Acquired resistance to tyrosine kinase inhibitors (TKI) represents a major challenge for personalized cancer therapy. Multiple genetic mechanisms of acquired TKI resistance have been identified in several types of human cancer. However, the possibility that cancer cells may also evade treatment by co-opting physiologically regulated receptors has not been addressed. Here, we show the first example of this alternate mechanism in brain tumors by showing that EGF receptor (EGFR)-mutant glioblastomas (GBMs) evade EGFR TKIs by transcriptionally de-repressing platelet-derived growth factor receptor β (PDGFRβ). Mechanistic studies show that EGFRvIII signaling actively suppresses PDGFRb transcription in an mTORC1- and extracellular signal- regulated kinase-dependent manner. Genetic or pharmacologic inhibition of oncogenic EGFR renders GBMs dependent on the consequently de-repressed PDGFRβ signaling for growth and survival. Importantly, combined inhibition of EGFR and PDGFRβ signaling potently suppresses tumor growth in vivo. These data identify a novel, nongenetic TKI resistance mechanism in brain tumors and provide compelling rationale for combination therapy. Significance: These results provide the first clinical and biologic evidence for receptor tyrosine kinase (RTK) "switching" as a mechanism of resistance to EGFR inhibitors in GBM and provide a molecular explanation of how tumors can become "addicted" to a nonamplified, nonmutated, physiologically regulated RTK to evade targeted treatment.
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