No abstract

No abstract

Activating transcription factor 3 (ATF3) is a transcriptional repressor that is rapidly induced in cells exposed

SDI1 is an inhibitor of DNA synthesis that we isolated by expression screening cDNAs prepared from senescent, terminally nondividing human cells. Other groups then cloned this gene as a cyclin-dependent kinase (cdk)-interacting protein (CIP1, p21) that inhibits cdks; the gene was also isolated by screening for genes transactivated by p53 (WAF1). p53 levels are low in senescent and quiescent contact-inhibited or serum-deprived normal human cells, which we have found express high levels of SDI1 mRNA. This indicates that alternate pathways for upregulation of message level of this gene may exist. We therefore proceeded with the study presented here, treating human cells with a variety of growth-arrest-inducing agents, including some that damaged DNA, and found that RNA levels of SDI1 were increased in all cases that resulted in growth inhibition. More important, with the exception of gamma-radiation, most of these agents were able to elevate SDI1 message levels in cells lacking wild-type p53. At least two distinct kinetic profiles for RNA induction were observed, one that implicated p53 transactivation and occurred early enough to cause arrest, and another that clearly was p53 independent and suggested a role for the SDI1 gene product in the maintenance rather than in the cause of inhibition of DNA synthesis.

No abstract

SummaryAfter an exposure to ionising radiation, cells can quickly repair damage to their genomes; however, a few unrepairable DNA doublestrand breaks (DSBs) emerge in the nucleus in a prolonged culture and perpetuate as long as the culture continues. These DSBs may be retained forever in cells such as non-dividing ageing tissues, which are resistant to apoptosis. We show that such unrepairable DSBs, which had been advocated by the classical target theory as the 'radiation hit', could account for permanent growth arrest and premature senescence. The unrepairable DSBs build up with repeated irradiation, which accounts for an accumulated dose. Because these DSBs tend to be paired, we propose that the untethered and 'torn-off' molecular structures at the broken ends of the DNA result in an alteration of chromatin structure, which protects the ends of the DNA from genomic catastrophe. Such biochemical responses are important for cell survival but may cause gradual tissue malfunction, which could lead to the late effects of radiation exposure. Thus, understanding the biology of unrepairable damage will provide new insights into the long-term effects of radiation.

In experimental organisms such as fruit flies and mice, increased frequencies in germ cell mutations have been detected following exposure to ionizing radiation. In contrast, there has been no clear evidence for radiation-induced germ cell mutations in humans that lead to birth defects, chromosome aberrations, Mendelian disorders, etc. This situation exists partly because no sensitive and practical genetic marker is available for human studies and also because the number of people exposed to large doses of radiation and subsequently having offspring was small until childhood cancer survivors became an important study population. In addition, the genome of apparently normal individuals seems to contain large numbers of alterations, including dozens to hundreds of nonfunctional alleles. With the number of mutational events in protein-coding genes estimated as less than one per genome after 1 gray (Gy) exposure, it is unsurprising that genetic effects from radiation have not yet been detected conclusively in humans.

IntroductionProgerin, the protein responsible for the Hutchinson-Gilford Progeria Syndrome (HGPS), is a partially deleted form of nuclear lamin A, and its expression has been suggested as a cause for dysfunctional nuclear membrane and premature senescence. To examine the role of nuclear envelop architecture in regulating cellular aging and DNA repair, we used ionizing radiation to increase the number of DNA double strand breaks (DSBs) in normal and HGPS cells, and analyzed possible relationship between unrepaired DSBs and cellular aging.ResultsWe found that HGPS cells are normal in repairing a major fraction of radiation-induced double strand breaks (M-DSBs)but abnormal to show increased amount of residual unrepaired DSBs (R-DSBs). Such unrepaired DSBs were 2.6 times (CI 95 %: 2.2–3.2) higher than that in normal cells one week after the irradiation, and 1.6 times (CI 95 %: 1.3–1.9) higher even one month after the irradiation. These damages tend to increase as the nuclear envelope become abnormal, a characteristic of both HGPS and normal human cells which undergo replicative senescence. The artificial, enforced over-expression of progerin further impaired the repair of M-DSBs, implying lamin A-associated nuclear membrane has an important role for DNA DSB repair. Introduction of telomerase gene function in HGPS cells reversed such aging phenotypes along with upregulation of lamin B1 and downregulation of progerin, which is a hallmark of young cells.ConclusionWe suggest that lamin A- or progerin-associated nuclear envelope is involved in cellular aging associated with DNA damage repair.Electronic supplementary materialThe online version of this article (doi:10.1186/s41021-015-0018-4) contains supplementary material, which is available to authorized users.