DLL4 and JAG1 in the Glioblastoma Response to Temozolomide Chemotherapy
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Introduction: Glioblastoma multiforme is the most common aggressive primary malignant brain tumour in adults. The current gold standard treatment comprises of surgery followed by radiotherapy and adjuvant temozolomide (TMZ) chemotherapy. Prognosis is poor, with a median survival of 14.6 months following diagnosis. Treatment often fails due to intrinsic or acquired TMZ resistance of a small population of cells termed glioma cancer stem cells (GCSCs). A promising target for glioblastoma therapy is the Notch signalling pathway, which can be suppressed using gamma-secretase inhibitors such as dibenzazepine (DBZ). We investigated the effects of the Notch ligands, Delta-like 4 (DLL4) and Jagged-1 (JAG1) on tumour resistance to TMZ and GCSCs.
Methods: U87 and U251 glioblastoma cells were transduced with empty vector-, DLL4, and JAG1-encoded retroviruses. Cells were cultured under 2D, 3D, and CSC culture conditions and response to single (TMZ/DBZ) and combination (TMZ and DBZ) treatment was assessed. A patient derived GCSC line, CSC-5, was also used to evaluate the effect of single and combination treatment by neurosphere recovery assay.
Results: DLL4 and JAG1 overexpression promotes resistance by increasing the TMZ IC50. Neurosphere formation, recovery, and secondary neurosphere formation is increased following DLL4 and JAG1 overexpression and is reversed upon combination TMZ and DBZ treatment. TMZ has little effect on the self-renewal of GCSCs, however single DBZ and combination DBZ and TMZ treatment significantly reduces GCSC self-renewal. Consequently, Notch inhibition reduces GCSC marker expression and may promote GCSC differentiation.
Conclusions: These data show the importance of the Notch pathway, in particular the ligands DLL4 and JAG1 in resistance to TMZ chemotherapy and GCSC self-renewal. The addition of Notch inhibitors to current treatment is a promising approach to overcome TMZ resistance and decrease brain tumour recurrence and encourages further translational and clinical studies.