ABSTRACT: The neuropeptide α-melanocyte stimulating hormone (α-MSH) is recognized as a potent mediator of immune and inflammatory reactions. Accordingly, α-MSH in vitro, as well as in vivo, antagonizes the proinflammatory activities of cytokines such as interleukin-1 (IL-1), IL-6, and tumor necrosis factor a (TNFα). Since the molecular basis of these antiinflammatory effects is not well known, the influence of α-MSH on IL-1β-induced chemokine production and transcription factor activation was investigated in human keratinocytes. α-MSH, in a dose-dependent manner, after 48 h, significantly reduced the IL-1b mediated secretion of the C-X-C chemokines IL-8 and Groα. This was confirmed by semiquantitative RT-PCR, which revealed a marked downregulation in IL-8 and Groα mRNA expression. Furthermore, we determined the effect of α-MSH on the IL-1β-induced activation of the nuclear factor κB (NFκB)-a major transcription factor for chemokine genes. Electrophoretic mobility-shift-assays showed that α-MSH, in a dose range from 10−6 to 10−12 M, significantly downregulated the IL-1b-induced activation of NFκB 10 minutes after stimulation. Therefore, NFκB inactivation by α-MSH appears to be a crucial event, one that is responsible for the downregulation of cytokine gene transcription.