Thrombin binds to murine bone marrow-derived macrophages and enhances colony-stimulating factor-1-driven mitogenesis

The binding and mitogenic properties of thrombin have been established in various transformed cell lines. In such systems, thrombin induces cell division in the absence of exogenous growth factors, and the enzyme is considered to act directly as a mitogen. This study explores thrombin's interaction with nontransformed, growth factor-dependent cells. Binding of ¹²⁵I-α-thrombin to colony-stimulating factor (CSF)-1-dependent bone marrow-derived macrophages is saturable, time-dependent, and displaceable by both unlabeled α-thronibin, and esterolytically inactive thrombin. Both dissociation studies of pre-bound radiolabeled thrombin and Scatchard analysis assisted by the program "Ligand" suggest adherence of thrombinbinding data to a multi-site model. There are an estimated 2 × 10⁴ high affinity sites (K_d = 7 × 10^-9 M) and 2 × 10⁶ low affinity sites (K_d = 9 × 10^-7 M) per cell. Quiescent bone marrow-derived macrophages were cultured with either 10^-8 M thrombin, 1000 units of CSF-1/ml, or both and [³H]thymidine incorporation was determined. Thrombin alone did not induce mitogenesis. CSF-1 induced mitogenesis with peak [³H] thymidine incorporation occurring 24 h after addition of the mitogen. This CSF-1-dependent mitogenic influence was enhanced greater than 2-fold by treatment with thrombin.