Activation of STAT5 by lipopolysaccharide through granulocyte-macrophage colony-stimulating factor production in human monocytes

LPS is a potent stimulator of monocytes, inducing many of their functions. Although the details of how LPS exerts such functions remain largely unknown, transcription factors such as nuclear factor-κB, nuclear factor-IL-6, and activator protein-1 have been shown to be involved in this process. However, to date it has been thought that no known STAT molecule plays a role in the activation of monocytes by LPS. In this study we examined whether some known STAT molecule is stimulated by LPS, based on the finding that a GAS motif sequence is conserved in the promoter regions of human, mouse, and rat cyclo-oxygenase-2 (COX-2) genes. Consequently, LPS induced activation of STAT5 in human monocytes, and this STAT5 activation occurred in an indirect way via granulocyte-macrophage CSF (GM-CSF) secreted by LPS- stimulated monocytes. Expression of COX-2 protein was partially reduced by treatment of anti-human GM-CSF Ab. Activation of STAT5 was inhibited by either IL-10 or dexamethasone (Dex), but not by aspirin. IL-10 blocked activation of STAT5 indirectly by suppressing GM-CSF production, while Dex inhibited this activation both directly and indirectly. Taken together, these results suggest that in addition to other transcription factors, STAT5 plays an important role in activation of monocytes by LPS, and that STAT5 is another target for IL-10 and Dex to inhibit COX-2 expression in activated monocytes.