Large-conductance K+ channel openers induce death of human glioma cells

Large-conductance Ca²⁺-activated K⁺ channels (BK _Ca channels) are highly expressed in human glioma cells. It has been reported that BK_Ca channels are present in the inner mitochondrial membrane of the human glioma cell line LN229. In the present study we investigated whether BK_Ca-channel openers, such as CGS7181 (ethyl 2-hydroxy-1-[[(4-methylphenyl)amino]oxo]-6-trifluoromethyl-1H-indole-3- carboxylate) and CGS7184 (ethyl 1-[[(4-chlorophenyl) amino]oxo]-2-hydroxy-6- trifluoromethyl-1H-indole-3-carboxylate), affect the functioning of LN229 glioma cell mitochondria in situ. In the micromolar concentration range CGS7181 and CGS7184 induced glioma cell death. Morphological and cytometric analyses confirmed that both substances trigger the glioma cell death. This effect was not inhibited by the pan-caspase inhibitor z-VAD-fmk. Lack of DNA laddering, PARP cleavage, and caspase 3 activation suggested that glioma cell death was not of the apoptotic type. We examined the effect of CGS7184 on mitochondrial membrane potential and mitochondrial respiration. Potassium channel opener CGS7184 increased cell respiration and induced mitochondrial membrane depolarization. The latter was dependent on the presence of Ca²⁺ in the external medium. It was shown that CGS7184 induced an increase of cytosolic Ca²⁺ concentration due to endoplasmic reticulum store depletion. In conclusion, our results show that CGS7181 and CGS7184 induce glioma cell death by increasing the cytosolic calcium concentration followed by activation of calpains.