Relationship of EGFR Signal-Transduction Modulation by Tyrosine Kinase Inhibitors to Chemosensitivit

Epidermal growth factor receptor (EGFR) is a transmembrane glycoprotein of 170 kDa with an extracellular EGF-binding domain and intracellular domain possessing intrinsic tyrosine kinase activity (1 -3) . Overexpression of the EGFR has been reported in a wide range of human malignancies, including non-small cell lung cancer (NSCLC) (4 -6) . One early consequence of the binding of EGFR ligands (EGF or transforming growth factor-α [TGF-α]) to EGFR is the induction of receptor dimerization with the subsequent activation of tyrosine kinase activity and trans-phosphorylation at several tyrosine residues in the intracellular domain of the receptor (2 ,7) . The activated receptor transduces signals through tyrosine phosphorylation of itself and other adaptor proteins that mediate the activation of its downstream signal molecules such as mitogen-activated protein (MAP) kinase and phosphatidylinositol 3-kinase (PI3K ) (8 ,9) . These cascades can then regulate nuclear and cytoplasmic events controlling transformation, mitogenesis, and cell survival (10) . Because overexpression of EGFR is a common occurrence in many human cancers, including lung, and increased receptor levels are associated with metastasis and poor clinical prognosis, EGFR represents an attractive therapeutic target.