According to the traditional medicinal usage of Sonneratia caseolaris, we tested the extract of S. caseolaris for antioxidant activity using 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging effect on thin-layer chromatography. Following activity-oriented separation, two flavonoids, luteolin (1) and luteolin 7-O-β-glucoside (2), were isolated. Both of the compounds were found to possess antioxidant activity.

In search of bioactive natural products for overcoming TRAIL resistance from natural resources, we previously reported a number of active compounds. Bioassay-guided fractionation of mangrove, Amoora cucullata, collected from Sundarbans Mangrove Forest, Bangladesh, led to the isolation of four new compounds (1-4), along with seven known compounds (5-11). Of the isolates, compounds 1, 5, 8, and 9 showed TRAIL resistance-overcoming activity, among which 8 showed the most potent activity and enhanced TRAIL-induced apoptosis in TRAIL-resistant human gastric adenocarcinoma (AGS) cells through the activation of caspase-3/7, enhancing the expression of DR4 and DR5 mRNA in AGS cells. Cell death caused by the combined treatment of 8 and TRAIL was inhibited by human recombinant DR5/Fc and DR4/Fc chimera proteins, indicating that 8 sensitizes TRAIL-resistant AGS cells to TRAIL through the induction of DR4 and DR5.

Exploration of actinomycetes for isolation of natural products for abrogating TRAIL resistance led to the isolation of two new tyrosine derivatives (1 and 2) along with novobiocin (3). The structures of 1 and 2 were determined by spectroscopic methods, while the absolute configuration was determined by analyzing CD spectra and by a modified Marfey's method. Compounds 1 (150 μM) and 3 (37.5 and 75 μM) in combination with TRAIL showed synergistic activity in sensitizing TRAIL-resistant human gastric adenocarcinoma cells.

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