Both low and high concentrations of staurosporine induce G1 arrest throughdown-regulation of cyclin E and cdk2 expression

Staurosporine has been reported to cause arrest of cells in G1 phase at low concentration and in G2 phase at high concentration. This raises the quest ion of why the effects of staurosporine on the cell cycle depend on the app lied concentration. In order to verify these multiple functions of staurosp orine in Meth-A cells, we used cyclin E as a landmark of G1/S transition, c yclin B as a landmark of G2/M transition and MPM2 as a hallmark of M phase. We found that staurosporine arrested cells in G1 phase at a low concentrat ion (20 nM) and in G2/M phase at a high concentration (200 nM). However, 20 0 nM staurosporine increased the expression of cyclin B and cdc2 proteins, suggesting that the cells progressed through the G2/M transition, and incre ased the expression of MPM2. protein, indicating that the cells entered M p hase. Moreover, 200 nM staurosporine increased the expression of p53 and p2 1 proteins and inhibited the expression of cyclin E and cdk2 proteins, sugg esting that the cells were arrested in the G1 phase of the next cycle. Morp hological observation showed similar results as well. These data suggest th at the G2/M accumulation induced by 200 nM staurosporine does not reflect G 2 arrest, but rather results from M phase arrest, followed by progression f rom M phase to the G1 phase of the next cycle without cytokinesis, and fina lly arrest of the cells in G1 phase.