title: Mitochondrial amplification of death signals determines thymidine kinase/ganciclovir-triggered activation of apoptosis.
abstract: Previous clinical experience shows that the efficacy of suicide gene transfer in tumor therapy is limited, resulting from inefficient gene transfer or alternatively, from intrinsic resistance of the tumor in vivo. Herpes simplex virus thymidine kinase/ganciclovir (TK/GCV), a paradigmatic suicide gene therapy system, has been described to exert its cytotoxic effect, at least in part, by inducing apoptosis in target cells. Here, we report that mitochondria amplify TK/GCV-induced apoptosis by regulating p53 accumulation and the effector phase of apoptosis. Treatment with TK/GCV led to mitochondrial perturbations including loss of the mitochondrial membrane potential and release of cytochrome c from mitochondria into the cytosol, inducing caspase activation and nuclear fragmentation. Inhibition of TK/GCV-induced mitochondrial perturbations by Bcl-2 overexpression or by the mitochondrion-specific inhibitor bongkrekic acid also strongly inhibited TK/GCV-induced activation of caspases and apoptosis. TK/GCV-induced mitochondrial perturbations depended on caspase activity possibly initiated by death receptor signaling. Perturbation of mitochondrial function mediated accumulation of wild-type p53 protein, since Bcl-2 overexpression, bongkrekic acid, or inhibition of mitochondrial protein synthesis with chloramphenicol strongly reduced TK/GCV-induced accumulation of wild-type p53 protein. These findings suggest that TK/GCV therapy may be less efficient in tumors in which the mitochondrial amplification of TK/GCV-induced apoptosis is blocked, e.g., by Bcl-2 overexpression. Given the low efficacy of currently used gene therapy systems, our data on molecular mechanisms that regulate sensitivity or resistance toward TK/GCV-induced cytotoxicity might have important implications to improve the clinical application of suicide gene therapy.
authors.affiliation: University Children's Hospital, Ulm, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Beltinger
authors.forename: S
authors.initials: S
authors.lastname: Fulda
authors.forename: T
authors.initials: T
authors.lastname: Kammertoens
authors.forename: W
authors.initials: W
authors.lastname: Uckert
authors.forename: K M
authors.initials: KM
authors.lastname: Debatin
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects*; D015153:Blotting, Western; D001865:Bongkrekic Acid / Q000494:pharmacology*; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism; D002474:Cell-Free System; D002701:Chloramphenicol / Q000494:pharmacology; D003574:Cytochrome c Group / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D004249:DNA Damage / Q000187:drug effects; D053938:DNA Fragmentation / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D005434:Flow Cytometry; D015774:Ganciclovir / Q000494:pharmacology*; D015316:Genetic Therapy / Q000379:methods*; D006801:Humans; D008564:Membrane Potentials / Q000187:drug effects; D008928:Mitochondria / Q000187:drug effects* / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D018139:Simplexvirus / Q000201:enzymology; D013937:Thymidine Kinase / Q000378:metabolism*; D013997:Time Factors; D014162:Transfection; D014407:Tumor Cells, Cultured; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; D003574:Cytochrome c Group; D019253:Proto-Oncogene Proteins c-bcl-2; D016159:Tumor Suppressor Protein p53; D001865:Bongkrekic Acid; D002701:Chloramphenicol; D013937:Thymidine Kinase; D015774:Ganciclovir