K+ channel expression in human breast cancer cells: involvement in cell cycle regulation and carcinogenesis. J Membr Biol. 2008;221:1-6. [PMID: 18060344 DOI: 10.1007/s00232-007-9080-6]

Potassium channel ether à go-go1 is aberrantly expressed in human liposarcoma and promotes tumorigenesis

The ether à go-go1 (Eag1) channel is overexpressed in a variety of cancers. However, the expression and function of Eag1 in liposarcoma are poorly understood. In the present study, the mRNA expression of Eag1 in different adipose tissue samples was examined by real-time PCR. Then, the protein expression of Eag1 in 131 different adipose tissues from 109 patients was detected by immunohistochemistry. Next, the associations between Eag1 expression and clinicopathological features of liposarcoma were analyzed. In addition, the effects of Eag1 on liposarcoma cell proliferation and cycle were evaluated by CCK-8, colony formation, xenograft mouse model, and flow cytometry, respectively. Finally, the activation of p38 mitogen-activated protein kinase (MAPK) was detected by Western blot analysis to explain the detailed mechanisms of oncogenic potential of Eag1 in liposarcoma. It was found that Eag1 was aberrantly expressed in over 67% liposarcomas, with a higher frequency than in lipoma, hyperplasia, inflammation, and normal adipose tissues. However, Eag1 expression was not correlated with clinicopathological features of liposarcoma. Eag1 inhibitor imipramine or Eag1-shRNA significantly suppressed the proliferation of liposarcoma cells in vitro and in vivo, accompanying with accumulation of cells in the G1 phase. These results suggest that Eag1 plays an important role in regulating the proliferation and cell cycle of liposarcoma cells and might be a potential therapeutic target for liposarcoma.

Upregulation of the large conductance voltage- and Ca2+-activated K+ channels by Janus kinase 2

The iberiotoxin-sensitive large conductance voltage- and Ca(2+)-activated potassium (BK) channels (maxi-K(+)-channels) hyperpolarize the cell membrane thus supporting Ca(2+) entry through Ca(2+)-release activated Ca(2+) channels. Janus kinase-2 (JAK2) has been identified as novel regulator of ion transport. To explore whether JAK2 participates in the regulation of BK channels, cRNA encoding Ca(2+)-insensitive BK channels (BK(M513I+Δ899-903)) was injected into Xenopus oocytes with or without cRNA encoding wild-type JAK2, gain-of-function (V617F)JAK2, or inactive (K882E)JAK2. K(+) conductance was determined by dual electrode voltage clamp and BK-channel protein abundance by confocal microscopy. In A204 alveolar rhabdomyosarcoma cells, iberiotoxin-sensitive K(+) current was determined utilizing whole cell patch clamp. A204 cells were further transfected with JAK2 and BK-channel transcript, and protein abundance was quantified by RT-PCR and Western blotting, respectively. As a result, the K(+) current in BK(M513I+Δ899-903)-expressing oocytes was significantly increased following coexpression of JAK2 or (V617F)JAK2 but not (K882E)JAK2. Coexpression of the BK channel with (V617F)JAK2 but not (K882E)JAK2 enhanced BK-channel protein abundance in the oocyte cell membrane. Exposure of BK-channel and (V617F)JAK2-expressing oocytes to the JAK2 inhibitor AG490 (40 μM) significantly decreased K(+) current. Inhibition of channel insertion by brefeldin A (5 μM) decreased the K(+) current to a similar extent in oocytes expressing the BK channel alone and in oocytes expressing the BK channel and (V617F)JAK2. The iberiotoxin (50 nM)-sensitive K(+) current in rhabdomyosarcoma cells was significantly decreased by AG490 pretreatment (40 μM, 12 h). Moreover, overexpression of JAK2 in A204 cells significantly enhanced BK channel mRNA and protein abundance. In conclusion, JAK2 upregulates BK channels by increasing channel protein abundance in the cell membrane.

Calcitriol restores antiestrogen responsiveness in estrogen receptor negative breast cancer cells: a potential new therapeutic approach

BACKGROUND

Approximately 30% of breast tumors do not express the estrogen receptor (ER) α, which is necessary for endocrine therapy approaches. Studies are ongoing in order to restore ERα expression in ERα-negative breast cancer. The aim of the present study was to determine if calcitriol induces ERα expression in ER-negative breast cancer cells, thus restoring antiestrogen responses.

METHODS

Cultured cells derived from ERα-negative breast tumors and an ERα-negative breast cancer cell line (SUM-229PE) were treated with calcitriol and ERα expression was assessed by real time PCR and western blots. The ERα functionality was evaluated by prolactin gene expression analysis. In addition, the effects of antiestrogens were assessed by growth assay using the XTT method. Gene expression of cyclin D1 (CCND1), and Ether-à-go-go 1 (EAG1) was also evaluated in cells treated with calcitriol alone or in combination with estradiol or ICI-182,780. Statistical analyses were determined by one-way ANOVA.

RESULTS

Calcitriol was able to induce the expression of a functional ERα in ER-negative breast cancer cells. This effect was mediated through the vitamin D receptor (VDR), since it was abrogated by a VDR antagonist. Interestingly, the calcitriol-induced ERα restored the response to antiestrogens by inhibiting cell proliferation. In addition, calcitriol-treated cells in the presence of ICI-182,780 resulted in a significant reduction of two important cell proliferation regulators CCND1 and EAG1.

CONCLUSIONS

Calcitriol induced the expression of ERα and restored the response to antiestrogens in ERα-negative breast cancer cells. The combined treatment with calcitriol and antiestrogens could represent a new therapeutic strategy in ERα-negative breast cancer patients.

Autocrine regulation of glioma cell proliferation via pHe-sensitive K(+) channels

Since the seminal studies of Otto Warburg in the 1920s, it has been widely recognized that cancers grow glycolytically, even in the presence of oxygen. This generates an abundance of protons in a gradient across most solid tumors with an acidic core and an alkaline rim. Whether and how this proton gradient may also serve in an autocrine fashion in these tumors is unclear. We demonstrate that human glioma cells form spheroids that act as a viable three-dimensional tumor model, forming physiologically relevant extracellular pH (pHe) and cell proliferation gradients. Using fluorescent cell cycle trackers, we determined that the rate of cell proliferation is directly dependent on pHe and that cells adjust their growth rate according to their position within the pH gradient. We further show that glioma cells sense pH via H(+)-sensitive K(+) channels, which translate changes in pH into changes in membrane voltage. These channels are tonically active and blocked by acidic pHe, quinine, and ruthenium red. Blockade of this K(+) conductance by acidic pHe or drug inhibition depolarized glioma cells and tumor spheroids and prevented their passage through the hyperpolarization-dependent G1-to-S phase cell cycle checkpoint, thereby inhibiting cell division. In this way, pHe directly determines the proliferative state of glioma cells.

The TREK2 Channel Is Involved in the Proliferation of 253J Cell, a Human Bladder Carcinoma Cell

Bladder cancer is the seventh most common cancer in men that smoke, and the incidence of disease increases with age. The mechanism of occurrence has not yet been established. Potassium channels have been linked with cell proliferation. Some two-pore domain K⁺ channels (K2P), such as TASK3 and TREK1, have recently been shown to be overexpressed in cancer cells. Here we focused on the relationship between cell growth and the mechanosensitive K2P channel, TREK2, in the human bladder cancer cell line, 253J. We confirmed that TREK2 was expressed in bladder cancer cell lines by Western blot and quantitative real-time PCR. Using the patch-clamp technique, the mechanosensitive TREK2 channel was recorded in the presence of symmetrical 150 mM KCl solutions. In 253J cells, the TREK2 channel was activated by polyunsaturated fatty acids, intracellular acidosis at -60 mV and mechanical stretch at -40 mV or 40 mV. Furthermore, small interfering RNA (siRNA)-mediated TREK2 knockdown resulted in a slight depolarization from -19.9 mV±0.8 (n=116) to -8.5 mV±1.4 (n=74) and decreased proliferation of 253J cells, compared to negative control siRNA. 253J cells treated with TREK2 siRNA showed a significant increase in the expression of cell cycle boundary proteins p21 and p53 and also a remarkable decrease in protein expression of cyclins D1 and D3. Taken together, the TREK2 channel is present in bladder cancer cell lines and may, at least in part, contribute to cell cycle-dependent growth.

Role of ion channels in the development and progression of hepatocellular carcinoma

Ion channels are pore-forming membrane proteins which play regulatory roles in a variety of biological processes. Their abnormality in expression or activity has a close relationship with the proliferation and apoptosis of cancer cells. This article will describe the role of four ion channels in the development and progression of hepatocellular carcinoma. Our review suggests that ion channels might be a new therapeutic target for hepatocellular carcinoma.

Expression profiling of ion channel genes predicts clinical outcome in breast cancer

Background

Ion channels play a critical role in a wide variety of biological processes, including the development of human cancer. However, the overall impact of ion channels on tumorigenicity in breast cancer remains controversial.

Methods

We conduct microarray meta-analysis on 280 ion channel genes. We identify candidate ion channels that are implicated in breast cancer based on gene expression profiling. We test the relationship between the expression of ion channel genes and p53 mutation status, ER status, and histological tumor grade in the discovery cohort. A molecular signature consisting of ion channel genes (IC30) is identified by Spearman’s rank correlation test conducted between tumor grade and gene expression. A risk scoring system is developed based on IC30. We test the prognostic power of IC30 in the discovery and seven validation cohorts by both Cox proportional hazard regression and log-rank test.

Results

22, 24, and 30 ion channel genes are found to be differentially expressed with a change in p53 mutation status, ER status, and tumor histological grade in the discovery cohort. We assign the 30 tumor grade associated ion channel genes as the IC30 gene signature. We find that IC30 risk score predicts clinical outcome (P < 0.05) in the discovery cohort and 6 out of 7 validation cohorts. Multivariate and univariate tests conducted in two validation cohorts indicate that IC30 is a robust prognostic biomarker, which is independent of standard clinical and pathological prognostic factors including patient age, lymph node status, tumor size, tumor grade, estrogen and progesterone receptor status, and p53 mutation status.

Conclusions

We identified a molecular gene signature IC30, which represents a promising diagnostic and prognostic biomarker in breast cancer. Our results indicate that information regarding the expression of ion channels in tumor pathology could provide new targets for therapy in human cancers.

K(+) channels and cell cycle progression in tumor cells

K⁺ ions play a major role in many cellular processes. The deregulation of K⁺ signaling is associated with a variety of diseases such as hypertension, atherosclerosis, or diabetes. K⁺ ions are important for setting the membrane potential, the driving force for Ca²⁺ influx, and regulate volume of growing cells. Moreover, it is increasingly recognized that K⁺ channels control cell proliferation through a novel signaling mechanisms triggered and modulated independently of ion fluxes. In cancer, aberrant expression, regulation and/or sublocalization of K⁺ channels can alter the downstream signals that converge on the cell cycle machinery. Various K⁺ channels are involved in cell cycle progression and are needed only at particular stages of the cell cycle. Consistent with this idea, the expression of Eag1 and HERG channels fluctuate along the cell cycle. Despite of acquired knowledge, our understanding of K⁺ channels functioning in cancer cells requires further studies. These include identifying the molecular mechanisms controlling the cell cycle machinery. By understanding how K⁺ channels regulate cell cycle progression in cancer cells, we will gain insights into how cancer cells subvert the need for K⁺ signal and its downstream targets to proliferate.

Membrane potential and cancer progression

Membrane potential (V_m), the voltage across the plasma membrane, arises because of the presence of different ion channels/transporters with specific ion selectivity and permeability. V_m is a key biophysical signal in non-excitable cells, modulating important cellular activities, such as proliferation and differentiation. Therefore, the multiplicities of various ion channels/transporters expressed on different cells are finely tuned in order to regulate the V_m. It is well-established that cancer cells possess distinct bioelectrical properties. Notably, electrophysiological analyses in many cancer cell types have revealed a depolarized V_m that favors cell proliferation. Ion channels/transporters control cell volume and migration, and emerging data also suggest that the level of V_m has functional roles in cancer cell migration. In addition, hyperpolarization is necessary for stem cell differentiation. For example, both osteogenesis and adipogenesis are hindered in human mesenchymal stem cells (hMSCs) under depolarizing conditions. Therefore, in the context of cancer, membrane depolarization might be important for the emergence and maintenance of cancer stem cells (CSCs), giving rise to sustained tumor growth. This review aims to provide a broad understanding of the V_m as a bioelectrical signal in cancer cells by examining several key types of ion channels that contribute to its regulation. The mechanisms by which V_m regulates cancer cell proliferation, migration, and differentiation will be discussed. In the long term, V_m might be a valuable clinical marker for tumor detection with prognostic value, and could even be artificially modified in order to inhibit tumor growth and metastasis.

Membrane potential mediates the cellular binding of nanoparticles

The use of nanoparticles for cellular therapeutic or sensing applications requires nanoparticles to bind, or adhere, to the cell surface. While nanoparticle parameters such as size, shape, charge, and composition are important factors in cellular binding, the cell itself must also be considered. All cells have an electrical potential across the plasma membrane driven by an ion gradient. Under standard conditions the ion gradient will result in a -10 to -100 mV potential across the membrane with a net negative charge on the cytosolic face. Using a combination of flow cytometry and fluorescence microscopy experiments and dissipative particle dynamics simulations, we have found that a decrease in membrane potential leads to decreased cellular binding of anionic nanoparticles. The decreased cellular binding of anionic nanoparticles is a general phenomenon, independent of depolarization method, nanoparticle composition, and cell type. Increased membrane potential reverses this trend resulting in increased binding of anionic nanoparticles. The cellular binding of cationic nanoparticles is minimally affected by membrane potential due to the interaction of cationic nanoparticles with cell surface proteins. The influence of membrane potential on the cellular binding of nanoparticles is especially important when considering the use of nanoparticles in the treatment or detection of diseases, such as cancer, in which the membrane potential is decreased.

Glibenclamide inhibits cell growth by inducing G0/G1 arrest in the human breast cancer cell line MDA-MB-231

BACKGROUND

Glibenclamide (Gli) binds to the sulphonylurea receptor (SUR) that is a regulatory subunit of ATP-sensitive potassium channels (KATP channels). Binding of Gli to SUR produces the closure of KATP channels and the inhibition of their activity. This drug is widely used for treatment of type 2-diabetes and it has been signaled as antiproliferative in several tumor cell lines. In previous experiments we demonstrated the antitumoral effect of Gli in mammary tumors induced in rats. The aim of the present work was to investigate the effect of Gli on MDA-MB-231 breast cancer cell proliferation and to examine the possible pathways involved in this action.

RESULTS

The mRNA expression of the different subunits that compose the KATP channels was evaluated in MDA-MB-231 cells by reverse transcriptase-polymerase chain reaction. Results showed the expression of mRNA for both pore-forming isoforms Kir6.1 and Kir6.2 and for the regulatory isoform SUR2B in this cell line. Gli inhibited cell proliferation assessed by a clonogenic method in a dose dependent manner, with an increment in the population doubling time. The KATP channel opener minoxidil increased clonogenic proliferation, effect that was counteracted by Gli. When cell cycle analysis was performed by flow cytometry, Gli induced a significant cell-cycle arrest in G0/G1 phase, together with an up-regulation of p27 levels and a diminution in cyclin E expression, both evaluated by immunoblot. However, neither differentiation evaluated by neutral lipid accumulation nor apoptosis assessed by different methodologies were detected. The cytostatic, non toxic effect on cell proliferation was confirmed by removal of the drug.Combination treatment of Gli with tamoxifen or doxorubicin showed an increment in the antiproliferative effect only for doxorubicin.

CONCLUSIONS

Our data clearly demonstrated a cytostatic effect of Gli in MDA-MB-231 cells that may be mediated through KATP channels, associated to the inhibition of the G1-S phase progression. In addition, an interesting observation about the effect of the combination of Gli with doxorubicin leads to future research for a potential novel role for Gli as an adjuvant in breast cancer treatment.

Unconventional role of the inwardly rectifying potassium channel Kir2.2 as a constitutive activator of RelA in cancer

The constitutive activation of NF-κB is a major event leading to the initiation, development, and progression of cancer. Recently, we showed that the size of preestablished tumors was reduced after the depletion of Kir2.2, an inwardly rectifying potassium channel. To determine the precise mechanism of action of Kir2.2 in the control of tumor growth, we searched for interacting proteins. Notably, NF-κB p65/RelA was identified as a binding partner of Kir2.2 in a yeast two-hybrid analysis. Further analyses revealed that Kir2.2 directly interacted with RelA in vitro and coimmunoprecipitated with RelA from cell lysates. Kir2.2 increased RelA phosphorylation at S536 and facilitated its translocation from the cytoplasm to the nucleus, thereby activating the transcription factor and increasing the expression level of NF-κB targets, including cyclin D1, matrix metalloproteinase (MMP)9, and VEGF. Kir2.2 was overexpressed in human cancer and the expression level was correlated with increased colony formation and tumor growth in mouse tumor models. On the basis of these findings, we propose an unconventional role for Kir2.2 as a constitutive RelA-activating protein, which is likely to contribute to tumor progression in vivo.

Astemizole synergizes calcitriol antiproliferative activity by inhibiting CYP24A1 and upregulating VDR: a novel approach for breast cancer therapy

Background

Calcitriol antiproliferative effects include inhibition of the oncogenic ether-à-go-go-1 potassium channel (Eag1) expression, which is necessary for cell cycle progression and tumorigenesis. Astemizole, a new promising antineoplastic drug, targets Eag1 by blocking ion currents. Herein, we characterized the interaction between calcitriol and astemizole as well as their conjoint antiproliferative action in SUM-229PE, T-47D and primary tumor-derived breast cancer cells.

Methodology/Principal Findings

Molecular markers were studied by immunocytochemistry, Western blot and real time PCR. Inhibitory concentrations were determined by dose-response curves and metabolic activity assays. At clinically achievable drug concentrations, synergistic antiproliferative interaction was observed between calcitriol and astemizole, as calculated by combination index analysis (CI <1). Astemizole significantly enhanced calcitriol’s growth-inhibitory effects (3–11 folds, P<0.01). Mean IC₂₀ values were 1.82±2.41 nM and 1.62±0.75 µM; for calcitriol (in estrogen receptor negative cells) and astemizole, respectively. Real time PCR showed that both drugs alone downregulated, while simultaneous treatment further reduced Ki-67 and Eag1 gene expression (P<0.05). Astemizole inhibited basal and calcitriol-induced CYP24A1 and CYP3A4 mRNA expression (cytochromes involved in calcitriol and astemizole degradation) in breast and hepatoma cancer cells, respectively, while upregulated vitamin D receptor (VDR) expression.

Conclusions/Significance

Astemizole synergized calcitriol antiproliferative effects by downregulating CYP24A1, upregulating VDR and targeting Eag1. This study provides insight into the molecular mechanisms involved in astemizole-calcitriol combined antineoplastic effect, offering scientific support to test both compounds in combination in further preclinical and clinical studies of neoplasms expressing VDR and Eag1. VDR-negative tumors might also be sensitized to calcitriol antineoplastic effects by the use of astemizole. Herein we suggest a novel combined adjuvant therapy for the management of VDR/Eag1-expressing breast cancer tumors. Since astemizole improves calcitriol bioavailability and activity, decreased calcitriol dosing is advised for conjoint administration.

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.

Overexpression of Delayed Rectifier K(+) Channels Promotes In situ Proliferation of Leukocytes in Rat Kidneys with Advanced Chronic Renal Failure

Leukocytes, such as lymphocytes and macrophages, predominantly express delayed rectifier K⁺ channels (Kv1.3), and the channels play crucial roles in the activation and proliferation of the cells. Since lymphocytes are activated in patients with end-stage renal disease (ESRD), the channels expressed in those cells would contribute to the progression of renal fibrosis in advanced-stage chronic renal failure (CRF). In the present study, using a rat model with advanced CRF that underwent 5/6 nephrectomy followed by a 14-week recovery period, we examined the histopathological features of the kidneys and the leukocyte expression of Kv1.3-channels and cell cycle markers. Age-matched sham-operated rats were used as controls. In the cortical interstitium of advanced CRF rat kidneys, leukocytes proliferated in situ and overexpressed Kv1.3 channel protein in their cytoplasm. Treatment with margatoxin, a selective Kv1.3-channel inhibitor, significantly suppressed the number of leukocytes and the progression of renal fibrosis with a significant decrease in the cortical cell cycle marker expression. This study demonstrated for the first time that the number of leukocytes was dramatically increased in rat kidneys with advanced CRF. The overexpression of Kv1.3 channels in the leukocytes was thought to contribute to the progression of renal fibrosis by stimulating cell cycling and promoting cellular proliferation.

Activation of BK(Ca) channels in zoledronic acid-induced apoptosis of MDA-MB-231 breast cancer cells

BACKGROUND

Zoledronic acid, one of the most potent nitrogen-containing biphosphonates, has been demonstrated to have direct anti-tumor and anti-metastatic properties in breast cancer in vitro and in vivo. In particular, tumor-cell apoptosis has been recognized to play an important role in the treatment of metastatic breast cancer with zoledronic acid. However, the precise mechanisms remain less clear. In the present study, we investigated the specific role of large conductance Ca(2+)-activated potassium (BK(Ca)) channel in zoledronic acid-induced apoptosis of estrogen receptor (ER)-negative MDA-MB-231 breast cancer cells.

METHODOLOGY/PRINCIPAL FINDINGS

The action of zoledronic acid on BK(Ca) channel was investigated by whole-cell and cell-attached patch clamp techniques. Cell apoptosis was assessed with immunocytochemistry, analysis of fragmented DNA by agarose gel electrophoresis, and flow cytometry assays. Cell proliferation was investigated by MTT test and immunocytochemistry. In addition, such findings were further confirmed with human embryonic kidney 293 (HEK293) cells which were transfected with functional BK(Ca) α-subunit (hSloα). Our results clearly indicated that zoledronic acid directly increased the activities of BK(Ca) channels, and then activation of BK(Ca) channel by zoledronic acid contributed to induce apoptosis in MDA-MB-231 cells. The possible mechanisms were associated with the elevated level of intracellular Ca(2+) and a concomitant depolarization of mitochondrial membrane potential (Δψm) in MDA-MB-231 cells.

CONCLUSIONS

Activation of BK(Ca) channel was here shown to be a novel molecular pathway involved in zoledronic acid-induced apoptosis of MDA-MB-231 cells in vitro.

Reduction of breast cancer cell migration via up-regulation of TASK-3 two-pore domain K+ channel

AIM

Many kinds of K(+) channels are expressed in a variety of cells, including cancer cells. However, only a small amount of research has explored the relationship between voltage-independent K(+) channels and breast cancer. This study was performed to investigate whether changes in two-pore domain K(+) (K(2P) ) channel expression levels are related to the migration of human breast cancer cells.

METHODS

K(2P) channel gene/protein expression levels were compared between MCF-7 (a non-invasive cell) and MDA-MB-231 (an invasive cell) using reverse transcriptase (RT)-polymerase chain reaction (PCR), real-time PCR, Western blotting and immunocytochemistry. The relationship between K(2P) channel expression level and cell migration was analysed using gene overexpression and knock-down techniques. Functional expression of TASK-3 in MCF-7 and MDA-MB-231 cells was recorded using patch-clamp technique.

RESULTS

Of K(2P) channels, TASK-3 mRNA and protein were highly expressed in MCF-7 cells compared with those in MDA-MB-231 cells. Overexpression of TASK-3 in breast cancer cells reduced migration and invasion, whereas silencing of TASK-3 increased the migration and invasion. The TASK-3 expression level was decreased by phorbol myristate acetate (PMA), a PKC activator. PMA also enhanced the cell migration in MDA-MB-231 cells.

CONCLUSION

These results show that an increase in TASK-3 expression levels, which could be modulated by PKC activation, reduces cell migration/invasion in breast cancer cells and suggest that modulation of TASK-3 expression may regulate metastasis of breast cancer cells.

Biopotential signals of breast cancer versus tumor types and proliferation stages

Clinical studies have shown compelling data of elevated biopotential signals recorded noninvasively from the breasts of women with breast cancer. While these data are compelling and show a strong potential for use in the noninvasive early detection of breast cancer, there remains significant knowledge gaps which must be addressed before this technology can be routinely used for breast cancer detection. A diffusion-drift model is developed to study the spatial and temporal characteristics of the biopotential signals of breast tumors taking into account the morphology and cell division stages. The electric signals of the most common tumor types-papillary, compact, and comedo-are also considered. The largest biopotential signal is observed from the compact tumor, while the smallest signal is observed from the papillary type. The results also show an increase in the time duration of the generated biopotential signals when cancer cells start their transitions at different time instants. The spatial and temporal variations of the biopotential signals are correlated with the tumor pattern which can have important implications for breast cancer detection.

Regulation of cell behavior and tissue patterning by bioelectrical signals: challenges and opportunities for biomedical engineering

Achieving control over cell behavior and pattern formation requires molecular-level understanding of regulatory mechanisms. Alongside transcriptional networks and biochemical gradients, there functions an important system of cellular communication and control: transmembrane voltage gradients (V(mem)). Bioelectrical signals encoded in spatiotemporal changes of V(mem) control cell proliferation, migration, and differentiation. Moreover, endogenous bioelectrical gradients serve as instructive cues mediating anatomical polarity and other organ-level aspects of morphogenesis. In the past decade, significant advances in molecular physiology have enabled the development of new genetic and biophysical tools for the investigation and functional manipulation of bioelectric cues. Recent data implicate V(mem) as a crucial epigenetic regulator of patterning events in embryogenesis, regeneration, and cancer. We review new conceptual and methodological developments in this fascinating field. Bioelectricity offers a novel way of quantitatively understanding regulation of growth and form in vivo, and it reveals tractable, powerful control points that will enable truly transformative applications in bioengineering, regenerative medicine, and synthetic biology.

Rapid internalization of the oncogenic K+ channel K(V)10.1

K(V)10.1 is a mammalian brain voltage-gated potassium channel whose ectopic expression outside of the brain has been proven relevant for tumor biology. Promotion of cancer cell proliferation by K(V)10.1 depends largely on ion flow, but some oncogenic properties remain in the absence of ion permeation. Additionally, K(V)10.1 surface populations are small compared to large intracellular pools. Control of protein turnover within cells is key to both cellular plasticity and homeostasis, and therefore we set out to analyze how endocytic trafficking participates in controlling K(V)10.1 intracellular distribution and life cycle. To follow plasma membrane K(V)10.1 selectively, we generated a modified channel of displaying an extracellular affinity tag for surface labeling by α-bungarotoxin. This modification only minimally affected K(V)10.1 electrophysiological properties. Using a combination of microscopy and biochemistry techniques, we show that K(V)10.1 is constitutively internalized involving at least two distinct pathways of endocytosis and mainly sorted to lysosomes. This occurs at a relatively fast rate. Simultaneously, recycling seems to contribute to maintain basal K(V)10.1 surface levels. Brief K(V)10.1 surface half-life and rapid lysosomal targeting is a relevant factor to be taken into account for potential drug delivery and targeting strategies directed against K(V)10.1 on tumor cells.

Ion channels and transporters in cancer. 1. Ion channels and cell proliferation in cancer

Progress through the cell mitotic cycle requires precise timing of the intrinsic molecular steps and tight coordination with the environmental signals that maintain a cell into the proper physiological context. Because of their great functional flexibility, ion channels coordinate the upstream and downstream signals that converge on the cell cycle machinery. Both voltage- and ligand-gated channels have been implicated in the control of different cell cycle checkpoints in normal as well as neoplastic cells. Ion channels mediate the calcium signals that punctuate the mitotic process, the cell volume oscillations typical of cycling cells, and the exocytosis of autocrine or angiogenetic factors. Other functions of ion channels in proliferation are still matter of debate. These may or may not depend on ion transport, as the channel proteins can form macromolecular complexes with growth factor and cell adhesion receptors. Direct conformational coupling with the cytoplasmic regulatory proteins is also possible. Derangement or relaxed control of the above processes can promote neoplasia. Specific types of ion channels have turned out to participate in the different stages of the tumor progression, in which cell heterogeneity is increased by the selection of malignant cell clones expressing the ion channel types that better support unrestrained growth. However, a comprehensive mechanistic picture of the functional relations between ion channels and cell proliferation is yet not available, partly because of the considerable experimental challenges offered by studying these processes in living mammalian cells. No doubt, such studies will constitute one of the most fruitful research fields for the next generation of cell physiologists.

K(+) channels as therapeutic targets in oncology

Ion channels are involved in a variety of tumors. In particular, potassium channels are expressed abnormally in many cancer types, where their pharmacologic manipulation impairs tumor progression. Since this group of molecules has been successfully targeted for decades in other therapeutic areas, there is a significant body of knowledge on the pharmacology of potassium channels. Several groups of potassium channels with defined molecular identities have been proposed as candidates for therapeutic intervention. The strategies put forward range from classical small molecule blockade to gene therapy approaches, and include the use of potassium channels as targets for adjuvant therapy. We will discuss the reasons for these proposals and explore possible future developments.

The life and death of breast cancer cells: proposing a role for the effects of phytoestrogens on potassium channels

Changes in the regulation of potassium channels are increasingly implicated in the altered activity of breast cancer cells. Increased or reduced expression of a number of K(+) channels have been identified in numerous breast cancer cell lines and cancerous tissue biopsy samples, compared to normal tissue, and are associated with tumor formation and spread, enhanced levels of proliferation, and resistance to apoptotic stimuli. Through knockout or silencing of K(+) channel genes, and use of specific or more broad pharmacologic K(+) channel blockers, the growth of numerous cell lines, including breast cancer cells, has been modified. In this manner it has been proposed that in MCF7 breast cancer cells proliferation appears to be regulated by the activity of a number of K(+) channels, including the Ca(2+) activated K(+) channels, and the voltage-gated K(+) channels hEAG and K(v)1.1. The effect of phytoestrogens on K(+) channels has not been extensively studied but yields some interesting results. In a number of cell lines the phytoestrogen genistein inhibits K(+) current through several channels including K(v)1.3 and hERG. Where it has been used, structurally similar daidzein has little or no effect on K(+) channel activity. Since many K(+) channels have roles in proliferation and apoptosis in breast cancer cells, the impact of K(+) channel regulation by phytoestrogens is of potentially great relevance.

The two-pore domain potassium channel KCNK5: induction by estrogen receptor alpha and role in proliferation of breast cancer cells

The growth of many human breast tumors requires the proliferative effect of estrogen acting via the estrogen receptor α (ERα). ERα signaling is therefore a clinically important target for breast cancer prevention and therapeutics. Although extensively studied, the mechanism by which ERα promotes proliferation remains to be fully established. We observed an up-regulation of transcript encoding the pH-sensitive two-pore domain potassium channel KCNK5 in a screen for genes stimulated by 17β-estradiol (E2) in the ERα(+) breast cancer cell lines MCF-7 and T47D. KCNK5 mRNA increased starting 1 h after the onset of E2 treatment, and protein levels followed after 12 h. Estrogen-responsive elements are found in the enhancer region of KCNK5, and chromatin immunoprecipitation assays revealed binding of ERα to the KCNK5 enhancer in E2-treated MCF-7 cells. Cells treated with E2 also showed increases in the amplitude of pH-sensitive potassium currents, as assessed by whole-cell recordings. These currents are blocked by clofilium. Although confocal microscopy suggested that most of the channels are located in intracellular compartments, the increase in macroscopic currents suggests that E2 treatment increases the number of active channels at the cell surface. Application of small interfering RNA specific for KCNK5 decreased pH-sensitive potassium currents and also reduced the estrogen-induced proliferation of T47D cells. We conclude that E2 induces the expression of KCNK5 via ERα(+) in breast cancer cells, and this channel plays a role in regulating proliferation in these cell lines. KCNK5 may therefore represent a useful target for treatment, for example, of tamoxifen-resistant breast cancer.

Dendrotoxin-κ suppresses tumor growth induced by human lung adenocarcinoma A549 cells in nude mice

Voltage-gated K(+) (Kv) channels have been considered to be a regulator of membrane potential and neuronal excitability. Recently, accumulated evidence has indicated that several Kv channel subtypes contribute to the control of cell proliferation in various types of cells and are worth noting as potential emerging molecular targets of cancer therapy. In the present study, we investigated the effects of the Kv1.1-specific blocker, dendrotoxin-κ (DTX-κ, on tumor formation induced by the human lung adenocarcinoma cell line A549 in a xenograft model. Kv1.1 mRNA and protein was expressed in A549 cells and the blockade of Kv1.1 by DTX-κ, reduced tumor formation in nude mice. Furthermore, treatment with DTX-κ significantly increased protein expression of p21(Waf1/Cip1), p27(Kip1), and p15(INK4B) and significantly decreased protein expression of cyclin D3 in tumor tissues compared to the control. These results suggest that DTX-κ has anti-tumor effects in A549 cells through the pathway governing G1-S transition.

Involvement of Kv4.1 K(+) channels in gastric cancer cell proliferation

Voltage-gated potassium (Kv) channels are expressed not only in excitable cells but also in non-excitable cells such as epithelial cells. Recent studies have demonstrated that several subtypes of Kv channels are expressed in epithelial tumor cells, including human gastric cancer cells, and are associated with cell proliferation. In the present study, we examined the expression of Kv4.1 in human gastric cancer cell lines and the effects of suppressed expression of Kv4.1 on cell proliferation and cell cycle distribution. We found that Kv4.1 mRNA and protein are expressed in the human gastric cancer cell lines MKN-45 and SNU-638. Moreover, Kv4.1-targeted small interference RNA (siRNA) treatment inhibited gastric cancer cell proliferation. Flow cytometric analysis revealed that suppressed expression of Kv4.1 induced a G1-S transition block of cell cycle progression. These results reveal that Kv4.1 plays a role in the proliferation of the human gastric cancer cell lines MKN-45 and SNU-638 and can be considered as a therapeutic target for human gastric cancer.

Transmembrane potential of GlyCl-expressing instructor cells induces a neoplastic-like conversion of melanocytes via a serotonergic pathway

Understanding the mechanisms that coordinate stem cell behavior within the host is a high priority for developmental biology, regenerative medicine and oncology. Endogenous ion currents and voltage gradients function alongside biochemical cues during pattern formation and tumor suppression, but it is not known whether bioelectrical signals are involved in the control of stem cell progeny in vivo. We studied Xenopus laevis neural crest, an embryonic stem cell population that gives rise to many cell types, including melanocytes, and contributes to the morphogenesis of the face, heart and other complex structures. To investigate how depolarization of transmembrane potential of cells in the neural crest's environment influences its function in vivo, we manipulated the activity of the native glycine receptor chloride channel (GlyCl). Molecular-genetic depolarization of a sparse, widely distributed set of GlyCl-expressing cells non-cell-autonomously induces a neoplastic-like phenotype in melanocytes: they overproliferate, acquire an arborized cell shape and migrate inappropriately, colonizing numerous tissues in a metalloprotease-dependent fashion. A similar effect was observed in human melanocytes in culture. Depolarization of GlyCl-expressing cells induces these drastic changes in melanocyte behavior via a serotonin-transporter-dependent increase of extracellular serotonin (5-HT). These data reveal GlyCl as a molecular marker of a sparse and heretofore unknown cell population with the ability to specifically instruct neural crest derivatives, suggest transmembrane potential as a tractable signaling modality by which somatic cells can control stem cell behavior at considerable distance, identify a new biophysical aspect of the environment that confers a neoplastic-like phenotype upon stem cell progeny, reveal a pre-neural role for serotonin and its transporter, and suggest a novel strategy for manipulating stem cell behavior.

Cloning and characterisation of GIRK1 variants resulting from alternative RNA editing of the KCNJ3 gene transcript in a human breast cancer cell line

The aim of this study was to investigate the impact of increased mRNA levels encoding GIRK1 in breast tumours on GIRK protein expression. mRNA levels encoding hGIRK1 and hGIRK4 in the MCF7, MCF10A and MDA-MB-453 breast cancer cell lines were assessed and the corresponding proteins detected using Western blots. cDNAs encoding for four hGIRK1 splice variants (hGIRK1a, 1c, 1d and 1e) were cloned from the MCF7 cell line. Subcellular localisation of fluorescence labelled hGIRK1a-e and hGIRK4 and of endogenous GIRK1 and GIRK4 subunits was monitored in the MCF7 cell line. All hGIRK1 splice variants and hGIRK4 were predominantly located within the endoplasmic reticulum. Heterologous expression in Xenopus laevis oocytes and two electrode voltage clamp experiments together with confocal microscopy were performed. Only the hGIRK1a subunit was able to form functional GIRK channels in connection with hGIRK4. The other splice variants are expressed, but exert a dominant negative effect on heterooligomeric channel function. Hence, alternative splicing of the KCNJ3 gene transcript in the MCF7 cell line leads to a family of mRNA's, encoding truncated versions of the hGIRK1 protein. The very high abundance of mRNA's encoding GIRK1 together with the presence of GIRK1 protein suggests a pathophysiological role in breast cancer.

Molecular background of leak K+ currents: two-pore domain potassium channels

Two-pore domain K(+) (K(2P)) channels give rise to leak (also called background) K(+) currents. The well-known role of background K(+) currents is to stabilize the negative resting membrane potential and counterbalance depolarization. However, it has become apparent in the past decade (during the detailed examination of the cloned and corresponding native K(2P) channel types) that this primary hyperpolarizing action is not performed passively. The K(2P) channels are regulated by a wide variety of voltage-independent factors. Basic physicochemical parameters (e.g., pH, temperature, membrane stretch) and also several intracellular signaling pathways substantially and specifically modulate the different members of the six K(2P) channel subfamilies (TWIK, TREK, TASK, TALK, THIK, and TRESK). The deep implication in diverse physiological processes, the circumscribed expression pattern of the different channels, and the interesting pharmacological profile brought the K(2P) channel family into the spotlight. In this review, we focus on the physiological roles of K(2P) channels in the most extensively investigated cell types, with special emphasis on the molecular mechanisms of channel regulation.

Ion channels and the hallmarks of cancer

Plasma membrane (PM) ion channels contribute to virtually all basic cellular processes and are also involved in the malignant phenotype of cancer cells. Here, we review the role of ion channels in cancer in the context of their involvement in the defined hallmarks of cancer: 1) self-sufficiency in growth signals, 2) insensitivity to antigrowth signals, 3) evasion of programmed cell death (apoptosis), 4) limitless replicative potential, 5) sustained angiogenesis and 6) tissue invasion and metastasis. Recent studies have indicated that the contribution of specific ion channels to these hallmarks varies for different types of cancer. Therefore, to determine the importance of ion channels as targets for cancer diagnosis and treatment their expression, function and regulation must be assessed for each cancer.

Transient receptor potential channelopathies

In the past years, several hereditary diseases caused by defects in transient receptor potential channels (TRP) genes have been described. This review summarizes our current knowledge about TRP channelopathies and their possible pathomechanisms. Based on available genetic indications, we will also describe several putative pathological conditions in which (mal)function of TRP channels could be anticipated.

Cell cycle-dependent expression of Kv3.4 channels modulates proliferation of human uterine artery smooth muscle cells

AIMS

Vascular smooth muscle cell (VSMC) proliferation is involved in cardiovascular pathologies associated with unwanted arterial wall remodelling. Coordinated changes in the expression of several K+ channels have been found to be important elements in the phenotypic switch of VSMCs towards proliferation. We have previously demonstrated the association of functional expression of Kv3.4 channels with proliferation of human uterine VSMCs. Here, we sought to gain deeper insight on the relationship between Kv3.4 channels and cell cycle progression in this preparation.

METHODS AND RESULTS

Expression and function of Kv3.4 channels along the cell cycle was explored in uterine VSMCs synchronized at different checkpoints, combining real-time PCR, western blotting, and electrophysiological techniques. Flow cytometry, Ki67 expression and BrdU incorporation techniques allowed us to explore the effects of Kv3.4 channels blockade on cell cycle distribution. We found cyclic changes in Kv3.4 and MiRP2 mRNA and protein expression along the cell cycle. Functional studies showed that Kv3.4 current amplitude and Kv3.4 channels contribution to cell excitability increased in proliferating cells. Finally, both Kv3.4 blockers and Kv3.4 knockdown with siRNA reduced the proportion of proliferating VSMCs.

CONCLUSION

Our data indicate that Kv3.4 channels exert a permissive role in the cell cycle progression of proliferating uterine VSMCs, as their blockade induces cell cycle arrest after G2/M phase completion. The modulation of resting membrane potential (V(M)) by Kv3.4 channels in proliferating VSMCs suggests that their role in cell cycle progression could be at least in part mediated by their contribution to the hyperpolarizing signal needed to progress through the G1 phase.

An investigation of the occurrence and properties of the mitochondrial intermediate-conductance Ca2+-activated K+ channel mtKCa3.1

The mitochondrial intermediate-conductance Ca2+-activated K+ channel mtKCa3.1 has recently been discovered in the HCT116 colon tumor-derived cell line, which expresses relatively high levels of this protein also in the plasma membrane. Electrophysiological recordings revealed that the channel can exhibit different conductance states and kinetic modes, which we tentatively ascribe to post-translational modifications. To verify whether the localization of this channel in mitochondria might be a peculiarity of these cells or a more widespread feature we have checked for the presence of mtKCa3.1 in a few other cell lines using biochemical and electrophysiological approaches. It turned out to be present at least in some of the cells investigated. Functional assays explored the possibility that mtKCa3.1 might be involved in cell proliferation or play a role similar to that of the Shaker-type KV1.3 channel in lymphocytes, which interacts with outer mitochondrial membrane-inserted Bax thereby promoting apoptosis (Szabò, I. et al., Proc. Natl. Acad Sci. USA 105 (2008) 14861-14866). A specific KCa3.1 inhibitor however did not have any detectable effect on cell proliferation or death.

The intermediate conductance Ca2+-activated K+ channel inhibitor TRAM-34 stimulates proliferation of breast cancer cells via activation of oestrogen receptors

BACKGROUND AND PURPOSE

K(+) channels play a role in the proliferation of cancer cells. We have investigated the effects of specific K(+) channel inhibitors on basal and oestrogen-stimulated proliferation of breast cancer cells.

EXPERIMENTAL APPROACH

Using the mammary adenocarcinoma cell line MCF-7 we assayed cell proliferation by radiolabelled thymidine incorporation in the absence or presence of various K(+) channel inhibitors with or without 17beta-oestradiol.

KEY RESULTS

Inhibitors of K(v)10.1 and K(Ca)3.1 K(+) channels suppressed basal proliferation of MCF-7 cells, but not oestrogen-stimulated proliferation. TRAM-34, a specific inhibitor of K(Ca)3.1 channels increased or decreased cell proliferation depending on the concentration. At intermediate concentrations (3-10 microM) TRAM-34 increased cell proliferation, whereas at higher concentrations (20-100 microM) TRAM-34 decreased cell proliferation. The enhancement of cell proliferation caused by TRAM-34 was blocked by the oestrogen receptor antagonists ICI182,780 and tamoxifen. TRAM-34 also increased progesterone receptor mRNA expression, decreased oestrogen receptor-alpha mRNA expression and reduced the binding of radiolabelled oestrogen to MCF-7 oestrogen receptor, in each case mimicking the effects of 17beta-oestradiol.

CONCLUSIONS AND IMPLICATIONS

Our results demonstrate that K(+) channels K(v)10.1 and K(Ca)3.1 play a role in basal, but not oestrogen-stimulated MCF-7 cell proliferation. TRAM-34, as well as inhibiting K(Ca)3.1, directly interacts with the oestrogen receptor and mimics the effects of 17beta-oestradiol on MCF-7 cell proliferation and gene modulation. Our finding that TRAM-34 is able to activate the oestrogen receptor suggests a novel action of this supposedly specific K(+) channel inhibitor and raises concerns of interpretation in its use.

Role of membrane potential in the regulation of cell proliferation and differentiation

Biophysical signaling, an integral regulator of long-term cell behavior in both excitable and non-excitable cell types, offers enormous potential for modulation of important cell functions. Of particular interest to current regenerative medicine efforts, we review several examples that support the functional role of transmembrane potential (V(mem)) in the regulation of proliferation and differentiation. Interestingly, distinct V(mem) controls are found in many cancer cell and precursor cell systems, which are known for their proliferative and differentiation capacities, respectively. Collectively, the data demonstrate that bioelectric properties can serve as markers for cell characterization and can control cell mitotic activity, cell cycle progression, and differentiation. The ability to control cell functions by modulating bioelectric properties such as V(mem) would be an invaluable tool for directing stem cell behavior toward therapeutic goals. Biophysical properties of stem cells have only recently begun to be studied and are thus in need of further characterization. Understanding the molecular and mechanistic basis of biophysical regulation will point the way toward novel ways to rationally direct cell functions, allowing us to capitalize upon the potential of biophysical signaling for regenerative medicine and tissue engineering.

Deregulation of 2 potassium channels in pancreas adenocarcinomas: implication of KV1.3 gene promoter methylation

OBJECTIVES

The aim of this study was to examine expression of 2 potassium (K) channels in pancreatic adenocarcinoma.

METHODS

The immunohistochemical and mRNA expression of GIRK1 (G-protein inwardly rectifying K channel 1) and KV1.3 channel (voltage-dependent K channel) was studied in 55 and 18 adenocarcinomas and 33 and 8 normal pancreas specimens, respectively. The methylation status of KV1.3 promoter was studied by methyl-specific polymerase chain reaction in 33 pancreatic adenocarcinomas. The results were correlated with the patients' prognosis.

RESULTS

GIRK1 was highly expressed in 80% (44/55) of adenocarcinoma samples versus 57.6% (19/33) of normal samples (P=0.03), as confirmed by reverse transcriptase-polymerase chain reaction results (P=0.007). KV1.3 expression was decreased in pancreatic adenocarcinomas compared with normal tissue (7.3% vs 39.4%; P=0.0005). KV1.3 down-expression was associated with metastatic tumors (P=0.018). KV1.3 promoter methylation was observed in 69.7% (22/33) of adenocarcinomas.

CONCLUSION

This is the first report of deregulation of 2 K channels in pancreatic adenocarcinoma. GIRK1 was highly expressed in pancreatic adenocarcinomas, corresponding to its role in cell proliferation. Methylation of KV1.3 gene promoter could explain the decrease of KV1.3 expression in adenocarcinomas. New therapeutic agents, such as DNA methylation inhibitors, could be useful against this dramatic cancer.

Bioelectric mechanisms in regeneration: Unique aspects and future perspectives

Regenerative biology has focused largely on chemical factors and transcriptional networks. However, endogenous ion flows serve as key epigenetic regulators of cell behavior. Bioelectric signaling involves feedback loops, long-range communication, polarity, and information transfer over multiple size scales. Understanding the roles of endogenous voltage gradients, ion flows, and electric fields will contribute to the basic understanding of numerous morphogenetic processes and the means by which they can robustly restore pattern after perturbation. By learning to modulate the bioelectrical signals that control cell proliferation, migration, and differentiation, we gain a powerful set of new techniques with which to manipulate growth and patterning in biomedical contexts. This chapter reviews the unique properties of bioelectric signaling, surveys molecular strategies and reagents for its investigation, and discusses the opportunities made available for regenerative medicine.

Intermediate conductance Ca2+-activated potassium channel (KCa3.1) in the inner mitochondrial membrane of human colon cancer cells

Patch-clamping mitoplasts isolated from human colon carcinoma 116 cells has allowed the identification and characterization of the intermediate conductance Ca(2+)-activated K(+)-selective channel K(Ca)3.1, previously studied only in the plasma membrane of various cell types. Its identity has been established by its biophysical and pharmacological properties. Its localisation in the inner membrane of mitochondria is indicated by Western blots of subcellular fractions, by recording of its activity in mitochondria made fluorescent by a mitochondria-targeted fluorescent protein and by the co-presence of channels considered to be markers of the inner membrane. Moderate increases of mitochondrial matrix [Ca(2+)] will cause mtK(Ca)3.1 opening, thus linking inner membrane K(+) permeability and transmembrane potential to Ca(2+) signalling.