Interleukin-6 signal transduction in human intestinal epithelial cells

The gut is an important source of inflammatory cytokines, but there is scan t information on the mechanisms of cytokine action in gut epithelium. We hy pothesized that in human Caco-2 cells, IL-6 acts directly through stimulati on of Stat phosphorylation and that bacterial lipopolysaccharide (LPS) caus es Stat activation indirectly because of its ability to cause the autocrine secretion and action of interleukin (IL)-6. Stat1, Stat5a, and Stat5b, but not Stat3, were detected in Caco-2 cells. DNA-binding activity correspondi ng to activated Stat5 was stimulated in a biphasic manner by IL-6, with a t ransient early phase, followed by sustained activation between 8 and 48 h. LPS also stimulated Stat5-like binding, but there was no early phase of act ivation. Functional tests of Stat5 activation showed that IL-6 stimulated S tat5-dependent reporter gene transcription but had no effect on Stat1-depen dent transcription. LPS did not stimulate Stat-dependent transcription, nor did it alter the transcriptional response to IL-6. Tyrosine phosphorylatio n of both Stat5a and Stat5b was induced by IL-6. We infer from these data t hat IL-6 acts on intestinal epithelia through a Stat5-mediated transcriptio nal mechanism, whereas LPS does not induce gene expression through autocrin e activation of enterocyte Stat signaling. These data provide a basis for t esting the in vivo regulation of gut signaling and the interaction of gut r eticuloendothelial cells with epithelial signal transduction.