Cells of some cultured lymphoma lines are killed rapidly by x-rays and by bleomycin

Elevated levels of wild-type p53 induced by radiolabeling of cells leads to apoptosis or sustained growth arrest

BACKGROUND

The tumor suppressor protein p53 regulates progression through the checkpoint between the G1 and S phases of the cell cycle in response to radiation- or drug-induced DNA damage. We have examined potential p53-mediated effects of metabolically labeling cultured mammalian cells with [35S]methionine and [3H]thymidine, methods that are commonly used to study the biochemical properties, synthesis, processing and degradation of proteins and the replication of DNA in proliferating cells.

RESULTS

Wild-type p53 protein concentrations rapidly increased to high levels following metabolic radiolabeling of cells, as determined by four distinct assays. The increased concentration of wild-type p53 resulted in apoptosis of normal human peripheral blood lymphocytes and of murine T-cell acute lymphoblastic leukemia cells. In leukemia cells containing no p53, or only mutant p53 alleles, p53 protein levels were not increased and the cells did not undergo apoptosis in response to radioactive labeling. Radiolabeling of human diploid fibroblasts resulted in a prolonged growth arrest that was maintained for nearly three weeks.

CONCLUSIONS

The results of experiments employing radiolabeling techniques to characterize various aspects of cellular physiology may be seriously influenced by the induction of aberrant cell-cycle arrest and/or apoptosis mediated by wild-type p53. Furthermore, our observations suggest that stabilization of wild-type p53 in response to irradiation may not act primarily to facilitate the repair of DNA damage by inducing a transient G1-phase arrest, but rather to ensure genetic stability through sustained cell-cycle arrest or apoptotic death of the damaged cells.

Continued cell cycling ability of HL-60 cells following 3HaraC incorporation: a combination of "double-labeling" and sister chromatid analysis

This study was designed to determine if HL-60 cells could undergo one or more cycles of DNA synthesis despite containing 3H-cytosine arabinoside (3HaraC) in their genome. HL-60 cells were incubated with 3HaraC for 2 hours, washed and maintained in a medium containing bromodeoxyuridine (BrdU). At fixed time points, cells were arrested in metaphase and prepared for chromosomal analysis. Treatment of the sample by an immunofluorescent monoclonal anti-BrdU antibody allowed us to determine the differential fluorescent pattern of sister chromatids in metaphase cells that had undergone two or more rounds of DNA synthesis in the presence of BrdU. Processing the samples by autoradiography demonstrated the presence of black grains (3HaraC) overlying the chromosomes. Thus, we were able to examine each metaphase for the presence of 3HaraC as well as the number of cycles it had completed in the presence of BrdU. We showed that despite the presence of 3HaraC in their DNA, some HL-60 cells were able to undergo two or more complete rounds of DNA replication.

Parameters of the labeling of mitogen-activated murine lymphocytes by [35S]methionine for two-dimensional gel electrophoresis. I. Effect of culture conditions

Labeling with [35S]methionine at a high specific activity is essential to the facile preparation of 2-dimensional gel electrophoretograms with the analytical 2-dimensional charge-size separation procedure (Anderson's ISODALT system). Mitogen-activated T and B lymphocytes subjected to low methionine concentrations would not proceed through cell cycle. In the case of activated B lymphocytes, the use of fetal bovine serum (FBS), dialyzed to lower endogenous methionine concentrations, prevented B cell growth even in the presence of otherwise satisfactory levels of methionine. High concentrations of [35S]methionine (greater than 300 mCi/1) induced B cell death, apparently by radiation damage. Despite these problems, good radioautograms and radiofluorograms of 2D electrophoretograms could be prepared by labeling activated B or T cells in bulk (10(6) cells/ml) with high specific activity [35S]methionine. The polypeptides labeled may be a biased sample since lymphoid cells do not proceed through cell cycle under these conditions. Small numbers (10(3] of activated T cells also yielded satisfactory samples but labeling of small numbers of activated B cells was not possible.