When mouse erythroleukemia (MEL) cells were induced to differentiate by growth in the presence of dimethyl sulfoxide, hexamethylene bisacetamide (HMBA), or hemin, the apparent activity of DNA ligase extractable from inducer-treated cells decreased 70 to 80% when compared to untreated cells. Earlier work had indicated that these changes did not occur in a differentiation-resistant MEL cell variant and suggested that the decrease in the level of DNA ligase activity might be related to the differentiation process. Since the MEL cells accumulate high levels of both hemoglobin-bound and non-hemoglobin-bound heme, the effect of both hemoglobin and hemin on DNA ligase activity of MEL cell extracts was tested. When cell-free extracts containing DNA ligase activity were preincubated with hemin at concentrations up to 150 µm, an 80% or greater inhibition of the DNA ligase activity resulted. The ATP-dependent DNA ligase from bacteriophage T4 was also inhibited by hemin, but the NAD-dependent DNA ligase from Escherichia coli was not sensitive to this treatment. Preincubation of these same extracts with hemoglobin at levels comparable to those present in differentiating cells did not result in inhibition of any of the ATP-dependent DNA ligases tested. Culturing the cells with dimethyl sulfoxide in the presence of imidazole resulted in a marked decrease in globin chain accumulation but did not reverse the dimethyl sulfoxide-related decrease in DNA ligase activity. These data suggest the possibility that heme or its metabolites, but not globin or hemoglobin, could serve to modify the process of DNA replication and/or repair in differentiating MEL cells via inhibition of DNA ligase activity. These data are consistent with the findings of Lo et al. (S. C. Lo, R. Aft, and G. C. Mueller, Cancer Res., 41:864–870, 1981) which correlated the onset of differentiation-related terminal cell division in MEL cells with the levels of nonhemoglobin heme present in these cells.

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This work was supported by American Cancer Society Grants CH-144 and CH-303; USPHS Grants AM16690-06 and CA24402-03; the Chemotherapy Foundation, Inc.; Gar Reichman Foundation; Charles E. Merrill Trust; Herman Goldman Foundation; and Samuel Waxman Cancer Foundation. Part of this work was presented at the Conference on Cell Proliferation, Cancer, and Cancer Therapy, New York Academy of Sciences, 1982, and the American Association for Cancer Research annual meeting, 1986.

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