TNF activates an IRF1-dependent autocrine loop leading to sustained expression of chemokines and STAT1-dependent type I interferon-response genes.

Rapid induction of inflammatory genes by tumor necrosis factor (TNF) has been well studied, but little is known about delayed and chronic TNF responses. Here we investigated the kinetics of primary macrophage responses to TNF and discovered that TNF initiates an interferon-beta-mediated autocrine loop that sustains expression of inflammatory genes and induces delayed expression of interferon-response genes such as those encoding the transcription factors STAT1 and IRF7, which enhance macrophage responses to stimulation of cytokines and Toll-like receptors. TNF-induced interferon-beta production depended on interferon-response factor 1, and downstream gene expression was mediated by synergy between small amounts of interferon-beta and canonical TNF-induced signals. Thus, TNF activates a 'feed-forward' loop that sustains inflammation but avoids the potential toxicity associated with the high interferon production induced by stimulation of Toll-like receptors.

Interleukin-10-induced gene expression and suppressive function are selectively modulated by the PI3K-Akt-GSK3 pathway.

Interleukin-10 (IL-10) is an immunosuppressive cytokine that inhibits inflammatory gene expression. Phosphatidylinositol 3-kinase (PI3K) -mediated signalling regulates inflammatory responses and can induce IL-10 production, but a role for PI3K signalling in cellular responses to IL-10 is not known. In this study we investigated the involvement of the PI3K-Akt-GSK3 signalling pathway in IL-10-induced gene expression and IL-10-mediated suppression of Toll-like receptor-induced gene expression in primary human macrophages. A combination of loss and gain of function approaches using kinase inhibitors, expression of constitutively active Akt, and RNA interference in primary human macrophages showed that expression of a subset of IL-10-inducible genes was dependent on PI3K-Akt signalling. The effects of PI3K-Akt signalling on IL-10 responses were mediated at least in part by glycogen synthase kinase 3 (GSK3). In accordance with a functional role for PI3K pathways in contributing to the suppressive actions of IL-10, PI3K signalling augmented IL-10-mediated inhibition of lipopolysaccharide-induced IL-1, IL-8 and cyclo-oxygenase-2 expression. The PI3K signalling selectively modulated IL-10 responses, as it was not required for inhibition of tumour necrosis factor expression or for induction of certain IL-10-inducible genes such as SOCS3. These findings identify a new mechanism by which PI3K-mediated signalling can suppress inflammation by regulating IL-10-mediated gene induction and anti-inflammatory function.

IL-10 suppresses calcium-mediated costimulation of receptor activator NF-kappa B signaling during human osteoclast differentiation by inhibiting TREM-2 expression.

Induction of effective osteoclastogenesis by RANK (receptor activator of NF-kappaB) requires costimulation by ITAM-coupled receptors. In humans, the TREM-2 (triggering receptor expressed on myeloid cells 2) ITAM-coupled receptor plays a key role in bone remodeling, as patients with TREM-2 mutations exhibit defective osteoclastogenesis and bone lesions. We have identified a new rapidly induced costimulatory pathway for RANK signaling that is dependent on TREM-2 and mediated by calcium signaling. TREM-2-dependent calcium signals are required for RANK-mediated activation of calcium/calmodulin-dependent protein kinase (CaMK)II and downstream MEK and ERK MAPKs that are important for osteoclastogenesis. IL-10 inhibited RANK-induced osteoclastogenesis and selectively inhibited calcium signaling downstream of RANK by inhibiting transcription of TREM-2. Down-regulation of TREM-2 expression resulted in diminished RANKL-induced activation of the CaMK-MEK-ERK pathway and decreased expression of the master regulator of osteoclastogenesis NFATc1. These findings provide a new mechanism of inhibition of human osteoclast differentiation. The results also yield insights into crosstalk between ITAM-coupled receptors and heterologous receptors such as RANK, and they identify a mechanism by which IL-10 can suppress cellular responses to TNFR family members.

Lipopolysaccharide-induced expression of matrix metalloproteinases in human monocytes is suppressed by IFN-gamma via superinduction of ATF-3 and suppression of AP-1.

Matrix metalloproteinases (MMPs) are induced during inflammatory responses and are important for immune regulation, angiogenesis, wound healing, and tissue remodeling. Expression of MMPs needs to be tightly controlled to avoid excessive tissue damage. In this study, we investigated the regulation of MMP expression by inflammatory factors in primary human monocytes and macrophages. IFN-gamma, which augments inflammatory cytokine production in response to macrophage-activating factors such as TLR ligands, instead broadly suppressed TLR-induced MMP expression. Inhibition of MMP expression was dependent on STAT1 and required de novo protein synthesis. IFN-gamma strongly enhanced TLR-induced expression of the transcriptional repressor activating transcription factor (ATF-3) in a STAT1-dependent manner, which correlated with recruitment of ATF-3 to the endogenous MMP-1 promoter as detected by chromatin immunoprecipitation assays. RNA interference experiments further supported a role for ATF-3 in suppression of MMP-1 expression. In addition, IFN-gamma suppressed DNA binding by AP-1 transcription factors that are known to promote MMP expression and a combination of supershift, RNA interference and overexpression experiments implicated AP-1 family member Fra-1 in the regulation of MMP-1 expression. These results define an IFN-gamma-mediated homeostatic loop that limits the potential for tissue damage associated with inflammation, and identify transcriptional factors that regulate MMP expression in myeloid cells in inflammatory settings.

Regulation of macrophage phenotype by long-term exposure to IL-10.

Macrophages are chronically exposed to IL-10 in a variety of physiological and pathological settings. Macrophage responses to short-term stimulation with IL-10 have been extensively studied, but the effects of chronic exposure to IL-10 on macrophage function are not known. Herein we used transcriptional profiling and functional studies to characterize the phenotype of macrophages after long-term culture with IL-10. Classical activation of macrophages by LPS and IFNgamma was suppressed by IL-10. In contrast, IL-10 activated expression of genes that suggested acquisition of functions important for cell trafficking, tissue remodeling, recognition of microbial pathogens and responsiveness to the T cell-derived cytokines IL-2 and IL-21. These results demonstrate that IL-10 induces a differentiation program in macrophages and characterize a novel alternatively activated macrophage phenotype.

Kinetics of IL-10-induced gene expression in human macrophages.

IL-10 is a strong inhibitor of macrophage (M(phi)) activation and inflammatory cytokine and chemokine production. To gain insight into mechanisms by which IL-10 suppresses inflammation, we performed a kinetic analysis of IL-10-induced gene expression in primary human M(phi). IL-10 induced rapid and transient expression of genes encoding transcription factors, followed by sustained elevation or suppression of gene expression. IL-10 suppressed basal expression of interferon-inducible genes, suggesting that IL-10 interrupts autocrine interferon-mediated priming. IL-10 induced the expression of prostaglandin dehydrogenase (PGDH), the major catabolic enzyme involved in prostaglandin degradation. Concomitant with PGDH expression, IL-10 induced increased degradation of the inflammatory PGE2 and suppressed PGE2-mediated effects on M(phi) morphology and gene expression. These results identify catabolism of inflammatory PGs as a mechanism of IL-10 anti-inflammatory action.

Dysregulation of interleukin-10-dependent gene expression in rheumatoid arthritis synovial macrophages.

OBJECTIVE: The inflammatory cytokines tumor necrosis factor alpha (TNFalpha) and interleukin-1 (IL-1) are produced by activated macrophages, are key mediators of pathogenesis, and are validated therapeutic targets in rheumatoid arthritis (RA) and seronegative spondylarthritis (SpA). IL-10 is a potent antiinflammatory cytokine that suppresses macrophage TNFalpha and IL-1 production, yet is not effective in suppressing inflammatory arthritis. To gain insight into IL-10 responses in inflammatory arthritis, we used microarray analysis to determine the patterns of IL-10-inducible gene expression in freshly isolated RA and seronegative SpA synovial macrophages. METHODS: Macrophages from the synovial fluid of 5 patients with RA and 3 with seronegative SpA (2 with psoriatic arthritis and 1 with ankylosing spondylitis) were isolated by positive selection and stimulated ex vivo with IL-10 or interferon-gamma (IFNgamma). Gene expression was analyzed using Affymetrix microarrays and protocols. Real-time polymerase chain reaction was used to confirm changes in gene expression. RESULTS: The number of genes induced by IL-10 in arthritic macrophages was markedly smaller than that induced in control macrophages, and the strength of induction was lower in arthritic macrophages for most genes. The residual response of arthritic macrophages to IL-10 stimulation was qualitatively altered, such that IL-10 preferentially increased expression of IFNgamma-inducible genes. In contrast, arthritic macrophages expressed many IFNgamma-inducible genes prior to stimulation, and their response to IFNgamma remained mostly intact. CONCLUSION: These results demonstrate that IL-10 responses are dysregulated in RA synovial macrophages. An altered biologic response to IL-10, with attenuation of its antiinflammatory function and a concomitant retention of IFNgamma-like activating functions, provides a basis for the lack of efficacy of IL-10 in suppressing inflammatory arthritis.

Identification of a repressor in the first intron of the human alpha2(I) collagen gene (COL1A2).

The human and mouse genes that code for the alpha2 chain of collagen I (COL1A2 and Col1a2, respectively) share a common chromatin structure and nearly identical proximal promoter and far upstream enhancer sequences. Despite these homologies, species-specific differences have been reported regarding the function of individual cis-acting elements, such as the first intron sequence. In the present study, we have investigated the transcriptional contribution of the unique open chromatin site in the first intron of COL1A2 using a transgenic mouse model. DNase I footprinting identified a cluster of three distinct areas of nuclease protection (FI1-3) that span from nucleotides +647 to +760, relative to the transcription start site, and which contain consensus sequences for GATA and interferon regulatory factor (IRF) transcription factors. Gel mobility shift and chromatin immunoprecipitation assays corroborated this last finding by documenting binding of GATA-4 and IRF-1 and IRF-2 to the first intron sequence. Moreover, a short sequence encompassing the three footprints was found to inhibit expression of transgenic constructs containing the COL1A2 proximal promoter and far upstream enhancer in a position-independent manner. Mutations inserted into each of the footprints restored transgenic expression to different extents. These results therefore indicated that the unique open chromatin site of COL1A2 corresponds to a repressor, the activity of which seems to be mediated by the concerted action of GATA and IRF proteins. More generally, the study reiterated the existence of species-specific difference in the regulatory networks of the mammalian alpha2(I) collagen coding genes.

Identification of the key regions within the mouse pro-alpha 2(I) collagen gene far-upstream enhancer.

Studies using transgenic mice have shown that the mouse pro-alpha2(I) collagen gene contains a far-upstream enhancer, which directs expression in the majority of collagen I-producing cells during development and in response to tissue injury. In this study, we have investigated the minimal functional region required for the enhancer effect and studied the role of the three hypersensitive sites (HS3-HS5) that overlap this region. The results of deletion experiments indicate that the minimal functional unit of this enhancer is a 1.5-kb region between -17.0 and -15.45 kb from the transcription start site. This region includes the core sequences of HS3 and HS4 but not HS5. The HS4 sequences are essential for the functional integrity of the enhancer, whereas HS3 represents tissue-specific elements that direct expression in mesenchymal cells of internal tissues and body wall muscles. The HS3 region appears to bind a complex of transcription factors illustrated by large regions of protected sequences. A 400-bp sequence located between -17.0 and -16.6 is also essential for the enhancer because its deletion results in increased susceptibility to the chromatin environment.

Cooperativity between far upstream enhancer and proximal promoter elements of the human {alpha}2(I) collagen (COL1A2) gene instructs tissue specificity in transgenic mice.

Interaction between the proximal (-378) promoter and the far upstream (-20 kb) enhancer is essential for tissue-specific expression of the human alpha2(I) collagen gene (COL1A2) in transgenic mice. Previous in vitro studies have shown that three Sp1 binding sites (around -300) are part of a cytokine-responsive element and that two TC-rich boxes (around -160 and -125) and a CBF/NFY consensus sequence (around -80) confer optimal promoter activity by interacting among themselves and with the upstream Sp1 sites. Here we report that mutations of the Sp1 binding sites, TC-rich boxes or CBF/NFY consensus sequence lead to reduced transgene activity, thus underscoring the functional interdependence of the proximal promoter elements. Loss of the Sp1 binding sites was associated with loss of transgene expression in osteoblasts, whereas elimination of the CBF/NFY binding site (alone or in combination with the TC-rich boxes) was correlated with a lack of activity in the ventral fascia and head dermis and musculature. Additionally, transgene expression in skin fascia fibroblasts depended on the integrity of the Sp1 binding sites and TC-rich boxes, and on their physical configuration. Evidence is also presented suggesting cooperativity between cis-acting elements of the far upstream enhancer and proximal promoter in assembling tissue-specific protein complexes. This study thus reiterates the complex and highly combinatorial nature of the regulatory network governing COL1A2 transcription in vivo.

Sensitization of IFN-gamma Jak-STAT signaling during macrophage activation.

A general paradigm in signal transduction is ligand-induced feedback inhibition and the desensitization of signaling. We found that subthreshold concentrations of interferon-gamma (IFN-gamma), which did not activate macrophages, increased their sensitivity to subsequent IFN-gamma stimulation; this resulted in increased signal transducer and activator of transcription 1 (STAT1) activation and increased IFN-gamma#150;dependent gene activation. Sensitization of IFN-gamma signaling was mediated by the induction of STAT1 expression by low doses of IFN-gamma that did not effectively induce feedback inhibition. IFN-gamma signaling was sensitized in vivo after IFN-gamma injection, and STAT1 expression was increased after injection of lipopolysaccharide and in rheumatoid arthritis synovial cells. These results identify a mechanism that sensitizes macrophages to low concentrations of IFN-gamma and regulates IFN-gamma responses in acute and chronic inflammation.