CONSTITUTIVE ACTIVATION OF PHOSPHATIDYLINOSITOL 3-KINASE BY A NATURALLY-OCCURRING MUTANT EPIDERMAL GROWTH-FACTOR RECEPTOR

The most frequently found alteration of the epidermal growth factor re ceptor (EGFR) in human tumors is a deletion of exons 2-7. This recepto r, termed EGFRvIII, can transform NIH 3T3 cells, and the frequent expr ession of this variant implies that it confers a selective advantage u pon tumor cells in vivo. Although EGFRvIII is a constitutively activat ed tyrosine kinase, there is no increase in Ras GTP levels and low lev els of mitogen-activated protein kinase activity in NIH 3T3 cells expr essing this variant. We investigated whether phosphatidylinositol (PI) 3-kinase was an effector in transformation by the EGFRvIII. High leve ls of PI 3-kinase activity were constitutively present in EGFRVIII-tra nsformed cells and were dependent upon the kinase activity of the rece ptor. While mitogen-activated protein kinase activity was quickly down -regulated to basal levels after 12 h of continuous EGFR activation, t here was a 3-fold increase in PI 3-kinase activity in cells expressing normal EGFR and an 8-fold increase in cells expressing EGFRvIII after 48 h. This increased activity may reflect enhanced binding to EGFRvII I and the presence of novel PI 3-kinase isoforms. Treatment with the P I 3-kinase inhibitors wortmannin and LY294002 blocked both anchorage-i ndependent growth and growth in low serum media and also resulted in m orphological reversion of EGFRvIII-transformed cells. These results su pport an essential role for PI 3-kinase in transformation by this EGFR variant.