ACTH(4-12) is the minimal message sequence required to induce the differentiation of mouse epidermal melanocytes in serum-free primary culture

It is well known that a-melanocyte stimulating hormone (MSH) induces the di fferentiation of mouse epidermal melanocytes in vivo and in vitro. Although adrenocorticotropic hormone (ACTH) possesses the same amino acid sequence as MSH does, it is not clear whether the peptide and its fragments induce t he differentiation of mouse epidermal melanocytes. In this study, the diffe rentiation-inducing potencies of human ACTH and its fragments were investig ated by adding them into a culture medium (0.001-1,000 nM) from the initiat ion of primary culture of epidermal cell suspensions. Their potencies were compared with the potency of alpha-MSH. After 2-4 days of primary cultures with ACTH(1-13), ACTH(1-17), ACTH(1-24), ACTH(1-39), ACTH(4-12), ACTH(4-13) , and alpha-MSH, pigment granules appeared in the cytoplasms and dendrites of melanoblasts that were in contact with the adjacent keratinocyte colonie s. By 14 days, cultures contained mostly pigmented melanocytes. The order o f potencies of ACTH fragments and alpha-MSH shown by the ED50 value was as follows: alpha-MSH = ACTH(1-13) = ACTH(1-17) = ACTH(4-12) = ACTH(4-13) > AC TH(1-24) > ACTH(1-39). The length of their peptide chains was inversely pro portional to the potency. On the contrary, ACTH(1-4), ACTH(11-24), and ACTH (18-39) failed to induce the differentiation of melanocytes. In contrast, A CTH(1-10), ACTH(4-10), ACTH(4-11), and ACTH(5-12) possessed a weak potency at high doses only (100 and 1,000 nM). These results suggest that ACTH(4-12 ) is the minimal message sequence required to induce the differentiation of mouse epidermal melanocytes in culture completely. The amino acids of Met( 4) and Pro(12) are suggested to be important for its potency. J Exp. Zool. 286:632-640, 2000. (C) 2000 Wiley-Liss, Inc.