Intracellular elevations of free calcium induced by activation of histamine H1 receptors in interphase and mitotic HeLa cells: hormone signal transduction is altered during mitosis

Forms and functions of store-operated calcium entry mediators, STIM and Orai

Calcium signals arise by multiple mechanisms, including mechanisms of release of intracellular stored Ca²⁺, and the influx of Ca²⁺ through channels in the plasma membrane. One mechanism that links these two sources of Ca²⁺ is store-operated Ca²⁺ entry, the most commonly encountered version of which involves the extensively studied calcium-release-activated Ca²⁺ (CRAC) channel. The minimal and essential molecular components of the CRAC channel are the STIM proteins that function as Ca²⁺ sensors in the endoplasmic reticulum, and the Orai proteins that comprise the pore forming subunits of the CRAC channel. CRAC channels are known to play significant roles in a wide variety of physiological functions. This review discusses the multiple forms of STIM and Orai proteins encountered in mammalian cells, and discusses some specific examples of how these proteins modulate or mediate important physiological processes.

Fucoidan inhibits Ca2+ responses induced by a wide spectrum of agonists for G‑protein‑coupled receptors

Fucoidan, a sulfated polysaccharide extracted from brown seaweed, has been used in traditional Chinese herbal medicine to treat thyroid tumors for many years. Although a number of its cellular effects have been investigated, the role of fucoidan in molecular signaling, particularly in Ca²⁺ signaling, remains largely unknown. In the present study, the effects of fucoidan on Ca²⁺ responses in HeLa cells, human umbilical vein endothelial cells and astrocytes were investigated using a wide range of receptor agonists. Fucoidan inhibited the increase in intracellular free calcium concentration that was induced by histamine, ATP, compound 48/80 and acetylcholine. The responses induced by the same agonists in the absence of extracellular Ca²⁺ were also markedly suppressed by fucoidan. Reverse transcription-polymerase chain reaction demonstrated that 0.5 and 1.0 mg/ml fucoidan treatment for 3 h decreased histamine receptor 1 expression in HeLa cells. Similarly, the expressions of purinergic receptor P2Y, G-protein coupled (P2YR)1, P2YR2 and P2YR11 were significantly downregulated within cells pretreated with 1.0 mg/ml fucoidan for 3 h, and 0.5 mg/ml fucoidan significantly inhibited P2YR1 and P2YR11 expression. The results demonstrated that fucoidan may exert a wide spectrum of inhibitory effects on Ca²⁺ responses and that fucoidan may inhibit a number of different G-protein coupled receptors associated with Ca²⁺ dynamics.

Pharmacoperone rescue of vasopressin 2 receptor mutants reveals unexpected constitutive activity and coupling bias

Pharmacoperones are small molecules that diffuse into cells and rescue misfolded, mistrafficked protein mutants, restoring their function. These substances act with high target specificity, serving as templates to fold (or refold) receptors, enzymes, ion channels or other proteins and enable them to pass the scrutiny of the cellular quality control system ("rescue"). In the present study we demonstrate that a rescued mutant (L83Q) of the vasopressin type 2 receptor (V2R), shows a strong bias for Gs coupling unlike the WT V2 receptor, which couples to both Gs and Gq/11. Failure of the mutant to couple to Gq/11 was not due to a limiting quantity of G-proteins since other Gq/11-coupled receptors (WT V2R, histamine receptor and muscarinic receptor) responded appropriately to their ligands. Transfection with DNA encoding Gq enabled the V2 receptor mutant to couple to this G protein, but only modestly compared with the WT receptor. Fourteen V2R mutant pharmacoperones, of multiple chemical classes, obtained from a high throughput screen of a 660,000 structure library, and one V2R peptidomimetic antagonist rescues L83Q. The rescued mutant shows similar bias with all pharmacoperones identified, suggesting that the bias is intrinsic to the mutant protein's structure, rather than due to the chemical class of the pharmacoperone. In the case of V2R mutant Y128S, rescue with a pharmacoperone revealed constitutive activity, also with bias for Gs, although both IP and cAMP were produced in response to agonist. These results suggest that particular rescued receptor mutants show functional characteristics that differ from the WT receptor; a finding that may be important to consider as pharmacoperones are developed as therapeutic agents.

Label-Free Imaging of Dynamic and Transient Calcium Signaling in Single Cells

Cell signaling consists of diverse events that occur at various temporal and spatial scales, ranging from milliseconds to hours and from single biomolecules to cell populations. The pathway complexities require the development of new techniques that detect the overall signaling activities and are not limited to quantifying a single event. A plasmonic-based electrochemical impedance microscope (P-EIM) that can provide such data with excellent temporal and spatial resolution and does not require the addition of any labels for detection has now been developed. The highly dynamic and transient calcium signaling activities at the early stage of G-protein-coupled receptor (GPCR) stimulation were thus studied. It could be shown that a subpopulation of cells is more responsive towards agonist stimulation, and the heterogeneity of the local distributions and the transient activities of the ion channels during agonist-activated calcium flux in single HeLa cells were investigated.

Reticulon 4 is necessary for endoplasmic reticulum tubulation, STIM1-Orai1 coupling, and store-operated calcium entry

Despite recent advances in understanding store-operated calcium entry (SOCE) regulation, the fundamental question of how ER morphology affects this process remains unanswered. Here we show that the loss of RTN4, is sufficient to alter ER morphology and severely compromise SOCE. Mechanistically, we show this to be the result of defective STIM1-Orai1 coupling because of loss of ER tubulation and redistribution of STIM1 to ER sheets. As a functional consequence, RTN4-depleted cells fail to sustain elevated cytoplasmic Ca(2+) levels via SOCE and therefor are less susceptible to Ca(2+) overload induced apoptosis. Thus, for the first time, our results show a direct correlation between ER morphology and SOCE and highlight the importance of RTN4 in cellular Ca(2+) homeostasis.

Calcium channels, external calcium concentration and cell proliferation

Evidence for a role for calcium channel proteins in cell proliferation is numerous suggesting that calcium influx is essential in this physiological process. Several studies in the past thirty years have demonstrated that calcium channel expression levels are determinant in cell proliferation. Voltage-gated, store-operated, second messengers and receptor-operated calcium channels have been associated to cell proliferation. However, the relationship between calcium influx and cell proliferation can be uncoupled in transformed and cancer cells, resulting in an external calcium-independent proliferation. Thus, protein expression could be more important than channel function to trigger cell proliferation suggesting that additional channel functions may be responsible to reconcile calcium channel expression and cell proliferation. When needed, external calcium concentration is obviously important for calcium channel function but it also regulates calcium sensing receptor (CaSR) activity. CaSR can up- or down-regulate cell proliferation depending on physiological conditions. CaSR sensitivity to external calcium is within the 0.5 to 5 mM range and therefore, the role of these receptors in cell proliferation must be taken into account. We therefore suggest here that cell proliferation rates could depend on the relative balance between calcium influx and CaSR activation.

ALG-2 attenuates COPII budding in vitro and stabilizes the Sec23/Sec31A complex

Coated vesicles mediate the traffic of secretory and membrane cargo proteins from the endoplasmic reticulum (ER) to the Golgi apparatus. The coat protein complex (COPII) involved in vesicle budding is constituted by a GTPase, Sar1, the inner coat components of Sec23/Sec24 and the components of the outer coat Sec13/Sec31A. The Ca(2+)-binding protein ALG-2 was recently identified as a Sec31A binding partner and a possible link to Ca(2+) regulation of COPII vesicle budding. Here we show that ALG-2/Ca(2+) is capable of attenuating vesicle budding in vitro through interaction with an ALG-2 binding domain in the proline rich region of Sec31A. Binding of ALG-2 to Sec31A and inhibition of COPII vesicle budding is furthermore dependent on an intact Ca(2+)-binding site at EF-hand 1 of ALG-2. ALG-2 increased recruitment of COPII proteins Sec23/24 and Sec13/31A to artificial liposomes and was capable of mediating binding of Sec13/31A to Sec23. These results introduce a regulatory role for ALG-2/Ca(2+) in COPII tethering and vesicle budding.

Regulation of store-operated calcium entry during cell division

Store-operate Ca2+ channels gate Ca2+ entry into the cytoplasm in response to the depletion of Ca2+ from endoplasmic reticulum Ca2+ stores. The major molecular components of store-operated Ca2+ entry are STIM (stromal-interacting molecule) 1 (and in some instances STIM2) that serves as the endoplasmic reticulum Ca2+ sensor, and Orai (Orai1, Orai2 and Orai3) which function as pore-forming subunits of the store-operated channel. It has been known for some time that store-operated Ca2+ entry is shut down during cell division. Recent work has revealed complex mechanisms regulating the functions and locations of both STIM1 and Orai1 in dividing cells.

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.

Role of the STIM1 C-terminal domain in STIM1 clustering

Store-operated Ca(2+) entry (SOCE) represents a ubiquitous Ca(2+) influx pathway activated by the filling state of intracellular Ca(2+) stores. SOCE is mediated by coupling of STIM1, the endoplasmic reticulum Ca(2+) sensor, to the Orai1 channel. SOCE inactivates during meiosis, partly because of the inability of STIM1 to cluster in response to store depletion. STIM1 has several functional domains, including the Orai1 interaction domain (STIM1 Orai Activating Region (SOAR) or CRAC Activation Domain (CAD)) and STIM1 homomerization domain. When Ca(2+) stores are full, these domains are inactive to prevent constitutive Ca(2+) entry. Here we show, using the Xenopus oocyte as an expression system, that the C-terminal 200 residues of STIM1 are important to maintain STIM1 in an inactive state when Ca(2+) stores are full, through predicted intramolecular shielding of the active STIM1 domains (SOAR/CAD and STIM1 homomerization domain). Interestingly, our data argue that the C-terminal 200 residues accomplish this through a steric hindrance mechanism because they can be substituted by GFP or mCherry while maintaining all aspects of STIM1 function. We further show that STIM1 clustering inhibition during meiosis is independent of the C-terminal 200 residues.

Regulation of store-operated Ca2+ entry during the cell cycle

Cytoplasmic Ca(2+) signals are central to numerous cell physiological processes, including cellular proliferation. Historically, much of the research effort in this area has focused on the role of Ca(2+) signals in cell-cycle progression. It is becoming clear, however, that the relationship between Ca(2+) signaling and the cell cycle is a 'two-way street'. Specifically, Ca(2+)-signaling pathways are remodeled during M phase, leading to altered Ca(2+) dynamics. Such remodeling probably better serves the large variety of functions that cells must perform during cell division compared with during interphase. This is clearly the case during oocyte meiosis, because remodeling of Ca(2+) signals partially defines the competence of the egg to activate at fertilization. Store-operated Ca(2+) entry (SOCE) is a ubiquitous Ca(2+)-signaling pathway that is regulated during M phase. In this Commentary, we discuss the latest advances in our understanding of how SOCE is regulated during cell division.

Phosphorylation of STIM1 underlies suppression of store-operated calcium entry during mitosis

Store-operated Ca(2+) entry (SOCE) and Ca(2+) release-activated Ca(2+) currents (I(crac)) are strongly suppressed during cell division, the only known physiological situation in which Ca(2+) store depletion is uncoupled from the activation of Ca(2+) influx [corrected]. We found that the endoplasmic reticulum (ER) Ca(2+) sensor STIM1 failed to rearrange into near-plasma membrane puncta in mitotic cells, a critical step in the SOCE-activation pathway. We also found that STIM1 from mitotic cells is recognized by the phospho-specific MPM-2 antibody, suggesting that STIM1 is phosphorylated during mitosis. Removal of ten MPM-2 recognition sites by truncation at amino acid 482 abolished MPM-2 recognition of mitotic STIM1, and significantly rescued STIM1 rearrangement and SOCE response in mitosis. We identified Ser 486 and Ser 668 as mitosis-specific phosphorylation sites, and STIM1 containing mutations of these sites to alanine also significantly rescued mitotic SOCE. Therefore, phosphorylation of STIM1 at Ser 486 and Ser 668, and possibly other sites, underlies suppression of SOCE during mitosis.

PKD2 Interacts and Co-localizes with mDia1 to Mitotic Spindles of Dividing Cells

Mutations in pkd2 result in the type 2 form of autosomal dominant polycystic kidney disease, which accounts for approximately 15% of all cases of the disease. PKD2, the protein product of pkd2, belongs to the transient receptor potential superfamily of cation channels, and it can function as a mechanosensitive channel in the primary cilium of kidney cells, an intracellular Ca(2+) release channel in the endoplasmic reticulum, and/or a nonselective cation channel in the plasma membrane. We have identified mDia1/Drf1 (mammalian Diaphanous or Diaphanous-related formin 1 protein) as a PKD2-interacting protein by yeast two-hybrid screen. mDia1 is a member of the RhoA GTPase-binding formin homology protein family that participates in cytoskeletal organization, cytokinesis, and signal transduction. We show that mDia1 and PKD2 interact in native and in transfected cells, and binding is mediated by the cytoplasmic C terminus of PKD2 binding to the mDia1 N terminus. The interaction is more prevalent in dividing cells in which endogenous PKD2 and mDia1 co-localize to the mitotic spindles. RNA interference experiments reveal that endogenous mDia1 knockdown in HeLa cells results in the loss of PKD2 from mitotic spindles and alters intracellular Ca(2+) release. Our results suggest that mDia1 facilitates the movement of PKD2 to a centralized position during cell division and has a positive effect on intracellular Ca(2+) release during mitosis. This may be important to ensure equal segregation of PKD2 to the daughter cell to maintain a necessary level of channel activity. Alternatively, PKD2 channel activity may be important in the cell division process or in cell fate decisions after division.

Vascular endothelial cell adherens junction assembly and morphogenesis induced by sphingosine-1-phosphate

Vascular endothelial cells undergo morphogenesis into capillary networks in response to angiogenic factors. We show here that sphingosine-1-phosphate (SPP), a platelet-derived bioactive lipid, activates the EDG-1 and -3 subtypes of G protein-coupled receptors on endothelial cells to regulate angiogenesis. SPP induces the Gi/mitogen-activated protein kinase/cell survival pathway and the small GTPase Rho- and Raccoupled adherens junction assembly. Both EDG-1-and EDG-3-regulated signaling pathways are required for endothelial cell morphogenesis into capillary-like networks. Indeed, SPP synergized with polypeptide angiogenic growth factors in the formation of mature neovessels in vivo. These data define SPP as a novel regulator of angiogenesis.

Bradykinin regulates the histamine-induced Ca2+ mobilization via protein kinase C activation in human gingival fibroblasts

We previously demonstrated that histamine and bradykinin evoke an increase in intracellular Ca2+ ([Ca2+]i) in human gingival fibroblasts by using a fluorescent Ca2+ indicator Fura-2. In this paper, we further demonstrate the regulation of the histamine-induced Ca2+ mobilization by bradykinin. In fibroblasts stimulated with bradykinin (1 microM), subsequent stimulation with histamine (100 microM) failed to mobilize Ca2+, whereas bradykinin induced an increase in [Ca2+]i in the cells pre-stimulated with histamine. The attenuation of the histamine response was dependent on the concentration of bradykinin for the first stimulation. Histamine also failed to induce the formation of inositol 1,4,5-trisphosphate in fibroblasts pretreated with bradykinin. In fibroblasts pretreated with bradykinin (1 microM) for 3 min and then washed with fresh medium, the effect of histamine on [Ca2+]i quickly returned to the control level. The activation of protein kinase C by phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (PMA) elicited a marked decrease in histamine-induced Ca2+ mobilization. When the protein kinase C activity was inhibited with H7, a protein kinase C inhibitor, or was down-regulated by pretreatment with PMA for 20 h, the inhibitory effect of PMA on the histamine response was relieved. In the fibroblasts pretreated with H7 or PMA for 20 h, histamine evoked Ca2+ mobilization even after bradykinin stimulation. These results suggest that the histamine response is regulated by bradykinin receptor activation via the activation of protein kinase C in human gingival fibroblasts.

Role of cdc25-C phosphatase in the immediate G2 delay induced by the exogenous factors epidermal growth factor and phorbolester

Studies on the link between cellular signalling and cell cycle control at the G2 checkpoint have shown that, in HeLa cells, epidermal growth factor (EGF) and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) rapidly inhibit the G2-M transition by preventing the key component of mitosis-promoting factor (MPF), p34cdc2, from expressing protein kinase activity. The kinase activity of active MPF is not inhibited; rather, the conversion of pre-MPF to MPF, i.e., the activating dephosphorylation of p34cdc2, at tyrosine is rapidly blocked (Barth and Kinzel, 1994, Exp. Cell Res. 212:383-388; 1995, J. Cell. Physiol., 162:44-51). The phosphatase responsible, cdc25-C, is activated by phosphorylation in mitotic cells starting at the G2-M transition in an autocatalytic loop with MPF (Hoffmann et al., 1993, EMBO J. 12:53-63). We now show that, concomitant with the prevention of MPF activation, EGF and TPA induced a reduction of the activity of cdc25-C in synchronized cultures. Furthermore, treatment of mitotic HeLa cells with TPA did not influence the kinase activity of MPF but caused a rapid decrease of the specific enzyme activity of cdc25-C, probably due to dephosphorylation of the enzyme, as indicated by reduced binding of monoclonal MPM-2 antibody specific for phosphoepitopes in M phase. Because of its inability to induce signalling during division, EGF failed to influence the activity of cdc25-C in mitotic cells. The scenario in cells late in G2 that are committed to enter mitosis may be as follows: In those cells where the signalling pathways responding to EGF as well as those responding to TPA are still open, cdc25-C is prevented by dephosphorylation from exceeding the threshold level of activity required to initiate the activation of and the autocatalytic feedback loop with p34cdc2 and to enter mitosis. Therefore, cdc25-C appears to represent part of an interface between cellular signalling and cell cycle control in G2 phase.

The histamine H1 receptor in GT1-7 neuronal cells is regulated by calcium influx and KN-62, a putative inhibitor of calcium/calmodulin protein kinase II

1. In GT1-7 cells, histamine stimulated the initial [Ca2+]i transient in a dose-dependent manner with a best-fit EC50 value of 4.2 +/- 4.2 microM (mean +/- s.e.mean, n = 4) and a best-fit maximal effect of 138 +/- 56 nM (n = 4) increase above basal calcium levels. 2. Pretreatment of cells with 30 microM histamine for 30 min desensitized the population mean peak calcium signal by 53% to 75 +/- 9 nM, (n = 3, P < 0.04). Analysis of the individual cells revealed that 39 +/- 7% (n = 94 cells from 8 experiments) of pretreated cells exhibited desensitized histamine-stimulated [Ca2+]i transients of < or = 1 standard deviation below the control cells mean calcium transient level. 3. The desensitization induced by histamine was prevented (P < 0.01) by KN-62 (10 microM), a putative inhibitor of the calcium/calmodulin-dependent protein kinase II (CaMKII). KN-62 (10 microM) alone did not induce [Ca2+]i mobilization, nor did it antagonize the histamine-stimulated [Ca2+]i signal. In addition, KN-62 did not appear to have its effect by hastening the rate of recovery from desensitization. 4. Histamine pretreatment in nominal (zero calcium + 0.2 mM EGTA) or in low (0.3 mM) extracellular calcium did not induce histamine receptor desensitization, supporting a role for extracellular calcium in the homologous H1 receptor desensitization process. 5. Histamine (30 microM) stimulated at least four different types of [Ca2+]i signals in GT1-7 cells. The majority (61%) were of single spikes with the remaining cells showing some form of calcium oscillatory behaviour. The proportion of GT1-7 cells showing histamine-induced calcium oscillations was histamine concentration-dependent and significantly reduced after acute desensitization. KN-62, when present during histamine pretreatment, prevented this fall in calcium oscillation. Under the conditions of nominal or 0.3 mM extracellular calcium the proportion of cells exhibiting histamine-stimulated calcium oscillations was not significantly different from the controls. 6. Bradykinin stimulated a [Ca2+]i transient in GT1-7 cells with a population mean peak response of 147 +/- 8 nM (n = 5) over basal levels. The bradykinin-induced [Ca2+]i signal was without any calcium oscillatory activity. Histamine pretreatment caused the heterologous desensitization of the bradykinin [Ca2+]i signal (44% reduction, P < 0.007), which was unaffected by KN-62. 7. The results presented here suggest that the histamine-mediated homologous H1 receptor desensitization process involves extracellular calcium and can be blocked by KN-62, a putative inhibitor of CaMKII. In contrast, KN-62 does not appear to prevent the histamine-mediated heterologous desensitization cascade. These findings suggest fundamental differences in the mechanisms underlying homologous and heterologous H1 receptor desensitization pathways in GT1-7 neuronal cells.

Capacitative calcium entry

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Arachidonic acid mobilization is suppressed during mitosis: role of cytosolic phospholipase A2 activation

In interphase HeLa cells, incubation with histamine or thapsigargin led to the rapid release of arachidonic acid. The release was absolutely dependent on Ca2+, consistent with the activation of an 85 kDa cytosolic phospholipase A2 (cPLA2). In metaphase-arrested HeLa cells, by contrast, the stimulation of arachidonate release by these agents was inhibited by more than 90%. The lack of arachidonic acid release by mitotic cells was at least partly expected, since histamine- or thapsigargin-induced Ca2+ influx and elevations of cytosolic free Ca2+ are known to be strongly inhibited during mitosis [Preston, Sha'afi and Berlin (1991) Cell Regul. 2, 915-925]. Indeed, incubation of interphase cells with the Ca2+ ionophore A23187 alone induced a high level of arachidonate release. However, even A23187 failed to elicit release from mitotic cells. Since the Ca(2+)-dependent release of arachidonate by many cell types is promoted by preincubation with ligands that activate receptors of the tyrosine kinase class, and tumour promoters that lead to the phosphorylation of cPLA2, we determined if the responses of mitotic HeLa cells could be modified by this 'priming' process. We first established that epidermal growth factor and phorbol 12-myristate 13-acetate were effective priming agents in interphase cells: cells preincubated with the hormone or tumour promoter showed a 2-fold stimulation of thapsigargin- or A23187-induced arachidonic acid release. However, none of the priming agents reversed the lack of mitotic cell response. This refractoriness was not caused by destruction of cPLA2 during mitosis: by Western blotting, cPLA2 of interphase and mitotic cells was shown to be present in comparable amounts. Moreover, cPLA2 activities measured in extracts of interphase and mitotic cells were also comparable. Surprisingly, mitotic cPLA2 appeared to be constitutively phosphorylated in non-hormone-treated (control) cells. The results indicate a novel mechanism of regulation by cPLA2 activity in mitotic cells.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Desensitization of histamine H1 receptor-mediated inositol phosphate production in HeLa cells

1. Histamine stimulated the accumulation of total [3H]-inositol phosphates (IPn) in control HeLa cells with an EC50 of 3.7 +/- 0.7 microM in the presence of 10 mM LiCl. The maximum response to histamine after 15 min incubation was 43 +/- 5% over basal accumulation and occurred at a concentration of 1 mM histamine. 2. The histamine-induced IPn production in HeLa cells was confirmed as H1 receptor-mediated, since the H1 antagonist mepyramine (10(-6) M) inhibited the histamine response (10(-4) M) by 83 +/- 7%, whereas the H2 antagonist, ranitidine (10(-4) M), and H3 antagonist, thioperamide (10(-6) M), were ineffective. 3. Histamine (10(-4) M) pretreatment of HeLa cells for 30 min desensitized the subsequent histamine-induced IPn accumulation. The desensitized cells accumulated IPn in response to histamine with an EC50 of 1.7 +/- 0.7 microM after 15 min incubation. The maximum histamine-induced IPn accumulation at 10(-4) M was 19 +/- 5% over basal and was significantly lower (P < 0.03) than the maximum response in control cells. 4. The desensitization of histamine-induced IPn accumulation was time-dependent and, at a desensitizing histamine concentration of 10(-4) M, the half-maximal attenuation occurred after approximately 9 min and maximum desensitization was achieved by 15-20 min. The desensitization of the IPn accumulation was a reversible phenomenon and full recovery of the response occurred 150 min after the removal of the desensitizing histamine-containing medium. The half-time for the recovery of the histamine-induced response was estimated at 120 min. 5. Bradykinin stimulated IPn, accumulation in HeLa cells, and the ECm in control cells of 1.9 +/- 0.2 nM was not significantly different from the EC50 value from histamine-pretreated cells of 1.6 +/- 0.9 nM. The bradykinin response at 1 microM was 194 +/- 48% over basal IPn accumulation in control cells and this value was significantly different (P <0.04) from the 1 microM bradykinin-induced IPn accumulation in histamine pretreated HeLa cells of 143 +/- 38% over basal.6. NaF stimulated IP,, accumulation in control HeLa cells in a dose-related manner, with the maximum effect occurring at 15-20 mM. The EC50 value for NaF-stimulated IPn accumulation in control cells was 10.5 +/- 1.1 mm and the maximum response was 136 +/- 41% over basal after 20 min incubation. In histamine desensitized HeLa cells the EC50 value for NaF was 12.3 +/- 0.4 mM after 20 min stimulation,which was not significantly different from the value obtained in control cells. The maximum NaF stimulated IPn formation in desensitized cells of 68 +/- 23% over basal occurred at 15 -20 mM and was significantly lower (P<0.01) than that obtained in control cells.7. We show here that the acute histamine pretreatment of HeLa cells results in the desensitization of histamine H1 receptor-mediated IPn production. The desensitization was not restricted to the H1 receptor-mediated signal transduction pathway, but also includes both the bradykinin- and NaF mediated responses, supporting a heterologous desensitization mechanism. Our results are consistent with the site of attenuation being at or distal to the G-protein and the underlying mechanism being a slowed time-course for the production of inositol phosphates.

Okadaic acid uncouples calcium entry from depletion of intracellular stores

The mechanism by which the depletion of intracellular Ca2+ stores stimulates Ca2+ influx is poorly understood. However, the coupling of depletion to influx is broken during mitosis [Preston, S.F. et. al., (1991) Cell Regul., 2, 915-925]. Thus, in interphase HeLa cells, activation of the histamine H1 receptor, or incubation with thapsigargin, which inhibits the Ca(2+)-ATPase of storage vesicles and depletes Ca2+ stores, strongly stimulate Ca2+ influx. In mitotic cells, however, neither histamine nor thapsigargin stimulate Ca2+ influx. Since it has been found that okadaic acid treatment of interphase cells induces a mitotic-like state with respect to a number of other membrane processes, we have asked if okadaic acid might also uncouple Ca2+ depletion from stimulated influx. We show that okadaic acid specifically does suppress this coupling: thapsigargin and histamine deplete stores in control and okadaic-acid-treated HeLa cells, but after treatment with okadaic acid, stimulation of Ca2+ influx is barely detectable. This suggests that a protein phosphorylation/dephosphorylation event controls the coupling of Ca2+ stores to influx, and that there may be a physiological mechanism for control of the Ca2+ response to hormonal signals at the level of coupling.

Intracellular free Ca2+ in the cell cycle in human fibroblasts: transitions between G1 and G0 and progression into S phase

Intracellular free calcium ([Ca2+]i) has been proposed to play an important part in the regulation of the cell cycle. Although a number of studies have shown that stimulation of quiescent cells with growth factors causes an immediate rise in [Ca2+]i (Rabinovitch et al., 1986; Vincentini and Villereal, 1986; Hesketh et al., 1988; Tucker et al., 1989, Wahl et al., 1990), a causal relationship between the [Ca2+]i transient and the ability of the cells to reenter the cell cycle has not been firmly established. We have found that blocking the mitogen-induced elevation of [Ca2+]i with the cytoplasmic [Ca2+]i buffer dimethyl BAPTA (dmBAPTA) also blocks subsequent entry of cells into S phase. The dose response curves for inhibition of serum stimulation of [Ca2+]i and DNA synthesis by dmBAPTA are virtually identical including an anomalous stimulation observed at low levels of dmBAPTA. Reversal of the [Ca2+]i buffering effect of dmBAPTA by transient exposure of the cells to the Ca2+ ionophore ionomycin also reverses the inhibition of DNA synthesis 20-24 h later. Ionomycin by itself does not stimulate DNA synthesis. These data are consistent with the conclusion that a transient increase in [Ca2+]i occurring shortly after serum stimulation of quiescent fibroblasts is necessary but not sufficient for subsequent entry of the cells into S phase. This study is the first to show a direct relationship between early serum stimulated Cai2+ increase and subsequent DNA synthesis in human cells. It also goes beyond recent studies on BALB/3T3 cells by providing dose response data and demonstrating reversibility, which are strong indications of a cause and effect relationship.

Short-term desensitization of the histamine H1 receptor in human HeLa cells: involvement of protein kinase C dependent and independent pathways

1. In this study we have investigated the effects of short-term exposure of cells to histamine on the subsequent H1 receptor responsiveness in HeLa cells, using Ca2+ fluorescence microscopy and video digital imaging. 2. In HeLa cells, histamine (100 microM) induces an immediate H1 receptor-mediated biphasic elevation of the intracellular Ca2+ concentration ([Ca2+]i) (basal [Ca2+]i: 81 +/- 30 nM, histamine-induced Ca2+ response: first phase: 1135 +/- 79 nM; second phase: 601 +/- 52 nM, n = 11). 3. The histamine H1 receptors on HeLa cells are readily susceptible to desensitization since repetitive exposure of the same group of cells to histamine (100 microM) markedly affected the release and influx component of the induced Ca2+ response (second application of histamine: first phase: 590 +/- 92 nM, second phase: 279 +/- 47 nM; third application of histamine: first phase: 454 +/- 127 nM, second phase: 240 +/- 45 nM, n = 6). Video digital imaging revealed an increase in the lag time between stimulation and monitoring of the Ca2+ response and a reduced increase in [Ca2+]i after desensitization with histamine. 4. Neither the release component of the ATP response (50 microM) nor the caffeine (3 mM)-induced Ca2+ release were found to be affected by desensitization with 100 microM histamine. However, the second phase of the ATP response was significantly reduced after desensitization with histamine (control cells: 516 +/- 33 nM; desensitized cells: 331 +/- 96 nM, n = 4, P < 0.05).5. Activation of protein kinase C (PKC) by phorbol-12-myristate-1 3-acetate was found to inhibit the histamine as well as ATP-induced Ca2" response in a dose-dependent manner.6. In PKC downregulated cells the second phase of the histamine-induced Ca2+ response was significantly elevated, indicating the involvement of PKC in the negative feedback on the Ca2+ influx(control cells: second phase: 601 +/- 52 nM (n = 11); PKC downregulated cells: second phase:890 +/- 90nM, n = I0, P<0.05).7. Homologous desensitization of H, receptor responsiveness was still observed in PKC downregulated cells, implying the rapid activation of a regulatory mechanism other than PKC.8. Based on our experimental data we suggest that short-term desensitization of the histamine H,receptor evolves from two different processes: a selective reduction of the histamine-induced Ca2+ release, mediated by a PKC-independent pathway, and a non-selective inhibition of the receptormediated Ca2+ influx activated by a PKC-dependent pathway.

Effect of thapsigargin and caffeine on Ca2+ homeostasis in HeLa cells: implications for histamine-induced Ca2+ oscillations

Several studies have already established that the stimulation of H1 receptors by exogenous histamine induces intracellular Ca2+ oscillations in HeLa cells. The molecular mechanism underlying this oscillatory process remains, however, unclear. A series of fura-2 experiments was undertaken in which the nature of the Ca2+ pools involved in the histamine-induced Ca2+ oscillations was investigated using the tumour promoter agent thapsigargin (TG) and the Ca(2+)-induced Ca(2+)-release promoter, caffeine. The results obtained indicate first that TG causes a gradual increase in cytosolic Ca2+ without inducing internal Ca2+ oscillations, and second that TG and histamine share common internal Ca2+ storage sites. The latter conclusion was derived from experiments performed in the absence of external Ca2+, where the addition of TG before histamine resulted in a total inhibition of the Ca2+ response linked to H1 receptor stimulation, whereas the addition of histamine before TG decreased by more than 90% the TG-induced Ca2+ release. Finally; TG was found to inhibit irreversibly histamine-induced Ca2+ oscillations when added to the bathing medium during the oscillatory process. The effect of caffeine at concentrations ranging from 1 mM to 10 mM on intracellular Ca2+ homeostasis was also investigated. The results obtained show that caffeine does not affect systematically the internal Ca2+ concentration in resting and TG-stimulated HeLa cells, but increases the Ca2+ sequestration ability of inositol-trisphosphate (InsP3)-related Ca2+ stores.(ABSTRACT TRUNCATED AT 250 WORDS)

An intracellular calcium store regulates protein synthesis in HeLa cells, but it is not the hormone-sensitive store

There is considerable evidence, reviewed by Brostrom and Brostrom [1], that Ca2+ stores are involved in the regulation of protein synthesis. We provide evidence in HeLa cells that is consistent with their findings that depletion of Ca2+ stores and not changes in cytosolic free Ca2+ ([Ca2+]i) inhibit protein synthesis, but we also show that the mechanism leading to depletion is critical. Specifically, depletion of stores by the Ca(2+)-mobilizing hormone histamine does not inhibit protein synthesis. In assessing the role of Ca2+ stores in protein synthesis, experiments in certain cell types have been complicated by the use of Ca2+ ionophores, which simultaneously elevate [Ca2+]i and deplete Ca2+ stores. We have measured total cell Ca2+, [Ca2+]i and protein synthesis in HeLa cells under conditions that allowed evaluation of the separate contributions of stores and [Ca2+]i. Using 1,2-bis(2-aminophenoxyethane)-N,N,N'N'-tetraacetic acid (BAPTA) as an intracellular Ca2+, chelator and thapsigargin, which inhibits the membrane Ca(2+)-ATPase of storage vesicles, total cell Ca2+ can be depleted and this depletion is enhanced by extracellular EGTA which blocks Ca2+ influx; [Ca2+]i is actually lowered by BAPTA under these conditions. Protein synthesis is inhibited by BAPTA in the presence of EGTA and by thapsigargin with or without EGTA. However, histamine which with EGTA, affects an equal degree of Ca2+ depletion does not inhibit protein synthesis. Thus, it is suggested that Ca2+ stores are not homogeneous, and that the hormone-sensitive store specifically does not play a role in the regulation of protein synthesis. In this respect, the hormone-sensitive and insensitive stores do not functionally communicate and may be separately regulated.

Increase in intracellular calcium induced by stimulating histamine H1 receptors in macrophage-like P388D1 cells

The addition of histamine to macrophage-like P388D1 cells resulted in a dose-dependent increase in intracellular calcium [Ca2+]i measured by fura-2 in single cells. The maximum level of [Ca2+]i was obtained by addition of 1 x 10(-4) M histamine. The increase was primarily due to release from the intracellular store. The addition of an H1 specific antagonist pyrilamine before histamine treatment inhibited the increase reversibly, while an H2 specific antagonist cimetidine had no inhibitory effect. Histamine also resulted in a dose-dependent increase in cGMP but not in cAMP. These data suggest the existence of histamine H1 receptors in these cells and histamine may have some biological effect on the function of macrophages via [Ca2+]i and cGMP as the second messengers.

Regulation of Ca2+ influx during mitosis: Ca2+ influx and depletion of intracellular Ca2+ stores are coupled in interphase but not mitosis

Activation of a wide variety of membrane receptors leads to a sustained elevation of intracellular Ca2+ ([Ca2+]i) that is pivotal to subsequent cell responses. In general, in nonexcitable cells this elevation of [Ca2+]i results from two sources: an initial release of Ca2+ from intracellular stores followed by an influx of extracellular Ca2+. These two phases, release from intracellular stores and Ca2+ influx, are generally coupled: stimulation of influx is coordinated with depletion of Ca2+ from stores, although the mechanism of coupling is unclear. We have previously shown that histamine effects a typical [Ca2+]i response in interphase HeLa cells: a rapid rise in [Ca2+]i followed by a sustained elevation, the latter dependent entirely on extracellular Ca2+. In mitotic cells only the initial elevation, derived by Ca2+ release from intracellular stores, occurs. Thus, in mitotic cells the coupling of stores to influx may be specifically broken. In this report we first provide additional evidence that histamine-stimulated Ca2+ influx is strongly inhibited in mitotic cells. We show that efflux is also strongly stimulated by histamine in interphase cells but not in mitotics. It is possible, thus, that in mitotics intracellular stores are only very briefly depleted of Ca2+, being replenished by reuptake of Ca2+ that is retained within the cell. To ensure the depletion of Ca2+ stores in mitotic cells, we employed the sesquiterpenelactone, thapsigargin, that is known to affect the selective release of Ca2+ from intracellular stores by inhibition of a specific Ca(2+)-ATPase; reuptake is inhibited. In most cells, and in accord with Putney's capacitative model (1990), thapsigargin, presumably by depleting intracellular Ca2+ stores, stimulates Ca2+ influx. This is the case for interphase HeLa cells. Thapsigargin induces an increase in [Ca2+]i that is dependent on extracellular Ca2+ and is associated with a strong stimulation of 45Ca2+ influx. In mitotic cells thapsigargin also induces a [Ca2+]i elevation that is initially comparable in magnitude and largely independent of extracellular Ca2+. However, unlike interphase cells, in mitotic cells the elevation of [Ca2+]i is not sustained and 45Ca2+ influx is not stimulated by thapsigargin. Thus, the coupling between depletion of intracellular stores and Ca2+ influx is specifically broken in mitotic cells. Uncoupling could account for the failure of histamine to stimulate Ca2+ influx during mitosis and would effectively block all stimuli whose effects are mediated by Ca2+ influx and sustained elevations of [Ca2+]i.

Variety of Ca(2+)-permeable channels in human carcinoma A431 cells

Patch-clamp methods were used to search for and characterize channels that mediate calcium influx through the plasma membrane of human carcinoma A431 cells. Here we present four Ca(2+)-permeable channel types referred to as SG, G, 1 and BI. With 105 mM Ca2+ as the charge carrier, at 30-33 degrees C their mean unitary conductances (in pS) are: 1.3 (SG), 2.4 (G), 3.7 (I) and 12.8 (BI). SG and G channels are activated by nonhydrolyzable analogues of guanosine 5-triphosphate (GTP) applied to the inside of the membrane, suggesting an involvement of G-proteins in the control of their activity. I and BI channels are activated by inositol 1,4,5-trisphosphate (InsP3). G, I, BI and possibly SG channels are activated from the extracellular side of the membrane by epidermal growth factor (EGF) and histamine. It is assumed that all identified Ca2+ channels take part in the generation of the agonist-induced intracellular Ca2+ signal. The variety of Ca-channel types seems to be necessary to tune cell responses according to the respective type and level of an external signal, on the one hand, and to the functional state of the cell, on the other.

The secretory pathway is blocked between the trans-Golgi and the plasma membrane during meiotic maturation in Xenopus oocytes

Protein secretion is blocked in Xenopus oocytes arrested at second meiotic metaphase. In this report, we show that secretion becomes blocked coincident with germinal vesicle breakdown (GVBD). Transport through the metaphase-arrested oocyte's secretory pathway continues unimpeded until proteins reach the trans-Golgi. These conclusions are drawn from experiments using exogenous prolactin and vesicular stomatitis virus G protein (VSV G) encoded by SP6 transcripts and endogenous glycosaminoglycan (GAG) chains initiated on beta-D-4-methylumbelliferyl-xyloside. From the initiation of maturation with progesterone until GVBD, secretion of prolactin synthesized before the start of maturation is comparable to secretion in immature oocytes, but after GVBD secretion of prolactin declines approximately 63% in the first hour. Not all steps in the secretory pathway are blocked when oocytes mature. Since VSV G protein acquires resistance to endo H digestion with equal efficiency in immature oocytes (arrested in first meiotic prophase) and matured oocytes (arrested in second meiotic metaphase), we conclude that transport of this protein from the ER to the Golgi is not inhibited at meiotic metaphase. Using [35S]sulfate to label xyloside-initiated GAG chains we find that transport of GAG chains from the trans-Golgi to the cell surface is 15-fold lower in matured oocytes than in immature oocytes. Examination of the size of GAG chains by SDS-PAGE and HPLC indicates that matured oocytes produce GAG chains significantly larger than GAG chains from immature oocytes. This increase in size suggests that GAG chains from matured oocytes have a longer residence time in the trans-Golgi than GAG chains from immature oocytes. Hence, part of the block to secretion in metaphase-arrested oocytes could be an inhibition of vesicle budding from the trans-Golgi.

Membrane traffic between secretory compartments is differentially affected during mitosis

Membrane traffic has been shown to be regulated during cell division. In particular, with the use of viral membrane proteins as markers, endoplasmic reticulum (ER)-to-Golgi transport in mitotic cells has been shown to be essentially blocked. However, the effect of mitosis on other steps in the secretory pathway is less clear, because an early block makes examination of following steps difficult. Here, we report studies on the functional characteristics of secretory pathways in mitotic mammalian tissue culture cells by the use of a variety of markers. Chinese hamster ovary cells were transfected with cDNAs encoding secretory proteins. Consistent with earlier results following viral membrane proteins, we found that the overall secretory pathway is nonfunctional in mitotic cells, and a major block to secretion is at the step between ER and Golgi: the overall rate of secretion of human growth hormone is reduced at least 10-fold in mitotic cells, and export of truncated vesicular stomatitis virus G protein from the ER is inhibited to about the same extent, as judged by acquisition of endoglycosidase H resistance. To ascertain the integrity of transport from the trans-Golgi to plasma membrane, we followed the secretion of sulfated glycosaminoglycan (GAG) chains, which are synthesized in the Golgi and thus are not subject to the earlier ER-to-Golgi block. GAG chains are valid markers for the pathway taken by constitutive secretory proteins; both protein secretion and GAG chain secretion are sensitive to treatment with n-ethyl-maleimide and monensin and are blocked at 19 degrees C. We found that the extent of GAG-chain secretion is not altered during mitosis, although the initial rate of secretion is reduced about twofold in mitotic compared with interphase cells. Thus, during mitosis, transport from the trans-Golgi to plasma membrane is much less hindered than ER-to-Golgi traffic. We conclude that transport steps are not affected to the same extent during mitosis.