Epidermal growth factor-activated calcium and potassium channels

Optical estimation of absolute membrane potential using fluorescence lifetime imaging

All cells maintain ionic gradients across their plasma membranes, producing transmembrane potentials (V_mem). Mounting evidence suggests a relationship between resting V_mem and the physiology of non-excitable cells with implications in diverse areas, including cancer, cellular differentiation, and body patterning. A lack of non-invasive methods to record absolute V_mem limits our understanding of this fundamental signal. To address this need, we developed a fluorescence lifetime-based approach (VF-FLIM) to visualize and optically quantify V_mem with single-cell resolution in mammalian cell culture. Using VF-FLIM, we report V_mem distributions over thousands of cells, a 100-fold improvement relative to electrophysiological approaches. In human carcinoma cells, we visualize the voltage response to growth factor stimulation, stably recording a 10-15 mV hyperpolarization over minutes. Using pharmacological inhibitors, we identify the source of the hyperpolarization as the Ca²⁺-activated K⁺ channel K_Ca3.1. The ability to optically quantify absolute V_mem with cellular resolution will allow a re-examination of its signaling roles.

The Role of Cell Membrane Information Reception, Processing, and Communication in the Structure and Function of Multicellular Tissue

Investigations of information dynamics in eukaryotic cells focus almost exclusively on heritable information in the genome. Gene networks are modeled as “central processors” that receive, analyze, and respond to intracellular and extracellular signals with the nucleus described as a cell’s control center. Here, we present a model in which cellular information is a distributed system that includes non-genomic information processing in the cell membrane that may quantitatively exceed that of the genome. Within this model, the nucleus largely acts a source of macromolecules and processes information needed to synchronize their production with temporal variations in demand. However, the nucleus cannot produce microsecond responses to acute, life-threatening perturbations and cannot spatially resolve incoming signals or direct macromolecules to the cellular regions where they are needed. In contrast, the cell membrane, as the interface with its environment, can rapidly detect, process, and respond to external threats and opportunities through the large amounts of potential information encoded within the transmembrane ion gradient. Our model proposes environmental information is detected by specialized protein gates within ion-specific transmembrane channels. When the gate receives a specific environmental signal, the ion channel opens and the received information is communicated into the cell via flow of a specific ion species (i.e., K⁺, Na⁺, Cl⁻, Ca²⁺, Mg²⁺) along electrochemical gradients. The fluctuation of an ion concentration within the cytoplasm adjacent to the membrane channel can elicit an immediate, local response by altering the location and function of peripheral membrane proteins. Signals that affect a larger surface area of the cell membrane and/or persist over a prolonged time period will produce similarly cytoplasmic changes on larger spatial and time scales. We propose that as the amplitude, spatial extent, and duration of changes in cytoplasmic ion concentrations increase, the information can be communicated to the nucleus and other intracellular structure through ion flows along elements of the cytoskeleton to the centrosome (via microtubules) or proteins in the nuclear membrane (via microfilaments). These dynamics add spatial and temporal context to the more well-recognized information communication from the cell membrane to the nucleus following ligand binding to membrane receptors. Here, the signal is transmitted and amplified through transduction by the canonical molecular (e.g., Mitogen Activated Protein Kinases (MAPK) pathways. Cytoplasmic diffusion allows this information to be broadly distributed to intracellular organelles but at the cost of loss of spatial and temporal information also contained in ligand binding.

SK3/TRPC1/Orai1 complex regulates SOCE-dependent colon cancer cell migration: a novel opportunity to modulate anti-EGFR mAb action by the alkyl-lipid Ohmline

Background

Barely 10-20% of patients with metastatic colorectal cancer (mCRC) receive a clinical benefit from the use of anti-EGFR monoclonal antibodies (mAbs). We hypothesized that this could depends on their efficiency to reduce Store Operated Calcium Entry (SOCE) that are known to enhance cancer cells.

Results

In the present study, we demonstrate that SOCE promotes migration of colon cancer cell following the formation of a lipid raft ion channel complex composed of TRPC1/Orai1 and SK3 channels. Formation of this complex is stimulated by the phosphorylation of the reticular protein STIM1 by EGF and activation of the Akt pathway. Our data show that, in a positive feedback loop SOCE activates both Akt pathway and SK3 channel activity which lead to SOCE amplification. This amplification occurs through the activation of Rac1/Calpain mediated by Akt. We also show that Anti-EGFR mAbs can modulate SOCE and cancer cell migration through the Akt pathway. Interestingly, the alkyl-lipid Ohmline, which we previously showed to be an inhibitor of SK3 channel, can dissociated the lipid raft ion channel complex through decreased phosphorylation of Akt and modulation of mAbs action.

Conclusions

This study demonstrates that the inhibition of the SOCE-dependent colon cancer cell migration trough SK3/TRPC1/Orai1 channel complex by the alkyl-lipid Ohmline may be a novel strategy to modulate Anti-EGFR mAb action in mCRC.

Activation of epidermal growth factor receptor mediates reperfusion arrhythmias in anaesthetized rats

AIMS

Epidermal growth factor receptor (EGFR) plays a critical role in the development and function of the heart. Previous studies have demonstrated that EGFR is involved in regulating electrical excitability of the heart. The present study was designed to investigate whether EGFR activation would mediate cardiac arrhythmias induced by reperfusion in anaesthetized rats.

METHODS AND RESULTS

Reperfusion arrhythmias were induced by 10 min ligation of the left anterior descending coronary artery, followed by a 30 min reperfusion in anaesthetized rats. The incidence and severity of cardiac arrhythmias were significantly reduced by pre-treatment with the EGFR kinase inhibitor AG556. The phosphorylation level of myocardial EGFR was increased during ischaemia and at early reperfusion. Intramyocardial transfection of EGFR siRNA reduced EGFR mRNA and protein, and decreased the incidence of ventricular fibrillation induced by reperfusion. Interestingly, tyrosine phosphorylation levels of cardiac Na(+) channels (I(Na)) and L-type Ca(2+) channels (I(Ca,L)) were significantly increased at time points corresponding to the alteration of EGFR phosphorylation levels during reperfusion. AG556 pre-treatment countered the increased tyrosine phosphorylation level of Na(+) and L-type Ca(2+) channels induced by reperfusion. Patch-clamp studies proved that AG556 could inhibit I(Na) and I(Ca,L) in rat ventricular myocytes. No significant alteration was observed in tyrosine phosphorylation levels of cardiac Kv4.2 and Kir2.1 channels during reperfusion.

CONCLUSION

These results demonstrate for the first time that EGFR plays an important role in the genesis of arrhythmias induced by reperfusion, which is likely mediated at least in part by enhancing tyrosine phosphorylation of cardiac Na(+) and L-type Ca(2+) channels.

Mechanism of Cancer Growth Suppression of Alpha-Fetoprotein Derived Growth Inhibitory Peptides (GIP): Comparison of GIP-34 versus GIP-8 (AFPep). Updates and Prospects

The Alpha-fetoprotein (AFP) derived Growth Inhibitory Peptide (GIP) is a 34-amino acid segment of the full-length human AFP molecule that inhibits tumor growth and metastasis. The GIP-34 and its carboxy-terminal 8-mer segment, termed GIP-8, were found to be effective as anti-cancer therapeutic peptides against nine different human cancer types. Following the uptake of GIP-34 and GIP-8 into the cell cytoplasm, each follows slightly different signal transduction cascades en route to inhibitory pathways of tumor cell growth and proliferation. The parallel mechanisms of action of GIP-34 versus GIP-8 are demonstrated to involve interference of signaling transduction cascades that ultimately result in: (1) cell cycle S-phase/G2-phase arrest; (2) prevention of cyclin inhibitor degradation; (3) protection of p53 from inactivation by phosphorylation; and (4) blockage of K+ ion channels opened by estradiol and epidermal growth factor (EGF). The overall mechanisms of action of both peptides are discussed in light of their differing modes of cell attachment and uptake fortified by RNA microarray analysis and electrophysiologic measurements of cell membrane conductance and resistance. As a chemotherapeutic adjunct, the GIPs could potentially aid in alleviating the negative side effects of: (1) tamoxifen resistance, uterine hyperplasia/cancer, and blood clotting; (2) Herceptin antibody resistance and cardiac (arrest) arrhythmias; and (3) doxorubicin's bystander cell toxicity.

Calmodulin-mediated regulation of the epidermal growth factor receptor

In this review, we first describe the mechanisms by which the epidermal growth factor receptor generates a Ca(2+) signal and, subsequently, we compile the available experimental evidence regarding the role that the Ca(2+)/calmodulin complex, formed after the rise in cytosolic free Ca(2+) concentration, exerts on the receptor. We focus not only on the indirect action that Ca(2+)/calmodulin exerts on the epidermal growth factor receptor, as a result of the activation of distinct calmodulin-dependent kinases, but also, and more extensively, on the direct interaction of Ca(2+)/calmodulin with the receptor. We also describe several mechanistic models that could account for the Ca(2+)/calmodulin-mediated regulation of epidermal growth factor receptor activity. The control exerted by calmodulin on distinct epidermal growth factor receptor-mediated cellular functions is also discussed. Finally, the phosphorylation of this Ca(2+) sensor by the epidermal growth factor receptor is highlighted.

Heregulinβ activates store-operated Ca2+ channels through c-erbB2 receptor level-dependent pathway in human breast cancer cells

The heregulinbeta (HRGbeta) is a ligand to activate c-erbB2/c-erbB3 interaction and can subsequently increases cytosolic [Ca(2+)](i). In the two human breast cancer cell lines, MCF-7 shows a low c-erbB2 expression level, whereas SK-BR-3 overexpress c-erbB2 receptor. In this article, we have found that in MCF-7, HRGbeta induced Ca(2+) release from the endoplasmic reticulums (ER) and subsequently activated Ca(2+) entry via store-operated Ca(2+) channel (SOC). However, in SK-BR-3, HRGbeta failed to induce Ca(2+) release and Ca(2+)entry. RNA interference to decrease c-erbB2 level in SK-BR-3 resulted in reactivation of HRGbeta-evoked Ca(2+) release and Ca(2+) entry via SOC, which was similar to that of MCF-7. In addition, in the absence of HRGbeta, a constitutive activation of SOC was observed in SK-BR-3 rather than in MCF-7 and c-erbB2-siRNA treated SK-BR-3. Compared to the cells with low c-erbB2 level, c-erbB2 might tend to interact with c-erbB3 in the resting state in the cells with high c-erbB2 level, which resulted in different [Ca(2+)](i) responses to HRGbeta. In SK-BR-3, the Ca(2+) mobilization in the presence or in the absence of HRGbeta was completely blocked by PLC inhibitor U73122. In summary, our results indicate that HRGbeta-induced SOC was regulated by c-erbB2 level and dependent on activation of PLC in human breast cancer cells.

Differentiated astrocytes acquire sensitivity to hydrogen sulfide that is diminished by the transformation into reactive astrocytes

Hydrogen sulfide (H2S) enhances the induction of hippocampal long-term potentiation (LTP) and induces calcium waves in astrocytes. Based on these observations, H2S has been proposed to be a synaptic modulator in the brain. Here we show that differentiated astrocytes acquire sensitivity to H2S that is diminished by their transformation into reactive astrocytes. Although sodium hydrosulfide hydrate (NaHS), a donor of H2S, did not increase the intracellular concentration of Ca2+ in progenitors, exposure of progenitors to leukemia inhibitory factor (LIF), which induces differentiation into glial fibrillary acidic protein (GFAP)-positive astrocytes, greatly increased the sensitivity to NaHS. In contrast, epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), dibutyryl cyclic AMP (db cAMP) and interleukin-1beta (IL-1beta) induced the conversion to reactive astrocytes with diminished sensitivity to NaHS. This suppressive effect of EGF on the sensitivity to NaHS was inhibited by cycloheximide, indicating that de novo protein synthesis was required for the suppression of H2S sensitivity.

Cell-Cycle-Dependent Regulation of Ca2+-Activated K+ Channel in Jurkat T-Lymphocyte

Small-conductance Ca2+-activated K+ (SK2) channel plays an important role in the activation of Jurkat T-lymphocytes by maintaining electrical gradients for the sustained Ca2+ influx. Apamin-sensitive K+ current was significantly decreased with cell-cycle progression from G0/G1 into G2/M phases, and protein expression of SK2 channels showed parallel down-regulation, with its highest expression at early G0/G1 phase. In the G0/G1 phase, the apamin-sensitive component of thapsigargin-induced Ca2+ influx was significantly larger than that in the G2/M phase. These observations suggest that SK2-channel activation may largely contribute to the sustained Ca2+ influx in the G0/G1 phase in comparison of that in the G2/M phase in Jurkat T-lymphocytes.

Epidermal growth factor induces vasoconstriction through the phosphatidylinositol 3-kinase-mediated mitogen-activated protein kinase pathway in hypertensive rats

We investigated whether increased contractile responsiveness to epidermal growth factor (EGF) is associated with altered activation of mitogen-activated protein kinase (MAPK) in the aortic smooth muscle of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. EGF induced contraction and MAPK activity in aortic smooth muscle strips, which were significantly increased in tissues from the DOCA-salt hypertensive rats compared with those from sham-operated rats. AG1478, PD98059, and LY294002, inhibitors of EGF receptor (EGFR) tyrosine kinase, MAPK/extracellular signal-regulated kinase (ERK) kinase, and phosphatidylinositol 3-kinase (PI3K), respectively, inhibited the contraction and the activity of ERK1/2 that were elevated by EGF. Y27632 and GF109203X, inhibitors of Rho kinase and protein kinase C, respectively, attenuated EGF-induced contraction, with no diminution of ERK1/2 activity. Although EGF also elevated the activity of EGFR tyrosine kinase in both sham-operated and DOCA-salt hypertensive rats, the expression and the magnitude of activation did not differ between strips. These results strongly suggest that EGF induces contraction by the activation of ERK1/2, which is regulated by the PI3K pathway in the aortic smooth muscle of DOCA-salt hypertensive rats.

Inhibition of tumor growth and prolonged survival of rats with intracranial gliomas following administration of clotrimazole

OBJECT

Clotrimazole, an imidazole derivative and inhibitor of cytochrome P-450, inhibits the proliferation of cancer cells by downregulating the movement of intracellular Ca++ and K+ and by interfering with the translation initiation process. Clotrimazole inhibits the proliferation of human glioblastoma multiforme cells; it induces morphological changes toward differentiation and blocks the cell cycle in the G1/G1 phase. In vitro, clotrimazole enhances the antitumor effect of cisplatin by inducing wild-type p53-mediated apoptosis. The authors examined the effect of clotrimazole on tumor growth, sensitivity to cisplatin, and survival of rats with intracranial gliomas.

METHODS

Cultured C6 and 9L glioma cells were exposed to clotrimazole, and cell growth was assessed using the 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl tetrazolium bromide colorimetric assay. Clotrimazole produced a dose- and time-dependent inhibition of cell proliferation. The growth inhibitory effect of clotrimazole could not be overcome by exogenous stimulation with epidermal growth factor. Both C6 and 9L glioma cells were implanted into the rat brain and after 5 days, the animals were treated with a daily single dose of clotrimazole for 8 consecutive days. Clotrimazole treatment caused a significant inhibition of intracranial tumor growth. The survival of rats with 9L gliomas was analyzed after 10 days of treatment with clotrimazole, cisplatin, or a combination of clotrimazole and cisplatin. Rats treated with either drug displayed a significantly prolonged survival time; however, the combination treatment resulted only in an additional survival benefit.

CONCLUSIONS

Clotrimazole effectively inhibits cell proliferation and tumor growth, and prolongs survival of rats with intracranial gliomas. Clotrimazole may be considered a potential anticancer drug for treatment of intracranial gliomas.

Potential pathways for intercellular communication within the calbindin subnucleus of the hamster suprachiasmatic nucleus

In mammals, the suprachiasmatic nucleus (SCN) is the master circadian pacemaker. Within the caudal hamster SCN, a cluster of neurons containing the calcium binding protein, calbindin-D28K (CB), has been implicated in circadian locomotion. However, calbindin-immunoreactive (CB+) neurons in the calbindin subnucleus (CBsn) do not display a circadian rhythm in spontaneous firing [Eur J Neurosci 16 (2002) 2469]. Previously, we proposed that intercellular communication might be essential in integrating outputs from rhythmic (CB-) neurons and nonrhythmic (CB+) neurons to produce a circadian output in the intact animal. The primary aim of this study is to provide a neuroanatomical framework to better understand intercellular communication within the CBsn. Using reconstructions of previously recorded neurons, we demonstrate that CB+ neurons have significantly more dendrites than CB- neurons. In addition, CBsn neurons have dorsally oriented dendritic arbors. Using double-label confocal microscopy, we show that GABA colocalizes with CB+ neurons and GABA(A) receptor subunits make intimate contacts with neurons in the CBsn. Transforming growth factor alpha (TGFalpha), a substance shown to inhibit locomotion [Science 294 (2001) 2511], is present within the CBsn. In addition, neurons in this region express the epidermal growth factor receptor, the only receptor for TGFalpha. Lastly, we show that CB+ neurons are coupled to CB+ and CB- neurons by gap junctions. The current study provides a structural framework for synaptic communication, electrical coupling, and signaling via a growth factor within the CBsn of the hamster SCN. Our results reveal connections that have the potential for integrating cellular communication within a subregion of the SCN that is critically involved in circadian locomotion.

Effects of Prolactin on Ionic Membrane Conductances in the Human Malignant Astrocytoma Cell Line U87-MG

Prolactin (PRL) is involved in numerous biological processes in peripheral tissues and the brain. Although numerous studies have been conducted to elucidate the signal transduction pathways associated with the PRL receptor, very few have examined the role of ion conductances in PRL actions. We used the patch-clamp technique in “whole cell” configuration and microspectrofluorimetry to investigate the effects of PRL on membrane ion conductances in the U87-MG human malignant astrocytoma cell line, which naturally expresses the PRL receptor. We found that a physiological concentration (4 nM) of PRL exerted a biphasic action on membrane conductances. First, PRL activated a Ca2+-dependent K+current that was sensitive to CTX and TEA. This current depended on PRL-induced Ca2+mobilization, through a JAK2-dependent pathway from a thapsigargin- and 2-APB-sensitive Ca2+pool. Second, PRL also activated an inwardly directed current, mainly due to the stimulation of calcium influx via nickel- and 2-APB-sensitive calcium channels. Both phases resulted in membrane hyperpolarizations, mainly through the activation of Ca2+-dependent K+channels. As shown by combined experiments (electrophysiology and microspectrofluorimetry), the PRL-induced Ca2+influx increased with cell membrane hyperpolarization and conversely decreased with cell membrane depolarization. Thus PRL-induced membrane hyperpolarizations facilitated Ca2+influx through voltage-independent Ca2+channels. Finally, PRL also activated a DIDS-sensitive Cl-current, which may participate in the PRL-induced hyperpolarization. These PRL-induced conductance activations are probably related to the PRL proliferative effect we have already described in U87-MG cells.

Regulation of Ca2+-dependent K+ current by alphavbeta3 integrin engagement in vascular endothelium

Interactions between endothelial cells and extracellular matrix proteins are important determinants of endothelial cell signaling. Endothelial adhesion to fibronectin through alpha(v)beta(3) integrins or the engagement and aggregation of luminal alpha(v)beta(3) receptors by vitronectin triggers Ca2+ influx. However, the underlying signaling mechanisms are unknown. The electrophysiological basis of alpha(v)beta(3) integrin-mediated changes in endothelial cell Ca2+ signaling was studied using whole cell patch clamp and microfluorimetry. The resting membrane potential of bovine pulmonary artery endothelial cells averaged -60 +/- 3 mV. In the absence of intracellular Ca2+ buffering, the application of soluble vitronectin (200 microg/ml) resulted in activation of an outwardly rectifying K+ current at holding potentials from -50 to +50 mV. Neither a significant shift in reversal potential (in voltage clamp mode) nor a change in membrane potential (in current clamp mode) occurred in response to vitronectin. Vitronectin-activated current was significantly inhibited by pretreatment with the alpha(v)beta(3) integrin antibody LM609 by exchanging extracellular K+ with Cs+ or by the application of iberiotoxin, a selective inhibitor of large-conductance, Ca2+-activated K+ channels. With intracellular Ca2+ buffered by EGTA in the recording pipette, vitronectin-activated K+ current was abolished. Fura-2 microfluorimetry revealed that vitronectin induced a significant and sustained increase in intracellular Ca2+ concentration, although vitronectin-induced Ca2+ current could not be detected. This is the first report to show that an endothelial cell ion channel is regulated by integrin activation, and this K+ current likely plays a crucial role in maintaining membrane potential and a Ca2+ driving force during engagement and activation of endothelial cell alpha(v)beta(3) integrin.

Epidermal growth factor activates store-operated Ca2+ channels through an inositol 1,4,5-trisphosphate-independent pathway in human glomerular mesangial cells

One of the fastest cellular responses following activation of epidermal growth factor receptor is an increase in intracellular Ca2+ concentration. This event is attributed to a transient Ca2+ release from internal stores and Ca2+ entry from extracellular compartment. Store-operated Ca2+ channels are defined the channels activated in response to store depletion. In the present study, we determined whether epidermal growth factor activated store-operated Ca2+ channels and further, whether depletion of internal Ca2+ stores was required for the epidermal growth factor-induced Ca2+ entry in human glomerular mesangial cells. We found that 100 nm epidermal growth factor activated a Ca2+-permeable channel that had identical biophysical and pharmacological properties to channels activated by 1 microm thapsigargin in human glomerular mesangial cells or A431 cells. The epidermal growth factor-induced Ca2+ currents were completely abolished by a selective phospho-lipase C inhibitor, U73122. However, xestospongin C, a specific inositol 1,4,5-trisphosphate receptor inhibitor, did not affect the membrane currents elicited by epidermal growth factor despite a slight reduction in background currents. Following emptying of internal Ca2+ stores by thapsigargin, epidermal growth factor still potentiated the Ca2+ currents as determined by the whole-cell patch configuration. Furthermore, epidermal growth factor failed to trigger measurable Ca2+ release from endoplasmic reticulum. However, another physiological agent linked to phospholipase C and inositol 1,4,5-trisphosphate cascade, angiotensin II, produced a striking Ca2+ transient. These results indicate that epidermal growth factor activates store-operated Ca2+ channels through an inositol 1,4,5-trisphosphate-independent, but phospholipase C-dependent, pathway in human glomerular mesangial cells.

Cancer cell motility--on the road from c-erbB-2 receptor steered signaling to actin reorganization

Cell migration depends mainly on actin polymerization and intracellular organization, which are influenced by a vast variety of actin binding proteins (ABPs). Regulation of ABP activity is mediated by second messengers such as phosphoinositides and calcium. Signaling via these second messengers is initiated and regulated by membrane receptors, e.g., receptor tyrosine kinases (RTKs), and by adhesion molecule interactions (e.g., integrins and selectins) and focal adhesion kinases. A major role in steering second-messenger signaling and thus in actin cytoskeleton reorganization and motility of cancer cells is played by the RTK c-erbB-2. This occurs through a number of signaling pathways which involve mainly enzymes, e.g., phospholipase Cgamma1 and GTPases, which modify signaling molecules. Furthermore large multiprotein complexes including actin-related protein 2/3, Wiskott-Aldrich syndrome protein, profilin, and capping protein among others play an important role in regulating actin reorganization. The complex picture of the mode of actin reorganization, which is involved in tumor cell migration, is slowly emerging from the mists of cellular signaling pathways, but this is still by no means a clear view.

Regulation of ion channels by protein tyrosine phosphorylation

Ion channels are regulated by protein phosphorylation and dephosphorylation of serine, threonine, and tyrosine residues. Evidence for the latter process, tyrosine phosphorylation, has increased substantially since this topic was last reviewed. In this review, we present a comprehensive summary and synthesis of the literature regarding the mechanism and function of ion channel regulation by protein tyrosine kinases and phosphatases. Coverage includes the majority of voltage-gated, ligand-gated, and second messenger-gated channels as well as several types of channels that have not yet been cloned, including store-operated Ca2+ channels, nonselective cation channels, and epithelial Na+ and Cl- channels. Additionally, we discuss the critical roles that channel-associated scaffolding proteins may play in localizing protein tyrosine kinases and phosphatases to the vicinity of ion channels.

Corneal epithelial wound healing

One of the important functions of the cornea is to maintain normal vision by refracting light onto the lens and retina. This property is dependent in part on the ability of the corneal epithelium to undergo continuous renewal. Epithelial renewal is essential because it enables this tissue to act as a barrier that protects the corneal interior from becoming infected by noxious environmental agents. Furthermore, the smooth optical properties of the corneal epithelial surface are sustained through this renewal process. The rate of renewal is dependent on a highly integrated balance between the processes of corneal epithelial proliferation, differentiation, and cell death. One experimental approach to characterize these three aspects of the renewal process has been to study the kinetics and dynamics of corneal re-epithelialization in a wound-healing model. This effort has employed in vivo and in vitro studies. From such studies it is evident that the appropriate integration and coordination of corneal epithelial proliferation, adhesion, migration, and cell demise is dependent on the actions of a myriad of cytokines. Our goal here is to provide an overview into how these mediators and environmental factors elicit control of cellular proliferation, adhesion, migration, and apoptosis. To this end we review the pertinent literature dealing with the receptor and the cell signaling events that are responsible for mediating cytokine control of corneal epithelial renewal. It is our hope that a better appreciation can be obtained about the complexity of the control processes that are responsible for assuring continuous corneal epithelial renewal in health and disease.

Regulation of ClC-2 chloride channels in T84 cells by TGF-α

The almost ubiquitously expressed ClC-2 chloride channel is activated by hyperpolarization and osmotic cell swelling. Osmotic swelling also activates a different class of outwardly rectifying chloride channels, and several reports point to a link between protein tyrosine phosphorylation and activation of these channels. This study examines the possibility that transforming growth factor-α (TGF-α) modulates ClC-2 activity in human colonic epithelial (T84) cells. TGF-α (0.17 nM) irreversibly inhibited ClC-2 current in nystatin-perforated whole cell patch-clamp experiments, whereas a superimposed reversible activation of the current was observed at 8.3 nM TGF-α. Both effects required activation of the intrinsic epidermal growth factor receptor (EGFR) tyrosine kinase activity, of phosphoinositide 3-kinase, and of protein kinase C. With microspectrofluorimetry of the pH-sensitive fluorescent dye 2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, TGF-α was shown to reversibly alkalinize T84 cells at 8.3 nM but not at 0.17 nM, suggesting that 8.3 nM TGF-α-induced alkalinization activates ClC-2 current. This study indicates that ClC-2 channels are targets for EGFR signaling in epithelial cells.

The essential role of H2O2 in the regulation of intracellular Ca2+ by epidermal growth factor in rat-2 fibroblasts

We have investigated a new mechanism by which epidermal growth factor (EGF) increases intracellular Ca(2+) ([Ca(2+)](i)) in Rat-2 fibroblasts. EGF induced a transient increase of [Ca(2+)](i), and sustained Ca(2+) increase disappeared in the absence of extracellular Ca(2+). However, EGF had no effect on the formation of inositol phosphates. Expression of N17Rac or scrape-loading of C3 transferase blocked the elevation of [Ca(2+)](i) by EGF, but not by lysophosphatidic acid (LPA). EGF increased intracellular H(2)O(2), with a maximal increase at 5 min, which was blocked by catalase, scrape-loading of C3 transferase, or expression of N17Rac. H(2)O(2) scavengers, catalase and N-acetyl-L-cysteine, also blocked the Ca(2+) response to EGF, but not to LPA. In the presence of EGTA, preincubation with EGF completely inhibited subsequent Ca(2+) response to extracellular H(2)O(2) and vice versa. Incubation with EGF or phosphatidic acid abolished subsequent elevation of [Ca(2+)](i) by phosphatidic acid or EGF, respectively. Furthermore, preincubation with LPA inhibited the subsequent Ca(2+) response to EGF, but not vice versa. These results suggested that intracellular H(2)O(2) regulated by Rac and RhoA, but not inositol phosphates, was responsible for the EGF-stimulated elevation of [Ca(2+)](i). It was also suggested that EGF cross talked with LPA in the regulation of [Ca(2+)](i) by producing intracellular H(2)O(2).

Acceleration of trophoblast differentiation by heparin-binding EGF-like growth factor is dependent on the stage-specific activation of calcium influx by ErbB receptors in developing mouse blastocysts

Heparin-binding EGF-like growth factor (HB-EGF) is expressed in the mouse endometrial epithelium during implantation exclusively at sites apposed to embryos and accelerates the development of cultured blastocysts, suggesting that it may regulate peri-implantation development in utero. We have examined the influence of HB-EGF on mouse trophoblast differentiation in vitro and the associated intracellular signaling pathways. HB-EGF both induced intracellular Ca2+ signaling and accelerated trophoblast development to an adhesion-competent stage, but only late on gestation day 4 after ErbB4, a receptor for HB-EGF, translocated from the cytoplasm to the apical surface of trophoblast cells. The acceleration of blastocyst differentiation by HB-EGF was attenuated after inhibition of protein tyrosine kinase activity or removal of surface heparan sulfate, as expected. Chelation of intracellular Ca2+ blocked the ability of HB-EGF to accelerate development, as did inhibitors of protein kinase C or calmodulin. The absence of any effect by a phospholipase C inhibitor and the requirement for extracellular Ca2+ suggested that the accrued free cytoplasmic Ca2+ did not originate from inositol phosphate-sensitive intracellular stores, but through Ca2+ influx. Indeed, N-type Ca2+ channel blockers specifically inhibited the ability of HB-EGF to both induce Ca2+ signaling and accelerate trophoblast development. We conclude that HB-EGF accelerates the differentiation of trophoblast cells to an adhesion-competent stage by inducing Ca2+ influx, which activates calmodulin and protein kinase C. An upstream role for ErbB4 in this pathway is implicated by the timing of its translocation to the trophoblast surface.

Activity of voltage-gated K+ channels is associated with cell proliferation and Ca2+ influx in carcinoma cells of colon cancer

Cell proliferation of carcinoma cells DLD-1 derived from colon cancer as measured by [3H] thymidine incorporation was drastically reduced in the presence of 4-aminopyridine, an inhibitors of voltage-gated K channel. A number of nonspecific K+ channel inhibitors including TPeA, TEA, verapamil and diltiazem also inhibited [3H] incorporation at the concentration reported to inhibit voltage-gated K+ channels. The presence of voltage-gated K+ channels was confirmed by reverse transcription-PCR and cDNA sequencing. Charybdotoxin and iberiotoxin, inhibitors for Ca2+-sensitive K+ channel, and glibenclamide, a specific inhibitor for ATP-sensitive K+ channel, did not have effect on cell proliferation. These experiments suggested a critical role of voltage-gated K+ channels in proliferation of colon cancer cells. Mechanism of action of K+ channel activity in cell proliferation was explored by studying the relationship between the K+ channel activity and Ca2+ entry. The results from experiments indicated that K+ channel inhibitors blocked [Ca2+]i influx. Therefore, it is likely that K+ channel activity may modulate Ca2+ influx into colon cancer cells, and subsequently modulate the proliferation of these cells.

Inhibition of EGF-dependent calcium influx by annexin VI is splice form-specific

Annexin VI is a widely expressed calcium- and phospholipid-binding protein that lacks a clear physiological role. We now report that A431 cells expressing annexin VI are defective in their ability to sustain elevated levels of cytosolic Ca(2+) following stimulation with EGF. Other aspects of EGF receptor signaling, such as protein tyrosine phosphorylation and induction of c-fos are normal in these cells. However, EGF-mediated membrane hyperpolarization is attenuated and Ca(2+) entry abolished in cells expressing annexin VI. This effect of annexin VI was only observed for the larger of the two annexin VI splice forms, the smaller splice variant had no discernable effect on either cellular phenotype or growth rate. Inhibition of Ca(2+) influx was specific for the EGF-induced pathway; capacitative Ca(2+) influx initiated by emptying of intracellular stores was unaffected. These results provide the first evidence that the two splice forms of annexin VI have different functions.

Effects of clotrimazole on the growth, morphological characteristics, and cisplatin sensitivity of human glioblastoma cells in vitro

OBJECT

Clotrimazole, an antimycotic drug, inhibits proliferation of normal and cancer cells by downregulating the movement of intracellular Ca++ and K+. The authors examined the effect of clotrimazole on the growth and sensitivity to cisplatin of two human glioblastoma cell lines--A172, which has the wild-type p53 gene, and T98G, which has the mutant p53 gene in vitro.

METHODS

The A172 and T98G glioblastoma cells were exposed to clotrimazole and cell growth was assessed using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium chloride colorimetric assay. Clotrimazole produced a dose-dependent inhibition of cell proliferation and caused changes in cellular structure toward a well-differentiated form. The growth inhibitory effect of clotrimazole was reversible. Western immunoblot analysis revealed a marked increase in cellular glial fibrillary acidic protein and wild-type p53 and a decrease in c-myc and c-fos oncoproteins in both cell lines treated with clotrimazole. Flow cytometric analysis revealed that clotrimazole-treated cells accumulated in the G0/G1 phase with a marked decrease in cells in the S phase; when clotrimazole was washed out from the culture medium, cells again started to proliferate, with a marked decrease in cells in the G0/G1 phase and an increase in cells in the S phase. The growth inhibitory effect of clotrimazole could not be overcome by exogenous stimulation with either epidermal growth factor or c-myc peptide. A combined treatment with clotrimazole and cisplatin significantly enhanced cell cytotoxicity compared with treatment using either drug alone. A DNA fragmentation assay showed that both clotrimazole and cisplatin induced apoptosis, which was increased in cells treated by both drugs.

CONCLUSIONS

The present study indicates that clotrimazole inhibits cell proliferation accompanied by morphological changes toward differentiation of glioblastoma cells and that this drug synergistically enhances the antitumor effect of cisplatin by inducing wild-type p53-mediated apoptosis.

Desensitization of the histamine H1-receptor and transcriptional down-regulation of histamine H1-receptor gene expression in bovine tracheal smooth muscle

We have investigated the role of protein kinase C (PKC) in the desensitization of histamine H1-receptors and in the expression of the histamine H1-receptor gene in airway smooth muscle. Prolonged 4beta-phorbol 12,13 dibutyrate (PDBu) pretreatment (4 h, 100 nM-1 microM) of bovine trachealis caused a concentration-dependent loss of contraction in response to histamine H1-receptor stimulation, which was associated with a concentration-dependent decrease in histamine-induced total [3H]-inositol phosphates accumulation. In contrast, the responses to sodium fluoride, a direct G-protein activator, were unalterd by PDBu (100-300 nM) pre-incubation and only slightly reduced following incubation with 1 microM PDBu. A selective PKC inhibitor, GF 109203X, partially blocked the PDBu (1 microM)-induced desensitization and completely blocked the effect of 100 nM PDBu, confirming the involvement of PKC. Binding experiments using [3H]-pyrilamine revealed a class of high-affinity binding sites within the range for the histamine H1 receptor in airway smooth muscle. PDBu (1 microM) pretreatment for 4 h did not change the number of histamine H1 receptors. PDBu (1 microM) exposure caused a time-dependent reduction in the steady-state levels of histamine H1-receptor mRNA, which was inhibited by pre-incubation with GF 109203X and by cycloheximide, a protein synthesis inhibitor. Nuclear run-on assays revealed a 50% reduction in the rate of histamine H1-receptor gene transcription after 17 h PDBu pretreatment, whereas mRNA stability was not affected by PDBu pretreatment (17 h). In conclusion, we have shown a PKC-mediated desensitization of the histamine H1-receptor in BTSM and a transcriptional down-regulation of the histamine H1-receptor gene expression, which requires new protein synthesis.

Stimulation of Ca2+ entry in lactotrophs and somatotrophs from immature rat pituitary by N-terminal fragments of proopiomelanocortin

We have previously shown that 10-12 kDa N-terminal fragments of rat proopiomelanocortin (POMC) and human POMC1-76 stimulate mitosis and/or differentiation in lactotrophs of early postnatal rat pituitary. A truncated form, POMC1-26, mimics the differentiation-inducing but not the mitogenic action of the former peptides. To further characterize these two biological responses, the present study compared changes in the intracellular free calcium concentration ([Ca2+]i) in response to POMC1-76 and POMC1-26 in isolated pituitary cells from 14-day-old female rats. Calcium (Ca2+) responses were also used as a guide to determine whether the responsive cells belong to the lactosomatotroph lineage. Application of POMC1-76 or POMC1-26 induced a maintained oscillating [Ca2+]i increase in a small population of cells. Increasing doses of the peptides did not affect the magnitude and the frequency of [Ca2+]i oscillations but clearly augmented the number of responding cells. Approximately 2% of the cells responded at 0.1 nM POMC1-76 or 5 nM POMC1-26, and 11-13% of the cells responded at 10 nM and 500 nM of the respective peptides. About one-third of the cells responsive to these peptides also showed a [Ca2+]i increase in response to growth hormone-releasing peptide-6 (GHRP-6) while, in a small number of responsive cells, [Ca2+]i was depressed by dopamine, suggesting that the former cells are somatotrophs and the latter lactotrophs. This interpretation was confirmed by immunocytochemical identification of prolactin and growth hormone (GH) in the cells. Of the cells showing Ca2+ response to POMC1-76, approximately one-third contained GH and another third prolactin. The remainder contained neither GH nor prolactin. Comparable results were obtained with POMC1-26. The rise of [Ca2+]i induced by the N-terminal POMC peptides persisted after depletion of intracellular Ca2+ stores by thapsigargin. Removal of Ca2+ from the extracellular medium or addition of cadmium completely abolished both the POMC1-76- and POMC1-26-induced [Ca2+]i increase. Nifedipine inhibited the Ca2+ response to both peptides, although only in 55% of the responsive cells. Depletion of some isoforms of protein kinase C by preincubation with the phorbol ester PMA for 24 h did not modify the Ca2+ responses. In contrast, blockade of the protein kinase A pathway with Rp-cAMPs partially inhibited the POMC1-76- or POMC1-26-induced [Ca2+]i increase. The present data show that, in immature pituitary cells, POMC1-76 induces an increase in [Ca2+]i through extracellular Ca2+ influx, possibly mediated in part by protein kinase A activation. The active domain of POMC1-76 seems to comprise its N-terminal moiety. The data support the hypothesis that POMC1-76 exerts a specific function in the development of different members of the lactosomatotroph lineage and that the peptide mobilizes different subsets of cells within this lineage, by a mechanism determined by its concentration.

Generation of reactive oxygen species in a human keratinocyte cell line: role of calcium

In the human keratinocyte cell line HaCaT, reactive oxygen species (ROS) were generated in a dose- and time-dependent manner in response to epidermal growth factor (EGF), bradykinin, thapsigargin, and the Ca(2+)-ionophore A23187, agonists that interact with different primary cell targets. ROS formation was assessed by both chemiluminescence- and fluorescence-based methods. The ROS evoked by EGF and bradykinin decayed within 8 and 4 min, respectively, this transient effect resulting probably from down-regulation of the specific agonist receptors or dissipation of the secondary signals. In contrast, the response to thapsigargin and A23187 was sustained for at least 15 min. Extracellular Ca2+ and a rise in intracellular Ca2+ concentration ([Ca2+]i) proved essential for ROS production. Chelation by BAPTA suppressed ROS formation. Direct measurement of [Ca2+]i using fura fluorescence revealed that EGF and bradykinin evoked a modest, transient [Ca2+]i elevation of less than twofold, whereas with thapsigargin and A23187 there was a sustained two- to fourfold elevation. For each agonist, the kinetics of the rise and decay of [Ca2+]i were similar to those of ROS. The enzyme(s) involved in ROS formation were inhibited by diphenyleneiodonium, indicating dependence on FAD. Our results suggest a close link between ROS and changes in [Ca2+]i generated by growth factors and hormones. This is a particularly interesting connection because elevation of ROS and/ or [Ca2+]i has been linked to cell proliferation, differentiation, and apoptosis.

Cloning and expression of a novel mammalian homolog of Drosophila transient receptor potential (Trp) involved in calcium entry secondary to activation of receptors coupled by the Gq class of G protein

Hormonal stimulation of Gq-protein coupled receptors triggers Ca2+ mobilization from internal stores. This is followed by a Ca2+ entry through the plasma membrane. Drosophila Trp and Trpl proteins have been implicated in Ca2+ entry and three mammalian homologues of Drosophila Trp/Trpl, hTrp1, hTrp3 and bTrp4 (also bCCE) have been cloned and expressed. Using mouse brain RNA as template, we report here the polymerase chain reaction-based cloning and functional expression of a novel Trp, mTrp6. The cDNA encodes a protein of 930 amino acids, the sequence of which is 36.8, 36.3, 43.1, 38.6, and 74. 1% identical to Drosophila Trp and Trpl, bovine Trp4, and human Trp1 and Trp3, respectively. Transient expression of mTrp6 in COS.M6 cells by transfection of the full-length mTrp6 cDNA increases Ca2+ entry induced by stimulation of co-transfected M5 muscarinic acetylcholine receptor with carbachol (CCh), as seen by dual wavelength fura 2 fluorescence ratio measurements. The mTrp6-mediated increase in Ca2+ entry activity was blocked by SKF-96365 and La3+. Ca2+ entry activity induced by thapsigargin was similar in COS cells transfected with or without the mTrp6 cDNA. The thapsigargin-stimulated Ca2+ entry could not be further stimulated by CCh in control cells but was markedly increased in mTrp6-transfected cells. Records of whole cell transmembrane currents developed in response to voltage ramps from -80 to +40 mV in control HEK cells and HEK cells stably expressing mTrp6 revealed the presence of a muscarinic receptor responsive non-selective cation conductance in Trp6 cells that was absent in control cells. Our data support the hypothesis that mTrp6 encodes an ion channel subunit that mediates Ca2+ entry stimulated by a G-protein coupled receptor, but not Ca2+ entry stimulated by intracellular Ca2+ store depletion.

Modulation of the Kv1.3 potassium channel by receptor tyrosine kinases

The voltage-dependent potassium channel, Kv1.3, is modulated by the epidermal growth factor receptor (EGFr) and the insulin receptor tyrosine kinases. When the EGFr and Kv1.3 are coexpressed in HEK 293 cells, acute treatment of the cells with EGF during a patch recording can suppress the Kv1.3 current within tens of minutes. This effect appears to be due to tyrosine phosphorylation of the channel, as it is blocked by treatment with the tyrosine kinase inhibitor erbstatin, or by mutation of the tyrosine at channel amino acid position 479 to phenylalanine. Previous work has shown that there is a large increase in the tyrosine phosphorylation of Kv1.3 when it is coexpressed with the EGFr. Pretreatment of EGFr and Kv1.3 cotransfected cells with EGF before patch recording also results in a decrease in peak Kv1.3 current. Furthermore, pretreatment of cotransfected cells with an antibody to the EGFr ligand binding domain (alpha-EGFr), which blocks receptor dimerization and tyrosine kinase activation, blocks the EGFr-mediated suppression of Kv1.3 current. Insulin treatment during patch recording also causes an inhibition of Kv1.3 current after tens of minutes, while pretreatment for 18 h produces almost total suppression of current. In addition to depressing peak Kv1.3 current, EGF treatment produces a speeding of C-type inactivation, while pretreatment with the alpha-EGFr slows C-type inactivation. In contrast, insulin does not influence C-type inactivation kinetics. Mutational analysis indicates that the EGF-induced modulation of the inactivation rate occurs by a mechanism different from that of the EGF-induced decrease in peak current. Thus, receptor tyrosine kinases differentially modulate the current magnitude and kinetics of a voltage-dependent potassium channel.

Effects of epidermal growth factor on isolated digestive gland cells from mussels

Effects of epidermal growth factor (EGF) and possible mechanisms of EGF-mediated signal transduction were investigated in isolated cells of the digestive gland of the mussel (Mytilus galloprovincialis Lam. ). EGF induced a cytosolic Ca2+ transient and subsequently stimulated DNA synthesis; both effects were dose-dependent in the nanomolar range and inhibited by pretreatment with an inhibitor of tyrosine kinase activity, suggesting specific EGFR-like receptors. The EGF-induced cytosolic Ca2+ transient was mainly due to a Ca2+ influx through the plasma membrane, possibly involving voltage-insensitive Ca2+ channels. Such a Ca2+ response was abolished by pretreatment with indomethacin and NDGA, inhibitors of arachidonic acid metabolism; similarly, the EGF-stimulated increase in DNA synthesis was significantly reduced. Indomethacin, a cyclooxygenase inhibitor, had the greatest effect on both EGF-induced responses. Results suggest the presence of EGF-responsive cells in the mussel digestive gland. A possible role for arachidonic acid and its metabolites in mediating the effects of EGF is also indicated.

IGF-1 modulates N and L calcium channels in a PI 3-kinase-dependent manner

Receptor tyrosine kinases (RTKs) have long been associated with proliferation in non-neural cells, although they are also expressed in postmitotic neurons. We demonstrate that insulin-like growth factor-1 (IGF-1) induces within seconds a large, tyrosine-kinase-dependent increase in calcium channel currents in cerebellar granule neurons. Separation of channel subtypes reveals that, while P, Q, and R channels are unaffected, N and L channel activities are strongly potentiated at specific membrane voltages: N currents triple at depolarized potentials, while L currents rapidly increase 4-fold at hyperpolarized potentials. Moreover, transient expression of dominant-negative and wild-type phosphatidylinositol 3-OH kinase (PI 3-kinase) subunits, as well as application of specific inhibitors, demonstrates that PI 3-kinase is an essential and rate-limiting messenger in this signaling pathway. Our results indicate that N and L calcium channels are downstream targets of neuronal RTKs and suggest that RTK modulation may control calcium-dependent processes, such as neurotransmitter release and IGF-1-dependent differentiation or survival.

Transforming growth factor alpha treatment alters intracellular calcium levels in hair cells and protects them from ototoxic damage in vitro

To determine if transforming growth factor alpha (TGF alpha) pretreatment protects hair cells from aminoglycoside induced injury by modifying their intracellular calcium concentration, we assayed hair cell calcium levels in organ of Corti explants both before and after aminoglycoside (i.e. neomycin, 10(-3) M) exposure either with or without growth factor pretreatment. After TGF alpha (500 ng/ml) treatment, the intracellular calcium level of hair cells showed a five-fold increase as compared to the levels observed in the hair cells of control cultures. After ototoxin exposure, calcium levels in hair cells of control explants showed an increase relative to their baseline levels, while in the presence of growth factors pretreatment, hair cells showed a relative reduction in calcium levels. Pretreatment of organ of Corti explants afforded significant protection of hair cell stereocilia bundle morphology from ototoxic damage when compared to explants exposed to ototoxin alone. This study correlates a rise in hair cell calcium levels with the otoprotection of hair cells by TGF alpha in organ of Corti explants.

Salivary mucins in oral mucosal defense

1. Salivary mucins are well recognized as an important factor in the preservation of the health of the oral cavity. These large glycoproteins play a major role in the formation of protective coatings covering tooth enamel and oral mucosa, which act as a dynamic functional barrier capable of modulating the untoward effects of oral environment, and are of significance to the processes occurring within the epithelial perimeter of mucosal defense. 2. Based on macromolecular characteristics, the mucins in saliva fall into high (> 1000 kDa) and low (200-300 kDa) molecular weight forms. The two forms, although differ with respect to bacterial clearance ability, display virtually identical carbohydrate chain make-up, ranging in size from 3 to 16 sugar units. 3. Of the two mucin forms, the low molecular weight form more efficient in bacterial aggregation, predominates in saliva and oral mucosal mucus coat of caries-resistant individuals, while the level of the high molecular weight form is higher in caries-susceptible subjects. The saliva of caries-resistant individuals also exhibits greater activity of protease capable of conversion of the high molecular weight mucin to the low molecular weight form. 4. The bacterial aggregating activity of salivary mucins appears to be associated with sulfomucins rather than sialomucins. While the removal of sialic acid causes only partial loss in mucin aggregating capacity, a complete loss in the bacterial aggregating activity occurs following mucin desulfation. 5. The mucins in oral mucosal mucus coat interact with the epithelial surfaces through specific membrane receptors. This interaction apparently involves the carbohydrate moiety of mucin molecule and may be rendered vulnerable to disruption by opportunistic bacteria colonizing the oral mucosa. 6. Salivary sulfo- and sialomucins actively participate in the modulation of the oral mucosal calcium channel activity through the inhibition of EGF-stimulated channel protein tyrosine phosphorylation. This function of salivary mucins is of paramount importance to mucosal calcium homeostasis.

Rac-dependent and -independent pathways mediate growth factor-induced Ca2+ influx

We report that expressing interfering mutants of the small Ras-related GTPase Rac, using either recombinant vaccinia virus or stable DNA transfection, eliminates epidermal growth factor-induced Ca2+ signaling, without affecting Ca2+ mobilization or influx from G protein-coupled receptors. Platelet-derived growth factor-dependent Ca2+ influx, however, is only partly sensitive to dominant negative Rac proteins. Thus, whereas epidermal growth factor-induced Ca2+ influx is completely mediated by Rac proteins, platelet-derived growth factor-induced Ca2+ influx involves Rac-dependent and -independent signaling pathways.

Differential regulation of Ca2+ release-activated Ca2+ influx by heterotrimeric G proteins

The least understood aspect of the agonist-induced Ca2+ signal is the activation and regulation of the Ca2+ release-activated Ca2+ influx (CRAC) across the plasma membrane. To explore the possible role of heterotrimeric G proteins in the various regulatory mechanisms of CRAC, continuous renal epithelial cell lines stably expressing alpha 13 and the constitutively active alpha qQ209L were isolated and used to measure CRAC activity by the Mn2+ quench technique. Release of intracellular Ca2+ by agonist stimulation or thapsigargin was required for activation of CRAC in all cells. Although the size of the internal stores was similar in all cells, CRAC was 2-3-fold higher in alpha 13- and alpha qQ209L-expressing cells. However, the channel was differentially regulated in the two cell types. Incubation at low [Ca2+]i, inhibition of the NOS pathway, or inhibition of tyrosine kinase inhibited CRAC activity in alpha 13 but not alpha qQ209L cells. Treatment with okadaic acid prevented inhibition of the channel by low [Ca2+]i and the protein kinase inhibitors in alpha 13 cells. These results suggest that expression of alpha qQ209L dominantly activates CRAC by stabilizing a phosphorylated state, whereas expression of alpha 13 makes CRAC activation completely dependent on phosphorylation by several kinases. G proteins may also modulate CRAC activity independently of the phosphorylation/dephosphorylation state of the pathway to increase maximal CRAC activity. Furthermore, our results suggest a general mechanism for regulation of CRAC that depends on coupling of receptors to specific G proteins.

Role of tyrosine phosphorylation in potassium channel activation. Functional association with prolactin receptor and JAK2 tyrosine kinase

Chinese hamster ovary (CHO) cells, stably transfected with the long form of the prolactin (PRL) receptor (PRL-R) cDNA, were used for PRL-R signal transduction studies. Patch-clamp technique in whole cell and cell-free configurations were employed. Exposure of transfected CHO cells to 5 nM PRL led to the increase of Ca(2+)- and voltage-dependent K+ channel (KCa) activity. The effect was direct as it was observed also in excised patch experiments. A series of tyrosine kinase inhibitors was studied to investigate the possible involvement of protein tyrosine kinases in KCa functioning and its stimulation by PRL. Genistein, lavendustin A, and herbimycin A decreased in a concentration and time-dependent manner the amplitude of the KCa current in whole cell and the open probability of KCa channels in cell-free experiments. The subsequent application of PRL was ineffective. The protein tyrosine phosphatase inhibitor orthovanadate (1 mM) stimulated KCa channel activity in excised patches, indicating that channels can be modulated in opposite directions by protein tyrosine kinase and protein tyrosine phosphatase. Moreover, in whole cell experiments as well as in excised patch recordings, anti-JAK2 tyrosine kinase antibody decreased the KCa conductance and the open probability of the KCa channels. Subsequent application of PRL was no longer able to stimulate KCa conductance. Immunoblotting studies using the same anti-JAK2 antibody, revealed the constitutive association of JAK2 kinase with PRL-R. Preincubation of anti-JAK2 antibody with the JAK2 Immunizing Peptide abolished the effects observed using anti-JAK2 antibody alone in both electrophysiological and immunoblotting studies. We conclude from these findings that these KCa channels are regulated through tyrosine phosphorylation/dephosphorylation; JAK2 tyrosine kinase, constitutively associated with PRL-R, is implicated in PRL stimulation of KCa channels.

Epidermal growth factor (EGF) induces serine phosphorylation-dependent activation and calcium-dependent translocation of the cytosolic phospholipase A2

Phospholipase A2 (PLA2) is a key enzyme in the release of arachidonic acid and subsequent production of eicosanoids, which play an important role in a variety of biological processes, including mitogenic signalling by epidermal growth factor (EGF). In a previous study [Spaargaren, M. et al. (1992) Biochem J. 287, 37-43] we identified the EGF-activated PLA2 as being similar to the recently cloned high-molecular-mass cytosolic phospholipase A2 (cPLA2). In the present study we demonstrate a rapid transient EGF-induced activation of this cPLA2 and an EGF-induced increase in phosphorylation of the cPLA2. The EGF-induced activation of cPLA2 is reversed upon phosphatase treatment showing phosphorylation-dependent activation of the cPLA2. No direct association of the cPLA2 to the EGF receptor was detected under conditions where such an association with phospholipase C-gamma was demonstrated. Phosphoamino acid analysis of this cPLA2 showed that EGF induced an increase in serine phosphorylation exclusively, no tyrosine phosphorylation being observed. EGF treatment of the cells resulted in a Ca(2+)-dependent translocation of the cPLA2 from the cytosol to the membrane fraction. This is due to an EGF-induced [Ca2+]i rise which is dependent on the influx of extracellular Ca2+ via voltage-independent Ca2+ channels. It is shown that the Ca(2+)-dependent association of cPLA2 to membranes does not require accessory membrane molecules.

Activation of Ca2+ influx by transforming Ha-ras

The effect of an induction of transforming Ha-ras on Ca2+ influx into NIH3T3 cells was studied employing Fura-2 quenching by Mn2+. The expression of transforming p21Ha-ras caused a significant increase in Mn2+ influx which was blocked by Cd2+, La3+, niguldipine and the Ca(2+)-channel blocker SK&F96365. This effect was specific for transforming Ha-ras and was not seen after overexpression of the Ha-ras proto-oncogene or v-mos. In addition to the enhanced Mn2+ influx, transforming p21Ha-ras elicited an increased efflux of the K(+)-congener 86Rb+ which was inhibitable by Ca(2+)-channel blockers and charybdotoxin, a selective inhibitor of high and intermediate conductance Ca(2+)-dependent K+ channels. Charybdotoxin did not reduce the increase in Mn2+ influx by ras, demonstrating that the activation of Ca(2+)-dependent K+ channels was not required for the sustained Mn2+/Ca2+ influx in the presence of transforming Ha-ras. In ras-expressing cells, the bradykinin-induced Mn2+ influx and charybdotoxin sensitive 86Rb+ efflux were markedly potentiated. The increase in the inositol- 1,4,5-trisphosphate and inositol-1,3,4,5-tetrakisphosphate levels by ras is not sufficient to explain the elevated Mn2+ influx. The mitogenic response to an expression of transforming Ha-ras was inhibited by the Ca(2+)-channel blockers not, however, by charybdotoxin. These data suggest the existence of an agonist-independent activation of a receptor- or second messenger-operated Ca2+ channel by transforming Ha-ras which is necessary for the mitogenic response to the activation of the oncogene.

Basic fibroblast growth factor activates calcium channels in neonatal rat cardiomyocytes

Basic fibroblast growth factor (bFGF) is a potent mitogen for many cell lineages including fetal cardiomyocytes. Furthermore, bFGF has been shown to modify gene expression, in vitro, in adult nonproliferative ventricular myocytes. This effect is suspected to be partly responsible for the genetic modifications that occur in vivo under pathophysiological conditions such as ischemia or pressure overload and that lead to myocardial hypertrophy. However, little is known about the first steps of the molecular mechanisms that take place soon after cell activation by bFGF. In this study, using biochemical and electrophysiological approaches, we have established, on cardiomyocytes cultured from neonatal rat ventricles, that (i) differentiated beating cells express at least two classes of bFGF-receptors having high and low affinity (Kd = 10 +/- 2 pM and 1 +/- 0.5 nM); (ii) the stimulation of these bFGF receptors promotes an increase in the beating frequencies of cultured cardiomyocytes (40 +/- 10%); (iii) bFGF provokes the activation of poorly specific and voltage-independent calcium channels (12pS); (iv) inositol 1,4,5-trisphosphate enhances similar bFGF-induced Ca2+ currents and is therefore suspected to be a second messenger triggering this activation. These results support the presence, in cultured cardiomyocytes, of new calcium channels whose activation after bFGF binding may be partly responsible for the cell response to this growth factor.

The role of calcium influx in cellular proliferation induced by interaction of endogenous ganglioside GM1 with the B subunit of cholera toxin

The B subunit of cholera toxin, which binds specifically to ganglioside GM1, is mitogenic for quiescent Swiss 3T3 fibroblasts. It was previously shown that the B subunit had no effect on cAMP, protein kinase C or phosphoinositide turnover, but did cause an increase in the influx of calcium from extracellular sources (Spiegel, S. and Panagiotopoulos, C. (1988) Exp. Cell Res. 177, 414-427). In contrast to the action of known growth factors, the B subunit induced significant DNA synthesis after only a 1-3 h treatment. We utilized this unique property to determine whether the increase in calcium influx plays a role in B subunit-induced mitogenicity. Cells were briefly treated with the B subunit in the presence of calcium channel blockers, followed by removal of the blockers and further incubation in B subunit-free medium for the remaining time required to measure DNA synthesis. When 1 mM cobalt was only present during the first 3 h incubation. DNA synthesis induced by either the B subunit or fetal bovine serum was completely abolished. However, both nickel (1 mM) adn the L-type voltage-gated calcium channel inhibitor nicardipin (10 microM) inhibited B subunit-induced cell proliferation without abrogating the response to fetal bovine serum. Using a gel retardation assay, we found that the B subunit markedly stimulated specific DNA-binding activity of the transcription factor, activator protein-1 (AP-1), which functions as a major convergence point coupling early events induced by a variety of mitogens to long term growth responses. Presence of c-Fos protein in the AP-1 complex was demonstrated as a supershift band in the gel mobility assay using c-Fos polyclonal antibody. Cobalt, which markedly inhibited B subunit-induced DNA synthesis, also completely abolished AP-1 DNA-binding activity stimulated by the B subunit. In sharp contrast, cobalt had no effect on DNA-binding activity of AP-1 induced by the tumor promoter, 12-O-tetradecanoylphorbol 13-acetate. Our results suggest that calcium influx is a key element for both DNA-binding activity of AP-1 and cell proliferation induced by binding of the B subunit of cholera toxin to cell surface ganglioside GM1.

Clotrimazole inhibits cell proliferation in vitro and in vivo

Cell proliferation is critically dependent on the regulated movement of ions across various cellular compartments. The antimycotic drug clotrimazole (CLT) has been shown to inhibit movement of Ca2+ and K+ across the plasma membrane. Our results show that CLT inhibits the rate of cell proliferation of normal and cancer cell lines in a reversible and dose-dependent manner in vitro. Moreover, CLT depletes the intracellular Ca2+ stores and prevents the rise in cytosolic Ca2+ that normally follows mitogenic stimulation. In mice with severe combined immunodeficiency disease (SCID) and inoculated intravenously with MM-RU human melanoma cells, daily subcutaneous injections of CLT induced a significant reduction in the number of lung metastases. Modulation of early ionic mitogenic signals and potent inhibition of cell proliferation both in vitro and in vivo are new and potentially useful clinical effects of CLT.

Role of heterotrimeric G-proteins in epidermal growth factor signalling

Since in 1986 it was reported that a pertussis toxin-sensitive substrate was involved in the Ca2+ signal induced by epidermal growth factor (EGF) in rat hepatocytes, much evidence accumulated to implicate heterotrimeric G-proteins in EGF action. EGF can also induce a cyclic AMP signal, but while the generation of a Ca2+ signal appears to be quite general in EGF action, the increase in cyclic AMP occurs only in few cell types. In non-transformed cell types these effects appear to involve G-proteins. EGF not only induces cell proliferation but also interacts with hormones in the short-term control of cell function in quiescent cells. Most of the known interactions are on cyclic AMP mediated hormone effects, and in many cases, the interaction between EGF and hormones involves G-proteins. Here we review the evidence accumulated in recent years that implicate G-proteins in EGF action. An understanding of the mechanisms involved may reveal new mechanisms of G-protein regulation and will contribute to our knowledge of EGF function and signal transduction.

Evaluation of single-channel gating kinetics produced after amplitude-based separation of unitary currents

Cell-attached patch-clamp recording has been established as a major technique for investigating ion channel behaviour in a physiological setting, despite the problems which arise in analysing records containing more than one type of unitary current. To circumvent these problems, single-channel amplitude-based assignment of discrete single-channel events to different channel types becomes increasingly necessary. Surprisingly, a systematic evaluation of the validity of this method in determining single-channel parameters has not been performed to date. Using computer-stimulated single-channel traces, and recordings from a biological preparation containing well-characterized ion channels (N1E-115 neuroblastoma cells), we have explored the accuracy by which amplitude-based separation recovers ion-channel parameters. Determination of gating kinetics after separation revealed that even a very small contamination in the selected population yields additional time constants in the probability density functions. Therefore, our results demonstrate that, whereas the use of amplitude-based separation is straightforward for determining slope conductance and reversal potential, it is prone to incorporate errors in establishing gating kinetics. Ways of identifying such errors are described.

Role of epidermal growth factor-induced membrane depolarization and resulting calcium influx in osteoblastic cell proliferation

This study investigated the effects of epidermal growth factor (EGF) on the membrane potential of rat calvarial osteoblasts, in order to understand the mechanism responsible for calcium influx and the role these EGF-induced events have in osteoblastic cell proliferation. Changes in plasma membrane potential were measured using patch clamp techniques in isolated cells. EGF induced changes in plasma membrane potential only after cells had been in culture for at least 6 days. EGF induced membrane depolarization in 55% of rat calvarial osteoblasts studied after 6 to 8 days in culture. This membrane event was dependent on extracellular calcium, therefore, one or more calcium conductances were involved. Nifedipine, a voltage-activated calcium channel blocker, significantly reduced membrane depolarization, and demonstrated the existence of a nifedipine-insensitive conductance. Osteoblastic cell proliferation was measured by cell count. The EGF-dependent increase in cell proliferation was blocked by addition of 10 microM nifedipine. Therefore, it appears that the mechanism of action of EGF-induced osteoblastic cell proliferation is mediated by changes in plasma membrane potential which result in extracellular calcium influx.

Inhibition of ATP-sensitive potassium channels causes reversible cell-cycle arrest of human breast cancer cells in tissue culture

The purpose of this study was to determine if potassium channel activity is required for the proliferation of MCF-7 human mammary carcinoma cells. We examined the sensitivities of proliferation and progress through the cell cycle to each of nine potassium channel antagonists. Five of the potassium channel antagonists produced a concentration-dependent inhibition of cell proliferation with no evidence of cytotoxicity following a 3-day or 5-day exposure to drug. The IC50 values for these five drugs, quinidine (25 microM), glibenclamide (50 microM), linogliride (770 microM), 4-aminopyridine (1.6 mM), and tetraethylammonium (5.8 mM) were estimated from their respective concentration-response curves. Four other potassium channel blockers were tested at supra-maximal channel blocking concentrations, including charybdotoxin (200 nM), iberiotoxin (100 nM), margatoxin (10 nM), and apamin (500 nM), and they had no effect on MCF-7 cell proliferation, viability, or cell cycle distribution. Of the five drugs that inhibited proliferation, only quinidine, glibenclamide, and linogliride also affected the cell cycle distribution. Cell populations exposed to each of these drugs for 3 days showed a statistically significant accumulation in G0/G1 phase and a significant proportional reduction in S phase and G2/M phase cells. The inhibition of cell proliferation correlated significantly with the extent of cell accumulation in G0/G1 phase and the threshold concentrations for inhibition of growth and G0/G1 arrest were similar. The G0/G1 arrest produced by quinidine and glibenclamide were reversed by removing the drug, and cells released from arrest entered S phase synchronously with a lag period of approximately 24 hours. Based on the differential sensitivity of cell proliferation and cell cycle progression to the nine potassium channel antagonists, we conclude that inhibition of ATP-sensitive potassium channels in these human mammary carcinoma cells, reversibly arrests the cells in the G0/G1 phase of the cell cycle, resulting in an inhibition of cell proliferation.