Background
Breast cancer is the second most common cancer type worldwide, and is the major cause of cancer death among women according to the GLOBOCAN 2012 project [
1]. Breast cancer is a very complex and heterogeneous disease that can be divided into several subtypes based on gene expression profiles and immunohistochemical markers. These subgroups also differ in their clinical outcomes, therapy responses and metastatic potential. Approximately 75% of breast cancers belong to the luminal A or B subtype, which are more differentiated breast tumors and express estrogen receptor alpha (ER-α) and/or progesterone receptor (PR). These cancers respond well to anti-estrogen therapy; however, recurrence is frequent. About 20% of breast cancers, that overexpress human epidermal growth factor receptor 2 (HER2) can be treated with targeted therapies against HER2. The most aggressive, triple negative (ER-, PR-, HER2-) basal and claudin-low subtypes still lack druggable targets; the only option for the systemic treatment of these tumors is chemotherapy [
2,
3]. Therefore, breast cancer research aims to find new targets and new biomarkers that can predict therapy response or resistance. Recent studies suggest that targeting the PI3K/Akt/mTOR and FGFR or IGFR pathways, using PARP inhibitors, epigenetic modulators such as histone deacetylase (HDAC) inhibitors, or immunotherapies are the most promising pharmacological therapeutic opportunities [
4‐
6]. HDAC inhibitors alone or in combination with other treatment options are under clinical investigation for the treatment of several solid malignancies, including breast cancer. These compounds exert their anticancer effects through inhibition of cell proliferation and induction of differentiation and cell death. Although the results are promising, especially in case of the combined therapies, the mechanisms of action are not completely understood [
6‐
8].
The remodeling of cellular calcium (Ca
2+) homeostasis is an important step in cancer progression, because Ca
2+ signaling is linked either directly or indirectly to the main processes altered in tumorigenesis such as regulation of proliferation, cell survival, migration or invasion [
9,
10]. The expression of several Ca
2+ channels or pumps show characteristic changes during these processes, resulting in altered Ca
2+ signal patterns affecting further downstream Ca
2+-sensitive signaling pathways [
9,
10]. Certain members of the transient receptor potential (TRP) family of the Ca
2+ channels are frequently upregulated in various cancer types. Several studies have revealed the remodeling of store-operated Ca
2+ entry (SOCE) and showed altered STIM and Orai protein expressions. Reduced expression of various forms of Ca
2+ pumps was also observed in a variety of cancer types [
9,
10]. The alteration of the sarco/endoplasmic reticulum Ca
2+ ATPase 3 (SERCA3) – a SERCA-type Ca
2+ pump – during tumorigenesis was confirmed in several different cancer types such as breast, lung, brain, colon, gastric carcinomas or leukemias, using either cancer cell lines or tissues [
11‐
16].
Plasma membrane type Ca
2+ pumps or Ca
2+ ATPases (PMCAs) are responsible for the expulsion of Ca
2+ from the cytosol into the extracellular space, to maintain a low intracellular Ca
2+ concentration [
17]. In mammals, four separate genes code for the major PMCA isoforms (
ATP2B1 ATP2B4 genes code PMCA1-PMCA4 proteins) and alternative RNA splicing can generate additional PMCA variants. The more than 20 splice variants show tissue- and cell type-specific expression and differ in their cellular localization and activity [
18‐
20]. Since PMCA is a major regulator of Ca
2+ signaling in many non-excitable cell types, it plays an essential role in the regulation of cell proliferation, differentiation and apoptosis, processes closely related to tumorigenesis [
9,
20]. However, limited data are available on PMCAs related to cancer. Studies on colon cancer showed lower PMCA4 expression in tumors compared to normal tissue [
21,
22], while PMCA1 was found to be downregulated in oral cancers [
23]. Previous in vitro studies showed upregulated
ATP2B1 and
ATP2B2 mRNA and downregulated
ATP2B4 mRNA expression in some breast cancer cell lines [
24,
25]. Moreover, high
ATP2B2 expression in breast cancer was found to be associated with specific tumor subtypes [
26‐
28].
Previous studies have suggested that PMCAs can impact on Ca
2+ signaling in an isoform specific manner [
19]. Moreover, we found considerable PMCA4b upregulation during HDAC inhibitor treatment of MCF-7 breast cancer cells, and this effect was further enhanced by phorbol 12-myristate 13-acetate (PMA). The altered PMCA4b expression led to enhanced Ca
2+ clearance, suggesting that the protein plays an important role in the Ca
2+ homeostasis of MCF-7 cells. Moreover, immunohistochemical analysis of normal breast tissue showed high PMCA4 expression in breast ductal epithelial cells, suggesting that PMCA4 is an essential component of the Ca
2+ signaling toolkit in the normal breast epithelium [
29]. More recently, our group has found that HDAC inhibitors upregulated PMCA4b expression in melanoma cell lines [
30]. In addition, we showed that inhibition of mutant B-Raf enhanced PMCA4b expression in
BRAF mutant melanoma cells, and that PMCA4b abundance (induced either by overexpression or drug treatments) was coupled with decreased migration and metastatic activity of
BRAF mutant melanoma cells [
31]. These observations also highlight the importance of PMCA4b in the development and progression of these tumor types.
In this study we investigated the expression of the
ATP2B genes in publicly available breast cancer gene expression datasets and studied the modulation of the expression of various Ca
2+ pumps at the protein level by HDAC inhibitor and/or 17β-estradiol (E2) treatments in a variety of breast cancer cell lines. The examined cell line panel represents the non-tumorigenic breast epithelium, the luminal A, luminal B and HER2 expressing, as well as the triple negative, basal subtype breast tumors (see Additional file
1: Table S1) [
2,
3,
32‐
34]. Publicly available data [
35] revealed significantly lower
ATP2B4 mRNA expression in breast carcinomas when compared to normal breast tissue. Protein levels of the PMCA and SERCA isoforms showed high variability among the cell lines, and distinct regulatory mechanisms of PMCA expression were observed upon drug treatments or ER-α activation, depending on tumor subtype.
Methods
Cell culture
The MCF-10A, MCF-7, ZR-75-1, BT-474, AU-565, SK-BR-3 and Hs578T cell lines were obtained from the American Type Culture Collection (ATCC). The T-47D, MDA-MB-468, BT-549 and MDA-MB-231 cell lines were obtained from NCI Development Therapeutics Program (DCTD Tumor Repository, National Cancer Institute at Frederick, MD). Stocks of frozen viable cells were generated immediately after one or two passages, and low passage number cells were used for all experiments. All cell lines were tested for mycoplasma infection with MycoSensor PCR Assay kit (Agilent Technologies) and only mycoplasma free cells were used for the experiments. The MCF-7, GCaMP2-MCF-7, SK-BR-3 and Hs578T cells were cultured in DMEM supplemented with 10% FBS (Gibco, Thermo Scientific), 100 U/ml penicillin, 100 μg/ml streptomycin and 2 mM glutamine. The T-47D, ZR-75-1, BT-474, AU-565, MDA-MB-468, BT-549, MDA-MB-231 and GCaMP2-MDA-MB-231 cells were cultured in RPMI 1640 supplemented with 10% FBS (Gibco, Thermo Scientific), 100 U/ml penicillin, 100 μg/ml streptomycin and 2 mM glutamine. The MCF-10A cell line was cultured in MEGM according to the instructions of ATCC. Cells were incubated at 37 °C and 5% CO2 in a humidified atmosphere.
Stable GCaMP2 expressing cell lines
The GCaMP2-MCF-7 cell line was established previously as described in [
29]. For further studies a similar GCaMP2-MDA-MB-231 cell line was generated using the Sleeping Beauty transposon system, as described earlier [
29].
Reagents and treatments
Valproic acid sodium salt (VPA; Sigma-Aldrich) was dissolved in sterile distilled water, membrane filtered and stored at − 20 °C. Suberoylanilide hydroxamic acid (SAHA; Sigma-Aldrich), 17β-estradiol (E2; Sigma-Aldrich) and fulvestrant (ICI 182,780; Sigma-Aldrich) stock solutions were made in DMSO and stored at − 20 °C. The final DMSO concentration did not exceed 0.01% in all experiments, DMSO vehicle was included in controls and did not interfere with the experiments.
For HDAC inhibitor treatments, exponentially growing cells were seeded in 6-well plates for Western blotting or in 8-well chambers (Nunc Lab-Tek II chambered coverglass, Thermo Scientific) for immunocytochemistry and Ca2+ signal measurements, and incubated for 3 days until cultures reached ~ 80% confluency. Culture medium was then replaced by fresh medium, and VPA or SAHA was added from concentrated stock solutions. During SAHA treatment the medium was replaced daily. After 4 days of treatment, protein expressions were analyzed by Western blotting or by immunocytochemistry, or Ca2+ signal measurements were performed. In the case of E2 treatments, cells were incubated in E2-free culture medium (DMEM w/o phenol red or RPMI 1640 w/o phenol red, supplemented with 10% charcoal-stripped FBS (Gibco, Thermo Scientific), 100 U/ml penicillin, 100 μg/ml streptomycin and 2 mM glutamine). For Western blotting or Ca2+ signal measurements cells were seeded in 6-well plates or in 8-well chambers (Nunc Lab-Tek II chambered coverglass, Thermo Scientific) respectively, in normal growth medium, and after 1 day medium was replaced by E2-free medium, and cells were incubated for 2 days until cultures reached ~ 80% confluency. E2-free medium was then renewed, and cells were treated with 1 nM E2 with or without 100 nM fulvestrant and the indicated amount of HDAC inhibitors. After 4 days of treatment, protein expressions were analyzed or Ca2+ signal measurements were performed.
Western blot analysis
Total protein extraction from the cells was obtained by precipitation with 6% TCA, and Western blotting was performed as described previously [
36]. Equal amounts of total cellular protein were loaded on polyacrylamide gels (7.5, 10 or 15% depending on the examined protein), electrophoresed and electroblotted onto PVDF membranes (Bio-Rad). Immunostainings were performed with the following primary antibodies: mouse monoclonal anti-
pan PMCA (5F10), rabbit polyclonal anti-PMCA1 (NR1), rabbit polyclonal anti-PMCA2 (NR2), rabbit polyclonal anti-PMCA3 (NR3), mouse monoclonal anti-PMCA4 (JA9), and mouse monoclonal anti-PMCA4b (JA3) described in [
37,
38], mouse monoclonal anti-SERCA2 antibody (IID8; Sigma-Aldrich), mouse monoclonal anti-SERCA3 (PL/IM430; [
39]), mouse monoclonal anti-β-actin (AC-15; Sigma-Aldrich), mouse monoclonal anti-ER-α (6F11; Invitrogen), rabbit polyclonal anti-ER-β (Invitrogen) and rabbit polyclonal anti-acetyl-histone H3 (Lys9/Lys14) (Cell Signaling). Signals of the secondary, HRP-conjugated anti-mouse or anti-rabbit antibodies (Jackson ImmunoResearch) were detected using the Pierce ECL Western Blotting Substrate (Thermo Scientific) and luminography on CL-XPosure Film (Thermo Scientific) or Amersham Hyperfilm ECL (GE Healthcare) films. Densitometric analyses were carried out using the ImageJ software v1.51j8, and data were processed with the Prism 4 software v4.01 (GraphPad Software) and expressed as means ± SEM.
Immunocytochemistry
The procedure of immunocytochemical staining was performed as described previously [
29] with a mouse monoclonal anti-PMCA4b (JA3) primary antibody [
37] and an Alexa Fluor 488-conjugated anti-mouse IgG secondary antibody (Invitrogen). Images were taken by a Zeiss LSM710 confocal laser scanning microscope equipped with a Plan-Apochromat 63×/1.40 oil immersion objective and Zeiss ZEN software. Images of control and treated wells from the same cell line were taken with the same microscope settings.
Ca2+ signal measurement
GCaMP2-MCF-7 and GCaMP2-MDA-MB-231 cell lines were used for Ca2+ signal measurements after VPA treatment. Cells were treated with 4 mM VPA for 4 days. In the case of E2 treatments, GCaMP2-MCF-7 cells were preincubated in E2-free DMEM (DMEM w/o phenol red, supplemented with 10% charcoal-stripped FBS (Gibco), 100 U/ml penicillin, 100 μg/ml streptomycin and 2 mM glutamine) for 2 days and then treated with 1 nM E2 for 4 days. Immediately before the Ca2+ signal measurement, medium was replaced by HBSS supplemented with with 0.9 mM MgCl2, 2 mM CaCl2 and 20 mM HEPES; pH 7.4. Ca2 influx was triggered by the addition of 2 μM A23187 (Sigma-Aldrich). Fluorescent signal of the GCaMP2 Ca2+ sensor was detected by confocal imaging with a Zeiss LSM710 confocal laser scanning microscope and Plan-Apochromat 63×/1.40 oil immersion objective. Time lapse images were recorded every 0.3 s at room temperature with the Zeiss ZEN software. Data were analyzed with the ImageJ software v1.51j8 and Prism 4 software v4.01 (GraphPad Software). The relative fluorescence intensities were calculated as F/F0 (where F0 was the average initial fluorescence) and are expressed as means ±95% CI. Statistical significance was calculated by t-test, *** means P < 0.001, ** means P < 0.01, * means P < 0.05, n.s. means not significant.
Analysis of publicly available ChIP-seq data
ChIP-seq data analysis was performed using the Cistrome Data Browser [
40,
41]. Used raw data from the Gene Expression Omnibus (GEO) [
42,
43] are: GSM798428 (ER-α Chip-seq data in T-47D cells, this data sample is in the GEO data series GSE32222 [
44]), GSM798424 (ER-α Chip-seq data in MCF-7 cells, this data sample is in the GEO data series GSE32222 [
44]), GSM986089 (ER-α Chip-seq data in ZR-75-1 cells, this data sample is in the GEO data series GSE40129 [
45]), GSM986063 (ER-α Chip-seq data in MCF-7 cells, this data sample is in the GEO data series GSE40129 [
45]), GSM2040043 (ER-α Chip-seq data in MCF-7 cells [
46]), GSM2467223 (ER-α Chip-seq data in MCF-7 cells [Dzida et al., unpublished]). Chip-seq data were visualized using the UCSC Genome Browser [
47,
48] on Human Dec. 2013 (GRCh38/hg38) Assembly.
Discussion
Altered expression of the members of the Ca
2+ signaling toolkit frequently occurs during cancer progression. However, the literature does not contain much published research on Ca
2+ pumps in tumors. In breast cancer cells upregulated
ATP2B1,
ATP2B2 mRNA and downregulated
ATP2B4 mRNA expressions were described previously [
24,
25]. Using bioinformatic tools we found that the expression of the
ATP2B4 gene is significantly lower in invasive breast cancer tissue samples than in normal breast tissue, while that of the
ATP2B1 and
ATP2B2 genes did not show considerable differences. However, in vitro examination of Ca
2+ pump protein levels in different breast cancer cell lines showed various expression levels of PMCA4b and other PMCA isoforms. The function of isoform 2 in the normal physiology of breast epithelium during lactation is well documented [
52,
55‐
57]. More recent studies examined the role of PMCA2 in breast cancer [
26‐
28]. Comparing different breast cancer subtypes we found that
ATP2B2 mRNA expression was elevated in basal type cancers, and correlated positively with survival [
28]. Controversially, elevated
ATP2B2 expression was found to be associated with poor clinical outcome in breast cancer in another study [
27]. PMCA2 also regulates HER2 signaling in HER2 positive breast cancer cells [
26]. HER2 was found to interact with PMCA2 in specific membrane domains where the low local Ca
2+ concentration supports sustained HER2 signaling and tumor growth [
26]. In our in vitro experiments PMCA2 protein expression was low in all examined breast cancer cell lines, with PMCA1 and PMCA4b being the main isoforms, although both were expressed at variable levels. We found considerable SERCA3 protein (encoded by the ATP2A3 gene) expression in all the examined luminal and HER2 overexpressing cell lines, while it was not detectable in the triple negative, basal subtype cells including MCF-10A. This observation is in accordance with former studies that showed loss of SERCA3 expression during tumorigenesis, and decreased SERCA3 expression in triple negative breast cancers [
15].
HDAC inhibitors such as VPA or SAHA are currently being developed for various types of malignancies, including breast cancer [
6,
58], and are already in clinical use for peripheral T cell lymphomas [
59,
60]. In this work VPA and SAHA, two structurally different HDAC inhibitors increased PMCA4b expression in breast cancer cells, and enhanced PMCA4b expression induced by E2 in MCF-7 cells. Unlike that of E2, the effect of VPA and SAHA on PMCA4b expression was not inhibited by fulvestrant. These observations taken together indicate that PMCA4b expression in MCF-7 cells is controlled by ER-α-, as well as by HDAC-dependent chromatin remodeling. MCF-7 cells constitute a unique in vitro model for the study of the E2-dependent control of PMCA4b expression and of the cross-talk between sex hormone and Ca
2+ signaling pathways.
The effect of the HDAC inhibitors was much less pronounced in the triple negative cells, where the initial PMCA4b expression was higher. We found that in the triple negative MDA-MB-231 cells a considerable amount of PMCA4b protein was located in intracellular compartments, suggesting that PMCA4b cannot perform its plasma membrane-associated function, similarly to that seen in non-confluent HeLa cells [
51]. It is important to note that mislocalization of other proteins, including those with tumor suppressor function has been reported in association with cancer development and metastasis [
61]. It has been shown that ER-α positive breast tumors express HDAC proteins at higher amounts [
49,
50], and that the antiproliferative effect of HDAC inhibitors is also more potent in the ER-α positive breast cancer cell lines [
62]. Therefore, it is not surprising that in our experiments HDAC inhibitors showed higher effect on PMCA4b expression in the luminal type cell lines. However, histone acetylation levels and PMCA4b upregulation did not correlate tightly in the HER2 overexpressing or basal cell lines.
ATP2A3 mRNA was found to be overexpressed after HDAC inhibitor treatment both in the MCF-7 and MDA-MB-231 cell lines [
63] but we could not find any significant changes in SERCA3 protein expression in response to VPA or SAHA. Several studies aim to elucidate the mechanism of HDAC inhibition to develop more potent strategies for cancer treatment. Some of these studies showed that HDAC inhibitors induced expression of ER-α in triple negative breast cancer cell lines or reversed hormone resistance of the ER-α positive cells [
49,
64,
65]. These ER-α negative or tamoxifen/aromatase inhibitor-resistant breast tumors could be sensitized to anti-estrogen therapies by HDAC inhibitors that induce the expression of the epigenetically repressed ER-α receptor. Clinical trials have also been performed with HDAC inhibitors alone or in combination with tamoxifen [
64,
66]. Although, some results were promising, controversial data were also reported in other studies, in which HDAC inhibitors did not induce ER-α expression in the triple negative breast cancer cells [
6,
67]. In our experiments ER-α protein expression in MDA-MB-231 cells was also not affected by VPA or SAHA.
As we already mentioned above, here we describe for the first time that activation of the ER-α pathway increases PMCA4b protein expression in MCF-7 cells. However, the expression of the protein did not change in the other examined ER-α positive breast cancer cell lines ZR-75-1, T-47D, BT-474 or in the ER-α negative MDA-MB-231 cells. Our results are in line with those studies that report complex regulation of the ER-α-induced transcription of target genes [
34,
64,
68]. The in-depth analysis of a gene chip assay revealed upregulation of the
ATP2B4 gene in response to E2 stimulation of MCF-7 cells [
68]. Interestingly,
ATP2B4 was also upregulated by E2 in MDA-MB-231 cells stably expressing exogenous ER-α [
68]. Stender et al. discussed that the ability of ER-α to regulate gene expression in different cell lines in a different way depended on many factors, such as varying transcription factor expression and activity, different chromatin structure or epigenetic modifications [
68]. Hilborn et al. examined the effect of E2 on the expression of hydroxysteroid 17β-dehydrogenase (
HSD17B) 1 and 2 [
69]. The protein products of
HSD17B1 and
HSD17B2 play an important role in controlling E2 activity. In their experiments
HSD17B2 expression was upregulated after a 7-days E2 treatment in MCF-7 but not in T-47D and ZR-75-1 cells [
69]. This result further supports the idea that different ER-α positive breast cancer cell lines use different regulatory pathways in response to ER-α activation, and are in accordance with our results indicating that among the ER-α positive cell lines only MCF-7 is E2-responsive in terms of elevation of PMCA4b expression.
The G protein-coupled estrogen receptor 1 (GPER/GRP30) plays a role in non-genomic ER-α signaling [
64]. Besides mediating a wide range of cellular processes, such as activation of the cAMP, ERK1/2 and PI3K pathways or intracellular Ca
2+ mobilization [
70], GPER/GRP30 can affect metastasis progression, as it was reported to regulate the cell-matrix adhesion of MCF-7 cells through the ERK1/2-calpain pathway [
71]. Previously, PMCA4 was found to form a protein complex with the GPER/GRP30, in which GPER/GPR30 inhibits PMCA4 activity and PMCA4 also affects GPER/GRP30 function [
72]. GPER/GPR30 was described as a E2-binding receptor, but it can be activated both by E2 and the pure ER-α antagonist fulvestrant [
70]. In our experimental system, fulvestrant completely inhibited the E2-induced PMCA4 upregulation, suggesting that GPER/GPR30 is not involved in this process. Further studies are needed to clarify the exact mechanisms of E2 on the regulation of Ca
2+ pump expression.
E2-dependent signaling is central for the physiological regulation of normal breast epithelial cell function, growth, differentiation and survival, and estrogen directs the growth also of ER-α positive breast cancer cells [
73]. Antiestrogen therapy by agents such as fulvestrant is therefore a mainstay of ER-α positive breast cancer therapy [
74]. Data presented in this work show that ER signaling, and its pharmacological modulation modify the Ca
2+ homeostasis of cells via the regulation of PMCA4b expression. It has been shown earlier that, due to its slow activation/inactivation kinetics, the presence or the absence of PMCA4b determines the spatiotemporal characteristics of SOCE-type Ca
2+ transients and oscillations [
19], which in turn affect cell survival, motility and proliferation. In addition, PMCA4b expression has been shown earlier in normal ductal mammary epithelial cells in situ [
29]. The demonstration of the E2 dependency of PMCA4b expression is therefore an interesting new aspect of mammary epithelial cell differentiation present in ER-α positive breast cancer cells, that constitutes a previously unknown mechanism of cross-talk between E2- and Ca
2+-dependent signaling. This may be exploited in the future, for example for devising new combination anticancer therapies whereby E2- and Ca
2+-dependent signaling mechanisms are simultaneously targeted in breast cancer cells.
It is also well known that the expression of various PMCA isoforms is tightly regulated in breast tissue, especially during pregnancy and lactation [
55‐
57], further highlighting the physiological importance of our findings. The main PMCA isoform is PMCA4b in the developing rat mammary tissue, and its expression is increased during pregnancy. However, PMCA4b protein shows a major rapid downregulation after parturition when PMCA2b is significantly upregulated, and plays an essential role in transporting Ca
2+ into the milk during lactation [
52]. The changes in PMCA4b abundance coincided with gradually increasing serum E2 levels during pregnancy that drastically drop prior to parturition [
75]. While further evidence is needed to prove a direct interaction between the ER and PMCA4, these results suggest that the effect of E2 on PMCA4 expression is physiologically relevant. Moreover, the altered PMCA4 expression and the reshaped Ca
2+ signal pattern in breast tumor cells suggest that the protein might have an important role in neoplasia.