No abstract

This study demonstrates that combined treatment with subtoxic doses of quercetin (3 0 ,3 0 ,4 0 ,5,7-pentahydroxyflavone), a flavonoid found in many fruits and vegetables, plus tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces rapid apoptosis in TRAIL-resistant hepatocellular carcinoma (HCC) cells. Effective induction of apoptosis by the combined treatment with quercetin and TRAIL was not blocked by overexpression of Bcl-xL, which is known to confer resistance to various chemotherapeutic agents. These results suggest that this combined treatment may provide an attractive strategy for treating resistant HCCs. While the proteolytic processing of procaspase-3 by TRAIL was partially blocked in various HCC cells treated with TRAIL alone, co-treatment with quercetin efficiently recovered TRAIL-induced caspase activation. We found that quercetin treatment of HCC cells significantly up-regulated the mRNA and protein levels of DR5, a death receptor of TRAIL, in a transcription factor Sp1-dependent manner. Furthermore, treatment with quercetin significantly decreased the protein levels of c-FLIP, an inhibitor of caspase-8, through proteasome-mediated degradation. Finally, administration of small interfering RNA against DR5 or overexpression of c-FLIP S , but not c-FLIP L , significantly attenuated quercetin-stimulated TRAIL-induced apoptosis. Collectively, these findings show that quercetin recovers TRAIL sensitivity in various HCC cells via up-regulation of DR5 and down-regulation of c-FLIP S . J. Cell. Biochem. 105: 1386-1398. ß 2008 KEY WORDS: QUERCETIN; TRAIL; HEPATOMA; DR5; c-FLIP S H epatocellular carcinoma (HCC) is the fifth most frequent neoplasm worldwide (>500,000 death/year) [Bruix et al., 2004]. Most hepatocellular carcinoma patients are diagnosed at advanced stages that are unsuitable for the current curative therapies of resection and transplantation [Avila et al., 2006]. The current chemotherapeutic options yield poor responses and low patient survival, making them ineffective for treatment of advanced HCC [Avila et al., 2006]. Therefore, new therapeutic alternatives are needed for more effective treatment of this malignancy.Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL), a member of the TNF family, is considered a promising anticancer agent due to its ability to induce apoptosis in a variety of tumor cell types while having only negligible effects on normal cells [Ashkenazi et al., 1999]. Binding of TRAIL to either DR4 or DR5, two death receptors of TRAIL, leads to oligomerization and clustering of their intracellular death domains. The subsequent interaction of DR4 or DR5 with the adaptor molecule, FADD (Fas-associated death domain), via their respective death domains leads to recruitment and activation of caspase-8 [ Thomas et al., 2004]. Finally, caspase-8 activates the executioner caspases (e.g., caspase-3 and caspase-7), leading to apoptotic cell death [Srivastava, 2001]. Although TRAIL has garnered considerable attention as a novel anticancer agent...

No abstract

No abstract

No abstract

No abstract

No abstract

Gambogic acid (GA), a xanthonoid extracted from the resin of the tree, Garcinia hanburyi, was recently shown to exert anticancer activity in multiple studies, but the underlying action mechanism remains unclear. Here, we show that GA induces cancer cell death accompanied by vacuolation in vitro and in vivo. This GA-induced vacuolation in various cancer cells was derived from dilation of the endoplasmic reticulum (ER) and mitochondria, and was blocked by cycloheximide. These findings suggest that GA kills cancer cells by inducing paraptosis, a vacuolization-associated cell death. We found that megamitochondria formation, which arose from the fusion of swollen mitochondria, preceded the fusion of ER-derived vacuoles. GA-induced proteasomal inhibition was found to contribute to the ER dilation and ER stress seen in treated cancer cells, and megamitochondria formation was followed by mitochondrial membrane depolarization. Interestingly, GA-induced paraptosis was effectively blocked by various thiol-containing antioxidants, and this effect was independent of ROS generation. We observed that GA can react with cysteinyl thiol to form Michael adducts, suggesting that the ability of GA to covalently modify the nucleophilic cysteinyl groups of proteins may cause protein misfolding and subsequent accumulation of misfolded proteins within the ER and mitochondria. Collectively, our findings show that disruption of thiol proteostasis and subsequent paraptosis may critically contribute to the anti-cancer effects of GA.