1. Galangin caused increased cell death in a renal cancer cell line that is resistant to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), the protein mimicked in certain cancer therapies.
2. The combination of galangin and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) increased cell death in resistant renal carcinoma cells by decreasing the levels of anti-apoptotic proteins.
Evidence rating level: 3 (Average)
Study Rundown: TRAIL can induce apoptosis, or programmed cell death, by binding to death receptors, proteins that are more highly expressed on the cell surface of cancer cells than normal cells. Cancer cells can become resistant to apoptosis through many different mechanisms, including down-regulating death receptors and up-regulating anti-apoptotic proteins. This study found that galangin sensitized TRAIL-resistant Caki cells. Compared to treatment with galangin or TRAIL alone, combination treatment resulted in an increased percentage of cells undergoing apoptosis, as measured by the number of cells in the sub-G1 phase of the cell cycle and the cleavage of PARP, a family of proteins involved in DNA repair. Galangin increased apoptosis by decreasing the levels of the anti-apoptotic proteins Bcl2, Mcl1, cFLIP and survivin. While galangin decreased Bcl2 levels by inhibiting its expression, galangin affected the levels of Mcl1, cFLIP, and survivin by increasing the protein degradation activity of the proteasome. Although a striking finding, the sensitizing effect of galangin was not tested in vivo and requires further evaluation in an animal model. Ntevertheless, the results in this study highlight the potential for combination therapies to treat apoptosis-resistant carcinomas.
In Depth [in vitro study]: Comparing the TRAIL-resistant Caki human carcinoma cell line against the control TMK-1 mouse kidney cell line, the researchers examined the efficacy of varying galangin and TRAIL concentrations in inducing apoptosis. Using flow cytometry and Western blotting, apoptosis was analyzed by looking at the percentage of cells in the sub-G1 phase of the cell cycle as well as PARP levels. Only a combination of galangin and TRAIL was effective in inducing significant apoptosis in Caki cells (p<0.01); furthermore, the killing effect was dose-dependent. Caki cells receiving combination treatment also showed other measures of apoptosis including increased chromatin damage and DNA fragmentation as measured by DAPI staining, as well as significantly increased caspase activation as measured by a DEVDase activity assay (p<0.01). The same experiments in the TMK-1 cell line showed no morphologic changes and no increase in the sub-G1 population.
The mechanisms of galangin-induced apoptosis were then elucidated using Western blots to detect changes in the protein levels of Bcl-2, Mcl-1, cFLIP, and survivin. RT-PCR was also used to evaluate RNA levels. Bcl-2 mRNA and protein levels both decreased following galangin treatment, indicating that galangin inhibited protein expression at the transcriptional level. In contrast, for Mcl-1, cFLIP, and survivin, protein levels decreased while mRNA levels remained constant. A proteasome activity assay showed an increase in proteasome activity, indicating that galangin affected the protein levels via increased protein degradation. Finally, Caki cells were transfected with the 4 anti-apoptotic proteins in order to determine the effect of their overexpression on galangin therapeutic efficacy. Consistent with their predicted role in TRAIL-resistance, overexpression of these proteins inhibited the function of galangin.
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