HGF induces novel EGFR functions involved in resistance formation to tyrosine kinase inhibitors

The epidermal growth factor receptor (EGFR) is overexpressed and activated in many human cancers and predicts poor patient prognosis. Targeting the kinase domain with specific EGFR tyrosine kinase inhibitors (TKIs) like gefitinib and erlotinib has been used in anticancer treatments. However, patient response rates in different human cancers were initially low. Only a subgroup of non-small-cell lung cancer (NSCLC) patients harboring EGFR-activating mutations responds to EGFR TKI treatment, but most of these responders relapse and acquire resistance. Recent clinical studies have demonstrated that MET proto-oncogene overexpression correlates with resistance to EGFR TKI treatment. Similarly to MET overexpression, the tumor microenvironment-derived ligand hepatocyte growth factor (HGF) was shown to activate Met and thereby induce short-term resistance to EGFR TKI treatment in gefitinib-sensitive NSCLC cell lines in vitro. However, only little is known about the HGF/Met-induced EGFR TKI resistance mechanism in other human cancer types. Therefore, in order to develop possible new anticancer strategies for diverse human cancers, we screened 12 carcinoma cell lines originating from the breast, kidney, liver and tongue for HGF-induced EGFR tyrosine kinase (TK)-inhibition. In addition, in order to advance our understanding of a TK-inactive EGFR, we used EGFR co-immunoprecipitation, followed by mass spectrometry to identify novel HGF-induced EGFR binding partners, which are potentially involved in tyrosine kinase-independent EGFR signaling mechanisms. Here we show for the first time that HGF-induced EGFR TK-inhibition is a very common mechanism in human cancers, and that the kinase-inactive EGFR directly interacts with and stabilizes several cancer-relevant proteins, including the receptor tyrosine kinases Axl and EphA2, and the CUB domain-containing protein-1. This study has strong implications for the development of new anticancer strategies.