α‐MSH production, receptors, and influence on neopterin in a human monocyte/macrophage cell line

α‐Melanocyte‐stimulating hormone (α‐MSH), a tridecapeptide derived from pro‐opiomelanocortin, has potent antiinflammatory activity in laboratory animals. α‐MSH inhibits nitric oxide production by murine macrophages, an influence believed to reflect activation of an autocrine circuit in these cells, one that is based on production and release of α‐MSH and subsequent stimulation of melanocortin receptors. We found that THP‐1 cells, human monocytic cells, produced α‐MSH; this production was increased by interleukin‐6, tumor necrosis factor α, or concanavalin A. These cells also expressed the gene for the human α‐MSH receptor MC1. Unlike murine macrophages, THP‐1 cells produced little nitrite in response to interferon‐γ(IFN‐γ) and lipopolysaccharide, and a‐MSH inhibited this production only slightly. However, production of neopterin, a presumed primate homologue of nitric oxide in lower animals, was increased in THP‐1 cells stimulated with IFN‐γ plus TNF‐α and α‐MSH significantly inhibited this production. The evidence indicates that an autocrine regulatory circuit based on α‐MSH occurs in human monocyte/macrophages much as in murine macrophages. α‐MSH‐induced modulation of specific inflammatory mediators/cytotoxic agents appears to differ depending on the importance of the mediators in the myelomonocytic cells of different species.