D3193: Iron uptake by cultured Sg4 cells from Drosophila melanogaster

Iron is an essential micronutrient, but it can initiate harmful chemical reactions in cells; therefore, animal lineages have evolved iron uptake mechanisms that supply an adequate amount of iron to cells while limiting iron toxicity. Surprisingly little is understood about how this process occurs in insects. The goal of this study was to learn how iron uptake occurs in the embryonically-derived Sg4 cell line from Drosophila melanogaster. We found that Sg4 cells express ferritin (an iron export and storage protein), Malvolio (a divalent metal transporter that is thought to transfer iron from endosomes to the cytoplasm), and three candidate ferric reductases. The cells did not express transferrin, which is a critical component of mammalian iron uptake mechanisms. To test the hypothesis that iron uptake by Sg4 cells requires endocytosis, we used RNAi and chemical inhibitors to block endocytosis and then measured cellular iron content. These treatments did not decrease iron content; therefore, endocytosis must not be essential for iron uptake by Sg4 cells. Consistent with this conclusion was our finding that knockdown of Malvolio had no influence on iron content. To test the hypothesis that Sg4 cells take up ferrous ions, we treated cells with a membrane-impermeant ferrous chelator and then measured the effect on iron content. This treatment significantly decreased the iron content of cells. Our results suggest that, rather than relying on endocytic uptake of iron, Sg4 cells may reduce ferric ions in the medium and then take up the resulting ferrous ions via an unknown ferrous transporter.