Abstract
Epidermal growth factor family of receptor tyrosine kinases (ERBB) family cell surface receptors, including epidermal growth factor receptor (EGFR/ERBB1), are phosphorylated upon binding by various EGF family ligands and signal via multiple kinase pathways. EGFR signaling is enhanced because of mutational activation of EGFR in almost half of glioblastomas, the most common malignant primary brain tumor. Therapeutic targeting of EGFR in glioblastoma has remained largely unsuccessful. Here, we profiled nine long-term (LTC) and five glioma-initiating (GIC) cell lines for expression and activation of ERBB family receptors and expression of their ligands. Receptors and ligands were abundantly expressed, with patterns overall similar to glioblastoma expression profiles in vivo as deposited in The Cancer Genome Atlas database. No differences between LTC and GIC emerged. Irrespective of ligand or receptor expression, neither an EGFR antibody, erbitux, nor an EGFR tyrosine kinase inhibitor, gefitinib, were particularly active against LTC or GIC at clinically relevant concentrations. Self-renewal capacity of GIC was severely compromised by epidermal growth factor (EGF) withdrawal, but rescued by transforming growth factor alpha (TGF-α), although not by neuregulin-1 (NRG-1). Subcellular fractionation indicated high levels of nuclear phosphorylated EGFR in all LTC and GIC. In LN-229 cells, pERBB2 and pERBB3 were also detected in the nucleus. Nuclear pERBB2 was less sensitive, whereas pERBB3 was induced, in response to gefitinib. This study provides an extensive characterization of human glioma cell models, including stem-like models, with regard to ERBB receptor/ligand expression and signaling. Redundant signaling involving multiple ERBB family ligands and receptors may contribute to the challenges of developing more effective EGFR-targeted therapies for glioblastoma.