Therapeutic targeting of osteosarcoma stem cells
CSCs exploit various mechanisms to deregulate apoptotic signaling pathways, including the ability to enter quiescence and slow the rate of proliferation [86]. This can be combined with overexpression of ABC drug efflux transporters to enhance chemoresistance [87]. Inhibition of drug efflux transporters was found to enhance cellular uptake of doxorubicin and initiate apoptosis in OSCs; whereas, efflux activity of drug transporters P-glycoprotein and breast-cancer related proteins facilitated OSCs entering quiescence through activation of anti-apoptotic mediators Bcl-2 and Bcl-xL, downregulation of Bak, and caspase-3/7 inactivation [88]. The anticancer agent salinomycin has been shown to be an effective inhibitor of OSCs in vivo and in vitro. Salinomycin inhibited expressions of Oct4 and Sox2 and suppressed the sphere-forming capacity and chemoresistance in OSCs without severe side effects. Salinomycin can inhibit Wnt pathway activity suggesting the involvement of Wnt/β-catenin signaling [34]. Bufalin is the active component in the Chinese medicine Chan Su. Bufalin treatment of immunodeficient mice and OSCs derived from the MG63 cell line resulted in their inability to differentiate. Further investigation revealed the expression of stem cell markers CD133 and Oct4. Treatment with bufalin also inhibited sphere forming and proliferation in these cells [25].
Signaling pathways associated with OSC niche functions may present novel targets for the treatment of osteosarcoma: studies have shown that the Hedgehog and Notch pathways participate in the perivascular niche; and VEGF/VEGFR and SDF-1/CXCR4 contributed to angiogenesis and reprograming of endothelial cells in the GSC niche. Conversely, anti-VEGF and CXCR4 could suppress tumor angiogenesis and may break down the perivascular niche.
TSSC3 is an imprinting gene that is primarily regulated by methylation. As reported above, expression of TSSC3 was found to be significantly lower in OSCs than in differentiated osteosarcoma cells; conversely overexpression of TSSC3 could increase apoptosis and inhibit drug resistance in OSCs [35]. These findings suggest that TSSC3 expression in osteosarcoma could be targeted by demethylation drugs. MST312 [53] and caffeine are telomerase and DNA repair inhibitors, respectively, suggesting they may also be candidates for OSC-targeted therapies [13]. Telomerase is partially responsible for immortalization in malignant tumor cells, and elevated telomerase activity has been implicated in self-renewal and chemoresistance in OSCs [89]; caffeine enhanced the efficacy of doxorubicin and cisplatin, indicating that drug resistance in sarcosphere cells was related to an efficient DNA repair capacity in OSCs. Other potential therapeutic agents for TSC (Tumor stem cells) ablation in osteosarcoma include the epigenetic regulator (5-azacytidine), anti-microtubule drug (vincristine) and telomerase inhibitor (RHPS4) [55].