EGF-ricin A conjugates: kinetic profiles of cytotoxic effects and resistant cell variants.

We have conjugated epidermal growth factor (EGF) with the A subunit of ricin (RICa) via a dithiopropionyl linkage. The EGF-RICa conjugate was competitive with [125I]EGF for binding to cell surface EGF receptors. Entry of the conjugate into cells was seen within 10 min at 37 degrees C and inhibition of protein synthesis was seen within 90 min. The blockage of protein synthesis continued for more than 20 h in sensitive cells. Protein synthesis in EGF receptor-deficient cells was not affected. The conjugate killed human epidermoid carcinoma (A431) cells, mouse 3T3 fibroblasts and Chinese hamster lung (CHL) cells with identical efficiencies (ED50 = 2 X 10(-9) M). The EGF-RICa conjugate was used as a selection agent and several resistant variants were isolated from CHL cells. These variants showed various degrees of [125I]EGF binding capacity. Some other characteristics of the variants are also described.