Hormone activity of a synthetic decapeptide with the adrenocorticotropin-like sequence of human immunoglobulin G1

The antiproliferative and immunosuppressive in vitro effects of immunocortin, a synthetic adrenocorticotropin-like (ACTH-like) decapeptide H-Val-Lys-Lys-Pro-Gly-Ser-Ser-Val-Lys-Val-OH, whose sequence corresponds to segment 11-20 of the variable part of the human IgG1 heavy chain, were studied. At concentrations of 10^-11-10^-7 M, immunocortin was found to inhibit the growth of the human MT-4 T-lymphoblastoid cell line, to suppress the blast transformation of thymocytes, and to decrease the spontaneous mobility of peritoneal macrophages and their bactericidal action toward the virulent strain Salmonella typhimurium 415. By using a ¹²⁵I-labeled "addressing" fragment of ACTH {[¹²⁵I]ACTH-(13-24)}, we showed that MT-4 cells express specific receptors for ACTH (K_d 97 pM). Immunocortin and human ACTH (but not the heavy chain of IgG1) competitively inhibited the binding of [¹²⁵I]ACTH-(13-24) to these receptors with K_i1 of 0.38 and K_i2 of 0.34 nM, respectively. Specific receptors for ACTH (K_d 5.8 nM) on mouse thymocytes were detected and characterized. The unlabeled immunocortin was shown to compete with labeled ACTH-(13-24) for binding to these receptors (K_i = 1.8 nM) and this binding of immunocortin to receptors on thymocytes activates adenylate cyclase from these cells and increases the intracellular concentration of cAMP.