Induction of lethal bystander effects in human breast cancer cell cultures by DNA-incorporated Iodine-125 depends on phenotype

Abstract

Purpose: This study uses a three-dimensional cell culture model to investigate lethal bystander effects in human breast cancer cell cultures (MCF-7, MDA-MB-231) treated with 125I-labeled 5-iodo-2 -deoxyuridine (125IdU). These breast cancer cell lines respectively form metastatic xenografts in nude mice in an estrogen-dependent and independent manner.Materials and methods: In the present study, these cells were cultured in loosely-packed three-dimensional architecture in a Cytomatrix™ carbon scaffold. Cultures were pulse-labeled for 3 h with 125IdU to selectively irradiate a minor fraction of cells, and simultaneously co-pulse-labeled with 0.04 mM 5-ethynyl-2′-deoxyuridine (EdU) to identify the radiolabeled cells using Click-iT® EdU and flow cytometry. The cultures were then washed and incubated for 48 h. The cells were then harvested, serially diluted, and seeded for colony formation. Aliquots of cells were subjected to flow cytometry to determine the percentage of cells labeled with 125IdU/EdU. Additional aliquots were used to determine the mean 125I activity per labeled cell. The percentage of labeled cells was about 15% and 10% for MCF-7 and MDA cells, respectively. This created irradiation conditions wherein the cross-dose to unlabeled cells was small relative to the self-dose to labeled cells. The surviving fraction relative to EdU-treated controls was measured.Results: Survival curves indicated significant lethal bystander effect in MCF-7 cells, however, no significant lethal bystander effect was observed in MDA-MB-231 cells.Conclusions: These studies demonstrate the capacity of 125IdU to induce lethal bystander effects in human breast cancer cells and suggest that the response depends on phenotype.