Translation Regulation of UV-induced Transcription Factor NF-κB and Oncogene COX-2

NF-κB plays an important role in ultraviolet light-induced skin tumorigenesis. Activation of NF-κB by UV-irradiation is composed of two phases. The early-phase culminates with maximal levels of DNA binding ability at 4 hours post-irradiation and is dependent on translational inhibition. The late-phase activation of NF-κB occurs between 16 and 48 hours post-irradiation and the mechanism is not clear due to the fact that NF-κB was activated even in the presence of high level of its inhibitor protein, IκBα. Here, we provide evidence that in the late-phase of UV-induced NF-κB activation, IκBα depletion is the combined result of regulation at both transcriptional and translational levels. Neither ubiquitination nor proteasomal degradation have detectable attributions to IκBα breakdown. We also demonstrate that UV only induces phosphorylation of p65 at Ser 276, while TNFα induced phosphorylation at both Ser 276 and 536 sites of p65. Based upon our results, we propose a novel mechanism for translation-regulated IκBα depletion and MSK-mediated NF-κB activation at 24 hours post UV-irradiation.

Besides NF-κB activation, ultraviolet light also induces a prolonged expression of COX-2. While transcriptional regulation of COX-2 expression is intensively studied, the role of translational regulation of COX-2 synthesis upon UV-irradiation is not yet clear. Here, we show that phosphorylation of the alpha subunit of the eukaryotic initiation factor 2 (eIF2α) plays an important role in the regulation of COX-2 expression after UV-irradiation. Our data shows that UV light induces COX-2 expression in wild-type mouse embryo fibroblasts (MEF^S/S) and that the inducibility is reduced in MEF^A/A cells in which the phosphorylation site, Ser-51 in the eIF2α, is replaced with a nonphosphorylatable Ala (S51A). UV light-induced transcription of COX-2 is delayed in MEF^A/A cells, which correlates with NF-κB activation, as previously reported. Our data also shows that translational efficiency of COX-2 is higher in MEF^A/A cells than in MEF^S/S cells, but not at the late stage of UV-irradiation. This may be due to the translational regulation of COX-2 binding protein TIAR expression, which is reduced in MEF^S/S cells but not in MEF^A/A cells at 24 hours post-UV. In addition, our data indicates that newly synthesized COX-2 protein is more stable in MEF^A/A cells than in MEF^S/S cells. These results suggest that translation initiation plays a role in the complex and dynamic regulation of COX-2 expression. Based on our results, and the use of Ingenuity Pathway Analysis™, we propose a novel eIF2α phosphorylation-centered network for the regulation of COX-2 expression after UV-irradiation.