Interleukin-6 trans-signaling and colonic cancer associated with inflammatory bowel disease

Interleukin-6 (IL-6) is a cytokine largely induced during infection, inflammation, and cancer. In the liver, IL-6 induces the synthesis of acute-phase proteins, which are believed to support the response of the body during infection and inflammation. Moreover, IL-6 has been reported to be a growth factor in multiple myeloma. IL-6 on cells binds to an IL-6 receptor (IL-6R) forming an IL-6/IL-6R complex, which associates with a homodimer of a second receptor subunit, gp130, to initiate signaling. Gp130 is present on all cells of the body, whereas IL-6R is only expressed on hepatocytes, some leukocytes, and some epithelial cells. Since gp130 has no measurable affinity for IL-6, cells which do not express IL-6R are unresponsive to the cytokine. A soluble form of IL-6R, which is found in the blood, can still bind IL-6 and the complex of IL-6/sIL-6R can bind to cellular gp130 also on cells without IL-6R expression. This signaling mechanism has been called trans-signaling. Interestingly, a soluble form of gp130 (sgp130) blocks IL-6 trans-signaling without affecting classic IL-6 signaling via the membrane-bound IL-6R. We used a dimerized version of sgp130 fused to the Fc portion of an IgG1 antibody (sgp130Fc) to discriminate between classic and trans-signaling of IL-6. It turned out that proinflammatory activities of IL-6 are mediated via trans-signaling, whereas anti-inflammatory or regenerative activities are mediated via classic signaling. These results are important for strategies to inhibit IL-6 signaling in autoimmune diseases such as rheumatoid arthritis, inflammatory bowel disease, and inflammation-associated colorectal cancer.