Submitted to: Biological Trace Element Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 1998
Publication Date: N/A
Interpretive Summary: Manganese (Mg) is an essential trace element, and the amount of Mn that remains in the body is regulated, to a large extent, by the liver because the liver removes excess Mn from the blood and excretes it through the bile into the gut. Therefore it is important to understand how the liver handles a load of Mn. Rats have traditionally been used to study this aspect of Mn metabolism, but a recent study has shown that rats may be quite different than humans in the way they metabolize Mn. Also, recent concerns about the overuse of laboratory animals have led to the search for models that could replace laboratory animals; cultured cells are such a potential model. In this study, we have shown that cultured human liver cancer cells can be used to study many aspects of Mn metabolism in the liver. We have shown that these cells actively accumulate Mn and then release it from the cell. We have studied the mechanism of Mn uptake and release and have suggested that calcium and intra-cellular structures are involved. Further research with this cell line should help elucidate many of the mechanisms by which the liver regulates Mn in humans, which should help establish the safe and adequate intake of this element.
Technical Abstract: The liver is the primary organ involved in manganese (Mn) homeostasis. The human hepato-carcinoma cell line, Hep-G2, shows many liver specific functions; consequently, Hep-G2 cells were investigated as a possible model of hepatic metabolism of Mn. Initial experiments showed that the concentration of Mn in the diet, or culture medium, similarly affected the retention of Mn by isolated rat hepatocytes and Hep-G2 cells. Manganese was taken up by Hep-G2 cells in a manner that suggested uptake was followed by release from the cell. Uptake was saturable and half-maximal at 1.99 umol Mn/1, and was inhibited by iodoacetate, vanadate, cold and bepridil. The cations Fe2+, Cu2+, Ni2+, Cd2+, and Zn2+ decreased Mn uptake. Uptake was dependent on calcium (Ca) concentration in a manner that resembled saturation kinetics. Cells that were pulsed with 54Mn and then placed into non-radioactive medium quickly released a large portion of their internalized Mn. Release of internalized Mn could be inhibited by cold, nocodozole, quinacrine and sodium azide. These data show that Hep-G2 cells are a potentially good model of hepatic Mn metabolism. Mn is taken up by a facilitated process that may be related to Ca uptake. Release apparently is an active, controlled process, that may involve microtubules and lysosomes.