Agouti signaling protein and other factors modulating differentiation and proliferation of immortal melanoblasts

The melanocyte lineage potentially forms an attractive model system for stu dies in cell differentiation, developmental genetics, cell signaling, and m elanoma, because differentiated cells produce the visible pigment melanin. Immortal lines of murine melanoblasts (melanocyte precursors) have been des cribed previously, but induction of differentiation involved a complex cult ure system with keratinocyte feeder cells. Here we describe conditions for both growth and induced differentiation of the melanoblast line melb-a, wit hout feeder cells, and analyze factors that directly control proliferation and differentiation of these pure melanoblasts. Several active factors are products of developmental and other coat color genes, including stem cell f actor (SCF), melanocyte-stimulating hormone (alpha MSH), and agouti signali ng protein (ASP), a natural antagonist at the MSH receptor (melanocortin 1 receptor, MC1R) encoded by the agouti gene. A stable analog of alpha MSH (N DP-MSH) stimulated differentiation and inhibited growth. ASP in excess inhi bited both effects of NDP-MSH, that is, ASP could inhibit pigmentation and stimulate growth. These effects provide an explanation for the interactions in mice of melanocyte developmental mutations with yellow agouti and Mc1r alleles, and a role for embryonic expression patterns of ASP. (C) 2001 Wile y-Liss, Inc.