REGULATION OF IRON-METABOLISM IN HEPG2 CELLS - A POSSIBLE ROLE FOR CYTOKINES IN THE HEPATIC DEPOSITION OF IRON

In chronic inflammation it is reported that serum iron is depleted and hepatic iron is increased because of reticuloendothelial system iron blockade. However, recent studies indicate that hepatic parenchymal ce lls increase the uptake of transferrin-bound iron after in vivo stimul ation with bacterial lipopolysaccharide, suggesting that endotoxemia i tself or lipopolysaccharide-induced production of inflammation-related cytokines may also be responsible for this phenomenon. In this study the actions of inflammation-related cytokines on the synthesis of iron -binding proteins (transferrin and ferritin) and transferrin receptor and the uptake of transferrin-bound iron were investigated in a human hepatoblastoma cell line, HepG2, which is the most commonly used cell line for examining the regulation of hepatic protein synthesis by cyto kines. The cells were exposed to interleukin-1beta, interleukin-6 or t umor necrosis factor-alpha separately for 24 hr. In each cytokine trea tment group, the level of transferrin, which is secreted into the cond itioned medium, was found to be decreased compared with that of untrea ted cells. On the other hand, the biosynthesis of ferritin was markedl y elevated after the same treatment. This increase in ferritin by cyto kine treatment was diminished when deferoxiamine was used concomitantl y to deplete intracellular chelatable iron. After stimulation with int erleukin-1beta, interleukin-6 or tumor necrosis factor-alpha, Fe-59-la beled transferrin uptake into the cells was increased by 36%, 48%, or 18%, respectively, and this uptake was inhibited by the addition of ex cess unlabeled transferrin. A binding study with I-125-labeled diferri c transferrin revealed that the three cytokines increased the number o f transferrin receptors on the cell surface by 1.15-fold to 1.35-fold. A metabolic labeling study using 35S-methionine showed that the incre ase of transferrin-receptor expression was not caused by the redistrib ution between the cell surface and internal pool but by de novo synthe sis. These findings suggest that not only reticuloendothelial system b lockade of iron release, but also enhanced liver cell uptake of transf errin-bound iron by cytokine stimulation as well as endotoxin is respo nsible for the hypoferremia seen during inflammation.