Cytotoxicity and Modulation of Cancer-Related Signaling by (Z)- and (E)-3,4,3',5'-Tetramethoxystilbene Isolated from Eugenia rigida

Bioassay-guided fractionation of the leaves of Eugenia rigida yielded three stilbenes, (Z)-3,4,3',5'-tetramethoxystilbene (1), (E)-3,4,3',5'-tetramethoxystilbene (2), and (E)-3,5,4'-trimethoxystilbene (3). The sterically hindered Z-stereoisomer 1, a new natural product, was prepared by time-dependent photoisomerization of the E- isomer (2) under UV irradiation at λ ₂₅₄ nm, while 2,3,5,7-tetramethoxyphenanthrene (5) was identified at λ ₃₆₅ nm by UHPLC/APCI-MS and NMR spectroscopy. Compounds 1 – 3 were tested against a panel of luciferase reporter gene assays that assesses the activity of many cancer-related signaling pathways, and the Z- isomer (1) was found to be more potent than the E- isomer (2) in inhibiting the activation of Stat3, Smad2/3, myc, Ets, Notch and Wnt signaling, with IC₅₀ values between 40 – 80µM. However, both compounds showed similar inhibition against Ap-1 and NF-κB signaling. In addition, 1 demonstrated cytotoxic activity towards human leukemia cells, as well as solid tumor cells of epidermal, breast and cervical carcinomas, and skin melanoma, with IC₅₀ values between 3.6 – 4.3 µM, while 2 was weakly active against leukemia, cervical and skin melanoma cells.