DIFFERENTIAL EXPRESSION OF THE MELATONIN RECEPTOR IN HUMAN MONOCYTES

Earlier studies have shown that the pineal hormone melatonin activates human monocytes. It is reported here that melatonin induces the secre tion of IL-1, IL-6, and TNF in fresh and 1-day in vitro cultured monoc ytes that also express the melatonin receptor (K-d = 270 +/- 60 pM; 42 ,000-48,000 receptors/cell). However, when monocytes were cultured in vitro for 2 days, the number of receptors decreased to 11,000 receptor s/cell, with the same K-. LPS activation of fresh or 1-day cultured mo nocytes did not result in any increase in melatonin receptor number. L PS activation of 2-day cultured monocytes led to an increase in the nu mber of melatonin receptors, from 11,000 receptors/cell to the plateau of 42,000 to 48,000 receptors/cell, The loss of receptors by 2-day cu ltured monocytes was not irreversible, Melatonin did not induce the re lease of IL-1, TNF, or IL-6 in monocytes cultured in vitro for 3 days and for up to 15 days, and these long time cultured monocytes did not express the melatonin receptors even after activation by LPS. The loss of melatonin receptors by monocytes cultured in vitro for 3 days and for up to 15 days was irreversible. Therefore, it is shown for the fir st time that human monocytes express melatonin receptors. Furthermore, human monocytes ex-press melatonin receptors differentially depending on their state of maturation, and it appears that in vitro monocyte d ifferentiation and maturation negatively affect human monocyte melaton in receptor expression.