UVB and Proinflammatory Cytokines Synergistically Activate TNF-α Production in Keratinocytes through Enhanced Gene Transcription

UVB irradiation potently induces cytokines in the skin, including IL-1 alpha and tumor necrosis factor-alpha (TNF-alpha). The mechanism for TNF-alpha induction in UVB-irradiated keratinocytes is not clear. In this study, we explored the effects of UVB and cytokines, alone or in combination in human keratinocytes. Keratinocytes were sham-or UVB-irradiated with 30 mJ cm(-2), and then incubated in the absence or presence of IFN-alpha 2b, TNF-alpha, or IL-1 alpha. UVB and IL-1 alpha treatment synergistically enhanced TNF-alpha secretion and mRNA levels in human keratinocytes, similar to the findings reported previously in human fibroblasts. Exogenous recombinant TNF-alpha up-regulates its own mRNA level. However, addition of IFN-alpha 2b did not show any additive effect on TNF-alpha mRNA induction. To understand the regulation of TNF-alpha mRNA by UVB, with or without IL-1 alpha, we examined the transcription rate and half-life of TNF-alpha mRNA. Treatment of keratinocytes with IL-1 alpha or UVB alone increased TNF-alpha gene transcription 4- to 5-fold over sham treatment, and TNF-alpha gene transcription increased 11-fold in cells treated with UVB plus IL-1 alpha over sham. UVB with IL-1 alpha did not enhance the half-life of TNF-alpha mRNA over that seen with UVB alone. In conclusion, TNF-alpha expression in primary keratinocytes is upregulated transcriptionally by UVB and IL-1 alpha.