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EGFR signaling defines Mcl-1 survival dependency in neuroblastoma

The pediatric solid tumor neuroblastoma (NB) often depends on the anti-apoptotic protein, Mcl-1, for survival through Mcl-1 sequestration of pro-apoptotic Bim. High affinity Mcl-1 inhibitors currently do not exist such that novel methods to inhibit Mcl-1 clinically are in high demand. Receptor tyrosine kinases (RTK) regulate Mcl-1 in many cancers and play a role in NB survival, yet how they regulate Bcl-2 family interactions in NB is unknown. We found that NB cell lines derived to resist the Bcl-2/-xl/-w antagonist, ABT-737, acquire a dependence on Mcl-1 and show increased expression and activation of the RTK, EGFR. Mcl-1 dependent NB cell lines derived at diagnosis and from the same tumor following relapse also have increased EGFR expression compared to those dependent on Bcl-2. Inhibition of EGFR by shRNA or erlotinib in Mcl-1 dependent NBs disrupts Bim binding to Mcl-1 and enhances its affinity for Bcl-2, restoring sensitivity to ABT-737 as well as cytotoxics in vitro. Mechanistically treatment of NBs with small molecule inhibitors of EGFR (erlotinib, cetuximab) and ERK (U0126) increases Noxa expression and dephosphorylates Bim to promote Bim binding to Bcl-2. Thus, EGFR regulates Mcl-1 dependence in high-risk NB via ERK-mediated phosphorylation of Bim such that EGFR/ERK inhibition renders Mcl-1 dependent tumors now reliant on Bcl-2. Clinically, EGFR inhibitors are ineffective as single agent compounds in patients with recurrent NB, likely due to this transferred survival dependence to Bcl-2. Likewise, EGFR or ERK inhibitors warrant further testing in combination with Bcl-2 antagonists in vivo as a novel future combination to overcome therapy resistance in the clinic.