title: Tumor senescence and radioresistant tumor-initiating cells (TICs): let sleeping dogs lie!
abstract: Preclinical data from cell lines and experimental tumors support the concept that breast cancer-derived tumor-initiating cells (TICs) are relatively resistant to ionizing radiation and chemotherapy. This could be a major determinant of tumor recurrence following treatment. Increased clonogenic survival is observed in CD24-/low/CD44+ TICs derived from mammosphere cultures and is associated with (a) reduced production of reactive oxygen species, (b) attenuated activation of γH2AX and CHK2-p53 DNA damage signaling pathways, (c) reduced propensity for ionizing radiation-induced apoptosis, and (d) altered DNA double-strand or DNA single-strand break repair. However, recent data have shed further light on TIC radioresistance as irradiated TICs are resistant to tumor cell senescence following DNA damage. Taken together, the cumulative data support a model in which DNA damage signaling and repair pathways are altered in TICs and lead to an altered mode of cell death with unique consequences for long-term clonogen survival. The study of TIC senescence lays the foundation for future experiments in isogenic models designed to directly test the capacity for senescence and local control (that is, not solely local regression) and spontaneous metastases following treatment in vivo. The study also supports the targeting of tumor cell senescence pathways to increase TIC clonogen kill if the targeting also maintains the therapeutic ratio.
authors.forename: Gaetano
authors.initials: G
authors.lastname: Zafarana
authors.forename: Robert G
authors.initials: RG
authors.lastname: Bristow
mesh_terms: D017209:Apoptosis / Q000528:radiation effects; D001943:Breast Neoplasms / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D051927:CD24 Antigen / Q000378:metabolism; D016922:Cellular Senescence; D064447:Checkpoint Kinase 2; D004249:DNA Damage; D004260:DNA Repair; D005260:Female; D006657:Histones / Q000378:metabolism; D006801:Humans; D018960:Hyaluronan Receptors / Q000378:metabolism; D008954:Models, Biological; D014411:Neoplastic Stem Cells / Q000378:metabolism / Q000473:pathology* / Q000528:radiation effects; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D017382:Reactive Oxygen Species / Q000378:metabolism; D015398:Signal Transduction; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism
chemical_list: D051927:CD24 Antigen; C468783:H2AX protein, human; D006657:Histones; D018960:Hyaluronan Receptors; D017382:Reactive Oxygen Species; D016159:Tumor Suppressor Protein p53; D064447:Checkpoint Kinase 2; C578634:CHEK2 protein, human; D017346:Protein Serine-Threonine Kinases