Investigating the anti-cancer mechanism of action of Tanshinone IIA through mTORC1 regulation

Glioblastoma multiforme (GBM) is a highly aggressive form of brain cancer with poor prognosis. As a result, novel compounds that provide effective inhibition of GBM growth and development are in demand. Tanshinone IIA is a lipophilic polyphenol compound derived from the root of red sage and has been used for centuries in Chinese herbal medicine. In this study, the model organism, D. discoideum was employed to validate the effect of tanshinone IIA on the inhibition of mTORC1 activity, thus mediating an anti-proliferative, anti-oxidant and pro-autophagy response that may function to inhibit cancer development and progression. In order to determine the mechanism of action of tanshinone IIA, mutants of mTORC1 regulators, protein kinase B (pkb-/- ), sesn- and mios-, were employed to identify a key role of the SESN/GATOR2/GATOR1 pathway in the intracellular effects of tanshinone IIA. Additionally, combinatory treatment of tanshinone IIA with phosphoinositide-3 kinase (PI3K) inhibitor, LY294002, was shown to inhibit PI3K activity in a synergistic manner, suggesting a potential therapeutic benefit in this combination. GBM cell lines were used to translate these findings in a clinically relevant model, showing that combinatory treatment of tanshinone IIA and a novel PI3K inhibitor, Paxalisib, synergistically inhibited 2D and 3D cell proliferation. Further analysis demonstrated that tanshinone IIA provided an antioxidant and proautophagy effect in D. discoideum, that was reliant upon the activation of SESN. However in mammalian cells, SESN is commonly regulated by p53, a transcription factor, which is dysregulated in many cancer cells. Thus, novel compounds that can by-pass the upregulation of sesn and directly inhibit GATOR2 may provide robust treatments in all cancer types. A drug discovery protocol, screening 400 million compounds against the MIOS protein subunit of GATOR2, identified Mi3, a polyphenol compound, capable of inhibiting MIOS and inhibiting cell proliferation in both D. discoideum and GBM cell lines. To conclude, tanshinone IIA was found to function synergistically to inhibit GBM cell proliferation in both 2D and 3D cell culture with PI3K inhibitors, which was reliant upon the upregulation of SESN, and was also involved in the antioxidant and pro-autophagy role of tanshinone IIA. However, the targeted inhibition of GATOR2 has the potential to provide better therapeutic effects.