Triterpenoids from the leaves of Synsepalum dulcificum and their antitumor activities against hepatocellular carcinoma cells

Miracle fruit (Synsepalum dulcificum) is an edible berry, and is known for its ability to turn sour to sweet. The leaf of S. dulcificum has been used as additives in food, medicine and cosmetic industries. In this study, two new pairs of enantiomeric coumarinolignans, synsepalumins A and B (1 and 2), three new pentacyclic triterpenoids, 3-O-cis-p-coumaroyl-2 alpha,3 beta,29-trihydroxy-olean-12-en-28-oic acid (3), 3-O-trans-p-coumaroyl-2 alpha,3 beta,29-trihydroxyolean-12-en-28-oic acid (4), and 3-O-cis-p-coumaroyl-3 beta,19 alpha-hydroxy-urs-12-en-28-oic acid (6), along with twenty-five known compounds (5, 7 30) were isolated from the leaves of S. dulcificum. Compounds 1 and 2 are two rare tricyclic 7-hydroxycoumarins featuring a classic bicyclic 7-hydroxycoumarin fused with a (1,4)-dioxane ring. Their structures were elucidated by the spectroscopic data analyses and quantum chemical electronic circular dichroism calculations. Meanwhile, compound 4 (SMG-1) significantly inhibited proliferation of HepG2 (IC50 = 13.9 +/- 0.3 mu M) and HCCLM3 (IC50 = 11.8 +/- 0.2 mu M) cells. We further explored the underlying mechanism of SMG-1 for hepatocellular carcinoma (HCC) cells death, and found that SMG-1 induced reactive oxygen to increase the level of BNIP3 through JNK/p53 pathway and decreased mitochondrial membrane potential, resulting in activating mitophagy. Moreover, SMG-1 up-regulated the Bax and cytoplasm level of Cyt-c to promote Caspase-9/Caspase-3-mediated apoptosis in HCC cells. Therefore, the leaves of S. dulcificum possess the ability to treat HCC.