Background: Ferumoxides-protamine sulfate (Fe-Pro) complexes are used for intracellular magnetic labeling of mesenchymal stem cells (MSCs) to non invasively monitor cell trafficking and tissue distribution by in vivo magnetic resonance imaging (MRI). It has been previously shown that there was no short or long- term toxic effects on cell’s viability, proliferation, reactive oxygen species and biological functions including differentiation capacity of MSCs, however the effects of dextran coated superparamagnetic iron oxide (SPIO) nanoparticles labeling on cellular iron metabolism and storage is unknown.

Objective: Evaluate the effect of using Fe-Pro complexes for magnetic cell labeling in MSCs on the expression of transferrin receptor (TfR-1) and ferritin, proteins involved in intracellular iron metabolism and storage.

Methods: Confluent MSCs were labeled with Fe-Pro complexes at the ratio of 100 ug/mL to 6 ug/mL. After labeling, cells were washed and cultured for 2 months. TfR-1 and ferritin expression were evaluated at the gene and protein levels in total cell lysates, on day 1, 3, 7, 14, 28 and 35 days post-labeling, by real-time polymerase chain reaction (RT-PCR) and Western blotting. MSCs were evaluated for the presence of iron by Prussian blue stain and light microscopy. In addition, TfR-1 and ferritin levels were determined for Fe-Pro labeled and unlabeled MSCs, allowed to slowly multiply, as a result of dividing the cells in half when they reached confluence.

Results: All cells were Prussian blue positive on light microscopy following labeling with Fe-Pro. Labeling of MSCs grown to confluence resulted in short-term increase of TfR-1 mRNA (day 1 and day 3, p= 0.0056) without changes in TfR-1 protein levels. Fe- Pro labeled MSC demonstrated an increase in ferritin gene expression on day 7 and 14 (p=0.0003) compared to unlabeled cells, while protein levels were higher compared to unlabeled MSC at each time point (p=0.005). However, ferritin protein levels from Fe-Pro labeled MSCs were not significantly increased after 7 passages.

Conclusions: Magnetic labeling of cells with FDA approved ferumoxides complexed to protamine sulfate elicited short-term changes in cellular iron metabolism and storage that was dependent on ability to divide. Fe-Pro labeling did not have any long-term implications validating the safety of Fe-Pro labeling technique for MSCs trafficking studies by MRI.

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