Cell death, growth arrest, proliferative recovery and DNA damage repair as components of the response to ionizing radiation in breast tumor cells

p53 wild type MCF-7 breast tumor cells are relatively resistant to ionizing radiation (IR); we hypothesize that this is due to their ability to arrest growth and repair double-strand breaks. The predominant response of MCF-7 cells to IR is prolonged growth arrest characterized by beta-galactosidase staining, indicative of cellular senescence. The senescence response was shown to be dependent on functional p53 and independent of telomere length or any alterations in telomerase activity or in its components, hTERT or hTR. We hypothesize that senescence may be due to telomere dysfunction and telomeric or subtelomeric breaks allowing for the fusion of chromosome ends.; Following exposure to 10 Gy IR, instead of apoptotic cell death, following a prolonged period of growth arrest, MCF-7 cells regain proliferative capacity and resume proliferation. In contrast, p53 mutated MDA-MB231 breast tumor cells demonstrate a transient growth arrest followed by apoptosis after IR. Although no direct role for p53 has been identified in repair of double-strand breaks by the predominant mammalian repair pathway (Non Homologous End Joining [NHEJ]), p53 has been shown to be involved in maintenance of growth arrest, which may allow adequate time to properly repair DNA damage. We hypothesize that loss of p53 function prevents G1 growth arrest and the maintenance of G2 growth arrest, resulting in cells re-entering the cell cycle with unrepaired DNA damage, eventually succumbing to apoptosis.; Since it is common knowledge that incomplete or misrepaired DNA double-strand breaks can result in cell death, it seems likely that inhibiting DNA damage repair will sensitize tumor cells to IR. Studies were performed to inhibit DNA damage repair by NHEJ in an attempt to sensitize breast tumor cells to IR. An XRCC4 protein fragment was constructed that serves as a dominant negative protein able to homodimerize and compete with full length XRCC4 for binding to ligase IV, preventing ligase IV activity, and inhibiting the final steps in NHEJ. Using adenovirus for the delivery of the XRCC4 protein fragment, we have shown that the XRCC4 protein fragment binds to ligase IV and sensitizes MDA-MB231 breast tumor cells to IR as determined by clonogenic survival assays.