Impaired Ag-presenting function in dendritic cells (DCs) due to abnormal differentiation is an important mechanism of tumor escape from immune control. A major role for vascular endothelial growth factor (VEGF) and its receptors, VEGFR1/Flt-1 and VEGFR2/KDR/Flk-1, has been documented in hemopoietic development. To study the roles of each of these receptors in DC differentiation, we used an in vitro system of myeloid DC differentiation from murine embryonic stem cells. Exposure of wild-type, VEGFR1−/−, or VEGFR2−/− embryonic stem cells to exogenous VEGF or the VEGFR1-specific ligand, placental growth factor, revealed distinct roles of VEGF receptors. VEGFR1 is the primary mediator of the VEGF inhibition of DC maturation, whereas VEGFR2 tyrosine kinase signaling is essential for early hemopoietic differentiation, but only marginally affects final DC maturation. SU5416, a VEGF receptor tyrosine kinase inhibitor, only partially rescued the mature DC phenotype in the presence of VEGF, suggesting the involvement of both tyrosine kinase-dependent and independent inhibitory mechanisms. VEGFR1 signaling was sufficient for blocking NF-κB activation in bone marrow hemopoietic progenitor cells. VEGF and placental growth factor affect the early stages of myeloid/DC differentiation. The data suggest that therapeutic strategies attempting to reverse the immunosuppressive effects of VEGF in cancer patients might be more effective if they specifically targeted VEGFR1.

We explored the role of the transcription factor c-Fos in lipopolysaccharide (LPS)-induced cytokine response using mice lacking c-Fos (Fos-/- mice). Compared with wild-type controls, Fos-/- macrophages and mice showed significantly enhanced production of tumour necrosis factor (TNF)-alpha, interleukin (IL)-6 and IL-12 p40, but reduced production of the anti-inflammatory cytokine IL-10. Bandshift analysis revealed that LPS-induced NF-kappaB binding activity to a functional site in the TNF-alpha promoter was significantly higher in Fos-/- than in wild-type macrophages. Using telemetry, we monitored body temperature and heart rate after LPS injection and found that Fos-/- mice undergo more severe hypothermia and bradycardia than wild-type mice. Such shock responses in Fos-/- mice were significantly reversed by neutralizing TNF-alpha. These data reveal a novel in vivo role for c-Fos as an anti-inflammatory transcription factor acting through suppression of NF-kappaB activity.

Receptor activator of NF-κB ligand (RANKL) is a membrane-bound or soluble cytokine essential for osteoclast differentiation, whereas the decoy receptor osteoprotegerin (OPG) masks RANKL activity. In mouse serum, both soluble RANKL and OPG are detectable. We observed that mice injected with LPS showed significantly down-regulated serum RANKL levels, whereas serum OPG levels were up-regulated. However, the roles of RANKL and OPG in innate immunity remain obscure. We found that RANKL pretreatment suppressed production of proinflammatory cytokines in macrophages in response to stimulation by bacteria and their components. Furthermore, such RANKL-induced tolerance in macrophages was inhibited by GM-CSF treatment, which blocks RANKL signaling. RANKL-induced tolerance occurred in the absence of c-Fos, which is essential for osteoclast differentiation. In mice lacking OPG, LPS-induced production of proinflammatory cytokines was reduced, whereas in mice lacking RANKL, it was increased, and lethality following LPS injection was also elevated, suggesting that constitutive activities of RANKL suppress cytokine responsiveness to LPS in vivo. Strikingly, prior administration of RANKL protected mice from LPS-induced death. These data reveal prophylactic potential of RANKL in acute inflammatory diseases.

Abstract. Osteoimmunology is the emerging concept that certain molecules link the skeletal and immune systems. The transcription factor c-Fos, a component of activator protein-1 (AP-1), is essential for osteoclast differentiation. Mice lacking c-Fos are osteopetrotic owing to impaired osteoclast development. Recent studies suggest that in contrast to this positive role in osteoclastogenesis, c-Fos expression inhibits differentiation and activation of mononuclear phagocytes. Here, we focus on the contrasting roles of c-Fos in the bone and immune lineages. Both osteoclasts and mononuclear phagocytes are derived from common myeloid precursors. Osteoclasts resorb bone, whereas macrophages and myeloid dendritic cells phagocytose microbial pathogens, initiating innate and adaptive immunity. Differentiation of the common precursors into either bone or immune lineage is determined by ligand binding to cell-surface receptors, particularly receptor activator of NF-kB (RANK) for osteoclasts, or Toll-like receptors (TLRs) for mononuclear phagocytes. Both RANK and TLRs activate the dimeric transcription factors NF-kB and AP-1. Yet, c-Fos/AP-1 plays a positive role in osteoclasts but a negative role in macrophages and dendritic cells. Further study is necessary to clarify this dual role of c-Fos.

The Fos family proteins, c-Fos and Fra-1, are components of the dimeric transcription factor AP-1, which is typically composed of Fos and Jun family proteins. We have previously shown that mice lacking c-Fos (Fos−/− mice) respond more strongly to LPS injection than do wild-type (wt) controls. We then examined the sensitivity of Fos−/− mice to acute inflammatory stress in a dextran sulfate sodium (DSS)-induced colitis model. We found that Fos−/− mice exhibited more severe weight loss, bleeding, diarrhea, and colon shortening than did wt mice, in association with higher TNF-α production and NF-κB activity in colon segments of DSS-treated Fos−/− mice. Furthermore, NF-κB inhibition suppressed severe DSS-induced colitis in Fos−/− mice. In contrast, Fra-1 transgenic (Tg) mice responded poorly to LPS injection, and Fra-1–overexpressing macrophages and fibroblasts showed reduced production of proinflammatory cytokines, NO, and NF-κB activity. Remarkably, in the DSS-induced colitis model, Fra-1 Tg mice showed less severe clinical scores of colitis than did wt mice. Consistently, proinflammatory cytokine production and NF-κB activity in colon segments of DSS-treated Fra-1 Tg mice were lower than in wt controls. These findings reveal that the absence of c-Fos and overexpression of Fra-1 respectively enhance and suppress the activation of NF-κB in DSS-induced inflammatory stress. In this paper, we propose that AP-1 transcription factors containing c-Fos or Fra-1 are negative regulators of NF-κB–mediated stress responses.

c-Fos is a component of transcription factor AP-1. We show that macrophages lacking c-Fos exhibit enhanced production of proinflammatory cytokines, potentiated NF-B phosphorylation, and increased cell death following Salmonella enterica serovar Typhimurium infection. Furthermore, mice lacking c-Fos are highly susceptible to infection, suggesting that c-Fos confers resistance to Salmonella infection in mice.

Background Inflammatory Bowel Disease (IBD) is characterised by chronic inflammation and increased oxidative stress in the intestinal mucosa of patients. NF-E2-related factor 2 (Nrf2) plays a key role in the antioxidant response by regulating the transcription of antioxidant genes as well as reducing expression of inflammatory mediators. A role for the Nrf2 pathway in IBD has been demonstrated in rodent models previously but these studies have predominantly used compounds such as Dimethyl Fumarate that are covalent Nrf2 activators that have the potential for significant off-target activities. C4X Discovery has identified C4X_6746, a novel potent and selective non-covalent Nrf2 activator, that disrupts the interaction between Nrf2 and its repressor Keap1 and is being investigated as an oral treatment for inflammatory disorders. Methods Male Han Wistar rats were given 3% Dextran Sodium Sulphate (DSS) in drinking water for 8 days in order to induce colitis and were dosed once daily by oral gavage with vehicle, C4X_6746 (0.3, 3 or 30 mg/kg) or Cyclosporine A (CsA, 20 mg/kg). The Disease Activity Index (DAI) was assessed daily based on stool consistency, faecal blood and body weight. On study termination, the colon weight and length were measured, and additional endpoints assessed (colon macroscopic score, histology, colon markers of inflammation and antioxidant response). Results C4X_6746 caused a dose-dependent decrease in the DAI with 30 mg/kg showing similar efficacy to the reference comparator CsA. C4X_6746 (3 and 30 mg/kg) resulted in a statistically significant reduction in the colon weight:length ratio and the macroscopic score. The compound also reduced the DSS-induced histological changes including inflammatory cell infiltrate, epithelial damage and mucosal architecture changes. Analysis of the colon tissue showed that C4X_6746 reduced inflammation (MPO activity) and upregulated the antioxidant response (SOD activity, ratio of reduced:oxidised glutathione). Efficacy of C4X_6746 in the model correlated with the dose-dependent induction of NQO1 mRNA in the colon and blood as a biomarker for Nrf2 pathway activation. Conclusion The efficacy of C4X_6746 in the rat DSS model supports development of non-covalent oral Nrf2 activators to significantly reduce oxidative tissue damage and chronic inflammation in IBD.