Discriminating between capacitative and arachidonate-activated Ca(2+) entry pathways in HEK293 cells

Regulation and organization of adenylyl cyclases and cAMP

Adenylyl cyclases are a critically important family of multiply regulated signalling molecules. Their susceptibility to many modes of regulation allows them to integrate the activities of a variety of signalling pathways. However, this property brings with it the problem of imparting specificity and discrimination. Recent studies are revealing the range of strategies utilized by the cyclases to solve this problem. Microdomains are a consequence of these solutions, in which cAMP dynamics may differ from the broad cytosol. Currently evolving methodologies are beginning to reveal cAMP fluctuations in these various compartments.

Phospholipase C and phosphatidylinositol 3-kinase signaling are involved in the exogenous arachidonic acid-stimulated respiratory burst in human neutrophils

To define the role of phospholipase C (PLC) and phosphatidylinositol 3-kinase (PI-3K), signaling pathways in arachidonic acid (AA)-stimulated respiratory burst in human neutrophils, the AA-stimulated respiratory burst, Ins(1,4,5)P(3) production, PI-3K activation, and cytoplasmic Ca(2+) mobilization were investigated. It was found that Ins(1,4,5)P(3) production and PI-3K activity in AA-stimulated cells were increased in a dose-dependent manner. U73122, the PLC inhibitor, effectively inhibited the AA-stimulated respiratory burst and Ca(2+) release from the intracellular calcium store but not the activity of PI-3K, indicating the independence of PI-3K signaling on PLC activation. Wortmannin, the PI-3K inhibitor, at the concentration sufficient to inhibit PI-3K activity, can only partially inhibit Ca(2+) release from the internal store, indicating a partial regulation of PLC signaling by PI-3K and the existence of two pathways initiated by different PLC subfamilies. One is regulated by PI-3K activation, and the other is independent of PI-3K signaling. It was observed that AA could still induce a noncapacitative Ca(2+) entry in the cells when Ca(2+) release from the intracellular store was blocked by a PLC inhibitor, or a capacitative Ca(2+) entry was induced by preincubation with thapsigargin. However, the AA-mediated, noncapacitative Ca(2+) entry seems to play a little, if any, role in the stimulated respiratory burst. The present study suggests that the PLC signaling pathway, which may be activated by PLC(beta) and PLC(gamma), respectively, and the PI-3K signaling pathway are involved in the AA-stimulated respiratory burst in human neutrophil.

Evidence that store-operated Ca2+ channels are more effective than intracellular messenger-activated non-selective cation channels in refilling rat hepatocyte intracellular Ca2+ stores

Liver cells possess store-operated Ca2+ channels (SOCs) with a high selectivity for Ca2+ compared with Na+, and several types of intracellular messenger-activated non-selective cation channels with a lower selectivity for Ca2+ (NSCCs). The main role of SOCs is thought to be in refilling depleted endoplasmic reticulum Ca2+ stores [Cell Calcium 7 (1986) 1]. NSCCs may be involved in refilling intracellular stores but are also thought to have other roles in regulating the cytoplasmic-free Ca2+ and Na+ concentrations. The ability of SOCs to refill the endoplasmic reticulum Ca2+ stores in hepatocytes has not previously been compared with that of NSCCs. The aim of the present studies was to compare the ability of SOCs and maitotoxin-activated NSCCs to refill the endoplasmic reticulum in rat hepatocytes. The experiments were performed using fura-2FF and fura-2 to monitor the free Ca2+ concentrations in the endoplasmic reticulum and cytoplasmic space, respectively, a Ca2+ add-back protocol, and 2-aminoethyl diphenylborate (2-APB) to inhibit Ca2+ inflow through SOCs. In cells treated with 2,5-di-t-butylhydroquinone (DBHQ) or vasopressin to deplete the endoplasmic reticulum Ca2+ stores, then washed to remove DBHQ or vasopressin, the addition of Ca2+ caused a substantial increase in the concentration of Ca2+ in the endoplasmic reticulum and cytoplasmic space due to the activation of SOCs. These increases were inhibited 80% by 2-APB, indicating that Ca2+ inflow is predominantly through SOCs. In the presence of 2-APB (to block SOCs), maitotoxin induced a substantial increase in [Ca2+](cyt), but only a modest and slower increase in [Ca2+](er). Under these conditions, Ca2+ inflow is predominantly through maitotoxin-activated NSCCs. It is concluded that SOCs are more effective than maitotoxin-activated NSCCs in refilling the endoplasmic reticulum Ca2+ stores. The previously developed concept of a specific role for SOCs in refilling the endoplasmic reticulum is consistent with the results reported here.

Regulation of TRP channels via lipid second messengers

In Drosophila photoreceptors, the light-sensitive current is mediated downstream of phospholipase C by TRP (transient receptor potential) channels. Recent evidence suggests that Drosophila TRP channels are activated by diacylglycerol (DAG) or its metabolites (polyunsaturated fatty acids), possibly in combination with the reduction in phosphatidyl inositol 4,5 bisphosphate (PIP2). Consistent with this view, diacylglycerol kinase is identified as a key enzyme required for response termination. Signaling is critically dependent upon efficient PIP2 synthesis; mutants of this pathway in combination with genetically targeted PIP2 reporters provide unique insights into the kinetics and regulation of PIP2 turnover. Recent evidence indicates that a growing number of mammalian TRP homologues are also regulated by lipid messengers, including DAG, arachidonic acid, and PIP2.

Interleukin-1beta induces calcium transients and enhances basal and store operated calcium entry in human myometrial smooth muscle

We have previously reported increased protein expression of sarcoplasmic reticulum calcium ATPase (SERCA) 2b in myometrium from women in labor at term, but the stimulus for this change is unknown. Proinflammatory cytokines have been implicated in the cascade of events leading to preterm and term labor, and we hypothesize that interleukin (IL)-1beta may induce changes in key calcium homeostatic mechanisms and, in turn, augment myometrial contractility before labor. The aim of the present study was to investigate the long-term effects of IL-1beta on SERCA 2b protein expression, calcium mobilization from intracellular stores, and store-operated calcium entry. Myometrial biopsies were obtained, with consent, from women undergoing elective cesarean section at term. Primary-cultured human myometrial smooth muscle (HMSM) cells were exposed to IL-1beta (10 ng/ml) for 24 h or to culture medium alone (control). Cells were subsequently used in Western blot studies or loaded with fura-2 to assess calcium dynamics using fluorescent digital imaging. The present study clearly demonstrated that IL-1beta significantly increased SERCA 2b protein expression in HMSM cells. Cyclopiazonic acid-induced calcium transients were also augmented, predominantly by activation of lanthanum-sensitive, store-operated calcium entry. HMSM cell excitability was enhanced, as evidenced by increased basal calcium entry and the initiation of spontaneous calcium transients in 37% of IL-1beta-treated cells. IL-1beta modulation of calcium mobilization may be an important mechanism in the cascade of events preparing the pregnant uterus for labor.

Mitochondrial Ca2+ uptake requires sustained Ca2+ release from the endoplasmic reticulum

We analyzed the role of inositol 1,4,5-trisphosphate-induced Ca(2+) release from the endoplasmic reticulum (ER) (i) in powering mitochondrial Ca(2+) uptake and (ii) in maintaining a sustained elevation of cytosolic Ca(2+) concentration ([Ca(2+)](c)). For this purpose, we expressed in HeLa cells aequorin-based Ca(2+)-sensitive probes targeted to different intracellular compartments and studied the effect of two agonists: histamine, acting on endogenous H(1) receptors, and glutamate, acting on co-transfected metabotropic glutamate receptor (mGluR1a), which rapidly inactivates through protein kinase C-dependent phosphorylation and thus causes transient inositol 1,4,5-trisphosphate production. Glutamate induced a transient [Ca(2+)](c) rise and drop in ER luminal [Ca(2+)] ([Ca(2+)](er)), and then the ER refilled with [Ca(2+)](c) at resting values. With histamine, [Ca(2+)](c) after the initial peak stabilized at a sustained plateau, and [Ca(2+)](er) decreased to a low steady-state value. In mitochondria, histamine evoked a much larger mitochondrial Ca(2+) response than glutamate ( approximately 15 versus approximately 65 microm). Protein kinase C inhibition, partly relieving mGluR1a desensitization, reestablished both the [Ca(2+)](c) plateau and the sustained ER Ca(2+) release and markedly increased the mitochondrial Ca(2+) response. Conversely, mitochondrial Ca(2+) uptake evoked by histamine was drastically reduced by very transient ( approximately 2-s) agonist applications. These data indicate that efficient mitochondrial Ca(2+) uptake depends on the preservation of high Ca(2+) microdomains at the mouth of ER Ca(2+) release sites close to mitochondria. This in turn depends on continuous Ca(2+) release balanced by Ca(2+) reuptake into the ER and maintained by Ca(2+) influx from the extracellular space.

Ca2+-independent phospholipase A2 is a novel determinant of store-operated Ca2+ entry

Store-operated cation (SOC) channels and capacitative Ca(2+) entry (CCE) play very important role in cellular function, but the mechanism of their activation remains one of the most intriguing and long lasting mysteries in the field of Ca(2+) signaling. Here, we present the first evidence that Ca(2+)-independent phospholipase A(2) (iPLA(2)) is a crucial molecular determinant in activation of SOC channels and store-operated Ca(2+) entry pathway. Using molecular, imaging, and electrophysiological techniques, we show that directed molecular or pharmacological impairment of the functional activity of iPLA(2) leads to irreversible inhibition of CCE mediated by nonselective SOC channels and by Ca(2+)-release-activated Ca(2+) (CRAC) channels. Transfection of vascular smooth muscle cells (SMC) with antisense, but not sense, oligonucleotides for iPLA(2) impaired thapsigargin (TG)-induced activation of iPLA(2) and TG-induced Ca(2+) and Mn(2+) influx. Identical inhibition of TG-induced Ca(2+) and Mn(2+) influx (but not Ca(2+) release) was observed in SMC, human platelets, and Jurkat T-lymphocytes when functional activity of iPLA(2) was inhibited by its mechanism-based suicidal substrate, bromoenol lactone (BEL). Moreover, irreversible inhibition of iPLA(2) impaired TG-induced activation of single nonselective SOC channels in SMC and BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid)-induced activation of whole-cell CRAC current in rat basophilic leukemia cells. Thus, functional iPLA(2) is required for activation of store-operated channels and capacitative Ca(2+) influx in wide variety of cell types.

Ca2+ selectivity and fatty acid specificity of the noncapacitative, arachidonate-regulated Ca2+ (ARC) channels

The arachidonate-regulated, Ca(2+)-selective ARC channels represent a novel receptor-activated pathway for the entry of Ca(2+) in nonexcitable cells that is entirely separate from the widely studied store-operated, Ca(2+) release-activated Ca(2+) channels. Activation of ARC channels occurs specifically at the low agonist concentrations typically associated with oscillatory Ca(2+) signals and appears to provide the predominant mode of Ca(2+) entry under these conditions (Mignen, O., Thompson, J. L., and Shuttleworth, T. J. (2001) J. Biol. Chem. 276, 35676-35683). In this study we demonstrate that ARC channels are present in a variety of different cell types including both cell lines and primary cells. Examination of their pharmacology revealed that currents through these channels are significantly inhibited by low concentrations (< 5 microm) of Gd(3+), are unaffected by 100 microm 2-aminoethyoxydiphenyl borane, and are not activated by the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (100 microm). Their selectivity for Ca(2+) was assessed by determining the EC(50) for external Ca(2+) block of the monovalent currents observed in the absence of external divalent cations. The value obtained (150 nm) indicates that the Ca(2+) selectivity of ARC channels is extremely high. Examination of the ability of various fatty acids, including arachidonic acid, to activate the ARC channels demonstrated that activation does not reflect any nonspecific membrane fluidity or detergent effects, shows a high degree of specificity for arachidonic acid over other fatty acids (especially monounsaturated and saturated fatty acids), and is independent of any arachidonic acid metabolite. Moreover, studies using the charged analogue arachidonyl coenzyme A demonstrate that activation of the ARC channels reflects an action of the fatty acid specifically at the internal face of the plasma membrane. Whether this involves a direct action of arachidonic acid on the channel protein itself or an action on some intermediary molecule is, at present, unclear.

Nitric oxide co-ordinates the activities of the capacitative and non-capacitative Ca2+-entry pathways regulated by vasopressin

In A7r5 vascular smooth muscle cells vasopressin, via arachidonic acid, regulates two Ca(2+)-entry pathways. Capacitative Ca(2+) entry (CCE), activated by empty Ca(2+) stores, is inhibited by arachidonic acid, and non-capacitative Ca(2+) entry (NCCE) is stimulated by it. This reciprocal regulation ensures that all Ca(2+) entry is via NCCE in the presence of vasopressin, while CCE mediates a transient Ca(2+) entry only after removal of vasopressin. We demonstrate that type III NO synthase (NOS III) is expressed in A7r5 cells and that NO inhibits CCE. Inhibition of CCE by vasopressin requires NOS III and the requirement lies downstream of arachidonic acid. Activation of soluble guanylate cyclase by NO and subsequent activation of protein kinase G are required for inhibition of CCE. Stimulation of NCCE by vasopressin also requires NOS III, but the stimulation is neither mimicked by cGMP nor blocked by inhibitors of soluble guanylate cyclase or protein kinase G. We conclude that arachidonic acid formed in response to vasopressin stimulates NOS III. NO then directly stimulates Ca(2+) entry through NCCE and, via protein kinase G, it inhibits CCE. The additional amplification provided by the involvement of guanylate cyclase and protein kinase G ensures that CCE will always be inhibited when vasopressin activates NCCE.

TRPM7 provides an ion channel mechanism for cellular entry of trace metal ions

Trace metal ions such as Zn(2+), Fe(2+), Cu(2+), Mn(2+), and Co(2+) are required cofactors for many essential cellular enzymes, yet little is known about the mechanisms through which they enter into cells. We have shown previously that the widely expressed ion channel TRPM7 (LTRPC7, ChaK1, TRP-PLIK) functions as a Ca(2+)- and Mg(2+)-permeable cation channel, whose activity is regulated by intracellular Mg(2+) and Mg(2+).ATP and have designated native TRPM7-mediated currents as magnesium-nucleotide-regulated metal ion currents (MagNuM). Here we report that heterologously overexpressed TRPM7 in HEK-293 cells conducts a range of essential and toxic divalent metal ions with strong preference for Zn(2+) and Ni(2+), which both permeate TRPM7 up to four times better than Ca(2+). Similarly, native MagNuM currents are also able to support Zn(2+) entry. Furthermore, TRPM7 allows other essential metals such as Mn(2+) and Co(2+) to permeate, and permits significant entry of nonphysiologic or toxic metals such as Cd(2+), Ba(2+), and Sr(2+). Equimolar replacement studies substituting 10 mM Ca(2+) with the respective divalent ions reveal a unique permeation profile for TRPM7 with a permeability sequence of Zn(2+) approximately Ni(2+) >> Ba(2+) > Co(2+) > Mg(2+) >/= Mn(2+) >/= Sr(2+) >/= Cd(2+) >/= Ca(2+), while trivalent ions such as La(3+) and Gd(3+) are not measurably permeable. With the exception of Mg(2+), which exerts strong negative feedback from the intracellular side of the pore, this sequence is faithfully maintained when isotonic solutions of these divalent cations are used. Fura-2 quenching experiments with Mn(2+), Co(2+), or Ni(2+) suggest that these can be transported by TRPM7 in the presence of physiological levels of Ca(2+) and Mg(2+), suggesting that TRPM7 represents a novel ion-channel mechanism for cellular metal ion entry into vertebrate cells.

Residence of adenylyl cyclase type 8 in caveolae is necessary but not sufficient for regulation by capacitative Ca(2+) entry

Ca(2+)-sensitive adenylyl cyclases (ACs) depend on capacitative Ca(2+) entry (CCE) for their regulation. Residence of the endogenous Ca(2+)-inhibitable adenylyl cyclase of C6-2B glioma cells in cholesterol-enriched caveolae is essential for its regulation by CCE (Fagan, K. A., Smith, K. E., and Cooper, D. M. F. (2000) J. Biol. Chem. 275, 26530-26537). In the present study, we established that depletion of cellular cholesterol ablated the regulation by CCE of a Ca(2+)-stimulable adenylyl cyclase, AC8, heterologously expressed in HEK293 cells. We considered the possibility that a calmodulin-binding domain in the N terminus of AC8, which is not required for in vitro regulation by Ca(2+), might play a targeting role. Deletion and mutation of the N terminus did attenuate the enzyme's sensitivity to CCE without altering its in vitro responsiveness to Ca(2+)/calmodulin. Both N terminus-deleted AC8 and wild type AC8 were expressed at the plasma membrane, as shown by imaging analysis of green fluorescence protein-tagged constructs. However, not only wild type AC8 but also the CCE-insensitive mutants occurred in caveolar fractions of the plasma membranes, even though a Ca(2+)-insensitive adenylyl cyclase, AC7, was excluded from caveolae. Finally, the AC8 mutants were no more responsive to nonphysiological elevation of Ca(2+) than the wild type. We conclude that (i) not all adenylyl cyclases reside in caveolae, (ii) the calmodulin-binding domain in the N terminus of AC8 does not play a role in caveolar targeting, (iii) the N terminus does play a role in associating AC8 with factors that confer sensitivity to CCE, and (iv) residence of Ca(2+)-sensitive adenylyl cyclases in caveolae is essential but not sufficient for regulation by CCE.

Reciprocal regulation of capacitative and non-capacitative Ca2+ entry in A7r5 vascular smooth muscle cells: only the latter operates during receptor activation

In A7r5 vascular smooth muscle cells, Arg(8)-vasopressin (AVP) stimulates phospholipase C leading to activation of two distinct Ca(2+) entry pathways. The capacitative Ca(2+) entry (CCE) pathway is activated by depletion of Ca(2+) stores, is permeable to Mn(2+), Ba(2+) and Ca(2+), and is selectively blocked by Gd(3+)(1 microM). A7r5 cells also express a non-capacitative Ca(2+) entry (NCCE) pathway, which is activated by arachidonic acid that is released by the sequential activities of phospholipase C and diacylglycerol lipase. This pathway is permeable to Sr(2+), Ba(2+) and Ca(2+) and selectively blocked by (R,S)-(3,4-dihydro-6,7-dimethoxy-isochinolin-1-yl)-2-phenyl-N,N-di[2-(2,3,4-trimethoxyphenyl)ethyl]acetamid mesylate ("LOE-908"). We use these selective tools to show that AVP, via the same signalling pathway that leads to activation of NCCE, also inhibits CCE and that the inhibition is not due to depolarization of the plasma membrane. Using the selective inhibitors to resolve the contributions of each Ca(2+) entry pathway during stimulation with AVP, we establish that reciprocal regulation of CCE and NCCE by arachidonic acid ensures that only NCCE is active in the presence of AVP, whereas CCE is active only after its removal. NCCE and CCE are therefore activated in a strict temporal sequence: NCCE first and then CCE. Because Ca(2+) passing through different Ca(2+) entry pathways can selectively regulate different responses, reciprocal regulation of CCE and NCCE may allow a stimulus to first evoke a response and then recruit actively a different response when the stimulus is removed.

Reciprocal regulation of capacitative and arachidonate-regulated noncapacitative Ca2+ entry pathways

Receptor-activated Ca(2+) entry is usually thought to occur via capacitative or store-operated Ca(2+) channels. However, at physiological levels of stimulation, where Ca(2+) store depletion is only transient and/or partial, evidence has suggested that an arachidonic acid-dependent noncapacitative Ca(2+) entry is responsible. Recently, we have described a novel arachidonate-regulated Ca(2+)-selective (ARC) conductance that is entirely distinct from store-operated conductances in the same cell. We now show that these ARC channels are indeed specifically activated by low agonist concentrations and provide the predominant route of Ca(2+) entry under these conditions. We further demonstrate that sustained elevations in cytosolic Ca(2+), such as those resulting from activation of store-operated Ca(2+) entry by high agonist concentrations, inhibit the ARC channels. This explains earlier failures to detect the presence of this noncapacitative pathway in experiments where store-operated entry had already been fully activated. The result is that the respective activities of ARC and store-operated Ca(2+) channels display a unique reciprocal regulation that is related to the specific nature of the [Ca(2+)](i) signals generated at different agonist concentrations. Importantly, these data show that at physiologically relevant levels of stimulation, it is the noncapacitative ARC channels that provide the predominant route for the agonist-activated entry of Ca(2+).

Recent developments in non-excitable cell calcium entry

Influx of calcium into cells following stimulation of cell surface receptors is a key process controlling cellular activity. However, despite intensive research, there is still no consensus on precisely how calcium entry is controlled in electrically no n-excitable cells. In particular, the regulation of depletion-activated or 'capacitative' calcium entry continues to be a focus of debate. Work published in the last 2 years has lent new impetus to the so-called 'conformational coupling' theory, although evidence for the existence of soluble messengers between the ER and the plasma membrane also continues to appear. In addition, there remains disagreement on whether intra-store [Ca(2+)] has to fall below a threshold before Ca(2+)entry is activated. A further major question is the identity of the putative depletion-operated Ca(2+)channel or channels. Here discussion has largely focussed on whether homologue(s) of the Drosophila TRP ('Transient Receptor Potential') protein is/are the elusive channel, or at least a part of it. Finally, it remains possible that Ca(2+)entry mechanisms other than depletion-activated channels may be important in agonist-evoked Ca(2+)influx. This commentary summarizes recent developments in the field, and highlights both current debates and critical unsolved questions.

The organisation and functions of local Ca2+ signals

Calcium (Ca2+) is a ubiquitous intracellular messenger, controlling a diverse range of cellular processes, such as gene transcription, muscle contraction and cell proliferation. The ability of a simple ion such as Ca2+ to play a pivotal role in cell biology results from the facility that cells have to shape Ca2+ signals in space, time and amplitude. To generate and interpret the variety of observed Ca2+ signals, different cell types employ components selected from a Ca2+ signalling ‘toolkit’, which comprises an array of homeostatic and sensory mechanisms. By mixing and matching components from the toolkit, cells can obtain Ca2+ signals that suit their physiology. Recent studies have demonstrated the importance of local Ca2+ signals in defining the specificity of the interaction of Ca2+ with its targets. Furthermore, local Ca2+ signals are the triggers and building blocks for larger global signals that propagate throughout cells.

Permeation of monovalent cations through the non-capacitative arachidonate-regulated Ca2+ channels in HEK293 cells. Comparison with endogenous store-operated channels

In a manner similar to voltage-gated Ca(2+) channels and Ca(2+) release-activated Ca(2+) (CRAC) channels, the recently identified arachidonate-regulated Ca(2+) (ARC) channels display a large monovalent conductance upon removal of external divalent cations. Using whole-cell patch-clamp recording, we have characterized the properties of these monovalent currents in HEK293 cells stably transfected with the m3 muscarinic receptor and compared them with the corresponding currents through the endogenous store-operated Ca(2+) (SOC) channels in the same cells. Although the monovalent currents seen through these two channels displayed certain similarities, several marked differences were also apparent, including the magnitude of the monovalent current/Ca(2+) current ratio, the rate and nature of the spontaneous decline in the currents, and the effects of external monovalent cation substitutions and removal of internal Mg(2+). Moreover, monovalent ARC currents could be activated after the complete spontaneous inactivation of the corresponding SOC current in the same cell. We conclude that the non-capacitative ARC channels share, with voltage-gated Ca(2+) channels and store-operated Ca(2+) channels (e.g. SOC and CRAC the general property of monovalent ion permeation in the nominal absence of extracellular divalent ions. However, the clear differences between the properties of these currents through ARC and SOC channels in the same cell confirm that these represent distinct conductances.

Mutual antagonism of calcium entry by capacitative and arachidonic acid-mediated calcium entry pathways

In nonexcitable cells, the predominant mechanism for regulated entry of Ca(2+) is capacitative calcium entry, whereby depletion of intracellular Ca(2+) stores signals the activation of plasma membrane calcium channels. A number of other regulated Ca(2+) entry pathways occur in specific cell types, however, and it is not know to what degree the different pathways interact when present in the same cell. In this study, we have examined the interaction between capacitative calcium entry and arachidonic acid-activated calcium entry, which co-exist in HEK293 cells. These two pathways exhibit mutual antagonism. That is, capacitative calcium entry is potently inhibited by arachidonic acid, and arachidonic acid-activated entry is inhibited by the pre-activation of capacitative calcium entry with thapsigargin. In the latter case, the inhibition does not seem to result from a direct action of thapsigargin, inhibition of endoplasmic reticulum Ca(2+) pumps, depletion of Ca(2+) stores, or entry of Ca(2+) through capacitative calcium entry channels. Rather, it seems that a discrete step in the pathway signaling capacitative calcium entry interacts with and inhibits the arachidonic acid pathway. The findings reveal a novel process of mutual antagonism between two distinct calcium entry pathways. This mutual antagonism may provide an important protective mechanism for the cell, guarding against toxic Ca(2+) overload.

Regulation of a Ca2+-sensitive adenylyl cyclase in an excitable cell. Role of voltage-gated versus capacitative Ca2+ entry

In nonexcitable cells, we had previously established that Ca(2+)-sensitive adenylyl cyclases, whether expressed endogenously or heterologously, were regulated exclusively by capacitative Ca(2+) entry (Fagan, K. A., Mahey, R. and Cooper, D. M. F. (1996) J. Biol. Chem. 271, 12438-12444; Fagan, K. A., Mons, N., and Cooper, D. M. F. (1998) J. Biol. Chem. 273, 9297-9305). Relatively little is known about how these enzymes are regulated by Ca(2+) in excitable cells, where they predominate. Furthermore, no effort has been made to determine whether the prominent voltage-gated Ca(2+) entry, which typifies excitable cells, overwhelms the effect of any capacitative Ca(2+) entry that may occur. In the present study, we placed the Ca(2+)-stimulable, adenylyl cyclase type VIII in an adenovirus vector to optimize its expression in the pituitary-derived GH(4)C(1) cell line. In these cells, a modest degree of capacitative Ca(2+) entry could be discerned in the face of a dramatic voltage-gated Ca(2+) entry. Nevertheless, both modes of Ca(2+) entry were equally efficacious at stimulating adenylyl cyclase. A striking release of Ca(2+) from intracellular stores, triggered either by ionophore or thyrotrophin-releasing hormone, was incapable of stimulating the adenylyl cyclase. It thus appears as though the intimate colocalization of adenylyl cyclase with capacitative Ca(2+) entry channels is an intrinsic property of these molecules, regardless of whether they are expressed in excitable or nonexcitable cells.

Protein kinase C activates non-capacitative calcium entry in human platelets

1. In many non-excitable cells Ca2+ influx is mainly controlled by the filling state of the intracellular Ca2+ stores. It has been suggested that this store-mediated or capacitative Ca2+ entry is brought about by a physical and reversible coupling of the endoplasmic reticulum with the plasma membrane. Here we provide evidence for an additional, non-capacitative Ca2+ entry mechanism in human platelets. 2. Changes in cytosolic Ca2+ and Sr2+ were measured in human platelets loaded with the fluorescent indicator fura-2. 3. Depletion of the internal Ca2+ stores with thapsigargin plus a low concentration of ionomycin stimulated store-mediated cation entry, as demonstrated upon Ca2+ or Sr2+ addition. Subsequent treatment with thrombin stimulated further divalent cation entry in a concentration-dependent manner. 4. Direct activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate or 1-oleoyl-2-acetyl-sn-glycerol also stimulated divalent cation entry, without evoking the release of Ca2+ from intracellular stores. Cation entry evoked by thrombin or activators of PKC was abolished by the PKC inhibitor Ro-31-8220. 5. Unlike store-mediated Ca2+ entry, jasplakinolide, which reorganises actin filaments into a tight cortical layer adjacent to the plasma membrane, did not inhibit divalent cation influx evoked by thrombin when applied after Ca2+ store depletion, or by activators of PKC. Thrombin also activated Ca2+ entry in platelets in which the release from intracellular stores and store-mediated Ca2+ entry were blocked by xestospongin C. 6. These results indicate that the non-capacitative divalent cation entry pathway is regulated independently of store-mediated entry and does not require coupling of the endoplasmic reticulum and the plasma membrane. These results support the existence of a mechanism for receptor-evoked Ca2+ entry in human platelets that is independent of Ca2+ store depletion. This Ca2+ entry mechanism may be activated by occupation of G-protein-coupled receptors, which activate PKC, or by direct activation of PKC, thus generating non-capacitative Ca2+ entry alongside that evoked following the release of Ca2+ from the intracellular stores.

Regulation of the Ca2+-inhibitable adenylyl cyclase type VI by capacitative Ca2+ entry requires localization in cholesterol-rich domains

The endogenous Ca(2+)-inhibitable adenylyl cyclase type VI of C6-2B glioma cells is regulated only by capacitative Ca(2+) entry and not by a substantial elevation of [Ca(2+)](i) from either intracellular stores or via ionophore-mediated Ca(2+) entry (Chiono, M., Mahey, R., Tate, G., and Cooper, D. M. F. (1995) J. Biol. Chem. 270, 1149-1155; Fagan, K. A., Mons, N., and Cooper, D. M. F. (1998) J. Biol. Chem. 273, 9297-9305). The present studies explored the role of cholesterol-rich domains in maintaining this functional association. The cholesterol-binding agent, filipin, profoundly inhibited adenylyl cyclase activity. Depletion of plasma membrane cholesterol with methyl-beta-cyclodextrin did not affect forskolin-stimulated adenylyl cyclase activity and did not affect capacitative Ca(2+) entry. However, cholesterol depletion completely ablated the regulation of adenylyl cyclase by capacitative Ca(2+) entry. Repletion of cholesterol restored the sensitivity of adenylyl cyclase to capacitative Ca(2+) entry. Adenylyl cyclase catalytic activity and immunoreactivity were extracted into buoyant caveolar fractions with Triton X-100. The presence of adenylyl cyclase in such structures was eliminated by depletion of plasma membrane cholesterol. Altogether, these data lead us to conclude that adenylyl cyclase must occur in cholesterol-rich domains to be susceptible to regulation by capacitative Ca(2+) entry. These findings are the first indication of regulatory significance for the localization of adenylyl cyclase in caveolae.

Characteristics of the Ca(2+)-dependent inhibition of cyclic AMP accumulation by histamine and thapsigargin in human U373 MG astrocytoma cells

1. Histamine, acting on H(1)-receptors, caused a Ca(2+)-dependent inhibition of forskolin- and isoprenaline-induced cyclic AMP accumulation in monolayers of human U373 MG cells (IC(50) 1.3+/-0.3 microM, maximum inhibition 66+/-3%). The inhibition was not reversed by the protein kinase inhibitor K-252A. 2. Thapsigargin also inhibited cyclic AMP accumulation (IC(50) 6.0+/-0.3 nM, maximum inhibition 72+/-1%). In the absence of extracellular Ca(2+) 5 microM thapsigargin caused only a 12+/-2% inhibition of cyclic AMP accumulation. 3. The inhibitory effect of 100 nM thapsigargin on forskolin-stimulated cyclic AMP accumulation was blocked by La(3+) (best-fit maximum inhibition 81+/-4%, IC(50) 125+/-8 nM). In contrast, the inhibitory action of 10 microM histamine was much less sensitive to reversal by 1 microM La(3+) (33+/-5% reversal, compared with 78+/-6% reversal of the inhibition by thapsigargin measured concurrently). However, in the presence of both thapsigargin and histamine the inhibition of cyclic AMP accumulation was reversed by 1 microM La(3+) to the same extent as the inhibition by thapsigargin alone. 4.++Thapsigargin (5 microM)+1 microM La(3+) caused only a 20+/-1% inhibition of histamine-stimulated phosphoinositide hydrolysis. 5. There was no indication from measurement of intracellular Ca(2+) of any persistent La(3+)-insensitive Ca(2+) entry component activated by histamine. 6. The results provide evidence that Ca(2+) entry is required for the inhibition by histamine and thapsigargin of drug-induced cyclic AMP accumulation in U373 MG astrocytoma cells. The differential sensitivity of the inhibitory action of the two agents to block by La(3+) suggests that more than one pathway of Ca(2+) entry is involved.

I(ARC), a novel arachidonate-regulated, noncapacitative Ca(2+) entry channel

Along with the inositol trisphosphate-induced release of stored Ca(2+), a receptor-enhanced entry of Ca(2+) is a critical component of intracellular Ca(2+) signals generated by agonists acting at receptors coupled to the activation of phospholipase C. Although the simple emptying of the intracellular Ca(2+) stores is known to be capable of activating Ca(2+) entry via the so-called "capacitative" mechanism, recent evidence suggests that Ca(2+) entry at physiological agonist concentrations, where oscillatory Ca(2+) signals are typically observed, does not conform to such a model. Instead, a noncapacitative Ca(2+) entry pathway regulated by arachidonic acid appears to be responsible for Ca(2+) entry under these conditions. Using whole-cell patch clamp techniques we demonstrate that low concentrations of arachidonic acid activate a Ca(2+)-selective current that is superficially similar to the store-operated current I(CRAC), but which also demonstrates certain distinct features. We have named this novel current I(ARC) (for arachidonate-regulated calcium current). Importantly, I(ARC) can be readily activated in cells whose Ca(2+) stores have been maximally depleted. I(ARC) represents a novel Ca(2+) entry pathway that is entirely separate from those activated by store depletion and is specifically activated at physiological levels of stimulation.

A Ca(2+)-independent activation of a type IV cytosolic phospholipase A(2) underlies the receptor stimulation of arachidonic acid-dependent noncapacitative calcium entry

The oscillatory [Ca(2+)](i) signals typically seen following physiologically relevant stimulation of phospholipase C-linked receptors are associated with a receptor-activated entry of Ca(2+), which plays a critical role in driving the oscillations and influencing their frequency. We have recently shown that this receptor-activated entry of Ca(2+) does not conform to the widely accepted "capacitative" model and, instead, reflects the activity of a distinct, novel Ca(2+) entry pathway regulated by arachidonic acid (Shuttleworth, T. J., and Thompson, J. L. (1998) J. Biol. Chem. 273, 32636-32643). We now show that the generation of arachidonic acid under these conditions results from the activity of a type IV cytosolic phospholipase A(2) (cPLA(2)). Although cPLA(2) activation commonly involves a Ca(2+)-dependent translocation to the membrane, at these low agonist concentrations cPLA(2) activation was independent of increases in [Ca(2+)](i), and no detectable translocation to the membrane occurs. Nevertheless, stimulation of cPLA(2) activity was confined to the membrane fraction, where an increase in phosphorylation of the enzyme was observed. We suggest that, at the low agonist concentrations associated with oscillatory [Ca(2+)](i) signals, cPLA(2) activation involves an increased phosphorylation of a discrete pool of the total cellular cPLA(2) that is already localized within the membrane fraction at resting [Ca(2+)](i).