Alternating Electric Fields (TTFields) Activate Ca(v)1.2 Channels in Human Glioblastoma Cells

Tumor treating fields (TTFields) represent a novel FDA-approved treatment modality for patients with newly diagnosed or recurrent glioblastoma multiforme. This therapy applies intermediate frequency alternating electric fields with low intensity to the tumor volume by the use of non-invasive transducer electrode arrays. Mechanistically, TTFields have been proposed to impair formation of the mitotic spindle apparatus and cytokinesis. In order to identify further potential molecular targets, here the effects of TTFields on Ca(2+)-signaling, ion channel activity in the plasma membrane, cell cycle, cell death, and clonogenic survival were tested in two human glioblastoma cell lines in vitro by fura-2 Ca(2+) imaging, patch-clamp cell-attached recordings, flow cytometry and pre-plated colony formation assay. In addition, the expression of voltage-gated Ca(2+) (Ca(v)) channels was determined by real-time RT-PCR and their significance for the cellular TTFields response defined by knock-down and pharmacological blockade. As a result, TTFields stimulated in a cell line-dependent manner a Ca(v)1.2-mediated Ca(2+) entry, G(1) or S phase cell cycle arrest, breakdown of the inner mitochondrial membrane potential and DNA degradation, and/or decline of clonogenic survival suggesting a tumoricidal action of TTFields. Moreover, inhibition of Ca(v)1.2 by benidipine aggravated in one glioblastoma line the TTFields effects suggesting that Ca(v)1.2-triggered signaling contributes to cellular TTFields stress response. In conclusion, the present study identified Ca(v)1.2 channels as TTFields target in the plasma membrane and provides the rationale to combine TTFields therapy with Ca(2+) antagonists that are already in clinical use.