Anti-proliferative effect of cardamonin on mTOR inhibitor-resistant cancer cells

Abstract

A number of mammalian target of rapamycin (mTOR) inhibitors have been approved for the treatment of certain types of cancer or are currently undergoing clinical trials. However, mTOR targeted therapy exerts selective pressure on tumour cells, which leads to the preferential growth of resistant subpopulations. There are two classes of mTOR inhibitors: i) The rapalogs, such as rapamycin, which bind to the 12-kDa FK506-binding protein/rapamycin-binding domain of mTOR; and ii) the ATP-competitive inhibitors, such as AZD8055, which block the mTOR kinase domain. Cardamonin inhibits mTOR by decreasing the expression of regulatory-associated protein of mTOR (Raptor), a mechanism of action which differs from the currently available mTOR inhibitors. The present study investigated the inhibitory effects of cardamonin on mTOR inhibitor-resistant cancer cells. HeLa cervical cancer cells and MCF-7 breast cancer cells were exposed to high concentrations of mTOR inhibitors, until resistant clones emerged. Cytotoxicity was measured using the MTT and colony forming assays. The inhibitory effect of cardamonin on mTOR signalling was assessed by western blotting. The resistant cells were less sensitive to mTOR inhibitors compared with the parental cells. Consistent with the anti-proliferation effect, rapamycin and AZD8055 had no effect on the phosphorylation of rapamycin-sensitive sites on ribosomal protein S6 kinase B1 (S6K1) and AZD8055-sensitive sites on protein kinase B and eukaryotic translation initiation factor 4E binding protein 1 (Thr 37/46), respectively, in rapamycin- and AZD8055-resistant cells. Cardamonin inhibited cell proliferation and decreased the phosphorylation of mTOR and S6K1, as well as the protein level of raptor, in the mTOR inhibitor-resistant cells. Therefore, cardamonin may serve as a therapeutic agent for patients with cervical and breast cancer resistant to mTOR inhibitors.