BK channel blockers inhibit potassium-induced proliferation of human astrocytoma cells

Bioactive peptides from scorpion venoms: therapeutic scaffolds and pharmacological tools

Evolution and natural selection have endowed animal venoms, including scorpion venoms, with a wide range of pharmacological properties. Consequently, scorpions, their venoms, and/or their body parts have been used since time immemorial in traditional medicines, especially in Africa and Asia. With respect to their pharmacological potential, bioactive peptides from scorpion venoms have become an important source of scientific research. With the rapid increase in the characterization of various components from scorpion venoms, a large number of peptides are identified with an aim of combating a myriad of emerging global health problems. Moreover, some scorpion venom-derived peptides have been established as potential scaffolds helpful for drug development. In this review, we summarize the promising scorpion venoms-derived peptides as drug candidates. Accordingly, we highlight the data and knowledge needed for continuous characterization and development of additional natural peptides from scorpion venoms, as potential drugs that can treat related diseases.

Role for calcium-activated potassium channels (BK) in migration control of human hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is a leading cause of cancer‐related death worldwide. Its high metastasis rate is significantly correlated with poor patient prognosis. Elucidating the molecular mechanism underlying HCC metastasis is essential for HCC treatment. Owing to their high conductance, large‐conductance calcium‐activated potassium channels (BK channels) play a critical role in the control of membrane potential and have repeatedly been proposed as potential targets for cancer therapy. Emerging evidence suggests that BK channels are involved in the progression of cancer malignancies. The present study investigated the role of BK channels in mediating the hypoxia‐stimulated migration of HCC cells both in vitro and in vivo in the absence and presence of various BK channels modulators. We found that BK channels were functionally expressed on the membranes of the SMMC‐7721 and Huh7 HCC cell lines. Furthermore, blockage or activation of BK channels on the surface of HCC cells correspondingly inhibited or promoted HCC cell proliferation, migration and invasion in hypoxia conditions, with altered expression and distribution of cell‐cell adhesion molecule E‐cadherin and typical marker of mesenchymal cells, Vimentin, but not N‐cadherin. Hypoxia conditions did not alter BK channels expression but increased its open probability. Moreover, BK channels blocker IbTX significantly inhibited HCC cell remote colonization in HCC cell xenografted mice. In conclusion, the results of this study suggest that blocking BK channels offers an attractive strategy for treating HCC.

Deeper and Deeper on the Role of BK and Kir4.1 Channels in Glioblastoma Invasiveness: A Novel Summative Mechanism?

In the last decades, increasing evidence has revealed that a large number of channel protein and ion pumps exhibit impaired expression in cancers. This dysregulation is responsible for high proliferative rates as well as migration and invasiveness, reflected in the recently coined term oncochannelopathies. In glioblastoma (GBM), the most invasive and aggressive primary brain tumor, GBM cells modify their ionic equilibrium in order to change their volume as a necessary step prior to migration. This mechanism involves increased expression of BK channels and downregulation of the normally widespread Kir4.1 channels, as noted in GBM biopsies from patients. Despite a large body of work implicating BK channels in migration in response to an artificial intracellular calcium rise, little is known about how this channel acts in GBM cells at resting membrane potential (RMP), as compared to other channels that are constitutively open, such as Kir4.1. In this review we propose that a residual fraction of functionally active Kir4.1 channels mediates a small, but continuous, efflux of potassium at the more depolarized RMP of GBM cells. In addition, coinciding with transient membrane deformation and the intracellular rise in calcium concentration, brief activity of BK channels can induce massive and rapid cytosolic water loss that reduces cell volume (cell shrinkage), a necessary step for migration within the brain parenchyma.

Molecular Choreography and Structure of Ca2+ Release-Activated Ca2+ (CRAC) and KCa2+ Channels and Their Relevance in Disease with Special Focus on Cancer

Ca2+ ions play a variety of roles in the human body as well as within a single cell. Cellular Ca2+ signal transduction processes are governed by Ca2+ sensing and Ca2+ transporting proteins. In this review, we discuss the Ca2+ and the Ca2+-sensing ion channels with particular focus on the structure-function relationship of the Ca2+ release-activated Ca2+ (CRAC) ion channel, the Ca2+-activated K+ (KCa2+) ion channels, and their modulation via other cellular components. Moreover, we highlight their roles in healthy signaling processes as well as in disease with a special focus on cancer. As KCa2+ channels are activated via elevations of intracellular Ca2+ levels, we summarize the current knowledge on the action mechanisms of the interplay of CRAC and KCa2+ ion channels and their role in cancer cell development.

Novel Therapeutic Approaches of Ion Channels and Transporters in Cancer

The expression and function of many ion channels and transporters in cancer cells display major differences in comparison to those from healthy cells. These differences provide the cancer cells with advantages for tumor development. Accordingly, targeting ion channels and transporters have beneficial anticancer effects including inhibition of cancer cell proliferation, migration, invasion, metastasis, tumor vascularization, and chemotherapy resistance, as well as promoting apoptosis. Some of the molecular mechanisms associating ion channels and transporters with cancer include the participation of oxidative stress, immune response, metabolic pathways, drug synergism, as well as noncanonical functions of ion channels. This diversity of mechanisms offers an exciting possibility to suggest novel and more effective therapeutic approaches to fight cancer. Here, we review and discuss most of the current knowledge suggesting novel therapeutic approaches for cancer therapy targeting ion channels and transporters. The role and regulation of ion channels and transporters in cancer provide a plethora of exceptional opportunities in drug design, as well as novel and promising therapeutic approaches that may be used for the benefit of cancer patients.

Potassium channels and their role in glioma: A mini review

K⁺ channels regulate a multitude of biological processes and play important roles in a variety of diseases by controlling potassium flow across cell membranes. They are widely expressed in the central and peripheral nervous system. As a malignant tumor derived from nerve epithelium, glioma has the characteristics of high incidence, high recurrence rate, high mortality rate, and low cure rate. Since glioma cells show invasive growth, current surgical methods cannot completely remove tumors. Adjuvant chemotherapy is still needed after surgery. Because the blood-brain barrier and other factors lead to a lower effective concentration of chemotherapeutic drugs in the tumor, the recurrence rate of residual lesions is extremely high. Therefore, new therapeutic methods are needed. Numerous studies have shown that different K⁺ channel subtypes are differentially expressed in glioma cells and are involved in the regulation of the cell cycle of glioma cells to arrest them at different stages of the cell cycle. Increasing evidence suggests that K⁺ channels express in glioma cells and regulate glioma cell behaviors such as cell cycle, proliferation and apoptosis. This review article aims to summarize the current knowledge on the function of K⁺ channels in glioma, suggests K⁺ channels participating in the development of glioma.

Expression of potassium channel genes predicts clinical outcome in lung cancer

Lung cancer is the most common cause of cancer deaths worldwide and several molecular signatures have been developed to predict survival in lung cancer. Increasing evidence suggests that proliferation and migration to promote tumor growth are associated with dysregulated ion channel expression. In this study, by analyzing high-throughput gene expression data, we identify the differentially expressed K⁺ channel genes in lung cancer. In total, we prioritize ten dysregulated K⁺ channel genes (5 up-regulated and 5 down-regulated genes, which were designated as K-10) in lung tumor tissue compared with normal tissue. A risk scoring system combined with the K-10 signature accurately predicts clinical outcome in lung cancer, which is independent of standard clinical and pathological prognostic factors including patient age, lymph node involvement, tumor size, and tumor grade. We further indicate that the K-10 potentially predicts clinical outcome in breast and colon cancers. Molecular signature discovered through K⁺ gene expression profiling may serve as a novel biomarker to assess the risk in lung cancer.

KCNMA1 cooperating with PTK2 is a novel tumor suppressor in gastric cancer and is associated with disease outcome

BACKGROUND

Inactivation of tumor suppressor genes by promoter hypermethylation plays a key role in the tumorgenesis. It is necessary to uncover the detailed pattern of whole genome-wide abnormal DNA methylation during the development of gastric cancer (GC).

METHOD

We performed a genome-wide methylation detection using 12 paired of GC tissues and their corresponding normal tissues. Methylation-specific PCR (MSP) and bisulphite sequencing (BSP) were used to measure methylation status of specific CpG site. Based on the bioinformatic analysis, the cell phenotypes and mouse model experiments were constructed to detect effect of the target gene. Using the Kaplan-Meier survival curve, the clinical value of KCNMA1 was assessed in GC patients.

RESULTS

The CpG site cg24113782 located at the promoter of KCNMA1 showed the most significant difference, contributing to the commonly silenced KCNMA1in gastric cancer cells and primary GC tissues. The promoter methylation of KCNMA1 was detected in 68.7% (77/112) of tumor tissues, compared with 16.2% (18/112) of normal tissues (P < 0.001). The survival curve indicated that KCNMA1 hypermethylation was significantly associated with the shortened survival in GC patients (P = 0.036). KCNMA1 significantly inhibited biological malignant behavior of gastric cancer cell by inducing cell apoptosis in vitro, and suppressed xenograft tumor growth in subcutaneous mouse models (both P < 0.001). Furthermore, the anti-tumor effect of KCNMA1was mediated through suppressing the expression of PTK2.

CONCLUSION

KCNMA1 is a critical tumor suppressor in gastric carcinogenesis and its hypermethylation is an independent prognostic factor in patients with gastric cancer.

Characterizing the biological and biochemical profile of six different scorpion venoms from the Buthidae and Scorpionidae family

The objective of this study was to characterize six different scorpion venoms using biological and biochemical methods, including a preliminary MS/MS and a post-translational modifications analysis. Despite the diversity of scorpion species of medical importance in Africa and Colombia, the venoms of these arachnids have been poorly studied in these two regions. We report the biochemical, electrophoretic, chromatographic profile, internal peptide sequences with a post-translational modification report, and a preliminary antitumor activity of five different scorpions of the Buthidae family, Androctonus amoreuxi, Babycurus jacksoni, Grosphus grandidieri, Hottentotta gentili and Tityus fuhrmanni, and one of the Scorpionidae family Pandinus imperator. No L-amino oxidase activity was detected in the evaluated venoms. Proteolytic activity using azocasein was detected only in G. grandidieri and P. imperator, indicating the possible presence of metalloproteinases in these two venoms. Proteolytic activity using NOBA was detected in all venoms indicating the possible presence of serine-proteinases. Phospholipase A₂ activity was detected in the venoms of P. imperator, G. grandidieri, H. gentili and A. amoreuxi, with P. imperator venom being the most active. All venoms analyzed contained defensin-like proteins, alpha toxins, metalloproteinases, neuropeptides, DBP affecting ion channels, DBP with antimicrobial activity, among others. Venoms from P. imperator, G. grandidieri and T. fuhrmanni showed a dose-dependent cytotoxic activity over MCF-7 cells. Only two isolated RP-HPLC fractions from P. imperator and T. fuhrmanni showed cytotoxic activity over MCF-7. No cytotoxic activity was found in the venoms from A. amoreuxi, B. jacksoni, and H. gentili.

The role of ion channels in malignant brain tumors

Malignant gliomas are the most common primary brain tumors and have poor clinical prognosis, despite multimodal therapeutic strategies. In recent years, ion channels have emerged as major players in tumor pathophysiology regarding all hallmarks of cancer. Since ion channels are easily accessible structures, they may prove to be effective targets for canner therapy, although their broad expression pattern and role in physiological processes should be taken into consideration. This review summarizes the current knowledge on the role of ion channels in the pathophysiology of malignant gliomas, especially glioblastoma, and evaluates their potential role in targeted antiglioma therapy.

High expression of KCa3.1 in patients with clear cell renal carcinoma predicts high metastatic risk and poor survival

Background

Ca²⁺-activated K⁺ channels have been implicated in cancer cell growth, metastasis, and tumor angiogenesis. Here we hypothesized that high mRNA and protein expression of the intermediate-conductance Ca²⁺-activated K⁺ channel, KCa3.1, is a molecular marker of clear cell Renal Cell Carcinoma (ccRCC) and metastatic potential and survival.

Methodology/Principal Findings

We analyzed channel expression by qRT-PCR, immunohistochemistry, and patch-clamp in ccRCC and benign oncocytoma specimens, in primary ccRCC and oncocytoma cell lines, as well as in two ccRCC cell lines (Caki-1 and Caki-2). CcRCC specimens contained 12-fold higher mRNA levels of KCa3.1 than oncocytoma specimens. The large-conductance channel, KCa1.1, was 3-fold more highly expressed in ccRCC than in oncocytoma. KCa3.1 mRNA expression in ccRCC was 2-fold higher than in the healthy cortex of the same kidney. Disease specific survival trended towards reduction in the subgroup of high-KCa3.1-expressing tumors (p<0.08 vs. low-KCa3.1-expressing tumors). Progression-free survival (time to metastasis/recurrence) was reduced significantly in the subgroup of high-KCa3.1-expressing tumors (p<0.02, vs. low-KCa3.1-expressing tumors). Immunohistochemistry revealed high protein expression of KCa3.1 in tumor vessels of ccRCC and oncocytoma and in a subset of ccRCC cells. Oncocytoma cells were devoid of KCa3.1 protein. In a primary ccRCC cell line and Caki-1/2-ccRCC cells, we found KCa3.1-protein as well as TRAM-34-sensitive KCa3.1-currents in a subset of cells. Furthermore, Caki-1/2-ccRCC cells displayed functional Paxilline-sensitive KCa1.1 currents. Neither KCa3.1 nor KCa1.1 were found in a primary oncocytoma cell line. Yet KCa-blockers, like TRAM-34 (KCa3.1) and Paxilline (KCa1.1), had no appreciable effects on Caki-1 proliferation in-vitro.

Conclusions/Significance

Our study demonstrated expression of KCa3.1 in ccRCC but not in benign oncocytoma. Moreover, high KCa3.1-mRNA expression levels were indicative of low disease specific survival of ccRCC patients, short progression-free survival, and a high metastatic potential. Therefore, KCa3.1 is of prognostic value in ccRCC.

Scorpion venom components as potential candidates for drug development

Scorpions are well known for their dangerous stings that can result in severe consequences for human beings, including death. Neurotoxins present in their venoms are responsible for their toxicity. Due to their medical relevance, toxins have been the driving force in the scorpion natural compounds research field. On the other hand, for thousands of years, scorpions and their venoms have been applied in traditional medicine, mainly in Asia and Africa. With the remarkable growth in the number of characterized scorpion venom components, several drug candidates have been found with the potential to tackle many of the emerging global medical threats. Scorpions have become a valuable source of biologically active molecules, from novel antibiotics to potential anticancer therapeutics. Other venom components have drawn attention as useful scaffolds for the development of drugs. This review summarizes the most promising candidates for drug development that have been isolated from scorpion venoms.

Big Potassium (BK) ion channels in biology, disease and possible targets for cancer immunotherapy

The Big Potassium (BK) ion channel is commonly known by a variety of names (Maxi-K, KCNMA1, slo, stretch-activated potassium channel, KCa1.1). Each name reflects a different physical property displayed by this single ion channel. This transmembrane channel is found on nearly every cell type of the body and has its own distinctive roles for that tissue type. The BKα channel contains the pore that releases potassium ions from intracellular stores. This ion channel is found on the cell membrane, endoplasmic reticulum, Golgi and mitochondria. Complex splicing pathways produce different isoforms. The BKα channels can be phosphorylated, palmitoylated and myristylated. BK is composed of a homo-tetramer that interacts with β and γ chains. These accessory proteins provide a further modulating effect on the functions of BKα channels. BK channels play important roles in cell division and migration. In this review, we will focus on the biology of the BK channel, especially its role, and its immune response towards cancer. Recent proteomic studies have linked BK channels with various proteins. Some of these interactions offer further insight into the role that BK channels have with cancers, especially with brain tumors. This review shows that BK channels have a complex interplay with intracellular components of cancer cells and still have plenty of secrets to be discovered.

Cl- and K+ channels and their role in primary brain tumour biology

Profound cell volume changes occur in primary brain tumours as they proliferate, invade surrounding tissue or undergo apoptosis. These volume changes are regulated by the flux of Cl(-) and K(+) ions and concomitant movement of water across the membrane, making ion channels pivotal to tumour biology. We discuss which specific Cl(-) and K(+) channels are involved in defined aspects of glioma biology and how these channels are regulated. Cl(-) is accumulated to unusually high concentrations in gliomas by the activity of the NKCC1 transporter and serves as an osmolyte and energetic driving force for volume changes. Cell volume condensation is required as cells enter M phase of the cell cycle and this pre-mitotic condensation is caused by channel-mediated ion efflux. Similarly, Cl(-) and K(+) channels dynamically regulate volume in invading glioma cells allowing them to adjust to small extracellular brain spaces. Finally, cell condensation is a hallmark of apoptosis and requires the concerted activation of Cl(-) and Ca(2+)-activated K(+) channels. Given the frequency of mutation and high importance of ion channels in tumour biology, the opportunity exists to target them for treatment.

Cell proliferation, potassium channels, polyamines and their interactions: a mini review

Polyamines, which are obligatory molecules involved in cell cycling and proliferation, are subject to a change in their free intracellular concentrations during the cell cycle. Potassium (K(+)) channels are also considered, but less well recognized, to be necessary for cell proliferation by either hyperpolarizing or depolarizing cells during the cell cycle. A block of polyamine synthesis as well as block or knockout of K(+) channels can halt cell proliferation. K(+) channels like BK (maxi calcium (Ca(2+))-activated K(+)), Kir (inward rectifier), M-type K(+)-and TASK (two-pore domain K(+)) channels or the delayed rectifier K(+) channels are modulated in their electrical properties by polyamines. Polyamines are most effective in blocking these channels when applied to the intracellular face of these channels except for TASK channels where they act only from the extracellular side. Quinidine, a general K(+) channel blocker, was found to reduce putrescine concentrations, to block the ornithine decarboxylase and halt cell proliferation. From these results, the question arises if there is an interaction between polyamines, K(+) channels and proliferation. It might be speculated that a decrease of intracellular polyamines allows more K(+) channels to be active, thus inducing hyperpolarization, while an increase of the polyamine concentration may block K(+) channel activity leading to depolarization of the membrane potential. On the other hand, a block or a deletion of K(+) channels may cause a decrease of the polyamine concentration in cells. More research is needed to test these hypotheses.

Role of KCNMA1 in breast cancer

KCNMA1 encodes the α-subunit of the large conductance, voltage and Ca²⁺-activated (BK) potassium channel and has been reported as a target gene of genomic amplification at 10q22 in prostate cancer. To investigate the prevalence of the amplification in other human cancers, the copy number of KCNMA1 was analyzed by fluorescence-in-situ-hybridization (FISH) in 2,445 tumors across 118 different tumor types. Amplification of KCNMA1 was restricted to a small but distinct fraction of breast, ovarian and endometrial cancer with the highest prevalence in invasive ductal breast cancers and serous carcinoma of ovary and endometrium (3–7%). We performed an extensive analysis on breast cancer tissue microarrays (TMA) of 1,200 tumors linked to prognosis. KCNMA1 amplification was significantly associated with high tumor stage, high grade, high tumor cell proliferation, and poor prognosis. Immunofluorescence revealed moderate or strong KCNMA1 protein expression in 8 out of 9 human breast cancers and in the breast cancer cell line MFM223. KCNMA1-function in breast cancer cell lines was confirmed by whole-cell patch clamp recordings and proliferation assays, using siRNA-knockdown, BK channel activators such as 17ß-estradiol and the BK-channel blocker paxilline. Our findings revealed that enhanced expression of KCNMA1 correlates with and contributes to high proliferation rate and malignancy of breast cancer.

Glioma big potassium channel expression in human cancers and possible T cell epitopes for their immunotherapy

Big potassium (BK) ion channels have several spliced variants. One spliced variant initially described within human glioma cells is the glioma BK (gBK) channel. This isoform consists of 34 aa inserted into the intracellular region of the basic BK ion channel. PCR primers specific for this inserted region confirmed that human glioma cell lines and freshly resected surgical tissues from glioblastoma multiforme patients strongly expressed gBK mRNA. Normal human brain tissue very weakly expressed this transcript. An Ab specific for this gBK isoform confirmed that human glioma cells displayed this protein in the cell membrane, mitochondria, Golgi, and endoplasmic reticulum. Within the gBK region, two putative epitopes (gBK1 and gBK2) are predicted to bind to the HLA-A*0201 molecule. HLA-A*0201-restricted human CTLs were generated in vitro using gBK peptide-pulsed dendritic cells. Both gBK1 and gBK2 peptide-specific CTLs killed HLA-A2⁺/gBK⁺ gliomas, but they failed to kill non-HLA-A2-expressing but gBK⁺ target cells in cytolytic assays. T2 cells loaded with exogenous gBK peptides, but not with the influenza M1 control peptide, were only killed by their respective CTLs. The gBK-specific CTLs also killed a variety of other HLA-A*0201⁺ cancer cells that possess gBK, as well as HLA-A2⁺ HEK cells transfected with the gBK gene. Of clinical relevance, we found that T cells derived from glioblastoma multiforme patients that were sensitized to the gBK peptide could also kill target cells expressing gBK. This study shows that peptides derived from cancer-associated ion channels maybe useful targets for T cell-mediated immunotherapy.

Ion channels in glioblastoma

Glioblastoma is the most common primary brain tumor with the most dismal prognosis. It is characterized by extensive invasion, migration, and angiogenesis. Median survival is only 15 months due to this behavior, rendering focal surgical resection ineffective and adequate radiotherapy impossible. At this moment, several ion channels have been implicated in glioblastoma proliferation, migration, and invasion. This paper summarizes studies on potassium, sodium, chloride, and calcium channels of glioblastoma. It provides an up-to-date overview of the literature that could ultimately lead to new therapeutic targets.

Quercetin-induced Growth Inhibition in Human Bladder Cancer Cells Is Associated with an Increase in Ca-activated K Channels

Quercetin (3,3',4',5,7-pentahydroxyflavone) is an attractive therapeutic flavonoid for cancer treatment because of its beneficial properties including apoptotic, antioxidant, and antiproliferative effects on cancer cells. However, the exact mechanism of action of quercetin on ion channel modulation is poorly understood in bladder cancer 253J cells. In this study, we demonstrated that large conductance Ca(2+)-activated K(+) (BK(Ca)) or MaxiK channels were functionally expressed in 253J cells, and quercetin increased BK(Ca) current in a concentration dependent and reversible manner using a whole cell patch configuration. The half maximal activation concentration (IC(50)) of quercetin was 45.5±7.2 µM. The quercetin-evoked BK(Ca) current was inhibited by tetraethylammonium (TEA; 5 mM) a non-specific BK(Ca) blocker and iberiotoxin (IBX; 100 nM) a BK(Ca)-specific blocker. Quercetin-induced membrane hyperpolarization was measured by fluorescence-activated cell sorting (FACS) with voltage sensitive dye, bis (1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC(4)(3); 100 nM). Quercetin-evoked hyperpolarization was prevented by TEA. Quercetin produced an antiproliferative effect (30.3±13.5%) which was recovered to 53.3±10.5% and 72.9±3.7% by TEA and IBX, respectively. Taken together our results indicate that activation of BK(Ca) channels may be considered an important target related to the action of quercetin on human bladder cancer cells.

Transient receptor potential canonical channels are essential for chemotactic migration of human malignant gliomas

The majority of malignant primary brain tumors are gliomas, derived from glial cells. Grade IV gliomas, Glioblastoma multiforme, are extremely invasive and the clinical prognosis for patients is dismal. Gliomas utilize a number of proteins and pathways to infiltrate the brain parenchyma including ion channels and calcium signaling pathways. In this study, we investigated the localization and functional relevance of transient receptor potential canonical (TRPC) channels in glioma migration. We show that gliomas are attracted in a chemotactic manner to epidermal growth factor (EGF). Stimulation with EGF results in TRPC1 channel localization to the leading edge of migrating D54MG glioma cells. Additionally, TRPC1 channels co-localize with the lipid raft proteins, caveolin-1 and β-cholera toxin, and biochemical assays show TRPC1 in the caveolar raft fraction of the membrane. Chemotaxis toward EGF was lost when TRPC channels were pharmacologically inhibited or by shRNA knockdown of TRPC1 channels, yet without affecting unstimulated cell motility. Moreover, lipid raft integrity was required for gliomas chemotaxis. Disruption of lipid rafts not only impaired chemotaxis but also impaired TRPC currents in whole cell recordings and decreased store-operated calcium entry as revealed by ratiomeric calcium imaging. These data indicated that TRPC1 channel association with lipid rafts is essential for glioma chemotaxis in response to stimuli, such as EGF.

Disruption of transient receptor potential canonical channel 1 causes incomplete cytokinesis and slows the growth of human malignant gliomas

Despite decades of research, primary brain tumors, gliomas, lack effective treatment options and present a huge clinical challenge. Particularly, the most malignant subtype, Glioblastoma multiforme, proliferates extensively and cells often undergo incomplete cell divisions, resulting in multinucleated cells. We now present evidence that multinucleated glioma cells result from the functional loss of transient receptor potential canonical 1 (TRPC1) channels, plasma membrane proteins involved in agonist-induced calcium entry and reloading of intracellular Ca(2+) stores. Pharmacological inhibition or shRNA mediated suppression of TRPC1 causes loss of functional channels and store-operated calcium entry in D54MG glioma cells. This is associated with reduced cell proliferation and, frequently, with incomplete cell division. The resulting multinucleated cells are reminiscent of those found in patient biopsies. In a flank tumor model, tumor size was significantly decreased when TRPC1 expression was disrupted using a doxycycline inducible shRNA knockdown approach. These results suggest that TRPC1 channels play an important role in glioma cell division most likely by regulating calcium signaling during cytokinesis.

Calcium-activated potassium channels BK and IK1 are functionally expressed in human gliomas but do not regulate cell proliferation

Gliomas are morbid brain tumors that are extremely resistant to available chemotherapy and radiology treatments. Some studies have suggested that calcium-activated potassium channels contribute to the high proliferative potential of tumor cells, including gliomas. However, other publications demonstrated no role for these channels or even assigned them antitumorogenic properties. In this work we characterized the expression and functional contribution to proliferation of Ca²⁺-activated K⁺ channels in human glioblastoma cells. Quantitative RT-PCR detected transcripts for the big conductance (BK), intermediate conductance (IK1), and small conductance (SK2) K⁺ channels in two glioblastoma-derived cell lines and a surgical sample of glioblastoma multiforme. Functional expression of BK and IK1 in U251 and U87 glioma cell lines and primary glioma cultures was verified using whole-cell electrophysiological recordings. Inhibitors of BK (paxilline and penitrem A) and IK1 channels (clotrimazole and TRAM-34) reduced U251 and U87 proliferation in an additive fashion, while the selective blocker of SK channels UCL1848 had no effect. However, the antiproliferative properties of BK and IK1 inhibitors were seen at concentrations that were higher than those necessary to inhibit channel activity. To verify specificity of pharmacological agents, we downregulated BK and IK1 channels in U251 cells using gene-specific siRNAs. Although siRNA knockdowns caused strong reductions in the BK and IK1 current densities, neither single nor double gene silencing significantly affected rates of proliferation. Taken together, these results suggest that Ca²⁺-activated K⁺ channels do not play a critical role in proliferation of glioma cells and that the effects of pharmacological inhibitors occur through their off-target actions.

High expression of large-conductance Ca2+-activated K+ channel in the CD133+ subpopulation of SH-SY5Y neuroblastoma cells

Solid tumors contain a population of cancer stem cells (CSCs), and CD133 is widely used as a CSCs marker. We investigated the differences between CD133(+) and CD133(-) cells from the neuroblastoma cell line SH-SY5Y in terms of the expressions of voltage-gated ion channels. A CD133(+) enriched (>60%) population was isolated, and a subsequent whole-cell voltage-clamp study showed that these cells predominantly express TEA-sensitive outward K(+) currents (I(K,TEA)) and TTX-sensitive voltage-gated inward Na(+) currents (I(Na)). Cell-attached single channel recordings demonstrated higher density of large-conductance (155pS) channel in CD133(+) cells than in CD133(-) cells. The TEA-sensitivity and single channel conductance indicated the large-conductance Ca(2+)-activated K(+) channels (BK(Ca)). Furthermore, RT-PCR analysis of 22 transcripts of voltage-gated ion channels in SH-SY5Y cells showed the expressions of Cav1.3, Kir2.1, Kv1.4, Kv2.1, Kv4.2, Kv7.1, BK(Ca), and Nav1.7, and those of BK(Ca) and Nav1.7 were higher in CD133(+) than in CD133(-) cells. In addition, the increase of cytosolic Ca(2+) concentration ([Ca(2+)](c)) in response to ionomycin (a Ca(2+) ionophore) was higher and more sustained in CD133(+) than in CD133(-) cells. Plausibly membrane hyperpolarization via BK(Ca) might be responsible for the augmented Ca(2+) influx observed in CD133(+) cells. The physiological implications of the differential expression of BK(Ca) and Nav1.7 in CSCs require further investigation.

Functional characterization of two isoforms of the P2Y-like receptor GPR17: [35S]GTPgammaS binding and electrophysiological studies in 1321N1 cells

The previously "orphan" G protein-coupled receptor GPR17 is structurally related to both P2Y nucleotide receptors and to receptors for cysteinyl leukotrienes. Genomic analysis revealed two putative open reading frames encoding for a "short" and a "long" receptor isoform of 339- and 367-amino acids, respectively, with the latter displaying a 28-amino acid longer NH(2) terminus. The short isoform has been recently "deorphanized," revealing dual responses to uracil nucleotides and cysteinyl leukotrienes. No information regarding the ligand specificity, tissue distribution, or pathophysiological roles of the long receptor isoform is available. In the present study, we cloned human long-GPR17, determined its tissue distribution, and characterized its pharmacological specificity in 1321N1 cells by [35S]GTPgammaS binding (which measures the ability of G protein-coupled receptor agonists to increase GTP binding to G proteins) and whole cell patch-clamp recording measuring receptor coupling to K+ channels. [35S]GTPgammaS binding in long-GPR17-expressing 1321N1 cells revealed concentration-dependent responses to uracil nucleotides (UDP-galactose = UDP > UDP-glucose) and cysteinyl leukotrienes (LTC4 > LTD4), which were counteracted by a purinergic (cangrelor) and a cysteinyl leukotriene antagonist (montelukast), respectively. The nonhydrolyzable ATP analog ATPgammaS also acted as an antagonist. GPR17 coupled to Gi and, to a lesser extent, Gq proteins. UDP-glucose and LTD(4) also induced increases in overall outward K+ currents, which were antagonized by the purinergic antagonists MRS2179 and cangrelor and by montelukast. We conclude that the previously uncharacterized long-GPR17 isoform is a functional receptor that is stimulated by both uracil nucleotides and cysteinyl leukotrienes. We also show that the signaling pathway of GPR17 involves the generation of outward K+ currents, an important protective mechanism that, in brain, is specifically aimed at reducing neuronal hyperexcitability and resultant neuronal injury.

Inhibition of transient receptor potential canonical channels impairs cytokinesis in human malignant gliomas

OBJECTIVES

Glial-derived primary brain tumours, gliomas, are among the fastest growing malignancies and present a huge clinical challenge. Research suggests an important, yet poorly understood, role of ion channels in growth control of normal and malignant cells. In this study, we sought to functionally characterize Transient Receptor Potential Canoncial (TRPC) channels in glioma cell proliferation. TRPC channels form non-selective cation channels that have been suggested to represent a Ca(2+) influx pathway impacting cellular growth.

MATERIALS AND METHODS

Employing a combination of molecular, biochemical and biophysical techniques, we characterized TRPC channels in glioma cells.

RESULTS

We showed consistent expression of four channel family members (TRPC-1, -3, -5, -6) in glioma cell lines and acute patient-derived tissues. These channels gave rise to small, non-voltage-dependent cation currents that were blocked by the TRPC inhibitors GdCl(3), 2-APB, or SKF96365. Importantly, TRPC channels contributed to the resting conductance of glioma cells and their acute pharmacological inhibition caused an approximately 10 mV hyperpolarization of the cells' resting potential. Additionally, chronic application of the TRPC inhibitor SKF96365 caused near complete growth arrest. A detailed analysis, by fluorescence-activated cell sorting and time-lapse microscopy, showed that growth inhibition occurred at the G(2)+ M phase of the cell cycle with cytokinesis defects. Cells underwent incomplete cell divisions and became multinucleate, enlarged cells.

CONCLUSIONS

Nuclear atypia and enlarged cells are histopathological hallmarks for glioblastoma multiforme, the highest grade glioma, suggesting that a defect in TRPC channel function may contribute to cellular abnormalities in these tumours.

KCNMA1 gene amplification promotes tumor cell proliferation in human prostate cancer

Molecular mechanisms of prostate cancer progression are poorly understood. Here, we studied gene amplification of the large conductance calcium-activated potassium channel alpha subunit (KCNMA1), which is located at the chromosomal region 10q22. Fluorescence in situ hybridization (FISH) revealed KCNMA1 amplification in 16% of 119 late-stage human prostate cancers and in the hormone-insensitive prostate cancer cell line PC-3. In contrast, KCNMA1 amplification was absent in 33 benign controls, 32 precursor lesions and in 105 clinically organ-confined prostate cancers. Amplification was associated with mRNA and protein overexpression as well as increased density of BK channel protein and beta-estradiol-insensitive BK currents in PC-3 cells as compared to non-amplified control cell lines. Specific blockade of BK channels by iberiotoxin or RNA(i) significantly inhibited K(+) currents and growth of PC-3 cells. The data demonstrate that 10q22 amplification drives KCNMA1 expression and cell proliferation. Thus, KCNMA1 qualifies as a promising diagnostic and therapeutic target in patients with prostate cancer.

Expression and function of calcium-activated potassium channels in human glioma cells

Ca(2+)-activated K(+) (K(Ca)) channels are a unique family of ion channels because they are capable of directly communicating calcium signals to changes in cell membrane potential required for cellular processes including but not limited to cellular proliferation and migration. It is now possible to distinguish three families of K(Ca) channels based on differences in their biophysical and pharmacological properties as well as genomic sequence. Using a combination of biochemical, molecular, and biophysical approaches, we show that human tumor cells of astrocytic origin, i.e. glioma cells, express transcripts for all three family members of K(Ca) channels including BK, IK, and all three SK channel types (SK1, SK2, and SK3). The use of selective pharmacological inhibitors shows prominent expression of currents that are inhibited by the BK channel specific inhibitors iberiotoxin and paxilline. However, despite the presence of transcripts for IK and SK, neither clotrimazole, an inhibitor of IK channels, nor apamin, known to block most SK channels inhibited any current. The exclusive expression of functional BK channels was further substantiated by shRNA knockdown experiments, which selectively reduced iberiotoxin sensitive currents. Western blotting of patient biopsies with antibodies specific for all three KCa channel types further substantiated the exclusive expression of BK type KCa channels in vivo. This finding is in sharp contrast to other cancers that express primarily IK channels.

Potassium channels: new targets in cancer therapy

BACKGROUND

Potassium channels (KCh) are the most diverse and ubiquitous class of ion channels. KCh control membrane potential and contribute to nerve and cardiac action potentials and neurotransmitter release. KCh are also involved in insulin release, differentiation, activation, proliferation, apoptosis, and several other physiological functions. The aim of this review is to provide an updated overview of the KCh role during the cell growth. Their potential use as pharmacological targets in cancer therapies is also discussed.

METHODS

We searched PubMed (up to 2005) and identified relevant articles. Reprints were mainly obtained by on line subscription. Additional sources were identified through cross-referencing and obtained from Library services.

RESULTS

KCh are responsible for some neurological and cardiovascular diseases and for a new medical discipline, channelopathies. Their role in congenital deafness, multiple sclerosis, episodic ataxia, LQT syndrome and diabetes has been proven. Furthermore, a large body of information suggests that KCh play a role in the cell cycle progression, and it is now accepted that cells require KCh to proliferate. Thus, KCh expression has been studied in a number of tumours and cancer cells.

CONCLUSIONS

Cancer is far from being considered a channelopathy. However, it seems appropriate to take into account the involvement of KCh in cancer progression and pathology when developing new strategies for cancer therapy.

Modulation of glioma BK channels via erbB2

Glioma cells show up-regulation and constitutive activation of erbB2, and its expression correlates positively with increased malignancy. A similar correlation has been demonstrated for the expression of gBK, a calcium-sensitive, large-conductance K(+) channel. We show here that glioma BK channels are a downstream target of erbB2/neuregulin signaling. Tyrphostin AG825 was able to disrupt the constituitive erbB2 activation in a dose-dependent manner, causing a 30-mV positive shift in gBK channel activation in cell-attached patches. Conversely, maximal stimulation of erbB2 with a recombinant neuregulin (NRG-1beta) caused a 12-mV shift in the opposite direction. RT-PCR studies reveal no change in the BK splice variants expressed in treated glioma cells. Furthermore, isolation of surface proteins through biotinylation did not show a change in gBK channel expression, and probing with phospho-specific antibodies showed no alteration in channel phosphorylation. However, fura-II Ca(2+) fluorescence imaging revealed a 35% decrease in the free intracellular Ca(2+) concentration after erbB2 inhibition and an increase in NRG-1beta-treated cells, suggesting that the observed changes most likely were due to alterations in [Ca(2+)](i). Consistent with this conclusion, neither tyrphostin AG825 nor NRG-1beta was able to modulate gBK channels under inside-out or whole-cell recording conditions when intracellular Ca(2+) was fixed. Thus, gBK channels are a downstream target for the abundantly expressed neuregulin-1 receptor erbB2 in glioma cells. However, unlike the case in other systems, this modulation appears to occur via changes in [Ca(2+)](i) without changes in channel expression or phosphorylation. The enhanced sensitivity of gBK channels in glioma cells to small, physiological Ca(2+) changes appears to be a prerequisite for this modulation.

Ion Channels and Cancer

Membrane ion channels are essential for cell proliferation and appear to have a role in the development of cancer. This has initially been demonstrated for potassium channels and is meanwhile also suggested for other cation channels and Cl- channels. For some of these channels, like voltage-gated ether à go-go and Ca2+-dependent potassium channels as well as calcium and chloride channels, a cell cycle-dependent function has been demonstrated. Along with other membrane conductances, these channels control the membrane voltage and Ca2+ signaling in proliferating cells. Homeostatic parameters, such as the intracellular ion concentration, cytosolic pH and cell volume, are also governed by the activity of ion channels. Thus it will be an essential task for future studies to unravel cell cycle-specific effects of ion channels and non-specific homeostatic functions. When studying the role of ion channels in cancer cells, it is indispensable to choose experimental conditions that come close to the in vivo situation. Thus, environmental parameters, such as low oxygen pressure, acidosis and exposure to serum proteins, have to be taken into account. In order to achieve clinical application, more studies on the original cancer tissue are required, and improved animal models. Finally, it will be essential to generate more potent and specific inhibitors of ion channels to overcome the shortcomings of some of the current approaches.

Scorpion venom induces glioma cell apoptosis in vivo and inhibits glioma tumor growth in vitro

Malignant gliomas are the main brain tumors notoriously resistant to currently available therapies, since they fail to undergo apoptosis upon anticancer treatment. Recent progress on enhanced studies of ion channels involved in glioma cells shed new light on the investigation of glioma cell growth and proliferation. Here we report BmK scorpion venom, a rich resource of various ion channels blockers/modulators, induces cell death of cultured malignant glioma U251-MG cells in vitro specifically at a dose of 10 mg/ml while shows no effect on human hepatocellular carcinoma cells and Chinese hamster ovary cells. The glioma cell death was then determined as apoptosis using 4,6-diamidino-2-phenylindole staining and fluorescence-activated cell sorting analysis. After incubation with BmK venom for 32 and 40 h, 36.20% and 63.08% of U251-MG cells showed apoptosis. Furthermore, BmK venom could significantly inhibit the tumor growth in vitro, which was assessed using U251-MG tumor xenografts on severe combined immunodeficiency mice. The tumor volume of the BmK venom treated mice is nearly 1/8 of that of control after 21 days, and the tumor weight is less than half of that of control. That BmK venom induces apoptosis and inhibits growth of glioma may result from the inhibition and/or modulation of various ion channels in glioma cells.

Role for calcium-activated potassium channels (BK) in growth control of human malignant glioma cells

Voltage-dependent large-conductance Ca(2+)-activated K(+) channels, often referred to as BK channels, are a unique class of ion channels coupling intracellular chemical signaling to electrical signaling. BK channel expression has been shown to be up-regulated in human glioma biopsies, and expression levels correlate positively with the malignancy grade of the tumor. Glioma BK channels (gBK) are a splice variant of the hslo gene, are characterized by enhanced sensitivity to [Ca(2+)](i), and are the target of modulation by growth factors. By using the selective pharmacological BK channel inhibitor iberiotoxin, we examined the potential role of these channels in tumor growth. Cell survival assays examined the ability of glioma cells to grow in nominally serum-free medium. Under such conditions, BK channel inhibition by iberiotoxin caused a dose- and time-dependent decrease in cell number discernible as early as 72 hr after exposure and maximal growth inhibition after 4-5 days. FACS analysis shows that IbTX treatment arrests glioma cells in S phase of the cell cycle, whereupon cells undergo cell death. Interestingly, IbTX effects were nullified when cells were maintained in 7% fetal calf serum. Electrophysiological analysis, in conjunction with biotinylation studies, demonstrates that serum starvation caused a significant translocation of BK channel protein to the plasma membrane, corresponding to a two- to threefold increase in whole-cell conductance, but without a change in total gBK protein. Hence, expression of functional gBK channels appears to be regulated in a growth-factor-dependent manner, with enhanced surface expression promoting tumor cell growth under conditions of growth factor deprivation as might occur under in vivo conditions.

Cell-cycle-dependent expression of the large Ca2+-activated K+ channels in breast cancer cells

In a previous work, we have reported that the ionic nature of the outward current recorded in MCF-7 cells was that of a K+ current. In this study, we have identified a Ca2+-activated K+ channel not yet described in MCF-7 human breast cancer cells. In cells arrested in the early G1 (depolarized cells), increasing [Ca2+]i induced both a shift in the I-V curve toward more negative potentials and an increase in current amplitude at negative and more at positive potential. Currents were inhibited by r-iberiotoxin (r-IbTX, 50 nM) and charybdotoxin (ChTX, 50 nM). These data indicate that human breast cancer cells express large-conductance Ca2+-activated K+ (BK) channels. BK current-density increased in cells synchronized at the end of G1, as compared with those in the early G1 phase. This increased current-density paralleled the enhancement in BK mRNA levels. Blocking BK channels with r-IbTX, ChTX or both induced a slight depolarization in cells arrested in the early G1, late G1, and S phases and accumulated cells in the S phase, but failed to induce cell proliferation. Thus, the expression of the BK channels was cell-cycle-dependent and seems to contribute more to the S phase than to the G1 phase. However, these K+ channels did not regulate the cell proliferation because of their minor role in the membrane potential.

Inhibitory action of ICI-182,780, an estrogen receptor antagonist, on BK(Ca) channel activity in cultured endothelial cells of human coronary artery

ICI-182,780 is known to be a selective inhibitor of the intracellular estrogen receptors. The effect of ICI-182,780 on ion currents was studied in cultured endothelial cells of human coronary artery. In whole-cell current recordings, ICI-182,780 reversibly decreased the amplitude of K(+) outward currents. The decrease in outward current caused by ICI-182,780 could be counteracted by further application of magnolol or nordihydroguaiaretic acid, yet not by 17beta-estradiol. Under current-clamp condition, ICI-182,780 (3microM) depolarized the membrane potentials of the cells, and magnolol (10 microM) or nordihydroguaiaretic acid (10 microM) reversed ICI-182,780-induced depolarization. In inside-out patches, ICI-182,780 added to the bath did not alter single-channel conductance of large-conductance Ca(2+)-activated K(+) channels (BK(Ca) channels), but decreased their open probability. ICI-182,780 reduced channel activity in a concentration-dependent manner with an IC(50) value of 3 microM. After BK(Ca) channel activity was suppressed by 2-methoxyestradiol (3 microM), subsequent application of ICI-182,780 (3 microM) did not further reduce the channel activity. The application of ICI-182,780 shifted the activation curve of BK(Ca) channels to positive potentials. Its decrease in the open probability primarily involved a reduction in channel open duration. ICI-182,780 also suppressed the proliferation of these endothelial cells with an IC(50) value of 2 microM. However, in coronary smooth muscle cells, a bell-shaped concentration-response curve for the ICI-182,780 effect on BK(Ca) channel activity was observed. This study provides evidence that ICI-182,780 can inhibit BK(Ca) channels in vascular endothelial cells in a mechanism unlikely to be linked to its anti-estrogen activity. The inhibitory effects on these channels may partly contribute to the underlying mechanisms by which ICI-182,780 affects endothelial function.

BK channel openers inhibit migration of human glioma cells

Large-conductance Ca(2+)-activated K(+) channels (BK channels) are highly expressed in human glioma cells. However, less is known about their biological function in these cells. We used the patch-clamp technique to investigate activation properties of BK channels and time-lapse microscopy to evaluate the role of BK channel activation in migration of 1321N1 human glioma cells. In whole cells, internal perfusion with a solution containing 500 nM free Ca(2+) and external application of the BK channel opener phloretin (100 micro M) shifted the activation threshold of BK channel currents toward more negative voltages of about -30 mV, which is close to the resting potential of the cells. The concentration of intracellular Ca(2+) in fura-2-loaded 1321N1 cells was measured to be 235+/-19 nM and was increased to 472+/-25 nM after treatment with phloretin. Phloretin and another BK channel opener NS1619 (100 micro M) reduced the migration velocity by about 50%. A similar reduction was observed following muscarinic stimulation of glioma cells with acetylcholine (100 micro M). The effects of phloretin, NS1619 and acetylcholine on cell migration were completely abolished by co-application of the specific BK channel blockers paxilline (5 micro M) and iberiotoxin (100 nM). The phloretin-induced increase in intracellular Ca(2+) was unaffected by the removal of extracellular Ca(2+) and co-application of paxilline. These findings indicate that glioma cell migration was inhibited through BK channel activation, independent of intracellular Ca(2+).