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

Abstract

Voltage-dependent large-conductance Ca 2+ -activated K + channels, often referred to as BK channels, are a unique class of ion channels coupling intracellular chemical signaling to electrical signaling. BK channel expression has been shown to be up-regulated in human glioma biopsies, and expression levels correlate positively with the malignancy grade of the tumor. Glioma BK channels (gBK) are a splice variant of the hslo gene, are characterized by enhanced sensitivity to [Ca 2+ ] i , and are the target of modulation by growth factors. By using the selective pharmacological BK channel inhibitor iberiotoxin, we examined the potential role of these channels in tumor growth. Cell survival assays examined the ability of glioma cells to grow in nominally serum-free medium. Under such conditions, BK channel inhibition by iberiotoxin caused a dose-and time-dependent decrease in cell number discernible as early as 72 hr after exposure and maximal growth inhibition after 4-5 days. FACS analysis shows that IbTX treatment arrests glioma cells in S phase of the cell cycle, whereupon cells undergo cell death. Interestingly, IbTX effects were nullified when cells were maintained in 7% fetal calf serum. Electrophysiological analysis, in conjunction with biotinylation studies, demonstrates that serum starvation caused a significant translocation of BK channel protein to the plasma membrane, corresponding to a two-to threefold increase in whole-cell conductance, but without a change in total gBK protein. Hence, expression of functional gBK channels appears to be regulated in a growth-factor-dependent manner, with enhanced surface expression promoting tumor cell growth under conditions of growth factor deprivation as might occur under in vivo conditions. Keywords BK channel; glioma; proliferation; iberiotoxin; cell growth Gliomas are primary brain tumors believed to originate from normal glial cells or their progenitors. They account for 20% of all brain malignancies and are characterized by relentless growth and aggressive invasion into the brain parenchyma. These features make surgical treatment of these cancers difficult, resulting in a dim prognosis for affected patients. Much has been learned regarding the unusual growth characteristics of gliomas. Most notably, it has been demonstrated that these tumors can grow under conditions that would be adverse to growth of nonmalignant cells (Rouzaire-Dubois et al., 1993). Thus, whereas normal cell growth depends on the continuous presence of growth factors, glioma cells have developed autocrine and paracrine signaling cascades that support their growth in the absence of exogenous growthstimulating factors (Rozengurt, 1999;Heasley, 2001;Gerber and Ferrara, 2003;Yu et al., 2003). Such factors include, for example, neuregulin-1, which is synthesized and released by , 1997) and which signals through heterodimeric erb-B2/erb-B4 receptors that are overexpressed in glioma cells (Westphal et al., 1997). In at least 50% of human gliomas, the epidermal growth factor receptor (EGF-R...

KCNMA1 gene amplification promotes tumor cell proliferation in human prostate cancer

“…The oncogenic role of KCNMA1 found in this study is in line with the previously reported growth inhibition of human astrocytoma by IBTX (Basrai et al, 2002;Weaver et al, 2004). Interestingly, the intermediate conductance K þ channel (IK) rather than KCNMA1 has been proposed to be important for PC-3 and LNCaP cells by Parihar et al (2003).…”

A role for ion channels in glioma cell invasion

“…Our data indicate that Ca 2+ -activated K + channels might participate in this process (Weaver et al, 2004) and we have recently identified two water channels (AQP 1 and 4) in glioma cells (McCoy and Sontheimer, 2005). In most cells, water permeability does not limit the regulation of cell size and cell volume (King et al, 2000) and we assume that this is also the case here.…”

Silencing of hSlo potassium channels in human osteosarcoma cells promotes tumorigenesis

“…4,32,33 Our in vitro experiments using either paxilline or hSlo knockdown indicate that BK channels do not play an essential role in Cal72 cell proliferation.…”

“…Indeed, we and others have shown that BK channels are a unique class of ion channels that are activated by a change in intracellular calcium, in voltage 2 and by a wide variety of molecules and molecular mechanisms including growth factors and hypoxia that, interestingly, are involved in carcinogenesis. [3][4][5][6] BK channels are expressed in a broad spectrum of cells in which they play a crucial role in neurotransmitter release, muscle relaxation 7,8 and diverse activities involved in tumor-associated processes such as cell proliferation, survival, secretion and migration. [9][10][11] Importantly, in human glioma, BK channels are significantly up-regulated, and their expression levels seem to correlate positively with tumor grade and poor patient prognosis.…”

“…In vitro studies have suggested a role for BK channels in the control of glioma cell growth, survival and migration. 4,11 In addition glioma, we have shown a high level of BK expression in osteosarcoma (OS). 12 Altogether, these studies suggest an important role for BK in carcinogenesis and point to its potential value as a prognostic marker and/or a therapeutic target.…”