Clinical Relevance of Ion Channels for Diagnosis and Therapy of Cancer

Sodium ion channel mutations in glioblastoma patients correlate with shorter survival

Background

Glioblastoma Multiforme (GBM) is the most common and invasive astrocytic tumor associated with dismal prognosis. Treatment for GBM patients has advanced, but the median survival remains a meager 15 months. In a recent study, 20,000 genes from 21 GBM patients were sequenced that identified frequent mutations in ion channel genes. The goal of this study was to determine whether ion channel mutations have a role in disease progression and whether molecular targeting of ion channels is a promising therapeutic strategy for GBM patients. Therefore, we compared GBM patient survival on the basis of presence or absence of mutations in calcium, potassium and sodium ion transport genes. Cardiac glycosides, known sodium channel inhibitors, were then tested for their ability to inhibit GBM cell proliferation.

Results

Nearly 90% of patients showed at least one mutation in ion transport genes. GBM patients with mutations in sodium channels showed a significantly shorter survival compared to patients with no sodium channel mutations, whereas a similar comparison based on mutational status of calcium or potassium ion channel mutations showed no survival differences. Experimentally, targeting GBM cells with cardiac glycosides such as digoxin and ouabain demonstrated preferential cytotoxicity against U-87 and D54 GBM cells compared to non-tumor astrocytes (NTAs).

Conclusions

These pilot studies of GBM patients with sodium channel mutations indicate an association with a more aggressive disease and significantly shorter survival. Moreover, inhibition of GBM cells by ion channel inhibitors such as cardiac glycosides suggest a therapeutic strategy with relatively safe drugs for targeting GBM ion channel mutations. Key Words: glioblastoma multiforme, ion channels, mutations, small molecule inhibitors, cardiac glycosides.

Monovalent ions control proliferation of Ehrlich Lettre ascites cells

Channels and transporters of monovalent ions are increasingly suggested as putative anticarcinogenic targets. However, the mechanisms involved in modulation of proliferation by monovalent ions are poorly understood. Here, we investigated the role of K+, Na+, and Cl− ions for the proliferation of Ehrlich Lettre ascites (ELA) cells. We measured the intracellular concentration of each ion in G0, G1, and S phases of the cell cycle following synchronization by serum starvation and release. We show that intracellular concentrations and content of Na+ and Cl− were reduced in the G0–G1 phase transition, followed by an increased content of both ions in S phase concomitant with water uptake. The effect of substituting extracellular monovalent ions was investigated by bromodeoxyuridine incorporation and showed marked reduction after Na+ and Cl− substitution. In spectrofluorometric measurements with the pH-sensitive dye BCECF, substitution of Na+ was observed to upregulate the activity of the Na+/H+ exchanger NHE1 as well as of Na+-independent acid extrusion mechanisms, facilitating intracellular pH (pHi) recovery after acid loading and increasing pHi. Results using the potential sensitive dye DiBaC4( 3 ) showed a reduced Cl− conductance in S compared with G1 followed by transmembrane potential ( Em) hyperpolarization in S. Cl− substitution by impermeable anions strongly inhibited proliferation and increased free, intracellular Ca2+ ([Ca2+]i), whereas a more permeable anion had little effect. Western blots showed reduced chloride intracellular channel CLIC1 and chloride channel ClC-2 expression in the plasma membrane in S compared with G1. Our results suggest that Na+ regulates ELA cell proliferation by regulating intracellular pH while Cl− may regulate proliferation by fine-tuning of Em in S phase and altered Ca2+ signaling.

Expression and clinical significance of the Kv3.4 potassium channel subunit in the development and progression of head and neck squamous cell carcinomas

The concept of ion channels as membrane therapeutic targets and diagnostic/prognostic biomarkers has attracted growing attention. We therefore investigated the expression pattern and clinical significance of the Kv3.4 potassium channel subunit during the development and progression of head and neck squamous cell carcinomas (HNSCCs). KCNC4 mRNA levels were determined by real-time RT-PCR in both HNSCC tissue specimens and derived cell lines. Kv3.4 protein expression was evaluated by immunohistochemistry in paraffin-embedded tissue specimens from 84 patients with laryngeal/pharyngeal squamous cell carcinomas and 67 patients with laryngeal dysplasias. Molecular alterations were correlated with clinicopathological parameters and patient outcome. Increased KCNC4 mRNA levels were found in 15 (54%) of 28 tumours, compared to the corresponding normal epithelia and varied mRNA levels were detected in 12 HNSCC-derived cell lines analysed. Increased Kv3.4 protein expression was observed in 34 (40%) of 84 carcinomas and also at early stages of HNSCC tumourigenesis. Thus, 35 (52%) of 67 laryngeal lesions displayed Kv3.4-positive staining in the dysplastic areas, whereas both stromal cells and normal adjacent epithelia exhibited negligible expression. No significant correlations were found between Kv3.4-positive expression in HNSCC and clinical data; however, Kv3.4 expression tended to diminish in advanced-stage tumours. Interestingly, patients carrying Kv3.4-positive dysplasias experienced a significantly higher laryngeal cancer incidence than did those with negative lesions (p = 0.0209). In addition, functional studies using HNSCC cells revealed that inhibition of Kv3.4 expression by siRNA leads to the inhibition of cell proliferation via selective cell cycle arrest at the G2/M phase without affecting apoptosis. Collectively, these data demonstrate for the first time that Kv3.4 expression is frequently increased during HNSCC tumourigenesis and correlated significantly with a higher cancer risk. Our findings support a role for Kv3.4 in malignant transformation and provide original evidence for the potential clinical utility of Kv3.4 expression as a biomarker for cancer risk assessment.

Dual modulation of inward rectifier potassium currents in olfactory neuronal cells by promiscuous G protein coupling of the oxytocin receptor

Oxytocin receptor is a seven transmembrane receptor widely expressed in the CNS that triggers G(i) or G(q) protein-mediated signaling cascades leading to the regulation of a variety of neuroendocrine and cognitive functions. We decided to investigate whether and how the promiscuous receptor/G protein coupling affects neuronal excitability. As an experimental model, we used the immortalized gonadotropin-releasing hormone-positive GN11 cell line displaying the features of immature, migrating olfactory neurons. Using RT-PCR analysis, we detected the presence of oxytocin receptors whose stimulation by oxytocin led to the accumulation of inositol phosphates and to the inhibition of cell proliferation, and the expression of several inward rectifier (IR) K+ channel subtypes. Moreover, electrophysiological and pharmacological inspections using whole-cell patch-clamp recordings evidenced that in GN11 cells, IR channel subtypes are responsive to oxytocin. In particular, we found that: (i) peptide activation of receptor either inhibited or stimulated IR conductances, and (ii) IR current inhibition was mediated by a pertussis toxin-resistant G protein presumably of the G(q/11) subtype, and by phospholipase C, whereas IR current activation was achieved via receptor coupling to a pertussis toxin-sensitive G(i/o) protein. The findings suggest that neuronal excitability might be tuned by a single peptide receptor that mediates opposing effects on distinct K+ channels through the promiscuous coupling to different G proteins.

Intracellular chloride regulates cell proliferation through the activation of stress-activated protein kinases in MKN28 human gastric cancer cells

Recently, we reported that reduction of intracellular Cl(-) concentration ([Cl(-)](i)) inhibited proliferation of MKN28 gastric cancer cells by diminishing the transition rate from G(1) to S cell-cycle phase through upregulation of p21, cyclin-dependent kinase inhibitor, in a p53-independent manner. However, it is still unknown how intracellular Cl(-) regulates p21 expression level. In this study, we demonstrate that mitogen-activated protein kinases (MAPKs) are involved in the p21 upregulation and cell-cycle arrest induced by reduction of [Cl(-)](i). Culture of MKN28 cells in a low Cl(-) medium significantly induced phosphorylation (activation) of MAPKs (ERK, p38, and JNK) and G(1)/S cell-cycle arrest. To clarify the involvement of MAPKs in p21 upregulation and cell growth inhibition in the low Cl(-) medium, we studied effects of specific MAPKs inhibitors on p21 upregulation and G(1)/S cell-cycle arrest in MKN28 cells. Treatment with an inhibitor of p38 or JNK significantly suppressed p21 upregulation caused by culture in a low Cl(-) medium and rescued MKN28 cells from the low Cl(-)-induced G(1) cell-cycle arrest, whereas treatment with an ERK inhibitor had no significant effect on p21 expression or the growth of MKN28 cells in the low Cl(-) medium. These results strongly suggest that the intracellular Cl(-) affects the cell proliferation via activation of p38 and/or JNK cascades through upregulation of the cyclin-dependent kinase inhibitor (p21) in a p53-independent manner in MKN28 cells.

A novel native store-operated calcium channel encoded by Orai3: selective requirement of Orai3 versus Orai1 in estrogen receptor-positive versus estrogen receptor-negative breast cancer cells

Store-operated calcium (Ca(2+)) entry (SOCE) mediated by STIM/Orai proteins is a ubiquitous pathway that controls many important cell functions including proliferation and migration. STIM proteins are Ca(2+) sensors in the endoplasmic reticulum and Orai proteins are channels expressed at the plasma membrane. The fall in endoplasmic reticulum Ca(2+) causes translocation of STIM1 to subplasmalemmal puncta where they activate Orai1 channels that mediate the highly Ca(2+)-selective Ca(2+) release-activated Ca(2+) current (I(CRAC)). Whereas Orai1 has been clearly shown to encode SOCE channels in many cell types, the role of Orai2 and Orai3 in native SOCE pathways remains elusive. Here we analyzed SOCE in ten breast cell lines picked in an unbiased way. We used a combination of Ca(2+) imaging, pharmacology, patch clamp electrophysiology, and molecular knockdown to show that native SOCE and I(CRAC) in estrogen receptor-positive (ER(+)) breast cancer cell lines are mediated by STIM1/2 and Orai3 while estrogen receptor-negative (ER(-)) breast cancer cells use the canonical STIM1/Orai1 pathway. The ER(+) breast cancer cells represent the first example where the native SOCE pathway and I(CRAC) are mediated by Orai3. Future studies implicating Orai3 in ER(+) breast cancer progression might establish Orai3 as a selective target in therapy of ER(+) breast tumors.

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.

Receptor channel TRPC6 is a key mediator of Notch-driven glioblastoma growth and invasiveness

Glioblastoma multiforme (GBM) is the most frequent and incurable type of brain tumor of adults. Hypoxia has been shown to direct GBM toward a more aggressive and malignant state. Here we show that hypoxia increases Notch1 activation, which in turn induces the expression of transient receptor potential 6 (TRPC6) in primary samples and cell lines derived from GBM. TRPC6 is required for the development of the aggressive phenotype because knockdown of TRPC6 expression inhibits glioma growth, invasion, and angiogenesis. Functionally, TRPC6 causes a sustained elevation of intracellular calcium that is coupled to the activation of the calcineurin-nuclear factor of activated T-cell (NFAT) pathway. Pharmacologic inhibition of the calcineurin-NFAT pathway substantially reduces the development of the malignant GBM phenotypes under hypoxia. Clinically, expression of TRPC6 was elevated in GBM specimens in comparison with normal tissues. Collectively, our studies indicate that TRPC6 is a key mediator of tumor growth of GBM in vitro and in vivo and that TRPC6 may be a promising therapeutic target in the treatment of human GBM.

Expression and prognostic roles of Eag1 in resected esophageal squamous cell carcinomas

Expression of Eag1 was detected in resected esophageal squamous cell carcinomas tissues and matched tissues by immunohistochemistry and RT-PCR. Positive expression of Eag1 protein was 75% (51/68), and mRNA was 73% (8/11) in primary cancer tissues. Eag1 protein positively stained in all 10 metastatic lymph nodes. Eag1 protein and mRNA were negatively expressed in all non-cancerous matched tissues. Eag1 protein was associated with depth of penetration (P = 0.023), but not associated with other clinicopathological factors. Eag1 protein positive group had a significantly shorter survival time than the negative group (P = 0.005). Survival rates at each time-point for the positive group were lower than that for the negative group (P = 0.006), and Eag1 was identified as an independent prognostic factor of long-term survival (P = 0.016). In conclusion, Eag1 was aberrantly expressed in ESCC and correlated with poor prognosis after surgery.

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.

Aberrant expression of Eag1 potassium channels in gastric cancer patients and cell lines

Recently, an interesting relationship between potassium channels and cancer has evolved. The aim of this study is to investigate expression of Eag1 potassium channel in gastric cancer and its role in cancer cells growth. The expression of Eag1 for gasric cancer patients and cell lines as well as gastric adenoma was investigated by immunohistochemistry and reverse transcription polymerase chain reaction. In addition, imipramine was used to identify the involvement of Eag1 in the growth of SGC-7901 and BGC-823 cells. Frequency of positive expression of Eag1 protein was 70.5% (67/95) and Eag1 mRNA was 68.2% (15/22) in gastric cancer primary tissues. Eag1 mRNA was positively expressed in two gastric cell lines. Eag1 protein and mRNA were negatively expressed in paired non-cancerous matched tissues and 5 cases of adenoma tissues. The expression level of Eag1 protein was associated with lymph node metastasis (P = 0.049) and stage (P = 0.039), but had no correlation with sex, age, differentiation grades, and other organs metastases. Imipramine significantly inhibited the proliferation of SGC-7901 and BGC-823 cells at 12 h and 24 h detected by cells number counting and MTT assay (P < 0.01). The study indicates Eag1 is aberrantly expressed in gastric cancer tissues and cell lines and associated with cancer lymph node metastasis and stage and play an important role in the proliferation of gastric cancer cells.

Ether à go-go potassium channel and malignant tumors

Recently, increasing evidence from cell biology and pharmacology demonstrates that cancer cells exhibit ion channel expression patterns, ion conductances and electric properties that are very different from those of resting cells. These peculiar properties are functionally involved in cancer pathogenesis. In particular, because of its oncogenic properties, distribution, modulation and pharmacology, human ether à go-go potassium channel (Eag1, K_v10.1, KCNH1) is considered a critical ion channel-encoding gene involved in the establishment and maintenance of neoplastic growth. This review summarizes most of the findings regarding Eag1 channels and malignant tumors, focusing on cellular mechanisms, mRNA and protein expression in tissues, oncogenic properties, modulation and pharmacology.

The human corneal endothelium: new insights into electrophysiology and ion channels

The corneal endothelium is a monolayer that mediates the flux of solutes and water across the posterior corneal surface. Thereby, it plays an essential role to maintain the transparency of the cornea. Unlike the epithelium, the human endothelium is an amitotic cell layer with a critical cell density and the risk of corneal decompensation. The number of endothelial cells subsequently decreases with age. Moreover, the endothelial cell loss is accelerated after various impairments such as surgical trauma (e.g. cataract extraction) and following corneal transplantation. This cell loss is associated with programmed cell death (apoptosis) and changed ion channel activity. However, little is known about the electrophysiology and ion channel expression (in particular Ca2+ channels) in corneal endothelial cells. This article reviews our current knowledge about the electrophysiology of the corneal endothelium. It highlights ion channel expression, which may have a major role in corneal cell physiology and pathological events. A better understanding of the (electro)physiological function of the cornea may lead to the development of clinical relevant new therapeutic and preventive measures.

Advantages of voltage-gated ion channels as drug targets

Many human diseases result from over- or underactivity in one or more critical physiologic systems. One of the foremost challenges in modern drug discovery is the identification and selection of cellular proteins that can be specifically targeted with therapeutic agents in order to normalize aberrant processes/systems. Suitable drug targets must be validated in the human disease state and ideally, the targeted protein will fulfill similar physiologic and pathologic functions in humans and at least one animal species so that in vivo efficacy and toxicology assays with some predictive clinical relevance may be developed. Nowadays, drug targets must also be amenable to high-throughput screening so that novel molecules, which are capable of modifying cellular protein function, can be identified in large libraries of compounds. Voltage-gated ion channels satisfy many of these requirements and, as a class, are viewed as promising drug targets. Nevertheless, despite their relevance to human disease, voltage-gated ion channels remain considerably underexploited. Therein lie some of the opportunities and advantages associated with voltage-gated ion channels as drug targets.

Calcium channel antagonists: clinical uses--past, present and future

The calcium channel antagonists are a mature group of drugs directed at cardiovascular diseases including hypertension, angina, peripheral vascular disorders and some arrhythmic conditions. Their sites and mechanisms of actions have been well explored over the past two decades and their interactions at the alpha(1) subunit of L-type channels (Ca(V)1.1-1.4) have made them valuable molecular tools for channel classification and localization. With the realization that other members of the voltage-gated calcium channel family exist--Ca(V)2.1-2.3 and Ca(V)3.1-3.3--considerable effort has been directed to drug discovery at these channel types where therapeutic prospects exist for a variety of disorders including pain, epilepsy, affective disorders, neurodegenerative disorders, etc. In contrast to the situation with the L-type channel antagonists success in developing small molecule antagonists of therapeutic utility for these other channel types has thus far been lacking. The reasons for this are explored and potential new directions are indicated including male fertility, bone growth, immune disorders, cancer and schistosomiasis.

Voltage-gated K+ channels support proliferation of colonic carcinoma cells

Plasma membrane potassium (K+) channels are required for cell proliferation. Evidence is growing that K+ channels play a central role in the development and growth of human cancer. Here we examine the contribution and the mechanism by which K+ channels control proliferation of T84 human colonic carcinoma cells. Numerous K+ channels are expressed in T84 cells, but only voltage-gated K+ (Kv) channels influenced proliferation. A number of Kv channel inhibitors reduced DNA synthesis and cell number, without exerting apoptotic or toxic effects. Expression of several Kv channels, such as EagI, Kv 3.4 and Kv 1.5, was detected in patch clamp experiments and in fluorescence-based assays using a voltage sensitive dye. The contribution of EagI channels to proliferation was confirmed by siRNA, which abolished EagI activity and inhibited cell growth. Inhibition of Kv channels did not interfere with the ability of T84 cells to regulate their cell vol, but it restricted intracellular pH regulation. In addition, inhibitors of Kv channels, as well as siRNA for EagI, attenuated intracellular Ca2+ signaling. The data suggest that Kv channels control proliferation of colonic cancer cells by affecting intracellular pH and Ca2+ signaling.

Activation of hEAG1 potassium channels by arachidonic acid

The depolarisation activated human ether à go-go (hEAG) potassium channels are primarily expressed in neuronal tissue but their appearance in various tumour entities is also indicative of an oncogenic role. Because upregulation of hEAG channels may yield to an enhanced cell proliferation, interventions increasing hEAG1 currents may serve similar purposes. We therefore investigated the effects of polyunsaturated fatty acids on hEAG1 channels. Arachidonic acid (AA) lowered their activation threshold, accelerated the activation kinetics and increased the open probability with a half-maximal concentration of about 4 microM. This effect correlated with the number of double bonds (db) in the fatty acids, increasing from oleic acid (1 db), linolenic acid (3 db), AA (4 db) to eicosapentaenoic acid (5 db). Unlike other voltage-gated K(+) channels, hEAG1 channels are not blocked by arachidonic acid. Therefore, in particular at typical resting potentials of tumour cells (-30 mV), AA potently activated hEAG1 channels in a reversible manner. Proliferation and metabolic activity of hEAG1-expressing human melanoma cells increased when cells were exposed to AA concentrations of 5 microM and this effect was suppressed in the presence of the hEAG1 blocker LY97241 suggesting that the proliferative effect of AA is in part mediated by activation of hEAG channels.

Overexpression of Eag1 potassium channels in clinical tumours

Background

Certain types of potassium channels (known as Eag1, KCNH1, Kv10.1) are associated with the production of tumours in patients and in animals. We have now studied the expression pattern of the Eag1 channel in a large range of normal and tumour tissues from different collections utilising molecular biological and immunohistochemical techniques.

Results

The use of reverse transcription real-time PCR and specifically generated monoclonal anti-Eag1 antibodies showed that expression of the channel is normally limited to specific areas of the brain and to restricted cell populations throughout the body. Tumour samples, however, showed a significant overexpression of the channel with high frequency (up to 80% depending on the tissue source) regardless of the detection method (staining with either one of the antibodies, or detection of Eag1 RNA).

Conclusion

Inhibition of Eag1 expression in tumour cell lines reduced cell proliferation. Eag1 may therefore represent a promising target for the tailored treatment of human tumours. Furthermore, as normal cells expressing Eag1 are either protected by the blood-brain barrier or represent the terminal stage of normal differentiation, Eag1 based therapies could produce only minor side effects.

TRP proteins and cancer

Cancer is the second most common cause of death in western countries. It is therefore of fundamental importance to improve the treatment of patients with malignant tumors. This goal can only be achieved if we get closer insight in the various mechanisms leading to tumor formation. Significant progress in the understanding of carcinogenesis has been made during the last couple of years. Ion channels contribute to the regulation of cell proliferation which has initially been shown for K+ channels. Meanwhile, other ion channels such as Cl-, Na+ and Ca2+ channels seem to influence cellular function like growth, migration and invasion. In addition, cation channels of the transient receptor potential (TRP) superfamily are implicated in cancer formation. Most recent data concerning TRP vanilloid (TRPV) type 6, TRP melastatin (TRPM) type 1 and 8 channels and their relevance for common human cancer types will be highlighted in this review. Furthermore, TRP channel structure and function will be discussed in the light of their possible importance as prognostic markers and targets for drug discovery.

Ca2+-activated K+ channels in human melanoma cells are up-regulated by hypoxia involving hypoxia-inducible factor-1alpha and the von Hippel-Lindau protein

Under chronic hypoxia, tumour cells undergo adaptive changes involving hypoxia-inducible factors (HIFs). Here we report that ion currents mediated by Ca2+-activated K+ (K(Ca)) channels in human melanoma IGR1 cells are increased by chronic hypoxia (3% O2), as well as by hypoxia mimetics. This increase involves the HIF system as confirmed by overexpression of HIF-1alpha or the von Hippel-Lindau tumour suppressor gene. Under normoxic conditions the K(Ca) channels in IGR1 cells showed pharmacological characteristics of intermediate conductance K(Ca) subtype IK channels, whereas the subtype SK2 channels were up-regulated under hypoxia, shown with pharmacological tools and with mRNA analysis. Hypoxia increased cell proliferation, but the K(Ca) channel blockers apamin and charybdotoxin slowed down cell growth, particularly under hypoxic conditions. Similar results were obtained for the cell line IGR39 and for acutely isolated cells from a biopsy of a melanoma metastasis. Thus, up-regulation of K(Ca) channels may be a novel mechanism by which HIFs can contribute to the malignant phenotype of human tumour cells.

Furosemide, a Blocker of Na+/K+/2Cl− Cotransporter, Diminishes Proliferation of Poorly Differentiated Human Gastric Cancer Cells by Affecting G0/G1 State

Furosemide, a blocker of Na(+)/K(+)/2Cl(-) cotransporter (NKCC), is often used as a diuretic to improve edema, ascites, and pleural effusion of patients with cancers. The aim of the present study was to investigate whether an NKCC blocker affects cancer cell growth. If so, we would clarify the mechanism of this action. We found that poorly differentiated gastric adenocarcinoma cells (MKN45) expressed the mRNA of NKCC1 three times higher than moderately differentiated ones (MKN28) and that the NKCC in MKN45 showed higher activity than that in MKN28. A cell proliferation assay indicates that furosemide significantly inhibited cell growth in MKN45 cells, but not in MKN28 cells. Using flow cytometrical analysis, we found that the exposure to furosemide brought MKN45 cells to spend more time at the G(0)/G(1) phase, but not MKN28 cells. Based on these observations, we indicate that furosemide diminishes cell growth by delaying the G(1)-S phase progression in poorly differentiated gastric adenocarcinoma cells, which show high expression and activity of NKCC, but not in moderately differentiated gastric adenocarcinoma cells with low expression and NKCC activity.

Functional expression of voltage-gated sodium channels in primary cultures of human cervical cancer

Cervical cancer (CaC) is the third most frequent cause of death from cancer among women in the world and the first in females of developing countries. Several ion channels are upregulated in cancer, actually potassium channels have been suggested as tumor markers and therapeutic targets for CaC. Voltage-gated sodium channels (VGSC) activity is involved in proliferation, motility, and invasion of prostate and breast cancer cells; however, the participation of this type of channels in CaC has not been explored. In the present study, we identified both at the molecular and electrophysiological level VGSC in primary cultures from human cervical carcinoma biopsies. With the whole cell patch clamp technique, we isolated and identified a voltage-gated Na(+) current as the main component of the inward current in all investigated cells. Sodium current was characterized by its kinetics, voltage dependence, sensitivity to tetrodotoxin (TTX) block and dependence to [Na(+)](o). By analyzing the expression of mRNAs encoding TTX-sensitive Na(+) channel alpha subunits with standard RT-PCR and specific primers, we detected Na(v)1.2, Na(v)1.4, Na(v)1.6, and Na(v)1.7 transcripts in total RNA obtained from primary cultures and biopsies of CaC. Restriction enzyme analysis of PCR products was consistent with the molecular nature of the corresponding genes. Notably, only transcripts for Na(v)1.4 sodium channels were detected in biopsies from normal cervix. The results show for the first time the functional expression of VGSC in primary cultures from human CaC, and suggest that these channels might be considered as potential molecular markers for this type of cancer.