Cytokine regulation of the interleukin‐1 receptor antagonist protein in U937 cells

A naturally occurring receptor-level antagonist of interleukin-1 (IRAPorIL-1 ra) has recently been cloned. To determine what stimuli might regulate this inhibitor, cytokines were tested for their effects on the steady-state level of IRAP mRNA in phorbol ester-differentiated U937 cells. The cytokines tested fell into one of three groups: (a) inducers: granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-4, (b) weak inducers (<2-fold stimulation): [IL-lα, IL-Iγ, and transforming growth factor-β (TGF-γ)] and (c) cytokines with no effect: (IL-2, platelet-derived growth factor, acidic fibroblast grouth factor, basic fibroblast growth factor, epidermal growth factor, granulocyte colony-stimulating factor, IL-3, IL-5, IL-6, interferon-y, multi-colony stimulating factor, tumor necrosis factor -a and IRAP itself. One hundred U/ml of either GM-CSF or IL-4 was the dose inducing peak IRAP mRNA expression; that peak expression occurred 12 h after addition of cytokine. GM-CSF induced a 34 ±15-fold increase in IRAP mRNA, and IL-4 induced a 15± 6-fold increase. In the same RNA samples, GM-CSF increased IL-ip mRNA 5.9 ± 1.7-fold, but IL-4 decreased IL-Iγ mRNA to half that of control levels (0.45 ± 0.17). Thus, a single stimulus (IL-4) decreased the expression of an agonist (IL-1) while it increased the expression of an antagonist (IRAP). When U937 cells were treated with both IL-4 and GM-CSF, the level of IRAP mRNA induced was additive, suggesting that the cytokines acted differently to increase IRAP mRNA levels. The level of IL-1 mRNA in cells treated with both IL-4 and GM-CSF was intermediate. Dexamethasone and cycloheximide inhibited all mRNA increases and did not reverse IL-4-induced decreases in IL-1 mRNA. These studies have identified two cytokines which induce IRAP in the monocytic cells studied, and have partially characterized the differential regulation of IL-1 and its antagonist, IRAP.