Signaling Control by Epidermal Growth Factor Receptor and MET: Rationale for Cotargeting Strategies in Lung Cancer

Abstract

In a report accompanying our article, Spigel et al present their findings on combining erlotinib, an epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI), with onartuzumab (MetMAb; Genentech, South San Francisco, CA), a humanized monovalent monoclonal antibody that blocks ligand activation of MET receptor tyrosine kinase (RTK). MET-positive patients (defined by immunohistochemistry) treated with erlotinib and onartuzumab demonstrated improved progression-free and overall survival, whereas MET-negative patients had worse outcomes. This article provides an overview of signaling control by EGFR and MET and the biologic rationale for cotargeting strategies. EGFR and MET are membrane-bound RTKs consisting of extracellular domains that bind to growth factors and intracellular domains that contain a kinase domain (Fig 1). MET is activated by its cognate ligand, hepatocyte growth factor (HGF), which is produced by stromal cells, also in an autocrine fashion. EGFR is activated by several ligands, including EGF, transforming growth factor alpha (TGF ), and heparin-binding EGF. On receptor binding of the ligands, the kinase domain becomes activated, leading to transautophosphorylation of tyrosine residues on the cytoplasmic tail. Phosphorylation of these tyrosine residues creates intramolecular docking sites for proteins containing Src homology 2 domains that serve as readers for tyrosine phosphorylation. This includes adaptor proteins such as growth factor receptor–bound protein 2 (GRB2), GAB1 (GRB2-associated binding protein 1), and Src homology 2–containing transforming protein 1 (SHC1). These proteins serve as scaffolds to support signal transduction to the mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K), phospholipase C (PLC), and signal transducer and activator of transcription (STAT) pathways. EGFR and MET use a highly overlapping repertoire of signaling adaptors and downstream effector pathways, highlighting their combined ability to codrive oncogenic signaling. Three general approaches are being employed in clinic to target EGFR and MET. The first uses small-molecule TKIs, such as erlotinib for EGFR and crizotinib for MET, that bind the ATP-binding pocket in the kinase domain and inhibit the kinase function. The second approach uses antibody therapy directed against the extracellular domain of the MET receptor (onartuzumab) or the HGF ligand (ficlatuzumab). The third approach targets downstream proteins involved in signaling, such as inhibitors of PI3K, or molecules regulating receptor trafficking and degradation, such as heat shock proteins. The EGFR pathway can be activated in lung cancer through genomic events, such as activating EGFR mutations or gene amplification, or through tumor environmental events, such as autocrine production of EGFR-activating ligands. Likewise, MET activation occurs through HGF produced in the tumor microenvironment and through MET gene amplification. MET amplification by fluorescent in situ hybridization occurs in approximately 5% to 15% of lung adenocarcinomas. Increased MET is associated with advanced stage of disease, poor outcome, brain metastasis, and high levels of EGFR. Coactivation of EGFR and MET occurs in subsets of lung cancer cell lines and tumor tissues and is associated with aggressive disease. In cells lacking EGFR mutation or high levels of MET, activation of either receptor can produce extensive signaling crosstalk to the opposing RTK and downstream substrates. EGFR activation can induce accumulation of activated MET protein through a c-SRC– dependent mechanism, and dual MET/EGFR inhibition has shown increased activity in xenograft models. Phosphoproteomic analysis of lung tumors and cell lines can identify tumors with concomitant EGFR and MET activation. EGFR, either through mutation or ligand-mediated activation, can also regulate MET levels through hypoxia-inducible factor-1 . Importantly, MET signaling is one actionable mechanism driving resistance to EGFR TKIs. MET activation can drive intrinsic as well as acquired resistance to EGFR through genomic and tumor microenvironment–mediated mechanisms. Primary resistance to anti-EGFR antibody therapy has been associated with MET activation in primary explant models of lung cancer. Approximately 5% of EGFR mutation–positive tumors with acquired resistance to EGFR TKIs are found to have MET gene amplification. Mechanistically, amplified MET co-opts use of ERBB3 to maintain the PI3K pathway activation. Combining MET and EGFR TKIs or EGFR/HSP90 inhibition has been effective in mouse models of lung cancer with T790M EGFR and MET amplification. High levels of HGF are associated with intrinsic or acquired resistance to EGFR TKIs in patients with lung cancer with activating EGFR mutations and in those with other solid tumors. HGF production by stromal fibroblasts leads to fibroblast-induced JOURNAL OF CLINICAL ONCOLOGY U N D E R S T A N D I N G T H E P A T H W A Y VOLUME 31 NUMBER 32 NOVEMBER 1