Inhibitory Effect of Mitoxantrone on Activity of Protein Kinase C and Growth of HL60 Cells1

Mitoxantrone, a new anthraquinone, showed inhibitory an effect on protein kinase C (PKC) activity. Its IC60 value was 4.4 /ug/ml (8.5 /iM), which is much lower than those of the well-known anthracyclines daunorubicin and doxorubicin, the IC ^ values of which are more than 100 fig/ml (> 170 pM). Kinetic studies demonstrated that mitoxantrone inhib-ited PKC in a competitive manner with respect to histone HI, and its JEi value was 6.3 fiM (K, values of daunorubicin and doxorubicin were 0.89 and 0.15 mM, respectively), and in a non-competitive manner with respect to phosphatidylserine and ATP. Inhibition of phosphorylation by mitoxantrone was observed with various substrates including S6 peptide, myelin basic protein and its peptide substrate derived from the amino-terminal region. Their IC60 values were 0.49/ag/ml (0.95/iM), 1.8/ig/ml (3.5 fM), and 0.82/ig/ml(1.6 /iM), respectively. Mitoxantrone did not markedly inhibit the activity of cyclic AMP-dependent protein kinase, casein kinase I or casein kinase II, at concentrations of less than 10 fig/ml. On the other hand, brief exposure (5 min) of HL60 cells to mitoxantrone caused the inhibition of cell growth with an IC50 value of 52 ng/ml (0.1 /iM). In HL60 cells, most of the PKC activity (about 90%) was detected in the cytosolic fraction. When HL60 cells exposed to 10 /ig/ml mitoxantrone for 5 min were observed with fluorescence microscopy