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Human conditions of elevated interleukin-6 (IL-6) and transgenic mice overexpressing IL-6 have increased proteolytic degradation of insulin-like growth factor binding protein (IGFBP)-3. In addition, IL-6 alters the hepatic expression of insulin-like growth factor-I (IGF-I) and the IGFBPs in vitro. The aim of the present study was to investigate whether moderately elevated IL-6 levels have short-term effects on circulating IGF-I, IGFBP-1 and IGFBP-3 proteolysis in vivo. Healthy men received a 3-h IL-6 (n = 6) or saline (n = 6) infusion and blood samples were collected prior to and up to 8 h after the start of infusion. Free IGF-I, total IGF-I, IGFBP-1, insulin and cortisol were measured using immunoassays. Serum IGFBP-3 proteolysis was analyzed by Western immunoblot and by in vitro degradation of 125I-IGFBP-3. We found that IL-6 concentrations reaching approximately 100 pg/ml significantly increased IGFBP-1 after the end of infusion in the absence of changes in insulin. In addition, plasma levels of cortisol were increased in response to IL-6 during and after infusion compared to saline. There was no effect of IL-6 on IGFBP-3 proteolysis, total IGF-I or free dissociable IGF-I. These data suggest that moderately elevated levels of IL-6 such as in the post-operative state or after exercise may contribute to increased levels of IGFBP-1. Although this study does not exclude that high levels and/or prolonged exposure to IL-6 may induce IGFBP-3 proteolysis in sepsis or chronic inflammatory disease, it suggests that IL-6 released from exercising skeletal muscle is not directly involved in proteolysis of circulating IGFBP-3. Copyright © 2006 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]