Regulation of GLI3 expression by TLR4 signaling

Abstract

Abstract While acute inflammation is an important host response to infections, chronic inflammation has emerged as a mechanism driving many diseases including autoimmune and malignant diseases. Monocytes are a subset of immune cells that secrete inflammatory cytokines in response to infections through toll-like receptor (TLR) signaling. The cross-talk between TLR signaling and other pathways has been shown to be important for downstream effector functions. We identify a novel role for the transcription factor GLI3, a negative regulator of Hedgehog signaling, in mediating TLR4-induced inflammation. Stimulation of monocytes with LPS increases GLI3 mRNA and protein expression. This occurred in a dose- and time-dependent manner. Investigation of the signaling mechanism suggests that LPS induced GLI3 occurs through the adaptor protein TRIF (but not MyD88). Targeting of transcription factors NFkb, AP-1 and IRF3 by pharmaceutical inhibitors suggests that TLR4-mediated GLI3 is regulated by IRF3. Transfection of our monocyte cell lines with shRNA targeting GLI3 suggests a regulatory role of transcription factor GLI3 in mRNA expression of essential cytokines involved in inflammation including IL-6 and CCL2. We generated conditional GLI3 knockout in mice myeloid cells using LysM-cre mediated recombination. IFA-elicited macrophages from mice with conditional GLI3 knockout driven by LysM-cre stimulated with LPS show reduced IL-6 and CCL2 secretion compared to macrophages from wild-type littermates. Taken together, these data suggest that GLI3 plays a role in innate immunity by regulating TLR4-induced inflammation.