HeLa cell mutants resistant to epidermal growth factor ricin A‐chain conjugate

Epidermal growth factor (EGF) was linked to the toxic A chain of ricin toxin (RTA) to produce an EGF-receptor-specific cytotoxic agent, EGF-RTA. Three EGF-RTA-resistant mutants of the human HeLa cell line were selected. These mutant cell lines are 10-fold to more than 100-fold more resistant to EGF-RTA when compared to HeLa cells. The EGF-RTA-resistant mutants have at least as many EGF receptors as parent cells; the basis for the EGF-RTA-resistant phenotype must be distal to EGF binding. The EGF-RTA-resistant cells are not cross-ressitant to ricin or to diphtheria toxin; their mutant phenotype appears to be EGF specific. The EGF-RTA-resistant mutants are able to internalize and degrade EGF. However, the mutants have altered EGF receptor down-regulation and phorbol 12-tetradecanoate 13-acetate modulation properties. EGF-RTA/ammonium chloride and EGF-RTA/adenovirus co-treatment data suggest that the mutant defect(s) which confers EGF-RTA resistance is either in the endosome or at a step(s) in the intracellular EGF processing pathway between the endosome and the lysosome.