Growth arrest and non-apoptotic cell death associated with the suppression of c-myc expression in MCF-7 breast tumor cells following acute exposure to doxorubicin.
In the MCF-7 human breast [correction of beast] adenocarcinoma cell line, acute exposure to 1 muM doxorubicin inhibited cell proliferation by approximately 75%. Analysis of cell cycle distribution indicated that within 24 hr, the G(2)/M fraction increased more than 3-fold and the S-phase population declined by >50%. In addition to growth arrest, there was an approximately 40% reduction in the viable cell population after 72 hr. Gel electrophoretic resolution of low molecular weight DNA immediately after exposure of cells to doxorubicin failed to demonstrate "laddered" oligonucleosomal profiles associated with apoptosis. The absence of intracellular DNA fragments or release of fragmented DNA into the incubation medium was confirmed by spectrofluorophotometry over a 72 hr interval following exposure of cells to 1 muM doxorubicin. In addition, there was no evidence of the morphological features associated with apoptosis during this period. Acute exposure to 1 muM doxorubicin also produced a transient increase in c-myc message expression (within the first hour) followed by a decline to 70% of control levels within 2-4 hr. The reduction in c-myc mRNA levels was concentration dependent and corresponded closely with growth arrest (as well as with inhibition of DNA synthesis). These findings (as well as similar reports demonstrating a correspondence between reduced c-myc expression and growth inhibition by VM-26 and m-AMSA in MCF-7 cells) suggest that the down-regulation of c-myc expression may reflect perturbations in regulatory processes contributing to growth arrest in MCF-7 cells exposed to topoisomerase II inhibitors.