REGULATION OF FERRITIN AND TRANSFERRIN RECEPTOR EXPRESSION BY IRON IN HUMAN HEPATOCYTE CULTURES

HepG2 cell cultures and human hepatocyte primary cultures were used to develop appropriate hepatocytic in vitro models of iron load in order to further understand the pathophysiological mechanisms occurring in the liver of patients with hemochromatosis. The first step of this study was to obtain an efficient iron supply in conditions of minimal toxicity. It was demonstrated that iron complexed to citrate entered efficiently into HepG2 cells and human hepatocytes. This iron load was obtained with minimal toxicity in both culture models as evaluated by the intracellular LDH activity and the total protein content. The second step was to study the effect of iron on ferritin and transferrin receptor expression. In HepG2 cell cultures, intracellular and extracellular ferritin concentrations were strikingly increased by iron in dose- and time-dependent manners. However, the relative amounts of H and L ferritin mRNAs were not significantly affected by iron, suggesting that ferritin regulation occurred at a translational level. On the other hand, in human hepatocyte cultures, the increase of intracellular and extracellular ferritin concentrations was accompanied by an increase in the amounts of H and L ferrritin mRNAs. In this model, iron-induced ferritin biosynthesis seemed to be more complex than in HepG2 cells and to be governed by transcriptional and/or post-transcriptional regulatory mechanisms. However, an additional translational level of regulation could not be excluded. In contrast, transferrin receptor expression was decreased by iron in HepG2 cells as well as in human hepatocyte cultures. This decrease was associated with a decrease in the mRNA steady-state level. In both culture models, transferrin receptor regulation seemed to occur at a transcriptional or post-transcriptional level. These results demonstrate that normal human hepatocytes in primary culture respond to iron in a manner close to that observed in vivo and thereby provide a promising experimental model for further understanding pathophysiological mechanisms involved in human hemochromatotic liver.