Plasma membrane and intracellular pools of transferrin receptors decline during in vitro cultivation of U937 cells

Transferrin receptor expression in the monocyte-like cell line U937 was investigated during in vitro cultivation. U937 cells expressed a single class of high affinity surface transferrin receptors (KD approximately 4 nM), with apparent subunit Mr of 90-95,000 Da as determined by SDS-reducing PAGE. 125I]-transferrin binding studies on detergent-solubilized cells revealed that half to two-thirds of the total functional binding sites were located intracellularly. Radioligand binding, immunofluorescence and flow cytometry studies were performed on intact, detergent-solubilized, or saponin-permeabilized cells, using either transferrin or the anti-transferrin receptor monoclonal antibody OKT9 IgG. These studies demonstrated that functional and antigenic transferrin receptor levels were maximal on cells 24 h after subculture at low density and declined during the culture period. Scatchard analysis of radioligand binding data suggested that the decline in functional transferrin binding sites resulted from a decline in the number of available receptors. These results demonstrate that in U937 cells there is a density-dependent regulation of transferrin receptor expression, resulting in a loss of functional and antigenic receptors from both plasma membrane and intracellular locations.