A novel MEK2/PI3Kδ pathway controls the expression of IL-1 receptor antagonist in IFN-β-activated human monocytes

IFN-β and sIL-1Ra play crucial roles in the regulation of innate immunity and inflammation. IFN-β, which is widely used to improve the course of relapsing, remitting multiple sclerosis, induces the production of sIL-1Ra in human monocytes through mechanisms that remain largely unknown. In this study, we identified PI3Kδ and MEK2 as key elements that control sIL-1Ra production in isolated human monocytes activated by IFN-β. Blockade of MEK2, but not of MEK1, by inhibitors and siRNA prevented IFN-β-induced PI3Kδ recruitment to the membrane, Akt phosphorylation, and sIL-1Ra production, suggesting that MEK2 acted upstream of PI3Kδ. Furthermore, ERK1/2, the only identified substrates of MEK1/2 to date, are dispensable for sIL-1Ra production in response to IFN-β stimulation. Upon IFN-β activation, MEK2 and PI3Kδ are translocated to monocyte membranes. These data suggest that MEK1 and MEK2 display different, nonredundant functions in IFN-β signaling. That neither MEK1 nor ERK1/2 play a part in this mechanism is also an unexpected finding that gives rise to a better understanding of the MAPK signaling network. Together, these findings demonstrate that IFN-β triggers an atypical MEK2/PI3Kδ signaling cascade to regulate sIL-1Ra expression in monocytes. The premise that MEK1 and MEK2 play a part in the induction of the proinflammatory cytokine, IL-1β in human monocytes provides a rationale for an alternative, IFN-β-mediated pathway to induce/enhance sIL-1Ra production and thus, to dampen inflammation.