Curcumin exhibits anti-inflammatory and antitumor activities. Although its functional mechanism has not been elucidated so far, numerous studies have shown that curcumin induces apoptosis in cancer cells. In the present study, we show that subtoxic concentrations of curcumin sensitize human renal cancer cells to the tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-mediated apoptosis. This apoptosis induced by the combination of curcumin and TRAIL is not interrupted by Bcl-2 overexpression. We found that treatment with curcumin significantly induces death receptor 5 (DR5) expression both at its mRNA and protein levels, accompanying the generation of the reactive oxygen species (ROS). Not only the pretreatment with N-acetylcystine but also the ectopic expression of peroxiredoxin II, an antioxidative protein, dramatically inhibited the apoptosis induced by curcumin and TRAIL in combination, blocking the curcumin-mediated DR5 upregulation. Taken together, the present study demonstrates that curcumin enhances TRAIL-induced apoptosis by ROS-mediated DR5 upregulation.

Proteolytic degradation of the extracellular matrix and tumor metastasis correlate with the expression of endopeptidases known as matrix metalloproteinases (MMPs). The expression of MMPs is regulated by cytokines and signal transduction pathways, including those activated by phorbol myristate acetate (PMA). We found that resveratrol, a phytoalexin present in grapes, significantly inhibits the PMA-induced increase in MMP-9 expression and activity. These effects of resveratrol are dose dependent and correlate with the suppression of MMP-9 mRNA expression levels. PMA caused about a 23-fold increase in MMP-9 promoter activity, which was suppressed by resveratrol. Transient transfection utilizing MMP-9 constructs, in which specific transcriptional factors were mutagenized, indicated that the effects of PMA and resveratrol were mediated via an activator protein-1 and nuclear factor-kappaB response element. Resveratrol inhibited PMA-mediated activation of c-Jun N-terminal kinase (JNK) and protein kinase C (PKC)-delta activation. Therefore, we conclude that the MMP-9 inhibition activity of resveratrol and its inhibition of JNK and PKC-delta may have a therapeutic potential, given that a novel means of controlling growth and invasiveness of tumors.

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Tumor necrosis factor-related apoptosis-induced ligand (TRAIL) is preferentially cytotoxic to cancer cells over normal cells. However, many cancer cells, including malignant glioma cells, tend to be resistant to TRAIL. Monensin (a polyether ionophore antibiotic that is widely used in veterinary medicine) and salinomycin (a compound that is structurally related to monensin and shows cancer stem cell-inhibiting activity) are currently recognized as anticancer drug candidates. In this study, we show that monensin effectively sensitizes various glioma cells, but not normal astrocytes, to TRAIL-mediated apoptosis; this occurs at least partly via monensin-induced endoplasmic reticulum (ER) stress, CHOP-mediated DR5 upregulation and proteasome-mediated downregulation of c-FLIP. Interestingly, other polyether antibiotics, such as salinomycin, nigericin, narasin and lasalocid A, also stimulated TRAIL-mediated apoptosis in glioma cells via ER stress, CHOP-mediated DR5 upregulation and c-FLIP downregulation. Taken together, these results suggest that combined treatment of glioma cells with TRAIL and polyether ionophore antibiotics may offer an effective therapeutic strategy.