RAPID INDUCTION OF HEPARIN-BINDING EPIDERMAL GROWTH-FACTOR DIPHTHERIA-TOXIN RECEPTOR EXPRESSION BY RAF AND RAS ONCOGENES

We have used differential display PCR to search for mRNAs induced by Delta Raf-1:ER, an estradiol-dependent form of the Raf-1 kinase. Through this approach the gene encoding heparin-binding epidermal growth factor (HB-EGF) was identified as an immediate-early transcriptional target of oncogenic Raf kinases. Activation of Delta Raf-1:ER and a conditional oncogenic form of B-Raf, Delta B-Raf:ER, resulted in rapid and sustained induction of HB-EGF mRNA expression and secretion of mature HB-EGF from cells. Neutralizing anti-HB-EGF antisera prevented the delayed activation of the c-Jun amino-terminal kinases that is observed in cells transformed by Delta Raf-1:ER. These results demonstrate that distinct signaling pathways can cross talk via the secretion of polypeptide growth factors. Furthermore, cells transformed by oncogenic Ras, which also induced HB-EGF expression, demonstrated a marked increase in sensitivity to the cytotoxic action of diphtheria toxin, for which the membrane anchored HB-EGF precursor acts as a cell-surface receptor.