Disruption of ATM in p53-null cells causes multiple functional abnormalities in cellular response to ionizing radiation.
(25/1510)
ATM is a member of the large phosphatidylinositol-3 kinase family and plays an important role in cellular response to DNA damage. To further define the physiological roles of ATM at the cellular level, we created an isogenic set of stable cell lines differing only in their ATM status from the chicken B cell line DT40 by targeted integration. These stable DT40 cell lines, as most of transformed chicken cell lines, do not express p53. However, ATM-/- DT40 cells displayed retarded cellular proliferation, defective G2/M checkpoint control and radio-resistant DNA synthesis. Furthermore, ATM-/- DT40 cells were sensitive to ionizing radiation and showed highly elevated frequencies of both spontaneous and radiation-induced chromosomal aberrations. In addition, a slight but significant reduction in targeted integration frequency was observed in ATM-/- DT40 cells. These results suggest that ATM has multiple p53-independent functions in cell cycle checkpoint control and in maintenance of chromosomal DNA. These ATM deficient DT40 clones therefore provide a useful model system for analysing p53-independent ATM functions. (+info)
Rapid ATM-dependent phosphorylation of MDM2 precedes p53 accumulation in response to DNA damage.
(26/1510)
The p53 tumor-suppressor protein, a key regulator of cellular responses to genotoxic stress, is stabilized and activated after DNA damage. This process is associated with posttranslational modifications of p53, some of which are mediated by the ATM protein kinase. However, these modifications alone may not account in full for p53 stabilization. p53's stability and activity are negatively regulated by the oncoprotein MDM2, whose gene is activated by p53. Conceivably, p53 function may be modulated by modifications of MDM2 as well. We show here that after treatment of cells with ionizing radiation or a radiomimetic chemical, but not UV radiation, MDM2 is phosphorylated rapidly in an ATM-dependent manner. This phosphorylation is independent of p53 and the DNA-dependent protein kinase. Furthermore, MDM2 is directly phosphorylated by ATM in vitro. These findings suggest that in response to DNA strand breaks, ATM may promote p53 activity and stability by mediating simultaneous phosphorylation of both partners of the p53-MDM2 autoregulatory feedback loop. (+info)
Translocation of SAPK/JNK to mitochondria and interaction with Bcl-x(L) in response to DNA damage.
(27/1510)
Activation of the stress-activated protein kinase (SAPK/JNK) by genotoxic agents is necessary for induction of apoptosis. We report here that ionizing radiation ionizing radiation exposure induces translocation of SAPK to mitochondria and association of SAPK with the anti-apoptotic Bcl-x(L) protein. SAPK phosphorylates Bcl-x(L) on threonine 47 (Thr-47) and threonine 115 (Thr-115) in vitro and in vivo. In contrast to wild-type Bcl-x(L), a mutant Bcl-x(L) with the two threonines substituted by alanines (Ala-47, Ala-115) is a more potent inhibitor of ionizing radiation-induced apoptosis. These findings indicate that translocation of SAPK to mitochondria is functionally important for interactions with Bcl-x(L) in the apoptotic response to genotoxic stress. (+info)
Atm and Bax cooperate in ionizing radiation-induced apoptosis in the central nervous system.
(28/1510)
Ataxia-telangiectasia is a hereditary multisystemic disease resulting from mutations of ataxia telangiectasia, mutated (ATM) and is characterized by neurodegeneration, cancer, immune defects, and hypersensitivity to ionizing radiation. The molecular details of ATM function in the nervous system are unclear, although the neurological lesion in ataxia-telangiectasia becomes apparent early in life, suggesting a developmental origin. The central nervous system (CNS) of Atm-null mice shows a pronounced defect in apoptosis induced by genotoxic stress, suggesting ATM functions to eliminate neurons with excessive genomic damage. Here, we report that the death effector Bax is required for a large proportion of Atm-dependent apoptosis in the developing CNS after ionizing radiation (IR). Although many of the same regions of the CNS in both Bax-/- and Atm-/- mice were radioresistant, mice nullizygous for both Bax and Atm showed additional reduction in IR-induced apoptosis in the CNS. Therefore, although the major IR-induced apoptotic pathway in the CNS requires Atm and Bax, a p53-dependent collateral pathway exists that has both Atm- and Bax-independent branches. Further, Atm- and Bax-dependent apoptosis in the CNS also required caspase-3 activation. These data implicate Bax and caspase-3 as death effectors in neurodegenerative pathways. (+info)
Male sterility and enhanced radiation sensitivity in TLS(-/-) mice.
(29/1510)
TLS (also known as FUS) is an RNA-binding protein that contributes the N-terminal half of fusion oncoproteins implicated in the development of human liposarcomas and leukemias. Here we report that male mice homozygous for an induced mutation in TLS are sterile with a marked increase in the number of unpaired and mispaired chromosomal axes in pre-meiotic spermatocytes. Nuclear extracts from TLS(-/-) testes lack an activity capable of promoting pairing between homologous DNA sequences in vitro, and TLS(-/-) mice and embryonic fibroblasts exhibit increased sensitivity to ionizing irradiation. These results are consistent with a role for TLS in homologous DNA pairing and recombination. (+info)
Deficiency of gamma-ray excision repair in skin fibroblasts from patients with Fanconi's anemia.
(30/1510)
The capacity of preparations of skin fibroblasts from normal individuals and patients with Fanconi's anemia to excise gamma-ray products of the 5,6-dihydroxydihydrothymine type from exogenous DNA was investigated. The excision capacity of whole-cell homogenates of fibroblasts from two of four patients with Fanconi's anemia was substantially below normal. This repair deficiency was further pronounced in nuclear preparations from cells of the same two patients. (+info)
Combined effect of tumor necrosis factor-related apoptosis-inducing ligand and ionizing radiation in breast cancer therapy.
(31/1510)
Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potent endogenous activator of the cell death pathway and functions by activating the cell surface death receptors 4 and 5 (DR4 and DR5). TRAIL is nontoxic in vivo and preferentially kills neoplastically transformed cells over normal cells by an undefined mechanism. Radiotherapy is a common treatment for breast cancer as well as many other cancers. Here we demonstrate that ionizing radiation can sensitize breast carcinoma cells to TRAIL-induced apoptosis. This synergistic effect is p53-dependent and may be the result of radiation-induced up-regulation of the TRAIL-receptor DR5. Importantly, TRAIL and ionizing radiation have a synergistic effect in the regression of established breast cancer xenografts. Changes in tumor cellularity and extracellular space were monitored in vivo by diffusion-weighted magnetic resonance imaging (diffusion MRI), a noninvasive technique to produce quantitative images of the apparent mobility of water within a tissue. Increased water mobility was observed in combined TRAIL- and radiation-treated tumors but not in tumors treated with TRAIL or radiation alone. Histological analysis confirmed the loss of cellularity and increased numbers of apoptotic cells in TRAIL- and radiation-treated tumors. Taken together, our results provide support for combining radiation with TRAIL to improve tumor eradication and suggest that efficacy of apoptosis-inducing cancer therapies may be monitored noninvasively, using diffusion MRI. (+info)
Radiation carcinogenesis.
(32/1510)
Research on radiation carcinogenesis during the past 2 decades has focused on cellular and molecular mechanisms for the effects of radiation in mammalian cells. This paper will review several of these areas of research, as they may relate specifically to the induction of cancer by ionizing radiation. Knowledge of the critical DNA damage of biologic importance, and how this damage is repaired, will be discussed in relation to its role in the induction of mutations by radiation. The search for the initiating event in radiation carcinogenesis, as well as other genetic events that may be involved, is discussed in terms of the possible role of the activation of oncogenes or tumor suppressor genes and the loss of cell-cycle checkpoints. Finally, evidence will be described indicating that important genetic consequences of radiation may arise in cells that in themselves receive no direct nuclear irradiation. It has been shown that radiation can, by itself, induce a type of genomic instability in cells, which enhances the rate at which mutations and other genetic changes arise in the descendants of the irradiated cell after many generations of replication. Preliminary evidence has been presented that irradiation targeted to the cytoplasm yields a significant increase in the frequency of mutations. Finally, genetic events including the induction of mutations and changes in gene expression may occur in neighboring cells that receive no direct radiation exposure at all. This 'bystander effect' involves gap junction mediated cell-cell communication, and activation of the p53 damage response pathway. The possible role of these phenomena in radiation carcinogenesis is discussed. (+info)