Characterization of a high-affinity receptor for phorbol esters in rat alveolar macrophages

Abstract

Macrophages display varied responses to the tumor promoter, TPA. In this study, a high-affinity receptor for phorbol ester is characterized in a viable alveolar macrophage population. The binding assay is performed using tritiated PDBu and specific binding is demonstrated to be temperature-sensitive. At 37°C, the level of bound ligand reaches maximal binding within 2–5 min but rapidly decays to within 30% of the original specific binding. Equilibrium, however, can be established when the assay is carried out at 4°C. The data indicate that at this temperature maximal binding is reached within 2 h and remains constant there-after. Scatchard analysis shows that the receptor has an apparentK _d of 21 nM and each macrophage possesses 2×10⁵ binding sites. Active phorbol derivatives such as TPA and PDBu compete with the labeled ligand for the receptor, whereas the inactive phorbol alcohol does not modulate the specific binding. Mezerein, a related diterpene which has been shown to share some of the properties of phorbol esters, also competes for the binding site. The high-affinity receptor is not affected by zymosan or EIgG phagocytosis. Inflammatory mediators such as prostaglandins E₂ and F_2a and platelet-activating factor do not compete for the receptor.