Erythropoietin Mediated Increase in Monocyte-Derived Liver Macrophages; A Role for Kupffer Cells?

Erythropoietin (EPO) is the major hormone that drives mammalian erythropoiesis, via its surface receptor, EPO-R. It is mainly used for treating anemia associated with chronic renal failure and certain malignancies, although this latter indication is currently disputed. EPO-Rs were also found in non-erythroid cells, including dendritic cells and bone marrow macrophages (Lifshitz, 2008; 2010). Here we addressed the effect of EPO on hepatic-macrophages, namely resident liver macrophages (Kupffer cells) and liver monocyte-derived macrophages (MFs). Utilizing the rat Kupffer cell line (RKC-2) we demonstrate that these cells express EPO-R transcripts and cell surface EPO-R, as detected by our novel EPO-R antibody (GM1012; Maxwell 2015). EPO treatment of the RKC-2 cells led to a 1.5±0.06 fold increase (p<0.05) in EPO-R mRNA levels and a 2±0.01 fold decrease (p<0.01) in surface EPO-R levels. Stimulation of the cells with EPO induced a 15%±0.06 (p=0.01) increase in transcript levels of CCL-2 (a chemo attractant for monocytes) and a 15%±0.6 (p<0.05) increase in the levels of the secreted chemokine. EPO treatment also enhanced cellular activity of the RKC-2 cells as manifested in a 50%±0.13 (p<0.01) increase in cell migration, an increase in phagocytosis of microbeads (40%±0.08, p<0.01) and of E.coli (13%±0.05, p<0.01).

Finally, in vivo experiments in which C57BL/6 mice were injected 3 times a week with 180U recombinant Human EPO (rHuEPO), demonstrated an EPO-induced selective increase in MFs (19.5%±0.01, p=0.05), but not in Kupffer cells. Elevated CCL-2 in sera of EPO-injected mice (2.1 fold increase, p<0.01) supports a mechanism by which EPO stimulates Kupffer cells to increase secretion of CCL-2, which in turn enhances recruitment of monocytes to the liver and their subsequent differentiation into MFs.

The present study points to a new as hitherto unexplored action of EPO on two separate liver macrophage populations (Kupffer cells and MFs) which play crucial and distinct roles in liver homeostasis and immunity as well as in liver pathologies. EPO selective actions on liver macrophages thus call for future studies on the effects of this hormone on inflammation in the liver with a therapeutic goal to enhance liver immunity.