XB-IMG-136330
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FIGURE 5:. Inhibition of Wee1/Myt1 and Cdc25 in synchronized cells causes mitotic collapse. (A) HeLa cells were synchronized at the S/G2 border after double thymidine block and then treated with the Wee1/Myt1 inhibitor, PD0166285, Cdc25 inhibitor, NSC663284, and the combination of the two drugs. Nocodazole was added to the medium to prevent mitotic exit. Cells were then collected at indicated time points, fixed and stained with antibody to phospho-histone H3 (mitotic marker) conjugated with Alexa Fluor 647, and processed by flow cytometry. In cells treated with vehicle only (DMSO, blue line), the mitotic index progressively increased, with more than half the cells being in mitosis by the end of the experiment. Cdc25 inhibitor, NSC663284, blocked mitotic entry (brown line). Wee1 inhibitor, PD0166285 (green line), caused rapid mitotic entry during the first hour after its addition. In cells treated with both PD0166285 and NSC663284 (orange line), the mitotic index first increased then fell. (B) HeLa cells were treated as in (A), lysed and analyzed by SDS–PAGE. In cells not treated with inhibitors (blue lanes), phosphorylations on histone H3 and nucleolin appeared by 8 h after second thymidine release and increased for the duration of the experiment. Phosphorylation of Cdk1 on inhibitory T14 and Y15 decreased over time, indicating the activation of the Cdk1/cyclin B complex. As cells were entering mitosis, a portion of Wee1, Myt1 Cdc25C, Cdc27, and MastL acquired an electrophoretic mobility shift. Cyclin B1 levels were increasing, and cyclin A2 levels dropped slightly as cells accumulated in mitosis. Inhibition of Wee1 and Myt1 kinases with PD0166285 (green lanes) resulted in rapid phosphorylation of Nucleolin and histone H3 that peaked 2 h after the drug addition and remained steadily high for the duration of the experiment. Cdk1 was rapidly dephosphorylated on inhibitory T14 and Y15. Wee1, Myt1, Cdc25, and Cdc27 rapidly shifted up. By 1 h after drug addition, Cyclin A2 was largely degraded and cyclin B1 was stable. Inhibition of Wee1 and Myt1 together with Cdc25 by addition of both PD0166285 and NSC 663284 (orange lanes) triggered the a weak phosphorylation on Nucleolin and histone H3 that peaked at 1–2 h and disappeared at 3–4 h after addition of the two drugs. Reduced mitotic phosphorylation shifts of Wee1, Myt1, Cdc25, and Cdc27 indicated that these proteins were not fully phosphorylated. Note that cyclin B and most of the cyclin A were not degraded in these cells. Panels on the right show quantifications of indicated Western blots. All values were adjusted for loading and normalized to the 4-h time point of DMSO-treated cells. Image published in: Potapova TA et al. (2011) © 2011 Potapova et al. This image is reproduced with permission of the journal and the copyright holder. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike license Larger Image Printer Friendly View |