ApoER2‐Mediated Endocytosis of Long‐Isoform Selenoprotein P (Sepp1) Supplies Skeletal Muscle Cells with Selenium

Sepp1 transports selenium from the liver to tissues such as testis, brain and kidney. Apolipoprotein E receptor‐2 (apoER2) and megalin, both members of the low‐density lipoprotein receptor (LDLR) family, have been identified as Sepp1 receptors in those tissues. The mechanism of selenium acquisition by skeletal muscle cells is not known. We studied selenium uptake by a rat myoblast cell line. L8 cells took up Sepp1 from the medium and utilized its selenium for the synthesis of glutathione peroxidase (Gpx). However, L8 cells did not take up Gpx3 or the Sepp1 isoform that lacked the selenium‐rich C‐terminal domain. To identify Sepp1 receptors on L8 cells, a solubilized membrane fraction from them was passed over a Sepp1 column. ApoER2 and Lrp1, another LDLR family member, were identified in the column eluate by mass spectrometry. RNAi experiments showed that knockdown of apoER2 inhibited 75Se uptake from 75Se‐labeled Sepp1 while knockdown of Lrp1 had no effect on it. Inhibition of clathrin‐mediated endocytosis suppressed 75Se uptake. Blocking of lysosome acidification did not inhibit uptake of 75Se from 75Selabeled Sepp1 but it suppressed development of Gpx activity by blocking digestion of the endocytosed Sepp1. We conclude that long‐isoform Sepp1 binds apoER2 on L8 cells and is endocytosed and digested in lysosomes to provide selenium to the cells. Supported by NIH grants ES02497 and DK82813. Grant Funding Source : NIH