Spontaneous calcium oscillations and mechanically and chemically induced calcium responses in mammary epithelial cells

Diversity of Intercellular Communication Modes: A Cancer Biology Perspective

From the moment a cell is on the path to malignant transformation, its interaction with other cells from the microenvironment becomes altered. The flow of molecular information is at the heart of the cellular and systemic fate in tumors, and various processes participate in conveying key molecular information from or to certain cancer cells. For instance, the loss of tight junction molecules is part of the signal sent to cancer cells so that they are no longer bound to the primary tumors and are thus free to travel and metastasize. Upon the targeting of a single cell by a therapeutic drug, gap junctions are able to communicate death information to by-standing cells. The discovery of the importance of novel modes of cell-cell communication such as different types of extracellular vesicles or tunneling nanotubes is changing the way scientists look at these processes. However, are they all actively involved in different contexts at the same time or are they recruited to fulfill specific tasks? What does the multiplicity of modes mean for the overall progression of the disease? Here, we extend an open invitation to think about the overall significance of these questions, rather than engage in an elusive attempt at a systematic repertory of the mechanisms at play.

The Mechanical Microenvironment in Breast Cancer

Mechanotransduction is the interpretation of physical cues by cells through mechanosensation mechanisms that elegantly translate mechanical stimuli into biochemical signaling pathways. While mechanical stress and their resulting cellular responses occur in normal physiologic contexts, there are a variety of cancer-associated physical cues present in the tumor microenvironment that are pathological in breast cancer. Mechanistic in vitro data and in vivo evidence currently support three mechanical stressors as mechanical modifiers in breast cancer that will be the focus of this review: stiffness, interstitial fluid pressure, and solid stress. Increases in stiffness, interstitial fluid pressure, and solid stress are thought to promote malignant phenotypes in normal breast epithelial cells, as well as exacerbate malignant phenotypes in breast cancer cells.

Intracellular signaling dynamics and their role in coordinating tissue repair

Tissue repair is a complex process that requires effective communication and coordination between cells across multiple tissues and organ systems. Two of the initial intracellular signals that encode injury signals and initiate tissue repair responses are calcium and extracellular signal-regulated kinase (ERK). However, calcium and ERK signaling control a variety of cellular behaviors important for injury repair including cellular motility, contractility, and proliferation, as well as the activity of several different transcription factors, making it challenging to relate specific injury signals to their respective repair programs. This knowledge gap ultimately hinders the development of new wound healing therapies that could take advantage of native cellular signaling programs to more effectively repair tissue damage. The objective of this review is to highlight the roles of calcium and ERK signaling dynamics as mechanisms that link specific injury signals to specific cellular repair programs during epithelial and stromal injury repair. We detail how the signaling networks controlling calcium and ERK can now also be dissected using classical signal processing techniques with the advent of new biosensors and optogenetic signal controllers. Finally, we advocate the importance of recognizing calcium and ERK dynamics as key links between injury detection and injury repair programs that both organize and execute a coordinated tissue repair response between cells across different tissues and organs. This article is categorized under: Models of Systems Properties and Processes > Mechanistic Models Biological Mechanisms > Cell Signaling Laboratory Methods and Technologies > Imaging Models of Systems Properties and Processes > Organ, Tissue, and Physiological Models.

Spontaneous calcium signaling of cartilage cells: from spatiotemporal features to biophysical modeling

Intracellular calcium ([Ca²⁺]_i) oscillation is a fundamental signaling response of cartilage cells under mechanical loading or osmotic stress. Chondrocytes are usually considered as nonexcitable cells with no spontaneous [Ca²⁺]_i signaling. This study proved that chondrocytes can exhibit robust spontaneous [Ca²⁺]_i signaling without explicit external stimuli. The intensity of [Ca²⁺]_i peaks from individual chondrocytes maintain a consistent spatiotemporal pattern, acting as a unique "fingerprint" for each cell. Statistical analysis revealed lognormal distributions of the temporal parameters of [Ca²⁺]_i peaks, as well as strong linear correlations between their means and sds. Based on these statistical findings, we hypothesized that the spontaneous [Ca²⁺]_i peaks may result from an autocatalytic process and that [Ca²⁺]_i oscillation is controlled by a threshold-regulating mechanism. To test these 2 mechanisms, we established a multistage biophysical model by assuming the spontaneous [Ca²⁺]_i signaling of chondrocytes as a combination of deterministic and stochastic processes. The theoretical model successfully explained the lognormal distribution of the temporal parameters and the fingerprint feature of [Ca²⁺]_i peaks. In addition, by using antagonists for 10 pathways, we revealed that the initiation of spontaneous [Ca²⁺]_i peaks in chondrocytes requires the presence of extracellular Ca²⁺, and that the PLC-inositol 1,4,5-trisphosphate pathway, which controls the release of calcium from the endoplasmic reticulum, can affect the initiation of spontaneous [Ca²⁺]_i peaks in chondrocytes. The purinoceptors and transient receptor potential vanilloid 4 channels on the plasma membrane also play key roles in the spontaneous [Ca²⁺]_i signaling of chondrocytes. In contrast, blocking the T-type or L-type voltage-gated calcium channel promoted the spontaneous calcium signaling. This study represents a systematic effort to understand the features and initiation mechanisms of spontaneous [Ca²⁺]_i signaling in chondrocytes, which are critical for chondrocyte mechanobiology.-Zhou, Y., Lv, M., Li, T., Zhang, T., Duncan, R., Wang, L., Lu, X. L. Spontaneous calcium signaling of cartilage cells: from spatiotemporal features to biophysical modeling.

Real-time scratch assay reveals mechanisms of early calcium signaling in breast cancer cells in response to wounding

Aggressive cellular phenotypes such as uncontrolled proliferation and increased migration capacity engender cellular transformation, malignancy and metastasis. While genetic mutations are undisputed drivers of cancer initiation and progression, it is increasingly accepted that external factors are also playing a major role. Two recently studied modulators of breast cancer are changes in the cellular mechanical microenvironment and alterations in calcium homeostasis. While many studies investigate these factors separately in breast cancer cells, very few do so in combination. This current work sets a foundation to explore mechano-calcium relationships driving malignant progression in breast cancer. Utilizing real-time imaging of an in vitro scratch assay, we were able to resolve mechanically-sensitive calcium signaling in human breast cancer cells. We observed rapid initiation of intracellular calcium elevations within seconds in cells at the immediate wound edge, followed by a time-dependent increase in calcium in cells at distances up to 500μm from the scratch wound. Calcium signaling to neighboring cells away from the wound edge returned to baseline within seconds. Calcium elevations at the wound edge however, persisted for up to 50 minutes. Rigorous quantification showed that extracellular calcium was necessary for persistent calcium elevation at the wound edge, but intercellular signal propagation was dependent on internal calcium stores. In addition, intercellular signaling required extracellular ATP and activation of P2Y₂ receptors. Through comparison of scratch-induced signaling from multiple cell lines, we report drastic reductions in response from aggressively tumorigenic and metastatic cells. The real-time scratch assay established here provides quantitative data on the molecular mechanisms that support rapid scratch-induced calcium signaling in breast cancer cells. These mechanisms now provide a clear framework for investigating which short-term calcium signals promote long-term changes in cancer cell biology.

Map and correlate intracellular calcium response and matrix deposition in cartilage under physiological oxygen tensions

Face to the limited repair capability of cartilage, we intended to find out signaling responsible for its matrix synthesis. Since spontaneous calcium response likes a label of cell status, here it was mapped in fresh and 24 hr cultured in situ chondrocytes under oxygen tensions of 20%, 5%, and 1% as well as mimic hypoxia conditions. The calcium source was traced using ethylene glycol-bis (β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) and thapsigargin (TG) to treat cartilage. Their relative matrix of type II collagen (COLL-II) and glycosaminoglycan (GAG) were quantified after cultured for 3 and 7 days. We disclosed the specific fingerprint of calcium response and matrix deposition along the histological zones under various oxygen tensions, from which the effects of hyperoxia, normoxia, and hypoxia conditions on as well as the optimal oxygen tensions for maintenance of various zones of cartilage or chondrocytes were derived and obtained. Our results revealed that cytoplasm calcium was conducive to synthesize COLL-II but detrimental to synthesize GAG. These results provide correlation in addition to details of intracellular calcium response and matrix deposition in in situ cartilage along its histological zones under physiological oxygen tensions.

An endocannabinoid system is present in the mouse olfactory epithelium but does not modulate olfaction

Endocannabinoids modulate a diverse array of functions including progenitor cell proliferation in the central nervous system, and odorant detection and food intake in the mammalian central olfactory system and larval Xenopus laevis peripheral olfactory system. However, the presence and role of endocannabinoids in the peripheral olfactory epithelium have not been examined in mammals. We found the presence of cannabinoid type 1 (CB1) and cannabinoid type 2 (CB2) receptor protein and mRNA in the olfactory epithelium. Using either immunohistochemistry or calcium imaging we localized CB1 receptors on neurons, glia-like sustentacular cells, microvillous cells and progenitor-like basal cells. To examine the role of endocannabinoids, CB1- and CB2- receptor-deficient (CB1(-/-)/CB2(-/-)) mice were used. The endocannabinoid 2-arachidonylglycerol (2-AG) was present at high levels in both C57BL/6 wildtype and CB1(-/-)/CB2(-/-) mice. 2-AG synthetic and degradative enzymes are expressed in wildtype mice. A small but significant decrease in basal cell and olfactory sensory neuron numbers was observed in CB1(-/-)/CB2(-/-) mice compared to wildtype mice. The decrease in olfactory sensory neurons did not translate to impairment in olfactory-mediated behaviors assessed by the buried food test and habituation/dishabituation test. Collectively, these data indicate the presence of an endocannabinoid system in the mouse olfactory epithelium. However, unlike in tadpoles, endocannabinoids do not modulate olfaction. Further investigation on the role of endocannabinoids in progenitor cell function in the olfactory epithelium is warranted.

The effect of chemically defined medium on spontaneous calcium signaling of in situ chondrocytes during long-term culture

Chemically defined serum-free medium has been shown to better maintain the mechanical integrity of articular cartilage explants than serum-supplemented medium during long-term in vitro culture, but little is known about its effect on cellular mechanisms. We hypothesized that the chemically defined culture medium could regulate the spontaneous calcium signaling of in situ chondrocytes, which may modulate the cellular metabolic activities. Bovine cartilage explants were cultured in chemically defined serum-free or serum-supplemented medium for four weeks. The spontaneous intracellular calcium ([Ca(2+)]i) signaling of in situ chondrocytes was longitudinally measured together along with the biomechanical properties of the explants. The spontaneous [Ca(2+)]i oscillations in chondrocytes were enhanced at the initial exposure of serum-supplemented medium, but were significantly dampened afterwards. In contrast, cartilage explants in chemically defined medium preserved the level of calcium signaling, and showed more responsive cells with higher and more frequent [Ca(2+)]i peaks throughout the four week culture in comparison to those in serum medium. Regardless of the culture medium that the explants were exposed, a positive correlation was detected between the [Ca(2+)]i responsive rate and the stiffness of cartilage (Spearman's rank correlation coefficient=0.762). A stable pattern of [Ca(2+)]i peaks was revealed for each chondrocyte, i.e., the spatiotemporal features of [Ca(2+)]i peaks from a cell were highly consistent during the observation period (15 min). This study showed that the beneficial effect of chemically defined culture of cartilage explants is associated with the spontaneous [Ca(2+)]i signaling of chondrocytes in cartilage.

Real-time luminescence imaging of cellular ATP release

Extracellular ATP and other purines are ubiquitous mediators of local intercellular signaling within the body. While the last two decades have witnessed enormous progress in uncovering and characterizing purinergic receptors and extracellular enzymes controlling purinergic signals, our understanding of the initiating step in this cascade, i.e., ATP release, is still obscure. Imaging of extracellular ATP by luciferin-luciferase bioluminescence offers the advantage of studying ATP release and distribution dynamics in real time. However, low-light signal generated by bioluminescence reactions remains the major obstacle to imaging such rapid processes, imposing substantial constraints on its spatial and temporal resolution. We have developed an improved microscopy system for real-time ATP imaging, which detects ATP-dependent luciferin-luciferase luminescence at ∼10 frames/s, sufficient to follow rapid ATP release with sensitivity of ∼10 nM and dynamic range up to 100 μM. In addition, simultaneous differential interference contrast cell images are acquired with infra-red optics. Our imaging method: (1) identifies ATP-releasing cells or sites, (2) determines absolute ATP concentration and its spreading manner at release sites, and (3) permits analysis of ATP release kinetics from single cells. We provide instrumental details of our approach and give several examples of ATP-release imaging at cellular and tissue levels, to illustrate its potential utility.

Shear stress-induced ATP-mediated endothelial constitutive nitric oxide synthase expression in human lymphatic endothelial cells

To clarify the roles of lymphatic endothelial cells (LEC) in the regulation of endothelial constitutive nitric oxide synthase (ecNOS) expression, we examined the effects of shear stress on ecNOS immunohistochemical staining and mRNA and protein expression in human LEC as well as on ATP release from these cells. Shear stress at 0.5 or 1.0 dyn/cm2 increased ecNOS immunohistochemical staining and ecNOS mRNA and protein expression in cultured LEC. The same strength of shear stress produced a significant release of ATP from the LEC. Exogenous ATP ranging in concentration from 10−9 to 10−6 M produced a significant increase in ecNOS immunohistochemical expression in a dose-dependent manner. The increase in ecNOS expression mediated by 10−6M ATP was significantly reduced by 10−5 M suramin. Suramin (10−5 M) caused a significant reduction in the shear stress-mediated increases in ecNOS immunohistochemical staining and mRNA expression. The shear stress-mediated increases in ecNOS expression were significantly reduced by 3 mM tetraethylammonium, 10−4 M apamin, 10−9 M iberiotoxin, 10−5 M 2-aminoethoxydephenyl borate, or 10−5M xestospongin C, but not 10−5 M glybenclamide or 10−5 M nifedipine. The shear stress-mediated increases in ecNOS expression were significantly potentiated by pinacidil or NS1619 in a dose-dependent manner. The immunohistochemical expression of small- (SKCa) and big-conductance (BKCa) Ca2+-activated K+ channels was confirmed on the surfaces of human LEC. These findings suggest that shear stress produces a significant release of ATP from LEC, which activates the purinergic P2X/2Y receptor, thereby facilitating ecNOS mRNA and protein expression through inositol 1,4,5-trisphosphate-mediated release of intracellular Ca2+ ions and the activation of Ca2+-activated K+ channels in LEC.

A dual system of intercellular calcium signaling in glial nets associated with lanceolate sensory endings in rat vibrissae

The lanceolate sensory endings that form palisades around the hair follicle associate with networks of branched Schwann cells. To define the properties of these glial networks as possible conduits of Ca2+ signals, lanceolate endings isolated from rat vibrissae were observed by confocal microscopy while the signaling was locally activated by mechanical stimulation. Intercellular coupling by gap junctions was also assessed by a technique employing fluorescence recovery after photobleaching (FRAP) and by transmission electron microscopy (TEM). Results showed that the glial Ca2+ signals can spread among the arrays of lanceolates in two forms: rapid signals that originate in individual Schwann processes covering the lanceolate axon terminals around the locus of mechanical stimulation, and delayed ones that travel from the stimulation locus through cytoplasmic arborization of the primarily activated cell to the adjacent cell processes. The former signaling was suppressed by the antipurinergic agents suramin and apyrase, whereas the latter was sensitive to the gap junction blocker carbenoxolon. FRAP experiments and TEM observations corroborated the presence of gap junction communications between the Schwann processes of different cell origins. These findings show that, in the Schwann networks, purinergically induced Ca2+ signals and those dependent on gap junctions are propagated in their own spatiotemporal patterns to constitute two distinct forms of communication among the mechanoreceptor palisades.

P2Y receptor regulation of sodium transport in human mammary epithelial cells

Primary human mammary epithelial (HME) cells were immortalized by stable, constitutive expression of the catalytic subunit of human telomerase. Purinergic receptors were identified by RT-PCR and quantitative RT-PCR from mRNA isolated from primary and immortalized cells grown to confluence on membrane filters. Several subtypes of P2Y receptor mRNA were identified including P2Y(1), P2Y(2), P2Y(4), and P2Y(6) receptors. RT-PCR experiments also revealed expression of A(2b) adenosine receptor mRNA in primary and immortalized cells. Confluent monolayers of HME cells exhibited a basal short-circuit current (I(sc)) that was abolished by amiloride and benzamil. When monolayers were cultured in the presence of hydrocortisone, mRNA expression of Na(+) channel (ENaC) alpha-, beta-, and gamma-subunits increased approximately threefold compared with that in cells grown without hydrocortisone. In addition, basal benzamil-sensitive Na(+) transport was nearly twofold greater in hydrocortisone-treated monolayers. Stimulation with UTP, UDP, or adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) produced increases in intracellular calcium concentration that were significantly reduced following pretreatment with the calcium-chelating agent BAPTA-AM. Concentration-response relationships indicated that the rank order of potency for these agonists was UTP > UDP > ATPgammaS. Basolateral stimulation with UTP produced a rapid but transient increase in I(sc) that was significantly reduced if cells were pretreated with BAPTA-AM or benzamil. Moreover, basolateral treatment with either charybdotoxin or clotrimazole significantly inhibited the initial UTP-dependent increase in I(sc) and eliminated the sustained current response. These results indicate that human mammary epithelial cells express multiple P2 receptor subtypes and that Ca(2+) mobilization evoked by P2Y receptor agonists stimulates Na(+) absorption by increasing the activity of Ca(2+)-activated K(+) channels located in the basolateral membrane.

Ultrasound-induced calcium oscillations and waves in Chinese hamster ovary cells in the presence of microbubbles

This study investigated the effects of ultrasound on the intracellular [Ca(2+)] of Chinese hamster ovary cells in the presence of albumin-encapsulated Optison microbubbles. Cells were exposed to 1 MHz ultrasound (tone burst of 0.2 s duration, 0.45 MPa peak pressure) while immersed in solution of 0.9 mM Ca(2+). Calcium imaging of the cells was performed using digital video fluorescence microscopy and Ca(2+)-indicator dye fura-2AM. Experimental evidence indicated that ultrasound caused a direct microbubble-cell interaction resulting in the breaking and eventual dissolution of the microbubble and concomitant permeabilization of the cells to Ca(2+). These cells exhibited a large influx of Ca(2+) over 3-4 s and did not return to their equilibrium levels. Subsequently, some cells exhibited one or more Ca(2+) oscillations with the onset of oscillations delayed by 10-80 s after the ultrasound pulse. A variety of oscillations were observed including decaying oscillations returning to the baseline value over 35-100 s, oscillations superimposed on a more gradual recovery over 150-200 s, and oscillations continued with increased amplitude caused by a second ultrasound tone burst. The delays in onset appeared to result from calcium waves that propagated across the cells after the application of the ultrasound pulse.

Spontaneous oscillation and mechanically induced calcium waves in chondrocytes

The characteristics of spontaneous calcium (Ca(2+)) oscillation and mechanically induced Ca(2+) waves in articular chondrocytes were studied. In some, but not all, chondrocytes in sliced cartilage and primary cultures, we observed spontaneous oscillation of intracellular Ca(2+) that never spread to adjacent cells. In contrast, a mechanical stimulus to a single cell by touching with a glass rod induced an increase of intracellular Ca(2+) that spread to neighboring cells in a wave-like manner, even though there was no physical contact between the cells. This indicated the release of some paracrine factor from the mechanically stimulated cells. Application of ultrasonic vibration also induced an oscillation of intracellular Ca(2+). The application of a uridine 5'-triphosphate (UTP), UTP, induced a transient increase in intracellular Ca(2+) and the release of adenosine 5'-triphosphate (ATP) in cultured chondrocytes. A P2 receptor antagonist (suramin) and blockers of Cl(-) channels, niflumic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), reduced the UTP-induced ATP release. The results indicated that Cl(-) channels were involved in the extracellular release of ATP following mechanical or P2Y receptor stimulation. Thus, ATP stimulation of P2Y receptors elicits an increase in intracellular Ca(2+), triggering further release of ATP from adjacent cells, thereby expanding the Ca(2+) wave in chondrocytes.

Characteristics of cultured subepithelial fibroblasts in the rat small intestine. II. Localization and functional analysis of endothelin receptors and cell-shape-independent gap junction permeability

Subepithelial fibroblasts form a cellular network with gap junctions under the epithelium of the gastrointestinal tract. Previously, we have reported their unique characteristics, such as reversible rapid cell-shape changes from a flat to a stellate configuration induced by dBcAMP and endothelins (ETs), and Ca2+ responses to, for example, ETs, ATP, and substance-P. We have now investigated the subtypes of ET receptors both in the rat small intestine and in primary cultured subepithelial fibroblasts isolated from rat duodenal villi. Their properties were compared between wild-type and endothelin-B-receptor-mutant sl/sl rats. Light- and electron-microscopic immunohistochemistry showed intense ETA immunoreactivity in the subepithelial fibroblasts from the small intestine and colon of both wild-type and sl/sl rats. In culture, immunocytochemistry, reverse transcription/polymerase chain reaction analysis, Ca2+ response measurements, and cell-shape change analysis indicated functional ETA and ETB receptors in the wild-type cells, but only ETA in the sl/sl cells. However, wild-type cells were more sensitive to ET-1 than to ET-3 by about one order of magnitude. ETA seemed to be dominant both in vivo and in vitro. The relationship between cell-shape change and gap junction permeability was examined by fluorescence recovery after photobleaching; the gap junctions were usually open but were blocked by carbenoxolone. Permeability did not change significantly with cell-shape change. This network of differentiated subepithelial fibroblasts may maintain intercellular communication via gap junctions to transduce signals evoked in the local network to the whole network. The cell-shape change of the cells through ETA activation may play an important role as a barrier and for intercellular signaling in the intestinal villi.

Biomechanical approaches for studying integration of tissue structure and function in mammary epithelia

The structure and function of each individual mammary epithelial cell (MEC) is largely controlled by a bidirectional interchange of chemical and mechanical signals with the microenvironment. Most of these signals are tissue-specific, since they arise from the three-dimensional (3D) tissue organization and are modulated during mammary gland development, maturation, pregnancy, lactation, and involution. Although the important role played by structural and mechanical signals in mammary cell and tissue function is being increasingly recognized, quantitative biomechanical approaches are still scarce. Here we review currently available biomechanical tools that allow quantitative examination of individual cells, groups of cells or full monolayers in two-dimensional cultures, and cells in 3D cultures. Current technological limitations and challenges are discussed, with special emphasis on their potential applications in MEC biology. We argue that the combination of biomechanical tools with current efforts in mathematical modeling and in cell and molecular biology applied to 3D cultures provides a powerful approach to unravel the complexity of tissue-specific structure-function relationships.

[Extracellular ATP mediated mechano-signaling in mammary glands]

ATP, an important and ubiquitous extracellular signaling molecule, is often released by mechanical stimuli and plays an essential role in mechano-signaling. In lactating mammary glands, secretory epithelial (SE) cells form alveoli in which milk is held, and myoepithelial (ME) cells surrounding the alveoli contract in response to oxytocin to expel milk. Previously we found that the contraction of ME cells worked as a mechanical stress to SE cells and caused ATP-release in cultured mammary epithelial cells. The released ATP activated P2Y2 in surrounding SE cells and P2Y1 in ME cells. We already reported that ATP synergistically enhanced oxytocin response in ME cells. These findings mean that ME and SE cells interact mutually via released ATP to enhance the milk ejection. Recently, we found that cell-stretch also induced Ca(2+)-increases and ATP-release. The stretching of alveoli should occur by milk filling. So, only the milk-filled alveoli (but not empty alveoli) are surrounded by ATP. The ATP lowers the threshold of the oxytocin receptors and enables the milk-filled alveoli to contract in response to oxytocin at a concentration in the blood. Slight but apparent constitutive-ATP-release was observed in non-stimulated cells and the release was enhanced in Ca(2+)-free solution. The pathway of ATP-release is not yet clear, but pharmacologically, there seems to be two or more pathways.

Evidence that Ca(2+) cycling by the plasma membrane Ca(2+)-ATPase increases the 'excitability' of the extracellular Ca(2+)-sensing receptor

The extracellular Ca(2+)-sensing receptor (CaR) is a widely expressed G-protein-coupled receptor that translates information about [Ca(2+)] in the extracellular milieu to the interior of the cell, usually via intracellular Ca(2+) signaling pathways. Using fura-2 imaging of cytoplasmic [Ca(2+)], we observed that HEK293 cells expressing CaR oscillated readily under conditions permissive for CaR activation. Spiking was also triggered in the absence of external Ca(2+) by the CaR agonist spermine (1 mM). Oscillating cells were typically located in clusters of closely apposed cells, but Ca(2+) spiking was insensitive to the gap junction inhibitor 18alpha-glycyrrhetinic acid. We hypothesized that Ca(2+) signals might be amplified, in part, through a positive feedback loop in which Ca(2+) extrusion via the plasma membrane Ca(2+)-ATPase (PMCA) activates CaRs on the same cell or adjacent cells through local increases in [Ca(2+)](out). In support of this idea, addition of exogenous Ca(2+) buffers (keeping free [Ca(2+)](out) constant) attenuated or eliminated Ca(2+) signals (manifested as oscillations), as did PMCA inhibitors (HgCl(2), orthovanadate and Caloxin 2A1). Measurement of extracellular [Ca(2+)] using the near membrane probe fura-C(18) revealed that external [Ca(2+)] rose following receptor activation, sometimes displaying an oscillatory pattern. Our data suggest that PMCA-mediated cycling of Ca(2+) across the plasma membrane leads to localized increases in [Ca(2+)](out) that increase the excitability of CaR.

P2 purinoceptors regulate calcium-activated chloride and fluid transport in 31EG4 mammary epithelia

It has been reported that secretory mammary epithelial cells (MEC) release ATP, UTP, and UDP upon mechanical stimulation. Here we examined the physiological changes caused by ATP/UTP in nontransformed, clonal mouse mammary epithelia (31EG4 cells). In control conditions, transepithelial potential (apical side negative) and resistance were -4.4 +/- 1.3 mV (mean +/- SD, n = 12) and 517.7 +/- 39.4 Omega. cm(2), respectively. The apical membrane potential was -43.9 +/- 1.7 mV, and the ratio of apical to basolateral membrane resistance (R(A)/R(B)) was 3.5 +/- 0.2. Addition of ATP or UTP to the apical or basolateral membranes caused large voltage and resistance changes with an EC(50) of approximately 24 microM (apical) and approximately 30 microM (basal). Apical ATP/UTP (100 microM) depolarized apical membrane potential by 17.6 +/- 0.8 mV (n = 7) and decreased R(A)/R(B) by a factor of approximately 3. The addition of adenosine to either side (100 microM) had no effect on any of these parameters. The ATP/UTP responses were partially inhibited by DIDS and suramin and mediated by a transient increase in free intracellular Ca(2+) concentration (427 +/- 206 nM; 15-25 microM ATP, apical; n = 6). This Ca(2+) increase was blocked by cyclopiazonic acid, by BAPTA, or by xestospongin C. 31EG4 MEC monolayers also secreted or absorbed fluid in the resting state, and ATP or UTP increased fluid secretion by 5.6 +/- 3 microl x cm(-2) x h(-1) (n = 10). Pharmacology experiments indicate that 31EG4 epithelia contain P2Y(2) purinoceptors on the apical and basolateral membranes, which upon activation stimulate apical Ca(2+)-dependent Cl channels and cause fluid secretion across the monolayer. This suggests that extracellular nucleotides could play a fundamental role in mammary gland paracrine signaling and the regulation of milk composition in vivo.

ATP-induced calcium oscillations and change of P2Y subtypes with culture conditions in HeLa cells

ATP, UTP, ADP and UDP induced intracellular Ca(2+) responses and oscillations in HeLa cells that sometimes lasted over 1 h. The response is due to the activation of P2Ys, G-protein coupled ATP receptors, because the oscillations persisted for several minutes even in Ca(2+)-free solution, and suramin and PPADS, antagonists of ATP receptors, partially inhibited the response. The potency of these nucleotides varied with the culture or cell conditions, i.e. UTP was generally most potent but in some cases UDP was more potent; responses to UDP were variable while those to ATP were constant. In addition, Ca(2+) responses to ATP and UDP were additive. These findings suggested the existence of two or more subtypes of P2Ys in HeLa cells. RT-PCR experiments revealed the existence of P2Y(2), P2Y(4) and P2Y(6). Recovery from starvation (culture in FBS-free medium overnight and re-addition of FBS) increased the responses to UTP and UDP but not to ATP, suggesting that the number or activity of P2Y(6) and/or P2Y(4) receptors may increase with cell proliferation in HeLa cells.

Polarized calcium and calmodulin signaling in secretory epithelia

This review examines polarized calcium and calmodulin signaling in exocrine epithelial cells. The calcium ion is a simple, evolutionarily ancient, and universal second messenger. In exocrine epithelial cells, it regulates essential functions such as exocytosis, fluid secretion, and gene expression. Exocrine cells are structurally polarized, with the apical region usually dedicated to secretion. Recent advances in technology, in particular the development of videoimaging and confocal microscopy, have led to the discovery of polarized, subcellular calcium signals in these cell types. The properties of a rich variety of local and global calcium signals have now been described in secretory epithelial cells. Secretagogues stimulate apical-to-basal waves of calcium in many exocrine cell types, but there are some interesting exceptions to this rule. The shapes of intracellular calcium signals are determined by the distribution of calcium-releasing channels and mechanisms that limit calcium elevation. Polarized distribution of calcium-handling mechanisms also leads to transcellular calcium transport in exocrine epithelial cells. This transport can deliver considerable amounts of calcium into secreted fluids. Multicellular polarized calcium signals can coordinate the activity of many individual cells in epithelial secretory tissue. Certain particularly sensitive cells serve as pacemakers for initiation of intercellular calcium waves. Many calcium signaling pathways involve activation of calmodulin. This ubiquitous protein regulates secretion in exocrine cells and also activates interesting feedback interactions with calcium channels and transporters. Very recently it became possible to directly study polarized calcium-calmodulin reactions and to visualize the process of hormone-induced redistribution of calmodulin in live cells. The structural and functional polarity of secretory epithelia alongside the polarity of its calcium and calmodulin signaling present an interesting lesson in tissue organization.

Suppression of EGF-induced cell proliferation by the blockade of Ca2+ mobilization and capacitative Ca2+ entry in mouse mammary epithelial cells

The role of intracellular Ca2+ stores and capacitative Ca2+ entry on EGF-induced cell proliferation was investigated in mouse mammary epithelial cells. We have previously demonstrated that EGF enhances Ca2+ mobilization (release of Ca2+ from intracellular Ca2+ stores) and capacitative Ca2+ entry correlated with cell proliferation in mouse mammary epithelial cells. To confirm their role on EGF-induced cell cycle progression, we studied the effects of 2,5-di-tert-butylhydroquinone (DBHQ), a reversible inhibitor of the Ca2+ pump of intracellular Ca2+ stores, and SK&F 96365, a blocker of capacitative Ca2+ entry, on mitotic activity induced by EGF. Mitotic activity was examined using an antibody to PCNA for immunocytochemistry. SK&F 96365 inhibited capacitative Ca2+ entry in a dose-dependent manner (I50: 1-5 microM). SK&F 96365 also inhibited EGF-induced cell proliferation in the same range of concentration (I50: 1-5 microM). DBHQ suppressed [Ca2+]i response to UTP and thus depleted completely Ca2+ stores at 5 microM. DBHQ also inhibited EGF-induced cell proliferation at an I50 value of approximately 10 microM. The removal of these inhibitors from the culture medium increased the reduced mitotic activity reversibly. Using a fluorescent assay of DNA binding of ethidium bromide, no dead cells were detected in any of the cultures. These results indicate that the inhibitory effects of SK&F 96365 and DBHQ on cell proliferation were due to the inhibition of capacitative Ca2+ entry and Ca2+ mobilization suggesting the importance of capacitative Ca2+ entry and Ca2+ mobilization in the control of EGF-induced cell cycle progression in mouse mammary epithelial cells.

Intercellular communication upon mechanical stimulation of CPAE- endothelial cells is mediated by nucleotides

Intercellular Ca(2+)-signaling, after mechanical stimulation of calf pulmonary artery endothelial cells (CPAE), was investigated with fluorescence video imaging. Mechanical stimulation evoked an intracellular Ca(2+)-response in the mechanically stimulated (MS) cell, proceeding to the neighboring (NB) cells as a Ca(2+)-wave. The intercellular propagation of the Ca(2+)-wave was unaffected by the gap junction blockers halothane or heptanol. Therefore the intercellular communication (IC) pathway of the Ca(2+)-wave in CPAE cells does not depend on gap junctional communication but is most likely mediated by release of an extracellular mediator. Continuous unilateral flow experiments confirmed the presence of a diffusible mediator: the Ca(2+)-rise in upstream NB cells is significantly lower than in control experiments. After desensitization of purinergic receptors by pretreatment of CPAE cells with ATP (100mM), UTP (100 microM), 2MeSATP (100microM) or ADPbS (100 microM), the propagation of the intercellular Ca(2+)-wave upon mechanical stimulation was significantly inhibited. Also suramin (200 and 400 microM), a non-specific purinergic receptor blocker, reduced the IC. Application of the nucleotidase apyrase VI (10U/ml), which has a high ATPase/ADPase ratio, enhanced Ca(2+)-signaling and IC. In contrast, apyrase VII (10U/ml), which has a high ADPase/ATPase ratio, significantly depressed the propagation of the intercellular Ca(2+)-wave upon mechanical stimulation. Our experiments therefore demonstrate that the IC, evoked by a mechanical stimulus of CPAE cells, is mediated via release of nucleotides in the extracellular space. The data indicate that the diffusible messenger, responsible for the propagation of a Ca(2+)-wave, is mainly ADP or a combination of ADP/ATP.

Mechanism of intracellular Ca(2+)-wave propagation elicited by mechanical stimulation in cultured endothelial CPAE cells

Intra- and intercellular Ca(2+)-signaling during mechanical stimulation in calf pulmonary artery endothelial cells (CPAE) was investigated with digital fluorescence microscopy. Mechanical stimulation of a CPAE cell in a Ca(2+)-containing solution revealed a rise of the free intracellular Ca(2+)-concentration ([Ca(2+)](i)) in the mechanically stimulated cell (MS) proceeding to the neighboring (NB) cells as an intercellular Ca(2+)-wave. Experiments in Ca(2+)-free solution, containing 2mM EGTA, demonstrated that a detectable [Ca(2+)](i)-transient in the MS cell is not always a requisite for intercellular communication (IC). The Ca(2+)-wave propagation was not affected by changes in membrane potential and was not mediated by voltage-dependent Ca(2+)-channels. Ca(2+)-influx through the Ni(2+)-sensitive Ca(2+)-pathway occurred in the MS as could be assessed by Mn(2+)-quenching experiments. The intra- and intercellular Ca(2+)-wave was triggered by the release of thapsigargin-sensitive intracellular Ca(2+)-stores. Phospholipase C (PLC) inhibition by U73122 reduced the Ca(2+)-amplitude of the MS cell and almost completely inhibited the IC, indicating that the Ca(2+)-release in the MS and NB cells is PLC/inositol 1,4,5-trisphosphate (IP(3)) mediated.

EGF enhances Ca(2+) mobilization and capacitative Ca(2+) entry in mouse mammary epithelial cells

Effects of epidermal growth factor (EGF) on the intracellular Ca(2+) ([Ca(2+)](i)) responses to nucleotides, Ca(2+) release from thapsigargin-sensitive stores and capacitative Ca(2+) entry were investigated in cultured mouse mammary epithelial cells. EGF treatment induced proliferation of mammary epithelial cells. We checked for mitotic activity by immunocytochemistry with an anti-PCNA (proliferating cell nuclear antigen) antibody, which stains nuclei of the cells in S-phase of cell cycle. EGF treatment apparently increased the number of PCNA-stained cells compared to those treated with differentiating hormones (insulin, prolactin and cortisol) or without any hormone. Application of EGF did not induce any acute [Ca(2+)](i) response. EGF treatment for 1-2 days in culture, however, enhanced [Ca(2+)](i) responses including [Ca(2+)](i) increase by ATP, UTP and other nucelotides, Ca(2+) release from thapsigargin-sensitive stores, as well as capacitative Ca(2+) entry. Genistein, a tyrosine kinase inhibitor, prevented EGF-induced cell proliferation and the [Ca(2+) ](i) responses in a dose-dependent manner. These results indicate that EGF treatment enhances Ca(2+) mobilization and capacitative Ca(2+) entry, well correlated with cellular proliferation in mammary epithelial cells.

A mechanism for action of oscillating electric fields on cells

The biological effects of electromagnetic fields have seriously concerned the scientific community and the public as well in the past decades as more and more evidence has accumulated about the hazardous consequences of so-called "electromagnetic pollution." This theoretical model is based on the simple hypothesis that an oscillating external electric field will exert an oscillating force to each of the free ions that exist on both sides of all plasma membranes and that can move across the membranes through transmembrane proteins. This external oscillating force will cause a forced vibration of each free ion. When the amplitude of the ions' forced vibration transcends some critical value, the oscillating ions can give a false signal for opening or closing channels that are voltage gated (or even mechanically gated), in this way disordering the electrochemical balance of the plasma membrane and consequently the whole cell function.

Developmental changes in capacitative Ca(2+) entry in mouse mammary epithelial cells

Developmental changes in capacitative Ca(2+) entry and Ca(2+) release from intracellular stores were measured using fura-2 fluorescence method during the pregnancy period (day 3-;18) in mouse mammary epithelial cells. Ca(2+) release was identified with the transient intracellular Ca(2+) ([Ca(2+)](i)) increase induced by thapsigargin addition in a Ca(2+)-free solution. Capacitative Ca(2+) entry was measured by the transient [Ca(2+)](i) increase induced by re-addition of extracellular Ca(2+) after depletion of Ca(2+) stores by thapsigargin. The capacitative Ca(2+) entry was greatest at the early stage of pregnancy (i.e. day 3 of pregnancy) and decreased as pregnancy progressed, while Ca(2+) release remained unchanged throughout the developmental stages. These findings indicate that in contrast to Ca(2+) release, a close correlation exists between capacitative Ca(2+) entry and pregnancy-induced development in mammary epithelial cells.

Spontaneous calcium oscillations in embryonic stem cell-derived primitive endodermal cells

In vitro differentiation of mouse embryonic stem cells within three-dimensional cell aggregates called embryoid bodies parallels the development of postimplantation embryos at the egg cylinder stage, where visceral and parietal endoderm diverge from the primitive endoderm. We have investigated spontaneous [Ca2+]i oscillations by means of confocal laser-scanning microscopy in primitive endodermal cell layers of embryoid bodies during their differentiation to parietal and visceral endoderm. The frequency of [Ca2+]i oscillations increased from day 4 to day 19 of development, whereas their duration decreased from day 3 to days 16-17. Oscillations depended on both extracellular Ca2+ and Ca2+ release from intracellular stores as they were abolished in Ca(2+)-free solution and in the prescence of Ni2+ and thapsigargin. Signal transduction operated via the phospholipase C (PLC)-mediated inositol 1,4,5-triphosphate (InsP3) pathway with a negative feedback loop via protein kinase C (PKC) as U73,122, a blocker of PLC; bisindolylmaleimide 1, staurosporine, and H-7, blockers of PKC; and 10 mM caffeine totally inhibited [Ca2+]i spiking. Thimerosal, which hypersensitizes the InsP3 receptor, as well as vasopressin and bradykinin, which act via the InsP3 pathway, increased the frequency of [Ca2+]i spikes. In the prescence of brefeldin A (50 microM) or monensin (20 microM), which both inhibit endo/exocytotic vesicle pathways, an immediate transient increase in spiking activity was followed by a decline within 1 to 2 h. In the presence of brefeldin A or thapsigargin or in the absence of extracellular Ca2+, endocytotic vesicles were absent, suggesting that oscillating [Ca2+]i transients are involved in the exo/endocytotic vesicle shuttle.

Intracellular calcium oscillations in cell populations of ras-transfected I-7 subline of human HaCaT keratinocytes

We have observed oscillations of intracellular Ca2+ (Ca[i]) concentration in populations of ras-transfected HaCaT keratinocytes of I-7 subline. In postconfluent monolayers of I-7 keratinocytes, an increase in extracellular Ca2+ (Ca[o]) concentration to 0.25-0.5 mM induced sinusoidal Ca(i) oscillations, which persisted longer than 1 h with amplitudes of 50-150 nM and periods of 5-10 min. Thapsigargin, which depletes internal Ca2+ stores, did not prevent Ca(o)-induced Ca(i) oscillations, and it also induced Ca(i) oscillations in the ras-transfected I-7 line. Removal of extracellular Ca2+ or addition of Ca2+-entry blocker La3+ or SK&F 96365 inhibited Ca(i) oscillations, suggesting that Ca(i) oscillations in ras-transfected HaCaT keratinocytes were dependent on Ca2+ influx across the plasma membrane. Because the Ca(o)-induced Ca(i) oscillations have been observed only in ras-transfected I-7 subline and not in its nontransfected parental HaCaT line, this may provide a partial explanation for the divergent responses of ras-transfected and nontransfected keratinocytes to Ca(o) signal for control of growth and differentiation.

Lysophosphatidic acid sensitises Ca2+ influx through mechanosensitive ion channels in cultured lens epithelial cells

We investigated the effect of lysophosphatidic acid (LPA), a bioactive phospholipid, on the response in cytosolic free Ca2+ concentration ([Ca2+]i) to mechanical stress in cultured bovine lens epithelial cells. Spritzing of bath solution onto cells as mechanical stress caused marked increase in [Ca2+]i in the presence of LPA and this increase was concentration-dependent (1-10 microM), whereas neither addition of LPA alone nor the mechanical stress in the absence of LPA affected [Ca2+]i. The mechanical stress-induced increase in [Ca2+]i in the presence of LPA was inhibited by removing extracellular Ca2+ or by addition of Gd3+, a blocker of mechanosensitive cation channels, but not by nicardipine, thapsigargin, an inhibitor of endoplasmic reticulum-ATPase pump, or U73122, a phospholipase C inhibitor. These results show that LPA sensitises Ca2+ influx through cation-selective mechanosensitive channels, but does not sensitise Ca2+ release from intracellular stores, triggered by changes in mechanical stress. On the other hand, phosphatidic acid had less of a sensitising effect than LPA, and neither lysophosphatidylcholine nor chlorpromazine had any effect. Also Ca2+ mobilising agonists, ATP, histamine and carbachol, did not sensitise Ca2+ response to the mechanical stress. These results show that LPA sensitises mechanoreceptor-linked response in lens epithelial cells, suggesting that it plays a role in the development of cataracts due to increases in [Ca2+]i induced by mechanical stress.

Temperature dependency of calcium responses in mammary tumour cells

Changes of intracellular calcium concentration ([Ca2+]i) induced by the extracellular application of ATP and bradykinin in mouse mammary tumour cells (MMT060562) were investigated by image analysis of fluo-3 fluorescence at 24 degrees C and 35 degrees C. ATP (0.1-100 microM) and bradykinin (0.1 nM-1 microM) induced the increase of [Ca2+]i at both temperatures and Ca(2+)-depletion did not affect these [Ca2+]i responses. Both [Ca2+]i responses became more sensitive at 35 degrees C than at 24 degrees C. A clear latency of [Ca2+]i increase after the application of the agonists was observed, and it changed with the concentration of the agonist. As concentrations of ATP or bradykinin became lower, the latency and rise time became longer. At higher concentrations, the latency and rise time approached a constant value. The latency shortened remarkably at 35 degrees C. These results suggested the involvement of a regenerative or threshold process in the [Ca2+]i responses in mammary tumour cells.

Lysophosphatidic acid sensitizes mechanical stress-induced Ca2+ response via activation of phospholipase C and tyrosine kinase in cultured smooth muscle cells

We previously reported that lysophosphatidic acid (LPA) sensitized mechanical stress-induced intracellular free Ca2+ concentration response (Biochem. Biophys. Res. Commun. 208, 19-25, 1995). In the present study, the signal transduction pathway of the sensitizing effect of LPA was investigated in cultured longitudinal muscle cells from guinea pig ileum. Suramin, a putative LPA receptor antagonist, did not affect the response in the presence of 30 nM LPA, suggesting that the response is induced via activation of suramin-insensitive LPA receptor. Neither pertussis toxin nor wortmannin inhibited the LPA-sensitized response, indicating that G(i/o)- and phosphatidylinositol 3-kinase (PI3-kinase)-mediated pathways are not involved in the sensitizing effect. C3 ADP ribosyltransferase had no effect on the response, whereas formation of actin-stress fiber in the presence of LPA was completely inhibited, suggesting rho-related cytoskeletal change is not involved in the response. In contrast, a phospholipase C (PLC) inhibitor, U73122, completely inhibited the response, but broad spectrum kinase inhibitors, staurosporine and H7, had no effect on the response. In addition, tyrosine kinase inhibitor, genistein, but not tyrphostin partially inhibited the response. These results suggest that LPA sensitizes the mechanical stress-induced response via activation of PLC, but not protein kinase C. Additionally, tyrphostin-insensitive tyrosine kinase, which is related to other pathway than G(i/o)- and rho-mediated pathways, may be involved in the response.

Feedback control of milk secretion from milk

Extracellular storage allows biologically-active substances in milk to influence mammary function. Among these factors is one which regulates the rate of milk secretion acutely according to frequency or completeness of milk removal in each mammary gland. The active factor in goat's milk has been identified by screening milk constituents for their ability to inhibit milk constituent secretion in tissue and cell culture bioassays, and found to be a novel milk protein. The proteins identified by bioassy in vitro, also inhibited milk secretion in lactating goats in a reversible, concentration-dependent manner. This protein, termed FIL (feedback inhibitor of lactation), acts by reversible blockade of constitutive secretion in the mammary epithelial cell. As the inhibitor is synthesized in the same epithelial cells, feedback inhibition is, therefore, an autocrine mechanism. FIL's unusual mechanism of action also influences other aspects of mammary function. Acute disruption of mammary membrane trafficking is associated with downregulation of prolactin receptors and followed by a decrease in epithelial cell differentiation. Thus, in addition to acutely-regulating milk secretion, FIL may induce the adaptation in mammary cell differentiation which acts in vivo to sustain the secretory response to a sustained change in milk removal. In the long term, matching of milk output to demand is achieved by a change in mammary cell number. This developmental response is also local in nature. Whether it too is due to autocrine modulation by FIL of mechanisms influencing cell proliferation or survival, or elicited by another milk-borne factor, remains to be determined.

Mechanisms of mechanical stress-induced Ca(2+)-mobilization sensitized by lysophosphatidic acid in cultured smooth muscle cells

We have previously reported that lysophosphatidic acid (LPA) sensitizes mechanical stress-induced cytosolic free Ca2+ concentration ([Ca2+]i) response related to Ca2+ entry through Gd(3+)-sensitive ion channels (Biochem. Biophys. Res. Commun. 208 19-25 1995). Here we examined the contribution of Ca2 release from intracellular stores to the mechanical stress-induced [Ca2+]i response sensitized by LPA in cultured longitudinal muscle cells from guinea pig ileum. Although the percentage of responsive cells to the mechanical stress in the presence of 30 nM LPA declined by decreasing extracellular Ca2+ concentration to less than 20 microM, the amplitude of the mechanical stress-induced [Ca2+]i transient did not depend on extracellular Ca2+ concentrations (10 microM-1.8 mM). The [Ca2+]i transient was completely abolished by treatment with thapsigargin. In addition, the amplitude of the [Ca2+]i transient gradually decreased after ryanodine and caffeine treatment. These results indicate that the mechanical stress-induced [Ca2+]i transient in the presence of LPA is mainly due to Ca2+ release from ryanodine-sensitive intracellular stores and may be triggered by Ca2+ influx through Gd(3+)-sensitive ion channels.

Release of arachidonic acid via Ca2+ increase stimulated by pyrophosphonucleotides and bradykinin in mammary tumour cells

The relationship between the increase of intracellular Ca2+ and the release of arachidonic acid by bradykinin and pyrophosphonucleotides was studied in cultured mammary tumour cells, MMT060562. Bradykinin, ATP, UTP and UDP induced an increase of intracellular Ca2+ and the release of arachidonic acid from phospholipids into the extracellular fluid. Release of arachidonic acid was also induced by the application of the Ca2+ ionophore, A23187. Liberation of arachidonic acid by bradykinin and ATP was reduced by mepacrine, a blocker of phospholipase A2 and W-7, a calmodulin antagonist. It is suggested that the increase in cytosolic Ca(2+)-induced release of arachidonic acid occurs through activation of calmodulin-dependent phospholipase A2.

Lysophosphatidic acid sensitizes mechanical stress-induced Ca2+ mobilization in cultured human lung epithelial cells

We conformed that lysophosphatidic acid (LPA), which is known to be released from activated platelets, sensitizes response in cytosolic free Ca2+ concentration ([Ca2+]i) to mechanical stimulation in cultured epithelial cells (REPF-LC-AI cells) from human lung carcinoma. [Ca2+]i was transiently increased by spritzing of bath solution onto cells as mechanical stimulation in the presence of LPA with concentration-dependent manner (10-100 nM). The transient increase induced by the mechanical stimulation in the presence of LPA was inhibited by 10 microM Ga3+ or removing extracellular Ca2+, but not by 10 microM nicaridipine, suggesting that LPA sensitizes mechanical stimulation-induced Ca2+ influx through stretch-activated ion channels. Phosphatidic acid (1 microM), but not lysophosphatidycholine (10 microM), histamine (100 nM), bradykinin (10 nM), nor ionomycin (100 nM), caused the same effect as that of LPA. This effect was observed in confluent cells, but not in subconfluent cells. These results show that LPA sensitizes mechanoreceptor-linked response in human lung epithelial cells, suggesting a possibility that LPA affects lung function, in particular, during pathological state.

A role for phospholipase C activity but not ryanodine receptors in the initiation and propagation of intercellular calcium waves

Mechanical stimulation of a single cell in an airway epithelial culture initiates an increase in intracellular Ca2+ concentration ([Ca2+]i) that propagates from cell to cell as an intercellular Ca2+ wave. These Ca2+ waves appear to require an increase in intracellular inositol 1,4,5-trisphosphate (IP3) concentration ([IP3]i) in the stimulated cell and are propagated between cells by the diffusion of IP3 through gap junctions. To test the hypothesis that the activation of phospholipase C (PLC) contributes to the elevation of [IP3]i and initiation of an intercellular Ca2+ wave, changes in [Ca2+]i induced by mechanical stimulation were measured by digital fluorescence microscopy in the presence of the PLC inhibitor, aminosteroid U73122. Following exposure to U73122 mechanical stimulation elevated [Ca2+]i of the stimulated cell, but did not initiate the propagation of an intercellular Ca2+ wave. By contrast, in the presence of U73343, a similar aminosteroid that does not inactivate PLC, mechanical stimulation increased the [Ca2+]i of the stimulated cell and initiated an intercellular Ca2+ wave. U73122 also blocked the elevation of [Ca2+]i of airway epithelial cells in response to ATP, a P2-receptor agonist that activates PLC to elevate [IP3]i and [Ca2+]i. In addition, the propagation of intercellular Ca2+ waves was not affected by the ryanodine-receptor agonists, caffeine or ryanodine. The hypotheses that: (1) an elevation of [IP3]i is required to initiate intercellular Ca2+ waves; (2) mechanical stimulation activates PLC; and (3) Ca2+ wave propagation in airway epithelial cells involves Ca2+ release from intracellular stores primarily via IP3 receptors are supported by these results.

Effect of extracellular ATP on breast tumor cell growth, implication of intracellular calcium

We studied the effects of purine nucleotides and particularly adenosine triphosphate (ATP) in two (one hormonosensitive, MCF7 and one hormonoinsensitive, MDA-MB 231) human breast tumor cell lines. As described in other cells, we observed that purine nucleotides produced transient elevations in intracellular calcium ions, [Ca2+]i, in both types of cells as determined from Indo-1 fluorescence of loaded cells. In the absence of external calcium the [Ca2+]i transients consisted of single narrow peaks while an extension of peak duration along with a biphasic appearance were observed in the presence of external calcium. The potency of different purine nucleotides in elevating [Ca2+]i was ATP > ADP >> AMP > adenosine (which was inefficient) proving the presence of P2 purinergic receptor subtypes. Suramin, a compound known to compete with ATP for its binding sites, nearly abolished the effect of ATP on [Ca2+]i increase. while verapamil, a calcium channel blocker, was unable to abolish such an an ATP-induced [Ca2+]i increase. The concentrations of ATP required to increase [Ca2%bdi ranged from 10(-7) M to 10(-3) M, the maximal effect being obtained with 10(-4) M ATP. At this latter concentration, ATP induced cell growth inhibition which was dose-independent as triggered only when maximal elevation of [Ca2+]i was attained. This ATP concentration also induced maximal apoptotic features in both types of cells. Together, our results highlighted an 'all or none' effect of ATP on breast tumor cell growth mediated by its effect on [Ca2+]i liberation from intracellular stores, the first rise of [Ca2+]i being further amplified by an influx of calcium from extracellular space. The attainment of sufficient [Ca2+]i level then triggers cellular events.

Intracellular calcium responses and shape conversions induced by endothelin in cultured subepithelial fibroblasts of rat duodenal villi

Subepithelial fibroblasts of rat duodenal villi were cultured and the physiological characteristics were studied using fura-2 fluorescence. The intracellular calcium concentration (Ca2+i) responded to various substances, i.e., endothelins (ET1 and ET3), substance P, serotonin, angiotensin II, ATP, and bradykinin. The Ca2+i responses to ET1 (> 0.1 nM) and ET3 (> 1 nM) were transient and sometimes followed oscillations that consisted of an initial Ca2+ release from the intracellular store and a sustained Ca2+ influx. Simultaneously with Ca2+i measurement, changes in the cell shape were monitored using fluorescence intensity upon 360-nm excitation. Stellate cells (with thick cell body and slender processes), formed as a result of 1 mM dibutyryl(Bt2)-cAMP treatment, began to change immediately after the short-term application of the endothelin and became flat about 20 min later. This process was not affected by the depletion of extracellular Ca2+ or by the treatment with BAPTA acetoxymethyl ester that completely suppressed the Ca2+i response. Substance P (> 100 nM) increased Ca2+i, but did not induce any morphological changes. The conversion of the shape from flat to stellate, induced by Bt2cAMP treatment, was not accompanied by any Ca2+i change. BQ-123, a specific blocker of the ETA-type receptor, did not block either Ca2+i change or shape conversion at low (100 nM) concentration. The results indicated that shape conversion in subepithelial fibroblasts did not require any Ca2+i response. Our findings regarding the characteristics of subepithelial fibroblasts in intestinal villi imply a functional similarity to astrocytes in the brain.

The increase in the intracellular Ca2+ concentration induced by mechanical stimulation is propagated via release of pyrophosphorylated nucleotides in mammary epithelial cells

Mechanical stimulation of one mammary tumor cell in culture induced an increase in its intracellular calcium concentration which spread to surrounding cells. The increase in calcium can also be induced by addition of a solution in which cultured mammary tumor cells were stimulated by repeated pipetting (solution after pipetting cells, SAPC). The activity of the SAPC was completely abolished by treatment with snake venom phosphodiesterase or pyrophosphatase. Uridine triphosphate (UTP), uridine diphosphate (UDP) and ATP (1 microM each) were detected in the SAPC, whereas 5'-UMP and 5'-AMP were produced by phosphodiesterase digestion. A mixture of UTP, UDP and ATP (1 microM each) elicited a calcium response which was comparable to that induced by SAPC, while UTP, UDP or ATP alone at 1 microM elicited a small increase in calcium concentration in mammary tumor cells. Suramin, a competitive antagonist of P2 purinoceptors, diminished the spreading of the calcium wave induced by mechanical stimulation. It also blocked the responses to SAPC, UTP, UDP and ATP. These findings suggest that the mechanical stimulation results in the release of UTP, UDP and ATP into the extracellular space which mediates induction of the spreading calcium response via P2U-type purinoceptors.

Induction of differentiation by radiation and hyperthermia in neuroblastoma-glioma hybrid cells

The effects of either radiation or hyperthermia on the differentiation potential of NG108-15, a neuroblastoma-glioma hybrid cell line, were studied. After radiation and hyperthermia, the outgrowth of neurites from NG108-15 cells was potentiated, and polarizing current and voltage pulses induced a distinct action potential and a diphasic (inward following outward) current, respectively. An increase in the specific activity of acetylcholinesterase was also observed. In addition, both treatments induced an elevation of the concentration of intracellular calcium in some cells. The increase in intracellular calcium concentration caused by applying the calcium ionophore, A23187, induced differentiation. It is suggested that both the radiation- and the hyperthermia-induced increases of electrical excitability and acetylcholinesterase activity may have originated from an increase in intracellular Ca2+ concentration.

Activation of Ca(2+)-dependent K+ and Cl- currents by UTP and ATP in CFPAC-1 cells

Activation of Cl- and K+ conductances by nucleotide receptor-operated mobilization of intracellular Ca2+ was investigated in CFPAC-1 cells with the perforated-patch technique. Adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) caused a dose-dependent fast and transient membrane hyperpolarization. UTP was more effective than ATP. In voltage-clamped cells, two currents with different ionic permeability and kinetics were activated by the nucleotides. The first one was carried by Cl- ions, peaked in the first few seconds after addition of nucleotides, and lasted for 1 +/- 0.3 min. Its amplitude was about 2.7 nA at -100 mV with 100 mumol/l of either ATP or UTP. The second current was carried by K+ ions and was blocked by Cs+. This current peaked more slowly and had a mean duration of 4.6 +/- 0.7 min. Its amplitude was 0.9 nA and 0.5 nA at -20 mV with 100 mumol/l UTP and ATP, respectively. Activation of the nucleotide receptor caused a transient increase in intracellular Ca2+ concentration ([Ca2+]i) that was similar in the presence or absence of extracellular Ca2+. The ED50 for UTP was 24 mumol/l and that for ATP was 94 mumol/l. Depletion of the inositol 1,4,5-trisphosphate-sensitive Ca2+ store by thapsigargin prevented both the nucleotide-induced [Ca2+]i increase and the activation of membrane currents. Addition of 2 mmol/l Ca2+ to thapsigargin-treated cells produced a sustained increase of Cl- and K+ currents, which was reversed by Ca2+ removal. The present study demonstrates that CFPAC-1 cells respond to nucleotide receptor activation with a transient increase in [Ca2+]i that stimulates Ca(2+)-dependent Cl- and K+ currents.(ABSTRACT TRUNCATED AT 250 WORDS)

Spatial and temporal signalling by calcium

Recent research has shown the importance of the spatial and temporal aspects of calcium signals, which depend upon regenerative properties of the inositol trisphosphate and ryanodine receptors that regulate the release of calcium from internal stores. Initiation sites have been found to spontaneously release calcium, recognized as 'hot spots' or 'sparks', and can trigger a wave that spreads through a process of calcium-induced calcium release.

Mechanisms and function of intercellular calcium signaling

Intercellular Ca2+ waves initiated by mechanical or chemical stimuli propagate between cells via gap junctions. The ability of a wide diversity of cells to display intercellular Ca2+ waves suggests that these Ca2+ waves may represent a general mechanism by which cells communicate. Although Ca2+ may permeate gap junctions, the intercellular movement of Ca2+ is not essential for the propagation of Ca2+ waves. The messenger that moves from one cell to the next through gap junctions appears to be IP3 and a regenerative mechanism for IP3 may be required to effect multicellular communication. Extracellularly mediated Ca2+ signaling also exists and this could be employed to supplement or replace gap junctional communication. The function of intercellular Ca2+ waves may be the coordination of cooperative cellular responses to local stimuli.