Modulation of iron metabolism in monocytic THP-1 cells and cultured human monocytes by the acute-phase protein...

The reticuloendothelial (RE) system plays an important role in the changes in iron metabolism associated with the anemia of chronic disease (ACD). We previously reported that the acute-phase protein alpha1-antitrypsin (alpha1-AT) reduced growth and proliferation in cells of the erythroid cell system by interfering with transferrin (Tf)-mediated iron uptake. The regulation of iron metabolism in cells of the RE system is distinctly different from that in other cell systems; moreover, monocytes and macrophages play an essential part in the regulation of the production and clearance of alpha1-AT. In the present study we examined the effect of alpha1-AT on cells of the monocyte-macrophage lineage. Alpha1-AT completely inhibited the binding of Tf to its receptor (TfR) on THP-1 human myelomonocytic cells and cultured human monocytes. Results of equilibrium saturation and kinetic studies indicated that this inhibition was competitive. No other acute-phase protein demonstrated the same inhibitory potency. Furthermore, alpha1-AT almost completely prevented internalization of the Tf-TfR complex in a dose-dependent manner. Interestingly, and in sharp contrast to the results of our studies with erythroid cells, this inhibition did not reduce the growth and proliferation of THP-1 cells. Furthermore, alpha1-AT significantly increased the concentration of intracellular ferritin in THP-1 cells and monocytes, whereas the number of TfR remained unchanged. Because alpha1-AT showed no enhancing effect on ferritin transcription and translation, we believe that an as-yet unidentified posttranslational mechanism may be responsible for this phenomenon. In addition, our results indicate that the increase in ferritin concentration caused by alpha1-AT is mediated independently of iron supply, as has previously been shown for several proinflammatory cytokines. These data provide further evidence that alpha1-AT is a mediator of the alterations in iron metabolism characteristic of ACD.