Long Isoform Mouse Selenoprotein P (Sepp1) Supplies Rat Myoblast L8 Cells with Selenium via Endocytosis Mediated by Heparin Binding Properties and Apolipoprotein E Receptor-2 (ApoER2)

• Published in: The Journal of biological chemistry 2012-08, Vol.287 (34), p.28717-28726
• Main Authors: Kurokawa, Suguru, Hill, Kristina E., McDonald, W.Hayes, Burk, Raymond F.
• Format: Article
• Language: English
• Subjects: Animals; ApoER2; Cell Biology; Cell Line; Chlorates - pharmacology; Endocytosis; Endocytosis - physiology; Glutathione Peroxidase - genetics; Glutathione Peroxidase - metabolism; Heparin-binding Protein; LDL-Receptor Related Proteins - genetics; LDL-Receptor Related Proteins - metabolism; Low Density Lipoprotein Receptor-Related Protein-1; Lysosomes - genetics; Lysosomes - metabolism; Metabolism; Mice; Mice, Knockout; Myoblasts - cytology; Myoblasts - metabolism; Physiology; Protein Isoforms - genetics; Protein Isoforms - metabolism; Rats; Receptors, LDL - genetics; Receptors, LDL - metabolism; Selenium; Selenium Transport; Selenoprotein; Selenoprotein P; Selenoproteins - genetics; Selenoproteins - metabolism; Skeletal Muscle Metabolism; Tumor Suppressor Proteins - genetics; Tumor Suppressor Proteins - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M112.383521

In vivo studies have shown that selenium is supplied to testis and brain by apoER2-mediated endocytosis of Sepp1. Although cultured cell lines have been shown to utilize selenium from Sepp1 added to the medium, the mechanism of uptake and utilization has not been characterized. Rat L8 myoblast cells were studied. They took up mouse Sepp1 from the medium and used its selenium to increase their glutathione peroxidase (Gpx) activity. L8 cells did not utilize selenium from Gpx3, the other plasma selenoprotein. Neither did they utilize it from Sepp1Δ240–361, the isoform of Sepp1 that lacks the selenium-rich C-terminal domain. To identify Sepp1 receptors, a solubilized membrane fraction was passed over a Sepp1 column. The receptors apoER2 and Lrp1 were identified in the eluate by mass spectrometry. siRNA experiments showed that knockdown of apoER2, but not of Lrp1, inhibited 75Se uptake from 75Se-labeled Sepp1. The addition of protamine to the medium or treatment of the cells with chlorate also inhibited 75Se uptake. Blockage of lysosome acidification did not inhibit uptake of Sepp1 but did prevent its digestion and thereby utilization of its selenium. These results indicate that L8 cells take up Sepp1 by an apoER2-mediated mechanism requiring binding to heparin sulfate proteoglycans. The presence of at least part of the selenium-rich C-terminal domain of Sepp1 is required for uptake. RT-PCR showed that mouse tissues express apoER2 in varying amounts. It is postulated that apoER2-mediated uptake of long isoform Sepp1 is responsible for selenium distribution to tissues throughout the body. Background: ApoER2 endocytosis of Sepp1 supplies testis and brain with selenium, but the mechanism of supply to other tissues is not known. Results: Sepp1 supplies selenium to heart and skeletal muscle cell lines via apoER2 and many tissues express apoER2. Conclusion: ApoER2 endocytosis of Sepp1 supplies selenium to many tissues. Significance: ApoER2 uptake of Sepp1 likely regulates selenium distribution in the whole body.