Mouse myeloid leukemia cells (M1) were induced to differentiate into mature macrophages and granulocytes by glucocorticoids or a protein inducer in ascitic fluid from tumor-bearing rats. Addition of nonsteroidal antiinflammatory agents to M1 cells in suspension cultures inhibited the induction of differentiation by glucocorticoid (dexamethasone) or the protein inducer. The inhibition was unrelated to cytotoxicity and was reversible. The nonsteroidal antiinflammatory agent indomethacin inhibited dexamethasone-induced differentiation only when added before the time of commitment of the cells to differentiation.

The indomethacin-mediated inhibition was counteracted by prostaglandins E1 or E2 but not by prostaglandins F or F. Prostaglandin E stimulated phagocytosis induced by a suboptimal concentration of dexamethasone, but prostaglandin F did not. Moreover, lysozyme activity, which is a typical biochemical marker of macrophages, was induced in M1 cells by prostaglandin E alone, as well as by inducers of differentiation. These results suggest that prostaglandin E may be important in the induction of differentiation of myeloid leukemia cells.

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This work was partly supported by Grants-in-Aid for Cancer Research from the Ministry of Education, Science and Culture, and from the Ministry of Health and Welfare, Japan.

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