Interleukin-6 is a cytokine not only involved in inflammation and infection responses but also in the regulation of metabolic, regenerative, and neural processes. In classic signaling, interleukin-6 stimulates target cells via a membrane bound interleukin-6 receptor, which upon ligand binding associates with the signaling receptor protein gp130. Gp130 dimerizes, leading to the activation of Janus kinases and subsequent phosphorylation of tyrosine residues within the cytoplasmic portion of gp130. This leads to the engagement of phosphatase Src homology domains containing tyrosin phosphatase-2 (SHP-2) and activation of the ras/raf/Mitogen-activated protein (MAP) kinase (MAPK) pathway. In addition, signal transducer and activator of transcription factors are recruited, which are phosphorylated, and consequently dimerize whereupon they translocate into the nucleus and activate target genes. Interestingly, only few cells express membrane bound interleukin-6 receptor whereas all cells display gp130 on the cell surface. While cells, which only express gp130, are not responsive to interleukin-6 alone, they can respond to a complex of interleukin-6 bound to a naturally occurring soluble form of the interleukin-6 receptor. Therefore, the generation of soluble form of the interleukin-6 receptor dramatically enlarges the spectrum of interleukin-6 target cells. This process has been named trans-signaling. Here, we review the involvement of both signaling modes in the biology of interleukin-6. It turns out that regenerative or anti-inflammatory activities of interleukin-6 are mediated by classic signaling whereas pro-inflammatory responses of interleukin-6 are rather mediated by trans-signaling. This is important since therapeutic blockade of interleukin-6 by the neutralizing anti-interleukin-6 receptor monoclonal antibody tocilizumab has recently been approved for the treatment of inflammatory diseases. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Summary Colitis associated cancer (CAC) is the most serious complication of inflammatory bowel disease. Pro-inflammatory cytokines were suggested to regulate pre-neoplastic growth during CAC tumorigenesis. Interleukin 6 (IL-6) is a multifunctional NF-κB–regulated cytokine which acts on epithelial and immune cells. Using genetic tools we now demonstrate that IL-6 is a critical tumor promoter during early CAC tumorigenesis. In addition to enhancing proliferation of tumor initiating cells, IL-6 produced by lamina propria myeloid cells protects normal and pre-malignant intestinal epithelial cells (IEC) from apoptosis. The proliferative and survival effects of IL-6 are largely mediated by transcription factor STAT3, whose IEC-specific ablation has profound impact on CAC tumorigenesis. Thus, the NF-κB-IL-6-STAT3 cascade is an important regulator of the proliferation and survival of tumor initiating IEC.

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Cytokine receptors, which exist in membrane-bound and soluble forms, bind their ligands with comparable affinity. Although most soluble receptors are antagonists and compete with their membrane-associated counterparts for the ligands, certain soluble receptors are agonists. In these cases, complexes of ligand and soluble receptor bind on target cells to second receptor subunits and initiate intracellular signaling. The soluble receptors of the interleukin (IL)-6 family of cytokines (sIL-6R, sIL-11R, soluble ciliary neurotrophic factor receptor) are agonists capable of transmitting signals through interaction with the universal signal-transducing receptor for all IL-6 family cytokines, gp130. In vivo, the IL-6/sIL-6R complex stimulates several types of cells, which are unresponsive to IL-6 alone, as they do not express the membrane IL-6R. We have named this process trans-signaling. The generation of soluble cytokine receptors occurs via two distinct mechanisms-limited proteolysis and translation-from differentially spliced mRNA. We have demonstrated that a soluble form of the IL-6 family signaling receptor subunit gp130, which is generated by differential splicing, is the natural inhibitor of IL-6 trans-signaling responses. We have shown that in many chronic inflammatory diseases, including chronic inflammatory bowel disease, peritonitis, rheumatoid arthritis, asthma, as well as colon cancer, IL-6 trans-signaling is critically involved in the maintenance of a disease state, by promoting transition from acute to chronic inflammation. Moreover, in all these models, the course of the disease can be disrupted by specifically interfering with IL-6 trans-signaling using the soluble gp130 protein. The pathophysiological mechanisms by which the IL-6/sIL-6R complex regulates the inflammatory state are discussed.

Eukaryotes possess mechanisms to limit crossing over during mitotic homologous recombination, thus avoiding possible chromosomal rearrangements. We show here that budding yeast Mph1, an ortholog of human FancM helicase, utilizes its helicase activity to suppress spontaneous unequal sister chromatid exchanges and DNA double-strand break-induced chromosome crossovers. Since the efficiency and kinetics of break repair are unaffected, Mph1 appears to channel repair intermediates into a noncrossover pathway. Importantly, Mph1 works independently of two other helicases-Srs2 and Sgs1-that also attenuate crossing over. By chromatin immunoprecipitation, we find targeting of Mph1 to double-strand breaks in cells. Purified Mph1 binds D-loop structures and is particularly adept at unwinding these structures. Importantly, Mph1, but not a helicase-defective variant, dissociates Rad51-made D-loops. Overall, the results from our analyses suggest a new role of Mph1 in promoting the noncrossover repair of DNA double-strand breaks.[Keywords: Genome instability; recombination; DNA helicase; crossing over; Fanconi anemia] Supplemental material is available at http://www.genesdev.org. Received September 8, 2008; revised version accepted November 12, 2008. DNA double-strand breaks (DSBs) that arise during DNA replication or are induced by DNA damaging agents, such as ionizing radiation, are frequently repaired by homologous recombination (HR). In yeast and other eukaryotes, the RAD52 epistasis group of proteins mediate homologous recombination (for reviews, see Paques and Haber 1999;Krogh and Symington 2004). In this process, DSB ends are resected by nucleases to create 39 ssDNA that becomes coated with the recombinase protein Rad51. The Rad51-ssDNA nucleoprotein filament then carries out a search for a homologous donor DNA sequence and promotes strand invasion of the donor molecule to form a D-loop (Sung and Klein 2006). After D-loop formation, there appear to be two alternative pathways that result in DSB repair. One pathway involves the formation of a double Holliday junction (dHJ) that can be resolved by symmetrical strand cleavage into either a crossover or noncrossover gene conversion (Szostak et al. 1983). An alternative mechanism, called synthesis-dependent strand annealing (SDSA), posits formation mostly of noncrossovers, and in most cases does not involve a dHJ intermediate (for review, see Paques and Haber 1999). In support of the SDSA model of gene conversion, we showed recently that both newly synthesized strands are inherited by the broken recipient DNA molecule (Ira et al. 2006). The choice between crossover and noncrossover is tightly regulated in both mitotic and meiotic cells. In meiotic S. cerevisiae cells the correct number of crossovers are required for proper chromosome segregation; the proportion of gene conversion accompanied by crossing over varies between 25% and 50% depending on the locus (Roeder 1995). In mitotic cells, the proportion of crossovers is much lower, ranging between <1% and 7 These authors c...

Molecular danger signals attract neutrophilic granulocytes (polymorphonuclear leukocytes (PMNs)) to sites of infection. The G protein-coupled receptor (GPR) 43 recognizes propionate and butyrate and is abundantly expressed on PMNs. The functional role of GPR43 activation for in vivo orchestration of immune response is unclear. We examined dextrane sodium sulfate (DSS)-induced acute and chronic intestinal inflammatory response in wild-type and Gpr43-deficient mice. The severity of colonic inflammation was assessed by clinical signs, histological scoring, and cytokine production. Chemotaxis of wild-type and Gpr43-deficient PMNs was assessed through transwell cell chemotactic assay. A reduced invasion of PMNs and increased mortality due to septic complications were observed in acute DSS colitis. In chronic DSS colitis, Gpr43−/− animals showed diminished PMN intestinal migration, but protection against inflammatory tissue destruction. No significant difference in PMN migration and cytokine secretion was detected in a sterile inflammatory model. Ex vivo experiments show that GPR43-induced migration is dependent on activation of the protein kinase p38α, and that this signal acts in cooperation with the chemotactic cytokine keratinocyte chemoattractant. Interestingly, shedding of L-selectin in response to propionate and butyrate was compromised in Gpr43−/− mice. These results indicate a critical role for GPR43-mediated recruitment of PMNs in containing intestinal bacterial translocation, yet also emphasize the bipotential role of PMNs in mediating tissue destruction in chronic intestinal inflammation.

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