Yijiang Shi

Mammalian target of rapamycin inhibitors activate the AKT kinase in multiple myeloma cells by up-regulating the insulin-like growth factor receptor/insulin receptor substrate-1/phosphatidylinositol 3-kinase cascade

Mammalian target of rapamycin (mTOR) inhibitors, such as rapamycin and CCI-779, have shown preclinical potential as therapy for multiple myeloma. By inhibiting expression of cell cycle proteins, these agents induce G1 arrest. However, by also inhibiting an mTOR-dependent serine phosphorylation of insulin receptor substrate-1 (IRS-1), they may enhance insulin-like growth factor-I (IGF-I) signaling and downstream phosphatidylinositol 3-kinase (PI3K)/AKT activation. This may be a particular problem in multiple myeloma where IGF-I-induced activation of AKT is an important antiapoptotic cascade. We, therefore, studied AKT activation in multiple myeloma cells treated with mTOR inhibitors. Rapamycin enhanced basal AKT activity, AKT phosphorylation, and PI3K activity in multiple myeloma cells and prolonged activation of AKT induced by exogenous IGF-I. CCI-779, used in a xenograft model, also resulted in multiple myeloma cell AKT activation in vivo. Blockade of IGF-I receptor function prevented rapamycins activation of AKT. Furthermore, rapamycin prevented serine phosphorylation of IRS-1, enhanced IRS-1 association with IGF-I receptors, and prevented IRS-1 degradation. Although similarly blocking IRS-1 degradation, proteasome inhibitors did not activate AKT. Thus, mTOR inhibitors activate PI3-K/AKT in multiple myeloma cells; activation depends on basal IGF-R signaling; and enhanced IRS-1/IGF-I receptor interactions secondary to inhibited IRS-1 serine phosphorylation may play a role in activation of the cascade. In cotreatment experiments, rapamycin inhibited myeloma cell apoptosis induced by PS-341. These results provide a caveat for future use of mTOR inhibitors in myeloma patients if they are to be combined with apoptosis-inducing agents.

AKT Activity Determines Sensitivity to Mammalian Target of Rapamycin (mTOR) Inhibitors by Regulating Cyclin D1 and c-myc Expression

Prior work demonstrates that AKT activity regulates sensitivity of cells to G1 arrest induced by mammalian target of rapamycin (mTOR) inhibitors such as rapamycin and CCI-779. To investigate this, a novel high-throughput microarray polysome analysis was performed to identify genes whose mRNA translational efficiency was differentially affected following mTOR inhibition. The analysis also allowed the assessment of steady-state transcript levels. We identified two transcripts, cyclin D1 and c-myc, which exhibited differential expression in an AKT-dependent manner: High levels of activated AKT resulted in rapamycin-induced down-regulation of expression, whereas low levels resulted in up-regulation of expression. To ectopically express these proteins we exploited the finding that the p27kip1 mRNA was efficiently translated in the face of mTOR inhibition irrespective of AKT activity. Thus, the p27kip1 5′-untranslated region was fused to the cyclin D1 and c-myc coding regions and these constructs were expressed in cells. In transfected cells, expression of cyclin D1 or c-myc was not decreased by rapamycin. Most importantly, this completely converted sensitive cells to a phenotype resistant to G1 arrest. Furthermore, the AKT-dependent differential expression patterns of these two genes was also observed in a mouse xenograft model following in vivo treatment with CCI-779. These results identify two critical downstream molecular targets whose expression is regulated by AKT activity and whose down-regulation is required for rapamycin/CCI-779 sensitivity.

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

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 post-translational 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 mTOR-induced 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.

Effect of autophagy on multiple myeloma cell viability

Because accumulation of potentially toxic malfolded protein may be extensive in immunoglobulin-producing multiple myeloma (MM) cells, we investigated the phenomenon of autophagy in myeloma, a physiologic process that can protect against malfolded protein under some circumstances. Autophagy in MM cell lines that express and secrete immunoglobulin and primary specimens was significantly increased by treatment with the endoplasmic reticulum stress–inducing agent thapsigargin, the mammalian target of rapamycin inhibitor rapamycin, and the proteasome inhibitor bortezomib. Inhibition of basal autophagy in these cell lines and primary cells by use of the inhibitors 3-methyladenine and chloroquine resulted in a cytotoxic effect that was associated with enhanced apoptosis. Use of small interfering RNA to knock down expression of beclin-1, a key protein required for autophagy, also inhibited viable recovery of MM cells. Because the data suggested that autophagy protected MM cell viability, we predicted that autophagy inhibitors would synergize with bortezomib for enhanced antimyeloma effects. However, the combination of these drugs resulted in an antagonistic response. In contrast, the autophagy inhibitor 3-methyladenine did synergize with thapsigargin for an enhanced cytotoxic response. These data suggest that autophagy inhibitors have therapeutic potential in myeloma but caution against combining such drugs with bortezomib. [Mol Cancer Ther 2009;8(7):1974–84]

Targeting TORC2 in multiple myeloma with a new mTOR kinase inhibitor

Although preclinical work with rapalogs suggests potential in treatment of multiple myeloma (MM), they have been less successful clinically. These drugs allostearically inhibit the mammalian target of rapamycin kinase primarily curtailing activity of the target of rapamycin complex (TORC)1. To assess if the mammalian target of rapamycin within the TORC2 complex could be a better target in MM, we tested a new agent, pp242, which prevents activation of TORC2 as well as TORC1. Although comparable to rapamycin against phosphorylation of the TORC1 substrates p70S6kinase and 4E-BP-1, pp242 could also inhibit phosphorylation of AKT on serine 473, a TORC2 substrate, while rapamycin was ineffective. pp242 was also more effective than rapamycin in achieving cytoreduction and apoptosis in MM cells. In addition, pp242 was an effective agent against primary MM cells in vitro and growth of 8226 cells in mice. Knockdown of the TORC2 complex protein, rictor, was deleterious to MM cells further supporting TORC2 as the critical target for pp242. TORC2 activation was frequently identified in primary specimens by immunostaining for AKT phosphorylation on serine 473. Potential mechanisms of up-regulated TORC2 activity in MM were stimulation with interleukin-6 or insulin-like growth factor 1, and phosphatase and tensin homolog or RAS alterations. Combining pp242 with bortezomib led to synergistic anti-MM effects. These results support TORC2 as a therapeutic target in MM.

Role of the AKT kinase in expansion of multiple myeloma clones: effects on cytokine-dependent proliferative and survival responses.

IL-6 is an established growth factor for multiple myeloma tumor cells, stimulating proliferative and survival responses. Recent work indicates that IL-6 can activate the AKT kinase in myeloma cells. Thus, to test a potential role for AKT in IL-6-induced cellular responses, we transfected myeloma cell lines with an active ‘E40K’ or dominant negative ‘PH’ AKT construct using an adenoviral vector. Transfection of the E40K into myeloma cells resulted in enhanced tumor cell growth and expression of the PH dominant negative AKT resulted in both inhibition of the IL-6-dependent proliferative response and a decrease in S phase distribution. While transfection of E40K protected myeloma cells from dexamethasone-induced apoptosis, the dominant negative PH had no effect on the ability of IL-6 to protect these cells from dexamethasone. These results clearly demonstrate that AKT activation is critical for the IL-6 proliferative response. In addition, although the level of AKT activation can regulate sensitivity to dexamethasone-induced apoptosis, additional cytokine-induced AKT-independent pathways can mediate IL-6 protection against dexamethasone.

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

We recently demonstrated that the mammalian target of rapamycin (mTOR) inhibitor, CCI-779, curtailed the growth of a subcutaneous challenge of multiple myeloma (MM) cells in immunodeficient mice. This antitumor effect was associated with prevention of cell proliferation, induction of apoptosis and inhibition of angiogenesis. Interestingly, myeloma tumors with heightened AKT activation were particularly sensitive to a CCI-779-induced antitumor response. To investigate whether part of the differential sensitivity was due to an AKT-regulated effect on angiogenesis, we compared the effects of mTOR inhibitors against isogenic MM cell lines that only differ by their degree of AKT activity. In this model, heightened AKT activity significantly sensitized MM cells to the following inhibitory effects of mTOR inhibition: angiogenesis in vivo, vascular endothelial growth factor (VEGF) expression in vitro and in vivo and VEGF translation (but not transcription). Assessment of p70S6 kinase activity indicated that rapamycin induced comparable mTOR inhibition in both cell lines suggesting that an adverse effect on VEGF cap-dependent translation would be comparable. Internal ribosome entry site (IRES)-mediated cap-independent translation is a salvage pathway for protein expression when mTOR is inhibited, so we analyzed a possible regulatory role of AKT on VEGF IRES activity. We found that elevated AKT activity inhibited VEGF IRES function. These results support a mechanism whereby AKT prevents VEGF IRES activity in myeloma cells during mTOR inhibition resulting in a more complete abrogation of VEGF translation, and ultimately, angiogenesis.

Interleukin-6 activates phosphoinositol-3' kinase in multiple myeloma tumor cells by signaling through RAS-dependent and, separately, through p85-dependent pathways.

The IL-6-induced activation of the phosphatidylinositol-3′ kinase (PI3-K)/AKT cascade in multiple myeloma (MM) cells is critical for tumor cell proliferation and viability. Since the IL-6 receptor does not contain binding sites for the p85 regulatory portion of PI3-K, intermediate molecules must play a role. Coimmunoprecipitation studies in MM cell lines demonstrated the IL-6-induced formation of two independent PI3-K-containing complexes: one containing p21 RAS but not STAT-3 and a second containing STAT-3 but not RAS. Both complexes demonstrated IL-6-induced lipid kinase activity. IL-6 also generated kinase activity in a mutant p110 molecule that could not bind p85. Use of dominant-negative (DN) constructs confirmed the presence of two independent pathways of activation: a DN RAS prevented the IL-6-induced generation of lipid kinase activity in the mutant p110 molecule but had no effect on activity generated in the STAT-3-containing complex. In contrast, a DN p85 prevented the generation of kinase activity in the STAT-3-containing complex but had no effect on activity generated in the p110 molecule. Both DN constructs significantly prevented the IL-6-induced activation of AKT. MM cells expressing activating RAS mutations demonstrated enhanced IL-6-independent growth and constitutive PI3-K activity. These data indicate two potential independent pathways of PI3-K/AKT activation in MM cells: one mediated via signaling through RAS which is independent of p85 and a second mediated via p85 and due to a STAT-3-containing complex.

IL-6-Induced Stimulation of c-Myc Translation in Multiple Myeloma Cells Is Mediated by Myc Internal Ribosome Entry Site Function and the RNA-Binding Protein, hnRNP A1

Prior work indicates that c-myc translation is up-regulated in multiple myeloma cells. To test a role for interleukin (IL)-6 in myc translation, we studied the IL-6-responsive ANBL-6 and IL-6-autocrine U266 cell lines as well as primary patient samples. IL-6 increased c-myc translation, which was resistant to rapamycin, indicating a mechanism independent of mammalian target of rapamycin (mTOR) and cap-dependent translation. In contrast, the cytokine enhanced cap-independent translation via a stimulatory effect on the myc internal ribosome entry site (IRES). As known IRES-trans-activating factors (ITAF) were unaffected by IL-6, we used a yeast-three-hybrid screen to identify novel ITAFs and identified hnRNP A1 (A1) as a mediator of the IL-6 effect. A1 specifically interacted with the myc IRES in filter binding assays as well as EMSAs. Treatment of myeloma cells with IL-6 induced serine phosphorylation of A1 and increased its binding to the myc IRES in vivo in myeloma cells. Primary patient samples also showed binding between A1 and the IRES. RNA interference to knock down hnRNP A1 prevented an IL-6 increase in myc protein expression, myc IRES activity, and cell growth. These data point to hnRNP A1 as a critical regulator of c-myc translation and a potential therapeutic target in multiple myeloma.

The PP242 Mammalian Target of Rapamycin (mTOR) Inhibitor Activates Extracellular Signal-regulated Kinase (ERK) in Multiple Myeloma Cells via a Target of Rapamycin Complex 1 (TORC1)/ Eukaryotic Translation Initiation Factor 4E (eIF-4E)/RAF Pathway and Activation Is a Mechanism of Resistance

Background: Although the active site mTOR inhibitor pp242 overcomes feedback activation of AKT, it may still be complicated by feedback ERK activation. Results: In myeloma cell models, pp242 was more potent than rapamycin for activating ERK, causing resistance. Conclusion: Activation of ERK is a complication of pp242. Significance: PP242 would be more effective if used in combination with inhibitors of the ERK pathway. Activation of PI3-K-AKT and ERK pathways is a complication of mTOR inhibitor therapy. Newer mTOR inhibitors (like pp242) can overcome feedback activation of AKT in multiple myeloma (MM) cells. We, thus, studied if feedback activation of ERK is still a complication of therapy with such drugs in this tumor model. PP242 induced ERK activation in MM cell lines as well as primary cells. Surprisingly, equimolar concentrations of rapamycin were relatively ineffective at ERK activation. Activation was not correlated with P70S6kinase inhibition nor was it prevented by PI3-kinase inhibition. ERK activation was prevented by MEK inhibitors and was associated with concurrent stimulation of RAF kinase activity but not RAS activation. RAF activation correlated with decreased phosphorylation of RAF at Ser-289, Ser-296, and Ser-301 inhibitory residues. Knockdown studies confirmed TORC1 inhibition was the key proximal event that resulted in ERK activation. Furthermore, ectopic expression of eIF-4E blunted pp242-induced ERK phosphorylation. Since pp242 was more potent than rapamycin in causing sequestering of eIF-4E, a TORC1/4E-BP1/eIF-4E-mediated mechanism of ERK activation could explain the greater effectiveness of pp242. Use of MEK inhibitors confirmed ERK activation served as a mechanism of resistance to the lethal effects of pp242. Thus, although active site mTOR inhibitors overcome AKT activation often seen with rapalog therapy, feedback ERK activation is still a problem of resistance, is more severe than that seen with use of first generation rapalogs and is mediated by a TORC1- and eIF-4E-dependent mechanism ultimately signaling to RAF.