IK1 channel activity contributes to cisplatin sensitivity of human epidermoid cancer cells

Atrioventricular re-entrant tachycardia and atrioventricular node re-entrant tachycardia in a patient with cancer under chemotherapy: a case report and literature review

Cardio-oncology is a new field of interest in cardiology focusing on the detection and treatment of cardiovascular diseases, such as arrhythmias, myocarditis, and heart failure, as side-effects of chemotherapy and radiotherapy. The association between chemotherapeutic agents and arrhythmias has previously been established. Atrial tachyarrhythmias, particularly atrial fibrillation, are most common, but ventricular arrhythmias, including those related to treatment-induced QT prolongation, and bradyarrhythmias can also occur. However, the association between chemotherapeutic agents and atrioventricular re-entrant tachycardia (AVRT)/atrioventricular node re-entrant tachycardia (AVNRT) remains poorly understood. Here, we report a patient with new-onset AVRT/AVNRT and lung cancer who underwent chemotherapy. We considered that chemotherapy or cancer itself may have been a trigger for the initiation of paroxysmal AVRT/AVNRT, and that radiofrequency catheter ablation was effective in treating this type of tachycardia. Here, possible mechanisms and potential genes (mostly ion channels) involved in AVRT/AVNRT are summarized and the mechanisms underlying the possible regulatory patterns of cancer cells and chemotherapy on ion channels are reviewed. Finally, we considered that ion channel abnormalities may link cancer or chemotherapy to the onset of AVRT/AVNRT. The aim of the present study was to highlight the association between chemotherapeutic agents and AVRT/AVNRT and to provide new insights for future research. Understanding the intermediate mechanisms between chemotherapeutic agents and AVRT/AVNRT may be beneficial in preventing chemotherapy-evoked AVRT/AVNRT (and/or other arrhythmias) in future.

Challenges in the Therapeutic Targeting of KCa Channels: From Basic Physiology to Clinical Applications

Calcium-activated potassium (KCa) channels are ubiquitously expressed throughout the body and are able to regulate membrane potential and intracellular calcium concentrations, thereby playing key roles in cellular physiology and signal transmission. Consequently, it is unsurprising that KCa channels have been implicated in various diseases, making them potential targets for pharmaceutical interventions. Over the past two decades, numerous studies have been conducted to develop KCa channel-targeting drugs, including those for disorders of the central and peripheral nervous, cardiovascular, and urinary systems and for cancer. In this review, we synthesize recent findings regarding the structure and activating mechanisms of KCa channels. We also discuss the role of KCa channel modulators in therapeutic medicine. Finally, we identify the major reasons behind the delay in bringing these modulators to the pharmaceutical market and propose new strategies to promote their application.

Interplay of Ca2+ and K+ signals in cell physiology and cancer

The cytoplasmic Ca2+ concentration and the activity of K+ channels on the plasma membrane regulate cellular processes ranging from mitosis to oriented migration. The interplay between Ca2+ and K+ signals is intricate, and different cell types rely on peculiar cellular mechanisms. Derangement of these mechanisms accompanies the neoplastic progression. The calcium signals modulated by voltage-gated (KV) and calcium-dependent (KCa) K+ channel activity regulate progression of the cell division cycle, the release of growth factors, apoptosis, cell motility and migration. Moreover, KV channels regulate the cell response to the local microenvironment by assembling with cell adhesion and growth factor receptors. This chapter summarizes the pathophysiological roles of Ca2+ and K+ fluxes in normal and cancer cells, by concentrating on several biological systems in which these functions have been studied in depth, such as early embryos, mammalian cell lines, T lymphocytes, gliomas and colorectal cancer cells. A full understanding of the underlying mechanisms will offer a comprehensive view of the ion channel implication in cancer biology and suggest potential pharmacological targets for novel therapeutic approaches in oncology.

Imaging of KCa 3.1 Channels in Tumor Cells with PET and Small-Molecule Fluorescent Probes

The Ca²⁺ activated K⁺ channel K_Ca 3.1 is overexpressed in several human tumor cell lines, e. g. clear cell renal carcinoma, prostate cancer, non-small cell lung cancer. Highly aggressive cancer cells use this ion channel for key processes of the metastatic cascade such as migration, extravasation and invasion. Therefore, small molecules, which are able to image this K_Ca 3.1 channel in vitro and in vivo represent valuable diagnostic and prognostic tool compounds. The [¹⁸ F]fluoroethyltriazolyl substituted senicapoc was used as positron emission tomography (PET) tracer and showed promising properties for imaging of K_Ca 3.1 channels in lung adenocarcinoma cells in mice. The novel senicapoc BODIPY conjugates with two F-atoms (9 a) and with a F-atom and a methoxy moiety (9 b) at the B-atom led to the characteristic punctate staining pattern resulting from labeling of single K_Ca 3.1 channels in A549-3R cells. This punctate pattern was completely removed by preincubation with an excess of senicapoc confirming the high specificity of K_Ca 3.1 labeling. Due to the methoxy moiety at the B-atom and the additional oxyethylene unit in the spacer, 9 b exhibits higher polarity, which improves solubility and handling without reduction of fluorescence quantum yield. Docking studies using a cryo-electron microscopy (EM) structure of the K_Ca 3.1 channel confirmed the interaction of 9 a and 9 b with a binding pocket in the channel pore.

CRAC and SK Channels: Their Molecular Mechanisms Associated with Cancer Cell Development

Cancer represents a major health burden worldwide. Several molecular targets have been discovered alongside treatments with positive clinical outcomes. However, the reoccurrence of cancer due to therapy resistance remains the primary cause of mortality. Endeavors in pinpointing new markers as molecular targets in cancer therapy are highly desired. The significance of the co-regulation of Ca²⁺-permeating and Ca²⁺-regulated ion channels in cancer cell development, proliferation, and migration make them promising molecular targets in cancer therapy. In particular, the co-regulation of the Orai1 and SK3 channels has been well-studied in breast and colon cancer cells, where it finally leads to an invasion-metastasis cascade. Nevertheless, many questions remain unanswered, such as which key molecular components determine and regulate their interplay. To provide a solid foundation for a better understanding of this ion channel co-regulation in cancer, we first shed light on the physiological role of Ca²⁺ and how this ion is linked to carcinogenesis. Then, we highlight the structure/function relationship of Orai1 and SK3, both individually and in concert, their role in the development of different types of cancer, and aspects that are not yet known in this context.

Exploring the Therapeutic Potential of Membrane Transport Proteins: Focus on Cancer and Chemoresistance

Improving the therapeutic efficacy of conventional anticancer drugs represents the best hope for cancer treatment. However, the shortage of druggable targets and the increasing development of anticancer drug resistance remain significant problems. Recently, membrane transport proteins have emerged as novel therapeutic targets for cancer treatment. These proteins are essential for a plethora of cell functions ranging from cell homeostasis to clinical drug toxicity. Furthermore, their association with carcinogenesis and chemoresistance has opened new vistas for pharmacology-based cancer research. This review provides a comprehensive update of our current knowledge on the functional expression profile of membrane transport proteins in cancer and chemoresistant tumours that may form the basis for new cancer treatment strategies.

Impaired actin filaments decrease cisplatin sensitivity via dysfunction of volume-sensitive Cl- channels in human epidermoid carcinoma cells

Cisplatin is a widely used platinum-based anticancer drug in the chemotherapy of numerous human cancers. However, cancer cells acquire resistance to cisplatin. So far, functional loss of volume-sensitive outwardly rectifying (VSOR) Cl^- channels has been reported to contribute to cisplatin resistance of cancer cells. Here, we analyzed protein expression patterns of human epidermoid carcinoma KB cells and its cisplatin-resistant KCP-4 cells. Intriguingly, KB cells exhibited higher β-actin expression and clearer actin filaments than KCP-4 cells. The β-actin knockdown in KB cells decreased VSOR Cl^- currents and inhibited the regulatory volume decrease (RVD) process after cell swelling. Consistently, KB cells treated with cytochalasin D, which depolymerizes actin filaments, showed smaller VSOR Cl^- currents and slower RVD. Cytochalasin D also inhibited cisplatin-triggered apoptosis in KB cells. These results suggest that the disruption of actin filaments cause the dysfunction of VSOR Cl^- channels, which elicits resistance to cisplatin in human epidermoid carcinoma cells.

KCa3.1 Channels Confer Radioresistance to Breast Cancer Cells

K_Ca3.1 K⁺ channels reportedly contribute to the proliferation of breast tumor cells and may serve pro-tumor functions in the microenvironment. The putative interaction of K_Ca3.1 with major anti-cancer treatment strategies, which are based on cytotoxic drugs or radiotherapy, remains largely unexplored. We employed K_Ca3.1-proficient and -deficient breast cancer cells derived from breast cancer-prone MMTV-PyMT mice, pharmacological K_Ca3.1 inhibition, and a syngeneic orthotopic mouse model to study the relevance of functional K_Ca3.1 for therapy response. The K_Ca3.1 status of MMTV-PyMT cells did not determine tumor cell proliferation after treatment with different concentrations of docetaxel, doxorubicin, 5-fluorouracil, or cyclophosphamide. K_Ca3.1 activation by ionizing radiation (IR) in breast tumor cells in vitro, however, enhanced radioresistance, probably via an involvement of the channel in IR-stimulated Ca²⁺ signals and DNA repair pathways. Consistently, K_Ca3.1 knockout increased survival time of wildtype mice upon syngeneic orthotopic transplantation of MMTV-PyMT tumors followed by fractionated radiotherapy. Combined, our results imply that K_Ca3.1 confers resistance to radio- but not to chemotherapy in the MMTV-PyMT breast cancer model. Since K_Ca3.1 is druggable, K_Ca3.1 targeting concomitant to radiotherapy seems to be a promising strategy to radiosensitize breast tumors.

Ion Channels: New Actors Playing in Chemotherapeutic Resistance

In the battle against cancer cells, therapeutic modalities are drastically limited by intrinsic or acquired drug resistance. Resistance to therapy is not only common, but expected: if systemic agents used for cancer treatment are usually active at the beginning of therapy (i.e., 90% of primary breast cancers and 50% of metastases), about 30% of patients with early-stage breast cancer will have recurrent disease. Altered expression of ion channels is now considered as one of the hallmarks of cancer, and several ion channels have been linked to cancer cell resistance. While ion channels have been associated with cell death, apoptosis and even chemoresistance since the late 80s, the molecular mechanisms linking ion channel expression and/or function with chemotherapy have mostly emerged in the last ten years. In this review, we will highlight the relationships between ion channels and resistance to chemotherapy, with a special emphasis on the underlying molecular mechanisms.

Cancer-Associated Intermediate Conductance Ca2+-Activated K⁺ Channel KCa3.1

Several tumor entities have been reported to overexpress K_Ca3.1 potassium channels due to epigenetic, transcriptional, or post-translational modifications. By modulating membrane potential, cell volume, or Ca²⁺ signaling, K_Ca3.1 has been proposed to exert pivotal oncogenic functions in tumorigenesis, malignant progression, metastasis, and therapy resistance. Moreover, K_Ca3.1 is expressed by tumor-promoting stroma cells such as fibroblasts and the tumor vasculature suggesting a role of K_Ca3.1 in the adaptation of the tumor microenvironment. Combined, this features K_Ca3.1 as a candidate target for innovative anti-cancer therapy. However, immune cells also express K_Ca3.1 thereby contributing to T cell activation. Thus, any strategy targeting K_Ca3.1 in anti-cancer therapy may also modulate anti-tumor immune activity and/or immunosuppression. The present review article highlights the potential of K_Ca3.1 as an anti-tumor target providing an overview of the current knowledge on its function in tumor pathogenesis with emphasis on vasculo- and angiogenesis as well as anti-cancer immune responses.

The combined activation of KCa3.1 and inhibition of Kv11.1/hERG1 currents contribute to overcome Cisplatin resistance in colorectal cancer cells

Background:

Platinum-based drugs such as Cisplatin are commonly employed for cancer treatment. Despite an initial therapeutic response, Cisplatin treatment often results in the development of chemoresistance. To identify novel approaches to overcome Cisplatin resistance, we tested Cisplatin in combination with K⁺ channel modulators on colorectal cancer (CRC) cells.

Methods:

The functional expression of Ca²⁺-activated (K_Ca3.1, also known as KCNN4) and voltage-dependent (K_v11.1, also known as KCNH2 or hERG1) K⁺ channels was determined in two CRC cell lines (HCT-116 and HCT-8) by molecular and electrophysiological techniques. Cisplatin and several K⁺ channel modulators were tested in vitro for their action on K⁺ currents, cell vitality, apoptosis, cell cycle, proliferation, intracellular signalling and Platinum uptake. These effects were also analysed in a mouse model mimicking Cisplatin resistance.

Results:

Cisplatin-resistant CRC cells expressed higher levels of K_Ca3.1 and K_v11.1 channels, compared with Cisplatin-sensitive CRC cells. In resistant cells, K_Ca3.1 activators (SKA-31) and K_v11.1 inhibitors (E4031) had a synergistic action with Cisplatin in triggering apoptosis and inhibiting proliferation. The effect was maximal when K_Ca3.1 activation and K_v11.1 inhibition were combined. In fact, similar results were produced by Riluzole, which is able to both activate K_Ca3.1 and inhibit K_v11.1. Cisplatin uptake into resistant cells depended on K_Ca3.1 channel activity, as it was potentiated by K_Ca3.1 activators. K_v11.1 blockade led to increased K_Ca3.1 expression and thereby stimulated Cisplatin uptake. Finally, the combined administration of a K_Ca3.1 activator and a K_v11.1 inhibitor also overcame Cisplatin resistance in vivo.

Conclusions:

As Riluzole, an activator of K_Ca3.1 and inhibitor of K_v11.1 channels, is in clinical use, our results suggest that this compound may be useful in the clinic to improve Cisplatin efficacy and overcome Cisplatin resistance in CRC.

Basolateral Kir4.1 activity in the distal convoluted tubule regulates K secretion by determining NaCl cotransporter activity

PURPOSE OF REVIEW

Renal potassium (K) secretion plays a key role in maintaining K homeostasis. The classic mechanism of renal K secretion is focused on the connecting tubule and cortical collecting duct, in which K is uptaken by basolateral Na-K-ATPase and is secreted into the lumen by apical ROMK (Kir1.1) and Ca-activated big conductance K channel. Recently, genetic studies and animal models have indicated that inwardly rectifying K channel 4.1 (Kir4.1 or Kcnj10) in the distal convoluted tubule (DCT) may play a role in the regulation of K secretion in the aldosterone-sensitive distal nephron by targeting the NaCl cotransporter (NCC). This review summarizes recent progresses regarding the role of Kir4.1 in the regulation of NCC and K secretion.

RECENT FINDINGS

Kir4.1 is expressed in the basolateral membrane of the DCT, and plays a predominant role in contributing to the basolateral K conductance and in participating in the generation of negative membrane potential. Kir4.1 is also the substrate of src-family tyrosine kinase and the stimulation of src-family tyrosine kinase activates Kir4.1 activity in the DCT. The genetic deletion or functional inhibition of Kir4.1 depolarizes the membrane of the DCT, inhibits ste20-proline-alanine rich kinase, and suppresses NCC activity. Moreover, the downregulation of Kir4.1 increases epithelial Na channel expression in the collecting duct and urinary K excretion. Finally, mice with low Kir4.1 activity in the DCT are hypomagnesemia and hypokalemia.

SUMMARY

Recent progress in exploring the regulation and the function of Kir4.1 in the DCT strongly indicates that Kir4.1plays an important role in initiating the regulation of renal K secretion by targeting NCC and it may serves as a K sensor in the kidney.

Low-dose photon irradiation alters cell differentiation via activation of hIK channels

To understand the impact of ionizing irradiation from diagnostics and radiotherapy on cells, we examined K(+) channel activity before and immediately after exposing cells to X-rays. Already, low dose in the cGy range caused in adenocarcinoma A549 cells within minutes a hyperpolarization following activation of the human intermediate-conductance Ca(2+)-activated K(+) channel (hIK). The response was specific for cells, which functionally expressed hIK channels and in which hIK activity was low before irradiation. HEK293 cells, which do not respond to X-ray irradiation, accordingly develop a sensitivity to this stress after heterologous expression of hIK channels. The data suggest that hIK activation involves a Ca(2+)-mediated signaling cascade because channel activation is suppressed by a strong cytosolic Ca(2+) buffer. The finding that an elevation of H2O2 causes an increase in the concentration of cytosolic Ca(2+) suggests that radicals, which emerge early in response to irradiation, trigger this Ca(2+) signaling cascade. Inhibition of hIK channels by specific blockers clotrimazole and TRAM-34 slowed cell proliferation and migration in "wound" scratch assays; ionizing irradiation, in turn, stimulated the latter process presumably via its activation of the hIK channels. These data stress an indirect radiosensitivity of hIK channels with an impact on cell differentiation.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

KCNJ10 mutations display differential sensitivity to heteromerisation with KCNJ16

BACKGROUND/AIMS

Mutations in the inwardly-rectifying K(+)-channel KCNJ10/Kir4.1 cause autosomal recessive EAST syndrome (epilepsy, ataxia, sensorineural deafness and tubulopathy). KCNJ10 is expressed in the distal convoluted tubule of the kidney, stria vascularis of the inner ear and brain glial cells. Patients diagnosed clinically with EAST syndrome were genotyped and mutations in KCNJ10 were studied functionally.

METHODS

Patient DNA was amplified and sequenced, and new mutations were identified. Mutant and wild-type KCNJ10 constructs were cloned and heterologously expressed in Xenopus oocytes. Whole-cell K(+) currents were measured by 2-electrode voltage clamping and channel expression was analysed by Western blotting.

RESULTS

We identified 3 homozygous mutations in KCNJ10 (p.F75C, p.A167V and p.V91fs197X), with mutation p.A167V previously reported in a compound heterozygous state. Oocytes expressing wild-type human KCNJ10 showed inwardly rectified currents, which were significantly reduced in all of the mutants (p < 0.001). Specific inhibition of KCNJ10 currents by Ba(2+) demonstrated a large residual function in p.A167V only, which was not compatible with causing disease. However, co-expression with KCNJ16 abolished function in these heteromeric channels almost completely.

CONCLUSION

This study provides an explanation for the pathophysiology of the p.A167V KCNJ10 mutation, which had previously not been considered pathogenic on its own. These findings provide evidence for the functional cooperation of KCNJ10 and KCNJ16. Thus, in vitro ascertainment of KCNJ10 function may necessitate co-expression with KCNJ16.

General Sensitization of melanoma cells for TRAIL-induced apoptosis by the potassium channel inhibitor TRAM-34 depends on release of SMAC

The death ligand TRAIL represents a promising therapeutic strategy for metastatic melanoma, however prevalent and inducible resistance limit its applicability. A new approach is presented here for sensitization to TRAIL. It is based on inhibition of the membrane potassium channel KCa3.1 (IK1), which serves fundamental cellular functions related to membrane potential. The selective inhibitor TRAM-34 did not induce apoptosis by itself but synergistically enhanced TRAIL sensitivity and overrode TRAIL resistance in a large panel of melanoma cell lines. Expression of IK1 was also found in mitochondria, and its inhibition resulted in mitochondrial membrane hyperpolarization and an early activation of Bax. The combination of TRAM-34 and TRAIL resulted in massive release of mitochondrial factors, cytochrome c, AIF and SMAC/DIABLO. Bax knockdown and Bcl-2 overexpression abolished apoptosis. Overexpression of XIAP diminished apoptosis by two-fold, and SMAC knockdown almost completely abolished apoptosis. These data uncover the existence of a rheostat in melanoma cells, consisting of inhibitor of apoptosis proteins and SMAC, which regulates TRAIL sensitivity. Thus, a new strategy is described based on mitochondrial membrane channels, which correspond to Bax activation. As both TRAIL and IK1 inhibitors had shown only minor side effects in clinical trials, a clinical application of this combination is conceivable.

Uncoupling of K+ and Cl- transport across the cell membrane in the process of regulatory volume decrease

It is accepted that K(+) and Cl(-) flows are coupled tightly in regulatory volume decrease (RVD). However, using self referencing microelectrodes, we proved that K(+) and Cl(-) transport mainly by channels in RVD was uncoupled in nasopharyngeal carcinoma CNE-2Z cells, with the transient K(+) efflux activated earlier and sustained Cl(-) efflux activated later. Hypotonic challenges decreased intracellular pH (pH(i)), and activated a proton pump-dependent H(+) efflux, resulting in a decline of extracellular pH (pH(o)). Modest decreases of pH(o) inhibited the volume-activated K(+) outflow and RVD, but not the Cl(-) outflow, while inhibition of H(+) efflux or increase of pH(o) buffer ability promoted K(+) efflux and RVD. The results suggest that the temporal dynamics of K(+) channel activities is different from that of Cl(-) channels in RVD, due to differential sensitivity of K(+) and Cl(-) channels to pH(o). H(+) efflux may play important roles in cell volume regulation, and may be a therapeutic target for human nasopharyngeal carcinoma.

Mechanism of cisplatin resistance in human urothelial carcinoma cells

An isogenic pair of cisplatin-susceptible (NTUB1) and -resistant (NTUB1/P) human urothelial carcinoma cell lines was used to elucidate the mechanism of cisplatin resistance. The significantly lower intracellular platinum (IP) concentration, which resulted from the decreased cisplatin uptake, was found in NTUB1/P cells. The enhancement of IP concentration did not increase the susceptibility of NTUB1/P cells to cisplatin treatment. The reduction of IP concentration as well was unable to enhance the cisplatin-resistance in susceptible NTUB1 cells. This indicated that reduction of IP concentration was not the account for the development of cisplatin resistance here. Instead, the over expression of anti-apoptotic Bcl-2, anti-oxidative heme oxygenase-1 (HO-1) and cell cycle regulator p16INK4 seemed to be more important for the gaining of cisplatin in these human urothelial carcinoma cell.

CBP501-calmodulin binding contributes to sensitizing tumor cells to cisplatin and bleomycin

CBP501 is an anticancer drug currently in randomized phase II clinical trials for patients with non-small cell lung cancer and malignant pleural mesothelioma. CBP501 was originally described as a unique G(2) checkpoint-directed agent that binds to 14-3-3, inhibiting the actions of Chk1, Chk2, mitogen-activated protein kinase-activated protein kinase 2, and C-Tak1. However, unlike a G(2) checkpoint inhibitor, CBP501 clearly enhances the accumulation of tumor cells at G(2)-M phase that is induced by cisplatin or bleomycin at low doses and short exposure. By contrast, CBP501 does not similarly affect the accumulation of tumor cells at G(2)-M that is induced by radiation, doxorubicin, or 5-fluorouracil treatment. Our recent findings point to an additional mechanism of action for CBP501. The enhanced accumulation of tumor cells at G(2)-M upon combined treatment with cisplatin and CBP501 results from an increase in intracellular platinum concentrations, which leads to increased binding of platinum to DNA. The observed CBP501-enhanced platinum accumulation is negated in the presence of excess Ca(2+). Some calmodulin inhibitors behave similarly to, although less potently than, CBP501. Furthermore, analysis by surface plasmon resonance reveals a direct, high-affinity molecular interaction between CBP501 and CaM (K(d) = 4.62 × 10(-8) mol/L) that is reversed by Ca(2+), whereas the K(d) for the complex between CBP501 and 14-3-3 is approximately 10-fold weaker and is Ca(2+) independent. We conclude that CaM inhibition contributes to CBP501's activity in sensitizing cancer cells to cisplatin or bleomycin. This article presents an additional mechanism of action which might explain the clinical activity of the CBP501-cisplatin combination.

The relationship between cisplatin-induced reactive oxygen species, glutathione, and BCL-2 and resistance to cisplatin

Cisplatin (cDDP) is an anticancer agent that is widely used in the treatment of many solid tumors. A major obstacle to successful cDDP-based chemotherapy, however, is the intrinsic and acquired resistance of tumor cells to this drug. Greater insight into the molecular mechanisms underlying the modulation of cellular responses to cDDP will aid in the development and optimization of new therapeutic strategies. Apart from induction of DNA damage, recent data have suggested that cDDP also induces the formation of reactive oxygen species that can trigger cell death. Cell death occurs as the result of several simultaneously activated signaling pathways. The specific pathway responsible for cell death depends on the cell type and the treatment conditions. This review focuses on the relationship between glutathione and BCL-2 and their protective role in cDDP-induced reactive oxygen species formation and cDDP resistance.

Identification and functional characterization of the intermediate-conductance Ca(2+)-activated K(+) channel (IK-1) in biliary epithelium

In the liver, adenosine triphosphate (ATP) is an extracellular signaling molecule that is released into bile and stimulates a biliary epithelial cell secretory response via engagement of apical P2 receptors. The molecular identities of the ion channels involved in ATP-mediated secretory responses have not been fully identified. Intermediate-conductance Ca(2+)-activated K(+) channels (IK) have been identified in biliary epithelium, but functional data are lacking. The aim of these studies therefore was to determine the location, function, and regulation of IK channels in biliary epithelial cells and to determine their potential contribution to ATP-stimulated secretion. Expression of IK-1 mRNA was found in both human Mz-Cha-1 biliary cells and polarized normal rat cholangiocyte (NRC) monolayers, and immunostaining revealed membrane localization with a predominant basolateral signal. In single Mz-Cha-1 cells, exposure to ATP activated K(+) currents, increasing current density from 1.6 +/- 0.1 to 7.6 +/- 0.8 pA/pF. Currents were dependent on intracellular Ca(2+) and sensitive to clotrimazole and TRAM-34 (specific IK channel inhibitors). Single-channel recording demonstrated that clotrimazole-sensitive K(+) currents had a unitary conductance of 46.2 +/- 1.5 pS, consistent with IK channels. In separate studies, 1-EBIO (an IK activator) stimulated K(+) currents in single cells that were inhibited by clotrimazole. In polarized NRC monolayers, ATP significantly increased transepithelial secretion which was inhibited by clotrimazole. Lastly, ATP-stimulated K(+) currents were inhibited by the P2Y receptor antagonist suramin and by the inositol 1,4,5-triphosphate (IP3) receptor inhibitor 2-APB. Together these studies demonstrate that IK channels are present in biliary epithelial cells and contribute to ATP-stimulated secretion through a P2Y-IP3 receptor pathway.

The eNOS enhancer AVE 9488: a novel cardioprotectant against ischemia reperfusion injury

Nitric oxide (NO) is an important regulator of vascular and myocardial function. Cardiac ischemia/reperfusion injury is reduced in mice overexpressing endothelial NO synthase (eNOS) suggesting cardioprotection by eNOS. Novel pharmacological substances, so called eNOS enhancers, upregulate eNOS expression and thereby increase NO production. We tested the effects of the eNOS enhancer AVE 9488 on cardiac ischemia/reperfusion injury in vivo in mice. After treatment with the eNOS enhancer AVE 9488 (30 mg/kg/day) or placebo for one week mice underwent 30 min of coronary artery ligation and 24 h of reperfusion in vivo. Ischemia-reperfusion damage was significantly reduced in mice treated with the eNOS enhancer when compared to placebo treated mice (infarct/area at risk 65.4 +/- 4.1 vs. 36.9 +/- 4.0%, placebo vs. eNOS enhancer, P = 0.0002). The protective effect was blunted in eNOS knockout mice treated with the eNOS enhancer (infarct/area at risk 64.1 +/- 6.2%, eNOS knockout + eNOS enhancer vs. WT + eNOS enhancer, P = ns). Reactive oxygen species were significantly reduced in mice treated with the eNOS enhancer as indicated by significantly lower malondialdehyde-thiobarbituric acid levels (placebo vs. eNOS enhancer, 3.2 +/- 0.5 vs. 0.8 +/- 0.07 micromol/l, P = 0.0003). Thus pharmacological interventions addressed to increase eNOS-derived NO production constitute a promising therapeutic approach to prevent myocardial ischemia/reperfusion injury.

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.

Protein kinase G-dependent cardioprotective mechanism of phosphodiesterase-5 inhibition involves phosphorylation of ERK and GSK3beta

Sildenafil, a potent inhibitor of phosphodiesterase-5 (PDE-5) induces powerful protection against myocardial ischemia-reperfusion injury. PDE-5 inhibition increases cGMP levels that activate cGMP-dependent protein kinase (PKG). However, the cause and effect relationship of PKG in sildenafil-induced cardioprotection and the downstream targets of PKG remain unclear. Adult ventricular myocytes were treated with sildenafil and subjected to simulated ischemia and reoxygenation. Sildenafil treatment significantly decreased cardiomyocyte necrosis and apoptosis. The PKG inhibitors, KT5823, guanosine 3',5'-cyclic monophosphorothioate, 8-(4-chloro-phenylthio) (R(p)-8-pCPT-cGMPs), or DT-2 blocked the anti-necrotic and anti-apoptotic effect of sildenafil. Selective knockdown of PKG in cardiomyocytes with adenoviral vector containing short hairpin RNA of PKG also abolished sildenafil-induced protection. Furthermore, intra-coronary infusion of sildenafil in Langendorff-isolated mouse hearts prior to ischemia-reperfusion significantly reduced myocardial infarct size after 20 min ischemia and 30 min reperfusion, which was abrogated by KT5823. Sildenafil significantly increased PKG activity in intact hearts and cardiomyocytes. Sildenafil also enhanced the Bcl-2/Bax ratio, phosphorylation of Akt, ERK1/2, and glycogen synthase kinase 3beta. All these changes (except Akt phosphorylation) were significantly blocked by KT5823 and short hairpin RNA of PKG. These studies provide the first evidence for an essential role of PKG in sildenafil-induced cardioprotection. Moreover, our results demonstrate that sildenafil activates a PKG-dependent novel signaling cascade that involves activation of ERK and inhibition of glycogen synthase kinase 3beta leading to cytoprotection.