Tumor necrosis factor–related apoptosis-inducing ligand (TRAIL) is one of the most promising candidates for new cancer therapeutics. A current problem is that some cancers still remain resistant to TRAIL. We show for the first time that a naturally occurring flavonoid, baicalein, overcomes TRAIL resistance in cancer cells. The combination of baicalein and TRAIL effectively induced apoptosis in TRAIL-resistant colon cancer SW480 cells. Baicalein up-regulated the expression of death receptor 5 (DR5) among TRAIL receptors at the mRNA and protein levels. Suppression of this up-regulation with small interfering RNA (siRNA) efficiently reduced the apoptosis induced by TRAIL and baicalein, suggesting that the sensitization was mediated through DR5 induction. Moreover, baicalein also overcame TRAIL resistance with DR5 up-regulation in prostate cancer PC3 cells. Of note, the combination of TRAIL and baicalein hardly induced apoptosis in normal human cells, such as blood cells and hepatocytes. Baicalein increased DR5 promoter activity, and this enhanced activity was diminished by mutation of a CCAAT/enhancer-binding protein homologous protein (CHOP)–binding site in SW480 cells. In SW480 cells, CHOP siRNA blocked both functions of baicalein. CHOP expression was induced by baicalein in SW480 cells; however, in PC3 cells, baicalein scarcely induced CHOP and mutation of the CHOP-binding site did not abrogate the DR5 promoter activation by baicalein. Interestingly, baicalein induced reactive oxygen species (ROS) and a ROS scavenger prevented DR5 expression and TRAIL sensitization in PC3 but not SW480 cells. These results indicate that, using two different pathways, baicalein exposes cancer surveillance of TRAIL and overcomes TRAIL resistance in cancer cells. [Cancer Res 2008;68(21):8918–27]

Allopurinol has been used for the treatment of gout and conditions associated with hyperuricemia for several decades. We explored the potential of allopurinol on cancer treatment. Allopurinol did not expose cytotoxicity as a single treatment in human hormone refractory prostate cancer cell lines, PC-3 and DU145. However, allopurinol drastically induced apoptosis of PC-3 and DU145 in combination with tumor necrosis factor–related apoptosis-inducing ligand (TRAIL), which is a promising candidate for anticancer agent but its efficacy is limited by the existence of resistant cancer cells. We examined the underlying mechanism by which allopurinol overcomes the resistance of prostate cancer cells to TRAIL. Allopurinol up-regulated the expression of a proapoptotic TRAIL receptor, death receptor 5 (DR5). Allopurinol increased DR5 protein, mRNA, and promoter activity. Using DR5 small interfering RNA (siRNA), we showed that allopurinol-mediated DR5 up-regulation contributed to the enhancement of TRAIL effect by allopurinol. Furthermore, we examined the mechanism of allopurinol-mediated DR5 up-regulation. DR5 promoter activity induced by allopurinol was diminished by a mutation of a CAAT/enhancer binding protein homologous protein (CHOP)-binding site. In addition, allopurinol also increased CHOP expression, suggesting that allopurinol induced DR5 expression via CHOP. Allopurinol possesses the activity of a xanthine oxidase (XO) inhibitor. We used XO siRNA instead of allopurinol. XO siRNA also up-regulated DR5 and CHOP expression and sensitized the prostate cancer cells to TRAIL-induced apoptosis. Here, we show the novel potential of allopurinol in cancer treatment and indicate that the combination of allopurinol with TRAIL is effective strategy to expand the TRAIL-mediated cancer therapy. (Mol Cancer Res 2008;6(12):1852–60)

Methylglyoxal is an essential component in glycolysis and is known to be an inducer of apoptosis. Glyoxalase I (GLO1) metabolizes and inactivates methylglyoxal. GLO1 is known to be overexpressed in cancer cells and causes resistance to anticancer agents. We show for the first time that methylglyoxal treatment or the silencing of GLO1 enhances sensitivity to the promising anticancer agent TRAIL in malignant tumor cells. Methylglyoxal suppressed the expression of antiapoptotic factors, X-linked inhibitor of apoptosis protein (XIAP), survivin, cIAP1, Bcl-2, and Bcl-xL, without affecting TRAIL receptors, DR4 and DR5. Knockdown of XIAP or survivin by siRNA also enhanced TRAIL-induced apoptosis, indicating that downregulation of XIAP and survivin expression by methylglyoxal contributes to the enhancement of TRAIL activity. Furthermore, methylglyoxal decreased NF-κB activity with or without TRAIL treatment. On the other hand, the knockdown of GLO1 by siRNA enhanced TRAIL-induced apoptosis via the downregulation of XIAP and survivin expression. In conclusion, our results strongly suggest that sensitivity to TRAIL is increased by inhibition of the glyoxalase pathway and that the combination of TRAIL with methylglyoxal or glyoxalase inhibitors may be useful for a novel combination chemotherapy. Mol Cancer Ther; 11(10); 2294–300. ©2012 AACR.