Abstract 4122: Role of Grb2-associated binder 1 (Gab1) in epidermal growth factor receptor (EGFR)-induced signaling in head and neck squamous cell carcinoma (HNSCC) in vitro
Authors: Hoeben Ann , Martin Daniel , Clement Paul M. , Schöffski Patrick , Gutkind Silvio J. 
Published: February 2011

Abstract
Introduction: EGFR plays an important role in the pathogenesis of HNSCC. Despite the overexpression of this molecule, EGFR inhibitors have only limited activity in the clinic. The Grb2-associated binder (Gab) family of adaptor proteins act as docking/scaffolding molecules downstream of tyrosine kinase receptors. Gab1 can be phosphorylated by EGFR in addition to the c-Met receptor. We hypothesized that Gab1 amplifies EGFR-induced signaling in HNSCCs and therefore may play a role in the reduced sensitivity to EGFR inhibitors (e.g. Gefitinib).
Methods: Gene expression analysis of a panel of human HNSCC cell lines allowed the identification of cell lines with differential EGFR and GAB1 expression profile. The head and neck cell line HN13 overexpresses wild type EGFR while expressing only one member of the Gab family (Gab1) making it suitable for further molecular analysis. We transiently transfected these cells with Gab1 and control siRNA and confirmed the knock down of Gab1 by Western blotting 72 hours after transfection. Proliferation was quantified using thymidine incorporation assays and changes in EGF-induced signaling were analyzed using Western blots. We focused on the AKT/mTOR and MAPK pathways.
Results: In HN13, Gab1 is activated by EGFR independently of c-Met, as judged by siRNA-mediated knock down of c-Met. In addition, basal Akt/mTOR and MAPK signaling were reduced in HN13 cells in which Gab1 was knocked down, in comparison to HN13 cells transfected with control siRNA. Maximal EGF-induced activation of the Akt/mTOR and MAPK pathway showed delayed kinetics in HN13 cells with knocked down Gab1. Moreover, the duration of the EGF-induced activation of these pathways was reduced after knocking down Gab1, while the sensitivity to the EGFR tyrosine kinase inhibitor Gefitinib was increased. This results support the idea that Gab1 acts as an amplifier of EGFR-induced signaling. Further analysis confirmed this observations in the HN6 HNSCC cell line, which shows a comparable ‘EGFR-GAB1 interdependence’.
Conclusion: Gab1 is an amplifier of EGFR-induced activation of the MAPK and AKT/mTOR pathway in the chosen in vitro models. We propose that this adaptor protein may amplify the EGFR-initiated pro-survival and proliferative pathways. This may explain the limited clinical benefit of therapeutic strategies aimed at inhibiting EGFR in HNSCC, despite the fact that HNSCC largely over-express EGFR.
Consequently, therapies aimed at achieving a simultanous EGFR and Gab1 inhibition could be potentially more effective that the current alternatives.
Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4122.