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Store-operated Ca²⁺ entry (SOCE) is a major pathway for Ca²⁺ entry that regulates several cellular functions. SOCE remodeling mediated by changes in the expression and/or function of the Orai channels results in the reorganization of intracellular Ca 2+ homeostasis leading to a variety of pathologies, including cancer. Notably, a significant alteration of Orai function has been reported in breast cancer cells, where the dysregulation of the Notch1 signaling pathway plays a role in the development and progression of cancer hallmarks. Here, we have investigated the possible role of Notch1 in the regulation of the expression of Orai1 and Orai3 in different breast cancer cell lines. Expression of the active form of Notch1, as well as cell stimulation with the Notch1 agonist Jagged-1 (Jag-1), demonstrates a differential role of Notch1 in the regulation of Orai expression in non-tumoral breast epithelial cells and triple negative or luminal breast cancer cells. The role of Notch1 was confirmed using DAPT, a γ-secretase inhibitor that prevents activation of the Notch pathway. Modulation of Orai1 and Orai3 expression by Notch1 was paralleled by changes in SOCE. The effect in Orai expression mediated by activation of Notch1 signaling pathway was mimicked by the expression of HEY1 or the non-phosphorylatable HEY1-S68A mutant; by contrast, expression of the phosphomimetic HEY1-S68D mutant was without effect on Orai expression. Understanding the Notch1-HEY1-Orai axis might provide insights into the development of subtype-specific therapeutic strategies targeting breast cancer.

Orai1 is the pore-forming subunit of the Ca 2+ -release activated Ca 2+ channels that mediate store-operated Ca 2+ entry (SOCE) in excitable and non-excitable cells. Two Orai1 forms have been identified in mammalian cells, the full-length variant Orai1α, and the short form Orai1β, lacking the N-terminal 63 amino acids. Stem cells were isolated from non-tumoral breast epithelial cells of the MCF10A cell line, and the most representative ER+ , HER2 or triple negative breast cancer cell lines MCF7, SKBR3 and MDA-MB-231, respectively. Orai and TRPC family members expression was detected by RT-PCR and Western blotting. Changes in cytosolic Ca 2+ concentration were analyzed by confocal microscopy using Fluo 4 and the spheroid-forming ability and self-renewal was estimated in culture plates coated with pHEMA using a cell imaging system. Here, we have characterized the expression of Orai family members and several TRPC channels at the transcript level in breast stem cells (BSC) derived from the non-tumoral breast epithelial cell line MCF10A and breast cancer stem cells (BCSC) derived from the well-known estrogen receptor positive (ER+), HER2 and triple negative cell lines MCF7, SKBR3 and MDA-MB-231, respectively. Furthermore, we have evaluated the mammosphere formation efficiency and self-renewal of the BSC and BCSC. Next, through a combination of Orai1 knockdown by iRNA and the use of MDA-MB-231 KO cells, missing the native Orai1, transfected with plasmids encoding for either Orai1α or Orai1β, we show that Orai1 is essential for mammosphere formation and self-renewal efficiency in BCSC derived from triple negative and HER2 subtypes cell cultures, while this channel has a negligible effect in BCSC derived from ER+ cells as well as in non-tumoral BSC. Both, Orai1α, and Orai1β support SOCE in MDA-MB-231-derived BCSC with similar efficiency, as well as COX activation and mammosphere formation. These findings provide evidence of the functional role of Orai1α and Orai1β in spheroid forming efficiency and self-renewal in breast cancer stem cells.

Transient receptor potential channels convey signaling information from a number of stimuli to a wide variety of cellular functions, mainly by inducing changes in cytosolic Ca2+ concentration. Different members of the TRPC, TRPM and TRPV subfamilies have been reported to play a role in tumorigenesis. Here we show that the estrogen receptor positive and triple negative breast cancer cell lines, MCF7 and MDA-MB-231, respectively, exhibit enhanced expression of the TRPC6 channel as compared to the non-tumoral MCF10A cell line. In vitro TRPC6 knockdown using shRNA impaired MCF7 and MDA-MB-231 cell proliferation, migration and invasion detected by BrdU incorporation, wound healing and Boyden chamber assays, respectively. Using RNAi-mediated TRPC6 silencing as well as overexpression of the pore-dead dominant-negative TRPC6 mutant we have found that TRPC6 plays a relevant role in the activation of store-operated Ca2+ entry in the breast cancer cell lines but not in non-tumoral breast cells. Finally, we have found that TRPC6 interacts with Orai1 and Orai3 in MCF7 and MDA-MB-231 cells and is required for the translocation of Orai1 and Orai3 to the plasma membrane in MDA-MB-231 and MCF7 cells, respectively, upon Ca2+ store depletion. These findings introduce a novel mechanism for the modulation of Ca2+ influx and the development of different cancer hallmarks in breast cancer cells.

Transient receptor potential (TRP) proteins form non-selective Ca2+ permeable channels that contribute to the modulation of a number of physiological functions in a variety of cell types. Since the identification of TRP proteins in Drosophila, it is well known that these channels are activated by stimuli that induce PIP2 hydrolysis. The canonical TRP (TRPC) channels have long been suggested to be constituents of the store-operated Ca2+ (SOC) channels; however, none of the TRPC channels generate Ca2+ currents that resemble ICRAC. STIM1 and Orai1 have been identified as the components of the Ca2+ release-activated Ca2+ (CRAC) channels and there is a body of evidence supporting that STIM1 is able to gate Orai1 and TRPC1 in order to mediate non-selective cation currents named ISOC. STIM1 has been found to interact to and activate Orai1 and TRPC1 by different mechanisms and the involvement of TRPC1 in store-operated Ca2+ entry requires both STIM1 and Orai1. In addition to the participation of TRPC1 in the ISOC currents, TRPC1 and other TRPC proteins might play a relevant role modulating Orai1 channel function. This review summarizes the functional role of TRPC channels in the STIM1–Orai1 scenario.

Breast cancer is a heterogeneous disease from the histological and molecular expression point of view, and this heterogeneity determines cancer aggressiveness. Store-operated Ca2+ entry (SOCE), a major mechanism for Ca2+ entry in non-excitable cells, is significantly remodeled in cancer cells and plays an important role in the development and support of different cancer hallmarks. The store-operated CRAC (Ca2+ release-activated Ca2+) channels are predominantly comprised of Orai1 but the participation of Orai2 and Orai3 subunits has been reported to modulate the magnitude of Ca2+ responses. Here we provide evidence for a heterogeneous expression of Orai2 among different breast cancer cell lines. In the HER2 and triple negative breast cancer cell lines SKBR3 and BT20, respectively, where the expression of Orai2 was greater, Orai2 modulates the magnitude of SOCE and sustain Ca2+ oscillations in response to carbachol. Interestingly, in these cells Orai2 modulates the activation of NFAT1 and NFAT4 in response to high and low agonist concentrations. Finally, we have found that, in cells with high Orai2 expression, Orai2 knockdown leads to cell cycle arrest at the G0-G1 phase and decreases apoptosis resistance upon cisplatin treatment. Altogether, these findings indicate that, in breast cancer cells with a high Orai2 expression, Orai2 plays a relevant functional role in agonist-evoked Ca2+ signals, cell proliferation and apoptosis resistance.

The STIM family of proteins plays a crucial role in a plethora of cellular functions through the regulation of store-operated Ca2+ entry (SOCE) and, thus, intracellular calcium homeostasis. The two members of the mammalian STIM family, STIM1 and STIM2, are transmembrane proteins that act as Ca2+ sensors in the endoplasmic reticulum (ER) and, upon Ca2+ store discharge, interact with and activate the Orai/CRACs in the plasma membrane. Dysregulation of Ca2+ signaling leads to the pathogenesis of a variety of human diseases, including neurodegenerative disorders, cardiovascular diseases, cancer, and immune disorders. Therefore, understanding the mechanisms underlying Ca2+ signaling pathways is crucial for developing therapeutic strategies targeting these diseases. This review focuses on several rare conditions associated with STIM1 mutations that lead to either gain- or loss-of-function, characterized by myopathy, hematological and immunological disorders, among others, and due to abnormal activation of CRACs. In addition, we summarize the current evidence concerning STIM2 allele duplication and deletion associated with language, intellectual, and developmental delay, recurrent pulmonary infections, microcephaly, facial dimorphism, limb anomalies, hypogonadism, and congenital heart defects.

Background/Aims: STIM1 and Orai1 are the key components of store-operated Ca 2+ entry (SOCE). Among the proteins involved in the regulation of SOCE, SARAF prevents spontaneous activation of SOCE and modulates STIM1 function. Methods: Cytosolic Ca 2+ mobilization was estimated in fura-2-loaded cells using an epifluorescence inverted microscope. STIM1 interaction with Orai1, EFHB (EF-hand domain family member B, also known as CFAP21) and SARAF was detected by immunoprecipitation followed by Western blotting using specific antibodies. The involvement of EFHB in the translocation of NFAT to the nucleus was detected by confocal microscopy. Results: Here, we report the identification of EFHB as a new SOCE regulator. EFHB interacts with STIM1 upon store depletion and dissociates through a Ca 2+ -dependent mechanism. RNAi-mediated silencing as well as overexpression studies revealed that EFHB plays a relevant role in the interaction of STIM1 and Orai1 upon store depletion, the activation of SOCE and NFAT translocation from the cytosol to the nucleus. Silencing EFHB expression abolished the dissociation of SARAF from STIM1, which indicates that EFHB might play an important role in the dynamic interaction between both proteins, which is relevant for the activation of Orai1 channels upon Ca 2+ store depletion and their subsequent modulation via slow Ca 2+ -dependent inactivation. Conclusion: Our results indicate that EFHB is a new SOCE regulator that modulates STIM1-SARAF interaction.

The systemic effects of physical activity are mediated by the release of IL-6 and other myokines from contracting muscle. Although the release of IL-6 from muscle has been extensively studied, the information on the cellular mechanisms is fragmentary and scarce, especially regarding the role of Ca2+ signals. The aim of this study was to characterize the role of the main components of Ca2+ signals in human skeletal muscle cells during IL-6 secretion stimulated by the Ca2+ mobilizing agonist ATP. Primary cultures were prepared from surgical samples, fluorescence microscopy was used to evaluate the Ca2+ signals and the stimulated release of IL-6 into the medium was determined using ELISA. Intracellular calcium chelator Bapta, low extracellular calcium and the Ca2+ channels blocker La3+ reduced the ATP-stimulated, but not the basal secretion. Secretion was inhibited by blockers of L-type (nifedipine, verapamil), T-type (NNC55-0396) and Orai1 (Synta66) Ca2+ channels and by silencing Orai1 expression. The same effect was achieved with inhibitors of ryanodine receptors (ryanodine, dantrolene) and IP3 receptors (xestospongin C, 2-APB, caffeine). Inhibitors of calmodulin (calmidazolium) and calcineurin (FK506) also decreased secretion. IL-6 transcription in response to ATP was not affected by Bapta or by the T channel blocker. Our results prove that ATP-stimulated IL-6 secretion is mediated at the post-transcriptional level by Ca2+ signals, including the mobilization of calcium stores, the activation of store-operated Ca2+ entry, and the subsequent activation of voltage-operated Ca2+ channels and calmodulin/calcineurin pathways.

Tumors are composed by a heterogeneous population of cells. Among them, a sub-population of cells, termed cancer stem cells, exhibit stemness features, such as self-renewal capabilities, disposition to differentiate to a more proliferative state, and chemotherapy resistance, processes that are all mediated by Ca2+. Ca2+ homeostasis is vital for several physiological processes, and alterations in the patterns of expressions of the proteins and molecules that modulate it have recently become a cancer hallmark. Store-operated Ca2+ entry is a major mechanism for Ca2+ entry from the extracellular medium in non-excitable cells that leads to increases in the cytosolic Ca2+ concentration required for several processes, including cancer stem cell properties. Here, we focus on the participation of STIM, Orai, and TRPC proteins, the store-operated Ca2+ entry key components, in cancer stem cell biology and tumorigenesis.

The role of cAMP in spermatic functions was classically thought to be mediated exclusively through the activation of Protein Kinase A (PKA). However, it has recently been shown that cAMP also exerts its effects through a PKA-independent pathway activating a family of proteins known as Epac proteins. Therefore, many of the spermatic functions thought to be regulated by cAMP through the activation of PKA are again under study. We aimed to identify and to investigate the role of Epac proteins in spermatozoa using a specific permeable analog (8-Br-2'-O-Me-cAMP). Also, we aimed to study its relationship with E-cadherin, an adhesion protein involved in fertility. Our results demonstrate the presence and sub-cellular distribution of Epac 1 and Epac 2 in mammalian spermatozoa. Capacitation and the acrosome reaction induced a change in the localization of Epac proteins in sperm. Moreover, incubation with 8-Br-2'-O-Me-cAMP prompted an increase in Rap1 activation, in the scrambling of plasma membrane phospholipids (necessary for the capacitation process), the acrosome reaction, motility, and calcium mobilization, when spermatozoa were incubated in acrosome reaction conditions. Finally, the activation of Epac proteins induced a change in the distribution of E-cadherin. Therefore, the increase in the acrosome reaction, together with the increase in calcium (which is known to be essential for fertilization) and the Epac nteraction with E-cadherin, might indicate that Epac proteins have an important role in gamete recognition and fertilization.