The interaction of 125I-colony-stimulating factor-1 with bone marrow-derived macrophages.

The colony-stimulating factor, CSF-1, stimulates cultured quiescent murine bone marrow-derived macrophages (BMM) to enter DNA synthesis with a lag phase of 10-12 h. The binding, dissociation, internalization, and degradation of lZ5I-CSF-1 by BMM during the lag phase were investigated. Quiescent BMM express -5 x lo4 cell surface receptor sites/cell but contain additional cryptic sites (-lO"/cell) that can appear at the cell surface within 10 min at 37 "C. Studies of the binding reaction at both 2 OC (& I 2 x 1O-l' M) and 37 O C (& -4 X 10"' M) are consistent with the existence of a single class of cell surface sites. The disappearance of cell surface 12SI-CSF-1 following a 2-37 OC temperature shift results from two, competitive, first order processes, internalization and dissociation. Internalization (tllz = 1.6 min) is 6 times more frequent than dissociation ($Ilz = 9.6 min). Following internalization, 10-15% of the intracellular CSF-1 is rapidly degraded whereas the remaining 85-90% is slowly degraded by a chloroquin-sensitive first order process (tl,z > 3.5 h). These findings were confirmed and extended by studies of the uptake of 12SI-CSF-1 at 37 "C. Following addition of 12BI-CSF-1, cell surface receptors are rapidly down-regulated (tllz -7 min) and their replacement does not commence until 20-60% of pre-existing surface receptor sites have disappeared. Despite receptor replacement, initially from the cryptic pool and later by de novo synthesis and/or receptor recycling (4 molecules/cell/s at steady state), the number of receptors at the cell surface remains low. The process results in the intracellular accumulation of large amounts of lZ6I-CSF-1 (>lo5 molecules/cell) by BMM. Thus, whereas the kinetics of association, dissociation, and internalization of CSF-1 with BMM and peritoneal exudate macrophages are similar, BMM, which exhibit a higher proliferative response, degrade growth factor 12 times more slowly.