AN EGF-PSEUDOMONAS EXOTOXIN-A RECOMBINANT PROTEIN WITH A DELETION IN TOXIN BINDING DOMAIN SPECIFICALLY KILLS EGF RECEPTOR-BEARING CELLS

We constructed two chimeric toxins; one composed of epidermal growth f actor (EGF) and pseudomonas exotoxin A (PE), designated EGF-PE and the other composed of EGF and PE with a deletion of the la domain (cell-b inding domain), designated EGF-PE (DELTAIa). Both chimeric toxins reac ted with anti-EGF and anti-PE antibodies. The cell-killing experiments showed that EGF-PE, but not EGF-PE(DELTAIa), was cytotoxic to the mur ine fibroblast cell line NR6, which carried the PE receptor, but not t he EGF receptor. However, after NR6 was transfected with DNA for the e xpression of human EGF receptor, the transfected cell line, designated NRHER5, over-expressed human EGF receptors and became sensitive to EG F-PE(DELTAIA). The cytotoxicity of EGF-PE(DELTAIa), but not EGF-PE, to NRHER5 can be completely blocked by an excess amount of EGF. To compl etely reverse the cytotoxicity of EGF-PE on NRHER5, both the EGF recep tor pathway and the PE receptor pathway need to be blocked. These resu lts suggest that EGF-PE exhibits both EGF and PE binding activities, w hile EGF-PE(DELTAIA) possesses only EGF binding activity. Thus, EGF-PE (DELTAIa) may be a better chimeric toxin than EGF-PE in terms of targe t specificity to EGF receptor bearing cells. We, therefore, examined t he cytotoxicity of EGF-PE(DELTAIa) to various human cancer cell tines. We find that human cancer cells containing more EGF receptors are mor e sensitive to EGF-PE(DELTAIa).