Cytotoxic and Cytostatic Properties of Rapamycin: Implications for Antitumor Efficacy
Item
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Title
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Cytotoxic and Cytostatic Properties of Rapamycin: Implications for Antitumor Efficacy
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Identifier
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d_2009_2013:75675eb410e9:11591
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identifier
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12034
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Creator
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Yellen, Paige B.,
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Contributor
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David Foster
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Date
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2012
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Language
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English
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Publisher
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City University of New York.
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Subject
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Cellular biology | Molecular biology | Oncology | Pharmacy sciences
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Abstract
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mTOR (mammalian target of rapamycin) is a central regulator of cell growth and proliferation. Frequently dysregulated in cancer cells, it is an attractive therapeutic target, named for its first inhibitor, rapamycin. Low (nanomolar) doses of rapamycin treatment inhibit phosphorylation of mTORC1 (mTOR complex 1) substrate, S6 kinase, thwarting protein synthesis and subsequently, proliferation. Though highly potent and specific, rapamcyin has lacked clinical success because it lacks universal anti-tumor effects and only a fraction of patients respond.;We have revealed rapamycin as a site-specific, cytotoxic anticancer drug in the absence of serum. Our studies demonstrate that high-dose rapamcyin induces apoptosis because it effectively inhibits all phosphorylation sites on 4E-BP1, subsequently inactivating eIF4E. Importantly, we demonstrate that cancer cells are resistant to high-dose rapamycin upon 4E-BP1 knockdown, confirming that the drug retains its site-specific property regardless of dose. Furthermore, we show that high-dose rapamycin irreversibly compromises the integrity of the mTORC1 complex, as it pertains to mTOR-raptor association. We acknowledge the inefficacy of rapamycin in cancer cells that activate Akt upon mTORC1 inhibition. Under these circumstances, an inhibitor of both mTORC1 and mTORC2 complexes is necessary and sufficient to induce apoptosis.;Our current study is to further investigate the surprising observation that while high-dose rapamycin treatment (indirect inactivation of eIF4E) induces apoptosis only in the absence of serum, knockdown of eIF4E (direct inactivation of eIF4E) induces apoptosis in both the presence and absence of serum. Thus, there is a mechanistic difference between the indirect and direct inactivation of eIF4E.;Since high-dose rapamycin treatment also inhibits phosphorylation of S6K and does not induce apoptosis in the presence of serum, we reasoned that protection from apoptosis induced by high-dose rapamycin could be due to suppression of S6K. Consistent with this hypothesis, dual S6K and eIF4E knockdown prevents cell death otherwise induced by knockdown of eIF4E alone.;We hypothesize that simultaneous inhibition of both S6K and eIF4E results in coordinate induction of transforming growth factor-beta (TGF-beta) signaling that is sufficient to induce arrest. This is predicated on prior work in the lab that shows high-dose rapamycin causes G1 arrest rather than apoptosis, if and only if, TGF-beta signaling is intact. TGF-beta mediates a cytostatic response by activation of Smad signaling and cyclin-dependent kinase inhibitor (CDK) p27.;Altogether, these data reveal the complexity of high-dose rapamycin, with implications for both cytotoxic and cytostatic effectiveness in the absence and presence of serum, respectively.
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Type
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dissertation
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Source
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2009_2013.csv
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degree
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Ph.D.
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Program
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Biology
