Differentiation of human monoblastic leukemia U937 cells induced by inhibitors of myosin light chain kinase and prevention of differentiation by granulocyte-macrophage colony-stimulating factor.

Inhibitors of protein kinase activities are useful for the study of intracellular signal transduction and some of these inhibitors are reported to induce differentiation of human leukemia cells. We examined effects of granulocyte-macrophage colony-stimulating factor (GM-CSF) in combination with several kinase inhibitors on differentiation of human leukemia U937 cells. Nitroblue tetrazolium (NBT)-reducing activity, a typical marker of myelomonocytic differentiation, of U937 cells was induced by genistein and GM-CSF enhanced this activity. GM-CSF also induced the NBT-reducing activity of the cells in combination with 2,5-dihydroxycinnamic acid methyl ester, psi-tectorigenin and staurosporine, although each of them did not induce the activity. Inhibitors of myosin light chain kinase, 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-9) and 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-7), induced in U937 cells NBT-reduction, and lysozyme and alpha-naphthyl acetate esterase activities. GM-CSF inhibited this differentiation and counteracted the anti-proliferation effect of the kinase inhibitors. These results suggest that some protein kinases are involved in differentiation of U937 cells and the kinases inhibited by ML-9 and ML-7 are associated with signal transduction of GM-CSF.