Effects of repeated exposure to cyclophosphamide on drug resistance and biological properties of a newly autonomy-acquired mouse mammary tumor (T4-OI320).

T4-OI320 tumors, the autonomous but estrogen receptor (ER-)-positive subline recently established from the pregnancy-dependent TPDMT-4 mouse mammary tumor, were characterized by moderate sensitivity to 1 mg cyclophosphamide (CY) and high sensitivity to 20 micrograms mitomycin C (MMC) and 1.5 mg 5-fluorouracil (5FU) twice weekly, the modal chromosome number of 40, the mean ER level of 20.8 fmol/mg protein, and a population of cells with one exogenous mouse mammary tumor virus (MMTV) genome. Serial tumor passages under CY treatment were conducted to investigate the effects of continued chemotherapy on growth habit, drug resistance, ER, karyotype, proviral MMTV genome and 22 oncogenes including int-1 and int-2. The growth rate was 2.9- and 4.5-fold increased after 10 and 20 passages, respectively. Along with this, the tumors acquired greater or complete resistance to CY and MMC similar to each other in antitumor mechanism, but maintained similarly high sensitivity to 5FU different from CY in antitumor mechanism. The ER level remained the same as but declined to half of the initial level after 10 and 20 passages, respectively. Tumor cells became more heterogenous in karyotype and the proportion of hyperdiploid cells with 41 to 47 chromosomes increased, resulting in shift of the modal number to 42 after 20 passages: Cells with 42 chromosomes accounted for 41% of the population. The exogenous MMTV bands in the Southern blotting became more intense after CY treatment accompanying neither amplification nor rearrangement of any cellular oncogenes. The tumors transplanted under no treatment for 20 generations had 1.5-fold higher growth rate, slightly stronger resistance to CY and MMC, similarly high sensitivity to 5FU, 4-fold lower ER level, the same distribution of chromosome numbers compared to the original tumors and similar intensity of the exogenous MMTV bands. Thus, acquisition of higher growth potential and greater drug resistance in the course of CY treatment appeared to be associated with chromosomal changes and selective overgrowth of the particular cell population with exogenous MMTV information in this model.