Naringenin up-regulates the expression of death receptor 5 and enhances TRAIL-induced apoptosis in human lung cancer A549 cells

Scope: While TRAIL is relatively non-toxic to normal cells, it can selectively induce apoptosis in many types of transformed cells. Nevertheless, some non-small cell lung cancer (NSCLC) cells are particularly resistant to the effects of TRAIL. Here, we report that in combination with naringenin exposure to TRAIL induced apoptosis in TRAIL-resistant NSCLC A549 cells with no detectable inhibitory effects on cell proliferation of normal lung fibroblast cells. Methods and results: Cytotoxicity was evaluated by MTT assay. Apoptosis was detected using DAPI staining, and flow cytometry. The protein levels were determined by Western blot analysis. Caspase activity was measured using a colorimetric assay. For knockdown of Bid and DR5 expression, Bid and DR5 siRNAs were transfected into cells via lipofection. We could show that following exposure to naringenin, DR5 proteins were up-regulated and knockdown of DR5 expression by siRNA attenuated naringenin plus TRAIL-induced apoptosis. Naringenin and TRAIL effectively induced Bid cleavage and siRNA-mediated silencing of Bid reduced the sensitizing effect of naringenin. Furthermore, co-treatment with naringenin and TRAIL resulted in reduction of the clonogenic capacity of A549 cells, and surviving clones could be re-sensitized for repeated TRAIL treatment. Conclusion: Our results indicate that treatment with a combination of TRAIL and naringenin may be a safe strategy for treatment of resistant NSCLC.