| dc.description.abstract | Macrophages and dendritic cells (DCs) are important in innate immune system, where they recognize pathogens and initiate inflammatory responses. Elevation of cAMP by inhibition of phosphodieasterases 4 (PDE4), enzymes that degrade cAMP with high affinity, in these cells is known to suppress various inflammatory responses. Among the four PDE4 isoforms (PDE4A-4D), PDE4B has been shown to play a major role in some of these responses. In this study, using PDE4 inhibitors and PDE4-deficient mice and cells, we demonstrate that PDE4B is involved in modulating additional immune responses in these cells under endotoxin lipopolysaccharide (LPS) stimulation.
Activation of TLR4 by LPS is known to induce both pro-inflammatory and anti-inflammatory cytokine production in macrophages. Here in the first part of the study, we show that PDE4 inhibitors, such as rolipram, enhance the anti-inflammatory cytokine interleukin-1 receptor antagonist (IL-1Ra) secretion in LPS-stimulated mouse peritoneal macrophages, and this response was regulated at the transcriptional level rather than an increased IL-1Ra mRNA stability. Studies with PDE4-deficient macrophages revealed that the IL-1Ra upregulation elicited by LPS alone is PKA-independent, whereas the rolipram-enhanced response was mediated by inhibition of only PDE4B, one of the three PDE4 isoforms expressed in macrophages, and it requires PKA but not Epac activity. However, both pathways activate CREB to induce IL-1Ra expression. PDE4B ablation also promoted STAT3 phosphorylation (Tyr705) to LPS stimulation, but this STAT3 activation is not entirely responsible for the IL-1Ra upregulation in PDE4B-deficient macrophages. In a model of LPS-induced sepsis, only PDE4B-deficient mice displayed an increased circulating IL-1Ra, suggesting a protective role of PDE4B inactivation in vivo. These findings demonstrate that PDE4B negatively modulates anti-inflammatory cytokine expression in macrophages. In the second part of the study, we demonstrate that during in vitro differentiation of mouse bone marrow (BM) cells to immature DCs (imDCs), rolipram or ablation of PDE4B, but PDE4A or PDE4D, significantly reduced CD11c+ imDC population and the cell surface level of CD11c. The CD11c+ population was further decreased by rolipram and PDE4B ablation following LPS induction of DC maturation. Quantitative PCR analysis revealed that the mRNA expression of Toll-like receptor (TLR) 1, 2, 3, 6, 7, and 9 in BMDCs was markedly upregulated at 4 h of LPS stimulation and then declined to near or lower than basal levels at 24 or 36 h. Among these TLRs, the induction of TLR1, 6, 7, and 9 mRNA was greatly inhibited by rolipram and this inhibition was shown to be mediated mainly by inhibition of PDE4B, indicating an essential role of PDE4B in the expression of these innate immune receptors. In a murine model of TLR7 agonist-induced psoriasis, we further showed that PDE4 inhibitor significantly attenuated the severity of psoriatic symptoms in mice treated with imiquimod. These data demonstrate an important role of PDE4, particularly PDE4B, in regulation of DC development and functions under inflammatory conditions. Taken together, our findings provide further evidence that the development of PDE4B-selective inhibitor should retain the therapeutic benefits but devoid the side effects of non-selective PDE4 inhibitors. | en_US |