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Ascorbic acid selectively targets glucose metabolism of osteosarcoma stem cells

Introduction Osteosarcoma (OS) is the most common primary bone sarcoma that mainly occurs in children and adolescents. The existence of drug resistant cancer stem cells (CSCs) with progenitor properties is responsible for OS relapse and metastasis. Thus, development of specific therapies targeting OS-CSCs is necessary to increase the long-term survival rate. Although ascorbic acid (AA) has controversial history as anticancer agent, recently it has been re-evaluated revealing more cytotoxic effect to cancer than normal cells. The aim of the study was to analyze AA as potential therapeutic for selective targeting of OS-CSCs. Material and Methods To establish primary tumor cultures, tumor samples were mechanically dissected and enzymatically digested. Sarcosphere assay was used to isolate OS-CSCs. The cytotoxic effect of AA was determined by MTT assay as well as relationship between cell concentration and AA. OS-CSCs were treated with different concentrations of AA (2.5 - 55 µg/ml) during 72 h. Concentrations of AA used for further experiments were 30 µg/ml and 40 µg/ml, respectively. Effect of AA on sarcosphere-forming ability was measured under low-attachment condition during 28 days. Cell death type was determined by Annexin V/PI staining using flow cytometry. Levels of GAPDH were determined by western blot while ROS were measured by DCFH-DA assay. Seahorse XF analyzer was used to measure glycolysis and oxidative phosphorylation. Results and Discussions While AA did not have any effect on hMSCs, U2OS and Hek 293, respectively, AA efficiently induced dose-dependent viability reduction of OS-CSCs. Further, it can be concluded that IC50 values of AA depend on the number of seeded OS-CSCs. AA successfully reduced sarcosphere formation on 6th day. High cytotoxicity of AA was further confirmed by Annexin V/PI staining. Prevalent death mode induced by AA was apoptotic since more than 70% of Annexin V-positive cells were detected. In addition, AA inhibited the activity of the key glycolytic enzyme GAPDH and induced ROS levels. Following the treatment with AA, extracellular acidification rate as a measure of glycolysis, was reduced significantly. Moreover, AA increased metabolic potential of OS-CSCs implying cells’ ability to meet an energy demand via respiration and glycolysis. Conclusion Based on the obtained results, it can be concluded that AA selectively targets OS-CSCs. The death mechanism is based on the blockage of glycolytic cycle and increased intracellular levels of ROS.

osteosarcoma stem cells, ascorbic acid, cytotoxicity, metabolic potential

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