ENHANCED TUMORIGENIC BEHAVIOR OF GLIOBLASTOMA CELLS EXPRESSING A TRUNCATED EPIDERMAL GROWTH-FACTOR RECEPTOR IS MEDIATED THROUGH THE RAS-SHC-GRB2 PATHWAY

A mutant epidermal growth factor receptor (Delta EGFR) containing a de letion of 267 amino acids from the extracellular domain is common in h uman glioblastomas. We have previously shown that the mutant receptor fails to bind EGF, is constitutively phosphorylated, and confers upon U87MG glioblastoma cells expressing it (U87MG.Delta EGFR), an increase d ability to form tumors in mice. Here we demonstrate that the constit utively phosphorylated Delta EGFR enhances growth of glioblastoma cell s through increased activity of Ras: 1) there was an increase in the p roportion of Ras present in the GTP-bound form, and 2) introduction of neutralizing anti-has 259 antibodies into U87MG and U87MG.Delta EGFR cells by microinjection inhibited DNA synthesis to the same low level in both cell populations, We also show that the truncated EGF receptor constitutively associates with the adapter proteins Shc and Grb2 whic h are involved in the recruitment of Ras to activated receptors, Sever al derivatives of Delta EGFR containing single, or multiple mutations at critical autophosphorylation sites were constructed and used to dem onstrate that the major She binding site is Tyr-1148, and that Grb2 as sociation occurs primarily through Tyr-1068, We conclude that the incr eased tumorigenic potential of glioblastoma cells expressing the trunc ated EGF receptor is due at least in part to has activation presumably involving the Shc and Grb2 adapter proteins.