Ultraviolet-A-Induced Transactivation of the Vascular Endothelial Growth Factor Gene in HaCaT Keratinocytes is Conveyed by Activator Protein-2 Transcription Factor

Abstract

Ultraviolet-A radiation represents a significant proportion of the ultraviolet solar spectrum that was recently shown to affect gene expression of epidermal keratinocytes by molecular mechanisms distinct from ultraviolet-B radiation. As ultraviolet-A either alone or in combination with ultraviolet-B may contribute to photocarcinogenesis, we aimed to explore the biologic effects of ultraviolet-A radiation on vascular endothelial growth factor gene expression by the immortalized keratinocyte cell line HaCaT. As keratinocyte-derived vascular endothelial growth factor not only provides the major cutaneous angiogenic activity but may also augment the malignant phenotype of tumor cells, we studied the molecular mechanisms of ultraviolet-A-induced vascular endothelial growth factor expression in HaCaT cells, serving as a transformed preneoplastic epithelial cell line. Whereas ultraviolet-B-mediated vascular endothelial growth factor expression has been previously indicated to be conveyed by indirect mechanisms, ultraviolet-A rapidly induced vascular endothelial growth factor mRNA expression in a fashion comparable to that seen with the transforming growth factor alpha, representing a direct and potent activator of vascular endothelial growth factor gene transcription. Ultraviolet-A was found to readily induce vascular endothelial growth factor promoter-based reporter gene constructs through a consensus element for activator protein-2 transcription factor. The critical role of activator protein-2 was substantiated by demonstration of ultraviolet-A-induced activator-protein-2-dependent nuclear DNA binding activity to this site, and by inhibition of ultraviolet-A-mediated vascular endothelial growth factor gene transcription through insertion of a critical mutation within the activator protein-2 sequence. Together, our data further elucidate photobiologic aspects of ultraviolet-A-induced gene expression by characterizing mechanisms of vascular endothelial growth factor upregulation at the molecular level. In addition, our experiments support the concept of a more general importance of activator protein-2 in ultra- violet-A-mediated responses by keratinocytes or keratinocyte-derived cell lines.