Hunting for Hemopexin Receptors: One Saga

ABSTRACT: Maintaining hemopexin (HPX) plasma levels protects against heme-activated inflammation and the toxicity of heme and its iron during hemolysis. Plasma heme regulates HPX turnover in hepatocytes, thus changing plasma HPX. However, the cell biology of hepatic HPX-mediated heme delivery is not fully elucidated. The low density-lipoprotein related protein 1 (LRP1) binds heme-HPX and targets it for lysosomal degradation. Nevertheless, heme from heme-HPX is delivered to the liver, and apo-HPX recycles without degradation. Heme-HPX endocytosis also occurs in mouse embryonic fibroblast LRP1-/-PEA 13 cells. Here, we characterize HPX receptors and HPX “interactomes” in a human neutrophil model, promyelocytic HL-60 cells, hepatoma HepG2 cells and primary human hepatocytes. Methods and Results: Immunoblots revealed that HL-60 cells lack LRP1; and immunocytochemistry data established that in HL60 cells, HPX trafficked with transferrin and transferrin receptor 1(TfR1) in Rab5-positive early endosomes supporting following a clathrin-mediated endocytotic pathway. Heme-HPX affinity chromatography of whole cell extracts isolated TfR1 and LRP1 as heme-HPX binding proteins and other proteins of novel HPX interactomes identified by their peptide sequences. TfR1 down-regulation in HL-60 cells decreased surface binding and intracellular HPX, implicating TfRs in heme-HPX endocytosis. In LRP+/+ HepG2 cells, HPX trafficked in endosomes with LRP1 and TfR1, or with TfR1 alone, and with TfR2, supporting that TfR2 potentially provides liver targeting of heme-HPX; Tfr1 and 2 could both account for apo-HPX recycling. LRP1 and TfR1 were identified in heme-HPX Affi-isolates from primary human hepatocytes, and this HPX interactome also included proteins associated with hemostasis, inflammation control, coagulation regulation, iron transport and body fluid regulation. Discussion: After heme-HPX uptake by human hepatocytes, HPX is recycled or degraded depending upon the relative expression of surface TfR1, TfR2 and LRP1. The overlapping and distinct roles of these receptors are discussed. TfR1 is a scavenger receptor like LRP1; nevertheless, specific hepatic HPX receptors may exist. Increasing knowledge of HPX biology will elucidate the causes that change plasma HPX, thus improving care in the clinic and in veterinary medicine, with increased understanding of the hematological adaptations to weightlessness that lead to clotting or anemia from hemolysis in astronauts and, potentially, space tourists.

INSTRUMENT(S):

ORGANISM(S): Homo Sapiens (human)

TISSUE(S): Hepatocyte, Liver

SUBMITTER: Kalyani Dhusia  

LAB HEAD: Ann Smith, Ph.D.

PROVIDER: PXD060789 | Pride | 2025-12-22

REPOSITORIES: Pride