Inhibition of agonist-induced Ca2+ entry in endothelial cells by myosin light-chain kinase inhibitor

Neuronal Store-Operated Calcium Channels

The endoplasmic reticulum (ER) is the major intracellular calcium (Ca²⁺) storage compartment in eukaryotic cells. In most instances, the mobilization of Ca²⁺ from this store is followed by a delayed and sustained uptake of Ca²⁺ through Ca²⁺-permeable channels of the cell surface named store-operated Ca²⁺ channels (SOCCs). This gives rise to a store-operated Ca²⁺ entry (SOCE) that has been thoroughly investigated in electrically non-excitable cells where it is the principal regulated Ca²⁺ entry pathway. The existence of this Ca²⁺ route in neurons has long been a matter of debate. However, a growing body of experimental evidence indicates that the recruitment of Ca²⁺ from neuronal ER Ca²⁺ stores generates a SOCE. The present review summarizes the main studies supporting the presence of a depletion-dependent Ca²⁺ entry in neurons. It also addresses the question of the molecular composition of neuronal SOCCs, their expression, pharmacological properties, as well as their physiological relevance.

Store-operated calcium entry: From physiology to tubular aggregate myopathy

Store-Operated Ca²⁺ entry (SOCE) is recognized as a key mechanism in muscle physiology necessary to refill intracellular Ca²⁺ stores during sustained muscle activity. For many years the cell structures expected to mediate SOCE in skeletal muscle fibres remained unknown. Recently, the identification of Ca²⁺ Entry Units (CEUs) in exercised muscle fibres opened new insights into the role of extracellular Ca²⁺ in muscle contraction and, more generally, in intracellular Ca²⁺ homeostasis. Accordingly, intracellular Ca²⁺ unbalance due to alterations in SOCE strictly correlates with muscle disfunction and disease. Mutations in proteins involved in SOCE (STIM1, ORAI1, and CASQ1) have been linked to tubular aggregate myopathy (TAM), a disease that causes muscle weakness and myalgia and is characterized by a typical accumulation of highly ordered and packed membrane tubules originated from the sarcoplasmic reticulum (SR). Achieving a full understanding of the molecular pathways activated by alterations in Ca²⁺ entry mechanisms is a necessary step to design effective therapies for human SOCE-related disorders.

Regulation of Store-Operated Ca2+ Entry by SARAF

Calcium (Ca²⁺) signaling plays a dichotomous role in cellular biology, controlling cell survival and proliferation on the one hand and cellular toxicity and cell death on the other. Store-operated Ca²⁺ entry (SOCE) by CRAC channels represents a major pathway for Ca²⁺ entry in non-excitable cells. The CRAC channel has two key components, the endoplasmic reticulum Ca²⁺ sensor stromal interaction molecule (STIM) and the plasma-membrane Ca²⁺ channel Orai. Physical coupling between STIM and Orai opens the CRAC channel and the resulting Ca²⁺ flux is regulated by a negative feedback mechanism of slow Ca²⁺ dependent inactivation (SCDI). The identification of the SOCE-associated regulatory factor (SARAF) and investigations of its role in SCDI have led to new functional and molecular insights into how SOCE is controlled. In this review, we provide an overview of the functional and molecular mechanisms underlying SCDI and discuss how the interaction between SARAF, STIM1, and Orai1 shapes Ca²⁺ signaling in cells.

Development of Store-Operated Calcium Entry-Targeted Compounds in Cancer

Store-operated Ca²⁺ entry (SOCE) is the major pathway of Ca²⁺ entry in mammalian cells, and regulates a variety of cellular functions including proliferation, motility, apoptosis, and death. Accumulating evidence has indicated that augmented SOCE is related to the generation and development of cancer, including tumor formation, proliferation, angiogenesis, metastasis, and antitumor immunity. Therefore, the development of compounds targeting SOCE has been proposed as a potential and effective strategy for use in cancer therapy. In this review, we summarize the current research on SOCE inhibitors and blockers, discuss their effects and possible mechanisms of action in cancer therapy, and induce a new perspective on the treatment of cancer.

Structural and Mechanistic Insights of CRAC Channel as a Drug Target in Autoimmune Disorder

BACKGROUND

Calcium (Ca2+) ion is a major intracellular signaling messenger, controlling a diverse array of cellular functions like gene expression, secretion, cell growth, proliferation, and apoptosis. The major mechanism controlling this Ca2+ homeostasis is store-operated Ca2+ release-activated Ca2+ (CRAC) channels. CRAC channels are integral membrane protein majorly constituted via two proteins, the stromal interaction molecule (STIM) and ORAI. Following Ca2+ depletion in the Endoplasmic reticulum (ER) store, STIM1 interacts with ORAI1 and leads to the opening of the CRAC channel gate and consequently allows the influx of Ca2+ ions. A plethora of studies report that aberrant CRAC channel activity due to Loss- or gain-of-function mutations in ORAI1 and STIM1 disturbs this Ca2+ homeostasis and causes several autoimmune disorders. Hence, it clearly indicates that the therapeutic target of CRAC channels provides the space for a new approach to treat autoimmune disorders.

OBJECTIVE

This review aims to provide the key structural and mechanical insights of STIM1, ORAI1 and other molecular modulators involved in CRAC channel regulation.

RESULTS AND CONCLUSION

Understanding the structure and function of the protein is the foremost step towards improving the effective target specificity by limiting their potential side effects. Herein, the review mainly focusses on the structural underpinnings of the CRAC channel gating mechanism along with its biophysical properties that would provide the solid foundation to aid the development of novel targeted drugs for an autoimmune disorder. Finally, the immune deficiencies caused due to mutations in CRAC channel and currently used pharmacological blockers with their limitation are briefly summarized.

Integration of TRPC6 and NADPH oxidase activation in lysophosphatidylcholine-induced TRPC5 externalization

Lipid oxidation products, including lysophosphatidylcholine (lysoPC), activate canonical transient receptor potential 6 (TRPC6) channels, and the subsequent increase in intracellular Ca²⁺ leads to TRPC5 activation. The goal of this study is to elucidate the steps in the pathway between TRPC6 activation and TRPC5 externalization. Following TRPC6 activation by lysoPC, extracellular regulated kinase (ERK) is phosphorylated. This leads to phosphorylation of p47^phox and subsequent NADPH oxidase activation with increased production of reactive oxygen species. ERK activation requires TRPC6 opening and influx of Ca²⁺ as evidenced by the failure of lysoPC to induce ERK phosphorylation in TRPC6^-/- endothelial cells. ERK siRNA blocks the lysoPC-induced activation of NADPH oxidase, demonstrating that ERK activation is upstream of NADPH oxidase. The reactive oxygen species produced by NADPH oxidase promote myosin light chain kinase (MLCK) activation with phosphorylation of MLC and TRPC5 externalization. Downregulation of ERK, NADPH oxidase, or MLCK with the relevant siRNA prevents TRPC5 externalization. Blocking MLCK activation prevents the prolonged rise in intracellular calcium levels and preserves endothelial migration in the presence of lysoPC.

AMPA Receptors Are Involved in Store-Operated Calcium Entry and Interact with STIM Proteins in Rat Primary Cortical Neurons

The process of store-operated calcium entry (SOCE) leads to refilling the endoplasmic reticulum (ER) with calcium ions (Ca²⁺) after their release into the cytoplasm. Interactions between (ER)-located Ca²⁺ sensors (stromal interaction molecule 1 [STIM1] and STIM2) and plasma membrane-located Ca²⁺ channel-forming protein (Orai1) underlie SOCE and are well described in non-excitable cells. In neurons, however, SOCE appears to be more complex because of the importance of Ca²⁺ influx via voltage-gated or ionotropic receptor-operated Ca²⁺ channels. We found that the SOCE inhibitors ML-9 and SKF96365 reduced α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced [Ca²⁺]_i amplitude by 80% and 53%, respectively. To assess the possible involvement of AMPA receptors (AMPARs) in SOCE, we used their specific inhibitors. As estimated by Fura-2 acetoxymethyl (AM) single-cell Ca²⁺ measurements in the presence of CNQX or NBQX, thapsigargin (TG)-induced Ca²⁺ influx decreased 2.2 or 3.7 times, respectively. These results suggest that under experimental conditions of SOCE when Ca²⁺ stores are depleted, Ca²⁺ can enter neurons also through AMPARs. Using specific antibodies against STIM proteins or GluA1/GluA2 AMPAR subunits, co-immunoprecipitation assays indicated that when Ca²⁺ levels are low in the neuronal ER, a physical association occurs between endogenous STIM proteins and endogenous AMPAR receptors. Altogether, our data suggest that STIM proteins in neurons can control AMPA-induced Ca²⁺ entry as a part of the mechanism of SOCE.

Store-operated CRAC channel inhibitors: opportunities and challenges

Aberrant Ca(2+) release-activated Ca(2+) (CRAC) channel activity has been implicated in a number of human disorders, including immunodeficiency, autoimmunity, occlusive vascular diseases and cancer, thus placing CRAC channels among the important targets for the treatment of these disorders. We briefly summarize herein the molecular basis and activation mechanism of CRAC channel and focus on discussing several pharmacological inhibitors of CRAC channels with respect to their biological activity, mechanisms of action and selectivity over other types of Ca(2+) channel in different types of cells.

Estrogen Enhances Linkage in the Vascular Endothelial Calmodulin Network via a Feedforward Mechanism at the G Protein-coupled Estrogen Receptor 1

Estrogen exerts many effects on the vascular endothelium. Calmodulin (CaM) is the transducer of Ca(2+) signals and is a limiting factor in cardiovascular tissues. It is unknown whether and how estrogen modifies endothelial functions via the network of CaM-dependent proteins. Here we show that 17β-estradiol (E2) up-regulates total CaM level in endothelial cells. Concurrent measurement of Ca(2+) and Ca(2+)-CaM indicated that E2 also increases free Ca(2+)-CaM. Pharmacological studies, gene silencing, and receptor expression-specific cell studies indicated that the G protein-coupled estrogen receptor 1 (GPER/GPR30) mediates these effects via transactivation of EGFR and subsequent MAPK activation. The outcomes were then examined on four distinct members of the intracellular CaM target network, including GPER/GPR30 itself and estrogen receptor α, the plasma membrane Ca(2+)-ATPase (PMCA), and endothelial nitric-oxide synthase (eNOS). E2 substantially increases CaM binding to estrogen receptor α and GPER/GPR30. Mutations that reduced CaM binding to GPER/GPR30 in separate binding domains do not affect GPER/GPR30-Gβγ preassociation but decrease GPER/GPR30-mediated ERK1/2 phosphorylation. E2 increases CaM-PMCA association, but the expected stimulation of Ca(2+) efflux is reversed by E2-stimulated tyrosine phosphorylation of PMCA. These effects sustain Ca(2+) signals and promote Ca(2+)-dependent CaM interactions with other CaM targets. Consequently, E2 doubles CaM-eNOS interaction and also promotes dual phosphorylation of eNOS at Ser-617 and Ser-1179. Calculations using in-cell and in vitro data revealed substantial individual and combined contribution of these effects to total eNOS activity. Taken together, E2 generates a feedforward loop via GPER/GPR30, which enhances Ca(2+)/CaM signals and functional linkage in the endothelial CaM target network.

Regulation of calcium channels in smooth muscle: new insights into the role of myosin light chain kinase

Smooth muscle myosin light chain kinase (MLCK) plays a crucial role in artery contraction, which regulates blood pressure and blood flow distribution. In addition to this role, MLCK contributes to Ca(2+) flux regulation in vascular smooth muscle (VSM) and in non-muscle cells, where cytoskeleton has been suggested to help Ca(2+) channels trafficking. This conclusion is based on the use of pharmacological inhibitors of MLCK and molecular and cellular techniques developed to down-regulate the enzyme. Dissimilarities have been observed between cells and whole tissues, as well as between large conductance and small resistance arteries. A differential expression in MLCK and ion channels (either voltage-dependent Ca(2+) channels or non-selective cationic channels) could account for these observations, and is in line with the functional properties of the arteries. A potential involvement of MLCK in the pathways modulating Ca(2+) entry in VSM is described in the present review.

The Inhibitory Mechanism of Gentamicin on Electrical Field Stimulation Response in Rat Bladder Smooth Muscle

To see the inhibitory mechanism of gentamicin in response to electrical field stimulation (EFS) using the rat bladder smooth muscle, atropine or guanethidine was treated but had no effect. Methylsergide, a non-selective 5-HT1, 5-HT2 receptor antagonist was also treated but had on effect. Kinase inhibitors, such as chelerythrine (PKC inhibitor), ML-9 (MLCK inhibitor), or Y27632 (rho kinase inhibitor) were pretreated before gentamicin treatment, but did not have effect. For U73122, a phospholipase C (PLC) inhibitor however, the inhibitory effect to gentamicin was significantly attenuated in all frequencies given by the EFS. Therefore gentamicin induced inhibitory effect on EFS response in rat bladder smooth muscle was not mediated by the activation of adrenergic, cholinergic, or serotonergic receptor. The inhibition of gentamicin might be mediated through the PLC dependent pathway, but not through the PKC, MLCK or rho kinase dependent pathway.

Blockage of LMP1-modulated store-operated Ca(2+) entry reduces metastatic potential in nasopharyngeal carcinoma cell

Epstein-Barr virus (EBV)-encoded latent membrane proteins (LMPs) expedite progression of EBV-relevant cancers. Of the full set of LMPs, latent membrane protein 1 (LMP1) was identified to uniquely augment store-operated Ca(2+) entry (SOCE). Previously, we reported that the suppression of SOCE exhibited inhibitory effects on cell migration and the extravasation from vasculature in EBV-negative nasopharyngeal carcinoma (NPC) cells. In this follow-up study, we aimed to expand our understanding of the modulation of SOCE by LMP1 and test the possibility that blockage of LMP1-modulated SOCE affects the LMP1-promoted metastatic potential. Here we showed that suppressions of the LMP1-boosted SOCE blunted the LMP1-promoted cell migration, VEGF-mediated angiogenesis and permeabilization in vitro. Blockage of SOCE inhibited vasculature-invasion of circulating cells and distant metastatic colonization in vivo. Notably, utilizing VEGFR2-EGFP-tag zebrafish we revealed that the LMP1-expressing cells arrested in a small-caliber vessel mobilized surrounding endothelial cells to facilitate vasculature-invasion. Thus, the LMP1-boosted SOCE promotes metastatic potential of NPC cells by solidifying their collaborations with the nearby non-cancer cells through the manipulation of oncogenic Ca(2+) signaling. Our study highlights the advantage of using both conventional mammal and transgenic zebrafish for developing a novel therapeutic strategy targeting the multiple steps of invasion-metastasis cascade.

Identification of ML-9 as a lysosomotropic agent targeting autophagy and cell death

The growing number of studies suggested that inhibition of autophagy enhances the efficacy of Akt kinase inhibitors in cancer therapy. Here, we provide evidence that ML-9, a widely used inhibitor of Akt kinase, myosin light-chain kinase (MLCK) and stromal interaction molecule 1 (STIM1), represents the ‘two-in-one' compound that stimulates autophagosome formation (by downregulating Akt/mammalian target of rapamycin (mTOR) pathway) and inhibits their degradation (by acting like a lysosomotropic agent and increasing lysosomal pH). We show that ML-9 as a monotherapy effectively induces prostate cancer cell death associated with the accumulation of autophagic vacuoles. Further, ML-9 enhances the anticancer activity of docetaxel, suggesting its potential application as an adjuvant to existing anticancer chemotherapy. Altogether, our results revealed the complex effect of ML-9 on autophagy and indentified ML-9 as an attractive tool for targeting autophagy in cancer therapy through dual inhibition of both the Akt pathway and the autophagy.

Molecular pharmacology of store-operated CRAC channels

Calcium influx through store-operated Ca(2+) release-activated Ca(2+) channels (CRAC channels) is a well-defined mechanism of generating cellular Ca(2+) elevations that regulates many functions including gene expression, exocytosis and cell proliferation. The identifications of the ER Ca(2+) sensing proteins, STIM1-2 and the CRAC channel proteins, Orai1-3, have led to improved understanding of the physiological roles and the activation mechanism of CRAC channels. Defects in CRAC channel function are associated with serious human diseases such as immunodeficiency and auto-immunity. In this review, we discuss several pharmacological modulators of CRAC channels, focusing specifically on the molecular mechanism of drug action and their utility in illuminating the mechanism of CRAC channel operation and their physiological roles in different cells.

Linoleic acid attenuates endothelium-derived relaxing factor production by suppressing cAMP-hydrolyzing phosphodiesterase activity

BACKGROUND

Linoleic acid (LA) promotes monocyte chemotaxis and cell adhesion molecules such as MCP-1 and VCAM-1, which contribute to atherosclerogenesis. These molecules are restrained by endothelium-derived relaxing factors (EDRFs), such as nitric oxide (NO) and prostaglandin I2 (PGI2). Hence, the expressions of MCP-1 and VCAM-1 upregulated by LA may be partly attributable to decreased EDRF production. However, effect of LA on EDRF production remains controversial.

METHODS AND RESULTS

The present study aimed to examine the effects of LA and other free fatty acids on EDRF production and the endothelial Ca(2+) responses that mediate EDRF production, using primary cultured porcine aortic endothelial cells (PAECs). LA at 0.1-5 μmol/L attenuated bradykinin (BK)-induced NO and PGI2 production while suppressing the BK-induced Ca(2+) response dose-dependently. The inhibitory effect of LA on the Ca(2+) response was eliminated by adenylate cyclase inhibitor SQ22536, boosted by cAMP-hydrolyzing phosphodiesterase (PDE) inhibitor, rolipram, and mimicked by plasma membrane permeable 8-bromo-cAMP. Moreover, LA was confirmed to dose-dependently increase intracellular cAMP levels and selectively inhibit cAMP-hydrolyzing PDE activity in vitro. In contrast, none of palmitic, stearic, or oleic acid affected BK-induced EDRF production or Ca(2+) responses, or induced intracellular cAMP accumulation.

CONCLUSIONS

LA induced intracellular cAMP accumulation by inhibiting cAMP-hydrolyzing PDE activity, thus resulting in attenuation of Ca(2+) responses and EDRF production in PAECs.

Inhibition of store-operated Ca2+ entry suppresses EGF-induced migration and eliminates extravasation from vasculature in nasopharyngeal carcinoma cell

Store-operated Ca(2+) entry (SOCE) mediates Ca(2+) responses evoked by extracellular signaling molecules to promote increases in cytosolic Ca(2+), thereby triggering downstream signal transduction. Here we demonstrated that either the pharmacological blockage of Ca(2+) influx through SOCE or the knockdown of Orai1, a key molecule of SOCE, suppressed the epidermal growth factor-induced migration by disturbing Ca(2+) signaling in nasopharyngeal carcinoma (NPC) cell. Furthermore, Orai1 depletion led to a delayed cell attachment to the extracellular matrix surface in vitro and eliminated the extravasation of microinjected cells from vasculature in a zebrafish hematogenous metastasis model. Our findings thus indicate that SOCE acts as a predominant Ca(2+) signaling involved in NPC cell metastasis, and may serve as a candidate target for anti-metastasis therapy in NPC.

Purification and neuroprotective effects of polysaccharides from Opuntia Milpa Alta in cultured cortical neurons

Opuntia is a traditional plant from China with medicinal applications. In this experiment, polysaccharides from Opuntia Milpa Alta (MAPs) were analyzed using gas chromatograph-mass spectrometer (GC-MS) method and result showed that MAPs contained mannose (6.37%), rhamnose (14.94%), xylose (1.99%), arabinose (24.07%), galactose (38.25%), ribose (2.63%) and glucose (11.48%). The neuroprotective effects of MAPs were evaluated at the mechanistic level in vitro models of cerebral ischemic injury. In vitro oxygen/glucose deprivation (OGD) model, MAPs (0.5 μg/ml, 5 μg/ml, 50 μg/ml) effectively increased cell viability by methyl thiazolyl tetrazolium (MTT) assay, inhibited cell cytotoxicity by lactate dehydrogenase (LDH) assay, reduced neuronal cell death, suppressed the production of intracellular reactive oxygen species (ROS), decreased of intracellular free Ca(2+) concentrations ([Ca(2+)](i)), and reduced extracellular glutamate level. Therefore, MAPs might prevent intracellular calcium overload and decreased glutamate excitotoxicity, both of which can cause neuronal injury and death in vitro models of cerebral ischemic injury.

Activation and regulation of store-operated calcium entry

The process of store-operated Ca²⁺ entry (SOCE), whereby Ca²⁺ influx across the plasma membrane is activated in response to depletion of intracellular Ca²⁺ stores in the endoplasmic reticulum (ER), has been under investigation for greater than 25 years; however, only in the past 5 years have we come to understand this mechanism at the molecular level. A surge of recent experimentation indicates that STIM molecules function as Ca²⁺ sensors within the ER that, upon Ca²⁺ store depletion, rearrange to sites very near to the plasma membrane. At these plasma membrane-ER junctions, STIM interacts with and activates SOCE channels of the Orai family. The molecular and biophysical data that have led to these findings are discussed in this review, as are several controversies within this rapidly expanding field.

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.

Neuroprotective effects of Dendrobium alkaloids on rat cortical neurons injured by oxygen-glucose deprivation and reperfusion

In this study we investigated the protective effects of alkaloids from Dendrobium spez. on cortical neurons injured by oxygen-glucose deprivation/reperfusion (OGD/RP) in vitro. Rat primary cultured cerebral cortical neurons were investigated at different time points of OGD/RP. The MTT assay and the lactate dehydrogenase (LDH) release were used to determine cell viability. The concentration of intracellular free calcium [Ca(2+)](i) and mitochondrial membrane potential (MMP) were determined to evaluate the degree of neuron damage. Morphologic changes of neurons following OGD/RP were examined by electron microscope. To evaluate neuron apoptosis, flow cytometry was performed and the expressions of caspase-3 and caspase-12 mRNA were examined by real-time quantitative PCR during OGD 2h/RP 12h. Treatment with Dendrobium alkaloids (0.025 approximately 2.5mg/l) significantly attenuated neuronal damage, with evidence of increased cell viability, decreased cell apoptosis, and decreased cell morphologic impairment. Furthermore, Dendrobium alkaloids inhibited [Ca(2+)](i) elevation, increased MMP and decreased the expressions of caspase-3 and caspase-12 in a concentration-dependent manner at OGD 2h/RP 12h. Dendrobium alkaloids have significantly protective effects on OGD/RP-induced neuronal damages in rat primary neuron cultures. The protection against OGD/RP-induced apoptosis appears to be mediated through blocking the decrease in MMP and increase in [Ca(2+)](i), as well as by down-regulating mRNA expression of caspase-3 and caspase-12.

Role of phosphoinositides in STIM1 dynamics and store-operated calcium entry

Ca2+ entry through store-operated Ca2+ channels involves the interaction at ER-PM (endoplasmic reticulum-plasma membrane) junctions of STIM (stromal interaction molecule) and Orai. STIM proteins are sensors of the luminal ER Ca2+ concentration and, following depletion of ER Ca2+, they oligomerize and translocate to ER-PM junctions where they form STIM puncta. Direct binding to Orai proteins activates their Ca2+ channel function. It has been suggested that an additional interaction of the C-terminal polybasic domain of STIM1 with PM phosphoinositides could contribute to STIM1 puncta formation prior to binding to Orai. In the present study, we investigated the role of phosphoinositides in the formation of STIM1 puncta and SOCE (store-operated Ca2+ entry) in response to store depletion. Treatment of HeLa cells with inhibitors of PI3K (phosphatidylinositol 3-kinase) and PI4K (phosphatidylinositol 4-kinase) (wortmannin and LY294002) partially inhibited formation of STIM1 puncta. Additional rapid depletion of PtdIns(4,5)P2 resulted in more substantial inhibition of the translocation of STIM1-EYFP (enhanced yellow fluorescent protein) into puncta. The inhibition was extensive at a concentration of LY294002 (50 microM) that should primarily inhibit PI3K, consistent with a major role for PtdIns(4,5)P2 and PtdIns(3,4,5)P3 in puncta formation. Depletion of phosphoinositides also inhibited SOCE based on measurement of the rise in intracellular Ca2+ concentration after store depletion. Overexpression of Orai1 resulted in a recovery of translocation of STMI1 into puncta following phosphoinositide depletion and, under these conditions, SOCE was increased to above control levels. These observations support the idea that phosphoinositides are not essential but contribute to STIM1 accumulation at ER-PM junctions with a second translocation mechanism involving direct STIM1-Orai interactions.

Physiological function and molecular basis of STIM1-mediated calcium entry in immune cells

Calcium signals in immune cells regulate a variety of physiological responses such as cell activation, differentiation, gene transcription, and effector functions. Surface receptor stimulation induces an increase in the concentration of cytosolic calcium ions (Ca2+), which are derived mainly from two sources, intracellular endoplasmic reticulum (ER) Ca2+ stores and the extracellular space. The major cascade for Ca2+ entry in immune cells is through store-operated Ca2+ entry (SOCE) and Ca2+ release-activated Ca2+ (CRAC) channels. Activation of SOCE is triggered by depletion of intracellular ER Ca2+ stores, but the molecular mechanism was a long-standing issue. With the recent molecular identification of the ER Ca2+ sensor [stromal interacting molecule-1 (STIM1)] and a pore-forming subunit of the CRAC channel (Orai1), our understanding of the SOCE activation pathway has increased dramatically. These advances have now made it possible to shed some light on important questions: what is the physiological significance of SOCE, and what is its molecular basis? This review focuses on the recent progress in the field and the exciting opportunities for understanding how SOCE influences diverse immune functions.

Dependence of STIM1/Orai1-mediated calcium entry on plasma membrane phosphoinositides

Recent studies identified two main components of store-operated calcium entry (SOCE): the endoplasmic reticulum-localized Ca2+ sensor protein, STIM1, and the plasma membrane (PM)-localized Ca2+ channel, Orai1/CRACM1. In the present study, we investigated the phosphoinositide dependence of Orai1 channel activation in the PM and of STIM1 movements from the tubular to PM-adjacent endoplasmic reticulum regions during Ca2+ store depletion. Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) levels were changed either with agonist stimulation or by chemically induced recruitment of a phosphoinositide 5-phosphatase domain to the PM, whereas PtdIns4P levels were decreased by inhibition or down-regulation of phosphatidylinositol 4-kinases (PI4Ks). Agonist-induced phospholipase C activation and PI4K inhibition, but not isolated PtdIns(4,5)P(2) depletion, substantially reduced endogenous or STIM1/Orai1-mediated SOCE without preventing STIM1 movements toward the PM upon Ca2+ store depletion. Patch clamp analysis of cells overexpressing STIM1 and Orai1 proteins confirmed that phospholipase C activation or PI4K inhibition greatly reduced I(CRAC) currents. These results suggest an inositide requirement of Orai1 activation but not STIM1 movements and indicate that PtdIns4P rather than PtdIns(4,5)P2 is a likely determinant of Orai1 channel activity.

Extracellular acidosis suppresses endothelial function by inhibiting store-operated Ca2+ entry via non-selective cation channels

AIMS

Hypoxia, ischaemia, and exogenous chemicals can induce extracellular and intracellular acidosis, but it is not clear which of these types of acidosis affects endothelial cell function. The synthesis and release of endothelium-derived relaxing factors (EDRFs) are linked to an increase in cytosolic Ca(2+) concentration, and we therefore examined the effects of extracellular and intracellular acidosis on Ca(2+) responses and EDRF production in cultured porcine aortic endothelial cells.

METHODS AND RESULTS

Cytosolic pH (pH(i)) and Ca(2+) were measured using fluorescent dyes, BCECM/AM (pH-indicator) and fura-2/AM (Ca(2+)-indicator), respectively. EDRFs, nitric oxide (NO) and prostaglandin I(2) (PGI(2)) were assessed using DAF-FM/DA (NO-indicator dye) fluorometry and 6-keto PGF(1alpha) enzyme immunoassay, respectively. HEPES buffers titrated to pH 6.4, 6.9, and 7.4 were used to alter extracellular pH (pH(o)), and propionate (20 mmol/L) was applied to cause intracellular acidosis. Extracellular acidosis strongly suppressed bradykinin (BK, 10 nmol/L)- and thapsigargin (TG, 1 micromol/L)-induced Ca(2+) responses by 30 and 23% at pH(o) 6.9, and by 80 and 97% at pH(o) 6.4, respectively. During the examinations, there were no significant differences in pH(i) among the three groups at pH(o) 7.4, 6.9, and 6.4. Extracellular acidosis also inhibited BK-stimulated PGI(2) production by 55% at pH(o) 6.9 and by 77% at pH(o) 6.4, and NO production by 38% at pH(o) 6.9 and by 91% at pH(o) 6.4. The suppressive effects of extracellular acidosis on Ca(2+) responses and NO production were reversible. Propionate changed pH(i) from 7.3 to 6.9, without altering pH(o) (7.4). Intracellular acidosis had no effect on BK- and TG-induced Ca(2+) responses or NO production.

CONCLUSION

These results indicate that extracellular, but not intracellular, acidosis causes endothelial dysfunction by inhibiting store-operated Ca(2+) entry, so helping to clarify the vascular pathophysiology of conditions such as ischaemia, hypoxia, acidosis, and ischaemia-reperfusion.

STIM and Orai: the long-awaited constituents of store-operated calcium entry

Rapid changes in cytosolic Ca(2+) concentrations [Ca(2+)](i) are the most commonly used signals in biology to regulate a whole host of cellular functions including contraction, secretion and gene activation. A widely utilized form of Ca(2+) influx is termed store-operated Ca(2+) entry (SOCE) owing to its control by the Ca(2+) content of the endoplasmic reticulum (ER). The underlying molecular mechanism of SOCE has eluded identification until recently when two groups of proteins, the ER Ca(2+) sensors stromal interaction molecule (STIM)1 and STIM2 and the plasma-membrane channels Orai1, Orai2 and Orai3, have been identified. These landmark discoveries have enabled impressive progress in clarifying how these proteins work in concert and what developmental and cellular processes require their participation most. As we begin to better understand the biology of the STIM and Orai proteins, the attention to the pharmacological tools to influence their functions quickly follow suit. Here, we briefly summarize recent developments in this exciting area of Ca(2+) signaling.

Capacitative Ca2+ entry via Orai1 and stromal interacting molecule 1 (STIM1) regulates adenylyl cyclase type 8

Capacitative Ca(2+) entry (CCE), which occurs through the plasma membrane as a result of Ca(2+) store depletion, is mediated by stromal interacting molecule 1 (STIM1), a sensor of intracellular Ca(2+) store content, and the pore-forming component Orai1. However, additional factors, such as C-type transient receptor potential (TRPC) channels, may also participate in the CCE apparatus. To explore whether the store-dependent Ca(2+) entry reconstituted by coexpression of Orai1 and STIM1 has the functional properties of CCE, we used the Ca(2+)-calmodulin stimulated adenylyl cyclase type 8 (AC8), which responds selectively to CCE, whereas other modes of Ca(2+) entry, including those activated by arachidonate and the ionophore ionomycin, are ineffective. In addition, the Ca(2+) entry mediated by previous CCE candidates, diacylglycerol-activated TRPC channels, does not activate AC8. Here, we expressed Orai1 and STIM1 in HEK293 cells and saw a robust increment in CCE, and a proportional increase in CCE-stimulated AC8 activity. Inhibitors of the CCE assembly process ablated the effects on cyclase activity in both AC8-overexpressing HEK293 cells and insulin-secreting MIN6 cells endogenously expressing Ca(2+)-sensitive AC isoforms. AC8 is believed to be closely associated with the source of CCE; indeed, not only were AC8, Orai1, and STIM1 colocalized at the plasma membrane but also all three proteins occurred in lipid rafts. Together, our data indicate that Orai1 and STIM1 can be integral components of the cAMP and CCE microdomain associated with adenylyl cyclase type 8.

Ca2+-store-dependent and -independent reversal of Stim1 localization and function

Stim1 responds to depletion of ER Ca2+ stores by rearranging from tubular structures throughout the ER into punctate structures near the plasma membrane, where it activates Orai store-operated Ca2+ entry (SOCE) channels. However, the mechanism and structural determinants of the localization and reversal of Stim1 puncta formation are poorly understood. Using HEK293 cells expressing Stim1 tagged with enhanced yellow fluorescent protein (EYFP-Stim1), we show that the basis for SOCE termination is the reversal of the punctate Stim1 localization, which absolutely depends on SOCE-dependent store refilling. We also describe rapid, store-independent reversal of EYFP-Stim1 punctae by the ML-9 inhibitor of myosin-light-chain kinase (MLCK). ML-9 similarly inhibited SOCE and the Ca2+-release-activated Ca2+ (CRAC) current. Reversal by ML-9 resulted in full re-establishment of the tubular EYFP-Stim1 localization. A constitutively active EF-hand mutant of EYFP-Stim1 was also reversed by ML-9, regardless of the Ca2+ store content. Inhibition by ML-9 was not due to MLCK inhibition as other inhibitors of MLCK had no effect. Finally, we provide evidence that EYFP-Stim1 punctae form in specific predetermined cellular loci. We conclude that SOCE is tightly coupled to formation of Stim1 puncta, and both SOCE and puncta formation involve a dynamic, reversible signaling complex that probably consists of components in addition to Stim1 and Orai channels.

Ca2+ signaling in hypoxic pulmonary vasoconstriction: effects of myosin light chain and Rho kinase antagonists

Antagonists of myosin light chain (MLC) kinase (MLCK) and Rho kinase (ROK) are thought to inhibit hypoxic pulmonary vasoconstriction (HPV) by decreasing the concentration of phosphorylated MLC at any intracellular Ca(2+) concentration ([Ca(2+)](i)) in pulmonary arterial smooth muscle cells (PASMC); however, these antagonists can also decrease [Ca(2+)](i). To determine whether MLCK and ROK antagonists alter Ca(2+) signaling in HPV, we measured the effects of ML-9, ML-7, Y-27632, and HA-1077 on [Ca(2+)](i), Ca(2+) entry, and Ca(2+) release in rat distal PASMC exposed to hypoxia or depolarizing concentrations of KCl. We performed parallel experiments in isolated rat lungs to confirm the inhibitory effects of these agents on pulmonary vasoconstriction. Our results demonstrate that MLCK and ROK antagonists caused concentration-dependent inhibition of hypoxia-induced increases in [Ca(2+)](i) in PASMC and HPV in isolated lungs and suggest that this inhibition was due to blockade of Ca(2+) release from the sarcoplasmic reticulum and Ca(2+) entry through store- and voltage-operated Ca(2+) channels in PASMC. Thus MLCK and ROK antagonists might block HPV by inhibiting Ca(2+) signaling, as well as the actin-myosin interaction, in PASMC. If effects on Ca(2+) signaling were due to decreased phosphorylated myosin light chain concentration, their diversity suggests that MLCK and ROK antagonists may have acted by inhibiting myosin motors and/or altering the cytoskeleton in a manner that prevented achievement of required spatial relationships among the cellular components of the response.

Calcium signalling in the endothelium

Elevations in cytosolic Ca2+ concentration are the usual initial response of endothelial cells to hormonal and chemical transmitters and to changes in physical parameters, and many endothelial functions are dependent upon changes in Ca2+ signals produced. Endothelial cell Ca2+ signalling shares similar features with other electrically non-excitable cell types, but has features unique to endothelial cells. This chapter discusses the major components of endothelial cell Ca2+ signalling.

F-actin-based Ca signaling-a critical comparison with the current concept of Ca signaling

A short comparative survey on the current idea of Ca signaling and the alternative concept of F-actin-based Ca signaling is given. The two hypotheses differ in one central aspect, the mechanism of Ca storage. The current theory rests on the assumption of Ca-accumulating endoplasmic/sarcoplasmic reticulum-derived vesicles equipped with an ATP-dependent Ca pump and IP3- or ryanodine-sensitive channel-receptors for Ca-release. The alternative hypothesis proceeds from the idea of Ca storage at the high-affinity binding sites of actin filaments. Cellular sites of F-actin-based Ca storage are microvilli and the submembrane cytoskeleton. Several specific features of Ca signaling such as store-channel coupling, quantal Ca release, spiking and oscillations, biphasic and "phasic" uptake kinetics, and Ca-induced Ca release (CICR), which are not adequately described by the current concept, are inherent properties of the F-actin system and its dynamic state of treadmilling.

Cilostazol suppresses adhesion of human neutrophils to HUVECs stimulated by FMLP and its mechanisms

The interaction between neutrophils and endothelial cells (ECs) is of great importance in many physiological and pathological progresses. Although cilostazol (CLZ), a novel selective phosphodiesterase (PDE) type 3 inhibitor, has been proved to be useful in vasodilatation and inhibition of platelet aggregation, its effect on adhesion is not clearly known. In this study, we examined the effects and investigated the mechanisms of cilostazol on neutrophil adhesion to human umbilical endothelial cells (HUVECs) triggered by N-formyl-methionyl-leucyl-phenylal-anine (FMLP), a chemotactic peptide. The soluble vascular cell adhesive molecule-1 (sVCAM-1) release from FMLP (10 microM)-stimulated HUVECs was determined by ELISA kits. Fluo-2, a fluorescent indicator, was used to investigate intracellular free calcium concentration ([Ca2+]i) in HUVECs. HL-60 cells were induced to be neutrophilic by DMSO and loaded with Fluo-3, another fluorescent indicator, to detect [Ca2+]i, and CLA was used as a chemiluminescent indicator to determine superoxide production in neutrophilic cells. The result showed that CLZ (1-100 microM) significantly inhibited neutrophil adhesion to FMLP-stimulated HUVECs. In HUVECs, CLZ obviously downregulated sVCAM-1 level, while it had no meaningful influence [Ca2)]i. But in neutrophils, FMLP-activated superoxide generation and [Ca2+]i increase were found being inhibited by exposure to CLZ . Furthermore, we also demonstrated that Ca2+ increase was preceded to the superoxide generation in neutrophils. The results suggest that CLZ involves in adhesion reactions between neutrophil and ECs, partly via VCAM-1 expression in ECs, and decreasing [Ca2+]i induced activation of neutrophils, which means a lot to prevent atherosclerosis and other cardiovascular diseases.

Modulation of intracellular Ca2+ release and capacitative Ca2+ entry by CaMKII inhibitors in bovine vascular endothelial cells

The effects of inhibitors of CaMKII on intracellular Ca2+ signaling were examined in single calf pulmonary artery endothelial (CPAE) cells using indo-1 microfluorometry to measure cytoplasmic Ca2+ concentration ([Ca2+]i). The three CaMKII inhibitors, KN-93, KN-62, and autocamtide-2-related inhibitory peptide (AIP), all reduced the plateau phase of the [Ca2+]i transient evoked by stimulation with extracellular ATP. Exposure to KN-93 or AIP alone in the presence of 2 mM extracellular Ca2+ resulted in a dose-dependent increase of [Ca2+]i consisting of a rapid and transient Ca2+ spike followed by a small sustained plateau phase of elevated [Ca2+]i. Exposure to KN-93 in the absence of extracellular Ca2+ caused a transient rise of [Ca2+]i, suggesting that exposure to CaMKII inhibitors directly triggered release of Ca2+ from intracellular endoplasmic reticulum (ER) Ca2+ stores. Repetitive stimulation with KN-93 and ATP, respectively, revealed that both components released Ca2+ largely from the same store. Pretreatment of CPAE cells with the membrane-permeable inositol 1,4,5-trisphosphate (IP3) receptor blocker 2-aminoethoxydiphenyl borate caused a significant inhibition of the KN-93-induced Ca2+ response, suggesting that exposure to KN-93 affects Ca2+ release from an IP3-sensitive store. Depletion of Ca2+ stores by exposure to ATP or to the ER Ca2+ pump inhibitor thapsigargin triggered robust capacitative Ca2+ entry (CCE) signals in CPAE cells that could be blocked effectively with KN-93. The data suggest that in CPAE cells, CaMKII modulates Ca2+ handling at different levels. The use of CaMKII inhibitors revealed that in CPAE cells, the most profound effects of CaMKII are inhibition of release of Ca2+ from intracellular stores and activation of CCE.

Intracellular Ca2+ homeostatic regulation and 4-hydroxynonenal-induced aortic endothelial dysfunction

The aldehydic lipid peroxidation product 4-hydroxynonenal (HNE) is known to compromise erythrocyte passive Ca2+ permeability and to irreversibly inhibit the plasma membrane (Ca2+ + Mg2+)-ATPase and Ca2+-transport. To measure the effects of HNE on passive and active Ca2+ transport in endothelial cells, we first characterized 45Ca2+ uptake and efflux in cultured porcine aortic endothelial cells (PAEC). PAEC exchanged 45Ca2+ to a cumulative near-isotopic equilibrium of about 4.5 pmole 45Ca2+/10(6) cells in 120 min at 37 degrees C. This Ca2+ pool was diminished by thapsigargin, cyclopiazonic acid, oligomycin B, and sodium azide. In contrast, ouabain enhanced Ca2+ uptake capacity from 5.17 to 5.77 pmole/10(6) cells. Accumulated 45Ca2+ was extruded at rate of 8.7 fmole 45Ca2+/10(6) cells/min or shunted rapidly by the ionophore A23187. HNE increased total 45Ca2+ accumulation in a time- and concentration-dependent manner by as much as 562% with an EC50 of 64.0 wM. Concomitant morphological analysis of PAEC revealed vacuolization, nuclear swelling, cell shrinking, and cell detachment. Initial structural changes, such as vacuolization, began well before any changes in Ca2+ accumulation were observed. These functional and morphological changes indicate that HNE significantly increases intracellular Ca2+ accumulation in vascular endothelium, which may explain the cytotoxic effects associated with HNE exposure and provide further evidence that atherogenic effects of HNE may, in part, be caused by disturbances in Ca2+ homeostasis.

On the endothelial cell I(SOC)

Ca2+ store depletion activates both Ca2+ selective and non-selective currents in endothelial cells. Recently, considerable progress has been made in understanding the molecular make-up and regulation of an endothelial cell thapsigargin-activated Ca2+ selective current, I(SOC). Indeed, I(SOC) is a relatively small inward Ca2+ current that exhibits an approximate +40mV reversal potential and is strongly inwardly rectifying. This current is sensitive to organization of the actin-based cytoskeleton. Transient receptor potential (TRP) proteins 1 and 4 (TRPC1 and TRPC4, respectively) each contribute to the molecular basis of I(SOC), although it is TRPC4 that appears to be tethered to the cytoskeleton through a dynamic interaction with protein 4.1. Activation of I(SOC) requires association between protein 4.1 and the actin-based cytoskeleton (mediated through spectrin), suggesting protein 4.1 mediates the physical communication between Ca2+ store depletion and channel activation. Thus, at present findings indicate a TRPC4-protein 4.1 physical linkage regulates I(SOC) activation following Ca2+ store depletion.

Integrity of actin-network is involved in uridine 5'-triphosphate evoked store-operated Ca2+ entry in bovine adrenocortical fasciculata cells

Store-operated Ca(2+) entry channels (SOCs) play an important role in the regulation of diverse non-excitable cell functions. However, the precise mechanism of SOCs activation is still controversial. Uridine 5'-triphosphate (UTP) was shown to induce Ca(2+) entry in a dihydropyridines-insensitive manner and accelerated steroidogenesis in bovine adrenocortical fasciculata cells (BAFCs) via the Gq/11 protein-coupled P2Y(2) receptor. Therefore we investigated whether UTP is involved in SOCs activation and the mechanism of UTP-induced SOCs activation. Fura 2-loaded BAFCs were used for the measurement of intracellular concentration of Ca(2+) ([Ca(2+)](i)) mobilization. Extracellular UTP evoked Ca(2+) release from intracellular stores followed by an increase in Ca(2+) entry. The Ca(2+) influx elicited by UTP was inhibited not by nifedipine, but by Zn(2+), Cd(2+), and Ni(2+) (potency order: Zn(2+) > Cd(2+) >> Ni(2+)), and the effect of UTP was also attenuated by a phospholipase C inhibitor (U73122). These results indicate that UTP activates SOCs in BAFCs. The increase in [Ca(2+)](i) by UTP was attenuated by ML-9, a myosin-light chain kinase inhibitor, and calmodulin inhibitors, W-7 and E6 berbamine, in a concentration-dependent manner. These reagents depolymerized actin filaments with rhodamine staining in BAFCs. Cytochalasin D also inhibited UTP-activated SOCs and depolymerized actin filaments. From these results, we proposed that calcium/calmodulin dependent myosin-light chain kinase is involved in the mobilization of actin filaments and the integrity of actin-network plays an important role in UTP-induced SOCs activation in BAFCs.

Phosphatidylinositol 3-kinase inhibitor failed to reduce cerebral vasospasm in dog model of experimental subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Phosphatidylinositol 3-kinase (PI3-kinase) is involved in smooth muscle contraction induced by growth factors and/or G protein-coupled receptor agonists. To evaluate the role of PI3-kinase in the pathogenesis of delayed vasospasm, we applied 2 PI3-kinase inhibitors to an established canine double-hemorrhage model of experimental subarachnoid hemorrhage.

METHODS

Twenty-four dogs underwent double blood injections via the cisterna magna on days 0 and 2. The dogs were killed on day 7. Dogs were treated with either vehicle (dimethyl sulfoxide), wortmannin, or LY294002 once per day on day 2 through day 6. Angiography was performed before blood injection and before the dogs were killed. The basilar arteries were collected for morphology, Western blot analysis, and PI3-kinase activity.

RESULTS

The residual diameter of the basilar arteries in the dimethyl sulfoxide treatment group, which was compared with day 0 angiogram, decreased markedly on day 7 (the percentage of the residual diameter was 47.8+/-0.8%). Wortmannin and LY294002 did not significantly change residual diameter on day 7. Both PI3-kinase inhibitors abolished PI3-kinase activity compared with the vehicle treatment group. However, both PI3-kinase inhibitors failed to significantly attenuate PI3-kinase protein expression (Western blot) (P>0.05, ANOVA).

CONCLUSIONS

Delayed treatment, which was to mimic the clinical situation, with PI3-kinase inhibitors failed to reverse vasospasm. PI3-kinase may not play an important role in the delayed vasospasm. The possible effect of PI3-kinase inhibitors in the early stage of vasospasm was not investigated in the present study.

Myosin light chain kinase regulates capacitative ca(2+) entry in human monocytes/macrophages

Monocytes/macrophages are present in all stages of atherosclerosis. Although many of their activities depend to various extents on changes in intracellular Ca(2+) concentration ([Ca(2+)](i)), mechanisms regulating [Ca(2+)](i) in these cells remain unclear. We aimed to explore the role of myosin light chain kinase (MLCK) in Ca(2+) signaling in freshly isolated human monocytes/macrophages. Large capacitative Ca(2+) entry (CCE) was observed under fura 2 fluoroscopy in human monocytes/macrophages treated with thapsigargin and cyclopiazonic acid. ML-9 and wortmannin, 2 structurally different inhibitors of MLCK, dose-dependently (1 to 100 micromol/L) prevented CCE and completely did so at 100 micromol/L, whereas inhibitors of tyrosine kinase and protein kinase C had only partial effects. Western blotting showed that thapsigargin significantly caused myosin light chain phosphorylation, which was almost completely blocked by ML-9 (100 micromol/L) and wortmannin (100 micromol/L). ML-9 also dose-dependently (1 to 100 micromol/L) inhibited this phosphorylation, which was well correlated with its inhibition of CCE. Transfection with MLCK antisense completely prevented CCE in response to thapsigargin and cyclopiazonic acid, whereas MLCK sense had no effect. These data strongly indicate that MLCK regulates CCE in human monocytes/macrophages. The study suggests a possible involvement of MLCK in many Ca(2+)-dependent activities of monocytes/macrophages.

Store-operated calcium entry and increased endothelial cell permeability

We hypothesized that myosin light chain kinase (MLCK) links calcium release to activation of store-operated calcium entry, which is important for control of the endothelial cell barrier. Acute inhibition of MLCK caused calcium release from inositol trisphosphate-sensitive calcium stores and prevented subsequent activation of store-operated calcium entry by thapsigargin, suggesting that MLCK serves as an important mechanism linking store depletion to activation of membrane calcium channels. Moreover, in voltage-clamped single rat pulmonary artery endothelial cells, thapsigargin activated an inward calcium current that was abolished by MLCK inhibition. F-actin disruption activated a calcium current, and F-actin stabilization eliminated the thapsigargin-induced current. Thapsigargin increased endothelial cell permeability in the presence, but not in the absence, of extracellular calcium, indicating the importance of calcium entry in decreasing barrier function. Although MLCK inhibition prevented thapsigargin from stimulating calcium entry, it did not prevent thapsigargin from increasing permeability. Rather, inhibition of MLCK activity increased permeability that was especially prominent in low extracellular calcium. In conclusion, MLCK links store depletion to activation of a store-operated calcium entry channel. However, inhibition of calcium entry by MLCK is not sufficient to prevent thapsigargin from increasing endothelial cell permeability.

Mechanisms of the thapsigargin-induced Ca(2+) entry in in situ endothelial cells of the porcine aortic valve and the endothelium-dependent relaxation in the porcine coronary artery

The mechanisms of the thapsigargin (TG)-induced capacitative Ca(2+) entry in in situ endothelial cells and its role in the regulation of arterial tone were investigated using front-surface fluorimetry and fura-2-loaded strips of porcine aortic valve and coronary artery. In the presence of extracellular Ca(2+), TG induced an initial rapid and a subsequent sustained elevation of cytosolic Ca(2+) concentration ([Ca(2+)](i)) in valvular strips. In the absence of extracellular Ca(2+), TG induced only a transient increase in [Ca(2+)](i). The TG-induced sustained elevation of [Ca(2+)](i) in endothelial cells was inhibited completely by 1 mM Ni(2+) and partly by 10 microM econazole and 30 microM ML-9, but not by 900 ng ml(-1) pertussis toxin or 100 microM wortmannin. Therefore, cytochrome P450 and protein phosphorylation are suggested to be involved in the TG-induced Ca(2+) influx in in situ endothelial cells. TG induced an endothelium-dependent large relaxation consisting of an initial and a late sustained relaxation in coronary arterial strip precontracted with U46619 (a thromboxane A2 analogue). Indomethacin alone had no effect, while indomethacin plus N(omega)-nitro-L-arginine (L-NOARG) markedly inhibited the sustained phase and slightly inhibited the initial phase of the TG-induced relaxation. TG induced a smaller but sustained relaxation during the 40 mM K(+)-induced precontraction than that seen during the U46619-induced precontraction. This relaxation was completely abolished by the pretreatment with indomethacin plus L-NOARG. In conclusion, both nitric oxide (NO) and endothelium-derived hyperpolarizing factor were suggested to mediate the TG-induced relaxation, while NO plays a major role in the sustained relaxation. The TG-induced sustained [Ca(2+)](i) elevation in endothelial cells was thus suggested to be mainly linked to the sustained production of NO.

Mechanism of ATP-induced [Ca(2+)](i) mobilization in rat basilar smooth muscle cells

BACKGROUND AND PURPOSE

We have previously reported that extracellular ATP activates P(2u) receptors and increases intracellular free Ca(2+) ([Ca(2+)](i)) by G protein/phospholipase C/inositol 1,4,5-triphosphate pathways in cerebral artery smooth muscle cells. However, the possible contribution of other signaling pathways remains unclear. This study was undertaken to investigate the role of protein tyrosine kinase (PTK) and mitogen-activated protein kinase (MAPK) in mediating ATP-induced Ca(2+) mobilization in rat basilar artery smooth muscle cells (RBASMCs).

METHODS

RBASMCs were freshly isolated, and [Ca(2+)](i) was monitored by fura 2 microfluorimetry. MAPK phosphorylation was studied by the Western blot technique.

RESULTS

ATP produced a biphasic [Ca(2+)](i) response, which consists of releasing Ca(2+) from internal stores and influx from extracellular space. PTK inhibitors tyrphostin 51 and genistein inhibited [Ca(2+)](i) response to ATP. Tyrphostin A1, an inactive analogue of tyrphostins, failed to reduce the ATP-induced response. MAPK kinase inhibitor PD98059, but not U0126, reduced the ATP-induced [Ca(2+)](i) response. Phosphatidylinositol 3-kinase (PI3-K) tyrosine kinase inhibitor wortmannin, but not janus tyrosine kinase (JAK2) inhibitor AG490, partially inhibited the [Ca(2+)](i) response induced by ATP. In addition, ATP enhanced MAPK phosphorylation in a concentration- and time-dependent manner, and genistein, tyrphostin 51, PD98059, and U0126 inhibited MAPK phosphorylation.

CONCLUSIONS

Extracellular ATP produced [Ca(2+)](i) elevation and MAPK phosphorylation in RBASMCs, and the effect was regulated by PTK. The role of MAPK in ATP-induced [Ca(2+)](i) elevation is not clear. PI3-K tyrosine kinase and JAK2 tyrosine kinase may not play an important role in the ATP-induced [Ca(2+)](i) response in RBASMCs.

Pharmacological and immunohistochemical characterization of calmodulin-stimulated (Ca(2+)+Mg(2+))-ATPase in cultured porcine aortic endothelial cells

Plasma membrane (Ca(2+)+Mg(2+))-ATPase and Ca(2+) transport activities, best characterized in human erythrocytes, are stimulated by calmodulin and thought to play a crucial role in the termination of cellular Ca(2+) signaling in all cells. In plasma membranes isolated from cultured porcine aortic endothelial cells, the (Ca(2+)+Mg(2+))-ATPase was not readily measured. This is in part because of an overabundance of nonspecific Ca(2+)- and/or Mg(2+)-activated ecto-5'-nucleotide phosphohydrolases. Moreover, addition of exogenous calmodulin (10(-9) to 10(-6) mol/L) produced no measurable stimulation of ATPase activities, suggesting a permanently activated state or, alternatively, a complete lack thereof. To establish and verify the presence of a calmodulin-regulated (Ca(2+)+Mg(2+))-ATPase activity in these endothelial cells, immunohistochemical localization using a monoclonal mouse anti-(Ca(2+)+Mg(2+))-ATPase antibody (clone 5F10) was applied to intact pig aorta endothelium, cultured endothelial monolayers, and isolated endothelial plasma membrane fractions. This approach clearly demonstrated Ca(2+) pump immunoreactivity in each of these preparations. To confirm functional calmodulin stimulation of the (Ca(2+)+Mg(2+))-ATPase, 10(-5) mol/L calmidazolium (R24571) was added to the isolated plasma membrane preparation, which lowered the (Ca(2+)+Mg(2+))-ATPase activity from 143.0 to 78.15 nmol P(i)/mg protein x min(-1). This calmidazolium-reduced activity could then be stimulated 113.1+/-0.8% in a concentration-dependent manner by the addition of exogenous calmodulin (10(-7) to 2 x 10(-6) mol/L) with an EC(50) of 3.45+/-0.04 x 10(-7) mol/L (n=4). This represents a competitive lowering of the apparent calmodulin affinity by approximately 100 compared with other unopposed calmodulin-stimulated processes. Together, these findings support evidence for the presence of a calmodulin-stimulated plasma membrane (Ca(2+)+Mg(2+))-ATPase activity in cultured porcine aortic endothelial cells.

Increased cytosolic Ca(2+) concentration in endothelial cells by calmodulin antagonists

Many functions of endothelial cells are Ca(2+)/calmodulin dependent, whereas the role of calmodulin in the regulation of cytosolic Ca(2+) ([Ca(2+)](i)) remains largely unexplained. In the present study, effects of various calmodulin antagonists on [Ca(2+)](i) were investigated in cultured aortic endothelial cells loaded with the Ca(2+)-sensitive dye fura-2/AM, and were compared with those of calmodulin-dependent protein kinase II (CaM kinase II) inhibitors. The calmodulin antagonists W-7, calmidazolium and fendiline provoked dose-dependent increases in [Ca(2+)](i). However, the CaM kinase II inhibitors KN-93 and lavendustin C had no effect on [Ca(2+)](i). In the absence of extracellular Ca(2+), pretreatment of cells with bradykinin (BK) and thapsigargin completely prevented W-7-stimulated increase in [Ca(2+)](i). Alternatively, pretreatment with W-7 also completely blocked BK- and thapsigargin-stimulated increases in [Ca(2+)](i). The time course of the Ca(2+)-response in W-7 treated cells was identical to that in thapsigargin-treated cells, but not that in BK-stimulated cells, suggesting that calmodulin antagonists could share a common signaling pathway with thapsigargin to increase [Ca(2+)](i) in endothelial cells. These findings indicate that calmodulin is involved in the regulation of [Ca(2+)](i), and may play an important role in the uptake of Ca(2+) to intracellular stores.

Light-evoked recovery from wortmannin-induced inhibition of catecholamine secretion and synaptic transmission

Wortmannin (WT) is known to inhibit catecholamine (CA) secretion in chromaffin cells. This effect was found to be sensitive to UV light in experiments designed to perform simultaneous monitoring of changes in [Ca2+]i and CA secretion in perfused rat adrenal medullas. When the change in [Ca2+]i was measured using calcium green-1 (490 nm excitation), a 35-min treatment with 10 microM WT caused a 69% inhibition of CA secretion evoked by excess (30 mM) extracellular K+ and a moderate inhibition of the [Ca2+]i response. In contrast, the same treatment of fura-2-loaded cells with WT caused only an 11% inhibition of the high-K+-evoked secretion and no significant attenuation of the [Ca2+]i response. However, during interruption of fluorometry with fura-2, the inhibitory effect of WT developed at a rate similar to that exhibited in calcium green-1-loaded cells. The WT-induced inhibition of high-K+- or bradykinin-evoked secretory responses, which was otherwise irreversible, was reversed by exposing WT-treated chromaffin cells to 380-nm light. When WT was reapplied to the cells of which the secretory ability had been restored by light irradiation, the secretory response was inhibited with a time course similar to that shown during the initial treatment with WT. The photosensitive effect of WT was also demonstrated using bullfrog sympathetic ganglia in which WT-induced inhibition of synaptic transmission was reversed by irradiation with 380-nm light. These results suggest that UV light removes the inhibitory effects of WT by disrupting the covalent bond formed between WT and a target molecule which remains to be determined, although myosin light chain kinase has been reported as the target molecule in both cases examined in this study.

Inhibition of Ca2+-dependent contraction in swine carotid artery by myosin kinase inhibitors

Experiments were designed to examine the efficacy of the MLCK inhibitors wortmannin and ML-9 in intact smooth muscle to determine whether contractile agonists can induce a Ca(2+) and myosin light chain phosphorylation-independent contraction. Both wortmannin and ML-9 reduced active stress in a dose-dependent manner. Both inhibitors interfered with Ca2+ mobilization in either the K(+)-depolarized or agonist activated swine carotid media at concentrations greater than 10 microM. Wortmannin reduced MRLC phosphorylation and stress to resting levels in stimulated tissues while Ca2+ remained above resting levels. There was no evidence for Ca2+ and MRLC phosphorylation-independent stress generation in swine arterial smooth muscle.

An essential role of myosin light-chain kinase in the regulation of agonist- and fluid flow-stimulated Ca2+ influx in endothelial cells

Cytosolic Ca2+ ([Ca2+]i) plays an important role in endothelial cell signaling. Although it has been suggested that the influx of Ca2+ can be triggered by depletion of intracellular Ca2+ stores, the mechanism (or mechanisms) underlying this phenomenon needs further elaboration. In the present study, involvement of myosin light-chain kinase (MLCK) in the regulation of Ca2+ signaling was investigated in agonist- and fluid flow-stimulated endothelial cells loaded with Ca2+-sensitive dyes. Bradykinin (BK) and thapsigargin caused an increase in [Ca2+]i followed by a sustained rise due to Ca2+ influx from extracellular space and shifted total myosin light-chain (MLC) from the unphosphorylated to the diphosphorylated form. ML-9 (100 microM), an inhibitor of MLCK, abolished Ca2+ influx and prevented MLC diphosphorylation in BK- and thapsigargin-treated cells, but did not affect Ca2+ mobilization from internal stores. Fluid flow stimulation (shear stress=5 dynes/cm2) increased [Ca2+]i and enhanced MLC phosphorylation. ML-9 also inhibited Ca2+ response and MLC phosphorylation in fluid flow-stimulated cells. The Ca2+ influx in response to BK was linearly correlated with the diphosphorylation of MLC in ML-9 treated cells. Effects of ML-5 and ML-7, analogs of ML-9, to inhibit Ca2+ influx paralleled their potencies to inhibit MLCK activity. These findings demonstrate that MLCK plays an essential role in regulating the plasmalemmal Ca2+ influx in agonist- and fluid flow-stimulated endothelial cells. This study is the first to report the close relationship between Ca2+ influx and MLC diphosphorylation.

Alzheimer's alpha-secretase may be a calcium-dependent protease

Proteolytic processing of beta-amyloid precursor protein (APP) is believed to be fundamental to the understanding of Alzheimer's disease. The identities and the regulatory elements of the proteases involved in the process, known as alpha/beta/gamma secretases, are unclear. In this study, by examining reported data, we found some indications suggesting that the putative alpha-secretase may be a calcium-dependent protease, and that this enzyme may play a primary role in the regulation of APP processing. Based on this, we proposed a model for the membrane orientations of the secretases for further discussions.

Roles of inhibitors of myosin light chain kinase and tyrosine kinase on cation influx in agonist-stimulated endothelial cells

Agonist-stimulated Ca2+ influx is critically important to mediate the function of endothelial cells. It has been suggested that release of Ca2+ from internal stores activates Ca2+ influx across the plasma membrane. In the present study, we investigated the effects of ML-9, a myosin light-chain kinase (MLCK) inhibitor, and genistein, a tyrosine kinase inhibitor, on the agonist stimulated Ca2+ response in porcine aortic endothelial cells loaded with a Ca2+-sensitive dye, fura-2. ML-9 almost completely abolished Ca2+ influx, whereas genistein only partially attenuated Ca2+ entry. Both of them did not affect the mobilization of Ca2+ from internal stores. In contrast, genistein was more potent in the inhibition of Mn2+ influx than ML-9. These findings indicate the different selectivity for Ca2+ and Mn2+ in the cation entry pathway in agonist-stimulated endothelial cells.