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

Abstract

Voltage‐dependent large‐conductance Ca²⁺‐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 [Ca²⁺]_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.