of LTCCs in human breast cancer cells and investigate the effect of androgens in regulating the expression of α1C subunit (Cav1.2) of LTCCs and Ca(2+)-binding protein RGN. PCR, Western blot, immunofluorescence and electrophysiological experiments demonstrated the expression and activity of Cav1.2 subunit in MCF-7 cells. The MCF-7 cells were treated with 1, 10 or 100 nM of 5α-dihydrotestosterone (DHT) for 24-72 h. The obtained results showed that 1 nM DHT up-regulated the expression of Cav1.2 subunit while diminishing RGN protein levels, which was underpinned by reduced cell viability. These findings first confirmed the presence of LTCCs in breast cancer cells and opened new perspectives for the development of therapeutic approaches targeting Ca(2+) signaling.

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).

Mango Fruit Extracts Differentially Affect Proliferation and Intracellular Calcium Signalling in MCF-7 Human Breast Cancer Cells

The assessment of human cancer cell proliferation is a common approach in identifying plant extracts that have potential bioactive effects. In this study, we tested the hypothesis that methanolic extracts of peel and flesh from three archetypal mango cultivars, Irwin (IW), Nam Doc Mai (NDM), and Kensington Pride (KP), differentially affect proliferation, extracellular signal-regulated kinase (ERK) activity, and intracellular calcium ([Ca2+]I) signalling in MCF-7 human breast cancer cells. Mango flesh extracts from all three cultivars did not inhibit cell growth, and of the peel extracts only NDM reduced MCF-7 cell proliferation. Mango cultivar peel and flesh extracts did not significantly change ERK phosphorylation compared to controls; however, some reduced relative maximal peak[Ca2+]Iafter adenosine triphosphate stimulation, with NDM peel extract having the greatest effect among the treatments. Our results identify mango interfruit and intrafruit (peel and flesh) extract variability in antiproliferative effects and[Ca2+]Isignalling in MCF-7 breast cancer cells and highlight that parts of the fruit (such as peel and flesh) and cultivar differences are important factors to consider when assessing potential chemopreventive bioactive compounds in plants extracts.

Mammary gland specific knockdown of the physiological surge in Cx26 during lactation retains normal mammary gland development and function

Connexin26 (Cx26) is the major Cx protein expressed in the human mammary gland and is up-regulated during pregnancy while remaining elevated throughout lactation. It is currently unknown if patients with loss-of-function Cx26 mutations that result in hearing loss and skin diseases have a greater susceptibility to impaired breast development. To investigate if Cx26 plays a critical role in mammary gland development and differentiation, a novel Cx26 conditional knockout mouse model was generated by crossing Cx26^fl/fl mice with mice expressing Cre under the β-Lactoglobulin promoter. Conditional knockdown of Cx26 from the mammary gland resulted in a dramatic reduction in detectable gap junction plaques confirmed by a significant ∼65-70% reduction in Cx26 mRNA and protein throughout parturition and lactation. Interestingly, this reduction was accompanied by a decrease in mammary gland Cx30 gap junction plaques at parturition, while no change was observed for Cx32 or Cx43. Whole mount, histological and immunofluorescent assessment of breast tissue revealed comparatively normal lobuloalveolar development following pregnancy in the conditionally knockdown mice compared to control mice. In addition, glands from genetically-modified mice were capable of producing milk proteins that were evident in the lumen of alveoli and ducts at similar levels as controls, suggesting normal gland function. Together, our results suggest that low levels of Cx26 expression throughout pregnancy and lactation, and not the physiological surge in Cx26, is sufficient for normal gland development and function.

Mitochondrial calcium uniporter activity is dispensable for MDA-MB-231 breast carcinoma cell survival

Calcium uptake through the mitochondrial Ca²⁺ uniporter (MCU) is thought to be essential in regulating cellular signaling events, energy status, and survival. Functional dissection of the uniporter is now possible through the recent identification of the genes encoding for MCU protein complex subunits. Cancer cells exhibit many aspects of mitochondrial dysfunction associated with altered mitochondrial Ca²⁺ levels including resistance to apoptosis, increased reactive oxygen species production and decreased oxidative metabolism. We used a publically available database to determine that breast cancer patient outcomes negatively correlated with increased MCU Ca²⁺ conducting pore subunit expression and decreased MICU1 regulatory subunit expression. We hypothesized breast cancer cells may therefore be sensitive to MCU channel manipulation. We used the widely studied MDA-MB-231 breast cancer cell line to investigate whether disruption or increased activation of mitochondrial Ca²⁺ uptake with specific siRNAs and adenoviral overexpression constructs would sensitize these cells to therapy-related stress. MDA-MB-231 cells were found to contain functional MCU channels that readily respond to cellular stimulation and elicit robust AMPK phosphorylation responses to nutrient withdrawal. Surprisingly, knockdown of MCU or MICU1 did not affect reactive oxygen species production or cause significant effects on clonogenic cell survival of MDA-MB-231 cells exposed to irradiation, chemotherapeutic agents, or nutrient deprivation. Overexpression of wild type or a dominant negative mutant MCU did not affect basal cloning efficiency or ceramide-induced cell killing. In contrast, non-cancerous breast epithelial HMEC cells showed reduced survival after MCU or MICU1 knockdown. These results support the conclusion that MDA-MB-231 breast cancer cells do not rely on MCU or MICU1 activity for survival in contrast to previous findings in cells derived from cervical, colon, and prostate cancers and suggest that not all carcinomas will be sensitive to therapies targeting mitochondrial Ca²⁺ uptake mechanisms.

Calcium influx pathways in breast cancer: opportunities for pharmacological intervention

Ca(2+) influx through Ca(2+) permeable ion channels is a key trigger and regulator of a diverse set of cellular events, such as neurotransmitter release and muscle contraction. Ca(2+) influx is also a regulator of processes relevant to cancer, including cellular proliferation and migration. This review focuses on calcium influx in breast cancer cells as well as the potential for pharmacological modulators of specific Ca(2+) influx channels to represent future agents for breast cancer therapy. Altered expression of specific calcium permeable ion channels is present in some breast cancers. In some cases, such changes can be related to breast cancer subtype and even prognosis. In vitro and in vivo models have now helped identify specific Ca(2+) channels that play important roles in the proliferation and invasiveness of breast cancer cells. However, some aspects of our understanding of Ca(2+) influx in breast cancer still require further study. These include identifying the mechanisms responsible for altered expression and the most effective therapeutic strategy to target breast cancer cells through specific Ca(2+) channels. The role of Ca(2+) influx in processes beyond breast cancer cell proliferation and migration should become the focus of studies in the next decade.

The diverse roles of calcium-binding protein regucalcin in cell biology: from tissue expression and signalling to disease

Regucalcin (RGN) is a calcium (Ca(2+))-binding protein widely expressed in vertebrate and invertebrate species, which is also known as senescence marker protein 30, due to its molecular weight (33 kDa) and a characteristically diminished expression with the aging process. RGN regulates intracellular Ca(2+) homeostasis and the activity of several proteins involved in intracellular signalling pathways, namely, kinases, phosphatases, phosphodiesterase, nitric oxide synthase and proteases, which highlights its importance in cell biology. In addition, RGN has cytoprotective effects reducing intracellular levels of oxidative stress, also playing a role in the control of cell survival and apoptosis. Multiple factors have been identified regulating the cell levels of RGN transcripts and protein, and an altered expression pattern of this interesting protein has been found in cases of reproductive disorders, neurodegenerative diseases and cancer. Moreover, RGN is a serum-secreted protein, and its levels have been correlated with the stage of disease, which strongly suggests the usefulness of this protein as a potential biomarker for monitoring disease onset and progression. The present review aims to discuss the available information concerning RGN expression and function in distinct cell types and tissues, integrating cellular and molecular mechanisms in the context of normal and pathological conditions. Insight into the cellular actions of RGN will be a key step towards deepening the knowledge of the biology of several human diseases.

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

BACKGROUND

The entry of calcium ions into mammary gland epithelial cells is one of the least well-understood processes in the transport of calcium into milk during lactation. The store-operated calcium entry channel ORAI1, has been suggested as a potential mechanism for the entry of Ca(2+) into mammary gland epithelial cells from the maternal blood supply during lactation. The down regulation of the canonical ORAI1 activator STIM1 during lactation suggests that other known ORAI activators such as STIM2 and SPCA2 may be important during lactation.

RESULTS

Differentiation of HC11 mammary gland epithelial cells was associated with enhanced basal Ca(2+) influx. Silencing of Orai1 abolished this enhancement of Ca(2+) influx. Stim2 had a modest effect on Ca(2+) influx in this in vitro model of lactation, whereas Stim1 and Spca2 silencing had no effect. Despite pronounced increases in Spca2 mRNA during lactation there was no change in the generation of the alternative splice product generated by Mist1, which increases during lactation.

CONCLUSIONS

These studies support the hypothesis that lactation is associated with a remodelling of Ca(2+) influx and this is associated with enhancement of basal Ca(2+) influx. This enhanced Ca(2+) influx appears to occur through the calcium channel Orai1.

Cellular calcium dynamics in lactation and breast cancer: from physiology to pathology

Breast cancer is the second leading cause of cancer mortality in women, estimated at nearly 40,000 deaths and more than 230,000 new cases diagnosed in the U.S. this year alone. One of the defining characteristics of breast cancer is the radiographic presence of microcalcifications. These palpable mineral precipitates are commonly found in the breast after formation of a tumor. Since free Ca(2+) plays a crucial role as a second messenger inside cells, we hypothesize that these chelated precipitates may be a result of dysregulated Ca(2+) secretion associated with tumorigenesis. Transient and sustained elevations of intracellular Ca(2+) regulate cell proliferation, apoptosis and cell migration, and offer numerous therapeutic possibilities in controlling tumor growth and metastasis. During lactation, a developmentally determined program of gene expression controls the massive transcellular mobilization of Ca(2+) from the blood into milk by the coordinated action of calcium transporters, including pumps, channels, sensors and buffers, in a functional module that we term CALTRANS. Here we assess the evidence implicating genes that regulate free and buffered Ca(2+) in normal breast epithelium and cancer cells and discuss mechanisms that are likely to contribute to the pathological characteristics of breast cancer.

Support Vector Machine classifier for estrogen receptor positive and negative early-onset breast cancer

Two major breast cancer sub-types are defined by the expression of estrogen receptors on tumour cells. Cancers with large numbers of receptors are termed estrogen receptor positive and those with few are estrogen receptor negative. Using genome-wide single nucleotide polymorphism genotype data for a sample of early-onset breast cancer patients we developed a Support Vector Machine (SVM) classifier from 200 germline variants associated with estrogen receptor status (p<0.0005). Using a linear kernel Support Vector Machine, we achieved classification accuracy exceeding 93%. The model indicates that polygenic variation in more than 100 genes is likely to underlie the estrogen receptor phenotype in early-onset breast cancer. Functional classification of the genes involved identifies enrichment of functions linked to the immune system, which is consistent with the current understanding of the biological role of estrogen receptors in breast cancer.

New insights into store-independent Ca(2+) entry: secretory pathway calcium ATPase 2 in normal physiology and cancer

Recent studies in secretory pathway calcium ATPases (SPCA) revealed novel functions of SPCA2 in interacting with store-operated Ca(2+) channel Orai1 and inducing Ca(2+) influx at the cell surface. Importantly, SPCA2-mediated Ca(2+) signaling is uncoupled from its conventional role of Ca(2+)-ATPase and independent of store-operated Ca(2+) signaling pathway. SPCA2-induced store-independent Ca(2+) entry (SICE) plays essential roles in many important physiological processes, while unbalanced SICE leads to enhanced cell proliferation and tumorigenesis. Finally, we have summarized the clinical implication of SICE in oral cancer prognosis and treatment. Inhibition of SICE may be a new target for the development of cancer therapeutics.

The role of calcium in the activation of estrogen receptor-alpha

Environmental estrogen mimics, including metalloestrogens that can activate estrogen receptor-alpha (ERα), may contribute to breast cancer risk. However, the underlying mechanisms through which these molecular mimics activate the ERα are generally poorly understood. With concern to this important question, we investigated whether intracellular calcium may mediate the cross-talk between signaling pathways that activate ERα and the ligand-binding domain of ERα. MCF-7 cells treated with EGF, ATP, extracellular calcium, or caffeine to increase intracellular calcium triggered a rapid recruitment of ERα to estrogen-responsive promoters and stimulated expression of estrogen-responsive genes including pS2, complement C3, and progesterone receptor. Induction was blocked by an antiestrogen but also by the chelation of intracellular calcium. Treatment with extracellular calcium also increased the growth of MCF-7 cells through an ER-dependent mechanism. We found that EGF and extracellular calcium activated the C-terminus of ERα and the activation was blocked by the antiestrogen. Mechanistic investigations identified four potential sites on the solvent-accessible surface of the ERα ligand-binding domain as important for calcium activation of the receptor. Taken together, our results suggest that calcium mediates the cross-talk between ERα-activating signaling pathways and the ligand-binding domain of ERα providing a potential explanation for the ability of certain environmental metalloestrogens to activate the receptor.

Store-independent activation of Orai1 by SPCA2 in mammary tumors

Ca(2+) is an essential and ubiquitous second messenger. Changes in cytosolic Ca(2+) trigger events critical for tumorigenesis, such as cellular motility, proliferation, and apoptosis. We show that an isoform of Secretory Pathway Ca(2+)-ATPase, SPCA2, is upregulated in breast cancer-derived cells and human breast tumors, and suppression of SPCA2 attenuates basal Ca(2+) levels and tumorigenicity. Contrary to its conventional role in Golgi Ca(2+) sequestration, expression of SPCA2 increased Ca(2+) influx by a mechanism dependent on the store-operated Ca(2+) channel Orai1. Unexpectedly, SPCA2-Orai1 signaling was independent of ER Ca(2+) stores or STIM1 and STIM2 sensors and uncoupled from Ca(2+)-ATPase activity of SPCA2. Binding of the SPCA2 amino terminus to Orai1 enabled access of its carboxyl terminus to Orai1 and activation of Ca(2+) influx. Our findings reveal a signaling pathway in which the Orai1-SPCA2 complex elicits constitutive store-independent Ca(2+) signaling that promotes tumorigenesis.

Golgi calcium pump secretory pathway calcium ATPase 1 (SPCA1) is a key regulator of insulin-like growth factor receptor (IGF1R) processing in the basal-like breast cancer cell line MDA-MB-231

Calcium signaling is a key regulator of pathways important in tumor progression, such as proliferation and apoptosis. Most studies assessing altered calcium homeostasis in cancer cells have focused on alterations mediated through changes in cytoplasmic free calcium levels. Here, we show that basal-like breast cancers are characterized by an alteration in the secretory pathway calcium ATPase 1 (SPCA1), a calcium pump localized to the Golgi. Inhibition of SPCA1 in MDA-MB-231 cells produced pronounced changes in cell proliferation and morphology in three-dimensional culture, without alterations in sensitivity to endoplasmic reticulum stress induction or changes in global calcium signaling. Instead, the effects of SPCA1 inhibition in MDA-MB-231 cells reside in altered regulation of calcium-dependent enzymes located in the secretory pathway, such as proprotein convertases. Inhibition of SPCA1 produced a pronounced alteration in the processing of insulin-like growth factor receptor (IGF1R), with significantly reduced levels of functional IGF1Rβ and accumulation of the inactive trans-Golgi network pro-IGF1R form. These studies identify for the first time a calcium transporter associated with the basal-like breast cancer subtype. The pronounced effects of SPCA1 inhibition on the processing of IGF1R in MDA-MB-231 cells independent of alterations in global calcium signaling also demonstrate that some calcium transporters can regulate the processing of proteins important in tumor progression without major alterations in cytosolic calcium signaling. Inhibitors of SPCA1 may offer an alternative strategy to direct inhibitors of IGF1R and attenuate the processing of other proprotein convertase substrates important in basal breast cancers.

PMCA2 regulates apoptosis during mammary gland involution and predicts outcome in breast cancer

After lactation, weaning causes mammary epithelial cell (MEC) apoptosis. MECs express the plasma membrane calcium-ATPase 2 (PMCA2), which transports calcium across the apical surface of the cells into milk. Here we show that PMCA2 is down-regulated early in mammary involution associated with changes in MEC shape. We demonstrate that loss of PMCA2 expression raises intracellular calcium levels and sensitizes MECs to apoptosis. In contrast, overexpression of PMCA2 in T47D breast cancer cells lowers intracellular calcium and protects them from apoptosis. Finally, we show that high PMCA2 expression in breast cancers is associated with poor outcome. We conclude that loss of PMCA2 expression at weaning triggers apoptosis by causing cellular calcium crisis. PMCA2 overexpression, on the other hand, may play a role in breast cancer progression by conferring resistance to apoptosis.

Localization of plasma membrane and secretory calcium pumps in the mammary gland

Until recently the mechanism for the enrichment of milk with calcium was thought to be almost entirely via the secretory pathway. However, recent studies suggest that a plasma membrane calcium ATPase, PMCA2, is the primary mechanism for calcium transport into milk, highlighting a major role for apical calcium transport. We compared the expression of the recently identified secretory calcium ATPase, SPCA2, and SPCA1, in the mouse mammary gland during development. SPCA2 levels increased over 35-fold during lactation with expression localized to luminal secretory cells, while SPCA1 increased only a modest 2-fold and was expressed throughout the cells of the mammary gland. We also observed major differences in the localization of PMCA2 and PMCA1. Our studies highlight the likely specific roles of PMCA2 and SPCA2 in lactation and indicate that calcium transport into milk is a complex interplay between apical and secretory pathways.

Calcium transport by mammary secretory cells: mechanisms underlying transepithelial movement

The secretion of calcium into milk by mammary epithelial cells is a fundamentally important process. Despite this, the mechanisms which underlie the movement of calcium across the lactating mammary gland are still poorly understood. There are, however, two models which describe the handling of calcium by mammary epithelial cells. On the one hand, a model which has existed for several decades, suggests that the vast majority of calcium enters milk via the Golgi secretory vesicle route. On the other hand, a new model has recently been proposed which implies that the active transport of calcium across the apical membrane of mammary secretory cells is central to milk calcium secretion. This short review examines the strengths and weaknesses of both models and suggests some experiments which could add to our understanding of mammary calcium transport.

Regucalcin is expressed in rat mammary gland and prostate and down-regulated by 17beta-estradiol

Regucalcin is involved in maintenance of calcium homeostasis due to the activation of Ca2+ pumping enzymes in the plasma membrane. It has a suppressive effect in cell proliferation, DNA and RNA synthesis, and may be associated with the abnormal cell division on tumor tissues. On the other hand both estrogens and Ca2+ are implicated in breast and prostate cancer but there are no studies focused on the expression of regucalcin in rat mammary gland or prostate. Furthermore, it is known that the expression of regucalcin in rat liver and kidney is regulated by 17beta-estradiol (E2). The aim of this study is to analyze if regucalcin is expressed in rat mammary gland and prostate and if it is regulated by E2 in these tissues. We demonstrated for the first time that regucalcin mRNA and protein are present in rat mammary gland and prostate by in situ hybridization and immunohistochemistry, respectively. Furthermore, we show by Real-time PCR that E2 down-regulates regucalcin expression in rat mammary gland and prostate.

The calcium-sensing receptor regulates plasma membrane calcium adenosine triphosphatase isoform 2 activity in mammary epithelial cells: a mechanism for calcium-regulated calcium transport into milk

The calcium-sensing receptor (CaR) regulates transepithelial calcium transport into milk by mammary epithelial cells. Using a genome-wide screening strategy, we identified the plasma membrane calcium ATPase isoform 2 (PMCA2) as a potential downstream target of the CaR. We show that PMCA2 expression in the mouse mammary gland increases during lactation and that PMCA2 is localized solely to the apical plasma membrane of mammary epithelial cells. In milk from deafwaddler mice, which have mutations in the gene encoding PMCA2, calcium concentrations were reduced, confirming its importance in calcium transport into milk. Furthermore, in cultured primary and EpH4 mouse mammary epithelial cells, CaR stimulation up-regulated calcium-dependent ATPase activity in plasma membrane preparations. By small interfering RNA-mediated gene knockdown of PMCA2, we show that PMCA2 accounts for the preponderance of calcium-ATPase activity. We also show that reduction of CaR expression with small interfering RNA eliminates the ability of extracellular calcium to elicit an increase in calcium-dependent ATPase activity in EpH4 cell membranes. These results demonstrate that activation of the CaR increases PMCA2 activity in mouse mammary epithelial cells, providing a mechanism for the regulation of transepithelial calcium transport by calcium in the lactating mouse mammary gland.