Other investigators have demonstrated by transfer of medium from irradiated cells and by irradiation with low-fluence alpha particles or microbeams that cells do not have to be directly exposed to ionizing radiation to be detrimentally affected, i.e. bystander effects. In this study, we demonstrate by transfer of medium from X-irradiated human CGL1 hybrid cells that the killing of bystander cells reduces the plating efficiency of the nonirradiated CGL1 cells by 33 +/- 6%. In addition, we show that the amount of cell death induced by bystander effects is not dependent on X-ray dose, and that the induction of apoptosis does not appear to be responsible for the cell death. Furthermore, we found that the reduction in plating efficiency in bystander cells is evident for over 18 days, or 22 cell population doublings, after medium transfer, despite repeated refeeding of the cell cultures. Finally, we report the novel observation that bystander effects induced by the transfer of medium from irradiated cells can induce neoplastic transformation. Exposing unirradiated CGL1 cells to medium from cells irradiated with 5 or 7 Gy increased the frequency of neoplastic transformation significantly from 6.3 x 10(-6) in unirradiated controls to 2.3 x 10(-5) (a factor of nearly four). We conclude that the bystander effect induces persistent, long-term, transmissible changes in the progeny of CGL1 cells that result in delayed death and neoplastic transformation. The data suggest that neoplastic transformation in bystander cells may play a significant role in radiation-induced neoplastic transformation at lower doses of X rays.

Studies on nontumorigenic and tumorigenic human cell hybrids derived from the fusion of HeLa (a cervical cancer cell line) with GM00077 (a normal skin fibroblast cell line) have demonstrated "functional" tumor-suppressor activity on chromosome 11. It has been shown that several of the neoplastically transformed radiation-induced hybrid cells called GIMs (gamma ray induced mutants), isolated from the nontumorigenic CGL1 cells, have lost one copy of the fibroblast chromosome 11. We hypothesized, therefore, that the remaining copy of the gene might be mutated in the cytogenetically intact copy of fibroblast chromosome 11. Because a cervical cancer tumor suppressor locus has been localized to chromosome band 11q13, we performed deletion-mapping analysis of eight different GIMs using a total of 32 different polymorphic and microsatellite markers on the long arm (q arm) of chromosome 11. Four irradiated, nontumorigenic hybrid cell lines, called CONs, were also analyzed. Allelic deletion was ascertained by the loss of a fibroblast allele in the hybrid cell lines. The analysis confirmed the loss of a fibroblast chromosome 11 in five of the GIMs. Further, homozygous deletion (complete loss) of chromosome band 11q13 band sequences, including that of D11S913, was observed in two of the GIMs. Detailed mapping with genomic sequences localized the homozygous deletion to a 5.7-kb interval between EST AW167735 and EST F05086. Southern blot hybridization using genomic DNA probes from the D11S913 locus confirmed the existence of homozygous deletion in the two GIM cell lines. Additionally, PCR analysis showed a reduction in signal intensity for a marker mapped 31 kb centromeric of D11S913 in four other GIMs. Finally, Northern blot hybridization with the genomic probes revealed the presence of a novel >15-kb transcript in six of the GIMs. These transcripts were not observed in the nontumorigenic hybrid cell lines. Because the chromosome 11q13 band deletions in the tumorigenic hybrid cell lines overlapped with the minimal deletion in cervical cancer, the data suggest that the same gene may be involved in the development of cervical cancer and in radiation-induced carcinogenesis. We propose that a gene localized in proximity to the homozygous deletion is the candidate tumor-suppressor gene.

Loss of active tumor suppressor alleles on fibroblast chromosomes 11 and 14 are involved in radiation-induced neoplastic transformation of human hybrid CGL1 cells. Loss of either chromosome 11 or 14 alone is not sufficient for neoplastic transformation. To gain insight into the potential functions of these tumor suppressor loci, we have investigated the effects of chromosome 11 or 14 loss on radiation-induced neoplastic transformation. We recently demonstrated that loss of chromosome 11 increases the susceptibility to X-ray induced cell killing, neoplastic transformation and the expression of delayed death. The data suggested that one possible function of the chromosome 11 tumor suppressor gene may be to help maintain genome stability after radiation damage. We postulated that if the chromosome 14 allele is functioning in a similar manner, then the loss of chromosome 14 may also make the hybrid cells more susceptible to radiation-induced cell killing and neoplastic transformation. A hybrid cell line which has lost one copy of chromosome 14 was isolated and designated CON3(-14). CON3(-14) cells were more sensitive to X-ray-induced cell killing when compared with parental CGL1 cells. However, the susceptibility to radiation-induced neoplastic transformation was significantly reduced (by a factor of two) compared with the parental CGL1 cells. The expression of delayed death in the progeny of the irradiated CON3(-14) cells, growing in transformation flasks, was similar to CGL1 cells during the 21 day assay period. Taken together, the data indicate that loss of chromosome 14 alone increased the X-ray sensitivity of the hybrid cells but reduced their susceptibility to radiation-induced neoplastic transformation. These data suggest that the tumor suppressor alleles on chromosomes 11 and 14 may be functionally distinct in terms of their regulation of genomic instability and neoplastic transformation after radiation exposure.

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