Apigenin Prevents UVB-Induced Cyclooxygenase 2 Expression: Coupled mRNA Stabilization and Translational Inhibition

Abstract

Cyclooxygenase 2 (COX-2) is a key enzyme in the conversion of arachidonic acid to prostaglandins, and COX-2 overexpression plays an important role in carcinogenesis. Exposure to UVB strongly increased COX-2 protein expression in mouse 308 keratinocytes, and this induction was inhibited by apigenin, a nonmutagenic bioflavonoid that has been shown to prevent mouse skin carcinogenesis induced by both chemical carcinogens and UV exposure. Our previous study suggested that one pathway by which apigenin inhibits UV-induced and basal COX-2 expression is through modulation of USF transcriptional activity in the 5 upstream region of the COX-2 gene. Here, we found that apigenin treatment also increased COX-2 mRNA stability, and the inhibitory effect of apigenin on UVB-induced luciferase reporter gene activity was dependent on the AU-rich element of the COX-2 3-untranslated region. Furthermore, we identified two RNA-binding proteins, HuR and the T-cell-restricted intracellular antigen 1-related protein (TIAR), which were associated with endogenous COX-2 mRNA in 308 keratinocytes, and apigenin treatment increased their localization to cell cytoplasm. More importantly, reduction of HuR levels by small interfering RNA inhibited apigenin-mediated stabilization of COX-2 mRNA. Cells expressing reduced TIAR showed marked resistance to apigenin's ability to inhibit UVB-induced COX-2 expression. Taken together, these results indicate that in addition to transcriptional regulation, another mechanism by which apigenin prevents COX-2 expression is through mediating TIAR suppression of translation.Cyclooxygenases are key enzymes in the conversion of arachidonic acid to prostaglandins. The inducible isoform of cyclooxygenase, cyclooxygenase 2 (COX-2), is an early response gene and is regulated by growth factors, cytokines, and tumor promoters (64). Another cyclooxygenase isoform, COX-1, is constitutively expressed and mediates many physiological functions (54). COX-2 is overexpressed in many transformed cells and various malignancies (5, 31). COX-2-deficient mice exhibit a decreased intestinal tumor incidence (47). Moreover, inhibitors of cyclooxygenases, e.g., nonsteroidal antiinflammatory drugs, reduce the formation, development, and metastasis of tumors in many animal models as well as risk of colorectal carcinoma in humans (27, 59). Together, these studies clearly demonstrate a significant role of COX-2 in carcinogenesis.Every year over 1 million cases of nonmelanoma skin cancer are diagnosed in the United States alone, and chronic UV light exposure is widely believed to be the cause (20). Solar UV radiation reaching the earth's surface is a mixture of UVB and UVA, and it is generally believed that UVB irradiation plays a causative role in skin cancer (20). Multiple lines of evidence demonstrate that induction of COX-2 expression and the subsequent increase in prostaglandins contribute to skin cancer development. For example, transgenic mice overexpressing COX-2 in basal epidermal cells develop skin tumors (46), selective COX-2 inhi...

Apoptotic Cell-Derived Sphingosine-1-Phosphate Promotes HuR-Dependent Cyclooxygenase-2 mRNA Stabilization and Protein Expression

“…Our data confirm previous findings that mRNA stabilizing, and thus posttranscriptional COX-2 regulatory mechanisms do operate in RAW264.7 macrophages (16). At present, a multiplicity of mRNA-stabilizing and -destabilizing factors such as TIAR, AUF1, HuR, and TIA-1 are known (15) and implicated in COX-2 regulation (16,17,35). Previous work also suggested a role of HuR in COX-2 mRNA stabilization after treatment of human mammary epithelial cells with taxanes (13).…”

Enhancement of UVB-Induced Apoptosis by Apigenin in Human Keratinocytes and Organotypic Keratinocyte Cultures

“…Our group and others have shown that apigenin treatment of cells results in a wide variety of antitumorigenic and chemopreventive actions (13)(14)(15)(16)(17)(18). More recently, we have focused our investigative efforts on determining the effects of apigenin treatment on keratinocytes exposed to UVB radiation (19,20), the primary causative agent of skin cancers.…”

Natural Flavonoids in StAR Gene Expression and Testosterone Biosynthesis in Leydig Cell Aging

“…At the translational level, apigenin increased the localization to cytoplasm of two proteins, HuR and T-cell-restricted intracellular antigen 1-related protein (TIAR). HuR and TIAR then bound to the AU-rich elements in the 3'-untranslated region of COX2 mRNA and inhibited COX2 translation (Tong et al, 2007). In addition to its inhibitory effects on COX2 expression, apigenin acts as a natural antagonist of the TBX A2 receptors.…”