During systemic inflammation the liver becomes unresponsive to GH resulting in decreased plasma insulin-like growth factor-I (IGF-I) with concomitant reductions in lean body mass. Transgenic mice that overexpress IL-6 also demonstrate impaired growth and decreased IGF-I. To determine whether IL-6 directly inhibits GH-inducible gene expression, CWSV-1 hepatocytes were incubated with IL-6 (10 ng/ml), then stimulated with rhGH (500 ng/ml, 18 h). The increase in IGF-I and serine protease inhibitor 2.1 (Spi 2.1) mRNA in GH-treated cells was inhibited by treatment with IL-6 for 24 h. To investigate potential mechanisms we examined the effects of IL-6 on GH receptor (GHR) expression and GH signaling via the JAK/STAT and MAPK pathways. Incubation of cells with IL-6 (10 ng/ml, 24 h) had no effect on GHR abundance or signaling proteins JAK2, Stat5b and ERK1/2. Although GH transiently increased (2 to 5-fold) the tyrosine phosphorylation of GHR, JAK2, Stat5b and ERK1/2, IL-6 did not alter these phosphorylation events. However, nuclear protein from IL-6-treated cells demonstrated reduced Stat5-DNA binding (by EMSA) at 15 min (-20%) and 60 min (-43%) after GH-stimulation. To determine whether IL-6 inhibits GH-inducible promoter activity CWSV-1 cells were transfected with Spi 2.1 or prolactin (PRL) receptor (PRLr) promoter luciferase vectors, incubated ± IL-6, then stimulated with GH. The induction of both Spi 2.1 (7.5-fold) and PRLr (4-fold) promoter activity by GH was inhibited by IL-6. In summary, IL-6 mediates hepatic GH resistance by a time-dependent inhibition of GH-inducible promoter activity that is associated with reductions in Stat5-DNA binding.