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Endoplasmic Reticulum Stress in the Proapoptotic Action of Edelfosine in Solid Tumor Cells

¹ Centro de Investigación del Cáncer, Instituto de Biología Molecular y Celular del Cáncer, Consejo Superior de Investigaciones Científicas-Universidad de Salamanca; ² Unidad de Investigación, Hospital Universitario de Salamanca, Campus Miguel de Unamuno, Salamanca, Spain; and ³ Departamento de Bioquímica y Biología Molecular y Fisiología, Instituto de Biología y Genética Molecular, Facultad de Medicina, Universidad de Valladolid-Consejo Superior de Investigaciones Científicas, Valladolid, Spain

Requests for reprints: Faustino Mollinedo, Centro de Investigación del Cáncer, Instituto de Biología Molecular y Celular del Cáncer, Consejo Superior de Investigaciones Científicas-Universidad de Salamanca, Campus Miguel de Unamuno, E-37007 Salamanca, Spain. Phone: 34-923-294806; Fax: 34-923-294795; E-mail: fmollin{at}usal.es.

The endoplasmic reticulum (ER) has been posited as a potential anticancer target. The synthetic antitumor alkyl-lysophospholipid analogue edelfosine accumulates in the ER of solid tumor cells. This ER accumulation of the drug leads to the inhibition of phosphatidylcholine and protein synthesis, G₂-M arrest, depletion of ER-stored Ca²⁺, Bax up-regulation and activation, transcriptional factor growth arrest and DNA damage–inducible gene 153 up-regulation, caspase-4 and caspase-8 activation, and eventually to apoptosis. Edelfosine prompted ER stress apoptotic signaling, but not the survival unfolded protein response. Edelfosine also induced persistent c-Jun NH₂-terminal kinase (JNK) activation. Gene transfer–mediated overexpression of apoptosis signal–regulating kinase 1, which plays a crucial role in ER stress, enhanced edelfosine-induced JNK activation and apoptosis. Inhibition of JNK, caspase-4, or caspase-8 activation diminished edelfosine-induced apoptosis. Edelfosine treatment led to the generation of the p20 caspase-8 cleavage fragment of BAP31, directing proapoptotic signals between the ER and the mitochondria. bax^–/–bak^–/– double-knockout cells fail to undergo edelfosine-induced ER-stored Ca²⁺ release and apoptosis. Wild-type and bax^–/–bak^–/– cells showed similar patterns of phosphatidylcholine and protein synthesis inhibition, despite their differences in drug sensitivity. Thus, edelfosine-induced apoptosis is dependent on Bax/Bak-mediated ER-stored Ca²⁺ release, but phosphatidylcholine and protein synthesis inhibition is not critical. Transfection-enforced expression of Bcl-X_L, which localizes specifically in mitochondria, prevented apoptosis without inhibiting ER-stored Ca²⁺ release. These data reveal that edelfosine induces an ER stress response in solid tumor cells, providing novel insights into the edelfosine-mediated antitumor activity. Our data also indicate that mitochondria are indispensable for this edelfosine-induced cell death initiated by ER stress. [Cancer Res 2007;67(21):10368–78]

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