Epidermal Growth Factor-Induced Tumor Cell Invasion and Metastasis Initiated by Dephosphorylation and Downregulation of Focal Adhesion Kinase

Abstract

Upregulated epidermal growth factor (EGF) receptor (EGFR) expression and EGFR-induced signaling havebeen correlated with progression to invasion and metastasis in a wide variety of carcinomas, but the mechanism behind this is not well understood. We show here that, in various human carcinoma cells that overexpress EGFR, EGF treatment induced rapid tyrosine dephosphorylation of focal adhesion kinase (FAK) associated with downregulation of its kinase activity. The downregulation of FAK activity was both required and sufficient for EGF-induced refractile morphological changes, detachment of cells from the extracellular matrix, and increased tumor cell motility, invasion, and metastasis. Tumor cells with downregulated FAK activity became less adherent to the extracellular matrix. However, once cells started reattaching, FAK activity was restored by activated integrin signaling. Moreover, this process of readhesion and spreading could not be abrogated by further EGF stimulation. Interruption of transforming growth factor alpha-EGFR autocrine regulation with an EGFR tyrosine kinase inhibitor led to a substantial increase in FAK tyrosine phosphorylation and inhibition of tumor cell invasion in vitro. Consistent with this, FAK tyrosine phosphorylation was reduced in cells from tumors growing in transplanted, athymic, nude mice, which have an intact autocrine regulation of the EGFR. We suggest that the dynamic regulation of FAK activity, initiated by EGF-induced downregulation of FAK leading to cell detachment and increased motility and invasion, followed by integrin-dependent reactivation during readhesion, plays a role in EGF-associated tumor invasion and metastasis.The invasive and metastatic stage of cancer progression correlates with poor clinical prognosis and represents the most formidable barrier to successful treatment. Cell motility and invasiveness are defining characteristics of tumors, which enable tumor cells to migrate into adjacent tissues or through limiting basement membranes and extracellular matrices. Invasive tumor cells are characterized by dysregulated cell motility in response to extracellular signals from growth factors and cytokines. In addition to roles in organ morphogenesis, maintenance, and repair, epidermal growth factor (EGF)-induced signaling has often been associated with tumor invasion and metastasis (75). EGF receptor (EGFR) overexpression has been found in many human tumors, including lung, colon, breast, prostate, brain, head and neck, thyroid, ovarian, and bladder, gliomas, and renal carcinoma (4,20,39,40,63,71), and has been correlated with an advanced tumor stage and a poor clinical prognosis. In addition, EGFR overexpression in tumor cells is often accompanied by production of transforming growth factor alpha (TGF-␣) or other EGF family ligands (73), and autocrine regulation through EGFR by such ligands has also been implicated in tumor progression. It has been reported that EGF promotes tumor cell motility and invasion (58,62,66). However, the basis for initiation and maintenanc...

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