We have shown that erythropoietin (epo), the primary regulator of erythrocyte formation, diminished the binding to peritoneal exudate macrophages (PEM) of the principal macrophage growth regulator, colony-stimulating factor (CSF-1). The effect of epo on 125I-CSF-1 binding was dose-dependent; at a concentration of 1-2 U of epo/ml (10(-10) M), CSF-1 binding was almost completely suppressed. Erythropoietin did not compete with CSF-1 for occupancy of the latter's receptors. The effect of epo on CSF-1 binding occurred at 37 degrees C but not at 2 degrees C, and during the continuous exposure of PEM to epo at 37 degrees C we found that CSF-1 binding reached a nadir at 1 h and recovered to pre-exposure levels in 7 h. Our novel results are consistent with the notion that specific receptors for epo exist on the cell surface of PEM and that binding of epo sets in motion a series of cellular events resulting in the internalization of CSF-1 receptors. Thus epo causes down regulation of CSF-1 receptors on PEM. We have previously shown that epo causes suppression of CSF-induced granulocyte-macrophage colony formation by mouse bone marrow cells. The results we present here provide a possible mechanism for these results.

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