Hypophosphorylation of residue Y1045 leads to defective downregulation of EGFRvIII.

Downregulation of the EGF receptor is the net result of receptor degradation and recycling. Cbl functions by specifically targeting activated ErbB receptors for ubiquitination, facilitating ligand-induced desensitization of EGFR. The interaction between EGFR and c-Cbl has been shown to depend upon receptor phosphorylation at tyrosine residue 1045, the major docking site for c-Cbl. To better understand the biological consequences of EGFR mutants in human cancers, we compared wild-type EGFR and EGFRvIII internalization, as well as gefitinib sensitive and resistant EGFR kinase mutations found in non-small cell lung carcinoma. We observed that Cbl failed to associate with EGFRvIII as well as an inability of the receptor to undergo ubiquitination and degradation. The most intriguing observation is that EGFRvIII tyrosine 1045 residue is either un-phosphorylated or hypophosphorylated. This is in contrast to other tyrosine residues in EGFRvIII, such as Y1173, which exhibit levels of phosphorylation comparable to those of wild-type EGFR. These results suggest that hypophosphorylation of tyrosine residue 1045 is likely to be the cause for EGFRvIII escape from c-Cbl-induced ubiquitination and degradation, enhancing EGFRvIII's ability to increase proliferation in breast cancer cells. Interestingly, inefficient degradation was only observed in the gefitinib resistant EGFR kinase mutant, despite the fact that this mutant receptor is capable of recruiting c-Cbl and undergoes ubiquitination. The gefitinib sensitive EGFR kinase mutant exhibits similar ubiquitination and degradation patterns as the wild-type EGFR. Collectively, different EGFR mutations exert various negative mechanisms that have the potential to modify receptor internalization and degradation, and may play a critical role in resistance to tyrosine kinase inhibitory treatments.