Expression and Role of the Intermediate-Conductance Calcium-Activated Potassium Channel KCa3.1 in GlioblastomaJournalsPublish with usPublishing partnershipsAbout usBlogJournal of Signal Transduction+Journal MenuPDFTable of ContentsSpecial IssuesSubmitJournal of Signal Transduction / 2012 / ArticleArticle SectionsOn this pageAbstractIntroductionReferencesCopyrightSpecial IssueSignal Transduction Alterations in Glioma: Implications for Diagnosis and TherapyView this Special IssueReview Article | Open AccessVolume 2012 |Article ID 421564 | https://doi.org/10.1155/2012/421564Luigi Catacuzzeno, Bernard Fioretti, Fabio Franciolini, Expression and Role of the Intermediate-Conductance Calcium-Activated Potassium Channel KCa3.1 in Glioblastoma , Journal of Signal Transduction, vol. 2012, Article ID 421564, 11 pages, 2012. https://doi.org/10.1155/2012/421564Show citationExpression and Role of the Intermediate-Conductance Calcium-Activated Potassium Channel KCa3.1 in GlioblastomaLuigi Catacuzzeno,1 Bernard Fioretti,1 and Fabio Franciolini11Dipartimento di Biologia Cellulare e Ambientale, Universita’ di Perugia, Via Pascoli 1, I-06123 Perugia, ItalyShow moreAcademic Editor: Laura CerchiaReceived05 Feb 2012Accepted15 Mar 2012Published17 May 2012AbstractGlioblastomas are characterized by altered expression of several ion channels that have important consequences in cell functions associated with their aggressiveness, such as cell survival, proliferation, and migration. Data on the altered expression and function of the intermediate-conductance calcium-activated K (KCa3.1) channels in glioblastoma cells have only recently become available. This paper aims to (i) illustrate the main structural, biophysical, pharmacological, and modulatory properties of the KCa3.1 channel, (ii) provide a detailed account of data on the expression of this channel in glioblastoma cells, as compared to normal brain tissue, and (iii) critically discuss its major functional roles. Available data suggest that KCa3.1 channels (i) are highly expressed in glioblastoma cells but only scantly in the normal brain parenchima, (ii) play an important role in the control of glioblastoma cell migration. Altogether, these data suggest KCa3.1 channels as potential candidates for a targeted therapy against this tumor.1. IntroductionGlioblastomas are the most common and aggressive among primary brain tumors. In spite of the intensive basic and clinical studies, only minor successes have been witnessed over the last decades. One-third of patients keep surviving no longer than one year from diagnosis, and average life expectancy remains dismal (12–15 months), even when radical surgical resection, chemo- and radiotherapy can be applied. The major problem with glioblastomas is their highly migratory and invasive potential into the normal brain tissue that prevents complete surgical removal of tumor cells and the extreme resistance of these cells to standard treatments [1]. To worsen the outcome of the disease is the presence in the tumor mass of a recently identified subpopulation of highly tumorigenic stem-like glioblastoma cells possessing even more invasive power, chemo- and radio-resistance than nonstem tumor cells, that are also thought to be responsible for the commonly observed tumor relapses [2–4].Glioblastomas are characterized by a large number and variety of genetic mutations that heavily disregulate the major signaling pathways controlling cell survival, proliferation, differentiation, and invasion [5]. Among the disregulated pathways found in glioblastoma cells there are those controlling the expression of ion channels, transmembrane proteins endowed with a permeation pore that allows the passage of ions. Usually ion channels are selectively permeable to one particular ion and can open and close their permeation pore in response to chemical and physical stimuli, such as neurotransmitters, modulators, and changes in the membrane potential [6]. Ion channels have been found to be involved in several cellular functions, hallmarks of cancer cell aggressiveness, such as proliferation, apoptosis, and migration. In most cases their contribution consists in regulating two important cellular parameters, the cell volume and the intracellular Ca2+ concentration ([Ca2+]i) [7, 8].By allowing the movement of K and Cl ions through the plasmamembrane, and the osmotically driven water flux, ion channels critically control the changes of cell volume that are functionally relevant for glioblastoma cells. For example, a premitotic volume condensation (PVC) is required for glioblastoma cells to switch from a bipolar into a round cell morphology just prior cell division. Notably, this process requires the opening of Cl-selective ClC-3 channels, that are markedly upregulated in glioblastoma cells as compared to healthy astrocytes [9–12]. Similarly, a cell volume reduction, the so-called apoptotic volume decrease (AVD), was observed during the staurosporine- or TRAIL (TNF-alpha-related apoptosis inducing ligand)-induced apoptosis of glioblastoma cells, and also in this case it was found to be sustained by a Cl channel flux, being prevented by inhibitors of Cl channels [13]. Cell migration and invasion through the narrow extracellular spaces of the brain parenchyma also require major changes in cell volume. These processes in addition to the ClC-3 channels discussed above require the activity of Ca2+-activated K-selective BK channels, likewise markedly upregulated in glioblastoma cells as compared to healthy astrocytes [14–16].The important role of the Ca2+ signals in the development of glioblastoma has recently been reviewed [17]. Notably, ion channels play a critical role to this regard; besides sustaining directly the Ca2+ influxes (through Ca2+-permeable channels) they can influence the entry of extracellular Ca2+ ions by modulating the membrane potential that controls the driving force for Ca2+ influx. Ca2+ influx through the TRPC family of Ca2+-permeable channels has indeed been shown to modulate glioblastoma cell cycle progression [18–20] and to induce a CaMKII-dependent activation of ClC-3 during premitotic volume condensation [12]. In addition, glioblastoma cell migration has been shown to be accompanied by intracellular Ca2+ oscillations that are instrumental to promote the kinase-dependent detachment of focal adhesions during cell rear retraction [21, 22], and these intracellular Ca2+ oscillations can be significantly affected by the membrane hyperpolarization determined by the activity of K channels [23].Perhaps the best suited ion channels to play a role in tumor development are the Ca2+-activated K (KCa) channels, as they are at the cell crossroad where Ca2+ influx, membrane potential, and outward ion fluxes, all processes governed by KCa channels, integrate to modulate a large array of cellular processes [24]. KCa channels are subdivided into three major classes according to their single channel conductance: large conductance (150–300 pS) K channels (BKCa or KCa1), small conductance (2–20 pS) K channels (SK or KCa2.1, KCa2.2, KCa2.3), and intermediate conductance (20–60 pS) K channels (IKCa or KCa3.1). Each subclass has specific biophysical and pharmacological properties that allow to identify them. KCa1 channels, encoded by the Kcnma1 gene, are broadly expressed in various tissues. They are regulated by cytoplasmic Ca2+ but also by membrane potential. In the absence of Ca2+, KCa1 channels can be activated only with extreme (nonphysiological) depolarizations. Elevations in cytoplasmic [Ca2+] shift the range of activating voltages to more negative potentials. Near resting potentials, the EC50 of the KCa1 is in the micromolar range. Paxilline, iberiotoxin, and low concentrations of tetraethyl ammonium are potent and specific inhibitors of the KCa1 channel. The KCa2.x channels are voltage independent but more sensitive to Ca2+ (EC50 in submicromolar range) due to the presence of calmodulin associated with the C-terminus that works as Ca2+ sensor. Apamine, but not paxilline or iberiotoxin, can selectively block the KCa2.x channels. The KCa3.1 channels, like the KCa2.x channels, are voltage independent but gated by intracellular Ca2+ that binds to calmodulin and opens the channel. Clotrimazole and its derivative TRAM-34 are potent inhibitors of the KCa3.1 channels, discriminating them from other KCa channels.KCa3.1 channels are expressed in a variety of normal and tumor cells, where they participate in important cell functions such as cell cycle progression, migration, and epithelial transport, by controlling the cell volume and the driving force for Ca2+ influx [25–27]. Here we review the major progresses that have led to our present understanding of the expression and role of the KCa3.1 channels in glioblastoma.2. General Properties of the KCa3.1 ChannelThe KCa3.1 channel has the overall architecture of the voltage-gated K (Kv) channel superfamily, with four subunits, each containing six transmembrane domains (S1–S6) and a pore domain (P loop) located between S5 and S6. The S4 domain, which confers voltage sensitivity to the Kv channels, shows in KCa3.1 channels only two positively charged aminoacids, as compared to the 4–7 charged residues of voltage-gated K channels. Channel activation is, therefore, voltage independent. The KCa3.1 channel is gated instead by the binding of intracellular Ca2+ to calmodulin, a Ca2+-binding protein that is constitutively associated with the C terminus of each channel subunit [28–30]. This Ca2+-dependent gating is similar to that displayed by the KCa2.x channel family but distinct from KCa1 channels, where the Ca2+-dependent module is intrinsic to the channel α subunit [24]. Patch-clamp experiments in several cell types, including glioblastoma, give IC50s for KCa3.1 channel activation by Ca2+ of 200–400 nM [31, 32], consistent with those found for the cloned channel [33–35]. The high Ca2+ sensitivity of the KCa3.1 channel allows its activation by submicromolar Ca2+ levels, easily reached upon Ca2+ release from intracellular stores or influx through Ca2+ permeable channels. A four-state gating scheme was proposed for KCa3.1 channels, with Ca2+-dependent transitions dependent on the [Ca2+]i in a nonlinear manner [36]. This peculiarity, not shared by the KCa2.x channel family [37], is related to the channel behaviour at saturating [Ca2+]i, as elevated divalent concentrations have been reported to block the channel [36, 38]. The most studied KCa3.1 mRNA is the 2.1 kb form, but other transcripts have been reported in humans [34, 35]. Three distinct Kcnn4 cDNAs that are designated as Kcnn4a, Kcnn4b, and Kcnn4c encoding 425, 424, and 395 aminoacid proteins, respectively, were isolated from the rat colon, and several differences in the functional expression and pharmacological properties of the different isoforms were found [39].The KCa3.1 channels are target for several inhibitory and activatory agents (for an exhaustive review see [40]). Two structurally distinct groups of KCa3.1 channel blockers, peptidic and nonpeptidic, have been found which also differ for their binding site on the channel protein. Among the peptidic blockers, maurotoxin and charybdotoxin display the strongest potency. Maurotoxin, is a 34-aminoacid toxin cross-linked by four disulfide bridges [41]. Lys23 of the toxin binds to the pore filter of the channel from the extracellular side, and a