Abstract #33: In vitro and in vivo synergistic anti-tumor efficacy of two naturally available dietary polyphenols EGCG and luteolin

Because of the drug-associated toxicities of most of the currently used chemotherapeutic drugs and safety of the natural dietary components, combination of dietary compounds either with other dietary compounds or existing chemotherapeutic agents are gaining increasing attention. In the current study, we have found that two dietary polyphenols epigallocatechin-3-gallate (EGCG) from green tea, and luteolin from green vegetables, exhibited synergistic anti-tumor effects against a panel of cell lines established from lung cancers and squamous cell carcinoma of the head and neck (SCCHN). To elucidate the underlying mechanism of this combination, annexin V-PE staining and western blotting were performed to study apoptosis and the expression pattern for the cell-cycle and apoptosis regulatory proteins, respectively. Lenti-virus based expression system using short hairpin RNA (shRNA) was employed to ablate the expression of specific proteins in this study. Nude mouse xenograft model was used to study the in vivo efficacy of this combined regimen. Treatment of cells with EGCG or luteolin alone induced minimal apoptosis (3-10%) in most of the cell lines tested. In contrast, simultaneous treatment with combination of the two compounds tremendously increased apoptosis (60-80%) as evidenced by annexin V-PE staining and cleavage of PARP. Treatments with the two compounds also cleaved caspase-8 and -3, and induced the expression of DR5, but not release of cytochrome C. Moreover, the expression of p53 and p73 and their transcriptional targets p21 and PUMA were increased after treatment with the compounds. Ablation of p53 with shRNA specific for p53 or inactivation of p73 with dominant negative p73 plasmid protected cells from apoptosis. In addition, combined treatment with the two compounds synergistically inhibited the expression of the p65 subunit of NF-\#954;B, its transcriptional target Bcl-2, and phosphorylation of AKT, a kinase responsible for NF-\#954;B activation. However, in cells with ablated p53, the compounds failed to inhibit NF-\#954;B. Combination of the two compounds more potently inhibited Tu212 xenograft growth in nude mice as compared with any of the single compound. Our results for the first time identified that combination of EGCG and luteolin had synergistic anti-tumor effects both in vitro and in vivo. These results suggest that the compounds induced extrinsic pathway of apoptosis by activating p53 family members and p53-dependent inhibition of NF-\#954;B signaling. Thus, the combination of EGCG and luteolin is highly promising for the further development of cancer prevention and therapy particularly for head and neck and lung cancers (Supported by U01CA101244, RO1CA112643, and P50CA128613 to DMS).

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 33.