Mechanism by Which Mammalian Target of Rapamycin Inhibitors Sensitize Multiple Myeloma Cells to Dexamethasone-Induced Apoptosis

Abstract

Mammalian target of rapamycin (mTOR) inhibitors curtail cap-dependent translation. However, they can also induce post-translational modifications of proteins. We assessed both effects to understand the mechanism by which mTOR inhibitors like rapamycin sensitize multiple myeloma cells to dexamethasone-induced apoptosis. Sensitization was achieved in multiple myeloma cells irrespective of their PTEN or p53 status, enhanced by activation of AKT, and associated with stimulation of both intrinsic and extrinsic pathways of apoptosis. The sensitizing effect was not due to posttranslational modifications of the RAFTK kinase, Jun kinase, p38 mitogen-activated protein kinase, or BAD. Sensitization was also not associated with a rapamycin-mediated increase in glucocorticoid receptor reporter expression. However, when cap-dependent translation was prevented by transfection with a mutant 4E-BP1 construct, which is resistant to mTORinduced phosphorylation, cells responded to dexamethasone with enhanced apoptosis, mirroring the effect of coexposure to rapamycin. Thus, sensitization is mediated by inhibition of cap-dependent translation. A high-throughput screening for translational efficiency identified several antiapoptotic proteins whose translation was inhibited by rapamycin. Immunoblot assay confirmed rapamycin-induced down-regulated expressions of XIAP, CIAP1, HSP-27, and BAG-3, which may play a role in the sensitization to apoptosis. Studies in a xenograft model showed synergistic in vivo antimyeloma effects when dexamethasone was combined with the mTOR inhibitor CCI-779. Synergistic effects were associated with an enhanced multiple myeloma cell apoptosis in vivo. This study supports the strategy of combining dexamethasone with mTOR inhibitors in multiple myeloma and identifies a mechanism by which the synergistic effect is achieved. (Cancer Res 2006; 66(4): 2305-13)

Apoptosis inhibition by Bcl-2 gives way to autophagy in glucocorticoid-treated lymphocytes

“…[32][33][34] At first this may seem paradoxical, as rapamycin is reported to induce autophagy through inhibition of mTOR. 35 However, mTOR also reduces expression of a number of anti-apoptotic proteins, including Bcl-2 and Mcl-1.…”

A Sequential Blockade Strategy for the Design of Combination Therapies to Overcome Oncogene Addiction in Chronic Myelogenous Leukemia

“…The mammalian target of rapamycin (mTOR) facilitates the assembly of the translation preinitiation complex. mTOR inhibition by rapamycin or its analogues has been shown to decrease protein synthesis (42).…”

AKT activity regulates the ability of mTOR inhibitors to prevent angiogenesis and VEGF expression in multiple myeloma cells

“…Confirmation of a true regulatory effect of AKT on sensitivity was obtained when an activated AKT allele was stably transfected into U266 cells. This stably transfected myeloma line was considerably more sensitive in vivo to the antitumor effects of mTOR inhibition than its empty vector transfected control (Frost et al, 2004;Yan et al, 2006).…”

“…In a xenograft model, we showed that the mTOR inhibitor, CCI-779, was an effective anti-myeloma drug, inhibiting in vivo tumor growth of OPM-2, 8226 and U266 MM tumors (Frost et al, 2004;Yan et al, 2006). Interestingly, the level of AKT activity was correlated with antitumor response to CCI-779, with OPM-2, which expresses constitutively active AKT owing to a phosphatase and tensin homolog deleted on chromosome ten (PTEN) mutation (Hyun et al, 2000), being the most sensitive.…”