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 cultured glioblastoma cells. The current study aimed at identifying the molecular mechanism. Staining for annexin V/PI or carboxyfluorescein succinimidyl ester was performed in order to determine the induction of apoptosis, necrosis or cytostasis in established and primary cultured glioblastoma cells. Moreover, expression of Ki-67 was determined by immunofluorescence, and the expression of cell cycle proteins was analysed by Western blot. Our data show that combined treatment with erlotinib and NSC23766 resulted in a reduced number of cell divisions, a significantly decreased Ki-67 expression, increased apoptosis and autophagy when compared to single agent treatments. 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. In conclusion, we demonstrate that combined treatment of human glioma cell lines in vitro with erlotinib and NSC23766 markedly inhibits cell division, induces apoptosis independent of caspase-3 activation and induces autophagy concomitant with suppression of survivin.
Keywords: Apoptosis; Cytostasis; Glioma; HER1/EGFR; RAC1; Survivin; Synergy.