Phosphorylation of β-actin by protein kinase C-delta in camptothecin analog-induced leukemic cell apoptosis

Aim： This study was conducted to reveal new proteins involved in acute myeloid leukemia （AML） cell apoptosis. Methods： Using camptothecin analog NSC606985- induced leukemic U937 cell apoptosis as a model, this study performed a differential proteomic analysis during apoptosis induction. The significantly modulated protein was underwent further investigation in the apoptotic process. Results： We found that β-actin protein presented two different spots on the two-dimensional electrophoresis （2-DE） map, which shared similar molecular weight and different pI. Those two spots demonstrated contrary changes （disappeared on the basic-end and increased on the acid-end spot） during apoptosis induction, although the total level of β-actin kept constant. This observation was further confirmed by immunoblot analysis on 2-DE gel. When NSC606985-treated cell lysate was incubated with alkaline phosphotase, β-actin on the basic-end spot was restored, indicating increased phosphorylation of β-actin during NSC606985- induced apoptosis. Moreover, the polymerization of actin also decreased after NSC606985 treatment. The increased β-actin phosphorylation and decreased actin polymerization was antagonized by pre-treatment of rottlerin, a specific protein kinase C-delta （PKCδ） inhibitor. Condusion： All these results indicate that β-actin was phosphorylated during apoptosis induction, which was mediated by activated PKCδ.