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 D elta Raf-1:ER, an estradiol-dependent form of the Raf-1 kinase. Throug h this approach the gene encoding heparin-binding epidermal growth fac tor (HB-EGF) was identified as an immediate-early transcriptional targ et of oncogenic Raf kinases. Activation of Delta Raf-1:ER and a condit ional oncogenic form of B-Raf, Delta B-Raf:ER, resulted in rapid and s ustained induction of HB-EGF mRNA expression and secretion of mature H B-EGF from cells. Neutralizing anti-HB-EGF antisera prevented the dela yed activation of the c-Jun amino-terminal kinases that is observed in cells transformed by Delta Raf-1:ER. These results demonstrate that d istinct signaling pathways can cross talk via the secretion of polypep tide growth factors. Furthermore, cells transformed by oncogenic Ras, which also induced HB-EGF expression, demonstrated a marked increase i n sensitivity to the cytotoxic action of diphtheria toxin, for which t he membrane anchored HB-EGF precursor acts as a cell-surface receptor.