Evidence for the Inhibition by Temozolomide, an Imidazotetrazine Family Alkylator, of Intermediate-Conductance Ca 2+ -Activated K + Channels in Glioma Cells

Abstract

Background: Temozolomide (TMZ), an oral alkylator of the imidazotetrazine family, is used to treat glioma. Whether this drug has any ionic effects in glioma cells remains largely unclear. Methods: With the aid of patch-clamp technology, we investigated the effects of TMZ on the ionic currents in U373 glioma cells. The mRNA expression of KCNN4 (K Ca 3.1) in U373 glioma cells and TMZ's effect on K + currents in these KCNN4 siRNA-transfected U373 cells were investigated. Results: In whole-cell recordings, TMZ decreased the amplitude of voltage-dependent K + currents (I K ) in U373 cells. TMZ-induced I K inhibition was reversed by ionomycin or 1-ethyl-2-benzimidazolinone (1-EBIO). In cell-attached configuration, TMZ concentration-dependently reduced the activity of intermediate-conductance Ca 2+ -activated K + (IK Ca ) channels with an IC 50 value of 9.2 μM. Chlorzoxazone or 1-EBIO counteracted the TMZ-induced inhibition of IK Ca channels. Although TMZ was unable to modify single-channel conductance, its inhibition of IK Ca channels was weakly voltage-dependent and accompanied by a significant prolongation in the slow component of mean closed time. However, neither large-conductance Ca 2+ -activated (BK Ca ) nor inwardly rectifying K + (Kir) channels were affected by TMZ. In current-clamp mode, TMZ depolarized the cell membrane and 1-EBIO reversed TMZ-induced depolarization. TMZ had no effect on I K in KCNN4 siRNA-transfected U373 cells. Conclusion: In addition to the DNA damage it does, its inhibitory effect on IKCa channels accompanied by membrane depolarization could be an important mechanism underlying TMZ-induced antineoplastic actions.