Abstract
Granulocyte macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) control the proliferation of human acute myeloid leukemia (AML) cells in vitro. Previously, we have shown that receptors for GM-CSF and IL-3 are often coexpressed on AML cells. Here we present experiments with purified AML blasts, normal monocytes, and granulocytes that were conducted to analyze the properties of GM-CSF and IL-3 binding proteins in more detail. On AML cells from eight cases we demonstrate two types of GM-CSF receptors: one with low affinity (dissociation constant [kd] 5.1 to 24.8 nmol/L) and one with a high affinity (kd 31 to 104 pmol/L). These AML cells also expressed high affinity receptors for IL-3 (kd 24 to 104 pmol/L). Cross-competition experiments showed that an excess concentration of nonlabeled IL-3 completely prevented the high affinity binding of radiolabled GM-CSF. This competition occurred at 37 degrees C as well as 4 degrees C. Low affinity GM-CSF binding was not affected by IL-3. Binding of radiolabeled IL-3 could be prevented by nonlabeled GM-CSF. In certain cases, this competition was complete, whereas in others only partial (49% to 77%) reduction of the radiolabeled IL-3 binding was seen. On the basis of these ligand binding features, we propose the existence of three receptor types on AML cells: (1) low affinity GM-CSF receptors that do not bind IL-3, (2) dual high affinity GM-CSF/IL-3 receptors, and (3) high affinity IL-3 receptors that do not bind GM-CSF. We could also demonstrate these receptor types on normal monocytes. In addition, a fourth type of receptor was apparent on normal granulocytes (4), incapable of binding IL-3 and with an intermediate affinity for GM-CSF (approximately 400 pmol/L). Chemical crosslinking showed that GM-CSF and IL-3 both bind to proteins with molecular weight values of 130, 105, and 75, which provides additional evidence for the existence of a common GM-CSF/IL-3 receptor complex.
