hERG1 channels are overexpressed in glioblastoma multiforme and modulate VEGF secretion in glioblastoma cell lines

State-dependent block of HERG potassium channels by R-roscovitine: implications for cancer therapy

Human ether-a-go-go-related gene (HERG) potassium channel acts as a delayed rectifier in cardiac myocytes and is an important target for both pro- and antiarrhythmic drugs. Many drugs have been pulled from the market for unintended HERG block causing arrhythmias. Conversely, recent evidence has shown that HERG plays a role in cell proliferation and is overexpressed both in multiple tumor cell lines and in primary tumor cells, which makes HERG an attractive target for cancer treatment. Therefore, a drug that can block HERG but that does not induce cardiac arrhythmias would have great therapeutic potential. Roscovitine is a cyclin-dependent kinase (CDK) inhibitor that is in phase II clinical trials as an anticancer agent. In the present study we show that R-roscovitine blocks HERG potassium current (human embryonic kidney-293 cells stably expressing HERG) at clinically relevant concentrations. The block (IC(50) = 27 microM) was rapid (tau = 20 ms) and reversible (tau = 25 ms) and increased with channel activation, which supports an open channel mechanism. Kinetic study of wild-type and inactivation mutant HERG channels supported block of activated channels by roscovitine with relatively little effect on either closed or inactivated channels. A HERG gating model reproduced all roscovitine effects. Our model of open channel block by roscovitine may offer an explanation of the lack of arrhythmias in clinical trials using roscovitine, which suggests the utility of a dual CDK/HERG channel block as an adjuvant cancer therapy.

Eag1 expression interferes with hypoxia homeostasis and induces angiogenesis in tumors

Ether-á-go-go-1 (Eag1) is a CNS-localized voltage-gated potassium channel that is found ectopically expressed in a majority of extracranial solid tumors. While circumstantial evidence linking Eag1 to tumor biology has been well established, the mechanisms by which the channel contributes to tumor progression remain elusive. In this study, we have used in vivo and in vitro techniques to identify a candidate mechanism. A mutation that eliminates ion permeation fails to completely abolish xenograft tumor formation by transfected cells, indicating that Eag1 contributes to tumor progression independently of its primary function as an ion channel. Our data suggest that Eag1 interferes with the cellular mechanism for maintaining oxygen homeostasis, increasing HIF-1 activity, and thereby VEGF secretion and tumor vascularization.

ERG voltage-gated K+ channels regulate excitability and discharge dynamics of the medial vestibular nucleus neurones

The discharge properties of the medial vestibular nucleus neurones (MVNn) critically depend on the activity of several ion channel types. In this study we show, immunohistochemically, that the voltage-gated K(+) channels ERG1A, ERG1B, ERG2 and ERG3 are highly expressed within the vestibular nuclei of P10 and P60 mice. The role played by these channels in the spike-generating mechanisms of the MVNn and in temporal information processing was investigated electrophysiologically from mouse brain slices, in vitro, by analysing the spontaneous discharge and the response to square-, ramp- and sinusoid-like intracellular DC current injections in extracellular and whole-cell patch-clamp studies. We show that more than half of the recorded MVNn were responsive to ERG channel block (WAY-123,398, E4031), displaying an increase in spontaneous activity and discharge irregularity. The response to step and ramp current injection was also modified by ERG block showing a reduction of first spike latency, enhancement of discharge rate and reduction of the slow spike-frequency adaptation process. ERG channels influence the interspike slope without affecting the spike shape. Moreover, in response to sinusoid-like current, ERG channel block caused frequency-dependent gain enhancement and phase-lead shift. Taken together, the data demonstrate that ERG channels control the excitability of MVNn, their discharge regularity and probably their resonance properties.

Identification of a posttranslational mechanism for the regulation of hERG1 K+ channel expression and hERG1 current density in tumor cells

A common feature of tumor cells is the aberrant expression of ion channels on their plasma membrane. The molecular mechanisms regulating ion channel expression in cancer cells are still poorly known. K(+) channels that belong to the human ether-a-go-go-related gene 1 (herg1) family are frequently misexpressed in cancer cells compared to their healthy counterparts. We describe here a posttranslational mechanism for the regulation of hERG1 channel surface expression in cancer cells. This mechanism is based on the activity of hERG1 isoforms containing the USO exon. These isoforms (i) are frequently overexpressed in human cancers, (ii) are retained in the endoplasmic reticulum, and (iii) form heterotetramers with different proteins of the hERG family. (iv) The USO-containing heterotetramers are retained intracellularly and undergo ubiquitin-dependent degradation. This process results in decreased hERG1 current (I(hERG1)) density. We detailed such a mechanism in heterologous systems and confirmed its functioning in tumor cells that endogenously express hERG1 proteins. The silencing of USO-containing hERG1 isoforms induces a higher I(hERG1) density in tumors, an effect that apparently regulates neurite outgrowth in neuroblastoma cells and apoptosis in leukemia cells.

Expression and functional role of HERG1, K+ channels in leukemic cells and leukemic stem cells

In order to investigate the expression and functional role of HERG1 K+ channels in leukemic cells and leukemic stem cells (LSCs), RT-PCR was used to detect the HERG1 K+ channels expression in leukemic cells and LSCs. The functional role of HERG1 K+ channels in leukemic cell proliferation was measured by MTT assay, and cell cycle and apoptosis were analyzed by flow cytometry. The results showed that herg mRNA was expressed in CD34+/CD38-, CD123+ LSCs but not in circulating CD34+ cells. Herg mRNA was also up-regulated in leukemia cell lines K562 and HL60 as well as almost all the primary leukemic cells while not in normal peripheral blood mononuclear cells (PBMNCs) and the expression of herg mRNA was not associated with the clinical and cytogenetic features of leukemia. In addition, leukemic cell proliferation was dramatically inhibited by HERG K+ channel special inhibitor E-4031. Moreover, E-4031 suppressed the cell growth by inducing a specific block at the G1/S transition phase of the cell cycle but had no effect on apoptosis in leukemic cells. The results suggested that HERG1 K+ channels could regulate leukemic cells proliferation and were necessary for leukemic cells to proceed with the cell cycle. HERG1 K+ channels may also have oncogenic potential and may be a biomarker for diagnosis of leukemia and a novel potential pharmacological target for leukemia therapy.

The hERG K+ channel: target and antitarget strategies in drug development

The human ether-à-go-go related gene (hERG) K+ channel is of great interest for both basic researchers and clinicians because its blockade by drugs can lead to QT prolongation, which is a risk factor for torsades de pointes, a potentially life-threatening arrhythmia. A growing list of agents with "QT liability" have been withdrawn from the market or restricted in their use, whereas others did not even receive regulatory approval for this reason. Thus, hERG K+ channels have become a primary antitarget (i.e. an unwanted target) in drug development because their blockade causes potentially serious side effects. On the other hand, the recent identification and functional characterization of hERG K+ channels not only in the heart, but also in several other tissues (e.g. neurons, smooth muscle and cancer cells) may have far reaching implications for drug development for a possible exploitation of hERG as a target, especially in oncology and cardiology.

Predicting Treatment Response of Malignant Gliomas to Bevacizumab and Irinotecan by Imaging Proliferation With [18F] Fluorothymidine Positron Emission Tomography: A Pilot Study

Purpose

Evaluation of treatment effects in malignant brain tumors is challenging because of the lack of reliable response predictors of tumor response. This study examines the predictive value of positron emission tomography (PET) using [18F] fluorothymidine (FLT), an imaging biomarker of cell proliferation, in patients with recurrent malignant gliomas treated with bevacizumab in combination with irinotecan.

Patients and Methods

Patients with recurrent malignant gliomas treated with biweekly cycles of bevacizumab and irinotecan were prospectively studied with FLT-PET at baseline, after 1 to 2 weeks, and after 6 weeks from start of treatment. A more than 25% reduction in tumor FLT uptake as measured by standardized uptake value was defined as a metabolic response. FLT responses were compared with response as shown by magnetic resonance imaging (MRI) and patient survival.

Results

Twenty-one patients were included, and 19 were assessable for metabolic response evaluation with FLT-PET. There were nine responders (47%) and 10 nonresponders (53%). Metabolic responders survived three times as long as nonresponders (10.8 v 3.4 months; P = .003), and tended to have a prolonged progression-free survival (P = .061). Both early and later FLT-PET responses were more significant predictors of overall survival (1 to 2 weeks, P = .006; 6 weeks, P = .002), compared with the MRI responses (P = .060 for both 6-week and best responses).

Conclusion

FLT-PET as an imaging biomarker seems to be predictive of overall survival in bevacizumab and irinotecan treatment of recurrent gliomas. Whether FLT-PET performed as early as 1 to 2 week after starting treatment is as predictive as the study indicates at 6 weeks warrants further investigation.

Relationship between survival and edema in malignant gliomas: role of vascular endothelial growth factor and neuronal pentraxin 2

PURPOSE

Vascular endothelial growth factor (VEGF) is a potent mediator of vascular permeability. VEGF inhibition reduces edema and tumor burden in some patients with malignant glioma, whereas others show no response. The role of VEGF expression in edema production and the relationship to survival is not well understood.

EXPERIMENTAL DESIGN

Using DNA microarray analysis, we examined VEGF and related gene expression in 71 newly diagnosed malignant gliomas and analyzed the relationship to edema and survival.

RESULTS AND CONCLUSIONS

VEGF expression was predictive of survival in tumors with little or no edema [Cox proportional hazard model, 6.88; 95% confidence interval (95% CI), 2.61-18.1; P<0.0001], but not in tumors with extensive edema. The expression of several proangiogenic genes, including adrenomedullin (correlation coefficient, 0.80), hypoxia-inducible factor-1A (0.51), and angiopoietin-2 (0.44), was correlated with VEGF expression (all with P<0.0001), whereas that of several antiangiogenic genes was inversely correlated. The expression of six genes was increased greater than 3-fold in edematous versus nonedematous tumors in the absence of increased VEGF expression. The most increased, neuronal pentraxin 2 (NPTX2, 7-fold change), was predictive of survival in tumors with the highest levels of edema, in contrast to VEGF (hazard ratio, 2.73; 95% CI, 1.49-5.02; P=0.049). NPTX2 was tightly correlated with expression of the water channel aquaporin-3 (0.74, P<0.0001). These results suggest that there are both VEGF-dependent and VEGF-independent pathways of edema production in gliomas and may explain why edema is not reduced in some patients following anti-VEGF treatment.

VEGFR-1 (FLT-1), beta1 integrin, and hERG K+ channel for a macromolecular signaling complex in acute myeloid leukemia: role in cell migration and clinical outcome

Leukemia cell motility and transendothelial migration into extramedullary sites are regulated by angiogenic factors and are considered unfavorable prognostic factors in acute leukemias. We have studied cross talk among (1) the vascular endothelial growth factor receptor-1, FLT-1; (2) the human eag-related gene 1 (hERG1) K(+) channels; and (3) integrin receptors in acute myeloid leukemia (AML) cells. FLT-1, hERG1, and the beta(1) integrin were found to form a macromolecular signaling complex. The latter mostly recruited the hERG1B isoform of hERG1 channels, and its assembly was necessary for FLT-1 signaling activation and AML cell migration. Both effects were inhibited when hERG1 channels were specifically blocked. A FLT-1/hERG1/beta(1) complex was also observed in primary AML blasts, obtained from a population of human patients. The co-expression of FLT-1 and hERG1 conferred a pro-migratory phenotype to AML blasts. Such a phenotype was also observed in vivo. The hERG1-positive blasts were more efficient in invading the peripheral circulation and the extramedullary sites after engraftment into immunodeficient mice. Moreover, hERG1 expression in leukemia patients correlated with a higher probability of relapse and shorter survival periods. We conclude that in AML, hERG1 channels mediate the FLT-1-dependent cell migration and invasion, and hence confer a greater malignancy.

The hERG potassium channel as a therapeutic target

The hERG (human ether-à-go-go-related gene) potassium channel has elicited intense scientific interest due to its counter-intuitive kinetics and its association with arrhythmia and sudden death. hERG blockade is involved in both antiarrhythmic pharmacotherapy and the pathogenesis of familial and acquired long QT syndrome (LQTS). Short QT syndrome (SQTS), muscular atrophy and many forms of cancer have also been associated with hERG as a target. Molecular models of both the channel and its blocker pharmacophores exist, revealing methods to design hERG liability out of potential drug molecules. Future developments will synthesise preclinical data on hERG with other criteria to determine net arrhythmogenic risk. Also, the molecular actions of hERG and its genetics will be elucidated in detail to allow clinical risk reduction.

Toxins interacting with ether-à-go-go-related gene voltage-dependent potassium channels

The critical role that ether-à-go-go-related gene (erg) K(+) channels play in mating in Caenorhabditis elegans, neuronal seizures in Drosophila and cardiac action potential repolarization in humans has been well documented. Three erg genes (erg1, erg2 and erg3) have been identified and characterized. A structurally diverse number of compounds block these channels, but do not display specificity among the different channel isoforms. In this review we describe the blocking properties of several peptides, purified from scorpion, sea anemone and spider venoms, which are selective for certain members of the ERG family of channels. These peptides do not behave as classical pore blockers and appear to modify the gating properties of the channel. Genomic studies predict the existence of many other novel peptides with the potential of being more selective for ERG channels than those discussed here.

hERG1 channels in human esophagus: evidence for their aberrant expression in the malignant progression of Barrett's esophagus

Ion channels regulate a broad range of cellular activities. Alteration in ion channel function has been reported in different human pathologies, such as cardiac, neuromuscular, autoimmune diseases, and cancer. We investigated the expression of hERG1 K+ channels in the human upper gastrointestinal tract, focusing our attention on the lower esophagus. In particular, we analyzed by both Reverse transcription and polymerase chain reaction (RT-PCR) and immunohistochemistry (IHC) endoscopic samples obtained from normal subjects, from patients suffering from gastroesophageal reflux, associated or not with esophagitis, and from patients affected by Barrett's esophagus (BE), that is, intestinal metaplasia. None of the normal samples, nor those from patients with gastro-esophageal reflux symptoms and reflux esophagitis expressed the hERG1 protein. On the other hand, 69% of patients with BE expressed hERG1. Since BE is a preneoplastic lesion, dysplasias (Ds) and adenocarcinomas (ADKs) arising on a previously diagnosed BE were also analyzed, and all the samples showed a high expression of the hERG1 protein. The surveillance of patients with BE showed that 89% of those who later developed ADKs displayed hERG1 expression. Data here reported, support the hypothesis that hERG1 expression marks an early step of the progression of normality to cancer in the human esophagus through a metaplastic and dysplastic stage.

Convection-enhanced delivery of immunotoxins and radioisotopes for treatment of malignant gliomas

✓ The treatment of malignant gliomas has advanced significantly in the past 15 years. The simultaneous development of new targeting agents and techniques to deliver these high-molecular-weight compounds has led to improved efficacy and promising results in Phase III trials. Convection-enhanced delivery (CED) of macromolecules has emerged as the leading delivery technique for the treatment of malignant gliomas. A summary of the basic principles of CED and a review of the current human trials of protein targeting agents are provided.