Nickel and skin irritants up-regulate tumor necrosis factor-α mRNA in keratinocytes by different but potentially synergistic mechanisms

A critical role of tumor necrosis factor (TNF)-α 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-α in these reactions, we studied the mechanisms of TNF-α mRNA induction in keratinocytes by agents that induce contact dermatitis. Murine Ia^-;/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-α mRNA production. In contrast, trinitrobenzenesulfonic acid and trinitrochlorobenzene did not significantly up-regulate TNF-α 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-α promoter, increased promoter activity was observed upon stimulation with PMA and DMSO. In addition, PMA stimulation did not affect the stability of TNF-α mRNA. The PMA- but also the DMSO- and SDSinduced up-regulation of TNF-α mRNA was abolished by an inhibitor of protein kinase C (PKC). In contrast, NISO4 up-regulated TNF-α mRNA by a PKC-independent mechanism, did not increase TNF-α promoter activity, but markedly increased the stability of the TNF-α mRNA. Co-stimulation with PMA and NISO₄ induced a marked increase in TNF-a mRNA over that obtained with each agent alone. Thus, whereas PKC-dependent irritants act by up-regulating TNF-α promoter activity, nickel acts via post-transcrlptional regulation. Our results, also establish that some irritants and irritant sensitizers directly induce TNF-α in keratinocytes without intermediate Langerhans cell derived signals.