A diffusible resistance factor(s) in spontaneous mitomycin resistant mammalian cancer cells

We developed a Mitomycin C (MMC)-resistant mouse breast cancer cell subline, R-FM3A, from FM3A parent cells(W-FM3A) by continuous exposure to a concentration of 0.01 microgram/ml MMC for over two months. R-FM3A cells were resistant to MMC and decreased in response to the combined effects of ionizing irradiation and MMC. The resistance to MMC and ionizing irradiation was mediated by a cell-free medium, in which R-FM3A cells were cultured for 12 hours without MMC (Re-medium). The cell cycles measured by means of a flow cytometer were not influenced by the Re-medium when W-FM3A cells were treated by MMC. But the cell cycles measured by means of a flow cytometer were influenced by the Re-medium when W-FM3A cells were irradiated at a dose of 4 Gy.