Liver sinusoidal endothelial cells constitute a major route for hemoglobin clearance

Abstract

Mild rupture of aged erythrocytes occurs physiologically in the spleen, leading to the release of hemoglobin (Hb), while pathological hemolysis characterizes several diseases. The detoxification of Hb has traditionally been attributed to the sequestration of Hb-haptoglobin complexes by macrophages. However, this process remains incompletely studied in animal models or primary cells, leaving the precise mechanisms of Hb clearance elusive. Using mice and primary liver cell cultures (murine and human), we uncovered that Hb uptake is chiefly performed by liver sinusoidal endothelial cells (LSECs) and involves macropinocytosis. Consistently, mouse LSECs displayed proteomic signatures indicative of active heme catabolism, ferritin iron storage, antioxidant defense, and macropinocytic capacity. LSECs also exhibited high iron content and the expression of hepcidin-regulated iron exporter ferroportin. Using erythrocyte/Hb transfusion assays in mice, we demonstrated that while splenic macrophages excel in phagocytosis of erythrocytes, LSECs primarily scavenge Hb and Kupffer cells clear erythrocyte membranes, the spleen-borne hemolysis products delivered to the liver via the portal circulation. High-dose Hb injections resulted in transient hepatic iron retention, early LSEC-specific induction of heme-catabolizing Hmox1 and iron-sensing Bmp6, culminating in hepcidin-mediated temporary hypoferremia. Transcriptional induction of Bmp6 in mice was phenocopied by erythrocyte lysis upon phenylhydrazine or iron citrate injection, although the latter elicited a distinct LSEC transcriptional signature compared to Hb. In conclusion, we identify LSECs as key Hb scavengers, a function that establishes the spleen-to-liver axis for iron recycling and contributes to heme detoxification during hemolysis, coupled with the induction of the BMP6-hepcidin axis to restore iron homeostasis.