Evidence that inhibition of hemojuvelin shedding in response to iron is mediated through neogenin

Hemojuvelin (HJV), encoded by the gene HFE2, is a critical upstream regulator of hepcidin expression. Hepcidin, the central iron regulatory hormone, is secreted from hepatocytes, whereas HFE2 is highly expressed in skeletal muscle and liver. Previous studies demonstrated that HJV is a GPI-anchored protein, binds the proteins neogenin and bone morphogenetic proteins (BMP2 and BMP4), and can be released from the cell membrane (shedding). In this study, we investigated the physiological significance and the underlying mechanism of HJV shedding. In acutely iron-deficient rats with markedly suppressed hepatic hepcidin expression, we detected an early phase increase of serum HJV with no significant change of either HFE2 mRNA or protein levels in gastrocnemius muscle. Studies in both C2C12 (a mouse myoblast cell line) and HepG2 (a human hepatoma cell line) cells showed active HJV shedding, implying that both skeletal muscle and liver could be the source of serum HJV. In agreement with the observations in iron-deficient rats, HJV shedding in these cell lines was down-regulated by holo-transferrin in a concentration-dependent manner. Our present study showing that knock-down of endogenous neogenin, a HJV receptor, in C2C12 cells suppresses HJV shedding and that overexpression of neogenin in HEK293 cells markedly enhances this process, suggests that membrane HJV shedding is mediated by neogenin. The finding that neither BMP4 nor its antagonist, noggin, was able to alter HJV shedding support the lack of involvement of BMP signaling pathway in this process.