Epithelial cells both produce and are affected by interleukin-6 (IL-6). Experiments with an adenocarcinoma-derived cell line (HeLa) reveal that activation of the transfected human IL-6 promoter occurs largely through two partially overlapping second messenger (cAMP, phorbol ester)- and cytokine (IL-1, TNF, serum)-responsive enhancer elements (MRE 1, -173 to -151 and MRE II, -158 to -145). MRE I contains the typical GACGTCA cAMP and phorbol ester-responsive (CRE-TRE) motif, whereas MRE II defines a new CRE/TRE motif that contains an imperfect dyad repeat. The mechanism of dexamethasone-mediated repression of IL-6 gene expression in epithelial cells involves occlusion of the entire MRE enhancer region and of the core-promoter elements (TATA-box and RNA start site) by ligand-activated glucocorticoid receptor. Enhanced levels of IL-6 expression are observed in many solid tumors and in the hyperproliferative (and glucocorticoid-suppressible) lesions of psoriasis. In cell culture, IL-6 enhances, inhibits, or has no effect on the proliferation of epithelial cells depending upon the cell-type examined. IL-6 enhances proliferation of keratinocytes but inhibits that of breast carcinoma cell lines ZR-75-1 and T-47D. In these breast carcinoma cells, IL-6 elicits a major change in cell phenotype which is characterized by a fibroblastoid morphology, enhanced motility, increased cell-cell separation, and decreased adherens type junctions (desmosomes and focal adhesions). The new data identify IL-6 as a regulator of epithelial cell growth and of cell-cell association.