Induction of heparin‐binding EGF‐like growth factor and activation of EGF receptor in imatinib mesylate‐treated squamous carcinoma cells

Imatinib mesylate is a tyrosine kinase inhibitor of the ABL, platelet‐derived growth factor receptor (PDGFR), and c‐kit kinases. Inhibition of BCR‐ABL and c‐kit accounts for its clinical activity in leukemia and sarcoma, respectively. In this report, we describe other cellular targets for imatinib. Treatment of head and neck squamous carcinoma cells with clinically relevant concentrations of imatinib‐induced changes in cell morphology and growth similar to changes associated with epidermal growth factor receptor (EGFR) activation. Imatinib‐induced changes were blocked with the EGFR antagonist cetuximab, which suggested direct involvement of EGFR in this process. Western blot analysis of cells incubated with imatinib demonstrated activation of EGFR and downstream signaling that was reduced by inhibition of mitogen‐activated protein/extracellular signal‐regulated kinase kinase 1 (MEK1) and EGFR, but not Her2/ErbB2. An in vitro kinase assay showed that imatinib did not directly affect EGFR kinase activity, suggesting involvement of EGFR‐activating molecules. Inhibitors and neutralizing antibodies against heparin‐binding epidermal growth factor‐like growth factor (HB‐EGF), and to a lesser extent transforming growth factor‐α, reduced imatinib‐mediated mitogen activated protein kinase (MAPK) activation. Imatinib stimulated the rapid release of soluble HB‐EGF and the subsequent induction of membrane‐bound HB‐EGF, which correlated with biphasic MAPK activation. Together, these results suggested that imatinib affects EGFR activation and signaling pathways through rapid release and increased expression of endogenous EGFR‐activating ligands. Although, imatinib primarily inhibits tyrosine kinases, it also stimulates the activity of EGFR tyrosine kinase in head and neck squamous tumors. This finding demonstrates the need for careful use of this drug in cancer patients. © 2005 Wiley‐Liss, Inc.