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

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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 apparentKd 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 F2a and platelet-activating factor do not compete for the receptor.