Two pathways of transferrin recycling evident in a variant of mouse LMTK- cells

We describe here the properties of a variant cell line, termed AF192, selected by exposing mouse LMTK- cells to a cytotoxic form of transferrin prepared by conjugating transferrin to diphtheria toxin. AF192 cells were mildly resistant to the transferrin-diphtheria toxin conjugate and were cross-resistant to the protein toxins modeccin, abrin, ricin, and Pseudomonas aeruginosa exotoxin A. AF192 cells had an aberrant transferrin cycle characterized by an approximately 50% reduction in the rate of iron uptake from diferric transferrin, an approximately 25% reduction in the number of surface transferrin receptors, and a time course for transferrin recycling that resolved into two apparent first-order rate processes. The aberrant transferrin cycle was not the result of a failure of endocytosed transferrin to discharge iron; rather, part, but not all, of the transferrin taken up by AF192 cells was diverted to an intracellular site from which it was recycled very slowly.