Pro-oxidant activity of sulforaphane and cisplatin potentiates apoptosis and simultaneously promotes autophagy in malignant mesothelioma cells

Sulforaphane (SFN) is an isothiocyanate compound derived from glucoraphanin, which is found in cruciferous vegetables, and has been heralded as a chemopreventive and/or chemotherapeutic agent. The present study investigated the effects of SFN on enhancing the anticancer role of cisplatin (cis-dichlorodiammineplatinum; CDDP) in H-28 malignant mesothelioma cells. At concentrations demonstrating limited toxicity in MeT-5A normal human mesothelial cells, combination treatment with the two compounds exhibited synergistic growth-inhibiting and apoptosis-promoting activities, as demonstrated by a series of proapoptotic events, including reactive oxygen species accumulation, loss of mitochondrial membrane potential, upregulation of p53 expression, increased B-cell lymphoma 2 (Bcl-2) associated X protein/Bcl-2 ratio, activation of caspase-3, the occurrence of a sub-G(0)/G(1) peak and an increase in cells with pyknotic and fragmented nuclei, Annexin V-phycoerythrin-positive staining and G(2)/M phase-transition delay in the cell cycle. The phosphorylation levels of Akt and mammalian target of rapamycin were reduced by the combination treatment, which was accompanied by a significant increase in the level of autophagosomal marker protein microtubule-associated protein 1 light chain 3B-II and the accumulation of acidic vesicular organelles. Pretreatment with the antioxidant N-acetylcysteine attenuated both apoptosis and autophagy, whereas inhibition of autophagy by bafilomycin A1 potentiated apoptotic cell death following the combination treatment with SFN and CDDP. Considering the pro-oxidant-based combinational approach, the results of the present study provide a rationale for targeting cytoprotective autophagy as a potential therapeutic strategy for malignant mesothelioma.