Liver sinusoidal endothelial cells constitute a major route for hemoglobin clearance
Hemolysis of senescent erythrocytes occurs in the spleen, resulting in hemoglobin (Hb) release. Several diseases are associated with an increased risk of erythrocyte rupture. Iron recycling from Hb and Hb detoxification have been attributed to the sequestration of Hb-haptoglobin complexes by macrophages, however, other routes of Hb clearance might exist. We identified liver sinusoidal endothelial cells (LSECs) as the major cell type that sequesters Hb. Hb uptake involves macropinocytosis and is independent of haptoglobin and the Hb-haptoglobin receptor CD163. LSECs expressed heme oxygenase 1 and hepcidin-controlled ferroportin and were the most efficient cellular scavengers of Hb at doses below the haptoglobin binding capacity. Erythrocyte transfusion assays demonstrated that while splenic red pulp macrophages are efficient in erytrophagocytosis, liver Kupffer cells and LSECs are primarily involved in the clearance of erythrocyte ghosts and Hb, respectively. Using time-course injections of high doses of Hb in mice, we detected transient hepatic iron retention, early induction of Hmox1 specifically in LSECs, and elevation of ferroportin levels. This was associated with transcriptional activation of LSEC Bmp6, followed by hepcidin-mediated transient hypoferremia. Injection of Hb and iron-citrate elicited distinct transcriptional signatures in LSECs and the early Bmp6 induction was phenocopied by erythrocyte lysis upon phenylhydrazine. Collectively, we propose that LSECs provide a key mechanism for Hb clearance, a function that establishes the spleen-liver axis for physiological iron recycling from Hb and contributes to heme detoxification during hemolysis, coupled with the induction of the BMP6-hepcidin axis that restores homeostasis. Overall design: We analyzed in total 4 versus 4 RNA samples derived from FACS-sorted LSECs obtained from young female Balb/c mouse mice, intravenously injected with PBS or free hemoglobin (10 mg), respectively.