Modulation of Transferrin Receptors in Bone Marrow Cells by Changes in Lipid Fluidity

Summary. The hypothesis that modulation of transferrin receptors during haematopoiesis is mediated by changes in the lipid fluidity of the cell membranes was tested in this study with bone marrow cells. Binding conditions were first established, under which internalization of fluorescently‐labelled transferrin was reduced to a minimum during the binding assays. Cholesterol depletion, which was achieved by a mild physiological treatment, and which increased the membrane‐lipid fluidity, resulted in a substantial increase in the average number of the available transferrin receptors per bone marrow cell. Cholesterol enrichment mediated a converse effect. These findings correlate well with the changes observed in the availability of transferrin receptors and membrane microviscosity during differentiation of erythroid cells. In line with the notion of passive modulation, it is proposed that the transferrin receptor is a cross‐membrane protein with a substantial portion facing the cytoplasm.