A critical role of tumor necrosis factor (TNF)-alpha in irritant contact dermatitis and in the challenge phase of allergic contact dermatitis has recently been demonstrated in vivo. As in situ hybridization studies have indicated that keratinocytes were the cellular source of TNF-alpha in these reactions, we studied the mechanisms of TNF-alpha mRNA induction in keratinocytes by agents that induce contact dermatitis. Murine la-/CD3- epidermal cells were stimulated with phorbol myristate acetate (PMA), dimethylsulfoxide (DMSO), sodium dodecyl sulfate (SDS) and NiSO4, all of which up-regulated epidermal cell TNF-alpha mRNA production. In contrast, trinitrobenzenesulfonic acid and trinitrochlorobenzene did not significantly up-regulate TNF-alpha mRNA. These results were confirmed with murine keratinocyte cell lines. In keratinocytes transfected with a chloramphenicol acetyltransferase construct containing the -1059 to +138 base pair TNF-alpha promoter, increased promoter activity was observed upon stimulation with PMA and DMSO. In addition, PMA stimulation did not affect the stability of TNF-alpha mRNA. The PMA- but also the DMSO- and SDS- induced up-regulation of TNF-alpha mRNA was abolished by an inhibitor of protein kinase C (PKC). In contrast, NiSO4 up-regulated TNF-alpha mRNA by a PKC-independent mechanism, did not increase TNF-alpha promoter activity, but markedly increased the stability of the TNF-alpha mRNA. Co-stimulation with PMA and NiSO4 induced a marked increase in TNF-alpha mRNA over that obtained with each agent alone. Thus, whereas PKC-dependent irritants act by up-regulating TNF-alpha promoter activity, nickel acts via post-transcriptional regulation. Our results also establish that some irritants and irritant sensitizers directly induce TNF-alpha in keratinocytes without intermediate Langerhans cell-derived signals.