gms | German Medical Science

Objective: We have previously reported that combined inhibition of the epidermal growth factor receptor by erlotinib and of RAC1 by NSC23766 yielded a synergistic antiproliferative effect on established and primary glioblastoma cells. The current study aimed at identifying the cellular and molecular mechanisms underlying this observation.

Method: Staining for annexin/PI or carboxyfluorescein succinimidyl ester was performed prior to flow cytometric analysis in order to determine the induction of apoptosis, necrosis or cytostasis in U87 and A172 glioblastoma cell lines. Moreover, expression of Ki67 was determined by immunofluorescence and microscopic analysis. Induction of senescence was examined by staining for β-galactosidase, and the expression of cell cycle proteins was analysed by Western blot. All analyses were performed after 144 h of continuous exposure to erlotinib, NSC23766 or both at the respective inhibitory concentration 50.

Results: Combined treatment with erlotinib and NSC23766 resulted in a reduced number of cell divisions and a significantly decreased Ki67 expression when compared to single agent treatments. Intrinsic cellular programs such as senescence, apoptosis or necrosis were not engaged. On the molecular level, concomitant treatment with both agents resulted in a pronounced downregulation of cyclin D1, cyclin-dependent kinases 2, 4 and 6, as well as of survivin when compared to treatments with either agent alone.

Conclusions: In this study, we demonstrate that the combined treatment with erlotinib and NSC23766 markedly inhibits cell cycle progression which is most likely responsible for the antiproliferative synergism of both agents. Overall, our data suggest that the combination treatment with erlotinib and a RAC1 inhibitor may represent a promising novel therapeutic approach in glioblastoma.