| Dendritic cells bridge innate and adaptive immunity, and are active participants in both responses. Upon capture of pathogens, dendritic cells release inflammatory cytokines and chemokines, which attract other immune cells to the site of infection. Endogenous agents such as anti-inflammatory cytokines, glucocorticoids, anti-inflammatory neuropeptides, and lipid mediators such as prostaglandin E2 (PGE2) limit and control the inflammatory response. PGE2 was reported to inhibit inflammatory chemokine release from activated macrophages and microglia. In this study we report on the inhibition by PGE2 of CCL3 (MIP-1alpha) and CCL4 (MIP-1beta) expression and release from murine bone marrow-derived dendritic cells stimulated with either lipopolysaccharide (LPS), a TLR4 ligand, or peptidoglycan, a TLR2 ligand. The inhibition is dose-dependent, and occurs at both mRNA and protein levels. Intraperitoneal administration of PGE2 together with LPS results in a reduction in the levels of CCL3 and CCL4 released in the peritoneal fluid, a reduction in the number of dendritic cells accumulating in the peritoneal cavity, and a reduction in CCL3 amount per cell in the peritoneal dendritic cell population. These results suggest that one of the mechanisms by which endogenous PGE2 acts as an anti-inflammatory agent, is the inhibition of inflammatory chemokine release from activated dendritic cells, preventing the excess accumulation of activated immune cells.; The inhibitory effect of PGE2 on CCL3/4 expression is mediated through EP2 and EP4 receptors. Bone marrow-derived dendritic cells express both EP-2 and EP-4 receptors and increase intracellular cAMP formation following PGE2 challenge. Intracellular cAMP activates the Epac → PI3K → PKB → GSK-3 pathway, resulting in phosphorylation and thereby inactivation of GSK-3. Active GSK-3 is required for the phosphorylation of the CCAAT displacement protein (CDP), a potent mammalian transcriptional repressor. Phosphorylated CDP cannot bind DNA and loses the repressor activity. The inactivation of GSK-3 by PGE2 leads to the accumulation of nonphosphorylated CDP, and subsequent CDP binding to specific DNA regions in the CCL3/4 promoters and repression of CCL3/4 transcription. The direct link between CDP and CCL3/4 transcription was established in experiments in which CDP expression was knocked-down by using CDP siRNA which led to the reversal of the inhibitory effects of PGE2. |
