Tumor necrosis factor-α induces distinctive NF-κB signaling within human dermal fibroblasts

The TNF-alpha receptor-associated factor 2 (TRAF2) and its downstream mediator, the NF-kappaB-inducing kinase (NIK), have been shown to induce NF-kappaB activation in 293 cells. Investigating the role these mediators play in human skin, we found that both NIK and TRAF2 failed to evoke transcription from NF-kappaB dependent promoters linked to the CAT reporter in human dermal fibroblast cultures, while epidermal keratinocyte cultures demonstrated NIK-dependent signaling. Further, NF-kappaB activation by TNF-alpha was unaffected by overexpression of a dominant negative mutant NIK in fibroblasts, despite detection of endogenous TRAF2 and NIK by Western analysis. To explore alternative signaling mechanisms in dermal fibroblasts, we found that the intracellular calcium chelator, 3,4,5-trimethoxybenzoic acid, and the calpain inhibitor, N-acetyl-Leu-Leu-norleucinal, both blocked NF-kappaB activation; however, the specific proteosome inhibitor, lactacystin, failed to do so. Furthermore, TNF-alpha receptor mutants lacking a functional death domain failed to stimulate NF-kappaB, while phosphatidylcholine-phospholipase C inhibition and alkalization of endolysosomal compartments blocked its activation by TNF-alpha. These data indicate that, while epidermal keratinocytes utilize previously defined, NIK-dependent NF-kappaB pathways, dermal fibroblasts demonstrate unique NIK/TRAF2 independent signal transduction, where both acidic sphingomyelinase and calpain activity act as surrogate mediators for NF-kappaB activation.