Combined Bcl-2/Bcl-xL and RAC1 inhibition yields a synergistic antineoplastic activity in glioblastoma

Objective:

The purpose of this study was to examine whether inhibition of RAC1 would enhance the pro-apoptotic reprogramming of glioblastoma's cellular circuitry following selective Bcl-2/Bcl-xL inhibition by BH-3 mimetics.

Methods:

Preclinical drug testing and molecular profiling was performed in different glioblastoma model systems including established, primary and glioma stem-like cells.

Results:

Combined inhibition of RAC1 and Bcl-2/Bcl-xL resulted in synergistic anti-proliferative and pro-apoptotic effects in a panel of different glioblastoma cells. RAC1 inhibition lead to decreased expression of deubiquitinase Usp9X and depletion of endogenous Mcl-1 through a post-translational mechanism. The combination treatment diminished the expression of the anti-apoptotic Bcl-2 family proteins Bcl-2 and Bcl-xL. In addition, the migratory activity of glioblastoma cells was significantly inhibited by the combination treatment. Lastly, tumor formation on the chorion allantoic membrane of chicken embryos was significantly impaired by simultaneous inhibition of RAC1 and Bcl-2/Bcl-xL.

Conclusion:

Our data suggest that inhibition of RAC1 strongly enhances the pro-apoptotic shift in glioblastoma cells induced by BH-3-mimetics via counteracting mechanisms of resistance such as upregulation of Mcl-1. From a mechanistic point of view, RAC1 inhibition affects protein stability of Mcl-1 which is likely to be subsequent to decreased expression of Usp9X. Overall, the promising biological anti-cancer activity of this multi-targeting strategy warrants further in vivo testing.