br Our study on program
Our study on program cell death revealed that ( ± )-kusukokinin and its derivative, ( ± )-bursehernin, induced apoptosis through multi-caspase activity including caspase-1, -3, -4, -5, -6, -7, -8, and -9. Previous report showed that (-)-kusunokinin induced cleaved caspase-7 and -8 . Similarly, many lignan compounds induce caspase-3 ac-tivity such as 7´-hydroxy-3´,4´,5,9,9´-pentamethoxy-3,4-methylene dioxy lignan , vitexins , hinokinin  and arctigenin . Finally, we proposed the mechanism of synthetic ( ± )-kusunokinin and ( ± )-bursehernin as shown in Fig. 8. From recently result, both compounds down-regulated STAT3 and decreased cyclin D1 protein.
T. Rattanaburee, et al.
STAT3 involves in cancer development through the promotion of cell proliferation, survival, angiogenesis and metastasis . The STAT3 transcription factor binds to cyclin D1 promoter and induces protein synthesis . Then, cyclinD1-Cdk4 complex serves as a regulator for cell progression from G1 to S phase in Herboxidiene . Therefore, down-regulation of STAT3 and cyclin D1 resulted in cell cycle arrest at S phase. Another targeted protein that could explain the action of ( ± )-kusunokinin and ( ± )-bursehernin on cell cycle arrest at G2/M was MEK1. Previously, we reported that (-)-bursehernin may inhibit MEK1 and FMS which related to cell growth and proliferation . STAT3 can be phosphorylated by multiple kinases in the MAPK cas-cades including MEKK1, ERK/JNK and MEK1 . STAT3 up-regulated many molecules such as bcl-2, bcl-xL, mcl-1, cyclin B and cyclin D1 . Cyclin B-CDC25C complex drives cell cycle progression from G2 to M phase , therefore inhibition of cyclin B1 protein could cause cell cycle arrest at G2/M phase. In addition, ( ± )-kusunokinin and ( ± )-bursehernin also down-regulated topoisomerase II which had an eﬀect on induction of p53, followed by induction of p21. Previously, we found that extracted (-)-kusunokinin up-regulated p53, p21, bax, cy-tochrome C, caspase -3, -7, -8, and -9, and down-regulated topoi-somerase II and bcl-2 , similar to this present result. Topoisomerase
II is an enzyme which capable of breaking and resealing double-stranded DNA. Therefore, inhibition of topoisomerase II generates DNA damage [49,50]. ATR and ATM are activated by DNA damage and DNA replication stress . Then, CHK1 and CHK2 are activated by ATR and ATM resulting to inhibition on binding of CDC25C with cyclin B . Moreover, ATM activates p53 via phosphorylation of CHK2 leading to inhibition of CDC25C. ATR also inhibits CDC25C via phosphorylation of CHK1. Finally, inactivated CDC25C lead to cell cycle arrest at G2/M phase . Therefore, these results conclude that ( ± )-kusukokinin and its derivative, ( ± )-bursehernin, their anticancer eﬀect could be through suppression of STAT3 and topoisomerase II inducing cell cycle arrest and apoptosis. These both compounds exhibited promising po-tency for anticancer treatment in future.
Declaration of Competing Interest
The authors declare that there is no conflict of interest.
This work was supported by The Royal Golden Jubilee Ph.D. Program (PHD/0062/2559), the Thailand Research Fund, Thailand; the Graduate School (Ph.D. Scholarship), Prince of Songkla University, Thailand; Faculty of Medicine (58-070-04-2), Prince of Songkla University, Thailand; the Higher Education Research Promotion and National Research University Project of Thailand, Oﬃce of the High Education Commission, Thailand.
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