Glucocorticoids increase interleukin-6-dependent gene induction by interfering with the expression of the suppressor of cytokine signaling 3 feedback inhibitor.

UNLABELLED: Glucocorticoids are known to be potent regulators of inflammation and have been used pharmacologically against inflammatory, immune, and lymphoproliferative diseases for more than 50 years. Due to their possible and well-documented side effects, it is crucial to understand the molecular mechanisms and targets of glucocorticoid action in detail. Several modes of action have been discussed; nevertheless, none of them fully explain all the functions of glucocorticoids. Therefore, we analyzed the cross-talk between glucocorticoids and interleukin-6 (IL-6) in the liver. IL-6 exerts pro-inflammatory as well as anti-inflammatory properties and is a main inducer of the acute-phase response. The balance between the proinflammatory and anti-inflammatory activities of IL-6 is tightly regulated by suppressor of cytokine signaling 3 (SOCS3), a well-known feedback inhibitor of IL-6 signaling. Here, it is demonstrated that glucocorticoids enhance IL-6-dependent γ-fibrinogen expression. Studying of the underlying mechanism revealed prolonged activation of signal transducer and activator of transcription 3 (STAT3) caused by down-regulation of SOCS3 protein expression. Consequently, in SOCS3-deficient cells glucocorticoids do not affect IL-6-induced signal transduction. Moreover, in hepatocytes lacking the SOCS3 recruiting motif within gp130, IL-6-dependent γ-fibrinogen expression is not influenced by glucocorticoid treatment. CONCLUSION: Glucocorticoids interfere with IL-6-induced expression of the feedback inhibitor SOCS3, thereby leading to enhanced expression of acute-phase genes in hepatocytes. This mechanism contributes to the explanation of how glucocorticoids affect inflammation and acute-phase gene induction.

Distinct functions of the mitogen-activated protein kinase-activated protein (MAPKAP) kinases MK2 and MK3: MK2 mediates lipopolysaccharide-induced signal transducers and activators of transcription 3 (STAT3) activation by preventing negative regulatory effects of MK3.

In LPS-treated macrophages, activation of STAT3 is considered to be crucial for terminating the production of inflammatory cytokines. By analyzing the role of MAPK-activated protein kinase (MK) 2 and MK3 for LPS-induced STAT3 activation in macrophages, the present study provides evidence that MK2 is crucial for STAT3 activation in response to LPS because it prevents MK3 from impeding IFNß gene expression. Accordingly, LPS-induced IFNß gene expression is down-regulated in MK2-deficient macrophages and can be reconstituted by additional ablation of the MK3 gene in MK2/3(-/-) macrophages. This is in contrast to LPS-induced IL-10 expression, which essentially requires the presence of MK2. Further analysis of downstream signaling events involved in the transcriptional regulation of IFNß gene expression suggests that, in the absence of MK2, MK3 impairs interferon regulatory factor 3 protein expression and activation and inhibits nuclear translocation of p65. This inhibition of p65 nuclear translocation coincides with enhanced expression and delayed degradation of IκBß, whereas expression of IκBα mRNA and protein is impaired in the absence of MK2. The observation that siRNA directed against IκBß is able to reconstitute IκBα expression in MK2(-/-) macrophages suggests that enhanced expression and delayed degradation of IκBß and impaired NFκB-dependent IκBα expression are functionally linked. In summary, evidence is provided that MK2 regulates LPS-induced IFNß expression and downstream STAT3 activation as it restrains MK3 from mediating negative regulatory effects on NFκB- and interferon regulatory factor 3-dependent LPS signaling.

Oncostatin M regulates SOCS3 mRNA stability via the MEK-ERK1/2-pathway independent of p38(MAPK)/MK2.

The induction of suppressor of cytokine signalling (SOCS)3 expression context dependently involves regulation of SOCS3 transcript stability as previously demonstrated for MAPK activated protein kinase (MK)2-dependent regulation of SOCS3 expression by TNFα (Ehlting et al., 2007). In how far the IL-6-type cytokine OSM, which in contrast to IL-6 is a strong activator of p38(MAPK)/MK2 signalling, also involves regulation of transcript stability and activation of MK2 to induce SOCS3 expression is unclear. In contrast to IL-6, OSM induces SOCS3 expression in murine fibroblasts and in primary human and murine hepatocytes, but not in macrophages because the latter lack the OSM receptor (OSMR)ß subunit. Evidence is provided that regulation of OSM-induced expression of SOCS3 involves MEK1- and Erk1/2-mediated stabilization of the SOCS3 transcript. Consistently, OSM-induced stabilization of the SOCS3 transcript is impaired in the presence of inhibitors that specifically block activation of MEK1/2 (U0126) and ERK1/2 (FR180204) or upon knock-down of ERK1/2 expression using specific siRNA. As a potential target site that integrates the stability regulating effect of OSM and OSM-induced activation of MEK1/2 and ERK1/2 a region containing three copies of a pentameric AUUUA motif located within position 2422 and 2541 in closed proximity to the 3' UTR of the SOCS3 transcript has been identified. Unexpectedly, activation of the p38(MAPK)/MK2 pathway, which apart from STAT3 and ERK1/2, is also strongly activated by OSM in human and murine hepatocytes and murine fibroblasts is dispensable for stabilization of the SOCS3 transcript as suggested from inhibitor studies using the p38(MAPK) inhibitor SB203580 or from the analysis of MK2-deficient hepatocytes. However, analysis of MK2-deficient macrophages and hepatocytes revealed that, although MK2 is dispensable for regulation of OSM-induced SOCS3 expression, MK2 is essential for LPS-induced OSM production in macrophages and limits the overall availability of the OSMRß subunit in hepatocytes. Thus MK2 plays a role for the induction and sensing of OSM-mediated intercellular signalling between macrophages and hepatocytes during LPS-induced inflammation.