Release of soluble transferrin receptor from the surface of human leukemic
HL60 cells
CR Chitambar and Z Zivkovic
Department of Medicine, Medical College of Wisconsin, Milwaukee 53226.
Information regarding transferrin (Tf) receptor degradation is largely
incomplete. HL60 cells were shown to release to their growth medium a
Tf-binding protein which could be immunoprecipitated by anti-Tf receptor
monoclonal antibodies (MoAbs) B3/25 and OKT9. Soluble Tf receptor was
detected in the medium within one hour of replating of cells, and its
release was inhibited at 4 degrees C. The affinity of Tf for the soluble
receptor released by cells (kd = 2.3 x 10(-10) mol/L) was slightly lower
than its affinity for the detergent-solubilized cellular receptor (kd = 1.2
x 10(-10) mol/L). 125I-Tf internalized and released by cells subsequently
bound to Tf receptor released by the same cells, and soluble Tf receptor in
the conditioned medium (CM) inhibited 125I-Tf binding to intact cells. The
soluble Tf receptor isolated from the CM was smaller (78,000 daltons) than
the cell surface receptor (94,000 daltons) when analyzed by gel
electrophoresis under reducing conditions. Isolated cell membranes readily
released soluble receptor; however, this release could be blocked by
protease inhibitors. The soluble Tf receptor may represent the
extracytoplasmic domain of the cellular Tf receptor released from the
surface of HL60 cells through proteolytic cleavage by a membrane-based
protease.
Volume 74,
Issue 2,
pp. 602-608,
08/01/1989
Copyright © 1989 by The American Society of Hematology
