Evidence for STIM1- and Orai1-dependent store-operated calcium influx through ICRAC in vascular smooth muscle cells: role in proliferation and migration

Nonsteroidal anti-inflammatory drugs inhibit vascular smooth muscle cell proliferation by enabling the Ca2+-dependent inactivation of calcium release-activated calcium/orai channels normally prevented by mitochondria

Abnormal vascular smooth muscle cell (VSMC) proliferation contributes to occlusive and proliferative disorders of the vessel wall. Salicylate and other nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit VSMC proliferation by an unknown mechanism unrelated to anti-inflammatory activity. In search for this mechanism, we have studied the effects of salicylate and other NSAIDs on subcellular Ca(2+) homeostasis and Ca(2+)-dependent cell proliferation in rat aortic A10 cells, a model of neointimal VSMCs. We found that A10 cells displayed both store-operated Ca(2+) entry (SOCE) and voltage-operated Ca(2+) entry (VOCE), the former being more important quantitatively than the latter. Inhibition of SOCE by specific Ca(2+) released-activated Ca(2+) (CRAC/Orai) channels antagonists prevented A10 cell proliferation. Salicylate and other NSAIDs, including ibuprofen, indomethacin, and sulindac, inhibited SOCE and thereby Ca(2+)-dependent, A10 cell proliferation. SOCE, but not VOCE, induced mitochondrial Ca(2+) uptake in A10 cells, and mitochondrial depolarization prevented SOCE, thus suggesting that mitochondrial Ca(2+) uptake controls SOCE (but not VOCE) in A10 cells. NSAIDs depolarized mitochondria and prevented mitochondrial Ca(2+) uptake, suggesting that they favor the Ca(2+)-dependent inactivation of CRAC/Orai channels. NSAIDs also inhibited SOCE in rat basophilic leukemia cells where mitochondrial control of CRAC/Orai is well established. NSAIDs accelerate slow inactivation of CRAC currents in rat basophilic leukemia cells under weak Ca(2+) buffering conditions but not in strong Ca(2+) buffer, thus excluding that NSAIDs inhibit SOCE directly. Taken together, our results indicate that NSAIDs inhibit VSMC proliferation by facilitating the Ca(2+)-dependent inactivation of CRAC/Orai channels which normally is prevented by mitochondria clearing of entering Ca(2+).

Local Ca²+ entry via Orai1 regulates plasma membrane recruitment of TRPC1 and controls cytosolic Ca²+ signals required for specific cell functions

Store-operated Ca²⁺ entry (SOCE) has been associated with two types of channels: CRAC channels that require Orai1 and STIM1 and SOC channels that involve TRPC1, Orai1, and STIM1. While TRPC1 significantly contributes to SOCE and SOC channel activity, abrogation of Orai1 function eliminates SOCE and activation of TRPC1. The critical role of Orai1 in activation of TRPC1-SOC channels following Ca²⁺ store depletion has not yet been established. Herein we report that TRPC1 and Orai1 are components of distinct channels. We show that TRPC1/Orai1/STIM1-dependent I_SOC, activated in response to Ca²⁺ store depletion, is composed of TRPC1/STIM1-mediated non-selective cation current and Orai1/STIM1-mediated I_CRAC; the latter is detected when TRPC1 function is suppressed by expression of shTRPC1 or a STIM1 mutant that lacks TRPC1 gating, STIM1(⁶⁸⁴EE⁶⁸⁵). In addition to gating TRPC1 and Orai1, STIM1 mediates the recruitment and association of the channels within ER/PM junctional domains, a critical step in TRPC1 activation. Importantly, we show that Ca²⁺ entry via Orai1 triggers plasma membrane insertion of TRPC1, which is prevented by blocking SOCE with 1 µM Gd³⁺, removal of extracellular Ca²⁺, knockdown of Orai1, or expression of dominant negative mutant Orai1 lacking a functional pore, Orai1-E106Q. In cells expressing another pore mutant of Orai1, Orai1-E106D, TRPC1 trafficking is supported in Ca²⁺-containing, but not Ca²⁺-free, medium. Consistent with this, I_CRAC is activated in cells pretreated with thapsigargin in Ca²⁺-free medium while I_SOC is activated in cells pretreated in Ca²⁺-containing medium. Significantly, TRPC1 function is required for sustained K_Ca activity and contributes to NFκB activation while Orai1 is sufficient for NFAT activation. Together, these findings reveal an as-yet unidentified function for Orai1 that explains the critical requirement of the channel in the activation of TRPC1 following Ca²⁺ store depletion. We suggest that coordinated regulation of the surface expression of TRPC1 by Orai1 and gating by STIM1 provides a mechanism for rapidly modulating and maintaining SOCE-generated Ca²⁺ signals. By recruiting ion channels and other signaling pathways, Orai1 and STIM1 concertedly impact a variety of critical cell functions that are initiated by SOCE.

Store-operated Ca²⁺ entry is present in all cell types and determines sustained cytosolic [Ca²⁺] increases that are critical for regulating a wide variety of physiological functions. This Ca²⁺ entry mechanism is activated in response to depletion of Ca²⁺ in the endoplasmic reticulum (ER). When ER [Ca²⁺] is decreased, the Ca²⁺-sensor protein STIM1 aggregates in the ER membrane and moves to regions in the periphery of the cells where it interacts with and activates two major types of channels that contribute to store-operated Ca²⁺ entry: CRAC and SOC. While gating of Orai1 by STIM1 is sufficient for CRAC channel activity, both Orai1 and transient receptor potential channel 1 (TRPC1) contribute to SOC channel function. The molecular composition of SOC channels and the critical role of Orai1 in activation of TRPC1 have not yet been established. In this study, we demonstrate that TRPC1 and Orai1 are components of distinct channels, both of which are regulated by STIM1. Importantly, we show that Orai1-mediated Ca²⁺ entry triggers plasma membrane insertion of TRPC1 which is then gated by STIM1. Ca²⁺ entry via functional TRPC1-STIM1 channels provides additional increase in cytosolic [Ca²⁺] that is required for regulation of specific cell functions such as K_Ca activation. Together, our findings elucidate the critical role of Orai1 in TRPC1 channel function. We suggest that the regulation of TRPC1 trafficking provides a mechanism for rapidly modulating cytosolic [Ca²⁺] following Ca²⁺ store depletion.

Role of STIM1/Orai1-mediated store-operated Ca²⁺ entry in airway smooth muscle cell proliferation

Hyperplasia of airway smooth muscle cells (ASMCs) is a characteristic change of chronic asthma patients. However, the underlying mechanisms that trigger this process are not yet completely understood. Store-operated Ca(2+) (SOC) entry (SOCE) occurs in response to the intracellular sarcoplasma reticulum (SR)/endoplasmic reticulum (ER) Ca(2+) store depletion. SOCE plays an important role in regulating Ca(2+) signaling and cellular responses of ASMCs. Stromal interaction molecule (STIM)1 has been proposed as an ER/SR Ca(2+) sensor and translocates to the ER underneath the plasma membrane upon depletion of the ER Ca(2+) store, where it interacts with Orai1, the molecular component of SOC channels, and brings about SOCE. STIM1 and Orai1 have been proved to mediate SOCE of ASMCs. In this study, we investigated whether STIM1/Orai1-mediated SOCE is involved in rat ASMC proliferation. We found that SOCE was upregulated during ASMC proliferation accompanied by a mild increase of STIM1 and a significant increase of Orai1 mRNA expression, whereas the proliferation of ASMCs was partially inhibited by the SOC channel blockers SKF-96365, NiCl(2), and BTP-2. Suppressing the mRNA expression of STIM1 or Orai1 with specific short hairpin RNA resulted in the attenuation of SOCE and ASMC proliferation. Moreover, after knockdown of STIM1 or Orai1, the SOC channel blocker SKF-96365 had no inhibitory effect on the proliferation of ASMCs anymore. These results suggested that STIM1/Orai1-mediated SOCE is involved in ASMC proliferation.

The physiological function of store-operated calcium entry

Store-operated Ca(2+) entry is a process whereby the depletion of intracellular Ca(2+) stores signals the opening of plasma membrane Ca(2+) channels. It has long been thought that the main function of store-operated Ca(2+) entry was the replenishment of intracellular Ca(2+) stores following their discharge during intracellular Ca(2+) signaling. Recent results, however, suggest that the primary function of these channels may be to provide direct Ca(2+) signals to recipients localized to spatially restricted areas close to the sites of Ca(2+) entry in order to initiate specific signaling pathways.

Down-regulation of Orai3 arrests cell-cycle progression and induces apoptosis in breast cancer cells but not in normal breast epithelial cells

Breast cancer (BC) is the leading cancer in the world in terms of incidence and mortality in women. However, the mechanism by which BC develops remains largely unknown. The increase in cytosolic free Ca(2+) can result in different physiological changes including cell growth and death. Orai isoforms are highly Ca(2+) selective channels. In the present study, we analyzed Orai3 expression in normal and cancerous breast tissue samples, and its role in MCF-7 BC and normal MCF-10A mammary epithelial cell lines. We found that the expression of Orai3 mRNAs was higher in BC tissues and MCF-7 cells than in normal tissues and MCF-10A cells. Down-regulation of Orai3 by siRNA inhibited MCF-7 cell proliferation and arrested cell cycle at G1 phase. This phenomenon is associated with a reduction in CDKs 4/2 (cyclin-dependent kinases) and cyclins E and D1 expression and an accumulation of p21(Waf1/Cip1) (a cyclin-dependent kinase inhibitor) and p53 (a tumor-suppressing protein). Orai3 was also involved in MCF-7 cell survival. Furthermore, Orai3 mediated Ca(2+) entry and contributed to intracellular calcium concentration ([Ca(2+)](i)). In MCF-10A cells, silencing Orai3 failed to modify [Ca(2+)](i), cell proliferation, cell-cycle progression, cyclins (D1, E), CDKs (4, 2), and p21(Waf1/Cip1) expression. Our results provide strong evidence for a significant effect of Orai3 on BC cell growth in vitro and show that this effect is associated with the induction of cell cycle and apoptosis resistance. Our study highlights a possible role of Orai3 as therapeutic target in BC therapy.

STIM2 Contributes to Enhanced Store-operated Ca Entry in Pulmonary Artery Smooth Muscle Cells from Patients with Idiopathic Pulmonary Arterial Hypertension

Pulmonary vasoconstriction and vascular remodeling are two major causes for elevated pulmonary vascular resistance and pulmonary arterial pressure in patients with idiopathic pulmonary arterial hypertension (IPAH). An increase in cytosolic free Ca²⁺concentration ([Ca²⁺]_cyt) in pulmonary artery smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction and an important stimulus for PASMC proliferation, which causes pulmonary vascular remodeling. Store-operated Ca²⁺ entry (SOCE), induced by depletion of stored Ca²⁺ in the sarcoplasmic reticulum (SR), can increase [Ca²⁺]_cyt in PASMC, independent of other means of Ca²⁺ entry. Stromal interaction molecule (STIM) proteins, STIM1 and STIM2, were both recently identified as sensors for store depletion and also signaling molecules to open store-operated Ca²⁺ channels. We previously reported that SOCE was significantly enhanced in PASMC from IPAH patients compared to PASMC from normotensive control subjects. Enhanced SOCE plays an important role in the pathophysiological changes in PASMC associated with pulmonary arterial hypertension. In this study, we examine whether the expression levels of STIM1 and STIM2 are altered in IPAH-PASMC compared to control PASMC, and whether these putative changes in the STIM1 and STIM2 expression levels are responsible for enhanced SOCE and proliferation in IPAH-PASMC. Compared to control PASMC, the protein expression level of STIM2 was significantly increased in IPAH-PASMC, whereas STIM1 protein expression was not significantly changed. In IPAH-PASMC, the small interfering RNA (siRNA)-mediated knockdown of STIM2 decreased SOCE and proliferation, while knockdown of STIM2 in control PASMC had no effect on either SOCE or proliferation. Overexpression of STIM2 in the control PASMC failed to enhance SOCE or proliferation. These data indicate that enhanced protein expression of STIM2 is necessary, but not sufficient, for enhanced SOCE and proliferation of IPAH-PASMC.

SERCA2a controls the mode of agonist-induced intracellular Ca2+ signal, transcription factor NFAT and proliferation in human vascular smooth muscle cells

In blood vessels, tone is maintained by agonist-induced cytosolic Ca(2+) oscillations of quiescent/contractile vascular smooth muscle cells (VSMCs). However, in synthetic/proliferative VSMCs, Gq/phosphoinositide receptor-coupled agonists trigger a steady-state increase in cytosolic Ca(2+) followed by a Store Operated Calcium Entry (SOCE) which translates into activation of the proliferation-associated transcription factor NFAT. Here, we report that in human coronary artery smooth muscle cells (hCASMCs), the sarco/endoplasmic reticulum calcium ATPase type 2a (SERCA2a) expressed in the contractile form of the hCASMCs, controls the nature of the agonist-induced Ca(2+) transient and the resulting down-stream signaling pathway. Indeed, restoring SERCA2a expression by gene transfer in synthetic hCASMCs 1) increased Ca(2+) storage capacity; 2) modified agonist-induced IP(3)R Ca(2+) release from steady-state to oscillatory mode (the frequency of agonist-induced IP(3)R Ca(2+) signal was 11.66 ± 1.40/100 s in SERCA2a-expressing cells (n=39) vs 1.37 ± 0.20/100 s in control cells (n=45), p<0.01); 3) suppressed SOCE by preventing interactions between SR calcium sensor STIM1 and pore forming unit ORAI1; 4) inhibited calcium regulated transcription factor NFAT and its down-stream physiological function such as proliferation and migration. This study provides evidence for the first time that oscillatory and steady-state patterns of Ca(2+) transients have different effects on calcium-dependent physiological functions in smooth muscle cells.

Thapsigargin activates Ca²+ entry both by store-dependent, STIM1/Orai1-mediated, and store-independent, TRPC3/PLC/PKC-mediated pathways in human endothelial cells

The ER Ca²+ sensor STIM1 and the Ca²+ channel Orai1 are key players in store-operated Ca²+ entry (SOCE). In addition, channels from the TRPC family were also shown to be engaged during SOCE, while their precise implication remains controversial. In this study, we investigated the molecular players involved in SOCE triggered by the SERCA pump inhibitor thapsigargin in an endothelial cell line, the EA.hy926. siRNA directed against STIM1 or Orai1 reduced Ca²+ entry by about 50-60%, showing that a large part of the entry is independent from these proteins. Blocking the PLC or the PKC pathway completely abolished thapsigargin-induced Ca²+ entry in cells depleted from STIM1 and/or Orai1. The phorbol ester PMA or the DAG analog OAG restored the Ca²+ entry inhibited by PLC blockers, showing an involvement of PLC/PKC pathway in SOCE. Using pharmacological inhibitors or siRNA revealed that the PKCeta is required for Ca²+ entry, and pharmacological inhibition of the tyrosine kinase Src also reduced Ca²+ entry. TRPC3 silencing diminished the entry by 45%, while the double STIM1/TRPC3 invalidation reduced Ca²+ entry by more than 85%. Hence, in EA.hy926 cells, TG-induced Ca²+ entry results from the activation of the STIM1/Orai1 machinery, and from the activation of TRPC3.

Pharmacology of store-operated calcium channels

Store-operated calcium entry is a process by which the depletion of calcium from the endoplasmic reticulum activates calcium influx across the plasma membrane. In the past few years, the major players in this pathway have been identified. STIM1 and STIM2 function as calcium sensors in the endoplasmic reticulum and can interact with and activate plasma membrane channels comprised of Orai1, Orai2, or Orai3 subunits. This review discusses recent advances in our understanding of this widespread signaling mechanism as well as the mechanisms by which a number of interesting pharmacological agents modify it.

Ca(2+) signalling by P2Y receptors in cultured rat aortic smooth muscle cells

Background and purpose:

P2Y receptors evoke Ca²⁺ signals in vascular smooth muscle cells and regulate contraction and proliferation, but the roles of the different P2Y receptor subtypes are incompletely resolved.

Experimental approach:

Quantitative PCR was used to define expression of mRNA encoding P2Y receptor subtypes in freshly isolated and cultured rat aortic smooth muscle cells (ASMC). Fluorescent indicators in combination with selective ligands were used to measure the changes in cytosolic free [Ca²⁺] in cultured ASMC evoked by each P2Y receptor subtype.

Key results:

The mRNA for all rat P2Y receptor subtypes are expressed at various levels in cultured ASMC. Four P2Y receptor subtypes (P2Y1, P2Y2, P2Y4 and P2Y6) evoke Ca²⁺ signals that require activation of phospholipase C and comprise both release of Ca²⁺ from stores and Ca²⁺ entry across the plasma membrane.

Conclusions and implications:

Combining analysis of P2Y receptor expression with functional analyses using selective agonists and antagonists, we isolated the Ca²⁺ signals evoked in ASMC by activation of P2Y1, P2Y2, P2Y4 and P2Y6 receptors.

Orai1 interacts with STIM1 and mediates capacitative Ca2+ entry in mouse pulmonary arterial smooth muscle cells

Previous studies in mouse pulmonary arterial smooth muscle cells (PASMCs) showed that cannonical transient receptor potential channel TRPC1 and stromal interaction molecule 1 (STIM1) mediate the sustained component of capacitative Ca(2+) entry (CCE), but the molecular candidate(s) that mediate the transient component of CCE remain unknown. The aim of the present study was to examine whether Orai1 mediates the transient component of CCE through activation of STIM1 in mouse PASMCs. In primary cultured mouse PASMCs loaded with fura-2, cyclopiazonic acid (CPA) caused a transient followed by a sustained rise in intracellular Ca(2+) concentration ([Ca(2+)](i)). The transient but not the sustained rise in [Ca(2+)](i) was partially inhibited by nifedipine. The nifedipine-insensitive transient rise in [Ca(2+)](i) and the increase in Mn(2+) quench of fura-2 fluorescence caused by CPA were both reduced in cells treated with Orai1 siRNA. These responses to CPA were further reduced in cells treated with Orai1 and STIM1 small interfering (si)RNA. Moreover, overexpression of STIM1 enhanced the rise in [Ca(2+)](i) and the increase in Mn(2+) quench of fura-2 fluorescence caused by CPA, and these responses were reduced in cells treated with Orai1 siRNA. RT-PCR revealed Orai1 and STIM1 mRNAs, and Western blot analysis identified Orai1 and STIM1 proteins in mouse PASMCs. Furthermore, Orai1 was found to coimmunoprecipitate with STIM1, and the precipitation level of Orai1 was increased in cells subjected to store-depletion. Immunostaining revealed colocalization of Orai1 and STIM1 proteins, and the colocalization of these proteins was more apparent after store-depletion. These data provide direct evidence that the transient component of CCE is mediated by Orai1 channel as a result of STIM1 activation in mouse PASMCs.

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.

The calcium store sensor, STIM1, reciprocally controls Orai and CaV1.2 channels

Calcium signals, pivotal in controlling cell function, can be generated by calcium entry channels activated by plasma membrane depolarization or depletion of internal calcium stores. We reveal a regulatory link between these two channel subtypes mediated by the ubiquitous calcium-sensing STIM proteins. STIM1 activation by store depletion or mutational modification strongly suppresses voltage-operated calcium (Ca(V)1.2) channels while activating store-operated Orai channels. Both actions are mediated by the short STIM-Orai activating region (SOAR) of STIM1. STIM1 interacts with Ca(V)1.2 channels and localizes within discrete endoplasmic reticulum/plasma membrane junctions containing both Ca(V)1.2 and Orai1 channels. Hence, STIM1 interacts with and reciprocally controls two major calcium channels hitherto thought to operate independently. Such coordinated control of the widely expressed Ca(V)1.2 and Orai channels has major implications for Ca(2+) signal generation in excitable and nonexcitable cells.

Orai1 contributes to the establishment of an apoptosis-resistant phenotype in prostate cancer cells

The molecular nature of calcium (Ca²⁺)-dependent mechanisms and the ion channels having a major role in the apoptosis of cancer cells remain a subject of debate. Here, we show that the recently identified Orai1 protein represents the major molecular component of endogenous store-operated Ca²⁺ entry (SOCE) in human prostate cancer (PCa) cells, and constitutes the principal source of Ca²⁺ influx used by the cell to trigger apoptosis. The downregulation of Orai1, and consequently SOCE, protects the cells from diverse apoptosis-inducing pathways, such as those induced by thapsigargin (Tg), tumor necrosis factor α, and cisplatin/oxaliplatin. The transfection of functional Orai1 mutants, such as R91W, a selectivity mutant, and L273S, a coiled-coil mutant, into the cells significantly decreased both SOCE and the rate of Tg-induced apoptosis. This suggests that the functional coupling of STIM1 to Orai1, as well as Orai1 Ca²⁺-selectivity as a channel, is required for its pro-apoptotic effects. We have also shown that the apoptosis resistance of androgen-independent PCa cells is associated with the downregulation of Orai1 expression as well as SOCE. Orai1 rescue, following Orai1 transfection of steroid-deprived cells, re-established the store-operated channel current and restored the normal rate of apoptosis. Thus, Orai1 has a pivotal role in the triggering of apoptosis, irrespective of apoptosis-inducing stimuli, and in the establishment of an apoptosis-resistant phenotype in PCa cells.

STIM proteins: integrators of signalling pathways in development, differentiation and disease

The stromal interaction molecules STIM1 and STIM2 are endoplasmic reticulum Ca²⁺ sensors, serving to detect changes in receptor-mediated ER Ca²⁺ store depletion and to relay this information to plasma membrane localized proteins, including the store-operated Ca²⁺ channels of the ORAI family. The resulting Ca²⁺ influx sustains the high cytosolic Ca²⁺ levels required for activation of many intracellular signal transducers such as the NFAT family of transcription factors. Models of STIM protein deficiency in mice, Drosophila melanogaster and Caenorhabditis elegans, in addition to the phenotype of patients bearing mutations in STIM1 have provided great insight into the role of these proteins in cell physiology and pathology. It is now becoming clear that STIM1 and STIM2 are critical for the development and functioning of many cell types, including lymphocytes, skeletal and smooth muscle myoblasts, adipocytes and neurons, and can interact with a variety of signalling proteins and pathways in a cell- and tissue-type specific manner. This review focuses on the role of STIM proteins in development, differentiation and disease, in particular highlighting the functional differences between STIM1 and STIM2.

TRPC channels in smooth muscle cells

Transient receptor potential canonical (TRPC) proteins constitute a family of seven (TRPC1-7) nonselective cation channels within the wider TRP superfamily. TRPC1, TRPC3, TRPC4, TRPC5 and TRPC6 channels are expressed in vascular smooth muscle cells from human vessels of all calibers and in smooth muscle from organs such as the uterus and the gastrointestinal tract. TRPC channels have recently emerged as important players in the control of smooth muscle function. This review will focus on the retrospective analysis of studies proposing contributions of TRPC channels to native calcium entry pathways in smooth muscle and to physiological and pathophysiological responses with emphasis on the vascular system.

ORAI-mediated calcium entry: mechanism and roles, diseases and pharmacology

ORAI1 is a protein located on the plasma membrane that acts as a calcium channel. Calcium enters via ORAI1 as a mechanism to refill the sarcoplasmic/endoplasmic reticulum calcium stores, the depletion of which can be detected by the sensor protein STIM1. Isoforms of these proteins ORAI2, ORAI3 and STIM2 also have roles in cellular calcium homeostasis but are less well characterized. This pathway of filling the calcium stores is termed store-operated calcium entry and while the pathway itself was proposed in 1986, the identity of the key molecular components was only discovered in 2005 and 2006. The characterization of the ORAI and STIM proteins has provided clearer information on some calcium-regulated pathways that are important in processes from gene transcription to immune cell function. Recent studies have also suggested the importance of the components of ORAI-mediated calcium entry in some diseases or processes significant in disease including the migration of breast cancer cells and thrombus formation. This review will provide a brief overview of ORAI-mediated calcium entry, its role in physiological and pathophysiological processes, as well as current and potential pharmacological modulators of the components of this important cellular calcium entry pathway.

A novel native store-operated calcium channel encoded by Orai3: selective requirement of Orai3 versus Orai1 in estrogen receptor-positive versus estrogen receptor-negative breast cancer cells

Store-operated calcium (Ca(2+)) entry (SOCE) mediated by STIM/Orai proteins is a ubiquitous pathway that controls many important cell functions including proliferation and migration. STIM proteins are Ca(2+) sensors in the endoplasmic reticulum and Orai proteins are channels expressed at the plasma membrane. The fall in endoplasmic reticulum Ca(2+) causes translocation of STIM1 to subplasmalemmal puncta where they activate Orai1 channels that mediate the highly Ca(2+)-selective Ca(2+) release-activated Ca(2+) current (I(CRAC)). Whereas Orai1 has been clearly shown to encode SOCE channels in many cell types, the role of Orai2 and Orai3 in native SOCE pathways remains elusive. Here we analyzed SOCE in ten breast cell lines picked in an unbiased way. We used a combination of Ca(2+) imaging, pharmacology, patch clamp electrophysiology, and molecular knockdown to show that native SOCE and I(CRAC) in estrogen receptor-positive (ER(+)) breast cancer cell lines are mediated by STIM1/2 and Orai3 while estrogen receptor-negative (ER(-)) breast cancer cells use the canonical STIM1/Orai1 pathway. The ER(+) breast cancer cells represent the first example where the native SOCE pathway and I(CRAC) are mediated by Orai3. Future studies implicating Orai3 in ER(+) breast cancer progression might establish Orai3 as a selective target in therapy of ER(+) breast tumors.

Wilms tumor suppressor 1 (WT1) and early growth response 1 (EGR1) are regulators of STIM1 expression

Store-operated calcium entry (SOCE) is a key evolutionarily conserved process whereby decreases in endoplasmic reticulum Ca(2+) content lead to the influx of Ca(2+) across the plasma membrane. How this process is regulated in specific tumor cell types is poorly understood. In an effort to address this concern, we obtained and tested primary Wilms tumor cells, finding no detectable SOCE in this cell type. Analysis of the expression levels of STIM1 and ORAI1 (the molecular mediators of SOC) revealed poor STIM1 expression. Analysis of the STIM1 promoter using the TESS search system (University of Pennsylvania) revealed four putative response elements to the zinc-finger proteins WT1 (Wilms tumor suppressor 1) and EGR1 (early growth response 1). Either overexpression of WT1 or knockdown of EGR1 resulted in loss of STIM1 expression and a resultant decrease in SOCE. Furthermore, examination of Egr1 knock-out animals revealed loss of STIM1 expression in multiple tissues. Finally, using chromatin immunoprecipitation, we reveal direct binding of both WT1 and EGR1 to putative response elements located within 500 bp of the transcriptional start site of STIM1. Considering that WT1 and EGR1 are well described oncogenes and tumor suppressors, these observations may reveal new mechanisms responsible for distinct Ca(2+) signals in cancer cells.

Ca2+ signaling and STIM1

An increase in the intracellular calcium ion concentration ([Ca(2+)]) impacts a diverse range of cell functions, including adhesion, motility, gene expression and proliferation. Elevation of intracellular calcium ion (Ca(2+)) regulates various cellular events after the stimulation of cells. Initial increase in Ca(2+) comes from the endoplasmic reticulum (ER), intracellular storage space. However, the continuous influx of extracellular Ca(2+) is required to maintain the increased level of Ca(2+) inside cells. Store-operated Ca(2+) entry (SOCE) manages this process, and STIM1, a newly discovered molecule, has a unique and essential role in SOCE. STIM1 can sense the exhaustion of Ca(2+) in the ER, and activate the SOC channel in the plasma membrane, leading to the continuous influx of extracellular Ca(2+). STIM1 senses the status of the intracellular Ca(2+) stores via a luminal N-terminal Ca(2+)-binding EF-hand domain. Dissociation of Ca(2+) from this domain induces the clustering of STIM1 to regions of the ER that lie close to the plasma membrane, where it regulates the activity of the store-operated Ca(2+) channels/entry (calcium-release-activated calcium channels/entry). In this review, we summarize the mechanism by which STIM1 regulates SOCE, and also its role in the control of mast cell functions and allergic responses.

Interplay between calcium and reactive oxygen/nitrogen species: an essential paradigm for vascular smooth muscle signaling

Signaling cascades initiated or regulated by calcium (Ca(2+)), reactive oxygen (ROS), and nitrogen (RNS) species are essential to diverse physiological and pathological processes in vascular smooth muscle. Stimuli-induced changes in intracellular Ca(2+) regulate the activity of primary ROS and RNS, producing enzymes including NADPH oxidases (Nox) and nitric oxide synthases (NOS). At the same time, alteration in intracellular ROS and RNS production reciprocates through redox-based post-translational modifications altering Ca(2+) signaling networks. These may include Ca(2+) pumps such as sarcoplasmic endoplasmic reticulum Ca(2+)-ATPase (SERCA), voltage-gated channels, transient receptor potential canonical (TRPC), melastatin2 (TRPM2), and ankyrin1 (TRPA1) channels, store operated Ca(2+) channels such as Orai1/stromal interaction molecule 1 (STIM1), and Ca(2+) effectors such as Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). In this review, we summarize and highlight current experimental evidence supporting the idea that cross-talk between Ca(2+) and ROS/RNS may represent a well-integrated signaling network in vascular smooth muscle.

Ion channels and the hallmarks of cancer

Plasma membrane (PM) ion channels contribute to virtually all basic cellular processes and are also involved in the malignant phenotype of cancer cells. Here, we review the role of ion channels in cancer in the context of their involvement in the defined hallmarks of cancer: 1) self-sufficiency in growth signals, 2) insensitivity to antigrowth signals, 3) evasion of programmed cell death (apoptosis), 4) limitless replicative potential, 5) sustained angiogenesis and 6) tissue invasion and metastasis. Recent studies have indicated that the contribution of specific ion channels to these hallmarks varies for different types of cancer. Therefore, to determine the importance of ion channels as targets for cancer diagnosis and treatment their expression, function and regulation must be assessed for each cancer.

CRAC channelopathies

Store-operated Ca2+ entry (SOCE) is an important Ca2+ influx pathway in many non-excitable and some excitable cells. It is regulated by the filling state of intracellular Ca2+ stores, notably the endoplasmic reticulum (ER). Reduction in [Ca2+]ER results in activation of plasma membrane Ca2+ channels that mediate sustained Ca2+ influx which is required for many cell functions as well as refilling of Ca2+ stores. The Ca2+ release activated Ca2+ (CRAC) channel is the best characterized SOC channel with well-defined electrophysiological properties. In recent years, the molecular components of the CRAC channel, long mysterious, have been defined. ORAI1 (or CRACM1) acts as the pore-forming subunit of the CRAC channel in the plasma membrane. Stromal interaction molecule (STIM) 1 is localized in the ER, senses [Ca2+]ER, and activates the CRAC channel upon store depletion by binding to ORAI1. Both proteins are widely expressed in many tissues in both human and mouse consistent with the widespread prevalence of SOCE and CRAC channel currents in many cells types. CRAC channelopathies in human patients with mutations in STIM1 and ORAI1 are characterized by abolished CRAC channel currents, lack of SOCE and-clinically-immunodeficiency, congenital myopathy, and anhydrotic ectodermal dysplasia. This article reviews the role of ORAI and STIM proteins for SOCE and CRAC channel function in a variety of cell types and tissues and compares the phenotypes of ORAI1 and STIM1-deficient human patients and mice with targeted deletion of Orai and Stim genes.

Essential role for STIM1/Orai1-mediated calcium influx in PDGF-induced smooth muscle migration

We recently demonstrated that thapsigargin-induced passive store depletion activates Ca(2+) entry in vascular smooth muscle cells (VSMC) through stromal interaction molecule 1 (STIM1)/Orai1, independently of transient receptor potential canonical (TRPC) channels. However, under physiological stimulations, despite the ubiquitous depletion of inositol 1,4,5-trisphosphate-sensitive stores, many VSMC PLC-coupled agonists (e.g., vasopressin and endothelin) activate various store-independent Ca(2+) entry channels. Platelet-derived growth factor (PDGF) is an important VSMC promigratory agonist with an established role in vascular disease. Nevertheless, the molecular identity of the Ca(2+) channels activated by PDGF in VSMC remains unknown. Here we show that inhibitors of store-operated Ca(2+) entry (Gd(3+) and 2-aminoethoxydiphenyl borate at concentrations as low as 5 microM) prevent PDGF-mediated Ca(2+) entry in cultured rat aortic VSMC. Protein knockdown of STIM1, Orai1, and PDGF receptor-beta (PDGFRbeta) impaired PDGF-mediated Ca(2+) influx, whereas Orai2, Orai3, TRPC1, TRPC4, and TRPC6 knockdown had no effect. Scratch wound assay showed that knockdown of STIM1, Orai1, or PDGFRbeta inhibited PDGF-mediated VSMC migration, but knockdown of STIM2, Orai2, and Orai3 was without effect. STIM1, Orai1, and PDGFRbeta mRNA levels were upregulated in vivo in VSMC from balloon-injured rat carotid arteries compared with noninjured control vessels. Protein levels of STIM1 and Orai1 were also upregulated in medial and neointimal VSMC from injured carotid arteries compared with noninjured vessels, as assessed by immunofluorescence microscopy. These results establish that STIM1 and Orai1 are important components for PDGF-mediated Ca(2+) entry and migration in VSMC and are upregulated in vivo during vascular injury and provide insights linking PDGF to STIM1/Orai1 during neointima formation.

ORAI1 deficiency and lack of store-operated Ca2+ entry cause immunodeficiency, myopathy, and ectodermal dysplasia

BACKGROUND

Defects in the development or activation of T cells result in immunodeficiency associated with severe infections early in life. T-cell activation requires Ca2+ influx through Ca2+-release activated Ca2+ (CRAC) channels encoded by the gene ORAI1.

OBJECTIVE

Investigation of the genetic causes and the clinical phenotype of immunodeficiency in patients with impaired Ca2+ influx and CRAC channel function.

METHODS

DNA sequence analysis for mutations in the genes ORAI1, ORAI2, ORAI3, and stromal interaction molecule (STIM) 1 and 2, as well as mRNA and protein expression analysis of ORAI1 in immunodeficient patients. Immunohistochemical analysis of ORAI1 tissue distribution in healthy human donors.

RESULTS

We identified mutations in ORAI1 in patients from 2 unrelated families. One patient is homozygous for a frameshift nonsense mutation in ORAI1 (ORAI1-A88SfsX25), and a second patient is compound heterozygous for 2 missense mutations in ORAI1 (ORAI1-A103E/L194P). All 3 mutations abolish ORAI1 expression and impair Ca2+ influx and CRAC channel function. The clinical syndrome associated with ORAI1 deficiency is characterized by immunodeficiency with a defect in the function but not in the development of lymphocytes, congenital myopathy, and anhydrotic ectodermal dysplasia with a defect in dental enamel calcification. In contrast with the limited clinical phenotype, we found ORAI1 protein expression in a wide variety of cell types and organs.

CONCLUSION

Ca2+ influx through ORAI1 is crucial for lymphocyte function in vivo. Despite almost ubiquitous ORAI1 expression, the channel has a nonredundant role in only a few cell types judging from the limited clinical phenotype in ORAI1-deficient patients.

ORAI1 and STIM1 deficiency in human and mice: roles of store-operated Ca2+ entry in the immune system and beyond

Store-operated Ca2+ entry (SOCE) is a mechanism used by many cells types including lymphocytes and other immune cells to increase intracellular Ca2+ concentrations to initiate signal transduction. Activation of immunoreceptors such as the T-cell receptor, B-cell receptor, or Fc receptors results in the release of Ca2+ ions from endoplasmic reticulum (ER) Ca2+ stores and subsequent activation of plasma membrane Ca2+ channels such as the well-characterized Ca2+ release-activated Ca2+ (CRAC) channel. Two genes have been identified that are essential for SOCE: ORAI1 as the pore-forming subunit of the CRAC channel in the plasma membrane and stromal interaction molecule-1 (STIM1) sensing the ER Ca2+ concentration and activating ORAI1-CRAC channels. Intense efforts in the past several years have focused on understanding the molecular mechanism of SOCE and the role it plays for cell functions in vitro and in vivo. A number of transgenic mouse models have been generated to investigate the role of ORAI1 and STIM1 in immunity. In addition, mutations in ORAI1 and STIM1 identified in immunodeficient patients provide valuable insight into the role of both genes and SOCE. This review focuses on the role of ORAI1 and STIM1 in vivo, discussing the phenotypes of ORAI1- and STIM1-deficient human patients and mice.

Orai1, a critical component of store-operated Ca2+ entry, is functionally associated with Na+/Ca2+ exchanger and plasma membrane Ca2+ pump in proliferating human arterial myocytes

Ca(2+) entry through store-operated channels (SOCs) in the plasma membrane plays an important role in regulation of vascular smooth muscle contraction, tone, and cell proliferation. The C-type transient receptor potential (TRPC) channels have been proposed as major candidates for SOCs in vascular smooth muscle. Recently, two families of transmembrane proteins, Orai [also known as Ca(2+) release-activated Ca(2+) channel modulator (CRACM)] and stromal interacting molecule 1 (STIM1), were shown to be essential for the activation of SOCs mainly in nonexcitable cells. Here, using small interfering RNA, we show that Orai1 plays an essential role in activating store-operated Ca(2+) entry (SOCE) in primary cultured proliferating human aortic smooth muscle cells (hASMCs), whereas Orai2 and Orai3 do not contribute to SOCE. Knockdown of Orai1 protein expression significantly attenuated SOCE. Moreover, inhibition of Orai1 downregulated expression of Na(+)/Ca(2+) exchanger type 1 (NCX1) and plasma membrane Ca(2+) pump isoform 1 (PMCA1). The rate of cytosolic free Ca(2+) concentration decay after Ca(2+) transients in Ca(2+)-free medium was also greatly decreased under these conditions. This reduction of Ca(2+) extrusion, presumably via NCX1 and PMCA1, may be a compensation for the reduced SOCE. Immunocytochemical observations indicate that Orai1 and NCX1 are clustered in plasma membrane microdomains. Cell proliferation was attenuated in hASMCs with disrupted Orai1 expression and reduced SOCE. Thus Orai1 appears to be a critical component of SOCE in proliferating vascular smooth muscle cells, and may therefore be a key player during vascular growth and remodeling.

ASIC1 contributes to pulmonary vascular smooth muscle store-operated Ca(2+) entry

Acid-sensing ion channels (ASIC) are voltage-insensitive, cationic channels that have recently been identified in vascular smooth muscle (VSM). It is possible that ASIC contribute to vascular reactivity via Na(+) and Ca(2+) conductance; however, their function in VSM is largely unknown. In pulmonary VSM, store-operated Ca(2+) entry (SOCE) plays a significant role in vasoregulatory mechanisms such as hypoxic pulmonary vasoconstriction and receptor-mediated arterial constriction. Therefore, we hypothesized that ASIC contribute to SOCE in pulmonary VSM. We examined SOCE resulting from depletion of intracellular Ca(2+) stores with cyclopiazonic acid in isolated small pulmonary arteries and primary cultured pulmonary arterial smooth muscle cells by measuring 1) changes in VSM [Ca(2+)](i) using fura-2 indicator dye, 2) Mn(2+) quenching of fura-2 fluorescence, and 3) store-operated Ca(2+) and Na(+) currents using conventional whole cell patch-clamp configuration in voltage-clamp mode. The role of ASIC was assessed by the use of the ASIC inhibitors, amiloride, benzamil, and psalmotoxin 1, or siRNA directed towards ASIC1, ASIC2, or ASIC3 isoforms. We found that store-operated VSM [Ca(2+)](i) responses, Mn(2+) influx, and inward cationic currents were attenuated by either pharmacological ASIC inhibition or treatment with ASIC1 siRNA. These data establish a unique role for ASIC1 in mediating SOCE in pulmonary VSM and provide new insight into mechanisms of VSM Ca(2+) entry and pulmonary vasoregulation.