Apoptosis resistance of MCF-7 breast carcinoma cells to ionizing radiation is independent of p53 and cell cycle control but caused by the lack of Caspase-3 and a caffeine-inhibitable event

• Published in: Cancer research (Chicago, Ill.) Vol. 64; no. 19; pp. 7065 - 7072
• Main Authors: ESSMANN, Frank, ENGELS, Ingo H, TOTZKE, Gudrun, SCHULZE-OSTHOFF, Klaus, JÄNICKE, Reiner U
• Format: Journal Article
• Language: English
• Published: Philadelphia, PA American Association for Cancer Research 01.10.2004
• Subjects: Antineoplastic agents; Apoptosis - drug effects; Apoptosis - radiation effects; Biological and medical sciences; Breast Neoplasms - drug therapy; Breast Neoplasms - genetics; Breast Neoplasms - pathology; Breast Neoplasms - radiotherapy; Caffeine - pharmacology; Caspase 3; Caspase 9; Caspases - deficiency; Caspases - metabolism; Cell Cycle - drug effects; Cell Cycle - physiology; Cell Cycle - radiation effects; Cell Line, Tumor; Enzyme Activation - drug effects; Enzyme Activation - radiation effects; G2 Phase - drug effects; G2 Phase - radiation effects; Humans; Medical sciences; Mitochondria - drug effects; Mitochondria - radiation effects; Mitosis - drug effects; Mitosis - radiation effects; Pharmacology. Drug treatments; Radiation Tolerance - drug effects; Transcriptional Activation - drug effects; Transcriptional Activation - radiation effects; Transfection; Tumor Suppressor Protein p53 - genetics; Tumor Suppressor Protein p53 - physiology; Tumors; Enzyme; Cysteine endopeptidases; Malignant tumor; Resistance; Mammary gland diseases; Peptidases; Breast carcinoma; Ionizing radiation; Regulation(control); TP53 Gene; Cell death; Cell cycle; Hydrolases; Xanthine derivatives; Apoptosis; Caffeine; Tumor suppressor gene
• ISSN: 0008-5472; 1538-7445
• DOI: 10.1158/0008-5472.CAN-04-1082

We have shown previously that ionizing radiation (IR) induces a persistent G(2)-M arrest but not cell death in MCF-7 breast carcinoma cells that harbor functional p53 but lack caspase-3. In the present study, we investigated the mechanisms of apoptosis resistance and the roles of p53, caspase-3, and cell cycle arrest in IR-induced apoptosis. The methylxanthine caffeine and the staurosporine analog UCN-01, which can inhibit ATM and Chk kinases, efficiently abrogated the IR-induced G(2)-M arrest and induced mitochondrial activation as judged by the loss of the mitochondrial membrane potential and the release of cytochrome c and Smac/Diablo. However, despite these proapoptotic alterations, cell death and activation of the initiator caspase-9 were not induced in MCF-7 cells but were interestingly only observed after reexpression of caspase-3. Sensitization to IR-induced apoptosis by caffeine or UCN-01 was abrogated neither by cycloheximide nor by pifithrin-alpha, an inhibitor of the transcriptional activity of p53. Furthermore, suppression of p53 by RNA interference could not prevent caffeine- and IR-induced mitochondrial alterations and apoptosis but resulted in an even more pronounced G(2)-M arrest. Collectively, our results clearly show that the resistance of MCF-7 cells to IR-induced apoptosis is caused by two independent events; one of them is a caffeine- or UCN-01-inhibitable event that does not depend on p53 or a release of the G(2)-M arrest. The second event is the loss of caspase-3 that surprisingly seems essential for a fully functional caspase-9 pathway, even despite the previous release of mitochondrial proapoptotic proteins.