Vitamin C inhibits granulocyte macrophage–colony-stimulating factor–induced signaling pathways

Vitamin C is present in the cytosol as ascorbic acid, functioning primarily as a cofactor for enzymatic reactions and as an antioxidant to scavenge free radicals. Human granulocyte macrophage–colony-stimulating factor (GM-CSF) induces an increase in reactive oxygen species (ROS) and uses ROS for some signaling functions. We therefore investigated the effect of vitamin C on GM-CSF–mediated responses. Loading U937 cells with vitamin C decreased intracellular levels of ROS and inhibited the production of ROS induced by GM-CSF. Vitamin C suppressed GM-CSF–dependent phosphorylation of the signal transducer and activator of transcription 5 (Stat-5) and mitogen-activated protein (MAP) kinase (Erk1 and Erk2) in a dose-dependent manner as was phosphorylation of MAP kinase induced by both interleukin 3 (IL-3) and GM-CSF in HL-60 cells. In 293T cells transfected with alpha and beta GM-CSF receptor subunits (αGMR and βGMR), GM-CSF–induced phosphorylation of βGMR and Jak-2 activation was suppressed by vitamin C loading. GM-CSF–mediated transcriptional activation of a luciferase reporter construct containing STAT-binding sites was also inhibited by vitamin C. These results substantiate the importance of ROS in GM-CSF signaling and indicate a role for vitamin C in downmodulating GM-CSF signaling responses. Our findings point to vitamin C as a regulator of cytokine redox-signal transduction in host defense cells and a possible role in controlling inflammatory responses.