A COMMON MUTANT EPIDERMAL GROWTH-FACTOR RECEPTOR CONFERS ENHANCED TUMORIGENICITY ON HUMAN GLIOBLASTOMA CELLS BY INCREASING PROLIFERATION AND REDUCING APOPTOSIS

Alterations of the EGFR gene occur frequently in human gliomas where t he most common is an in-frame deletion of exons 2-7 from the extracell ular domain, resulting in a truncated mutant receptor (Delta EGFR or d e 2-7 EGFR). We previously demonstrated that introduction of Delta EGF R into human U87MG glioblastoma cells(U87MG.Delta EGFR) conferred rema rkably enhanced tumorigenicity in vivo. Here, we show by cell-mixing e xperiments that the enhanced tumorigenicity conferred by Delta EGFR is attributable to a growth advantage intrinsic to cells expressing the mutant receptor. We analyzed the labeling index of the proliferation m arkers Ki-67 and bromodeoxyuridine and found that tumors derived from U87MG.Delta EGFR cells had significantly higher labeling indexes than those of tumors derived from U87MG cells that were either naive, expre ssed kinase-deficient mutants of Delta EGFR, or overexpressed exogenou s wild-type EGFR. We also utilized terminal deoxynucleotidyl transfera se-mediated nick end-labeling assays and showed that the apoptotic ind ex of U87MG.Delta EGPR tumors was more than 4-fold lower than that of parental U87MG tumors. This decrease in cell death was inversely corre lated with the expression level of Bcl-X(L), a negative regulator of a poptosis, which was more than 3-fold higher in U87MG.Delta EGFR-derive d tumors than in those derived from parental cells. Similar observatio ns were obtained in vitro in serum-free conditions. These results sugg est that Delta EGFR exerts its pronounced enhancement of glioblastoma tumorigenicity by stimulating proliferation and inhibiting apoptosis a nd that the effects are directly attributable to its constitutively ac tive signal.