Apigenin restores TSP-1 expression in UVB-irradiated keratinocytes

Non-melanoma skin cancer (NMSC) is a major health problem in the United States, with over two million new cases diagnosed yearly, making it the most common cancer in this country (1). Ultraviolet B (UVB) radiation is the major carcinogen for non-melanoma skin cancer. Like all solid tumors, NMSC promotes growth of new blood vessels (angiogenesis) in order to grow and metastasize. Thrombospondin-1 (TSP-1) is the first protein inhibitor of angiogenesis to be identified (2), and its expression is markedly downregulated in epidermis following UVB irradiation and throughout distinct steps of skin carcinogenesis (3-6). Apigenin (Api, 4', 5, 7- trihydroxyflavone), a nonmutagenic flavonoid found in fruits and vegetables, inhibits NMSC induced by UV exposure and chemical carcinogens (7,8). We and others have shown that Api induces many antitumorigenic and chemopreventive actions in multiple cell types (9-14), however, the role of TSP-1 in chemoprevention by Api has never been reported. Our preliminary results show that Api blocks downregulation of TSP-1 by UVB in cultured keratinocytes and skin in vivo. We show that regulation of TSP-1 expression in UVB/Api-treated keratinocytes occurs post-transcriptionally and is mediated by RNA-binding protein HuR. Importantly, HuR has recently been demonstrated to play an important role in translation of TSP-1 mRNA. In addition we have shown that Api promotes nuclear to cytoplasmic translocation of HuR (15), where polyribosomal translation of TSP-1 occurs. Furthermore, we have shown that siRNA knockdown of HuR expression abrogates the ability of Api to restore normal TSP-1 levels in UVB-irradiated keratinocytes. Our compelling preliminary results in an established in vivo model of UVB-induced skin carcinogenesis, confirms in vivo that UVB inhibited TSP-1 protein expression in epidermis and Api restored TSP-1 expression. Furthermore, Api inhibited UVB-induced up-regulation of stromal inflammation and microvascular density (MVD) in superficial dermis. Given the well-known anti-angiogenic and less studied anti-inflammatory action of TSP-1, it is likely that induction of TSP-1 expression by Api is the cause of decreased MVD and inflammatory infiltrates. Based on these findings, we propose to test the novel hypothesis that Api restores TSP-1 expression in UVB-irradiated epidermis via upregulation of TSP-1 translation. We further hypothesize that this induction of TSP-1 expression by Api leads to inhibition of inflammation and angiogenesis in the stromal compartment, with overall chemoprevention of NMSC. We propose to test this hypothesis in cell-based and in vivo studies using wild- type and Thrombospondin-1-null (TSP-1 -/-) mice and keratinocytes exposed to UVB and/or Api. Identifying TSP-1 as a key target of apigenin and demonstrating its important role in chemoprevention by Api will provide a new target and rationale for improved treatment and prevention strategies for NMSC.