Transferrin reduces the production of soluble transferrin receptor

The effect of homologous diferric transferrin from which contaminating transferrin receptor has been removed by monoclonal antibody affinity chromatography on soluble transferrin receptor concentrations was studied in K562 cells and HL60 cells in culture. Diferric transferrin in K562 cells caused a dose-dependent decrease in cellular receptor expression, a dose-dependent increase in cellular ferritin content, and a reduction in soluble receptor concentration which was of greater proportional magnitude than the reduction in cell receptor content. In HL60 cells, while there was a dose-dependent increase in cellular ferritin, cellular receptor content was relatively unaffected, while there was a consistent reduction in soluble receptor concentration. In both cells, the inhibitory effect of diferric transferrin on soluble receptor concentration was evident as early as 3 hr into the incubation. Apotransferrin, by contrast, did not reduce soluble receptor concentration. While elemental iron was capable of producing similar changes in cellular receptor and ferritin content, it had no inhibitory effect on proportional soluble receptor content. Studies employing other proteins, including human and bovine serum albumin, human lactoferrin, and rat ferritin, had no inhibitory effect on soluble receptors concentration, thus confirming the specificity of the findings. Control studies excluded an assay artifact as the explanation for the current findings. Prior contrary reports appear completely explained by the combination of soluble transferrin receptor contaminating the transferrin employed for study and a systematic difference in the assays employed between free and transferrin-bound receptor.