A Combination of Ursolic Acid and Oleanolic Acid Disrupts Bioenergetic Homeostasis in Prostate Cancer and Inhibits Cancer Growth by Cytotoxic Autophagy

Oleanolic acid (OA) and ursolic acid (UA) are bioactive pentacyclic triterpenoids (PTTs) and potential therapeutics for cancers. If tested individually, their therapeutic efficacy is limited. The antitumor potential of OA, UA, and their combination was tested against castration-resistant prostate cancer cells (CRPCs) in vitro and in vivo. UA, OA, and UA + OA (1:1) inhibited the cell proliferation and clonogenic growth of CRPC cells. Compared to the potency of individual OA and UA on these cells, their potency was synergistically enhanced when tested as an OA + UA mix. Neither of these compounds inhibited the proliferation of normal prostate epithelial cells. Further, in a CRPC (22Rv1) tumor model, the gavage of UA and, more effectively, the OA + UA mix inhibited tumor growth ≥ 60%, without measurable systemic toxicity. Mechanistic studies showed that these PTTs increase cyto-plasmic and mitochondrial oxidative stress. Cytotoxicity was predominantly triggered by high levels of reactive oxygen species (ROS). It was abolished when cells were co-treated with N-acetylcysteine (NAC), an oxygen scavenger and precursor for glutathione synthesis. The PTTs caused cell cycle arrest at the G1/S boundary, reduced glucose utilization, reprogrammed mitochondrial bioenergetics, and cytotoxic autophagy. Further, these data showed that cytotoxic autophagy is a significant contributor to PTT-induced tumor inhibition and OA + UA, at a 1:1 mix, selectively targets bioenergetics in prostate cancer cells, leading to ROS-mediated cytotoxic autophagy and robust tumor inhibition without significant systemic toxicity.