Publications (9) View all
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Article: High-dose rapamycin induces apoptosis in human cancer cells by dissociating mTOR complex 1 and suppressing phosphorylation of 4E-BP1.
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ABSTRACT: mTOR, the mammalian target of rapamycin, has been widely implicated in signals that promote cell cycle progression and survival in cancer cells. Rapamycin, which inhibits mTOR with high specificity, has consequently attracted much attention as an anti-cancer therapeutic. Rapamycin suppresses phosphorylation of S6 kinase at nano-molar concentrations, however at higher micro-molar doses, rapamycin induces apoptosis in several human cancer cell lines. While much is known about the effect of low dose rapamycin treatment, the mechanistic basis for the apoptotic effects of high-dose rapamycin treatment is not understood. We report here that the apoptotic effects of high-dose rapamycin treatment correlate with suppressing phosphorylation of the mTOR complex 1 substrate, eukaryotic initiation factor 4E (eIF4E) binding protein-1 (4E-BP1). Consistent with this observation, ablation of eIF4E also resulted in apoptorsis in MDA-MB 231 breast cancer cells. We also provide evidence that the differential dose effects of rapamycin are correlated with partial and complete dissociation of Raptor from mTORC1 at low and high doses, respectively. In contrast with MDA-MB-231 cells, MCF-7 breast cancer cells survived rapamycin-induced suppression of 4E-BP1 phosphorylation. We show that survival correlated with a hyper-phosphorylation of Akt at S473 at high rapamycin doses, the suppression of which conferred rapamycin sensitivity. This study reveals that the apoptotic effect of rapamycin requires doses that completely dissociate Raptor from mTORC1 and suppress that phosphorylation of 4E-BP1 and inhibit eIF4E.Cell cycle (Georgetown, Tex.) 11/2011; 10(22):3948-56. · 5.36 Impact Factor -
Article: AKT inhibition relieves feedback suppression of receptor tyrosine kinase expression and activity.
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ABSTRACT: Activation of the PI3K-AKT pathway in tumors is modulated by negative feedback, including mTORC1-mediated inhibition of upstream signaling. We now show that AKT inhibition induces the expression and phosphorylation of multiple receptor tyrosine kinases (RTKs). In a wide spectrum of tumor types, inhibition of AKT induces a conserved set of RTKs, including HER3, IGF-1R, and insulin receptor. This is in part due to mTORC1 inhibition and in part secondary to a FOXO-dependent activation of receptor expression. PI3K-AKT inhibitors relieve this feedback and activate RTK signaling; this may attenuate their antitumor activity. Consistent with this model, we find that, in tumors in which AKT suppresses HER3 expression, combined inhibition of AKT and HER kinase activity is more effective than either alone.Cancer cell 01/2011; 19(1):58-71. · 25.29 Impact Factor -
Article: mTOR kinase inhibition causes feedback-dependent biphasic regulation of AKT signaling.
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ABSTRACT: mTOR kinase inhibitors block mTORC1 and mTORC2 and thus do not cause the mTORC2 activation of AKT observed with rapamycin. We now show, however, that these drugs have a biphasic effect on AKT. Inhibition of mTORC2 leads to AKT serine 473 (S473) dephosphorylation and a rapid but transient inhibition of AKT T308 phosphorylation and AKT signaling. However, inhibition of mTOR kinase also relieves feedback inhibition of receptor tyrosine kinases (RTK), leading to subsequent phosphoinositide 3-kinase activation and rephosphorylation of AKT T308 sufficient to reactivate AKT activity and signaling. Thus, catalytic inhibition of mTOR kinase leads to a new steady state characterized by profound suppression of mTORC1 and accumulation of activated AKT phosphorylated on T308, but not S473. Combined inhibition of mTOR kinase and the induced RTKs fully abolishes AKT signaling and results in substantial cell death and tumor regression in vivo. These findings reveal the adaptive capabilities of oncogenic signaling networks and the limitations of monotherapy for inhibiting feedback-regulated pathways. SIGNIFICANCE: The results of this study show the adaptive capabilities of oncogenic signaling networks, as AKT signaling becomes reactivated through a feedback-induced AKT species phosphorylated on T308 but lacking S473. The addition of RTK inhibitors can prevent this reactivation of AKT signaling and cause profound cell death and tumor regression in vivo, highlighting the possible need for combinatorial approaches to block feedback-regulated pathways.Cancer discovery. 06/2011; 1(3):248-59. -
Article: Phosphorylated 4E-BP1 is associated with poor survival in melanoma.
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ABSTRACT: Both phosphatidylinositol 3-kinase/AKT and RAS/mitogen-activated protein kinase signal transduction pathways mediate 4E-BP1 phosphorylation, releasing 4E-BP1 from the mRNA cap and permitting translation initiation. Given the prevalence of PTEN and BRAF mutations in melanoma, we first examined translation initiation, as measured by phosphorylated 4E-BP1 (p-4E-BP1), in metastatic melanoma tissues and cell lines. We then tested the association between amounts of total and p-4E-BP1 and patient survival. Seven human metastatic melanoma cells lines and 72 metastatic melanoma patients with accessible metastatic tumor tissues and extended follow-up information were studied. Expression of 4E-BP1 transcript, total 4E-BP1 protein, and p-4E-BP1 was examined. The relationship between 4E-BP1 transcript and protein expression was assessed in a subset of patient tumors (n = 41). The association between total and p-4E-BP1 levels and survival was examined in the larger cohort of patients (n = 72). 4E-BP1 was hyperphosphorylated in 4 of 7 melanoma cell lines harboring both BRAF and PTEN mutations compared with untransformed melanocytes or RAS/RAF/PTEN wild-type melanoma cells. 4E-BP1 transcript correlated with 4E-BP1 total protein levels as measured by the semiquantitative reverse-phase protein array (P = 0.012). High levels of p-4E-BP1 were associated with worse overall and post-recurrence survival (P = 0.02 and 0.0003, respectively). Our data show that translation initiation is a common event in human metastatic melanoma and correlates with worse prognosis. Therefore, effective inhibition of the pathways responsible for 4E-BP1 phosphorylation should be considered to improve the treatment outcome of metastatic melanoma patients.Clinical Cancer Research 04/2009; 15(8):2872-8. · 7.74 Impact Factor -
Article: Myc stabilization in response to estrogen and phospholipase D in MCF-7 breast cancer cells.
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ABSTRACT: Estrogen, which has been strongly implicated in breast cancer, suppresses apoptosis in estrogen receptor (ER) positive MCF-7 breast cancer cells. Phospholipase D (PLD), which is commonly elevated in ER negative breast cancer cells, also suppresses apoptosis. Survival signals generated by both estrogen and PLD are dependent upon elevated Myc expression. We report here that estrogen- and PLD-induced increases in Myc expression are due to reduced turnover of Myc protein. Estrogen and PLD suppressed phosphorylation of Myc at Thr58--a site that targets Myc for degradation by the proteasome. The data provide a mechanism for elevated Myc expression in hormone-dependent and hormone-independent breast cancer.FEBS Letters 11/2006; 580(24):5647-52. · 3.54 Impact Factor
