cis-Urocanic acid initiates gene transcription in primary human keratinocytes

It is well established that solar UV radiation (UVR) suppresses cutaneous cell-mediated immunity in humans. trans-Urocanic acid (trans-UCA) is a major UVR-absorbing skin molecule that undergoes a photoisomerization to its cis-isomer following UVR exposure. Animal studies have demonstrated that cis-UCA plays a role in UVR-induced immune suppression, but the molecular mechanisms of action of cis-UCA are not fully understood. In this study, we examined changes in gene expression and synthesis of cytokines and PGE2 following UCA treatment of primary human keratinocytes. A limited microarray analysis of keratinocytes from two donors indicated that approximately 400 genes were induced by solar-simulated radiation (SSR), 16 of which were also up-regulated by cis-UCA. In contrast, trans-UCA had little or no effect on gene expression. The genes up-regulated by both cis-UCA and SSR were associated with apoptosis, cell growth arrest, cytokines, and oxidative stress. Further studies using primary keratinocytes from four new donors showed that PG-endoperoxide synthase-2 was dramatically induced by cis-UCA, resulting in an enhanced secretion of PGE2 into the cell culture supernatant. cis-UCA also increased cytokine protein production such as that of TNF-alpha, IL-6, and IL-8 in a dose-dependent manner. SSR had the same effect as cis-UCA, but trans-UCA had no effect. In addition, activation of NF-kappaB and lipid peroxidation were induced by cis-UCA and SSR, but not trans-UCA, suggesting possible upstream events of the gene expression changes. The data suggest that the induction of immune suppression by cis-UCA may involve the initiation of gene transcription of immunomodulatory mediators in primary human keratinocytes.