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Perturbation of intracellular K+ homeostasis with valinomycin promotes cell death by mitochondrial swelling and autophagic processes

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Abstract

Perturbation of cellular K+ homeostasis is a common motif in apoptosis but it is unknown whether a decrease in intracellular K+ alone is sufficient to replicate apoptotic hallmarks. We investigated, which mode of cell death is induced by decreasing the intracellular K+ concentration using valinomycin, a highly K+-selective ionophore. Valinomycin treatment induced mitochondrial swelling and minor nuclear changes in cell lines (BV-2, C6, HEK 293), and in primary mouse microglia and astrocytes. In the microglial cell line BV-2, we identified and quantified three phenotypes in valinomycin-exposed cells. The first and most prevalent phenotype (62 ± 2%) was characterized by swollen mitochondria and no chromatin condensation, and the second (25 ± 3%) by swollen mitochondria and slight chromatin condensation. Only the third phenotype (11 ± 4%) fulfilled criteria of apoptosis by having normal-sized mitochondria and strongly condensed chromatin. Valinomycin-induced swelling of mitochondria was not altered by the adenine nucleotide translocase inhibitor bongkrekic acid (BA), the pan caspase inhibitor Z-VAD-FMK, changing extracellular K+ or Cl− concentrations, or the membrane-permeable Ca2+ chelator BAPTA-AM. Only co-exposure of cells to valinomycin and the Ca2+ ionophore ionomycin in high K+ Cl−-free extracellular solution suppressed mitochondrial swelling. Ionomycin alone caused shrinkage of mitochondria. Additionally, valinomycin promoted autophagic processes, which were further enhanced by preincubation with BA or with Z-VAD-FMK. Valinomycin-dependent chromatin condensation was inhibited by BA, Z-VAD-FMK, BAPTA-AM, and ionomycin. Our findings demonstrate that mitochondrial swelling and autophagy are common features of valinomycin-exposed cells. Accordingly, valinomycin promotes an autophagic cell death mode, but not apoptosis.

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Acknowledgments

We thank our colleagues Hannelore Bauer and Sebastian Töpfer for generously providing us with HEK 293 cells, and Anton Hermann and Thomas Weiger for C6 cells.

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  1. Department of Cell Biology, University of Salzburg, Hellbrunnerstr. 34, 5020, Salzburg, Austria

    Barbara Klein, Katrin Wörndl, Ursula Lütz-Meindl & Hubert H. Kerschbaum

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  1. Barbara Klein
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Correspondence to Hubert H. Kerschbaum.

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Klein, B., Wörndl, K., Lütz-Meindl, U. et al. Perturbation of intracellular K+ homeostasis with valinomycin promotes cell death by mitochondrial swelling and autophagic processes. Apoptosis 16, 1101–1117 (2011). https://doi.org/10.1007/s10495-011-0642-9

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