Cytotoxic Autophagy: A Novel Treatment Paradigm against Breast Cancer Using Oleanolic Acid and Ursolic Acid

Background: Oleanolic acid (OA) and Ursolic acid (UA) are bioactive triterpenoids. Reported activities vary with the dose used for testing their activities in vitro. Studies using doses of ≥20 µM showed Apoptosis activities in Cancer cells. However, reported drug levels in circulation achieved by oral administration of UA and OA are ≤2 µM, thus limiting their use for treatment or delivering a combination treatment.

Materials and methods: The present report demonstrates the efficacy of OA, UA, and OA + UA on tumor cell-specific cytotoxicity at low doses (5 µM to 10 µM) in breast Cancer (BrCa) cell lines MCF7 and MDA-MB231.

Results: The data show that both OA and UA killed BrCa cells at low doses, but were significantly less toxic to MCF-12A, a non-tumorigenic cell line. Moreover, OA + UA at ≤10 µM was lethal to BrCa cells. Mechanistic studies unraveled the significant absence of Apoptosis, but their cytotoxicity was due to the induction of excessive Autophagy at a OA + UA dose of 5 µM each. A link to drug-induced cytotoxic Autophagy was established by demonstrating a lack of their cytotoxicity by silencing the autophagy-targeting genes (ATGs), which prevented OA-, UA-, or OA + UA-induced cell death. Further, UA or OA + UA treatment of BrCa cells caused an inhibition of PI3 kinase-mediated phosphorylation of Akt/mTOR, the key pathways that regulate Cancer cell survival, metabolism, and proliferation.

Discussion: Combinations of a PI3K Inhibitor (LY294002) with OA, UA, or OA + UA synergistically inhibited BrCa cell survival. Therefore, the dominance of cytotoxic Autophagy by inhibiting PI3K-mediated Autophagy may be the primary mechanism of PTT-induced Anticancer activity in BrCa cells.

Conclusion: These results suggest it would be worthwhile testing combined OA and UA in clinical settings.

PI3 kinase inhibition; bioactive triterpenoids; combination therapy; mitophagy; triple-negative breast cancer.