ALBERT

Description: Tumor necrosis factor (TNF)–related apoptosis-inducing ligand (TRAIL) induces apoptosis in many transformed cells; however, not all human tumors respond to TRAIL, potentially limiting its therapeutic utility. Although there is substantial evidence that cytotoxic drugs can augment sensitivity to TRAIL, it has become important to know what kinds of nontoxic drugs can be used together with TRAIL. We thus screened several natural compounds that can overcome resistance to TRAIL and found that a cycloanthranilylproline derivative, Fuligocandin B (FCB), an extract of myxomycete Fuligo candida, exhibited significant synergism with TRAIL. Treatment of the TRAIL-resistant cell line KOB with FCB and TRAIL resulted in apparent apoptosis, which was not induced by either agent alone. FCB increased the production of 15-deoxy-Δ12,14 prostaglandin J2 (15d-PGJ2), an endogenous PPARγ ligand, through activation of cyclooxygenase-2 (COX-2). This unique mechanism highlighted the fact that 15d-PGJ2 directly enhanced sensitivity to TRAIL by inhibiting multiple antiapoptotic factors. More importantly, similar effects were observed in other leukemia cell lines irrespective of their origin. The enhancement was observed regardless of PPARγ expression and was not blocked even by peroxisome proliferator-activated receptor-γ (PPARγ) siRNA. These results indicate that 15d-PGJ2 sensitizes TRAIL-resistant cells to TRAIL in a PPARγ-independent manner and that the use of 15d-PGJ2 or its inducers, such as FCB, is a new strategy for cancer therapy.

Description: TNF-related apoptosis-inducing ligand (TRAIL) induces apoptosis in many transformed cells but not in normal cells, hence TRAIL has recently emerged as a novel anti-cancer agent. Adult T-cell Leukemia/Lymphoma (ATLL) is a neoplasm of T-lymphocyte origin etiologically associated with HTLV-I, and is known to be resistant to standard anti-cancer therapies. In a previous study, we showed that although most ATLL cells express TRAIL-death receptors DR4 and/or DR5, they are resistant to TRAIL. To overcome this resistance, we tried to find natural materials that increase the sensitivity to TRAIL in the present study. Here, we report for the first time that dihydroflavonol from a plant increases TRAIL sensitivity by enhancing death receptor expression. Materials and methods: We used a TRAIL-resistant ATLL cell line, KOB, for screening, and peripheral blood mononuclear cells (PBMCs) from normal healthy donors. The cell surface expression of DR4, DR5, and decoy-receptors DcR1 and DcR2 was examined using flow cytometry (FCM). Cell viability was assessed by MTS assay after incubation with various combinations of natural extracts and TRAIL for 48 hours. Apoptosis was evaluated by Annexin V binding and propidium iodide (PI) staining. We also analyzed caspase−8, − 9 and −3 activation and pro- and anti-apoptotic proteins by Western blot. Results and Discussion: We screened more than 500 natural products. Among them, the most striking synergism with TRAIL was observed in a dihydroflavonol that was extracted from the Blumea Balsamifera, a family of chrysanthemum from Thailand. It was a methyldihydroquercetin (molecular weight, 318), and we named it BB-1. The viability of KOB cells treated with either TRAIL or BB-1 was 98% and 97%, respectively, and decreased to 60% with concurrent treatment. Interestingly, sequential treatment, pre-treatment with BB-1 followed by TRAIL, further augmented the synergism, and the viability decreased to 38%. Using Annexin V and PI, we confirmed that these are the results of apoptosis. Although it has been reported that anti-apoptotic proteins FLIP, Bcl-2, Bcl-xL and XIAP play key roles in TRAIL resistance, we did not find any change in these factors in the BB-1-treated cells. Instead, we found that BB-1 increased DR5 expression. Subsequent activation of caspase−8, −9 and −3 was observed in a time-dependent manner in the sequentially treated cells, but not in cells treated with BB-1 or TRAIL alone. More importantly, BB1 did not induce DR5 expression or enhance TRAIL sensitivity in PBMCs. Thus, BB-1 augmented TRAIL-induced apoptosis by up-regulating DR5 expression without any cytotoxic activity on normal PBMCs. These results suggest that combined treatment with BB-1 and TRAIL can be a new strategy for cancer immunotherapy against TRAIL-resistant cells.