Differential Effects of Tyrosine Kinase Inhibitors on Normal and Oncogenic EGFR Signaling and Downstream Effectors.

UNLABELLED: Constitutive activation of EGFR due to overexpression or mutation in tumor cells leads to dysregulated downstream cellular signaling pathways. Therefore, EGFR as well as its downstream effectors have been identified as important therapeutic targets. The FDA-approved small-molecule inhibitors of EGFR, gefitinib (Iressa) and erlotinib (Tarceva), are clinically effective in a subset of patients with non-small cell lung cancer (NSCLC) whose tumors harbor activating mutations within the kinase domain of EGFR. The current study examined effects of these drugs in 32D cells expressing native (WT) or oncogenic (L858R) EGFR as well as in cancer cell lines A431 and H3255. Distinct patterns for gefitinib and erlotinib inhibition of EGFR autophosphorylation at individual tyrosines were revealed for wild-type (WT) and L858R EGFR. Phosphorylation of Y845 has been shown to be important in cancer cells and Y1045 phosphorylation is linked to Cbl-mediated ubiquitination and degradation. Dramatic differences were observed by greater potency of these drugs for inhibiting downstream effectors for L858R EGFR including Cbl and STAT5. Selective targeting of Cbl may play a role in oncogene addiction and effects on STAT5 identify features of signaling circuitry for L858R EGFR that contribute to drug sensitivity and clinical efficacy. These data provide new understanding of the EGFR signaling environment and suggest useful paradigms for predicting patient response to EGFR-targeted therapy as well as combination treatments.

IMPLICATIONS: This study offers fundamental insights for understanding molecular mechanisms of drug sensitivity on oncogenic forms of EGFR and downstream signaling components as well as considerations for further drug optimization and design of combination therapy.