title: Plasma membrane sequestration of apoptotic protease-activating factor-1 in human B-lymphoma cells: a novel mechanism of chemoresistance.
abstract: Burkitt lymphoma (BL) is a highly aggressive B-cell neoplasm harboring chromosomal rearrangements of the c-myc oncogene. BL cells frequently resist apoptosis induction by chemotherapeutic agents; however, the mechanism of unresponsiveness has not been elucidated. Here, we show that cytochrome c fails to stimulate apoptosome formation and caspase activation in cytosolic extracts of human BL-derived cell lines, due to insufficient levels of apoptotic protease-activating factor-1 (Apaf-1). Enforced expression of Apaf-1 increased its concentration in the cytosolic compartment, restored cytochrome c-dependent caspase activation, and rendered the prototypic Raji BL cell line sensitive to etoposide- and staurosporine-induced apoptosis. Surprisingly, in nontransfected BL cells, the bulk of Apaf-1 was found to associate with discrete domains in the plasma membrane. Disruption of lipid raft domains or the actin cytoskeleton of Raji cells liberated Apaf-1 and restored sensitivity to cytochrome c-dependent apoptosis, indicating that constitutive Apaf-1 retained its ability to promote caspase activation. Moreover, disruption of lipid rafts sensitized BL cells to apoptosis induced by etoposide. Together, our findings suggest that ectopic (noncytosolic) localization of Apaf-1 may constitute a novel mechanism of chemoresistance in B lymphoma.
authors.affiliation: Division of Toxicology, Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Yu
authors.initials: Y
authors.lastname: Sun
authors.forename: Sten
authors.initials: S
authors.lastname: Orrenius
authors.forename: Shazib
authors.initials: S
authors.lastname: Pervaiz
authors.forename: Bengt
authors.initials: B
authors.lastname: Fadeel
mesh_terms: D017209:Apoptosis / Q000187:drug effects; D053477:Apoptotic Protease-Activating Factor 1; D020169:Caspases / Q000378:metabolism; D045744:Cell Line, Tumor; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D045304:Cytochromes c / Q000378:metabolism; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D019008:Drug Resistance, Neoplasm / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D005047:Etoposide / Q000494:pharmacology; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D016393:Lymphoma, B-Cell / Q000235:genetics / Q000378:metabolism* / Q000473:pathology*; D008928:Mitochondria / Q000187:drug effects; D021381:Protein Transport / Q000187:drug effects; D011506:Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: C506592:APAF1 protein, human; D053477:Apoptotic Protease-Activating Factor 1; D011506:Proteins; D005047:Etoposide; D045304:Cytochromes c; D020169:Caspases