Quercetin potentiates UVB-induced c-Fos expression: Implications for its use as a chemopreventive agent

Quercetin (Qu) is currently being investigated as a chemopreventive agent for several cancers, including nonmelanoma skin cancer induced by UV light. We previously reported that Qu degradation has important consequences on signaling and cell biology. In the current study, we report that Qu induces c-Fos mRNA and protein expression through activation of p38 and cAMP-responsive element binding protein (CREB), and Qu potentiates UVB-induced c-Fos expression. Inclusion of ascorbic acid (AA) in cell culture medium stabilizes Qu and completely prevents both Qu- and UVB-induced p38 and CREB activation, leading to a blockade of c-fos gene expression through reduced CREB/cAMP-responsive element binding. AA stabilizes c-Fos mRNA, increasing steady-state levels even when c-fos gene expression is suppressed, but this has no effect on c-Fos protein levels in either mock- or UVB-irradiated cells. We report that Qu blocks mammalian target of rapamycin signaling and inhibits c-Fos protein expression directly through this mechanism because cotreatment with Qu and AA resulted in the complete suppression of UVB-induced c-Fos protein expression even in the presence of significantly increased mRNA levels. We further confirmed that this was not due to increased protein turnover because inhibition of proteasome activity with MG-132 did not raise c-Fos protein levels in Qu+AA-treated cells. Together, these data indicate that although Qu has been reported to have some beneficial properties as a chemopreventive agent, it is also capable of inducing c-fos expression, a cellular event important for the promotion phase of tumor development, if it is not stabilized.