HYPOXIA DOWREGULATES HEPCIDIN AND UPREGULATES FGL1 EXPRESSION IN MICE

Repeated administration of erythropoietin (EPO) decreases liver hepcidin expression; the exact mechanism of this effect is unknown. Possible mediators include secreted proteins such as erythroferrone. Recently, it has been proposed that hepcidin downregulation by bleeding is partially mediated by the hypoxia-responsible hepatokine FGL1. The purpose of this study was to examine the response of Fgl1 mRNA to hypoxia in vivo, and to determine whether FGL1 protein could contribute to hepcidin downregulation by EPO.

Male C57BL/6J mice were subjected to 10% oxygen hypoxia for one week. The treatment increased hematocrit and spleen size; hepcidin expression decreased by about two PCR cycles. In addition, hypoxia increased Fgl1 mRNA content (Δ CT 2.32 vs 3.08; n=6, p˂0.05). These data support the possible role of FGL1 in hepcidin downregulation by hypoxia.

To examine the contribution of FGL1 to hepcidin suppression by EPO, groups of intact and splenectomized male C57BL/6J mice were administered 50 U of EPO for four days. Hepcidin dramatically decreased in intact mice (Δ CT 6.49 vs 0.25); in splenectomized mice, the decrease was much less pronounced (2.06 vs 0.13; n=5). Fgl1 mRNA content was unaffected by EPO treatment. These results suggest that the main signal responsible for EPO-induced hepcidin downregulation in mice originates in the spleen.    

Both erythroferrone and FGL1 act by binding the BMP6 protein. However, EPO administration to female Bmp6-/- mice still markedly decreased hepcidin expression (9.15 vs 0.25). These data suggest that additional factors other than erythroferrone and FGL1 contribute to hepcidin downregulation by EPO in mice.