(P 85) PTEROSTILBENE SENSITIZES OSTEOSARCOMA CELLS TO KILLING BY C-MYC INHIBITORS

Objective: Mitochondria are places of energy production, and where reactive oxygen species are produced as well. In recent years, it has been reported that oxidative metabolism (OXPHOS) is superior to glycolysis in the metabolism of cancer stem cells, and it is expected to be a new therapeutic target. Pterostilbene (PTE) is a methylated resveratrol of plant origin and has been reported for its anti-cancer cell effects in various types of cancers. As a mechanism of action, we have previouly reported the inhibitory action of PTE against F0-F1 ATP synthase which is an energy producing enzyme in mitochondria, and will present here the synergistic effects of PTE with cMyc inhibitors against osteosarcomas in vitro and in vivo.

Methods: Using SaOS2, U2OS, and MG63 cells, cell viability with and without PTE was assessed, and the expression of stem cell markers Oct3, NS, and CD44 was examined. Next, the amount of ATP was measured and mitochondrial ATP synthase activity was measured. In addition, synergistic effect of PTW with JQ1 or Honokiol, both the cMYC transcription inhibitor, was assessed by MTS assay. Finally, their synergistic effects in vivo were investigated with xenograft model of MG63 and SaOS2 in nude mice.

Results: PTE treatment reduced the viability of all cells in a dose-dependent manner. The expression of stem cell markers was also reduced. The activity of F0-F1 ATP synthase was mainly reduced, and the amount of ATP synthesis was also reduced in spheroid culture. This suggests that PTE inhibits ATP synthesis in osteosarcoma stem cells via F0-F1 ATP synthase inhibition. The cytotoxicity of PTE to osteosarcoma cells was significantly enhanced by both JQ1 and Honokiol. In vivo experiments, the combination of PTE with cMyc inhibitors Honokiol or JQ1 synergistically suppressed the tumor growth.

Conclusion: Prognosis of the patients with osteosarcoma has reached to plateau without any breakthroughs over the last quarter century, and nearly 30% of patients still have to face very severe poor prognosis, especially with metastatic disease. In recent years, it has been reported that cancer stem cells are dependent on OXPHOS. Contrary, it is known that cMYC is highly expressed in non-cancer stem cells and carries out glycolysis-dominant metabolism. cMYC inhibitors were thought to induce metabolic fluxes to OXPHOS and were thought to have a synergistic effect with OXPHOS inhibition. Current study suggests that modulation of metabolic flux by cMyc and OXPHOS inhibitors showed a greater synergistic effect, and it could be a novel therapeutic strategy against osteosarcoma, possibly targeting both stem-like cell population and general tumor cell population.