BIM mediates EGFR tyrosine kinase inhibitor-induced apoptosis in lung cancers with oncogenic EGFR mutations

Background: Epidermal growth factor receptor (EGFR) mutations are present in the majority of patients with non-small cell lung Cancer (NSCLC) responsive to the EGFR tyrosine kinase inhibitors (TKIs) gefitinib or erlotinib. These EGFR-dependent tumors eventually become TKI resistant, and the common secondary T790M mutation accounts for half the tumors with acquired resistance to gefitinib. However, the key proapoptotic proteins involved in TKI-induced cell death and Other secondary mutations involved in resistance remain unclear. The objective of this study was to identify the mechanism of EGFR TKI-induced Apoptosis and secondary resistant mutations that affect this process.

Methods and findings: To study TKI-induced cell death and mechanisms of resistance, we used lung Cancer cell lines (with or without EGFR mutations), Ba/F3 cells stably transfected with EGFR mutation constructs, and tumor samples from a gefitinib-resistant patient. Here we show that up-regulation of the BH3-only polypeptide Bim (also known as BCL2-like 11) correlated with gefitinib-induced Apoptosis in gefitinib-sensitive EGFR-mutant lung Cancer cells. The T790M mutation blocked gefitinib-induced up-regulation of Bim and Apoptosis. This blockade was overcome by the irreversible TKI CL-387,785. Knockdown of Bim by small interfering RNA was able to attenuate Apoptosis induced by EGFR TKIs. Furthermore, from a gefitinib-resistant patient carrying the activating L858R mutation, we identified a novel secondary resistant mutation, L747S in cis to the activating mutation, which attenuated the up-regulation of Bim and reduced Apoptosis.

Conclusions: Our results provide evidence that Bim is involved in TKI-induced Apoptosis in sensitive EGFR-mutant cells and that both attenuation of the up-regulation of Bim and resistance to gefitinib-induced Apoptosis are seen in models that contain the common EGFR T790M and the novel L747S secondary resistance mutations. These findings also suggest that induction of Bim may have a role in the treatment of TKI-resistant tumors.