Introduction
Tea (Camellia sinensis L.) is one of the most widely consumed beverages in the world. (-)-Epigallocatechin-3-O-gallate (EGCG), which is the major green tea catechin present in the leaves, is believed to the compound most responsible for the health benefits attributed to tea. EGCG was reported to have antioxidative [1], [2], antimutagenic [3], anti-inflammatory [4], and anticarcinogenic activities [5].
Although the EGCG concentrations required to elicit the anticancer activity have been shown to be more than 1 µM, the blood level of EGCG after consuming the equivalent of 2–3 cups of green tea was 0.1–0.6 µM and for an equivalent of 7–9 cups was still lower than 1 µM [6], [7]. In a cohort study, daily consumption of ten cups of green tea was required for the cancer preventive effect [8]. Moreover, adverse effects of green tea, mainly hepatitis, by consumption of high doses of green tea have been reported [9]. Therefore, it is important to enhance the pharmacologic effect of EGCG to obtain the health benefit in reasonable concentration in daily life.
We have reported that the cell-surface binding of EGCG and its derivatives is involved in their biological activities [10]–[15]. We have identified the 67-kDa laminin receptor (67LR) as a cell surface receptor for EGCG that mediates the anticancer activity of EGCG [16]. 67LR has been shown to be overexpressed on the cell surface of various tumor cells [17]. It was postulated that 67LR plays a significant role in the tumor progression and speculated that studies conducted to define the function of 67LR could provide a new approach to cancer prevention. Indeed, expression of 67 LR confers EGCG responsiveness to tumor cells in vivo [18].
Vitamin A, also known as retinol, participates in physiological activities related to the immune system, maintenance of epithelial and mucosa tissues, growth, reproduction, and bone development. It comes from animal sources, such as eggs, meat, milk, cheese, cream, liver, kidney, cod and halibut fish oil. In vitro and in animal models, it has been demonstrated that vitamin A is involved in the regulation and promotion of growth and differentiation of many cells [19]. The visual function of vitamin A depends on its natural and synthetic derivatives, retinoids [20]. All-trans-retinoic acid (ATRA), the active derivative of vitamin A, has been well documented as a growth and differentiation factor in many tissues and cells, and proved to be an effective treatment to many diseases including cancers [21], [22].
Retinoids exert their physiological activities through retinoid receptor nuclear proteins that belong to the superfamily of steroid/thyroid hormone receptors, of which there are two classes, retinoic acid receptors (RARs) and the retinoic-X receptors (RXRs), each of which has three subtypes, α, β, and γ [23], [24]. The natural ligands for the RARs are ATRA and its stereoisomers 9-cis-RA and 13-cis-RA, whereas RXRs are activated by 9-cis-RA only. ATRA acts through RAR to transcriptionally activate target genes, such as cytochrome P450 and CRABI [24].
This study was designed to identify a food component that could be effectively used in combination with EGCG and to investigate the mechanism of action of this combination. By using in vitro and in vivo systems involving a highly metastatic mouse B16 melanoma cell line [25], we found that ATRA enhances the antitumor activity of EGCG by upregulating the 67 LR expression through RAR.