Release of specific proteins from nuclei of HL-60 and MOLT-4 cells by antitumor drugs having affinity to nucleic acids.

The binding sites for mitoxantrone (MIT), Ametantrone (AMT), doxorubicin (DOX), actinomycin D (AMD) and ethidium bromide (EB) in nuclei from exponentially growing and differentiating human promyelocytic HL-60 and lymphocytic leukemic MOLT-4 cells were studied by gel electrophoresis of proteins selectively released during titration of these nuclei with the drugs. Each drug at different drug: DNA binding ratios resulted in a characteristic pattern of protein elution and/or retention. For example, in nuclei from exponentially growing HL-60 cells, MIT affected 44 nuclear proteins that were different from those affected by EB; of these 29 were progressively released at increasing MIT:DNA ratios, 11 were transiently released (i.e. only at a low MIT:DNA ratio) and 4 entrapped. Patterns of proteins displaced from nuclei of exponentially growing HL-60 cells differed from those of cells undergoing myeloid differentiation as well as from those of exponentially growing MOLT-4 cells. The first effects were seen at a binding density of approximately one drug molecule per 10-50 base pairs of DNA. The observed selective displacement of proteins may reflect drug-altered affinity of the binding sites for those proteins, for example due to a change of nucleic acid or protein conformation upon binding the ligand. The data show that the binding site(s) for each of the ligands studied is different and the differences correlate with variability in chemical structure between the ligands. The nature of the drug-affected proteins may provide clues regarding antitumor or cytotoxic mechanisms of drug action.