Apigenin, a chemopreventive flavonoid, enhances p53 expression through RNA-binding protein HuR

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Basal cell carcinomas (BCC) and squamous cell carcinomas (SCC), commonly referred to as nonmelanoma skin cancers, are the most diagnosed cancers in the United States. Many generic and environmental factors contribute to these keratinocyte-derived tumors, and exposure to ultraviolet (UV) radiation is considered as the major causal factor. Apigenin is a naturally occurring and nonmutagenic flavonoid abundantly present in fruits and vegetables. Previous studies have shown that apigenin applied topically prevents mouse skin carcinogenesis induced by both chemical carcinogens and UV exposure. We subsequently demonstrated that one important aspect of apigenin’s chemopreventive action is due to its ability to induce p53 expression by increasing protein stability. In the present report, we further demonstrated that the increase in p53 protein level induced by apigenin treatment of mouse 308 keratinoyctes was not due to enhanced p53 transcription, mRNA stabilization or cytoplasmic export of p53 mRNA. Instead, we found that apigenin increased nascent p53 protein synthesis, indicating an enhancement of p53 translation. The AU-rich element (ARE) within the 3’-untranslated region of p53 mRNA was found to be a target of HuR, an ARE-containing RNA-binding protein, and apigenin treatment augmented the binding of HuR to endogenous p53 mRNA. Apigenin treatment also increased HuR translocation into the cytoplasm. Over-expression of HuR in 308 cells enhanced apigenin-induced p53 expression, whereas siRNA-mediated HuR reduction suppressed apigenin-induced p53 expression and nascent p53 protein synthesis. Furthermore, apigenin treatment induced the expression of p16, a cyclin-dependent kinase inhibitor, which in turn regulated the cytoplasmic localization of HuR induced by apigenin. In short, these results indicate that, in addition to modulating p53 protein stability, one of the mechanisms by which apigenin induces p53 expression is enhancement of translation through RNA binding protein HuR.