To understand the impact of ionizing irradiation from diagnostics and radiotherapy on cells, we examined K(+) channel activity before and immediately after exposing cells to X-rays. Already, low dose in the cGy range caused in adenocarcinoma A549 cells within minutes a hyperpolarization following activation of the human intermediate-conductance Ca(2+)-activated K(+) channel (hIK). The response was specific for cells, which functionally expressed hIK channels and in which hIK activity was low before irradiation. HEK293 cells, which do not respond to X-ray irradiation, accordingly develop a sensitivity to this stress after heterologous expression of hIK channels. The data suggest that hIK activation involves a Ca(2+)-mediated signaling cascade because channel activation is suppressed by a strong cytosolic Ca(2+) buffer. The finding that an elevation of H2O2 causes an increase in the concentration of cytosolic Ca(2+) suggests that radicals, which emerge early in response to irradiation, trigger this Ca(2+) signaling cascade. Inhibition of hIK channels by specific blockers clotrimazole and TRAM-34 slowed cell proliferation and migration in "wound" scratch assays; ionizing irradiation, in turn, stimulated the latter process presumably via its activation of the hIK channels. These data stress an indirect radiosensitivity of hIK channels with an impact on cell differentiation.

Ionizing radiation is a universal tool in tumor therapy but may also cause secondary cancers or cell invasiveness. These negative side effects could be causally related to the human-intermediate-conductance Ca2+-activated-K+-channel (hIK), which is activated by X-ray irradiation and affects cell proliferation and migration. To analyze the signaling cascade downstream of ionizing radiation we use genetically encoded reporters for H2O2 (HyPer) and for the dominant redox-buffer glutathione (Grx1-roGFP2) to monitor with high spatial and temporal resolution, radiation-triggered excursions of H2O2 in A549 and HEK293 cells. The data show that challenging cells with ≥1 Gy X-rays or with UV-A laser micro-irradiation causes a rapid rise of H2O2 in the nucleus and in the cytosol. This rise, which is determined by the rate of H2O2 production and glutathione-buffering, is sufficient for triggering a signaling cascade that involves an elevation of cytosolic Ca2+ and eventually an activation of hIK channels.

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Radiation therapy efficiently eliminates cancer cells and reduces tumor growth. To understand collateral agonistic and antagonistic effects of this treatment on the immune system, we examined the impact of x-ray irradiation on human T cells. We find that, in a major population of leukemic Jurkat T cells and peripheral blood mononuclear cells, clinically relevant radiation doses trigger delayed oscillations of the cytosolic Ca2+ concentration. They are generated by store-operated Ca2+ entry (SOCE) following x-ray–induced clustering of Orai1 and STIM1 and formation of a Ca2+ release–activated Ca2+ (CRAC) channel. A consequence of the x-ray–triggered Ca2+ signaling cascade is translocation of the transcription factor nuclear factor of activated T cells (NFAT) from the cytosol into the nucleus, where it elicits the expression of genes required for immune activation. The data imply activation of blood immune cells by ionizing irradiation, with consequences for toxicity and therapeutic effects of radiation therapy.

Radiation therapy is efficiently employed for eliminating cancer cells and reducing tumor growth. To further improving its therapeutic application it is mandatory to unravel the molecular effects of ionizing irradiation and to understand whether they support or counteract tumor therapy. Here we examine the impact of X-ray irradiation on immune activation of human T cells with single doses typically employed in tumor therapy. We discover that exposing cells to radiation triggers in a population of leukemic Jurkat T cells and in peripheral blood mononuclear cells (PBMCs) a canonical Ca2+ signaling cascade, which elicits immune activation of these cells. An early step in the signaling cascade is the initiation of sustained oscillations of the cytosolic Ca2+ concentration, an event mediated by store operated Ca2+ entry (SOCE) via an X-ray induced clustering of the Calcium Release-Activated Calcium Modulator 1 with the stromal interaction molecule 1 (Oari1/STIM1). A functional consequence of the Ca2+ signaling cascade is the translocation of the transcription factor nuclear factor of activated T cells (NFAT) from the cytosol into the nucleus where it elicits the expression of genes required for immune activation. These data imply that a direct activation of blood immune cells by ionizing irradiation has an impact on toxicity and therapeutic effects of radiation therapy.