SUMMARY The retinoblastoma (Rb) tumor suppressor is often inactivated in cancers. To identify genes that can be used to specifically target such cancers, we carried out a genetic screen in Drosophila. We identified gig (fly TSC2) and found that inactivation of rbf (fly Rb) and gig synergistically induced cell death. Interestingly, inactivation of TSC2 specifically kills Rb mutant cancer cells under stress conditions, which is correlated with an inhibition of tumor growth. We show that cancer cell killing induced by concomitant inactivation of Rb and TSC2 is mediated by increased cellular stress, including oxidative stress. Inactivation of TSC2 and Rb synergistically induce oxidative stress via increased protein synthesis, inhibited de novo lipid synthesis, and decreased ROS scavenger enzyme SOD2 induction.

Ginseng is one of the most commonly used herbs that is believed to have a variety of biological activities, including reducing blood sugar and cholesterol levels, anti-cancer, and anti-diabetes activities. However, little is known about the molecular mechanisms involved. In this study, we showed that protopanaxadiol (PPD), a metabolite of the protopanaxadiol group ginsenosides that are the major pharmacological constituents of ginsengs, significantly altered the expression of genes involved in metabolism, elevated Sestrin2 (Sesn2) expression, activated AMPK, and induced autophagy. Using CRISPR/CAS9-mediated gene editing and shRNA-mediated gene silencing, we demonstrated that Sesn2 is required for PPD-induced AMPK activation and autophagy. Interestingly, we showed that PPD-induced Sesn2 expression is mediated redundantly by the GCN2/ATF4 amino acid-sensing pathway and the PERK/ATF4 endoplasmic reticulum (ER) stress pathway. Our results suggest that ginseng metabolite PPD modulates the metabolism of amino acids and lipids, leading to the activation of the stress-sensing kinases GCN2 and PERK to induce Sesn2 expression, which promotes AMPK activation, autophagy, and metabolic health.

Protopanaxadiol (PPD), a ginseng metabolite generated by the gut bacteria, was shown to induce colorectal cancer cell death and enhance the anticancer effect of chemotherapeutic agent 5‐FU. However, the mechanism by which PPD promotes cancer cell death is not clear. In this manuscript, we showed that PPD activated p53 and endoplasmic reticulum (ER) stress and induced expression of BH3‐only proteins Puma and Noxa to promote cell death. Induction of Puma by PPD was p53‐dependent, whereas induction of Noxa was p53‐independent. On the other hand, PPD also induced prosurvival mechanisms including autophagy and expression of Bcl2 family apoptosis regulator Mcl‐1. Inhibition of autophagy or knockdown of Mcl‐1 significantly enhanced PPD‐induced cell death. Interestingly, PPD inhibited expression of genes involved in fatty acid and cholesterol biosynthesis and induced synergistic cancer cell death with fatty acid synthase inhibitor cerulenin. As PPD‐induced ER stress was not significantly affected by inhibition of new protein synthesis, we suggest PPD may induce ER stress directly through causing lipid disequilibrium.