Mutant KB cells with decreased EGF receptor expression: Biochemical characterization

Mutants of the human KB carcinoma cell line resistant to a cytotoxic conjugate of epidermal growth factor and Pseudomonas exotoxin (EGF‐PE) express a pleiotropic phenotype, which includes reduced levels of ¹²⁵I‐EGF binding, without altered affinity for EGF (Lyall et al., 1987). Here, the EGF‐toxin (ET) resistant mutants were further characterized with respect to the amount and size of the EGF receptor and the level of EGF receptor RNA. These data indicate that decreased binding of ¹²⁵I‐EGF in the mutants is due to reduced amounts of EGF receptor, which is associated with decreased mRNA levels. Changes in other proteins in the ET mutants were also examined. Five of the six ET mutants had a decrease in a 78,000 M_r⁻ membrane glycoprotein. In addition, an increase in a protein with a M_r⁻ of 40,000 and a pl = 8.0 was found in all the mutants, and an increase in a series of proteins with a M_r⁻ of 36,000 and a pl of 6.3–6.5 was found in some of the mutants. These results confirm the pleiotropic nature of the EGF‐PE resistant mutants and show that reduced EGF binding is due to altered expression of the EGF receptor gene in the mutants.