IDENTIFICATION OF P90(RSK) AS THE PROBABLE CREB-SER(133) KINASE IN HUMAN MELANOCYTES

Normal human melanocytes proliferate in vitro only in response to cost imulation by at least two selected peptide growth factors. In the pres ence of only one mitogen, melanocytes become quiescent or die. These m itogens also enhance expression of differentiated functions, since in their presence the proliferating melanocytes become progressively more pigmented. To assess the intermediates participating in this dual res ponse, we have determined the activated state of several known ligand- induced signal transducers. We demonstrate that hepatocyte growth fact or/scatter factor, mast/stem-cell growth factor, basic fibroblast grow th factor, and endothelin-l induce phosphorylation of Ser(133) within the KID domain of the cAMP-responsive element binding protein, a modif ication necessary for transcriptional activation of all members of thi s family of transcription factors, including also cAMP-responsive elem ent modulator tau and activating transcription factor 1. The costimula tion with synergistic growth factors prolonged the phosphorylated stat e and activity of the mitogen-activated protein kinase 2 cascade. cAMP -responsive element binding protein Ser(133) phosphorylation in respon se to synergistic growth factors was due probably to the activation of p90(RSK) and, to a lesser extent, to p70(S6K). Our findings support t he concept that signals initiated at the cell surface converge on regu latory proteins that sustain both cell division and differentiation.