Signal transduction by receptor tyrosine kinases

Inhibition of the epidermal growth factor receptor family of tyrosine kinases as an approach to cancer chemotherapy: progression from reversible to irreversible inhibitors

The rationale to inhibit the epidermal growth factor receptor (EGFR) tyrosine kinase family as an approach to cancer chemotherapy has continued to grow stronger over the last 10 years. Both preclinical and clinical data strongly support the involvement of these receptors in the formation and progression of human cancers, as well as establish a high correlation in cancer patients between receptor/ ligand expression and poor prognosis. During the past 4 years, significant progress has been made in the area of EGFR tyrosine kinase inhibitors, and new structural classes have emerged that exhibit enormous improvements with regard to potency, specificity, and in vitro and in vivo activity. Very recently, further advancements in this field have been made whereby very specific, irreversible inhibitors of the EGFR family have been synthesized that provide unique pharmacological properties and exceptional efficacy. The in vivo performance of these modern kinase inhibitors has improved to the point where several compounds are either in clinical trials or very near to that point in their development. This review will briefly address the justification for targeting the EGFR family for cancer therapeutics, and then will highlight some of the more promising kinase inhibitors that are in development.

EGF stimulates tyrosine phosphorylation of focal adhesion kinase (p125FAK) and paxillin in rat pancreatic acini by a phospholipase C-independent process that depends on phosphatidylinositol 3-kinase, the small GTP-binding protein, p21rho, and the integrity of the actin cytoskeleton

Epidermal growth factor (EGF) is a potent mitogen in many cell types including pancreatic cells. Recent studies show that the effects of some growth factors on growth and cell migration are mediated by tyrosine phosphorylation of the cytosolic tyrosine kinase p125 focal adhesion kinase (p125FAK) and the cytoskeletal protein, paxillin. The aim of the present study was to determine whether EGF activates this pathway in rat pancreatic acini and causes tyrosine phosphorylation of each of these proteins, and to examine the intracellular pathways involved. Treatment of pancreatic acini with EGF induced a rapid, concentration-dependent increase in p125FAK and paxillin tyrosine phosphorylation. Depletion of the intracellular calcium pool or inhibition of PKC activation had no effect on the response to EGF. However, inhibition of the phosphatidylinositol 3-kinase (PI3-kinase) or inactivation of p21rho inhibited EGF-stimulated phosphorylation of p125FAK and paxillin by more than 70%. Finally, cytochalasin D, a selective disrupter of the actin filament network, completely inhibited EGF-stimulated tyrosine phosphorylation of both proteins. All these treatments did not modify EGF receptor autophosphorylation in response to EGF. These results identify p125FAK and paxillin as components of the intracellular pathways stimulated after EGF receptor occupation in rat pancreatic acini. Activation of this cascade requires activation of PI3-kinase and participation of p21rho, but not PKC activation and calcium mobilization.