Mitochondria-targeted derivative of pterostilbene, a dietary phytoestrogen, exhibits superior cancer cell cytotoxicity via mitochondrial superoxide mediated induction of autophagy

Pterostilbene (trans-3,5-dimethoxy-4-hydroxystilbene;PTS), a dimethyl ether analogue of resveratrol, has been shown to possess anti-bacterial, anti-cancer, anti-inflammatory and antioxidant properties. In the present study, we have synthesized a mitochondria targeted derivative of Pterostilbene (M-PTS) with aim to enhance its anti-cancer efficacy. M-PTS was synthesized by substituting the free hydroxyl group of PTS with a propyl linker and Triphenylphosphonium (TPP+, a membrane-permeable lipophilic cation that readily accumulates and penetrates through the mitochondrial membrane). We observed that mitochondrial uptake of M-PTS in A549 cells was higher (∼8.5-fold) as compared to PTS and M-PTS was more potent in killing human lung (A549) and breast (MCF-7) cancer cells as compared to PTS. IC₅₀ of M-PTS was found to be 0.17 µM and 0.76 µM, respectively, while IC₅₀ of PTS was found to be 40.45 µM and 53.15 µM in A549 and MCF-7 cells, respectively. M-PTS was superior in inhibiting the clonogenic potential of A549 and MCF-7 cell and M-PTS induced apoptosis in A549 cells. M-PTS specifically increased both cytosolic ROS and mitochondrial superoxide levels whereas PTS did not have any effect on mitochondrial superoxide. M-PTS treatment led to autophagy induction in cancer cells. M-PTS induced cytotoxicity was attenuated on pre-treatment with mitochondria-targeted antioxidant (mitoTEMPO) as well as inhibitor of autophagy (chloroquine) signifying the involvement of mitochondrial ROS and autophagy in the cytotoxic effects of M-PTS. In conclusion, the superior cytotoxic efficacy of M-PTS via specific induction of mitochondrial superoxide illustrates the application of mitochondria targeted redox-active natural products as potent anti-cancer drugs.