A potent synergy between FOXG1 overexpression and Wnt signaling drives cell cycle re-entry in quiescent glioblastoma stem cells


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Faye Robertson1,Eoghan O'Duibhir1,Harry Bulstrode2,Steve Pollard1
1University of Edinburgh, Edinburgh, UK,2University of Cambridge, Cambridge, UK

Abstract

Background

Glioblastoma is a malignant brain tumour which is universally fatal. Stem cells within the tumour exist in a quiescent state, evade destruction and reactivate, causing relapse. These stem cells are known to overexpress the transcription factor FOXG1.

 

 

 

Method

We use an in vitro model of quiescence in mouse neural stem cells, incorporating a doxycycline inducible human FOXG1 overexpression cassette, to identify, through high content pharmacological screening, a synergistic relationship between high FOXG1 expression and inhibition of glycogen synthase kinase 3 (GSK3) in driving cells into an active, proliferative state.

We quantify this effect using EdU incorporation assays and demonstrate the high efficiency of cell cycle re-entry with colony forming assays. We use pharmacological and genetic approaches to demonstrate that the synergy is effected by canonical Wnt signaling and that it is relevant in human glioblastoma stem cells (GSCs).

Results

The effect of GSK3 inhibition can be phenocopied both by Wnt3a and by inducible constitutively active beta-catenin, suggesting that the synergy is effected through beta-catenin, the key downstream effector of canonical Wnt signaling. Furthermore, the combined effect of FOXG1 overexpression and GSK3 inhibition on exit from quiescence can be abrogated by Wnt inhibitors.

This effect is present in patient-derived human GSCs and it is abolished by excision of FOXG1. Additionally, we show that the groucho-binding domain of the FOXG1 protein is necessary for the response of human GSCs to GSK3 inhibition, consistent with a putative mechanism whereby FOXG1 may sequester TLE1/groucho, a co-repressor at Wnt target genes.

 

Finally, we have developed a mouse model of glioblastoma by excision of NF-1 and Pten and overexpression of EGFRvIII in cells with inducible active beta-catenin and FOXG1.

Conclusion

Targeting the synergistic relationship between FOXG1 and beta-catenin may provide an exciting therapeutic opportunity in preventing relapse and improving the prognosis of glioblastoma.