Targeted molecular therapies against the
epidermal growth factor receptor (EGFR) are novel, promising and potentially radiosensitising therapeutic approaches in the treatment of
glioblastoma, a highly malignant and treatment-refractory brain tumour. Despite a solid rational basis, specific EGFR inhibition has rendered only disappointing clinical results to date. We therefore evaluated the efficacy of additional inhibition of
human epidermal growth factor receptor 2 (HER2), the 'non-autonomous amplifier' of EGFR signalling.
Glioblastoma cells (LN-18, LN-229) with different co-expression levels of EGFR and HER2 were treated with specific EGFR and bispecific EGFR/HER2
tyrosine kinase inhibitors (TKIs) (
AG1478,
AEE788) and experimental
radiotherapy, followed by assessment of growth inhibition. Activity of the major downstream signalling pathways Akt and MAPK was determined by immunoblotting. EGFR-overexpressing LN-18 cells (EGFR++++/HER2+) showed resistance and HER2-overexpressing LN-229 cells (EGFR+/HER2++) showed sensitivity to EGFR-specific inhibition. Interestingly, resistance of LN-18 to EGFR inhibition was overcome by
AEE788 treatment, supposedly due to its additional HER2 inhibition. Application of
AEE788 resulted in blockage of
EGF-dependent EGFR/HER2-heterodimer activation in LN-18 cells, disclosing a possible mediating mechanism for overcoming EGFR-resistance. TKI treatment resulted in significant blockage of both Akt and MAPK signalling pathways, but an incomplete inhibition of PI3K/Akt paralleled the resistance of cells to TKI-induced growth inhibition. Furthermore, the bispecific EGFR/HER2 inhibitor
AEE788 showed a radio-sensitising effect in EGFR-overexpressing cells. Taken together, we conclude that inhibition of HER2 in EGFR-overexpressing tumours may harbour the potential to overcome resistance to EGFR-targeted
therapy and exert radio-sensitising properties. We suggest that responsiveness to EGFR targeted
therapy is mediated through impairment of EGFR/ HER2 heterodimer signalling, and thus depends on the ratio of EGFR to HER2 rather than on the amount of individual receptors.