In this study, we found that the marine cytotoxic product 13-acetoxysarcocrassolide (13-AC), recently isolated from the alcyonacean coral Lobophytum crassum [1], exhibited potent inhibitory activity on HSP90. 13-AC induced apoptosis in oral cancer cells Ca9-22 through the disruption of mitochondrial membrane potential and the stimulation of reactive oxygen species generation. However, the pretreatment of Ca9-22 cells with N-acetylcysteine, an antioxidant, inhibited ROS production resulting in the attenuation of the cytotoxic activity of 13-AC. Under stressful conditions, Ca9-22 cells treated with 13-AC showed a rapid induction of Keap1-Nrf2 pathway and an increase in the expression of p62/SQSTM1, but a suppression in antioxidative function of Nrf2 with immunoprecipitation, immunocytofluorecent and western blotting analysis. Inhibition of p62 expression by siRNA considerably attenuated the growth-inhibited by 13-AC treatment. Moreover, 13-AC exerted antitumor effect against oral cancer cells as demonstrated by the in vivo xenograft animal model. It significantly reduced tumor volume (55.29%) and tumor weight (90.33%). T he molecular docking analysis demonstrated that 13-AC binds to N- terminal domain of HSP90 protein showing binding affinity more than 17-allylaminogeldanamycin (17-AAG), a HSP90 inhibitor of N- terminal ATP binding site and suppressed HSP90 client proteins including p-Akt, CDK4, HIF-1 a , and MMP-2. On the proteins level, 13-AC increased the expression of apoptosis related proteins such as cleaved caspases-3 and -9 as well as cleaved PARP in a dose- and time-dependent manner. Moreover, the results suggested that 13-AC exerted its cytotoxi c activity through the promotion of ROS generation and the suppression of antioxidant enzyme activities. Altogether, the apoptotic effect of 13-AC was found to be mediated through the inhibition of HSP90 suggesting its potential future application as an anticancer agent.
[1] Peng, B.-R.; Lu, M.-C.; El-Shazly, M.; Wu, S.-L.; Lai, K.-H.; Su, J.-H. Mar. Drugs 2018, 16, 15.