Specific binding of a ¹²⁵I-secretoneurin analogue to a human monocytic cell line

Secretoneurin (SN) is a novel neuropeptide expressed in the central and peripheral nervous system as well as in various endocrine tissues. SN inhibits growth of aortic pulmonary and endothelial cells and is a potent chemoattractant for endothelial cells, skin fibroblasts and monocytes. We investigated here the presence of specific high affinity binding sites for SN on a target tissue SN was iodinated with the Bolton-Hunter (BH) reagent and purified by isocratic reversed phase chromatography. Specific binding sites for ¹²⁵I-BHSN were identified on human Mono Mac 6 cells, a monocytic cell line. Scatchard analysis revealed a single class of binding sites with a K(d) value of 7.3 nM and a B(max), of 322 (fmol/mg protein). Competition studies demonstrated that the 15 C-terminal amino acids of SN could displace authentic SN, whereas shorter fragments were inactive. Other sensory neuropeptides like substance P, calcitonin gene-related peptide or galamin as well as the chemokine receptor ligand Rantes or the typical chemoattractant FMLP could not displace SN. Our studies demonstrate specific high affinity binding sites for SN on a monocytic cell line. Since SN exerts a potent chemotactic activity towards monocytes and increases cytosolic calcium in these cells, these binding sites might well represent a putative functional plasma membrane receptor for SN.