Submitted July 7, 2005
Accepted June 2, 2006
Molecular events contributing to cell death in malignant
human hematopoietic cells elicited by an IgG3-avidin
fusion protein targeting the transferrin receptor
Patrick P Ng, Gustavo Helguera, Tracy R Daniels, Simon Z Lomas, Jose A Rodriguez, Gary Schiller, Benjamin Bonavida, Sherie L Morrison, and Manuel L Penichet*
Department of Microbiology, Immunology & Molecular Genetics, University of California Los Angeles,CA
Division of Surgical Oncology, Department of Surgery, UCLA, CA, USA
Division of Hematology & Oncology, Department of Medicine, UCLA, CA, USA
* Corresponding author; email: penichet{at}microbio.ucla.edu.
We have previously reported that an anti-human
transferrin receptor IgG3-avidin fusion protein [anti-
hTfR-(IgG3-Av)] inhibits the proliferation of an
erythroleukemia cell line. We have now found that anti-
hTfR-(IgG3-Av) also inhibits the proliferation of
additional human malignant B and plasma cells. Anti-hTfR-
(IgG3-Av) induces internalization and rapid degradation
of the TfR. These events can be reproduced in cells
treated with anti-hTfR-IgG3 cross-linked with a
secondary Ab, suggesting that they result from increased
TfR cross-linking. Confocal microscopy of cells treated
with anti-hTfR-(IgG3-Av) shows that the TfR is directed
to an intracellular compartment expressing the lysosomal
marker LAMP-1. The degradation of TfR is partially
blocked by cysteine protease inhibitors. Furthermore,
cells treated with anti-hTfR-(IgG3-Av) exhibit
mitochondrial depolarization and activation of caspases
9, 8, and 3. The mitochondrial damage and cell death can
be prevented by iron supplementation, but cannot be
fully blocked by a pan-caspase inhibitor. These results
suggest that anti-hTfR-(IgG3-Av) induces lethal iron
deprivation, but the resulting cell death does not
solely depend on caspase activation. This report
provides insights into the mechanism of cell death
induced by anti-TfR Abs such as anti-hTfR-(IgG3-Av), a
molecule that may be useful in the treatment of B-cell
malignancies such as multiple myeloma.
