Apoptosis and antitumor effects induced by the combination of an mTOR inhibitor and an autophagy inhibitor in human osteosarcoma MG63 cells

Abstract

The inhibition of the mammalian target of rapamycin (mTOR) signaling pathway promotes the initiation of autophagy. Although it remains under debate whether chemotherapy-induced autophagy in tumor cells is a protective response or is invoked to promote cell death, recent studies indicate that autophagy is a self-defense mechanism of cancer cells that are subjected to antitumor agents and that blocking autophagy can trigger apoptosis. The aim of this study was to examine the effects of rapamycin, an mTOR inhibitor, on MG63 osteosarcoma cells. We further examined whether the combination of rapamycin and the small molecule inhibitor of autophagy Spautin-1 (specific and potent autophagy inhibitor-1) enhanced the rapamycin-induced apoptosis in MG63 cells. We examined the effects of rapamycin treatment on cell proliferation, phosphorylation of mTOR pathway components, and autophagy by western blot analysis. Furthermore, we examined the effects of rapamycin with or without Spautin-1 on the induction of apoptosis by western blot analysis and immunohistochemical staining. We found that rapamycin inhibited cell proliferation and decreased the phosphorylation of mTOR pathway components in MG63 cells. Rapamycin induced the apoptosis of MG63 cells, and this apoptosis was enhanced by Spautin-1. It was considered that Spautin-1 suppressed the protective mechanism induced by rapamycin in tumor cells and induced apoptosis. Therefore, the combination of an mTOR inhibitor and an autophagy inhibitor may be effective in the treatment of osteosarcoma because it effectively induces the apoptotic pathway.

The Role of Deoxycytidine Kinase (dCK) in Radiation-Induced Cell Death

“…Spautin-1 is a specific and potent autophagy inhibitor-1 that can inhibit USP10 and USP13, and promote the degradation of Beclin-1 [44]. As an autophagy inhibitor, 3-MA (3-methyladenine) can suppress class III PtdIns 3-kinase [45].…”

Cell apoptosis, autophagy and necroptosis in osteosarcoma treatment

“…NSC185058 could effectively inhibit ATG4B activity in vitro and in cells, while having no effect on MTOR and PtdIns3K activities; suggesting that NSC185058 is effective in suppressing autophagy in vivo and in attenuating osteosarcoma tumor growth [117]. Rapamycin inhibited cell proliferation and decreased the phosphorylation of mTOR pathway components in MG63 cells, while Spautin-1 suppressed the protective mechanism induced by rapamycin in osteosarcoma cells and effectively induced apoptosis [118]. Dendropanoxide (DP) newly isolated from leaves and stem of Dendropanax morbifera Leveille could induce autophagy through ERK1/2 activation in MG-63 human osteosarcoma cells, but inhibiting autophagy enhanced dendropanoxide-induced apoptosis [119].…”

Suppression of Autophagy Sensitizes Osteosarcoma to mTOR Inhibition

“…Thus far, there have been reports on the effect of Rapamycin in osteosarcoma, but there has been a dearth of reports on the combined mTOR/autophagy inhibition [10]. We intended to address the role of this combination in osteosarcoma in the current study, providing novel perspectives of treatments.…”