Effects of protein kinase inhibitors on the progression of the cell cycle. especially on the initiation of M phase.

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Ammonium sulfate fraction of maturation-promoting factor (MPF) was prepared from unfertilized eggs of Xenopus laevis. The inhibitory effects of K252 derivatives and staurosporine on MPF and cdc2 Kinase activities of the MPF fraction were examined. K252a and staurosporine inhibited the MPF activity with IC50 of 15 and 1.5 ug/ml, respectively and the cdc2 Kinase activity with IC50 of 3.5 and 0.4 ug/ml, respectively. K252b and KT5926 inhibited the cdc2 kinase activity with IC50 of 0.15 and 60 ug/ml, respectively. The decreasing order of the protein kinase inhibitors in the potency of the inhibitory actions was K252b> staurosporine> K252a> KT5926. Thus, the property of cdc2 kinase is remarkably different from that of the other protein kinases, especially in that its sensitivity to K252b is greater by more than 20 times than to K252a. The effects of the protein kinase inhibitors on the processes leading to the activation of MPF were also examined. Although 30 ug/ml K252a and 1.5 ug/ml K252b inhibited cdc2 kinase approximately to the same extent, the former inhibited the dephosphorylation of cdc2 protein and the thiophosphorylation of p110, whereas the latter did not. Therefore, the protein kinase responsible for the activation of MPF is suggeted to be different from cdc2 kinase itself. As to the inhibitory effects on the initiation of M phase of cultured cells staurosporine was the strongest, followed by K252a and K252b. This may be due to the low permeability of K252b across the cell membrane, but it is also possible that the inhibitors act on other targets than cdc2 kinase itself.