It has previously been demonstrated that interleukin-1 (IL-I) is expressed in a variety of fibroblast cell lines. In this study, we investigated the mechanisms involved in the regulation of IL-1 p production by cultured human dermal fibroblasts. We have shown that IL-I p is constitutively expressed as a cell-associated form, with no soluble form detectable in control cell or in stimulated cell supernatants. IL-la and tumor necrosis factor-a (TNF-a) exerted a dose-dependent stimulation on the production of the cell-associated IL-1 p, as estimated using a specific enzyme linked immunosorbent assay (ELISA). As expected, this effect was accompanied by a huge release of prostaglandin E, (PGE, ) and a transient rise in intracellular cyclic AMP. Furthermore, IL-1 p production was elevated to a lesser extent by the addition of increasing concentrations of the protein kinase C activator phorbol myristate acetate or by low concentration (0.001 kg/ml) of PGE,. In contrast, higher concentrations (0.1 and 1 pg/ml) of PGE,, as well as exogenous dibutyryl-cyclic AMP, were clearly inhibitory. H7, an inhibitor of protein kinases also reduced the stimulatory effect of IL-la and TNF-a. Together with the results obtained with phorbol myristate acetate, these data suggest that protein kinase C may play a role in the upregulation of IL-1 p expression in normal skin fibroblasts. The addition of indomethacin not only suppressed prostaglandin synthesis, but also dramatically reduced cyclic AMP formation, probably because the PGE,-induced stimulation of adenylate cyclase was abolished. This resulted in a strong potentiation of the stimulatory effect of I L -l a and TNF-a, supporting the role of both the cyclooxygenase and adenylate cyclase pathways in the endogenous downregulation of IL-1 p induction by the two cytokines studied.