Liver sinusoidal endothelial cells constitute a major route for hemoglobin clearance

Area of Science:

• Hematology
• Cell Biology
• Iron Metabolism

Background:

• Red blood cell aging leads to mild hemoglobin release in the spleen.
• Pathological hemolysis occurs in various diseases, necessitating efficient hemoglobin clearance.
• Current understanding attributes hemoglobin detoxification primarily to macrophages, leaving other clearance routes unexplored.

Purpose of the Study:

• To identify the primary cells responsible for hemoglobin uptake and clearance.
• To elucidate the mechanisms and pathways involved in hemoglobin catabolism and iron recycling.
• To understand the role of liver sinusoidal endothelial cells (LSECs) in hemoglobin scavenging.

Main Methods:

• Proteomic analysis of LSECs to identify functional signatures.
• Erythrocyte and hemoglobin transfusion assays in vivo.
• Assessment of iron metabolism markers, including ferroportin and hepcidin.
• Transcriptional profiling of LSECs in response to erythrocyte lysis.

Main Results:

• Liver sinusoidal endothelial cells (LSECs) primarily uptake hemoglobin via macropinocytosis.
• LSECs exhibit proteomic evidence of heme catabolism, iron storage, and antioxidant defense.
• Splenic macrophages perform erythrophagocytosis, while LSECs and Kupffer cells clear hemoglobin and erythrocyte membranes, respectively.
• High hemoglobin loads induce hepatic iron retention, LSEC-specific Hmox1 expression, and activate the Bmp6-hepcidin axis, leading to hypoferremia.

Conclusions:

• LSECs are identified as crucial hemoglobin scavengers, complementing the role of splenic macrophages.
• A spleen-to-liver axis for iron recycling and heme detoxification is established.
• This pathway is critical for managing hemoglobin overload during hemolysis.