Apotransferrin can elevate intracellular free calcium ion and stimulate mitogenesis in human leukemic HL60 cells

In order to resolve the question whether or not transferrin could have a growth-promoting effect on cells independent of its action in iron transport, we investigated the effect of the iron-free form of transferrin, apotransferrin on cell activation and proliferation in the human leukemic HL60 cell line. Within a minute of its addition to HL60 cells, apotransferrin caused a rise in intracellular free calcium concentration ([Ca2+]i) in a dose-dependent manner and the higher the apotransferrin, the quicker it was to attain the calcium peak, showing the physiological characteristics of an agonist-induced [Ca2+]i elevation. The source of calcium appears to be extracellular since this signal could be abolished by nickel or when the reaction was carried out in calcium-free medium. Addition of apotransferrin in the serum-free medium could markedly promote DNA synthesis whereas addition of iron citrate could not. However, apotransferrin could not sustain cell proliferation and hypertrophism without other growth or nutritional factors. Antitransferrin receptor antibody inhibited the growth of HL60 cells cultured in serum-free medium supplemented with transferrin and insulin in a dose-dependent manner, whereas addition of ferric citrate could not reverse cell growth. Generation of the calcium signal probably reflects the initiation of the cell activation processes which could culminate into mitogenesis. Hence, our results suggest that apotransferrin, not iron, is bioactive in HL60 cells.