Cytotoxic effects of selenium conjugated transferrins on leukemia cell lines

Human leukemia is a malignancy of hematopoietic stem cells with impaired differentiation and uncontrolled proliferation. Many leukemia cell lines over express the membrane transferrin receptor (TfR), a trans membrane protein for the endocytosis of iron by holo transferrin (Holo-Tf). Over expression of the TfR has permitted transferrin- mediated drug delivery targeting leukemia cells. Selenium (Se) selenides (RSe-) generate free radicals, i.e.; superoxide (O2.-) and other ROS by redox cycling oxidizing glutathione (GSH) following endocytosis within cells. The initial O2.- generation by Se was reported to cause cell apoptosis due to increased endogenous oxidative stress. In the present study, selenium as ROCH2CH2SeCN was covalently attached to Apo-, Holo-Tf, and Human Serum Albumin (HuSA) to target the TfR of K562 and THP-1 cell lines. The rate of cell proliferation and the pathway of cell death by selenite and the Se-conjugated proteins were investigated. Selenium conjugation to the proteins was performed using a Se modified Bolton-Hunter Reagent. Selenium conjugated to the proteins was quantified, by ICP-MS (µg Se/mg protein). K562 and THP-1 leukemia cell lines were treated in 24-well plates with SeApo-, SeHolo-Tf, SeHuSA, and selenite. The cytotoxic effects of SeApo-Tf (IC50 3.28 and 2.03 µg Se/mg protein for K562 and THP-1 cells), SeHolo-Tf (IC50 3.98 and 2.12 µg Se/mg protein for K562 and THP-1) and SeHuSA (IC50 4.76 and 2.29 µg Se/mg protein for K562 and THP-1) against K562 and THP-1 were measured at concentrations of 0, and 1.2 to 19.2 µg Se/mg of protein. Cells were assayed at 24, 48, 72, 96, and 120 hours post Se treatments. Se-labeled Apo-Tf, Holo-Tf, HuSA, and selenite all demonstrated a Se concentration and time dependent cytotoxicity to K562 and THP-1 cells in culture as determined by photographic morphology and a positive MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. An increase in Caspase 3 release was observed for both K562 and THP-1 cells after all Se treatments, indicating the arrest of cell proliferation was at least partially apoptotic.