The number of the Trf receptors on human hematopoietic cell lines is influenced by membrane phospholipids

The effect of phospholipases and of inhibitors of phospholipases on the binding of 125I-Trf to human hematopoietic cell lines was investigated. The results showed that incubation of the cells with phospholipases or inhibitors of phospholipases elicited a significant reduction of the Trf binding. This phenomenon was related to a reduction of Trf binding sites without affecting the affinity of the receptors for Trf. These results suggest that the phospholipids of the cell membrane may modulate the binding of transferrin to its receptor

Inhibition of transferring binding and iron uptake of hematopoietic cell lines by phorbol esters

Phorbol esters inhibit cell growth and the binding of transferrin to receptors on K 562, HL 60 and U 937 human leukemic cell lines. Exposure of these cells to 12-0-tetradecanoyl phorbol-13-acetate (TPA) at 37 degrees C results in a 40% reduction of the specific binding of 125I-transferrin, which is apparent within 15 min. Half-maximal inhibition occurs at about 1 nM. Other tumor promoting phorbol esters also inhibit 125I-transferrin binding in a dose-dependent manner which parallels their known promoting activity in vivo. TPA reduces the number of transferrin receptors, and does not alter the degradation or the internalization of transferrin. In addition, TPA inhibits iron uptake by these cell lines. These effects are specific, since phorbol esters do not affect either cell growth or the binding of transferrin to Friend erythroleukemia cells and Raji cell line. On the basis of these findings it is suggested that the inhibition of transferrin binding may represent one of the mechanisms by which phorbol esters affect the growth and the differentiation of hematopoietic cell lines