The Signal Transduction Mechanisms Of UVB-induced VEGF Expression

Ultraviolet B (UVB, 280-320 nm) irradiation is a key environmental factor for skin photodamage. Short-timed exposure to UVB results in the formation of erythema, edema and peeling while prolonged exposure would contribute to the acceleration of skin aging and carcinogenesis. Besides the major role in causing DNA damages, UVB may also regulate the secretion and expression of several inflammation factors. As an important cytokine among these factors, vascular endothelial growth factor (VEGF) has well been known to play a major role in the formation of new blood vessels, increase of vascular permeability and production of vascular. Therefore, VEGF plays a critical role in UVB-induced photodamages. Though the molecular mechanisms of UVB-induced VEGF expression are still largely unknown, more and more results show that the blockade of VEGF activity could significantly decrease skin photosensitivity to UVB. So the discovery of the related signaling pathways and elucidation of molecular mechanisms of UVB-induced VEGF expression will be greatly help for clinical application for acute and chronic UVB irradiation.In this study, we investigated the signal transduction pathways of VEGF expression in UVB-treated mouse embryonic fibroblasts (MEFs). Currently, two mechanisms (direct and indirect) of UVB-induced VEGF expression are suggested. According to the direct manner, UVB could directly up-regulate VEGF expression by activating several transcription factors. As to the indirect manner, it was implicated that TNFαmediate VEGF expression without the connection to its receptor.In the first part, we demonstrated that UVB could significantly induce VEGF expression in mouse embryo fibroblasts (MEFs). We found that the induction of VEGF upon UVB exposure is IKKα-dependent but independent of the activation of IKKαand NF-αB. By bioinformatics analysis we identified three potential AP-1-binding sites within the vegf promoter. We demonstrate that AP-1 is one of the downstream targets of IKKαin mediating VEGF induction after UVB exposure, and the transcription factors c-Jun and c-Fos are the major components of activated AP-1 proteins. However, HIF1α, which plays a critical role in upregulating the VEGF expression in tumor cells, is not involved in UVB-induced VEGF expression. Subsequently, we investigated the relationship between IKKαand AP-1 activation in mediating VEGF expression under UVB exposure. Our results showed that IKKαplays a critical role in c-Fos induction upon UVB exposure, and c-Fos and c-Jun could bind to AP-1-binding sites within the vegf promoter. Thus our results reveal that IKKα/c-Fos/AP-1 pathway is involved in VEGF induction in response to UVB exposure.Previous studies indicate that IKKαcould solely control the expression of target genes through its nuclear localization, direct association with the NF-αB-responsive promoters and induction of the phosphorylation of histone H3, leading to the activation of the NF-αB-regulated gene expression without the involvement of IKKα. We speculated that IKKαmight also act similarly in UVB-induced VEGF expression.In our study, we found that IKKαwas remarkably phosphorylated after UVB exposure and interacted with c-Fos. By ChIP assays, we demonstrated that IKKαand c-Fos could be simultaneously recruited to three potential AP-1-binding sites in the mouse vegf promoter. Because nuclear kinase could associate with the chromatin and function in the nucleus, we suggested that IKKαcould induce the expression of VEGF by phosphorylating the vegf promoter-associated histone H3 in response to UVB irradiation.Taken together, we reveal a novel pathway though which IKKαmediate VEGF induction after UVB exposure. This discovery not only benefits to the basic research in the field of IKK/NF-κB signal transduction, but also provides a new model of IKKα-regulated inflammation.As a dominant regulatory subunit of PI-3 kinase, p85αis more abundant than the catalytic subunit p110 of PI-3 kinase. Though under most conditions p85αinteract with p110 to activate their downstream targets, monomeric p85αhas also been proven to play specific biological functions independently. Our previous study have disclosed a p85α-dependent but PI-3K-independent apoptotic response upon UVB exposure and revealed a novel p85α/NFAT3/TNFαsignaling pathway.In the second part, we demonstrated that TNFαsecretion induced by p85αis also involved in UVB-induced VEGF expression. We found that p85αmediated VEGF expression in response to UVB exposure, and NFAT3 activity was also involved in the process of p85α-mediated VEGF expression. In addition, the induction of TNFαaffected UVB-induced VEGF expression. Our results indicated that UVB could up-regulate VEGF expression by activating p85α/NFAT3/TNFαsignal transduction pathway as well. Thus, the data reveal the role of monomeric p85αin inflammatory responses and related mechanisms.In conclusion, this thesis reveals two novel signal transduction pathways related to UVB-induced VEGF expression. The data illustrate the new function models of two important proteins, IKKαand p85α, in the process of VEGF induction. Our study may provide new clues for the defend strategy for UVB irradiation-damages.