Studies of the Transferrin Receptor on both Human Reticulocytes and Nucleated Human Cells in Culture

Abstract

A B S T R A C T The transferrin receptor, present on reticulocytes and nucleated cells in tissue culture, has been measured with both immunoassay techniques and transferrin binding studies. The total cellular immunoreactive receptor is rapidly lost from erythrocytes during the process of reticulocyte maturation (from as many as 400,000 molecules to <20,000 molecules/ reticulocyte). This event parallels the loss of cell surface transferrin binding sites and RNA content, and correlates with previous studies that have measured the decline in hemoglobin synthesis.Nonhemoglobin-producing normal human fibroblasts, which appear to have a much lower iron requirement than reticulocytes, contain similar numbers of immunoreactive receptors per cell (400,000 receptor molecules), when in an active state of proliferation. Although receptor density on fibroblasts is directly related to cell proliferation, our studies demonstrate that nonproliferating fibroblasts still retain significant numbers of immunoreactive receptors (150,000 molecules/ cell) and transferrin binding sites. Since additional studies indicate that proliferating cells have increased iron uptake, a simple hypothesis would predict that the parallel increase in transferrin binding sites and total cellular immunoreactive receptor associated with proliferation is related to an increased cellular iron requirement. However, the number of immunoreactive receptor molecules and transferrin binding sites is not changed when cells are grown in iron-deficient media, or in media with added transferrin-iron. This result and the lack of marked differences in receptor number on both hemoglobin-producing and nonhemoglobin-producing cells indicate that other factors besides receptor density play major roles in the regulation of cellular iron uptake, retention, and loss.

Human Erythrocyte Membranes Contain a Cytochrome b561 That May Be Involved in Extracellular Ascorbate Recycling

“…Because DCytb shows Asc-dependent FeCN reductase activity when expressed in yeast (27), it is possible that it includes part of the erythroblast iron reduction/uptake system. However, all components of the hemoglobin-synthesizing system, as well as the transferrin receptor and the ability to take up iron, are lost during the maturation of reticulocytes (52)(53)(54)(55)(56). Furthermore, DMT-1 (divalent metal transporter 1), which works in concert with DCytb for iron uptake (57), was not detected in the membrane fractions of mature human erythrocytes by ion trap mass spectrometry (45).…”

Effects of different transferrin forms on transferrin receptor expression, iron uptake, and cellular proliferation of human leukemic HL60 cells. Mechanisms responsible for the specific cytotoxicity of transferrin-gallium.

“…'25l-sodium iodide and 59FeC13 were obtained from New England Nuclear (Boston, MA) and Amersham Corp. (Arlington Heights, IL), respectively. Saturation ofapoTfwith 59Fe and iodination of'9Fe-Tfwere performed as previously described (8).…”

“…'25I-Tf binding studies performed on these cells utilized a previously described assay (8,18). Incubations were carried out at 4°C for 4 h, a time when equilibrium of binding was reached.…”

“…The receptor for transferrin (Tf),' the plasma Fe transport protein (1), was initially characterized on hemoglobin-producing cells and placental cells (2)(3)(4), but the more recent finding ofincreased Tf receptors on rapidly proliferating cells (5)(6)(7)(8)(9) has generated considerable interest. This latter finding underscores the importance ofTf for cellular proliferation (10).…”

Failure of red blood cell maturation in mice with defects in the high-density lipoprotein receptor SR-BI

“…23 Both can be readily measured by means of fluorescent stains and flow cytometry. 12 As expected from the morphologic studies, we could detect neither RNA nor Trfr in most of the fully differentiated erythrocytes circulating in wild-type and apoE Ϫ/Ϫ mice (lower left quadrant of cytograms in Figure 4Q,R).…”