mTORC1 is essential for leukemia propagation but not stem cell self-renewal
01 Jun 2012-Journal of Clinical Investigation (American Society for Clinical Investigation)-Vol. 122, Iss: 6, pp 2114-2129
TL;DR: Transplantation of Raptor-deficient undifferentiated AML cells in a limiting dilution revealed that mTORC1 is essential for leukemia initiation, and it was demonstrated that the reactivation of m TORC1 in those cells restored their leukemia-initiating capacity.
Abstract: Although dysregulation of mTOR complex 1 (mTORC1) promotes leukemogenesis, how mTORC1 affects established leukemia is unclear. We investigated the role of mTORC1 in mouse hematopoiesis using a mouse model of conditional deletion of Raptor, an essential component of mTORC1. Raptor deficiency impaired granulocyte and B cell development but did not alter survival or proliferation of hematopoietic progenitor cells. In a mouse model of acute myeloid leukemia (AML), Raptor deficiency significantly suppressed leukemia progression by causing apoptosis of differentiated, but not undifferentiated, leukemia cells. mTORC1 did not control cell cycle or cell growth in undifferentiated AML cells in vivo. Transplantation of Raptor-deficient undifferentiated AML cells in a limiting dilution revealed that mTORC1 is essential for leukemia initiation. Strikingly, a subset of AML cells with undifferentiated phenotypes survived long-term in the absence of mTORC1 activity. We further demonstrated that the reactivation of mTORC1 in those cells restored their leukemia-initiating capacity. Thus, AML cells lacking mTORC1 activity can self-renew as AML stem cells. Our findings provide mechanistic insight into how residual tumor cells circumvent anticancer therapies and drive tumor recurrence.
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Journal Article•
TL;DR: Research data show that more resistant stem cells than common cancer cells exist in cancer patients, and to identify unrecognized differences between cancer stem cells and cancer cells might be able to develop effective classification, diagnose and treat for cancer.
Abstract: Stem cells are defined as cells able to both extensively self-renew and differentiate into progenitors. Research data show that more resistant stem cells than common cancer cells exist in cancer patients.To identify unrecognized differences between cancer stem cells and cancer cells might be able to develope effective classification,diagnose and treat ment for cancer.
2,194 citations
TL;DR: It is shown that phosphorylation of the autophagy-adaptor protein p62 markedly increases p62's binding affinity for Keap1, an adaptor of the Cul3-ubiquitin E3 ligase complex responsible for degrading Nrf2, and that inhibitors of the interaction between phosphorylated p62 and Keap 1 have potential as therapeutic agents against human HCC.
824 citations
Cites background from "mTORC1 is essential for leukemia pr..."
...Phosphorylation of p62 and induction of Nqo1 both decreased upon As(III) exposure in MEFs lacking Raptor (Hoshii et al., 2012), a component of the mTORC1 complex (Figures S2B and S2C)....
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Dissertation•
01 Jan 2010
262 citations
TL;DR: A comprehensive review of the current understanding of the mechanisms and regulation of autophagy in embryonic stem cells, several tissue stem cells (particularly hematopoietic stem cells), as well as a number of cancer stem cells is provided.
Abstract: Autophagy is a highly conserved cellular process by which cytoplasmic components are sequestered in autophagosomes and delivered to lysosomes for degradation. As a major intracellular degradation and recycling pathway, autophagy is crucial for maintaining cellular homeostasis as well as remodeling during normal development, and dysfunctions in autophagy have been associated with a variety of pathologies including cancer, inflammatory bowel disease and neurodegenerative disease. Stem cells are unique in their ability to self-renew and differentiate into various cells in the body, which are important in development, tissue renewal and a range of disease processes. Therefore, it is predicted that autophagy would be crucial for the quality control mechanisms and maintenance of cellular homeostasis in various stem cells given their relatively long life in the organisms. In contrast to the extensive body of knowledge available for somatic cells, the role of autophagy in the maintenance and function of stem cells is only beginning to be revealed as a result of recent studies. Here we provide a comprehensive review of the current understanding of the mechanisms and regulation of autophagy in embryonic stem cells, several tissue stem cells (particularly hematopoietic stem cells), as well as a number of cancer stem cells. We discuss how recent studies of different knockout mice models have defined the roles of various autophagy genes and related pathways in the regulation of the maintenance, expansion and differentiation of various stem cells. We also highlight the many unanswered questions that will help to drive further research at the intersection of autophagy and stem cell biology in the near future.
255 citations
Cites background from "mTORC1 is essential for leukemia pr..."
...Conversely, the deletion of the MTORC1 component RAPTOR, therefore theoretically causing an increase in autophagy, results in a decrease of this myeloid population.(91) However, it remains to be shown definitively that loss or gain of autophagy contributes to this phenotype, as MTOR inhibition signals for many other important cellular functions such as inhibition of protein translation, mitochondrial biogenesis, cell growth, motility and proliferation....
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TL;DR: It is shown that glutamine removal inhibits mTORC1 and induces apoptosis in AML cells, and that l-ases upregulate glutamine synthase expression in leukemic cells and that a GS knockdown enhances l-ase-induced apoptotic response in someAML cells.
238 citations
Cites background from "mTORC1 is essential for leukemia pr..."
...Recently, it was shown in a raptor deficiency mouse model thatmTORC1 inactivation induces apoptosis in differentiated leukemic cells and maintains immature leukemic cells with leukemia initiation potential in a dormant state, underlying the critical role of mTORC1 in leukemia.(4) In vitro, in primary AML cells, mTORC1 inhibition with rapamycin has cytostatic effects but does not induce apoptosis,(2-5) mainly because it does not inhibit 4E-BP1 phosphorylation on ser65....
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References
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TL;DR: PDK1 is required for activation of members of the AGC kinase family; it is shown that two such kinases, p70 S6 kinase (regulated via mTOR) and p90RSK1 (activated by Erk) phosphorylate eEF2k at a conserved serine and inhibit its activity.
Abstract: Elongation factor 2 kinase (eEF2k) phosphorylates and inactivates eEF2. Insulin induces dephosphorylation of eEF2 and inactivation of eEF2 kinase, and these effects are blocked by rapamycin, which inhibits the mammalian target of rapamycin, mTOR. However, the signalling mechanisms underlying these effects are unknown. Regulation of eEF2 phosphorylation and eEF2k activity is lost in cells in which phosphoinositide‐dependent kinase 1 (PDK1) has been genetically knocked out. This is not due to loss of mTOR function since phosphorylation of another target of mTOR, initiation factor 4E‐binding protein 1, is not defective. PDK1 is required for activation of members of the AGC kinase family; we show that two such kinases, p70 S6 kinase (regulated via mTOR) and p90 RSK1 (activated by Erk), phosphorylate eEF2k at a conserved serine and inhibit its activity. In response to insulin‐like growth factor 1, which activates p70 S6 kinase but not Erk, regulation of eEF2 is blocked by rapamycin. In contrast, regulation of eEF2 by stimuli that activate Erk is insensitive to rapamycin, but blocked by inhibitors of MEK/Erk signalling, consistent with the involvement of p90 RSK1 .
787 citations
"mTORC1 is essential for leukemia pr..." refers background in this paper
...Although S6 and eEF2K are substrates of p70S6K, it was also reported that p90 RSK can phosphorylate these proteins (28, 30)....
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TL;DR: This study provides a mechanistic basis for enhancing mTOR-targeted cancer therapy by combining an mTOR inhibitor with a PI3K or Akt inhibitor and shows that rapamycin combined with LY294002 exhibited enhanced inhibitory effects on the growth and colony formation of cancer cells.
Abstract: The mammalian target of rapamycin (mTOR) has emerged as an important cancer therapeutic target. Rapamycin and its derivatives that specifically inhibit mTOR are now being actively evaluated in clinical trials. Recently, the inhibition of mTOR has been shown to reverse Akt-dependent prostate intraepithelial neoplasia. However, many cancer cells are resistant to rapamycin and its derivatives. The mechanism of this resistance remains a subject of major therapeutic significance. Here we report that the inhibition of mTOR by rapamycin triggers the activation of two survival signaling pathways that may contribute to drug resistance. Treatment of human lung cancer cells with rapamycin suppressed the phosphorylation of p70S6 kinase and 4E-BP1, indicating an inhibition of mTOR signaling. Paradoxically, rapamycin also concurrently increased the phosphorylation of both Akt and eIF4E. The rapamycin-induced phosphorylation of Akt and eIF4E was suppressed by the phosphatidylinositol-3 kinase (PI3K) inhibitor LY294002, suggesting the requirement of PI3K in this process. The activated Akt and eIF4E seem to attenuate rapamycin's growth-inhibitory effects, serving as a negative feedback mechanism. In support of this model, rapamycin combined with LY294002 exhibited enhanced inhibitory effects on the growth and colony formation of cancer cells. Thus, our study provides a mechanistic basis for enhancing mTOR-targeted cancer therapy by combining an mTOR inhibitor with a PI3K or Akt inhibitor.
784 citations
"mTORC1 is essential for leukemia pr..." refers background in this paper
...org Volume 122 Number 6 June 2012 (32, 33), neither Raptor deficiency nor rapamycin resulted in hyperphosphorylation of AKT (S473) (Figure 6A and Supplemental Fig-...
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TL;DR: The unexpected discovery that rapamycin's ability to regulate cap-dependent translation varies significantly among cell types is reported, and it is shown that mTOR catalytic inhibitors are effective inhibitors of theRapamycin-resistant phenotype.
Abstract: The mammalian translational initiation machinery is a tightly controlled system that is composed of eukaryotic initiation factors, and which controls the recruitment of ribosomes to mediate cap-dependent translation. Accordingly, the mTORC1 complex functionally controls this cap-dependent translation machinery through the phosphorylation of its downstream substrates 4E-BPs and S6Ks. It is generally accepted that rapamycin, a specific inhibitor of mTORC1, is a potent translational repressor. Here we report the unexpected discovery that rapamycin's ability to regulate cap-dependent translation varies significantly among cell types. We show that this effect is mechanistically caused by rapamycin's differential effect on 4E-BP1 versus S6Ks. While rapamycin potently inhibits S6K activity throughout the duration of treatment, 4E-BP1 recovers in phosphorylation within 6 h despite initial inhibition (1-3 h). This reemerged 4E-BP1 phosphorylation is rapamycin-resistant but still requires mTOR, Raptor, and mTORC1's activity. Therefore, these results explain how cap-dependent translation can be maintained in the presence of rapamycin. In addition, we have also defined the condition by which rapamycin can control cap-dependent translation in various cell types. Finally, we show that mTOR catalytic inhibitors are effective inhibitors of the rapamycin-resistant phenotype.
784 citations
"mTORC1 is essential for leukemia pr..." refers background or result in this paper
...blasts (Supplemental Figure 1E), which is consistent with previous reports showing that 4E-BP1 is a rapamycin-insensitive mTORC1 target (24, 25)....
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...However, 4E-BP1, a direct target of mTORC1, is reportedly a rapamycin-insensitive substrate (24, 25)....
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...the phosphorylation of 4E-BP1, which is consistent with previous reports (24, 25)....
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TL;DR: A search for substrates of a growth-promoting kinase revealed a regulatory feedback loop involved in tumor suppression, and large-scale quantitative phosphoproteomics experiments were used to define the signaling networks downstream of mTORC1 and m TORC2.
Abstract: The evolutionarily conserved serine-threonine kinase mammalian target of rapamycin (mTOR) plays a critical role in regulating many pathophysiological processes. Functional characterization of the mTOR signaling pathways, however, has been hampered by the paucity of known substrates. We used large-scale quantitative phosphoproteomics experiments to define the signaling networks downstream of mTORC1 and mTORC2. Characterization of one mTORC1 substrate, the growth factor receptor-bound protein 10 (Grb10), showed that mTORC1-mediated phosphorylation stabilized Grb10, leading to feedback inhibition of the phosphatidylinositol 3-kinase (PI3K) and extracellular signal-regulated, mitogen-activated protein kinase (ERK-MAPK) pathways. Grb10 expression is frequently down-regulated in various cancers, and loss of Grb10 and loss of the well-established tumor suppressor phosphatase PTEN appear to be mutually exclusive events, suggesting that Grb10 might be a tumor suppressor regulated by mTORC1.
772 citations
TL;DR: It is shown that the Wnt/β-catenin signaling pathway is required for self-renewal of LSCs that are derived from either hematopoietic stem cells (HSC) or more differentiated granulocyte-macrophage progenitors (GMP).
Abstract: Leukemia stem cells (LSCs) are capable of limitless self-renewal and are responsible for the maintenance of leukemia. Because selective eradication of LSCs could offer substantial therapeutic benefit, there is interest in identifying the signaling pathways that control their development. We studied LSCs in mouse models of acute myelogenous leukemia (AML) induced either by coexpression of the Hoxa9 and Meis1a oncogenes or by the fusion oncoprotein MLL-AF9. We show that the Wnt/beta-catenin signaling pathway is required for self-renewal of LSCs that are derived from either hematopoietic stem cells (HSC) or more differentiated granulocyte-macrophage progenitors (GMP). Because the Wnt/beta-catenin pathway is normally active in HSCs but not in GMP, these results suggest that reactivation of beta-catenin signaling is required for the transformation of progenitor cells by certain oncogenes. beta-catenin is not absolutely required for self-renewal of adult HSCs; thus, targeting the Wnt/beta-catenin pathway may represent a new therapeutic opportunity in AML.
726 citations
"mTORC1 is essential for leukemia pr..." refers background or result in this paper
...Previous reports on this AML model have indicated that c-Kit marks undifferentiated AML cells (17, 18, 31)....
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...on this AML model have shown that AML stem cells are highly enriched in the c-Kit+Gr-1– (K+G–) population (31)....
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